Selective recognition of Zn2+ by a triazole linked thiophene-methylimine based derivative of calix[6]arene and the secondary sensing of biothiols by the corresponding zinc complex

A 1,3,5-tris-triazole linked thiophene-methylimino appended calix[6)arene conjugate, L has been shown to bind to Zn2+ selectively in ethanol among the metal ions studied which include, alkali and alkaline earth ions, transition metal ions and lanthanides by fluorescence, absorption and ESI-MS. The conjugate, L exhibited-25 fold increase in its fluorescence intensity at lambda(em) = 450 nm in presence of Zn2+ upon exciting at 330 nm with a limit of detection of 25 4 ppb while the Ka is (2.31 +/- 0.1)x10(5) M-1. The control molecules, C-1 and C-2 which lack iminophenolic core and calix[6]arene platform respectively have been studied. These do not show considerable changes in their fluorescence emission in presence of Zn2+ implying the essential role of both the arene cavity as well as the role of the three arms to bind to Zn2+. Absorption studies further support the binding of Zn2+ to L by exhibiting isosbestic points at 294 and 344 nm and stoichiometry of the 1:1 complex formed between L and Zn2+ was confirmed by ESI-MS. The computationally optimized structure of {L+Zn2+} complex exhibited distorted octahedral geometry about Zn2+ with N(3)O(0)3 core. The in situ generated complex, {L+Zn2+} interacts with biologically relevant thiols by exhibiting fluorescence quenching of the complex due to the release of Zn2+ from its complex which is further supported by absorption and ESI-MS studies.