Carbamodithioate-based fluorescent chemosensor for Hg(II): a staged response approach and investigation into the sensing mechanism.

Carbamodithioate-based compound C1 was designed and synthesized as a new fluorescent probe for Hg2+ ions. Upon the addition of Hg2+ ions, it displayed a rare staged response: the emission spectra of C1 first showed an apparent red-shift, followed by a dramatic decrease. To investigate the sensing mechanism, control compounds C2 with the same phenanthroimidazole unit and C3 with the same carbamodithioate functionality were synthesized. On comparison, the first step sensing process was ascribed to decreasing photoinduced electron transfer on the coordination of Hg2+ with the lone pair electrons of the nitrogen atom on the phenanthroimidazole ring. The affinity of Hg2+ and the carbamodithioate unit with four sulfur atoms then induced changes in intramolecular charge transfer efficiency and the second step fluorescent response.