Spectral measurement on selected 7-aminocoumarin derivatives demonstrates that both their absorption and fluorescence emission in a variety of solvents exhibit no detectable change in spectral profile upon addition of certain organic compounds such as phenyl halides

benzene

toluene and cyclohexane. In several cases

however

the fluorescence intensity and lifetime are increased

accompanied by a small shift of emission peak to shorter wavelength

and this fluorescence enhancement is clearly depended on the substitution pattern of aminocoumarin at 7-position as well as on the solvents used

i.e

the fluorescence enhancement can be observed for coumarin with rotatable amino substituent in polar solvents. As a typical example

the fluorescence intensity of 4-trifluoromethyl-7-diethyl-aminocoumarin in methanol is enhanced by a factor of two upon addition of 1

5M phenyl iodide

which is contrary to the general consideration of so-called external heavy atom effect. Moreover

the similar enhancement is also observed in tht presence of compounds without halogen atom

although the fluorescence intensity increases in a lesser extent. This finding can not be interpreted in terms of changing polarity of the solvent due to addition of this compounds

otherwise it. becomes difficult to understand why a different enhancement can be observed in the presence of phenyl halides of the same polarity at the same concentration

although it is not excluded tht possible contribution from changing polarity of the medium in general

particularly

when the compounds of low polarity are used as the additives. On the other hand

the dependence of observed fluorescence enhancement on both the substitution pattern of aminocoumarins and the solvents used leads us to realise this unexpected phenomenon by an argument based on the possible effect of additive on the dynamic equilibrium between the excited cou-marin molecules in fluorescent intramolecular charge transfer state(ICT) and its non-emissive rotamer with twisted conformation(TICT)

which have been suaggested by us and G.Jones to be established for nonrigid aminocoumarins in polar solvents. In this connection we postulate that the formation of inter-molecular charge transfer complex between excited aminocoumarin and additive molecule shifts the dynamic equilibrium to the fluorescent ICT state.This postulate for understanding the fluorescence enhancement observed is verified by the kinetic analysis of relevant species decay and the dependence of the observed fluorescence enhancement on the oxidation potential cf the additive used.