Sensitizing dyes electrochemical potentials

The relations between structure and electrochemical potential are an important aspect in the study of dyes, since effective sensitizers require both the correct absorption wavelength and suitable electrochemical potentials. 

It has been necessary to understand the relationship between molecular fine structure of cyanine dyes and important properties such as colour, dye aggregation, adsorption on silver halide and electrochemical potentials in order to design and prepare sensitizers with optimum performance. For general discussion of these topics and the mechanism of spectral sensitization, the reader is referred to recent surveys on the subject (B-77MI11401, 77HC(30)441). 

The dye radical formed by reduction of the dye molecule would have an additional electron, would not have the same electronic configuration, and possibly not the same geometric configuration compared to the excited dye molecule. Moreover, the electrochemical measurements contain contributions from solvation energy differences between the parent dye and its reduced or oxidized radicals (43). These contributions do not appear in the dye s optical transition energy. In addition, many cyanine dyes undergo irreversible redox reactions in solution and the potentials, as commonly measured, are not strictly reversible. Nevertheless, Loutfy and Sharp (260) showed that the absorption maxima of more than 50 sensitizing dyes in solution conformed approximately to the equation... 

J. R. Lenhard, A New Method for Measuring Electrochemical Potentials of Cyanine Dyes, in "Preprints The International East-West Symposium on the Factors Influencing Photographic Sensitivity (Oct. 28-Nov. 2, 1984, Maui,... 

Figure 13. Schematic outline of a dye-sensitized photovoltaic cell, showing the electron energy levels in the different phases. The system consists of a semiconducting nanocrystalline Ti02 film onto which a Ru-complex is adsorbed as a dye and a conductive counterelectrode, while the electrolyte contains an I /Ij redox couple. The cell voltage observed under illumination corresponds to the difference, AF, between the quasi-Fermi level of Ti02 and the electrochemical potential of the electrolyte. S, S, and S+ designate, respectively, the sensitizer, the electronically excited sensitizer, and the oxidized sensitizer. See text for details. Adapted from [69], A Flagfeldt and M. Gratzel, Chem Rev. 95, 49 (1995). 1995, American Chemical Society.

Fig. 6 Schematic of DSSC operation for conversion of light into electrical energy, a Represents the different steps involved in energy conversion. The horizontal line represents the Fermi level/redox potential/electrochemical potential of the electron. Step 1 represents the regeneration of iodide by the reduction of tri-iodide at the counter electrode, step 2 diffusion of 1 to dye-sensitized electrode, step 3 restoration of dye to the original state through electron donation by the electrolyte, step 4 photoexcitation of the dye and the resulting injection into the conduction band on the semi-conductiong oxide, and finally, step 5 recombination, b Represents the dye molecule adsorbed on nanoparticles (based on appropriate semiconducting oxide) of the nanostructured electrode...

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. 

Recently, room temperature ionic liquids (RT-ILs) have attracted much attention for their excellent properties, e.g., wide temperature range of liquid phase, ultra-low vapor pressure, chemical stability, potential as green solvents, and high heat capacities [64,65]. These properties make them good candidates for the use in many fields, such as thermal storage [66], electrochemical applications, homogeneous catalysis [67], dye sensitized solar cells [68], and lubricants [69,70]. 

The adsorbed sensitizers in the excited state inject an electron into the conduction band of the semiconductor substrate, provided that the excited state oxidation potential is above that of the conduction band. The excitation of the sensitizer involves transfer of an electron from the metal t2g orbital to the 7r orbital of the ligand, and the photo-excited sensitizer can inject an electron from a singlet or a triplet electronically excited state, or from a vibrationally hot excited state. The electrochemical and photophysical properties of both the ground and the excited states of the dye play an important role in the CT dynamics at the semiconductor interface. 

The formal reduction potentials, E°, for a sensitizer, S, that are most relevant to dye sensitization correspond to the ground, reduced and excited states. The first two can be directly measured by electrochemical techniques, such as cyclic voltammetry, and often in situ at the sensitized electrode of interest [7]. The excited state reduc-... 

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