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

Ultrafast time-resolved pump-probe spectroscopy is an ultimate method for revealing fundamental photophysical and photochemical processes that govern the evolution of molecular systems. This method can be used to study organic, inorganic, biological molecules, as well as materials, unraveling the response of the sample to photoexcitation on very rapid timescales from femtoseconds (10-15 s) to picoseconds (10-12 s), which response frequently defines molecular properties and functions. Excited-state relaxation dynamics of a paradigm ruthenium nitrosyl complex, an important sub-class of nitric oxide carriers, is studied by means of ultrafast dispersed, broadband transient absorption spectroscopy. A computational extension is performed for related NO-releasers such as trans [RuNOL2Cl3] complexes which possess coordinated derivatives of biologically-relevant nicotinic and isonicotinic acids. Further studies to develop NO releasers, including those involving covalent linkage sites as isonicotinic/nicotinic derivatives for potential application as photochemical drugs, can rely on the findings in the ultrafast study of [RuNOCl5]2- dynamics as involvement of the triplet states rather than linkage isomers. Further, molecular properties of compounds related to the perovskite-based photovoltaic were computationally investigated. The electron-rich series of [I3]-, [TeI4]2-, and [BiI6]3- compounds were discussed in detail with a focus on three-center four-electron bond, which plays an important role in the compounds containing heavy-atoms. Excited-state relaxation dynamics in a polyhalogenated compound (CH2BrI) were investigated by means of computational dynamics on the earliest timescale of 100 fs following excitation of this molecule into two electronic states of spectroscopic interest. The computed pump probe spectra yield the time occurrence and spectral position of the absorption and stimulated emission transitions of the involved product states. The results are instrument (open full item for complete abstract)

Committee: Alexander N. Tarnovsky Ph.D. (Committee Chair); Amelia Carr Ph.D. (Other); John R. Cable Ph.D. (Committee Member); Alexis D. Ostrowski Ph.D. (Committee Member) Subjects: Chemistry

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

Our overall goal here in this dissertation is to develop silicon-based hybrid materials that are potential high stability materials replacements for those in current electronics systems. To design the hybrid structures, a unique class of silicon-based compounds, silsesquioxanes (SQ) was used as the building block. SQs are three dimensionally compact Si-O bonded, cage-type compounds that can be synthesized to contain a variety of functional groups on each of the cage vertices. They offer useful properties such as thermal and photo stability, a high degree of functionalization, solution processability, and facile synthesis. The works in this dissertation focus on mixed functional (vinyl/phenyl) SQs of different sized cages containing 8, 10, and 12 silicon atoms. They are synthesized by fluoride catalyzed rearrangement reaction in a statistically controlled manner to achieve the desired vinyl groups for oligomerization. Spectroscopic measurements in picosecond/subpicosecond timeframes were performed before evaluating their potential applications. In chapter 2, vinyl/phenylSQs are cross-coupled by 4-di-bromo-aromatic linkers: Benzothiadiazole (BT), Phenanthrenequinone (PQ), Ethyl-carbazole (EC) and Phenyl-carbazole (PC). To compare photophysical properties between caged and non-caged structures, bis-tri-alkoxysilyl (linker) model compounds are synthesized. Luminescence quantum yields for oligomers are generally lower than the corresponding model compounds (except for PQ) which denotes non-radiative energy transfer possibility in oligomer. In addition, rapid transient absorption anisotropy decay (10's ps in oligomers) provide signatures for excitation energy transfer between linker chromophores in oligomers. In chapter 3, we have designed hybrid oligomers with a vinyl/phenylSQ cage backbone linked with cross-linkers including 2,7-dibromo-9-fluorenone, 2,7-dibromo-9,9-dimethylfluorene, 1,4-dibromo-2,5-dimethoxybenzene, 2,5-dibromopyridine, 2,6-dibromopyridine, 2, (open full item for complete abstract)

Committee: Joseph Furgal Ph.D. (Committee Chair); Robyn Miller Ph.D. (Other); H. Peter Lu Ph.D. (Committee Member); Xiaohong Tan Ph.D. (Committee Member) Subjects: Organic Chemistry; Physical Chemistry

Doctor of Philosophy, The Ohio State University, 2021, Physics

In this thesis, we lay the groundwork for performing attosecond transient absorption spectroscopy (ATAS) measurements in the condensed phase using mid-infrared (MIR) lasers. This was accomplished by designing, building and testing several pieces of home-built experimental equipment, including a MIR / extreme ultraviolet (XUV) Mach-Zehnder interferometer and a two-dimensional XUV spectrometer. A home-made bright XUV light source was designed and demonstrated to be nearly two orders of magnitude brighter than existing sources. Finally, the equipment was used to study ultrafast dynamics in germanium, a technologically important indirect bandgap semiconductor. This thesis is organized as follows. Chapter 1 introduces the relevant background, including ultrafast dynamics and the tools required to observe them. Chapter 2 details the commercial laser system, the home-built transient absorption beamline (TABLe) and the XUV spectrometer. In Chapter 3, we design and optimize the XUV light source for high flux, which is a general requirement for ATAS measurements and especially needed at longer wavelengths with poor high harmonic generation (HHG) quantum efficiency. Also covered in Chapter 3 are basic diagnostics of the XUV & IR optics, as well as our XUV detector. In Chapter 4, we present the results of a MIR ATAS experiment in germanium, an experimental first. Chapter 5 concludes the dissertation with a roadmap for future condensed matter studies. An appendix provides instructions on how to operate some aspects of the home-built experimental apparatus.

Committee: Louis DiMauro (Advisor); Robert Baker (Committee Member); Jay Gupta (Committee Member); Yuri Kovchegov (Committee Member) Subjects: Condensed Matter Physics; Optics; Physics

Doctor of Philosophy, Case Western Reserve University, 2018, Chemistry

Whether interrogating biomolecules for the reactivity as a photosensitizer or metal complexes for their potential for charge separation in a photovoltaic; understanding of the excited state dynamics is critical. Ultrafast broadband transient absorption spectroscopy was used in conjunction with steady-state spectroscopies and time-dependent density functional theory level computations in order to propose excited-state relaxation mechanisms. The characterization and quantification that coincides with the excited-state relaxation mechanism is useful for proposing avenues for in situ reactivity. For the class of biomolecules, pterins, it was shown that UV light populates a vibrationally-hot fluorescent 1pp* state. While the excess vibrational energy is redistributed (ca. 0.2 ps), intersystem crossing (ISC) occurs to an isoenergetic 3np* state from which the population either internally converts to the 3pp* state or undergos electron transfer to generate a neutral radical species. It is proposed that the radical species is responsible for the unimolecular and bimolecular photodegradation that has been observed of the pterins. Similarly, zinc(II)-chelating tetraphenylazadipyrrolmethene and derivatives utilizing different metal centers (cobalt(II) and nickel(II)) and ligands with expanded conjugation and fluorination were explored. The general relaxation mechanism is that visible light populates two nearly degenerate ligand-to-ligand pp* states (2S+1¿). Internal conversion populates the lower energy state which undergoes some energy redistribution, either vibrational cooling or relaxation dynamics, before undergoing ground-state recovery by charge recombination. It was determined that an addition ultrafast ground state recovery pathway exists between when cobalt(II) and nickel(II) are used as the central metal that is inaccessible when zinc(II) is used. The charge recombination lifetime correlates with the power conversion efficiency acquired for these complexes when used (open full item for complete abstract)

Committee: Carlos Crespo-Hernández (Advisor); Geneviève Sauvé (Committee Chair); Emily Pentzer (Committee Member); Mary Barkley (Committee Member); Nancy Oleinick (Committee Member) Subjects: Analytical Chemistry; Chemistry; Physical Chemistry

Master of Science, The Ohio State University, 2016, Chemistry

The conductive oligomer (E)-1,2-bis(2,2'-bithiophene-5-yl)ethylene (abbreviated T2VT2) was studied in its excited state using transient absorption spectroscopy and femtosecond infrared spectroscopy. The resulting spectra were interpreted and compared to previous experimental data taken using diphenyl stilbene. A single broad transient electronic absorption band was observed to decay according to a biexponential equation with time components on the order of 30-80 ps and 800-1500 ps. Femtosecond stimulated IR spectroscopy reveals no excited state vibrational data, but does have a broad background signal which decays according to a biexponential equation as well, with time constants on the order of 5-20 ps and 700-1500 ps. Early time oscillations were observed but remain unexplored. These results indicate that T2VT2 has a long lived excited state with a conformationally constrained structure. The origin of the transient absorption bands was unclear, and could be attributed to between one and three singlet energy levels, or the generation of a free electron by the photoexcitation of the compound.

Committee: Terry Gustafson (Advisor); Robert Baker (Committee Member) Subjects: Chemistry

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

Ruthenium and osmium polypyridyl sulfoxide complexes that undergo photochemical S-O linkage isomerization have gained sufficient research attention due to the potential application in solar energy storage, information storage and in development of new photorefractive materials. Among those sulfoxide complexes, the bis-sulfoxide complexes are of specific interest since some of them features two isomerizations upon absorbing one photon. However, the excited state isomerization details as well as how to change the yield of photoproducts are not fully understood. Ultrafast transient absorption spectroscopy was utilized to investigate the excited state isomerization mechanism of bis-sulfoxide complex [Ru(bpy)2(dmso)2]2+. It was found that single photon excitation triggers two sulfoxide isomerizations. Meanwhile, the photoproduct yield of S,O- and O,O-bonded isomers was found to be controlled by the photoexcitation wavelength. The spectroscopic data as well as the kinetics further our understanding of the excited state relativities for the bis-sulfoxide complexes. In order to learn more about the motion of the sulfoxide group during isomerization process, polarization transient absorption anisotropy measurements were performed to monitor this motion optically. Polarization anisotropy measurements report on the relative orientation of the transition dipole to the excitation dipole. The motion of the sulfoxide group was concluded to occur in the time scale of ~1 ps. In addition, the ultrafast study of a new class of highly fluorescent boron difluoride dyes was resented.

Committee: Jeffrey Rack (Advisor) Subjects: Chemistry

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2015, Photochemical Sciences

A molecular level understanding of photodynamics in condensed media is one of the recent challenges to chemical physics. This is because of the intrinsic complexity of liquid-phase photophysical and photochemical singularities arising from competing intra- and intermolecular processes. Such processes often take place on a timescale of a few femtoseconds (10-15 s) to several tens of picoseconds (10-12 s). In this work, the model photochemical processes used to investigate ultrafast photo-induced reaction dynamics in solution. The model compounds are non-metal/metal polyhalogenated small molecules. The gas-phase photochemistry of these small molecules is thoroughly examined, which also enables to establish the connection between liquid and gas phase dynamics. Furthermore, contrary to the scrupulously investigated di- and triatomic molecular systems, more vibrational degrees of freedom are accessible both for the model parent molecules, nascent polyatomic radical species, and isomer photoproducts. Therefore, a detailed mapping of the photochemical reaction paths of these molecular systems can possibly reveal different couplings between the reactive modes and other dark states in a far-from-equilibrium situation. The complexity of the encountered ultrafast events requires the utilization of several experimental and computational approaches. Results of femtosecond transient absorption, picosecond transient resonance Raman, excited state ab initio calculations are discussed in this context.

Committee: Alexander Tarnovsky Dr. (Advisor); Haowen Xi Dr. (Other); Marshall Wilson Dr. (Committee Chair); Alexey Zayak Dr. (Committee Chair) Subjects: Chemistry; Physical Chemistry

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