Synthesis and Hierarchical Superstructures of Side-Chain
Liquid Crystal Polyacetylenes Containing
Galactopyranoside End-Groups
MING-SHOU HO, CHAIN-SHU HSUDepartment of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010, Republic of China
Received 23 July 2009; accepted 24 August 2009 DOI: 10.1002/pola.23702
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Three kinds of chiral saccharide-containing liquid crystalline (LC) acetylenic monomers were prepared by click reaction between 2-azidoethyl-2,3,4,6-tetraacetyl-b-D-galactopyranoside and 1-biphenylacetylene 4-alkynyloxybenzoate. The
obtained monomers were polymerized by WCl6-Ph4Sn to form three side-chain LC
polyacetylenes containing 1-[2-(2,3,4,6-tetraacetyl-b-D
-galactopyranos-1-yl)-ethyl]-1H-[1,2,3]-triazol-40-biphenyl 4-alkynyloxybenzoate side groups. All monomers and poly-mers show a chiral smectic A phase. Self-assembled hiearchical superstructures of the chiral saccharide-containing LCs and LCPs in solution state were studied by field-emission scanning electron microscopy. Because of the LC behavior, the LC mol-ecules exhibit a high segregation strength for phase separation in dilute solution (THF/H2O ¼ 1:9 v/v). The self-assembled morphology of LC monomers was
depend-ent upon the alkynyloxy chain length. Increasing the alkynyloxy chain length caused the self-assembled morphology to change from a platelet-like texture (LC-6) to helical twists morphology (LC-11 and LC-12). Furthermore, the helical twist morphological structure can be aligned on the polyimide rubbed glass substrate to form two-dimen-sional ordered helical patterns. In contrast to LC monomers, the LCP-11 self-assembled into much more complicate morphologies, including nanospheres and heli-cal nanofibers. These nanofibers are evolved from the heliheli-cal cables ornamented with entwining nanofibers upon natural evaporation of the solution in a mixture with a THF/methanol ratio of 3:7.VVC2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6596–6611, 2009
Keywords: liquid-crystalline polymers (LCP); polyacetylenes; self-assembly
INTRODUCTION
Liquid crystalline (LC) materials have large ani-sotropy of optical, electrical, magnetic, and physi-cal properties because of their anisotropy molecu-lar structure and molecumolecu-lar alignment. They can
show a variety of characteristic behavior on appli-cation of electric and magnetic fields.1 On the other hand, conjugated polymers with highly extended p-conjugation in their main chain have many potential applications such as sensors,2 light-emitting diodes,3 and photovoltaic devices.4 Recently, a variety of LC conjugated polymers having a mesogenic part in their side-chains were synthesized and characterized. Such kinds of new conjugated polymers, which can be aligned under LC phase, are useful for the application in organic Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 6596–6611 (2009)
V
VC2009 Wiley Periodicals, Inc.
Correspondence to: C.-S. Hsu (E-mail: [email protected]. edu.tw)
electronics.5 In particular, much attention has been paid to LC polyacetylenes, which have been widely studied by several research groups.6–14
Helical polymers such as naturally occurring proteins and genes have precisely ordered hier-archical architectures and have broad potential applications including molecular recognition (chi-ral separation, sensory functions) and LC forma-tion through well-ordered molecular align-ment.15–17The helix can be found among the most sophisticated and fundamental structures of the polymer chain because its characteristic features can be expected for synthetic helical polymers. Therefore, based on the optically active helical structure, synthetic helical polymers have been investigated during the past decade because of their unique properties.18
Among the synthetic helical polymers, polyace-tylenes have been extensively studied. Moore et al.19reported that polyacetylenes with a chiral substituent have predominantly a single sense of helix due to the nonplanar conformation of the polyene structure. For the polyacetylene-based helical polymers, previous studies have mostly focused on poly(phenylacetylenes) with chiral sub-stituents. This is because high stereoregularity is indispensable for the construction of well-ordered helical polymers.20 Tang and coworkers21 synthe-sized several series of amphiphilic poly(phenyl-acetylenes) carrying amino acid pendants which were self-assembled into micellar spheres, helical cables, and nanofibers in solution. Yashima et al. reported several unique chirality-responsive heli-cal polymers, such as poly(phenylacetylenes) bearing functional pendant groups as an excess of a single-handed helix through noncovalent bond-ing interaction, which provides an efficient chiral-ity-sensing system.
In this study, we will focus on the synthesis and self-assembled superstructures of poly(alkyl-acetylenes). Three LC polymers were prepared by polymerization of acetylenic monomers using transition metals, such as WCl6-Ph4Sn as an ini-tiator. The polymers comprise of conjugated poly-acetylene backbones, 4-biphenylbenzoate mesogen, and galactopyranoside end group. Self-assembled superstructures of the obtained saccharide-con-taining LCs and LCPs in solution were studied. To our knowledge, this is the first example of a LC polyacetylene containing a saccharide chiral entity used to explore its self-assembly behavior.
In addition, we aim to study orientation control of helical morphology formed by the chiral LC molecules. The fabrication of two-dimensional
(2D) ordered alignment of chiral molecules on substrates is the great challenges in materials sci-ence.22The supramolecular self-assembly concept has been intensively applied to create 2D chiral surfaces through epitaxial adsorptions and align-ments of chiral molecules on metals and highly oriented pyrolytic graphite (HOPG).23 Here, we utilized rubbed polyimide (PI) alignment layers to control the orientation of the self-assembled heli-ces. This method has been widely used in the pro-duction of liquid-crystal displays in which LC molecules can be aligned homogeneously by the rubbed PI layers. In the similar manner, we expect the self-assembled morphology to be aligned epitaxially on the PI alignment layers to form 2D ordered superstructures.24
EXPERIMENTAL Materials
1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI), 4-Dimethylaminopyridine (DMAP) (from Aldrich), tungstun(VI) chloride,
tetraphenyltin (from Strem) were used as
received. The solvents were dried according to standard procedures. Tetrahydrofuran (THF) was distilled under a nitrogen atmosphere over sodium benzophenone ketyl just before use. Dichloromethane (DCM) and toluene were dried over calcium hydride and then distilled under nitrogen. Triethylamine (Et3N) was distilled and dried over potassium hydroxide.
Characterization Techniques 1
H and 13C NMR spectra (300 MHz) were
recorded on a Varian VXR-300 spectrometer. Mass spectra were obtained using a JEOL JMS-HX 110 mass spectrometer. Thermal transitions and thermodynamic parameters were determined by using a Seiko EXSTAR 6000 differential scan-ning calorimeter (DSC) equipped with a liquid nitrogen cooling accessory. DSC unit operated at heating and cooling rates of 10C/min. A 5–10 mg sample of purified material was placed on an alu-minum pan and measured against another empty pan as a reference. Thermogravimetric analysis (TGA) was undertaken using a Perkin-Elmer Pyris 1 instrument. The thermal decomposition temperature (Td) was read at a temperature at which 5% weight loss occurred while heating at a rate of 10C/min under a nitrogen atmosphere. A Carl-Zeiss Axiphot optical polarized microscope
equipped with a Mettler FP82 hot stage and a FP80 central processor was used to observe the thermal transitions and to analyze the anisotropic textures. Gel permeation chromatography (GPC) assembled from a Viscotek T50A differential vis-cometer and a LR125 laser refractometer were used to measure the molecular weights of poly-mers relative to polystyrene standards. The oven temperature was set at 35C, and THF was used as eluent with flow rate of 1 mL/min.
UV–vis absorption spectra were recorded on a Hitachi U-3300 spectrophotometer. Circular dichroism (CD) measurements were conducted on a JASCO J-720 spectrophotometer. UV–vis and CD spectroscopy measurements were performed to investigate the specific absorption bands and chiral expressions of the samples in solution. The CD curve has a convex shape in the vicinity of the maximum of a UV–vis absorption band. This kind of characteristic-shape phenomenon is known as the Cotton effect. This effect, which provides structural information, is characterized by the position, magnitude, sign, and shape of the curve. The spectroscopic experiments were carried out in THF solution (1 104M) in quartz tubes.
Self-assembly process of LC monomers and LCP polymers is undergone in solution. The chiral saccharide-containing acetylenic LC monomers were aggregated in dilute solution by using 1 mg of compound dissolved in 1 mL of THF, and then introducing 9 mL of deionized water into the well-dissolved solution. For the LC polymers, the sol-vent mixture was performed in the THF/methanol solution with a ratio of 3:7 (v/v). After sufficient time for aggregation, a drop of the mixture was transferred to a glass substrate. The cast samples were then examined by field-emission scanning electron microscopy (FESEM). FESEM was per-formed on a JEOL JSM-7100F using accelerating voltages of 0.3 keV. The samples were mounted to brass shims using carbon adhesive and then sput-ter-coated with 2–3 nm of Pt (the Pt coating thick-ness was estimated from the calculated deposition rate and experimental deposition time).
Synthesis of Monomers
The synthesis of chiral galactopyranoside-contain-ing acetylenic monomers is outlined in Scheme 1. 2-Azidoethanol (1) and 2-azidoethyl-2,3,4,6-tet-raacetyl-b-D-galactopyranoside (2) were prepared
according to previous literature25,26 procedures and fully characterized.
40-(2-(Trimethylsilyl)ethynyl)-biphenyl-4-ol (3) 40-Bromo-4-hydroxybiphenyl (10.0 g, 40.1 mmol), CuI (0.61 g, 3.2 mmol), bis(tripheny1phosphine)-palladium dichloride [PdCl2(PPh3)2] (0.56 g, 0.8 mmol), and PPh3 (0.84 g, 3.2 mmol) were dis-solved in Et3N (150 mL), and the mixture was stirred under nitrogen. Once all catalysts had been dissolved, (trimethylsilyl) acetylene (6.8 mL, 48.2 mmol) was added. The resulting solution was reacted at 70C for 15 h. After Et3N was removed
under reduced pressure, the product was
extracted with diethyl ether. The crude product was isolated by evaporating the solvent and puri-fied by column chromatography (silica gel, ethyl acetate/n-hexane 1/4 as eluent) to afford white crystals in 95% yield (10.17 g). Purity: 99þ% (HPLC). mp 160 C. Rf ¼ 0.43 (silica gel, ethyl acetate/n-hexane 1/4).
1
H NMR (CDCl3): d ¼ 7.47 (m, 6H, aromatic pro-tons), 6.89 (d, 2H, J¼ 3.30 Hz, aromatic protons), 4.94 (s, 1H, AOH), 0.24 (s, 9H, ASi(CH3)3). 13C NMR (CDCl3): d ¼ 155.41, 141.34, 131.21, 129.92, 128.34, 126.48, 120.29, 115.25, 105.53, 93.01, 0.50. MS (EI, observed m/z): 266. HRMS (m/z) Calcd for C17H18OSi: 266.4097, found: 266.4501.
40-(1-Ethynyl)-biphenyl-4-ol (4)
Tetrabutylammonium fluoride (TBAF) (21 mL, 1 M/THF, 21.0 mmol) was added to a stirred solution of 3 (5.02 g, 18.8 mmol) in THF (5 mL) at ambient temperature. The reaction mixture was stirred for 1 h. Once solvent had been evaporated, the brown residue was partitioned (CH2Cl2/NaHCO3(aq)). The organic layer was washed (brine), dried (MgSO4), and evaporated. The residue was purified by col-umn chromatography (ethyl acetate/n-hexane 1/8) to give 3.17 g of white crystals (87% yield). Purity: 99þ% (HPLC). mp 168 C. Rf ¼ 0.33 (silica gel, ethyl acetate/n-hexane 1/4).
1
H NMR (CDCl3): d ¼ 7.47 (m, 6H, aromatic protons), 6.89 (d, 2H, J¼ 8.40 Hz, aromatic pro-tons), 4.91 (s, 1H, AOH), 3.09 (s, 1H, ACBCH). 13
C NMR (CDCl3): d ¼ 155.39, 141.07, 133.00, 132.52, 128.34, 126.48, 120.29, 115.75, 83.63, 77.55. MS (EI, observed m/z): 194. HRMS (m/z) Calcd for C14H10O3: 194.2286, found: 194.2296.
1-[2-(2,3,4,6-Tetraacetyl- b-D-galactopyranos-1-yl)-ethyl]-1H-[1,2,3]-triazol-40-bi-phenyl-4-ol (5) Compound 2 (3.24 g, 7.8 mmol), 4 (1.51 g, 7.8 mmol, 1 eq), and CuI (0.74 g, 3.9 mmol) were
dissolved in dry THF. To this mixture diisopropy-lethylamine (DIPEA) (1.5 mL, 7.76 mmol) was added and the vial was capped. After stirring for 24 h at room temperature, the mixture was fil-tered over Celite and the solvent was evaporated under reduced pressure. Further purification was performed using column chromatography (silica gel, ethyl acetate/n-hexane 2/1 as eluent) to give
3.1 g of white crystals (65% yield). Purity: 99þ% (HPLC). mp 105C. Rf¼ 0.30 (silica gel, ethyl ac-etate/n-hexane 2/1). [a]29 D ¼ 2.16 (c 1, CHCl3). 1 H NMR (CDCl3): d ¼ 7.90 (d, 2H, J ¼ 8.10 Hz), 7.89 (s, 1H), 7.59 (d, 2H, J ¼ 8.10 Hz), 7.49 (d, 2H, J ¼ 8.40 Hz), 6.96 (d, 2H, J ¼ 8.40 Hz), 5.39 (d, 1H, J¼ 3.30 Hz), 5.23 (m, 1H), 4.98 (dd, 1H, J¼ 10.50 Hz, J ¼ 3.60 Hz), 4.72 (m, 1H), 4.57
Scheme 1. Synthesis of saccharide-containing acetylenic LC monomers (LC-6, LC-11, and LC-12).
(m, 1H), 4.45 (d, 1H, J¼ 7.80), 4.31 (m, 1H), 4.14 (m, 2H), 3.92 (m, 2H), 2.15 (s, 3H), 2.05 (s, 3H), 1.97 (s, 3H), 1.75 (s, 3H). 13C NMR (CDCl3): d ¼ 170.48, 170.23, 170.08, 169.91, 156.01, 147.36, 140.58, 132.54, 128.43, 128.07, 126.90, 126.04, 121.58, 115.85, 100.79, 70.79, 70.52, 68.49, 67.66, 66.83, 61.17, 50.11, 20.64, 20,59, 20.55, 20.50. MS (EI, observed m/z): 611. HRMS (m/z) Calcd for C30H33N3O11: 611.5965, found: 611.5971.
1-[2-(2,3,4,6-Tetraacetyl- b-D-galactopyranos-1-yl)-ethyl]-1H-[1,2,3]-triazol-40-bi-phenyl
4-alkynyloxybenzoate (LC-6, LC-11, and LC-12) All three monomers were prepared by esterifica-tion of compound 5 with corresponding 4-(alkyny-loxy)benzoic acid. An example for the synthesis of LC-11 is described as follows.
A solution of EDCI (0.3 g, 1.57 mmol) in dichloromethane (30 mL) was added dropwise to a solution of 5 (0.8 g, 1.31 mmol), 4-(undec-10-yny-loxy)benzoic acid (0.41 g, 1.44 mmol) and DMAP (0.02 g, 0.16 mmol) in dichloromethane (10 mL) at 0 C. After complete addition, the reaction mix-ture was allowed to stir overnight at room tem-perature. The reaction progress was monitored by TLC. The resulting mixture was washed twice with an 1 M sodium hydroxide solution and two times with distilled water. The organic layer was dried with anhydrous magnesium sulfate and then evaporated under reduced pressure to give the crude product. Further purification was per-formed using column chromatography (silica gel, ethyl acetate/n-hexane 1/1 as eluent) to give 0.9 g of white crystals (90% yield). Purity: 99þ% (HPLC). Rf¼ 0.10 (silica gel, ethyl acetate/n-hex-ane 1/1). [a]29
D ¼ 3.03 (c 1, CHCl3). 1
H NMR (CDCl3): d ¼ 8.14 (d, 2H, J ¼ 8.70 Hz, aromatic protons), 7.96 (d, 2H, J¼ 7.80 Hz, aro-matic protons), 7.90 (s, 1H, AC2N3HA), 7.65 (d, 4H, J¼ 8.70 Hz, aromatic protons), 7.27 (d, 2H, J ¼ 8.70 Hz, aromatic protons), 6.96 (d, 2H, J ¼ 9.00 Hz, aromatic protons), 5.38 (d, 1H, J¼ 3.30 Hz), 5.22 (m, 1H), 4.96 (dd, 1H, J¼ 10.50 Hz, J ¼ 3.30 Hz), 4.70 (m, 1H), 4.58 (m, 1H), 4.43 (d, 1H, J ¼ 7.80), 4.30 (m, 1H), 4.17 (m, 2H), 4.05 (m, 2H), 3.90 (m, 2H), 2.20–2.15 (m, 5H, ACOOCH3 and ACH2CBCH), 2.04 (s, 3H, ACOOCH3), 1.99–1.91 (m, 4H, ACOOCH3 and ACBCH), 1.84–1.73 (m, 5H, ACOOCH3 and AOCH2CH2), 1.54–1.32 (m, 12H,A(CH2)6CH2CBCH). 13C NMR (CDCl3): d ¼ 170.35, 170.12, 169.98, 169.72, 164.97, 163.56, 150.64, 147.16, 140.09, 138.12, 132.29, 129.43, 127.95, 127.44, 126.12, 122.16, 121.63, 121.44, 114.29, 100.86, 84.72 (1C,ACBCH), 70.85, 70.52, 68.47, 68.28, 68.04, 67.57, 66.83, 61.17, 50.12, 29.36, 29.26, 29.06, 28.99, 28.68, 28.43, 25.93, 20.65, 20,62, 20.55, 20.51, 18.37. MS (FAB; observed m/z): 882. HRMS (m/z) Calcd for C48H55N3O13: 881.9626, found: 881.9659.
LC-6. White crystals. Yield 85%. Purity: 99þ% (HPLC). Rf¼ 0.50 (silica gel, ethyl acetate/n-hex-ane 2/1). [a]29
D ¼ 3.13 (c 1, CHCl3). 1
H NMR (CDCl3): d ¼ 8.17 (d, 2H, J ¼ 9.00 Hz, aromatic protons), 7.98 (d, 2H, J¼ 8.10 Hz, aro-matic protons), 7.92 (s, 1H, AC2N3HA), 7.68 (d, 4H, J¼ 8.10 Hz, aromatic protons), 7.30 (d, 2H, J ¼ 8.40 Hz, aromatic protons), 6.99 (d, 2H, J ¼ 9.00 Hz, aromatic protons), 5.40 (d, 1H, J¼ 3.00 Hz), 5.24 (m, 1H), 4.98 (dd, 1H, J¼ 10.50 Hz, J ¼ 3.30 Hz), 4.70 (m, 1H), 4.58 (m, 1H), 4.46 (d, 1H, J ¼ 8.10), 4.31 (m, 1H), 4.16 (m, 2H), 4.09 (m, 2H), 3.92 (m, 2H), 2.31 (td, 2H, J ¼ 6.90 Hz, J ¼ 2.40 Hz, ACH2CBCH), 2.16 (s, 3H, ACOOCH3), 2.06 (s, 3H, ACOOCH3), 2.00 (t, 1H, J ¼ 2.70 Hz,
ACBCH), 1.97–1.93 (m, 5H, ACOOCH3 and
AOCH2CH2), 1.80–1.75 (m, 5H, ACOOCH3 and ACH2CH2CBCH). 13C NMR (CDCl3): d ¼ 170.34, 170.11, 169.97, 169.69, 164.91, 163.34, 150.55, 147.14, 139.96, 138.10, 132.27, 129.52, 127.91, 127.39, 126.05, 122.21, 121.58, 121.53, 114.23, 100.81, 83.86 (1C, ACBCH), 70.79, 70.49, 68.78, 68.42, 67.69, 67.55, 66.80, 61.15, 50.02, 28.03, 24.87, 20.64, 20,60, 20.55, 20.49, 18.08. MS (FAB; observed m/z): 812. HRMS (m/z) Calcd for C43H45N3O13: 811.8297, found: 811.8217.
LC-12. White crystals. Yield 90%. Purity: 99þ% (HPLC). Rf¼ 0.45 (silica gel, ethyl acetate/n-hex-ane 3/1). [a]29
D ¼ 3.07 (c 1, CHCl3). 1
H NMR (CDCl3): d ¼ 8.17 (d, 2H, J ¼ 8.70 Hz, aromatic protons), 7.98 (d, 2H, J¼ 8.10 Hz, aro-matic protons), 7.92 (s, 1H, AC2N3HA), 7.68 (d, 4H, J¼ 8.40 Hz, aromatic protons), 7.30 (d, 2H, J ¼ 8.40 Hz, aromatic protons), 6.99 (d, 2H, J ¼ 9.00 Hz, aromatic protons), 5.40 (d, 1H, J¼ 3.30 Hz), 5.24 (m, 1H), 4.98 (dd, 1H, J¼ 10.50 Hz, J ¼ 3.30 Hz), 4.72 (m, 1H), 4.59 (m, 1H), 4.46 (d, 1H, J ¼ 8.10), 4.33 (m, 1H), 4.16 (m, 2H), 4.05 (m, 2H), 3.92 (m, 2H), 2.19–2.13 (m, 5H, ACOOCH3 and ACH2CBCH), 2.03 (s, 3H, ACOOCH3), 1.98–1.91 (m, 4H, ACOOCH3 and ACBCH), 1.84–1.72 (m, 5H, ACOOCH3 and AOCH2CH2), 1.53–1.30 (m, 14H,A(CH2)7CH2CBCH). 13C NMR (CDCl3): d ¼ 170.33, 170.11, 169.96, 169.69, 164.96, 163.55, 150.62, 147.18, 140.03, 138.11, 132.28, 129.50, 127.93, 127.41, 126.09, 122.15, 121.59, 121.42,
114.28, 100.85, 84.75 (1C,ACBCH), 70.83, 70.51, 68.46, 68.28, 68.05, 67.68, 66.83, 61.16, 50.07, 29.44, 29.37, 29.29, 29.04, 28.69, 28.55, 28.43, 25.93, 20.63, 20,60, 20.55, 20.49, 18.36. MS (FAB; observed m/z): 896. HRMS (m/z) Calcd for C49H57N3O13: 895.9892, found: 895.9815.
General Polymerization Procedure
The monomers were polymerized using WCl6 as initiator and Ph4Sn as coinitiator. All polymeriza-tion reacpolymeriza-tions and manipulapolymeriza-tions were carried out under nitrogen using either an inert-atmosphere glovebox or Schlenk techniques in a vacuum line system, except for the purification of the poly-mers, which was done in a fume hood. A typical experimental procedure for the polymerization of monomer LC-11 is given below as an example.
Monomer LC-11 (0.6 g, 0.68 mmol) was added into a Schlenk tube with a three-way stopcock on the sidearm. The tube was evacuated under vac-uum and then flushed with dried N2 three times through sidearm. Dried THF (8.5 mL) was injected into the tube through a septum to dis-solve the monomer. The initiator solution was pre-pared in another tube by dissolving WCl6 (55.52 mg, 0.14 mmol) and Ph4Sn (59.79 mg, 0.14 mmol) in 1.5 mL of toluene. Both tubes were aged at room temperature for 30 min. The monomer solu-tion was then transferred to the initiator solusolu-tion using a hypodermic syringe. The reaction mixture was stirred at 60C under N2for 24 h. The solu-tion was then cooled to room temperature, diluted with 2 mL of THF and added dropwise to 300 mL of diethyl ether through a cotton filter with stir-ring. The polymer was separated by filtration, purified by several reprecipitations from THF so-lution into diethyl ether, and then dried in a vac-uum oven to yield 0.4 g (67% yield) of the final product LCP-11. 1H NMR (CDCl 3): d ¼ 8.18 (d, 2nH), 7.96 (d, 2nH), 7.92 (s, nH), 7.64 (d, 4nH), 7.27 (d, 2nH), 6.98 (d, 2nH), 6.81 (s, nH),5.40 (m, nH), 5.22 (m, nH), 4.94 (m, nH), 4.70 (m, nH), 4.58 (m, nH), 4.43 (m, nH), 4.32 (m, nH), 4.17 (m, 2nH), 4.03 (m, 2nH), 3.90 (m, 2nH), 2.23 (m, 2nH), 2.10–2.02 (m, 6nH), 1.99–1.91 (s, 3nH), 1.85–1.62 (m, 5nH), 1.54–1.22 (m, 12nH). GPC: Mn ¼ 251,000, Mw¼ 410,500, PDI¼ 1.64. Tg¼ 97.1C. Td¼ 265.3C. LCP-6 Yield 65%. 1H NMR (CDCl3): d ¼ 8.18 (d, 2nH), 7.97 (d, 2nH), 7.90 (s, nH), 7.65 (d, 4nH), 7.32 (d, 2nH), 6.95 (d, 2nH), 6.82 (s, nH), 5.44 (d, nH), 5.22 (m, nH), 4.91 (m, nH), 4.73 (m, nH), 4.56 (m, nH), 4.44 (m, nH, J¼ 8.10), 4.32 (m, nH), 4.17 (m, 2nH), 4.10 (m, 2nH), 3.94 (m, 2nH), 2.31–2.21 (m, 2nH), 2.16 (s, 3nH), 2.06 (s, 3nH), 1.95–1.90 (m, 5nH), 1.85-1.63 (m, 5nH). GPC: Mn ¼ 99,300, Mw ¼ 118,200, PDI¼ 1.19. Tg¼ 57.6C. Td¼ 277.9C. LCP-12 Yield 60%. 1H NMR (CDCl3): d ¼ 8.19 (d, 2nH), 7.98 (d, 2nH), 7.93 (s, nH), 7.63 (d, 4nH), 7.30 (d, 2nH), 6.99 (d, 2nH), 6.81 (s, nH), 5.42 (m, nH), 5.21 (m, nH), 4.93 (m, nH), 4.72 (m, nH), 4.59 (m, nH), 4.46 (m, nH), 4.33 (m, nH), 4.16 (m, 2nH), 4.08 (m, 2nH), 3.92 (m, 2nH), 2.22 (m, 2nH), 2.18-2.01 (m, 6nH), 1.98-1.91 (s, 3nH), 1.87–1.65 (m, 5nH), 1.56–1.22 (m, 14nH). GPC: Mn ¼ 73,400, Mw ¼ 148,300, PDI ¼ 2.02. Tg ¼ 65.5 C. Td ¼ 270.3C.
Bulk Orientation Control
To control the bulk orientation of the helical mor-phology formed by LC-11, a cotton rubbed PI film on the glass was prepared. A drop of the aggre-gated LC-11 was then transferred to the rubbed PI coated glass and then the glass slide was put in a vessel for slow solvent evaporation.
RESULTS AND DISCUSSION Synthesis of Monomers
Scheme 1 outlines the synthetic steps to prepare the saccharide-containing LC acetylenic deriva-tives. A saccharide entity was prepared using b-D
-galactopyranoside with the acetyl protecting group, since the hydroxyl groups (i.e., acidic pro-tons) in D-galactose are poisoning to the
transi-tion-metal catalysts used during the polymeriza-tions of acetylenes.27 2-Azidoethyl-2,3,4,6-tetraa-cetyl-b-D-galactopyranoside (2) was synthesized
by glycosylation of b-D-galactose pentaacetate
with 2-azidoethanol. The coupling of (trimethylsi-lyl)acetylene to biphenyl halide with a palladium catalyst followed by desilylation of the trimethyl-silyl protecting group appeared to be a convenient method to prepare hydroxybiphenylacetylene (4) in good yields. The immediate compound 5 was prepared by copper(I)-catalyzed regiospecific 1,3-cycloaddition between 2 and 4. Finally, the esteri-fication of compound 5 with 4-alkynyloxybenzoic acid yielded the LC monomers with different
alkynyloxy chain lengths. All the monomers were fully characterized by standard spectroscopic methods, from which satisfactory analysis data corresponding to their expected molecular struc-tures were obtained.
Polymerization Reaction
W- and Rh-based compounds are the most widely used initiators for the polymerization of acetylene-based and phenylacetylene-acetylene-based monomers.28 Tang et al. found that [Rh(nbd)-Cl]2 can initiate polymerization of phenylacetylene-based mono-mers with ester functionalities, but is not an effective initiator for the polymerization of acety-lene-based monomers.29 They also found that WCl6-Ph4Sn is the best initiating system for the polymerization of acetylene-based monomers with polar functionalities.30,31 In this study, all mono-mers belong to acetylene-based monomono-mers. There-fore, we chose a WCl6-Ph4Sn mixture to polymer-ize the synthespolymer-ized LC monomer. It is noted that no polymeric product was obtained when using WCl6-Bu4Sn to replace WCl6-Ph4Sn to initiate the polymerization under the same reaction condi-tions. The polymerization of the chiral saccharide-containing acetylenic LCs was carried out in anhy-drous toluene and THF. In toluene, polymerization proceeded with a brown oligomer precipitated, whereas the reaction system was homogeneous so-lution throughout the polymerization when a mixed solvent of THF and toluene was used. The present results indicate that toluene is not a good solvent, especially for the polymerization of alkyla-cetylene containing hydrophilic saccharide end groups. The results of the polymerization with WCl6-Ph4Sn catalyst are summarized in Table 1. Structure Characterization of Polymers
The polymers are characterized by spectroscopic techniques, and all of them give satisfactory
anal-ysis data corresponding to their molecular struc-tures. Analyses by13C NMR spectroscopy confirm that the resonance peaks of acetylenic triple bonds of the monomers have been converted to the olefinic double bonds of the polymers. As shown in Figure 1 as an example, the acetylene carbon atoms of LC-12 resonate at d 84.7 ppm; the peak is completely absent in the spectrum of LCP-12. This proves that the polymerization is realized via the transformation of triple bond to double bond. All other peaks in the spectra of the monomer and the polymer can be assigned with confidence.
The stereoregularity of the homopolymers was investigated by lH NMR spectroscopy. As for the structure of polyacetylenes, at least four possible conformers exist: cis-transoid, cis-cisoid, trans-transoid, and trans-cisoid. The chemical shift and line shape of the main chain’s olefinic protons and aromatic protons are considered to be sensitive to the conformers.32 According to Percec and co-workers33 and Tang et al.,30the chemical shift of
Table 1. Polymerization of Saccharide-Containing LC Monomersa
Initiator Solvent Yield (%) Mwb Mw/Mnb
LCP-6 WCl6/Ph4Sn Toluene 0
LCP-6 WCl6/Bu4Sn THF/Toluene 0
LCP-6 WCl6/Ph4Sn THF/Toluene 65 118,200 1.19
LCP-11 WCl6/Ph4Sn THF/Toluene 67 410,500 1.64
LCP-12 WCl6/Ph4Sn THF/Toluene 60 148,300 2.02
aCarried out under nitrogen for 24 h at 60C; [M]
0¼ 0.2 M, [Int.] ¼ [coInt.] ¼ 0.2 mM. bDetermined by GPC in THF relative to a polystyrene calibration.
Figure 1. 13C NMR spectra of (a) monomer LC-12 and (b) its polymer LCP-12 in chloroform-d. The reso-nance peaks of the acetylene carbon atoms are marked with solid arrows, while the solvents peak are labeled with asterisks (*).
cis-olefin proton appears in the region 5.88–6.26 ppm for mesogen-containing poly(1-alkynes). Fig-ure 2 depicts the 1H NMR spectra of monomer LC-12 and polymer LCP-12. In the spectra of LCP-12, no peak is observed in this region. Therefore, we conclude that the trans-olefin con-tent of polymer LCP-12 is 100%. Similar observa-tion is found for polymers LCP-6 and LCP-11.
Liquid Crystallinity
The mesomorphic phase behaviors of monomers and polymers were characterized by both differ-ential scanning calorimetry (DSC) and polarized optical microscopy (POM). Their phase transi-tion temperatures are summarized in Table 2. Based on the polarized optical texture, all three monomers exhibit a characteristic texture of chiral SAphase (SA*). Figure 3 displays the typi-cal fan texture exhibited by the monomers. Since SAand SA* show similar texture, it is diffi-cult to differentiate only by optical microscopic observation. However, the monomer contains a chiral saccharide end group, a SA* phase should be built up. Just like our previously studied sac-charide-containing LCs,34 all three monomers exhibit a glassy state. Their glass transition temperatures are ranging from 57.8 to 66.2 C. LC-6 which contains the shortest spacer length shows the highest glass transition temperature (66.2 C) and isotropic temperature (165.9 C). All the polymers also show SA* mesomorphism. POM micrographs show that all polymers ex-hibit a homogeneous texture of the SA* phase (Fig. 4). However, all samples undergo
decompo-sition at a temperature over 250 C, which agrees with the thermogravimetry measure-ment. It should be noted that, despite the degradation which occurred both before and simultaneously with the transition, the disor-dered mesophase re-appeared upon cooling.
Optical Activity
To examine the chiral character of the saccharide-based entity, UV–vis and CD experiments of chiral saccharide-based LC monomers were per-formed first. Figure 5(a) shows the UV–vis spec-tra of three monomers with various alkynyloxy chain lengths in THF. A maximum absorption band at 283 nm appears as a result of p–p* transi-tions related to their mesogenic unit, biphenyl benzoate and triazole moieties. The absorption of the biphenyl mesogen containing triazole ring was 37 nm blue-shifted from that biphenyl (246 nm),35 indicating that this kind mesogenic group was polarized. The monomers did not show any absorption peak at longer wavelengths (above ca. 325 nm), which the absorption peak is assignable to the conjugated polyacetylene backbone. The UV–vis spectra of three polyacetylene LC poly-mers showed absorption peaks originating from their alternating double-bond backbones. The absorption, however, was quite weak [Fig. 5(b)]. The steric crowding cause by the bulky mesogenic pendant groups and the aggregate of saccharide pendants might have forced the polymer chain coiled, nonplanar conformation, leading to a
Table 2. Thermal Properties of LC Monomers and LC Polyacetylenes
Phase Transitions,Ca(Kcal/mol)
Tdb Heating LC-6 G 66.2 S* 165.9 (10.73) IA 283.2 LC-11 G 57.8 SA* 165.9 (6.90) I 362.9 LC-12 G 59.1 SA* 165.9 (8.46) I 373.8 LCP-6 G 57.6 SA* [ 250 Ic 277.9 LCP-11 G 97.1 SA* [ 250 Ic 265.3 LCP-12 G 65.5 SA* [ 250 Ic 270.3
G, glass transition; SA, smectic A phase; I, isotropic phase.
aData taken from the first cycle at 10C/min.
bDetermined by TGA. T
d¼ temperature at which weight loss of 5% occurred.
cData obtained from polarizing optical microscopy, since it is affected by degradation.
Figure 2. 1H NMR spectra of (a) monomer LC-12
and (b) its polymer LCP-12 in chloroform-d. The sol-vents peaks are marked with asterisks (*).
reduction in the effective conjugation length of the polyene backbone (i.e., reducing the intensity of the absorption).36 In comparison with those of their corresponding monomers, all three polymers show wider UV spectra with a maximum at 280 nm, due to overlap of the p–p* transitions of the conjugated main chain. Figure 5(c) shows their corresponding CD spectra of the three monomers. As shown from the spectra, three monomers dis-play a typical negative Cotton effect in THF, which comes from b-D-galactopyranoside chiral
end group. It means that the bending and twist-ing forces driven by the chiral saccharide entity can be anticipated in the self-assembly process of three LC monomers.
To characterize the chiroptical properties of polyacetylenes having chiral saccharide pendants, we also used CD spectra to check whether poly-mer chains have the tendency to induce helical morphology.37 All chiral polymers in dilute THF solution exhibit intense CD bands with both nega-tive and posinega-tive Cotton effects at about 270 and 300 nm [Fig. 5(d)]. In comparison with monomers, negative Cotton effect also comes from b-D
-galac-topyranoside chiral end group. In addition, all
chi-ral monomers are CD-inactive at wavelengths longer than 290 nm, giving an almost flat line up to the visible spectral region. The positive Cotton effect at300 nm thus must be due to the absorp-tion of the polyacetylene backbone, unambigu-ously confirming that the main chain of the poly-mer is a helical conformation with a preferred screw sense. Therefore, the main chain of the polymer seems to be chiral, probably due to a pre-dominant one-handed helical sense.
The CD signal reflects both chirality of the main-chain and side-chain aggregation. The inter-play between the macromolecular helicity, the chirality of the pendant groups, and the chirality at a macroscopic level are of great interest from fundamental and biological viewpoints. To this end, we have studied the structures of the helical polymers in dilute solution with SEM.
Self-Assembled Hierarchical Superstructures of Saccharide-Containing LC Monomers in Solution The self-assembled morphology of the chiral sac-charide-containing acetylenic LC monomers in so-lution was examined by FESEM. An interesting
Figure 3. Polarized optical microscopy of the fan-shaped textures exhibited by (a) LC-6, (b) LC-11, and (c) LC-12 at 100C.
Figure 4. Polarized optical micrographs displayed by (a) LCP-6, (b) LCP-11, and (c) LCP-12 at 150C.
morphological evolution was identified according to the change in the alkynyloxy chain length. LC-6 exhibits a mixed morphology including a plate-let-like morphology and right-handed helical twists, as shown in Figure 6(a). However, the micrographs of LC-11 and LC-12 exhibit a right-handed helical morphology in the majority [Fig. 6(b,c)]. The hierarchical aggregates composed of several helical strands were further visualized in the FESEM micrographs. It is also noted that the pitch length of the helical twists decreases with increasing alkynyloxy chain length. These results agree with our previous investigation on the saccharide-containing LC molecules.34 The helicity of the LC molecules is primarily by the chiral saccharide end group. However, the helic-ity is amplified in LC state because rod seg-ments p–p interaction leads to liquid-crystal-line-like aggregation. This will stabilize the helical morphology.
Orientation Control of the Helical Morphology Exhibited by LC Monomers
We investigated the rubbed induced helical align-ment of LC-11. Figure 7(b–d) shows the polarized optical micrographs with gypsum plate of the ori-ented LC-11 film observed under crossed polar-izers. Flat and elongated film is seen. The LC-11 film is oriented parallel to the rubbing direction, as evidenced by the fact that the counterclockwise [Fig. 7(b)] and clockwise [Fig. 7(d)] images tilted at45andþ45with respect to the transmission axis of the polarizer, respectively, showed a clear change of birefringence.
The helical morphology in order 2D arrays is investigated by FESEM. The LC-11 molecules spontaneously self-assembled into a highly or-dered monolayer without helical structure as evi-denced by parallel thin plates [Fig. 8(a), red arrow]. This indicates that the molecules lie flat on the surface with a planar conformation,
Figure 5. Corresponding UV–vis spectra and CD results of LC monomers (a,b) and LC polymers (c,d) in THF. The spectral data below 250 nm are not taken because of the interfering absorption. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
probably because of the strong and epitaxial adsorption of molecules on the surface. The orien-tation of these plates in the first layer was antici-pated to be influenced by that rubbed PI and was nearly parallel to the rubbing direction.
Although the second layer of molecules depos-ited on this 2D ordered first monolayer appeared to be formless with no specific aggregations [blue arrow in Fig. 8(a)], they further self-assembled into well-defined helix. In the SEM image, 2D
Figure 6. FESEM micrographs of the self-assembled morphology exhibited by monomers (a) LC-6, (b) LC-11, and (c) LC-12. The bar represents 100 nm.
Figure 7. Alignment of helical LC-11 on rubbed PI substrate. (a) Schematic repre-sentation of the rubbed induced alignment of LC-11 molecules. (b–d) Polarized opti-cal micrographs with gypsum plate of oriented LC-11 film under crossed polarizers. The substrate was tilted at 45 (b, counterclockwise), 0 (c, parallel), and þ45 (d, clockwise) to the transmission axis of the polarizer. The green arrows indicate the rubbing direction. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
smectic arrangement on the rubbed PI substrate can be clearly resolved into individual right-handed helix packed parallel to each other. The
elongate helices can be seen over lengths larger than 2 lm [Fig. 9(a)]. The oriented helical mor-phology also exhibited a highly birefringent
Figure 9. FESEM images of self-assembled helical structures of LC-11 and their respective zoomed in images on each of following substrates: (a,b) rubbed PI sub-strate; (c,d) nonrubbed PI subsub-strate; (e,f) HOPG substrate.
Figure 8. FESEM observation of molecular ordering of helical LC-11 on rubbed PI substrate. (a) FESEM image of the epitaxial LC-11. The green arrow indicates the rub-bing direction. The red arrows indicate the direction of epitaxial growth molecules. The blue arrow indicates the formless aggregations. The 2D self-assembled LC-11 with right-handed helices can be clearly seen. (b) Schematic illustration of the hierarchical superstructure of the self-assembled LC-11 on a rubbed PI substrate. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
texture with a characteristic banded pattern per-pendicular to the rubbing direction [Fig. 9(b)]. Therefore, the first layer of the LC-11 epitaxially deposited on the basal plate of the rubbed PI sub-strate could not only prevent unfavorable direct interactions between molecules and surface but also serve as a template38 for subsequent forma-tion of the upper-layer helix with controlled helic-ity and packing.
To confirm the effect of the rubbing process on the orientation control to the PI surface, we used nonrubbed PI substrate as a comparison. The cor-responding images from FESEM are shown in Fig-ure 9. A nanostructFig-ure of helical morphology simi-lar to that obtained from the rubbed PI substrate is observed [Fig. 9(d)]. However, in terms of the perfection of the ordering, the nanostructure pro-duced on the nonrubbed PI substrate is much more defective with many more arced dislocations than when the rubbed substrate is used [Fig. 9(c)]. It was reported that rubbing treatment enhanced the degree of regularity and crystallinity of poly-mer chains at the surface region,39though studied PI is an amorphous polymer. We consider that the rubbing process is an effective way to induce mo-lecular orientation onto a polymer layer. We also studied that LC-11 self-assemble into 2D layered alignment on HOPG with ordered planar carbon lattices [Fig. 9(e,f)]. The observations for the mor-phology and degree of orientation on HOPG are not as well as on rubbed PI substrate. This result can be explained by considering the substrate sur-face energy (c). According to the literature results,40the surface energy of PI substrate (cPI¼ 0.038 J m2) is much smaller than that of HOPG substrate (cHOPG¼ 0.2 J m2). In general, LC mol-ecules prefer to stay on the low surface energy
sur-face. The van der Waals dispersion interaction between LC molecules and the rubbed PI sub-strate is much stronger than that of LC molecules and HOPG substrate. Consequently, the rubbed PI film is found to be more effective to orient the epi-taxial molecules in monolayer than HOPG.
Self-Assembled Hierarchical Superstructures of LC Polymers in Solution
The self-assembled hierarchical superstructure of LC polyacetylenes in solution was also investi-gated with FESEM. LCP-11 aggreinvesti-gated to form helical nanofibers at primary stage [Fig. 10(a)] and some nanospheres at intermediate stage [Fig. 10(b)] when the nonsolvent, methanol, was added gradually into THF solution. This structural tran-sition was due to the fact that THF tends to induce the polymer chains to twist into extended fibers, while methanol is inclined to promote the polymer chains to twine into micellar spheres. The balance of these two repulsive forces leads to the formation of the intermediate structures. In the final stage, the nanofibers are evolved from the helical cables ornamented with entwining nanofibers upon natu-ral evaporation of the solution in a mixture of THF/methanol ¼ 3/7 (v/v) [Fig. 10(c)]. Both the main chain helixes and chiral character of saccha-ride units are the driving forces for the polymer molecules to form complicated helical cables. LCP-12 formed very similar helical morphology as LCP-11 in methanol/THF solution while LCP-6 did not form any helical morphology.
CONCLUSION
We have synthesized a series of chiral galacto-pyranoside-based 1,2,3-triazole liquid crystal
Figure 10. FESEM micrographs of the self-assembled morphologies exhibited by LCP-11 in solution (a) primary stage, (b) intermediate stage, and (c) final stage. The bar represents 100 nm.
poly(alkylacetylenes) with different alkynyloxy chain lengths. Both LC monomers and polymers clearly show a chiral SA mesomorphism. All the monomers exhibited liquid-crystalline-like behav-ior for assembly so as to create tunable self-assembled morphologies including platelets and helical twists. The self-organized polymers exhibit a mixed intermediate morphology, including nanospheres and helical nanofibers except for LC-6. This structural transformation seems to originate from the balance between the repulsive interactions of the interstrand association in a methanol solution and extended force in a THF solution of polymer chains. Subsequently, these fibers are coiled around to form the helical cables ornamented with entwining nanofibers. Consider-ing the conventional self-assemblConsider-ing systems such as copolymers, homopolymers have been com-monly believed to be difficult to self-assemble into well-defined morphological structures.41 The results described here demonstrate that chiral homopolymers carrying saccharide pendants can assemble into well-defined supramolecular nano-structures. We also have demonstrated that heli-cal LC-11 could be highly aligned on the rubbed PI substrate. To the best of our knowledge, this may be the first example of the spontaneous for-mation of a large area orientation of helical mor-phology due to a rubbed induced alignment of molecules. In conclusion, the synthesized
saccha-ride-containing LC monomers and polymers
which will aggregate to form helical superstruc-tures have potential applications in optical switching and sensors devices.
The authors thank the National Science Council and Ministry of Education (MOE ATU Program) for finan-cial support.
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