Tactfully revealing the working mechanisms on a tetraarylimidazole derivative: AIE characteristic, ESIPT process and ICT effect integrating in one molecule

Recently, a novel tetraarylimidazole derivative 2-(benzo[d]thiazol-2-yl)-4-(4,5-bis(4-methoxyphenyl)-1-phenyl-1H-imidazol-2-yl)-phenol (be called MHBT herein) was architectured by our research group showing the fascinating synergy of aggregation-induced emission (AIE) characteristic, excited-state intramolecular proton transfer (ESIPT) mechanism and intramolecular charge transfer (ICT) effect. Nevertheless, a detailed and reasonable interpretation of its mechanisms both in theory is urgently needed. Consequently, to unveil the working mechanism meticulously, herein, we tactfully applied density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods to illuminate the underlying mechanisms in different solvent conditions. After optimizing the structures, the geometric parameters of hydrogen bonds (HBs), the infrared (IR) vibrational spectrum, the reduced density gradient (RDG) isosurfaces were calculated in detail, vividly explaining how the enhancement of HBs behaved as the driving force to proceed ESIPT process. Simultaneously, the frontier molecular orbitals (FMOs) combined with the potential energy curves (PECs) were conducted to interpretate the role and character of ICT and ESIPT in molecule MHBT. Further, the PECs of MHBT for dihedral angles in different organic solvents were calculated to compare the dominant torsion degree, rationalizing the AIE phenomenon from the view of the restriction of intramolecular rotation process. This work may well underpin the understanding of the interaction between different mechanisms in fluorescent dyes and thereby provide meaningful guideline for the design and construction of ideal molecules