Porphyrins heterodimers

As mentioned earlier, a great deal of literature has dealt with the properties of heterogeneous liquid systems such as microemulsions, micelles, vesicles, and lipid bilayers in photosynthetic processes [114,115,119]. At externally polarizable ITIES, the control on the Galvani potential difference offers an extra variable, which allows tuning reaction paths and rates. For instance, the rather high interfacial reactivity of photoexcited porphyrin species has proved to be able to promote processes such as the one shown in Fig. 3(b). The inhibition of back ET upon addition of hexacyanoferrate in the photoreaction of Fig. 17 is an example of a photosynthetic reaction at polarizable ITIES [87,166]. At Galvani potential differences close to 0 V, a direct redox reaction involving an equimolar ratio of the hexacyanoferrate couple and TCNQ features an uphill ET of approximately 0.10 eV (see Fig. 4). However, the excited state of the porphyrin heterodimer can readily inject an electron into TCNQ and subsequently receive an electron from ferrocyanide. For illumination at 543 nm (2.3 eV), the overall photoprocess corresponds to a 4% conversion efficiency. 

Quantum Mechanical Studies of Charge-Transfer States in Porphyrin Heterodimers... 

Fig. 11 Co corrole-Fe/Mn porphyrin heterodimers. Ring substituents removed for clarity...

FIG. 17 Reversal of the photocurrent sign upon replacing the electron donor DCMFc (a) by the electron acceptor TCNQ (b) in the presence of the porphyrin heterodimer ZnTMPyP-ZnTPPS at the water-DCE interface. The strong back electron-transfer features in the photoreduction of TCNQ were diminished upon addition of an equimolar ratio of ferri/ferrocyanide acting as supersensitizer in the aqueous phase (b). The mechanism of supersensitization is described in Fig. 11. From the potential relationship between these redox couples (Fig. 4), these phenomena can be regarded as interfacial photosynthetic processes as defined in Fig. 3(b). (Reprinted with permission from Ref. 87. Copyright 1999 American Chemical Society and from Ref. 166 with permission from Elsevier Science.)... 

Figure 21. Temporal response of photocurrent density to illuminaion obtained with the zinc porphyrin heterodimer and dimethylferrocene (Me2Fc), butylferrocene (BuFc), and ferrocene (Fc) dissolved in the organic phase. (Reprinted with permission from Ref. 156 copyright 2002, American Chemical Society.)...

Figure 6.6.3 (a) Model of the vesicle permeability test using fluorescence quenching in a porphyrin heterodimer made of two charged porphyrins, which (b) cannot pass the intact vesicle membrane, (c) Typical pathway of the corresponding heterodimerization through a vesicle membrane perturbed by a membrane disrupter. (Endisch et al, 1999.)... 

The photoinduced reduction of TCNQ by the porphyrin heterodimer ZnTPPS-ZnTMPyP provides a good illustration of these concepts [56,109]. Figure 12(a) displays photocurrent transients at the water DCE junction in the presence and absence of an equimolar ratio of Fe(CN)g /Fe(CN)g. The photocurrent relaxation in the absence of the aqueous redox couples is associated with the back electron transfer from TCNQ to the oxidized porphyrin complex. The substantial decrease in back electron transfer on addition of Fe(CN)g /Fe(CN)g is associated with the supersensitization phenomenon schematically depicted in Fig. 12(b). The back electron transfer from the radical TCNQ to the oxidized porphyrin complex is in competition with the regeneration of the dye by ferrocyanide. In the absence of back electron transfer, the overall reaction involves electron transfer from the redox species in the aqueous phase to TCNQ. In this case, the energetic balance is determined by the Galvani potential difference across the interface. 

Sheets of noncovalent polymers, which would be analogous to the p-pleated sheets of silk, are commonplace on water and solid surfaces. The molecular weight and extension of these surface mono- and bilayers is unlimited. Of current interest are well-defined rigid gaps within such monolayers. They provide small reactive domains which can be manipulated on a molecular scale. It was found possible, for example, to fixate porphyrin heterodimers with a plane-to-plane distance of 10 or 20 A within such gaps and to position tyrosine monomers as electron relays between them. These sheets can then be fixated on spherical colloids (see Fig. 3.4) and we are back to spherical polymers. ... 

Subbaiyan NK, D Souza F (2012) Light-to-electron converting panchromatic supramolecular solar cells of phthalocyanine-porphyrin heterodimers adsorbed onto nanocrystalline Sn02 electrodes. Chem Commun 48(30) 3641-3643... 

Ruhlmann L, Nakamura A, Vos JG, Fuhrhop J-H (1998) Manganese porphyrin heterodimers and -trimers in aqueous solution. Inorg Chem 37 6052-6059... 

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