Organic Synthesis Mediated by Carbon Nitride Photocatalysis and Photocharged Carbon Nitrides

ConspectusOrganic synthesis mediated by graphitic carbon nitrides (g-CNs) became a research hotspot primarily due to a combination of the following aspects: (1) the simple and convenient preparation of the material on a gram scale from inexpensive precursors, (2) the heterogeneous nature of the material, which allows for its easy recovery from the reaction mixture, chemical and thermal stability, and possibility to create nanostructures, such as membranes and thin films, and (3) the effective utilization of sustainable energy─photons in the UVA-vis range, which may be used to drive chemical reactions that are endergonic in the dark. By combining various spectroscopic techniques and the results of theoretical modeling, our group identified three modes of substrate activation by g-CN via (1) photoinduced electron transfer, (2) energy transfer, and (3) proton-coupled electron transfer/hydrogen atom transfer. In this Account, we discuss the chemical structure of the electronically excited state of g-CN. This information may be used to design rational pathways for substrate activation via the above-mentioned mechanisms. Using elemental sulfur (S₈) as a nearly 100% atom-efficient sulfurating agent, we developed a set of methods to incorporate sulfur atom(s) into the organic scaffold by means of g-CN photocatalysis. On the other hand, we identified S₈ as a more selective, compared to O₂, sacrificial oxidant to mediate a few net-oxidative photocatalytic transformations. Among the products of the developed synthetic methods are highly fluorescent heterocycles, artificial flavoring agents, and precursors for organic synthesis. While g-CNs are typically used by the community as photocatalysts under continuous light illumination (the sensitizer is regenerated many times in the catalytic cycle), our group contributed to understanding the ability of this class of materials to undergo photocharging, i.e., to store charges by forming long-lived radical species. We applied photocharged carbon nitrides as donors of electrons and protons in the dark in a series of organic transformations. We outline the current challenges and future development prospects of carbon nitride-mediated organic synthesis. At the same time, we provide guidance on the development of organic catalytic systems and material design at the molecular level.