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 diastereoisomericcis- andtrans-forms of the rhodamine derivative were separated and thecis-form (cis-1-S) was structurally characterized by X-ray crystallography. Upon treatment with Hg(2+)ion,cis-1-Swas 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 pi-conjugation across the fused six-membered rings upon the two rings-opening processes in the presence of Hg(2+)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 thecis-1-Shave been studied by confocal microscopy.