Chemical insight from crystallographic disorder-structural studies of supramolecular photochemical systems. Part 2. The β-cyclodextrin–4,7-dimethylcoumarin inclusion complex: a new β-cyclodextrin dimer packing type, unanticipated photoproduct formation, and an examination of guest influence on β-CD dimer packing

Abstract

As part of an ongoing structural study of supramolecular photochemical β-cyclodextrin(β-CD)–coumarin derivatives systems the crystal structure of the β-CD–4,7-dimethylcoumarin complex has been determined at 13 K and complemented with ab initio molecular orbital calculations on selected guest coumarin molecules. The 4,7-dimethylcoumarin molecules form a crystalline 2∶2 host–guest (H–G) complex with β-CD that is appropriately described as a “reaction nano-vessel” in which the inter-molecular interactions of import to the outcome of the reaction are confined to a single β-CD dimer host. Structure determination of the isolated photoproduct showed the product formed is the anti-HT photodimer. Production of this dimer is only possible with much rearrangement of the guests. This appears to occur because of a preferential spatial fit of this product to the β-CD dimer cavity. The crystal structure reveals a new β-CD dimer packing pattern, designated tetrad type, with reduced reaction tube intermolecular interactions among guest molecules. Tetrad packing is characterized in comparison with the previously reported β-CD dimer packing types. Guest influence on packing of β-CD dimers is examined in β-CD inclusion complexes with different derivatized coumarins. To probe basic molecular properties giving rise to inter-molecular interactions influencing crystal packing, ab initio molecular orbital calculations for the guest coumarin molecules were carried out to examine electrostatic interactions.