Charge‐Transfer Mechanisms between Gold Clusters

Abstract

Abstract Electron‐transfer processes in complex chemistry are usually described by Taube’s “outer sphere” and “inner sphere” mechanisms. A behaviour related to these electron‐transfer mechanisms in complex chemistry can be observed for charge‐transfer transport between ligand protected Au 55 nanoclusters. A linear dependence between cluster‐cluster distance and activation energy for electron transfer is observed if a noncovalent linkage exists; this interaction is independent of the distance in covalently linked clusters. Instead, the activation energy depends on the electronic nature of the linking molecules. The dithiols 1,5‐dithionaphthaline ( 1 ), 4,4′‐thiobis(benzenethiol) ( 2 ), and 2,8‐dithio‐6‐hydroxypurine ( 3 ) have been used as bifunctional covalent linkers either in their monomeric form ( 2 ) or, in the presence of air, dimerized via S−S bonds ( 1 , 2 , 3 ), causing an increase in the distance between the clusters and leading to the cluster networks 4 − 7 . Noncovalent cluster networks are formed either by pellets of clusters with monodentate ligands in 8 and 11 , linked only by van der Waals forces, or by using bifunctional spacers that interact with the clusters by ion attractions ( 9 , 10 ). A study of the activation energies clearly indicates that in the case of noncovalently organized nanoparticles only the cluster spacing is of relevance, even if conjugated π‐systems like in 10 are used. This behaviour corresponds to an outer‐sphere mechanism. On the contrary, for covalently linked clusters the distances between them does not play a visible role. The activation energies all lie below those of the noncovalent examples, and a relation between cluster‐cluster distance and activation energy is not obvious. In those cases the relationship suggests an inner‐sphere mechanism where the transport properties of the spacer play a decisive role. These findings possibly help to explain contradictory reports on the conductivity behaviour of organic molecules. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)