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Unveiling the Mechanisms in Surface Chemical Reactions of 2D MoS2 for Crystallographic and Chemical Engineering

2023-12-31T15:00:00Z

Title: Unveiling the Mechanisms in Surface Chemical Reactions of 2D MoS2 for Crystallographic and Chemical Engineering Author(s): Younghee Park Abstract: Two-dimensional materials have a layered structure, and all atoms are exposed on the surface. This makes surface chemical reactions one of the most important factors in the study of 2D materials. In particular, in the chemical engineering and crystallographic epitaxial growth of MoS2, which is a representative material of Transition metal dichalcogenides, the surface chemical reaction control plays a large role in controlling the properties of the material. This study focused on investigating the surface chemical reaction of 2H-MoS2 from two perspectives: chemical engineering and crystallographic epitaxial growth. In the chemical engineering, a study was conducted to form a covalent bond directly on the surface of 2H-MoS2 through a photochemical reaction using highly reactive diazonium salt and alkyl halide as reactants, and a reaction mechanism was proposed. In addition, in the crystallographic epitaxial growth of MoS2, a systematic investigation Demonstrated that the growth temperature has a significant effect on the crystallographic orientation of MoS2 during its crystallographic epitaxial growth. The study revealed that the surface terminal group of the sapphire substrate is strongly affected by temperature, which determines the crystallographic orientation selectivity of MoS2. These results are anticipated to be applied to various 2D materials by discovering and clarifying the chemical engineering of the surface chemical reaction of MoS2 and the fundamental mechanism of crystallographic epitaxial growth.

Ultrafast Intramolecular Charge and Proton Transfer coupled with Solvation Dynamics of DMSO

2020-12-31T15:00:00Z

Title: Ultrafast Intramolecular Charge and Proton Transfer coupled with Solvation Dynamics of DMSO Author(s): Kooknam Jeon Abstract: Excited-state proton and charge transfer are one of the fundamental processes and play a key role in various chemical and biological systems. It has been received huge interest during the last decades and has been extensively studied by many time-resolved electronic and vibrational spectroscopic techniques. Solvation dynamics play important roles in charge transfer and proton transfer reactions and has been extensively studied by numerous experimental and theoretical methods. The reaction dynamics occurring in the solution phase are inherently dependent on the solvent properties. Solvation may facilitate chemical reactions by opening an energetically favored reaction channel and can stabilize the product state by dissipating the excess vibrational energy after the chemical reaction. FSRS has been successfully applied to many ESIPT and ICT processes occurring in ultrafast time scales due to the high spectral and temporal resolutions. In this dissertation, we will discuss the excited-state intramolecular charge and proton transfer reaction of 1-amino anthraquinone (AAQ) and 1,2-dihydroxyanthraquinone (alizarin) with solvation dynamics of DMSO via femtosecond stimulated Raman spectroscopy (FSRS). In chapter 3, we will discuss the ultrafast intramolecular proton transfers of 1,2-dihydroxyanthraquinone (alizarin-h2) and its deuterated product (alizarin-d2) in dimethyl sulfoxide (DMSO) by femtosecond stimulated Raman spectroscopy. The population dynamics in the solute vibrational mode of νC=O and the coherent oscillations observed in all of the skeletal vibrational modes νC=O and νC=C clearly show the ultrafast excited-state intramolecular proton transfer dynamics of 110 and 170 fs for alizarin-h2 and alizarin-d2, respectively. Interestingly, we have observed that the solvent vibrational modes νS=O and νCSC may also be applied to ultrafast structural dynamics at these frequencies for its “free” or “aggregated” species. From the kinetic analysis of the νS=O and νCSC modes of DMSO, the ultrafast changes in the solvation, or intermolecular interactions between DMSO molecules initiated by the structural changes of solute molecules, have been thoroughly investigated. In chapter 4, we will discuss transient Raman spectra of AAQ in the dimethylsulfoxide (DMSO) solution by femtosecond stimulated Raman spectroscopy for the twisted ICT state of AAQ. An ultrafast (~110 fs) ICT dynamics of AAQ was observed from the major vibrational modes of AAQ including the νC-N + δCH and νC=O modes. The coherent oscillations in the vibrational bands of AAQ strongly coupled to the nuclear coordinate for the ICT process have been observed, which showed anharmonic coupling to the low frequency out of plane deformation modes. The vibrational mode of solvent DMSO, νS=O showed a decrease in intensity especially in the hydrogen-bonded species of DMSO, which clearly shows that solvation dynamics of DMSO including hydrogen bonding are crucial in understanding the reaction dynamics of AAQ with the ultrafast structural changes accompanying the ICT.

Ultrafast Chemical Reactions Probed by Time-Resolved Vibrational Spectroscopy

2021-12-31T15:00:00Z

Title: Ultrafast Chemical Reactions Probed by Time-Resolved Vibrational Spectroscopy Author(s): Myungsam Jen Abstract: A chemical reaction refers a series of atomic rearrangements between the reactant and product molecules, where the rearrangements of single chemical bonds occur on the ultrafast time scales of femtosecond and picoseconds. The chemical reactions are generally described by the energy difference between the reactant and products, activation barriers, reaction rates, etc. The reaction rates of a certain chemical reaction can be obtained by experimentally observe the concentration changes of reactant or product molecules. In order to measure the reaction rates of the ultrafast chemical reactions such as intramolecular proton transfer, ultrafast spectroscopic methods are inevitable where ultrashort laser pulses initiate the chemical reactions in the excited state and time-resolved spectroscopy measures the spontaneous concentration changes of reactants and/or products. The reaction dynamics study by the time-resolved spectroscopy would be beneficial in many important applications including artificial photosynthesis, dye-sensitized solar cell, or molecular optoelectronics. Simple photochemical reactions of excited state proton transfers and photoinduced charge transfers with subsequent structural changes in the electron donor or acceptor groups are considered as one of the fundamental processes in many chemical and biological systems. Therefore, the excited-state reaction dynamics probed by time-resolved spectroscopic measurements can be used to understand the ultrafast bond breaking and formation of reactant and product molecules, or the or the instantaneous structural changes accompanying various excited state photophysical processes. In this thesis, transient absorption spectroscopy and femtosecond stimulated Raman spectroscopy (FSRS) has been adopted as the time-resolved electronic and vibrational probes, respectively. Time-resolved electronic and vibrational probes would be complementary to each other in analyzing the ultrafast photophysical and photochemical processes in the excited state. The structural changes of 1,2-dihydroxyanthraquinone (alizarin) in dimethyl sulfoxide (DMSO) solution upon the excited-state intramolecular proton transfer (ESIPT) reaction have been studied by FSRS measurements. Previous time-resolved electronic measurements may provide the detailed excited-state dynamics upon the proton transfer. However, the detailed structural changes of alizarin or related molecules upon the ESIPT has not been reported. The ESIPT dynamics of alizarin of 70-80 fs has been observed from the vibrational modes of ν(C═C) and ν(C═O) in the singlet excited state, where the population changes and opposite peak shifts of these modes occurring with the common 70-80 fs time constant are compatible to the proposed transition state of six-membered ring structure with intramolecular hydrogen bonding between the carbonyl and adjacent hydroxyl group. The ESIPT dynamics of alizarin has been updated by improving the temporal resolution of FSRS measurements with the pulse compression of actinic pump. The vibrational probe of ν(C═C) and ν(C═O) modes in the excited state have provided more detailed ESIPT dynamics (110 fs) of alizarin with the population transfer between the locally-excited (LE) and proton-transferred state, and the coherent oscillation signals in these vibrational probes show that the reaction coordinate of the ESIPT reaction is strongly coupled to several low-frequency vibrational modes of intramolecular hydrogen bonding. The solvent vibrational modes of DMSO, ν(S=O) and ν(CSC) are strongly changed upon the ESIPT of the solute alizarin. Although the vibrational modes of DMSO are inseparable from the nonlinear cross-phase modulation and long-lasting hot ground state Raman signals, the ν(S=O) and ν(CSC) modes of DMSO shows instantaneous (60-120 fs) increase in the “free (isolated)” or “aggregated (dimer)” bands indicating the solvation changes of DMSO upon the ESIPT of alizarin. The solvent vibrational modes of DMSO, ν(S=O) and ν(CSC), may “probe” the ultrafast chemical reactions of the solute indirectly since these modes are very sensitive to the instantaneous solvation changes resulting from the structural changes of the solute. The photoinduced charge transfer dynamics of curcumin in DMSO solution has also been investigated by FSRS measurements. Curcumin is one of the well-known antioxidants, and shows ultrafast intramolecular charge transfer (ICT) in the excited stat, where the excited state lifetime and fluorescence quantum yield are strongly dependent on solvent polarity and hydrogen bonding with solvent. The vibrational modes of curcumin in the LE and charge-transferred (CT) states are separately observed from the FSRS measurements, where an ultrafast ICT (0.6-0.8 ps) and subsequent vibrational relaxation (6-9 ps) dynamics in the CT state have been retrieved. The ground-state vibrational modes of curcumin, ν8a and ν(C=C,C=O) and the solvent vibrational modes of DMSO, ν(CSC) and ν(S=O) appear strongly coupled to the ICT dynamics of curcumin, which supports the strong solvation interactions including the hydrogen bonding. Especially, the ν(CSC) and ν(S=O) modes of DMSO represent the ultrafast (20-50 fs) dynamics with the hydrogen-bonded species upon the ICT of curcumin. However, further explorations on the detailed spectral changes between the “free” and “hydrogen-bonded” species in the ν(CSC) and ν(S=O) mode of DMSO are required to explain the solvation changes of DMSO in the solvation shells with the ultrafast ICT dynamics of chromophores. Lastly, up-to-date investigations and future directions for the solvation dynamics study with the ultrafast excited-state processes of chromophores are summarized. All the FSRS measurements described in the thesis are based on the stimulated Raman “gain” measurements, where the Raman pump centered at higher frequency and the Raman probe with the lower frequencies complete the stimulated Raman process. Similarly, the stimulated Raman “loss” measurements requires the Raman pump at lower and the Raman probe at higher frequencies. It has been known that the modulations of the Raman probe by the Stokes and anti-Stokes Raman processes in the stimulated Raman “loss” measurements are observed in the opposite contribution while the Stokes and anti-Stokes Raman signals are inseparable in the stimulated Raman “gain” measurements. We propose the possible separation of the long-lasting thermal signals in the solvent vibrational modes of DMSO (observed in the excited-state dynamics of alizarin and curcumin) by combining the stimulated Raman “gain” and “loss” measurements. The “hot” ground-state transitions of the ν(S=O) and ν(CSC) modes of DMSO by the anti-Stokes Raman process can be subtracted with some experimental control between the stimulated Raman “gain” and “loss” measurements. Similarly, the nonlinear cross-phase modulation artifacts can be minimized by the control experiment only with the solvent. Further experimental developments are required, however, for more accurate determination of the solvation dynamics of DMSO, which may indirectly “probe” the ultrafast chemical reaction dynamics of chromophores in the excited state including the proton and charge transfer. In this thesis research, the ultrafast chemical reactions of intramolecular proton and charge transfers have been observed by time-resolved electronic and vibrational spectroscopy. FSRS presents numerous advantages in the reaction dynamics study in the excited state due to its high temporal and spectral resolutions and multimodal vibrational probes in a wide spectral range. The ultrafast chemical reaction dynamics in the excited state can also be probed by the instantaneous changes in the solvent vibrational modes of DMSO including the hydrogen bonding interactions when the strong solvation dynamics exists between the chromophore and solvent molecules.

Ultrafast chemical reactions and energy transfers investigated by time-resolved electronic and vibrational spectroscopy

2020-12-31T15:00:00Z

Title: Ultrafast chemical reactions and energy transfers investigated by time-resolved electronic and vibrational spectroscopy Author(s): Sebok Lee Abstract: Photo-induced chemical reaction dynamics of chromophores has been of great interest in chemistry and related fields for decades. Understanding the dynamics of molecules including excited state proton transfer, charge transfer (CT), energy transfer provides important keys to the chemical reactions in various chemical and biological systems. This can also be beneficial in many applications including the artificial photosynthesis, dye-sensitized solar cell, or molecular optoelectronics. To investigate these photochemical reactions rapidly occurring on picosecond to femtosecond time scales, a femtosecond time-resolved spectroscopic method have been widely used. In my dissertation, time-resolved electronic and vibrational spectroscopy including transient absorption, femtosecond stimulated Raman, impulsive stimulated Raman were used to seek detailed understanding of the excited-state dynamics of chromophores with the structural changes. In Chapter 1 and Chapter 2, the overall introduction and experimental setups used in this dissertation were described. With a transient absorption setup, the excited-state intramolecular proton transfer (ESIPT) process of 1,2-dihydroxyanthraquinone (alizarin), the photo-induced electron transfers from the S2 state of carotenoid molecules to TiO2 semiconductor nanoparticles, and the energy transfer process in porphyrin-peptoid complexes were explored. In Chapter 3, the population dynamics (87 ps) and vibrational relaxation (0.35 and 8.3 ps) during the ESIPT process of alizarin in ethanol solution were described. The ESIPT reaction of alizarin seems to be blocked by the strong hydrogen bond formation between water and alizarin. Even though the ultrafast ESIPT of time constant (<100 fs) was not observed due to the overlapped several contributions in transient absorption signal, the structural dynamics of alizarin accompanied by the ESIPT process will be further described by femtosecond stimulated Raman measurements. In Chapter 4, the ultrafast excited-state dynamics including the electron and recombination dynamics between ACOA and TiO2 nanoparticles were described. The electron injection of 240 fs from ACOA into TiO2 nanoparticles with quantum yields of ~33 % were observed with no dependence on excitation wavelengths. In Chapter 5, high efficiencies (96.3-97.6 %) through the energy transfer rate (66 ps-1 ~ 101 ps-1) for five porphyrin-peptoid complexes were investigated depending on the relative spatial arrangements between the zinc porphyrin (donor) and free base porphyrin (acceptor) with the transient absorption measurements. Two porphyrins were synthesized at the defined residues of peptoids with varying the distances and orientations. With the control of spatial arrangements of porphyrins, the precisely tuneable energy transfer efficiency of porphyrin-peptoid conjugates were demonstrated. In Chapter 6-8, the structural changes of push-pull emitters including DCM and nitroaromatic molecules have been investigated by using the femtosecond stimulated Raman spectroscopy with high temporal (<50 fs) and high spectral resolution (<10 cm-1). Many TDDFT simulations estimated twisted intramolecular charge transferred states of push-pull dyes with rotation of electron donating or withdrawing groups. With the experimental evidence, the structures of push-pull emitters during the ICT process were investigated. For 4-dicyanomethylene-2-methyl-6- (p-dimethylaminostyryl)-4H-pyran (DCM), the ICT formation with ~1 ps time constant followed by the vibrational relaxations of 4~7 ps was observed in dimethyl sulfoxide (highly polar) solution from the population changes, frequency shifts, and bandwidth changes of the major vibrational modes including ν(C≡N), ν(C=C/C-C), and δ(CH3) (Chapter 6). In chloroform (slightly polar) solution, CT state simultaneously populated with the locally excited (LE) state decays to the LE state with a fast (~300 fs) lifetime followed by the vibrational relaxation in the LE state (4~7 ps). The time-dependent density functional theory (TDDFT) simulations were performed with comparison of two distinct Raman spectra of DCM in the LE state and CT state, and a twisted geometry of the dimethylamino group of DCM during the ICT process was proposed. In Chapter 7, the accompanied structural changes of nitroaromatic molecules including 4-dimethylamino-4'-nitrobiphenyl (DNBP) and 4-dimethylamino-4′-nitrostilbene (DMANS) during the ICT process were explored. During the ICT process of nitroaromatic molecules, the vibrational resonance at stretching vibrational modes (ν8a) between two phenyl rings observed in the region of 1550-1650 cm-1 were broken with the twisted biphenyl group. Based on the TDDFT simulations, a twisted geometry of the nitrophenyl group of nitroaromatic molecules during the ICT process was proposed. In Chapter 8, LD 688 dye which has similar molecular structure of DCM with restricted rotation around dimethylamino group was investigated. Upon photoexcitation, the ICT with the 1.0 ps time constant followed by ~5 ps vibrational relaxation in the product CT state was observed, which are evidenced in several vibrational modes including δ(CH/CH3), ν(C=C), and ν(C≡N) as the changes in the Raman intensity and frequency shift. Based on the experimental results and the TDDFT simulations, a twisted molecular geometry with rotated julolidine group of LD 688 during the ICT process was suggested. In Chapter 9, the setup of time-resolved impulsive stimulated Raman spectroscopy (ISRS) has been described, in which the frequency-domain vibrational spectra are obtained by a fast Fourier transform of the temporal coherent nuclear wavepacket motions which are impulsively excited by ultrashort Raman pump. In order to observe the fingerprint region over the 3000 cm-1 of molecules, ultrashort (<20 fs) and broadband pulses were generated from a noncollinear optically parametric amplifier (NOPA). To get the ISRS signal with high signal to noise ratio, the detection noise level was significantly improved as <25 μOD per 1000 pulses by minimizing the fluctuation level with the reference detector. With this technique, several preliminary results on including the protonation dynamics of a photoacid HPTS and the solvation dynamics of dimethyl sulfoxide during the ESIPT process of alizarin will be introduced.