Synthesis and fluorescence properties of substituted 7-aminocoumarin-3-carboxylate derivatives

Article abstract of DOI:10.1002/jhet.5570370608

4-Trifluoromethyl- or 6-bromo-substituted 7-diethylaminocoumarin-3-carboxamide derivatives 2 and 3, each containing a maleimide have been synthesized as potential fluorescent labeling reagents for thiol groups in proteins and their fluorescence properties have been determined. The 4-trifluoromethyl substituted compound 2 has a significantly greater Stokes shift than the comparable compound lacking this group, but both the new coumarins have low fluorescence quantum yields (φ_f). When a 4-trifluoromethyl substituent is present, the 3-carboxamide is unusually labile to hydrolysis. Bromination of ethyl 7-diethylaminocoumarin-3-carboxylate 17 gave the 6- and 8-bromo derivatives 18 and 19 respectively, and also the 8-bromo-7-monoethylamino compound 20. φ_f for the latter compound is 100-fold greater than for its diethylamino analogue 19. Fluorescence lifetime measurements support an interpretation based on the twisted intramolecular charge transfer (TICT) model to explain these large differences in φ_f.