Interfacial proton transfer

Elementary interfacial proton transfer and configuration energy along the reaction path, identified in ab initio calculations. 

Based on the oriented dipole and the interfacial proton transfer mechanism described above, an equivalent circuit was established which described the relaxation time course of a photoelectric current generated by a single chemical reaction step of charge separation and recombination regardless of whether the charge separation is confined within the membrane or takes place across a membrane-water interface [21]. This equivalent-circuit analysis is notable for the absence of any adjustable parameters each and every parameter used for the computation can be measured experimentally (figure 10.2). An example of the... 

Figure 1. Two types of photoinduced charge separation (upper diagrams). The interfacial proton transfer (IPT) mechanism applies both to cytoplasmic proton binding and extracellular proton release at the membrane surface (only proton binding is shown). The oriented dipole (OD) mechanism applies to charge separation inside the membrane (or rather, inside bacteriorhodopsin). The thick curve across the membrane shows the space charge density profile, which, together with the potential profile across the membrane (not shown), allows us to deduce the two microscopic equivalent circuits shown in the lower diagrams. The two slightly different microscopic equivalent circuits are equivalent to the same irreducible equivalent circuit. (Reproduced with permission from reference...

The interfacial proton transfer reactions responsible for both the B2 and the B2 component are bimolecular reactions. According to the law of mass action, the relaxation of the photosignal is dependent on three factors ... 

There is httle doubt that fast photoelectric signals are electrical manifestation of Hght-induced charge separation and recombination. For a macromolecule with a multistep reaction sequence as complex as rhodopsin or bacteriorhodopsin, there are many candidates for generating a fast photoelectric signal component. In principle, each step of reversible reaction can contribute a component. Fast photokinetic measurements have a tendency to pick up faster processes. We identified three fast components in reconstituted bacteriorhodopsin membranes Bl, B2, and a B2-like signal, which we called B2 component. Like B2, the B2 component is generated by interfacial proton transfer — the proton release and reuptake at the extracellular surface. This does not mean that slower components do not exist. In fact. 

The catalyst layer is located between the PEM and the gas diffusion layer (GDL). Protons transfer between the CL and the PEM, and electrons transfer between the catalyst layer and the GDL. Both require good interfacial contact. 

Molecular modeling of PT at dense interfacial arrays of protogenic surface groups in PEMs needs ab initio quantum mechanical calculations. In spite of fhe dramafic increase in computational capabilihes, it is still "but a dream" to perform full ab initio calculations of proton and water transport within realistic pores or even porous networks of PEMs. This venture faces two major obstacles structural complexity and the rarity of proton transfer events. The former defines a need for simplified model systems. The latter enforces the use of advanced compufahonal techniques that permit an efficient sampling of rare evenfs. ... 

Picosecond time-resolved total internal reflection fluorescence spectroscopy was applied to analyze the proton-transfer reaction of INpOH in water-sapphire interface layers [206], The rate constant of the proton-transfer reaction from excited neutral species became slow in the interface layer as compared with that in the bulk aqueous solution and decreased smoothly with increasing penetration depth in the interfacial layer up to 100 nm. The anomaly was interpreted in terms of rotational fluctuations of water aggregates in the interface layer. 

Election transfer remains one of the most important processes explored when using interfacial supramolecular assemblies and given the emerging area of molecular electronics, this trend is set to continue. Therefore, Chapter 2 outlines the fundamental theoretical principles behind the electiochemically and photochemi-cally induced processes that are important for interfacial supramolecular assemblies. In that chapter, homogeneous and heterogeneous electron transfer, photoinduced proton transfer and photoisomerizations are considered. 

Interfacial monolayer, multilayer and polymer species which exhibit interesting examples of light and electrically stimulated functions such as isomerization and proton transfer in ISAs are also presented in this chapter. Such materials may represent the precursors for electrooptic switches and addressable molecular-based machines. 

Electrochemical and photochemical processes are the most convenient inputs and outputs for interfacial supramolecular assemblies in terms of flexibility, speed and ease of detection. This chapter provides the theoretical background for understanding electrochemical and optically driven processes, both within supramolecular assemblies and at the ISA interface. The most important theories of electron and energy transfer, including the Marcus, Forster and Dexter models, are described. Moreover, the distance dependence of electron and energy transfer are considered and proton transfer, as well as photoisomerization, are discussed. 

Electron, energy and proton transfer or molecular rearrangements are the most important events that occur in interfacial supramolecular assemblies. In this chapter, the general theories of electron transfer, both within ISAs and across the film/electrode interface, are described. Moreover, photoinduced electron, energy and proton transfer processes are discussed. As this book focuses on supramolecular species, the treatment is restricted to intramolecular or interfacial processes without the requirement for prior diffusion of reactants. 

The most prevalent photoinduced processes in supramolecular and interfacial systems are electron transfer, energy transfer and nuclear motion, such as proton transfer and isomerization. Before discussing these processes, it is important to outline the fundamental properties of electronically excited states. 

Like proton transfer, photoisomerization is a fundamentally important photochemical process. The two most important forms of photoisomerization are valence isomerization and stereoisomerization. The latter is probably the most common photoinduced isomerization in supramolecular chemistry. It may occur in systems in which the photoactive component has unsaturated bonds which can be excited, and this effect may be exploited for optical switching applications. A number of interfacial supramolecular complexes capable of undergoing cis-trans photoisomerization have been studied from this perspective - some examples are outlined in Chapter 5. 

These interfacial pH effects have been investigated by probing the voltammetry in buffered solutions. Figure 5.16 shows that for 1.0 < pFl < 10.6, E0 depends linearly on pH, with a slope of 63 3 mV. This value is indistinguishable from the slope of 59 mV pH-1 expected for a coupled proton/electron transfer and indicates that the H2Q species is produced when the monolayer is reduced. Between pH 10.6 and 12.0, the slope decreases to 25 4 mV pH-1, which compares favorably with the slope expected (29.5 mV pH-1) for a two-electron, one-proton transfer reaction. Therefore, over this pH range Q is reduced to HQ- and the p/12.0, E0 is independent of pH, thus indicating that the pKa of the HQ-/Q2- couple is 12.0 0.2. 

The main reason of appearance of exponential multiplier eq in Eqs. (2.7) and (2.8) for the rate of interfacial electron transfer from semiconductor particles to dissolved oxygen molecules or protons seems to the electric charge of these electrons. This causes considerable changes in the potential of the nanoparticles double layer, which, in turn, affects directly the rate of electron transfer [12-15]. 

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