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Conjugated Polymers with Main Chain Chirality. 1. Synthesis of an Optically Active Poly(arylenevinylene)† Qiao-Sheng Hu,1a Dilrukshi Vitharana,1a Gang-Yu Liu,1b Vijay Jain,1b Michael W. Wagaman,1c Lei Zhang,1d T. Randall Lee,1d and Lin Pu*,1a Departments of Chemistry, North Dakota State University, Fargo, North Dakota 58105, Wayne State University, Detroit, Michigan 48202-3489, and University of Houston, Houston, Texas 77204-5641, and the Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received September 19, 1995 Revised Manuscript Received November 22, 1995
Conjugated polymers such as polyacetylene, poly(pphenylene), and poly(p-phenylenevinylene) have attracted considerable attention during the past two decades. These materials have demonstrated remarkable electronic and optical properties including electroluminescence, nonlinear optical effects, and high conductivities upon doping.2 Optically active conjugated polymers are potentially useful materials. For example, they can be used to prepare ferroelectric liquid crystals and to prepare chiral electrodes for asymmetric electrosynthesis. However, remarkably few studies in this area have been reported.3 Recently, Grubbs et al. described the synthesis of soluble chiral polyacetylenes.3a In their system (Scheme 1), a cyclooctatetraene substituted with an optically active alkyl group undergoes ring-opening metathesis polymerization by reaction with a tungsten carbene catalyst to give a chiral conjugated polyacetylene. The chiral side groups in the polymer are observed to twist the main chain in predominantly one screw sense to generate a disymmetric chromophore. Other examples of chiral conjugated polymers have been prepared including polypyrroles and polythiophenes containing optically active functional groups.3b-d In all cases, these polymers have their chirality derived from chiral side groups. In our laboratory, we are interested in the synthesis of conjugated polymers with main chain chiral configuration. We anticipate that these novel materials will provide enhanced chiral induction in asymmetric electrosynthesis due to their main chain chirality. We chose 1,1′-bi-2-naphthol, 1, and its derivatives as building blocks in the construction of main chain chiral conjugated polymers because of their outstanding asymmetric differentiation properties.4 The chirality of binaphthyl compounds is derived from the restricted rotation of the two naphthalene rings. The angle between the two naphthalene rings ranges from 60° to 120°.4d The chiral configuration of binaphthyls is remarkably stable. For certain optically active binaphthyl molecules, no significant racemization in solution is observed at temperatures in excess of 200 °C.5 These molecules have been used in a number of asymmetric processes either as chiral ligands for metal catalysts or as chiral reagents.4 Optically active binaphthyls have also been applied to make chiral packing materials for enantioselective chromatographic separations.6 Racemic 1 (rac-1) can be obtained by using a copper(II)catalyzed oxidative coupling of 2-naphthol in air.7 By † Dedicated to Professor Henry Taube at Stanford University on the occasion of his eightieth birthday.
0024-9297/96/2229-1082$12.00/0
Figure 1. CD spectrum of (R)-7.
using (8S,9R)-(-)-N-benzylcinchonidinium chloride, 2, as an inclusion crystallization agent, optically pure (R)-1 can be obtained from rac-1.8 We have improved this method to obtain both (R)-1 and (S)-1 with high optical purities and on a large scale.9 In a typical experiment, 60 g of rac-1 is resolved to give 22.5 g of (R)-1 (>99% ee) and 21 g of (S)-1 (>99% ee). The chiral resolving agent 2 is recovered in 80% yield. We generate chiral conjugated polymers from (R)-2,2′bis(hexyloxy)-1,1′-binaphthyl-6,6′-diboronic acid, (R)-5, which can be readily prepared from (R)-1 (Scheme 2). Bromination of (R)-1 gives 6,6′-dibromo-1,1′-bi-2-naphthol, (R)-3, in 95% yield.6a,10 Reaction of (R)-3 with 1-iodohexane in the presence of base produces the alkylated product (R)-4 {[R]D ) 25.9° (c ) 0.52, THF)} in 95% yield. (R)-4 can be converted to (R)-5 in 50% yield by reaction with magnesium to form a bifunctional Grignard reagent followed by treatment with trimethyl borate and acidic hydrolysis.11,12 The optical rotation of (R)-5 is [R]D ) 35.6° (c ) 0.22, DMSO). The palladium-catalyzed Suzuki coupling of (R)-5 with 1,4-bis(p-bromostyryl)benzene (E:Z ) 1:1.2),13 6, leads to an optically active conjugated polymer (R)-7 (Scheme 3).14 Analysis of the polymer by GPC (THF, polystyrene standard) shows Mn ) 20 000 and Mw ) 67 000 (PDI ) 3.4). This polymer is soluble in common organic solvents (e.g., THF, benzene, toluene, pyridine, chlorobenzene, methylene chloride, chloroform, and 1,2dichloroethane). Self-standing films can be prepared by evaporation of the solvent from a polymer solution. The polymer contains a mixture of cis and trans double bonds. In the 1H NMR spectrum of (R)-7, a broad peak at δ 6.61 is assigned to the cis protons of the vinylenes and the trans proton signals overlap with aromatic signals (>δ 7). The NMR integration indicates a trans: cis ratio of 1:0.45 for the polymer. The optical rotation of the polymer is [R]D ) -351° (c ) 0.38, THF). Figure 1 is the circular dichroism (CD) spectrum of (R)-7 in THF (2.73 × 10-5 M).15 The observed CD effects are consistent with a chiral conjugated π system for (R)7.3a Following a similar procedure, rac-7 has been prepared from the Suzuki coupling of the racemic monomer, rac-5, and 6. rac-7 is most likely composed of a mixture of diastereomeric polymers containing both R and S binaphthyl units. Analysis of rac-7 by GPC shows Mn ) 17 000 and Mw ) 48 000 (PDI ) 2.8). The trans:cis ratio of rac-7 is the same as that of (R)-7 based on 1H NMR analysis. Both (R)-7 and rac-7 are green solids. The UV spectra of (R)-7 and rac-7 show an absorption maximum at λmax ) 390-392 nm. The fluorescence spectra of the polymers show a maximum emission at λ ) 468 nm when excited at 390 nm. These polymers emit strong blue light under a UV lamp. The fluores© 1996 American Chemical Society
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Scheme 1. Ring-Opening Metathesis Polymerization of an Optically Active Cyclooctatetraene
Scheme 2. Synthesis of the Chiral Monomer (R)-5
cence quantum yield of rac-7 in toluene solution is about 50% by comparison to that of 9-anthracenecarboxylic acid.16 Interestingly, the fluorescence quantum yield of (R)-7 in toluene is greatly enhanced to 80%. Thus, the optically active polymer has a much greater fluorescence quantum efficiency than the polymer derived from the racemic monomer. Thin films of the polymers that have been spin-coated onto glass slides can be oxidatively doped by exposure to an acetonitrile solution of NOBF4. The doped conductivities for both (R)-7 and rac-7 range from 4 × 10-5 to 7 × 10-5 S cm-1. Thermogravimetric analysis (TGA) shows that (R)-7 is stable up to 340 °C under N2. Differential scanning
calorimetry (DSC) indicates a glass transition temperature (Tg) for (R)-7 at 210 °C. Analysis of rac-7 by TGA suggests that (R)-7 is less stable than rac-7, which begins to lose mass at 390 °C. The Tg of rac-7 is observed at 203 °C. To characterize the packing and microscopic structure of these polymers, we have conducted a systematic atomic force microscopy (AFM) study.17 Figure 2 shows the AFM images of thin films of (R)-7 and rac-7 spin-coated on mica. Mica is chosen because of its atomically flat (0001) surface.18 Therefore, any corrugations observed in the AFM images can be attributed to the polymers instead of the substrate.17b Each mica surface is freshly cleaved before spin-coating to avoid contamination. As can be seen from the topographic images of (R)-7 and rac-7, the two films exhibit different surface morphologies. Although both are somewhat rough and porous, (R)-7 films have larger domains (thousands of angstroms) separated by pits or holes ranging from 50 to >200 Å in depth. rac7, on the other hand, is composed of smaller domains, only hundreds of angstroms, and the corrugation is only tens of angstroms. Scans of areas smaller than those shown in Figure 2 are taken systematically by zooming into the polymer domains. No long-range order (or periodicity) is found, however, on either of the films.
Figure 2. AFM images (2 × 2 µ scans) of rac-7 (top) and (R)-7 (bottom). Images were taken under ambient conditions using a home-built AFM18 and a Digital cantilever with a force constant of 0.58 N/m. The force (or load) used for the two images is 6 nN. Both images were raw data with only background correction.
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Scheme 3. Synthesis of the Optically Active Poly(arylenevinylene), (R)-7
In summary, we have synthesized the first optically active conjugated polymer with main chain chiral configuration.19 This polymer and its optically inactive analog are soluble in common organic solvents and can be cast into self-standing films. The high thermal stability, high glass transition temperature, and good processability of these polymers make them attractive for practical applications. These polymers are blue light emitting materials with very good fluorescence quantum efficiencies. We are currently exploring their use in asymmetric electrosynthesis and in chiral column chromatography. A detailed investigation of the nonlinear optical properties and the electroluminescent properties of these new materials is in progress. Studies of other chiral conjugated polymers constructed by the optically active binaphthyl units will be reported shortly. Acknowledgment. This work was supported by the Department of Chemistry, North Dakota State University. We thank Professor Steven Meinhardt at North Dakota State University, Professor Jin Zhang, Ms. Elizabeth Goka, and Ms. Janet L. McGowen at the University of California Santa Cruz, and Dr. Mark C. Lonergan at the California Institute of Technology for their assistance on the optical experiments and conductivity measurements. We also thank Professor Robert H. Grubbs at Caltech for his generous help. Supporting Information Available: Detailed experimental procedures and characterizations involving (R)-4, (R)5, 6, (R)-7, and rac-7 (3 pages). Ordering information is given on any current masthead page.
References and Notes (1) (a) North Dakota State University. (b) Wayne State University. (c) California Institute of Technology. (d) University of Houston. (2) (a) Skotheim, T. A., Ed. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986; Vols. 1 and 2. (b) Bredas, J. L.; Silbey, R. Conjugated Polymers: the Novel Science and Technology of Highly Conducting and Nonlinear Optically Active Materials; Kluwer Academic Publishers: Boston, 1991. (c) Kiess, H. G., Ed. Conjugated Conducting Polymers; Springer-Verlag: New York, 1992. (3) (a) Moore, J. S.; Gorman, C. B.; Grubbs, R. H. J. Am. Chem. Soc. 1991, 113, 1704. (b) Kotkar, D.; Joshi, V.; Ghosh, P. J. Chem. Soc., Chem. Commun. 1988, 917. (c) Lemaire, M.; Delabouglise, D.; Garreau, R.; Guy, A.; Roncali, J. J. Chem. Soc., Chem. Commun. 1988, 658. (d) Salmon, M.; Biden, G. J. Electrochem. Soc., Electrochem. Sci. Technol. 1985, 1897.
(4) (a) Rosini, C.; Franzini, L.; Raffaelli, A.; Salvadori, P. Synthesis 1992, 503. (b) Whitesell, J. K. Chem. Rev. 1989, 89, 1581. (c) Bringmann, G.; Walter, R.; Weirich, R. Angew. Chem., Int. Ed. Engl. 1990, 29, 977. (d) Mason, S. F. Molecular Optical Activity and the Chiral Discriminations; Cambridge University Press: New York, 1982; p 72. (5) Kyba, E. P.; Gokel, G. W.; de Jong, F.; Koga, K.; Sousa, L. R.; Siegel, M. G.; Kaplan, L.; Sogah, G. D. Y.; Cram, D. J. J. Org. Chem. 1977, 42, 4173. (6) (a) Sogah, G. D. Y.; Cram, D. J. J. Am. Chem. Soc. 1979, 101, 3035. (b) Yamashita, J.; Numakura, T.; Kita, H.; Suzuki, T.; Oi, S.; Miyano, S.; Hashimoto, H.; Takai, N. J. Chromatogr. 1987, 403, 275. (7) Noji, M.; Nakajima, M.; Koga, K. Tetrahedron Lett. 1994, 7983. (8) (a) Toda, F.; Tanaka, K.; Stein, Z.; Golderberg, I. J. Org. Chem. 1994, 59, 5748. (b) Tanaka, K.; Okada, T.; Toda, F. Angew. Chem., Int. Ed. Engl. 1993, 32, 1147. (9) Hu, Q.-S.; Vitharana, D.; Pu, L. Tetrahedron: Asymmetry 1995, 6, 2123. (10) Pradellok, W.; Kotas, A.; Walczyk, W.; Jedlinski, Z. Chem. Abstr. 1979, 90, 121289t. (11) Coutts, I. G. C.; Goldschmid, H. R.; Musgrave, O. C. J. Chem. Soc. 1970, 488. (12) The optical purity of (R)-5 and (R)-4 is supported by the following experiment. The bifunctional Grignard reagent prepared from the reaction of (R)-4 with Mg was hydrolyzed in the absence of air to give 2,2′-dihexoxy-1,1′-binaphthalene. The hexyl groups of the obtained 2,2′-dihexoxy-1,1′binaphthalene were then removed with BBr3 to regenerate (R)-1. (R)-1 was purified by column chromatography without recrystallization in order to avoid possible resolution during the crystallization process. The optical rotation of the recovered (R)-1 indicated >99% ee, which suggests that no racemization occurs in the course of this reaction sequence. (13) McDonald, R. N.; Campbell, T. W. J. Am. Chem. Soc. 1960, 82, 4669. (14) (a) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981, 11, 513. (b) Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1991, 113, 7411. (c) Suzuki, A. Acc. Chem. Res. 1982, 15, 178. (d) Huber, J.; Scherf, U. Macromol. Rapid Commun. 1994, 15, 897. (15) The molarity of (R)-7 is calculated on the basis of the repeat unit of the polymer. (16) Kamat, P. V.; Ford, W. E. Chem. Phys. Lett. 1987, 135, 421. (17) For examples of AFM study, see: (a) Kolbe, W. F.; Ogletree, D. F.; Salmeron, M. B. Ultramicroscopy 1992, 42-44, 1113. (b) Magonov, S. N.; Cantow, H.-J. J. Appl. Polym. Sci., Appl. Polym. Symp. 1992, 51, 3. (c) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607. (18) Liu, G.-Y.; Fenter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301. (19) A co-oligomer of a multifunctional binaphthyl and pbiphenylene has been reported: Bedworth, P. V.; Tour, J. M. Macromolecules 1994, 27, 622.
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