Five orthogonal smectic phases (Sm-A, Sm-Ab, Sm-APR, Sm-APAR, and Sm-APA) were observed in a 4-cyanoresorcinol bisbenzoate with twoterephthalate–based wings achiral bent-core(BC) compound. The phase behavior was investigated by polarising optical microscopy, second harmonic electro-optic response(EO2) and dielectric spectroscopy. The ﬁeld dependent biaxiality of polar and non-polar Sm-A phases was studied. It is obviously of enormous scientiﬁc signiﬁcance and practical interest to ﬁnd ﬁve different orthogonal smectic phases in a low molecular weight bent-core (BC) compound at relatively low temperatures

Lehmann, A.

Panarin, Yuri

Sreenilayam, S.

Tschierske, C.

Vij, J. K.

English

Arrow@TUDublin

Technological University Dublin ARROW@TU Dublin Articles School of Electrical and Electronic Engineering 2015 Occurrence of Five Different Orthogonal Smectic Phases in a Bent-Core (BC) Liquid Crystal S. Sreenilayam Trinity College Dublin, Ireland Yuri Panarin Technological University Dublin, yuri.panarin@tudublin.ie J. K. Vij Trinity College Dublin, Ireland, jvjj@tcd.ie A. Lehmann Martin Luther University Halle-Wittenberg C. Tschierske Martin Luuther University Halle-Wittenberg Follow this and additional works at: https://arrow.tudublin.ie/engscheleart2  Part of the Electrical and Computer Engineering Commons Recommended Citation Lehmann, A. et al. (2015). Occurrence of five different orthogonal smectic phase in a bent-core (BC) liquid crystal. Molecular Crystals and Liquid Crystals , vol. 610, pp. 116-121. DOI:10.1080/15421406.2015.1025606 This Article is brought to you for free and open access by the School of Electrical and Electronic Engineering at ARROW@TU Dublin. It has been accepted for inclusion in Articles by an authorized administrator of ARROW@TU Dublin. For more information, please contact yvonne.desmond@tudublin.ie, arrow.admin@tudublin.ie, brian.widdis@tudublin.ie. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License Mol. Cryst. Liq. Cryst., Vol. 610: pp. 116–121, 2015Copyright © Taylor & Francis Group, LLCISSN: 1542-1406 print/1563-5287 onlineDOI: 10.1080/15421406.2015.1025606Occurrence of Five Different Orthogonal SmecticPhases in a Bent-Core (BC) Liquid CrystalS. SREENILAYAM,1 YU. P. PANARIN,1,2,∗ J. K. VIJ,1,∗A. LEHMANN,3 AND C. TSCHIERSKE31Department of Electronic and Electrical Engineering, Trinity College,University of Dublin, Dublin, Ireland2School of Electronic & Communication Engineering, Dublin Institute ofTechnology, Dublin, Ireland3Institute of Chemistry, Organic Chemistry, Martin-Luther-UniversityHalle-Wittenberg, GermanyFive orthogonal smectic phases (Sm-A, Sm-Ab, Sm-APR, Sm-APAR, and Sm-APA) wereobserved in a 4-cyanoresorcinol bisbenzoate with two terephthalate –based wings achi-ral bent-core (BC) compound. The phase behavior was investigated by polarising opticalmicroscopy, second harmonic electro-optic response (EO2) and dielectric spectroscopy.The field dependent biaxiality of polar and non-polar Sm-A phases was studied. It isobviously of enormous scientific significance and practical interest to find five differentorthogonal smectic phases in a low molecular weight bent-core (BC) compound atrelatively low temperatures.Keywords bent-core molecule; orthogonal Smectic; electro-optic effects; dielectricrelaxation1. IntroductionA considerable interest has been shown by the liquid crystalline community in thestudy of ferroelectricity and antiferroelectricity in liquid crystalline (LC) materials com-posed of achiral molecules [1–3]. In 1992, Brand et al. introduced a model of ferro-electricity for an orthogonal smectic phase (CP) formed by non-chiral bent-core (BC)molecules [4] and Niori et al. reported the first experimental evidence of ferroelectricity in1996 [5].Recently, many orthogonal smectics composed of BC, LC molecules showing differentphase structures and phase transitions have been synthesized and investigated with a largeshape biaxiality and strong polarity [6-9]. Some of the studies in this direction includean investigation of orthogonal polar smectic mesophases such as Sm-APA [10–13], Sm-APR [14, 15] and Sm-APAR [16]. In all these achiral Sm-A phases, the molecular rotation∗Address correspondence to Yu. P. Panarin and J. K. Vij, Department of Electronic and ElectricalEngineering, Trinity College, University of Dublin, Dublin 2, Ireland. E-mail: yuri.panarin@dit.ie;jvij@tcd.ieColor versions of one or more of the figures in the article can be found online atwww.tandfonline.com/gmcl.116Downloaded by [Leabharlann Choláiste na Tríonóide/Trinity College Library & IReL], [Jagdish K. Vij] at 07:43 07 July 2015 Different Orthogonal Smectic Phases in Bent-Core LC 117Figure 1. Molecular structure of the bent core LC compound with chain length n = 16around its principal axis within a smectic layer at least to some extent is biased. Hence,the molecules are packed in the BC direction which is also the polar-axis direction in asingle layer. In Sm-APR phase, the polarization direction from layer to layer is randomizedleading to the macroscopic non-birefringent texture under crossed polarizers and in Sm-APA phase, the polarization direction between the adjacent layers are anti-parallel leading toan antiferroelectric phase which is biaxial. Ferroelectric phase (Sm-APF) with polarizationdirection in adjacent layers parallel to each other has also been observed [17]. The existenceof non-polar biaxial Sm-Ab phase or CM (McMillan) first suggested by de Gennes [1] wasrecently observed in LCs built from the non-symmetrical BC molecules and in their mixtureswith calamitic molecules [18, 19].Here we report a new BC material with symmetric terminal chain lengths which exhibitsa phase sequence of three different polar orthogonal smectic phases (Sm-APA, Sm-APARand Sm-APR) in addition to optically biaxial non-polar Sm-Ab on cooling down from theoptically uniaxial Sm-A. Occurrence of these phases was confirmed by biaxiality measure-ments [20–23], second harmonic electro-optic response [23] and dielectric spectroscopy[24]. It is obviously of enormous to significance to find the five different mesophases atrelatively lower temperature region in a low molar pure BC compound having a highlypolar cyano group attached to the middle of the core.2. ExperimentsThe smectic BC compound under study, 4-cyanoresorcinol bisbenzoate with twoterephthalate–based wings was synthesized in Halle Germany. The molecular structurealong with the phase transition temperatures and phase sequences are shown in Fig. 1. Thematerial under investigation has symmetric terminal alkyl chains on both sides with thenumber of carbon atoms 16 (chain length = 16). It exhibits the following phase sequence:Cr 75◦C- Sm-APA 90◦C- Sm-APAR 98◦C- Sm-APR 107◦C- Sm-Ab 125◦C- Sm-A 164◦C -Ion cooling from the isotropic temperature and Cr 110◦C- Sm-Ab 125◦C- Sm-A 162◦C-I, onheating from the crystalline state. It is interesting to note that the homologue series withvery similar molecular structure, with C O bonds pointed in the same direction, exhibitonly nematic phases [25].For electro-optic measurements homeotropic alignment is obtained by two glass sub-strate coated with polymer AL60702 (JSR Korea). The electrodes are arranged with aseparation of ∼80 μm for applying the in-plane electric field at a frequency of 110 Hzby etching indium tin oxide (ITO) stripes on the bottom glass substrate. The dielectricstudy is carried out using broadband high resolution dielectric spectrometer (Novo controlGmbH, Germany) on planar (homogenous) cells. Homogeneous cell configuration wasobtained by coating with polymer RN 1175 (Nissan chemicals, Japan). Sample thicknessis adjusted using appropriate spacers and interference technique is used to measure the cellthickness.Downloaded by [Leabharlann Choláiste na Tríonóide/Trinity College Library & IReL], [Jagdish K. Vij] at 07:43 07 July 2015 118 S. Sreenilayam et al.3. Results and Discussions3.1. Biaxiality MeasurementsBiaxiality measurements were performed using a tilting optical compensator. The effectof applied field on different Sm-A phases was studied in a 5.5 μm homeotropic cell undercrossed polarizers with an angle of 45o between the electric field and the polarizer axes.Magnitude of biaxiality measured as a function of voltage for different phases with areduction in temperature is shown in Fig. 2.On cooling the cell from the optically uniaxial Sm-A to 125◦C, it first transforms toan optically biaxial Sm-Ab phase. By the application of electric field (0.4 V/μm) the initiallow-biaxiality (δn = 1.61×10−4) with a Schlieren texture becomes ordered and on a furtherincrease in the field (8.75 V/μm) the magnitude of biaxiality increases to δn = 0.0094 at118◦C. On further cooling this optically biaxial phase (Sm-Ab) turns to optically uniaxialSm-APR phase at 107◦C. The initial uniaxial state goes to biaxial state (δn = 0.0056) onlyat a higher field (8.75 V/μm). It shows an increasing trend of biaxiality by the applica-tion of higher electric field. The device containing the sample turns to Sm-APAR phase at98◦C, the macroscopically uniaxial state (δn = 0) goes to biaxial antiferroelectric state(δn = 0.0019 at 1.5 V/μm) and then to ferroelectric ordered saturated state (δn = 0.0140, at6.25 V/μm) through uniaxial order (δn = 0 at 2.75 V/μm). At 90◦C, the initial Schliren tex-ture of Sm-APA phase (δn = 0.0026, at 0 V/μm) goes to the ordered biaxial antiferroelectricstructure (δn = 0.0073, at 1.25 V/μm) and on further application of field; it turns to orderedferroelectric saturated state (δn = 0.0168, at 5.25 V/μm) through an optically uniaxial state(δn = 0, at 3.5 V/μm). At a particular temperature (78◦C), δn value of both antiferroelectricand ferroelectric states are similar. From Fig. 2 it is clearly seen that the magnitude offield dependent biaxiality of all smectic phases gradually increases with a reduction intemperature.Figure 2. The field dependent biaxiality measurement of orthogonal smectic LC phases, Sm-Ab(118◦C–112◦C), Sm-APR (106◦C-103◦C), Sm-APAR (100◦C–91◦C) and Sm-APA (88◦C–78◦C), oncooling down from the uniaxial Sm-A phase. Measurements are carried out on a 5.5 μm homeotropiccell under an in-plane electric field. The various phases are marked on the left and right. The curvesfor various colors may not all be clear in the Figure.Downloaded by [Leabharlann Choláiste na Tríonóide/Trinity College Library & IReL], [Jagdish K. Vij] at 07:43 07 July 2015 Different Orthogonal Smectic Phases in Bent-Core LC 119Figure 3. The 2nd harmonic electro-optic response (EO2) of orthogonal smectic phases observed inBCLC compound (PAL29) with a reduction in temperature from the isotropic phase. The inset figure(a) shows the Sm-Ab at 112◦C (), Sm-APR at 107◦C (◦C;), Sm-APAR at 98◦C (◦73;) and Sm-APA at90◦C (). The inset figure (b) shows the EO2 response only in Sm-Ab phase at 112◦C () and redfilled circle SmAPR.3.2. Second Harmonic Electro-optic ResponseThe second harmonic electro-optic response (EO2) was recorded [23] by applying a square-wave ac voltage at a f = 80 Hz across the in-plane electrodes having electrode distance∼80 μm. The magnitude of EO2 response is proportional to the sin2θ , where, θ = πδnd/λ.If θ << 1, the linear dependence of δn(E) on E would give a quadratic dependence of EO2on E (i.e., EO2 α E2) due to EO2 α θ2. Figure 3 shows the temperature dependent EO2response while the sample is cooled from the SmA phase.The plot in Fig. 3 reveals the presence of all orthogonal phases (Sm-Ab, Sm-APR, Sm-APAR and Sm-APA) and it is in good agreement with the biaxiality measurement results. Theinset figure (a) of Fig. 3 shows the phases Sm-Ab at 112◦C, Sm-APR, at 107◦C Sm-APAR at98◦C and Sm-APA at 90◦C. As the magnitude of EO2 response is very small for Sm-Ab andSm-APR when compared to the other Sm-APAR and Sm-APA phases, the field dependentEO2 response of both Sm-Ab and Sm-APR are repeated on an extended scale separately ininset figure (b) of Fig. 3. The EO2 response from the material under study is fitted to theequation EO2 = k.Eγ to determine the exponent γ . In Sm-APR and Sm-APAR, γ = 2 forthe lower values of E, which implies the linear dependence of δn(E) on E due to the polarinteraction with E. When we consider Sm-APA phase, γ shows a cross over behavior (2.5 to3.5) due to the combination of polar and dielectric interactions. In optically biaxial Sm-Ab,the fitting result shows γ = 4.2, which assumes that this phase is non-polar in nature.3.3. Dielectric MeasurementsThe dielectric relaxation measurement over a frequency range 1Hz to 1MHz was performedin a homogeneous cell having 7.3 μm thickness. Low resistance (20 /) indium tin oxide(ITO) glass substrates were used to make cells for the study. The measurement was carriedDownloaded by [Leabharlann Choláiste na Tríonóide/Trinity College Library & IReL], [Jagdish K. Vij] at 07:43 07 July 2015 120 S. Sreenilayam et al.Figure 4. Dielectric relaxation strength (δε) and the relaxation frequency (fR) as a function oftemperature measured in a planar cell configuration.out over a temperature range of 170◦C–70◦C (varied in steps of 0.5◦C) and the dielectricspectrum is fitted to the Havriliak-Negami equation,ε∗(ω) = ε′ − iε′′ = ε∞ +n∑j=1δεi[1 + (iωτj)αi ]βi −iσdcε0ω(1)where, ε∞ is the high frequency dielectric permittivity, j varying from 1 to n (number ofrelaxation process), ω is the angular frequency, ε0 is the permittivity in free space, ιj is therelaxation time of the jth relaxation process, δεj is the dielectric relaxation strength and αjand β j are the symmetric and asymmetric broadening parameters of the jth process. Thefitting parameters of the dominant process, e.g. dielectric strength, δε (amplitude) and therelaxation frequency, fR are shown in Fig. 4 (see [26]).While cooling down from the isotropic phase, the relaxation processes, P1 is movingtowards the lower frequency part. In polar phases, the high frequency relaxation mode isdue to the non collective motion of polar molecules and the low frequency relaxation modeis due to the collective movement of dipole moments. It is clearly seen in Fig. 4 that, thedielectric amplitude ε increases with a reduction in temperature and reaches maximumat the Sm-APR to Sm-APAR phase transition.4. Conclusions and PerspectivesIn summary, the occurrence of five different orthogonal smectic phases (Sm-A, Sm-Ab, Sm-APR, Sm-APAR, and Sm-APA) by field dependent biaxiality measurements were confirmedby the second harmonic electro-optic response and the dielectric spectroscopy. The Sm-Abphase exhibits very small values of biaxiality when compared to all other orthogonal smecticphases observed. Biaxiality measurement results are in agreement with the second harmonicelectro-optic response measurement results. The dielectric study reveals an increasing trendof dielectric amplitude with a reduction in temperature and it reaches the summit at Sm-APRto Sm-APAR phase transition region.Downloaded by [Leabharlann Choláiste na Tríonóide/Trinity College Library & IReL], [Jagdish K. Vij] at 07:43 07 July 2015 Different Orthogonal Smectic Phases in Bent-Core LC 121FundingWe thank the Science Foundation of Ireland for the grant 13/US/I2866 under Ireland-US co-operative research program and by US-Ireland R&D Partnership Program jointlyadministrated with the United States NSF (NSFDMR-1410649).References[1] De Gennes, P. G. & Prost, J. (1993). The Physics of Liquid Crystals, Clarendon: Oxford.[2] Chandrasekhar, S. (1992). Liquid Crystals, Cambridge: Cambridge University Press.[3] Brand, H. R., Cladis, P. E., & Pleiner, H. (1992). Macromolecules, 25, 7223.[4] Brand, H. R., Cladis, P. E., & Pleiner, H. (1998). Eur. Phys. J. B, 6, 347.[5] Niori, T., Sekine, T., Watanabe, J., Furukawa, T., & Takezoe, H. 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Occurrence of Five Different Orthogonal Smectic Phases in a Bent-Core (BC) Liquid Crystal

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Occurrence of Five Different Orthogonal Smectic Phases in a Bent-Core (BC) Liquid Crystal

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