Research
Following what I did during my PhD thesis (copy of the summary). For further information see Publications.
The main aim of this doctoral thesis was the synthesis of a series of monodisperse poly(triacetylene) (PTA) oligomers based on (E)-hex-3-ene-1,5-diyne (E)-1,2-diethynylethene, DEE) monomer units. These new oligomers extend the available range of monodisperse PTA oligomers synthesized in our research group. Oligomers and polymers, built up entirely from carbon atoms, have drawn attention in the last few years, particularly with respect to their potential applications in materials science, and led to the Nobel-prize for chemistry in the year 2000 for electrically conducting polymers. For structure-property-relationship investigations it is important to gain secured and reproducible physical data with oligomers of defined chain length. Pi-Chain polymers display a high molecular mass and are therefore only moderately or not at all soluble in organic solvents. This makes their characterization and further physical investigations difficult. Furthermore, structural defects prevent the access to secured physical data. Chapter 2 shows the synthesis and characterization of a series PTA oligomers, ranging from monomer up to tetracosamer.
The Si--Si-distance gives a value of 18.2 nm for tetracosamer, establishing a new world record length for monodisperse oligomers exclusively built up of carbon atoms within a nonaromatic backbone. The 24mer consists of 24 double and 48 triple bonds and possesses therefore 144 (!) linearly conjugated carbon atoms. The synthesis proceeded via an efficient and rapid protocol: "Statistical" deprotection and oxidative Glaser-Hay oligomerization yielded the desired oligomers that were used as macromonomers in a subsequent cycle. All oligomers were stable under standard laboratory conditions and highly soluble in aprotic organic solvents. They were characterized by melting point, size-exclusion chromatography, UV/Vis-, IR-, 1H-, and 13C-NMR-spectroscopy and mass spectrometry. The crystal structure of tetramer was determined by X-ray crystallography, which displayed the planar backbone with all C=C-bonds adopting an s-trans configuration with respect to the buta-1,3-diynediyl fragments.
The electronic absorption spectra of the oligomers gave rise to absorption bands with exceptionally high extinction: the tetracosamer displayed a molar extinction coefficient up to 285'000 M-1cm-1. The UV/Vis spectra of the oligomeric series clearly displayed a saturation of the longest-wavelength absorption maximum. The effective conjugation length was determined on the basis of these data, showing saturation for the decamer in agreement with previously determined values. An analogous saturation behavior for the oligomeric series was observed by means of Raman spectra, which offered an opportunity to calculate the effective conjugation length based on a data set determined by a different method: saturation was observed for a nonamer. That value agreed well with the previously determined value (10-mer).
New oligomers bearing sulfide end-groups were synthesized for measuments of the electrical conductivity in poly(triacetlyene) oligomers along the pi-conjugated backbone. These oligomers should be placed between gold electrodes, allowing the measurement on a single molecular rod. Although the endcap and subsequently the end-capped oligomers were successfully synthesized and obtained in monodisperse form up to the octamer, they were not stable and decomposed in solution as well as in the solid state.
In Chapter 3 is shown the synthesis of a series of hybrid oligomers for continuation of a previous study on the modulation of pi-electron conjugation of oligo(triacetylene)s by insertion of central hetero-spacer fragments between two (E)-hex-3-ene-1,5-diyne ((E)-1,2-diethynylethene, DEE) moieties. A series of transition metal complexes were prepared as potential precursors for nanoscale scaffolding based on both covalent acetylenic coupling and supramolecular assembly. The UV/VIS spectra revealed that the majority of spacers provided hetero-trimers featuring extended pi-electron delocalization. The new hybrid chromophores showed a dramatically enhanced fluorescence compared with the DEE dimer and homo-trimer. This increase in emission intensity appeared as a general feature of these systems: even if the spacer molecule was non-fluorescent, the corresponding hetero-trimer may show a strong emission. The redox properties of the new hybrid chromophores were determined by cyclic voltammetry (CV) and rotating disk voltammetry (RDV). In each case, the first one-electron reduction step in the hetero-trimers appeared anodically shifted compared with DEE dimer and homo-trimer. With larger spacer chromophore extending into two dimensions, the anodic shift (by 240-490 mV) seemed to originate from inductive effects of the two strongly electron-accepting DEE substituents rather than from extended pi-electron conjugation along the oligomeric backbone, as had previously been observed for DEE-substituted porphyrins.
