Room‐Temperature Paramagnetic‐to‐Diamagnetic Switching Behavior in an Open‐Shell Ionic Liquid with a Tetracyanoquinodimethane Radical Anion Salt

An ionic liquid consisting of a 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion and a tetra-n-octylphosphonium ion, 1⁺•TCNQ^•-, undergoes a transformation near room temperature from paramagnetic liquid to diamagnetic solid accompanied by significant changes in photophysical properties. A blue solid of 1⁺•TCNQ^•- melts upon heating above 328 K, yielding a green liquid. The green liquid exhibits a magnetic susceptibility of 5 × 10^-4 emu mol^- ¹. Upon cooling, the liquid retains its paramagnetic properties until the temperature reaches approximately 300 K. Upon further cooling below 300 K, the paramagnetic liquid reverts to the initial blue solid. The solid form of 1⁺•TCNQ^•- exhibits near-zero magnetic susceptibility. The thermal hysteresis loop with magnetic bistability is observed without decomposition over the temperature range from 300 to 328 K. Crystal structure analysis, electron spin resonance (ESR), and electronic spectra indicate that the drastic changes in magnetic properties are attributable to the reversible π-dimerization of TCNQ^• ^- species during the solid-liquid/liquid-solid phase transitions. Additionally, the association/dissociation behaviors of the TCNQ^• ^- dimers also induce significant changes in photophysical properties, especially in the short-wave infrared region (1000-2500 nm), suggesting the potential application of 1⁺•TCNQ^•- as a bifunctional material.