ACYCLIC TETRAETHYNYLETHENE MOLECULAR SCAFFOLDING - MULTINANOMETER-SIZED LINEARLY CONJUGATED RODS WITH THE POLY(TRIACETYLENE) BACKBONE AND CROSS-CONJUGATED EXPANDED DENDRALENES

Derivatives of fully cross-conjugated tetraethynylethene (3,4-diethyny lhex-3-ene-1,5-diyne) 1 are versatile precursors to multinanometer-siz ed molecular rods with all-C-backbones. Oxidative polymerization (CuCl , N,N,N',N'-tetramethylethylenediamine (TMEDA), O-2) of the trans-bis- deprotected trans-bis(triisopropylsilyl)-protected tetraethynylethene 2 yielded, after end-capping with phenylacetylene, the remarkably stab le, soluble oligomers 3-7 with a persilylethynylated poly(triacetylene ) (PTA) backbone [-(C=C-CR=CR-C=C)(n)-] and a length of 19.4 (3), 26.8 (4), 34.3 (5), 41.8 (6), and 49.2 (7) Angstrom (Scheme I). These comp ounds underwent facile one-electron reductions with the number of reve rsible reduction steps being equal to the number of tetraethynylethene moieties in each molecular rod. Oxidative Eglinton-Glaser home-coupli ng of tetraethynylethenes 8-10 with a single free ethynyl group provid ed the fully silyl-protected 3,4,9,10-tetraethynyl-substituted dodeca- 3,9-diene-1,5,7,11-tetraynes 11-13 (Scheme 2) and, after alkyne deprot ection, the novel hydrocarbon 14, a C20H6 isomer, and its partially si lyl-protected derivative 15. Oxidative hetero-coupling between two dif ferent tetraethynylethene derivatives, one with a single and the other with two free terminal ethynyl groups, yielded the extended chromopho res 16-21 composed of 3 or 4 tetraethynylethene moieties (Scheme 3). T he linearly conjugated oligomers 16 and 17 with the PTA backbone are i someric to 19 and 20, respectively, which are members of the cross-con jugated expanded dendralenes, i.e., dendralenes with butadiynediyl fra gments inserted between each pair of double bonds [-(C=C-C(=CR(2))-C=C )(n)-]. The electronic absorption spectra of these compounds were comp ared and analyzed in terms of the competition between linear and cross -conjugation in determining the extent of pi-electron delocalization. Although steric factors on pi-electron conjugation remain to be clarif ied, this analysis strongly suggests that cross-conjugation is not an efficient mechanism for pi-electron delocalization. All extended acety lenic-olefinic chromophores considered in this study exhibited remarka bly high stability and did not decompose when exposed to laboratory ai r and light for months. In agreement with this observation, electroche mical studies demonstrated that the compounds are difficult to oxidize with the oxidation potentials in THF (0.1M (Bu(4)N)PF6) being higher than 1.0 V (vs. the ferrocene/ferrocenium couple).