Guest inclusion by dynamic processes diffusion

Non-covalent bonding of guest molecules in the interior of a dendrimer molecule can also proceed dynamically. In micelle-like amphiphilic dendrimers, guests can diffuse into and out of the interior of the dendrimer scaffold via hydrophilic and hydrophobic interactions (Fig. 6.18). 

Balzani, Vogtle et al. followed the kinetics of complexation of metal ions (Co2+, Cu2+) for the example of dansyl-bearing POPAM dendrimers (see Chapter 4, Fig. 4.3) [33]. In the first step the cations interacted with the dimethylami-no nitrogen of the dansyl units, as can be recognised from the quenching of their fluorescence - which is complete at high ion concentrations (Fig. 6.19a). 

If such solutions are allowed to stand for several hours, their fluorescence surprisingly reappears. This is interpreted by assuming that the metal ions slowly penetrate into the interior of the dendrimer to the N atoms of the PO-PAM-core, again freeing up the fluorescing dansyl units (Fig. 6.19b). 

It goes without saying that a non-dendritic monodansyl amino compound as reference substance will be unable to show analogous behaviour because it cannot form intramolecular niches for inclusion of cations. Thus dendritic architectures lead to specific properties which are unattainable or attainable only with difficulty in the absence of branching units. 

Newkome et al. were the first to synthesise symmetrical, quater-directionaF cascade molecules with a carbon scaffold bearing 36 terminal carboxyl groups -all at an equal distance from the neopentyl core (Fig. 6.20a). The carboxyls were converted into the corresponding ammonium and tetramethylammonium car-boxylates. Synthesis of these dendritic unimolecular micelles with hydrophobic core and hydrophilic shell was accomplished up to the fourth generation by coupling of a dendritic hypercore (constructed from 4,4-bis(4 -hydroxyphenyl)-pentanol monomer) and PEG mesylate (PEG = polyethylene glycol). Dyes such 

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