Dendrimer porphyrins

These researchers also described [93] the design and synthesis of iron(II) porphyrin dendrimers with triethylene glycol monomethyl, ether surface groups (e.g., 31) which render them soluble in a wide range of organic solvents and water. The potential difference between the first (1 FeCl) and second generation (2 FeCl) Fe-porphryin dendrimers was found to increase more in water than in dichloromethane (0.42 vs 0.08 V). This remarkable potential difference between 2 FeCl and 1 FeCl in water was comparable with that found between cytochrome c and a similarly ligated, more solvent-exposed cytochrome c heme model compound. 

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... 

Energy transfer in light-harvesting Zn porphyrin dendrimers... 

The scaffold of the two porphyrin dendrimers used is identical (Fig. 5.16) they differ solely in the functionalisation of their periphery. The dendrimer shown schematically in red in Fig. 5.15 corresponds to this basic scaffold, for example with R = CONH(CH2)2N+Me3 the dendrimer shown in green bears the group R = COCTK+. 

Fig. 5.16 I onic porphyrin dendrimers (according to Fr chet and Aida et a. )...

Fig. 6. 38 Metallo-porphyrin dendrimers (according to Moore, Suslick et al.)...

Porphyrin dendrimers are suitable as sensors for small molecular and ionic analytes. Unsubstituted metal-free porphyrins often show poor solubility in water. However, if it proves possible to envelop them in hydrophilic dendrimers they can be used in water as fluorescent pH indicators because they exhibit distinct changes of their absorption and emission bands owing to protonation of... 

Construction of organic nanotubes starting from porphyrin dendrimers with core/shell architecture is also feasible. Figure 8.29 also shows how covalent nanotubes can be produced by removal of the dendritic component of the molecule. A coordination polymer is first synthesised from a dendritic metallopor-phyrin with alkene end groups. This is subjected to intramolecular and intermo-lecular crosslinking by ring-closing metathesis at the periphery. 

Fig. 13.23 Porphyrin dendrimers and the reference compound employed for construction of organic solar cells composed of multi-porphyrin/C60 supramolecular assemblies [91].

The photocurrent generation in the present system is initiated by photoinduced charge separation from the porphyrin excited singlet state (1H2P /H2P+ = -0.7 V vs. NHE) [78] in the dendrimer to C60 (C60/Cf>0 = -0.2 V vs. NHE) [78] in the porphyrin dendrimer-C60 complex rather than direct electron injection to conduction band of Sn02 (0 V vs. NHE) system [91] The reduced C60 injects electrons into the Sn02 nanocrystallites, whereas the oxidized porphyrin (H2P/H2P+ = 1.2 V vs. NHE) [78] undergoes electron-transfer reduction with iodide (I3 /I = 0.5 V vs. NHE) [78] in the electrolyte system [91]. 

In contrast with the porphyrin dendrimers in Fig. 13.23, porphyrin oligomers with a polypeptidic backbone in Fig. 13.25 are flexible enough to accommodate... 

Examination of the cyclic voltammetry (CV) of the Zn-porphyrin dendrimers in THF and CH2C12 with Bu4N+PF6 (0.1 m) electrolyte revealed first oxidation potentials up to 300 mV (THF) less positive than the corresponding values obtained for the unshielded tetraester, Zn-porphyrin core. These preliminary electrochemical experiments suggested dendritic encapsulation of redox-active chromophores can effectively influence the electrophone environment controlled and well-conceived cascade architecture can lead to new avenues of selective redox catalyst design. 

Introducing ionized groups onto the outer surface of a dendrimer induces the adsorption of counterionized functional molecules. A porphyrin dendrimer with anionic groups on the outer surface has been shown to adsorb the electron accepter methylviologen (MV +) (Fig. 3.15). Photoexcitation of the central porphyrin induces electron transfer to the adsorbed methylviolo-gens. As the size of the dendrimer was increased, electron back-transfer was effectively suppressed and a long-lived electron separation state was achieved. 

Electron transfer quenching of the excited metalloporphyrin core in compounds 43 46 was investigated in DMF using benzylviologen as electron acceptor [121], The estimated electron transfer rate constants are similar in the four dendrimers, indicating that small molecules have no difficulties in approaching the porphyrin core. Thus, these porphyrin dendrimers may potentially be employed as catalysts. 

Higher (and supramolecular) ligands based on sugar, porphyrin, dendrimers, cyclodextrins, calix[4]arenes, etc., have also been tested for water-soluble conversions, the hydroformylation of water-insoluble olefins included [219]. In some cases the water-soluble, macromolecular cpds. act as inverse phase-transfer catalysts, e. g., when crown ethers are involved [269]. 

The periphery of convergently synthesized den-drimers has also been modified to allow the assembly of monolayers,494 to support dendritic catalysts,495 to control the intermolecular assembly of porphyrin dendrimers,246 to probe the effect of photo isomerization,319 and to enable cross-linking of the periphery followed by removal of the core.496 These studies in peripheral modification highlight the versatility of the convergent synthesis. In particular, the ability to selectively modify the periphery and focal functionalities of a dendron enables the design of complex macromolecules that involve the interaction between multiple functional components. 

Scheme 18 Porphyrin dendrimers containing poly(phenylene) dendrons by Shirai and coworkers [67]...

Depending on the type of charges, polycationic PEG-P(Lys) and polyanionic PEG-P(Asp) are used for the preparation of such PIC micelles (Fig. 6). Based on recent research, core-shell type PIC micelles with a 52 nm diameter were prepared from a Zn-porphyrin-dendrimer with 32 carboxylate groups on the periphery, 32(-)DPZn, and PEG-P(Lys) block copolymers (56,57). On the contrary, when the dendrimer structure is cationic, a third-generation porph3rrin-dendrimer with 32 primary amine groups on the periphery, 32(+)DPZn, and PEG-P(Asp) were used to prepare a PIC micelle, and had a spherical structure with a 55 nm-sized diameter (58). Both... 

Figure 6 The PIC micelles loading photosensitizers for photodynamic therapy. PEG-P(Asp) (block polyanions) and PEG-P(Lys) (block polycations) are selected according to the surface charge of Zn-porphyrin-dendrimer s.

Representative examples of dendrimers with metal-containing cores include the materials 8.1-8.6, which were reported in the mid- to late 1990s. For example, first- (8.1), second- (8.2), third- (8.3), and fourth- (8.4) generation zinc porphyrin dendrimers (M = Zn) have been prepared by a convergent synthesis approach [18, 19]. Although the electrochemical and photophysical nature of the metalloporphyr-in core was found to be preserved in the resultant materials, the rate of interfacial electron transfer was found to be reduced due to the separation of the electroactive centers from the electrode surface. Second- (8.5) and third-generation (8.6)... 

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