Photodimerization of 1-alkylthymines crystallized from acetonitrile solution

Crystallization of 1-octylthymine from acetonitrile solution gave needles and plates. The needles gave the photodimer by irradiation of UV light, but the plates did not give the photodimer. Crystal structures of needles and plates were determined by X-ray analysis. There were two pairs of thymine, which gave the photodimers in the needles of 1-octylthymine. The isomer ratio of the photodimers indicated that thymine rotated disrotatory during the photodimerization reaction to give Irans-anti, cis-anti, and trans-syn isomers. The crystal structure of the plates from acetonitrile solution was identical to the structure of plates obtained from ethanol solution, which was inactive for the photodimerization as reported in the previous paper. When the alkyl group became long, the formation of trans-syn photodimer decreased because strong interaction with the alkyl group inhibited moving of molecules. The photodimerization of I-alkylthymine was concluded to occur when the rotation of the thymine is possible in the crystal.