Understanding H-aggregates crystallization induced emissive behavior: insights from theory.

We conducted a theoretical investigation into how the molecular stacking effect impacts the photophysical properties in solid phases. Our findings indicated that in the aggregated state, the out-of-plane distorted vibration and imidazole ring stretching vibration of triimidazo-[1,3,5] triazinethe are significantly suppressed, which decreased the Huang-Rhys factor and the corresponding reorganization energy of the photophysical process, as a result, this restricted intramolecular motions and dissipation pathways of excess energy in the excited state, therefore, aggregation induced enhancement emission (AIEE) was found for the title compound from dichloromethane solution to solid state. Analysis of the emission spectrum through discrete spectral lines revealed that the main peak was affected by the vibrational modes with lower frequencies, while the middle-frequency modes influenced the shoulder peak. 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 due 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 of the title compound, our theoretical data declared this process can be ignored due to its low transfer rate.