Ultrafast Chemical Reactions Probed by Time-Resolved Vibrational Spectroscopy

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.