Luminescent alumina-based aerogels modified with tris(8-hydroxyquinolinato)aluminum

A facile procedure was developed for the production of Al₂O₃ aerogels modified with tris(8-hydroxyquinolinato)aluminum complex (AlQ₃). The addition of 8-hydroxyquinoline during the stage of alumina lyogel formation was found to increase gelation duration. The translucent monolithic aerogels were obtained by supercritical drying of lyogels in CO₂. The composition and properties of aerogels were analyzed using low-temperature nitrogen adsorption, helium pycnometry, IR spectroscopy, UV–visible spectroscopy, luminescence spectroscopy, powder X-ray diffraction, scanning and transmission electron microscopy, and thermogravimetry combined with mass spectrometry. The obtained aerogels had low density (0.15–0.18?g?cm^?3), high specific surface area (480–550?m²/g), high porosity (90–95%), and showed bright luminescence upon excitation in the UV range.

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Excitation energy transfer in tris(8-hydroxyquinolinato)aluminum doped with a pentacene derivative

We investigate the excitation energy transfer in a guest-host molecular system consisting of a pentacene derivative, namely 6,13-bis(2,6-dimethylphenyl)pentacene (DMPP), doped into tris(8-hydroxyquinolinato)aluminum (Alq(3)) using steady-state and time-resolved photoluminescence (PL) spectroscopy. The concentration dependent energy transfer rate and efficiency are calculated and analyzed in terms of the F?rster resonance energy transfer model. A relatively long excitation transfer time ( approximately 0.6-3.4 ns depending on the DMPP concentration) and a large transfer radius (31-36 A) are obtained. The F?rster radius calculated directly from the Alq(3) PL-DMPP absorption spectral overlap (26 A) is smaller than the transfer radii obtained from the PL studies, which suggests that excitation energy migration within Alq(3) plays an important role in the energy transfer process, effectively elongating the transfer radius and increasing the transfer rate and efficiency.     

Characterisation of different polymorphs of tris(8-hydroxyquinolinato)aluminium(III) using solid-state NMR and DFT calculations

Background  

Organic light emitting devices (OLED) are becoming important and characterisation of them, in terms of structure, charge distribution, and intermolecular interactions, is important. Tris(8-hydroxyquinolinato)-aluminium(III), known as Alq₃, an organomettalic complex has become a reference material of great importance in OLED. It is important to elucidate the structural details of Alq₃ in its various isomeric and solvated forms. Solid-state nuclear magnetic resonance (NMR) is a useful tool for this which can also complement the information obtained with X-ray diffraction studies.     

Theoretical study of isomerism/phase dependent charge transport properties in tris(8-hydroxyquinolinato)aluminum(III)

The charge carrier transporting ability in the polymorphism of tris(8-hydroxyquinolinato)aluminum(III) (Alq(3)) has been studied using density functional theory (DFT) and Marcus charge transport theory. α- and β-Alq(3) composed of mer-Alq(3) molecules have stronger electron-transporting property (n-type materials) compared with their hole-transporting ability. In contrast, γ- and δ-Alq(3) formed by fac-Alq(3) molecules possess stronger hole-transporting character than their electron-transporting ability. The detailed theoretical calculations indicate the reason lies in the differences of HOMO and LUMO distribution states of the two kinds of isomers, and the different molecular packing modes of charge-transporting pathways for different phases.     

Charge transport properties of tris(8-hydroxyquinolinato)aluminum(III): why it is an electron transporter

The charge transport properties of mer-tris(8-hydroxyquinolinato)aluminum(III) (mer-Alq), which is the most widely used electron transport material in OLED, were investigated by quantum chemistry calculations within the framework of the charge hopping model and Marcus electron transfer theory. Internal reorganization energies of 0.276 and 0.242 eV were calculated by the DFT-B3LYP method employing a 6-31 G* basis set for the electrons lambdai(e) and holes lambdai(h), respectively. The relative distances and orientations of Alq molecules in amorphous film were simulated by those in the beta-phase. The intermolecular charge-transfer integrals, Hda(h) and Hda(e), along all 14 hopping pathways were then calculated by the Koopmans Theorem in conjunction with the Hartree-Fock method employing a 6-31 G* basis set as well as by the direct coupling method. The results showed that there were some Hda(e) that were 1 order of magnitude larger than any Hda(h), because hopping pathways with effective overlaps of LUMOs can occur and, thus, large Hda(e). On the other hand, effective overlap of HOMO was absent in all pathways, resulting in a relatively small Hda(h). This difference in the magnitudes of Hda(e) and Hda(h) would predict a 2 orders of magnitude difference in the electron-transfer rate constants and account for the observed 2 orders of magnitude difference in the mobilities of electrons and holes.     

Thermal change of Alq3, tris(8-hydroxyquinolinato) aluminum(III) studied by TG and XRD-DSC

The thermal change of the tris(8-hydroxyquinolinato)aluminum (Alq₃) is currently investigated by XRD-DSC and TG. The phase transition of Alq₃ from α-phase to γ-phase takes place at 643–669 K. A very sharp peak with the peak temperature at approx. 709 K is ascribed to the melting of the Alq₃. The decomposition of the Alq₃ was observed accompanied with the melting and evaporation at >703K. The effect of the atmospheres on the mass loss procedure was studied by TG. It was found that thermal process of Alq₃ was strongly influenced by the partial pressure of water vapor in the atmosphere instead of oxygen.     

The formation of tris(2-methyl-8-hydroxyquinolinato)-aluminum(III)

The compound Al(C₁₀H₃ON)₃ can be precipitated from a 20 % ($\text{v}\text{v}$) acetone—water medium in the absence of auxiliary complexing agents. Precipitation from an initially acidic solution yields a product containing hydrolyzed forms of aluminum(III) but precipitation from an initially basic solution results in the pure tris compound. Under appropriate conditions, solid-phase aluminum hydroxide can be completely converted to Al(C₁₀H₃ON)₃. Stability constants for the formation of aluminum(III) complexes of 2-methyl-8-hydroxyquinoline are given.     

Femtosecond depolarization dynamics of tris(8-hydroxyquinoline) aluminum films

For vapor-deposited tris(8-hydroxyquinoline) aluminum thin films, steady-state and subpicosecond transient optical anisotropy are investigated. It is found that the transient absorption anisotropy decays within tens of picoseconds. With a simple model calculation, the excited state population and anisotropy decay dynamics are disentangled, and the latter signal, the depolarization of the excited state, is explained by the energy transfer between the non-orthogonally-coordinated quinolate ligands. It is also shown that there are two pathways for this fast interligand energy transfer.     

Relativistic band structure calculations

The inclusion of relativistic effects in band theory is discussed. Examples showing how these affect optical properties, Fermi surface properties, and cohesive properties are shown. With respect to the latter, it is pointed out that the mass–velocity and Darwin shifts are most important, whereas the SO coupling can be omitted since it does not shift the bands (quenching of angular momentum). The band theoretical aspects are presented in the terminology of the LMTO -ASA method which offers a physical transparent picture of the formation of bands. The volume and energy dependences of the spin-orbit splittings are explained in this picture. As examples are considered calculations for W, Xe, Au, and the copper halides.     

Performance of tris(2-methyl-8-quinolinolato)aluminum as fluorescent anionophore.

An X-ray crystallographic study demonstrated that the tris complex of Al3+ with 2-methyl-8-quinolinol (Hmq), [Al(mq)3], had a discrete octahedral geometry in a meridional configuration with respect to mq-; the Al-N bonds were appreciably elongated. In organic solvents, the complex reacted with water to give a lower species of a mu-oxo dimer, [Al2O(mq)4], and a protonated ligand, depending on the concentrations of water and of excess ligand. None of Cl-, Br-, I-, ClO4-, and CH3COO- showed any reactions with [Al(mq)3]. The anions having high affinities to Al3+, such as H2PO4- and F-, replaced all the ligand. The anion having a lower affinity, HSO4-, exclusively formed a mixed-ligand complex of [Al(mq)2HSO4], which had a more intense fluorescence than the others. In contrast, the corresponding 8-quinolinol (Hq) complex, [Alq3], showed no reactions with any anions.     

Chemistry between magnesium and multiple molecules in tris(8-hydroxyquinoline) aluminum films

Metal organic contacts are at the basis of devices such as organic light emitting diodes (OLEDs). Here, we report a theoretical investigation of the chemical interaction between a Mg atom and an organic film made of tris(8-hydroxyquinoline)aluminum (Alq3) molecules. The latter is modeled either by an isolated molecule or by a bulk crystal. Using first-principles molecular dynamics for structural optimization, we find that an isolated Alq3 molecule and a Mg atom form an ion-pair. However, when the metal atom interacts with molecules in a bulk crystalline environment, we find that an organometallic complex is energetically preferred over the ion-pair. The complex formation is an effect of the environment which makes possible the interaction of the metal atom with several adjacent molecules. Here, our calculated O(1s) and N(1s) core level shifts agree well with recent experimental data on Alq3 films exposed to Mg. Our results resolve the apparent contradiction between experiment and predictions made in previous calculations in which a single Alq3 molecule was used to model a thin film.     

Poly(1,4-phenylene-1,2-diphenylvinylene) and tris(8-quinolinolato)aluminum bilayer light-emitting diodes

A bilayer polymer light-emitting diode has been demonstrated. This bilayer structure is composed of two different materials with nearly identical luminescent spectra, but different band positions. The band structure of one material, poly(1,4-phenylene-1,2-diphenylvinylene), favors hold injection, while the band structure of the other material, tris-(8-quinolinolato)aluminum, favors electron injection. Therefore in this device structure, a heterojunction forms at the bilayer interface. Charges injected from the anode and cathode accumulated at this interface and subsequently radiative recombine. Compared with the single-layer devices made from each of these two materials, the device performance of this bilayer LED is significantly improved owing to this carrier localization. Device quantum efficiency is enhanced by a factor of 20 to reach ⪅1% photons/electron. Unlike some other bilayer systems, there is no color shift of this bilayer structure, because the luminescent spectra are the same for both materials. © 1997 John Wiley & Sons, Ltd.     

Photophysical properties of dioxolane-substituted pentacene derivatives dispersed in tris(quinolin-8-olato)aluminum(III)

Two novel dioxolane-substituted pentacene derivatives, namely, 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (TP-5) and 2,2,10,10-tetraethyl-6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5), have been synthesized and spectroscopically characterized. Here, we examine the steady-state and time-resolved photoluminescence (PL) of solid-state composite films containing these pentacene derivatives dispersed in tris(quinolin-8-olato)aluminum(III) (Alq(3)). The films show narrow red emission and high absolute photoluminescence quantum yields (phi(PL) = 59% and 76% for films containing approximately 0.25 mol % TP-5 and EtTP-5, respectively). The F?rster transfer radius for both guest-host systems is estimated to be approximately 33 A. The TP-5/Alq(3) thin films show a marked decrease in phi(PL) with increasing guest molecule concentrations, accompanied by dramatic changes in the PL spectra, suggesting that intermolecular interactions between pentacene molecules result in the formation of weakly radiative aggregates. In contrast, a lesser degree of fluorescence quenching is observed for EtTP-5/Alq(3) films. The measured fluorescence lifetimes of TP-5 and EtTP-5 are similar (approximately 18 ns) at low concentrations but deviate at higher concentrations as aggregation begins to play a role in the TP-5/Alq(3) films. The onset of aggregation in EtTP-5/Alq(3) films occurs at higher guest molecule concentrations (>1.00 mol %). The addition of ethyl groups on the terminal dioxolane rings leads to an increase in the intermolecular spacing in the solid, thereby reducing the tendency for pi-pi molecular stacking and aggregation.     

Configuration-specific synthesis of the facial and meridional isomers of tris(8-hydroxyquinolinate)aluminum (Alq3)

Treatment of AlO(OH) with 3 equiv of 8-hydroxyquinolinol in refluxing deionized water provided the meridional and facial isomers of tris(8-hydroxyquinolinate)aluminum (Alq3) with good yields as solid deposits after 1 and 90 h, respectively. X-ray diffraction and solid-state 13C NMR studies revealed that mer-Alq3 is formed in the early stage of the reaction and then gradually converts to fac-Alq3, which is thermodynamically less stable, although no existence of a catalyst substance is implied.     

Synthesis and Structure of Tris(8-mercaptoquinolinato)thallium and Tris(8-hydroxyquinolinato)thallium Monohydrate

Tris(8-mercaptoquinolinato)thallium Tl(C₉H₆NS)₃ and tris(8-hydroxyquinolinato)thallium monohydrate Tl(C₉H₆NO)₃·H₂O have been synthesized. Their structures have been confirmed by X-ray crystallographic analysis.     

High-efficiency tris(8-hydroxyquinoline)aluminum (Alq3) complexes for organic white-light-emitting diodes and solid-state lighting

Combinations of electron-withdrawing and -donating substituents on the 8-hydroxyquinoline ligand of the tris(8-hydroxyquinoline)aluminum (Alq(3)) complexes allow for control of the HOMO and LUMO energies and the HOMO-LUMO gap responsible for emission from the complexes. Here, we present a systematic study on tuning the emission and electroluminescence (EL) from Alq(3) complexes from the green to blue region. In this study, we explored the combination of electron-donating substituents on C4 and C6. Compounds 1-6 displayed the emission tuning between 478 and 526 nm, and fluorescence quantum yield between 0.15 and 0.57. The compounds 2-6 were used as emitters and hosts in organic light-emitting diodes (OLEDs). The highest OLED external quantum efficiency (EQE) observed was 4.6%, which is among the highest observed for Alq(3) complexes. Also, the compounds 3-5 were used as hosts for red phosphorescent dopants to obtain white light-emitting diodes (WOLED). The WOLEDs displayed high efficiency (EQE up to 19%) and high white color purity (color rendering index (CRI≈85).     

An assessment of polymer band structure calculations

A brief comparison is made of four band structure calculations for the polyethylene chain and it is found that the status of the results bears little relation to the sophistication of the method. A criterion is suggested that may enable one to determine in advance whether this situation is liable to abtain in other systems.