Spectroscopy is a powerful tool for the identification, study, and selective control of molecular processes. Raman scattering is particularly useful in observing the vibrational properties of molecules, and identifying compounds based on structure. While the experimental measurements provide structural information on the vibrational transitions, the detailed interpretation of Raman spectra for complex molecules requires assignment of the observed Raman bands to specific vibrational motions. As a result, experimental spectra are often compared with calculated vibrational frequencies and Raman intensities. Therefore, it is necessary that the simulated spectra correctly reproduce the experimental Raman spectra. Although simulations of Raman spectra generally have good agreement with experiment, additional work is required to understand the effects of intramolecular electronic and nuclear structure on the Raman intensities of molecules with extended π conjugation.
Conjugated thiophene derivatives have a variety of interesting optical and electronic properties, which makes them ideal targets for studies of charge-transport processes. Additionally, the delocalized π electron distribution gives these compounds relatively large scattering cross-sections, making them excellent model compounds for Raman spectroscopic studies. In particular, time-resolved techniques are able to measure transient Raman spectra in electronically excited states, allowing for the direct observation of structural dynamics following optical excitation, which is instrumental in understanding the charge separation processes in these molecules. Due to small excited-state population, transient Raman measurements typically rely on electronic resonance enhancement, which increases the Raman transition intensities for specific vibrational modes by up to several orders of magnitude. This mode-specific enhancement occurs for vibrations with large displacements along the potential energy surfaces of the higher-lying electronic states, therefore, detailed information can be obtained about the structure of higher-lying states based on which vibrations have enhanced Raman scattering intensity. Although resonance-enhanced excited-state Raman measurements are often used to increase signal, the role of the resonant electronic transition has been largely overlooked. By combining experimental measurements and theoretical simulations of excited-state Raman spectra, it is possible to gain a more comprehensive understanding of the structure of the higher-lying electronic states.
In this dissertation, we measured resonance-enhanced Raman spectra in excited singlet and triplet electronic states for a set of conjugated thiophene derivatives, and compared the experiments with calculated Raman intensities for the excited states. Using relatively inexpensive computational methods, we were able to assign the experimental Raman bands to specific vibrational motions by considering the resonance enhancement condition in detail. Additionally, it was found that the experimental resonance Raman intensities can be qualitatively reproduced by calculations of the energy gradient of the higher-lying electronic state along vibrational displacements, particularly for vibrations that have relatively large resonance enhancements. We also investigated the effect of inter-ring torsion on the Raman intensities of aryl-substituted benzene and thiophene compounds in the ground state, for which density functional theory calculations tend to overestimate the delocalization of the π electron distribution between aryl rings. In addition to providing a benchmark for the accuracy of theoretical methods, the combination of experimental and simulated Raman spectra provided more detailed insight into the electronic and structural properties of the conjugated thiophene molecules than could be obtained by either approach alone. Finally, we studied the excited-state dynamics of 2,5-diphenylthiophene (DPT) following photoexcitation. We found that, by examining the potential energy gradients of higher-lying electronic states, it was possible to characterize vibrational coherences observed in the excited-state absorption spectrum, ultimately providing a more complete interpretation of the excited-state dynamics of this model compound.
|Advisor:||Elles, Christopher G., Caricato, Marco|
|Commitee:||Johnson, Carey K., Thompson, Ward H., Laird, Brian B., Marshall, Craig P.|
|School:||University of Kansas|
|School Location:||United States -- Kansas|
|Source:||DAI-B 81/8(E), Dissertation Abstracts International|
|Subjects:||Physical chemistry, Computational chemistry|
|Keywords:||Conjugated molecules, Excited state, Raman spectroscopy, Resonance Raman spectroscopy, Thiophene|
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