Reversible iodine absorption of nonporous coordination polymer Cu(TCNQ)

Polycrystalline powders of Cu(TCNQ) (TCNQ = 7,7',8,8'-tetracyanoquinodimethane) absorb iodine to form Cu(TCNQ)I-4 upon solid grinding with iodine or immersion in a hexane solution of iodine. Of the two polymorphs of Cu(TCNQ), phase II Cu(TCNQ) exhibits a much slower iodine-absorption rate than that of phase I Cu(TCNQ) in the liquid-phase reaction, whereas the solid grinding reaction results in efficient absorption for both phases. The valence state of the iodine-containing salt is [Cu+I-(TCNQ(0))](I-2)(1.5), where the copper ion is coordinated with an iodide anion and neutral TCNQ. The salt is a semiconductor (sigma(RT) = 3 x 10(-3) S cm(-1), compaction pellet) with an electrical conductivity one order lower than that of Cu(TCNQ). The salt releases iodine by annealing to regenerate the original phases of Cu(TCNQ) via an intermediate Cu(TCNQ) I state. A solid-state reaction of TCNQ, CuI, and iodine also produces the iodine-containing salt. The iodine absorption-desorption mechanism of Cu(TCNQ) differs from that of alkali-TCNQ salts that we reported previously.