Room-temperature hyperpolarization of polycrystalline samples with optically polarized triplet electrons: pentacene or nitrogen-vacancy center in diamond?

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