Phthalocyanines electron transfer processes

Similarly, the dipyridyl perylenediimide 45 also axially binds to zinc(II) 1,8, 15,22-tetrakis(2,4-dimethyl-3-pentoxy)phthalocyanine (25) to form the corresponding 1 1 and 1 2 supramolecular complexes [48], The coordination of these two compounds has been monitored by UV-Vis, fluorescence, and 1H NMR spectroscopic methods. It has been found that the ligation of pyridyl ligand to zinc phthalocyanine is relatively weak and labile with a binding constant of 2,080 M-1 in CDCI3. The two components can mutually quench the fluorescence of their partner through an electron transfer process. 

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. 

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. 

Porphyrin-like structures received considerable attention because of their interesting chemical and physical behaviors, and because of nature s ubiquitous use of porphyrins in electron-transfer processes. The phthalocyanine (hereafter referred to as Pc) is a porphyrin-like dye that has been known for many years. The word phthalocyanine, from the Creek for naphtha (rock oil) and cyanine (blue), was first used by Linstead in 1933 to describe a new class of organic compounds.Phthalocyanine was probably discovered by accident in 1907. as a by-product during the synthesis of o-cyanobenzamide, but it was not until almost 20 years later that a patent was filed describing a manufacturing process. Linstead and coworkers showed that a vast range of phthalocyanines, the metal (M)-substituted forms of the molecules (hereafter referred to as MPcs), were all based on the structure depicted in Fig. 1. In a classic series of papers, starting in 1935, Robertson and coworkers showed that the enviromnent of the metal atom in MPcs was square planar and coordinated with four pyrrolic N atoms, and moreover, that the entire Pc-molecule was flat within the limits of uncertainty. ... 

PTs 244 [437], 245 [438], and 246 [439] with pendant anthraquinone or tetracyanoanthraquinodi-methane moieties have been synthesized as possible active materials PV devices. The polymers were reported to be soluble and to exhibit a photoinduced electron transfer process in solution. Photoinduced electron transfer from PT to phthalocyanine units was observed in copolymer 247 [440]. 

Bravo-Diaz C —> Gonzalez-Romero E Briscoe WH, Horn RG Electrical double layer interactions in a non-polar liquid measured with a modified surface force apparatus 147 Brunner M, Bechinger C Colloidal systems in intense, two-dimensional laser fields 156 Buckin V Lehmann L Burrows HD, Kharlamov AA About energy and electron transfer processes in Cgo/phthalocyanine films 52 Burrows HD Kharlamov AA Burrows HD Hungerford G... 

The formation of dimetal face-to-face macrocycles can effectively promote a 4-electron transfer pathway from oxygen to water. For example, such catalysts include dicobalt face-to-face porphyrins [15], and pillared dicobalt cofacial porphyrins [16], as well as other binuclear and polynuclear Co phthalocyanines [17]. In this catalyst, two Co centers can provide two adsorption sites for O2 to form a bridge adsorption, facilitating a 4-electron transfer process, as suggested by Anson et al. [18, 19] ... 

Porphyrins and phthalocyanines can be used as core, branches, and terminal groups of dendrimers that have been extensively used as bio-inspired models or mimics of natural systems such as hemoproteins and chlorophyll in photosynthesis, but also in host-guest chemistry, biosensors, photodynamic therapy, and catalysis. Aida et al. demonstrated the amplitude of this field in a review of 2009 [98]. In the following paragraphs, some historical backgrounds will be presented before focusing in a topic specifically concerned on the porphyrin-dendrimer electrochemistry and photoinduced electron transfer processes. 

The fluorescence of the zinc-phthalocyanine core in dendrimer 49 is completely quenched [67]. On the basis of energetic considerations, this quenching process was attributed to electron transfer from the connected ferrocenyl units to the excited phthalocyanine core. The photophysical behavior of 50 and 51 was not mentioned. 

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. 

The template synthesis represents an elegant method that uses metal ions to direct reactions of ligands and provides a useful route to macrocyclic structures. Several books159-161 describe the template processes that involve reactions on matrices used to synthesize polyazamacrocyles, crown ethers, cryptands, rotaxanes, knots,159 clathrochelates,160 phthalocyanines,161 etc. which are applied, e.g., as molecular switches, in ion exchange, electron transfer or catalysis. An example of clathrochelate synthesis is given in Chapter 1.33... 

When considering organic dyes for use in DSSCs, porphyrins and phthalocyanines have attracted particular attention, the former because of the analogy with natural photosynthetic processes, the latter because of their photochemical and phototherapeutic applications. However, porphyrins cannot compete with the N3 or black dye sensitiser due to their lack of red light and near-lR absorption. Phthalocyanines, on the other hand, show intense absorption bands in this spectral region. However, problems with aggregation and the unsuitable energetic position of the LUMO level, which is too low for electron transfer to the TiOa conduction band, have turned out to be intractable for the moment. 

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. 

Phthalonitrile was reduced at controlled potential in alcoholic solution at a C cathode in a two-compartment cell in which cathode and anode were separated by a Nafion cation-exchange membrane. The anolyte contained metallic salts such as CUSO4, NiS04, etc. The products formed in the cathode compartment were metal phthalocyanine complexes. Low yields were obtained when the metal ions were placed into the cathode compartment. The phthalonitrile radical anion formed by a 1-electron transfer was invoked as being important in the cyclization process leading to the metal complexes. Several intermediates... 

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