Dendrimers phthalocyanine core

The effect of core shielding of a porphyrin moiety by peripheral dendrons has been carefully investigated on two series of Zn-phthalocyanine-cored dendrimers with aryl-ether branches [60]. Generation 0,1, and 2 (dendrimer 27) species, terminated with ester groups, are soluble in organic solvents, while the species terminated with carboxylate units (e.g., 28) are soluble in water. 

A phthalocyanine core was functionalized with four, eight, and 16 ferrocenyl units (compounds 49-51) [67]. Cyclic voltammetry experiments in DMF or CH2CI2 showed the presence of many quasi-reversible waves. All the ferrocenyl units behave independently, giving rise in all three compounds to a single oxidation wave, which is accompanied by one or two oxidation and by two or three reduction processes due to the phthalocyanine rt-system. For 51, the presence of a stripping peak indicates precipitation of the oxidized product in CH2CI2, as often observed in ferrocene-functionalized dendrimers (see Section 9.4.1). 

A similar quenching by methylviologen through the dendrimer architecture was also observed in the Zn-phthalocyanine-cored dendrimer 54 [125]. 

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. 

FIGURE 5.6 Metallated phthalocyanine-cored dendrimers with eight arms as described by Majoral and coworkers. 

Macrocyclic metal complexes are the principal materials incorporated in the dendrimer structure (for reviews see [192,193]). Examples are metallo dendrimers with a porphyrin core [194-204], with a phthalocyanine core [205-209], with crown ethers in the repeat unit [210-212] or with a cyclic amine core [213],... 

In addition to porphyrins, phthalocya-nines have been placed in the middle of hyperbranched molecules. Cobalt phthalo-cyanines with dendritic branches have been synthesized, and the analysis of their electrochemical behavior suggests that electron transfer between the phthalocya-nine core and an electrode is considerably hindered by the dendritic structure [51]. Dendrimers with phthalocyanine cores and electroactive tetrathiafulvalene (TTF) branches have also been synthesized and examined [52]. 

A water-soluble hydroformylation catalyst was developed by Xi and co-workers [65]. Third generation PAMAM dendritic ligands, with hydrophilic amine or sulfonic acid end groups, were phosphonated and the rhodium complexes thus formed were found to catalyse efficiently the hydroformylation of 1-octene and styrene, under very mild conditions. Water-soluble dendritic cobalt phthalocyanines that exhibited catalytic activities and oxidised thiols in the presence of oxygen, have been synthesised by Kimura and co-workers [66]. The catalytic activity of the phthalocyanines was influenced by a egation of the catalytic sites that results fi om strong intermolecular cohesive forces. It was proposed that steric isolation, enforced by the addition of a bulky dendritic coaf around the active phthalocyanine unit, could improve the catalytic activity. Acid terminated polyamide dendrimers were coupled to a phthalocyanine core to produce the desired water-soluble cobalt phthalocyanines, which were tested subsequently for catalytic activity and stability. The results obtained showed that the aggregation of phthalocyanines was reduced the catalytic activity was improved and the stability of the catalyst was improved by addition of the dendritic substituents. 

El-Khouly, M. E. et al. Silicon-phthalocyanine-cored fuUerene dendrimers Synthesis and prolonged charge-separated states with dendrimer generations. Chem. Ear. J. 13, 2854—2863, 2007. 

There has been considerable investigation of dendritic species with redox-active moieties placed at the center of the molecule. Various metal-polypyridyl complexes, as well as porphyrins and phthalocyanines, have been used as cores around which dendrimers have been built. Dendritic molecules with metallocenes at or near the core and dendrimers with central metal clusters have also been synthesized. In addition to metal complexes, electroactive organic moieties have been placed at the cores of various types of dendrimers. The rate of electron transfer between redox-active species and a working electrode, and... 

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. 

Leclaire J, Coppel Y, Caminade AM, Majoral JP (2004) Nanometric sponges made of water-soluble hydrophobic dendrimers. J Am Chem Soc 126 2304-2305 Leclaire J, Dagiral R, Fery-Foigues S, Coppel Y, Donnadieu B, Caminade AM, Majoral JP (2005) Octasubstituted metal-free phthalocyanine as core of phosphoms dendrimers a probe for the properties of the internal sOucture. J Am Chem Soc 127 15762-15770 Galliot C, Prevote D, Caminade AM, Majoral JP (1995) Polyaminophosphines containing dendrimers—syntheses and characterizations. J Am Chem Soc 117 5470-5476... 

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