Scolastica Serroni

Dendrimers based on photoactive metal complexes. Recent advances

Abstract Recent advances in the field of photoactive dendrimers containing metal complexes are reviewed. Dendrimers with [Ru(bpy)3]2+ as a core exhibit the characteristic [Ru(bpy)3]2+-type luminescence that can be (i) protected from external quenchers by the dendrimer branches and (ii) sensitized by chromophoric groups contained in the periphery of the dendrimer (antenna effect). Several examples of dendrimers fully based on transition metal complexes (i.e., containing a metal at each branching point of the dendrimer structure) have been investigated with the purpose of light harvesting. Dendrimers containing one or more free base and metal porphyrin units have been investigated for light harvesting and for a variety of other purposes. Scattered examples of other types of photoactive dendrimers are also reviewed.

Dendrimers based on ruthenium(II) and osmium(II) polypyridine complexes and the approach of using complexes as ligands and complexes as metals

The use of the “complexes as ligands and complexes as metals” synthetic strategy for the preparation of luminescent and redox-active Ru(II) and Os(II) dendrimers, the dominant synthetic approach for this novel class of compounds, is reviewed. A few comments on the photophysical and redox properties of the metal dendrimers are provided and an overview of alternative synthetic approaches is also presented.

ELECTROCHEMICAL AND PHOTOCHEMICAL PROPERTIES OF METAL-CONTAINING DENDRIMERS

Metal complexes are characterized by a precise molecular geometry related to the characteristic coordination number of the metal ion and also, in some cases, to the rigid structure of the ligands. Furthermore, they can exhibit valuable properties such as absorption of visible light, luminescence, and reduction and oxidation levels at accessible potentials. By using metal complexes to construct a dendrimer it is therefore possible to incorporate in the dendritic structure many “pieces of information”. In this paper the available results on the electrochemical and photochemical properties of metal-containing dendrimers are reviewed. It is shown that by a suitable choice of the metal-based building blocks, it is possible to control the number of exchanged electrons at a fixed potential and the pattern of migration of electronic energy. These properties can be exploited for multielectron catalysis and light harvesting.

Harvesting sunlight by artificial supramolecular antennae

Abstract We have designed a divergent synthetic strategy, based on the “complexes-as-metals and complexes-as-ligands” procedure, to prepare polynuclear metal compounds of nanometer size and dendritic structure. Such a synthetic strategy is modular, very flexible, efficient, and characterized by a full, step-by-step control of the growing process. It allows us to obtain supramolecular arrays where different metal ions, bridging ligands, and terminal ligands can occupy predetermined sites. In this way, the light absorption, luminescence, and redox properties of these polynuclear compounds can be varied. In particular, it is possible to obtain a synthetic control of the direction(s) of electronic energy transfer after light absorption. This is a step towards the construction of nanometer-sized antennae for harvesting solar energy.

Recent advances in luminescent polymetallic dendrimers containing the 2,3-bis(2′-pyridyl)pyrazine bridging ligand

Abstract Some new developments in the area of metal-based light-harvesting dendrimers based on the 2,3-bis(2′-pyridyl)pyrazine bridging ligand are presented, with particular regard to unidirectional energy transfer, enhanced light absorption, coupling dendritic structures with electron donors, and measurement of some of the ultra-fast processes occurring in this class of compounds.

Polynuclear metal complexes of nanometre size. A versatilesynthetic strategy leading to luminescent and redox-active dendrimers madeof an osmium(II)-based core and ruthenium(II)-basedunits in the branches

A docosanuclear metal complex of nanometric size and dendritic shape made of an osmium(ii)-based core and containing 21 ruthenium(ii)-based units in the branches has been prepared. The key building blocks are the [Os(2,3-dpp) 3 ] 2+ ‘complex ligand’, and the [Ru(2,3-Medpp) 2 Cl 2 ] 2+ and [{Ru(bpy) 2 (µ-2,3-dpp)} 2 RuCl 2 ] 4+ ‘complex metals’ {2,3-dpp=2,3-bis(2-pyridyl)pyrazine; 2,3-Medpp + =2-[2-(1-methylpyridiniumyl)]-3-(2-pyridyl)pyrazine; bpy=2,2′-bipyridine}. The first step of the synthesis is the formation of the tetranuclear [Os{(µ-2,3-dpp)Ru(2,3-Medpp) 2 } 3 ] 14+ species in which the peripheral ligands 2,3-Medpp + are 2,3-dpp ligands with the second chelating site inactivated (protected) by methylation. This species is obtained from the reaction of the [Os(2,3-dpp) 3 ] 2+ ‘complex ligand’ core, which contains three open chelating positions, with three equivalents of the [Ru(2,3-Medpp) 2 Cl 2 ] 2+ ‘complex metal’, where the labile Cl - ligands can be replaced by the chelating units of the core. Successive demethylation (deprotection) of the tetranuclear compound opens the six peripheral chelating sites. At this stage, the divergent synthesis can be iterated {reaction with six equivalents of the [Ru(2,3-Medpp) 2 Cl 2 ] 2+ ‘complex metal’} with formation of the protected decanuclear compound [Os{(µ-2,3-dpp)Ru[(µ-2,3-dpp)Ru(2,3-Medpp) 2 ] 2 } 3 ] 32+ . Alternatively, in a convergent approach, the reaction of the deprotected tetranuclear species with six equivalents of the trinuclear [{Ru(bpy) 2 (µ-2,3-dpp)} 2 RuCl 2 ] 4+ ‘complex metal’ leads to the docosanuclear [Os{(µ-2,3-dpp)Ru[(µ-2,3-dpp)Ru{(µ -2,3-dpp)Ru(bpy) 2 } 2 ] 2 } 3 ] 44+ species. The absorption spectra, luminescence properties, and electrochemical behaviour of [Os(2,3-dpp) 3 ] 2+ , [Os(µ-2,3-dpp) 3 {Ru(2,3-Medpp) 2 } 3 ] 14+ , [Os{(µ-2,3-dpp)Ru[(µ-2,3-dpp)Ru(2,3-Medpp) 2 ] 2 } 3 ] 32+ , and [Os{(µ-2,3-dpp)Ru[(µ-2,3-dpp)Ru{(µ -2,3-dpp)Ru(bpy) 2 } 2 ] 2 } 3 ] 44+ have been investigated.

Bottom-up strategy to obtain luminescent and redox-active metal complexes of nanometric dimensions

Abstract By using the “complexes as metals and complexes as ligands” synthetic strategy, it has been possible to obtain oligonuclear metal complexes which contain up to 22 metal ions. Complexes containing two different types of metal ions (Ru and Os; Ru and Rh; Os and Rh; Ru and Ir) have also been prepared. The light absorption, luminescence, and redox properties of these polynuclear compounds can be varied by changing (i) the nuclearity, (ii) the nature of metal ions, bridging ligands and/or terminal ligands, and (iii) the position of the various components in the supramolecular structure. Because of their strong absorption in the visible spectral region and the possibility to predetermine the direction of energy migration, these compounds could be used as photochemical molecular devices for harvesting solar energy.

Electrochemistry and spectroelectrochemistry of ruthenium(II)-bipyridine building blocks. Different behaviour of the 2,3- and 2,5-bis(2-pyridyl)pyrazine bridging ligands

We report the results of an investigation, using electrochemical and spectroelectrochemical techniques, into redox properties of uncoordinated free bis-chelating 2,3- and 2,5-bis(2-pyridyl)pyrazine ligands (2,3- and 2,5-dpp) and of the complexes of the [Ru(2,3-dpp)n(bpy)3−n]2+ and [Ru(2,5-dpp)n(bpy)3−n]2+ families (bpy=2,2′-bipyridine), which are used as building blocks for obtaining polynuclear complexes. For comparison purposes, the electrochemical behaviour of the [Ru(2,3-dpp)(DCE-bpy)2]2+complex, where DCE-bpy is 5,5′-dicarboxyethyl-2,2′-bipyridine, has also been investigated. Correlations of the E1/2 values observed for the compounds examined (genetic diagrams) have allowed us to assign all the ligand-based reduction processes as well as to discuss electronic interactions. The localisation of the first three reduction processes for each complex has also been established on the basis of the spectroelectrochemical results. Theoretical calculations (AM1 semiempirical and ab-initio level) carried out for the 2,5-dpp and 2,3-dpp ligands show that, in the uncoordinated state, the former ligand does not exhibit any substantial conformation arrangement, whereas the latter has a stable conformation for a large (56°) dihedral angle between the pyridyl and pyrazine rings. The changes in conformation upon mono- and bis-coordination of 2,3-dpp can account for its peculiar electrochemical behaviour consisting in a change of the number of redox processes with varying coordination state.

CONTROLLING THE DIRECTION OF PHOTOINDUCED ENERGY TRANSFER IN MULTICOMPONENT SPECIES

The direction of the efficient photoinduced energy transfer which takes place in two new multicomponent species that contain RuII-polypyridine chromophores and anthracene subunits is controlled by the temperature. The switch in the direction of the energy-transfer process arises from the quite different intrinsic lifetimes of the luminophores and the relatively small energy gap between the lowest energy excited states located in each subunit. Novel molecular-level devices based on this particular effect are foreseen.

Ni(0) catalysed homo-coupling reactions: a novel route towards the synthesis of multinuclear ruthenium polypyridine complexes featuring made-to-order properties

Abstract A new synthetic procedure for the efficient preparation of dinuclear ruthenium(II) polypyridyl complexes is reported. The compounds synthesised are [(bpy)2Ru(BPBT)Ru(bpy)2](PF6)2 and [(bpy)2Ru(BPZBT)Ru(bpy)2](PF6)2 (bpy=2,2′-bipyridine; H2BPBT=5,5′-bis(pyridin-2-yl)-3,3′-bis(1,2,4-triazole); H2BPZBT=5,5′-bis(pyrazin-2-yl)-3,3′-bis(1,2,4-triazole). Electrochemical experiments show that the two dinuclear systems investigated exhibit pH switchable intercomponent interactions.