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Materials Chemistry and Physics 146 (2014) 18e25

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Effect of a bulky lateral substitution by chlorine atom and methoxygroup on self-assembling properties of lactic acid derivatives

Maja Stojanovi�c a,*, Alexej Bubnov b, Du�sanka �Z. Obadovi�c a, V�era Hamplová b,Miroslav Cvetinov a, Miroslav Ka�spar b

aDepartment of Physics, Faculty of Sciences, University of Novi Sad, Trg D.Obradovi�ca 4, 21000 Novi Sad, Serbiab Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague, Czech Republic

h i g h l i g h t s

� Chiral liquid crystalline materials derived from the lactic acid have been studied.� Effect of bulky lateral substituents on self-assembling properties has been established.� Bulky methoxy substitution suppresses spontaneous polarisation but increases the melting point.� The compounds might have a strong potential for many advanced electro-optic applications.

a r t i c l e i n f o

Article history:Received 5 June 2013Received in revised form14 February 2014Accepted 23 February 2014

Keywords:Liquid crystalsDifferential scanning calorimetry (DSC)Optical microscopyDielectric propertiesX-ray scattering

* Corresponding author.E-mail address: maja.stojanovic@df.uns.ac.rs (M. S

http://dx.doi.org/10.1016/j.matchemphys.2014.02.0360254-0584/� 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Several chiral liquid crystalline materials derived from the lactic acid have been studied with the aim toestablish the effect of bulky lateral substituents on their self-assembling properties. A chlorine atom andmethoxy group have been used as lateral substituents in ortho position to ether group position on phenylring far from the chiral centre. All the studied materials possess tilted ferroelectric smectic C* phase in abroad temperature range. In dependence on the molecular structure namely type of lateral substituentand length of the chiral chain, the cholesteric mesophase, orthogonal paraelectric smectic A* and crystalmesophases have been detected. Lateral chlorine substitution results in decrease of both the clearingpoint and crystallisation temperature as well as in a distinct increase of spontaneous polarization. Bulkymethoxy substitution slightly suppresses the spontaneous polarisation but strongly increases themelting point that results in monotropic peculiarity of the SmC* phase. Mesomorphic, spontaneous,structural and dielectric properties of the substituted compounds were established and compared tothose of the non-substituted ones in order to contribute to better understanding of the structureeproperty relationship for such chiral self-assembling materials.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Materials that are able to self-assemble into supramolecularsmart structures with desirable functionality and physical proper-ties at nano- and meso-scopic length scales represent currently anexciting area of intense research, which provide a highlightedapproach for design and synthesis of new functional materials [1,2].One of the most fascinating classes of organic materials that areable to self-assemble are those possessing the liquid crystalline (LC)properties. The combination of fluidity and sensitivity to electric/

tojanovi�c).

magnetic fields or illumination by UV-light makes polar fluids idealfor photonics, telecommunications, non-linear optics, etc [3e5].From the fundamental point, chiral smectic liquid crystals i.e. thosepossessing self-assembling into the layered structure of nanometredimensions, have attracted strong attention of the soft matterresearch community for the last three decades [6,7] while in ourdays the nematic LC materials are already extensively applied inmass production of various display devices. Possible applicability ofsmectic LCs put high demands on understanding their basicmesomorphic and physical properties and also on mixture formu-lation rules while reaching the properties responding definite de-mands [8e10].

Lactate based chiral liquid crystallinematerials (or the lactic acidderivatives) have been intensively studied for the last two decades

M. Stojanovi�c et al. / Materials Chemistry and Physics 146 (2014) 18e25 19

due to their attractive properties [2,11e21] but also due to areasonably low production (synthetic) costs with respect to morecommonly used LC materials with chiral part derived from (S)-2-octanol. Varying the molecular architecture [16,22e24], makingthe LC materials polymerisable by attaching various functionalgroups [23,25e29] and also mixing various LC materials[8,23,30,31] are the most commonly used tools to reach the desir-able properties. However, design of LC monomers/polymers as wellas mixing similar homologues or LC materials with quite differentstructure are out of scope of the present work. While designingchiral molecule special courtesy should be paid for the type andnumber of the attached chiral centres [23,32], structure of themolecular core and linkage groups [21,32,33], length and type ofthe end chains [19e21,34,35] and also for the place and type of thelateral substituents [17,36e38]. For chiral liquid crystalline mate-rials bulky methyl [17,36,39,40] and methoxy [17,38] groups alongwith chlorine [17,41,42], bromine [37,43,44], fluorine [37,45e47]and even iodine atoms are the most commonly used lateral sub-stituents that strongly affect the mesomorphic, spontaneous andstructural properties of LC compounds with respect to that of thenon-substituted related structures [16,17].

The aim of this work is to study and discuss properties of theferroelectric liquid crystalline materials derived from the lactic acidin relation to their molecular structure and type of lateral sub-stituents [17] and, thus to contribute to better understanding of themolecular structureemesomorphic property relationship for theself-assembling materials possessing polar mesophases. In order tostudy the effect of bulky lateral substitution placed far from thechiral centre in comparison to that for similar non-substitutedmaterials [16] the methoxy group and chlorine atom have beenchosen as a lateral substituents. General chemical formula of thematerials under study is:

The materials without lateral substitution (X ¼ H) are 40-(2-(pentyloxy)propanoyloxy)biphenyl-4-yl 4-(dodecyloxy)benzoatedenoted as H 12/5 and 40-(2-(heptyloxy)propanoyloxy)biphenyl-4-yl 4-(dodecyloxy)benzoate denoted as H 12/7; laterally substitutedby chlorine atom 40-(2-(pentyloxy)propanoyloxy)biphenyl-4-yl 3-chloro-4-(dodecyloxy)benzoate denoted as Cl 12/5 (X ¼ Cl) andfour materials laterally substituted by a methoxy group MO 12/n(X ¼ OCH3, n ¼ 5, 7, 8, 10), namely 40-(2-(pentyloxy)propanoyloxy)biphenyl-4-yl 3-methoxy-4-(dodecyloxy)benzoate; 40-(2-(hepty-loxy)propanoyloxy)biphenyl-4-yl 3-methoxy-4-(dodecyloxy)ben-zoate; 40-(2-(oktyloxy)propanoyloxy)biphenyl-4-yl 3-methoxy-4-(dodecyloxy)benzoate and 40-(2-(decyloxy)propanoyloxy)biphenyl-4-yl 3-methoxy-4-(dodecyloxy)benzoate.

2. Synthetic procedure for the studied materials

Details on synthetic procedure of studied compounds have beenpresented earlier e for non-substituted compounds H 12/5 and H12/7 in Ref. [15], for laterally substituted by chlorine atom Cl 12/5compound in Ref. [48] and for compounds MO 12/5, MO 12/7, MO12/8 and MO 12/10 with methoxy group as a lateral substituent inRef. [16]. The chemical purity of LC materials was checked by highpressure liquid chromatography (HPLC), which was carried outusing a silica gel column (Biosphere Si 100-5 mm, 4�250, Watrex)with a mixture of 99.8% of toluene and 0.2% of methanol used aseluent. The detection of the eluting products was done by UVeVISdetector (l ¼ 290 nm). For all materials under the study, chemical

purity was found between 99.7% and 99.9% under these conditions.The structures of final compounds have been checked andconfirmed by 1H NMR (300 MHz, Varian) as it is shown below.

The optical rotations were measured using Polarimeter OpticalActivity Ltd. For these compounds in spite of distinct differences intheir molecular core structure very similar values of optical rotation½a�20D have been found: H 12/5 e ½a�20D ¼ �25.0 (c ¼ 0.10, CHCl3), Cl12/5 ½a�20D ¼ �22.5 (c ¼ 0.10, CHCl3) and MO 12/5 ½a�20D ¼ �23.0(c ¼ 0.10, CHCl3).

1H NMRofH 12/5 (CDCl3): 8.17 (d, 2H, ortho toeCOOe); 7.60 (m,4H, ortho toeAr); 7.28 (d, 2H, ortho to ArCOO); 7.20 (d, 2H, ortho toeC*COO); 6.98 (d, 2H, ortho to OCH2); 4.22 (q, 1H, C*H); 4.06 (t, 2H,CH2OAr); 3.50 and 3.70 (m, 2H, CH2OC*); 1.83 (quint., 2H,CH2CH2OAr); 1.60 (d, 3H, CH3C*); 1.60e1.20 (m, 24H, CH2); 0.90 (m,6H, CH3).

1H NMRofH 12/7 (CDCl3): 8.17 (d, 2H, ortho toeCOOe); 7.60 (m,4H, ortho toeAr); 7.28 (d, 2H, ortho to ArCOO); 7.20 (d, 2H, ortho toeC*COO); 6.98 (d, 2H, ortho to OCH2); 4.22 (q, 1H, C*H); 4.06 (t, 2H,CH2OAr); 3.50 and 3.70 (m, 2H, CH2OC*); 1.83 (quint., 2H,CH2CH2OAr); 1.60 (d, 3H, CH3C*); 1.60e1.20 (m, 28H, CH2); 0.90 (m,6H, CH3).

1H NMR of Cl 12/5 (CDCl3): 8.24 (d, 1H, ortho to eCl); 8.09 (dd,1H, para to Cl); 7.60 (m, 4H, ortho to eAr); 7.28 (d, 2H, ortho toArCOO); 7.20 (d, 2H, ortho toeC*COO); 7.01 (d,1H, meta to Cl); 4.22(q, 1H, C*H); 4.14 (t, 2H, CH2OAr); 3.50 and 3.70 (m, 2H, CH2OC*);1.90 (quint., 2H, CH2CH2OAr); 1.60 (d, 3H, CH3C*); 1.60e1.20 (m,24H, CH2); 0.90 (m, 6H, CH3).

1H NMR of MO 12/5 (CDCl3): 7.86 (dd, 1H, para to eCH3O); 7.69(d, 1H, ortho to eCH3O); 7.61 (m, 4H, ortho to eAr); 7.28 (d, 2H,ortho to ArCOO); 7.19 (d, 2H, ortho toeC*COO); 6.96 (d, 1H, meta toeCH3O); 4.22 (q, 1H, C*H); 4.12 (t, 2H, CH2OAr); 3.96 (s, 3H,CH3OAr); 3.50 and 3.70 (m, 2H, CH2OC*); 1.90 (quint., 2H,

CH2CH2OAr); 1.59 (d, 3H, CH3C*); 1.50e1.20 (m, 24H, CH2); 0.90 (m,6H, CH3).

1H NMR of MO 12/7 (CDCl3): 7.86 (dd, 1H, para to eCH3O); 7.69(d, 1H, ortho to eCH3O); 7.61 (m, 4H, ortho to eAr); 7.28 (d, 2H,ortho to ArCOO); 7.19 (d, 2H, ortho toeC*COO); 6.96 (d, 1H, meta toeCH3O); 4.22 (q, 1H, C*H); 4.12 (t, 2H, CH2OAr); 3.96 (s, 3H,CH3OAr); 3.50 and 3.70 (m, 2H, CH2OC*); 1.90 (quint., 2H,CH2CH2OAr); 1.59 (d, 3H, CH3C*); 1.50e1.20 (m, 28H, CH2); 0.90 (m,6H, CH3).

1H NMRofMO12/8 (CDCl3): 7.86 (dd,1H, para toeCH3O); 7.69 (d,1H, ortho toeCH3O); 7.61 (m, 4H, ortho toeAr); 7.28 (d, 2H, ortho toArCOO); 7.19 (d, 2H, ortho to eC*COO); 6.96 (d, 1H, meta to eCH3O);4.22 (q, 1H, C*H); 4.12 (t, 2H, CH2OAr); 3.96 (s, 3H, CH3OAr); 3.50 and3.70 (m, 2H, CH2OC*); 1.90 (quint., 2H, CH2CH2OAr); 1.59 (d, 3H,CH3C*); 1.50e1.20 (m, 30H, CH2); 0.90 (m, 6H, CH3).

1H NMR ofMO 12/10 (CDCl3): 7.86 (dd, 1H, para to eCH3O); 7.69(d, 1H, ortho to eCH3O); 7.61 (m, 4H, ortho to eAr); 7.28 (d, 2H,ortho to ArCOO); 7.19 (d, 2H, ortho toeC*COO); 6.96 (d, 1H, meta toeCH3O); 4.22 (q, 1H, C*H); 4.12 (t, 2H, CH2OAr); 3.96 (s, 3H,CH3OAr); 3.50 and 3.70 (m, 2H, CH2OC*); 1.90 (quint., 2H,CH2CH2OAr); 1.59 (d, 3H, CH3C*); 1.50e1.20 (m, 34H, CH2); 0.90 (m,6H, CH3).

3. Experimental

Sequence of phases and phase transition temperatures weredetermined on cooling from the isotropic phase from characteristic

M. Stojanovi�c et al. / Materials Chemistry and Physics 146 (2014) 18e2520

textures and their changes observed on planar 6, 12, 25 mm thickcells in the polarising microscopes (NIKON ECLIPSE E600POL andAmplival Pol-U with a Boetius heating/cooling stage). The heating/cooling rate was 5 K min�1. The LINKAM LTS E350 heating stagewith TMS 93 temperature programmer, which enabled tempera-ture stabilisation within �0.1 K was used for the temperaturecontrol during electro-optic studies and dielectric spectroscopymeasurements. The DSC measurements (Pyris Diamond PerkineElmer 7) were carried out on cooling and heating runs at temper-ature rate of 5 K min�1 on the samples of 3e8 mg placed in a ni-trogen atmosphere and hermetically sealed in aluminium pans.Values of spontaneous polarisation (Ps) have been evaluated fromP(E) hysteresis loop detected during Ps switching in a.c. electric fieldE of frequency 60 Hz on 12e25 mm thick a sample with an orientinglayer for planar orientation of the director. The Ps measurementswere done on cooling. Values of tilt angle (qs) have been deter-mined on cooling optically from the difference between extinctionpositions at crossed polarisers under opposite d.c. electric fields�40 kV cm�1.Well aligned samples were used for qs measurements.Temperature dependences of the real part, ε0, of complex permit-tivity (ε* ¼ ε

0 � iε00) have been measured on cooling at a frequencyof 130 Hz using a Schlumberger 1260 impedance analyzer. Fre-quency dispersion of the complex permittivity (ε* ¼ ε

0 � iε00) hasbeen measured on cooling in the frequency range of 1 Hze1 MHzkeeping the temperature stable within�0.1 K during the frequencysweep. Non-oriented samples were investigated by the small(SAXS) and wide (WAXS) angle X-ray diffraction in transmissiongeometry by means of a conventional powder diffractometer (Sei-fert V-14, CuKa radiation at 0.154 nm) with an automated hightemperature kit (A. Paar HTK-10). From the X-ray diffraction studiesthe smectic layer spacing (d) in the paraelectric SmA*, ferroelectricSmC* and crystal mesophases have been calculated from the po-sition of the small angle diffraction peaks. In case of cholesteric (N*)phase the detected d values corresponds to the average molecularlength. The average intermolecular distance between the long axesof neighbouring parallel molecules (D) have been calculated fromthe position of the large angle diffraction peaks, for all the phases.For these calculations the Bragg’s law: nl ¼ 2xsinq, were x ¼ d orD were used. Depending on the molecular structure one of thelow temperature crystal mesophase (CrX) was identified. In case ofthe hexagonal ordering of the long molecular axis in the smecticlayer plane, the average intermolecular distance between thelong axes of neighbouring parallel molecules, is the function ofD, and hence might be calculated as b ¼ 2D/31/2, where D istypical intermolecular distance calculated from Bragg’s law[45,49,50].

Table 1Sequence of phases and phase transition temperatures (�C) measured on cooling (5 K m(5 K min�1) and respective phase transition enthalpies [J g�1] by determined DSC for thebrackets [J g�1]. (“e” phase does not exist).

Comp. C.p. M.p. Phase �C Phase

H 12/5 137.4[þ11.0]

75.0[þ30.2]

Cr 73.5[�17.2]

e

H 12/7 149.1[þ1.4]

85.3[þ45.1]

Cr 57.9[�15.4]

CrX

Cl 12/5 113.1[þ8.2]

75.4[þ45.1]

Cr 18.3[�18.6]

e

MO 12/5 88.4[þ60.8]

88.4[þ60.8]

Cr 53.2[�27.2]

CrX

MO 12/7 91.7[þ68.8]

91.7[þ68.8]

Cr 52.7[�56.9]

e

MO 12/8 93.2[þ53.2]

93.2[þ53.2]

Cr 39.1[�18.0]

e

MO 12/10 86.0[þ2.6]

68.5[þ15.6]

Cr 48.6[�6.3]

e

4. Experimental results

In this section we present results of experimental studies ofmesomorphic, spontaneous, structural and dielectric propertiesdone on the compounds under investigation together with thediscussion on the effect of the molecular structure on self-assembling properties.

4.1. Mesomorphic and self-assembling properties

For the studied materials, sequence of phases and phase tran-sition temperatures were determined by characteristic textures andtheir changes observed in polarizing optical microscope and fromDSC measurements. Sequence of phases and phase transitiontemperatures of all investigated materials are shown in Table 1. Allthe studied materials possess the tilted ferroelectric smectic C*phase over a broad temperature range. In dependence on type ofthe lateral substituent and length of the chiral chain, the cholestericmesophase, orthogonal paraelectric smectic A* and crystal meso-phase have been also detected. Lateral substitution by the chlorineatom (Cl 12/5) strongly decrease the clearing point and also thecrystallisation temperature with respect to the non-substituted H12/5 compound. Similar behaviour has been detected on thehalogen substituted compoundswithmolecular core different fromthat studied in this work [36,37,41e43]. The difference in some datapresented in Table 1 with respect to the data presented inRefs. [15,17,48] might be due to different temperature calibration.

For the methoxy substituted MO 12/5, MO 12/7, MO 12/8 andMO 12/10 compounds, DSC plots obtained on second cooling fromthe isotropic phase are shown on Fig. 1. It is quite unexpected thaton cooling from the isotropic phase MO 12/5 and MO 12/8 com-pounds possess the SmA* phase while for MO 12/7 and MO 12/10compounds the phase is absent. Nevertheless the length of thechiral chain has negligible effect on the clearing point while thelateral methoxy substitution strongly suppresses the phase tran-sition temperatures with respect to the non-substituted H 12/ncompounds. Since lowering of the phase transition temperaturesfrom the isotropic state is smaller for the MO 12/n than for non-substituted compounds; it is obvious that the methoxy group fa-vours the packing of the molecules into a more ordered structuresimilarly as in Ref. [17]. This can be understood as an increase of theintermolecular interactions in the MO 12/n series due to themesomeric effect, which can compensate the disordering stericeffect of a bulky lateral group. However, the influence of themesomeric effect of the methoxy group on the phase transitiontemperatures to the crystalline state is much weaker.

in�1); melting points, m.p. (�C) and clearing points, c.p. (�C) measured on heatingstudied compounds. Values of the phase transition enthalpies are shown in square

�C Phase �C Phase �C Phase

SmC* 136.0[�10.6]

e Iso

80.7[�22.1]

SmC* 144.7[�4.2]

N* 148.0[�2.5]

Iso

SmC* 112.0[�8.1]

e Iso

65.3[�19.7]

SmC* 82.6[�0.3]

SmA* 85.4[�6.1]

Iso

SmC* 85.8[�8.4]

e Iso

SmC* 79.7[�0.4]

SmA* 88.0[�3.6]

Iso

SmC* 85.1[�2.1]

e Iso

Fig. 1. DSC plots obtained on second cooling from the isotropic phase for:MO 12/5,MO12/7, MO 12/8 and MO 12/10 compounds. Vertical arrows indicate the peaks corre-sponding to the phase transitions.

M. Stojanovi�c et al. / Materials Chemistry and Physics 146 (2014) 18e25 21

The optical microphotographs of characteristic textures for theselected compounds as indicated are shown on Fig. 2 for theorthogonal SmA* phase (fan-like texture on Fig. 2a and b) and forthe ferroelectric SmC* phase (broken fan texture with clearly seendechiralization lines, which indicate the helical pitch length i.e. the

Fig. 2. Optical microphotographs of studied compounds: (a) fan-shaped texture of the SmA*of the SmA* phase for MO 12/5 obtained on 25 mm thick planar cell at 84.0 �C; (c) fan-shastructure for MO 12/5 obtained on 25 mm thick planar cell at 79.0 �C; (d) broken fan texturplanar cell at 102.0 �C; (e) texture of the crystal mesophase for H 12/7 obtained on 25 mm thi25 mm thick cell at 45.0 �C. Width of the microphotographs (a)e(c) is about 300 mm and (d

periodicity of the helical structure [23,48] e see Fig. 2c and d). Thetextures of the crystal mesophase for H 12/7 and a low temperaturecrystal phase for MO 12/10 are shown on Fig. 2e and f, respectively.For H 12/7 and MO 12/5, the type of the low temperature crystalmesophase couldn’t be fully elucidated by X-ray studies. Numerousequidistant reflections on small angles (001) indicate preservedlong-range order of molecules in smectic layers. The second sharpreflection in the wide angle range is rather strong, indicating non-hexagonal in-layer arrangement of the molecules. According to thePOM observations, this phase was not found to be fluid.

4.2. Spontaneous quantities

Values of the spontaneous polarisation (Ps) and tilt angle (qs) ofmolecules measured optically are summarised in Table 2. Thelateral substitution by a chlorine atom results in a strong increase ofthe Ps values with respect to the non-substituted compounds. ThePs increase can be ascribed to the increase of the transverse mo-lecular dipole-moment and to the increase of the material densitydue to the mesomeric effect, which strengthens the intermolecularinteractions. Similar behaviour could be expected also in case of thesubstitution by the methoxy group due to its polar properties.However, the results show an opposite behaviour: the lateralmethoxy group strongly suppresses the spontaneous polarisationvalues, which is not clearly understood so far. For the methoxysubstituted compounds, the increase of the chiral chain lengthcauses the increase of both the spontaneous polarisation and tilt

phase for MO 12/5 obtained on 6 mm thick planar cell at 84.5 �C; (b) fan-shaped textureped texture of the SmC* phase with equidistant line pattern due to the helical super-e of the SmC* phase with equidistant line pattern for Cl 12/5 obtained on 25 mm thickck planar cell at 74.0 �C; (f) texture of a low temperature crystal phase for MO 12/10 on)e(f) is about 200 mm.

Table 2Values of the spontaneous polarization Ps [nC cm�2] and tilt angle measured opti-cally qs [degree] measured on cooling 5 K and 15 K below the temperature, Tc, of thephase transition to the SmC* phase, respectively.

Comp. Ps (Tc � 5 K) qs (Tc � 15 K)

H 12/5 95 38H 12/7 107 40Cl 12/5 148 34MO 12/5 27 25MO 12/7 61 31MO 12/8 68 34MO 12/10 79 36

M. Stojanovi�c et al. / Materials Chemistry and Physics 146 (2014) 18e2522

angle. Such an increase of the spontaneous values has been alreadyobserved for the liquid crystalline compounds with differentstructure of the molecular core [34].

4.3. Structural properties

X-ray diffraction studies have been carried out on all theinvestigated compounds. The examples of the X-ray diagrams ob-tained on the non-oriented samples at indicated temperatures areshown in Fig. 3. All the mesophases which were detected bypolarizing optical microscopy and DSC are clearly distinguishable.While decreasing the temperature, the position of the small anglereflection is somewhat shifted on angular scale indicating a changein the smectic layer spacing with temperature. The comparison ofthe layer spacing calculated from the X-ray diagrams for the non-substituted H 12/5, chlorine substituted Cl 12/5 and methoxysubstituted MO 12/5 compounds are shown in Fig. 4a. It can beobserved that the lateral substitution strongly decreases thed values. The layer spacing versus temperature for the methoxysubstituted compounds is presented in Fig. 4b. The layer spacing isstrongly increases with elongation of the chiral chain length. ForMO 12/5 andMO 12/8 compounds there is a gradual decrease in thelayer spacing values due to the increase of the molecular tilt angleat the SmA*eSmC* phase transition.

Fig. 3. X-ray diffraction profiles for: (a) MO 12/8

The position of the diffuse scattering centres at wide Braggangles (see Fig. 3) gives the average intermolecular distance (orintermolecular distance) D between long axes of the neighbouringparallel molecules. The intermolecular distance D at indicatedtemperatures for the studied compounds is shown in Table 3. Theposition of the diffuse scattering centres (at wide Bragg angles)consistently shifts toward higher angles with temperaturedecrease. Hence, the decrease in D values indicates an increase inthe packing density while cooling the material from the isotropicphase through the liquid crystalline phases to a crystal state.

The type of the lateral substituent has a considerable effect onthe intermolecular distance D. The degree of packing density in-creases while the lateral methoxy group is present and decreaseseven more in case of the chlorine substitution. This effect is clearlyillustrated in Table 3 while comparing the D values in the SmC*phase for theH12/5, Cl 12/5 andMO12/5 compounds. However, thelength of the chiral chain does not affect much the molecularintermolecular distance. For themethoxy substituted compounds, asmall decrease of D values in the SmC* phase can be observed withincrease of the length of the chiral chain (see Table 3).

The MOPAC/RM1 model was used to calculate the length of H12/5 and H 12/7, as well as Cl 12/5 and all molecules from MO 12/nseries, in the energy-optimized conformation. The molecularstructure with the principal axis of minimum moment of inertia(‘longmolecular axis’) is presented in Fig. 5 for Cl 12/5 compound asan example. Taking into account the most extended conformer, thelengths of molecules L are found as 41.36�A, 41.40�A and 41.37�A forH 12/5, Cl 12/5 and MO 12/5, respectively. For H 12/5 and Cl 12/5compounds, the calculated molecular lengths correspond very wellwith the values of the layer spacing. Minor dissimilarities of areprobably related to non-perfect orientational order. For MO 12/5compound calculated length (41.37�A) of most extended conformerdoes not matches the layer spacing value (32.7 �A) determinedexperimentally in the SmA* phase. In that case the presence oflaterally substituted bulky methoxy group hindered the smecticordering, therefore facilitating rotation and subsequent shorteningof terminal alkyl chain which could results in intercalation of themolecules in the neighbouring smectic layers.

and (b) Cl 12/5 for all detected mesophases.

Fig. 4. Temperature dependence of the layer spacing d: (a) for H 12/5,MO 12/5 and Cl 12/5 and (b) for studied methoxy substituted compounds as indicated. Vertical arrows indicatethe phase transition temperature to the SmC* phase.

Table 3The average intermolecular distance D [�A] between long axes of the neighbouringparallel molecules for all observed isotropic and smectic phases at indicated tem-perature T [�C]. (The error of measurements dD is estimated as �0.002 �A).

Comp. Phase T D

H 12/5 Iso 149 6.220SmC* 91 6.028

Cl 12/5 Iso 120 5.394SmC* 100 5.337

MO 12/5 Iso 99 5.513SmA* 84 5.423SmC* 80 5.394

MO 12/7 Iso 100 5.513SmC* 70 5.280

MO 12/8 Iso 99 5.482SmA* 82 5.308SmC* 73 5.225

MO 12/10 Iso 99 5.453SmC* 55 5.225

M. Stojanovi�c et al. / Materials Chemistry and Physics 146 (2014) 18e25 23

4.4. Dielectric spectroscopy

Broadband dielectric spectroscopy is a powerful tool to studythe non-collective and collective relaxation processes in a liquidcrystalline phases formed by the chiral molecules [51,52]. For thechiral molecules, the non-collective relaxation processes, namelythe rotation around long and short molecular axis and intra-molecular rotations can be detected in the frequency range of 107e1011 Hz that is out of scope of the present work. At lower fre-quencies (101e106 Hz), beside the non-collective processes, thedielectric spectrum of the chiral smectic liquid crystals contains thecollective processes connected with the fluctuations of the mole-cule director in the directions of the tilt and azimuthal angles calledthe soft mode and the Goldstone mode, respectively.

Temperature dependences of the real part of complex permit-tivity measured at 130 Hz on cooling on 25 mm thick planar cells forcompounds without lateral substitution and with chlorine atomand methoxy group as lateral substituents are shown on Fig. 6a. A

Fig. 5. Conformation of Cl 12/5 molecules after energy minimization using MOP

strong contribution of the Goldstone mode has been clearlyobserved in the ferroelectric SmC* phase. The type and evenpresence of lateral substitution also strongly affects the phasetransition temperatures: the phase transition to the SmC* phaseshifts to lower temperatures for the laterally substituted com-pounds with respect to the non-substituted one. Effect of the chiralchain length on permittivity for methoxy substituted compounds isshown on Fig. 6b. There is a great increase of permittivity on coolingfrom the SmA* (or from isotropic) phases to the SmC* phase due tothe strong contribution of the Goldstone mode. The length of thechiral chain does not influence much the temperature of the phasetransition to the ferroelectric phase.

Besides, the frequency dispersion of the real and imaginaryparts of permittivity has been measured in the frequency rangefrom 1 Hz to 1 MHz in the SmC* phase. As an example, dielectricabsorption ε

00 spectra at zero bias electric field measured on 25 mmthick sample cell in the ferroelectric SmC* phase for non-substituted H 12/5 compound (see Fig. 7) show a strong Gold-stone mode with relaxation frequency of several kHz and a lowfrequency mode detected at much low frequency related to struc-tural fluctuations [51]. More detailed results of dielectric spec-troscopy incorporating the discussion on the behaviour of therelaxation frequency and dielectric strength of the detected modeswith respect to the molecular structure will be presentedelsewhere.

5. Summary of results and conclusion

The results of study done on chiral liquid crystalline materialsderived from the lactic acid have been presented with the aim toestablish the effect of bulky lateral substituents and contribute tobetter understanding of molecular structureeself-assemblingproperty relationship for this class of functional materials [36e38,44]. A chlorine atom and the methoxy group have beenselected as bulky lateral substituents that strongly influence theself-assembling properties. All studied materials possess tiltedferroelectric smectic C* phase in a broad temperature range. On

AC/RM1 method. The length of the most extended conformer L is 41.40 �A.

Fig. 6. Temperature dependence of the real part of complex permittivity measured at 130 Hz on cooling on 25 mm thick planar cells for indicated compounds: (a) the effect of lateralsubstituent and (b) the effect of the chiral chain length.

M. Stojanovi�c et al. / Materials Chemistry and Physics 146 (2014) 18e2524

cooling, the phase transition temperature to the SmC* phase de-creases in a lateral substituent sequence as H > Cl > CH3O, whichcorresponds well with the van der Waals diameters of the atom/group used as a lateral substituents, namely H e 1.20�A, Cl e 1.85�A,and the calculated diameters for methoxy (2.60�A) substituent [37].This effect can be explained in terms of the steric influence of lateralsubstitutes on molecular packing.

Lateral chlorine substitution results in decrease of the clearingpoint and crystallization temperature as well as in increase of thespontaneous polarization. Bulky methoxy substitution slightlysuppresses the values of spontaneous polarisation and stronglyincreases the melting point that results in partially monotropiccharacter of the SmC* phase. For the methoxy substituted com-pounds, the values of spontaneous polarization are smaller incomparison to those of the non-substituted ones. Moreover, the Psvalues increase with increase of the chiral chain length. The highestvalue of spontaneous polarization corresponds to chlorinesubstituted compound. Values of the spontaneous tilt angle are

Fig. 7. Dielectric absorption ε00 spectra at zero bias electric field measured on 25 mm

thick sample cell in the ferroelectric SmC* phase for non-substituted H 12/5 compoundshow a strong Goldstone mode and a low frequency mode related to structuralfluctuations.

smaller in the case of substituted compounds compare to those ofthe non-substituted ones. Moreover, for the methoxy substitutedcompounds the tilt angle increases with increase of the chiral chainlength. Values of the tilt angle measured optically for Cl 12/5compound are smaller than that for compounds from MO 12/nseries, which can be explained by a higher flexibility of methoxysubstituted molecules.

The smectic layer spacing strongly increases with elongation ofthe chiral chain length while the lateral substitution considerablydecreases the layer spacing [2,22,23]. The average intermoleculardistance D between long axes of the neighbouring parallel mole-cules decreases: (i) with decrease of temperature; (ii) with increaseof the chiral chain length [18,19] and (iii) also if any bulky lateralsubstituent is present. The interaction between molecules starts tobe stronger and the packing density is bigger if the OCH3 group isreplaced with chlorine atom.

The studied compounds (especially with chlorine substitution)can be very useful as chiral dopants for mixtures aimed for thespecific applications due to a broad range of the ferroelectric SmC*phase down to room temperatures and relatively high values ofspontaneous polarisation. Authors strongly believe that in futuresuch type of self-assembling smectic mixtures might have a strongpotential for many advanced electro-optic applications.

Acknowledgements

This work was supported by Grant no. 171015 from the Ministryof Education and Science of the Republic of Serbia and by researchprojects CSF 13-14133S and AS CR M100101204.

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