Nature uses organic molecules for light harvesting and photosynthesis but most man-made water splitting catalysts are inorganic semiconductors. Organic photocatalysts, while attractive because of their synthetic tunability, tend to have low quantum efficiencies for water splitting. Here we present a crystalline covalent organic framework (COF) based on a benzobis(benzothiophene sulfone) moiety that shows a much higher activity for photochemical hydrogen evolution than its amorphous or semi-crystalline counterparts. The COF is stable under long-term visible irradiation and shows steady photochemical hydrogen evolution with a sacrificial electron donor for at least fifty hours. We attribute its high quantum efficiency of FS-COF to its crystallinity, its strong visible light absorption, and its wettable, hydrophilic 3.2 nm mesopores. These pores allow the framework to be dye sensitized, leading to a further 61% enhancement in the hydrogen evolution rate up to 16.3 mmol g-1 h-1. The COF also retained its photocatalytic activity when cast as a thin film onto a support. Photocatalytic solar hydrogen production-or water splitting-offers an abundant clean energy source for the future. The use of dispersed, powdered photocatalysts or thin catalyst films is attractively simple, but so far, no catalyst satisfies the combined requirements of cost, stability and solar-to-hydrogen efficiency. Since the first report of TiO2 as a photocatalyst, 1 many inorganic semiconductors have been explored for water splitting, both in photoelectrochemical cells and as photocatalyst suspensions. 2-4 Recently, organic semiconductors have emerged as promising materials for photocatalytic hydrogen and oxygen evolution. 5-7 Poly(p-phenylene) was first reported as a photocatalyst for hydrogen evolution in 1985, 8,9 but its activity was poor and limited to the ultraviolet spectrum. Since then, more active organic materials have been reported as visible light photocatalysts for hydrogen production using sacrificial donors. This started with carbon nitrides 5,10 followed by poly(azomethine)s, 11 conjugated microporous polymers (CMPs), 6,12,13 linear conjugated polymers, 12,14-16 and covalent triazine-based frameworks (CTFs). 17-19 Carbon nitrides were further developed into hybrid systems that facilitate overall water splitting to produce both hydrogen and oxygen, for example by including metal co-catalysts. 20 CMPs were also claimed to exhibit overall photocatalytic water splitting. 21 However, while it is possible to tune semiconductor properties such as band gap by modular copolymerization strategies, 6 organic materials such as carbon nitrides, conjugated polymers and CTFs lack long-range order: they are amorphous or semi-crystalline. 17,22 This lack of order might limit the transport of photoactive charges to the catalyst surface. 23 More generally, it is challenging to construct atomistic structure-property relationships for materials where the three-dimensional architecture is poorly defined. Covalent organic frameworks (COFs) 24-26 are a cla...

Photocatalytic hydrogen production from water offers an abundant, clean fuel source, but it is challenging to produce photocatalysts that use the solar spectrum effectively. Many hydrogen-evolving photocatalysts are active in the ultraviolet range, but ultraviolet light accounts for only 3% of the energy available in the solar spectrum at ground level. Solid-state crystalline photocatalysts have light absorption profiles that are a discrete function of their crystalline phase and that are not always tunable. Here, we prepare a series of amorphous, microporous organic polymers with exquisite synthetic control over the optical gap in the range 1.94-2.95 eV. Specific monomer compositions give polymers that are robust and effective photocatalysts for the evolution of hydrogen from water in the presence of a sacrificial electron donor, without the apparent need for an added metal cocatalyst. Remarkably, unlike other organic systems, the best performing polymer is only photoactive under visible rather than ultraviolet irradiation.

Technologies such as batteries, biomaterials, and heterogeneous catalysts have functions that are defined by mixtures of molecular and mesoscale components. As yet, this multi-length scale complexity cannot be fully captured by atomistic simulations, and the design of such materials from first principles is still rare 1-5. Likewise, experimental complexity scales exponentially with the number of variables, restricting most searches to narrow areas of materials space. Robots can assist in experimental searches 6-14 but their widespread adoption in materials research is challenging because of the diversity of sample types, operations, instruments and measurements that is required. Here we use a mobile robot to search for improved photocatalysts for hydrogen production from water 15. The robot operated autonomously over 8 days, performing 688 experiments within a 10-variable experimental space, driven by a batched Bayesian search algorithm 16-18. This autonomous search identified photocatalyst mixtures that were six times more active than the initial formulations, selecting beneficial components and deselecting negative ones. Our strategy uses a dexterous 19,20 free-roaming robot 21-24 , automating the researcher rather than the instruments. This modular approach could be deployed in conventional laboratories for a range of research problems beyond photocatalysis. Leverhulme Research Centre for Functional Materials Design, the Engineering and Physical Sciences Research Council (EPSRC) (EP/N004884/1), the Newton Fund (EP/R003580/1), and CSols Ltd. X.W. and Y.B. thank the China Scholarship Council for a Ph.D. studentship. We thank KUKA Robotics for help with gripper design and initial implementation of the robot. Author contributions. B.B. developed the workflow, developed and implemented the robot positioning approach, wrote the control software, designed the bespoke photocatalysis station, and carried out experiments. P.M.M. and V.V.G. developed the optimiser and its interface to the control software. X.L. advised on the photocatalysis workflow. C.M.A., Y.B. and X.L. synthesized materials. Y.B. performed kinetic photocatalysis experiments. X.W. performed NMR analysis and synthesized materials. B.L. carried out initial scavenger screening. R.C. and N.R. helped to build the bespoke stations in the workflow. B.H. analysed the robustness of the system, assisted with the development of control software, and operated the workflow during some experiments. B.A. helped to supervise the automation work. R.S.S. helped to supervise the photocatalysis work. A.I.C. conceived the idea, set up the five hypotheses with BB, and coordinated the research team. Data was interpreted by all authors and the manuscript was prepared by A.I.

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 that their electronic properties are easily tuneable through molecular engineering. Polymeric photocatalysts have developed rapidly over the last 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: nonstandardized 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.

Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.