Photochemistry in an organic capsule: communication across a molecular wall

Host-guest chemistry has witnessed a renaissance under the name 'supramolecular chemistry' since the award of Nobel Prize to Cram, Lehn and Pederson in 1987. During the last five decades, threading several concepts of host-guest chemistry and physical organic chemistry that have existed for over a century, 'supramolecular chemistry' has enabled altering the chemical and physical behavior of molecules, not only in the ground but also in the excited state. One of the problems of this approach is that most common host systems are 'porous (partially open)' resulting in reactions occurring both within and outside the host in organic solvents, leading to poor selectivity. Given the current thrust on performing molecular transformations under environmentally friendly and sustainable conditions, light as a reagent and water as the medium is a good approach. Water insolubility and porosity in host-guest complexes could be overcome with a water-soluble host that fully surrounds the reactant molecule. In this context, octa acid (OA) that is water soluble and forms a closed capsule by including organic guest molecules within it is noteworthy. One of the drawbacks of confining a molecule is that it loses its freedom to interact with other molecules in solution. In this Perspective using OA as the host, we provide examples where the confined molecules are able to communicate with free unconfined molecules in water leading to spin, electron and energy transfer. The proof of principle established here should work with even larger capsules when they become available.