Single-crystal linear conjugated polyenes through light-induced inclusion polymerization: a possible organic superconductor*

Polyacetylene, (CH)_n, does not dissolve in any solvent and does not melt, prohibiting formation of ordered crystals, thereby hindering its study as a potentially superconductive material. We report a new route for (CH)_n chain synthesis and control over its structural order using the photochemical reaction of a reactive “guest” molecule, diiodohexatriene (DIHT), within the channels of a urea “host” crystal. Broadband light irradiation results in C-I bond scission followed by C–C bond formation and iodine (atoms or molecules) elimination from the urea structure. Continuation results in a complete loss of iodine and formation of extended, non-interacting (CH)_n chains within parallel urea channels. Raman spectroscopy and mass-loss measurements are used to probe DIHT-to-(CH)_n conversion and monitor iodine loss. As the urea-enclosed oligomers grow, Raman signals consistent with conjugated polyenes of known lengths are observed. With extensive irradiation, Raman signals associated with both DIHT and the growing polymer diminish, while iodine is completely expelled from the crystalline urea. These two observations indicate that polymerization goes to completion, leading to an array of isolated, fully-extended (CH)_n chains within the channels. The physical properties of (CH)_n in this idealized extended form, including its conductivity, may differ considerably from those of the material prepared by standard methods. The argument for high-temperature superconductivity will be discussed.