The interplay of host−guest interactions and controlled modulation of spin-crossover (SCO) behavior is one of the most exploited topics regarding data storage, molecular sensing, and optical technologies. In this work, we demonstrate the experimental approach of tuning the SCO behavior via controlled modulation of the spin-state cooperativity in a 2D Hofmann coordination polymer, [Fe II Pd-Removal of the solvent changes the four-step transition to a complete one-step spin transition with an enhanced hysteresis width (∼20 K). Structural analysis of solvated (1•1.3MeOH) and partially desolvated (1•0.3MeOH) compounds reveals that the crystal system changes from a monoclinic C2/c space group to an orthorhombic Imma space group, where the Fe II sites are present in a more symmetrically equivalent environment. Consequently, the axial ligand-field (LF) strength and face-to-face interactions of the ligands were increased by removing the guest, which is reflected in the highly cooperative SCO in 1 (desolvated compound). Also, the shift of the CN bond stretching frequencies and decrease of their relative intensities from the variable-temperature Raman spectroscopic measurements further corroborate the SCO behavior. Besides, theoretical calculations reveal that the singlet ( 1 Γ) LF terms decrease by removing guest molecules, enhancing the molecular population in the low-spin state at low temperature, as experimentally observed for 1. Notably, fine tuning of the SCO behavior via host−guests interactions provides a great opportunity to design potential chemosensors.
Spin crossover complexes that reversibly interconvert between two stable states imitate a binary state of 0 and 1, delivering a promising possibility to address the data processing concept in smart materials. Thus, a comprehensive understanding of the modulation of magnetic transition between high spin and low spin and the factors responsible for stabilizing the spin states is an essential theme in modern materials design. In this context, the present review attempts to provide a concise outline of the design strategy employed at the molecular level for fine-tuning the spin-state switching in Fe II -based Hofmann-type coordination polymers and their effects on the optical and magnetic response. In addition, development towards the nanoscale architectures of HCPs, i. e., in terms of nanoparticles and thin films, are emphasized to bridge the gap between the laboratory and reality.
Spin–Spin interactions between unpaired electrons in organic radicals are of utter importance from the viewpoint of molecular magnetism and the development of smart materials. The diamagnetic to paramagnetic phase transition observed in some radicals often leads to “magnetic bistability,” sometimes associated with a thermally accessible structural phase transition. The noncovalent interactions determining the solid-state packing arrangement are highly susceptible to external stimuli (temperature, pressure, light, electric field, etc.) and allow the radicals to respond reversibly. Thus, a qualitative understanding of the communication pathway of the spin centers and factors determining the solid-state packing arrangement for the radicals is most important. In this perspective, we mainly discuss the effect of noncovalent interactions rearranging the radicals’ position with temperature determining the mechanistic pathway of such phase transitions. We focus on the importance of electronic parameters stabilizing different polymorphic phases of the radicals, secondary dynamic effects arising from the π-stacking in solid-state, and their role in a magnetic phase transition, along with the consequences of different external stimuli in fine-tuning the magnetic bistable states.
As a consequence of increasing global energy demand and depleting fossil fuels, artificial photosynthesis, i.e., subsequent photocatalytic water oxidation and CO2 reduction, has become significant for providing renewable energy. To...
Fabrication of bench-stable radical ions under ambient conditions is of utmost significance from the perspective of materials and structural (solid-state) chemistry. Two exceptionally stable benzotriazinyl radical cationic salts of 1-phenyl-3-(phenylamino)-1,2,4-benzotriazin-4-ium-1-ylium (A and B) have been prepared and structurally characterized for the first time, in which the hydrogen bonding controls their supramolecular arrangement, and thus, their magnetism is exploited. Introduction of intrinsically disordered trifluoroacetate counteranion (A) leads to a reversible phase transition (PT) at ca. ∼119 K, associated with order−disorder structural transformation of the magnetically innocent counteranion. In turn, no such transition was observed using a nondisordered 2-nitrobenzoate counteranion (B). Variable temperature crystallography along with molecular dynamics simulations quantitatively demonstrates that order−disorder structural transformation in A leads to a cooperative change in the dynamic motion of the radical pairs. Consequently, this changes the π−π stacking interactions (d) and latitudinal and longitudinal slippage angles (ϕ) and modifies the distribution of the magnetic exchange couplings (J) in A upon thermal vibration. Overall, it is a demonstration of a new mechanism to introduce subtle molecular changes to regulate the magnetism of organic open shell components.
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