Two substituted phenanthrolines (L = DEP and PiPhen) (DEP = 5-Diethylamino-1,10-phenanthroline, PiPhen = 5-Piperidine-1,10-phenanthroline) were successfully prepared and europium complexes Eu(TTA)3(L) (Eu-L) based on these ligands were synthesized from EuCl3, 2-thenoyltrifluoroacetone (TTA) and L in good yields. The europium complexes emit a strong sharp red band at ~ 612 nm in solution and in the solid state. The HOMO levels of these europium complexes are at ca. 5.6 eV. Several electroluminescent devices using these two europium complexes as dopant emitters were fabricated. The results show that some of these devices successfully emit saturated red light.
Keywords

Substituted Phenanthroline, Europium Complex, Electroluminescence, Red Emitter, OLED
Introduction

Studies on the use of rare-earth complexes as the emitter materials in OLEDs (organic light emitting devices) have attracted considerable attention recently [1-8]. Unlike common fluorescent and phosphorescent compounds, rare-earth complexes show high luminance efficiency with sharp emission bands involving electrons associated with inner f orbitals of the central rare-earth metal ions. Of the rare-earth complexes reported, europium complexes appear most studied due to the strongly red-emission ability that are widely exploited in full-color displays [2]. Several europium complexes have been employed as red emitters in electroluminescent devices [1-8]. Due to the unique mechanism of excitation, europium complexes used in electroluminescent devices emit red light at the same frequency, although the ligands on the europium center are different to each other [9]. In the fabrication of electroluminescent device using europium complexes as dopants, the choice of host materials and hole blocker is very limited because of the very low-lying HOMO levels for most europium complexes. Several groups have successfully produced saturated red light by using CBP (4,4’-N,N’-dicarbazole-biphenyl) as the host and BCP (2,9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline), TAZ ( 3 - [biphenyl-4-yl]- 4- phenyl- 5- [4-tert-butyl] phenyl-1,2,4-triazole) or the europium complex itself as the hole blocker [1,2,4-6].

In a recent paper [10] reported, we used europium complex Eu(TTA)3(DPPz), where TTA = 2-thenoyltrifluoroacetonate and DPPz = dipyrido[3,2-a:2’,3’-c]phenazine, as the red emitters for electroluminescent devices. Some of these devices emit saturated red light with brightness greater than 1000 cd/m2. In this paper, we prepared phenanthroline derivatives with a strong electron donating amino group and new europium complexes based on these new ligands and the use of these complexes as red emitters in electroluminescent devices.
Experimental
General Information

All reactions were carried out under nitrogen atmosphere in sealed reaction vessels. 1,10-Phenanthroline-5,6-epoxide was synthesized according to a literature method [11]. Piperidine, diethylamine and other chemicals were used as purchased. Melting points were measured on a Fargo MP-2D melting point apparatus and were uncorrected. 1H NMR spectra were recorded with a Varian Mercury 400 spectrometer. High resolution mass spectra were obtained on a Finnigan MAT-95XL instrument. Elemental analysis was carried out using a Heraeus CHN-O Rapid instrument. UV-vis spectra were recorded on a Hitachi U-3300 model while photoluminescence (PL) spectra were taken using a Hitachi F-4500 fluorescence spectrophotometer.
OLED Fabrication and Measurement

EL devices based on europium complexes were fabricated through vacuum deposition of the materials onto clean glass precoated with a layer of indium tin oxide (sheet resistance 25 ohm/square) at 10-6 Torr. The rate of deposition of each organic compound was 0.1–0.3 nm/s and for the europium complex was 0.005–0.01 nm/s. The cathode was formed through co-evaporation of Mg and Ag with Mg:Ag ratio 10:1 and total thickness 55 nm, followed by vacuum deposition of Ag (100 nm). Rates of co-deposition of Mg and Ag were 0.5–0.6 and 0.05–0.06 nm/s, respectively; the rate of deposition of Ag was 0.3 nm/s. The emitting diode has an effective area 9.00 mm2. Current, voltage and light intensity were measured simultaneously using a source meter (Keithley 2400) and an optical meter (Newport 1835-C) equipped with a silicon photodiode (Newport 818-ST). EL spectra were measured on a fluorescence spectrophotometer (Hitachi F-4500). The Commission Internationale de l’Éclairage (CIE) values were calculated based on the data of EL spectra [12]. According to the CIE 1931 system, X = SP(l)x(l)dl, Y = SP(l)y(l)dl and Z = SP(l)z(l)dl; where P(l) was the emission intensity of the EL spectrum, x(l), y(l) and z(l) were the color-matching function. Therefore, the CIE values of x and y of a device were equal to X/(X + Y + Z) and Y/(X + Y + Z), separately. Oxidation potentials were obtained on a model CHI600A electrochemical analyzer. The HOMO levels of the Eu complexes were calculated based on their oxidation potentials, while LUMO was calculated based on the HOMO energy level and the lowest-energy absorption edge of the UV-vis absorption spectrum.