SINGLET TRIPLET INTERSYSTEM CROSSING IN 2,2' 5',2''-TERTHIOPHENE AND SOME OF ITS DERIVATIVES

During the course of a study devoted to the investigation of the existence of a relationship between the phototoxic and photophysical properties of 2,2':5',2''-terthiophene (1a) and related compounds, the singlet-to-triplet intersystem crossing quantum yields of compound la and six of its monosubstituted derivatives (1b-1g) were determined in 95% ethanol solution. The variations in the phototoxicity of these compounds against some insect and mite species are large in comparison with changes in the quantum yields of triplet state formation. The latter were very similar in the compounds studied (range, 0.88-0.99), with the exception of 5-formyl-2,2':5',2''-terthiophene. (1c) (PHI(T)=0.50). Moreover, a complete characterization of the S1 decays of compounds la-1g was obtained by combining these data with the corresponding fluorescence quantum yields and lifetimes. Finally, the consequences of the nature and position of the substituents on the measured triplet-triplet absorption and rate constants of the two S1 decay paths, i.e. fluorescence emission and intersystem crossing, are qualitatively discussed.