Triply Branched Viologen Stars: Synthesis and Polymerization by Peripheral Benzyl Coupling

Abstract

The synthesis of 0th (n = 1) and 1st (n = 2) generation, monodisperse, star-shaped oligoviologens, C[(BnV++PhV++)nCH2X]3, which consist of a phenyl or a 2,4,6-trimethylphenyl core (C) with triple 1,3,5-branches, in which each branch consists of a linear alternating series of diphenyl- (PhV++) and dibenzyl viologens (BnV++) and a variety of peripheral groups X (CH3, OH, Br), is described. According to MM+ modeling, the stars adopt a flat structure with the branches pointing towards the corners of an equilateral triangle with a centrecorner distances of 2.9 (n = 1) and 5.8 nm (n = 2, repulsive charge interaction). Discrete redox chemistry from the benzyl- (0.27, 0.70 V) and phenyl viologen subunits(0.12, 0.42 V vs. Ag/AgCl) is observed as a set of four interdigitated reduction waves, which involve 3?x?4 = 12 (0th generation) and 3?x?8 = 24 electrons (1st generation) as shown by cyclic voltammetry and spectroelectrochemistry and supported by semiempirical PM6 calculations. Viologen stars C[(BnV++PhV++)1-CH2Br]3 (7) and C[(BnV++PhV++)2CH2Br]3 (12) undergo electrochemical or Zn-induced polymerization by benzylbenzyl radical coupling. The polymer-modified electrodes (average film thickness of ca. 60 nm) exhibit the same electrochemistry as the monomers. They were characterized by cyclic voltammetry, spectroelectrochemistry, and STM, and those produced by Zn reduction by IR spectroscopy. Viologen stars in their monomeric and polymeric state represent a new multielectron redox material (12 e and 24 e per molecule for the 0th and 1st generation viologen stars are exchangeable, respectively) with localized redox orbitals and electrochemical activity, which exhibits four waves within a potential range of 0.58 V.