Quantum mechanical phenomena are playing tremendous roles in many areas of chemistry and materials science research. In recent years, the potential role of coupling between electronic and vibrational degrees of freedom in photochemical processes has been widely researched. Despite its potential to improve molecular technologies, the lack of studies focusing on small, highly controlled molecular systems has precluded efforts to harness vibrational-electronic (vibronic) coupling. Understanding the generalized functions of and tunable parameter space for vibronic coupling is vital for integrating molecular vibrations as a design element in next-generation optoelectronic devices.This dissertation focuses on investigations of vibronic phenomena in the photodriven dynamics of model dimeric compounds. We begin in Chapter 4 by exploring the role of steric hindrance between chromophores in driving changes to vibronic coherences in a series of substituted perylenediimide (PDI) cyclophane dimers. Using a combination of two-dimensional electronic spectroscopy (2DES) and femtosecond stimulated Raman spectroscopy (FSRS), we report differences in wavepacket evolution between these systems that are attributable to sterically driven distortions of the PDI cores. Chapter 5 focuses on the vibronic mechanism of sub-50 fs singlet fission (SF) in a linear terrylenediimide (TDI) dimer. We observed that ultrafast SF in this dimer is accompanied by the transfer of low-frequency coherent wavepackets which, in conjunction with theoretical analysis of the vibronic eigenstates, reveals that interactions between low-frequency singlet modes and high-frequency correlated triplet pair motions lead to mixing of the diabatic electronic states. This work highlights how multi-mode vibronic couplings can impact ultrafast singlet fission. In Chapter 6, we transition to a joint experimental and theoretical study of quantum beats in 2DES signals, which are well-known to provide direct insight into the intra- and interchromophoric couplings within a chemical system. We expand traditional theories to account for multiple Franck-Condon active vibrations and compared simulated spectra directly to experimental results from two organic semiconductors and biomedical dyes. We find that coupling between purely harmonic vibrational wavepackets can significantly impact signatures and interpretations of coherences measured by 2DES. This dissertation contributes insight into the factors that can impact vibronic coupling in multichromophoric systems and the influence that vibronic coupling may have on photophysical dynamics in organic molecular assemblies. In addition, the analytical and theoretical tools developed for these studies serve as quality starting points for future researchers utilizing nonlinear spectroscopy to investigate quantum phenomena in chemistry.

Creator

• Schultz, Jonathan David

DOI

• https://doi.org/10.21985/n2-2kqn-qr81

Subject

• Physical chemistry
• Molecular physics
• Quantum physics

Language

• en

Alternate Identifier

• etdadmin_upload_929572
• http://dissertations.umi.com/northwestern:16278

Keyword

• Coherence
• Quantum
• Excitons
• Vibronic coupling
• Spectroscopy
• Photophysics

Date created

• 2022-01-01

Resource type

• Dissertation

Rights statement

• In Copyright