Supramolecular photochemistry concepts highlighted with select examples

Abstract

‘Supramolecular photochemistry’ (SP) deals with a study of the properties of molecules in their excited states where the medium plays a significant role. While ‘molecular photochemistry’ (MP) deals with studies in isotropic solution, the SP deals with reactant molecules that interact weakly with their surroundings. The surroundings in general are highly organized assemblies such as crystals, liquid crystals, micelles, and host–guest structures. The behavior of exited molecules in SP unlike in isotropic solution is controlled not only by their inherent electronic and steric properties but also by the immediate surroundings. The weak interactions that control the chemistry include van der Walls, hydrophobic, CH⋯π, π⋯π and several types of hydrogen bonds. In this review the uniqueness of SP compared to MP is highlighted with examples chosen from reactions in crystals, micelles and host–guest assemblies. In spite of distinctly different structures (crystals, micelles, etc.) the influence of the medium could be understood on the basis of a model developed by G.M.J. Schmidt for photoreactions in crystals. The principles of reaction cavity model are briefly outlined in this review. There are a few important features that are specific to SP. For example, highly reactive molecules and intermediates could be stabilized in a confined environment; they enable phosphorescence to be observed at room temperature and favor chiral induction in photochemical reactions. Using such examples the uniqueness of SP is highlighted. The future of SP depends on developing efficient and unique catalytic photoreactions using easily available reaction ‘containers’. In addition, their value in artificial photosynthesis should be established for SP to occupy a center stage in the future.