Reversible iodine absorption by alkali-TCNQ salts with associated changes in physical properties

Alkali metal salts of 7,7′,8,8′-tetracyanoquinodimethane (TCNQ) reversibly absorb iodine forming the ternary salts M(TCNQ)I (M = Li, Na, K) and M2(TCNQ)3I2 (M = Rb). The ternary salts are also obtained by solid-state reactions of TCNQ with alkali iodides. These salts are paramagnetic and have high electrical conductivities, ∼10−1 S cm−1, for compacted pellets, whereas the alkali metal salts of TCNQ are diamagnetic insulators. The ternary salts further absorb iodine to give over-doped salts M(TCNQ)In (n ≈ 6, M = Na, K), which gradually release iodine to give M(TCNQ)I. In contrast, the solid-state reaction of F4TCNQ and sodium iodide produces Na(F4TCNQ), which does not exhibit iodine absorption.