  Quantifying the spin-spin interactions which influence electronic transitions
in organic semiconductors is crucial for understanding their
magneto-optoelectronic properties. By combining a theoretical model for three
spin interactions in the coherent regime with pulsed electrically detected
magnetic resonance experiments on MEH-PPV diodes, we quantify the spin-coupling
within complexes comprising three spin-half particles. We determine that these
particles form triplet-exciton:polaron pairs, where the polaron:exciton
exchange is over 5 orders of magnitude weaker (less than 170 MHz) than that
within the exciton. This approach providing a direct spectroscopic approach for
distinguishing between coupling regimens, such as strongly bound trions, which
have been proposed to occur in organic devices.
