Effect of hyperfine interactions in highly anisotropic organic light-emitting diode magnetoelectroluminescence resolved in the singlet and triplet recombination channels

Organic light-emitting diodes (OLEDs) can show a remarkable spatial anisotropy in the strength of magnetoelectroluminescence (MEL) at geomagnetic field strengths, serving as models of the electron-hole radical-pair process invoked to explain some forms of biological magnetoreception. We examine this effect in dual singlet-triplet-emitting devices by quantifying the modifications to both the fluorescence and phosphorescence intensity. Although the changes in the yields of singlet and triplet excitations with magnetic-field strength are generally anticorrelated, with an increase in singlet population accompanied by a decrease in triplet population, and vice versa, we find that, unexpectedly, MELs in the singlet and triplet channels display strikingly different anisotropies with respect to the orientation of the magnetic field. By deuterating the OLED compounds, we control the strength of the anisotropic hyperfine interactions, providing insight into the underlying magnetic-field effects. We examine magnetoresistance and MEL in the ultralow field regime of below a few hundred microtesla, comparing the behavior of protonated and deuterated compounds. The functional form of the anisotropy differs markedly between singlet and triplet channels, which we rationalize quantitatively by means of quantum-stochastic simulations of the spin dynamics.