Electron transfer in the confined environments of metal-organic coordination supramolecular systems

The incorporation of electron transfer pairs in both ground and excited states (including electron transfer pairs in excited states and hydrogen or oxygen transfer pairs in ground states) into redox-active hosts with electronic acceptor or donor guests has led to development of a novel method of mimicking the photophysical properties and redox reactions of naturally occurring enzymatic systems. This occurs within the confined microenvironments of metal-organic capsules and metal-organic frameworks. These two types of coordination supramolecular host-guest systems can dock and separate electronic donor-acceptor pairs via closed through-space separation. Electron transfer within confined cavities, which is mainly controlled by spatial and kinetic effects, does not utilize a through-bond electron transfer pathway. In this review, we provide an overview of significant progress in the photophysical and catalytic applications of supramolecular host-guest systems with electron-transfer processes in confined environments. Special emphasis is placed on the action modes and regulatory factors that affect electron transfer between different components to produce enhanced photophysical or redox catalytic performance. Finally, the prospects for confined-environment electron transfer, its application to photophysics and catalysis, and the remaining challenges in this field are highlighted.