New rhodamines with changeable π-conjugation for lengthening fluorescence wavelengths and imaging peroxynitrite

Rhodamines, a group of excellent fluorophores, have been widely used to design various fluorescent probes; however, their maximum emission wavelengths are relatively short for bioimaging. Herein, we report a new kind of rhodamine derivatives characterized by the dibenzo[1,4]oxazepine core, which can selectively react with peroxynitrite (ONOO−), generating oxazines with much longer analytical wavelengths. These derivatives are readily synthesized in one step via a similar Stieglitz rearrangement; the presence of the 1,4-oxazepine ring is confirmed by single-crystal X-ray. Most importantly, the derivative (ON-RB) from rhodamine B, upon reaction with ONOO−, releases the oxazine moiety that emits in the near-infrared wavelength region. Using ON-RB, we have monitored the generation of ONOO− in macrophage-derived foam cells, revealing that ONOO− might be formed mainly in lipid droplets. The new chemistry of the dibenzo[1,4]oxazepine ring formation, together with its specific reaction with ONOO−, may provide a convenient strategy to develop rhodamine-based probes with longer wavelengths.