Nonlinear Excitation of Photoactivated Molecules: Two-Photon Absorption Spectroscopy, Dynamics, and Quantum Yields

Abstract

Higher-lying excited electronic states of model, photoactivated molecules are studied using nonlinear excitation to explore the electronic spectroscopy, excited-state dynamics, and reaction quantum yields. The photoactivated molecules studied in this dissertation include two photochromic molecules, stilbene and 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluoro-cyclopentene, that reversibly convert between different isomers following irradiation, as well as a photoactivated protecting group, para-hydroxyphenacyl, that photochemically releases a bound substrate. Studying higher-lying excited states above S1 following nonlinear excitation provides information about the initially excited state, the subsequent excited-state dynamics, and the reaction quantum efficiency. Much less is known about the higher-lying excited states as compared to the well-studied ground and lowest-lying excited states, which motivates the work in this dissertation to investigate the higher-lying excited states of photoactivated molecules following nonlinear excitation. The measurements of the higher-lying excited states reported here include a broadband pump-probe technique that is used to measure the two-photon absorption spectroscopy, as well as the excited-state dynamics following linear and nonlinear excitation of the studied photoactivated molecules. The broadband two-photon absorption spectroscopy measurements reveal the two-photon accessible states and their absolute two-photon absorption cross sections. Separate measurements of the excited-state dynamics and of the reaction quantum yields following nonlinear excitation collectively provide information about the behavior of the higher-lying excited-states, and how the identity of the excited states affect the outcome of the photochemical reactions. Probing the spectroscopy, dynamics, and quantum yields of the studied photoactivated molecules is important to develop a fundamental understanding of photochemical reactions from higher-lying excited states. The spectroscopy, dynamics, and quantum yield measurements in this dissertation can also serve as new benchmarks for computational studies of these model molecules.