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 angstrom) immobilized on conductive substrates. By applying a theoretical model to the chronoamperometric responses of three Fc-MOFs, the electron and ion diffusion coefficients (D-e approximate to 10(-12)-10(-2) cm(2) s(-1); D-i approximate to 10(-16)-10(-12) cm(2) s(-1)) and electron- and ion-transfer rate constants (k(e-hop) approximate to 10(3)-10(7) s(-1); k(i-hop) approximate to 10(-3)-10(1) s(-1)) were quantified independently Increasing MOF pore size led to an increase in k(i-hop) and a decrease in k(e-hop). The overall charge-transfer rate constant, k(hop), increased when MOF pore size increased, confirming the ability to enhance charge-transfer rates through control of MOF pores size.