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

Structure of Co-Doped Alq3 thin films investigated by grazing incidence X-ray absorption fine structure and Fourier transform infrared spectroscopy

The structural properties of Co-doped tris(8-hydroxyquinoline)aluminum (Alq(3)) have been studied by grazing incidence X-ray absorption fine structure (GIXAFS) and Fourier transform infrared spectroscopy (FTIR). GIXAFS analysis suggests that there are multivalent Co-Alq(3) complexes and the doped Co atoms tend to locate at the attraction center with respect to N and O atoms and bond with them. The FTIR spectra indicate that the Co atoms interact with the meridional (mer) isomer of Alq(3) rather than forming inorganic compounds.

Functional properties of metallomesogens modulated by molecular and supramolecular exotic arrangements

New concepts for the synthesis of metallomesogens have been recently developed in order to use the metal centre as a scaffold for grafting different functionalities and inducing non-conventional shapes and properties in the resulting complexes. Our strategy was based on the synthesis of mesogenic coordination complexes whose molecular architectures are controlled by the modulation of different and tunable molecular motifs: the nature of the metal ion and the surrounding ligands as central unit, the number of flexible chains at the periphery, and the nature of counter-ions in ionic complexes. The appropriate choice of molecular construction motifs allows control at global architectures and induces pre-selected properties from the level of single molecule to supramolecular network, confirming that metal coordination provides a helpful tool for obtaining multifunctional soft materials.

Crystalline Gaq(3) Nanostructures: Preparation, Thermal Property and Spectroscopy Characterization

Crystalline Gaq(3) 1-D nanostructures and nanospheres could be fabricated by thermal evaporation under cold trap. The influences of the key process parameters on formation of the nanostructures were also investigated. It has been demonstrated that the morphology and dimension of the nanostructures were mainly controlled by working temperature and working pressure. One-dimensional nanostructures were fabricated at a lower working temperature, whereas nanospheres were formed at a higher working temperature. Larger nanospheres could be obtained when a higher working pressure was applied. The XRD, FTIR, and NMR analyses evidenced that the nanostructures mainly consisted of delta-phase Gaq(3). Their DSC trace revealed two small exothermic peaks in addition to the melting endotherm. The one in lower temperature region was ascribed to a transition from delta to beta phase, while another in higher temperature region could be identified as a transition from beta to delta phase. All the crystalline nanostructures show similar PL spectra due to absence of quantum confinement effect. They also exhibited a spectral blue shift because of a looser interligand spacing and reduced orbital overlap in their delta-phase molecular structures.

Hydroxynaphthyridine-derived group III metal chelates: wide band gap and deep blue analogues of green Alq3 (tris(8-hydroxyquinolate)aluminum) and their versatile applications for organic light-emitting diodes

A series of group III metal chelates have been synthesized and characterized for the versatile application of organic light-emitting diodes (OLEDs). These metal chelates are based on 4-hydroxy-1,5-naphthyridine derivates as chelating ligands, and they are the blue version analogues of well-known green fluorophore Alq(3) (tris(8-hydroxyquinolinato)aluminum). These chelating ligands and their metal chelates were easily prepared with an improved synthetic method, and they were facially purified by a sublimation process, which enables the materials to be readily available in bulk quantity and facilitates their usage in OLEDs. Unlike most currently known blue analogues of Alq(3) or other deep blue materials, metal chelates of 4-hydroxy-1,5-naphthyridine exhibit very deep blue fluorescence, wide band gap energy, high charge carrier mobility, and superior thermal stability. Using a vacuum-thermal-deposition process in the fabrication of OLEDs, we have successfully demonstrated that the application of these unusual hydroxynaphthyridine metal chelates can be very versatile and effective. First, we have solved or alleviated the problem of exciplex formation that took place between the hole-transporting layer and hydroxynaphthyridine metal chelates, of which OLED application has been prohibited to date. Second, these deep blue materials can play various roles in OLED application. They can be a highly efficient nondopant deep blue emitter: maximum external quantum efficiency eta(ext) of 4.2%; Commision Internationale de L'Eclairage x, y coordinates, CIE(x,y) = 0.15, 0.07. Compared with Alq(3), Bebq(2) (beryllium bis(benzoquinolin-10-olate)), or TPBI (2,2',2''-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole), they are a good electron-transporting material: low HOMO energy level of 6.4-6.5 eV and not so high LUMO energy level of 3.0-3.3 eV. They can be ambipolar and possess a high electron mobility of 10(-4) cm(2)/V s at an electric field of 6.4 x 10(5) V/cm. They are a qualified wide band gap host material for efficient blue perylene (CIE(x,y) = 0.14, 0.17 and maximum eta(ext) 3.8%) or deep blue 9,10-diphenylanthracene (CIE(x,y) = 0.15, 0.06 and maximum eta(ext) 2.8%). For solid state lighting application, they are desirable as a host material for yellow dopant (rubrene) in achieving high efficiency (eta(ext) 4.3% and eta(P) 8.7 lm/W at an electroluminance of 100 cd/m(2) or eta(ext) 3.9% and eta(P) 5.1 lm/W at an electroluminance of 1000 cd/m(2)) white electroluminescence (CIE(x,y) = 0.30, 0.35).

Synthesis and Characterization of Hemicage 8-Hydroxyquinoline Chelates with Enhanced Electrochemical and Photophysical Properties

A new hexadentate, tripodal 8-hydroxyquinoline ligand (QH3) and its trivalent metal chelates (MQ, M=Al3+, Ga3+, In3+) with hemicage structures have been prepared and the electrochemical and photophysical properties systematically studied. The hemicage structure of the metal complexes was characterized by 1H NMR, indicating a pure facial geometry, in contrast to their uncaged cousins with 8-hydroxyquinoline (Mq3) and 3-methyl-8-hydroxyquinoline (M(3Meq)3), which all exist only as the meridional form in fluid solutions at room temperature. The photoluminescence quantum efficiency for the three hemicage complexes is 1.48, 1.79, and 1.26 times higher for AlQ, GaQ, and InQ, respectively, than their corresponding 3-methyl-8-hydroxyquinoline complexes, likely due to the rigidity of the ligand system, which can efficiently decrease the nonradiative decay of the excited states. The improved electrochemical stability of the hemicage complexes has been demonstrated by cyclic voltammetry, showing an increasingly reversible behavior from InQ to GaQ to AlQ (Ered=-2.15, -2.17, and -2.22 V vs Fc/Fc+ in DMSO). We infer that the degree of reversibility and redox potential result from the metal-ligand bond strength, which is largest in the case of aluminum.

Diboron and Triboron Compounds Based on Linear and Star-Shaped Conjugated Ligands with 8-Hydroxyquinolate Functionality: Impact of Intermolecular Interaction and Boron Coordination on Luminescence

New 8-R-quinoline functionalized linear and star-shaped conjugated molecules have been synthesized using Suzuki-Miyaura coupling methods (R = MeO, L1-L5; R = CH3OCH2O, L1'-L5'). When treated with HCl, L1'-L5' are converted readily to the corresponding 8-hydroxyquinoline compounds L(OH)1-L(OH)5 which react readily with BPh3 in refluxing THF to produce the corresponding polyboron chelate compounds B1-B5 in good yields. L1-L5 and B1-B5 display similar thermal stability with Td at approximately 300 degrees C. Experimental and molecular orbital calculation results showed that the chelation by boron stabilizes the LUMO level of the ligand and narrows the HOMO-LUMO gap, resulting in the blue emission of the ligands and the green or orange emission of the boron compounds. Crystal structures of L1, L3, and L5 showed that these molecules have layered arrangements in the solid state with significant intermolecular pi-pi interactions. The linear diboron B5 displays concentration and temperature-dependent emission in solution, attributable to intermolecular interactions. The properties of a monoboron compound BPh2(5-Ph-8-MeO-q) (B0) and its corresponding free ligand L0 were investigated and compared to the closely related diboron compound B1 and the ligand L1, which revealed that the increase of the number of chromophores linked by an aromatic group has a significant impact on thermal stability and the HOMO and LUMO energy levels.

Effective Manipulation of the Electronic Effects and Its Influence on the Emission of 5-Substituted Tris(8-quinolinolate) Aluminum(III) Complexes

The unique electron-transport and emissive properties of tris(8-quinolinolate) aluminum(III) (Alq(3)) have resulted in extensive use of this material for small molecular organic light-emitting diode (OLED) fabrication. So far, efforts to prepare stable and easy-to-process red/green/blue (RGB)-emitting Alq(3) derivatives have met with only a limited success. In this paper, we describe how the electronic nature of various substituents, projected via an arylethynyl or aryl spacer to the position of the highest HOMO density (C5), may be used for effective emission tuning to obtain blue-, green-, and red-emitting materials. The synthetic strategy consists of four different pathways for the attachment of electron-donating and electron-withdrawing aryl or arylethynyl substituents to the 5-position of the quinolinolate ring. Successful tuning of the emission color covering the whole visible spectrum (lambda=450-800 nm) was achieved. In addition, the photophysical properties of the luminophores were found to correlate with the Hammett constant of the respective substituents, providing a powerful strategy with which to predict the optical properties of new materials. We also demonstrate that the electronic nature of the substituent affects the emission properties of the resulting complex through effective modification of the HOMO levels of the quinolinolate ligand.

Di- and Tetranuclear Metal Complexes with Phenoxo Bridges: Synthesis, Structures, and Photoluminescent and Electroluminescent Properties

Dinuclear and tetranuclear copper 2,6-bis(2-hydroxyphenyl)pyridine (H(2)L) complexes Cu2(L)2(py)2 (1) and Cu4(L)4(DMF) (2) were synthesized. The complexes 1 and 2 were characterized by elemental analyses, mass spectrometry, and single-crystal X-ray diffraction analyses. 1 crystallizes in the monoclinic space group P2(1)/n with a = 13.330(2) Angstroms, b = 9.361(1) Angstroms, c = 14.676(1) Angstroms, beta = 100.94(1) degrees, V = 1798.1(3) Angstroms(3), and Z = 2. 2 crystallizes in the monoclinic space group P2(1)/n with a = 13.360(1) Angstroms, b = 14.884(1) Angstroms, c = 15.462(2) Angstroms, beta = 97.50(4) degrees, V = 3048.4(1) Angstroms(3), and Z = 2. Tetranuclear zinc complex Zn4(L)4(py)4 (3) was prepared and characterized by X-ray diffraction. 3 crystallizes in the triclinic space group P with a = 13.770(1) Angstroms, b = 15.465(1) A, c = 16.409(2) Angstroms, alpha = 88.877(9) degrees, beta = 88.035(4) degrees, gamma = 82.956(3) degrees, V = 3465.6(5) Angstroms(3), and Z = 2. The di- and tetranuclear complexes 1-3contain phenoxo bridges. 1 is a dinuclear complex with two Cu(II) centers, two py ligands, and two L ligands, and each L ligand donates its pyridyl ring and one of the phenolate groups to one metal and shares the other phenolate group between both metals, affording a Cu(2)(mu-O)(2) core. 2, in contrast, is a tetranuclear complex with four Cu(II) centers and four L ligands. Two of the L ligands have the same coordination mode as 1, and the other two L ligands donate their pyridyl rings to one metal and share both phenolate groups between four metals, resulting in three four-membered Cu2(mu-O)2 rings, which joined each other and showed great distortion from planarity. 3 is a tetranuclear complex with four Zn(II) centers, four pyridine ligands, and four L ligands, and the L ligands have the same coordination modes as those of 2. Single-crystal X-ray analysis showed that hydrogen-bonding and pi-pi stacking interactions exist in complexes 1 and 2 resulting in two- and three-dimensional molecular arrangements, and the parallel arrangement of the ligand in the crystal of complex 3 resulted in a close inter- and intramolecular pi-pi interactions. Investigation of the crystals, as well as an amorphous thin film and powder of 3, by photoluminescence (PL) allowed the effect of the molecular packing on the emission properties to be elucidated. Furthermore, the electroluminescent (EL) properties of 3 were examined by fabricating a multilayer device with structure of [ITO/NPB/(ZnL)(n)/Alq3/LiF/Al] (NPB = N,N'-bis(alpha-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine, Alq3 = tris(8-hydroxyquinolinato)aluminum).

Characterization of molecular disorder in vapor-deposited thin films of aluminum tris(quinoline-8-olate) by one-dimensional Al27 NMR under magic angle spinning

Aluminum tris (quinoline-8-olate) (Alq3) is used as an electron-transport layer in organic light-emitting diodes. The material can be obtained in a wide range of different solid phases, both crystalline and amorphous, by deposition from the vapor phase or from solution under controlled conditions. While the structure of the crystalline polymorphs of Alq3 has been investigated thoroughly by x-ray diffraction as well as solid-state NMR, very little information is currently available on the amount of structural disorder in the amorphous forms of Alq3. In the present contribution, we report the use of 27Al NMR spectroscopy in the solid state under magic angle spinning to extract such information from amorphous vapor deposits of Alq3. The NMR spectra obtained from these samples exhibit different degrees of broadening, reflecting distributions of the electric-field gradient tensor at the site of the aluminum ion. These distributions can be obtained from the NMR spectra by solving the corresponding inverse problem. From these results, the magnitude of structural disorder in terms of molecular geometry has been estimated by density-functional theory calculations. It was found that the electric-field gradient anisotropy delta follows a bimodal distribution. Its majority component is centered around delta values comparable to the meridianal alpha crystal polymorph and has a width of about 10%, corresponding to distortions of the molecular geometry of a few degrees in the orientation of the ligands. Alq3 samples obtained at higher deposition rates exhibit higher degrees of disorder. The minor component, present at about 7%, has a much smaller anisotropy, suggesting that it may be due to the facial isomer of Alq3.

Investigation of a binuclear gallium complex with bipolar charge transporting capability for organic light-emitting diodes

Detailed theoretical and experimental investigations have been presented on a novel binuclear gallium complex Ga2(saph)2q2, a potentially active and novel material for organic light-emitting diodes (OLEDs). The density-functional theory has been employed to shed light on the reason why this complex exhibits bipolar character. Theoretical results, such as ionization potential and electron affinity, and optimized geometries are in good agreement with what we have obtained experimentally. Our theoretical analysis provides new insight into the origin of bipolar features. Such character enables the gallium complex to become a prosperous hole and electron transporter used to fabricate single-layer OLEDs, which exhibit better electroluminescence compared with those originated from the previously well-adopted tri(8-hydroxyquinoline)aluminum (Alq3). An OLED device of single Ga2(saph)2q2 layer exhibiting desirable performances has been fabricated, which is in good agreement with the theoretical analysis.