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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(Cu4 I4 )(ina)3 (DMF)] (1) (ina=isonicotinate). As revealed by single-crystal X-ray crystallography, the uptaken I2 molecules directly contact the {Cu4 I4 }n chains, virtually forming an electronically polarizable tetraiodide anion (I42- ) through strong I- ⋅⋅⋅I2 ⋅⋅⋅I- interaction. As such, a quasi-copper-iodide layer of {Cu4 I5 }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 .