Photodimerization of 4,5',8-trimethylpsoralen

Abstract

The photodimerization of 4, 5, 8-trimethylpsoralen (TMP) in dichloromethane solution has been investigated. Three products have been isolated and characterized: 1) a non-fluorescent homodimer resulting from the C$_4$-cycloaddition at the pyrone end, with a trans-anti configuration, 2) a bicyclomer resulting from a double cycloaddition between pyrone and furan moieties, and 3) a fluorescent heterodimer resulting from the C$_4$-cycloaddition between the furan end of one TMP moiety and the pyrone end of the other, with cis-syn configuration. The stereochemical structures of photoproducts were assigned by NMR utilizing a shift reagent (EuFOD) and NOE experiments. The pyrone$<>$pyrone, trans-anti dimer was formed from the non-stacked species in organic solution and the mechanism of bicyclomer formation is similar to that of the photoreaction between psoralens and pyrimidine bases. The formation of the furan$<>$pyrone dimer represents an unusual type of dimerization. Only 6,5-dimethylangelicin and 3-carbethoxypsoralen seem to show the similar behavior. Unusual Lewis acid effect was observed in the photodimerization of TMP. Increasing concentration of $BF_3-OET_2$ leads to an increase of foluorescence intensity and a decrease of quantum yield for the photodimerization of TMP. These results indicate that the energy level ordering is reversed in the TMP-Lewis acid complex so that the intersystem crossing becomes competitive with the fluorescence. Thus the triplet-state photodimerization is decreased.