Modeling the Modulation of Emission Behavior in E/Z Isomers of Dipyrrolyldiphenylethene: From Molecules to Nanoaggregates

The dipyrrolyldiphenylethene (DPYDPE) molecule shows a switching between aggregation-induced-emission (AIE) and crystallization-induced emission (CIE) upon modification of the stereochemistry of the molecule. Herein, we propose a theoretical study based on molecular dynamics, time-dependent (TD-) density functional theory (DFT), and QM/QM′ calculations to investigate the structural and optical properties of the E and Z isomers in three different phases (solution, crystal, and aggregate). By computing the Huang–Rhys factors and the reorganization energies, we demonstrate that the fluorescence quenching observed in solution for both isomers is due to a nonradiative decay process involving low-frequency vibrational modes assigned to scissoring motions. In the crystal and in the aggregates, the effects of steric hindrance strongly modify the topology of the potential energy surface of the first excited state, and this results in a restriction of the vibrational modes involved in the energy dissipation. The modulation of the emission behavior (CIE/AIE) as a function of the stereochemistry can be rationalized by analysis of the inter- and intramolecular interactions.