Zuqiang Bian

Functional IrIII Complexes and Their Applications

Iridium complexes are drawing great interest because they exhibit high phosphorescence quantum efficiency. Extensive efforts have been devoted to the molecular design of ligands to achieve phosphorescent emission over a wide range of wavelengths that is compatible with many applications. In this research news article, we focus on materials design to improve the performance of phosphorescent Ir(III) complexes for organic light-emitting diodes (OLEDs), luminescence sensitizers, and biological imaging.

Direct Observation of Long Electron-Hole Diffusion Distance in CH3NH3PbI3 Perovskite Thin Film

In high performance perovskite based solar cells, CH3NH3PbI3 is the key material. We carried out a study on charge diffusion in spin-coated CH3NH3PbI3 perovskite thin film by transient fluorescent spectroscopy. A thickness-dependent fluorescent lifetime was found. By coating the film with an electron or hole transfer layer, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) respectively, we observed the charge transfer directly through the fluorescence quenching. One-dimensional diffusion model was applied to obtain long charge diffusion distances in thick films, which is ~1.7u2009μm for electrons and up to ~6.3u2009μm for holes. Short diffusion distance of few hundreds of nanosecond was also observed in thin films. This thickness dependent charge diffusion explained the formerly reported short charge diffusion distance (~100u2009nm) in films and resolved its confliction to thick working layer (300–500u2009nm) in real devices. This study presents direct support to the high performance perovskite solar cells and will benefit the devices’ design.

Promoting near-infrared emission of neodymium complexes by tuning the singlet and triplet energy levels of β-diketonates

A series of neodymium complexes with various modified β-diketonates and their corresponding gadolinium complexes have been synthesized and characterized. The singlet and triplet energy levels of the coordinated ligands were measured and compared. These complexes emit characteristic Nd3+ light at about 880, 1060, 1330 nm with long-lived lifetimes. The relationship between the structure and photophysical properties has been discussed and the energy transfer process in these complexes has been studied.

Multisignaling detection of cyanide anions based on an iridium(III) complex: remarkable enhancement of sensitivity by coordination effect

A new molecule, (Z)-4-(4-(dimethylamino)benzylidene)-3-methyl-1-(pyridin-2-yl)-1H-pyrazol-5(4H)-one (dmpp), and its novel heteroleptic iridium(III) complex, [(ppy)2Ir(dmpp)]PF6, were synthesized and characterized. Both dmpp and [(ppy)2Ir(dmpp)]PF6 could be used as highly selective and sensitive chemodosimeters for the cyanide anion by the naked eye, owing to the addition of the CN− to the vinyl group of dmpp. The addition of CN− to a solution of dmpp induced a change in the solution color from yellow to colorless, and to [(ppy)2Ir(dmpp)]PF6 induced a change in the solution color from pink to colorless. Moreover, [(ppy)2Ir(dmpp)]PF6 could also act as a phosphorescent and electrochemical probe of CN−. The green phosphorescence band at 520 nm and a new irreversible oxidation wave at 0.423 V showed up upon the addition of CN−, indicating that [(ppy)2Ir(dmpp)]PF6 was an excellent multisignaling chemodosimeter of CN−. Importantly, the Ir complex-based cyanide chemodosimeter [(ppy)2Ir(dmpp)]PF6 has much easier and faster detection of CN− than pure organic molecule dmpp.

Energy transfer pathways in the carbazole functionalized β-diketonate europium complexes

Two novel Eu3+ complexes Eu(CDBM)3·2H2O and Eu(CCDBM)3·2H2O have been synthesized (CDBM = 1-(4-(9-carbazol)phenyl)-3-phenyl-1,3-propanedione, CCDBM = 1-((4-(9-carbazol)methyl)phenyl)-3-phenyl-1,3-propanedione). Eu(CDBM)3·2H2O showed strong internal ligand charge transfer (ILCT) fluorescence and sensitized emission of Eu3+. Due to the charge transfer character, the fluorescence band of CDBM shifted from 403 nm in cyclohexane to 559 nm in acetonitrile. Photophysical studies demonstrated that no energy was migrated from the ILCT excited state of the ligands to Eu3+, and that Eu3+ was sensitized by the triplet state which was localized in the 1,3-diphenyl-1,3-propanedione (DBM) part. The quantum efficiencies of Eu3+ in the three complexes are in the order Eu(DBM)3·2H2O > Eu(CCDBM)3·2H2O > Eu(CDBM)3·2H2O in both solution and the solid state. The energy transfer pathways in the three Eu3+ complexes were discussed in detail. Based on the systematic photophysical studies, a new guideline for the organo-lanthanide light emitting materials has been proposed: ILCT should be avoided during molecular modification.

The influence of triplet energy levels of bridging ligands on energy transfer processes in Ir(III)/Eu(III) dyads†

A series of N^N,O^O-bridging ligands based on substituted 1-(pyridin-2-yl)-3-methyl-5-pyrazolone and their corresponding heteroleptic iridium(III) complexes as well as Ir-Eu bimetallic complexes were synthesized and fully characterized. The influence of the triplet energy levels of the bridging ligands on the energy transfer (ET) process from the Ir(III) complexes to Eu(III) ions in solution was investigated at 77 K in Ir(III)/Eu(III) dyads. Photophysical experiment results show the bridging ligands play an important role in the ET process. Only when the triplet energy level of the bridging ligand was lower than the triplet metal-to-ligand charge transfer ((3)MLCT) energy level of the Ir moiety, was pure emission from the Eu(III) ion observed, implying complete ET took place from the Ir moiety to the Eu(III) ion.

Synthesis and electroluminescent property of novel europium complexes with oxadiazole substituted 1,10-phenanthroline and 2,2′-bipyridine ligands

Several 1,10-phenanthroline derivatives (PhoR), 2,2′-bipyridine derivatives (BpoR) and related europium complexes Eu(TTA)3PhoR and Eu(TTA)3BpoR were synthesized (HTTA = 2-thenoyltrifluoroacetone). Single crystal X-ray diffraction of Eu(TTA)3Php (Php = 2-(pyridyl)-1,10-phenanthroline) shows that Php acts as a tridentate N⁁N⁁N ligand, leading to a high stability of the complex for vacuum evaporation. When an oxadiazole moiety is incorporated into the ligand, the corresponding europium complexes show improved carrier-transporting abilities as well as thermal stabilities under vapor deposition for electroluminescence (EL) applications. Experiments revealed that these complexes have high photoluminescence (PL) quantum yields due to suitable triplet energy levels (ET) of the ligands, between 19u2006724 and 22u2006472 cm−1, for the sensitization of Eu(III) (5D0: 17u2006500 cm−1). Utilizing Eu(TTA)3PhoB (PhoB = 2-(5-phenyl-1,3,4-oxadiazol-2-yl)-1,10-phenanthroline) as the dopant emitter in CBP (4,4′-N,N′-dicarbazolebiphenyl), EL devices with a structure of TPD (4,4′-bis[N-(p-tolyl)-N-phenylamino] biphenyl, 30 nm)/Eu(TTA)3PhoB:CBP (7.5%, 20nm)/BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, 20 nm)/Alq3 (tris(8-hydroxyquinoline), 30 nm) exhibited a pure emission from europium ions. The highest efficiency obtained was 5.5 lm W−1, 8.7 cd A−1 and the maximum brightness achieved was 1086 cd m−2. At a practical brightness of 100 cd m−2, the efficiency remains above 2.0 cd A−1.

Progresses in electroluminescence based on europium(III) complexes

Abstract The research on electroluminescence based on europium(III) complexes has come to an important phase. This article reviewed the progresses in photoluminescence and electroluminescence of Eu(III) complexes in recent years from the views of the design of Eu(III) complexes and optimization of device structures, and discussed some important factors influencing electroluminescence performance. The problems existing in the practical application such as the volatility and thermal stability of Eu(III) complexes in this area were discussed, and the possible corresponding solutions were briefly prospected.

Photophysical Properties of Heteroleptic Iridium Complexes Containing Carbazole-Functionalized β-Diketonates

Twelve iridium complexes with general formula of Ir(C;N)(2)(LX) [C;N represents the cyclometalated ligand, i.e. 2-(2,4-difluorophenyl) pyridine (dfppy), 2-phenylpyridine (ppy), dibenzo{f, h}quinoxaline (DBQ); LX stands for beta-diketonate, i.e. acetyl acetonate (acac), 1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate (CBDK), 1-(carbazol-9-yl)-5,5,6,6,7,7,7-heptafluoroheptane-2,4-diketonate (CHFDK), 1-(N-ethyl-carbazol-3-yl)-4,4,5,5,6,6,6-heptafluorohexane-1,3-diketonate (ECHFDK)] are synthesized, characterized and their photophysical properties are systemically studied. In addition, crystals of Ir(DBQ)(2)(CHFDK) and Ir(DBQ)(2)(acac) are obtained and characterized by single crystal X-ray diffraction. The choice of these iridium complexes provides an opportunity for tracing the effect of the triplet energy level of ancillary ligands on the photophysical and electrochemical behaviors. Data show that if the triplet energy level of the beta-diketonate is higher than that of the Ir(C;N)(2) fragment and there is no superposition on the state density map, strong (3)LC or (3)MLCT-based phosphorescence can be obtained. Alternatively, if the state density map of the two parts are in superposition, the (3)LC or (3)MLCT-based transition will be quenched at room temperature. Density functional theory calculations show that these complexes can be divided into two categories. The lowest excited state is mainly determined by C;N but not beta-diketonate when the difference between the triplet energy levels of the two parts is large. However, when this difference is very small, the lowest excited state will be determined by both sides. This provides a satisfactory explanation for the experimental observations.

Highly efficient terbium(III)-based organic light-emitting diodes obtained by exciton confinement

We present highly efficient Tb(III)-based organic light-emitting diodes optimized by the subtle choice of bipolar hosts, adjacent layers and double emitting structures. By introducing di(9H-carbazol-9-yl)(phenyl)phosphine oxide (DCPPO) as the host for the first emitting layer, and 9-(4-tert-butylphenyl)-3,6-bis(diphenylphosphine oxide)-carbazole (DPPOC) for the second emitting layer for Tb(PMIP)3 (PMIP stands for 1-phenyl-3-methyl-4-isobutyryl-pyrazol-5-one), the excitons can be well confined within the double-emitting layer. When 4,4′,4′′-tris(N-carbazolyl)triphenylamine (TCTA) and tris-[3-(3-pyridyl)mesityl]borane (3TPYMB) with high triplet energy levels are used as a hole transporting layer (HTL) and an electron transporting layer (ETL), respectively, the optimized device reaches a maximum efficiency of 52 lm W−1, 57 cd A−1, i.e. a maximum external quantum efficiency (EQE) of 15%. At a practical brightness of 100 cd m−2 (4.6 V) the efficiency remains at around 20 lm W−1, 30 cd A−1.