Poly(triacetylene) oligomers: Conformational analysis by X-ray crystallography and synthesis of a 17.8-nm-long monodisperse 24-mer

Starting from the octameric poly(triacetylene) (PTA) oligomer 1e as a large 'macromonomer', the monodisperse tetracosamer (24-mer) 1h was prepared by a previously introduced statistical deprotection-oligomerization sequence ( Scheme). It is the longest known molecular rod featuring a fully conjugated , non-aromatic all-carbon backbone. Matrix assisted laser-desorption-ioniza tion time-of-flight (MALDI-TOF) mass spectrometry was particularly useful i n the characterization of oligomer 1h and clearly demonstrated its monodisp ersity (Fig. 1). In an effort to further clarify the conformational prefere nces of PTA oligomers, the X-ray crystal structure of the 3.2-nm-long tetra mer 1c was solved (Figs. 2-4). In the solid start, the C=C bonds in 1c all adopt the s-trans conformation with respect to the buta-1,3-diynediyl moiet ies. The pi -conjugated system is perfectly planar, with the squared sum of the deviations of the backbone C-atoms from the best plane amounting to 0. 077 Angstrom (2). Analysis of the crystal lattice revealed a layered struct ure, in which the pi -conjugated backbone of one oligomer is insulated by t he trialkylsilyl groups of adjacent oligomers in neighboring layers.