  The geminate annihilation of two triplet excitons created by singlet exciton
fission is affected by the dimensionality of transport as determined by
typically anisotropic triplet exciton mobilities in organic molecular crystals.
We analyze this process using a random-walk model where the time-dynamics of
the geminate annihilation probability is determined by the average exciton
hopping times along the crystallographic directions. The model is then applied
to the geminate fluorescence dynamics in rubrene, where the main channel for
triplet-triplet annihilation is via triplet fusion and subsequent photon
emission, and we identify the transitions between transport in one, two, and
three dimensions.
