Electron spin resonance studies of trityl OX063 at a concentration optimal for DNP

We have performed temperature-dependent electron spin resonance (ESR) measurements of the stable free radical trityl OX063, an efficient polarizing agent for dissolution dynamic nuclear polarization (DNP), at the optimum DNP concentration (15 mM). We have found that (i) when compared to the W-band electron spin-lattice relaxation rate T_1e^-1 of other free radicals used in DNP at the same concentration, trityl OX063 has slower T _1e^-1 than BDPA and 4-oxo-TEMPO. At T > 20 K, the T _1e^-1vs. T data of trityl OX063 appears to follow a power law dependence close to the Raman process prediction whereas at T < 10 K, electronic relaxation slows and approaches the direct process behaviour. (ii) Gd³⁺ doping, a factor known to enhance DNP, of trityl OX063 samples measured at W-band resulted in monotonic increases of T_1e ^-1 especially at temperatures below 20-40 K while the ESR lineshapes remained essentially unchanged. (iii) The high frequency ESR spectrum can be fitted with an axial g-tensor with a slight g-anisotropy: g_x = g _y = 2.00319(3) and g_z = 2.00258(3). Although the ESR linewidth D monotonically increases with field, the temperature-dependent T _1e^-1 is almost unchanged as the ESR frequency is increased from 9.5 GHz to 95 GHz, but becomes faster at 240 GHz and 336 GHz. The ESR properties of trityl OX063 reported here may provide insights into the efficiency of DNP of low-γ nuclei performed at various magnetic fields, from 0.35 T to 12 T.