Unveiling the Aggregation-Induced Emission (AIE) Mechanism and the Effect of Substituents on Luminescence Properties for Salicylaldehyde Azine Derivatives with Intramolecular Hydrogen Bond

In recent years, salicylaldehyde azine derivatives with high quantum yield and large Stokes shift for aggregation-induced emission (AIE) have attracted wide attention. To understand the fluorescence quenching and AIE mechanism, as well as reveal the effect of distinct substituents on the luminescence properties, herein we carried out a comparative study on two salicylaldehyde azine derivatives (namely, DPAS and FAS) using the QM(MS-CASPT2//TD-DFT) and ONIOM(QM:MM) calculations. Computational results show that the ESIPT processes for DPAS and FAS are unfavorable both in THF solution and in the solid phase (the barriers for DPAS are 14.5 and 19.6 kcal/mol, respectively, and those for FAS are 17.1 and 16.9 kcal/mol, respectively). The fluorescent quenching in solution is caused by the N–N assisted C═N double bond rotation. Moreover, the participation of an intramolecular H-bond in solution is favorable for the nonradiative transitions, while the molecular motion with a hula-twist mode in crystal correspondingly prompts the radiative transitions back to the ground state. Compared with diphenyl-substituted DPAS, fluorenyl-substituted FAS is more difficult to undergo nonradiative relaxation in solution, resulting in a slightly stronger emission; however, FAS in the solid phase has a lower S1–S0 gap (1.24 eV) for the fluorescence emission structure and a stronger π–π stacking interaction in the face-to-face stacking pattern, which leads to an overall lower fluorescence quantum yield. Our study on the AIE mechanism and structure–property relationship of salicylaldehyde azine derivatives is of great significance for understanding such compounds and developing high-performance AIE molecules.