Photoinduced processes electron injection

Photoinduced electron transfer reactions between surface bound dye molecules and semiconductor electrodes are important for practical as well as fundamental reasons. Absorption of light by the dye can improve the spectral response of the semiconductor and these systems are models for the photographic process (47-511. MDC surfaces are excellent substrates for studying electron injection into the conduction band of the semiconductor. 

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. 

Photoinduced electron injection is by no means a new development. This process has already been applied in areas such as silver halide photography. In this discussion, only sensitized TiC>2 surfaces will be considered. Many experiments have shown that the charge injection into the semiconductor surface is very fast. In order to study these processes, fast spectroscopic techniques are preferred. Whether or not charge injection takes place can be studied conveniently on the nanosecond time-scale by using transient absorption spectroscopy. However, to address the injection process directly, experiments are carried out on the femtosecond time-scale, while recombination and charge separation require the nanosecond to microsecond range. 

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. 

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. 

Figure 49. Some possible photoinduced electron transfer processes involving dyes adsorbed to the surface of crystalline silver halide. CB and VB refer to the conduction and valence bands of the silver halide, AgX, and HOMO and LUMO refer to the highest occupied and lowest unoccupied molecular orbitals of the sensitizing dye. a) Electron injection from the excited state of the dye b) hole injection from the excited state of the dye c) electron transfer to the valence band after excitation of the silver halide d) desensitization by an adsorbed dye.

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... 

The importance of carotenoids in photobiology is well known and there is a continuing interest in their photochemistry. Although they do not normally participate in PET processes because of the short lifetimes of their singlet states, incorporation into organized systems with suitable electron acceptors can lead to photoactivity. The dynamics of photoinduced electron injection and recombination between all-trans-8 -apo-p-caroten-8 -oic acid (125) and a Ti02 colloidal nanoparticle have been studied by means of transient absorption spectroscopy. " " An ultrafast ( 360 fs) electron injection from the initially excited S2 state of the carotenoic acid into the Ti02 conduction band with a quantum... 

Figure 1 Photoinduced charge transfer processes in semiconductor nanoclusters, (a) Under bandgap excitation and (b) sensitized charge injection by exciting adsorbed sensitizer (S). CB and VB refer to conduction and valence bands of the semiconductor and et and ht refer to trapped electrons and holes, respectively.

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