Journal
TETRAHEDRON LETTERS
Volume 55, Issue 30, Pages 4222-4226Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.tetlet.2014.05.124
Keywords
2 ',7 '-Diphenyl rhodamine dyes; Emission; Absorption; MSNT coupling; Lactonization
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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 (pK(a) = 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 (pK(a) = 6.95). (C) 2014 Elsevier Ltd. All rights reserved.
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