2, MV Lomonosov MSU, Moscow, , Russian Federation
3, Institute of Problems of Chemical Physics of Russian Academy of Sciences, Chernogolovka, , Russian Federation
4, National Chiao Tung University, Hsinchu, , Taiwan
The molecular systems exhibiting the bistable behavior are considered to be promising for application in molecular electronics. One of the examples of molecular bistability is spin-crossover (SCO) phenomena, meaning that the system can be stabilized in different electronic states by varying the external conditions (pressure, temperature or magnetic field, by irradiation exposing). For example, the molecular complexes with SCO are appealing for application as sensors or molecular switchers.
The presented study is focused on the investigation of temperature induced SCO phenomena in [Mn(5-Cl-sal-N-1,5,8,12)]TCNQ1.5 2CH3CN (I) and [Mn(5-Br-sal-N-1,5,8,12)]TCNQ1.5 2CH3CN complexes (II). There are several aspects making these objects are particularly interesting. First, SCO is commonly observed in molecular complexes based on Fe (II), Fe (III) and Co (II) and not so often in Mn (III) -based compounds. Second, the SCO with wide thermal hysteresis loop is relatively rare phenomena and it is difficult to predict. Third, the studied systems combine the SCO behavior and conductive properties, that makes them the first conducting SCO compound based on Mn (III) cation complex.
Here we present the results of studying the magnetic properties, heat capacity and structural characterization of compounds I and II. Examination of the temperature dependence of magnetic susceptibility (χ(T)) revealed the SCO in both compounds. However, the character of SCO observed in compounds I and II are substantially different. Specifically, the complex I exhibits a sharp SCO at about 120 K with wide thermal hysteresis loop in χ(T) suggesting the significant cooperativity for the transition. The magnitude and form of χ(T) hysteretic loop observed in compound I reproduces after several thermal cycling thus proving the reproducibility of observed SCO behavior in complex I. The values of effective magnetic moment (µeff=(8C)1/2µB) calculated from the fitting of the experimental χ (T) data with Curie-Weiss law (χ= χ0+C/(T- Θ), where χ0 - temperatue independent term, C - Curie constant, Θ – Curie temperature) at temperatures below and above the temperature of transition are consistent with the theoretical values (µeff=gµB(S(S+1))1/2) for S=1 and S=2 electronic states. The SCO phenomena in the compound II was observed at lower temperatures (about 50K) than in compound I and was not so pronounced as in the compound I indicating the substantional role of the ligands on the cooperativity of the SCO effect in studied objects.
The EXAFS examination conducted at temperatures below and above the SCO manifests the significant changes in the local geometry of Mn (III), thus proving the SCO nature of observed magnetic susceptibility behavior.