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.
A range of linear conjugated polymers is reported that promote simultaneous photocatalytic CO2 reduction and proton reduction with a sacrificial hole-scavenger.
Oxidation of a nonaromatic Siamese-twin porphyrin, a pyrazole-containing expanded porphyrin with two porphyrinlike binding pockets, with a stoichiometric amount of the two-electron, two-proton oxidizing agent 2,3-dichloro-5,6-dicyano-1,4-benzochinone led to the formation of a single N(pz) -C(o-Ph) linkage between the pyrazole unit with a neighboring meso-phenyl group, forming a pyrazolo- [1,5-a]indole moiety. Repeated treatment with a second equivalent of the oxidant yielded a doubly N-fused species, involving the second pyrazole moiety. The conversion products were characterized by variable-temperature and multinuclear 1D and 2D NMR spectroscopy. The fusions strongly alter the conformation of the macrocycles, as shown by X-ray diffraction analyses of all three compounds, eventually leading to a folded structure. UV/Vis and NMR-spectroscopic investigations indicated the presence of highly delocalized but nonmacrocycle-aromatic π systems. This behavior of the Siamese-twin porphyrin in response to oxidation is in contrast to the behavior of related all-pyrrole-based expanded macrocycles that switch, by redox processes and protonation, between Hückel and Möbius aromatic states.
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