Unraveling AIE in zinc(ii) coordination complexes: role of ligand structure and mechanistic insights

Abstract

The study presents the synthesis, structural characterization, and optical properties of two novel ligands, L¹ and L², along with their corresponding zinc(II) complexes, ZnL¹ and ZnL². Ligand L¹, featuring a flexible C–C single bond linkage between benzo[b]thiophene and a 2,6-bis(pyrazol-1-yl)pyridine (bpp) core, was synthesized via a four-step protocol. Ligand L², incorporating a conjugated vinyl bridge (CC) between the same fragments, was obtained through a six-step synthetic route. While both ligands and their zinc(II) complexes were studied, the structural features of ZnL² were confirmed through single-crystal X-ray diffraction, revealing a distorted octahedral coordination geometry. Comparative optical studies demonstrate that the extended conjugation in L² and ZnL² induces a bathochromic shift in their absorption and emission spectra relative to L¹ and ZnL¹. Interestingly, ZnL² exhibits aggregation-induced emission (AIE) behaviour in acetonitrile/water mixtures, with significantly intensified fluorescence at high water fractions (f_w ≥ 80%). The AIE properties are attributed to the restriction of intramolecular motions in the aggregated state, enhancing radiative decay pathways. Time-resolved fluorescence spectroscopy further supports the role of aggregation in prolonging fluorescence lifetimes. Notably, neither L¹, L², nor ZnL¹ exhibit AIE properties, highlighting the unique structural and electronic contributions of ZnL².