Free Energy Profile Analysis for the Aggregation-Induced Emission of Diphenyldibenzofulvene.

An improved understanding of the contributions of various factors to aggregation-induced emission (AIE) is required to advance the development and application of highly efficient AIE luminogens. Herein, the AIE of diphenyldibenzofulvene (DPDBF), which is nonemissive in dilute solutions but becomes highly emissive in aggregated states, was investigated using molecular simulations. Electronic structure calculations showed that the ground and first singlet excited states of DPDBF were degenerate after rotation of the ethylenic C═C bond, which results in fluorescence quenching via conical intersections (CIs). In this study, free energy (FE) profiles were used to quantify the extent to which the intramolecular motions of DPDBF required to reach the CIs were restricted in condensed phases. In acetonitrile solution, the FE profile showed that the ethylenic C═C bond of DPDBF could easily rotate to reach the CIs, which enabled nonradiative internal conversion. In contrast, in an aggregated state, the FE profile showed that the rotation around the ethylenic C═C bond of DPDBF was markedly restricted, thus preventing quenching through the CIs. These findings provide quantitative insights into the AIE mechanism of DPDBF.