A low-dimensional molecular spin system with two steps of magnetic transitions and liquid crystal property

A low-dimensional compound [C₆-Apy][Ni(mnt)₂] (1, where mnt^2- = maleonitriledithiolate, C₆-Apy⁺ = 4-amino-1-hexylpyridinium) has been designed and synthesized, which has layer arrangement of anions and cations and shows two steps of magnetic transitions. The low temperature magnetic transition has an uncommon hysteresis loop, while the crystal structure investigations disclosed no structural transition with the magnetic transition. The high temperature magnetic transition exhibits two remarkable features: (1) it synchronously occurs with a crystalline-to-mesophase transition in the first heating process and (2) the structural changes that accompany the solid-mesophase transition are irreversible. A diamagnetic and isostructural compound, [C₆-Apy][Cu(mnt)₂], is further characterized by structure, DSC and POM techniques, which revealed also the existence of an irreversible crystalline-to-mesophase transition in the same temperature interval of [C₆-Apy][Ni(mnt)₂]. Therefore, the higherature magnetic transition in 1 is driven by release of the structural strains, but not magnetoelastic interactions. The mesophase exhibits the characteristic of smectic A phase, and the alkyl chain melting in the cation layers probably lead to the formation of mesophase. It is noticeable that the finding of a mesophase occurring in a hexyl hydrocarbon chain molecular system is in contrast to a suggested rule that at least a dodecyl chain is required. Our results will shed a light on the design and preparation of a new low-dimensional molecular system combining magnetic transition and liquid crystal properties.