The use of hydrogen as a fuel, when generated from water using semiconductor photocatalysts and driven by sunlight, is a sustainable alternative to fossil fuels. Polymeric photocatalysts are based on Earth-abundant elements and have the advantage over their inorganic counterparts in that their electronic properties are easily tuneable through molecular engineering. Polymeric photocatalysts have developed rapidly over the past decade, resulting in the discovery of many active materials. However, our understanding of the key properties underlying their photoinitiated redox processes has not kept pace, and this impedes further progress to generate cost-competitive technologies. Here, we discuss state-of-the-art polymeric photocatalysts and our microscopic understanding of their activities. We conclude with a discussion of five outstanding challenges in this field: non-standardized reporting of activities, limited photochemical stability, insufficient knowledge of reaction mechanisms, balancing charge carrier lifetimes with catalysis timescales and the use of unsustainable sacrificial reagents.

A metal-decorated alkene-linked covalent organic framework is a robust, selective photocatalyst for CO ₂ reduction.

A range of linear conjugated polymers is reported that promote simultaneous photocatalytic CO ₂ reduction and proton reduction with a sacrificial hole-scavenger.

Conjugated ladder polymers (cLaPs) are introduced as organic semiconductors for photocatalytic hydrogen evolution from water under sacrificial conditions. Starting from a linear conjugated polymer (cLiP1), two ladder polymers are synthesized via post-polymerization annulation and oxidation techniques to generate rigidified, planarized materials bearing dibenzo[b,d]thiophene (cLaP1) and dibenzo[b,d]thiophene sulfone subunits (cLaP2). The high photocatalytic activity of cLaP1 (1307 μmol h⁻¹ g⁻¹) in comparison to that of cLaP2 (18 μmol h⁻¹ g⁻¹) under broadband illumination (λ > 295 nm) in the presence of a hole-scavenger is attributed to a higher yield of long-lived charges (μs to ms timescale), as evidenced by transient absorption spectroscopy. Additionally, cLaP1 has a larger overpotential for proton reduction and thus an increased driving force for the evolution of hydrogen under sacrificial conditions.

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