Effect of the Functional Groups of Racemic Rodlike Schiff Base

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The Effect of Functional Groups of Racemic Rodlike Schiff Base Mesogens on the Stabilization of Blue Phase in the Binary Mixture Systems Chiung-Cheng Huang, Zong-Ye Wu, Bing-Han Sie, We-Hao Chou, Yu-Chang Huang, Mei-Ching Yu, Bo-Hao Chen, I-Jui Hsu, Lai-Chin Wu, and Jey-Jau Lee J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b09823 • Publication Date (Web): 12 Nov 2016 Downloaded from http://pubs.acs.org on November 15, 2016

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The Journal of Physical Chemistry

The Effect of Functional Groups of Racemic Rodlike Schiff Base Mesogens on the Stabilization of Blue Phase in the Binary Mixture Systems Chiung-Cheng Huang,*a Zong-Ye Wu,a Bing-Han Sie,a We-Hao Chou,a Yu-Chang Huang,a Mei-Ching Yu,a Bo-Hao Chen,b I-Jui Hsu,*b Lai-Chin Wuc and Jey-Jau Leec a

Department of Chemical Engineering, Tatung University, Taipei 104, Taiwan a

E-mail:[email protected] a

b

Tel.: +886-2-77364681

Department of Molecular Science and Engineering, National Taipei University of Technology, Taipei 106, Taiwan b

b

c

E-mail: [email protected]

Tel.: +886-2-27712171 # 2420

National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan Abstract

Four series of rodlike racemic Schiff base mesogens possessing different alkyl chains and two types of linkage, ester and alkynyl groups were synthesized and applied to induce cubic blue phases (BPs) in simple binary mixture systems. The mesophases of these Schiff base mesogens were confirmed by variable-temperature X-ray diffraction (XRD) and the characteristic texture of POM. In general, when chiral additive S-(+)-2-Octyl 4-(4-hexyloxybenzoyloxy)benzoate (S811) with the 1

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ratio of 20-40 wt% is added into the rodlike racemic salicylaldimine-based mesogens, the temperature range of cubic BPs could be induced more than 20 K. The widest temperature range of cubic BP (35 K) presents in the blending mixture composed of rodlike racemic salicylaldimine-based mesogen OH-TIn possessing alkynyl linkage and 35-40 wt% S811. However, Schiff base mesogens possessing alkynyl linkage show direct isotropic to chiral nematic transition when equal chiral dopant is added. Notably, the termination temperature of BPs is very close to room temperature (ca. 35 °C) after 40.0 wt% S811 is added into the salicylaldimine-based mesogens possessing terminal alkyl chain and ester linkage. Interestingly, wide BPs (>30

K)

also

can

be

induced

by

1,4:3,6-dianhydro-2,5-bis[4-(n-hexyl-1-oxy)benzoic

adding

chiral

additive

acid]sorbitol (ISO(6OBA)2)

with high HTP into the racemic Schiff base mesogen possessing ester linkage. Cubic BPI and BPII can be confirmed by reflectance spectra and polarized optical microscopy (POM). The results of reflectance spectra indicate that the binary mixture composed of salicylaldimine-based mesogens and S811 easily exhibits super-cooling effect and induces BPI. However, only BPII can be observed in all binary mixtures containing Schiff base mesogen. Based on our experimental results and the molecular modeling, we suppose that the values of biaxiality, the polarizability and the dipole moment of molecular geometry are the main factors to 2


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affect the BP stabilization.

1. Introduction Thermodynamically stable blue phases (BPs) are well-known mesophoric behaviors in frustrated liquid crystals (LCs). On cooling process, three types of BPs, BPIII, BPII and BPI emerge from the isotropic phase to the chiral nematic phase.1 Amorphous BPIII is believed to be a local cubic lattice structure in the director field and as the isotropic phase with arbitrary orientation. However, as a result of having a fluid three-dimensional periodic structure in the director field, BPII and BPI possess higher ordered simple cubic and body-centered cubic packing structures, respectively. Since the lattice periods of cubic BPs (BPII and BPI) are the order of the wavelength of visible light, cubic BPs exhibit no birefringence but Bragg reflections of circularly polarized light. Consequently they are potentially useful for many applications such as fast light modulators, tunable photonic crystals, larger-screen flat panel display and three dimensional lasers.2-5 However, the defect of their lattice structure (disclination) causes the narrow temperature range to limit their practical applications.6 Therefore, modern researches are developing several methodologies to broaden and stabilize the temperature range of BPs. For synthetic chemists, novel single molecules can be designed and synthesized by incorporating 3



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optically pure groups, various linkages and lateral substituents into liquid crystalline structures.7-9

Thus several bent-core molecules, banana-shaped, T-shaped, and

U-shaped molecules have been designed and prepared.4 stabilization,10 hydrogen

bond

In addition, polymer

inducing,11-14 nanoparticle

doping,15-17

light

induction18-20 and blending mixture techniques with chiral dopants have been also reported.4,21 According to the literatures mentioned above and reviews, chirality, elasticity, flexoelectricity, lateral fluoro-substituent, and molecular biaxiality are important factors responsible for BPs stabilization. Schiff base (azomethine) has been utilized widely as a linking group in the synthesis

of

many

types

of

LC

molecules.22

salicylaldimine[N-(2-hydroxy-4-alkoxybenzylidene)

aniline]

Its

derivative,

possesses

quasi

six-membered ring resulting from intramolecular hydrogen (H)-bonding between the H-atom of the hydroxy group and the nitrogen (N)-atom of the imine linkage to exhibit more stable to moisture and heat than Schiff base analogues. With their synthetic versatility and ability to coordinate metals, they also have been applied in the metallomesogens. General speaking, nematic and/or smectic mesophases present in achiral rodlike Schiff base mesogens. New phase sequences and frustrated mesophases could be discovered by introducing asymmetric carbons centers in the form of chiral alkyl tail or presenting within Schiff base molecular structures. Such 4


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as the first chiral metallomesogen, a monosubstituted ferrocene-based Schiff base derivative tethering (S)-2-methylbutoxy tail,23 exhibited two frustrated phases, BP and TGBA* phases. Moreover, enantiotropic BPIII with broad temperature range (22 K) could be presented in unsymmetrical Schiff base linking dimers featuring an achiral bent core tethered to cholesteryl ester segment through a flexible odd-parity spacer.24 Yelamagged et al. also reported the symmetric Schiff base dimer with (R)-2-octyloxy tail exhibited cubic BPs range with approximately 9 K.25 In addition, they also prepared chiral ferroelectric Schiff base mesogens possessing BP with less than 1K.26-27

Recently, Takezoe, et al. also prepared an asymmetric dimer with

wide BPs range by using a flexible spacer with nine carbon to link a rodlike Schiff base mesogen and a cholesterol mesogenic unit.28 On the other hand, some bent-core molecules with Schiff base linkage have been also utilized to blend with conventional nematic LCs to induce or stabilize BPs and decrease the formation temperature of BPs even though they could not be as a single molecular form to exhibit BPs.29 It suggested that the bent-core molecules possessing biaxialities and small bend elastic constants29-32 could enhance twisting power in N* phase and stabilize the DTC structures.9,

33-34

For instance, Takezoe et al. reported that an

amorphous BPIII materials with wide thermal range (~20 K) and large electrooptical Kerr effect was induced by doping a Schiff base bent-core molecule and high HTP 5



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chiral dopant ISO(6OBA)2.29 In the meanwhile, Takezoe and Choi et al. incorporated azo linkage into Schiff base bent-core structure to obtain a new photoresponsive bent-core molecule.20 The UV stimulus can convert chiral nematic phase to cubic BPs through its photoisomerization. However, only a few racemic rodlike Schiff base molecules were utilized to induce BPs. Recently, we prepared a series of chiral and racemic rodlike Schiff base mesogens with tolane moiety.35 Notably, they possessed two specific soft crystal phases that could be confirmed by the POM texture and variable-temperature X-ray diffraction (XRD). Subsequently they could be applied in the stabilization of the BP temperature range by adding chiral dopants ISO(6OBA)2, S811 or R811. In general, BPs can be induced by doping chiral dopant R811 or S811 into racemic salicylaldimine-based mesogens. In addition, BPs are not stabilized in these binary mixture systems composed of the Schiff base mesogens possessing alkynyl linkage and no hydroxyl group. Among these binary mixture systems, the widest temperature range of cubic BP (35 K) can be induced by adding rodlike chiral dopant R811 or S811 into rodlike racemic salicylaldimine-based mesogen. By molecular modeling, it can be suggested that the appearance and temperature range of BPs are affected by dipole moment and biaxiality of molecular geometry. Accordingly, we demonstrated that the hydroxyl group and the methyl branch in this type of Schiff base mesogen play important 6


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roles in stabilization of BPs. In this work, four series of racemic Schiff base mesogens possessing terminal alkoxy/alkyl chains, two types of linkages between two rigid cores and the hydroxyl group at inner-core position were prepared. Subsequently these Schiff base mesogens are doped with variable ratio of chiral dopant S811 and ISO(6OBA)2 to investigate the effect of functional groups on the stabilization of blue phase. In addition, we also prepared racemic Schiff base compounds H-TIIn, H-EIn and H-EIIn that are in the absence of intramolecular hydrogen

bonding

and

structurally

similar

with

their

salicylaldimine-based analogues to further investigate the

respective

effect of the

intramolecular hydrogen bonding between the hydroxyl group and the imine group on the stabilization of BPs in blending mixture system. Furthermore, in order to understand the reasons of the BPs stabilization, we utilize molecular modeling calculation for these types of Schiff base mesogens to find the correlation between the BPs stabilization and structural variations.

Figure 1 Chemical structures of chiral dopants ISO(6OBA)2 and S811

7



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Br O

1.(COCl)2/CH2Cl2 R

R

COOH 2. HO

Y

CHO

CHO X

Et3N/THF

R

R

Pd(PPh3)2Cl2 / CuI THF / Et3N

OH a: R = CnH2n+1O, Y = COO b: R = CnH2n+1, Y = COO c: R = CnH2n+1O, Y = d: R = CnH2n+1,

H2N

X

O

C6H13

Y=

MeOH

Y N

O

C6H13

X Salicylaldimines: X = OH

Schiff bases: X = H

OH-TIn series : X = OH, Y = , R = OCnH2n+1 OH-TI6 : n = 6 ; OH-TI7 : n = 7 ; OH-TI8 : n = 8 OH-TIIn series : X = OH, Y = , R = CnH2n+1 OH-TII6 : n = 6 ; OH-TII7 : n = 7 ; OH-TII8 : n = 8

H-TIn series : X = OH, Y = H-TI7 : n = 7

, R = OCnH2n+1

H-TIIn series : X = OH, Y = H-TII6 : n = 6

, R = CnH2n+1

OH-EIn series : X = OH, Y = COO, R = OCnH2n+1 OH-EI6 : n = 6 ; OH-EI7 : n = 7 ; OH-EI8 : n = 8

H-EIn series : X = OH, Y = COO, R = OCnH2n+1 H-EI6 : n = 6 ; H-EI7 : n = 7 ; H-EI8 : n = 8

OH-EIIn series : X = OH, Y = COO, R = CnH2n+1 OH-EII6 : n = 6 ; OH-EII7 : n = 7 ; OH-EII8 : n = 8

H-EIIn series : X = OH, Y = COO, R = CnH2n+1 H-EII6 : n = 6 ; H-EII7 : n = 7 ; H-EII8 : n = 8

Scheme 1. Synthetic route of the rodlike racemic Schiff base mesogens

2. Experimental 2.1. Spectroscopic analysis The chemical structure of the target materials were identified by proton nuclear magnetic resonance (1H NMR) spectroscopy using a Bruker Avance DRX 500 NMR spectrometer (Bruker Co., Karlsruhe, Germany). The purity of the final compounds was assessed by thin layer chromatography (TLC), and further confirmed by elemental analysis using a Heraeus Vario EL III analyzer (Elementar Analysenyteme GmbH Co., Hanau, Germany). The carbon and hydrogen analytical data agreed with calculated results within ±1%. Variable-temperature x-ray diffraction (XRD) experiments were performed at the wiggler beam-line BL17A in National 8


H

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Synchrotron Radiation Research Center (NSRRC), Taiwan. The experimental wavelength is 1.3214 Å, and the sample was packed into a 0.5 mm capillary. A heat gun was equipped at this beamline and the temperature controller was programmable by a PC with a PID feedback system. The experimental XRD pattern was indexing by DICVOL program to obtain the crystal system and cell constants. 36-38

2.2. Liquid-crystalline and physical properties The initial phase sequence and corresponding transition temperature of the compounds were determined by the polarizing optical microscopy (POM). Mesophases were principally identified by microscopic texture of the materials sandwiched between two glass plates under crossed polarizing microscope using Nikon Microphoto-FXA optical microscopy in conjunction with hot stage controlled by control processor. The phase transition temperatures and corresponding phase transition enthalpies of compounds were determined by differential scanning calorimeter (DSC) using Perkin Elmer Diamond calorimeter under running rates of 3°C min-1. The Bragg reflection spectra of BPs were examined with USB2000 spectrometer in reflection mode, and the temperature of the samples was controlled accurately by the hot stage. 9



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2.3. Preparation of materials All of starting materials were purchased from Sigma-Aldrich with purity greater than 99%. Thin layer chromatography was performed with TLC sheets coated with silica; spots were detected by UV irradiation. Silica gel (Merck silica gel 60, 63-200 mesh) was used for column chromatography. The organic solvents were dried and distilled before use. The Schiff base mesogens were synthesized according to Scheme 1. 1-Ethynyl-4-(alkyloxy)benzene39 and 1-ethynyl-4-alkylbenzene40 were synthesized according to the literature. The intermediates, compounds c, and 4-(1-methylheptoxy)aniline were synthesized by our currently reported method.35 In addition, compounds OH-TI7 and H-TI7 have been prepared and characterized by our previous report. Chemical characterization data of new Schiff base mesogens are provided below. 2.4. Synthesis of racemic Schiff base compounds A mixture of 4-substitued-2-hydroxybenzaldehyde, a-d (0.90 mmol) and 4-(1-methylheptoxy)aniline (1.10 mmol) in mixed solvent of ethanol (10 mL) was refluxed for 4 h. After cooling room temperature, the yellow precipitation was obtained and collected by filtration. The crude product was further purified by repeated re-crystallization from the ethanol to give yellow powder in 80-85% yield. Compound OH-EI6 : 1H-NMR (CDCl3): δ (ppm) 13.79 (s, Ar-OH, 1H), 8.61 (s, 10


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-CH=N-, 1H), 8.13 (d, Ar-H, 2H, J = 8.8 Hz), 7.40 (d, Ar-H, 1H, J = 8.4 Hz), 7.25 (d, Ar-H, 2H, J = 8.8 Hz), 6.97 (d, Ar-H, 2H, J = 8.9 Hz), 6.94 (d, Ar-H, 2H, J = 9.0 Hz), 6.88 (d, Ar-H, 1H), 6.82 (dd, Ar-H, 1H, J = 2.5 Hz, J = 2.0 Hz), 4.38-4.35 (m, -OCH, 1H), 4.05 (t, -OCH2, 2H, J = 6.5 Hz), 1.83-1.25 (m, -CH2-, -CH3, 21H), 0.91 (m, -CH3, 6H). 13C-NMR (CDCl3): 164.4, 163.6, 162.4, 159.4, 157.6, 154.4, 140.8, 132.7, 132.3, 122.3, 121.2, 117.3, 116.5, 114.3, 112.9, 110.5, 74.3, 68.3, 36.4, 31.8, 31.5, 29.3, 29.0, 25.6, 25.5, 22.6, 19.7, 14.1, 14.0. FT-IR (KBr): 3421, 2927, 2856, 1720, 1612, 1508, 1464, 1252cm-1. Elemental analysis for C34H43NO5(percent): calculated C, 74.83, H, 7.94, N, 2.57; found C, 74.96, H, 7.70, N 2.55. Compound OH-EI7 : 1H-NMR (CDCl3): δ (ppm) 13.80 (s, Ar-OH, 1H), 8.62 (s, -CH=N-, 1H), 8.15 (d, Ar-H, 2H, J = 9.0 Hz), 7.41 (d, Ar-H, 1H, J = 8.5 Hz), 7.31 (d, Ar-H, 2H, J = 9.0 Hz), 6.98 (d, Ar-H, 2H, J = 8.5 Hz), 6.94 (d, Ar-H, 2H, J = 9.0 Hz), 6.89 (d, Ar-H, 1H), 6.82 (dd, Ar-H, 1H, J = 2.5 Hz, J = 2.0 Hz), 4.40-4.36 (m, -OCH, 1H), 4.06 (t, -OCH2, 2H, J = 6.5 Hz), 1.85-1.46 (m, -CH2-, -CH3, 23H), 0.91 (m, -CH3, 6H).

13

C-NMR (CDCl3): 164.4, 163.6, 162.4, 159.4, 157.6, 154.4, 140.8,

132.71, 132.4, 122.3, 121.2, 117.3, 116.5, 114.3, 112.9, 110.5, 74.3, 68.3, 36.4, 31.8, 31.7, 29.3, 29.1, 29.0, 25.9, 25.5, 22.6, 19.7, 14.1. FT-IR (KBr): 3423, 2927, 2854, 1728, 1600, 1579, 1466, 1246cm-1. Elemental analysis for C35H45NO5(percent): calculated C, 75.10, H, 8.10, N, 2.50; found C, 75.27, H, 8.14, N, 2.47. 11



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Compound OH-EI8 : 1H-NMR (CDCl3): δ (ppm) 13.80 (s, Ar-OH, 1H), 8.62 (s, -CH=N-, 1H), 8.15 (d, Ar-H, 2H, J = 9.0 Hz), 7.41 (d, Ar-H, 1H, J = 8.5 Hz), 7.31 (d, Ar-H, 2H, J = 9.0 Hz), 6.98 (d, Ar-H, 2H, J = 8.5 Hz), 6.94 (d, Ar-H, 2H, J = 9.0 Hz), 6.90 (d, Ar-H, 1H), 6.82 (dd, Ar-H, 1H, J = 2.5 Hz, J = 2.0 Hz), 4.40-4.36 (m, -OCH, 1H), 4.06 (t, -OCH2, 2H, J = 6.5 Hz), 1.85-1.46 (m, -CH2-, -CH3, 25H), 0.91 (m, -CH3, 6H). 13C-NMR (CDCl3): 164.4, 163.6, 162.4, 159.3, 157.6, 154.4, 140.74, 132.7, 132.3, 122.3, 121.2, 117.3, 116.5, 114.3, 112.9, 110.5, 74.3, 68.3, 36.4, 31.77, 29.3, 29.2, 26.0, 25.5, 25.1, 22.6, 22.5, 19.7, 14.1. FT-IR (KBr): 3452, 2925, 2856, 1728, 1610, 1574, 1464, 1248cm-1. Elemental analysis for C36H47NO5(percent): calculated C, 75.36, H, 8.26, N, 2.44; found C, 75.48, H, 8.33, N, 2.50. Compound H-EI6 : 1H-NMR (CDCl3): δ (ppm) 8.50 (s, -CH=N-, 1H), 8.16 (d, Ar-H, 2H, J = 8.7 Hz), 7.96 (d, Ar-H, 2H, J = 8.6 Hz), 7.33 (d, Ar-H, 2H, J = 8.6 Hz), 7.23 (d ,Ar-H, 2H, J = 8.9 Hz), 6.99 (d, Ar-H, 2H, J = 9.1 Hz), 6.93 (d, Ar-H, 2H, J = 8.9 Hz), 4,39-4.35 (m, -OCH-, 1H), 4.06 (t, -CH2-, 2H, J = 6.5 Hz), 1.85-1.81 (m, -CH2-, 2H), 1.34-1.26 (m, -CH2-, -CH3, 19H), 0.93-0.88 (m, -CH3-, 6H).

13

C-NMR

(CDCl3): 164.6, 163.7, 157.0, 153.2, 144.5, 134.1, 132.3, 129.7 122.2, 121.2, 116.4, 114.3, 74.3, 68.3, 36.5, 31.8, 31.7, 29.3, 29.1, 29.0, 25.9, 25.5, 22.6, 19.8, 14.1. FT-IR (KBr): 2925, 2854, 1726, 1602, 1506, 1265, 1163, 1119 cm-1. Elemental analysis for C36H47NO4(percent): calculated C, 77.09, H, 8.18, N, 2.64; found C, 12


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77.22, H, 8.27, N, 2.54. Compound H-EI7 : 1H-NMR (CDCl3): δ (ppm) 8.50 (s, -CH=N-, 1H), 8.16 (d, Ar-H, 2H, J = 8.8 Hz), 7.96 (d, Ar-H, 2H, J = 8.6 Hz), 7.33 (d, Ar-H, 2H, J = 8.6 Hz), 7.23 ( d, Ar-H, 2H, J = 8.8 Hz), 6.99 (d, Ar-H, 2H, J = 9.0 Hz), 6.93 (d, Ar-H, 2H, J = 9.0 Hz), 4.39-4.35 (m, -OCH-, 1H), 4.06 (t, -CH2-, 2H, J = 6.6 Hz), 1.87-1.80 (m, -CH2-, 2H), 1.51-1.27 (m, -CH2-, -CH3-, 21H), 0.93-0.88 (m, -CH3, 6H).

13

C-NMR

(CDCl3): 164.6, 163.7, 157.0, 153.2, 144.5, 134.1, 132.3, 129.7, 122.2, 121.2, 116.4, 114.32, 74.3, 68.3, 36.5, 31.8, 31.7, 29.3, 29.1, 29.0, 25.9, 25.5, 22.6, 19.8, 14.1. FT-IR (KBr): 2925, 2854, 1726, 1602, 1506, 1265, 1166, 1119 cm-1. Elemental analysis for C36H47NO4(percent): calculated C, 77.31, H, 8.34, N, 2.58; found C, 77.42, H, 8.31, N, 2.57. Compound H-EI8 : 1H-NMR (CDCl3): δ (ppm) 8.50 (s, -CH=N-, 1H), 8.16 (d, Ar-H, 2H, J = 8.9 Hz), 7.96 (d, Ar-H, 2H, J = 8.7 Hz), 7.33 (d, Ar-H, 2H, J = 8.6 Hz), 7.23 (d, Ar-H, 2H, J = 8.9 Hz), 6.99 (d, Ar-H, 2H, J = 9.0 Hz), 6.93 (d, Ar-H, 2H, J = 8.8 Hz), 4.39-4.35 (m, -OCH, 1H), 4.06 (t, -CH2-, 2H, J = 6.5 Hz), 1.89-1.24 (m, -CH2-, -CH3, 25 H), 0.92-0.88 (m, -CH3, 6H). 13C-NMR (CDCl3): 164.6, 163.7, 157.0, 153.2,

144.5, 134.1, 132.3, 129.7, 122.2, 121.2, 116.4, 114.3, 74.3, 68.3, 36.5, 31.8, 29.3, 29.2, 29.1, 25.6, 25.5, 22.6, 22.6, 19.8, 14.1.

FT-IR (KBr): 2925, 2854, 1726, 1602,

1506, 1265, 1163, 1119 cm-1. Elemental analysis for C36H47NO4(percent): calculated 13



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C, 77.52, H, 8.49, N, 2.51; found C, 77.60, H, 8.33, N, 2.45. Compound OH-EII6 : 1H-NMR (CDCl3): δ (ppm) 13.82 (s, Ar-OH, 1H), 8.63 (s, -CH=N-, 1H), 8.12 (d, Ar-H, 2H, J = 8.2 Hz), 7.42 (d, Ar-H, 1H, J = 8.5 Hz), 7.33 (d, Ar-H, 2H, J = 8.2 Hz), 7.27 (d, Ar-H, 2H, J = 8.7 Hz), 6.94 (d, Ar-H, 2H, J = 8.8 Hz), 6.88 (d, Ar-H, 1H, J = 2.1 Hz), 6.82 (dd, Ar-H, 1H, J = 8.4 Hz, J = 2.1 Hz), 4.41-4.35 (m, -OCH, 1H), 2.71 (t, -CH2-, 2H, J = 7.7 Hz), 1.80-1.24 (m, -CH2-, -CH3, 21H), 13

0.92-0.88 (m, -CH3, 6H).

C-NMR (CDCl3): 164.7, 162.4, 159.3, 157.6, 154.3,

149.6, 140.7, 132.7, 130.3, 128.7, 126.7, 122.3, 117.4, 116.5, 113.0, 110.5, 74.3, 36.4, 36.1, 31.8, 31.6, 31.1, 29.1, 28.9, 25.5, 22.6, 19.7, 14.1. FT-IR (KBr): 3440, 2925, 2850, 1726, 1614, 1508, 1265, 1180, 1119 cm-1. Elemental analysis for

C34H43NO4(percent): calculated C, 77.09, H, 8.18, N, 2.64; found C, 77.02, H, 8.10, N, 2.58. Compound OH-EII7 : 1H-NMR (CDCl3): δ (ppm) 13.82 (s, Ar-OH, 1H), 8.63 (s, -CH=N-, 1H), 8.12 (d, Ar-H, 2H, J = 8.3 Hz), 7.42 (d, Ar-H, 1H, J = 8.5 Hz), 7.33 (d, Ar-H, 2H, J = 8.3 Hz), 7.27 (d, Ar-H, 2H, J = 8.8 Hz), 6.94 (d, Ar-H, 2H, J = 9.0 Hz) , 6.88 (d, Ar-H, 1H, J = 2.3 Hz), 6.82 (dd, Ar-H, 1H, J = 8.4 Hz, J = 2.3 Hz), 4.41-4.35 (m, -OCH, 1H), 2.71 (t, -CH2-, 2H, J = 7.7 Hz), 1.80-1.24 (m, -CH2-, -CH3, 23H), 0.90 (t, -CH3, 6H, J = 6.9 Hz).

13

C-NMR (CDCl3): 164.7, 162.4, 159.3, 157.6, 154.3,

149.6, 140.7, 132.7, 130.3, 128.7, 126.7, 122.3, 117.4, 116.5, 112.9, 110.5, 74.3, 14


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36.4, 36.1, 31.8, 31.1, 29.3, 29.2, 29.1, 25.5, 22.6, 22.5, 19.7, 14.1. FT-IR (KBr): 3440, 2924, 2852, 1726, 1614, 1508, 1265, 1180, 1119 cm-1. Elemental analysis for

C35H45NO4(percent): calculated C, 77.31, H, 8.34, N, 2.58; found C, 77.36, H, 8.39, N, 2.55. Compound OH-EII8 : 1H-NMR (CDCl3): δ (ppm) 13.82 (s, Ar-OH, 1H), 8.63 (s, -CH=N-, 1H), 8.12 (d, Ar-H, 2H, J = 8.1 Hz), 7.42 (d, Ar-H, 1H, J = 8.4 Hz), 7.33 (d, Ar-H, 2H, J = 8.2 Hz), 7.27 (d, Ar-H, 2H, J = 8.8 Hz), 6.94 (d, Ar-H, 2H, J = 8.9 Hz), 6.89 (d, Ar-H, 1H, J = 2.1 Hz), 6.82 (dd, Ar-H, 1H, J = 8.4 Hz, J = 2.2 Hz), 4.41-4.35 (m, -OCH, 1H), 2.71 (t, -CH2-, 2H, J = 7.7 Hz), 1.80-1.24 (m, -CH2-, -CH3, 25 H), 0.90 (t, -CH3, 6H, J = 6.8 Hz).

13

C-NMR (CDCl3): 164.9, 157.0, 156.9, 153.1, 149.6,

144.5, 134.1, 130.3, 129.7, 128.7, 126.6, 122.2, 122.3, 116.4, 74.3, 36.5, 36.1, 31.8, 31.8, 31.1, 29.4, 29.3, 29.2, 25.5, 22.6, 22.6, 19.8, 14.1. FT-IR (KBr): 3440, 2924, 2852, 1726, 1614, 1508, 1267, 1180, 1119 cm-1. Elemental analysis for

C36H47NO4(percent): calculated C, 77.52, H, 8.49, N, 2.51; found C, 77.43, H, 8.41, N, 2.45. Compound H-EII6 : 1H-NMR (CDCl3): δ (ppm) 8.50 (s, -CH=N-, 1H), 8.13 (d, Ar-H, 2H, J = 8.2 Hz), 7.96 (d, Ar-H, 2H, J = 8.6 Hz), 7.33 (dd, Ar-H, 4H, J = 8.5 Hz, J = 1.9 Hz), 7.23 (d, Ar-H, 2H, J = 8.8 Hz), 6.93 (d, Ar-H, 2H, J = 8.8 Hz), 4.40-4.34 (m, -OCH, 1H), 2.72 (t, -CH2-, 2H, J = 7.7 Hz), 1.80-1.25 (m, -CH2-, -CH3, 15



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21 H), 0.92-0.88 (m, -CH3, 6H). 13C-NMR (CDCl3): 164.9, 157.0, 156.9, 153.1, 149.6,

144.5, 134.1, 130.3, 129.7, 128.7, 126.6, 122.2, 122.1, 116.4, 74.3, 36.5, 36.1, 31.8, 31.6, 31.1, 29.3, 28.9, 25.5, 22.6, 22.5, 19.8, 14.1. FT-IR (KBr): 2925, 2850, 1728, 1606, 1504, 1273, 1198, 1110 cm-1. Elemental analysis for C34H43NO3(percent):

calculated C, 79.49, H, 8.44, N, 2.73; found C, 79.35, H, 8.48, N, 2.70. Compounds H-EII7 : 1H-NMR (CDCl3): δ (ppm) 8.50 (s, -CH=N-, 1H), 8.13 (d, Ar-H, 2H, J = 8.1 Hz), 7.96 (d, Ar-H, 2H, J = 8.7 Hz), 7.33 (dd, Ar-H, 4H, J = 8.6 Hz, J = 1.75 Hz), 7.23 (dd, Ar-H, 2H, J = 8.5 Hz, J = 0.6 Hz), 6.93 (d, Ar-H, 2H, J = 8.8 Hz), 4.40-4.35 (m, -OCH, 1H), 2.71 (t, -CH2-, 2H, J = 7.7 Hz), 1.80-1.23 (m, -CH2-, -CH3, 23 H), 0.90 (t, -CH3, 6H, J = 6.8 Hz). 13C-NMR (CDCl3): 164.9, 157.0,

156.9, 153.1, 149.6, 144.4, 134.1, 130.3, 129.7, 128.7, 126.6, 122.2, 122.1, 116.4, 74.3, 36.5, 36.1, 31.8, 31.7, 31.1, 29.3, 29.2, 29.1, 25.5, 22.6, 22.6, 19.7, 14.1. FT-IR (KBr): 2925, 2850, 1738, 1610, 1502, 1257, 1198, 1057 cm-1. Elemental analysis for C35H45NO3(percent): calculated C, 79.66, H, 8.59, N, 2.65; found C, 79.53, H, 8.41, N, 2.55. Compounds H-EII8 : 1H-NMR

(CDCl3): δ (ppm) 8.50 (s, -CH=N-, 1H), 8.13 (d,

Ar-H, 2H, J = 8.1 Hz), 7.96 (d, Ar-H, 2H, J = 8.5 Hz), 7.33 (dd, Ar-H, 4H, J = 8.0 Hz, J = 0.9 Hz), 7.23 (d, Ar-H, 2H, J = 8.7 Hz), 6.93 (d, Ar-H, 2H, J = 8.8 Hz), 4.40-4.34 (m, -OCH, 1H), 2.71 (t, -CH2-, 2H, J = 7.7 Hz), 1.80-1.27 (m, -CH2-, -CH3, 16


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25 H), 0.90 (t, -CH3, 6H, J = 6.8 Hz). 13C-NMR (CDCl3): 164.9, 157.0, 156.9, 153.1,

149.6, 144.5, 134.1, 130.3, 129.7, 128.7, 126.6, 122.2, 122.3, 116.4, 74.3, 36.5, 36.1, 31.8, 31.8, 31.1, 29.4, 29.3, 29.2, 25.5, 22.6, 22.6, 19.8, 14.1. FT-IR (KBr): 2922, 2854, 1726, 1608, 1506, 1265, 1178, 1119 cm-1. Elemental analysis for

C36H47NO3(percent): calculated C, 79.81, H, 8.74, N, 2.59; found C, 79.62, H, 8.89, N, 2.57. Compounds OH-TI6 : 1H-NMR (CDCl3): δ 13.56 (s, Ar-OH, 1H), 8.60 (s, -CH=N-, 1H), 7.48 (d, Ar-H, 1H, J = 8.4 Hz), 7.33 (d, Ar-H, 2H, J = 8.4 Hz), 7.27 (d, Ar-H, 2H, J = 8.4 Hz), 7.15 (s, Ar-H, 1H), 7.07 (dd, Ar-H, 1H, J = 7.8 Hz, J = 1.2 Hz), 6.94 (d, Ar-H, 2H, J = 8.4 Hz), 6.89 (d, Ar-H, 2H, J = 8.4 Hz), 4.41-4.36 (m, -OCH*, 1H), 3.99 (t, -OCH2, 2H, J = 6.6 Hz), 1.83-1.28 (m, -CH2-, -CH3, 21H), 0.91 (m, -CH3, 6H).

13

C-NMR (CDCl3): 160.7, 159.5, 159.2, 157.7, 140.8, 133.3, 131.7,

127.7, 122.4, 122.2, 119. 7, 119.1, 116.5, 114.7, 114.6, 92.0, 88.0, 74.3, 68.1, 36.5, 31.8, 31.6, 29.3, 29.2, 25.7, 25.5, 22.6, 19.7, 14.1. FT-IR (KBr): 3444, 2932, 2858, 2206,

1602,

1516,

1496,

1287,

1245

cm-1.

Elemental

analysis

for

C35H43NO3(percent): calculated C, 79.96, H, 8.24, N, 2.66; found C, 79.87, H, 8.16, N, 2.53. Compound OH-TI8 : 1H-NMR (CDCl3): δ 13.56 (s, Ar-OH, 1H), 8.61 (s, -CH=N-, 1H), 7.48 (d, Ar-H, 1H, J = 8.4 Hz), 7.33 (d, Ar-H, 2H, J = 8.4 Hz), 7.27 (d, Ar-H, 17



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2H, J = 8.4 Hz), 7.15 (s, Ar-H, 1H), 7.07 (dd, Ar-H, 1H, J = 8.4 Hz, J = 1.2 Hz), 6.94 (d, Ar-H, 2H, J = 8.4 Hz), 6.89 (d, Ar-H, 2H, J = 8.4 Hz), 4.42-4.37 (m, -OCH*, 1H), 4.00 (t, -OCH2, 2H, J = 6.6 Hz), 1.84-1.30 (m, -CH2-, -CH3, 25H), 0.90 (m, -CH3, 6H).

13

C-NMR (CDCl3): 160.7, 159.5, 159.2, 157.7, 140.8, 133.3, 131.7,

127.7, 122.4, 122.2, 119.7, 119.1, 116.5, 114.7, 114.6, 92.0, 88.0, 74.3, 68.1, 36.5, 31.8, 31.6, 29.3, 29.2, 25.7, 25.5, 22.6, 19.7, 14.1. FT-IR (KBr): 3436, 2929, 2856, 2206,

1615,

1517,

1496,

1288,

1245

cm-1.

Elemental

analysis

for

C37H47NO3(percent): calculated C, 80.25, H, 8.55, N, 2.53; found C, 80.36, H, 8.66, N, 2.67. Compound OH-TII6 : 1H-NMR (CDCl3): δ (ppm) 13.56 (s, Ar-OH, 1H), 8.61 (s, -CH=N-, 1H), 7.47 (d, Ar-H, 2H, J = 8.1 Hz), 7.34 (d, Ar-H, 1H, J = 8.0 Hz), 7.28 (d, Ar-H, 2H, J = 7.2 Hz), 7.18 (d, Ar-H, 2H, J = 8.0 Hz) 7.16 (d, Ar-H, 1H, J = 1.1 Hz), 7.08 (dd, Ar-H, 1H, J = 7.8 Hz, J = 1.4 Hz), 6.94 (d, Ar-H, 2H, J = 8.8 Hz), 4.41-4.35 (m, -OCH, 1H), 2.63 (t, -OCH2, 2H, J = 7.8 Hz), 1.80-1.26 (m, -CH2-, -CH3, 21H), 0.90 (t, -CH3, 6H, J = 6.8 Hz). 13C-NMR (CDCl3): 160.7, 159.2, 157.72, 143.9, 140.8, 131.7, 128.5, 127.5, 122.4, 122.3, 112.0, 119.9, 119.3, 116.5, 92.0, 88.6, 74.3, 36.5, 36.0, 31.8, 31.7, 31.2, 29.3, 28.9, 52.5, 22.6, 19.7, 14.1. FT-IR (KBr): 3682, 2957, 2924, 2856, 2243, 1616, 1520, 1467, 1380, 1244 cm-1. Elemental analysis for C35H43NO2(percent): calculated C, 82.47, H, 8.50, N, 2.75; found C, 18


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82.51, H, 8.43, N, 2.70. Compound OH-TII7 : 1H-NMR (CDCl3): δ (ppm) 13.56 (s, Ar-OH, 1H), 8.61 (s, -CH=N-, 1H), 7.47 (d, Ar-H, 1H, J = 8.0 Hz), 7.34 (d, Ar-H, 2H, J = 7.9 Hz), 7.28 (d, Ar-H, 2H, J = 7.1 Hz), 7.18 (d, Ar-H, 2H, J = 8.2 Hz), 7.16 (d, Ar-H, 1H, J = 1.0 Hz) 7.08 (dd, Ar-H, 1H, J = 7.9 Hz, J = 1.3 Hz), 6.94 (d, Ar-H, 2H, J = 8.8 Hz), 4.41-4.36 (m, -OCH, 1H), 2.63 (t, -OCH2, 2H, J = 7.7 Hz), 1.82-1.24 (m, -CH2-, -CH3, 24H), 0.90-0.88 (m, -CH3, 6H).

13

C-NMR (CDCl3): 160.6, 159.2, 157.7,

143.9, 140.7, 131.7, 128.5, 127.4, 122.4, 122.3, 119.9, 119.8, 119.2, 116.5, 91.9, 88.6, 74.3, 36.41, 35.9, 31.8, 31.2, 29.3, 29.2, 29.1, 25.5, 22.6, 22.6, 19.7, 14.1. FT-IR (KBr): 3672, 2956, 2925, 2851, 2203, 1615, 1506, 1467, 1380, 1244 cm-1. Elemental analysis for C36H45NO2(percent): calculated C, 82.65, H, 8.66, N, 2.67; found C, 82.75, H, 8.50, N, 2.64. Compounds OH TII8 : 1H-NMR (CDCl3): δ (ppm) 13.56 (s, Ar-OH, 1H), 8.61 (s, -CH=N-, 1H), 7.47 (d, Ar-H, 1H, J = 8.0 Hz), 7.34 (d, Ar-H, 2H, J = 8.1 Hz), 7.28 (d, Ar-H, 2H, J = 7.2 Hz), 7.18(d, Ar-H, 2H, J = 8.1 Hz), 7.16 (d, Ar-H, 1H, J = 1.0 Hz), 7.08 (dd, Ar-H, 1H, J = 7.9 Hz, J = 1.3 Hz), 6.94 (d, Ar-H, 2H, J = 8.8 Hz), 4.41-4.35 (m, -OCH, 1H), 2.63 (t, -OCH2, 2H, J = 7.8 Hz), 1.82-1.23 (m, -CH2-, -CH3, 25H), 0.91-0.88 (m, -CH3, 6H).

13

C-NMR (CDCl3): 160.6, 159.2, 143.9,

140.7, 131.6, 128.5, 127.4, 122.3, 122.3, 119.9, 119.8, 119.2, 116.5, 91.9, 88.6, 74.3, 19



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36.4, 35.3, 31.8, 31.7, 31.2, 29.4, 29.2, 25.5, 22.6, 22.5, 19.7, 14.1. FT-IR (KBr): 3672, 2954, 2920, 2848, 2209, 1615, 1506, 1496, 1380, 1243 cm-1. Elemental analysis for C37H47NO2(percent): calculated C, 82.64, H, 8.81, N, 2.60; found C, 82.80, H, 8.85, N, 2.64. Compound H-TII6 : 1H-NMR (CDCl3): δ (ppm) 8.49 (s, -CH=N-, 1H), 7.87 (d, Ar-H, 2H, J = 8.2 Hz), 7.61 (d, Ar-H, 2H, J = 8.3 Hz), 7.47 (d, Ar-H, 2H, J = 8.1 Hz), 7.24 (dd, Ar-H, 2H, J = 2.4 Hz, J = 2.3 Hz), 7.18 (d, Ar-H, 2H, J = 8.2 Hz), 7.0 6.92 (dd, Ar-H, 2H, J = 2.5 Hz, J = 2.2 Hz), 4.40-4.34 (m, -OCH, 1H), 2.63 (t, -OCH2, 2H, J = 7.8 Hz), 1.82-1.22 (m, -CH2-, -CH3, 21H), 0.90-0.88 (m, -CH3, 6H). 13

C-NMR (CDCl3): 157.2, 144.4, 143.8, 135.6, 131.6, 131.5, 128.6, 1258.4, 126.1,

122.3, 120.1, 119.4, 92.0, 88.7, 74.3, 36.5, 36.0, 31.8, 31.7, 31.2, 29.3, 29.0, 25.6, 22.6, 19.8, 14.1. FT-IR (KBr): 3436, 2929, 2856, 2206, 1615, 1517, 1496, 1288, 1245 cm-1. Elemental analysis for C35H43NO(percent): calculated C, 85.14, H, 8.78, N, 2.84; found C, 85.27, H, 8.92, N, 2.80. Theoretical calculation All Density Functional Theory (DFT) calculations were performed in the Gaussian09 package.41 The calculation strategy is based on our previous report.35 The coordinates used for geometry optimization of all Schiff base compounds were initially built in Avogadro program42 then optimized by long-range corrected hybrid functional CAM-B3LYP43 in G09 program. The basis sets of 20


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6-311G(d,p) was used to calculate the dipole moment and polarizability. In order to compare the dipole moment of all molecules, the origin was set at the center of phenyl ring, and the x-axis is parallel with the longitudinal direction and z-axis is perpendicular to the phenyl plane. The molecular structures, dipole moments, and isosurface plots of the molecular orbitals were generated using the GaussView5.0.44

3. Results and discussion 3.1. The Phase Transition Behaviors and X-ray Powder Diffraction Study of Schiff Base Mesogens As shown in Table 1, the phase transition temperatures and enthalpies of four series of salicylaldimine-based mesogens were determined by DSC (Figure 1S) and POM. Compounds OH-TIn and OH-TIIn with alkynyl linkage exhibit phase sequence of isotropic (Iso)-nematic (N)-smetic C (SmC)- soft crystal G (CrG)- soft crystal H (CrH). Figure 2a exhibits schlieren texture can be observed at the appearance of nematic phase for compound OH-TII6 on cooling process. On further cooling, the striated texture or transition bar is observed at the phase transition, indicating the characteristic transition of nematic to smectic C phase (Figure 2b). The SmC phase can be characterized by the schlieren texture of four brush singularities (Figure 2c). After SmC phase disappears, the texture of a soft crystal G 21



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phase is mosaic kind and retains schlieren characteristics of SmC phase (Figure 2d). On further cooling, mosaic terrace-like relief with larger domains immediately appears (CrH), as shown in Figure 2e.45-46 The smectic phase and the soft crystal assignments of these Schiff base compounds were also confirmed by XRD measurements. Figure 3b shows that the lattice constants a and c in the hexagonal phase (CrG) are 5.15 Å and 25.66 Å, respectively. The weak and wide angle peak q3 becomes sharper from SmC phase to CrG phase, which may indicate that the packing of the mesogenic molecules is getting more order. In Figure 3c, the XRD pattern can be indexed by monoclinic crystal system with cell constants, a = 25.58(6) Å, b = 4.393(6) Å, c = 5.607(8) Å, and β = 96.0(1)º. To correlate the phase transition from hexagonal to monoclinic system, this standard unit cell setting can be transformed into a = 4.393(6) Å, b = 5.607(8) Å, c = 25.58(6) Å, and α = 96.0(1)º. The cell constants of CrH at 34°C indicate slight distortion in comparison with that of CrG at 50°C. The indexation of both standard and transformation settings are listed in Table 2. However, salicylaldimine-based mesogens OH-EIn and OH-EIIn possessing more flexible ester linkage between two rigid cores exhibit phase sequence of Iso-N-SmC-Cr. In addition, their phase transition temperatures are lower than that those of OH-TIn and OH-TIIn. Table 3 shows the phase transition temperatures and enthalpies of racemic three series of Schiff base mesogens H-TIIn, 22


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H-EIn and H-EIIn without hydroxyl group. Compounds H-TII6 with alkynyl linkage and terminal alkyl chain shows the same phase sequence with compound H-TI7. Notably, compounds H-EIIn with ester linkage and terminal alkyl chain also exhibit two soft crystal G and H phases in addition to N and SmC phases. The temperature dependent XRD patterns obtained from the powder sample of compound H-EII8 are shown in Figure S6. Figure S6a shows that a layer structure with a periodicity d spacing ~ 28.89 Å, and the molecular width w0 ~ 4.55 Å, indicating the packing characteristics of smectic C phase of H-EII8. In addition, Figure 4d shows H-EII8 at 36 °C retains schlieren texture of SmC phase and exhibits mosaic patches, implying the characteristics of the hexagonal CrG phase with cell constants, a =5.18 Å and c =28.99 Å.35 On further cooling, the characteristics of schlieren texture disappear and irregular grain boundary emerges in POM texture as the characteristic of soft crystal H (Figure 4e). The XRD patterns of CrH can be indexed by monoclinic crystal system with cell constants, a = 28.72(4) Å, b = 2.621(2) Å, c = 5.276(6) Å, and β = 94.67(9)º. This standard unit cell can be transformed into a = 2.621(2) Å, b = 5.276(6) Å, c = 28.72(4) Å, and α = 94.67(9)º. The indexation of both standard and transformation settings are listed in Table S1. The cell constants of CrH indicate the molecular packing becomes more ordering with roughly half of cell volume in comparison with that of CrG at 34°C. Based on 23



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the results of the POM textures, DSC (Figure S1f) and XRD patterns (Figure S6), it can be inferred that Schiff base mesogens H-EIIn with ester linkage possess soft crystal G and H phases.

Table 1. Phase transition temperature and corresponding transition enthalpies of salicyla1dimine-based mesogens a phase sequenceb

a b

(°C, ∆H/ kJmol-1)

compounds

Cooling

Heating

OH-TI6

Iso 165.5 (1.4) N 103.2 (1.9) SmC 80.1 (3.3) CrG 70.8 (1.0) CrH

Cr 86.0 (21.5) SmC 103.5 (2.5) N 168.8 (1.3) Iso

OH-TI7

Iso 160.6 (1.0) N 110.6 (1.7) SmC 81.2 (1.8) CrG 75.9 (1.4) CrH

Cr. 82.2 (13.8) SmC 112.9 (2.1) N 163.7 (0.8) Iso

OH-TI8

Iso 161.7 (1.0) N 118.0 (2.2) SmC 84.3 (2.2) CrG 78.0 (1.4) CrH

Cr. 84.2 (13.7) SmC 118.7 (2.2) N 163.2 (1.2) Iso

OH-TII6

Iso 134.5 (0.79) N 72.9 (1.51) SmC 52.5 (0.62) CrG 48.5 (0.73) CrH

Cr. 72.3 (49.4) N 134.9 (0.7) Iso

OH-TII7

Iso 137.9 (1.03) N 84.5 (1.45) SmC 61.8 (0.72) CrG 59.3 (0.51) CrH 41.5 (0.81) Cr.

Cr. 47.5 (9.6) CrH 60.80 (0.6) CrG 62.72 (0.6) SmC 85.11 (1.4) N 138.92 (1.0) Iso

OH-TII8

Iso 134.2 (0.84) N 72.9 (1.39) SmC 52.6 (0.65) CrG 48.7 (0.73) CrH

Cr. 62.3 (47.2) SmC 89.6 (1.4) N 132.6 (0.7) Iso

OH-EI6

Iso 148.0 (0.86) N 81.8 (1.17) SmC 34.4 (0.05) Cr.

Cr. 65.9 (18.4) SmC 81.3 (1.1) N 146.0 (0.9) Iso

OH-EI7

Iso 146.5 (1.06) N 95.1 (1.99) SmC 41.7 (0.31) Cr.

Cr. 77.3 (25.0) SmC 96.0 (2.0) N 147.5 (1.0) Iso

OH-EI8

Iso 146.6 (1.07) N 104.5 (1.64) SmC 51.3 (16.6) Cr.

Cr. 76.1 (25.0) SmC 104.9 (1.7) N 147.5 (1.0) Iso

OH-EII6

Iso 116.9 (0.62) N 54.0 (0.83) SmC

Effect of the Functional Groups of Racemic Rodlike Schiff Base

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