  In organic semiconductors, bi-excitons are key intermediates in
carrier-multiplication and exciton annihilation. Their local geometry governs
their electronic properties and yet has been challenging to determine. Here, we
access the structure of the recently discovered $S=2$ quintet bi-exciton state
in an organic semiconductor using broadband optically detected magnetic
resonance. We correlate the experimentally extracted spin structure with the
molecular crystal geometry to identify the specific molecular pairings on which
bi-exciton states reside.
