4856
Langmuir 2006, 22, 4856-4862
Meta-Linked Poly(phenylene ethynylene) Conjugated Polyelectrolyte Featuring a Chiral Side Group: Helical Folding and Guest Binding Xiaoyong Zhao and Kirk S. Schanze* Department of Chemistry, UniVersity of Florida, GainesVille, Florida 32611-7200 ReceiVed January 4, 2006. In Final Form: March 14, 2006 A water soluble, meta-linked poly(phenylene ethynylene) featuring chiral and optically active side groups based on L-alanine (mPPE-Ala) has been studied by using absorption, fluorescence, and circular dichroism spectroscopy. Studies of mPPE-Ala in methanol/water solvent mixtures show that the polymer folds into a helical conformation, and the extent of helical folding increases with the volume % water in the solvent. The presence of the helical conformation is signaled by the appearance of a broad, excimer-like visible fluorescence band, combined with a strong bisignate circular dichroism signal in the region of the π,π* absorption of the polymer backbone. The circular dichroism signal exhibits negative chirality, suggesting that the left-handed (M-form) of the helix is in enantiomeric excess. Binding of the metallointercalator [Ru(bpy)2(dppz)]2+ (where bpy ) 2,2-bipyridine and dppz ) dipyrido[3,2-a: 2′,3′-c]phenazine) with the helical polymer is accompanied by the appearance of the orange-red photoluminescence from the metal complex. This effect is directly analogous to that observed when [Ru(bpy)2(dppz)]2+ binds to DNA via intercalation, suggesting that the metal complex binds to mPPE-Ala by intercalating between the π-stacked phenylene ethynylene residues. Cationic cyanine dyes also bind to the periphery of the helical polymer in a manner that is interpreted as “groove binding”. A circular dichroism signal is observed that is believed to arise from exciton coupling within the chiral cyanine dye chromophore aggregate that is formed as the dye molecules are oriented by the helical mPPE-Ala “template”.
Introduction There has been considerable interest in the development of synthetic helical polymers. One motivation for this work is the notion that the study of helix formation in nonbiological polymers will enhance our understanding of the mechanism for helix formation in biopolymers.1,2 In addition to this interest, there is also the hope that helical polymers may find application in areas such as the development of circularly polarized photo- and electroluminescent materials and enantioselective sensors and/ or catalysts.1,3 Recent interest in nonbiological, helical polymers has focused on π-conjugated materials that feature primary structures programmed to allow folding of the backbone into a helical secondary structure. An early example of this class of materials was an optically active, chiral polyacetylene synthesized via polymerization of (S)-4-methyl-1-hexyne.4 Since this early report, various optically active monosubstituted5-9 and disubstitued10-13 poly* Corresponding author. Tel: 352-392-9133. Fax: 352-392-2395. E-mail:
[email protected]. Web site: http://chem.ufl.edu/∼kschanze. (1) Nakano, T.; Okamoto, Y. Chem. ReV. 2001, 101, 4013-4038. (2) Cornelissen, J.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk, N. Chem. ReV. 2001, 101, 4039-4070. (3) Pu, L. Chem. ReV. 1998, 98, 2405-2494. (4) Ciardell, F.; Benedett, E.; Pieroni, O. Makromol. Chem. 1967, 103, 1-18. (5) Moore, J. S.; Gorman, C. B.; Grubbs, R. H. J. Am. Chem. Soc. 1991, 113, 1704-1712. (6) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1995, 117, 11596-11597. (7) Nakako, H.; Mayahara, Y.; Nomura, R.; Tabata, M.; Masuda, T. Macromolecules 2000, 33, 3978-3982. (8) Kwak, G.; Masuda, T. Macromolecules 2000, 33, 6633-6635. (9) Shinohara, K.; Yasuda, S.; Kato, G.; Fujita, M.; Shigekawa, H. J. Am. Chem. Soc. 2001, 123, 3619-3620. (10) Aoki, T.; Shinohara, K.; Kaneko, T.; Oikawa, E. Macromolecules 1996, 29, 4192-4198. (11) Aoki, T.; Kobayashi, Y.; Kaneko, T.; Oikawa, E.; Yamamura, Y.; Fujita, Y.; Teraguchi, M.; Nomura, R.; Masuda, T. Macromolecules 1999, 32, 79-85. (12) Lam, J. W. Y.; Dong, Y. P.; Cheuk, K. K. L.; Tang, B. Z. Macromolecules 2003, 36, 7927-7938. (13) Lam, J. W. Y.; Dong, Y. P.; Cheuk, K. K. L.; Law, C. C. W.; Lai, L. M.; Tang, B. Z. Macromolecules 2004, 37, 6695-6704.
acetylenes have been prepared from chiral monomers. Helical polyacetylenes with achiral side groups14-19 have also been shown to exhibit induced optical activity via interaction with chiral molecules that bind selectively to one form of the helix. In addition to the polyacetylenes, oligo- and poly-thiophenes,20-26 oligo- and polyfluorenes,27-29 poly(p-phenylenes),30,31 polycarbazoles,32 poly(p-phenylene ethynylene)s,33-36 and poly(14) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1997, 119, 6345-6359. (15) Yashima, E.; Maeda, K.; Okamoto, Y. Nature 1999, 399, 449-451. (16) Yashima, E.; Nimura, T.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1996, 118, 9800-9801. (17) Saito, M. A.; Maeda, K.; Onouchi, H.; Yashima, E. Macromolecules 2000, 33, 4616-4618. (18) Nonokawa, R.; Yashima, E. J. Am. Chem. Soc. 2003, 125, 1278-1283. (19) Maeda, K.; Morino, K.; Okamoto, Y.; Sato, T.; Yashima, E. J. Am. Chem. Soc. 2004, 126, 4329-4342. (20) Sakurai, S.; Goto, H.; Yashima, E. Org. Lett. 2001, 3, 2379-2382. (21) Leclere, P.; Surin, M.; Viville, P.; Lazzaroni, R.; Kilbinger, A. F. M.; Henze, O.; Feast, W. J.; Cavallini, M.; Biscarini, F.; Schenning, A.; Meijer, E. W. Chem. Mater. 2004, 16, 4452-4466. (22) Lemaire, M.; Delabouglise, D.; Garreau, R.; Guy, A.; Roncali, J. J. Chem. Soc., Chem. Commun. 1988, 658-661. (23) Yashima, E.; Goto, H.; Okamoto, Y. Macromolecules 1999, 32, 79427945. (24) Nilsson, K. P. R.; Olsson, J. D. M.; Konradsson, P.; Inganas, O. Macromolecules 2004, 37, 6316-6321. (25) Lermo, E. R.; Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W. Chem. Commun. 1999, 791-792. (26) Goto, H.; Yashima, E. J. Am. Chem. Soc. 2002, 124, 7943-7949. (27) Geng, Y. H.; Trajkovska, A.; Katsis, D.; Ou, J. J.; Culligan, S. W.; Chen, S. H. J. Am. Chem. Soc. 2002, 124, 8337-8347. (28) Oda, M.; Nothofer, H. G.; Scherf, U.; Sunjic, V.; Richter, D.; Regenstein, W.; Neher, D. Macromolecules 2002, 35, 6792-6798. (29) Craig, M. R.; Jonkheijm, P.; Meskers, S. C. J.; Schenning, A.; Meijer, E. W. AdV. Mater. 2003, 15, 1435-1438. (30) Chen, H. P.; Katsis, D.; Mastrangelo, J. C.; Marshall, K. L.; Chen, S. H.; Mourey, T. H. Chem. Mater. 2000, 12, 2275-2281. (31) Fiesel, R.; Scherf, U. Acta Polym. 1998, 49, 445-449. (32) Zhang, Z. B.; Motonaga, M.; Fujiki, M.; McKenna, C. E. Macromolecules 2003, 36, 6956-6958. (33) Fiesel, R.; Scherf, U. Macromol. Rapid Commun. 1998, 19, 427-431. (34) Wilson, J. N.; Steffen, W.; McKenzie, T. G.; Lieser, G.; Oda, M.; Neher, D.; Bunz, U. H. F. J. Am. Chem. Soc. 2002, 124, 6830-6831. (35) Zahn, S.; Swager, T. M. Angew. Chem., Int. Ed. 2002, 41, 4225-4230.
10.1021/la060031t CCC: $33.50 © 2006 American Chemical Society Published on Web 04/12/2006
mPPE Conjugated Polyelectrolyte
Langmuir, Vol. 22, No. 10, 2006 4857 Chart 1
(p-phenylenevinylene)s37-39 with optically active side groups have been suggested to form helical structures in poor solvents or in the solid state. In some of the most enlightening work in this area, Moore and co-workers demonstrated that oligo(m-phenylene ethynylene)s with chain lengths >8 repeat units fold into a helical conformation (foldamers) in a process which is thermodynamically driven by solvophobic effects.40,41 Evidence for the helical conformation was afforded by the observation of a bisignate circular dichroism signal produced when the foldamers were mixed with the chiral guest (-)-R-pinene.42 Other approaches were also used to obtain an enantiomeric excess in one of the helical forms, including incorporating chiral binaphthyl groups in the m-phenylene ethynylene backbone or by appending optically active, chiral side chains.43,44 More recently, Tew and co-workers reported that amphiphilic meta-linked poly(phenylene ethynylene)s (mPPEs) undergo a conformational change with spectroscopic features similar to Moore’s results; they pointed out that the spectroscopic changes were consistent with folding of the mPPE chains into a helical conformation.45 We have an ongoing interest in the synthesis and study of water soluble, conjugated polyelectrolytes (CPEs).46-48 These CPEs feature ionic side groups,49 such as sulfonate (R-SO3-), carboxylate (R-CO2-), phosphate (R-PO32-), or ammonium (R-NR3+) tethered to the polymer backbone via short alkoxy chains. On the basis of studies of solvent effects on the absorption and fluorescence properties of para-linked, linear chain CPEs, it was concluded that, in polar organic solvents such as MeOH, DMF, and DMSO, these materials exist as “molecularly dissolved” chains, but in water, they form aggregates due to the hydrophobic effect, combined with weak, attractive interchain π-π interactions in the aggregates.46 In a related investigation, we synthesized and studied the optical spectroscopic properties of a meta-linked CPE that features sulfonate side groups (mPPE(36) Fiesel, R.; Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.; StuderMartinez, S. L.; Scherf, U.; Bunz, U. H. F. Macromol. Rapid Commun. 1999, 20, 107-111. (37) Peeters, E.; Delmotte, A.; Janssen, R. A. J.; Meijer, E. W. AdV. Mater. 1997, 9, 493. (38) Satrijo, A.; Swager, T. M. Macromolecules 2005, 38, 4054-4057. (39) Peeters, E.; Christiaans, M. P. T.; Janssen, R. A. J.; Schoo, H. F. M.; Dekkers, H. P. J. M.; Meijer, E. W. J. Am. Chem. Soc. 1997, 119, 9909-9910. (40) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science 1997, 277, 1793-1796. (41) Prince, R. B.; Saven, J. G.; Wolynes, P. G.; Moore, J. S. J. Am. Chem. Soc. 1999, 121, 3114-3121. (42) Prince, R. B.; Barnes, S. A.; Moore, J. S. J. Am. Chem. Soc. 2000, 122, 2758-2762. (43) Gin, M. S.; Yokozawa, T.; Prince, R. B.; Moore, J. S. J. Am. Chem. Soc. 1999, 121, 2643-2644. (44) Prince, R. B.; Brunsveld, L.; Meijer, E. W.; Moore, J. S. Angew. Chem., Int. Ed. 2000, 39, 228-230. (45) Arnt, L.; Tew, G. N. Macromolecules 2004, 37, 1283-1288. (46) Tan, C. Y.; Pinto, M. R.; Schanze, K. S. Chem. Commun. 2002, 446-447. (47) Pinto, M. R.; Kristal, B. M.; Schanze, K. S. Langmuir 2003, 19, 65236533. (48) Tan, C. Y.; Alas, E.; Muller, J. G.; Pinto, M. R.; Kleiman, V. D.; Schanze, K. S. J. Am. Chem. Soc. 2004, 126, 13685-13694. (49) Pinto, M. R.; Schanze, K. S. Synthesis-Stuttgart 2002, 1293-1309.
SO3-, Chart 1).50 The solvent effect on the absorption and fluorescence of mPPE-SO3- strongly suggests that the metalinked polymer folds into a helical conformation in water. The existence of a helix that is partially stabilized by π-stacking interactions is supported by the observation that the metallointercalator [Ru(bpy)2(dppz)]2+ (where bpy ) 2,2-bipyridine and dppz ) dipyrido[3,2-a:2′,3′-c]phenazine) binds to mPPE-SO3via intercalation, in direct analogy with the mode by which the complex binds to double-stranded deoxyribonucleic acid (DNA). The work described herein is a follow-up on our initial study of mPPE-SO3-. The objective of this work was to prepare and study the absorption, fluorescence, and chiroptical properties of a meta-linked PPE which features a chiral, optically active side group. To meet this objective, we designed the meta-linked conjugated polyelectrolyte mPPE-Ala (Scheme 1) which features a chiral side group derived from L-alanine. It was anticipated that, if mPPE-Ala folds into a helical conformation, the optically active side group would induce an enantiomeric excess in the twist sense of the helix. This would provide proof for the existence of the helical conformation via the observation of a bisignate circular dichroism signal in the wavelength region corresponding to the polymer backbone absorption. The current manuscript describes the results of the study of mPPE-Ala and a related model compound. The results demonstrate that the polymer exists in a helical conformation in waterrich solution, and circular dichroism spectroscopy provides an unambiguous probe of the solvent effect on the folding process. The helical polymer also binds to the metallointercalator, [Ru(bpy)2(dppz)]2+; the bound metal complex features strong photoluminescence, in direct analogy to its behavior when it is bound to DNA. Finally, we also demonstrate that cationic cyanine dyes groove-bind to the helical polymer, forming a supramolecular, chiral dye aggregate that features a circular dichroism signal. Experimental Section Instrumentation. NMR spectra were obtained with a Varian VXR300 or a Varian Gemini-300. Gel permeation chromatography (GPC) analyses were carried out on a system comprised of a Rainin Dynamax SD-200 pump and a Beckman Instruments Spectroflow 757 absorbance detector. UV-visible absorption spectra were obtained either on a Varian Cary 100 or a PerkinElmer Lambda 25 dual beam absorption spectrometer using 1-cm quartz cells. Corrected steadystate fluorescence spectra were recorded on either a SPEX Fluorolog-2 or a SPEX Fluorolog-3 fluorescence spectrometer. Circular dichroism (CD) spectra were obtained on an Aviv model 202 CD spectrometer, with temperature set as 25 °C using 1 cm quartz cell. For optical measurements on polymer solutions, the reported concentrations refer to the concentration of polymer repeat units. Solutions for spectroscopic studies were prepared by dilution of stock solutions. Titrations were carried out by adding microliter aliquots of stock solutions to the sample solution that was contained in a quartz optical (50) Tan, C. Y.; Pinto, M. R.; Kose, M. E.; Ghiviriga, I.; Schanze, K. S. AdV. Mater. 2004, 16, 1208-1212.
4858 Langmuir, Vol. 22, No. 10, 2006
Zhao and Schanze
Scheme 1. (a) NaNO2, H2SO4, KI, 55%; (b) L-alanine benzyl ester‚HCl, DPTS, DIPC, DMF, 67%; (c) phenylacetylene, Pd(PPh3)4, CuI, THF, Et3N, 91%; (d) 0.1 M NaOH, DMF, quant.; (e) Pd(PPh3)4, CuI, THF, Et3N, 71%.
cuvette. Luminescence studies on [Ru(bpy)2(dppz)]2+ were carried out on samples that had been deoxygenated by bubbling with argon for 20 min. Materials. Triethylamine (Et3N), tetrahydrofuran (THF), and CH2Cl2 were purified by distillation over sodium hydride. Anhydrous dimethylformamide (DMF) was used as supplied by Acros Chemical Co. Palladium catalysts were used as received from Strem Chemical Co. 4-(Dimethylamino)pyridinium-4-toluenesulfonate (DPTS), 1,4diethynylbenzene, and [Ru(bpy)2(dppz)][BF4-]2 were prepared according to literature procedures.51-54 All other chemicals were purchased from either Acros or Aldrich and used as received. The polymer stock concentration was 0.47 mg‚mL-1, which corresponds to [PRU] ) 1.4 mM. The stock solution was diluted as needed to prepare solutions just before the spectroscopic experiments. 3,5-Diiodobenzoic Acid (2).55 To an ice-cooled, stirred suspension of 3,5-diaminobenzoic acid (6.0 g, 39.5 mmol) in 90 mL of a mixture of concentrated sulfuric acid and water (2:1 v:v) was added sodium nitrite powder (6.5 g, 94 mmol) with the temperature controlled between -5 and 0 °C. After 1 h, urea (0.50 g, 8.3 mmol) was added, and then a cold solution of potassium iodide (65.6 g, 395 mmol) in 60 mL of water was added dropwise. The black-brown mixture was stirred for another 3 h with the temperature between -5 and 0 °C and heated to 60 °C for 30 min. Then the warm suspension was poured into 300 mL of ice-cold water, and the resulting brown precipitate was collected by vacuum filtration, dried, and then dissolved in diethyl ether. The resulting dark brown diethyl ether solution was washed with aqueous Na2S2O3 until a pale yellow color was observed. After evaporation of the diethyl ether, the resulting yellow solid was purified by recrystallization from toluene, yield 9.6 g (65%). 1H NMR (DMSO-d6): δ ) 8.30 (s, 1H, 4-H), 8.17 (s, 2H, 2, 6-H). 13C NMR (DMSO-d6): 165.3 (-COOH), 148.9 (C-4), 135.0 (C-1), 96.8 (C-3). Benzyl 2-(3,5-diiodobenzamido)propanoate (3). 3,5-Diiodobenzoic acid (1.3 g, 3.5 mmol), l-alanine benzyl ester (0.59 g, 3.3 mmol), DPTS (1.2 g, 4.1 mmol), and 15 mL of DMF were combined in an oven-dried flask with a stirring bar. Then diisopropylcarbodiimide (0.57 g, 4.6 mmol) was added, and the reaction was allowed to stir at 25 °C for 12 h. Then the reaction mixture was diluted with CH2Cl2, washed three times with H2O, and concentrated in vacuo. Flash chromatography using CH2Cl2/hexane (7:3) as eluent affords the pure product as a white solid, yield 1.2 g (64%). M.p. 142-144 (51) Moore, J. S.; Stupp, S. I. Macromolecules 1990, 23, 65-70. (52) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. SynthesisStuttgart 1980, 627-630. (53) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17, 33343341. (54) Amouyal, E.; Homsi, A.; Chambron, J. C.; Sauvage, J. P. J. Chem. Soc., Dalton Trans. 1990, 1841-1845. (55) Endres, A.; Maas, G. Tetrahedron 2002, 58, 3999-4005.
°C. 1H NMR (CDCl3): δ ) 1.52 (d, 3H), 4.80 (m, 1H), 5.23 (q, 2H), 6.79 (d, 1H), 7.38 (m, 5H), 8.05 (s, 2H), 8.20 (s, 1H). 13C NMR (CDCl3): 173.0 (-COO-), 164.0 (-CONH-), 67.7 (-CH2-), 18.7 (-CH3). FTIR (νmax, KBr pellet): 696, 752, 861, 912, 1140, 1166, 1207, 1348, 1384, 1450, 1533, 1627, 1637, 1732, 1760, 2980, 3056, 3258. HRMS (EI): calcd for C17H16I2NO3 [M+H+] 535.9214, found 535.9229. Elemental analysis: Calcd for C17H15I2NO3: C, 38.16; H, 2.83; N, 2.62. Found: C, 37.97; H, 2.51; N, 2.61. Benzyl 2-(3,5-bis(phenylethynyl)benzamido)propanoate (mPEAla-OBn). A solution of 3 (1 g, 1.8 mmol), phenylacetylene (0.95 g, 93 mmol), Pd(PPh3)4 (62 mg, 54 µmol), and CuI (12 mg, 32 µmol) in 10 mL of a mixture of a THF/Et3N was stirred at ambient temperature for 6 h under argon. The solution was then filtered, the solvent was removed under reduced pressure, and the remaining solid was purified by flash chromatography (silica, CH2Cl2/hexane (8:2)). The product was isolated as a white solid, yield 0.79 g (91%). 1H NMR (CDCl ): δ ) 7.91 (d, 2H), 7.83 (t, 1H), 7.56 (m, 4H), 3 7.39 (m, 11H), 6.80 (d, 1H), 5.15 (t, 2H), 4.87 (q, 1H), 1.56 (d, 3H). 13C NMR (CDCl ): 173.2 (-COO-), 165.6 (-CONH-), 91.3 (-Ct 3 C-), 67.6 (-CH2-). 2-((3,5-Bisphenylethynyl)benzamido)propanoic Acid, Sodium Salt (mPE-Ala). A solution of mPE-Ala-OBn (0.2 g) in 5 mL of DMF was added dropwise into 5 mL of an aqueous 0.1 M NaOH solution. The reaction mixture was stirred at ambient temperature for 15 min. Then the solvents were evaporated under reduced pressure, and the residue was washed with CH2Cl2, acetone, and cold water. The product was purified by recrystallization from H2O/MeOH (7:3 v:v). The material was obtained as a white solid, yield 0.15 g (87%). 1H NMR (DMSO-d ): 8.27 (d, 1H), 7.99 (d, 2H), 7.86 (t, 1H), 7.61 6 (m, 4H), 7.45 (m, 6H), 4.05 (q, 1H), 1.30 (d, 3H). 13C NMR (DMSOd6): 175.3 (-COO-), 163.6 (-CONH-), 91.3 (-CtC-), 19.7 (-CH3). FTIR (νmax, KBr pellet): 532, 691, 757, 886, 958, 1027, 1070, 1154, 1174, 1365, 1407, 1442, 1456, 1491, 1532, 1586, 1599, 1624, 2212, 2989, 3080, 3307, 3411. HRMS (EI): calcd for C16H29NNaO3 [M+H+] 416.1257, found 416.1276. mPPE-Ala-OBn. Benzyl 2-(3,5-diiodobenzamido)propanoate (535 mg, 1 mmol) and 1,4-diethynyl-benzene (126 mg, 1 mmol) were dissolved in 10 mL of dry DMF/Et3N (3:1) in a small Schlenk flask. The solution was outgassed with argon for 30 min, and then Pd(PPh3)4 (57.7 mg, 50 µmol) and CuI (9.5 mg, 50 µmol) were added under argon. The reaction mixture was stirred at ambient temperature for 24 h, and then poured into 150 mL of methanol. The polymer precipitated as a fine yellow powder, and it was further purified by repeated dissolution in DMF followed by precipitation from methanol and acetone, yield 288 mg (71%). GPC (THF, polystyrene standards), Mn ) 12,700, Mw ) 23,700, PDI ) 1.86. 1H NMR (DMSO-d ): 9.11 (br, d, 1H), 8.10 (br, m, 2H), 7.95 (br, 6
mPPE Conjugated Polyelectrolyte m, 1H), 7.68 (br, m, 4H), 7.36 (br, m, 6H), 5.16 (br, s, 2H), 4.58 (br, m, 1H), 1.45 (br, d, 3H). mPPE-Ala. A solution of 250 mg of mPPE-Ala-OBn was dissolved in 30 mL of DMF, then 10 mL of aqueous 0.1 M NaOH was added dropwise, and the resulting solution was stirred at room temperature. During the course of the hydrolysis reaction, a few drops of water were added as necessary to keep the polymer in solution. After 30 min, the reaction mixture was poured into 500 mL of methanol/ acetone/ether (1:4:5 v:v:v), and mPPE-Ala precipitated as a light yellow solid. Final purification was accomplished by dialysis of an aqueous solution of the polymer into deionized water (Millipore Simplicity Water System) using a 12-14 kD MWCO cellulose membrane (Fisher Scientific). After dialysis, the polymer solution was filtered through a 0.8 µm nylon membrane filter. The polymer was stored as an aqueous stock solution and diluted as necessary for spectroscopic studies. 1H NMR (DMSO-d6): 8.05-8.10 (br, m, 1H), 8.00 (s, 2H), 7.75 (s, 4H), 4.0-4.08 (br, m, 1H), 1.24 (d, 3H).
Results and Discussion Structures and Synthesis. This investigation focuses on the properties of the two materials shown in Scheme 1. In particular, mPE-Ala is a model compound that contains the base chromophore which is the repeat unit structure in the polymer mPPEAla. Both mPE-Ala and mPPE-Ala contain an amide-linked propanoic acid side chain that is derived from L-alanine. The side chain has a single chiral carbon (the alanine R-carbon) and the materials are optically active, since they are prepared from optically active L-alanine. The model compound and polymer were prepared via the sequence of reactions illustrated in Scheme 1. Thus, starting from commercially available 3,5-diaminobenzoic acid (1), L-alanine benzyl ester substituted 3,5-diiodobenzamide 3 was prepared in two steps. Compound 3 was subsequently used in Sonagashira coupling reactions to prepare the benzyl ester protected model compound mPE-Ala-OBn (Scheme 1) and benzyl ester functionalized polymer mPPE-Ala-OBn. In each case, the benzyl ester protecting groups were removed by mild basic hydrolysis affording the carboxylic acid substituted model mPE-Ala and polymer mPPE-Ala. An advantage of the synthetic approach used to prepare mPPEAla is that the precursor polymer, mPPE-Ala-OBn, is soluble in common organic solvents such as THF, CHCl3, and CH2Cl2 and consequently, its molecular weight can be determined by organicphase gel permeation chromatography (GPC). Molecular weight analysis of mPPE-Ala-OBn indicated a number average molecular weight of 12 kD (polydispersity ) 1.9), suggesting a number average degree of polymerization (Xn) of ca. 30 repeats, which corresponds to 60 phenylene ethynylene units. Subsequent hydrolysis of the benzyl ester groups affords mPPE-Ala, which is insoluble in THF and CHCl3 but is quite soluble in polar solvents such as methanol, water, and DMSO. Although GPC molecular weight analysis of mPPE-Ala is not possible, since hydrolysis is not expected to alter the polymer’s backbone structure, the Xn of the water soluble polymer is believed to be the same as that of the precursor, mPPE-Ala-OBn. Solvent Dependence of Absorption and Fluorescence Spectra. Figure 1 illustrates the UV region absorption spectra of mPPE-Ala in MeOH, in water, and in mixtures of these two solvents at various compositions. First, the absorption of the polymer in MeOH is characterized by a broad maximum at λmax ≈ 326 nm and a second band that appears as a shoulder at λ ≈ 350 nm. These bands have been previously assigned as arising from the cisoid and transoid conformations of the mPPE backbone.41 Interestingly, as the amount of water in the solvent mixture increases, the 350 nm absorption band blue-shifts and decreases in intensity (hypochromic effect), whereas the 350 nm shoulder red-shifts and decreases in intensity. The spectral changes
Langmuir, Vol. 22, No. 10, 2006 4859
Figure 1. UV-visible absorption of mPPE-Ala (c ) 10 µM polymer repeat units, path length 1 cm) in methanol, water, and methanol/ water mixtures. The arrow shows the direction of change with increasing volume % water.
Figure 2. Fluorescence spectra (λexc ) 320 nm) of mPPE-Ala (c ) 10 µM polymer repeat units) in methanol, water, and methanol/ water mixtures. The spectra were obtained under the same conditions, so the spectral intensities reflect the approximate change in fluorescence quantum efficiency with solvent composition.
for mPPE-Ala that are induced by the change of solvent from methanol to water are very similar, but slightly attenuated, relative to those observed for the sulfonate-substituted meta-linked PPE polymer (mPPE-SO3-).50 By analogy to the previously studied system, the change in absorption that accompanies an increase in the volume fraction of water in the solvent is believed to arise due to an increase in the fraction of the polymer which exists in the helical conformation. Specifically, the absorption spectrum of mPPE-Ala exhibits a hypochromic effect with increasing water in the solvent; this effect is likely caused by π-stacking of the aromatic chromophores in the folded helical conformation. The solvent effects on the absorption of mPPE-Ala are attenuated relative to those reported previously for mPPE-SO3-,50 presumably because in mPPE-Ala a fraction of the polymer exists in a helical conformation even in pure MeOH solution (vide infra). Fluorescence spectra (Figure 2) of mPPE-Ala obtained in MeOH, water, and MeOH/water mixtures provide additional information regarding the influence of solvent composition on the polymer’s conformation. First, in pure MeOH, the fluorescence of mPPE-Ala is dominated by a structured band with a 0-0 transition at λmax ) 353 nm with weaker vibronic bands in the 360-400 nm region. In addition to the structured near-UV fluorescence seen in MeOH, the polymer also exhibits a weak and very broad emission band centered at λ ≈ 500 nm. The structured near-UV emission is believed to emanate from the
4860 Langmuir, Vol. 22, No. 10, 2006
random coil conformation of the polymer, whereas the broad, structureless “excimer like” visible emission band likely arises from the folded, helical conformation of mPPE-Ala.50 The fact that the structured emission dominates in MeOH solution suggests that in this solvent mPPE-Ala exists predominantly in the random coil conformation in this medium; however, the appearance of the broad excimer like emission indicates that in MeOH a fraction of mPPE-Ala is folded such that π-π stacking of adjacent chains is possible. Interestingly, as the volume fraction of water in the solvent increases, the intensity of the structured near-UV emission attributed to the random coil conformation decreases significantly. For solutions that contain g60% water, the structured emission is completely quenched, and the only emission observed is the broad, visible region fluorescence. This observation is consistent with the conclusions based on the solvent-induced shift in the absorption spectrum; that is, in MeOH, mPPE-Ala exists mainly in a random coil conformation, whereas in solvent mixtures that contain g60% water, the polymer is mostly folded into the helical conformation. Circular Dichromism Spectroscopy. Circular dichroism is a powerful probe for chiral molecules that contain UV or visible absorbing chromophores. The method is especially sensitive for systems that contain two or more closely spaced chromophores with high oscillator strength that are held in a chiral environment. The CD sensitivity in such systems arises due to exciton coupling between the chromophores leading to the appearance of a characteristic bisignate signal in which the splitting between the two bands is related to the Davydov splitting in the exciton state. The appearance of a CD signal requires (1) the existence of a chiral environment and (2) an enantiomeric excess. Circular dichroism has been widely used to study conjugated oligomers and polymers that fold into helical conformations. The method is particularly useful in these systems because the helical conformation is inherently chiral (either right-handed or left-handed, P and M form, respectively) and also because the π-conjugated chromophores have a high oscillator strength giving rise to strong CD signals. In a series of ingenious studies, Moore42 and co-workers used CD to demonstrate helical folding of metalinked phenylene ethynylene oligomers. To produce an enantiomeric excess in the helical conformation, chiral guest molecules were mixed with achiral conjugated oligomers, or chiral sidegroups were appended to the conjugated oligomer backbone. The alanine-derived side group in mPPE-Ala is chiral and optically active, and we anticipated that its presence would induce an enantiomeric excess in the helical conformation of the mPPE backbone. To probe this effect, CD spectroscopy was used to study the optical activity of mPPE-Ala and the model compound mPE-Ala in solutions of methanol, water, and methanol/water mixtures. Figure 3 shows the CD spectra of mPPE-Ala in a series of solvent mixtures along with the spectrum of mPE-Ala in water. First, it is quite evident that the model compound is CD inactive. This finding is not surprising in view of the fact that the conjugated chromophore in mPE-Ala is inherently achiral. It clearly shows that the perturbation provided by the chiral center in the alanine side group alone is insufficient to induce a measurable chiroptical effect on the conjugated chromophore. By contrast to the model, mPPE-Ala exhibits a strong bisignate CD spectrum in methanol, water, as well as mixtures of the two solvents. Since the model compound mPE-Ala is CD-inactive, the bisignate CD signal observed for mPPE-Ala clearly arises because the conjugated mPPE backbone of the polymer is in a chiral conformation (i.e., a helix) and the chiral sidegroup in the alanine side chain induces an enantiomeric excess in one of the
Zhao and Schanze
Figure 3. Circular dichroism spectra of model compound mPE-Ala in water (c ) 15 µM, solid black line) and mPPE-Ala (c ) 15 µM polymer repeat units) in methanol, water and in methanol/water mixtures.
helical conformers. The longer wavelength Cotton effect is negative (λ ≈ 368 nm), whereas the short wavelength couplet is positive (λ ≈ 325 nm). This relationship is termed “negative chirality” and suggests that the left-handed helical conformation (i.e., an M-helix) of the mPPE backbone is in excess.56 Interestingly, the amplitude of the CD signal for mPPE-Ala varies with solvent composition. In particular, the spectrum is relatively weak in MeOH, and it increases approximately 8-fold with increasing water content, reaching a maximum at 60 volume-% water. Further addition of water results in a slight decrease (ca. 20%) in the amplitude of the CD signal. First, the observation of a CD signal for mPPE-Ala in MeOH indicates that in this solvent the polymer adopts a helical conformation to some extent. This fact is consistent with the fluorescence of the polymer in MeOH, where the excimer-like emission attributed to the helical conformation is observed. The fact that mPPE-Ala exists in part in a helical conformation in pure MeOH contrasts with the behavior of mPPE-SO3-, which on the basis of fluorescence and absorption spectroscopy is believed to exist exclusively in a random coil conformation in MeOH. The helical conformation in mPPE-Ala may be stabilized in MeOH by hydrogen bond formation between proximal amide units in the helix. The increase in CD intensity with increasing water content in the solvent indicates that the hydrophobic effect is important in inducing the polymer to adopt the helical conformation to a greater extent. The loss in CD intensity for volume fractions of water >60% may signal a loss of helical conformation, or more likely it is due to a change in the structure of the helix (e.g., a decrease in the helical pitch) which leads to a change in the strength of the exciton coupling along the meta-linked π system. Guest Binding with the mPPE-Ala Helix. A number of investigations have probed the interaction of cationic dyes and metal complexes with the DNA polyanion. Cationic dyes that feature large planar aromatic systems typically bind to DNA via intercalation, wherein the planar dye “slips” between stacked DNA bases in the double helix. A well-known example of a DNA intercalating dye is the metal complex [Ru(bpy)2(dppz)]2+, which binds to DNA via intercalation of the dppz ligand.57,58 This complex is particularly useful, because it is nonluminescent (56) Berova, N.; Nakanishi, K.; Woody, R., Eds. Circular Dichroism: Principles and Applications, 2nd ed.; Wiley-VCH: New York, 2000. (57) Friedman, A. E.; Chambron, J. C.; Sauvage, J. P.; Turro, N. J.; Barton, J. K. J. Am. Chem. Soc. 1990, 112, 4960-4962. (58) Hartshorn, R. M.; Barton, J. K. J. Am. Chem. Soc. 1992, 114, 5919-5925.
mPPE Conjugated Polyelectrolyte
Langmuir, Vol. 22, No. 10, 2006 4861 Scheme 2
Figure 4. Emission spectra of [Ru(bpy)2(dppz)]2+ in the absence (solid black line) and presence of mPPE-Ala (concentration of mPPEAla ranges from 0 to 120 µM polymer repeat units, excitation wavelength is 450 nm). Solutions are deoxygenated by argon bubbling. The inset illustrates the variation of [Ru(bpy)2(dppz)]2+ emission intensity with increasing concentration of mPPE-Ala.
in water or alcohol solution, but it displays a strong orange-red photoluminescence when it is intercalated within DNA. Cationic cyanine dyes, which are relatively long and slightly curve-shaped molecules, bind within the major or minor groove of the DNA helix. In an elegant series of investigations, Armitage and co-workers59-61 demonstrated that DNA can act as a template for the creation of cyanine dye aggregates. In particular, when multiple cyanine dyes bind within the minor-groove of DNA, they exhibit a strong CD signal which arises due to exciton coupling between the dye chromophores which are oriented into a chiral, supramolecular aggregate by the DNA helix. Given the qualitative similarity between the secondary structure of the helical conformation of anionic mPPEs and double-helical DNA (i.e., both are helical polyanions which feature aromatic units that are π-stacked along the helical axis), we have an interest in exploring whether cationic dyes will interact with mPPEs via intercalative or groove-binding motifs. Along these lines, in a previous report, we demonstrated that the complex [Ru(bpy)2(dppz)]2+ binds to the helical conformation of mPPE-SO3- via intercalation of the dppz ligand into the π stack of the adjacent phenylene ethynylene units.50 Intercalation of [Ru(bpy)2(dppz)]2+ within the polymer helix is signaled by the appearance of the strong orange-red photoluminescence characteristic of the intercalated form of the complex. In the present investigation, we have explored the interaction of mPPE-Ala with [Ru(bpy)2(dppz)]2+ and the cationic cyanine dye HMIDC (Chart 1) in order to determine if these dyes will interact with the polymer via intercalation and/or groove-binding, and if so, if the chiral environment of the polymer helix will give rise to a CD transition for the intercalated dye. First, we examined the effect of addition of mPPE-Ala on the photoluminescence of the cationic metallo-intercalator [Ru(bpy)2(dppz)]2+. As shown in Figure 4, in an argon deoxygenated aqueous solution, [Ru(bpy)2(dppz)]2+ (c ) 15 µM) does not display any photoluminescence. However, upon addition of mPPE-Ala to the deoxygenated solution of [Ru(bpy)2(dppz)]2+ the orange-red photoluminescence (λmax ≈ 640 nm) of the metal complex is observed. The intensity of the photoluminescence (59) Seifert, J. L.; Connor, R. E.; Kushon, S. A.; Wang, M.; Armitage, B. A. J. Am. Chem. Soc. 1999, 121, 2987-2995. (60) Wang, M. M.; Silva, G. L.; Armitage, B. A. J. Am. Chem. Soc. 2000, 122, 9977-9986. (61) Garoff, R. A.; Litzinger, E. A.; Connor, R. E.; Fishman, I.; Armitage, B. A. Langmuir 2002, 18, 6330-6337.
increases very sharply over the polymer concentration range 0-50 µM, and then it increases more slowly until it saturates at [mPPE-Ala] ≈ 150 µΜ (concentrations in polymer repeat units). This behavior is interesting and suggests that at a polymer: Ru concentration ratio of ca. 3 most of the [Ru(bpy)2(dppz)]2+ is bound to the mPPE-Ala helix. This stoichiometry suggests that the “binding site size” for the intercalated metal complex is approximately three mPPE-Ala repeat units; moreover, since one turn of the helix requires ca. 6 polymer repeat units, this corresponds to two metal complexes bound per turn of the helix (e.g., one on either side as shown in Scheme 2). Addition of polymer above the 50 µM level leads to an additional but less pronounced increase in the [Ru(bpy)2(dppz)]2+ photoluminescence. The additional increase in emission intensity is likely due to a small increase in the number of bound complexes but more importantly due to tighter intercalation of the complexes as they are able to bind less densely on the mPPE-Ala helix. An aqueous solution that contained 100 µM mPPE-Ala and 15 µM [Ru(bpy)2(dppz)]2+ was explored by using circular dichroism spectroscopy. This solution exhibited the near-UV bisignate CD signal characteristic of the helical polymer, however, no CD was observed in the 400-500 nm region which corresponds to the metal-to-ligand absorption (MLCT) of the metal complex. This finding indicates that the chiral environment provided by the helix is insufficient to induce a CD signal in the MLCT transition localized on the metal complex chromophore. In a second series of studies, we explored the effect of addition of mPPE-Ala on the optical spectroscopy of two cationic cyanine dyes HMIDC and DISC2(5). In general, similar results were obtained for both dyes, so only results for the former dye are presented herein. Data for DISC5(5) interacting with mPPE-Ala is provided in the Supporting Information. In an aqueous solution, HMIDC features an absorption band due to the π,π* transition of the polymethine chromophore at λmax ≈ 635 nm. As shown in Figure 5a, addition of mPPE-Ala to the aqueous solution of HMIDC induces significant changes in the dye’s absorption. In particular, as mPPE-Ala is added, the 635 nm absorption decreases, and it is replaced by a red-shifted absorption at 660 nm. The red-shift of the HMIDC absorption upon addition of mPPE-Ala clearly signals that the dye binds to the polymer helix. The red-shift likely arises because the polymer-bound dye experiences an effectively lower solvent polarity than when it is in aqueous solution. A plot of the HMIDC absorbance at 660 nm as a function of the polymer:dye molar ratio reveals that dye binding is 80% complete at a 1:1 stoichoimetry and it is saturated at a ratio of ca. two mPPE-Ala repeat units per HMIDC (see the Supporting Information for plot). The nearly stoichiometric binding ratio suggests that the dye-polymer interaction is driven by electrostatics (ion-pairing) and supports the notion that HMIDC binds to the periphery of the mPPE-Ala helix (i.e., “groove binding”). Addition of mPPE-Ala to HMIDC also elicits changes in the dye’s fluorescence. As shown in Figure 6, addition of mPPE-Ala to an aqueous solution of HMIDC leads to a decrease
4862 Langmuir, Vol. 22, No. 10, 2006
Zhao and Schanze Scheme 3
polymer-bound dye. Although the bisignate signal is unsymmetrical, we believe that it arises due to exciton coupling within a chiral dye-chromophore aggregate that is formed as the dye molecules are oriented by the helical mPPE-Ala “template”. Scheme 3 shows an idealized model for this structure. Individual HMIDC molecules bound within the mPPE-Ala “groove” are oriented by the helical turn of the polymer. The individual HMIDC molecules are thus oriented relative to one another such that the dipole-dipole coupling between them will follow a helical path, which affords the condition necessary for the exciton-coupled CD signal to be observed.
Summary and Conclusions Figure 5. (a) UV-visible absorption of HMIDC in water (c ) 5 µM) titrated with mPPE-Ala. Concentration range of mPPE-Ala is 0-5 µM polymer repeat units, in 0.5 µM increments. Arrows show direction of change of spectrum with increasing polymer concentration. (b) Circular dichroism spectra of HMIDC (c ) 7 µM) alone and HMIDC with mPPE-Ala (c ) 7 µM and 10 µM, respectively) in aqueous solution.
Figure 6. Fluorescence spectra of HMIDC (c ) 5 µM) in water titrated with mPPE-Ala, excitation wavelength is 610 nm. Concentration range of mPPE-Ala is 0-5 µM polymer repeat units, in 0.5 µM increments. Arrows show the direction of the change of the spectrum with increasing polymer concentration.
in the fluorescence from the free dye (λem ) 667 nm) with concomitant appearance of a red-shifted fluorescence with λmax ≈ 685 nm. Interestingly, the fluorescence of the free dye is completely quenched at a relatively low polymer:dye ratio. This suggests that most of the HMIDC is bound to the polymer even at low polymer concentration and that energy transfer occurs to the fully groove-bound dyes which emit at longer wavelength. Figure 5b illustrates the circular dichroism spectrum obtained for solutions that contain HMIDC only (c ) 7 µM) or HMIDC and mPPE-Ala (c ) 7 µM and 10 µM, respectively). As expected, HMIDC alone features no CD signal; however, the polymerHMIDC mixture features a distinct unsymmetrical, bisignate CD signal in the region corresponding to the absorption of the
This report describes the synthesis and characterization of a meta-linked PPE-type conjugated polyelectrolyte that features chiral, optically active side chains. This polymer undergoes a transition from a random coil to a helical conformation upon the addition of water (poor solvent) to MeOH (good solvent). The helical conformation of the polymer exhibits significantly different spectroscopic properties compared to the random coil conformation, including absorption hypochromicity, quenching of the nearUV fluorescence characteristic of the isolated conjugated chains and the appearance of a visible region, excimer like fluorescence. The presence of the optically active, chiral side group based on L-alanine induces an enantiomeric excess in the M-form of the helical conformation, as indicated by the negative chirality in the bisignate CD spectrum exhibited by the polymer in solution. The helical polymer interacts in a manner similar to DNA with the metallointercalator [Ru(bpy)2(dppz)]2+; binding of the metal complex with the polymer is assumed to involve the dppz ligand intercalating within adjacent π-stacked phenylene ethynylene residues in the helical conformation. The helical polymer is also able to template the formation of supramolecular helical aggregates of cyanine dyes, which are believed to groove-bind to the periphery of the polymer helix. Work in progress seeks to take advantage of the specific interaction of small molecules with the helical polymer to develop a novel family of fluorescence based biosensors. Work in this area will be reported in a forthcoming manuscript. Acknowledgment. We thank Professor Stephen Hagen of the University of Florida, Physics Department for the use of the CD spectrometer. This work was supported by the United States Department of Energy, Office of Basic Energy Sciences (DEFG-02-96ER14617). Supporting Information Available: 1H and 13C NMR spectra of compound 3, mPE-Ala-OBz, and mPE-Ala, additional absorption and fluorescence data for interaction of HMIDC with mPPE-Ala and absorption, fluorescence and circular dichroism data for interaction of DiSC2(5) with mPPE-Ala. This material is available free of charge via the Internet at http://pubs.acs.org. LA060031T
