Bottom-up-Synthese neuartiger 3D-Kohlenstoff-Allotrope abgeleitet von 6.8² P und 6.8² D Polybenzol
Zusammenfassung der Projektergebnisse
The central goal of this postdoctoral research study was to develop the synthetic methodologies required to access molecular building blocks that can be assembled into the three-dimensional structure of both 6.82 D and 6.82 P polybenzene, which can be added to the family of synthetic carbon allotropes (SCA). I successfully established two synthetic sequences that enable the accessibility of potential polybenzene-precursors derived from cycloocta-1,3,5,7-teraone with two 1,3-dioxolane ketal protecting groups (syn and anti). While a 12-step synthesis allows for the generation of syn-di-ketal protected monomer on multi gram scale, the developed route towards the anti-di-ketal protected building block consists of only six steps but with limited scalability. Attempts to isolate the initially proposed deprotected precursor cycloocta-1,3,5,7-teraone were not successful but the in-situ generation of the reactive species during acidic crystal growth conditions should serve as a slow supply of monomers that is a strategy known to increase crystallinity in reticular chemistry. I investigated the self-assembly of both key molecular building blocks by conducting acid catalysed aldol-cyclotrimerization reaction, focusing on numerous Lewis (LA) or Brønsted acids (H+) as well as the optimisation of solvent mixture, temperature and reaction time. Any insoluble precipitation was extensively analysed by IR, pXRD, Raman-spectroscopy or SEM-techniques. Two promising samples were isolated, but the insufficient amount of less than 5 mg material each inhibited a complete and unambiguous analysation. Attempts to reproduce more were not successful. As a side-project I was also working on the reticular bottom-up synthesis of a Kagome lattice of [4]triangulenes, a two-dimensional covalent organic framework imbued with a deliberate single particle band instability with the aim to realise an excitonic insulator (EI) state. I successfully synthesised halogenated [4]triangulene-monomers suitable for investigations on the on-surface polymerization and cyclodehydrogenation. First-principles calculations and scanning tunnelling spectroscopy (STS) reveal a mixing of valence (VB) and conduction (CB) flat-bands (FBs) along with a non-trivial semiconducting gap that can only be explained by invoking many-body theory. Our findings directly corroborate the excitonic nature of a flat-band induced insulator ground state. These interesting results provide a robust yet highly tuneable platform for the exploration of many-boson physics in quantum materials.
