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

Hydrogen-end-capped polyynes, H(C≡C)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 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., CH + nI → C HI, was determined to be n = 3 from the concentration-dependence experiment. C NMR spectra for 1:3 mixture of polyyne and iodine molecules, namely CH/3I and CH/3I, exhibited five and six lines, respectively, all shifted to lower fields compared to those in the case without iodine. After removing excess I by reductive reagent of NaSO, recovery of the missing absorption bands for the components of CH and I was observed. These observations strongly support the formation of an unique molecular complex of CHI (n = 5-7) upon photoabsorption by an I molecule within a cluster of polyyne and iodine molecules, C H(I) (m ≥ 3). © 2011 American Chemical Society.