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.