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

In this study, aggregation-induced emission (AIE) of a cyanostilbene derivative, called 1-cyano-1,2-bis-(4′-methylbiphenyl)­ethylene (CN-MBE), was investigated using molecular simulations. The <i>E</i>-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 <i>Z</i>-form is non-emissive even in its crystalline form. In addition, upon UV light irradiation, the <i>Z</i>-form exhibits a <i>Z</i>-to-<i>E</i> isomerization, whereas the <i>E</i>-form keeps its conformation at room temperature and undergoes a nonradiative <i>E</i>-to-<i>Z</i> 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 (S₀) and first excited (S₁) states were degenerate at a twisted conformation around the ethylenic CC π-bond, which led to fluorescence quenching of the molecule. Molecular dynamics simulations and free-energy analyses revealed that the <i>E</i>-form molecules assembled closely, with the CC bond rotation markedly restricted. This, in turn, prevented fluorescence quenching via the S₀/S₁ conical intersection. In contrast, the <i>Z</i>-form molecules aggregated relatively sparsely, allowing for the nonradiative <i>Z</i>-to-<i>E</i> isomerization to proceed. All in all, the theoretical insights presented herein give a clear picture on the AIE and photo/thermal isomerization mechanisms of CN-MBE