Radio Frequency Magnetic Field Effects on Electron-Hole Recombination

Woodward, Jonathan; Timmel, Christiane R.; McLauchlan, K.A.; Hore, P. J.

doi:10.1103/physrevlett.87.077602

Cited by 64 publications

(66 citation statements)

References 21 publications

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“…Magnetic fields oscillating at frequencies in the range 1-100 MHz, with or without a static field present, are expected to alter the yields of radical pair reactions by modifying the spin dynamics (69). This has been observed for reactions of organic radicals in solution when the frequency of the applied field matches a S 7 T interconversion frequency arising from hyperfine and/or Zeeman interactions (70)(71)(72). Such resonances are diagnostic for radical pairs: Magnetite particles of sufficient size to function as a compass are too large to reorient in magnetic fields fluctuating as rapidly as 1 MHz (73,74).…”

Section: Evidence For Radical Pair Magnetoreceptionmentioning

confidence: 99%

Chemical magnetoreception in birds: The radical pair mechanism

Rodgers

Hore²

2009

Proc. Natl. Acad. Sci. U.S.A.

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449

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Migratory birds travel vast distances each year, finding their way by various means, including a remarkable ability to perceive the Earth's magnetic field. Although it has been known for 40 years that birds possess a magnetic compass, avian magnetoreception is poorly understood at all levels from the primary biophysical detection events, signal transduction pathways and neurophysiology, to the processing of information in the brain. It has been proposed that the primary detector is a specialized ocular photoreceptor that plays host to magnetically sensitive photochemical reactions having radical pairs as fleeting intermediates. Here, we present a physical chemist's perspective on the ''radical pair mechanism'' of compass magnetoreception in birds. We outline the essential chemical requirements for detecting the direction of an Earth-strength Ϸ50 T magnetic field and comment on the likelihood that these might be satisfied in a biologically plausible receptor. Our survey concludes with a discussion of cryptochrome, the photoactive protein that has been put forward as the magnetoreceptor molecule.

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Section: Evidence For Radical Pair Magnetoreceptionmentioning

confidence: 99%

Chemical magnetoreception in birds: The radical pair mechanism

Rodgers

Hore²

2009

Proc. Natl. Acad. Sci. U.S.A.

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483

449

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“…23 Even at zero static field a resonant oscillating field drives singlet-triplet interconversion, hence further reducing the singlet yield from its zero static field value. From previously published OMFE spectra of the Py-d 10 /1,3-DCB system, 7 it is clear that OMFE spectra of these systems are relatively broad meaning a 36 MHz oscillating field will fall within the resonance feature arising from 1,3-DCB •− , a eff = 29.2 MHz.…”

Section: Rydmr-b 0 -Orthogonal Fieldsmentioning

confidence: 99%

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

Wedge

Lau

Ferguson

et al. 2013

Phys. Chem. Chem. Phys.

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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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“…1b because the hyperfine-broadened resonance narrows 18,21 . Parallels to this RF effect exist in solution-based reaction-yield-detected magnetic resonance of pair processes [24][25][26] , with the crucial difference being that the OLED current reveals absolute population changes, allowing steady-state detection in magnetoresistance. This signal amplitude enables time-resolved excitation and detection to uncover spin-Rabi oscillations and spin beating due to correlated precession of electrons and holes 27 .…”

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confidence: 99%

The spin-Dicke effect in OLED magnetoresistance

et al. 2015

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Pairs of charge-carrier spins in organic semiconductors constitute four-level systems that can be driven electromagnetically 1 . Given appropriate conditions for ultrastrong coupling 2 -weak local hyperfine fields B hyp , large magnetic resonant driving fields B 1 and low static fields B 0 that define Zeeman splittingthe spin-Dicke e ect, a collective transition of spin states, has been predicted 3 . This parameter range is challenging to probe by electron paramagnetic resonance spectroscopy because thermal magnetic polarization is negligible. It is accessed through spin-dependent conductivity that is controlled by electron-hole pairs of singlet and triplet spin-permutation symmetry without the need of thermal spin polarization 4 . Signatures of collective behaviour of carrier spins are revealed in the steady-state magnetoresistance of organic light-emitting diodes (OLEDs), rather than through radiative transitions. For intermediate B 1 , the a.c.-Zeeman e ect appears. For large B 1 , a collective spin-ensemble state arises, inverting the current change under resonance and removing power broadening, thereby o ering a unique window to ambient macroscopic quantum coherence.Macroscopic phase coherence is a hallmark of many exotic states of matter such as superconductivity, ferromagnetism or BoseEinstein condensation. Such coherence may also emerge between two-level systems, where it is mediated by electromagnetic fields, as described by the Dicke effect in collisional narrowing 5 and superradiance 6 . Collective behaviour may already arise within a pair of interacting two-level systems 7 , an observation that can potentially be extended to the prototypical two-level system of an electron spin. For pairs of charge-carrier spins in organic semiconductors, with driving fields B 1 exceeding the hydrogen-induced random local hyperfine field 1 B hyp and approaching the magnitude of the static magnetic field B 0 , a collective macroscopic spin phase has been predicted to emerge 3 . Under these conditions, when the spin-Rabi splitting becomes comparable to the Zeeman splitting, the electromagnetic field links individually resonant spin pairs together, forming a spin-Dicke state analogous to that in the dipolar Dicke effect [5][6][7] . These macroscopic effects are observable through measurements of electronic recombination rates, which depend on spin-permutation symmetry of the pair 8 .We monitor the electron-hole recombination current in an OLED, where positive and negative charges are injected into a thin film of an organic semiconductor from opposite electrodes. As the charges drift through the material, they can capture each other on intermolecular length scales owing to weak dielectric screening. These weakly coupled intermolecular electron-hole pairs 9 can ultimately recombine on individual molecules to form a molecular excited state, or exciton, which gives rise to electroluminescence. The subsequent discussion focuses on carrier pairs and not on excitons, which have spin S = 0 or 1. Because the carriers possess spi...

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Radio Frequency Magnetic Field Effects on Electron-Hole Recombination

Cited by 64 publications

References 21 publications

Chemical magnetoreception in birds: The radical pair mechanism

Chemical magnetoreception in birds: The radical pair mechanism

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

The spin-Dicke effect in OLED magnetoresistance

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