When excited with light many organic compounds, such as Eosin Y, benzophenone, as well as metal oxoclusters, such as decatungstate, abstract hydrogen atom from organic molecules. Such a feature is employed in organic synthesis, specifically to generate C-, N-, and O-centered radicals.
Abstraction of a hydrogen atom may be considered as the transfer of an electron and a proton – it is oxidation and deprotonation of a molecule. Therefore, strong basicity and highly positive reduction potential of the sensitizer's electronically excited state provide strong driving force for hydrogen atom transfer.
The basicity of nitrogen-rich heterocycles decreases in the series: pyridine > pyrimidine > 1,3,5-triazine. In other words, the more sp2-hybridized nitrogen atoms added to the structure, the weaker base the heterocycle is. From this standpoint, heptazine, having seven (sp2)N atoms, must be a weak base and, as such, should not be a suitable photocatalyst for reactions involving hydrogen atom transfer. But is this the case for heptazine-based graphitic carbon nitrides? We addressed this question in our recent article in Angewandte Chemie.
Graphitic carbon nitrides are a class of 2D materials in which heptazine units are covalently linked to form layers, while these layers are assembled via weak van der Waals forces into a graphite-like structure. Such assembly of heptazine units into a 2D material has a profound impact on the material's basicity compared to single heptazine units. Similar to the well-known proton sponge, two adjacent heptazine units chelate a proton. Therefore, the basicity of mesoporous graphitic carbon nitride (mpg-CN) is higher than it would be for molecular heptazine derivatives. mpg-CN has plenty of protonation sites – several (sp2)N atoms, NH- and NH2-groups. We measured 'average' pKa value of protonated mpg-CN to be approximately 6.6. This means that mpg-CN holds protons stronger compared to, for example, pyridine.
On the other hand, extended π-conjugated structure allows for stronger stabilization of the electron added upon reductive quenching of the mpg-CN excited state.
These two factors allow for mpg-CN to abstract an electron and a proton, i.e., hydrogen atom, from substrates upon excitation with light. In the recent work, we applied this feature of mpg-CN to oxygenate endo-CH2-groups in oxazolidinones.
Our work outlines general guidelines for designing semiconductors that would be optimal for employing them as photocatalysts to activate organic molecules via homolytic cleavage of X–H bonds.
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