Iolinda Aiello

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.

Synthesis and spectroscopic characterization of organometallic chromophores for photoluminescent materials: cyclopalladated complexes

Abstract A homologous series of isostructural and isoelectronic palladium acetylacetonato complexes containing different cyclometalated ligands HL (HL=2-phenylpyridine H(PhPy), benzo[h]quinoline H(BhQ), 9-diethylamino-benzo[a]phenoxazin-5-one H(NR), azobenzene H(Azo), 5-[(4′-methyl)-phenylazo]-8-methoxyquinoline H(QAzo), 2-benzoylpyridine H(BKPy), phenyl-2-pyridylketone-2,4-dinitrophenylhydrazone, H(BHPy)) has been prepared. The photophysical characterization of the newly synthesized products shows that many of them are luminescent in solution at room temperature. The best performances are exhibited by the H(NR) derivative (Φem=0.23, λem=660, in dichloromethane solution), which improves the emission features displayed by the unmetalated ligand (Φem=0.13, λem=602, in dichloromethane solution). H(NR) is a well known laser dye commonly named nile red and to the best of our knowledge [(NR)Pd(acac)] is the first example reported up to now of its incorporation in a transition metal complex. Interestingly, the results of the present investigation prove that “[(NR)Pd]” can be an useful building block for the preparation of luminescent organometallic complexes.

Tuning solid state luminescent properties in a hydrogen bonding-directed supramolecular assembly of bis-cyclometalated iridium(III) ethylenediamine complexes.

Synthesis, crystal structural determination and photophysical properties of a series of heteroleptic cationic cyclometalated iridium(III) derivatives of general formula [(ppy)(2)Ir(en)]X (X = ClO(4)(-) (1), PF(6)(-) (2), Cl(-) (3), BPh(4)(-) (4)), are described. The assembly of the common molecular building block allows to get highly luminescent crystalline materials or to assemble poorly luminescent supramolecular channelled architectures, for which the additional contribution of oxygen quenching effects has been observed. Moreover, the high reproducibility of the preparations of the crystalline materials in their specific crystalline phases, makes the control of the supramolecular organization of photo-active iridium(III) complexes within the crystalline structures a useful synthetic procedure for the construction of highly luminescent materials.

Self-incorporation of a luminescent neutral iridium(III) complex in different mesoporous micelle-templated silicas

The neutral luminescent tris-cyclometallated 2-phenylpyridine iridium(III) complex, (fac-Ir(ppy)3, [Ir]) following a self-assembling procedure, has been successfully located in the cavities of mesostructured silica materials through a surfactant-mediated process. Two different structure-directing agents, the cationic cetyltrimethyl ammonium bromide (CTAB) and the non-ionic poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) were tested. The structural features induced by the metal complex incorporation can be explained by comparing the newly synthesized hybrid mesostructured materials with the corresponding undoped samples which were similarly prepared. X-Ray powder diffraction (XRD), nitrogen sorption, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and UV-Vis spectroscopy were used to characterize the investigated materials. Medium-sized spherical particles of 800 nm were obtained using CTAB as a structure-directing agent whereas larger monolithic aggregates with a minimum dimension of 5–8 μm were obtained using P123. The new hybrids showed the typical hexagonal symmetry of analogously prepared materials. Moreover high luminescence quantum yield values were obtained for both hybrids as a result of a very good dispersion of the chromophore in the mesostructured matrices, thereby avoiding dramatic self-quenching phenomena. The approach described in this paper provides a simple synthetic way to prepare new luminescent silica-based materials by the inclusion of neutral metal-containing luminophores into the pores of a mesoporous hosting skeleton.

Synthesis and crystal structure of dinuclear cyclopalladated 1,2- and 1,3-bridged squarato complexes

Abstract Two new dinuclear Pd(II) cyclometalated complexes of formula [L2Pd2(μ-1,2-sq)(C5H11N)2] (1) and [L2Pd2(μ-1,3-sq)(C5H11N)2]·2[(C5H12N)BF4] (1a) [HL=2,6-dimethylazobenzene], have been synthetized from a reaction of the mononuclear [LPd(acetone)2][BF4] with the piperidinium salt of squaric acid, [C5H12N]2[sq]. The single crystal X-ray analysis of complexes 1 and 1a shows that the sq ligand is found to bridge in both μ-1,2- and μ-1,3-coordination modes. These are the first examples of squarato Pd(II) complexes characterized structurally up to now.

UV/Vis to NIR photoconduction in cyclopalladated complexes.

The incorporation of a rigid core, formed by a cyclopalladated azobenzene fragment bonded to an ancillary Schiff base ligand, into molecules with 12 or 11 peripheral alkyl chains has been successfully achieved. These new complexes, 1 and 2, respectively, are columnar liquid crystals between room temperature and about 50 degrees C. Both cyclometallated and ancillary ligands have been polyalkylated through either aryl ester (electron-withdrawing group) or aryl ether (electron-releasing group) linkages, in order to tune the HOMO/LUMO energy levels. The photoconductive properties of 1 and 2 have been studied as a function of their absorption properties before and after annealing, from the UV/Vis to NIR region. Compared with the reference compounds, tris-alkynyl benzene discotics, these new materials gave similar performances (sigma/I approximately 8x10(-13) S cm W(-1) with E = 10 V microm(-1) at lambda = 370 nm). Moreover, complex 2 shows a normalized photoconductivity sigma/I = 8.5x10(-13) S cm W(-1) at lambda = 760 nm. Organic photoconductors in such a high wavelength spectral range are not common and are usually assembled by mixing dyes with organic semiconductors.

Organometallic red-emitting chromophores: a computational and experimental study on cyclometallated Nile Red complexes of palladium(II) and platinum(II) acetylacetonates and hexafluoroacetylacetonates

A comparative photophysical, electrochemical and computational study of a series of Nile Red cyclopalladated and newly synthesised cycloplatinated complexes is reported. All complexes are luminescent at room temperature and the emission properties are governed by the nature of the cyclometallated dye. However, the presence of platinum(II) as metal centre decreases severely the emission quantum yield, while the electronegativity of the ancillary hexafluoroacetylacetonato ligand enhanced the emission properties of the Nile Red complexes.

Monomeric and polymeric oxovanadium(IV) complexes containing 5-(4′-alkyl-phenylazo)-8-hydroxy-quinoline ligands

Abstract A number of bis-[5-(4′-R-phenylazo)-8-hydroxy-quinolinato] vanadyl(IV) compounds (R=H, CH 3 , C 2 H 5 , n-C 3 H 7 , n-C 4 H 9 ) have been synthesized and characterized by elemental analyses and IR, EPR and UV–Vis spectroscopies. Data from the above show that they exist in two isomeric solid forms (which were separated for R=C 2 H 5 , n-C 3 H 7 and n-C 4 H 9 ) whose IR and EPR spectra account for isolated molecules (IR: ν (VO)≅961 cm −1 ; EPR: g ≅1.980) or for polymers (IR: ν (VO) about 890 cm −1 ; EPR: g x = g y =1.975(5), g z =1.960(2), A x = A y =40(5), A z =160(3)×10 −4 cm −1 ). The polymeric structure results from intermolecular VO⋯VO interactions.

Copper(II) and Nickel(II) Complexes of a Tetradentate Ligand Containing an N,N′-Bis(Salicylidene)Dodecane-1, 10-Diamine Core

The synthesis of mesomorphic Cu(II) and Ni(II) coordination compounds arising from a new series of tetradentate Schiff bases containing two salicylaldimine fragments connected by a very long spacer,-(CH2)12-, (H2Ln): L1 = p-(C6H13O)-C6H4COO, L2 = p-(C12H25O)-C6H4COO, is described. The self-assembly and liquid-crystalline properties of these complexes have been investigated by variable temperature powder X-ray diffraction and the relationship between the global molecular shape and the mesogenic performances is discussed. The reported Cu(II) compounds are molecular materials which could be of interest for applications in the field of magnetic molecule-based devices.