PHOTOPHYSICAL PROPERTIES OF 4-ALKYLCOUMARIN AND 7-ALKOXYCOUMARIN DERIVATIVES - ABSORPTION AND EMISSION-SPECTRA, FLUORESCENCE QUANTUM YIELD AND DECAY TIME

4-Alkyl- and 7-alkoxycoumarin derivatives have been investigated for t heir spectroscopic properties in three different solvents. The substit uents are methyl, ethyl, propyl and butyl groups. Absorption and emiss ion spectra, quantum yields and mean fluorescence decay times have bee n determined. The spectroscopic studies together with calculations of the radiative transition probabilities indicate that the 4-alkyl- and 7-alkoxy substituents do not change the symmetry of the electronic clo ud of the coumarin skeleton. The changes observed for the rates of rad iative transitions and also for the positions of the absorption and em ission maxima can be explained by changes in the molecular electronic transition moment, caused by the weak donation potential of the substi tuents. A strong dependence of the quantum yields and mean fluorescenc e decay times on solvent viscosity has been found, leading to the sugg estion that torsional rotations of the alkyl and alkoxy substituents s ignificantly increase the internal non-radiative energy conversion rat e.