Magnetic Sponge with Neutral–Ionic Phase Transitions

Phase transitions caused by the charge instability between the neutral and ionic phases of compounds, i.e., N–I phase transitions, provide avenues for switching the intrinsic properties of compounds related to electron/spin correlation and dipole generation as well as charge distribution. However, it is extremely difficult to control the transition temperature (T_c) for the N–I phase transition, and only chemical modification based on the original material have been investigated. Here, a design overview of the tuning of N–I phase transition by interstitial guest molecules is presented. This study reports a new chain coordination-polymer [Ru₂(3,4-Cl₂PhCO₂)₄TCNQ(EtO)₂]∙DCE (1-DCE; 3,4-Cl₂PhCO₂ ⁻ = 3,4-dichlorobenzoate; TCNQ(EtO)₂ 2,5-diethoxy-7,7,8,8-tetracyanoquinodimethane; and DCE = 1,2-dichloroethane) that exhibits a one-step N–I transition at 230 K (= T_c) with the N- and I-states possessing a simple paramagnetic state and a ferrimagnetically correlated state for the high- and low-temperature phases, respectively. The T_c continuously decreases depending on the content of DCE, which eventually disappears with the complete evacuation of DCE, affording solvent-free compound 1 with the N-state in the entire temperature range (this behavior is reversible). This is an example of tuning the in situ T_c for the N–I phase transition via the control of the interstitial guest molecules.