Aggregation-induced emission mechanism of styrene derivative: a theoretical study

The aggregation-induced emission (AIE) mechanism of the fluorescent styrene derivative 4-dimethylamino-2-benzylidene malonic acid dimethyl ester (BIM) in methanol solution is theoretically investigated using spin-flip long-range corrected time-dependent density functional theory (SF-LC-TDDFT). The potential energy surfaces (PESs) for the ground (S₀) and first singlet excited (S₁) states of BIM were calculated along the rotation of the aryl main axis (α angle rotation), consistent with experimental observations. For the monomer, our findings reveal a significant reduction in oscillator strength, approaching zero at the optimized geometry in the S₁ state. As this state corresponds to a charge transfer state, it suggests that the BIM monomer operates as a twisted intramolecular charge transfer (TICT) system, undergoing quenching through α angle rotation. The restriction of TICT, and consequently the inhibition of fluorescence quenching in the aggregate state, is also investigated by extracting the coordinates of 13 monomers from the crystal structure of BIM. The α-torsional angle of the central monomer was manually rotated in both clockwise and anti-clockwise directions to assess the intramolecular restrictions within the constrained environment. This analysis reveals that even a 10° rotation of the α-torsional angle, in either direction, causes the atoms of the central monomer to come into close contact with the atoms of the neighboring monomers. These short contacts effectively inhibit the TICT process, thereby leading to aggregation-induced emission.