Perdeuterated Conjugated Polymers for Ultralow‐Frequency Magnetic Resonance of OLEDs

Abstract

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.

Measuring the Magnetic Field Amplitude of rf Radiation by the Quasistatic Magnetic Field Effect in Organic Light-Emitting Diodes

“…In Ref. [3], we noted the similarity of these signals to the curvature of the static magnetic field effect, i.e., the magnetoresistance [13]. We note that this near-zero-frequency regime of oscillating magnetic field effects was previously predicted theoretically in radicalpair effects in reaction-yield-detected magnetic resonance (RYDMR) [14] and observed experimentally [15,16].…”

“…By exploring a significantly larger part of the full parameter space of the EDMR experiments at lower frequencies than in Ref. [3], including parallel driving conditions where B 1 B 0 , we are now able to provide a conclusive reasoning for the observed zero-field phenomena in OLED EDMR. Following an intuitive picture of the quasistatic magnetic field effect, a numerical procedure can be performed to compute the line shape and signal amplitude expected in a low-frequency magneticresonance experiment without the need of any quantitative microscopic assumptions or parameters.…”

“…As a consequence, for excitation frequencies f >10 MHz, spectra consisting of two Gaussian lines are observed, reporting on the hyperfine field distributions experienced by electron and hole spins [11]. Previously, we showed that a low-frequency limit for resolving such Zeeman resonances in OLEDs exists [3,12], and is around 3 MHz for devices based on the perdeuterated version of poly[2-methoxy-5-(2 -ethylhexyloxy)-1,4phenylenevinylene] (d-MEHPPV). At even lower values of the excitation frequency, the shape of the EDMR spectra changes, exhibiting a single peak centered around B 0 = 0 mT accompanied by an inversion of the signal for nonzero B 0 .…”

“…This characteristic can be directly visualized by exciting resonant transitions between the two spin states, up or down, and monitoring the Rabi oscillations of the state occupation by pulsed magnetic-resonance experiments [1,2]. Even static magnetic fields B 0 as small as 120 µT, barely three times the geomagnetic field strength, can lead to distinctly resolved resonance peaks at room temperature under radio frequency (rf) excitation, although the energy of the Zeeman splitting is more than seven orders of magnitude smaller than the thermal energy [3]. The reason that such a high resolution can be achieved lies in the fact * john.lupton@physik.uni-regensburg.de that the resonance occurs in nonequilibrium states, electrically injected electron-hole pairs bound by the Coulomb interaction, which can exist either in a singlet or a triplet spin configuration.…”

Rearrangement Strategy for the Preparation of Polymers With π-Conjugated Structures

“…The application of these diverse π-conjugated polymers represents a potentially fertile field for chemistry and materials research (Dai et al, 2020 ; Saito et al, 2020 ). Recently, there is an increasing interest in conjugated polymers, and their applications are becoming more widespread, such as intrinsically conducting polymers (Aydemir et al, 2016 ), solar cells (Zhang et al, 2019 ; Liu et al, 2020 ), and organic light emitting diodes (OLEDs) (Lozano-Hernández et al, 2020 ; Milster et al, 2020 ). Besides, conjugated double bonds have more excellent reactivity compared to isolated double bonds, so conjugated polymers are widely used as substrates to prepare high-performance materials through chemical modification.…”