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An azomethin-zinc complex for organic electroluminescence: Crystal structure, thermal stability and optoelectronic properties
Inorganica Chimica Acta 358 (2005) 4451–4458 www.elsevier.com/locate/ica
An azomethin-zinc complex for organic electroluminescence: Crystal structure, thermal stability and optoelectronic properties Junfeng Xie, Juan Qiao *, Liduo Wang, Jing Xie, Yong Qiu Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Beijing 100084, China Department of Chemistry, Tsinghua University, Beijing 100084, China Received 30 March 2005; received in revised form 11 August 2005; accepted 11 August 2005 Available online 26 September 2005
Abstract An azomethin-zinc complex, bis[salicylidene(4-dimethylamino)aniline]zinc(II) (Zn(sada)2) was synthesized and structurally characterized by single-crystal X-ray crystallography. Crystal data for Zn(C15H15N2O)2 was determined as follows: space group, triclinic, P 1; ˚ , b = 16.5008(14) A ˚ , c = 17.5984(15) A ˚ , a = 114.830(2), b = 96.579(2), c = 97.674(2), Z = 4. Through thermal anala = 10.2791(9) A ysis characterization and FT-IR spectra, this complex was proved to have good thermal stability. The vapor-deposited films exhibited uniform and environment-stable morphology. The light emission and charge transporting performance of Zn(sada)2 in organic light emitting diodes (OLEDs) were investigated preliminarily, and the results indicated the superior electron transporting property of this complex. Compared with the typical bilayer device of N,N 0 -diphenyl-N,N 0 -bis(1-naphthyl)-benzidine (NPB)/tris-(8-hydroxyquinoline)aluminum (Alq3), the device with Zn(sada)2 as the electron transporting layer exhibited a much lower turn-on voltage of 2.5 V (it is usually 3.5 V for an NPB/Alq3 device). 2005 Elsevier B.V. All rights reserved. Keywords: Zinc complexes; Organic electroluminescence; X-ray crystal structures; Thermal stability; Optoelectronic properties
1. Introduction Since the first vacuum-deposited OLEDs using tris-(8hydroxyquinoline) aluminum (Alq3) were reported by Tang and VanSlyke [1], organic metal-chelate compounds have drawn a great deal of attention. These materials offer many attractive properties such as the dual function of electrontransporting and light emission, higher environmental stability, and ease of sublimation. By varying the central metal atom (Al, Ga, In, Be, Mg, Ca, Zn, Cu, etc.), a variety of other metal complexes with 8-hydroxyquinoline have been designed and synthesized for use in OLEDs. Amongst these, the zinc complex bis-(8-hydroxyquinoline)zinc (Znq2) was found to have strong yellow fluorescence and *
Corresponding author. Tel.: +86 10 6277 3109; fax: +86 10 6279 5137. E-mail address: [email protected] (J. Qiao).
0020-1693/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.08.018
was demonstrated to be a useful emitter material or electron-transporting hosts in OLEDs [2]. It is well understood that the charge balance in OLEDs is one of the most important factors necessary to achieve high device efficiency. Alq3 is the most widely used electron-transporting material in OLEDs. However, the electron mobility of Alq3 is lower than the hole mobility of traditional hole transporting materials (HTMs), N,N 0 -diphenyl-N,N 0 -bis(3methyl)-benzidine (TPD) and N,N 0 -diphenyl-N,N 0 -bis(1naphthyl)-benzidine (NPB). Zinc complexes have been used in OLEDs for more than a decade [2,3], but the best electroluminescent performance of these materials as emitters is just comparable with that of Alq3. However, in many instances, the electron-transporting mobility of zinc complexes goes beyond that of Alq3. So zinc complexes may be potential candidates to enhance the electron-transporting properties for OLEDs.
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Since 1993, when OLEDs with Znq2 were reported [2], studies of zinc complexes as active materials for OLEDs have focused on improving electron mobility and/or producing a blue shift, relative to Znq2, in the emission wavelength maximum. Zn2+ ion is the only oxidation state of zinc atom and has no unoccupied valence electron orbits. Therefore, the oxidation and reduction of zinc complexes are mainly carried out in the ligands, i.e., luminescence and charge transporting may be ascribed to intraligand electron transition. One simple way to tune the emission wavelength is to manipulate substituents in the 8-hydroxyquinoline rings. 2-Position substituted bis(2-methyl-8-hydroxyquinoline)zinc (ZnMq2) [3], 5-position substituted bis(8-hydroxy-5piperidinylsulfonamidoquinolate)zinc (Zn(QS)2) [4] and (ZnLn)2 Æ 2H2O (L = 5-amido-substituted-8-hydroxyquinolinate ligand) [5], etc., have been reported one after the other. These complexes showed more or less either blue-shifting or red-shifting with reference to Znq2. The other way is to introduce non-oxidate ligands, including Schiff base ligands [2,6,7], polycyclic aromatic ligands [8–11] and polypyridylamines [12–14]. In addition, metalloporphyrin [15] and metal clusters [16,17] were also used to obtain fine EL materials. Most of these zinc complexes have been reported as potential OLED materials. However, few of them have been used for practical OLEDs. One of the aims of molecular design is to obtain devicequality materials. Studying structure–property relationships provides guidance in molecular design for improved molecules. The molecular structure of anhydrous Znq2 was determined in 1985 [18], and this complex was introduced in OLEDs in 1993, but it was not until 2002 that the relationship between the structure of tetrameric Znq2 and its performance in OLEDs was clearly studied by Sapochak et al. [19]. A detailed investigation of zinc(II) 2-(2-hydroxyphenyl)benzothiazolate (Zn(BTZ)2) was just accomplished last year [20], although it was reported as an excellent white EL material as early as 1996 [9]. Recently, our group has focused on metal complexes based on Schiff base ligands, which have proven to be highly efficient luminescent materials for OLEDs [21,22]. Azomethin-zinc complexes are typical metal complexes based on Schiff base ligands and early studies were focused on their structures and reactions [23–27]. As early as 1993, they were reported as potential OLED materials by Hamada et al. [2]. However, there are few investigations on the relationships between the molecular structure and molecular packing, the morphological properties in thin films and the corresponding optoelectronic properties of these complexes. In this study, we used an azomethine ligand bearing an electron-donating substituent, salicylidene(4-dimethylamino)aniline (sada), and synthesized the corresponding azomethin-zinc complex bis[salicylidene(4-dimethylamino)aniline]zinc(II) (Zn(sada)2) (Fig. 1(a)). By using X-ray single crystal diffraction, Zn(sada)2 was structurally characterized as a racemic compound, and this complex has very good thermal stability in both the single crystal and thin film forms. Herein, we describe the molecular and crystal struc-
ture of Zn(sada)2, the thermal stability of the racemic structure and its optoelectronic properties. 2. Experimental 2.1. Synthesis of sada A mixture of salicylaldehyde and N,N-dimethyl-benzene-1,4-diamine (purchased from Aldrich Chemical Co.) in a 1:1 molar ratio was heated and the following recrystallization with ethanol gave an orange-red precipitate with a yield of about 87%. MS (EI) [m/z] 240. 1H NMR (400 MHz, CD3Cl), d = 2.99 (s, 6H), 6.80–6.70 (m, 2H), 6.91 (t, 1H, J = 8.6 Hz), 7.00 (d, 1H, J = 8.7 Hz), 7.27 (d, 2H, J = 9.0 Hz), 7.30 (ddd, 1H, J = 8.6, 7.0, 1.8 Hz), 7.34 (dd, 1H, J = 7.8, 1.8 Hz), 8.61 (s, 1H), 13.73 (s, 1H). 2.2. Synthesis of Zn(sada)2 First, a solution of ZnCl2 (0.6815 g, 5 mmol) in ethanol (30 ml) was gradually added to a solution of sada (1.20 g, 5 mmol) and piperidine (1.0 ml, 10 mmol) in 120 ml ethanol. After the mixture was stirred for 0.5 h while being heated at reflux, and stirred again for 24 h at room temperature, a yellow precipitate was produced. The crude product was collected by filtration and washed with ethanol and finally dried under an infrared lamp. The material was further purified by vacuum train sublimation before analysis and the fabrication of device. Yield: 86%. MS (EI) [m/z] 542. Anal. Calc. for C30H30N4O2Zn: C, 66.24; H, 5.56; N, 10.30. Found: C, 66.29; H, 5.43; N, 10.50%. 1H NMR (400 MHz, [D6]DMSO), d = 2.85 (s, 12H), 6.70–6.62 (m, 8H), 7.22 (dd, 4H, J = 7.0, 1.9 Hz), 7.30 (ddd, 2H, J = 8.7, 7.0, 1.9 Hz), 7.50 (dd, 2H, J = 8.0, 1.8 Hz), 8.78 (s, 2H). IR (cm1): 2898, 1612, 1579, 1531, 1518, 1463, 1442, 1387, 1365, 1348, 1331, 1317, 1255, 1226, 1190, 1180, 1149, 1125, 1064, 1032, 947, 922, 862, 818, 757, 717. 2.3. X-ray crystallography A single crystal of Zn(sada)2 suitable for X-ray crystallography was obtained by gradient-temperature vacuum sublimation. Zn(sada)2 was heated incrementally in zone 1 of a two zone furnace from 220 to 270 C under a N2 atmosphere of about 2 Pa for 48 h. The single crystal was collected at 245 C and had a typical size of 0.3 · 0.2 · 0.2 mm3. The solid-state structure was further confirmed by single-crystal X-ray diffraction analysis. Room temperature (294 ± 1 C) single-crystal X-ray experiments were performed on a Bruker SMART APEX CCD diffractometer equipped with graphite-monochromatized Mo Ka radiation (k = ˚ ). Direct phase determination yielded the positions 0.71073 A of Zn, O, N and most of the C atoms, and the other C atoms were located in successive-difference Fourier syntheses. Hydrogen atoms were generated theoretically and rode on their parent atoms in the final refinement. All non-hydrogen atoms were subjected to anisotropic refinement. The final
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Fig. 1. (a) Molecular formula of Zn(sada)2; (b) ORTEP drawing of molecule 1, with 35% probability ellipsoids, showing the atomic numbering scheme; (c) ORTEP drawing of molecule 2, with 35% probability ellipsoids, showing the atomic numbering scheme.
full-matrix least-square refinement on F2 converged with R1 = 0.0385 and wR2 = 0.0822 for 7715 observed reflections [I P 2r(I)]. The final difference electron density map shows no features. The structural solutions and refinements were performed using the SHELXTL NT ver. 5.10 program package (Bruker, 1997) [28]. 2.4. Equipment 1
H NMR spectra were recorded on a Bruker ARX400 NMR spectrometer with tetramethylsilane as the internal standard. Infrared spectra were recorded on a Nicolet Magna-IR 750 (USA) FT-IR microscope system. Absorption spectra were recorded with a UV–Vis spectrophotometer (HP8452A), and PL spectra were obtained with a fluorospectrophotometer (HITACHI, F4500). Relative PL quantum efficiencies (UPL) were determined from degassed dimethyl formamide (DMF) solutions, and the concentrations of the samples were carefully adjusted so that the optical densities at 390 nm (excitation wavelength) were <0.1 absorption units. PL quantum yields were finally calculated relative to the known value of Alq3 in DMF (UPL = 0.116) [29] and normalized to Alq3. The glass tran-
sition temperature (Tg) and melting point (Tm) were determined by differential scanning calorimetry (DSC) performed on a TA thermal analysis instrument (DSC 2910 Modulated DSC). Cyclic voltammetry measurements were conducted on a model CH 600 voltammetric analyzer with a platinum plate as the working electrode, a silver wire as the pseudo-reference electrode, a polished platinum wire as the counter electrode, and ferrocene as an internal [30], at a scan rate of 50 mV/s. The supporting electrolyte was 0.1 mol/dm3 tetrabutylammonium tetrafluoroborate in DMF. Prior to electrochemical measurements, the solution was deoxygenated with bubbling nitrogen for 15 min. 2.5. Fabrication of EL devices Devices were grown on glass slides precoated with indium tin oxide (ITO) with 30 X/square. The substrates were ultrasonically cleaned in detergent solution for 1 min, followed by thorough rinsing in de-ionized water. They were then rinsed in acetone and methanol, and then dried in pure nitrogen gas. The organic films were deposited layer by layer onto the ITO surface. After deposition of the organic layers, a Mg:Ag (10:1 mass ratio) electrode was deposited
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onto the organic layer without breaking the vacuum. The chamber pressure was below 5 · 104 Pa during the deposition of organic materials and metals. The active area of the devices was 0.35 cm2. Devices were tested in air with ITO as the positive electrode in the forward bias configuration without further encapsulation. Current–voltage–luminance (I–V–L) curves of OLEDs were measured using a Keithley 4200 semiconductor characterization system. The EL spectra were measured with a Photo Research PR650 spectrophotometer. 3. Results and discussion 3.1. Molecular structure The coordination numbers of Zn2+ ion can vary from 2 to 6, and the change of the ligands may result in distinct molecular structures. Recent reports involved dimeric [20], trimeric [11] and tetrameric structures [19]. The molecular structure of Zn(sada)2 was elucidated as a monomer by X-ray crystallography. The crystal data and selected bond lengths and angles are listed in Tables 1 and 2, respectively. A perspective drawing of the structure is shown in Fig. 1. There are two independent molecules, 1 and 2 in the Zn(sada)2 single crystal. It was considered that this phenomenon comes from the restriction in solid state. As seen in Table 2, the averages of Zn–O and Zn–N bond lengths are ˚ , respectively. The zinc ion is about 1.91 and 2.01 A four-coordinate to the Schiff base ligands and has distorted tetrahedron geometry. In both molecules, the bond angle (96.30–97.71) formed by oxygen and nitrogen atoms of the same ligand together with zinc atom (O–Zn–N) is much less than 109.5, because of the tension of the Zn1–O1–C1– Table 1 Crystallographic data Formula C30H30N4O2Zn Formula weight 543.95 Color yellow prism Crystal size (mm3) 0.2 · 0.2 · 0.3 Crystal system triclinic Space group P 1 ðNo. 2Þ ˚) a (A 10.2791(9) ˚) b (A 16.5008(14) ˚) c (A 17.5984(15) a () 114.830(2) b () 96.579(2) c () 97.674(2) ˚ 3) V (A 2636.0(4) Z 4 Dcalc (g cm3) 1.371 l (cm1) 0.966 2hmax () 50 F(0 0 0) 1136 Reflections measured 9216/7715 Rint 0.0148 Goodness-of-fit on F2 1.023 R1, wR2 [I P 2r(I)]a 0.0385, 0.0822 0.0468, 0.0846 R1, wR2 (all data)a P P P P a 2 2 2 R1 ¼ jjF o j jF c jj= jF o j; wR2 ¼ ½ wðF o F c Þ = wðF 2o Þ2 1=2 .
Table 2 ˚ ) and angles () Selected bond lengths (A Molecule 1 Zn(1)–O(1) Zn(1)–O(2) Zn(1)–N(1) Zn(1)–N(3) O(1)–Zn(1)–O(2) O(2)–Zn(1)–N(3) O(1)–Zn(1)–N(3) O(2)–Zn(1)–N(1) O(1)–Zn(1)–N(1) N(1)–Zn(1)–N(3) Molecule 2 Zn(1A)–O(1A) Zn(1A)–O(2A) Zn(1A)–N(1A) Zn(1A)–N(3A) O(1A)–Zn(1A)–O(2A) O(2A)–Zn(1A)–N(3A) O(1A)–Zn(1A)–N(3A) O(2A)–Zn(1A)–N(1A) O(1A)–Zn(1A)–N(1A) N(1A)–Zn(1A)–N(3A)
1.9164(19) 1.908(2) 2.020(2) 2.012(2) 115.88(9) 97.15(8) 113.87(9) 113.00(9) 97.71(8) 120.47(9) 1.907(2) 1.914(2) 2.007(2) 2.006(2) 109.28(9) 96.30(8) 123.28(9) 118.38(9) 96.74(9) 114.40(8)
C6–C7–N1 hexatomic ring. The coordinate bond lengths and bond angles are similar to the mentioned azomethinzinc complexes [23–27]. The distance of the two aniline rings in molecule 2 is closer than that in molecule 1, resulting in an increase of the torsion of the C(8A)–N(1A) and C(23A)– N(3A) bonds, i.e., the dihedral angle of the two aromatic rings of the same ligand in molecule 2 is bigger than that in molecule 1. In fact, the dihedral angles between phenol and aniline groups of the same sada ligand in molecule 1 are 9.7 and 19.8, which are almost planar, while those in molecule 2 are 42.0 and 45.6, respectively. The C2 symmetry of the Zn(sada)2 molecule in the ideal state disappears in single crystals because of the molecule stacking effect.
Fig. 2. Crystal packing view of Zn(sada)2 along the a direction.
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3.2. Thermal stability
3.3. Absorption and photoluminescence
As seen in Fig. 2, the Zn(sada)2 crystal is a racemic compound. Experimental evidence for the stability of this racemic compound was obtained from DSC/TGA thermal analysis. The complex only had a 2% weight loss at 317 C and finally decomposed at temperatures over 400 C. The melting point was obtained at the first DSC heating of about 274 C. By rapid quenching of the melting sample, the glass transition was observed at 112 C for the second heating, and no recrystallization peak was found. In addition, the DSC scans at different heating rates for Zn(sada)2 powders were measured. The single endothermic transition (DHfusion = 54.94 J/mol) peak observed at 274 C narrowed as the heating rate decreased from 20 to 2 C/min. There was no additional thermal transition detected, indicating that no phase transition happened in the heating process. This behavior is in contrast to Alq3, which shows several enthalpic transitions assigned to different polymorphic phases. Furthermore, there were no detectable changes in the FT-IR spectra for powder samples obtained by vacuum train sublimation or deposited from the acetone solution and vapor-deposited film sample on the silicon substrate. These results indicate that the Zn(sada)2 complex is a stable racemic compound. An atomic force microscope (AFM) was utilized to observe the surface morphology of the vapor-deposited films on a silica substrate. The as-deposited film of Zn(sada)2 exhibited an entirely amorphous and uniform surface and a negligible change after being stored for 5 days at room temperature in air. This environment-stable amorphous morphology may come from the non-planar molecular structure caused by the dimethylamino substituents and an increased number of conformers in this molecule which may prevent easy packing of molecules and hence ready crystallization [31].
The UV–Vis absorption and PL spectra of Zn(sada)2 and sada in DMF solutions are shown in Fig. 3. The maximum absorption peak of sada is at 390 nm, and that of Zn(sada)2 is bathochromically shifted to 412 nm. The broad absorption band closely matches the absorption spectrum of the protonated ligand precursor and thus was assigned as a ligand-based p–p* transition. The maximum emission peak of sada is at 553 nm, and that of Zn(sada)2 is at 542 nm which does not change at different excitation wavelengths. It is obvious that the sada ligand acts as absorber and emitter in this molecule. The rotational and vibrational degrees of freedom due to the dimethylamino substitutes could increase the probability of non-radiative relaxation processes, resulting in low fluorescence quantum efficiency of Zn(sada)2 (0.007 in DMF solution). Compared with the solution sample, the absorption and emission of the film exhibited a small red shift of only several nanometers. This suggests that molecules of Zn(sada)2 in the solid film have a weak intermolecular interaction and the bulky dimethylamino substitutes may prevent easy aggregation. 3.4. Electrochemical properties Energy level is an important property for organic materials used in OLEDs. The charge injection (transport) barriers, which are one of the key conditions that influence the operating voltage and efficiency of an OLED, are mainly due to energetic differences between the work function of the electrodes and the HOMO or LUMO energy of organic materials. Commonly, the corresponding energy level can be easily obtained by cyclic voltammetry. The oxidation and reduction potentials of Zn(sada)2 were determined in DMF solution using ferrocene as an internal reference.
1.2 Zn(sada)2 Absorption
sada
Emission
1.0
Zn(sada)2 film
Intensity (a.u.)
0.8
0.6
0.4
0.2
0.0 300
350
400
450
500
550
600
650
700
750
Wavelength (nm) Fig. 3. UV–Vis absorption spectra and normalized PL spectra of Zn(sada)2 and sada.
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There are two oxidation maxima (0.704 and 0.863 V) in the oxidation process, because of continuous oxidation of the two ligands. As the potential is swept back, a cation reduction peak appears at 0.643 V, which shows that the molecule oxidation process is somewhat reversible. The reduction peak of Zn(sada)2 is 1.871 V, but this process is irreversible as observed from the cyclic voltammogram. Thus the HOMO and LUMO energy were calculated as 5.20 and 2.63 eV, respectively, with an Eg of 2.57 eV. The optical band gap evaluated from the absorption spectrum is about 2.59 eV, which agrees with that obtained from cyclic voltammetry. 3.5. OLED studies To investigate the emission and electron transporting characteristics of Zn(sada)2 in OLEDs, a device using Zn(sada)2 as the electron transporting/emissive layer with a configuration of ITO/NPB (60 nm)/Zn(sada)2 (60 nm)/ Mg:Ag (device A) was fabricated, where NPB was used as a hole transporting material. As shown in Fig. 4, this device (A) emitted yellow light centered at 544 nm, which
is similar to the PL spectrum of the Zn(sada)2 film. As we expected, inferior EL performance with a maximum brightness of 1470 cd/m2 (at 11.8 V) and a luminous effi-
1.0
Normalized EL Intensity
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Alq3 Zn(sada )2
0.8
Alq3/Zn(sada)2 0.6
0.4
0.2
0.0 400
500
600
700
800
Wavelength (nm) Fig. 4. EL spectra at 10 V of the type A, B and C devices, using Zn(sada)2, Alq3/Zn(sada)2, and Alq3 as electron-transporting layers, respectively.
Fig. 5. Luminance, current and voltage characteristics of the type A, B and C device. (a) Luminance–voltage characteristics; (b) current–voltage characteristic; (c) luminance efficiency–current density characteristics.
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4. Summary
Fig. 6. Energy level of the OLED of ITO/NPB/Alq3/Zn(sada)2/Mg:Ag/ Ag.
ciency of 0.5 cd/A (at 20 mA/cm2) was obtained. The lower EL efficiency is consistent with the significantly lower PL efficiency in solution, and may be even lower in the solid state. Because of the poor luminous property of Zn(sada)2, device A could not indicate anything about the electron transporting characteristics for this complex. Two additional types of devices were designed and fabricated: ITO/NPB (60 nm)/Alq3 (60 nm)/Mg:Ag (device B), and ITO/NPB (60 nm)/Alq3 (30 nm)/Zn(sada)2 (30 nm)/Mg:Ag (device C). The EL spectra of these devices are shown in Fig. 4, and are almost identical at different operating voltages (from 4 to 12 V). Devices B and C both emitted strong green light centered at 520 nm, originating from the intrinsic emission of Alq3, with higher luminescent efficiencies of 2.5 and 3.0 cd/A (at 20 mA/cm2), respectively (Fig. 5(c)). As seen in Fig. 5(b), the current of device C with Zn(sada)2 as the electron transport layer is much higher than that of device B with Alq3 at the same voltage. Furthermore, the turn-on voltages of devices B and C were about 3.5 and 2.5 V, respectively. (Herein, the turn-on voltage is defined as the voltage required to give a brightness of 1 cd/m2.) It is well-known that charge injection and transport are the limiting factors in determining current density and operating voltage. In both devices, Alq3 served as an emitter. The only difference between devices B and C is the electron transporting layer: Alq3 for B and Zn(sada)2 for C. From the energy level diagram (Fig. 6) it is clear that Zn(sada)2 has the higher electron injection barrier with regard to Alq3. Thus, the possibility of enhanced injection efficiency for device C could be ruled out, and the decrease of the turn-on voltage in device C could be ascribed to the better electron transporting property of the Zn complex. Actually, the introduction of the electron-donating dimethylamino substituents led to a relatively high HOMO energy level of Zn(sada)2, which may not have been enough to block the holes within the Alq3 emitting layer. Thus, the redundant current coming from the holes flowing into the electron transporting layer would probably result in a lower QE. Above all, the results indicate that Zn(sada)2 might be a good electron transporting material for OLEDs. Further optimization of the device structure is underway.
In conclusion, we successfully designed and synthesized an azomethin-zinc complex, Zn(sada)2, which was structurally characterized by single-crystal X-ray crystallography. Further thermal analyses proved it has good thermal stability. The study of the vapor-deposited thin film gave strong evidence of Zn(sada)2 possessing excellent film-forming capability. By using Zn(sada)2 as an electron transporting layer, three types of organic light emitting devices were fabricated. Compared with the typical bilayer device of NPB/ Alq3, the device with Zn(sada)2 showed much lower turnon voltage of 2.5 V (compared to 3.5 V for NPB/Alq3 devices), which indicates that Zn(sada)2 may serve as a good electron transporting material in OLEDs. By changing the substituents on the ligand, we have also recently synthesized a series of azomethin-zinc complexes. The preliminary studies demonstrated that the substituents on the ligand can also finely tune the luminescent properties of the corresponding metal complexes. A detailed study is in process. 5. Supplementary material Crystallographic data for the structure of Zn(sada)2 reported in this paper has been deposited with the Cambridge Crystallographic Data Center as Supplementary Publication No. CCDC-252862. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: +44 1223 336 033; e-mail: [email protected]). Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 50403001 and 50325310). The authors thank associate Professor Ruji Wang for the analysis of the crystal structure. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] Y. Hamada, T. Sano, M. Fujita, T. Fujii, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 32 (1993) L514. [3] Y. Hamada, T. Sano, M. Fujita, T. Fujii, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 32 (1993) L511. [4] T.A. Hopkins, K. Meerholz, S. Shaheen, M.L. Anderson, A. Schmidt, B. Kippelen, A.B. Padias, H.K. Hall Jr., N. Peyghambarian, N.R. Armstrong, Chem. Mater. 8 (1996) 344. [5] M. Ghedini, M. La Deda, I. Aiello, A. Grisolia, Inorg. Chim. Acta 357 (2004) 33. [6] G. Yu, Y. Liu, Y. Song, X. Wu, D. Zhu, Synth. Met. 117 (2001) 211. [7] P. Wang, Z. Hong, Z. Xie, S. Tong, O. Wong, C.S. Lee, N. Wong, L. Hung, S. Lee, Chem. Commun. (2003) 1664. [8] N. Nakamura, S. Wakabayashi, K. Miyairi, T. Fujii, Chem. Lett. (1994) 1741. [9] Y. Hamada, T. Sano, H. Fujii, Y. Nishio, H. Takahashi, K. Shibata, Jpn. J. Appl. Phys. 35 (1996) L1339. [10] H. Tanaka, S. Tokito, Y. Taga, A. Okada, J. Mater. Chem. 8 (1998) 1999.
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[11] T.S. Kim, T. Okubo, T. Mitani, Chem. Mater. 15 (2003) 4949. [12] K.Y. Ho, W.Y. Yu, K.K. Cheung, C.M. Che, J. Chem. Soc., Dalton Trans. (1999) 1581. [13] W. Yang, H. Schmider, Q. Wu, Y.S. Zhang, S. Wang, Inorg. Chem. 39 (2000) 2397. [14] S. Wang, Coord. Chem. Rev. 215 (2001) 79. [15] C.J. Walsh, T. Sooksimuang, B.K. Mandal, J. Porphyr. Phthalocyan. 5 (2001) 803. [16] C.F. Lee, K.F. Chin, S.M. Peng, C.M. Che, J. Chem. Soc., Dalton Trans. (1993) 467. [17] Y. Ma, T. Lai, Y. Wu, Adv. Mater. 12 (2000) 433. [18] Y. Kai, M. Morita, N. Yasuoka, N. Kasai, Bull. Chem. Soc. Jpn. 58 (1985) 1631. [19] L.S. Sapochak, F.E. Benincasa, R.S. Schofield, J.L. Baker, K.K.C. Riccio, D. Fogarty, H. Kohlmann, K.F. Ferris, P.E. Burrows, J. Am. Chem. Soc. 124 (2002) 6119. [20] G. Yu, S. Yin, Y. Liu, Z. Shuai, D. Zhu, J. Am. Chem. Soc. 125 (2003) 14816.
[21] Y. Shao, Y. Qiu, W.H. Hu, X.Y. Hong, Adv. Mater. Opt. Electron. 10 (2000) 285. [22] J. Qiao, Y. Qiu, L.D. Wang, L. Duan, Y. Li, D.Q. Zhang, Appl. Phys. Lett. 81 (2002) 4913. [23] M. Dreher, H. Elias, Z. Naturforsch. 42b (1987) 707. [24] T. Sogo, J. Romero, A. Sousa, A. de Blas, M.L. Duran, Z. Naturforsch. 43b (1988) 611. [25] F.A. Bottino, P. Finocchiaro, E. Libertini, G. Mattern, Zeitschrift fur Kristallographie 187 (1989) 71. [26] X.L. Liu, H.L. Chen, F.M. Miao, Youji Huaxue 12 (1992) 522. [27] F.A. Bottino, P. Finocchiaro, Polyhedron 4 (1985) 1507. [28] G.M. Sheldrick, SHELXTL, Version 5.1, Bruker Analytical X-ray System, Inc., Madison, WI, 1997. [29] F.E. Lytle, D.R. Storey, M.E. Juricich, Spectrochim. Acta, Part A 29 (1973) 1357. [30] R.R. Gange, C.A. Koval, G.C. Lisensky, Inorg. Chem. 19 (1980) 2854. [31] P. Strohriegl, J.V. Grazulevicius, Adv. Mater. 14 (2002) 1439.
An azomethin-zinc complex for organic electroluminescence: Crystal structure, thermal stability and optoelectronic properties Junfeng Xie, Juan Qiao *, Liduo Wang, Jing Xie, Yong Qiu Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Beijing 100084, China Department of Chemistry, Tsinghua University, Beijing 100084, China Received 30 March 2005; received in revised form 11 August 2005; accepted 11 August 2005 Available online 26 September 2005
Abstract An azomethin-zinc complex, bis[salicylidene(4-dimethylamino)aniline]zinc(II) (Zn(sada)2) was synthesized and structurally characterized by single-crystal X-ray crystallography. Crystal data for Zn(C15H15N2O)2 was determined as follows: space group, triclinic, P 1; ˚ , b = 16.5008(14) A ˚ , c = 17.5984(15) A ˚ , a = 114.830(2), b = 96.579(2), c = 97.674(2), Z = 4. Through thermal anala = 10.2791(9) A ysis characterization and FT-IR spectra, this complex was proved to have good thermal stability. The vapor-deposited films exhibited uniform and environment-stable morphology. The light emission and charge transporting performance of Zn(sada)2 in organic light emitting diodes (OLEDs) were investigated preliminarily, and the results indicated the superior electron transporting property of this complex. Compared with the typical bilayer device of N,N 0 -diphenyl-N,N 0 -bis(1-naphthyl)-benzidine (NPB)/tris-(8-hydroxyquinoline)aluminum (Alq3), the device with Zn(sada)2 as the electron transporting layer exhibited a much lower turn-on voltage of 2.5 V (it is usually 3.5 V for an NPB/Alq3 device). 2005 Elsevier B.V. All rights reserved. Keywords: Zinc complexes; Organic electroluminescence; X-ray crystal structures; Thermal stability; Optoelectronic properties
1. Introduction Since the first vacuum-deposited OLEDs using tris-(8hydroxyquinoline) aluminum (Alq3) were reported by Tang and VanSlyke [1], organic metal-chelate compounds have drawn a great deal of attention. These materials offer many attractive properties such as the dual function of electrontransporting and light emission, higher environmental stability, and ease of sublimation. By varying the central metal atom (Al, Ga, In, Be, Mg, Ca, Zn, Cu, etc.), a variety of other metal complexes with 8-hydroxyquinoline have been designed and synthesized for use in OLEDs. Amongst these, the zinc complex bis-(8-hydroxyquinoline)zinc (Znq2) was found to have strong yellow fluorescence and *
Corresponding author. Tel.: +86 10 6277 3109; fax: +86 10 6279 5137. E-mail address: [email protected] (J. Qiao).
0020-1693/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.08.018
was demonstrated to be a useful emitter material or electron-transporting hosts in OLEDs [2]. It is well understood that the charge balance in OLEDs is one of the most important factors necessary to achieve high device efficiency. Alq3 is the most widely used electron-transporting material in OLEDs. However, the electron mobility of Alq3 is lower than the hole mobility of traditional hole transporting materials (HTMs), N,N 0 -diphenyl-N,N 0 -bis(3methyl)-benzidine (TPD) and N,N 0 -diphenyl-N,N 0 -bis(1naphthyl)-benzidine (NPB). Zinc complexes have been used in OLEDs for more than a decade [2,3], but the best electroluminescent performance of these materials as emitters is just comparable with that of Alq3. However, in many instances, the electron-transporting mobility of zinc complexes goes beyond that of Alq3. So zinc complexes may be potential candidates to enhance the electron-transporting properties for OLEDs.
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Since 1993, when OLEDs with Znq2 were reported [2], studies of zinc complexes as active materials for OLEDs have focused on improving electron mobility and/or producing a blue shift, relative to Znq2, in the emission wavelength maximum. Zn2+ ion is the only oxidation state of zinc atom and has no unoccupied valence electron orbits. Therefore, the oxidation and reduction of zinc complexes are mainly carried out in the ligands, i.e., luminescence and charge transporting may be ascribed to intraligand electron transition. One simple way to tune the emission wavelength is to manipulate substituents in the 8-hydroxyquinoline rings. 2-Position substituted bis(2-methyl-8-hydroxyquinoline)zinc (ZnMq2) [3], 5-position substituted bis(8-hydroxy-5piperidinylsulfonamidoquinolate)zinc (Zn(QS)2) [4] and (ZnLn)2 Æ 2H2O (L = 5-amido-substituted-8-hydroxyquinolinate ligand) [5], etc., have been reported one after the other. These complexes showed more or less either blue-shifting or red-shifting with reference to Znq2. The other way is to introduce non-oxidate ligands, including Schiff base ligands [2,6,7], polycyclic aromatic ligands [8–11] and polypyridylamines [12–14]. In addition, metalloporphyrin [15] and metal clusters [16,17] were also used to obtain fine EL materials. Most of these zinc complexes have been reported as potential OLED materials. However, few of them have been used for practical OLEDs. One of the aims of molecular design is to obtain devicequality materials. Studying structure–property relationships provides guidance in molecular design for improved molecules. The molecular structure of anhydrous Znq2 was determined in 1985 [18], and this complex was introduced in OLEDs in 1993, but it was not until 2002 that the relationship between the structure of tetrameric Znq2 and its performance in OLEDs was clearly studied by Sapochak et al. [19]. A detailed investigation of zinc(II) 2-(2-hydroxyphenyl)benzothiazolate (Zn(BTZ)2) was just accomplished last year [20], although it was reported as an excellent white EL material as early as 1996 [9]. Recently, our group has focused on metal complexes based on Schiff base ligands, which have proven to be highly efficient luminescent materials for OLEDs [21,22]. Azomethin-zinc complexes are typical metal complexes based on Schiff base ligands and early studies were focused on their structures and reactions [23–27]. As early as 1993, they were reported as potential OLED materials by Hamada et al. [2]. However, there are few investigations on the relationships between the molecular structure and molecular packing, the morphological properties in thin films and the corresponding optoelectronic properties of these complexes. In this study, we used an azomethine ligand bearing an electron-donating substituent, salicylidene(4-dimethylamino)aniline (sada), and synthesized the corresponding azomethin-zinc complex bis[salicylidene(4-dimethylamino)aniline]zinc(II) (Zn(sada)2) (Fig. 1(a)). By using X-ray single crystal diffraction, Zn(sada)2 was structurally characterized as a racemic compound, and this complex has very good thermal stability in both the single crystal and thin film forms. Herein, we describe the molecular and crystal struc-
ture of Zn(sada)2, the thermal stability of the racemic structure and its optoelectronic properties. 2. Experimental 2.1. Synthesis of sada A mixture of salicylaldehyde and N,N-dimethyl-benzene-1,4-diamine (purchased from Aldrich Chemical Co.) in a 1:1 molar ratio was heated and the following recrystallization with ethanol gave an orange-red precipitate with a yield of about 87%. MS (EI) [m/z] 240. 1H NMR (400 MHz, CD3Cl), d = 2.99 (s, 6H), 6.80–6.70 (m, 2H), 6.91 (t, 1H, J = 8.6 Hz), 7.00 (d, 1H, J = 8.7 Hz), 7.27 (d, 2H, J = 9.0 Hz), 7.30 (ddd, 1H, J = 8.6, 7.0, 1.8 Hz), 7.34 (dd, 1H, J = 7.8, 1.8 Hz), 8.61 (s, 1H), 13.73 (s, 1H). 2.2. Synthesis of Zn(sada)2 First, a solution of ZnCl2 (0.6815 g, 5 mmol) in ethanol (30 ml) was gradually added to a solution of sada (1.20 g, 5 mmol) and piperidine (1.0 ml, 10 mmol) in 120 ml ethanol. After the mixture was stirred for 0.5 h while being heated at reflux, and stirred again for 24 h at room temperature, a yellow precipitate was produced. The crude product was collected by filtration and washed with ethanol and finally dried under an infrared lamp. The material was further purified by vacuum train sublimation before analysis and the fabrication of device. Yield: 86%. MS (EI) [m/z] 542. Anal. Calc. for C30H30N4O2Zn: C, 66.24; H, 5.56; N, 10.30. Found: C, 66.29; H, 5.43; N, 10.50%. 1H NMR (400 MHz, [D6]DMSO), d = 2.85 (s, 12H), 6.70–6.62 (m, 8H), 7.22 (dd, 4H, J = 7.0, 1.9 Hz), 7.30 (ddd, 2H, J = 8.7, 7.0, 1.9 Hz), 7.50 (dd, 2H, J = 8.0, 1.8 Hz), 8.78 (s, 2H). IR (cm1): 2898, 1612, 1579, 1531, 1518, 1463, 1442, 1387, 1365, 1348, 1331, 1317, 1255, 1226, 1190, 1180, 1149, 1125, 1064, 1032, 947, 922, 862, 818, 757, 717. 2.3. X-ray crystallography A single crystal of Zn(sada)2 suitable for X-ray crystallography was obtained by gradient-temperature vacuum sublimation. Zn(sada)2 was heated incrementally in zone 1 of a two zone furnace from 220 to 270 C under a N2 atmosphere of about 2 Pa for 48 h. The single crystal was collected at 245 C and had a typical size of 0.3 · 0.2 · 0.2 mm3. The solid-state structure was further confirmed by single-crystal X-ray diffraction analysis. Room temperature (294 ± 1 C) single-crystal X-ray experiments were performed on a Bruker SMART APEX CCD diffractometer equipped with graphite-monochromatized Mo Ka radiation (k = ˚ ). Direct phase determination yielded the positions 0.71073 A of Zn, O, N and most of the C atoms, and the other C atoms were located in successive-difference Fourier syntheses. Hydrogen atoms were generated theoretically and rode on their parent atoms in the final refinement. All non-hydrogen atoms were subjected to anisotropic refinement. The final
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Fig. 1. (a) Molecular formula of Zn(sada)2; (b) ORTEP drawing of molecule 1, with 35% probability ellipsoids, showing the atomic numbering scheme; (c) ORTEP drawing of molecule 2, with 35% probability ellipsoids, showing the atomic numbering scheme.
full-matrix least-square refinement on F2 converged with R1 = 0.0385 and wR2 = 0.0822 for 7715 observed reflections [I P 2r(I)]. The final difference electron density map shows no features. The structural solutions and refinements were performed using the SHELXTL NT ver. 5.10 program package (Bruker, 1997) [28]. 2.4. Equipment 1
H NMR spectra were recorded on a Bruker ARX400 NMR spectrometer with tetramethylsilane as the internal standard. Infrared spectra were recorded on a Nicolet Magna-IR 750 (USA) FT-IR microscope system. Absorption spectra were recorded with a UV–Vis spectrophotometer (HP8452A), and PL spectra were obtained with a fluorospectrophotometer (HITACHI, F4500). Relative PL quantum efficiencies (UPL) were determined from degassed dimethyl formamide (DMF) solutions, and the concentrations of the samples were carefully adjusted so that the optical densities at 390 nm (excitation wavelength) were <0.1 absorption units. PL quantum yields were finally calculated relative to the known value of Alq3 in DMF (UPL = 0.116) [29] and normalized to Alq3. The glass tran-
sition temperature (Tg) and melting point (Tm) were determined by differential scanning calorimetry (DSC) performed on a TA thermal analysis instrument (DSC 2910 Modulated DSC). Cyclic voltammetry measurements were conducted on a model CH 600 voltammetric analyzer with a platinum plate as the working electrode, a silver wire as the pseudo-reference electrode, a polished platinum wire as the counter electrode, and ferrocene as an internal [30], at a scan rate of 50 mV/s. The supporting electrolyte was 0.1 mol/dm3 tetrabutylammonium tetrafluoroborate in DMF. Prior to electrochemical measurements, the solution was deoxygenated with bubbling nitrogen for 15 min. 2.5. Fabrication of EL devices Devices were grown on glass slides precoated with indium tin oxide (ITO) with 30 X/square. The substrates were ultrasonically cleaned in detergent solution for 1 min, followed by thorough rinsing in de-ionized water. They were then rinsed in acetone and methanol, and then dried in pure nitrogen gas. The organic films were deposited layer by layer onto the ITO surface. After deposition of the organic layers, a Mg:Ag (10:1 mass ratio) electrode was deposited
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onto the organic layer without breaking the vacuum. The chamber pressure was below 5 · 104 Pa during the deposition of organic materials and metals. The active area of the devices was 0.35 cm2. Devices were tested in air with ITO as the positive electrode in the forward bias configuration without further encapsulation. Current–voltage–luminance (I–V–L) curves of OLEDs were measured using a Keithley 4200 semiconductor characterization system. The EL spectra were measured with a Photo Research PR650 spectrophotometer. 3. Results and discussion 3.1. Molecular structure The coordination numbers of Zn2+ ion can vary from 2 to 6, and the change of the ligands may result in distinct molecular structures. Recent reports involved dimeric [20], trimeric [11] and tetrameric structures [19]. The molecular structure of Zn(sada)2 was elucidated as a monomer by X-ray crystallography. The crystal data and selected bond lengths and angles are listed in Tables 1 and 2, respectively. A perspective drawing of the structure is shown in Fig. 1. There are two independent molecules, 1 and 2 in the Zn(sada)2 single crystal. It was considered that this phenomenon comes from the restriction in solid state. As seen in Table 2, the averages of Zn–O and Zn–N bond lengths are ˚ , respectively. The zinc ion is about 1.91 and 2.01 A four-coordinate to the Schiff base ligands and has distorted tetrahedron geometry. In both molecules, the bond angle (96.30–97.71) formed by oxygen and nitrogen atoms of the same ligand together with zinc atom (O–Zn–N) is much less than 109.5, because of the tension of the Zn1–O1–C1– Table 1 Crystallographic data Formula C30H30N4O2Zn Formula weight 543.95 Color yellow prism Crystal size (mm3) 0.2 · 0.2 · 0.3 Crystal system triclinic Space group P 1 ðNo. 2Þ ˚) a (A 10.2791(9) ˚) b (A 16.5008(14) ˚) c (A 17.5984(15) a () 114.830(2) b () 96.579(2) c () 97.674(2) ˚ 3) V (A 2636.0(4) Z 4 Dcalc (g cm3) 1.371 l (cm1) 0.966 2hmax () 50 F(0 0 0) 1136 Reflections measured 9216/7715 Rint 0.0148 Goodness-of-fit on F2 1.023 R1, wR2 [I P 2r(I)]a 0.0385, 0.0822 0.0468, 0.0846 R1, wR2 (all data)a P P P P a 2 2 2 R1 ¼ jjF o j jF c jj= jF o j; wR2 ¼ ½ wðF o F c Þ = wðF 2o Þ2 1=2 .
Table 2 ˚ ) and angles () Selected bond lengths (A Molecule 1 Zn(1)–O(1) Zn(1)–O(2) Zn(1)–N(1) Zn(1)–N(3) O(1)–Zn(1)–O(2) O(2)–Zn(1)–N(3) O(1)–Zn(1)–N(3) O(2)–Zn(1)–N(1) O(1)–Zn(1)–N(1) N(1)–Zn(1)–N(3) Molecule 2 Zn(1A)–O(1A) Zn(1A)–O(2A) Zn(1A)–N(1A) Zn(1A)–N(3A) O(1A)–Zn(1A)–O(2A) O(2A)–Zn(1A)–N(3A) O(1A)–Zn(1A)–N(3A) O(2A)–Zn(1A)–N(1A) O(1A)–Zn(1A)–N(1A) N(1A)–Zn(1A)–N(3A)
1.9164(19) 1.908(2) 2.020(2) 2.012(2) 115.88(9) 97.15(8) 113.87(9) 113.00(9) 97.71(8) 120.47(9) 1.907(2) 1.914(2) 2.007(2) 2.006(2) 109.28(9) 96.30(8) 123.28(9) 118.38(9) 96.74(9) 114.40(8)
C6–C7–N1 hexatomic ring. The coordinate bond lengths and bond angles are similar to the mentioned azomethinzinc complexes [23–27]. The distance of the two aniline rings in molecule 2 is closer than that in molecule 1, resulting in an increase of the torsion of the C(8A)–N(1A) and C(23A)– N(3A) bonds, i.e., the dihedral angle of the two aromatic rings of the same ligand in molecule 2 is bigger than that in molecule 1. In fact, the dihedral angles between phenol and aniline groups of the same sada ligand in molecule 1 are 9.7 and 19.8, which are almost planar, while those in molecule 2 are 42.0 and 45.6, respectively. The C2 symmetry of the Zn(sada)2 molecule in the ideal state disappears in single crystals because of the molecule stacking effect.
Fig. 2. Crystal packing view of Zn(sada)2 along the a direction.
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3.2. Thermal stability
3.3. Absorption and photoluminescence
As seen in Fig. 2, the Zn(sada)2 crystal is a racemic compound. Experimental evidence for the stability of this racemic compound was obtained from DSC/TGA thermal analysis. The complex only had a 2% weight loss at 317 C and finally decomposed at temperatures over 400 C. The melting point was obtained at the first DSC heating of about 274 C. By rapid quenching of the melting sample, the glass transition was observed at 112 C for the second heating, and no recrystallization peak was found. In addition, the DSC scans at different heating rates for Zn(sada)2 powders were measured. The single endothermic transition (DHfusion = 54.94 J/mol) peak observed at 274 C narrowed as the heating rate decreased from 20 to 2 C/min. There was no additional thermal transition detected, indicating that no phase transition happened in the heating process. This behavior is in contrast to Alq3, which shows several enthalpic transitions assigned to different polymorphic phases. Furthermore, there were no detectable changes in the FT-IR spectra for powder samples obtained by vacuum train sublimation or deposited from the acetone solution and vapor-deposited film sample on the silicon substrate. These results indicate that the Zn(sada)2 complex is a stable racemic compound. An atomic force microscope (AFM) was utilized to observe the surface morphology of the vapor-deposited films on a silica substrate. The as-deposited film of Zn(sada)2 exhibited an entirely amorphous and uniform surface and a negligible change after being stored for 5 days at room temperature in air. This environment-stable amorphous morphology may come from the non-planar molecular structure caused by the dimethylamino substituents and an increased number of conformers in this molecule which may prevent easy packing of molecules and hence ready crystallization [31].
The UV–Vis absorption and PL spectra of Zn(sada)2 and sada in DMF solutions are shown in Fig. 3. The maximum absorption peak of sada is at 390 nm, and that of Zn(sada)2 is bathochromically shifted to 412 nm. The broad absorption band closely matches the absorption spectrum of the protonated ligand precursor and thus was assigned as a ligand-based p–p* transition. The maximum emission peak of sada is at 553 nm, and that of Zn(sada)2 is at 542 nm which does not change at different excitation wavelengths. It is obvious that the sada ligand acts as absorber and emitter in this molecule. The rotational and vibrational degrees of freedom due to the dimethylamino substitutes could increase the probability of non-radiative relaxation processes, resulting in low fluorescence quantum efficiency of Zn(sada)2 (0.007 in DMF solution). Compared with the solution sample, the absorption and emission of the film exhibited a small red shift of only several nanometers. This suggests that molecules of Zn(sada)2 in the solid film have a weak intermolecular interaction and the bulky dimethylamino substitutes may prevent easy aggregation. 3.4. Electrochemical properties Energy level is an important property for organic materials used in OLEDs. The charge injection (transport) barriers, which are one of the key conditions that influence the operating voltage and efficiency of an OLED, are mainly due to energetic differences between the work function of the electrodes and the HOMO or LUMO energy of organic materials. Commonly, the corresponding energy level can be easily obtained by cyclic voltammetry. The oxidation and reduction potentials of Zn(sada)2 were determined in DMF solution using ferrocene as an internal reference.
1.2 Zn(sada)2 Absorption
sada
Emission
1.0
Zn(sada)2 film
Intensity (a.u.)
0.8
0.6
0.4
0.2
0.0 300
350
400
450
500
550
600
650
700
750
Wavelength (nm) Fig. 3. UV–Vis absorption spectra and normalized PL spectra of Zn(sada)2 and sada.
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There are two oxidation maxima (0.704 and 0.863 V) in the oxidation process, because of continuous oxidation of the two ligands. As the potential is swept back, a cation reduction peak appears at 0.643 V, which shows that the molecule oxidation process is somewhat reversible. The reduction peak of Zn(sada)2 is 1.871 V, but this process is irreversible as observed from the cyclic voltammogram. Thus the HOMO and LUMO energy were calculated as 5.20 and 2.63 eV, respectively, with an Eg of 2.57 eV. The optical band gap evaluated from the absorption spectrum is about 2.59 eV, which agrees with that obtained from cyclic voltammetry. 3.5. OLED studies To investigate the emission and electron transporting characteristics of Zn(sada)2 in OLEDs, a device using Zn(sada)2 as the electron transporting/emissive layer with a configuration of ITO/NPB (60 nm)/Zn(sada)2 (60 nm)/ Mg:Ag (device A) was fabricated, where NPB was used as a hole transporting material. As shown in Fig. 4, this device (A) emitted yellow light centered at 544 nm, which
is similar to the PL spectrum of the Zn(sada)2 film. As we expected, inferior EL performance with a maximum brightness of 1470 cd/m2 (at 11.8 V) and a luminous effi-
1.0
Normalized EL Intensity
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Alq3 Zn(sada )2
0.8
Alq3/Zn(sada)2 0.6
0.4
0.2
0.0 400
500
600
700
800
Wavelength (nm) Fig. 4. EL spectra at 10 V of the type A, B and C devices, using Zn(sada)2, Alq3/Zn(sada)2, and Alq3 as electron-transporting layers, respectively.
Fig. 5. Luminance, current and voltage characteristics of the type A, B and C device. (a) Luminance–voltage characteristics; (b) current–voltage characteristic; (c) luminance efficiency–current density characteristics.
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4. Summary
Fig. 6. Energy level of the OLED of ITO/NPB/Alq3/Zn(sada)2/Mg:Ag/ Ag.
ciency of 0.5 cd/A (at 20 mA/cm2) was obtained. The lower EL efficiency is consistent with the significantly lower PL efficiency in solution, and may be even lower in the solid state. Because of the poor luminous property of Zn(sada)2, device A could not indicate anything about the electron transporting characteristics for this complex. Two additional types of devices were designed and fabricated: ITO/NPB (60 nm)/Alq3 (60 nm)/Mg:Ag (device B), and ITO/NPB (60 nm)/Alq3 (30 nm)/Zn(sada)2 (30 nm)/Mg:Ag (device C). The EL spectra of these devices are shown in Fig. 4, and are almost identical at different operating voltages (from 4 to 12 V). Devices B and C both emitted strong green light centered at 520 nm, originating from the intrinsic emission of Alq3, with higher luminescent efficiencies of 2.5 and 3.0 cd/A (at 20 mA/cm2), respectively (Fig. 5(c)). As seen in Fig. 5(b), the current of device C with Zn(sada)2 as the electron transport layer is much higher than that of device B with Alq3 at the same voltage. Furthermore, the turn-on voltages of devices B and C were about 3.5 and 2.5 V, respectively. (Herein, the turn-on voltage is defined as the voltage required to give a brightness of 1 cd/m2.) It is well-known that charge injection and transport are the limiting factors in determining current density and operating voltage. In both devices, Alq3 served as an emitter. The only difference between devices B and C is the electron transporting layer: Alq3 for B and Zn(sada)2 for C. From the energy level diagram (Fig. 6) it is clear that Zn(sada)2 has the higher electron injection barrier with regard to Alq3. Thus, the possibility of enhanced injection efficiency for device C could be ruled out, and the decrease of the turn-on voltage in device C could be ascribed to the better electron transporting property of the Zn complex. Actually, the introduction of the electron-donating dimethylamino substituents led to a relatively high HOMO energy level of Zn(sada)2, which may not have been enough to block the holes within the Alq3 emitting layer. Thus, the redundant current coming from the holes flowing into the electron transporting layer would probably result in a lower QE. Above all, the results indicate that Zn(sada)2 might be a good electron transporting material for OLEDs. Further optimization of the device structure is underway.
In conclusion, we successfully designed and synthesized an azomethin-zinc complex, Zn(sada)2, which was structurally characterized by single-crystal X-ray crystallography. Further thermal analyses proved it has good thermal stability. The study of the vapor-deposited thin film gave strong evidence of Zn(sada)2 possessing excellent film-forming capability. By using Zn(sada)2 as an electron transporting layer, three types of organic light emitting devices were fabricated. Compared with the typical bilayer device of NPB/ Alq3, the device with Zn(sada)2 showed much lower turnon voltage of 2.5 V (compared to 3.5 V for NPB/Alq3 devices), which indicates that Zn(sada)2 may serve as a good electron transporting material in OLEDs. By changing the substituents on the ligand, we have also recently synthesized a series of azomethin-zinc complexes. The preliminary studies demonstrated that the substituents on the ligand can also finely tune the luminescent properties of the corresponding metal complexes. A detailed study is in process. 5. Supplementary material Crystallographic data for the structure of Zn(sada)2 reported in this paper has been deposited with the Cambridge Crystallographic Data Center as Supplementary Publication No. CCDC-252862. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: +44 1223 336 033; e-mail: [email protected]). Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 50403001 and 50325310). The authors thank associate Professor Ruji Wang for the analysis of the crystal structure. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] Y. Hamada, T. Sano, M. Fujita, T. Fujii, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 32 (1993) L514. [3] Y. Hamada, T. Sano, M. Fujita, T. Fujii, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 32 (1993) L511. [4] T.A. Hopkins, K. Meerholz, S. Shaheen, M.L. Anderson, A. Schmidt, B. Kippelen, A.B. Padias, H.K. Hall Jr., N. Peyghambarian, N.R. Armstrong, Chem. Mater. 8 (1996) 344. [5] M. Ghedini, M. La Deda, I. Aiello, A. Grisolia, Inorg. Chim. Acta 357 (2004) 33. [6] G. Yu, Y. Liu, Y. Song, X. Wu, D. Zhu, Synth. Met. 117 (2001) 211. [7] P. Wang, Z. Hong, Z. Xie, S. Tong, O. Wong, C.S. Lee, N. Wong, L. Hung, S. Lee, Chem. Commun. (2003) 1664. [8] N. Nakamura, S. Wakabayashi, K. Miyairi, T. Fujii, Chem. Lett. (1994) 1741. [9] Y. Hamada, T. Sano, H. Fujii, Y. Nishio, H. Takahashi, K. Shibata, Jpn. J. Appl. Phys. 35 (1996) L1339. [10] H. Tanaka, S. Tokito, Y. Taga, A. Okada, J. Mater. Chem. 8 (1998) 1999.
4458
J. Xie et al. / Inorganica Chimica Acta 358 (2005) 4451–4458
[11] T.S. Kim, T. Okubo, T. Mitani, Chem. Mater. 15 (2003) 4949. [12] K.Y. Ho, W.Y. Yu, K.K. Cheung, C.M. Che, J. Chem. Soc., Dalton Trans. (1999) 1581. [13] W. Yang, H. Schmider, Q. Wu, Y.S. Zhang, S. Wang, Inorg. Chem. 39 (2000) 2397. [14] S. Wang, Coord. Chem. Rev. 215 (2001) 79. [15] C.J. Walsh, T. Sooksimuang, B.K. Mandal, J. Porphyr. Phthalocyan. 5 (2001) 803. [16] C.F. Lee, K.F. Chin, S.M. Peng, C.M. Che, J. Chem. Soc., Dalton Trans. (1993) 467. [17] Y. Ma, T. Lai, Y. Wu, Adv. Mater. 12 (2000) 433. [18] Y. Kai, M. Morita, N. Yasuoka, N. Kasai, Bull. Chem. Soc. Jpn. 58 (1985) 1631. [19] L.S. Sapochak, F.E. Benincasa, R.S. Schofield, J.L. Baker, K.K.C. Riccio, D. Fogarty, H. Kohlmann, K.F. Ferris, P.E. Burrows, J. Am. Chem. Soc. 124 (2002) 6119. [20] G. Yu, S. Yin, Y. Liu, Z. Shuai, D. Zhu, J. Am. Chem. Soc. 125 (2003) 14816.
[21] Y. Shao, Y. Qiu, W.H. Hu, X.Y. Hong, Adv. Mater. Opt. Electron. 10 (2000) 285. [22] J. Qiao, Y. Qiu, L.D. Wang, L. Duan, Y. Li, D.Q. Zhang, Appl. Phys. Lett. 81 (2002) 4913. [23] M. Dreher, H. Elias, Z. Naturforsch. 42b (1987) 707. [24] T. Sogo, J. Romero, A. Sousa, A. de Blas, M.L. Duran, Z. Naturforsch. 43b (1988) 611. [25] F.A. Bottino, P. Finocchiaro, E. Libertini, G. Mattern, Zeitschrift fur Kristallographie 187 (1989) 71. [26] X.L. Liu, H.L. Chen, F.M. Miao, Youji Huaxue 12 (1992) 522. [27] F.A. Bottino, P. Finocchiaro, Polyhedron 4 (1985) 1507. [28] G.M. Sheldrick, SHELXTL, Version 5.1, Bruker Analytical X-ray System, Inc., Madison, WI, 1997. [29] F.E. Lytle, D.R. Storey, M.E. Juricich, Spectrochim. Acta, Part A 29 (1973) 1357. [30] R.R. Gange, C.A. Koval, G.C. Lisensky, Inorg. Chem. 19 (1980) 2854. [31] P. Strohriegl, J.V. Grazulevicius, Adv. Mater. 14 (2002) 1439.