This thesis presents applications, and developments, in pulsed electron paramagnetic resonance (EPR) spectroscopy at microwave frequencies ranging from ca. 9 to 95 GHz. Both permanent, and light-induced, paramagnetic centres are investigated, primarily via the use of arbitrary waveform generated (AWG) pulses, implementing a variety of pulsed dipolar and hyperfine techniques. The conformational flexibility of a molecular multi-qubit model system composed of dipolar coupled Cu(II), Cr7Ni ring, and nitroxide spin centres, was investigated by established orientation-selective pulsed dipolar spectroscopy (PDS) methods. Numerical simulations identified the relative orientations of the spin qubit centres and demonstrated low molecular flexibility of the system. Disparity in T1 of the electron spin qubit centres allowed for switchability of the inter-qubit interactions, and further minimised multi-spin effects (MSEs). A multi-qubit array composed of four Cr7Ni rings covalently linked to a zinc porphyrin was characterised; a potential route towards photo-switchable quantum information processing (QIP) devices. Despite the lack of an observable triplet echo, and the potential formation of an unusual strongly coupled higher spin state, laser-induced magnetic dipole (LaserIMD) spectroscopy could measure the interaction between the Cr7Ni ring and the optically excited porphyrin centre. The lower distance limit that can be accessed by LaserIMD is only limited by the excitation bandwidth of the employed microwave pulses. Preliminary studies investigating direct laser excitation of an isolated Cr7Ni ring gave first evidence of the formation of an optically excited state of the ring. A family of multi-nitroxide labelled free base and Cu(II) porphyrins was synthesised, and the relative effects of orientation selection and MSEs investigated in order to determine the dominant factor to deviations of the dipolar spectrum from a complete Pake pattern. PDS techniques, using AWG controlled microwave pulses, validated the inter-spin distances predicted by density functional theory (DFT) calculations. Orientation selection effects were most prominent in the systems studied, and MSEs were limited by the inversion efficiency. A previously reported orientation selective PDS simulation routine was adapted to account for multiple spin contributions to the dipolar trace, the results of which were in excellent agreement with experiment. AWG frequency swept wideband, uniform rate, smooth truncation (WURST) amplitude pulses were developed to enhance the echo sensitivity of the higher spin transitions of Gd(III) spin labels by polarisation transfer from the central transition (CT). Hahn, and stimulated, echo enhancements up to a factor of 2 were observed. Polarisation transfer was further demonstrated in 1H electron-nuclear double resonance (ENDOR) experiments, and the enhancements were consistent with spin dynamic simulations. A recently reported ELDOR-detected NMR (EDNMR) simulation routine was applied to the all-manganese cluster of the oxygen evolving complex (OEC) in photosystem II (PSII). Broken-symmetry DFT (BS-DFT) calculations allowed the dominant coupling topology of the last semi-stable S-state of the Kok cycle before O2 release to be determined. The simulations highlighted the importance of considering the EDNMR experimental conditions during spectral assignment, particularly when analysing small magnitude hyperfine couplings.
- Electron Paramagnetic Resonance
Pulsed Electron Paramagnetic Resonance Spectroscopy: From Quantum Information Processing to Structural Biology
Rogers, C. (Author). 31 Dec 2023
Student thesis: Phd