Magnetic Switching by the In Situ Electrochemical Control of Quasi-Spin-Peierls Singlet States in a Three-Dimensional Spin Lattice Incorporating TTF-TCNQ Salts

Magnetic phase switching in a coordination polymer is reported, which is demonstrated by combining two processes: (A) the pre-organization of magnetic/redox-active molecules into a framework, and (B) a post-treatment through electrochemical tuning of the pre-organized molecules. A TTF^.+–TCNQ^.− salt (TTF=tetrathiafulvalene; TCNQ=7,7,8,8-tetracyano-p-quinodimethane) was incorporated into a three-dimensional framework with paddlewheel-type dimetal(II, II) units ([M₂ ^II,II]; M=Ru with S=1, 1; and Rh with S=0, 2), where the [M₂ ^II,II] and TCNQ^.− units form the coordinating framework, and TTF^.+is located in the pores of framework, forming an irregular π-stacking alternating column with the TCNQ^.− in the framework. In 1, the spins of [Ru₂ ^II,II] and TCNQ^.− units make a magnetic correlation through the framework upon decreasing the temperature from 300 K, which is, however, suddenly suppressed below 137 K (=T_d (1)) by the formation of a spin singlet in the TTF^.+–TCNQ^.− columns, as seen in the spin-Peierls transition (T_d (2)=200 K). This material was incorporated as a cathode in a Li-ion battery (LIB); a long-range ferrimagnetic correlation was formed through the three-dimensional [{Ru₂ ^II,II}₂TCNQ]⁻ framework at T_c=78 K in the discharge process. The reversible magnetic phase switching between the non-volatile ferrimagnetic and paramagnetic states, resulting from the local spin tuning of quasi-spin-Peierls singlet, is demonstrated through the discharge/charge cycling of the LIB.