Dendritic Effect on Supramolecular Self-Assembly: Organogels with

Mar 16, 2009 - Photographs of the self-supporting gels, phase transitions of 2a and 2b, more AFM, SEM, and TEM images of 2a−c gels, rheological data...
1 downloads 9 Views 3MB Size
Download PDF
pubs.acs.org/Langmuir © 2009 American Chemical Society

Dendritic Effect on Supramolecular Self-Assembly: Organogels with Strong Fluorescence Emission Induced by Aggregation† Yulan Chen,‡ Yuxia Lv,§ Yang Han,‡ Bo Zhu,‡ Fan Zhang,‡ Zhishan Bo,*,‡ and Chen-Yang Liu*,§ ‡ Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China and §Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China

Received October 16, 2008. Revised Manuscript Received December 13, 2008 A novel class of dumbbell-shaped dendritic molecules with a p-terphenylene core was synthesized, and their self-assembling properties were investigated. The incorporation of bulky dendritic wedges to the central stiff aromatic scaffolds could finely tune their solubility in many organic solvents. Unlike the self-assembly behavior of p-terphenylen-1,400 -ylenebis(dodecanamide), the p-terphenylene cored different generation dendritic molecules could form gels in several kinds of organic solvents through a cooperative effect of the π-π stacking, hydrogen-bonding, and van der Waals forces. Interestingly, significant fluorescence enhancement was observed after gelation. Extensive investigations with atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), rheological measurements, UV-vis absorption spectroscopy, FT-IR spectroscopy, 1H NMR, and X-ray powder diffraction (XRD) revealed that these dendritic molecules self-assembled into elastically interpenetrating one-dimensional nanostructures in organogels.

Introduction Recently, the bottom-up fabrication of ordered superstructures using specific noncovalent interactions has received immense interest.1-4 By the rational design of simple building blocks, the information (such as molecular shape, rigidity, amphiphilicity, etc.) programmed on a molecular scale could be transcribed to a macroscopical scale by hierarchical supramolecular self-assembly. Thus, new functional materials are achievable by nanometer-scale manipulation.5-7 The incorporation of multinoncovalent interactions can enhance the stability and robustness of the ordered superstructures,8-10 and the reversible and dynamic nature of these noncovalent interactions can also endow supramolecular materials with intelligence to external stimuli.11 Utilizing this facile approach, well-defined nanostructures such as nanofibers, † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding authors: Fax +86-10-82618587, e-mail zsbo@iccas. ac.cn (Z.B.); Fax +86-10-62558903, e-mail [email protected] (C.-Y.L.).

(1) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, 1995. (2) Whitesides, G. W.; Grzybowski, B. Science 2002, 295, 2418–2421. (3) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. Rev. 2001, 101, 4071–4098. (4) Elemans, J. A. A. W.; Rowan, A. E.; Nolte, R. J. M. J. Mater. Chem. 2003, 13, 2661–2670. (5) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz, A. Science 1997, 276, 384–389. (6) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119–1122. (7) Ajayaghosh, A.; Praveen, V. K. Acc .Chem. Res. 2007, 40, 644–656. (8) Kamikawa, Y.; Kato, T. Org. Lett. 2006, 8, 2463–2466. (9) Li, X. Q.; Stepanenko, V.; Chen, Z. J.; Prins, P.; Siebbeles, L. D. A.; Wurthner, F. Chem. Commun. 2006, 3871–3873. (10) Song, B.; Wei, H.; Wang, Z. Q.; Zhang, X.; Smet, M.; Dehaen, W. Adv. Mater. 2007, 19, 416–420. (11) Ishi-I, T.; Shinkai, S. Top. Curr. Chem. 2005, 258, 119–160.

8548 DOI: 10.1021/la803436h

nanoribbons, helical structures, nanotubes, etc., have been created.12-17 Coil-rod-coil molecules consisting of a central rigid-rodlike core, two flexible coil-like segments at the two ends, and two secondary amido functional groups are considered to be powerful molecular architectures to form strong anisotropic ordering via self-assembling.18-22 This concept has also been used in our recent paper:23 the hierarchical supramolecular self-assembly of p-terphenylen-1,400 -ylenebis(dodecanamide) (TB) to form a new type of rolled-up organic nanotube. A synergistic effect of the π-π interaction and the intermolecular translation-related hydrogen bonding resulted in the formation of extremely stable aggregations. Supramolecular gels are fascinating self-organized soft matters because of their architectural elegance and potential (12) Shimizu, T. Macromol. Rapid Commun. 2002, 23, 311–331. (13) Messmore, B. W.; Hulvat, J. F.; Sone, E. D.; Stupp, S. I. J. Am. Chem. Soc. 2004, 126, 14452–14458. (14) Jang, W. D.; Jiang, D. L.; Aida, T. J. Am. Chem. Soc. 2000, 122, 3232–3233. (15) Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science 1999, 284, 785–788. (16) Zubarev, E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. J. Am. Chem. Soc. 2001, 123, 4105–4106. (17) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem., Int. Ed. 2001, 40, 988–1011. (18) Lee, M.; Cho, B.-K.; Zin, W.-C. Chem. Rev. 2001, 101, 3869–3892. (19) Ryu, J.-H.; Hong, D.-J.; Lee, M. Chem. Commun. 2008, 1043–1054. (20) Kim, J.-K.; Lee, E.; Huang, Z.; Lee, M. J. Am. Chem. Soc. 2006, 128, 14022–14023. (21) Kim, H.-J.; Lee, J.-H.; Lee, M. Angew. Chem., Int. Ed. 2005, 44, 5810 –5814. (22) Han, K.-H.; Lee, E.; Kim, J. S.; Cho, B.-K. J. Am. Chem. Soc. 2008, 130, 13858–13859. (23) Chen, Y. L.; Zhu, B.; Zhang, F.; Han, Y.; Bo, Z. S. Angew. Chem., Int. Ed. 2008, 47, 6015–6018.

Published on Web 03/16/2009

Langmuir 2009, 25(15), 8548–8555

Chen et al.

applications in template synthesis,24,25 controlled release,26 separations,27 and biomimetics.28 However, complex interactions of gelator-gelator and gelator-solvent make the gelation process poorly understood, and the contemporary design of supramolecular gelators is still on a somewhat serendipitous stage.29-31 To better understand the relationship between molecular structure and gel-phase behavior, there has been considerable focus on the development of diverse chemical structures and exerting them on the structure-function studies.32-34 Herein, we extend our focus from solution to this quasi-solid self-assembly system by effective molecular modification. On account of the unique structures and properties of dendritic molecules,13,14,35-37 bulky dendritic wedges were incorporated at the two ends of the central stiff aromatic scaffolds to fine-tune the intermolecular interactions. As expected, the steric bulky effect imposed by the dendritic wedges provided a delicate balance between the crystallization, precipitation, and solubility; as a result, a novel type of dumbbell-shaped dendritic gelator was designed and synthesized. With a combination of the π-π stacking, hydrogenbonding, and van der Waals forces, the three dendritic molecules could gel several organic solvents. A negative dendritic effect38,39 on the gelation ability was observed, and the morphologies as well as the packing structures of the organogels were significantly influenced by the dendritic generation. In particular, the three dendritic molecules in gel phase exhibited unusually enhanced fluorescence emission, which is a novel supramolecular version of the aggregation-induced emission that is of growing interest in recent years.40-47 (24) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980–999. (25) Xue, P. C.; Lu, R.; Li, D. M.; Jin, M.; Tan, C. H.; Bao, C. Y.; Wang, Z. M.; Zhao, Y. Y. Langmuir 2004, 20, 11234–11239. (26) Friggeri, A.; Feringa, B. L.; van Esch, J. J. Controlled Release 2004, 97, 241–248. (27) Ghosh, Y. K.; Bhattacharya, S. Chem. Commun. 2001, 185–186. (28) Carnahan, M. A.; Middleton, C.; Kim, J.; Kim, T.; Grinstaff, M. W. J. Am. Chem. Soc. 2002, 124, 5291–5293. (29) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159. (30) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263–2266. (31) Hirst, A. R.; Smith, D. K. Top. Curr. Chem. 2005, 256, 237–273. (32) Huang, X.; Terech, P.; Raghavan, S. R.; Weiss, R. G. J. Am. Chem. Soc. 2005, 127, 4336–4344. (33) Percec, V.; Peterca, M.; Yurchenko, M. E.; Rudick, J. G.; Heiney, P. A. Chem.;Eur. J. 2008, 14, 909–918. (34) Lu, L.; Cocker, T. M.; Bachman, R. E.; Weiss, R. G. Langmuir 2000, 16, 20–34. (35) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665–1688. (36) Zeng, F. W.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681–1712. (37) Percec, V.; Ahn, C. H.; Ungar, G.; Yeardley, D. J. P.; Moller, M.; Sheiko, S. S. Nature (London) 1998, 391, 161–164. (38) Love, C. S.; Andrew, R. H.; Chechik, V.; Smith, D. K.; Ashworth, I.; Brennan, C. Langmuir 2004, 20, 6580–6585. (39) Kim, C.; Lee, S. J.; Lee, I. H.; Kim, K. T. Chem. Mater. 2003, 15, 3638–3642. (40) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410–14415. (41) Luo, J. D.; Xie, Z. L.; Lam, J. W. Y.; Cheng, L.; Chen, H. Y.; Qiu, C. F.; Kwok, H. S.; Zhan, X. W.; Liu, Y. Q.; Zhu, D. B.; Tang, B. Z. Chem. Commun. 2001, 1740–1741. (42) Ryu, S. Y.; Kim, S.; Seo, J.; Kim, Y. W.; Kwon, O. H.; Jang, D. J.; Park, S. Y. Chem. Commun. 2004, 70–71. (43) Bao, C. Y.; Lu, R.; Jin, M.; Xue, P. C.; Tan, C. H.; Xu, T. H.; Liu, G. F.; Zhao, Y. Y. Chem.;Eur. J. 2006, 12, 3287–3294. (44) Wang, C.; Zhang, D.; Xiang, J.; Zhu, D. Langmuir 2007, 23, 9195–9200. (45) Palui, G.; Banerjee, A. J. Phys. Chem. B 2008, 112, 10107–10115. (46) Yang, H.; Yi, T.; Zhou, Z. G.; Zhou, Y.; Wu, J. C.; Xu, M.; Li, F. Y.; Hang, C. H. Langmuir 2007, 23, 8224–8230. (47) Babu, S. S.; Praveen, V. K.; Prasanthkumar, S.; Ajayaghosh, A. Chem.;Eur. J. 2008, 14, 9577–9584.

Langmuir 2009, 25(15), 8548–8555

Article

Experimental Section General Methods. Unless otherwise noted, all chemicals were purchased from commercial suppliers and used without further purification. Dichloromethane (CH2Cl2) was distilled over CaH2. Tetrahydrofuran (THF) was distilled over sodium and benzophenone. All reactions were performed under an atmosphere of nitrogen and monitored by TLC with silica gel 60 F254 (Merck, 0.2 mm). Column chromatography was carried out on silica gel (200-300 mesh). The catalyst precursor Pd(PPh3)4 was prepared according to the literature48 and stored in a Schlenk tube under nitrogen. Characterization. 1H and 13C NMR spectra were recorded on an AV400 and DM300 spectrometer in CDCl3. The matrixassisted laser desorption ionization time-of-flight (MALDITOF) mass spectroscopy measurements were carried out with a Bruker BIFLEXIII mass spectrometer with R-cyano-4hydroxycinnamic acid as the matrix. Elemental analyses were performed on a Flash EA 1112 analyzer. Melting points were measured with an XT4-100X apparatus and uncorrected. Scanning electron microscopy (SEM) was performed on a JEOL model JSM-6700F FE-SEM operating at 5 kV with Pt coated. Samples for SEM measurement were prepared by wiping a small amount of gel onto a silicon wafer followed by naturally evaporating the solvent. Transmission electron microscopy (TEM) was recorded on a Hitachi H-800 electron microscope operating at 100 kV. Gels were diluted 10-fold in corresponding solvents before been drop-casting onto a 200-mesh carboncoated copper grid for TEM measurement. AFM images were recorded under ambient conditions using a Digital Instrument Multimode Nanoscope IIIA operating in the tapping mode. Samples for AFM measurement were prepared by dipping a silicon wafer to the gel phase for 15 min. Differential scanning calorimetry (DSC) measurements were carried out with a Mettler DSC822e under a nitrogen atmosphere at a heating rate of 10 °C/min. Polar optical microscopy (POM) was performed on an Olympus BH-2 optical microscope with a Mettler hot stage (FP-52) and an automatic camera (heating rate: 3 °C/min). Rheological characterization was performed by using a TA AR2000ex stress-controlled rheometer with 40 mm parallel plates geometry. A hot solution of the compound was transferred onto the plate kept at 25 °C and allowed to form a uniform layer. In order to avoid the solvent evaporation, the surface of sample between two plates was covered with glycerol. Oscillatory dynamic shear experiments were performed in the frequency range of 0.1-100 rad/s, using a constant strain (0.2%) determined with a strain sweep to lie within the linear viscoelastic regime. The evolution of moduli (G0 and G00 ) vs time was tested at 25 °C, with a frequency of 1 Hz and a strain of 0.2%. The temperature ramp was measured at 1 Hz frequency and a strain of 0.2%, with a fixed heating or cooling rate of 5 °C/min. UV-vis absorption spectra were obtained on a Shimadzu UV-vis spectrometer model UV-1601 PC. Fluorescence emission and excitation spectra were recorded on a Hitachi F-4500 spectrometer. Quartz cell with 0.1 cm path length was used. FT-IR spectra of gels and solutions were measured at room temperature using a Bruker Tensor 27 FT-IR spectrometer and an IFS-66v/S FT-IR spectrometer. Gel samples were prepared by casting gels onto a KBr plate, and solutions were placed in CaF2 cells. Wide-angle X-ray diffraction (WAXRD) and small-angle X-ray diffraction (SAXRD) data were collected using monochromated Cu KR radiation (λ = 1.540 56 A˚) on a Rigaku D/max-2500 diffractometer. Samples for WAXRD measurement were prepared by casting the gels on glass slide and dried at room temperature. Freeze-dried xerogels from benzene were used for SAXRD measurement. Fluorescence lifetimes were obtained (48) Tolman, C. A.; Seidel, W. C.; Gerlach, D. H. J. Am. Chem. Soc. 1972, 94, 2669–2676.

DOI: 10.1021/la803436h

8549

Article using a time-correlated single-photon-counting spectrometer FLS920. Gelation Test of Organic Fluids. A weighed amount of the compound (5 mg) in an appropriate solvent (0.5 mL) was placed in a glass vial, which was sealed and heated until the compound was dissolved. After the solution was allowed to stand at room temperature for about 12 h, the state of the solution was evaluated by the “stable to inversion of a test tube” method. The reversibility of the gelation was confirmed by repeated heating and cooling. The critical gelator concentration (CGC) is determined from the minimum amount of gelator required for the formation of gel at room temperature. 1a: A mixture of [G1]-COOH (2.8 g, 5.70 mmol), 4-bromobenzenamine (1.47 g, 8.54 mmol), 1-hydroxybenzotriazole (HOBt) (0.92 g, 6.81 mmol), 1-ethyl-3-[3-(dimethylamino) propyl] carbodiimide hydrochloride (EDC 3 HCl) (1.97 g, 10.28 mmol), dry CH2Cl2 (60 mL), and triethylamine (2.0 mL) was stirred at room temperature for 24 h. Water was added; the organic layer was separated, washed successively with 5 mol/L aqueous HCl solution and water, dried over anhydrous Na2SO4, and evaporated to dryness. Chromatography on silica gel eluting with hexane/CH2Cl2 (2:3, v/v) afforded as a white solid (3.2 g, 87%). 1H NMR (CDCl3, 400 MHz): δ 7.73(s, 1H, NH), 7.54 (d, 2H, ArH), 7.48 (d, 2H, ArH), 6.93 (s, 2H, ArH), 6.61 (s, 1H, ArH), 3.98 (t, 4H, OCH2), 1.78 (m, 4H, CH2), 1.47-1.26 (m, 36H, CH2), 0.88 (t, 6H, CH3). 13C NMR (CDCl3, 100 MHz): δ 165.59, 160.61, 137.04, 136.65, 132.05, 121.61, 117.09, 105.36, 104.76, 68.43, 31.93, 29.67, 29.65, 29.61, 29.59, 29.38, 29.36, 29.19, 26.02, 22.70, 14.12. Anal. Calcd for C37H58NBrO3: C, 68.92; H, 9.07; N, 2.17. Found: C, 69.00; H, 9.02; N, 2.50. 1b was synthesized according to a similar method as that of 1a; yield: 79%. 1H NMR (CDCl3, 400 MHz): δ 7.67 (s, 1H, NH), 7.53 (d, 2H, ArH), 7.48 (d, 2H, ArH), 7.04 (s, 2H, ArH), 6.77 (s, 1H, ArH), 6.55 (s, 4H, ArH), 6.42 (s, 2H, ArH), 5.00 (s, 4H, OCH2), 3.94 (t, 4H, OCH2), 1.76 (m, 8H, CH2), 1.44-1.26 (m, 72H, CH2), 0.88 (t, 12H, CH3). 13C NMR (CDCl3, 100 MHz): δ 165.30, 160.61, 160.18, 138.43, 136.98, 136.78, 132.07, 121.54, 117.14, 106.13, 105.79, 105.56, 100.91, 70.43, 68.13, 31.93, 29.68, 29.65, 29.62, 29.60, 29.42, 29.36, 29.28, 26.07, 22.70, 14.12. Anal. Calcd for C75H118NBrO7: C, 73.50; H, 9.70; N, 1.14. Found: C, 73.41; H, 9.78; N, 1.69. 1c was synthesized according to a similar method as that of 1a; yield: 66%. 1H NMR (CDCl3, 400 MHz): δ 7.72 (s, 1H, NH), 7.54 (d, 2H, ArH), 7.45 (d, 2H, ArH), 7.03 (s, 2H, ArH), 6.75 (s, 1H, ArH), 6.67 (s, 4H, ArH), 6.57 (s, 2H, ArH), 6.54 (s, 8H, ArH), 6.40 (s, 4H, ArH), 5.02 (s, 4H, OCH2), 4.95 (s, 8H, OCH2), 3.91 (t, 16H, OCH2), 1.75 (m, 16H, CH2), 1.43-1.26 (m, 144H, CH2), 0.87 (t, 24H, CH3). 13C NMR (CDCl3, 100 MHz): δ 165.47, 160.55, 160.22, 160.01, 138.91, 138.75, 137.18, 136.87, 131.98, 121.74, 117.07, 106.41, 106.19, 105.76, 105.37, 101.70, 100.83, 76.78, 70.17, 68.08, 31.98, 29.74, 29.70, 29.66, 29.49, 29.42, 29.34, 29.17, 26.28, 22.74, 14.16. Anal. Calcd for C151H238NBrO15: C, 75.97; H, 10.05; N, 0.59. Found: C, 76.05; H, 10.28; N, 0.44. [G1]-TP-[G1] (2a): 1a (0.63 g, 0.98 mmol), 1,4-phenyldiboronic acid (0.1 g, 0.4 mmol), Pd(PPh3)4 (14 mg, 0.01 mmol), NaHCO3 (1.37 g, 16.31 mmol), H2O(15 mL), and THF (40 mL) were charged sequentially into a Schlenk flask and heated to reflux under a nitrogen atmosphere for 24 h. The organic layer was separated; the aqueous layer was extracted with Et2O (2  30 mL); the combined organic layers were dried over anhydrous Na2SO4 and evaporated to dryness. Chromatography on silica gel eluting with hexane/CH2Cl2 (1:3, v/v) afforded 2a as a white solid (0.48 g, 98%); mp: 146 °C. 1H NMR (CDCl3, 400 MHz): δ 7.83 (s, 2H, NH), 7.73 (d, 4H, ArH), 7.68 (s, 4H, ArH), 7.66 (d, 4H, ArH), 6.98 (s, 4H, ArH), 6.62 (s, 2H, ArH), 4.01 (t, 8H, OCH2), 1.80 (m, 8H, CH2), 1.53-1.27 (m, 72H, CH2), 0.88 (t, 12H, CH3). 13 C NMR (CDCl3, 100 MHz): δ 165.59, 160.61, 139.24, 137.33, 136.99, 136.78, 127.56, 127.22, 120.42, 105.40, 104.67, 68.44, 8550 DOI: 10.1021/la803436h

Chen et al. 31.93, 29.68, 29.65, 29.62, 29.60, 29.40, 29.36, 29.21, 26.04, 22.70, 14.12. Anal. Calcd for C80H120N2O6: C,79.68; H,10.03; N,2.32. Found: C, 79.33; H, 9.93; N, 2.65. MALDI-TOF, m/z: calcd, 1205.8; found, 1206.0 ([M + H]+). [G2]-TP-[G2] (2b) was synthesized according to a similar method as that of 2a; yield: 86%; mp: 145 °C. 1H NMR (CDCl3, 400 MHz): δ 7.77 (s, 2H, NH), 7.72 (d, 4H, ArH), 7.68 (s, 4H, ArH), 7.66 (d, 4H, ArH), 7.09 (s, 4H, ArH), 6.78 (s, 2H, ArH), 6.57 (s, 8H, ArH), 6.42 (s, 4H, ArH), 5.02 (s, 8H, OCH2), 3.94 (t, 16H, OCH2), 1.77 (m, 16H, CH2), 1.45-1.26 (m, 144H, CH2), 0.88 (t, 24H, CH3). 13C NMR (CDCl3, 100 MHz): δ 165.96, 161.27, 160,84, 139.90, 139.14, 137.93, 137.77, 137.49, 128.23, 127.87, 121.01, 106.82, 106.46, 106.11, 101.60, 71.11, 68.78, 32.58, 30.33, 30.30, 30.28, 30.26, 30.08, 30.01, 29.94, 26.73, 23.34, 14.77. Anal. Calcd for C156H240N2O14: C, 79.14; H, 10.22; N, 1.18. Found: C, 79.66; H, 10.19; N, 1.65. MALDITOF, m/z: calcd, 2367.6; found, 2390.8 ([M + Na]+). [G3]-TP-[G3] (2c) was synthesized according to a similar method as that of 2a; yield: 74%; mp: 75 °C. 1H NMR (CDCl3, 400 MHz): δ 7.89 (s, 2H, NH), 7.76 (d, 4H, ArH), 7.68 (s, 4H, ArH), 7.66 (d, 4H, ArH), 7.10 (s, 4H, ArH), 6.78 (s, 2H, ArH), 6.70 (s, 8H, ArH), 6.60 (s, 4H, ArH), 6.57 (s, 16H, ArH), 6.41 (s, 8H, ArH), 5.04 (s, 8H, OCH2), 4.97 (s, 16H, OCH2), 3.94 (t, 32H, OCH2), 1.76 (m, 32H, CH2), 1.44-1.27 (m, 288H, CH2), 0.89 (t, 48H, CH3). 13C NMR (CDCl3, 100 MHz): δ 165.27, 160.55, 160.26, 160.13, 138.96, 138.89, 138.72, 137.33, 137.18, 136.80, 127.54, 127,19, 120.37, 106.47, 106.35, 106.24, 105.77, 101.76, 100.87, 76.72, 70.23, 68.10, 31.94, 29.70, 29.66, 29.64, 29.62, 29.44, 29.38, 29.30, 26.09, 22.71, 14.14. Anal. Calcd for C308H480N2O30: C, 78.86; H, 10.31; N, 0.60. Found: C, 78.14; H, 10.21; N, 0.47. MALDI-TOF, m/z: calcd, 4691.1; found, 4714.8 ([M + Na]+).

Results and Discussion Synthesis. The synthetic routes for compounds [Gn]TP-[Gn] 2a-c are shown in Scheme S1 (Supporting Information). Carboxylic acid-terminated poly(benzyl ether) dendrons, [Gn]-COOH (n = 1-3)49 and 1,4-phenyldiboronic acid50 were prepared according to literature procedures. 1a-c were prepared by acylation of 4-bromobenzenamine with the corresponding [Gn]-COOH (n = 1-3). Suzuki cross-coupling of 1,4-phenyldiboronic acid and 1a-c was carried out in a biphasic mixture of THF and aqueous NaHCO3 with Pd(PPh3)4 as a catalyst precursor to afford the final dendritic molecules 2a-c in yields of 74-98%. All the compounds were characterized unambiguously with 1 H and 13C NMR spectroscopy, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy, and elemental analysis. Gelation. The p-terphenylene cored dendritic molecules comprise three parts: the central p-terphenylene core, two amido functional groups, and two dendritic wedges at the two ends. The terphenylene segment and two amido functional groups have a strong tendency to form supramolecular aggregation through the π-π stacking and the intermolecular translation-related hydrogen bonding. The two peripheral dendritic wedges not only adjust the distance (the strength of supramolecular interactions) among the p-terphenylene segments via bulk effect but also endow the supramolecular aggregates with certain solubility in some organic solvents. Unlike p-terphenylen-1,400 -ylenebis (dodecanamide) (TB), which is almost insoluble in any (49) Fujigaya, T.; Jiang, D. L.; Aida, T. J. Am. Chem. Soc. 2005, 127, 5484–5489. (50) Lauter, U.; Meyer, W. H.; Wegner, G. Marcromolecules 1997, 30, 2092–2101.

Langmuir 2009, 25(15), 8548–8555

Chen et al.

Figure 1. Possible packing style of dendritic molecules in gels. solvent at room temperature, these dendritic molecules 2a-c are soluble in many organic solvents such as CH2Cl2, CHCl3, THF, and DMF. The controlled supramolecular self-assembling of 2a-c is depicted in Figure 1. The p-terphenylene segment has a strong tendency to aggregate through π-π overlapping, and the directional intermolecular hydrogen bonding between the bridges strengthens the longitudinal assemblies. Meanwhile, van der Waals forces between the flexible shells, and the steric constraints imposed by dendritic wedges would control the balance between solubility and crystallization. With the synergistic effect of these noncovalent forces, an interpenetrating 3-D network of one-dimensional nanostructures would be eventually established entrapping a large amount of solvent, thereby forming a gel. The gelation abilities of these molecules were estimated in different solvents. As summarized in Table 1, the lowest generation dendritic molecule 2a exhibited the strongest gelation ability: it formed self-supporting gels in many apolar organic solvents such as benzene, toluene, xylene, octane, carbon tetrachloride, etc. All the gels were thermoreversible; i.e., the gelators were insoluble at room temperature, they turned into clear and low-viscosity solution upon heating, and after cooling to room temperature the turbid gels appeared (Figure S1 in Supporting Information). This cycle could be repeated for hundreds of times, and all the gels could stably exist for more than a year. The critical gelator concentrations (CGC) of 2a in xylene was tested using “stable to inversion of a test tube” method, and the value was determined as 1.6 mmol/L (mM), which meant that 2a could entrap about 5000 molecules of xylene per gelator molecule. 2b was also an effective gelator for nonpolar aliphatic solvents, whereas for aromatic solvents, the gelation process needed a longer time and partial gels were formed in some cases. Phase transitions of two dendritic gelators in bulk were measured by DSC and POM to identify their different thermodynamics. Figure S2 (Supporting Information) shows the DSC curves (first heating scan) for xerogels 2a and 2b. A broad endothermic peak at about 60 °C for 2a and 55 °C for 2b suggested their supramolecular polymer characteristic.51 For 2b, the lower transition temperature, the broader and less clear endothermic peak revealed a (51) Qu, S.; Li, F.; Wang, H.; Bai, B.; Xu, C.; Zhao, L.; Long, B.; Li, M. Chem. Mater. 2007, 19, 4839–4846.

Langmuir 2009, 25(15), 8548–8555

Article

less cooperative transition compared with that of 2a.38,52 The decreased thermodynamic stability of 2b relative to that of 2a was further identified by the fact that 2b formed only a monotropic mesophase in the DSC curve,33 while for 2a, an additional endothermic peak (109 °C) before the melting point was observed. Combined with POM examination (Figure S3 in Supporting Information), this peak was in accordance with the transition from the crystalline solid phase to the LC mesophase.46 However, under the same condition as 2a and 2b, the highest generation dendritic molecule 2c could not gel most of these solvents efficiently. Only partial gels were formed in the same organic solvents. With the increase of the dendritic generation, their gelation ability decreased. This negative dendritic effect could be attributed to the steric effects that hindered and slowed the stacking process of the dumbbellshaped molecules. The increase of the number of the peripheral alkyl chains that weakened the intermolecular interactions, resulting in a good solubility of the dendritic molecules in many organic solvents. Microscopy. To obtain visual images of the gels, AFM, SEM, and TEM images were taken. As shown in Figure 2a and Figure S4a (Supporting Information), AFM images of 2a gel displayed a network structure composed of nanoscale fibrous aggregates with a high aspect ratio. These fibers are several micrometers long and with the diameters in the range of 30-300 nm, and they are formed by hierarchical selforganization, namely, the dendritic molecules 2a first form thin fibrillar aggregates by π-π stacking and translated intermolecular hydrogen bonding and then further entangled together to fibers and bundles and finally the interconnected network. As shown in Figure S4b (Supporting Information), 2b could also form the elongated fibers by supramolecular self-assembling. However, AFM image showed that the fibers of 2b were not as rigid as those of 2a, which could be attributed to relatively weaker interactions between individual fibrils. The partial gel of 2c also showed the presence of heavily entangled fibrous assemblies (Figure S4c in Supporting Information), whereas the dimension of most fibers from AFM observation was much smaller than those of 2a and 2b. SEM and TEM studies offered a closer look at the self-assembled structures. The results (Figure 2b,c and Figure S4 in Supporting Information) provided a full support of fibrous morphology of the hierarchical self-assembly, just as the AFM analyses. Furthermore, the precipitate of 2b from benzene showed a crystalline structure (Figure S4e in Supporting Information). This difference indicated that the gelation process changed the gelators’ aggregation mode from the 3D aggregate to the 1D aggregate.53 Rheological Properties.54 Rheological experiments on 2a and 2b gels formed in octane were conducted to study their viscoelastic properties, which are determined by the (52) Brinksma, J.; Feringa, B. L.; Kellogg, R. M.; Vreeker, R.; van Esch, J. Langmuir 2000, 16, 9249–9255. (53) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 42, 1229–1233. (54) The reproducibility of the absolute values of G0 and G00 is not very good because they are sensitive to factors that are very difficult to control, such as the history of the sample, the presence of nucleating sites (e.g., microparticulates and small scratches on the rheometer plates), and the method of placement of the samples between the rheometer plates. However, the trends are clear and reproducible. See also: Huang, X.; Raghavan, S. R.; Terech, P.; Weiss, R. G. J. Am. Chem. Soc. 2006, 128, 15341–15352. And for 2c is not an effective gelator, rheological study of 2c gel was not performed here.

DOI: 10.1021/la803436h

8551

Article

Chen et al. Table 1. Gelation Properties of 2a-c in Organic Solventsa

gelator

benzene

PhCl

toluene

xylene

hexane

octane

heptane

CCl4

2a G G G G G G G G 2b PG PG PG G G G G G 2c S S S PG PG PG PG S a Gelator = 10 mg/mL; G: gel, PG: partial gel, S: soluble, P: precipitation. b vol/vol = 2:1. c Slight ultrasound is needed.

Figure 2. (a) AFM height image of xerogel formed from the gel phase of 2a in xylene (scan size: 80  80 μm). (b) SEM image of xerogel formed from the gel phase of 2b in xylene. (c) TEM image of the self-assembled nanofibers from 2a in xylene. dynamics and strength of the network structure.32,47,52 The frequency sweep curve of 2a gel (4.2 mM) (Figure 3a) was consistent with the behaviors as a true gel: the elastic modulus G0 and the viscous modulus G00 values do not depend strongly on oscillation frequency in the frequency range of 0.1-100 rad/s, and the G0 . G00 at all tested frequencies, thereby indicating that the gel is elastically stronger and dominates the viscous properties. The variation of dynamic moduli (G0 and G00 ) versus time at 25 °C is shown in Figure 3b. The modulus values were almost unchanged within a long time, indicating the stability of the gel network once it was formed. Similar rheological behaviors of 2a and 2b samples with different concentration (Figure S5 in Supporting Information) also confirmed their viscoelastic nature as a gel. To monitor the sol-gel phase transition, the modulus evolution as a function of temperature was recorded (Figure 3c). The sol-gel transition temperature could be obtained from the temperature sweep because of obvious modulus changes at the phase transition point of an organogel. By analyzing results (as to the elastic modulus G0 eq of the gels at 25 °C and the gel-to-sol transition temperature Tg-s) from 2a and 2b gels with different concentration, generation- and concentration-dependent gelation behaviors are summarized in Figure 4. Both the G0 eq and Tg-s for 2a gel were higher than those of 2b at the same concentration as 4.2 mM, suggesting that the former is elastically stronger and more stable than the latter. This trend, namely, the negative dendritic effect, was in agreement with the observation from gelation test as mentioned above. While for the same gelator (2a or 2b), when the concentration increased, these two values increased. These results revealed a gradual improvement in the viscoelastic solidlike behavior of the gels with the increase of concentration as well as the decrease of generation. When several cycles of heating and cooling were repeated, the sol-gel transition was reproducible, indicating that the gel formation was thermoreversible. UV-vis, FT-IR, 1H NMR, and XRD Spectra.55 UV-vis, FT-IR, and 1H NMR spectroscopy and X-ray diffraction (XRD) technique are very powerful tools to study the intermolecular interactions and the driving force for the (55) For 2b and 2c have the similar self-assembly behavior, we here take 2a as an example. Detailed information about 2b and 2c is shown in the Supporting Information.

8552 DOI: 10.1021/la803436h

CH2Cl2/CH3OHb Gc G P

formation of organogels.34,56-61 As shown in Figure 5, the π-π* transition bands of 2a in the dilute xylene solution appeared at 317 nm, which shifted to 313 nm in film and 304 nm in the gel state. The blue shift of the UV absorption indicated the formation of the π-π stacking by p-terphenylene cores during the gelation, which were forced to adopt an H-aggregation mode (face-to-face mode). The FT-IR spectrum of the xerogel 2a from xylene (Figure S7a in Supporting Information) showed an N-H stretch band at 3283 cm-1 and an amide I band at 1649 cm-1. The relatively low wavenumbers (from the FT-IR spectrum of dilute tetrachloride solution for 2a, the hydrogen-bonding-free N-H stretch band at 3438 cm-1, and amide I band at 1687 cm-1 were observed;62 see Supporting Information) suggested the presence of strong intermolecular hydrogen bonding.45,57,58 A control experiment was performed to further illustrate the role of hydrogen bonding in the formation of gels: when treated with a droplet of acetic acid, the self-sorting gels turned into partial gels and the sol-gel transition then became thermo-irreversible. As shown in Figure S8 (Supporting Information), the SEM image of partial gels revealed that thinner and more pliable fibers were formed compared to the self-supporting 2a gels from xylene. FT-IR results of the self-supporting 2a gel also showed CH2 stretching vibrations locating at lower frequencies (υ anti: 2921 cm-1; υ sym: 2851 cm-1) which could be ascribable to an all-trans conformation, indicating that the alkyl chains were closely packed to form a crystalline domain.58,63 The variable temperature 1H NMR measurement further confirmed these results. As a typical example, the 1H NMR spectra of 2a in [D4] o-dichlorobenzene at 25 and 80 °C are shown in Figure S9 (Supporting Information). When recorded at room temperature (25 °C), the gel sample displayed broad and lessresolved signals for both the aromatic and aliphatic protons due to the immobilization of the organic molecules in the gelphase network. At elevated temperature (80 °C) as the gel sample converted into sol, the signals corresponding to aromatic protons became well-resolved, and the change of chemical shift of the p-terphenylene protons was observed (Figure S9a,b in Supporting Information). These temperature-dependent changes of chemical shifts of aromatic protons implied the existence of aromatic interactions in the gel state. Notably, the proton signals of the N-H groups shifted from 8.01 ppm at 25 °C to 7.67 ppm at 80 °C. This upfield (56) Shirakawa, M.; Kawano, S.; Fujita, N.; Sada, K.; Shinkai, S. J. Org. Chem. 2003, 68, 5037–5044. (57) Shimizu, T.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 2812–2818. (58) Masuda, M.; Shimizu, T. Langmuir 2004, 20, 5969–5977. (59) George, M.; Weiss, R. G. Langmuir 2003, 19, 8168–68176. (60) Menger, F. M.; Yamasaki, Y.; Catlin, K. K.; Nishimi, T. Angew. Chem., Int. Ed. 1995, 34, 585–586. (61) George, S. J.; Ajayaghosh, A. Chem.;Eur. J. 2005, 11, 3217–3227. (62) From the FT-IR spectra of dilute tetrachloride solution for 2a and 2b, N-H stretch band and amide I band attributed to the hydrogen-bonded amides coexisted with those hydrogen-bonding-free ones, which implied that in the free-flowing state large aggregates could still exist. See also ref 34. (63) Hill, J. P.; Jin, W.; Kosaka, A.; Fukushima, T.; Ichihara, H.; Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004, 304, 1481–1483.

Langmuir 2009, 25(15), 8548–8555

Chen et al.

Article

Figure 4. Elastic modulus G0 eq (square symbols) at 25 °C and gelto-sol phase transition temperature Tg-s (circle symbols) for the 2a (filled symbols) and 2b (open symbols) gels in octane as a function of concentration.

Figure 5. UV-vis spectra of 2a in dilute xylene solution (2.5  10-5 mol/L), film, and xylene gel.

Figure 3. Elastic modulus G0 (filled symbols) and the viscous modulus G00 (open symbols) for the 2a gel formed in octane (4.2 mM) as a function of (a) oscillation frequency at 25 °C with a strain of 0.2%. (b) Time at 25 °C with a frequency of 1 Hz and a strain of 0.2%. (c) Temperature (circle symbols for the heating scan, square symbols for the cooling scan). shift implied that hydrogen-bond interactions became weaker at elevated temperature, and such interactions probably played an important role in mediating the assembly of dendritic gelators. Further heating the sample to 100 °C did not produce any significant change, except for a small, upfield shift of N-H proton from 7.67 to 7.60 ppm. XRD studies further revealed the formation of aggregation in gel state. WAXRD results of the xerogel 2a from xylene are shown in Figure 6a. The d-spacing of 3.9 A˚ was probably corresponding to the center-to-center distance between the p-terphenylene units in 2a gels,61 while for 2b Langmuir 2009, 25(15), 8548–8555

and 2c (Figure S10 in Supporting Information), this value increased to 4.2 and 4.3 A˚, respectively, indicating a less compact packing mode in gels due to the increase of dendritic generation. Also, several sharp peaks in the range of 3°-15° for 2a reflected crystalline properties while those broad ones for both 2b and 2c indicated a less ordered and less closed packing in gels. All these differences could be attributed to the steric effect caused by bulky dendritic wedges that hampered longitudinal ordering of gelators. The d-spacing of 4.4 A˚ was possibly caused by the ordered packing of the alkyl chains.64 Well-resolved SAXRD patterns of the nanostructures from 2a were observed, demonstrating the longrange ordering of molecules (Figure 6b).16 The SAXRD profiles indicated that ribbonlike structures were predominant in the gel state, for which the prominent diffraction peak corresponding to a d-spacing of 1.69 nm was probably due to the inter-ribbon distance in the fiber.65 The other two reflections with a q-spacing ratio of 1:2 indicated a lamellar mesophase. The periodic value was determined to be 2.13 nm for 2a and increased to 2.8 nm for 2b with the generation of dendron increased (Figure S11 in Supporting Information). Considering the dimension of the dendron segments (64) Jung, J. H.; Shinkai, S.; Shimizu, T. Chem.;Eur. J. 2002, 8, 2684–2690. (65) Aggeli, A.; Nyrkova, I. A.; Bell, M.; Harding, R.; Carrick, L.; McLeish, T. C. B.; Semenov, A. N.; Boden, N. Proc. Natl. Acad. Sci. U.S. A. 2001, 98, 11857–11862.

DOI: 10.1021/la803436h

8553

Article

Chen et al.

Figure 6. (a) WAXRD spectrum of 2a xerogel from xylene. (b) SAXRD spectrum of 2a xerogel from benzene. (1.6 nm for 2a and 2.3 nm for 2b by the CPK molecular modeling) and the rodlike segment (1.4 nm for the rigid core by the CPK molecular modeling), these signals were consistent with the distances from the central aromatic core to the dendritic wedges, which suggested that the dendron segments in the lamellar mesophase self-assemble into interdigitated layers.61,66-68 Besides, as for 2b, the presence of a relatively broad reflection and the absence of higher-order reflections in the small-angle region suggested a low degree of ordering in this case. On the basis of the above results, it was clear that π-π interaction, hydrogen bonding, and van der Waals forces played a cooperative role in the formation of gel and a possible packing mode of 2a in the gel state as illustrated in Figure 7. Aggregation-Induced Fluorescence Emission. Gelation-induced fluorescence enhancement was observed for all three dendritic molecules 2a-c. Herein, we typically discuss the fluorescence properties of 2b in detail; the spectral data of 2a and 2c are summarized in the Supporting Information. As shown in Figure 8a, for dendritic molecule 2b, its solution was nonfluorescent under the illumination of UV light, whereas its gel was highly fluorescent. The gelation process of 2b was monitored in situ by fluorescence spectroscopy. The increase of fluorescence intensity for dendritic molecule 2b was recorded during its gelation. For 2b, the fluorescence intensity of its gel is more than 800 times stronger than that of its solution. This interesting property would provide an opportunity to modulate the fluorescent emission by conveniently controlling the aggregation degree, which would be useful in fluorescent labels,69 optical sensors,70 etc. Aggregation-induced fluorescence emission (AIE) has been explored recently; however, only a few examples of fluorescent H-aggregates were reported.42,44,71,72 Generally, H-aggregates tended to induce the nonradiative process, and the fluorescence should be strongly quenched.72,73 :: (66) Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Hagele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; Tosoni, M. Angew. Chem., Int. Ed. 2007, 46, 4832–4887. (67) Chung, Y.-W.; Lee, J.-K.; Zin, W.-C.; Cho, B.-K. J. Am. Chem. Soc. 2008, 130, 7139–7147. (68) Yagai, S.; Kubota, S.; Iwashima, T.; Kishikawa, K.; Nakanishi, T.; Karatsu, T.; Kitamura, A. Chem.;Eur. J. 2008, 14, 5246–5257. (69) Camerel, F.; Bonardi, L.; Ulrich, G.; Charbonniere, L.; Donnio, B.; Bourgogne, C.; Guillon, D.; Retailleau, P.; Ziessel, R. Chem. Mater. 2006, 18, 5009–5021. (70) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18–52. (71) Zeena, S.; Thomas, K. G. J. Am. Chem. Soc. 2001, 123, 7859–7865. (72) Rosch, U.; Yao, S.; Wortmann, R.; Wurthner, F. Angew. Chem., Int. Ed. 2006, 44, 7026–7030. (73) Whitten, D. G. Acc. Chem. Res. 1993, 26, 502–509.

8554 DOI: 10.1021/la803436h

Figure 7. Schematic representation of a possible packing of 2a in the gel state.

For a better understanding the nature of emission, fluorescence decay and excitation spectra were measured. The fluorescence decay was well fitted with a monoexponential decay in all the three gel samples, indicating the presence of only one type of fluorophore in the medium.74 Figure 8b showed the fluorescence decay profile at 400 nm for the 2b gel in xylene (4.2 mM). The lifetime of 1.4 ns was close to the reported value for p-terphenylene (1 ns).75 The measurement of the fluorescence decay for the corresponding solution of 2b was not carried out because of the rather weak fluorescence intensity. The excitation spectra of the 2b gel in xylene (Figure S14 in Supporting Information) did not correspond in shape or position to its absorption spectra: an additional new band at long wavelength region (360 nm) and a plateau region appeared in the excitation spectra. These differences provided evidence for the “trapping” or localization of excitation in the H-aggregate.34,73 Unlike the exciton-like emitting character as previously found in TB aggregates, excitonic interactions were not detected in gel phase, as supported by the fact that the fluorescence emission and excitation spectra were excitation or emission wavelength independent.34,76 On the other hand, molecular models of 2a-c reveal a twisted conformation in the isolated state (Figure S15 in Supporting Information), which suppresses the radiation process. However, according to previous reports, a more planar conformation is favored (74) Mille, M.; Lamere, J.-F.; Rodrigues, F.; Fery-Forgues, S. Langmuir 2008, 24, 2671–2679. (75) Rosso, P. G. D.; Almassio, M. F.; Aramendia, P.; Antollini, S. S.; Garay, R. O. Eur. Polym. J. 2007, 43, 2584–2593. (76) Lewis, F. D.; Yang, J.-S.; Stern, C. L. J. Am. Chem. Soc. 1996, 118, 2772–2773.

Langmuir 2009, 25(15), 8548–8555

Chen et al.

Article

Figure 8. (a) PL spectra of 2b in xylene sol, partial gel, and gel state at the same concentration (4.2 mM) exited at 350 nm. Inset shows the fluorescence image of 2b taken under the illumination with 365 nm UV light: dilute solution in xylene (left); gel formed in xylene (4.2 mM, right). (b) Fluorescence decay profile of 2b gel formed in xylene (4.2 mM), monitored at 400 nm, λex = 350 nm.

for PPP oligomers in a crystalline state.40,77 Therefore, we assume p-terphenylene cored dendritic molecules are likely to show similar planarization in the gel state: the aromatic moieties of the neighboring gelator molecules come close to each other due to strong intermolecular hydrogen bonds and π-π interactions, and this favors planarization of the p-terphenylene unit. Thus, the radiation process is activated and fluorescence enhancement occurs. Also, the intermolecular hydrogen bonds would probably reduce the bond rotation within the p-terphenylene unit and thus prohibits the nonradiative transitions to some extent;44,45 therefore, the lifetime is elongated and fluorescence emission is enhanced.

Conclusion We have synthesized a new series of dumbbell-shaped dendritic molecules with a p-terphenylene core. These dendritic molecules exhibited generation-dependent gelation (77) Bakera, K. N.; Fratinia, A. V.; Rescha, T.; Knachela, H. C.; Adamsb, W. W.; Soccic, E. P.; Farmer, B. L. Polymer 1993, 34, 1571–1587.

Langmuir 2009, 25(15), 8548–8555

ability, which was attributed to the steric effects of the dendritic branches and the solubility induced by peripheral alkyl chains. Aggregation-induced fluorescence emission was discovered after the formation of organogels. Acknowledgment. This work is supported by Nature Science Foundation of China Grants 20834006, 20423003, and 50521302 and the Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM200706). Supporting Information Available: Photographs of the self-supporting gels, phase transitions of 2a and 2b, more AFM, SEM, and TEM images of 2a-c gels, rheological data for 2a and 2b gels with different concentration, variable temperature 1H NMR spectra of 2a in [D4] o-dichlorobenzene, SEM image for the control experiment, UV-vis, FT-IR, XRD, and fluorescence excitation spectra, AIEE data of 2a and 2c, molecular models of 2a-c. This material is available free of charge via the Internet at http://pubs. acs.org.

DOI: 10.1021/la803436h

8555