At the Molecular Level through Photophysical Studies: Structural Implications on the Reactivity of Dual-Site Sensitive Positional Isomers Toward a Gasotransmitter (H2S)

Abstract

A combined experimental and theoretical approach has enabled us to understand at the molecular level the importance of positional and electronic effects of chemical functionality present in molecular system that acts as an optical signaling agent. The present study demonstrates the structural implications of isomeric dual-site reactive (nitro and sulfonte ester groups) molecular probes (P1, P2, and P3) on optical signaling of hydrogen sulfide (H2S), a known emerging mediator in human physiological activities and diseases. The reactivity of these probes toward H2S was established using fluorescence signaling studies. The reductive interaction of H2S with nitro functionality of P2 resulted in the formation of orange fluorescent amine derivative P2′, while the nucleophilic S–O bond cleavage of sulfonate ester group of P3 produced sulfonothionic acid derivative P3′ as a green emissive fluorescent species. Crystal structure determination and structure–reactivity relationship studies demonstrated positional as well as electronic effects of nitro functionality on the reactivity of these probes. While the electronic effect is responsible for increasing the reactivity of sulfonate functionality, the accessibility of the reactive site by H2S is dictated by the steric factor. Although both −M (mesomeric) and −I (inductive) effects of nitro functionality are supposed to be prominent in P1 and P3, crystal structure analysis revealed a steric crowding on P1created by nitro group as well as out-of-plane arrangement of nitro group, which in turn makes P1 much less reactive than P3. In the case of P2, the probe is free from steric effect, but the weak −I effect and the absence of −M effect made sulfonate functionality nonreactive toward H2S. At the same time, slow reductive interaction of nitro group of P2 yielded orange emissive fluorescent species P2′.