Theoretical calculations. The two TADF molecules were designed using TXO moiety as an acceptor coupled with one or two 1,3,6,8-tetramethylcarbazole (MCz) as electron-donating units, namely, MCz-TXO and DMCz-TXO (Fig. 1a), respectively. Here, TXO fragment is likely to realize stronger SOC due to the heavy atom effect introduced by sulfur, thus endowing the molecules with larger kRISC and higher TADF efficiency. Considering that kr is also important to control the exciton lifetimes,22 the D-A-D structure is adopted to expect higher kr.16,35 To verify our design strategy, density functional theory (DFT) and time-dependent DFT with Tamm-Dancoff approximation (TDA-DFT) calculations were carried out by Amsterdam Density Functional program package (ADF2019.302).36 We used PBE0 functional with triple zeta basis set for geometry optimization of S0. The highest occupied natural transition orbitals (HONTOs) and the lowest unoccupied natural transition orbitals (LUNTOs) were visualized to understand the nature of multiple excited states, together with the corresponding energy levels of the states (Fig. 1, Supplementary Fig. S4 and Supplementary Table S1). SOCMEVs between S1 and Tn (where n = 1–5) were also calculated as shown in Table 1.
MCz-TXO and DMCz-TXO have very small ΔEST of 0.03 eV and 0.01 eV, respectively, due to the well-separated HOMO and LUMO as shown in Fig. 1a. Both molecules possess CT-type S1 and T1. Figure 1b shows HONTO and LUNTO of MCz-TXO. T2 has LE character and thus large SOCMEV between T2 (3LE) and S1 (1CT) is expected. In addition, T2 is close to S1 in energy with the downhill energy gap from T2 to S1 (ΔES1−T2) of 0.20 eV. As shown in Table 1, a considerably large SOCMEV of 4.68 cm− 1 is found between the T2 (3LE) and S1 (1CT) in MCz-TXO. For DMCz-TXO (Fig. 1c), both T1 and T2 are CT-type but T3 is LE with a small downhill energy gap between T3 and S1, ΔES1−T3, of 0.18 eV. A simiarly large SOCMEV of 6.01 cm− 1 is found between T3 (3LE) and S1 (1CT) in DMCz-TXO.
To investigate the heavy atom effect introduced by the sulfur atom in TXO, we also conducted calculations on a reported molecule, MCz-XT (see Supplementary Fig. S3), for comparison. MCz-XT has the same chemical structure as MCz-TXO except that the oxygen atom is replaced with sulfur.34 Therefore, the heavy atom effect of sulfur can be investigated directly by the comparison. As shown in Supplementary Fig. S3, similar to MCz-TXO, MCz-XT exhibited well separated HOMO and LUMO with a similarly large torsion angle of 81°. For MCz-XT (see Supplementary Table S1), S1 and T1 are CT-type. T2 is hybridized local and charge-transfer (HLCT) type, but is a CT-dominant character. In contrast, T3 is LE-dominant HLCT type, showing the largest SOCMEV in MCz-XT of 1.04 cm− 1 for T3 (3LE)→S1 (1CT) transition (Table 1). For MCz-TXO, HONTO and LUNTO of S1 and T1 are very similar to those of MCz-XT, but HONTO of T2 is significantly different. T2 of MCz-TXO is LE-type and the distribution on sulfur is very large (see Supplementary Table S1), resulting in the very large SOCMEV of 4.68 cm− 1 for T2 (3LE)→S1 (1CT) transition in MCz-TXO, far larger than the largest SOCMEV in MCz-XT, 1.04 cm− 1. Besides, the energy difference between 3LE and 1CT of MCz-TXO, 0.20 eV, is much smaller than that of MCz-XT, 0.59 eV. Although both the transitions are downhill in energy, the energy mismatch for MCz-XT makes the internal conversion from T1 (3CT) to T3 (3LE) ineffective (the energy difference: 0.62 eV), resulting in the difficulty of the participation of T3 (3LE) in the RISC process. Therefore, only the direct 3CT→1CT transition with SOCMEV of 0.13 cm− 1 and energy difference of 0.03 eV is expected for MCz-XT (the experimental kRISC was ~ 2×106 s− 1). On the basis of the discussion, we can expect a far larger kRISC value for MCz-TXO, because the LE-dominant T2 can be incorporated into the RISC process due to the relatively small energy difference (0.20 eV) and the large SOCMEV of 4.68 cm− 1 between T2 (3LE) and S1 (1CT). Similarly, DMCz-TXO (SOCMEV of 6.01 cm− 1 and energy difference of − 0.18 eV for T3→S1 transition) is also expected to show large kRISC.
Table 1. The calculated SOCMEVs together with the energy difference between S1 and the triplet states (Tn) for MCz-XT, MCz-TXO, and DMCz-TXO. aThe nature of the state is shown in parentheses.
Photophysical properties. Photophysical measurements of MCz-TXO and DMCz-TXO in 10− 4 M oxygen-free toluene solution were conducted to evaluate their TADF efficiencies. The ultraviolet-visible (UV-vis) absorption spectra, photoluminescence (PL) spectra of MCz-TXO and DMCz-TXO were shown in Fig. 2. The absorption band in the range of 300 to 350 nm and the range of 350 to 380 nm in the UV-vis spectra correspond to the absorption of MCz and TXO, respectively.37,38 The absorption around 400–450 nm can be attributed to intramolecular CT (ICT) absorption from MCz to TXO and the intensity of ICT absorption of DMCz-TXO is much larger than that of MCz-TXO, although it overlapped partly with the absorption of the TXO at a shorter wavelength. The emission peak maximum wavelength (λMAX) was 480 nm for MCz-TXO and 491 nm for DMCz-TXO. The PL quantum yield (PLQY or ФPL) and transient PL measurements were also conducted for two compounds in 10− 4 M oxygen-free toluene solution at ambient temperature. The transient PL data, in Fig. 2b, exhibited double exponential decay curves with τPF/τDF = 0.8 ns/1.2 µs for MCz-TXO and 1.8 ns/0.8 µs for DMCz-TXO, respectively. Here, τPF and τDF are the lifetimes of prompt and delayed fluorescences, respectively. To understand the origin of the very fast delayed fluorescence, we performed a detailed analysis of related rate constants for the MCz-TXO and DMCz-TXO according to the reported equation, which is suitable for the TADF molecules showing large kRISC.18 The experimental and the analyzed data are summarized in Table 2. For comparison, the reported photophysical performance of 5 vol% MCz-XT:PPF film was also cited in Table 2. As we expected, both MCz-TXO and DMCz-TXO showed very large kRISC, exceeding 107 s− 1. When compared to the kRISC value of MCz-XT (~ 2×106 s− 1), MCz-TXO achieved thirty times larger kRISC of 6.4×107 s− 1, confirming the heavy atom effect introduced by sulfur. We assume that our strategy can be widely adopted in various TADF molecular designs for further RISC enhancement. DMCz-TXO also showed a large kRISC of 3.4×107 s− 1 and large kr of 1.2×107 s− 1, providing a sub-microsecond-scale τDF of 0.8 µs. To our best knowledge, DMCz-TXO is the first organic molecule showing both kr and kRISC exceeding 107 s− 1, simultaneously.
The photophysical performance of the doped films were also examined. Here, MCz-TXO and DMCz-TXO were doped in 9-(4-tertbutylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) and 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP), respectively. As shown in Supplementary Table S2, the MCz-TXO:CzSi film with an optimal concentration of 5 vol% provided ФPL of 64.7% and λMAX of 475 nm with Commission Internationale de l’Eclairage (CIE) coordinates of (0.16, 0.22). The transient PL curve of 5 vol% MCz-TXO:CzSi film showed multi-delayed fluorescence and the detailed analysis was not carried out. For DMCz-TXO-doped films (see Supplementary Table S3), ФPL of 100% was obtained for 5 vol% DMCz-TXO:mCBP film with λMAX of 492 nm and CIE of (0.21, 0.40). The transient PL decay curves (Fig. 2c) showed a very fast prompt fluorescence with nanosecond-scale τPF of 3.0 ns and a very fast delayed fluorescence with a sub-microsecond-scale τDF of 0.8 µs. The detailed analysis provides rate constants of kr = 1.5×107 s− 1, kRISC = 2.9×107 s− 1, and kISC = 3.0×108 s− 1 with completely suppressed non-radiative decay from S1 (PLQY of 100%) (see Supplementary Table S3). From an Arrhenius plot of kRISC for the 5 vol% DMCz-TXO:mCBP film (Fig. 2d), the activation energy was estimated to be 53 meV. Moreover, DMCz-TXO still possessed a high kRISC of 8.5×106 s− 1 even at low temperature of 200 K (see Supplementary Table S4), which is higher than that of most of TADF materials at 300 K, indicating that the
inclusion of 3LE, together with the heavy atom effect by sulfur, enhance RISC process effectively.
Table 2
The photophysical performance of MCz-TXO and DMCz-TXO in 10− 4 M oxygen-free toluene solution, and 5 vol% DMCz-TXO:mCBP film. The values for 5 vol% MCz-XT:PPF film are also listed for comparison. aThe related data were obtained in 10− 4 M oxygen-free toluene solution. bThe total (ФPL), prompt (ФPF), and delayed (ФDF) photoluminescence efficiency yield. cThe rate constant of reverse intersystem crossing (kRISC), intersystem crossing (kISC), radiative decay (kr), and nonradiative decay (knr). dPLQY was measured under excitation wavelength at 405 nm for DMCz-TXO and 365 nm for MCz-TXO, respectively. ePhotophysical properties of MCz-XT in a PPF host matrix at doping concentrations of 5 wt%, cited from ref. 34. fThe rate constants were calculated using the reported method in ref. 18. gThe data were for the 5 vol% DMCz-TXO:mCBP film. hPLQY was determined under nitrogen atmosphere at the excitation wavelength of 380 nm.
Emitter | Environment | λMAX (nm) | τPF (ns) | τDF (µs) | ФPLb (%) | ФPFb (%) | ФDFb (%) | kRISCc (107 s− 1) | kISCc (108 s− 1) | krc (107 s− 1) | knrc (106 s− 1) |
DMCz-TXOa | Toluene Solution | 491 | 1.8 | 0.8 | 60.2d | 2.3 | 57.9 | 3.4 | 5.1 | 1.2 | 8.1 |
MCz-TXOa | Toluene Solution | 480 | 0.8 | 1.2 | 40.3d | 0.6 | 39.7 | 6.4 | 11.7 | 0.7 | 9.9 |
MCz-XTe | Doped in PPF | / | 33.3 | 1.2 | 98.0 | 41.4 | 56.6 | ~ 0.2f | ~ 0.2f | 1.2f | ~ 0.2f |
DMCz-TXOg | Doped in mCBP | 492 | 3.0 | 0.8 | 100.0h | 4.7 | 95.3 | 2.9 | 3.0 | 1.5 | 0.0 |
OLED properties. To evaluate the potential of our designed molecules as TADF emitters, the MCz-TXO based device is prepared using the following structure: indium-tin-oxide (ITO) (50 nm)/4,4ʹ-cyclohexylidenebis[N,Nbis(4-methylphenyl) benzenamine] (TAPC) (60 nm)/1,3-bis(9,9-dimethylacridin-10(9H)-yl)benzene (mAP) (10 nm)/5 vol% MCz-TXO:CzSi (20 nm)/2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF) (10 nm)/1,3-bis[3,5-di(pyridin-3-yl)phenyl]benzene (BmPyPhB)39 (30 nm)/lithium quinolin-8-olate (Liq) (1 nm)/Al (80 nm) (Supplementary Fig. S7). The structure of DMCz-TXO based device (as shown in Fig. 3a) is ITO
Table 3
The summary of OLED performances using MCz-TXO, DMCz-TXO, and the reported TXO-based TADF materials as emitters. aat 100 cd m− 2. bEQEMAX means maximum EQE value. EQEY means EQE value at Y cd m− 2. cN.A. means that data was not provided in the related reference. d/ means no data available.
Emitter | CIE(x, y)a | EQEMAXb (%) | EQE100b (%) | EQE1,000b (%) | EQE10,000b (%) | EQE20,000b (%) | LMAX b (cd m− 2) | Ref. |
5 vol% DMCz-TXO:mCBP | (0.20, 0.45) | 20.5 | 20.5 | 19.1 | 14.0 | 9.8 | 23,180 | This work |
5 vol% MCz-TXO:CzSi | (0.15, 0.21) | 17.4 | 14.3 | 7.8 | /d | / | 2,097 | This work |
10 wt% CzTXO:DPEPO | (0.16, 0.20) | 11.2 | N.A.c | / | / | / | 183 | 40 |
10 wt% DCzTXO:DPEPO | (0.18, 0.37) | 11.5 | N.A. | / | /c | / | 627 | 40 |
10 wt% 1,6-2TPA-TX:CBP | (0.41, 0.55) | 2.2 | 0.8 | 0.4 | / | / | < 5,000 | 41 |
3 wt% 3,6-2TPA-TX:CBP | (0.14, 0.28) | 23.7 | 4.4 | 3.2 | / | / | < 10,000 | 41 |
(50 nm)/TAPC (60 nm)/mAP (10 nm)/5 vol% DMCz-TXO:mCBP (30 nm)/PPF (10 nm)/BmPyPhB (35 nm)/Liq (1 nm)/Al (80 nm). TAPC and BmPyPhB were used as hole and electron transport layers, respectively. The mAP and PPF possessing high T1 energy (3.0 eV and 3.1 eV, respectively) were inserted in both sides of the emitting layers to confine excitons. The electroluminescent performances were listed in Fig. 3 and Table 3.
The device using 5 vol% MCz-TXO:CzSi as an emitter layer exhibited sky-blue emission with the peak maximum wavelength at 469 nm and CIE of (0.15, 0.21) at 100 cd m− 2. The device achieved EQEMAX of 17.4% owing to a relatively high ФPL of 64.7%. DMCz-TXO based device with an optimal doping concentration of 5 vol% achieves EQEMAX of 20.5% and high luminance (> 20,000 cd m− 2) without any outcoupling treatment. Besides, the device showed suppressed efficiency roll-off; the EQEs were 20.5% and 19.1% (the efficiency roll-offs were only 0.2% and 7.0%) at the luminance of 100 and 1,000 cd m− 2, respectively. The EQEs at 10,000 and 20,000 cd m− 2 were still high, namely 14.0% and 9.8% respectively. The high kRISC (> 107 s− 1) and kr (> 107 s− 1) of DMCz-TXO can prevent the accumulation of triplet excitons, thus suppressing singlet-triplet annihilation and triplet-triplet annihilation process, giving rise to reduced efficiency roll-off.
Previously, there are two reports about the TXO-based TADF molecules and the related OLED performances were also summarized in Table 3.40 Among these OLEDs using TXO as an acceptor of TADF emitters, MCz-TXO-based OLED showed blue emission with slightly reduced efficiency roll-off. Besides, the DMCz-TXO-based OLED also exhibited greatly improved roll-off suppression, clearly exemplifying the great advantage of simultaneous realization of very fast RISC and radiative decay with the rate constants both over 107 s− 1.
In conclusion, a newly-designed molecule, MCz-TXO, showed (1) good energy level matching of the three states, 1CT, 3CT, 3LE; (2) heavy atom effect introduced by sulfur, thus possessing considerably large kRISC of 6.4×107 s− 1, one of the largest kRISC among all reported pure organic TADF emitters. The value is thirty times larger than that of the molecule which has the same chemical structure as MCz-TXO except that the oxygen atom is replaced with sulfur, confirming the effectiveness of our design concept. We believe that this concept, namely, good matching of 1CT, 3CT, and 3LE, together with the incorporation of heavier organic atoms (such as S, Se), can be applied to various TADF materials, for further improvement of kRISC and thus the enhancement of TADF efficiency. Moreover, a higher radiative decay rate constant is also of vital importance to shorten the delayed lifetime and realize a small efficiency roll-off for OLEDs. As demonstrated in this study, DMCz-TXO, possessed kRISC of 3.4×107 s− 1 and kr of 1.2×107 s− 1, offering a sub-microsecond-scale delayed lifetime. The DMCz-TXO-based OLED exhibited great device performance with suppressed efficiency roll-off when applied to OLEDs. In the future, these kinds of TADF molecules showing very fast triplet-to-singlet conversion can also be promisingly used as are TADF-assistant dopants in hyperfluorescent OLEDs for realizing various emission color or achieving better performance.12,18,42,43