Design of organic TADF molecules. The role of E(S 1 -T 1 ): From fluorescence to TADF and beyond - towards the fourth generation OLED mechanism.

Transcription

1 Design of organic TADF molecules. The role of E(S -T ): From fluorescence to TADF and beyond - towards the fourth generation OLED mechanism. H. Yersin, L. Mataranga-Popa, R. Czerwieniec University of Regensburg, Germany

2 Outline General focus: Introduction how to design organic TADF molecules Short introduction to Singlet Harvesting for 00% 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

3 TADF and Singlet Harvesting in OLEDs for 00% exciton use Spins and electron-hole recombination three triplet paths 75 % singlet path 25 % - kbt up-/downisc S t(t) k(s) - DE(S-T) < 0 cm (0.2 ev) k BT T S0 e DE(S-T) t(tadf) TADF: Parker 96 OLEDs: Yersin 2006

4 Introduction - TADF of organic molecules LUMO CT HOMO D Transition energy Example D A 0 0 E(CT ) e(eox (D) - ERed (A)) N N E O0 x = V > > Suitable energy range A N E R0 ed = V - E(CT).04 ev (24 00 cm )

5 Concept - Schematic structure with no sterical hindrance DONOR ACCEPTOR

6 Concept - Emission 465 nm T = 00 K, cw, PMMA N N N 00 em exc nm 700 prompt fluorescence t = 4 ns FPL (deg) = 76 % FPL (air) = 76 % > Indication : No TADF

7 Concept - Emission T = 77 K, time resolved, PMMA T = 77 K, cw, PMMA 466 nm 466 nm 580 nm t = ms Dt = 900 ms t = ns Dt = 00 ns exc FPL = 84 % em 4400 cm 0.55 ev prompt fluor. t = 5 ns 600 phos.? nm prompt fluor. t = 5 ns 600 phos. t=s nm 700

8 Concept - Energy level diagram S T 4-5 ns s (77 K) 466 nm 580 nm S0 - DE 4400 cm (550 mev) Assignment No TADF Reason: Planarization in the excited state due to double bond formation between the rings (quinoid form) > Fluorescent molecule Requirement Breaking/reducing the conjugation

9 Concept 2 - Schematic structure with sterical hindrance DONOR ACCEPTOR DE A DE A WO A WO A

10 Concept 2 - Emission spectrum - Blue light emitter 465 nm N N CIE{0.64, 0.58} t = 6 ns N sterical hinderance em. exc. 00 DE A DE A WO A WO A T = 00 K, cw, PMMA prompt fluorescence + TADF FPL (deg) = 68 % FPL (air) = 58 % > Indication: TADF 700

11 Concept 2 - Emission decay behavior, T = 00K, PMMA Long time range Short time range ns 5 ns 0-0 ns ns Intensity Intensity ms 900 ms ms ms Time [ms] Time [ns] Non-monoexponential decay > Strong inhomogeneity > Variation of DE(S - T) very distinct 0 2

12 Concept 2 - Time-resolved spectra, T = 00K, PMMA Prompt fluorescence shifts with time Reason: Different DE(S - T) values in the inhomogeneous matrix Small DE(S - T) > long t(s) > good spectral overlap of prompt fluor. and TADF t = 0 ns t = 20 ns Dt = 0 ns Dt = 40 ns prompt fluorescence 600 t = 5 ms Dt = 500 ms TADF [nm] 700 No phos.

13 Concept 2 - Time-resolved spectra, T = 77K, PMMA 470 nm 580 nm t =.2 s t = 6 ns - DE (S - T) = 500 cm 40 mev cm prompt fluor. t = 20 ns Dt = 40 ns 600 [nm] phosphorescence t = 500 ms Dt = 800 ms 700

14 Concept 2 - Energy level diagram Inhomogeneous distribution of molecules S small DE large DE t(tadf): 7 ms... ms T > > TADF long t(s) no TADF short t(tadf) S0 Ranges of decay times at T = 00K t(s) : 6 ns...5 ns > More rigid structure required

15 Concept - Schematic structure with rigid, non-conjugated bridge(s) BRIDGE ACCEPTOR DONOR BRIDGE DE A WO A DE Electronically not strongly coupled > Small DE(S-T) expected TADF decay tails > Reduced expected

16 Concept - TADF molecule with very small DE(S-T) Calculations TD-DFT; BLYP TD-DFT; MO6 bridge N N LUMO N donor acceptor - DE( CT- CT) 0 cm (.5 mev) T geometry optimization Small overlap of HOMO and LUMO HOMO Some hyper-conjugation through the bridge: H H C

17 Concept - Emission properties in polystyrene (PS) at 00K 448 nm {0.74; 0.54} Prompt fluorescence 000 Intensity FPL(deg.) = 0 % FPL(air) = 5 %» 00 ns TADF 0 ms > 0 00 Decay: two components no distinct tail less inhomogeneity 600 [nm] t = 0 ns Dt = 00 ns l(det)= 448 nm 5 0 Time [ms] t = ms Dt = 00 ms 45

18 Concept - Time resolved spectra at T = 00K, PS 448 nm prompt fluor. t= 0 ns Dt = 00 ns TADF t = ms Dt = 00 ms [nm] 700 fit of prompt fluorescence and TADF emission type: CT

19 Concept - Time-resolved emission in PS at T = 2 K 450 nm 425 nm t = ms Dt = 00 ms t = 0 ns Dt = 00 ns t(ct) t ( LE) order of 00 ns 400 ms 0-0 S0 S T S0 400 order 2-0 cm [nm] 700 low temperature phos. spectral structure >localized state LE DE (S-T) = cannot be determined from emission spectra >Localized state slightly, below CT state, in PS.

20 Concept - Emission properties in toluene at 00K (high polarity) cw 476 nm Intensität FPL(deg.) = 0 % FPL(air) = 5 % Prompt fluorescence 270 ns TADF 9 ms nm 700 Higher polarity than PS - - Red shift 00 cm (60 mev) Decay - No fast equilibration - t( CT) = 270 ns t(tadf) = 0 t = 0 ns Dt = 80 ns 0 TRES 20 0 ms 40 t = 5 ms Dt = 0 ms 9 ms

21 Concept - Time resolved spectra at T = 00K, toluene 476 nm fit of prompt fluorescence and TADF emission type at T = 00 K CT(prompt) + CT(TADF) Polarity: toluene PS Question: LE state nearby? prompt fluorescence t = 0 ns Dt = 80 ns 600 [nm] 700 TADF t = 5 ms Dt = 0 ms

22 Concept - Tentative energy level diagram, PS/toluene DE( CT CT LE slow prompt TADF quench. fluor 00K 00K 00K 9 ms several 400 ms (PS) 2 2 K. 0 ns S0 no phos. at 00K - CT - CT) = 0 cm (.5 mev) Long fluor. decay time Estimate, Turro r k (S-S0) n. f(s-s0) f(s-s0) 0.00 (BLYP) (calc.) n FPL 2 - = cm = 0% (meas.) (meas.) > t(prompt) 500 ns Magnitude fits to exp. value, CT LE slow vanishing FC factors Remark: CT, CT red shift with increasing polarity from PS to - toluene by 00 cm (60 mev)

23 Concept - Summary Emission quantum yields PL = 0%: too small (TADF) = 9 0 s: too long Chemical stability can be improved => New concept required.

24 Concept 4 - Schematic structure with functionalized bridge(s) BRIDGE AROMATIC RINGS Reducing hyper-conjugation reduction of DE(S-T) > ACCEPTOR DONOR BRIDGE DE EP Introduction of an additional LC state Chemical stabilization > New OLED concept for fourth generation OLEDs

25 Concept 4 - Functionalized bridge functionalized bridge N N LUMO Hyper-conjugation strongly reduced Extremely small HOMO-LUMO overlap - DE( CT- CT) 7 cm (0.8 mev) (MO6 and BLYP) N LE (functionalized bridge) (gas phase calculation: 0 cm donor below acceptor HOMO, CT) -

26 Concept 4 - Emission properties in toluene at 00K nm No shorter component 000 t(fluorescence) Counts t = 420 ns t(calc. gas phase) = 940 ns 00 No TADF component [nm] 600 FPL(deg.) = 65 % FPL(air) = 2 % = 420 ns TRES equal for all time ranges Time [ms] 5

27 Concept 4 - Tentative energy level scheme CT lower CT higher fluor 420 ns FPL= 65% polarity Important parameter, S0 CT shift LE no shift DE( CT- CT) kbt prompt fluorescence long Rate: ISC prompt fluor. one emission > only component Can we learn more?

28 , Concept 4 - Energy shifts of CT states due to polarity change with temperature, toluene - e cm CT emission energy dielectric constant cm blue shift of the Freezing induces a strong, CT states Ref.: JACS 998, 20, 988 (similar to hexane) frozen K liquid

29 Concept 4 - Emission spectra: temperature, polarity, toluene, 77K 00 K LE CT, CT Freezing t 420 ns Strong blue shift of Decrease FPL (00K) = 65 % FPL (77K) = 0 % s 00 ns CT states LE structured phosphorescence t t, 500 DE( CT- LE) 500 cm nm 600

30 Concept 4 - Energy level schemes at different temperatures, toluene 77 K CT CT ISC order: 02ns LE 00 K cm (40 mev) LE, observed at 77K 00 ns S0 s FPL = 0% not observed at T = 00K dark state CT LE fluorescence 420 ns S0 FPL = 65%, 2 CT: order 0 ns

31 Concept 4 - Quantum yield and polarity, degassed, 00K Hexane 96 nm FPL = 6 % Toluene 468 nm FPL = 65 % t(prompt) = 420 ns e DE( - CT - LE) 000 cm 70 mev > weak CT emission > LE quenches 4. DE(, polarity scale TADF matrix (concept ) 525 nm FPL = 80 %? PMMA 485 nm FPL = 65 % PS 40 nm FPL = 4 %, Diethyl ether 55 nm FPL = 70 % t(prompt) = 960 ns CT - LE) 0 > Intense CT emission, no TADF > Direct singlet harvesting

32 Concept 4 - Energy levels at higher polarity, state mixings, and dynamics LE SOC CT fast ISC CI CT CT fast ISC almost iso-energetic equilibrated fluor ms S0 CT CT and CT states large FC factors state mixings also with higher lying localized states LE dark state

33 Conclusion / Highlight Spins and electron-hole recombination > three paths 75 % triplet Systematic photophysical studies new OLED harvesting mechanism Direct Singlet Harvesting All excitons are harvested singlet path 25 % in the CT state CT fast CT equilibrated fluor ms S0 No TADF Emission as equilibrated fluorescence > Fourth generation mechanism for OLEDs

34 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.

35 , Concept 4 - Energy shifts of CT states for different solvent polarities at T = 00 K hexane e =.9 toluene e = nm diethyl ether e = 4. chloroform e = nm 55 nm 576 nm Results Increase of polarity, > distinct red shift of CT states No distinct shifts for LE expected nm 800

36 Concept 4 - Assignment of LE emission Phosphorescence spectra Functionalized bridge Toluene, 77K l(exc) = 0 nm shifted by 0 nm to the red TRES t = ms Dt = 500 ms LE emission N N N nm 700