Direct and Indirect Probing of Higher-Lying Excited-States of Photoactive Molecules

Abstract

It is often assumed that optically accessible excited-states above S1 play only a marginal role in the photochemistry and photodynamics of molecular systems. However, many classes of molecules have been found in which the higher-lying electronic states above S1 can significantly impact the photochemistry as it occurs on the S1 potential energy surface. Using the resonance condition associated with transient Raman spectroscopy, the higher-lying excited-states are probed directly via the vibration specific enhancements. The vibrational enhancements reveal the forces that the higher-lying resonant state applies in terms of the vibrational coordinates. In turn, the vibrational enhancements directly probe how the electronic potential shifts between the two electronic states, leading to optical control of the excited-state dynamics. Pump-probe and pump-repump- probe spectroscopies indirectly probe higher-lying states by showing the overall impact when those states are accessed either by directly via one-photon excitation or by sequential multiphoton excitation. By combining transient electronic and vibrational spectroscopies, a detailed picture of the excited-state landscape emerges with dynamic information about the higher-lying electronic states. In this dissertation, the role and structure of higher-lying excited electronic states are studied to understand how those states provide selective optical control. The photoactive molecules studied in this dissertation are related to diarylethene-based photoswitches. These photoswitches provide interesting photochemistry to study with these spectroscopic methods, and a detailed understanding of the optical control of these molecules will aid in their integration into optically active material systems. The primary photoswitch of study is 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl) perfluorocyclopentene (DMPT-PFCP) which reversibly isomerizes following irradiation and acts as a model system to study photochemical dynamics. Two additional photochromic switches and a phenylthiophene derivative related to the aryl side groups of DMPT-PFCP are investigated to study the effect of structural changes on the excited-state dynamics.