Dendritic first generation

The first-generation dendrimer 51 was directly observed by transmission electron microscopy (TEM). The TEM image showed that the dimensions of individual molecules are about 50 A, which is consistent with the calculated one [36]. Third-order NLO measurements showed a significant enhancement of two-photon absorption upon proceeding from the constituent molecules to the dendritic complex [35]. 

FIGURE 5.42 Chemical structures of first-generation (42) and second-generation (43) self-immolative, receiver-amplifier dendritic molecules with an enzymatic trigger (blue), cleaved by PGA and 6-aminoquinoline (red) reporter groups. 

As the energy of the excited states and the redox levels of each metal-polypyridine unit depend on metal and ligands in a predictable way, the simultaneous presence of different metals in a dendritic structures gives rise to intramolecular energy transfer processes as well to different redox patterns with multielectron processes. In particular, the tetranuclear [Os(2,3-dpp)3 (2,3-dpp)Ru(bpy)2 3]8+ (OsRu3) shown in Fig. 5.3 has been designed to achieve an efficient antenna effect. This species can also be considered a first-generation mixed-metal dendrimers.31... 

Fig. 81. (Top) Cyclam-based dendrimer of first generation. (Bottom) Dendritic 9,10-diphenylanthracene ligands and...

In a continuous set-up using Go-5, a considerable decrease in activity was also observed in comparison with the monomeric models. This effect was partly explained by deactivation of the catalyst due to the reaction of the catalytic units with the membrane or to some dendritic effect (due to the proximity of the unit centers leading to double or multiple phosphine complexation with the same palladium center), since the formation of a palladium black precipitate was observed on the membrane. Tests with the Go dendrimer model ligand (without palladium) showed a retention degree of 85%. This fact could also partly explain the decrease in activity due to the washout of catalyst during the reaction. However, a first-generation Gi-5 catalyst, with a higher retention, showed almost the same deactivation behavior. Thus, catalyst decomposition is most probably the main reason for the observed deactivation. 

Using the first-generation dendritic ALB as catalyst, the Michael adduct was obtained with 91% ee and in 63% yield after 48 h. Under similar conditions, the G2 dendritic ALB gave the product with 91% ee in 59% yield. The dendritic catalysts could be recycled and reused twice, giving comparable results. It is notable that a catalyst derived from randomly introduced BINOLs on polystyrene resin only gave an essentially racemic product. 

In conclusion it has turned out to be difficult to polymerize all but the first generation dendritic macromonomer. Higher generation dendrimers need to be attached through a long series of bonds to the polymer backbone. This in turn prevents any strong conformational influence on the backbone and no interesting new properties have been detected so far. 

These curves thus suggest an open and vacuous structure characterized by dynamic channels and pockets. The solvent accessible surface area, as measured for the first generation 12-acid, indicates a non-porous structure possessing little or no void regions as evidenced by experimental values falling below the ideal surface area. Data for the second generation 36-acid reveal a structure intermediate between dendritic and simply branched . 

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