Temperature Dependence of Solid-State Electron Exchanges of Mixed-Valent Ferrocenated Au Monolayer-Protected Clusters

Abstract

Electron transfers (ETs) in mixed-valent ferrocene/ferrocenium materials are ordinarily facile. In contrast, the presence of ∼1:1 mixed-valent ferrocenated thiolates in the organothiolate ligand shells of <2 nm diameter Au₂₂₅, Au₁₄₄, and Au₂₅ monolayer-protected clusters (MPCs) exerts a retarding effect on ET between them at and below room temperature. Near room temperature, in dry samples, bimolecular rate constants for ET between organothiolate-ligated MPCs are diminished by the addition of ferrocenated ligands to their ligand shells. At lower temperatures (down to ∼77 K), the thermally activated (Arrhenius) ET process dissipates, and the ET rates become temperature-independent. Among the Au₂₂₅, Au₁₄₄, and Au₂₅ MPCs, the temperature-independent ET rates fall in the same order as at ambient temperatures: Au₂₂₅ > Au₁₄₄ > Au₂₅. The MPC ET activation energy barriers are little changed by the presence of ferrocenated ligands and are primarily determined by the Au nanoparticle core size