Exploring the Mechanism of Fluorescence Quenching and Aggregation-Induced Emission of a Phenylethylene Derivative by QM (CASSCF and TDDFT) and ONIOM (QM:MM) Calculations

• Published in: Journal of physical chemistry. C Vol. 120; no. 38; pp. 21850 - 21857
• Main Authors: Wang, Bin, Wang, Xiaojuan, Wang, Wenliang, Liu, Fengyi
• Format: Journal Article
• Language: English
• Published: American Chemical Society 29-09-2016
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b07963

We report a QM (including TD-DFT and CASSCF) and ONIOM (QM:MM) study on the fluorescence quenching in methanol solution and fluorescence enhancement in crystal for a styrene derivative, namely 4-diethylamino-2 benzylidene malonic acid dimethyl ester (BIM) that possesses push–pull structure and AIE properties. The results showed that in methanol solution the weakening of ethylenic CC bond after photoexcitation initiates a barrierless relaxation via one-bond rotation around it, until the reactive molecule reaches a low-energy intermediate with strong charge-transfer character, then a S1/S0 conical intersection optimized near the charge-transfer intermediate is responsible for the fluorescence quenching in the dilute solution. The existences of charge-transfer intermediate as well as the conical intersection in the vicinity, which has not been observed in other symmetric (or less polar) phenylethylene-based luminophores, are the major features of BIM in solution. While in crystalline phase, the excited-state deactivation channels via torsional motions, either via one-bond rotation or via hula-twist mechanism, are restricted by steric hindrance and electrostatic repulsion from surrounding molecules, and thus fluorescence is enhanced.