What Can Be Studied with Electron Paramagnetic Resonance?

Brustolon, Marina

doi:10.1002/9780470432235.ch3

Cited by 35 publications

(67 citation statements)

References 59 publications

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“…These temperature-dependent lineshape changes are reversible. The data obtained are in line with 16DSA being positioned in the rather flexible, lipophilic environment toward the center of the bilayer, while 5DSA resides in a more ordered, rigid, and hydrophilic region close to the polar head group. ,− Our experimental data correspond well with the MD simulations referred to in the introduction . Simulations evidence that all the EPR signals measured represent single species.…”

Section: Resultssupporting

confidence: 85%

Systematic Quantification of Electron Transfer in a Bare Phospholipid Membrane Using Nitroxide-Labeled Stearic Acids: Distance Dependence, Kinetics, and Activation Parameters

et al. 2020

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In this report, we present a method to characterize the kinetics of electron transfer across the bilayer of a unilamellar liposome composed of 1,2-dimyristoyl- sn -glycero-3-phosphocholine. The method utilizes synthetic phospholipids containing noninvasive nitroxide spin labels having the >N–O• moiety at well-defined distances from the outer surface of the liposome to serve as reporters for their local environment and, at the same time, permit measurement of the kinetics of electron transfer. We used 5-doxyl and 16-doxyl stearic acids. The paramagnetic >N–O• moiety is photo-oxidized to the corresponding diamagnetic oxoammonium cation by a ruthenium electron acceptor formed in the solution. Electron transfer is monitored by three independent spectroscopic methods: by both steady-state and time-resolved electron paramagnetic resonance and by optical spectroscopy. These techniques allowed us to differentiate between the electron transfer rates of nitroxides located in the outer leaflet of the phospholipid bilayer and of those located in the inner leaflet. Measurement of electron transfer rates as a function of temperature revealed a low-activation barrier (ΔG ‡ ∼ 40 kJ/mol) that supports a tunneling mechanism.

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Section: Resultssupporting

confidence: 85%

Systematic Quantification of Electron Transfer in a Bare Phospholipid Membrane Using Nitroxide-Labeled Stearic Acids: Distance Dependence, Kinetics, and Activation Parameters

et al. 2020

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“…Various 2-pyridyl derivatives and metal complexes have been developed so far, where a persistent radical is substituted, such as nitronyl nitroxide, , verdazyl, and dithiadiazolyl groups (Figure ). The spin density of the ligating atom in a nitroxide group is assumed to be almost twice as large as that of the other spin-delocalized radicals; the delocalization area is almost half (N 1 O 1 in nitroxides versus N 2 O 2 in nitronyl nitroxides, N 4 in verdazyls, and N 2 S 2 in dithiadiazolyls), as quantified by means of electron spin resonance, polarized neutron diffraction studies, and so on. , The exchange interaction is proportional to the spin densities at the contacting atoms . For the realization of spin transition, strongly exchange coupled systems in an intrasite or intersite manner seem to be needed for competition with the aufbau principle.…”

Section: Discussionmentioning

confidence: 99%

High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly Chelated [Ni(L)₂Br₂] (L = tert-Butyl 5-Phenyl-2-pyridyl Nitroxide)

2019

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Complexation of nickel(II) bromide with tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) gave two morphs of doubly chelated [Ni(phpyNO)2Br2] as a 2p–3d–2p heterospin triad. The α phase crystallizes in the orthorhombic space group Pbcn. An asymmetric unit involves a half-molecule. The torsion angle around Ni–O–N–C2py is as small as 6.5(3)° at 100 K and 7.0(6)° at 400 K, guaranteeing an orthogonal arrangement between the magnetic radical π* and metal 3d x 2–y 2 and 3d z 2 orbitals. Magnetic study revealed the high-spin ground state with the exchange coupling constant 2J/k B = +288(5) K, on the basis of a symmetrical spin Hamiltonian. The β phase crystallizes in the monoclinic space group P21/n. The whole molecule is an independent unit. The Ni–O–N–C2py torsion angles are 24.2(6) and 37.2(5)° at 100 K and 10.4(7) and 25.9(6)° at 400 K. A magnetic study revealed a very gradual and nonhysteretic spin transition. An analysis based on the van’t Hoff equation gave a successful fit with the spin-crossover temperature of 134(1) K, although the susceptibility did not reach the theoretical high-spin value at 400 K. Density functional theory calculation on the β phase showed ground S total = 0 in the low-temperature structure while S total = 2 in the high-temperature structure, supporting the synchronized exchange coupling switch on both sides. Consequently, the β phase can be recognized as an “incomplete spin crossover” material, as a result of conflicting thermal depopulation effects in a high-temperature region.

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“…Nonetheless, zinc treatment of O-Se affords paramagnetic species with a similar EPR signal to that of the thiophene analogue ( g ∼ 2.0022, Figure c and Table S1). Since selenium substitution of donor units does not significantly shift the EPR signal to higher g -factors, as would be expected with heavy atom substitution, unpaired electrons do not couple significantly to donor units. Instead, hyperfine splitting of O-Th/Zn and O-Se/Zn EPR signals suggests coupling between unpaired electrons and the naphthyl protons and imide nitrogens of acceptor units.…”

Section: Resultsmentioning

confidence: 74%

Anion-Radical Polymerization of Sulfur- and Selenium-Substituted N-Type Conjugated Polymers

Pahlavanlu

Panchuk

et al. 2021

Macromolecules

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While controlled chain-growth polymerizations have been developed for a handful of p-type conjugated polymers, most n-type conjugated polymers are still synthesized through step-growth methods that offer little to no control over the polymerization. The anion-radical polymerization of thiophene-flanked naphthalene diimides has been shown to exhibit non-living chain-growth behavior; however, anion-radical polymerizations are limited to a few examples that exhibit varying degrees of control. Strategies to improve and expand the scope of this promising methodology have not been developed. In this study, we investigate the anion-radical polymerization of a series of oxygenated and thionated, thiophene-and selenophene-flanked naphthalene diimides. We show, through optical and spectroelectrochemical studies, that anion-radical monomers likely consist of a complex mixture of organic radicals with varying oxidation states. Surprisingly, subtle changes in monomer structure afforded by sulfur and selenium substitution result in significant changes in polymer synthesis. Notably, selenophene-flanked naphthalene diimides polymerize more rapidly to reach higher degrees of polymerization when compared to thiophene-flanked analogues. While thionated naphthalene diimides form anion-radical complexes and exhibit signs of catalyst insertion, they do not undergo polymerization. These results provide insights into the further development of anion-radical polymerization as a promising route to well-defined n-type conjugated polymers.

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What Can Be Studied with Electron Paramagnetic Resonance?

Cited by 35 publications

References 59 publications

Systematic Quantification of Electron Transfer in a Bare Phospholipid Membrane Using Nitroxide-Labeled Stearic Acids: Distance Dependence, Kinetics, and Activation Parameters

Systematic Quantification of Electron Transfer in a Bare Phospholipid Membrane Using Nitroxide-Labeled Stearic Acids: Distance Dependence, Kinetics, and Activation Parameters

High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly Chelated [Ni(L)₂Br₂] (L = tert-Butyl 5-Phenyl-2-pyridyl Nitroxide)

Anion-Radical Polymerization of Sulfur- and Selenium-Substituted N-Type Conjugated Polymers

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What Can Be Studied with Electron Paramagnetic Resonance?

Cited by 35 publications

References 59 publications

Systematic Quantification of Electron Transfer in a Bare Phospholipid Membrane Using Nitroxide-Labeled Stearic Acids: Distance Dependence, Kinetics, and Activation Parameters

Systematic Quantification of Electron Transfer in a Bare Phospholipid Membrane Using Nitroxide-Labeled Stearic Acids: Distance Dependence, Kinetics, and Activation Parameters

High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly Chelated [Ni(L)2Br2] (L = tert-Butyl 5-Phenyl-2-pyridyl Nitroxide)

Anion-Radical Polymerization of Sulfur- and Selenium-Substituted N-Type Conjugated Polymers

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High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly Chelated [Ni(L)₂Br₂] (L = tert-Butyl 5-Phenyl-2-pyridyl Nitroxide)