  Identifying multi-exciton states generated from singlet fission is key to
understanding the carrier multiplication process, which presents a strategy for
improving the efficiency of photovoltaics and bio-imaging. Broadband optically
detected magnetic resonance is a sensitive technique to detect multi-exciton
states. Here we report a dominant species emerging under intense light
excitation corresponding to a weakly exchange coupled triplet pair located on
adjacent molecules oriented by nearly 90 degrees, contrasting to the pi-stacked
triplet pair under low excitation intensity. The weakly coupled species model
precisely reproduces the intricate spin transitions in the Hilbert space of the
triplet pair. Combining the magneto photoluminescence and high-magnetic field
ODMR, we also identify a strongly exchange-coupled state of three triplet
excitons formed by photoexcited V2, which manifests through the magnetic field
induced level crossings between its quintet and triplet manifolds. The
excellent agreement between the experimental Zeeman fan and the two-triplet
spin Hamiltonian highlights the potential of multi-exciton states for quantum
information processing.
