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.