Synthesis of Two-Dimensional Polymers by On-Surface Photopolymerization

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The goal of this project is to advance the synthesis of organic two-dimensional polymers (2DP) by topochemical photopolymerization on solid surfaces. Our synthetic route is based on a two-staged protocol: (1) self-assembly of monomers into a polymerizable monolayer with photoactive positioning of functional moieties; (2) covalent cross-linking into the 2DP by photochemical excitation of intermolecular cycloaddition reactions. Structural characterization is performed by high-resolution scanning tunneling microscopy. Due to the strong change in the local electronic structure caused by the cycloaddition, individual newly formed covalent bonds can be clearly identified. This is the basis for studies of polymerization at the molecular level. In addition, the photoreaction is detected by local infrared absorption spectroscopy (nano-FTIR). The first part of the project builds on our extensive experience with the fantrip (fluorinated anthracene triptycene) monomer. In preliminary experiments, we have demonstrated the reversibility of cycloaddition by photoexcited energetic charge carriers in graphite. Detailed mechanistic studies will investigate fundamental aspects, not least to exploit the bond reversibility to improve structural quality through self-healing processes. Photolithography beyond the diffraction limit will be achieved by plasmonic field enhancement under metallized probes of near-field microscopes. Preliminary studies on passivated Au(111) surfaces showed interesting results: The reaction rate is enhanced by two orders of magnitude, but the degree of polymerization stagnates at 25%. These unexpected results can be interpreted in terms of hot charge carrier excitation in the metal. We expect fundamental insights from studies with variable wavelength and illumination intensity, not least to facilitate complete polymerization. Experiments with local injection of charge carriers promise additional insights, as they allow the separate study of electron- and hole-induced chemistry. The exploration of other 2DP is an important milestone for on-surface photopolymerization. Preliminary experiments have identified a very suitable and promising monomer for further studies. In terms of applications, the synthesis of macroscopic monocrystalline 2DP is an important milestone. Our targeted approach is to optimize the self-assembly on graphitic substrates. This will be followed by synthesis on bulk insulator surfaces. Finally, the applicability of established methods for transferring 2DP to alternative substrates will be evaluated. This is a very important first step for the future exploration of application relevant properties of 2DP.