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.