Observing Coupled Nuclear and Electronic Motions Involved in Intramolecular Hydrogen Bonding and Proton Transfer with Ultrafast Multicolor Spectroscopy

Abstract

The use and development of of multidimensional spectroscopy have allowed scientists to uncover coupled motions of electrons and nuclei in solution-state systems. Wavelengths ranging from X-ray to infrared offer both localized and delocalized pictures of the coupled degrees of freedom in solution and their influence on one another. As multidimensional, multicolor spectroscopy develops further, experiments and calculations in tandem have aided in understanding the coupled nuclear and electronic motions involved in fundamental chemical processes such as intramolecular hydrogen bonding (IHB) and proton transfer. IHB mediates many solution-phase reactions in chemistry and biology such as protein folding, DNA replication, proton transfer, and more, and is a key component of many molecular structures. Model complexes offer a convenient approach to systematically investigate IHB and proton transfer because they are much smaller than many systems found in nature and used in industry. 10-Hydroxybenzo[h]quinoline (HBQ) is a particularly useful, synthetically tunable, model complex for studying both proton transfer and IHB. It undergoes excited state intramolecular proton transfer (ESIPT), while the proton donor and acceptor participate in a strong intramolecular hydrogen bond. To understand the relationship between hydrogen bonding, proton transfer and the electronic and nuclear structure of HBQ, we must be able to examine the whole molecule and the microscopic interactions within the molecule. Infrared and X-ray spectroscopy provide a complementary understanding of the atomic and electronic fluctuations affected by and involved in hydrogen bonding. Electronic spectroscopy in the UV-visible region reports on the entire delocalized electronic structure of the molecule, while X-ray spectroscopy also offers deeper insight to the local electronic structure in an atom of interest. As a combination of femtosecond pulses used in the form of pulsed light, these three wavelength regions (infrared, UV-Vis, and X-ray) serve as powerful spectroscopic tools to investigate ultrafast chemical reactions. This dissertation presents the use of transient X-ray absorption spectroscopy (t-XAS) calculations and multidimensional vibrational-electronic (VE) spectroscopy to investigate the coupled electronic and nuclear motions involved in IHB and ESIPT in HBQ, as well as recent advances in VE experimental development. One- and two-dimensional vibrational-electronic (1D and 2D VE) spectroscopy utilize two vibrationally-resonant pump pulses to excite ground state vibrations, and an electronically- resonant visible or near-UV probe pulse to observe the changes in the electronic absorption spectrum caused by the interaction of the pump with a sample of interest. This work details improvements made to the recently developed 1D and 2D VE experiments and their ongoing experimental challenges. In particular, the addition of a broadband visible probe source and a broader infrared pump with improved stability and increased pulse energies has enabled the study of new systems with electronic absorptions in the near-UV. Experimental protocols have also been optimized to improve data collection times, replicability, and processing. Polarization-selective 1D and 2D VE spectroscopy are used to investigate the coupled low- and high-frequency modes in the S1 ← S0 electronic transition of HBQ. Coherent low-frequency oscillations are observed in the 1D VE spectra at 242 cm−1 and 386 cm−1, coupled to the electronic transition through the high-frequency OH stretch. 2D VE spectra at three time delays (τ2) reveal that regions of the ground state OH stretch couple differently to the S1 ← S0 transition, and likely oscillate at the same low frequencies as observed in the 1D experiments. On the electronic excited state, intramolecular hydrogen bonding mediates the ESIPT in HBQ. The proton donor and acceptor atoms undergo significant changes to their local electronic environments during and following proton transfer. Transient X-ray absorption calculations at the oxygen (proton donor) and nitrogen (acceptor) K-edges reveal that the local electronic environment of each is influenced by the coherent oscillations as HBQ relaxes through the excited vibrational manifold of the electronic potential. Shifting X-ray absorption peak energies report on the changes to each local electronic environment in the proton transfer moiety. This is the first successful example of using transient X-ray spectroscopy calculations to track an ESIPT, and it paves the way for future experiments at X-ray freeelectron laser facilities.