Structural Basis for the Phase Switching of Bisaminecopper(II) Cations at the Thermal Limits of Lattice Stability

The structural grounds of the decrease of point and lattice symmetries coupled with switching of the exchange
interaction in single crystals of a highly strained, coordinationally unsaturated bisdiaminecopper(II) cation are described.
The combined magnetic susceptibility and X-ray diffraction results indicate that the interplay between the inherent
vibronic instability and ligand-field strain imposed by moderately flexible, coordinationally shielding ligands enables
effective switching of the pseudo-Jahn−Teller d9 centers between states with different exchange interaction in the
low-temperature regime and valence orbital orientation and coordination geometry in the high-temperature regime.
Within the low-temperature hysteresis region, the phase transition can also be induced by excitation of the ligand-to-metal charge-transfer bands, resulting in overall shrinkage of the lattice. The compound is a prototype of weakly
electronically coupled one-dimensional Jahn−Teller systems, which can undergo phase transitions induced by light,
in addition to heating, cooling, and change of pressure, and it represents a prospective basis for the design of
switching materials capable of multimode external control.

本工作阐述了高应变、配位不饱和双二胺合铜(II)（bisdiaminecopper(II)）阳离子单晶中，点群与晶格对称性降低与交换相互作用切换耦合的结构基础。结合磁化率与X射线衍射（X-ray diffraction）的测试结果表明，固有振动-电子不稳定性与适度柔性配位遮蔽配体所施加的配体场应变之间的相互作用，使得赝姜-泰勒（pseudo-Jahn−Teller）d⁹构型中心可在低温区实现不同交换相互作用状态间的有效切换，并在高温区实现价轨道取向与配位几何构型的切换。在低温磁滞区内，相转变也可通过激发配体到金属电荷转移带（ligand-to-metal charge-transfer bands）诱导产生，进而导致晶格整体收缩。该化合物是弱电子耦合一维姜-泰勒（Jahn−Teller）系统的原型体系，此类系统除可通过加热、降温与压力变化诱导相转变外，还可受光激发触发相转变；其为设计具备多模式外部调控能力的切换型材料提供了极具应用前景的基础。