A benzothiazole-coupled NS4-donor macrocycle and its complexation-based dual-channel sensing for Hg2+: the influence of anions and structure–function relationship

Abstract

A benzothiazole-coupled NS₄-macrocycle derivative L was synthesized via a three-step route for the detection of Hg²⁺. Chemosensor L, as a dual channel (UV-vis and fluorescence) receptor, undergoes a significant hypsochromic shift and fluorescence quenching in the presence of Hg(ClO₄)₂ in ethanol/DMSO (v/v, 9 : 1), showing remarkable selectivity towards Hg²⁺ over other metal ions both in single and competitive metal ion experiments. The observed chelation-enhanced quenching (CHEQ) effect seems to be attributed to the combination of the heavy atom effect and Hg²⁺⋯benzothiazole interaction. However, such Hg²⁺ sensing responses in both channels were not observed for non-perchlorate anions. To explain the anion-dependent Hg²⁺ sensing, systematic studies on the complexations of L with Hg(ClO₄)₂ and HgI₂ were performed via UV-vis/fluorescence titrations, comparative NMR, and anion exchange. Taken together, the Hg²⁺ sensing critically depends on the Hg–N_tert bonding which triggers the hypsochromic shift or the CHEQ effect. A weaker coordinating ClO₄⁻ allows an endocyclic complex to form with the Hg–N_tert bond being responsive, while a stronger coordinating I⁻ leads to exocyclic complexation without the Hg–N_tert bond being non-responsive. The crystal structure of the mercury(II) iodide complex [Hg₂(L)I₄] featuring an exocyclic dinuclear form also supports this explanation. This result could present a new perspective on the design of macrocycle-based chemosensors.