tesis con dft y td-dft

PHOTOPHYSICAL PROPERTIES OF METALLOTETRAPHENYLTETRABENZOPORPHYRINS: INSIGHTS FROM

EXPERIMENTAL AND THEORETICAL STUDIES

G. V. Nepali Rajapakse

A Dissertation

Submitted to the Graduate College of Bowling Green State University in partial fulfillment of

the requirements for the degree of

DOCTOR OF PHILOSOPHY

August 2008

Committee:

Michael A. J. Rodgers, Advisor

Sri Kolla Graduate Faculty Representative

Michael Y. Ogawa

Deanne L. Snavely

ii

ABSTRACT

Michael A. J. Rodgers, Advisor

Porphyrins are the most widely studied tetrapyrrole-macrocycles because of their

diverse structures with unique properties and wide distribution in nature. Variations of the

peripheral substituents on the porphyrin ring and the insertion/change of metal atoms into the

macrocycle usually change the visible absorption spectrum. For this reason, in recent years,

(metallo)porphyrins have become of major interest for applications in opto-electronics, data

storage, solar cells and photomedicine. Metallo-tetraphenyltetrabenzoporphyrins (MTPTBPs)

are different from other porphyrins because of the combination of electronic and structural

factors, such as extension of the π-system, meso-substitution and highly distorted macrocycle.

Recent development in photothermal therapy (PTT) has drawn extra attention to this type of

molecules. For a compound to be an effective photothermal agent, it should possess high

photostability, high molar absorption coefficient in the red spectral region and fast

radiationless decay of the excited states, converting the photon energy into the thermal event.

Metallotetrapyrroles with first row transition metal centers have been shown to

undergo fast radiationless deactivation of their excited states since the metal incorporated into

the macrocycle cavity introduces a manifold of electronic states with metal-centered and/or

metal+π–system character, some of which can be situated at lower energy than the pure π,π*

states that are populated by a photo-excitation process. These low-lying states provide rapid

non-radiative channels for the excited deactivation, which is an optimum situation for PTT.

Within this context, this dissertation has focused on tetrabenzoporphyrins coordinated

with different metals to investigate central metal effects on the photophysical properties.

Special attention has given to first row transition metals Cr(III), Mn(III), Co(II), Cu(II) and

iii

Zn(II) while 3rd row Pt (II), analogue has been studied for comparison purposes. Only the

theoretical investigations of Ni(II) and Pd(II) analogues have performed to give a full picture

about the nature of the deactivation mechanism. Previously, Ni(II)TPTBP has studied in this

laboratory in both experimental and theoretical points of view.

Zn(II)TPTBP was investigated in order to provide experimental evidence for the

spectral properties of the π-localized singlet and triplet states of the tetrapyrrole macrocycle.

It showed that the formation of the singlet (π,π*) state decayed through fluorescence to the

ground state and the intersystem crossing to the triplet state(π,π*). The produced singlet state

was vibrationally hot and after cooling it decayed to the triplet state having ca. 340 ps

lifetime in pyridine. Triplet state of Zn(II)TPTBP deactivated to the ground state with ca. 236

μs lifetime in pyridine.

Compared with Zn(II)TPTBP, Pt(II)TPTBP showed that π localized S1 state

undergoes fast intersystem crossing (ca.500 fs lifetime) to triplet state. Low fluorescence

quantum yield (0.0003) was observed compare to Zn analogue and it showed a high yield of

phosphorescence. The triplet state lifetime found to be ca. 41 μs and it was shorter than

Zn(II) triplet state showing that faster intersystem crossing initiated from the spin orbit

interaction introduced by heavy Pt atom in the third row transition series.

Co(II), Cu(II) ,Mn(IIІ), Cr(IIІ) has introduced different features to the picture of the

excited state deactivation mechanism having initiated by the unpaired metal electron.

Co(II)TPTBP, excited singlet (π,π*) has converted to the π localized triplet state within the

instrument response time and then it was converted to hot d,d state, wherein intramolecular

cooling has occurred and completed within 3 ps. After cooling the d,d state decayed into the

ground state in an exponential manner having 17 ps lifetime in hexane solution. The TDDFT

results indicated that the lowest d,d transition arises from a fully occupied metal dπ orbital to

a partially occupied metal dz2 orbital and could be responsible for the observed d,d state.

Cu(II)TPTBP ground state repopulation was occurred through the set of trip-doublet 2T1 and

iv

trip-quartet 4T1 states which is in a equilibrium via a lower lying LMCT states. The

dependence of the observed lifetime on solvent polarity confirmed the participation of the

LMCT states in the overall deactivation process. The repopulation was completed within 500

ps in toluene solution.

After 640 nm excitation in Cr(ΙΙΙ)TPTBPCl, the S1 state underwent fast intersystem

crossing (4S1→4T1) within a very short period of time (ca.0.05 ps lifetime) to 4T1 state.

The 4T1 state of Cr(ΙΙΙ)TPTBPCl in toluene deactivated with a lifetime of 224 ps, resulting the

4T1↔ 6T1 equilibrium.

In Mn(ΙΙΙ)TPTBPCl, the excited singquintet, 5S1(π,π*) state deactivated to the

tripquintet, 5T1(π,π*) within the instrument response time. After short time period, it

generated the hot d,d state wherein cooling had occurred within 4 ps and cooled d,d state

repopulated the ground state having 120 ps lifetime in toluene.

Transient absorption spectrometry with femtosecond and nanosecond time resolution

has been employed along with DFT/TDDFT theoretical examinations to investigate the

sequence of events that follow Q band photo-excitation. Overall, the results of the present

investigation reported a complete picture of the nature and energies of all electronic states

induced by the different metals on the photophysical properties

tetraphenytetrabenzoporphyrins.

v

To my parents

vi

ACKNOWLEDGMENTS

The years of graduate studies have been the most fruitful period of my life. During

this period, I have grown both as a scientist and as a person. All the experience I gained from

here helped me develop into the person I am now. Bowling Green was my home away from

home. I would like to acknowledge all those that helped with this adventure by shining the

light on my path and making sure that I did not get lost.

My heartiest and foremost gratitude goes to my adviser Dr. Michael A. J. Rodgers.

You paved the way for my career. Your support, guidance and encouragement helped me to

develop myself as a scientist as well as a strong person to be able to face the challenges in

this society. Dr. Rodgers, thank you very much for believing in me and giving me the

freedom to work independently. I am sure the experience of having you as a great adviser

will be in my mind for the rest of my life.

I would like to extend my gratitude to my committee, Dr. Michael Ogawa, Dr.

Deanne Snavely and Dr. Sri Kolla. You have supported me in enormous ways. You were

with me when I was in trouble and I am fortunate to have all of you as my committee

members.

My special thanks go to Dr. Alexandra Soldatova for giving me a jump start to my

research career. I really appreciate all of your help to succeed my thesis work within a very

short period of time. I have learned so many things from you and I hope your valuable advice

as a colleague and friend will continue in the future.

I would like to thank the Department of Chemistry and the Center for Photochemical

Sciences for giving me this great opportunity to pursue my Ph.D. career. Especially, I want to

thank Nora and Alita for your valuable advice and for making the department a warm and

friendly place. Also I want to thank Doug, Craig and Larry for their technical support.

Also, thank you to all the members of Dr. Rodgers’ group, past and present. Special

thanks go to Dr. Eugene Danilov.

vii

Apart from making a way for my career, the other most important asset I’ve gained

here was my friends. I want to thank all of my friends who helped me make my journey at

BG a pleasant and enjoyable place. Especially, I want to thank Chinthaka, Atty and Lilani for

sharing my sorrows and laughter together.

I wish to express my deepest thanks to my parents without whom I would not be the

person I am now. Your inspiration and encouragement helped me to succeed and I hope I

made your dream come true.

viii

TABLE OF CONTENTS

Page

CHAPTER I. INTRODUCTION.......................................................................................... 1

Basic Photophysics .................................................................................................... 1

Electronic structure of Metallotetrapyrroles .............................................................. 2

Metal-Macrocycle interaction and UV-visible spectra ................................. 2

Metallotetrapyrroles: Background ............................................................................. 8

Bond-Type in metalloporphyrins ................................................................... 13

Symmetry in Metalloporphyrins..................................................................... 14

Excited states of porphyrins and characterization of excited states

of Porphyrins............................................................................................................. 15

π,π* state of the ring ...................................................................................... 15

d,d ligand field states ..................................................................................... 16

(π,d) and (d, π*) metal ↔ ring CT states....................................................... 16

Use of tetrapyrrolic photosensitizers for cancer treatment ........................................ 17

The effect of the extension of porphyrin ring and meso-phenyl substitution ............ 20

Effect of saddling ........................................................................................... 21

DFT/TDDFT Methods: Background ......................................................................... 22

Glossary of acronyms ................................................................................................ 24

References .................................................................................................................. 24

CHAPTER II. MATERIALS AND METHODS.................................................................. 30

Materials .................................................................................................................... 30

Instrumentation and Methods .................................................................................... 30

UV-visible Absorption Spectrometry ............................................................. 30

Steady-State Fluorescence Spectroscopy....................................................... 31

Femtosecond Transient Absorption Spectroscopy......................................... 31

ix

Nanosecond transient Absorption.................................................................. 34

Computational Details ............................................................................................... 36

Estimation of the MTPTBP triplet (π,π*) energies from the oxygen quenching

bimolecular rate constant ........................................................................................... 38

References ................................................................................................................ 39

CHAPTER III. THE PHOTOPHYSICAL PROPERTIES OF ZINC AND NICKEL

TETRAPHNYLTETRABENZOPORPHYRINS: A COMBINED EXPERIMENTAL

AND THEORETICAL INVESTIGATION........................................................................... 41

Abstract ................................................................................................................... 41

Introduction................................................................................................................ 41

Theoretical characterization of Zn(ΙΙ)TPTBP............................................................ 44

Ground State Molecular Structure Analysis.................................................. 44

Ground State Electronic Structure Analysis.................................................. 46

Excited States and Ground State Absorption Spectra ................................... 48

Optically Silent Excited States Below the S1(ππ*) State ................................ 50

Steady-State Fluorescence and Transient Absorption Experiments .......................... 51

Steady-State Fluorescence Measurements..................................................... 51

Transient Absorption Experiments ............................................................................ 53

Femtosecond Transient Absorption ............................................................... 53

Nanosecond Transient Absorption Experiment ............................................. 59

Summary and Conclusions ........................................................................................ 61

References .................................................................................................................. 62

x

CHAPTER IV. THE PHOTOPHYSICAL BEHAVIOR OF OPEN-SHELL FIRST-ROW

TRANSITION METAL TETRAPHENYLTETRABENZOPORPHYRINS: COPPER

AND COBALT ..................................................................................................................... 67

Abstract ..................................................................................................................... 67

Introduction................................................................................................................ 68

Theoretical characterization of Co(ΙΙ)TPTBP and Cu(ΙΙ)TPTBP.............................. 70



a) HOMO and LUMO relative positions ........................................... 73

b) the metal d levels ........................................................................... 75

c) Spin unrestricted calculations ....................................................... 76


Ground State Absorption Spectra ...................................................... 77

Excited States Below the S1(π,π*)State .............................................. 79

Transient Absorption Experiments: Spectral observations and

Dynamic properties.................................................................................................... 82

Co(ΙΙ)TPTBP in Hexane ................................................................................ 82

Cu(ΙΙ)TPTBP in Toluene................................................................................ 87

Cu(II)TPTBP in Benzonitrile ......................................................................... 88


References .................................................................................................................. 94

CHAPTER V. THE PHOTOPHYSICAL PROPERTIES OF CHROMIUM(ΙΙΙ) AND

MANGANESE(ΙΙΙ) TETRAPHNYLTETRABENZOPORPHYRINS: AN INSIGHT

INTO EXCITED STATE DECAYDYNAMICS .................................................................. 97

Abstract ...................................................................................................................... 97

Introduction ............................................................................................................... 98

xi

Photophysical Properties of Cr(ΙΙΙ)TPTBPCl............................................................ 101

Ground State Absorption Spectra .................................................................. 101

Transient Absorption: Spectral Observations and Dynamic Properties of

Cr(ΙΙΙ)TPTBPCl ............................................................................................. 103

Cr(ΙΙΙ)TPTBPCl in toluene and DMF................................................ 103

Cr(ΙΙΙ)TPTBPCl in Benzonitrile ........................................................ 106

Photophysical Properties of Mn(ΙΙΙ)TPTBPCl ......................................................... 108

Ground State Absorption Spectra ..................................................................... 108

Transient Absorption: Spectral Observations and Dynamic properties of

Mn(ΙΙΙ)TPTBPCl ............................................................................................... 110

Mn(ΙΙΙ)TPTBPCl in Toluene .......................................................................... 110

Mn(ΙΙΙ)TPTBPCl in Benzonitrile ................................................................... 113


References .................................................................................................................. 116

CHAPTER VI.HEAVY –METAL EFFECT ON THE PHOTOPHYSICAL

PROPERTIES OF PLATINUM TETRAPHYNEYTETRABENZOPORPHYRINS:

INSIGHT FROM EXPERIMENTAL AND RELATIVISTIC DFT/TDDFT STUDIES...... 118

Abstract ..................................................................................................................... 118

Introduction................................................................................................................ 118

Theoretical characterization of Pt(ΙΙ)TPTBP: heavy- metal effect............................ 121



a) HOMO and LUMO relative positions ........................................... 124

b) the metal d levels ........................................................................... 127


Ground State Absorption Spectra ...................................................... 128

xii

Optically Silent Excited States Below the S1(ππ*) State .................... 131

Steady State Luminescence and Transient Absorption Experiments ........................ 131

Steady State Luminescence ............................................................................ 131

Transient Absorption Experiments ................................................................ 134

Femtosecond Transient Absorption ................................................... 134

Nanosecond Transient Absorption..................................................... 136

Oxygen quenching of the triplet state ........................................................................ 139

Summary and Conclusions ................................................................ 140

References.......................................................................................... 142

CHAPTER VII. CONCLUSIONS ......................................................................................... 145

APPENDIX A: ENERGIES AND PERCENTAGE COMPOSITION OF THE HIGHEST

OCCUPIED AND LOWEST UNOCCUPIED MOLECULAR ORBITALS OF

M(ІІ)TPTBP, M= Zn, Co, Cu, Pt, Pd EXPRESSED INTERMS OF

INDIVIDUAL ATOMS.................................................................................................... 148

xiii

LIST OF FIGURES

Page

1.1 Jablonski diagram to depict the energy relationship and rate processes

between electronic states .......................................................................................... 1

1.2 Structures of different types of tetrapyrrole macrocycles.......................................... 3

1.3 Porphyrin HOMOs and LUMOs................................................................................ 4

1.4 Schematic Orbital diagram showing possible transitions .......................................... 5

1.5 A typical absorption spectrum of a metalloporphyrin ............................................... 6

1.6 Origin of the hypsochromic shift of the Q band ........................................................ 7

1.7 d metal orbitals in porphyrins .................................................................................... 14

1.8 Mechanism of action of PDT..................................................................................... 17

1.9 Definition of rotation angle θ of the phenyl plane between an upward

tilted pyrrole and a downward tilted pyrrole ............................................................. 21

2.1 The schematic representation of the ultrafast transient absorption setup .................. 34

2.2 Schematic diagram of the nanosecond laser flash photolysis system........................ 36

3.1 Top view of the DFT-optimized molecular structure of Zn(ΙΙ)TPTBP ..................... 44

3.2 Energy level scheme for Zn(ΙΙ)TPTBP and Ni(ΙΙ)TPTBP......................................... 46

3.3 Contour plots of the Zn(ΙΙ)TPTBP, together with the energy levels ......................... 47

3.4 Ground state absorption spectra of M(ΙΙ)TPTBP complexes in toluene

M= Zn, Ni .................................................................................................................. 48

3.5 Normalized absorption and steady state fluorescence spectra of Zn(ΙΙ)TPTBP

in pyridine at room temperature................................................................................. 52

3.6 Femtosecond transient absorption difference spectra of Zn(ΙΙ)TPTBP in pyridine .. 53

3.7 Femtosecond transient absorption difference spectra of Zn(ΙΙ)TPTBP in

benzonitrile at 2 ps and 1000 ps................................................................................. 54

xiv

3.8 Femtosecond kinetics profiles of the transient absorption signal at

various probe wavelengths for Zn(ΙΙ)TPTBP in pyridine, excited at 640 nm ........... 55

3.9 Zn(ΙΙ)TPTBP in pyridine; first lifetime/wavelength dependence.............................. 57

3.10 Normalized kinetic profiles of the transient absorption signal at 520 nm for

Zn(ΙΙ)TPTBP in benzonitrile and pyridine solutions after 640 nm

excitation.................................................................................................................... 58

3.11 Nanosecond transient absorption difference spectra of Zn(ΙΙ)TPTBP in Ar

saturated pyridine_toluene solution at different delay times followed by

460 nm excitation....................................................................................................... 59

3.12 Nanosecond kinetic profile of the transient absorption signal in Ar saturated

pyridine_toluene solution, λexc= 460 nm .................................................................. 60

3.13 Kinetic diagram for the excited state deactivation of Zn(ΙΙ)TPTBP ......................... 62

4.1 Side view of the DFT optimized molecular structure of M(ІІ)TPTBP,

M= Cu, Co ................................................................................................................. 70

4.2 Energy level diagram for Co-, Ni- and CuTPTBP. Orbital energies are obtained

from spin-restricted calculations................................................................................ 72

4.3 Energy level scheme for Co(ΙΙ)TPTBP together with contour plots of orbitals....... 74

4.4 Energy level scheme for Cu(ΙΙ)TPTBP together with contour plots of orbitals........ 74

4.5 The energy level diagram obtained from spin unrestricted calculations for

Co(ΙΙ)TPTBP and Cu(ΙΙ)TPTBP................................................................................ 76

4.6 The 23a1 orbital in Co(ΙΙ)TPTBP and the 23b2 orbital Cu(ΙΙ)TPTBP ....................... 77

4.7 Normalized ground state absorption spectra of M(ΙΙ)TPTBP complexes with

M = Cu, Ni in toluene and Co in hexane ................................................................... 78

4.8 The theoretically predicted dark states lying below the Q state ................................ 80

4.9 Transient absorption behavior of Co(ΙΙ)TPTBP in hexane excited at 400 nm .......... 83

xv

4.10 Two time regimes of the transient absorption spectra of Co(ΙΙ)TPTBP in

hexane excited at 635 nm........................................................................................... 84

4.11 Kinetic profiles of the transient absorption signal at 500 nm and 680 nm probe

wavelengths of Co(ΙΙ)TPTBP in hexane after 635 nm excitation ............................. 85

4.12 Spectra recorded immediately after a laser pulse at 400 nm and at 3 ps later in

Co(II)TPTBP in hexane ............................................................................................. 86

4.13 Temporal evolution of the transient absorption spectra of Cu(ΙΙ)TPTBP in

toluene (2.8 μM), excited at 640 nm.......................................................................... 88

4.14 Evolution of the absorption spectra of Cu(ΙΙ)TPTBP in benzonitrile, after

excitation at 640 nm................................................................................................... 89

4.15 Time profiles of the transient absorption signal at two probe wavelengths for

Cu(ΙΙ)TPTBP in toluene after photoexcitation at 640 nm ......................................... 90

4.16 Normalized kinetic profiles of the transient absorption signal at 653 nm for

Cu(ΙΙ)TPTBP in two different solvents after 640 nm excitation ............................... 90

4.17 Proposed schematic diagrams for the excited state relaxation pathways of

Co(ІІ)TPTBP and Cu(ІІ)TPTBP................................................................................ 94

5.1 Orbital energy levels for Cr and Mn porphyrin complexes with trivalent metal

chlorides by iterative extended Hückel method......................................................... 101

5.2 Normalized ground state absorption of Cr(ΙΙΙ)TPTBPCl in toluene, DMF and

benzonitrile solutions................................................................................................. 102

5.3 Transient absorption spectra of Cr(ΙΙΙ)TPTBPCl in Toluene, excited at 400 nm;

Transient absorption spectra of Cr(ΙΙΙ)TPTBPCl in DMF, excited at 640 nm.......... 103

5.4 Kinetic behavior of Cr(ΙΙΙ)TPTBPCl in toluene at 530 nm after 640 nm

excitation.................................................................................................................... 105

5.5 Transient absorption spectra of Cr(ΙΙΙ)TPTBPCl in benzonitrile solution

following 640 nm excitation ...................................................................................... 106

xvi

5.6 The kinetic profile of Cr(ΙΙΙ)TPTBPCl in benzonitrile solution following 640 nm

excitation.................................................................................................................... 106

5.7 Absorption spectra of Mn(ΙΙΙ)TPTBPCl in toluene, benzonitrile and in DMF ......... 109

5.8 Transient absorption spectra of Mn(ΙΙΙ)TPTBPCl in toluene solution following

400 nm excitation....................................................................................................... 111

5.9 Kinetic behavior of the Mn(ΙΙΙ)TPTBPCl in toluene solution at 530 nm

and 686 nm followed by 400 nm excitation............................................................... 112

5.10 The spectral cuts taken from early time and later times of spectral evolution of

Mn(ΙΙΙ)TPTBPCl in benzonitrile solution following 400 nm excitation................... 113

5.11 Femtosecond kinetic profiles of the transient absorption signals of

Mn(ΙΙΙ)TPTBPCl in benzonitrile solution after excited at 640 nm ........................... 114

5.12 Proposed schematic diagram for the excited state relaxation pathways

of Cr(ΙΙΙ)TPTBPCl and Mn(ΙΙΙ)TPTBPCl ................................................................. 115

6.1 Top view and side view of the DFT-optimized molecular structure of

Pt(ΙΙ)TPTBP together with atom labeling.................................................................. 118

6.2 Energy level scheme for M(ΙΙ)TPTBP, M= Ni, Pd, Pt .............................................. 124

6.3 Contour plots of the MOs of Pt(ΙΙ)TPTBP ................................................................ 127

6.4 Ground state absorption of M(ΙΙ)TPTBP (M= Pt and Zn) in toluene solution;

extinction coefficients are given for absorbance data................................................ 129

6.5 Normalized room temperature absorption and fluorescence spectra and

phosphorescence spectra in toluene solution ............................................................. 132

6.6 Evolution of the transient absorption spectra of Pt(ΙΙ)TPTBP in toluene

excited at 400 nm and 610 nm ............................................................................... 134

6.7 Kinetic profile of the Pt(ΙΙ)TPTBP in toluene solution reported at 530 nm

upon excitation at 400 nm; kinetic trace at 584 nm upon excitation at 610 nm ........ 135

xvii

6.8 Nanosecond transient absorption spectra of Pt(ΙΙ)TPTBP in Ar-saturated toluene

at different delay times followed by 430 nm excitation ............................................ 137

6.9 Nanosecond kinetic profiles of the transient absorption signals of Pt(ΙΙ)TPTBP,

excited at 430 nm....................................................................................................... 138

6.10 Proposed excited state deactivation mechanism for Pt(ΙΙ)TPTBP............................. 141

xviii

LIST OF TABLES Page

1.1 Symmetry presentation of metal and porphyrin orbitals in metal

porphyrinates ............................................................................................................. 15

3.1 Selected bond distances (Ao) and bond angles(deg) calculated for

M(ΙΙ)TPTBP in the D2d confirmation ........................................................................ 45

3.2 Vertical excitation energies (Eva) and oscillator strengths (f) computed for the

lowest optically allowed excited states of Zn(ΙΙ) and Ni(ΙΙ)TPTBP.......................... 49

3.3 Excitation energies (eV) obtained for the low-lying excited states of

Zn(ΙΙ)TPTBP ............................................................................................................. 51

3.4 The photophysical properties of Zn(ΙΙ)TPTBP.......................................................... 61

4.1 Selected bond lengths (Å) and bond angles (deg) calculated for MTPTBP in the

saddled D2d conformation .......................................................................................... 79

4.2 Calculated vertical excitation energies (Eva) and oscillator strengths (f) for the

optically allowed excited states of MTPTBP (M = Co(II), Ni(II), and Cu(II))

in the D2d confirmation and compared to experimental data ..................................... 80

4.3 Excitation Energies (eV), composition and character of the lowest excited

states of Co(ΙΙ)TPTBP ............................................................................................... 81

4.4 Excitation Energies (eV), composition and character of the lowest excited

states of Cu(ΙΙ)TPTBP ............................................................................................... 82

6.1 Comparison of the optimized theoretical values of the selected bond lengths (Ao)

and bond angles (deg) with x-ray data for the Pt(ΙΙ)TPTBP...................................... 122

6.2 Selected bond distance and bond angles calculated for MTPTBP, M=Pd, Pt, Ni ..... 123

xix

6.3 Vertical excitation energies and oscillator strengths (f) computed for the

optically allowed excited states of M(ΙΙ)TPTBP(M= Zn, Pt) responsible for the

appearance of the Q band and B band and compared with experimental data .......... 130

6.4 Excitation energies (eV) calculated for the low-lying excited states of

Pt(ΙΙ)TPTBP ............................................................................................................... 131

6.5 Photophysical properties of M(ΙΙ)TPTBP(M = Zn, Pd, Pt) ....................................... 133

6.6 Equilibrium derived triplet state energies for Pt and Zn complexes. Pt is

in toluene solution and Zn in 1% pyridine-toluene solution...................................... 140

1

CHAPTER 1: INTRODUCTION

Basic Photophysics

The absorption of a photon creates excited states of the molecules that are unstable

and the excess of energy will be dissipated in various routes as the molecule returns to its

ground state. Figure 1.1 illustrates the Jablonski diagram1 that is useful compilation of the

states and processes that may be involved when excited states deactivate to the ground state.

The singlet ground state is depicted by the horizontal thick line labeled as S0 and higher

singlet electronic states are represented as S1, S2 and Sn. Triplet states are labeled with T in a

similar manner. Within each electronic state, vibrational energy states exist and are

represented using thin lines. The absorption of light by ground state molecules are depicted

by upward blue arrows.

A molecule in its ground state can be excited to any number of singlet states (S1, S2).

After excitation to S2, the molecule usually rapidly and non-radiatively relaxes to the lowest

vibrational level of S1.

Figure 1.1. Jablonski diagram to depict the energy relationship and rate processes between

electronic states.

Gro

und

stat

e

S0

S1

S2

Sn

C

ISC IFl

uore

scen

ce

Phos

phor

esce

nc

ISC

VR

VR

IC

2

The vibrational cascade of S1 continues until there is a crossing between S1 and T1. In

that crossing region an isothermal intersystem crossing process can occur resulting in

population of the triplet state. Relaxation to the ground state takes place either radiatively

(fluorescence and phosphorescence) or non-radiatively (internal conversion-IC and

intersystem crossing-ISC). In fact, absorption and fluorescence are inverse processes.

Fluorescence is the radiative transition between states of like multiplicity e.g.S1→ S0.

Phosphorescence is a radiative transition between states of unlike multiplicity e.g.T1→ S0.

Internal conversion (IC) is the radiationless transition between states of the same multiplicity

(S2→S1) and intersystem crossing process occurs when it occurs between the different spin

states (S1→T1). Vibrational relaxation is also a radiationless transition and it is from higher

vibronic level to the lower one within the same electronic state.

Electronic structure of Metallotetrapyrroles

Metal-Macrocycle interaction and UV-visible spectra

Complexes with various tetrapyrrole ligands namely porphyrins and porphyrazines

have attracted considerable interest because of diverse potential applications. Some

porphyrins are naturally occurring and are involved in a wide variety of biological processes.

Porphyrins consist of four pyrrolic units, linked together by four methine (=CH-) bridging

groups. Figure 1.2 illustrates the different compounds derived from the basic structure of

cyclic tetrapyrroles. Porphyrazines are synthetic porphyrins differ from porphyrins due to the

presence of aza bridges (nitrogen atoms) instead of methine links.

3

NH

HN

N

NN

N

N

N

Cmeso

CαNH

N

N

HN

Cβ

Phthalocyanine Porphyrin

NH

HN

N

N

NH

HN

N

N

Tetrabenzoporphyrin Tetraphenyltetrabenzoporphyrin

Figure 1.2. Structures of different types of tetrapyrrole macrocycles.

Phthalocyanines and naphthalocyanines also contain the basic structure of

porphyrazines, but they differ by the presence of benzo rings fused to the pyrrole rings.

Porphyrins are aromatic and they conform to the Hückel aromaticity condition in which 4n+2

π electrons (n=5) are delocalized over the macrocycle. A porphyrin has eight β positions and

four meso- positions available for substitution.

Porphine, the macrocycle having four pyrrole rings is the parent structural compound

for porphyrins. Following removal of the two protons the four pyrrolic nitrogen atoms

become equivalent and the compound can act as a tetradentate ligand for binding a variety of

4

metal ions. The cavity size of the macrocycle is such that most metal ions can be coordinated

therein. Many of the resulting metalloporphyrins show important biological activities: as iron

complexes in the hemoproteins, as magnesium complexes in the chlorophylls and as a cobalt

complex in Vitamin B12.2

By varying the choice of the metal center, the bonding and characteristics of the

metalloporphyrin may be critically affected. This diversity is one reason why

metalloporphyrins have found many applications in modern life, such as magnetic materials,3,

4 photoconductive materials, non-linear optical materials5 and tumor photo-therapeutic

drugs.6 Metalloporphyrins (MPs) generally have high thermal stability and show strong

electronic transitions in the visible and ultraviolet regions. Their electronic states deactivate

on picosecond, nanosecond and millisecond time scales and a variety of intermediate states

have been identified.7, 8 In order to evaluate the applications of MPs, it is necessary to

understand the electronic structure and photophysical properties, particularly the mechanism

of excited state deactivation.

Figure 1.3. Porphyrin HOMOs (bottom) and LUMOs(top); adapted from reference 9 Electron

densities are shown in red and blue.

5

To interpret their various electronic states, optical absorption spectra and

luminescence properties, Martin Gouterman first proposed a four-orbital model in the 1960s

and it remains an effective model for explaining the absorption spectra of porphyrns.7 This

model is depicted in Figure 1.3 and it illustrates the electronic density distributions in the

orbitals. The a1u and a2u orbitals are HOMOs and the two LUMOs are identified as eg(π*). As

seen in the Figure 1.3, the HOMO a2u orbital is mainly localized on the pyrrolic nitrogen and

meso- carbon atoms while HOMO-1 a1u has contribution mainly from the Cα and Cβ atoms.

The LUMOs are delocalized on the porphyrin ring. For a molecule with D4h symmetry, the

eg(π*) orbitals are strictly degenerate whereas the two a orbitals are nearly degenerate.10 The

electronic configuration for porphyrin in the ground state is (a1u)2 (a2u)2 with 1A1g character.

a1u(π)

a2u(π)

eg(π∗)

dxy

dyz

dxz

dz2

dx2

-y2

π−π∗d-π∗

π-d

d-d

Porphyrin Metal

Figure 1.4. Schematic orbital diagram showing possible transitions of metalloporphyrin

adapted from ref 11

6

Figure 1.4 describes the schematic diagram showing the possible transitions of

metalloporphyrins. The absorption bands arise from transition between two HOMOs and two

LUMOs (π→π*). Accordingly the lowest singlet excited configurations are 1(a1u, eg) and

1(a2u,eg) both having the Eu character. The Q bands are the result of the transition dipoles

nearly canceling each other out, resulting in a weaker absorption band. The higher energy B

band transition results from a linear combination of the two transitions adding the transition

dipoles and it is intense.11 In the free base porphyrins, the central protons lift the degeneracy

of the eg orbitals causing the Q bands to split into Qx and Qy.10 Typical ground electronic

state absorption spectrum of a metalloporphyrin is shown in Figure 1.5.

300 350 400 450 500 550 600

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Q bands

Soret or B band

Abso

rban

ce

Wavelength/nm

Figure 1.5. A typical absorption spectrum of a metalloporphyrin.

The four orbital model has been very effective in explaining the effects of various

substituents, central metal ions and extraneous ligands.10 As shown in Figure 1.3, MO

calculations of the porphine core have shown that the a1u orbital has nodes at the pyrrole

nitrogens and it cannot directly interact with metal. It also has nodes at the meso- carbons and

is not expected to be influenced by meso substitution. In contrast, the a2u orbital should be

strongly affected by meso substituents. On the other hand the a2u orbital puts less charge at

7

the β carbons of the pyrrole rings than a1u orbital does. As a result the a1u level is shifted

when compared to the pyrrole substituted porphyrins. Less electronegative metals shift a2u to

higher energy and more electronegative metals stabilize a2u.

Hypso porphyrins show blue shifts of the absorption spectra compared to regular

porphyrins. If the metal possesses filled d orbitals, dπ electron donation from the dxz and dyz

orbitals to the empty eg- π*- orbitals of the porphyrin may occur, raising the eg π* orbitals and

lowering the dπ orbitals which have become bonding. (metal to ring π back bonding, Figure

1.6). As a result the energy gap between the LUMO and HOMO will increase showing the

hypsochromic shift.

a1u

a2u

eg

dxy

Figure 1.6. Origin of the hypsochromic shift of the Q band. Taken from ref 12

Normally the replacement of a 3d metal ion by one having 4d or 5d orbitals

gradually enhances back-bonding because the 4d or 5d orbitals have a larger radial expansion

and thus penetrate the antibonding orbitals more. Indeed a large increase in the hypsochromic

shift of the Q band in the series NiΙΙ < PdΙΙ < PtΙΙ can be seen indicating the increase in metal-

to- porphyrin back-donation.

Hyper absorption spectra show extra absorption bands in addition to Q and B bands.

dyzdxzπ−π∗

Normal Metal(d6)

π∗

π

eg

Hypso-Type

8

Metallotetrapyrroles: Background

Metallo-tetraphenyltetrabenzoporphyrins (MTPTBPs) are an interesting class of

tetrapyrrole chromophores because of the extension of the π-system and meso-substitution. In

these porphyrins, a combination of electronic and structural factors, such as extension of the

π-system, meso-substitution and highly distorted macrocycle core causes a red-shift and

intensification of the Q band absorption as compared to other MPs13 making these complexes

potentially useful in diverse practical applications. One such application is photothermal

therapy (PTT), which is a promising approach to tumor treatment.8, 14 In addition to having a

high molar decadic extinction coefficient in the red spectral region, where light penetration of

tissue is optimal, efficient photothermal sensitizers for PTT should also show fast

radiationless decay of the excited states, converting the photon energy into vibrational energy

of the ground state, thereby initiating thermal events that can be lethal to the cell.

The ability of a compound to photothermally destroy cancer cells depends on how

rapidly the initial photo-produced excited state deactivates to the ground state surface. This,

in turn, depends on the nature and the lifetime of the electronic states lying between S1 and

the ground state -the so-called silent states. The energy and nature of these states are critically

dependent on the central metal.

Metallotetrapyrroles with first row transition metal centers (e.g. Co, Fe, Ni, Cu) have

been shown to undergo fast radiationless deactivation of their excited states because the metal

incorporated into the macrocycle cavity introduces a manifold of electronic states with metal-

centered and/or metal+π–system character, some of which can be situated at lower energy

than the pure π,π* states that are populated by a photo-excitation process.15 These low-lying

states may provide rapid non-radiative channels for excited state deactivation, which is an

optimum situation for PTT.

9

The effect of the central metal on the photophysical properties of MPs, especially the

Ni(II) variants, has been investigated intensively.10, 16-34 The results of extended Hückel

calculations and the lack of luminescence supported the idea that the normally emissive

1,3(π,π*) excited states of the porphyrin ring deactivate rapidly through low-lying metal

centered 1,3(dz2,dx2-y2) states.7, 16 Holten and et al showed that in nickel porphyrins such as

Ni(II)OEP and Ni(II)TPP, the (d,d) state forms within <1 ps, followed by deactivation to the

ground state in 200-500 ps.28-30, 35

In porphyrins with Zn(II) centers where the metal has a d10 electronic configuration,

the interactions between the porphyrin π-orbitals and the metal are negligibly small.6 The

deactivation of Zn(II)P singlet state S1 occurs largely through the T1 triplet state with some

raditive process. The quantum yield of triplet state for Zn(II)TMPyP was found to be 0.98

and for Zn(II)TPPS and Zn(II)TPP, it was 0.96.36 The triplet state deactivates with the

lifetime of 1.3 ms for Zn(II)TMPyP, 1.4 ms for Zn(II)TPPS and 1.2 ms for Zn(II)TPP.36 The

phosphorescence quantum yields for these compounds are <10-4 at 298 K due to efficient

non-radiative relaxation of the triplet state. Chen and et al33 have investigated the transient

spectra and kinetics of different benzoporphyrin derivatives including Zn(II)TPTBP in

benzene solution by means of transient spectroscopy with ca. 35 ps resolution and pointed out

that ZnTPTBP exhibited both 1(π,π*) and 3(π,π*) transitions. Rogers and et al8 also

investigated meso tetraphenylmetalloporphyrins, using TDDFT calculations and nanosecond

flash photolysis and characterized the T1→Tn absorption spectra and fluorescence quantum

efficiencies.

The fact that Co(II), d7, and Cu(II), d9, are paramagnetic introduces complications to

the excited state picture. The unpaired d electron interacts with the macrocycle orbitals

generating doublet multiplicity in the ground and π,π∗ excited states, and splitting the triplet

states to trip-doublet (2T1) and trip-quartet (4T1) states that lie very close in energy.13 High

10

time resolution studies on various Cu porphyrins have shown that intersystem crossing from

the primary excited 2S1 state into the triplet manifold occurs in less than 350 fs and the main

cause for the ground state repopulation involves an equilibrium set of triplet states

2T1(π,π*)/4T1(π,π*).29, 37 Calculations showed that in Cu complexes the half-filled dx2-y2

orbital lies within the HOMO-LUMO gap giving rise to many charge transfer transitions.38

Observations suggest that a CT state lies close in energy above the 2T and 4T in non-

coordinating solvents and that a non-luminescent CT state is thermally accessible.15

Several ultrafast transient absorption experiments and extended Hückel calculations

predicted that a low-lying charge transfer state a2u(π)→ Co(dz2) participates in the

deactivation process in Co(ІІ) porphyrins such as octaethyl and tetraphenyl complexes.32, 34

Determination of d-orbital energies of low lying excited states from ESR data on Co(II)

porphyrins suggest that up to seven possible metal centered (d,d) states may lie below 1.24

eV, which is well below the 2T1( π,π*) state and the lowest lying (π,d) charge transfer state.38

These possibly participate in the deactivation process.32

Chromium(ΙΙΙ) and Manganese(ΙΙΙ) porphyrin complexes are interesting compounds

mainly because of their unique absorption spectra due to the unusual electronic structure

induced by half-filled metal d orbitals. Irvine et al.39, 40 who used 1-ps excitation at 597 nm to

examine Mn(ΙΙΙ)TPPCl in CH2Cl2 and pyridine, reported that the lifetime for the decay of the

strongly absorbing species near 500 nm was 17 ps and assigned to the decay of a ring (π,π*)

“tripmultiplet” state. Holten et al41 reported the excited state kinetics at 500-900 nm of

Mn(ΙΙΙ)TPPCl and Mn(ΙΙΙ)OEPCl in CH2Cl2 and pyridine and reported two transients having

fast and slower components. The fast decay had 5-30 ps lifetime and the slower components

had an 80 ps lifetime for TPP and 140 ps lifetime for OEP and they assigned the short-lived

component to the “tripquintet” 5T1(π,π*), a fraction of which relaxes to the longer lived

11

trpseptet, 7T1(π,π*). And also they suggested that decay of both tripmultiplets possibly

proceeds via lower energy CT or d,d excited states.

Gouterman et al. reported that there were two luminescence bands at 815 nm and 850

nm from ClCr(ΙΙΙ)TPP at 77 K, and those bands were assigned to the emission originated

from tripquartet 4T(π,π*) and tripsextet 6T(π,π*) states.42 And also they showed that the

emissions from the quartet and sextets were in thermal equilibrium. Photophysical studies on

CrΙΙΙ porphyrins were carried out to elucidate the photodissociation mechanism of the axial

ligand. Hoshino et al.43, 44 reported laser flash photolysis of ClCr(ΙΙΙ)TPP(L) (L= sixth axial

ligand) in various solvents and suggested that 4S1(π,π*) excited state acts as the main route

for the photodissociation process. Jeoung et al.45 found out that the temporal evolutions of

photoinduced absorption and bleaching signals of XCr(ΙΙΙ)TPP(X= Cl, Br) in benzene exhibit

biphasic decay profiles with time constants 1 and 20 ms. The faster decay was assigned to the

four coordinated CrΙΙΙTPP* species and the slower decay component to the recombination

process returning to the original five-coordinate XCr(ΙΙΙ)TPP species. A significant reduction

in the lifetime of photoexcited ClCr(ΙΙΙ)TPP in THF was observed as compared with that in

benzene. An investigation on the luminescent properties of Cr(ΙΙΙ) porphyrins showed that the

lifetime of the photexcited state is relatively short ~295 ps in ethanol and also depends on the

solvent.46

The photophysical properties of the second and third row transition metal tetrapyrrole

complexes (Ru, Pd, Pt, etc) differ greatly from the first row metals showing intense

phosphorescence and long lived triplet T1 state localized on the π system.10, 27 The effects of

heavy metals on spectroscopic properties have been attributed to the spin-orbit interaction

that raises the energy of the unoccupied d orbitals. Coordination of a heavy metal atom into

the π system increases the rate of the intersystem crossing between singlet and triplet states of

the metalloporphyrins, thereby enhancing the rate of radiative decay. For example, Pd(ІІ)

12

porphyrins show a weak fluorescence (ФF< 10-4) 47 and in Pt complexes only delayed

fluorescence could be observed.10

The research presented in this dissertation is focused on an investigation of the

photophysical properties of the metallotetrabenzoporphyrins with first row transition metals

Cr(ΙΙΙ), Mn(ΙΙΙ), Co(ІІ), Cu(ІІ), Zn(ІІ), and the third row Pt(ІІ), analogue. The effect of the

different central metal on the electronic properties and the excited state dynamics of

tetrabenzoporphyrins are discussed from both experimental and theoretical points of view.

Theoretical-only investigations of Ni(ІІ) and Pd(ІІ) analogues have been performed to give a

full picture about the nature of the deactivation mechanism. The first two chapters discuss

some of the aspects of photophysics and tetrapyrrole molecules together with the methods

and materials used in the experiments and calculations. Chapter 3 describes the ground and

excited state properties of Zn(ΙΙ)TPTBP using a combination of DFT/TDDFT and transient

absorption spectrometry in order to provide experimental and theoretical evidence on the

spectral properties of the π-localized singlet and triplet states of the

tetraphenyltetrabenzoporhyrin macrocycle. The results are compared to Ni(ΙΙ)TPTBP.

Accumulated experimental and theoretical evidence on the excited state deactivation

of Cu(II)TPTBP and Co(II)TPTBP complexes are described in chapter 4. In chapter 5, the

photophysics of Cr(ΙΙΙ)TPTBPCl and Mn(ΙΙΙ)TPTBPCl are described using transient

absorption spectrometry to provide data necessary in understanding the influences that metals

with partially occupied d orbitals have on the photophysical properties of the porphyrin

macrocycle. Chapter 6 is introduced to describe the heavy metal effect on the photophysics of

tetrabenzoporphyrins. The experimental results from femtosecond and nanosecond transient

absorption studies have combined with relativistic DFT and TDDFT calculations to see the

effect of heavy metals on photodeactivation. Only calculations were performed with

Pd(ΙΙ)TPTBP and compared the metal influence on the Ni→ Pd→ Pt in transition series.

13

Bond-Type in metalloporphyrins

On chelating with a metal ion the porphyrin nucleus loses two protons from its pyrrole

nitrogen atoms. Thus the chelating species is a di-anion and electrostatic forces undoubtedly

contribute to the binding.48 When porphyrin is coordinated to a metal as a tetradentate ligand

there are four six-membered chelate rings with delocalized π –bonds, hence, there are

additional rings with delocalized π-systems.

The most important geometrical type in porphyrins is the square planar

tetracoordinate porphyrin chelate with D4h symmetry having low spin state.49 Square-

pyramidal penta-coordinate complexes are formed when a further ligand is added

perpendicularly on one side of the porphyrin plane. The further addition of another out-of

plane ligand results in octahedral complexes and become high spin due to the electronic

rearrangement being forced by the increasing energy of the dz2 orbital induced by the axial

ligation.50 Among the five d orbitals in metal porphyrins, the dx2-y

2 orbital lies along the x and

y axes pointing towards the four nitrogen atoms and the dz2 orbital lies along the molecular

fourfold symmetry z axis, perpendicular to the plane. In the plane, the dxy orbital lies

transecting the dx2

-y2 orbital, and the dxz and dyz orbitals lie between the xz and yz axes

respectively. If the dx2

-y2 and dz

2 orbitals are not filled with electrons, unpaired electrons may

be transferred to the ligands through different types of metal-ligand bonding interactions such

as ligand-to-metal σ donation, ligand to metal π donation and metal- to- ligand π back

bonding.51 The spatial disposition of the dxy, dxz and dyz orbitals means that they do not play a

part in σ bonding with ligands, but they can overlap with vacant π orbitals to form π bonding.

In the square planar porphyrin chelates the dxz and dyz orbitals of the metal ion are sterically

able to overlap with vacant p orbitals of the ring nitrogen atoms of the porphyrin whereas in

square pyramidal or octahedral complexes the dxz and dyz orbitals of the metal ion can overlap

with vacant π orbitals in the fifth and sixth positions.48 For four and six coordinate

14

metalloporphyrins with D4h symmetry, the dx2

-y2 orbital interacts with porphyrin σ type

molecular orbitals with nodal planes passing through meso-carbons. The dz2 orbital has

electron density along the z axis and will interact mostly with axial ligand orbitals of σ

symmetry.52 Figure 1.7 illustrates these orbital types.

Figure 1.7. d metal orbitals in porphyrins, taken from ref 52

Symmetry in Metalloporphyrins

Many porphyrins that are important in biological applications are observed to be non-

planar. Non-planar distortion may be caused by peripheral steric crowding, electronic

interactions involving axial ligands, crystal packing effects, the size of the central ion and

specific metal-ligand orbital interaction.53 For a planar metalloporphyrin with D4h symmetry

the a1u and a2u HOMOs are orthogonal to each of the five metal d orbitals.

With a ruffling deformation introduced, the symmetry of a six coordinate

metalloporphyrin will be lowered from D4h to D2d ; both dxy and a2u orbitals will be of b2

representation making it symmetry-allowed and can therefore interact.54 Saddling makes the

metal dx2

-y2 and porphyrin b2 orbital interaction while a five-coordinate metalloporphyrin

with C4v symmetry, both dz2 and a2u will be in a1 representation to interact.

15

Orbitals Point Group

D4h D2h D2d C4v C2v Cs planar ruffled saddled domed metal dx

2-y

2 b1g ag b1 b2 b1 a1 a’ dz

2 a1g ag a1 a1 a1 a1 a’ dπ eg b2g, b3g e e E b2,b1 a’ dxy b2g b1g b2 b1 b2 a2 a’ porphyrin LUMO eg b2g, b3g eg eg a’ HOMO a1u au b1 b1 a2 a2 a’ a2u b1u b2 b2 a1 b1 a’

Table 1.1. Symmetry presentation of metal and porphyrin orbitals in metal porphyrinates,

adapted from ref 52, 54, 55

Excited states of porphyrins and characterization of excited states of porphyrins

Transient absorption measurements allow measuring the difference in absorption

spectra between an excited state and the ground state. Those absorption difference spectra

such as π,π*, d-d, CT (π, d) or d, π* appear to have distinct features that can be used to

identify them and they are most important in explaining their photophysical and

photochemical behavior.

π,π* state of the ring

Both singlet and triplet π,π* spectra contain a broad absorption extending from 500

nm throughout the near infrared. The 1(π,π*) spectra could also display a characteristic Q

band stimulated emission which appears as a negative feature to the red of the Q band

bleaching. The position of Q(0,1) stimulated emission coincides with the position of Q(0,1)

spontaneous emission.(fluorescence). Therefore, stimulated emission occurs in the vicinity of

650 nm for the metalloporphyrins and near 700 nm for the free-base complexes.29 The triplet

16

3(π,π*) spectrum is marked by a distinct near-infrared absorption peak and always appears at

the long wavelength edge of the absorption.

d,d ligand field states

The absorption spectra of low-lying d,d excited states of metalloporphyrins appear to

be similar to but red shifted from the ground state spectra. Picosecond studies on Ni16-21 and

Co32 porphyrins have shown absorption difference spectra in the visible and the Soret region

that have 15-20 nm to the red of ground state bleachings. d,d state states can be thought of as

a combination of porphyrin ground state π-system with an electronically excited metal

center. The optical absorption spectrum of such an entity will be dominated by the Soret and

Q bands of the π –ring and it will differ from the original ground state spectrum slightly

because of the electronic effects imparted by the changed configuration of the d-electrons. In

general, the spectrum of metalloporphyrin d,d state exhibits derivative –shaped absorption

changes. In general, one expects the spectrum of metalloporphyrin d,d state to exhibit

derivative –shaped absorption changes since the porphyrin π system has returned to its

ground state electronic configuration. Thus, the absorption spectrum of d,d state should be

dominated by the same π π* transitions governing the spectrum of the ground electronic state

of the complex but shifted in energy and possibly intensity due to the new metal electronic

configuration.56

(π,d) and (d, π*) metal ↔ ring CT states

Low lying charge transfer states are also formed by radiationless deactivation of

higher energy excited states. Accordingly the energies of these CT states and their transient

behavior are expected to be dependent on the electronic properties of the ring, the central

metal ion and the axial ligands.10 These CT excited states should show little difference to

17

those observed upon production of metalloporphyrin ring π π* transitions since the most of

the oscillator strength for these excited states come from the porphyrin ring.57

Use of tetrapyrrolic photosensitizers for cancer treatment

Metallo-tetrapyrroles are potential candidates for diverse applications due to the

inherent chemical flexibility of the metallomacrocycle that allows modulation of their

physico-chemical properties through ligand and metal modification. Among them, a

significant use of these types of molecules is in the area of cancer treatment.

Different types of therapeutic modalities are available and here, photodynamic

therapy (PDT) and photothermal therapy (PTT) will be discussed in detail. With these kinds

of treatments, a non-toxic photosensitizing agent will be injected to the targeted tissue and

followed by the specific irradiation using selected visible or near IR light, usually from a

laser. The sensitizing agents should accumulate and be retained primarily in the tumor cells

rather than in healthy tissue.58

The mechanism of PDT is described in Figure 1.8. There, light, O2 and a

photosensitizing drug are combined to produce a selective therapeutic effect.

Figure 1.8. Mechanism of action of PDT modified from ref. 59

Light Photosensitizer (ground state)

Photosensitizer (excited state)

Tissue oxygen

Free radicals, singlet

Cellular toxicity

18

PDT is based on the concept that certain photosensitizers can be localized in

neoplastic tissue and subsequently they can be activated with the appropriate wavelength

(energy) of light to generate active molecular species such as free radicals and singlet oxygen

(1O2) that are toxic to cells and tissues.14, 59-62

In general, photosensitizers with longer absorbing wavelengths and higher molar

absorption coefficients at these wavelengths are more effective photodynamic agents. The

photophysical process involved in PDT can be described as follows. The ground state

electronic state of the photosensitizer is a singlet state (S0) and upon excitation at the

appropriate wavelength, it is excited to the short-lived first excited singlet state (S1).The

photosensitizer can return to the S0 state by emitting the absorbed energy as fluorescence or

by internal conversion. Alternatively the S1 state can convert to the first excited triplet state

(T1) by intersystem crossing. The T1 state is sufficiently long-lived to take part in chemical

reactions and therefore photodynamic action is mostly mediated by the T1 state. When the

photosensitizer returns to the ground state it can generate a manifold concentration of reactive

intermediates as a result of an oxidation of biomolecules in the cell. There are two types of

photodynamic reactions.63 Type Ι photoprocess are electron or hydrogen transfer reactions

between the T1 photosensitizer and target molecules generating reactive intermediates which

are harmful to cells such as superoxide, hydroperoxyl and hydroxyl radicals. The type ΙΙ

photoprocess is an electron spin exchange between the T1 photosensitizer and 3O2. It

produces the first excited-singlet state of oxygen (O21Δg ) which is regarded as the main

mediator of phototoxicity while the photosensitizer returns to S0 state. A good

photosensitizing agent should not be toxic in the absence of light and should be localized in

the target tissue as selectively as possible and be eliminated from the body quickly to avoid

generalized skin photosensitization.60

19

PDT is a binary modality for tumor treatment which requires the presence of two

independent substituents to initiate the treatment process. On the other hand Photothermal

therapy (PTT) process only requires the presence of photosensitizer alone. A potential

advantage of PTT over PDT is the lack of need of the presence of oxygen in the irradiated

tissue and the lack of the necessity that the irradiating photon has sufficient energy to

generate singlet oxygen.14 For a compound to be an effective photothermal agent, it should be

able to very rapidly return to its ground state after photoexcitation, generating a vibrationally

hot ground state molecule. Therefore the most useful photosensitizers for PTT would be

chromophores with excited electronic states that only decay non-radiatively to the ground

state converting the photon energy into the thermal event.

Following the proposal of PTT for tumor treatment, tetrapyrroles with first row

transition metal centers have attracted much attention due to their high extinction coefficient

in the near IR region where light penetrates into tissues more effectively, their high

photostability and the short lifetime of the excited states allowing for the generation of high

local temperatures. Metallotetrapyrroles with first row transition metal centers have been

shown to undergo fast radiationless deactivation of their excited states since the metal

incorporated into the macrocycle cavity introduces a manifold of electronic states with metal-

centered and/or metal+π–system character, some of which can be situated at lower energy

than the pure π,π* states that are populated by a photo-excitation process.15 These low-lying

states provide rapid non-radiative channels for the excited state deactivation, which is an

optimum situation for PTT. Here, the excited state dynamics of Metallo-

tetraphenyltetrabenzoporphyrins (MTPTBPs) have been studied with the intent to formulate

an understanding of how such dynamics depend on molecular characteristics to explore the

ability of these kinds of molecules to be used as photothermal agent for future.

20

The effect of the extension of porphyrin ring and meso-phenyl substitution

This study is all about tetraphenyltetrabenzoporphyrins and it is fruitful to compare

how these molecules deviate from the unsubstituted porphyrins. To know the structure –

property relationship, it is important to understand how chemical changes affect the

photophysical properties. Extension of the porphyrin ring by benzo-annulation results in red

shifts in both the ground (Q and B bands) and triplet excited state (T1-Tn) absorption

spectra.13 Rosa et al investigated the effect of benzo-annulation in Ni tetrapyrroles and found

that NiTBP showed enhanced oscillator strength and significant red shift of the Q band

compared to regular porphyrins.64

The red shifts of the ground state spectra may be due to some extent to the

conformational changes as well as electronic effects due to the increased π conjugation.65-69

Rogers and et. al have studied the difference in energy (ΔE) relative to the planar porphyrin

with extended macrocycle and meso substituted porphyrins to have an idea about the overall

contribution of each type of substitution. The data showed that a large percentage of the red

shifts observed is due to electronic effects which results from increased π conjugation from

the added benzo groups.13

There is significant separation in energy between the Gouterman HOMO and HOMO-

1 orbitals that are nearly degenerate in unsubstituted porphyrins64,70, 71 It has been shown that

different substituents at the meso-carbon or at the periphery of the porphyrin core can alter

the position of these orbitals, removing degeneracy to some extent, and thus giving rise to

intensification and/or shifting of the Q band.13, 64, 68

As seen in Figure 1.3, the HOMO a2u orbital is mainly localized on the pyrrolic

nitrogen and meso- carbon atoms while HOMO-1 a1u has contribution mainly from the Cα and

Cβ atoms. To selectively red shift the B band, an addition of π conjugated groups at the meso-

21

position should be made and to selectively red-shift the Q band the addition of π conjugated

groups to the β position should be made and the end result will be lowering the energy.

The near –degeneracy of the G-HOMO and G-HOMO-1 that exists in porphyrins does

not occur in benzoporphyrin derivatives since the electronic effect of benzo-annulation

partially removes the mixing of the Gouterman one-electron transitions.15, 64, 71 This makes

the cancellation of the transition dipole moments in the Q band less effective, resulting in the

enhanced oscillator strength of the Q band.

Effect of saddling

Saddling is effected by the simultaneous tilting upward of two opposite pyrrole rings

and the tilting downward of the two other opposite pyrrole rings. Figure 1.9 presents a

schematic diagram to show the tilting of pyrrole rings and the plane of the drawing is the

plane perpendicular to the porphyrin plane.

N

N N

N

M

θ

Cmeso

Pyrrole

Pyrrole

Figure 1.9. Definition of rotation angle θ of the phenyl plane between an upward tilted

pyrrole and a downward tilted pyrrole from ref.64

The degree of saddling projections can be defined as the saddling φ angle that pyrrole

rings makes with the porphyrin plane. The rotation of meso phenyl rings is described by the

22

angle θ between the phenyl plane and the porphyrin plane. Saddled meso-aryl porphyrins

shows a quite acute aryl-porphyrin dihedral angles θ < 600 72. Rosa and et al reported that the

saddling distortion of the porphyrin core and the tilting of the meso-aryl groups are coupled

through a synergic mechanism that is entirely governed by intrinsic electronic factors.64

Calculation have showed that the energy of HOMO increases with increasing the number of

phenyl groups while the change of LUMO is irregular. Detailed analysis has further

indicated that the sequential decreasing of the HOMO-LUMO gap going from H2P to H2TPP

with increasing phenyl numbers.73

DFT/TDDFT Methods: Background

Density Functional Theory (DFT) has become a major tool to predict and rationalize

the nature of the electronic ground state of transition metal compounds. The basic notion in

DFT is that the energy of an electronic system can be expressed as a function of its electron

density.

The formal proof that the energy of an N- electron system can be defined without any

approximation as a function of its density ρ(r) was given in 1964 by Hohenberg and Kohn74.

Thus DFT is an exact reformulation of the Schrödinger equation based on the Hohenberg-

Kohn theorem according to which the ground state energy of an electronic system is uniquely

determined by the electronic density ρ(r). DFT differs from Hartree-Fock ab initio methods in

that it uses the electronic density rather than one electron wave functions.

DF theory is turned into practical application after the Kohn-Sham (KS) theory75 was

developed. The general DFT energy can be expressed as

[ ] [ ] [ ]S N C XCE( ) T V V E ( )ρ = + + + ρ

where ρ is the electronic density.

[ ]ST is the kinetic energy of the non-interacting electrons with the density of ρ

23

[ ]NV is the interaction energy of the external potential VN with ρ

11 1N N[V ] V ( ) ( )dr= ρ∫

[ ]CV is the Coulomb energy due to the interaction between two charge distributions of ρ(1)

and ρ(2)

1 212

12

1 2C [V ] = dr dr

r( ) ( )ρ ρ

∫

XCE ( )ρ , is the exchange-correlation energy term and its exact functional dependence on ρ is

unknown.

To use DF theory effectively the exchange-correlation part has to be approximated.

To find an accurate continues to be great challenge in density functional theory. The

simplest and widely used approximation is Local Density Approximation(LDA)

XCE ( )ρ

76 which uses

the homogeneous electron gas formula for the exchange and correlation energy, EXC which is

written as 11C C (E ( ) ( ) )drρ = ρ ε ρ∫

where εC is the correlation energy per electron in a homogeneous gas. The most

successful and widely used analytical expressions of εc(ρ) is developed by Vosko, Wilk and

Nusair (VWN)77. Recently, LDA and VWN have become synonyms in the literature78.

Slater neglects the correlation energy and the exchange functional term is given as

3 1 13 18 3 3

XE ( ) ( )⎛ ⎞ ⎡ ⎤ ⎡ ⎤ρ = − α ρ⎜ ⎟ ⎢ ⎥ ⎢π⎝ ⎠ ⎣ ⎦ ⎣ ⎦⎥

and now is known as the Hartree-Fock-Slater or Xα method.

More sophisticated approximations to the exchange-correlation energy incorporate

non-local corrections, in which the exchange and correlation were improved by non-local

gradient correction, due to Becke79 for the exchange and Perdew80 for correlation.

Time dependent density functional theory(TDDFT) is popular for predicting excited

state and response properties of molecules and is based on a Runge- Gross theorem, the time

24

dependent analog of the Hohenberg-Kohn theorem,81 which is a simple consequence of the

time-dependent Schrödinger equation.

All the DFT/TDDFT calculations performed here used the ADF (Amsterdam Density

Functional) suite of programs together with BP functional and the ADF largest available 3ζ-

STO basis set. The BP functional has provided accurate results for transition metal systems to

predict the positions of the metal-based molecular orbitals relative to the ring orbitals and in

describing pure valence excitations.82

The combination of DFT and TDDFT is capable of reaching accuracies ~ 100 meV in

energy calculations. The combination of TDDFT and transient spectroscopy methods is

becoming a promising strategy for excited state structure elucidation in larger molecular

systems containing metals.

Glossary of acronyms

OEP = octaethylporphyrin

TPP = tetraphenylporphyrin

TMPyP = tetrakis(N-methylpyridyl)porphyrin

TPPS=tetrakis(sulfonatophenyl)porphyrin

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30

CHAPTER 2: MATERIALS AND METHODS

Materials

All the solvents used were HPLC grade and used without further purification.

(Toluene (99.5+%, Aldrich, Spectrophotometric grade), benzonitrile (99.9%, Sigma, HPLC

grade), pyridine(99.9%, Sigma, HPLC grade), DMF (99.9%, Sigma, HPLC grade)). All

metallotetraphenyltetrabenzoporphyrins(MTPTBP), M= Cr, Mn, Co, Ni, Cu and Zn were

bought from Frontier Scientific, Inc. and used as received.

Instrumentation and Methods

UV-visible Absorption Spectrometry

The ground state electronic absorption UV-visible spectra were recorded using a Varian Cary

50 Bio (Varian Corporation) single beam spectrophotometer. All spectra were recorded at

room temperature (295 K) using 2 mm or 1 cm path length quartz cuvettes.

Steady state Fluorescence Spectrometry Steady state fluorescence spectra were measured at room temperature using a PTI

Instruments spectrofluorimeter (QM-4/2006-SE) equipped with a 75 W Xe lamp as excitation

source and a R-928 PMT detector for the UV-Vis region. Near-IR signals were monitored at

the right angle geometry using a Peltier cooled InGaAs detector with lock-in amplification.

Luminescence spectra were acquired at room temperature and all solutions in 10 mm

x 10 mm path length quartz cuvettes were Ar-saturated prior to luminescence experiments.

Appropriate filters were used to avoid scattered light entering the detection system. All

spectra were corrected for the sensitivity of the detector. All the photophysical experiments

were performed using optically dilute solutions (OD = 0.08–0.1). For near IR emission the

concentrations of the samples were ~ 6 μm, providing optical absorption in the Q band

maximum of ~ 1.2.

31

The fluorescence quantum yields (ΦF) were measured using Zn meso-

tetraphenylporphyrin (ZnTPP) as a standard with a known quantum yield of 0.33 in deaerated

benzene. The reference and the samples were excited at the same wavelength, where they had

the same absorbance. (A = 0.05 a.u.) The areas under the fluorescence spectra (G) were

measured and fluorescence quantum yields were calculated using the following equation.

2

( ) .( )2

( ) .

=Φ Φsample ZnTPP ref sam solutionsample ZnTPP ref

sample ZnTPP ref ref solution

G GX X X

A Aηη

G= Area/Intensity

A=Absorbance of the sample at excited wavelength

Φref= Quantum yield of the standard

η = viscosity of the solvent; η benzene=1.50112; η pyridine = 1.51016; η toluene = 1.4969

Transient Absorption Experiments

The early time events that follow photoexcitation of studied compounds were

examined by femtosecond transient absorption spectrometry with 100 fs excitation pulses

and the transient absorption signal at times longer than 10ns was monitored using nanosecond

flash photolysis on Ar saturated solutions using excitation pulses of 8 ns duration.

Femtosecond Transient Absorption Spectroscopy

The set up for the femtosecond pump-probe experiments at the Ohio Laboratory for

Kinetic Spectrometry is represented in Figure 2.1. A Spectra-Physics Hurricane laser system

was used as the excitation source. This includes a Ti:sapphire seed laser (Mai Tai, cw diode

pumped, mode-locked pulsed laser), a pump Nd:YLF laser (Evolution, diode-pumped Q

switched), a pulse stretcher, a Ti:sapphire regenerative amplifier and a pulse compressor. The

32

seed laser generated femtosecond pulses as short as 60 fs at an 80 MHz repetition rate with

average power 700 Mw. The pulses are stretched using diffraction gratings and then

amplified in a Ti-sapphire regenerative amplifier pumped by an Evolution Q-switched

Nd:YLF laser. The fundamental output of the laser system consists of pulses of 800 nm, 1

mJ/pulse, 100 fs (FWHM) operating at a repetition rate of 1 kHz.

This output was first split into two parts to generate pump (95%) and probe (5%). The

pump beam generates the excitation pulse and the 5% generates a white light continuum that

probes the absorption of the excited molecules generated by the pump pulse.

The pump beam is converted to selected excitation wavelengths by coupling it into a

second harmonic generator (CSX) for 400 nm excitation or into an optical parametric

amplifier (Spectra-Physics OPA 800C) for wavelengths in the 320-700 nm. The pump beam

is passed through an optical chopper (DigiRad c-980) rotating at a frequency of 100 Hz in

order to switch the sample between excited and ground states.

The probe beam is sent through a computer controlled delay line (Newport Corp.

MTL250 PP 1250 mm linear positioning stage). The delay line provides an experimental time

window of about 1.6 ns with a step resolution of 6.6 fs. Subsequently the 800 nm probe is

focused onto a 3 mm thick sapphire plate (Crystal Systems, Inc., HEMLUX grade) that

generates a white light continuum (450-750 nm). Prior to white light generation, the beam

was passed through an iris diaphragm, a polarizer and a half wave plate in order to adjust the

beam intensity delivered to the continuum generator. A spherical (f = 10 cm) mirror focuses

the beam to the back of the sapphire plate to avoid additional chirp1 and another spherical

mirror (f = 5 cm) after the sapphire plate collimates the white light beam prior to the sample

cell. The mirrors were used to minimize the chirp of the white light continuum. A horizontal

XY translation stage was employed to optimize the position of the sapphire plate.

33

The white light continuum probe beam was directed into the sample cell,

superimposed on the pump beam at an angle of approximately 5o. The energy of the probe

pulses was < 5 μJ at the sample. After passing through the sample cell, the probe continuum

was coupled into a 400 μm optical fiber and then connected to a CCD spectrograph (Ocean

Optics, PC 2000) for time resolved spectral information. (450-850 nm) The CCD

spectrograph was externally trigged by the chopper in order to distinguish between the

continuum spectra corresponding to the ground and excited states of the sample. The delay

line and the CCD spectrograph were computer-controlled by LabVIEW(National

Instruments) software developed by Ultrafast Systems LLC (www.ultrafastsystems.com)

allowing an automatic spectral acquisition over a series of delay times.

The samples were continuously flowed during the experiments using a 2 mm flow cell

(Quartz, Starna Cells) and the absorption spectra of the solutions were measured before and

after the experiments to ascertain that minimal sample decomposition had occurred during the

experiment. The instrument rise time was ca.150 fs (measured by an anti-Stokes Raman

scattered peak width in toluene). At each time delay setting, dispersed absorption spectra

were recorded for 2 seconds and the set averaged. Data points were recorded every 50 fs for

the first 55 ps, or so and less frequently for longer delay times.

34

M FM

M

M M

M

BSFM

Delay stage

Sample

CCD spectrograph

Continuum generator

Chopper

CSK super tripler

PM

PM

O

PA

800

CF

M

1

Mai Tai diode-pumped mode-locked Ti: sapphire laser

Stre

tche

r

Rege

nera

tive

ampl

ifier

Q sw

itche

d fr

eque

ncy

diod

e N

d:YL

F ev

olut

ion

lase

r

Figure 2.1. The schematic representation of the ultrafast transient absorption setup. M-

Mirror; FM-Flipping mirror; PM-Parabolic mirror; BS-Beam splitter

Nanosecond transient Absorption

Figure 2.2 shows a schematic of the nanosecond laser flash photolysis system. The set

up included a Q-switched Nd:YAG laser (Spectra Physics Quanta Ray GCR-230) and a

computer aided kinetic spectrometer. The laser can be switched to obtain the frequency

doubled output at 532 nm and frequency tripled output at 355 nm (6 ns, 10 Hz) and the

energy was kept between 0.5 and 3 mJ per pulse. The kinetic spectrophotometer is described

in details elsewhere2-4 and a brief description with recent modifications are as follows. The

transient absorption was monitored at right angle using white light (probe) from a 150 W

xenon CW arc lamp (Oriel Corporation). The probe beam was focused through the sample

and re-imaged on the entrance slit of a Spex 1681 (0.22 m) monochromator. The

35

monochromatic light was detected with a modified Hamamatsu R928 PMT (5 instead of

typical 9 stage dynode amplification) operating with a Keithley 247 high voltage supply. The

resulting signal was routed through a DC-coupled back-off circuit that stored and displayed

digital readouts for the intensity of the monitoring beam.

A fast shutter (Uniblitz VMM-D1) was used to minimize the exposure of the sample

to the probe light. The real time signal from the PMT was sent through a back-off circuit to

eliminate a DC offset from the PMT and then was sent to a digital oscilloscope which

averaged the signal over a pre-determined number of shots (typically 16-32). The

counter/timer board (National Instruments NI-TIO) synchronized the Nd:YAG laser, fast

shutter driver and oscilloscope and data were collected using in-house software routine

written in LabVIEW (National Instruments), which generated the absorption difference

spectra. The kinetic profiles were obtained in the same manner collecting data at one

particular wavelength and averaging over 16-32 shots. Appropriate filters were used to avoid

scattered light from laser beam entering the PMT.

To obtain transient absorption spectra, solutions were Ar saturated prior to the

experiment. Kinetic profiles were recorded in Ar, air and O2 saturated solution which enabled

the measurements of oxygen quenching rate constant when necessary.

The data analyses were done using standard data analysis software using Origin (Origin Lab

Corporation).

36

Xe lamp PMT

Shutter

Third harmonic generator

Sample

Nd:YAG laser

Monochromator

back- off

oscilloscope

Figure 2.2. Schematic diagram of the nanosecond laser flash photolysis system

Computational Details

For all calculations the Amsterdam Density Functional ADF suite of Programs

(release 2004.01 or 2005.01.11,12, SCM, Amsterdam) have been used. The calculations

exploited the local density approximation (LDA) functional of Vosko-Wilk-Nusair (VWN)5

together with the generalized gradient approximation (GGA), employing Becke's6 gradient

approximation for exchange and Perdew's7 for correlation (BP). Corrections for relativistic

effects were employed in calculation for Pd and Pt complexes by using the Zeroth Order

Regular Approximation (ZORA) scalar Hamiltonian.8, 9 The excitation energies and oscillator

37

strengths were calculated using time-dependent density functional theory (TDDFT) as

implemented in the ADF code.10, 11 In brief, the solution of the TDDFT response equations is

obtained as a result of iterations starting from the usual ground state or zeroth-order Kohn–

Sham (KS) equations.11 approximate to the usual static exchange-correlation (xc) potential νxc

(r). After the solution to the ordinary KS equations has been obtained, the first order density

change has been calculated from an iterative solution to the first order KS equations. There,

an approximation to the first functional derivative of the time-dependent xc potential νxc (r,t)

with respect to the time-dependent density ρ(r′, t′), the so-called xc kernel was used.12 For

the xc kernel, the Adiabatic Local Density Approximation (ALDA) was employed. When this

approximation is used the time dependence (or frequency dependence referring to the

Fourier-transformed kernel) should be neglected, so that the differentiated static LDA

expression can be used. In this work the Vosko-Wilk-Nusair parameterization was utilized.5

For the exchange correlation potentials which appear in the zeroth-order KS equations, the

same GGA as in the DFT calculations was employed.

For calculations the standard all electron ADF TZ2P basis set, which is an

uncontracted triple-ζ STO basis set with one 3d and one 4f polarization function for C, N and

O atoms, one 2p and one 3d polarization function for H, a triple-ζ 3d, 4s basis with one 4p

and one 4f function for Co, Ni, Cu and Zn, a triple-ζ 4d, 5s basis with one 5p and one 4f

function for Pd, and a triple-ζ 4f, 5d, 6s basis with one 6p and one 5f function for Pt was

used.

For the Pd and Pt complexes the core densities were computed relativistically by the

Dirac auxiliary program. Geometry optimizations were performed for the ground and selected

triplet excited states of the studied compounds. Open-shell DFT calculations for the triplet

states were performed within a spin-unrestricted formalism. The optimized ground state and

excited state structures were verified to be true minima by frequency calculations. All

38

calculations were performed in the gas phase. The molecular orbitals were visualized with the

Graphical User Interface for the ADF package (ADF GUI).

Estimation of the MTPTBP triplet (π,π*) energies from the oxygen quenching bimolecular rate constant

The energies of the triplet state of porphyrins were calculated using bimolecular rate

constant of the quenching of the T1 by molecular oxygen. Following scheme has been used to

represent the energy transfer process from the triplet state localized on the π system to the

ground state oxygen.13-15

kq3(M(ΙΙ)TPTBP )∗+ O2 (3 Σ −g) 1(M(ΙΙ)TPTBP )+ O2 ( 1Δg )

kbkT kΔ

1(M(ΙΙ)TPTBP) O2 (3Σ −g)

- kq = the rate constant for the energy transfer from the triplet state porphyrins to ground state

molecular oxygen ( from Table 6.4);

- kT, kΔ = the intrinsic rates of decay of macrocycles T1 and O2( 1Δg )

- kb = rate constant for the back reaction. The back reaction can be considered as diffusion –

limited reaction. It depends on the solvent viscosity and the temperature and can be

calculated from the Debye equation:

K diff = 83

BTkη

= 1.1 x 1010 M-1S-1

For toluene η = 0.589 Cp at 298 K from ref.16

-equilibrium constant for the T1 quenching reaction, Keq = kq/kdiff, where kb=kdiff)

From Keq, triplet energy ET can be obtained, according to;

ET – EΔ + RT ln(1/9) = RT ln Keq

39

Where; EΔ = 22.54 kcal/mol for the O2 ( 1Δg ) → O2 (3Σ −g) (0,0) transition in oxygen

RT ln(1/9) = 1.3 kcal/mol ; ln 1/9 is the spin-statistical factor and it accounts that only

one-ninth of the intervening collision complexes are of overall singlet multiplicity17

The calculated data are collected in Table 6.5.

References

1. Cerullo, G.; De Silvestri, S. Rev. Sci. Instrum. 2003, 74, 1.

2. Nikolaitchik, A. V.; Korth, O.; Rodgers, M. A. J. J. Phys. Chem. A 1999, 103.

3. Pelliccioli, A. P.; Henbest, K.; Kwag, G.; Carvagno, T. R.; Kenney, M. E.; Rodgers,

M. A. J. J. Phys. Chem. A 2001, 105, 1757.


112, 8064.

5. Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.

6. Becke, A. D. Phys. Rev. A 1998, 38, 3098.

7. Perdew, J. P. Phys. Rev. A 1986, B33, 8822.

8. van Lenthe, E.; Barends, E. J.; Snijders, J. G. J. Chem. Phys 1993, 99, 4597.

9. van Lenthe, E.; van Leeuwen, R.; Barends, E. J. Quantum Chem 1996, 57, 281.

10. van Gisbergen, S. J. A.; Snijders, J. G.; Baerends, E. J. Comput. Phys. Commun. 1999,

118, 119.

11. van Gisbergen, S. J. A.; Snijders, J. G.; Baerends, E. J. J. Chem. Phys. 1995, 103,

9347.

12. Rosa, A.; Ricciardi, G.; Baerends, E. J.; van Gisbergen, S. J. A. J. Phys. Chem. A

2001, 105, 3311.

13. Firey, P. A.; Ford, W. E.; Sounik, J. R.; Kenney, M. E.; Rodgers, M. A. J. J. Am.

Chem. Soc. 1988, 110, (23), 7626.

40

14. Gunaratne, T.; Kennedy, V. O.; Kenney, M. E.; Rodgers, M. A. J. J. Phys. Chem. A

2004, 108, 2576.


115, 8146.

16. Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T., Handbook of Photochemistry.

Taylor & Francis Group, LLC: New York, 2006.

17. Soldatova, A. Ph.D Thesis, Bowling Green State University, Bowling Green, 2007.

41

CHAPTER 3: THE PHOTOPHYSICAL PROPERTIES OF ZINC AND NICKEL

TETRAPHENYLTETRABENZOPORPHYRINS: A COMBINED EXPERIMENTAL

AND THEORETICAL INVESTIGATION

Abstract

The ground and excited state properties of Zn(ΙΙ)TPTBP have been investigated by a

combination of DFT/TDDFT and transient absorption spectrometry in order to provide

experimental and theoretical evidence on the spectral properties of the π-localized singlet and

triplet states of the tetraphenyltetrabenzoporhyrin macrocycle. The results were compared

with Ni(ΙΙ)TPTBP. Specifically Zn(ΙΙ) and Ni(ΙΙ), were chosen to study the effect of filled

and half- filled d orbitals on the photophysical properties on porphyrin macrocycle.

In Zn(ΙΙ)TPTBP, 640 nm excitation produced a vibrationally hot π-localized S1 state.

After cooling, it decayed to the 3π,π* excited state where ground state repopulated with

ca.236 μs lifetime in pyridine. The lifetime of the singlet state 1π,π* was found to be ca.235

ps in benzonitrile and ca.340 ps in pyridine. Fluorescence from the S1 state was observed

with quantum yield of 0.008. Transient absorption spectrometry with femtosecond time

resolution was employed along with DFT/TDDFT calculations to provide a detailed

understanding of the sequence of events that follow Q band photo-excitation.

Introduction

Zn porphyrins are generally employed as reference complexes for other

metallotetrapyrroles. And also these type of complexes have been investigated for potential

components in various molecular electronic devices.1-7 For example, Zn porphyrins have

been utilized as photosensitizers in self-assembled supramolecular dyads1 or triads2, 8 together

42

with fullerene. Water-soluble zinc porphyrins have been used to photosensitize water

reduction and oxidation.9, 10

The metal ion to which the tetrapyrrole macrocycle is ligated plays a vital role in the

excited state deactivation. Having a d10 electronic configuration, Zn has completely filled d-

orbitals and belongs to a class of regular porphyrins according to Gouterman’s

classification.11 The optical absorption and emission spectra of regular porphyrins are

determined by the π electrons of the ring, with only minor perturbation from the electrons of

the central metal. As a result, the interactions between the porphyrin π-system and the metal

are small and the photophysical properties are mainly governed by the macrocycle’s π

system. Zn(ΙΙ) has been selected for study as the prototype to investigate the photophysical

properties that are mainly controlled by the π system.

When the central metal is a transition metal having vacant or partially filled d-orbitals,

such as Ni, Co, Fe, Pd, etc., the excited state deactivation can be governed by the electronic

states that are formed by the interactions between π orbital and the metal d-orbital manifold

that can be situated at lower energy than the pure π π* states. Such low-lying states can be

intermediate in the cascade of deactivation process by providing the series of low energy

radiationless transitions.12

The deactivation of the Zn porphyrin S1 state occurs predominantly through the

formation of the π-localized triplet state and through the fluorescence to the ground state. The

lifetime of the S1(π,π*) of Zn(ΙΙ) compounds has been found to be ~ 2.5 ns.13

Examination of the Zn porphyrins’ literature reveals information about the above

mechanism and some examples (in fluid solutions at room temperature) are as follows;

The quantum yield of triplet state for Zn(ΙΙ)TPP was found to be 0.82 while the fluorescence

quantum yield was 0.04. The triplet state lifetime in the absence of oxygen was 1.2 ms and

the fluorescence lifetime was 2.7 ns.14 For Zn(ΙΙ)TMPy, the quantum yield of triplet state was

43

found to be 0.90 and the fluorescence quantum yield was 0.02. The triplet lifetime was 1.30

ms in deaerated solution and the fluorescence lifetime was 1.3 ns.14 For Zn(ΙΙ)OEP, the

fluorescence quantum yield was 0.04 and the fluorescence lifetime was 2.2 ns.15

Photophysics of the higher electronic states (S2) of zinc porphyrin have also been

studied and it was reported that the S2 state deactivates with the life time 1-2 ps via internal

conversion into the S1 state.16-18

Ni(ΙΙ) has a d8 electronic configuration. When there are no axial ligands attached (in

non-coordinating solvents), Ni porphyrins are low spin with dz2 being the highest occupied d

orbital and dx2-y

2 is the lowest unoccupied d orbital. One-electron excitation, related to the

transition of an electron from dz2 orbital to the higher dx

2-y

2 orbital, gives rise to singlet and

triplet d,d states. When the central metal ion has unfilled d orbitals, these can couple with the

π-system and provide additional deactivation pathways. Extended Hückel calculations and

the lack of luminescence suggested that the normally emissive (π,π*) excited state of the

porphyrin macrocycle becomes deactivated to produce the lower lying singlet or/and triplet

metal-centered (dz2, dx

2-y

2) excited state.11, 19 The mechanism of deactivation of the excited

states of planar porphyrins proceeds via a sequence of electronic deactivation steps,

(π,π*)→(d,d)→ground state.20, 21 For example, the (π,π*) excited states of Ni(II)TPP and

Ni(II)OEP have been shown to form (d,d) states within 1 ps13, 22, 23; these d,d states repopulate

their ground states in 200-500 ps. 24-28

Apart from the coupled conformational and electronic changes that take place

subsequent to excitation of Ni porphyrins, the other interesting feature is that the excited

metal-centered state is generated vibrationally hot. Holten et al observed that excited d,d

states in non-coordinating solvents displayed a blue-shift and wavelength dependent kinetics

on the 5-25 ps time scale as vibrational cooling occurred.13, 22, 23

44

In this chapter, the photophysical properties of Zn(ΙΙ)TPTBP are investigated by

means of transient absorption spectrometry and using theoretical characterization of the

molecule and electronic structure of the ground state and the excited state. A comparison with

Ni(ΙΙ)TPTBP12 (already published) is made in order to have insight of the effect of the central

atom on the deactivation pathway.

Theoretical characterization of Zn(ΙΙ)TPTBP

Ground State Molecular Structure Analysis

The DFT optimized molecular structure of the Zn(ΙΙ)TPTBP complex is presented in

Figure 3.1.

Figure 3.1. Top view of the DFT-optimized molecular structure of Zn(ΙΙ)TPTBP with atom

labeling.

The calculations reveal that the complex adopts a D2d saddled confirmation with the

phenyl groups twisted out of the porphyrin plane by 70o (the porphyrin-phenyl dihedral angle-

45

< (C2C1Cmeso)) was similar to the other compounds studied. The dihedral angle between the

adjacent pyrrolic nitrogens that characterizes saddling of the macrocycle plane is < (Cα

NNCα)ad 25o. Ni(ΙΙ)TPTBP also has adopted a D2d structure with a saddled conformation.12

The optimized geometrical parameters for both complexes are listed in Table 3.1. Similar to

tetraphenytetrabenzoporphyrins, the Zn and Ni octabutoxy phthalocyanines and

naphthalocyanines (ZnNc(OBu)8/ ZnPc(OBu)8 and Ni Nc(OBu)8/ NiPc (OBu)8) have been

shown to have D2d symmetry in order to minimize the steric repulsion between the lone pair

of facing oxygen atoms belonging to the methoxy groups.29, 30 The tilting of phenyl groups is

necessary to relieve the steric hindrance making possible for phenyl rings to rotate toward the

mean porphyrin plane.31, 32

Parameter Ni(ΙΙ)TPTBP Zn(ΙΙ)TPTBP

M – N 1.922 2.048 Cα – N 1.384 1.377 Cα – Cβ 1.454 1.461 Cβ – Cβ 1.413 1.418 Cβ – Cm 1.405 1.405 Cm – Co 1.395 1.390 Co – Co 1.406 1.404

Cα – Cmeso 1.395 1.409 Cmeso – C1 1.494 1.496

C1 – C2 1.403 1.402 C2 – C3 1.402 1.396 C3 – C4 1.399 1.397

∠ CαNCα 107.4o 109.2o

∠ NCαCβ 109.4o 108.7o

∠ CαCmesoCα 121.4o 125.3o

∠ C2C1CmesoCα 70o 70o

∠ (CαNNCα )op 0o 0o

∠ (CαNNCα )ad 25.2o 21.5o

Table 3.1. Selected Bond Distances (Ao) and Bond Angles (deg) calculated for M(ΙΙ)TPTBP

in the D2d confirmation.

46

The central metal-pyrrolic nitrogen distance for Zn(ΙΙ)TPTBP (2.048 Ao) is larger

than that found for the first row transition metal complexes with partially filled d shells. This

is a result of the repulsion between the filled 3dx2

-y2

metal orbital and the lone pairs of the

pyrrolic nitrogens.

Ground State Electronic Structure Analysis

The ground state energies of the highest occupied and the lowest unoccupied

molecular orbitals calculated for both Ni and Zn complexes in the D2d confirmation are

shown in the Figure 3.2. The atomic orbital population analysis for the orbitals represented in

Figure 3.2, calculated for Zn complex is given in Appendix A. For the Ni complex the data

are taken from ref.12.

-6.5-6.0-5.5-5.0-4.5-4.0-3.5-3.0-2.5-2.0-1.5

dπ

dz2

dx2-y2

NiTPTBP ZnTPTBP

38e23a1

23b216a2

39e

22b2

18b1

40e

17a2

22a1

16a2

38e39e

23a1

22b2

18b1

23b2

40e

E/eV

Figure 3.2. Energy level scheme for Zn(ΙΙ)TPTBP and Ni(ΙΙ)TPTBP. Gouterman’s HOMO,

HOMO-1 and LUMO orbitals are depicted in red.

47

Metal d orbitals lying in close proximity to Gouterman HOMO and LUMO are likely

to be responsible for fast non-radiative decay and it is worth paying attention to those

orbitals in the Ni(ΙΙ)TPTBP. The 23b2 dx2

-y2, virtual orbital lies between the HOMO and

LUMO. The Zn 3d orbitals lie too low in energy to play any role in the deactivation of the S1

state. Contour plots of the Zn(ΙΙ)TPTBP, together with the energy levels are given in Figure

3.3.

-6

-5

-4

-3

-2

-1

38e23a1

23b216a2

39e

22b2

18b1

40e

17a2

E/e

V

17a2 40e:yz 40e:xz

18b1

23b2

39e:yz 39e:xz22b2

23a1

16a2

38e:yz 38e:xz-6

-5

-4

-3

-2

-1

38e23a1

23b216a2

39e

22b2

18b1

40e

17a2

E/e

V

17a2 40e:yz 40e:xz

18b1

23b2

39e:yz 39e:xz22b2

23a1

16a2

38e:yz 38e:xz

Figure 3.3. Contour plots of the Zn(ΙΙ)TPTBP, together with the energy levels.

The four orbitals that are responsible for the appearance of the main spectroscopic

bands on the ground state absorption spectrum, according to Gouterman’s four –orbital

model33 are HOMO 18b1, HOMO-1 22b2 and an unoccupied set of degenerate 40e orbitals.

The HOMO 18b1 has contributions mainly from the Cα, Cm and Co, while the electronic

density on the HOMO-1 is largely localized on the pyrrolic nitrogen and meso- carbon atoms.

(Appendix A) The G-LUMO, which is an unoccupied set of degenerate 40e orbitals are

48

primarily located on the Cα and Cβ atoms. In Zn it is composed of a small contribution from

pyrrolic nitrogen (12%) while with Ni it has a small contribution from the metal 3dπ orbital.

(4%)

Excited States and Ground State Absorption Spectra

Figure 3.4 shows the room-temperature ground state absorption spectra of zinc and

nickel tetrabenzoporphyrins in toluene solution normalized to unit absorbance at the B band

maximum.

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

652nm

645nm

448nm 462nm

norm

aliz

ed a

bsor

banc

e

wavelength/nm

NiTPTBP ZnTPTBP

Figure 3.4. Ground state absorption spectra of M(ΙΙ)TPTBP complexes in toluene, M=Zn, Ni

The Q band corresponds to the transition to the macrocycle’s lowest 1(π,π*) excited

state with a weak vibrational progression to the blue; the intense B band corresponds to the

transition to the second excited singlet state. The position of the B and Q bands were shifted

to higher energy when going from Ni to Zn complexes. Red shifts of the Q band have been

observed with metallo-octabutoxynaphthalocyanines along the Co, Ni, Cu series.34-36

TDDFT calculations were performed for the lowest optically allowed excited states of

Zn complex and their excitation energies and oscillator strengths related to the appearance of

the Q band are presented and compared to the experimental data for the Ni complex in Table

3.2. For both compounds 11E state is responsible for the appearance of the Q band. This state

49

has substantial oscillator strength (on the contrary to common porphyrins) and is mainly

derived from the one electron transition from the G-HOMO 18b1 to the set of unoccupied 40e

orbitals. Due to considerable separation in energy between the Gouterman HOMO and

HOMO-1, the contribution from the 22b2 → 40e one electron transition to this state is minor.

For Zn, the energy gap between HOMO and HOMO-1 is 0.37 eV and has 27% contribution.

Ni has a higher energy gap between HOMO and HOMO-1 (0.55 eV) resulting in even less

contribution to the composition of the Q band (16%). For the Zn complex, the Q band is

computed at 1.89 eV and experimentally it can be seen at 1.90 eV. The theoretical results are

consistent with the observed red shifs of the B and Q band when going from Ni to Zn.

TDDFT Exp.

state Composition(%) Eva(eV/nm) f E(eV/nm)a

Zn(ΙΙ)TPTBP 11E(Q) 18b1→ 40e(71) 1.89(656) 0.131 1.90(652)

22b2→ 40e(27)

21E(B) 22b2→ 40e(59)

18b1→ 40e(21)

23b2→ 40e(11)

2.62(473) 1.52 2.68(462)

Ni(ΙΙ)TPTBP 11E(Q) 18b1→ 40e(82) 1.94(640) 0.21 1.93(643)

22b2→ 40e(16)

51E(B) 22b2→ 40e(58)

38e→ 23b2(21)

18b1→ 40e(10)

2.72(456) 0.95 2.78(447)

Table 3.2. Vertical excitation energies (Eva) and oscillator strengths (f) computed for the

lowest optically allowed excited states of Zn(ΙΙ) and Ni(ΙΙ)TPTBP.a data taken in toluene at

room temperature.

50

The near-degeneracy of the G-HOMO and G-HOMO-1 that exists in porphyrins,37, 38

does not occur in the benzoporphyrin derivatives due to the electronic effect of benzo-

annulation and it partially removes mixing of the Gouterman one-electron transitions. This

makes cancellation of the transition dipole moments in the Q band less effective, resulting in

the enhanced oscillator strength of the Q band. In the Zn complex, the large oscillator

strength, of 1.52 at 2.62 eV (473 nm) accounts for the intense B band at 462 nm in the ground

state absorption spectrum. This state arises from the mixing of the Gouterman one-electron

transitions 22b2 → 40e (59%), 18b1→ 40e (21%) and 23b2→ 40e (11%). In the Ni complex,

the 51E is responsible for the B band with similar transitions as Zn. In addition, the Ni

complex has a small contribution from an LMCT transition.

There are no states between the Q band and B band related with Zn complex while an

abundance of states can be found with Ni complex.

Optically Silent Excited States Below the S1(ππ*) State

Since the excited state deactivation pathway in the Zn complex is mainly governed by

the triplet (π,π*) state, the triplet excited states below the Q state were also been examined.

Table 3.3 lists their vertical absorption energies, Eva, related to the Franck-Condon transitions

from the ground state, together with adiabatic, Eadia and vertical emission Eve energies. The

relative energies and characters are important to understand the fast non-radiative decay of

the optically produced excited states, especially with Ni case. In addition to 3(π,π*) T1 state,

there are six states below the Q state can be found in Ni(ΙΙ)TPTBP. The details of the states

were gathered in reference 12. The lowest triplet state in Zn(ΙΙ)TPTBP is the 13E(ππ*) state,

which can be found 1.52 eV vertically above the S0 state.

51

TDDFT

state Composition(%) character Eva

Zn(ΙΙ)TPTBP 23E 23b2→ 40e(99) ππ* 1.86

13E 18b1→ 40e(99) ππ* 1.52

Table 3.3. Excitation energies (eV) obtained for the low-lying excited states of Zn(ΙΙ)TPTBP

Many metalloporphyrins undergo pseudo Jahn-Teller distortions because of the near-

degeneracy of the a1u and a2u orbitals in the HOMO.39 The Jahn-Teller postulate says any

non-linear molecular system in a degenerate electronic state will undergo distortion to form a

system of lower energy removing the degeneracy.40 Normally Zn porphyrins are expected to

undergo Jahn-Teller distortions into a D2 or C2v structure along the b1 or b2 active modes.30, 41,

42 Therefore to examine the effect of vibrational relaxation shift of the 13E(ππ*)→ S0

transition, geometry optimization was performed for the 13E, the lowest triplet state. It was

found that the C2v was the most stable structure.

In the Ni complex, the 21E has mixed dπ,dx2

-y2/MLCT and its triplet 23E is doubly

degenerate and has undergone a Jahn-Teller distortion into a C2v structure.12 Unlike the Zn

complex, the 1B2/3B2 splitting was found to be larger than 1E/3E splitting because of the more

prominent d,d character associated with one electron transitions.43

Steady-State Fluorescence and Transient Absorption Experiments

Steady-State Fluorescence Measurements

Steady-state fluorescence and ground state absorption spectra in the Q band region for

Zn(ΙΙ)TPTBP in pyridine solution were normalized to unit amplitude at their respective

maxima and displayed in Figure 3.5.

52

550 600 650 700 750 8000.0

0.2

0.4

0.6

0.8

1.0

norm

aliz

ed a

bsor

banc

e an

d flu

ores

cenc

e

wavelength/nm

absorption fluorescence

Figure 3.5. Normalized absorption (black) and steady state fluorescence (red) spectra of

Zn(ΙΙ)TPTBP in pyridine at room temperature. The fluorescence spectrum was obtained in

argon saturated pyridine with 442 nm excitation.

The fluorescence spectrum of Zn(ΙΙ)TPTBP has a mirror-symmetry relationship to the

ground state absorption band in the Q band region showing that it undergoes minimal nuclear

geometry change on excitation. The fluorescence originated from the first excited singlet

state(Q(0,0)) with maxima at 668 nm and a weak fluorescence band(Q(0,1)) at ca. 1290 cm-1

to the red of the Q(0,0) maxima.

The fluorescence quantum yield for the Zn(ΙΙ)TPTBP was estimated according to

equation 1 in chapter 2 using Zn meso-tetraphenylporphyrin (ZnTPP) as a standard with a

known quantum yield of 0.33 in deaerated benzene.34 The radiative lifetime was calculated

from the Strickler-Berg equation,44 as described in chapter 2. The Stokes’ shift between the

fluorescence and absorption maxima is 250 cm-1. When transitions are allowed and little

changes in the geometry of the excited state are observed, the Strickler-Berg equation has

proved to calculate the radiative lifetimes with good precision. For Zn complex, the (π→ π*)

transition responsible for the Q band is highly allowed making it more appropriate in

calculating the radiative lifetimes.45 Fluorescence quantum yield (ФF) of Zn(ΙΙ)TPTBP was

53

found to be 0.008 and the calculated radiative lifetime was found to be ~ 2 ns, in good

agreement with the published 1.5 ns lifetime, measured in benzene.34 Phosphorescence of

Zn(ΙΙ)TPTBP at room temperature in deoxygenated pyridine was not observed.


Femtosecond Transient Absorption

The Zn complex was investigated in order to provide experimental evidence for the

spectral properties of the π-localized singlet and triplet states of the

tetraphenyltetrabenzoporhyrin macrocycle keeping in mind that the metal orbitals do not

contribute to the π,π* basic electronic condition.

Since Zn(ΙΙ)TPTBP was unstable in toluene, transient absorption spectra were carried

out in pyridine solution. Figure 3.6 shows the temporal evolution of the transient absorption

signal for Zn(ΙΙ)TPTBP in pyridine upon excitation at 640 nm at different delay times.

500 550 600 650 700 750 800

-0.05

0.00

0.05

0.10

ΔA

wavelength/nm

2.69 ps 107.49 ps 394.71 ps 1107.05 ps

Figure 3.6. Femtosecond transient absorption difference spectra of Zn(ΙΙ)TPTBP in pyridine

at different delay times in the wavelength region 500-800 after 640 nm excitation.

Figure 3.7 shows the temporal evolution of the transient absorption signal for

Zn(ΙΙ)TPTBP in benzonitrile at 2 ps and 1000 ps upon excitation at 640 nm.

54

500 550 600 650 700 750 800

-1.0

-0.5

0.0

0.5

1.0

norm

aliz

ed Δ

A

wavelength/nm

2.29 ps 1075.1 ps

Figure 3.7. Femtosecond transient absorption difference spectra of Zn(ΙΙ)TPTBP in

benzonitrile at 2 ps and 1000 ps, normalized to the positive absorption maxima in the

wavelength region 500-800 nm after 640 nm excitation.

The spectrum formed within the instrument response time (200 fs) showed a broad

positive absorption interrupt by the bleaching signal due to the Q band region of

Zn(ΙΙ)TPTBP centered near 660 nm. A strong transient absorption at 500 nm was observed.

As the time evolved the positive absorption maxima shifted slightly to the red with the

maximum at ca.530 nm having a different shape indicating the formation of a new transient

species.

As with the other complexes, ultrafast experiments on Zn(ΙΙ)TPTBP were also done in

benzonitrile solution. The spectra related to 2 ps and 1000 ps normalized to the positive

absorption maxima in the wavelength region 500-800 nm after 640 nm excitation are

depicted in Figure 3.7. Scrutiny of the figure reveals that the differences in spectral shape

remained the same as in pyridine.

Photoexcitation at 640 nm directly generates the Q state (see the ground state optical

absorption spectrum in Figure 3.4) and the first observed transient will undoubtedly be the

absorption spectrum of the Q state. The ca.3 ps spectrum of the Zn complex is thus assigned

55

to the S1 → Sn transition of the π system. As time passes, the S1 state undergoes intersystem

crossing to the T1 state, as represented by the 1000 ps spectrum in Figure 3.6, peaking at 550

nm. Figure 3.8 shows the temporal profiles of the transient absorption displayed by

Zn(ΙΙ)TPTBP in pyridine solution following 640 nm excitation taken at four spectral regions.

0 200 400 600 800 1000 1200 1400

0.00

0.02

0.04

0.06

0.08

0.10

τ1 = 1.88 ±0.29 ps (13%)

τ2 = 406.37 ±5.52 ps (87%)ΔA_5

07nm

wavelength/nm

0 200 400 600 800 1000 1200 1400

0.000

0.005

0.010

0.015

0.020

0.025

τ1 = 0.7± 0.07 ps (10%)

τ2 = 338.39 ± 1.68 ps (90%)

ΔA_6

10nm

wavelength/nm

0 200 400 600 800 1000 1200 1400-0.002

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

τ1 = 1.98 ± 0.07 ps (47%)

τ2 = 344.53 ± 4.57 ps (53%)ΔA

_692

nm

wavelength/nm0 200 400 600 800 1000 1200 1400

0.000

0.005

0.010

0.015

0.020

τ1 = 1.58 ± 0.08 ps (25%)

τ2 = 326.19 ±3.51 ps (75%)

ΔA

_710

nm

wavelength/nm

Figure 3.8. Femtosecond kinetics profiles of the transient absorption signal at various probe

wavelengths for Zn(ΙΙ)TPTBP in pyridine, excited at 640 nm.

The decay of the positive absorption signal as well as the recovery of the negative

signal were fitted by a double exponential function providing two lifetimes. The kinetic data

for probe wavelengths ranging from 490 to 720 nm were analyzed in details. It was found

56

that the lifetime of the fastest component and its relative contribution to the overall decay

showed strong wavelength dependence. A plot of this first lifetime with different

wavelengths in the region 580 to 720 nm are displayed in Figure 3.8 with the transient

absorption spectral cuts taken at 2 ps and 8 ps time delays.

As can be seen the second component having longer lifetime also showed similar

wavelength dependence; but it can be assigned as ca.340 ps. During the 10 ps or so the

spectral shape of the positive absorption stretching from 500 nm to 640 nm does not change

even though slight red shift can be observed where the bleaching occurs. (Figure 3.8 lower

panel)

The studies of solvent-induced vibrational relaxation processes of the S1 states with

porphyrins have showed that the vibrational relaxation the rates are observed on the time

scale of picoseconds to tens of picoseconds. 46-48

Given the consideration that exciting to the blue of the Q band generates vibrationally

hot species, it is reasonable to assign the first exponential lifetime component to the cooling

of the S1 state. The excess vibrational energy is first distributed among all the vibrational

modes of the molecule and the cooling process occurs due to the solvent induced dissipation

of excess vibrational energy within the S1 excited state.

Figure 3.9 shows that the short lifetime lies close to the wavelength where the

negative and positive absorptions cross. The wavelength dependent kinetics arise from the

detailed nature of the upper and lower potential energy surfaces. When the origin state is hot,

upward transitions originating closer to the classical turning points become increasingly

probable.29 The vibrational states with high quantum numbers have a higher density of

vibrational oscillators below them than states with lower quantum numbers. Therefore they

will cool resulting in different wavelength regions having different cooling lifetime.

57

600 625 650 675 700 725-0.06

-0.04

-0.02

0.00

0.02

0.04

0

1

2

3

4

5

ΔA

wavelength/nm

1.46 ps 8.49 ps

τ 1/p

s

Figure 3.9. Upper panel: first lifetime/wavelength dependence, Lower panel: two transient

absorption spectral cuts taken at 2 ps and 8 ps time delays excited at 640 nm in pyridine

solution.

The kinetic profiles in benzonitrile solution showed that it also follows the

wavelength dependent kinetics as pyridine. And also it showed a bi-exponential decay

kinetics having slower rise lifetime and the subsequent decay lifetime. Figure 3.9 shows the

comparison of the kinetic profile at 520 nm of Zn(ΙΙ)TPTBP in the solvents toluene and

benzonitrile. It shows that in benzonitrile solution the lifetime for the S1 state is lower than

that of pyridine.

58

0 200 400 600 800 1000 1200 14000.0

0.2

0.4

0.6

0.8

1.0

τ2 = ~ 235 ps

τ2 = ~ 344 ps

ΔA_5

20 n

m

wavelength/nm

pyridine benzonitrile

Figure 3.10. Normalized kinetic profiles of the transient absorption signal at 520 nm for

Zn(ΙΙ)TPTBP in benzonitrile and pyridine solutions after 640 nm excitation.

The properties of solvents such as polarity and coordinating ability may alter the

absorption as well as the decay characteristic of the excited states. For example the N atoms

in pyridine and benzonitrile can bind to Zn center to form five and /or six coordinated

species.49, 50 Additionally, the lone pairs on nitrogen or oxygen in the above solvents, give

them the potential of coordinating to the central metal, stabilizing the chromophore by

lowering the energy. The electron-donating ability increase in the order benzonitrile <

pyridine.50

An exponential fit to the early part of the decay profile in Figure 3.7, generated a

lifetime of 200-300 ps for the S1 state depending on the solvent corresponding to intersystem

crossing to T1. The TDDFT calculations have confirmed that the (ππ*) triplet is the only state

below the S1. (Table 3.3) Wavelength dependent kinetics for vibrational relaxation has been

described for various Ni porphyrins and phthalocyanines.13, 20, 22, 23, 43, 51

The experimental results of Ni(ΙΙ)TPTBP demonstrated that the (π,π*) excited states

deactivate through a metal-centered state. The excess vibrational energy was dissipated by

59

intra-molecular and intermolecular modes of vibrational relaxation and the cooled d,d state

repopulated the initial ground state via a lower-lying charge transfer state.12

Nanosecond Transient Absorption Experiment

To determine the lifetime of the π localized triplet state, transient absorption

measurements on the nanosecond scale were performed on deaerated solution of

Zn(ΙΙ)TPTBP in toluene solution and excited at 460 nm.(it was necessary to add 1% of

pyridine by volume to the toluene solution in order to stabilize the complex.)

400 500 600 700-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

657nm

542nm

470nm

ΔA

wavelength/nm

3μs 20μs 50μs 100μs 200μs 300μs 400μs 500μs

Figure 3.11. Nanosecond transient absorption difference spectra of Zn(ΙΙ)TPTBP in Ar

saturated toluene (1% pyridine was added) solution at different delay times followed by 460

nm excitation. Vertical arrows indicate the evolution of the transient absorption signal.

Figure 3.11 shows the characteristic difference spectrum of the triplet state localized

on the porphyrin macrocycle.34 The negative absorption bands in the B and Q band regions

are due to the ground state bleaching. The maximum of the T1-Tn absorption band is around

542 nm, which corresponds to the maximum in the 1000 ps spectrum in figure 3.6.

60

There are clearly isosbestic points between the positive and negative absorption,

indicating the simultaneous behavior of the transitions. Figure 3.12 LH panel, illustrates the

decay profile of the transient absorption signal of the ZnTPTBP in pyridine/toluene solution

at the absorption at 540 nm.

0 100 200 300 400 500 600 700 800

0.000

0.005

0.010

0.015

0.020

τ = 236.72 ± 2.0μs

ΔA_5

40nm

_Ar

time/μs0 2 4 6 8

0

500

1000

1500

2000

2500

3000

10

Ksv = (190.14 ± 2.5)*103 M-1

τ Ar/τ

concentration of O2 / mM

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

-0.03

-0.02

-0.01

0.00

0.01

0.02

τ = 0.40 ± 0.004 μs

ΔA_5

00nm

_air

time/μs0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

τ1= 0.083 ± 0.001 μs

ΔA_5

00nm

_O2

time/μs

Figure 3.12. Left Upper: Nanosecond kinetic profile of the transient absorption signal in Ar

saturated pyridine_toluene solution, λexc = 460 nm. Solid red line is the fit to the experimental

points and blue line represents the residuals of the fit. Right, Upper: determination of the rate

constant for the quenching of the triplet 3(π,π*) state of Zn(ΙΙ)TPTBP by oxygen. Left Lower:

Nanosecond kinetic profile of the transient absorption signal in air saturated pyridine_toluene

solution, λexc = 460 nm Right Lower, Nanosecond kinetic profile of the transient absorption

signal in air saturated pyridine_toluene solution, λexc = 460 nm.

61

The triplet state lifetime was 237 μs and it decayed directly to the S0 surface with

isosbestic behavior at 442 nm, 492 nm, 627 nm and 680 nm. The decay of the triplet state

was enhanced in the presence of oxygen, and the bimolecular oxygen quenching rate constant

was obtained by measuring kobs in solution at different oxygen concentrations. The

competition plot for the decay of the triplet state in the presence of oxygen is shown in the

RH panel of Figure 3.12. The bimolecular rate constant was 1.2 x109 M-1s-1.

The photophysical properties of Zn(ΙΙ) TPTBP are summarized in Table 3.4.

Photophysical Properties λQ band/nm* 652 10-5εQband/ mM-1cm-1 47 λfluor /nm 668 ΦF 0.008Δstokes/cm-1 250 τS1/ps 380 λmax T1→ Tn/nm 560 τT1 in Ar/μs 236.710-9 kq 1.2

Table 3.4. The photophysical properties of Zn(ΙΙ)TPTBP. * Extinction coefficient in toluene

with Q band maxima 652 nm. Triplet lifetimes were measured in 1% of pyridine_ toluene

solution. All the other measurements were carried out in pyridine solution

Summary and Conclusions

There are only two electronic states involved in the photo deactivation of

Zn(ΙΙ)TPTBP complex. The lowest excited state, which is identified as 11E is the photo-

generated Q state and it has π,π* character. The singlet state is generated in high vibrational

levels and after vibrational cooling; the deactivation takes place mainly through the T1 triplet

state in competition with a low quantum yield of fluorescence. The decay of the triplet state

62

occurs on the microsecond time scale. There are no metal-associated states such as low-lying

metal centered (d,d) or charge-transfer excited states available as compared to Ni(ΙΙ)TPTBP.

2S0

T1

236 μs

S1

340 ps

Reaction coordinate

700fs

Figure 3.13. Kinetic diagram for the excited state deactivation of Zn(ΙΙ)TPTBP.

Theoretical and experimental results gave enough evidence to explore about the

influence of the central metal atom in the π system on excited state deactivation.

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67

CHAPTER 4: THE PHOTOPHYSICAL BEHAVIOR OF OPEN-SHELL

FIRST-ROW TRANSITION METAL

TETRAPHENYLTETRABENZOPORPHYRINS: COPPER AND COBALT

Abstract

The ground and excited state properties of Co(ΙΙ)TPTBP and Cu(ΙΙ)TPTBP have been

investigated by a combination of DFT/TDDFT and transient absorption spectrometry in order

to draw a complete picture of the excited state deactivation. Cu(ΙΙ) and Co(ΙΙ) were chosen to

investigate the impact from the half-filled d orbitals on the photophysical properties of the

porphyrin macrocycle.

For Co(ІІ)TPTBP, the first observed transient which is a π localized triplet state was

converted to a hot d,d state, wherein intramolecular cooling occurred that was completed

within 3 ps. After cooling the d,d state decayed into the ground state in an exponential

manner having a 17 ps lifetime.

Similar to Co(ІІ)TPTBP, the first observed transient in Cu(ІІ)TPTBP was assigned to

the triplet state that equilibrate with an LMCT state; ground state repopulation occurred via a

lower lying LMCT state. The dependence of the observed lifetime on solvent polarity

confirmed the participation of the LMCT state in the overall deactivation process. The

repopulation was completed within 500 ps.

Investigating the photophysical behavior of the cobalt and copper complexes proved

useful to highlight the relationship between the central metal on the rates of the deactivation

pathways.

68

Introduction Transition metal centers in tetrapyrrolic systems can introduce d orbitals at energies

within the HOMO-LUMO gap, giving rise to ligand → metal charge transfer (CT), metal →

ligand CT and/or dd excited states at energies between that of the lowest excited ππ* state

and the ground state, helping the deactivation of the photo-excited state. Copper(ΙΙ) and

Cobalt(ΙΙ) porphyrins have been a subject of many investigations because of their unusual

photophysical properties which stem from the interaction of the half-filled d orbital with a π-

electronic system of the porphyrin ring. As a result of coupling, the ground and singlet

excited states 1(π,π*) become doublet 2S(ππ*),where as the excited triplet state 3(π,π*) is split

into a trip-doublet, 2T1(ππ*) and a trip-quartet, 4T1(ππ*) which lie very close in energy to each

other.1, 2 The energy separation (ΔE) between the two triplet states depend on the d- π

exchange integral and the small energy gap can lead to establishment of equilibrium between

those two states.3

Cu has a d9 configuration with an unpaired electron on the dx2

-y2 orbital. As a result

the excited singlet states of the porphyrin are quenched by the paramagnetic d9 Cu(ΙΙ) ion

leading to very rapid intersystem crossing to low-energy trip-multiplet states.4 The excited

state relaxation process in Cu porphyrins has been extensively studied by picosecond

transient absorption and found that the rate of intersystem crossing occurs with the lifetime of

<350 fs, thereby obviating porphyrin fluorescence.5, 6

Calculations have shown that in Cu complexes the half-filled dx2-y2 orbital lies within

the HOMO-LUMO gap giving rise to many charge transfer transitions.7 Interaction between

the filled dz2 metal orbital and the empty eg

* porphyrin orbitals creates a low-lying metal to

ring (d,π*) charge transfer excited state and interaction between the half filled dx2-y

2 metal

orbital and the filled a2u porphyrin orbital creates a low-lying ring-to-metal (π,d) CT state.

These charge transfer states can lie between the singlet and triplet manifold.4, 8

69

In many cases, these charge transfer states are higher in energy than the set of triplets,

as shown by the absence of any experimental spectral evidence during the excited state

decay. However, in some cases this state can become accessible thermally or by changing the

solvent polarity resulting shorter lifetimes.

Transient absorption measurements on copper(ΙΙ) porphyrins in coordinating solvents

(oxygen or nitrogen containing solvents) have shown that the excited state lifetimes are

reduced from nanosecond scale to picosecond scale.1, 9, 10 Kim and et al1 suggested when an

axial ligand is attached to a square-planar species to make five-coordinated complex, ring-to-

metal charge transfer lowers the energy closer to or below the triplet manifold upon the

formation of the exciplex and it acts as a strong quencher of the 2T-4T manifold, thereby

shortening the lifetime.11, 12

Co(ΙΙ) porphyrins have a d7 electronic configuration with the unpaired electron in a

dz2 orbital. The ground state of these molecules are doublet while the lowest excited states are

doublets and quartets, formed by the interaction of the unpaired electron with the porphyrin π

system.13 Iterative extended Hückel calculations predicted the participation of a charge

transfer state, a2u(π)→Co(dz2), in the deactivation process.2 This was supported by ultrafast

transient absorption experiments8, 13 and near-IR absorption spectra. Picosecond kinetic

studies of Co(ΙΙ)octaethylporphyrin (OEP) and Co(ΙΙ)tetraphenylporphyrin (TPP) in different

solvents have been studied by Tait and et al13 who suggested that in non-coordinating

solvents, the excited-state relaxation of the complex proceeds via the trip-doublet, 2T(ππ*),

and the 2(π,d) charge transfer (CT) states with a lifetime less than 35 ps. These proposed

states are doublets without any changes in multiplicity and this may be accounted for the

short lifetimes observed.

The determination of d-orbital energies and low-lying excited states from ESR data on

cobalt(ΙΙ) porphyrins indicate that several d,d states14 lie below the 2T1(ππ*) state and the

70

lowest charge transfer state and might participate in the excited state deactivation process.15

In this case it seems that CT states are deactivated through lower-lying dd states. Lopponow

et al15 reported a 12 ps lifetime for excited states of Co(OEP) and a class of donor-appended

Co(ІІ)OEP molecules and suggested that the observed transient is not the charge transfer

state, but rather a dd excited state.

This chapter describes the photophysical properties of the

tetraphenyltetrabenzoporphyrins with Cu(ІІ) and Co(ІІ) as central metals. The relaxation

dynamics of the excited states have been investigated by ultrafast transient absorption

spectrometry and the results compared with DFT/TDDFT calculations of the nature and

energies of the excited states. The properties of the quartet excited states, which involve spin-

flip (SF) transitions have been studied using a TDDFT formalism based on the non-collinear

representation of the exchange-correlation (XC) potential,16, 17 recently implemented in the

Amsterdam Density Functional (ADF) code.18

Theoretical characterization of Co(ΙΙ)TPTBP and Cu(ΙΙ)TPTBP


DFT calculations in the spin unrestricted formalism, of the Co(ІІ) and Cu(ІІ) analogs

showed that these compounds adopt a D2d saddle conformation (Figure 4.1).

N M N

Figure 4.1. Side view of the DFT optimized molecular structure of M(ІІ)TPTBP, M = Cu or Co

71

Table 4.1 summarizes the optimized bond lengths, bond angles and geometrical

parameters for the two complexes. The dihedral angle between the adjacent pyrrolic nitrogen

(CαNNCα)ad that defines the saddling of macrocycle is ~25o and the twisting of the meso-

phenyl groups relative to the porphyrin core (the porphyrin-phenyl dihedral angle),

(C2C1CmesoCα) is ~ 700 in both complexes in support of the above confirmation.

parameter Co(ΙΙ)TPTBP Cu(ΙΙ)TPTBP

M – Np 1.938 2.005 Cα – N 1.385 1.377 Cα – Cβ 1.455 1.459 Cβ – Cβ 1.414 1.417 Cβ – Cm 1.405 1.404 Cm – Co 1.391 1.391 Co – Co 1.405 1.404 Cα – Cmeso 1.397 1.404 Cmeso – C1 1.495 1.494 C1 – C2 1.402 1.402 C2 – C3 1.396 1.396 C3 – C4 1.397 1.397 ∠CαNCα 107.5o 108.6o

∠NCαCβ 109.3o 109.0 ∠CαCmesoCα 122.1 123.3 ∠C2C1CmesoCα 70.8 67.2

∠(CαNNCα )op 0o 0o

∠(CαNNCα )adb 26.9o 25.0

aGeometry optimization was performed using all electrons basis set. The unpaired electron in

Co(ΙΙ)TPTBP is on the dz2 (23a1) orbital; in Cu(ΙΙ)TPTBP, it is on the dx2-y2 (23b2). bDihedral

angle (deg) between adjacent pyrrole ring planes.

Table 4.1. Selected bond lengths (Å) and bond angles (deg) calculated for M(ΙΙ)TPTBP in

the saddled D2d conformation.a

72

The difference in the central metal has a significant effect on the metal-pyrrolic

nitrogen distance. The distance is larger in Cu than Co, Co-Np(1.938Ao) < Cu-Np (2.005Ao).

Correspondingly, the CαNCα angle is larger indicating expansion of the macrocycle

core because of the single occupancy of the Cu dx2-y

2 orbital. The electron in this orbital is

involved in σ bonding with the nitrogen lone pairs of the pyrrolic rings. In order to relieve

electrostatic interaction, the Cu-Np distance increases, as a result of which the macrocycle

expands.


The ground state one electron levels obtained from the spin-restricted calculations are

shown in Figure 4.2 for Co(ΙΙ) and Cu(ΙΙ). The data for Ni(ΙΙ) also included for the

comparison purpose. Atomic population analysis is given in Appendix A.

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

CuTPTBPNiTPTBPCoTPTBP

16a216a2

23a138e38e

23a139e39e

22b222b2

18b118b1

23b2

23b2

40e 40e

17a2

dz2

dπ

16a2

38e

17b123a1

22b2

39e

18b1

40e

23b2dx2-y2

E/eV

Figure 4.2. Energy level diagram for Co-, Ni- and CuTPTBP. Orbital energies are obtained

from spin-restricted calculations. In the numbering of the MOs, the core electrons (C, N, O,

1s; Co, Ni, Cu 1s-2p) are not included. HOMO,HOMO-1 and LUMO are depicted in red.

73

a) Relative HOMO and LUMO positions

The highest fully occupied molecular orbital for all three complexes is 18b1, which is

a purely macrocycle orbital having contribution mainly from the Cα and Cβ. A small

stabilization of the HOMO can be seen going from Co(ΙΙ) to Cu(ΙΙ). It is possible that this is

due to the enlargement of the macrocycle cavity by Cu that results in an increased distance

between the two Cα on the pyrrolic rings. In Co(ΙΙ)TPTBP, this distance is equal to 2.234 Ao,

while in the Cu(ΙΙ)TPTBP complex it is 2.238 Ao.

As can be visualized in the contour plots in Figures 4.3 and 4.4, the electron densities

on the α-carbons in the G-HOMO (18b1) are alternating in sign. Thus, due to the expansion

of the macrocycle cavity in the Cu complex the overlap between the Cα 2pz orbitals

diminishes, decreasing the antibonding interaction which, in turn, causes the lowering of the

energy of the G-HOMO. A similar effect has been seen in a comparable series of

MNc(OMe)8 complexes (Nc = naphthalocyanine).19

The HOMO -1 is the 22b2 orbital where the electron density is mainly localized on the

pyrrolic nitrogen atoms and the meso-carbon atoms. The G-LUMOs are the degenerate 40e

orbitals that have contributions from Cmeso, Cα, Cβ and N orbitals. Moving along the series

Co-Ni-Cu, the energy of the LUMOs experiences a small downward shift as the size of the d

orbital decreases. As seen from Tables in Appendix A, there is a small contribution to 40e

from the 3dπ orbitals in Co (similar to the Ni analog) while the contribution of the 3dπ metal

orbital to the LUMO in Cu(ΙΙ)TPTBP is zero (Appendix A). The interaction between this dπ

orbital and the π system is anti-bonding as inferred from the contour plots.20 Thus smaller

orbitals have lesser overlap with the π orbital, lowering the energy of the resulting molecular

orbital. The stabilization of the LUMOs is greater than the downward shift of the HOMOs

leading to decrease the HOMO-LUMO gap causing the red shift of the Q band in the ground

state absorption spectra going along the series.

74

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.023b2

40e

18b1

39e

22b223a117b1

38e

16a2

E/eV

23b2

40e:xz 40e:yz18b1

39e:xz 39e:yz

22b2

23a1

17b1

16a2 38e:xz 38e:yz-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.023b2

40e

18b1

39e

22b223a117b1

38e

16a2

E/eV

23b2

40e:xz 40e:yz18b1

39e:xz 39e:yz

22b2

23a1

17b1

16a2 38e:xz 38e:yz

Figure 4.3. Energy level scheme for Co(ΙΙ)TPTBP together with contour plots of orbitals.

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

16a2

38e23a1

39e

22b2

18b1

23b2

40e

17a2

E/eV

17a240e:xz 40e:yz

23b2

18b1

22b2

39e:xz 39e:yz

23a1

38e:xz 38e:yz16a2

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

16a2

38e23a1

39e

22b2

18b1

23b2

40e

17a2

E/eV

17a240e:xz 40e:yz

23b2

18b1

22b2

39e:xz 39e:yz

23a1

38e:xz 38e:yz16a2

Figure 4.4. Energy level scheme for Cu(ΙΙ)TPTBP together with contour plots of orbitals.

75

b) The metal d levels

The molecular orbitals that have significant contribution from the central metal are

23b2(dx2-y2), 39e(dπ), 23a1(dz2) and 17b1(dxy). The 23b2 orbital, lies between the HOMO and

the LUMO in the Ni and Cu complexes, while with Co center, it is above. The Cu complex

has an unpaired electron in 23b2, an orbital that is mainly composed of the 3d(dx2-y2) orbital

and the 2px orbitals of the pyrrolic nitrogen atoms. According to figure 4.2, the metal orbitals

show stabilization when moving from Co(ΙΙ) to Cu(ΙΙ). It is more obvious for the 23b2 orbital,

which becomes occupied with an unpaired electron in Cu(ΙΙ). The empty 23b2 orbital in the

Co(ΙΙ) complex lies above the LUMO. This MO arises from the antibonding σ interaction

between the empty metal dx2-y2 orbital and the lone pairs of pyrrolic nitrogens. The contour

plots in Figure 4.3 and 4.4 clearly show that the dz2

orbital in Co(ΙΙ)TPTBP and Ni(ΙΙ)TPTBP

is mainly a pure metal orbital (82%), with some (8%) 4s character. But in Cu(ΙΙ)TPTBP, this

orbital mixes better with the macrocycle orbital (only 2%) due to the close proximity of the

interacting orbitals in energy and thus it is stabilized more.

c) Spin unrestricted calculations

In recognition of the fact that both the Co(d7) and Cu(d9) complexes have doublet

ground states, spin-unrestricted DFT calculations were carried out to ascertain the open shell

effects. Figure 4.5 shows the molecular orbital energy level diagram obtained from spin

unrestricted calculations for both Co(ΙΙ) and Cu(ΙΙ) compounds. The calculations indicate that

the highly delocalized π macrocycle orbital senses little exchange effect and the splitting

between the spin up and spin down configurations is very small. However, the orbitals that

are localized on the metal experience considerable splitting. For the Co(ΙΙ) complex the

largest splitting occurs in 23a1 and in the Cu(ΙΙ) complex it happens in 23b2, both being

singly occupied molecular orbitals (SOMO).

76

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

SOMO8% dz2

92% dxy

3% dπ

92% dxy

63% dπ

86% dz2

61% dx2-y2

56% dx2-y2

spin spin

22b2

18b1

23a117b1

16a2

23b2

38e

17b139e

22b2

39e

18b1

23a1

40e

23b2

E/eV

-6

-5

-4

-3

-2

-1

3% dxy

spin spin

E/eV

4% dπ 5% d

π38e

16a2 16a2

23a1

5% dπ

8% dπ39e

13% dxy

21% dx2-y2

22b2

18b1

23b2

46% dx2-y223b2

40e

17a2

Figure 4.5. The energy level diagram obtained from spin unrestricted calculations for

Co(ΙΙ)TPTBP(upper) and Cu(ΙΙ)TPTBP(lower).

According to the DFT results, the unpaired electron resides on the 23a1↑ in Co

complex, which is mainly a pure d orbital (82% 3dz2) with some 8% 4s character. Figure 4.6

77

displays the electron density distribution in the 23a1 orbital in Co(ΙΙ)TPTBP(left) and the

23b2 orbital Cu(ΙΙ)TPTBP (right). Analysis of the electron density distribution in the Co

complex indicates that the spin density is mostly on the cobalt atom, with small pockets of

opposite spin on the Np atoms.

Figure: 4.6. The 23a1 orbital in Co(ΙΙ)TPTBP(left) and the 23b2 orbital Cu(ΙΙ)TPTBP(right).

In Cu(ΙΙ)TPTBP, 23b2 becomes singly occupied. The spin density in the complex is

almost equally distributed between the copper atom and the pyrrolic nitrogens. The metal

levels are significantly stabilized going from the cobalt to the copper complex, attributable to

the increased nuclear charge of the metal; as a result, there are no metal states in the upper

valence region, apart from 23b2.


Ground State Absorption Spectra

Figure 4.7 shows the normalized ground state absorption spectra of Cu(ΙΙ)TPTBP in

toluene and Co(ΙΙ)TPTBP in hexane. The absorption spectrum of Ni(ΙΙ)TPTBP in toluene is

also presented for comparison. Since Co(ΙΙ)TPTBP proved not to be stable in toluene, all the

experiments on this compound were carried out in hexane.

78

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0648nm643nm637nm

Nor

mal

ized

abs

orba

nce

wavelength/nmN

orm

aliz

ed a

bsor

banc

e

wavelength/nm

Co Ni Cu

Figure 4.7. Normalized ground state absorption spectra of M(ΙΙ)TPTBP complexes with M =

Cu, Ni in toluene and Co in hexane. Inset: normalized absorption in the Q band region.

The Q band corresponds to the macrocycle’s lowest (π,π*) excited state with a weak

vibrational progression to the blue; the intense B band corresponds to the transition to the

second excited state. The position of the B and Q bands were shifted to lower energy going

from the Co to the Cu complex. Similar red shifts of the Q band have been observed with

metallo-octabutoxynaphthalocyanines along the Co, Ni, Cu series. 21, 22

Table 4.2 shows the calculated vertical excitation energies and oscillator strengths for

the optically allowed Q band of the Co and Cu variants with Ni23 for comparison. Also listed

are the experimental values for the Q band absorption maxima.

79

TDDFT expa

state Eva(eV/nm) f eV/nm CoTPTBP 42E 1.95/639 0.212 1.94/640 NiTPTBPb 11E 1.94/640 0.210 1.93/643 CuTPTBP 42E 1.92/648 0.216 1.92/648

ain toluene solution, this work, bdata taken from ref 24.

Table 4.2 Calculated vertical excitation energies (Eva) and oscillator strengths (f) for the

optically allowed excited states of MTPTBP (M = Co(II), Ni(II), and Cu(II)) in the D2d

confirmation and compared to experimental data.

The Q band is assigned to the 42E state in both Co and Cu complexes based on the

substantial oscillator strength. The TDDFT calculations reproduce the red shift of the Q band

in the series Co-Ni- Cu complexes. A bathchromic shift is consistent with stabilization of the

G-LUMO relative to the G-HOMO along the series, caused by the reducing contribution of

the Ni and Cu dπ metal orbitals to the G-LUMO.(Appendix A)

Excited States Below the S1(π,π*) State

The rates of return to the ground state of photo-excited compounds such as those

investigated here are critically determined by the nature and energies of the states lying below

the photo-produced state. To assist understanding the excited state deactivation, the manifold

of states lying between the Q state and the ground state has been examined by TDDFT

calculations. Vertical absorption energies (Eva) related to the Frank-Condon transition from

the ground state are listed in Tables 4.3 and 4.4 for the Co- and CuTPTBP complexes,

respectively. Figure 4.8 shows the theoretically predicted “dark” states lying below the Q

state for both Co and Cu complexes.

80

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Cu(ΙΙ)TPTBP

12A2

12A1

2T1/4T1

44E32E

2S1

12B1

12E

12B2

24B1

4T1

2T1

14A2

12A2

44E32E

24A2

E/eV

2S122A2

Co(ΙΙ)TPTBP

Figure 4.8. The theoretically predicted “dark” states lying below Q state for both Co and Cu

complexes. The d,d states are indicated by green lines, the LMCT states are indicated by red

lines, and the π states.

The Co(ІІ) and Cu(ІІ) complexes have an abundance of states of doublet and quartet

multiplicity between the Q state (42E) and the ground state surface. In the Co(ІІ) complex

there are six excited states that lie vertically between the 2S1(π,π*) and the 2,4T1(π,π*) states

(22E/ 34E) that in turn lie vertically above the ground state. They are 22A2, 24A2, 32E, 44E,

12A2, 14A2. The 44E has pure (d,d) character while others have LMCT character. 32E state has

very little d character as it involves a transition out of the 38e orbital which has only 3%

contribution from Co-3dπ.

81

statea compositiona (%) type Evab

42E(2S1) 18b1↑ → 40e↑ (38) 18b1↓ → 40e↓ (43) π,π* 1.95(0.2118)

22A2 16a2↓ → 23a1↓ (99) LMCT 1.94 24A2 16a2↑ → 23a1↓ (100) LMCT 1.93 32E 38e↓ → 23a1↓ (99) LMCT/dπ,dz2 1.82 44E 39e↑ → 23b2↓ (98) dπ,dx2-y2 1.77 12A2 18b1↑ → 23b2↑ (99) LMCT 1.63 14A2 18b1↑ → 23b2↓ (99) LMCT 1.62

32E (2T1) 18b1↑ → 40e↑ (52) 18b1↓ → 40e↓ (47) π,π* 1.58(0.0003)

24E (4T1) 18b1↓ → 40e↑ (99) π,π* 1.57

24BB1 17b1↑ → 23a1↓ (99) dxy,dz2 1.40 12BB2 22b2↓ → 23a1↓ (99) LMCT 1.00(0.0003) 12E 39e↓ → 23a1↓ (99) dπ,dz2 0.81(0.0003) 12BB1 18b1↓ → 23a1↓ (99) LMCT 0.46

aThe numbering and composition of the excited states refer to TDDFT calculations in the gas

phase; boscillator strengths in parentheses; ccomputed at the geometry optimized in the gas-

phase (D2d symmetry)

Table 4.3. Excitation energies (eV), composition and character of the lowest excited states of

Co(ІІ)TPTBP.

There are several pure d,d states lying below the 2,4T1(π,π*) set, one being 12E at 0.81

eV. The lowest LMCT states involves the transition from G-HOMO to 23a1 which has 86%

contribution from metal dz2 orbital and can be considered as having some d character. For the

Cu(ІІ) complex, four LMCT states are seen between the Q state (42E) and ground state in

addition to the 2,4T1(π,π*) states (12E / 14E) that lie vertically at ~ 1.5 eV. Represented are

two nearly degenerate 2,4LMCT states (32E and 44E) described as the 39e → 23b2 and two

pairs of states lying below the T1 states. These LMCT states arise from excitation from the G-

HOMO (18b1) and G-HOMO-1 (22b2) to the empty Cu dx2-y2 spin orbital.

82

statea compositiona (%) type Evab

42E(2S1) 18b1↑ → 40e↑ (25) 18b1↓ → 40e↓ (23) π,π* 1.91 (0.216)

32E 39e↓ → 23b2 ↓ (81) LMCT 1.88(0.019) 44E 39e↑ → 23b2↓ (94) LMCT 1.86

12E (2T1) 18b1↑ → 40e↑ (53) 18b1↓ → 40e↓ (45) π,π* 1.53 (0.004)

14E(4T1) 18b1↑ → 40e↓ (99) π,π* 1.53 12A1 22b2↓ → 23b2↓ (99) LMCT 1.24 12A2 18b1↓ → 23b2↓ (100) LMCT 0.75

aThe numbering and composition of the excited states refer to TDDFT calculations in the gas

phase; boscillator strengths in parentheses; ccomputed at the geometry optimized in the gas-

phase (D2d symmetry)

Table 4.4. Excitation energies (eV), composition and character of the lowest excited states of

CuTPTBP.

Transient Absorption Experiments: Spectral Observations and Dynamic Properties

Co(ΙΙ)TPTBP in Hexane

As with the other metal-centered analogs, excitation at 400 nm directly generates the

S2(π,π*) excited state, whereas excitation at 640 nm directly generates the lowest S1 (π,π*)

excited state. A series of absorption spectra of a solution of Co(ΙΙ)TPTBP in hexane (7.4 μM)

recorded over 40 ps after 400 nm (B band) excitation is shown in Figure 4.9. The

characteristic spectral features of these absorption spectra include (i) two immediately-

formed negative absorption bands with peaks at 437 nm and 640 nm, (ii) immediate

formation of positive absorption bands in the 350-400 nm, 480-580 nm, and 680-750 nm

ranges and (iii) the delayed growth of positive absorption near 450nm and 650 nm.

83

350 400 450 500 550 600 650 700 750 800

-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

41.17 ps 29.57 ps 21.21 ps 16.54 ps 11.34 ps 8.35 ps 5.43 ps 4.03 ps 3.36 ps 2.89 ps 2.59 ps

ΔA

wavelength/nm

Figure 4.9. Transient absorption behavior of Co(ΙΙ)TPTBP in hexane (7.4 μM), excited at

400 nm. The probe continuum was generated in a CaF2 plate.

The first observed transient (FOT), the 2.59 ps spectrum, shows a broad featureless

positive band across the whole spectral window interrupted by two negative peaks in the

regions covered by the intense B and Q bands of the ground state. Significant spectral

changes are seen as time evolves, the early positive and negative bands diminish, and the

positive absorbances at the red edges of the negative regions undergo blue shifts over several

picoseconds. Subsequently all features returned to zero amplitude as ground state recovery

occurred.

To ascertain whether the results of B band excitation (400 nm) were different from Q

band excitation and to obtain further information about decay dynamics, the compound was

excited by light pulses at 635 nm. In addition, the data were collected using a probe

continuum generated in a sapphire plate, which has the advantage of providing better signal-

to-noise at the expense of spectral coverage in the B band region. Figure 4.10 shows the

evolution with time of the induced absorption spectra. The LH panel shows the first 4 ps and

the RH panel shows the remainder. No significant differences in spectral shape and temporal

84

evolution could be discerned to those found with B band excitation, indicating no excitation

wavelength dependence and the internal conversion is extremely rapid and complete within

the instrument response (ca 200 fs).

In the LH panel, the blue shift of the positive signal to the red of the Q band is clearly

revealed. At ca 5 ps the spectrum has a derivative-like shape. As seen in the RH panel, this

derivative spectrum decays with time showing no further wavelength shifts. It has the

characteristics of a d,d spectrum as observed in many tetrapyrroles, such as the nominally

planar nickel porphyrins.5, 25, 26

600 625 650 675 700 725 750

-8

-6

-4

-2

0

2

0 10 20 30 40 50 60 70 80-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

time/ps

ΔA_6

40 n

m

τ1= 1.46 ±0.04 ps (31%)

τ2= 22.57±0.17 ps (69%)norm

aliz

ed Δ

A

wavelength/nm

4.2 ps 2.8 ps 2.1 ps 1.8 ps 1.5 ps

550 575 600 625 650 675 700 725 750-0.08

-0.06

-0.04

-0.02

0.00

0.02

wavelength/nm

ΔA

5.6 ps 8.4 ps 12.7 ps 16.8 ps 23 ps

Figure 4.10. Two time regimes of the transient absorption spectra of Co(ΙΙ)TPTBP in hexane

(8.1 μM) excited at 635 nm. Left: Transient absorption evolution during the first four

picoseconds of the experiment. Inset: the decay kinetics at 640 nm after 400 nm excitation.

Right: Transient absorption spectra at later time.

Figure 4.11 shows kinetic profiles of the Co(ΙΙ)TPTBP transient absorption in hexane

solution time profiles at 500 nm and 680 nm probe wavelengths. The former is in the spectral

region between the bleached B and Q bands; the latter is on the red side of the Q band.

85

The time profiles of the Co(ΙΙ)TPTBP transient absorption signal at all wavelengths

were fitted best by a pair of exponentials, the shortest lifetime of which showed significant

wavelength dependence. Similar kinetic behavior for the decay of the excited states has also

been observed for other Ni-centered tetrapyrroles.5, 26, 27 Note that at the bluer wavelength the

first lifetime is ca one half of that at 680 nm, but the slower phase has the same lifetimes.

Also it is evident that at 680 nm the time-dependent blue spectral shift contributes to the

rising edge of the time profile and has an apparent lifetime of near 400 fs.

0 10 20 30 40 50 600.00

0.01

0.02

0.03

0.04

time/ps

ΔA_5

00nm τ2 = 0.99 ± 0.01 ps (85%)

τ3 = 16.57 ± 0.50 ps (15%)

0 5 10 15 20 25 30 35 40 45 50 55 60

0.000

0.005

0.010

0.015

0.020

time/ps

ΔA_6

80nm

τ1= 0.40 ± 0.04 ps (100%)τ2= 2.10 ± 0.08 ps (64%)τ3= 16.70 ± 0.40 ps (36%)

Figure 4.11. Kinetic profiles of the transient absorption signal at 500 nm (left) and 680 nm

(right) probe wavelengths of Co(ΙΙ)TPTBP in hexane after 635 nm excitation.

The time profiles shown in above figures indicate that there are two phases in the

relaxation of the FOT to the ground state, the earlier one of which (ca 2.2 ps lifetime) is

accompanied by wavelength shifts and results in the readily identified difference spectrum of

a d,d state (Figure 4.10). This state decays into the ground state in an exponential manner (17

ps lifetime), with no further wavelength dependence and no spectral shifts, indicating that it

lacks vibrational excitation. The TDDFT calculations (Table 4.2) show that there are two d,d

states lying below the triplet state (1.57 eV) being a (dπ,dz2), 12E as the lowest, situated

0.81eV vertically above the ground state with the same multiplicity as the ground state. This

86

state arises from the transition from the anti-bonding 39e molecular orbital with 60% on the

dπ metal character to the empty dz2 spin orbital. It is not unreasonable to assign the observed

d,d state having a 17 ps lifetime, to the 12E. In Ni(ІІ)porphyrins and phthalocyanines, d,d

states have been reported with lifetimes in the hundreds of picoseconds region,27-29 but these

states in Co porphyrins and octabutoxypthalocyanines have been reported as having lifetimes

below 20 ps.22, 30 The presence of an abundance of lower-lying d,d and LMCT states (Table

4.2) and an unpaired electron probably all contribute to this rapid rate of decay.

It is readily apparent in Figures 4.9 and 4.10 that the conversion of the FOT into the

vibrationally cold d,d state over a few picoseconds is accompanied by significant spectral

blue shifting. This is apparent at both the B and Q bands of the ground state absorption

(Figure 4.8). This is symptomatic of a cooling process within the electronic envelope and has

been seen in ultrafast studies of metallophthalocyanines and metalloporphyrins in this

laboratory and elsewhere.5, 23, 26, 31, 32

To make for clearer argument about the nature of the FOT and subsequent species two

of the spectra are presented again in Figure 4.12. It shows two of the spectra extracted from

Figure 4.8; the blue curve is immediately post pulse (labeled 2 ps) and the red one is at 3 ps

later.

350 400 450 500 550 600 650 700 750 800

-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

5.0 ps

2.0 ps

ΔA

wavelength/nm

Figure 4 0 nm (blue curve) and at 3 .12. Spectra recorded immediately after a laser pulse at 40

ps later (red curve) in Co(II)TPTBP in hexane.

87

This latter is the characteristic spectrum of the cooled d,d state and the former is the

FOT. The FOT, except for the superposed bleaching bands, is broad and featureless and

state

al

iminis gy

sity

.

A series of absorption spectra of Cu(ΙΙ)TPTBP in toluene (2.8 μM) recorded over 120

itation is shown in Figure 4.13. The first observed transition

(F T)

shows no resemblance to that of a d,d state. Rather it is reminiscent of the optical absorption

spectrum of a (π,π*) state (cf the Cu complex). One such state listed in Table 4.3 is the Q

(42E) which is the one populated by Q band absorption. The other listings are the 2,4T1 pair

just short of 1.6 eV above the ground state. Since no stimulated emission was observed

unlike the case of the Ni23 complex, it is most likely that the FOT is one or both of the 2,4T1

pair. Presumably within a very short time this crosses into the higher regions of the d,d

vibrational manifold wherein intramolecular cooling occurs and is complete within 3 ps.

As vibrationally hot molecular species relax to the lower reaches of the vibration

ladder, their low energy transitions at the red edge of their optical absorption spectrum

d h while the higher energy transitions at the blue edge increase. Thus the higher ener

(blue) part of the white probe beam monitors vibrationally cooler species, where the den

of lower-lying vibrational states is less than the red-absorbing region. Thus it is not surprising

that lifetimes at the blue edge of transient spectra are longer than at the red edge, as seen here

Similar effects have been seen in the Ni analog.23

Cu(ΙΙ)TPTBP in Toluene

ps after 640 nm (Q band) exc

O (at 2 ps) displays a broad positive absorption band across the spectral window with a

superimposed negative band with a minimum at 652 nm coincident with the Q band of the

ground state. As the time delay between pump and probe was increased, the intensity of the

transient absorption signal decreased, but no spectral changes in shape were detected.

88

500 550 600 650 700 7

Figure 4.13. Temporal evolution of the transient absorption spectra of Cu(ΙΙ)TPTBP in

toluene (2.8 μM), excited at 640 nm. The inset shows an enlargement of the region near 675

nm.

50 800

-0.15

-0.10

-0.05

0.00

0.05

ΔA

wavelength/nm

121.11 ps 86.86 ps 69.04 ps 53.89 ps 40.63 ps 28.87 ps 20.57 ps 15.64 ps 11.77 ps 6.99 ps 5.13 ps 3.20 ps 2.10 ps

660 670 680 690 700 710 720

-0.10

-0.05

0.00

ΔA

wavelength/nm

B C D E F G H I J K L M N

Scrutiny of Figure 4.13 and in particular, the inset thereto reveals that, unlike the Co

and Ni23 cases, there is no hint of spectral blue shifts in the early picoseconds; in fact at 675

nm and at 450 nm there are clean isosbestic points. It is also clear, within the spectral

window limitations that the same positive to negative switching is occurring in both Q and B

band regions.

Cu(II)TPTBP in Benzonitrile

Figure 4.14 shows the spectral changes induced in a solution of the Cu complex in

benzonitrile (2.9 μM) after 640 nm excitation. The spectral images replicate those seen in

Figure 4.13.

89

500 550 600 650 700 750 800-0.15

-0.10

-0.05

0.00

0.05

ΔA

wavelength/nm

72.97 ps 54.63 ps 38.25 ps 27.21 ps 17.76 ps 12.03 ps 8.13 ps 5.01 ps 2.20 ps

Figure 4.14. Evolution of the absorption spectra of Cu(ΙΙ)TPTBP in benzonitrile, after

excitation at 640 nm.

A time profile of the repopulation dynamics at 653 nm presented in Figure 4.15 LH

panel, reveals that ground state recovery occurs in a single exponential process with a

lifetime of 54.0 ps. Within the regions of positive absorption the majority of the total decay

had the same lifetime as the ground state repopulation signal, however, there was an

additional small component that grew and decayed within the instrument response (e.g.

Figure 4.15, RH panel).

90

0 100 200 300 400 500

0.00

0.02

0.04

0.06

0.08

ΔA_

494n

m

time/ps

τ1= 6.20 ± 0.29 ps (11%)

τ2= 53.1 ± 0.24 ps (89%)

0 50 100 150 200 250

-0.15

-0.10

-0.05

0.00

time/ps

ΔA_6

53nm τ1 = 54.0 ± 0.13 ps

Figure 4.15. Time profiles of the transient absorption signal at two probe wavelengths for

Cu(ΙΙ)TPTBP in toluene after photoexcitation at 640 nm.

Figure 4.16 shows the comparison of the dynamics of the ground state repopulation

process of Cu(ΙΙ)TPTBP in the solvents toluene and benzonitrile.

0 50 100 150 200

-1.0

-0.8

-0.6

-0.4

-0.2

0.0τbenzonitrile~ 34ps

τtoluene~ 54 ps

time/ps

norm

alis

ed Δ

A_65

3nm

Toluene Benzonitrile

Figure 4.16. Normalized kinetic profiles of the transient absorption signal at 653 nm for

Cu(ΙΙ)TPTBP in two different solvents after 640 nm excitation.

The optical difference spectra shown in Figures 4.13 and 4.14 for the Cu complex in

toluene and benzonitrile, respectively, reveal that the FOT has positive absorption values

91

throughout the spectral window, superimposed on which are regions of intense negative

absorption caused by bleaching of the ground state absorption. And also those two figures

reveal that as the FOT (near 2 ps) decays the spectral shape does not change; in fact there are

distinct isosbestic points at 450 and 675 nm. During the decay period the negative absorption

is replenished with a lifetime of 54 ps (Figure 4.15, LH panel) in a single exponential

process. These observations would indicate that the FOT is converting into the ground state

of the complex in a one-step process. Nowhere during the spectral evolution are indications

of spectral blue shifts or spectral narrowing that accompany vibrational cooling. Thus it

would seem that the FOT is the unique precursor of the ground state in the deactivation

scheme.

However, the temporal profile in Figure 4.15, RH panel, does not support of this

conclusion. This shows that the decay profile at 494 nm (and other wavelengths, not shown)

is a composite of two exponential components, one having 6 ps lifetime (11%); the other

(89%) a 53 ps lifetime that is coincident with the ground state recovery lifetime. It appears

then, that there are two kinetically distinct populations of the FOT, both spectrally identical.

It is to be noted that there is no corresponding 6 ps component of ground state recovery.

It is possible that the resolution of this paradox is that the FOT (yet unassigned)

becomes involved in a rapid reversible reaction with another state that lies close by energy. In

such case the 6 ps lifetime component would be the decay of the initially produced FOT into

the equilibrium state and the 53 ps lifetime process would be the decay of the equilibrium

state into the ground state.33

An essential requirement for this scheme to be in accord with the experimental

observations is for the equilibrium state to have an absorption spectrum that is

indistinguishable from that of the FOT, i.e. the partner state contributes negligibly to the total

absorbance within the experimental spectral window. In this condition it is the temporal

92

profile of the precursor state (here the FOT) that tracks the kinetics of the equilibrium rise

and decay.

With this discussion in mind it is appropriate to turn to a consideration of the state

identities. In Table 4.4 the energies, composition and character of the lowest-lying states of

Cu(II)TPTBP are listed, as generated by the unrestricted TDDFT calculations. The highest

lying state of (π,π*) character is 42E at 1.91 eV with a high oscillator strength. This is the Q

state and it is the state that is generated directly by the 640 nm pump pulse.

The only other states of (π,π *) character below the Q state are the 12E/14E degenerate

pair at 1.53 eV. For this lower-lying π –localized states to be the FOT requires that the

2S 2T conversion (formally an internal conversion) could be accomplished within the

instrument response function (ca 200 fs). This has been described for various Cu(ΙΙ)

porphyrins and the conversion to the triplet state from the Q state is greatly enhanced by the

presence of the unpaired electron.34 Table 4.4 reports that there is a pair of LMCT state lies

adiabatically between the 2S(ππ*) and the 2T1(ππ*) that can facilitate the internal conversion.

In support of this is the fact that no stimulated emission was observed post-pulse,

hence no emissive states (e.g. 2S1) were present in the sample post-pulse, as would be the

case if all 2S1 had indeed collapsed into 12E within the pulse duration. So, the transient

observed at the delay time of 2ps (FOT) is assigned to the porphyrin triplet states. As seen in

Table 4.4, there is a LMCT state lies close in energy to the pair of triplet states and that can

be in equilibrium with those triplet states and 6 ps lifetime component would be the decay of

the initially produced FOT into the equilibrium LMCT state.

Figure 4.15 shows that the process of repopulation of the ground state (LH panel) has

the same lifetime as the decay of the equilibrium state (53 ps). This, together with the

appearance of the spectral isosbestic behavior would imply that the equilibrium state returns

to the ground state without the intrusion of intervening species, OR, that the intervening

93

species have lifetimes much shorter than the equilibrium state and therefore the rate of loss of

the equilibrium state is the rate determining process in the repopulation of the ground state.

The energy gap from the 12E/14E pair to the ground state is ca 1.2 eV and the rate of the

direct process would most certainly be very much lower than the 54 ps lifetime measured

here. The latter is the explanation favored here, because a direct, rapid radiationless

transition between the Q and ground states is unfavorable because the 1.91 eV energy

difference would lead to very small Franck-Condon overlaps, and if it happened the resulting

ground state would be vibrationally very hot and no indications of vibrational cooling (cf Co

complex) were observed. On the other hand, (Table 4.4) there is another LMCT state lies at

0.75 eV. Thus it is concluded that the equilibrated state proceeds through even the lower

lying state of LMCT character in a cascade with all the intermediate species having

ephemeral existence. In support it is noted that Chen et al35 reported that in a derivative of

CuTBP (copper meso-tetrakis(p-methoxyphenyl)tetrabenzoporphyrin-CuTMPTBP)

relaxation to the ground state occurred with a 50 ps lifetime, but no details of intervening

states were offered.

In benzonitrile solution (Figure 4.16), the repopulation of the ground state is more

rapid with a lifetime of ca 34 ps. The close proximity of these LMCT states to the ground

state would expect to shorten the lifetime in comparison with the 54 ps lifetime in toluene.

This supports the notion that LMCT states are involved in the deactivation pathway, since in

high dielectric media LMCT states move to lower energies, aiding the deactivation.


The excited state spectral and dynamic behavior of Co(ΙΙ)TPTBP and Cu(ΙΙ)TPTBP

has been investigated using transient absorption spectrometry with femtosecond time

resolution and DFT/TDDFT computational methods. Figure 4.17 summarizes the mechanism

of excited state deactivation pathways for both Co(ΙΙ)TPTBP and Cu(ΙΙ)TPTBP complexes.

94

2S0

2T1/4T1

15-20 ps

2 ps

>200 fs

2S1

LMCT

LMCT

Reaction coordinate

2S0

2S1

2T1/4T1 >200 fs

50 ps 6 ps

Reaction coordinate

Figure 4.17. Proposed schematic diagrams for the excited state relaxation pathways of

Co(ІІ)TPTBP (LH panel) and Cu(ІІ)TPTBP (RH panel).

For both complexes, it was found that the internal conversion from B band excitation

to S1 state is extremely rapid. For Co(ІІ)TPTBP, the FOT as recognized as the π localized

triplet state which is next converted to the vibrationally hot d,d state. It converted to cool d

d,d state over a few picoseconds accompanied by significant spectral blue shifting. d,d state

decayed into the ground state in an exponential manner having 17 ps lifetime.

In Cu(ІІ)TPTBP, ground state repopulation occurred through the set of (π, π*) triplet

states via a lower lying set of LMCT states.

References



95

3. Kalyanasundaram, K., In Photochemistry of Polypyridine and Porphyrin Complexes,

Academic Press: San Diego, CA: 1992.

4. de Paula, J. C.; Walters, A. V.; Jackson, A. B.; Cardozo, K. J. Phys. Chem 1995, 99,

4373.


6. Kruglik, S. G.; Apanasevich, P. A.; Chirvony, V. S.; Kvach, V. V.; Orlovich, V. A. J.

Phys. Chem 1995, 99, 2978.

7. Antipas, A.; Dolphin, D.; Gouterman, M.; Johnson, E. C. J. Am. Chem. Soc. 1978,

100, 7705.


Phys. Lett. 1998, 293, 1.

9. Yan, X.; Holten, D. J. Phys. Chem 1988, 92, 5982.

10. Holten, D.; Gouterman, M. In Optical Properties and Structure of Tetrapyrroles,

Blauer, G.; Sund, H., Eds. Walter de Gruyter, Berlin, New York: 1985; p 63.

11. Jeoung, S. C.; Takeuchi, S.; Tahara, T.; Kim, D. Chem. Phys. Lett 1999, 309, 369.

12. Jeoung, S. C.; Kim, D.; Cho, W. D.; Yoon, M. J. Phys. Chem 1996, 100, 3075.

13. Tait, C. D.; Holten, D.; Gouteman, M. J. Am. Chem. Soc. 1984, 106, 6653.

14. Lin, W. C. Inog. Chem 1976, 15, (5), 1114.

15. Loppnow, G. R.; Melamed, D.; Leheny, A. R.; Hamilton, A. D.; Spiro, T. G. J. Phys.

Chem 1993, 97, 8969.

16. Wang, F.; Ziegler, T. J. Chem. Phys. 2005, 122, 074109.

17. Wang, F.; Ziegler, T. J. Chem. Phys. 2004, 121, 12191.

18. Davis, W. B.; Ratner, M. A.; Wasielewski, M. R. Chem. Phys. 2002, 281, 333.

19. Ricciardi, G.; Soldatova, A. V.; Rosa, A. J. Inog. Bio 2008, 102, (3), 406.


96


22. Soldatova, A. V.; Kim, J.; A., R.; Ricciardi, G.; E., K. M.; Rodgers, M. A. J.

submitted for publication, 2007.

23. Zamyatin, A. V.; Soldatova, A. V.; Rodgers, M. A. J. Inog.Chim.Acta 2006, 360 (3),

857.

24. Zamyatin, A. V.; Soldatova, A. V.; Rodgers, M. A. J. Inog.Chim.Acta 2006, 360, (3),

857-868.

25. Rodriguez, J.; Holten, D. J. Chem. Phys. 1989, 91, (6), 3525.


27. Rosa, A.; Ricciardi, G.; Baerends, E. J.; Zimin, M.; Rodgers, M. A. J.; Matsumoto, S.;

Ono, N. Inog.Chem 2005, 44, (19), 6609.

28. Kim, D. H.; Holten, D. Chem. Phys. Lett. 1983, 98, 584.

29. Kim, D. H.; Kirmaier, C.; Holten, D. Chem. Phys. 1983, 75, 305.

30. Loppnow, G. R.; Melmed, D.; Leheny, A. R.; Hamilton, A. D.; Spiro, T. G. 1993, 97,

8969.

31. Gunaratne, T. C.; Gusev, A. V.; Peng, X.; Rosa, A.; Ricciardi, G.; Baerends, E. J.;

Rizzoli, C.; Kenney, M. E.; Rodgers, M. A. J. J. Phys. Chem. A 2005, 109, 2078.

32. Soldatova, A. V.; Kim, J.; Peng, X.; Rosa, A.; Ricciardi, G.; Kenny, M. E.; Rodgers,

M. A. J. Inog.Chem 2007, 46, 2080.

33. Hoshino, M.; Nagamori, T.; Seki, H.; Chihara, T.; Tase, T.; Wakatsuki, Y. J. Phys.

Chem. A 1998, 102, 1297.

34. Soldatova, A. V. Bowling Green State University, Bowling Green, 2007.

35. Chen, P.; Tomov, I. V.; Dvornikov, A. S.; Nakashima, M.; Roach, J. F.; Alabran, D.

M.; Rentzepis, P. M. J. Phys. Chem. A 1996, 100, 17507.

97

CHAPTER 5: THE PHOTOPHYSICAL PROPERTIES OF CHROMIUM(ΙΙΙ) AND

MANGANESE(ΙΙΙ) TETRAPHENYLTETRABENZOPORPHYRINS: AN INSIGHT

INTO EXCITED STATE DECAY DYNAMICS

Abstract

The photophysics of Cr(ΙΙΙ)TPTBPCl and Mn(ΙΙΙ)TPTBPCl have been investigated

using transient absorption spectrometry to provide data necessary for understanding the

influences that metals with partially occupied d orbitals have on the photophysical properties

of the porphyrin macrocycle.

After 640 nm excitation in Cr(ΙΙΙ)TPTBPCl, the S1 state undergoes fast intersystem

crossing (4S1→4T1), within a very short period of time (ca.0.05 ps lifetime) to 4T1 state.

The 4T1 state of Cr(ΙΙΙ)TPTBPCl in toluene deactivated with a lifetime of 224 ps, resulting the

4T1↔ 6T1 equilibrium. In benzonitrile solution Cr(ΙΙΙ)TPTBPCl showed a lifetime of 90 ps to

result the above mentioned equilibrium. A substantial decrease in the lifetimes of photo-

excited Cr(ΙΙΙ)TPTBPCl in benzonitrile indicated the possible existence of a CT state that

acts as a quenching state of the trip-multiplet state.

In Mn(ΙΙΙ)TPTBPCl , the excited sing-quintet, 5S1(π,π*) state deactivated to the

triplet trip-quintet, 5T1(π,π*) within the instrument response time. Within short time period

afterwards, it generated a hot d,d state wherein cooling occurred within 4 ps. Subsequently

the cooled d,d state repopulated the ground state with a 120 ps lifetime. It showed the same

kinetics in both coordinating and non coordinating solvents indicating that no CT states were

involved in the deactivation process.

Neither Cr(ΙΙΙ)TPTBPCl nor Mn(ΙΙΙ)TPTBPCl showed emission at room temperature.

98

Introduction

Chromium(ΙΙΙ) and Manganese(ΙΙΙ) porphyrin complexes are of considerable interest

for several reasons. Foremost is that they show unique absorption spectra, most probably due

to the unusual electronic structure induced by half filled metal d orbitals. Cr(ΙΙΙ) and Mn (ΙΙΙ)

are classified as d- type hyper porphyrins with paramagnetic metal ions with stable lower

oxidation states.1 Both Manganese(ΙΙΙ) and Chromium(ΙΙΙ) porphyrin complexes have been

studied for photonic and optoelectronic materials.2

Although chromium(ΙΙΙ) porphyrins are not naturally occurring substances in

biological systems, their chemical and photochemical properties have been widely studied in

relation to their photoinduced axial ligand dissociation and association reactions.3-5 Such

studies can provide information related to the biological functions of axial ligands in heme

proteins.6

The Cr(ΙΙΙ) metal ion has a d3 (S=3/2) electronic configuration with half-filled dxy, dyz

and dxz orbitals and empty dx2-y2 and dz2 orbitals. As a result of coupling between the

porphyrin and metal electronic configurations, the ground and singlet excited 1(π,π*) states

become quartet 4S(π,π*), where as the excited triplet 3(π,π*) is split into trip-doublet 2T(π,π*),

trip-quartet 4T(π,π*) and trip-sextet 6T(π,π*) states. In addition intramolecular CT transitions

between porphyrin π and metal d orbitals and d,d transitions within the metal orbitals are also

prossible.7 Gouterman et al. reported that there were two luminescence bands at 815 nm and

850 nm from ClCr(ΙΙΙ)TPP at 77K, and those bands were assigned to the emission originated

from trip-quartet 4T(π,π*) and trip-sextet 6T(π,π*) states. They also showed that the quartet

and sextet states to be in thermal equilibrium.8

Photophysical studies on CrΙΙΙ porphyrins have carried out to elucidate the photo-

dissociation mechanism of the axial ligand. Hoshino et al. reported laser flash photolysis of

ClCr(ΙΙΙ)TPP(L) (L= sixth axial ligand) in various solvents and suggested that the 4S1(π,π*)

99

excited state acts as the main route for the photodissociation process.3-5, 9-11 Jeoung et al. also

studied the photo-dissociation of halochromium(ΙΙΙ) tetraphenylporphyrin using transient

absorption and Resonance Raman Investigations and explained the phenomenon in terms of

electron density changes in the metal d orbitals, which is sensitive to the interaction with σ

donor axial ligands.12 They found that the temporal evolutions of photoinduced absorption

and bleaching signals of XCr(ΙΙΙ)TPP in benzene exhibit biphasic decay profiles with time

constants of 1 and 20 ms. The faster decay was assigned to the four coordinated CrΙΙΙTPP*

species and the slower decay component to the recombination process returning to the

original five-coordinate XCr(ΙΙΙ)TPP species. A significant reduction in the lifetime of

photoexcited ClCr(ΙΙΙ)TPP in THF was observed as compared with that in benzene.

An investigation on the luminescent properties of Cr(ΙΙΙ) porphyrins showed that the lifetime

of the photo-excited state is relatively short ~295 ps in ethanol and also depends on the

solvent.13

Manganese porphyrin complexes serve as model compounds for the manganese

dependent oxygen evolution in green plant photosynthesis.14 As a basis for understanding

their possible role in photosynthesis, many studies have carried out on the photochemistry of

manganese porphyrins.15-17

The Mn(ΙΙΙ) metal ion has a high spin d4 ground state electronic configuration(S= 2)

and only the high energy dx2-y2 orbital is unoccupied in the ground state. Because of the

coupling of the unpaired metal electrons with the ring π electrons, the ground state is a

quintet (5S0). The quintet excited state (5S0) is derived from the lowest excited ring (π,π*)

singlet and the trip-multiplet (3T1, 5T1, 7T1 ) set is derived from the lowest ring (π,π*) triplet.7

According to Hund’s rule the lowest energy state have the highest multiplicity.

Mn(ΙΙΙ) porphyrins are generally considered as “non-luminescent”. This is usually

attributed to quenching of the normally emissive (π,π*) excited states by CT or d,d states at a

100

lower energy. 1, 7, 18 Irvine et al.19 used 1-ps excitation at 597 nm with Mn(ΙΙΙ)TPPCl in

CH2Cl2 and pyridine and reported a 17 ps lifetime for the decay of the strong absorption near

500 nm; Chirovonyl and et al 20 measured a 55 ps lifetime for Mn(ΙΙΙ)Meso(Cl) in the 450-

600 nm region following photoexcitation. Both the above time constants were assigned to the

decay of a ring (π,π*) “trip-multiplet” state. Holten et al reported the kinetics of the transient

absorption in the 500-900 nm region using Mn(ΙΙΙ)TPPCl and Mn(ΙΙΙ)OEPCl in CH2Cl2 and

pyridine and reported two transients having fast and slower components. The fast decay had

5-30 ps lifetime and they mentioned that Chirovonyl’s values is an average for the slow and

fast components that they have observed with Mn(ΙΙΙ)OEPCl. For the slower component

Holten reported an 80 ps lifetime for TPP and 140 ps lifetime for OEP and they assigned the

short-lived component to the “trip-quintet” 5T1(π,π*), a fraction of which relaxes to the longer

lived trip-septet, 7T1(π,π*). They also suggested that the decays of both trip-multiplets

proceeds via lower energy CT or d,d excited states.21

There is a scarcity of quantum chemical studies on manganese porphyrins and there

are so many unresolved questions to be answered, such as how the unpaired electron spins are

distributed in the different electronic states.22, 23 Gunter et al has mentioned that the

experimental chemistry of manganese porphyrins greatly needs theoretical support to

interpret the experimental data.24

Figure 5.1.displays the orbital energy levels for Cr(ΙΙΙ) and Mn(ΙΙΙ) porphyrin

complexes with trivalent metal chlorides by iterative extended Hückel calculations.

101

-12

-11

-10

-9

-8

-7

Mn111Cr111

dx2-y2

dx2-y2

eg(π*)eg(π*)

a2ua1u

dxy

dπ

dΖ2

dπ

dxy

a2u

a1u

orbi

tal e

nerg

y/eV

Figure 5.1. Orbital energy levels for Cr and Mn porphyrin complexes with trivalent metal

chlorides by iterative extended Hückel calculations. Data for the figure are taken from

reference 8.

The figure shows that the dz2 orbital in Cr(ΙΙΙ) is vacant and half filled in Mn(ΙΙΙ).

The half filled dxy, dyz and dxz orbitals are located between the LUMO and HOMO of the

porphyrin ring whereas an empty dx2

-y2 orbital lies well above the eg orbital giving rise to

CT(dπ*,πd) or d,d state that would participate in the deactivation of photo-excited Cr(III) and

Mn(ΙΙΙ) porphyrins.

PHOTOPHYSICAL PROPERTIES OF Cr(ΙΙΙ)TPTBPCl


Figure 5.2 shows the normalized ground state absorption of Cr(ΙΙΙ)TPTBPCl in

different solvents to show the effect of coordinating solvent vs. non-coordinating solvents.

Toluene and DMF serve as non-coordinating solvents while benzonitrile is a coordinating

solvent.

102

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0ΔA

_nor

mal

ized

to B

ban

d

wavelength/nm

toluene benzonitrile

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

ΔA_n

orm

aliz

ed to

B b

and

wavelength/nm

toluene DMF

Figure 5.2. Normalized ground state absorption of Cr(ΙΙΙ)TPTBPCl in toluene, DMF and

benzonitrile solutions.

Cr(ΙΙΙ)TPTBPCl showed strong B band absorptions near 500 nm, and a Q band

absorption in the 600-700 nm region. These main bands can be allocated to transitions a1u(π),

a2u(π) →eg(π*). In addition there are other bands in the region between 350 nm to 450 nm.

For metal atoms with partly filled d orbitals, allowed charge transfer transitions a1u(π), a2u(π)

→eg(dπ) are also expected and they might be account for the extra bands seen near 400 nm.

The B band absorption maximum in toluene becomes slightly red-shifted in benzonitrile

solution. The absorption spectra in toluene and DMF showed no difference.

Iterative extended Hückel calculations performed by Gouterman8 showed that the

energy gap between the metal dπ orbitals and the a2u orbital decreases along the series of

transition metals Cr(ΙΙΙ), Mn(ΙΙΙ) and Fe(ΙΙΙ) (Figure 5.1). As a consequence, CT transitions

from the ground state; a1u(π), a2u(π) → eg(dπ) become more favorable along this series. The

energy of this CT state is higher than that of the B band for Cr(ΙΙΙ), and the additional

absorption peaks of Cr(ΙΙΙ)TPTBPCl in toluene and benzonitrile to the blue of the B band can

be assigned to those CT transitions.

103


Cr(ΙΙΙ)TPTBPCl

Femtosecond transient absorption spectrometry with excitation wavelength in the Q

and B band regions was employed in order to explore the influence of the central metal on the

excited state deactivation. Different spectra were measured using different solvents.

Cr(ΙΙΙ)TPTBPCl in toluene and DMF

Figure 5.3 displays the overlaid transient absorption spectra of Cr(ΙΙΙ)TPTBPCl in

toluene at different time delays following 640 nm excitation. To ascertain whether the results

of Q band excitation were different from B band excitation and to obtain further information

about decay dynamics, the compound was excited by light at 400 nm, which generates the 4S2

(π,π*) state, whereas excitation at 640 nm produced the 4S1 (π,π*). Transient spectra of both

excitations showed that there is no difference proving that internal conversion from S2 → S1

is fast.

500 550 600 650 700 750

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

ΔA

wavelength/nm

2.61 ps 83.18 ps 294.30 ps 776.30 ps

500 550 600 650 700 750

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

ΔA

wavelength/nm

2.90 ps 3.94 ps 24.50 ps 161.14 ps 459.73 ps 1155.73 ps

Figure 5.3. Left: Transient absorption spectra of Cr(ΙΙΙ)TPTBPCl in Toluene, excited at 400

nm. Right: Transient absorption spectra of Cr(ΙΙΙ)TPTBPCl in DMF, excited at 640 nm.

104

The first observed transient (ca.2 ps) displays the characteristic features of the excited

states localized on the porphyrin macrocycle, exhibiting a broad positive absorption band

with maximum at 530 nm and two negative bands at 495 nm and 665 nm due to ground state

depopulation. When the time delay between the pump and probe was further increased, the

intensity was decreased having the same spectral shape. Two isosbestic points, 515 nm and

680 nm were maintained.

Figure 5.3, RH panel displays the evolution of transient absorption spectra of

Cr(ΙΙΙ)TPTBPCl in DMF solution after excitation in the Q band. Transient absorption spectra

in DMF replicate the spectral images those seen in Figure 5.3 in toluene showing that the

broad positive absorption interrupted by two negative bands at 495 nm and 665 nm. It also

shows the small changes in intensity of the signal at early evolution time and as the time

evolves the signal decreased without changing spectral shape.

In Figure 5.4 the LH panel displays the temporal decay profile taken at 530 nm

excited at 640 nm in toluene solution. The RH panel shows the time profile at the same

wavelength during the first 5 ps. On the same figure the time profile of a Raman signal

generated from the solvent is superimposed to show the instrument response profile.

The experimental time window limits the observation range and the kinetic profile at 530 nm

showed a bi-exponential fit having 0.05 ps and 224 ps lifetimes.

105

0 200 400 600 800 1000-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07ΔA

_530

nm

time/ps

τ1 = 0.05 ± 0.01 ps (23%)

τ2 = 224.47 ± 2.53 ps (77%)

1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

6

solvent

0.05 ps

ΔA_5

30nm

time/ps

Figure 5.4. Left: kinetic behavior of Cr(ΙΙΙ)TPTBPCl in toluene at 530 nm after 640 nm

excitation. Right: first 6 ps of kinetic trace at 530 nm upon excitation at 640 nm. The dashed

blue curve is the instrument response to a solvent Raman signal. Both curves are normalized

to unity.

It has been reported that for various Cu(ΙΙ) porphyrins and phthalocyanines the

intersystem crossing from the S1 state to the T1 state occurs very rapidly (<350 fs), followed

by the establishment of the trip-doublet/trip-quartet equilibrium.25, 26 Presumably, the

interaction between the excited porphyrin ligand and paramagnetic central copper might be

shortening the lifetime of the 2S1 state. It has also known that the paramagnetic Cr(ΙΙΙ)TPP

complex does not show normal (π, π*) fluorescence and that extremely weak emission from

4T1 is observed in ethanol.13 These findings were interpreted as arising from extremely rapid

intersystem crossing from 4S1 to 4T1. Moreover, excitation to any quartet excited state should

lead to rapid radiationless relaxation to the lowest excited state quartet, 4T1.8 Considering the

time profile in Figure 5.4, the short lifetime ~ 0.05 ps can be assigned to the conversion from

(4S1→4T1) that generates the π-localized triplet state.

106

The energy gap (ΔE) separating the 4T1 and 6T1 trip-multiplet state is ca.550 cm-1, and

it could result in equilibration between the 4T1↔ 6T1 states.13 The same study by Harriman

demonstrated that 4T1 state of ClCrΙΙΙTPP in hexane deactivates with a lifetime of 295 ps,

resulting the above mentioned equilibrium. The slower lifetime in figure 5.4 can be assigned

to the 4T1 state of the ClCrΙΙΙTPTBP which is in equilibrium with 6T1 states.

Cr(ΙΙΙ)TPTBPCl in Benzonitrile

In Figure 5.5 the upper panel shows the temporal evolution of a solution of

Cr(ΙΙΙ)TPTBPCl in benzonitrile following 640 nm excitation. The lower panel shows spectral

cuts taken from early time evolution in toluene and benzonitrile solution excited at 640 nm to

display the blue shift of spectra in toluene solution.

500 550 600 650 700 750 800-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

ΔA

wavelength/nm

3.67 ps 21.59 ps 82.38 ps 228.29 ps 660.38 ps 1404.23 ps

500 550 600 650 700 750

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2.61 ps

2.23 ps

ΔA

wavelength/nm

benzonitrile toluene

Figure 5.5. Left: Transient absorption spectra of Cr(ΙΙΙ)TPTBPCl in benzonitrile solution

following 640 nm excitation. Right: The early time evolution of Cr(ΙΙΙ)TPTBPCl in

benzonitrile and toluene solutions normalized to the maximum positive band.

107

Similar to the spectral profiles in DMF and Toluene, Cr(ΙΙΙ)TPTBPCl in benzonitrile

shows the characteristic features of excited states localized on the porphyrin macrocycle, with

a broad positive absorption interrupted by the Q band bleaching. It has a positive maximum at

537 nm interrupted by two negative peaks at 505 nm and 669 nm. Figure 5.6 displays the

kinetics of Cr(ΙΙΙ)TPTBPCl in benzonitrile solution at 535 nm following 640 nm excitation.

0 25 50 75 100 125 15

0.00

0.02

0.04

0.06

0.08

0

ΔA_5

35nm

time / ps

τ2 = 0.29 ± 0.03ps (19%)

τ1 = 83.97 ± 1.87ps (81%)

Figure 5.6.The kinetic profile of Cr(ΙΙΙ)TPTBPCl in benzonitrile solution followed by the

640 nm excitation.

It also showed biexponential kinetics having 0.29 ps and 84 ps lifetimes. The short

lifetime can be viewed as faster intersystem crossing process as seen in previously.

When comparing the changes in spectra and kinetics in benzonitrile with toluene, there are

two scenarios to consider. Firstly the slower lifetime has shortened, secondly the spectra

shifted to the blue in benzonitrile.

Axial ligands in CrΙΙΙ porphyrins are known to be anomalously labile compared with

those of other chromium(ΙΙΙ) complexes having non-porphyrin ligands.9, 27-29 Transient

absorption and resonance Raman investigations on the axial ligand photodissociation studies

of Halochromium(ΙΙΙ) Tetraphenylporphyrin (XCr ΙΙΙTPP, X=Cl, Br) in coordinating solvents

such as THF have showed that the axial halogen ligand photodissociates to form the five-

108

coordinate CrΙΙΙTPP(THF) on photoexcitation. Significant reduction in the lifetime compared

to non coordinating solvents was assigned to the five-coordinate photoexcited CrΙΙΙ

TPP(THF)* species, which decays rapidly due to the participation of low energy states below

the tripmultiplet (π,π*) states. Five coordinate CrΙΙΙ TPP(THF)* species was suggested to

possess (π,dπ) charge transfer character based on the comparison with those of other

paramagnetic metalloporphyrins.

With that in mind, when return to photoexcitation to the Cr(ΙΙΙ)TPTBPCl in

benzonitrile, the red shifted transient that has seen compare to the transient in toluene

solution can also be assigned to six co-ordinated Cr(ΙΙΙ)TPTBPCl(benzonitrile)* species. A

substantial decrease in the lifetimes of photoexcited Cr(ΙΙΙ)TPTBPCl in benzonitrile can be

ascribed to the possible existence of a CT state that acts as a quenching state of the

tripmultiplet state.

The ground state absorption spectra of Cr(ΙΙΙ)TPTBPCl in benzonitrile and toluene

exhibit absorption maxima at 505 nm and 496 nm, respectively, in the Soret band region as

shown in (Figure 5.2) The absorption maxima of Cr(ΙΙΙ) compound in toluene and DMF

showed to be no significant difference. This proposes that toluene and DMF does not affect

the axial ligand nature and the ground-state electronic structure of Cr(ΙΙΙ) porphyrin, whereas

benzonitrile solvent acts as a σ-donor to form a six-coordinate CrΙΙΙ porphyrin

(ClCrΙΙΙTPTBPCl(benzonitrile)), in the ground state.

PHOTOPHYSICAL PROPERTIES OF Mn(ΙΙΙ)TPTBPCl


Figure 5.7 displays the ground state absorption spectra of Mn(ΙΙΙ)TPTBPCl in three

different solvents. The absorption spectra of Mn(ΙΙΙ)TPTBPCl are dissimilar to the normal

absorption spectra of porphyrins in the visible region. It shows a splitting of the B band into

109

bands at 400 nm and 530 nm. The additional bands can be attributed to charge transfer bands

assigned to π,d transition.

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

684nm

532nm390nm

abso

rban

ce

wavelength/nm

benzonitrile toluene

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

670nm622nm

404nm

470nm

ΔA

wavelength/nm

Figure 5.7. LH panel: Absorption spectra of Mn(ΙΙΙ)TPTBPCl in toluene and benzonitrile;

RH panel: Absorption spectra of Mn(ΙΙΙ)TPTBPCl in DMF.

Iterative extended Hückel calculations have shown that the energy of the CT state is at

approximately the same energy in Mn(ΙΙΙ)TPP.19 The red ground state absorption bands in the

“octaalkyl substituted” complexes such as Mn(ΙΙΙ)OEP(Cl) had been ascribed to transitions

a2u(π)→ a1g(dz2). The bands near 680 nm and 620 nm in Mn(ΙΙΙ)TPPCl had been described as

a representation of a CT transition.21 In addition, there may be large number of CT((π,d),

( d,π*)) and d,d states available which may not evident from the ground state visible

spectrum.

Even though the spectral pattern is the same for toluene and DMF, some of the

maxima are sharpened and red shifted when going from toluene to DMF. Boucher noted the

similar spectral behavior with halide complexes of the manganese(ΙΙΙ) porphyrins going from

benzene to chloroform and which was ascribed to a solvent effect involving the hydrogen

bonding ability of the chloroform.30 The same thought can be applied the difference of the

110

porphyrins dissolve in coordinating solvent, the spectra were identical to the spectra observed

with Mn(ΙΙΙ)TPTBPCl in toluene and benzonitrile.14, 30


Mn(ΙΙΙ)TPTBPCl

Mn(ΙΙΙ)TPTBPCl in Toluene

Figure 5.8 shows the overlaid transient absorption spectra at a series of delay times

after 400 nm excitation of Mn(ΙΙΙ)TPTBPCl in toluene solution. There can be seen strong

excited-state absorptions to the blue of 500 nm, bleaching of the absorption bands in the B

and Q band regions and positive absorption between 550 nm to 650 nm and from 725 nm

extending to longer wavelengths. The spectrum of the first observed transient (Figure 5.9,

lower panel, spectrum at 1.79 ps) has more positive absorption in the 550 to 650 nm region

which is red of the Soret band and may be assigned to a trip-multiplet state. More specifically

it can be assigned that the trip-quintet, 5T1(π,π*), based on the fact that in first row

paramagnetic complexes the intersystem crossing process within π localized system occurs

extremely rapidly due to the presence of the unpaired electron. Moreover, Manganese(ΙΙΙ)

porphyrins are identified as the essentially non-luminescent which implies the Q State

deactivates within the instrument response time to the triplet from the lowest sing-quintet,

5S1(π,π*).

111

500 550 600 650 700 750

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04Δ

A

wavelength/nm

1.79 ps 2.19 ps 2.59 ps 3.23 ps 4.83 ps 8.22 ps 16.52 ps 30.32 ps 58.42 ps

510 525 540 555 570

-0.04

-0.03

-0.02

-0.01

0.00

0.01

ΔA

wavelength/nm

44.62 ps 21.02 ps 15.02 ps 10.42 ps 7.62 ps 5.63 ps 4.03 ps

500 550 600 650 700 750-0.08

-0.06

-0.04

-0.02

0.00

0.02

ΔA

wavelength/nm

4.83 ps 1.79 ps

Figure 5.8. LH panel: Transient absorption spectra of Mn(ΙΙΙ)TPTBPCl in toluene solution

following 400 nm excitation. RH panel: spectral evolution after 4 ps in the wavelength region

500-570 nm showing decay of dd state. The lower panel shows spectral cuts taken at two

time delays.

As time progressed, the positive absorption signal red of the B and Q bands shifted to

the blue evolving a spectrum having a derivative like shape and no isosbestic behavior. At

later times, the derivative-shaped spectrum decayed to the ground state having isosbestic

points at 500 nm and 454 nm, reflecting complete ground-state recovery.

Presumably within a very short time the trip-quintet, 5T1(π,π*) state, crosses into the

higher regions of the d,d vibrational manifold (Figure 5.8, lower panel, 5 ps transient),

112

wherein intramolecular cooling occurs and is complete within 4 ps. After the cooling it

repopulates the ground state.

The spectral evolution described above was accompanied by complex temporal

behavior with wavelength dependent lifetimes. The temporal decay profiles taken at 530 nm

and 686 nm are shown in Figure 5.9.

0 100 200 300 400 500

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

τ3 = 0.94 ± 0.01 ps (39%)τ1 = 16.04 ± 0.29 ps (33%)τ2 = 120.53 ± 1.71 ps (28%)ΔA

_530

nm

wavelength/nm0 150 300 450 600

-0.035

-0.028

-0.021

-0.014

-0.007

0.000

ΔA_6

86nm

wavelength/nm

τ1 = 0.69 ± 0.03 ps (23%)τ

3 = 13.66 ± 0.23 ps (50%)

τ2 = 117.51 ± 2.43 ps (27%)

Figure 5.9. Kinetic behavior of Mn(ΙΙΙ)TPTBPCl in toluene solution at 530 nm and 686 nm

following 400 nm excitation.

The kinetics could be described with a three exponential fit throughout the spectral

window implying that there are three components related to the deactivation from photo-

excited Mn(ΙΙΙ)TPTBPCl. The first phase is characterized by a fast decay (~ 1 ps lifetime)

and it is probably the relaxation of the vibrationally hot species within the molecule (IVR),

the second phase (~ 16 ps lifetime) represents thermal losses to solvent oscillators. After the

vibrational cooling the d,d state deactivates to the ground state as shown by the isosbestic

behavior at 500 nm and 454 nm, with a lifetime of 120 ps.

113

Mn(ΙΙΙ)TPTBPCl in Benzonitrile

Holten et al21 mentioned that the rapid deactivation from trip-multiplets in Mn(ΙΙΙ)

porphyrins probably proceeds via lower energy CT or d,d excited states. To check the

possibility of involving a CT state in the deactivation process, femtosecond transient

absorption studies have done in benzonitrile solution since the involvement of a charge

transfer state should be sensitive to the solvent polarity.

Figure 5.10 shows the spectral cuts taken from early and later times of the spectral

evolution of Mn(ΙΙΙ)TPTBPCl in benzonitrile solution following 400 nm excitation. It also

replicates the spectral difference as seen in toluene. The first observed transient shows a

broad positive absorption band to the red of the B band interrupted by bleaching of B and Q

bands.

500 550 600 650 700 750-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

0.02

ΔA

wavelength/nm

18.76 ps 3.06 ps 1.99 ps

Figure 5.10. The spectral cuts taken from early time and later times of spectral evolution of

Mn(ΙΙΙ)TPTBPCl in benzonitrile solution following 400 nm excitation.

Figure 5.11 shows the kinetics profiles of the transient absorption signal at different

wavelengths of Mn(ΙΙΙ)TPTBPCl in benzonitrile solution excited at 640 nm.

114

0 100 200 300 400 500-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00ΔA

_530

nm

wavelength/nm

τ1 = 0.67 ± 0.02 ps (54%)τ2 = 12.37 ± 0.82 ps (18%)τ3 = 120.75 ± 3.72 ps (28%)

0 100 200 300 400 500

-0.08

-0.06

-0.04

-0.02

0.00

τ1 = 1.17 ± 0.11 ps (24%)τ

2 = 15.34 ± 0.96 ps (44%)

τ3 = 127.15 ± 7.60 ps (32%)

ΔA_6

88nm

wavelength/nm

Figure 5.11. Femtosecond kinetic profiles of the transient absorption signals of

Mn(ΙΙΙ)TPTBPCl in benzonitrile solution after excited at 640 nm.

The kinetics were best fitted by a three exponential decay with wavelength dependent

kinetics similar to the toluene case. The slowest phase (ground state recovery) remained

unchanged in benzonitrile solution with the wavelength, in support of the conjecture that the

excited state deactivation occurs through a d,d state rather than a CT state.

Following Kim and et al 18 there is another possible decay pathway that could be

involved. The 5T1(π,π*) decay might give rise to the 7T1(π,π*). The rapid 5-20 ps decay could

be viewed, in part, as the time for equilibrium of the trip-multiplets. Kim et al have

rationalized the putative longer lifetime for 7T1(π,π*) on the basis of spin selection.

Foremost, direct deactivation from the trip-septet to the quintet ground state is spin forbidden.

Next, no low energy septet CT or d,d excited state to which 7T1(π,π*) could decay rapidly are

expected. Moreover no septet d,d excited states are existent because no metal ↔ ring CT

transition involving the half-filled d orbitals can lead to the septet excited state. As a result,

the only possible septet CT states are 7(π,dx2-y2) and these will be very high in energy and

unlikely to participate in decay of 7T1(π,π*). With the experiments performed here related to

115

Mn(ΙΙΙ)TPTBPCl, it is thought to be most likely that the deactivation occurs through the d,d

state.


The excited state spectral and dynamic behavior of Mn(ΙΙΙ)TPTBPCl and Cr(ΙΙΙ)TPTBPCl

have been investigated by transient absorption spectrometry with femtosecond time

resolution in an effort to understand the paramagnetic nature on the photodeactivation.

It has been found that photoexcitation of the Cr(ΙΙΙ)TPTBPCl generates 4T1 state

within very short period of time and the excited species deactivates resulting the 4T1↔ 6T1

equilibrium state. It was found out that there may be possible involvement of LMCT state

during the deactivation process. In the Mn(ΙΙΙ)TPTBPCl, the triplets have formed during the

instrument response time and forms the vibrationally hot d,d state. Cooled d,d state has

decayed to the ground state. To summarize the above conclusions, a schematic diagram for

the deactivation of the photoexcited Cr(ΙΙΙ)TPTBPCl and Mn(ΙΙΙ)TPTBPCl in toluene is

depicted in Figure 5.12.

Reaction coordinate Reaction coordinate

5S0

7T1

>200 fs 5T1

5S1

16 ps

d, d

120 ps

4S0

4T1

4S1

>100 fs 6T1

LMCT

224 ps

Figure 5.12. Proposed schematic diagram for the excited state relaxation pathways of

Cr(ΙΙΙ)TPTBPCl and Mn(ΙΙΙ)TPTBPCl.

116

References

1. Kalyanasundaram, K. Photochemistry of Polypyridine and Porphirin Complexes,

1992.

2. Zhang, J.; Wang, D.; Shi, T. S.; Wang, B.; Sun, J.; Li, T. Thin Solid Films 1996, 284-

285, 596.

3. Inamo, M.; Eba, K.; Nakano, K.; Itoh, N.; Hoshino, M. Inorg. Chem. 2003, 42, (19),

6095.

4. Inamo, M.; Nakaba, H.; Nakajima, K.; Hoshino, M. Inorg. Chem. 2000, 39, 4417.

5. Inamo, M.; Hoshino, M. Photochem. Photobiol. 1999, 70, (4), 596.

6. Alden, R. G.; Chavez, M. D.; Ondrias, M. R.; Courtney, S. H.; Friedman, J. M. J. Am.

Chem. Soc. 1990, 112, (8), 3241.

7. Gouterman, M., In The Porphyrins, Dolphin, D., Ed. Academic Press: New York:

1978; Vol. 3A, p 62.

8. Gouterman, M.; Hanson, L. K.; Khalil, G. E.; Leenstra, R. J. Chem. Phys 1975, 62,

2343.

9. Hoshino, M.; Nagamori, T.; Seki, H.; Chihara, T.; Tase, T.; Wakatsuki, Y.; Inamo, M.

J. Phys. Chem. A. 1998, 102, (8), 1297.

10. Hoshino, M.; Tezuka, N.; Inamo, M. J. Phys. Chem. 1996, 100, (2), 627.

11. Inamo, M.; Okabe, C.; Nakabayashi, T.; Nishi, N.; Hoshino, M. Chem. Phys.Lett.

2007, 445, 167.

12. Jeoung, S. C.; Kim, D.; Cho, D. W.; Yoon, M. J. Phys. Chem. A. 2000, 104, (21),

4816.

13. Harriman, A. J. Chem. Soc., Faraday Trans. 1982, 1, (78), 2727.

14. Boucher, L. J. J. Am. Chem. Soc. 1970, 92, (9).

15. Suslick, K. S.; Watson, R. A. New. J. Chem 1992, 16, 633.

117

16. Suslick, K. S.; Watson, R. A.; Wilson, S. R. Inorg. Chem. 1991, 30, 2311.

17. Jeoung, S. C.; Kim, D.; Cho, D. W. J. Raman Spectrosc. 2000, 31, 319.

18. Kim, Y.; Choi, J. R.; Yoon, M.; Furube, A.; Asahi, T.; Masuhara, H. J. Phys. Chem.

B. 2001, 105, (36), 8513.

19. Irvine, M. P.; Harrison, R. J.; Strahand, M. A.; Beddard, G. S. Ber. Bunsenges. Phys.

Chem 1985, 89, 226.

20. Chirvonyi, V. S.; Dzhagarov, B. M. Fuusika Mat. 1982, 31, 129-132.

21. Yan, X.; Kirmaier, C.; Holten, D. Inorg. Chem. 1986, 25, (26), 4774.

22. Ghosh, A.; Gonzalez, E. Isr. J. Chem. 1999, 40, 1.

23. Ghosh, A.; Vangberg, T.; Gonzalez, E.; Taylor, P. J. Porphyrins Phthalocyanines

2001, 5, 345.

24. Gunter, M. J.; Turner, P. Inorg. Chem. 1994, 33, 1406.


26. Yan, X.; Holten, D. J. Phys. Chem. 1988, 92, 5982.

27. Fleisher, E. B.; Krishnamurthy, M. J. Am. Chem. Soc. 1971, 93, 3784.

28. Summerville, D. A.; Jones, R. D.; Hoffman, B. M.; Basolo, F. J. Am. Chem. Soc.

1977, 99, (25), 8195.

29. Yamaji, M.; Hama, Y.; Miyazaki, Y.; Hoshino, M. Inorganic Chemistry 1992, 31, (5),

932.

30. Boucher, L. J. J. Am. Chem. Soc. 1968, 90, (24).

118

CHAPTER 6: HEAVY –METAL EFFECT ON THE PHOTOPHYSICAL PROPERTIES

OF PLATINUM TETRAPHENYLTETRABENZOPORPHYRINS:

INSIGHT FROM EXPERIMENTAL AND RELATIVISTIC DFT/TDDFT STUDIES

Abstract

The ultrafast photodynamics of Pt(ΙΙ)TPTBP has been investigated and results are

compared with relativistic DFT and TDDFT calculations in order to explore the heavy metal

effect on the photophysics of tetrabenzoporphyrins. The Pt analog showed a blue shift of the

position of the Q band compared with first-row Zn-centeredcompound. It was found that a π

localized S1 state undergoes fast intersystem crossing (ca.500 fs lifetime) to a triplet 3(π,π*) state.

A low fluorescence quantum yield (0.0003) was observed and it showed a high yield of

phosphorescence. The triplet state lifetime was found to be ca. 41 μs significantly shorter than

Zn triplet state indicating that faster intersystem crossing due to spin orbit interaction introduced

by the heavy Pt atom in the third row transition series.

Introduction

Pt(ΙΙ)and Pd(ΙΙ) porphyrins are typical members of the “phosphorescent” classification of

porphyrins that have gained attention for use in some practical applications. The

phosphorescence of those metalloporphyrins is quenched by molecular oxygen(O2) and this

property has been utilized to make optical sensors to detect oxygen.1 Recent studies have

reported the potential use of Pt porphyrins as photosensitizers for conversion and storage of solar

energy2, and as potential molecular conductors.3 Moreover they can been used as photodynamic

and photothermal sensitizers for tumor treatment because of the excitation energy of these

systems can be transferred from their T1 state to ground state molecular oxygen, thereby

119

generating the chemically reactive singlet state of oxygen that can initiate oxidative damage in

cellular media, including tumor cells.4

The inclusion of heavy atoms into molecular systems leads to changes in photophysical

parameters due to the enhancement of inter-combination transitions, known as the heavy-atom

effect. Coordination of a heavy metal atom into the π system increases the rate of the intersystem

crossing between singlet and triplet states of the metalloporphyrins, thereby enhancing the rate of

radiative decay.5

Pt (ΙΙ)porphyrins have been classified as hypsoporphyrins (irregular porphyrins) as the

Soret band undergo a shift to shorter wavelength (hypsochromic shift) with respect to regular

metalloporphyrin systems.6 The hypsochromic shift is interpreted as the strong dπ (dxz,dyz) metal

orbital interaction with the porphyrin eg (π*)orbital (metal- to- ligand π- back- bonding). This

interaction increases the separation between the porphyrin HOMO and LUMO causing the

electronic transitions to occur at shorter wavelengths.7 It is known that Pt(ΙΙ) in a square planar

coordination environment displayed little tendency to bind axial ligands owing to the low-spin d8

configuration in which the filled dz2 orbital repels axial ligands approaching to the metal ion.7

The Co(ΙΙ), Ni(ΙΙ) (d7,d8) porphryns show no luminescence and it has been postulated

that this is due to the presence of d,d states lying between the porphyrin π,π* triplet and the

ground states. The Pd(ΙΙ)and Pt(ΙΙ) porphyrins which are also d8 strongly phosphoresce implying

that the d,d state lies higher in energy than the 3(π,π*). 8

Examination of the literature reports on Pt(ΙΙ), and Pd(ΙΙ) porphyrins revealed information

about the mechanism of excited state deactivation. It mainly occurs through the formation of the

π-localized triplet state, and the radiative processes, fluorescence and phosphorescence.

Pd(ΙΙ)porphyrins such as Pd(ΙΙ)OEP(octaethylporphyrin) and Pd(ΙΙ)TPP(tetraphenylporphyrin)

120

show a weak fluorescence, giving an estimated lifetime for the 1Q(π,π*) state of ca.20 ps.

Pt(ΙΙ)OEP showed the lifetime of S1 ≤ 15 ps.9 Both Pd(ΙΙ)and Pt(ΙΙ)porphyrins phosphoresce

strongly with quantum yields between 0.2 Pd(ΙΙ) and 0.9 (Pt(ΙΙ).6 Picosecond studies on

Pt(ΙΙ)TPP and Pd(ΙΙ)TPP have shown triplet (π,π*) lifetimes of > 10 ns while Pt(ΙΙ)OEP has

shown a triplet lifetime of > 50 ns .10 The TBP derivative of Pt showed strong phosphorescence

at 745 nm with a quantum yield 0.18 in pyridine solution. The phosphorescence spectrum and

the lifetime of Pt(ΙΙ)TBP varied with the temperature.2

The photophysical and structural properties of Pt and Pd complexes are affected by

relativistic effects that dominate for elements of atomic number greater than 50. Relativistic

effects on the physicochemical properties of metals have been studied widely.11,12 It has been

shown that only through consideration of relativistic effect could the difference between

oxidation states be explained. For example platinum exhibits both the Pt(ΙΙ) and Pt(IV) states, the

higher oxidation state in palladium is less stable than in platinum and the majority of palladium

compounds involve Pd0 or PdΙΙ. 13 Non-relativistic and relativistic TDDFT calculations

performed on Pt compounds proved that the larger HOMO-LUMO gap observed in heavy metal

complexes can be predicted theoretically only when relativistic effects were taken into account.14

In this chapter, the photophysical properties of Pt(ΙΙ)TPTBP are investigated by means of

femtosecond and nanosecond transient absorption spectrometry. In addition relativistic DFT and

TDDFT calculations have been performed to obtain the information necessary to justify the

mechanism proposed by experiments. Whenever necessary, Zn, Ni and Pt metals are brought into

the discussion to compare the data to have a relative picture on the photodeactivation

mechanism.

121

Theoretical characterization of Pt(ΙΙ)TPTBP: heavy metal effect


Relativistic density functional ground-state geometry optimization has been carried out

for the Pt(ΙΙ)TPTBP complex in order to investigate the effect of the heavy metal atom on the

geometrical parameters of the macrocycle and to provide basic structures for the calculations of

the excited states. Geometry optimization has shown that it adopts a D2d saddled conformation in

the ground state, as is the case for the first-row transition metal complexes presented in previous

chapters. Analysis of the X-ray crystallographic structure provided by Mark Thompson at USC

Chemistry Department reveals the non-planar structure of Pt(ΙΙ)TPTBP with saddle-type

distortion. The optimized structure and atom labeling of Pt(ΙΙ)TPTBP are illustrated in Figure 6.1

with atom labeling.

figure 6.1.

Figure 6.1. Top view (above) and side view (below) of the DFT-optimized molecular structure

of Pt(ΙΙ)TPTBP together with atom labeling.

Pt N Cα

Cβ Cm Co

C4 C3 C2

C1

Cmes

122

Selected optimized bond lengths and angles for Pt(ΙΙ)TPTBP obtained from calculation and x-ray

data are summarized in Table 6.1.

parameter Pt(ΙΙ)TPTBPcal Pt(ΙΙ)TPTBPx-ray

M – N 2.018 2.011 Cα – N 1.383 1.377 Cα – Cβ 1.457 1.458 Cβ – Cβ 1.415 1.399 Cβ – Cm 1.405 1.405 Cm – Co 1.390 1.382 Co – Co 1.403 1.387

Cα – Cmeso 1.400 1.400 Cmeso – C1 1.495 1.495

C1 – C2 1.401 - C2 – C3 1.396 - C3 – C4 1.397 -

∠ CαNCα 108.9o 108.3o ∠ NCαCβ 108.6o 108.9o

∠ CαCmesoCα 124.3o 124.0o ∠ C2C1CmesoCα

a 71.2o 71.2o ∠ (CαNNCα)op 0o 0.83o ∠ (CαNNCα)ad

b 21.5o 20.1o a Dihedral angle between the phenyl group and the plane of the porphyrin ring;

b Dihedral angle (deg) between adjacent pyrrole ring planes.

Table 6.1. Comparison of the optimized theoretical values of the selected bond lengths (Å) and

bond angles (deg) with x-ray data for the Pt(ΙΙ)TPTBP.

To compare the changes in bond lengths and bond angles along the Ni→Pd→Pt series,

calculated values are given in the Table 6.2. It can be seen that the most changes occur in the

porphinato nitrogen-metal bond (M-Np) distance when changing the metal from Ni to Pt. For

example Ni-Np distance was 1.922 Å (Table 6.2), whereas in the Pt complexes this value was

found to be 2.018Å.

123

Parameter Ni(ΙΙ)TPTBP Pd(ΙΙ)TPTBP Pt(ΙΙ)TPTBP M – N 1.922 2.022 2.018 Cα – N 1.384 1.380 1.383 Cα – Cβ 1.454 1.459 1.457 Cβ – Cβ 1.413 1.416 1.415 Cβ – Cm 1.405 1.405 1.405 Cm – Co 1.395 1.391 1.390 Co – Co 1.406 1.403 1.403 Cα – Cmeso 1.395 1.400 1.400 Cmeso – C1 1.494 1.495 1.495 C1 – C2 1.403 1.401 1.401 C2 – C3 1.402 1.396 1.396 C3 – C4 1.399 1.397 1.397 ∠CαNCα 107.4o 109.0o 108.9o ∠NCαCβ 109.4o 108.6o 108.6o ∠CαCmesoCα 121.4o 124.3o 124.3o ∠C2C1CmesoCα 70o 70.9o 71.20 ∠(CαNNCα)op 0o 0o 0o ∠(CαNNCα)ad 25.2o 22.4o 21.5o

Table 6.2. Selected Bond Distances (Ao) and Bond Angles (deg) calculated for M(ΙΙ)TPTBP in

the D2d confirmation, M = Ni, Pd, Pt.

The size of the metal ion is increasing when going from the first to the third row

transition metal complexes and as a result the size of the porphyrin cavity is also increased. Due

to the enlargement of the macrocycle, the dihedral angle between the Cα-N bonds on the adjacent

pyrrole rings that defines the degree of saddling is decreased along the series Ni-Pt (from 25.2o

to 21.5o respectively). It is worth noting that the Zn-Np distance is 2.047 as seen in Table 3.1. Zn

has d10 electronic configuration with dx2

-y2 being filled. The repulsion between nitrogen lone

pairs and the d electrons causes an increase in Zn-Np compared with Pt-Np (2.018 Å).

124


The highest occupied and the lowest unoccupied one electron levels for the ground states

of Pt(ΙΙ)TPTBP and Pd(ΙΙ)TPTBP in the D2d saddled confirmation are shown in Figure 6.2. The

highest occupied and the lowest unoccupied levels are depicted in red to highlight the differences

in the relative position of the frontier orbitals that are likely to cause changes in photophysical

behavior when heavier atoms are introduced into the macrocycle cavity. The data for

Ni(ΙΙ)TPTBP are also included in the figure. The energies and percentage composition of the

molecular orbitals are given in the Appendix A.

-6.5-6.0-5.5-5.0-4.5-4.0-3.5-3.0-2.5-2.0-1.5

dπ

dz2

dz236a1

22a2

57e37a1

58e

24a2

63e

dπdz2

dz239a1

23a261e

40a1

62e35b2

32b2

29b128b1

59e

33b2

22a1

16a2

dπ

dz2

38e39e

23a122b2

18b1

dx2-y223b2

40e

PdTPTBP PtTPTBPNiTPTBP

E/e

V

Figure 6.2. Energy level scheme for M(ΙΙ)TPTBP, M= Ni, Pd, Pt.

(a) HOMO and LUMO relative positions

The highest fully-occupied Gouterman molecular orbital in the Ni, Pd and Pt complexes

is of b1 symmetry. The composition of this orbital is the same for all three metal complexes. It is

a purely macrocycle orbital with large amplitude on the Cα, Cm and Co atoms. As can be inferred

from the data in Appendix A (and Figure 6.3), the central metal has no contribution to this

orbital. But Figure 6.2 reveals that going from Ni → Pd → Pt, there is a small stabilization in the

125

HOMO. It is possible that when a larger metal is introduced into the macrocycle, the cavity

expands and the antibonding interaction between adjacent Cα atoms decreases resulting in the

stabilization of HOMO.

The lowest unoccupied Gouterman orbital in the Ni-Pd and Pt series is a pair of

degenerate e orbitals. As can be seen from Appendix A (and Figure 6.3), it has larger

contribution from the macrocycle ring with some contribution from pyrrolic nitrogen. It also has

small contribution from the metal dπ orbital. The contribution from the dπ orbital is increased on

going from Ni (3%) to Pt (5%). Going from Ni to Pd, the G- LUMO is stabilized and going from

Pd to Pt, the LUMO is upshifted again. The interaction between the empty macrocycle orbital

and the dπ orbital of the central metal is an antibonding one and it introduces the metal-

macrocycle π- back donation.15 In heavy metal complexes, it is more effective due to the larger

and more diffuse character of the 4d and 5d metals that results in the upshift of the LUMOs.16

The central metal does not contribute to the composition of the HOMO as seen in the

contour plots in Figure 6.3. But inserting an atom of larger radius into the porphyrin cavity

causes an expansion of the macrocycle and a concomitant lowering of the energy of the HOMO.

Moreover, the change in the position of LUMO results in the blue shift of the Q band absorption

along the Ni to Pt series.

126

24a2 G-LUMO

63e:yz 63e:xz

62e:yz 62e:xz G-HOMO 29b1

35b2

40a1-side view

40a1

61e: xy 61e: xz 23a2

39a1

Figure 6.3. Contour plots of the MOs of Pt(ΙΙ)TPTBP.

127

(b) the metal d levels

It is worth commenting on the strength of the σ interaction between the tetrapyrrole Np

lone pairs and the metal- dx2

-y2, and the out-of-plane interaction between the tetrapyrrole Np Pz

and the metal-dπ. The relative positions of the metal d levels are determined mainly from these

two influences, followed by the size of the interacting orbitals and on the size of the macrocycle

cavity.17,18 In first row transition metals, the virtual dx2

-y2 orbital plays an important role in the

excited state deactivation since it lies very close in energy to the HOMO.

Larger spin-orbit coupling in the second and third row transition metals locates the

energy of the virtual dx2

-y2orbital well above that of the G-LUMOs. The virtual dx

2-y

2orbital

participates in the antibonding σ interaction with the pyrrolic Np lone pairs to form the 23b2

orbital in Ni(ΙΙ)TPTBP where it is composed of 51% of the dx2

-y2 orbital and 24% of the Np Px

orbital. Its bonding counterpart 17b1 lies 1.8 eV below the HOMO. In Pt(ΙΙ)TPTBP, the virtual

dx2

-y2 orbital lies much higher in energy. Its bonding counterpart 27b1 (composed of 76% of the

dx2

-y2 orbital and 4% of the Np Px) lies 2.23 eV below the HOMO. The relativistic destabilization

of the 5d orbitals also allows for better metal-to-ligand back donation. Thus the unfilled dx2

-y2

orbital that controls the excited state deactivation in the first row transition metal complexes is

not expected to play any role in the deactivation of the optically produced S1 excited state in the

second and third row metal complexes.

The highest occupied metal d orbitals are the dz2 (23a1 in the Ni complex, 37a1 in the Pd

complex and 40a1 in the Pt complex) and the dπ (39e in the Ni complex, 58e in the Pd, 62e in the

Pt). The dz2 orbital in Ni(ΙΙ)TPTBP is nearly a pure metal orbital (84% dz

2), whereas in

Pd(ΙΙ)TPTBP the contribution from dz2 is 61% and in Pt(ΙΙ)TPTBP it has a 63% dz

2 contribution.

128

This shows that the dz2 orbital is stabilized by the π interaction with the macrocycle when going

from Zn to Pt.

Metal dπ orbitals play a significant role when considering the out-of-plane interaction.

The metal contribution to these orbitals changes as in the Ni complex, 56%, in Pd 26% and in Pt

it has 33% contribution from the dπ bonding orbital. In the Pt complex the contribution from the

metal dπ slightly has increased owing to the larger size of the metal 5d orbital. The spacing

between the resulting π bonding/antibonding pair (39e/38e in Ni and 62e/61e in Pt) is very small.

These orbitals experience downward shift when going from Ni to Pd and upshifted again when

going further to the Pt macrocycle, similar to the metal contribution to these orbitals. When

going from Pt to Pd, the increased repulsive interaction between the metal and macrocycle

orbitals results in these MOs moving to a higher level.

The remaining occupied 3d orbital, the in-plane dxy orbital is an almost pure 3d orbital.

In Ni(ΙΙ)TPTBP, it is 19b2, in Pd(ΙΙ)TPTBP it is 26b1 and in Pt(ΙΙ)TPTBP it is 29b2. In the Pt

complex the contribution from the occupied dxy metal orbital to this MO is the largest due to the

more diffusive 5d orbital in Pt.

The upshift of the empty dx2

-y2orbital, together with the stabilization of the occupied

metal-levels remove any possibility of involvement of ligand field excited states in the

deactivation of the optically produced (π,π*) excited state in the Pt complex.


Figure 6.4 shows the ground state absorption spectra of Pt(ΙΙ)TPTBP together with Zn

analog in toluene solution. Extinction coefficients are given for absorbance data. The extinction

coefficient of Pt(ΙΙ)TPTBP εQband with λmax = 615 nm is 2.02x105 M-1cm-1. The extinction

129

coefficient for Pd analogue has been reported as 1.05x105 M-1cm-1 at λmax = 629 nm in benzene

solution.19Compared to the Q band and B band absorption maxima in the Zn complex, Pt

complex showed a blue shift; where the Q band gain intensity while the B bands lose intensity.

(Figure 6.4.)

300 400 500 600 700 8000

1

2

3

4

5

6

ε x

10-5 (M

-1cm

-1)

wavelength/nm

ZnTPTBP PtTPTBP

Figure 6.4. Ground state absorption spectra of M(ΙΙ)TPTBP (M = Pt and Zn) in toluene solution.

Extinction coefficients are given for absorbance data.

Table 6.3 lists the vertical absorption energies and oscillator strengths calculated for the

Q band and B band and compared with experimental data for both Pt and Zn complexes in

toluene. It also includes the composition of the BP/ALDA solution vectors in terms of the major

one-electron MO transitions. According to the TDDFT excitation energies, the Q band on the

ground state absorption spectra in Pt(ΙΙ)TPTBP, can be assigned to the 11E state, computed at

616 nm. It has considerable oscillator strength and is mainly derived from the promotion of one

130

electron from the G-HOMO to the G-LUMO. It also has some minor contribution from HOMO-

1 to LUMO.

TDDFT Exp.

state Composition(%) Eva(eV/nm) f E(eV/nm)

Zn(ΙΙ)TPTBP 11E(Q) 18b1→ 40e(71) 1.89(656) 0.13 1.90(652)

22b2→ 40e(27)

21E(B) 22b2→ 40e(59)

18b1→ 40e(21)

23b2→ 40e(11)

2.62(473) 1.52 2.68(462)

Pt(ΙΙ)TPTBP 11E(Q) 29b1→ 63e(83) 2.016(616) 0.24 2.019(615)

35b2→ 63e(14)

21E(B) 35b2→ 63e(72)

29b1→ 63e(10)

2.79(445) 1.08 2.89(430)

Table 6.3. Vertical excitation energies and oscillator strengths (f) computed for the optically

allowed excited states of M(ΙΙ)TPTBP(M= Zn, Pt) responsible for the appearance of the Q band

and B band and compared with experimental data.

The calculated blue shift of the Q band of the Pt complex compared to the Zn analog is in

excellent agreement with the experimental data. With heavier metals, it was necessary to take

into account relativistic effects in the geometry optimization process.14 Table 6.3 also reveals the

increase of the HOMO-LUMO gap in the Pt complex. It can be interpreted as a result of G-

HOMO stabilization and destabilization of the G-LUMO due to the stronger π –back bonding

repulsion between the Pt dπ orbital and the macrocycle orbital.7,20 The intense B band at 2.79 eV

(445 nm) arises mainly(72%) from the one electron transition from HOMO-1 to LUMO.

131

Optically Silent Excited States Below the S1(ππ*) State

In order to provide the insights on the excited state deactivation for Pt(ΙΙ)TPTBP

subsequent to Q band excitation, the manifold of singlet and triplet symmetry forbidden states

lying below the Q band were also examined. The vertical absorption energies calculated for these

are summarized in Table 6.4.

TDDFT

state Composition(%) character Eva

Pt(ΙΙ)TPTBP

11E

29b1→ 63e(83)

ππ*

2.01

13E 29b1→ 63e (99) ππ* 1.63

Table 6.4. Excitation energies (eV) calculated for the low-lying excited states of Pt(ΙΙ)TPTBP

The TDDFT calculations indicate that there is only one silent state lies between the Q

state and the ground state. It is the emissive triplet (ππ*) state and is doubly degenerate. It also

undergoes a Jahn-Teller distortion as previously described in chapter 3.

Steady State Luminescence and Transient Absorption Experiments

Steady State Luminescence

Being a heavy atom, Pt influences the production of the triplet state, resulting in faster

intersystem crossing through enhanced spin-orbit coupling.3 An increase in spin-orbit coupling

changes the photophysical properties in such a way that can increase the phosphorescence

quantum yield, decrease the fluorescence quantum yield and increase the rate constant for

intersystem crossing.

132

Pt(ΙΙ)TPTBP showed a weak fluorescence having λmax at 620 nm with quantum yield

(ΦF) of 0.0003 in toluene measured as outlined in chapter 2. The fluorescence spectrum of

Pd(ΙΙ)TPTBP was previously published having similar quantum yield to Pt complex. The

normalized ground state absorption spectrum of Pt(ΙΙ)TPTBP in the Q band region, the

fluorescence spectrum along with the phosphorescence spectrum, excited at 442 nm in toluene

solution are shown in Figure 6.5.

540 570 600 630 700 800 9000.0

0.2

0.4

0.6

0.8

1.0

0

5

10

15

20615 620

768

norm

aliz

ed a

bsor

banc

e an

d flu

ores

cenc

e

phos

phor

esce

nce

inte

nsity

/ V

wavelength / nm

Figure 6.5. Normalized room temperature absorption (black solid line) and fluorescence spectra

(red dashed line) and phosphorescence spectra (red solid line) in toluene solution. Excitation

wavelength for emission spectra was 442 nm.

A small Stokes’ shift of 131cm-1 between the fluorescence and absorption maxima was

observed. The phosphorescence spectrum of Pt(ΙΙ)TPTBP in thoroughly degassed toluene

solution showed a maximum at 768 nm at room temperature. It has been reported that in aerated

solutions at room temperature the intense signal at 1270 nm can be attributed to the emission

133

arising from the 1Δg→3Σg transition in oxygen, generated by energy transfer from triplet state

precursors.21 (see later) Phosphorescence was the major emission from this porphyrin similar to

what has been previously observed for Pt(ΙΙ)tetrabenzoporphyrin (TBP)2 and Pd(ΙΙ)TPTBP.19

Coordination of a heavy metal increases the rate of intersystem crossing between singlet and

triplet states enhancing the rate of radiative decay from the triplet state.22,23

All the photophysical properties are summarized in Table 6.5 and compared with

reported Pd data.

M(ΙΙ)TPTBP Z λQ band/

nm 10-5 εQband / mM-1cm-1 λfluor /nm ΦF τT1 in Ar/μs 10-9 kq/ M-1s-1

Zna 30 652 47 668 0.008 236 1.2 Pdb 46 629 105 669 0.0003 195 Ptc 78 615 202 620 0.0003 42 2.26 a extinction coefficient in toluene. triplet lifetimes were measured in 1% of pyridine/toluene

solution. All the other measurements were carried out in pyridine solution;

b data taken from ref;19

c this work; all in toluene solution.

Table 6.5. Photophysical properties of M(ΙΙ)TPTBP (M = Zn, Pd, Pt)

The Q band maximum of the Pt complex shows a hypsochromic shift compared to Zn

and Pd. In view of that the T1-S0 energy gap calculated by TDDFT (Table 6.3) is higher in the Pt

complex compare to the Zn analog. The bimolecular rate constant for quenching of the T1 state

by molecular oxygen (see later) is also higher with the Pt complex.

134


Femtosecond Transient Absorption

Femtosecond pump-probe absorption experiments were carried out to illustrate the early

events that follow photoexcitation (see Figure 6.6). The LH panel shows the spectra of a solution

of Pt(ΙΙ)TPTBP in toluene solution excited at 400 nm and probed using a white light continuum

pulse generated in a CaF2. plate In addition the experiment was repeated using 610 nm excitation

and a probe continuum pulse generated in a sapphire plate which has the advantage of providing

better signal-to-noise at the expense of spectral coverage in the B band region (see RH panel).

350 400 450 500 550 600 650 700 750

-0.20

-0.15

-0.10

-0.05

0.00

0.05

7.8 ps 4.2 ps 2.3 ps

ΔA

wavelength / nm

500 550 600 650 700 750 800

-0.3

-0.2

-0.1

0.0

0.1

0.2

ΔA

wavelength/nm

67.47 ps 21.67 ps 2.36 ps

Figure 6.6. Left: evolution of the transient absorption of Pt(ΙΙ)TPTBP in toluene, excited at 400

nm, using a. probe continuum generated in a CaF2 plate. Right: evolution of the transient

absorption of Pt(ΙΙ)TPTBP in toluene, excited at 610 nm, white light sapphire.

Excitation at 400 nm directly generates the S2 (π,π*) excited state, whereas excitation at

610 nm directly generates the lowest S1 (π,π*) excited state. In the former case the S1 (π,π*)

135

excited state is expected to result from internal conversion from the S2 (π,π*). In fact, the early

time scales in Figure 6.6 showed to be no difference between the two types of excitation, leading

to the conclusion that internal conversion is extremely rapid and is complete within the

instrument response (ca.200 fs).

Scrutiny of Figure 6.6 shows that the first observed transition (FOT) is observed by a

broad featureless positive band from 350 nm to 800 nm interrupted by negative bands at 430 nm

and 615 nm in the regions of the intense B and Q bands of the ground state. By analogy to the

other porphyrins studied, this spectral feature can be assigned to the S1 →Sn absorption within

the π system. The subsequent spectral changes were minor during the first few picoseconds

(Figure 6.6 LH panel) after which there were no changes (Figure 6.6 RH panel) during the time

window of the ultrafast spectrometer (~1.5 ns).

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0-0.2

0.0

0.2

0.4

0.6

0.8

1.0

ΔA_5

84nm

time / ps

τ1 = 0.53 ± 0.02 ps

Figure 6.7. Left: kinetic profile of the Pt(ΙΙ)TPTBP in toluene solution measured at 530 nm upon

excitation at 400 nm. Right: kinetic trace at 584 nm upon excitation at 610 nm. The dashed blue

curve is the instrument response to a solvent Raman signal. Both curves are normalized to unity.

0 2 4 6 8 10

0.00

0.02

0.04

0.06

0.08

ΔA_

530n

m

time/ps

τ1 = 0.40 ± 0.07 ps

136

Given the similarity of the absorption spectra between S1 and T1 observed for other

metalloporphyrins studied earlier, this process can be identified as the intersystem crossing

(S1→T1) which generates the π localized triplet.

In Figure 6.7 the LH panel displays the time dependence of the photoinduced transient

absorption signal at 530 nm after 400 nm excitation. The RH panel shows the time profile at 584

nm following 610 nm excitation. On the same figure the time profile of a Raman signal

generated from the solvent is superimposed to show the instrument response profile. It shows

that the intensity of the transient absorption signal at 584 nm decreased more slowly than the

solvent response. Experimental points were fitted starting from the time when Raman signal had

disappeared. A single exponential fit lead to an estimated ca.0.5 ps lifetime for the decay, which

can be assigned to the intersystem crossing, S1→T1, process. The LH panel also shows a single

exponential fit to the decaying absorption signal at 530 nm having a lifetime of 0.5 ps. It is

closely similar to the observed single exponential decay that was seen after excitation at 610 nm.

The nanosecond transient absorption spectra closely resemble the ultrafast transient spectra on

the longer time scale.

Nanosecond Transient Absorption

Figure 6.8 displays the transient absorption spectra excited at 430 nm in Ar-saturated

toluene solution at various time delays. At early times the data show a broad photoinduced

positive absorption signal with a superimposed strong bleaching of the ground state absorption.

137

400 450 500 550 600 650 700 750

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

ΔA

wavelength / nm

1.5 μs 5.0 μs 10.0 μs 15.0 μs 20.0 μs 30.0 μs 40.0 μs 50.0 μs 70.0 μs

Figure 6.8. Nanosecond transient absorption spectra of Pt(ΙΙ)TPTBP in Ar-saturated toluene at

different delay times followed by 430 nm excitation.

This first observed transient spectrum closely resembles the last transient on the

picosecond time scale (Figure 6.6; RH panel) and as such it can be assigned to the absorption of

the T1 state. These transient absorption features, both positive and negative decayed

concomitantly with isosbestic points, indicating that the triplet state decayed by repopulating the

ground state surface.

The time profile for the Q band bleaching recovery in Ar-saturated toluene solution is

shown in the upper LH panel in figure 6.9. The measured T1 lifetime was 41.7 μs. In the

presence of oxygen, the decay of the triplet state was enhanced, remaining single exponential.

The rate constant of the decay was directly proportional to the concentration of oxygen (Figure

6.9; upper right). In air saturated solution, the lifetime was 0.5 μs (lower RH panel) and in O2

saturated solution this lifetime reduced to 0.055 μs.

138

Earlier experimental data on the Pd analogue19 showed triplet excited state lifetime as

195 μs in de-oxygenated pyridine. This is ca.5 times longer than that obtained for Pt(ΙΙ)TPTBP;

this lifetime shortening can be presumably attributed to faster intersystem crossing due to the

higher atomic number(Z) with Pt center.

0 50 100 150 200 250 300 350 400

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

τ = 41.68 ± 0.15μs

ΔA_4

30nm

_Ar

time/μs

0 2 4 6 8 10

0

200

400

600

800

Ksv = (93.20 ± 2.24)*103 M-1

concentration of O2 / mM

τ Ar/τ

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-0.025

-0.020

-0.015

-0.010

-0.005

0.000

τ1 = 0.054 ± 0.0004 μs

ΔA_6

20nm

_O2

time/μs0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

-0.020

-0.015

-0.010

-0.005

0.000

ΔA_6

20nm

_air

τ = 0.500 ± 0.002 μs

time/μs

Figure 6.9. Nanosecond kinetic profiles of the transient absorption signals of Pt(ΙΙ)TPTBP,

excited at 430nm. Solid red lines depict the fits to the experimental data while blue lines

represent the residual of the fit.

139

The bimolecular rate constant (kq) for quenching of the triplet in toluene by molecular

oxygen was obtained by measuring the triplet state lifetime in solutions with different oxygen

concentrations (Figure 6.9; upper right) and the plot of triplet lifetime vs. [O2] was linear with a

slope that yielded a bimolecular rate constant for oxygen quenching of 2.26 x109 M-1 s-1 for

Pt(ΙΙ)TPTBP.

Oxygen quenching of the triplet state

The energies of the triplet states can be extracted from the bimolecular rate constant of

the quenching of the triplet state by molecular oxygen. An understanding of the transference of

electronic energy to molecular oxygen from the triplet states of large molecules has become

significant because of its impact in photodynamic therapy, as discussed in Chapter 1. Singlet

molecular oxygen, O2(1Δg) is considered a leading candidate for the initiation of tissue damage in

the presence of light, oxygen and an absorber.25 To detect singlet molecular oxygen, the S0 → T1

energy gap should be below the energy gap of oxygen 1Δg → 3Σ −g transition ( 22. 5

kcal/mol/(1269 nm)) and a phosphorescence quantum efficiency is much higher than that of

singlet oxygen.26 It has been demonstrated that exergonic energy transfer from triplet states to

oxygen occurs with rate constants that are close to 2 x109 M-1 s-1. Rate constant values lower than

that are found when the energy transfer from the triplet state to O2(1Δg) is endoergic.27

In the presence of oxygen the triplet states of both Zn(ΙΙ)TPTBP and Pt(ΙΙ)TPTBP

decayed exponentially to a zero baseline. The first order rate constants of the decay were

proportional to the O2 concentration. Bimolecular rate constants were found to be 1.20 x109 M-

1s-1(Table 3.4 in Chapter 3) for Zn(ΙΙ)TPTBP and 2.26 x109 M-1s-1 for Pt(ΙΙ)TPTBP (Table 6.5.).

The values of near 2 x109 s-1 are as anticipated for oxygen quenching of a triplet state having an

140

energy level higher than that of O2(1Δg) such that the reaction proceeds down the energy

gradient.26 At room temperature, Zn(ΙΙ)TPTBP clearly showed a luminescence signal due to

singlet oxygen to the red of fluorescence (at 1270 nm) in aerated pyridine solution. The energy

of the triplet of Zn(ΙΙ) and Pt(ΙΙ)TPTBP complexes were calculated according to the scheme and

equation described in chapter 2 and gathered in Table 6.5.

M(ΙΙ)TPTBP 10-9 kq/ M-1s-1 Keq ET/kcal.mol-1 ET/(eV/nm) Zn 1.20 0.0109 21.16 0.917/1352

Pt

2.26

0.205

22.90

0.993/1248

Table 6.6. Equilibrium derived triplet state energies for Pt and Zn complexes. Pt is in toluene

solution and Zn in 1% pyridine-toluene solution.

The values in the table represent the adiabatic values corresponding to the energies of the

triplet states after relaxation of the molecule to the minimum T1 potential energy surface and can

be compared with Eadia values obtained by calculation.


The excited state spectral and dynamic behavior of the Pt(ΙΙ)TPTBP complex has been

investigated by ultrafast transient spectrometry and interpreted with relativistic DFT and TDDFT

calculations to see the heavy metal effect on the photophysics of metallotetrabenzoporphyrins.

141

Reaction coordinate

Figure 6.10. Proposed excited state deactivation mechanism for Pt(ΙΙ)TPTBP.

It was found that the optically produced S1 state is rapidly deactivated through the

macrocycle’s T1 state localized on the π system. The TDDFT calculations has shown that triplet

π,π* was the only state below S1.

Heavy metal effects on the photophysics of third row transition metals can be attributed

to the spin-orbit interaction that raises the energy of the unoccupied d orbital and enhance the

intersystem crossing process. The lifetime of the triplet state was decreased in Pt complex

compare to Zn. There is no metal-associated d,d or charge transfer excited state available in

Pt(ΙΙ)TPTBP complex. The higher energy gap between d,d/LMCT states and the singlet (π,π*)

excited state obviated fast non-radiative deactivation as had been observed in the first row

transition metal complexes.

2S0

T1

42 μs

S1

0.5 ps

142

The Pt complex showed a hypsochromic shift of the Q band and corresponding triplet

(π,π*) state energy relative to the first row metallotetrabenzoporphyrins. Comparison of the Pt

with first row transition metal gives an insight of the effect of 3d orbitals on the photophysics of

tetrabenzoporphyrins.

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F. A.; Isaia, F.; Lelj, F.; Lippolis, V.; Tommasi, R. J. Phys. Chem. A 2003, 107, 9679.

(15) Rosa, A.; Baerends, E. J. Inog.Chem 1994, 33, 584.

(16) Soldatova, A. In Department of Chemistry; Bowling Green State University:

Bowling Green, 2007; Vol. Ph.D.

(17) Rosa, A.; Baerends, E. J. Inog.Chem 1992, 31, 4717.

(18) Rosa, A.; Ricciardi, G.; Baerends, E. J.; van Gisbergen, S. J. A. J. Phys. Chem. A

2001, 105, 3311.

(19) Rogers, J. E.; Nguyen, K. A.; D. C. Hufnagle; D. G. McLean; Su, W.; M.Gossett,

K.; Burke, A. R.; A.Vinogradov, S.; Pachter, R.; Fleitz, P. A. J. Phys. Chem. A 2003, 107,

11331.

(20) Brown, R. J. C.; Kucernak, A. R.; Long, N. J.; Mongay-Batalla, C. New J. Chem.

2004, 28, 676.

(21) Rihter, B. D.; Kenney, M. E.; Ford, W. E.; Rodgers, M. A. J. J. Am. Chem. Soc.

1993, 115, 8146.

(22) Borek, C.; Hanson, K.; Djurovich, P. I.; Thompson, M. E.; Aznavour, K.; Bau, R.;

Sun, Y.; Forrest, S. R.; Brooks, J.; Michalski, L.; Brown, J. Angew. Chem. Int. Ed. 2007, 46,

1109.

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2003, 42, 4253.

144

(24) Callis, J. B.; Knowles, J. M.; Gouterman, M. J. Phys. Chem. 1973, 77, 154.

(25) Firey, P. A.; Ford, W. E.; Sounik, J. R.; Kenney, M. E.; Rodgers, M. A. J. J. Am.

Chem. Soc. 1988, 110, 7626.

(26) Pelliccioli, A. P.; Henbest, K.; Kwag, G.; Carvagno, T. R.; Kenney, M. E.;

Rodgers, M. A. J. J. Phys. Chem. A. 2001, 105, 1757.

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145

CHAPTER 7: SUMMARY AND CONCLUSIONS

In this work, the investigation of the photophysical properties of the metallo-

tetraphenyltetrabenzoporphyrins coordinated with first row transition metals with Cr(III),

Mn(III), Co(II), Ni(II), Cu(II) and Zn(II) have been reported. Comparison has been made

with Pt(II) and Pd(II) analogues to see the effect of the heavy metals to the deactivation

kinetics. Transient absorption spectrometry with femtosecond and nanosecond time

resolution has been employed along with DFT/TDDFT calculations to provide a detailed

understanding of the sequence of events that follow Q band photo-excitation. The comparison

has been done to address the effect of the nature of the central metal on the deactivation

schemes for the excited states of metal-centered meso-tetraphenyltetrabenzoporphyrins.

The first row transition metal complexes of tetraphenyltetrabenzoporphyrins have

advantageous properties. Among them, a high molar absorption coefficient in the red spectral

region and a fast radiationless decay of the excited states converting the photon energy into

the thermal event make them useful in potential applications such as photothermal therapy

which is a promising approach to tumor treatment.

The ground and excited state properties of Zn(ΙΙ)TPTBP have been investigated and

the results were described in chapter 3. It showed the excitation to Q band produced

vibrationally hot π localized singlet (π,π*) and after cooling it decayed to the ground state

through triplet state and fluorescence. TDDFT results indicated that below the singlet (π,π*),

there are only π,π* states can be seen and the lowest energy is responsible for the triplet state.

For Co(ІІ)TPTBP, the first observed transient which is a π localized triplet state was

converted to a hot d,d state, wherein intramolecular cooling occurred and completed within 3

ps. After cooling, the d,d state decayed into the ground state in an exponential manner having

a 17 ps lifetime. The TDDFT calculations showed that there are two d,d states lying below

the triplet state (1.57 eV) being a (dπ,dz2), 12E as the lowest, situated 0.81 eV vertically above

146

the ground state with the same multiplicity as the ground state. This state arises from the

transition from the anti-bonding 39e molecular orbital with 60% on the dπ metal character to

the empty dz2 spin orbital. So, the observed d,d state having a 17 ps lifetime was assigned to

the 12E.

Similar to Co(ІІ)TPTBP, the first observed transient in Cu(ІІ)TPTBP was assigned to

the triplet state that equilibrate with an LMCT state; ground state repopulation occurred via a

lower lying LMCT state. The dependence of the observed lifetime on solvent polarity

confirmed the participation of the LMCT state in the TDDFT calculations overall

deactivation process. The repopulation was completed within 500 ps. And also TDDFT

calculations showed that below the triplet state there are two LMCT states that can be

responsible for the observed lifetime shortening.

After 640 nm excitation in Cr(ΙΙΙ)TPTBPCl, the S1 state undergoes fast intersystem

crossing (4S1→4T1), within a very short period of time (ca.0.05 ps lifetime) to 4T1 state. The

4T1 state of Cr(ΙΙΙ)TPTBPCl in toluene deactivated with a lifetime of 224 ps, resulting the

4T1↔ 6T1 equilibrium. In benzonitrile solution 4T1 of Cr(ΙΙΙ)TPTBPCl showed a lifetime of

90 ps. A substantial decrease in the lifetimes of photoexcited Cr(ΙΙΙ)TPTBPCl in benzonitrile

indicated the possible existence of a CT state that acts as a quenching state of the trip-

multiplet state. 6T1 state should have a longer lifetime which was unable to find using the

nanosecond transient absorption set up here.

In Mn(ΙΙΙ)TPTBPCl , the excited sing-quintet, 5S1(π,π*) state deactivated to the trip-

quintet, 5T1(π,π*) within the instrument response time. Within short time period afterwards, it

generated a hot d,d state wherein cooling occurred within 4 ps. Subsequently the cooled d,d

state repopulated the ground state with a 120 ps lifetime.

With Pt analog, it was found that a π localized S1 state undergoes fast intersystem

crossing (ca.500 fs lifetime) to a triplet 3(π,π*) state. A low fluorescence quantum yield

147

(0.0003) was observed and it showed a high yield of phosphorescence. The triplet state

lifetime was found to be ca. 41 μs significantly shorter than Zn triplet state indicating that

faster intersystem crossing due to spin orbit interaction introduced by the heavy Pt atom in

the third row transition series. Vertical excitation energies computed for the lowest optically

allowed and silent states showed that there is only the triplet state lie below the singlet state

proving the above mentioned mechanism.

In summary, with the aim of exploring the role of the central metal on the

photophysics of MTPTBPs, transient absorption spectrometry with femtosecond and

nanosecond time resolution has been employed. Experimental results have been interpreted

with the help of DFT/TDDFT calculations with special emphasis on the nature and energies

of the low lying excited states along the relaxation pathway of the photo-generated π* state.

The two approaches were brought together to generate a detailed understanding of the

sequence of events that follow photo-excitation. The first row transition metal introduced

low-lying electronic states that can serve as intermediates in the deactivation sequence by

providing a series of low energy radiationless transition. A fast dissipation of the photon

energy by the thermal radiationless event can be useful to implement those complexes as

photothermal sensitizers.

APP

EN

DIX

A: E

NE

RG

IES

AN

D P

ER

CE

NT

AG

E C

OM

POSI

TIO

N O

F T

HE

HIG

HE

ST O

CC

UPI

ED

AN

D L

OW

EST

UN

OC

CU

PIE

D

MO

LE

CU

LA

R O

RB

ITA

LS

OF

M(ІІ)

TPT

BP,

M=

Zn, C

o, C

u, P

t, Pd

EX

PRE

SSE

D IN

TE

RM

S O

F IN

DIV

IDU

AL

AT

OM

S Zn

(ІІ)

TPTB

P

MO

E,

eV

Zn

N

Cm

eso

Cα

Cβ

Cm

Co

C1

C2

C3

C4

Un

occu

pied

17

a 2-1

.787

-

- 14

(2p z

) 5(

2pz)

18(2

p z)

- 11

(2p z

) 4(

2pz)

8(2p

y)

3(2P

y)

8(2p

y)

2(2p

y)

2(2p

y)

3(2p

x)

2(

2px)

6(

2py)

2(

2pz)

2(2P

z) 5(

2pz)

2(2p

y)

2(

2py)

40

e -2

.863

-

12(2

p z)

44

(2p z

) 10

(2p z

) 4(

2pz)

7(2p

z) -

2(2p

x)

- -

4(

2py)

3(

2px)

occu

pied

18

b 1-4

.473

-

- -

52(2

p z)

4(2p

z) 14

(2p z

) 10

(2p z

) -

- -

-

7(2p

y)

3(

2px)

1(

2px)

1(2p

x)

2(

2py)

1(

2py)

22

b 2-4

.843

2(

3Pz)

18(2

p z)

45(2

p z)

3(2p

z) 2(

2pz)

- 1(

2pz)

- 4(

2py)

-

1(2p

y)

7(

2px)

6(

2py)

1(

2py)

1(

2py)

39

e -5

.673

14(2

p z)

11(2

p z)

7(2p

z) 5(

2pz)

23(2

p z)

10(2

p z)

- 2(

2py)

-

1(

2py)

1(2p

y)

1(2p

y)

2(2p

y)

4(

2px)

1(

2px)

23

b 2-5

.687

17

(3d x

y)

38(2

p x)

1(2p

y)

5(2p

x)

5(2p

y)

- -

- -

- -

13

(2p z

)

2(

2s)

11

(2s)

23a 1

-5.8

51

- 23

(2p z

) -

- 28

(2p z

) -

25(2

p z)

- -

- -

5(2p

x)

4(

2px)

5(

2py)

4(2p

y)

16a 2

-5.8

97

- -

6(2p

z) 3(

2pz)

6(2p

z) 35

(2p z

) 10

(2p z

) 3(

2pz)

4(2p

y)

- 3(

2py)

1(2p

y)

1(2p

x)

7(2p

x)

2(2p

x)

4(2p

y)

1(2p

z)

2(2p

z)

3(2p

y)

2(2p

y)

148

Co(ІІ

)TPT

BP

MO

E,

eV

Co

N

Cm

eso

Cα

Cβ

Cm

Co

C1

C2

C3

C4

un o

ccup

ied

23b 2

-2.4

12

60(3

d x2-

y2)

20(2

p x)

- 2(

3px)

2(

3s)

- -

- -

- -

8(

3s)

2(2P

y)

3(

2pz)

40

e -2

.751

5(

3dπ)

12

(2p z

) 17

(2p z

) 21

(2p z

) 10

(2p z

) -

11(2

p z)

- 2(

2px)

-

-

2(2p

y)

5(2p

x)

5(2p

y)

2(2p

y)

1(

2px)

1(

2py)

oc

cupi

ed

18b 1

-4.4

11

- -

- 47

(2p z

) 4(

2pz)

- 23

(2p z

) -

- -

-

11(2

p y)

5(2p

x)

1(

2px)

3(

2py)

39

e -4

.938

60

(3d π

) 7(

2pz)

- 7(

2pz)

4(2p

z) 2(

2pz)

5(2p

z) -

- -

-

2(2p

x)

3(2p

z)

1(

2py)

22

b 2-4

.959

2(

2pz)

22(2

p z)

40(2

p z)

4(2p

z) 3(

2pz)

- 2(

2pz)

- 3(

2py)

-

-

2(2p

x)

10(2

p y)

1(3d

π)

1(2p

y)

23

a 1-5

.126

82

(3d z

2)

3(2p

z) -

1(2p

x)

- -

- -

- -

-

8(

4s)

1(2p

x)

17

b 1-5

.476

92

(dxy

) 1(

2px)

-

3(2p

x)

- -

- -

- -

-

1(d x

2-y2

)

38e

-5.7

78

3(3d

π)

1(2p

z) 10

(2p z

) 4(

2pz)

5(2p

z) 27

(1p z

) 9(

2pz)

2(2p

z) 5(

2py)

-

2(2p

y)

4(2p

y)

2(2p

y)

1(2p

y)

6(2p

x)

2(2p

y)

2(

2pz)

2(

2py)

1(

2px)

16

a 2-5

.893

-

- 5(

2pz)

2(2p

z) 5(

2pz)

33(2

p z)

9(2p

z) 4(

2pz)

6(2p

y)

- 3(

2py)

2(2p

y)

1(2p

x)

8(2p

x)

6(2p

y)

2(

2pz)

2(

2px)

2(

2px)

149

MO

E,

eV

Cu

N

Cm

eso

Cα

Cβ

Cm

Co

C1

C2

C3

C4

Un

occu

pied

17

a 2-1

.823

-

- 13

(2p z

) 5(

2pz)

16(2

p z)

- 10

(2p z

) 5(

2pz)

7(2p

y)

3(2P

y)

7(2p

y)

3(2p

y)

3(2p

y)

3(2p

x)

3(

2px)

5(

2py)

3(

2pz)

2(2P

z) 7(

2pz)

3(2p

y)

2(

2py)

40

e -2

.835

-

11(2

p z)

19(2

P z)

23(2

p z)

10(2

p z)

3(2p

z) 6(

2pz)

- 2(

2px)

-

-

2(2p

y)

3(2P

x)

5(2p

y)

2(2p

y)

1(

2py)

2(2p

x)

1(2p

x)

1(

2pz)

occu

pied

23b 2

-4.0

58

41

(3d x

2-y2

) 29

(2p x

) 6(

2pz)

2(3p

x)

2(2p

y)

- -

- -

- -

4 (4

d x2-

y2)

8(3s

)

2(

3s)

150C

u(ІІ

)TPT

BP

18b 1

-4.4

52

- -

- 49

(2p z

) 4(

2pz)

13(2

p z)

9(2p

z) -

- -

-

9(2p

y)

3(

2px)

2(

2px)

2(2p

x)

2(

2py)

1(

2py)

22

b 2-4

.916

3(

3dx2

-y2)

26

(2p z

) 40

(2p z

) 4(

2pz)

3(2p

z) -

- -

4(2p

y)

- 1(

2py)

1(

3Pz)

1(3s

) 7(

2py)

1(

3dπ)

39e

-5.5

70

10(3

d π)

20(2

p z)

6(2p

z) 7(

2pz)

8(2p

z) 12

(2p z

) 11

(2p z

) -

- -

1(

2px)

1(

2px)

2(

2py)

2(

2py)

2(

2py)

1(

2px)

2(

2px)

3(

2px)

38

e -5

.840

5(

3dπ)

4(

2pz)

5(2p

z) 2(

2pz)

16(2

p z)

6(2p

z) 23

(2p z

) 3(

2pz)

1(2p

z) 2(

2py)

2(

2pz)

1(

2py)

3(

2py)

3(

2px)

4(

2py)

2(

2py)

2(

2py)

2(

2px)

1(

2py)

2(

2px)

23

a 1-5

.843

2(

3dz2 )

24(2

p z)

- -

24(2

p z)

- 22

(2p z

) -

- -

-

6(

2py)

4(2p

x)

6(2p

x)

4(

2py)

16

a 2-5

.890

-

- 5(

2pz)

2(2p

z) 5(

2pz)

32(2

p z)

9(2p

z) 4(

2pz)

4(2p

y)

- 3(

2py)

2(2p

y)

1(2p

x)

8(2p

x)

2(2p

x)

3(2p

y)

2(2p

z)

2(2p

z)

4(2p

y)

2(2p

y)

MO

E,

eV

Pt

N

Cm

eso

Cα

Cβ

Cm

Co

C1

C2

C3

C4

un o

ccup

ied

24a 2

-1.7

79

- -

14(2

p z)

5(2p

z) 17

(2p z

) -

11(2

p z)

5(2p

z) 8(

2py)

3(

2Py)

9(

2py)

2(

2py)

2(

2py)

3(

2px)

2(2p

x)

7(2p

y)

2(2p

z) 2(

2Pz)

5(2p

z)

2(

2py)

2(2p

y)

63e

-2.7

83

5(d y

z) 12

(2p z

) 19

(2p z

)

23(2

p z)

11(2

p z)

3(2p

z) 8(

2pz)

2(2p

x)

- -

-

2(2p

y)

3(2p

x)

3(2p

y)

1(2p

y)

1(

2py)

oc

cupi

ed

29b 1

-4.4

80

- -

- 50

(2p z

) 4(

2pz)

15(2

p z)

10(2

p z)

- -

- -

7(

2py)

3(2p

x)

1(2p

x)

2(

2px)

2(2p

y)

1(2p

y)

35b 2

-5.0

60

1(2P

z) 20

(2p z

) 44

(2p z

) 4(

2pz)

4(2p

z) -

2(2p

z) -

4(2p

y)

- 1(

2py)

7(

2py)

1(

d xz)

62

e -5

.189

33

(5d x

z) 14

(2p z

) -

8(2p

z) 12

(2p z

) 4(

2pz)

12(2

p z)

- -

-

1(

2px)

2(

2py)

1(2p

y)

2(2p

x)

40a 1

-5.6

54

63(5

d z2 )

7(2p

z) -

2(2p

x)

3(2p

z) 4(

2pz)

- -

- -

-

15

(s)

2(

2py)

1(

2px)

61

e -5

.793

1(

5dxz

) -

9(2p

z) 3(

2pz)

8(2p

z) 13

(2p z

) 26

(2p z

) 2(

2py)

3(

2py)

-

2(2p

y)

1(2p

y)

1(2p

y)

1(2p

x)

5(2p

x)

1(2p

x)

2(2p

z)

1(

2pz)

2(2p

y)

3(2p

y)

2(2p

y)

23a 2

-5.8

87

- -

5(2p

z) 2(

2pz)

6(2p

z) -

47(2

p z)

3(2p

z) 3(

2py)

-

1(2p

z)

1(2p

y)

1(2p

x)

9(

2px)

3(

2py)

1(

2pz)

5(2p

y)

39a 1

-6.0

44

13(5

d z2 )

14(2

p z)

- -

25(2

p z)

- 23

(2p z

) -

- -

-

3

(6s)

2(

2px)

4(

2px)

4(2p

y)

3(2p

y)

4(

2px)

151Pt

(ІІ)

TPTB

P

Pd(ІІ)

TPTB

P

MO

E,

eV

Pd

N

Cm

eso

Cα

Cβ

Cm

Co

C1

C2

C3

C4

un o

ccup

ied

33b 2

-2.2

78

43(3

d xy)

28

(2p x

) 2(

2pz)

1(2p

z) 2(

2py)

-

- -

- -

-

3(

4dxy

) 10

(3s)

1(

2py)

2(

2px)

2(

3s)

1(5d

xy)

2(2p

z)

59e

-2.8

01

3(d y

z) 12

(2p z

) 20

(2p z

)

24(2

p z)

10(2

p z)

- 11

(2p z

) -

2(2p

x)

- -

2(

2py)

3(

2px)

3(

2py)

1(

2py)

oc

cupi

ed

28b 1

-4.4

83

- -

- 50

(2p z

) 3(

2pz)

2(2p

x)

15(2

p z)

- -

- -

7(

2py)

2(2p

y)

10(2

p x)

2(

2px)

1(

2py)

32

b 2-5

.017

-

23(2

p z)

43(2

p z)

4(2p

z) 3(

2pz)

- -

- 4(

2py)

-

1(2p

y)

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