Structure-property correlation of a hockey stick-shaped compound exhibitingN-SmA-SmCa phase transitions

P. Sathyanarayana1, S. Radhika2, B. K. Sadashiva2 and Surajit Dhara1 ∗1School of Physics, University of Hyderabad, Hyderabad-500046, India

2Raman Research Institute, C. V. Raman Avenue, Bangalore-560080, India(Dated: November 21, 2011)

We report measurement of birefringence, dielectric constant, splay, bend elastic constants androtational viscosity in the nematic phase (N) of a hockey stick-shaped compound exhibiting smectic-A (SmA) and anticlinic smectic-C (SmCa) phase transitions at lower temperature. It is found thatthe physical properties significantly different than calamitics and partially similar to the bent-coreliquid crystals. It exhibits positive dielectric and negative elastic anisotropy. Optical and thermalmeasurements show that all the transitions are first order. Rotational viscosity is comparativelyhigher than calamitic liquid crystals and exhibits weak pretransitional divergence. Temperaturedependent static dielectric measurements indicate antiparallel correlation of both the longitudinaland transverse components of dipole moments in both the smectic phases. Parallel component ofdielectric constant exhibits a single Debye type relaxation in all the phases. The activation energy inthe N, SmA and SmCa phases are comparatively larger than the respective phases of calamitic liquidcrystals and can partially be attributed to the higher rotational viscosity of the hockey stick-shapedcompound.

PACS numbers:

I. INTRODUCTION

The structure-property correlation in conventional liq-uid crystals made of rod-like molecules has been stud-ied extensively and mostly well established. Efforts havealways been made to design new molecules with im-proved physical property that enhances the display per-formance. In this context liquid crystals made of bent-core molecules have created worldwide interests after thediscovery of electrooptical switching in achiral bent-corecompounds [1]. Subsequently several groups reportedthe synthesis of new liquid crystal compounds with var-ious molecular shapes like T, H and W [2–4]. It hasbeen established that molecular shape has significantrole on their phase structures and physical properties[5–7]. Commonly bent-core molecules exhibit so calledbanana mesophases, B1 - B8 and occasionally they alsoexhibit some phases of calamitic liquid crystals like ne-matic, SmA and SmC etc. To understand the effect ofmolecular shape nematic has always been preferred forits simple structure and easier to perform experiments.It has been reported that bent-core nematic liquid crys-tals exhibit sign inversion of elastic anisotropy [8, 9],unusual electroconvection [10–12], giant flexoelectricity[13, 14], large viscosity [15, 16], Kerr coefficient [17], in-duced [18] and spontaneous biaxiality [19, 20]. In case ofcompounds with T shaped molecule it was reported thatthe splay and bend elastic constants are almost equal (i.e., K11 ≃ K33) in the entire N phase [21]. As men-tioned these compounds also exhibit occasionally varioussmectic phases at lower temperature and the structure-

∗Corresponding author: sdsp@uohyd.ernet.in

property correlation of such unconventional liquid crys-tals in these phases are meagre. In this paper we re-port measurement of birefringence, static dielectric con-stant and dielectric dispersion in the N, SmA and SmCaphases and in addition splay, bend elastic constants, ro-tational viscosity of a hockey stick-shaped compound as afunction of temperature. The results are compared withconventional calamitic liquid crystals and interpreted interms of the molecular structure, shape and dipolar cor-relation in the mesophase.

II. EXPERIMENTAL

Two indium tin oxide (ITO) coated glass plates werethoroughly cleaned to prepare liquid crystal cells. Theseplates were spin coated with polyimide (AL-1254) andcured at 180 ◦C for 1h and rubbed in antiparallel man-ner for planar or homogeneous alignment of the sample.ITO plates were spin coated with JALS-204 and werecured at 200 ◦C for homeotropic alignment of the sam-ple. The cell thickness was measured by using a fibreoptic spectrometer (Ocean Optics, HR-4000). Typicalcell thickness used in the experiment was ∼ 5µm. Theempty cell was heated and filled with the sample in theisotropic phase. The phase transition temperatures andenthalpies of the compound were measured using differ-ential scanning calorimetry (DSC) (Perkin-Elmer, PyrisID). The textures were observed using a polarising opticalmiscroscope (Nikon, LV100POL). The perpendicular andparallel components of the static dielectric constant weremeasured in planar and homeotropic cells respectively byusing an impedance analyser (Novocontrol, Alpha-A). Incase of planar cell the measuring voltage was below theFreedericksz threshold voltage.

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The temperature dependent optical retardation in pla-nar cell was measured by using a phase modulation tech-nique [22] with the help of a Helium-Neon laser ( HTPS-Ec-1, Thor Labs), photoelastic modulator (PEM-100,Hinds Instruments) and a lock-in amplifier (Ametek, SR-7265). A sinusoidal voltage of frequency 4.111 kHz andamplitude 0.5 V was used for the temperature dependentdielectric measurement. Dielectric dispersion was mea-

sured at various temperatures in the frequency range of100 Hz to 10 MHz. The splay elastic constant (K11) isobtained from the Freedericksz threshold voltage (Vth)and is given by K11 = ϵo∆ϵ(Vth/π)

2, where ∆ϵ = ϵ||−ϵ⊥is the dielectric anisotropy. At strong surface anchoringcondition, the voltages above the threshold voltage Vth,the retardation (∆ϕ) is given by the parametric equations[23]:

V

Vth=

2

π

√1 + γ sin2(ϕm)

∫ ψ=π2

ψ=0

√1 + κ sin2(ϕm) sin2(ψ)(

1 + γ sin2(ϕm) sin2(ψ)) (

1− sin2(ϕm) sin2(ψ))dψ (1)

∆ϕ = 2πned

λ

∫ ψ=π2

ψ=0

√(1+γ sin2(ϕm) sin2(ψ))(1+κ sin2(ϕm) sin2(ψ))(1−sin2(ϕm) sin2(ψ))(1+ν sin2(ϕm) sin2(ψ))

dψ∫ ψ=π2

ψ=0

√1+γ sin2(ϕm) sin2(ψ))(1+κ sin2(ϕm) sin2(ψ))

((1−sin2(ϕm) sin2(ψ))dψ

− none

(2)

where d is the cell thickness, ϕm is the tilt angle at themiddle of the cell and the other terms of the reducedquantities are: γ = (ϵ||/ϵ⊥) − 1, κ = (K33/K11) − 1,

ν = (n2e/n2o) − 1. ϕ is the z-dependent tilt angle and

ϕm is the maximum tilt angle at the middle of the cell(z = d/2) and substituted with sin(ϕ) = sin(ϕm) sin(ψ).The sample retardation at higher voltage are fitted tothe above equations by an iterative procedure to get thebend elastic constant K33 [24, 25]. To verify the anchor-ing condition we estimated the pretilt angle (α) in planarcell by putting the lower limit of the integration ψ = α in-stead of zero in equations (1) and (2) as a fit parameter.It was found that α ≤ 1◦ at all temperatures indicat-ing the strong anchoring at the surface hence α was as-sumed to be zero in present analysis. It has been reportedthat the pretilt angle of bent-core nematic liquid crystals(BCN) on planar alignment layer is very low (≤ 1◦) dueto the presence of flexible alkyl chains on both sides [26].Recently we have found that a wide temperature BCNprovides very low pretilt angle on the present alignmentlayer (AL-1254) [16]. Further it may be mentioned thatthe flexoelectric effect in unconventional liquid crystalscould be significant and can affect K33 measurements.From voltage dependent dielectric measurements it wasshown by Brown et al. [27] that it can slightly under-estimate K33 when the flexoelectric effect is neglected.In our experiment we measured retardation over a smallregion (< 1 × 10−6 m2) using He-Ne laser and expectedthat such effect is minimum and hence not included inthe present analysis.

The rotational viscosity (γ1) was measured in a planarcell by using a phase-decay-time measurement technique.An arbitrary signal generator (Tektronix, AFG3102) wasused to apply sinusoidal voltage at a frequency of 4.111kHz and a photomultiplier tube (Hamamatsu, H6780-01)

was used to measure the time dependent transmitted in-tensity. A small voltage (Vb) corresponding to the firstmaxima or minima was applied depending on transmis-sion intensity such that the total phase retardation ofsample was nπ, where n is an integer. At time t=0,the bias voltage (Vb) was switched off and the relaxationtransmission intensity change of the liquid crystal cell wasmeasured with an oscilloscope (Tektronix, TDS 2012B).This procedure was repeated at various temperatures inthe entire nematic phase.The time dependent transmittance at a particular tem-perature in a planar cell is given by [28]

I(t) = Io sin2{[∆tot − δ(t)]/2}, (3)

where Io is the maximum intensity change and ∆tot isthe total phase difference. The optical phase differenceδ(t), for small director distortion can be approximated asδ(t) = δo exp(−2t/τo), where δo is total phase differenceof liquid crystal under bias voltage (Vb) that is not farfrom the Freedericksz threshold voltage (Vth). In case δois close to nπ, δ(t) becomes δ(t) = δo exp(−4t/τo). Theslope of the variation ln[δo/δ(t)] with time (t) yields therelaxation time (τo). The rotational viscosity (γ1) of theliquid crystal is given by γ1 = τoK11π

2/d2, where d isthe thickness of liquid crystal sample.

III. RESULTS AND DISCUSSION

A. Optical and static dielectric measurements

The hockey stick-shaped compound we studiedis 4-n-butyloxyphenyl[4-(4-n-heptyloxybenzoyloxy4-benzoyloxy)]biphenyl-3-carboxylate and exhibits

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metastable liquid crystalline phase transitions. Thechemical structure and the phase transition temper-atures of the compound is shown in Fig. 1(a). Thesynthesis and some preliminary characterisation isreported recently [29].

(i) (ii)

(iii) (iv)

116 °C 113 °C

109.5 °C 103.5 °C

(a)

(b)

FIG. 1: (a) Chemical structure of the compound and thephase transition temperatures (heating). (b) Photomicro-graphs obtained in untreated cell under cooling at differenttemperatures (i) N phase (ii) at N-SmA transition (iii) SmA(iv) SmCa.

The molecule is achiral, asymmetric, hockey stick-shaped and has several strong dipolar groups. The tex-ture obtained in between two glass plates without anysurface treatment is shown in Fig. 1(b). In the ne-matic phase a typical schlieren texture with four brushdefects are observed (Fig. 1(b) (i)). SmA is sponta-neously aligned homeotropically below the N-SmA tran-sition temperature (Fig. 1(b) (iii)) and again the darkregion of the texture is transformed into schlieren textureas the temperature is reduced to SmCa phase. SmCaexhibits two and four brush defects with dynamic fluc-tuations (Fig. 1(b) (iv)), suggesting it to be an anti-clinic SmC (SmCa) phase. Recently it has been reportedthat the bent-core liquid crystals with ∆ϵ > 0 can bealigned homeotropically on conventional alignment layerswhereas compounds with ∆ϵ < 0 in general do not alignhomeotropically [30]. The results were explained due tothe stronger dipolar interaction of resultant dipole mo-ment with the surfaces. The present compound exhibits∆ϵ > 0 (see later) and provides good homeotropic align-ment both in the N and SmA phases and supports infavour of the explanation i.e., the strong dipolar interac-tion between the substrate surface and molecule of mate-rials with ∆ϵ > 0 stabilise homeotropic alignment. Thetextures obtained in planar and homeotropic cells under

(ii)

(iii) (iv)

(v) (vi)

119 °C 119 °C

112 °C 112 °C

104 °C 104 °C

SmCa

SmA

N

(v)

(i)

(iii)

FIG. 2: Photomicrographs obtained in planar cell (i, iii, v)and in homeotropic cell (ii, iv, vi) at various temperaturesunder polarising optical microscope. White arrows indicatethe rubbing directions.

optical polarising microscope is shown in Fig. 2. It isnoticed that all the phases can be aligned well in planarcells. In homeotropic cell both N and SmA phases arewell aligned. In case of SmCa phase the overall textureis dark in homeotropic cell except some comparativelybrighter and smaller regions are seen due to the tilting ofthe molecules with respect to the layer normal.

The temperature dependent variation of ∆n and adifferential scanning calorimetric (DSC) thermogramare shown in Fig. 3(a). The enthalpies of I to N, N toSmA and SmA to SmCa transitions are 0.38, 0.98 and0.23 kJ/mol respectively. ∆n jumps to ≃ 0.06 from 0 atnematic-isotropic (NI) transition and gradually increasedas the temperature is lowered. At N-SmA transition itshows significant increase (≃ 0.02) and tends to saturatein the SmA phase. A similar jump (≃ 0.02) in ∆n is alsoseen at SmA- SmCa transition and further it increases asthe temperature is decreased. Since ∆n ∝ S, where S isthe orientational order parameter, it is anticipated thatorientational order parameter increases with decreasingtemperature showing finite jumps across the transi-tions. Hence, these are first order transitions. Further,N-SmA transition (Fig. 3(a)) is comparatively lesssharp than SmA-SmCa transition due to the presence ofsmectic short-range fluctuations in the N phase. Finiteenthalpies of these transitions also suggest that thetransitions are first order. It may be pointed out thatusually the measurement of ∆n in lower symmetricsmectic phases are difficult and rare due to poor align-ment. Moreover SmCa is biaxial because of tilting of

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the molecules from the layer normal. The narrow linesalong the rubbing direction in Fig. 2(v) could be dueto the local biaxial order of SmCa phase. Neverthelessin the present system it was possible to measure ∆nbecause the director in the SmA phase is well aligned(Fig. 2(iii)) and the effective principal axis in the SmCaphase still remains parallel to the rubbing direction be-cause of opposite tilt of the molecules in successive layers.

Δn

DSC

NSmASmCa I

Δn

0

0.1

0.2

Heat fl

ow

(mW

)

19

20

21

22

23

T-TNI (°C)

−20 −10 0

NSmASmCa Iε

ε||

ε⊥

ε

6

7

8

T-TNI (°C)

−25 −20 −15 −10 −5 0

(a)

(b)

FIG. 3: (a) A DSC thermogram in cooling cycle (rate 5◦/min)and variation of birefringence (∆n) (b) Variation of dielectricconstant (ϵ||, ϵ⊥ and ϵ) as a function of temperature. Dottedvertical lines are drawn to indicate the transitions. Dashedline denotes the extrapolated value of isotropic dielectric con-stant.

Temperature dependent variation of dielectric con-stants ϵ||, ϵ⊥ and average value, ϵ (= (ϵ|| + 2ϵ⊥)/3) areshown in Fig. 3(b). ϵ in the nematic phase is less andfurther reduces in smectic phases from the extrapolatedvalue of the isotropic dielectric constant. Usually ϵ isexpected to increase as the temperature is reduced dueto the increase in density and contribution from orien-tational polarisation [31]. In the present compound theopposite behaviour especially in the smectic phases aredue to the strong antiparallel correlation of the longi-tudinal components of dipole moments. The dielectricanisotropy ∆ϵ (= ϵ|| − ϵ⊥) is positive in all the phases.ϵ|| increases as the temperature is lowered in the N phaseand starts to decrease sharply at the N-SmA transitionand decreases rather slowly in the SmCa phase. The de-crease in ϵ|| in conventional and in unconventional com-

pounds with similar molecular structure is also reportedand attributed to the increase in antiparallel correlationbetween the long axis components of dipoles in the SmAlayer [8, 21]. The slow decrease of ϵ|| as the tempera-ture is reduced in the SmCa phase is due to the tiltingof the molecules in addition to the antiparallel correla-tion. ϵ⊥ decreases in the N phase as the temperatureis decreased from the I phase. It is further lower in theSmA phase than both N and SmCa with slope change atthe transitions. The lower value of ϵ⊥ in the SmA phasesuggests that there is also an antiparallel correlation be-tween the short axis components of the dipoles in theSmA layer. This result is opposite to the behaviour seenin other unconventional liquid crystals reported recently[8, 21]. It may be mentioned that since the moleculeshave anticlinic tilt in successive layers of SmCa phase, ϵ||in this phase refers to the dielectric constant when theelectric field is parallel to the layer normal rather thanthe director. Further both the planar and homeotropicalignment of the SmCa phase is relatively poor than theSmA phase due to the reason mentioned above and theabsolute accuracy in the measurements of ∆n, ϵ|| and ϵ⊥in this phase is expected to be somewhat less than bothin N and SmA phases.

B. Viscoelastic measurements

To measure K11 and K33 we measured voltage depen-dent optical retardation (∆ϕ) at various temperatures inthe nematic phase. The temperature variation of K11

and K33 is shown in Fig. 4(a). A typical variation of ∆ϕtogether with the best fit of equations (1) and (2) is alsoshown in the inset of Fig. 4(a). K11 is significantly higherthan K33 in the entire N phase except very close to theNI transition hence elastic anisotropy (∆K = K33−K11)is negative. K33 increases much slower than K11 withdecreasing temperature and shows strong pretransitionaldivergence as the SmA phase is approached. In case ofcalamitic liquid crystals with rigid rod-like molecules ∆Kis mostly positive whereas in pure bent-core compoundsand in their mixtures it has been found to be negative[8, 32, 33]. It has been shown that the bent shape ofthe molecules facilitate bend distortion as a result K33 islower than K11. The hockey stick-shaped molecule alsohas asymmetric bent-shape and the present result is themanifestation of the same effect.We also measured rotational viscosity (γ1) in the N phaseas a function of temperature by using the time depen-dent phase-decay method. Representative variation ofln[δo/δ(t)] with time (t) at two different temperaturestogether with the best fits are shown in the inset of Fig.4(b). γ1 increases as the temperature is lowered in theN phase and a change in curvature is observed at T-TNI≃-5◦C suggesting a pretransitional effect of smecticshort-range order in the N phase. γ1 in this compound isslightly larger than in pure bent-core nematic liquid crys-tals [16] and significantly higher than that of many known

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K11

K33

K (

pN

)

2

4

6

8

T-TNI (°C)−8 −6 −4 −2 0

Δφ

0

5

Voltage (V)0 5 10

γ1

γ1 (

Pa

s)

0.2

0.4

0.6

0.8

T-TNI (°C)−8 −6 −4 −2 0

T-TNI=-2.5°C

T-TNI=-7.5°C

ln[δ

o/δ

(t)]

0

0.5

Time (ms)0 100 200 300

(a)

(b)

FIG. 4: (a) Variation of splay (K11) and bend (K33) elas-tic constants in the N phase as a function of temperature.(Inset) Retardation (∆ϕ) as a function of applied voltage atT-TNI=-4.5◦C. Continuous line in the inset is a theoretical fitto Eqs.(1) and (2). (b) Variation of rotational viscosity (γ1)as a function of temperature. (Inset) Variation of ln[δo/δ(t)]at two temperatures namely T-TNI=-7.5◦C and -2.5◦C withbest fits. Continuous red and green lines are drawn as a guideto the eye.

calamitic nematic liquid crystals and in their mixtures[16, 32]. The larger rotational viscosity in this hockeystick-shaped compound partially may arise due to theeffect of molecular weight compared to the calamitics.However in case of bent-core compounds it was attributedto the existence of temporarily fluctuating smectic clus-ters of a few molecules in the N phase that are expected tooriginate from the restricted free rotation of the bent-coremolecules along the long axis [9, 15]. Existence of suchtemporary clusters in the nematic phase is also revealedin rheological [34] and dynamic light scattering [35] stud-ies and more recently in small angle X-ray studies [36–39].The hockey-stick shaped molecule can be considered ashighly asymmetric bent-core molecule and hence the en-hanced rotational viscosity is believed to originate fromthe temporarily fluctuating clusters.

C. Dielectric relaxation

The hockey stick-shaped molecule has several dipolargroups which are oriented in different directions. Since∆ϵ > 0, it is anticipated that the molecule has relativelystrong longitudinal dipole moment along the long axis asa result ϵ|| can relax at relatively low frequency. In caseof ϵ⊥ no dipolar relaxation was found up to 10 MHz.Variation of ϵ|| as a function of frequency in the three

119 °C (N)

112 °C (SmA)

104 °C (SmCa)

ε||"

0

1

2

3

ε||'3 4 5 6 7 8

119 °C (N)112 °C (SmA)104 °C (SmCa)ε||' ε||"

4

6

8

0

1

2

3

Frequency (Hz)103 104 105 106 107

(a)

(b)

FIG. 5: (a) Frequency dispersion of real (ϵ′||)(open symbols)

and imaginary (ϵ′′

||) (filled symbols) parts of the dielectricconstant at three different temperatures namely 119◦C (N),112◦C (SmA) and 104◦C (SmCa). Continuous lines are theo-retical fit to the equation (4). (b) Cole-Cole plot for ϵ|| relax-ation at the same temperatures. The centers of the semicir-

cles lies on the ϵ′|| axis suggesting that the relaxation is Debye

type. Dashed lines are theoretical fit to the Cole-Cole equa-tion. The high frequency portion of the semicircles are dueto the ITO cell relaxations.

phases at three different temperatures are shown in Fig.5(a). It is observed that there are two relaxations, one istemperature dependent (below ∼1 MHz) and the relax-ation frequency decreases with decreasing temperature.The other one is above ∼1 MHz frequency and tempera-ture independent. The high frequency relaxation is tem-perature independent and is attributed to the relaxationdue to the finite resistance of ITO which is even presentin empty cell. The low frequency relaxation is due tothe longitudinal component of the molecular dipole mo-ment of the long axis. Cole-Cole plots of the dielectricconstant are shown in Fig. 5(b). They are semicircularwith the centers on the x-axis suggesting that the relax-ation is Debye type. Similar low frequency relaxations

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are also seen in other bent-core compounds [40]. Sincethe relaxation region due to ITO is partially overlappedwith dipolar relaxation we fitted complex dielectric con-stant (ϵ(ω) = ϵ

′(ω)−iϵ′′(ω)) as a function of frequency to

find the exact dipole relaxation frequency. The complexdielectric function is given by

ϵ(ω)∗ = ϵ2 +ϵo − ϵ1

1 + (iωτ1)1−α1+

ϵ1 − ϵ21 + (iωτ2)1−α2

(4)

where ϵo is the static dielectric permittivity, ϵi and τiare the frequency limits and relaxation times of the ith

mode. αi is the Cole-Cole distribution parameters of therespective modes.

N SmA SmCa

τ 1 (μ

s)

2

5

10

20

50

1000/T (K-1)

2.55 2.60 2.65 2.70

NSmASmCa Iε 0-ε∞

2

3

T-TNI (°C)

−25 −20 −15 −10 −5 0

(a)

(b)

FIG. 6: (a) Variation of dielectric strength (ϵ0 − ϵ∞) withtemperature (b) Variation of relaxation time τ1 as a func-tion of 1/T . Dashed lines are theoretical fit to the equationτ1 = τ0exp(U/RT ). Vertical lines indicate the phase transi-tion temperatures.

It is found that both α1 and α2 are very small(< 0.01) at all temperatures and the relaxation timeτ2 is independent of temperature as expected. Thetemperature variations of dielectric strength (ϵ0 − ϵ∞)and relaxation time τ1 with temperature are shown inFig. 6. The dielectric strength increases rapidly in theN phase due to the increase in the orientational orderand decreases sharply at the N-SmA transition (Fig.6(a)). The sharp decrease below N-SmA transition isdue to strong antiparallel correlation of the longitudinal

components of the dipoles in the smectic phases asdiscussed earlier. τ1 increases and varies linearly withthe inverse of temperature (Fig. 6(b)) and can be fittedwell with the Arrhenius equation, τ1 = τ0exp(U/RT ).The activation energy U of the N, SmA and SmCaphases are 158, 117 and 127 kJ/mol respectively. Thesevalues are comparatively higher than many calamaticsin their respective phases [41, 42]. Similar behaviouris also reported very recently in a binary liquid crystalmixture (rod and bent-core) showing N-SmCa phasetransition [40]. Since the activation energy represent theenergy barrier associated with the flip-over of the longmolecular axis it suggests that the flip-over motion isrelatively uneasier in hockey-stick or bent-core systemsthan calamitic liquid crystals. Presumably this couldarise due to the shape and larger rotational viscosity(Fig. 4(b)) of the hockey stick-shaped and bent-corecompounds. However further investigations are neededto shine more light on this aspect.

IV. CONCLUSION

In conclusion, we have measured detailed physicalproperties of an unconventional liquid crystal withhockey stick-shaped molecules for the first time. Allphase transitions are detected to be first order fromoptical and thermal (DSC) measurements. Dielectricmeasurements indicate that there exists antiparalleldipolar correlation of both the longitudinal and trans-verse components of dipole in the SmA and SmCaphases. Bend elastic constant is significantly lower thansplay elastic constant as seen in other bent-core nematicliquid crystals. Rotational viscosity is comparativelylarger than calamitics and are influenced by pretransi-tional smectic fluctuations. Dielectric relaxation of thelongitudinal components of dipole moments is Debyetype and the relaxation frequency exhibits Arrheniusbehaviour with comparatively higher activation energy.Finally it may be asserted that many the physical prop-erties of hockey stick-shaped compound are noticeablydifferent than that of calamitic liquid crystals and theshape is an important parameter for tuning the physicalproperties.

Acknowledgments: One of the authors S. D. grate-fully acknowledges the financial support from UGC-CASSchool of Physics and DST, Govt. of India (Projectno: SR/NM/NS-134/2010). P. Sathyanarayana acknowl-edges CSIR-UGC for the fellowship. BKS thanks IndianNational Science Academy, New Delhi for financial sup-port.

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