This article was downloaded by: [University of Limerick] On: 04 May 2013, At: 18:13 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Liquid Crystals Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tlct20 Methylene-linked liquid crystal dimers and the twist- bend nematic phase Peter A. Henderson a & Corrie T. Imrie a a Chemistry, School of Natural and Computing Sciences, University of Aberdeen, Aberdeen, UK Published online: 22 Nov 2011. To cite this article: Peter A. Henderson & Corrie T. Imrie (2011): Methylene-linked liquid crystal dimers and the twist-bend nematic phase, Liquid Crystals, 38:11-12, 1407-1414 To link to this article: http://dx.doi.org/10.1080/02678292.2011.624368 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
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This article was downloaded by: [University of Limerick]On: 04 May 2013, At: 18:13Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
Liquid CrystalsPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tlct20
Methylene-linked liquid crystal dimers and the twist-bend nematic phasePeter A. Henderson a & Corrie T. Imrie aa Chemistry, School of Natural and Computing Sciences, University of Aberdeen, Aberdeen,UKPublished online: 22 Nov 2011.
To cite this article: Peter A. Henderson & Corrie T. Imrie (2011): Methylene-linked liquid crystal dimers and the twist-bendnematic phase, Liquid Crystals, 38:11-12, 1407-1414
To link to this article: http://dx.doi.org/10.1080/02678292.2011.624368
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form toanyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses shouldbe independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly inconnection with or arising out of the use of this material.
Methylene-linked liquid crystal dimers and the twist-bend nematic phase
Peter A. Henderson and Corrie T. Imrie*
Chemistry, School of Natural and Computing Sciences, University of Aberdeen, Aberdeen, UK
(Received 7 September 2011; final version received 13 September 2011)
The transitional properties of three methylene-linked liquid crystal dimers are reported, namely,1,5-bis(4-cyanoanilinebenzylidene-4′-yl)pentane (CN-5-CN), 1,5-bis(4-methoxyanilinebenzylidene-4′-yl)pentane(1O-5-O1), and 1,5-bis(4-ethoxyanilinebenzylidene-4′-yl)pentane (2O-5-O2). Each dimer exhibits two monotropicmesophases. The higher temperature mesophase is a normal nematic phase while the lower temperature phaseis assigned as a twist-bend nematic phase. The assignment of the twist-bend nematic phase was based on thestrong similarities in the optical textures observed to those reported recently for a structurally similar dimer. Thecomplete miscibility of the mesophases exhibited by CN-5-CN and 1O-5-O1 has been established. The analogoushexamethylene-linked dimers exhibit only the normal nematic phase as do the corresponding ether-linkeddimers. A review of the literature reveals another five methylene-linked odd-membered dimers that exhibit anematic–nematic transition and, in each, the lower temperature nematic phase exhibits similar properties to thosereported for the twist-bend nematic phase. The formation of this new nematic phase has been attributed to anegative bend elastic constant which results from the bent geometry of methylene-linked odd-membered dimers.
There is a rich complexity of molecular structuresbased around rod-like mesogenic units now knownto support liquid crystallinity, highlighting the cen-tral importance molecular shape has in determiningliquid crystalline behaviour [1]. Flexible fragmentswithin the molecule may control the molecular shapeand thus the transitional properties. Perhaps the moststriking example is liquid crystal dimers [2–4]. Theseconsist of molecules containing two mesogenic unitsconnected by a flexible spacer, most commonly analkyl chain, and their transitional properties exhibita pronounced dependence on the length and parityof the spacer connecting the mesogenic groups. Suchbehaviour has been accounted for, by theory, in termsof the different shapes of the conformers having oddor even membered spacers and their associated confor-mational distributions [5]. In essence, dimers having anodd-membered spacer show a bent average shape whilethose with an even-membered spacer are linear [5].
Over the last 30 years, liquid crystal dimers haveproved to be a rich source of new smectic phasesincluding the intercalated phases [6–8], but recentlyit has been the nematic behaviour of dimers that hasaroused most interest. Specifically, dimers containingan odd-membered spacer linked to the mesogenic unitsvia methylene groups have been reported to exhibit a
new nematic phase [9, 10]. Unusual nematic phaseshave also been observed for other bent molecules, mostnotably the bis-(phenyl)oxadiazole derivatives [11], butthe discussion here is restricted to a new nematic phaseobserved for a small number of odd-membered liquidcrystal dimers.
Cestari et al. [10] recently undertook an extensiveinvestigation of the phase behaviour and properties of1,7-bis(4-cyanobiphenyl-4′-yl)heptane (CB7CB):
CB7CB exhibits a weak first-order transition froma normal nematic phase, N, to a second nematic phase,NX, at 103◦C. The optical textures seen for the NX
phase included a broken focal conic fan texture typi-cal of tilted smectic phases, a polygonal texture and arope-like texture indicative of chiral domains. In addi-tion, deuterium NMR spectroscopy also revealed theNX phase to be chiral, either globally or locally. Theauthors interpreted their observations in terms of anew nematic phase having a twist-bend director dis-tribution. Such a phase was previously predicted byDozov [12], who showed that a negative value of thebend elastic constant could result in two different
spontaneously modulated nematic phases [12]. Thefirst of these has a splay-bend modulation but this isnot chiral. The second contains a twist-bend direc-tor distribution in which the director is tilted withrespect to z, the average director, at a constant angleθ0 and rotates about z. This enables the local bendto always adopt its spontaneous value. The twist-bendnematic phase is sketched in Figure 1. In an achiralsystem, such as CB7CB, the left-handed and right-handed twists are degenerate and so equal domains ofboth are expected and globally the phase is achiral.
Cestari et al. [13] developed a methodology forthe calculation of the elastic properties of liquid crys-tal dimers and showed that, for bent odd-membereddimers, unusually low bend elastic constants are pre-dicted which decrease with increasing order. In fact,CB7CB was calculated to have a negative bend elas-tic constant over the entire measured range of orderparameters such that the twist-bend nematic phaseshould appear directly from the isotropic phase [10].This is not the case and the authors suggest that eitherthe theory may overestimate the contribution of thebent molecular structure to the bend elastic constantor that there is another elastic contribution whichcompetes with the bend deformation preventing theformation of a deformed director [10].
Panov et al. [9] investigated the behaviour ofanother member from the same homologous series asCB7CB, namely, CB11CB:
CB11CB exhibits a transition from a normalnematic phase to a second nematic phase at 109◦C.They also reported the transitional properties of twoother odd-membered dimers, both of which exhibiteda nematic–nematic transition.
M2 n = 3; M3 n = 5
This was observed at 137◦C for the ethyl homo-logue and 123◦C for the pentyl homologue. The lowertemperature nematic phases exhibited by each of thesedimers show optical textures consisting of periodicstripe domains in planar aligned cells, similar to thosereported for CB7CB [10]. Panov et al. [9] proposedthat the new nematic phase and the observed striped
z
y
x
b(z)
b(z)
n(z)
θ0
Figure 1. Sketch of the twist-bend nematic phase.Reproduced with permission from reference [12].
textures require at least the splay or twist elastic con-stant to be negative. This suggestion is contrary to therecent calculations that show that, for odd-membereddimers, the splay and twist elastic constants are actu-ally positive and increase with increasing order [13].
Šepelj et al. [14] also reported a nematic–nematictransition for an odd-membered liquid crystal dimer,7-4. This weak first order transition occurred at96.6◦C. The lower temperature nematic phase againexhibited similar textures to those later reported byCestari et al. [10] and included fan-like and rope-liketextures.
The common structural feature in each of thedimers reported to exhibit this new nematic phaseare the methylene links between the spacer and therigid mesogenic groups. If we compare the transitionalproperties of liquid crystal dimers in which the spaceris changed from being ether-linked, i.e. O(CH2)nO, tomethylene-linked, i.e. (CH2)n+2, then we see a decreasein the nematic–isotropic transition temperatures; thisreduction is most pronounced for odd-memberedspacers. In contrast, the entropy changes associatedwith the nematic–isotropic transition increases for the
even-membered dimers but decreases for the odd-membered dimers. These observations are completelyin accord with the predictions of a theoretical modeldescribed by Luckhurst and co-workers in which theonly difference between the dimers is their shape[15, 16]. Specifically, the bond angle between the
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para-axis of the mesogenic unit and the first bondin the spacer is 113.5◦ for methylene-linked dimersand 126.4◦ for ether-linked dimers. Thus the all-transconformation of an odd-membered methylene linkeddimer is more bent than that of the correspondingether-linked dimer.
Initial interest in methylene-linked dimers wasstimulated by the prediction made by Ferrarini et al.[17, 18] that systems containing high concentra-tions of bent conformers in the isotropic phase (i.e.dimers containing short odd-membered methylene-linked spacers) should exhibit a nematic-nematic tran-sition. This intriguing possibility stimulated us tosynthesise a range of methylene-linked liquid crys-tal dimers and amongst these identified three odd-members as showing a monotropic phase, whichwas tentatively assigned as being smectic in nature[19, 20]. In the light of the reports of the twist-bend nematic phase, we have revisited the behaviourof these dimers and focussed our attention on:
CN-5-CN
and
n = 1: 1O-5-O1
n = 2 : 2O-5-O2
The properties of these dimers are compared totheir even-membered counterparts containing a hex-amethylene spacer. We also report the phase behaviourof selected binary mixtures.
2. Experimental
The syntheses of all the dimers described here arereported in detail elsewhere [19, 20]. Binary mixtures
of dimers were prepared by co-dissolving pre-weighedamounts in chloroform and allowing the solventto evaporate slowly at room temperature. Phasecharacterisation was performed by polarised lightmicroscopy using an Olympus BH2 polarising lightmicroscope equipped with a Linkam TMS 92 hotstage.
3. Results and discussion
Figure 2 shows the differential scanning calorime-try (DSC) traces obtained for CN-5-CN on initialheating, cooling and second heating. The two heat-ing traces contain a strong endotherm with a peakmaximum of 147◦C corresponding to the crystal–isotropic transition. On cooling, two weak endothermsare observed at 125◦C and 113◦C, respectively, prior tocrystallisation. On cooling under the polarising lightmicroscope, a nematic schlieren texture develops at
125◦C (see Figure 3). Thus, this transition is assignedas a nematic–isotropic transition and the associatedentropy change, �SNI/R = 0.13, is consistent with thisassignment. On further cooling, a poorly defined focalconic fan texture develops in coexistence with regionsof schlieren-like texture (see Figure 4). On shearing thesample, a schlieren texture is formed (see Figure 5).
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25
Hea
t flo
w10
mW
50 75 100 125Temperature (°C)
150 175
(a)
(b)
(c)
exo^
200
Figure 2. DSC traces obtained for CN-5-CN: (a) initialheating; (b) cooling; and (c) second heating.
Figure 3. Schlieren texture of the higher temperaturenematic phase exhibited by CN-5-CN (T = 123◦C).
In a cell with planar surface alignment, a well-definedelliptical polygonal domain texture developed (seeFigure 6). In other regions of the cell, a rope-like tex-ture formed (see Figure 7), which changed to focalconic domains on further cooling. The lower tempera-ture mesophase is considerably more viscous than thenormal nematic phase and director fluctuations arenot observed. These observations are similar to thosereported by Cestari et al. [10].
The observation of focal conics implies that thereis an optical periodicity in the phase, while the rope-like textures strongly suggests the presence of chiraldomains. The entropy change associated with the tran-sition from the normal nematic phase to the lower tem-perature mesophase (�S/R) is 0.16, which is similarin magnitude to that seen for CB7CB (0.31) [10] andCB11CB (0.04) [9] for the reported nematic–nematictransition.
Figure 4. Fan-like texture of the lower temperaturemesophase exhibited by CN-5-CN (T = 111◦C).
Figure 5. Texture obtained on shearing the sample shownin Figure 4 (T = 111◦C).
Figure 6. Elliptical polygonal domain texture formed in acell with planar surface alignment by the lower temperaturemesophase of CN-5-CN (T = 104◦C).
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Figure 7. Rope-like texture formed in a cell with planarsurface alignment by the lower temperature mesophase ofCN-5-CN (T = 108◦C).
The strong similarity in the optical textures exhib-ited by the lower temperature mesophases shownby CN-5-CN and CB7CB, as well as the sharedhighly bent molecular structure, leads us to assignthe lower temperature phase as a nematic phase,NX, containing a twist-bend director distribution(see Figure 1). Unfortunately, the monotropic natureof this phase precluded its study by X-ray diffrac-tion. Microscopic observations of the lower tem-perature mesophases exhibited by 1O-5-O1 and2O-5-O2 revealed identical textures to those shownfor CN-5-CN and hence these are also assigned asNX phases. Their transitional properties are listedin Table 1. The corresponding hexamethylene-linkeddimers exhibit only a conventional nematic phase[19, 20].
To further investigate the phase behaviour ofthese dimers, we constructed binary phase diagramsfor selected mixtures of CN-5-CN, 1O-5-O1 and theclosely related compounds:
X = CN, Y = O: m = 4 CN-O4O-CN, m = 3 CN-O3O-CNX = CN, Y = CH2, m = 4 CN-6-CN
X = OCH3, Y = O: m = 4 1O-O4O-O1, m = 3 1O-O3O-O1X = OCH3, Y = CH2, m = 4 1O-6-O1.
The phase diagrams were constructed by determin-ing the transitional properties of mixtures and not bythe contact method.
We start the discussion by considering thebehaviour of even-membered dimers. Thus, Figure 8(a)
shows the phase diagram for binary mixtures of 1O-6-O1 and CN-6-CN. The mixtures exhibited onlynematic behaviour and the nematic–isotropic phaseboundary shows a linear dependence on composition.Essentially identical behaviour is seen in Figure 8(b)for the phase diagram of the corresponding ether-linked dimers, 1O-O4O-O1 and CN-O4O-CN. Thebehaviour seen in Figures 8(a) and 8(b) is thoughtof as being ideal and may be understood withinthe framework of a molecular field theory developedto predict phase diagrams for binary mixtures ofnematics [21]. The theory requires three intermolec-ular energy parameters to be defined: εAA, εBB andthe mixed parameter εAB. The parameters describ-ing interactions between the like species, εAA andεBB, are proportional to the nematic–isotropic transi-tion temperatures of the individual components. If theinteraction parameter between the unlike species, εAB,is assumed to be the geometric mean of εAA and εBB,then the nematic-isotropic transition temperature ofthe mixture is simply the weighted average of thoseof the components i.e. a linear dependence of thenematic-isotropic temperature on composition is pre-dicted. As we have seen, such behaviour is observedin Figures 8(a) and 8(b) indicating that there are nospecific interactions between the two unlike mesogenicgroups. This is a slightly surprising result given thatthe phase diagrams of similar Schiff’s base dimerswith cyanobiphenyl-based dimers showed nematic–isotropic boundaries exhibiting upward curvaturessuggesting a positive deviation in εAB away from thegeometric mean approximation [7]. Such diagramshave been interpreted in terms of a specific favourableinteraction between the unlike mesogenic units and,although the precise nature of this interaction has yetto be determined, it has been suggested to be an elec-trostatic quadrupolar interaction between groups withquadrupole moments that are opposite in sign [22].Presumably the introduction of the Schiff’s base link-ages into the cyanobiphenyl unit and the removal ofthe ether linkages to give the CN-O4O-CN and CN-6-CN dimers, respectively, have changed the electrondistribution in the mesogenic units in such a way so asto reduce the strength of this specific interaction. Thiswould also account for the absence of induced smecticbehaviour, which is often seen in mixtures containingstructurally similar electron rich and electron deficientcomponents [23, 24].
The phase diagram for a pair of odd-memberedether linked dimers, CN-O3O-CN and 1O-O3O-O1, isshown in Figure 8(c) and again only nematic behaviouris observed. The nematic-isotropic phase boundaryshows a very small but consistent downward cur-vature suggesting a negative deviation in εAB awayfrom the geometric mean approximation [21]. Again
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(°C
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Figure 8. Binary phase diagrams for mixtures of: (a) CN-6-CN and 1O-6-O1; (b) CN-O4O-CN and 1O-O4O-O1; (c) CN-O3O-CN and 1O-O3O-O1; and (d) CN-5-CN and 1O-5-O1.
this is a slightly surprising result as a positive devia-tion has been seen for mixtures of structurally simi-lar odd-membered dimers [7]. Figure 8(d) shows thephase diagram for the corresponding methylene-linkeddimers, CN-5-CN and 1O-5-O1; both the nematicphase and the twist-bend nematic phase are seen overthe whole composition range. The nematic–isotropicphase boundary again shows a very small downwardcurvature while the N–NX phase boundary varies lin-early with composition. The downward curvaturesof the nematic–isotropic phase boundaries seen inFigures 8(c) and 8(d) are very small and it would beunwise to speculate on the molecular significance ofsuch a small effect. The miscibility of the NX phaseacross the whole composition range confirms that thedimers do indeed exhibit the same phase.
We have seen that mixing dimers having thesame linkages between the spacers and meso-genic units and the same spacer length resultsin what may be described as essentially idealbehaviour, although a very small downward curva-ture was seen for the nematic–isotropic phase bound-aries for the mixtures of odd-membered dimers.
Figure 9 shows the phase diagram for mixtures ofCN-5-CN and 1O-O3O-O1 and now quite differ-ent behaviour is observed. At low concentrationsof CN-5-CN, the nematic–isotropic phase boundaryexhibits essentially a linear dependence on composi-tion. On increasing CN-5-CN concentration, however,
Cr
I
(Nx)
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(°C
)
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(N)
0 0.25
Mole fraction CN-5-CN
0.750.5 1
Figure 9. Binary phase diagram from mixtures of CN-5-CNand 1O-O3O-O1.
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Table 1. Transitional properties of liquid crystal dimers suggested to exhibit the twist-bend nematic phase.
aB6N transition at 84.4◦C. bNot reported. cCrystallisation precluded measurement.dOverlapping peaks.
the nematic–isotropic phase boundary exhibits amarked negative curvature and reaches a minimumvalue for the mixture containing 0.75 mole fractionCN-5-CN. On further increasing the CN-5-CN con-centration, the nematic–isotropic transition tempera-ture increases and reaches a limiting value at 0.89 molefraction CN-5-CN.
The origin of this strong negative deviation fromideal behaviour is unclear but is presumably linkedto the difference in the molecular geometry arisingfrom the change in linking group between the spacerand mesogenic units. Ferrarini et al. [18] have shownthat the transitional properties of dimers are particu-larly sensitive to changes in the bond angle betweenthe two mesogenic groups. It is not clear, however,why this should have such a striking effect on thenematic–isotropic phase boundary shown in Figure(9). The N–NX phase boundary appears to mirrorthe nematic–isotropic phase boundary in that littlechange in the transition temperature is seen on ini-tially decreasing the mole fraction CN-5-CN to 0.89.The subsequent decrease to 0.75 mole fraction CN-5-CN must be accompanied by a rapid decrease in TNNX
such that the NX phase is no longer observed, althoughwe note that we are dealing with strongly monotropicmixtures. As we have seen already, the formation ofthe twist-bend nematic phase has been attributed toa negative bend elastic constant and presumably theaddition of 1O-O3O-O1 serves to increase this in apositive sense and so destabilises the phase.
In this paper we have identified three new exam-ples of odd-membered methylene-linked dimers whichexhibit a nematic–nematic transition. Table 1 sum-marises the transitional data for the eight dimers wesuggest exhibit the same NX phase, based primarily onthe similarities in their optical textures and molecu-lar structures. The X-ray diffraction patterns obtainedfor both nematic phases shown by five of these liq-uid crystal dimers reveal another structural similarity
[9, 10, 14]. Specifically, the diffuse scattering in thesmall angle region seen for both nematic phases ofeach dimer correspond to a repeat distance of approx-imately half the molecular length, implying an interca-lated arrangement of the molecules. As noted earlier,the monotropic nature of the nematic phases reportedhere preclude their study by X-ray diffraction. It wouldbe tempting to directly link this observation to the for-mation of the NX phase but it is also the case thatether-linked dimers exhibit an intercalated arrange-ment of the molecules in the nematic phase [25].
Table 1 also lists the scaled transition temperature,TNX N/TNI , for each dimer. Six of the dimers showscaled transition temperatures of 0.95 or higher. Thesehigh values are consistent with the view that the bendelastic constants for these odd-membered methylenelinked dimers are negative for a wide range of orderparameters as calculations have indicated for CB7CB[10]. The two terphenyl-based dimers, M2 and M3,show somewhat lower scaled transition temperaturesimplying that for these dimers the bend elastic con-stants are positive at low order and show a weakdecreasing tendency with increasing order. Recent cal-culations have shown that the bend elastic constant isextremely sensitive to changes in molecular structure[13] and much work is now required to understandthese structure–property relationships. It would beunwise, therefore, to speculate further on the originsof the differences in the scaled transition temperatureslisted in Table 1.
4. Conclusions
The study reported here was stimulated by a predic-tion made by Cestari et al. [10] that the twist-bendnematic phase should be exhibited by dimers havingstructures related to that of CB7CB and which have asufficiently bent shape to give a negative bend elastic
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constant, suggesting that the requisite bent shape maybe achieved by using an odd-membered methylene-linked spacer. This prediction appears to have beenborne out by our study and has led to the identificationof three further examples of dimers which exhibitthe twist-bend nematic phase. We suggest that thisbrings the total to eight dimers, all containing an odd-methylene linked spacer, which are now thought toexhibit this new nematic phase.
Acknowledgements
We are especially grateful to Professor G.R. Luckhurst(University of Southampton) for discussions concerning thetwist-bend nematic phase.
