Radiofrequency polarization effects in low-field electron paramagnetic resonance

Wedge, Christopher J.; Rodgers, Christopher T.; Norman, Stuart A.; Baker, Neville; Maeda, Kiminori; Henbest, Kevin B.; Timmel, Christiane R.; Hore, P. J.

doi:10.1039/b907915g

“…5 In the high-field limit it is convenient to make the rotating frame approximation in order to remove the time-dependence of the problem. Such an approximation cannot be used in the low-field limit 6 or when the static and oscillating fields are not orthogonal. A modified form of the γ-COMPUTE algorithm [11][12][13] is used which exploits the periodicity of the problem and efficiently averages over the initial phase of the RF field.…”

Section: Simulation Methodsmentioning

confidence: 99%

“…[4][5][6] The breakdown of high-field models is gradual, parallel transitions becoming significant at higher frequencies than those at which the rotating frame approximation breaks down. 6 Extending previous studies, orientation effects are now investigated in the case of strong B 1 fields.…”

Section: Orientation-dependent Effectsmentioning

confidence: 99%

“…[4][5][6] The breakdown of high-field models is gradual, parallel transitions becoming significant at higher frequencies than those at which the rotating frame approximation breaks down. 6 Extending previous studies, orientation effects are now investigated in the case of strong B 1 fields. According to the model proposed by Koptyug et al 24 for spin-locking it should not be possible to cause a spin-locking type inversion for parallel fields, an assertion that is tested below, alongside discussion of higher order resonances.…”

Section: Orientation-dependent Effectsmentioning

confidence: 99%

“…6,7 The reagent solution flowed rapidly in a recirculating loop through a 5 mm optical path-length quartz cuvette, with radical pair generation achieved by photolysis under continuous UV irradiation from a 1 kW arc lamp. The resultant fluorescence signal perpendicular to the irradiation source was collected and focussed onto a photomultiplier tube for detection, with an interference filter used to select exciplex fluorescence in a 100 nm pass band centred at 548 nm.…”

Section: Experimental Methodsmentioning

confidence: 99%

“…This problem has been circumvented by utilizing reagents in which photolytic radical generation leads to a magnetically sensitive fluorescence, allowing an optically detected low-field EPR technique to be developed. [4][5][6] …”

Section: Introductionmentioning

confidence: 99%

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The purported effects of weak magnetic fields on various biological systems from animal magnetoreception to human health have generated widespread interest and sparked much controversy in the past decade. To date the only well established mechanism by which the rates and yields of chemical reactions are known to be influenced by magnetic fields is the radical pair mechanism, based on the spin-dependent reactivity of radical pairs. A diagnostic test for the operation of the radical pair mechanism was proposed by Henbest et al. [J. Am. Chem. Soc., 2004, 126, 8102] based on the combined effects of weak static magnetic fields and radiofrequency oscillating fields in a reaction yield detected magnetic resonance experiment. Here we investigate the effects on radical pair reactions of applying relatively strong oscillating fields, both parallel and perpendicular to the static field. We demonstrate the importance of understanding the effect of the strength of the radiofrequency oscillating field; our experiments demonstrate that there is an optimal oscillating field strength above which the observed signal decreases in intensity and eventually inverts. We establish the correlation between the onset of this effect and the hyperfine structure of the radicals involved, and identify the existence of 'overtone' type features appearing at multiples of the expected resonance field position.

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Perdeuterated Conjugated Polymers for Ultralow‐Frequency Magnetic Resonance of OLEDs

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The formation of excitons in OLEDs is spin dependent and can be controlled by electron‐paramagnetic resonance, affecting device resistance and electroluminescence yield. We explore electrically detected magnetic resonance in the regime of very low magnetic fields (<1 mT). A pronounced feature emerges at zero field in addition to the conventional spin‐1/2 Zeeman resonance for which the Larmor frequency matches that of the incident radiation. By comparing a conventional π‐conjugated polymer as the active material to a perdeuterated analogue, we demonstrate the interplay between the zero‐field feature and local hyperfine fields. The zero‐field peak results from a quasistatic magnetic‐field effect of the RF radiation for periods comparable to the carrier‐pair lifetime. Zeeman resonances are resolved down to 3.2 MHz, approximately twice the Larmor frequency of an electron in Earth's field. However, since reducing hyperfine fields sharpens the Zeeman peak at the cost of an increased zero‐field peak, we suggest that this result may constitute a fundamental low‐field limit of magnetic resonance in carrier‐pair‐based systems. OLEDs offer an alternative solid‐state platform to investigate the radical‐pair mechanism of magnetic‐field effects in photochemical reactions, allowing models of biological magnetoreception to be tested by measuring spin decoherence directly in the time domain by pulsed experiments.

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Perdeuterated Conjugated Polymers for Ultralow‐Frequency Magnetic Resonance of OLEDs

Milster

¹

,

Grünbaum

²

,

Bange

³

et al. 2020

View full text Add to dashboard Cite

The formation of excitons in OLEDs is spin dependent and can be controlled by electron‐paramagnetic resonance, affecting device resistance and electroluminescence yield. We explore electrically detected magnetic resonance in the regime of very low magnetic fields (<1 mT). A pronounced feature emerges at zero field in addition to the conventional spin‐1/2 Zeeman resonance for which the Larmor frequency matches that of the incident radiation. By comparing a conventional π‐conjugated polymer as the active material to a perdeuterated analogue, we demonstrate the interplay between the zero‐field feature and local hyperfine fields. The zero‐field peak results from a quasistatic magnetic‐field effect of the RF radiation for periods comparable to the carrier‐pair lifetime. Zeeman resonances are resolved down to 3.2 MHz, approximately twice the Larmor frequency of an electron in Earth's field. However, since reducing hyperfine fields sharpens the Zeeman peak at the cost of an increased zero‐field peak, we suggest that this result may constitute a fundamental low‐field limit of magnetic resonance in carrier‐pair‐based systems. OLEDs offer an alternative solid‐state platform to investigate the radical‐pair mechanism of magnetic‐field effects in photochemical reactions, allowing models of biological magnetoreception to be tested by measuring spin decoherence directly in the time domain by pulsed experiments.

show abstract

Radiofrequency polarization effects in low-field electron paramagnetic resonance

Cited by 10 publications

References 30 publications

Spin-locking in low-frequency reaction yield detected magnetic resonance

Spin-locking in low-frequency reaction yield detected magnetic resonance

Perdeuterated Conjugated Polymers for Ultralow‐Frequency Magnetic Resonance of OLEDs

Perdeuterated Conjugated Polymers for Ultralow‐Frequency Magnetic Resonance of OLEDs

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