Bis(pyrrole-benzimidazole) conjugates as novel colorimetric sensor for anions

Abstract

Abstract: Novel bis(pyrrole-benzimidazole) (PYBI) conjugates were investigated as colorimetric anion recognition motif by employing multiple donor sites. In this regard, PYBI derivatives where pyrrole unit is connected to the C2 position of benzimidazole via its α-position were synthesized and their anion recognition behavior were evaluated by UV-Vis, fluorescence and 1H NMR spectroscopy. PYBI selectively interacts with fluoride ion, whereas introduction of nitro group on the benzimidazole moiety enhances the binding affinity by at least one order, albeit at the expense of the selectivity. Bridging of two PYBIs led to interesting cooperative effect between the two subunits, which gets enhanced upon changing the spacer between the two from non-conjugating type (sp3-C) to conjugating one (quinoxaline). This also affects the way they interact with anions, with the latter moiety displaying very interesting stepwise double deprotonation of benzimidazole protons upon addition of fluoride ions with strong colorimetric as well as fluorometric response. Further, acidity of the H-bond donor sites was found to play pivotal role in the anion complexation mechanism and selectivity. Graphical abstract : SYNOPSIS Novel bis(pyrrole-benzimidazole) (PYBI) conjugates where pyrrole units are connected to the C2 position of benzimidazole via its α-position were synthesized and their anion recognition behavior were evaluated. Anion selectivity and mode of interaction was found to depend on the nature of the substituent and the spacer between the two PYBI units. [Figure not available: see fulltext.].