Sensitizing dyes energy levels

Based on correlations between energy level positions and electrochemical redox potentials, it has been estabHshed that polymethine dyes with reduction potentials less than —1.0 V (vs SCE) can provide good spectral sensitization (95). On the other hand, dyes with oxidation potentials lower than +0.2 V ate strong desensitizets. 

Fig. 21. Energy level diagram for a semiconductor photoanode sensitized by a dye (D) layer. The black arrow indicates the optical transition on the dye. The semiconductor is on the left-hand side...

Figure 4 Operating principles and energy level diagram of a dye-sensitized solar cell. S/S+/S = Sensitizer in the ground, oxidized and excited state, respectively. R/R = redox mediator (I3 / I-).

Two principal ways exist to use a dye as a sensor of local polarity (or of microscopic electric fields) (1) monitoring the polarity-induced shift of the energy levels, e.g., the red shift of the fluorescence and (2) monitoring changes in fluorescence intensity induced by the polarity- or field-induced modulation of nonradiative rates. As these compete with the fluorescence emission, the fluorescence intensity (and lifetime) is correspondingly modulated. (3) In some cases, the radiative rates are also solvent sensitive this is usually connected with the formation of luminescent products. 

Fig. 32 (a) Energy level diagram, (b) Four layers from bottom to top are Au, dye-sensitized heterojunction, compact T102, and graphene film. (Reprinted with permission from [267])... 

One method used to position the energy levels of the dyes relative to those of the silver halide depends on the measurement of ionization potentials of the dyes adsorbed by a substrate. Nelson (251,252) measured ionization potentials of several sensitizing dyes coated on a variety of substrates ... 

The pronounced dependence of the crossover potential on chemical sensitization, and particularly on the gaseous environment, shows that the crossover does not represent a division between dyes that can cause the appearance of conduction electrons in the silver halide and dyes that cannot. Instead, it represents an energy level determined by a kinetic balance between the formation and loss of photoelectrons and/or silver in the silver halide, a kinetic balance between sensitization and desensitization (259,265). One cancels the other and the net formation of latent image is zero. The actions of oxy-gen/moisture and of mobile holes are important sources of desensitization. 

For Class 3 dyes, both photoelectron and photohole injection can occur because of the distribution among the adsorbed molecules of the energy levels relative to the silver halide. Some molecules sensitize by the same mechanism as Class 1 dyes, others by the same mechanism as Class 2 dyes. 

The assumption that some dyes can spectrally sensitize latent image formation in silver halides by direct electron transfer from the excited dye to the conduction band and other dyes by indirect electron transfer from the dye radical formed by photoinjection of a hole into the valence band is in good accord with experiment. The locations of the highest filled and lowest vacant energy levels of the dye relative to the valence and conduction bands of the silver halide determine which mode of sensitization will occur, or whether both can occur. 

In order to achieve efficient sensitization of a nano-crystalline TiC>2 electrode, the sensitizer requires not only proper energy levels but also directionality for their excited states. The directionality should be arranged to provide an efficient electron transfer from the excited dye to the TiC>2 conduction band by good electronic... 

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