極性スピンクロスオーバー錯体に対する磁場効果

概要

九州大学学術情報リポジトリ
Kyushu University Institutional Repository

Magnetic Field Effects on Polar Spin Crossover
Complexes
張, 暁鵬

https://hdl.handle.net/2324/7157290
出版情報：Kyushu University, 2023, 博士（理学）, 課程博士
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権利関係：Public access to the fulltext file is restricted for unavoidable reason (3)

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氏

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論 文 名

：張 暁鵬 (Zhang Xiaopeng)
： Magnetic Field Effects on Polar Spin Crossover Complexes
（極性スピンクロスオーバー錯体に対する磁場効果）

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：甲

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Spin crossover (SCO) systems has been extensively studied regarding their responsiveness to
various stimuli such as light, pressure, etc. Development of SCO complexes with polarization
change further extends their application to the novel memory devices and sensors. Despite the
growing interest in thermal, light, and magnetic field-induced polarization changes
accompanied by SCO, the SCO process induced by magnetic fields has not been as thoroughly
investigated. This scarcity of research is especially noticeable in molecular magnetoelectric
materials, where SCO, when instigated by magnetic fields, is accompanied by a polarization
change. Nevertheless, these materials hold substantial significance due to the demand for
miniaturization in the memory device, a key application for magnetoelectric materials. From a
development perspective, the limited availability of complexes, small polarization changes, and
high magnetic field requirements remain significant constraints.
In this thesis, we investigated the magnetoelectric properties of two dinuclear complexes,
[(A(RR-cth))(B(SS-cth))(μ-dhbq)](AsF6)3 and [(C(RR-cth))(D(SS-cth))(μ-dhbq)](PF6)3 noted as
1(AsF6)3 and 2(PF6)3, respectively. Our findings demonstrate the largest polarization change
induced by a magnetic field in the 1(AsF6)3 and improved sensitivity to the magnetic field in
2(PF6)3. We also proposed strategies to design new complexes, potentially resolving the
primary limitations in molecular magnetoelectric materials: small polarization change and high
magnetic field requirement.
In Chapter 2, we report the synthesis and physical properties of the 1(AsF6)3 complex.
Temperature-dependent measurements of the single crystal structure, Mössbauer spectra, IR
spectra, and magnetometry demonstrate an abrupt SCO process followed by a gradual
temperature-dependent electron transfer process, similar to the transition behavior observed in
the 1(PF6)3 complex. Its polar structure, accompanied with the dipole moment change during
SCO, results in a significant macroscale polarization change, as validated by pyroelectric

measurements. Nevertheless, the 1(AsF6)3 complex exhibits some unique properties compared
to the 1(PF6)3 complex that make it more suitable for investigating magnetic field-induced
polarization switching. These differences mainly relate to the lower SCO transition temperature,
larger magnetic susceptibilities difference (∆χm) at lower temperatures, and narrower SCO
transition temperature. Additionally, the asymmetric coordination sphere leads to a significant
change in the molecular dipole moment during the SCO process, and the strong metal-ligand
covalency allows for a directional shift of electron density toward the metal site. Consequently,
the polarization change reaches 0.45 µC cm−2 during the SCO process, inducible by both
thermal and magnetic fields. To our knowledge, this represents the largest polarization change
induced by a magnetic field in molecular systems to date, underscoring the potential of our
approach for practical applications.
In Chapter 3, we successfully synthesized the isostructural 2(PF6)3 complex exhibiting
similar electron dynamics, as verified by X-ray crystallography, IR spectroscopy, magnetic, and
pyroelectric measurements. After annealing treatment, the 2(PF6)3 complex exhibits an SCO
process at lower transition temperatures of 35 K (heating) and 15 K (cooling). This SCO
process also induces a polarization change, with a maximum value of 0.27 μC cm−2 observed in
this complex. The 2(PF6)3 complex demonstrates higher magnetic field sensitivity due to its
larger spin quantum number and lower transition temperature. Furthermore, the complex reacts
differently to pulsed and DC magnetic fields due to the slower SCO transition speed compared
to the pulsed magnetic field timescale. The pulsed magnetic field induces the largest
temperature shift of about 40 K in the SCO system, detected at a temperature of 1.4 K.
The central focus of this thesis is the exploration of magnetic effects on polar SCO
complexes, including enhancing the polarization change coupled with SCO induced by the
magnetic field and reducing the magnetic field requirement to induce this change. The
asymmetric coordination sphere structure and the electron density shift offer a plausible
solution for a larger polarization change induced by magnetic fields in the 1(AsF6)3 complex.
Additionally, increasing spin quantum number in the 2(PF6)3 complex enhances the magnetic
susceptibilities difference (∆χm) between low and high spin states, leading to a highly
magnetically sensitive complex. These approaches could provide the blueprint for designing
magnetoelectric materials with a larger polarization change induced by magnetic fields and
higher magnetic field sensitivity, which may help achieving the practical threshold for
polarization change and decrease the magnetic field requirement, moving us closer to the
development of next-generation magnetoelectric memory devices.