Understanding the charge transport properties of redox active metal–organic conjugated wires. Issue 14 (12th March 2018)

• Record Type: Journal Article
• Authors: Bu, Donglei; Xiong, Yingqi; Tan, Ying Ning; Meng, Miao; Low, Paul J.; Kuang, Dai-Bin; Liu, Chun Y.
• Is Part Of: Chemical science. Volume 9:Issue 14(2018)
• Page Start: 3438
• Page End: 3450
• Publication Date: 2018-03-12
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c7sc04727d
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : For Rh2 -organic molecular wires, we found that weaker coupling systems built using longer bridging ligands exhibit better electrical conductance. Abstract : Layer-by-layer assembly of the dirhodium complex [Rh2 (O2 CCH3 )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, (Rh2 LS ) n @Au and (Rh2 LL ) 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, (Rh2 LL ) 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 (Rh2 LS ) 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 (Rh2 LL ) n @Au has a smaller energy gap between the filled π*(Rh2 ) and the empty π*(L) orbitals. Thus, for (Rh2 LL ) n @Au, electron hopping across the bridge is facilitated by the decreased metal to ligand charge transfer gap, while in (Rh2 LS ) 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 Rh2 L subunits in mixed-valence states alternately arranged along the wire serve as the hopping sites.