Polar Order and Symmetry Breaking at the Boundary between Bent-Core and Rodlike Molecular Forms: When 4-Cyanoresorcinol Meets the Carbosilane End Group

DOI: 10.1002/chem.201503901

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&

Liquid Crystals

Polar Order and Symmetry Breaking at the Boundary between

Bent-Core and Rodlike Molecular Forms: When 4-Cyanoresorcinol

Meets the Carbosilane End Group

Eduard Westphal,^{[a, b]}Hugo Gallardo,^[b]Giovanni Finoto Caramori,^[b]Nerea Sebastiµn,^[c]

Maria-Gabriela Tamba,^[c]Alexey Eremin,*^[c]Susumu Kawauchi,^[d]Marko Prehm,^[a]and

Carsten T s chierske*^[a]

Dedicated to Prof. Heinrich Lang on the occasion of his 60th birthday

Abstract: Two isomeric achiral bent-core liquid crystals in-

volving a 4-cyanoresorcinol core and containing a carbosilane

unit as nanosegregating segment were synthesized and

were shown to form ferroelectric liquid-crystalline phases. In-

version of the direction of one of the COO groups in these

molecules leads to a distinct distribution of the electrostatic

potential along the surface of the molecule and to a strong

change of the molecular dipole moments. Thus, a distinct

degree of segregation of the carbosilane units and conse-

quent modification of the phase structure and coherence

length of polar order result. For the compound with larger

dipole moment (CN1) segregation of the carbosilane units is

suppressed, and this compound forms paraelectric SmA and

SmC phases; polar order is only achieved after transition to

a new LC phase, namely, the ferroelectric leaning phase

(SmCL_sP_S) with the unique feature that tilt direction and

polar direction coincide. The isomeric compound CN2 with

a smaller dipole moment forms separate layers of the carbo-

silane groups and shows a randomized polar SmA phase

(SmAP_AR) and ferroelectric polydomain SmC_sP_Sphases with

orthogonal combination of tilt and polar direction and much

higher polarizations. Thus, surprisingly, the compound with

the smaller molecular dipole moment shows increased polar

order in the LC phases. Besides ferroelectricity, mirror-sym-

metry breaking with formation of a conglomerate of macro-

scopic chiral domains was observed in one of the SmC

phases of CN1. These investigations contribute to the gener-

al understanding of the development of polar order and

chirality in soft matter.

Introduction

long-range polarization of magnetic and electric dipoles, re-

spectively, are of significant importance for the further devel-

opment of technology, information storage, and information

processing. In contrast to solid-state inorganic magnetic and

ferroelectric materials, organic counterparts can be soft or

even fluid, which provides them with the additional advantage

of being easily processible, self-healing, and responsive to dif-

ferent types of stimuli.^[1–8]Especially liquid-crystalline (LC) or-

ganic ferroelectrics combining molecular mobility with long-

range order attract significant practical interest for microdis-

plays, energy conversion, and information storage.^[4–6]However,

the softness of LC ferroelectrics requires sophisticated molecu-

lar design to avoid randomization of polar order in the fluid

state, which still remains a challenging task. It is just as impor-

tant to know how structural modifications affect the molecular

properties, which allows the development of more efficient

materials. One important class of soft ferroelectrics is based on

bent-core mesogens or banana-shaped LCs involving nonlinear

units such as 1,3-disubstituted aromatics or other shape-persis-

tent bent building blocks.^[9–11]A small change of the molecular

shape from linear, as in classical rodlike (calamitic) LCs,^[4]to

bent, in these bent-core mesogens, hampers molecular rota-

tion around the mesogenic long axis, and provides polar order

Multifunctional materials responding to magnetic or electrical

fields, especially ferromagnetic and ferroelectric materials with

[a] Dr. E. Westphal, Dr. M. Prehm, Prof. Dr. C. Tschierske

Institute of Chemistry, Organic Chemistry

Martin Luther University Halle-Wittenberg

Kurt-Mothes-Strasse 2, 06120 Halle/Saale (Germany)

E-mail: carsten.tschierske@chemie.uni-halle.de

[b] Dr. E. Westphal, Prof. Dr. H. Gallardo, Prof. Dr. G. F. Caramori

Department of Chemistry

Federal University of Santa Catarina

88040-900, Florianópolis (Brazil)

[c] Dr. N. Sebastiµn, Dr. M.-G. Tamba, Dr. A. Eremin

Department of Nonlinear Phenomena, Institute for Experimental Physics

Otto von Guericke University Magdeburg

Universitätsplatz 2, 39106 Magdeburg (Germany)

E-mail: alexey.eremin@ovgu.de

[d] Prof. Dr. S. Kawauchi

Department of Organic & Polymeric Materials

Tokyo Institute of Technology

O-okayama, Meguro-ku, Tokyo 152-8552 (Japan)

Supporting information and ORCID number from the author for this article

are available on the WWW under htt p ://dx.doi.org/10.1002/

chem.201503901.

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P along the molecular-bending direction and parallel to the

layers (along the a axis in Figure 1a). This allows denser pack-

ing of the bent cores and, in most cases, induces a tilted or-

ganization in layers (SmC phases). The resulting tilt b is per-

pendicular to the polar direction P, that is, P is along the a axis

and b is along the b axis in Figure 1a, whereas c is the layer

ent color, respectively. This layer chirality can couple through

diastereomeric relations with helical molecular conformations

and helical superstructures, leading to spontaneous mirror

symmetry breaking with formation of macroscopic chirali-

ty.^[13–22]

In recent years, the investigation of molecules having inter-

mediate structures between those of the typical bent-core

molecules and linear rodlike molecules has received significant

attention, as this provides new phase structures and phenom-

ena.^[23–25]One class of compounds with reduced molecular

bend are acylated 4-cyanoresorcinols,^[26,27]for which numerous

unusual LC phases were reported.^[28–35]The polar cyano group

is known to affect the conformation of the adjacent carboxyl

group by dipole coupling with the C=O group, which favors

helical molecular conformations^[36]and reduces the molecular

bend by increasing the angle a (see Figure 2) from 120 to

130–1408.^[27]In addition, the lateral molecular dipole of the CN

group is known to favor an antiparallel alignment of adjacent

molecules.^[37,38]This competes with the parallel alignment of

the bend directions, which is favored by the packing of the

bent cores with minimized excluded volume and provides

polar order.^[9,10]

Figure 1. Schematic presentation of the distinct structures resulting from

the combination of tilt and polar order in the SmCP phases (a, b) and in the

SmCLP phases (c, d) of bent-core molecules; a), c) show the packing in

a single layer and b), d) the distinct modes of correlation between adjacent

layers; the direction of the resulting polar components P parallel and per-

pendicular to the layers are indicated by arrows; SmC_sP_Sand SmC_aP_Aare ho-

mogeneously chiral structures, whereas SmC_sP_Aand SmC_aP_Sare racemic; see

Figure S22 of the Supporting Information for an explanation of the origin of

layer chirality; for a color version showing chirality more clearly, see Fig-

ure S23 in the Supporting Information, in which blue and red indicate oppo-

site chiralities of the superstructural layer chirality and gray indicates achiral

structures; note that some combinations of leaning direction and polar di-

rection lead to bilayer phases (L₂; b: molecular tilt).

Figure 2. Structure of the compounds under investigation; red arrows indi-

cate the direction of the COO groups; in bent-core mesogens the angle a is

significantly smaller than 1808.

It was previously shown that siloxane^[13,39]and carbosilane

units^[40,41]attached to one end of bent-core mesogens could

result in ferroelectric switching in otherwise antiferroelectric LC

phases of bent-core mesogens due to layer decoupling by

nanosegregated layers involving the silyl groups.^[42]The carbo-

silane unit, which provides improved chemical stability com-

pared to related siloxanes, is especially advantageous for this

purpose.^[40,41,43]These silyl groups also allow the design of new

phase structures with reduced symmetry. Reduction of the

phase symmetry is an important way to increase structural

complexity of soft-matter self-assembly.^[44]Thus, double-tilted

SmC_Gphases^[45–47]with triclinic C₁symmetry, in which the po-

larization is neither in the tilt plane nor perpendicular to it,

were recently reported for silylated, hydrogen-bonded, bent-

core aggregates.^[48]

normal. This orthogonal combination of b and P allows four

possible combinations of tilt direction and polar direction. In

adjacent layers the tilt can be synclinic (uniform, C_s) or anticlin-

ic (opposite, C_a), and polar order can be synpolar (ferroelectric,

P_S) or antipolar (antiferroelectric, P_A), providing the four phase

structures designated SmC_sP_S, SmC_sP_A, SmC_aP_A, and SmC_aP_S

(Figure 1b; see also color Figure S23b in the Supporting Infor-

mation).^[12]In addition, the orthogonal combination of polar

order, tilt, and layer normal provides an inherently chiral struc-

ture with C₂symmetry, which leads to superstructural chirality

of the layers (layer chirality, see Figure S22 of the Supporting

Information), inverted by tilt inversion or inversion of the polar

direction.^[12]Therefore, some of the above mentioned phase

structures are homogeneously chiral (SmC_sP_S, SmC_aP_A), whereas

others are racemic (SmC_sP_Aand SmC_aP_F), as identified in Fig-

ure S23b of the Supporting Information by uniform and differ-

Herein, we report two new bent-shaped LC compounds CN1

and CN2 combining a 4-cyanoresorcinol-based bent core with

a carbosilane end group (Figure 2). The only difference be-

tween CN1 and CN2 is the inversion of the direction of one

ester linkage unit (X)^[49,50]on the carbosilane-terminated side of

the bent aromatic core (see Figure 2). Investigation of CN1

provided evidence for the formation of a new LC phase desig-

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nated as the “leaning” phase (SmCLP).^[51,52]In this LC phase

with C_ssymmetry the direction of tilt, in this case designated

as leaning L, and direction of polar order P coincide (both are

along direction a in Figure 1b).^[53]For this reason these LC

phases are achiral and can have polar order along the layers

and perpendicular to the layers, depending on the kind of

layer correlation (see Figure 1c,d and Figure S23c,d of the Sup-

porting Information). Though a leaning-phase structure was

predicted theoretically,^[53,54]and recently a leaning organization

was proposed for a modulated polar LC phase^[55]constituting

the first example of a nonmodulated leaning phase. In con-

trast, the isomeric compound CN2 forms a proper ferroelectric

SmC phase in which the directions of tilt and polar order are

perpendicular to each other (SmCP, Figure 1a,b). Moreover,

mirror-symmetry breaking was observed, but only for one of

the isomeric compounds (CN1) in a limit temperature range.

Besides experimental investigations, theoretical calculations

were also employed to elucidate how the small molecular

modification by reversing only one COO group affected the

molecular self-assembly and materials characteristics so much.

route by hydrosilylation of methyl 4-(10-undecenyloxy)ben-

zoate (1) and 1-benzyloxy-4-(10-undecenyloxy)benzene (5)

with the thus-obtained Si3 in the presence of the Karstedt cat-

alyst.^[41]The olefinic precursors 1 and 5 were obtained by alky-

lation of methyl-4-hydroxybenzoate and 4-benzyloxyphenol

with 11-bromoundecene, respectively. Deprotection by saponi-

fication or Pd-catalyzed hydrogenolysis led to benzoic acid 2

and phenol 6, respectively. Esterification of 2 with 4-hydroxy-

benzaldehyde and 6 with 4-formylbenzoic acid yielded the

substituted benzaldehydes 3 and 7 (not shown in Scheme 1,

see Schemes S1 and S2 of the Supporting Information), which

after oxidation of the formyl groups with KMnO₄^[56]gave carbo-

silane-functionalized benzoic acids 4 and 8. In parallel, phenol

12, which provides the molecular bend, was prepared by

esterification of 4-formylbenzoic acid with 4-hexyloxyphenol

(9)^[57]followed by oxidation of the formyl group of the result-

ing benzaldehyde 10 (see Scheme S3 of the Supporting Infor-

mation) with KMnO₄. The obtained benzoic acid 11 was con-

verted to the respective benzoyl chloride by treatment with

thionyl chloride and further esterified with 4-benzyloxy-2-hy-

droxybenzonitrile;^[28a]hydrogenolysis then yielded the phenolic

intermediate 12. The final step was an esterification between

the acid chlorides derived from silyl-substituted benzoic acids

4 and 8 and phenol 12, resulting in the target compounds

CN1 and CN2, respectively. Detailed experimental procedures

and characterization are fully described in the Supporting In-

formation.

Results and Discussion

Synthesis

Compounds CN1 and CN2 were synthesized as briefly illustrat-

ed in Scheme 1. First, 2,2,4,4,6-pentamethyl-2,4,6-trisilaheptane

(Si3) was prepared by following a published procedure in

which freshly prepared trimethylsilylmethylmagnesium chlo-

ride reacts with chloro(chloromethyl)dimethylsilane, followed

by conversion to the respective Grignard reagent and final re-

action with chlorodimethylsilane (see Scheme S1 of the Sup-

porting Information).^[41]In our synthetic strategy, the carbosi-

lane unit was introduced in an early step of the synthetic

Liquid-crystalline self-assembly

The molecular self-assembly in LC phases was investigated by

polarizing microscopy, differential scanning calorimetry (DSC),

XRD, electro-optical and dielectric methods, and second har-

monic generation measurements (SHG), as described in the Ex-

perimental Section. Both isomeric compounds CN1 and CN2

show a series of different LC phases (Table 1). The bulky carbo-

silane unit attached to only one end of each mesogen reduces

the LC–Iso transition temperature only marginally, approxi-

mately 10 K, compared with related nonsilylated compounds,

and modifies the observed LC phase types.^{[26,28,30,33]}For exam-

ple, a compound related to CN1, but having an octadecyloxy

chain instead of the carbosilane chain, has a phase sequence

Cr 116 8C SmAP_R1618C Iso^[58](SmAP_R=nontilted smectic phase

with polar domains having randomized polar direction^[59]) and

a compound related to CN2, but having two tetradecyloxy

chains (i.e., approximately the same total lipophilic chain

volume) has a sequence Cr 1048C SmC_sP_A1368C SmCP_R1468C

SmA 1928C Iso^[34,60](SmCP_R=tilted smectic phase with polar

domains having randomized polar direction^[16]). For both CN1

and CN2 a uniaxial SmA phase is found as a high-temperature

phase, which changes to a biaxial, synclinic, tilted SmC_sphase

at similar transition temperatures of 139 and 1378C for CN1

and CN2, respectively, with relatively small transition enthal-

pies (0.2–0.3 and 1.0–1.1 kJmol^À1, respectively; see Figure 3).

Both compounds show additional continuous transitions that

are not associated with measurable transition enthalpies

(dotted lines in Figure 3). The two compounds are described

Scheme 1. Synthetic route to CN1 and CN2. i) 11-Bromo-1-undecene, K₂CO₃,

18-crown-6, butanone; ii) 2,2,4,4,6-Pentamethyl-2,4,6-trisilaheptane (Si3), Kar-

stedt catalyst (ꢀ2% Pt solution in xylene), dry toluene; iii) KOH, THF, MeOH,

H₂O; iv) HCl; v) 4-Hydroxybenzaldehyde, N,N’-dicyclohexylcarbodiimide, 4-di-

methylaminopyridine, dry CH₂Cl₂; vi) KMnO₄, acetone; vii) SOCl₂, dry DMF, dry

CH₂Cl₂; viii) Dry triethylamine (TEA), dry CH₂Cl₂; ix) H₂, Pd/C (10%), THF; x) 4-

Formylbenzoic acid, DCC, DMAP, dry CH₂Cl₂; xi) 4-Benzyloxy-2-hydroxybenzo-

nitrile, dry TEA, dry CH₂Cl₂.

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Table 1. Mesophases, transition temperatures and associated enthalpies of CN1 and CN2.^[a]

Compound

T/8C [DH/kJmol^À1

H!

]

CN1

(X=OOC)

Cr 119 [33.0] SmC_s139 [0.3] SmA 150 [6.0] Iso

Cr 101 [33.4] SmCL_sP_S115 [–] SmC_s139 [0.2] SmA 150 [6.4] Iso

SmCL_sP_S127 [–] SmC_s139 [0.3] SmA 150 [6.0] Iso

!

C

[b]

H!

CN2

(X=COO)

H!

Cr 88 [20.4] SmC_sP_S129 [–] SmC_sP_S^SR137 [1.0] SmAP_ARꢀ146 [–] SmA 183 [8.5] Iso

!

SR

C

Cr <25 SmC_sP_S129 [–] SmC_sP_S136^[c][1.1] SmAP_ARꢀ146 [–] SmA 183 [9.4] Iso

[a] Transitions determined by DSC (peak temperatures) during the first heating (H)/cooling cycle (C) at a heating/cooling rate of 10 Kmin^À1; transitions

without enthalpy changes were determined by polarizing microscopy; abbreviations: Cr=crystalline phase; SmA=uniaxial smectic phase; SmAP_AR=anti-

ferroelectric switching SmA phase with randomized polar direction; SmC_s=synclinic tilted smectic phase; SmC_sP_S=polar switching polydomain SmC_s

phase; SmC_sP_S^SR=SmC_sphase with polar polydomain structure having shorter coherence length of polar order than SmC_sP_S; SmCL_sP_S=synclinic polar lean-

ing phase (see Figure 1d); Iso=isotropic liquid (see Figure 18 for models of the phase structures). [b] Observed by heating from the SmCL_sP_Sphase with-

out previous crystallization. [c] This value corresponds to the rounded value of the DSC peak temperature on cooling at 10 Kmin^À1; since for most experi-

ments much smaller cooling rates were used, close to equilibrium conditions, and the practically observed transition temperature corresponds in most

cases to that observed on heating (1378C).

Compound CN1

Optical observations and DSC

First, phase characterization was carried out by optical micros-

copy between crossed polarizers. On cooling a sample of CN1

sandwiched between untreated glass plates from the isotropic

liquid state, initial formation of bâtonnets is observed, and the

fan texture of the SmA phase easily aligns homeotropically

and then appears completely dark between crossed polarizers

(Figure 4d, left). Around 1398C, the homeotropic areas form

a low-birefringence schlieren texture (Figure 4d, right), indicat-

ing a transition from the uniaxial SmA phase to a biaxial smec-

tic phase, in line with the onset of the tilt in the SmC phase.

This SmA–SmC transition is associated with a small peak in the

DSC traces (DH=0.2 kJmol^À1, Figure 3a and Table 1). A further

change of the schlieren texture, associated with a decrease in

the birefringence, was observed at 115 8C (Figure 4e); this tran-

sition (SmC_S–SmCL_sP_S) does not show up in the DSC traces. On

heating, the reverse transition, associated with an increase in

birefringence, takes place at 1278C; this indicates significant

hysteresis of this transition.

Cooling CN1 in a planar-alignment cell resulted in a typical

SmA fanlike texture (Figure 4a) at T=1508C, confirming the

observations on homeotropic samples. At 1398C the extinc-

Figure 3. First DSC heating and cooling scans for a) CN1 and b) CN2

tions become inclined to the directions of polarizer and ana-

(10 Kmin^À1); for second heating and cooling scans, see Figure S12 of the

lyzer (Figure 4b), in line with the onset of a synclinic tilt (SmC_s

Supporting Information; continuous phase transitions without visible enthal-

phase) with an optical tilt angle of about 358, as estimated

py are indicated by dashed lines; in a) the continuous transition SmCL_sP_S–

from the angle of the dark extinction crosses with respect to

the direction of polarizer and analyzer (Figure 4b). The SmA–

SmC_stransition is associated with an increase in birefringence,

as indicated by the color change of the texture from blue to

green (Figure 4a, b). On further cooling, the birefringence fur-

ther increases in the SmC_sregion (Figure 4c). This indicates

a growing order parameter of the aromatic cores due to

SmC_son heating is only observed if the sample is heated from the SmCL_sP_S

phase before crystallization takes place.

separately in more detail in the following sections and com-

pared in the Conclusion section.

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at T=1208C in freely suspended (FS) films.^[51]The phase transi-

tion SmC_s–SmCL_sP_Shas a large hysteresis; the SmCL_sP_Sphase

forms below the melting point and hence is monotropic

(metastable) on cooling. Though the SmCL_sP_S–SmC_stransition

on heating is above the melting point, in practice the

SmCL_sP_S–SmC_stransition at 1278C can only be observed if crys-

tallization is avoided. Once the sample has crystallized, melting

directly leads to the SmC_sphase without formation of SmCL_sP_S.

The crystallization strongly depends on the boundary condi-

tions. In most cases, in thin glass cells CN1 immediately crystal-

lizes around the SmC_s–SmCL_sP_Stransition at about 115 8C,

whereas in thicker cells and in FS films it can be cooled to

about 958C without crystallization. The crystallization tempera-

ture and the long-term stability of the SmCL_sP_Sphase are also

influenced by the type of applied electric field (ac or dc), its

frequency, and strength. Therefore, different crystallization

temperatures were found in the distinct investigations, and

under some conditions no data could be obtained due to crys-

tallization.

A remarkable feature of the SmC_sphase is the presence of

a chiral conglomerate structure. This can be identified in ho-

meotropic samples by slightly uncrossing the direction of the

analyzer by a small angle clockwise or counterclockwise, which

leads to dark and bright domains exchanging their brightness

on changing the direction of the analyzer (see Figure 5a,c,d).^[64]

Rotating the sample between crossed polarizers does not lead

to any change of brightness in the distinct domains, which

confirms that the alignment of the tilt direction in the domains

Figure 4. Textures of the mesophases of CN1 in a 17.4 mm PI-coated ITO cell

(a–c, f; planar alignment) and between untreated glass plates (d, e; homeo-

tropic alignment) as observed on cooling. a) SmA phase at T=1458C.

b) SmC_sphase at T=1308C. c) SmC_sphase at T=1208C. d) SmA-to-SmC_s

transition at T=1398C. e) Transition from SmC_s(left) to SmCL_sP_S(right) at

T=115 8C. d), e) Snapshots of the phase transitions; there is a small tempera-

ture gradient with slightly higher T on the left and slightly lower T on the

right; the phase boundary moves quickly from right to left; the dashed line

in e) indicates the position of the phase boundary. f) SmCL_sP_Sphase at

T=110 8C; for a discussion of the texture in relation to the molecular pack-

ing, see text; width of the photographs is 750 mm; for additional textures,

see Figure S17 of the Supporting Information.

denser molecular packing. The increase of the birefringence at

the SmA–SmC_stransition is a first indication of a de Vries char-

acter of the SmA phase.^[30,61]This means that in the SmA phase

of CN1 the layers consist of tilted molecules like in the SmC

phase, but in contrast to the SmC phase, there is no long-

range orientational order of the molecular tilt within the

layers; thus, the phase is uniaxial. At the SmA–SmC_stransition,

the 2D isotropic distribution of the tilt director becomes long-

range orientationally ordered.^[30,61]

A comparatively small

change of the layer thickness at this transition hardly reflects

a change of the tilt angle, in line with XRD results described

below. For bent-core molecules having an intrinsic tendency

for cluster formation,^[28,62]the de Vries phases are considered

to be of the “cluster diffusion cone” type,^[63]that is, they are

composed of tilt-randomized SmC_sdomains adopting uniform

director orientation at the SmA–SmC transition.

At 115 8C the texture changes again forming a fan texture

with parallel stripes across the fans (broken fan texture, see

Figure 4 f and Figure S17f of the Supporting Information). Si-

multaneously, the extinction crosses apparently flip back to

the positions coinciding with the directions of the polarizers. It

is proposed that the transition observed at T=115 8C on cool-

ing and at T=1278C on heating corresponds to the transition

between the SmC_sphase at higher temperature and the lean-

ing phase (SmCL_sP_S) at lower temperature, which was observed

Figure 5. Chiral domains observed in the SmC_sphase of CN1 at T=1338C

(homeotropic alignment between untreated glass plates. a)–c)–d) As ob-

served by rotating the analyzer and b)–c)–e) as observed by rotation of the

sample between crossed polarizers; in the series a)–c)–d) inversion of the

brightness of the domains is observed, whereas there is no change in the

series b)–c)–e); width of the photographs is 700 mm.

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cannot be the origin of this observation (see Figure 5b,c,e).^[65]

This spontaneous chirality is reproducibly observed in homeo-

tropically aligned samples between glass surfaces and is

thought to arise from a locally polar, and hence chiral,^[64]

SmC_sP_Sdomain structure, fixed by pinning of the tilt director

by polar surface alignment.^[16,18]In line with this, chiral domains

are absent in FS films. Optical rotation may result directly from

the layer chirality^[66]or from the coupling of the layer chirality

with the enantiomorphic molecular conformations of the tran-

siently chiral molecules, which leads to chirality synchroniza-

tion and twisted organization of the involved chromophores.^[20]

The presence of local polar ordering in the SmC_sphase of CN1

is confirmed by electro-optical experiments, dielectric investi-

gations, and field-induced SHG activity (see below). The bire-

fringent domain walls (bright lines in Figure 5) between the

homogeneously chiral areas start to expand significantly below

1208C and, with further decreasing temperature, an achiral

schlieren texture develops, which replaces the vanishing chiral

domains (Figure 6). This means that the chiral domains formed

in the SmC_sphase become unstable at the transition to the

leaning phase (SmCL_sP_S), in line with the absence of layer chir-

ality in the leaning phase, which has a C_ssymmetry due to the

coincidence of polar direction and tilt direction (Figure 1c).

Figure 6. Vanishing chiral domains in the SmCL_sP_Sphase of CN1 at

T=114 8C, as observed immediately after cooling from the homeotropic

SmC_sphase between untreated glass plates with the analyzer a) rotated

clockwise or b) anticlockwise; the domain walls (Figure 5) expand and form

achiral regions with schlieren texture; the small hysteresis in the formation

of the achiral state is assumed to be due to polar surface stabilization ef-

fects; the bar corresponds to 50 mm.

Figure 7. XRD patterns of CN1. a), b) 2D patterns of surface-aligned samples

after subtraction of the scattering in the isotropic liquid state. a) SmA phase

at T=1458C and b) SmC_sphase at T=1358C (the SmCL_sP_Sphase could not

be investigated, as crystallization takes place under these conditions). c),

d) Data of powder samples measured in thin capillaries; c) diffuse wide-

angle scattering in the SmCL_sP_Sphase at T=115 8C, in the SmC_sphase at

T=1358C, and in the SmA phase at T=1458C; d) temperature dependence

of the layer distance d; see Figures S13, S15 and Table S1 of the Supporting

Information for more details.

X-ray investigations

X-ray scattering provides further fundamental insight into de-

tails of the phase structure. XRD patterns of all three LC

phases of CN1 are characterized by a single sharp layer reflec-

tion, corresponding to a layer distance of d=5.03–5.23 nm

without any visible higher-order reflections in the small-angle

range (Figure 7a,b). The maximum length of the molecules in

a conformation assuming a 1208 bend of the aromatic core

and stretched terminal chains (measured with CPK models, see

Figure 8a) is L_mol=5.9 nm. This value is only a bit larger than

chains, aliphatic spacers, and carbosilane chains, in line with an

average antiparallel packing of the molecules on average and

nearly complete interdigitation of the bulky carbosilane seg-

ments with the aliphatic spacers of adjacent molecules, as

shown in Figure 8a. Figure 8a also shows that the mixed or-

ganization of the carbosilane groups between the aliphatic

spacers requires some longitudinal shift of the aromatic cores

to provide sufficient space for the n-hexyl chains. This shift re-

duces the sharpness of the interfaces between the core layers

and the layers of the terminal chains, which is in line with the

absence of higher-order layer reflections.

the layer spacing

d (d/L_mol=0.85–0.89), which indicates

a single-layer structure. There is only one symmetrical, diffuse

wide-angle scattering peak with a maximum of the scattering

at d=0.5 nm (Figure 7c) corresponding to an average lateral

distance of 0.45 nm, typical for hydrocarbon chains, and about

0.6 nm, corresponding to the mean distance between the car-

bosilane chains.^[41]This indicates mixed organization of alkyl

No significant changes of the shape and position of the dif-

fuse wide-angle scattering were observed in the whole meso-

morphic temperature range, that is, the fundamental organiza-

tion of the molecules is the same in all LC phases. In 2D XRD

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(SmCL_sP_S), where a tiny increase is observed. Thus, the overall

change of the d value at the SmA–SmC_stransition is very

small, as is typically observed for de Vries phases,^[61]and con-

firms that the tilt is already present in the SmA phase, but

randomized, and becomes uniform at the transition to the

SmC_sphase.

SHG, dielectric, and electro-optical investigations

Second harmonic generation (SHG) is one of the most power-

ful and reliable tools to identify polar order in molecular struc-

tures. It is based on the fact that a noncentrosymmetric molec-

ular organization provided by polar order leads to a nonlinear

optical effect, manifested in the generation of light with dou-

bled frequency compared to the irradiating light beam. Dielec-

tric spectroscopy and investigation of the switching under an

applied triangular-wave field support these investigations and

provide additional information.

In SHG experiments in an applied field, a detectable field-in-

duced SHG signal is already observed in the Iso phase and it

increases slightly in intensity in the temperature range of the

SmA and SmC_sphases (Figure 9a). On approaching the SmC_s–

SmCL_sP_Stransition there is a steep increase of the slope of the

SHG curve, but due to rapid crystallization at 116 8C the SHG

signal could not be recorded in the SmCL_sP_Sphase under

these conditions. In the whole LC temperature range, SHG ac-

tivity is only observed under applied field, and switching off

the field immediately removes SHG activity. Only in the crystal-

line phase is SHG activity observed without applied field, and

this indicates a polar crystalline phase (Figure S19 of the Sup-

porting Information). As shown in Figure 9b, in the Iso, SmA,

and SmC_sphases SHG intensity continuously increases with in-

creasing field strength. No saturation was observed up to

20 Vmm^À1. A small SHG signal can be explained by enhanced

polar correlations and formation of polar domains that persist

even in the isotropic phase. In the low-temperature SmCL_sP_S

phase (Figure 9c) the signal is about twice as strong and ex-

hibits a saturation behavior not found in the high-temperature

phase. This indicates enhanced polar correlation and the pres-

ence of polar order, at least locally. Although the SHG relaxes

in the field-free state in glass cells, the experiments in FS films

proved the presence of a spontaneous polarization.^[51]

Figure 8. CPK models of CN1 and CN2 and organization of the molecules in

the LC phases. a) Molecular length L_molof CN1 in the most extended confor-

mations with all-trans alkyl chains (CN2 has the same length) and its organi-

zation in the smectic phases; additional carbosilane units of two neighbor-

ing molecules are added (encircled); schematic presentations are shown

below the models. b) Segregation of the carbosilane groups with end-to-

end packing of the carbosilane groups leads to d_modbeing a bit larger than

the molecular length, shown for the organization of CN2; in this arrange-

ment additional folding of the spacers is required. c) Segregated organiza-

tion of CN2 with partial overlap of the carbosilane groups (for color version,

see Figure S24 of the Supporting Information).

patterns of aligned samples the diffuse wide-angle scatterings

become inclined by an angle of 228 with respect to the equa-

tor at the transition from the SmA to the SmC_sphase, while

the layer reflection retains its position on the meridian. This

confirms the onset of a uniform synclinic tilt of b=228 (Fig-

ure 7b and Figure S13c of the Supporting Information). This

“XRD tilt” (average tilt of aromatics and aliphatics) is smaller

than that observed optically from the position of the extinc-

tion crosses (ꢀ358, determined by the orientation of the p

system), which indicates that the major contribution to the op-

tical tilt arises from the tilting of the p-conjugated aromatic

cores.

Dielectric data obtained at 1 kHz as a function of tempera-

ture are shown in Figure 10. At the Iso–SmA transition the per-

mittivity slightly increases. On further cooling, the dielectric

constant continuously increases over the temperature range of

the SmA, SmC_S, and SmCL_sP_Sphases. Such behavior is in line

with the SHG results and polarization curves, and supports the

presence of polar domains in the SmA, SmC_S, and SmCL_sP_S

mesophases, the size of which increases significantly on cool-

ing.

The development of a tilt is in line with the temperature de-

pendence of the d values of the layer spacing, shown in Fig-

ure 7d. On cooling from the isotropic phase, the layer distance

in the SmA region increases, reaches its maximum value at the

SmA–SmC transition at T=1398C, and slightly declines until T

ꢀ1358C is reached. The increase of the layer spacing in the

SmA range is attributed to an increase of the lateral packing

density of the bent-core units at lower temperature, which

provides additional chain stretching. Below 1358C, in the tem-

perature range of the tilted smectic phases (SmC_s, SmCL_sP_S),

the layer spacing remains nearly constant with only slight

changes at the transition to the low-temperature phase

In line with the SHG and dielectric investigations a weak

single current peak (A) is already observed in the isotropic

liquid state under an applied ac triangular-wave field (Fig-

ure 11 a). In the SmA phase a second peak (B) becomes visible,

very close to peak A (Figure 11 b). Peak B grows and becomes

sharper in the temperature range of the SmA phase, whereas

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Figure 11. Switching current response curves of CN1 depending on temper-

ature. a) Iso phase at T=1558C, b) SmA phase at T=1408C, and c) SmC_s

phase at T=1258C, as measured in a 6 mm PI-coated ITO cell at 10 Hz,

50 V_ppmm^À1, 5 kW; P_svalues are listed in Table S2 of the Supporting Informa-

tion.

Figure 9. (a) SHG signal of CN1 as a function of temperature measured in

a 6 mm PI coated ITO cell under an applied field of 12.5 Vmm^À1on cooling

from Iso; under these conditions crystallization takes place at 1168C, just

before the transition to the SmCL_sP_Sphase. Field dependence of the intensi-

ty of the SHG signal of CN1 b) in the Iso phase at T=1578C, in the SmA

phase at T=1478C and in the SmC_sphase at 1308C (6 mm PI coated ITO

cell) and c) in the SmCL_sP_Sphase at T=113 8C (25 mm PI coated ITO cell), for

comparison the values in the other phases are also added.

peak A decreases. In the temperature range of the SmC_sphase

only one relatively sharp single peak B is observed (Figure 11 c),

for which a polarization value of about 50 nCcm^À2can be cal-

culated (see Table S2 of the Supporting Information). On the

basis of SHG and dielectric results the current peaks can be as-

signed to a polarization. In the leaning phase no reliable polar-

ization curves could be recorded, due to partial crystallization

under the applied triangular wave field. On field removal and

reversal in dc-field experiments, the texture does not change;

this indicates switching through collective rotation of the

mesogens around the long axis (Figure S18 of the Supporting

Information).

Discussion of LC phase structures of CN1

As is evident from XRD investigations, the bulky carbosilane

groups of compound CN1 do not segregate in their own

layers, and these bulky groups mainly provide a significant

steric effect that separates the bent aromatic cores. This re-

quires relatively strong tilting of these cores (indicated by the

position of the extinction brushes in Figure 4b) for dense pack-

Figure 10. Dielectric permittivity of CN1 as a function of temperature in

a 17.4 mm antiparallel treated ITO cells measured at 1 kHz.

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ing, and the reduced packing hampers formation of macro-

scopic polar order. Nevertheless, local polar domains can al-

ready be found in the isotropic liquid phase and persist in the

following smectic phases. These domains are responsible for

the de Vries-like character of the SmA phase and for the specif-

ic properties of the tilted smectic phases. Although there are

local polar domains, their coherence length is short in the SmA

and SmC_sphases, and therefore the complete SmC_srange is

only paraelectric and a larger coherence length of polar order

develops in the leaning phase, in which tilt direction coincides

with the polar direction.

The leaning phase has single-layer structure (Table S1 of the

Supporting Information), and thus SmCL_2sP_Aand SmCL_2aP_S

structures are excluded (Figure 1d). The leaning direction is

synclinic and there is in-plane polarization, as indicated by in-

vestigation of FS films;^[51]thus, the structure is SmCL_sP_S(Fig-

ure 1d).^[67]A polydomain structure composed of stacks or do-

mains of SmCL_sP_Slayers with alternating leaning direction be-

tween adjacent layer stacks would allow compensation of the

polar moments parallel to the layer normal, and lead to a struc-

ture with exclusively in-plane polarization. The small domain

size with a coherence length shorter than the optical wave-

length leads to textures with extinctions parallel to the polariz-

ers (see Figure 4 f and Figure S17f of the Supporting Informa-

tion). The presence of a stripe pattern across the fans might be

due to some thicker layer stacks. Dielectric investigations sup-

port increasing polar-domain size from SmA to SmC_sto

SmCL_sP_S(see Figure 10) and are in line with the proposed poly-

domain structure of the smectic phases of CN1.

Thus, the leaning phase is a ferroelectric SmCL_sP_Sphase with

macroscopic uniform polar order in FS films and an apolar

polydomain structure with the bow planes anchored parallel

to the substrate if confined between solid surfaces. In this case

a macroscopically polar phase is formed above a threshold

field of 8 Vmm^À1(Figure 9b), required for the anchoring transi-

tion (see Section 2.4.1 and Figure S18 in the Supporting Infor-

mation). The unique organization of this compound might be

associated with the relatively small molecular bend and large

dipole moment of mꢀ7 D, which is directed nearly parallel to

the CN dipole (see Figure 12).

Figure 12. Contour plots of molecular electrostatic potential and approxi-

mate directions of the molecular dipole moments in two distinct views (top:

front view, bottom: top view) of a) CN1 and b) CN2; the electrostatic poten-

tials are displayed on the 0.002 a.u. isodensity surfaces; the maximum (dark-

est blue) and minimum (red) values correspond to 0.02 a.u. The dipole mo-

ments were calculated for the stable conformations with the orientation of

the two carbonyl groups in the terephthalate moieties being anti. Although

dipole moments depend on the conformations, CN1 has always larger

dipole moments than CN2; for details of simulations, see Experimental Sec-

tion.

tion comes from SHG activity which is already observed in the

Iso phase (Figure 9). This means that small disordered SmC_sP_S

clusters occur already in the isotropic phase (cybotactic iso-

tropic phases)^[20a,68]and could lead to polar surface layers pro-

viding strong surface pinning. In the SmC_sphase this surface

pinning could contribute to the development of a chiral con-

glomerate, which is absent in FS films (see Figure 5).

The large dipole moment might also be responsible for an-

other remarkable feature of CN1 associated with the isotropic

liquid phase occurring immediately above the SmA–Iso transi-

tion. In planar samples, the fanlike texture can still be observed

as a grayish shadow in the Iso phase (Figure S17a,b of the Sup-

porting Information), which only slowly fades with further in-

creases in temperature. Even at T=1808C this texture could be

recognized. It appears to result from LC surface layers persist-

ing above the SmA–Iso transition. This is an indication of a cy-

botactic structure of the isotropic liquid in this temperature

range, which in addition was confirmed by the XRD patterns.

This confirmation comes from the presence of diffuse scatter-

ing in the small-angle region, close to the position of the layer

reflection in the adjacent smectic phases, which appears

around 1808C in the Iso phase and grows in intensity on ap-

proaching the Iso–SmA phase-transition temperature (Fig-

ure S13a of the Supporting Information). Additional confirma-

Compound CN2

Compound CN2 is distinguished from CN1 only by the inver-

sion of one of the carboxyl groups (X in Figure 2). Thus, the

orientation of the COO groups in both rod-like molecular seg-

ments is symmetric with respect to the central resorcinol unit,

and both attached rodlike units (the “wings”) are phenyl tere-

phthalates. This leads to compensation of the dipole moments

of the COO groups in each of the rodlike units and to an alter-

nation of electron-poor and electron-rich benzene rings in

both rodlike units. Figure 12 shows the surface electrostatic

potential maps of CN1 and CN2. Besides the distinct electron-

density distribution, there is a huge effect on the dipole

moment, which has the same direction, but is much smaller

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for CN2 (m=4 D) compared to CN1. As shown in Table 1, this

structural variation increases mesophase stability by more than

30 K compared to CN1, whereas the reduced melting point

and crystallization tendency result in almost 1008C range of

the enantiotropic mesophase. The DSC trace for CN2 (see Fig-

ure 3b) shows, besides the melting and the LC–Iso peak, only

one phase transitions in the LC range at T=136–1378C.

On further cooling, the planar texture becomes yellowish

and then orange, and thus evidences a significant increase of

Dn in this high-temperature range of the SmC_sphase between

SR

137 and 1298C (Figure 13c,d), tentatively assigned as SmC_sP_S

,

where superscript SR indicates a restricted (“short-range”) co-

herence length of polar order. There are no further textural

changes below 1298C, at the transition to the phase range as-

signed as SmC_sP_S. All textural changes occurring below 1378C

are continuous and take place without visible enthalpy

changes. The rather strong increase of birefringence between

137 and 1298C points to an increasing order parameter of the

aromatic cores, in line with a growing coherence length of

polar order in the SmC_sP_S^SRrange, as confirmed by electro-opti-

cal and SHG studies (see below).

Optical investigations

The high-temperature SmA phase exhibits a fan-shaped tex-

ture in planar cells (Figure 13a) and a homeotropic texture be-

XRD studies

XRD investigations (Figure 14) confirmed the presence of

smectic phases with slightly larger d values (d=5.4–5.85 nm)

compared to CN1 (d=5.03–5.23 nm). However, these values

are still smaller than the molecular length (L_mol=5.9 nm), in

agreement with a monolayer structure. The larger d value com-

pared to CN1 partly results from the smaller tilt (average tilt in

the SmA phase and synclinic tilt in the SmC_sphases). In con-

trast to CN1, higher orders of the layer reflections can be ob-

served in the small-angle range of the XRD patterns. The

number of higher-order reflections increases from only one in

SmA, two in SmAP_AR, three in SmC_sP_S^SR, to five in the SmC_sP_S

phase (Figure 14a,b and Figure S14 of the Supporting Informa-

tion), and this indicates sharpening of the interfaces. Two-di-

mensional XRD patterns of aligned samples confirm the onset

Figure 13. Textures of CN2 in planar-aligned samples (10 mm ITO cell with PI

layer) and insets of a homeotropic sample between untreated glass plates.

SR

of tilting b at the transition to the SmC_sP_Sphase at 1378C,

SR

a) SmA phase at T=1708C. b) Transition from SmAP_AR(top) to SmC_sP_S

(bottom) at 1378C. c) SmC_sP_S^SRphase at T=1358C. d) SmC_sP_Sphase at

T=1298C; arrows show the orientation of polarizer and analyzer, and

dotted lines show the change of the orientation of the dark extinctions.

which reaches b=218 at T=1308C and remains nearly con-

stant in the SmC_sP_Sphase with b=228 at T=1008C (XRD tilt,

as determined from the splitting of the wide-angle scattering;

tween untreated substrates (inset in Figure 13a). As discussed

below in more detail, growth of polar domains occurs with de-

creasing temperature, so that the high-temperature region of

the SmA phase is a paraelectric SmA phase, whereas the low-

temperature region below 1468C is designated SmAP_AR. No en-

thalpy change or significant textural change is associated with

this continuous transition, and only a tiny increase in birefrin-

gence can be observed in the planar samples (Figure 13a,b

top area). On cooling the homeotropic samples, a transition to

a biaxial LC phase takes place at T=1378C by development of

a low-birefringence texture (see inset in Figure 13c). In planar

samples the extinction brushes become inclined with respect

to the directions of the polarizers (Figure 13c); this indicates

the transition to a SmC_sphase at this temperature. The optical

tilt of about 20–258 is a bit smaller than that found for the

SmC_sphase of CN1. This SmA–SmC_stransition is accompanied

by the formation of a fine stripe texture (Figure 13b, bottom),

which could indicate temporary formation of a helical super-

structure with a helix axis parallel to the layer normal in

a small temperature range at the SmAP_ARto SmC_sP_Stransition.

Figure 14. XRD patterns of a surface-aligned sample of CN2. a) 2q scan in

the SmA phase at T=1508C. b) 2q scan in the SmC_sP_Sphase at T=1008C.

c) Original pattern in the SmC_sP_Sphase at T=1008C. d) Temperature de-

pendence of d; see Figure S14, S16 and Table S1 of the Supporting Informa-

tion for further details.

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see Figure 14c and Figures S14 and S16 of the Supporting In-

formation). The XRD tilt is comparable to the optically deter-

mined tilt of about 20–258. The 2q scans show a broad and

nonsymmetrical shape of the wide-angle scattering, which is

already present in the SmA phase and becomes more pro-

SR

nounced with decreasing temperature in the SmC_sP_Sand

SmC_sP_Sranges. In the SmC_sP_Srange there is a clear splitting of

the wide-angle scattering into a maximum at d=0.47 nm, cor-

responding to the usually observed lateral mean distance be-

tween hydrocarbon-based mesogens in LC phases, and

a shoulder at d=0.61 nm (2qꢀ14.58), attributed to the mean

distance between the carbosilane segments (Figure 14b). This

evidences that for CN2 the silyl groups are segregated into

their own layers. By correlating the d values with the molecular

dimensions, an antiparallel and fully intercalated organization

of the aromatic cores with an end-to-end packing of the silyl

groups can be assumed (see Figure 8b,c). This predominately

non-intercalated packing of the bulky carbosilane units (Fig-

ure 8b) reduces steric layer frustration and additionally contrib-

utes to the increase in the layer distance. The formed carbosi-

lane sublayers reduce out-of-plane interlayer fluctuations and

give rise to sharper interlayer interfaces, in line with the occur-

rence of higher orders of the layer reflection (Figure 14b,c).

The change of d spacing depending on temperature is

shown in Figure 14d. On cooling from the isotropic liquid,

there is an almost linear increase of d in the SmA phase be-

tween 160 and 1468C from d=5.45 to 5.7 nm, which suggests

a growing packing density, which leads to chain stretching in

combination with increasing carbosilane segregation with de-

creasing temperature, in line with the birefringence increase

observed optically. The slope of the d=f(T) curve decreases

a bit already in the SmA range around 1468C, corresponding

to the SmA–SmAP_ARtransition, but there is no significant

Figure 15. Switching-current response curves of CN2 in a 6 mm PI-coated

ITO cell as a function of temperature. a) SmAP_ARregion at T=1428C and

b) at T=1408C. c) SmC_sP_S^SRphase at T=1348C. d) Dependence of P_son tem-

perature.

gether at this temperature, forming a single peak positioned at

zero-voltage crossing (Figure 15c); this single peak is retained

on further cooling without changes. The variation in switching

polarization on cooling, deduced from the current response, is

shown in Figure 15d. It exhibits a continuous increase across

the whole mesophase region and reaches a polarization as

high as about 600 nCcm^À2in the SmC_sP_Sphase. Bistable (ferro-

electric) switching in the SmC_sP_Sphase was confirmed by opti-

cal investigation of the in-plane switching process (IPS) in ho-

meotropic cells (the applied field and the layers are parallel to

the substrate surfaces, in contrast to the usually used ITO cells,

in which layers and applied field are directed perpendicular to

the substrate surfaces) and also in FS films.^[52]For more details

of the electro-optical investigation, see the Supporting Infor-

mation (Section 2.4.2 and Figure S20).

SR

change at the SmA–SmC_Sphase transition at T=1378C. Only

at the SmC_sP_S^SR–SmC_sP_Stransition at 1298C is the maximum d

value reached (d=5.83 nm), and on further cooling the layer

spacing remains nearly constant in the temperature range of

the SmC_sP_Sphase. These changes of the slope of the d=f(T)

curve coincide with the optical investigations described above

and are in line with a significant increase of the packing densi-

The major temperature region of the SmA phase does not

show any SHG signal in an electric field up to 20 Vmm^À1. Only

below 1468C, in the SmAP_AR, SmC_sP_S^SR, and SmC_sP_Sregions,

a field-induced SHG signal was found, the intensity of which

SR

ty in the SmC_sP_Srange between 137 and 1298C, which satu-

continuously increases

with decreasing

temperature

rates in the SmC_sP_Srange. This also indicates the presence of

a pretransitional region at the high-temperature side of the

SmA–SmC_stransition between 146 and 1378C, associated with

the growth of the SmC_sP_Sdomains in this temperature range

of the SmA phases assigned as SmAP_AR.

(Figure 16). To distinguish between surface and bulk effects,

SHG was measured in cells of different thicknesses (6 and

10 mm polyimide (PI)-coated cells). The field-induced signal

nearly doubles as the cell thickness is increased from 6 to

10 mm, which suggests that it is based on a bulk effect. A dis-

tinct feature of the SmC_sP_Sphase occurring below T=1378C is

the presence of a residual signal, which remained after the

field was turned off. There was a tiny residual SHG signal even

when no field was previously applied to the sample; this is in

Electro-optical investigations, SHG, and dielectric

investigations

SR

Under a triangular-wave driving voltage there is no current

peak in the SmA region. The transition into the SmAP_ARphase

at 1468C is marked by appearance of a double-peak current re-

sponse in each half-period of the applied triangular-wave field.

On further cooling the polarization peaks become sharper on

line with ferroelectric switching. At the transition from SmC_sP_S

to SmC_sP_Sat T=1298C the dependences of the field-induced

and residual SHG intensities on temperature become less

steep. It is assumed that in the temperature range 137–1298C,

designated as SmC_sP_S^SR, the ferroelectric domains have reached

appreciable size, but they are not long-range and grow further

with decreasing temperature, in line with the dielectric results

SR

approaching the SmAP_AR–SmC_sP_Stransition at T=1378C (Fig-

ure 15a,b). Simultaneously the peaks get closer and merge to-

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Figure 17. Static dielectric permittivity of CN2 as a function of temperature

in a 12 mm gold-coated, untreated cell.

1508C the permittivity sharply increases, indicating the onset

of collective behavior. The discontinuity in the permittivity

curve around 146–1508C might be due to hysteresis of the for-

mation of antipolar correlations between the domain in the

SmAP_ARphase after reaching a critical size of the SmC_sP_Sdo-

mains. Detailed inspection of the dielectric spectra (Figure S21

of the Supporting Information) shows that at higher tempera-

tures the SmA mesophase is characterized by a single dielectric

relaxation mode, whereas close to 1468C two relaxation pro-

cesses with very similar frequencies start to contribute to the

dielectric response corresponding to the transition to the

SmAP_ARphase. These contributions can be associated with the

collective fluctuations of the phase (antiphase phason mode)

and amplitude of the polarization (antiphase amplitudon

mode).^[24,70]Unfortunately, the two relaxation modes appear

too close in frequency, which prevents any further analysis of

the dynamics. On further cooling, the combined contribution

SR

of both modes continues to increase over the SmC_sP_Sand

Figure 16. a) SHG signal of CN2 as a function of temperature in a 6 mm cell.

The squares were obtained under an applied electric square-wave field

(E_pp=2 Vmm^À1, f=10 Hz). The triangles correspond to a residual signal in

the field-free state after removing the applied field. b) Field dependence of

the SHG signal in the SmAP_ARphase at T=1358C and c) in the SmC_sP_Srange

at T=1268C.

SmC_sP_Smesophases without any significant changes

(Figure 17), and this evidences the continuous increase of the

polar correlation. After the transition to the SmC_sP_Sphase, the

permittivity reaches a maximum at Tꢀ1238C and then steadily

decreases. Such behavior, which is in line with SHG observa-

tions, points to the slow development of an antipolar organiza-

tion promoted by the necessity to escape from the increasing

macroscopic polarization.

(see below). Below T=1298C, in the temperature region desig-

nated as SmC_sP_S, the polar domain size has become nearly

long-range, but a polydomain structure is still retained, and

with further decreasing temperature the domains continuously

grow further. SHG experiments indicate a reduced residual

signal at T<110 8C (Figure 16a). It appears that the growing

polar domain size (see Figure 15d) at further reduced tempera-

ture leads to an increasing desire to escape from the growing

macroscopic polarization, and favors an antipolar organization

in the bulk sample; the residual signal might result from re-

maining ferroelectric surface layers.

Discussion of LC phase structures of CN2

Compound CN2 forms SmA and synclinic SmC_sphases with

a transition between them at T=1378C. Tilt and polar order

are already present locally in the SmA phase, which thus is

a special kind of paraelectric de Vries-like phase composed of

SmC_sP_Sdomains, that is, tilt and polar order have short coher-

ence length. Though tilt and polar correlation are still short-

range the SmC_sP_Sdomains reach appreciable size in the SmA

region between about 146 and 1378C, assigned as SmAP_AR. In

the temperature range of the SmAP_ARphase a preference for

an antipolar organization between adjacent polar SmC_sP_Sdo-

mains develops, which leads to antiferroelectric-like switch-

ing.^[71]At 1378C the SmC_sP_Sdomains reach a critical size, tilt

correlation becomes long-range, and phase biaxiality results.

Dielectric spectroscopy measurements were carried out for

CN2 in the frequency range of 10 Hz to 10 MHz. The static di-

electric constant, plotted in Figure 17, was obtained as the

sum of the different contributions to the dielectric permittivity

by fitting the spectra to the Havriliak–Negami equation.^[69]The

value of the permittivity in the SmA phase slightly grows over

a broad temperature range. However, for temperatures below

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At this transition, a denser packing is achieved and the polar

molecular conformations and intermolecular interaction pa-

rameters provided by the reversal of the direction of only one

COO group. Also, the reduced dipole moment of CN2 could

reduce the coupling with polar surfaces, and thus decrease the

surface stabilization of the chiral domains. In addition, an en-

hanced tendency for antipolar correlation of the SmC_sP_Sdo-

mains in the smectic phases of CN2, indicated by formation of

the SmAP_ARphase and reduced SHG signal in the SmC_sP_S

phase at lower temperatures, could inhibit the formation of

macroscopic chiral domains.

SR

coherence length increases substantially. In this SmC_sP_S

range, the polar domains have still limited size (somewhere be-

tween types C and D in Figure 18) and further grow to

Conclusion

Two isomeric bent-core LCs, CN1 and CN2, both based on

a weakly bent 4-cyanoresorcinol core and having a carbosilane

unit as potential nanosegregating unit at one end, were syn-

thesized and were investigated with respect to LC phase for-

mation, polar order, and mirror-symmetry breaking. At high

temperature both compounds form uniaxial SmA phases in-

volving local SmC_sor SmC_sP_Sdomains without long-range ori-

entational order of the molecular tilt. With decreasing temper-

ature, increases in the coherence length of tilt and polar order

lead to synclinic tilted biaxial LC phases (SmC_s) and develop-

ment of polar order. For the terephthalate-based CN2 the seg-

regation of the carbosilane sublayers is stronger than for CN1,

as indicated by the distinct shape of the XRD wide-angle scat-

terings and the number of higher harmonics of the layer reflec-

tions in the SmC_sphases. The inversion of the direction of one

of the COO groups leads to a change of the surface electro-

static potential distribution and to a strong increase of the

dipole moment from about 4 D for CN2 to about 7 D for CN1

(Figure 12). This obviously favors a longitudinal shift of the aro-

matic cores in the LC phases of CN1, which then reduces the

segregation of the carbosilane units from the aliphatic seg-

ments, increases steric layer distortion, and thus increases the

tilt and decreases the coherence length of polar order and

layer spacing. For CN2, with segregated carbosilane groups,

less tilt is required to achieve dense core packing, and hence it

shows a stronger preference for polar order in the smectic

phases compared to CN1. Therefore, the coherence length of

polar order is larger for the less polar CN2, the dielectric con-

stants and polarization values are significantly higher, and

polar order develops before tilt correlation, whereas for the

more polar compound CN1 uniform tilt develops first, before

appreciable polar order is established. Thus the SmA and SmC

phases of CN1 are paraelectric (B in Figure 18), and polar order

(C!D) is only formed after the onset of a leaning organization

(SmCL_sP_S). In contrast, for CN2 local polar order is already

found in the SmAP_ARphase, and polar SmC_sP_Sdomains domi-

nate the properties of the SmC phases, and this finally leads to

a proper ferroelectric-switching SmC_sP_Sphase.

Figure 18. Development of polar order in (tilted) smectic phases and sche-

matic presentations of the distinct phase types formed by rodlike (A),

weakly bent (B–E), and strongly bent (F, G) molecules. In principle the same

phase types can be considered for the related SmA phases and phase se-

quences involving SmA–SmC–SmCL transitions. Color indicates the polar di-

rection pointing inwards (crosses) or outwards (dots), and gray means that

the polar direction does not coincide with the two preferred directions.

become nearly long-range below 1298C in the polydomain fer-

roelectric SmC_sP_Srange (structure D in Figure 18). With the fur-

ther growth of the domain size and increasing polarization, it

appears that polar order becomes unstable and antipolar layer

correlation becomes increasingly favorable, which would even-

tually lead to SmCP_Aphases with alternating polarization be-

tween layer stacks (E) or individual layers (F in Figure 18).

Though a stable SmCP_Aphase is not obtained for CN2, there

are first indications of emerging antipolar organization from

SHG and dielectric investigations. In Figure 18 the develop-

ment of the distinct phase structures depending on polar co-

herence length is schematically summarized.

With regard to chirality, in contrast to CN1, no signature of

SR

chiral segregation can be observed in the SmC_sP_Sand SmC_sP_S

phases of CN2 with much larger SmC_sP_Sdomain size. As the

SmC_sP_Sstructure is chiral, the formation of chiral domains is ex-

pected to be more likely in these polar SmC phases of CN2 in-

volving larger domains and thus providing increased coopera-

tivity for chirality synchronization.^[20c]That this is not the case

could have different reasons. For example, homochiral segre-

gation could be less favorable for CN2 due to the changed

Another unique feature of CN1 is that at reduced tempera-

ture the tilt becomes coincident with the polar direction, and

thus it is the first example of a nonmodulated leaning phase.

This represents a third kind of tilted and polar smectic phases

besides the usually observed polar SmC phases (SmCP), in

which the tilt is perpendicular to the polar direction (Fig-

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ure 1a,b), and the rarely observed SmCG phases, in which the

polarization is neither in the tilt plane nor perpendicular to it.

Regarding the tilt orientation, the SmCG phase is considered

to be an intermediate type of structure between the SmCP

and SmCLP phases. It is likely that the reduced bend and the

high dipole moment of CN1 are favorable for formation of the

leaning phase with polydomain SmCL_sP_Sstructure.^[53]The

major contribution is assumed to arise from steric stabilization

of the packing in the leaning phases. The mixed organization

of the carbosilane groups with the hydrocarbon chains, as evi-

denced by XRD, reduces the packing density of the aromatic

cores. However, as shown in Figure 1c, by leaning, a denser

packing of the more tilted rod-like units of the bent cores can

be retained. The simultaneously developing additional space

between the less tilted and more widely separated rodlike

units of the bent-core molecules is easily filled by the excess

volume provided by mixing of carbosilanes and alkyl chains.

This mode of packing, involving distinct packing densities for

the wings is hardly possible for other bent-core molecules, and

therefore leaning phases have not been observed previously.

Nevertheless, the fact that SmCG phases were found for other

silylated bent-core mesogens indicates a tendency for moving

the polar director from perpendicular to parallel to the tilt di-

rection for bent-core mesogens with bulky end groups. More-

over, the bulky carbosilane groups give rise to relatively low

melting points, so that sufficiently low temperatures can be

reached to allow the observation of this phase in a certain

temperature range before crystallization.

In addition, symmetry breaking with formation of chiral con-

glomerate structures was observed. This is proposed to be the

result of a cooperative chirality synchronization of the helical

conformers of the transiently chiral mesogens, associated with

the formation of a local helical superstructure by self-assembly

of these helical conformers.^[20c,72]For tilted smectic phases of

bent-core mesogens there are additional diastereomeric rela-

tions with the superstructural layer chirality (resulting from the

orthogonal combination of tilt and polar order^[12]), leading to

energetically favored and disfavored combinations of molecu-

lar helix sense and sign of layer chirality. For the investigated

compounds the chirality synchronization apparently requires

a short-range SmC_sP_Sdomain structure as found in paraelectric

or randomized SmC phases.^[16,34]As expected, it is removed at

the transition to the achiral SmCL_sP_Sphase, lacking the stabili-

zation by layer chirality. Surprisingly, it is also absent in the

SmC phases of CN2, which have increased polar coherence

length. It appears that a large molecular dipole moment, im-

proving the surface pinning, is more important for chirality

synchronization than the increased cooperativity provided by

the larger size of the polar domain. Another possible reason

could be that the helical twist arising by chirality synchroniza-

tion competes with the lamellar organization.^[20c]Only if the la-

mellar ordering is weak (diffuse interlayer interfaces), as found

for CN1, sufficient local helical twist can be retained in the sur-

face-aligned samples. In smectic phases with sharp interfaces

(CN2) the development of a local helical twist is restricted and

chirality synchronization becomes less favored.

Compounds CN1 and CN2 are weakly bent molecules, form-

ing their own class of mesogenic molecules with properties

and phase structures distinct from those of the “classical”

bent-core mesogens with about 1208 molecular bend and also

from those of the linear rodlike LCs. Long-range polar order in

the layers and antipolar correlation between adjacent layers

(B2 phases, SmCP_Aphases) are dominant for the strongly bent

molecules. If synpolar correlation of the layers is preferred,

then polarization-modulated and undulated smectic phases

(B1, B6, and B7 phases) provide escape from macroscopic

polar order.^[10,12]At the other extreme, nematic and nonpolar

smectic phases are typical for achiral rodlike mesogens. Smec-

tic phases composed of polar SmC_sP_Sdomains and growing

coherence length of polar order determine the phase sequen-

ces of the weakly bent molecules. Typical phase sequences of

this kind of molecules involve cybotactic nematic phases,^[28]

de Vries-type SmA phases, paraelectric and randomized polar

smectic phases with polydomain structure (P_AR/P_R), and eventu-

ally ferroelectric SmC phases (Figure 18A–D). In general, for

these weakly bent molecules easier rotation around the molec-

ular long axis leads to reduced coupling of the tilt and the

polar direction, and thus favors polar domain structures, which

are entropically favorable over uniformly polar layers. Layer

modulations are avoided and soft ferroelectrics with macro-

scopic polar order can more easily be achieved under the influ-

ence of weak external stimuli, for example, by alignment

layers. For CN2 the nanosegregated carbosilane layers provide

layer decoupling and additionally stabilize the SmC_sP_Sstruc-

ture.^[13]

Overall, this work reports a new type of polar smectic

phases, contributes to the general understanding of the devel-

opment of polar order, ferroelectricity, and chirality in self-as-

sembled soft matter systems in relation to the molecular struc-

ture, and provides general rules for the design of soft ferro-

electrics.

Experimental Section

Synthesis

The synthesis of the final compounds and intermediates

(Scheme 1) and analytical data of the intermediates are reported in

detail in the Supporting Information. Purification was performed

by column chromatography (silica gel 60, Merck, pore size 60 ,

230–400 mesh) and/or by crystallization from the solvents given in

1

the Supporting Information. H, ¹³C, and ²⁹Si NMR spectra were re-

corded with Varian Unity 500 and Varian Unity 400 spectrometers

in CDCl₃solutions by using the solvent peak as internal standard,

and IR spectra were recorded with a Perkin Elmer model 283 spec-

trometer by using KBr disks. High-resolution mass spectra were re-

corded with a Bruker micrOTOF-Q II APPI mass spectrometer. Mi-

croanalyses were performed with a Leco CHNS-932 elemental ana-

lyzer.

2-[4-(4-Hexyloxyphenoxycarbonyl)benzoyloxy]-4-{4-[4-

(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)-

benzoyloxy]benzoyloxy}benzonitrile (CN1): ¹H NMR (400 MHz,

CDCl₃, 258C, TMS): d=8.34–8.40 (m, 4H; ArH), 8.25–8.29 (m, 2H;

ArH), 8.13–8.17 (m, 2H; ArH), 7.82 (d, J=8.6 Hz, 1H; ArH), 7.57 (d,

J=2.2 Hz, 1H; ArH), 7.39–7.42 (m, 2H; ArH), 7.35 (dd, J=2.2 Hz

and 8.6 Hz, 1H; ArH), 7.13–7.17 (m, 2H; ArH), 6.97–7.01 (m, 2H;

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ArH), 6.93–6.97 (m, 2H; ArH), 4.06 (t, J=6.6 Hz, 2H; OCH₂), 3.98 (t,

J=6.6 Hz, 2H; OCH₂), 1.82 (m, 4H; CH₂CH₂O), 1.48 (m, 4H; CH₂),

1.25–1.40 (m, 18H; CH₂), 0.92 (t, J=7.0 Hz, 3H; CH₃), 0.48 (broad,

2H; CH₂CH₂Si), 0.05 (s, 6H; Si(CH₃)₂), 0.02 (s, 9H; Si(CH₃)₃), 0.00 (s,

6H; Si(CH₃)₂), À0.26 (s, 2H; SiCH₂Si), À0.28 ppm (s, 2H; SiCH₂Si);

¹³C NMR (100.6 MHz, CDCl₃, 258C, TMS): d=164.43, 164.20, 163.92,

163.26, 162.82, 157.14, 155.97, 154.86, 153.22, 144.00, 134.82,

134.04, 132.44, 132.23, 132.01, 130.63, 130.45, 125.64, 122.37,

122.21, 120.78, 120.27, 117.35, 115.19, 114.46, 104.25, 68.47, 68.43,

33.70, 31.58, 29.62, 29.60, 29.56, 29.38, 29.23, 29.10, 25.99, 25.72,

23.99, 22.61, 18.07, 14.03, 5.81, 4.06, 2.45, 1.45, À0.45 ppm; ²⁹Si

NMR (99.3 MHz, CDCl₃, 258C, TMS): d=1.27, 0.57, 0.02 ppm; IR

(KBr): nÜ= >_max=3112, 3070, 2951, 2923, 2853, 2233 (CꢁN), 1929,

1732, 1605, 1509, 1469, 1407, 1277, 1256, 1216, 1203, 1159, 1120,

Switching experiments and electro-optical investigations were per-

formed in 6, 10, 17.4, and 25 mm PI-coated indium tin oxide (ITO)

cells (EHC, Japan) with a measuring area of 1 cm²(antiparallel rub-

bing in the PI-coated cells). The cells were filled in the isotropic

state. Switching experiments were carried out by the triangular-

wave method^[73]with a combination of function synthesizer (Agi-

lent, model 33220) and amplifier (FLC Electronics, model A400),

and the current response traces were recorded with an oscillo-

scope (Tektronix, model TDS2014) across a 5 kW resistance.

Measurements of optical SHG were performed with an Nd:YAG

laser operating at l=1064 nm (10 ns pulse width and 10 Hz repeti-

tion rate). The primary beam was incident at an angle of 308 to the

cell normal. The SHG signal was detected in transmission by a pho-

tomultiplier tube (Hamamatsu). The acquired signal was calibrated

with a 50 mm reference quartz plate.

1085, 1061, 1016, 838, 810 cm^À1

; Q-TOF MS: m/z calcd for

C₆₁H₈₃N₂O₁₀Si₃[M+NH₄]⁺: 1087.5350; found: 1087.5365; elemental

analysis calcd (%) for C₆₁H₇₉NO₁₀Si₃: C 68.44, H 7.44, N 1.31; found:

C 68.21, H 7.36, N 1.26.

Complex dielectric permittivity was measured on cooling runs in

the frequency range of 10 Hz to 10 MHz with a Solartron 1260A

impedance analyzer. Capacitance of the cells was carefully calibrat-

ed before being filled in the isotropic state.

2-[4-(4-Hexyloxyphenoxycarbonyl)benzoyloxy]-4-{4-[4-

(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)-

phenoxycarbonyl]benzoyloxy}benzonitrile

(CN2):

¹H NMR

Calculations

(400 MHz, CDCl₃, 258C, TMS): d=8.30–8.40 (m, 8H; ArH), 7.84 (d,

J=8.6 Hz, 1H; ArH), 7.61 (d, J=2.2 Hz, 1H; ArH), 7.38 (dd, J=

2.2 Hz and 8.6 Hz, 1H; ArH), 7.13–7.17 (m, 4H; ArH), 6.93–6.97 (m,

4H; ArH), 3.98 (t, J=6.6 Hz, 2H; OCH₂), 3.97 (t, J=6.6 Hz, 2H;

OCH₂), 1.80 (m, 4H; CH₂CH₂O), 1.47 (m, 4H; CH₂), 1.25–1.39 (m,

18H; CH₂), 0.92 (t, J=6.9 Hz, 3H; CH₃), 0.48 (br, 2H; CH₂CH₂Si), 0.05

(s, 6H, Si(CH₃)₂), 0.02 (s, 9H; Si(CH₃)₃), 0.00 (s, 6H; Si(CH₃)₂), À0.25

(s, 2H; SiCH₂Si), À0.28 ppm (s, 2H; SiCH₂Si); ¹³C NMR (100.6 MHz,

CDCl₃, 258C, TMS): d=164.40, 163.13, 162.81, 157.16, 157.13,

154.56, 153.24, 143.98, 143.97, 134.87, 134.76, 134.12, 132.53,

132.14, 130.63, 130.47, 130.41, 122.19, 122.16, 120.14, 117.26,

115.19, 68.48, 68.47, 33.70, 31.58, 29.64, 29.61, 29.58, 29.41, 29.39,

29.27, 29.23, 26.05, 25.71, 23.98, 22.60, 18.06, 14.02, 5.80, 4.04, 2.46,

1.46, À0.43 ppm; ²⁹Si NMR (99.3 MHz, CDCl₃, 258C, TMS): d=1.27,

0.57, 0.02 ppm; IR (KBr): nÜ= >_max= 3110, 3072, 2953, 2923, 2853,

2235 (CꢁN), 1740, 1607, 1509, 1469, 1407, 1273, 1246, 1199, 1159,

1108, 1081, 1059, 1016, 836, 812 cm^À1; Q-TOF MS: m/z calcd for

C₆₁H₈₃N₂O₁₀Si₃[M+NH₄]⁺: 1087.5350; found: 1087.5346; elemental

analysis calcd (%) for C₆₁H₇₉NO₁₀Si₃: C 68.44, H 7.44, N 1.31; found:

C 68.39, H 7.26, N 1.31.

For calculation of the surface potential maps, the geometries of

compounds CN1 and CN2 used for MEP calculations (Figure 12a,b)

were optimized at the DFT level of theory, by employing the ex-

change functional of Becke^[74]and the correlation functional of

Perdew,^[75]in conjunctional with a triple-z-quality basis set TZV^[76]

(model BP86/TZV), as implemented in GAMESS-US package.^[77]The

contours of MEP were obtained by using MacMolPlt v. 7.5 code.^[78]

For calculation of the dipole moments two stable conformations

each for CN1 and CN2, with the carbonyl groups of the terephtha-

late unit being syn or anti, were picked up from MMFF force-field

calculations implemented in SPARTAN’14 program. Then the mo-

lecular geometries were optimized by DFT methods with long-

range and dispersion-corrected wB97X-D functional^[79]and 6-

31G(d,p) split valence double-zeta polarized basis set.^[80]All calcula-

tions were carried out with the Gaussian 09 program.^[81]

Acknowledgements

This work was supported by the DFG (ER 467/8-1 and TS 39/

24-1); E.W. is grateful to CNPq/DAAD for the support during

the “PhD Sandwich” exchange program realized at the Martin-

Luther-Universität Halle-Wittenberg in Germany. The authors

also thank CEBIME-UFSC for the Q-TOF HRMS analysis. N.S.

thanks the Alexander von Humboldt Foundation for a Postdoc-

toral Research Fellowship. Some numerical calculations were

carried out on the TSUBAME2.5 supercomputer at the Tokyo

Institute of Technology, Tokyo, Japan, and on the supercom-

puter at the Research Center for Computational Science, Oka-

zaki, Japan. We thank H. Takezoe (Emeritus Prof. from TIT

Tokyo) and R. Stannarius (Uni. Magdeburg) for discussions.

Investigations

Measurements of transition temperatures and optical inspection of

the LC phases were performed on samples between ordinary glass

slides by polarized-light optical microscopy (Optiphot 2, Nikon) in

conjunction with a heating stage (FP82HT, Mettler).

DSC (TA Instruments Q2000 Module) was performed in 30 mL pans

for 2–5 mg samples with heating and cooling rates of 10 Kmin^À1

peak temperatures from the first heating and cooling scans are

listed in Table 1. The mesophases were assigned on the basis of

optical textures, DSC data, and XRD data.

;

XRD measurements were done using Cu_Karadiation (l=1.54 ) of

a standard Coolidge tube source with an Ni filter. Investigations of

oriented samples were performed with a 2D detector (HI-Star, Sie-

mens or Vantec 500, Bruker Siemens AG). Alignment was achieved

by slow cooling (0.1 Kmin^À1) of a small droplet on a glass surface;

the beam was applied parallel to the surface. Powder samples

were investigated in thin capillaries (Ø 1 mm). The sample-to-de-

tector distances were 8.8 and 26.9 cm for the wide-angle and

small-angle measurements, respectively, and the exposure time

was 15–60 min.

Keywords: bent-core mesogens

crystals · mesophases · self-assembly

·

ferroelectrics

·

liquid

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Received: September 29, 2015

Published online on May 3, 2016

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Article Doi

Polar Order and Symmetry Breaking at the Boundary between Bent-Core and Rodlike Molecular Forms: When 4-Cyanoresorcinol Meets the Carbosilane End Group

DOI: 10.1002/chem.201503901

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