Molecular synthesis and computational results
The structure of the targeted D-MR-A molecule, 5-(3,11-bis(trifluoromethyl)-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene-7-yl)-11-phenyl-5,11-dihydroindolo[3,2-b]carbazole (1BOICz), was provided in Fig. 1a, constructed by attaching a trifluoromethyl group substituted oxygen-bridged boron (BO) group on to a 5,11-dihydroindolo[3,2-b]carbazole (32bICz) segment. The BO derivatives have been widely reported to possess obvious MR properties16,21 and thus were adopted as MR-A units. While 32bICz was a well-known D group. For comparison, we also constructed a D-π-A type TADF emitter, 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-11-phenyl-5,11-dihydroindolo[3,2-b]carbazole (1TICz). We firstly performed the density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations based on B3LYP/6-31G* basis set, aiming to analyze the distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of those molecules. Derived from the sterically uncrowded structures of both donor and acceptor planes, only moderate dihedral angles (θ) in the range of 46‒52o were observed for all three compounds (Fig. 1a). As a result, in addition to the mainly separated frontier molecular orbitals (FMOs) on the donor and acceptor groups, the moderate twist motifs render obvious both HOMO and LUMO residence on their phenylene bridge but in rather different behaviors (see Fig. 1b). For 1TICz, a clear localized π-bonding orbital distribution was observed on the phenylene bridge with significant HOMO-LUMO overlap, thus creating a hybrid orbital distribution combining local and long-range D/A-CT characteristics. Therefore, only a moderate ΔEST of 0.193 eV together with a rather high f value of 0.2361 was obtained for 1TICz. In terms of 1BOICz, its FMO distribution on the phenylene bridge showed a clear MR behavior with HOMO on the attached carbon atom and the carbon atoms positioned meta to it while the LUMO on the other carbon atoms (see Fig. 1b). Such alternative electron-rich and electron-poor regions on single atoms would thereof form a short-range MR-CT transition for 1BOICz.21 As a consequence, compared with 1TICz, the HOMO-LUMO overlap of 1BOICz was limited and thus generated a clearly reduced ΔEST of only 0.1 eV. More importantly, benefiting from the short-range MR-CT, a decent f value of 0.1428 was still obtained for 1BOICz, which was much larger than most donor-acceptor type TADF emitters, particularly those with twisted structures.22 The hybrid orbital distribution combining short-range MR-CT and long-range D/A-CT characters of such D-MR-A molecules renders them more advantageous than the D-π-A emitters in integrating a small ΔEST and a large f value.
Furthermore, to compensate the sacrifice of f value for 1BOICz, 5,11-bis(3,11-bis(trifluoromethyl)-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-5,11-dihydroindolo[3,2-b]carbazole (2BOICz) was developed. The FMOs of 2BOICz inherit the hybrid orbital distribution characters of 1BOICz, except for the dual MR-type distributions on the two phenylene bridges. Noting that the theoretical results revealed a similar S1 and ΔEST for 2BOICz compared with that of 1BOICz, but a doubled f value of 0.3498 (see Fig. 1b). The plausible reason should be assigned to the dual equivalent MR acceptors of 2BOICz, which can create dual equivalent emitting channels to double the f value. Considering that the f value of 2BOICz is even higher than 1TICz, this means the decreased FMO overlap by MR-type distribution can be compensated by the equivalent acceptors without sacrificing a small ΔEST. In our previous works, TADF emitters with multiple but not equivalent donors and/or acceptors have been developed and thus no equivalent multiple emitting channels are being formed.23 The high f of 2BOICz, reminiscent of locally excited (LE) states, and its small ΔEST should lead to both fast radiative decay and RISC process, which is exactly the aim of this molecular design.
To further clarify the superiority of the D-MR-A motif, we simulated the corresponding ΔEST and f values with varied θ between donor and acceptor (see Fig. 1c and 1d). For the D-π-A type 1TICz, the ΔEST value strongly depends on the θ value and a small θ would greatly enlarge the ΔEST. This is in correspondence with the conventional wisdom that a large sterically structure, namely a large θ, is required for D-π-A type TADF emitters, which, however, simultaneously lead to a small f value. Therefore, it is difficult to balance a small ΔEST and a large f for a D-π-A emitter. Intriguingly, in terms of the D-MR-A type 1BOICz, the ΔEST only slightly varied and remained at a small value below 0.2 eV together with a decent f value in a large θ range of 0 ~ 90o (Fig. 1e). A more interesting result is observed for 2BOICz that a small ΔEST comparable to 1BOICz is accompanied by a large f value similar to 1TICz, successfully breaking the mutual exclusion between a small ΔEST and a large f value. It is also worth noting that a small θ is necessary to maintain this D-MR-A effect as too large θ will greatly suppress the extension of HOMO from the donor to the phenyl linkage. In fact, BO-type acceptors have been adopted in previous works, which, nevertheless, usually adopted steric N-donors to enlarge θ to narrow ΔEST.15,24 As a consequence, the FMOs were completely separated for a rather small f value. Therefore, to guarantee the hybrid MR and CT distribution, a sterically uncrowded donor is crucial for D-MR-A emitters, making this strategy clearly different from the conventional D-π-A motif. Those findings provide a screen that one can construct TADF emitters with equivalent multiple acceptors (or donors) to break the mutual exclusion of a large f and a small ΔEST. To the best of our knowledge, Adachi et al unveiled that delocalizing the frontier molecular orbital distributions would also balance a large f and a small ΔEST. However, the proof-of-the-concept emitters they developed only exhibited tens of microsecond-scale τD and moderate device performances.25,26 Our molecular design principle proposed here is clearly distinguished from their strategy and opens a new pathway towards ideal TADF emitters.
Photophysical And Electronic Properties
The photophysical properties of both D-MR-A emitters were first studied in toluene with a concentration of 10–5 M as depicted in Fig. S3 and Table S2. Similar ultraviolet-visible (UV) absorption spectra were observed for both materials, with strong, high-energy narrowband absorption peaking at 323 nm, 339 nm, 383 nm and 403 nm, while wide, relatively weak absorption bands at 425 nm. The former should arise from the intrinsic n-π* or π-π* transition of acceptor and donor units while the latter can be assigned to the intramolecular CT transition from donor to acceptor. The fluorescent spectra exhibited wide and structureless green emission with peaks at 527 nm and 518 nm for 1BOICz and 2BOICz, respectively. And the redshifted spectra were observed for both emitters in higher polar solvents, which indicated the pronounced positive solvatochromism, evidencing their CT character emissions. The onset of UV-absorption spectra also defined the optical energy gap (Eg) of 1BOICz (2.63 eV) and 2BOICz (2.64 eV). Meanwhile, we determined the HOMO energy levels of both compounds by ultraviolet photoemission spectroscopy (UPS) in pure neat films as illustrated in Fig. S5. HOMO levels were estimated to be − 5.98 eV for 1BOICz and − 6.02 eV for 2BOICz, the LUMO energy levels thereafter can be deduced from the HOMO and Eg to be − 3.38 and − 3.38 eV for 1BOICz and 2BOICz, respectively, as illustrated in Table S2.
The TADF characters of both emitters were fully characterized by being dispersed in mCPBC matrix with a concentration of 20 wt%, where mCPBC is 9-(3-(9H-carbazol-9-yl)phenyl)-9H-3,9′-bicarbazole.27 For comparison, the properties of 1TICz in the same conditions were also evaluated. Figure 2a‒c provided the fluorescence and phosphorescence spectra of all doped films and wide structureless emissions were observed, indicating the CT characters of their S1 and T1. Therefore, the spectra onset defines the energies of S1 and T1, being 2.78 eV and 2.61 eV for 1TICz, 2.65 eV and 2.60 eV for 1BOICz and 2.71 eV and 2.65 eV for 2BOICz, respectively. Compared with the much larger ΔEST value of 1TICz (0.17 eV), strikingly small values of 0.05 eV for 1BOICz and 0.06 eV for 2BOICz were obtained. Those results are inconsistent with the theoretical results aforementioned, suggesting that the smaller ΔEST values of 1BOICz and 2BOICz arise from their limited HOMO-LUMO overlap by the MR type orbital distribution.
The PL decay curves of the three doped films were also measured under an excitation wavelength of ~ 400 nm as illustrated in Fig. 2d‒f, all exhibiting clear TADF behaviors with both prompt and delayed components. Interestingly, unlike most TADF emitters, only rather weak delayed parts were observed and over 90% were from the prompt components. Under photoluminescence (PL) excitation, only S1 excitons can be formed. The large ratio of the prompt part evidenced that most excitons are directly radiative decay to the ground states rather than to the triplet states via ISC process. Combining with the prompt PL efficiency (ΦP) and lifetimes (τP) of those three compounds, large kr values of 8.27×107 s− 1, 4.60×107 s− 1 and 5.94×107 s− 1 can be recorded for 1TICz, 1BOICz and 2BOICz, respectively. The corresponding kISC values of 0.82×107 s− 1 for 1TICz, 0.40×107 s− 1 for 1BOICz and 0.31×107 s− 1 for 2BOICz were also obtained. For all compounds, the kr is even tenfold higher than kISC. This situation is rare for most donor-acceptor type TADF emitters, which should arise from their large f values. In terms of the delayed components, different from the microsecond-scale delayed lifetime of 1TICz (3.22 µs), sub-microsecond-scale delayed components were observed for 1BOICz (0.97 µs) and 2BOICz (0.88 µs), respectively. The kRISC values of those three compounds were obtained to be 0.28×106 s− 1, 1.12×106 s− 1 and 1.18×106 s− 1 for 1TICz, 1BOICz and 2BOICz, respectively. The kRISC of 1TICz is in agreement with previous reports and the relatively smaller value should be due to its larger ΔEST. Meanwhile, the emitters with MR acceptors showed nearly five times larger kRISC values, which naturally benefit from their small ΔESTs.
Table 1
Photophysical properties of 1TICz, 1BOICz and 2BOICz in doped films.
Compound
|
λPL
(nm)a
|
ES1
(eV)b
|
ET1
(eV)b
|
ΔEST
(eV)b
|
ΦP
(%)c
|
ΦD
(%)c
|
τP
(ns)d
|
τD
(ns)d
|
kr
(107 s− 1)e
|
kISC
(107 s− 1)e
|
kRISC
(106 s− 1)e
|
1TICz
|
495
|
2.78
|
2.61
|
0.17
|
0.91
|
0.09
|
11
|
3222
|
8.27
|
0.82
|
0.28
|
1BOICz
|
534
|
2.65
|
2.60
|
0.05
|
0.92
|
0.08
|
20
|
968
|
4.60
|
0.40
|
1.12
|
2BOICz
|
528
|
2.71
|
2.65
|
0.06
|
0.95
|
0.05
|
16
|
884
|
5.94
|
0.31
|
1.18
|
aMeasured in mCPBC matrix with a concentration of 20 wt%; bSinglet (ES1) and triplet (ET1) energies were estimated from onsets of the emission spectra at 298 and 77 K in 20 wt% doped films, respectively. Singlet-triplet energy gap (ΔEST), ΔEST=ES1−ET1; cFractional quantum yields for prompt (ΦP) and delayed fluorescence (ΦD); dEmission lifetime for prompt (τP) and delayed (τD) fluorescence; eRate constant of fluorescence radiative decay (kr), intersystem crossing (kISC) and reverse intersystem crossing (kRISC), kr = ΦPF/τPF, kISC=(1‒ΦPF)/τPF, kRISC = ΦDF/(kISC.τPF.τDF.ΦPF). |
Interestingly, though their kRISC values are not the cutting-edge ones, τDs of 1BOICz and 2BOICz are even shorter than the ones with kRISC>107 s−1.28,29 As aforementioned, in addition to kRISC, the competition between kr and kISC is also the decisive factor that controls the exciton lifetimes. The rate constants of the three compounds are illustrated in Fig. 2g. For the three compounds, the kr values are over tenfold higher than kISC values and thus most S1 will directly decay to the ground state, rather than repeating the S1↔T1 spin-flip transition cycles. Under this circumstance, the kRISC is the true rate-determining process to control exciton lifetimes and a rate constant in the range of 106 s−1 will lead to a sub-microsecond delayed lifetime, as is the case of 1BOICz and 2BOICz. And the even shorter τD of 2BOICz should arise from its larger kr, which further reduces the spin-flip transitions compared with 1BOICz. In comparison, the inefficient kRISC of 1TICz generated a much longer delayed lifetime. Those results validate that kr>>kISC~kRISC>106 s−1 is an effective dynamic model to achieve a sub-microsecond-scale delayed lifetime. Different from previous works that pursue extremely large kRISC values, our work here provides an alternative strategy to shorten the delayed lifetime of TADF emitters. Notably that similar to 1TICz, previous works have revealed other TADF emitters possessing kr larger than kISC when adopting a carbazole similar to sterically uncrowded donors.30 However, the kRISC values of those emitters are rather slow owing to the large ΔEST. Taking the most representative DACT-II as an example9, an extremely large kr of approaching 108 s−1 has been obtained, which is also tenfold higher than kISC, but a much slow kRISC in the order of <105 s−1. Based on the above findings, we envision that simply adopting an MR acceptor for such classic TADF emitters may unlock the full potential of their performances.
Device Characterization And Performance
The electroluminescence (EL) performances of both emitters were further evaluated using the following device architecture: Indium tin oxide (ITO)/ TAPC (4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine]) (40 nm)/ TCTA (tris(4-(9H-carbazol-9-yl)phenyl)amine) (10 nm)/ mCPBC: emitters (24 nm)/ CzPhPy (4,6-bis(3-(9H-carbazol-9-yl)phenyl)pyrimidine) (10 nm)/ DPPyA (9,10-bis(6-phenylpyridin-3-yl)anthracene) (30 nm)/ LiF (lithium fluoride) (0.5 nm)/ Al (150 nm). The concentrations of those emitters were optimized to be 20 wt% and the diagram of the device architectures is illustrated in Fig. 3a and Fig. S6. For comparison, the control device with 1TICz was constructed. The EL spectra recorded at 1,000 cd m− 2 were provided in Fig. 3b and showed wide emission with peaks at 534 nm for 1BOICz, 528 nm for 2BOICz and 504 nm for 1TICz, corresponding to Commission Internationale de l´Eclairage (CIE) coordinates of (0.381, 0.566), (0.383, 0.550) and (0.247, 0.499), respectively, which consisted with the PL spectra of the doped films, indicating the complete energy transfer. Fig. S7 depicts the current density-voltage-luminance (J-V-L) characters of those devices, exhibiting operation voltages of 4.2, 4.0 and 4.2 V at 1,000 cd m− 2 for 1BOICz, 2BOICz and 1TICz based devices, respectively. The relatively lower operation voltages of 2BOICz than that of 1BOICz should arise from its dual acceptors, which enhance the electron transporting abilities.
The EQE versus luminance plots of those devices were illustrated in Fig. 3c and a significantly improved EQEmax up to 35.6% was obtained for 1BOICz, much higher than that of 1TICz (26.9%). A greatly alleviated efficiency roll-off was also noted for 1BOICz-based device, with EQE values of 34.8% and 32.6% at high luminance of 100 cd m− 2 and 1,000 cd m− 2. On the contrary, the EQE values of 1TICz-based device sharply decreased to 25.6% and 17.2% at 100 cd m− 2 and 1,000 cd m− 2. The differences in efficiency roll-off behaviors were believed to be originated from the TADF emissive dynamics of the two emitters under electrical excitation, which will be depicted later by EL decay curves. Notably, an unprecedented high EQEmax of 41.2% was recorded for 2BOICz-based devices, which was maintained at 41.1% and 36.6% at 100 cd m− 2 and 1,000 cd m− 2, respectively. To the best of our knowledge, this is the highest value at this specific color. A maximum power efficiency (PEmax) of 124.9 lm W− 1 was also observed for 2BOICz as illustrated in Fig. 3d, obviously outperforming those of 1BOICz (97.4 lm W− 1) and 1TICz (73.9 lm W− 1), respectively.
Table 2
Summary of the EL performance of TADF-OLEDs.
Compound
|
λEL a
[nm]
|
Von b
[V]
|
Lmax c
[cd m− 2]
|
CEmax/100/1,000 d
[cd A− 1]
|
PEmax/100/1,000 e
[lm W− 1]
|
EQEmax/100/1,000 f
[%]
|
CIE
(x,y) g
|
1TICz
|
504
|
3.1
|
68,910
|
80.0/76.6/51.1
|
73.9/63.5/38.2
|
26.9/25.6/17.2
|
(0.247, 0.499)
|
1BOICz
|
534
|
3.1
|
77,310
|
112.3/112.0/103.6
|
97.4/94.9/77.5
|
35.6/34.8/32.6
|
(0.381, 0.566)
|
2BOICz
|
528
|
3.0
|
106,800
|
130.4/129.7/116.3
|
124.9/116.7/91.4
|
41.2/41.1/36.6
|
(0.383, 0.550)
|
aEL peak wavelength; bTurn-on voltage (Von); cMaximum luminescence (L ); dMaximum current efficiency (CE), value at 100 and 1,000 cd cm− 2; eMaximum power efficiency (PE), value at 100 and 1,000 cd cm− 2; fMaximum external quantum efficiency (EQE), value at 100 and 1,000 cd cm− 2; gCIE coordinates at 1,000 cd cm− 2. |
To better understand the origin of the impressive efficiencies of those devices, we investigated the angle-dependent p-polarization-resolved PL intensity measurement of the corresponding emitting layers. The varied PL intensity measured with different angles was given in Fig. 3e and was analyzed with the classical dipole optical simulations. A high ratio of horizontal emitting dipole orientation (Θ//) of 80% and 81% were obtained for 1BOICz and 1TICz, which was further increased to 88% for 2BOICz. Owing to the moderate dihedral angles between donor and acceptors, those molecules possess large quasi-planar structures, which were further enlarged by the dual equivalent acceptors. The large molecular planarity should render the molecular orientation parallel to the base plane during the evaporation process. Meanwhile, the calculated S0-S1 transition dipole moment as illustrated in Fig. 3f and Fig. S14 is nearly parallel to the molecular orientation for all three compounds, thus generating those high Θ//s. Previous work has validated the critical role of a high Θ// in enhancing the light outcoupling efficiency for OLEDs.31-33 The highest Θ// of 2BOICz among the three compounds also accounts for its highest efficiency. Though essentially favoring high Θ//, quasi-planar structures commonly face the formidable challenge of narrowing ΔEST as a moderate θ goes against the highly separated FMO distribution. Our molecular design using MR acceptors to modulate FMO overlap and separation not only balances exciton emissive dynamics for a short-delayed lifetime but also enhances high Θ//, unlocking the full potential of TADF emitters particularly under high brightness.
We further evaluated the operational stabilities of those devices at a constant current with an initial luminance of 5,000 cd m−2 as shown in Fig. 4a. Decent half-lifetimes (LT50s) of 89 h, 196h and 302 h were obtained for 1TICz, 1BOICz and 2BOICz based devices, respectively. Using a commonly adopted degradation acceleration factor (n) of 1.75, a LT50 at an initial luminance of 1,000 cd m−2 can be extrapolated with the equation of LT50 (1,000 cd m−2) = LT50 (5,000 cd m−2)×(5,000 cd m−2/1,000 cd m−2)n, being 1,486 h for 1TICz, 3,272 h for 1BOICz and 5,043 h for 2BOICz, respectively. Notably that the longest lifetime was observed for 2BOICz, over three times longer than that of 1TICz, evidencing that the molecular design strategy possesses the potential to improve device stability compared with the conventional ones.34
To understand the underlying physics of the alleviated efficiency roll-off and extended operational stability of the two novel emitters under electrical excitation, the EL decay curves of those devices under an operation voltage of 6 V were recorded. It should be further pointed out that different to the situation in PL excitations, 75% of the generated excitons under EL excitations would be triplet ones. Therefore, the ratio of the delayed component will greatly be enlarged and a balance in kr, kISC and kRISC to short τD would matter more for device performances. As provided in Fig. 4b, the τDs of all three compounds showed a trend of 1TICz > 1BOICz > 2BOICz, which is exactly inverse to their LT50s. As aforementioned, various bimolecular annihilation processes have been acknowledged as the dominant reasons for not only the efficiency roll-off under high luminance but also the operation stability.11,35 Briefly, those bimolecular annihilation processes strongly depend on both concentrations and residence time of excitons. The small efficiency roll-off and the longer LT50s of both 1BOICz and 2BOICz should arise from their shorter delayed lifetimes than 1TICz, which can greatly suppress exciton annihilations under EL excitation.