Multi-channel receptors based on thiopyrylium functionalised with macrocyclic receptors for the recognition of transition metal cations and anions

Abstract

We report herein the synthesis and characterization of a family of ligands containing different cation binding sites covalently connected to a thiopyrylium signalling reporter. The receptors L¹–L⁶ are able to signal the presence of certain metal cations via three different channels; i.e. electrochemically, fluorogenically and chromogenically. An acetonitrile solution of L¹–L⁶ shows a bright blue colour due to a charge-transfer band in the 575–585 nm region. The colour variation in acetonitrile of L¹–L⁶ in the presence of the metal cations Ag⁺, Cd²⁺, Cu²⁺, Fe³⁺, Hg²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ has been studied. A selective hypsochromic shift of the blue band was found for the systems L⁴-Pb²⁺ and L⁵-Hg²⁺. Additionally, L¹–L⁶ are poorly fluorescent but coordination with certain metal cations induces an enhancement of the fluorescence at ca 500 nm. For instance, the presence of Cu²⁺ and Fe³⁺ induced a remarkable 42-fold and 45-fold enhancement in the emission intensity of L¹ centred at 500 nm, respectively. Also remarkable was the 18-fold enhancement observed for L⁴ and L⁵ in the presence of Fe³⁺ and Cu²⁺, respectively. The electrochemical behaviour of receptors L¹–L⁶ was studied in acetonitrile using platinum as a working electrode and [Bu₄N][BF₄] as a supporting electrolyte. This family of receptors showed a one-electron reversible redox process at ca. -0.46 V versus sce attributed to the reduction of the thiopyrylium group. A moderate anodic shift in the presence of certain metal cations was observed. The effect in the UV-visible spectra of acetonitrile solutions of receptor L¹–L⁶ in the presence of anions was also studied. A remarkable bleaching was found in the presence of cyanide.