Dendritic phthalocyanines

Cobalt complexes of dendritic phthalocyanines (Fig. 6.37) showed a 20% lower catalytic activity (TON 339 min-1 for G2 dendrons) as catalysts for the oxidation of 2-mercaptoethanol than non-dendritic phthalocyanines [56]. By way of compensation, however, the dendritic catalysts proved to be more stable than non-dendritic ones, which is probably attributable to enclosure of the metallo-phthalocyanine core unit by the dendrons. This also prevents molecular aggregation of the phthalocyanines in polar solvents and thin films. 

Fig. 6.37 Dendritic phthalocyanine 10 and supramolecular host-guest system 11 a,b (according to Kimura et al.)...

A poly(propylenamine) dendrimer (11, Fig. 6.37) functionalised with poly-(N-isopropylacrylamide) (PIPAAm) (see Section 4.1.2) was used as dendritic host for anionic cobalt(II)-phthalocyanine complexes (a, b) as guests, which are held together by supramolecular (electrostatic and hydrophobic) interactions [57]. These dendritic complexes were investigated as catalysts in the above-mentioned oxidation of thiols, where they show a remarkable temperature dependence the reaction rate suddenly increases above 34°C. One attempted explanation assumes that the dendritic arms undergo phase separation and contraction above the Lower Critical Solubility Temperature (LCST). At this temperature the phthalocyanine complex site is more readily accessible for substrates and the reaction rate is therefore higher. 

Although strictly not a dendritic system, Agar et al.[75] have reported the preparation of copper(n) phthalocyaninate substituted with eight 12-membered tetraaza macrocycles as well as its nickel(n), copper(n), cobalt(n), and zinc(n) complexes. Thus, the use of the l,4,7-tritosyl-l,4,7,10-tetraazacyclododecane offers a novel approach to the 1 — 3 branching pattern and a locus for metal ion encapsulation. 

A series of ruthenium(II) phthalocyanines with one or two pyridyl dendritic olig-othiophene axial substituent(s) have also been reported (compounds 50 and 51) [50], The dendritic ligands absorb in the region from 380 to 550 nm, which complements the absorptions of the phthalocyanine core. This combination results in better light harvesting property and enhancement in efficiency of the corresponding solar cells. The solution-processed photovoltaic devices made with these compounds and fullerene acceptor give efficiencies of up to 1.6%. These represent the most efficient phthalocyanine-based bulk heterojunction solar cells reported so far. 

Similar behavior has been very recently observed in cobalt phthalocyanines bearing dendritic substituents [122], In 47, Co is oxidized in methanol in a mono-electronic reversible wave, whereas in 48 no clear oxidative wave is observed, suggesting that the electroactive core is encapsulated in the dendritic structure. The ability of the Co-phthalocyanine core of 47 and 48 to catalyze oxidation of 2-mer-captoethanol in the presence of oxygen was also investigated. The two compounds exhibit similar catalytic activity, indicating that the dendritic structure does not hinder penetration of small molecules 48 showed, however, an enhanced catalytic stability which was ascribed to the encapsulation of the core. 

Axial dendritic substituents are an alternative way to functionalize these hyper-branched macromolecules and to avoid stacking. McKeown s group synthesized dendritic silicon phthalocyanines based on Frechet s poly(aryl ether) dendrons, and used them to create emissive thin films by spin-coating. The disappearance of stacking rearrangement was clearly seen in the UV-vis spectra [45],... 

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... 

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. 

Consequently, electroconductivity (including the many papers of Hanack ) and photovoltaic characteristics are not mentioned. The development of new phthalocyanine compounds with expanded pi-electron systems, such as two-dimensionally polymerized phthalocyanine compounds and dendritic structures, are also outside the scope of this subject matter. 

Phthalocyanine compounds exhibit many kinds of electronic, optical, magnetic, and optoelectronic characteristics. To hyperbolize, phthalocyanine compounds can do anything. But, viewed in practical terms, every characteristic of phthalocyanine compounds meets the competitive materials in the market. We have referred principally to application fields where phthalocyanine compounds have superiority over competition. So, for example, electroconductivity, including many works of Hanack [29] and photovoltaic characteristics are not mentioned. The development of new phthalocyanine compounds with expanded 7t-electron systems, such as two-dimensionally polymerized phthalocyanine compounds [30] and dendritic structures [31], have also not been considered. 

Brewis M, Clarkson GJ, Goddard V, HelliwellM, Holder AM, McKeown NB (1998) Silicon phthalocyanines with axial dendritic substiments. Angew Chem Int Ed 37(8) 1092-1094... 

Ozeelik S, Koca A, Gul A (2012) Synthesis and electrochemical investigation of phthalocyanines with dendritic bulky ethereal substituents. Polyhedron 42(l) 227-235... 

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