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 (I₄^2- ) 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 (Δ_σ =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 (Δ_σ =10² times) and by interactions between I₂ and a redox-active metal centre (Δ_σ =10⁴ times). The drastic enhancement in electrical conductivity was used to switch on/off an LED bulb, suggesting the possibility of electrically sensing I₂ .