Porphyrins electron transfer processes

FIG. 4 (a) Latimer diagram of the water soluble zinc tetra-Af-methyl-4-pyridium porphyrin (ZnTMPyP ). (Reprinted with permission from Ref. 47.) (b) Schematic representation of a photosynthetic process based on porphyrin sensitized water-organic interface. Dotted line corresponds to the back electron-transfer process. (Reprinted from Ref. 51 with permission from Elsevier Science.)... 

Although the correlation between ket and the driving force determined by Eq. (14) has been confirmed by various experimental approaches, the effect of the Galvani potential difference remains to be fully understood. The elegant theoretical description by Schmickler seems to be in conflict with a great deal of experimental results. Even clearer evidence of the k t dependence on A 0 has been presented by Fermin et al. for photo-induced electron-transfer processes involving water-soluble porphyrins [50,83]. As discussed in the next section, the rationalization of the potential dependence of ket iti these systems is complicated by perturbations of the interfacial potential associated with the specific adsorption of the ionic dye. 

Towards understanding biological electron transfer processes many researchers have reported the synthesis of dendrimers with electroactive cores (e.g., porphyrin). Dendrimers with organic dendrons attached tetrahedrally around an inorganic, electroactive iron-sulfur core were reported by Gorman and coworkers [109]. These are the first examples of dendrimers with a hybrid... 

Based on studies showing that the close proximity between the porphyrin and the Cm is essential for the observation of an electron transfer process, Fukuzumi and co-workers have prepared the porphyrin-C6o diad 41, in which the C6o-pyrrolidinyl moiety is directly connected to the meso position of the porphyrin macrocycle (Scheme 12) <03JPC(A)8834>. The strategy adopted for the synthesis of the starting porphyrin involved the 2+2 condensation of a maso-unsubstituted dipyrrylmethane with 3,5-di-tert-butylphenyl-substituted dipyrrylmethane and 3,5-di-tert-butylbenzaldehyde, to give 39, in 11.5% yield. Subsequent Ni(II) metallation, followed by Vilsmeier-Haack formylation and demetallation, gave rise to 40 which was used as the 1,3-dipole precursor this dipole in the presence of N-methylglycine and C6o, yielded the expected diad 41. 

Figure 6.27 Electron transfer processes in a zinc porphyrin-Q0 dyad...

In confirmation that these redox changes are centred on the porphyrin ligand, the free porphyrin (i.e. [H2TPP]) exhibits the same electron transfer processes (2sjy+ = +0.90 N E+j2+ = +1.23 V Eqj =... 

Many spectroscopic methods have been employed for the investigation of such systems For example, wide-band, time-resolved, pulsed photoacoustic spectroscopy was employed to study the electron transfer reaction between a triplet magnesium porphyrin and various quinones in polar and nonpolar solvents. Likewise, ultrafast time-resolved anisotropy experiments with [5-(l,4-benzoquinonyl)-10,15,20-triphenylpor-phyrinato]magnesium 16 showed that the photoinduced electron transfer process involving the locally-excited MgP Q state is solvent-independent, while the thermal charge recombination reaction is solvent-dependent . Recently, several examples of quinone-phtha-locyanine systems have also been reported . 

The TTF-porphyrin dyad 3 was described by the group of Odense.11 The fluorescence of 3 is significantly quenched by the photoinduced electron transfer process. Notably, the fluorescence intensity of dyad 3 increases largely after addition of Fe3 + that oxidizes TTF into TTF" +. Successive reduction of TTF" + is not reported. Nevertheless, it is anticipated that the fluorescence of dyad 3 can be reversibly modulated by redox reactions. In fact, the fluorescence of the supramolecule 4, formed between Zn-tetraphenylporphyrin and a pyridine-substituted TTF (TTF- ), can be reversibly tuned by sequential oxidation and reduction of the TTF moiety in 4.12 It should be noted in this context that the synthetically challenging system associating a porphyrin ring fused to four TTFs (5) was also reported.13... 

Pioneering works by Harriman and Sauvage reported the zinc(II)-gold(III) bis(porphyrin)-type complexes with diimine linkers, where the photoinduced electron transfer process from the zinc(II) porphyrin excited state to the... 

Inspecting the solvent dependence of the quenching indicates electron-transfer processes in all oligomers of 21 and 22. With increasing solvent polarity from toluene (e = 2.4) to THF (e = 7.6) and benzonitrile (e = 24.9), the quenching of the porphyrin emission increases (Fig. 9.56). 

In a supramolecular approach to fullerene-porphyrin hybrids, the assembly of a rigidly connected dyad, in which a zinc tetraphenylporphyrin, Zn(TPP), is noncovalently linked to a C60 derivative via axial pyridine coordination to the metal, was reported [219-222]. Photo excitation of the dyad Zn-complex led to electron transfer with very long lifetimes of the charge-separated pairs, as revealed by optical spectroscopy and confirmed by time-resolved electron paramagnetic resonance spectroscopy. Accordingly, two different solvent-dependent pathways can be considered for the electron-transfer processes. Either the excitation of the porphyrin chromophore is followed by fast intramolecular electron transfer inside the complex, or alternatively the free porphyrin is excited undergoing intermolecular electron transfer when the acceptor molecules ap-... 

The best molecule mimicking multi-step electron-transfer processes in the photo synthetic reaction center so far reported is a ferrocene-meso, meso-linked porphyrin trimer-fullerene pentad [Fc-(ZnP)3-C60] in Fig. 13.16b, where the C60 and the ferrocene (Fc) are tethered at both the ends of (ZnP)3 (R = 46.9 A)... 

In the next example, a mixed SAM is discussed which aims to utilize photoinduced energy and electron transfer processes to create a photocurrent in an approach which is reminiscent of the natural photosynthetic process. Figure 5.33 illustrates the molecular structures of the components of interest, i.e. the molecular triad ferrocene-porphyrin-fullerene (Fc-P-C6o) and a boron dipyrrin thiol (BoDy) [67]. Mixed monolayers were generated by coadsorption onto vacuum-deposited gold... 

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