Synthesis and structure-property aspects of poly(carbon diselenide)

Poly(carbon diselenide), (CSe₂)_x, is obtained as a black powder either by thermal polymerization of CSe₂ solutions at high pressure, slow thermal polymerization under ambient conditions of neat CSe₂, or photopolymerization of CSe₂ solutions. X-ray diffraction measurements indicate that the as-prepared polymer is highly disordered and does not contain free crystalline or amorphous selenium. This result contrasts with the extensive formation of free selenium in the high pressure, high temperature polymerization of neat CSe₂. The electrical conductivity is below 10^-6 S cm^-1, which is consistent with a band gap of ∼2 eV obtained by diffuse reflection spectroscopy. Details of the thermal properties, and the optical absorption, electron spin resonance, infrared absorption, Raman scattering, and extended x-ray absorption fine structure (EXAFS) spectra of the polymer are discussed. The EXAFS data clearly indicate that Se-Se bonds (but not Se chains) are present. The resonance-enhanced Raman and infrared absorption spectra are consistent with the presence of conjugated carbon-carbon and C=Se bonds, respectively. Based on these results, the structure indicated for (CSe ₂)_x is the head-to-head polymer Se ∥ [-Se-Se-C-C-]_n, ∥ Se rather than the head-to-tail polymer Se Se ∥ ∥ [-C-Se-C-Se-]_n, proposed by previous workers. This polymer thermally decomposes above about 130°C to form trigonal selenium as a by-product.