Ultrafast Excited State Proton Transfer in the Condensed Phase and Nanoconfinement

Photoacids are studied by a combination of time-resolved and steady-state spectroscopic techniques in order to understand the driving forces responsible for excited state proton transfer and to probe environments in nanoscopic cavities where proton transfer reactions are important. The electronic properties of pyrene derivative photoacids, which are by far the most widely used photoacids for condensed phase research, are investigated to assign the electronic states from which excited state proton transfer takes place. A combination of MCD and fluorescence polarization spectroscopy is used to separate the overlapping 1La and 1Lb transitions of the lowest lying absorption band. The interesting spectral evolution of the photoacid pyranine in different solvents is shown to fit very well to a Brownian oscillator model. Ultrafast proton transfer dynamics of pyranine are studied by pump-probe spectroscopy to uncover the identity of an intermediate state formed in the reaction. Solvation dynamics are decoupled from proton transfer dynamics through a global spectral analysis. By comparison to other similar photoacids, the intermediate state is shown to be formed from an intramolecular charge transfer process. The charge transfer is believed to involve the acidic functional group of the photoacid donating electron density to the aromatic system. Intramolecular charge transfer occurs on a timescale significantly longer than general solvation and it is proposed to be triggered by specific rearrangements to the hydrogen bonding structure that facilitate the proton transfer process, which may not be favorable under normal equilibrium conditions. Differences in charge distribution between the ground and excited state, which are the ultimate driving forces for proton transfer in the excited state, are investigated with Stark spectroscopy. The amount of charge transfer occurring upon photo-excitation is shown to be highly dependent upon the electronic state (1La or 1L b) involved in the transition. It is also found that the amount of excited state charge transfer is similar when comparing the photoacid with its conjugate base so long as the transition is made to the same electronic state. Proton transfer in nanoconfinement is studied in AOT reverse micelle systems and Nafion fuel cell membranes. The size of the aqueous pools that form in these materials can be controlled by adjusting the water content. Changes to the hydrogen bonding network of water from confinement on a nanometer length scale can have dramatic effects on proton transfer reactions. Pyranine is incorporated into the two materials and excited state proton transfer kinetics and anisotropy relaxation dynamics are measured. The two systems demonstrate surprisingly similar behavior when the samples are prepared at the same hydration level. These results indicate that the local water environment the probe molecule samples is approximately the same size in both materials. The proton concentration profile is observed in the water pools of Nafion using photoacids and rhodamine-6G. The proton concentration at the edge of the water pools is calculated to be three times higher than the interior water region at maximum hydration. The proton concentration in the center of the water pools is shown to be much more sensitive to the membrane hydration than the proton concentration at the water interface region.