A novel process developed by researchers in Germany has overcome solubility and light penetration issues generally associated with photochemical reactions and allowed the regioselective solid-state photo-mechanochemical synthesis of nanographenes. This, they believe, opens the door to the discovery of completely new reactivities.
Based at Ruhr-Universität, Bochum, Germany, the researchers replaced the large volume of toxic solvents usually needed for light-driven chemical reactions with the mechanical energy in ball mills.
“This provides a sustainable alternative to established synthesis methods,” says team leader and chair of inorganic chemistry, Lars Borchardt.
Essentially, the researchers used ball mills as reactors. Starting materials were placed in vessels with milling balls and shaken at high frequencies. The high energy impacts provided the mechanical energy for the reaction and thoroughly mixed the substances. Using a specially adapted photoreactor, the researchers conducted the ball milling process under UV irradiation. This led to the solid-state photo-mechanochemical synthesis of nanographenes.
The vessel consists of a quartz tube containing two polytetrafluoroethylene (PTFE) balls with a perfluoroalkoxy alkane (PFA) cap at either end. UV lights were mounted on an aluminum frame around the tube.
In the first experiments, triphenylene was synthesized by two routes, the Mallory photochemical-cyclization-elimination reaction and cyclodehydrochlorination (CDHC), resulting in yields of 81% and 92%, respectively.
On investigation, the researchers determined the driving force of the reaction is a mechanically assisted photochemical reaction, while mechanical grinding is critical for mixing the substrates.
In the next step, the researchers successfully scaled up the reaction to gram scale and demonstrated the robustness of the method by applying it to a wide range of substrates, including substituted triphenylenes, a heterocycle, a five-membered ring and larger systems.
“This shows that our method is well suited for the synthesis of triphenylene derivatives, a substance class that has hardly been accessible by mechanochemical methods so far,” they wrote in a recent issue of Angewandte Chemie.
Finally, using the larger compounds, the researchers showed that nanographenes could be formed regioselectively for the first time via mechanochemistry.
“This new process enabled us to carry out specific reactions and synthesize chemical substances in a much more sustainable way,” says Borchardt. “We reduced reaction times by up to 56% while using 98% less solvent than in equivalent syntheses done with conventional methods. Last but not least, the new photoreactor consumes almost 80% less energy than conventional equipment,” he concludes.