Coordination-Directed Stacking and Aggregation-Induced Emission Enhancement of the Zn(II) Schiff Base Complex

Abstract

2-(Trityliminomethyl)-quinolin-8-ol (HL) and its Zn(II) complex were synthesized and characterized by single-crystal X-ray diffraction. HL is an unsymmetrical molecule and coordinated with Zn(II) ion to form ZnL₂ in the antiparallel-mode arrangement via Zn-O (hydroxyl group) and Zn-N (quinoline ring) of HL. A high degree of ZnL₂ molecules ordering stacking is formed by the coordination bonds and intermolecular π-π interactions, in which head-to-tail arrangement (J-mode stacking) for L^- is found. HL is nonfluorescent and ZnL₂ is weakly fluorescent in THF. The fluorescence emission of ZnL₂ enhances in THF/H₂O as H₂O% (volume %) is above 60% and aggregates particles with several hundred nanometers are formed, which is confirmed by DLS data and TEM images. The J-aggregates stacking for L^- in ZnL₂ results in aggregation-induced emission enhancement (AIEE) for ZnL₂ in THF/H₂O. Theoretical computations based on B3LYP/6-31G(d, p) and TD-B3LYP/6-31G(d, p) methods were carried out. ESIPT is the supposed mechanism for fluorescent silence of HL, and fluorescence emission of ZnL₂ is attributed to the restriction of ESIPT process. The oscillator strength of ZnL₂ increases from 0.017 for monomer to 0.032 for trimer. It indicates that a high degree of ZnL₂ molecules ordering stacking in THF/H₂O is of benefit to fluorescence enhancement. HL is an ESIPT-coupled AIEE chemosensor for Zn(II) with high selectivity and sensitivity in aqueous medium. HL can efficiently detect intracellular Zn(II) ions because of ESIPT-coupled AIEE property of ZnL₂ in mixed solvent.