A fluorene-bridged double carbonyl/amine multiresonant thermally activated delayed fluorescence emitter for eﬃcient green OLEDs †‡

Herein, we report a fluorene-bridged double carbonyl/amine-based MR TADF emitter DDiKTa-F, formed by locking the conformation of the previously reported compound DDiKTa. Using this strategy, DDiKTa-F exhibited narrower, brighter, and red-shifted emission. The OLEDs with DDiKTa-F emitted at 493 nm and showed an EQE max of 15.3% with an eﬃciency roll-oﬀ of 35% at 100 cd m (cid:2) 2 . Thermally activated delayed fluorescence (TADF) materials have demonstrated great potential as next-generation emitters in organic light-emitting diodes (OLEDs) due to their ability to harness 100% of the excitons to produce light without the need for noble metals, present in phosphorescent OLEDs. TADF compounds convert non-emission triplet excitons into emissive singlets by an endothermic upconversion reverse intersystem crossing (RISC) process. 1–4 The efficiency of the RISC process is governed in part by the singlet– triplet energy gap, D E ST . 5 A strongly twisted structure that effectively reduces the conjugation between donor and acceptor moieties is one strategy to achieve a small D E ST as the exchange integral of the frontier molecular orbitals (FMOs) is small. 4 However, a twisted structure exhibits significant excited-state structural relaxation, resulting in a broad emission characterized by a full width at half maximum (FWHM) higher than 70 nm. 6 To compensate for the broad emission, filters or microcavities are required to improve color purity; however, this can, unfortunately, reduce the device efficiency. 7

Thermally activated delayed fluorescence (TADF) materials have demonstrated great potential as next-generation emitters in organic light-emitting diodes (OLEDs) due to their ability to harness 100% of the excitons to produce light without the need for noble metals, present in phosphorescent OLEDs.2][3][4] The efficiency of the RISC process is governed in part by the singlettriplet energy gap, DE ST . 5A strongly twisted structure that effectively reduces the conjugation between donor and acceptor moieties is one strategy to achieve a small DE ST as the exchange integral of the frontier molecular orbitals (FMOs) is small. 4However, a twisted structure exhibits significant excited-state structural relaxation, resulting in a broad emission characterized by a full width at half maximum (FWHM) higher than 70 nm. 6To compensate for the broad emission, filters or microcavities are required to improve color purity; however, this can, unfortunately, reduce the device efficiency. 7ultiresonant TADF (MR-TADF) emitters have emerged as a potential solution as these rigid structures exhibit narrowband emission.First reported by Hatakeyama et al., these compounds are p-and n-doped polycyclic aromatic hydrocarbons (PAHs). 8By employing this approach, the singlet and triplet excited states possess an alternating pattern of increasing and decreasing electron density compared to the ground state, thus enabling a small exchange integral and consequently a small DE ST . 9The rigid structure and the short-range charge transfer (SRCT) nature of the excited states endow the MR-TADF compounds with bright, narrowband emission.Since the first report of MR-TADF emitters used in OLEDs in 2016, there has been intense research focused on expanding the chemical space encompassed by this class of emitters. 10In the original works of Hatakeyama et al., the n-dopants were boron atoms.It is possible to replace these with carbonyl groups, and the groups of Zysman-Colman 11 Zhang, 12 and Jiang and Liao 13 were among the first to report examples of MR-TADF emitters containing this motif.Expanding the MR-TADF skeleton has been demonstrated to be an effective strategy for improving the performance of MR-TADF emitters, 14 which has been less explored in carbonyl/amine systems.
We have shown that the dimerization of the MR-TADF emitter, DiKTa, in DDiKTa, leads to a modest red-shift of the emission and the OLED showed an improved performance. 15In an attempt to further improve the device performance and reduce the structural motion inherent in DDiKTa, here, we envisioned annealing together two DiKTa units through a central 9,9-dimethyl-9H-fluorene bridge, DDiKTa-F.An analogue without the tert-butyl groups was also synthesized; however, purification proved too difficult owing to its poor solubility, likely due to its strong propensity to aggregate.Therefore, two tert-butyl groups were added to improve the solubility of this compound.DDiKTa-F was found to be brighter (photoluminescence quantum yield, F PL , of 78%) and emits with a narrower FWHM, of 49 nm compared to DDiKTa (F PL of 68% and FWHM of 59 nm) in 2 wt% doped films in 1, 3-bis(carbazolyl)benzene (mCP).The device with DDiKTa-F showed an EQE max of 15.3% emitting at a l EL of 493 nm (FWHM of 46 nm) with an improved efficiency roll-off at 100 cd m À2 of 35% compared to the devices with DDiKTa (56%) 15 and DiKTa (44%) (Fig. 1). 11heoretical calculations were out to investigate the effect of the incorporation of the fluorene bridge on the optoelectrical properties of the emitter compared to those of the reference, DiKTa.The geometry in the ground state was first optimized using density functional theory at the PBE0/6-31G(d,p) level.The frontier molecular orbitals (FMOs) are delocalized over the entire p-conjugated system, and the HOMO and LUMO show an alternating distribution pattern similar to that of DDiKTa, which is emblematic of MR-TADF compounds. 15The calculated HOMO and LUMO levels of DDiKTa-F are À5.94 and À2.32 eV, respectively.The HOMO-LUMO gap of 3.62 eV for DDiKTa-F is smaller than that of DDiKTa (DE HOMO-LUMO = 3.70 eV), reflecting an increased conjugation in the former.The locked structure of the molecule contributed to small geometric changes between the S 0 and S 1 states, as depicted in Fig. S15 (ESI †).Thus, it is expected that the emission spectrum will be narrow and that there will be a small Stokes shift.The emission spectra of both DiKTa and DDiKTa-F under vacuum were simulated using Frank-Condon analysis based on the S 1 -S 0 transition at the TDA-DFT-PBE0/6-31G(d,p) level (Fig. S16, ESI †).The simulated spectrum of DiKTa shows an emission band peaking at l PL = 428 nm and a small FWHM = 14 nm, which closely aligns with the emission in hexane at l PL = 436 nm (FWHM = 21 nm).By contrast, the simulated emission spectrum of DDiKTa-F is red-shifted at l PL = 474 nm and is slightly broader (FWHM = 18 nm).We previously demonstrated that DFT calculations do not accurately predict the excited-state properties of MR-TADF emitters. 16Here, we employed SCS-ADC(2)/cc-pVDZ calculations to accurately model the excited states of DDiKTa-F. 16ifference density plots provide information on the changes in the electron density distribution in the excited states compared to that of the ground state.The difference density plots between S 0 and each of the S 1 and T 1 states, calculated for the S 0 optimized geometry, reveal that these excited states have SRCT characteristics typical of MR-TADF emitters.The calculated energies of the S 1 and T 1 states are 3.34 and 3.08 eV, respectively, which are lower than those of DiKTa (S 1 /T 1 = 3.45/3.18eV) and DDiKTa (S 1 /T 1 = 3.39/3.12eV), indicating that the emission in this compound should be red-shifted compared to the two reference emitters.The calculated DE ST for DDiKTa-F is 0.26 eV, which is similar to those of DiKTa (0.26 eV) 17 and DDiKTa (0.27 eV) (Fig. 2). 15he calculated spin-orbit coupling matrix element (SOCME) value between S 1 and T 1 is 0.37 cm À1 based on the T 1 -optimized geometry, while the SOCME values between S 1 and the four closely lying higher triplet excited states range from 0.07 to 5.93 cm À1 .In particular, the large hS 1 |H ˆSOC |T 3 i value of 5.93 cm À1 is attributed to an n-p* transition localized on the carbonyl groups (Fig. S18, ESI †). 17 These closely lying intermediate triplet states can participate in the RISC mechanism between T 1 and S 1 mediated by spin-vibronic coupling. 18he electrochemical properties of DDiKTa-F and DiKTa were investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in deaerated DCM with 0.1 M tetra-n-butylammonium hexafluorophosphate as the supporting electrolyte (Fig. S19, ESI †).The CV results show that the oxidation is irreversible while the reduction is a quasi-reversible process.The oxidation and reduction potentials, E ox and E red , determined, respectively, from the first oxidation and reduction peaks of the DPV, are 1.34 and À1.48 V vs. SCE.The corresponding HOMO/LUMO levels and energy gap (DE) are À5.68/À2.86 and 2.82 eV, respectively.The HOMO/LUMO are both destabilized compared to those of DDiKTa (À5.97/À3.07eV) 15 and DiKTa (À6.10/À2.99eV), implying that the fluorene bridge acts as an electron donor.As a result, DE was smaller than those of DDiKTa (2.90 eV) and DiKTa (3.03 eV).
The absorption spectrum of the diluted toluene solution (10 À5 M), shown in Fig. 3, exhibits two major bands.The band between 300 and 400 nm is linked to a p-p* transition delocalized over the whole skeleton, and the band at 375 nm is associated with the absorption of the central fluorene unit, both assigned from analysis of the TDA-DFT calculations (Fig. S17, ESI †).The lower energy band at 453 nm and shoulder at 431 nm are characteristics of an SRCT excited state transition for MR-TADF emitters (Fig. S17, ESI †).The SRCT band of DDiKTa-F is red-shifted and more intense (e = 25 Â 10 3 M À1 cm À1 ) than those of DDiKTa (l abs = 440 nm and e = 10.4Â 10 3 M À1 cm À1 ) and DiKTa (l abs = 433 nm and e = 21 Â 10 3 M À1 cm À1 ) due in part to its larger p-conjugation. 15The photoluminescence (PL) spectrum of DDiKTa-F in toluene, shown in   3a, has a peak maximum, l PL , of 476 nm, a shoulder at 511 nm, and an FWHM of 32 nm.The shoulder peak arises from the vibrational energy levels of the molecule, a typical characteristic of MR-TADF emitters. 19,20This emission is redshifted compared to those of DDiKTa (l PL = 470 nm) and DiKTa (l PL = 453 nm). 15The emission of DDiKTa-F shows a modest positive solvatochromism (Fig. S20, ESI †), which is consistent with the emissive excited state of SRCT.The energies of the S 1 and T 1 states, determined from the onsets of the steady-state PL and phosphorescence spectra at 77 K in 2-MeTHF glass are 2.63 and 2.43 eV, respectively (Fig. 3b); thus, DE ST = 0.20 eV.This value is similar in magnitude to those of DiKTa (0.22 eV in frozen toluene) and DDiKTa (0.21 eV in frozen toluene).The photoluminescence quantum yield, F PL , in toluene is 34%, which decreases to 31% upon exposure to air (Fig. S21, ESI †).
No delayed emission was observed in toluene and the lifetime of the emission decay, t PL , was 4.5 ns (Fig. S21, ESI †), which is similar to that of DiKTa (t PL = 5.1 ns). 11ith a view to employ DDiKTa-F as an emitter in OLEDs and to cross-compare their device performance with those of DDiKTa and DiKTa, we next investigated the photophysical properties of this emitter as doped films in mCP.The 2 wt% doped film of DDiKTa-F in mCP emits at 494 nm with a FWHM of 49 nm (Fig. 4a), an emission that is red-shifted compared to those of DDiKTa (l PL = 491 nm) and DiKTa (l PL = 467 nm) in 2 wt% doped films in mCP. 17We identified that 2 wt% doping provided the highest F PL of 78%, while the F PL decreased to 43% and the PL spectrum showed a pronounced red-shift from 491 to 507 nm when the doping concentration increased from 1 wt% to 10 wt% (Fig. S22, ESI †), implying that aggregation becomes an issue at this higher doping concentration.The F PL of the 2 wt% doped film in mCP decreased to 65% in air.The F PL of DDiKTa-F is slightly higher than those of both DiKTa (F PL = 46%) and DDiKTa (F PL = 65%) in 2 wt% doped films in mCP.At the same doping concentration, the F PL in the phosphine oxide-based hosts DPEPO and PPT are similar at 74 and 61% but the l PL are red-shifted at 510 and 511 nm, respectively, due to their higher polarity (Fig. S23, ESI †).The S 1 /T 1 energies, determined from the onsets of the steady-state PL and delayed emission spectra at 77 K in the 2 wt% doped films in mCP, are 2.58/2.40eV, resulting in a DE ST of 0.18 eV (Fig. S26, ESI †), which is similar to that measured for 2-MeTHF glass.Temperaturedependent transient PL decay analysis reveals the expected increase in the delayed emission with increasing temperature, which confirms the TADF in the 2 wt% doped film in mCP (Fig. S24, ESI †).The emission decays with the associated average prompt (t p ) and delayed (t d ) lifetimes are 5.6 ns and 188 ms (Table 1), respectively.These values are intermediate to those of DDiKTa (t p = 5.9 ns and t d = 159 ms) and DiKTa (t p = 4.8 ns and t d = 242 ms); in air, the delayed emission of DDiKTa-F was not completely quenched (Fig. S25, ESI †).From these photophysical measurements, the RISC rate constant (k RISC ) of DDiKTa-F was determined to be 2.16 Â 10 4 S À1 (Table S2, ESI †), 21,22 which is intermediate to those of DDiKTa (k RISC = 1.77Â 10 4 S À1 ) and DiKTa (k RISC = 2.52 Â 10 4 S À1 ).
The electroluminescence peak of the OLED, l EL of 493 nm and FWHM of 46 nm match those of the PL spectrum of the 2 wt% films in mCP (l PL = 494 nm and FWHM = 49 nm).The EL is narrower compared to the previously reported device with   a F PL was measured using an integrating sphere under nitrogen (l exc = 340 nm).b Obtained from the onset of the SS PL spectrum at 77 K. c Obtained from the onset of the delayed emission spectrum (1-10 ms) at 77 K (l exc = 340 nm).
DDiKTa (9 wt% in DPEPO), which emitted at a l EL of 500 nm and had an FWHM of 59 nm. 15 This small red-shifted emission compared to the SS PL in 2 wt% mCP film can be attributed to a combination of the use of the higher polarity DPEPO host and higher doping concentrations.By contrast, the EL is red-shifted compared to the device with DiKTa (3.5 wt% in mCP), which emitted at a l EL of 465 nm and had a FWHM of 59 nm. 11The corresponding Commission Internationale de l'E ´clairage (CIE) coordinates are (0.16, 0.50) for the device with DDiKTa-F, which are close to those of the device with DDiKTa (0.18, 0.53), yet are red-shifted compared to the device with DiKTa (0.14, 0.18).The device with DDiKTa-F exhibited an EQE max of 15.3%, which is similar to those of DDiKTa (19.0%) and DiKTa (14.7%).Gratifyingly, the efficiency roll-off was less severe, with an EQE of 100 cd m À2 at 9.9% for the device with DDiKTa-F, which was higher than those of DDiKTa (EQE 100 = 8.1%) and DiKTa (EQE 100 = 8.3%).This modestly improved efficiency roll-off can be explained by a higher figure of  In conclusion, we demonstrated an easy-to-access synthetic route for constructing a p-extended dimeric MR-TADF emitter by fusing two DiKTa units onto a fluorene bridge.Through this strategy, the structural motion was reduced compared to that of the parent dimeric emitter DDiKTa.This led to an improved F PL of 79% and a red-shifted and narrower emission at 494 nm (FWHM = 49 nm) in 2 wt% doped films in mCP.Moreover, the DE ST decreased to 0.18 eV, which led to a modest improvement in k RISC from 1.77 Â 10 4 S À1 to 2.16 Â 10 4 S À1 .The device with DDiKTa-F exhibited an EQE max of 15.3% and emission at 493 nm.Owing to the faster k RISC , the device exhibited a smaller efficiency roll-off of 35% at 100 cd m À2 than the devices with DDiKTa (56%) and DiKTa (44%).This emitter design, annelating multiple MR-TADF cores about a central fluorene, provides a simple method to maintain narrowband emission in MR-TADF compounds while simultaneously enhancing the F PL and k RISC . S

Fig. 1
Fig. 1 Chemical structures, photophysical properties and device properties of DDiKTa and DDiKTa-F.Fig. 2 (a) Distribution of FMOs of DDiKTa-F, calculated at the PBE0/ 6-31G(d,p) level.(b) Difference density plots of S 1 /S 2 and T 1 /T 2 excited states, calculated at the SCS-ADC(2)/cc-pVDZ level for DDiKTa-F, where f is the oscillator strength.The dashed lines in each figure reference the calculated values of DiKTa at the same level of theory. 17

Fig.
Fig.3a, has a peak maximum, l PL , of 476 nm, a shoulder at 511 nm, and an FWHM of 32 nm.The shoulder peak arises from the vibrational energy levels of the molecule, a typical characteristic of MR-TADF emitters.19,20This emission is redshifted compared to those of DDiKTa (l PL = 470 nm) and DiKTa (l PL = 453 nm).15The emission of DDiKTa-F shows a modest positive solvatochromism (Fig.S20, ESI †), which is consistent with the emissive excited state of SRCT.The energies of the S 1 and T 1 states, determined from the onsets of the steady-state PL and phosphorescence spectra at 77 K in 2-MeTHF glass are 2.63 and 2.43 eV, respectively (Fig.3b); thus, DE ST = 0.20 eV.This value is similar in magnitude to those of DiKTa (0.22 eV in frozen toluene) and DDiKTa (0.21 eV in frozen toluene).The photoluminescence quantum yield, F PL , in toluene is 34%, which decreases to 31% upon exposure to air (Fig.S21, ESI †).No delayed emission was observed in toluene and the lifetime of the emission decay, t PL , was 4.5 ns (Fig.S21, ESI †), which is similar to that of DiKTa (t PL = 5.1 ns).11With a view to employ DDiKTa-F as an emitter in OLEDs and to cross-compare their device performance with those of DDiKTa and DiKTa, we next investigated the photophysical properties of this emitter as doped films in mCP.The 2 wt% doped film of DDiKTa-F in mCP emits at 494 nm with a FWHM of 49 nm (Fig.4a), an emission that is red-shifted compared to those of DDiKTa (l PL = 491 nm) and DiKTa (l PL = 467 nm) in 2 wt% doped films in mCP.17We identified that 2 wt% doping provided the highest F PL of 78%, while the F PL decreased to 43% and the PL spectrum showed a pronounced red-shift from 491 to 507 nm when the doping concentration increased from 1 wt% to 10 wt% (Fig.S22, ESI †), implying that aggregation becomes an issue at this higher doping concentration.The F PL of the 2 wt% doped film in mCP decreased to 65% in air.The F PL of DDiKTa-F is slightly higher than those of both DiKTa (F PL = 46%) and DDiKTa (F PL = 65%) in 2 wt% doped films in mCP.At the same doping concentration, the F PL in the phosphine oxide-based hosts DPEPO and PPT are similar at 74 and 61% but the l PL are red-shifted at 510 and 511 nm, respectively, due to their higher polarity (Fig.S23, ESI †).The S 1 /T 1 energies, determined from the onsets of the steady-state PL and delayed emission spectra at 77 K in the 2 wt% doped films in mCP, are 2.58/2.40eV, resulting in a DE ST of 0.18 eV (Fig.S26, ESI †), which

Table 1
Photophysical data of DDiKTa-F and DDiKTa in 2 wt% doped films in mCP . W. thanks the China Scholarship Council (201906250199) for support.D.S. acknowledges support from the Royal Academy of Engineering Enterprise Fellowship (EF2122-13106).E. Z.-C.thanks the Engineering and Physical Sciences Research Council (EP/W015137/1, EP/W007517) for support.X.-H.Z. acknowledges support from the National Natural Science Foundation of China (Grant No. 52130304, 51821002) and the Collaborative Innovation Center of Suzhou Nano Science & Technology.