Template-Assisted Visible Light-Induced [2 + 2] Photodimerization in a Pseudopolymorphic Binary Solid: Topotactic Transformation vs Photoinduced Crystal Melting

[2 + 2] Photodimerization has been an evergreen topic in organic crystal engineering for a long time. Recent exploitation of an age-old molecule of benzilic acid (BZA) as a template to control such reactions in the solid state accidentally led to a photoinduced crystal-to-liquid transition, also known as photoinduced melting. In this work, we describe two pseudopolymorphs of a BZA-based multicomponent system that shows drastically different photo responses upon photoirradiation. While the native form exhibits topotactic single-crystal to single-crystal transformation, the DCM-solvate is sustained through a photoinduced melting upon photoirradiation. The observation of such drastically different photoresponses in a pseudopolymorphic system is unique and offers an opportunity to investigate responsible structural factors, which can pave the way for future crystal engineering strategies.