Time-Dependent Fluorescence (TDF) in Nanoconfined Solvents

How are solvation dynamics different in a nanoconfined solvent than in a bulk solvent?

Time-dependent fluorescence measurements are often used to probe solvation dynamics - the same dynamics that can determine electron and proton transfer reaction rates.  In these experiments, described schematically below, a dye molecule (with a charge-transfer electronic transition) dissolved in the solvent is excited and the changing energy of the emitted fluorescence is monitored as a function of the time after excitation.  In the electronic excitation the nuclei of the solvent molecules are unchanged (according to the Franck-Condon principle) leaving the solvent molecule dipoles out of equilibrium with the new, excited-state charge distribution of the dye molecule.  The changes in the time-dependent fluorescence energy are thus due to solvent molecules responding to the change in the charge distribution on the dye molecule.

The results of such a TDF experiment provide quite direct information on the solvation dynamics. The measurements are particularly interesting in nanoconfined solvents where the solvent molecules are constrained by the tight quarters and interactions with the pore/cavity surface.  Among the key results of TDF measurements in nanoconfined solvents (mostly reverse micelles and sol-gels) are significantly longer time-scales and multiple time-scales compared to experiments in bulk liquids.

We are using theory and simulation to understand the solvation dynamics and model time-dependent fluorescence experiments in nanoconfined solvents.  Our past and current work includes:

  • Proposing and testing explanations for the multiple, long time-scales observed in the TDF measurements.  See J. Chem. Phys. 120, 8125 (2004).
  • Investigating the energetic and entropic driving forces for dye molecule positions in nanoconfining frameworks.  See J. Phys. Chem. C 111, 11991 (2007).
  • Testing and understanding linear-response approximations, an area where nanoconfined solvents are a promising, but underutilized, platform.  See J. Chem. Phys. 126, 211104 (2007) and J. Chem. Phys. 135, 084511 (2011).
  • Implementing and validating an approximate (Smoluchowski equation) approach for simulating TDF and using it to rapidly explore the general effects of framework and solvent properties.  See J. Phys. Chem. C 111, 18060 (2007) and J. Phys. Chem. C 114, 4279 (2010).

This work is ongoing in several respect. There remain many unanswered questions and fruitful avenues for exploration of both fundamental and applied importance.

Relevant References:

Brian B. Laird and Ward H. Thompson,
Journal of Chemical Physics 135, 084511 (2011). Abstract
"Time-Dependent Fluorescence in Nanoconfined Solvents: Linear Response Approximations and Gaussian Statistics"

Ward H. Thompson,
Annual Reviews of Physical Chemistry 62, 599-619 (2011). Abstract
"Solvation Dynamics and Proton Transfer in Nanoconfined Liquids"

Xiaobing Feng and Ward H. Thompson,
Journal of Physical Chemistry C 114, 4279-4290 (2010). Abstract
"Time-Dependent Fluorescence in Nanoconfined Solvents.  A Smoluchowski Equation Model Study"

Xiaobing Feng and Ward H. Thompson,
Journal of Physical Chemistry C 111, 18060-18072 (2007). Abstract
"Smoluchowski Equation Description of Solute Diffusion Dynamics and Time-Dependent Fluorescence in Nanoconfined Solvents"

Katie R. Mitchell-Koch and Ward H. Thompson,
Journal of Physical Chemistry C 111, 11991-12001 (2007). Abstract
"How Important is Entropy in Determining the Position-Dependent Free Energy of a Solute in a Nanoconfined Solvent?"

Brian B. Laird and Ward H. Thompson,
Journal of Chemical Physics 126, 211104 (2007). Abstract & Full Text
"On the Connection between Gaussian Statistics and Excited-State Linear Response for Time-Dependent Fluorescence"

J. A. Gomez and Ward H. Thompson,
Journal of Physical Chemistry B 108, 20144 - 20154 (2004). Abstract
"Monte Carlo Simulations of Absorption and Fluorescence Spectra in Ellipsoidal Nanocavities'"

Ward H. Thompson,
Journal of Chemical Physics 120, 8125-8133 (2004). Abstract & Full Text
"Simulations of Time-Dependent Fluorescence in Nano-Confined Solvents'"

Ward H. Thompson,
Journal of Chemical Physics 117, 6618-6628 (2002). Abstract & Full Text
"A Monte Carlo Study of Spectroscopy in Nano-Confined Solvents'"

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