Proton uptake vs. redox driven release from metal-organic-frameworks: Alizarin red S reactivity in UMCM-1

Small redox active molecules such as alizarin red S are readily adsorbed and bound into redox-inactive metal-organic framework hosts such as UMCM-1. Redox activity of the bound guest molecule is of interest for electrochemical conversions and electrocatalysis within pores. For the reduction of alizarin red S charge compensating proton uptake into pores is expected. However, it is shown here that alizarin red S redox processes in UMCM-1 immersed in aqueous electrolyte, ethanolic electrolyte, and in acetonitrile electrolyte media are dominated instead by surface processes and the potential driven reductive release of leuco-alizarin red S into the surrounding solution. Self-mediation via released alizarin red S occurs, whereas in acetonitrile a cobaltocene redox mediator is employed for the potential driven redox release of alizarin red S from the UMCM-1 host lattice.