published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

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This thesis is an investigation of the vacuum uv spectra of the phosphorus, phosphoryl, and thiophosphoryl trichlorofluorides in the region 1250 to 3000A. Assignments for absorption bands are made utilizing results from photoelectron spectra and ab initio calculations, oscillator strengths for absorption bands, and CNDO/2 molecular orbital calculations. Results from CNDO/2 calculations are compared with theoretical calculations, and experimental data are discussed with regard to the bonding in the compounds.

Hybrid perovskites are a new class of semiconductor which have proven to be an ideal material for making thin film solar cells. They have the advantages of flexibility, low cost, and easy processing, whilst achieving efficiencies competitive with monocrystalline silicon. Many of the properties which make them ideal for solar cells are also applicable to light emitting devices, and there is now increasing interest in their application for light emitting diodes (LEDs) and lasers. This thesis aims to use a range of novel spectroscopy techniques to investigate the origin of these favourable properties, and to exploit these properties to produce high performance distributed feedback lasers. A detailed understanding of the origins of the excellent properties of hybrid perovskites is of crucial importance in the search for new variations with improved performance or lowered toxicity. This thesis uses Kelvin probe, air photoemission, and resonant ultrasound spectroscopy to probe deeply into the underlying physics of hybrid perovskite single crystals and devices. Using these techniques, we are able to produce detailed maps of the energy levels in a common perovskite solar cell, and we also gain strong insight into the underlying strains and instabilities in the perovskite structure that give rise to their elastic properties. The strong light emission of hybrid perovskites is then exploited to produce high quality distributed feedback lasers emitting in the green and infrared part of the spectrum. These lasers are observed to have superior stability, good thresholds, and many interesting beam parameters owing to their high refractive index. We explore a wide range of processing methods in order to achieve the lowest lasing threshold and the best stability. Finally, we investigate the properties of low dimensional perovskites and investigate their potential in optoelectronic applications.

The reactions of cobalt halides with lithium dialkylamides have been investigated. The reaction with lithium diethylamide proved to be very unusual. This reaction was fully studied and the main products were shown to be lithium chloride, cobalt metal and bis(4-ethylimino-2-butene-2-ethylamino)Cob~lt(II). The latter product which was a volatile brown liquid was identified by its physical and . . chemical properties and the physical properties of derivatives prepared from the ligand after removal of the cobalt. Bis-(4-ethylimino-2-butene-2-ethylamino)cobalt(II) is a nitrogen analogue of the metal acetylacetonates and shows many similar properties. The ligand 4-ethylimino-2-ethylamino-2-butene • was shown, by the magnetic moment and electronic spectra of the tetrahedral cobalt compound, to produce a very high ligand field. A reaction mechanism has been proposed to explain the products 'of the reaction of cobalt chloride with lithium diethylamide. The reactions of other lithium dialkylamides with cobalt halides haye shown that dialkylamino compounds of cobalt(II) exist but are generally unstable thermally and very oxygen sensitive. The only stable (thermally) dialkylamino cobalt(II) compound prepared was bis-hexamethyldisilylamino cobalt(II). In solution this compound was shown· to be a linear two coordinate compound of cobalt(II). This unusual stereochemistry for cobalt(II) presents many theoretical problems before the magnetic and spectral properties ,are fully understood. It was found that the other dialkylamino cobalt(II) compounds produced were u!lstable.thermally decomposing above room temperature. The most stable compounds were formed when the dialkyl- group produced large steric hindrance. Two new alkolides were prepared from the dialkyamino cobalt(II) compounds. The magnetic and spectral properties of these compounds indicate that they are polymeric. possibly three coordinate compounds of cobalt.

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The photodebromination of selected bromoarenes has been studied at 300 nm to determine the the possible mechanistic pathways leading to product. Irradiation of 4-bromobiphenyl (BpBr) at this wavelength leads to the product biphenyl. The quantum yield of intersystem crossing (0.98) and quenching studies with cis-1 ,3-pentadiene suggest that the reaction occurs only via the triplet state. The observed increase of quantum yield of reaction with increasing concentration of BpBr suggests formation of a triplet excimer between the triplet state and ground state of BpBr as the key reactive intermediate. The log of the rate constant for excimer formation showed a linear increase with solvent polarity. The correlation of rate constants for excimer formation with linear solvation energy parameters indicates only a weak polarization of the excimer species. In order to understand the extent of radical anion character in the excimer, the regiochemistry of the photo-debromination of 3,4-dibromobiphenyl (3,4-BpBr) was studied. 3,4-BpBr was irradiated with and without an electron donor (triethylamine). 3,4-BpBr was also chemically reduced with lithium biphenylide (LiDBB). The difference in the regiochemistries under these conditions has been interpreted in terms of reaction via a free radical anion in the case of reactions with triethylamine and LiDBB and a weakly polarized excimer in the case of direct irradiation. In order to understand the extent of polarization in the excimers and their conformation, it was decided to study systems where the structure of the molecule would fix the geometry of potential intramolecular charge-transfer complexes. Towards this goal, brominated [2.2]paracyclophanes, 4-bromo[2.2]para-cylophane (CpBr), pseudo-para-dibromo[2.2]cyclophane (ps-p-CpBr) and pseudo-ortho-dibromo[2.2]cyclophane (ps-o-CpBr), were studied. The effect of substitution pattern of the bromines on the efficiency of excimer formation was also investigated. The brominated [2.2]para-cyclophanes showed varying efficiencies of formation of intermolecular excimer and intramolecular charge-transfer. A probable conformation for the excimer from BpBr has been proposed based on these results. / Graduation date: 1991

Thesis (Ph. D.)--University of Michigan, 1928. / "By Willard H. Bennett and Charles F. Meyer." From Physical review, v. 32, Dec., 1928.

Thesis (Ph. D.)--Florida State University, 2002. / Advisor: Dr. James G. Skofronick, Florida State University, College of Arts and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed Oct. 3, 2003). Includes bibliographical references.

Thesis--Ph. D., University of Hong Kong, 1981. / Cover title.