Anthracene-Arrangement-Dependent Emissions of Crystals of 9

(c) McGrady , G. S.; Odlyha , M.; Prince , P. D.; Steed , J. W. CrysEngComm 2002, 4, 271. [Crossref], [CAS]. 5. Why do polymorphs form? A single cryst...
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DOI: 10.1021/cg9008569

Anthracene-Arrangement-Dependent Emissions of Crystals of 9-Anthrylpyrazole Derivatives

2009, Vol. 9 5069–5076

Zuolun Zhang, Yu Zhang, Dandan Yao, Hai Bi, Iqbal Javed, Yan Fan, Hongyu Zhang,* and Yue Wang* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China Received July 23, 2009; Revised Manuscript Received October 15, 2009

ABSTRACT: We have designed and synthesized a series of 9-anthrylpyrazole derivatives 1,4-bis(3-(9-anthryl)-1-pyrazolylmethyl)benzene (1), 1-(3-(9-anthryl)-1-pyrazolylmethyl)-4-(5-(9-anthryl)-1-pyrazolylmethyl)benzene (2), 1,4-bis(3-(9-anthryl)1-pyrazolyl)benzene (3), and 1-(3-(9-anthryl)-1-pyrazolyl)-4-(5-(9-anthryl)-1-pyrazolyl)benzene (4). All compounds formed two types of crystals that exhibited anthracene-arrangement-dependent emission colors. For instance, crystal 1a with strong π-overlap between anthracene moieties exhibited an emission maximum at 515 nm, while 1b with no such interchromophore interactions displayed an emission band at 424 nm. The fluorescence quantum yield (ΦF) measurements showed that the blueemitting crystals have high quantum yields (ΦF = 0.46 for 1b, 0.90 for 2a, 0.91 for 2b, 0.77 for 3b, and 0.51 for 4a), suggesting their potential as blue emitters in optoelectronics.

Introduction The exploitation of highly emissive organic solids is of immense interest due to their widespread and direct applications in the field of optoelectronics such as organic electroluminescent devices.1,2 Generally, there are two approaches to achieve luminescent materials with desired photophysical properties. One of which is the modification of molecular structure. Emission properties of luminescent materials can be efficiently modulated by this method; however, it is timeconsuming and expensive because of the possibility of requiring difficult synthesis. The other approach is the control of solid-state structure, namely, tuning of the packing and arrangement manner of chromophores in the solid state. This method can also endow required emission properties since the properties of luminescent materials in the solid state are determined by the whole collective rather than by individual molecules.3-6 The arrangement of chromophores exerts great influence on the luminescent properties, and consequently, the performance of optical devices strongly depends on the molecular assembly structures.7 The latter approach toward different emission properties is of particular interest considering that there are no requirements of further synthesis. To extensively utilize this approach, it is necessary to demonstrate well the relationship between molecular arrangement and emission properties. The past decade witnessed a lot of efforts, experimental as well as theoretical, toward understanding this relationship; however, systematic investigations remain elusive.8,9 Solid-state blue-emitting materials are deemed critical and attract a considerable amount of interest because these materials not only provide blue emission but also white and other emission colors through energy transfer.10,11 Among the most frequently used emission materials in this field, anthracene derivatives are one of the promising families due to their excellent properties, such as intensive luminescence as well as

high thermal stabilities.12-14 Unfortunately, many anthracenebased organic emission materials that exhibit bright blue fluorescence in solution usually shift their emission maxima to the longer wavelength region in the solid state, which is accompanied by decreased fluorescence efficiencies, due to the formation of molecular aggregates.15 We have previously reported that 3(5)-(9-anthryl)pyrazole (ANP) can produce five polymorphs and demonstrated that the fluorescence of the produced polymorphs was relative to the π-overlap degree of anthracene moieties.16 To evaluate the generality of these types of anthracene-arrangementdependent emission properties and construct a model system for understanding how anthracene packing affects the luminescent properties, we are now interested in designing a series of anthracene derivatives and studying their solid-state structures as well as emission properties. In this study, four ANP-contained compounds 1-4 were synthesized. The former two compounds can be regarded as a model with molecular flexibility, while the latter two are π-conjugated molecules with rigid frameworks. Two types of crystals were obtained for each compound. The fluorescence of crystalline samples as well as the anthracene-arrangement modes in the packing structures was carefully investigated. Another goal of this study was to explore highly blue-emissive organic solids. Therefore, the potential of these crystalline materials as blue emitters in optoelectronics are also discussed. Experimental Section

*Authors to whom correspondence should be addressed. E-mail: [email protected] (H.Z.); [email protected] (Y.W.).

General Information. Starting materials were purchased from Aldrich Chemical Co. and used without further purification. The solvents for syntheses were freshly distilled over appropriate drying reagents except for methanol which was analytical reagent grade. All synthesis experiments were performed under a nitrogen atmosphere by using standard Schlenk techniques. NMR spectra were recorded on a Bruker AVANCE 500 MHz spectrometer (500 MHz for 1H, 126 MHz for 13C) with CDCl3 as solvents and tetramethylsilane as internal standard. Mass spectra were recorded on a Shimadzu AXIMA-CFR MALDI-TOF mass spectrometer. Elemental analyses were performed on a flash EA 1112 instrument. UV-vis absorption

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Scheme 1. Chemical Structures of Compounds 1-4

spectra were recorded using a PE UV-vis lambdazo spectrometer. The emission spectra were recorded using a Shimadzu RF-5301 PC spectrometer. The fluorescence quantum yields (ΦF) of the crystals were measured using a calibrated integrating sphere in terms of the previously reported procedures.17 Differential scanning calorimetric (DSC) measurements were performed on a NETZSCH DSC204 instrument. X-ray powder diffraction (XRPD) patterns were measured on a Rigaku D/Max 2550 diffractometer with Cu KR radiation (λ = 1.5418). ANP was synthesized according to the literature procedure.16 Syntheses of 1,4-Bis(3-(9-anthryl)-1-pyrazolylmethyl)benzene (1) and 1-(3-(9-Anthryl)-1-pyrazolylmethyl)-4-(5-(9-anthryl)-1-pyrazolylmethyl)benzene (2). A solution of 1,4-bis(bromomethyl)benzene (235 mg, 0.89 mmol) in THF (40 mL) was added dropwise to a refluxed mixture of ANP (478 mg, 1.96 mmol), NaH (171 mg, 7.12 mmol) and THF (60 mL) under stirring. After addition, the reaction mixture was refluxed for 35 h, then, cooled to room temperature, and methanol (15 mL) was added dropwise to it. THF and the excess methanol were removed by vacuum, and the resulting residue was extracted with dichloromethane (3  40 mL). The dichloromethane extract was washed with water, dried over Na2SO4, and filtered. The filtrate was evaporated to dryness, and the resulting residue was purified by column chromatography twice (silica gel, dichloromethane/diethyl ether = 16:1) to give 1 (Rf = 0.73, 226 mg, 43% yield) and 2 (Rf = 0.58, 173 mg, 33% yield). Compound 1: 1H NMR (CDCl3, ppm): δ 8.51 (s, 2 H), 8.03 (d, J = 8.5 Hz, 4 H), 7.92 (d, J = 9 Hz, 4 H), 7.70 (d, J = 2.5 Hz, 2 H), 7.45-7.43 (m, 8 H), 7.40-7.37 (m, 4 H), 6.58 (d, J = 2 Hz, 2 H), 5.57 (s, 4 H). 13C NMR (CDCl3, ppm): δ 149.01, 136.84, 131.38, 131.18, 130.09, 128.82, 128.30, 128.21, 127.42, 126.60, 125.61, 125.03, 109.61, 55.88. Ms m/z: 590.2 [M]þ (calcd: 590.3). Anal. Calcd (%) for C42H30N4: C, 85.40; H, 5.12; N, 9.48. Found: C, 85.28; H, 5.21; N, 9.43. Compound 2: 1H NMR (CDCl3, ppm): δ 8.59 (s, 1 H), 8.50 (s, 1 H), 8.04 (t, J = 9.5 Hz, 4 H), 7.91-7.88 (m, 3 H), 7.46-7.44 (m, 7 H),7.39-7.32 (m, 4 H), 6.98 (d, J = 8 Hz, 2 H), 6.71 (d, J = 8 Hz, 2 H), 6.57-6.52 (m, 2 H), 5.30 (s, 2 H), 4.98 (s, 2 H). 13C NMR (CDCl3, ppm): δ 148.63, 139.39, 139.18, 136.23, 135.84, 131.36, 131.28, 131.15, 131.09, 129.80, 128.85, 128.74, 128.45, 128.41, 128.27, 127.57, 127.36, 126.63, 126.40, 125.66, 125.55, 125.39, 125.01, 124.28, 109.44, 109.38, 55.73, 53.51. Ms m/z: 589.9 [M]þ (calcd: 590.3). Anal. Calcd (%) for C42H30N4: C, 85.40; H, 5.12; N, 9.48. Found: C, 85.43; H, 5.36; N, 9.64. Syntheses of 1,4-Bis(3-(9-anthryl)-1-pyrazolyl)benzene (3) and 1-(3-(9-Anthryl)-1-pyrazolyl)-4-(5-(9-anthryl)-1-pyrazolyl)benzene (4). DMF (6 mL) was added to a mixure of ANP (600 mg, 2.46 mmol), 1,4-dibromobenzene (232 mg, 0.98 mmol), CuI (63 mg, 0.33 mmol), 1,10-phenanthroline (119 mg, 0.66 mmol), and Cs2CO3 (1.30 g, 3.99 mmol). After degassing via three freeze-thaw cycles, the reaction mixture was heated under reflux for 18 h. When cooled to room temperature, the mixture was poured into 50 mL of dichloromethane, treated with ultrasonication for 10 min, and filtered. The filtrate was concentrated by vacuum to yield a brown residue which was purified

by column chromatography (silica gel, dichloromethane/diethyl ether = 20:1) to successively give 3 (Rf = 0.86, 393 mg, 71% yield) and 4 (Rf = 0.62, 60 mg, 11% yield). Compound 3: 1H NMR (CDCl3, ppm): δ 8.55 (s, 2 H), 8.28 (d, J = 2.5 Hz, 2 H), 8.06 (d, J = 8.5 Hz, 4 H), 8.03 (d, J = 8.5 Hz, 4 H), 7.99 (s, 4 H), 7.50-7.42 (m, 8 H), 6.77 (d, J = 2.5 Hz, 2 H). 13C NMR (CDCl3, ppm): δ 150.93, 138.39, 131.42, 131.16, 128.40, 128.13, 127.83, 127.36, 126.53, 125.85, 125.17, 119.95, 111.51. Ms m/z: 562.3 [M]þ (calcd: 562.2). Anal. Calcd (%) for C40H26N4: C, 85.38; H, 4.66; N, 9.96. Found: C, 85.55; H, 4.77; N, 9.68. Compound 4: 1H NMR (CDCl3, ppm): δ 8.54 (s, 1 H), 8.48 (s, 1 H), 8.03 (d, J = 1.5 Hz, 1 H), 8.00 (t, J = 9 Hz, 4 H), 7.97 (d, J = 2.5 Hz, 1 H), 7.85 (d, J = 8.5 Hz, 2 H), 7.69 (d, J = 9 Hz, 2 H), 7.47-7.40 (m, 8 H), 7.35-7.33 (m, 2 H), 7.23 (d, J = 9 Hz, 2 H), 6.69 (d, J = 2 Hz, 1 H), 6.60 (d, J = 2.5 Hz, 1 H). 13C NMR (CDCl3, ppm): δ 150.73, 140.69, 140.00, 139.27, 138.45, 138.16, 131.29, 131.11, 131.00, 128.92, 128.64, 128.29, 128.01, 127.69, 127.17, 126.75, 126.41, 125.71, 125.53, 125.44, 125.07, 124.55, 124.04, 118.98, 111.38, 111.30. Ms m/z: 562.3 [M]þ (calcd: 562.2). Anal. Calcd (%) for C40H26N4: C, 85.38; H, 4.66; N, 9.96. Found: C, 85.51; H, 4.87; N, 9.72. Crystal Growth. Crystals 1a, 2a, 3a, 3b, 4a, and 4b were obtained by slowly diffusing petroleum ether vapor into a chloroform solution of 1, a benzene solution of 2, a chloroform solution of 3, a pyridine solution of 3, a chloroform solution of 4, and a THF solution of 4 at room temperature, respectively. Crystals 1b and 2b were obtained by slowly diffusing petroleum ether vapor into a pyridine solution of 1, and a THF solution of 2 at about 4 °C, respectively. Crystals 1a, 1b, 2a, 2b, 3a, 4a, and 4b were blockshaped with typical sizes of 0.17 mm  0.31 mm  0.44 mm, 0.16 mm  0.23 mm  0.25 mm, 0.18 mm  0.18 mm  0.19 mm, 0.26 mm  0.31 mm  0.33 mm, 0.13 mm  0.17 mm  0.20 mm, 0.10 mm  0.10 mm  0.10 mm, and 0.23 mm  0.33 mm  0.37 mm, respectively. Crystal 3b was needle-shaped with a typical size of 0.33 mm  0.08 mm  0.06 mm. X-ray Crystallography. The crystal structures were determined by single-crystal X-ray diffraction experiments. Diffraction data were collected on a Rigaku RAXIS-PRID diffractometer using the ω-scan mode with graphite-monochromator Mo 3 KR radiation. The structures were solved with direct methods using the SHELXTL programs and refined with full-matrix least-squares on F2. Non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated and refined isotropically. A summary of crystal data and refinement parameters can be found in Tables S1 and S2, Supporting Information. The selected bond lengths and angles of the crystals are listed in Tables S3 and S4, Supporting Information.

Results and Discussion Synthesis and Crystal Growth. The chemical structures of the materials used in this study are shown in Scheme 1. Compound 1 with two anthracene rings lying at 3-positions

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Figure 2. Fluorescence spectra of the crystals of 1-4.

Figure 1. UV-vis absorption (a) and fluorescence spectra (b) of compounds 1-4 in CHCl3 (3  10-6 M).

and 2 with two anthracene rings lying at 3- and 5-positions of pyrazole rings were simultaneously synthesized from the reaction of ANP and 1,4-bis(bromomethyl)benzene in the presence of sodium hydride and easily separated by common column chromatography in 43 and 33% yields, respectively. Compounds 3 and 4 were obtained simultaneously in 71 and 11% yields, respectively, by copper-catalyzed Ullmann-type coupling reaction of ANP with 1,4-dibromobenzene. The molecular structures of these compounds were characterized by 1H and 13C NMR, mass spectrometry, elemental analysis, and finally confirmed by single crystal X-ray crystallographic data. Single crystals were obtained by slowly diffusing petroleum ether vapor into the solution of compounds 1-4. This class of molecules forms polymorphs or pseudo-polymorphs in different solvents and, hence, it was possible to obtain two types of single crystals for each compound by changing solvents, such as CHCl3, THF, pyridine, benzene, and so on. Emission Properties of Compounds 1-4. The photophysical properties of anthracene derivatives 1-4 in solution are first studied. The absorption and fluorescence spectra were measured for the dilute chloroform solution, as shown in Figure 1. All the compounds exhibit similar absorption bands typically in the range of 325-420 nm. The characteristic vibrational fine structure of the absorption spectra can be attributed to the π-π* transitions of the anthracene

group.18 The emission maxima of these compounds are located in the range of 425-450 nm. The observed emission profiles of 1 and 2, and 3 and 4 are identical, which indicates that the positions of anthracene groups relative to pyrazole rings only have little impact on their luminescent properties. The emission properties of crystals are very interesting (Figure 2). The emission band of crystal 1a (λmax = 515 nm) lies in the green region, while that of 1b (λmax = 424 nm) is distinctly in the blue region, and similar observations were found in crystals 3a (λmax = 505 nm) and 3b (λmax = 426 nm). Crystal 4a shows sky-blue emission with a peak centered at 465 nm, while 4b exhibits a deep blue emission at 432 nm. The fluorescence spectra of 2a and 2b are almost fully overlapping, showing an emission maximum at 447 nm for the former and 448 nm for the latter. The differences of emission maxima between the two crystals of compounds 1, 3, and 4 are 91, 79, and 33 nm, respectively. These values are really large in the cases of 1 and 3 considering the fact that both crystals are constructed by the same molecule. The spectra of crystals 1b, 3b, and 4b with vibrational bands can be assigned to the emission from anthracene monomer. On the other hand, other five crystals show structureless emission profiles, indicating that interchromophore interactions between anthracene moieties exist in their packing structures.15d To gain a deep insight into the solid-state emission behaviors of these anthracene derivatives, it is highly important to know the details of molecular packing modes, especially the arrangement of anthracene moieties, in their single crystals. This is because the molecular packing modes are normally related to the nature of weak interactions that typically decide the bulk alignment of the molecules and thereby the photophysical properties of the materials. X-ray Crystal Structures. Figure 3 shows the conformation and packing structure of the molecules in crystal 1a. The torsion angles that could basically describe the molecular conformation are listed in Table 1. The N1-C18-C19-C20 torsion angle is -89.05°, resulting in that the nitrogen atoms N1 and N1A deviate from central benzene plane by a large distance of 1.27 A˚ and, consequently, both ANP units are located at the same side of central benzene plane. The molecule exhibits a cleft structure with two anthracene rings showing an interplanar angle of 70.77° and a centroidcentroid distance of 9.85 A˚. In the packing structure, both anthracene rings are π-overlapped with other anthracene

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Figure 3. (a) Molecular structure of 1a; (b) molecular packing structure of 1a (yellow anthracene rings show the π 3 3 3 π stacking). Table 1. Selected Torsion Angles (deg) for Compounds 1-4 Crystal 1a N(1)-C(18)-C(19)-C(20) N(2)-N(1)-C(18)-C(19) C(2)-C(1)-C(15)-C(16) Crystal 1b N(1)-C(18)-C(19)-C(20) N(2)-N(1)-C(18)-C(19) C(2)-C(1)-C(15)-C(16) Crystal 2a N(1)-C(18)-C(19)-C(20) N(2)-N(1)-C(18)-C(19) C(2)-C(1)-C(15)-C(16) N(4)-C(25)-C(22)-C(21) N(3)-N(4)-C(25)-C(22) C(30)-C(29)-C(28)-C(27) Crystal 2b N(1)-C(18)-C(19)-C(20) N(2)-N(1)-C(18)-C(19) C(2)-C(1)-C(15)-C(16) N(4)-C(25)-C(22)-C(21) N(3)-N(4)-C(25)-C(22) C(30)-C(29)-C(28)-C(27) Crystal 3a C(17)-N(1)-C(18)-C(19) C(2)-C(1)-C(15)-C(16) Crystal 4a C(17)-N(1)-C(18)-C(19) C(2)-C(1)-C(15)-C(16) C(20)-C(21)-N(4)-N(3) C(25)-C(26)-C(27)-C(28) Crystal 4b C(17)-N(1)-C(18)-C(19) C(2)-C(1)-C(15)-C(16) C(20)-C(21)-N(4)-N(3) C(25)-C(26)-C(27)-C(28)

-89.05 -71.36 -102.18 178.76 89.46 -71.77 140.33 -64.51 -104.88 -120.65 -89.74 -83.35 -140.92 65.13 103.10 122.23 88.55 83.36 -15.21 119.34 22.26 106.03 -26.24 -83.22 -12.36 104.84 -140.03 -108.01

rings of neighboring molecules with an interplanar angle of 0.00° and an interplanar distance of about 3.45 A˚; thus, a one-dimensional molecular chain was formed. In 1b (Figure 4), the molecules adopt a totally different conformation compared with those in 1a. The N1-C18-C19-C20 torsion angle is 178.76°, and thus nitrogen atoms N1 and N1A are almost in the central benzene plane. The two anthracene rings are parallel with a centroid-centroid distance of 15.63 A˚, which is obviously larger than that in crystal 1a. Although molecules in 1b have more outspread geometry

Figure 4. (a) Molecular structure of 1b; (b) C-H 3 3 3 π hydrogen bond in 1b (H 3 3 3 π-ring centroid = 2.77 A˚, H 3 3 3 π-ring plane = 2.72 A˚, C-H 3 3 3 π-ring centroid = 163.71°, interplanar angle between interacting rings = 66.20°); (c) C-H 3 3 3 N hydrogen bond in 1b (H 3 3 3 N = 2.61 A˚, C-H 3 3 3 N = 166.37°).

compared with those in 1a, surprisingly, the major interactions in the crystal packing are C-H 3 3 3 π and C-H 3 3 3 N hydrogen bonding in nature, and no π 3 3 3 π interactions were observed between neighboring anthracene rings. As for crystals 2a and 2b, the molecular structures are similar with two ANP groups located at the same side of central benzene plane (Figure 5). The only difference between these two crystals is that no solvent molecules were found in the crystalline lattice of 2b, while benzene molecules exist in 2a. In their packing structures, C-H 3 3 3 N hydrogen bonding as well as C-H 3 3 3 π interactions are predominant between neighboring molecules. There is only a very weak π-overlap between anthracene rings with the overlapping area of about 1/3 of an anthracene ring and the interplanar angle of 0.00°. In addition, each molecule only donates one anthracene group to participate in the π-overlap. The arrangement of the molecules can be basically described as a one-dimensional double molecular chain formed through C-H 3 3 3 N hydrogen bonding and edge-to-face interactions. Compared with 1 and 2, compounds 3 and 4 have a more rigid framework. In crystal 3a, the two pyrazole rings are conjugated with the linker benzene ring. The dihedral angle between pyrazole and benzene rings is 14.58°; therefore, the central three rings (a benzene and two pyrazole) have a nearly coplanar skeleton, as shown in Figure 6a. In the packing structure, there are strong π 3 3 3 π stacking interactions between neighboring anthracene rings, as evidenced by the fact that about 2/3 of an anthracene ring participates in the overlap with an interplanar angle of 0.00° and a short

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Figure 6. (a) Molecular structure of 3a; (b) molecular packing structure of 3a (yellow anthracene rings show the π 3 3 3 π stacking).

Figure 7. (a) Molecular structure of 4a; (b) molecular packing structure of 4a, green line: C-H 3 3 3 N hydrogen bond (H 3 3 3 N = 2.41 A˚, C-H 3 3 3 N = 165.98°).

Figure 5. (a) Molecular structure of 2a; (b) molecular packing structure of 2a, pink line: C-H 3 3 3 π hydrogen bond (H 3 3 3 π-ring centroid = 3.45 A˚, H 3 3 3 π-ring plane = 3.08 A˚, C-H 3 3 3 π-ring centroid = 159.80°, interplanar angle between interacting rings = 80.79°), green line: C-H 3 3 3 N hydrogen bond (H 3 3 3 N = 2.75 A˚, C-H 3 3 3 N = 148.25°); (c) molecular structure of 2b; (d) molecular packing structure of 2b, pink line: C-H 3 3 3 π hydrogen bond (H 3 3 3 π-ring centroid = 3.34 A˚, H 3 3 3 π-ring plane = 3.04 A˚, C-H 3 3 3 π-ring centroid = 163.47°, interplanar angle between interacting rings = 79.72°), green line: C-H 3 3 3 N hydrogen bond (H 3 3 3 N = 2.74 A˚, C-H 3 3 3 N = 150.41°).

interplanar distance of 3.48 A˚ (Figure 6b). Compound 3 produced needle-like crystal 3b; however, its crystal structure has not yet been determined due to the weak diffraction intensity as well as the small crystal size.

Molecules in crystals 4a and 4b have different conformations. In 4a (Figure 7), both anthracene rings lie on the same side of pyrazole-benzene-pyrazole conjugated core. There is a weak π 3 3 3 π stacking interaction between anthracene rings of neighboring molecules with the π-overlapping area of about 1/3 of an anthracene ring, interplanar distance of 3.45 A˚, and interplanar angle of 0.00°. Similar to the cases of 2a and 2b, for each molecule, only one anthracene ring participates in this interaction. In 4b (Figure 8), the two anthracene rings are on either side of the central conjugated core. No obvious π-stacking interactions between anthracene moieties were observed; instead, C-H 3 3 3 N hydrogen bonding becomes a dominant force to determine the packing structure. Relationship between Emission Properties and Crystal Structures. Before the discussion of structure-property relationship, it should be confirmed that the samples used for fluorescence measurement possess a unique crystalline phase. Considering this point, the X-ray powder diffraction (XRPD) of crystals 1a, 1b, 2a, 2b, 3a, and 4a were performed

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Figure 8. (a) Molecular structure of 4b; (b) molecular packing structure of 4b, green line: C-H 3 3 3 N hydrogen bond (H 3 3 3 N = 2.74 A˚, C-H 3 3 3 N = 155.48°). Table 2. Anthracene Packing Modes and Emission Data of the Crystals

b

a Quantum yield were determined by a calibrated integrating sphere. Upon irradiation of UV light (λ = 365 nm). c Not determined.

using a large amount of crystalline samples. The experimental powder diffraction patterns of these crystals are consistent with the simulated powder diffraction patterns from the single crystal data (Supporting Information: Figure S9), suggesting the unique crystalline phase of these crystals. The XRPD of 3b and 4b were not measured, since the crystal structure was not determined for 3b, and it was difficult to get a large amount of samples for 4b. An interesting correlation was observed between the solidstate emission color and the degree of π 3 3 3 π interaction between anthracene moieties (Table 2). As the contact distance and interplanar angle between the π-stacked

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anthracene rings are very similar, we therefore assume that the degree of π 3 3 3 π interaction between anthracene moieties is only overlapping-area dependent in our system. The emission colors of the crystalline samples exhibit a red shift with increasing overlapping area. About 2/3 of an anthracene ring participates in the overlap in 1a, and no overlapping anthracene rings were found in 1b. Thus, crystal 1a with strong interchromophore π 3 3 3 π interactions exhibits green emission, while crystal 1b with no interchromophore π 3 3 3 π interactions shows deep blue emission under UV irradiation. Crystals 2a and 2b having almost the same small π-stacking degree of anthracene moieties with the overlapping area of about 1/3 of an anthracene ring exhibit identical blue-emission profiles. Crystal 3a, which possesses a large interchromophore π-stacking area similar to that in crystal 1a, displays green emission under the irradiation of UV light. In the cases of 4a and 4b, weak π 3 3 3 π interactions (about 1/3 of an anthracene ring participates in the overlap) between anthracene rings exist in 4a, and no such interactions were observed in 4b. Correspondingly, 4a emits skyblue color while 4b exhibits deep-blue emission. Such a direct correlation between the crystal packing and the emission property is indeed noteworthy. Corresponding to the emission of crystal 3b, a conclusion can be drawn that there is no overlapping between neighboring anthracene rings, although its crystal structure was not determined. These results clearly demonstrate that the emission properties of these compounds are anthracene-arrangement-dependent, which is consistent with the observations in the ANP-based crystal system.16 Some of the crystals in the present system contain solvent molecules; however, the possibility of solvent-induced emission change can be excluded because crystals 2a (solvent molecules contained) and 2b (no solvent molecules) having similar molecular conformations as well as overlapping degrees of anthracene moieties exhibit almost the same emission profile. Crystal Photostability. It is well-known that anthracene and some of its derivatives can undergo [4 þ 4] photocycloaddition reactions.19 We found that crystals 1a and 3a are responsive to UV light. Their fluorescence gradually changed from green to blue under irradiation of 365 nm light (Supporting Information: Figure S10). We suggest that the photocycloaddition took place, since the common feature in the packing structure of these two crystals is the face-to-face arrangement of anthracene moieties, which is thought to facilitate this type of reaction.20 The blue emission should originate from the isolated, unreacted anthracene rings maintained in the crystalline-phase destruction process caused by the photocycloaddition reactions.20a,21 The product of the photochemical reaction could not dissolve in common organic solvents, which is presumably due to the formation of polymerized species considering the infinite one-dimensional molecular chain formed by the π 3 3 3 π stacking of anthracene rings in the crystals 1a and 3a. The blue-emissive crystals 1b, 2a, 2b, 3b, and 4a were also treated by 365 nm UV irradiation (4b was not checked, due to its very small amount). The fluorescence of 2a, 2b, 3b, and 4a did not exhibit an obvious variation (Supporting Information: Figure S11), which is reasonable because the arrangement of anthracene rings in these crystals impedes the occurrence of the photocycloaddition reactions. This result indicates their good photostability. As for 1b, its emission spectrum showed an increase in the intensity of the shoulder at longer wavelength (445 nm). The reason for this spectra variation is

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not very clear at this moment. Some phase transformation may take place during UV irradiation. Crystal Thermal Stability. DSC experiments were carried out to determine the thermal stability of these crystals (4b was not measured, due to its very small amount). The first peak on DSC thermograms for all these crystals corresponds to an endothermic process starting at about 218, 47, 120, 180, 268, 86, and 152 °C for 1a, 1b, 2a, 2b, 3a, 3b, and 4a, respectively (Supporting Information: Figure S12). This process may be attributed to the melting or solvent loss or phase transition of the crystals. No matter what happened, the process will be accompanied by the destruction of the original crystal structure. In another word, these crystals are stable until the endothermic process starts. It was observed that the fluorescence color of these crystals did not exhibit obvious change before the start of the endothermic process. Crystal Fluorescence Quantum Yields. In addition to the interesting structure-emission color relationship, another notable feature of the present system is that all the blueemissive crystals possess high ΦF values (except for 4b whose ΦF was not measured because the amount of this crystal we can obtain was too little), being 0.46, 0.90, 0.91, 0.77, and 0.51 for 1b, 2a, 2b, 3b, and 4a, respectively. This is the fundamental and essential property for blue-emitting materials. For compounds 1 and 3, the ΦF value of blue-emissive crystal is obviously higher than that of the green-emissive crystal, as summarized in Table 2. The difference in the anthracene packing modes might be responsible for the difference in the solid-state ΦF values. This result demonstrates that, for the present anthracence derivatives, the suppression of π-stacking between anthracene rings is crucial for achieving bright fluorescent solid. Conclusions Four anthracene derivatives with flexible (1 and 2) or rigid (3 and 4) frameworks have been synthesized, and two types of crystals were obtained for each compound by solution diffusion method. The emission colors of crystalline samples are strictly related to the intermolecular packing degree of the anthrancene moieties: the larger overlapping area, the greater red shift of emission peak. We proposed that chromophore arrangements play a crucial role in optical properties, which may have significant implications on the understanding of structure-property relationship. Moreover, the fluorescence quantum yield measurements showed that the produced blueemissive crystals possess high quantum yields, suggesting their potentials as blue emitters in optoelectronics. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50733002 and 20772045), the Major State Basic Research Development Program (2009CB623600), the 863 Project (2006AA03A162) and 111 Project (B06009). Supporting Information Available: 1H and 13C NMR spectra of compounds 1-4, crystallographic data for 1-4 in PDF and CIF formats, XRPD patterns, DSC thermograms, and fluorescence variation of the crystals under UV irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.

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