Iodine Adsorption in a Redox-Active Metal–Organic Framework: Electrical Conductivity Induced by Host−Guest Charge-Transfer

Abstract

We report a comparative study of the binding of I₂ (iodine) in a pair of redox-active metal-organic framework (MOF) materials, MFM-300(V^III) and its oxidized, deprotonated analogue, MFM-300(V^IV). Adsorption of I₂ in MFM-300(V^III) triggers a host-to-guest charge-transfer, accompanied by a partial (∼30%) oxidation of the V^III centers in the host framework and formation of I₃^- species residing in the MOF channels. Importantly, this charge-transfer induces a significant enhancement in the electrical conductivity (Δ_σ = 700000) of I₂@MFM-300(V^III/IV) in comparison to MFM-300(V^III). In contrast, no host-guest charge-transfer or apparent change in the conductivity was observed upon adsorption of I₂ in MFM-300(V^IV). High-resolution synchrotron X-ray diffraction of I₂@MFM-300(V^III/IV) confirms the first example of self-aggregation of adsorbed iodine species (I₂ and I₃^-) into infinite helical chains within a MOF.