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Electron transfer in two and three dimensions

Authors
Publisher
Dublin City University. School of Chemical Sciences
Publication Date
Keywords
  • Chemistry
  • Oxidation-Reduction Reaction
  • Charge Transfer
Disciplines
  • Chemistry
  • Communication

Abstract

A number of osmium bipyndyl complexes have been synthesised and characterised using spectroscopic, chromatographic and electrochemical techniques. The complexes formed are [Os(bpy)2 4-tet C1]C104, [Os(bpy)2 4-bpt C1]PF6 and [Os(bpy)2 Cl 4-bpt Os(bpy) 2 C1](PF6)2, where bpy is 2 ,2 ’-bipyridyl, 4-tet is 3,6-bis(4-pyndyl)-l,2,455-tetrazine and 4- bpt is 3,5-bis(pyridin-4-yl)-l,2,4-tnazole Monolayers of [Os(bpy) 2 4-tet C1]C1 0 4 have been formed by spontaneous adsorption onto clean gold microelectrodes. The tetrazine bridge between the [Os(bpy)2Cl]+ head group and the metal electrode surface undergoes a reversible protonation/deprotonation reaction depending on the pH of the contacting electrolyte solution High speed cyclic voltammetry reveals that the redox switching mechanism is best described as a non-adiabatic, through-bond tunnelling mechanism Significantly, while protonating the bridging ligand does not influence the free energy of activation, 10 3±1 1 kJ mol *, k° decreases by 1 order of magnitude from 1 1 x 104 s 1 to 1 2 x 103 s 1 upon going from a deprotonated to a protonated bridge. These observations are interpreted in terms of a through-bond tunnelling mechanism m which protonation decreases the electron density on the bridge and reduces the strength of electronic coupling between the redox centre and the electrode. Solid deposits of the dimeric complex [Os(bpy) 2 Cl 4-bpt Os(bpy)2 C1](PF6)2 have been deposited on platinum microelectrodes by mechanical attachment. The electrochemical response exhibited by these deposits is unusually ideal over a wide range of electrolyte compositions and pH values Dct, the charge transport diffusion coefficient, is independent of the electrolyte concentration, indicating that electron self-exchange between adjacent redox centres limits the overall rate of charge transport through the solid In 1 0 M L1CIO4 and 1 0 M HCIO4, Dct values are 2 0±0 lxlO10 and 1 7±0 4x10 10 cm2 s corresponding to second order electron transfer rate constants of 18 x l07 and 3xl07 M 1 s 1. The standard rate of heterogeneous electron transfer across the electrode/deposit interface is 1 08+0 05x10 cm s. This value is approximately one order of magnitude lower than that found for a similar monomeric complex in which the bridging ligand is attached directly to the electrode surface, indicating that the 4-bpt ligand does not promote strong electronic communication between the [Os(bpy)2CI]+ head group and the electrode surface. Monolayers of [Os(bpy)2 4-bptCl]PF6 have been formed by spontaneous adsorption onto platinum microelectrodes. These monolayers are extremely stable under a wide range of electrolyte compositions and pH values Significantly, the 4-bpt ligand is capable of undergoing a protonation/deprotonation reaction depending on the pH of the contacting electrolyte solution. High speed chronoamperometry reveals that protonation of the 4-bpt bridging ligand causes the standard rate of heterogeneous electron transfer to decrease by at least an order of magnitude from 2 67 x 106 to 4 5 x 104 s' 1 for the oxidation process and from 1 60 x 106 to 1 9 x 105 for the reduction process Consistent with a superexchange mechanism, these observation are interpreted in terms of a hole superexchange process, the rate of which decreases with increasing energy gap between the osmium metal dn orbitals and the highest occupied molecular orbital of the bridge.

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