Iodine adsorption in a redox-active metal-organic framework: Electrical conductivity induced by host-guest charge-transfer

We report a comparative study of the binding of I2 (iodine) in a pair of redox-active metal-organic framework (MOF) materials, MFM-300(VIII) and its oxidized, deprotonated analogue, MFM-300(VIV). Adsorption of I2 in MFM-300(VIII) triggers a host-to-guest charge transfer, accompanied by a partial (~30%) oxidation of the VIII centres 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 (Δσ = 700,000) of I2@MFM-300(VIII/IV) in comparison to MFM-300(VIII). In contrast, no host-guest charge-transfer or apparent change in conductivity was observed on adsorption of I2 in MFM-300(VIV). High resolution synchrotron X-ray diffraction of I2@MFM-300(VIII/IV) confirms the first example of self-aggregation of adsorbed iodine species (I2 and I3-) into infinite helical chains within a MOF.