The Role of Molecular Structure And Conformation in TADF

Transcription

1 II The Role of Molecular Structure And Conformation in TADF Physics Fernando Dias Paloma dos Santo Lays Marc Etherington Heather Cole Przemyslaw Data David Graves Chemistry Martin Bryce Jonathan Ward Vandana Bhalla José Santos Mark Fox University of Newcastle Tom Penfold Jamie Gibson Stuart Thompson

2 devices Photophysics of a TADF emitter So we need resonance between CT and 3 LE states for good risc and we can tune the CT states independently by solvatochromism

3 Host Effect Paloma Lays Dos Santos et al, J. Phys. Chem. Lett. 2016, 7, CT and 3 LE emission in zeonex ΔE ST = 0.15 ev Still get strong but slow TADF

4 Host Effect In non polar host, zeonex ΔE ST = 0.15 ev In polar dielectric DPEPO ΔE ST ~ 0.05 ev Nearly an order of magnitude more DF Far higher ISC rate as predicted by our model Complete triplet harvesting even at 80K No phosphorescence measurable

5 Host Effect Zeonex ΔE ST = 0.15 ev DEPEPO ΔE ST ~ 0.05 ev PLQY Air Nitrogen (18 ±3) % (30 ±3) % PLQY Air Nitrogen (93 ±3) % (95 ±3) % If risc very fast oxygen does not the DF!

6 Host Effect Deep blue TADF OLED EL green tail due to exciplex formation with TPBi In a device with DPEPO host the singlet triplet splitting decreases to < 5 mev as seen from the onset of EL and phosphorescence (measured in zeonex)

7 Host Effect iving excellent deep blue TADF devices Low roll of at 1000 nits 3V turn on >2500 cd/m 2

8 Host Effect iving excellent deep blue TADF devices 22.4% EQE at 130 cd/m 2

9 devices Photophysics of a TADF emitter What does D A bonding conformation do? M Etherington et al Nature Communications 8, (2017)

10 The not so subtle effect of conformation on TADF meet the 2K isomers 2K (MA) Linear 2K (MB)

11 DFT gives the molecule a few options a b c Possible conformations of the 3,7-DPTZ- DBTO 2 isomer (a) ax-ax (b) eq-eq (c) eq-ax a b Possible conformations of the 2,8- DPTZ-DBTO 2 isomer (a) ax-ax (b) eqax (c) eq-eq. c

12 a X-tal structure revels two conformers of the donor group in the different isomers b c Equitorial-Equitoial eq-eq eq Equitorial-Axial ax-eq ax

13 D-A conformations come from the phenothiazine H-extra (eq) Conformers of phenothiazine H-intra (ax)

14 MB isomer has more conjugation between D and A see the oscillator shift to the lowest ππ*

15 Dual CT emission in linear 3,7 MB has two stable CT states with different solvatochromism so different degrees of charge transfer. The quasi axial CT state has much weaker CT character

16 Only a small difference in the Oxidation wave energy Cyclic voltammetry of 2,8-DPTZ-DBTO2 and 3,7-DPTZ-DBTO2 at 1 mm concentration in 0.1 M Bu4NBF4/DCM.. Measurement conditions: scan rate 50 mv/s, Ag/AgCl quasi-reference electrode, calibrated against a ferrocene/ferrocenium redox couple.

17 2,8-DPTZ-DBTO2 3,7-DPTZ-DBTO2 2,8-DPTZ-DBTO2 band edge shows a clear direct CT absorption that blue shifts with solvent polarity identifying it s n-π* character 3,7-DPTZ-DBTO2 has a much weaker band with π=π* character showing the delocalised nature of the nitrogen lone pair via the H-extra configuration so we have weaker CT character and higher energy CT ax in line with echem.

18 Tom s colour scheme to match our laser labs 2,8-DPTZ-DBTO2 3,7-DPTZ-DBTO2

19 MB absorption band has much less mixing of n-π* character and so little direct CT absorption More conjugation is a bad way to go

20 a b 2,8-DPTZ-DBTO 2 3,7-DPTZ-DBTO 2 in zeonex Comparison of the degassed and non-degassed emission in zeonex (a) 2,8-DPTZ-DBTO 2 higher onset emission than 1 CT which is attributed to 3 LE. (a) 3,7-DPTZ-DBTO 2 with the CT ax emission observed at onset ca. 3 ev and also the contribution of 3 LE giving a higher onset than the expected 2.50 ev for CT eq.

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22 MB has phosphorescence from one low energy 3 LE state only From photoinduced absorption we can see both axial and equitorial 3 LE states in MB

23 Energy Energy TADF 3 LE ax acts as a triplet energy trap and reduces the TADF efficiency 1 CT= 2.61 ev 3 LE A = 2.70 ev 1 CT ax = 3.10 ev 1 CT ax = 2.95 ev 3 LE eq D= 2.58 ev Also, as the CT ax - 3 LE ax gap is much larger there is no TADF from the axial conformer MCH 3 LE D = 2.58 ev 1 CT= 2.43 ev Toluene 1 CT eq = 2.51 ev MCH 3 LE ax D= 2.54 ev 1 CT eq = 2.43 ev Toluene Polarity Polarity 2,8-DPTZ-DBTO 2 3,7-DPTZ-DBTO 2

24 In 2,7 we observe dual Phosphorescence from D and A showing strong mixing between the respective D and A triplets, this is why the two species have the same measured lifetime, whereas in 3,7 the coupling is much weaker and only D triplet emits.

25 3 LE ax trap states reduce device performance in terms of both absolute efficiency and roll-off behavior

26 devices Photophysics of a TADF emitter with ax-ax conformation Paloma Lays dos Santos et al., J.Phys.Chem accepted

27 eq-eq ax-ax

28 ax-ax DMePT looses all the direct CT absorption band oscillator strength pushed way up in energy as less electron density on the bridging nitrogen atom. eq-eq DPT has much more density here through the H-intra configuration giving Strong n π mixing and strong CT absorption. In both cases in the D-A-D systems the oscillator strengths of the lower energy bands are greatly increased, not via conjugation (orthogonal D-A) but because of the n and π mixing.

29 eq-eq ax-ax Strong solvatochromism of strong CT eq band, weak CT ax emission at 400 nm? Strong CT ax emission, in non or weakly polar solvent, then CT eq stabalised in stronger polarity but always see dual CT emission Given excess energy CT ax disappears

30 MCH ax-ax gets converted and see strong eq emission after excess energy to cause flip of conformation (about 0.25 ev)

31 Here we see typical polarity tuning of the CT 3 LE gap so we observe strong DF in DCB Now when ax conformer dominates we see no DF as the CT 3 LE gap is so large, only in high polarity (DCB) when ex-ax forms do we see weak DF

32 ex-ex configuration gives well behaved DF From TCSPC, CT ax lifetime <3 ps ax-ex still strongly dominated by the ax conformer but it has a much shorter lifetime than ex conformer

33 ax-ax in zeonex In zeonex we observe much more ax emission as the barrier to flip to the ex configuration is hindered. Even excess excitation energy doesn t help.

34 DPT-TXO2 in zeonex Nicely see phosphorescence at 79 ms and large CT 3 LE D gap so poor DF and weak thermal activation in non polar zeonex. At 290 K the DF is stronger and the phosphorescence weaker showing we are harvesting some triplets

35 DMePT-TXO2 in zeonex Very weak and long lived DF at high temperature very mixed state emission i.e. weak ex at >20 ns

36 The phosphorescence of DMePT is complex, we observe the 3 LE eq at early ms times and then the grow-in of the blue shifted 3 LE ax at later times showing an unquenched 3 LE ax population which does not contribute to DF even though it is in resonance with CT ex emission showing that there is no CT 3 LE coupling between different conformers

37 At 355 nm we observe 3 LE ax and 3 LE eq emission At 266 nm we observe strong 3 LE A as well as 3 LE D showing these triplet states are decoupled as well

38 So axial conformer is higher energy state than equitorial 3 LE ax to high energy to couple with CT ax 3 LE ax doesn t couple with CT eq either Excess energy or high polarity cause ax to eq transition This is hindered in solid state so a real D rotation D and A triplets don t couple either