Synthesis and Optical Characterization of a Rhodamine B Spirolactam Dimer

Amide derivatives of xanthene dyes such as rhodamine B are useful in a variety of sensing applications due to their colorimetric responses to stimuli such as acidity changes and UV light. The optical properties of these molecules can be influenced by intermolecular associations into dimeric structures, but the exact impact can be hard to predict. We have designed a covalently linked intramolecular dimer of the dye rhodamine B utilizing p-phenylene diamine to link the two dyes via amide bonds. The doubly closed spirolactam version of this dimer, RSL2, is isolated as a colorless solid. Under acidic conditions or UV exposure, RSL2 solutions develop a pink color that is expected for the ring-opened form of the molecule. However, NMR and single crystal diffraction data show that the equilibrium still prefers the closed dimer state. Interestingly, the emission profile of RSL2 shows solvatochromic blue fluorescence. Control studies of model compounds with similar structural motifs do not display similar blue fluorescence, indicating that this optical behavior is unique to the dimeric form. This behavior may lend itself to applications of such xanthene dimers to more sophisticated sensors beyond those with traditional binary on/off fluorescence profiles.