Solid-State Photochemical Reaction of Multisubstituted Thymine Derivatives

Solid-state photochemical reactions in crystals, known as topochemical reactions, are solvent-free green chemical reactions that produce stereospecific molecules. The photoreaction of thymine is interesting because the dimeric photoproduct can form four types of stereoisomers and when the dimer is formed in DNA helices it can cause skin cancers. We investigated the photoreaction of five multisubstituted thymine derivatives in the solid-state, which were designed with crystal engineering concepts to promote π–π stacking of benzene rings in the crystal. Powder X-ray diffraction analysis revealed that a <i>para</i>-substituted bis-thymine derivative was aligned along the <i>c</i> axis in the crystal and was susceptible to topochemical reaction to form a polymer, as previously reported. <i>Ortho</i>- and <i>meta</i>-substituted bis-thymine derivatives and a tetrakis-substituted derivative were found to be topochemically unreactive using both gel permeation chromatography (GPC) and X-ray crystal structural analysis. The tris-substituted thymine derivative was found to be topochemically reactive due to favorable crystal packing, which included ethanol molecules to form hydrogen bonding with one of the thymine moieties and stabilize the crystal packing. GPC and crystal structural analysis revealed that it could form tetramer at most via topochemical [2+2]-cycloaddition upon UV irradiation. Based on the crystal structure of the tris-substituted thymine derivative, the structure of the tetrameric photoproduct is expected to link via <i>cis–syn</i>, <i>trans–anti</i> and <i>cis–syn</i> cyclobutane isomers