Conjugated microporous polymers for heterogeneous visible light photocatalysis in aqueous medium
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Abstract
Visible light-driven photochemistry and photocatalysis have found significant broad utility in organic synthesis in recent years. Among the visible light-active photocatalytic systems, conjugated microporous polymers (CMPs) offer a promising alternative in comparison to traditional molecular transition metal photocatalysts due to their visible light activity, heterogeneous nature, high stability, ease of reusability and highly tunable electronic and optical properties. However, there are two main drawbacks which impair the broader application of CMPs: (1) their insoluble nature limits their processability in solvents and (2) the highly hydrophobic backbone reduces their compatibility with aqueous reaction media, which is considered as the cheapest and more sustainable reaction environment. In this work, we aim to address the above-mentioned problems. A number of different structural designs of CMPs are presented and thoroughly investigated as promising new approaches for the use of CMPs as water-compatible visible light-active and heterogeneous photocatalysts in aqueous media. First, a model of dispersed CMP nanoparticles (NPs) is presented as processability modification method. It is demonstrated that the nanostructure morphology can be finely adjusted by molecular structural design between various aromatic building block moieties in the polymer backbone. Second, it is shown that different molecular combinations can largely influence the energy band levels of the CMP NPs and result in great impacts on the photocatalytic performance, especially for enhancement in the generation of singlet oxygen, which in the best case, the production rate lies at ca. 0.14 mmol g-1 s-1. Third, CMP NPs have shown ability for the photocatalytic regeneration of nicotinamide cofactors under visible light irradiation. At last, another processability improvement method is achieved by dispersing conventional CMPs in water under the use of hydrophilic phase transfer agent during the photocatalytic process. Furthermore, the impact of different geometric settings on the photocatalytic efficiency of CMPs is also investigated. The study of the architectural variation of CMPs demonstrates that one dimensional polymer offers the highest photocatalytic efficiency for H2 evolution from water with a significant evolution rate achieved under visible-light irradiation at 420 nm. We believe that this study will pave the way for a broader application field for CMPs as a visible light metal-free photocatalyst.