Formation of stable naphthalenediimide radical anion: Substituent-directed synergistic effects of hydrogen bonding and charge transfer interactions on chromogenic response towards hydrazine

Abstract

Naphthalimide-based chromogenic probes have been developed with different electron-withdrawing terminal functional groups. All the compounds showed a dose-dependent gradual change in solution color from pale yellow to dark yellow to light brown and finally dark brown upon the addition of hydrazine hydrate. Concomitantly, the appearance of intense green fluorescence is noticed. The mechanistic investigations, as supported by 1H NMR and ESR studies, indicate the hydrazine induces formation of radical anion species in the reaction medium. The presence of electron-withdrawing substituents not only improves the degree of chromogenic response but also the stability of the radical species. It is found that the substituents take an active role in the adsorption of hydrazine via hydrogen bonding interactions, which facilitates the subsequent charge transfer from hydrazine to probe molecules. In this context, a detailed density functional theory (DFT) based first-principle calculations are performed to theoretically study the cooperative effect of hydrogen bonding interaction in the sensing of hydrazine. It is observed that on changing substituents, significant changes in the numbers of possible chemisorption sites and binding energies at individual sites for hydrazine, as well as the magnitude of charge transfer from the hydrazine at individual binding sites. Finally, chemically modified biodegradable paper strips have been developed on on-location naked-eye sensing of hydrazine.