OLEDs as models for bird magnetoception: detecting electron spin resonance in geomagnetic fields

Abstract

Certain species of living creatures are known to orientate themselves in the geomagnetic field. Given the small magnitude of approximately 48 µT, the underlying quantum mechanical phenomena are expected to exhibit coherence times approaching the millisecond regime. In this contribution, we show sensitivity of organic light-emitting diodes (OLEDs) to magnetic fields far below Earth's magnetic field, suggesting that coherence times of the spins of charge-carrier pairs in these devices can be similarly long. By electron paramagnetic resonance (EPR) experiments, a lower bound for the coherence time can be assessed directly. Moreover, this technique offers the possibility to determine the distribution of hyperfine fields within the organic semiconductor layer. We extend this technique to a material system exhibiting both fluorescence and phosphorescence, demonstrating stable anticorrelation between optically detected magnetic resonance (ODMR) spectra in the singlet (fluorescence) and triplet (phosphorescence) channel. The experiments demonstrate the extreme sensitivity of OLEDs to both static as well as dynamic magnetic fields and suggest that coherent spin precession processes of Coulombically bound electron spin pairs may play a crucial role in the magnetoreceptive ability of living creatures.

Two-photon absorption in a two-level system enabled by noise

“…As a consequence, magnetic resonance can even occur at zero external field, mediated alone by the local hyperfine fields. 30 Electrically detected magnetic resonance (EDMR) in OLEDs bears some resemblance to magnetic-field effects and magnetic resonance phenomena in radical-pair-based chemical reactions 31,32 but offers more flexibility in terms of choosing the crucial ratio between carrier-wave frequency (B 0 ), Rabi frequency (B 1 ) and the noise amplitude, which is determined by the local hyperfine fields. These fields, in turn, can be tuned chemically by controlling the isotopic ratio of hydrogen and deuterium.…”

“…33 Since spin-orbit coupling and resulting shifts of the gyromagnetic ratio are negligible at low frequencies, 35 hyperfine coupling pro-vides the only source of inhomogeneous broadening of the resonance. Recently, clear Zeeman resonances in EDMR were reported at frequencies as low as 5 MHz for a hydrogenated organic semiconductor; 30 for deuterated materials, this limit is expected to be much lower. It should therefore be straightforward to find a suitable parameter space where the resonance frequency is of order the typical noise frequency.…”

Perdeuterated Conjugated Polymers for Ultralow‐Frequency Magnetic Resonance of OLEDs

“…Details of the fabrication process are discussed in the Supporting Information. The OLEDs were placed on a stripline to generate the linearly polarized RF radiation of oscillating magnetic‐field amplitude B 1 , and positioned inside three pairs of Helmholtz coils to provide static magnetic fields B 0 of ≈50 nT precision and arbitrary orientation . Figure b shows a sketch of the arrangement.…”

“…Previously, we introduced a stripline design, with which it became possible to resolve resonances at around 100 MHz, a frequency range initially thought to be limited by the local hyperfine‐field strengths . With careful sampling, however, we recently succeeded in resolving Zeeman resonances down to 5.5 MHz in a commercially available conjugated polymer, a frequency already quite close to the electron's Larmor frequency in geomagnetic fields of ≈0.7–1.74 MHz. However, in addition to the anticipated Zeeman resonances, a further feature emerges at low RF frequency: a zero‐field peak reminiscent in structure to what has also been reported in reaction‐yield detected magnetic resonance (RYDMR) .…”

“…With careful sampling, however, we recently succeeded in resolving Zeeman resonances down to 5.5 MHz in a commercially available conjugated polymer, a frequency already quite close to the electron's Larmor frequency in geomagnetic fields of ≈0.7–1.74 MHz. However, in addition to the anticipated Zeeman resonances, a further feature emerges at low RF frequency: a zero‐field peak reminiscent in structure to what has also been reported in reaction‐yield detected magnetic resonance (RYDMR) . Following the results of quantum‐dynamical simulations, these features have been attributed to both the RF‐induced resonances in the local hyperfine fields as well as to the quasistatic nature of the low‐frequency RF magnetic fields experienced by short‐lived radical pairs .…”