有机添加物对7-胺基香豆素分子发光的增强效应

测定了在有卤代苯等存在时,一些7-胺基香豆素在不同溶剂中的吸收和发射谱及荧光寿命。所得结果表明,具有给电子能力的有机添加物可对非刚性7-胺基香豆素衍生物在极性溶剂中的荧光起增强作用。基于荧光衰变动力学的分析及添加物的荧光增强能力与其氧化电位的线性关系,提出荧光增强作用的机理是添加物通过分子间电荷转移作用使香豆素分子激发态不发光的TICT态向发光的ICT态转化所致。

ORGANIC ADDITIVE-INDUCED FLUORESCENCE ENHANCEMENT OF 7-AMINOCOUMARIN DERIVATIVES

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.