vibrational coupling in tadf and how molecular structure ... · tadf generation ii monkman oem...

OEM Research Group

http://www.dur.ac.uk/OEM.group

[email protected]

Fernando Dias

Paloma dos Santo Lays

Marc Etherington

Heather Cole

Przemyslaw Data

David Graves

University of Newcastle

Tom Penfold

Jamie Gibson

Stuart Thompson

Martin Bryce

Jonathan Ward

Vandana Bhalla

José Santos

Mark Fox

Physics Chemistry

Vibrational coupling in TADF and how

molecular structure can control this

complex triplet harvesting



Monkman OEM Research Group

100% internally efficient OLEDs using thermally activated

delayed fluorescence; Photophysics of a complex triplet

harvesting process

Introduction


E-type delayed fluorescence

! "$55*+1*&./-&+/<*5&/-1$+#)&*A #F *-&G&Y' OR&

int pext Pr pL! ! ²! !! != =

~100% 25+# % ~100% ~30%

!"#$

$%&'()*+),+)

- . ' */ . ' () *+) , +)

0%) +1(2+3%405+2136' ,78

79

: ; <

=; <! 8 Small EST

%: e/h injection, transport and recom. eff.

! r: exciton production efficiency

! PL: PL quantum efficiency

! p: light out-coupling efficiency

R5: /-*3)*+)*&

2"/3?"/-*3)*+)*

YIY&: ?)/+<*-3#/+

YI[&: ?)/+<*-3#/+

TADF: Thermally Activated Delayed Fluorescence E-type delayed fluorescence

so named as it was first observed

in eosin dye by Parker in 1964

this is a type of delayed emission

(singlet decay) which is thermally

activated.

Beans and Fredericks first predicted

that this should be the case for an

exciplex

Adachi rechristen this TADF

“TADF’ however first used by

Wilkinson and Horrocks in “ Luminescence in

Chemistry” 1968


TADF devices


How to use a range

of spectroscopic

measurements to gain

an understanding of

TADF


2d

Rotation between the D and A

controls the degree of CT character

and thus the exchange energy

TD-DFT calculation

In solid state, small perturbations

of the torsion angle will introduce

large heterogeneity in optical

properties

ICT TADF These first generation molecules show large heterogeneity

in physical properties

TADF generation II


! 3!

acceptor connected to the N-10 nitrogen strongly influences the ordering of energy levels

and lifetimes of excited states.14,15

Photophysics in solution

The conformation of the D and A sub-units in 2k yields negligible overlap between the

HOMO and LUMO orbitals, and gives rise to singlet (S1) excited state with strong charge

transfer character, thereafter identified as the 1CT state. Emission from 1CT, appears at

lower energies, with onset at 2.53 eV, than the emission from the localized phenothiazine

donor unit singlet state in 2k, 1LE, with emission peaking at 2.63 eV (470 nm). A 2k triplet

state is also localized on the phenothiazine unit, 3LE, identified by its well-structured

phosphorescence, first vibronic peaking at 2.61 eV (475 nm), ca. 0.1 eV above the 1CT

singlet. The energies of the 1LE and

3LE states are in excellent agreement with previous

findings for other substituted phenothiazines.16 Thus the CT states are the lowest energy

states identified in 2k. An extremely small energy splitting between 1CT and 3CT will be

shown supporting the recycling between singlet and triplet states via activated RISC.17

a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)

C-C-N-C Dihedral Angle (o)

ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)

dibenzothiophenedioxide

phenothiazine

2k

A

Figure 1- a) Simplified energy diagram showing the thermal assisted equilibrium between

singlet and triplet excited states; chemical structure and X-ray molecular structure of

2k,.2CDCl3, CCDC-1034115. Molecule 2k has crystallographic C2 symmetry, the

‘An archetypal D-A-D material’, 2K

PTZ-DBTO2

dibenzothiophene-S,S-dioxide core acceptor

phenathiazine donors

D-A bonded through the N atoms Dias et al

Advanced Science

3 (2016) 1-10

4




phenothiazine units are folded by 34o along N…S vectors and on average are inclined by

82o to the planar dibenzothiophene-S,S-dioxide system. Absorption and emission

spectra of 2k (black), donor (red) and acceptor (blue) moieties in methylcyclohexane

(MCH) solution at RT. The absorption of 2k matches the linear combination of

phenothiazine (D) and dibenzothiophenedioxide (A) absorptions, and the emission is

strongly red shifted relatively to the D and A emissions, even in this non-polar solvent. b)

HOMO and LUMO 2k orbitals, and 1CT and 3CT energies obtained from TD-DFT

calculations, as a function of the D-A-D relative dihedral angles, and assuming that singlet

and triplet excited state geometries are the same.

The HOMO and LUMO energy levels of 2k were measured by cyclic-voltammetry, -5.2±0.1

eV and -3.0±0.1 eV, respectively, and for the individual D (HOMO: -5.2±0.1 eV) and A

(LUMO: -2.9±0.1 eV) units (SI 2), in excellent agreement with the corresponding 2k

energies, showing that the HOMO-LUMO gap in 2k is determined by the LUMO and

HOMO levels of the acceptor and donor, respectively.

The absorption spectrum of 2k clearly reflects the sum of phenothiazine (D) and

dibenzothiophenedioxide (A) contributions, indicating negligible conjugation across the

donor-acceptor units, which are almost entirely electronically decoupled. This is further

a)

HOMO LUMO

b)

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)

C-C-N-C Dihedral Angle ( )

ES1

TADF generation II


All geometry optimizations of 2k, phenothiazine and

dibenzothiophene-S,S-dioxide were carried out at B3LYP/6-

31G(d) using the GAUSSIAN09

! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




Basic TD-DFT calculation of frontier orbitals

TADF generation II


! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




! 3!











singlet. The energies of the 1LE and 3LE states are in excellent agreement with previous




a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




From the absorption spectrum

of 2K we see that there is little

interaction between the D and A

fragments (in MCH)

However strong CT formation is

seen even in non-polar MCH

solution

TADF generation II


! 3!















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

N

orm

aliz

ed

In

ten

sity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




Emission feature onset

1CT 2.53 eV

1LE 3.0 eV

3LE 2.61 eV

Local exciton states arise

from the uncoupled phenothiazine

donor units

τph = 80 ms and efficient phosphorescence

thus it must be from an 3(n-π)* state

3LE is above 1CT perfect

excitation 337 nm

! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




1μs

TADF generation II


400 450 500 550 600 650 7000

4

8

Inte

nsityx1

06 (

cp

s)

wavelength (nm)

degassed

O2

2k/MCH

Idg

/IO2

=12.67

If TADF is active in 2K then triplet states must play an important

role in the steady state photophysics…….they do!

Integrated emission

in air and vacuum

! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




In vacuum a small contribution on the blue

edge resulting from 3LE phosphorescence is

observed even at RT.

Thus triplets contribute ca. 92% to the total

luminescence of 2K

PLQY

in-air 0.03 oxygen free 0.33

TADF generation II


Cyclic voltammetry confirms

isolated nature of D and A

fragments

We cannot observe

phenothiazine reduction

or dibenzothiophene oxidation

Electrochemical studies were conducted in 0.1 M solutions of Bu4NBF4, 99% (Sigma Aldrich) in

dichloromethane (DCM) solvent, CHROMASOLV®, 99.9% (Sigma Aldrich) for oxidation and in

tetrahydrofuran (THF) solvent, 99.9% (Sigma Aldrich) for reduction at room temperature

! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




TADF generation II


! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




iCCD measurement system

Time

resolved

emission

400 500 600 7000.0

0.4

0.8

1.2

A

Norm

alized In

ten

sity (

A.U

.)Wavelength (nm)

D+A

PTZ-DBTO2

(b)

D

2.73 eV

400 450 500 550 600 6500

2

4

6

8

10

LE

2.66 eV

14 s

Are

a N

orm

. In

ten

sity x

10

-3 (

A.U

.)

Wavelength (nm)

1.1 ns

CT(a)

TADF spin flip mechanism

The molecule emits dual phosphorescence from both D and A local triplet states both with the

same lifetime which we assume indicates that the A triplet is much longer lived than the D triplet

More on this at the end (remind me!)

The D 3LE is of lower energy than the 1CT state


Dual phosphorescence in ‘2K’

Nobuyashu et al Adv Opt Mat 2016

2.63 eV

TADF generation II


! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




Time evolution of 2K emission in MCH

Observe 2 decay regions

Lifetime Species 5 ns 1CT

15 μs delayed CT

At very long times > 10 ms

we observe 3LE

phosphorescence

DF/PF = 10.3

excellent agreement

with steady state

oxygen dependency

φT= 0.91

3LE states

3LEA 2.61 eV

3LED 2.54 eV

TADF generation II


400 500 600 7000.0

0.4

0.8

1.2

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)

steady-state

DF

2k/MCH

Match of DF and steady state

emission

From the intensity dependence

of the DF we readily confirm

a TADF not Triplet Triplet

Annihilation (TTA) mechanism

! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




in MCH solution

solid state

in CBP film

TADF generation II


! 12!

phenothiazine donor 1LE fluorescence, peaking around 470 nm (2.63 eV). Due to the

strong decoupling between orthogonal D and A units, and possibly better packing in the

more rigid CBP matrix, the initial D/A electron transfer rate is sufficiently slowed for the

radiative decay of the D phenothiazine fluorescence to compete with the electron transfer

process and ISC to the 3LE state.

Following the decay of the initial 1LE fluorescence, the 2k excited state decay proceeds to

1CT prompt fluorescence, which again completely decays within the first 100 ns. From

100 ns onwards the 2k emission is dominated by 1CT delayed fluorescence which initially

peaks at 560 nm, then progressively blue shifts to 512 nm, due the underlying 3LE

phosphorescence which dominates at later times.

10-1

100

101

102

103

104

105

106

107

108

10-1

100

101

102

103

104

105

106

107

108

10910

-110

010

110

210

310

410

510

610

710

8

2.52

2.54

2.56

2.58

2.60

2.62

2.64

2.66

2.68

2.70

2.72

time (ns)

Pe

ak

po

sit

ion

(e

V)

PL

In

ten

sit

y (

A.U

.)

time (ns)

2k/CBP at RT

10-1

100

101

102

103

104

105

106

107

108

109

10-3

10-2

10-1

100

101

102

103

104

105

106

107

108

PL

In

ten

sit

y (

A.U

.)

time (ns)

60K

100K

160K

220K

298K

prompt fluorescence

delayed fluorescence

3D phosphorescence

b) 2k/CBP

Figure 4-a)!Plot of the 2k emission intensity decay in CBP, over a time interval spanning 9

orders of magnitude, obtained at RT. Prompt and delayed fluorescence components are

clearly separated in time, showing exponential decays. As in solid zeonex matrix, the

excited state dynamics of 2k in CBP is complex, with the emission peak shifting in time,

due to underlying contributions of different species. b) temperature dependence of the 2k

emission decay in CBP matrix. From 298 K to 220 K, the emission decay appears almost

independent of temperature, in line with the small energy barrier for triplet harvesting in

this material. However, below 220 K, the 1CT delayed fluorescence is progressively

quenched, and at latter times, the long lived 3LE phosphorescence becomes dominant.

From these measurements we are able to propose the full kinetic scheme shown in figure

5 to describe the 2k excited state dynamics. Upon optical excitation of the phenothiazine

donor (1LE), and depending on the medium, the population of the 1CT state occurs by fast

(solution), or relatively slower (solid films), electron transfer (2). In the latter, initial

This indicates that electron transfer from

D to A is very slow indicative of the

extreme decoupling of the D and A

Time and temperature evolution of the emission from 2K in CBP

! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




In first few ns we see emission from 1LE at ca. 3 eV

TADF generation II


400 450 500 550 600 650

0.0

5.0x104

1.0x105

1.5x105

2.0x105

2.5x105

3.0x105

3.5x105

PL

In

ten

sit

y (

A.U

.)

Wavelength (nm)

1.1 ns

3.4 ns

6.0 ns

Initial observations on the electron transfer step

Approximate ET half life is 3-4 ns

which is very slow for intramolcular CT

V is the electronic coupling between

D and A

KET typically 1012s-1, here it’s 108s-1

For 2K this is small as no spectral

overlap (and dipoles orthogonal) or

wavefunction overall

4




phenothiazine units are folded by 34o along N…S vectors and on average are inclined by

82o to the planar dibenzothiophene-S,S-dioxide system. Absorption and emission

spectra of 2k (black), donor (red) and acceptor (blue) moieties in methylcyclohexane

(MCH) solution at RT. The absorption of 2k matches the linear combination of

phenothiazine (D) and dibenzothiophenedioxide (A) absorptions, and the emission is

strongly red shifted relatively to the D and A emissions, even in this non-polar solvent. b)

HOMO and LUMO 2k orbitals, and 1CT and 3CT energies obtained from TD-DFT

calculations, as a function of the D-A-D relative dihedral angles, and assuming that singlet

and triplet excited state geometries are the same.

The HOMO and LUMO energy levels of 2k were measured by cyclic-voltammetry, -5.2±0.1

eV and -3.0±0.1 eV, respectively, and for the individual D (HOMO: -5.2±0.1 eV) and A

(LUMO: -2.9±0.1 eV) units (SI 2), in excellent agreement with the corresponding 2k

energies, showing that the HOMO-LUMO gap in 2k is determined by the LUMO and

HOMO levels of the acceptor and donor, respectively.

The absorption spectrum of 2k clearly reflects the sum of phenothiazine (D) and

dibenzothiophenedioxide (A) contributions, indicating negligible conjugation across the

donor-acceptor units, which are almost entirely electronically decoupled. This is further

a)

HOMO LUMO

b)

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)

C-C-N-C Dihedral Angle ( )

ES1

This slow rate of electron transfer

is highly significant

! 3!















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




TADF generation II


! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




Simple 2K devices

EQE 18.8 %

at 10 cd/m2

TADF generation II


! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




Pure 2K emission

NO 3LE

Phosphorescence

observed

Roll-off not too bad very low turn on voltage

TADF devices


EQE =hout ×f fl ×g ×x fr

Low turn on voltage 3.5 V for a trilayer device, EQE 18.8%

If we assume 0.3 out coupling

PLQY 0.3 (as measured in oxygen free)

charge recombination yield 1

singlet yield , ξfr = 1

Then EQE should be 9%

To achieve the measured EQE of 19% we have to have a PLQY of 0.65

! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




TADF generation II


Hole and electron injection can only occur directly into the decoupled donor LUMO

and acceptor HOMO DIRECTLY forming the ICT state NOT an excited

phenothiazine

! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




Thus we do not populate 1LE so we avoid non radiative decay from this state.

TADF generation II


! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




300 400 500 60010

-2

10-1

100

101

102

103

104

x=475 nm

x=450 nm

x=425 nm

x=400 nm

x=380 nm

phenothiazine/toluene (=2.43)

1,2 DichloBz (=10.36)

Anisole (=4.45)

toluene (=2.43)

(

M-1cm

-1)

wavelength (nm)

4a) PTZ-DBTO2

400 500 600 700 8000

4

8

12

16

20

4b) PTZ-DBTO2/toluene

x=380 nm

x=400 nm

x=425 nm

x=450 nm

x=475 nm

Ab

s.

No

rm.

Inte

nsity x

10

6 (

cp

s)

wavelength (nm)

350 375 400 425 450 475 5000.0

0.2

0.4

0.6

0.8

1.0

4B) PTZ-DBTO2/toluene

Flu

ore

sce

nce

yie

ld

excitation wavelength (nm)

Direct excitation of the lowest lying n-π* state

Very high concentration solutions

We observe new absorption below the π-π* of either 2K or

phenothiazine fragment, blue shifts with increasing polarity

Extinction coeff ca 100 cm-1

Site specific excitation yields CT emission

Normalising to absorption the relative PLQY of this new

transition is found to be ca 0.60-0.65

Dias et al

Advanced Science

1600080 (2016)

TADF devices


Photophysics of a

TADF emitter

Can we control

molecular geometry

to control RISC?

TADF generation III


Consequences of this new model

2 | Chem. Commun., 2014, 50, 1–3 This journal is © The Royal Society of Chemistry 2014

Fig. 1 Structures of D–A–D and D–A molecules

Compound 2

Compound 3

Compound 5

Fig. 2 X-ray molecular structures of compounds 2, 3 and 5 displayed with

thermal ellipsoids at 50% probability.

The dibenzothiophene moiety in 2 and 5 is practically planar;

in 3 it is slightly twisted and in 7 folded, with the two arene rings

forming a dihedral angle of 6.9° and 12.5°, respectively. The

molecules of 3 and 5 possess a crystallographic two-fold axis,

hence the phenothiazine substituents have transoid orientation

with respect to the dibenzothiophene plane, i.e. their S atoms lie

on opposite sides of this plane. 2 has a similar conformation,

although the molecule has no crystallographic symmetry.

Interestingly, one of the phenothiazine moieties [at C(9)] is

disordered in a 0.6:0.4 ratio between two conformations, with the

methyl substituent on opposite sides of N(2) and the tricyclic

system librating around the N(2) as the pivot, to provide room for

the methyl group. The phenothiazine moiety is always folded

along the N…S vector, forming a dihedral angle of 128.7° (3),

135.5° (5) and 135.3° (7). In 2, the ordered phenothiazine moiety

is folded by 131.1° and the disordered one by 133.6° or 135.0°. In

every case, the N atom is substantially pyramidalised; its lone

pair is oriented favourably for conjugation with the

dibenzothiophene π-system rather than with the arene rings of

phenothiazine itself. Therefore, the N-C(dibenzothiophene) bond

is shorter than N-C(phenothiazine), on average by 0.035 Å in 3

and 5 and 0.039 Å in 7 (s.u. 0.001-0.002 Å); the similar

difference (0.038 Å) in 2 is not statistically significant due to

disorder and high s.u. The packing of 2 and 5 is characterised by

stacking (π-π) interactions between phenothiazine arene rings

(related via inversion centres and therefore parallel) with

interplanar separations of 3.50 and 3.60 Å (intermittently) in 2, or

3.66 Å (uniformly) in 5. These interactions link molecules into

infinite chains (SI S8). No stacking is evident in the structures of

3 and 7.

The 1H NMR spectra of 1,

16a 2, 5–8 at 298 K display sharp

peaks showing that there is one species in solution at this

temperature on the NMR timescale. In contrast, compounds 3 and

4 which have a bulkier iPr or

tBu substituent on each

phenothiazine ring exhibit restricted rotation. The spectrum of

compound 3 at 298 K clearly shows two species in solution: upon

heating the peaks coalescence to reveal only one species at 373 K

(Figure 2). 1H NOESY and ROESY NMR experiments strongly

suggest that the two species are interconverting with each other

(See SI S4). For the t-butyl analogue 4, at 298 K selected peaks

are extremely broad and the spectra at 348 K and 373 K are

comparable to 3 at 298 K. Rotation in compound 4 is still

partially restricted even at 373 K (Fig. 3). 1H NOESY NMR

experiments with 4 also demonstrate there is some exchange

occurring between the two very broad peaks at 298 K (see SI S4).

The presence of these two species is likely to be due to up-up and

up-down configurations of the iPr or t

Bu groups with respect to

the acceptor.

Fig. 2 500 MHz Variable temperature

1H NMR experiments for 3 from

298 – 373 K in DMSO-d6.

Photophysical measurements on 2–8 reveal how the increasingly

bulky phenothiazine donors affect their optical properties. It has

been previously shown that 1 and 6 emit strongly via a TADF

mechanism16

(see SI S6).



Compound 2

Compound 3

Compound 5
































3 and 7.





















the acceptor.



298 – 373 K in DMSO-d6.




mechanism16

(see SI S6).

compound 3

We use bulky side groups

to fix a 90o dihyedral angle

This slows down D-A rocking such that

NMR can resolve the motion

Ward et al

Chem. Commun., 2016,52, 2612-2615

TADF generation III


When we hinder rocking and torsional motion about the C-N bond

we kill all DF but observe strong room temp phosphorescence



Compound 2

Compound 3

Compound 5
































3 and 7.





















the acceptor.



298 – 373 K in DMSO-d6.




mechanism16

(see SI S6).

TADF


But which excited states

are really involved in rISC

1CT – 3CT as originally thought

OR some other states?


Polarity effects on DF

2,7-bis(phenoxazin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide

DPO-TXO2

DPO-TXO2 behaves like most D-A-D TADF emitters showing a

clear CT excited state and strong solvatochromism. But also it

shows a new absorption band which red shifts with polarity so

predominantly π-π* character

Paloma Lays Dos Santos et al, J Mat Chem C. 4, 3815, 2016

TADF generation II



In non-polar MCH solution

Time and temperature dependent emission show

TADF character with rapid quenching of the DF

component with temperature indicative of a large

rISC energy barrier

Looking at the TR spectra;

At 1 ns we observe donor emission plus emission

from a dimer of the donor at 450 nm

CT emission clear after 15 ns

CT relaxes and at 1µs phosphorescence from

the donor at the red edge appears

Phosphorescence lasts into ms

∆EST = (0.16± 0.03) eV



TR emission in more polar toluene

Clear decay regions and isoemissive points

Below 50 ns ; Prompt 1CT→CT0

Region A 50ns to 5 µs; DF 1CT→CT0

Region B 5 µs to 60 µs; mixed DF 1CT→CT0 3LE →S0

60 µs to 1 ms; donor phosphorescence 3LED > 1CT

BUT phosphorescence observed, therefore 3LED still at the same energy as in MCH

Region A DF increases with increasing Temp (TADF)

Region B DF decreases with Temp (phosphorescence)

Competition between TADF and phosphorescence

rates and triplet lifetime

∆EST = -(0.07 ± 0.03) eV



In more polar toluene

The DF is much stronger and very

clearly defined

DF/PF ratio 4.81

KrISC = 1.04x106 s-1

∆EST = -(0.07± 0.03) eV

rISC barrier closes

because CT energies

stabilise by the solvent

polarity whereas the

local triplet is unaffected



We see that the 3LED energy is independent

of polarity, as it should be, but 1CT states

shift to lower energy in polar environment;

Thus ∆EST (MCH) + 160 meV

∆EST (toluene) – 70 meV

Because of this rISC increase strongly

BUT in both cases, ∆EST both +ve or –ve

we observe strong rISC

TADF


So we needed a new model


TADF rISC Thoery

T he Pot ent ials

0.5

0.4

0.3

0.2

0.1

0.0

Rel

ativ

e En

ergy

(eV

)

-6 -4 -2 0 2 4 6

Nuclear Coordinate

n1

0.5

0.4

0.3

0.2

0.1

0.0

Rela

tiv

e E

ne

rgy

(e

V)

-6 -4 -2 0 2 4 6

Nuclear Coordinate

n11

0.5

0.4

0.3

0.2

0.1

0.0

Rela

tiv

e E

ne

rgy

(e

V)

-6 -4 -2 0 2 4 6

Nuclear Coordinate

n23

• TDA-TDDFT(M062X) /

Def2-SVP basis.

Thomas Penfold (Newcast le University) June 16, 2016 13 / 25

Modes reasonable to thermally activate

3LE 3CT

1CT

Solid lines are the fit from the model vibronic

Hamiltonian used to yield coupling constants

Vibrational modes

that mix 3LE and 3CT

in 2K


TADF rISC Thoery

W hat is t he M echanism?

k / h f |Hint | i i +h f |Hint | n i h n|Hint | i i

En − Ei

2

δ(Ef − Ei )

3LE

1CT

3CT

ĤSOC=&2&cm*1&

ĤHFI&=&0.2&cm*1&

Ĥvib&≈&65&cm*1&

First step is rIC into 3CT:

krIC / h 3CT |Hvib | 3LE i2

δ(E3CT − E3LE )

and then the second order term of the form:

krISC /h 1CT |Hsoc | 3LE i h 3LE |Hvib| 3CT i

E3CT − E3LE

2

δ(E1CT − E3LE )

Thomas Penfold (Newcast le University) June 16, 2016 18 / 25

Causing a thermal equilibrium

between the two states

Couples 3CT to 1CT

Via the intermediary 3LE state

From TR EPR 1CT-3CT splitting

is ca. 2 μeV

so the optically

measured gap

is 1CT-3LE

thermal gap 3LE-3CT

TADF


Photophysics of a

TADF emitter with the

Vibronic coupling model

What observations

can we see because of the

differential solvatochromic

effect on CT and 3LE?


reorientationandtheCTenergyandthemolecularstructureoftheguestarefixed.Atthispolarity

the CT states will lie above the local donor triplet, found at 2.58 eV, extracted from

phosphorescencepreviouslymeasuredwithinthisgroup.[20]Thisplacesthisarrangementasan

exampleofTypeITADFinFigure1b).

Toinvestigatethepolaritydependenceoftheseexcitedstatespectra,20μMsolutionsof2Kin

toluene(ε=2.38)and2-methyltetrahydrofuran(MeTHF)(ε=6.97)werestudied.Figure3showsthe

steadystateemissionspectraof2KintolueneandMeTHFalongsidethatof2Kfilmsmadeinzeonex

andpolyethyleneoxide(PEO).Theemissionpeakof2KintolueneandMeTHFisredshiftedcompared

tothatinzeonexby0.2eVand0.4eVrespectively,inlinewiththepolarityofthesolvents.

Figure3:a)Photoluminescenceof2Kinavarietyofhosts.ThetransparentcurvesoftolueneandMeTHFsolution(2x10-5M)

correspondtotheleftscaleandtheboldlines(λex=400nm)ofZEONEXandPEO(at295K,220Kand150K)correspondtotherightscale(λex=375nm).ThereductioninmagnitudeforMeTHFispolarityrelated,whiletheshiftofemissioninPEOisrelatedtotheglasstransitiontemperature.b)Photoluminescenceofphenothiazine(PTZ)intolueneandMeTHFsolutionsataconcentrationof2x10

-5M.λex=320nm

Thispolarityeffectonthephotophysicsof2Kisfurtheremphasisedinthequasi-CWPIA

investigatedintheaforementionedsolvents(seeFigure4)atconcentrationsof10mM.Thein-phase

spectrashowthattheCTemissiondominatesandthatintolueneitisstrongerthaninMeTHF,a

resultmirroredinthesteadystatePL(seeFigure3a).However,theout-of-phasespectraforboth

solventsystemsaredominatedbyabroad,longlivedCTstateinducedabsorption,againnolocal

tripletstateinducedabsorptionisseen.ThisisattributedtothefactthattheenergyoftheCTstates

arestabilisedinthemorepolarenvironmentssuchthattheynowliebelowthe3LE(unaffectedby

hostpolarity),whichprovidesanextradecaypathway,internalconversion,from3LEto3CT,and

sinceinterconversionfrom3CTto1CTisforbiddenbySOCT,3CTbecomesaneffectivelong-livedtrap.

Themeasuredinducedabsorptionsignalisthusidentifiedasthe3CTstateduetoitbeingtheonly

featureseenintheout-of-phasemeasurement,concomitantwithithavingamuchlongerlifetime

thantheemissive1CTstate(in-phasesignal).ThisisanexampleofTypeIIITADF,Figure1b)and

confirmsthatthePIAspectracanidentifytheenergeticarrangementsintheTADFsystem.

a) b)

! 3!
















a)

HOMO LUMO

b)

0 10 20 30 40 50 60 70 80 90

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

ET1

En

erg

y (

eV

)


ES1

300 400 500 600 7000.0

0.4

0.8

1.2

D

No

rma

lize

d I

nte

nsity (

A.U

.)

wavelength (nm)


phenothiazine

2k

A




Polyethylene oxide

host useful as it is

polar and has a low

glass transition

temperature and its

polarity decreases as

temperature

decreases

Tg 220 K

TADF in a polar solid

Etherington et al

Nature Communications

7, (2016) 13680

Lets look at a different D-A-D system a different way

295K

150K


TADF in a polar solid Conversion to energy scale

•

Mooney, J. & Kambhampati, P. Get the Basics Right: Jacobian Conversion of Wavelength and Energy Scales for Quantitative Analysis of Emission Spectra. J. Phys. Chem. Lett. 4, 3316–3318 (2013).


2K behaviour in PEO TADF in a polar solid

We can tune 1CT in

solid state with temp

In the PEO host

1CT plateaus at ca 200 K, the glass

transition of PEO


Figure4:Thequasi-CWphotoinducedabsorptionof2KintolueneandMeTHFatconcentrationsof10mM.Thein-phasespectraaretheboldlinesandtheout-of-phasethetransparentlines.Thebroadabsorptionbetween500nmand1000nmisrelatedtothe

3CTstates.

TostudypolaritydependenceinthesolidstatethepolarhostPEOwaschosen,whichhasadielectric

constantofε=5.However,thispolarityismeasuredatmicrowavefrequencies[29]andatoptical

frequenciesPEOhasasimilarpolaritytotolueneatroomtemperature(ε=2.38).Onedifference

betweenthetwosolidstatehostsystemsusedhoweveristhattheglasstransitiontemperatureof

PEOis220K,muchlowerthanthatofzeonex.[29–31]Bypassingthroughthistransitionwecan

restrictvibrationalmotionoftheguestandfurtheraffecttheenergeticsofthesystem.

Figure5showshowpropertiesofthe2KinPEOchangeasafunctionoftemperature,especially

aroundtheglasstransitiontemperature.TheCTenergyofthemoleculeblueshiftsastheTgis

approachedandthenstabiliseswhenthePEObecomesrigidbelowTg.WiththisincreaseinCT

energy the intensity of the emission also increases before reducing at low temperatures.

EnergeticallythisrelatestotheCTenergylevelshiftingfrombelowthe3LEstate(TypeIII),passing

throughresonanceat220K(TypeII)andthenincreasingfurtherandstabilisingabovethe3LEstate

(TypeI).ThisisrepresentedinFigure1b)asthesystemgoesfromTypeIIIthroughresonance,Type

IItoTypeITADF.TheshiftintheCTenergyonsetisfrom2.50eVto2.58eVbringingthestatesmore

intoresonanceandthenoutofresonancewiththe3LEstate(againunaffectedbyTg).Thisisshown

intheshapeoftheintensitycurve.Thedecreaseinintensitytowardsevenlowertemperaturesmay

alsobetheresultofthethermallyactivatednatureofTADF,aphenomenonalreadywell

documentedinliterature.[1–3,18]

Figure5:Thetemperaturedependenceoftheintensity(blackline)andCTonsetenergy(purpledash).ThechangeinCTonsetenergyplateausbelowtheTg,representativeofthePEOfilmbecomingrigid.Theblackdashedlinerepresentstheenergyofthe

3LE,withthepeakinintensityapparentastheCTenergycrossesresonance.

2K behaviour in PEO

The DF emission reaches a max with 1CT is

resonant with 3LE

We tune 1CT with respect to 3LE changing the

ΔEST as polarity of PEO decreases

TADF in a polar solid



a b

Model prediction DF contribution

Tuning the resonance between 3LE and CT states



Consequence of

the model

As the CT-3LE gap closes ISC and rISC rates increase but

the prompt 1CT lifetime and DF lifetime ALSO increase as a

consequence of the stronger S-T coupling i.e. strong state

mixing

a b

This we believe is the real TADF mechanism

Thus near degenerate 1CT, 3CT and a 3LE state is critical

for efficient RISC and thud TADF


1D

3A 1CT krISC106 s-1

kPH<<106 s-1

EST~25-50 meV

kisc108 s-1

kTADF106 s-1

kET >108 s-1

kFl108 s-1

kFl107 s-1

3CT

3D

kIC107 s-1

e- + h+

recombination in the device


Key new observations on TADF

i. BOTH 1CT - 3LE and 3CT - 3LE energy gaps controls rISC and TADF

ii. Second order vibrational coupling SOC mediates the rISC

iii. CT’s and 3LE tune independently

iv. Both TADF emitter and host material together dictate device efficiency

v. Emitter PLQY does not tell you about efficiency in a device; see Adv Sci paper

vi. When the gaps are near resonance rISC is near100% yielding efficient OLEDS

vii. But at resonance, the excited state lifetimes get longer which might cause a

problem in a device through charge excited state quenching?

TADF

Thank you


[email protected]

OEM Research Group

Andy Monkman

Fernando Dias

David Graves

Vygintas Jankus

Przemyslaw Data

Paloma Dos Santos

Roberto Nobuyasu

Marc Etherington

Heather

Higginbotham

Martin R. Bryce

Vandana Bhalla

José Santos

Mark Fox

Jonathan Ward

Andrei Batsanov

Theory

Tom Penfold

Jamie Gibson

Stuart Thompson

Physics Chemistry

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