Quantized Electron‐Transfer Pathways at Nanoparticle–Redox Centre Hybrids

Abstract

Hexanethiolate gold monolayer-protected clusters (C6-MPCs) with an average core diameter of 1.8 nm and a capacitance of 0.6 aF are synthesised by a two-phase method. These clusters are functionalised with (6-ferrocenyl)-1-hexanethiol by a place exchange reaction at different molar ratios. The average number of ferrocene centres per cluster determined by (1)H NMR is ten, seven and four. Differential pulse voltammetry and cyclic voltammetry measurements for cluster solutions in 0.1 M TBAPF(6)/Tol:AN (2:1) clearly show the response of the Fc(+)/Fc redox couple and of quantized double layer (QDL) charging events of the gold core. A transition from single to multiple electron-transfer response for the redox couple is observed as the number of ferrocene units per cluster is increased. The distances between the redox moieties are estimated considering a homogeneous distribution of the redox sites on the nanoparticle ligand shell. In all the cases, the inter-ferrocene average separation is too large to observe self-exchange reactions and the most likely electron-transfer pathway is by fast rotational diffusion. The oxidation of the ferrocene groups results in an electrostatic switching-off of electron transfers between the electrode and the nanoparticle core.

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

“…Multiple ET in solutions of hexanethiolate-protected Au nanoparticles has previously been observed to increase with larger populations of ferrocenes in their ligand shells. 12 While rotational diffusion would be slowed in the solid state, multiple ferrocene sites equally distributed between Fc 1+ and Fc 0 should still permit multiple ET pathways between neighboring particles. This view is consistent with the lack of change in the ET E A with the number of ferrocene-containing ligands.…”

“…The ET rate constant in Au 225 Fc 7 is larger than that for Au 225 Fc 2 by over an order of magnitude. The increase in k EX with the number of ferrocenes present in the ligand shell suggests that ET is accelerated by the higher probability of adjacency and favorable orbital overlap ,, of ferrocenes on neighboring nanoparticles. Multiple ET in solutions of hexanethiolate-protected Au nanoparticles has previously been observed to increase with larger populations of ferrocenes in their ligand shells .…”

“…The increase in k EX with the number of ferrocenes present in the ligand shell suggests that ET is accelerated by the higher probability of adjacency and favorable orbital overlap ,, of ferrocenes on neighboring nanoparticles. Multiple ET in solutions of hexanethiolate-protected Au nanoparticles has previously been observed to increase with larger populations of ferrocenes in their ligand shells . While rotational diffusion would be slowed in the solid state, multiple ferrocene sites equally distributed between Fc 1+ and Fc 0 should still permit multiple ET pathways between neighboring particles.…”

“…10 Early work 11 showed that functionalization of Au MPCs with a ferrocene (Fc) thiol can lead to multielectron transfer reactions for both freely diffusing nanoparticles and electrode-adsorbed clusters. More recently, Schiffrin and co-workers 12 synthesized 1.8 nm diameter Au MPCs containing 4, 7, or 10 Fc thiolate ligands, using ligand exchanges, and observed their electron transfer (ET) reactions.…”

Faradaic Quantized Capacitance as an Ideal Pseudocapacitive Mechanism

“…Our simulations indicate that a nanosphere of radius r = 3.5 nm in DCM (or another solvent with similar dielectric properties) should satisfy both the aforementioned criteria, as presented in Figure . When U is sufficiently large compared to λ, a peaked voltammetric profile results (see the Supporting Information for details). − …”