Excited state decay on semiconductor surfaces

Monitoring of the fluorescence of the dye could provide a direct evidence for operation of such a mechanism [49-59]. Muenter [49], for example, measured the fluorescence lifetime and quantum yields of two carbocyanine dyes in a photographically inert medium such as gelatin and in the adsorbed state on silver chloride, silver bromide microcrystals. Both the fluorescence lifetime and yield showed a large decrease upon change of the dye environment from that of gelatin to silver halides. Rate constants in the rate of 109-10 0 s-i were determined for the charge injection process. 

Willig and coworkers examined the fluorescence features and photocurrent yields of rhodamine B adsorbed on phenanthrene, anthracene and naphthalene crystals [50]. In this series of hydrocarbon crystals, the ionization energy increases and the charge injection process as monitored by photocurrent decreases. On anthracene crystals where the charge injection is 

Hashimoto et al. [56-58] followed the luminescence decay of Ru(bpy)32+ adsorbed onto Ti02 particles and found the luminescence to decay rapidly but in a complex manner. The luminescence behaviour was different in vacuo and upon introduction of water vapour. The non-exponential decay was fitted using a sum of four exponentials. The decay rates on Ti02 are considered to be determined mainly by electron transfer rates from excited Ru(bpy)32 to Ti02 by comparison of the decay curve on Ti02 to that on Si02 or porous vycor glass (PVG). 

The charge injection and recombination dynamics have been studied for various dyes on colloidal semiconductors using luminescence and laser photolysis in conjunction with time-resolved optical spectroscopy. The transparent/translucent nature ofthe finely dispersed colloids allow facile measurements of the luminescence of the sensitizer and the excited state quenching by the semiconductor [60-65]. Sensitization of Ti02 membrane films by the anionic porphyrin [tetrakis(p-carboxyphenyl)porphyrinato Zn(II)], ZnTPPC. Examination of the photophysical behaviour of the porphyrin in the presence of colloidal Ti02 allows identification of the excited state involved in the charge injection process. 

The above observations on the pH-dependence of the ZnP fluorescence lead to the conclusion that, in acidic solutions, the dye adsorbs onto Ti02 

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