Article pubs.acs.org/JPCB
Stable Ferroelectric Liquid Crystals Derived from SalicylaldimineCore Bhyranalyar N. Veerabhadraswamy, D. S. Shankar Rao, and Channabasaveshwar V. Yelamaggad* Centre for Nano and Soft Matter Sciences, Prof. U. R. Rao Road, Post Box No. 1329, Jalahalli, Bangalore 560 013, India S Supporting Information *
ABSTRACT: Five pairs of enantiomers derived from salicylaldimine-core have been prepared by condensing (R)- or (S)-4-(octan-2-yloxy)anilines with 4-formyl-3hydroxyphenyl 4-(n-alkoxy)benzoates. They have been designed to probe the correlation between molecular structure and mesomorphism, and especially to provide stable mesogens having potential for applications in ferroelectric liquid crystal devices. Thus, they have been substituted with a chiral tail at one end and by n-alkoxy chains of varying length at the other terminal. A detailed study confirms an indistinguishable behavior of all ten mesogens exhibiting an enantiotropic chiral smectic C (SmC*) phase besides blue phase (BP) and chiral nematic (N*) phase. The SmC* phase occurring over a 50−70 °C thermal width shows ferroelectric switching with spontaneous polarization (Ps) value crossing over 100 nC/cm2. Circular dichroism spectroscopic study of the mesophases confirms the chromophores of the molecules being in the macroscopic chiral (helical) environment.
1. INTRODUCTION One of the most fascinating themes of the contemporary research involves bestowing molecular chirality and investigating its effect on the self-organization property of materials.1 Such interest stems from the fact that the chirality, which enables the spontaneous mirror symmetry-breaking of molecules, generally stimulates molecular self-assembly into highly ordered mesoscopic- /or macroscopic-structures that hold great promise for many areas of basic science and applied research. The design and facile synthesis of optically active (enantiopure) liquid crystals (LCs) bearing asymmetric carbon atom/s, called chiral (mesogens) LCs, represent one such innovative area;2−29 notably, here the imbued molecular chirality generally favors a macroscopic twist in the organization of the constituent mesogens leading to complex ordered LC phases, which includes frustrated2−17 and helical phases.2,3,18−29 Indeed, chiral LCs also stabilize layered phases.20 In fact, these exotic and highly organized LC phases can also be realized effectively following an alternative protocol where chiral guest molecules (dopants, which need not necessarily be LC) are dissolved in the host achiral LC phases.2,3,11,30−34 The examples of frustrated LC phases, which are also called defect phases, include blue phases (BPs)2−13 and twist grain boundary (TGB)14−17 phases where the structural frustration, originating from the competition between the chiral forces and packing topology, is eventually relived by the formation of lattice of defects. They generally exist over a narrow thermal regime. Blue phases (BPI, BPII, and BPIII) occur between the N*/smectic (Sm) phase and the isotropic liquid (I) phase, whereas the TGB (TGBA, TGBC, and TGBC*) phases are seen during the phase transition from the isotropic liquid/chiral nematic (N*) to smectic A (SmA)/chiral smectic (SmC*) © 2015 American Chemical Society
phases. While the TGB phases, being the LC analogues of the Abrikosov phases, have attracted intense scientific interest, the BPs hold potential for next-generation display and photonics device applications; consequently, there have been some successful attempts lately to widen the thermal range of such phases.11−13 On the other hand, the N* and SmC* phases are the classical examples of defect-free helical LC structures exhibiting a wide range of unique and fascinating phenomenon. For example, the N* phase shows temperature-dependent selective reflection (thermochromism), and thus, it is used in thermochromic devices.2,3,35,36 The SmC* phase shows ferroelectric switching, which is being well-used in realizing advanced display devices featuring high image quality, high resolution, wide-viewing angles, and fast response time.2,3,35−37 In essence, chiral LCs exhibit a dizzying array of special properties and structures that have great significance in the fields of both fundamental research and applied science. In view of such a marked thermal behavior of chiral LCs, the authors have been engaged for more than a decade in the rational design, synthesis, and characterization of a vast array of optically active LCs,30−54 including polymers.55 The experimental results of these investigations have clearly demonstrated the inherent capability of such chiral LCs in stabilizing a variety of novel phase sequences and frustrated/technologically important LC phases over a wide thermal range. The present work described herein relates to the design, preparation, and evaluation of LC and chiroptical properties of five enantiomeric pairs of three-ring calamatics derived from the Received: January 8, 2015 Revised: March 3, 2015 Published: March 3, 2015 4539
DOI: 10.1021/acs.jpcb.5b00209 J. Phys. Chem. B 2015, 119, 4539−4551
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The Journal of Physical Chemistry B Scheme 1. Synthesis of Three-Ring Rodlike LCs Derived from Salicylaldimine-Corea
a
Reagents & conditions: (i) (S)-(+)-2-octanol, Ph3P, DIAD, THF, rt, 18 h; (ii) H2, (1 atm, ballon), 10% Pd−C, THF, rt, 4 h. (iii) (R)-(−)-2Octanol, Ph3P, DIAD, THF, rt, 18 h; (iv) n-bromodecane, anhydrous K2CO3, DMF, 80 °C, 12 h; (v) Jone’s reagent, acetone, rt, 1 h; (vi) 2,4Dihydroxybenzaldehyde, DCC, DMAP, THF, rt, 4 h; (vii) EtOH, AcOH, ref lux, 2 h.
placing asymmetric carbon atom (stereocenter) closer to the rigid core; such a substitution improves the magnitude of polarization due to hindered rotation of the chiral moiety. The general molecular structure of the target mesogens, incorporating the aforesaid features, is shown in Scheme 1.
salicylaldimine-core. The target molecular architecture with three benzene rings linked in an end-to-end manner has been especially chosen, owing to its inherent ability to support the stabilization of smectic phase(s).2,3,20−29,56−65 The mesogens derived from an electron rich salicylaldimine-core have attracted an enormous amount of research attention owing to their sturdiness to heat and moisture, ease of derivatization and synthesis, and remarkable thermal properties. Specifically, this core has been incorporated in the fabrication of mesomorphic compounds ranging from conventional LCs viz., calamitics,66−71 discotics,72,73 and polymers74 to nonconvetional LCs such as dimers,74 hexacatenars,67,68 and banana-shaped LCs.78−82 Notably, their appealing LC behavior has been attributed to the presence of salicylaldimine segment. Thus, in the present work, a salicylaldimine-core in the three-ring molecular architecture has been incorporated with the rationale that the resultant LCs ought to be displaying interesting LC behavior with the thermal stability and hydrolytic resistance. Furthermore, it was deliberated to join the salicylaldimine-core to a (n-alkoxy)benzene ring via an ester (−COO−) linkage as it induces, similar to an imine (−CHN−) group, lateral polarity quite efficiently;2,3,20−29,56−65 here, it was planned to vary the length of n-alkoxy tail from n-octyloxy to n-dodecyloxy to explore its effect on the phase transitional behavior of the compounds. Importantly, the insertion of chiral tails, such as (R)-2-octyloxy and (S)-2-octyloxy chains, in the molecular design not only accounts for the molecular chirality but helps in
2. RESULTS AND DISCUSSION 2.1. Synthesis and Molecular Structural Characterization. The proposed mesogenic compounds were prepared in excellent yields and fully characterized with the help of UV− vis, FTIR, NMR, MS, and elemental analyses (see Experimental Section for details). Scheme 1 illustrates the stepwise synthesis of intermediates and target salicylaldimine-based mesogens. The essential amines, (R)- and (S)-4-(octan-2-yloxy)anilines (2 and 4) were prepared starting from commercially available 4nitrophenol; it was subjected to Mitsunobu O-alkylatation83 with (S)- and (R)-2-octanol to get (R)- and (S)-4-(octan-2yloxy)nitrobenzenes (1 and 3),84,85 which upon reduction by catalytic hydrogenation using H2 balloon (1 atm) and Pd/C (10%) in THF afforded the corresponding anilines 2 and 4.84,85 4-(n-Alkoxy)benzaldehydes (5a−5e),45,46,52,54 readily prepared by Williamson ether synthesis with 4-hydroxybenzaldehyde and n-bromoalkanes, were subjected to Jones oxidation giving the required 4-(n-alkoxy)benzoic acids (6a−6e).86 These acids 6a− 6e were subsequently reacted with 2, 4-dihydroxybenzaldehyde in the presence of N,N′-dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino)pyridine (DMAP)87 to obtain 4-formyl-34540
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The Journal of Physical Chemistry B hydroxyphenyl 4-(n-alkoxy)benzoates (7a−e). In the concluding step, the salicylaldehydes 7a−7e and the amines 2 and 4 were refluxed together in the presence of a solvent, absolute ethanol, and a catalytic amount of acetic acid to get the final mesogens. The characterization data obtained for all the intermediates and target mesogens were found to be in complete agreement with their proposed structure (see Experimental Section and Supporting Information for details). The UV−Vis spectra of all ten salicylaldimines, recorded for their dilute solutions (∼3 × 10−3 M; cell length = 1 mm) in spectroscopic grade dichloromethane, were found to be nearly the same comprising two broad but intense peaks at ∼277 and ∼355 nm due to π−π* and n-π* transitions, respectively. FTIR (KBr) spectra of the mesogens showed characteristic CO and CN stretching vibration bands in the regions of 1719− 1727 and 1622−1624 cm−1, respectively. 1H NMR spectra measured in dilute CDCl3 solutions at room temperature exhibited well-resolved signals of aromatic and other nonaromatic protons. It is worth mentioning that the spectra showed two typical singlets at δ 8.6 and δ 13.8 resonating due to imine and phenolic protons, respectively. It may be noted here that the signal of phenolic-proton appears in the same downfield-position (δ 13.8) regardless of the variation made in the concentration of the mesogens. This is expected given the fact that the chemical shift of protons participating in the strong intramolecular H-bonding is nearly independent of the concentration88 and such protons are deshielded, relative to TMS, as they become more positive in their electronic nature causing their signal to shift to a downfield.88 The 13C NMR spectra of the mesogens recorded in CDCl3 confirmed the structure of the basic carbon skeleton. Each spectrum, downfield from the CDCl3 peaks (a 1:1:1 triplet at δ = 77), showed the expected 16 signals, at ca δ = 164.4, 163.7, 162.5, 159.4, 157.7, 154.5, 140.9, 132.7, 132.4, 122.3, 121.3, 117.4, 116.6, 114.4, 113, and 110.6, belonging to sp2-hybridized carbon atoms. Among these, the farthest downfield peak with the chemical shift of 164.4 ppm can be assigned to the most deshielded carbonyl-carbon. While the oxymethylene (−OCH2−) and oxymethine (−OCH−) carbons resonated at 74.4 and 68.4 ppm, respectively, the sp3-hybridized carbons of methylene and methyl groups of paraffinic tails appeared in the region of 37 to 14 ppm. Thus, the spectral data coupled with elemental analyses established molecular structure of the target mesogenic compounds indubitably. 2.2. Evaluation of Liquid Crystallinity. 2.2.1. Optical Microscopic and Calorimetric Studies. Five pairs of enantiomers synthesized were examined in detail for their mesomorphic behavior mainly with the aid of thermal polarized light optical microscopy (POM) and differential scanning calorimetry (DSC). Powder X-ray diffraction (XRD) measurements and electrical switching studies were also carried out to elucidate the mesophase structure of some representative compounds. DSC and POM experimental results complemented each other and not only evidenced the identical thermal behavior but also confirmed remarkable thermal and hydrolytic stability of the mesogens. Table 1 gives the summary of the phase transitional behavior of the compounds synthesized; the sequences and transitions temperatures mentioned therein have been deduced from both optical and calorimetric studies. The enthalpies of transitions presented have been derived from DSC traces of the first heating−cooling cycles at a rate of 5 °C/min. When the thermal signatures were not observed in DSC thermograms, the transition temperatures
Table 1. Phase Sequences, Phase Transition Temperatures (°C)b and Associated Enthalpies (kJ/mol−1) of Transitions of Three-Ring Rodlike SA(S)-n and SA(R)-n Compoundsa phase sequence compounds SA(S)-8 SA(R)-8 SA (S)-9 SA (R)-9 SA (S)-10 SA (R)-10 SA (S)-11 SA (R)-11 SA (S)-12 SA (R)-12
heating
cooling
Cr 81.2 (24.6) SmC* 107.4 (2.1) N* 148.6 (1.2) I Cr 83.2 (23.5) SmC* 107.6 (2.0) N* 149.0 (1.3) I Cr 86.3 (33.4) SmC* 113.4 (2.5) N* 145.4 (1.0) I Cr 85.5 (25) SmC* 113.2 (2.4) N* 145.2 (0.9) I Cr 90.7 (24.8) SmC* 117.1 (2.0) N* 144.4 (0.8) I Cr 92.1 (31.0) SmC* 117.9 (2.1) N* 144.5 (0.9) I Cr 80.3 (29.5) SmC* 120.4 (2.7) N* 141 (1.1) I Cr 80.9 (28.3) SmC* 120.2 (2.7) N* 141.1 (1) I Cr 68.4 (24.4) SmC* 122.7 (2.6) N* 139.6 (1.1) I Cr 66.9 (23.3) SmC* 121.8 (2.5) N* 138.8 (0.9) I
I 148.2 BP 147.9 (1.0) N* 106.5 (2) SmC* 52.6 (20.3) Cr I 148.3 BP 148.0 (1.0) N* 106.5 (1.9) SmC* 54.1 (19.5) Cr I 144.9 BP 144.5 (1.1) N* 112.4 (2.5) SmC* 59.1 (25.5) Cr I 144.8 BP 144.3 (1.0) N* 112.3 (2.3) SmC* 59.9 (14.7) Cr I 143.1 BP 142.6 (0.8) N* 116.2 (2.3) SmC* 62.2 (20.1) Cr I 143.4 BP 142.4 (0.8) N* 116.8 (2.2) SmC* 61.5 (23.5) Cr I 140.4 BP 140.0 (1.1) N* 119.4 (2.8) SmC* 48.1 (22.1) Cr I 140.6 BP 140.3 (1.1) N* 119.5 (2.8) SmC* 49.9 (21.9) Cr I 138.4 BP 137.9 (1.1) N* 120.6 (2.7) SmC* 44.3 (20.1) Cr I 138.2 BP 137.5 (1.1) N* 120.3 (2.6) SmC* 43.9 (19.0) Cr
a Acronyms: Cr, Crystal; SmC*, Chiral Smectic C Phase; N*, Chiral Nematic Phase; BP = Blue Phase-I or Blue Phase-II. bPeak temperatures in the DSC traces obtained during the first heating− cooling cycles at 5 °C/min rate.
including thermal range of the mesophase have been taken from the microscopic observations. As shown in Figure 1, DSC
Figure 1. DSC thermograms recorded during the first heating−cooling cycles at a rate of 5 °C for the ten (five pairs of enantiomers) mesogens.
traces of all the mesogens were found to be virtually identical, each comprising three endothermic/exothermic peaks in heating/cooling cycles. The calorimetric results coupled with microscopic observations led us to clearly identify the presence of three enantiotropic LC phases in these compounds: detailed textural observations under variable temperature POM and analysis of the textures suggested the presence of BP, N*, and SmC* phases as elucidated in the following. The pristine solid samples held between an untreated glass slide and a coverslip were heated to their isotropic phase and cooled slowly. As shown in Figure 2a, a platelet textural pattern, 4541
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Figure 2. Photomicrographs of the textures, seen under a polarizing microscope, of different LC phases of SA(R)-12 placed between two untreated glass substrates: (a) platelet texture of the BP-I/II phase (T − 138.1 °C); (b) pseudo-focal conic texture of the N* phase (T − 135 °C); (c) schlieren texture with only four-brush disclinations (strength S = ± 1) of the SmC* phase (T − 108 °C); the inset shows a uniform, dull-greyish pattern observed in another portion of the same sample slide. T = temperature.
Figure 3. Microphotographs of the various optical textures observed for the SmC* phase under different conditions: (a) the texture with broken-focal conic (left), schlieren (middle), and dull-greyish (rightbottom) patterns observed for the sample SA(R)-10 held between microscopic (untreated) glass slides (T − 108 °C); (b) the iridescent (Grandjean) texture of the homeotropically oriented sample of SA(S)8 (T − 66 °C); (c) the texture comprising undulation lines and crosshatched pattern (right and left portions) seen for the sample SA(R)-10 sandwiched between microscopic (untreated) glass slides (T − 108 °C).
comprising blue and green plates, grows rapidly over black background of the isotropic liquid (I) phase. Upon subjecting to gentle mechanical stress, it instantly transforms into a Grandjean planar pattern of the N* phase. These observations signify the presence of a BP-I/BP-II phase.2−13,89 It may be noted here that in DSC traces, the signature due to I-BP transition could not be detected. Upon cooling the samples further, the N* phase appears rapidly with a pseudo-focal conic optical texture (Figure 2b), which changes to a typical Grandjean plane texture (Figure 2c) when the top glass substrate (coverslip) is pressed gently; in this planar texture, the helical axis of the phase lies along the viewing direction. In other words, it remains perpendicular to the confining (glass plate) surfaces. The enthalpy (ΔH) values, ranging from 0.8 to 1 kJ/mol−1, shown in Table 1, represent the combined ΔH for the I-BP and BP-N* transitions as the BP exists over a short thermal width of 0.3 to 1 °C. On cooling these samples further, a third phase transition occurs below the N* phase; this transition was clearly identified based on the observation that the Grandjean plane pattern of the N* phase sharply changes to a schlieren texture comprising four-brush disclinations (strength S = 1) exclusively; as shown in the inset of Figure 2c, in several regions of the same slide, a greyish uniform pattern adjoining the schlieren texture also existed. As is wellknown,89 for long pitch SmC* phase where light propagates along the helix axis, the textural patterns seen are analogous to those identified for the achiral smectic C (SmC) phase. For instance, one of the optical textures observed for the SmC phase is the low birefringent (due to light propagation along the direction of the optic axis) greyish pattern which normally accompanies a schlieren texture having four-brush defects.89 Thus, these optical results hint the presence of a SmC* phase below the N* phase. This assignment was further substantiated by the observation of other appealing textural patterns. For example, a texture comprising broken-focal conic fan, schlieren, and dull-greyish patterns was seen; as a representative case, such a pattern observed for the mesogen SA(R)-10, held between the two untreated glass plates, is shown in Figure 3a. Here, broken focal conic and dull-greyish patterns stem respectively from the planar and homeotropic orientation of the constituent mesogens; the latter pattern obviously suggests the layered structure of the phase. In the regions of pseudo-homeotropic alignment of the sample SA(S)-8, for example, the Grandjean texture expectedly appears iridescent as shown in Figure 3b. Such textures are known to result from a helical macrostructure with the tilting of molecules in smectic layers.2,3,63,90 That is, these textures are reported to be seen for the SmC* phase.63,90
Notably, a texture consisting of both undulation lines and crosshatched pattern was seen for the sample SA(R)-10 held between the two ordinary glass slides (Figure 3c).63,91 Thus, these textural patterns observed under a polarizing microscope evidently indicate the presence of a SmC* phase below the N* phase. XRD and electro-optical measurements strongly supported this assignment which we discuss in the sections to follow. 2.2.2. X-Ray Powder Diffraction (XRD) Study. As is wellknown, in the SmC* phase, the constituent mesogens tilt at an angle with respect to an axis perpendicular to the layer planes and it is the precession of the tilt that yields the helical macroscopic structure. Given the tilted orientation of mesogens within the smectic layers, the measured spacing (d) value derived from powder XRD data is expected to be smaller than the molecular length (L) of the mesogens. To demonstrate this, XRD measurements were carried out on unaligned specimens of representative compounds SA(R)-8, SA(R)-11, and SA(S)12. The samples were heated to their isotropic phase and filled into the Lindemann capillary (1 mm diameter) tube by capillary action and both the ends of the tube were flamesealed. The XRD patterns in the region of SmC* phase were recorded as a function of reducing temperature; all the profiles obtained were found to be qualitatively identical. Specifically, the 1D intensity versus diffraction (2θ) profiles obtained for SmC* phase at different temperatures were found to be nearly the same. Figure 4a represents such profiles obtained at 101, 114, and 112 °C for SA(R)-8, SA(R)-11, and SA(S)-12, respectively. Each pattern comprised a sharp reflection, with the layer thickness d, in the small angle region (0 < 2θ < 5°) and a diffuse peak in the wide-angle area (2θ ≈ 20°), clearly establishing the layer structure of the phase. The diffuse peak in the wide-angle region with the spacing ∼4.6 Å corresponds to the intermolecular separation within the smectic layer arising due to the liquidlike positional correlation within the layer. Figure 4b illustrates the temperature dependence of the layer spacings derived from the low-angle Bragg reflections obtained in the smectic region of compounds SA(R)-8, SA(R)-11, and SA(S)-12. The spacings d of the low-angle reflections, wideangle peak positions, the molecular length (L) of the three mesogens in their all-trans conformation, the d/L ratio and the tilt angle (θ), estimated using the expression θ = cos−1(d/L), are given in Table S1 of the Supporting Information. It apparent from Figure 4b and Table S1 of the Supporting Information that the layer spacing values ranging between 26.04 4542
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Figure 4. (a) 1D intensity vs 2θ profiles obtained for the SmC* phase of mesogens SA(R)-8, SA(R)-11, and SA(S)-12; note that in each trace, a diffuse peak in the wide-angle region (indicating liquidlike order), and a sharp reflection in the low-angle region are commonly seen. (b) The layer spacing (d) recorded as a function of temperature in the SmC* phase of SA(R)-8, SA(R)-11 and SA(S)-12.
Figure 5. Current response peaks obtained by applying a triangular-wave filed for the SmC* phase of compounds (a) SA(R)-8 (90 °C; 46.8 Vrms, 20 Hz; 9 kΩ; Ps = 121.1 nC cm−2) and (b) SA(S)-12 (90 °C; 46.8 Vrms, 20 Hz; 9 kΩ; Ps = 111.1 nC cm−2). (c) Profile showing the temperature dependence of the Ps in the SmC* phase of mesogens SA(R)-8 (red trace) and SA(S)-12 (blue trace).
Although such a trend of reversing the layer spacing (or tilt angle) is rather unusual but can be interpreted in terms of the negative thermal expansion of the layer structure, which stems from the stretching of the paraffinic tails, counterbalances the decrease in the layer spacing due to the tilted nature of the phase. Thus, the foregoing XRD measurement data and the results of the optical textural pattern observation unequivocally support the presence of a SmC* phase. As discussed below, electrical switching studies confirmed ferroelectric behavior of the phase. 2.2.3. Electro-Optic Measurements. The SmC* phase is a helical superstructure that results from the self-assembly of the constituent chiral mesogens, as discussed previously. In particular, such a novel structure originates from a gradual change in molecular tilt direction from layer-to-layer, about an
and 26.4 Å, 28.6−28.9 Å, and 29.7−29.8 Å are much less than the estimated all-trans molecular length (L) 37.5, 41.2, and 42.5 Å of the respective compounds SA(R)-8, SA(R)-11, and SA(S)-12 and thus, the d/L ratio are less than one. These results point to tilted monolayer organization of the mesogens within the smectic layers. The tilt angle, θ, of the molecules with respect to the layer normal was found to be in the range of 45.3−46.1. Thus, the XRD study evidence the tilted organization of the molecules within the smectic layers. It is clear from Figure 4b that for mesogens SA(R)-8 and SA(R)-11, the layer spacing decreases on cooling the phase, as expected, suggesting that the tilt angle of the mesogens within the smectic layers increases upon reducing the temperature. But for mesogen SA(S)-12, the layer spacing decreases initially and then increases marginally upon cooling (Figure 4b, blue trace). 4543
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The Journal of Physical Chemistry B axis perpendicular to the layer planes.2,3,20−29 Notably, in this phase, which lacks mirror symmetry due to the presence of chiral mesogens, the tilting of mesogens in each smectic layer induces a bias in their rotation around their long axes. This leads to spontaneous polarization (Ps) in the plane of each layer when the mesogens possess a transverse dipole moment parallel to the layer planes. Due to the helix, the tilt direction is rotated about the normal to the layer planes, and accordingly, the layer Ps averages out to zero. However, the macroscopic Ps can be realized by unwinding the helix by surface action or by an external stimulus such as the electrical field.2,3,20−29 In fact, experimental results have well-demonstrated that the SmC* phase possesses distinct polarization and optical properties that are practically apt for various technological applications.2,3,20−29 Thus, the SmC* phase formed by the compounds SA(R)-8 and SA(S)-12, as representative cases, was subjected to a detailed electrical switching study. For this purpose, cells were fabricated using a pair of electrically conducting indium tin oxide (ITO) glass plates treated with polyimide solution and rubbed unidirectionally to promote planar alignment. Samples were carefully filled into cells in their isotropic phase, cooled slowly to the SmC* phase, and a triangular wave field was applied. The current response profiles of the samples SA(R)-8 (Figure 5a) and SA(S)-12 (Figure 5b), obtained upon applying a voltage of 46.8 Vrms and a frequency of 20 Hz, showed a single polarization current peak for each half period confirming the intrinsic ferroelectric switching characteristics of the SmC* phase. The switching current peaks were found to be existing without any significant change over the whole thermal range of the SmC* phase. Importantly, such switching peaks were not seen in the isotropic liquid phase, suggesting that the current profiles are real and not stemming, owing to the presence of ionic impurities. In the Supporting Information, the optical textural patterns seen under these experimental conditions are given (Figures S1 of the Supporting Information). The macroscopic Ps values obtained by integrating the area under the peaks of the respective profiles of SA(R)-8 and SA(S)-12 were found to be 121.1 and 111.1 nC cm−2. The temperature dependence of Ps was measured, and the corresponding profiles obtained are shown in Figure 5c. It is immediately apparent that the Ps increases progressively with decreasing temperature and vanishes across the SmC*-N* transition, as expected. Thus, the preceding experimental data clearly point to the ferroelectric nature of the SmC* phase.
The helical sense (left or right) of the N* phase illustrates the direction in which rotation of the director occurs from layer-to-layer. Fundamentally, the molecular chirality imparts the form chirality (handedness) to the structure; that is, the helical twist sense is determined by the molecule’s chirality. As is well-known, two enantiomeric forms of a chiral compound display optical activities of equal magnitudes but opposite signs. Thus, CD study facilitates in ascertaining the handedness as the circularly polarized light (CPL), being a chiral system, interacts differently with the N*/SmC* phase formed by the two enantiomers of a chiral mesogen. That is to say, the N* phase shows CD at a given well-defined wavelength band where the incident unpolarized light beam (comprising equal amounts of left and right handed circularly polarized components) will be split into its right and left circularly polarized (r-CPL and lCPL) components. When white light propagates through a right-handed (RH) N* phase, the RH circularly polarized component with wavelength of the N* pitch is selectively reflected while the left-handed (LH) component is completely transmitted. Therefore, the helical screw sense of the N*/ SmC* phase of a pair of mirror-image isomers, the enantiomers, ought to be opposite to each other. Of particular interest is to know what happens to the helix sense of the N* and SmC* phases when their phase transition occurs in succession, which is the case with all compounds synthesized in the present study. In order to examine these characteristics, the CD experiments were carried out in the entire thermal range of SmC* and N* phases formed by two pairs of enantiomers, namely, SA(S)-10 and SA(R)-10 and SA(S)-12 and SA(R)-12. Initially, with the aim of quantifying the measurements, prefabricated cells of known thickness were used. However, the stronger CD signals obtained were found to be out of the measurable range of the instrument. Thus, to work within the upper (saturation) limits of measurement, the usage of very thin films of the samples was found be essential, and thus, our CD measurements presented herein, needless to say, are qualitative. The above-mentioned representative samples, held between two quartz plates, were heated slightly above their isotropization temperatures and pressed-hard to obtain evenly spread very thin films. This process was repeated until the films showed the genuine CD signals without saturation of the detector. However, despite several attempts, we could not circumvent the saturation problem for the samples SA(S)-12 and SA(R)12, especially in the SmC* phase (see Figure S2 of the Supporting Information) the intensity of the bands was found to exceed the range of the CD instrument. The samples were cooled from their isotropic liquid state using a programmable hot stage having accuracy of about ±0.5 °C. CD spectra were recorded as a function of decreasing temperature from isotropic state to the temperature range covering both the N* and SmC* phases. It is important to mention here that none of the samples displayed the CD effect in the isotropic state, whereas both the mesophases showed very strong characteristic CD signals under the identical experimental conditions. The presence of CD signals is a direct proof for chiral orientation of chromophores. This implies that the chromophores are not influenced by the molecular chirality. In other words, these results evidently suggest the origin of optical activity of these phases from the chiral induction in the LC structure but not from the molecular chirality. The CD spectra obtained for SA(S)-10 and SA(R)10, as a representative case, are shown in Figure 6. As shown in
3. CIRCULAR DICHROISM SPECTROSCOPIC STUDY The N* and SmC* phases, being helical structures, exhibit special optical properties such as circular dichroism (CD) and Bragglike light reflection. The CD measurement has been proven to be one of the best techniques to probe the chiroptical properties of both N* and SmC* phases.45,46,53,92−104 In a planarly well aligned N* phase, the constituent mesogens adopt a macroscopic helical order (perpendicular to the substrate) with a pitch p; that is, the director rotates in a helical way around a perpendicular axis. Iridescent colors due to selective reflection of an incident light beam, remaining parallel to the helix axis, can be observed. Bragg reflections occur when the helix pitch is on the order of the wavelength of the incident light beam passing along the helix axis according to the equation p = nλ (where n is the refractive index and λ is the wavelength of incident light). 4544
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Figure 6. Temperature-dependent CD spectra recorded in the N* (a and c) and SmC* (b and d) phases formed by the mirror-image isomers SA(S)-10 and SA(R)-10.
counterpart enantiomer SA(R)-10 shows CD signals of opposite sign with some variations in their magnitude (Figure 6c); the notable differences in the peak positions and intensity of the CD signals, when compared to that of enantiomer SA(S)-10, can be ascribed to the thickness of the sample. It may be recalled here that the UV−Vis (solution) spectra of these enantiomers show an intense broad band at 355 nm due to the n−π* transition, and thus, among the aforesaid CD signals, the intense maximum (430 nm) can be ascribed to the n−π* transition. The weak signals at 707 and 540 nm of samples SA(S)-10 and SA(R)-10, respectively, must be arising from a very weak n−π* state embedded in the lowest energy absorption system and therefore are not seen in the UV−Vis spectra. This means that intrinsically weak n−π* transitions are reasonably sensed in CD spectral profiles than in the solution UV−Vis spectra. The CD bands appearing below 300 nm are apparently due to electronic transitions from a bonding π orbital to an antibonding π* orbital. Thus, the presence of CD peaks in the LC phase corresponds to chromophores that are organized in a specific chiral orientation. In other words, CD activity originates because of intermolecular interactions between two or more electronic transition dipoles arranged in a helical order. Indeed, when compared with the absorption behavior of the solution, the films of the neat N* phase exhibit a strong bathochromic effect (red shift) that can be attributed to the presence of auxochrome, hydroxyl (−OH) group, and/or strong cofacial proximity of chiral aggregates. It must be remarked here that the CD spectra of both the N* phases of the samples were found to be highly reproducible with only minor (negligible) variations in position and intensity (mdeg) of the CD signals. The broadening of the CD-band must be related to the distribution (inconsistently or homogeneously)97 of chiral aggregates. Another important feature apparent from Figure 6 (panels a and c) is that the CD intensity increases with
Figure S2 of the Supporting Information, the CD spectral patterns obtained for the samples SA(R)-12 and SA(S)-12 were found to be virtually indistinguishable when compared to those of the samples SA(S)-10 and SA(R)-10 In fact, in the absorption regime, the visible CD band combines the genuine CD signal, linear dichroism (LD), and birefringence. LD effect was completely ruled out by employing a well-known simple protocol that involves rotating the cell through 360o perpendicular to the incident light.105−110 Specifically, the CD activity observed in the mesophases was ascertained by recording the spectra at different temperatures by rotating the sample cells (in the plane normal to the incident light beam). The spectra were found to be practically the same to those obtained in the original position of the samples. Given the fact that the CD spectrometer used in our study also measures LD, the samples were subjected to such a measurement directly. As expected, no LD activity was apparent, clearly signifying that the peaks seen in the spectra are due to genuine CD activity. The peak position (λmax/nm), sign (+ve/−ve) and intensity (mdeg) of the CD signals obtained as a function of temperature for the two LC phases of the samples SA(S)-10 and SA(R)-10 are shown in Table 2. Similar data obtained for the mesophases of SA(R)-12 and SA(S)-12 are given in Table S2 of Supporting Information. As can be seen in Figure 6 (panels a and c), the CD spectra of the N* phase obtained at different temperatures for enantiomers SA(S)-10 and SA(R)-10 show nearly mirrorimage CD spectral patterns wherein multiple CD signals in each spectrum can be seen. The origin of such multiple bands in each CD spectrum points to different excitonically coupled chromophores capable of exhibiting several electronic transitions. For instance, the N* of an enantiomer SA(S)-10 shows three negative peaks at ∼266, 430, and 707 nm; besides, it shows two very weak positive bands at ca. 265 and 295 nm (see Figure 6a and Table 2). As expected, the N* phase of the 4545
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SA(R)-10, respectively, can be attributed to π−π* transitions. It is well-known that a CD couplet results from the throughspace coupling of two or more chromophores in chiral substrates, leading to an exciton-coupled CD. In fact, as mentioned earlier, coupling also happens when more than two identical or different chromophores are present. It is instantly apparent from Figure 6b of enantiomer SA(S)-10 that within each bisignate CD couplet (with zero crossing at ca. 450) nm), a −ve Cotton-effect at higher wavelength (in the range of 588− 632 nm; see Table 2) and a +ve Cotton-effect at lower wavelength (∼400 nm) exist, whereas the CD spectral features of the antipode SA(R)-10 were nearly equal in magnitude but opposite in sign, as expected. Thus, the SmC* phase of enantiomers SA(S)-10 and SA(R)-10 show a positive-couplet and a negative-couplet, respectively (see Table 2). The intense sharp peak observed in the 395−400 nm regions of the CD spectra of both the samples should be arising from the n−π* transition as the UV absorption spectra show a peak in the corresponding region assigned to the n−π* transition. The broad and relatively strong peak seen in the range of 600−720 nm indicates the presence of a weak n−π* transition and thus supports a similar occurrence in the N* phase. As noted earlier, the CD spectra of the N* phase belonging to enantiomers SA(S)-10 and SA(R)-10 show intense CD bands in the wavelength (λ) range of 424−430 nm that correspond to the absorption wavelength of n−π* transition of the chiral chromophore. Likewise, the intense but sharp CD bands corresponding to the absorption wavelength of the n−π* transition also exist for the SmC* phase of the chiral compounds SA(S)-10 and SA(R)-10. However, for the sample SA(S)-10, the CD band (λ ∼ 400 nm) obtained in the SmC* phase is positive (Figure 6b) whereas it is a −ve CD peak (λ ∼ 425 nm) for the N* phase (Figure 6a). It can therefore be assumed that the sense of the helix in the SmC* phase is righthanded, while it is left-handed in the case of the N* phase. The counterpart CD bands are positive (Figure 6c) and negative (Figure 6d) for the N* and SmC* phases, respectively, formed by SA(R)-10. These observations imply that reversal of helical handedness from left-to-right and vice versa occurs during the N* to SmC* phase transition. Aptly, the screw sense of the helical array of the N* and SmC* phases of an enantiomer is opposite.
Table 2. CD Spectroscopic Data* Obtained in N* and SmC* Phases As a Function of Temperature for the Enantiomers SA(S)-10 and SA(R)-10 CD compounds SA(S)-10
chiral LC phase N*
temperature (°C) 122 124 126 128 130
SA(R)-10
N*
132 121 123 126 129 131 134
SA(S)-10
SmC*
104 107 110 113 116
SA(R)-10
SmC*
103 107 111 113 115
λmax (nm) 266, 297, 380, 430, 707 265, 295, 377, 428, 709 266, 295, 374, 427, 705 266, 295, 374, 428, 708 266, 295, 374, 427, 717 426, 721 259, 294, 376, 430, 569 259, 294, 376, 428, 543 258, 296, 375, 426, 539 257, 293, 372, 424, 572 258, 372, 426, 537 258, 372, 426, 539 265, 296, 401, 632 265, 295, 401, 630 265, 294, 401, 608 264, 295, 401, 589 264, 294, 400, 588 255, 291, 396, 722 255, 291, 396, 713 255, 293, 397, 702 255, 293, 396, 672 255, 292, 397, 653
CD (mdeg) −5, +3, +9, −65, −9 −5, +2, +7, −62, −11 −4, +2, +5, −57, −10 −3, +2, +3, −52, −11 −2, +1, +2, −44, −11 −35, −9 +18, −6, −19, +179, +27 +16, −3, −14, +168, +27 +16, −3, −13, +145, +26 +17,-6, −17, +116, +31 +15, −8, +105, +25 +15, −8, +103, +25 −20, +5, +80, −116 −20, +6, +80, −120 −20, +5, +79, −124 −20, +5, +79, −126 −20, +6, +77, −125 +4, −7, −33, +19 +4, −7, −32, +21 +4, −7, −32. +22 +5, −6, −31, +25 +5, −5, −29, +26
4. CONCLUSIONS Summarizing, the facile synthesis and characterization of ten optically active mesogenic compounds, in the form of five pairs of enantiomers, are reported. The salicylaldimine-core has been especially incorporated in the molecular design to maximize the probability of ferroelectric properties as well as to provide resistance to heat and moisture to these new monomeric motifs. Chiral mesogens that are nonsuperimposible mirror images, making up a pair of enantiomers, have been derived from (R)-octyloxy and (S)-octyloxy tails. The length of the nalkoxy chain substituted at the other end has been varied from n-octyloxy to n-dodecyloxy. This variation influences the clearing temperatures of the mesogens; that is, the N*-I (clearing) transition temperatures decrease with the increase in the length of the terminal tail; this behavior can be interpreted in terms of enhanced molecular flexibility with the increase in the length of the terminal tail. All the mesogens behave identically, exhibiting BP, N*, and SmC* phases, meaning that the alteration in the length of the n-alkoxy tail has no significant effect on their LC behavior. The thermal width of the SmC*
*
The significant (intense) peaks have been shown in the table for simplicity.
decreasing temperature which can be perhaps ascribed to the higher mobility of the terminal alkyl chains. On the basis of the literature guidelines,97,105−110 one may assign a right-handed screw sense to the helix of the N* phase of SA(R)-10 as it shows a positive CD peak and a left-handed screw sense to the helix of the N* phase of SA(S)-10 as it shows a negative CD band. Figure 6 (panels b and d) show the temperature-dependent CD spectra recorded, under the similar experimental conditions, in the thermal range of the SmC* phase of SA(S)-10 and SA(R)-10. Both the spectra in the wavelength range of 400−800 nm (Table 2), exhibit quasi-mirror imaged bisignate CD curves, called CD couplets which are defined as the through progression of two intensive Cotton-effects with inverted signs. In addition, weak peaks at ca. 265 (−), 295 (+), and 265 (+), 296 (−) seen in the spectra of SA(S)-10 and 4546
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The Journal of Physical Chemistry B phase widens upon elongating the length of the n-alkoxy tail. Notably, the SmC* phase exhibits ferroelectric switching with high polarization value. The circular dichroism study, apart from confirming the helical structure of the N* and SmC* phases, also serve to demonstrate the reversal of the helix-sense during the N*−SmC* phase transition.
FP90 Central Processor. To corroborate the phase transition temperatures of microscopic study and to determine the enthalpies of phase transitions, differential scanning calorimeter (DSC), PerkinElmer Diamond DSC with the PC system operating on Pyris software, was employed. Indeed, prior to use, the DSC was calibrated using pure indium. DSC traces of both heating and cooling cycles were obtained at a rate of 5 °C/ min under the constant flow of nitrogen gas. X-ray diffraction studies were performed on powder samples in Lindemann capillaries with Cu Kα (λ = 0.15418 nm) radiation using the PANalytical X’Pert PRO MP X-ray diffractometer comprising a focusing elliptical mirror and a fast resolution detector (PIXCEL). 5. 2. Synthesis. 5.2.1. Synthesis and Characterization of Intermediates (1−4, 5a−5e, 6a−6e, and 7a−7e). The synthetic procedures and characterization data of all the intermediates namely, (S)- and (R)-1-nitro-4-(octan-2-yloxy)benzenes (1 and 3), (R)- and (S)-4-(octan-2-yloxy)-anilines (2 and 4), 4-(n-alkoxy)benzaldehydes (5a−5e), 4-(n-alkoxy)benzoic acids (6a−6e), and 4-formyl-3-hydroxyphenyl 4(alkoxy)benzoates (7a−7e) have been given in the Supporting Information. 5.2.2. General Procedure for the Synthesis of (S)-3Hydroxy-4-{[(4-(octan-2-yloxy)phenyl)-imino]methyl}phenyl 4-(n-alkoxy)benzoates. A mixture of 4-formyl-3-hydroxyphenyl-4-(alkoxy)benzoate (0.4 mmol,1eq ) and (S)-4-(octan-2yloxy)aniline (0.33 mmol, 1.2eq ), and a catalytic amount of acetic acid in ethanol (10 mL) was refluxed under inert atmosphere for 2 h. The yellow solid separated upon cooling the reaction mixture was collected by filtration, washed with ethanol, and air-dried. The crude product was purified by repeated recrystallizations using absolute ethanol. SA(S)-8. A yellow solid; yield: 0.208 g (96%); IR (KBr pellet): νmax in cm−1 2922, 2852, 1725, 1609, 1574, 1506, 1466, 1392, and 1169; UV−vis: λmax = 354.15 nm, mol. conc. = 6.45 × 10−3 M in CH2Cl2, ε = 2.806 × 102 L mol−1 cm−1; 1H NMR (400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H, −CHN), 8.14 (d, J = 9.2 Hz, 2H, Ar), 7.40 (d, J = 8.4 Hz, 1H, Ar), 7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H, Ar), 6.93 (d, J = 8.8 Hz, 2H Ar), 6.87(d, J = 2 Hz, 1H, Ar), 6.82 (dd, J1 = 8.4 Hz, J2 = 2.4 Hz, 1H, Ar), 4.39 (m, 1H, −OCH), 4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.85−0.86 (m, 31H, 3 × CH3, 11 × CH2); 13C NMR (100 MHz): 164.44, 163.71, 162.48, 159.42, 157.65, 154.51, 140.88, 132.74, 132.39, 122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.40, 36.49, 31.82, 29.34, 29.30, 29.24, 29.12, 26.01, 25.53, 22.67, 22.62, 19.76, 14.11, and 14.09; anal. Calcd for C36H47NO5: C, 75.36; H, 8.26; N, 2.44. Found: C, 75.43; H, 8.36; N, 2.35. SA(S)-9. A yellow solid; yield: 0.180g (91%); IR (KBr pellet): νmax in cm−1 2923, 2853, 1725, 1611, 1506, 1465, 1391, and 1165; UV−vis: λmax = 353.46 nm, mol. Conc. = 8.85 × 10−3 M in CH2Cl2, ε = 2.565 × 102 L mol−1 cm−1; 1H NMR (400 MHz, CDCl3): δ 13.79(s, 1H, Ar−OH), 8.61 (s, 1H, −CH N), 8.14 (d, J = 8.8 Hz, 2H, Ar), 7.38 (d, J = 8.4 Hz, 1H, Ar), 7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 9.2 Hz, 2H, Ar), 6.93 (d, J = 8.8 Hz, 2H Ar), 6.87(d, J = 2.4 Hz, 1H, Ar), 6.82 (dd, J1 = 8.4 Hz, J2 = 2.4 Hz, 1H, Ar), 4.38 (m, 1H, −OCH), 4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.85−0.86 (m, 33H, 3 × CH3, 12 × CH2); 13C NMR (100 MHz): 164.44, 163.71, 162.48, 159.42, 157.65, 154.51, 140.88, 132.74, 132.39, 122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.39, 36.49, 31.89, 31.82, 29.54, 29.39, 29.30, 29.27, 29.12, 26, 25.53, 22.69,
5. EXPERIMENTAL SECTION 5.1. Materials and Methods. Chemicals such as (R)-2octanol, (S)-2-octanol, triphenylphosphine, diisopropyl-azodicarboxylate, 10% Pd−C, 2,4-dihydroxy benzaldehyde, n-alkyl bromide, N,N′-dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), 4-nitrophenol, and acetic acid purchased either from overseas companies or local sources were used as received. Solvents were dried as per the standard procedures: DMF was stored over KOH pellets for 12 h and then distilled carefully under reduced pressure; it was then stored over molecular sieves (4 Å) for 48 h prior to use. Commercially available, ultra pure, dry nitrogen was generally employed to maintain an inert atmospheric condition. The reactions that are sensitive to air- and/or moisture were carried out under a nitrogen atmosphere in oven-dried glassware. HPLC-grade THF was distilled from sodium and benzophenone under inert atmosphere. Reagent grade acetone was stored over anhydrous potassium carbonate for 24 h and then distilled. Absolute alcohol was distilled with magnesium ethoxide (obtained by treating ethanol with magnesium turnings and iodine). Silica gel (60−120 and 100−200 mesh) and basic aluminum oxide were used as the stationary phases used in column chromatography. Industrial grade ethyl acetate and hexanes used for column chromatography were distilled twice prior to use. Purity of the compounds and progress of the reaction were examined with the help of thin layer chromatography (TLC); for this purpose, aluminum sheets precoated with silica gel (Merck, Kieselgel 60, F254) were used. UV-light and iodine vapors were used for visualization. The final compounds were dissolved in CH2Cl2 and microfiltered to eliminate the fine particles, cotton threads, filter paper fibers, etc. UV−vis spectra were recorded on a PerkinElmer Lambda 20 UV−vis spectrometer (1 cm path length, CH2Cl2). FTIR spectra of the compounds were recorded on KBr pellets or in neat form using a PerkinElmer Spectrum 1000 FTIR spectrometer, and the spectral positions are given in a wavenumber (cm−1) unit. 1H and 13C NMR spectra were recorded in a CDCl3 solvent at ambient temperature using a Bruker AMX-400 (400 MHz) spectrometer. The coupling constants (J) are given in Hz. The chemical shifts are reported in ppm on a scale downfield from TMS regarded as an internal standard. The pattern (splitting or multiplicity) of the 1H NMR signals are presented as s = singlet, d = doublet, dd = doublet of doublet, brs = broad singlet, and t = triplet. Elemental microanalysis was performed using Eurovector E300 elemental analyzer. The molecular lengths of target LCs were measured using the energy minimized (all-trans) space filling models deduced from the ChemBio3D Ultra 12.0 program. Spacefilling models of molecular structures were minimized using MM2 computations. CD spectra were recorded with the help of the Jasco J-810 spectropolorimeter. The mesophase assignments and determination of transition temperatures of the ten new chiral compounds were carried out with the help of an Olympus BX50 (BX50F4 model) optical polarizing transmitted light microscope equipped with a digital camera and a Mettler FP82HT hot stage programmed by an 4547
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SA(R)-8. A yellow solid; yield: 0.202 g (93%); IR (KBr pellet): νmax in cm−1 2926, 2855, 1726, 1607, 1573, 1506, 1465, 1393, and 1169; UV−vis: λmax = 354.05 nm, mol. Conc. = 8.89 × 10−3 M in CH2Cl2, ε = 2.205 × 102 L mol−1 cm−1; 1H NMR (400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H, −CHN), 8.14 (d, J = 8.8 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz, 1H, Ar), 7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H, Ar), 6.94 (d, J = 8.8 Hz, 2H Ar), 6.87 (d, J = 2.4 Hz, 1H,Ar), 6.82 (dd, J1 = 8.4 Hz, J2 = 2.4 Hz, 1H,Ar), 4.40 (m, 1H, −OCH), 4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.86−0.86 (m, 31H, 3 × CH3, 11 × CH2); 13C NMR (100 MHz): 164.44, 163.71, 162.48, 159.42, 157.65, 154.51, 140.88, 132.74, 132.39, 122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.40, 36.49, 31.82, 29.34, 29.30, 29.24, 29.12, 26.01, 25.53, 22.67, 22.62, 19.76, 14.11 and 14.09; anal. Calcd for C36H47NO5: C, 75.36; H, 8.26; N, 2.44. Found: C, 75.30; H, 8.03; N, 2.37. SA(R)-9. A yellow solid; yield: 0.2 g (92%); IR (KBr pellet): νmax in cm−1 2923, 2852, 1724, 1605, 1574, 1505, 1467, 1394 and 1169; UV−vis: λ max = 355.86 nm, mol. Conc. = 6.98 × 10−3 M in CH2Cl2, ε = 2.064 × 102 L mol−1 cm−1; 1H NMR (400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H, −CHN), 8.14 (d, J = 9.2 Hz, 2H, Ar), 7.40 (d, J = 8.4 Hz, 1H, Ar), 7.25 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H, Ar), 6.93 (d, J = 8.8 Hz, 2H Ar), 6.87 (d, J = 2 Hz, 1H, Ar), 6.81 (dd, J1 = 8.4 Hz, J2 = 2 Hz, 1H, Ar), 4.40 (m, 1H, −OCH), 4.06 (t, J = 6.4 Hz, 2H, −OCH2), and 1.85−0.86 (m, 33H, 3 × CH3, 12 × CH2); 13C NMR (100 MHz): 164.44, 163.71, 162.48, 159.42, 157.64, 154.51, 140.88, 132.74, 132.39, 122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.58, 74.41, 68.39, 36.49, 31.89, 29.53, 29.38, 29.30, 29.27, 29.12, 26, 25.53, 22.69, 22.62, 19.76, 14.12 and 14.09; anal. Calcd for C37H49NO5: C, 75.60; H, 8.40; N, 2.38. Found: C, 75.92; H, 8.43; N, 2.44. SA(R)-10. A yellow solid; yield: 0.213 g (94%); IR (KBr pellet): νmax in cm−1 2922, 2852, 1724, 1605, 1574, 1505, 1467, 1394, and 1169; UV−vis: λmax = 354.88 nm, mol. Conc. = 3.66 × 10−3 M in CH2Cl2, ε = 3.11 × 102 L mol−1 cm−1; 1H NMR (400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H, −CHN), 8.14 (d, J = 8.8 Hz, 2H, Ar), 7.40 (d, J = 8.4 Hz, 1H, Ar), 7.25 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H, Ar), 6.93 (d, J = 9.2 Hz, 2H Ar), 6.87 (d, J = 2.4 Hz, 1H,Ar), 6.82 (dd, J1= 8.4 Hz, J2= 2 Hz, 1H, Ar), 4.37 (m, 1H, −OCH), 4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.84−0.87 (m, 35H, 3 × CH3, 13 × CH2); 13C NMR (100 MHz): 164.44, 163.71, 162.48, 159.42, 157.64, 154.51, 140.88, 132.74, 132.39, 122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.39, 36.49, 31.92, 31.82, 29.57, 29.38, 29.33, 29.30, 29.12, 26, 25.53, 22.70, 22.62, 19.76, 14.12, and 14.09; anal. Calcd for C38H51NO5: C, 75.84; H, 8.54; N, 2.33. Found: C, 75.88; H, 8.39; N, 2.18. SA(R)-11. A yellow solid; yield: 0.208 g (90%); IR (KBr pellet): νmax in cm−1 2922, 2852, 1725, 1607, 1573, 1506, 1466, 1392, and 1169; UV−vis: λmax = 354.98 nm, mol. Conc. = 4.38 × 10−3 M in CH2Cl2, ε = 1.733 × 102 L mol−1 cm−1; 1H NMR (400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H, −CHN), 8.14 (d, J = 8.8 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz, 1H, Ar), 7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H, Ar), 6.94 (d, J = 8.8 Hz, 2H Ar), 6.87 (d, J = 2.4 Hz, 1H, Ar), 6.82 (dd, J1 = 8.4 Hz, J2 = 2 Hz, 1H, Ar), 4.37 (m, 1H, −OCH), 4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.84−0.86 (m, 37H, 3 × CH3, 14 × CH2); 13C NMR (100 MHz): 164.44, 163.71, 162.48, 159.42, 157.65, 154.51, 140.88, 132.74, 132.39, 122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.39,
22.62, 19.76, 14.12, and 14.09; anal. Calcd for C37H49NO5: C, 75.60; H, 8.40; N, 2.38. Found: C, 75.74; H, 8.25; N, 2.15. SA(S)-10. A yellow solid; yield: 0.194g (93%); IR (KBr pellet): νmax in cm−1 2923, 2853, 1725, 1611, 1574, 1506, 1465, 1392, and 1166; UV−vis: λmax = 354.04 nm, mol. Conc. = 3.66 × 10−3 M in CH2Cl2, ε = 2.922 × 102 L mol−1 cm−1; 1H NMR (400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H, −CHN), 8.14 (d, J = 9.2 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz, 1H, Ar), 7.26 (d, J = 8.8 Hz Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H, Ar), 6.94 (d, J = 9.2 Hz, 2H Ar), 6.87 (d, J = 2.4 Hz, 1H,Ar), 6.82 (dd, J1 = 8.4 Hz, J2= 2 Hz, 1H, Ar), 4.37 (m, 1H, −OCH), 4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.84−0.87 (m, 35H, 3 × CH3, 13 × CH2); 13C NMR (100 MHz): 164.44, 163.71, 162.48, 159.42, 157.64, 154.51, 140.88, 132.74, 132.39, 122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.39, 36.49, 31.92, 31.82, 29.57, 29.38, 29.33, 29.30, 29.12, 26, 25.53, 22.70, 22.62, 19.76, 14.12, and 14.09; anal. Calcd for C38H51NO5: C, 75.84; H, 8.54; N, 2.33. Found: C, 75.81; H, 8.63; N, 2.25. SA(S)-11. A yellow solid; yield: 0.181g (91%); IR (KBr pellet): νmax in cm−1 2922, 2851, 1738, 1719, 1605, 1574, 1508, 1468, 1395, and 1169; UV−vis: λmax = 352.02 nm, mol. Conc. = 5.36 × 10−3 M in CH2Cl2, ε = 2.929 × 102 L mol−1 cm−1; 1H NMR (400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H, −CHN), 8.14 (d, J = 8.8 Hz, 2H, Ar); 7.40 (d, J = 8.4 Hz, 1H, Ar); 7.26 (d, J = 8.8 Hz, 2H, Ar); 6.98 (d, J = 8.8 Hz, 2H, Ar), 6.93 (d, J = 8.8 Hz, 2H Ar), 6.87 (d, J = 2 Hz, 1H,Ar), 6.82 (dd, J1= 8.4 Hz, J2= 2 Hz, 1H, Ar), 4.40 (m, 1H, −OCH), 4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.85−0.86 (m, 37H, 3 × CH3, 14 × CH2); 13C NMR (100 MHz): 164.44, 163.71, 162.48, 159.42, 157.64, 154.51, 140.88, 132.75, 132.39, 122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.39, 36.49, 31.93, 31.82, 29.63, 29.61, 29.57, 29.38, 29.36, 29.30, 29.12, 26, 25.53, 22.71, 22.62, 19.76, 14.13, and 14.09; anal. Calcd for C39H53NO5: C, 76.06; H, 8.67; N, 2.27. Found: C, 75.84; H, 8.56; N, 2.19. SA(S)-12. A yellow solid; yield: 0.192g (90%); IR (KBr pellet): νmax in cm−1 2922, 2852, 1720, 1605, 1574, 1507, 1467, 1394, and 1169; UV−vis: λmax = 354.71 nm, mol. Conc. = 5.72 × 10−3 M in CH2Cl2, ε = 3.691 × 102 L mol−1 cm−1; 1H NMR (400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H, −CHN), 8.14 (d, J = 9.2 Hz, 2H, Ar), 7.40 (d, J = 8.4 Hz, 1H, Ar), 7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 8.8 Hz, 2H, Ar), 6.93 (d, J = 8.8 Hz, 2H Ar), 6.87(d, J = 2.4 Hz, 1H,Ar), 6.82 (dd, J1 = 8.4 Hz, J2 = 2.2 Hz,1H, Ar), 4.37 (m, 1H, −OCH), 4.06 (t, J = 6.4 Hz, 2H, −OCH2), and 1.84−0.87 (m, 39H, 3 × CH3, 15 × CH2); 13C NMR (100 MHz): 164.43, 163.71, 162.48, 159.42, 157.64, 154.51, 140.88, 132.74, 132.39, 122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.39, 36.49, 31.94, 31.82, 30.93, 29.68, 29.66, 29.58, 29.38, 29.30, 29.12, 26, 25.53, 22.71, 22.62, 19.76, 14.13 and 14.09; Anal. Calcd for C40H55NO5: C, 76.27; H, 8.80; N, 2.22. Found: C, 76.07; H, 8.87; N, 2.14. 5.2.3. General Procedure for the Synthesis of (R)-3Hydroxy-4-{[(4-(octan-2-yloxy)phenyl)-imino]methyl}phenyl 4-(n-Alkoxy)benzoates. A mixture of 4-formyl-3-hydroxyphenyl-4-(alkoxy)benzoate (0.4 mmol, 1eq ) and (R)-4-(octan-2yloxy)aniline (0.33 mmol, 1.2 eq), catalytic amount of acetic acid in ethanol (10 mL) was refluxed under inert atmosphere for 2 h. The yellow solid separated upon cooling the reaction mixture was collected by filtration, washed with ethanol and airdried. The crude product was purified by repeated recrystallizations using absolute ethanol. 4548
DOI: 10.1021/acs.jpcb.5b00209 J. Phys. Chem. B 2015, 119, 4539−4551
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(9) Castles, F.; Day, F. V.; Morris, S. M.; Ko, D.-H.; Gardiner, D. J.; Qasim, M. M.; Nosheen, S.; Hands, P. J. W.; Choi, S. S.; Friend, R. H.; et al. Blue-Phase Template Fabrication of Three-Dimensional Nanostructures for Photonic Applications. Nat. Mater. 2012, 11, 599−603. (10) Lin, T.-H.; Li, Y.; Wang, C.-T.; Jau, H.-C.; Chen, C.-W.; Li, C.C.; Krishna Bisoyi, H. T.; Bunning, J.; Li, Q. Red, Green and Blue Reflections Enabled in an Optically Tunable Self-Organized 3D Cubic Nanostructured Thin Film. Adv. Mater. 2013, 25, 5050−5054. (11) Coles, H. J.; Pivnenko, M. N. Liquid Crystal Blue Phases with a Wide Temperature Range. Nature 2005, 436, 997−1000. (12) Kikuchi, H.; Yokota, M.; Hisakado, Y.; Yang, H.; Kajiyama, T. Polymer-Stabilized Liquid Crystal Blue Phases. Nat. Mater. 2002, 1, 64−68. (13) Yoshizawa, A. Material Design for Blue Phase Liquid Crystals and Their Electro-Optical Effects. RSC Adv. 2013, 3, 25475−25497. (14) Goodby, J. W.; Waugh, M. A.; Stein, S. M.; Chin, E.; Pindak, R.; Patel, J. S. Characterization of a New Helical Smectic Liquid Crystal. Nature 1989, 337, 449−452. (15) Goodby, J. W.; Waugh, M. A.; Stein, S. M.; Chin, E.; Pindak, R.; Patel, J. S. A New Molecular Ordering in Helical Liquid Crystals. J. Am. Chem. Soc. 1989, 111, 8119−8125. (16) Goodby, J. W. In Structure and Bonding: Liquid Crystals II; Mingos, D. M. P., Ed.; Springer-Verlag: Berlin, 1999; p 83. (17) Goodby, J. W. Twist grain boundary and frustrated liquid crystal phases. Curr. Opin. Colloid Interface Sci. 2001, 7, 326−332. (18) Handbook of Liquid Crystals: Volume 3: Nematic and Chiral Nematic Liquid Crystals, Part III. Discotic, Biaxial and Chiral Nematic Liquid Crystals, 2nd edition; Goodby, J. W.; Collings, P. J.; Kato, T.; Tschierske, C.; Gleeson, H.; Raynes, P., Eds; Wiley-VCH: Weinheim, Germany, 2014. (19) Liquid Crystals beyond Displays: Chemistry, Physics, and Applications; Li, Q., Ed.; John Wiley & Sons: Hoboken, NJ, 2012. (20) Handbook of Liquid Crystals: Volume 4: Smectic and Columnar Liquid Crystals, Part II: Chiral Smectic Liquid Crystals, 2nd ed.; Goodby, J. W., Collings, P. J., Kato, T., Tschierske, C., Gleeson, H., Raynes, P., Eds.; Wiley-VCH: Weinheim, Germany, 2014. (21) Nanoscience with Liquid Crystals: From Self-Organized Nanostructures to Applications; Li, Q., Ed.; Springer: Heidelberg, 2014. (22) Bisoyi, H. K.; Li, Q. Light-directing chiral liquid crystal nanostructures: From 1D to 3D. Acc. Chem. Res. 2014, 47, 3184−3195. (23) Wang, Y.; Li, Q. Light-driven chiral molecular switches or motors in liquid crystals. Adv. Mater. 2012, 24, 1926−1945. (24) Rameshbabu, K.; Urbas, A.; Li, Q. Synthesis and Characterization of Thermally Irreversible Photochromic Cholesteric Liquid Crystals. J. Phys. Chem. B 2011, 115, 3409−3415. (25) Lagerwall, S. T. Ferroelectric and Antiferroelectric Liquid Crystals; Lagerwall, S. T., Ed.; Wiley-VCH: Weinheim, Germany, 1999. (26) Hird, M. Ferroelectricity in Liquid Crystals: Materials, Properties and Applications. Liq. Cryst. 2011, 38, 1467−1493 and references cited therein. (27) Goodby, J. W.; Blinc, R.; Clark, N. A.; Lagerwall, S. T.; Osipov, M. A.; Pikin, S. A.; Sakurai, T.; Yoshino, K.; Zenks, B. Ferroelectric Liquid Crystals: Principle and Applications; Gordon and Breach: Philadelphia, 1991. (28) Takezoe, H. Ferroelectric, Antiferroelectric and Ferrielectric Liquid Crystals: Applications, Encyclopedia of Materials, Science and Technology; Elsevier Science Ltd.: Amsterdam, 2001. (29) Lagerwall, S. T. Ferroelectric Liquid Crystals: Low Molecular Weight Liquid Crystals II; Demus, D., Goodby, J. W., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH, Weinheim, 1998; Vol. 2B, pp 515−664. (30) Solladie, G.; Zimmermann, R. G. Liquid Crystals: A Tool for Studies on Chirality. Angew. Chem., Int. Ed. 1984, 23, 348−362. (31) Chojnowsk, O.; Dabrowski, R.; Kula, P.; Sczucinski, Ł.; Yan, J.; Wu, S.-T. Liquid Crystalline Blue Phase in Mixtures of Fluorinated Compounds with Positive and Negative Dielectric Anisotropy and Its Electro-Optic Performance. Liq. Cryst. 2014, 41, 15−24 and references cited therein.
36.49, 31.93, 31.82, 29.63, 29.61, 29.57, 29.38, 29.36, 29.30, 29.12, 26, 25.53, 22.71, 22.62, 19.76, 14.13, and 14.09; anal. Calcd for C39H53NO5: C, 76.06; H, 8.67; N, 2.27. Found: C, 76.01; H, 8.77; N, 2.29. SA(R)-12. A yellow solid; yield: 0.214 g (91%); IR (KBr pellet): νmax in cm−1 2923, 2851, 1724, 1605, 1574, 1505, 1467, 1391 and 1169; UV−vis: λmax = 354.77 nm, mol. Conc. = 8.26 × 10−3 M in CH2Cl2, ε = 2.398 × 102 L mol−1 cm−1; 1H NMR (400 MHz, CDCl3): δ 13.79 (s, 1H, Ar−OH), 8.61 (s, 1H, −CHN), 8.14 (d, J = 8.8 Hz, 2H, Ar), 7.41 (d, J = 8.4 Hz, 1H, Ar), 7.26 (d, J = 8.8 Hz, 2H, Ar), 6.98 (d, J = 9.2 Hz, 2H, Ar), 6.94 (d, J = 8.8 Hz, 2H Ar), 6.87 (d, J = 2 Hz, 1H, Ar), 6.82 (dd, J1 = 8.4 Hz, J2 = 2.4 Hz, 1H, Ar), 4.40 (m, 1H, −OCH), 4.06 (t, J = 6.6 Hz, 2H, −OCH2), and 1.84−0.87 (m, 39H, 3 × CH3, 15 × CH2); 13C NMR (100 MHz): 164.44, 163.71, 162.48, 159.42, 157.65, 154.51, 140.88, 132.74, 132.39, 122.31, 121.32, 117.36, 116.60, 114.38, 112.96, 110.57, 74.41, 68.39, 36.49, 31.94, 31.82, 29.68, 29.66, 29.61, 29.58, 29.37, 29.30, 29.12, 26, 25.53, 22.71, 22.62, 19.76, 14.13, and 14.09; anal. Calcd for C40H55NO5: C, 76.27; H, 8.80; N, 2.22. Found: C, 76.13; H, 8.84; N, 2.29.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures, structural characterizations, powder X-ray data, the microphotographs of textural changes seen under microscope during the electric field studies and CD spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] and
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS C.V.Y. sincerely thanks the DST, New Delhi, Govt. of India, for financial support through SERB project no. SR/S1//OC-04/ 2012.
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REFERENCES
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