Alkyl and Dendron Substituted Quinacridones: Synthesis, Structures

Apr 18, 2007 - DtBQA (4.2 g, 10 mmol) reacted with 1-bromohexane (9.9 g, 60 mmol) according to the ..... Inset is magnified curves of the 0→1 transi...
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Alkyl and Dendron Substituted Quinacridones: Synthesis, Structures, and Luminescent Properties Jia Wang, Yunfeng Zhao, Chuandong Dou, Hui Sun, Peng Xu, Kaiqi Ye, Jingying Zhang, Shimei Jiang, Fei Li, and Yue Wang* Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: December 16, 2006; In Final Form: March 6, 2007

The synthesis of two alkyl substituted quinacridone derivatives, N,N′-di(n-hexyl)-1,3,8,10-tetramethylquinacridone (1) and N,N′-di(n-hexyl)-2,9-di(t-butyl)quinacridone (2), and four dendritic quinacridone derivatives, N,N′-didendritic-1,3,8,10-tetramethylquinacridones (3-G1 and 3-G2) and N,N′-didendritic-2,9-di(tert-butyl)quinacridones (4-G1 and 4-G2) are reported. X-ray crystal structure and thermal analysis revealed that the quinacridone derivatives reported in this paper exhibit the evolution from crystalline phase to amorphous phase upon varying from alkyl substituted quinacridones to dendritic quinacridones. The concentrationdependent 1H NMR, UV-vis, and photoluminescence (PL) spectroscopic studies demonstrated the aggregation properties of the quinacridone derivatives in solution. For dendritic quinacridones with the sufficient shield of dendrons, the fluorescence concentration quenching can be significantly suppressed and emission intensity in concentrated solution and solid state could be greatly enhanced. Compound 4-G2 displays good solution process property and higher PL yield in concentrated solution, suggesting that it is a potential candidate for the fabrication of high-performance organic electroluminescent devices (OLEDs) on the basis of low-cost solution process technique.

Introduction Quinacridone (QA) and its derivatives 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 electroluminescent 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.5,6 Moreover, some studies on QA have been devoted to investigate the assembly and structure 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.7,8 Nakahara and co-workers have synthesized quinacridone derivatives with alkyl chains and used them in Langmuir-Blodgett films to control the orientation and packing of the chromophores.3,4 Armstrong and co-workers have demonstrated that dendrimerization of QA could efficiently inhibit the aggregation and self-absorption between core molecules in the solid state.9 We have reported that highly ordered 2-D structures based on quinacridone derivatives could be obtained at highly oriented pyrolytic graphite (HOPG).10-12 Recently, we have carried out studies aimed at understanding how supramolecular organization in the solid state and assembly in a solution system affects the photoluminescent (PL) and electroluminescent (EL) properties of quinacridone derivatives. It is demonstrated that molecular packing properties have dramatic effect on the PL and EL properties of solid thin films, and it is possible to optimize the PL and EL properties of quinacridone derivatives through the regulation of molecular * Corresponding author. Fax: +86-431-85193421. E-mail: yuewang@ jlu.edu.cn.

structure and the molecular 3-D packing feature.13 To fully exploit the potential applications of this kind of functional compound in organic materials based devices, it is necessary to design and synthesize some new type QA derivatives with different molecular structures and to understand the relationship between molecular structures and aggregation, film formation, and optical and electronic properties in solution and condensed states. These will enable the development of new strategies for the preparations of high-performance organic optical and electronic materials. Furthermore, controlling self-quenching and aggregation properties, and in turn achieving desirable performance in devices, remain a challenge for QA derivatives. In this contribution, we report the synthesis characterizations of six quinacridone derivatives 1, 2, 3-G1, 3-G2, 4-G1, and 4-G2 (Scheme 1) with different substituents. The detailed thermal behaviors, film formation, crystals structure, and aggregation and luminescence properties will be presented. Experimental Section General Method. 1H NMR spectra were recorded on a Bruker AVANVE 500 MHz spectrometer with tetramethylsilane as the internal standard. Mass spectra were recorded on a GC/ MS mass spectrometer. Element analyses were performed on a Flash EA 1112 spectrometer. Absorption spectra were obtained using a PE UV-vis Lambda 20 spectrometer. Photoluminescence spectra were collected by a Shimadzu RF-5301PC spectrophotometer. An Olympus BX51 fluorescence microscope was used to obtain digital images of spin-coating films. The atomic force microscope (AFM) images were recorded at a Nanoscope IIIa AFM Multimode (Digital Instruments, Santa Barbara, CA) under ambient conditions. AFM was operated in the tapping mode with an optical readout using Si3N4 cantilevers (Nanoprobes, Digital Instruments). The room-temperature lu-

10.1021/jp068646m CCC: $37.00 © 2007 American Chemical Society Published on Web 04/18/2007

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SCHEME 1: Chemical Structures of the Six Quinacridone Derivatives

SCHEME 2: Synthesis Procedure of the Six Quinacridone Derivatives

minescence quantum yields of 1, 2, 3-G1, 3-G2, 4-G1, and 4-G2 in 1,1,2,2-tetrachloroethane solution were measured at a single excitation wavelength (365 nm) referenced to quinine sulfate in sulfuric acid aqueous solution (φ ) 0.546) and calculated according to literature.13 1,3,8,10-Tetramethylquinacridone (TMQA)13 and benzyl aryl ether dendron14 (first generation and second generation) are synthesized according to literature methods. All starting materials (Acros) were used as received. X-ray Crystallography. Single crystals suited for X-ray structural analysis were obtained by slow diffusion of petroleum ether into chloroform solution of 2. Diffraction data were collected on a Rigaku R-AXIS RAPID diffractometer (Mo KR radiation, graphite monochromator) in the Ψ rotation scan mode. The structure determination was done with direct methods by using SHELXTL 5.01v and refinements with full-matrix least squares on F2. The positions of hydrogen atoms were calculated and refined isotropically. 2,9-Di(tert-butyl)quinacridone. 2,9-Di(tert-butyl)quinacridone (DtBQA) was synthesized according to the procedures reported in refs 7 and 13. The 1H NMR measure of DtBQA was limited due to its relatively poor solubility in common organic solvents such as chloroform and dimethyl sulfoxide (DMSO). MS: m/z: 424.4 [M]+. Anal. Calcd (%) for C28H28N2O2 (424.2): C, 79.21; H, 6.65; N, 6.60. Found: C, 79.24; H, 6.55; N, 6.68. N,N′-Di(n-hexyl)-1,3,8,10-tetramethylquinacridone (1). Under nitrogen, sodium hydride (2.0 g, 70 mmol) was added to a suspension of TMQA (3.7 g, 10 mmol) in 100 mL of dry tetrahydrofuran (THF). The mixture was heated to reflux for 1 h, and then 1-bromohexane (9.9 g, 60 mmol) was added. The reaction mixture was continued to heat to reflux overnight. After

distilling off excess 1-bromohexane and THF, methanol (50 mL) was added dropwise into the reaction mixture. The resulting suspension was stirred for 1 h. The generated orange precipitate was filtered off and washed three times with petroleum ether. After drying in air, crude 1 was obtained in 90% yield, which was purified by column chromatography using silica gel with chloroform as eluent to yield 4.7 g of 1 (85%). 1H NMR (CDCl3): δ 8.67 (s, 2H), 7.13 (s, 2H), 6.87 (s, 2H), 4.43-4.45 (t, 4H), 3.00 (s, 6H), 2.50 (s, 6H), 2.00 (m, 4H), 1.61-1.64 (m, 4H), 1.39-1.49 (m, 8H), 0.94-0.97 (t, 6H). MS: m/z 535.9 [M]+. Anal. Calcd for C36H44N2O2 (536.75): C, 80.56; H, 8.26; N, 5.22. Found: C, 79.84; H, 8.19; N, 5.06. N,N′-Di(n-hexyl)-2,9-di(t-butyl)quinacridone (2). DtBQA (4.2 g, 10 mmol) reacted with 1-bromohexane (9.9 g, 60 mmol) according to the procedure described for the synthesis of 1 to yield 4.8 g (81%) of 2. 1H NMR (CDCl3): δ 8.81 (s, 2H), 8.588.59 (d, 2H), 7.84-7.86 (m, 2H), 7.49-7.51 (d, 2H), 4.504.54 (t, 4H), 1.99-2.02 (m, 4H), 1.60-1.64 (m, 4H), 1.371.47 (m, 8H), 1.42 (s, 18H), 0.92-0.95 (t, 6H). MS: m/z 592.0 [M]+. Anal. Calcd for C40H52N2O2 (592.85): C, 81.04; H, 8.84; N, 4.73. Found: C, 81.13; H, 8.79; N, 4.65. N,N′-Di(first generation dendritic substituent)TMQA (3G1) and N,N′-Di(second generation dendritic substituent)TMQA (3-G2). Excess G1 and G2 reacted with TMQA according to the procedure described for the synthesis of 1 to give 3-G1 (97%) and 3-G2 (96%), respectively. Compound 3-G1. 1H NMR (CDCl3): δ 8.48 (s, 2H), 7.227.32 (m, 20H), 6.92 (s, 2H), 6.87 (s, 2H), 6.54 (s, 2H), 6.46 (s, 4H), 5.59 (s, 4H), 4.96 (s, 8H), 2.96 (s, 6H), 2.34 (s, 6H). MS: m/z 972.7 [M]+. Anal. Calcd for C66H56N2O6 (973.18): C, 81.46; H, 5.80; N, 2.88. Found: C, 81.50; H, 5.68; N, 2.82. Compound 3-G2. 1H NMR (CDCl3): δ 8.43 (s, 2H), 7.277.37 (m, 40H), 6.90 (s, 2H), 6.79 (s, 2H), 6.57 (s, 8H), 6.50 (s, 2H), 6.47 (s, 4H), 6.42 (s, 4H), 5.52 (s, 4H), 4.94 (s, 16H), 4.87 (s, 8H), 2.89 (s, 6H), 2.31 (s, 6H). MS: m/z 1821.4 [M]+. Anal. Calcd for C122H104N2O14 (1822.15): C, 80.42; H, 5.75; N, 1.54. Found: C, 80.37; H, 5.85; N, 1.51. N,N′-Di(first generation dendritic substituent)DtBQA (4G1) and N,N′-Di(second generation dendritic substituent)DtBQA (4-G2). Compounds 4-G1 and 4-G2 were prepared by the same procedures as those for 3-G1 and 3-G2 with high yields (96% for 4-G1 and 97% for 4-G2). Compound 4-G1. 1H NMR (CDCl3): δ 8.67 (s, 2H), 8.53 (d, 2H), 7.70-7.71 (m, 2H), 7.22-7.32 (m, 22H), 6.54 (s, 2H), 6.48 (s, 4H), 5.69 (s, 4H), 4.95 (s, 8H), 1.40 (s, 18H). MS: m/z 1029.0 [M]+. Anal. Calcd for C70H64N2O6 (1029.28): C, 81.69; H, 6.27; N, 2.72. Found: C, 81.73; H, 6.40; N, 2.70. Compound 4-G2. 1H NMR (CDCl3): δ 8.61 (s, 2H), 8.508.51 (d, 2H), 7.70-7.71 (m, 2H), 7.28-7.38 (m, 42H), 6.57 (s, 8H), 6.49 (s, 2H), 6.46 (s, 4H), 6.42 (s, 4H), 5.58 (s, 4H), 4.94 (s, 16H), 4.86 (s, 8H), 1.34 (s, 18H). MS: m/z 1877.2 [M]+. Anal. Calcd for C126H112N2O14 (1878.27): C, 80.57; H, 6.01; N, 1.49. Found: C, 80.46; H, 6.09; N, 1.43.

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Figure 2. Stacking diagram showing π‚‚‚π interactions in 2. Figure 1. ORTEP view of 2 (50% probability displacement ellipsoids).

TABLE 1: X-ray Crystallographic Data for 2 empirical formula FW temp, K wavelength, Å cryst syst, space group unit cell dimens a, Å b, Å c, Å R, deg β, deg γ, deg vol, Å3 Z; calcd dens, mg/m3 abs coeff, mm-1 F(000) cryst size, mm3 θ range for data collecn, deg limiting indices reflecns collected/unique completeness to θ ) 27.48, % abs correcn max and min transmn refinement method data/restraints/params GOF on F2 final R indices [I > 2σ(I)] R indices (all data) extinction coeff largest diff peak; hole, e‚A-3

C42 H54 Cl6 N2 O2 831.57 293(2) 0.710 73 monoclinic, P21/c 5.67940(10) 18.1714(8) 21.44410(10) 90 96.8770(7) 90 2197.17(10) 2; 1.257 0.427 876 0.52 × 0.16 × 0.13 1.47-27.48 -7 E h E 7, 0 E k E 23, -27 E l E 6 4532/4532 [R(int) ) 0.047 177] 89.80 semiempirical from equivalents 0.9478 and 0.8092 full-matrix least squares on F2 4532/0/235 0.679 R1 ) 0.0475, wR2 ) 0.1053 R1 ) 0.2711, wR2 ) 0.1782 0 0.277; -0.324

Results and Discussions Synthesis and Characterization. The synthetic procedure of 1, 2, 3-G1, 3-G2, 4-G1, and 4-G2 is displayed in Scheme 2, which generally affords superior yields for substituted quinacridones. 1H NMR spectra of the six compounds reveal that their molecular geometries are all centrosymmetric in solution. Single-Crystal and Powder X-ray Characterizations and Molecular Packing Properties. Single-crystal X-ray diffraction studies reveal the planar geometry for the rigid π-cores of molecule 2 (Figure 1). Table 1 summarizes the relevant structural parameters of 2. For crystal 2, the molecular geometries are centrosymmetric and the two n-hexyl chains adopt a regular all-trans conformation. In the solid of 2, there are faceto-face intermolecular π‚‚‚π stacking interactions accompanied by hydrogen bonding interactions, which result in the formation

Figure 3. View along the cell R-axis of the packing structure in 2.

TABLE 2: Selected Bond Lengths (Å) and Angles (deg) of 2 O(1)-C(8) N(1)-C(5) N(1)-C(6) N(1)-C(11) C(5)-N(1)-C(6) C(5)-N(1)-C(11) C(6)-N(1)-C(11) O(1)-C(8)-C(9) O(1)-C(8)-C(7)

1.236(5) 1.367(5) 1.394(5) 1.472(5) 119.9(4) 120.2(4) 119.8(4) 122.8(5) 122.1(5)

of one-dimensional molecular columns. The rigid π-cores adopt a ladder-like arrangement in a molecular column (Figure 2). The separation distance between adjacent molecular planes of 2 is 3.5697 Å. Intermolecular hydrogen bonding interactions between the oxygen atom on the CdO group and the hydrogen atom on CHCl3 occur in solid and the hydrogen bonding distance of CdO‚‚‚H is 2.016 Å. (Figure 3). Table 2 illustrates the selected bond lengths and angles of 2. The experimental results for 2 reveal that the face-to-face aggregated behavior exists in the crystalline phase, suggesting that alkyl substituent groups cannot efficiently inhibit aggregation between chromophores in the solid state. The powder X-ray diffraction patterns of 1, 2, 3-G1, 3-G2, 4-G1, and 4-G2 are shown in Figures 4 and 5, respectively. For the set of 1, 3-G1, and 3-G2,

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Figure 4. Powder XRD patterns of 1, 3-G2, and 3-G2.

Figure 5. Powder XRD patterns of 2, 4-G1, and 4-G2.

the three samples clearly display crystalline characteristic and their powder X-ray patterns (Figure 4) are different from each other, suggesting that the three samples belong to different crystalline phases. The set of 2, 4-G1, and 4-G2 undergoes progressive changes from crystalline phase to amorphous phase (Figure 5). Compounds 2 and 4-G2 possess typical crystalline and amorphous features, respectively, while 4-G1 is an intermediate between 2 and 4-G2. We can easily understand the difference of the solid phase of the different substituted quinacridone derivatives, although it is impossible to obtain detailed molecular packing structures from the powder X-ray patterns.

Thermal Behaviors and Film Formation Characteristics. The differential scanning calorimetry (DSC) curves of the six quinacridone derivatives are shown in Figures 6 and 7, respectively. After preheating treatment compound 1 still displays a sharp endothermic peak and an exothermic peak due to melting and crystalline behavior, respectively. The DSC curves of 2 presents the obvious melting endothermic peak and the absence of crystalline peak. It is clear that the melting point of 2 is lower than that of 1. This phenomenon should be attributed to the introduction of branched tert-butyl groups that can lead to the decrease of crystallization capability. For 3-G2, the unremarkable and broad melting and crystalline points

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Figure 6. DSC scans of 1, 3-G1, and 3-G2 with heating and cooling at a rate of 10 °C/min after preheating, respectively.

Figure 8. Fluorescent microscopic (left) and AFM (right) images of the samples of 1, 2, and 4-G2 fabricated by spin-coating approach.

Figure 7. DSC scans of 2, 4-G1, and 4-G2 with heating and cooling at a rate of 10 °C/min after preheating, respectively.

appear at 254.7 and 176 °C, respectively. In contrast, the DSC curves of 4-G2 do not display any melting and crystalline behaviors, suggesting that 4-G2 is a typical amorphous glass with Tg around 73.2 °C. With an increase in the size and branched degree of substituents, the series of quinacridone derivatives clearly exhibit a systematic variation process from a typical crystal phase to a typical amorphous phase. This phenomenon is also confirmed by powder X-ray diffraction characterizations (Figures 4 and 5). The results are in agreement with the concept that the introduction of nonrigid-branched substituents could lead to the formation of amorphous glasses. Figure 8 presents the fluorescence microscopy and atomic force microscopy (AFM) images of the spin-coated samples of 1, 2, and 4-G2. The morphology of the 4-G2 film shows roughness of 0.575 nm, suggesting that uniform film based on 4-G2 could be prepared with solution process. The excellent film formation property of 4-G2 suggests the possibility to develop solutionprocessable small molecular emitting materials based on quinacridone derivatives. Solution process has been regarded as a lower cost approach for the fabrication of large-area full color display.15 The spin-coated samples of 1 and 2 are composed of microcrystals on the substrates, which is attributed to the strong

Figure 9. Concentration-dependent 1H NMR measurements of 1 in CDCl3 at room temperature.

crystallization property of the two compounds. Therefore, the thermodynamic and the film-forming properties in solution process reach good agreement. Compound 4-G2 with tert-butyl and dendritic groups displays perfect solubility in common organic solvents such as chloroform, dichloromethane, and THF, which is a critical property for the solution process. 1H NMR, Absorption, and Photoluminescence Properties. Figures 9-11 show the concentration-dependent 1H NMR spectra of compounds 1, 2, and 4-G2 in CDCl3 solution. Increasing the concentrations of 1, 2, and 4-G2 at a constant temperature (298 K) result in the upfield shift of aromatic protons. The changes in the chemical shifts are attributed to the formation of the face-to-face stacking aggregates with different degrees for 1, 2, and 4-G2, respectively, in solution.16-18 When the concentration varies from 1.5 × 10-3 to 3.0 × 10-2 M, the ∆δH values of the proton H6 (Scheme 2) are 0.071, 0.043, and 0.021 ppm for compounds 1, 2, and 4-G2, respec-

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Figure 10. Concentration-dependent 1H NMR measurements of 2 in CDCl3 at room temperature. Figure 13. Concentration-dependent emission spectra of 1 in chloroform.

Figure 11. Concentration-dependent 1H NMR measurements of 4-G2 in CDCl3 at room temperature.

Figure 14. Concentration-dependent emission spectra of 4-G2 in chloroform.

Figure 12. Concentration-dependent absorption spectra of 1 in chloroform. Inset is magnified curves of the 0f1 transition.

tively. The weaker aggregation tendency of 4-G2 relative to 1 can be attributed to the enhanced steric hindrance in former. The ∆δH values orderly decrease with the increasing size of the substituents. Compound 1 displays more obvious aggregation behavior when the concentrations are in the range of 1.5 × 10-3 to 3.0 × 10-2 M. Figure 12 displays the normalized absorption spectra of 1 recorded at different concentrations. The electronic absorption of 1 exhibits obvious coupling with the vibronic features corresponding to the V ) 0 to V′ ) 0, 1, and 2. 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 lead to a blue shift of the absorption band.13,18,19 For 1, the A0f0/A0f1 (ratio of the intensities of A0f0 to A0f1) slightly decreases when the concentration changes from 6.0 × 10-6 to 1.0 × 10-3 M, suggesting that within this concentration region 1 displays slight aggregation tendency. The concentration-dependent absorption spectra demonstrate that the other five quinacridone derivatives are nearly unaggregated within the concentration range of 6.0 × 10-6 to 1.0 × 10-3 M (see Supporting Information). Figure 13 presents the PL spectra of 1 recorded at different concentrations. Upon increasing concentration, a remarkable red shift of emission band is observed. For the dilute solution (6.0 × 10-6 to 6.0 × 10-5 M) of 1, the PL spectra have a sharp emission peak around 533 nm accompanied by a weak shoulder around 570 nm. When the concentration varies from 3.0 × 10-6 to 6.0 × 10-5 M, the emission intensity around 530 nm increases progressively and reaches a maximum value at a concentration of 6.0 × 10-5 M. A further increase in concentration leads to the decrease of emission intensity around 530 nm and the red shift of the emission spectrum of 1, suggesting the existence of self-absorption in the concentrated solution. The PL spectra of the other five compounds show analogous concentration-dependent properties to that of 1 (Figure 14 and

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TABLE 3: PL Quantum Yields of Six Synthesized Quinacridone Derivatives PL quantum yield (%) concn (M)

compd 1

compd 2

compd 3-G1

compd 3-G2

compd 4-G1

compd 4-G2

3 × 10-6 6 × 10-6 3 × 10-5 6 × 10-5 3 × 10-4 6 × 10-4 1 × 10-3

95 94 91 87 36 23 5

88 85 82 82 58 37 13

99 99 98 98 74 42 a

99 99 99 98 78 38 13

99 99 98 97 77 48 25

99 99 99 98 76 58 34

a The attempt to measure the PL quantum yield of 3-G1 in the concentration of 1 × 10-3 M has failed due to the relatively poor solubility of 3-G1 in 1,1,2,2-tetrachloroethane.

function of the light-harvesting and the energy transfer to the center chromophore, in turn resulting in the enhancement of PL efficiency.21 Figure 15 presents the photographs of the bulk powders of 1, 2, 3-G1, 3-G2, 4-G1, and 4-G2 under UV light (365 nm). It is clear that 3-G2 and 4-G2 display obviously strong emission compared with 1, 2, 3-G1, and 4-G1. In the neat 3-G2 and 4-G2 solids, the emission quenching is also decreased. Conclusions

Figure 15. Digital images of the bulk powder samples under UV light (λ ) 365 nm).

see Supporting Information). It is worthwhile to mention that the critical concentration (3.0 × 10-4 M) of 4-G2, at which emission intensity begins to drop significantly, is obviously higher than that (6.0 × 10-5 M) of 1. This is due to the dendritic substitute that can efficiently decrease the fluorescence quenching induced by self-absorption and excimer emission. Table 3 summarizes the PL quantum yields of all synthesized compounds in different concentrations. The PL quantum yields of the six compounds in solution decrease with the increase of their concentration. The PL quantum yield data in Table 3 clearly reveal that 1 and 2 display similar concentrationdependent PL efficiency property, while the four dendritic quinacridone compounds (3-G1, 3-G2, 4-G1, and 4-G2) exhibit analogous concentration-dependent PL efficiency properties. Compounds 3-G1, 3-G2, 4-G1, and 4-G2 show higher PL quantum yields than 1 and 2 in a relatively higher concentration region (3 × 10-4 to 1 × 10-3 M). The concentration quenching has been efficiently suppressed by the introduction of simple dendritic groups. With the sufficient shield of large dendrons, the intermolecular quenching between encapsulated chromophores can be significantly decreased, leading to great enhancement of the emission intensity in concentrated solutions. It is worthwhile to note that 2 displays a relatively lower PL quantum yield than 1 in dilute solution (3 × 10-6 to 6 × 10-5 M). This result should be attributed to the tert-butyl group having stronger rotational vibration property compared with 1 and the rotational vibration can lead to the emission quenching. For 4-G2 with two tert-butyl groups, the PL quantum yields in dilute solution (3 × 10-6 to 6 × 10-5 M) approach 100%; the tert-butyl group inducing the decrease of PL quantum yield is completely eliminated. This phenomenon may be due to the dendron encapsulation inhibiting the rotational vibration of tertbutyl groups. On the other hand peripheral dendrons have a

In summary, two alkyl substituted quinacridone derivatives (1 and 2) and four dendron substituted quinacridone derivatives (3-G1, 3-G2, 4-G1, and 4-G2) have been synthesized. The single-crystal structure of 2 is reported. Detailed spectroscopic studies have been performed. It is demonstrated that in the crystal structure of 2 there are intermolecular π‚‚‚π stacking and hydrogen bonding interactions. The DSC analysis revealed that 1 and 2 display a typical crystal feature, while 4-G2 is a typical amorphous glass. The dendritic 4-G2 compound exhibits good solubility and solution-process film-forming properties. The concentration-dependent 1H NMR characterization demonstrated that 1 displays a stronger aggregation property than 4-G2 within the concentration range of 1.5 × 10-3 to 3 × 10-2 M. The dendrimers 3-G1, 3-G2, 4-G1, and 4-G2 exhibit a higher PL quantum yield than 1 and 2 in the concentrated solution (3 × 10-4 to 1 × 10-3 M), suggesting that the introduction of dendritic groups could efficiently suppress the fluorescence quenching at relatively higher concentration region. Compound 4-G2 displays good solubility, perfect solution process characteristic, weaker aggregation property, and higher PL efficiency at concentrated solution and is a potential candidate that may be applicable for the fabrications of OLEDs based on solution process. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grants 50573029 and 50520130316), the Major State Basic Research Development Program (Grant 2002CB613401), the Program for Changjiang Scholars, Innovative Research Team in University (Grant IRT0422), the 111 project (Grant B06009), and the Natural Science Foundation of Jilin Province (Grant 20050120). Supporting Information Available: Crystallographic information (CIF), concentration-dependent absorption spectra of compounds 2, 3-G1, 3-G2, 4-G1, and 4-G2, and concentrationdependent PL spectra of 2, 3-G1, 3-G2, and 4-G1. 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.

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