Dendrimers amine coordinating units

Dendrimers with Multiple Amine Coordinating Units... 

In most cases, metal ion coordination by a dendrimer takes place by units that are present along the dendrimer branches (e.g., amine, imine, or amide groups) or appended at the dendrimer periphery (e.g., terpyridine, cathecolamide ligands). When multiple identical coordinating units are present, dendrimers give rise to metal complexes of variable stoichiometry and unknown structures. Luminescent dendrimers with a well defined metal-coordinating site have been reported so far [16, 17], and the most used coordination site is 1,4,8,11-tetraazacyclotetradecane (cyclam). 

It has been demonstrated that dendrimers can be used also as fluorescent sensors for metal ions. Poly(propylene amine) dendrimers functionalized with dansyl units at the periphery like 34 can coordinate metal ions by the aliphatic amine units contained in the interior of the dendrimer [80]. The advantage of a dendrimer for this kind of application is related to the fact that a single analyte can interact with a great number of fluorescent units, which results in signal amplification. For example, when a Co ion enters dendrimer 34, the fluorescence of all the 32 dansyl units is quenched with a 32-fold increase in sensitivity with respect to a normal dansyl sensor. This concept is illustrated in Fig. 3. 

Vogtle F, Gestermann S, Kauffmann C et al (2000) Coordination of C02+ ions in the interior of polypropylene amine) dendrimers containing fluorescent dansyl units in the periphery. J Am Chem Soc 122 10389-10404... 

A first generation poly(amido amine) dendrimer has been functionalized with three calyx[4]arenes, each carrying a pyrene fluorophore (4) [30]. In acetonitrile solution the emission spectrum shows both the monomer and the excimer emission band, typical of the pyrene chromophore. Upon addition of Al3+ as perchlorate salt, a decrease in the excimer emission and a consequent revival of the monomer emission is observed. This can be interpreted as a change in the dendrimer structure and flexibility upon metal ion complexation that inhibits close proximity of pyrenyl units, thus decreasing the excimer formation probability. 1H NMR studies of dendrimer 4 revealed marked differences upon Al3+ addition only in the chemical shifts of the CH2 protons linked to the central amine group, demonstrating that the metal ion is coordinated by the dendrimer core. MALDI-TOF experiments gave evidence of a 1 1 complex. Similar results have been obtained for In3+, while other cations such as Ag+, Cd2+, and Zn2+ do not affect the luminescence properties of... 

For historical reasons, examples of case 2d, containing multiple amine or amide groups in their branches, will be discussed first. These types of dendrimers give rise to metal complexes of variable stoichiometry and unknown structures since they contain several more or less equivalent ligand units and not well-defined coordination sites. 

Fig. 9. Formula of a dendrimer constituted by a poly(propylene amine) structure functionalized with 32 dansyl units at the periphery, capable of coordinating Co ions, and the corresponding scheme (Fig. 2d).

Kim and coworkers adapted the periphery of PPIs for a similar, albeit more ionic, coordination with cylindrical cucurbituril (CB 38) units to give a stable, poly(pseudorotaxane) architecture. The required l w(am-monium)-coated scaffolding was accessed via treatment of the dendriiner amine surface with the perfluorophenyl-activated ester of a Ijw(protected) diamine-modified acetate 3 9 followed by deprotection (HBr) of the diamine chains. Dissolution of the polyprotonated framework in H2O followed by the addition of CB afforded the self-assembled product 40. Preliminary studies aimed at decomplexation via treatment with base yielded partial separation. T] (spin-lattice) relaxation experiments with the CB-dendrimer complex showed a sharp increase with higher generations, suggesting that the outer shell possessed attributes of a solid phase such behavior has also been noted by Jansen et al. ... 

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