Polymeric graphitic carbon nitride (for simplicity, g-C3N4) is a layered material similar to graphene, being composed of only C, N, and some impurity H. Contrary to graphenes, g-C3N4 is a medium band gap semiconductor and an effective photocatalyst for a broad variety of reactions, and it possesses a high thermal and chemical stability In this Perspective, we describe the polycondensation of this structure, how to modify band positions and band gap by doping and copolymerization, and how to texture the organic solid to make it an effective photocatalyst. We then describe the photochemical splitting of water and some mild and selective photooxidation reactions catalyzed by g-C3N4.

Mesoporous carbon nitride (mpg-C(3)N(4)) polymer can function as a metal-free photocatalyst to activate O(2) for the selective oxidation of benzyl alcohols with visible light, avoiding the cost, toxicity, and purification problems associated with corresponding transition-metal systems. By combining the surface basicity and semiconductor functions of mpg-C(3)N(4), the photocatalytic system can realize a high catalytic selectivity to generate benzaldehyde. The metal-free photocatalytic system also selectively converts other alcohol substrates to their corresponding aldehydes/ketones, demonstrating a potential pathway of accessing traditional mild radical chemistry with nitroxyl radicals.

The development of stable systems to generate chemical fuels through water splitting by sunlight is a key challenge of modern materials chemistry, one that is driven by increasing energy demands and climate change. The central problems are to design chemically stable light-harvesting antenna molecules and co-factors, and then to assemble these active components into an integrated photosystem. Various substances have been examined as visible-light converters, including metal-organic dyes, [1] inorganic semiconductors, [2] and conjugated polymers. [3] The last are of particular interest, as they are much closer to biological systems in composition and are potentially sustainable, as well as cheap and easily processable. Principally, properties such as the HOMO and LUMO position and the resulting band gap can be precisely chemically engineered by synthesizing the constituents. To date, in spite of the wide availability of conducting polymers developed for photovoltaics, [4] the impact of these polymers on water splitting technology remains minor, because ordinary conducting polymers are usually unstable to visible light irradiation in conjunction with exposure to oxygen and water.A recent study has explored metal-free, polymeric graphitic carbon nitride (g-C 3 N 4 ) for the generation of hydrogen from a protic solution with visible light. [5] This carbon nitride (CN) polymer was not only found to be a stable semiconductor, but also to be capable of achieving both half reactions of water splitting, meaning that the band-gap covers both the water reduction and water oxidation potentials. This is indeed a rare and lucky case. However, the activity of pristine g-C 3 N 4 remains moderate. [5] Several strategies, such as nanostructuring, [6] doping, [7] cocatalyzing, [5,8] and copolymerization, [9] have been exploited to improve the photocatalytic activity and selectivity of carbon nitride. Indeed, it was shown that by copolymerizing simple barbituric acid with the carbon nitride precursor through a Schiff base reaction, the performance of g-C 3 N 4 could be enhanced, which is an effect resulting from extension of the optical absorption of the polymer to cover more of the visible light range. [9] However, in this case the HOMO was decreased in energy, thus lowering the oxidation potential, which is presumably the most difficult step in achieving overall water splitting with organic semiconductors. This is why the design of appropriate comonomers with diverse chemical composition and structure to allow for modification of the band structure and optoelectronic properties of carbon nitride is still a relevant and promising task.Herein, we advance this strategy by employing a variety of new monomer building blocks with the desired compositions and electronic structures for chemical incorporation into the conjugated polymeric network of g-C 3 N 4 . Most precursors of carbon nitride polymers contain cyano groups, amino groups, or both, with the simplest case being cyanamide, which can undergo multiple thermal condensations to ...