Design Rules for Efficient Charge Transfer in Metal-Organic Framework Films: The Pore Size Effect.

In redox-active metal-organic frameworks (MOFs), charge transfer can occur by a redox hopping mechanism, i.e., electron hopping coupled with ion diffusion to balance electroneutrality. To elucidate the correlation between MOF structure and electron and ion diffusion, we prepared three ferrocene-doped MOF (Fc-MOF) films with different pore sizes (15-47 Å) immobilized on conductive substrates. By applying a theoretical model to the chronoamperometric responses of three Fc-MOFs, the electron and ion diffusion coefficients (De ≈ 10-12-10-7 cm2 s-1; Di ≈ 10-16-10-12 cm2 s-1) and electron- and ion-transfer rate constants (ke-hop ≈ 103-107 s-1; ki-hop ≈ 10-3-101 s-1) were quantified independently. Increasing MOF pore size led to an increase in ki-hop and a decrease in ke-hop. The overall charge-transfer rate constant, khop, increased when MOF pore size increased, confirming the ability to enhance charge-transfer rates through control of MOF pore size.