The effect of the solvent viscosity dependence of time-resolved magnetoluminescence (ML) on the delayed fluorescence of 9,10-diphenylanthracene (DPA) sensitized by platinum octaethylporphyrin has clarified the structure and dynamics of the triplet triplet pair (TT), i.e., the transition state of triplet fusion. Phase inversion of the ML effect with time provides evidence for the recycle dynamics of the excited triplet state for DPA in triplet fusion. The electron spin-relaxation by random molecular rotation causes intersystem crossing among the different spin states of the triplet-triplet pair and allows the (TT)-T-3,5 to engage in triplet fusion. Therefore, slow-down of the molecular diffusion by an increase in the solvent viscosity can enhance the triplet fusion yield. However, the reduction of the ML effect observed in quite high viscosity solvents suggests that the substantially slow rotational motion decreases the triplet fusion yield due to steric factors in electron exchange from the triplet-triplet pair.