Rigid Planar Aggregation-Induced Emission-Active Conjugated Molecule

Aggregation-induced emission (AIE) materials are in high demand for various practical applications in organic optoelectronics, sensorics, and bioimaging applications. Typically, these materials were designed to have nonplanar molecular structures with at least one-rotor moiety and intramolecular motion/rotation. Here, we designed, synthesized, and comprehensively studied 1,4-bis((9H-(1,8-diazafluoren)-9-ylidene)methyl)phenylene (1,8-BDFMP), demonstrating a unique and counterintuitive behavior. Despite the rigid and planar molecular structure caused by the effective conjugation and intramolecular N···H interactions coupled with strong H-aggregation, it clearly demonstrated AIE activity. The photoluminescence quantum yield of the luminophore in solution was only 0.04%, whereas its single crystals, despite strong π-stacking intermolecular interactions, were emissive with a photoluminescence quantum yield of 10%. The charge transport in 1,8-BDFMP single crystals and drop-cast films was evaluated. The detailed photophysics of 1,8-BDFMP was studied both experimentally and computationally. The conical intersection of the S1–S0 states was demonstrated to be the main nonradiative deactivation pathway in the monomeric state.