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 (I₄ ²⁻) 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 (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.