A Multifunctional High-Spin Biradical Pyrazolylbipyridine-bisnitronylnitroxide

Synthesis and UV−vis, IR, and EPR spectroscopic characterizations, together with X-ray structural analysis, of functional nitronyl- and iminonitroxides attached to pyrazolylbipyridine are described. The exchange interactions between the nitronylnitroxides are found to be stronger than for the iminonitroxides. Although the substitution of a pyridine with the pyrazole ring leads to shorter distances and larger dipolar couplings, the exchange interaction is diminished. While compound 1 forms dimers in the solid state, the terpyridine 3 leads to supramolecular π-stacking

A Multifunctional High-Spin Biradical Pyrazolylbipyridine-bisnitronylnitroxide

Synthesis and UV−vis, IR, and EPR spectroscopic characterizations, together with X-ray structural analysis, of functional nitronyl- and iminonitroxides attached to pyrazolylbipyridine are described. The exchange interactions between the nitronylnitroxides are found to be stronger than for the iminonitroxides. Although the substitution of a pyridine with the pyrazole ring leads to shorter distances and larger dipolar couplings, the exchange interaction is diminished. While compound 1 forms dimers in the solid state, the terpyridine 3 leads to supramolecular π-stacking

A Multifunctional High-Spin Biradical Pyrazolylbipyridine-bisnitronylnitroxide

Synthesis and UV−vis, IR, and EPR spectroscopic characterizations, together with X-ray structural analysis, of functional nitronyl- and iminonitroxides attached to pyrazolylbipyridine are described. The exchange interactions between the nitronylnitroxides are found to be stronger than for the iminonitroxides. Although the substitution of a pyridine with the pyrazole ring leads to shorter distances and larger dipolar couplings, the exchange interaction is diminished. While compound 1 forms dimers in the solid state, the terpyridine 3 leads to supramolecular π-stacking

Magnetic Interactions in Supramolecular NO···HC⋮C Type Hydrogen-Bonded Nitronylnitroxide Radical Chains

Two paramagnetic building blocks, 2-(4-ethynyl-1-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (3) and 2-(5-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (4) were synthesized and crystallized. Single crystal X-ray studies of 3 and 4 show the formation of supramolecular head-to-tail one-dimensional H-bonded (NO···HC⋮C type) chain structures with O···C distances of 3.181 and 3.155 Å, respectively. High-resolution isotropic liquid state (c ≤ 10-4 M) electron spin resonance (ESR) spectroscopy studies of the well-isolated molecules confirmed the intramolecular spin polarization from the nitronylnitroxide radical group (acceptor, NO) to the acetylenic proton (donor, HC⋮C), which is mediated by the π-conjugated backbone. The influence of the heteroatom (pyridine nitrogen−14N) in the ESR hyperfine splitting pattern was clearly seen in radical 4, with an additional number of lines appearing in the MI = 0 line of the total five-line spectrum. The solution state paramagnetic 1H NMR investigation of radicals 2-(4-trimethylsilylethynyl-1-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (1) and 3 clearly support the intramolecular spin density propagation from the acceptor to the donor groups as well as the proton hyperfine coupling (hfc) values of the conjugated backbone determined by ESR studies. Bulk magnetic investigations of the polycrystalline chain compounds (3 and 4) in the temperature range 300 down to 4.5 K display antiferromagnetic exchange interactions at very low temperature. The experimental bulk magnetic data were found to be fit by using the dimer model with exchange coupling 2J/KB values of −3.10 ± 1.16 and −8.00 ± 3.83 K for 3 and 4, respectively, as well as by adopting the Heisenberg-chain model with 2J/KB values of −0.62 ± 0.02 and −2.21 ± 0.13 K for 3 and 4, respectively

A Multifunctional High-Spin Biradical Pyrazolylbipyridine-bisnitronylnitroxide

Synthesis and UV−vis, IR, and EPR spectroscopic characterizations, together with X-ray structural analysis, of functional nitronyl- and iminonitroxides attached to pyrazolylbipyridine are described. The exchange interactions between the nitronylnitroxides are found to be stronger than for the iminonitroxides. Although the substitution of a pyridine with the pyrazole ring leads to shorter distances and larger dipolar couplings, the exchange interaction is diminished. While compound 1 forms dimers in the solid state, the terpyridine 3 leads to supramolecular π-stacking

Modulation of Ligand-Field Parameters by Heme Ruffling in Cytochromes <i>c</i> Revealed by EPR Spectroscopy

Electron paramagnetic resonance (EPR) spectra of variants of <i>Hydrogenobacter thermophilus</i> cytochrome <i>c</i>₅₅₂ (<i>Ht c</i>-552) and <i>Pseudomonas aeruginosa</i> cytochrome <i>c</i>₅₅₁ (<i>Pa c</i>-551) are analyzed to determine the effect of heme ruffling on ligand-field parameters. Mutations introduced at positions 13 and 22 in <i>Ht c</i>-552 were previously demonstrated to influence hydrogen bonding in the proximal heme pocket and to tune reduction potential (<i>E</i>_m) over a range of 80 mV [Michel, L. V.; Ye, T.; Bowman, S. E. J.; Levin, B. D.; Hahn, M. A.; Russell, B. S.; Elliott, S. J.; Bren, K. L. <i>Biochemistry</i> <b>2007</b>, <i>46</i>, 11753–11760]. These mutations are shown here to also increase heme ruffling as <i>E</i>_m decreases. The primary effect on electronic structure of increasing heme ruffling is found to be a decrease in the axial ligand-field term Δ/λ, which is proposed to arise from an increase in the energy of the d_<i>xy</i> orbital. Mutations at position 7, previously demonstrated to influence heme ruffling in <i>Pa c</i>-551 and <i>Ht c</i>-552, are utilized to test this correlation between molecular and electronic structure. In conclusion, the structure of the proximal heme pocket of cytochromes <i>c</i> is shown to play a role in determining heme conformation and electronic structure

Fe₃O₄ Nanocrystals Tune the Magnetic Regime of the Fe/Ni Molecular Magnet: A New Class of Magnetic Superstructures

A new class of organometallic–inorganic magnetic material was engineered by a sonochemically assisted self-assembly process between magnetite nanoparticles (biogenic Fe₃O₄, hard constituent) functionalized with isonicotinic acid and a metamagnetic organometallic complex ([Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O, soft constituent). In such bottom-up methodology, hard and soft counterparts form well-organized microdimensional clusters that showed morphological fingerprints and magnetic behavior clearly distinct from those of the initial building units. In the engineered soft–hard material, the magnetite nanocrystals induced ferromagnetic ordering at room temperature of closer contact layers of [Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O, thus demonstrating the ability to sensibly modify the [Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O paramagnetic regime. The magnetic ordering of [Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O was triggered by the intrinsic local field of the hard magnetic nanocrystals, which resembled, to some extent, the effects promoted by large, external magnetic fields

Solvent Controlled Generation of Spin Active Polarons in Two-Dimensional Material under UV Light Irradiation

Polarons belong to a class of extensively studied quasiparticles that have found applications spanning diverse fields, including charge transport, colossal magnetoresistance, thermoelectricity, (multi)ferroism, optoelectronics, and photovoltaics. It is notable, though, that their interaction with the local environment has been overlooked so far. We report an unexpected phenomenon of the solvent-induced generation of polaronic spin active states in a two-dimensional (2D) material fluorographene under UV light. Furthermore, we present compelling evidence of the solvent-specific nature of this phenomenon. The generation of spin-active states is robust in acetone, moderate in benzene, and absent in cyclohexane. Continuous wave X-band electron paramagnetic resonance (EPR) spectroscopy experiments revealed a massive increase in the EPR signal for fluorographene dispersed in acetone under UV-light irradiation, while the system did not show any significant signal under dark conditions and without the solvent. The patterns appeared due to the generation of transient magnetic photoexcited states of polaronic character, which encompassed the net 1/2 spin moment detectable by EPR. Advanced ab initio calculations disclosed that polarons are plausibly formed at radical sites in fluorographene which interact strongly with acetone molecules in their vicinity. Additionally, we present a comprehensive scenario for multiplication of polaronic spin active species, highlighting the pivotal role of the photoinduced charge transfer from the solvent to the electrophilic radical centers in fluorographene. We believe that the solvent-tunable polaron formation with the use of UV light and an easily accessible 2D nanomaterial opens up a wide range of future applications, ranging from molecular sensing to magneto-optical devices

Solvent Controlled Generation of Spin Active Polarons in Two-Dimensional Material under UV Light Irradiation

Polarons belong to a class of extensively studied quasiparticles that have found applications spanning diverse fields, including charge transport, colossal magnetoresistance, thermoelectricity, (multi)ferroism, optoelectronics, and photovoltaics. It is notable, though, that their interaction with the local environment has been overlooked so far. We report an unexpected phenomenon of the solvent-induced generation of polaronic spin active states in a two-dimensional (2D) material fluorographene under UV light. Furthermore, we present compelling evidence of the solvent-specific nature of this phenomenon. The generation of spin-active states is robust in acetone, moderate in benzene, and absent in cyclohexane. Continuous wave X-band electron paramagnetic resonance (EPR) spectroscopy experiments revealed a massive increase in the EPR signal for fluorographene dispersed in acetone under UV-light irradiation, while the system did not show any significant signal under dark conditions and without the solvent. The patterns appeared due to the generation of transient magnetic photoexcited states of polaronic character, which encompassed the net 1/2 spin moment detectable by EPR. Advanced ab initio calculations disclosed that polarons are plausibly formed at radical sites in fluorographene which interact strongly with acetone molecules in their vicinity. Additionally, we present a comprehensive scenario for multiplication of polaronic spin active species, highlighting the pivotal role of the photoinduced charge transfer from the solvent to the electrophilic radical centers in fluorographene. We believe that the solvent-tunable polaron formation with the use of UV light and an easily accessible 2D nanomaterial opens up a wide range of future applications, ranging from molecular sensing to magneto-optical devices

Modulation of the Ligand-Field Anisotropy in a Series of Ferric Low-Spin Cytochrome <i>c</i> Mutants derived from Pseudomonas aeruginosa Cytochrome <i>c</i>-551 and Nitrosomonas europaea Cytochrome <i>c</i>-552: A Nuclear Magnetic Resonance and Electron Paramagnetic Resonance Study

Cytochromes of the c type with histidine−methionine (His-Met) heme axial ligation play important roles in electron-transfer reactions and in enzymes. In this work, two series of cytochrome c mutants derived from Pseudomonas aeruginosa (Pa c-551) and from the ammonia-oxidizing bacterium Nitrosomonas europaea (Ne c-552) were engineered and overexpressed. In these proteins, point mutations were induced in a key residue (Asn64) near the Met axial ligand; these mutations have a considerable impact both on heme ligand-field strength and on the Met orientation and dynamics (fluxionality), as judged by low-temperature electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectra. Ne c-552 has a ferric low-spin (S = 1/2) EPR signal characterized by large g anisotropy with gmax resonance at 3.34; a similar large gmax value EPR signal is found in the mitochondrial complex III cytochrome c1. In Ne c-552, deletion of Asn64 (NeN64Δ) changes the heme ligand field from more axial to rhombic (small g anisotropy and gmax at 3.13) and furthermore hinders the Met fluxionality present in the wild-type protein. In Pa c-551 (gmax at 3.20), replacement of Asn64 with valine (PaN64V) induces a decrease in the axial strain (gmax at 3.05) and changes the Met configuration. Another set of mutants prepared by insertion (ins) and/or deletion (Δ) of a valine residue adjacent to Asn64, resulting in modifications in the length of the axial Met-donating loop (NeV65Δ, NeG50N/V65Δ, PaN50G/V65ins), did not result in appreciable alterations of the originally weak (Ne c-552) or very weak (Pa c-551) axial field but had an impact on Met orientation, fluxionality, and relaxation dynamics. Comparison of the electronic fingerprints in the overexpressed proteins and their mutants reveals a linear relationship between axial strain and average paramagnetic heme methyl shifts, irrespective of Met orientation or dynamics. Thus, for these His-Met axially coordinated Fe(III), the large gmax value EPR signal does not represent a special case as is observed for bis-His axially coordinated Fe(III) with the two His planes perpendicular to each other