Exploiting EPR to elucidate fine and hyperfine interactions in complex chemical and biological systems
Keywords: molecular wires
zero field splitting
porphyrin
fine and hyperfine structure
spin-orbit coupling
spin-correlated radical pairs
Electro paramagnetic resonance
cryptochromes

The research presented in this thesis is concerned with the applications of Electron Paramagnetic Resonance (EPR) spectroscopy to the study of porphyrin systems and cryptochrome proteins. In particular, the fine and hyperfine interactions measured using EPR are exploited to further our understanding of both the intricate electronic and spatial structure of these systems as well as their potential future applications.
In a first project, the 1H and 14N hyperfine couplings in the radical cations of oligoporphyrin molecular wires were determined using continuous-wave EPR (cwEPR) as well as pulsed Electron-Nuclear Double Resonance (ENDOR). The degree of spin delocalisation, which quantifies the viability of these systems as nano-wires, can be elucidated via these hyperfine couplings.
A comparison of a previously investigated butadiyne-linked oligoporphyrin series with a novel series of ethyne-linked analogues revealed that the trends in the cwEPR spectral envelopes of the latter deviate from the trends expected for complete spin delocalisation, in contrast to the former series which follows the theoretical trend neatly. The consequences of these cwEPR envelope trends to the spin delocalisation in both series is explored in detail from the perspective of Density Functional Theory (DFT) calculations, complementary ENDOR measurements, and a re-examination of the original theory behind the cwEPR envelope analysis method.
The applicability of trEPR and ENDOR to studying complex spin density trends was further exploited in the investigation of the photo-excited triplet states of palladium(II), zinc(II), free-base, and mesoper fluoro oligoporphyrin systems. For these triplet states, the information obtained from ENDOR about the hyperfine couplings is complemented by the fine-structure parameters, i.e. the g-tensor and zero- field splitting (ZFS) tensor, and the non-Boltzmann (spin polarised) populations of the triplet spin sublevels, all of which dominate their trEPR spectral signature.
In the palladium(II) systems, the spin-orbit interaction due to this heavy metal ion is shown to dominate all the fine-structure and spin polarisation parameters. However, ENDOR and electronic structure calculations reveal that the spin density in the Pd2+ systems is conserved relative to the Zn2+ and free-base porphyrins. This means that the Pd2+ ion could replace the Zn2+ in molecular wires when a particular type of spin polarisation pattern is desired without compromising on spin delocalisation.
In view of the potential to control spin delocalisation in nano-wires, the trends in the fine-structure parameters of meso-perfluoro/acceptor substituted porphyrin systems was also investigated. The sigma- acceptor groups are shown to disturb the planarity of the porphyrin backbone, causing significant orthorhombic distortions in the symmetry of the spin density. The implications of these distortions on the future use of meso-perfluoro groups in molecular wires, as well as on the continuing role of EPR spectroscopy in their study, is reviewed in light of the experimental observations.
The last chapter continues the exploitation of light induced spin polarisation but turns its attention to spin-correlated radical pairs in a series of cryptochrome proteins, which could be responsible for the ability of songbirds to sense and exploit the Earth’s magnetic field for navigation. The fine and hyperfine interactions obtained by new simulation methods will be linked to the all-essential spin-correlation and to the intimate spatial structure of the radical pairs.