Reverse Hofmann-Type Spin-Crossover Compound Showing a Multichannel Controllable Color Change in an Ambient Environment

Abstract

Materials that demonstrate a multichannel controllable color change in response to external stimuli are fascinating for their potential applications in sensoring and displaying devices. Herein we report a FeII spin-crossover (SCO) compound that exhibits both solvatochromism and thermochromism under an ambient environment. This Hofmann-type compound possesses two different pores where the solvent guests can be removed in a two-step process. Because the loss of solvent guests modifies the spin state of magnetic centers, an unusual yellow–red–yellow two-step color change of crystals was detected. Moreover, because of the strong cooperativity of the spin centers, a dramatic red-to-yellow color change of crystals in response to a minute thermal perturbation around 303 K is triggered by an abrupt spin transition of the metal centers. The multichannel controllable dramatic color change demonstrated in the present compound highlights the sensoring and displaying roles of SCO materials.

Guest-Induced Multistep-to-One-Step Reversible Spin Transition with Enhanced Hysteresis in a 2D Hofmann Framework

“…In the absence of guest molecules, the stronger face-to-face interactions between the pyridyl groups of the ligand increase the cooperativity of the system, which is manifested by the increased hysteresis width in 1 . Besides, the steric factor induced by the guest molecules is presumably a contributing factor in stabilizing the HS sites at low temperature because of its internal chemical pressure effect and host–guest or guest–guest interactions, as observed in some earlier cases. − …”

“…The elusive nature of intermolecular interactions and the extreme sensitivity of the SCO phenomenon to minute structural changes impose a considerable challenge in the design of SCO materials with targeted properties. In this regard, the Fe II -based Hofmann-type coordination polymers (CPs) are of great interest because the polymeric network exhibits stronger cooperativity between the SCO sites, linked through covalent bonds to create a rigid lattice. − In addition, the presence of guest molecules in the pores of the CPs can significantly perturb SCO behavior by efficient communication − between the spin centers through changes in the ligand-field (LF) strength around Fe II centers, resulting in substantial changes in the cooperativity and T 1/2 . − …”

Redox-Programmable Spin-Crossover Behaviors in a Cationic Framework

“…, pH and guest molecules) with significant changes in structural architectures as well as optical, magnetic, and mechanical properties. − SCO materials possessing multi-responsive capabilities around room temperature are the most appealing for practical use. To this end, extensive efforts have been devoted to functionalize SCO MOFs by fine-tuning the guest effects or tailoring the host frameworks. − Among them, post-synthetic modification (PSM) is a judicious tool to modify targeted functionalities but is rarely reported in the SCO field up to now. For example, oxidative addition of halogens onto the [Pt­(CN) 4 ] 2– linker, inverse-electron-demand Diels–Alder reaction, and photochemical [2 + 2] cycloaddition reaction of pillar ligands had been implemented to control the spin transition temperature or regulate the SCO dynamics.…”

Thermally induced spin-crossover in the Fe(3-ethynylpyridine)2[M(CN)4] series with M = Ni, Pd, and Pt. The role of the electron density found at the CN 5σ orbital

“…Such is the case of pyrazine and pyridine and some of their derivatives. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] More recently, spin transition has been reported for 2D ferrous nitroprussides with these organic molecules as axial ligands for the iron atom. [33][34][35][36][37] In this last series of coordination polymers, the NO ligand modulates the charge density found at the CN 5s orbital, and by this way, the metal-NC bonding strength.…”