Pentachlorophenol (PCP) is one of the prominent environmental pollutant that has penetrated into food chain and is present in humans. Health concerns have been raised since daily intake of PCP by the US population is estimated to be 16-19 µg. PCP facilitates dissipation of electrochemical potential gradients of hydrogen ions across energy transducing membranes, which are the energy sources for the conversion of adenosine diphosphate into adenosine triphosphate. Closely linked to these dissipative effects is the development of electrical conductivity in lipid membranes, induced by the presence of PCP. Three modes of PCP - induced membrane electrical conductivity were theoretically analyzed and experimentally verifiable formulations of each models were developed. Experimental studies using the charge - pulse method involved characterization of the time dependent transmembrane voltage over a wide pH range, from 1.8 to 9.5, for 30 µM concentrations of PCP. Lipid membranes were prepared from dioleoyl phosphatidylcholine. It was shown that three PCP molecular species were determining the transmembrane transfer of hydrogen ions: electrically neutral PCP molecules (HA), negatively charged pentachlorophenolate ions (A⁻) and negatively charged heterodimers (AHA⁻). It was found that at pH>9 the membrane electrical conductivity was determined by the transmembrane movement of A⁻ ions, whenever pHAHA⁻ species. Two new membrane surface reactions were proposed as supplementary mechanisms for the generation of AHA⁻ in addition to the formation of AHA⁻ by the recombination of HA and A⁻, HA + A⁻→ AHA⁻. These new reactions are, (i) 2HA → H⁺ + AHA⁻, and (ii) H20 + 2A⁻ → OH' + AHA⁻. Reaction (i) provides formation of membrane permeable heterodimers AHA⁻ at pH < < 5.5 and reaction (ii) at pH> > 5.5. The maximum surface density of AHA" heterodimers was 0.09 pmol/cm² • The rate constant of formation of AHA' by recombination, HA + A⁻ → AHA' was estimated to be k[subscript f] = 2.6xl0⁹ cm² mol⁻¹ s⁻¹ and the dissociation rate constant for AHA⁻ Further, it was possible to determine the rate constants of transmembrane translocation for A' and AHA⁻ ions to be k[subscript a] = 6.6x10⁻⁵ s⁻¹ and k[subscript aha] = 1200 S⁻¹, respectively.

In this thesis, the photophysics and spin chemistry of donor-photosensitizer-acceptor triads were investigated. While all investigated triads comprised a TAA as an electron donor and a NDI as an electron acceptor, the central photosensitizers (PS) were different chromophores based on the dipyrrin-motif. The purity and identity of all target compounds could be confirmed by NMR spectroscopy, mass spectrometry and elemental analysis. The first part of the work dealt with dipyrrinato-complexes of cyclometalated heavy transition metals. The successful synthesis of novel triads based on Ir(III), Pt(II) and Pd(II) was presented. The optical and electrochemical properties indicated charge separation (CS), which was confirmed by transient absorption (TA) spectroscopy. TA-spectroscopy also revealed that the process of CS is significantly slower and less efficient for the triads based on Pt(II) and Pd(II) than for the analogous Ir(III) triads. This is mostly due to a much more convoluted reaction pathway, comprising several intermediate states before the formation of the final charge separated state (CSS2). On the other hand, CSS2 exhibits long lifetimes which are dependent on the central metal ion. While the Ir(III) triads show lifetimes of about 0.5 µs in MeCN, the Pt(II) and Pd(II) analogues show lifetimes of 1.5 µs. The magnetic field effect on the charge recombination (CR) kinetics of CSS2 was investigated by magnetic field dependent ns-TA spectroscopy and could be rationalized based on a classical kinetic scheme comprising only one magnetic field dependent rate constant k±. The behavior of k± shows a clear separation of the coherent and incoherent spin interconversion mechanisms. While the coherent spin evolution is due to the isotropic hyperfine coupling with the magnetic nuclei of the radical centers, the incoherent spin relaxation is due to a rotational modulation of the anisotropic hyperfine coupling tensor and is strongly dependent on the viscosity of the solvent. This dependence could be used to measure the nanoviscosity of the oligomeric solvent pTHF, which was found to be distinctly different from its macroviscosity. The second part of the work dealt with bisdipyrrinato complexes and their bridged porphodimethenato (PDM) analogues. Initially, the suitability of the different chromophores for the use as PS in donor-acceptor substituted triads was tested by a systematic investigation of their steady state and transient properties. While the PDM-complex of Zn(II) and Pd(II) exhibited promising characteristics such as a high exited state lifetime and relatively intense emission, the purely organic parent PDM and the non-bridged bisdipyrrinato-Pd(II) complex were less suitable. The difference between the two Pd(II) complexes could be explained by a structural rearrangement of the non-bridged complex which results in a non-emissive metal centered triplet state with disphenoidal geometry. This rearrangement is prevented by the dimethylmethylene-bridges in the bridged analogue resulting in higher phosphorescence quantum yields and excited state lifetimes. With the exception of the Zn(II)PDM-complex, the synthesis of novel donor acceptor substituted triads could be realized for all desired central chromophores. They were investigated equivalently to the cyclometalated triads described in the first part. The steady state properties indicate a stronger electronic coupling between the subunits due to the lack of unsaturated bridges between the donor and the central chromophore. Photoinduced CS occurs in all investigated triads. Due to the low exited state lifetimes of the central chromophores, CSS is formed less efficiently for the triads based on the unbridged Pd(II)-complex as well as the purely organic PDM. In the triad based on the bridged Pd(II) complex, the CR of CSS2 is faster than its formation resulting in low intermediate concentrations. For its elongated analogue, this is not the case and CSS2 can be observed clearly. Although the spin-chemistry of the triads based on bisdipyrrinato-Pd(II) and porphodimethenato-Pd(II) is less well understood, first interpretations of the magnetic field dependent decay kinetics gave results approximately equivalent to those obtained for the cyclometalated triads. Furthermore, the MFE was shown to be useful for the investigation of the quantum yield of CS and the identity of the observed CSSs. In both parts of this work, the influence of the central photosensitizer on the photophysics and the spin chemistry of the triads could be shown. While the process of CS is directly dependent on the PS, the PS usually is not directly involved in the final CSSs. None the less, it can still indirectly affect the CR and spin chemistry of the CSS since it influences the electronic coupling between donor and acceptor, as well as the geometry of the triads. / In der vorliegenden Arbeit wurden die Photophysik und Spinchemie von Donor-Photosensibilisator-Akzeptor-Triaden untersucht. In allen untersuchten Triaden wurden Triarylamin (TAA) bzw. Naphthalindiimid (NDI) als Elektronen-Donor bzw. -Akzeptor verwendet. Der zentrale Photosensibilisator (PS) hingegen wurde variiert, behielt allerdings das Dipyrrin-Chromophor als gemeinsames Strukturmotiv bei. Alle Zielverbindungen wurden mittels NMR-Spektroskopie, Massenspektrometrie und Elementaranalyse auf Identität und Reinheit geprüft. Der erste Abschnitt der Arbeit beschäftigte sich mit cyclometallierten Dipyrrinato-Komplexen schwererer Übergangsmetalle. Die erfolgreiche Synthese von neuartigen Triaden mit Ir(III)-, Pt(II)- und Pd(II)- Zentralionen wurde beschrieben. Die optischen und elektrochemischen Eigenschaften der Verbindungen deuteten auf das Auftreten von Ladungstrennung hin was mit Hilfe transienter Absorptionsspektroskopie (TA) bewiesen werden konnte. Weiterhin ergab die Untersuchung der Stoffe, dass die Ladungstrennung in den Triaden mit Pd(II) und Pt(II) als Zentralion wesentlich langsamer und weniger effizient abläuft als in den analogen Verbindungen mit Ir(III) als Zentralion. Der resultierende ladungsgetrennte Zustand (CSS2) weist eine hohe Lebenszeit auf, welche vom Zentralatom abhängt. Für Ir(III) beträgt diese ungefähr 0.5 µs, während sie für Pt(II) und Pd(II) auf ca. 1.5 µs ansteigt. Der Einfluss eines externen Magnetfeldes auf die Kinetik der Ladungsrekombination (CR) wurde mittels magnetfeldabhängiger ns-TA-Spektroskopie untersucht. Der auftretende Magnetfeldeffekt (MFE) konnte mit Hilfe eines klassischen Modells auf eine einzige magnetfeldabhängige Geschwindigkeitskonstante k± zurückgeführt werden. Der Verlauf von k± zeigt eine deutliche Trennung der kohärenten und inkohärenten Spin-Flip-Mechanismen. Während die kohärente Spin-Umwandlung auf die isotrope Hyperfeinkopplung mit den magnetischen Nuklei der Radikalzentren zurückgeht, wird die inkohärente Spin Relaxation durch die Rotationsmodulation des anisotropen Hyperfeinkopplung-Tensors hervorgerufen und ist daher stark abhängig von der Viskosität des Lösungsmittels. Dieser Umstand wurde dazu genutzt die Nanoviskosität des polymeren Lösungsmittels pTHF zu messen, welche stark von der angegebenen Makroviskosität abweicht. Im zweiten Teil dieser Arbeit wurden zunächst mehrere Bisdipyrrinato-Metall-Komplexe sowie ihre verbrückten Porphodimethenato (PDM)-Analoga hinsichtlich ihrer Eignung als PS in Donor-Akzeptor-substituierten Triaden getestet. Während die verbrückten Zn(II)- und Pd(II)-Komplexe vielversprechende Eigenschaften wie langlebige angeregte Zustände und relativ intensive Emission aufweisen, erscheinen der unverbrückte Pd(II)-Komplex sowie der nicht komplexierte PDM-Ligand weniger geeignet. Der große Unterschied zwischen den beiden augenscheinlich ähnlichen Pd(II)-Komplexen ist auf eine strukturelle Reorganisation der Geometrie des unverbrückten Komplexes zurück zu führen. Diese resultiert in einem Metall-zentrierten Triplet-Zustand mit bisphenoidaler Geometrie, welcher schnell und strahlungslos in den Grundzustand übergeht. Im verbrückten Komplex hingegen verhindern die Dimethylmethylen-Brücken zwischen den beiden Dipyrrin-Chromophoren die strukturelle Umordnung und erhöhen daher die Lebenszeit und Quantenausbeute der Phosphoreszenz. Mit Ausnahme des Zn(II)-PDM-Komplexes konnten für alle geeigneten PS die entsprechenden Donor-Akzeptor substituierten Triaden synthetisiert werden. Diese wurden mit den bereits für die cyclometallierten Triaden beschriebenen Methoden untersucht. Da keine ungesättigte Verbindung zwischen Donor und PS vorhanden ist, sind die elektronischen Wechselwirkungen zwischen den einzelnen Untereinheiten nicht vernachlässigbar. In allen untersuchten Triaden konnte photo-induzierte Ladungstrennung (CS) nachgewiesen werden. Diese ist weniger effizient für Photosensibilisatoren in welchen der angeregte Zustand eine kurze Lebenszeit aufweist. Während für die Triade mit dem verbrückten Pd(II)-PDM-Komplex als PS, die CR von CSS2 schneller ist als seine Bildung, ist die Lebensdauer von CSS2 im verlängerten Anlog deutlich höher und CSS2 kann klar nachgewiesen werden. Obwohl die Spin-Chemie der Bisdipyrrinato- und Porphodimethenato-Triaden weniger gut untersucht wurden, ergab die Analyse der durchgeführten Magnetfeld-abhängigen Messung der CR-Kinetik Ergebnisse, die gut mit den Befunden für die cyclometallierten Triaden übereinstimmen. In beiden Teilen dieser Arbeit konnte der Einfluss des Photosensibilisators auf die Photophysik und Spin-Chemie der Triaden gezeigt werden. Während die CS direkt vom PS abhängt, ist dieser normalerweise nicht direkt am finalen CSS beteiligt. Dennoch kann der PS die CR und Spin-Chemie des ladungsgetrennten Zustandes indirekt beeinflussen, da er die elektronische Wechselwirkung zwischen Donor und Akzeptor sowie die Struktur der Triaden bestimmt.

The world’s oceans absorb a significant percentage of anthropogenic carbon emissions, and CO2 levels have profound effects on the marine environment. Of primary concern is the acidification of the oceans due to dissolved CO2. The goal of this research is to design new sensing technologies for deployment in the marine environment to detect CO2 pollutant. A series of carbon dioxide (CO2) receptors that are complexed to fluorene oligomers were studied computationally. In chapter 1, an overview of CO2 chemistry and various CO2 sensors is discussed. A short overview of the method (Kohn-Sham density functional theory and time-dependent density functional theory (TDDFT)) employed in this work is given. Chapter 2 presents a study on the interaction energy and electronic excitations of fluorene-receptors as CO2 sensors. The aim of this work is to gain an understanding of the nature of interactions between these receptors and CO2. The structural, electronic, and optical properties of these receptor complexes have been determined computationally. The monomer-receptor complexes show remarkable redshifts in their absorption spectra, which decrease on moving to dimer and trimer-receptor complexes (all blue-shifted).

High-performance functional electrode materials are critical for the development of electrochemical energy conversion and storage technologies. Among various advanced materials, three-dimensional (3D) porous structures have attracted extensive interest due to their high surface area and capability for efficient mass transport. Metal-organic frameworks (MOFs) are a novel class of porous coordination polymers constructed with organic linkers connected by inorganic nodes. Their extraordinarily high surface area, permanent pores/channels, good thermal and chemical stabilities have made MOFs one of the most promising materials for various electrochemical applications, including electrocatalysis, supercapacitors, Lithium-ion batteries, chemical sensors, etc. The present dissertation focuses on the investigation of charge transfer mechanism in MOF films so as to establish design rules for future MOF design, and the exploration of MOF-based materials for electrochemical and photoelectrochemical applications. To promote the use of MOF-based materials in electrochemical applications, efficient charge transfer is a necessity. In redox-active MOFs, charge transfer can happen through redox hopping, i.e. site-to-site electron hopping coupled to diffusion of counter ions to balance electroneutrality. While the apparent diffusion coefficient (Dapp) has been employed to describe the overall charge transfer efficiency, independent elucidation of electron and ion diffusion is crucial for providing insights into the mechanism of charge transfer in MOFs. In Chapter 2, we investigated the MOF pore size effect on electron and ion diffusion. Three redox-active ferrocene-doped MOF (Fc-MOF) films with different pore sizes immobilized on conductive substrates were prepared, and electron and ion diffusion coefficients and rate constants were quantified by applying a theoretical model to chronoamperometric responses. Increasing MOF pore size led to an increase in ion diffusion rate constant and a decrease in electron diffusion rate constant. The overall charge transfer rate constant increased when MOF pore size increased, implying the ability of promoting efficient charge transfer through control of MOF pore size. As charge transfer via redox hopping proved to be feasible, Chapter 3 focused on the application of a ruthenium(II)-polypyridyl doped MOF film immobilized on a conductive substrate, UiO-67-Ru@FTO, for solid-state electrochemiluminescence (ECL). In the presence of tripropylamine as a coreactant, UiO-67-Ru@FTO exhibited higher ECL intensity and better reproducibility compared to corresponding solution-based ECL system. Subsequently, UiO-67-Ru@FTO was successfully used for dopamine detection, highlighting the great potential of using MOF-based materials as solid-state ECL detector for practical applications. Covalent-organic frameworks (COFs) are a recently emerging family of crystalline organic polymers constructed with organic building blocks linked by covalent bonds. In addition to advantages including high surface area and high porosity that are similar to MOFs, COFs possess low density due to the constitution of light-weighted elements and excellent stability owing to the robust covalent bonds. Therefore, it is of our interest to investigate the properties and potential applications of COFs. Two-dimensional (2D) COFs are composed of conjugated organic layers stacked via - interactions. Chapter 4 focused on understanding the effects of intraplanar -conjugation and interplanar -stacking on the photophysical properties of a 2D COF, TpBpy. Compared to the two building blocks, TpBpy exhibited a red-shifted emission, due to the - stacking. Density functional theory (DFT) calculations were performed on energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). It was found that the extended structure of the framework resulted in a decrease in the HOMO-LUMO gap. The experimental and computational studies reveal the important influence of intraplanar and interplanar interactions on photophysical properties in 2D COFs. In Chapter 5, we modified the COF TpBpy with nickel(II) and investigated its application as an electrocatalyst for 5-hydroxymethylfufural (HMF) oxidation. Unlike TpBpy characterized in Chapter 4, TpBpy thin films were prepared by an interfacial crystallization strategy. The films were transferred to conductive substrates and then post-synthetically modified by nickel acetate. Similar to redox-active MOFs, the resulting TpBpy-Ni COF film exhibited redox conductivity. TpBpy-Ni showed good catalytic activity for HMF oxidation under basic conditions. This study suggests the great potential of functionalized COFs for electrochemical applications. / Doctor of Philosophy / The increasing demand for clean and efficient energy has triggered a great deal of research interest in developing novel energy conversion and storage technologies. In particular, electrochemical (EC) systems including supercapacitors, Lithium-ion batteries, artificial photosynthetic system, fuel cells, etc. have drawn significant attention. The key component in high-performance EC energy conversion and storage devices is the functional electrode materials. Three-dimensional (3D) porous nanostructures have been widely applied as advanced electrode materials due to their high surface area that enables more liquid/solid interfacial interactions, and pores/channels that allows efficient mass diffusion and transport. Metal-organic frameworks (MOFs), made of organic ligands bridged by inorganic nodes, are a novel kind of porous materials with extraordinarily high surface area and permanent porosity. As a result, there is great potential in developing MOF-based electrode materials for EC applications. As the name itself suggests, EC systems rely on electrochemical reactions that involve transfer of charges (i.e. electrons and ions). Therefore, efficient charge transfer is vital for achieving high performance. While MOFs used for gas separation and storage have been reported, their electrochemical applications are still in early stages. The fundamental understanding of charge transfer in MOFs is in its infancy. As a result, there is an urgent demand for understanding the nature of charge transfer in MOFs. In this dissertation, we investigated the mechanism of charge transfer by independent quantification of electron and ion transfer rate constants. With a better understanding in hand, we also explored two electrochemical applications in MOFs, electrocatalysis and electrogenerated chemiluminescence.

An electropolymerizable zinc porphyrin carrying eight entities of peripheral bithiophene, 4 was newly designed and synthesized. In this design, the bithiophene entities were separated by a biphenyl spacer to minimize ground state interactions perturbing porphyrin π-electronic structure. By multi-cyclic voltammetry, thin-films of 4 were formed on transparent FTO electrode and were characterized by optical, electrochemical and STM measurements. Further, the ability of zinc porphyrin in 4 to axially coordinate phenyl imidazole functionalized fullerene, C60Im both in solution and on the film interface was performed and characterized. Fluorescence quenching of zinc porphyrin both in solution and in the film was observed upon binding of C60Im. Femtosecond transient absorption studies revealed excited state charge separation for the dyad in solution wherein the measured rate of charge separation, kCS and charge recombination, kCR were found to be 2 x 1010 s−1 and 1.2 x 109 s−1, respectively. In contrast, transient absorption studies performed on the dyad in the film were suggestive of energy transfer with minimal contributions from electron transfer. The present study brings out the importance of modulating photochemical reactivity of donor-acceptor dyad in film as compared to that in solution. The electro- and photocatalytic reduction of molecular nitrogen to ammonia (nitrogen reduction reaction, NRR) is of broad interest as an environmentally- and energy-friendly alternative to the Haber–Bosch process for agricultural and emerging energy applications. Herein, we review our recent findings from collaborative electrochemistry/surface science/theoretical studies regarding transition metal oxides, oxynitrides and sulfides as NRR catalysts. We found that, for all metal oxides and oxynitrides specifically, there is no Mars–van Krevelen mechanism and that the reduction of lattice nitrogen and N2 to NH3 occurs by parallel reaction mechanisms at O-ligated metal sites without incorporation of N into the oxide lattice. Additionally, the results highlight the importance of both O-ligation and the importance of N in stabilizing the transition metal cation in an intermediate oxidation state, for effective N≡N bond activation. For transition metal sulfides, various exfoliation treatments are known to yield Sulfur vacancies and DFT calculations corroborate N2 binding to S-vacancies, with substantial π-backbonding to activate dinitrogen. Most of our NRR catalysts were selective to ammonia production without appreciable competing production of H2.

Das Hauptaugenmerk der vorliegenden Arbeit richtet sich auf die Untersuchung der photophysikalischen Eigenschaften von Derivaten von N,N-Dimethylaminobenzonitril (DMABN) und N-Phenyl-pyrrolobenzonitril (PBN) als Donor-Akzeptor Verbindungen. Die untersuchten Verbindungen zeichnen sich durch Einführung von Fluor-Substituenten durch eine erhöhte Akzeptorstärke aus, wodurch neue Erkenntnisse bzgl. der intramolekularen Charge-Transfer-Zustände (ICT) gewonnen werden konnten. Hierbei wurden die Ergebnisse zum Verhalten der untersuchten Moleküle im angeregten Zustand mit denen der entsprechenden Basisverbindungen verglichen. Die spektroskopischen und photophysikalischen Eigenschaften wurden sowohl durch die Anwendung der stationären und zeitaufgelösten Fluoreszenzspektroskopie bei Raum- und Tieftemperatur als auch durch Nutzung der transienten Absorptionsspektroskopie in Kombination mit quantenchemischen Berechnungen untersucht. Im Unterschied zu den Basisverbindungen DMABN und PBN zeigen die Spektren der fluorierten Derivate nur eine einzige stark rotverschobene Fluoreszenzbande, die dem ICT-Zustand zugeordnet werden kann. Die extrem kleinen Quantenausbeuten, die typisch für alle fluorierten Derivate sind, können auf die Existenz eines weiteren strahlungslosen Deaktivierungskanals zurückgeführt werden. Der beobachtete ICT kann mit dem TICT-Modell (Twisted intramolecular Charge Transfer), bei dem von einer gegenseitigen Verdrillung der Donor- und Akzeptoreinheiten ausgegangen wird, erklärt werden. Weiterhin wurden die Variation der Verknüpfungsposition zwischen Donor- und Akzeptoreinheit sowie der Einfluss zusätzlicher Akzeptor-Substituenten auf die Eigenschaften der ICT-Zustände untersucht. Durch die Ergebnisse dieser Arbeit konnte ein vertieftes Verständnis über die Ladungstrennungsprozesse in Donor-Akzeptor-Systemen, die sich durch eine starke Solvatochromie und die Existenz von strahlungslosen Deaktivierungskanälen auszeichnen, entwickelt werden. Es konnte die Möglichkeit der Besetzung von zwei verschiedenen ICT-Zuständen (TICT – verboten, mesomerer ICT – erlaubt) gezeigt werden. / The focus of this work is mainly concerned with the investigation of photophysical properties of electron donor-acceptor compounds, namely, derivatives of N,N-dimethylamino benzonitrile (DMABN) and N-phenyl-pyrrolobenzonitrile (PBN). New insights into the intramolecular charge transfer (ICT) states were obtained while dealing with an acceptor moiety of increased strength in the form of fluorinated analogues of both these compounds. The molecules studied in this work have been compared with their corresponding parent compounds to get more useful information on the excited state behaviour. The spectroscopic and photophysical properties were studied using steady-state and time-resolved fluorescence at room and low temperature as well as with transient absorption spectroscopy in conjunction with quantum chemical calculations. Unlike in the parent compounds DMABN and PBN, their fluorinated derivatives show only a single strongly red-shifted fluorescence emitted from the ICT state, and possess low quantum yields. The nearly non-fluorescent behaviour for all of these fluorinated derivatives investigated is due to the presence of a photochemical mechanism additional to that of ICT, which acts as a new non-radiative funnel. The ICT observed in these compounds can be explained by twisting motion taking place between the donor and acceptor moieties. Thus, twisted intramolecular charge transfer (TICT) model supports the observations. Apart from the changes in the strength of the acceptor moieties, the ICT nature has been further explored by changing their linking positions as well as with additional acceptor substituents. From the findings obtained in this work, a deeper understanding of the charge separation processes occurring in donor-acceptor systems with high solvatochromism and non-radiative decay properties was obtained. The possibility for populating two different ICT states (of forbidden nature – TICT, and allowed nature – mesomeric ICT) has been exemplified.

One electron oxidation of DNA has been studied extensively over the years. When a charge is injected into a DNA duplex, it migrates through the DNA until it reaches a trap. Upon further reactions, damage occurs in this area and strand cleavage can occur. Many works have been performed to see what can affect this damage to DNA. Netropsin is a minor groove binder that can bind to tracts of four to five A:T base pairs. It has been used in the studies within to determine if it can protect DNA against oxidative damage, caused by one-electron oxidation, when it is bound within the minor groove of the DNA. By using a naphthacenedione derivative as a photosensitizer, several DNA duplexes containing netropsin binding sites as well as those without binding sites, were irradiated at 420 nm, analyzed, and visualized to determine its effect on oxidative damage. It has been determined netropsin creates a quenching sphere of an average of 5.8 * 108 Šwhether bound to the DNA or not. Herein we will show netropsin protects DNA against oxidative damage whether it is free in solutions or bound within the minor groove of a DNA duplex.

A series of anthraquinone-linked DNA oligonucleotides was prepared and the efficiency of long-distance radical cation migration was measured. In one set of oligonucleotides, two GG steps are separated by either a TATA or an ATAT bridge. In these two compounds, the efficiency of radical cation migration from GG to GG differs by more than an order of magnitude. Replacement of the thymines in the TATA or ATAT bridges with 3-methyl-2-pyridone (t, a thymine analog) results in the much more efficient radical cation migration across the bridge in both cases. This is attributed to a decrease in the oxidation potential of t to a value below that of A. In contrast, replacement of the thymines in the TATA or ATAT bridges with difluorotoluene (f, a thymine analog with high oxidation potential) does not measurably affect radical cation migration. These findings are readily accommodated by the phonon-assisted polaron-hopping mechanism for long-distance charge transfer in duplex DNA and indicate that DNA in solution behaves as a polaronic semiconductor. Oligomers containing thiophene-pyrrole-thiphene (SNS) monomers were covalently linked to the nucleobases of DNA. Treatment of these oligomers with horseradish peroxidase and hydrogen peroxide lead to the formation of conducting oligomers conjoined to the DNA. The DNA template aligns the oligomers along one strand of the duplex and limits the intermolecular reaction of monomers. This method enables utilization of the unique self-recognizing properties and programmability of DNA to create tailored oligomers.

This thesis is concerned with the material processing, photophysical and third-order nonlinear optical responses, and applications of a set of conjugated polymers in the telecommunication regions. Polyacetylene-based third-order nonlinear optical materials were chosen as candidates for all-optical signal and image processing. Substituted polyacetylenes were obtained using ring-opening metathesis polymerization of mono-substituted cyclooctatetraenes. Polymerization and processing conditions have been developed to generate thick, large-area films possessing large third-order nonlinearities in the telecommunication bands. The good optical quality of a 200 μm thick substituted polyacetylene film allowed for image correlation via off-resonant degenerated four-wave mixing with improved diffraction efficiency. Poly(2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)) (MEH-PPV) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) composites showed strong nonlinear absorption and potential as optical limiters in the region of 700-900 nm. High optical quality, thick film of MEH-PPV:PCBM with the plasticizer dioctylphthalate (DOP) were made. Optical limiting of femtosecond and nanosecond pulses in the near infrared on these composites showed strong power suppression over a broad temporal regime. Femtosecond and nanosecond transient studies on the same thick MEH-PPV:PCBM:DOP composite films and the experimental results showed evidence for the photogeneration of radical ions as being responsible for the enhanced nonlinear absorption and strong optical suppression in the near infrared. Dithienopyrrole-based donor-acceptor copolymers with narrow bandgap showed strong nonlinear absorption and potential as optical limiters in the telecommunication wavelengths. Molecular engineering was applied to manipulate the spectral overlap of two-photon absorption and subsequent nonlinear absorptions. Femtosecond transient spectroscopy showed near infrared transient absorption and 22 - 61% yields of photogenerated charge-transfer species depending on donor-acceptor coupling strength. Torsional fluctuations of the backbone structure potentially affected the excited state behavior. Evidence suggests that ultrafast relaxation occurs to ground state and to long-lived charge-transfer state from the initially excited state. The dispersion of nonlinear absorption measured using the Z-scan method revealed large two-photon absorption cross sections of these polymers in the telecommunication region. Large suppression of nanosecond pulses at 1064 nm was achieved.

Under the electric dipole approximation, second harmonic of the incident light is scattered by a collection of randomly oriented molecular dipoles in solution due to instantaneous orientational fluctuation which is directional. If two such dipoles are correlated in space through intermolecular or other interactions, the intensity of the second harmonic scattered light (SHSL) will be related to the extent of such interactions. If two dipoles are arranged in a particular geometry by design, the geometry will determine the intensity of the SHSL. If a molecule has no dipole moment, the intensity of the SHSL will be less and is only allowed by higher order electric multipoles. If two such zero-dipole molecules interact with each other and transfer some amount of electronic charge from one to the other, the induced dipole moment will give rise to an enhanced SHSL. However, along with the direction of the dipole moment from the donor to the acceptor, the actual geometry of such molecular dimer/complex should also play an important role to determine the nature of the SHSL response. If all the isotropic nonzero components of first hyperpolarizability (β) are taken into account, from the measurement of β and related quantities such as depolarization ratios, in solution it should be possible to derive information about the geometry of the dimer/complex. This is precisely the motivation behind this thesis. Chapter 1 gives a brief introduction of 1:1 charge transfer (CT) complexes between a donor and an acceptor and their importance in chemistry. It also contains an introduction to nonlinear optics, various spectroscopic techniques to characterize CT complexes, etc. The motivation of extracting the geometry of such complexes from hyper-Rayleigh scattering (HRS) measurements in solution is presented in this chapter. In Chapter 2, all the experimental details of the unpolarized and polarization resolved HRS measurements at various excitation wavelengths have been described. Generation of infrared wavelengths (1543 nm and 1907 nm) using stimulated Raman scattering in gases have also been discussed. In Chapter 3, the first hyperpolarizability (βHRS) for two series of 1:1 molecular complexes between methyl substituted benzene donors with tetrachloro-p-benzoquinone (CHL) and dicyanodichloro-p-benzoquinone (DDQ) acceptors in solution at 1543 nm have been presented. Enhancement of βHRS due to charge transfer from the donor to the acceptor molecule which was predicted theoretically has been verified. Using linearly (electric field vector along X direction) and circularly polarized incident light, respectively, two macroscopic depolarization ratios D = I2ω,X,X/I2ω,Z,X and D' = I2ω,X,C/I2ω,Z,C in the laboratory fixed XYZ frame by detecting the SHSL in a polarization resolved fashion have been measured. The experimentally obtained first hyperpolarizability (βHRS), D and D' values, are then matched with the theoretically calculated values from single and double configuration interaction calculations using the Zerner’s intermediate neglect of differential overlap and the self-consistent reaction field (ZINDO–SDCI– SCRF) approach by adjusting the geometrical parameters. It has been found that in most of the CT complexes studied here, there exists a significant twist in the equilibrium geometry at room temperature which is not a simple slipped parallel geometry as was believed. In chapter 4, the βHRS, D and D' values of 1:1 pyridine (PY)-chloranil (CHL) complex at 1064 nm have been described. Previous theoretical studies have shown that there is a tilt angle of 77.9 degree in the gas phase PY-CHL complex. In this chapter, this prediction about the geometry of 1:1 PY-CHL complex has been probed. The experimentally found βHRS, D and D' are matched well with theoretically calculated values, using ZINDO–SDCI–SCRF, for a cofacial geometry of PY-CHL complex in solution indicating that the solution geometry is different from the gas phase geometry. In Chapter 5, the βHRS, D and D' for a series of 1:1 complexes of tropyliumtetrafluoroborate and methyl-substituted benzenes in solution at 1064 nm have been reported. The measured D and D' values vary from 1.36 to 1.46 and 1.62 to 1.72, respectively and are much lower than the values expected from a typical sandwich or a T-shaped geometry. The lowering in D and D' indicates that these complexes have higher symmetry than C2v. The value of D close to 1.5 indicates there is a significant octupolar contribution in such complexes. In order to probe it further, βHRS, D and D' were computed using the ZINDO-SDCI-SCRF technique in the presence of BF4-anion. By arranging the three BF4-ions in a C3 symmetry around the complex in such a way that electrical neutrality is maintained, the computed values are brought to agreement with experiments. This unprecedented influence of the anion on the HRS, D and D' values of these complexes are discussed in this chapter. In Chapter 6, the effect of dipolar interactions, within a multichromophoric system, on the second order nonlinear optical properties have been studied. It has been found that the βHRS response of the multichromophoric system is always larger than expected for uncorrelated chromophores demonstrating that the dipole moment of individual chromophores are not merely additive within the multichromophoric system but contribute cooperatively to the SHSL signal. Also the relative orientation and nature of the chromophores and the angle of interaction between them alter the HRS values. Chapter 7 is the concluding chapter in which all the work done in the thesis has been summarized and future direction has been proposed.