Spectral properties of coumarin derivatives substituted at position 3. Effect of polymer matrix

The spectral properties of coumarin (2H-1-benzopyran-2-one) derivatives substituted at position 3 with bulky substituents, such as phenyl, phenylthio and benzylthio, were investigated in solvents of different viscosity and in polymer matrices. In order to evaluate the heavy atom effect, some derivatives were substituted by bromine at positions 6 and 8. The absorption spectra exhibit a broad band with a maximum at 340 nm (log epsilon approximate to 4) which is not influenced by the medium. Coumarin derivatives in solvents of low viscosity exhibit broad band fluorescence with a low quantum yield of 0.005 and a lifetime shorter than 0.1 ns. In solvents of higher viscosity and in polymer films, the quantum yields are 0.05-0.2 higher and the lifetime is increased by approximately 2 ns. On the basis of the spectral data, the rate constant of radiationless deactivation k(nr) is 10(11) s(-1) in solution and 4 x 10(8) s(-1) in the polymer matrix. The activation energy of the radiationless process in 3-phenylcoumarin is about 34 kJ mol(-1) in glycerol. The activation energy of viscosity change of glycerol is 72 kJ mol(-1). This comparison indicates that the torsional rotation of the substituent at position 3 is partially influenced by the friction viscosity of the medium. (C) 1997 Elsevier Science S.A.