Preparation of Ordered Polyacetylene by Solid-State Polymerization in Nanoscale Confinement

We report a novel, highly effective strategy for controlling the synthesis of polyacetylene as a guest in an organic host crystal by monitoring in situ an elimination-condensation polymerization reaction. Specifically, in this process, the polymer material is forced to have its chains extended and aligned such that translational periodicity applies, producing a bond alternation potential that has a symmetric double minimum. The synthetic approach used is photochemical elimination of iodine from a conjugated diene, (E,E)-1,4-diiodo-1,3-butadiene, which forms a commensurate and fully ordered urea inclusion compound. Photochemical cleavage of the terminal C-I bonds results in elimination of iodine from the single crystal and formation of C-C bonds between adjacent radicals to produce the conjugated 1,8-diiodo-1,3,5,7-octatetraene and subsequent longer polyene species. The combination of in situ crystal mass-loss measurements and vibrational Raman spectroscopy demonstrates clearly the presence of new polyene chains and loss of iodine from the urea substructure. The first few product oligopolyenes exhibit very strong Raman scattering with the most intense vibrational features decreasing in frequency for longer chains approaching an asymptotic limiting frequency that mimics the behavior of conjugated polyenes of known lengths from previous vibrational Raman studies. With extensive irradiation, the mass loss approaches that anticipated from the crystal stoichiometry and, at the same time of irradiation, the Raman intensity largely disappears. These results demonstrate that the reaction reported here proceeds to completion, leading to a quasi-one-dimensional array of isolated polyacetylene chains that are constrained to be in a continuous extended, all-trans conformation within the tunnels formed by the urea crystal lattice.