Electron paramagnetic resonance line shapes of photoexcited triplets with rotational diffusion

A new line shape formula is developed for magnetic resonance on systems which are initially far from thermal equilibrium. A formalism for treating electron paramagnetic resonance (EPR) of rotationally diffusing molecules is extended to include typical triplet states, in which intramolecular interactions are not very small compared with the Zeeman interaction. The two developments are combined to a new formalism for calculating line shapes of photoexcited triplet states undergoing rotational diffusion. Approximations are made for parts of the calculation, and their range of validity is examined. Simulations based on this theory for randomly oriented samples of chlorophyll a(Chl a) demonstrate the usefulness of the theory in calculating line shapes for both equilibrated and polarized photoexcited EPR triplets undergoing rotational diffusion. In particular we study the effect of anisotropy in the molecular diffusion tensor on the line shapes. A comparison is also made between simulated and experimental line shapes for two expanded porphyrinoids, cadmium-texaphyrin complex (TxP-Cd), and zincoctaethylporphycene (ZnOEPC). Finally, we present a series of dynamic line shapes for the photoexcited triplet state of C ₆₀ (buckminsterfullerene), discussing their correspondence with available experimental results.