ZnTPPC potential dependence

The potential dependent capacitance as estimated from Equations (11.17)-(11.22) for a molecule charged -4 (as in the case of ZnTPPC in neutral aqueous phase) is exemplified in Figure 11.13. The Gibbs energy of adsorption was taken as a constant value of A Gads = -38 kJ mol and the maximum surface concentration was F = 10 mol This simplified model predicts an increase... 

Figure 11.14. Capacitance-potential curves for various concentrations of ZnTPPC (a) and Cu-chlorophyllin (b) at the water/DCE interface. The changes in the potential dependence of the capacitance, as well as the shift of the potential of minimum capacitance reflect the specific adsorption of the water-soluble porphyrin derivatives. Figure (a) reprinted with permission from ref.[95]. Copyright (2003) American Chemical Society. Figure (b) reproduced from ref.[87] by permission of the Royal Society of Chemistry.

The potential dependence of the ZnTPPC coverage illustrates the typical trend observed for other anionic metalloporphyrin and chlorins as those shown in Figure 11.4. For instance, studies based on potential modulated fluorescence and capacitance curves have shown that ZnTPPS speciflcally adsorbs at the... 

Figure 11.19. Potential dependence of the ZnTPPC orientation with respect to the normal of the water/lXI E interface at various concentrations as estimated from Equation (11.34) and the results on Eigure 11.18(a). Reprinted with permission form ref. [35]. Copyright (2(X)3) American Chemical Society. Orientation dependence of SnTPPC (b), chlorin e-6 and protoIX (c) as a function of the bulk concentration of the photoactive species in the water phase. Similarly to the case of ZnTPPC, a substantial dependence of the SnTPPC orientation on the applied potential is observed. On the other hand, the orientation of chlorin e-6 and protoIX is effectively independent of the concentration and Galvani potential difference. Reprinted from ref.[38] with permission from Elsevier Science.

In part I of the series, it was demonstrated that photocurrent responses associated with the heterogeneous quenching of water-soluble ZnTPPC by ferrocene and DFCET are potential dependent, proportional to light intensity, and that the action spectrum followed the absorption spectrum of the porphyrin [220]. 

In part II, the photoresponses for the heterogeneous quenching of ZnTPPC by ferrocene derivatives were studied by intensity-modulated photocurrent spectroscopy (IMPS). The different contributions, that is, the electron injection, the recombination-product separation competition, and the attenuation due to the uncompensated resistance and interfacial capacitance (RC) time constant of the cell were deconvoluted in the frequency domain. The flux of electron injection was described as a competition between the relaxation of the porphyrin-excited state and the electron-transfer step. Experimental results confirmed that the electron-transfer rate increases with the Galvani potential difference (Butler-Volmer behavior), but the ZnTPPC coverage was potential-dependent. 

FIG. 20 Photocurrent dependence on the concentration of the porphyrin ZnTPPC" at the water-DCE interface in the presence of Fc at various Galvani potential differences. (Reprinted with permission from Ref 50. Copyright 1998 American Chemical Society.)... 

The behaviour observed for asymmetrically derivatised porphyrins such as chlorin e-6, protoIX and Cu-chlorophyllin is remarkably different to the ZnTPPC and SnTPPC cases. The results in Figure 11.19c indicate that the orientation of chlorin e-6 and protoIX are effectively independent of the bulk concentration of the porphyrin. Furthermore, no dependence on the Galvani potential difference was observed either. These results appear to indicate that lateral interactions between the adsorbed species do not play an important role in the molecular orientation of these species. The asymmetric distribution of the carboxyl groups around the chromophore ring appears to introduce amphiphillic properties to these dyes. This interpretation is consistent with the fact that chlorin e-6 and protoIX exhibit a more upright orientation than SnTPPC ... 

The photocurrent dependence on the porphyrin concentration is illustrated in Figure 11.22 for ZnTPPC " and the ion pair ZnTPPS-ZnTMPyP. In these experiments, the photo-oxidation of ferrocene derivatives were examined under the conditions where responses associated with back electron transfer are negligible. For both set of results, it is observed that the photocurrent increases with increasing Galvani potential difference. The lines also indicate that the concentration dependence can be rationalised in terms of expression (11.46). In Figure 11.22a, the photocurrent responses exhibit a stronger dependence on the ZnTPPC... 

Figure 4 shows the relative disposition of redox potentials of the anionic porphyrin ZnTPPC referred to earlier, with resepct to the valence and conduction band edges of Ti02. As an amphoteric oxide, the band edge positions are pH-dependent and shown are values at two pH values. Both the singlet and triplet excited states can, in principle, inject charges into the... 

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