Design of organic TADF molecules. The role of ∆E(S1-T1): From fluorescence to TADF and

beyond - towards the fourth generation OLED mechanism.

H. Yersin, L. Mataranga-Popa, R. Czerwieniec

University of Regensburg, Germany

Outline

General focus: Introduction how to design organic TADF molecules

Short introduction to Singlet Harvesting for 100% exciton use - based on TADF

Important requirements: Materials with short-lived TADF decay time without long decay tails Crucial for: - High emission quantum yields - Increase of device stability - Decrease of roll-off

Case studies demonstrating step by step improvements

Last step: New mechanism beyond TADF for fourth generation OLEDs

t(T )1

Spins and electron-hole recombination

3 -1DE(S -T ) < 10 cm (0.12 eV)1 1

threetriplet paths 75 % 25 %

singlet

up-/down-ISC

S0

S1

T1

path

k TB

t(TADF)k(S )1

TADF and Singlet Harvesting in OLEDs for 100% exciton use

TADF: Parker 1961OLEDs: Yersin 2006

eDE(S -T )1 1

k TB-

Introduction - TADF of organic molecules

(A)) - (D)( )( 0Red

0Ox EEeCTETransition energy ≈

Example D N AN

N

9 2.00O x =E 12.20

R ed -=E-1 E(CT) 3.04 eV (24 300 cm )≈═>

Suitable energy range

D A

HOMO

LUMO

V V

CT

═>

Concept 1 - Schematic structure with no sterical hindrance

DONOR ACCEPTOR

Concept 1 - Emission

T = 300 K, cw, PMMA

465 nm

prompt fluorescence t = 4 ns F (deg) = 76 %PL F (air) = 76 %PL Indication : No TADF═>

300 400 500 600 700nm

emexcN

N

N

400 500 600 700nm

466 nm 580 nm

-14400 cm0.55 eV

t = 1 msDt = 900 ms

prompt fluor.

t = 5 ns

phos. t = 1 s

Concept 1 - Emission T = 77 K, , PMMA cw

300 500400 600 700nm

exc em

466 nm

prompt fluor.

t = 5 ns

F = 84 %PL

phos.?

T = 77 K, , PMMA time resolved

t = 1 nsDt = 100 ns

300

Concept 1 - Energy level diagram

4-5 ns466 nm

S0

1 s (77 K)580 nm

-1

DE≈ 4400 cm (550 meV)

S1

T1

Assignment

No TADFReason: Planarization in the excited state due to double bond formation between the rings (quinoid form)═> Fluorescent molecule

RequirementBreaking/reducing the conjugation

Concept 2 - Schematic structure with sterical hindrance

DE 10 2014 106 987 A1DE 10 2014 106 986 A1WO 00 2015 121 239 A1WO 00 2015 121 241 A1

ACCEPTORDONOR

Concept 2 - Emission spectrum - Blue light emitter

prompt fluorescence + TADF F (deg) = 68 %PL F (air) = 58 %PL Indication: TADF═>

sterical hinderance

300 400 500 600 700

T = 300 K, , PMMAcw465 nm

em.

exc.

t = 6 ns

CIE{0.164, 0.158}

N

N

N

DE 10 2014 106 987 A1DE 10 2014 106 986 A1WO 00 2015 121 239 A1WO 00 2015 121 241 A1

0 20 40 60 80 1001

10

100

1000

10000

15 ns

6 ns

Inte

nsity

Time [ns]

Concept 2 - Emission decay behavior, T = 300K, PMMA

0 2 4 6 8 10 12

1

10

100

1000

3 ms

7 ms

Inte

nsity

Time [ms]

900 ms

Short time range Long time range

0 - 10 ns

20 - 40 ns5 - 500 ms

Non-monoexponential decay═> Strong inhomogeneity═> Variation of DE(S - T ) very distinct1 1

Concept 2 - Time-resolved spectra, , PMMAT = 300K

400 500 600 700 [nm]

t = 0 nsDt = 10 ns

t = 20 nsDt = 40 ns

t = 5 msDt = 500 ms

No phos.

prompt fluorescence TADF

Prompt fluorescence shiftswith timeReason: Different DE(S - T ) 1 1values in the inhomogeneous matrix

Small DE(S - T )1 1

═> long t(S )1═> good spectral overlap of prompt fluor. and TADF

Concept 2 - Time-resolved spectra, , PMMAT = 77K

400 500 600 700[nm]

t = 20 nsDt = 40 ns

t = 500 msDt = 800 ms

prompt fluor. phosphorescence

-1 DE (S - T ) = 3500 cm1 1 430 meV

-13500 cm

t = 1.2 st = 6 ns

470 nm 580 nm

Concept 2 - Energy level diagram

S0

large DE

S1

Ranges of decay times at T = 300Kt(S ) : 6 ns.....15 ns1

t(TADF): 7 ms.....3 ms

═> More rigid structure required

T1

small DE

Inhomogeneous distribution of molecules

═> ═>

no TADFTADFlong t(S )1

short t(TADF)

Concept 3 - Schematic structure with rigid, non-conjugated bridge(s)

═> Electronically not strongly coupled

Small expected Reduced TADF decay tails expected

DE(S -T )1 1

DE 10 WO 00 2017 017 205 A1

2015 112 501 A1

DE 10 2017 101 432.2

ACCEPTORDONOR

BRIDGE

BRIDGE

═>

HOMO

LUMO

donor

bridge

acceptor

CalculationsTD-DFT; TD-DFT;

B3LYP MO6

1 3 -1DE( CT- CT) ≈ 30 cm (3.5 meV)T geometry optimization1

Small overlap of HOMO and LUMO

Some hyper-conjugation through the bridge:

Concept 3 - TADF molecule with very small DE(S -T )1 1

N N

N

C

H H

Concept 3 - Emission properties in polystyrene (PS) at 300K

300 400 500 600 700 [nm]

448 nm {0.174; 0.154}

F (deg.) = 30 %PL

F (air) = 5 %PL

0 15 30 45

1

10

100

1000

Inte

nsity

Time [ms]

TADF10 ms

Prompt fluorescence » 100 ns

t = 0 nsDt = 100 ns

t = 1 msDt = 100 ms

Decay: two components no distinct tail less inhomo- geneity═>

l(det)= 448 nm

Concept 3 - Time resolved spectra at , PST = 300K

300 400 500 600 700 [nm]

t = 0 nsDt = 100 ns

t = 1 msDt = 100 ms

prompt fluor.

TADF fit of prompt fluorescence and TADF

emission type: CT

448 nm

Concept 3 - Time-resolved emission in PS at T = 2 K

300 400 500 600 700[nm]

t = 1 msDt = 300 ms

3t ( LE) 400 ms

425 nm 450 nm

1t( CT) order of 100 ns

t = 0 nsDt = 100 ns

0-0 0-0S1 S0 T1 S0

order≈

2 -110 cm

low temperature phos.spectral structure

3localized state LE

DE (S -T ) = cannot be1 1determined from emissionspectra

Localized state slightly 1,3 below CT state, in PS.

═>

═>

Concept 3 - Emission properties in toluene at (high polarity)

300K

cw

476 nm

F (deg.) = 30 %PL

F (air) = 5 %PL

TADF 9 ms

Prompt fluorescence 270 ns

t = 5 msDt = 30 ms

t = 0 nsDt = 80 ns

TRES

10

1

20 30 40ms

10100

1000000

0nm300 400 500 600

Higher polarity than PS-1

- Red shift 1300 cm (160 meV)Decay- No fast equilibration

1- t( CT) = 270 ns t(TADF) = 9 ms

1000

10000010000

Inte

nsitä

t

700

300 400 500 600 700[nm]

Concept 3 - Time resolved spectra at , tolueneT = 300K

t = 0 nsDt = 80 ns

t = 5 msDt = 30 ms

prompt fluorescence TADF

fit of prompt fluorescence and TADF

emission type at T = 300 K1 1CT(prompt) + CT(TADF)Polarity: toluene

LE state nearby?

˃ PS3

Question:

476 nm

prompt fluor 300Kseveral

210 ns

1CT

S0

3CT

TADF300K9 ms

quench.300K

400 ms (PS) 2 K.

no phos.at 300K

3LE

DE( (3.5 meV)

1 3 -1 CT - CT) = 30 cm

Concept 3 - Tentative energy level diagram, PS/toluene

Long fluor. decay timeEstimate, Turro

r 2k (S -S ) n f(S -S )1 0 1 0

≈

-1 n = 25000 cm (meas.)F = 30% (meas.)PL

t(prompt) ≈ 500 ns

31,3CT LE slow vanishing FC factors

1 3Remark: CT, CT red shift withincreasing polarity from PS to

-1toluene by 1300 cm (160 meV)

f(S -S ) 0.001 (B3LYP) (calc.)1 0 ≈

═> Magnitude fits to exp. value

slow

.

Concept 3 - Summary

Emission quantum yields PL = 30%: too small

(TADF) = 9 – 10 s: too long

Chemical stability can be improved => New concept required.

Concept 4 - Schematic structure with functionalized bridge(s)

═> Reducing hyper-conjugation reduction of

3 Introduction of an additional LC state Chemical stabilization

DE(S -T )1 1

═> New OLED concept for fourth generation OLEDs

DONOR

BRIDGE

BRIDGE

ACCEPTOR

DE 10 2017 101 432.2EP 17 17 06 82.3

AROMATIC RINGS

HOMO

LUMO

Concept 4 - Functionalized bridge

N

N

N

donor

functionalized bridge

acceptor

Hyper-conjugation stronglyreducedExtremely small HOMO-LUMOoverlap

1 3 -1 DE( CT- CT) ≈ 7 cm (0.8 meV)(MO6 and B3LYP)3LE (functionalized bridge)

3 -1(gas phase calculation: ≈10 cm1,3 below CT)

Concept 4 - Emission properties in toluene at 300K

[nm]

468 nm

400 500 600F (deg.) = 65 %PL

F (air) = 2 %PL

10000

1000

100

10

10 1 2 3 4 5

t = 420 ns

Time [ms]

Cou

nts

No shorter component

t(fluorescence) = 420 ns

t(calc. gas phase) = 940 ns

No TADF component

TRES equal for all timeranges

Concept 4 - Tentative energy level scheme

fluor420 nsF = 65%PL

1CT

S0

lower

higher polarity3

CT

Importantparameter1,3CT shift3 LE ≈ no shift

1 3DE( CT- CT) ˂˂ k TBprompt fluorescence longRate: ISC ˃˃ prompt fluor. only one emission component

Can we learn more?

═>

1,3Concept 4 - Energy shifts of CT states due to polarity change with temperature, toluene

50 100 150 200 250 300 K1.0

1.5

2.0

2.5

3.0

edi

elec

tric

cons

tant

(similar to hexane)

frozen liquid

21500

2100020200

21200

-1cm

24300 -1

cmFreezing induces a strong

1,3blue shift of the CT states

Ref.: JACS 1998, 120, 3988

1 CT

emis

sion

ene

rgy

350 500400 450 550 600nm

3LE

1,3CT

1,3CT

77K 300 K

t 420 ns

t100 ns

Concept 4 - Emission spectra: temperature, polarity, toluene

Freezing1,3Strong blue shift of CT states

3LE structured phosphorescence

Decrease F (300K) = 65 %PL

F (77K) = 30 %PL

1 3 -1 DE( CT- LE) 3500 cm≈

t3s

Concept 4 - Energy level schemes at different temperatures, toluene

fluorescence 420 ns

3LE

S0

1,3CT

100 ns

S0

3 s

3LE

1CT

3CT

not observedat T = 300K“dark“ state3 1,3LE CT:

2order 10 nsobservedat 77K

ISC 2 10 ns

order:-1

≈3500 cm (430 meV)

F = 30%PL F = 65%PL

77 K 300 K

Concept 4 - Quantum yield and polarity, degassed, 300K

polarity scale

1.9

Hexane F = 6 % PL

396 nm

2.4

Toluene F = 65 % PL

t(prompt) = 420 ns

468 nm

4.3

TADF matrix (concept 3) F = 80 % PL

525 nm ?

PS F = 14 % PL

430 nm

PMMA F = 65 % PL

485 nm

Diethyl ether F = 70 % PL

t(prompt) = 960 ns

515 nm

1,3 3DE( CT - LE) ˂ 0

1 Intense CT emission, no TADF

═>═> Direct singlet harvesting

-11,3 3DE( CT - LE) ≈ 3000 cm

370 meV1

weak CT emission ═>3

═> LE quenches

e

Concept 4 - Energy levels at higher polarity, state mixings, and dynamics

S0

equilibrated fluor. 0.4 - 1 ms

SOC CIfastISC

1CT

3LE3CT

1 3CT CT fast

almost iso-energetic 1 3CT and CT states

large FC factors

state mixings also withhigher lying localizedstates3LE dark state

ISC

Conclusion / Highlight

Spins and electron-hole recombination

threetriplet paths 75 % 25 %

singletpath

equilibrated fluor. 0.4 - 1 ms

1CT

S0

3CT

fast

Systematic photophysical studies new OLED harvesting mechanism ═>

═>

All excitons are harvested 1in the CT state

No TADF

Emission as equilibrated fluorescence

Direct Singlet Harvesting

Fourth generation mechanism for OLEDs

Thanks to my group

Dr. Larisa Mataranga-Popa

Dr. Rafal Czerwieniec

Dr. Thomas Hofbeck

Dr. Markus Leitl

Alexander Schinabeck, M. Sc.

Yan Dovbiy, M. Sc.

Shu-Wei Li, M. Sc.

Alfiya Suleymanova, M. Sc.

Marsel Shafikov, M. Sc.

396 nm468 nm

515 nm576 nm

hexanee = 1.9

toluenee = 2.4

diethyl ethere = 4.3

chloroforme = 4.8

nm300 400 500 600 700 800

1,3Concept 4 - Energy shifts of CT states for different solvent polarities at T = 300 K

Results Increase of polarity

1,3 distinct red shift of CT states3LE expected

═> No distinct shifts for

3 Assignment of LE emission - Phosphorescence spectraConcept 4 -

300 400 500 600 700nm

Toluene, 77Kl(exc) = 310 nm

TRESt = 1 msDt = 500 ms

N

N

N

shifted by 10 nm to the red

Functionalized bridge

3LE emission