Singlet oxygen and optical limiting applications of BODIPYs and other molecular dyes
- Authors: May, Aviwe Khanya
- Date: 2022-04-08
- Subjects: Dyes and dyeing Chemistry , Phthalocyanines , Photochemotherapy , Active oxygen , Nonlinear optics , Time-dependent density functional theory , Photochemistry
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/294618 , vital:57238 , DOI 10.21504/10962/294620
- Description: A series of structurally diverse novel and previously synthesized BODIPY core dyes are synthesized and characterized in this thesis. These BODIPYs were synthesized using 2-methylpyrrole, 2-ethylpyrrole, 2,4-dimethylpyrrole and 3-ethyl-2,4-dimethylpyrrole as the starting pyrroles. The combination of different pyrroles with the same aldehyde results in BODIPY core dyes that are structural analogues. These core dyes were used as precursors to synthesise halogenated BODIPYs and novel styrylBODIPY dyes, which were successfully characterized using FT-IR and 1H NMR spectroscopy. The halogenated BODIPY core dyes and the styrylBODIPY dyes were also characterized using MALDI-TOF mass spectrometry. The introduction of heavy atoms on the BODIPY core leads to a red shift of the main spectral. In the presence of styryl groups, the main spectral band red shifts to the far red end of the visible region. As expected, the halogenated BODIPY core dyes also had moderate singlet oxygen quantum yields. These halogenated core dyes were found to be suitable as photosensitizers as all the dyes reduced bacterial viability to below 50% during photodynamic antimicrobial chemotherapy (PACT) studies against Staphylococcus aureus. The structure-property relationships studied demonstrate that the presence of protons rather than methyls at the 1,7-positions or iodines at the 2,6-positions results in more favorable PACT activity. This is likely to be related to the greater ability of the meso-aryl to rotate into the plane of the dipyrromethene ligand and suggests that there should be a stronger focus on dyes of this type in future studies in this field. During nonlinear optical (NLO) studies, all the styrylBODIPYs exhibited favorable reverse saturable absorption (RSA) responses. In the absence of methyl groups at the 1,7-positions, the meso-aryl ring lies closer to the π-system of the BODIPY core, enhancing donor (D)–π–acceptor (A) properties and resulting in slightly enhanced optical limiting (OL) parameters. Additionally, there is no evidence that the introduction of heavy atoms at the 2,6-positions significantly enhances OL properties. In a similar manner, alkyl substituents at these positions also do not significantly enhance OL properties; this was studied for the first time using 15 with ethyl groups at the 2,6-positions. The combination of z-scan data and transient spectroscopy for 16 demonstrated that the main mechanism responsible for the NLO properties of nonhalogenated BODIPY dyes is one-photon absorption from the ground state followed by ESA in the singlet manifold. From the NLO studies of 25, OL parameters of 1,3,5-tristyrylBODIPY dyes were found to be similar in magnitude to properties of distyrylBODIPY dyes, but to have less favorable optical properties for OL applications. The OL properties of scandium phthalocyanines were assessed for the first time, since the Sc(III) ion, unusually for a first row transition metal ion, is known to readily form sandwich complexes. The presence of a Sc(III) ion does not significantly enhance the OL properties of phthalocyanines relative to those of rare earth metal ions that also form complexes of this type. Because BODIPYs and phthalocyanines typically absorb significantly in the visible region, transparent PBC polymer thin films of disilane-bridged compounds with minimal absorption in this region were studied and exhibited an excellent RSA response. These compounds may be useful in the design of OL materials that can protect the human eye. The optimized geometries and spectroscopic properties of selected BODIPYs were studied. As expected, the presence of bromine, iodine, ethyl and styryl groups at different positions of the BODIPY core leads to a narrowing of the HOMO–LUMO band gap, which results in a red-shift of the main spectral band. Partial atomic charges have also been calculated for some of the styrylBODIPY dyes studied for application in OL, and electrostatic potential energy maps were also visualized to better assess how the dipole moment of BODIPY dyes can be modulated since this can affect the OL properties. For all the BODIPYs studied, the electronegativity of the atoms present influences charge distribution on the BODIPY structure. , Thesis (PhD) -- Faculty of Science, Chemistry, 2022
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- Date Issued: 2022-04-08
Colonial power and the transformation of feudal relations in Buganda 1900-1962
- Authors: Sekiswa, Peter
- Date: 2020-12
- Subjects: Uganda , Uganda (South Africa) , South Africa
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10948/55218 , vital:49670
- Description: This thesis has engaged in a debate that the establishment of colonial administration in Buganda in the 19th century more than anything else saw the transformation the Buganda state into an African feudal state. Basic explanation for this state of affairs was the need by the colonial authority to establish a dependent commodity producing economy in Buganda. The African feudal state contextually referred to is the transformation of power, labour and economic relations in this period to suit circumstances that favoured the process of commodity production in the Buganda state; a process that involved the strengthening of the political power base, by creating a landed oligarchy, thus destroying the traditional patron–client relations and creating out of such a relation a class of civil servants, yet also extracting land rent from siting tenant in an agrarian economic system. This period saw an interplay of numerous actors like the Christian missionaries and their Buganda collaborators, the Christian elite (Abasomi), the Learners. The second phase of this study examines the creation of a colonial economic system based on commodity production and the colonial authority creating a frame work of operation, as to how the economy-based commodity production was to operate as to either allow a settler plantation or a peasant mode of production. Coupled with the effects of the wars of nations the deteriorating economic conditions and the age of the rise of the spirit of nationalism in the 1950s, created a class of agitators against the colonial authority. Using the historical methodology, the study employed the use of oral interviews, archives, primary printed journals, and secondary sources, to trace the evolution of labour, political and economic relations in the Buganda state. This was in order to understand the process leading to the creation of the modern Buganda colonial state by 1962. The study puts it that the intercourse between the British colonial administrators and the Buganda state leaders was responsible for the resultant state of affairs. Intentionally or unintentionally, the British colonial authority created a new nation state dominated by a landed class elite. It is this elite class that led the independence struggle not for the whole nation but for one region of the country - their kingdom Buganda - the cause of conflict between the British colonial authority and the Buganda Kingdom, but also a source of economic and political competition between Buganda and other regions of Uganda. , Thesis (PhD) -- Faculty of Humanities, School of Governmental and Social Sciences, 2020
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- Date Issued: 2020-12