Dendrimers stereoisomers

Figure 5. Stereoisomers of the high-spin dendrimer 5 (G = 1) with Dj and symmetries which differ in the helicity of their three propeller-like conformations adopted by each diphenylmethyl group.

Figure 6. Stereoisomers of dendrimer 6 (G = 1) differing in the helical senses of the four propeller-like subunits and in their relative orientations.

Figure 7. Representative HPLC separations of the stereoisomers of (a) dendrimer 5 (G = 1) at 298 K and (b) dendrimer 6 (G = 1) at 258 K using a HPLC achiral stationary phase of ODS and ACN/THF (55 45) as mobile phase.

Table 1. Structural and Electronic Characteristics of Stereoisomers of Dendrimer 5 (G = 1) Obtained by AMI Semiempirical Calculations...

Table 1 shows the aspect ratio values for the geometries of the stereoisomers of dendrimer 5 (G = 1) with Dj and Cj symmetries, obtained by semiempirical AMI optimizations. Other molecular characteristics, like their molar enthalpies of formation, entropies, and dipolar moments, which are relevant for a complete characterization of these isomers, are also given in Table 1. The aspect ratios of both isomers are very low, close to 1.3, indicating therefore spheroidal shapes in both cases. This fact is in complete agreement with the results previously described, suggesting that the first generations of both dendrimer series are close to the limit ones.

Such a relationship is illustrated in Figure 9 for the AMI optimized geometries of the stereoisomers of dendrimer 5 (G = 1) with and Cj symmetries. TTie slopes of both function in the two plots approach zero (corresponding... 

The same situation is expected for stereoisomers of the dendrimer 6 (G = 1), as revealed by a careful inspection of Figure 10 in which the dependence of the fractal dimension of the 12 isomers on probe radii is depicted. Due to the large number of atoms of this open-shell dendrimer, a low level theoretical method, the Dreiding force-field, was used for the optimizations of the geometries of these 12 isomers. ... 

As a main conclusion of this LSER study one might state that the relative stability of the stereoisomers of 5 (G = 1) in solution is controlled first by the differences in their shapes and sizes and, second, by their distinct polarities. Therefore, the overall size and shape are again one of the most important characteristics of this dendrimer since they play an major role in the relative population of its conformers. 

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