Photochemistry in a capsule: controlling excited state dynamics via confinement

Exerting control on excited state processes has been a long-held goal in photochemistry. One approach to achieve control has been to mimic biological systems in Nature (e.g., photosynthesis) that has perfected it over millions of years by performing the reactions in highly organized assemblies such as membranes and proteins by restricting the freedom of reactants and directing them to pursue a select pathway. The duplication of this concept at a smaller scale in the laboratory involves the use of highly confined and organized assemblies as reaction containers. This article summarizes the studies in the author's laboratory using a synthetic, well-defined reaction container known as octa acid (OA). OA, unlike most commonly known cavitands, forms a capsule in water and remains closed during the lifetime of the excited states of included molecules. Thus, the described excited state chemistry occurs in a small space with hydrophobic characteristics. Examples where the photophysical and photochemical properties are dramatically altered, compared to that in organic solvents wherein the molecules are freely soluble, are presented to illustrate the value of a restricted environment in controlling the dynamics of molecules on an excited state surface. While the ground state complexation of the guest and host is controlled by well-known concepts of tight-fit, lock and key, complementarity, etc., free space around the guest is necessary for it to be able to undergo structural transformations in the excited state, where the time is short. This article highlights the role of free space during the dynamics of molecules within a confined, inflexible reaction cavity.