The impact of dihedral angle in aryl groups on the photocyclization reactivity of inverse-type diarylethenes†

Photoreactivity in crystals is one of the essential properties for creating photo-functional crystalline materials. This study explores the impact of the dihedral angle in aryl groups on the photocyclization reactivity of inverse-type diarylethenes, both in solution and crystalline phases. By synthesizing various diarylethene derivatives with different dihedral angles, the relationship between structural geometry and photoreactivity is systematically examined. We find that larger dihedral angles between the thiophene and phenyl rings enhance photocyclization reactivity in solution, indicating that destabilized π-conjugation lowers the activation barrier. In fact, ultrafast spectroscopy confirms that the cyclization time constant decreases with larger dihedral angles. In the crystalline phase, X-ray crystallographic analysis shows that all diarylethene derivatives adopt ideally photoreactive anti-parallel conformations, but only crystals with a dihedral angle exceeding approximately 81° exhibit photocyclization. These findings indicate that a certain threshold dihedral angle is essential for photocyclization to occur in crystals. The results of this work provide new insights into the role of molecular geometry in photoreactivity and offer a strategy for designing functional photochromic materials that operate efficiently in the solid state.