Evidence for Aggregation-Induced Fluorescence Exhibited by Rhodamine B Spirolactams

Rhodamine B amide derivatives (RADs), in which an amide moiety replaces the carboxylic acid in rhodamine B, exhibit two conformers in solution: a ring-closed spirolactam, or a ring-opened, fluorescent amide. In this work, we show that multiple RAD species exhibit concentration-dependent blue fluorescence in solution conditions which favor ring-closed RAD conformers. All RAD species exhibited µM threshold concentration requirements for onset of blue fluorescence. Additionally, a density gradient separation performed on a RAD p-phenylenediamine-linked dimer successfully isolated blue (ring-closed) and orange (ring-opened) fluorescence, with the blue fluorescence band indicative of a larger macromolecular species relative to the orange band. Taken together, this data suggests that the blue fluorescence observed for RAD species in closed-ring conformers is aggregation-induced. Further control experiments and literature review suggest the blue fluorescence may be due to an aggregation-promoted intramolecular charge transfer mechanism.