Dendritic substituents

Dendrimers built around a metal complex as a core. These compounds can be considered metal complexes of ligands carrying dendritic substituents (Fig. 1 a). The most commonly used metal complex cores are porphyrin complexes, polypyridine complexes, and ferrocene-type compounds. 

Further studies have shown that the torsional angle of the binaphthyl unit depends not only upon the steric demands of the two dendritic substituents, but... 

Such enantiomerically pure chiral compounds, which are optically inactive in the generally investigated UV/Vis spectral range from 200 to 800 nm, can be designated as cryptochiraF according to Mislow et al., since the chiral information is hidden in the molecule (i.e. present in cryptic form) [12a, 20]. Cryptochirality of the dendrimer is explained on the one hand by the pronounced conformational mobility of the dendrimer branches and on the other by the slight electronic difference between the dendritic substituents. 

Fig. 8.26 Potentially switchable nanoimpeller (schematic) the cones stand for dendritic substituents azo groups (red) are in the E...

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. 

Three kinds of NLO chromophores 13-15 (Scheme 9) were obtained through the modification of one highly nonlinear tricyanofuran (FTC) acceptor-based chromophore with different shapes and sizes of substituents. When compared to the t-butyldimethylsilyl side group substituted in chromophore 13, the dendritic substituent on chromophore 15 has a much more flexible and wide spreading shape because of the branching alkyl chains that are connected to the phenyl... 

In 2002, Malmstrom and coworkers attached tlie first- to fourth-generation dendritic substituents based on 2,2-bis(hydroxymethyl)propinic acid and... 

The photodimerization of the anthracene (176) having a 9-dendritic substituent has been studied within the vaterite crystal of CaCOs (R = H),"" and in its liquid crystalline smectic phase (R = Me). The crystal phase of the CaCOs is... 

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],... 

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. 

A different approach to dendritic sensors involves modification of a sensor core unit with dendritic substituents to confer beneficial solubility properties. An example of a sensor core unit is the porphyrin macrocycle, a heterocycle that has been employed extensively in prototypical photochemical sensor systems. Vinogradov and co-workers have exploited the versatile photoactive porphyrin sensor unit as a fluorescence-based pH indicator for use in biological assays [73], by attaching acid terminated polyamide-ether dendrons as substituents (Figure 8.12). The two imino nitrogen atoms present in the free-base porphyrin are susceptible to stepwise protonation to afford initially a cation and then a dication, respectively. Upon protonation, both the emission and absorption fluorescence spectroscopic characteristics of the porphyrin core are subject to dramatic hypochromic shifts. This spectroscopic phenomenon formed the basis for an accurate pH indicator with potential applications in proton gradient determination studies in biological systems. 

Cyclotriveratrylene-based dendritic structures reveal a similar trend the association constant increases with the generation number of the surrounding dendritic substituents. 

More efficient OLEDs have been reported for phosphorescent tris-cyclometalated homoleptic Ir(III) complex Ir(TPSppy)3 (TPSppy = 2 — (4 -(triphenylsilyl)biphenyl-3-yl)pyridine) with a silane-based dendritic substituent (Fig. 32) [133]. This dendrimer has shown high luminescence efficiency both in solution (0p = 0.63) and in film (0p = 0.74). A polymer-based triplet OLED with this Ir(III) doped emitter exhibits a remarkable efficiency of 32.8cdA maximal brightness of... 

Fig. 32 Phosphorescent tris-cyclometalated homoleptic Ir(III) complex Ir(TPSppy)3 with a silane-based dendritic substituent...

All ligands 3c through e-H2 form coordination compounds with titanium(IV) ions that possess dendrimer structures. Due to the non-polar Frechet-type dendritic substituents, the compounds are soluble only in solvents with low polarities (mixtures of methanol and chloroform). Consequently, the dimeric complexes are observed as the major species in solution. Fignre 2.6 shows the structural formula of L Lij (30)5X12 ] and the structure derived from x-ray diffraction of the anion of the GO dendrimer [Lij (30)5X12)] . Xhe central triple lithium-bridged part and the dendritic substituents decorating its periphery may be seen. 

The synthetic utility of the Suzuki reaction for polymer synthesis has also been demonstrated through the synthesis of polymers with Frechet-type dendritic substituent groups (247) and through progress made toward the preparation of large oligophenylene cycles (248),... 

Jen a al. have developed dendronized polymeric NEO materials that have shown significantly improved poling efficiency by encapsulating chromophore with dendritic substituents that can electronically shield the core, ii-electrons, and form spherical molecular shapes." "" " Figure 6 illustrates different molecular architectures of dendronized side-chain NLO polymers with crosslinkers. The diverse selections of molecular architectures provide additional flexibility in the molecular engineering of high-performance polymeric NLO materials. Moreover, the unique nanoscale environment created by the shape and size, dielectric properties, and distribution of chromophores in crosslinkable polymers with dendrons and dendrimers can all play critical roles in maximizing the macroscopic EO properties of polymeric NEO materials. 

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