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

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 Cu(2+) 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 Cu(2+) to L. The binding has been elicited through the changes observed in the fluorescence, ESI MS and (1)H NMR titrations. All the other metal ions studied do not show any new band and further do not interfere with the recognition of Cu(2+) 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.