Importance of Electrostatic Interactions in The Mobility of Cations in Nafion

Solvation and protonation of coumarin 102 in aqueous media: a fluorescence spectroscopic and theoretical study

The ground- and excited-state protonation of Coumarin 102 (C102), a fluorescent probe applied frequently in heterogeneous systems with an aqueous phase, has been studied in aqueous solutions by spectroscopic experiments and theoretical calculations. For the dissociation constant of the protonated form in the ground state, pKa = 1.61 was obtained from the absorption spectra; for the excited-state dissociation constant, pKa* = 2.19 was obtained from the fluorescence spectra. These values were closely reproduced by theoretical calculations via a thermodynamic cycle (the value of pKa* also by calculations via the Förster cycle) using an implicit–explicit solvation model (polarized continuum model + addition of a solvent molecule). The theoretical calculations indicated that (i) in the ground state, C102 occurs primarily as a hydrogen-bonded water complex, with the oxo group as the binding site, (ii) this hydrogen bond becomes stronger upon excitation, and (iii) in the ground state, the amino nitrogen atom is the protonation site, and in the excited state, the carboxy oxygen atom is the protonation site. A comprehensive analysis of fluorescence decay data yielded the values kpr = 3.27 × 10(10) M(–1) s(–1) for the rate constant of the excited-state protonation and kdpr = 2.78 × 10(8) s(–1) for the rate constant of the reverse process (kpr and kdpr were treated as independent parameters). This, considering the relatively long fluorescence lifetimes of neutral C102 (6.02 ns) and its protonated form (3.06 ns) in aqueous media, means that a quasi-equilibrium state of excited-state proton transfer is reached in strongly acidic solutions.

Unique effects of aerosol OT lamellar structures on the dynamics of guest molecules

The behavior of lamellar structures of Aerosol OT (AOT) as hosts, vis-à-vis the flexible normal micelles and rigid nanochannels of Nafion membranes, has been investigated with two different fluorophores, [2,2'-bipyridyl]-3,3'-diol (BP(OH)2) and coumarin 102 (C102). Surprisingly, for BP(OH)2, a rise time is observed at intermediate emission wavelengths and not in the red edge of the fluorescence spectrum. A shoulder at 525 nm is observed in time resolved emission spectra (TRES) at initial times of BP(OH)2 in AOT lamellar structures. This feature is the signature of the monoketo (MK) tautomer, observed for the first time in a microheterogeneous medium. Also, the usually ultrafast single proton transfer in BP(OH)2 is retarded to an considerable extent in lamellar structures. The potential of this medium in promoting unusual intermediates is thus highlighted. This property may be ascribed to the rigidity of lamellar structures, compared to hosts such as regular micelles. However, studies using another fluorophore, coumarin 102 (C102), brings out the fact that these structures are significantly different from the rigid host, Nafion, as well. The absence of excited state proton transfer (ESPT) in this molecule in AOT lamellar structures indicates that it is not protonated, unlike in Nafion. Thus, the interfacial pH of lamellar structures is found to be significantly greater than that of Nafion nanochannels. From the time dependent Stokes shift (TDSS) of the emission spectra of C102, the relaxation time (0.85 ns) of interfacial water in lamellar structures is found to be an order of magnitude faster than that observed in Nafion nanochannels, in which H3O(+) ions have been substituted by different cations. Hence, this study demonstrates that AOT lamellar structures are rather unique hosts and that they behave very differently from conventional rigid and flexible hosts such as normal micelles and Nafion, respectively.

Transport in supported polyelectrolyte brushes

Functional polymers have a wide variety of applications ranging from energy storage to drug delivery. For energy storage applications, desirable material properties include low cost, high charge storage and/or mobility, and low rates of degradation. Isotropic thin films have been used for many of these types of applications, but research suggests that different structures such as polymer brushes can improve charge transport by an order of magnitude. Supported polymer brush structures produced by "grafting-from" polymerization methods offer a framework for a controlled study of these materials on the molecular scale. Using these materials, researchers can study the basis of hindered diffusion because they contain a relatively homogeneous polyelectrolyte membrane. In addition, researchers can use fluorescent molecular probes with different charges to examine steric and Coulombic contributions to transport near and within polymer brushes. In this Account, we discuss recent progress in using fluorescence correlation spectroscopy, single-molecule polarization-resolved spectroscopy, and a novel three-dimensional orientational technique to understand the transport of charged dye probes interacting with the strong polyanionic brush, poly(styrene sulfonate). Our preliminary experiments demonstrate that a cationic dye, Rhodamine 6G, probes the brush as a counterion, and diffusion is therefore dominated by Coulombic forces, which results in a 10,000-fold decrease in the diffusion coefficient in comparison with free diffusion. We also support our experimental results with molecular dynamics simulations. Further experiments show that, up to 50% of the time, Rhodamine 6G translates within the brush without significant rotational diffusion, which indicates a strong deviation from Fickian transport mechanisms (in which translational and rotational diffusion are related directly through parameters such as chemical potential, size, solution viscosity, and thermal properties). To understand this oriented transport, we discuss the development of an experimental technique that allows us to quantify the three-dimensional orientation on the time scale of intrabrush transport. This method allowed us to identify a unique orientational transport direction for Rhodamine 6G within the poly(styrene sulfonate) brush and to report preliminary evidence for orientational dye "hopping".

Microheterogeneity in native and cation-exchanged Nafion membranes

2-(4'-Pyridyl)benzimidazole (4PBI) has been used to investigate the microheterogeneity of water nanochannels of Nafion membranes at two different hydration levels. Native as well as cation-exchanged Nafion membranes are found to protonate one of the two monoprotonated forms of 4PBI selectively. In native membranes and in those in which H(3)O(+) ions are replaced by (CH(3))(4)N(+) ions, the pyridyl nitrogen is protonated preferentially. In Na(+)-exchanged membranes, however, the benzimidazole nitrogen is protonated selectively. Unlike other fluorescent probes used in earlier studies, 4PBI can differentiate between the two different cation-exchanged membranes at lower as well as higher hydration levels.