A new [2 + 2] photodimerization of 5-chloro- and 5-methyl-2-pyridone in their inclusion complexes with 1,1'-biphenyl-2,2'-dicarboxylic acid as a model for DNA damage by photodimerization of its thymine component.

As a model for DNA damage by photodimerization of its thymine component, a new [2 + 2] photodimerization of 5-chloro and 5-methyl-2-pyridone to the corresponding cis-anti-dimers as their inclusion complexes with 1,1'-biphenyl-2,2'-dicarboxylic acid was found, and the mechanism of this stereoselective solid state reaction was studied by X-ray analysis.