Metal-polypyridine dendrimers

Redox-Active Metal-Polypyridine Dendrimers as Light-Harvesting Antennae... 

Although the building blocks of the metal-polypyridine dendrimers are mononuclear species, the effective models for the high-nuclearity dendrimers are the dinuclear species. This is because the properties of the mononuclear and dinuclear compounds (absorption, luminescence, and redox properties) are significantly different, as a consequence of the bis-chelation of the dpp ligand.20-22 In the high-nuclearity dendrimers, dpp always plays the role of bridge, so the redox properties (and indeed also the spectroscopic properties) of the dendrimers are directly connected to the properties of the dinuclear species. Representative dinuclear species are discussed here. 

The hexanuclear Ru6 species has four outer and two inner metal centers oxidation active. Both in acetonitrile at room temperature ( 1/2 at + 1.52 V) and in liquid S02 at low temperature ( 1/2 at + 1.46 V), an oxidation process involving the practically simultaneous one-electron oxidation of the four outer Ru(II) centers is evidenced (Fig. 5.9 and Table 5.1). This confirms that the electronic interaction between metal centers that are not directly connected via a bridging ligand is negligible from an electrochemical viewpoint in the metal-polypyridine dendrimers. At more positive potentials, only recordable in liquid S02 at low temperature (Fig. 5.9), a bielectronic process, related to the simultaneous one-electron oxidation of the two inner metal centers at + 2.11 V, is found. This result was at a first sight surprising, since the redox... 

The reduction pattern of Ru6 warrants some additional comments it can be considered that the first electron probably enters the inner 2,3-dpp ligand, which should be the easiest site to be reduced however, upon second electron addition to one of the outer bridging ligands, the first electron should move to one of the other outer bridges, for coulombic reasons. This sort of redox-induced electron shift is not rare in this class of metal-polypyridine dendrimers.38... 

Electronic Energy Transfer in Metal-Polypyridine Dendrimers ... 

As the energy of the excited states and the redox levels of each metal-polypyridine unit depend on metal and ligands in a predictable way, the simultaneous presence of different metals in a dendritic structures gives rise to intramolecular energy transfer processes as well to different redox patterns with multielectron processes. In particular, the tetranuclear [Os(2,3-dpp)3 (2,3-dpp)Ru(bpy)2 3]8+ (OsRu3) shown in Fig. 5.3 has been designed to achieve an efficient antenna effect. This species can also be considered a first-generation mixed-metal dendrimers.31... 

Supramolecules containing metal-polypyridine units, especially the Ru(dpp)-based dendrimers, could be used as electron reservoirs or components of molecular-electronic devices. Supramolecules in which an electroactive M(N,N) group is attached to a receptor capable of molecular recognition (crown ethers, calixarenes, cryptands etc.) can work as electrochemical sensors. Electrochemical recognition of cations as well as anions has been reported [33-35, 257, 263]. 

The electroactive unit is the core of the dendrimer (Figure 2a). The most commonly used units in this category are metal-polypyridine complexes and porphyrins. 

The electroactive units are the peripheral groups (Figure 2b). These dendrimers are functionalized on their surface, and all the units are equivalent. The most common units of this type are ferrocene and metal-polypyridine complexes. 

Metal-polypyridine complexes have been used extensively as electroactive units in dendrimers [50, 51, 70-115]. For reasons of space, only representative examples are presented here. 

Electrochemical investigation of dendrimers containing metal-polypyridine units shows the presence of many redox processes, as usually oxidation is a metal-centered process, while reduction is centered on the polypyridine ligands. 

Balzani et al. [9] reported metal-containing dendrimers, where the core and branching unit are built up from ruthenium complexes of a polypyridine... 

Dendrimers Containing Polypyridine-Type Metal Complexes. 203... 

Dendrimers built around a metal complex as a core. These compounds can be considered metal complexes of ligands carrying dendritic substituents (Fig. 1 a). The most commonly used metal complex cores are porphyrin complexes, polypyridine complexes, and ferrocene-type compounds. 

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

Compound 12 is the largest photoactive dendrimer in which organic branches have been attached around a polypyridine metal core [29]. This dendrimer was obtained by reacting the metal precursor with a modified bpy carrying highly-... 

The aim of this chapter is limited to reviewing some recent developments concerning luminescent dendrimers that can play the role of ligands and sensors for luminescent and nonluminescent metal ions, mainly investigated in our laboratories, with particular references to transition metal or lanthanide ions. We will not discuss dendrimers constituted by polypyridine metal complexes [21] and porphyrins [22] since it is outside the scope of the present paper. 

---