Doctor of Philosophy (Ph.D.), Bowling Green State University, 2023, Photochemical Sciences

The very initial photoprocesses of relevant chromophores and organic molecular probes can provide important mechanistic insight into designing more robust and useful compounds for targeting in vivo applications, drug delivery, as well as an overall understanding of significant biological functions. Therefore, examining and comprehending these ultrafast processes is critical. In this dissertation, the elucidation of excited state dynamics of several molecular probes and organic systems is obtained from the results of multiple femtosecond transient absorption experiments. Chapters I and II detail the theoretical and experimental aspects, respectively, of this dissertation as fundamental and practical methods are addressed. The first chapter will cover laser spectroscopy and associated theories surrounding the technique relevant to the work discussed herein in general, while the second chapter will discuss specifics of experimental design and practices used for data analysis. The third chapter focuses on a photochromic system, trans-4,4'-azopyridine, capable of undergoing trans-cis isomerization upon irradiation and how similar and different this compound's dynamics are compared to trans-azobenzene and other azo dyes in general. An unusual trend in the quantum yield increasing upon exciting with higher excitation photon energies is linked to vibrational coherence observed for an in-plane bending mode. Chapter IV delves into a project on two polymethine cyanine dyes, which are utilized for deep tissue imaging due to their absorption and emission in the shortwave infrared region. The excited state dynamics in the fluorescent state and non-radiative relaxation mechanisms in this state, discovered to be competing photoisomerization and the energy gap law relaxation pathways, are analyzed and discussed. Finally, Chapter V describes work on a series of enaminones where the question of if and how excited state intramolecular proton transfer plays a role in the excited state m (open full item for complete abstract)

Committee: Alexander Tarnovsky Ph.D. (Committee Chair); Yuning Fu Ph.D. (Other); John Cable Ph.D. (Committee Member); Peter Lu Ph.D. (Committee Member) Subjects: Chemistry; Physical Chemistry

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2023, Photochemical Sciences

To correlate and be able to tune the photophysical properties of molecular scale chromophores with the mechanical properties of the macroscale supramolecular polymer such as viscosity, modulus etc. is a complex phenomenon. To tune these properties, we utilize dynamical coordination bonds such as Metal-Ligand interactions. In this work, we present how we can use the photophysical properties specifically probing excited state arising from these tunable dynamical interactions to probe the mechanical properties of the macroenvironment real time in-situ. Utilizing this approach, we were able to detect the viscosity of supramolecular polymeric assembly by probing emission lifetime from triplet electronic state of [Cu(diptmp)2]2+. However, this correlation is not quantitatively ubiquitous in every polymeric host. We probed the reversibility of the polymeric assembly by swelling and deswelling the polymer assembly which is attributed to micro-viscosity. The photophysical changes were of significant magnitude to be able to detect even small reversible changes during swelling and deswelling the polymer. Furthermore, worked on synthesizing a bio-based polymer and made a composite material with multiple ligand moieties. We present how these moieties interact with the polymeric environment and quantify them through their affinity to different metals. The photophysical changes in emission of a bio-based Cu-curcumin polymeric composite material upon applying stress to the composite material has been discussed. We also present to show whether we can induce the photophysical changes in NIR emission of a water soluble [Cr(ddpd)3]3+ complex termed as molecular ruby exhibiting upon subjecting it to thin supramolecular films with increasing modulus and looked at its stress-response.

Committee: Alexis D. Ostrowski Ph.D. (Committee Chair); Francisco Cabanillas Ph.D. (Other); Joseph C. Furgal Ph.D. (Committee Member); Alexander N. Tarnovsky Ph.D. (Committee Member) Subjects: Chemistry; Inorganic Chemistry; Physical Chemistry

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2023, Photochemical Sciences

Uncovering untrodden photochemical pathways often depends on manipulating reactive chromophores and channeling their excited state energy towards a desired chemical pathway. To alter an established photochemical reactivity, a clear comprehension of the dynamics of the excited states involved is quintessential. This thesis will present the development of a new class of photochemical reaction which was established by circumventing an established excited state process. We have designed chromophores in which a new type of photochemical reactivity was observed in excited 1,3-dicarbonyl compounds with activated alkene circumventing the traditional De Mayo pathway. In Chapter 2 we will discuss in detail regarding this newly observed reaction, which allows the conversion of completely planar reactants to a complex heterocyclic structure with multiple stereocenters in a single step. Photophysical investigations have enabled us to elucidate the mechanism of this new excited state process. To enhance the applicability of this newly discovered photoreaction, we have also evaluated the structural features that are crucial for controlling the excited state processes in Chapter 3. Our investigations have uncovered the role of cyclic dicarbonyl compounds in dictating the regiochemical outcome of this new photoreaction leading to the formation of complex heterocycles. By manipulating the reaction temperature in various solvents, the regioselectivity in different cyclic carbonyl compounds were evaluated allowing us to build complex heterocycles in a single step by considering the principle of green chemistry and atom economy. Based on detailed photophysical and spectroscopic studies, the mechanism for the observed difference in the regiochemistry of the photoproduct based on the ring size of the dicarbonyl compound employed for the photo transformation was uncovered. The final aspect of the thesis, Chapter 4 details a design to circumvent the known [2+2]-photodimerization in ma (open full item for complete abstract)

Committee: Jayaraman Sivaguru Ph.D. (Committee Chair); Lori Brusman Lovins Ph.D. (Committee Member); Pavel Anzenbacher Jr Ph.D. (Committee Member); Alexander Tarnovsky Ph.D. (Committee Member) Subjects: Chemistry; Organic Chemistry; Physical Chemistry

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2022, Photochemical Sciences

Halide complexes of heavy metals attract great attention due to their applications in photonics and light harvesting (collection of solar energy through photochemical processes with subsequent conversion into chemical energy). Ultrafast spectroscopic studies deliver the molecular level understanding of very initial physicochemical processes, which often define the useful functions these complexes provide, and can be used for rationally designing the materials in which these complexes are the building blocks. We aim to investigate ultrafast excited-state reaction dynamics and photophysical/photochemical relaxation mechanisms of hexahalide complexes of heavy metals. In the first part of this work, the excited-state relaxation pathways in a Bi(III) hexahalide, BiX63–, (X = I, Br) a paradigm complex for understanding the ns2 main group metals, are studied by means of femtosecond transient absorption spectroscopy supported by nanosecond transient absorption spectroscopy, steady-state absorption/emission measurements and DFT computations. Radiationless relaxation out of the initially excited, predominantly metal-centered (MC) triply degenerate 3T1u state, populates two lower-energy states: a ligand-to-metal-charge transfer (LMCT) excited state of the 3π LMCT I(npπ)Bi(6p) nature and a luminescent ‘trap' 3A1u(3P0) MC state. On a nanosecond timescale, the 3π LMCT population decays through other intermediates to generate the triplet species (acetonitrile solvent), which is identified as the η2 metal ligated dihalide-bismuth adduct with the intramolecularly formed X–X bond, [(η2-X2)Bi(II)X4]3−, the product of interest for applications in solar energy conversion and storage. In the second part of this work, the ultrafast dynamics of IrCl62-, a prototype low-spin d5 metal complex, excited into intraconfigurational short-wavelength-infrared MC and visible LMCT electronic states is studied by femtosecond transient absorption spectroscopy. Upon any excitation of the complex in wat (open full item for complete abstract)

Committee: Alexander Tarnovsky Ph.D (Committee Chair); Kei Nomaguchi Ph.D (Other); Malcolm Forbes Ph.D (Committee Member); H Peter Lu Ph.D (Committee Member) Subjects: Chemistry; Physical Chemistry

Doctor of Philosophy, Case Western Reserve University, 2022, Chemistry

Three research projects are described herein: the synthetic investigations into digold(II) complexes, the design and synthesis in the pursuit of gold(III) bidentate complexes and the excited state dynamics of iridium(III) azadipyrromethene complexes. The relationship between the structures of the complexes and their photophysical properties was examined, with an emphasis on the potential nonlinear optical mechanisms of two photon absorption and reverse saturable absorption. Following an introduction to nonlinear optics in chapter 1, the attempts to synthesize digold(II) complexes and the interest in studying the photophysical properties arising from the elusive +2-oxidation state are discussed in chapter 2. After multiple reaction attempts, it was concluded that the digold(II) precursor was not robust enough to withstand the coupling reaction conditions. Future work would have to utilize a stronger sigma donating ligand such as a ylide ligand, to add stability to the digold(II) complex. The ongoing study involving chromophoric gold(III) bidentate complexes, with varying heterocyclic and fluorenyl ligands, is discussed in chapter 3. It was hypothesized that the presence of the bidentate fluorenyl ligands may increase excited state absorption and the rate of intersystem crossing when compared to monodentate gold(III) complexes. The use of a microwave to synthesize the bidentate gold(III) precursor is required for the continuation of the project. Lastly, structurally characterized iridium(III) azadipyrromethene complexes with varying cyclometalating ligands were analyzed via femtosecond transient absorption spectroscopy and are examined in chapter 4. The transient absorption spectra were similar across the series regardless of variation in cyclometalating ligands. The ground and excited state photophysics were dominated by the azadipyrromethene moiety with excited state absorption being observed in the visible and near infrared regions. Nonradiative decay was the domi (open full item for complete abstract)

Committee: Thomas Gray (Advisor) Subjects: Chemistry

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2021, Photochemical Sciences

Analytical chemistry together with photophysical studies is a powerful tool for the detection and quantification of crucial chemical species involved in the environment and biological processes. Among analytical techniques, luminescent chemosensors are widely used in the development of new solutions to long-standing problems. Here, the main focus of this research is the recognition and quantification of important biological and organic/inorganic anions utilizing luminescent molecular sensors. The first part of this work is focused on the recognition of ATP due to its pivotal role in many biological processes. Bisantrene, a molecule comprising imidazolium hydrazone receptor moieties and anthracene as a central fluorophore, was employed as a new sensor for ATP in water at neutral pH by displaying amplified fluorescence. This process was selective to ATP while other nucleotide phosphates such as AMP, GTP, UTP, and CTP did not elicit sensor response. Also, this sensor was used to sense other small anions such as F-, Cl-, AcO-, H2PO4- and HP2O73- in an organic solvent, where the sensor shows fluorescence quenching. The second project is focused on the recognition and sensing of various phosphates and carboxylates in water using a metal-sensor coordination system. The sensors are based on fluorescent carboxamidequinoline chemosensors derivatized by phenol and benzothiazole. These sensors form non-fluorescent complexes with Eu3+ where the fluorescence is recovered when the phosphate anion sequesters the Eu3+ from the complex. The present sensors show selectivity for biological phosphates such as ATP, ADP, and AMP. Arsenate (HAsO42-) is a highly toxic analyte to living organisms, and efforts toward sensing arsenate are an important area of research. The third project is aimed at the detection of arsenate (HAsO42-) in water by luminescence spectroscopy using a lanthanide/transition metal dyad. We have synthesized a cryptand molecule which binds Eu3+and displays a li (open full item for complete abstract)

Committee: Pavel Anzenbacher, Jr. (Advisor); Ganming Liu (Other); Malcolm D. E. Forbes (Committee Member); H. Peter Lu (Committee Member) Subjects: Chemistry

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2021, Photochemical Sciences

We present studies that showcases metal-organic frameworks as suitable platforms that can mediate molecular interactions by its tunable structural architecture, to enhance the functionality of its incorporated molecules towards potential applications. Other projects include studies on inorganic complexes and other materials using electron paramagnetic resonance (EPR) as a chief tool to investigate these dynamic systems. Our first project aimed at designing novel types of fluorescent multivariate MOF to produce white light emitting materials. We highlighted the ability to carry over solution state luminescent properties of select push-pull fluorophores based on 2,5-disubstituted diacrylic benzene, 2,5-disubstituted distyrylbenzene, and 2,5-disubstituted diethylnylbenzene into a MOF structure. The SBU concept was utilized to achieve suitable topologies that can mitigate florescence quenching. Some of these MOFs were also used to isolate singular rotamers of two rotermer often mixed in solution state photophysics, and this could be a means to study such isolated rotamers. We were also able to design and construct a novel pillared MOF containing a 2D-array of well-ordered organic qubit candidate made from a diethynylpyridine isolidine nitroxides backbone as pillar linkers. Inter–qubit distances were tuned by mixing the pillar linkers with magnetic neutral linkers without nitroxides groups. Due to the anisotropic nature of these materials, steady-state electron paramagnetic resonance (SSEPR) technique was very useful to characterize properties of these materials. EPR variable temperature studies unveiled spin-spin interaction and its changes as the inter-qubit distances were tuned. Extent of spin-lattice and spin-spin interactions are important information needed when considering qubits for potential application in advancing quantum information science. Spin probing and spin trapping techniques were used to investigate different types of system. We were able to use (open full item for complete abstract)

Committee: Forbes Malcolm (Advisor); Alexis Ostrowski (Committee Member); Anzenbacher Pavel Jr. (Committee Member); Ju Ilyoung (Other) Subjects: Chemistry; Inorganic Chemistry; Materials Science; Organic Chemistry

Doctor of Philosophy, The Ohio State University, 2019, Chemistry

With over one million new cases predicted for the year 2020 and an average five-year survival rate of only 66%, cancer remains one of the most challenging medical issues of our lifetime. Currently anticancer drugs such as cisplatin, while effective, offer limited selectivity for cancerous cells over healthy cells and cause severe systemic side effects. Photoactivated drugs offer significant advantages over traditional therapies, as they allow for precise spatial and temporal control over the delivery of a biologically active species from a prodrug using light. Developing such photoactivated drugs, however, requires a detailed understanding of their photophysical and photochemical properties after the absorption of a photon. Polypyridyl Ru(II) complexes are particularly suited for use as photoactivated drugs, as they exhibit intense absorptions in the visible spectrum and reactivity from several different excited states. This work examines the photochemistry of a series of Ru(II) complexes capable of killing cells through both the generation of reactive oxygen species as well as photoinduced drug dissociation. In addition, the photophysical and photochemical properties of nitrile-containing Ru(II) complexes are also studied in detail. The results presented herein reveal efficient photoinduced dissociation of nitrile ligands from mixed triplet metal-to-ligand charge transfer (3MLCT/ligand) excited states, instead of from the triplet ligand field (3LF) states traditionally thought to be responsible for ligand dissociation in Ru(II) complexes. Moreover, the complexes in this work that exhibit the greatest quantum yields of ligand dissociation also contain the most red-shifted absorption spectra (important for therapeutic use), demonstrating that efficient ligand dissociation can occur using red light with judicious choice of ancillary ligands, instead of requiring the introduction of a large degree of steric distortion.

Committee: Claudia Turro (Advisor); Hannah Shafaat (Committee Member); Terry Gustafson (Committee Member) Subjects: Chemistry; Inorganic Chemistry; Physical Chemistry

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2019, Photochemical Sciences

This work aims at deeper understanding of chemical reaction dynamics, ultrafast photochemical and photophysical relaxation pathways, and energy redistribution and dissipation in the liquid phase on a femto-to-picosecond timescale. Breaking and making of chemical bonds, structural reorganization, and energy flow are all the essence of chemistry, and all occur on the time scale of molecular vibrations (tens of femtoseconds). Important insights about the structure of evolving molecules, relaxation phenomena, and ultrafast photochemical and photophysical population dynamics can be obtained using sub-50-fs time-resolved transient absorption spectroscopy. As a result, highly viable and/or unique information about the mechanism of relaxation dynamics and the nature of short-lived intermediates can be gained. Furthermore, ultrashort laser pulses with duration of 100 fs or shorter can help set up and detect coherent motion in vibronic excited and ground states. Coherence observed in product states provides the knowledge about electronic and nuclear motion along the relaxation path. This yields the information about the potential energy surfaces involved in the photochemical or photophysical process that occurs. The knowledge acquired from such experiments can be used as an entry point for control of femtochemical processes in solution. The technical aspect of this work focuses on developing and optimizing the ultrafast transient absorption setup for studying ultra-rapid photochemical relaxation dynamics in the molecules of interest, namely, geminal dibromocycloalkanes and halogen-containing transition metal complexes. Excitation pulses from deep-UV to near-IR were temporally compressed using prisms compressors to generate sub-40 fs laser pulses. Also, stable white-light continuum pulses used for probing were generated in the wide spectral range of 280 – 760 nm. This thesis is divided into two main parts. The first part provides information about ultrafast dynamics (10 (open full item for complete abstract)

Committee: Alexander Tarnovsky Dr. (Advisor); John Cable Dr. (Committee Member); R. Marshall Wilson Dr. (Committee Member); Kristen Rudisill Dr. (Other) Subjects: Chemistry; Physical Chemistry

Doctor of Philosophy, The Ohio State University, 2018, Chemistry

Photon energy fuels life on earth; efforts to better use sunlight for the creation of energy through solar photovoltaics and solar fuels have been of great interest across scientific disciplines. A key factor for using the energy stored in photons is the ability to characterize, to change, and ultimately, to tune the excited states of molecules to absorb wavelengths of light to better harness the solar spectrum while also designing molecules with have excited state energies that are favorable for electron transfer and photocatalytic reactions. This work demonstrates, using Ru(II) and Rh2(II,II) transition metal complexes, the rational synthetic modification of inorganic complexes to better harness low energy wavelengths of the solar spectrum and the tuning of those states to do useful chemical transformations. This is first demonstrated using excited state using Ru(II) complexes, which were explored by small modifications to the ligand geometry in fused donor-acceptor systems using quinone-based electron accepting groups. In this molecule, a side-on geometry was determined to more fully delocalize the electron density on the quinone-containing ligand. This delocalization was determined to both red-shift the absorption to the red (λmax = 546 nm) and increase the excited state lifetime from 0.35 ns to 19 ns. This excited state is capable of performing excited state electron transfer reactions to oxidize phenothiazine, lending the complex to p-type semiconductor applications. These findings outline how small changes to the ligand coordination environment impact the light absorption profile and the excited state dynamics. The effect of synthetic modifications on the excited state properties of transition metal complexes was further explored using dirhodium(II,II) formamidinate complexes. Ultrafast UV-Vis and infrared time-resolved spectroscopy demonstrated the relatively long singlet lifetimes (τS = 7 ps) and 25 ns triplet lifetimes of these complexes, which were then (open full item for complete abstract)

Committee: Claudia Turro (Advisor); Yiying Wu (Committee Member); Anne Co (Committee Member) Subjects: Chemistry; Inorganic Chemistry; Physical Chemistry

Master of Science (MS), Bowling Green State University, 2018, Physics

Highly luminescent few-atoms thick colloidal PbS nanosheets are a promising material for the applications in infrared optoelectronics and photonics since a large in-plane charge carrier mobility and a tunable energy gap are unified in a single material. The time-resolved photoluminescence of the excitons in the nanosheets shows two distinguished fast and slow decays, which is very different from the quantum dots. The slow decay might be due to the exciton recombination after exciton migration or dissociation. The time-resolved photoluminescence spectroscopy studies show that there is a significant redshift of the emission spectrum later after excitation. This time-dependent photoluminescence spectrum indicates the possible migrations of excitons from thin parts (large energy gaps) of the nanosheets to thick parts (small energy gaps) due to the motion of the charge carriers, or possible Forster resonance energy transfer (FRET) from a thin nanosheet to a thicker one. The shift of the emission spectrum occurs within 500 to 700 nanoseconds, indicating the average time of the exciton-migration or FRET is at the same scale. As a side project of this thesis, the electric dipole moment of a typical PbSe quantum dot in the reaction solution is derived. This helped in the calculation of the dipole-dipole interaction energy of the quantum dots to explain the growth mechanism of PbSe nanorods. Another side project of this thesis was focused on the calculation of the energy gap of the carbon dots consisting amide groups. The calculated energy gap is close to one of the energy gaps measured by optical absorption. It helped to understand the light-emitting mechanism of the carbon dots.

Committee: Liangfeng Sun (Advisor); Alexey Zayak (Committee Member); Haowen Xi (Committee Member) Subjects: Physics

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2016, Photochemical Sciences

The present work is the detailed study of photophysical and photochemical processes in molecules by means of femtosecond (fs) time-resolved transient absorption spectroscopy both in the gas and solution phases. This knowledge is important for an understanding of many light-driven mechanisms in photosensitive systems – building blocks of light-triggered molecular devices. Many light-driven reactions occur on a time scale comparable to that of vibration motion, and therefore, can only be “seen” by using advanced methods such as the fs transient absorption. This method provides ultrahigh temporal resolution on a vibrational period timescale, which is superior to other time-resolved methods and, importantly, can be utilized to study the dynamics of molecular systems. Specifically, ultrafast transient absorption spectroscopy was used to investigate radiationless relaxation dynamics of two paradigm transition metal complexes in octahedral (IrBr62 ) and tetrahedral (CuCl42-) environment. Following excitation at 2000-nm, both systems undergo internal conversion to the ground electronic state with significant difference in lifetimes (55-fs and 360 -ps for CuCl42- and IrBr62-, respectively). The difference was explained by the presence of conical intersection between the excited and ground electronic states in the Cu2+ system due to strong Jahn Teller effect and the strong spin-orbit coupling in the Ir4+ complex. Upon visible irradiation, IrBr62- was found to undergo cascade-like relaxation through the long-lived (360 ps) metal-centered 2Ug'(T2g) excited state. Transient absorption studies upon 330 nm excitation evidence ultrafast intersystem crossing in the initially-excited state to form an excited state in quartet multiplicity. This state decays via a series of internal conversion steps into the lowest-energy quartet excited state. We found that the electron transfer within the t2g metal orbital is found to be the rate-limited step for non-radiative relaxation of IrBr6 (open full item for complete abstract)

Committee: Alexander Tarnovsky (Advisor); Mikhail Zamkov (Other); H. Peter Lu (Committee Member); Alexis Ostrowski (Committee Member) Subjects: Chemistry; Physical Chemistry

Doctor of Philosophy (PhD), Ohio University, 2015, Chemistry (Arts and Sciences)

This dissertation reports and discusses the synthesis, characterization and photophysical/photochemical investigation of a variety of ruthenium(II) polypyridyl sulfoxide complexes which undergo photochemical and electrochemical intramolecular (S→O) linkage isomerization reactions. Relevant analytical spectroscopic techniques are discussed in detail. Molecules which undergo multiple isomerizations (bis-sulfoxides) have been studied to interrogate excited states which lead to sequential or concerted isomerizations, with the aim of optimizing reaction efficiency. Furthermore, molecules designed to be structurally (covalently or non-covalently) incorporated to develop photochromic materials have been synthesized and investigated and are discussed.

Committee: Jeffrey Rack PhD (Advisor); Hugh Richardson PhD (Committee Member); Michael Jensen PhD (Committee Member); Eric Masson PhD (Committee Member); David Drabold PhD (Committee Member) Subjects: Chemistry; Inorganic Chemistry; Physical Chemistry

Doctor of Philosophy, University of Akron, 2014, Chemistry

As the limitations of fossil fuels and the effects they have on the environment become more apparent, the need for new alternative energy sources is becoming increasingly obvious. One potential source of renewable energy converts solar energy using organic and inorganic based photovoltaic (PV) cells into electrical energy. In this dissertation, I have used fluorescence spectroscopy to study two photoactive organic compounds that have been shown to undergo self-assembly, and two organic-inorganic hybrids that were also designed for use in PV cells. I also developed a new technique to study sub-picosecond fluorescence anisotropy. The organic compounds studied in this work were amphiphilic N,N-disubstituted naphthalene diimides that were shown to self-assemble into highly ordered aggregates that eventually formed nanostructures such as twisted nanotapes, twisted nanoribbons, and nanotubes. In this work, picosecond time correlated single photon counting (TCSPC) was used to investigate the fluorescence lifetimes and time-dependent fluorescence anisotropies nanostructures. For two of these compounds (Dipeptide B and Bola 1), the fluorescence lifetimes (tfl ~153 ps for Dipeptide B and tfl ~313 ps for Bola 1) were over an order of magnitude longer than that of a simple naphthalene diimide (N,N-dibutyl naphthalene diimide, tfl ~16.4 ps). The lifetime results were consistent with energy migration within highly ordered nanostructures. Further, the time-resolved fluorescence anisotropy results indicated an ultrafast depolarization of the fluorescence signal that could only be attributed to energy migration along a twisted or circular structure. The second portion of this work involved organic-inorganic hybrids consisting of a capped cadmium sulfide nanoparticle forming nanocomposites with a homo-poly(phenylene-ethynylene) polymer and an alternating co-poly(phenylene-ethynylene) polymer. In this work, picosecond TCSPC was used to investigate the average lumin (open full item for complete abstract)

Committee: David Modarelli Dr. (Advisor); David Perry Dr. (Committee Member); Adam Smith Dr. (Committee Member); Claire Tessier Dr. (Committee Member); Sergei Lyuksyutov Dr. (Committee Member) Subjects: Chemistry

Doctor of Philosophy, The Ohio State University, 2011, Chemistry

The electronic and photophysical properties of multiply bonded dimetal paddlewheel complexes of Mo, W, and Re are discussed. The nature of the excited states are determined and the electronic delocalization in the excited states are explored.

Committee: Malcolm Chisholm (Advisor); Claudia Turro (Advisor) Subjects: Chemistry

Doctor of Philosophy, The Ohio State University, 2010, Biophysics

The cyclobutane pyrimidine dimer is the most prevalent mutagenic UV photoproduct in DNA. Of these, the thymine dimer is the most readily formed and assayed, and is therefore a useful model for this photodamage pathway. Based on insight into the excited-state dynamics and timescale of thymine dimer formation, we have conducted computational studies of thymine dimer formation in various thymine-only model compounds. Experimental evidence has suggested that the fate of a photoexcited thymine-thymine base step is determined by the conformation of the base step at the instant of photoexcitation. We have used a two-parameter heuristic model to reproduce quantum yields of dimer formation in dTpdT in various water:organic co-solvent mixtures. Furthermore, we have showed that specific precursor conformations exist where these two bonds are closely aligned, which we hypothesized will dimerize on an ultrafast timescale upon photoexcitation. We studied the mean dimer precursor conformation predicted above as well as the canonical B-form conformation using DFT and CIS methods. These results suggest that B-form DNA has primarily localized excited states, and would undergo internal conversion back to the ground state in a manner similar to isolated thymine bases. On the other hand, the mean dimer precursor conformations identified in our heuristic study has a distinct photoreactive excited state, featuring significant weakening of the C5-C6 double bonds as well as putative bonding patterns needed for formation of the cyclobutane ring. We propose that population of this excited state leads to concerted cyclobutane pyrimidine dimer formation as predicted in the literature. We have further shown that dimer formation can be predicted using CIS calculations for the atom-atom overlap-weighted density matrix elements using the natural bond orbital formalism. These have allowed us to develop semi-heuristic models, where we assume that dimer formation occurs if and only if an increase in t (open full item for complete abstract)

Committee: Dongping Zhong PhD (Advisor); Bern Kohler PhD (Advisor); Chenglong Li PhD (Committee Member); Sherwin Singer PhD (Committee Member) Subjects: Biochemistry; Biophysics; Chemistry

Doctor of Philosophy, The Ohio State University, 2008, Chemistry

Exposure to UV light, particularly from the sun, is the primary controllable risk factor for the development of skin cancer. The damaging effects of UV photons results from their ability to induced photochemistry in DNA bases. While the many possible photoproducts of DNA are well known, the formation mechanisms for these photoproducts are not. In order to better understand these processes, we seek to better understand the events that occur between photon absorption and photoproduct formation - the photophysics of DNA. Femtosecond UV pump/UV probe transient absorption spectroscopy was used to study the ground-state vibrational cooling of the DNA base derivative 9-methyladenine (9MA) in solution. Photoexcitation of 9MA to the lowest bright electronic excited state at 267 nm is followed by rapid (τ ≈ 0.4 ps) internal conversion to the electronic ground state, generating more than 30 000 1/cm of excess vibrational energy. Transfer of this excess vibrational energy to the solvent was monitored via changes in the ground-state electronic absorption band at 250 and 285 nm. The vibrational cooling time increases in H2O (2.4 ps), D2O (4.2 ps), methanol (4.5 ps) and acetonitrile (13.1 ps) solvents. The studies show that the initial vibrational energy transfer from the hot solute molecule to the first solvent shell determined the thermalization rate. The studies also suggest that energy transfer between high-frequency solute and solvent modes play a more important role in vibrational cooling than expected. While the majority of excitation lead to ultrafast internal conversion, in pyrimidine bases additional decay pathways exist involving long-lived, intermediate, 1nπ* and 3ππ* states. The 1nπ*, 1ππ*, 3ππ* and S0 states of single pyrimidine bases have strongly-overlapping electronic absorption spectra which complicates study of their dynamics with conventional UV and visible techniques. A UV-pump/mid-IR-probe femtosecond transient absorption spectrometer was constructed for the (open full item for complete abstract)

Committee: Bern Kohler PhD (Advisor) Subjects: Chemistry; Physics

Doctor of Philosophy, The Ohio State University, 2007, Chemistry

The excited state dynamics of four of the five naturally occurring nucleic acid bases and several of their derivatives were studied by femtosecond transient absorption spectroscopy. Two tautomers of Adenine were distinguished in solution, with the canonical 9H tautomer having a 1pipi* lifetime of ~200 fs. The 7H tautomer, that we determined accounts for 22 +/- 4% of the population, has a forty-fold longer lifetime. The 1pipi* lifetimes of 1-, 3-, and 9-methyladenine were found to be the same as the 9H tautomer. The lifetime of 7-methyladenine was ~4 ps, half the lifetime of the 7H tautomer. Neither the lifetimes nor the relative populations of the tautomers changed drastically with solvent. The effects of protonation on the excited state lifetimes were also studied. These results were interpreted in light of several proposals for the excited state decay mechanism. A new intermediate state between the excited 1pipi* state and the ground state was observed to be specific to the pyrimidine bases uracil, thymine, cytosine and their derivatives. This state, assigned to the lowest energy 1npi* state, absorbs between ~300 and 450 nm and displays a lifetime that is sensitive to solvent and varies strongly between pyrimidines, with the lifetime much longer in the nucleotides than in the bases. UV probe experiments showed that 50 to 90% of the excited population decays via direct internal conversion to the ground state. The remaining 10 – 50% ends up in the 1npi* state where it either decays directly to the ground state or intersystem crosses to the triplet state. The triplet yield for 1-cyclohexyluracil was measured as a function of solvent. The triplet yield was found to increase as the solvent hydrogen bond donating ability decreased. Triplet state formation was observed to be complete within ~10 ps. These observations in the pyrimidines lead to a new picture of the excited state dynamics of these molecules in solution.

Committee: Bern Kohler (Advisor) Subjects: Chemistry, Physical

Doctor of Philosophy, Case Western Reserve University, 2010, Chemistry

The past several decades have seen heightened interest in molecular devices such as molecular switches, organic light emitting diodes (OLED's), molecular wires, and numerous other molecularly engineered devices. Many of these devices are organic-based materials that exploit E-Z isomerization of carbon-carbon, nitrogen-nitrogen, and/or carbon-nitrogen double bonds. As intriguing as the current literature is on these compounds, the possibility of substituting heavier main group elements, such as phosphorus, into these potentially useful systems has not been much explored. Heavier atoms may introduce new properties that currently are not available with the lighter atom cousins. High-level computational methods, including time-dependent density functional theory (TD-DFT) and complete active space ab initio methods with (CASPT2) and without (CASSCF) perturbation theory, have been used to study the E-Z isomerization of aryl-diphosphenes (Ar-P=P-Ar) and aryl-phosphaalkenes (Ar-P=C(H)-Ar). Both the thermal and photo-excited processes (energies, geometries, transitions) have been explored for three proposed isomerization pathways. The pathways examined include (1) rotation about the central double bond (C-P=E-C, E = P or C(H)), (2) in-the-plane inversion about the E=P-C bond angle, and (3) dissociation of the phosphorus-phosphorus and phosphorus-carbon double bonds for diphosphenes and phosphaalkenes, respectively. In both, diphosphenes and phosphaalkenes, rotation dominates as the preferred thermal (ground state) pathway. The calculations are also consistent with rotation as the primary mechanism for photo-induced isomerization by excitation to the low-lying electronic states. The photoisomerization of diphosphenes shows strong similarities to recently proposed models of the analogous process in azobenzene; namely, it involves a doubly excited phantom state. In contrast, phosphaalkenes parallel stilbene's classic rotation photoisomerization pathway. These results provide i (open full item for complete abstract)

Committee: Robert C. Dunbar PhD (Committee Chair); Alfred B. Anderson PhD (Committee Member); Thomas G. Gray PhD (Committee Member); M. Cather Simpson PhD (Committee Member); John D. Protasiewicz PhD (Advisor) Subjects: Chemistry; Physics

Doctor of Philosophy, Case Western Reserve University, 1993, Chemistry

Infrared radiative cooling of vibrationally excited n-butylbenzene ions was studied in chapter III. This was done by a technique called ion thermometry, where the internal energy of the ions could be probed by the branching ratio of two competitive photoproducts. The infrared radiative cooling rate constant was observed to be 0.8 s-1 for ions with energies of only 0.3 eV above room temperature. Confidence in the thermometric data depended on the reliability of the measured branching ratios. In chapter IV a computer simulation was used to show that erroneous peak height ratios were produced by the Coulombic repulsion between ions during ion excitation, and reliable ratios could be obtained by working at low excitation, and reliable ratios could be obtained by working at low ion densities and using a short excitation pulse. Chapter V discussed our first attempt to describe infrared multiphoton dissociation (IRMPD) of trapped ions in a thermal framework. Using a computer simulation the laser intensity was associated with an internal ion temperature for ions undergoing continuous laser irradiation. An Arrhenius type plot was constructed, and the activation energy obtained from it seemed reasonable within the expectations from Tolman's theorem. To pursue the feasibility of the thermal analysis of IRMPD kinetics, the thermometric technique was used in chapter VI to observe the CO2 laser pumping process for n-butylbenzene ions. The data showed that the steady state distribution of the ion energies reached during laser pumping is very dependent on the rate of photon absorption and emission. The generalized thermal analysis which was done in chapter V was found to be unfeasible. Ion thermometry was also used in chapter V to observe the rate of photon emission from n-butylbenzene ions heated by the cw-CO2 laser. Although the ions contained 0.3 eV more energy than the ions studied in chapter 2, the observed rate of cooling was the same. Chapter VII describes the association re (open full item for complete abstract)

Committee: Robert Dunbar (Advisor) Subjects: