α,ω-diferrocenyl cumulene molecular wires studied by density functional theory

Density functional theory calculations for neutral and oxidized diferrocenyldiphenyl-cumulenes, [Fc(Ph)C-n(Ph)Fc] with n = 2, 4, 6, 8, were performed at the B3LYP/6-31G level without any restrictions in symmetry. Geometrically, of the four possible diasteromeric structures of the neutral cumulenes, no preferential conformation was revealed, and the calculated structures reproduce the experimental X-ray structural data quite well. The spin density of all mono- and dications is located on the iron atoms, but not on the cumulene chain. Calculations of the ionization potentials for monocations show a breakdown of Koopmans' theorem. A calibration procedure was proposed for evaluation of vertical ionization potentials, resulting in values with experimental accuracy. UV-visible absorptions are assigned in terms of their band maximums, transition energies, and bandwidths on the basis of deconvoluted experimental spectra and on the basis of comparison with structurally related compounds. Calculations of the characteristic high-frequency symmetric vibrations of the cumulene chain show a very good match with experimental Raman bands. Redox potentials in solution are calculated by a simplified model on the basis of empirical findings; the results reproduce the experimental half-wave potentials quite satisfactorily and allow an estimate of an electrochemical decay slope and an effective conjugation limit in these cumulene molecular wires. The structural, chemical, optical, electrochemical, and molecular orbital properties of 2-31 make ferrocenyl cumulenes very promising molecular objects with great potential in molecular-scale electronics in comparison with pure organic molecular wires.