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 T₁ and T₂ energy gap is quite large, and the T₂ is very close to S₁ in the energy level. The large gap is beneficial for inhibiting the internal conversion between T₁ and T₂, and quite closed S₁ and T₂ energies are favor for activating the T₂ → S₁ 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 T₁ and T₂ energy levels. However, the plots of S₁ PES display two kinds of results that the S₁ emissive state is dominated by charge-transfer (CT) or HLCT state. The CT emission state formation would decrease the S₁ energy level, enlarge the S₁ and T₂ gap, and impair the triplet exciton utilization. Therefore, understanding the relationship between the S₁ PES and molecular structures is important for designing high-performance luminescent materials utilizing HLCT state.