The aggregation-induced emission (AIE) of a cyanostilbene derivative, called CN-MBE, was studied theoretically by using electronic structure calculations, together with molecular dynamics (MD) simulations. The E-form of CN-MBE has been known to exhibit the AIE, which is non-emissive in dilute solutions but becomes highly emissive in solid or aggregated states; however, its Z-isoform is non-emissive even in a crystal. In this study, we have investigated the conformation-dependent AIE mechanism of CN-MBE. The electronic structure calculations showed that potential energies of CN-MBE for electronic ground (S0) and first excited (S1) states are degenerate at a conformational state with the twist angle of 90° around its ethylenic C=C bond, which can lead the fluorescence quenching of this molecule via an S0/S1 conical intersection (CI) in dilute solutions. The results of MD simulations revealed that the E-forms of CN-MBE tend to assemble in close contact, where the ethylenic C=C bond rotation is markedly restricted, preventing the fluorescence quenching via the S0/S1 CI. Its Z-isoforms, however, aggregate sparsely, proceeding the spontaneous relaxation in a barrierless manner from the vertical Franck-Condon point to the S0/S1 CI point on the S1 potential energy surface, leading the fluorescence quenching of the molecule. These results gave a clear picture of the conformation-dependent AIE mechanism of CN-MBE. The method combining the quantum chemistry calculation and the MD simulation used in this study can not only be utilized for analysis of various other various AIE phenomena but also in the development of new photochemical devices making use of the specificity of molecular crystals.

Theoretical insights into the mechanisms of aggregation-induced emission and photo/thermal E/Z isomerization of a cyanostilbene derivative

2019 Fall National Meeting

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