Peripherally modified dendrimers

Dye molecules can also be hosted into poly(propylene amine) dendrimers peripherally modified with OPV units [71]. In these systems, energy transfer from the OPV fluorescent units nm) to the enclosed dye molecules is... 

Near-infrared (NIR) absorption spectroscopy has been used to characterize the delocalized re-stacks on electronic conducting dendrimers by Miller and coworkers [47-49]. These dendrimers were prepared by peripherally modifying PAMAM dendrimers with cationically substituted naphthalene diimides, and then reduced with one electron per imide group to convert each imide into its anion radical. The re-stacking of these radical anions on these dendrimer surfaces was indicated by an absorbance band beyond 2000 nm in the NIR spectra. 

In particular, poly(amidoamine) dendrimers were peripherally modified with diimide moieties (see the structure shown in Scheme 1.43). After rednction with dithionite, this dendrimer was cast into a film, the electronic properties of which were isotropic. (This means that on the molecular and macroscopic levels, there is a three-dimensional (3-D) electron delocalization.) The conductivity was humidity dependent. Water molecules integrate into the material s crystal structure and take part in long-distance electron transfer. Such an effect of water was also observed to enhance electric... 

Figu re 4.1 Commonly encountered chiral catalyst immobilization on dendritic polymer supports (a) core-functionalized chiral dendrimers (b) peripherally modified chiral dendrimers (c) solid-supported dendritic chiral catalysts. 

The first example of a catalytically active metallodendrimer, having catalytic groups at the periphery, was reported by van Koten, van Leeuwen and coworkers [20]. These authors prepared the nickel(II) complexes containing carbosilane dendrimers, which were successfully employed in the homogeneous regioselective Kharasch addition of polyhalogenoalkanes to the terminal C=C double bonds. Since these early studies there has been a steadily increasing number of dendrimer catalysts which have been synthesized and studied [15]. In this section, the details of peripherally modified chiral dendrimer catalysts for different asymmetric catalytic reactions will be summarized. 

In this section, the electrochemical properties of dendrimers peripherally modified with redox-active moieties and their applications will be reviewed. 

Using peripherally modified PAMAM dendrimer-supported chiral salen Mn(III) complex, asymmetric epoxidation of unfunctionaUzed olefins was performed. Modification of dendrimer was achieved through reaction of the terminal primary amion group of three generations of PAMAM dendrimer, (G , n = 1,2,3)... 

Grabchev, L, Petkov, C., and Bojinov, V. Infrared spectral characterization of poly (ami-doamine) dendrimers peripherally modified with 1,8-naphthalimides. Dyes and Pigments, 62,229-234 (2004). 

For some examples, see e.g. (a) Hogan CF, Harris AR, Bond AM et al (2006) Electrochemical studies of porphyrin-appended dendrimers. PhysChemChemPhys 8 2058-2065 (b) Jang W-D, Nishiyama N, Zhang G-D et al (2005) Supramolecular nanocarrier of anionic dendrimer porphyrins with cationic block copolymers modified with polyethylene glycol to enhance intracellular photodynamic efficacy. Angew Chem Int Ed 44 419 -23 (c) Loiseau F, Campagna S, Hameurlaine A et al (2005) Dendrimers made of porphyrin cores and carbazole chromophores as peripheral units. Absorption spectra, luminescence properties, and oxidation... 

The physical properties of dendrimers such as solubility, arising from their hyperbranched globular shapes and the peripheral groups, can be modified by end-group modification. In core-functionalized dendrimers, the immiscibility of the wedges with a solvent enables precipitation and subsequent separation by filtration. 

Low generations of hydrophobic ally modified PAMAM dendrhners also exhibit a tendency to deform at air-water interfaces in order to present the hydrophilic dendrimer interior toward water and the aliphatic chains toward the air (Sayed-Sweet et al. 1997). Similarly, the tendency of poly(aryl ether) dendrhners with peripheral carbox-ylate end groups to aggregate in aqueous media decreased with increasing generation (Fig. 11.16 Laufersweiler et al. 2001). 

Dendrimers are large molecules, and their solubility is often an issue. Thus, it is not surprising that dendrimers can be good candidates to precipitate upon and thus modify the surface of an electrode. Dendrimers with ferrocenyl peripheral units were shown to display well-defined redox waves with the ferrocenyl units oxidizing independently.176-178 These molecules oxidatively precipitated onto the electrode surface and were characterized electrochemically and by atomic force microscopy.44 It was later shown that cyclodextrin complexation increased the solubility of these molecules.179 Similar results were obtained with dendrimers containing pendant ruthenium tris (bipyridine) and bis(terpyridine) groups.180... 

For these dendrimers, the morphology of the mesophase is thus determined by the number of alkyl chains grafted on the peripheral mesogenic group, i.e. the change in the number of terminal chains per end group modifies the relationships between the hard parts and the soft parts, and... 

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