Upconversion Luminescence in a Photostable Ion-Paired Yb-Eu Heteronuclear Complex.

Lanthanide-based upconversion molecular complexes have potential application in diverse fields and attracted considerable research interest in recent years. However, the similar coordination reactivity of lanthanide ions has constrained the designability of target molecule with well-defined structure, and many attempts obtained statistical mixtures. Herein, an ion-paired Yb-Eu heteronuclear complex [Eu(TpPy)2][Yb(ND)4] (TpPy = tris[3-(2-pyridyl)pyrazolyl]hydroborate, ND = 3-cyano-2-methyl-1,5-naphthyridin-4-olate) was designed and synthesized. Thanks to the radius difference between Eu3+ (1.07 Å) and Yb3+ (0.98 Å) ions, the hexadentate TpPy ligand was selected to coordinate with Eu3+ and the Yb3+ with a smaller radius was chelated by bidentate ND ligand. As a result, the sites of Eu3+ and Yb3+ in the complex can be clarified by high-resolution mass spectrometry and single-crystal structure analysis. Upon the excitation of Yb3+ at 980 nm, the upconversion emission of Eu3+ was realized through a cooperative sensitization process. Furthermore, [Eu(TpPy)2][Yb(ND)4] demonstrated excellent photostability during continuous high-power density 980 nm laser irradiation, with a LT95 (the time to 95% of the initial emission intensity) of 420 minutes. This work provides the first example of a pure ion-paired Yb-Eu heteronuclear complex upconversion system and may bring insights into rational design of lanthanide-based upconversion molecular complexes.

Warm-White-Light Perdeuterated Dy(III) Complex with a Photoluminescence Quantum Yield of up to 72% in Deuterated Chloroform.

Herein, a deuteration strategy is proposed to enhance the photoluminescence quantum yield (PLQY) of a Dy(III) complex. The perdeuterated Dy(III) complex Dy(D-DPPOP)3 (D-DPPOP = 6-[bis(phenyl-d5)phosphoryl]picolinate-d3) exhibits a high PLQY of up to 72% in deuterated chloroform, which is 4.8 times higher than that of the nondeuterated Dy(III) complex Dy(DPPOP)3. Then the corresponding ultraviolet-excited light-emitting diode is fabricated, showing a warm-white light with a Commission Internationale de l'Eclairage (CIE) of (0.36, 0.41) and a color temperature of around 4800 K. The deuteration strategy to improve the PLQY of the Dy(III) complex is proved in this work, and it will inspire the further design of white-emission Dy(III) complexes with high efficiency.

Delayed Doublet Emission in a Cerium(III) Complex.

Doublet emission from open-shell molecules has demonstrated its research and application value in recent years. However, understandings of the photoluminescence mechanism of open-shell molecules are far less than that of closed-shell molecules, leading to challenges in molecular design of efficient doublet emission systems. Here we report a cerium(III) 4-(9H-carbozol-9-yl)phenyl-tris(pyrazolyl)borate complex Ce(CzPhTp)3 with a new luminescence mechanism of delayed doublet emission, which also represents the first example with metal-centered delayed photoluminescence. The energy gap between the doublet and triplet excited states of Ce(CzPhTp)3 is reduced by the management of the inner and outer coordination spheres, thereby promoting efficient energy transfer between the two excited states and activating the delayed emission. The photoluminescence mechanism discovered may provide a new way for the design of efficient doublet emission and bring insights into rational molecular design and energy level regulation in open-shell molecules.

Complexes of Ce(III) and Bis(pyrazolyl)borate Ligands: Synthesis, Structures, and Luminescence Properties.

Luminescent cerium(III) complexes based on the d-f transition have characteristics of broad emission spectra, tunable emission colors, and short excited state lifetimes, showing potential applications in display, lighting, and other fields. Thus it is important to construct luminescent Ce(III) complexes with high photoluminescence efficiency and good stability. In this work, five Ce(III) complexes with dihydrobis(pyrazolyl)borate or diphenylbis(pyrazolyl)borate ligands, where pyrazolyl stands for pyrazolyl, 3-methylpyrazolyl, or 3,5-dimethylpyrazolyl, were designed and synthesized, showing emission colors from deep blue to yellow with a maximum wavelength in the range of 390-560 nm, short excited state lifetimes of 30-80 ns, and photoluminescence quantum yields exceeding 75% in solid powder. By comparing these complexes, it is found that higher photoluminescence efficiency and better thermal/air stability could be achieved in the complexes with dihydrobis(pyrazolyl)borate ligands.

Rare Earth Complexes of Europium(II) and Substituted Bis(pyrazolyl)borates with High Photoluminescence Efficiency.

Rare earth europium(II) complexes based on d-f transition luminescence have characteristics of broad emission spectra, tunable emission colors and short excited state lifetimes, showing great potential in display, lighting and other fields. In this work, four complexes of Eu(II) and bis(pyrazolyl)borate ligands, where pyrazolyl stands for pyrazolyl, 3-methylpyrazolyl, 3,5-dimethylpyrazolyl or 3-trifluoromethylpyrazole, were designed and synthesized. Due to the varied steric hindrance of the ligands, different numbers of solvent molecules (tetrahydrofuran) are participated to saturate the coordination structure. These complexes showed blue-green to yellow emissions with maximum wavelength in the range of 490-560 nm, and short excited state lifetimes of 30-540 ns. Among them, the highest photoluminescence quantum yield can reach 100%. In addition, when the complexes were heated under vacuum or nitrogen atmosphere, they finally transformed into the complexes of Eu(II) and corresponding tri(pyrazolyl)borate ligands and sublimated away.

Highly Efficient Heteroleptic Cerium(III) Complexes with a Substituted Pyrazole Ancillary Ligand and Their Application in Blue Organic Light-Emitting Diodes.

Compared with red and green organic light-emitting diodes (OLEDs), blue is the bottleneck that restricts the wide development of OLEDs from being the next-generation technology for displays and lighting. As a new type of emitter, a Ce(III) complex shows many satisfactory advantages, such as a short excited-state lifetime, 100% theoretical exciton utilization efficiency, and tunable emission color. Herein we synthesized three heteroleptic Ce(III) complexes Ce(TpMe2)2(dtfpz), Ce(TpMe2)2(dmpz), and Ce(TpMe2)2(dppz) with the hydrotris(3,5-dimethylpyrazolyl)borate (TpMe2) main ligand and different substituted pyrazole ancillary ligands, namely, 3,5-di(trifluomethyl)pyrazolyl (dtfpz), 3,5-dimethylpyrazolyl (dmpz), and 3,5-diphenylpyrazolyl (dppz), and studied their structures and luminescence properties. All the Ce(III) complexes exhibited a near-unity photoluminescence quantum yield both in solution and as a powder with maximum emission wavelengths in the range of 450-486 nm. The OLED employing Ce(TpMe2)2(dppz) as the emitter showed the best performance, including a turn-on voltage, maximum luminance, and external quantum efficiency of 3.2 V, 29 200 cd m-2, and 12.5%, respectively.

A Family of Highly Emissive Lanthanide Complexes Constructed with 6-(Diphenylphosphoryl)picolinate.

We report a novel family of lanthanide complexes Ln(DPPOP)3 (Ln = Pr, Nd, Sm, Eu, Tb, Dy, Er, and Yb) employing anionic tridentate (O∧N∧O) ligand 6-(diphenylphosphoryl)picolinate (DPPOP). Crystal structures of the complexes reveal that each lanthanide ion is nine-coordinated by three tridentate ligands. In the crystals, 1D channels are found, which can absorb and eliminate water reversibly. DPPOP possesses high triplet energy and can sensitize a series of lanthanide ions. An energy transfer mechanism is proposed through the higher excited states of the lanthanide ions. In the solid state, remarkably high quantum yields in the visible range are obtained: 81% for Eu(III), 97% for Tb(III), 13% for Dy(III), and 4% for Sm(III) complex.

Highly Efficient and Air-Stable Lanthanide EuII Complex: New Emitter in Organic Light Emitting Diodes.

Luminescent EuII complexes with a characteristic 5d-4f transition have potential applications in many fields. However, their instability in ambient conditions impedes further exploration and application. Herein, we report two new EuII complexes, bis[hydrotris(3-trifluoromethylpyrazolyl)borate]europium(II) (Eu-1) and bis[hydrotris(3-methylpyrazolyl)borate]europium(II) (Eu-2). Intriguingly, the blue emissive Eu-1 showed high air stability arising from fluorine protection and close molecular packing, as maintaining a photoluminescence quantum yield (PLQY) of 91 % (initial 96 %) upon exposure to air over 2200 hours. While the orange emissive Eu-2 showed a maximum luminance of 30620 cd m-2 , and a maximum external quantum efficiency (EQE) of 6.5 %, corresponding to an exciton utilization efficiency around 100 % in organic light-emitting diodes (OLEDs). These results could inspire further research on stable and efficient EuII complexes and their application in OLEDs.

Two-Coordinate Copper(I)/NHC Complexes: Dual Emission Properties and Ultralong Room-Temperature Phosphorescence.

As a kind of photoluminescent material, CuI complexes have many advantages such as adjustable emission, variable structures, and low cost, attracting attention in many fields. In this work, two novel two-coordinate CuI -N-heterocyclic carbene complexes were synthesized, and they exhibit unique dual emission properties, fluorescence and phosphorescence. The crystal structure, packing mode, and photophysical properties under different conditions were systematically studied, proving the emissive mechanism to be the locally excited state of the carbazole group. Based on this mechanism, ultralong room-temperature phosphorescence (RTP) with a lifetime of 140 ms is achieved by selective deuteration of the carbazole group. These results deepen the understanding of the luminescence mechanism and design strategy for two-coordinate CuI complexes, and prove their potential in applications as ultralong RTP materials.

Quantum Yields over 80% Achieved in Luminescent Europium Complexes by Employing Diphenylphosphoryl Tridentate Ligands.

Four tridentate europium(III) complexes containing a diphenylphosphoryl group are prepared with strong bonding between the ligands and centered ion, convinced by crystal structures. Compared to their parent bidentate complexes, the tridentate complexes display improved and exceptionally high photoluminescence quantum yields (PLQYs) in powder (all over 80%, best 91%), as well as in a CH2Cl2 solution and poly(methyl methacrylate) films, benefiting from compact, stable, and saturated coordination.

A Breakthrough Efficiency of 19.9% Obtained in Inverted Perovskite Solar Cells by Using an Efficient Trap State Passivator Cu(thiourea)I.

It is extremely significant to study the trap state passivation and minimize the trap states of perovskite to achieve high-performance perovskite solar cells (PSCs). Here, we have first revealed and demonstrated that a novel p-type conductor Cu(thiourea)I [Cu(Tu)I] incorporated in perovskite layer can effectively passivate the trap states of perovskite via interacting with the under-coordinated metal cations and halide anions at the perovskite crystal surface. The trap state energy level of perovskite can be shallowed from 0.35-0.45 eV to 0.25-0.35 eV. In addition, the incorporated Cu(Tu)I can participate in constructing the p-i bulk heterojunctions with perovskite, leading to an increase of the depletion width from 126 to 265 nm, which is advantageous for accelerating hole transport and reducing charge carrier recombination. For these two synergistic effects, Cu(Tu)I can play a much better role than that of the traditional p-type conductor CuI, probably due to its identical valence band maximum with that of perovskite, which enables to not only lower the trap state energy level to a greater extent but also eliminate the potential wells for holes at the p-i heterojunctions. After optimization, a breakthrough efficiency of 19.9% has been obtained in the inverted PSCs with Cu(Tu)I as the trap state passivator of perovskite.

Water-Soluble and Highly Luminescent Europium(III) Complexes with Favorable Photostability and Sensitive pH Response Behavior.

Two highly luminescent and water-soluble Eu(III) complexes, Eu1 and Eu2, based on novel carboxyl-functionalized 1,5-naphthyridine derivatives 8-hydroxy-1,5-naphthyridine-2-carboxylic acid (H2L1) and 7-cyano-8-hydroxy-1,5-naphthyridine-2-carboxylic acid (H2L2), respectively, are designed and synthesized. The crystal structure of Eu2 indicates that the central Eu(III) ion is nine-coordinated by three tridentate ligands (O^N^O). Both Eu1 and Eu2 show strong luminescence in aqueous solution with quantum yields (lifetimes) of 28% (1.1 ms) and 14% (0.76 ms), respectively. The chelates display unique UV-light stability in solution and remain highly emissive after 100 min of strong UV irradiation (∼300 W·m-2 at 345 nm). Moreover, they exhibit reversible luminescence intensity changes with varied pH values, and the response mechanism is investigated. "Turn-on" of the Eu(III) emission upon increasing pH is realized by ligand structure change from keto to enol anion form, resulting in red-shifted absorption band and suppressed quenching from solvents and N-H vibration upon deprotonating. The results show that these novel Eu(III) complexes are quite intriguing for potential application as bioimaging agents and pH probes.

CuSCN-Based Inverted Planar Perovskite Solar Cell with an Average PCE of 15.6%.

Although inorganic hole-transport materials usually possess high chemical stability, hole mobility, and low cost, the efficiency of most of inorganic hole conductor-based perovskite solar cells is still much lower than that of the traditional organic hole conductor-based cells. Here, we have successfully fabricated high quality CH3NH3PbI3 films on top of a CuSCN layer by utilizing a one-step fast deposition-crystallization method, which have lower surface roughness and smaller interface contact resistance between the perovskite layer and the selective contacts in comparison with the films prepared by a conventional two-step sequential deposition process. The average efficiency of the CuSCN-based inverted planar CH3NH3PbI3 solar cells has been improved to 15.6% with a highest PCE of 16.6%, which is comparable to that of the traditional organic hole conductor-based cells, and may promote wider application of the inexpensive inorganic materials in perovskite solar cells.

Lanthanide complexes with d-f transition: new emitters for single-emitting-layer white organic light-emitting diodes.

White organic light-emitting diodes (WOLEDs) is a new generation of lighting technology and has stimulated wide-ranging studies. Despite the advantage of simple device structure, single-emitting-layer WOLEDs (SEL-WOLEDs) still face the challenges of difficult material screening and fine energy level regulation. Herein, we report efficient SEL-WOLEDs with a sky-blue emitting cerium(III) complex Ce-TBO2Et and an orange-red emitting europium(II) complex Eu(Tp2Et)2 as the emitters, showing a maximum external quantum efficiency of 15.9% and Commission Internationale de l'Eclairage coordinates of (0.33, 0.39) at various luminances. Most importantly, the electroluminescence mechanism of direct hole capture and hindered energy transfer between the two emitters facilitate a manageable weight doping concentration of 5% for Eu(Tp2Et)2, avoiding the low concentration (<1%) of the low-energy emitter in typical SEL-WOLEDs. Our results indicate that d-f transition emitters may circumvent fine energy level regulation and provide development potential for SEL-WOLEDs.

4f → 3d sensitization: a luminescent EuII-MnII heteronuclear complex with a near-unity quantum yield.

A new heteronuclear EuII-MnII complex [Eu(N2O6)]MnBr4 (N2O6 = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) is designed and synthesized, which shows an intense green emission from MnII with a near-unity photoluminescence quantum yield. Measurement of excited-state dynamics demonstrated the sensitization process from EuII to MnII, which represents the first example of f → d molecular sensitization. Due to the large optical absorption cross-section of the EuII center, [Eu(N2O6)]MnBr4 shows an emission intensity 7 to 2500 times stronger than that of the SrII-MnII control complex [Sr(N2O6)]MnBr4 upon the excitation of near ultraviolet to blue light.

Ligand Engineering in Tin-Based Perovskite Solar Cells.

Perovskite solar cells (PSCs) have attracted aggressive attention in the photovoltaic field in light of the rapid increasing power conversion efficiency. However, their large-scale application and commercialization are limited by the toxicity issue of lead (Pb). Among all the lead-free perovskites, tin (Sn)-based perovskites have shown potential due to their low toxicity, ideal bandgap structure, high carrier mobility, and long hot carrier lifetime. Great progress of Sn-based PSCs has been realized in recent years, and the certified efficiency has now reached over 14%. Nevertheless, this record still falls far behind the theoretical calculations. This is likely due to the uncontrolled nucleation states and pronounced Sn (IV) vacancies. With insights into the methodologies resolving both issues, ligand engineering-assisted perovskite film fabrication dictates the state-of-the-art Sn-based PSCs. Herein, we summarize the role of ligand engineering during each state of film fabrication, ranging from the starting precursors to the ending fabricated bulks. The incorporation of ligands to suppress Sn2+ oxidation, passivate bulk defects, optimize crystal orientation, and improve stability is discussed, respectively. Finally, the remained challenges and perspectives toward advancing the performance of Sn-based PSCs are presented. We expect this review can draw a clear roadmap to facilitate Sn-based PSCs via ligand engineering.

Deep-blue emitting cerium(III) complexes with tris(pyrazolyl)borate and triflate ligands.

Red, green and blue emitting materials, the three primary colors, are very important in lighting and display. Red-emitting Eu(III) complexes and green-emitting Tb(III) complexes exhibit high color purity and photoluminescence (PL) efficiency. However, it is difficult to realize efficient blue emission based on f-f transition. Alternatively, Ce3+ with d-f transition can be used to construct blue-emitting lanthanide complexes. Herein, we synthesized two heteroleptic Ce(III) complexes Ce-1Me-OTf and Ce-2Me-OTf based on hydrotris(3-methylpyrazolyl)borate (TpMe) and hydrotris(3,5-dimethylpyrazolyl)borate (TpMe2) ligands, respectively, in which triflate is used as the ancillary ligand. Ce-1Me-OTf and Ce-2Me-OTf exhibit strong blue emission in dichloromethane and as powder with maximum emission wavelengths in the range of 424-436 nm. Both complexes demonstrate near-unity photoluminescence quantum yields (PLQYs) in powder and good sublimation properties. In particular, Ce-1Me-OTf emits deep blue light both in dichloromethane and as powder with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.07) and (0.15, 0.06), respectively, which are close to the standard blue points recommended by the National Television System Committee (NTSC) and the European Broadcast Union (EBU).

Rare Earth Complexes with 5d-4f Transition: New Emitters in Organic Light-Emitting Diodes.

Organic light-emitting diodes (OLEDs) are considered as next-generation displays and lighting technologies. During the past three decades, various luminescent materials such as fluorescence, phosphorescence, and thermally activated delayed fluorescence materials have been subsequently investigated as emitters. To date, blue OLEDs are still the bottleneck as compared to red and green ones because of the lack of efficient emitters with simultaneous high exciton utilization efficiency and long-term stability. Recently, d-f transition rare earth complexes have been reported as new emitters in OLEDs with potential high efficiency and stability. In this Perspective, we present a brief introduction to OLEDs and an overview of the previous electroluminescence study on d-f transition rare earth complexes. This is followed by our recent developments in cerium(III) complex- and europium(II) complex-based OLEDs. We finally discuss the challenges and opportunities for OLED study based on d-f transition rare earth complexes.

A mild liquid reduction route toward uniform blue-emitting EuCl2 nanoprisms and nanorods.

In this work, for the first time, uniform blue-emitting EuCl(2) nanoprisms and nanorods were synthesized from Eu(CCl(3)COO)(3)·2H(2)O [or Eu(CH(3)COO)(3)·H(2)O] by a novel mild liquid reduction route, using acetamidine hydrochloride (or picolinamidine hydrochloride) as the reductant in oleylamine. The synthetic reaction even can take place under an atmosphere in the absence of inert gas at around 300 °C. The EuCl(2) nanoprism dispersion in n-hexane showed an intense blue emission when excited by UV light.

Dual molecular light switches for pH and DNA based on a novel Ru(II) complex. A non-intercalating Ru(II) complex for DNA molecular light switch.

A new Ru(II) complex of [Ru(phen)(2)(Hcdpq)](ClO(4))(2) {phen = 1,10-phenanthroline, Hcdpq = 2-carboxyldipyrido[3,2-f:2',3'-h]quinoxaline} was synthesized and characterized. The spectrophotometric pH and calf thymus DNA (ct-DNA) titrations showed that the complex acted as a dual molecular light switch for pH and ct-DNA with emission enhancement factors of 17 and 26, respectively. It was shown to be capable of distinguishing ct-DNA from yeast RNA with this binding selectivity being superior to two well-known DNA molecular light switches of [Ru(bpy)(2)(dppz)](2+) {bpy =2,2'-bipyridine, and dppz = dipyrido-[3,2-a:2',3'-c]phenazine}and ethidium bromide. The complex bond to ct-DNA probably in groove mode with a binding constant of (4.67 ± 0.06) × 10(3) M(-1) in 5 mM Tris-HCl, 50 mM NaCl (pH = 7.10) buffer solution, as evidenced by UV-visible absorption and luminescence titrations, the dependence of DNA binding constants on NaCl concentrations, DNA competitive binding with ethidium bromide, and emission lifetime and viscosity measurements. To get insight into the light-switch mechanism, theoretical calculations were also performed by applying density functional theory (DFT) and time-dependent DFT.