Unraveling the Room-Temperature Spin Dynamics of Photoexcited Pentacene in Its Lowest Triplet State at Zero Field

Photoexcited pentacene, upon arriving via intersystem crossing into its lowest triplet state, has been extensively studied due to the large and relatively long-lived spin polarization that it exhibits. However, the spin dynamics of these triplets has not hitherto been accurately determined, with glaring inconsistencies between published values. Using zero-field transient electron paramagnetic resonance (ZF-trEPR), we here report the determination of a complete set of depopulation and spin-lattice relaxation rates for the lowest triplet state of pentacene doped at 0.1% into a p-terphenyl host crystal at room temperature in zero applied magnetic field. The rates of spin-lattice relaxation between the triplet's sublevels are found to be highly anisotropic (i.e., transition-specific) and not negligible compared to the rates of depopulation from the same three sublevels back to pentacene's ground state. The spin dynamics, as well as the ZF-trEPR technique reported here, can aid the rational, quantitative engineering of applications such as room-temperature masers and triplet dynamic nuclear polarization.