SPECTROSCOPIC STUDIES OF 7-DIETHYLAMINO-3-STYRYL COUMARINS

The absorption and fluorescence characteristics of four 7-diethylamino-3-styryl coumarins (dyes I-IV), containing either an extended formyl or acetyl substituent at the 3-position, in solvents of different polarity (chloroform, dimethylsulphoxide and dimethyiformamide) were studied with regard to the influence of the flexibility of the chromophoric system in both the ground and excited states. The photophysical measurements show that the dyes are very weakly fluorescent. The fluorescence lifetimes are in the region of 0.01 ns and the non-radiative rate constant is very high when compared with the radiative rate constant. These observations indicate that the reduction of the acceptor strength of the substituent at the 3-position, as in the present case, in a 7-diethylaminocoumarin dye does not always result in sustained fluorescence in polar, aprotic solvents. Therefore these dyes are not technically important as laser dyes.