Phase Structures and Self-assembled Helical Suprastructures via

Selinger, J. V.; MacKintosh, F. C.; Schnur, J. M. Phys. Rev. E 1996, 53, 3804. [Crossref], [PubMed], [CAS]. (13) . Theory of cylindrical tubules and h...
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Phase Structures and Self-assembled Helical Suprastructures via Hydrogen Bonding in a Series of Achiral 4-Biphenyl Carboxylic Acid Compounds Kwang-Un Jeong,† Shi Jin,‡ Jason J. Ge,† Brian S. Knapp,† Matthew J. Graham,† Jrjeng Ruan,† Mingming Guo,† Huiming Xiong,† Frank W. Harris,† and Stephen Z. D. Cheng*,† Maurice Morton Institute and Department of Polymer Science, The UniVersity of Akron, Akron, Ohio 44325-3909, and Department of Chemistry, College of Staten Island, The City UniVersity of New York, Staten Island, New York 10314 ReceiVed February 15, 2005. ReVised Manuscript ReceiVed April 7, 2005

A series of achiral 4-biphenyl carboxylic acid compounds (BPCA-Cn-PmOH) connected with alkoxyl chains having various carbon numbers (n ) 6-10) and terminated by phenyl groups with meta-positioned hydroxyl groups was synthesized. Different phase structures including nematic, smectic A (SmA), smectic C (SmC), and highly ordered smectic liquid-crystalline phases along with crystalline phases were identified based on wide-angle X-ray diffraction and electron diffraction experiments. It was found via infrared spectroscopy that the hydrogen (H)-bonds were formed between the carboxylic acids to construct headto-head dimers as the building blocks for these ordered structural formations. H-bonds formed via the meta-positioned hydroxyl groups also played an important role in forming ordered layers in these structures. The morphology of this series of BPCA-Cn-PmOH as observed under polarized light microscopy showed an oily streak (cylinder) texture with Myelin-figure in the SmA phase. When temperature cools to enter the SmC phase, these streaks (cylinders) started to twist into helical suprastructures, which were not only by the birefringence changes but also by the three-dimensional helical geometry observed in other microscopic techniques. The dynamic conformational changes of the aromatic and aliphatic parts in this series of BPCA-Cn-PmOH at different temperatures correspond well with the thermal transitions via solid-state carbon-13 nuclear magnetic resonance experiments. Computer simulation indicated that the head-to-head dimers possess a twisted rather than a bent conformation. It was deduced that the twisted conformation of the dimers and the terminal meta-substituted phenyl groups at both ends of the dimers are critically important in forming the helical suprastructures.

Introduction Chiral materials in nature such as DNA,1,2 RNA,3,4 and peptides5,6 play critical roles in biological functions.1-7 In these materials, the recognition of chiral structures on different length scales is essential.7 On the sub-nanometer scale, the atomic configurational chirality (the first level of chirality) forms chiral centers with covalent bonds having the lowest possible symmetry (C1). On the nanometer scale, noncovalent interactions such as hydrogen (H)-bonding cause the handedness of helical conformations (the second level of chirality). These helical molecules then pack to form the phase structure (the third level of chirality) and, thus, exhibit * To whom correspondence should be addressed. E-mail: [email protected]. † The University of Akron. ‡ The City University of New York.

(1) Barton, J. K. Science 1986, 233, 727. (2) Declercq, R.; Aerschot, A. V.; Read, R. J.; Herdewijn, P.; Meervelt, L. V. J. Am. Chem. Soc. 2002, 125, 928. (3) Rozners, E.; Katkevica, D.; Bizdena, E.; Stro¨emberg, R. J. Am. Chem. Soc. 2003, 125, 12125. (4) Tamura, K.; Schimmel, P. Science 2004, 305, 1253. (5) Mason, S. F. Nature 1984, 311, 19. (6) Bombelli, C.; Borocci, S.; Lupi, F.; Mancini, G.; Mannina, L.; Segre, A. L.; Viel, S. J. Am. Chem. Soc. 2004, 126, 13354. (7) Zubay, G. Biochemistry, 2nd ed.; Macmillan Publishing Company: New York, 1988; pp 845-1151.

phase chirality. These phase structures further aggregate to form object chirality (the fourth level of chirality), which is on the macroscopic scale. It is known that the formation and construction of chiral structures as well as their chirality transfer behaviors are critical to the design, synthesis, and construction of molecular and supramolecular chiral structures to achieve the macroscopic properties desired for specific bio-mimetic and optical applications.7-11 Chiral small molecule liquid crystals (LC) have been synthesized, many of which exhibit helical phase morphologies such as in the cholesteric and smectic C* (SmC*) phases.9-14 The helical handedness or chiral pitch distances of the phase chirality can change as a function of temperature (8) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science 1995, 268, 1860. (9) Goodby, J. W. Science 1986, 231, 350. (10) Kitzerow, H.-S.; Bahr, C. in Chriality in Liquid Crystals; Kitzerow, H.-S., Bahr, C., Eds.; Springer: London, 2001; pp 1-27. (11) Goodby, J. W. in Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 1, pp 115-132. (12) Katsaras, J.; Raghunathan, V. A. Phys. ReV. Lett. 1995, 74, 2022. (13) Selinger, J. V.; MacKintosh, F. C.; Schnur, J. M. Phys. ReV. E 1996, 53, 3804. (14) Gooby, J. W.; Slaney, A. J.; Booth, C. J.; Nishiyama, I.; Vuijk, J. D.; Styring, P.; Toyne, K. J. Mol. Cryst. Liq. Cryst. 1994, 243, 231.

10.1021/cm050338y CCC: $30.25 © 2005 American Chemical Society Published on Web 05/05/2005

Achiral 4-Biphenyl Carboxylic Acid Compounds

in these low-ordered LC phases, while their helicity is dependent upon the configurational chirality as well as the length of methylene units connected to the chiral molecules.10-14 In chiral-packing theory, the packing of molecules composing the phase is responsible for imparting a twist which ultimately leads to phase chirality.12,13 The formation of these phase chiralities was in the past exclusively attributed to the existence of atomic chiral centers in these molecules. Since the driving force in forming positional and/or bond orientational order requires parallel packing of molecules, it was thought that these low order helical chiral phases would disappear when the small molecule LCs further develop into highly ordered LC and crystalline phases.14 In polymers, it was recently reported that in a series of specifically designed polyesters synthesized from (R)-(-)4′-{omega-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1nonyloxy}-4-biphenyl carboxylic acid [abbreviated as PET(R*)-n, where n is the number of methylene units; n ) 7-11], helical single crystals were observed and the handedness of the helical sense was not only determined by the configurational chirality but also by the number of methylene units.15-23 For example, having the identical R* chiral center with different numbers of methylene units [PET(R*)-n, n: even versus odd numbers], the handedness of the helical single crystals could switch from right for n ) odd to left for n ) even. The chirality transfer between different length scales has been determined to be dependent upon the packing scheme within each of the length scales. On the other hand, the origin of phase chirality may not be attributed to chiral intermolecular interactions, but to a collective tilt of the molecules24 with respect to the layer normal like polyethylene25 twisted lamellae crystals. Additionally, different chain fold conformations in polymer crystals may generate unbalanced surface stresses for lamellar twisting/scrolling like Nylon 66.26 Similarly in low molecular weight organic molecules, a small increase in unit-cell dimensions near the lamellar surface is thought to cause lamellar twisting. It is worth considering the fact that twisting is more often (15) Li, C. Y.; Yan, D.; Cheng, S. Z. D.; Bai, F.; He, T.; Chien, L. C.; Harris, F. W.; Lotz, B. Macromolecules 1999, 32, 524. (16) Li, C. Y.; Yan, D.; Cheng, S. Z. D.; Bai, F.; Ge, J. J.; Calhoun, B. H.; He, T.; Chien, L. C.; Harris, F. W.; Lotz, B. Phys. ReV. B 1999; 60, 12675. (17) Li, C. Y.; Cheng, S. Z. D.; Ge, J. J.; Bai, F.; Zhang, J. Z.; Mann, I. K.; Harris, F. W.; Chien, L.-C.; Yan, D.; He, T.; Lotz, B. Phys. ReV. Lett. 1999, 83, 4558. (18) Li, C. Y.; Ge, J. J.; Bai, F.; Zhang, J. Z.; Calhoun, B. H.; Chien, L. C.; Harris, F. W.; Lotz, B.; Cheng, S. Z. D. Polymer 2000, 41, 8953. (19) Li, C. Y.; Cheng, S. Z. D.; Ge, J. J.; Bai, F.; Zhang, J. Z.; Mann, I. K.; Chien, L. C.; Harris, F. W.; Lotz, B. J. Am. Chem. Soc. 2000, 122, 72. (20) Li, C. Y.; Cheng, S. Z. D.; Weng, X.; Ge, J. J.; Bai, F.; Zhang, J. Z.; Calhoun, B. H.; Harris, F. W.; Chien, L. C.; Lotz, B. J. Am. Chem. Soc. 2001, 123, 2462. (21) Li, C. Y.; Ge, J. J.; Bai, F.; Calhoun, B. H.; Harris, F. W.; Cheng, S. Z. D.; Chien, L. C.; Lotz, B.; Keith, H. D. Macromolecules 2001, 34, 3634. (22) Li, C. Y.; Jin, S.; Weng, X.; Ge, J. J.; Zhang, D.; Bai, F.; Harris, F. W.; Cheng, S. Z. D.; Yan, D.; He, T.; Lotz, B.; Chien, L. C. Macromolecules 2002, 35, 5475. (23) Weng, X.; Li, C. Y.; Jin, S.; Zhang, J. J.; Zhang, D.; Harris, F. W.; Cheng, S. Z. D.; Lotz, B. Macromolecules 2002, 35, 9678. (24) Pang, J.; Clark, N. A. Phys. ReV. Lett. 1994, 73, 2332. (25) Lotz, B.; Cheng, S. Z. D. Polymer 2005, 46, 577, and references cited therein. (26) Cai, W.; Li, C. Y.; Li, L.; Lotz, B.; Keating, M.; Marks, D. AdV. Mater. 2004, 7, 600.

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observed when the unit-cell symmetry is low, such as in monoclinic or triclinic lattices.27 It has been reported in the past decade that bent-core molecules show LC behavior, and they form macroscopic helical structures.28,29 There are no atomic chiral centers in these bent-core molecules, but they form the helical structures in the B7 phase due to the geometric anisotropy (“banana”like shape) of the LC molecule. Chiral spontaneous electrooptical responses such as antiferro-electricity, previously thought to only originate from chiral molecules in the SmC* phase, has been observed in achiral bent-core molecules in B2 phases.30 These new observations from achiral molecules can lead to new optical switches, shutters, displays, and sensors. One of the new ideas in the development of the helical suprastructure is to self-assemble achiral molecules through noncovalent interactions.31-36 These include Hbonding, electrostatic interactions (ion-ion, ion-dipole, and dipole-dipole), π-π stacking interactions, van der Waals interactions, and hydrophobic-hydrophilic effects along with others. Among these interactions, H-bonding has become a topic of major research due to its numerous potential applications in the biological and material sciences.32-37 In this study, we report on self-assembled helical suprastructures from a series of novel achiral 4-biphenyl carboxylic acid compounds (BPCA-Cn-PmOH, n ) 6-10) in the lowordered smectic C (SmC) phase. On the basis of our experimental observations about the phase-transition properties, phase-structure identifications, and phase-texture evolutions, we can conclude that the self-assembled head-to-head dimers formed via the intermolecular H-bondings between carboxylic acids are essential to form a stable helical suprastructure in this system. Possible formation mechanism of the helical suprastructure from BPCA-Cn-PmOH is also speculated. Experimental Section Materials and Sample Preparation. A series of novel achiral 4-biphenyl carboxylic acid compounds (BPCA-Cn-PmOH) connected with alkoxyl chains having various carbon numbers (n ) 6-10) and terminated by phenyl groups with meta-positioned hydroxyl groups was synthesized using four-step substitution reactions. Final compounds were purified using column chroma(27) Zastavker, V. Y.; Asherie, N.; Lomakin, A.; Pande, J.; Donovan, J. M.; Schnur, J. M.; Benedek, G. B. Proc. Natl. Acad. Sci. 1999, 96, 7883. (28) Pelzl, G.; Diele, S.; Ja´kli, A.; Lischka, C.; Wirth, I.; Weissflog, W. Liq. Cryst. 1999, 26, 135. (29) Thisayukta, J.; Niwano, H.; Takezoe, H.; Watanabe, J. J. Am. Chem. Soc. 2002, 124, 3354. (30) Link, D. R.; Natale, G.; Shao, R.; Mclennan, J. E.; Clark, N. A.; Korblova, E.; Walba, D. M. Science 1997, 278, 1924. (31) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550. (32) Yang, W.; Chai, X.; Chi, L.; Liu, X.; Cao, Y.; Lu, R.; Jiang, Y.; Tang, X.; Fuchs, H.; Li, T. Chem. Eur. J. 1999, 5, 1144. (33) Hirschberg, J. H.; Brunsveld, L.; Ramzi, A.; Vekemans, J. A.; Sijbesma, R. P.; Meijer, E. W. Nature 2000, 407, 167. (34) Reinhoudt, D. N.; Drego-Calama, M. Science 2002, 295, 2403. (35) Keinan, S.; Ratner, M. A.; Marks, T. J. Chem. Mater. 2004, 16, 1848. (36) Jin, S.; Ma, Y.; Zimmerman, S. C.; Cheng, S. Z. D. Chem. Mater. 2004, 16, 2975. (37) Xue, C.; Jin, S.; Weng, X.; Ge, J. J.; Shen, Z.; Shen, H.; Graham, M. J.; Jeong, K.-U.; Wang, H.; Zhang, D.; Guo, M.; Harris, F. W.; Cheng, S. Z. D.; Li, C. Y.; Zhu, L. Chem. Mater. 2004, 16, 1014.

2854 Chem. Mater., Vol. 17, No. 11, 2005 tography, followed by repeated crystallization, and their purity was confirmed using 1H NMR along with thin-layer chromatography. The detailed synthetic procedures can be found in ref 38. The general chemical structure of this series of compounds is

For one-dimensional (1D) wide-angle X-ray diffraction (WAXD) powder measurements, film samples with a thickness of about 1 mm were prepared by melting the compounds in an aluminum cell. Oriented samples obtained by mechanical shearing in the lowordered SmC phase were used in 2D WAXD experiments for determining various phase structures after the samples were thermally treated at different temperatures. A typical sample thickness was about 0.3 mm. Samples were prepared for transmission electron microscopy (TEM) experiments by casting a BPCACn-PmOH/tetrahydrofuran solution (0.1 w/v %) on carbon-coated glass slides. After crystallization, the films were removed from the glass slide, floated on the water surface, and recovered using copper grids. The samples prepared for polarized light (PLM) and phase contrast optical microscopy (PCM) had a typical thickness of 5 µm, and they were melt-processed between two polyimide-coated and rubbed glass sides. Surface morphological observations were made using scanning electron microscopy (SEM) and atomic force microscopy (AFM) on film cast from solution onto polyimidecoated and rubbed glass surfaces. After the solvent was evaporated, the samples were heated to a high temperature, usually to the SmC phase, and held there for a few hours. The samples were then quickly quenched into an ice-water mixture. Samples for SEM were further coated with gold before making observations. The film samples for infrared spectroscopy (FT-IR) measurements were the same as those used in AFM experiments, but the substrates were KBr plates. The powder samples were used for solid-state 13C nuclear magnetic resonance (NMR). Equipment and Experiments. The thermal behavior of the phase transitions was studied using a Perkin-Elmer PYRIS Diamond DSC with an Intracooler 2P apparatus. The temperatures and heat flows were calibrated using material standards at cooling and heating rates ranging from 2.5 °C/min to 40 °C/min. The heating experiments always preceded the cooling experiments in order to eliminate previous thermal histories, and the cooling and heating rates were always kept identical. The transition temperatures were determined by measuring the onset and peak temperatures from both the cooling and heating scans at different rates. 1D WAXD powder experiments were conducted in the transmission mode of a Rigaku 12 kW rotating-anode X-ray (Cu KR radiation) generator coupled to a diffractometer. The diffraction peak positions and widths were calibrated with silicon crystals of known crystal size in the high 2θ-angle region (>15°) and silver behenate in the low 2θ-angle region. A hot stage was coupled to the diffractometer in order to study the LC structural evolutions with temperature changes during heating and cooling. The temperature of this hot stage was calibrated to be within (1 °C. Samples were scanned across a 2θ-angle range of 1.5-35° at a scanning rate of 4°/min. The oriented 2D WAXD patterns were obtained using a Rigaku X-ray imaging system with an 18 kW rotating anode X-ray generator. A hot stage was also used to obtain diffraction peaks from the LC structures at elevated temperatures. (38) Knapp, B. S. Ph.D. Dissertation, University of Akron, 2004.

Jeong et al. A 30 min exposure time was required for a high-quality pattern. In both 1D and 2D WAXD experiments, background scattering was subtracted from the sample scans. TEM (FEI Tacnai 12) experiments were carried out to examine crystal morphology using an accelerating voltage of 120 kV. Selected area electron diffraction (SAED) patterns of different zones within the samples were also obtained. Calibration of the SAED spacing smaller than 0.384 nm was carried out using evaporated thallous chloride, which has a largest first-order spacing diffraction of 0.384 nm. Spacing values larger than 0.384 nm were calibrated by doubling the d spacing values of the first-order diffractions. Optical textures of the LC phases at different temperatures were observed with a PLM and a PCM (both are Olympus BH-2) coupled with a Mettler heating stage (FP-90). A SEM (JEOL JSM-5310) was used to study the surface morphology of the samples. An AFM (Digital Instrument Nanoscope IIIa) was also used in tapping mode to examine the helical morphology of the samples. To limit the damage to the sample, the force applied on the cantilever was adjusted to the minimum limit, but good engagement was maintained between the AFM tip and the sample surface. A 1 Hz scanning rate was used at a resolution of 512 × 512 for a scan size of 10 µm. The operation and resonance frequencies were 290 kHz. The scanner was calibrated with the standard grid for both lateral size and height. H-bond formations between carboxylic acids and between hydroxyl groups were studied using a FT-IR (Digilab Win-IR Pro FTS 3000) equipped with a Bruker heating stage. The temperatures of this hot stage were calibrated to be within (0.5 °C. The dynamics of each carbon and the conformations of the alkoxyl chain in BPCACn-PmOH (n ) 8 and 9) were also studied using a solid-state 13C NMR (Chemagnetics CMX 200) operating at 201.13 and 50.78 MHz for 1H and 13C nuclei. The samples were spun in nitrogen gas at 4.5 kHz at the magic angle. The magic angle was optimized by the intensity calibration of the aromatic carbon resonance of hexamethylbenzene. The 13C cross polarization/magic angle spinning/dipolar decoupling (CP/MAS/DD) NMR spectra were acquired to selectively investigate the rigid components and the Bloch decay spectra while MAS/DD was used to selectively study the mobile components. The contact time was 2 ms and each spectrum consisted of an accumulation of 1000 scans. The temperature of the solid-sate 13C NMR experiment was controlled using a REXF900 VT unit covered the temperature range from 25 °C to 200 °C. The Cerius2 (Version 4.6) simulation software from Accelrys was used to calculate the head-to-head dimer minimal energy geometry of BPCA-Cn-PmOH compounds in the isolated gas phase utilizing the COMPASS force field. Overlapped CdO absorption peaks in FT-IR and β carbon peaks in solid-state 13C NMR were resolved using the PeakFit peak separation program (Jandel Scientific). Gaussian and/or Lorentzian functions were used to obtain the best fit.

Results and Discussion Thermal Transitions and Their Corresponding Structural Changes. Figure 1a shows a set of DSC cooling diagrams for BPCA-Cn-PmOH (n ) 6-10) at a rate of 2.5 °C/min. In this set of DSC diagrams, the odd-even effect of the transition temperatures is evident in that BPCA-CnPmOH having even numbers (n ) 6, 8, and 10) possess higher transition temperatures (Figure 1a).39,40 Furthermore, BPCA-Cn-PmOH (n ) 6 and 8) apparently do not possess a broad exothermic low transition process as observed in the cases of those having odd numbers. The set of subsequent

Achiral 4-Biphenyl Carboxylic Acid Compounds

Figure 1. Sets of DSC (a) cooling and (b) subsequent heating diagrams for BPCA-Cn-PmOH samples at a rate of 2.5 °C/min. Two insets are DSC cooling and subsequent heating diagrams for BPCA-C8-PmOH (in Figure 1a) and BPCA-C9-PmOH (in Figure 1b) samples at two different rates of 2.5 °C/min and 10 °C/min.

heating DSC diagrams shown in Figure 1b reflects the endothermic processes that correspond to those phase transitions observed during cooling. Different cooling and subsequent heating rates were also used and are shown in the insets of Figures 1a and 1b for BPCA-C8-PmOH (the even number) and BPCA-C9-PmOH (the odd number), respectively, at 2.5 and 10 °C/min as examples. The hightemperature transitions do not exhibit an undercooling dependence upon the cooling rates, while the most evident cooling-rate dependence is found for the lowest transition process as shown in these insets. The two expanded DSC cooling diagrams at a rate of 1.0 °C/min in the insets of Figures 2a and 2b (see below) show additional weak and broad transitions starting at 220 °C for BPCA-C8-PmOH and 213 °C for BPCA-C9-PmOH. (39) Demus, D. in Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 1, pp 133-187. (40) Seddon, J. M. in Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 1, pp 635-679.

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Figure 2. Sets of 1D WAXD patterns of (a) BPCA-C8-PmOH and (b) BPCA-C9-PmOH at a cooling rate of 1.0 °C/min at different temperatures. Two insets are the expanded DSC cooling diagrams for BPCA-C8-PmOH (in Figure 2a) and BPCA-C9-PmOH (in Figure 2b) at 1.0 °C/min.

1D WAXD cooling experiments at 1.0 °C/min for the BPCA-Cn-PmOHs were carried out in order to obtain structural information as shown in Figures 2a and 2b at different temperatures for BPCA-C8-PmOH and BPCA-C9PmOH, respectively. In both figures, structures with two different length scales can be identified. One structure is on the nanometer scale between 1.5° and 8°, and the other is on the sub-nanometer scale between 9° and 35°. For BPCAC8-PmOH, Figure 2a shows the structural changes which correspond to the five exothermic processes observed in the DSC cooling diagram. At Ti ) 222 °C, there is a sudden shift of the scattering halo in the WAXD patterns toward a higher 2θ angle from 18.9° to 19.1° (d spacing from 0.470 to 0.465 nm) in Figure 2a, indicating a transition from the isotropic melt (I) to a nematic (N) LC phase.41-47 Slightly (41) Unger, G.; Feijoo, J. L.; Percec, V.; Yourd, R. Macromolecules 1991, 24, 953. (42) Yandrasits, A.; Cheng, S. Z. D.; Zhang, A.; Cheng. J.; Wunderlich, B.; Percec, V. Macromolecules 1992, 25, 2112.

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lower than 220 °C, the low 2θ-angle reflection appears at 1.72° (d spacing of 5.14 nm) while the scattering halo at 2θ ) 19.1° shifts toward to a 2θ angle of 19.4° (d spacing of 0.458 nm). The broad exothermic process started at 220 °C (see the insert of Figure 2a) can be distinguished by the shift of the low 2θ-angle reflection to 2.0° (d spacing of 4.42 nm) coupled with a shift of amorphous halo from 2θ ) 19.4° to 19.6° (d spacing of 0.453 nm) in the high 2θ-angle region at 215 °C. A sharpening of the halo in the high 2θ-angle region corresponds to the small exothermic process at 194 °C in Figure 1a. Finally, sharp reflections develop in the high 2θ-angle region, and reflection peaks in the low 2θangle region also change in Figure 2a, reflecting the structural transformation caused by the exothermic transition at 158 °C (Figure 1a). In the case of BPCA-C9-PmOH, the I phase can be found above 214 °C. During cooling through this temperature, there is a sudden shift of the scattering halo in the WAXD patterns from 2θ ) 17.8° to 18.2° (d spacing of 0.498 to 0.487 nm, respectively) reflecting an I T N phase transition.41-47 Below 213 °C, a relatively broad reflection peak develops in the low 2θ-angle region. The sharpening of the scattering halo in the high 2θ-angle region is also evident and corresponds to the small exothermic process at 210 °C in the inset of Figure 2b. Below 179 °C, the significantly sharpened scattering halo in the high 2θ-angle region in Figure 2b is coupled with the small exothermic peak at 179 °C in Figure 1a. The strong exothermic peak at 146 °C (Figure 1a) is attributed to the development of reflections in the high 2θangle region and the appearance of the second-order reflection in the low 2θ-angle region. The reflection peaks in the high and low 2θ-angle regions sharpen as the sample passes through two more exothermic transitions at 118 °C and 82 °C in the DSC cooling diagrams (Figure 1a). These structural changes in Figures 2a and 2b can be characterized by the d spacing and changes in the width at half-height (WAHH) of the scattering halos with respect to temperature before the reflection peaks appeared in the high 2θ region of the BPCA-Cn-PmOHs. The correlation lengths can be calculated from the Scherrer equation using the WAHH.39,40,44-46 Both the results are shown in Figures 3a and 3b for BPCA-C8-PmOH and BPCA-C9-PmOH, respectively. The vertical dash lines are indications of the thermal transitions based on the DSC cooling results. The changes of these structural parameters correspond well to those thermal transitions. The WAXD patterns during heating are very similar to those during cooling. The behavior of n ) 8 and n ) 9 is very representative of the other even and odd BPCA-Cn-PmOHs, respectively. Table 1 lists the transition temperatures, transition enthalpies, and their phase identifica(43) Pardey, R.; Harris, F. W.; Cheng, S. Z. D.; Adduci, J.; Facinelli, J. V.; Lenz, R. W. Macromolecules 1992, 25, 5060. (44) Pardey, R.; Shen, D.; Gabori, P. A.; Harris, F. W.; Cheng, S. Z. D.; Adduci, J.; Facinelli, J. V.; Lenz, R. W. Macromolecules 1993, 26, 3687. (45) Yoon, Y.; Ho, R.-M.; Moon, B.; Kim, D.; McCreight, K. W.; Li, F.; Harris, F. W.; Cheng, S. Z. D.; Percec, V.; Chu, P. Macromolecules 1996, 29, 3421. (46) Ge, J. J.; Zhang, A.; McCreight, K. W.; Ho, R.-M.; Wang, S.-Y.; Jin, X.; Harris, F. W.; Cheng, S. Z. D. Macromolecules 1997, 30, 6498. (47) Zheng, R.-Q.; Chen, E.-Q.; Cheng, S. Z. D.; Xie, F.; Yan, D.; He, T.; Percec, V.; Chu, P.; Ungar, G. Macromolecules 1999, 32, 3574.

Jeong et al.

Figure 3. The relationship between the d spacing of the center scattering halos and the correlation lengths of the scattering halos of (a) BPCA-C8PmOH and (b) BPCA-C9-PmOH with temperature above the temperature where the crystallization takes place. The correlation lengths were calculated based on the Scherrer equation. The vertical dash lines are indications of the thermal transitions based on the DSC cooling results.

tions which will be further discussed in the following sections. Identifications of Crystal Structures. Figures 4a-4c show a set of 2D WAXD patterns for mechanically sheared BPCA-C8-PmOH along three incident X-ray beam directions (the through, the edge, and the end directions as illustrated in Figure 4d) at room temperature. In Figure 4a, the X-ray beam is in the through direction, and no diffraction peaks can be found in the low 2θ-angle region. However, when the X-ray beam is along the edge and end directions, as in both Figures 4b and 4c, a pair of diffractions at 2θ ) 2.37° (d spacing of 3.73 nm) is observed on the meridian. Even higher order diffractions (2θ ) 4.74° and 7.11°) can be clearly seen. The orientation of the diffractions in Figure 4b is better than those in Figure 4c, as judged by the azimuthal distribution of the diffraction spots. These observations indicate that the diffraction at 2θ ) 2.37° is attributed to the layer structure of which the layer normal is parallel to the through direction. Namely, the layer structure normal is parallel to the z-axis in Figure 4d. The layer is thus parallel to the x-y plane. The d spacing of the layer structure is 3.73 nm. Since one BPCA-C8-PmOH molecule is 2.63 nm in length as calculated after energy minimization in a vacuum using Cerius2 4.6 at 0 K, the layer structure is expected to

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Figure 4. Set of 2D WAXD patterns of a mechanically sheared BPCA-C8-PmOH sample along three directions: (a) the X-ray beam is along the through direction, (b) along the edge direction, and (c) along the end direction. (d) The sheared sample geometry and (e) a schematic illustration of molecular arrangement in the crystal. In Figures 4b and 4c, the inserts are the enlarged 2D WAXD patterns in the low 2θ-angle region. Table 1. Transition Temperatures and Transition Enthalpies for the BPCA-Cn-PmOH Series n

transition temperature in °C (transition enthalpy in kJ/mol) measured at 2.5 °C/min during cooling

6 7 8 9 10

Km 165 (17.1) SmI 193 (0.8) SmC 224 (0.7) SmA 227 (1.5) N 229 (11.2) I Kt 85 (8.1) Ktx 131 (1.3) Kt1 145 (9.7) SmF 175 (0.4) SmC 210 (0.6) SmA 214 (1.0) N 218 (11.1) I Km 158 (17.4) SmI 194 (1.3) SmC 215 (0.7) SmA 220 (1.3) N 222 (11.0) I Kt 82 (8.7) Ktx 118 (0.2) Kt1 146 (9.4) SmF 179 (0.6) SmC 210 (0.6) SmA 213 (0.8) N 214 (11.7) I Km 125 (18.0) Kmx 148 (4.0) Km1 154 (6.5) SmI 191 (1.4) SmC 212 (0.7) SmA 214 (1.2) N 215 (11.9) I

be constructed from more than one molecule. Moreover, this compound forms a head-to-head dimer via the H-bond formation between the carboxylic acid groups of two molecules as previously reported (also see the FT-IR results below).48-53 The length of this head-to-head dimer should be 5.26 nm. Since the layer spacing is 3.73 nm, the dimers in the layer must be tilted by an angle of 45° with respect to the layer normal. Figures 4a and 4c show symmetric 2D WAXD patterns for the oriented BPCA-C8-PmOH sample. However, in Figure 4b, the 2D WAXD pattern is asymmetric, and the diffraction peaks in the high 2θ-angle region are tilted 45° counterclockwise from the meridian direction (the layer normal). The diffractions only appear in the second and fourth quadrants of the 2D WAXD pattern. This reveals that all the head-to-head dimers are synclinically clockwise 45°tilted toward the shear direction within the layers in real space, as schematically illustrated in Figure 4e. This dimerpacking model for the crystal predicts symmetric 2D WAXD (48) Stepanian, S. G.; Reva, I. D.; Radchenko, E. D.; Sheina, G. G. Vib. Spectrosc. 1996, 11, 123. (49) Kutsumizu, S.; Kato, R.; Yamada, M.; Yano, S. J. Phys. Chem. B 1997, 101, 10666. (50) Dong, J.; Ozaki, Y.; Nakashima, K. Macromolecules 1997, 30, 1111. (51) Torgova, S. I.; Petrov, M. P.; Strigazzi, A. Liq. Cryst. 2001, 28, 1439. (52) Kutsumizu, S.; Yamada, M.; Yamaguchi, T.; Tanaka, K.; Akiyama, R. J. Am. Chem. Soc. 2003, 125, 2858. (53) Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy, 2nd ed.; Harcourt Brace College Publishers: Austin, 1996.

Table 2. Experimental and Calculated Crystallographic Parameters of the Km Monoclinic Phase of BPCA-C8-PmOH 2θ (deg)

d-spacing (nm)

hkl

expt

calca

020 100 110 120 001 002 003 024

22.8 16.1 19.7 28.1 2.4 4.7 7.1 24.5

22.8 16.0 19.7 28.0 2.4 4.7 7.1 24.7

expt

calca

0.39 0.55 0.45 0.32 3.73 1.86 1.24 0.36

0.39 0.55 0.45 0.32 3.73 1.86 1.24 0.36

a The calculated data listed are based on the K monoclinic unit cell m with a ) b ) 0.78 nm, c ) 5.27 nm, and β ) 45.0°.

patterns when the X-ray beam is along the through and end directions (as shown in Figures 4a and 4c). Detailed structural analysis indicates that this crystal possesses a monoclinic lattice (Km) with a ) b ) 0.78 nm, c ) 5.27 nm, and β ) 45.0°. Table 2 lists the experimentally observed d spacings and the calculated d spacings based on this unit cell lattice. With two dimers (four molecules) in one unit cell, its calculated crystallographic density is 1.27 g/cm3. The experimentally observed density is 1.26 g/cm3, which fits well with the calculated data. This BPCA-C8-PmOH structural determination is also supported by our SAED results obtained from lamellar crystals. The TEM morphology in Figure 5a shows multiple stacked lamellar crystals. Their SAED single-crystal patterns are shown in Figures 5b and 5c. Both SAED patterns are

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Figure 5. (a) TEM micrograph of the BPCA-C8-PmOH sample, (b) the SAED pattern of the [301] zone, and (c) the SAED pattern of the [001] zone. The [001] zone SAED pattern was obtained by 25°-counterclockwise tilting of the [301] zone sample around the b* axis toward the chain tilting direction.

Figure 6. Set of 2D WAXD patterns of a mechanically sheared BPCA-C9-PmOH sample along three directions: (a) the through direction, (b) the edge direction, and (c) the end direction. In Figures 6b and 6c, the insets are the enlarged 2D WAXD patterns in the low 2θ-angle region (the shear geometry is identical to that shown in Figure 4d).

identified to be the [301] zone and the [001] zone, respectively. The SAED pattern with the [301] zone is observed from the lamellar stack without sample tilting, while the SAED pattern with the [001] zone is observed by tilting the sample 25° counterclockwise around the b* axis toward the chain tilting direction from the [301] zone, which fully agrees with the dimer-packing model presented in Figure 4e. Figures 6a-6c show a set of 2D WAXD patterns for a mechanically sheared BPCA-C9-PmOH sample along the three incident X-ray beam directions at room temperature (illustrated in Figure 4d). These observations again show that the diffraction at 2θ ) 2.23° (d spacing of 3.96 nm) is attributed to the layer structure caused by the head-to-head BPCA-C9-PmOH dimers. The layer structure normal in Figure 6a is parallel to the X-ray beam when it is along the through direction. Since the dimer length is 5.46 nm and the layer spacing is 3.96 nm, these head-to-head dimers in the layer have to be tilted by 43.5° with respect to the layer normal direction. In Figure 6b, an asymmetric 2D WAXD pattern for the oriented BPCA-C9-PmOH sample is also observed, which is similar to the case of BPCA-C8-PmOH (Figure 4b). This reveals that all the head-to-head dimers again possess a synclinic packing in the crystalline phase, and that the chain is tilted 43.5° from the layer normal direction. The crystal structure of BPCA-C9-PmOH was, however, determined to be a large triclinic lattice (Kt) with a ) 2.18 nm, b ) 1.11 nm, c ) 4.02 nm, R ) 80.5°, β ) 89.6°, and γ ) 102.0°. Table 3 lists the experimentally observed d spacings and the

Table 3. Experimental and Calculated Crystallographic Parameters of the Kt Triclinic Phase of BPCA-C9-PmOH 2θ (deg)

d-spacing (nm)

hkl

expt

calca

expt

calca

020 100 1h20 120 200 2h20 220 300 3h20 320 400 420 500 5h20 001 1h01 002 1h02 003 1h03 5h23 004 1h04 3h24 3h25 5h25

16.4 4.2 16.2 17.5 8.3 16.8 20.1 12.5 18.3 22.7 16.7 26.0 20.9 23.5 2.2 4.8 4.5 6.3 6.7 7.9 24.3 8.9 10.1 19.4 20.5 25.6

16.5 4.2 16.2 17.8 8.3 16.9 20.0 12.5 18.5 22.8 16.7 26.0 20.9 23.8 2.2 4.8 4.5 6.2 6.7 8.0 24.2 8.9 10.0 19.5 20.4 25.5

0.54 2.13 0.55 0.51 1.06 0.53 0.44 0.71 0.48 0.39 0.53 0.34 0.43 0.38 3.96 1.84 1.98 1.41 1.32 1.12 0.37 0.99 0.88 0.46 0.43 0.35

0.54 2.13 0.55 0.50 1.06 0.53 0.44 0.71 0.48 0.39 0.53 0.34 0.43 0.37 3.96 1.84 1.98 1.42 1.32 1.10 0.37 0.99 0.88 0.45 0.43 0.35

a The calculated data listed are based on the K triclinic unit cell with t a ) 2.18 nm, b ) 1.11 nm, c ) 4.02 nm, R ) 80.5°, β ) 89.6°, and γ ) 102.0°.

calculated d spacings based on this unit cell lattice. The calculated crystallographic density is 1.27 g/cm3 based on

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Figure 7. (a) TEM micrograph of the BPCA-C9-PmOH sample and the SAED patterns of the [031h] zone (b) and of the [001] zone (c). The [001] zone was obtained by tilting the sample 48°-counterclockwise around the [1h10] axis toward the chain tilting direction.

Figure 8. Set of 2D WAXD patterns on mechanically sheared BPCA-C8-PmOH sample in the SmC LC phase at 200 °C along three directions: (a) the through direction, (b) the edge direction, (c) the end direction, (d) the azimuthal scan of the 2θ-angle between 19° and 22° in the through direction 2D WAXD pattern, and (e) a schematic illustration of the molecular arrangement. In Figures 8b and 8c, the inserts are the enlarged 2D WAXD patterns in the low 2θ-angle region (the shear geometry is identical to that shown in Figure 4d).

the lattice dimensions with 8 dimers in one unit cell. The experimentally observed density is 1.25 g/cm3, which fits well with the calculated data. Our SAED results also support the determined BPCAC9-PmOH crystal structure. In Figure 7a, stacked layers of lamellae can be observed. The SAED pattern shown in Figure 7b is without tilting and is identified to be the [031h] zone. The [001] zone can be obtained by tilting the sample 48° counterclockwise around the [1h 10] axis toward the chain tilting direction from the [031h] zone as shown in Figure 7c. In this case, the electron beam enters along the dimer chain direction. Detailed structural parameters for this series of BPCA-Cn-PmOH are listed in Table 4. It is evident that the crystal structures show an odd-even effect, in which the compounds having n ) odd numbers have the large triclinic lattices, while those with n ) even numbers possess small monoclinic lattices. Some of these compounds also show more than one crystalline phase above room temperature and are modifications of the crystal structures listed in Table 4.

Table 4. Crystal System, Axial System, and Density of BPCA-Cn-PmOHs at the Room Temperature axial system no. of n dimers 6 7 8 9 10

2 8 2 8 2

crystal system monoclinic triclinic monoclinic triclinic monoclinic

density

a b c R β γ (nm) (nm) (nm) (deg) (deg) (deg) expt calc 0.77 2.18 0.78 2.18 0.73

0.74 1.11 0.78 1.11 0.74

4.92 3.71 5.27 4.02 7.06

90.0 83.5 90.0 80.5 90.0

44.5 90.0 1.27 1.33 89.4 105.7 1.25 1.27 45.0 90.0 1.26 1.27 89.6 102.0 1.25 1.27 40.5 90.0 1.22 1.24

Identifications of Liquid Crystal Structures. Above their crystal melting temperature, BPCA-Cn-PmOHs undergo multiple LC phases with increasing temperature. For example, in BPCA-C8-PmOH, there are four LC phases between the crystal and I phase. The LC phase appearing between 222 °C and 220 °C is a N phase. The LC phase between 220 °C and 215 °C can be identified as a SmA phase since the low 2θ-angle reflection at 1.72° (d spacing of 5.14 nm) is the layer diffraction, which is close to the length of

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Figure 9. Set of 2D WAXD patterns of a mechanically sheared BPCA-C8-PmOH sample in the SmI LC phase at 170 °C along three directions: (a) the through direction, (b) the edge direction, (c) the end direction, (d) azimuthal scan of 2θ-angle between 19° and 22° in the through direction 2D WAXD pattern, and (e) a schematic illustration of the molecular arrangement in the layer. In Figures 9b and 9c, the inserts are the enlarged 2D WAXD patterns in the low 2θ-angle region (the shear geometry is identical to that shown in Figure 4d).

the dimer. In addition, the high 2θ-angle scattering halo suddenly shifts toward a higher 2θ angle. As the temperature decreases, another LC phase can be found between 215 °C and 194 °C. Figures 8a-8c show a set of 2D WAXD patterns at 200 °C along the three incident X-ray beam directions (as shown in Figure 4d). Figure 8a shows a scattering halo centered at 2θ ) 19.6° (d spacing of 0.453 nm), exhibiting a lateral liquidlike ordering within the layer. Its correlation length is ∼1.5 nm, as estimated by the Scherrer equation. No low 2θ-angle diffractions are observed. However, in Figures 8b and 8c, a pair of low 2θ-angle diffractions at 2θ ) 2.0° (d spacing of 4.42 nm) can be clearly identified with a second-order diffraction at 2θ ) 4.0°. On the other hand, a pair of scattering halos is located in the second and fourth quadrants in Figure 8b, which illustrates the 33°-clockwise dimer tilting with respect to the layer normal as shown in Figure 8e. This structure can thus be identified as a synclinically tilted SmC phase. On the basis of the tilting angle (33°) and the measured layer thickness (4.42 nm), the length of dimer in the layer of this SmC phase is calculated to be 5.25 nm, which agrees with the calculated length of the dimer (5.26 nm). Another LC phase is generated below the weak exothermic process at 194 °C. It has a significantly sharpened scattering halo in the high 2θ-angle region. Figures 9a-9c exhibit a set of 2D WAXD patterns at 170 °C along the three incident X-ray beam directions. Figure 9a shows three pairs of scattering halos centered at 2θ ) 20.0° (d spacing of 0.444 nm). One is on the meridian and two pairs are located at (50° away from the meridian. They can also be identified by an azimuthal scan of the 2θ-angle as shown in Figure 9d. No low 2θ-angle diffraction pairs can be observed in Figure 9a. In Figures 9b and 9c, the low 2θ-angle diffraction

at 1.98° (d spacing of 4.46 nm) can be identified with a second-order diffraction at 3.96°. A pair of scattering halos is located at 32°-counterclockwise to the meridian in the second and fourth quadrants in Figure 9b. The orthogonal head-to-head dimer packing in the layer can thus be identified as a SmI phase.40 Figure 9e schematically illustrates the dimer packing in this phase. Similar LC structural analyses are also carried out for the rest of the oriented BPCA-Cn-PmOHs. For example, with BPCA-C9-PmOH as a representative of the odd-number compounds, there are four LC phases between the crystal melting and the isotropic melt: a N phase between 214 °C and 213 °C, a SmA phase between 213 °C and 210 °C, a SmC phase between 210 °C and 179 °C, and a SmF phase between 179 °C and 146 °C. For identification of the SmF phase of BPCA-C9-PmOH, Figures 10a-10c exhibit 2D WAXD patterns at 165 °C along the three incident X-ray beam directions. Figure 10a shows only a pair of scattering halos centered at 2θ ) 19.8° (d spacing of 0.448 nm). We know that this compound has a triclinic crystal structure (Kt) at room temperature. If the packing in this phase still retains the molecular orientation and certain positional order, the head-to-head dimers in the layer should be tilted, not toward apex, but toward a side of the quasi-hexagonal lattice based on the SmI phase net. Therefore, this phase can be assigned as a SmF phase.40 Overall, the LC phase transformations of the other even-number compounds (n ) 6 and 10) follow the identifications of BPCA-C8-PmOH, while the LC phase transformations in BPCA-C7-PmOH follow those of BPCAC9-PmOH. All of the LC phase identifications are listed in Table 1. Hydrogen Bonding Associations and Molecular Mobility in the Phases. To understand the intermolecular H-

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Figure 10. Set of 2D WAXD patterns of a mechanically sheared BPCA-C9-PmOH sample in the SmF LC phase at 165 °C along three directions: (a) the through direction, (b) the edge direction, and (c) the end direction. In Figures 10b and 10c, the inserts are the enlarged 2D WAXD patterns in the low 2θ-angle region (the shear geometry is identical to that shown in Figure 4d).

bonding between carboxylic acids which form the dimers within the layers, and the H-bonding between hydroxyl groups at the end of dimers in BPCA-Cn-PmOHs, FT-IR measurements of BPCA-C8-PmOH and BPCA-C9-PmOH are shown in Figures 11a and 11b, respectively. The band at 3560 cm-1 can be assigned to the stretching vibration of free O-H groups.53,54 The H-bonded O-H stretching absorption bands appear in the broad 3500 cm-1 to 3200 cm-1 frequency range due to the different types of H-bonds between hydroxy groups at the end of dimers.54 Second, the free CdO stretching absorption in carboxylic acids appears at 1737 cm-1.47-52 The H-bonded CdO stretching absorptions from the H-bonded cyclic dimers are at 1683 cm-1 and 1677 cm-1.49,50 The two different frequencies indicate that the strengths of the H-bonds holding the two acid molecules together are different. By decreasing the temperature from the I state to the highly ordered LC phases in BPCA-CnPmOH (both odd and even numbers), the quantity of free O-H and CdO groups decrease due to the formation of dimers and the H-bonding between hydroxyl groups between the ends of the dimers. The percentage of H-bonded and free -COOH groups in BPCA-C8-PmOH and BPCA-C9-PmOH with respect to temperature is plotted in the insets of Figures 11a and 11b, respectively. The percentage of -COOH groups that form dimers starts to increase about 15 °C above the Ti in both BPCA-C8-PmOH and BPCA-C9-PmOH as reported in the previous studies.49,50 As the temperature is further decreased, the percentages of the H-bonded -COOH groups increase. Below the temperature where the crystalline phase forms (158 °C) in BPCA-C8-PmOH, the O-H stretching absorption of the hydroxyl groups does not change, yet the CdO stretching absorption at 1683 cm-1 shifts down to 1677 cm-1. This could be due to increased H-bond strength in the crystalline phase. In the case of BPCA-C9-PmOH, there are two more crystal-to-crystal transitions at 118 °C and 82 °C, which were already discussed in the DSC (Figure 1a) and WAXD experiments (Figure 2b). During the transition at 118 °C, both the O-H stretching absorption of the hydroxyl groups and the CdO stretching absorption change little, yet the intensities sharpen due to a H-bond population shift. In addition to the sudden intensity increase of the H-bonded CdO at 1677 cm-1 below 82 °C, the H-bonded (54) Huggins, K. E.; Son, S.; Stupp, S. I. Macromolecules 1997, 30, 5305.

-OH functions appear at 3200 cm-1. This could be a factor in the decrease of the c-axis in the Kt crystal lattices. Conformations of alkoxy chains and mobility of each carbon in both the phenyl and biphenyl groups in BPCAC8-PmOH (Figures 12b and 12c, and the inset of Figure 12c) and BPCA-C9-PmOH (Figure 13a and 13b, and the inset of Figure 13b) were studied using solid-sate 13C NMR CP/ MAS/DD and Bloch decay methods at different temperatures. The CP/MAS/DD method (Figures 12b and 13a) is sensitive to the rigid components, and the mobile components can easily be detected by the Bloch decay method (Figures 12c and 13b).55-59 Chemical shift identifications are carried out by both the solution 13C NMR (Figure 12a) and theoretical calculations based on the tabulated data.53 The calculated values agree well with those actually measured. The chemical shift at 172 ppm results from the carbon atoms in the carbonyl group, and chemical shifts above 100 ppm come from the carbon atoms in the phenyl and biphenyl groups. In addition, the 13C chemical shift at 68 ppm represents the carbon atoms at the R position, and the β and γ carbon atoms appeared at 31 and 27 ppm, respectively. Furthermore, the β carbon chemical shift in the Bloch decay method (insets of Figures 12c and 13b) consists of two values resulting from all trans sequences (31.3 ppm) and gauche/trans random sequences (30.7 ppm) of the alkoxyl chains. In the low-ordered SmC phase of BPCA-C8-PmOH (Figures 12b and 12c) above 194 °C, the mobility of all the carbon atoms including those in the carbonyl and the biphenyl groups is liquidlike. Furthermore, the percentage of the all trans conformation of the β carbons in the alkoxyl chains of the dimers is about 50%, as shown in the inset of Figure 12c. When the temperature is decreased to the highordered SmI phase, the carbon atoms in biphenyls and in H-bonded carbonyl group are frozen, while the mobility of the carbon atoms in the alkoxyl chains and in the phenyls at the ends of the head-to-head dimers remains. Additionally, (55) Cheng, J.; Jin, Y.; Wunderlich, B.; Cheng, S. Z. D.; Yandrasits, M. A.; Zhang, A.; Percec, V. Macromolecules 1992, 25, 5991. (56) Cheng, J.; Yoon, Y.; Ho, R.-M.; Leland, M.; Guo, M.; Cheng, S. Z. D.; Chu, P.; Percec, V. Macromolecules 1997, 30, 4688. (57) Ge, J. J.; Guo, M.; Zhang, Z.; Honigfort, P. S.; Mann, I. K.; Wang, S. Y.; Harris, F. W.; Cheng, S. Z. D. Macromolecules 2000, 33, 3983. (58) McElheny, D.; Grinshtein, J.; Frydman, V.; Frydman, L. Macromolecules 2002, 35, 3544. (59) Ishida, H.; Horii, F. Macromolecules 2002, 35, 5550.

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Figure 11. FT-IR spectra of BPCA-C8-PmOH (a) and BPCA-C9-PmOH (b) between 3650 and 3100 cm-1 and between 1770 and 1650 cm-1 at a scan rate of 2.5 °C/min. Two insets are the percentages of the dimeric -COOH of BPCA-C8-PmOH (in Figure 11a) and BPCA-C9-PmOH (in Figure 11b) at different temperatures at 1687 cm-1 vs the percentage of monomeric -COOH at 1732 cm-1. The vertical dash lines are indications of the thermal transitions based on the DSC cooling results.

the all trans conformation of the β carbons in the alkoxyl chains of the dimer increases to above 60% as shown in the inset of Figure 12c. After crystallization of BPCA-C8-PmOH, all the carbon atoms are frozen in the solid, leaving only vibrational motion. The percentage of β carbons in the all trans conformation in the alkoxyl chains in Km crystalline phase is above 90% (inset of Figure 12c). The carbon atom mobility and the conformation of the alkoxyl chains in all the ordered phases of BPCA-C9-PmOH (including the lowordered SmC, the high-ordered SmF, and the Kt crystalline phases) are similar to those in BPCA-C8-PmOH. The mobility of the carbon atoms and the conformations of the alkoxyl chains in the two modified Kt phases of BPCA-C9PmOH are between those of the SmF and the Kt as shown

Jeong et al.

Figure 12. (a) Solution 13C NMR spectrum and solid-state 13C NMR spectra of BPCA-C8-PmOH during cooling from 200 °C to room temperature: (b) CP/MAS/DD spectra and (c) Bloch decay spectra. The inset is the expanded solid-state 13C NMR Bloch decay spectra of BPCA-C8-PmOH (in Figure 12c) between 35 and 25 ppm. The two vertical dashed lines indicate approximately the chemical shifts for the β carbon atoms in the all trans sequences and gauche/trans random sequences of the alkoxyl chains.

in Figures 13a and 13b. The percentage of β carbons in the all trans conformation in the alkoxyl chains in these two modified Kt phases of BPCA-C9-PmOH are about 65% and 75%, respectively (the inset of Figure 13b). Liquid Crystalline Textures and Helical Morphologies. The phase identifications in Table 1 can also be supported by the observation of LC texture changes in PLM. Figures 14a-14f show changes in the optical textures of BPCA-C8PmOH in PLM within a polyimide-coated and rubbed sandwich-type cell. Upon cooling at the rate of 2.5 °C/min,

Achiral 4-Biphenyl Carboxylic Acid Compounds

Figure 13. Solid-state 13C NMR spectra of BPCA-C9-PmOH during cooling from 190 °C to room temperature: (a) CP/MAS/DD spectra and (b) Bloch decay spectra. The inset is the expanded solid-state 13C NMR Bloch decay spectra of BPCA-C9-PmOH (in Figure 13b) between 35 and 25 ppm. The two vertical dashed lines indicate approximately the chemical shifts for β carbon atoms in the all trans sequences and gauche/trans random sequences of the alkoxyl chains. A solution 13C NMR spectrum of BPCAC9-PmOH is identical to that of BPCA-C8-PmOH (Figure 12a).

the transition from the I melt to a low-ordered N phase occurs at 222 °C as exhibited by the birefringent N droplets (Figure 14a). Decreasing from 222 °C, these small N droplets start to merge, and some of them break and form lines (Figure 14b). These lines always possess double birefringent stripes, and they grow along the thread long axis while the width of the thread remains unchanged. Sometimes, the threads can laterally combine with others, becoming streaks. When the temperature reaches 218 °C, a fully developed oily streak texture (cylinders) with Myelin-figure (SmA) can be observed (Figure 14c). This phase is fairly stable at this temperature and is independent of the cooling history from the I melt, although the thread length is shorter when formed with a faster cooling rate. The backgrounds so far are always in a dark homeotropic state, similar to the case in the B7 phase of nitro-substituted banana-shaped molecules28,29 and

Chem. Mater., Vol. 17, No. 11, 2005 2863

the SmA phase in lyotropic LC molecules.60 When the temperature decreases to below the weak, broad SmA T SmC transition (215 °C), the cylinders start to twist along the long axis (Figures 14d and 15a). Both right- and lefthanded helical structures are observed, which reflects the racemic nature of achiral molecules.10,11,14 Note that BPCAC8-PmOH has neither a strong bend angle at the center of the dimers unlike conventional banana-shaped molecules28-30 nor configurational chirality in its chemical structure unlike conventional SmC* LC compounds.10-13,15-23 There is a possibility that the helical suprastructures observed are only due to a periodic change of optical birefringence along the long axis of the oily streaks (Figure 15a). Figures 15b-15d shows the observation of twists under AFM (Figure 15b), SEM (Figure 15c), and PCM (Figure 15d), respectively. It is evident that the optical birefringent twists shown in Figure 15a do represent 3D helical suprastructures. Since the samples were prepared by quenching the helical suprastructure grown in a one-free surface cell into an ice-water mixture, some parts of helical suprastructures were destroyed during the quenching. With further decreasing of the temperature, the helical suprastructures become broken and interspersed with granular textures developing from the dark background (Figure 14e). Finally, when the temperature enters the crystallization region, the residual helical suprastructures overlap with the marbled texture (Figure 14f). All the samples of BPCA-Cn-PmOH (n ) 6-10) develop the similar helical suprastructures as shown for BPCA-C8PmOH. Therefore, the odd-even effect of the methylene units and the number of carbon atoms in the BPCA-CnPmOHs are not the main cause of helical suprastructure formation. To understand the origin of these helical suprastructures in achiral BPCA-Cn-PmOHs, computer calculation of the conformational minimum free energy in the head-to-head BPCA-C8-PmOH dimer in the isolated vacuum state was carried out using Cerius2 4.6 software. The results are shown in Figures 16a and 16b. First, the monomer global equilibrium geometry of BPCA-C8-PmOH was constructed at 0 K using the COMPASS force field. The energy-minimized monomers were then used to construct the head-to-head dimer (Figure 16a). In this calculation the length of the H-bonding between the H-acceptor (O at the carbonyl) and the H-donor (H at the carboxylic acid) was 0.17 nm and the angle O-H‚‚‚O was 171°. The attractive interaction energy is ∼20 kJ/mol.61,62 In the head-to-head dimer minimal energy geometry of BPCA-C8-PmOH (Figure 16a), the three rings at the center of the dimer are on the same plane to form a mesogen. The axis of the alkoxy chains and substituted phenyls at the ends of the dimer are tilted 30° with respect to the long axis of mesogen at the center of the dimer. The alkoxy chain with zigzag conformations, the substituted phenyl at the ends of dimer, and the phenyl ring at the end of mesogen in the dimer are on another plane. These two planes outside of the dimer intersect at an angle of (39.5° (60) Tschierske, C. J. Mater. Chem. 1998, 8, 1485. (61) Gavezzotti, A. J. Mol. Struct. 2002, 615, 5. (62) Reddy, L. S.; Nangia, A.; Lynch, V. M. Cryst. Growth Design 2004, 4, 89.

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Figure 14. Set of PLM textures of the BPCA-C8-PmOH sample between polyimide-coated/rubbed glasses during cooling at a rate of 2.5 °C/min at different temperatures: (a) 221 °C, (b) 220 °C, (c) 218 °C, (d) 212 °C, (e) 170 °C, and (f) 140 °C.

with respect to the plane at the center of the dimer. If the rotational direction of these three planes in the dimer are along the same direction, two enantiomeric conformers (axial chiral molecules) can be constructed like twisted biphenyls10,63,64 as shown in Figure 16b. It is worthwhile to note the effect of the substituted phenyls at both ends of the dimers on the formation of the helical suprastructures. So far, none of the dimers created by achiral biphenyl carboxylic acids without the two substituted phenyls at both ends have been reported to exhibit the helical suprastructure in the low-ordered SmC phase. Based on competing chiral symmetry-breaking theories,12,13 the origin of the helical suprastructures (belonging to the phase chirality) do not originate from intermolecular interactions of chiral centers as in the cases of chiral molecules, but from a collective tilting of the molecules with respect to the layer (63) Lunkwitz, R.; Tschierske, C.; Langhoff, A.; Gieβelmann, F.; Zugenmaier, P. J. Mater. Chem. 1997, 7, 1713. (64) Vizitiu, D.; Lazar, C.; Halden, B. J.; Lemieux, R. P. J. Am. Chem. Soc. 1999, 121, 8229.

normal.24,25 Therefore, both the twisted dimeric conformations and the two substituted phenyls at the ends of the dimer are critically important to form the helical suprastructures in this series of compounds. The transition from the cylindershaped texture in SmA to a helix in SmC is speculated to involve a physical mechanism involving the molecular tilting. If the long axis of the dimers in the cylinders is oriented parallel to the cylinder surface normal in the SmA phase and it tilts along the long axis of the cylinder when entering the SmC phase, the cylinder elongates. This would lead to a reduction in the cylinder diameter. Additionally, the cylinder length is fixed between two neighboring junction points. The helix may then form containing the twisted dimers. The chemical and physical origins of the helical supramolecular structure formation are currently under investigation using WAXD and PLM. Conclusion A series of achiral 4-biphenyl carboxylic acid molecules (BPCA-Cn-PmOH) connected with alkoxyl chains having

Achiral 4-Biphenyl Carboxylic Acid Compounds

Figure 15. Helical morphology of the BPCA-C8-PmOH sample on the one-free surface glass substrate: (a) PLM at 218 °C, (b) atomic force microscopy tapping mode image, (c) scanning electron microscopy, and (d) phase contract optical microscopy.

various carbon numbers (n ) 6-10) and terminated by substituted phenyls with hydroxyls at the meta-positions exhibit a rich set of phase behaviors based on the DSC measurements. The phase transitions in this series of BPCACn-PmOH show a clear odd-even effect. All of these compounds possessed low-order and high-order LC phases as well as crystalline phases. The structure of these phases was characterized and identified with WAXD and SAED. The crystalline phase structures also possess an odd-even effect. It was found that head-to-head dimers are the basic building blocks for constructing these LC and crystalline phases. FT-IR and solid-state 13C NMR experimental results give evidence of the dimer formation and segmental mobility in these compounds at different temperatures. In particular, this series of compounds presents 3D helical suprastructures which are developed in the SmC phase based on our morphological observations with PLM, PCM, SEM, and AFM techniques. The helical suprastructures observed do not depend on the number of carbons in the alkoxyl chains of the BPCA-Cn-PmOHs. The computer calculations suggest that the head-to-head dimer building blocks may twist to

Chem. Mater., Vol. 17, No. 11, 2005 2865

Figure 16. The head-to-head dimer minimal energy geometry of BPCAC8-PmOH in the isolated gas phase at absolute zero Kelvin using Cerius2 4.6: (a) the calculated dimer length of the right-handed head-to-head dimer geometry and (b) the two enantiomeric conformers of the head-to-head dimer of BPCA-C8-PmOH.

minimize their energy. These twisted dimers then fit into a layered structure in the SmC phase. Both the twisting of dimers and the substituted phenyls at the ends of the dimers are critically important to form the helical suprastructures in this system. This new self-assembly of achiral molecules will open a new route toward the construction of helical supramolecular structures through the noncovalent H-bonding interactions between biphenyl carboxylic acid compounds and others. The optical properties of BPCA-Cn-PmOH and the helical supramolecular structure formation mechanisms are currently being investigated. Acknowledgment. This work was supported by NSF DMR0203994 and 0516602. We also acknowledge that the DSC experiments in this paper were carried out on a Perkin-Elmer PYRIS Diamond DSC that has been setup in our laboratory by Perkin-Elmer Co. CM050338Y