  Molecular spin systems based on photoexcited triplet pairs formed via singlet
fission (SF) are attractive as carriers of quantum information because of their
potentially pure and controllable spin polarization, but developing systems
that offer optical routes to readout as well as initialization is challenging.
Herein, we characterize the electron spin magnetic resonance change in
photoluminescence intensity for a tailored organic molecular crystal while
sweeping a microwave drive up to 10 GHz in a broadband loop structure. We
observe resonant transitions for both triplet and quintet spin sublevel
populations showing their optical sensitivity, and revealing zero-field
parameters for each. We map the evolution of these spectra in both microwave
frequency and magnetic field, producing a pattern of optically-detected
magnetic resonance (ODMR) peaks. Fits to this data using a suitable model
suggest significant spin polarization in this system with orientation
selectivity. Unusual excitation intensity dependence is also observed, which
inverts the sign of the ODMR signal for triplet features, but not for quintet.
These observations demonstrate optical detection of the spin sublevel
population dictated by SF and intermolecular geometry, and provide unique
insight into the dynamics of triplet pairs.
