Direct Observation of Confined I−⋅⋅⋅I2⋅⋅⋅I− Interactions in a Metal–Organic Framework: Iodine Capture and Sensing

Herein a strategy is reported for capturing and sensing iodine by strong I−⋅⋅⋅I2⋅⋅⋅I− interaction, confined in a metal‐organic framework, [Tb(Cu4I4)(ina)3(DMF)] (1) (ina=isonicotinate). As revealed by single‐crystal X‐ray crystallography, the uptaken I2 molecules directly contact the {Cu4I4}n chains, virtually forming an electronically polarizable tetraiodide anion (I42−) through strong I−⋅⋅⋅I2⋅⋅⋅I− interaction. As such, a quasi‐copper‐iodide layer of {Cu4I5}n with semiconducting characteristics results, leading to a significant enhancement (Δσ=107 times) in electrical conductivity over the I2‐free 1. The effect observed is several orders of magnitude higher than those reported due to iodine⋅⋅⋅aromatic interactions (Δσ=102 times) and by interactions between I2 and a redox‐active metal centre (Δσ=104 times). The drastic enhancement in electrical conductivity was used to switch on/off an LED bulb, suggesting the possibility of electrically sensing I2. I2 spy: A heterometallic metal‐organic framework {Tb(Cu4I4)(ina)3(DMF)} with 1D {Cu4I4}n building units shows a huge conductivity enhancement by ca. 107 times after uptake of guest iodine molecules, which can be used to sense iodine.