Effects of systematic methyl substitution of metal (III) tris(n-methyl-8-quinolinolato) chelates on material properties for optimum electroluminescence device performance

Experimental and computational evidence of the intermolecular motifs in the crystal packing of luminescent pentacoordinated gallium(iii) complexes

This paper reports the synthesis, characterization, photophysical and structural properties of the homologous series of good emitting pentacoordinated GaQ'2L complexes 1-3, where Q' is 2-methyl-quinolin-8-olate and L is a phenolate substituted in para position with respect to the oxygen donor atom. A combined approach between the experimental structural analysis (i.e. the molecular fragments involved in intermolecular pi-pi interactions) and the computational study (i.e. the nature of the molecular orbitals residing therein) is discussed in order to compare the charge transport aptitude of complexes 1-3, in relation to the facing of their LUMO/LUMO, HOMO/HOMO and HOMO/LUMO arising from the molecular packing. The different phenolate ligands significantly change the packing characteristics of 1-3, with indirect effects on the electron hopping kinetics responsible for conduction in amorphous thin films. The observed preference for pyridyl-pyridyl stacking proves a faster electron conduction in comparison to hole conduction in the three complexes studied.

Fluorinated Alq3 derivatives with tunable optical properties

This communication reports that not only the emission colour but also the photoluminescence quantum yield of Alq3 can be tuned by introducing fluorine atoms at different positions; with fluorination at C-5 the emission is red-shifted with a tremendously decreased intensity, fluorination at C-6 causes a blue-shift with a significantly increased intensity, and fluorination at C-7 has a minor effect on both the colour and intensity of Alq3's emission.

Organoboron Compounds with an 8-Hydroxyquinolato Chelate and Its Derivatives: Substituent Effects on Structures and Luminescence

Four new luminescent organoboron complexes have been synthesized and fully characterized. These compounds are four-coordinate boron chelated by either 8-hydroxyquinolato (q) or functionalized 8-hydroxylquinolato ligands, including BPh2(5-(1-naphthyl)-q) (1), BPh2(5-(2-benzothienyl)-q) (2), B(2-benzothienyl)2q (3), and B(2-benzothienyl)2(2-Me-q) (4). All four compounds have a tetrahedral geometry as established by X-ray diffraction analyses. In solution, compounds 1-4 have an emission maximum at 534, 565, 501, and 496 nm, respectively, at room temperature. They emit similar colors in the solid states without red shifts of the emission band due to the lack of significant intermolecular interactions in the crystal lattices. The substituent group at C5 or C2 position of the 8-hydroxyquinolato ligand has been observed to have a significant impact on the emission energy and the emission quantum efficiency of the boron complexes. Molecular orbital calculations (Gaussian 98) showed that the electronic transition of 1 and 2 is a pi-pi* transition centered on the functionalized 8-hydroxyquinolato group and the electronic transition of 3 and 4 is an interligand charge transfer from the 2-benzothienyl ligand to the hydroxyquinolato ring. A double-layer electroluminescent device using 3 as the emitter has been fabricated, which produced a broad emission band with a significant contribution of exciplex emission.

Strongly luminescent binuclear aluminium chelate with polymer-like molecular packing and solution-processibility

A binuclear aluminium(III) chelate with rigid and flexible mixed ligands has been synthesized and structurally characterized, which exhibits polymer-like molecular packing and solution-processiblity, as well as high photoluminescence quantum yield for organic light-emitting diodes (OLEDs); with this new compound as the emissive and host layer, the multi-layer OLEDs prepared via low-cost spin-coating showed encouraging performance.

Synthesis, Crystal Structure, and Luminescent Properties of a Binuclear Gallium Complex with Mixed Ligands

By introducing tridentate Schiff base ligands, a binuclear gallium complex with mixed ligands, bis(salicylidene-o-aminophenolato)-bis(8-quinolinolato)-bis-gallium(III) [Ga(2)(saph)(2)q(2)], has been synthesized and structurally characterized by single-crystal X-ray crystallography. Crystal data for C(44)H(30)Ga(2)N(4)O(6) are as follows: space group, triclinic, P; a = 11.357(3) A, b = 12.945(3) A, c = 12.947(3) A, alpha = 103.461(15) degrees, beta = 100.070(7) degrees, gamma = 96.107(18) degrees, Z = 2. This complex was identified as a dimeric complex of hexacoordinated gallium with strong intermolecular and intramolecular pi-pi stacking interactions between the pyridyl/pyridyl rings. The thermal analysis showed that Ga(2)(saph)(2)q(2) can readily form a stable amorphous glass with a high glass transition temperature (T(g) = 204 degrees C), which is 27 degrees C higher than that of tris(8-hydroxyquinolinolate)aluminum (Alq(3)). In addition, a high photoluminescence efficiency (phi(PL)) of 0.318 in DMF has been demonstrated, although the central gallium atom can result in heavy-atom quenching. Organic light-emitting diodes (OLEDs) based on this complex displayed a turn-on voltage as low as 2.5 V and a high efficiency. Even at a low doping concentration of 1%, the doped Ga(2)(saph)(2)q(2) devices with 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as the dopant exhibited excellent red emission centered at 628 nm with improved durability, compared with the case of Alq(3) as the host. These distinguishing properties of Ga(2)(saph)(2)q(2) make it a good candidate as a novel electron-transporting and emitting material for OLEDs.

Investigations on the electronic effects of the peripheral 4′-group on 5-(4′-substituted)phenylazo-8-hydroxyquinoline ligands: zinc and aluminium complexes

A new series of 5-(4'-substituted)phenylazo-8-hydroxyquinolines (H[L-R]; R = N(CH(3))(2), C(2)H(5), n-C(4)H(9), C(CH(3))(3), H, and F, ) has been prepared and the corresponding Zn[L-R](2) (1a-6a) and Al[L-R](3) (1b-6b) complexes successfully synthesized. These compounds have been studied in order to design new molecular materials with enhanced electron transport properties. The obtained species have been extensively characterized by absorption and emission spectra and by cyclic voltammetric measurements. Experimental and computational results show that the Zn[L-N(CH(3))(2)].2H(2)O (1a) and Al[L-N(CH(3))(2)](1b) complexes only feature luminescence (at 620 and 600 nm), respectively. The unique effects, which are induced by the N=N-C(6)H(4)-N(CH(3))(2) group, are further proved by a reversible electron transfer process detected by cyclic voltammetry. These outcomes, discussed on the basis of theoretical calculations performed on the (H[L-N(CH3)2])-, H[L-N(CH3)2] and (H[L-N(CH3)(2)])+ species, suggest that metal complexes formed by 5-(4'-N,N-dimethylamino)phenylazo-8-hydroxyquinoline should be considered as electron transport materials suitable for applications in photonic devices.

Structures, Electronic States, and Electroluminescent Properties of a Zinc(II) 2-(2-Hydroxyphenyl)benzothiazolate Complex

Bis(2-(2-hydroxyphenyl)benzothiazolate)zinc (Zn(BTZ)(2)) is one of the best white electroluminescent materials used in organic light-emitting diodes (LEDs). Despite a large number of studies devoted to this complex, very little is known about its basic molecular and electronic structures and electron transport properties in LEDs. Therefore, we investigate the structures and electroluminescent properties. The unsolvated single crystal of Zn(BTZ)(2) was grown and its crystalline structure was determined from X-ray diffraction data. The crystal is triclinic, space group P-1, a = 9.4890(19) A, b = 9.5687(19) A, c = 11.685(2) A, alpha = 84.38(3) degrees, beta = 78.94(3) degrees, gamma = 83.32(3) degrees. The structure of the chelate is dimeric [Zn(BTZ)(2)](2) with two isotropic Zn(2+) ion centers having five-coordinate geometry. The present study provides direct evidence for the sole existence of dimeric structure in the powder and the thin film. The dimer is energetically more stable than the monomer. Analysis of the electronic structure of [Zn(BTZ)(2)](2) calculated by density functional theory reveals a localization of orbital and the distribution of four orbital "tetrads". The structural stabilities of both anion and cation and the distribution of the hole in the cation and that of the excess electron in the anion are discussed in terms of theoretical calculations. Strong intermolecular interaction may be expected to enable good electron transport properties as compared with tris(8-hydroxyquinolinato)aluminum.

Blue luminescent rigid molecular rods bearing N-7-azaindolyl and 2,2'-dipyridylamino and their Zn(II) and Ag(I) complexes

To investigate the luminescent and thermal properties of organic compounds with rigid entities, a series of new blue/purple luminescent bridging ligands, 4,4'-(N-7-azaindolyl)diphenylacetylene (5), 4-(N-7-azaindolyl)-4'-(2,2'-dipyridylamino)diphenylacetylene (6), 4,4'-(2,2'-dipyridylamino)diphenylacetylene (7), and 4,4'-(dipyridylamino)diphenylbutadiyne (8) have been synthesized through Pd-mediated Sonogashira coupling. The structures of compounds 6 and 8 were determined by single-crystal X-ray diffraction analyses. Compounds 5-8 are luminescent in solution at room temperature, with emission lambda(max) = 361, 382, 386, and 405 nm, respectively. At 77 K, compounds 5-8 exhibit both fluorescent and phosphorescent emission. The 2,2'-dipyridylamino symmetrically substituted ligand 7 forms a linear dinuclear complex 9 with zinc(II) ions, in which the two pyridyl rings of the dipyridylamino portion are chelated to the metal center. However, with Ag(I) ions, ligand 7 forms a dinuclear complex (10), which displays a macrocyclic structure with only one of the two pyridyl rings from each dipyridylamino portion being coordinated to the silver atom. Both 9 and 10 exhibit luminescence in the near-UV region in CH(2)Cl(2) at room temperature with lambda(max) = 385 and 384 nm, respectively. The fluorescence of 7 can be partially quenched by either Zn(2+) or H(+). The behavior of ligands 8 toward Ag(I) and Zn(II) ions is similar to that of 7.