A Deep-Red to Near Infrared (NIR) Fluorescent Probe Based on a Sulfur-Modified Rhodamine Derivative with Two Spirolactone Rings.

We report the synthesis, characterization, and photophysical and ion-binding properties of deep-red to near-infrared (NIR) fluorescent rhodamine derivatives, bearing two spirolactone rings and substitution of the oxygen atoms in the xanthene ring with sulfur atoms (1-S). The diastereoisomeric cis- and trans-forms of the rhodamine derivative were separated and the cis-form (cis-1-S) was structurally characterized by X-ray crystallography. Upon treatment with Hg2+ ion, cis-1-S was converted into the dual spirolactone ring-opened species, resulting in significant color change and fluorescence enhancement. Substitution of the oxygen atoms with sulfur and extended π-conjugation across the fused six-membered rings upon the two rings-opening processes in the presence of Hg2+ ion led to a significant red-shift of absorption (623 nm) and fluorescence (706 nm) peaks, compared to the ordinary rhodamine. Furthermore, the intracellular Hg2+ -sensing properties of the cis-1-S have been studied by confocal microscopy.