Reverse dynamic nuclear polarisation for indirect detection of nuclear spins close to unpaired electrons

Abstract Polarisation transfer schemes and indirect detection are central to magnetic resonance. Using the trityl radical OX063 and a pulse electron paramagnetic resonance spectrometer operating in the Q-band (35 GHz, 1.2 T), we show here that it is possible to use pulsed dynamic nuclear polarisation (DNP) to transfer polarisation from electrons to protons and back. The latter is achieved by first saturating the electrons and then simply using a reverse DNP step. A variable mixing time between DNP and reverse DNP allows us to investigate the decay of polarisation on protons in the vicinity of the electrons. We qualitatively investigate the influence of solvent deuteration, temperature, and electron concentration. We expect reverse DNP to be useful in the investigation of nuclear spin diffusion and envisage its use in electron–nuclear double-resonance (ENDOR) experiments.


S 5 Details about RA-NOVEL
The following amplitude modulation function was used: with x = t/t max , a 0 = 0.27, ∆a = 0.2 and κ = 0.025.The scale corresponds to the digital output of the AWG. a 0 = 0.265 corresponded to the normal NOVEL condition.No compensation of the non-linearity of the TWT amplifier was used.When sweeping the pulse length, the waveform was simply stretched.

S 9 Saturation behaviour during depolarisation
As shown in the main text, the nuclear relaxation is much slower than T 1,e .This raises the concern that saturation effects could appear when repeating experiments with the "normal" repetition times used for EPR experiments.To investigate this we performed a simple experiment: The pulse sequence in Figure 1(a) of the main text was repeated several times with a repetition time of 10 ms, with the spinlock fulfilling the NOVEL conditions for a duration of 500 ns.This should lead to an accumulation of nuclear polarisation, if no phase-cycling is used.Single echoes were detected, and the intensity normalised to the first echo.The results of this experiment are shown in Figure S13 In the case of deuterated solvent, there is only a small number of protons in the sample per electron.After a few DNP contacts, the protons are significantly polarized.Since the effective Hamiltonian during NOVEL leads to an oscillation of the difference of polarisations, less and less polarisation is transferred for each repetition, until some steady-state is reached.This leads to an apparent increase in Figure S13.In other words, less polarisation is lost from the electron to the nuclei during the spinlock.The electron polarisation does not rise over its equilibrium value.
In the case of the protonated solvent, no such "saturation effect" is observed, most likely because the amount of polarisation transferred per proton is so small and spin diffusion is fast enough that the difference between proton and electron spin polarisation is effectively constant over the timescale of the experiment.We did not check what happens if the experiment is repeated for several minutes.
Note that in the main text, all experiments are performed with phase-cycling.This leads to opposite nuclear polarisation for every other repetition, and thus nearly no net nuclear polarisation.A significant nuclear polarisation at the beginning of the measurements might lead to unexpected effects during reverse DNP measurements.We cannot directly saturate the nuclei before, since we are currently using no rf-channel.Even if one would be available, directly coupled nuclei might be unaffected due to significant hyperfine coupling.However, saturating the electrons, then performing reverse DNP several times (see Figure S14) at least partially saturated the nuclei close to the electrons.The echo intensity after DNP and reverse DNP for the sample with deuterated solvent is shown in Figure S15.If no pre-saturation or phase-cycling is used, the intensity increases with each repetition, indicating an accumulation of nuclear polarisation.Presaturation alone reduces this accumulation, but is not sufficient to completely get rid of it.As expected, phase-cycling also does not lead to an accumulation of nuclear polarisation, because an opposite phase of the π/2 pulse leads to an opposite sign in nuclear polarisation.However, the first echo was always slightly more intense than the rest.If presaturation and phase-cycling are used together, the echo intensity is constant.no presat, no pc with presat, no pc with presat, with pc no presat, with pc Fig. S15: Electron spin echo intensity after DNP and reverse DNP, using the 100 µM sample in deuterated solvent.The experiment was simply repeated several times, and single shots were acquired.A combination of presaturation and phase-cycling leads to results without any saturation behaviour from one repetition to the next.
Fig. S1: Chemical structure of the OX063 radical.

Fig. S10 :
Fig.S10: Measurement of T 1ρ for different samples and conditions.The spinlock power was set to the maximum.Since only a few percent of the signal are lost, and there is no difference between samples, the influence of T 1,ρ can be neglected for the experiments presented in this work.
Fig.S12: Hahn echo decays measured at 50 K.There is no significant difference compared to 80 K.

Fig. S14 :
Fig. S14: Electron spin echo intensity after DNP and reverse DNP