Co-crystals of a salicylideneaniline: Photochromism involving planar dihedral angles

A study was conducted to demonstrate the application of co-crystallization to induce photochromism in a salicylideneaniline (SA) derivative that was thermochromic and photoinactive. The SA 2-((4-hydroxyphenylimino)methyl)phenol (SHA) was used as a hydrogen-bond donor through use of a hydroxyl group located in the para position to conduct the investigations. The origin of the photochromism was attributed to the presence of nearest-neighbor edge-to-face interactions between the SA and co-crystal former (CCF), which served to prohibit the generation of 'closed structures'. The photochromism arose from a structural mismatch between the CCF and SHA that supported an open structure. The study also investigated the incorporation of additional substituents on the rings of the SA and determined how additional CCFs affected photochromic and thermochromic properties of SA-based materials.