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

Photophysical properties and characteristics of electronic energy transfer of coumarin derivatives substituted by bulky group in position 3 and 7-diet hylamino-4-methyl coumarin (Coumarin 1) were investigated in solution and p olymer matrices. The bulky electron donating groups were: phenyl-, phenylth io-, 2-methylphenylthio-, 2,6-dimethylphenylthio-, dimethylamino- and benzo ylamino- in position 3. Fluorescence of coumarin derivatives was quenched b y polar methanol with bimolecular rate constant (k(q)) larger than the diff usion controlled Limit indicating static quenching. The increased polarity of mixed solvent prefers processes leading to intramolecular charge transfe r (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 radiu s (R-0) were determined for derivatives of coumarin as donors and N-oxyl ra dical as acceptor. For selected pairs, the experimental and theoretical val ues of the electronic energy transfer for k(ET) and R-0 were compared in va rious solvents like cyclohexane, heptadecane and methanol in order to deter mine the type of the electronic energy transfer, influence of the solvent a nd number of paramagnetic centers on this process. The resonance transfer s eems 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 qu enching in solid phase. There is strong indication that resonance energy tr ansfer is operative as well. (C)1999 Elsevier Science S.A. All rights reser ved.