Displacement assay detection by a dimeric lanthanide luminescent ternary Tb(iii)–cyclen complex: high selectivity for phosphate and nitrate anions

Abstract

The luminescent dimeric ternary lanthanide–cyclen complexes (2-(Ln.1)₂; Ln = Tb/Eu) were designed and both their self-assembly formation and their ability to detect anions via displacement assays were investigated using spectrophotometric titrations in MeOH solution. The formation of 2-(Tb.1)₂ and 2-(Eu.1)₂ was investigated in solution, and determination of the binding constants and stoichiometry showed that the former was formed almost exclusively over the 1 : 1 complex 2-(Tb.1) after the addition of two equivalents of 2; while for 2-(Eu.1)₂ a mixture of both stoichiometries existed even after the addition of four equivalents of 2. Of these two systems, 2-(Tb.1)₂ was studied in details as a probe for anions, where significant changes where observed in the photophysical properties of the complex; with the characteristic Tb(III)-centred emission being fully switched off upon the sensing of phosphates and nitrate, giving rise to the formation of a H₂PO₄⁻:Tb.1 complex in a 1 : 2 stoichiometry upon sensing of H₂PO₄⁻ by 2-(Tb.1)₂, while NO₃⁻ gave 1 : 1 complex formation and two equivalents of NO₃⁻·Tb.1.