Light-Induced Organic Transformations by Covalent Organic Frameworks as Reticular Platforms for Selective Photosynthesis

Photoassisted synthesis of value-added organic products has developed greatly in the last decades in response to the pressing need for a transition toward sustainable processes and renewable energy. One of the formidable challenges of the light-induced chemical steps is provided by the control of the catalytic efficiency and selectivity under photocatalytic conditions. An attractive perspective is foreseen by triggering the photoreaction events in confined spaces, wherein light harvesting and photocatalytic units are framed into functional architectures. Division of tasks among specialized compartments responds to a bioinspired strategy with the final aim to orchestrate the rate of concurrent and sequential events, to maximize performance while directing the reaction selectivity. Covalent organic frameworks (COFs) are a class of emerging materials that can meet these requirements, with the potential to bridge the existing gap between molecular and heterogeneous photocatalysis. Here, a rich pool of molecular building blocks and chemical linkages is available to afford crystalline porous solids with tailored photophysical properties emerging from the interconnected COF structure walls, while catalytic cofactors can be provided by engineering of the pore surface. In this Perspective, we highlight recent developments where COFs have been successfully employed as photocatalysts for selective organic transformations. The relationship between the COF reticular structure and its photocatalytic behavior is discussed, in terms of the light-conversion pathways and photoredox events, including electron and/or energy transfer mechanisms. The possible role of confinement effects, intrinsic in long-range order porous COF materials, remains largely unexplored in photocatalytic applications. New progress is expected to arise from close interdisciplinary cooperation involving synthetic chemistry and materials science communities.