No description available.

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No description available.

Exposure of a range of dilute solutions of halogenobenzenes in fluoro-trichloromethane to 60Co y-rays at 77 K gave the corresponding cations characterised by their ESR spectra. The approximate spin-densities on the halogens were greater than predicted by comparison with neutral x-bromo radicals and increased from ca, 8% for PhF+ to 23% for PhCl+, 30% for PhBr+ and 46% for PhI+ In accord with the fall in the ionization potential for this series of halogen. For PhBr+, replacement of para hydrogen by Br, OH and SH gave a steady fall in spin-density on Br reflecting increasing n delocallsation onto the para substituent. Evidence for dimer cation formation in concentrated solutions is presented. The major species obtained from benzyl chloride suggested a preferred conformation with the chlorine close to, but not in, the plane of the benzene ring, with a significant barrier for the in-plane site, in contrast with benzyl bromide with strong hyperfine coupling to bromine [Chapters 3, 4 and 9]. Exposure of dilute solutions of Me2C(Br)C(Br)Me2 In CD3OD and MeTHF to 60Co y-rays at 77 K gave the radical Me2CC(Br)Me2. From changes in the e.s.r. spectrum of this radical, it is deduced that the stable structure is asymmetric, but that the rate of migration of bromine between the two equivalent sites becomes fast on the e.s.r. time-scale at ca. 100-1 [Chapter 6]. Evidence is given for the 1,2-intramolecular proton shift in the interconversion of isobutyl to tert-butyl radicals. The parent material was dissolved in a variety of matrices and exposed to 60Co y-rays at 77 K. The reaction was observed using an in-cavity Proportional, Integral and Differential (PID) temperature control system [Chapter 5]. A single crystal of ethyl iodide was grown and irradiated at 77 K with a a 60Co y-ray source. The spectra were orientation dependent but it was not possible to determine the principal g-values and the elements of the hyperfine tensor [Chapter 7]. 1-Bromo adamantane was dissolved in a variety of deuterated matrices and exposed to 60Co y-rays at 77 K. No firm conclusions can be drawn regarding the single proton coupling of ca. 65 G to the bromine. Exposure of dilute solutions of m-dinitrobenzene or s-trinitrobenzene in methyltetrahydrofuran to 60Co y-rays at 77 K gave two species at 77 K. One, favoured at low doses, had features characteristic of mono-anions with the unpaired electron localized on one nitro-group. At high 7-ray doses a second species with triplet state characteristics grew at the expense of the first. The average separation between the two unpaired electrons was estimated to be 5-6 A.

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Organic semiconductors exhibit exceptionally long spin lifetimes, and recent observations of the inverse spin hall effect as well as micrometer spin diffusion lengths in conjugated polymers have spiked interest in employing such carbon-based materials in spintronics applications. The charge transport and photophysics of organic semiconductors have been intensely studied for optoelectronic applications, revealing subtle relationships between molecular geometry, morphology and physical properties. Similar structure-property relationships remain mostly unknown for spin dynamics, where the charge carrier spins couple to their environment through hyperfine (HFI) and spin-orbit interactions (SOC). HFIs provide a pathway for spin relaxation while SOC plays a dual role in such materials: it couples the spin to its angular momentum and therefore enables both spin-to-charge conversion and spin relaxation. Understanding the molecular SOC, and finding a means to tune its strength, therefore is fundamentally important for materials design and selection. However, quantifying SOC strengths indirectly through spin relaxation effects has proven difficult due to competing relaxation mechanisms. We initially present a systematic study of the g-tensor shift in molecular semiconductors and establish it as a probe for the SOC strength in a series of high mobility molecular semiconductors. The results demonstrate a rich variability of molecular g-shifts with the effective SOC, depending on subtle aspects of molecular composition and structure. We then correlate the above g-shifts to spin-lattice relaxation times over four orders of magnitude, from 200 µs to 0.15 µs, for isolated molecules in solution and relate our findings to the spin relaxation mechanisms that are likely to be relevant in solid state systems. Isolated molecules provide an ideal model system to investigate a spin's interactions with its environment but device applications commonly employ thin films. The second half of this thesis investigates polaron spin lifetimes in field effect transistors with high-mobility conjugated polymers as active layers. We use field-induced electron spin resonance measurements to demonstrate that spin relaxation is governed by the charges' hopping motion at low temperatures while Elliott-Yafet-like relaxation due to short-range, rapid spin density dynamics likely dominates high temperature spin lifetimes. Such a microscopic relaxation mechanism is highly sensitive to the local conformation of polymer backbones and we show its dependence on the degree of crystallinity in a polymer film.

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