Photo-excited 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 (triplet-DNP).

We demonstrate the cooling of a microwave mode at 2872 MHz through its interaction with optically spin-polarized NV− centers in diamond at zero applied magnetic field, removing thermal photons from the mode. By photo-exciting (pumping) a brilliant-cut red diamond jewel with a continuous-wave 532-nm laser, outputting 2 W, the microwave mode is cooled down to a noise temperature of 188 K. This noise temperature can be preserved continuously for as long as the diamond is optically excited and kept cool. The latter requirement restricted operation out to 10 ms in our preliminary setup. The mode-cooling performance of NV− diamond is directly compared against that of pentacene-doped para-terphenyl, where we find that the former affords the advantages of cooling immediately upon light excitation (whereas pentacene-doped para-terphenyl undesirably mases before it begins cooling) and being able to cool continuously at substantially lower optical pump power.

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Charge-transfer co-crystals have garnered interest for use in organic electronics and photovoltaics, where their ease of growth by solution/vapor deposition and flexibility in choice of constituent molecules allows tailoring of their band gaps and triplet spin dynamics. Here, we report on the triplet spin dynamics of one such charge-transfer co-crystal phenazine/1,2,4,5-tetracyanobenzene (PNZ/TCNB), as investigated using algorithm-assisted continuous-wave electron paramagnetic resonance and transient powder electron paramagnetic resonance (EPR) at X-band, as well as transient EPR at zero field (ZF-trEPR). The zero-field splitting parameters are |D| = 2118 ± 2 MHz and |E| = 297 ± 1 MHz, with initial triplet sublevel populations P x /P y /P z being 0.764:0.127:0.109. The triplet spin dynamics were found to have decay times in the sub-microsecond regime. Furthermore, with the aid of a Q-boosted dielectric cavity, PNZ/TCNB can output a maser burst at 2412 MHz at room temperature and zero field under 445 nm pulsed excitation. This demonstrates that it is feasible for PNZ/TCNB to mase if a cavity with a sufficiently high-quality factor is provided and opens up avenues toward investigating other charge-transfer co-crystals with similar triplet spin dynamics as potential maser gain media at room temperature and zero field.

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