Low Oxidation Potential Tetrathiafulvalene Analogues Based on 3,4-Dialkoxythiophene π-Conjugating Spacers

Tetrathiafulvalene analogues involving dihexyloxythiophene ( 1 ), ethylenedioxythiophene ( 2 ), and bis(3,4-dihexyloxy-2-thienyl)ethylene ( 3 ) as conjugating spacer and diversely substituted at the 1,3-dithiole ring (R) have been synthesized. Electronic absorption spectra show the expected decrease of HOMO−LUMO gap when increasing the electron-releasing power of R or the length of the conjugating spacer. Cyclic voltammetry (CV) shows that whereas compounds 1 and 2 are reversibly oxidized into their cation radical and dication through two one-electron steps, for compounds 3 the dication is formed directly via a two-electron transfer. Comparison of the data for compounds 2 and 3 with those of their respective analogues based on thiophene and dithienylethylene shows that introduction of the electron-donating alkoxy groups at the 3 and 4 positions of the thiophene ring produces a 150−200 mV negative shift of the first redox potential ( E ° 1 ). On the other hand, CV data for compounds 1 and 2 reveal several unusual features such as E ° 1 ≈ 0.10 V/SCE ranking among the lowest known to date and a Coulombic repulsion between positive charges in the dication larger than for the analogue π-donors based on unsubstituted thiophene. These results are interpreted by a major reorganization of the electronic distribution in the donor molecule due to alkoxy groups: the highest electron density moving from the 1,3-dithiole moiety toward the central thiophene ring.