There are advantages to performing the EPR measurements under varying viscosity conditions. In high viscosity solutions, the rotation of the molecules is slow enough so that the anisotropy by the ZFS is observed with the random orientations. As a result, the motional narrowing effect on the EPR line-shape is suppressed, and EPR spectra similar to those obtained in frozen solutions are observed, allowing for determination of ZFS parameters: D and E. As the viscosity of the solvent decreases, the degrees of freedom related to molecular motion, including rotation and vibrations, are increased. This leads to a high rotation frequency of the molecule comparable to the frequency of the ZFS interaction. Thermally activated intramolecular exciton hopping driven by the molecular vibrations may also induce pseudo-rotation as a temperature dependent process. These exciton dynamics can be observed as the motional narrowing effects on the EPR spectral line-shapes. Figure shows the TREPR spectra of PtOEP/TAB system and PtOEP/DPA system in solvents of varying viscosity at room temperature. Spectral line shapes reflect the spin populations of the triplet sublevels (|𝑇 8 ⟩, |𝑇 < ⟩, |𝑇 ? ⟩) of emitters, by the TET from PtOEP. Thermal equilibrium electron spin population according to the Boltzmann distribution is assumed here to interpret the microwaveabsorptive EPR spectra, although the absorptive spin polarization could possibly be created by the spin-selective reactions in the triplet-pairs during the TTA process, as reported previously. The slower rise and longer decay time of the EPR signal with increasing viscosity is attributed to the slower rate of TET and TTA respectively due to slower molecular diffusion.