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2085wileyonlinelibrary.com found to be effective in light harvesting and sensitizing the lanthanide emission through the "antenna effect." [1][2][3] Bright luminescence can be detected even if the LLCs are diluted into extremely low concentration. Therefore, LLCs are very promising for a wide range of applications, such as bioimaging, [ 4 ] immunoassay, [ 5 ] sensors, [ 6 ] light conversion fi lms for photovoltaic devices, [ 7,8 ] light emitting diode (LED) phosphors, [ 9 ] and organic LEDs (OLEDs). [ 2,10 ] Large quantities of LLCs have been synthesized and tremendous progress has been made to improve their luminescent properties. From the standpoint of materials engineering, convenient synthesis, high emission quantum yield (>0.5), and excellent photostability (no degradation >20 h under UV/blue excitation) are quite essential for successful application of such materials. [ 11 ] Up to now, photoinstability has become the major obstacle that limits the applicability of LLCs. In most conditions, lanthanide complexes (e.g., Eu 3+ and Tb 3+ chelates) harvest excitation energy from UV/blue light. However, organic ligands in the compounds are likely to be destroyed by the high energy photons. This photodegradation process has been observed in various kinds of, most currently known, lanthanide complexes. [12][13][14][15][16][17][18][19] Taking the most commonly used β-diketone derivatives as an example, although ligands like 2-thenoyltrifl uoroacetonate (TTA) and 1,3-diphenylpropane-1,3-dionate (DBM) are widely utilized for complexation of Eu 3+ ion and constructing luminescent material with high photoluminescence quantum yield (PLQY), [ 20,21 ] LLCs based on such kind of ligands suffer from serious photodegradation upon UV exposure, showing continuous decrease of the emission intensity. [ 22,23 ] To overcome this shortcoming, lanthanide complexes have been incorporated into polymers, [ 24,25 ] ionic liquids, [ 12 ] organic-inorganic hybrid systems, [ 13,19,[26][27][28][29][30][31] and so on, where the matrix plays the role of a protective cage for the complexes, strongly decreasing the UV degradation rate. However, such techniques always demand complicated synthesis process and may cause decreased PLQYs, [ 32 ] and these roles are still uncertain to remain effective for long enough time toward practical Antiphotobleaching is a critical challenge in the fi eld of luminescent lanthanide complexes (LLCs) as well as in many disciplines concerning organic luminescent processes. In this work, a type of structurally rigid organic ligand, 4-hydroxy-1,5-naphthyridine (ND), is developed, which can not only effi ciently sensitize the europium emission but also demonstrate unique photostability. A series of ND derivatives with different substituent groups are synthesized and their singlet and triplet excited state energy levels are systematically investigated. Photophysical characterizations of the corresponding europium complexes reveal that the sensitization effi ciencies ( η sens ) are close to 100% and the total photolumines...

Organic light-emitting diodes (OLEDs) have had commercial success in displays and lighting. Compared to red and green OLEDs, blue OLEDs are still the bottleneck because the high-energy and long-lived triplet exciton in traditional blue OLEDs causes the short operational lifetime of the device. As a new type emitter, lanthanide complexes with a 5d–4f transition could have short excited-state lifetimes on the order of nanoseconds. To achieve a high-efficiency 5d–4f transition, we systematically tuned the steric and electronic effects of tripodal tris­(pyrazolyl)­borate ligands and drew a full picture of their Ce­(III) complexes. Intriguingly, all of these complexes show bright blue emission with high photoluminescence quantum yields exceeding 95% and short decay lifetimes of 35–73 ns both in the solid powder and in dichloromethane solutions. Using the Ce­(III) complex emitter, we show a blue OLED with a maximum external quantum efficiency of 14.1% and a maximum luminance of 33,160 cd m–2, and the specific electroluminescence mechanism of direct exciton formation on the Ce­(III) ion with a near-unity exciton utilization efficiency is also confirmed. The discovered photoluminescence and electroluminescence property–structure relationships may shed new light on the rational design of highly efficient lanthanide-based blue emitters and their optoelectronic devices such as OLEDs.

A novel type of NIR-emitting lanthanide complexes Ln(PND)3 (Ln = Nd, Er and Yb) was designed and synthesized based on a tridentate monoanionic N,N,O-ligand 6-(pyridin-2-yl)-1,5-naphthyridin-4-ol (PND). Such complex owns definite charge-neutral, coordination-saturated and mononuclear structure that is proved by X-ray single crystal diffraction of Nd(PND)3. Photophysical studies on the ligand and complexes reveal that the PND ligand has suitable energy level to sensitize the near-infrared (NIR) emitting lanthanide ions (Nd(3+), Er(3+), and Yb(3+)). Among the three compounds, Yb(PND)3 shows the highest photoluminescence quantum yield up to 0.9% in a mixture of acetonitrile and methanol solution (10 : 1, v/v, 10(-4) M). Thermal measurements indicate that these compounds have high decomposition (Td) and glass transition (Tg) temperature up to 420 and 265 °C, respectively, implying great advantage for constructing organic electronic devices via vacuum deposition method. At last, NIR organic light-emitting diodes (OLEDs) with simple three-layer structure were fabricated to test their electroluminescent performance, showing maximum NIR irradiance and maximum external quantum efficiency (EQE) of 25 μW cm(-2) and 0.019% for Nd(3+), 0.46 μW cm(-2) and 0.004% for Er(3+), and 86 μW cm(-2) and 0.14% for Yb(3+), respectively.

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