Effect of chain extension on the electrochemical and electronic properties of pi-conjugated soluble thienylenevinylene oligomers

Thienylenevinylene oligomers (nTVs) containing up to 10 thiophene rings and bearing solubilizing hexyl groups at the alpha-position of the end thiophene rings or octyl or dibutyl chains at the 3-position or the 3- and 4-positions of the thiophene rings have been synthesized by a combination of formylation reaction, Wittig-Horner olefination, and McMurry dimerization. Owing to the good solubility imparted by alkyl chains, the electrochemical behavior of nTVs has been analyzed for the first time. Chain extension leads to a negative shift of the peak potentials corresponding to the formation of the cation radical and dication and to a decrease of their difference. For the octamers the dication is formed directly through a two-electron transfer while the system can be reversibly charged up to the tetracationic state. Electronic absorption spectra show the expected bathochromic shift of lambda(max) and decrease of the HOMO-LUMO gap with extension of the conjugation length. Thus, solution-cast films of octamers exhibit band-gap values comparable to or even smaller than that of the parent poly(thienylenevinylene) (PTV). Extrapolation of the electrochemical and optical data to infinite chain length suggests that the oxidation potential and intrinsic band gap of a defect-free PTV could be significantly smaller than the present experimental values.