Understanding H–aggregates crystallization induced emissive behavior：insights from theory

Abstract

Abstract The influence of molecular stacking effect on photophysical property in solid phase was investigated theoretically. We found the out–plane distorted vibration and imidazole ring stretching vibration of triimidazo-[1, 3, 5]triazinethe were obviously suppressed in the aggregated state, which decreased Huang–Rhys factor and the corresponding reorganization energy of photophysical process, as a result, it restricted intramolecular motion and restrained dissipation pathways of the excess energy in the excited state, therefore, aggregation induced enhancement emission (AIEE) was found for the title compound from difluorochloromethane solution to solid state. The simulated emission spectrum by discrete spectral lines indicated the main peak was affected by the vibrational modes with lower frequencies, meanwhile the shoulder peak of the emission spectrum was affected by the middle–frequency modes. Furthermore, the predicted intersystem crossing rate (kiosk) and reverse intersystem crossing rate (krisc) using Marcus theory confirmed that an electron can successfully shift from its S1 state to the T1 state, however, the reverse T1 → S1 process can not come into being duo to very small krisc (10–6 − 10–9 s–1), therefore the phosphorescence can be observed. At last, we explored the influence of charge transfer process on the delayed fluorescence phenomenon of the title compound, the prompt fluorescence and the delayed fluorescence phenomenon from photo-induced charge transfer of both the paths can explain the experimental three-exponential fluorescent spectrum.