Electronic Absorption Spectra of the Polyacetylene Chains HC₂nH, HC₂nH⁻, and HC₂n₋₁N⁻ (n = 6−12) in Neon Matrixes

Three electronic transitions of the isoelectronic species HC 2 n H - and HC 2 n - 1 N - and one of the neutral polyacetylenes HC 2 n H ( n = 6−12) have been observed by absorption spectroscopy in neon matrixes. Anions produced in an electron impact source were mass-selected and codeposited with excess of neon at 6 K. The HC 2 n H species were formed from the anions by photodetachment. A well-structured band system at 780 nm for HC 12 H -, and shifting by regular increments to 1325 nm for HC 24 H -, is assigned to two overlapping electronic transitions of 2 Π ← X 2 Π symmetry. Another 2 Π ← X 2 Π transition with broader features is observed in the 300−480 nm region. Corresponding transitions are observed for the isoelectronic cyanosubstituted anions HC 2 n - 1 N - . The neutral polyacetylenes show a vibrationally well-resolved UV band system attributed to the 1 ← X 1 electronic transition. The energy of the electronic transition shows a characteristic inverse dependence on the carbon chain length.