Photophysics and photodimerization of 6,5′-dimethylangelicin in different solvents

The photophysical and photochemical behaviour of 6,8-dimethyl-2H-furo[2,3-h]chromen-2-one (6,5′-dimethylangelicin, 6,5′-DMA) was studied by steady state and pulsed techniques and by semiempirical calculations. The fluorescence characteristics and the triplet state properties were determined in solvents of different polarity/proticity. The obtained values of fluorescence quantum yields and lifetimes together with the triplet formation quantum yield indicate that the energy gap between the two lowest excited states controls the decay pathways of the singlet. In non-polar and non-protic solvents the internal conversion is almost the sole decay process of the singlet, while in protic solvents the intersystem crossing reaches substantial quantum yields. The effect of ground state concentration on the fluorescence lifetime suggested the occurrence of aggregation while the triplet lifetime was quenched and the rate constant of self-quenching was determined. 5′-DMA photodimerization was investigated in different solvents and in different experimental conditions. Two pyrone-pyrone and one furan–pyrone photodimers of 6,5′-DMA were the main photoproducts detected in the irradiated solution. The effects of the 6,5′-DMA concentration and of oxygen on the amounts of the dimers obtained together with the photophysical behaviour gave indications on the formation mechanism of different dimers.