Metal-polypyridine units

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... 

Currently, metal-polypyridine units are the molecular building blocks of choice whenever a compound with special electro- and/or photo-activity is to be designed. The research emphasis has somewhat shifted from fundamental studies of electron transfer reactivity and excited state properties of individual complexes to the design of new functional molecules and supermolecules with predetermined properties. 

Redox Properties of Supermolecules Containing Metal-Polypyridine Units... 

Depending on the particular structure, the following supramolecular effects can be distinguished (i) encapsulation of a metal-polypyridine unit, (ii) interaction between linked metal-polypyridine units, (iii) interactions between metal-polypyridine units and chemically different redox centers, and, (iv) host-guest interactions where a metal-polypyridine unit is the part of the host. 

Another type of supramolecular systems contain a metal-polypyridine unit (Ru or Os bis- or tris-polypyridines, Re (CO)3(N,N)", or Cu (catphen)2" ) linked co-... 

It is also possible to build metal-polypyridines into supramolecular hosts and receptors. For example, one of the bpy ligands may be a part of a crown ether ring [34], or it may be capped with a calixarene [33, 257] attached at the 4,4 positions. Electrochemical behavior of the metal-polypyridine unit is then affected by an interaction with a guest , e.g. an alkali metal cation. 

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]. 

Several kinds of supramolecular effects on the redox behavior of metal-polypyridine units were mentioned in Section 5.3.6. Besides influencing photophysical properties, incorporation of metal-bipyridine chromophores into supramolecular structures enables new electron-transfer reactions. Since these processes are dealt with in detail in other chapters, only basic principles and links between the behavior of isolated and supramolecular metal-polypyridine units will be mentioned here. 

Intramolecular excited state electron transfer in bi- and polynuclear metal polypyridine complex is a much studied process, especially in relation with the development of molecular wires [38, 51, 52, 83, 87, 88, 318]. The rates are dependent on the nature of the bridging groups linking the metal-polypyridine units. These process will be discussed in detail elsewhere in this book. 

Multicomponent assemblies in which a photoredox-active metal-polypyridine unit is combined with electron acceptors and/or donors show very rich photo-induced electron transfer reactivity [38]. Such species are often called molecular dyads (triads, tetrads. ..). Electron transfer usually occurs from an excited metal-polypyridine unit M to an attached acceptor ... 

The electron injection from the MLCT-excited central metal-polypyridine unit M to an acceptor A seems to be kinetically preferred over an alternative D- M-A —> D+-M -A step, even if the energetics are similar. This is because the excited electron resides at the complex periphery, that is at the polypyridine ligand to which the acceptor group is attached. 

Polynuclear complexes, molecular dyads, triads, and other supermolecules composed of redox- and photo-active metal polypyridine units have a great promise as components of future molecular electronic or photonic devices as optical switches, relays, memories, etc. [38, 46],... 

When connected to other components of a supramolecular assembly or to a semiconductor electrode through a polypyridine ligand, a metal-polypyridine unit is kinetically especially suited to inject an electron from its MLCT excited state, that is to act as an excited state reductant. Ultrafast rates can be reached. 

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. 

Osawa et al. [ 112] described the Pd-catalyzed one-step graft reaction of metal-lodendron units with a core coordination (metal center=Ni, Cu, Ru) compound. This afforded a polypyridine-metal-based stiff dendritic architecture which... 

When only polypyridine-type ligands are present, each mononuclear metal-based unit exhibits intense LC bands in the UV region and moderately intense MLCT bands in the visible. As it is shown by the electrochemical behavior vide supra), in the polymetallic species there is some interaction among the neighboring metal-based units. To a first approximation, however, each building block carries its own absorption properties in the polynuclear species so that the molar absorption coefficients exhibited by the compounds of higher nuclearity are huge, as it is clear by a cursory examination of the absorption data reported in Table 3. For example, the spectra of the decanuclear compounds lOB and lOC (Scheme 1 and Table 1)... 

In supramolecular systems, electronic interactions between metal-polypyridine and other redox-active or units are too small to perturb ground-state electrochemical and spectroscopic properties but are sufficient to enable very fast intramolecular electron-transfer reactions upon excitation. 

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. 

Among outputs, luminescence emission is considered to be mie of the most attractive, owing to the ease of detection and the cheap fabrication of devices in which it is detected. Many examples of fluorescent photochromic molecules have been published, by combining a DTE unit with a fluorophore [4]. Incorporation of the DTE fragment into the ligands of transition-metal polypyridine complexes allows the photoreaction to proceed via a triplet state leading to a photoregulation of phosphorescence. 

Similar Ru(II) polypyridine units were connected to dirhodium(ll) tetracarboxylate platforms to form the supramolecular assemblies 80-82 (Figures 9.37 and 9.38) [166], exhibiting severed metal-centered oxidation and ligand-centered reduction processes, and intense LC and MLCT bands centered on the Ru(II) units. Efficient energy transfer from MLCT (Ru-based) to the lowest-energy... 

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. 

Balzani et al. prepared dendrimers with metal complexes serving both as core [36] and as branching unit The metallodendrimer in Fig. 2.10 is constructed solely from polypyridine ligands and transition metal ions. Such dendritic transition metal complexes can be synthesised both convergently and divergently and different transition metal ions (ruthenium/osmium) can be incorporated. This provides a means of influencing the luminescence properties of the den-drimer. Thus the energy transfer process proceeds from the inside outwards in... 

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