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...
