Proton transfer and proton concentrations in protonated Nafion fuel cell membranes.

Proton transfer in protonated Nafion fuel cell membranes is studied using several pyrene derivative photoacids. Proton transfer in the center of the Nafion nanoscopic water channels is probed with the highly charged photoacid 8-hydroxypyrene-1,3,6-trisulfonate (HPTS). At high hydration levels, both the time-integrated fluorescence spectrum and the fluorescence kinetics of HPTS permit the determination of hydronium concentration of the interior of the water pools in Nafion. The proton transfer kinetics of HPTS in protonated Nafion at maximum hydration are identical to the kinetics displayed by HPTS in a 0.5 M HCl solution. The hydronium concentration near the water interface in Nafion is estimated with rhodamine-6G to be 1.4 M. Excited state proton transfer (ESPT) is followed in the nonpolar side chain regions of Nafion with the photoacid 8-hydroxy-N,N,N',N',N'',N''-hexamethylpyrene-1,3,6-trisulfonamide (HPTA). Excited state proton transfer of HPTA is possible in protonated Nafion only at the highest hydration level due to a relatively high local pH.

Charge transfer in photoacids observed by stark spectroscopy.

The charge redistribution upon photoexcitation is investigated for a series of pyrene photoacids to better understand the driving force behind excited-state proton-transfer processes. The changes in electric dipole for the lowest two electronic transitions ( (1)L b and (1)L a) are measured by Stark spectroscopy, and the magnitudes of charge transfer of the protonated and deprotonated states are compared. For neutral photoacids studied here, the results show that the amount of charge transfer depends more upon the electronic state that is excited than the protonation state. Transitions from the ground state to the (1)L b state result in a much smaller change in electric dipole than transitions to the (1)L a state. Conversely, for the cationic (ammonium) photoacid studied, photoexcitation of a particular electronic state results in much smaller charge transfer for the protonated state than for the deprotonated state.

Charge redistribution and photoacidity: neutral versus cationic photoacids.

A series of pyrene photoacids is used to investigate excited-state proton transfer with time-dependent pump-probe spectroscopy. The deprotonation dynamics of a cationic photoacid, 8-aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS), shows single exponential dynamics( approximately 30 ps) in water. This is in contrast to what is observed for the neutral photoacids 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) and 8-hydroxy-N,N,N',N',N",N"-hexamethylpyrene-1,3,6-trisulfonamide, which display biexponential dynamics. For the cationic photoacid, the vast majority of the intramolecular charge redistribution does not occur in the protonated state. Instead, the charge redistribution, which is responsible for the photoacidity and the observed spectroscopic changes, occurs primarily following the excited-state proton transfer. The lack of charge redistribution prior to proton transfer causes APTS to display single exponential kinetics. In contrast, the dynamics for the neutral photoacids are multiexponential because major charge redistribution precedes proton transfer followed by additional charge redistribution that accompanies proton transfer. Previous studies of HPTS in water are discussed in terms of the results presented here.

Water dynamics and proton transfer in nafion fuel cell membranes.

The dynamics of water and its effect on proton transport kinetics in Nafion membranes are compared at several hydration levels. Nafion is the most widely used polyelectrolyte membrane in fuel cells. Ultrafast infrared spectroscopy of the O-D stretch of dilute HOD in H2O provides a probe of the local environment and hydrogen bond network dynamics of water confined in the hydrophilic regions of Nafion. The kinetics of proton transfer in Nafion are tracked by following the excited-state proton transfer and recombination kinetics of a molecular probe, pyranine (HPTS). The hydrophilic domains of Nafion grow with increased hydration, and the interfacial regions reorganize, leading to a changing local environment for water near the interface. Swelling is not uniform throughout the membrane, and heterogeneity is observed in the fluorescence anisotropy decays of the methoxy derivative of pyranine. Measurements of the time-dependent anisotropy of water in Nafion provide a direct probe of the hydrogen bond network dynamics. These dynamics, as well as the rate of proton transport over nanoscopic distances, are observed to slow significantly as the hydration level of the membrane decreases. The results provide insights into the influence of changes in the dynamics of water on the proton-transfer processes.

Observation of slow charge redistribution preceding excited-state proton transfer.

The photoacid 8-hydroxy-N,N,N',N',N',N'-hexamethylpyrene-1,3,6-trisulfonamide (HPTA) and related compounds are used to investigate the steps involved in excited-state deprotonation in polar solvents using pump-probe spectroscopy and time correlated single photon counting fluorescence spectroscopy. The dynamics show a clear two-step process leading to excited-state proton transfer. The first step after electronic excitation is charge redistribution occurring on a tens of picoseconds time scale followed by proton transfer on a nanosecond time scale. The three states observed in the experiments (initial excited state, charge redistributed state, and proton transfer state) are recognized by distinct features in the time dependence of the pump-probe spectrum and fluorescence spectra. In the charge redistributed state, charge density has transferred from the hydroxyl oxygen to the pyrene ring, but the OH sigma bond is still intact. The experiments indicate that the charge redistribution step is controlled by a specific hydrogen bond donation from HPTA to the accepting base molecule. The second step is the full deprotonation of the photoacid. The full deprotonation is clearly marked by the growth of stimulated emission spectral band in the pump-probe spectrum that is identical to the fluorescence spectrum of the anion.

Confinement or the nature of the interface? Dynamics of nanoscopic water.

The dynamics of water confined in two different types of reverse micelles are studied using ultrafast infrared pump-probe spectroscopy of the hydroxyl OD stretch of HOD in H2O. Reverse micelles of the surfactant Aerosol-OT (ionic head group) in isooctane and the surfactant Igepal CO 520 (nonionic head group) in 50/50 wt % cyclohexane/hexane are prepared to have the same diameter water nanopools. Measurements of the IR spectra and vibrational lifetimes show that the identity of the surfactant head groups affects the local environment experienced by the water molecules inside the reverse micelles. The orientational dynamics (time-dependent anisotropy), which is a measure of the hydrogen bond network rearrangement, are very similar for the confined water in the two types of reverse micelles. The results demonstrate that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.

Proton transport and the water environment in nafion fuel cell membranes and AOT reverse micelles.

The properties of confined water and diffusive proton-transfer kinetics in the nanoscopic water channels of Nafion fuel cell membranes at various hydration levels are compared to water in a series of well-characterized AOT reverse micelles with known water nanopool sizes using the photoacid pyranine as a molecular probe. The side chains of Nafion are terminated by sulfonate groups with sodium counterions that are arrayed along the water channels. AOT has sulfonate head groups with sodium counterions that form the interface with the reverse micelle's water nanopool. The extent of excited-state deprotonation is observed by steady-state fluorescence measurements. Proton-transfer kinetics and orientational relaxation are measured by time-dependent fluorescence using time-correlated single photon counting. The time dependence of deprotonation is related to diffusive proton transport away from the photoacid. The fluorescence reflecting the long time scale proton transport has an approximately t-0.8 power law decay in contrast to bulk water, which has a t-3/2 power law. For a given hydration level of Nafion, the excited-state proton transfer and the orientational relaxation are similar to those observed for a related size AOT water nanopool. The effective size of the Nafion water channels at various hydration levels are estimated by the known size of the AOT reverse micelles that display the corresponding proton-transfer kinetics and orientational relaxation.

Deprotonation dynamics and stokes shift of pyranine (HPTS).

The short and intermediate time scale dynamics of the photoacid pyranine (1-hydroxy-3,6,8-pyrenetrisulfonic acid, commonly referred to as HPTS) are studied with visible pump-probe spectroscopy in various solvents to elucidate the nature of its proton-transfer kinetics in water. The observed time dependences of HPTS are compared with those of the methoxy derivative, MPTS. A global fitting procedure is employed to model both the spectral shift (Stokes shift) caused by solvent reorganization and deprotonation of pyranine in water. Three distinct time-dependent features can be clearly identified. They are the Stokes shift (1 ps in H(2)O and 1.5 ps in D(2)O), followed by the deprotonation processes, which gives rise to a biexponential decay of the protonated species with time constants (in H(2)O) of 3 and 88 ps. By the use of a model previously discussed in the literature, the biexponential process can be interpreted as an initial deprotonation step followed by the longer time scale process which separates the resulting ion pair. The results presented here are consistent with some of the previous reports but unambiguously identify and quantitatively measure the Stokes shift as a separate and distinct phenomenon from the deprotonation process, in contrast to other reports that have suggested that all short time (a few picoseconds) dynamics are merely a Stokes shift.

Identification and properties of the 1La and 1Lb states of pyranine.

The spectroscopic locations of the 1La and 1Lb electronic states of pyranine (1-hydroxy-3,6,8-pyrenetrisulfonic acid, commonly referred to as HPTS), as well as several related compounds, are found using magnetic circular dichroism spectroscopy as well as absorption and fluorescence spectroscopies. These electronic states have been discussed in connection with the photoacid properties of HPTS. Polarization selective fluorescence spectroscopy is used to identify the transition dipole directions of the electronic states of the compounds studied. The issue of the origin for the changes in vibronic structure of HPTS in different solvents is addressed. It is demonstrated that a Brownian oscillator model, in which the strength of the coupling of the electronic states to the solvent changes with solvent, is sufficient to reproduce the trends in the shapes of the vibronic structure.

Testing the core/shell model of nanoconfined water in reverse micelles using linear and nonlinear IR spectroscopy.

A core/shell model has often been used to describe water confined to the interior of reverse micelles. The validity of this model for water encapsulated in AOT/isooctane reverse micelles ranging in diameter from 1.7 to 28 nm (w0 = 2-60) and bulk water is investigated using four experimental observables: the hydroxyl stretch absorption spectra, vibrational population relaxation times, orientational relaxation rates, and spectral diffusion dynamics. The time dependent observables are measured with ultrafast infrared spectrally resolved pump-probe and vibrational echo spectroscopies. Major progressive changes appear in all observables as the system moves from bulk water to the smallest water nanopool, w0 = 2. The dynamics are readily distinguishable for reverse micelle sizes smaller than 7 nm in diameter (w0 = 20) compared to the response of bulk water. The results also demonstrate that the size dependent absorption spectra and population relaxation times can be quantitatively predicted using a core-shell model in which the properties of the core (interior of the nanopool) are taken to be those of bulk water and the properties of the shell (water associated with the headgroups) are taken to be those of w0 = 2. A weighted sum of the core and shell components reproduces the size dependent spectra and the nonexponential population relaxation dynamics. However, the same model does not reproduce the spectral diffusion and the orientational relaxation experiments. It is proposed that, when hydrogen bond structural rearrangement is involved (orientational relaxation and spectral diffusion), dynamical coupling between the shell and the core cause the water nanopool to display more homogeneous dynamics. Therefore, the absorption spectra and vibrational lifetime decays can discern different hydrogen bonding environments whereas orientational and spectral diffusion correlation functions predict that the dynamics are size dependent but not as strongly spatially dependent within a reverse micelle.

Superfluidity in CH4-doped H2 nanoclusters.

We report a theoretical study of superfluidity in CH(4)-doped para-H(2) nanoclusters. Path integral simulations for clusters of 12-16 H(2) around a single CH(4) molecule were carried out at temperatures between 0.5 and 2 K to study the superfluid response of the cluster. The results indicate that a rapid increase in the superfluid response is expected to occur around 1 K. We analyzed the structures and statistics of these clusters and found that the larger permutation cycles which dominate the superfluid component tend to adopt ringlike structures on the surface of the CH(4) molecule.