  We investigate the physical mechanisms for spin-dependent recombination of a
strongly bound pair of triplet excitons generated by singlet fission and
forming a spin quintet (total spin of two) bi-exciton. For triplet excitons the
spin-dependent recombination pathways can involve intersystem crossing or
triplet-triplet annihilation back to the singlet ground state. However the
modeling of spin-dependent recombination for quintets is still an open
question. Here we introduce two theoretical models and compare their
predictions with the broadband optically detected magnetic resonance spectrum
of a long lived quintet bi-exciton with known molecular structure. This
spectrum measures the change in the fluorescence signal induced by microwave
excitation of each of the ten possible spin transitions within the quintet
manifold as function of the magnetic field. While most of the experimental
features can be reproduced for both models, the behavior of some of the
transitions is only consistent with the quintet spin-recombination model
inspired by triplet intersystem crossing which can reproduce accurately the
experimental two-dimensional spectrum with a small number of kinetic
parameters. Thus quantitative analysis of the broadband optically detected
magnetic resonance signal enables quantitative understanding of the dominant
spin-recombination processes and estimation of the out-of equilibrium spin
populations.
