Sensitization, excited states

Figure 1 Schematic representation of a Gratzel solar cell. Sub-band-gap light absorption leads to the formation of the sensitizer excited state, followed by electron injection into the conduction band of the high-area nanocrystalline semiconductor. The electrons can be drawn into a circuit to do useful work and returned to the system through the redox mediator, the I/Ij" couple, at the counterelectrode.

Based on extensive screening of hundreds of ruthenium complexes, we discovered that the sensitizer excited-state oxidation potential should be negative and at least - 0.9 V versus saturated calomel electrode (SCE), in order to inject electrons efficiently onto the TiO2 conduction band. The ground-state oxidation potential should be about 0.5 V versus SCE, in order to be regenerated rapidly via electron donation from the electrolyte (iodide/triiodide redox system) or an hole conductor. A significant decrease in electron-injection efficiencies will occur if the excited-and ground-state redox potentials are lower than these values. 

Both, strained and unsaturated organic molecules are known to form cation radicals as a result of electron transfer to photoexdted sensitizers (excited-state oxidants). The resulting cation radical-anion radical pairs can undergo a variety of reactions, including back electron transfer, nucleophilic attack on to the cation radical, electrophilic attack on the anion radical, reduction of anion radical, and addition of anion radical to the cation radical. This concept has been nicely demonstrated by Gassman et al. [103, 104], using the photoinduced electron-transfer cydization of y,8-unsatu-rated carboxylic add 232 to y-ladones 233 and 234 as an example (see Scheme 8.65). 

Quenching of the sensitizer excited state by a quencher molecule (Q) ... 

Competition between Type I and Type II photooxidations are affected by micellar media. Type I photooxidation involves initial quenching of the sensitizer excited state by substrate, while Type II photooxidations involve initial quenching of the sensitizer excited state by oxygen. Since, competition between Types I and II photooxidations are altered by the concentration of the substrate, local concentration effects in micelles play an important role. The photooxidation of tryptophan and tryptamine... 

Figure 1. Schematic representation of regenerative and photo-electrosynthetic cells. S is the sensitizer excited state, D is an electron donor, and D is the oxidized electron donor. Regenerative cells convert light into electricity while photoelectrosynthetic cells convert light into electricity and also produce chemical products.

In general, the absorption spectra of sensitizers bound to colloidal semiconductor films closely resemble those measured in fluid solution. In some cases small spectral shifts have been observed and attributed to Stark effects, acid-base chemistry or stabilization of the sensitizer excited states by the semiconductor surface. However, the effects are small, typically a few nanometers in the visible region. 

Figure 23. Proposal intermolecular energy migration between surface bound sensitizers. Excited states proximate to each other may annihilate and ultimately yield ground state products and heat.

Figure 32. The relative energetic positions of semiconductor conduction band edges, Ecb, for Sn02, ZnO, and Ti02 relative to a sensitizer excited state, S. AE is the apparent energy separation between the conduction band edge and the excited state sensitizers reduction potential.

Type I Substrate or solvent reacts with the sensitizer excited state (either singlet or triplet sens ) to give radicals or radical ions, respectively, by hydrogen atom or electron transfer, leading to oxygenated products. 

Since the energy of the sensitizer (excited state) is greater than the energy of the activator (excited state), the transition takes place only in the direction S A with the... 

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