Chemical insight from crystallographic disorder-structural studies of supramolecular photochemical systems. Part 3. The β-cyclodextrin–7-hydroxy-4-methylcoumarin inclusion complex: direct observation of photodimerization by X-ray crystallography

As part of an ongoing structural study of supramolecular photochemical β-cyclodextrin(β-CD)–coumarin derivatives systems the crystal structure of the β-CD–7-hydroxy-4-methylcoumarin complex has been determined for the unreacted crystal and for a crystal exposed to UV light for 6 days. The 7-hydroxy-4-methylcoumarin molecules form a crystalline 2∶2 host–guest (H–G) complex with β-CD. The guest molecules are disordered, with pairs of guest molecules similarly oriented with respect to each other distributed over three sites. The guest molecules are oriented with their reactive double bonds separated by about 3.5–3.7 A and oriented parallel to each other. Crystallographic analysis of the photoreacted crystal shows the reaction proceeds within the crystal to produce the topochemically predicted anti-HT photodimer. The system is appropriately described as a “reaction nano-vessel”, where the inter-molecular interactions of import to the outcome of the reaction are confined to a single β-CD dimer cavity. Ab initio molecular orbital calculations of the conformation of the anti-HT 7-hydroxy-4-methylcoumarin photodimer show that the preferred gas-phase geometry for the cyclobutyl ring is planar, the same as that observed in the β-CD complex. This provides further evidence for classifying the β-CD dimer environment as non-constraining.