Abstract
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.
Highlights
The remarkable performance, kinetic control, and selectivity of molecular transformations that occur in natural processes under mild conditions provide a unique paradigm for sustainable processes engineered under an abiotic environment
Chart 1. (a) Covalent organic frameworks (COFs)-5 Structure Based on the Reversible Formation of Boronate Ester. (b) Most Popular Dynamic Covalent Chemistry Employed for the Synthesis of COFs systems share a similar organization where the colocalization of multichromophoric antennas with reaction centers (RCs) allows a highly efficient light harvesting (LH) and energy conversion by orchestrating a cascade of photochemical events.[1,2]
An attractive strategy relies on photoredox catalysis requiring two fundamentals components: (i) an LH/ RC module consisting of a molecular sensitizer and/or a photoactive semiconductor, and (ii) a redox catalytic manifold targeting a specific reaction.[3−6] To this aim, the knowledge acquired on the light-induced events occurring in natural photosystems has been recently translated in the field of both homogeneous and heterogeneous photocatalysis applied to selective organic synthesis.[7,8]
Summary
The remarkable performance, kinetic control, and selectivity of molecular transformations that occur in natural processes under mild conditions provide a unique paradigm for sustainable processes engineered under an abiotic environment. A high chemically stable COF, Porsp2c-COF, fully sp[2] carbon conjugated, has been recently prepared through Knoevenagel condensation reaction by Wang and co-workers (Scheme 12b) and successfully tested as a heterogeneous photocatalyst in the visible-light aerobic oxidation of amines (Scheme 12a).[46] Por-sp2c-COF displayed higher catalytic activity than its imine-based COF homologue, 2D-PdPor-COF, which indicates that the carbon conjugated framework efficiently stabilizes the charge carrier and, facilitates the ET processes. The hydrazone-based COF, TFB-COF, has been employed by Yang and co-workers as visible-light photocatalyst for cyclization reactions, namely, (i) radical addition-cyclization of 2-aryl phenyl isocyanides for the synthesis of 6-substituted phenanthridines,[52] and (ii) the oxidative construction of N−S bond for the preparation of 1,2,4-thiadiazoles (Scheme 22).[53] From good to excellent product yields have been obtained in both reactions tested for a wide substrate scope making the TFB-COF as efficient as the molecular photocatalyst alternatives. The conversion of trans to cis-stilbene decreased from 90% to 8% when switching from the crystalline TpTt to the amorphous counterpart, which may indicate that the reaction is occurring within the COF pores or at the crystallite surface since the length of the substrate (∼1.1 nm) is not compatible with the determined TpTt pore size distribution (η ≈ 1.3 nm)
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