Hai-Zhu Sun

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.

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.

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.

Efficient non-doped phosphorescent orange, blue and white organic light-emitting devices

Efficient phosphorescent orange, blue and white organic light-emitting devices (OLEDs) with non-doped emissive layers were successfully fabricated. Conventional blue phosphorescent emitters bis [4,6-di-fluorophenyl]-pyridinato-N,C2′] picolinate (Firpic) and Bis(2,4-difluorophenylpyridinato) (Fir6) were adopted to fabricate non-doped blue OLEDs, which exhibited maximum current efficiency of 7.6 and 4.6 cd/A for Firpic and Fir6 based devices, respectively. Non-doped orange OLED was fabricated utilizing the newly reported phosphorescent material iridium (III) (pbi)2Ir(biq), of which manifested maximum current and power efficiency of 8.2 cd/A and 7.8 lm/W. The non-doped white OLEDs were achieved by simply combining Firpic or Fir6 with a 2-nm (pbi)2Ir(biq). The maximum current and power efficiency of the Firpic and (pbi)2Ir(biq) based white OLED were 14.8 cd/A and 17.9 lm/W.

Effect of alkyl chain length on piezochromic luminescence of iridium(III)-based phosphors adopting 2-phenyl-1H-benzoimidazole type ligands

In this work, a series of luminescent cationic iridium(III) complexes containing 2-phenyl-1H-benzimidazole-type ligands modified with n-alkyl chains of various lengths have been successfully synthesized and characterized. Their photophysical and electrochemical properties have been investigated in detail. Differences in n-alkyl chain length has negligible affect on their respective complexs emission spectra or on their excited-state characteristics in solution, which is supported by density functional theory calculations and cyclic voltammetry. In the solid state, these complexes exhibit piezochromic luminescence (PCL) behaviour which is visible to the naked eye. Their emission colour can be reversibly and quickly switched by grinding–fuming or grinding–heating processes with high contrast. Moreover, the n-alkyl chain lengths can effectively control their PCL and thermodynamic properties, showing chain length dependent emission behaviours: longer alkyl chains were shown to produce more marked mechanochromism. A reproducible and reversible two-colour emission writing/erasing process was achieved by employing the iridium(III) materials as a medium. Powder X-ray diffractometry and differential scanning calorimetric studies suggest that the reversible transformation between crystalline and amorphous states upon application of external stimuli is responsible for the observed piezochromism.

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.

Simultaneous modification of N-alkyl chains on cyclometalated and ancillary ligands of cationic iridium(III) complexes towards efficient piezochromic luminescence properties

In this study, we have designed and synthesized a series of multifunctional cationic iridium(III) complexes with different lengths of N-alkyl chains on 2-phenyl-1H-benzimidazole-based cyclometalated ligands and phenyl-pyridine type ancillary ligands. The photophysical properties of each complex have been investigated in detail and also ascertained by comprehensive density functional theory calculations. Despite showing negligible influence on their emission spectra and excited-state characteristics in solution, altering the N-alkyl chain length can efficiently modify their photophysical properties in the solid state. All complexes exhibit fascinating visible piezochromic luminescence (PCL) behaviour. Most interestingly, these iridium(III)-based luminophores with longer N-alkyl chains display significant emission colour changes and unique reversible features by mechanical grinding and solvent fuming. The powder X-ray diffraction (PXRD), 1H NMR and MALDI-TOF/TOF mass spectrometry data demonstrate that the phase transitions between crystalline and amorphous states are crucial to the present piezochromism. Moreover, with the merit of high quantum efficiency in aggregate states, the studied iridium(III) complex can serve as an efficient sensor for the sensitive and selective detection of the explosive, 2,4,6-trinitrophenol (TNP).

Hierarchical manipulation of uniform multi-nanoparticles by electrochemical coupling assembly

Assembling multiple nanomaterials into a single nanostructure is a promising way to obtain a multifunctionality derived from each building block. We address here the need for a general all-solution processed strategy to control the fabrication of multiple nanoparticles (NPs) at room temperature and under vacuum free conditions. The monodisperse multiple NPs were integrated successively into thin bulk-hybrid gradient or periodic tandem multilayer films through tuning the cycling number of cyclic voltammetry (CV), which are based on the quantitatively electrochemical deposition of each type of NPs thanks to the electrochemical coupling reaction of the N-alkylcarbazole ligand. This simple method yields nanoporous, transparent, stable and photoactive films with a hierarchical structure of multiple uniform NPs, exemplified by the prototype photodetector devices. Significantly, this strategy opens an avenue to fabricate low-cost wire and 3-dimensional NPs films on physically flexible conducting substrates.