  Several recent electron spin resonance studies have observed a quintet
multiexciton state during the singlet fission process. Here we provide a
general theoretical explanation for the generation of this state by invoking a
time-varying exchange coupling between pairs of triplet excitons, and
subsequently solving the relevant time-varying spin Hamiltonian for a range of
transition times. We simulate experimental ESR spectra and draw qualitative
conclusions about the adiabatic/diabatic transition between triplet pair spin
states.
