Low oxidation potential tetrathiafulvalene analogues based on 3,4-dialkoxythiophene pi-conjugating spacers

Tetrathiafulvalene analogues involving dihexyloxythiophene (1), ethylenedio xythiophene (2), and bis(3,4-dihexyloxy-2-thienyl)ethylene (3) as conjugati ng spacer and diversely substituted at the 1,3-dithiole ring (R) have been synthesized. Electronic absorption spectra show the expected decrease of HO MO-LUMO gap when increasing the electron-releasing power of R or the length of the conjugating spacer. Cyclic voltammetry (CV) shows that whereas comp ounds 1 and 2 are reversibly oxidized into their cation radical and dicatio n through two one-electron steps, for compounds 3 the dication is formed di rectly via a two-electron transfer. Comparison of the data for compounds 2 and 3 with those of their respective analogues based on thiophene and dithi enylethylene shows that introduction of the electron-donating alkoxy groups at the 3 and 4 positions of the thiophene ring produces a 150-200 mV negat ive shift of the first redox potential (E degrees(1)). On the other hand, C V data for compounds 1 and 2 reveal several unusual features such as E degr ees(1) approximate to 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 ct-donors based on unsubstituted thiophene. These results are interpreted by a major reorganization of the electronic distribution i n the donor molecule due to alkoxy groups: the highest electron density mov ing from the 1,3-dithiole moiety toward the central thiophene ring.