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

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.