UC Berkeley

The aim of this dissertation is to describe how Raman spectroscopies can be used to track photoinitiated reactivity. Spectroscopy is the primary tool that allows us to measure the states of matter to understand how they interact. We can then use this understanding to build and innovate systems that fulfill human needs. There is a paradox that studying systems that yield the most fundamental understanding (i.e. gas phase low temperature spectroscopy) are often farthest removed from useful applications (e.g. solar panels, transistors etc.). This work explores both practical and fundamental research where the more fundamental results are presented in the early chapters and more application based results appear in later chapters. In short, we have developed a powerful spectroscopic technique, two-dimensional excited state femtosecond stimulated Raman spectroscopy (2D-ES-FSRS). By pushing FSRS to higher time resolution we can measure couplings between Raman active vibrations that were previously undetectable. Despite our experimental capability, the theoretical interpretation of the signal remains a challenge.

Chapter 1 succinctly summarizes spectroscopic signals with focus on vibronic absorption, resonance Raman spectroscopy, and stimulated Raman. Chapter 2 presents an analysis of the two-dimensional excited state femtosecond stimulated Raman spectrum of the charge transfer complex H4-TCNQ:1,2,4,5-tetramethylbenzene. These initial results were interpreted in the context of excited state anharmonicity between low and high frequency Raman vibrations. Chapter 3 presents the high time resolution transient absorption and FSRS analysis of another charge transfer complex tetracyanoethylene:1-chloronapthalene and provides theoretical and experimental evidence for the prevalence of the difference bands in resonant FSRS of small symmetric chromophores. Chapter 4 presents a reevaluation of the analysis of H4-TCNQ:1,2,4,5-tetramethylbenzene from chapter two. Additional excited state data are presented on the fluorinated analogue of H4-TCNQ to confirm vibration reassignments. A formula for the fifth-order 2D-ES-FSRS polarization is derived.

We then apply vibronic analysis to systems with more useful applications. Chapter 5 investigates the resonance Raman intensities and vibronic properties of the promising singlet fission sensitizing material, crystalline tetracene. The reorganization energy is diminished in the crystalline system relative to the monomer by a factor of seven indicating that the exciton is delocalized onto about seven tetracene molecules. In Chapter 6 examples of the reciprocal relation between polaron binding energy and polaron size are discussed. In Chapter 7 transient absorption spectroscopy is used to help deduce the molecular mechanism of photocatalysis of an allylic 1,3-photorearrangement of 1-cinnamylalkylammonium ions by a super molecular assembly.

The main points of this dissertation can be summarized as follows:

•In efficient optical photochemical reactions in the condensed phase low frequency vibrational coherences and the multimode phase relations thereof often determine the branching ratio between reaction pathways.

•A resonance Raman spectrum projects the action of the lowest order derivative of the resonant excited state potential energy surface(s) at the ground state geometry onto the ground state vibrational coordinates. Through clever use of resonance conditions these projections allow us to determine the coordinates of proximate conical intersections, those that are reached first by the excited state Raman wavepacket.

•The amount of energy that is deposited into vibrational reorganization energy immediately after excitation scales as the reciprocal of the exciton delocalization. The exciton size and therefore the polaron size can sometimes be estimated by a comparison of the magnitude of the Stokes shift.

• The time resolution of the amplitudes observed in a femtosecond stimulated Raman spectrum is the cross correlation of the actinic pump and probe pulses and in principal can be sub-10 fs. The time resolution of a frequency observed in a stimulated Raman spectrum is convolved over the duration of the Raman pump pulse or the vibrational dephasing time where the shorter of the two is effectively limiting.

•A fifth-order 2D-FSRS signal can be resonantly enhanced relative to a cascading third-order FSRS signal if the last four transient dipole-field interactions are many times more intense than the first two. This is observed when an impulsive pump pulse is resonant with a weak ground state absorption while the Raman pump and Stokes probe pulses are resonant with an intense excited state absorption.