Introduction

Organic light-emitting diode (OLED) was widely studied since the first small molecules device was reported by Tang et al.1. The luminescent materials, as the most important component of OLED, were always the focused study by materials scientists2,3,4,5,6. One most important requirement of the luminescent materials was a high photoluminescence quantum yield (PLQY). The pure organic materials utilizing hybridized local and charge-transfer (HLCT) state7,8,9 become potential high-efficiency OLED materials considering the low cost, less toxicity, and high exciton utilization.

Numerous highly luminous HLCT-state compounds containing triphenylamine (TPA, Fig. 1) group have been reported10,11,12,13,14,15. For example, in 2014, a series of twisting donor–acceptor (D–A) molecules with high exciton utilization efficiencies and full-color-range emissions were reported16. To enhance the proportion of radiative exciton, methods such as constructing orthogonal cyano substituted D–A structure imidazole derivative17, designing butterfly-shaped D-A chromophore18, modulating a fine proportion of charge-transfer (CT) state and locally excited (LE) state19 and choosing an appropriate substituted position, were suggested20. Especially, a pure-blue emitting molecule consisting of phenanthroimidazole and phenylcarbazole-substituted anthracene (PAC, Fig. 1) was reported, and the external quantum efficiency (EQE) exceeds 10%21. Besides being the emitting materials, the 4-(2-(4-(diphenylamino)phenyl)-1H-phenanthro[9,10-d]imidazol-1-yl)benzonitrile (TPMCN, Fig. 1) was also used as host material with EQE up to 6.3% and exciton utilizing efficiency of 64%22. More recently, triphenylamine-acridine (TPA-9AC, Fig. 1) was reported to be a visualization and ultrasensitive fluorescence sensor for diethyl chlorophosphate vapor fast and accurate detection23.

Figure 1
figure 1

Molecular structures extracted from references.

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Taking the carbazole-dicyanobenzene molecule 4CzIPN (Fig. 1) and triphenylamine-thiadiazole molecule TPA-NZP (Fig. 1) as theoretical models, Pan et al. pointed out different reverse intersystem crossing (RISC) paths (T1 → S1 for 4CzIPN and T2 → S2 for TPA-NZP) by comparing the singlet exciton utilization formats using density functional theory calculation24. The systematic calculations on accurate description of HLCT state also demonstrate that the ωB97XD or optimally tuned range-separated functional can provide a better description of HLCT state25,26.

It is observed that molecules containing TPA group show a sharp emission wavelength dependence on solvent polarity27,28,29,30. For example, the work done by Xia et al. proposed the explanation that the emissions of cyano-substituted oligo α-phenylenevinylene-1,4-bis(R-cyano-4-diphenylaminostyryl)-2,5-diphenylbenzene (CNDPASDB, Fig. 1) were from the LE state in a low-polarity solvent but were from the CT state in the high-polarity solvent31. An overall theoretical investigation of TPA-NZP also concluded the excited state changed from HLCT character to complete intramolecular CT with increasing solvent polarities, followed by increased non-radiative decay rate32.

Huang et al. also suggested an aggregation-induced emission mechanism that a conversion is from dark twisted intramolecular charge transfer (TICT) excited state to emissive quasi-TICT state because that the restriction of molecular rotation was lower in the amorphous film than in power and crystalline33. The fluorescent molecules atomistic molecular dynamics simulation of deposition process showed that the torsion angles between donor and acceptor groups have a broadened distribution around 90 degrees due to the thermal fluctuation and intermolecular interaction34. Based on reported compounds (Fig. 1), namely 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN) and (4CzIPN), the theoretical simulation of bulk amorphous phase also indicated that the torsion angles between donor and acceptor are in a broad distribution, and the RISC is a dynamical process with the varying molecular structures35,36.

Ma et al. have proposed the hot-exciton RISC path of the higher triplet state to the single state (T2 → S1/S2) to explain the large proportion of radiative singlet exciton for HLCT-state molecules. However, we wonder whether the excited-state (S1, T1, and T2) energy levels would lead to the variations between S1 and T2 gaps and affect the RISC process with molecular rotation. Thus, in this work, taking reported HLCT-state molecules (1, 2, and 3) as examples (Fig. 2a), we have calculated the excited-state potential energy surface (PES) by twisting TPA (Fig. 1) or diphenylamine (PA, Fig. 1) using the time-dependent density functional theory (TD-DFT) method. The calculated results indicate that twisting TPA or PA almost has no influence on T1 and T2 energy levels, but would affect the S1 energy level and the T2 → S1 RISC process. Therefore, understanding the relationship between the S1 PES and molecular structures is important for designing high-performance luminescent materials utilizing HLCT state.

Figure 2
figure 2

(a) Molecular structures of 1, 2, and 3. (b) Natural transition orbitals of S1, T1, and T2 excited-state with M06-2X/Def2SVP based on ground-state structures. (c) Relative S1, T1, and T2 energy levels with M06-2X/Def2SVP and a structural comparison among S1, T1, and T2 structures. S1 (orange); T1 (green); T2 (cyan). (b) Drawn by GaussView 6.0 and Multiwfn programs; (c) is drawn by VMD 1.9.1. program.

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Computational details

In this work, the ground-state (S0) geometries of 1, 2, and 3 (Fig. 2a) were optimized at density functional theory (DFT) calculations M06-2X/Def2SVP level. The lowest singlet excited state (S1) was obtained with time-dependent density functional theory (TD-DFT) M06-2X/Def2SVP level. Two low-lying triplet excited-state geometries (T1 and T2) were optimized using the same M06-2X/Def2SVP considering the Tamm-Dancoff approximation (TDA)37,38 for avoiding the issue of triplet instability. The corrected linear response (cLR) approach39 was employed to obtain the total energies of S1, T1, and T2 excited-state structures. In all calculations, the dimethylsulfoxide (DMSO) solvent under the SMD model40 was used to consider the solvent effect because that the high polar DMSO was prone to stabilize the charge-transfer state. To analyze the excited-state (S1, T1, and T2) properties, the vertical singlet and triplet excited states with M06-2X/Def2SVP were calculated based on their S0 structures. Besides, the S1, T1 and T2 excited-state potential energy surface (PES) was performed by twisting TPA or PA group with a scan step of 10 degrees. All calculations were accomplished by Gaussian 16 program41. The natural transition orbitals, electron and hole analysis and structural comparison among S1, T1, and T2 excited states were obtained by Multiwfn program42.

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Result and discussion

Excited-state properties of 1, 2 and 3

Most reported works explain the high exciton utilization of HLCT-state molecules by analyzing the vertical singlet and triplet excited-state energy levels. In this work, we have optimized the ground-state, singlet, and triplet excited-state structures (S0, S1, T1, and T2) and calculated their relatively adiabatic energies. For better describing the excited-state properties, we also illustrate the natural transition orbitals (NTOs) and the associated weights (v) for the singlet (S1) and triplet (T1 and T2) states. However, the v values for some excited states are far away from 100% (in Fig. 2b), and the NOTs are not enough to describe the excited-state properties. Therefore, we further to depict the hole and electron distribution to supplement the NTOs. To analyze the excited states (S1, T1, and T2) properties, the hole and electron distribution were calculated with three methods (CAM-B3LYP, M06-2X, and ωB97XD) based on their S0 structures.

The overlap of hole and electron (Sr), separation degrees of hole and electron (t), distance between centroid of hole and electron (D), and overall mean span of hole and electron (H) (in Table S1-S3) were used to describe the quantitative representation of hole and electron distribution, and there are tiny differences in these parameters. However, three methods give similar excited-state characteristics for S1, T1, and T2 states. Figure S1 depicts the electron and hole distribution obtained with CAM-B3LYP, ωB97XD and M06-2X methods. According to the hole and electron distribution, the S1, T1 and T2 states of 1, 2, and 3 display main locally excited (LE) and very slight charge-transfer (CT) natures.

We take the plot (Fig. 2b) with M06-2X as an example to discuss the NTOs. As seen in Fig. 2b, the S1 states of 1, 2, and 3 display main LE and very slight CT natures. For example, the NTOs of 1 mainly locate on the anthracene group and the middle phenyl ring with little CT from the TPA to anthracene group. Both NOTs of 2 and 3 are delocalized over the whole molecules. Such high LE percentages in S1 states were beneficial for high fluorescence radiative rate. Similarly, the T1 states of 1, 2, and 3 also possess main LE and little CT natures. For example, the NTOs of 1 are mainly distributed on the anthracene group, and both 2 and 3 display the NTOs over 2-phenylquinazoline group and middle phenyl ring. As for T2, 1, 2, and 3 have dominant LE characteristics. 1 has NTOs over the TPA group, and the NTOs of 2 and 3 locate over the whole molecules.

Taking S0 as a reference, the relative S1, T1, and T2 energy levels are depicted in Fig. 2c. For all molecules (1, 2, and 3), the T2 is quite close to S1 in energy levels, and such a small T2 and S1 gaps are apt to realize T2 → S1 reverse intersystem crossing (RISC) path. Besides, compared to the S1 and T2 gaps, the T2 and T1 gaps of all molecules (1, 2, and 3) are relatively large and beneficial to suppress the T1 and T2 internal conversion, leading to more T2 population. The combination of more T2 population and negligible T2 and S1 gaps is advantage for T2 → S1 RISC, which is consistent with the explanation of the large proportion of radiation singlet exciton for HLCT-state molecules. The 1 and 2 have similar structures, but the T1 and T2 gap of 1 and 2 are 1.14 eV and 0.75 eV, indicating a slight geometric variation has a huge influence on T1 and T2 energy gap. Consequently, a careful group combination for designing novel luminescent materials utilizing HLCT state is important.

To quantify the geometric change, we also summarize the typical bond lengths (BL1 and BL2) and dihedral angles (DA1 and DA2) (shown in Fig. 2a and Table S4) in S0, S1, T1, and T2 structures. Upon electron excitation from S0 to S1, T1 and T2, the DA1 have more noticeable variation (− 45.9 to − 69.0° for 1) than DA2 (28.8°–38.9° for 1). Likewise, the BL2 variation between S0 and excited state (S1, T1, and T2) is smaller than BL1, and the differences are below 0.03 Å. The BL1 and BL2 variations show the opposite variations to DA1 and DA2. The larger rotations lead to shorter bond lengths between TPA (or PA) and various groups. Figure 2c depicts a geometric comparison among the S1, T1, and T2, and the structure deviation are mostly originated from the DA1 and DA2.

Singlet excited-state potential energy surface by twisting TPA or PA group

The results with non-restricted optimized S1, T1, and T2 structures are consistent with the hot-exciton RISC path proposed by Ma et al. However, upon the excited states (S1, T1, and T2), the stable CT state formation by twisting TPA or PA would affect the T2 and S1 gap and RISC process. On one hand, if the CT state is the most stable emission state, the decrease of the S1 state would affect the quantum efficiency by decreasing the fluorescence decay rate and aggravating the non-radiative relaxation. On the other hand, the enlarging S1 and T2 gap originated from the decreased S1 energy level would weaken the T2 exciton utilization. Like-TADF molecules based on donor and acceptor groups through a single bond, the molecular rotation was easily activated and would generate a large spectrum of conformations at room temperature43. For 1, 2, and 3 (Fig. 2a), the donor and acceptor groups are connected via a flexible single bond, therefore, the S1, T1, and T2 CT states by twisting TPA or PA were calculated.

At first, we performed the triplet excited-state (T1 and T2) potential energy surface (PES) (in Figure S2) with twisting TPA or PA. It can be found that almost all T1 and T2 energies are increased with twisting TPA or PA, meaning that molecular rotations are difficult to realize upon T1 and T2 PES. Consequently, the T1 and T2 energy levels are almost constant. Further, the singlet excited-state potential energy surface (S1 PES) with twisting TPA or PA with a scan step of 10 degrees has been performed.

The vertical S1 states of three HLCT-state molecules based on S0 structures (in Table S5) display large oscillator strength in a range from 0.3560 to 0.9085, indicating main LE transition and high fluorescence emission. Figure 3b depicts the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). All S1 states are attributed to a mixture of more HOMO → LUMO and less HOMO-1 → LUMO contributions (Fig. 3a). For example, 3 possesses a total HOMO → LUMO (80%) transition with HOMO mainly diffused over TPA and LUMO localized on the 2-phenylquinazoline group (Fig. 3b). The HOMO-1 of 1 is distributed over the whole molecules, and the HOMO-1 → LUMO transition presents the main LE characteristic. The S1 states of 1, 2, and 3 molecules display a combination of CT and LE features.

Figure 3
figure 3

(a) S1 transition configuration of 1, 2, and 3, (b) HOMO-1, HOMO and LUMO distribution, (c) S1 PES by twisting TPA, (d) S1 PES by twisting PA.

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In terms of twisting TPA (DA1) or PA (DA2) (Fig. 3c,d), the plots of S1 PES show three variation tendencies. It can be concluded that the CT state of 3 is very easy to realize by twisting TPA or PA. But the TPA or PA of 2 is very different to twist on the S1 PES. The CT state formation of 1 is in a barrierless process by twisting TPA, but not for PA. To illustrate the excited-state properties, Table S6-S13 lists the oscillator strength, transition configuration, and frontier molecular orbitals of 1, 2, and 3 with twisting TPA or PA.

On the S1 PES of 1 and 3 with twisting TPA, the CT state is more stable than the HLCT state and would become the finally stable emission state, suppressing small energy barriers (0.05 eV for 1 and 0.02 eV for 3). At small DA1, the S1 states of 1 contain a main HOMO → LUMO transition, but a mixture of HOMO → LUMO and HOMO-1 → LUMO transitions at 70°—80° is found. With increasing DA1, the HOMO shifts from the whole molecules to the TPA group, and LUMO moves from the anthracene and the middle phenyl ring to the whole anthracene (Table S8). Finally, the S1 state (DA1 = 90 degrees) is a total CT character with a low f (nearly zero). The S1 states of 3 are assigned to HOMO → LUMO in a percentage range of 20% → 90% with increased DA1. Considering orbital distribution (Table S10), the initial HOMO shows a high degree of localization of electron density on the whole molecule, and transfers to TPA with increasing DA1. Upon the TPA rotation process, LUMO is always distributed on the 2-phenylquinazoline and the middle phenyl ring. When DA1 is 90 degrees, a noticeably separated HOMO and LUMO for CT state is formed, and f is nearly approaching to zero. However, the S1 energies of 2 are increasing with twisting TPA (Fig. 3c). The energy is the highest point on the S1 PES (DA1 = 90 degrees), indicating that TPA rotation of 2 is restrained. Therefore, the relative S1 and T1 energy levels are almost unchanged and have no influence on the T2 → S1 RISC process. Different from other HLCT molecules, the S1 states have large f values in a range from 0.991 to 1.433 (Table S6). With twisting TPA group, the transition configurations (Table S9) change from HLCT to total LE, accompanied by high f.

As with twisting PA (Fig. 3d), the relative energies of 1 and 2 increase linearly with increasing DA2, indicating that the PA rotation of 1 and 2 on S1 PES need to overcome the high energy barrier to realize. The S1 states of 1 contain a major HOMO → LUMO (Table S7). As with twisting PA, the HOMO is initially delocalized over the whole molecules and shifts to the anthracene and middle phenyl ring, and the LUMO show almost complete localization on the anthracene and middle phenyl ring. Finally, the S1 state (DA2 = 90 degrees) of 1 contains the main LE state and has non-zero f (Table S6). The S1 states of 2 contain a major HOMO → LUMO and minor HOMO-1 → LUMO. Along twisting PA, the HOMO and HOMO-1 are always localized over the whole molecules, and LUMO is distributed on the phenanthrene and middle phenyl ring (Table S12). When the DA2 is 90 degrees, the S1 state displays the main LE characteristic and large f (Table S6). With increasing DA2, the S1 energy of 3 is decreasing, and a stable CT state would form in a barrierless process by twisting PA. The HOMO → LUMO (84–90%) transition gives dominant contributions to S1 states along twisting PA. At small DA2, the HOMO is mostly distributed on the TPA and a small part on the 2-phenylquinazoline but shift to the TPA group with increasing DA2 (Table S13). When the DA2 is 90 degrees, the HOMO is centered on the PA group, while LUMO are distributed on 2-phenylquinazoline and middle phenyl ring with f nearly to zero for S1 state.

In the excited state, the CT states (DA1 = 90 degrees) of 1 and 3 (DA1/DA2 = 90 degrees) are easy to populate via twisting TPA or PA, accomplished by decreased f and S1 emission efficiency. Moreover, the formation of stable CT states would enlarge the S1 and T2 gap, weaken the T2 to S1 RISC and decrease the triplet exciton utilization.

Conclusion

In conclusion, taking molecules 1, 2, and 3 as examples, we have investigated the excited-state potential energy surface (PES) by twisting triphenylamine (TPA) or diphenylamine (PA) by density functional theory and time-dependent density functional theory calculations. The calculated relative total energy indicated that the T2 and T1 energy gap is large, and T2 is closed to S1 in energy levels under the case of non-restricted excited-state optimizations. Meanwhile, the S1 states of 1, 2, and 3 have a mixture of locally excited (LE) and charge-transfer (CT) characteristics, high oscillator strength, and high fluorescence emission efficiency. Considering the excited state PES, the T1 and T2 energy levels are almost constant. However, for the S1 PES, it can be found that twisting TPA or PA would stabilize the CT state, which would seriously damage the quantum efficiency. For example, the CT states of 3 are very easy to realize no matter by twisting TPA or PA. The stable CT state formation of 3 would enlarge the energy gap between S1 and T2 and decrease the T2 exciton utilization. However, the TPA or PA group of 2 is very different to twist in the S1 PES, and S1 and T2 energy gap is almost unchanged. For 1, the CT states are formed in a barrierless process by twisting TPA, but not for PA. Therefore, understanding the relationship between the S1 PES and molecular structures is important for designing high-performance luminescent materials utilizing HLCT state.