Mechanisms of Aggregation-Induced Emission and Photo/Thermal E/Z Isomerization of a Cyanostilbene Derivative: Theoretical Insights

In this study, aggregation-induced emission (ME) of a cyanostilbene derivative, called 1-cyano-1,2-bis-(4'-methylbiphenyl)ethylene (CN-MBE), was investigated using molecular simulations. The E-form of CN-MBE has been known to exhibit AIE, which is non-emissive in dilute solutions but becomes highly emissive in aggregated states, whereas its Z-form is non-emissive even in its crystalline form. In addition, upon UV light irradiation, the Z-form exhibits a Z-to-E isomerization, whereas the E-form keeps its conformation at room temperature and undergoes a nonradiative E-to-Z isomerization only at a high temperature. The results from the electronic structure calculations employed in this work showed that the potential energies of CN-MBE for the electronic ground (S-0) and first excited (S-1) states were degenerate at a twisted conformation around the ethylenic C=C pi-bond, which led to fluorescence quenching of the molecule. Molecular dynamics simulations and free-energy analyses revealed that the E-form molecules assembled closely, with the C=C bond rotation markedly restricted. This, in turn, prevented fluorescence quenching via the S-0/S-1 conical intersection. In contrast, the Z-form molecules aggregated relatively sparsely, allowing for the nonradiative Z-to-E isomerization to proceed. All in all, the theoretical insights presented herein give a clear picture on the ME and photo/thermal isomerization mechanisms of CN-MBE.