科研之友
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摘要
Layer-by-layer assembly of the dirhodium complex [Rh-2(O2CCH3) 4] (Rh-2) with linear N, N0-bidentate ligands pyrazine (LS) or 1,2-bis(4-pyridyl) ethene (LL) on a gold substrate has developed two series of redox active molecular wires, (Rh2LS) n@ Au and (Rh2LL) n@ Au (n 1/4 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 1/4 4, irrespective of the nature and length of the wires. Whilst weak length dependence of electrical resistance is observed for both series, (Rh2LL) n@ Au wires exhibit smaller distance attenuation factors (b) in both the tunneling (b 1/4 0.044 degrees A(-1)) and hopping (b 1/4 0.003 degrees A (-1)) regimes, although in (Rh2LS) n@ Au the electronic coupling between the adjacent Rh-2 centers is stronger. DFT calculations reveal that these wires have a p-conjugated molecular backbone established through p(Rh-2)-p(L) orbital interactions, and (Rh2LL) n@ Au has a smaller energy gap between the filled p*(Rh-2) and the empty p*(L) orbitals. Thus, for (Rh2LL) n@ Au, electron hopping across the bridge is facilitated by the decreased metal to ligand charge transfer gap, while in (Rh2LS) 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 Rh2L subunits in mixed-valence states alternately arranged along the wire serve as the hopping sites.
