Charge transfer characteristics in one- and two-photon absorption of dibenzothiophene derivatives: roles of pedal rotation and donor alkyl chain length

Abstract

Dibenzothiophene (DBT)-based derivatives have attracted considerable interest for their excellent nonlinear optical (NLO) properties. However, the specific roles of configurational isomerization and alkyl chain length variation in modulating their NLO properties remain insufficiently understood. Herein, we employ density functional theory (DFT), time-dependent DFT (TD-DFT), and the sum-over-states (SOS) method to systematically investigate two series of trans-configured dibenzothiophene derivatives. Our results reveal that pedal-type rotation serves as a notable structural switch: upon a 180° rotation from the 0° conformation, the peak two-photon absorption (TPA) cross-section decreases consistently by approximately 20% (to 79.3 ∼ 81.6% of the original value) across all three molecules. In addition, elongation of the alkyl chain in the donor moiety leads to a modest one-photon absorption enhancement but a gradual decline in the peak TPA cross-section. Notably, the TPA response exhibits a clear state-dependent behavior: the S1 state, characterized by strong intramolecular charge transfer, benefits from enhanced donor strength, whereas the S10 state, characterized by mixed short-range charge-transfer/local-excitation character, suffers from steric hindrance and potential multichannel interference. These findings indicate that, beyond electronic effects, steric conformational changes play a pivotal and state-selective role in governing TPA performance. This work provides a theoretical reference for tuning the optical responses of related DBT-based NLO molecules.