Femtosecond to nanosecond transient absorption studies of aqueous solvation and deprotonation dynamics in confinement
Date
2011
Authors
Cole, Richard Leo, author
Levinger, Nancy E., advisor
Bernstein, E. R. (Elliot R.), committee member
Ladanyi, Branka M., committee member
Van Orden, Alan K., committee member
Bartels, Randy, committee member
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Abstract
We explore the use of logarithmic based optical delay in time-resolved data collection. We show that logarithmic spacing of data points provides an economical way to collect data over many decades of time which speeds data collection. We present a simple algorithm to generate time delay points for application in time-resolved data collection. We test the use of logarithmic vs. linear data collection over six orders of magnitude by measuring broadband femtosecond transient absorption (BFTA) spectra of HPTS in pH-7 water from femtoseconds to nanoseconds. Statistical analysis of logarithmic and linear data collection show that linear data collection shows a clear advantage by requiring a fewer number of time-delay points to achieve a given precision in subsequent data analysis. We investigate solvation dynamics (SD) via coumarin 343 (C343) in Aerosol OT (sodium bis(2-ethylhexyl) sulfosuccinate, AOT) reverse micelles with varying water content through broadband femtosecond transient absorption experiments. These studies build upon our previous studies of SD in the AOT reverse micelles through time-resolved fluorescence Stokes shift (TRFSS) experiments (J. Phys. Chem. B, 1998, 102, 2705) which limited data collection to approximately 100 ps. We extend the experimental time window to 2 nanoseconds and recover the entire solvation response. These results combined with steady-state spectra and reorientation dynamics indicate that C343 exists in two distinct populations within the reverse micelles which correlate with interfacial and core water. Our results suggest that translational motion of C343 may contribute to the total observed solvation response. We study excited state proton transfer (ESPT) of HPTS (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt) in cationic (cetyltrimethylammonium bromide, CTAB), anionic (AOT), and nonionic (polyoxyethylene (5) isooctylphenyl ether, IGE) reverse micelles by BFTA. For larger AOT RM, ESPT dynamics are found to be approximately equal to the dynamics found in bulk water. As the size of the AOT RM approaches the size of the probe molecule, ESPT becomes increasingly quenched. For all sizes of CTAB RM, HPTS ESPT is found to be 10-20 times slower than HPTS ESPT in bulk water. This result combined with reorientation measurements suggest that HPTS resides at the interfacial region in CTAB RM and thus remains immobilized. In IGE RM, ESPT is 4-10 times slower than bulk water behavior which we contribute to immobilization of HPTS in the micelle interface. HPTS reorientational motion is hindered with respect to bulk HPTS motion. We measure the kinetic isotope effect (KIE) on HPTS ESPT dynamics and results suggest that the solvent plays a significant role in the observed dynamics only in the largest IGE reverse micelles. Steady-state absorption measurements show that HPTS exists in a unique environment within IGE RM which contrasts with HPTS in other nonionic reverse micelle systems.
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Subject
proton transfer
ultrafast
transient absorption
solvation dynamics