Residual and exploitable fluorescence in micellar self-assembled ON–OFF sensors for copper(ii)

Abstract

A set of new ligands, L2H₂–L5H₂, containing the 1,4,8,11-tetraaza-5,7-dione framework has been prepared, The ligands feature lipophilic substituents either on the carbon atom in the 6 position or on the amino groups, or on both. The solution behaviour of the ligands when included in TritonX-100 micelles has been investigated by means of potentiometric titrations and protonation and complexation constants for the Cu²⁺ cation have been determined in micellar medium. Micellar assemblies containing the ligands and pyrene have been prepared, and coupled pH-metric and fluorimetric titrations allowed the determination of the response of the systems as ON–OFF fluorescent sensors for Cu²⁺. A correlation between the effective lipophilicity of the ligand and the residual fluorescence (i.e. the fluorescence of the OFF state) was observed, and with the more lipophilic ligand, L3H₂, we obtained a residual fluorescence as low as 8%, with a significant improvement with respect to other published systems. On the other hand, introduction of functionalities on the amino groups of 1,4,8,11-tetraaza-5,7-dione brings the drawback of a small but significant decrease of the exploitable fluorescence, i.e. the fluorescence of the system in the absence of added Cu²⁺, at the pH value suitable for full metal complexation.