D-f transition rare earth Eu (II) complex as well as preparation method and application thereof

Abstract

D-f transition rare earth Eu (II) complex with any one of following structures: # imgabs0 #

Description

Technical Field

[0001] The present invention belongs to the fields of organic fluorescent phosphors and organic electroluminescence. In particular, the present invention relates to a multi-dentate chelating d-f transition rare earth Eu(II) complex and its applications as a light conversion fluorescent phosphor and an electroluminescent material. Background Art

[0002] Compared with traditional f-f transition rare earth ions such as Eu(III) and Tb(III), d-f transition rare earth ions represented by Eu(II) have the characteristics of short excited state lifetime, high absorption intensity, and adjustable spectrum. However, Eu(II) ions have strong reducibility and are easily oxidized to Eu(III) ions in air. Therefore, their complexes are difficult to stably exist in air, which greatly limits the research and application of such materials.

[0003] If Eu(II) ions are confined in the lattice of inorganic materials, the erosion of oxygen in the environment can be effectively isolated. At present, a large number of commercial applications of inorganic fluorescent phosphors based on rare earth Eu(II) ions play an important role in the fields of lighting and display. However, the luminous efficiency of such inorganic fluorescent phosphors is dependent on the particle size, and most of the preparation methods require high-temperature processes. The compatibility of the products with organic matrices is low, and the processability is poor, which limits the application prospects of Eu(II) rare earth luminescent materials. In contrast, the luminescence of rare earth complexes comes from the rare earth ions themselves, without size-dependent effects. The synthesis process is relatively mild, and the compatibility with the organic system is good, which has broad application prospects in the fields of photoluminescence and electroluminescence.

[0004] On the other hand, the preparation of organic light emitting diodes (OLEDs) by spin coating has the advantages of simple method, low cost, and low requirement for the thermal stability of the complex. However, most of the currently commercially applied OLEDs are prepared by vacuum evaporation, and there is no report in the literature on spin-coated devices of d-f transition complexes. Therefore, synthesizing Eu(II) complexes with good solubility and then preparing OLEDs by spin coating is of great significance for the development of d-f transition complexes and OLEDs. Summary of the Invention

[0005] In the process of striving to solve problems such as the easy oxidation of Eu(II) and the preparation of organic light-emitting diodes by spin coating, the inventors of the present invention found that using a class of triaminoethylamine ligands to protect Eu(II) ions can largely isolate the erosion of Eu(II) ions by oxygen in the air, and then obtain Eu(II) complexes that can stably exist in the air. On this basis, by changing the substituents on the ligand, the regulation of luminescence properties can be achieved, and the solubility of the complex can be improved, thereby realizing the preparation of organic light-emitting diodes by spin coating.

[0006] Examples of the present invention provide a d-f transition rare earth Eu(II) complex having any one of the following structures:

[0007]

[0008] , where R1 and R2 independently selected from hydrogen, unsubstituted alkyl, halogen-substituted alkyl, alkoxy, unsubstituted alkenyl, halogen-substituted alkenyl, unsubstituted alkynyl, halogen-substituted alkynyl, unsubstituted aryl, substituted aryl, unsubstituted phenyl, substituted phenyl, alkyl or heterocycle containing O, N, S ligand sites;

[0009] R3, R3’, R4, R4’ independently selected from hydrogen, unsubstituted alkyl, halogen-substituted alkyl, alkoxy, unsubstituted alkenyl, halogen-substituted alkenyl, unsubstituted alkynyl, halogen-substituted alkynyl, unsubstituted aryl, substituted aryl, unsubstituted phenyl, substituted phenyl;

[0010] X is a monovalent negative ion; preferably, X is selected from at least one of trifluoromethanesulfonate, halogen, pseudohalogen, tetrafluoroborate, hexafluorophosphate;

[0011] Preferably, R1 and R2 independently selected from hydrogen, unsubstituted C1-C18 alkyl, halogen-substituted C1-C18 alkyl, C1-C18 alkoxy, unsubstituted C2-C18 alkenyl, halogen-substituted C2-C18 alkenyl, unsubstituted C2-C18 alkynyl, halogen-substituted C2-C18 alkynyl, unsubstituted C6-C30 aryl, substituted C6-C30 aryl, unsubstituted phenyl, substituted phenyl, alkyl or heterocycle containing O, N, S ligand sites;

[0012] Preferably, R3, R3’, R4, R4’ are independently selected from any one of hydrogen, unsubstituted C1-C18 alkyl, halogen-substituted C1-C18 alkyl, C1-C18 alkoxy, unsubstituted C2-C18 alkenyl, halogen-substituted C2-C18 alkenyl, unsubstituted C2-C18 alkynyl, halogen-substituted C2-C18 alkynyl, unsubstituted C6-C30 aryl, substituted C6-C30 aryl, unsubstituted phenyl, and substituted phenyl;

[0013] Preferably, R1 and R2 are independently selected from hydrogen, unsubstituted C1-C18 alkyl, halogen-substituted C1-C18 alkyl, unsubstituted C6-C30 aryl, substituted C6-C30 aryl, C1-C18 alkyl or heterocycle containing O, N, S coordination sites, and R3, R4 are independently selected from hydrogen and unsubstituted C1-C18 alkyl;

[0014] More preferably, R1 and R2 are independently selected from any one of hydrogen, methyl, methylene furan, methylene thiophene, benzyl, methylene carbazole, and methylene pyrazole, and R3, R4 are independently selected from hydrogen and methyl;

[0015] More preferably, R1 is hydrogen, R2 is selected from any one of methyl, methylene furan, methylene thiophene, and benzyl, and R3, R4 are hydrogen.

[0016] According to an embodiment of the present invention, for example, the structural formula of the d-f transition rare earth Eu(II) complex is:

[0017] Wherein, R1, R2, R3, R3’, R4, R4’ are all hydrogen, and X is I, Br or Cl.

[0018] According to an embodiment of the present invention, for example, the structural formula of the d-f transition rare earth Eu(II) complex is:

[0019] Wherein, X is I, and R1, R3, R3’, R4, R4’ are all hydrogen;

[0020] Or, X is I, R1, R3’, R4, R4’ are all hydrogen, and R3 is CH3;

[0021] Or, X is I, R3, R3’, R4, R4’ are all hydrogen, and R1 is CH3.

[0022] According to an embodiment of the present invention, for example, the structural formula of the d-f transition rare earth Eu(II) complex is:

[0023] Wherein, X is I, R1, R2 are CH3, and R3, R3’, R4, R4’ are hydrogen;

[0024] Alternatively, X is I, R1 is H, R2 is phenyl, and R3, R3', R4, R4' are hydrogen;

[0025] Alternatively, X is I, R1 is H, R2 is thienyl, and R3, R3', R4, R4' are hydrogen.

[0026] According to an embodiment of the present invention, for example, the structural formula of the d-f transition rare earth Eu(II) complex is:

[0027] Wherein, X is I, R1 is H, R2 is furyl, and R3, R3', R4, R4' are hydrogen.

[0028] An embodiment of the present invention further provides an electroluminescent device, which includes a cathode, an anode of the electroluminescent device, and a light-emitting layer located between the cathode and the anode, and the light-emitting layer includes the d-f transition rare earth Eu(II) complex as described above;

[0029] Preferably, the light-emitting layer includes a host material and a doping material, and the doping material includes the d-f transition rare earth Eu(II) complex as described above;

[0030] Preferably, the triplet energy level E of the host material T > 2.7 eV;

[0031] Preferably, the doping concentration of the d-f transition rare earth Eu(II) complex in the host material is 5-15 wt%, preferably 8-13 wt%, preferably 10-12 wt%, preferably 11 wt%;

[0032] Preferably, the electroluminescent device further includes a hole transport layer located between the anode and the light-emitting layer;

[0033] Preferably, the electroluminescent device further includes an electron transport layer located between the cathode and the light-emitting layer;

[0034] Preferably, the structure of the electroluminescent device is: ITO / PEDOT:PSS(40 nm) / PVK(Approx. Mw90000)(50 nm) / PCZAC:Eu5(11 wt%, 60 nm) / TmPyPB(40 nm) / LiF(0.7 nm) / Al.

[0035] According to an embodiment of the present invention, for example, both the light-emitting layer and the hole transport layer are prepared by spin coating.

[0036] An embodiment of the present invention also provides a light-emitting film, which includes a polymer material and the d-f transition rare earth Eu(II) complex as described above;

[0037] Preferably, the mass ratio of the d-f transition rare earth Eu(II) complex to the polymer material is 0.1-10:100, preferably 0.5-2:100, and more preferably 0.8-1.2:100;

[0038] Preferably, the polymer material is PMMA resin, and the mass ratio of the d-f transition rare earth Eu(II) complex to the PMMA resin is 1:100.

[0039] An embodiment of the present invention also provides an LED device, which includes an LED chip and a light-emitting layer attached to the LED chip, and the light-emitting layer includes a polymer material and the d-f transition rare earth Eu(II) complex as described above;

[0040] Preferably, the polymer material is a thermosetting resin;

[0041] Preferably, the mass ratio of the d-f transition rare earth Eu(II) complex to the polymer material is 0.1-10:10, preferably 0.5-2:10, and more preferably 0.8-1.2:10;

[0042] Preferably, the polymer material is a thermosetting silicone encapsulant; the mass ratio of the d-f transition rare earth Eu(II) complex to the polymer material is 1:10. Description of the Drawings

[0043] Figure 1 It is the crystal structures of complexes Eu1-3, Eu5-8, and Eu10 provided by the embodiments of the present invention, wherein Eu1-3, Eu5-8, and Eu10 are schematic diagrams of crystal structures in which atoms appear as spheres at the 50% probability level.

[0044] Figure 2 It is the curve of the quantum yield of Eu2 solid powder prepared in the embodiments of the present invention varying with the ultraviolet aging time.

[0045] Figure 3 It is the room temperature emission spectrum of the complex prepared in the embodiments of the present invention.

[0046] Figure 4 It is the photoluminescence quantum yield of the complex Eu5-doped film prepared in the embodiments of the present invention.

[0047] Figure 5 It is the electroluminescence emission spectrum of the electroluminescent device D1 prepared in the embodiments of the present invention.

[0048] Figure 6 It is the current density-voltage-luminance curve of the electroluminescent device D1 prepared in the embodiment of the present invention.

[0049] Figure 7 It is the power efficiency-luminance-EQE curve of the electroluminescent device D1 prepared in the embodiment of the present invention.

[0050] Figure 8 It is the curve of the luminance change of the LED device manufactured with the complex Eu3 of the embodiment of the present invention with respect to voltage. Detailed implementation manners

[0051] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. However, those skilled in the art know that the present invention is not limited to the drawings and the following embodiments.

[0052] The Eu(II) complexes of the present invention and their applications as electroluminescent materials and photoluminescent materials will be further described below with reference to specific embodiments. It should be understood that the following embodiments are only illustrative of and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0053] In the embodiments of the present invention, starting from triaminoethylamine, three Eu(II) complexes were synthesized, and their structures are shown in Formula 1 below. The obtained Eu(II) complexes have bright yellow light emission, and at the same time have relatively high quantum yield and air stability. Based on this, a series of Eu(II) complexes with luminescence ranging from blue light to yellow light were obtained by introducing large steric hindrance groups, new coordination groups into the triaminoethylamine ligand and changing the framework coordination atoms, realizing the spectral regulation of such complexes.

[0054]

[0055] Formula 1 Schematic formula of the coordination structure of triaminoethylamine and Eu(II).

[0056] Synthesis and characterization:

[0057]

[0058] Formula 2 Synthesis route of complexes Eu1 to Eu10

[0059] The synthetic route of the complexes is shown in Scheme 2 above. The ligands of Eu-1, Eu-2, Eu-3, Eu-4, Eu-8, and Eu-9 were directly purchased; the ligands of Eu-5, Eu-6, and Eu-7 were obtained by reacting triaminoethylamine with aldehyde to get Schiff base intermediates and then reducing them; the ligand of Eu-10 was obtained by deprotonating triethanolamine and reacting it with methyl iodide. The resulting ligands were reacted with europium(II) halide in tetrahydrofuran or dichloromethane to obtain the complexes.

[0060] The crystal structure of the complex is as Figure 1 shown.

[0061] Currently, single crystal structures of Eu1-3, Eu5-8, and Eu10 have been obtained, as Figure 1 shown. Among them, Eu1-3, Eu8, and Eu10 have a structure with a 2:1 coordination of the ligand to Eu(II), Eu5 and Eu7 are binuclear complexes bridged by iodide ions, and the ratio of the ligand to Eu(II) is 1:1. Due to the introduction of steric groups, the coordination of the second ligand is restricted, and finally the coordination number of Eu(II) is 7. Eu-6 is a complex with a direct 1:1 coordination of the ligand to Eu(II). The introduction of coordination groups increases the ligand coordination sites of the ligand, enabling the complex to achieve a high coordination number without the bridging of I ions.

[0062] In view of the fact that Eu(II) complexes are theoretically excellent electroluminescent materials, the inventors of the present invention synthesized ten Eu(II) complexes Eu1-10 (the structures of four compounds are shown in Scheme 2 above), and studied their photoluminescence and electroluminescence properties accordingly. The luminescence of the four complexes all comes from the d-f transition of the central Eu(II), and Eu(II) complexes with different luminescence colors were obtained by changing the ligand structure and thus changing the ligand field around Eu(II). The photoluminescence quantum yields (PLQY) of yellow-green light-emitting Eu2 and Eu3 in the solid powder state are both greater than 70%. Under the irradiation of an ultraviolet lamp with an irradiation intensity of 25 - 30 W / m² at a wavelength of 365 nm for 400 hours, the complex Eu2 can still maintain a quantum yield greater than 10%, proving that such complexes may have application potential in fields such as LED phosphors and agricultural light conversion films. At the same time, the inventors of the present invention also prepared OLEDs devices based on Eu5. Through device structure optimization, it was found that the maximum brightness of the optimal device is 1780 cd m -2 , and the maximum external quantum efficiency is 9.2%, indicating that this type of material is also a very promising OLEDs luminescent material.

[0063] Example 1 Preparation and Characterization of Complexes:

[0064] Synthesis of Eu1: 0.4058 g (1 mmol) of europium(II) iodide was dispersed in 20 mL of methanol in a glove box, and 0.3 mL (2 mmol) of tris(2-aminoethyl)amine was dispersed in 10 mL of methanol. The latter was slowly added dropwise to the methanol solution of europium(II) iodide, and the mixture was stirred at room temperature overnight. The solvent was removed under reduced pressure, and the remaining solid was washed with a mixed solvent (dichloromethane: n-hexane = 1:1) to obtain 0.4564 g of a yellow product. The yield was 65%. Calculated values for elemental analysis of C 12 H 36 EuI2N8: C, 20.64; N, 16.05; H, 5.20. Found: C, 20.85; N, 15.92; H, 5.14.

[0065] Synthesis of Eu2: 0.3118 g (1 mmol) of europium(II) bromide was dispersed in 30 mL of methanol in a glove box, and 0.3 mL (2 mmol) of tris(2-aminoethyl)amine was dispersed in 10 mL of methanol. The latter was slowly added dropwise to the methanol solution of europium(II) bromide, and the mixture was stirred at room temperature for 24 h. The system was filtered by suction, and the filter cake was washed with tetrahydrofuran. The filter cake was collected to obtain 0.3728 g of a light yellow product. The yield was 62%. Calculated values for elemental analysis: C 12 H 36 EuBr2N8: C, 23.85; N, 18.54; H, 6.01. Found: C, 23.70; N, 18.14; H, 5.98.

[0066] Synthesis of Eu3: 0.2229 g (1 mmol) of europium(II) chloride was dispersed in 30 mL of methanol in a glove box, and 0.3 mL (2 mmol) of tris(2-aminoethyl)amine was dispersed in 10 mL of methanol. The latter was slowly added dropwise to the methanol solution of europium(II) chloride, and the mixture was stirred at room temperature for 24 h. The system was filtered by suction, and the filter cake was washed with methanol. The filter cake was collected to obtain 0.4227 g of a light yellow product. The yield was 82%. Calculated values for elemental analysis: C 12 H 36 EuCl2N8: C, 27.97; N, 21.74; H, 7.04. Found: C, 27.85; N, 21.76; H, 7.02.

[0067] Synthesis of Eu4: 0.4058 g (1 mmol) of europium(II) iodide was dispersed in 20 mL of tetrahydrofuran in a glove box, and 0.27 mL (1 mmol) of tris(2-dimethylaminoethyl)amine was dispersed in 10 mL of tetrahydrofuran. The latter was slowly added dropwise to the tetrahydrofuran solution of europium(II) iodide, and the mixture was stirred at room temperature overnight. The solvent was removed under reduced pressure, and the remaining solid was washed with dichloromethane to obtain 0.5387 g of a white product. The yield was 85%. Calculated values for elemental analysis: C 12 H 30EuI2N4: C, 22.66; N, 8.81; H, 4.75. Found: C, 22.85; N, 8.38; H, 4.70.

[0068] Synthesis of Eu5: 0.4058 g (1 mmol) of europium(II) iodide was dispersed in 20 mL of tetrahydrofuran in a glove box, and 0.23 g (1 mmol) of the ligand was dispersed in 10 mL of tetrahydrofuran. The latter was slowly added dropwise to the europium(II) iodide tetrahydrofuran solution, and the mixture was stirred at room temperature overnight. The solvent was removed under reduced pressure, and the remaining solid was washed with a dichloromethane / n - hexane mixed solvent to obtain 0.260 g of a light yellow product. The yield was 52%. Calculated values for elemental analysis: C 54 H 72 Eu2I4N8: C, 39.43; N, 6.81; H, 4.41. Found: C, 39.75; N, 6.43; H, 4.40.

[0069] Synthesis of Eu6: 0.4058 g (1 mmol) of europium(II) iodide was dispersed in 20 mL of tetrahydrofuran in a glove box, and 0.387 g (1 mmol) of the ligand was dispersed in 10 mL of tetrahydrofuran. The latter was slowly added dropwise to the europium(II) iodide tetrahydrofuran solution, and the mixture was stirred at room temperature overnight. The solvent was removed under reduced pressure, and the remaining solid was washed with a dichloromethane / n - hexane mixed solvent to obtain 0.342 g of a white product. The yield was 43%. Calculated values for elemental analysis: C 21 H 30 EuI2N4O3: C, 31.84; N, 7.07; H, 3.82. Found: C, 32.03; N, 7.43; H, 4.02.

[0070] Synthesis of Eu7: 0.4058 g (1 mmol) of europium(II) iodide was dispersed in 20 mL of tetrahydrofuran in a glove box, and 0.435 g (1 mmol) of the ligand was dispersed in 10 mL of tetrahydrofuran. The latter was slowly added dropwise to the europium(II) iodide tetrahydrofuran solution, and the mixture was stirred at room temperature overnight. The solvent was removed under reduced pressure, and the remaining solid was washed with dichloromethane / n - hexane to obtain 0.323 g of a white product. The yield was 38%. Calculated values for elemental analysis: C 42 H 60 Eu2I4N8S6: C, 30.01; N, 6.67; H, 3.60. Found: C, 29.72; N, 6.44; H, 3.52.

[0071] Synthesis of Eu8: 0.4058 g (1 mmol) of europium(II) iodide was dispersed in 30 mL of tetrahydrofuran in a glove box, and 0.28 mL (1 mmol) of triethanolamine was dispersed in 10 mL of tetrahydrofuran. The triethanolamine solution was slowly added dropwise to the europium(II) iodide tetrahydrofuran solution, and the mixture was stirred at room temperature for 24 h. The system was filtered by suction, and the filter cake was washed with methanol. The filter cake was collected to obtain 0.6527 g of a light yellow product. The yield was 93%. Calculated values for elemental analysis: C 12 H 30 EuI2N2O6: C, 20.47; N, 3.98; H, 4.29. Found: C, 20.74; N, 3.86; H, 4.28.

[0072] Synthesis of Eu9: 0.4058 g (1 mmol) of europium(II) iodide was dispersed in 30 mL of tetrahydrofuran in a glove box, and 0.28 mL (2 mmol) of triethanolamine was dispersed in 10 mL of tetrahydrofuran. The triethanolamine solution was slowly added dropwise to the europium(II) iodide tetrahydrofuran suspension, and the mixture was stirred at room temperature for 3 d. The system was filtered by suction, and the filter cake was washed with tetrahydrofuran. The filter cake was collected to obtain 0.5773 g of a light yellow product. The yield was 95%. Calculated values for elemental analysis: C 18 H 42 EuI2N2O6: C, 27.43; N, 3.55; H, 5.37. Found: C, 27.14; N, 3.40; H, 4.82.

[0073] Synthesis of Eu10: 0.4058 g (1 mmol) of europium(II) iodide was dispersed in 30 mL of tetrahydrofuran in a glove box, and 0.383 g (1 mmol) of triethanolamine was dispersed in 10 mL of tetrahydrofuran. The triethanolamine solution was slowly added dropwise to the europium(II) iodide tetrahydrofuran suspension, and the mixture was stirred at room temperature for 3 d. The system was filtered by suction, and the filter cake was washed with tetrahydrofuran. The filter cake was collected to obtain 0.5773 g of a light yellow product. The yield was 95%. Calculated values for elemental analysis: C 18 H 42 EuI2N2O6: C, 27.43; N, 3.55; H, 5.37. Found: C, 27.39; N, 3.50; H, 5.43.

[0074] Crystal Structure of the Complex in Example 2

[0075] Table 1. Crystal Data of Complexes Eu1, 2, 3, 5, 6, 7, 8, 10

[0076]

[0077]

[0078]

[0079] Table 2. Eu-N, Eu-O, and Eu-I Coordination Bond Lengths of Complexes Eu1, 2, 3, 5, 6, 7, 8, 10

[0080]

[0081] The crystal structures of complexes Eu1, 2, 3, 5, 6, 7, 8, 10 were characterized by single-crystal X-ray diffraction. Among them, the coordination structures of Eu1-3, Eu8, and Eu10 are similar. Since the two ligands completely wrap the central Eu(II) from above and below, and the halogen ions do not participate in coordination, these complexes have an 8-coordination structure. The Eu-N bond lengths in these five complexes are respectively The Eu-O bond lengths in Eu8 and Eu10 are respectively Eu5 and Eu7 are binuclear complexes bridged by iodide ions, and the ratio of ligand to Eu(II) is 1:1. Due to the introduction of steric groups, the coordination of the second ligand is restricted, and the final coordination number of the complex is 7. Eu6 is a complex with a direct 1:1 coordination of ligand and Eu(II). The introduction of furan groups increases the coordination sites provided by a single ligand, enabling the ligand to form an 8-coordination structure with a single ligand and two iodide ions.

[0082] Photophysical Properties of the Complex in Example 3

[0083] Since the luminescence of these complexes in solution is extremely weak, only the powder photophysical properties were characterized. For complexes Eu1, Eu2, and Eu3, since the anions are not directly coordinated and the average Eu-N bond lengths are relatively close, their emission spectra are very similar, with emission peaks located at about 550 nm, showing bright yellow-green light emission. Among them, Eu2 and Eu3 have a quantum yield of 75%, and even after being placed in air for 3 months, they still maintain a quantum yield of more than 50%. After the introduction of steric groups, the emissions of Eu4, Eu5, and Eu7 all show a certain degree of blue shift, which can be attributed to the weakening of the coordination ability after the introduction of substituents on N, and the weakening of the ligand field due to the decrease in the coordination number of Eu(II), resulting in a decrease in the d-orbital splitting energy and an increase in the d-f transition energy. The emission of Eu6 after the introduction of coordination groups also shows a certain degree of blue shift compared to Eu1-3. In addition to the weakening of the coordination ability of N in this type of ligand itself, it is also related to the fact that atoms such as I and O with relatively weak coordination to Eu(II) replace N to participate in coordination. For Eu8, Eu9, and Eu10, since three N atoms on the ligand skeleton are replaced by O atoms with relatively weak coordination ability, their emissions also show a certain degree of blue shift. Among them, the emission peak of Eu10 is located at 440 nm, showing blue light emission.

[0084] The excited-state lifetimes of these complexes are all between 100 ns and 500 ns, which are shorter than those of phosphorescent complexes and TADF molecules (in the microsecond range). Among them, the complexes coordinated with O atoms generally have relatively shorter excited-state lifetimes, which can be attributed to the weaker coordination ability of O atoms with Eu(II), poorer overall rigidity of the complexes, and easier quenching of the excited state, which is consistent with their lower quantum yields (~20%).

[0085] To characterize the stability of the material against ultraviolet light, the complex Eu2 among them was subjected to an ultraviolet aging experiment. The ultraviolet aging experiment used a UVA340 type lamp tube with a power of 40 watts, and the irradiation intensity during the test was 25 - 30 watts per square meter. As Figure 2 shown, the solid powder of the Eu2 complex still has a quantum yield of about 10% after 400 h of ultraviolet aging experiment, indicating its good tolerance to ultraviolet radiation.

[0086] Table 3. Photophysical properties, air stability, and CIE color coordinates of the complexes

[0087]

[0088] The photophysical properties, air stability, and CIE color coordinates of the complexes are listed in Table 3 above, and the room-temperature emission spectra of the complexes are shown in the appendix Figure 3 . From Table 3 and Figure 2 it can be seen that among these complexes, Eu2 and Eu3 have good air stability. Even after being exposed to air for more than three months, the solid powder still has a relatively high quantum yield of 50%. In addition, by introducing steric and coordinating groups on the original ligands, a series of Eu(II) complexes with new structures were obtained, and their luminescence covers the range from blue light to yellow light. The above results show that highly stable Eu(II) complexes can be obtained through ligand structure design, and the spectrum can be adjusted by changing the ligand field. At the same time, combined with shorter excited-state lifetimes and higher PLQYs, these Eu(II) complexes have the potential to become highly efficient and stable OLED luminescent materials.

[0089] Example 4 Electroluminescence Research

[0090] Full names of compounds corresponding to the abbreviations:

[0091] CzSi 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole

[0092] mCP 1,3-bis(9H-carbazol-9-yl)benzene

[0093] TmPyPB 1,3,5-tris[(3-pyridinyl)-3-phenyl]benzene

[0094] TAPC 4,4′-Cyclohexylbis[N,N-bis(4-methylphenyl)aniline]

[0095] TmPyPB 1,3,5-Tris[(3-pyridinyl)-3-phenyl]benzene

[0096] TCTA Tris(4-(9-carbazolyl)phenyl)amine

[0097] oCBP 4,4-Di(carbazolyl)biphenyl

[0098] mCBP 3,3-Di(carbazolyl)biphenyl

[0099] PVK Poly(vinylcarbazole)

[0100] PEDOT:PSS Poly(3,4-ethylenedioxythiophene):Polystyrenesulfonate

[0101] PCZAC 9,9-Dimethyl-10-(9-phenyl-9H-carbazol-2-yl)-9,10-dihydroacridine

[0102] A complex Eu5 with both a relatively high photoluminescence quantum yield and good solubility was specifically selected for the preparation of solution-processed OLED devices. To study the electroluminescence properties of the complex Eu5, it was first doped into a variety of host materials with relatively high triplet energy levels (E T > 2.7 eV), and the photoluminescence quantum yields of these thin films under 270 nm excitation were measured using a Hamamatsu C9920-02 photoluminescence quantum yield measurement system, as Figure 4 shown. When Eu5 was doped into PCZAC at 11 wt%, the photoluminescence quantum yield of the thin film was relatively high, reaching 70%.

[0103] Given that high photoluminescence efficiency is a prerequisite for high electroluminescence efficiency, the inventors of the present invention prepared OLEDs devices using PCZAC as the host material and Eu5 as the luminescent material. The device structure is as follows:

[0104] D1: ITO / PEDOT:PSS(40 nm) / PVK(Approx. Mw 90000)(50 nm) / PCZAC:Eu5(11 wt%, 60 nm) / TmPyPB(40 nm) / LiF(0.7 nm) / Al

[0105] The main performance parameters of the device are listed in Table 4.

[0106] Table 4. Electroluminescence properties of electroluminescent device D1

[0107]

[0108] Figure 5, Figure 6 , Figure 7 respectively show the electroluminescence emission spectrum, current density-voltage-luminance curve, and power efficiency-luminance-EQE curve of device D1. The obtained device has a maximum EQE of 9.2%, while for traditional f-f transition Eu(III) complexes, after decades of development, the highest reported EQE in the literature is only 7.8%; at the same time, compared with the f-f transition red emission of traditional Eu(III) complexes, the emission color of d-f transition is easier to adjust. However, the performance of blue-light Eu(II) complex OLEDs reported in the literature is poor, with a maximum EQE of only 0.75% and a luminance of only 23 cd / m 2 . The main emission peak of this device is located at 485 nm, and both the EQE (9.2%) and luminance (1780 cd / m 2 ) are greatly improved, making it an indispensable blue-light material for full-color displays. In addition, the hole injection layer, hole transport layer, and light-emitting layer of this device are all prepared by spin coating. Compared with the vacuum evaporation method and spin coating method commonly used for traditional f-f transition rare-earth complex OLEDs in the literature, spin coating has the advantages of simple operation, low cost, and lower requirements for the sublimation performance of materials.

[0109] Example 5:

[0110] The rare-earth complex Eu10 is dispersed in the polymer PMMA and used as a light-emitting film

[0111] The rare-earth complex Eu10 and the polymer PMMA resin are mixed at a mass ratio of 1:100 and dissolved in dichloromethane solution. Then the obtained mixture is spin-coated on the surface of a clean quartz glass to form a uniform polymer film.

[0112] Under the irradiation of an ultraviolet lamp, the obtained film can be seen to emit bright blue light with the naked eye. The absolute quantum yield of photoluminescence measured using an integrating sphere can reach 80%, making it an efficient light conversion thin-film material. This result indicates that the complex provided in this embodiment of the present invention can be used as a light-emitting material for light conversion thin films.

[0113] Example 6:

[0114] The solid powder of the rare-earth complex Eu3 itself has high photoluminescence efficiency, and its quantum yield measured using an integrating sphere is 70%, making it an efficient and bright organic phosphor material. The powder of the complex is dispersed in a thermosetting silicone encapsulant at a mass percentage of 1:10 and coated on the surface of a commercial 365 nm ultraviolet LED chip. Then it is heated and cured at 120 °C for 1 hour to obtain a yellow-green light-emitting LED device excited by an ultraviolet chip.

[0115] A combined system of a computer-controlled Keithley 2400 measuring instrument and a PR650 spectrometer can measure the variation of the brightness of the yellow-green LED device with voltage (as Figure 8 shown). The measurement results of the device's emission spectrum show that the d-f yellow-green emission of rare-earth europium ions is obtained. At a driving voltage of 3.8 V, the maximum brightness of the device exceeds 1000 cd m -2 . Compared with rare-earth-containing inorganic phosphors, d-f transition rare-earth complex phosphors have a larger molar extinction coefficient (ε>1000 Lmol -1 cm -1 ) and a more easily adjustable emission range. It can effectively reduce the types and amounts of rare earths used, thus greatly reducing costs.

Claims

1. A d-f transition rare earth Eu(II) complex, characterized in that, The structural formula of the d-f transition rare earth Eu(II) complex is as follows: Among them, R1, R2, R3, R3', R4, and R4' are all hydrogen, and X is I, Br, or Cl.

2. A d-f transition rare earth Eu(II) complex, characterized in that, The structural formula of the d-f transition rare earth Eu(II) complex is as follows: Wherein, X is I, and R1, R3, R3', R4, and R4' are all hydrogen; Alternatively, X is I, R1, R3’, R4, and R4’ are all hydrogen, and R3 is CH3; Alternatively, X is I, R3, R3’, R4, and R4’ are all hydrogen, and R1 is CH3.

3. A d-f transition rare earth Eu(II) complex, characterized in that, The structural formula of the d-f transition rare earth Eu(II) complex is as follows: Wherein, X is I, R1 and R2 are CH3, and R3, R3’, R4 and R4’ are hydrogen; Alternatively, X is I, R1 is H, R2 is phenyl, and R3, R3’, R4, and R4’ are hydrogen; Alternatively, X is I, R1 is H, R2 is thienyl, and R3, R3’, R4, and R4’ are hydrogen.

4. A d-f transition rare earth Eu(II) complex, characterized in that, The structural formula of the d-f transition rare earth Eu(II) complex is as follows: Wherein, X is I, R1 is H, R2 is furyl, and R3, R3’, R4, and R4’ are hydrogen.

5. An electroluminescent device, characterized in that, The electroluminescent device includes a cathode, an anode, and a light-emitting layer located between the cathode and the anode. It is characterized in that the light-emitting layer includes the d-f transition rare earth Eu(II) complex according to any one of claims 1-4; Preferably, the light-emitting layer includes a host material and a doping material, and the doping material includes the d-f transition rare earth Eu(II) complex according to any one of claims 1-5; Preferably, the triplet energy level E of the host material T > 2.7 eV; Preferably, the doping concentration of the d-f transition rare earth Eu(II) complex in the host material is 5-15 wt%, preferably 8-13 wt%, preferably 10-12 wt%, preferably 11 wt%; Preferably, the electroluminescent device further includes a hole transport layer located between the anode and the light-emitting layer; Preferably, the electroluminescent device further includes an electron transport layer located between the cathode and the light-emitting layer; Preferably, the structure of the electroluminescent device is: ITO / PEDOT:PSS(40nm) / PVK(Approx.Mw 90000)(50nm) / PCZAC:Eu5(11wt%,60nm) / TmPyPB(40nm) / LiF(0.7nm) / Al.

6. A method for manufacturing an electroluminescent device according to claim 5, characterized in that, Both the light-emitting layer and the hole transport layer are prepared by spin coating.

7. A light-emitting film, characterized in that, The light-emitting film includes a polymer material and the d-f transition rare earth Eu(II) complex according to any one of claims 1-4; Preferably, the mass ratio of the d-f transition rare earth Eu(II) complex to the polymer material is 0.1-10:100, preferably 0.5-2:100, and further preferably 0.8-1.2:100; Preferably, the polymer material is PMMA resin, and the mass ratio of the d-f transition rare earth Eu(II) complex to the PMMA resin is 1:

100.

8. An LED device, characterized in that, The LED device includes an LED chip and a light-emitting layer attached to the LED chip. The light-emitting layer includes a polymer material and the d-f transition rare earth Eu(II) complex according to any one of claims 1-4; Preferably, the polymer material is a thermosetting resin; Preferably, the mass ratio of the d-f transition rare earth Eu(II) complex to the polymer material is 0.1-10:10, preferably 0.5-2:10, and further preferably 0.8-1.2:10; Preferably, the polymer material is a thermosetting silicone encapsulant; the mass ratio of the d-f transition rare earth Eu(II) complex to the polymer material is 1:10.

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