Electronic spectra of conjugated polyynes, cumulenes and related systems:: A theoretical study

Electronic spectra of conjugated polyynes [H(C equivalent to C)(nT)H, nT=1 to 7] and cumulenes [H-2(C)(nC)H-2, nC=1 to 9] were calculated by means of the time-dependent density functional theory (TD DFT) and, for a group of selected molecules, also by the symmetry-adapted cluster configuration interaction method (SAC-CI). A comparison was made between calculated and published experimental spectral data. It turned out that the TD DFT (with B3LYP) was a reliable tool for calculation of band positions in the longest-wavelength region. In the case of cumulenes, except allene, only data for alkyl and phenyl derivatives are available. The characteristics of cumulenes split into two distinctly separate classes, planar (with an even number of carbon atoms, D-2h) and non-planar (with an odd number of carbon atoms, D-2d). Special attention was paid to the influence of substituents of various types on the position of the first intensive bands of polyynes and polyenes. Plotting wavenumbers of these bands against the reciprocal number of the CC triple bonds (polyynes) or the CC double bonds (polyenes) lead to (partial) rectification and made the extrapolation to infinite chains easier. While the extrapolated value for infinite polyynes (and derivatives like dimethyl or dicyano) amounts to about 16000 cm(-1), for anions and radical anions derived from the parent polyynes, the extrapolated value tends to significantly smaller values or possibly to zero. The situation with polyenes and their derivatives and heteroanalogues is similar; small deviations from zero with chains without significant bond alternation seem likely.