Abstract
Layer-by-layer assembly of the dirhodium complex [Rh2(O2CCH3)4] (Rh2) with linear N,N'-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 = 1-6). 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, (Rh2LL) 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 (Rh2LS) n @Au the electronic coupling between the adjacent Rh2 centers is stronger. DFT calculations reveal that these wires have a π-conjugated molecular backbone established through π(Rh2)-π(L) orbital interactions, and (Rh2LL) n @Au has a smaller energy gap between the filled π*(Rh2) and the empty π*(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.
Highlights
The dirhodium complex [Rh2(O2CCH3)4] is a strong Lewis acid with respect to its axial coordination capability and readily forms [Rh2(m-O2CCH3)4L2] adducts with Lewis bases, L, and is stable under aerobic conditions. These features allow the facile assembly of Rh2 and linear N,N0-bidentate bridging ligands L (L 1⁄4 ligands pyrazine (LS) for pyrazine or LL for 1,2-bis(4-pyridyl)ethene), yielding a linear structure with the Rh–Rh bonds aligned with the long molecular axis
To immobilize the self-assembled monolayers (SAMs) onto the gold substrate, 2-(4-pyridyl) ethanethiol was utilized as a molecular anchor that is bonded to the Au surface with the S atom, preparing a pyridyl group for the complexation of the incoming Rh2 building block
Similar processes involving [Rh2(phen)2(m-O2CCH3)2(NCMe)2], pyrazine (LS) and 4-thiopyridine as a surface anchor,[46] and systems consisting of bis(Rh2) dimers, 1,2-bis(4-pyridyl)ethene (LL) and 2-(4-pyridyl) ethanethiol as anchors[40] have been reported by other groups
Summary
Molecular electronics has attracted great attention due to the potential applications of preprogrammed molecules as components in nanoscale circuits.[1,2] Whilst molecular junctions that offer electronic function equivalent to traditional components such as recti ers,[3] switches[1,4,5,6] and transistors[7] receive growing attention, the synthesis of linear molecules that can be embedded between two electrodes and function as molecular wires continues to be of primary importance for the development of science underpinning the operation of molecular electronic devices.[8,9,10,11] Generally speaking, the conductive performance of a molecular wire in a molecular junction is determined by a convolution of factors including the environment,[12,13,14] the nature of the molecule–electrode contacts[15,16] and the electronic con guration of the molecule.[17,18] DetailedIn both single molecular junctions[19,34] and self-assembled monolayers (SAMs),[17,20,25,26,27] two principal conductance mechanisms, charge tunneling for shorter wires and charge hopping for longer wires, have been identi ed. Molecular electronics has attracted great attention due to the potential applications of preprogrammed molecules as components in nanoscale circuits.[1,2] Whilst molecular junctions that offer electronic function equivalent to traditional components such as recti ers,[3] switches[1,4,5,6] and transistors[7] receive growing attention, the synthesis of linear molecules that can be embedded between two electrodes and function as molecular wires continues to be of primary importance for the development of science underpinning the operation of molecular electronic devices.[8,9,10,11] Generally speaking, the conductive performance of a molecular wire in a molecular junction is determined by a convolution of factors including the environment,[12,13,14] the nature of the molecule–electrode contacts[15,16] and the electronic con guration of the molecule.[17,18] Detailed.
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