Shape And Size

(E)-stilbene and (E,E)-1,4-distyrylbenzene, the building blocks of the stilbenoid dendrimers, are planar molecules, but their activation barriers for torsions are low because the energy hypersurface is very flat around the planar minima [50, 51]. Thus, one could have the idea that planar cores with stilbenoid dendrons yield altogether planar compounds. However, this is only true for low generations. An X-ray crystal structure analysis of the first generation 1a (R1 = R2 = OC3H7) proved planar molecules, whose torsional angles in the stilbenoid skeleton are, at most, ±6°. The free or dissolved molecule has a D3h symmetry [20, 39]. In order to imagine the shape of the higher generations, we developed molecular models of 1a-e with the CERIUS 2.0 program [20]. The models revealed that the second generation 1b is still a flat disk; however, from the third generation onwards, the steric crowding becomes so severe that the repulsions force an extension into the third dimension. The

Scheme 2. Coupled and convergent synthesis of the stilbenoid dendrimers 1a-e (R1 = R2 = OC6H13).

disk-like shape is transformed to a cylindrical shape. Figure 1 illustrates the model of the fifth generation.

The enormous steric interactions are in accord with the fast growing number of atoms and molecular masses with increasing generation n. Table 1 gives a survey over the series 1a-e with R1 = R2 = OC6H13.

The dependence of the diameter 0 and the height h of the disks or cylinders on the generation n is shown in Figure 2.

Good indications of the size of the particles can be obtained by gel permeation chromatography (GPC) measurements. The retention time t is an exponential function of the diameter 0 [Eq. (1)]. Figure 3 shows the corresponding curve. The excellent fit parameter (x2 = 0.012) reveals the consistency of the 0 values within the series.

where tO = (16.63 ± 0.49) min is the retention time corresponding to the total volume of the mobile phase, tt = (5.37 ± 0.47) min is the retention time corresponding to the volume of the stationary phase, e(^Gp/kT) = e-a97±a17 is the distribution coefficient for the substrate-sorbent interaction, and s = 0.66 ± 0.11 nm-1 is the surface area per unit pore volume.

Figure 1. Molecular model of the fifth generation 1e (R1 = R2 = OC6H13). View from the side and from above. Reprinted with permission from [20], H. Meier et al., Chem. Eur. J. 6, 2462 (2000). © 2000, Wiley-VCH.
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