http://www.nd.edu/~pkamat

Prashant V. KamatRadiation Laboratory,

University of Notre Dame, Notre Dame, Indiana 46556

Outline

I. Reactive Intermediates and Fast Kinetic Spectroscopy Techniques

Time-resolved photochemistry– Detection of singlet and triplet excited states

using picosecond and nanosecond laser flash photolysis

Radiolysis– Gamma radiolysis and product identification

– Pulse radiolysis for spectral characterization and kinetic evaluation

II. Photochemistry of Dyes in Surfactant Solution

Dye aggregation

Triplet-triplet energy transfer processes

Excited state interactions

Sensitization of Semiconductor Surfaces

What are they?Reactive intermediates are short-lived chemical species that interact with other molecules.

• Singlet and triplet excited states• Excited state charge transfer complex• Radical anions and radical cations• Trapped charge carriers

What are the reaction pathways?• Energy transfer in the excited state• Electron transfer to initiate chemical transformation• Dimerization, polymerization, fragmentation,

hydrolysis, etc.

Why are they important?• Understanding the problems associated with

photostability and degradation mechanism• Improving the stability of the molecules in

heterogeneous media

Reactive Intermediates

Mechanistic and Kinetic Aspects of Excited State and

Radical Reactions

Electrochemistry/ESR

Photochemistry Radiolysis

Product Analysis

Study of Reactive Intermediates

Fast Kinetic Spectroscopy (Pump-Probe Method)• Picosecond and Nanosecond Laser Flash

Photolysis • Pulse Radiolysis (Radiation Induced Processes)

Diffuse Reflectance Spectroscopy• In-situ photolytic studies of opaque samples• UV-VIS, FTIR and Emission Spectroscopy

Electrochemistry, Spectroelectrochemistry, Sonochemistry, -radiolysis and Analytical Techniques

Probe

Sample

Detector

Pulsed Laser

a. Laser flash photolysis (or pulse radiolysis)

Probe

Pulsed Laser

DetectorTo

Sample

b. Diffuse reflectance laser

flash photolysis

Experimental Techniques

(or e-pulse)

— The chemical events in these experiments are initiated by an ultrafast laser pulse (pump) and the photophysical and photochemical events are probed by another ultrafast laser probe pulse. (Mode-locked, Q-switched Continuum YG-501 DP Nd:YAG laser, pulse width ~18ps).

— Provides vital information on the mechanistic and kinetic details of chemical events that occur in the timescale of 20 picoseconds to 10 nanoseconds.

— The white continuum picosecond probe pulse is generated by passing the fundamental output through a D2O/H2O solution. An optical delay rail employed to control the delay time of the probe pulse enables detection of transients at desired time intervals after the sample excitation..

Picosecond Laser Flash Photolysis

tpump

probe

Optical delay rail

Pump

LaserProbe It

I0

H2O/D2O cell

Spectrograph/ Detector

Singlet Excited State

S+ T+ NR = 1

S= kf/(kf+knr)S1

T1

S0

S2

h h’

tpump

probe

pump

probe

•The transient absorption recorded immediately after the laser pulse excitation corresponds to singlet excited state

•Triplet excited states accumulate at longer times.

•Singlet excited state has a shorter lifetime of 420 ps while triplet excited state has a lifetime of ~10 ms.

•The singlet excited lifetimes can also be determined from the emission measurements.

400 450 500 550 600

-0.4

-0.2

0.0

c

a

b

d

A

Wavelength, nm

0 1 20.00

0.02

0.04

0.06

445 nm = 420 psA

Time, ns

Difference absorption spectrum recorded following 532 nm laser pulse excitation of thionine dye.

A0

Aex

A=Aex-A0

What is a difference absorption spectrum?

S

N

NH2H2N

Thionine

+

Nitrogen laser (337 nm / 6 ns) • kinetic absorption spectroscopy • fluorescence lifetimes • 2-pulse experiments

Excimer laser (308 nm / 20 ns) • kinetic absorption spectroscopy • 2-pulse experiments

YAG laser (266, 355, & 532 nm/ 6 ns) • kinetic absorption spectroscopy • fluorescence lifetimes • microwave conductivity • diffuse reflectance • 2-pulse experiments

S1

T1

S0

T2

S2

h h’

Time-resolved Raman Spectrometer

Nanosecond Laser Flash Photolysis

time

A

Since T1 S0 is a forbidden transition the triplet excited states are long-lived.

Triplet excited molecules undergo diffusion controlled electron transfer reactions with other solutes.

Triplet Excited State

S1

T1

S0

T2

h

Difference absorption spectrum recorded following 532 nm laser pulse excitation of thionine dye. S

N

NH2H2N

Thionine

+

TH+ + h 1TH+* 3TH+*

3TH+* TH+

3TH+* + ZnO TH•2+ + ZnO(e)

The reactivity of triplet excited thionine can be established using laser flash photolysis.

The dye molecules participate in the electron transfer with ZnO colloids.

Photoinduced electron transfer processes play an important role in determining the stability of dyes in different environments.

400 500 600 700

-75

-50

-25

0

25

50

A ×

10

3

Wavelength (nm)

thionine thionine + ZnO

400 500 600 700

-75

-50

-25

0

25

50

A ×

10

3

Wavelength (nm)

thionine thionine + ZnO

400 500 600 700

-75

-50

-25

0

25

50

A ×

10

3

Wavelength (nm)

1 s

A=0.01

500 nm

420 nm

1 sA=0.01

t

Thionine + ZnO System

1 s 3 s 5 s

400 500 600 700

-75

-50

-25

0

25

50

A ×

10

3

Wavelength (nm)

1 s

A=0.01

500 nm

420 nm

1 sA=0.01

t

Thionine + ZnO System

1 s 3 s 5 s

Radiolysis of Water

H2O —^^^ OH•, H•, eaq, H+, H2O2, H2

G(X) = number of molecules of X/100 eV absorbed

G(eaq)=G(•OH)= 2.7

G(H) =0.6; G(H2) =0.45; G(H2O2) = 0.7

At pH4, OH• and eaq are the major reactive species that

survive during the ionization of water

Reductive Conditions:……….alcohol as a hydroxyl radical scavenger

(CH3)3-COH + •OH — (CH3)2-•CH2-COH + H2O

(k=6.0x108 M-1s-1)

Oxidative Conditions:……….N2O as an electron scavenger

eaq + N2O + H2O — N2 + OH•+ OH

(k=9.1x109 M-1s-1)

Secondary Oxidizing Radicals:

OH•+ N3— N3

•+ OH

(k=1.2x1010 M-1s-1)

eaq + S2O82— SO4

•+ SO42

NDRL has three cobalt-60 gamma irradiators, with radiation intensities of about 2, 6 and 20 kilocuries, respectively.

These sources are programmable to give exposures ranging from minutes to days.

After irradiation, samples can be analyzed by a variety of methods, including optical and infrared absorption spectroscopy, high-performance liquid chromatography, ion chromatography and mass spectrometry.

Gamma Irradiators

—The short-lived reactive intermediates of water radiolysis for low LET radiation (- or X-rays with energies above 30 keV) are eaq,

•H and •OH. —In the presence of oxygen,

hydrated electrons and H atoms are converted into O2

and HO2.

•OH H+ + O (pKa 11.9) HO2 H+ + O2

(pKa 4.9)

—By adjusting the pH and O2 concentration one can produce eaq, •H, •OH, O2

O and HO2• species

Reaction with Hydroxyl Radicals

Chart 2

54

32

m/z =194

HOHO

SO3

NH2N=N

m/z = 372 and 186

HO

NH2N=N

m/z = 189

NH2

HO

O3S

m/z = 172.5

HO

O3S

SO3

SO3

SO3

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

f

e

d

c

b

a

Ab

sorb

ance

Wavelength, nm

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

f

e

d

c

b

a

Ab

sorb

ance

Wavelength, nm

t, mina 0b 5c 15d 40e 60f 90

Radiolysis of 5mM Acid Yellow 9 solution in N2O saturated aqueous solution

Four major products were identified from the Electron spray mass spectral analysis of the reaction mixture.

SO3

O3S NH2N=N

Das, Kamat, Padmaja, Au, Madison, J. Chem. Soc. Perkin Trans. 2, 1999, 1219-1224

—An 8 MeV linear electron accelerator is the experimental centerpiece of the radiation chemistry effort. This instrument is capable of delivering pulses of electrons ranging from 1 nanosecond to 1.5 microseconds in duration. These pulses are delivered to a sample cell where they ionize molecules in the sample, a process called pulse radiolysis.

—The ions and electrons rapidly recombine, but in the process produce large quantities of free radicals. If the sample is an aqueous solution, the radicals produced in greatest quantities are the hydroxyl radical ( •OH), the hydrogen atom and the hydrated electron (eaq–).

—The free radicals react with molecules dissolved in the water to produce the chemical species that are the subject of our studies.

Pulse Radiolysis

Electronbeam

Linear accelerator characteristics Nominal beam energy: 8 MeVRF source: 20 MW, 2856 Mhz klystronPulse duration: 2 to 100 nanosec, 1.5sPulse frequency: 1 to 60 HzMaximum beam current: 4 ampsNominal beam diameter: 5 mmPulse-to-pulse dose stability: ±1%Manufacturer: Titan Beta, Dublin CA

300 400 500 600 700-0.010

0.000

0.010

0.020d

c

b

a

A

Wavelength/nm

300 400 500 600 700-0.010

0.000

0.010

0.020d

c

b

a

A

Wavelength/nm

t, sa 2b 5c 8d 16

Reaction with Sulfate Radical Anions

eaq + S2O82— SO4

•+ SO42

dye + SO4•—dye • + SO4

2

500 nm

370 nm

Scheme 1

SO3NaN=NNH

H

SO4 +. .

SO3Na + SO42- N=NN.+

H

HSO3Na

SO3Na N=NN.

HSO3Na

-H+

SO3Na

.

67

+H+

8

ET

Acid Yellow 9 in water at pH 7

pKa 5.5

k= 11010 M-1s-1

-10 0 10 20 30 40

0.000

0.005

0.010

0.015

0.020

Time, s

A

0.000

0.005

0.010b

a

A

Das, Kamat, Padmaja, Au, Madison, J. Chem. Soc. Perkin Trans. 2, 1999, 1219-1224

2 3 4 5 6 7 8 9 10

0.000

0.010 b

a

A

pH

2 3 4 5 6 7 8 9 10

0.000

0.010 b

a

A

pH

370 nm500 nm

Scope of Future Research

Time-resolved transient studies of hair

colorants• Primary photochemical events

– Characterization of singlet and triplet excited states(Spectra, lifetimes, quenching rate constants,

pKa)

• Photochemistry in heterogeneous media– Effect of surfactants, polymers, colloids and

proteins(dye aggregation effects, excited state

properties)

• Photostability of dyes during long term exposure– Wavelength and energy dependence– Product analysis

• Reactivity of dyes with oxidizing and reducing radicals– Spectral characterization of transients using pulse

radiolysis – Kinetics and mechanistic details– Product analysis– Influence of heterogeneous media on the

reactivity of dyes