Metal complex dendrimers absorption spectra

It should be noted that the 1,3-dimethoxybenzene and 2-naphthyl chromophoric units contained in the branches of the dendrimer are not involved in metal coordination. In some way, they belong to a second coordination sphere. If is considered a large metal complex, the absorption and emission bands of the 1,3-dimethoxybenzene and 2-naphthyl chromophoric units can formally be classified as LC. However, can be more properly viewed as a supramolecular (multicomponent) species (12). In such species, each chromophoric imit displays its own absorption spectrum since there is no appreciable interactions among them in the ground state, but in the excited state even weak interactions can cause intercomponent energy or electron-transfer processes. This kind of reasoning can also be applied to all the other systems discussed in this chapter. 

Dendrimer-encapsulated bimetallic clusters can also be prepared by simultaneous co-complexation of two different metal ions, followed by a single reduction step. For example, the absorption spectrum of a solution containing G6-OH, PtCl , and PdCl is essentially the sum of the spectra of a solution containing G6-OH + PtCl and a second solution containing G6-OH -i- PdCl, which strongly suggests co-complexation of Pt + and Pd + within individual dendrimers. After reduction of these co-complexed materials, a new interband transition, which has an intensity different from that of either a pure Pt or Pd cluster, is observed. 

Dendrimer 1 (Fig. 5) is a classical example of a dendrimer built around a metal complex core. In this compound, the 2,2 -bipyridine ligands, that constitute the first coordination sphere of the Ru ion, carry branches containing 1,3-dimethoxybenzene and 2-naphthyl chromophoric imits separated by aliphatic connectors (10). Since the interchromophoric interactions are weak, the absorption spectrum of 1 is substantially equal to the summation of the spectra of [Ru(bpy)s], which is characterized by a broad spin-allowed Ru bpy metal-to-ligand (MLCT) band around 450 nm (11), and of the chromophoric groups contained in the branches, which show very intense bands in the near UV region. 

Upon titration with Zn(CF3S03)2, no change was observed in the absorption spectrum, whereas strong changes were observed in the emission spectrum. Such changes, qualitatively similar to those caused by protonation, indicate that a 1 1 complex, [Zn(6)], is first formed and then replaced by a 2 1 species [Zn2(6)] (log j6i i = 9.7 and log j 2 i = 16.1 for these two species, respectively). In the 1 1 complex [Zn(6)] , the metal ion is likely sandwiched between the two cyclam emits. As previously observed in the case of dendrimer 5, apparently, the dendrimer... 

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