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-center dot center dot center dot I-2 center dot center dot center dot 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 I-2 molecules directly contact the {Cu4I4}(n) chains, virtually forming an electronically polarizable tetraiodide anion (I-4(2-)) through strong I-center dot center dot center dot I-2 center dot center dot center dot I- interaction. As such, a quasi-copper-iodide layer of {Cu4I5}(n) with semiconducting characteristics results, leading to a significant enhancement (Delta(sigma)=10(7) times) in electrical conductivity over the I-2-free 1. The effect observed is several orders of magnitude higher than those reported due to iodine center dot center dot center dot aromatic interactions (Delta(sigma)=10(2) times) and by interactions between I-2 and a redox-active metal centre (Delta(sigma)=10(4) times). The drastic enhancement in electrical conductivity was used to switch on/off an LED bulb, suggesting the possibility of electrically sensing I-2.