Francesco Barigelletti

Photochemistry and Photophysics of Coordination Compounds: Iridium

Mononuclear Ir(III)-polyimine complexes show outstanding luminescence properties, i.e., high intensities, lifetimes in the μs time range, and emission wavelengths that can be tuned so as to cover a full range of visible colors, from blue to red. We discuss the approaches for the use of ligands that afford control on luminescence features. Emphasis is placed on subfamilies of cyclometalated complexes, whose recent enormous expansion is motivated by their potential for applications, including that as phosphorescent dopants in OLEDs fabrication. The interplay of the different excited states associated with the luminescence, usually of MLCT and/or LC nature, is examined and the possible detrimental role of MC levels toward the luminescence properties is outlined. Ir(III)-polyimine moieties can be incorporated within multicomponent arrays where they can play as photoactive and/or electroactive units in photoinduced energy and electron transfer processes. The field is reviewed with attention to the processes of light collection and conversion into chemical energy.

Photoactive molecular wires based on metalcomplexes

Molecular wires incorporating polypyridine metal complexes are amenable to studies of directional energy and electron transfer. The complexes are chromophores mainly based on Ru(II), Os(II), Rh(III), and Re(I) centres, which usually exhibit luminescence and can play as donor (D) or acceptor (A) units. A bridging ligand (B) provides both the structural and electronic connectivity between D and A and the DAB wires are flexible or rigid, depending on the spacers included within the bridge. Developments regarding multicentre systems and stereochemically interesting systems are taken into account.

A family of luminescent coordination compounds: iridium(III) polyimine complexes

30 years of IrIII coordination chemistry with polyimine-type ligands are summarized. Over the years, milder reaction conditions have been used for their synthesis, allowing the incorporation of various functional substituents. Complexes are described with bidentate and terdentate ligands, with both N- and C-donor sites. All complexes are luminescent, with predominantly charge-transfer or ligand-centred emissive states depending on the charge density donated from the ligands to the metal. IrIII excited state lifetimes range from nanoseconds to microseconds. A wide range of properties are obtained: [IrN6]3+ complexes are strong photooxidants while tris-cyclometallated [IrN3C3] complexes are strong photoreductants.

PHOTOINDUCED PROCESSES IN MULTICOMPONENT ARRAYS CONTAINING TRANSITION METAL COMPLEXES

Abstract The authors’ recent activity in the study of photoinduced energy and electron transfer in linear arrays containing porphyrins assembled around a Ru(II) ion, is reviewed. The effect of substituents and distance and the role of the heavy metal ion is discussed. The photophysical and electrochemical properties of Ir(III) terpyridine complexes indicate that Ir(III) ion is a good candidate to successfully replace Ru(II) in the construction of multiporphyrinic linear arrays to achieve efficient photoinduced electron transfer. Preliminary results on multi-porphyrinic systems based on Ir(III) bis-terpyridine are presented.

Metal complexes as light absorption and light emission sensitizers

3 Mediators of Electron Transfer Processes . . . . . . . . . . . . . . . 40 3.1 Thermal Reactions: Relays . . . . . . . . . . . . . . . . . . . 40 3.2 Photoinduced Reactions: Light Absorption Sensitizers (LAS) . . . . 40 3.3 Chemiluminescent Reactions: Light Emission Sensitizers (LES) . 41 3,4 Requirements Needed for LAS and LES . . . . . . . . . . . . . 43 3.5 Why Metal Complexes? . . . . . . . . . . . . . . . . . . . . 44

Dinuclear cyclopalladated azobenzene complexes : a comparative study on model compounds for organometallic liquid-crystalline materials

The series of dinuclear 4,4′-bis(hexyloxy)azobenzene, [H(Azo-6)], cyclopalladated complexes of general formula [Azo-6)Pd(µ-X)]2, (X = Cl, Br, I, N3, SCN, OAc) and [Azo-6)2Pd2(µ-Ox)] (Ox = oxalate) have been synthesized and investigated for mesomorphism and spectroscopic properties. Single-crystal X-ray analysis of the dinuclear bromo- and iodo-bridged complexes has been performed. The structural data, compared with those of the known homologous chloro compound, show that all the [Azo-6)Pd(µ-X)]2)] (X = Cl, Br, I) molecules crystallize in the monoclinic space group P21/c and are isomorphous. They are arranged in slipped pairs with intermolecular non-bonding Pd–Pd contacts ranging from 3.668(1) A(X = Cl) to 3.758(3) A(X = I). The different nature of the bridging group allows variation of the distance between the palladium atoms and the bond environment experienced by the metal centers. Thus, this comparative study reveals that the effectiveness of the bridging group in promoting thermotropic mesophases is greater for chloride, bromide, azide or oxalate than for iodide, thiocyanate or acetate. The greatest range of liquid-crystal behavior was displayed by [Azo−6)2Pd2(µ−Ox)]. Remarkably, this compound is the first example of a metallomesogen containing the bridging oxalate group. The bimetallic complexes exhibit different absorption spectra (i.e. colors) depending, in general terms, on the nature of the bridge connecting the two cyclometalated [H(Azo-6)] moieties, which can be varied so as to tune the optical properties. Blocking the azo group in the trans position results in several cases in weakly luminescent complexes, with luminescence efficiencies ϕ ≈10−4 and luminescence lifetimes of the order of nanoseconds. Using the data obtained from the 4,4′-bis(hexyloxy)azoxybenzene [H(Azoxy-6)] derivative, [Azoxy-6)Pd(µ -Cl)]2, from the mononuclear acetylacetonate (acac) complexes [(Azo-6)Pd(acac)] and [(Azoxy-6)Pd(acac)], and from the uncomplexed [H(Azo-6)] and [H(Azoxy-6)] ligands, the nature of the excited states relevant to the photophysical behavior are discussed. Copyright

Synthesis and characterization of a homologous series of mononuclear palladium complexes containing different cyclometalated ligands

Abstract New mononuclear palladium(II) complexes formed by with 2-hydroxy-4-( n -hexyloxybenzylidene)-4′- n -hexyaniline ( HL ) and 2-phenylpyridine ( I ), benzo[h]quinoline ( II ), azobenzene ( III ), 2-benzoylpyridine ( IV ) or phenyl-2-pyridylketone-2,4-dinitrophenylhydrazone ( V ) have been synthesized and characterized by analytical and spectroscopic methods and the single-crystal structures of [(IVa)Pd(L)] and [(Va)Pd(L)] have been established. The spectroscopic and diffractrometric data account for molecular structures wherein the palladium(II) center is part of two chelate rings involving HL and one of the I – V ligands which form a five-member N,C{[(Ia)PdL], [(IIa)Pd(L)] and [(IIIa)Pd(L)]}, a six-member N,C{[(IVa)Pd(L)]}, or a six-member N , N ′{[(Va)Pd(L)]} metallacycle. The electronic spectra of [(IIIa)Pd(L)] and [(Va)Pd(L)] show an absorption maxima, at 495 ( e ∼3×10 3 M −1 cm −1 ) and 531 nm ( e ∼2×10 4 M −1 cm −1 ), respectively, which results are significantly affected by the polarity of the solvent. These chromophores should therefore be of interest for the preparation of nonlinear optical materials.

Control of photoinduced energy transfer between metal-polypyridyl luminophores across rigid covalent, flexible covalent, or hydrogen-bonded bridges

Abstract This review describes four recent examples of how photoinduced energy-transfer in dinuclear complexes can be manipulated or controlled according to the nature of the bridging pathway between the metal-polypyridyl luminophores. In the first examples, the interacting fragments [polypyridyl complexes of Ru(II), Os(II) or Re(I)] are covalently linked by the bridging ligand 2,2′:3′,2′′:6′′,2′′′-quaterpyridine which has two inequivalent bipyridyl binding sites in close proximity. In heterodinuclear Ru–Os and Ru–Re complexes, efficient inter-component photoinduced energy-transfer occurs, with the emission characteristics being sensitive to the electronic difference between the two bipyridyl sites. This, in the Ru–Re diads either Ru→Re or Re→Ru energy transfer can occur depending on which metal fragment is in which binding site. The second example is of a supramolecular assembly in which the interacting Ru(II) and Os(II) mononuclear components are associated in CH 2 Cl 2 solution via a reversible hydrogen-bonding interaction between peripheral nucleobase groups. Watson–Crick base-pair formation results in a Ru–Os diad showing efficient Ru→Os energy-transfer across the hydrogen-bonded interface. The third example describes a Ru–Re diad in which the flexible bridging ligand incorporates a diazacrown macrocyclic unit. Binding of Ba 2+ into this macrocycle at 77 K results in a decrease in the rate of Re→Ru photoinduced energy-transfer by a factor of 30, probably because of a conformational change which causes the Ru and Re components to move further apart. The final example is of a Ru–Os diad in which the [Ru(bipy) 3 ] 2+ and [Os(bipy) 3 ] 2+ components are separated by a flexible poly(oxoethylene) 18-atom chain whose conformation is solvent dependent. Changing the solvent polarity results in a conformational change in the chain, and consequently a change in the Ru⋯Os separation and hence the Ru→Os energy-transfer rate. Thus, the long-range energy-transfer interaction can be controlled by the polarity of the solvent.

A room temperature luminescent cyclometallated ruthenium(II) complex of 6-phenyl-2,2'-bipyridine

Abstract The cyclometallated complex Ru(tt)(phbp) + (tt=4′-tolyl-2,2′:6′2″-terpyridine, phbp=6-phenyl-2,2′-bipyridine) with a (N,N,N)(C,N,N) coordination, has been synthesized and characterized by 1 H NMR, UV, FAB-MS spectral techniques and by elemental analysis. We have compared its electrochemical and photophysical properties with those of the non-orthometallated analogues Ru(tt) 2 2+ and Ru(terpy) 2 2+ (terpy=2,2′:6′,2″-terpyridine). The most remarkable feature of Ru(tt)(phbp) + is its ability to luminesce at room temperature in alcoholic and nitrile solvents. The lifetime of its 3 MLCT excited state is 60 ns in CH 3 CN.

Electrochemical and Spectroscopic Properties of Cyclometallated and Non-Cyclometallated Ruthenium(II) Complexes Containing Sterically Hindering Ligands of the Phenanthroline and Terpyridine Families

Two series of cyclometallated and noncyclometallated ruthenium(II) complexes incorporating mono- or disubstituted 1,10-phenanthroline- and 2,2′:6′,2′′-terpyridine-type ligands have been synthesized and characterized. An X-ray crystal structure for one of the complexes, Ru(ttpy)(mapH)-(Cl)(PF6), has been obtained (mapH = 2-p-anisyl-1,10-phenanthroline; ttpy = 4′-tolyl-2,2′:6′,2′′-terpyridine). Distinct electrochemical and photophysical properties have been observed for the two series: a remarkable feature is the observation of relatively long-lived MLCT excited states (from 70 to 106 ns at room temperature in CH3CN) for three of the cyclometallated complexes. A discussion is given on the role of factors like sigma donation by the cyclometallating ligands, interligand steric hindrance and interligand π-π interactions that affect the electrochemical and spectroscopic properties.