Electroactive Ferrocene at or near the Surface of Metal-Organic Framework UiO-66

Here, we describe the installation of a ferrocene derivative on and within the archetypal metal-organic framework (MOF), UiO-66, by solvent-assisted ligand incorporation. Thin films of the resulting material show a redox peak characteristic of the Fc/Fc⁺ couple, as measured by cyclic voltammetry. Consistent with restriction of redox reactivity solely to Fc molecules sited at or near the external surfaces of MOF crystallites, chronoamperometry measurements indicate that less than 20% of the installed Fc molecules are electrochemically active. Charge-transport diffusion coefficients, D_CT, of 6.1 ± 0.8 × 10^-11 and 2.6 ± 0.2 × 10^-9 cm²/s were determined from potential step measurements, stepping oxidatively and reductively, respectively. The 40-fold difference in D_CT values contrasts with the expectation, for simple systems, of identical values for oxidation-driven versus reduction-driven charge transport. The findings have implications for the design of MOFs suitable for delivery of redox equivalents to framework-immobilized electrocatalysts and/or delivery of charges from a chromophoric MOF film to an underlying electrode, processes that may be central to MOF-facilitated conversion of solar energy to chemical or electrical energy.

Ion Transfer Mechanisms Accompanying p-Doping of Poly(3,4-Ethylenedioxythiophene) Films in Deep Eutectic Solvents

Redox-driven ion transfer processes accompanying p-doping and undoping were investigated for poly(3,4-ethylenedioxythiophene) (PEDOT) films. The PEDOT films were potentiostatically grown from acetonitrile solutions of monomer onto the Au electrodes of 10 MHz AT-cut quartz crystal resonators. The films were then sequentially exposed to 0.1 M LiClO4/CH3CN solution and to two deep eutectic solvent (DES) media, Ethaline 200 (choline chloride and ethylene glycol in 1:2 molar ratio) and Propaline 200 (choline chloride and propylene glycol in 1:2 molar ratio). In each case, the electron and ion transfer processes accompanying PEDOT redox switching were investigated using the EQCM. The maintenance of film electroneutrality was achieved in quite different ways in the three media. In contrast to anion transfer domination in 0.1 M LiClO4/CH3CN solution, cation transfer (in the opposite direction) was dominant in Ethaline 200. In Propaline 200, a two stage mixed mechanism was observed, with cation transfer domination in the early stages of p-doping (later stages of undoping) and anion transfer domination in the later stages of p-doping (earlier stages of undoping). The roles of thermodynamic and kinetic factors in these competitive situations were explored by observing the effect of experimental timescale (potential scan rate) in voltammetric experiments.