Theoretical Insights into the Mechanisms of Aggregation-Induced Emission of a Tetraphenylethylene

The aggregation induced emission (AIE) of tetraphenylethylene (TPE) was studied theoretically. The TPE has been known to exhibit the AIE, which is non-emissive in dilute solutions but becomes highly emissive in solid or aggregated state. In this study, the AIE mechanism of TPE was investigated by using electronic structure calculations, together with molecular dynamics (MD) simulations. The results of electronic structure calculations showed that potential energies of TPE for electronic ground (S₀) and first excited (S₁) states are degenerated at a conformation with the twist angle of 90° around its ethylenic C=C bond, which can lead the fluorescence quenching of this molecule in dilute solutions. The results of MD simulations revealed that the TPE in aggregated state tends to assemble in close contact, where the ethylenic C=C bond rotation is markedly restricted, preventing the fluorescence quenching via the S₀/S₁ conical intersection; the TPE in THF solution, however, proceeds the barrierless non-radiative transition. These results gave a clear picture of the AIE mechanism of TPE.