Room-temperature hyperpolarization of polycrystalline samples with optically polarized triplet electrons: pentacene...

We demonstrate room-temperature $$C hyperpolarization by dynamic nuclear polarization (DNP) using optically polarized triplet electron spins in two polycrystalline systems: pentacene-doped [carboxyl-$$C] benzoic acid and microdiamonds containing nitrogen-vacancy (NV$$) centers. For both samples, the integrated solid effect (ISE) is used to polarize the $$C spin system in magnetic fields of 350-400 mT. In the benzoic acid sample, the $$C spin polarization is enhanced by up to 0.12 % through direct electron-to-$$C polarization transfer without performing dynamic $$H polarization followed by $$ cross-polarization. In addition, the ISE has been successfully applied to polarize naturally abundant $$C spins in a microdiamond sample to 0.01 %. To characterize the buildup of the $$C polarization, we discuss the efficiencies of direct polarization transfer between the electron and $$C spins as well as that of $$ spin diffusion, examining various parameters which are beneficial or detrimental for successful bulk dynamic $$C polarization.