Viologen Stars and Rods: Synthesis, electrochemical Investigations and Polymerization

• Main Author: Constantin, Veronica-Alina
• Other Authors: Prof. Dr. Lorenz Walder
• Format: Doctoral Thesis
• Language: English
• Published: 2012
• Subjects: viologen; polymerization; electrochemistry; rods; stars; 35.27 - Elektrochemische Analyse; 35.14 - Elektrochemie; 35.52 - Präparative Organische Chemie; 35.80 - Makromolekulare Chemie; 35.25 - Spektrochemische Analyse; ddc:540
• Online Access: https://repositorium.ub.uni-osnabrueck.de/handle/urn:nbn:de:gbv:700-2012042710149

This doctoral thesis focuses on synthesis of rigid star-shaped and rod-like viologen oligomers. The work is divided in two parts: synthesis, characterization and electropolymerization of star-shaped (i) and rod-like viologens oligomers (ii). In the first part the synthesis of viologen stars consisting of a phenyl core with triple 1,3,5-branching, each branch consisting of a linear alternating series of diphenyl (PhV++) and dibenzyl (BnV++) viologens and a variety of peripheral groups –X (Br, OH), is presented. A new electrochemical method of electrode modification (Gold, GC, ITO and CNTs) with viologen derivatives based on a benzyl radical coupling mechanism is described. The radicals are excessively generated at the star periphery resulting in a highly cross-linked polyviologen film with persisting star subunits. It is characterized by CV, STM and UV-Vis methods. Redox-titration experiments monitored by UV-Vis reveals that the reduction of the viologen stars begins at the periphery with the formation of PhV+•, continues with generation of BnV+• and ends with the reduction of the radical cations in the same sequence. Thus, viologen stars combine the unique redox and electrochromic property of isolated phenyl and benzyl viologen in one molecule. The second part of the thesis focuses on the step-wise synthesis of a library of rigid rod-like conjugated difunctional viologen/diphenyl oligomers with varying chain lengths including different side chain substitution. All oligomers are soluble in DMSO or MeOH depending on the counter anion (PF6- or Cl-). In order to tune the solubility of the oligomers, the side chains are tailored as methoxy, butoxy and oligo(ethylene oxide). The most solubilizing side-chains are of the oligo(ethylene oxide) type. All viologen oligomers are characterized by means of 1H-NMR, 13C-NMR, elemental analysis, optical spectroscopy and cyclic voltammetry. A simple surface functionalization and grafting technique has been developed for covalent binding of the viologen oligomers onto various conductive substrates e.g.: Au, GC and ITO. These modified electrodes are suitable for potential applications in designing field-effect transistors, sensors and supercapacitors. The polymer layers are characterized by means of FT-IR, STM, XPS and CV. The combined results presented in thesis represent a major advance in electrode functionalization by n-dopable viologen polymers and herald a variety of potential applications that make use of n-type semiconductors.