design of organic tadf molecules. the role of e(s 1): … of organic tadf molecules. the role of...
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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