Conformational Flexibility-Triggered Differential Sensing and Metric for the Naturally Abundant Transition Metal Ions by a Pyrrole-Functional Schiff Base

Abstract

A hitherto unknown Schiff base ligand (H₃L) is synthesized upon condensing benzhydrazide with 2,5-diformylpyrrole in 2 : 1 molar ratio. H₃L displays well-resolved UV-vis absorption bands with high molar extinction coefficients. It also exhibits photoluminescence (PL) with large Stokes shifts and high emission intensities. H₃L is found to be a remarkable chemosensor for commonly abundant transition metal ions through its UV-vis and photoluminiscence (PL) spectral change. It can not only detect and quantify various transition metal ions but can also distinguish them from their mixtures under restricted circumstances. Detailed computational analyses revealed that such a unique photophysical characteristic originates from ligand-centric internal electronic state rearrangement. H₃L can switch among an enormously large number of low-energy-barrier conformers in its free state. The degree of π-electron conjugation across the molecular skeleton of H₃L is dictated by its conformation. Its UV-vis/PL spectral signature is subject to an analyte metal ion. Treatment of H₃L with a transition metal ion leads to the formation of [2+2] self-assembled discrete coordination complex, which renders the restriction on the conformational freedom of H₃L. The commonly abundant alkali/alkaline earth metal ions and commonly abundant anions are found to cause no-to-negligible interference. The chemosensing is affirmed to occur practically instantaneously with excellent limit of detection.