Ratiometric fluorescence off-on-off sensor for Cu²⁺ in aqueous buffer by a lower rim triazole linked benzimidazole conjugate of calix[4]arene

Abstract

A benzimidazole appended triazole linked 1,3-diconjugate of calix[4]arene (L) has been synthesized and characterized. The conjugate L has been found to recognize Cu²⁺ among the thirteen different metal ions studied by exhibiting ratiometric fluorescence changes through newly generated excimer band at ∼380 nm. Fluorescence off–on–off behavior has been clearly demonstrated on the basis of the binding variability of Cu²⁺ to L. The binding has been elicited through the changes observed in the fluorescence, ESI MS and 1H NMR titrations. All the other metal ions studied do not show any new band and further do not interfere with the recognition of Cu²⁺ by L, even when these are present in the same medium. The structural features of both the mono- and di-nuclear complexes were established by DFT computational calculations and found to display highly distorted geometry about the copper centers that deviate from both the tetrahedral and the square planar.