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

4-Trifluoromethyl- or 6-bromo-substituted 7-diethylaminocoumarin-3-carboxam ide 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 co mparable compound lacking this group, but both the new coumarins have low f luorescence quantum yields (phi (f)). When a 4-trifluoromethyl substituent is present, the 3-carboxamide is unusually labile to hydrolysis. Brominatio n 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 c ompound 20. phi (f) for the latter compound is 100-fold greater than for it s diethylamino analogue 19, Fluorescence lifetime measurements support an i nterpretation based on the twisted intramolecular charge transfer (TICT) mo del to explain these large differences in phi (f).