Photoinduced reaction of hydrogen-end-capped polyynes with iodine molecules

Hydrogen-end-capped polyynes, H(C≡C)(n)H (n = 5-7), were photoirradiated in the presence of iodine molecules in nonpolar solvents to find a dramatic change in the UV/vis absorption spectrum. Absorption bands of polyynes in the UV and the band of I(2) in the visible region disappeared, whereas weaker bands of polyynes were intensified in the near UV region. The emerging features are associated with vibronic bands in the symmetry-forbidden transition of the linear polyyne molecule. Stoichiometry for the reaction, i.e., C(12)H(2) + nI(2) → C(12)H(2)I(2n), was determined to be n = 3 from the concentration-dependence experiment. (13)C NMR spectra for 1:3 mixture of polyyne and iodine molecules, namely C(10)H(2)/3I(2) and C(12)H(2)/3I(2), exhibited five and six lines, respectively, all shifted to lower fields compared to those in the case without iodine. After removing excess I(2) by reductive reagent of Na(2)SO(3), recovery of the missing absorption bands for the components of C(10)H(2) and I(2) was observed. These observations strongly support the formation of an unique molecular complex of C(2n)H(2)I(6) (n = 5-7) upon photoabsorption by an I(2) molecule within a cluster of polyyne and iodine molecules, C(2n)H(2)(I(2))(m) (m ≥ 3).