Temperature dependence of high field ¹³C dynamic nuclear polarization processes with trityl radicals below 35 Kelvin

In order to facilitate versatile applications with high field dynamic nuclear polarization (DNP), it is important to be able to optimize the DNP performance, i.e. reach high nuclear hyperpolarization within a short signal build up time. Given that the solid-state DNP process is strongly temperature-dependent, it is important to benchmark the temperature dependence of various DNP and electron paramagnetic resonance (EPR) parameters that can then be used to test and develop theories and models for high field DNP mechanisms. However, DNP and EPR experiments at high fields and cryogenic temperatures below 20 Kelvin usually require home built instrumentation, and therefore even basic experimental observations are lacking in the literature. DNP and EPR experiments at 7 T (197 GHz) and 8.5 T (240 GHz), respectively, were conducted at temperatures between 35 K and 3.7 K where the electron thermal polarization changes from 13.4% to 85.6%, respectively. The samples are frozen solutions of 15 mM OX063Me trityl radicals in various mixtures of [1- ¹³C]pyruvic acid, glycerol, and Gd³⁺-chelates. For all sample mixtures, the trityl EPR lines are found to be inhomogeneously broadened and the dominant DNP mechanism is shown to be the cross effect (CE). A 20%, 11%, and 6.77% ¹³C polarization is achieved at 3.7 K with a [1- ¹³C]pyruvic-glycerol-H₂O sample, the addition of 2 mM of Gd³⁺-chelates, and pure [1-¹³C]pyruvic acid, respectively. When T_1n is sufficiently long, our results seem to suggest T _1e is a key variable in the DNP process, where longer T_1e values correlate with larger DNP enhancements (ε_DNP). The experimental data reported here on the temperature dependence of T_1n, T_1e, T_m (electron phase memory time), the EPR linewidth, T_DNP and ε_DNP at high fields will be helpful for testing the mechanism and theory of DNP processes.