(Λ)-Shaped Pyridinium Salt Based on Tröger's Base - American

J. Phys. Chem. C 2007, 111, 12811-12816

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Synthesis, Structure, and Aggregation-Induced Emission of a Novel Lambda (Λ)-Shaped Pyridinium Salt Based on Tro1 ger’s Base Chun-Xue Yuan,† Xu-Tang Tao,*,† Yan Ren,† Yang Li,† Jia-Xiang Yang,‡ Wen-Tao Yu,† Lei Wang,† and Min-Hua Jiang† State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan, 250100, People’s Republic of China, and Department of Chemistry, Anhui UniVersity, Hefei, 230039, People’s Republic of China ReceiVed: February 11, 2007; In Final Form: June 18, 2007

A novel Λ-shaped pyridinium salt 2,8-(6H,12H-5,11-methanodibenzo[b,f]diazocineylene)-di(p-ethenyl-Nmethyl-pyridinium) ditosylate (abbreviated as DMDPS) based on Tro¨ger’s base was designed, synthesized and characterized. DMDPS exhibits a typical aggregation-induced emission (AIE) behavior that is virtually nonemissive in solution but highly luminescent in solid state. The difference between the structure and the optical properties of DMDPS and 1,4-phenyl-di(p-ethenyl-N-methyl-pyridinium) ditosylate (abbreviated as DPPS) are analyzed and compared. It is concluded that the loose stacking caused by twisted molecular configuration could reduce the distance-dependent intermolecular quenching effect to produce intense fluorescence in the aggregation state while the enantiomerization and/or the intramolecular vibrational motion which induce the nonradiative deactivation process could cause fluorescence quenching in the solution. As a water-soluble AIE-active material, DMDPS would have potential application in the area of optical and biological research.

Introduction Most organic chromophores are highly emissive in solution but become weakly luminescent in solid state.1 It is mainly attributed to the intermolecular vibronic interactions which induce the nonradiative deactivation process, that is, fluorescence quenching, such as excitonic coupling, excimer formation, and excitation energy migration to the impurity traps.2 Aggregation quenching has been the thorniest problem in the development of OLEDs because the luminescent materials are predominately used as thin solid films in OLEDs, in which aggregation is inherently accompanied with the film formation.1,3 Thus, development of luminescent materials, which can overcome this emission quenching problem or even show enhanced emission in the solid state, would be very rewarding. In recent years, several exceptional examples, including siloles,4 CN-MBE,5 NPAFN,6 DPDSB derivatives,7 DCM derivatives,8 conjugated polymers,9 DPDBF derivatives10 and others,11 have been found to show significant enhancements on light-emission upon aggregation or in the solid state. This intriguing phenomenon was named aggregation-induced emission (AIE). AIE-active materials are normally highly emissive in their crystalline forms, therefore becoming promising candidates for the fabrication of efficient OLEDs and electrically pumped lasers.4-12 Moreover, water-soluble AIE-active molecules can be used as bioprobes.11f However, AIE-active compounds reported previously are almost neutral molecules, and only a few ionic salts with AIE-active feature were known.11f In addition, Tro¨ger’s base (TB),13 first synthesized by Tro¨ger in 1887, has gained steady interest in recent years because of * To whom correspondence should be addressed. Tel: +86-53188364963. Fax: +86-531-88574135. E-mail: [email protected]. † Shandong University. ‡ Anhui University.

its C2 symmetry, chirality, and rigid concave shape.14 This special Λ-shaped geometry configuration is theoretically disadvantageous to form π-π close stacking, which commonly results in fluorescence quenching in the solid state. Organic pyridinium salts are historically of special interest in photoelectric field due to their good chemical and thermal properties.15 Considering these two aspects, we designed and synthesized a newly TB analogue DMDPS that combines the multifunctional properties of pyridinium salt and the structural characteristic of Λ-shaped molecule, in anticipation to obtain excellent optical performance. Exhilaratingly, it was found that the emission of DMDPS is unusually quenched in solution, but enhanced in bulk-solid or aggregated dispersions, or in other words, DMDPS is AIE-active. As a water-soluble AIE-active material, DMDPS would have potential application in the area of optical and biological research. In this paper, we present the molecular design, synthesis, and fluorescence property of a new Λ-shaped pyridinium salt DMDPS based on Tro¨ger’s Base. Meanwhile, its crystal structure is analyzed to obtain insights on the possible origins of aggregation-induced fluorescence enhancement. Experimental Section Materials and Instrumentation. All chemicals were purchased from Aldrich and Acros and used as received without further purification. 4-methyl-N-methylpyridinium tosylate (1),16 2,8-diformyl-6H,12H-5,11-methanodibenzo[b,f][1,5]-diazocine (2)17 were prepared according to literature procedures. The 1H and 13C NMR spectra were recorded on a Bruker Avance 400 spectrometer using tetramethylsilane (TMS; δ ) 0 ppm) as internal standard. Coupling constants J are given in Hertz. IR spectra were obtained on a Nicolet NEXUS 670 spectrometer (KBr pellet). The UV-visible absorption spectra were measured on a TU-1800 spectrophotometer using a quartz

10.1021/jp0711601 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/08/2007

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SCHEME 1: Synthetic Routes to Compounds DMDPS and DPPS

cuvette having 1 cm path length. The photoluminescence spectra were collected on a Hitachi F-4500 fluorescence spectrophotometer with a 150 W Xe lamp. Solvents were purified and dried according to standard procedures. The X-ray diffraction intensity data were collected at 293 K on a Bruker Smart Apex2 CCD area-detector diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.71069 Å). Processing of the intensity data was carried out using the Bruker SMART routine, and the structure was solved by direct methods and refined by a full-matrix least-squares technique on F2 using SIR-92 and SHELXL-97 programs.18 CCDC reference number 626558. For crystallographic data in CIF or other electronic format, see http://www.ccdc.cam.ac.uk. Synthesis. 2,8-(6H,12H-5,11-Methanodibenzo[b,f]diazocineylene)-di(p-ethenyl-N-meth yl-pyridinium)ditosylate (DMDPS). To a stirring of 4-methyl-N-methyl pyridinium tosylate 1 (0.278 g, 1 mmol) and 2,8-diformyl-6H,12H-5,11-methanodibenzo[b,f][1,5]-diazocine 2 (0.670 g, 2.4 mmol) in acetonitrile (30 mL) was added piperidine as a catalyst and refluxed for 10 h. After being cooled to room temperature, ethyl ether was added to the reaction mixture. The precipitate was filtered, washed with dichloromethane and dried under vacuum. Product DMDPS (0.24 g) was isolated in 30% yield. Mp: 250 °C. IR (cm-1): 3419, 3129, 3040, 2854, 1643, 1619, 1567, 1519, 1493, 1469, 1346, 1192, 1121, 1034, 1011, 965, 836, 816, 684. 1H NMR (400 MHz, MeOD), δ (ppm): 2.32 (s, 6H), 4.25 (s, 6H), 4.31 (d, 2H, J ) 16.8 Hz), 4.38 (s, 2H), 4.77 (d, 2H, J ) 16.8 Hz), 7.18 (d, 4H, J ) 8.0 Hz), 7.23 (d, 2H, J ) 2.1 Hz), 7.26 (d, 2H, J ) 5.6 Hz), 7.35 (d, 2H, J ) 1.4 Hz), 7.57 (dd, 2H, J ) 8.5 Hz), 7.67 (d, 4H, J ) 8.0 Hz), 7.77 (d, 2H, J ) 16.3 Hz), 8.04 (d, 4H, J ) 6.9 Hz), 8.62 (d, 4H, J ) 6.9 Hz). 13C NMR (100.57 MHz, MeOD), δ (ppm): 19.38, 45.73, 57.71, 65.78, 120.86, 122.84, 123.99, 124.84, 125.05, 126.51, 126.79, 127.89, 128.05, 130.61, 139.71, 140.65, 141.77, 144.04, 149.86, 153.42. 1,4-Phenyl-di(p-ethenyl-N-methyl-pyridinium)ditosylate(DPPS). A mixture of 4-methyl-N-methyl pyridinium tosylate 1 (1.120 g, 4 mmol), terephthaladehyde 3 (0.134 g, 1 mmol) and piperidine (3 drops) in fleshly distilled acetonitrile (10 mL) was refluxed for 6 h and cooled to room temperature. The yellow precipitate was filtered, washed with acetonitrile three times and dried under vacuum. Product DPPS (0.15 g) was isolated in 25% yield. Mp: 309 °C; IR (cm-1): 3446, 3117, 3044, 1642, 1619, 1567, 1519, 1473, 1460, 1423, 1339, 1187, 1121, 1035, 1012, 985, 854, 820, 684, 562, 551, 468. 1H NMR (400 MHz,

DMSO), δ (ppm): 2.28 (s, 6H), 4.27 (s, 6H), 7.09 (d, 3H, J ) 8.0 Hz), 7.48 (d, 3H, J ) 8.0 Hz), 7.60 (d, 2H, J ) 16.3 Hz), 7.86(s, 4H), 8.01 (d, 4H, J ) 16.3 Hz), 8.22 (d, 4H, J ) 6.7 Hz), 8.86 (d, 4H, J ) 6.7 Hz). 13C NMR (100.57 MHz, DMSO), δ (ppm): 20.75, 46.99, 123.70, 124.53, 125.48, 128.01, 128.76, 136.88, 137.51, 139.55, 145.22, 145.87, 152.12. Results and Discussion Synthesis. As outlined in Scheme 1, compounds DMDPS and DPPS were synthesized directly by condensation of 4-methyl-N-methylpyridinium tosylate 1 with corresponding aldehydes 2,8-diformyl-6H,12H-5,11- methanodibenzo[b,f][1,5]diazocine 2 and terephthaladehyde 3 in the presence of piperidine used as a catalyzer. Both DMDPS and DPPS were characterized by IR, 1H NMR, and 13C NMR methods. These two pyridinium salts are readily soluble in polar solvents such as methanol, ethanol, DMF and water etc., but completely insoluble in nonpolar solvents like ethyl ether, toluene, benzene, and so on. The yellow prism single crystal of DMDPS suitable for X-ray analysis was obtained by slow evaporation of its methanol-benzene mixture. Crystal Structure. To understand the relationship between optical properties and intermolecular interactions in the solid state, the crystal structure of compound DMDPS was determined. Its crystal data and intensity collection parameters are summarized in Table 1. The molecular structure of DMDPS and its packing arrangement in single crystal are given in Figure 1 and Figure 2, respectively. As shown in Figure 1, there are two inequivalent cations, two individual anions, and five water molecules in an asymmetric unit. For these two individual cations representing Λ-shaped conformation, the dihedral angles between the two benzene rings constituting the TB framework are 88.6° and 87.3°, respectively. Due to the twisted molecular configuration, the distance between two neighboring molecules, which is found to be approximately 3.8 Å, is larger than the normal π-π interaction distance (ca. 3.3 Å), indicating that there is an offset of π-π interaction between the quasi-plane of different molecules. Furthermore, there is only partial overlap between adjacent molecules (see from Figure 2 (a)) and a weak π-π interaction exists in DMDPS crystal. The unique structure may play important roles in suppressing excimer formation and closing nonradiative pathways in aggregation state. Torsion angles between the benzene rings and the adjacent pyridine rings for the two independent cations mentioned above

Novel Lambda (Λ)-Shaped Pyridinium Salt

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TABLE 1: Crystal Data, Diffraction Data, and Refinement Data of DMDPS empirical formula

C45H54N4O11S2

formula weight crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg crystal size volume, mm-3 Z Dc, g/cm3 F (000) T, K radiation (λ), Å Μ (Mo KR) mm-1 2θmax, deg (completeness) reflections collected/unique data/restraints/parameters R1, wR2 [I > 2σ (I)] R1, wR2 (all data) goodness of fit, F2 largest diff. peak/hole e- Å-3 transmission ratio

891.04 monoclinic C2/c 34.527(5) 10.543(5) 29.876(5) 90.000(5) 123.540(5) 90.000(5) 0.30 × 0.20 × 0.15 mm 9065(5) 8 1.306 3776 293(2) Mo KR, 0.71069 0.181 50.02 (99.7%) 33121/7980 [R(int) ) 0.0574] 7980/24/564 0.0742, 0.2197 0.1223, 0.2545 1.048 1.402/-0.377 0.9734/0.9477

are only 3.7° and 5.5°, respectively, indicating that the benzene rings and the adjacent pyridine rings are almost coplanar. Excellent coplanarity of conjugated moiety enables DMDPS to be a highly declocalized π-electron system. In addition, from the unit cell of DMDPS, we can see that there are two enantiomer forms (5S,11S)-DMDPS and (5R,11R)-DMDPS in the crystal of DMDPS. Aggregation-Induced Emission Property. In our experiment, DMDPS solid emits bright yellow light but becomes hardly emissive in solution when examined under illumination with a 365 nm UV lamp (Figure 3). To have a spectrometric verification, we measured the UV-visible absorption spectra and photoluminescence (PL) spectra of DMDPS in acetonitrile/ toluene mixtures with different volume fractions of toluene and the final concentrations being kept constant at 1 × 10-5 mol/L as shown in Figure 4(a) and 4(b), respectively. As shown in Figure 4(a), the absorption edge is located at λ ) 510 nm with an absorption maximum at λ ) 400 nm. No obvious spectral shift is observed in the absorption spectra when the dye molecules are aggregated, indicating that only weak intermolecular interactions exist in the dye aggregates.10c,11g The spectra of DMDPS in acetonitrile and 70% acetonitrile-toluene mixture are almost identical. However, there exists a sudden drop in the absorbance in 75% acetonitrile-toluene mixture, suggesting that the molecules began to aggregate at toluene volume fraction greater than 70%. The absorption of DMDPS gradually decreases upon continued aggregation with increasing

Figure 1. Molecular structure of DMDPS. The hydrogen atoms have been omitted for clarity.

Figure 2. (a) Unit cell of DMDPS as viewed along the b-axis. The hydrogen atoms have been omitted for clarity. (b) Perspective view of crystal packing of molecules of DMDPS. The hydrogen atoms, water molecules, and anions have been omitted for clarity. The distance between two molecules is 3.8 Å.

Figure 3. Fluorescence image of solution and solid of DMDPS (left) and DPPS (right) under illumination with a 365.0 nm UV lamp.

toluene volume fraction.8a,11c,d This is because that when the DMDPS molecules cluster together, the number of compound that dissolved in solvent decreases. Since the absorption intensity depends on the compound dissolved in solvent, the absorption of DMDPS decrease with aggregation.11h Figure 4(b) is the corresponding emission spectra of DMDPS in acetonitrile/toluene mixtures with different volume fractions of toluene. In comparison with Figure 4(a), it shows an opposite change trend. When a dilute acetonitrile solution of DMDPS was excited at 400 nm, almost no PL signals were recorded by a spectrofluorometer. However, when a large amount of toluene, a nonsolvent of DMDPS, was added to the acetonitrile solutions, a dramatic enhancement of luminescence emission was observed in the 75% acetonitrile-toluene mixture, while the PL intensity kept nearly unchanged for mixtures with lower toluene volume fractions. The abrupt increase in PL intensity for toluene volume fraction >70% agreed well with the sudden decrease in the absorbance shown in Figure 4(a), further confirming that DMDPS molecules started to aggregate when the volume fraction of toluene reached 70%. It also shown that the PL intensity was further intensified as the toluene volume fraction was increased. The dependence of the changes of PL peak

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Figure 4. (a) UV-visible absorption spectra of DMDPS (1.0 × 10-5 mol/L) in acetonitrile/toluene mixtures with different volume fractions of toluene. Inset depicts the changes of peak absorbance. (b) PL spectra of DMDPS (1.0 × 10-5 mol/L) in acetonitrile/toluene mixtures with different volume fractions of toluene; excitation wavelength: 400 nm. Inset depicts the changes of PL peak intensities.

Figure 5. (a) UV-visible absorption spectra of DPPS (mol/L) in acetonitrile/toluene mixtures with different volume fractions of toluene. Inset depicts the changes of peak absorbance. (b) PL spectra of DPPS (1.0 × 10-5 mol/L) in acetonitrile/toluene mixtures with different volume fractions of toluene; excitation wavelength, 385 nm. Inset depicts the changes of PL peak intensities.

intensities of DMDPS on the solvent composition of acetonitrile/ toluene mixture was depicted in the inset of Figure 4(b). The dramatic change of the PL spectrum caused by the aggregation may allow DMDPS to find potential applications as sensors. Possible Mechanism for AIE of DMDPS. Aggregation normally quenches emission. What then is the exact cause for the “abnormal” AIE phenomenon? Several models, such as exciton diffusion, rotational deactivation, and coplanarity, have previously been proposed for the enhanced emission in the solid state, compared with lower PL quantum yield or PL quenching in solutions. Belton et al. reported that the rigid packing produced severe torsional hindrances along the polymer backbone, which results in large energy barriers for excitons to diffuse to quenching sites, therefore raising the PL yield.9b For siloles, the AIE feature is believed to result from the restricted intramolecular motion rotation, shutting down nonradiative relaxation process and thus boosting their PL emissions.19 Twisted geometry configuration or noncoplanarity are favorable for PL emission in the solid state. Nevertheless, in contrast with the former mechanism, Park et al. deemed that noncoplanarity is responsible for the PL quenching of CN-MBE in solution while coplanarization in the solid-state causes the aggregationinduced emission.5a To clarify the structural effect on the “abnormal” AIE phenomenon of DMDPS, we designed and synthesized a planar pyridinium salt DPPS. In comparison with DMDPS, DPPS has a planar conformation. Our experiments show that DPPS has an opposite optical behavior with efficient fluorescence emission

in solution but weak emission in solid state. The totally diffrent fluorescent behaviors of these two pyridinium salts were well visualized through fluorescence imaging of solution and solid as shown in Figure 3. The UV absorption spectra and PL spectra of DPPS in acetonitrile/toluene mixtures with different volume fractions of toluene were shown in Figure 5(a) and 5(b), respectively. In contrast to the drastic increase of DMDPS emission, the emission intensity of DPPS showed a drastic decrease in aggregates (Figures 4(b) and 5(b), respectively). DPPS is AIE-inactive. On consideration of the difference in the chromophore structure is the core moiety, it is concluded that the Λ-shaped TB core might play a crucial role in overcoming flurescence quenching in the solid state, proving the value of our molecular-design strategy. The fluorescence quenching of DMDPS in solution might be understood as dominant nonradiative decay, enhanced by enantiomerization and/or intramolecular vibrational motion. According to literature report,20 TB undergos enantiomerization in the gas and liquid phases, and protonation will accelerate this process. DMDPS is only soluble in polar solvents due to its big molecular polarity that enantiomerization may occur easily under the influence of protic solvents. Moreover, as an organic salt, strong ionic interaction between the anion and cation probably leads relative vibration of the two wings of Λ-shaped framework. As for fluorescence enhancement in the aggregation state, several effects need to be taken into consideration: the freedom of enantiomerization and/or the vibrational motion, the inter-

Novel Lambda (Λ)-Shaped Pyridinium Salt molecular fluorescence-quenching interaction, and the effective intramolecular charge transfer. In solid state, the enantiomerization and/or vibrational motion may be unambiguously stopped by steric interaction, resulting in the closure of the nonradiative decay channel. More importantly, the efficient fluorescence in the aggregates suggested that the second effectsthat is, the quenching interactions among moleculessis suppressed to some extent by the increased intermolecular distances resulting from the twisted geometry configuration of DMDPS. Crystallographic analysis indicates that long molecular distance (∼3.8 Å) could possibly reduces the distance-dependent intermolecular quenching effects to produce intense fluorescence in the aggregation state. Furthermore, its pyridyl ring with positive charge and amidogen on Λ-bridge might act as electron acceptor and electron donor, respectively, therefore polarizing the whole molecule, and strengthening the ability of intramolecular charge transfer (ICT) through strong push-pull interaction. Therefore, most probably, the enhanced emission of DMDPS is attributed to the combined effects of twisted geometry configuration with partial coplanarity. But for DPPS with no Λ-shaped bridge, enantiomerization and/or vibrational process would not occur in solution. As a planar molecule, it is apt to form close stacking in the solid state. Therefore, DPPS exhibits usual “concentration quenching” property. Different structure results in different optical property. Conclusion We have synthesized a novel Λ-shaped ionic compound DMDPS based on Tro¨ger’s base and determined its crystal structure by X-ray diffraction. Anomalously, DMDPS is an AIEactive molecule: luminescence virtually invisible in solution while highly emissive in the aggregation state. By analyzing crystal structure of DMDPS and comparing the difference between the structure and the optical properties of DMDPS and DPPS, we speculate that the enhanced emission of DMDPS may mainly attribute to the twisted geometry configuration which sterically disturbs close packing by increasing intermolecular distances. DMDPS has high solubility in water, so we can prepare films using ink-jet printing technique which is environmentally friendly. DMDPS could also be used as bioprobe for which water-solubility is very important. Furthermore, we also demonstrate the potentials of Tro¨ger’s base analogue used as organic photoelectric materials. Further studies on DMDPS for photoelectric devices are in progress. Acknowledgment. We gratefully acknowledge the financial support from the state National Natural Science Foundation of China (Grants No. 50323006, 50325311, 50590403, 50603011) and 973 program of the People’s Republic of China (Grant No. 2004CB619002) Supporting Information Available: An X-ray crystallographic file (CIF) is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (b) Chen, C.-T. Chem. Mater. 2004, 16, 4389. (2) Birks, J. B. In Photophysics of Aromatic Molecules; Wiley: London, 1970. (3) (a) Schouwink, P.; Scha¨fer, A. H.; Seidel, C.; Fuchs, H. Thin Solid Films 2000, 372, 163. (b) Crenshaw, B. R.; Weder, C. Chem. Mater. 2003, 15, 4717. (c) Kwon, T. W.; Alam, M. M.; Jenekhe, S. A. Chem. Mater. 2004, 16, 4657.

J. Phys. Chem. C, Vol. 111, No. 34, 2007 12815 (4) (a) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740. (b) Chen, J.; Xie, Z.; Lam, J. W. Y.; Law, C. C. W.; Tang, B. Z. Macromolecules 2003, 36, 1108. (c) Chen, J.; Peng, H.; Law, C. C. W.; Dong, Y.; Lam, J. W. Y.; Williams, I. D.; Tang, B. Z. Macromolecules 2003, 36, 4319. (d) Chen, J.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y.; Lo, S. M. F.; Williams, I. D.; Zhu, D.; Tang, B. Z. Chem. Mater. 2003, 15, 1535. (e) Ha¨ussler, M.; Chen, J.; Lam, J. W. Y.; Tang, B. Z. J. Nonlin. Opt. Phys. Mater. 2004, 13, 335. (f) Chen, H.; Chen, J.; Qiu, C.; Tang, B. Z.; Wong, M.; Kwok, H. S. IEEE J. Select. Top. Quantum Electron. 2004, 10, 10. (g) Law, C. C. W.; Chen, J.; Lam, J. W. Y.; Peng, H.; Tang, B. Z. J. Inorg. Organomet. Polym. 2004, 14, 39. (h) Yu, G.; Yin, S.; Liu, Y.; Chen, J.; Xu, X.; Sun, X.; Ma, D.; Zhan, X.; Peng, Q.; Shuai, Z.; Tang, B. Z.; Zhu, D.; Fang, W.; Luo, Y. J. Am. Chem. Soc. 2005, 127, 6335. (i) Mi, B.; Dong, Y.; Li, Z.; Lam, J. W. Y.; Ha¨ussler, M.; Sung, H. H. Y.; Kwok, H. S.; Dong, Y.; Williams, I. D.; Liu, Y.; Luo, Y.; Shuai, Z.; Zhu. D.; Tang, B. Z. Chem. Commun. 2005, 3583. (5) (a) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. (b) Lim, S.-J.; An, B.-K.; Jung, S.-D.; Chung, M.A.; Park, S. Y. Angew. Chem., Int. Ed. 2004, 43, 6346. (c) Lim, S.-J.; An, B.-K.; Park, S. Y. Macromolecules 2005, 38, 6236. (d) Tong, X.; Zhao, Y.; An, B.-K.; Park, S. Y. AdV. Funct. Mater. 2006, 16, 1799. (6) Yeh, H.-C.; Yeh, S.-J.; Chen, C.-T. Chem. Commun. 2003, 2632. (7) (a) Xie, Z.; Yang, B.; Cheng, G.; Liu, L.; He, F.; Shen, F.; Ma, Y.; Liu, S. Chem. Mater. 2005, 17, 1287. (b) Xie, Z.; Yang, B.; Xie, W.; Liu, L.; Shen, F.; Wang, H.; Yang, X.; Wang, Z.; Li, Y.; Hanif, M.; Yang, G.; Ye, L.; Ma, Y. J. Phys. Chem. B 2006, 110, 20993. (c) Li, Y.; Li, F.; Zhang, H.; Xie, Z.; Xie W.; Xu, H.; Li, B.; Shen, F.; Ye, L.; Hanif, M.; Ma, D.; Ma, Y. Chem. Commun. 2007, 231. (8) (a) Tong, H.; Dong, Y.; Ha¨ussler, M.; Hong, Y.; Lam, J. W. Y.; Sung, H. H-Y.; Williams, I. D.; Kwok, H. S.; Tang, B. Z. Chem. Phys. Lett. 2006, 428, 326. (b) Tong, H.; Hong, Y.; Dong, Y.; Ren, Y.; Ha¨ussler, M.; Lam, J. W. Y.; Wong, K. S.; Tang, B. Z. J. Phys. Chem. B 2007, 111, 2000-2007. (9) (a) Deans, R.; Kim, J.; Machacek, M. R.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 8565. (b) Belton, C.; O’Brien, D. F.; Blau, W. J.; Cadby, A. J.; Lane, P. A.; Bradley, D. D. C.; Byrne, H. J.; Stockmann, R.; Ho¨rhold, H-H. Appl. Phys. Lett. 2001, 78, 1059. (c) Holzer, W.; Penzkofer, A.; Stockmann, R.; Meysel, H.; Liebegott, H.; Ho¨rhold, H-H. Polymer 2001, 42, 3183. (10) (a) Tong, H.; Dong, Y.; Ha¨ussler, M.; Lam, J. W. Y.; Sung, H. Y.; Williams, I. D.; Sun, J.; Tang, B. Z. Chem. Commun. 2006, 1133. (b) Dong, Y.; Lam, J. W. Y.; Qin, A.; Li, Z.; Sun, J.; Sung, H. H. Y.; Williams, I. D.; Tang, B. Z. Chem. Commun. 2007, 40. (c) Tong, H.; Dong, Y.; Hong, Y.; Ha¨ussler, M.; Lam, J. W. Y.; Sung, H. H.-Y.; Yu, X.; Sun, J.; Williams, I. D.; Kwok, H. S.; Tang, B. Z. J. Phys. Chem. C 2007, 111, 2287. (d) Tong, H.; Dong, Y.; Ha¨ussler, M.; Lam, J. W. Y.; Tang, B. Z. Nonlin. Opt. Quantum Opt., in press. (11) (a) Ryu, S. Y.; Kim, S.; Seo, J.; Kim, Y-W.; Kwon, O-H.; Jang, D-J.; Park, S. Y. Chem. Commun. 2004, 70. (b) Chen, J.; Xu, B.; Ouyang, X.; Tang, B. Z.; Cao, Y. J. Phys. Chem. A 2004, 108, 7522. (c) Bhongale, C. J.; Chang, C-W.; Lee, C-S.; Diau, E. W-G.; Hsu, C-S. J. Phys. Chem. B 2005, 109, 13472. (d) Wang, Z.; Shao, H.; Ye, J.; Tang, L.; Lu, P. J. Phys. Chem. B 2005, 109, 19627. (e) Tracy, H. J.; Mullin, J. L.; Klooster, W. T.; Martin, J. A.; Haug, J.; Wallace, S.; Rudloe, I.; Watts, K. Inorg. Chem. 2005, 44, 2003. (f) Tong, H.; Hong, Y.; Dong, Y.; Ha¨ussler, M.; Lam, J. W. Y.; Li, Z.; Guo, Z.; Guo, Z.; Tang, B. Z. Chem. Commun. 2006, 3705. (g) Kim, S.; Zheng, Q.; He, G. S.; Bharali, D. J.; Pudavar, H. E.; Baev, A.; Prasad, P. N. AdV. Funct. Mater. 2006, 16, 2317. (h) Liu, Y.; Tao, X.; Wang, F.; Shi, J.; Yu, W.; Ren, Y.; Zou, D.; Jiang, M. J. Phys. Chem. C 2007, 111, 6544. (12) (a) Chen, H.; Lam, J. W. Y.; Luo, J.; Ho, Y.; Tang, B. Z.; Zhu, D.; Wong, M.; Kwok, H. S. Appl. Phys. Lett. 2002, 81, 574. (13) Tro¨ger, J. J. Prakt. Chem. 1887, 36, 225. (14) (a) Crossley, M. J.; Try, A. C.; Walton, R. Tetrahedron Lett. 1996, 37, 6807. (b) Jensen, J.; Tejler, J.; Wa¨rnmark, K. J. Org. Chem. 2002, 67, 6008. (c) Solano, C.; Svensson, D.; Olomi, Z.; Jensen, J.; Wendt, O. F.; Wa¨rnmark, K. Eur. J. Org. Chem. 2005, 3510. (d) Abella, C. A. M.; Rodembusch, F. R.; Stefani, V. Tetrahedron Lett. 2004, 45, 5601. (e) Sergeyev, S.; Diederich, F. Angew. Chem., Int. Ed. 2004, 43, 1738. (15) (a) Marder, S. R.; Perry, J. W.; Schaefer, W. P. Science 1989, 245, 626. (b) Marder, S. R.; Perry, J. W.; Yakymyshyn, C. P. Chem. Mater. 1994, 6, 1137. (c) Zhao, C. F.; He, G. S.; Bhawalkar, J. D.; Park, C. K.; Prasad, P. N. Chem. Mater. 1995, 7, 1979. (d) He, G. S.; Yuan, L. X.; Cui, Y. P.; Li, M.; Prasad, P. N. J. Appl. Phys. 1997, 81, 2529. (e) Ren, Y.; Fang, Q.; Yu, W.; Lei, H.; Tian, Y.; Jiang, M.; Yang, Q.; Mak, T. C. W. J. Mater. Chem. 2000, 10, 2025. (f) Yang, Z.; Aravazhi, S.; Schneider, A.;

12816 J. Phys. Chem. C, Vol. 111, No. 34, 2007 Seiler, P.; Jazbinsek, M.; Gu¨nter, P. Adv. Funct. Mater. 2005, 15, 1072. (g) Ruiz, B.; Yang, Z.; Jazbinsek, M.; Gramlich, V.; Gu¨nter, P. J. Mater.Chem. 2006, 16, 2839. (16) Haja Hameed, A. S.; Yu, W. C.; Chen, Z. B.; Tai, C. Y; Lan, C. W. J. Cryst. Growth 2005, 282, 117. (17) Jensen, J.; Tejler, J.; Wa¨rnmark, K. J. Org. Chem. 2002, 67, 6008.

Yuan et al. (18) Sheldrick, G. M. SHELXL-97; University of Gottingen: Gottingen, Germany, 1997. (19) Li, Z.; Dong, Y.; Mi, B.; Tang, Y.; Ha¨ussler, M.; Tong, H.; Dong, Y.; Lam, J. W. Y.; Ren, Y.; Sung, H. H. Y.; Wong, K. S.; Gao, P.; Williams, I. D.; Kwok, H. S.; Tang, B. Z. J. Phys. Chem. B 2005, 109, 10061. (20) Trapp, O.; Schurig, V. J. Am. Chem. Soc. 2000, 122, 1424.