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.

Platinum-acetylide-based π-conjugated polymers and oligomers have attracted interest because their photophysics are dominated by long-lived and phosphorescent 3 π,π* excited states. 1,2 This characteristic leads to materials that may be used to fabricate highefficiency organic electroluminescent devices, 3 and for applications in laser protection. 4 Considerable insight concerning excited-state structure and delocalization in organic π-conjugated systems has been acquired through the study of monodisperse oligomers. 5 Comparatively less information is available concerning the effect of delocalization on 3 π,π* states in conjugated systems, because these states are spectroscopically "silent" due to the forbidden character of the S 0 T T 1 transitions.Herein we report the preparation and photophysical characterization of the series of linear Pt-acetylide oligomers (PAOs) shown in Scheme 1. PAOs with n > 2 luminesce from the 1 π,π* and 3 π,π* manifolds of the Pt-acetylide π-conjugated system, which allows us to probe in detail the effect of oligomer length on the spectroscopy, energetics, and dynamics of the long-lived excited states.PAOs are synthesized by an iterative-convergent approach that relies on the use of Pt-acetylide building blocks in which terminal acetylenes are protected using the trimethylsilyl protecting group. 5 Complete details of the synthesis and spectral characterization of the PAOs are available as Supporting Information.Absorption spectra of the PAOs (Figure 1a) exhibit three wellresolved bands. The spectra are dominated by an intense band (I) arising from the long-axis polarized π,π* transition. The maximum of band I red-shifts with increasing PAO length (Table 1), but the difference in λ max between Pt-5 and Pt-7 is small, indicating that the effective conjugation length as probed by the Franck-Condon absorption event is ∼6 repeat units. The width of band I also increases with PAO length, possibly signaling the existence of conformers that differ with respect to the relative orientations of the chromophoric units (i.e., the phenylene rings and the plane defined by the PtP 2 R 2 units) along the chain. Band broadening may occur because the number of different conformers increases with PAO length. Two other absorption bands (II and III) are resolved at shorter wavelengths. The intensity of these bands increases with PAO length, but their absorption maxima do not vary systematically. This observation is consistent with the notion that the high-energy bands arise from short-axis polarized transitions localized on the phenylene rings. Importantly, the spectra of the longer PAOs are very similar to the absorption of the analogous Pt-acetylide polymer, 1a,b which indicates that the oligomers are effective models for the excited-state properties of the corresponding polymer. Figure 1b illustrates the photoluminescence spectra of the series of PAOs Pt-2-Pt-7 obtained in degassed THF solutions. (At room temperature, Pt-1 is a nonemissive compound.) Each of the PAOs exhibits a very weak fluorescence b...

Metal-free hydrides are of increasing research interest due to their roles in recent scientific advances in catalysis, such as hydrogen activation with frustrated Lewis pairs and electrocatalytic CO2 reduction with pyridinium and other aromatic cations. The structural design of hydrides for specific applications necessitates the correct description of their thermodynamic and kinetic prowess using reliable parameters - thermodynamic hydricity (ΔGH-) and nucleophilicity (N). This review summarizes reported experimental and calculated hydricity values for more than 200 metal-free hydride donors, including carbon-, boron-, nitrogen- and silicon-based hydrides. We describe different experimental and computational methods used to obtain these thermodynamic and kinetic parameters. Furthermore, tabulated data on metal-free hydrides are discussed in terms of structure-property relationships, relevance to catalysis and contemporary limitations for replacing transition-metal hydride catalysts. Finally, several selected applications of metal-free hydrides in catalysis are described, including photosynthetic CO2 reduction and hydrogen activation with frustrated Lewis pairs.

The success of solar fuel technology relies on the development of efficient catalysts that can oxidize or reduce water. All molecular water-oxidation catalysts reported thus far are transition-metal complexes, however, here we report catalytic water oxidation to give oxygen by a fully organic compound, the N(5)-ethylflavinium ion, Et-Fl(+). Evolution of oxygen was detected during bulk electrolysis of aqueous Et-Fl(+) solutions at several potentials above +1.9 V versus normal hydrogen electrode. The catalysis was found to occur on glassy carbon and platinum working electrodes, but no catalysis was observed on fluoride-doped tin-oxide electrodes. Based on spectroelectrochemical results and preliminary calculations with density functional theory, one possible mechanistic route is proposed in which the oxygen evolution occurs from a peroxide intermediate formed between the oxidized flavin pseudobase and the oxidized carbon electrode. These findings offer an organic alternative to the traditional water-oxidation catalysts based on transition metals.