Cyclohexyl-diimine capped lower rim 1,3-di-derivatized calix[4]arene conjugate as sensor for Al3+ by spectroscopy, microscopy, titration calorimetry and DFT computations

Abstract

A cyclohexane-trans-1,2-diimine capped conjugate of 1,3-calix[4]arene (L) has been synthesized and characterized using different analytical and spectral techniques. L has been shown to be sensitive toward Al3+ by exhibiting ~45-fold enhancement in its emission intensity at 445 nm upon complexation. All the other 15 metal ions showed almost no or minimal change in the fluorescence intensity of the L supporting that none of those 15 ions is sensed by L. The complexation between L and Al3+ has been further confirmed by absorption spectroscopy, isothermal titration calorimetry and ESI MS. The isotopic peak pattern of the ESI MS peak clearly confirmed the presence of aluminum in the 1:1 complex formed. The need for the flexible cap moiety for bringing selectivity to Al3+ was proven by comparing the titration studies with the corresponding control molecules. The sensing of Al3+ by L in the solid powder was demonstrated by fluorescence microscopy. The supramolecular behavior of L changes from simple spherical type morphology in L to an aggregated micro pots and fibers upon Al3+ binding. The DFT computational study yielded a distorted tetrahedral complex of the dianionic receptor resulting in AlN2O2 core.