Dendrimers molecular modeling

It should be mentioned, however, that the radius of the dendrimers in the gas phase will be different Irom that in solution. Also, protonation will change the radius of the dendrimers. Molecular modelling shows a radius of 2.1 nm for the protonated fifth generation dendrimer, while for the unprotonated form 1.6 nm was calculated. 

Molecular modeling techniques are a powerful tool to obtain a very detailed insight in the three-dimensional structure of dendrimer molecules at the atomic level. They have been applied to calculate sizes of the polypropylene imine) dendrimers and radial density profiles in order to estimate the free volume inside the dendrimers, as well as to make predictions about de Gennes dense-packed generations. The molecular modeling work by Coussens and co-workers [20] was focused on the generations 1-5 of the DAB-dendr-(CN)n and DAB-dendr-(NH2)n (n = 4, 8, 16,32, 64). 

First, second, and third generation dendrimers based on THPE (core) and bis-MPA (repeating unit) have been synthesized and characterized. SEC and NMR analyses verified that the dendrimers were monodisperse. Diffusion NMR was used to determine the dendrimers hydrodynamic radii in deuterated chloroform. First, second, and third generation dendrimers were found to be 7.8, 10.3, and 12.6 A, respectively. These were in agreement with sizes as determined from molecular models (Biichi). 

Attempts to isolate the ferrocenyl dendrimers described above in a crystalline form suitable for X-ray structural determination have so far been unsuccessful. For this reason, we have used computer-generated molecular models in order to gain further information about the structural features of these materials. Figure 3 illustrates an energy-minimized structure determined from CAChe molecular mechanic calculations of the ferrocenyl dendrimer 2. From these studies, we have measured approximate diameters of 2 run for the first-generation dendrimers 1,3, and 5, and 3 nm for the second-generation dendrimers 2,4, and 6. 

Figure 3. Structure of dendrimer 2 from molecular modeling.

Most studies performed partly on molecular models [33] but also on real POPAM and PAMAM dendrimers support the latter model concept [34]. Careful studies on the three-dimensional structure of flexible dendrimers in solution were performed by Ballauff et al. by means of SANS (Small Angle Neutron Scattering) [35] (see Section 7.6). 

The functionalization worked smoothly with the new 27-NH2 dendrimer shown in Scheme 8 [100] giving the 27-Fe4 dendrimer 3. These new met-allodendrimers 1, 2 and 3 are air-stable, forest-green powders. They were characterized using standard spectroscopic and elemental analyses, and AFM of 2 and 3 (Fig. 10) shows that it forms monolayers of aggregated metalloden-drimers on the mica surface. Indeed, the height of 1.5 nm (2) and 2.4 nm (3) is reproducibly obtained by AFM and corresponds to the dimension of slightly flattened molecular models. 

Table 2 Molecular volumes and radii of gyration of the DAB-Je /r-(NH2) dendrimers as obtained by molecular modelling in the gas phase [34]...

Fig. 30 Schematic representation of the molecular model for dendrimers with one-terminal-chain mesogenic units. Model for the Smectic A supramolecular organization (here G3-Li dendrimer)...

Fig. 33 Schematic representation of the molecular model for the dendrimers bearing the L2 mesogenic unit. Model for the columnar supramolecular organization (D inter-columnar distance)...

The dendritic core adopts an extended conformation which is possible due to the great flexibility (conformational freedom) of the PAMAM and PPI skeletons. In this way, the molecular model of thick disks proposed for the LC dendrimers with two-terminal chain mesogenic units is transformed into a model consisting of a long cylinder, which is the result of the axial elonga-... 

Fig.40 Molecular model for the LC dendrimers with bulky mesogenic units...

Many of the physical properties of dendrimers may be predicted, at least qualitatively, from molecular modeling of the growth process. With the PAMAM... 

No such long decay component of 7.4 ns is observed for para-substituted dendritic structures p-CnPn. The absence of the long decay component is therefore due to the different position of substitution leading to a better spatial separation of the individual chromophores. This is also supported by a comparison of the molecular structures of the para- and meta-substituted dendrimers obtained from molecular modeling, since the average center-to-center distance among the chromophores is 2.9 nm for the para series but only 2.6 nm for the meta series in the first generation series. 

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