Small and not very small organic molecules are the foundation of our civilization. They are fuels, medical drugs, solvents and in general are building blocks for industry. The beginning of 21st century was heralded by the development of numerous photocatalytic reactions as alternatives to the traditional purely chemical synthetic procedures. Transition metal photoredox complexes from the group of platinum, e.g. Ir- and Ru-based, paved the way for organic photoredox catalysis. Considering that sustainability is a central point of our society today, it is allowed to raise the question: “How sustainable is using of two the rarest elements on Earth”?
Ru and Ir, the core of the homogeneous photoredox chemistry, gained 7.6 points out of 10 on the scale of the ‘relative supply risk’ by the Royal Society of Chemistry. Only some rare-earth elements, e.g. dysprosium (9.5 points), ‘excel’ in this regard Ru and Ir.
If we aim not only for momentary success by demonstrating breath-taking photocatalytic reactions, but to create a truly flexible chemical technology that will serve for many decades, it is essential to develop synthetic protocols free of rare elements. However, we must not forget about functionality of the photocatalysts, i.e. suitable redox potentials and optical band gap, easiness of handling under ambient conditions and recyclability are essential.
Both economic and functional requirements to the photocatalysts are met in medium gap organic semiconductors – carbon nitrides.
Innovative Heterogeneous Photocatalysis develops carbon nitride materials. Our target structure is potassium poly(heptazine imide), K-PHI. This is highly crystalline material.[J. Catal. 2017, 350, 203] Differently from graphitic carbon nitride (g-C3N4), K-PHI forms a long lived green radical upon light irradiation in the presence of electron donors.[Chem. Sci. 2018, 9, 3584] Schematic mechanism of this process is shown on the figure below. The tinted long-lived radical is a key intermediate in numerous photocatalytic reactions where K-PHI is employed.
Innovative Heterogeneous Photocatalysis uses K-PHI to redefine organic synthesis. We aim to make it simpler, universal and applied.
Complex reagents are replaced by small molecules activated photocatalytically by carbon nitrides. For example, instead of smelly sulfurating agents (P4S10, Lawesson reagent, etc.), we use elemental sulfur. By this we have synthesized dibenzyldisulfide from toluene and S8 at nearly room temperature.[Chem. Sci. 2018, 9, 3584]
The beauty of carbon nitrides is that they are universal photocatalysts. They work equally well in oxidation reactions and reduction reactions. The reaction path is defined by the reagents and appropriate scavenger, while K-PHI enables flow of electrons from one reaction partner to another. Thus, K-PHI mediates cyclization of N-acylhydrazones to oxadiazoles-1,3,4 under slightly oxidative conditions maintained by elemental sulfur.[Appl. Catal., B 2018, 228, 97]
On the other hand, K-PHI enables photocatalytic reduction of enones in the presence of convenient electron donors – amines. This photocatalytic mode gives cyclopentanoles and N-fused pyrroles.[ ACS Catalysis 2019, 9, 1531],[ Nat. Comm. 2019, doi: 10.1038/s41467-019-08652-w.]
Innovative Heterogeneous Photocatalysis is convinced that not random molecules, but molecules for certain application should be synthesized. For example, the abovementioned dibenzyldisulfide is a common flavoring agent in food industry (flavor of smoked bacon).[Chem. Sci. 2018, 9, 3584]
Furthermore, while discovering new reactions we obtain molecules with unusual structure. We investigate properties of these molecules in order to find a suitable application. The abovementioned N-fused pyrroles are highly fluorescent in blue region with IQY of up to 24% that makes them suitable candidates for optoelectronic applications or as fluorescent dyes.[Nat. Comm. 2019, doi: 10.1038/s41467-019-08652-w] Fluorescence spectra and appearance of the N-fused pyrrole solutions under UV irradiation and natural ambient light as insets are shown below.
Finally, photocatalytic methods are not only useful for making small organic molecules. We apply photocatalytic methods in the areas beyond organic chemistry, for example, in post functionalization of materials using carbon nitride photocatalysis.