Effect of hybridized local and charge transfer molecules rotation in excited state on exciton utilization

The fluorescent molecules utilizing hybridized local and charge-transfer (HLCT) state as potential organic light-emitting diodes materials attract extensive attention due to their high exciton utilization. In this work, we have performed the density functional theory method on three HLCT-state molecules to investigate their excited-state potential energy surface (PES). The calculated results indicate the T(1) and T(2) energy gap is quite large, and the T(2) is very close to S(1) in the energy level. The large gap is beneficial for inhibiting the internal conversion between T(1) and T(2), and quite closed S(1) and T(2) energies are favor for activating the T(2) → S(1) reverse intersystem crossing path. However, considering the singlet excited-state PES by twisting the triphenylamine (TPA) or diphenylamine (PA) group, it can be found that the TPA or PA group almost has no influence on T(1) and T(2) energy levels. However, the plots of S(1) PES display two kinds of results that the S(1) emissive state is dominated by charge-transfer (CT) or HLCT state. The CT emission state formation would decrease the S(1) energy level, enlarge the S(1) and T(2) gap, and impair the triplet exciton utilization. Therefore, understanding the relationship between the S(1) PES and molecular structures is important for designing high-performance luminescent materials utilizing HLCT state.