Three-Dimensional Metal− Organic Network Architecture with Large π

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CRYSTAL GROWTH & DESIGN

Three-Dimensional Metal-Organic Network Architecture with Large π-Conjugated Indolocarbazole Derivative: Synthesis, Supramolecular Structure, and Highly Enhanced Fluorescence

2008 VOL. 8, NO. 1 259–264

Hui-Jun Liu, Xu-Tang Tao,* Jia-Xiang Yang, Yun-Xing Yan, Yan Ren, Hua-Ping Zhao, Qian Xin, Wen-Tao Yu, and Min-Hua Jiang State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan, 250100, P. R. China ReceiVed March 22, 2007; ReVised Manuscript ReceiVed September 30, 2007

ABSTRACT: By combining a new, large π-conjugated bidentate ligand, 2,8-bis[2-(2-pyridyl)vinyl]-5,11-di(2-ethylhexyl)-indolo[3,2b]carbazole (BPVIC) with a [Cd(NCS)2]∞ polymer chain, a novel supramolecular coordination polymer, [Cd2(BPVIC)2(SCN)4]∞, with network structure has been prepared. The crystal structures of the ligand and the coordination polymer were determined by X-ray crystallography. The crystallographic analyses revealed that “drums” formed by the ligands, Cd2+ cations, and SCN- anions are interconnected, leading to the formation of infinite three-dimensional networks. The π-π and C-H · · · π interactions among the free ligands, acting as energy traps and increasing nonradiative decay, are eliminated effectively in the metal coordination polymer. The metal coordination polymer exhibits strong luminescence in the yellow region and therefore is a potential luminescent material. Introduction Large π-conjugated systems with plentiful delocalized π-electrons and optimum delocalization environments have received extensive attention due to their potential applications as luminescent materials.1 The properties of luminescent materials are influenced by both intramolecular and intermolecular electronic and steric factors. The intermolecular π-π and C-H · · · π interactions among large π-conjugated molecules exist widely in the solid state.2 However, the π-π and C-H · · · π interactions, acting as energy traps and increasing nonradiative decay in the aggregate state, can weaken or quench luminescent emission.3 It is known that, in a large π-conjugated system, the extended π-electron delocalization environment and strong induction effect can enhance and shift luminescent emission effectively.4 The skeleton of indolo[3,2-b] carbazole is a planar structure possessing abundant π-electrons and also in an optimized delocalization environment.5 Its derivatives exhibit different properties, however, and have seldom been mentioned as luminescent materials. On the other hand, Cd(II)-containing coordination polymers have attracted considerable interest owing to the variability in forming bonds with different donors, various coordination modes, and special physical properties of Cd(II) ion.6 One-, two-, and three-dimensional (1D, 2D, and 3D) Cd(II) coordination compounds exhibit many potential applications in catalysis, phase transformation, host–guest chemistry, NLO materials, and luminescent materials.7 To design highly luminescent materials, here we present the synthesis and characterization of a new multidimensional coordination polymer that was assembled from a new bidentate ligand, an indolo[3,2-b]carbazole derivative, and linear polymeric cadmium thiocyanate. The ordered chains can separate π-systems to limit the intermolecular π-π and C-H · · · π interactions, as spacers and controllers. Moreover, the metal ions have a strong induction effect toward the π-systems, and the π-electron delocalization environment can be extended and * To whom correspondence should be addressed. Phone: +86-0531-88364963. Fax: +86-0531-88574135. E-mail: [email protected].

Table 1. Crystallographic Data for BPVIC and [Cd2(BPVIC)2(SCN)4]∞ formula M crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) vol (Å3) Z dcalc (g/cm3) µ (cm-1) F(000) crystal size data/restraint/parameters reflns collected unique reflns goodness of fit R1[I > 2σ(I)] wR2[I > 2σ(I)] R1 (all data) wR2 (all data) residuals (e · Å-3)

BPVIC

[Cd2(BPVIC)2(SCN)4]∞

C48H54N4 686.95 orthorhombic Pna21 31.4928(10) 5.7534(2) 22.5512(6) 90 90 90 4086.1(2) 4 1.117 0.065 1480 0.16 × 0.11 × 0.06 4819/12/470 22001 4819 1.002 0.0788 0.1738 0.2528 0.2498 0.359, -0.239

C50H54CdN6S2 915.51 monoclinic C2/c 11.4021(2) 32.4814(6) 26.1045(6) 90 91.1640(10) 90 9666.0(3) 8 1.258 0.577 3808 0.31 × 0.28 × 0.22 13279/21/461 38352 13279 0.934 0.0971 0.2735 0.2051 0.3395 1.713, -0.631

enhanced. The luminescence of the ligand and the coordination polymer was investigated. Experimental Section General Remarks. All chemicals and solvents were dried and purified using common methods. The 2,8-dibromo-5,11-di-(2-ethylhexyl)-indolo[3,2-b]carbazole was synthesized according to the literature.5 1H NMR and 13C NMR spectra were carried out on a Bruker av400 MHz spectrometer with chloroform-d as solvent and with tetramethylsilane as internal standard. Elemental analyses were performed on a Vario EL III elemental analyzer. The electrospray mass spectrum (ES-MS) was recorded on a Finnigan LCQ mass spectrograph at a sample concentration of around 1.0 mmol/mL. The diluted solution was electrosprayed at a flow rate of 5 × 10-6 L/min with a needle voltage of 4.5 kV. Steady-state luminescence spectra and decay curves were measured with an Edinburgh FLS920 fluorescence spectrometer equipped with a 450 W Xe lamp and a time-correlated single photon

10.1021/cg070276j CCC: $40.75  2008 American Chemical Society Published on Web 12/13/2007

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Table 2. Selected Bond Lengths (Å) and Angles (°) for BPVIC and [Cd2(BPVIC)2(SCN)4]∞a BPVIC C(1)-N(1) C(33)-N(3) C(21)-N(3) C(32)-N(4) C(15)-N(2)-C(11) C(21)-N(3)-C(18)

1.325(14) 1.456(10) 1.367(10) 1.354(15) 108.9(7) 108.2(8)

C(5)-N(1) C(15)-N(2) C(41)-N(2) C(1)-N(1)-C(5) C(15)-N(2)-C(41) C(21)-N(3)-C(33)

2.32(5) 22.49(5) 2.30(5) 2.54(7) 80.7(19) 80.7(19) 93.6(13) 81(2) 81(2) 93.7(14)

Cd(1)-N(1) Cd(1)-S(4)#3 Cd(2)-N(2) Cd(2)-S(3)#3 N(1)-Cd(1)-N(3) N(3)-Cd(1)-N(3)#3 N(3)-Cd(1)-S(4) N(2)-Cd(2)-N(6)#3 N(6)#3-Cd(2)-N(6) N(6)-Cd(2)-S(3)#3

1.338(12) 1.376(12) 1.453(10) 117.4(11) 124.8(8) 126.4(8)

C(11)-N(2) C(18)-N(3) C(28)-N(4) C(28)-N(4)-C(32) C(11)-N(2)-C(41) C(18)-N(3)-C(33)

1.401(11) 1.408(12) 1.337(13) 114.8(11) 126.1(8) 125.1(8)

Cd(1)-N(3) Cd(1)-S(4) Cd(2)-N(6)#3 Cd(2)-S(3) N(1)#3-Cd(1)-N(3)#3 N(3)-Cd(1)-S(4)#3 N(3)#3-Cd(1)-S(4) N(2)#3-Cd(2)-N(6) N(6)-Cd(2)-S(3) N(6)#3-Cd(2)-S(3)

2.49(5) 2.684(15) 2.54(7) 2.688(15) 83.4(18) 103.9(11) 103.9(11) 84(2) 93.7(14) 103.5(13)

[Cd(BPVIC)(SCN)2]∞ Cd(1)-N(1)#3 Cd(1)-N(3)#3 Cd(2)-N(2)#3 Cd(2)-N(6) N(1)#3-Cd(1)-N(3) N(1)-Cd(1)-N(3)#3 N(3)#3-Cd(1)-S(4)#3 N(2)#3-Cd(2)-N(6)#3 N(2)-Cd(2)-N(6) N(6)#3-Cd(2)-S(3)#3 a

22.32(5) 2.684(15) 2.30(5) 2.688(15) 83.4(18) 157(2) 93.6(13) 84(2) 157(3) 103.5(13)

Symmetry code: #1: x + 1/2, y - 1/2, z; #2: -x + 3/2, y - 1/2, -z + 1/2; #3: -x + 1, y, -z + 1/2.

Scheme 1. Synthesis of the New Bidentate Ligand

counting (TCSPC) card. The PL lifetime measurement was performed on an Edinburgh FLS920 spectrofluorimeter with a Hydrogen flash lamp as the excitation source. IR diffuse reflectance spectra were recorded on a Nicolet NEXUS 670 FT-IR spectrometer photometer in the 4000–550 cm-1 range. Synthesis and Characterization. 2,8-Bis[2-(2-pyridyl)vinyl]-5,11di(2-ethylhexyl)-indo[3,2-b]carbazole (BPVIC). 2,8-Dibromo-5,11di(2-ethylhexyl)-indolo[3,2-b]carbazole (3.19 g, 5 mmol), tri-o-tolylphosphane (1.12 g, 5 mmol), 2-vinylpyridine (4.17 mL, 40 mmol), palladium(II) acetate (0.076 g, 0.25 mmol), and redistilled triethylamine (200 mL) were added under a nitrogen atmosphere to a three-necked flask equipped with a magnetic stirrer, a reflux condenser, and a nitrogen input tube. The reaction mixture was refluxed in an oil bath at 90 °C under nitrogen atmosphere. The resulting solution was refluxed for 72 h, and triethylamine was removed by reduced pressure distillation. The residue was extracted with dichloromethane (160 mL), washed three times with distilled water, and dried with anhydrous sodium sulfate, and filtered and concentrated, successively. The crude compound was purified by silica-gel column chromatography with ethyl acetate/ petroleum (1/10, v/v) as the eluent to give the product as a yellow solid. Elution with ethyl acetate and recrystallization from ethanol and dichloromethane produced light yellow crystals. Yield: 3.435 g, 91.0%. 1 H NMR (400 MHz, CDCl3): δ ) 0.91 (t, J ) 7.2 Hz, 6 H), 0.98 (t, J ) 7.3 Hz, 6 H), 1.51–1.29 (m, 16 H), 3.22–2.19 (m, 2 H), 4.23–4.27 (m, 4 H), 7.15–7.11 (m, 2 H), 7.26 (d, J ) 16.0 Hz, 2 H), 7.39 (d, J ) 8.5 Hz, 2 H), 7.45 (d, J ) 7.8 Hz, 2 H), 7.68 (t, J ) 7.7 Hz, 2 H), 7.74 (d, J ) 8.6 Hz, 2 H), 7.87 (d, J ) 16.0 Hz, 2 H), 8.00 (s, 2 H), 8.41 (s, 2 H), 8.62 (d, J ) 5.1 Hz, 2 H). 13C NMR (400 MHz, CDCl3): δ ) 155.99, 149.12, 141.97, 136.59, 135.96, 133.44, 126.56, 124.84, 124.44, 122.69, 122.41, 120.95, 120.87, 118.61, 108.45, 98.86, 47.44, 38.85, 30.62, 28.34, 24.35, 22.62, 10.52. FT-IR (cm-1): 3034 (w), 3003 (w), 2958 (s), 2924 (s), 1971 (w), 1870 (m), 1736 (m), 1613 (s), 1582 (s), 1511 (s), 1471 (s), 1367 (m), 1284 (m), 1222 (m), 1199 (m), 1163 (w), 1124 (m), 1090 (m), 987(m), 972 (s), 887 (m), 839 (s), 805 (s), 767 (m), 742 (m), 707 (w), 660 (w), 660 (w), 597 cm-1. ES-MS (Calcd for C48H54N4, 686.97) found, 687.9 (MH+). Anal. Calcd for C48H54N4 (686.97): C 83.92, H 7.92, N 8.16. Found C 84.09, H 7.87, N 8.00. [Cd2(BPVIC)2(SCN)4]∞. BPVIC (68.70 mg, 0.1 mmol) in CH2Cl2 solution (5 mL) was covered with a solution of Cd(SCN)2 (23.00 mg,

0.1 mmol) in MeOH (5 mL). After six days, yellow crystals of [Cd2(BPVIC)2(SCN)4]∞ were produced with a yield of 87%. FT-IR (cm-1): 2960 (vs), 2735 (m), 2611 (m), 2506 (w), 2131 (vs), 2072 (s), 1596 (m), 1455 (m), 629 (m), 603 (m). Anal. Calcd for C50H54CdN6S2 (916.29): C 65.59, H 6.10, N 9.18. Found C 64.96, H 5.74, N 9.97. X-ray Crystallography. Single-crystal X-ray diffraction measurements were conducted on a Bruker Smart APEX II CCD diffractometer equipped with a graphite crystal monochromator for data collection at room temperature. The determination of unit parameters and data collection were performed with Mo KR radiation (λ ) 0.71073 Å). Unit dimensions were obtained with least-squares refinements, and all structures were solved by direct methods using SHELXL-97.8 Other non-hydrogen atoms were located in successive difference Fourier syntheses. The final refinement was performed by full-matrix leastsquares methods with anisotropic thermal parameters for nonhydrogen atoms on F2. The hydrogen atoms were added theoretically and riding on the concerned atoms. Crystallographic data and processing parameters for the ligand and the ligand polymer are shown in Table 1; the selected bond lengths and bond angles are listed in Table 2. CCDC-626554 (for the ligand) and CCDC-626553 (for the coordination polymer) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

Results and Discussion Synthesis. The new bidentate ligand 2,8-bis[2-(2-pyridyl)vinyl]-5,11-di(2-ethylhexyl)-indolo[3,2-b] carbazole (BPVIC) was obtained via a Pd-catalyzed Mizoroki-Heck coupling reaction (MHR) between 2,8-dibromo-5,11-di(2-ethylhexyl)-indolo[3,2b]carbazole and 2-vinyl-pyridine in 91.0% yield (Scheme 1).9 The new ligand was characterized by elemental analyses, 1 13 electrospray mass spectroscopy (ES-MS), H NMR and C NMR spectroscopy. Crystal Structures. (A) BPVIC. The molecular architecture, intermolecular interaction, and molecular packing arrangement

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Figure 1. Molecular structure of the ligand (BPVIC): the whole skeleton structure possesses perfect planarity.

Figure 2. Packing diagram of the ligand (BPVIC). The adjacent molecules are stacked through strong π-π interactions along the parallel array at a distance of 3.330-3.380 Å and C-H · · · π interaction along the same direction at a distance of 2.782 Å.

Figure 3. A fragment of [Cd2(BPVIC)2(SCN)4]∞.

of the ligand (BPVIC) are shown in the X-ray crystal structure (Figures 1 and 2). The ligand BPVIC crystallizes in the orthorhombic space group Pna21, with one molecule per unit cell. The whole skeleton structure, including the trans-CH)CH bonds, two terminal pyridine rings, and the indolo[3,2-b] carbazole unit, possesses good planarity (as shown in Figure

1). The dihedral angles between the two pyridine rings and central indolo[3,2-b] carbazole unit are 8.76 and 6.87°, respectively, while the angle between the pyridine rings is 2.31°. For the central indolo[3,2-b] carbazole unit, the dihedral angles between the two terminal phenyls and the central phenyl are 6.41 and 6.87°. Moreover, the bond lengths of the whole skeleton range from the typical double-bond length of C)C to the typical single-bond length of C-C. The conjugated geometric configuration reveals that BPVIC is a highly delocalized π-electron system, which is an A-π-D · · · D-π-A type configuration formed with a strong π-electron donor (2-ethylhexyl), a conjugated π-electron skeleton (carbazolvinyl) and a π-electron acceptor (pyridyl). The single molecule is slightly S-curved, but the packing diagram shows that the parallel array is in good order. The adjacent molecules are stacked through strong π-π interactions along the parallel array at a short intermolecular distance of 3.330-3.380 Å and C-H · · · π interactions along the same direction at a short intermolecular distance of 2.782 Å. Moreover, the π-π interactions of edge-to-face and C-H · · · π interactions of point-to-face among the ligands are dominantly attractive.2a,10 These strong attractive interactions contribute to forming traps and channels for intermolecular electric charge and energy transfer. The probability of radiative transition may be reduced due to these interactions when the ligands are excited in some way.3 The nitrogen atoms of the two pyridine rings are located on the diagonal line of the molecular plane. This special molecular configuration is the base of the coordination polymer. (B) [Cd2(BPVIC)2(SCN)4]∞. The metal coordination polymer, [Cd2(BPVIC)2(SCN)4]∞ crystallizes in the monoclinic space group C2/c, with eight asymmetric units per unit cell. The crystallographic analyses reveal that the structure is a 3D polymeric building block structure and the constituent fragments are [Cd2(BPVIC)2(SCN)4]∞ (Figures 3 and 4). From these figures, some interesting structural characteristics can be observed. The BPVIC ligands, Cd2+ cations, and SCN- anions form drumlike patterns and are interconnected in an infinite 3D network. The [Cd(NCS)2]∞ chains run along the crystallographic a-axis as infinite zigzag chains. It can be seen that adjacent rows of zigzag chains are “antiparallel” to one another, resulting in the centrosymmetric space group C2/c. The opposing

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Figure 4. Packing diagram of [Cd2(BPVIC)2(SCN)4]∞. (a) View down the crystallographic a- axis: the adjacent ligands are arranged head-to-tail in each fragment. (b) View down the crystallographic b-axis: the ligand is perpendicular to the approximately vertical [Cd(NCS)2]∞ chains. (c) View down the crystallographic c-axis: interconnecting “drums” are formed by the ligands, Cd2+ cations, and SCN- anions, forming infinite 3D networks.

Figure 5. (a) A comparison of the solid-state luminescence spectra of BPVIC and [Cd2(BPVIC)2(SCN)4]∞. (b) Luminescence of BPVIC and [Cd2(BPVIC)2(SCN)4]∞ under 365 nm illumination in the solid state.

S4-Cd1-S4 and S3-Cd2-S3 angles are 82.51 and 80.52°, respectively. The ligands are assembled nearly vertically to the chains. The N3-Cd1-N1, N3-Cd1-S4, N6-Cd2-N2 and N6-Cd2-S3 angles are 83.49, 93.56, 83.76, and 83.72°, respectively. In the fragment shown in Figure 3, two ligands are simultaneously coordinated to two Cd cations. The N-Cd bonds are weak, with bond lengths of 2.507 and 2.547 Å. The N3-Cd1-N3 and N6-Cd2-N6 angles are 156.86 and 157.38°, respectively. The Cd2+ · · · Cd2+ distance, in other words, the distance between two adjacent [Cd(NCS)2]∞ chains, is 15.508 Å along the crystallographic b-axis. For the free ligand, the distance of N · · · N atoms of the two pyridine rings is 17.901 Å, which is larger than that of Cd2+ · · · Cd2+ in the coordination polymer. The coordinated ligand is obviously bent into a U-shaped arch away from the opposite side, and each ligand coordinates to two Cd cations. Moreover, the paired ligands are arranged in opposite orientations (head-to-tail) as they coordinate with the [Cd(NCS)2]∞ chains because the coordinating nitrogen atoms

are located at opposite ends of the ligand skeleton. Because of the space and structural factors, only the central phenyls of the paired ligands are superimposed, and the superposition distance is enlarged to 3.521-3.564 Å in the same unit cell. The distance between the two neighboring ligands in the complex is longer than that in the free ligand. Moreover, the face-to-face π-π interactions between the superposing phenyls are mainly repulsive, which hinders the formation of channels for intermolecular charge and energy transfer.2a,10 Thus, the π-π and C-H · · · π interactions among the ligands, which could act as energy traps and sources of nonradiative decay, are eliminated.3 Intermolecular π-π and C-H · · · π interactions among organic molecules exist widely in the solid state, and these interactions also frequently occur in coordination complexes.2,11,12 In this article, we designed a new bidentate ligand and chose a [Cd(NCS)2]∞ chain as the spacer and controller to synthesize a novel metal coordination polymer, in which the interactions among the ligands are effectively avoided. Every ligand is fixed and linked on both sides by Cd2+ cations. So the skeleton of

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Figure 6. PL lifetime spectra of BPVIC (a) and [Cd2(BPVIC)2(SCN)4]∞ (b) in the solid state. The PL lifetime data is fit to a single exponential and the fitting factors are both close to 1.

the coordination polymer is a completely rigid structure. Moreover, interactions between the ligand and the Cd cations should occur because the Cd cation possesses positive charge and can provide feedback electrons. These factors strongly favor the luminescence.3,4 Luminescent Properties. The polymer is strongly luminescent and is a potential light-emitting material. The luminescence of BPVIC and [Cd2(BPVIC)2(SCN)4]∞was investigated in the solid state at room temperature, as shown in Figure 5. A comparison of the solid-state luminescence spectra of the ligand with the metal coordination polymer is useful in studying the emission origin (as shown in Figure 5a). Under the same conditions, the free ligand exhibits a maximum emission at 477 nm with optimum excitation at 452 nm. With excitation at 480 nm, the emission wavelength of the coordination polymer is significantly red-shifted compared to that of the ligand (from 477 to 562 nm), and the emission intensity is almost 10 times enhanced. The excitation wavelength is also somewhat redshifted (from 452 to 480 nm). Cd(SCN)2 alone has negligible luminescence at these wavelengths (data not shown). At the same time, the PL lifetime of the coordination polymer is 3 times longer than that of the ligand (4.58 vs 1.36 ns). On the basis of the time-dependent perturbation theory of spectroscopy, the probability of radiative transition is apparently increased in the metal coordination polymer.13 The differences between the solid-state luminescence of BPVIC and [Cd2(BPVIC)2(SCN)4]∞ may be explained by the intramolecular and intermolecular electronic and steric factors. The polymer is strongly luminescent, indicating a strong interaction exists between the ligand and the metal, specifically, the π-electron system and the Cd2+ cations in the coordination polymer and forming a new π-electron system with more extended delocalization environment by the metal coordination. Moreover, in this system, the π-electrons are strongly influenced by the Cd2+ cations. The new π-electron system has an optimized delocalization environment and a narrow π-π* energy gap. These factors mean that the transition between S0 and S1 becomes relatively easy.14 On the other hand, the ligands are spaced and braced apart by the coordination. The π-π and C-H · · · π interactions between the ligands, which mainly act as energy traps and channels of nonradiative decay, are effectively eliminated.3 These factors decrease the probability of nonradiative decay. In the ligand, these electronic factors are absent, and there are extensive, strong intermolecular π-π

and C-H · · · π interactions; therefore, the free ligand shows only very weak solid-state luminescence. Conclusions In conclusion, a new supramolecular coordination polymer [Cd2(BPVIC)2(SCN)4]∞ has been synthesized and characterized. The crystal structures of the free ligand and the coordination polymer were determined by X-ray single-crystal diffraction analysis and their solid-state luminescence was investigated. The experimental results indicate that the luminescence maximum of the coordination polymer was obvious red-shifted compared to that of the free ligand, and the luminescence intensity is enhanced significantly. The strong interaction between the ligand and metal in the coordination polymer is one of the important factors for the strong luminescence of the polymer. The other factor is that in the coordination polymer, the π-π and C-H · · · π interactions are eliminated due to the infinite [Cd(NCS)2]∞ zigzag chain, which acts as a spacer and a controller for the arrangement and alignment of the large π-conjugated system. The results indicate that supramolecular assembly is a suitable way to design strongly luminescent materials. Supporting Information Available: X-ray crystallographic files in CIF format for the ligand (BPVIC) and the metal coordination polymer [Cd2(BPVIC)2(SCN)∞ are available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment. The authors are grateful to the State National Natural Science Foundation of China (Grant Nos. 50325311, 50590403, 50721002) and 973 program of P. R. China (Grant No. 2004CB619002) for financial support. The authors thank Dr. Pamela Holt for editing the manuscript.

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