Tetrathiafulvalene-oligo(para-phenyleneethynylene) conjugates: formation of multiple mixed-valence complexes upon electrochemical oxidation

Abstract

Short monodisperse oligo- (para-phenyleneethynylene) (pOPE) units bearing laterally attached tetrathio-substituted tetrathiofulvalene (TTF) units have been synthesised from functionalised aromatic building blocks by using the Sonogashira cross-coupling methodology. The unusual redox properties of these TTF-pOPE conjugates were observed by employing electrochemical methods, such as cyclic voltammetry and exhaustive electrolysis. We found that formally one half of the TTF units in the pOPE monomer 1, dimer 2, and trimer 3 (with 2, 4, and 6 TTF units, respectively) are electrochemically silent during the first-step oxidation at 0.49 V. We propose the formation of persistent mixed-valence complexes from the TTF and TTF(+·) units present in an equal ratio. Such mixed-valence dyads (single or multiple in the partially oxidised 1-3) exhibit an unusual stability towards oxidation until the potential of the second oxidation at 0.84 V is achieved. This finding suggests that below this potential the oxidation of the respective mix-valence complexes is extremely slow.