Tetrazines as versatile building blocks in polymer chemistry

The proposed research project is intended to investigate the scope of electron-deficient tetrazine monomers. These materials are expected to be highly interesting building blocks in polymer chemistry which will a) show high reactivity in inverse-electron demand Diels-Alder reactions and b) allow for low-bandgap conjugated polymers. Thus, novel copolymers with unique properties can be achieved. The inverse electron-demand Diels-Alder reaction has only recently gained attention in the polymer science community although it represents an efficient way to combine different polymer chains to yield block copolymers with exciting new properties. One key hypothesis within the proposed research is that this "click" reaction is orthogonal to two already well-established reactions (azide-alkyne and thiol-ene click chemistry) meaning that under mild, biocompatible conditions, all three click chemistries can be selectively accessed without the need for protecting groups. First indications for this orthogonality are given by results from our preliminary experiments. The electron-deficient nature of the targeted tetrazine structures, which is the leitmotif for enhanced reactivity in the ied-DA reaction, also renders them as useful alternative to electron-deficient monomers for conjugated donor- acceptor polymers. Therefore, as a second major part, tetrazine-containing electroactive copolymers will be investigated. Due to their tuneable electron-deficient nature, the developed tetrazines are expected to act as acceptors in donor-acceptor conjugated polymers and could therefore, in combination with the right electron-rich partner, lead to alternative low-bandgap p-type donor polymers with interesting properties for photovoltaic applications. An important feature of these materials is again the clickability which could be used for morphology stabilisation of bulk heterojunction photovoltaic devices. For this part of the project, collaboration with Professor Iain McCulloch (Imperial College London) is envisaged. Recently his group has been especially active in the development of novel electron-rich structures for electroactive polymers and DFT (density functional theory) studies to optimise the structure of such materials. In summary, this project is seen by the applicants as an opportunity to develop the fundamental knowledge which is needed for development of tetrazine-based materials which can be used in various applications.

The Hertha Firnberg project Tetrazines as versatile building blocks in polymer science has been dedicated to exploring different applications of 1,2,4,5-tetrazines (heterocyclic aromatic compounds with four nitrogens) related to polymer and materials science. Tetrazines are very electron-deficient which makes them interesting in two ways: Firstly, they can act as dienes in inverse-electron-demand Diels-Alder (iEDDA) reactions, which can be used to click two molecules together (i.e. link two functional groups selectively without any catalyst). Click reactions are widely applied in materials science for the preparation of novel functional materials which will be useful in diagnostics, imaging and other life-science related applications. It is now an intriguing concept to use multiple click reactions to further extend the scope of such approaches. Therefore one of the main research hypotheses which could be proven within the research project was that iEDDA reactions can be combined with other click reactions to perform three of these reactions in one pot. Furthermore, iEDDA reactions have been successfully applied for the modification of porous foams and glycopolymers. Secondly, the electron-deficient nature of tetrazines also makes them ideal candidates for acceptor monomers in donor-acceptor copolymers. There is an ongoing strive for new materials to be used in organic electronics. In particular, organic photovoltaics will contribute to the energy revolution and the aforementioned conjugated copolymers are about to replace silicon solar cells making solar energy affordable for a broader public and thus providing an exit strategy from nuclear and fossil-fuel energy. Thereby, two strategies have evolved in the recent years. Instead of further developing sophisticated units leading to smaller and smaller bandgaps, new device architectures (e.g. tandem solar cells) and smart combinations of materials have led to further improved efficiencies. Another important development leading to a paradigm shift in organic electronics were organic non-fullerene acceptor materials replacing cost-intensive fullerenes in organic solar cells. The Hertha Firnberg project was able to contribute to this field not only by designing new polymers with outstanding performance tailored for these new device architectures and acceptor types but also providing building blocks for this new material class of organic materials.