Selective Hg²⁺ Sensing Behaviors of Rhodamine Derivatives with Extended Conjugation Based on Two Successive Ring-Opening Processes

A novel class of rhodamine derivatives with two spirolactam moieties have been synthesized, and their two stereoisomers of cis- and trans-forms have been successfully separated and isolated, as well as structurally characterized by X-ray crystallography. Attributed to the successive ring-openings of two spirolactam moieties, different solution color, electronic absorption and emission responses were exhibited upon addition of various concentrations of mercury­(II) ion. Arising from two successive ring-opening processes in the presence of various concentration of Hg­(II) ion, two reporting states with different spectroscopic properties were suggested, that is, the first state showing pink color (absorption maximum at 496 nm) but no emission, while the second state giving purple color (absorption maximum at 593 nm) and red emission (emission maximum at 620 nm). The mechanism of such different spectroscopic responses was also proposed and has also been supported by computational studies. An extension of the present work to the study of the corresponding chemodosimeters from the compounds with two spirolactam groups has been made, in which a stoichiometric and irreversible Hg­(II)-promoted reaction of thiosemicarbazides was utilized to form 1,3,4-oxadiazoles.