α,ω-Diferrocenyl Cumulene Molecular Wires Studied by Density Functional Theory

Density functional theory calculations for neutral and oxidized diferrocenyldiphenylcumulenes, [Fc(Ph)Cn(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.