Femtosecond time-resolved transient absorption (fs-TA), nanosecond time-resolved transient absorption (ns-TA), and nanosecond time-resolved resonance Raman spectroscopy (ns-TR3) methods were used to study the behaviors of the transient intermediates involved in the photophysical and photochemical processes of 3-methylbenzophenone (3-MeBP), 3-(hydroxymethyl)benzophenone (m-BPOH), and 2-(1-hydroxyethyl) 9,10-anthraquinone (2-HEAQ). A particular focus of this work was to study the unusual meta methyl activation reactions of these compounds in water-containing solutions. Density functional theory (DFT) calculations were conducted to help make assignments of the observed experimental transient species and to better understand the reaction mechanisms. First, the photophysical and photochemical reactions of m-BPOH were investigated in selected solvents. In acetonitrile (MeCN) the formation of the triplet state of m-BPOH, (denoted as (m-BPOH)3 ), was detected via an intersystem crossing (ISC). In 2-propanol (IPA), (m-BPOH)3 abstracted a hydrogen atom from the solvent molecule to form an aryl ketyl radical. In an acidic mixed aqueous solution at pH 2, the photoredox reaction appeared to be the predominant reaction. In a more acidic aqueous solution with [H+] =1.0 M, the photoredox reaction faced some competition from the overall photohydration reaction. Second, the photophysical and photochemical reactions of 2-HEAQ in MeCN, IPA, and neutral, acid and basic aqueous solutions were studied. The ISC process of 2-HEAQ took place in MeCN with generation of the triplet excited state species of 2-HEAQ, (2-HEAQ)3. In IPA solvent, (2-HEAQ)3 underwent a hydrogen abstraction with the solvent. A photoredox reaction takes place via an initial protonation process of the AQ group that is followed by deprotonation of the methylene C-H bond in aqueous solutions within a pH range from 2 to 10. Under a stronger acidic aqueous condition with [H+] =1.0 M, the photohydration reaction becomes the major reaction. In strong basic solutions (pH=12) only ISC was observed to take place. The unusual photoredox reaction takes place via protonation of the carbonyl oxygen first followed by deprotonation of the C-H bond in the side chain for both m-BPOH and 2-HEAQ. The protonation of the excited carbonyl oxygen group has been widely studied. On the other hand, the deprotonation of methylene C-H bond is unusual. Therefore, this photoredox reaction for m-BPOH and 2-HEAQ is termed as a meta methyl activation reaction. Third, the photophysical and photochemical reactions of 3-MeBP were explored and compared to those of 4-methylbenzophenone (4-MeBP). This work found that 3-MeBP and 4-MeBP exhibit similar behaviors with m-BPOH and 2-HEAQ in MeCN and IPA. In MeCN, both 3-MeBP and 4-MeBP undergo an efficient ISC process producing triplet excited state species, (3-MeBP)3 and (4-MeBP)3, respectively. In IPA, the (3-MeBP)3 and (4-MeBP)3 intermediates were quenched by the hydrogen abstraction reaction with the solvent. In acidic aqueous solutions (pH  2), the protonated carbonyl oxygen species (3-MeBPH+)3 and (4-(MeBPH+)3 are directly observed by fs-TA spectra. In the case of 4-MeBP, a photohydration is detected and the m-(4-MeBPH2O)3 and o-(4-MeBPH2O)1 species are observed. In contrast, an unusual meta methyl activation reaction is observed for 3-MeBP. In a stronger acid aqueous solution ([H+] =1.0 M) the meta methyl activation reaction becomes the predominant reaction. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

This thesis presents time-dependent density functional theory studies on electronic and vibronically resolved linear and nonlinear optical absorption and dichroism spectra of organic molecules. Special attention has been paid to the influence of solvent environment and molecular vibrations on one-, two- and three-photon absorption and one- and two-photon circular dichroism. It is found that dielectric medium as described by polarizable continuum model can enhance remarkably three-photon absorption cross section of a highly conjugated fluorene derivative, for which the simplified two-state model is shown to be largely inadequate. Origin-invariant density functional calculations on one- and two-photon circular dichroisms of a chiral molecule confirm that the recently developed CAMB3LYP functional performs better than the popular B3LYP functional for Rydberg-states. The first experimental measurement of TPCD spectra is performed on an axial chiral system in tetrahydrofunan, where the double L-scan technique is applied. Theoretical calculations well reproduce the experimental profiles when both the electron correlation and the solvent effect are taken into account. Vibronically resolved one- and two-photon absorption spectra of charge-transfer molecules have been obtained using a Linear Coupling model, where the 'borrowing mechanism' for the so-called Herzberg-Teller contribution is analyzed in detail. It is shown that Herzberg-Teller contribution can introduce a change of sign to the chiral responses of a molecule without the involvement of different electronic states, which has important consequences for the assignment of absolute configurations of chiral molecules. Adiabatic harmonic Franck-Condon model is also applied to simulate vibronically resolved one- and two-photon circular dichroism spectra of the same chiral system, where the sign-inversion and the interference between Franck-Condon and Herzberg-Teller contributions are also observed. / QC 20100727

This thesis is an investigation of the photophysical processes that occur in organic photovoltaic blends in the time between light being absorbed and free charges being generated. The purpose of all solar cells is to generate a photocurrent. The free charges, as they flow out of the device, make up the photocurrent, so understanding the processes by which they are created is vitally important to organic photovoltaic research. The main experimental method used was time-resolved fluorescence spectroscopy. This technique was used to probe the exciton population with respect to time for a variety of blends of organic semiconductors, including the high performance photovoltaic materials PCDTBT, PTB7, C71-PCBM and P3HT. The main goal of the work was to characterise the exciton diffusion lengths of these materials by developing a technique called volume quenching. Volume quenching involves blending a small quantity of quenching material into a thin film of semiconducting material. These introduced quenching sites render excitons unemissive on contact. Thus, from the drop in fluorescence compared with the ‘unquenched' material, it was possible to work out what proportion of the initial excitons have encountered a quenching site in the blends. The results can then be fitted to quantify how diffusive the excitons are -i.e. how far they move. By looking at the rate constant of the quenching process and how it varies with respect to time, quencher concentration and quencher type, it was possible to generate a wealth of additional information, not just about exciton diffusion, but about all the inter-related processes that contribute to exciton harvesting. These processes included the measurement of long-range energy transfer from the donor to the acceptor, electron transfer at the interface with the acceptor and the understanding of nanomorphology of donor-acceptor heterojunctions.

Please read the abstract in the section 00front of this document / Dissertation (MSc (Applied Sciences:Chemistry))--University of Pretoria, 2005. / Chemistry / unrestricted

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Live-cell imaging with fluorescent protein-based sensors allows us to monitor many dynamic changes in situ. The first genetic manipulation of green fluorescent protein to increase brightness initiated a boom, with a myriad of fluorescent protein sensors now available that span the UV, visible and near-IR range; capable of detecting a great number of metabolites, ions, and other biological signaling components with increased spatial and temporal precision. Used for both steady-state and time-resolved approaches, fluorescent proteins can be used in a wide variety of quantitative approaches. Steady-state sensors are typically characterized as intensiometric or ratiometric; and intensiometric sensors are characterized by an increase or decrease in emission intensity in response to analyte. However, moving in vivo, concentration and intensity dependence of the fluorophore, sample thickness, and photobleaching are limiting factors. Ratiometric probes respond by an inverse change in excitation or emission profiles in response to analyte, normalizing for bleaching or protein expression effects. As an intrinsic property of fluorophores, fluorescence lifetime does not rely on protein concentration, method of measurement or fluorescence intensity. By monitoring changes in lifetime using fluorescence lifetime spectroscopy, no special ratiometric fluorophores are needed, opening up a wider selection of potential fluorescent sensors. Lifetime and other time-resolved approaches are becoming more and more popular due to ease of quantitation and increased signal to background. Here we present the in vitro and live-cell characterization of genetically encoded, ratiometric and lifetime optimized red fluorescent protein pH sensors, a methodology for quantifying receptor trafficking in real time, as well as a lanthanide time resolved imaging approach.

This thesis aimed to investigate the features and properties of the ground states, transient species and photoproducts involved in the photophysical and photochemical processes for four kinds of drug compounds: lomefloxacin (LF), norfloxacin (NF), tiaprofenic acid (TPA), and flurbiprofen (Fp). The investigation used femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), UV/Vis absorption spectra (UV/Vis), nanosecond transient resonance Raman (ns-TR2) and nanosecond time-resolved resonance Raman spectroscopy (ns-TR3), as well as density functional theory (DFT) calculations. Although many previous investigations have indicated that photo-defluorination or photo-decarboxylation reactions may account for the phototoxicity for these compounds, detailed information on the mechanisms remains unclear. In this thesis, the photo-defluorination reaction of LF was explored in neutral water at pH 7.2. The fs-TA results revealed that the lowest lying excited singlet state species (S1) partially decayed into the ground state through fluorescence emission and partially underwent cleavage of the carbon-fluorine bond at position 8 to generate into a singlet aryl cation. Subsequently, intersystem crossing (ISC) allowing the transformation from singlet cation to triplet carbene was observed. Finally, a cyclization reaction with the N-ethyl chain took place for the triplet carbene to generate the final product. The mechanism underlying NF phototoxicity involves a photo-defluorination reaction in neutral water (pH=7.2). The fs-TA spectra indicated that the S1 underwent efficient ISC to swiftly transform into lowest excited triplet (T1) The ns-TA gained under nitrogen-saturated condition observed a new transient species produced from T1 that was proposed to be a transient species derived from the photo-defluorination reaction involving a SN2Ar* mechanism. The photo-defluorinated product ultimately experienced an ISC process to produce the final product. The photo-decarboxylation mechanism of TPA was studied in a neutral phosphate buffered solution (PBS). The fs-TA data revealed that S1 went through an efficient ISC to rapidly transform into T1 that then undergoes a photo-decarboxylation reaction to produce a triplet biradical species (denoted as TB3). The ns-TA and ns-TR3 results supplied evidence of the protonation process of TB3 that produces the neutral species (denoted as TBP3) that then decayed through ISC to give rise to the singlet TBP species, which underwent further reaction to make the final product (DTPA). The photo-decarboxylation reaction of Fp was explored in pure acetonitrile (MeCN). The second excited singlet (S2) went through internal conversion (IC) to decay to S1. Intriguingly, three different pathways for S1 decay co-exist. One pathway is fluorescence emission and the second is an ISC process. The third pathway is the homolysis of the carbon α bond reaction that proceeds to generate two radical species, one being a carboxyl species and the other being the residual, denoted as FpR that was liable to be oxidized under an oxygen-saturated condition to yield a new radical species with the addition of one oxygen molecule which is denoted as FOR that then experienced intramolecular hydrogen transfer (IHT) and dehydroxylation (DHO) to produce the final product. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

The pure organic dye sensitized solar cells (DSSCs) were studied and a new organic dye of donor-π-2acceptors (D-π-2A configuration) was fabricated. This dye, denoted as B2, was investigated and applied in DSSCs. Density functional theory (DFT) was used to examine the electronic distribution of the frontier orbitals of the B2 dye. It was found that intramolecular charge transfer (ICT) between the donor moieties and acceptor moieties of the B2 dye may take place under photo irradiation. The LUMO, LUMO+1 and LUMO+2 of B2 are all distributed on the acceptor moieties and this is very helpful to enhance the intramolecular electron transfer from the donor moieties to the acceptor moieties, which will consequently promote the chance of electron injection into the semiconductor. DSSCs based on B2 demonstrated an power conversion efficiency of 3.62 %. This efficiency value is approximately half of the power conversion efficiency of DSSCs based on N719 (7.69 %) under the same conditions. Femtosecond transient absorption and nanosecond transient absorption (TA), and time-correlated single photon counting (TCSPC) technique were applied to examine the electron transfer processes occurring on the surface of B2/TiO2. B2 dye has life time of the excited states three orders in magnitude shorter than that of N719. The electron injection time from excited B2 to TiO2 is also three orders in magnitude shorter than that from excited N719 to TiO2. It was revealed that the delocalized electrons of π → π* transition for both the B2 dye and the N719 dye could be further guided into the semiconductor, while such injection processes may not happen for the localized electrons in π → π* transition of these dyes. The nanosecond transient absorption and transient emission spectroscopy of the ruthenium bipyridyl sensitizer N719 in different solvents were studied. Three kinds of ZnO nanoparticles were utilized to study the electron transfer process taking place on the interface of N719/ZnO with and without electrolyte by Time-Correlated Single Photon Counting (TCSPC) technique, TA and transient emission spectroscopy. Isopropanol was found to stabilize the singlet excited state of N719 and a related emission band centered at 460 nm was observed in nanosecond time scale. It was revealed that the electrolyte has a significant impact upon the electron transfer dynamics on the N719/ZnO interface. In the absence of electrolyte, the electron transfer process on the N719/ZnO interface is dependent upon the depth of defects in ZnO nanoparticles. Conversely, in the presence of electrolyte, the impact of ZnO defects upon the electron transfer process is eliminated and the effective electron injection happens from the excited states of N719 to ZnO, in spite of the ZnO particle sizes. The polymer based solar cells were studied and a polymer incorporated with a pyrenylcarbazole pendant was synthesized and applied in the functionalization of multi-wall carbon nanotubes (MWCNT) by noncovalent π-π interaction. The polymer/MWCNT hybrids were isolated and examined. The strong interaction between the polymer and MWCNT in a 1,1,2,2-tetrachloroethane (TCE) solution was investigated. The emission spectra demonstrated an effective quenching of emission from the polymer by the MWCNT. DFT calculations showed an electron delocalization phenomenon between the pyrene and carbazole moieties. The LUMO of the polymer is mainly located on the pyrene moiety while the LUMO+1 of the polymer is predominantly positioned on the carbazole moiety. The electronic transition of LUMO+1→LUMO results in intramolecular charge transfer (ICT) from the carbazole moieties to the pyrene moieties. Femtosecond TA determined the characteristic TA feature of the excited states, which are contributed from both the pyrene and carbazole moieties. The excited state lifetime of the polymer was calculated to be 659 ps and the photo excited electrons can inject into the MWCNT very fast on a time scale of 420 fs. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

The construction of a high-resolution optical microscope system for micro-photoluminescence (µ-PL) spectroscopy is described, and a range of time-integrated and time-resolved experimental work on single InGaN quantum dots (QDs) is presented. Time-integrated measurements demonstrate the existence of InGaN QDs in three different samples via the presence of sharp exciton recombination lines in the µ-PL spectra. The narrowest peaks display a linewidth Γ of ~230 µeV, implying a decoherence time T2 ≥5.7 ps. Time-resolved measurements on exciton recombination lines from single self-assembled InGaN QDs reveal typical lifetimes of ~2.0 ns (which decrease with increasing temperature), while typical lifetimes for excitons in single selectively-grown micropyramidal InGaN QDs are found to be ~0.4 ns. The shorter exciton recombination lifetime in selectively-grown QDs is believed to be due to a stronger coupling of these QDs to the underlying quantum well. Temporal fluctuations (on a timescale of seconds) in the energy, intensity and FWHM of µ-PL peaks arising from the recombination of excitons in single self-assembled InGaN QDs are observed. These are attributed to transient Stark shifts induced by a fluctuating local charge distribution as carriers become trapped in defect states in the vicinity of the QDs. Time-integrated power-dependent measurements are used to demonstrate the presence of biexciton states in single self-assembled InGaN QDs. The exciton–biexciton energy splitting is found to be ~41 meV, in agreement with values predicted by theoretical calculations. Time-resolved studies of the biexciton and exciton decay curves reveal a coupling as the exciton population is refilled by biexciton decays. The biexciton lifetime is found to be ~1.4 ns, compared to an exciton lifetime of ~1.0 ns. Lateral electric fields are applied to a single self-assembled InGaN QD using aluminium electrodes lithographically defined on the sample surface. Application of fields of the order of ~0.17 MVcm-1 is found to cause both a red-shift and a reduction in the intensity of the exciton recombination peak in the µ-PL spectrum.

Thesis (Ph. D.)--University of Hong Kong, 2008. / Includes bibliographical references (leaves 140-147) Also available in print.