COMMUNICATIONS 942  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998 1433-7851/98/3707-0942 $ 17.50+.50/0 Angew. Chem. Int. Ed. 1998, 37 , No. 7 Extended Thienylenevinylene Oligomers as Highly Efficient Molecular Wires** Isabelle Jestin, Pierre Fre Áre, Philippe Blanchard, and Jean Roncali* Linear p-conjugated oligomers with a well-defined chem- ical structure have been intensively investigated to simulate the electronic and electrochemical properties of the corre- sponding polydisperse polymers. [1] More recently, their pos- sible use as molecular wires in molecular electronics or nanoscopic systems has attracted particular attention. [2] Although most of the work in these two areas has involved oligomers of intermediate chain length (n  2 ± 10), [1, 2] the synthesis of highly extended p-conjugated oligomers is of crucial importance for several reasons. On one hand, the use of p-conjugated oligomers as molecular wires in nanoscopic systems requires dimensions approaching the present limit of nanopatterning techniques (namely the 100  gap regime). [2c] On the other hand, as the oligomeric model postulates that the properties of an oligomer series would approach those of the polymer as chain length increases, the analysis of the ultimate electronic properties of highly extended systems should permit assessment of the relevance of the oligomeric model and/or give evidence of novel properties. The recently synthesized hexadecamers of the oligothio- phenes, [3] oligo(phenylenethynylene)s [4] and oligo(thiophe- nethynylene)s [5] series are the longest clearly characterized linear p-conjugated systems. Although the size of these molecules (65 ± 120 ) make them potentially interesting as molecular wires, these various classes of oligomers show a rapid saturation of effective conjugation with chain extension. In other words, the ultimate values of the oxidation potential and HOMO ± LUMO gap are attained far before the max- imum chain length (at n  10 ± 12 for oligothiophenes [1, 3] and n  8 for oligo(phenylenethynylene)s and oligo(thienylene- thynylene)s). [4, 5] As this saturation originates from p-electron confinement related to structural factors such as rotational disorder or resonance stabilization energy of the aromatic ring, [6] oligomeric systems in which such effects are minimized may even surpass the present limit. Although the excellent electron transmission efficiency of oligo(thienylenevinylene)s (nTVs) has been demonstrated in nonlinear optical chromophores [7] or extended tetrathiafulva- lene analogues, [8] this class of conjugated oligomers has been scarcely considered until now. nTVs up to n  7 were first synthesized by Kossmehl et al. by a succession of formylation reactions and Wittig olefinations. [9] More recently a slightly different approach was reported by Nakayama et al.; how- ever, the insolubility of nTVs did not allow the n  7 limit to be surpassed. [10] We recently described the synthesis of alkyl-substituted soluble nTVs. [11] The chain length dependence of the electro- chemical and electronic properties gave no evidence of saturation up to the decamer (n  10), but further chain extension was hampered by solubility problems. We report here on the synthesis and characterization of novel soluble nTVs based on 3,4-dihexylthiophene (2a, 4a, 6a, 8a, 12 a, and 16 a) and on the analysis of the chain length dependence of their electrochemical and electronic proper- ties. Except for the dimer (2a) and tetramer (4a), all longer nTVs have been synthesized by twofold Wittig ± Horner olefination of dicarbaldehydes with phosphonate 2e (Scheme 1). Although McMurry dimerization [12] leads to doubling of the chain length at each iterative step, [10,11] we finally opted for the double Wittig ± Horner route because of the increasing difficulty to achieve monoformylation selec- tively as the chain length increases. S S S S S S m 6a, m = 1 8a, m = 2 12a, m = 4 16a, m = 6 Dithienylethylene 2a was prepared by McMurry coupling of 3,4-dihexylthiophene carbaldehyde (1b). Mono- and dia- ldehydes 2b and 2c were prepared by Vilsmeier formylation of 2a with appropriate amounts of reagents. McMurry coupling of 2b yielded the tetramer 4a, while the hexamer 6a was prepared by double Wittig ± Horner olefination of 2c with 2e. Vilsmeier bisformylation of 4a and twofold Wittig ± Horner olefination of the resulting dialdehyde (4b) gave the octamer 8a. Repetition of this procedure yielded the dodeca- mer 12 a and hexadecamer 16a (Table 1), which are the longest ever reported nTVs (70 and 94 sp 2 carbon atoms). Optimization of the geometry of 16 a by the MM  method (Hyperchem 5.0) gave a chain length of 95 , in good agreement with the value estimated from the X-ray structure of the tetramer. [13] Table 2 lists the main cyclic voltammetry (CV) data for nTVs. As expected, chain extension leads to a negative shift of the redox potentials and to an increase in the number of accessible redox states. Whereas the CV of 2a shows a first anodic peak indicative of an irreversible process (the sub- sequent polymerization), [14] that of 4a exhibits two well- defined reversible one-electron oxidation waves correspond- ing to the successive generation of the cation radical and dication (E pa 1 and E pa 2). Chain extension produces a decrease in the difference E pa 2  E pa 1, indicative of a reduction of Coulombic repulsion between positive charges in the dication. Thus, for 6a the two oxidation peaks are almost merged into a single wave, and a third peak corresponding to the generation [*] Prof. J. Roncali, Dr. I. Jestin, Dr. P. Fre Áre, Dr. P. Blanchard Inge  nierie Mole Âculaire et Mate  riaux Organiques CNRS UMR 6501, Universite  dAngers 2 Bd Lavoisier, F-49045 Angers (France) Fax: ( 33) 02-4173-5405 E-mail: jean.roncali@univ-angers.fr [**] We thank Dr. B. Illien (IMMO) for geometry optimization and Dr. J. C. Blais (LCSOB), University Paris VI, for recording the MALDI- TOF spectra.