Role of free space and weak interactions on geometric isomerization of stilbenes held in a molecular container.

Photochemical geometric isomerization of olefins is long known to depend on the medium in which it occurs. Highest selectivity occurs in flexible biological systems as well as in inflexible crystals. We present results in this report that suggest the isomerization is selective even in an isotropic flexible aqueous medium provided it occurs within an isolated water-soluble inflexible reaction cavity. By examining the photochemistry of twelve stilbenes (trans and corresponding cis isomers) included in an organic cavitand octa acid we have been able to probe the role of 'free volume', 'weak interactions' and 'supramolecular steric effects' on the geometric isomerization process. Geometric isomerization becomes selective when the olefin's mobility is restricted by the medium through weak interactions, supramolecular steric effects and controlled free space (free volume).