Characteristics of the excited states of 3-substituted coumarin derivatives and transfer of electronic energy to N-oxyl radicals

Abstract

Photophysical properties and characteristics of electronic energy transfer of coumarin derivatives substituted by bulky group in position 3 and 7-diethylamino-4-methyl coumarin (Coumarin 1) were investigated in solution and polymer matrices. The bulky electron donating groups were: phenyl-, phenylthio-, 2-methylphenylthio-, 2,6-dimethylphenylthio-, dimethylamino- and benzoylamino- in position 3. Fluorescence of coumarin derivatives was quenched by polar methanol with bimolecular rate constant ( k q) larger than the diffusion controlled limit indicating static quenching. The increased polarity of mixed solvent prefers processes leading to intramolecular charge transfer (ICT) or twisted intramolecular charge transfer (TICT) which effectively compete with fluorescence. The experimental and theoretical values for the rate constants of the electronic energy transfer ( k ET) and critical radius ( R 0) were determined for derivatives of coumarin as donors and N-oxyl radical as acceptor. For selected pairs, the experimental and theoretical values of the electronic energy transfer for k ET and R 0 were compared in various solvents like cyclohexane, heptadecane and methanol in order to determine the type of the electronic energy transfer, influence of the solvent and number of paramagnetic centers on this process. The resonance transfer seems to be the prevailing mechanism of energy transfer. In non-polar glassy polystyrene matrix at temperature lower than T g, the energy transfer from coumarin donor to N-oxyl acceptor follows the Perrin’s model for static quenching in solid phase. There is strong indication that resonance energy transfer is operative as well.