Multidecker Bis(benzene)chromium: Opportunities for Design of Rigid and Highly Flexible Molecular Wires

On the basis of density functional theory calculations, we have designed three classes of multidecker bis(benzene)chromium molecular wires with −(arene−chromium(0)−arene)− sandwich complexes as monomer units. The arene fragments of the wires are either [2.2]paracyclophane (class- 1 ), biphenylene (class- 2 ), or biphenyl (class- 3 ) compounds with two strongly coupled benzene rings. The wires are rigid (class- 1 ) or highly flexible (class- 3 ), and they are realistic synthetic targets as the bonding at each Cr (0) atom satisfies the 18-electron rule. The Cr (0) atoms couple strongly with the arene units giving a “quasi-band” that stems from the highest occupied molecular orbital (HOMO) of the monomers, a HOMO sub-band in which the orbitals are highly delocalized indicating metal/π-conjugation. Moreover, the HOMO energies are close to the Fermi energy of the metal electrodes used (Zn(111)), and therefore, injected electrons can easily tunnel through the wires. The metal of the electrodes was selected so that its Fermi level is located slightly above the HOMO energies of the wires. High conductivity and very slow decay of conductance with increased length are found for all three wire classes, making them suitable for molecular electronics applications. Class- 2 and class- 3 wires display high conformational flexibilities and, simultaneously, only modest conformational dependence of the conductance. These wires therefore function as molecular electrical cords, i.e., molecules which are easily twisted and coiled and for which the conductance displays only modest conformational dependence.