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J. Phys. Chem. B 2005, 109, 8008-8016
Supramolecular Structures and Assembly and Luminescent Properties of Quinacridone Derivatives Kaiqi Ye,† Jia Wang,† Hui Sun,† Yu Liu,† Zhongcheng Mu,† Fei Li,† Shimei Jiang,† Jingying Zhang,† Hongxing Zhang,‡ Yue Wang,*,† and Chi-Ming Che§ Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China, State Key Laboratory of Theoretical and Computation Chemistry, Jilin UniVersity, Changchun 130023, People’s Republic of China, and Department of Chemistry and The HKU-CAS Joint Laboratory on New Materials, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong S.A.R., People’s Republic of China ReceiVed: December 5, 2004; In Final Form: February 12, 2005
The synthesis and single-crystal X-ray structures of two quinacridone derivatives, N,N′-di(n-butyl)quinacridone (1) and N,N′-di(n-butyl)-1,3,8,10-tetramethylquinacridone (2), are reported, and the 1H NMR, absorption, photoluminescent (PL), and electroluminescent (EL) characteristics are presented. Both these crystal structures are characterized by intermolecular π‚‚‚π and hydrogen bonding interactions. The intermolecular π‚‚‚π interactions lead to the formation of molecular columns in the solids of 1 and 2, and the interplanar contact distances between two adjacent molecules are 3.48 and 3.55 Å, respectively. Crystals of 1 display shorter intermolecular π‚‚‚π contacts and higher density than 2. These results suggest that tighter intermolecular interactions exist in 1. The 1H NMR, absorption, and PL spectra of 1 and 2 in solutions exhibit concentrationdependent properties. The PL quantum yields of 1 in solutions decrease more quickly with the increase of concentration compared to that of 2 in solutions. For solid thin films of Alq3:1 (Alq3 ) tris(8hydroxyquinolinato)aluminum), emission intensities dramatically decrease and obvious red shifts are observed when the dopant concentration is above 4.2%, while for films of Alq3:2, a similar phenomenon occurs when the concentration is above 6.7%. EL devices with Alq3:1 as emitting layer only show high efficiencies (20.314.5 cd/A) within the narrow dopant concentration range of 0.5-1.0%. In contrast, high efficiencies (21.512.0 cd/A) are achieved for a wider dopant concentration range of 0.5-5.0% when Alq3:2 films are employed as emitting layer. The different PL and EL concentration-dependent properties of the solid thin films Alq3:1 and Alq3:2 are attributed to their different molecular packing characteristics in the solid state.
Introduction Quinacridone and its derivatives (QA) are widely used organic pigments that display excellent fastness properties as well as pronounced photovoltaic and photoconductive activities.1-4 High photoluminescent efficiency in dilute solution combined with good electrochemical stability in the solid state has allowed the fabrication of high-performance organic light emitting devices (OLEDs) based on QA. A great deal of effort has been invested in optimization of QA-based devices in terms of efficiency and lifetime.5-8 Moreover, some studies on QA have been devoted to investigating the assembly and structural properties of this class of pigments. Mu¨llen and co-workers have carefully studied the phase-formation behavior and aggregation properties of some soluble quinacridones.9,10 Nakahara and co-workers have synthesized quinacridone derivatives with four alkyl chains and used them in Langmuir-Blodgett films to control the orientation and packing of chromophores.3,4 Jones and co-workers have reported the single-crystal structure of quinacridone.11 We have reported * Corresponding author. Fax: +86-431-5193421. E-mail: yuewang@ jlu.edu.cn. † Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin University. ‡ State Key Laboratory of Theoretical and Computation Chemistry, Jilin University. § Department of Chemistry and The HKU-CAS Joint Laboratory on New Materials, The University of Hong Kong.
the aggregation behavior of a series of N,N′-dialkyl-substituted quinacridones at highly oriented pyrolytic graphite (HOPG).12 Recently, we have found that the coadsorption of quinacridone derivatives and fatty acids at the liquid/graphite interface leads to the formation of chiral arrays for quinacridone derivatives with methyl groups and chiral domains for that without methyl groups, respectively.13 We also demonstrated that ordered monolayers of N,N′-di(n-butyl)quinacridone were prepared by vacuum deposition on Ag(110), Au(111), and Cu(110) surfaces.14 To fully exploit the potential of these molecules in organic material based devices, it is necessary to understand, and if possible to control, the supramolecular packing in the solid state and assembly in solution or on substrates for a given chromophore. Recently, we carried out studies aimed at understanding how supramolecular organization in solid state and assembly in solution system affects the photoluminescent and electroluminescent properties of quinacridone derivatives. Here we report the synthesis, supramolecular structures, assembly, and luminescent properties of two quinacridone derivatives, 1 and 2 (Scheme 1). Experimental Section Materials. Quinacridone was purchased from Tokyo Kasei Kogyo Co. 3,5-Dimethylaniline (Acros), 1-bromobutane (Acros), diethyl-2,5-dihydroxy-1,4-dicarboxylate (Aldrich), copper phthalocyanine (CuPc) (Aldrich), and LiF (Aldrich) were used
10.1021/jp0444767 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/30/2005
Luminescent Properties of Quinacridone Derivatives SCHEME 1: Synthesis Procedure of 1 and 2
J. Phys. Chem. B, Vol. 109, No. 16, 2005 8009 excitation wavelength (360 nm) referenced to quinine sulfate in sulfuric acid aqueous solution (φ ) 0.546), and calculated according to the following equation:
Φunk ) Φstd
as received. Tris(8-hydroxyquinolinato)aluminum (Alq3) was purchased from Aldrich and purified by vacuum sublimation. 1,3,8,10-Tetramethylquinacridone10 and N,N′-di(R-naphthyl)N,N′-diphenyl(1,1′-biphenyl)-4,4′-diamine) (NPB)15 were synthesized according to literature procedures. Instrumentation. 1H NMR spectra were recorded on Bruker AVANVE 500 MHz spectrometer with tetramethylsilane as the internal standard. Mass spectra were recorded on a GC/MS mass spectrometer. Absorption spectra were obtained using a PE UVvis Lambda 20 spectrometer. Element analyses were performed on a Flash EA 1112 spectrometer. Photoluminescence spectra were collected by a Shimadzu RF-5301PC spectrophotometer. Electroluminescent spectra and Commission Internationale de l’Eclairage (CIE) coordinates were measured on a PR-650 Spectrascan Cholorimeter. Electroluminescence (EL). The devices were grown on glass substrates, which were precoated with indium tin oxide (ITO). The ITO glass substrates were routinely cleaned by ultrasonic treatment in detergent solution, followed by rinsing in acetone, boiling in 2-propanol, rinsing in methanol, and finally rinsing in deionized water. The glass was dried in a vacuum oven between each cleaning step above. The devices were fabricated by successive vacuum deposition of organic materials onto ITOcoated glass substrate. Prior to the deposition, all the organic materials were purified by vacuum sublimation method. The devices consist of a layer of CuPc to aid hole injection from ITO, a layer of the hole transport material NPB, and a layer of LiF to enhance electron injection from aluminum cathode. The Alq3:1 and Alq3:2 films were fabricated by vacuum coevaporation in a vacuum chamber with a base pressure of 4 × 10-6 Torr. The deposition sources Alq3 and 1 or 2 were set in separate quartz crucibles whose temperatures were independently controlled by coil heaters. EL spectra, luminance, and currentvoltage characteristics were measured at room temperature under ambient atmosphere. Photoluminescence (PL) Measurements. The solid thin films deposited on quartz substrate were employed to record the absorption and emission spectra. The room-temperature luminescence quantum yields were measured at a single
( )( )( ) Iunk Astd ηunk Aunk Istd ηstd
2
where Φunk is the radiative quantum yield of the sample; Φstd is the radiative quantum yield of the standard; Iunk and Istd are the integrated emission intensities of the sample and standard, respectively; Aunk and Astd are the absorptions of the sample and standard at the excitation wavelength, respectively; and ηunk and ηstd are the indexes of refraction of the sample and standard solutions (pure solvents were assumed), respectively. N,N′-Di(n-butyl)quinacridone (1). Under nitrogen, sodium hydride (2.0 g, 0.07 mol) was added to a suspension of quinacridone (3.1 g, 0.01 mol) in 50 mL of dry tetrahydrofuran (THF). The mixture was heated to reflux for 1 h and then 1-bromobutane (4.1 g, 0.03 mol) was added. The reaction mixture was continued to heat to reflux overnight. After distilling off excess 1-bromobutane and THF, methanol (50 mL) was added dropwise to the reaction mixture. The resulting suspension was stirred for 1 h. The generated orange precipitate was filtered and washed with methanol. After drying, crude 1 was obtained in 95% yield, which was purified by column chromatography using silica gel with chloroform as eluent to yield 3.9 g (91%) of 1. 1H NMR (CDCl3): δ ) 8.75 (s, 2H), 8.55 (dd, J ) 8.0 Hz, J ) 1.0 Hz, 2H), 7.74 (td, J ) 7.75 Hz, J ) 1.50 Hz, 2H), 7.50 (d, J ) 9.0 Hz, 2H), 7.25 (t, J ) 7.5 Hz, 2H), 4.5 (t, J ) 8.0 Hz, 4H), 2.0 (m, 4H), 1.67 (m, 4H), 1.12 (t, 7.5, 6H). MS: m/z 424.0 [M]+; element analysis calcd (%) for C28H28N2O2 (424.0): C 79.22, H 6.65, N 6.60; found: C 79.01, H 6.90, N 6.33. N,N′-Di(n-butyl)-1,3,8,10-tetramethylquinacridone (2). 1,3,8,10-Tetramethylquinacridone (3.7 g, 0.01 mol) reacted with 1-bromobutane (4.1 g, 0.03 mol) according to the procedure described for the synthesis of 1 to yield 4.1 g (85%) of 2. 1H NMR (CDCl3): δ ) 8.66 (s, 2H), 7.15 (s, 2H), 6.87 (s, 2H), 4.45 (t, J ) 5 Hz, 4H), 3.00 (s, 6H), 2.50 (s, 6H), 2.00 (m, 4H), 1.65 (m, 4H), 1.11 (t, J ) 7.5 Hz, 6H). MS: m/z 480.6 [M]+; element analysis calcd (%) for C32H36N2O2 (480.6): C 79.96, H 7.55, N 5.83; found: C 79.75, H 7.63, N 5.55. X-ray Crystallography. Single crystals suited for X-ray structural analysis were obtained by slow diffusion of petroleum ether into chloroform solutions of 1 or 2. Diffraction data were collected on a Rigaku R-AXIS RAPID diffractometer (Mo KR radiation, graphite monochromator) in the Ψ rotation scan mode. The structural determination was done with direct methods by using SHELXL 5.01v and refinements with full-matrix least squares on F2. The positions of hydrogen atoms were calculated and refined isotropically. Results and Discussion Synthesis and Characterization. The synthetic procedures of 1 and 2 are displayed in Scheme 1, which generally affords superior yields for substituted quinacridones. 1H NMR spectra of 1 and 2 both have nine sets of proton resonances, which demonstrates that the molecular geometries of 1 and 2 are all centrosymmetric in solution. X-ray Structures and Molecular Packing Properties. The molecular structures of 1 and 2 are shown in Figure 1. Singlecrystal X-ray diffraction studies reveal planar geometry for the rigid π-cores of molecules 1 and 2. Table 1 summarizes relevant structural parameters of 1 and 2. For crystal 1, the molecular
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Figure 2. Stacking diagram showing π‚‚‚π interactions in 1.
Figure 1. ORTEP views of 1 (top) and 2 (bottom) (30% probability displacement ellipsoids).
TABLE 1: X-ray Crystallographic Data for 1 and 2 compound
1
2
chemical formula formula weight crystal size [mm] crystal system space grouop a [Å] b [Å] c [Å] R [deg] β [deg] γ [deg] V [Å3] Z Fcalcd [g cm-3] µ [mm-1] 2θmax temperature [K] F[000] no. unique data (Rint) no. obsd data [I > 2σ(I)] R Rw GOF
C28H28N2O2 424.52 0.48 × 0.18 × 0.17 monoclinic P21/n 7.4962(2) 14.7065(4) 19.2131(4) 90 93.091(2) 90 2115.03(9) 4 1.333 0.084 27.46 293(2) 904 4430 (0.024) 2808 0.0491 0.1288 0.973
C32H36N2O2 480.63 0.42 × 0.09 × 0.08 monoclinic P21/c 5.0894(1) 16.0916(13) 15.8959(4) 90 98.732(2) 90 1286.73(11) 2 1.241 0.077 27.48 293(2) 516 2883 (0.064) 818 0.0619 0.1559 0.729
geometry is noncentrosymmetric and the two n-butyl chains do not favor the all-trans conformation and are bent. This may be attributed to the fact that crystal-packing forces are overriding the preference for the all-trans conformation. For crystal 2, the molecular geometry is centrosymmetric and the two n-butyl
Figure 3. View along the cell a-axis of hydrogen-bonded molecular sheet in 1.
chains adopt a regular all-trans conformation. In the solid of 1, there is strong face-to-face intermolecular π‚‚‚π stacking accompanied by hydrogen bonding interactions, which results in the formation of one-dimensional molecular columns (Figure 2). In each column intermolecular hydrogen bonding interactions occur between CdO and CH2 groups and the molecules of 1 slip alternately to lead to a “sawtooth” arrangement. The interplanar distance between adjacent molecules is 3.48 Å, which is similar to the values generally observed in molecular organic conducting materials.16,17 Although the molecules are “stepped” relative to one another, the step direction is parallel to the long axis of the conjugated ring system, so that there remains appreciable overlap of π-electron density on adjacent molecules. In solid 1 the molecular columns are held together through intermolecular hydrogen bonding interactions between oxygen atoms on CdO groups and hydrogen atoms on outer phenyl rings (Figure 3). The hydrogen bonding distance for CdO‚‚‚H is 2.523 Å. One-dimensional molecular stacking columns are also found in solid 2, and the molecules adopt a “straircase” arrangement in a molecular column (Figure 4), which is different from that found in solid 1. The separation distance between adjacent molecular planes of 2 is 3.55 Å, which is longer than that of solid 1. Intermolecular hydrogen bonding interactions between the oxygen atom on CdO group and the hydrogen atom on CH3 group occur in solid 2, and the hydrogen bonding distance of CdO‚‚‚H is 2.352 Å (Figure 5). It is worth noting that the crystal densities of 1 and 2 are 1.333 and 1.241 g cm-3, respectively. 1 exhibits higher density than 2. Table 2 illustrates the selected bond lengths and angles of 1 and 2. Using the Cerius
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Figure 6. Concentration-dependent 1H NMR measurements of 1 in CDCl3. Figure 4. Stacking diagram showing π‚‚‚π interactions in 2.
Figure 7. Concentration-dependent 1H NMR measurements of 2 in CDCl3.
Figure 5. View along the cell a-axis of hydrogen-bonded molecular sheet in 2.
TABLE 2: Selected Bond Lengths [Å] and Angles [deg] of 1 and 2 1
2
N(1)-C(7) N(1)-C(8) N(1)-C(11) O(1)-C(1) C(1)-C(2) C(1)-C(9) C(11)-C(12) C(12)-C(13) C(13)-C(14)
1.388(2) 1.392(2) 1.471(2) 1.238(2) 1.459(2) 1.475(2) 1.523(2) 1.525(3) 1.520(3)
N(1)-C(7) N(1)-C(8) N(1)-C(11) O(1)-C(1) C(1)-C(2) C(1)-C(9) C(11)-C(12) C(12)-C(13) C(13)-C(14)
1.379(4) 1.382(4) 1.468(4) 1.227(4) 1.451(5) 1.469(5) 1.522(4) 1.496(5) 1.513(5)
C(7)-N(1)-C(8) C(7)-N(1)-C(11) C(8)-N(1)-C(11) O(1)-C(1)-C(2) O(1)-C(1)-C(9) N(1)-C(11)-C(12) C(11)-C(12)-C(13) C(14)-C(13)-C(12)
120.8(1) 120.4(1) 118.8(1) 122.9(2) 121.8(2) 112.9(1) 112.4(1) 113.8(2)
C(7)-N(1)-C(8) C(7)-N(1)-C(11) C(8)-N(1)-C(11) O(1)-C(1)-C(2) O(1)-C(1)-C(9) N(1)-C(11)-C(12) C(13)-C(12)-C(11) C(12)-C(13)-C(14)
121.0(3) 120.9(3) 118.0(3) 123.7(4) 119.7(4) 114.0(3) 111.5(3) 113.9(4)
2 program package, the crystal packing energies of 1 and 2 were calculated. The calculated crystal packing energies of 1 and 2 are 274 and 233 kcal/mol, respectively. Crystal 1 has more crystal packing energy than 2, which is in agreement with the X-ray crystal studies described above. The experimental and
theoretical results for 1 and 2 reveal that intermolecular interaction in solid 1 is stronger than that in solid 2, suggesting that molecules of 1 should more easily undergo molecular aggregation compared with those of 2. Intermolecular π‚‚‚π and hydrogen bonding interactions are found in both solids 1 and 2, which reveals that there are similar molecular packing features in these two solids. However, 1 and 2 exhibit obviously different molecular packing parameters and crystal packing energies. The differences between the molecular packing properties of 1 and 2 suggest that 1 and 2 should exhibit different PL and EL properties, which will be discussed in the following paragraphs. 1H NMR, Absorption, and Photoluminescent Properties. Figure 6 shows the 1H NMR spectra of 1, which were recorded over a range of concentrations at room temperature. Increasing the concentrations of 1 leads to the upfield shifts of aromatic protons. The changes in the chemical shifts are attributed to the formation of face-to-face intermolecular π‚‚‚π stacking aggregates of 1 in solution. Experimental and theoretical studies demonstrated that face-to-face π‚‚‚π stacking interaction could result in the upfield shifts of aromatic protons.18-20 The concentration-dependent 1H NMR spectroscopic property of 2 also displays a tendency of aggregation in solution (Figure 7). The signals of C and C′ in Figure 7 are due to the coupling between 13C and H in CDCl3. Some detailed concentration- and temperature-dependent NMR experiments have been carried out for the solution systems of 1 and 2. Experimental results revealed that 1 and 2 exhibit π‚‚‚π stacking aggregates in solution, which is similar to the aggregate property in solid. The detailed results will be reported elsewhere. Figure 8 displays the normalized
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Figure 10. Absorption spectra of solid thin films of 1 and 2. Figure 8. Concentration-dependent normalized absorption spectra of 1 in chloroform.
Figure 11. Concentration-dependent PL spectra of 1 in chloroform.
Figure 9. Concentration-dependent normalized absorption spectra of 2 in chloroform.
absorption spectra of 1 at different concentrations. The electronic absorption of 1 exhibits obvious coupling with the vibronic features corresponding to V ) 0 to V′ ) 0, 1, and 2. The absorption spectra exhibit a marked blue shift with the increase of concentration. A0f1/A0f0 (ratio of the intensities of A0f1 to A0f0) increases obviously when the concentration is above 8.0 × 10-4 M. This blue shift is due to molecule 1 monomers assembling into higher oligomers in solution. Experimental and theoretical works have revealed that face-to-face stacking interaction of aromatic π-systems could result in an increase in 0f1 and 0f2 transitions, and in turn leaded to a blue shift of the absorption band.18 The absorption spectra of 2 solutions exhibit a concentration-dependent property similar to that of 1 (Figure 9). It is worth noting that the turning point of 1, at which concentration absorptions change obviously, is lower than that of 2, suggesting that 1 more easily forms aggregates in solution than 2. The stronger aggregation tendency of 1 relative to 2 can be attributed to the diminished steric hindrance in the former. The absorption spectra of dopant solid thin films of Alq3:1 and Alq3:2 are dominated by the host material of Alq3. In this paper we present the absorption spectra of the thin films of pure 1 and 2, which have absorption bands around 475 and 525 nm, respectively (Figure 10). Figure 11 presents the PL spectra of 1 at different concentrations. Upon increasing concentration, a remarkable red shift of the emission bands is observed. For dilute solution (3.0 × 10-5 M) of 1, the PL spectrum has a sharp emission peak at 538 nm companied by a weak shoulder around 573 nm. When concen-
tration varies from 3.0 × 10-5 to 2.4 × 10-4 M, the emission intensity around 540 nm increases progressively and reaches a maximum value at a concentration of 2.4 × 10-4 M. A further increase of concentration leads to the decrease of emission intensity around 540 nm. When the concentration is below 2.4 × 10-4 M, the PL spectra are governed by the monomer fluorescence of molecules of 1 and the emission intensity increases with the increase of free monomer of molecule 1 in solution. Aggregate formation induces the decrease of emission intensity around 540 nm when the concentration is above 2.4 × 10-4 M, and the PL spectra gradually display the molecular aggregate emission with increase of concentration. When the concentration is 3.9 × 10-3 M, the emission intensity ratio between the peak at 557 nm and that at 573 nm is 0.8. A further increase of concentration results in complete domination of emission around 577 nm. The PL spectra of 2 show an analogous concentration-dependent property compared with those of 1 (Figure 12), which supports the formation of faceto-face aggregates in concentrated solutions. It is worthwhile to mention that reabsorption of emitted light is also another possible reason leading to the observed variations in the emission band maximum and intensity upon concentration changes in solution systems. Table 3 summarizes the PL quantum yields of the 1 and 2 solutions under different concentrations. The PL quantum yields of 1 and 2 in solution decrease with the increase of concentration. The PL quantum yield of 1 is more sensitive to concentration and drops more quickly with the decrease of concentration than that of 2. These results indicate that 1 has stronger tendency for aggregation in solution compared with 2. The PL spectra of the solid thin films with different dopant concentrations of 1 and 2 in Alq3 display concentrationdependent features, and the emission intensities decrease with
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Figure 12. Concentration-dependent PL spectra of 2 in chloroform.
Figure 15. (top) PL spectra of doping thin films of Alq3:1 (1.6%) and solid 1. (bottom) PL spectra of doping thin films of Alq3:2 (2.0%) and solid 2.
Figure 13. Concentration-dependent PL spectra of doping thin films of Alq3:1.
Figure 14. Concentration-dependent PL spectra of doping thin films of Alq3:2.
the increase of dopant concentration (Figures 13 and 14). Upon the increase of dopant concentration in the solid thin films, molecular aggregation occurs. We cannot measure the PL quantum efficiencies of solid thin films due to the limitations of our fluorescent spectrometer. These results demonstrate that high concentration could result in fluorescence quenching. For Alq3:1 thin films, the emission intensity decreases quickly on increasing the dopant concentration and becomes very weak when the concentration is up to 4.2%. However, the emission intensity decreases more slowly with the increase of concentration for Alq3:2 thin films and is maintained at a higher level when the concentration reaches 6.7%. This means that, for 2, a higher concentration of dopant is required to reach the turning point and the thin films with dopant emitter 2 would exhibit
TABLE 3: PL Quantum Yields of 1 and 2 in Solution under Different Concentrations 1
2
concn (M)
PL yield (%)
concn (M)
PL yield (%)
4.0 × 10-4 2.0 × 10-4 1.0 × 10-4 5.0 × 10-5 5.0 × 10-6
20 48 85 90 91
4.0 × 10-4 2.0 × 10-4 1.0 × 10-1 5.0 × 10-5 5.0 × 10-6
41 72 87 88 90
higher PL efficiency for a wider concentration range compared with dopant emitter 1. The emission wavelengths of pure solid powders of 1 and 2 are all red shifted with respect to those of the films in which 1 and 2 are doped in Alq3 at low concentration. We note that the red shift is much greater for 1, presumably because there are stronger intermolecular interactions for molecules 1 than 2 (Figure 15). The different concentration-dependent PL properties of the thin film systems of 1 and 2 suggest that 1 and 2 will display different EL properties, which will be presented later. For quinacridone derivatives with no substitutes on the two N atoms, strong intermolecular hydrogen bond interactions of CdO‚‚‚H-N can form easily, which are crucial for aggregation formation.21-23 Although alkylation on the two N atoms of 1 and 2 eliminates the possibility of a strong hydrogen bond between CdO and H-N groups, obvious aggregations exist in these systems. Previous experimental and theoretical studies demonstrated that intermolecular π‚‚‚π stacking and hydrogen bonding interactions in molecular organic materials could result in the formation of delocalized excitons or excimers that can decrease the PL and EL efficiencies.24 The emission-quenching phenomena for higher concentration solutions and solid thin films are attributed to intermolecular π‚‚‚π stacking and hydrogen bonding interactions. It was demonstrated that, to
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Figure 16. Molecular structures used for EL device fabrication.
Ye et al.
Figure 18. Voltage-current (9) and voltage-luminance (2) characteristics of the device [ITO/CuPc/NPB/Alq3:1 (0.5%)/LiF/Al].
Figure 19. Voltage-current (9) and voltage-luminance (2) characteristics of the device [ITO/CuPc/NPB/Alq3:2 (0.5%)/LiF/Al]. Figure 17. EL spectra of devices with structures of [ITO/CuPc/NPB/ Alq3:1 (0.5%)/LiF/Al] and [ITO/CuPc/NPB/Alq3:2 (0.5%)/LiF/Al].
achieve highly efficient EL devices based on dopant thin films with quinacridone derivatives, the dopant concentration of quinacridone derivatives must be kept appropriately low.5-7 Electroluminescent Properties. The devices were grown on glass that was precoated with indium tin oxide (ITO). The EL devices were fabricated by high vacuum (5 × 10-6 Torr) thermal evaporation techniques. The device structure consists of a layer of copper phthalocyanine (CuPc), a layer of hole transport material, N,N′-di(R-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)4,4′-diamine (NPB), and a layer of Alq3 doped with varying weight concentrations of 1 or 2. Figure 16 shows the molecular structures of materials used in this study. Green luminescent complex Alq3 was employed as host material to construct the emitting layer. Films with different concentrations of 1 or 2 by weight were employed as emitting layers. The EL device structure is [ITO/CuPc (150 Å)/NPB (600 Å)/Alq3:1 or 2 (500 Å)/LiF (10 Å)/Al (2000 Å)]. The EL spectra (Figure 17) of devices with 1 or 2 displayed the expected emission features, indicating that energy transfer from Alq3 to 1 or 2 occurred completely. The emitting color of the devices was green, and the Commission Internationale de l’Eclairage (CIE) coordinates are (CIE: 0.32, 0.65) and (CIE: 0.34, 0.63) for 1 and 2, respectively. The peak luminance efficiency of device 1, in which the dopant concentration of 1 was 0.5%, was 20.3 cd/A with a luminance of 132.0 cd/m2 at a drive voltage of 6.5 V. The device showed a luminance of 2900 cd/m2 and an EL efficiency of 14.5 cd/A at a current density of 20 mA/cm2. The current density-voltage and
Figure 20. EL efficiency-voltage characteristics of devices [ITO/ CuPc/NPB/Alq3:1 (0.5%)/LiF/Al] and [ITO/CuPc/NPB/Alq3:2 (0.5%)/ LiF/Al].
luminance-voltage characteristics of device 1 are displayed in Figure 18. For device 2, in which the concentration of 2 was 0.5%, the maximum efficiency of 21.5 cd/A with a luminance of 1453 cd/m2 at a drive voltage of 8.5 V was recorded. Device 2 showed a luminance of 4100 cd/m2 and an EL efficiency of 20.9 cd/A at a current density of 20 mA/cm2. The current density-voltage and luminance-voltage characteristics of device 2 are displayed in Figure 19. The efficiency-voltage characteristics of devices 1 and 2 are presented in Figure 20, which clearly shows that the high efficiencies for devices 1 and 2 are maintained for a wide range of drive voltages. The EL efficiency-concentration characteristics of 1 and 2 are compared in Figure 21. Although the highest EL efficiencies of 1 and 2 are similar, the EL efficiencies of the two materials
Luminescent Properties of Quinacridone Derivatives
Figure 21. Concentration-dependent EL efficiency of devices [ITO/ CuPc/NPB/Alq3:1/LiF/Al] and [ITO/CuPc/NPB/Alq3:2/LiF/Al].
display obviously different concentration-dependent properties. When 1 is employed as emitting material, the EL efficiencies dramatically decrease with the increase of dopant concentration of 1. The OLEDs only exhibit high efficiency within a quite narrow concentration region. To obtain highly efficient devices, the dopant concentration of 1 must be accurately controlled within a narrow range. Because the efficiency of the devices with 1 is very sensitive to dopant concentration, reproducing the device fabrication process becomes very hard and precise fabrication technologies must be employed. When 2 is used as dopant emitter, the EL efficiencies maintain at a high level within a relatively wide range of dopant concentration. The devices with concentrations of 0.5%, 1.0%, and 2.0% of 2 display efficiencies of 21.5, 19.7, and 19.4 cd/A, respectively. Within the concentration range from 0.5% to 2.0%, the EL efficiency remains high and is relatively insensitive to concentration. If the dopant concentration is controlled in this range, high-performance EL devices will be achieved easily. The fabrication process of devices based on 2 can be controlled easily, and the reproducibility of the fabrication process is improved. These results demonstrate that 2 has some advantage for the fabrication of highly efficient devices compared with 1. We note that 1 and 2 have quite similar molecular structures; however, their concentration-dependent EL properties are significantly different. We believe that the difference can be attributed to the different molecular packing characteristics. The solid 2 displays less dense packing properties, which is thought to decrease aggregation and enhance EL efficiency for higher concentrations. Conclusions Two alkyl-substituted quinacridone derivatives 1 and 2 have been synthesized, and their supramolecular structures are reported. Detailed studies of their 1H NMR, absorption, PL, and EL characteristics are presented. It is demonstrated that the solids of 1 and 2 are characterized by intermolecular π‚‚‚π stacking and hydrogen bonding interactions. Experimental and theoretical studies reveal that 1 exhibits stronger intermolecular interactions in the solid state compared with 2. In solution, the 1H NMR, absorption, and PL spectra of 1 and 2 display concentrationdependent properties, which indicate the formation of face-toface assemblies in solutions. PL quantum yield-concentration characteristics demonstrate that 1 has stronger concentrationdependent quenching properties than 2. The thin films of Alq3:1 and Alq3:2 display PL quenching when the concentration of 1
J. Phys. Chem. B, Vol. 109, No. 16, 2005 8015 and 2 reaches 4.2% and 6.7%, respectively. Dopants 1 and 2 can be used to fabricate highly efficient EL devices. The EL devices based on 1 display high efficiency (>10 cd/A) within only a narrow concentration range from 0.5% to 1.0%, while EL devices based on 2 exhibit high efficiency (>10 cd/A) for a wider concentration range, from 0.5% to 5.0%. The different PL and EL concentration-dependent properties of solid thin films Alq3:1 and Alq3:2 are attributed to the different molecular packing characteristics of 1 and 2 in the solid state. It is demonstrated that molecular packing properties have a dramatic effect on the PL and EL properties of solid thin films, and it is possible to optimize the PL and EL performances of quinacridone derivatives through the regulation of molecular structure and molecular packing features. When 2 is employed as dopant, the device fabrication process is easier to control and the process reproducibility is improved. The methyl substitutes partly suppressed the aggregation of quinacridone core, suggesting that the introduction of more bulky steric hindrances should totally suppress the aggregation of quinacridone core. Further control of molecular packing in the solid state and assembly property in solution should be possible through the optimization of molecular structure. Stability studies of devices based on 1 and 2, and the synthesis and properties of other quinacridone derivatives with bulky substituents, are in progress. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50225313, 20174014, 5001161951) and the Major State Basic Research Development Program (2002CB613401). Supporting Information Available: Crystallographic information (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hiramoto, M.; Kawase, S.; Yokoyama, M. Jpn. J. Appl. Phys. 1996, 35, L349. (2) Shichiri, T.; Suezaki, M.; Inoue, T. Chem. Lett. 1992, 1717. (3) Nakahara, H.; Kitahara, K.; Nishi, H.; Fukuda, K. Chem. Lett. 1992, 711. (4) Nakahara, H.; Fukuda, K.; Ikeda, M.; Kitahara, K.; Nishi, H. Thin Solid Films 1992, 210/211, 555. (5) Shi, J.; Tang, C. W. Appl. Phys. Lett. 1997, 70, 1665. (6) Shaheen, S. E.; Kippelen, B.; Peyghambarian, N.; Wang, J. F.; Anderson, J. D.; Mash, E. A.; Lee, P. A.; Armstrong, N. R. J. Appl. Phys. 1999, 85, 9739. (7) Aziz, H.; Popovic, Z. D.; Hu, N. X. Appl. Phys. Lett. 2002, 81, 370. (8) Gross, E. M.; Anderson, J. D.; Slaterbeck, A. F.; Thayumanavan, S.; Barlow, S.; Zhang, Y.; Marder, S. R.; Hall, H. K.; Nabor, M. F.; Wang, J. F.; Mask, E. A.; Armstrong, N. R.; Wightman, R. M. J. Am. Chem. Soc. 2000, 122, 4972. (9) Keller, U.; Mu¨llen, K.; De Feyter, S.; De Schryver, F. C. AdV. Mater. 1996, 8, 490. (10) De Feyter, S.; Gesquie`re, A.; De Schryver, F. C.; Keller, U.; Mu¨llen, K. Chem. Mater. 2002, 14, 989. (11) Potts, G. D.; Jones, W.; Bullock, J. F.; Andrews, S. J.; Maginn, S. J. Chem. Commun. 1994, 2565. (12) Qiu, D.; Ye, K.; Wang, Y.; Zhou, B.; Zhang, X.; Lei, S. B.; Wan, L. J. Langmuir 2003, 19, 678. (13) Mu, Z.; Wang, Z.; Zhang, X.; Ye, K.; Wang, Y. J. Phys. Chem. B, in press. (14) Lin, F.; D. Zhong, Y.; L. Chi, F.; Ye, K.; Wang, Y.; Fuchs, H. Submitted for publication in Phys. ReV. B. (15) Koene, B. E.; Loy, D. E.; Thompson, M. E. Chem. Mater. 1998, 10, 2235.
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