Abstract

The use of 7-aminocoumarin dyes as efficient laser dyes in the blue-green spectral region and as fluorescent markers for synthetic peptides in biological control is well-known. The laser efficiency of 7-aminocoumarins laser dyes is due to high fluorescence quantum yields and to a large Stokes shift between the absorption spectrum and the fluorescence spectrum due to the large dipole moment of the fluorescent excited state. Surprisingly, the emissive properties of similar coumarins present very peculiar differences. For example, a high fluorescence quantum yield and a fairly long lifetime of 7-amino-4-methyl-coumarin (coumarin 120) which possesses an unsubstituted -NH₂ group are well recognized in most organic solvents and water. Conversely, in polar and hydrogen-bonding solvents, the fluorescence quantum yield and lifetime of 7-diethylamino-4-methyl-coumarin (coumarin 1) which is substituted by two ethyl groups on the nitrogen are strongly reduced. This effect is not yet understood although various models for radiationless deactivation have been proposed: relaxation to a nonradiative twisted internal charge-transfer (TICT) state [1], structural change of the amino group from a planar N⁺ sp² hybridation to a pyramidal N⁺ sp² hybridation, called open-closed umbrella-like motion (ULM) [2].

© 1996 IEEE