Electrochemical STM Break Junction Studies of Single Molecular Conductance for Ligand-Supported Tri-Metal Strings
Date Issued
2009
Date
2009
Author(s)
Shih, Kai-Neng
Abstract
In this thesis, we demonstrate that manipulation of the single molecular conductance by controlling the redox states of tri-metal string complexes. The target molecules are [Ru3(μ3-dpa)4(X)2] (dpa– = dipyridylamido anion, X = NCS–, CN–) and [M3(μ3-dpa)4(NCS)2] (M = Cr, Co, Ni, Cu). The metal atoms are collinear and wrapped helically by four ligands. The measurements were carried out by electrochemical STM break junction (ECSTM BJ), which forms the electrode gap of single molecular scale repeatedly under electrochemical conditions. We have proposed that the conductance is related to the bond order between adjacent metal ions. We expect the electronic configurations or structures for a molecular wire are different for the corresponding redox states. Therefore, the electrochemical control is an approach to tune the conductance of the molecules. The results show that (1) The conductance of [Ru3(μ3-dpa)4(NCS)2]0 is superior to that of [Ru3(μ3-dpa)4(CN)2]0. This indicates that the coupling between the axial ligand and electrode affects the conductivity of the metal string. (2) When [Ru3(μ3-dpa)4(CN)2]0 is oxidized, the conductance increases. This is due to the increase of the bond order. The formation of the π-backbonding from metal to cyanide reduces the π-antibonding energy, and thus the HOMO of this compound is π* orbital. In contrast, isothiocyanate doesn’t have this effect. The HOMO of [Ru3(μ3-dpa)4(NCS)2]0 is πnb orbital, so we can’t observe the change in conductance. (3) For [Cr3(μ3-dpa)4(NCS)2], the conductance in neutral state is superior to oxidized state. This can be ascribed to the degree of delocalization upon 1-e− oxidation. (4) With the viewpoint of bond order, plausible explanation regarding the conductance of the redox states of other metal strings will be provided.
Subjects
ECSTM
break junction
tri-metal strings
Type
thesis
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