Photoinduced reaction of methylpolyynes H(C≡C)_nCH₃ (n = 5-7) and polyyne H(C≡C)₅H with I₂ molecules

Abstract

Methylpolyynes, H(C≡C) _n CH₃ (n = 5-7), were photo-irradiated under the presence of iodine molecules in hexane to confirm the formation of a series of molecular complexes, H(C≡C) _n CH₃(I₆) (n = 5-7), by systematic changes in their UV absorption spectra. In order to investigate the reaction mechanism, solutions of hydrogen-capped polyyne, H(C≡C)₅H, and I₂ molecules in hexane were irradiated with a cw-laser beam at 532 nm, then their UV absorption spectra were recorded to obtain temporal decay profiles for the polyyne C₁₀H₂. At C₁₀H₂ concentrations of about 7 μmol L^-1, the rate of reaction in the decay profile for C₁₀H₂ increased with increasing I₂ concentration (8.2–126 μmol L^-1) as well as with increasing laser power (0.34–6.2 mW). Based on a kinetic model, the degradation of C₁₀H₂ was shown to compete with the formation of the polyyne-iodine molecular complex, H(C≡C)₅H(I₆). The formation of the complex was found to be efficient under the condition that the I₂ concentration is orders of magnitude higher than the C₁₀H₂ concentration as this suppresses the degradation process.