Chemistry Reference
In-Depth Information
Figure 11.30 (a) Structure of a 4th generation alkylammonium-terminated organopho-
sphorus dendrimer (G4T). (b) Optical micrograph of G4T (scale bar ¼ 100 mm). (c)
Scanning electron micrograph of one tip of a single macroscopic dendrimer fiber after dilution
with 1 M NaOH to show the fibrillar substructure (scale bar ¼ 100 mm). Reprinted from El
Ghzaoui et al. (2004). Copyright 2004 American Chemical Society.
Similarly, Majoral's group reported a series of cationic organophosphorous dendri-
mers that formed noncovalent gels capable of immobilizing aqueous solutions at
concentrations of 1.2-1.5 wt% after 11-13 days at 65 8C (Marmillon et al. 2001).
Gelation time was significantly reduced when various buffer and metal salts were
added. The impact of salt on the gelation process was attributed to a colloidal floccu-
lation process whereby added salts screened repulsive electrostatic interactions,
resulting in aggregation (El Ghzaoui et al. 2004). Freeze-fracture TEM and optical
microscopy revealed dendrimer aggregates and fiberlike networks within the gels
(Fig. 11.30).
11.4. FOLDED AMPHIPHILIC DENDRIMERS
The Parquette group developed a series of dendrimers that fold into a well-defined,
helical structure capable of amplifying chiral perturbations (Recker et al. 2000;
Gandhi et al. 2001; Huang and Parquette 2001; Parquette 2003; Lockman et al.
2005). The structural preorganization of the dendrimers arises from the preference
of the branched pyridine-2,6-dicarboxamide repeat unit to exist predominantly
in the syn-syn conformation rather than either the higher energy syn-anti or
anti-anti forms (Fig. 11.31; Gabriel et al. 2006).
This conformational preference in conjunction with the preference of the terminal
amides to exist in an s-trans conformation constrains the system such that the terminal
anthranilamide substituents are positioned above and below the plane of the
molecule, resulting in a local helical structure. The helical antipodes experience a
