Synthesis and smectogenic properties of novel phloroglucinol-based star-shaped liquid crystals containing three peripheral alkyloxylated Schiff base arms

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Synthesis and smectogenic properties of novel

phloroglucinol-based star-shaped liquid crystals

containing three peripheral alkyloxylated Schiff base

arms

Yew-Hong Ooi a , Guan-Yeow Yeap a , Chun-Chieh Han b , Hong-Cheu Lin b , Kenji Kubo c & Masato M. Ito c

a

Liquid Crystal Research Laboratory , School of Chemical Sciences, Universiti Sains Malaysia , 11800 , Penang , Malaysia

b

Department of Materials Science and Engineering , National Chiao Tung University , Taiwan , Republic of China

c

Department of Environmental Engineering for Symbiosis , Soka University , Tokyo , Japan Published online: 08 Jan 2013.

To cite this article: Yew-Hong Ooi , Guan-Yeow Yeap , Chun-Chieh Han , Hong-Cheu Lin , Kenji Kubo & Masato M. Ito (2013) Synthesis and smectogenic properties of novel phloroglucinol-based star-shaped liquid crystals containing three peripheral alkyloxylated Schiff base arms, Liquid Crystals, 40:4, 516-527, DOI: 10.1080/02678292.2012.761734

To link to this article: http://dx.doi.org/10.1080/02678292.2012.761734

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Vol. 40, No. 4, 516–527, http://dx.doi.org/10.1080/02678292.2012.761734

Synthesis and smectogenic properties of novel phloroglucinol-based star-shaped liquid crystals

containing three peripheral alkyloxylated Schiff base arms

Yew-Hong Ooia_{, Guan-Yeow Yeap}a_{*, Chun-Chieh Han}b_{, Hong-Cheu Lin}b_{, Kenji Kubo}c_{and Masato M. Ito}c

a_{Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia;}

b_{Department of Materials Science and Engineering, National Chiao Tung University, Taiwan, Republic of China;}c_Department

of Environmental Engineering for Symbiosis, Soka University, Tokyo, Japan

(Received 23 July 2012; final version received 19 December 2012)

A series of symmetrical trimeric liquid crystalline compounds of which the molecular structure with a central core of 1,3,5-benzene attached by three rod-like mesogenic Schiff base moieties via the propylene spacers and ether linkages has successfully been synthesised and characterised by infra red and nuclear magnetic resonance spectroscopic techniques. All the star-shaped compounds in this series exhibit predominantly SmC phase except the analogue possessing terminal C8H17group. It is apparent that the members with even parity from C10H21to

C16H33 show enantiotropic SmC phase while the member with longest terminal chain of C18H37 is inclined to

monotropic smectogen. The X-ray diffraction measurements reveal that the tilted smectic layer structures of the SmC phase are confirmed to have an obvious sharp peak at small angles of 2θ ∼ 1.03◦–1.48◦with d-spacing values of 4.01–4.58 nm, which are corresponding to tilt angles of∼48◦in the SmC phase.

Keywords: phloroglucinol; Schiff bases; star-shaped liquid crystals; smectic C; XRD

1. Introduction

The development in science and technology has led to the growing interest in the synthesis and investigation of unconventional liquid crystals. Apart from the typi-cal classes of conventional liquid crystals composed of rod-like (calamitic) and disc-like (discotic) molecules, the liquid crystals such as multi-arm mesogens [1–4], oligomers [5–10], bent-core mesogen [11–15] as well as supramolecular mesogens including the metallomeso-gens [16–22] and hydrogen-bonded mesometallomeso-gens [23–26] have also been widely studied. These compounds can be used as the fundamental prerequisite for the for-mation of unconventional thermotropic liquid crystals because their anisotropic properties play a vital role in this interesting state of soft material. Unconventional star-shaped liquid crystals consisting of a small central core connected with a few extended semi-rigid meso-genic units as the peripheral arm has attracted increas-ing attention and research interest [27–35]. Followincreas-ing the pioneering work by Attard et al. [36], the sim-plest star-shaped liquid crystals composed of a small aromatic core unit surrounded by three mesogenic units had become the most commonly studied liquid crystalline materials even though more sophisticated star-shaped liquid crystals consisting up to three meso-genic arms had been reported [1–4]. In general, the molecular design of star-shaped or disc-like liquid crystals have based on benzene core and 1,3,5-triazine

*Corresponding author. Email: [email protected]

core bearing long, flexible alkyl or alkoxyl chains at the periphery.

Phloroglucinol or 1,3,5-trihydroxybenzene is a sim-ple six-membered ring with three hydroxyl groups attached to 1, 3 and 5 positions in aromatic ring. It is an important class of organic compound which has commonly been used in cosmetics, pesticides, paints and dyeing [37] as well as in the synthesis of pharmaceuticals, explosives and polymeric sub-stances. In addition, researchers has once isolated phloroglucinol from natural sources such as the coastal woodfern Dryopteris arguta [38] and subse-quently some of its derivatives were found naturally in certain plant species [39–42].

On the other hand, phloroglucinol had also been employed in the synthesis of star-shaped and discotic liquid crystals [27–30] as well as hydrogen-bonded phloroglucinol-based system [23–25]. The incorpora-tion of different mesogenic side chain has conveniently led to the molecular rearrangement which entailed the crossing from rod-like to disc-like domains. Moreover, the mesogenic unit thus introduced could enhance the mesomorphic properties. Hence, star-shaped liq-uid crystals are considered as one of the important aspects for the theoretical understanding of the liquid crystalline phenomenon.

The earlier study showed that the direct esterifi-cation of ω-[4-(p-alkoxybenzoloxy)phenoxycarbonyl]

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C_nH2n + 1 C_nH2n + 1 CnH2n + 1 O COO COO COO O O O O O O O O O O O O O O (3a: n=1,3b: n=2,3c: n=3)

Figure 1. Molecular structures of the star-shaped liquid crystals tri[ω-4-(p-alkoxybenzoloxy)phenoxycarbonyl] valeric acid phloroglucinol ester, 3a–3c, prepared by Zhang et al. [28]

valeric acid (Figure 1) as mesogenic arms would result in a typical nematic phase. Besides, the documented liquid crystals did not undergo crystallisation upon cooling but vitrify into a glassy state. This observa-tion had received a considerable attenobserva-tion because the glass-forming liquid crystalline materials possess inter-esting optical, mechanical as well as thermal stable properties [43–46] which may be useful as soft mate-rials for fabrication of thermo-electro-optical storage and different optical components [1,28]. In relation to this study, the star-shaped liquid crystals incorporat-ing cholesterol unit as the chiral component had also been given great attention [29].

We have recently reported a series of three-armed star-shaped analogues containing (4-benzylidene-substituted-aniline-4-oxy)-6-bromohexane as periph-eral arm in which the 4-position at one end con-sisting different terminal substituent X (X = F, Cl, Br, I and C2H5) [47]. They were found to exhibit calamitic mesophase. Consequently, the question on how to modify the substitution of the peripheral arm of 1,3,5-trisubstituted-benzene central core in order to exhibit highly ordered smectic phases has driven us to investigate further. Therefore, as the continu-ation of our effort to further investigate the prop-erties of star-shaped compounds, we report in this article another new series of the homologous three-armed star-shaped liquid crystals wherein the three rod-like (p-alkyloxybenzylidene) anilines with differ-ent length of even parity alkyl chain ranging from 8 to 18 are attached to a disc-shaped benzene ring via a propylene spacer and ether linkages. Since the liquid crystalline properties of the star-shaped liquid crystals bearing three Schiff base fragments compris-ing different polarity and molecular size of terminal substituent has been explored, our present objective is to investigate the mesomorphic properties of the star-shaped liquid crystals consisting of three Schiff base moieties with different length of terminal alkyl chain.

2. Experimental

2.1 Chemicals and reagents

Phloroglucinol anhydrous, aminophenol and p-hydroxybenzaldehyde were purchased from Acros Organics (Geel, Belgium). Bromooctane, bromode-cane, bromododebromode-cane, bromotetradebromode-cane, bromohex-adecane, bromooctadecane and 1,3-dibromopropane were obtained from Merck (Darmstadt, Germany). While potassium carbonate was purchased from Systerm (Kielce, Poland), potassium iodide was obtained from Fischer Scientific (Waltham, MA, USA). All the chemicals as well as solvents were pur-chased commercially and used directly from the bottles without further purification.

2.2 Methods and characterisation

Structural elucidation of intermediary and title com-pounds was carried out by elemental analyses, Fourier transform infrared (FTIR) and NMR spectroscopies. The carbon, hydrogen and nitrogen (CHN) microanal-yses were conducted via Perkin-Elmer 2400 LS series CHN analyser. The IR spectra were recorded and measured on a Perkin-Elmer 2000 FTIR spectropho-tometer where the samples were prepared in the form of KBr pellet and the spectra were recorded within a frequency range of 400–4000 cm−1. NMR spec-tra were recorded by using Bruker Avance 300 MHz and 500 MHz Ultrashield FT-NMR spectrometers in which the deuterated chloroform (CDCl3) and DMSO-d6 with tetramethylsilane as internal stan-dard were used as the solvents. Complete 1H and 13_{C NMR assignments of the representative} com-pounds were obtained and substantiated by means of the two-dimensional spectroscopic measurement such as1_H–1_{H correlation spectroscopy,}1_H–13_C het-eronuclear multiple quantum correlation and1_H–13_C heteronuclear multiple bond correlation.

Phase-transition temperatures and enthalpy changes were measured using Differential Scanning

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Calorimeter Mettler Toledo DSC823e_{with the heating} and cooling rates of +2◦C/min and −2◦C/min, respectively. Carl Zeiss polarizing optical microscope (POM) equipped with Linkam heating stage in the School of Chemical Sciences, USM (Penang, Malaysia), was used for temperature-dependent stud-ies and texture observation. Samples were prepared as thin films sandwiched between a glass slide and a coverslip.

Synchrotron powder X-ray diffraction (XRD) measurements were performed at beamline BL17A of the National Synchrotron Radiation Research Center, Taiwan, where the wavelength of X-ray was 1.33366 Å. The powder samples were packed into capillary tubes and heated by a heat gun, whose temperature controller was programmable by a computer with a proportional, integral and differential feedback sys-tem. The scattering angleθ was calibrated by a mixture of silver behenate and silicon.

2.3 Synthesis

Scheme 1 illustrates the structures and the syn-thetic pathway towards the formation of intermediates

1a–1f, 2a–2f, 3a–3f and the symmetric star-shaped

compounds 4a–4f. The peripheral mesogenic units were prepared by alkylation of various bromoalkane with p-hydroxybenzaldehyde and then condensation of p-alkyloxybenzaldehyde with p-aminophenol to afford the Schiff bases 2a–2f. The flexible spacers were subsequently introduced by alkylation of com-pounds 2a–2f with the excess of 1,3-dibromopropane in the presence of potassium carbonate as a base to yield compounds 3a–3f. Excess of com-pounds 3a–3f was further reacted with phloroglucinol anhydrous (1,3,5-trihydroxybenzene) by Williamson etherification to produce the desired star-shaped molecule 2,4,6-tri{[ω-4-(alkoxybenzylidene)aniline-4 -oxy]propoxy}benzene, 4a–4f. The codes for com-pounds 3a–3f and 4a–4f were shown in Table 1.

2.3.1 Synthesis of compound 2

In a round-bottom flask containing an equimolar of compound 1 and p-aminophenol in 50 mL absolute ethanol, three drops of acetic acid were added. The reaction mixture was then heated at reflux for 3 h before it was cooled down to room temperature. The precipitate thus isolated was then recrystallised from ethanol to yield the desired intermediate 2. The ana-lytical data for compounds 2a–2f are summarised as follows:

2a: Yield: 78%. Beige. Elemental analysis: found,

C 77.72, H 8.40, N 4.33; calculated (C21H27NO2), C 77.50, H 8.36, N 4.30. IR (KBr) v/cm−1: 3392 (OH phenolic), 2954, 2920, 2851 (C–H apliphatic), 1609–1627 (C=N azomethine), 1262 (C–O ether). 1_{H-NMR (CDCl}

3) δ/ppm: 0.90 (t, 3H, CH3), 1.29–1.49 (m, 10H, CH2), 1.77–1.86 (m, 2H, CH2), 4.05 (t, 2H, OCH2), 5.01 (s, 1H, OH), 6.86 (d, 2H, Ar), 6.98 (d, 2H, Ar), 7.17 (d, 2H, Ar), 7.84 (d, 2H, Ar), 8.41 (s, 1H, CH=N).

2b: Yield: 81%. Beige. Elemental analysis: found,

C 78.48, H 8.81, N 4.09; calculated (C23H31NO2), C 78.15, H 8.84, N 3.96. IR (KBr) v/cm−1: 3392 (OH phenolic), 2955, 2917, 2851 (C–H apliphatic), 1609–1625 (C=N azomethine), 1254 (C–O ether).1 H-NMR (CDCl3) δ/ppm: 0.92 (t, 3H, CH3), 1.29–1.50 (m, 14H, CH2), 1.77–1.85 (m, 2H, CH2), 4.05 (t, 2H, OCH2), 4.97 (s, 1H, OH), 6.87 (d, 2H, Ar), 6.98 (d, 2H, Ar), 7.15 (d, 2H, Ar), 7.84 (d, 2H, Ar), 8.41 (s, 1H, CH=N).

2c: Yield: 83%. Beige. Elemental analysis: found,

C 78.88, H 9.30, N 3.73; calculated (C25H35NO2), C 78.70, H 9.25, N 3.67. IR (KBr) v/cm−1: 3392 (OH phenolic), 2854, 2918, 2850 (C–H apliphatic), 1609–1625 (C=N azomethine), 1254 (C–O ether).1 H-NMR (CDCl3) δ/ppm: 0.92 (t, 3H, CH3), 1.29–1.50 (m, 18H, CH2), 1.77–1.86 (m, 2H, CH2), 4.04 (t, 2H, OCH2), 4.99 (s, 1H, OH), 6.87 (d, 2H, Ar), 7.00 (d, 2H, Ar), 7.18 (d, 2H, Ar), 7.86 (d, 2H, Ar), 8.40 (s, 1H, CH=N).

2d: Yield: 84%. Beige. Elemental analysis: found,

C 79.45, H 9.80, N 3.56; calculated (C27H39NO2), C 79.17, H 9.60, N 3.42. IR (KBr) v/cm−1: 3397 (OH phenolic), 2955, 2918, 2851 (C–H apliphatic), 1610–1625 (C=N azomethine), 1255 (C–O ether).1 H-NMR (CDCl3) δ/ppm: 0.90 (t, 3H, CH3), 1.28–1.48 (m, 22H, CH2), 1.75–1.85 (m, 2H, CH2), 4.04 (t, 2H, OCH2), 5.07 (s, 1H, OH), 6.89 (d, 2H, Ar), 7.00 (d, 2H, Ar), 7.19 (d, 2H, Ar), 7.85 (d, 2H, Ar), 8.40 (s, 1H, CH=N).

2e: Yield: 89%. Beige. Elemental analysis: found,

C 79.77, H 9.94, N 3.21; calculated (C29H43NO2), C 79.59, H 9.90, N 3.20. IR (KBr) v/cm−1: 3399 (OH phenolic), 2954, 2917, 2849 (C–H apliphatic), 1610–1625 (C=N azomethine), 1255 (C–O ether).1 H-NMR (CDCl3) δ/ppm: 0.90 (t, 3H, CH3), 1.27–1.50 (m, 26H, CH2), 1.75–1.85 (m, 2H, CH2), 4.03 (t, 2H, OCH2), 5.05 (s, 1H, OH), 6.86 (d, 2H, Ar), 6.98 (d, 2H, Ar), 7.17 (d, 2H, Ar), 7.83 (d, 2H, Ar), 8.40 (s, 1H, CH=N).

2f: Yield: 88%. Beige. Elemental analysis: found,

C 80.15, H 10.15, N 3.14; calculated (C31H47NO2), C 79.95, H 10.17, N 3.01. IR (KBr) v/cm−1: 3399

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1a–1f 2a–2f 3a–3f 4a–4f 4a 4b 4c 4d 4e 4f

Scheme 1. Synthetic pathway towards the formation of intermediates and star-shaped compounds 4a–4f.

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Table 1. The Schiff base intermediates 3a–3f and the title star-shaped compounds 4a–4f.

R Intermediate Compound C8H17 3a 4a C10H21 3b 4b C12H25 3c 4c C14H29 3d 4d C16H33 C18H37 3e 3f 4e 4f

(OH phenolic), 2954, 2918, 2849 (C-H apliphatic), 1609–1625 (C=N azomethine), 1255 (C-O ether).1 H-NMR (CDCl3) δ/ppm: 0.90 (t, 3H, CH3), 1.28–1.51 (m, 30H, CH2), 1.77–1.85 (m, 2H, CH2), 4.03 (t, 2H, OCH2), 5.07 (s, 1H, OH), 6.87 (d, 2H, Ar), 6.98 (d, 2H, Ar), 7.18 (d, 2H, Ar), 7.85 (d, 2H, Ar), 8.41 (s, 1H, CH=N).

A mixture of compound 2 (0.015 mol) and excess of 1,3-dibromopropane (0.06 mol, 12.1 g) were dissolved in 50 mL acetone followed by the addition of anhy-drous potassium carbonate (0.045 mol, 6.2 g). The reaction mixture was refluxed for 8 h before it was left at room temperature to allow complete evaporation of the solvent. Fifty millilitre of distilled water was added and the resulting precipitate was filtered and rinsed with hexane to remove the excess dibromoalkane. The crude product was recrystallised from acetone to yield the pure compound 3. Summary of the analytical data for 3a–3f is shown below:

3a: Yield: 67%. White. Elemental analysis: found,

C 64.72, H 7.29, N 3.20; calculated (C24H32NO2Br), C 64.57, H 7.23, N 3.14. IR (KBr) v/cm−1: 2955, 2922, 2854 (C–H apliphatic), 1606–1620 (C=N azomethine), 1249 (C–O ether). 1_{H-NMR (CDCl} 3) δ/ppm: 0.90 (t, 3H, CH3), 1.29–1.49 (m, 10H, CH2), 1.77–1.87 (m, 2H, CH2), 2.31–2.40 (m, 2H, CH2), 3.64 (t, 2H, BrCH2), 4.05 (t, 2H, OCH2), 4.14 (t, 2H, OCH2), 6.95 (d, 2H, Ar), 7.00 (d, 2H, Ar), 7.24 (d, 2H, Ar), 7.86 (d, 2H, Ar), 8.41 (s, 1H, CH=N).

3b: Yield: 71%. White. Elemental analysis: found,

C 65.90, H 7.67, N 3.03; calculated (C26H36NO2Br), C 65.82, H 7.65, N 2.95. IR (KBr) v/cm−1: 2954, 2918, 2849 (C-H apliphatic), 1607–1622 (C=N azome-thine), 1251 (C–O ether).1_{H-NMR (CDCl}

3)δ/ppm: 0.90 (t, 3H, CH3), 1.30–1.52 (m, 14H, CH2), 1.78–1.85 (m, 2H, CH2), 2.31–2.38 (m, 2H, CH2), 3.64 (t, 2H, BrCH2), 4.04 (t, 2H, OCH2), 4.15 (t, 2H, OCH2), 6.94 (d, 2H, Ar), 6.98 (d, 2H, Ar), 7.22 (d, 2H, Ar), 7.84 (d, 2H, Ar), 8.41 (s, 1H, CH=N).

3c: Yield: 70%. White. Elemental analysis: found,

C 67.04, H 8.14, N 2.83; calculated (C28H40NO2Br), C 66.92, H 8.02, N 2.79. IR (KBr) v/cm−1: 2954, 2919, 2848 (C–H apliphatic), 1608–1623 (C=N azomethine), 1255 (C–O ether). 1_{H-NMR (CDCl} 3) δ/ppm: 0.90 (t, 3H, CH3), 1.29–1.52 (m, 18H, CH2), 1.78–1.86 (m, 2H, CH2), 2.31–2.39 (m, 2H, CH2), 3.64 (t, 2H, BrCH2), 4.04 (t, 2H, OCH2), 4.15 (t, 2H, OCH2), 6.95 (d, 2H, Ar), 7.00 (d, 2H, Ar), 7.24 (d, 2H, Ar), 7.86 (d, 2H, Ar), 8.41 (s, 1H, CH=N).

3d: Yield: 72%. White. Elemental analysis: found,

C 68.00, H 8.39, N 2.62; calculated (C30H44NO2Br), C 67.91, H 8.36, N 2.64. IR (KBr) v/cm−1: 2954, 2918, 2850 (C–H apliphatic), 1608–1623 (C=N azomethine), 1252 (C–O ether). 1H-NMR (CDCl3) δ/ppm: 0.90 (t, 3H, CH3), 1.28–1.51 (m, 22H, CH2), 1.78–1.87 (m, 2H, CH2), 2.31–2.39 (m, 2H, CH2), 3.64 (t, 2H, BrCH2), 4.04 (t, 2H, OCH2), 4.14 (t, 2H, OCH2), 6.95 (d, 2H, Ar), 6.99 (d, 2H, Ar), 7.24 (d, 2H, Ar), 7.86 (d, 2H, Ar), 8.41 (s, 1H, CH=N).

3e: Yield: 75%. White. Elemental analysis: found,

C 68.94, H 8.70, N 2.55; calculated (C32H48NO2Br), C 68.80, H 8.66, N 2.51. IR (KBr) v/cm−1: 2955, 2918, 2850 (C–H apliphatic), 1608–1622 (C=N azomethine), 1251 (C–O ether). 1_{H-NMR (CDCl} 3) δ/ppm: 0.90 (t, 3H, CH3), 1.28–1.51 (m, 26H, CH2), 1.78–1.87 (m, 2H, CH2), 2.30–2.39 (m, 2H, CH2), 3.64 (t, 2H, BrCH2), 4.03 (t, 2H, OCH2), 4.14 (t, 2H, OCH2), 6.94 (d, 2H, Ar), 6.98 (d, 2H, Ar), 7.21 (d, 2H, Ar), 7.83 (d, 2H, Ar), 8.41 (s, 1H, CH=N).

3f: Yield: 72%. White. Elemental analysis: found,

C 70.01, H 8.92, N 2.40; calculated (C34H52NO2Br), C 69.61, H 8.93, N 2.39 IR (KBr) v/cm−1: 2954, 2917, 2849 (C-H apliphatic), 1607–1622 (C=N azome-thine), 1252 (C-O ether). 1_{H-NMR (CDCl}

3) δ/ppm: 0.89 (t, 3H, CH3), 1.28–1.52 (m, 26H, CH2), 1.78–1.87 (m, 2H, CH2), 2.31–2.39 (m, 2H, CH2), 3.64 (t, 2H, BrCH2), 4.03 (t, 2H, OCH2), 4.14 (t, 2H, OCH2), 6.94 (d, 2H, Ar), 6.98 (d, 2H, Ar), 7.22 (d, 2H, Ar), 7.83 (d, 2H, Ar), 8.41 (s, 1H, CH=N).

Although the same reaction has previously been reported by Yoon et al. [48], the present compounds

4a–4f are obtained by using the method which has

been modified as follows.

In a round bottom flask, phloroglucinol anhydrous was dissolved in 10 mL dimethylformamide (DMF) followed by the addition of potassium carbonate anhy-drous. The mixture was stirred at 80◦C for an hour and then compounds 3a–3f, which were dissolved in DMF, were added into the reaction mixture with a catalytic amount of potassium iodide. The reaction mixture was heated at 80◦C for 48 h. When the reaction mixture

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was cooled to room temperature, 500 mL of cold water was added. The crude precipitate was collected by filtration and then recrystallised twice by using DMF to afford the desired star-shaped compounds

4a–4f.

4a: Yield: 24%. Beige. Elemental analysis: found,

C 76.69, H 8.20, N 3.48; calculated (C78H99N3O9), C 76.62, H 8.16, N 3.44. IR (KBr) v/cm−1: 2956, 2922, 2854 (C–H apliphatic), 1621 (C=N azomethine), 1249 (C–O ether). 1_{H-NMR (CDCl} 3) δ/ppm: 0.91 (t, 9H, CH3), 1.31–1.53 (m, 30H, CH2), 1.80–1.85 (m, 6H, CH2), 2.25–2.30 (m, 6H, CH2), 4.02 (t, 6H, OCH2), 4.13–4.19 (m, 12H, OCH2), 6.15 (s, 3H, Ar), 6.95 (d, 6H, Ar), 6.98 (d, 6H, Ar), 7.20 (d, 6H, Ar), 7.83 (d, 6H, Ar), 8.40 (s, 3H, CH=N). 13_{C-NMR (CDCl} 3)δ/ppm: 158.04 (CH=N), 161.64, 160.77, 157.22, 145.37, 130.25, 129.22, 122.08, 115.00, 114.66, 94.15 (Caromatic), 68.20, 64.70, 64.56 (OCH2), 22.71–31.93 (Caliphatic), 14.13 (CH3).

4b: Yield: 24%. Beige. Elemental analysis: found,

4c: Yield: 23%. Beige. Elemental analysis: found,

4d: Yield: 30%. Beige. Elemental analysis: found,

C 78.09, H 9.25, N 2.81; calculated (C96H135N3O9), C 78.17, H 9.22, N 2.85. IR (KBr) v/cm−1: 2954, 2918, 2850 (C–H apliphatic), 1622 (C=N azomethine), 1252 (C–O ether). 1_{H-NMR (CDCl} 3) δ/ppm: 0.90 (t, 9H, CH3), 1.29–1.51 (m, 66H, CH2), 1.80–1.85 (m, 6H, CH2), 2.23–2.28 (m, 6H, CH2), 4.02 (t, 6H, OCH2), 4.13–4.19 (m, 12H, OCH2), 6.15 (s, 3H,

Ar), 6.94 (d, 6H, Ar), 6.98 (d, 6H, Ar), 7.20 (d, 6H, Ar), 7.83 (d, 6H, Ar), 8.40 (s, 3H, CH=N). 13_{C-NMR (CDCl} 3)δ/ppm: 158.02 (CH=N), 161.64, 160.77, 157.22, 145.38, 130.25, 129.25, 122.08, 115.00, 114.67, 94.17 (Caromatic), 68.20, 64.70, 64.56 (OCH2), 22.72–31.95 (Caliphatic), 14.15 (CH3).

4e: Yield: 28%. Beige. Elemental analysis: found,

C 78.66, H 9.61, N 2.72; calculated (C102H147N3O9), C 78.57, H 9.50, N 2.69. IR (KBr) v/cm−1: 2954, 2918, 2850 (C–H apliphatic), 1622 (C=N azomethine), 1251 (C–O ether). 1_{H-NMR (CDCl} 3) δ/ppm: 0.90 (t, 9H, CH3), 1.29–1.51 (m, 78H, CH2), 1.80–1.85 (m, 6H, CH2), 2.23–2.38 (m, 6H, CH2), 4.02 (t, 6H, OCH2), 4.13–4.20 (m, 12H, OCH2), 6.15 (s, 3H, Ar), 6.95 (d, 6H, Ar), 6.98 (d, 6H, Ar), 7.20 (d, 6H, Ar), 7.83 (d, 6H, Ar), 8.41 (s, 3H, CH=N). 13_{C-NMR (CDCl} 3)δ/ppm: 158.02 (CH=N), 161.64, 160.77, 157.22, 145.38, 130.25, 129.24, 122.08, 115.00, 114.66, 94.17 (Caromatic), 68.20, 64.70, 64.56 (OCH2), 22.71–31.95 (Caliphatic), 14.14 (CH3).

4f: Yield: 33%. Beige. Elemental analysis: found,

3. Results and discussion

The thermal stability and mesomorphic properties of the peripheral mesogenic units 3a–3f have earlier been reported [49]. While the phase-transition tempera-tures, associated enthalpy and entropy changes for the SmC-I transition obtained from differential scanning calorimetry (DSC) analysis on heating and cooling for compounds 4a–4f are listed in Table 2, the DSC traces for a representative compound 4d on heating and cooling are depicted in Figure 2.

3.1 Thermal stability and mesomorphic properties

All the star-shaped compounds except for member possessing octyl terminal chain (compound 4a) exhibit liquid crystalline properties as revealed by DSC and POM. From the results thus obtained, compounds

4c, 4d and 4e are found to display enantiotropic

liquid crystals behaviour with the appearance of SmC

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Table 2. Phase transition temperature and the correspond-ing enthalpy changes for compounds 4a–4f durcorrespond-ing heatcorrespond-ing and cooling scan.

Compound

Phase transition temperature,◦C

(enthalpy change, kJ mol−1) SSmC-I/R

4a (R= C8H17) Cr 136.7 (93.3) I − I 132.4 (23.8) Cr1111.3 (51.2) Cr2 – 4b (R= C10H21) Cr 129.6 (58.9) SmC 134.8 (40.0) I 11.8 I 130.9 (23.4) SmC 111.4 (70.4) Cr 7.0 4c (R= C12H25) Cr 124.2 (86.1) SmC 137.5 (31.7) I 9.3 I 136.0 (19.1) SmC 100.7 (70.0) Cr 5.6 4d (R= C14H29) Cr 127.7 (102.2) SmC 144.6 (29.8) I 8.6 I 141.4 (30.4) SmC 103.7 (105.7) Cr 8.8 4e (R= C16H33) Cr 119.2 (123.7) SmC 141.0 (29.5) I 8.6 I 139.8 (29.8) SmC 106.4 (118.7) Cr 8.7 4f (R= C18H37) Cr 142.9 (238.3) I − I 140.8 (29.1) SmC 131.9 (230.1) Cr 8.4

Notes: Cr, crystal; SmC, smectic C; I, isotropic. Data in italic font denote the cooling cycle.

phase characterised by Schlieran texture (Figure 3) on both heating and cooling cycles. The SmC phase of compound 4b showed a broken fan-shaped texture (Figure 4) instead of Schlieren texture. One of the notable observations is the presence of both bro-ken fan-shaped and Schlieren textures in SmC phase (Figure 5).

Figure 3. Optical photomicrograph of compound 4d (R=C14H29) showing the Schlieran texture of SmC phase

during cooling cycle.

A correlation between the transition tempera-tures and number of carbon atom in the alkyloxy terminal chain is shown in Figure 6. It can be inferred from Figure 6 that the SmC phase range apparently increases as the length of the terminal alkyl chain is increased. However, the SmC phase range descends as the chain length reaches the maximum (R=C18H37). Larger SmC phase range is exhibited by the mem-bers possessing medium chain length. In short, on cooling, compound 4f exhibits the narrowest meso-genic region (8.9◦C), while compound 4d displays the widest mesophase temperature range of 37.7◦C. An interesting feature in relation to the liquid crys-talline properties for these star-shaped mesogens is that the SmC phase is less stable in the homologues

0.500 0.000 –0.500 –1.000 –1.500 –2.000 –2.500 –3.000 Hea t flow (mW) 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 Temperature (ºC) Cooling Cr Heating Cr SmC SmC I I

Figure 2. DSC trace for compound 4d (R=C14H29) on heating and cooling cycles at the rate of±2◦C min−1.

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Figure 4. Optical photomicrograph of compound 4b (R=C10H21) showing the broken fan-shaped texture of SmC

phase during cooling cycle.

Figure 5. Optical photomicrograph of compound 4f (R=C18H37) whereby simultaneous occurrence of Schlieran

texture and broken fan-shaped texture of SmC phase at the same time. 150 140 130 120 110 100 90 Temperature ( °C) 6 8 10 12 14 16 18 20

Number of carbon in the alkoxy group Isotropic

SmC

Cr

Figure 6. Plot of transition temperature against the number of carbon in the alkoxy chain during the cooling cycle.

possessing shortest and longest terminal chain length. This observation has earlier been reported for cyclic trimers derived from benzene-1,3,5-tricarboxylic acid

[36]. Although the present star-shaped molecule is derived from 1,3,5-trihydrobenzene, it also comprise of an aromatic benzene unit as the central core. Hence, it can be suggested that the terminal polarity group played an important factor in this phenomenon as it maintains the molecular orientation through the acting forces of molecular induction and polarisa-tion [28].

When the length of the terminal alkoxy chain reaches R=C14H29 (compound 4d), the attraction between the long alkyl chain leading to intertwining which is essential to facilitates the molecular pack-ing. As a result, it increases the structural anisotropy that gives widest mesogenic region detected in com-pound 4d.

However, compounds 4a and 4f exhibit an endotherm corresponding to the direct melting from crystal to isotropic liquid upon heating run. The subsequent cooling on compound 4a demonstrates an exotherm characteristic of the Iso-Cr and Cr-Cr transitions, which is indicative of the non-mesogenic properties. In contrary to compound 4a, compound

4f exhibits monotropic behaviour in which the SmC

phase is observed upon cooling cycle while the mesophase is absent upon heating.

It is worthwhile to mention that on cooling the present trimeric star-shaped mesogens exhibit I-SmC transition with the enthalpy changes ranging from 19.1 to 30.4 kJ mol−1. These values can be con-sidered high and comparable to those found in earlier reported unconventional trimeric analogues (27.1–43.1 kJ mol−1) [50]. The high enthalpy of I-SmC reveals a change of the star-shaped mesogens from high molecular order of the tilted arrangement to the diffuse isotropic state. In addition, the entropy changes associated with SmC-I transition of the star-shaped mesogens have also been determined and expressed in dimensionless quantity, SSmC-I/R in which R is the universal gas constant. The overall SSmC-I/R values of 4b–4f fall in the range 5.6–11.8. These values are similar to the earlier reported cyclic trimers homologues [36], even though the lat-ter exhibited I-SmA instead of I-SmC transition. The entropy change is seemed to be on the high side and this observation could be due to the higher order of the smectic phase, which has resulted from the specific interactions between the neighbouring molecules [36].

It can also be inferred from Table 2 that the entropy change with respect to I-SmC transition on cooling is dependence on the chain length of the methylene unit. A drastic change ofSSmC-I/R from 7.0 to 5.6 can be observed when the alkyloxy chain length increases from 4b to 4c. However, on ascend-ing alkyloxy chain from compounds 4d–4f, only small

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reduction of entropy values towards elongation of ter-minal chain length is observed. The differences of entropy values among 4d–4f are relatively small, which lie in the range of ± 0.4 only. This can probably be ascribed to the conformational degrees of free-dom of the terminal alkyloxy chain which affects the transition.

3.2 XRD analysis

In addition to two-phase transition peaks revealed by DSC, all liquid crystalline compounds (4b–4f) char-acterised as birefringent liquids by POM in their mesophase were also further investigated by powder XRD measurements. Figures 7–10 demonstrate typi-cal XRD patterns of 1- and 2-dimensional (1-D and 2-D) images of 4c and 4d, respectively, in the SmC

0 10 20 30 40 50 0 1000 2000 3000 4000 5000 6000 7000 Index 2θ

Figure 7. XRD data of compound 4c at 120◦C during the cooling process.

Figure 8. XRD data of compound 4c at 120◦C during the cooling process (2-D image).

0 10 20 30 40 50 0 1000 2000 3000 4000 5000 Index 2θ

Figure 9. XRD data of compound 4d at 120◦C during the cooling process.

Figure 10. XRD data of compound 4d at 120◦C during the cooling process (2-D image).

Table 3. XRD data (2θ and d-spacing values) of all liquid crystalline samples.

Compound Temperature 2_{θ (}◦) d-Spacing (nm)

4b 160◦C (cooling) 1.03 16.3 4.58 0.36

4c 120◦C (cooling) 1.48 17.4 4.01 0.34

4d 120◦C (cooling) 1.48 ₋ 4.01 ₋

4e 120◦C (cooling) 1.39 − 4.26 −

4f 160◦C (cooling) 1.36 16.4 4.35 0.36

phase with layered structures obtained during the cooling process. XRD data of all liquid crystalline compounds 4b–4f are illustrated in Table 3. All com-pounds 4b–4f show an obvious sharp peak at small angles of 2θ ∼1.03◦–1.48◦ (with d-spacing values of 4.01–4.58 nm, which are correspondent to tilt angles of ∼48◦_{with respect to a fully extended molecular chain} length of 5.8 nm), which could verify the tilted smectic

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layered structures of the SmC phase in compounds

4b–4f. The broad peaks at the wide angles of 2θ ∼

16.3◦–17.4◦ (with d-spacing values of 0.34–0.36 nm) are correspondent to the average distances between rigid rods of the SmC phase, which should be irrel-evant to their molecular chain lengths. Interestingly, as shown in Figure 10, we found that the 2-D image with two arcs of XRD data for compound 4d at 120◦C during the cooling process had a special aligned orien-tation, which was not observed in the 2-D images of the other compounds. Further XRD investigations for compound 4d will be needed to realise this particular aligned phenomenon.

The formation of smectic phase by the star-shaped mesogens 4b–4f is presumably due to a side-by-side organisation of the Schiff base peripheral units which lie parallel to each other. The conformation adopted by the trimeric compounds also explains why only smectic phase is present instead of a discotic phase because the propyl spacer linking the linearly aromatic peripherals and the benzene core is flexi-ble enough to allow the peripherals to rotate freely. Consequently, they favour the side-by-side interaction between neighbouring molecules. In short, the sym-metric star-shaped molecule is capable of folding an anisotropic shape to give rise to the appearance of smectic liquid crystalline order. The molecular struc-tures of rod-like Schiff base ether fragment as well as the non-polar terminal aliphatic chain are con-tributing factors towards the thermal stability as well as the structure of mesophase thus formed, that is, the smectic layers should be formed by the rod-like units and the benzene core acts only as a linking unit interconnecting the rods.

In the present series, a much easier conformational change is resulted when benzene core and peripheral Schiff base ether are linked by a flexible spacer. As a result, the rod-like fragments can easily align parallel to give a smectic layer structure. This kind of possible self-assembly by mesogens 4b–4f in the smectic phase is similar to the earlier statement used to rationalise the behaviour of other three-armed star-shaped homo-logues series which show smectic phases in their liquid crystalline state [25,36,51].

4. Conclusions

A novel series of trimeric star-shaped liquid crystals

4a–4f in which the phloroglucinol core incorporated

with peripheral (p-alkyloxybenzylidene) anilines pos-sessing different terminal alkyloxy chains is reported in the current article. The trimeric star-shaped com-pounds are found to be smectogenic exhibiting pre-dominantly the SmC phase. The homologue mem-ber with the shortest terminal alkyl chain was found

to be non-mesogenic, while member with terminal octadecyl chain exhibited the monotropic SmC phase. Initially, the Schlieran texture associated with com-pound 4d suggests the existence of nematic phase, but the ambiguity was then resolved by using XRD as the analysis confirms that SmC phase is the only mesophase present during cooling cycles for com-pounds 4b–4f. In addition, the appearance of bro-ken fan-shaped texture observed on compound 4b has further ascertained the SmC phase. The study also revealed that the length of the terminal alkyl chain exerts a significant effect upon the mesomorphic properties, whereby larger smectic phase range was observed for medium-chain derivatives (R=C12H25, C14H29 and C16H33). In addition, the generation of calamitic SmC phase by the star-shaped mesogens was accomplished by the assembly of the rod-shaped meso-genic Schiff base ether peripheral mesomeso-genic units.

Acknowledgements

The author (G.Y.Yeap) would like to thank Malaysian Ministry of Higher Eduation (MOHE) for funding this project through the Fundamental Research Grant Scheme (No. 203/PKIMIA/6711265). Y.H.Ooi would also like to acknowledge MOHE for the MyBrain15 fellowship.

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