- In this dissertation, my research work on excited state dynamics of small organic molecules in solutions is presented. Using femtosecond stimulated Raman spectroscopy (FSRS) as the main experimental tool, the excited state hydrogen-bonding dynamics (ESHBD) of Coumarin 102, a common laser dye molecule, in ethanol solution is examined with <150 fs time resolution. The dynamics extracted from our FSRS spectra brings new insights into the currently controversial picture of ESHBD of Coumarin 102, i.e. whether the hydrogen bond formed between the carbonyl oxygen of Coumarin 102 and a nearby hydrogen donor is strengthened or cleaved upon 400 nm photoexcitation. The dynamics of the spectator mode, i.e. the C=O stretching mode, is examined within a 600 ps time window. The excited state dynamics in the first few picoseconds after photoexcitation unequivocally supports a picture of excited state reaction pathway bifurcation, i.e. only a portion of the molecules in the initial excited state undergo hydrogen bond cleavage occurring within 160 ± 50 fs after 400 nm photoexcitation. In the first 200 fs after excitation, clear vibrational peaks of the hydrogen-bonded C=O stretching mode are observed for the first time, confirming that the ultrafast excited state hydrogen bond cleavage occurs after rather than during photoexcitation. Dynamics extracted from FSRS data on a longer timescale, when combined with data obtained from femtosecond transient absorption (TA) spectroscopy, suggests a minimal model of ESHBD of Coumarin 102 in which both the hydrogen bond coordinate and the solvation coordinate play critical roles.
The conformational dynamics of a photoacid pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, or HPTS) in pure water/D₂O and aqueous solution is also studied using FSRS. In the presence of acetate as the proton acceptor, it is observed that the marker bands of the deprotonated form of HPTS (1139 cm⁻¹) appears earlier and faster than the marker band of the monomer acetic acid (864 cm⁻¹), indicating the participation of water as an intervening molecule. Several low-frequency modes (106, 150, 195 and 321 cm⁻¹) have been identified in our FSRS spectra and their possible role of facilitating ESPT at different stages is discussed with the assistance of computational chemistry. Similar low-frequency modes (108, 191, 321, 362 cm⁻¹) have been observed in pure water, and the intricate relationship between the dynamics of these modes further supports the essential role they play during ESPT. A comparison of the dynamics of key vibrational modes in H₂O and D₂O yields a kinetic isotope effect (KIE) of 3-4 on the 5-200 timescale. The merit of FSRS as a powerful tool of deciphering excited state dynamics, whose complexity roots in the multidimensional nature of the chemical potential energy surface, is demonstrated through these experiments.