Hongze Gao

Luminescent Boron-Contained Ladder-Type π-Conjugated Compounds

Four diboron-contained ladder-type pi-conjugated compounds 1-4 were designed and synthesized. Their thermal, photophysical, electrochemical properties, as well as density functional theory calculations, were fully investigated. The single crystals of compounds 1 and 3 were grown, and their crystal structures were determined by X-ray diffraction analysis. Both compounds have a ladder-type pi-conjugated framework. Compounds 1 and 2 possess high thermal stabilities, moderate solid-state fluorescence quantum yields, as well as stable redox properties, indicating that they are possible candidates for emitters and charge-transporting materials in electroluminescent (EL) devices. The double-layer device with the configuration of [ITO/NPB (40 nm)/1 or 2 (70 nm)/LiF (0.5 nm)/Al (200 nm)] exhibited good EL performance with the maximum brightness exceeding 8000 cd/m(2).

Very high-efficiency red-electroluminescence devices based on an amidinate-ligated phosphorescent iridium complex

Highly efficient, low driving-voltage, red phosphorescent OLEDs with a wide range of doping concentrations or even without doping are successfully fabricated by use of an easily available amidinate-ligated iridium complex.

Fac‐Alq3 and Mer‐Alq3 Nano/Microcrystals with Different Emission and Charge‐Transporting Properties

Since the first efficient organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinoline)aluminum (Alq3) reported by Tang and VanSlyke, Alq3 has been employed as an electron-transporting or host-emitting material in OLEDs because of its excellent electron-transporting ability as well as film formation properties. In addition to the intensive studies on the optimization of Alq3-based devices for high performance, a great deal of effort has been invested in its polymorphs and photophysical properties. Generally, Alq3 molecules adopt mer-Alq3 (mer1⁄4meridional) isomeric form with C1 symmetry in both solution and solid state, and fac-Alq3 (fac1⁄4 facial) with C3 symmetry could only been obtained by annealingmer-Alq3 solid at a very high temperature (around 400 8C). These two isomeric forms exhibit quite different physical properties such as color and fluorescence, that is, green emission for light-yellowmer-Alq3 and blue emission for whitish fac-Alq3. Along with the progress on the studies of Alq3 bulk materials, increased effort has been directed toward the construction of Alq3-based nano or microstructured materials, particularly, 1D nanostructures, to further understand their photophysical properties in the nanoscale and explore novel optoelectronic applications. Although some green-emittingmer-Alq3 nanocrystals in a or b phases have been obtained via different processes, the preparation of blue-emitting fac-Alq3 nanocrystals in d or g phase is still an important issue. It was demonstrated that the fac-Alq3 crystalline phase could only be achieved by annealing mer-Alq3 at around 400 8C for at least several tens of minutes. Therefore, the reported approaches such as vapor deposition and a solution-based route, which have been employed to prepare mer-Alq3 nanocrystals, are not suitable for the fabrication of fac-Alq3 nanocrystals. From this viewpoint, the development of an efficient approach suitable to prepare fac-Alq3 nanocrystals is a highly important issue regarding the fundamental physical properties of fac-Alq3-based nanostructures. Since different phases of an organic functional material often have obviously different optical and electronic properties, it can be expected that fac-Alq3 will display distinct transporting properties compared to mer-Alq3. The mer-Alq3 has already been proven to be an excellent n-type semiconducting material; however, the charge-transporting property of the fac-Alq3 has not yet been studied in detail. To understand the transporting nature of fac-Alq3 and develop its potential applications for optoelectronics, the preparation of a fac-Alq3-based film is also urgently required. Following these points, we have mainly concentrated our attention on the development of an efficient approach to prepare fac-Alq3 nano/microstructures as well as a uniform thin film thereof. In this contribution, we report a facile method for the controllable fabrication of high-quality fac-Alq3 and mer-Alq3 1D nano/microstructures as well as their nano/microstructure-based films. This approach is significant because it not only enables the preparation of high-quality fac-Alq3 nanostructures and thin films but also provides the pathway to evaluate the transporting nature of the produced fac-Alq3 film. The uniform films comprised fac-Alq3 nano/microcrystals forming on substrates, which were pre-coated with electrodes, thus, allowing us to directly characterize their electrical properties such as carrier transport. Indeed, a single-carrier device based on fac-Alq3 film fabricated in situ exhibited an excellent hole-transporting character. To obtain mer-Alq3 and fac-Alq3 nano/microcrystals, a double-film annealing process was developed (Fig. 1). Firstly, two Alq3 films with a thickness of about 500 nm each n were prepared on glass substrates by a common vacuum deposition technique at room temperature; these Alq3 films were nearly amorphous, as verified by X-ray diffraction measurement (see Supporting Information, Fig. S1). Then, the two films were stacked together with the help of gravity in a face-to-face fashion and horizontally placed into a small chamber, which was filled

Theoretical Characterization of a Typical Hole/Exciton-Blocking Material Bathocuproine and Its Analogues

The structural, electronic, and carrier transport properties of bathocuproine (BCP), which is a typical hole/exciton-blocking material applied in organic light-emitting diodes (OLEDs), have been investigated based on density functional theory (DFT) and ab initio HF method. The detail characterizations of frontier electronic structure and lowest-energy optical transitions have been studied by means of time-dependent density functional theory (TD-DFT). Five BCP analogues, o-phenanthroline (1), 2,9-dimethyl-1,10-phenanthroline (2), 2,9-diphenyl-1,10-phenanthroline (3), 4,7-diphenyl-1,10-phenanthroline (4), and 2,9-bis(trifluoromethyl)-1,10-phenanthroline (5) have also been studied in order to select more suitable candidates of efficient hole-blocking materials. The calculated results showed that rigid planar structures, conjugate degrees, and substitute groups play crucial roles in the hole/exciton-blocking and electron-transport properties of these materials. The calculated geometries, ionization energies (IP), and energy gap between the singlet ground state and triplet excited state (E(T1)) were well in agreement with the experimental results. On the basis of the incoherent transport model, the calculated electron mobility of BCP is 1.79 x 10(-2) cm(2)/(V s), which is comparable to experimental results of 1.1 x 10(-3) cm(2)/(V s). The electron mobilities for compounds 1, 4, and 5 are 3.45 x 10(-2), 2.90 x 10(-2), and 1.40 x 10(-2) cm(2)/(V s), respectively. The calculated results indicated that compounds 1, 4, and 5 may be more effective hole/exciton-blocking materials than BCP.

Novel Urea-Functionalized Quinacridone Derivatives: Ultrasound and Thermo Effects on Supramolecular Organogels

In investigations into the effects of environmental factors on organogels, two urea-functionalized quinacridone derivatives 1a and 1b have been designed and synthesized. These two compounds can respond to ultrasound and thermal stimuli in the organic test solvents, and exhibit pronounced aggregation properties. The field-effect (FE)-SEM images of xerogels show the characteristic gelation morphologies of 3D fibrous network structures. The concentration- and temperature-dependent (1)H NMR spectra suggest that the intermolecular π-π and hydrogen-bonding interactions of gelators are the main driving forces for the supramolecular assembly process. X-ray diffraction (XRD), UV/Vis absorption, and photoluminescent spectroscopy studies have been carried out and provide more information to define the molecular packing model in gelation states.

Sonication-Induced Molecular Gels Based on Mono-Cholesterol Substituted Quinacridone Derivatives

A series of monocholesterol substituted quinacridone derivatives MCC(n) (n = 4, 6, 8) has been designed and synthesized. Compounds MCC(6) and MCC(8) can gelate a wide range of organic solvents upon ultrasound irradiation and afford intriguing well-defined nanostructures composed of three-dimensional sponge-like superstructures or fibrous networks. Interestingly, the gel produced from MCC(6) is sensitive to thermo-, aniline, and formic acid stimulus, giving obviously different aggregation behaviors as well as physical properties. Time-dependent spectroscopic data and theoretical calculation results provided explanation for the possible molecular aggregation mode during the formation of the gels.

Polymorphs and a pseudo-polymorphs based on a luminescent boron-containing compound: structural diversity arising from conformational isomers and noncovalent interactions

The synthesis of an organoboron compound 2,6-bis(5-methyl-2-hydroxylphenyl)pyridyl boron (p-methoxyl)benzene (MDPPBPM(p)) is reported. The crystallizations of MDPPBPM(p) gave two crystalline polymorphs A and B (A from THF/diethyl ether system and B from pyridine/diethyl ether system, respectively) and a chloroform solvate C (pseudo-polymorph from chloroform/diethyl ether system). It was demonstrated that in crystalline polymorphs A and B, MDPPBPM(p) molecules adopt two different conformations, a cis-conformer for A and a trans-conformer for B, respectively. The pseudo-polymorph C also exhibits a trans-conformer. The MDPPBPM(p) molecules in the three phases vary remarkably in the rotational position of methoxy groups. Apart from the different conformational characteristic and structural motifs, non-covalent intermolecular interactions such as C–H⋯O hydrogen bonds and π⋯π stacking interactions, physical properties such as shape, calculated density and thermal stability, cause significant differences between the three crystals. All the polymorphs and the pseudo-polymorph exhibit solid-state photoluminescence due to the intraligand π → π* electron transition.

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.

Influence of noncovalent intermolecular interactions on crystal packing: syntheses and crystal structures of three layered Zn(II)/1,2,4-triazole/carboxylate coordination polymers

Three 2D layered zinc coordination polymers: Zn(trz)(HBDC) (1), Zn4(trz)4(4-FBA)4·(H2O)0.5 (2) and Zn4(trz)4(4-CBA)4 (3) (trz = 1,2,4-triazole, H2BDC = 1,3-benzenedicarboxylic acid, 4-HFBA = 4-fluorobenzoic acid and 4-HCBA = 4-chlorobenzoic acid) have been synthesized by hydrothermal reactions and characterized by IR, elemental analysis, TGA measurements and X-ray single-crystal analysis. Compounds 1–3 have the same 2D layer constructed by zinc(II) with trz and carboxylate. The main difference in the structure of the three compounds is the interactions between the layers. Compound 1 shows unusual intermolecular hydrogen bonding and C–O⋯π interactions, compound 2 exhibits intermolecular hydrogen bonding and C–F⋯π interactions, while compound 3 only shows intermolecular C–Cl⋯π interactions. It is found that the noncovalent intermolecular interactions have an influence on crystal packing.