Abstract Compact domain features have been observed in spin crossover [Fe{H 2 B(pz) 2 } 2 (bipy)] molecular thin film systems via soft x-ray absorption spectroscopy and photoemission electron microscopy. The domains are in a mixed spin state that on average corresponds to roughly 2/3 the high spin occupation of the pure high spin state. Monte Carlo simulations support the presence of intermolecular interactions that can be described in terms of an Ising model in which interactions beyond nearest-neighbors cannot be neglected. This suggests the presence of short-range order to permit interactions between molecules beyond nearest neighbor that contribute to the formation of largely high spin state domains structure. The formation of a spin state domain structure appears to be the result of extensive cooperative effects.
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Probing the unpaired Fe spins across the spin crossover of a coordination polymer
For the spin crossover coordination polymer [Fe(L1)(bipy)] n (where L1 is a N 2 O 2 2− coordinating Schiff base-like ligand bearing a phenazine fluorophore and bipy = 4,4′-bipyridine), there is compelling additional evidence of a spin state transition. Both Fe 2p X-ray absorption and X-ray core level photoemission spectroscopies confirm that a spin crossover takes place, as observed by magnetometry. Yet the details of the temperature dependent changes of the spin state inferred from both X-ray absorption and X-ray core level photoemission, differ from magnetometry, particularly with regard to the apparent critical transition temperatures and the cooperative nature of the curve progression in general. Comparing the experimental spin crossover data to Ising model simulations, a transition activation energy in the region of 160 to 175 meV is indicated, along with a nonzero exchange J . Overall, the implication is that there may be perturbations to the bistability of spin states, that are measurement dependent or that the surface differs from the bulk with regard to the cooperative effects observed upon spin transition.
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- Award ID(s):
- 2003057
- NSF-PAR ID:
- 10226000
- Date Published:
- Journal Name:
- Materials Advances
- Volume:
- 2
- Issue:
- 2
- ISSN:
- 2633-5409
- Page Range / eLocation ID:
- 760 to 768
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The X-ray-induced spin crossover transition of an Fe (II) molecular thin film in the presence and absence of a magnetic field has been investigated. The thermal activation energy barrier in the soft X-ray activation of the spin crossover transition for [Fe{H2B(pz)2}2(bipy)] molecular thin films is reduced in the presence of an applied magnetic field, as measured through X-ray absorption spectroscopy at various temperatures. The influence of a 1.8 T magnetic field is sufficient to cause deviations from the expected exponential spin state transition behavior which is measured in the field free case. We find that orbital moment diminishes with increasing temperature, relative to the spin moment in the vicinity of room temperature.more » « less
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From X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS), it is evident that the spin state transition behavior of Fe( ii ) spin crossover coordination polymer crystallites at the surface differs from the bulk. A comparison of four different coordination polymers reveals that the observed surface properties may differ from bulk for a variety of reasons. There are Fe( ii ) spin crossover coordination polymers with either almost complete switching of the spin state at the surface or no switching at all. Oxidation, differences in surface packing, and changes in coordination could all contribute to making the surface very different from the bulk. Some Fe( ii ) spin crossover coordination polymers may be sufficiently photoactive so that X-ray spectroscopies cannot discern the spin state transition.more » « less
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Abstract The Fe(II) spin crossover complex [Fe{H2B(pz)2}2(bipy)] (pz = pyrazol‐1‐yl, bipy = 2,2′‐bipyridine) can be locked in a largely low‐spin‐state configuration over a temperature range that includes temperatures well above the thermal spin crossover temperature of 160 K. This locking of the spin state is achieved for nanometer thin films of this complex in two distinct ways: through substrate interactions with dielectric substrates such as SiO2and Al2O3, or in powder samples by mixing with the strongly dipolar zwitterionic
p ‐benzoquinonemonoimine C6H2(—⋯ NH2)2(—⋯ O)2. Remarkably, it is found in both cases that incident X‐ray fluences then restore the [Fe{H2B(pz)2}2(bipy)] moiety to an electronic state characteristic of the high spin state at temperatures of 200 K to above room temperature; that is, well above the spin crossover transition temperature for the pristine powder, and well above the temperatures characteristic of light‐ or X‐ray‐induced excited‐spin‐state trapping. Heating slightly above room temperature allows the initial locked state to be restored. These findings, supported by theory, show how the spin crossover transition can be manipulated reversibly around room temperature by appropriate design of the electrostatic and chemical environment. -
null (Ed.)Spin crossover (SCO) is a phenomenon observed for certain transition metal complexes with electronic configuration 3d4-3d7. The conversion between the low-spin (LS) and high-spin (HS) states is usually driven by a variety of external perturbations, such as temperature, pressure, or light. The switching between the enthalpically preferred LS state and entropically favorable HS state is accompanied by dramatic changes in the metal-ligand bond lengths, unit cell volume, optical absorption spectrum, and magnetic susceptibility.1 These changes make SCO materials suitable for applications in sensors, memory, and display devices. One of the central challenges in the SCO research is to initiate strongly cooperative interactions known to lead to abrupt spin transitions and thermal hysteresis that can be harvested as a memory effect. One of the strategies to enhance the cooperativity is to design SCO complexes with supramolecular interactions such as π-stacking of aromatic fragments or hydrogen bonding.2 In this work, we report syntheses and characterization of heteroleptic complexes of [Fe(tpma)(L)](ClO4)2 (tpma = tris(pyridin-2-ylmethyl)amine) with novel π-extended biimidazole-type ligands (L) bearing 2,3-dimethyl-naphthalene-, 6,7-dimethyl-2,3-diphenyl-quinoxaline, and 2,3-dimethyl-anthracene pendant fragments. Solvent-free naphthalene-functionalized complex [Fe(tpma)(xnap-bim)](ClO4)2 exhibits abrupt spin transition at T1/2 = 127K with a narrow 1 K hysteresis loop. In contrast, polymorph of this complex that contains one interstitial molecules of pyridine exhibits gradual SCO. Anthracene-functionalized complex [Fe(tpma)(anthra-bim)](ClO4)2 also crystallizes as two polymorphs. Structural studies at 100, 230, and 300 K revealed dramatic changes in the N-Fe-N biting angles and Fe-N distances, indicating the occurrence of temperature-induced SCO. Complex [Fe(tpma)(quin-bim)](ClO4)2 (quin-bim = 6,7-dimethyl-2,3-diphenyl-quinoxaline-2,2’-biimidazole) showed only HS state at 100 and 230 K. In the crystal packing the mononuclear cations form stacks along b axis. We discuss how the observed magnetic behavior correlates with changes in the crystal packing and interactions between the pendant aromatic substituents on the aforementioned complexes.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0MA00612B
