Dongxia Zhu

Reversible piezochromic behavior of two new cationic iridium(III) complexes

We demonstrate that two new cationic Ir(III) complexes exhibit an interesting piezochromism, and their emission color can be smartly switched by grinding and heating. This is the first example that the Ir(III) complexes display piezochromic phosphorescence.

Controllable synthesis of iridium(III)-based aggregation-induced emission and/or piezochromic luminescence phosphors by simply adjusting the substitution on ancillary ligands

Three multifunctional cationic iridium(III)-based materials with aggregation-induced emission (AIE) and piezochromic luminescence (PCL) behavior have been rationally designed with the help of the theoretical calculations and successfully synthesized. All complexes contain the same cyclometalated ligand, 1-(2,4-difluorophenyl)-1H-pyrazole (dfppz), while functionalized ancillary ligands with different substitution are used to control their photophysical properties. Complex 1 and 2 with ancillary ligands modified with aliphatic chains and carbazole end-capped alkyl groups, respectively, undergo remarkable and reversible changes in emission color in the solid state upon grinding or heating. In addition, 2, characterized as having the 3ILCT excited-state feature, simultaneously exhibits an interesting AIE behavior, showing almost non-emission in good solution but enhanced emission in its solid state. Further modification of 2 by attachment of a tert-butyl group on the ligand obtains complex 3, an amorphous material, which only displays AIE activity. More importantly, with the merits of reversible PCL and AIE properties of 2, the rare multi-channel color change and temperature-dependent emission behavior of the iridium(III) complex have been observed. Furthermore, the emissive nanoaggregates of 2 can be efficiently quenched by picric acid, making it a highly sensitive chemosensor for explosives, which is demonstrated in iridium(III)-based luminescent materials for the first time.

Piezochromic luminescent (PCL) behavior and aggregation-induced emission (AIE) property of a new cationic iridium(III) complex

A new cationic Ir(III) complex based on a dendritic ancillary ligand has been designed and synthesized, which simultaneously exhibits piezochromic luminescent (PCL) behavior and aggregation-induced emission (AIE) property for the first time.

Iridium(III) complexes adopting 1,2-diphenyl-1H-benzoimidazole ligands for highly efficient organic light-emitting diodes with low efficiency roll-off and non-doped feature

Two novel iridium(III) complexes (pbi)2Ir(mtpy) (1) and (pbi)2Ir(pbim) (2) adopting 1,2-diphenyl-1H-benzoimidazole (Hpbi) as cyclometalated ligands were successfully synthesized and characterized. Strong emissions at 501 and 536 nm with high photoluminescence quantum yields of 48% and 91% in CH2Cl2 at 298 K were obtained for 1 and 2, respectively. The quantum chemical calculations and the photophysical properties indicated that the dominant 3MLCT (metal-to-ligand charge-transfer) state mixed with 3LLCT (ligand-to-ligand charge-transfer) and 3LC (ligand-centered 3π–π*) characters contributed to their phosphorescence emissions. Doped organic light-emitting diodes (OLEDs) based on 1 and 2 showed a peak current efficiency of 45.0 cd A−1 and power efficiency of 47.9 lm W−1 accompanied by very low efficiency roll-off values. In their non-doped OLEDs, high efficiencies of 24.4 cd A−1 and 26.3 lm W−1 were achieved as well. These appealing results reveal that complexes 1 and 2 open interesting perspectives for the development of high-performance OLEDs in the future.

Intramolecular π-stacking in cationic iridium(III) complexes with a triazole–pyridine type ancillary ligand: synthesis, photophysics, electrochemistry properties and piezochromic behavior

To make Ir(III)-based complexes potentially multifunctional materials, two new cationic Ir(III) complexes with a 2-(5-phenyl-2-phenyl-2H-1,2,4-triazol-3-yl)pyridine (Phtz) ancillary ligand were designed and synthesized. By introducing the pendant phenyl ring into the ancillary ligand, the two complexes possess desired intramolecular π–π stacking between the pendant phenyl ring of the Phtz ligand and one of the phenyl rings of the cyclometalated ligand, which renders the complexes more stable. Density functional theory calculation indicates that the intramolecular π–π interactions in both complexes can reduce the degradation reaction in metal-centered (3MC) states to some extent, which further implies their stability. With these results in combination with their reversible oxidation and reduction processes as well as excellent photophysical properties, the stable light-emitting cells (LECs) would be expected. Furthermore, the two synthesized complexes exhibit reversible piezochromism. Their emission color can be smartly switched by grinding and heating, which is visible to the naked eye. In light of our experimental results, the present piezochromic behavior is due to interconversion between crystalline and amorphous states.

An orange iridium(III) complex with wide-bandwidth in electroluminescence for fabrication of high-quality white organic light-emitting diodes

A novel iridium(III) (pbi)2Ir(biq) with a strong orange emission at 573 nm was synthesized, which showed a high photoluminescence quantum yield (PLQY) of 45% in CH2Cl2 at 298 K. The quantum chemical calculations together with the photophysical properties indicated that the transition incorporation of 3MLCT (metal-to-ligand charge transfer), 3LLCT (ligand-to-ligand charge transfer) mixed with 3LC (ligand-centered 3π–π*) characters contributed to the emission of (pbi)2Ir(biq). Cyclic voltammetry (CV), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were then performed to investigate its electrochemical and thermal properties for fabrication of electroluminescent devices. Interestingly, complex (pbi)2Ir(biq) exhibited wide-bandwidth in electroluminescence (EL) of its orange electroluminescent devices, which made it effectively available to fabricate high-quality two-element white organic light-emitting diodes (WOLEDs). The resultant WOLED possessed a high current efficiency (ηc) of 22.1 cd A−1 and power efficiency (ηp) of 25.5 lm W−1 with a high color rendering index (CRI) of 80. Especially, the WOLED retained a favorable ηp of 10.9 lm W−1 and ηc of 16 cd A−1 at a high luminance of 1000 cd m−2. Moreover, it was worthwhile to note that this WOLED exhibited a qualified R9 of 13 and Duv of −0.0013, and these values together with the high CRI conformed to the required standard for lamp illumination of the ENERGY STAR.

A series of organopolymolybdate polymers linked by dual fuses: metal–organic moiety and organic ligand through Mo–N bonds

Three organopolymolybdate polymers, [Cu2(bte)2(H2O)(γ-Mo8O26)0.5] (1) (bte = 1,2-bis(1,2,4-triazol-1-yl)ethane), [Cu(btb)2(H2O)(γ-Mo8O26)0.5]·2H2O (2) (btb = 1,4-bis(1,2,4-triazol-1-y1)butane), and [Cu(bbtz)(H2O)2(γ-Mo8O26)0.5] (3) (bbtz = 1,4-bis(1,3,4-triazol-1-ylmethyl)benzene), were hydrothermally synthesized and structurally characterized. In compound 1, binuclear circles are exhibited, linked by [γ-Mo8O26]4− anions to construct one dimensional (1D) chains which connect with each other by Mo–N bonds to form two dimensional (2D) layers. These layers are linked through bte ligands to construct a three dimensional (3D) framework. In compound 2, two kinds of lines, inorganic lines [Cu1(γ-Mo8O26)]2n− and metal–organic lines [Cu2(btb)]2n+, are connectd to form a chain which is further reinforced by btb ligands through Mo–N and Cu1–N bonds, respectively. By sharing the Cu1 ions, these chains construct 2D layers, which are further fused by bi-dentate btb ligands to form a 3D framework. Compound 3 shows a 1D chain, in which the [γ-Mo8O26]4− anions and binuclear circles arrange alternately linked by Cu–O bonds. The linkages are further strengthened by Mo–N bonds. The dual fuses for linking polymolybdate anions, metal–organic moiety and organic bis(triazole) ligand, are discussed in detail.

Enhancing the luminescence properties and stability of cationic iridium(III) complexes based on phenylbenzoimidazole ligand: a combined experimental and theoretical study

Herein we designed and synthesized a series of cationic iridium(III) complexes with a phenylbenzoimidazole-based cyclometalated ligand, containing different numbers of carbazole moieties from zero to three (complexes 1-4). The photophysical and electrochemical properties of this series have been systematically investigated. The complexes exhibit strong luminescence in both solution and in neat films, as well as excellent redox reversibility. Introducing carbazole groups into the complexes is found to lead to substantially enhanced photoluminescence quantum efficiency in the neat film, but has little effect on the emitting color and excited-state characteristics as supported by density functional theory (DFT) results. DFT calculations also suggest that functionalized complexes 2-4 reveal better hole-transporting properties than 1. More importantly, all complexes effectively reduce the degradation reaction to some extent in metal-centered (³MC) excited-states, demonstrating their stability. Further studies indicate that restriction of opening of the structures in the ³MC state is caused by the unique molecular conformation of the phenylbenzoimidazole ligand, which is first demonstrated here in cationic iridium(III) complexes without intramolecular π-π stacking. These results presented here would provide valuable information for designing and synthesizing highly efficient and stable cationic iridium(III) complexes suitable for the optical devices.

New oxazoline- and thiazoline-containing heteroleptic iridium(III) complexes for highly-efficient phosphorescent organic light-emitting devices (PhOLEDs): colour tuning by varying the electroluminescence bandwidth

Two new homologous phosphorescent iridium complexes, bis-(2-phenylpyridine)(2-(2′-hydroxyphenyl)-2-oxazoline)iridium(III) [(ppy)2Ir(oz)] (1) and bis-(2-phenylpyridine)(2-(2′-hydroxyphenyl)-2-thiazoline)iridium(III) [(ppy)2Ir(thoz)] (2), have been obtained in good yields and characterized by single-crystal X-ray diffraction, cyclic voltammetry, photoluminescence and electroluminescence studies, and by time-dependent density functional theory (TD-DFT) calculations. Using the two complexes, which differ only by the heteroatom (O or S) substitution at the same site in the ancillary ligand, as the emitter, doped in a 4,4′-bis(N-carbazolyl)biphenyl (CBP) host, gave phosphorescent organic light-emitting diodes (PhOLEDs) with very efficient green and yellow emission, respectively. The turn-on voltages for both devices are low (3.5–3.7 V). The green-emitting (ppy)2Ir(oz) – based device has a maximum brightness of 61560 cd m−2 (at 16 V); maximum luminance efficiency of 66.2 cd A−1, 17.1% external quantum efficiency, 54 lm W−1 power efficiency and CIE coordinates of (0.35, 0.61) at a brightness of 10000 cd m−2. For the yellow-emitting (ppy)2Ir(thoz)-based device with a wide full spectral width at half maximum (FWHM) of 110 nm, the corresponding values are 21350 cd m−2 (at 14.5 V); 27.0 cd A−1, 8.5%, 18.0 lm W−1 and CIE coordinates of (0.46, 0.50). Colour tuning is primarily a consequence of the significantly wider emission bandwidth of complex 2 compared to complex 1.

Rational design and characterization of heteroleptic phosphorescent iridium(III) complexes for highly efficient deep-blue OLEDs

Two new deep-blue iridium(III) complexes, (dfpypy)2IrFptz (Ir1) and (Medfpypy)2IrFptz (Ir2), comprising difluoro-bipyridyl (dfpypy) derivatives as cyclometaling ligands and a chelated pyridyl-triazole (Fptz) ancillary ligand are reported. The bipyridyl ligands lead to a significantly increased HOMO–LUMO gap and a hypsochromic shift of the phosphorescence compared to phenylpyridyl analogs. Density function theory (DFT) calculations and electrochemical measurements for Ir1 and Ir2 support their genuine blue phosphorescent emission. The combination of ancillary and cyclometalating ligands in Ir1 and Ir2 significantly influences the molecular orbitals of both complexes, leading to clearly distinct electron density distributions of the HOMO and LUMO compared with other blue-emitting Ir(III) derivatives. Both complexes Ir1 and Ir2 show deep-blue emission with λmax values in the region of 435–465 nm with high PLQYs and short excited-state lifetimes. The phosphorescent organic light emitting diodes (PhOLEDs) based on Ir1 and Ir2 achieve remarkably high EL performance with low efficiency roll-off at high luminance. The bluest color (CIEx,y 0.14, 0.11) and the highest EL efficiency were achieved in the device based on Ir2 (Device 2), where the peak EQE/PE of 13.0%/11.2 lm W−1 together with the corresponding values of 12.6%/8.8 lm W−1 and 10.1%/5.0 lm W−1 at the practical luminances of 100 and 1000 cd m−2 respectively, strongly compete with those of any deep-blue fluorescent and/or phosphorescent OLEDs with similar CIE coordinates previously reported.