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 para-substituted bis-thymine derivative was aligned along the c axis in the crystal and was susceptible to topochemical reaction to form a polymer, as previously reported. Ortho- and meta-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 cis–syn, trans–anti and cis–syn cyclobutane isomers.

Solid-state photo-reversible polymerization mechanism in crystals was studied, which can produce stereoregular polymer molecules in environmentally friendly solvent-free systems.

Polymer crystallization affects structural and mechanical properties of polymer thin films. In this study, we focused on the thermal annealing-induced crystallization in semi-crystalline poly(l-lactic acid) (PLLA) ultrathin films (referred as nanosheets) was investigated in terms of interfacial interaction of PLLA with air and substrate. The surface structure of the PLLA nanosheets observed by atomic force microscopy showed that roughness of the air-side surface increased due to crystallization of PLLA under thermal annealing, whereas that of the substrate-side surface changed little. The elastic moduli and the physical adhesiveness of the nanosheets also changed only on the surface of the air side from crystallization, in contrast to the substrate side. The X-ray diffraction studies of the PLLA nanosheets with different thickness showed that the crystalline contents steeply increased below ca. 200 nm. These results indicated that the crystallization was enhanced near the surface of the air side and restricted near that of the substrate side due to the different interfacial association of the polymer chains in the nanosheet.

In this study, the conversion of self-assembled structures into continuous polymeric structures by linking the self-assembled molecules using the [2 + 2]-cycloaddition reaction was investigated. Synthesized bio-inspired thymine-based bolaamphiphilic molecules were designed to force the interactions between two molecules to engage two thymines in their self-assembled structure to undergo a cycloaddition reaction. Thymine-based bolaamphiphilic molecules were designed and synthesized using different phenylene spacers based on aromatic substituents (ortho-) (meta-) (para-). The formed self-assembled structures from these molecules were characterized and compared using molecular mechanical simulations. Simulations were performed to discuss the relationship between the inter-and intramolecular cycloaddition sensitivity to different substituents. This study provides a strategy for creating higher molecular weight linear polymers by controlling the photocyclization sites within the self-assembly by spacers between thymines.