Stepwise Neutral−Ionic Phase Transitions in a Covalently Bonded Donor/Acceptor Chain Compound

Neutral (N)−ionic (I) transitions in organic donor (D)/acceptor (A) charge-transfer complexes are intriguing because a ‘reservoir of functions’ is available. For systematically controlling N−I transitions, tuning the ionization potential of D and the electron affinity of A is extremely important. However, the effect of Coulomb interactions, which likely causes a number of charge-gap states at once in a system bringing about stepwise transitions, is a long-standing mystery. Here, we show definite evidence for stepwise N−I transitions caused by contributions from anisotropic interchain Coulomb interactions in a metal-complex-based covalently bonded DA chain compound, [Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)]·2(p-xylene) (1; 2,3,5,6-F4PhCO2− = 2,3,5,6-tetrafluorobenzoate; DMDCNQI = 2,5-dimethyl-N,N′-dicyanoquinonediimine), where the [Ru2II,II(2,3,5,6-F4PhCO2)4] moiety has a paddlewheel diruthenium(II,II) motif with a Ru−Ru bond. An intermediate-temperature phase involving self-organized N and I chains was observed in the temperature range between 210 K (= T2) and 270 K (= T1) with N phase at T > T1 and I phase at T T2. Accompanying the charge transitions, the spin−ground states as well as the ferrimagnetic ordering in the I phase vary. The stepwise feature of the N−I transition with a highly sensitive magnetic response should bring about new dynamical functionalities associated with charge, spin, and lattice.