Photodimerization of 5,7-dimethoxycoumarin

Abstract

Direct irradiation of 5,7-dimethoxycoumarin (DMC) under N$_2$ produced three dimers such as anti head-to-head, syn head-to-head, and syn headto-tail. Relative yields of the three dimers changed with solvent polarity varying. In nonpolar solvents such as benzene, relative yield of anti head-to-head dimer is larger, while in polar solvents such as methyl alcohol syn head-to-tail and syn head-to-head dimer. Also, as solvent polarity increases from benzene to methyl alcohol, overall yields of dimers increase. In nonpolar solvents, rata of self-quenching is faster, while in polar solvents the rate of dimerization relative to self-quenching is faster. In the presence of triplet sensitizers such as benzophenone, the anti head-to-head dimer is formed exclusively in non-polar solvents but with traces of syn dimers in polar solvents. From sensitization and quenching experiments using azulene. We concluded that anti head-to-head dimer was formed via excited singlet state and syn dimers were formed via both excited singlet and triplet states. MAC 89463 9127