Singlet probes based on coumarin derivatives substituted in position 3; spectral properties in solution and in polymer matrices

Spectral properties of coumarin derivatives (2H-1-benzopyran-2-one), substi tuted with a bulky group in position 3, were investigated in solvents and i n polymer matrices. The bulky electron donating groups in position 3 were p henyl-, phenyltio-, 2-methylphenyltio-, 2,6-dimethylphenyltio-, benzyl-, ph enoxy-, dimethylamino- and benzoylamino-. The absorption spectra of all the derivatives are dominated by a broad band with a maximum at 340 nm (log ep silon similar to 4.0), which were not influenced by the polarity or viscosi ty of the environment. The fluorescence of these derivatives is strongly in fluenced by the polarity of the solvent and viscosity of the surroundings. In high viscosity solvents and in polymer matrices, the quantum yields of a round 0.1 and a lifetime of around 2 ns was observed. In low viscosity non- polar solvents such as cyclohexane, the quantum yields lower than 0.01 were observed. The fluorescence of coumarin probes was quenched by polar methan ol with a bimolecular rate constant, k(q), larger than diffusion controlled limit indicating static quenching. The increased polarity of the mixed sol vent introduces processes such as intramolecular charge transfer or twisted intramolecular charge transfer which effectively compete with fluorescence . The dependence of quantum yield of fluorescence on temperature was determ ined in viscose solvents and polymer matrices. The activation energy of rad iation-less process (E-a) increased in going from phenyl to more the bulky 2,6-dimethylphenyltio group in non-polar high viscosity polybutene oil and polar glycerol supporting the idea that the radiation-less process is relat ed to rotation of the group in position 3. The E-a value is lower in rubber y matrices such as polyoctenamer or atactic polypropylene than in glassy po lymers such as polystyrene, poly(methyl methacrylate) or polyvinyl chloride . 3-Phenylcoumarin, due to its spectral properties, seems to be the most su itable probe for monitoring microviscosity in polymers. (C) 1999 Elsevier S cience Ltd. All rights reserved.