Formation of Cu3,4(TCA), making the TCA complex a highly selective probe for Cu2+ detection: a TDDFT study

Abstract

Previous experimental research on thia[4]calixarene (TCA) derivatives indicated that there were, at most, two Cu²⁺ anions that could coordinate to one TCA molecule, and that neither Cu₃(TCA) nor Cu₄(TCA) complexes could form in solution. However, we studied a new type of self-assembled molecule using an ON–OFF fluorescence probe for Cu²⁺ ion detection, made up of tert-butylthiacalix[4]arene, micelle and fluorescence group, by the DFT/TDDFT method. The signal response mechanism was attributed to fluorescence resonance energy transfer (FRET). Based on four basic triggering conditions for the FRET process, the electronic structure, excited states and spectral properties of TCA and TCA–metallic complexes, M_1–4(TCA)(MCu, Ni), were analyzed considering the effect on pH value. For all the metallic complexes studied, only Cu₄ (TCA) was shown to fulfill all four basic conditions for triggering the fluorescence quenching signal response of the FRET mechanism. The Cu₃,₄(TCA) complexes were inferred to coexist with Cu(TCA) and Cu₂(TCA), and would be in too low a proportion in solution to be found by some of the existing experiments.