Alkyl Chain Length Dependent Morphology and Emission Properties

Feb 10, 2009 - Alkyl Chain Length Dependent Morphology and Emission Properties of the Organic Micromaterials Based on Fluorinated Quinacridone ...
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Langmuir 2009, 25, 3264-3270

Alkyl Chain Length Dependent Morphology and Emission Properties of the Organic Micromaterials Based on Fluorinated Quinacridone Derivatives Yunfeng Zhao, Xiaoyue Mu, Chunxiao Bao, Yan Fan, Jingying Zhang, and Yue Wang* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed December 19, 2008. ReVised Manuscript ReceiVed January 17, 2009 The fluorinated quinacridone derivatives N,N′-dialkyl-2,9-difluoroquinacridone (Cn-DFQA, n ) 4, 8, 10, 16) with different alkyl chains were used as building blocks to assemble luminescent micromaterials. It was demonstrated that the morphology and emission of the Cn-DFQA-based micromaterials strongly depended on their alkyl chain length. C4-DFQA and C8-DFQA showed stronger tendency to form 1-D microstructures, while C10-DFQA and C16-DFQA displayed the aggregation properties to form diamond and hexagonal platelike microcrystals, respectively. The photoluminescent (PL) spectra of Cn-DFQA (n ) 4, 8, 10, 16) in THF dilute solutions displayed approximate profiles with a sharp emission peak at 533 nm and a shoulder at 573 nm, while the PL spectra of the Cn-DFQA-based micromaterials exhibited obviously red-shift emission bands at 622 nm for C4-DFQA, 627 nm for C8-DFQA, 614 nm for C10-DFQA, and 613 nm for C16-DFQA, respectively. The single-crystal X-ray structures of four Cn-DFQA compounds have been studied. In the C4-DFQA and C8-DFQA single crystals, there are 1-D molecular columns based on the intermolecular π · · · π and hydrogen bonding interactions. In the single crystals of C10-DFQA and C16-DFQA, the molecules assembled into 2-D molecular sheets based on the hydrogen bonds and C-H · · · π interactions. The molecular packing structures provide a reasonable explanation for the alkyl chain length dependent morphologies and emission properties of fluorinated quinacridone micromaterials.

Introduction Micro- and nanoscale materials such as nanowires, nanoparticles, and nanorods represent attractive building blocks for the fabrication of functional micro- and nanoscale devices.1 The quantum size effect of micro- and nanostructured inorganic semiconductors could induce new optical and electronic properties compared with those of common bulk materials.2 Especially, one-dimensional (1-D) nanostructures have attracted great attention due to that 1-D nanowires or nanofibers with a high length/diameter aspect ratio often display unusual physical and chemical properties.3 The 1-D assembly of organic functional molecules on various substrates offers a useful strategy for the construction of well-defined functional nanoscale materials that are potential candidates of active components in gas sensors, organic light-emitting diodes (OLED), organic field effect transistors (OFET), optical waveguide fibers, photodetectors, and solar cells, etc.4-9 Recently, it was demonstrated that some π-conjugated small organic molecules could be employed as * Authors to whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Shah, P. S.; Hanrath, T.; Johnston, K. P.; Korian, B. A. J. Phys. Chem. B 2004, 108, 9574. (b) Hayden, O.; Payne, C. K. Angew. Chem., Int. Ed. 2005, 44, 1395. (c) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66. (2) (a) Henglein, A. Chem. ReV. 1989, 89, 1861. (b) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (c) Moriarty, P. Rep. Prog. Phys. 2001, 64, 297. (d) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019. (e) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016. (3) (a) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (b) Hu, J. T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (4) (a) Naddo, T.; Che, Y.; Zhang, W.; Balakrishnan, K.; Yang, X.; Yen, M.; Zhao, J.; Moore, J. S.; Zang, L. J. Am. Chem. Soc. 2007, 129, 6978. (b) Che, Y.; Datar, A.; Balakrishnan, K.; Zang, L. J. Am. Chem. Soc. 2007, 129, 7234. (c) Che, Y.; Yang, X.; Loser, S.; Zang, L. Nano Lett. 2008, 8, 2219. (d) Sun, Y.; Ye, K.; Zhang, H.; Zhang, J.; Zhao, L.; Li, B.; Yang, G.; Yang, B.; Wang, Y.; Lai, S.-W.; Che, C.-M. Angew. Chem., Int. Ed. 2006, 45, 5610. (e) Che, Y.; Yang, X.; Zang, L. Chem. Commun. 2008, 1413.

building blocks to assemble supramolecular 1-D nanostructures.10 However, the controlling and understanding of anisotropic aggregation of small organic molecules used for constructing 1-D micro- and nanomaterials with well-defined structures remains a challenge.11 To obtain desirable organic nanomaterials for optical and electronic applications, the design and synthesis of functional organic building blocks and development of 1-D self-assembly approaches should be focused on. Quinacridone (5) (a) Dautel, O. J.; Wantz, G.; Almairac, R.; Flot, D.; Hirsch, L.; Lere-Porte, J.-P.; Parneix, J.-P.; Serein-Spirau, F.; Vignau, L.; Moreau, J. J. E. J. Am. Chem. Soc. 2006, 128, 4892. (b) Zhao, Y. S.; Di, C.; Yang, W.; Yu, G.; Liu, Y.; Yao, J. AdV. Funct. Mater. 2006, 16, 1985. (c) Zhao, Y. S.; Fu, H.; Hu, F.; Peng, A.; Yang, W.; Yao, J. AdV. Mater. 2008, 20, 79. (d) Datar, A.; Balakrishnan, K; Yang, X.; Zuo, X.; Huang, J.; Oitker, R.; Yen, M.; Zhao, J.; Tiede, D. M.; Zang, L. J. Phys. Chem. B 2006, 110, 12327. (6) (a) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Appl. Phys. Lett. 1996, 3066. (b) Briseno, A. L.; Mannsfeld, S. C. B.; Reese, C.; Hancock, J. M.; Xiong, Y.; Jenekhe, S. A.; Bao, Z.; Xia, Y. Nano Lett. 2007, 7, 2847. (c) Che, Y.; Datar, A.; Yang, X.; Naddo, T.; Zhao, J.; Zang, L. J. Am. Chem. Soc. 2007, 129, 6354. (7) (a) Takazawa, K.; Kitahama, Y.; Kimura, Y.; Kido, G. Nano Lett. 2005, 5, 1293. (b) Yanagi, H.; Morikawa, T. Appl. Phys. Lett. 1999, 75, 187. (c) Balzer, F.; Bordo, V. G.; Simonsen, A. C.; Rubahn, H. G. Appl. Phys. Lett. 2003, 82, 10. (d) Quochia, F.; Cordella, F.; Mura, A.; Bongiovanni, G.; Balzer, F.; Rubahn, H. G. Appl. Phys. Lett. 2006, 88, 041106. (e) Zhao, Y. S.; Xu, J.; Peng, A.; Fu, H.; Ma, Y.; Jiang, L.; Yao, J. Angew. Chem., Int. Ed. 2008, 47, 7301. (f) Zhao, Y. S.; Peng, A.; Fu, H.; Ma, Y.; Yao, J. AdV. Mater. 2008, 20, 1661. (8) (a) Yamamoto, Y.; Fukushima, T.; Sun, Y.; Ishii, N.; Saeki, A.; Seki, S.; Tagawa, S.; Taniguchi, M.; Kawai, T.; Aida, T. Science 2006, 1761. (b) Schwab, A. D.; Smith, D. E.; Bond-Watts, B.; Johnston, D. E.; Hone, J.; Johnson, A. T.; de Paula, J. C.; Smith, W. F. Nano Lett. 2004, 1261. (c) Zhang, X.; Jie, J.; Zhang, W.; Zhang, C.; Luo, L.; He, Z.; Zhang, X.; Zhang, W.; Lee, C.; Lee, S. AdV. Mater. 2008, 20, 2427. (d) Ji, H.-X.; Hu, J.-S.; Wan, L.-J. Chem. Commun. 2008, 2653. (9) (a) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (b) Hasobe, T.; Imahori, H.; Fukuzumi, S.; Kamat, P. V. J. Mater. Chem. 2003, 13, 2515. (10) (a) Wu¨rthner, F. Chem. Commun. 2004, 1564. (b) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491. (c) Grimsdale, A. C.; Mu¨llen, K. Angew. Chem., Int. Ed. 2005, 44, 5592. (d) Supramolecular Dye Chemistry; Wu¨rthner, F., Ed.; Topics in Current Chemistry, Springer: Berlin, 2005; Vol. 258. (e) Zang, L.; Che, Y.; Moore, J. S. Acc. Chem. Res. 2008, 41, 1596–1608. (f) Zhao, Y. S.; Fu, H.; Peng, A.; Ma, Y.; Xiao, D.; Yao, J. AdV. Mater. 2008, 20, 2859.

10.1021/la804182d CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

Micromaterials Based on Quinacridone DeriVatiVes

and its derivatives (QAs) have displayed excellent photovoltaic12,13 and organic electroluminescent14,15 properties. Moreover, the three-dimensional (3-D) supramolecular structures in crystals16-18 and the two-dimensional (2-D) assembly properties in films19-22 of QAs have been extensively investigated. Recently, we have successfully constructed one-dimensional (1-D) nanofibers based on QAs, and the size-dependent luminescence property of the QA-based nanofibers has been documented.23 In this contribution, we present four fluorinated quinacridone derivatives with different lengths of the alkyl chains (Cn-DFQA, n ) 4, 8, 10, 16). The 1-D and platelike micromaterials have been generated from Cn-DFQA through a self-assembly process, respectively. The Cn-DFQA-based micromaterials display alkyl chain length dependent morphology properties. Furthermore, the emission properties of the micromaterials showed obviously morphology dependent characteristics. The single-crystal structures revealed that the molecular packing structure is the critical element that determines the morphology and emission characteristics of CnDFQA micromaterials.

Experimental Section Materials. 4-Fluoroaniline, 1-brombutane, 1-bromooctane, 1-bromodecane, 1-bromohexadecane, and sodium hydride (NaH) were all obtained from Aldrich. These chemicals were used as received without further purification. Instrumentation. 1H NMR spectra were recorded on a Bruker AVANCE 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. An Olympus BX51 fluorescence microscope was used to obtain digital images of samples. Scanning electronic microscopy (SEM) images were performed on a JSM 6700F field emission scanning electronic microscope. Photoluminescence spectra were recorded on a Perkin-Elmer LS 55 spectrophotometer. UV-visible spectra of micromaterials were recorded on a Shimadzu UV-2550 PC spectrometer equipped with an integrating sphere attachment in the range 200-900 nm using BaSO4 as background. Quantum yields of micromaterials were determined with a PTI C-701 calibrated integrating sphere system. The X-ray diffraction patterns were performed by a Siemens D5005 diffractometer with Cu KR radiation (λ ) 1.5418 Å). Preparation of Micromaterials and Spectra Measurements. The micromaterials were prepared by injecting 100 µL of Cn-DFQA solution (1.0 × 10-3 M) in tetrahydrofuran (THF) into 5 mL of (11) (a) Balakrishnan, K.; Datar, A.; Naddo, T.; Huang, J.; Oitker, R.; Yen, M.; Zhao, J.; Zang, L. J. Am. Chem. Soc. 2006, 128, 7390. (b) Johnson, C. A.; Sharma, S.; Subramaniam, B.; Borovik, A. S. J. Am. Chem. Soc. 2005, 127, 9698. (c) Wang, Y.; Fu, H.; Peng, A.; Zhao, Y.; Ma, J.; Ma, Y.; Yao, J. Chem. Commun. 2007, 1623. (d) Su, W.; Zhang, Y.; Zhao, C.; Li, X.; Jiang, J. ChemPhysChem 2007, 8, 1857. (12) Hiramoto, M.; Kawase, S.; Yokoyama, M. Jpn. J. Appl. Phys. 1996, 35, L349. (13) Shichiri, T.; Suezaki, M.; Inoue, T. Chem. Lett. 1992, 21, 1717. (14) Shi, J.; Tang, C. W. Appl. Phys. Lett. 1997, 70, 1665. (15) 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. (16) Lincke, G. Dyes Pigm. 2000, 44, 101. (17) Ye, K. Q.; Wang, J.; Sun, H.; Liu, Y.; Mu, Z. C.; Li, F.; Jiang, S. M.; Zhang, J. Y.; Zhang, H. X.; Wang, Y.; Che, C. M. J. Phys. Chem. B 2005, 109, 8008. (18) Paulus, E. F.; Leusen, F. J. J.; Schmidt, M. U. CrystEngComm 2007, 9, 131. (19) Keller, U.; Mu¨llen, K.; De Feyter, S.; De Schryver, F. C. AdV. Mater. 1996, 8, 490. (20) (a) Qiu, D. L.; Ye, K. Q.; Wang, Y.; Zou, B.; Zhang, X.; Lei, S. B.; Wan, L. J. Langmuir 2003, 19, 678. (b) Yang, X. Y.; Mu, Z. C.; Wang, Z. Q.; Zhang, X.; Wang, J.; Wang, Y. Langmuir 2005, 21, 7225. (21) Lin, F.; Zhong, D. Y.; Chi, L. F.; Ye, K. Q.; Wang, Y.; Fuchs, H. Phys. ReV. B 2006, 73, 235420. (22) Trixler, F.; Markert, T.; Lackinger, M.; Jamitzky, F.; Heckl, W. M. Chem.-Eur. J. 2007, 13, 7785. (23) Wang, J.; Zhao, Y. F.; Zhang, J. H.; Zhang, J. Y.; Yang, B.; Wang, Y.; Zhang, D. K.; You, H.; Ma, D. G. J. Phys. Chem. C 2007, 111, 9177.

Langmuir, Vol. 25, No. 5, 2009 3265 vigorously stirred diethyl ether. THF is a good solvent, while the diethyl ether is a poor one for Cn-DFQA, so the aggregation of molecules Cn-DFQA occurs in consequence of the change in the solubility. The mixture solution was continually stirred for 3 min and sequentially was aged for 5 min at room temperature. The aged suspensions of microcrystals are transferred onto clean quartz substrates and dried in air for morphology and spectroscopy measurements. To measure the photoluminescent spectra of the micromaterials, the quartz substrates with micromaterials were fixed on a holder and placed in the optical path of the exciting light source. The angle between the substrate plane and the exciting light was maintained at 45°. A quartz cell was used to measure the solution emission spectra. The quantum yields of Cn-DFQA in THF 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 the literature.17 X-Ray Crystallography. Single crystals suited for X-ray structural analysis were obtained by slow diffusion of petroleum ether into chloroform solutions of Cn-DFQA (n ) 4, 8, 10, 16), respectively. 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. CCDC 699904 (C4-DFQA), 699905 (C8-DFQA), 699906 (C10-DFQA), and 699907 (C16DFQA) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. 2,9-Difluoroquinacridone (DFQA). DFQA was synthesized from 4-fluoroaniline according to the reported procedures.17 The 1H NMR measure of DFQA was limited due to its relatively poor solubility in common organic solvents such as chloroform and dimethyl sulfoxide (DMSO). MS: m/z 348.1 [M]+. Anal. Calcd (%) for C20H20F2N2O2 (348.3): C, 68.97; H, 2.89; N, 8.04. Found: C, 68.95; H, 2.86; N, 7.98. N,N′-Di(n-butyl)-2,9-difluoroquinacridone (C4-DFQA). Under nitrogen, sodium hydride (0.5 g, 20.8 mmol) was added to a suspension of DFQA (2.0 g, 5.8 mmol) in 100 mL of dry THF. The mixture was heated to reflux for 1 h, and then 1-bromobutane (2.1 g, 15 mmol) was added. The reaction mixture continued to heat to reflux overnight. After distilling off excess 1-bromobutane and THF, methanol (15 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 C4-DFQA was obtained in 90% yield, which was purified by column chromatography using silica gel with chloroform as eluent to yield 2.1 g of C4-DFQA (75%). 1 H NMR (CDCl3): δ 8.75 (s, 2H), 8.18-8.20 (d, 2H), 7.52 (s, 2H), 7.54 (s, 2H), 4.49-4.55 (t, 4H), 1.97-2.02 (m, 4H), 1.56-1.67 (m, 4H), 1.09-1.14 (t, 6H). MS: m/z 460.2 [M]+. Anal. Calcd for C28H26F2N2O2 (460.51): C, 73.03; H, 5.69; N, 6.08. Found: C, 73.26; H, 5.67; N, 6.05. N,N′-Di(n-octyl)-2,9-difluoroquinacridone (C8-DFQA). DFQA (2.0 g, 5.8 mmol) reacted with 1-bromooctane (2.9 g, 15 mmol) according to the procedure described for the synthesis of C4-DFQA to yield 2.3 g (70%) of C8-DFQA. 1H NMR (CDCl3): δ 8.67 (s, 2H), 8.12-8.15 (d, 2H), 7.49 (s, 2H), 7.48 (s, 2H), 4.45-4.51 (t, 4H), 1.95-1.97 (m, 4H), 1.32-1.61 (m, 20H), 0.88-0.92 (t, 6H). MS: m/z 571.8 [M]+. Anal. Calcd for C36H42F2N2O2 (572.72): C, 75.50; H, 7.39; N, 4.89. Found: C, 75.36; H, 7.41; N, 4.93. N,N′-Di(n-decyl)-2,9-difluoroquinacridone (C10-DFQA). DFQA (2.0 g, 5.8 mmol) reacted with 1-bromodecane (3.3 g, 15 mmol) according to the procedure described for the synthesis of C4-DFQA to yield 2.5 g (68%) of C10-DFQA. 1H NMR (CDCl3): δ 8.78 (s, 2H), 8.22-8.24 (d, 2H), 7.55 (s, 2H), 7.57 (s, 2H), 4.52-4.57 (t, 4H), 1.98-2.08, (m, 4H), 1.32-1.62 (m, 28H), 0.90-0.93 (t, 6H). MS: m/z 627.02 [M]+. Anal. Calcd for C40H50F2N2O2 (628.82): C, 76.40; H, 8.01; N, 4.45. Found: C, 76.35; H, 7.97; N, 4.42.

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Scheme 1. Chemical Structures of Cn-DFQA (n ) 4, 8, 10, 16)

N,N′-Di(n-hexadecyl)-2,9-difluoroquinacridone (C16-DFQA). DFQA (2.0 g, 5.8 mmol) reacted with 1-bromohexadecane (4.6 g, 15 mmol) according to the procedure described for the synthesis of C4-DFQA to yield 3.1 g (67%) of C16-DFQA. 1H NMR (CDCl3): δ 8.79 (s, 2H), 8.23-8.26 (d, 2H), 7.56 (s, 2H), 7.58 (s, 2H), 4.52-4.58 (t, 4H), 1.98-2.04 (m, 4H), 1.29-1.60 (m, 52H), 0.89-0.93 (t, 6H). MS: m/z 796.82 [M]+. Anal. Calcd for C52H74F2N2O2 (797.13): C, 78.35; H, 9.36; N, 3.51. Found: C, 78.30; H, 9.34; N, 3.48.

Figure 2. SEM images of Cn-DFQA micromaterials: (a) C4-DFQA, (b) C8-DFQA, (c) C10-DFQA, and (d) C16-DFQA.

Results and Discussion Compounds C4-DFQA, C8-DFQA, C10-DFQA, and C16DFQA were synthesized by N-alkylation of DFQA with a different length of the alkyls in high yields and characterized by 1H NMR spectroscopy, mass spectra, and element analysis. The molecular structures of the compounds are shown in Scheme 1, which reveal that all molecules contain planar π-conjugated rigid cores and two alkyl chains. It is well-known that such molecular structures containing a π-conjugated rigid core and soft alkyl chains could be suitable to prepare 1-D nanostructures for the application of photoelectric nanodevices.10 Cn-DFQA were employed to fabricate organic micromaterials by using a reprecipitation approach,24 injecting an amount of the Cn-DFQA solution in THF into a vigorously stirred diethyl ether. The aged suspension of nanocrystals was transferred onto clean quartz substrates for the measurements of fluorescence microscopy and SEM. Luminescence microscopy and SEM images reveal that 1-D fiber structures are generated from the compounds with shorter alkyl chains (C4-DFQA and C8-DFQA) as shown in Figure 1 and Figure 2, respectively. Compound C4-DFQA generates the beltlike fibers with a diameter of 1 µm and length up to about 200 µm. The belt structure is also fabricated from compound

Figure 3. Normalized fluorescence spectra (λex ) 480 nm) of Cn-DFQA (n ) 4, 8, 10, 16) in THF (1 × 10-6 M, dash lines) and micromaterials (solid lines).

Figure 4. Absorption spectra of the Cn-DFQA (n ) 4, 8, 10, 16) micromaterials.

Figure 1. Fluorescence microscopy images of Cn-DFQA micromaterials: (a) C4-DFQA, (b) C8-DFQA, (c) C10-DFQA, and (d) C16-DFQA.

C8-DFQA. The diameter is about 1 µm, and the length reaches about 1000 µm. In addition, the surfaces of the 1-D microbelts of C4-DFQA and C8-DFQA are smooth. However, compounds C10-DFQA and C16-DFQA with longer alkyl chains only form microparticle crystals. The diamond-like microcrystals with size of 10 × 10 µm2 are fabricated from C10-DFQA (Figure 2c), and

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Table 1. X-Ray Crystallographic Data for Cn-DFQA (n ) 4, 8 10, 16) Compound

C4-DFQA

C8-DFQA

C10-DFQA

C16-DFQA

chemical formula formula weight crystal size (mm) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g/cm3) µ (mm-1) 2θmax (deg) temperature (K) F (000) no. unique data (Rint) no. obsd data [I > 2σ(I)] R Rw GOF

C28H26F2N2O2 460.51 0.25 × 0.18 × 0.14 monoclinic P2(1)/n 5.0366(10) 19.092(4) 11.454(2) 90 99.39(3) 90 1086.7(4) 2 1.407 0.100 27.47 293(2) 484 10247 (0.0433) 2467 0.0453 0.1101 1.017

C36H42F2N2O2 572.72 0.34 × 0.21 × 0.16 triclinic P1j 8.0930(16) 8.8966(18) 12.017(2) 110.82(3) 101.89(3) 99.42(3) 764.3(3) 1 1.244 0.085 27.46 293(2) 306 7612 (0.0346) 3474 0.0516 0.1705, 0.709

C40H50F2N2O2 628.82 0.29 × 0.26 × 0.08 monoclinic C2/c 22.992(5) 9.991(2) 15.192(3) 90 98.67(3) 90 3450.1(12) 4 1.211 0.081 27.48 293(2) 1352 16646 (0.0677) 3943 0.0583 0.1843 0.771

C52H74F2N2O2 797.13 0.22 × 0.20 × 0.11 monoclinic P2(1)/c 16.163(3) 10.169(2) 15.187(3) 90 113.16(3) 90 2294.8(8) 2 1.154 0.074 27.47 293(2) 868 21696 (0.1293) 5222 0.0873 0.2357 0.864

hexagonal microcrystals are prepared from C16-DFQA (Figures 1d and 2d). The solid state emission properties of micromaterials, C4DFQA, C8-DFQA, C10-DFQA, and C16-DFQA, were characterized at room temperature. The Cn-DFQA micromaterials with different morphologies display remarkably different photoluminescence properties. Figure 3 presents the comparison of the emission spectra of the microcrystals and the dilute solutions

Figure 5. ORTEP drawings of compounds (a) C4-DFQA, (b) C8-DFQA, (c) C10-DFQA, and (d) C16-DFQA, respectively, with 30% probability ellipsoids.

(1.0 × 10-6 M) in THF. For the dilute solutions of Cn-DFQA (n ) 4, 8, 10, 16), the emissions spectra profiles are approximately the same with a sharp emission peak at 533 nm and a shoulder at around 573 nm, and the PL quantum yields (97% for C4DFQA, 97% for C8-DFQA, 98% for C10-DFQA, and 98% for C16-DFQA) are very approximate with each other. However, the photoluminescent quantum yields of the micromaterials are 0.19% for C4-DFQA, 0.33% for C8-DFQA, 0.57% for C10DFQA, and 1.12% for C16-DFQA, respectively. The strong intermolecular interactions should be responsible for the low solid state quantum yield.23 The emission spectra of the micromaterials show a remarkable red shift compared to that of the THF solutions. The micromaterials of C4-DFQA, C8-DFQA, C10-DFQA, and C16-DFQA exhibit emission maxima at around 622, 627, 614, and 613 nm, respectively. It is worth noting that the emission of C4-DFQA and C8-DFQA 1-D microfibers displays a stronger red shift compared with that of C10-DFQA and C16-DFQA microparticles. In Figure 3, besides the main peaks discussed above, there are also shoulders in the blue region for Cn-DFQA (n ) 8, 10, 16). A possible explanation for these phenomena is that the different emission peaks were attributed to the excitons delocalized on the Cn-DFQA molecular aggregates with different numbers of Cn-DFQA molecules in the solid state.25 The absorption spectra of the micromaterials are presented in Figure 4, which are similar to the solid absorption spectra of the QAs without fluorine atoms.17 To understand the alkyl chain length dependent morphology property and different emission feature of the Cn-DFQA-based microcrystals, the single crystal structures of four Cn-DFQA compounds have been characterized. The crystal data and refinement parameters are summarized in Table 1. The molecular geometries of these compounds are shown in Figure 5, respectively. The ORTEP drawings of these molecules reveal that all molecules are centrosymmetric, and two alkyl chains adopt a regular all-trans conformation. (24) (a) Kasai, H.; Kamatani, H; Okada, S.; Oikawa, H.; Matsuda, H.; Nakanishi, H. Jpn. J. Appl. Phys. Part 2 1996, (2B), L221. (b) Fu, H. B.; Yao, J. N. J. Am. Chem. Soc. 2001, 123, 1434. (c) Fu, H. B.; Loo, B. H.; Xiao, D. B.; Xie, R. M.; Ji, X. H.; Yao, J. N.; Zhang, B. W.; Zhang, L. Q. Angew. Chem., Int. Ed. 2002, 41, 962. (25) (a) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765. (b) Lim, S. H.; Bjorklund, T. G.; Spano, F. C.; Bardeen, C. J. Phys. ReV. Lett. 2004, 92, 107402(1).

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Figure 6. (a) Stacking diagram showing π · · · π interactions and CdO · · · H-C hydrogen bonds within a column in C4-DFQA along the a-axis. (b) View of the molecular columns connected by CdO · · · H-C and C-F · · · H-C hydrogen bonds in C4-DFQA.

Figure 7. (a) Stacking diagram showing π · · · π interactions within a column and (b) view of columns connected by CdO · · · H-C hydrogen bonds in C8-DFQA.

However, the molecular packing feature of the four crystals is remarkably different. In the single crystal of C4-DFQA, there are strong intermolecular π · · · π stacking interactions accompanied by hydrogen bonding contacts resulting in the formation of one-dimensional molecular columns (Figure 6a). In each column, hydrogen bonding interactions occur between CdO and CH2 groups, and the molecules pack into a “ladder” arrangement. The interplanar distance between adjacent molecules is 3.43 Å, and the overlap area between two neighboring quinacridone rigid cores is over three aromatic rings. The molecular columns are further held together along the c axis through intermolecular hydrogen bonding interactions between oxygen atoms on CdO groups and hydrogen atoms on the terminal CH3 of alkyl chains with a distance of 2.62 Å (Figure 6b). These columns are further held into a “herringbone” arrangement with intermolecular C-F · · · H-C hydrogen bonding interactions. The distance of C-F · · · H-C interaction is 2.53 Å which is shorter than the sum of the van der Waals radii of fluorine and hydrogen of 2.55 Å.26 One-dimensional molecular stacking columns are also found in the solid C8-DFQA, and the molecules adopt a “staircase” arrangement in a molecular column (Figure 7a). However, there is no hydrogen bonding interaction within a molecular column, and the molecules are only held into the columns by π · · · π stacking (3.38 Å) interactions with 1/3 rigid core overlap. The molecular columns are further held together through intermolecular hydrogen bonding interactions between oxygen atoms on CdO groups and hydrogen atoms on outer phenyl rings (Figure 7b). The hydrogen bonding distance for CdO · · · H-C is 2.46 Å. In single-crystal C10-DFQA, the molecules are assembled into molecular chains with intermolecular hydrogen bonding (26) (a) Ma, J. A.; Cahard, D. Chem. ReV. 2004, 104, 6119. (b) Babudri, F.; Farinola, G. M.; Naso, F.; Ragni, R. Chem. Commun. 2007, 1003.

Figure 8. (a) Stacking diagram showing the molecular chain driven by C-F · · · H-C hydrogen bonds and (b) view of hydrogen-bonded molecular sheet in C10-DFQA driven by CdO · · · H-C and C-H · · · π hydrogen bonds (hydrogen atoms are omitted for clarity).

Micromaterials Based on Quinacridone DeriVatiVes

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Figure 9. View of sandwich C-H · · · π stacking structure in (a) C10-DFQA and (b) C16-DFQA crystals (only one quinacridone core and two alkyl chains are shown for clarity).

Figure 11. Powder XRD patterns of Cn-DFQA (n ) 4, 8, 10, 16) measured from micromaterials (dash lines) and calculated from single crystals (solid lines). Black, C4-DFQA; Red, C8-DFQA; Green, C10DFQA; Blue, C16-DFQA.

Figure 10. (a) Stacking diagram showing the molecular chain driven by C-F · · · H-C hydrogen bonds and (b) view of hydrogen-bonded molecular sheet in C16-DFQA connected by CdO · · · H-C and C-H · · · π hydrogen bonds (hydrogen atoms are omitted for clarity).

interactions between fluorine atoms on the rigid core and hydrogen atoms on the CH2 of alkyl chains with a distance of 2.54 Å (Figure 8). The molecular chains are further connected into molecular sheets based on CdO · · · H-C hydrogen bonds and weak C-H · · · π interactions. The distance and angle of CdO · · · H-C interactions are 2.40 Å and 159.8°, respectively. A quinacridone core is sandwiched by two parallel decyl chains of the two adjacent C10-DFQA molecules through C-H · · · π interactions with contact distances of 2.735 to 3.238 Å (Figure 9a).27 The molecular arrangement and intermolecular interactions in solid C16-DFQA are similar to that of single-crystal C10DFQA. In single-crystal C16-DFQA, the molecular chains are held together with intermolecular hydrogen bonding interactions (27) Nishio, M. CrystEngComm 2004, 6, 130.

between fluorine atoms on the rigid core and hydrogen atoms on the CH2 of alkyl chains with a distance of 2.52 Å (Figure 10). The molecular sheets are connected from the molecular chains with intermolecular CdO · · · H-C hydrogen bonds and weak C-H · · · π interactions. The distance and angle of CdO · · · H-C interactions are 2.45 Å and 162.6°, respectively. A quinacridone core is sandwiched by two parallel hexadecyl chains of the two adjacent C16-DFQA molecules through C-H · · · π interactions. The contact distances of C-H · · · π ranged from 2.750 to 3.392 Å (Figure 9b). It is obvious, upon increasing the length of the alkyl chains, the interactions between the alkyl chains and aromatic cores appeared. Therefore, the steric effect of long alkyl chains also has an influence on the molecular packing in the solid state of the Cn-DFQA system. Therefore, the distinct morphologies of the microstructures generated from the four compounds may derive from the different molecular packing structures as observed in the single crystals of Cn-DFQA (n ) 4, 8, 10, 16). In the crystals C4-DFQA and C8-DFQA, the molecular packing structures reveal that 1-D molecular columns are formed based on the intermolecular π · · · π and hydrogen bonding interactions. Thus, the molecular arrangements in the crystals C4-DFQA and C8-DFQA display obviously anisotropic features suggesting that C4-DFQA and C8-DFQA should have the tendency to aggregate into a 1-D structure in nature. Unlike C4-DFQA and C8-DFQA, there is no obvious 1-D growth tendency in the crystals C10-DFQA and C16-DFQA, and the molecules are assembled into 2-D molecular sheets based on the hydrogen bonding and C-H · · · π interactions, which should benefit the formation of plate microcrystals. The common structural feature of crystals C4-DFQA and C8-DFQA

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is that there are intermolecular π · · · π stacking interactions. According to the research by our and other groups, the faceto-face intermolecular π · · · π interactions can have a dramatic impact on the luminescence properties of organic materials and lead to red shifts of emission spectra due to excimer emission formation.17,25a,28 In the crystals C10-DFQA and C16-DFQA, there is no intermolecular π · · · π stacking interaction. Therefore, the intermolecular interaction properties of C4-DFQA, C8-DFQA, C10-DFQA, and C16-DFQA provide a possible explanation for the emission spectra of these four organic micromaterials. Figure 11 represents the X-ray diffraction (XRD) patterns of micromaterials and that of the single crystals of Cn-DFQA, which are simulated by using the program of MERCURY based on the single-crystal X-ray diffraction data. The XRD patterns of the four kinds of micromaterials exhibit profiles similar to the corresponding simulated XRD patterns of the four single-crystal Cn-DFQA, although the crystalline degree of the micromaterials is not very high. Therefore, employing the single-crystal structures to explain the micromorphology formation properties of C4DFQA, C8-DFQA, C10-DFQA, and C16-DFQA is acceptable.

Zhao et al.

DFQA with shorter alkyl chains could assemble into 1-D micromaterials, while C10-DFQA and C16-DFQA with longer alkyl chains aggregate into diamond and hexagonal microparticle crystals, respectively. The emission spectra of the 1-D micromaterials formed by C4-DFQA or C8-DFQA exhibited a red shift compared with that of the microparticle crystals composed of C10-DFQA or C16-DFQA. The single-crystal structure analysis revealed that in the crystals C4-DFQA and C8-DFQA there are 1-D molecular columns based on intermolecular π · · · π and hydrogen bond interactions, while 2-D hydrogen bond molecular sheets are observed in the crystals C10-DFQA and C16-DFQA. The molecular packing properties of the four crystals suggest that C4-DFQA and C8-DFQA molecules have the tendency to form a 1-D structure, while C10-DFQA and C16DFQA molecules possess the characteristics to generate the sheet structures. The single-crystal structures give a rational explanation for the alkyl chain length dependent morphology properties of the Cn-DFQA based micromaterials. Therefore, it is possible to control the morphologies and emission properties of the organic micro- and nanomaterials through varying the molecular structures.

Conclusions In summary, four fluorinated quinacridone compounds (CnDFQA: C4-DFQA, C8-DFQA, C10-DFQA, and C16-DFQA) have been synthesized. It was demonstrated that Cn-DFQA could be used as building blocks to fabricate organic luminescent micromaterials. The assembly properties of Cn-DFQA have obvious alkyl chain length dependent characteristics. The alkyl chain length has a dramatic effect on the morphology of the resulting micromaterials. The molecules C4-DFQA and C8(28) Zhang, H. Y.; Zhang, Z. L.; Ye, K. Q.; Zhang, J. Y.; Wang, Y. AdV. Mater. 2006, 18, 2369.

Acknowledgment. This work was supported by the National Natural Science Foundation of China (50733002 and 50773027), the Major State Basic Research Development Program (2009CB623600), and Program for Changjiang Scholars and Innovative Research Team in University (IRT0422). Supporting Information Available: X-ray crystallographic information files (CIF files). This material is available free of charge via the Internet at http://pubs.acs.org. LA804182D