The Synthesis and Polymerization Behavior of Polyynes

Abstract

Polyynes are 1-dimensional rods composed of sp-hybridized carbon atoms, which have attracted much interest due to their electronic, optical, and physical properties. These carbon oligomers can serve as possible precursors to graphitic ribbons, as subjecting aggregates to heat and/or irradiation can lead to topochemical polymerization. Furthermore, polyynes can serve as models for understanding conjugation, and can be used to predict the properties of carbyne, the linear allotrope of carbon. In this work, the synthesis, characterization, and polymerization behavior of polyynes with various end groups have been thoroughly explored. An iterative synthetic strategy was developed to prepare tetraynes, pentaynes, hexaynes, and heptaynes, via a series of iodination and Stille coupling reactions. The advantage of this route over existing methods is that the required starting materials are simple, easily accessible, and relatively stable. This method is particularly effective for accessing polyynes with an odd number of carbon-carbon triple bonds, and has allowed for the preparation of the novel iodine-capped pentayne, diiododecapentayne. The topochemical polymerization of diiodohexatriyne was also achieved by applying a host-guest strategy that aligns the monomers in the proper orientation for controlled polymerization via halogen bonding interactions. Bis(nitrile) oxalamide hosts form co-crystals with the guest triyne in a stoichiometric ratio of 1:2, as determined by X-ray diffraction. Raman spectroscopy was used to confirm the polymerization of diiodohexatriyne, as the increased disorder in the crystal made it difficult to elucidate the structure using XRD. In addition, the synthesis of diynes containing nitrile end groups was also pursued, to determine the effects of electron withdrawing groups on polymerization behavior. Dicyanodiacetylene was initially targeted, since the 1,4-topochemical polymerization of this diyne would result in a unique, very electron poor polymer. Although efforts towards dicyanodiacetylene were unsuccessful, a synthetic route towards cyanoiodobutadiyne was developed using modified Cadiot-Chodkiewicz coupling conditions. The topochemical polymerization of this novel push-pull diyne monomer was pursued using both symmetric and asymmetric host molecules bearing oxalamide functionalities.