Transfer core-functionalized dendrimers

In these dye-functionalized dendrimers, light absorbed by the numerous peripheral coumarin-2 units is funneled to the coumarin-343 core with remarkably high efficiency (toluene solution 98% for the first three generations 93% for compound 8). Given the large transition moments and the good overlap between donor emission and acceptor absorption, energy transfer takes place by Forster mechanism [34]. 

Photoinduced electron transfer studies have been performed in a more limited number of cases, due to the fact that the simultaneous presence of two reaction partners with appropriate characteristics is necessary. The study of suitably functionalized dendrimers can furnish information on the accessibility of the inner shell or core to external reagents (such as molecular oxygen or other small molecules) and on interchromophoric interactions within the dendritic structures. Electro-inactive dendrimers can also be important in regulating the electron transfer reactions between hosted partners in this sense dendrimers may also constitute a peculiar medium to investigate electron transfer reactions in restricted spaces. 

Both energy and electron transfer quenchers have been employed to show that the quenching rates of the fullerene triplet state are decreased as a function of the size of the dendrimer shell [36]. These results further demonstrate that fullerene is an excellent functional group to probe the accessibility of a dendrimer core by external molecules. 

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

A current example of the value of shape persistent dendrimers in fundamental research is their use in the study of electron transfer processes between different redox centers. If one redox center is situated in the core and another redox center is located in the shell at a distinct distance from the other center, the distances which are necessary for an electron transfer to take place can be studied. This currently presents an objective of our research. Furthermore, dendrimers coated with two types of functionalities at the periphery are currently under intense investigation. The attachment of two different types of dyes at the periphery is of interest in the study of the energy transfer between these dyes (light harvesting antenna). 

Star poly(methylmethacrylates) were synthesized via atom transfer polymerization using a small carbosilane dendrimer functionalized with a tertiary bromide moiety as an initiator core (Figure 12)100,101. a convergent approach to star polymers with a carbosilane dendrimer core was described in a report by Allgaier and coworkers102, in which living poly(butadienyl-lithium) arms were coupled with various SiCl-terminated carbosilane den-drimers. Utilizing smaller dendrimers with lower functionality was found to yield nearly ideal results in terms of substitution and polydispersity. 

Within this context and by using Figs. 5 and 46 as conceptual references, let us examine the degree of mimicry offered by the more simplistic dendrimers in the transfer of genealogical information. First, as one assembles poly(amidoamine) (PAMAM) dendrimer (see Sect. 5.2.3.1) around an ammonia initiator core, it becomes readily apparent that important molecular ( genetic ) information about the dimensions, shape and multiplicity of NH3 are being transferred from the initiator core to the surface of the PAMAM dendrimers as a function of generation. This may be visualized as shown in Fig. 47. In this context, the dendrimer initiator... 

Fig. 47. Molecular information (genealogy) transferred from the initiator core (NH3) to the surface of Starburst dendrimers as a function of generation, using a divergent synthesis protocol...

Dendrimers have been used as large compartmented hosts for drug delivery, as carriers for catalytically active sites in flow reactors, and for charge or energy transfer purposes. New types of such dendrimers were prepared starting from functionalized poly-p-phcnylcncs as polymeric cores whose functional groups were further used to anchor dendrons [165]. The polymers were synthesized from dibromide 117 and diboronic acid 118 via a Suzuki crosscoupling the dendrons were attached via a Williamson ether synthesis [165]. 

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