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Chemical Physics Letters 453 (2008) 207–211

Molecular reorientations in liquid crystal 6O.4

Siddharth Gautam, R.R. Choudhury, Lata Panicker, S. Mitra, R. Mukhopadhyay *

Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India

Received 23 November 2007; in final form 22 January 2008Available online 30 January 2008

Abstract

Quasielastic neutron scattering (QENS) studies on the reorientational dynamics in the ordered phases (crystalline and the smectic H)of p-n-hexyloxybenzylidine-p0-n-butylaniline (6O.4) are reported here. Analysis of the QENS data showed that in the crystalline phase(room temperature �25 �C) the reorientational motion involves only the core of the molecule, whereas in the SH phase (at 45 �C) thewhole molecule is involved. This motion is found to persist even when the sample was cooled back to 25 �C (below the crystalline–SH transition temperature of 33.7 �C) suggesting existence of the liquid crystalline phase as a supercooled state. DSC measurementsand X-ray diffraction studies corroborate the QENS results.� 2008 Published by Elsevier B.V.

1. Introduction

Liquid crystals are generally geometrically anisotropicmolecules and exhibit a variety of mesophases [1]. Thesemesophases are characterized by the different orientationsof the molecules. Phase transition across these mesophasesis usually accompanied by a change in the dynamical prop-erties of the molecule. The compounds belonging to thenO.m (alkoxy benzylidine alkylanilenes) homologous seriesof liquid crystal where ‘n’ and ‘m’ represent the number ofcarbons in the alkoxy and alkyl chains, respectively, are ofparticular interest because, apart from their wide rangeapplications, one can vary their molecular structure sys-tematically by varying n and m and study the effects of suchvariations on the physical properties of these liquid crystals[2,3]. One of the problems in reorientational motion ofliquid crystals is whether the aliphatic chains posses a pos-sibility of additional rotation with respect to the centralpart, apart from the orientation of the molecule as a whole.This can be probed by studying the molecular dynamics inthese liquid crystals at different mesophases. This can bedone with the help of several experimental techniques [4]

0009-2614/$ - see front matter � 2008 Published by Elsevier B.V.

doi:10.1016/j.cplett.2008.01.058

* Corresponding author. Fax: +91 22 25505151.E-mail address: mukhop@barc.gov.in (R. Mukhopadhyay).

that include dielectric relaxation methods [5,6], nuclearmagnetic resonance [7–9] and quasielastic neutron scatter-ing (QENS) [10–13] among several others. QENS tech-nique is very suitable to study orientational conformationof molecules particularly for hydrogenous materials [13],providing the timescale of the motion (10�10–10�13 s),geometry of the motion and also the potential experiencedby the orienting species. We had earlier reported QENSstudies on the molecular motions in 4O.4 [10], 4O.8 and5O.6 [11] liquid crystals in the nO.m type liquid crystal fam-ily, demonstrating the successful description of dynamicalmotions as contributed from different parts of the moleculewithout using deuterated samples. Here we report theQENS studies on the reorientational motion of 6O.4 mol-ecules in the crystalline and a liquid crystalline phase. Alsoreported is the evidence of the existence of a supercooledliquid crystalline state at room temperature when the sam-ple is cooled from the high temperature phase (SH), as indi-cated in QENS studies and corroborated by differentialscanning calorimetry, and X-ray diffraction. We also reportthe structure in the ordered phases (crystalline and smecticH) as studied by X-ray diffraction.

The 6O.4 molecular configuration is schematized in itstrans conformation in Fig. 1. The molecule consists oftwo benzene rings, which form the core and that is attachedto two side chains on either side. The side chains are

Fig. 1. Scheme of the liquid crystal 6O.4 molecule. Three different possible axes of rotations as discussed in the text are shown. AB axis is the hexyloxychain axis, CD axis is the molecular or core axis and EF axis is the butyl chain axis referred in the text.

208 S. Gautam et al. / Chemical Physics Letters 453 (2008) 207–211

formed by hexyloxy group and a butyl group. The transi-tion scheme in 6O.4 as reported by Leadbetter et al. [14]is, K!30C

SH!57C

SB!59C

SA!70C

N!78CI.

2. Experimental details

6O.4 sample was prepared by refluxing mixture of p-n-hexyloxy-bezaldehyde (1 mol) and vacuum distilled p-n-butyl aniline (1 mol) azeotropically at 100 �C in benzenefor 5–6 h. Benzene was then distilled out. The productwas recrystallized using petroleum ether several times toget fine crystals, which gave sharp melting point. Thin layerchromatography of the final recrystallised 6O.4 compoundgave single spot confirming absence of any other impuri-ties. Neutron scattering experiments were performed atthe QENS spectrometer at Dhruva reactor, Trombay[15]. This spectrometer has an energy resolution of200 leV with incident neutron energy of 5.1 meV. QENSmeasurements were done at room temperature (in the crys-talline phase), and at 45 �C (in the smectic H phase). Sam-ple was put in an aluminum canister. QENS data at 60 �Cand above show very large broadening, beyond the sensi-tive range of the spectrometer, suggesting very high orien-tational disorder. Mettler Toledo DSC822 instrument wasused for thermal measurements of the 6O.4 samples (keptin a circular Al pan), with an empty pan as a reference.RUB 200 series DMax powder diffractometer coupled toa rotating anode generator (Rigaku make) was used forthe X-ray diffraction (XRD) studies. Sample was placedwithin a groove (0.5 mm depth) on a glass slide. XRD pat-tern was recorded initially at ambient condition. The sam-ple was then placed in an incubator set at 45 ± 0.1 �C foraround an hour, and once again pattern was recorded at25 �C.

3. Results and discussions

In a neutron scattering experiment on a hydrogenousmaterial the scattering due to all other atoms is overshad-owed by the incoherent scattering due to hydrogen becauseof exceedingly large incoherent scattering cross-section ofhydrogen as compared to other atoms. The information

about the dynamics can be obtained from the scatteringlaw S(Q,x), which is proportional to the double differentialcross-section, which in turn is proportional to the scatteredintensity [13]. In general the scattering law is expected tohave both elastic as well as quasielastic contributions andcan be written as

SðQ;xÞ ¼ AðQÞdðxÞ þ ½1� AðQÞ�LðC;xÞ ð1Þ

where the first term is the elastic contribution whereas thesecond term is quasielastic. Elastic incoherent structure fac-tor (EISF), defined as the fraction of elastic contribution tothe total spectra, provides information about the geometryof motion. In case of liquid crystals, which are expected tohave a significant degree of orientational order, EISFshould have a finite value. In the second term of Eq. (1),L(C,x) is a Lorentzian function with half width at halfmaximum (HWHM) C, related to the correlation timesassociated with the molecular motion in the system. Tostart with, the elastic and quasielastic contributions wereseparated from the background subtracted spectra by fit-ting the QENS data with Eq. (1), convoluted with instru-mental resolution, which was measured using a standardvanadium sample, by the method of least squares. The fitsthus obtained for three typical Q values at different temper-atures are shown in Fig. 2a (at RT), b (at 45 �C) and c (atRT after cooling from 45 �C). Data in the higher tempera-ture phases show that the system becomes so disorderedthat the quasielastic broadening went beyond the effectiverange of the present instrument so could not be used forany further analysis. The EISF values as obtained fromthe fit of the experimental data correspond to RT, 45 �Cand cooled back to RT are shown in Fig. 3. It is clear thatthe system does not go back to its room temperature phaseafter cooling from 45 �C. We shall discuss this feature indetail later. To explain the observed behavior of the exper-imentally obtained EISF various plausible models wereconsidered as described below.

In the ordered phases of liquid crystals as studied here(K and SH) the reorientational motion is expected to beuniaxial rather than isotropic because of a high degree oforientational order present. A molecule can have severalmolecular axes of rotation leading to different degrees of

0

200

400

600 (a)Q=0.8 Å-1

Energy Transfer (meV)

S(Q

,ω)(a

rb. u

nits

)

0

100

200

300

400 Q=1.32 Å-1

-1.0 -0.5 0.0 0.5 1.0 -1.0 -0.5 0.0 0.5 1.0-1.0 -0.5 0.0 0.5 1.00

50

100

150

200 Q=1.8 Å-1

0

100

200

300

400

500 (b)Q=0.8 Å-1

Energy Transfer (meV)

100

200

300 Q=1.32 Å-1

0

50

100

150

200Q=1.8 Å-1

0

100

200

300

400 (c)Q=0.8 Å-1

Energy Transfer (meV)

50

100

150

200

250Q=1.32 Å-1

0

50

100

150

200Q=1.8 Å-1

Fig. 2. Typical QENS data (symbols) along with the fits (solid curves) of Eq. (3) convoluted with instrumental resolution at three typical Q values at (a)RT, (b) 45 �C and (c) RT brought back after heating to higher temperatures. The elastic and quasielastic parts are shown as dash-dotted and dotted curves,respectively.

0.0 0.5 1.0 1.5 2.0 2.5

0.0

0.2

0.4

0.6

0.8

1.0

1.2 QENS data at room temperature QENS data at 45ºC QENS data at room temperature

after heating

EISF

Q (Å-1)

Fig. 3. Variation of EISF with Q. The lines correspond to model functionsand the symbols are the experimental data at different temperatures.

S. Gautam et al. / Chemical Physics Letters 453 (2008) 207–211 209

orientational disorder. The axis of rotation for uniaxialrotation in the case of an elongated molecule like 6O.4 isexpected to coincide with the molecular axis, line CD (seeFig. 1). In this case, the incoherent scattering law for theatoms of the molecule diffusing on a cylindrical surfaceof radius a, with a diffusion coefficient Dr, for a powdersample can be written as [16]

SðQ;xÞ ¼ A0ðQÞdðxÞ þ2

p

X1l¼1

AlðQÞl2Dr

ðl2DrÞ2 þ x2ð2Þ

where

AlðQÞ ¼1

p

Z p

0

j0ð2Qa sin xÞ cosð2lxÞdx ðl ¼ 0; 1; 2; . . . ;1Þ

As the hydrogen atoms are at different distances from theaxis of gyration one has to take average over the variousdistances. The average incoherent elastic factor for a pow-der sample can be written as

A0ðQÞ ¼1

Np

XN

i¼1

Z p

0

j0ð2Qai sin xÞdx ð3Þ

where ai is the distance of the ith hydrogen atom from theaxis of rotation and N is the number of hydrogen atomstaking part in the rotational motion.

To explain the observed EISF various dynamical situa-tions with respect to the core and chains belonging to 6O.4(Fig. 1) were considered, like, core of the molecule rotatingside chains static and vice versa, side chain can rotate aboutits own axis (line AB or EF) or about the molecular axis(line CD), core and chain move either independently orin unison, etc. Here, we only describe the ones that seemto describe the data. In the crystalline phase a model wherethe hydrogen atoms only belonging to the core undergoingrotation about the core axis (line CD in Fig. 1) describesthe data consistently. The model where the side chainsrotate about their own axes correspond to much higher dis-order as the number of protons involved is quite large vis-a-vis the core. However, since data show high degree of

0 10 20 30 40 50 60 70 80

-4

-2

0

2

4

Hea

t Fl

ow

1st cooling scan

2nd heating scan

1st heating scan

Temperature (ºC)

Fig. 4. DSC thermograms for 6O.4 with heating and cooling at 5 �C/min.

210 S. Gautam et al. / Chemical Physics Letters 453 (2008) 207–211

order in the crystalline phase as the quasielastic componentis quite weak, this type of motion is unlikely to occur. TheEISF for the model where only the core is rotating can bewritten as

EISF ¼ N c

N all

AccðQÞ þ

N all � N c

N all

ð4Þ

where Nc = 9 denotes the number of hydrogen atoms in thecore, Nall = 31 is the total number of hydrogen atoms in themolecule and Ac

cðQÞ is the structure factor corresponding tothe motion of only core hydrogen atoms about its axis. Thesecond term in the above equation accounts for the immo-bile hydrogen atoms.

Data corresponding to the SH phase show much higherdisorder compared to the crystalline phase as can be seenfrom Fig. 3. As noted earlier, the side chains also can rotateabout their respective axes in addition or independent ofthe core motion. In the case of smectic H phase of 6O.4the model in which the whole molecule (core + side chain)rotate about the molecular axis and the side chains rotateabout their respective axes, describes the behavior of EISFthe best. Assuming that all the individual motions areuncorrelated, the EISF can be written as,

EISF ¼ N h

N all

AhhðQÞAc

hðQÞ þN b

N all

AbbðQÞAc

bðQÞ þN c

N all

AccðQÞ

ð5ÞThe notation Ax

yðQÞ used above stands for the structure fac-tor corresponding to the rotation of the hydrogen atomsbelonging to group ‘y’ about an axes of rotation ‘x’. Sym-bols h, b, and c denote the hexyl, the butyl and the coregroup, respectively.

The EISF calculated as per the situations describedabove are shown in Fig. 3 along with the experimentaldata. As can be seen in the figure, in the crystalline phasethe data correspond to a model (shown by dashed curve)where only the core of the molecule undergoes rotationaldiffusion about its axis and in the SH phase (at 45 �C) thewhole molecule rotates about the molecular axis and inaddition the side chains rotate about their own axes inde-pendently. It is also found that when the sample was cooledfrom 45 �C to room temperature (below the K–SH transi-tion temperature) the data still correspond to that of theSH phase indicating that this phase exists as a metastablestate at room temperature. Variations of HWHM with Q

were found to be consistent with uniaxial rotational diffu-sion model [16].

To investigate the behavior observed in the QENS datawe have studied the phase transitional behavior in 6O.4 bydifferential scanning calorimetry (DSC) and X-ray diffrac-tion. In DSC run, the sample was initially cooled to–10 �C and heated up to 80 �C; all the transitions reportedin Ref. [15] were reproduced. In the cooling cycle all thetransitions were observed except the first one correspond-ing to the crystalline to SH. It was not observed in the2nd heating cycle either. Even cooling down to –100 �Cthe crystalline phase could not be observed. To Re-ascer-

tain the existence of K–SH transition (at 33.7 �C), a freshsample was taken and heated only up to 42 �C (just abovethe crystalline to liquid crystalline melting point). Whilesimilar to the first time, an endothermic peak was observedat 33.7 �C, but it was not observed in the cooling or in the2nd heating cycle indicating non-reversal of the parentphase. The DSC thermograms are shown in Fig. 4.

The existence of the metastable SH phase in 6O.4 atroom temperature was not reported earlier, however, wefound that delayed crystallization is not a very uncommonfeature in nO.m type of liquid crystals [17].

The XRD patterns of 6O.4 sample at 25 �C (before andafter heating to 45 �C) is shown in Fig. 5. The fact thatthere are well-defined diffraction peaks till 50� (Fig 5a) indi-cates that molecules have three-dimensional orderedarrangement suggesting that the sample is crystalline. Eigh-teen strong peaks in 2h range 3–25� were used for indexingusing the program DICVOL [18,19], the figure of merit forthis indexing are M (18) = 5.4, F (18) = 11.5. The crystalsystem was found to be monoclinic with cell parame-ters a = 19.07(2) A, b = 5.180(6) A, c = 17.28(3) A, b =117.6(1)� and cell volume = 1514.15 A3. When the sampleis heated to 45 �C and cooled back to 25 �C, a large degreeof order is lost as evident from the diffraction patternshown in Fig. 5b. The sample now is much disordered thanthe original 25 �C phase. This is corroborative to theQENS results and indicates that the higher temperaturephase above 33.7 �C remains arrested even cooling toRT. The most intense first peak corresponds to the distancebetween the two consecutive layers of 6O.4 molecules isfound to be shifted from 5.18� to 3.98� indicating that thereis a significant increase in the layer spacing (L), which canbe rationalized by the decrease in the tilt angle [20] of themolecules from crystalline to liquid crystalline phase. Thisis consistent with the fundamental fact that parallelarrangement of anisotropic objects leads to an increase inpositional entropy. Two kinds of reflections, in the patternshown in Fig. 5b, can be seen. First kind are strong sharpcrystalline reflections at 3.98�, 7.95�, 11.97� and 19.90�

10 20 30 40 50 60 700

2000

4000

6000

(a)

Inte

nsity

2θ

10 20 30 40 50 60 70

2000

4000

6000

Inte

nsity

2θ

Peaks corresponding to layer seperation(b)

17 18 19 20 21 22 230

1000

2000

3000 [10] (20.26º)[01]

20.48º(001)19.9º

Fig. 5. XRD pattern of 6O.4 taken at (a) 25 �C and (b) 25 �C after coolingfrom 45 �C.

S. Gautam et al. / Chemical Physics Letters 453 (2008) 207–211 211

corresponding to the 1st, 2nd, 3rd and 5th orders of (00 l)type reflections representing the layer separation of thesmectic phase and the second kind are the ones at 20.26�and 20.44� arising from two-dimensional near-hexagonalorder within the smectic plane. Assuming the molecule tobe a cylinder of length 23.2 A and diameter 5 A (obtainedby assuming that the entire molecule is rotating around themolecular axis in smectic H phase as indicated by ourQENS results) a hexagonal packing of these cylinders inthe smectic plane was obtained. The first order reflection[10] of such a hexagonal closed packed layer came to beat 20.48� which is very close to what we have observed inour X-ray diffraction pattern hence the peaks at 20.26�and 20.44� have been indexed as [10] and [01], the slightdifference between the two is because of the distortion ofthe hexagons due to the tilt of the molecules with respectto the layer normal.

4. Summary

We have studied the orientational conformation in theordered phases (crystalline and the smectic H) of p-n-hex-yloxybenzylidine-p0-n-butylaniline (6O.4). QENS data atRT (�25 �C) is consistent with the model where only thecore of the molecule rotates around the molecular axiswhereas at 45 �C in the SH phase it is the whole moleculethat rotates about the molecular axis and in addition the

two side chains spin about their own axes, a motion muchmore disordered than that at RT. Moreover this type ofmotion is found to persist when the system is brought backto RT (below the K–SH transition temperature) indicatingarrest of the liquid crystalline phase down to RT. DSCmeasurements corroborate that a physically stable liquidcrystalline phase (SH) is formed at 33.7 �C which doesnot go to the crystalline phase even after cooling the sampleup to �100 �C. The initial phase at 25 �C is found to becrystalline consistent with monoclinic structure and trans-forms to SH phase in which the packing is hexagonal asindicated in the XRD study. This demonstrates that whatwas observed in the microscopic measurement like QENSwas later confirmed by the macroscopic technique likeDSC. XRD studies reconfirmed both the results. So QENS,DSC and XRD results are consistent among themselves.The delayed crystallization in 6O.4 as observed here wasnot reported earlier, however it is not an uncommonfeature in nO.m type of liquid crystals as reported in Ref.[17].

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