Efficient syntheses of the novel, pyrene containing branched building blocks 10, 13, 22, 25 and of a pyrene based core molecule 16 for the construction of dendrimers are reported. The main tool is a Suzuki cross-coupling. The functional groups for further growth are amines and carboxylic acids, which were used in an orthogonally protected fashion. It was proven that the building blocks could be assembled to a low generation dendrimer 17.The spherically shape, dendrimers, 1 are unlike practically all other chemical compounds and can potentially be used to generate and investigate a directed charge transfer. 2,3 If the interior of dendrimers is equipped with electron acceptors at defined positions (generations), an electron transfer should be possible from acceptor to acceptor if there is a gradient, which provides the necessary driving force. We are presently pursuing a project to construct dendrimers with a polar gradient and to use them as components to investigate a photochemically induced energy and charge transfer. 4 The polarity gradient should serve as a driving force for either of these transfers and the large distance between the dendrimers' core and surface ought to increase the life-time of an eventual charge separated state. Both an outline of the concept 4 as well as first spectroscopical results 5 have been preliminarily published. We here report in detail the basic synthetic sequences involved. The aim is to construct pyrenes containing branched building blocks like compounds 10 and 13 for dendron and dendrimer synthesis and therefore carry an AB 2 functionality pattern (A: carboxylic acid; B: amine) to allow for application of repetitive growth schemes. The coupling between individual dendritic blocks is based on peptide derived amide bond formation, which has been proven to be very efficient in dendrimer chemistry. 6The concept involves the use of fluorescence probes and dummies, 7 whereby the latter should structurally resemble the former as closely as possible. This is why in the following some of the dendrons carry a methylene spacer between pyrene and acceptor-substituted branching unit (potential dummies) and others do not (potential probes). Tests on model compounds have shown that the incorporation of just a methylene spacer is sufficient for that purpose. 5The whole strategy rests upon compounds 4a and 4b (Scheme 1), which besides the ester (as the protected A of an AB 2 building block), carry two bromo functionalities and one iodo functionality. It was anticipated that, by utilizing the high iodo selectivity 8 of Suzuki cross-coupling (SCC) 9 that the required functionality could be incorporated into the molecule. If both iodinated and brominated sp 2 -hybridized C-atoms are present in the same molecule, the pyrene unit and two N-protected amine-terminated arms (as the B's of an AB 2 building block) could be incorporated at C-I and C-Br, respectively. Scheme 1 Reagents and conditions: (a) EtOH, toluene, H 2 SO 4 , reflux, 5 h, 78% (b) concd HOAc, Br 2 , 0 °C, 12 h, 87% (c) i. H 2 O, NaNO ...
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