Photoinduced processes interfacial

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. 

Dye-sensitized semiconductors continue to be the focus of considerable research as a consequence of their importance in photovoltaic technologies, with the theories and mechanisms behind their operation also emerging. Considerably less is known about photoinduced interfacial processes in large photochemically active adsorbates on metals. ISAs on metals are becoming increasingly prevalent and are possible precursors for suitable molecular electronic devices. It would seem likely that over the coming years fundamental studies on the photophysics and chemistry of these materials will become more widespread. 

While transient photocurrent and photoinduced discharge techniques are the conventional methods for measuring photogeneration efficiencies, these cannot be readily employed in the presence of trapping. A further limitation is that it is difficult to separate the field dependence of the photogeneration process from field dependencies associated with an injection or interfacial process (Seki, 1970, 1972a). As such, it is difficult to apply the method to dispersions or two-phase materials. Experimental methods that may avoid these limitations are... 

The timescales of the processes occurring after photo-generation of an electron hole pair can be shorter or longer than the RC time constant of the photo-electrochemical cell, Tceii- In many cases, the time scale for interfacial processes at a semiconductor/electrolyte junction is longer than Xceit, whereas electron-hole separation and electron transit through the depletion layer are much faster. The apparatus used to probe electron-hole separation is therefore different from that used to study the kinetics of photoinduced interfacial processes. 

Functionalized polyelectrolytes are promising candidates for photoinduced ET reaction systems. In recent years, much attention has been focused on modifying the photophysical and photochemical processes by use of polyelectrolyte systems, because dramatic effects are often brought about by the interfacial electrostatic potential and/or the existence of microphase structures in such systems [10, 11], A characteristic feature of polymers as reaction media, in general, lies in the potential that they make a wider variety of molecular designs possible than the conventional organized molecular assemblies such as surfactant micelles and vesicles. From a practical point of view, polymer systems have a potential advantage in that polymers per se can form film and may be assembled into a variety of devices and systems with ease. 

These three equations (11), (12), and (13) contain three unknown variables, ApJt kn and sr The rest are known quantities, provided the potential-dependent photocurrent (/ph) and the potential-dependent photoinduced microwave conductivity are measured simultaneously. The problem, which these equations describe, is therefore fully determined. This means that the interfacial rate constants kr and sr are accessible to combined photocurrent-photoinduced microwave conductivity measurements. The precondition, however is that an analytical function for the potential-dependent microwave conductivity (12) can be found. This is a challenge since the mathematical solution of the differential equations dominating charge carrier behavior in semiconductor interfaces is quite complex, but it could be obtained,9 17 as will be outlined below. In this way an important expectation with respect to microwave (photo)electro-chemistry, obtaining more insight into photoelectrochemical processes... 

The combination of photocurrent measurements with photoinduced microwave conductivity measurements yields, as we have seen [Eqs. (11), (12), and (13)], the interfacial rate constants for minority carrier reactions (kn sr) as well as the surface concentration of photoinduced minority carriers (Aps) (and a series of solid-state parameters of the electrode material). Since light intensity modulation spectroscopy measurements give information on kinetic constants of electrode processes, a combination of this technique with light intensity-modulated microwave measurements should lead to information on kinetic mechanisms, especially very fast ones, which would not be accessible with conventional electrochemical techniques owing to RC restraints. Also, more specific kinetic information may become accessible for example, a distinction between different recombination processes. Potential-modulation MC techniques may, in parallel with potential-modulation electrochemical impedance measurements, provide more detailed information relevant for the interpretation and measurement of interfacial capacitance (see later discus-... 

Photocyanations rely on photoinduced electron transfer [29]. This was demonstrated by monitoring cyanation yields as a function of the droplet size for oil-in-water emulsions. Hence increase in interfacial area is one driver for micro-channel processing. Typically, fluid systems with large specific interfacial areas tend to be difficult to separate and solutions for more facile separation are desired. 

Between 0.20 and 0.30 V, a decay of the initial photocurrent and a negative overshoot after interrupting the illumination are developed. This behavior resembles the responses observed at semiconductor-electrolyte interfaces in the presence of surface recombination of photoinduced charges [133-135] but at a longer time scale. These features are in fact related to the back-electron-transfer processes within the interfacial ion pair schematically depicted in Fig. 11. 

Figure 5 Photoinduced direct and remote charge-injection process leading to a common interfacial charge-separated state.

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. 

The Marcus theory is the most widely applied theory used to describe electron transfer reactions and is equally applicable to photoinduced, interfacial and thermally driven electron transfers. The fundamental difference between these processes lies primarily in the nature of the driving force. In the case of heterogeneous electron... 

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. 

After photoinduced electron injection, the strong interfacial electric field at the semiconductor solid-liquid junction draws the injected electron (or hole) into the semiconductor and towards the electrical contact. This process facilitates charge separation and reduces the chances of hole-electron recombination. The most important prerequisites for this process include, as described, good redox matching of the dye species excited state and the conduction band of the semiconductor, as well as strong orbital coupling between the immobilized dye and semiconductor. 

In the following, factors external to the actual interfacial supramolecular assembly, which are capable of modifying the photoinduced electron injection process, are considered. This discussion will concentrate on how the rate of charge injection can be manipulated by changing the composition of the electrolyte and by changing the external potential applied to the semiconductor film. 

In spite of a great number of investigations aimed at the preparation of photocatalysts and photoelectrodes based on the semiconductors surface-modified with metal nanoparticles, many factors influencing the photoelectrochemical processes under consideration are not yet clearly understood. Among them are the role of electronic surface (interfacial) states and Schottky barriers at semiconductor / metal nanoparticle interface, the relationship between the efficiency of photoinduced processes and the size of metal particles, the mechanism of the modifying action of such nanoparticles, the influence of the concentration of electronic and other defects in a semiconductor matrix on the peculiarities of metal nanophase formation under different conditions of deposition process (in particular, under different shifts of the electrochemical surface potential from its equilibrium value), etc. 

Kinetic analysis of photoinduced interfacial charge transfer processes in colloidal semiconductor systems is frequently complex. Indeed, it is diffi-... 

The long effective pathlength and high surface area afforded by these colloidal semiconductor materials allow spectroscopic characterization of interfacial electron transfer in molecular detail that was not previously possible. It is likely that within the next decade photoinduced interfacial electron transfer will be understood in the same detail now found only in homogeneous fluid solution. In many cases the sensitization mechanisms and theory developed for planar electrodes" are not applicable to the sensitized nanocrystalline films. Therefore, new models are necessary to describe the fascinating optical and electronic behavior of these materials. One such behavior is the recent identification of ultra-fast hot injection from molecular excited states. Furthermore, with these sensitized electrodes it is possible to probe ultra-fast processes using simple steady-state photocurrent action spectrum. 

These observations of an excitation wavelength dependence of the charge injection process show that photoinduced interfacial electron transfer from a molecular excited state to a continuum of acceptor levels can take place in competition with the relaxation from upper excited levels. The rather slow growth of the injection... 

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