2016 年 14 巻 6 号 p. 189-192
A high external quantum efficiency observed for organic light-emitting diodes using PTZ-BZP (PTZ: 10-hexyl-phenothiazin, and BZP:4-phenyl-2,1,3-benzothiadiazole) is attributed to fluorescence from S1 via reverse intersystem crossing from the T3 or T2 state under electrical excitation. The radiative and non-radiative transitions from these higher triplet states to the lower triplet states are suppressed because of their small overlap densities. In this study, a principle to design such an electronic structure is proposed.
Thermally activated delayed fluorescence (TADF) is delayed fluorescence via reverse intersystem crossing (RISC) from a thermally activated triplet T1 state. Recently, TADF has attracted significant attention as a novel light-emitting mechanism for third-generation organic light-emitting diodes (OLEDs) . There is a 25% probability for the generation of a singlet exciton under electrical excitation. On the other hand, there is a 75% probability for the generation of a triplet exciton under electrical excitation. Fluorescent OLEDs utilize singlet excitons, while phosphorescent OLEDs use triplet excitons. On the other hand, TADF OLEDs can exhibit high external quantum efficiencies (EQEs) as they utilize both singlet and triplet excitons.
Sato et al. have proposed a novel light-emitting mechanism for OLEDs based on fluorescence via RISC from triplet states higher than T1 employing selection rules of an electric dipole moment and spin-orbit coupling, caused by a high molecular symmetry .
Yao et al. have reported a high EQE of 1.54% for an OLED using PTZ-BZP (Figure 1) as an emitting molecule . In a fluorescent OLED, EQE is equal to 0.05×PLQY, where PLQY represents the photoluminescence quantum yield. As the PLQY of PTZ-BZP is 16%, the observed EQE cannot be explained as a conventional fluorescent OLED. In addition, as T1 is low, the emitting mechanism of PTZ-BZP is not TADF. Yao et al. have proposed a fluorescence emitting mechanism via RISC from T3.
Their mechanism is possible as long as the electric dipole and non-radiative transitions caused by vibronic couplings from T3 or T2 to the lower triplet states are suppressed [4,5]. In this Letter, we calculate the off-diagonal vibronic coupling constants (VCCs), which serve as the driving force for non-radiative transitions, and analyze overlap densities for elucidating the suppressed transition dipole moment and VC in PTZ-BZP.
The ground and excited states were calculated at the B3LYP/6-31G (d,p) and TD-B3LYP/6-31G (d,p) levels of theory using Gaussian09. The geometries of the relevant triplet states, T3 and T2, were optimized. VCC calculations and vibronic coupling density (VCD) analyses were performed using our in-house codes. The off-diagonal VCCs of the non-radiative T3 → T1 and T3 → T2 transitions were calculated at the optimized geometry for the T3 state, while those of the non-radiative T2 → T1 transition were calculated at the optimized geometry for T2.
Table 1 summarizes the calculated excitation energy and major electronic configurations with configuration interaction (CI) coefficients. The energy gap between T3 and S1, ,is −84 meV. Therefore, if the radiative and non-radiative transitions from the T3 state to the lower triplet states are suppressed, fluorescence from S1 via RISC from T3 is expected without temperature dependence because of the negative energy gap.
Table 2 lists the calculated results for the T2 optimized structure. The T2 state is close to S1 with a positive energy gap, meV. Therefore, TADF via RISC from T2 is expected if the radiative and non-radiative decays T2 → T1 are suppressed. Figure 2 shows the off-diagonal VCCs between (a) T3 and T1, (b) T3 and T2, and (c) T2 and T1 for vibrational modes. The off-diagonal VCCs of T3 → T2 and T2 → T1 are very small. Therefore, the non-radiative decay path T3 → T2 → T1 is suppressed. As the off-diagonal VCCs of T3 → T1 are intermediate as an organic π-conjugated molecule, the non-radiative decay path T3 → T1 is not significantly suppressed.
Off-diagonal vibronic coupling constants between (a) T3 and T1, (b) T3 and T2, and (c) T2 and T1 for vibrational modes at the B3LYP/6-31G (d,p) level of theory.
An off-diagonal VCC, Vα, and transition dipole moment, μ, are expressed in terms of overlap (transition) density between the electronic states n and m, :
The suppression of the radiative and non-radiative transitions from the T3 and T2 states to the lower states can be elucidated on the basis of overlap density. According to equations (1) and (2), if the overlap density between two electronic states is small, Vα and μnm are reduced. Therefore, non-radiative and radiative transitions are suppressed.
A TD-DFT wavefunction is written as
1. Overlap density between and is zero for and ,
2. Overlap density between and is equal to orbital overlap between unoccupied orbitals r and s for i = j, , and equal to orbital overlap between occupied orbitals i and j for , r = s.
According to Table 1, the T3 state can be approximately written as
Therefore, the overlap density between and vanishes and that between and is equal to the orbital overlap between the LUMO and NLUMO. As shown in Figure 3, as the LUMO and NLUMO are separately localized, the orbital overlap between them is small. Accordingly, the overlap density between T3 and T2 is very small. On the other hand, the overlap density between T3 and T1 is not so small. However, if the HOMO and NHOMO are pseudo-degenerate, and these orbitals are separately localized on the BZP units, the overlap density can decrease because of the relations of the CI coefficients: ab − ba = 0 and .
Frontier orbitals at the T3 optimized structure.
The high EQE observed in PTZ-BZP is attributed to fluorescence via RISC from the T3 or T2 states.
Fluorescence via RISC from a higher triplet state is possible even for asymmetric molecules as long as the transition dipole moments and vibronic couplings are small among the lower triplet states. A small overlap density results in the suppression of transition dipole moments and vibronic coupling. One design principle for realizing such an electronic state is to make use of a pseudo-degenerate electronic structure, namely using molecules in which the same fragments (BZPs in PTZ-BZP) are linked by a linker unit (PTZ in PTZ-BZP).
Numerical calculations were partly performed at the Supercomputer Laboratory of Kyoto University and at the Research Center for Computational Science, Okazaki, Japan. This study was also supported by a Grant-in-Aid for Scientific Research (C) (15K05607) from the Japan Society for the Promotion of Science (JSPS).