Investigation of the Role Nonadiabatic Energy Relaxation Plays in Excited State Intramolecular Proton Transfer using Multidimensional Electronic-Vibrational Spectroscopy

Abstract

Understanding the important mechanisms involved in photoinduced energy transfer in molecular systems is key to unlocking more efficient energy harvesting systems. Under specific conditions, nonadiabatic energy transfer, energy transfer that directly couples the electronic motion to that of the molecular vibrations is applied to explain how a 12 femtosecond proton transfer event occurs in a photoexcited molecular system. In this dissertation, the role of nonadiabatic coupling and nonadiabatic energy transfer on photoinduced intramolecular proton transfer molecular systems is explored using a subset of third order Fourier Transform techniques, termed electron-vibrational (EV) spectroscopies. These spectroscopic techniques combine an ultrabroadband near-ultraviolet (BBnUV) pump laser pulse which spans 370-440 nm and a broadband mid-infrared probe laser pulse which span the fingerprint molecular region (1200-1600 cm-1) on a class of organic molecules that undergo ultrafast intramolecular tautomerization reactions in the excited electronic state. These complexes are termed as excited state intramolecular proton transfer (ESIPT) complexes, and prove to be excellent reporters of the impact of nonadiabatic coupling on ultrafast structural transformations in molecular systems. The work presented here will advance the understanding of how coupled electronic and vibrational motions dictate ultrafast excited state proton transfer reactions. Ultrafast EV spectroscopy proves to be a viable method to explore the unique coupling of the electronic and vibrational degrees of freedom in complex molecular systems. To gain the best chemical insight, technical and experimental advancements of the EV spectrometer are implemented and discussed. The theory and design of a self-diffraction and transient grating cross correlation frequency resolved optical gating (FROG) pulse characterization method is introduced to characterize the pulses in the EV spectrometer. The generation of a BBnUV pump pulse that is used in the multidimensional spectroscopy experiments is improved. Lastly, a shot-to-shot pump-pulse phase cycling scheme is introduced and programmed into the EV spectrometer, which leads to a statistically-defined characterization of the noise floor used in the experiments. With the aid of EV spectroscopy and the advancements above, direct experimental evidence that nonadiabatic energy relaxation and transfer contribute to ESIPT in 10-hydroxybenzo[h]quinoline (HBQ), and its deuterated analog (DBQ) are discovered.