Luisa De Cola

Blue emitting iridium complexes : synthesis, photophysics and phosphorescent devices

Homoleptic Ir(Fnppy)3 and heteroleptic (Fnppy)2Ir(acac) complexes (n = 3: F3ppy = 2-(3′,4′,6′-trifluorophenyl)pyridine; n = 4: F4ppy = 2-(3′,4′,5′,6′-tetrafluorophenyl)pyridine; acac = acetylacetonate) have been synthesized and their spectroscopic properties investigated. The homoleptic complexes exist as two stereoisomers, facial (fac) and meridional (mer), that have been isolated and fully characterized. Their electrochemical and photophysical properties have been studied both in solution and in the solid state and electroluminescent devices have been fabricated. The emissive layers in devices have been obtained mixing the iridium complexes with a PVK [poly(9-vinylcarbazole)] host matrix, in the presence of the electron carrier Bu-PBD [2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole]. The application of a voltage (5.0–6.5 V) between the electrodes of devices leads to electro-generated blue luminescence which has similar energy to the solution emissions. Interestingly, the stability of the devices made with the homoleptic fluorinated iridium complexes strongly depends on the stereochemistry of these phosphors and high (up to 5.5%) external quantum efficiencies for the fac complexes are measured.

AZOBENZENE-FUNCTIONALIZED CASCADE MOLECULES : PHOTOSWITCHABLE SUPRAMOLECULAR SYSTEMS

Potential holographic materials are obtained from the azobenzene-functionalized dendrimers described herein (shown schematically on the right). Even in the largest species presented, which contains up to 32 azobenzene chromophores, each chromophoric unit behaves independently compared to “monomer” model compounds. Diffraction efficiencies larger than 20 % were achieved with holographic films of the azobenzene dendrimers – the second-generation dendrimer exhibits good thermal stability.

Iridium complexes containing p-phenylene units. The influence of the conjugation on the excited state properties

The synthesis, characterisation and photophysical properties of a series of heteroleptic iridium complexes are reported. The complexes contain two orthometalating ligands, 2-(2,4-difluorophenyl)pyridine and a bipyridine derivative as the third chelating unit, substituted by 1–3 oligo(para-phenylene) units. We also describe a dinuclear complex in which the two iridium units are connected by a tetra-p-phenylene bridging ligand. All the complexes are strongly luminescent at room temperature and, interestingly, for the longest complexes, the deaerated solution shows an excited state lifetime of several microseconds. The presence of dioxygen reduces the lifetime, dramatically suggesting their potential use as oxygen sensors. At low temperature the emission predominantly originates from the triplet excited state of the ligand involving the poly(phenylene) units and the phosphorescence lifetime increases to several hundreds of microseconds.

Molecular architecture in the field of photonic devices

Abstract We report the synthesis and photophysical properties of two different photonic devices. The first system describes dinuclear metal complexes with a rigid and linear bridging ligand (PAP) that contains an adamantane spacer. We discuss the correlation between the nature of the bridging ligand and the electrochemical as well as photophysical properties of the metal complexes. Two interesting observations can already be pointed out: (i) the lifetime of the intermediate electron-transfer product RuIII–PAP–OsII is very long (130 μs); and (ii) for the first time in a dinuclear Ru/Os system, the rate constant of energy transfer from the Ru(II) to the Os(II) unit is faster than the rate of the electron transfer from the Ru(II) to the Os(III) unit. The second system represents a photonic switch which is built up by two subunits, a rhenium complex as the active switching part and an anthracene moiety as detector. We discuss the synthesis, the reversibility of the switch and the energy transfer properties of the new system.

Mono- and di-nuclear iridium(III) complexes. Synthesis and photophysics

This paper reports the synthesis and photophysical characterization of heteroleptic mono- and di-nuclear iridium(III) complexes. The complexes contain two ortho-metalating ligands, 2-phenylpyridine, with a bipyridine derivative as the third chelating unit. In the case of the dinuclear complexes the two iridium moieties are connected by a conjugated bridging ligand containing three or four phenyl units. All the complexes emit at room temperature and steady state and time resolved spectroscopy demonstrates that the lowest excited state is a metal-to-ligand charge transfer involving the bipyridine ligand.

Photoinduced electron transfer between metal-coordinated cyclodextrin assemblies and viologens

Two novel tris(bipyridine)ruthenium(II) complexes bearing two and six beta-cyclodextrin binding sites on their ligands have been synthesised and characterised. Complex 1, bearing two cyclodextrins, adopts a conformation in aqueous solution where parts of the aromatic ligands are self-included into the cyclodextrin moieties. This results in a loss of symmetry of the complex and gives rise to a much more complicated 1H NMR spectrum than expected. Photophysical studies indicate that the appended cyclodextrins protect the luminescent ruthenium core from quenching by oxygen, which results in longer excited state lifetimes and higher emission quantum yields compared with the reference compound, the unsubstituted ruthenium tris(bipyridine). Inclusion of suitable guests such as dialkyl-viologens leads to a quenching of the luminescence of the central unit. In these supramolecular donor-acceptor dyads an efficient photoinduced electron transfer from the excited ruthenium moiety (the donor) to the viologen unit (the acceptor) is observed. The alkyl chain length of the acceptor plays an important role on the binding properties; when it exceeds a certain limit the binding becomes strong enough for electron transfer to occur. Interestingly, a viologen with only one long alkyl tail instead of two shows no efficient quenching; this indicates that cooperative interactions between two cyclodextrins binding one viologen are essential to raise the binding constant of the supramolecular dyad.

Photophysical, Electrochemical and Electrochromic Properties of Copper-Bis(4,4'-dimethyl,6,6'-diphenyl-2,2'- bipyridine) Complexes

Abstract The synthesis, solution and solid state structural characterization, photophysical and electrochemical properties of two redox forms of an electrochromic copper-bis(4,4′-dimethyl-6,6′-diphenyl-2,2′-bipyridine) complex, [Cu(3)2]n (n=+1, +2), are presented. Both complexes were characterized in the solid state by X-ray diffraction methods on single-crystals showing that both forms exist in a pseudo-tetrahedral coordination, and a comparison with other structures was made. Like most copper(I) complexes, the red [Cu(3)2]+ complex shows a rather weak emission (Φem=2.7×10−4, dichloromethane). The lifetime of the emitting MLCT state is 34±1 ns, as observed with time resolved emission, and transient absorption (in deoxygenated dichloromethane). Typical emission and transient absorption spectra are presented. The transient absorption spectra indicate that the MLCT state absorbs stronger than the ground state, which is relatively uncommon for metal bipyridine complexes, i.e. no ground state bleaching is observed. The green [(3)2Cu]2+ complex does not show any observable emission or transient absorption, which is a common feature for Cu(II) complexes of this type. The electronic absorption spectra of the chemically and electrochemically produced copper(I/II) complexes are identical. The repeated electrochemical conversion of the Cu(I) center into Cu(II) and vice versa does not cause any decomposition. This is consistent with a fully reversible Cu(I)/Cu(II) redox couple in the corresponding cyclic voltammogram, (E1/2 (Cu(I)/Cu(II))=+0.68 V vs. SCE=+0.23 V vs. Fc/Fc+). These observations indicate that no large structural reorganization occurs upon electrochemical timescales (sub second), and that the different ways of generating the complexes does not effect their final structure, apart from the small differences observed in the X-ray structures of both forms. These characteristics make these complexes rather well suited for their incorporation into an electrochromic display configuration.

Dinuclear RuII and/or OsII complexes of bis-bipyridine bridging ligands containing adamantane spacers: synthesis, luminescence properties, intercomponent energy and electron transfer processes

Abstract The ( E,E )-1,3-bis[2-(2,2′-bipyridine-5-yl)ethenyl]adamantane (BAB) and ( E,E )-3,3′-bis[2-(2,2′-bipyridine-5-yl)ethenyl]-1,1′-biadamantane (BAAB) bridging ligands made of two 2,2′-bipyridine groups (B) separated by spacers containing one and two adamantane (A) units have been synthesized. Their dinuclear complexes [(bpy) 2 Ru(BAB)Ru(bpy) 2 ] 4+ (Ru II .BAB.Ru II ), [(bpy) 2 Os-(BAB)Os(bpY) 2 ] 4+ (Os II .BAB.Os II ), [(bpy) 2 Ru(BAB)O9(bPY) 2 ] 4+ (ROSAB.Osn), [(bPY)2Ru(BAAB)-Ru(bpy)2] 4+ (Ru II .BAAB.Ru II ), and [(bpy) 2 Os(BAAB)Os(bpy) 2 ] 4+ (Os II .BAAB.Os II ) have been prepared as PF 6 − salts. In these novel compounds each Ru-based and Os-based unit displays its own absorption spectrum and electrochemical properties, regardless of the presence of a second metal-based unit. In the homodinuclear complexes also the luminescence properties of each unit are unaffected. In the mixed metal Ru II .BAB.Os II complex electronic energy transfer takes place from the Ru-based to the Os-based unit with rate constant 5.8 × 108 s −1 at room temperature, whereas at 77 K energy transfer takes place through two distinct processes with rate constants 1.4 × 108 S −1 and 2.0 × 107 s −1 , presumably because of the presence of two conformers (or families of conformers). Partial oxidation of the binuclear compounds Os II .BAB.Os II , Ru II .BAB.Os II , and Os II .BAAB.Os II by Ce IV in acetonitrile/water solutions leads to the mixed-valence Os II .BAB.Os III , Ru II .SAB.Os III , and Os II .BAAB.Os III species where the oxidized metal-based unit quenches by electron transfer the luminescent excited state of the unit that is not oxidized. At room temperature the rate constants for the excited state ∗ Os II .BAB.Os III → Os III .BAB.Os II , ∗ Os II .BAAB.Os III → Os III .BAAB.Os II processes are 4.0 × 109 s −1 and 8.8 × 108 si, respectively. For Ru II .BAB.Os III , the rate constants for the excited state ∗ Ru II .BAB.Os III → Ru III .BAB.Os II process is 2.8 × 109 s −1 and the rate constant for the back electron transfer process Ru III .BAB.Os II → Ru II .BAB.Os III is 4.0 × 107 s −1 .

SYNTHESIS AND PHOTOPHYSICAL PROPERTIES OF CHIRAL, BINUCLEAR METAL COMPLEXES

Abstract The synthesis of isomerically pure multicentred species is an important task for preparative chemists. Only stereochemically well-defined structures of polynuclear metal complexes lead to photophysical results that are clearly interpretable. We have developed several strategies to synthesise such compounds that have a predetermined chirality (Δ- or Λ-helix) at each metal center. The approximate local symmetry of the described metal complexes is D 3 (octahedral, tris-bidentate). The methods are the following: (a) resolving a racemic building block followed by substitution of the two labile monodentate ligands by a bidentate diimine ligand under total retention of configuration. (b) Synthesis of an optically pure precursor complex in which two chelating bipyridine ligands are bridged and each contain an optically active pinene unit. The remaining coordination sites are then replaced by a bidentate diimine ligand. (c) Use of ligands that have sterically demanding substituents perpendicular to the molecular plane. The use of such ligands leads direct to optically active compounds. Examples of these methods will be given, while considering their photophysical application.

Rigid Rod-like Molecular Wires of Nanometric Dimension. Electronic Energy Transfer from a Naphthyl to an Anthracenyl Unit Connected by a 1,4-Pentaphenylene Spacer

Abstract We have synthesized the multicomponent species N (ph) 5 A , where a 1-naphthyl ( N ) and a 9-anthryl ( A ) units are connected by a 1,4-pentaphenylene rod-like spacer ( (ph) 5 ). The overall length of the compound is 3.1 nm and the center-to-center distance between the naphthyl and anthryl units is 2.6 nm. The absorption and emission spectra of N (ph) 5 A have been investigated in cyclohexane solution at 293 K and compared with those of model compounds of the component units. Emission spectra, fluorescence quantum yields and excited state lifetimes show that in N (ph) 5 A the fluorescence of the naphtyl and oligophenyl units is completely quenched by energy transfer to the fluorescent excited state of the anthryl unit. The mechanism of the energy transfer process is discussed.