Sensitized photogeneration

Figure 5. The sensitized photogeneration model with electron transfer from photoexcited dye to hole transport molecule. (Reprinted with permission from Ref. [18d].)...

Microwave measurements are typically performed at frequencies between 8 and 40 Gc/s. The sensitivity with which photogenerated charge carriers can be detected in materials by microwave conductivity measurements depends on the conductivity of the materials, but it can be very high. It has been estimated that 109-1010 electronic charge carriers per cubic centimeter can be detected. Infrared radiation can, of course, also be used to detect and measure free electronic charge carriers. The sensitivity for such measurements, however, is several orders of magnitude less and has been estimated to be around 1015 electronic charge carriers per cubic centimeter.1 Microwave techniques, therefore, promise much more sensitive access to electrochemical mechanisms. 

Fig. 3.2a Electron transport in (natural) photosynthesis. P = chlorophyll that acts as a light sensitizer, from which a photogenerated electron travels to Q = Plastquinone that in combination with CO2 forms a carbohydrate. The photo-ejected electron from Peso is replenished by taking one from the Mn cluster through the redox active tyrosine linkage (or mediator), which in turn extracts an electron from water.

Only a limited number of reliable prediction tools are currently available for photoinduced toxicity. This is not surprising since establishing phototoxic potential is a complex task. Phototoxicity can be the consequence of various mechanisms such as photogeneration of reactive oxygen species, production of toxic photoproducts or sensitization of DNA damage by energy transfer. In addition, so far, there are no available universal descriptors (indicators) to predict the photodynamic potency of chemicals. 

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