The laboratory of Dr Miriam M. Unterlass at the Institute of Materials Chemistry at TU Wien, Austria has reported the synthesis of more than 20 different perylene bisimide dyes. The way they prepare these compounds is impressive. Conventionally, perylene bisimides are generated in highly toxic solvents and employing toxic and expensive catalysts. Moreover, classical reactions towards these dyes require an important excess of the starting compounds. Additionally, tedious purification is necessary for obtaining dye products of sufficient purity. All in all, the conventional route is a complex chemical synthesis.
“In our approach, we are using the starting compounds in a 1:1 ratio, i.e. without an excess of reactants. The starting compounds are dispersed in water inside a closed reactor. Then the mixture is heated to 200 °C and increased pressure is generated”, explains Dr Unterlass. The reactor works like a pressure cooker. The process of conducting reactions in hot water under pressure is called hydrothermal synthesis. After the reaction has completed, the final perylene bisimide dyes are obtained with high purity, thus removing the necessity for tedious purification. The novel hydrothermal synthesis bears the potential of enabling an easy access to these materials, which will help in realistic application.
Despite their small size, hydrothermal synthesis was very challenging for perylene bisimides. They are very non- polar, and therefore hydrophobic at room temperature. However, by heating the water to increased temperatures the challenge is met. The hydrothermal synthesis of perylene bisimides is highly efficient and environmentally friendly.