Isocyano-functionalized, 1,8-naphthalimide-based chromophore as efficient ratiometric fluorescence probe for Hg2+ in aqueous medium

A type of isocyano-functionalized, 1,8-naphthalimide-based chromophore ( M2 ) was successfully synthesized from its primary amine-derivatived precursor ( M1 ) compound. The chemical structure of M2 was verified by FT-IR, NMR and HRMS analyses. The presence of electron-withdrawing isocyano group brought significant influence to the optical properties of M2 . Distinct from the yellowish green emission of its precursor ( M1 ), M2 emitted ultraviolet light (with the emission peak at ∼389   nm, excited by 352   nm) in THF-water [V THF /V water   =   3/7, buffered with 5   mM sodium dihydrogen phosphate-disodium hydrogen phosphate (PB), pH   =   7.4] co-solvent system. Interestingly, M2 exhibited distinctive and selective optical response (with significant enhancement of emission at ∼560   nm) toward Hg 2+ . In Hg 2+ titration process, PL spectra of M2 displayed contiguous ratiometric alteration (simultaneous decreasing at 389   nm and increasing at 560   nm). Hg 2+ -assisted hydration of isocyano group to amino group on M2 was proposed as the plausible probing mechanism in this current case.