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‐diethynylhex‐3‐ene‐1,5‐diyne) 1 are versatile precursors to multinanometer‐sized molecular rods with all‐C‐backbones. Oxidative polymerization (CuCl, N,N,N′,N′‐tetramethylethylenthylenediamine (TMEDA), O₂) of the trans‐bis‐deprotected trans‐bis(triisopropylsilyl)‐protected tetraethynylethene 2 yielded, after end‐capping with phenylacetylene, the remarkably stable, 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) Å (Scheme 1). These compounds underwent facile one‐electron reductions with the number of reversible reduction steps being equal to the number of tetraethynylethene moieties in each molecular rod. Oxidative Eglinton‐Glaser homo‐coupling of tetraethynylethenes 8–10 with a single free ethynyl group provided 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 deprotection, the novel hydrocarbon 14, a C₂₀H₆ isomer, and its partially silyl‐protected derivative 15. Oxidative hetero‐coupling between two different tetraethynylethene derivatives, one with a single and the other with two free terminal ethynyl groups, yielded the extended chromophores 16–21 composed of 3 or 4 tetraethynylethene moieties (Scheme 3). The linearly conjugated oligomers 16 and 17 with the PTA backbone are isomeric to 19 and 20, respectively, which are members of the cross‐conjugated expanded dendralenes, i.e., dendralenes with butadiynediyl fragments inserted between each pair of double bonds [(CCC(CR₂)CC)_n]. The electronic absorption spectra of these compounds were compared and analyzed in terms of the competition between linear and cross‐conjugation in determining the extent of π‐electron delocalization. Although steric factors on π‐electron conjugation remain to be clarified, this analysis strongly suggests that cross‐conjugation is not an efficient mechanism for π‐electron delocalization. All extended acetylenic‐olefinic chromophores considered in this study exhibited remarkably high stability and did not decompose when exposed to laboratory air and light for months. In agreement with this observation, electrochemical studies demonstrated that the compounds are difficult to oxidize with the oxidation potentials in THF (0.1M(Bu₄N)PF₆) being higher than 1.0 V (vs. the ferrocene/ferrocenium couple).