Optically-generated Overhauser Dynamic Nuclear Polarization

  • Daniel Cheney

Student thesis: Doctoral Thesis


Nuclear magnetic resonance (NMR) is one of the most powerful analytical techniques currently in use, but it suffers from inherently low sensitivity. The sensitivity can be improved using dynamic nuclear polarization (DNP), and involves the transfer of magnetization from electron spins of a radical to nuclear spins. Although it is typically a microwave-driven technique, an alternative approach has recently been introduced, which instead uses visible light. A dye is added to the sample alongside the radical, and is photoexcited to the triplet state. The triplet is quenched by the radical, which induces electron spin polarization in the latter due to the radical-triplet pair mechanism (RTPM). This polarization is transferred to nuclear spins via cross-relaxation. This thesis discusses the development of this technique beyond its initial proof-of-concept study. For the first time, theoretical models are used to rationalize previous experimental data, and to aid in the optimization of experimental parameters, such as the laser power, dye concentration, sample volume, magnetic field, and viscosity. Most notably, it is predicted that enhancements should greatly increase if the sample volume is decreased, owing to the larger triplet concentration that can be generated with a higher photon density. This prediction is experimentally verified to be the case, which has significant implications for NMR studies of volume-limited samples.
The scope of the simulations is then expanded to other systems, including 13C DNP of organic solvents, and the use of tethered chromophore-radical polarizing agents, where electron hyperpolarization is driven by the reversed quartet mechanism (RQM). It is also demonstrated theoretically that TR-EPR could be used to rapidly screen potential polarizing agents. An extensive numerical study of pulsed illumination is carried out in order to determine the optimum pulse length and duty cycle while allowing for sufficient sample cooling between pulses in order to reduce heating effects. Finally, a combination of the optical DNP method with a shuttle set-up is presented, allowing for polarization of the sample at low magnetic field before transfer to higher fields for detection. This has enabled for the rst time the detection of optically-polarized NMR spectra of dissolved amino acids, measurement of glycerol enhancements, and a study into the magnetic field dependence of the optical DNP effect.
Date of Award25 Jul 2022
Original languageEnglish
SupervisorChristopher Wedge (Main Supervisor) & Paul Elliott (Co-Supervisor)

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