Synthesis, chemical reactivity, and photophysical properties of 2′,7′ phenylated rhodamine dyes

While exploring water soluble rhodamine based fluorescent polymeric systems for biological imaging applications we came across new rhodamine derivatives that possess interesting optical properties. We report the synthesis of three different 2′,7′-diphenylated rhodamine derivatives (1-3) with distinct photophysical properties. The three rhodamine derivatives differ by the number of methyl groups present on the nitrogens and their absorption maxima are red-shifted on increased methylation. We observed an unusual inertness of these compounds toward traditional DCC-DMAP esterification conditions, which we attribute to the ease of lactonization in the presence of even minute amounts of the nucleophile/base DMAP (pKa = 9.2). Synthesis of acrylate esters was successfully accomplished using MSNT (1-(Mesitylene-2-sulfonyl)-3-nitro-1,2,4-triazole) coupling conditions using a much milder nucleophile/base, for example, N-methyl imidazole (pKa = 6.95).