The solid line in the above figure corresponds to the fit of the function in eq. 1 to the data between 39 and 200 K with.
s-1 + 28.75 s-1exp(-501.5 K/T).
As in this temperature region the photoreaction renders only negligible contributions to the triplet decay rate kT(T), the data fit reflects the intramolecular acridine-d9 T1 decay rate in the fluorene-d8D2 host crystal. Such a weak temperature dependence is typical for intramolecular decay rates in organic molecular crystals.