Investigating Charge Transfer Dynamics in Organic Crystals and Photocatalytic Solutions with Femtosecond Stimulated Raman Spectroscopy

Abstract

Femtosecond stimulated Raman spectroscopy (FSRS) is a non-destructive vibrational technique that is used to track evolving structural dynamics over the course of a photoinduced reaction. Due to its high spectral (cm-1) and temporal (fs) resolution, FSRS is able to follow spectral signatures as a function of time relative to photoexcitation, discerning ultrafast reaction dynamics and providing chemically specific structural information on relevant timescales of chemical reactivity, thus enabling the identification of structure-function relationships. Historically, FSRS has been used to determine polymorph identity via phonon mode assignments, map exciton transport, and isolate essential molecular structures in charge transfer processes. This thesis discusses the improvements made to the FSRS technique, enhancing its ability to probe solid state materials, and how FSRS can be used to follow charge transfer processes in organic single crystals and homogeneous photocatalytic systems. Initial studies utilize FSRS to probe the effects of crystal orientation and laser polarization on the charge transfer process in betaine-30. This work demonstrated that changing the crystal orientation does not always have the same effect as changing the laser polarization and further examined differences in ultrafast chemical dynamics at various polarization and orientation combinations. Next, I explore intermolecular charge transfer dynamics in an organic co-crystal of perylene and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Using FSRS, I track the photoexcitation of perylene and subsequent charge transfer to TCNQ within a single crystal and examine the kinetics of the molecular dynamics tracked throughout the charge transfer process. Finally, I investigate the photoinduced reduction of carbon dioxide in a homogeneous photocatalytic system. In this chapter, I demonstrate my ability to spectroscopically detect methanol, the desired reduction product, and discuss my efforts to investigate the ultrafast dynamics and elucidate the mechanism of carbon dioxide in this specific system. Herein, I demonstrate the ability of FSRS to examine charge transfer processes in a variety of material states. This thesis focuses on the improvement to FSRS to better probe charge transfer events in solid state materials, discusses my results in both solid-state and liquid samples, and provides insight into future avenues for promising research studies.