Theoretical insights into aggregation-induced emission of bis(cyanostyryl)pyrrole derivatives

The molecular architecture and the positioning of the cyano group in cyanostilbene derivatives give rise to intriguing variations in their photophysical properties. The present study provides theoretical insights into the contrasting photoluminescence behaviors of bis(cyanostyryl)pyrrole derivatives with different cyano group positions. Using quantum mechanics/molecular mechanics (QM/MM) free energy perturbation methods, we investigated o-DCSP and i-DCSP isomers, which exhibited markedly different fluorescence quantum yields in the solution (Φ_f = 0.0036 vs. 0.43) and aggregated states (Φ_f = 0.15 vs. 0.12). We identified the minimum energy conical intersection (MECI) structures for both isomers, characterized by substantial rotation and pyramidalization of one ethylenic CC bond, and determined the minimum energy path (MEP) connecting the Franck-Condon point to the MECI using the string method. By calculating the free energy profiles along this MEP, we revealed significant differences in energy barriers: o-DCSP showed a low barrier in solution (0.57 eV), which dramatically increased upon aggregation (2.36 eV), explaining its aggregation-induced emission behavior, whereas i-DCSP maintains relatively high barriers in both states (1.40 eV and 1.67 eV), resulting in efficient emission regardless of the environment. These findings establish a quantitative molecular-level understanding of the structure-property relationships in fluorescent materials and provide design principles for developing high-performance luminescent compounds with tailored emission characteristics for specific applications.