Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

Thesis (Ph.D.)--Brown University, 2008. / Source: Dissertation Abstracts International, Volume: 69-06, Section: B, page: 3587. Adviser: Paul G. williard. Includes bibliographical references.

A series of photochromic dithienylethene-containing triarylboranes has been synthesized and characterized. The X-ray crystal structure of (4,5-bis(2,5-dimethylthiophen-3-yl)-thiophen-2-yl)dimesitylborane has been determined. An intense electronic absorption band was observed in all the triarylborane-containing compounds, corresponding to the intramolecular charge-transfer transition from the π-conjugated thiophene / fused thiophene / bithiophene to the vacant pπorbital of boron. This class of compounds was also found to bind fluoride ion such that the trigonal planar geometry of the boron center would be changed to tetrahedral, leading to a significant perturbation in the photophysical properties. Upon photo-excitation into the transition band, (4,5-bis(2,5-dimethylthiophen-3-yl)-thiophen-2-yl)dimesityl- borane and (4,5-bis-(2,5-dimethylthiophen-3-yl)-thieno[3,2-b]thiophen-2-yl)- dimesitylborane displayed multi-addressable photochromic reactivities upon addition of fluoride, in which a green solution was found in the closed form of the unbound compounds, while they changed to a purple color upon fluoride binding. Interestingly, both (4',5'-bis(2,5-dimethyl-thiophen-3-yl)-2,2'- bithiophen-5-yl)dimesitylborane and (4',5'-bis(2,5-dimethyl-thiophen-3-yl)-2,2'- bithiophen-3-yl)dimesitylborane displayed gated photochromic reactivities upon addition of fluoride, while only typical photochromic reactivity was observed in (4',5'-bis(2,5-dimethyl-thiophen-3-yl)-2,2'-bithiophen-4-yl)- dimesitylborane, indicative of a strong influence of the substituent effect on the photochromic reactivity upon incorporation of the -BMes2 group. A series of photochromic dithienylethene-containing β-diketones and the corresponding boron(III) compounds, appended with -BF2, -B(C6F5)2 and -BPh2, have been designed and synthesized. Upon photo-excitation into the transition band, typical photochromism was observed in all the dithienylethene- containing β-diketone ligands with the color changed from pale yellow to green in degassed benzene solution. Upon incorporation of the -BF2 and -B(C6F5)2 groups, near-infrared responsive photochromic behavior was observed, in which the absorption of the closed form was found to be at ca. 758-863 nm, suggesting that a pronounced perturbation of the dithienylethene was achieved by coordination of the boron(III) center. Moreover, a significant reduction in luminescence intensity of over 90 % was observed upon photocyclization, indicative of their potential application as photoswitchable luminescence materials. The thermal stability of the closed form and the activation energy for the thermal cycloreversion have also been studied on the representative compounds, 1-(4,5-bis(2,5-dimethylthiophen-3-yl)thiophen-2- yl)-butane-1,3-dione and difluoroboryl(1-(4,5-bis(2,5-dimethylthiophen-3-yl)- thiophen-2-yl)butane-1,3-dionate). A series of photochromic dithienylethene-containing BODIPY has been successfully prepared and characterized. The electronic absorption properties have been studied. These compounds showed an intense visible absorption band, corresponding to the 0-0 band of a strong S0→S1 transition of the BODIPY core, with a weak and broad absorption band at higher energy, corresponding to the S0→ S2 transition of the BODIPY core. An absorption band in the UV region was also observed in 2,3-bis(2,5-dimethyl- thiophen-3-yl)thiophene-containing derivatives, corresponding to the π→π* transitions of the dithienylethene moiety. The photochromic reactivities have also been studied such that the photostationary state was found to be reached upon irradiation in the UV region for over eight hours, while no observable changes would be obtained upon photo-excitation in the visible region. The slow conversion rate has been suggested to be attributed to the fact that a rapid excitation energy transfer could occur from the excited Th-DTE to the BODIPY moiety, rendering the suppression of the photocyclization. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

The first example of an N,N'-chelated [beta]-diketiminato phosphenium salt has been isolated by careful tuning of both the ligand architecture and the electronic characteristics of the phosphorus containing moiety. Using a two-electron reduction approach, the first example of a phosphinidene valence isomer has been isolated and structurally characterized. The mechanism of formation of this valence isomer was elucidated via Density Functional Theory (DFT) calculations. These calculations revealed that the formation of this valence isomer involves the intermediacy of a triplet diradical. In turn, the triplet diradical decays by two further steps to generate the observed product. A cationic phosphinous acid has been prepared via the hydrolysis of a cationic phosphenium complex in basic solution. DFT calculations reveals that the introduction of a positive charge stabilizes the phosphinous acid form. Two N,C-bonded phosphenium cation salts have also been obtained, the crystal structures of which reveal that the formation of these compounds involves C-H activation of a methyl group on the [beta]-diketiminate ligand. Three synthetic methods, namely halide abstraction, aluminum/boron halide exchange process and salt metathesis, have been explored for the preparation of boron cations supported by [beta]-diketiminate ligands. The first structurally characterized boron cation supported by a [beta]-diketiminate ligand has been isolated as its [Al₂Cl₇]⁻ salt. A Ga-Fe organometallic complex supported by a [beta]-diketiminate ligand has also been synthesized via salt metathesis reaction and fully characterized. Finally, several bisamidinate supported boron complexes have been synthesized and structurally characterized. Such complexes may have applications as bifunctional catalysts or as building blocks for novel polymers.

The objective of the research in this thesis was to develop high-efficiency n-type silicon solar cells at low-cost to reach grid parity. This was accomplished by reducing the electrical and optical losses in solar cells through understanding of fundamental physics and loss mechanisms, development of process technologies, cell design, and modeling. All these technology enhancements provided a 3.44% absolute increase in efficiency over the 17.4% efficient n-type PERT solar cell. Finally, 20.84% efficient n-type PERT (passivated emitter and rear totally diffused) solar cells were achieved on commercial grade 239cm2 n-type Cz silicon wafers with optimized front boron emitter without boron-rich layer and phosphorus back surface field, silicon dioxide/silicon nitride stack for surface passivation, optimized front grid pattern with screen printed 5 busbars and 100 gridlines, and improved rear contact with laser opening and physical vapor deposition aluminum. This thesis also suggested research directions to improve cell efficiency further and attain ≥21% efficient n-type solar cells which involves three additional technology developments including the use of floating busbars, selective emitters, and negatively charged aluminum oxide (Al2O3) film for boron emitter surface passivation.

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