ULTRAFAST EXCITED-STATE STRUCTURAL DYNAMICS AND CHARGE TRANSFER IN PHOTORESPONSIVE CONJUGATED MOLECULAR MATERIALS
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As many fundamental light-induced photochemical processes, e.g. isomerization, intramolecular electron or hydrogen transfer, happen on timescales ranging from femtoseconds to picoseconds, further understanding of these intriguing phenomena at the molecular level requires powerful tools to efficiently probe ultrafast excited-state dynamics. Femtosecond transient absorption spectroscopy (TAS) has proven to be a valuable method for interrogating excited-state evolution, providing significant insight about the nature of the elementary reactions that control the macroscopic properties of photo-activated systems and how they are influenced by the local environment. On the other hand, transient electronic spectroscopy reveals limited information about structural evolution, unlike vibrationally resolved spectroscopy. Femtosecond stimulated Raman spectroscopy (FSRS) provides access to this level of detail, and is also highly compatible and complementary to TAS. The scope of this dissertation involves application of both TAS and FSRS to the interrogation of structural and charge-transfer dynamics in various photo-responsive conjugated materials. Chapter 2 presents the study of S1 structural relaxation in two quaterthiophenes as probed by FSRS. This is a good example where a vibrationally resolved method is more suitable to examine the structural dynamics of a molecular material on sub-ps timescales, while TAS cannot specifically characterize such a detailed process merely on electronic level. Chapter 3 describes the interrogation of E/Z photoisomerization found in a novel diarylethene derivative. This photoswitchable molecule operates through state-selective (E and Z isomer) photoactivation with visible light. The ensuing mechanistic study by TAS reveals that this structural switching occurs via multiplicity-exclusive photoisomerization pathways. Chapter 4 focuses on the intramolecular charge transfer (ICT) and recombination in σ-π conjugated organosilanes upon photo excitation. Results from steady-state spectroscopies imply the occurrence of excited-state ICT in these donor-acceptor complexes, which has been definitively proven by FSRS. Meanwhile, the charge recombination process is comprehensively studied by TAS regarding the dependence of central oligosilane length and solvent. Chapter 5 describes excited-state intramolecular hydrogen transfer in benzoindolizine (bi). Two bi compounds with different substituents exhibit dramatically distinct fluorescence behaviors upon protonation. TAS reveals that the electron donating group in the protonated bi compound can facilitate hydrogen transfer in the excited state, thus quenching the radiative relaxation pathway.