Alberto Juris

Photochemistry and photophysics of Ru(II)polypyridine complexes in the Bologna group. From early studies to recent developments

Abstract The investigations carried out in the Bologna group on Ru(bpy)32+ (bpy=2,2′-bipyridine) and related systems are reviewed. The following topics are discussed: (i) bimolecular energy and electron-transfer processes (including the measure of the rate of self-exchange energy-transfer); (ii) tuning the excited state properties by changing the ligands (including a caged version of Ru(bpy)32+); (iii) chemi- and electrochemiluminescent processes (including the description of an artificial firefly); (iv) photochemistry without light; (v) molecular-level wires for energy and electron transfer; (vi) dendrimers for light harvesting; (vi) light-powered molecular machines.

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.

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.

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.

Can a functionalized phosphine ligand promote room temperature luminescence of the [Ru(bpy)(tpy)]2+ core?

Unexpected room temperature luminescence is observed and rationalized by highly challenging excited state calculations for a functionalized phosphine ligand coordinated on the [Ru(bpy)(tpy)](2+) core.

Ligand-centered luminescence from a ruthenium(II) complex

Abstract The complex Ru( i -biq) 2+ 3 , where i -biq is 2,2,′-bis-isoquinoline, was synthesized and studied spectroscopically and electro-chemically. The strong luminescence exhibited at 77 K in rigid glasses is characterized as a triplet ligand-centered emission. It is the first example of ligand-centered luminescence from a Ru(II) complex.

From a molecular to a supramolecular photochemistry

Abstract Following a current trend of chemical research, photochemical investigations are moving from molecular to supramolecular species. Some of the results obtained by the authors with supramolecular species containing metal complexes are briefly reviewed, with particular emphasis on (i) cage-type complexes, (ii) host-guest systems, (iii) metal catenates, and (iv) oligonuclear metal complexes.

Luminescent host–guest complexes involving molecular clips and tweezers and tetracyanobenzene

The molecular tweezer 1 and the clips 2 and 3, containing naphthalene or anthracene in the sidewalls, form host–guest complexes in CHCl3 solution with the guest TCNB (1,2,4,5-tetracyanobenzene). The interaction leading to formation of the adduct is essentially of CT (charge-transfer) nature. A luminescence emission of CT origin is observed from the host–guest complexes both in fluid solution at 298 K and in rigid matrix at 77 K; to our knowledge, this is the first case of CT luminescence from a host–guest complex. At room temperature, the luminescence maxima are observed at 570, 614, and 668 nm, respectively, for the complexes based on the hosts 1, 2, and 3. Spectrophotometric and spectrofluorimetric titrations were performed to investigate the association processes. In all cases 1∶1 complexes are formed, with association constants 7.3 × 105, 5.4 × 106, and 1.24 × 104 L mol−1, respectively, for the receptors 1, 2, and 3. In the case of 2, a species with a 2∶1 host∶guest ratio is also formed. The system 1+TCNB was further investigated by cyclic and differential pulse voltammetry, giving the rate constants for adduct formation (1.9 × 108 L mol−1 s−1) and disassembling (2.0 × 102 s−1). The association/dissociation dynamics between the receptors 1 and 2 and the guest TCNB is discussed in relation to the receptor topology.