Photocatalysis utilising carbon nitride (CN) polymeric compounds display the potential and capability of carbon nitride materials as efficient and sustainable alternatives to various heterogeneous photocatalysts, particularly with regards to the carbon-nitrogen (C-N) cross coupling reaction wherein the construction of a C-N bond occurs.
Via irradiation of the CN photocatalyst with light, it is able to serve as a sensitiser through excitation of its electrons, undergoing an electron transfer mechanism with a nickel (II) bromide salt in order to initiate the cross coupling reaction. Within the study, comparisons between three different synthesised CN photocatalysts was sought, with an aim to determine which proved most effective and efficient for the C-N cross coupling.
Synthesis of each photocatalyst required a melamine precursor, alongside cyanuric acid for the oxygen doped CN (O-CNx) and a trithiocyanuric acid precursor for the sulphur doped CN (S-CNx). Pristine carbon nitride required only the melamine starting material, with the simplest synthesis of the three. Each coupling reaction was operated at room temperature in a sealed vial containing an aryl bromide, nickel (II) bromide, pyrrolidine and a dimethylacetamide (DMAC) solvent with a nitrogen saturated environment. They were directly irradiated for approximately 18 hours with a blue LED light whilst continuous stirring was applied.
Assessing the efficiency of each photocatalyst was conducted via measurements of the overall conversion from starting material to product, using an internal standard to compare molar amounts following the reaction. Primarily, the comparisons were based on 1H NMR data which allowed for an appropriate conclusion to be drawn, utilising the molar ratios of each NMR signal. Focussing on the signals generated from the aryl protons allowed for a clear insight into the progress of each reaction and the ratios of the starting material, product and internal standard in the final solution. Generally, it was concluded that the doped photocatalysts revealed rather similar results for the time scale and scope of the experiment, whereas the pristine photocatalyst did not lead to as good a conversion. The most efficient photocatalyst was indeterminable, reasons for which are elucidated within the full study.
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