The Effect of an Active Guest on the Spin Crossover Phenomenon

摘要

A straightforward method for the reversible modification of solid-state properties is a goal being constantly pursued in the development of molecular switches, and molecular electronic and photonic devices. As one of the most attractive molecule-based switchable materials, spin crossover (SCO) complexes present different magnetic, optical, electrical and structural properties in response to external stimuli (such as temperature, pressure, light, or magnetic fields), driven by conversion of the electron configuration between high spin (HS) and low spin (LS) states. However, the extreme sensitivity of SCO behavior to subtle effects, including humidity or solvent inclusion in the crystal lattice, often impedes the ability to regulate these switchable properties at will. On the other hand, this sensitivity may be exploited if the host-guest chemistry of the material can be controlled. A recent demonstration of this is the incorporation of SCO centers into porous coordination polymers (PCPs), which allows the fine-tuning of SCO properties through the inclusion of guest molecules without disrupting the framework structure. It has been suggested that guest molecules could be used as a new perturbation to switch spin state at constant (ideally ambient) temperature. A few encouraging results have already been reported, especially for the three-dimensional Hofmann-like SCO-PCP {Fe(pz)[M(CN)4]} (M = Ni, Pt, Pt; pz = pyrazine). This includes the dynamic interplay between SCO and the host-guest function, the precise control of transition temperature upon I2 addition, and a significant enhancement in cooperativity through the inclusion of thiourea. Particularly, in the work of Kepert et al. and Ohba et al. , the spin state has been successfully switched at room temperature through the chemical response of the framework. However, the critical operating conditions are difficult to achieve in practical applications; in Ohba's study for example, several successive steps are required: the adsorption of one guest (benzene), desorption under vacuum, adsorption of another guest (CS2), and a subsequent desorption under vacuum. It is necessary to develop a more efficient approach to regulate SCO properties. Ligand-driven light-induced spin change (LD-LISC) is a promising strategy; spin conversion could be triggered by cis-trans isomerization of the ligand by means of light over a broad range of temperatures. However, the drastic structural rearrangement of the whole lattice that accompanies the isomerization makes it difficult to achieve in densely packed assemblies, and the current successful examples are limited to only a few mononuclear complexes.