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

Meltable Aluminum Molecular Rings with Fluorescence and Nonlinear Optical Properties

Crystal-liquid-glass, which combines the tunable properties of crystalline compounds with the processability of glasses, has emerged as a new class of materials for fabricating bulk-shapable devices in real applications. Inspired by the characteristics of deep eutectic solvent (DES) mixtures involving significant depressions in melting points compared to their neat constituent components, in this study, we designed and synthesized the first examples of meltable aluminum oxo clusters (AlOCs) via lattice doping with DESs at the molecular level. The abundant and strong hydrogen bonding between the aluminum molecular ring, DES components, and lattice solvents is postulated to be the root that affords melting point depressions and, thus, "melting" clusters. We prepared a transparent bubble-free glass film under autogenous pressure using a hot-press method. These cluster-based films exhibited luminescent and nonlinear optical properties similar to those of pristine crystalline compounds. Our study belongs to the interdisciplinary disciplines of chemistry and physics. It not only breaks the limitations of crystalline glass on metal and ligand types but also acts as a general guide for extending the range of meltable crystalline materials.

Fabrication of Super-Sized Metal Inorganic-Organic Hybrid Glass with Supramolecular Network via Crystallization-Suppressing Approach

Metal coordination compound (MCC) glasses [e.g., metal-organic framework (MOF) glass, coordination polymer glass, and metal inorganic-organic complex (MIOC) glass] are emerging members of the hybrid glass family. So far, a limited number of crystalline MCCs can be converted into glasses by melt-quenching. Here, we report a universal wet-chemistry method, by which the super-sized supramolecular MIOC glasses can be synthesized from non-meltable MOFs. Alcohol and acid were used as agents to inhibit crystallization. The MIOC glasses demonstrate unique features including high transparency, shaping capability, and anisotropic network. Directional photoluminescence with a large polarization ratio (≈47 %) was observed from samples doped with organic dyes. This crystallization-suppressing approach enables fabrication of super-sized MCC glasses, which cannot be achieved by conventional vitrification methods, and thus allows for exploring new MCC glasses possessing photonic functionalities.

A Highly Fluorescent Dinuclear Aluminium Complex with Near‐Unity Quantum Yield**

The high natural abundance of aluminium makes the respective fluorophores attractive for various optical applications, but photoluminescence quantum yields above 0.7 have yet not been reported for solutions of aluminium complexes. In this contribution, a dinuclear aluminium(III) complex featuring enhanced photoluminescence properties is described. Its facile one‐pot synthesis originates from a readily available precursor and trimethyl aluminium. In solution, the complex exhibits an unprecedented photoluminescence quantum yield near unity (Φ_absolute 1.0±0.1) and an excited‐state lifetime of 2.3 ns. In the solid state, J‐aggregation and aggregation‐caused quenching are noted, but still quantum yields of 0.6 are observed. Embedding the complex in electrospun non‐woven fabrics yields a highly fluorescent fleece possessing a quantum yield of 0.9±0.04.

Chiroptical switching behavior of heteroleptic ruthenium complexes bearing acetylacetonato and tropolonato ligands

Four types of tris-chelate ruthenium complexes bearing acetylacetonato (acac) and tropolonato (trop) ligands were synthesized and optically resolved into Δ and Λ isomers: [Ru(acac)₃] (Ru-0), [Ru(acac)₂(trop)] (Ru-1), [Ru(acac)(trop)₂] (Ru-2), and [Ru(trop)₃] (Ru-3). Chiral HPLC chromatograms, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) of the four ruthenium complexes were systematically investigated. As a result, the absolute configurations of the newly prepared enantiomeric complexes Ru-2 and Ru-3 were determined. For the case of Ru-2, its absolute configuration was also confirmed by single crystal X-ray diffraction analysis. The ECD changes upon chemical oxidation were further investigated for the four complexes. An ECD change in enantiomeric Ru-1 was observed upon oxidation, but the oxidized species soon returned to the neutral state within a few minutes. Enantiomers of Ru-3 also showed explicit ECD changes upon oxidation. Further, the lifetime of the oxidized state was the longest among the four investigated complexes, whereas they racemized in solution at room temperature. In contrast, the enantiomers of heteroleptic complexes (Ru-1 and Ru-2) concurrently exhibited ECD changes, relatively long lifetime of the oxidized state, and nil or quite slow racemization behavior. The coexistence of acac and trop ligands was key to making the competing factors compatible in the resultant ruthenium complexes.

Synthesis and Photophysical Properties of a Series of Dimeric Indium Quinolinates

A novel class of quinolinol-based dimeric indium complexes (1-6) was synthesized and characterized using ¹H and ¹³C(¹H) NMR spectroscopy and elemental analysis. Compounds 1-6 exhibited typical low-energy absorption bands assignable to quinolinol-centered π-π* charge transfer (CT) transition. The emission spectra of 1-6 exhibited slight bathochromic shifts with increasing solvent polarity (p-xylene < tetrahydrofuran (THF) < dichloromethane (DCM)). The emission bands also showed a gradual redshift, with an increase in the electron-donating effect of substituents at the C5 position of the quinoline groups. The absolute emission quantum yields (Φ_PL) of compounds 1 (11.2% in THF and 17.2% in film) and 4 (17.8% in THF and 36.2% in film) with methyl substituents at the C5 position of the quinoline moieties were higher than those of the indium complexes with other substituents.

Theoretical and experimental studies on the fluorescence properties of aluminum(III), cadmium(II), zinc(II), and copper(II) complexes of substituted 8-hydroxyquinoline

Fifty-five 8-hydroxyquinoline (8-HQ) derivatives are synthesized and the corresponding aluminum(III), cadmium(II), copper(II), and zinc(II) metal complexes are prepared and their fluorescent activities are evaluated. The results indicate that the aluminum complexes have the best fluorescence properties, followed by zinc and cadmium complexes, while almost no fluorescence is observed with the copper complexes. The relationship between the fluorescence properties and complex structure is summarized and a predictive three-dimensional quantitative structure–property relationship model is established using the multifit molecular alignment rule of Sybyl program. With the introduction of groups at the C-2 position or electron-withdrawing groups to the 8-hydroxyquinoline framework, fluorescence wavelength blue shifts are observed with the zinc, aluminum, and cadmium complexes. At the same time, a red shift of the fluorescence emission wavelength is detected for the aluminum and zinc complexes when C-5 of 8-hydroxyquinoline was substituted with halogens or a methyl group. Moreover, the zinc, cadmium, and aluminum complexes with 2,4-dimethyl substituents on the 8-hydroxyquinoline all show good fluorescence properties. The highest occupied molecular orbital and lowest unoccupied molecular orbital energies of the complexes are also calculated and the fluorescence properties of the metal complexes are analyzed from the viewpoint of molecular orbitals.

Absorption and emission properties of 5-phenyl tris(8-hydroxyquinolinato) M(III) complexes (M = Al, Ga, In) and correlations with molecular electrostatic potential

Substituent effect for a series of 5-phenyl tris(8-hydroxyquinolinato) M(III) complexes (Mq3) of aluminum, gallium, and indium are investigated using density functional theory (DFT) for the ground state properties and the time-dependent version of DFT (TDDFT) for their absorption and emission properties. A comparison between the ground state energy of mer and fac isomers of all the complexes revealed that the mer configuration is always more stable than fac. The substituent effect is significantly reflected at the fluorescence maximum (λ_F ) values whereas the effect is moderate at the absorption maximum (λ_abs ) values. The molecular electrostatic potential (MESP) at the metal center (V_M ) and the most electron rich region indicated by MESP minimum (V_min ), located at the oxygen of phenoxide ring exhibit excellent correlations with the λ_F and Stokes shift (λ_F -λ_abs ) values. The study suggests the use of Stokes shift as an experimental quantity to measure the excited state substituent effect while the V_min or V_M emerge as theoretical quantities to measure the same.

White light emitting diode based on purely organic fluorescent to modern thermally activated delayed fluorescence (TADF) and perovskite materials

White organic/polymer light emitting diode (WOLED/WPLED) processed from solution has attracted significant research interest in recent years due to their low device production cost, device flexibility, easy fabrication over large area including roll to roll and ability to print in various designs and shapes providing enormous design possibilities. Although WOLEDs fabricated using solution process lack their thermally evaporated counterparts in terms of device efficiency, remarkable progress has been made in this regard in recent years by utilizing new materials and device structures. In the present review, we have summarized and extrapolated an excellent association of old and modern concept of cost-effective materials and device structure for realization of white light. In particular, this article demonstrated and focused on design, and development of novel synthesis strategy, mechanistic insights and device engineering for solution process low cost WOLEDs device. Herein, an overview of the prevailing routes towards white light emitting devices (WLEDs) and corresponding materials used, including polymer based WLED, small molecules emitters based thermally activated delayed fluorescence (TADF), perovskite light-emitting diodes (PeLEDs) and hybrid materials based LEDs, color down-converting coatings with corresponding best efficiencies ever realized. We presume that this exhaustive review on WLEDs will offer a broad overview of the latest developments on white SSL and stonework the approach en route for innovations in the immediate future.

Study on Relationship Between Fluorescence Properties and Structure of Substituted 8-Hydroxyquinoline Zinc Complexes

Organic light-emitting diodes (OLEDs) produced from 8-hydroxyquinoline metal complexes play a vital role in modern electroluminescent devices. In this manuscript, a series of 8-hydroxyquinoline derivatives were synthesized by different methods and their corresponding zinc metal complexes were prepared. The UV and fluorescence properties of the complexes were measured aiming to understand the effect of substituents at the quinoline ring on the fluorescence properties of the complexes. When the C-5 of 8-hydroxyquinoline was replaced by halogen group, the fluorescence emission wavelengths had been red-shifted, at the same time, blue-shifted of fluorescence emission wavelength was observed when the C-5 position of 8-hydroxyquinoline was substituted by electron-withdrawing group. When the C-4 position of 8-hydroxyquinolie was substituted by methyl or the C-5 position was substituted by sulfonic acid group, the corresponding zinc complexes had higher fluorescence intensity than 8-hydroxyquinolie zinc.

To the homogeneity range of tris(8-hydroxyquinoline)gallium

The p_8-Hq–T diagram of non-stoichiometrical tris(8-hydroxyquinoline)gallium was plotted in the 408–665 K temperature range using a newly developed technique.

Substrate temperature controls molecular orientation in two-component vapor-deposited glasses

Vapor-deposited glasses can be anisotropic and molecular orientation is important for organic electronics applications. In organic light emitting diodes (OLEDs), for example, the orientation of dye molecules in two-component emitting layers significantly influences emission efficiency. Here we investigate how substrate temperature during vapor deposition influences the orientation of dye molecules in a model two-component system. We determine the average orientation of a linear blue light emitter 1,4-di-[4-(N,N-diphenyl)amino]styryl-benzene (DSA-Ph) in mixtures with aluminum-tris(8-hydroxyquinoline) (Alq3) by spectroscopic ellipsometry and IR dichroism. We find that molecular orientation is controlled by the ratio of the substrate temperature during deposition and the glass transition temperature of the mixture. These findings extend recent results for single component vapor-deposited glasses and suggest that, during vapor deposition, surface mobility allows partial equilibration towards orientations preferred at the free surface of the equilibrium liquid.

Push-pull effect on the geometrical, optical and charge transfer properties of disubstituted derivatives of mer-tris(4-hydroxy-1,5-naphthyridinato) aluminum (mer-AlND3)

To design innovative and novel optical materials with high mobility, two kinds of disubstituted derivatives for mer-tris(4-hydroxy-1,5-naphthyridinato) aluminum (mer-AlND3) with push (EDG)–pull (EWG) substituents have been designed. The structures of mer-tris(8-EDG-2-EWG-4-hydroxy-1,5-naphthyridinato) aluminum (type I) and mer-tris(8-EWG-2-EDG-4-hydroxy-1,5-naphthyridinato) aluminum (type II) in the ground and first excited states have been optimized at the B3LYP/6-31G(D) and CIS/6-31G(D) level of theory, respectively. It can be seen from frontier molecular orbitals analysis, in all these complexes, the highest occupied molecular orbital (HOMO) is localized on the pyridine-4-ol ring of A-ligand while lowest unoccupied molecular orbital (LUMO) is on the pyridyl ring of B-ligand in ground state irrespective of electron donor/acceptor substitution present on the ligands similar to that of mer-tris(8-hydroxyquinoline) aluminum (mer-Alq3) and parent mer-AlND3.The absorption and emission wavelengths have been evaluated at the TD-PBE0/6-31G(D) level and it can be see that all the type I derivatives show blue shift while most of the type II derivatives show red shift compared to mer-AlND3. All the disubstituted complexes have showed hypsochromic shifts in both the absorption and emission spectra when compared with the calculated absorption and emission spectra respectively of mer-Alq3. It can be seen that the reorganization energies of some of the disubstituted derivatives are comparable with mer-Alq3 and these derivatives might be good candidates for emitting materials in OLED.

Synthesis, characterization and fluorescent properties of 5-(aryliminomethyl)quinaldine-8-ol derivatives and their trinuclear zinc complexes

A series of 5-[1-(arylimino)methyl]quinaldine-8-ol derivatives L1-L5 and their trinuclear zinc(II) complexes (C1-C5) were synthesized. The compounds L1-L5 were fully characterized by the FT-IR spectra, NMR measurement and elemental analysis, meanwhile the zinc complexes C1-C5 were characterized by the FT-IR spectra and elemental analysis as well as the single crystal X-ray diffraction of a representative complex C3, which revealed a trinuclear zinc complex bearing six organic ligands. The fluorescent properties of both organic compounds and the zinc complexes have been carefully investigated by the UV-Vis absorption in various solvents, indicating the significant influences of the solvents and also double exponential decays.

Synthesis, characterization and DFT calculation of 4-fluorophenyl substituted tris(8-hydroxyquinoline)aluminum(III) complexes

New 4-fluorophenyl substituted 8-hydroxyquinoline derivatives, 5-(4-fluorophenyl)quinolin-8-ol and 5,7-bis(4-fluorophenyl)quinolin-8-ol, were synthesized and characterized by spectroscopic methods. The aluminum complexes of 5-(4-fluorophenyl)quinolin-8-ol (AlQF) and of 5,7-bis(4-fluorophenyl)quinolin-8-ol (AlQF2) exhibit strong fluorescence emission centered at 525 nm and 530 nm respectively. The quantum yield of both complexes were enhanced compared to the parent tris(8-hydroxyquinolinato)aluminum(III) complex. Electronic structures and photophysical properties of the new complexes were investigated theoretically by ab initio and density functional theory (DFT) and time dependent DFT (TD-DFT). Geometries of the ground state (S0) and the first excited state (S1) of the new complexes were optimized at the B3LYP/6-31G(d) functional and configuration interaction singles (CIS) method respectively. The aryl substituents were found to contribute significantly to the frontier molecular orbitals (FMOs). We have observed that in both cases the lowest occupied molecular orbital (LUMO) energy decreases while the energy of the highest occupied molecular orbital is slightly increased. The most significant increase was observed for AlQF2.

Charge transfer properties of two polymorphs of luminescent (2-fluoro-3-pyridyl)(2,2′-biphenyl)borinic 8-oxyquinolinate

First example of polymorphism and its impact on the charge transport properties of a model borinic quinolinate system.

Effect of high pressure on the crystal structure and charge transport properties of the (2-fluoro-3-pyridyl)(4-iodophenyl)borinic 8-oxyquinolinate complex

Compression of the crystal structure and its impact on the charge transport properties of a model borinic quinolinate system.

Recent progress in metal–organic complexes for optoelectronic applications

This critical review reports recent advances in the development of metal–organic complexes for optoelectronic applications.

Synthesis, photophysical, electrochemical and electroluminescent properties of a novel iridium(III) complex based on 2-phenylbenzo[d]oxazole derivative

A new phosphorescent iridium (III) complex based on 2-(4-tert-butylphenyl)-5-methylbenzo[d]oxazole as main ligand, i.e. bis(2-(4-tert-butylphenyl)-5-methylbenzo[d]oxazole-N,C(2'))iridium(acetylacetonate) [(tmbo)2Ir(acac)], was synthesized for organic light-emitting diodes (OLEDs), and its photophysical, electrochemical and electroluminescent properties were investigated. The complex displayed strong phosphorescence emission, high decomposition temperature, short phosphorescent lifetime and reversible redox electrochemical behavior. The OLEDs based on (tmbo)2Ir(acac) as dopant emitter exhibited maximum luminance efficiency of 26.1cdA(-1) and high luminance of 16,445 cd m(-2). Interestingly, highly doped device based on (tmbo)2Ir(acac) showed high efficiency with negligible roll-off under a wide range of driving current density, which was mainly attributed to the effect of bulky steric hindrance of multi-methyl groups on this complex and its short phosphorescent lifetime.