Understanding the charge transport properties of redox active metal–organic conjugated wires

Layer-by-layer assembly of the dirhodium complex [Rh(2)(O(2)CCH(3))(4)] (Rh(2)) with linear N,N′-bidentate ligands pyrazine (L(S)) or 1,2-bis(4-pyridyl)ethene (L(L)) on a gold substrate has developed two series of redox active molecular wires, (Rh(2)L(S))(n)@Au and (Rh(2)L(L))(n)@Au (n = 1–6). By controlling the number of assembling cycles, the molecular wires in the two series vary systematically in length, as characterized by UV-vis spectroscopy, cyclic voltammetry and atomic force microscopy. The current–voltage characteristics recorded by conductive probe atomic force microscopy indicate a mechanistic transition for charge transport from voltage-driven to electrical field-driven in wires with n = 4, irrespective of the nature and length of the wires. Whilst weak length dependence of electrical resistance is observed for both series, (Rh(2)L(L))(n)@Au wires exhibit smaller distance attenuation factors (β) in both the tunneling (β = 0.044 Å(–1)) and hopping (β = 0.003 Å(–1)) regimes, although in (Rh(2)L(S))(n)@Au the electronic coupling between the adjacent Rh(2) centers is stronger. DFT calculations reveal that these wires have a π-conjugated molecular backbone established through π(Rh(2))–π(L) orbital interactions, and (Rh(2)L(L))(n)@Au has a smaller energy gap between the filled π*(Rh(2)) and the empty π*(L) orbitals. Thus, for (Rh(2)L(L))(n)@Au, electron hopping across the bridge is facilitated by the decreased metal to ligand charge transfer gap, while in (Rh(2)L(S))(n)@Au the hopping pathway is disfavored likely due to the increased Coulomb repulsion. On this basis, we propose that the super-exchange tunneling and the underlying incoherent hopping are the dominant charge transport mechanisms for shorter (n ≤ 4) and longer (n > 4) wires, respectively, and the Rh(2)L subunits in mixed-valence states alternately arranged along the wire serve as the hopping sites.