Phthalocyanines electron transfer systems

As described before, the rr-electrons of porphyrin are delocalized over the molecule and the energy levels of the HOMO and the LUMO are high and low, respectively. The resultant narrow intramolecular HOMO-LUMO gap causes absorption of the entire region of visible light. Usually, porphyrins are red to purple and phthalocyanines are blue to green. Furthermore, the long lifetime of their excited states is appHcable to the construction of photo-induced electron and/or energy transfer systems. 

The cobalt(II)15 and zinc(II)16 complexes of phthalocyanine(Pc), octcyano-Pc, and tetrasulfon-ato-Pc incorporated in poly(4-vinylpyridine-co-styrene) or Nafion films coated on graphite have also been examined as catalytic devices for dihydrogen electrogeneration in phosphate buffer. These catalytic systems were strongly suggested to be dominated by the electron transfer within the polymer matrix. The best catalytic film is that constituted of the nonsubstituted Con-Pc complex in poly(4-vinylpyridine-co-styrene), giving a turnover number of 2 x 10s h-1 at an applied potential of —0.90 V vs. Ag Ag Cl. 

However, the changes in environment which occurred with the change from a reductive to an oxidative atmosphere rendered iron sulfide-based redox systems inconvenient, as they were very sensitive to (irreversible) oxidation. We saw in earlier chapters the facile formation of porphyrin and phthalocyanines from relatively simple precursors, and these systems were adopted for the final steps of electron transfer in oxidative conditions. The occurrence of iron centres in planar tetradentate macrocycles is ubiquitous, and metalloproteins containing such features are involved in almost every aspect of electron transfer and dioxygen metabolism. A typical example is seen in the electron transfer protein cytochrome c (Fig. 10-10). 

El-Khouly ME, Ito O, Smith PM, D Souza F. Intermolecular and supramolecular photoinduced electron transfer processes of fullerene-porphyrin/phthalocyanine systems. J Photochem Photobiol C Photochem Rev 2004 5 79-104. 

We reported the first example of such a compound, porphyrin-fullerene dyad 23, in 1994 [134, 135]. Since that paper, a number of examples of dyads consisting of fullerenes linked to porphyrins or phthalocyanines have been prepared [136, 137-159]. Some of these, such as 24, use amide linkages related to those employed in some porphyrin-quinone systems [137, 149, 152]. In dyad 25, the moieties are linked by an extended, rigid bridge, of the type that has been shown to facilitate rapid long-range electron transfer in other systems [146, 147]. A number of dyads feature pyrrolidine-functionalized fullerenes, as in 26 and 27 [90, 139, 145, 148]. 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system pd(II)-BQ-ML where ML" is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPp)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

Pulse radiolysis has been used to study elementary reactions of importance in photosynthesis. Early experiments provided rate constants for electron transfer reactions of carotenoid radical cations and radical anions with chlorophyll pigments.More recent experiments dealt with intramolecular electron transfer in covalently bound carotenoid-porphyrin and carotenoid-porphyrin-quinone compounds. Intramolecular electron transfer reactions within metalloproteins have been studied by various authors much of that work has been reviewed by Buxton, and more recent work has been published. Pulse radiolysis was also used to study charge migration in stacked porphyrins and phthalocyanines. Most of these studies were carried out by pulse radiolysis because this techruque allowed proper initiation of the desired processes and pemtitted determination of very high reaction rate constants. The distinct character of radiolysis to initiate reactions with the medium, in contrast with the case of photolysis, and the recent developments in pulse radiolysis techniques promise continued application of this technique for the study of porphyrins and of more complex chemical systems. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

The design of covalently linked donor-fullerene systems capable of undergoing photoinduced electron-transfer processes has been widely studied as a result of the remarkable photophysical [35] and electronic [36] properties of fullerenes. Porphyrins, phthalocyanines, tetrathiafulvalenes, carotenes, and ferrocene [37] have been covalently attached to the fullerene sphere, usually as pyrrolidine[ 60] fullerene derivatives by 1,3-dipolar cycloaddition reactions. 

A special situation is encountered with metal-porphyrin or metal-phthalo-cyanine molecules that can be either deposited by sublimation under UHV conditions or in solution environments. For these macrocyclic compounds, free-base species exist, i.e., the metal centers are not required per se as a construction unit. The building of supramolecular structures that incorporate porphyrin subunits is of great interest to many research groups. The rich photochemistry and redox properties (e.g., photoinduced electron transfer, luminescence, and light harvesting) of porphyrins have driven this interest. Porphyrins or phtalocyanines have a rich coordination chemistry that allows the inclusion of many different metal centers at their macrocycle. They serve in many respects as a model system since this constitutes a low-coordination complex. Recent STM studies report on the organization of metal-coordinated or free-base porphyrins as well as phthalocyanines on... 

Photoinduced electron transfer in supramolecular systems of fullerenes functionalized with ligands capable of binding to zinc porphyrins and zinc phthalocyanines 05CCR(249)1410. 

Various fullerenes and C70) were coordinated to zinc(II) and mag-nesium(II) porphyrins via functionalized pyridines or imidazoles (Fig. 7) by D Souza and Ito [10-22]. Both single-point [10,11] and two-point [12-14] binding strategies were employed, together with additional covalent functionalization of the porphyrins with ferrocene (Fc) [10] or boron dipyrrin (BDP) [16]. Similar systems were also studied by Guldi, Diederich, Nieren-garten and Schuster, and the results on the intermolecular and supramolecu-lar photoinduced electron transfer (PET) processes of fullerene-porphyrin and phthalocyanine systems were reviewed recently [23,24]. Since PET is... 

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