The hydrothermal route to functional organic frameworks

How can one synthesize high-performance materials addressing the global energy challenge by benign routes? The START project The hydrothermal route to functional organic frameworks by Miriam Unterlass is addressing this question by using a method that is inspired by natural processes taking place in the earths crust: hydrothermal polymerization. This technique allows for generating very large molecules, so-called polymers, of outstanding order in solely hot water. The procedures works like cooking in a pressure cooker: the starting compounds are enclosed in a steel vessel filled with water, and heated to elevated temperatures. At the end of the process, highly crystalline structures result. Within this START project, the applicant intends to extend hydrothermal polymerization to network structures, so-called frameworks, e.g. honeycomb structures. The target materials provide high ordered porosity, as well as extreme thermal and chemical stability and functionality that is potentially highly useful for applications in the context of sustainable energy. These features will allow for applying these materials in Lithium ion batteries, high-temperature fuel cells and organic photovoltaics. 1

The HYDROCOF project aimed at generating new ways to make advanced porous materials, called covalent-organic frameworks (COFs) using hot liquid water and autogenous pressure, i.e., hydrothermal synthesis (HTS), instead of harmful solvents and catalysts. COFs are strong, stable networks of lightweight building blocks with pores throughout their structure. In this project, the focus was on developing COFs that include ring-shaped heterocyclic chemical groups that give the materials unique properties making them useful for carbon dioxide capture, conducting charged carriers, and chemical sensing. During the project, the research expanded into new areas, including organic dyes and hybrid materials that combine organic and inorganic components. One remarkable achievement was the first one-step HTS of a hybrid material combining polyimides - a class of high-performance materials - to a silica matrix. The same approach was also used to produce silica materials embedded with fluorescent dyes, which are useful photocatalysts. The method was extended to other metal oxides - such as titanium, vanadium, and manganese oxides - yielding materials with unique structures. When pigments like perinones were incorporated in the main structure of the hybrid, they displayed promising features as electrodes in batteries. The team also developed the HTS of the so-called quinoxaline and imidazole scaffolds. These compounds displayed excellent light-emitting properties in both solution and solid form. Some were used to stain specific parts of living cells - such as the endoplasmic reticulum, Golgi apparatus, lysosomes, and cell membranes - showing potential for biomedical imaging. The sustainability of these synthetic methods was confirmed by calculation of green metrics and comparison to a huge number of conventional syntheses reported in the scientific literature. Overall, the developed HTS feature greenness, and do not require toxic solvents or catalysts to be conducted. To accelerate discovery, HYDROCOF began creating an automated hydrothermal synthesis platform. This system includes a custom-built autosampler, a continuous-flow reactor, and integrated analysis tools linked to a computer interface. It can run experiments, analyze results, and automatically adjust conditions, allowing efficient exploration of chemical possibilities and opening the door to faster discovery of new materials. By combining sustainable chemistry with advanced materials design, HYDROCOF has expanded the range of accessible complex architectures, hybrid materials, and fluorescent compounds. These innovations could provide solutions to today's need for technologies, improved environmental monitoring, and better biomedical tools, showing how bioinspired methods can drive progress in multiple fields.