In search of a new design strategy for solid single-component organic ferroelectrics: Polar crystalline phases formed by bent-core molecules

View Article Online / Journal Homepage / Table of Contents for this issue

PAPER

www.rsc.org/materials | Journal of Materials Chemistry

In search of a new design strategy for solid single-component organic

ferroelectrics: Polar crystalline phases formed by bent-core molecules†

a

W. Weissﬂog, G. Pelzl, H. Kresse, U. Baumeister, K. Brand, M. W. Schroder, M. G. Tamba,

a

*

€

S. Findeisen-Tandel,^aU. Kornek,^bS. Stern,^bA. Eremin,^bR. Stannarius^band J. Svoboda^c

Received 5th February 2010, Accepted 19th April 2010

DOI: 10.1039/c0jm00322k

A new design strategy for solid single-component organic ferroelectrics and pyroelectrics has been

developed. The subject is based on ﬁve-ring bent-core compounds which have the chemical structure

typical for banana-shaped liquid crystals, however, they do not form liquid crystalline phases. Starting

from a parent compound, systematic variation of different molecular moieties has lead to the chemical

structure forming liquid crystalline compounds and which results in materials exhibiting crystalline

modiﬁcations only. Calorimetric and polarizing microscopic investigations, X-ray diffraction

measurements, dielectric, electro-optic and SHG studies have been used to characterize the new

materials. More than 50% of the compounds which do not exhibit a liquid crystalline phase show

ferroelectric and/or pyroelectric behaviour in different solid modiﬁcations.

layers of poly(vinylidine ﬂuoride) can be utilized for nano-

lithography on organic substrates.⁵

Introduction

Ferroelectrics exhibit a spontaneous electric polarisation which

can be reversed by inverting an external electric ﬁeld.¹This effect

has long been an important topic in condensed matter science

from the theoretical and experimental point of view. Ferroe-

lectric materials can be widely used, for example, as semi-

conductors, electro-optical modules for data storage, and

substitutes for swing quartz. FeRAMs (ferroelectric random

access memory) and ferroelectric ﬁeld effect transistors are

practical applications of interest.²

Compared with the millions of existing organic compounds,

the number of low-molecular organic ferroelectrics is extremely

low.^6–9It is helpful to subdivide these into single-component

ferroelectrics and into those resulting from mixtures of two or

more components, because their ferroelectric behaviour is based

on different mechanisms. The functionality of multi-component

systems can be explained on the basis of charge-transfer inter-

action and/or H-bonded supramolecules, as well as non-ionic–

ionic transition systems. For example, proton-displasive

ferroelectricity is reported for co-crystals formed by phenazine

and halogen substituted anilic acids as well as of 5,5-dimethyl-

2,2-bipyridines and substituted quinones, both types are well-

investigated systems.^10,11Croconic acid was recently found to

have a hydrogen-bonded polar structure in a crystalline state.¹²

Salt-like derivatives of organic compounds belong to the

ferroelectrics described a long time ago. The discovery of ferro-

electrics started in 1920 with the investigation of Rochelle salt

(sodium potassium tartrate).¹³Another example is the study of

tris(sarcosine) calcium chloride and triglycin sulfate in the

1950s.¹⁴The number of real single-component polar organic

ferroelectrics may amount to about a dozen. Thiourea,^14,15

tanane (2,2,6,6-tetramethyl-1-pyridinyl-N-oxide),¹⁶cyclohexane-

1,1-diacetic acid,¹⁷diazabicyclooctane (monoprotonated),¹⁸

squaric acid,¹⁹DNP (1,6-bis(2,4-dinitrophenoxy)-2,4-hex-

adiyne),²⁰trichloroacetamide,²¹tricyclohexylmethanol²²belong

to this type. Two mesogenic compounds forming crystalline

modiﬁcations which not only have a polar structure (with pyro-

and piezoelectric properties) but also show ferroelectric

switching have been reported.²³It should be stressed that the

ferroelectric behaviour of all these organic single component

materials was discovered more or less by chance. General

structure-property relationships could not be found for single-

component polar organic crystals up to now.

Ferroelectricity can result from polar substances of either solid

or liquid crystals. The subject of the present paper are organic

solids, furthermore, we will show that the gap between ferro-

electric crystals and ferroelectric liquid crystals can be very small,

especially for bent-core compounds.

Most of the ferroelectrics reported up to now are inorganic

materials, e.g. sodium nitrite, potassium dihydrogen phosphates,

and barium titanate. Compounds belonging to the Perowskite

type ABX₃, also named PLZT materials (polycrystalline lead-

zirkonium-titanium-oxides) have been systematically studied

concerning the inﬂuence of the different elements in the position

A, B, and X.³

Looking at organic crystals, there are only a few ferroelectric

materials that can be high-molecular and low-molecular

compounds. Fluorine-containing crystalline polymers such as

copolymers of vinylidene ﬂuoride/triﬂuoroethylene are well-

known ferroelectrics.⁴Recently, Rankin et al. reported that thin

^aMartin-Luther-University, Halle-Wittenberg, Institute of Physical

Chemistry, Mu€hlpforte 1, D-06108 Halle, Germany. E-mail: wolfgang.

weissﬂog@chemie.uni-halle.de

^bOtto-von-Guericke-University, Institute of Experimental Physics,

Universita€tsplatz 2, 39106 Magdeburg, Germany

^cDepartment of Organic Chemistry, Institute of Chemical Technology,

Technickaꢀ 5, CZ-166 28 Prague 6, Czech Republik

Since 1975 it is known that smectic phases of liquid crystals

with a tilted arrangement of chiral calamitic molecules can show

† Electronic supplementary information (ESI) available: Supplementary

Figures 1–7. See DOI: 10.1039/c0jm00322k

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6057

View Article Online

ferroelectric and antiferroelectric properties.^24,25In the case of

banana-shaped liquid crystals—a topic in the liquid crystal

research since 1996—a polar arrangement is possible using non-

chiral compounds: if the bent molecules are preferably packed in

layers the rotation of the molecules around the long axis is

hindered resulting in a spontaneous layer polarization which is

usually oriented parallel to the layers.^26–28

Furthermore, a carbon–carbon triple bond was inserted in the

aliphatic chain.

- The direction of the different carboxylic groups which link

the aromatic rings together is of decisive importance on the phase

behaviour.

- The lateral attachment of small atoms and groups to different

positions of the central phenyl ring opens the greatest diversity in

the chemical variation of the basic compound.

Recently, we reported ferroelectric behaviour of a crystalline

modiﬁcation formed by bent-core molecules.²⁹Although the

chemical structure of the compound corresponds to the general

scheme usually resulting in ‘banana mesophases’ this compound

does not exhibit liquid crystalline phases, but several crystalline

modiﬁcations. Ferroelectric switching has been proved by elec-

tro-optical measurements in the high-temperature crystalline

phase. Furthermore we found that the longer chain homologues

behave similarly.³⁰

- Insertion of C]C bonds between the phenyl ring and the

carboxylic group results in cinnamic acid compounds which

exhibit a greater molecular length, but exhibit ﬁve aromatic rings

only. Another way to enlarge the aromatic core a little bit was the

substitution of the central 2-methylresorcinol fragment by a 1-

methylnaphthalin-2,7-diol moiety.

To characterize the new compounds beside calorimetric

measurements, polarization microscopy and X-ray diffraction

studies, electro-optical investigations have been performed

together with SHG measurements. Selected materials have been

additionally studied by dielectric methods. The compounds have

been prepared by esteriﬁcation of the corresponding 4-(4-subst.-

phenoxycarbonyl)benzoic acids and 4-(4-n-hexadecyloxy-

phenoxycarbonyl)-cinnamic acid, respectively, with substituted

resorcinols or 1-methyl-naphthalin-2,7-diol by means of dicy-

clohexylcarbodiimide in dichloromethane as described in detail

for compound 1, recently.²⁹

Starting from these results, it seems to be a unique opportunity

to search for connections between the chemical structure and

ferroelectricity in single-component organic crystals formed by

derivatives of the substance class under study by systematic

variation of different molecular fragments. So, the phenomenon

of the spontaneously formed polar order could be studied in the

same class of substances in two different aggregation states—the

solid state and/or the liquid crystalline state.

Results

The basic structure is that of the ﬁve-ring compound 1 synthe-

sized by esteriﬁcation of 2-methylresorcinol with two equivalents

4-(4-n-hexadecyloxyphenoxy)benzoic acid.²⁹The formula and

the phase transition behaviour are given in Fig. 1, together with

the molecular fragments which will be varied and discussed in the

work presented herein.

Description of experimental details

X-ray diffraction measurements on selected compounds were

carried out on powder-like and surface aligned samples. The

former were recorded with a Guinier ﬁlm camera (Huber) with

samples kept in glass capillaries (/ 1 mm) in a temperature-

controlled heating stage using quartz-monochromatized Cu-K_a

radiation. The ﬁlm patterns were calibrated with the powder

pattern of Pb(NO₃)₂. 2D patterns for samples on a glass plate

aligned at the sample–glass interface or at the sample–air inter-

face on a temperature-controlled heating stage were recorded

with an area detector (HI-STAR, Siemens) using Ni-ﬁltered Cu-

K_aradiation.

All synthetic activity in the ﬁeld of bent-core mesogens

described in the literature up to now was directed towards the

preparation of compounds exhibiting ‘banana-phases’ of a new

type and/or new phase sequences.^26–28In contrast, the work

presented here is based on another intention. However, we

searched for ﬁve-ring bent-core compounds which have the

chemical structure typical for banana-shaped liquid crystals,

which do not form liquid crystalline phases. Nevertheless we were

hopeful that these molecules could be able to construct a polar

arrangement in a crystalline modiﬁcation.

Dielectric investigations were carried out in the frequency

range from 1 Hz to 10 MHz using the Solartron-Schlumberger

Impedance Analyzer SI1260 and a Chelsea Interface. A brass cell

coated with gold (d ¼ 0.10 mm) was used as a capacitor which

was calibrated with cyclohexane. The measurements were per-

formed during stepwise cooling or heating with a mean rate of

about 0.2 K min^ꢀ1. The temperature was measured in the middle

electrode with a thermocouple. The real and imaginary part (3⁰

and 3⁰⁰) of the dielectric permittivities were calculated from the

measured capacitances and resistances. The data at different

frequencies did allow the calculation of the relaxation time using

the COLE–COLE equation.³¹

The molecular fragments which have been varied are sketched

in Fig. 1:

- Not only was the length of the hydrocarbon chain changed,

but also their connection group to the terminal phenyl ring.

Electro-optical investigation and P_Smeasurements were done in

commercially available sandwich cells (EHC Corp., Japan) with

a cell thickness of 5, 6 or 10 mm. The cells were mounted in the

heating stage of the polarizing microscope and a function

synthesizer (Keithley 3910) generated the voltage of different

shapes. The switching polarization was measured employing the

tri-angular wave voltage method.³²

Fig.

1 Phase transition temperatures and chemical formula of

compound 1 together with the molecular fragments which have been

varied.

6058 | J. Mater. Chem., 2010, 20, 6057–6079

This journal is ª The Royal Society of Chemistry 2010

View Article Online

Table 1 Phase behaviour of the derivatives 2–7 produced by variation of

the terminal wing groups—the chemical structure of the ﬁve-ring

aromatic bent-core remains unchanged

Second harmonic generation (SHG) measurements have been

performed in transmission geometry using a Nd:YAG laser

(wavelength l ¼ 1064 nm, pulse duration 10 ns, repetition rate

10 Hz). The fundamental light beam was incident at an angle of

40^ꢁto the normal of the cell. The SHG signal was detected in

transmission direction by a photomultiplier tube (Hamamatsu).

In order to improve the alignment of the crystal in the cell, we

prepared our samples by slow cooling of the material in the

electric ﬁeld. We used the arbitrary-wave generator TGA 1241

(TTi) and commercially available 5 and 10 mm ITO cells (E.H.C.,

Japan). However, this approach could not provide us with well-

aligned samples. The overall alignment in the direction of the cell

normal could be signiﬁcantly improved, in the transversal

direction, however, the sample was not oriented. This has been

checked using polarizing microscopy techniques during SHG

measurements and dependence of the SHG signal on the polar-

ization of the primary beam. The intensity of the second

harmonic has been calibrated using an alpha-quartz plate. The

effective d-value d_effis estimated using an expression:

Mesophase

behaviour^a

No. R

SHG^b

+

1²⁹–OC₁₆H₃₃

CrI 135 [19.0]

CrII 165 [58.0] Is

Cr 160.7 [50.9] Is

Cr 176.4 [55.4] Is

2

3

4

–COO–C₁₂H₂₅

–OOC–C₁₅H₃₁

—

+

–OC₆H₁₂C^CC₈H₁₇CrI 71.5 [12.9]

CrIII 131.0 [44.1] Is

CrI 68.9 [15.6]

5

–OC₁₅H₃₀COOCH₃

—

CrII 130.4 [11.5] Cr III 143.9

[50.8] Is

Cr 143.5 [51.6] Is

6

7

–CH₂O–C₁₄H₂₉

—

!

1=2

d_e²_ffQz²_Q

–CH₂CH₂OOCC₁₂H₂₅CrI 131.7 [55.9]

CrII 139.2 [37.4] Is

2

I_S

n_1Sn_2S

d_eff

¼

I_Qn²_1Qn_2Qz²_S

a

Transition temperatures ^ꢁC; transition enthalpies [in brackets] are given

in kJ/mole; CrI–III means different crystalline modiﬁcations. + means

b

where we assume that the coherence length is much larger than

the thickness of the cell. I_LCand I_Qare the absolute SHG

intensities in the sample and quartz plates, respectively; z_Qand z_S

are interaction-lengths of the quartz and the sample; n_1Qand n_1S

as well as n_2Qand n_2Sare the refractive indices of the quartz and

the sample for the ground wave (1064 nm) and the second

harmonic (532 nm), respectively. It must be pointed out, that the

estimated d-values are subjected to large errors since there is no

possibility to qualitatively estimate the degree of order in our

samples (crystals have poor orientation). The switching behav-

iour has been studied by applying a dc electric ﬁeld. The

behaviour of the SHG signal has been compared with the

textural changes in the polarizing microscopy.

a SHG intensity d_eff> 10^ꢀ6measured in the crystalline solid phase.

In compound 3, the ester group in the wing chains shows in

inverse direction in comparison to compound 2. Different

textures could be observed depending on the cooling rate: very

small needles and grainy textures (cooling rate 0.3 K min^ꢀ1), but

also nice patterns with arcs and bundles of stripes (cooling rate

2.0 K min^ꢀ1) as shown in Fig. ESI 2.† No spontaneous SHG

signal has been detected in the crystalline phase of the

compounds 2 and 3. Also the application of an external electric

ﬁeld did not induce SHG activity.

Compound 4 is comparable to the alkyloxy-substituted

ferroelectric parent compound 1 but contains unsaturated wing

groups. A carbon–carbon triple bond is inserted approximately

in the middle of the hydrocarbon chain, therefore the shape and

ﬂexibility of this aliphatic part is inﬂuenced. Calorimetric

measurements show one main peak on heating and cooling

beside a small transition peak near to 70 ^ꢁC. On cooling a texture

of a crystalline phase arises including some areas which strongly

remind of the images observed for the polar compound 1 and its

homologues, see Fig. 2a

Variation of the terminal aliphatic groups

It is well-known in liquid crystal chemistry that not only the

length of the terminal hydrocarbon chains inﬂuences the meso-

phase behaviour but also the type of linking to the outer phenyl

ring. This linking group can change the size of the mesomeric

system as well as the polarity and polarizability of the molecules.

In Table 1 compounds are listed having other wing groups than

the mostly used saturated alkyloxy chains. The hydrocarbon part

consists of 12 or more carbon atoms and, therefore it should be

sufﬁciently long to support a layer arrangement in the condensed

state. Liquid crystalline behaviour could not be observed for the

compounds 2–7 which corresponds to the strategy of our work.

In both compounds 2 and 3, the terminal hydrocarbon chains

are linked by carboxylic groups to the outer phenyl rings, but not

by ether oxygen as in the polar compound 1. On cooling

compound 2 which exhibits dodecyl benzoate fragments in both

terminal positions, two crystalline modiﬁcations appear and

grow at the same time, see Fig. ESI 1.† The texture of one of

these, CrII, is optically changed on applying an electric ﬁeld,

however, no spontaneous polarization could be found. The CrII

modiﬁcation transforms relatively fast into the CrI modiﬁcation.

The electro-optical results cannot be completely understood.

On applying an electric ﬁeld of 170 V_ppan optical switching can

be observed, however, the current response curve measured with

a ﬁeld up to 400 V_ppshows hardly any response, see Fig. 2b.

But, there is a strong SHG signal which occurs spontaneously

on the isotropic-crystal transition. The d_effvalues clearly increase

with decreasing temperature, see Fig. 3. This signal, however,

cannot be affected by an external electric ﬁeld: no switching

behaviour in the SHG signal can be found, which indicates

pyroelectric properties of this phase.

Dielectric investigations of substance 4 show a relaxation

range with an intensity of about 0.15 and a relaxation frequency

of about 0.3 MHz at 40 ^ꢁC that means in the CrI phase, see

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6059

View Article Online

Fig. 2 (a) Microphotograph of the crystalline modiﬁcation CrIII of compound 4 during its formation on cooling the isotropic liquid at 125 ^ꢁC; (b)

current response, cell thickness 5 mm, 400 Vpp, 124 ^ꢁC.

reﬂections in the whole theta range (Fig. 5b). The occurrence of

diffuse scattering and reﬂections which are smeared out parallel

to the meridian of the pattern suggests a comparatively high

degree of positional and/or dynamical disorder within the

structure.

The wing groups of compound 5 are changed in such a way

that a methoxycarbonyl group is attached at the u-position of

the long alkyloxy chains. Calorimetric measurements prove

crystalline trimorphism, see Table 1. Polarisation microscopic

studies without and with electric ﬁeld give no hint for any

unusual behaviour. No SHG activity has been found in this

compound, although the chemical structure is comparable to

that of the polar compound 1 apart from the methoxycarbonyl

groups at the end of the terminal alkyloxy chains.

For the compounds 6 and 7, the mesomeric effect between the

outer aromatic rings to the next oxygen atom in the aliphatic

Fig. 3 SHG measurements of compound 4: d_effvalues as a function of

chain is interrupted by one or two methylene units. No pecu-

liarity could be observed by microscopic studies. Electro-optical

and SHG measurements give no hint of pyro- or ferroelectric

behaviour.

the temperature.

Fig. 4. The activation energy of 140 kJ mol^ꢀ1means that the

relaxation process becomes too fast for our set up in the CrII

phase. No big change of the relaxation frequency but an increase

of the relaxation intensity was observed at the transition into the

CrII phase. The CrII phase has not been detected by calorimetric

measurements, which could be related to different cooling rates.

X-Ray diffraction measurements on powder-like and partially

surface-aligned samples prove the 3D long-range order, i.e. the

crystalline nature of the phase of compound 4. They show Bragg

Variation of the direction of the connecting ester groups

The inﬂuence of the direction of the four ester connecting groups

on the mesophase behaviour of isomeric ﬁve-ring bent-core

compounds (however without lateral methyl group at the central

phenyl ring) has been investigated some time ago. All these

compounds exhibit liquid crystalline phases. The behaviour of

isomeric compounds only differs in the type and stability of the

mesophases.³³

The relationships between the chemical structure and the

mesophase behaviour are strongly changed for the correspond-

ing 2-methyl-substituted compounds, the transition behaviour of

these compounds 8–12 is listed in Table 2. Compound 8 is the

short-chain homologue of compound 1 and did not exhibit

mesophases. No liquid crystalline behaviour could be observed

in the case of the terminally dodecyloxy-substituted 2-methyl-

resorcinol ester 9, but should latently exist because the hexa-

decyloxy-substituted homologue exhibits an oblique columnar

phase with unusual electro-optic response.³⁴Esters of the 2-

methylisophthalic acid (no. 10–11) show very high clearing

temperatures. Surprisingly, mesophases typical for calamitic

mesogens are detected, just as for the derivative of 3-hydroxy-

benzoic acid (no. 12). Preliminary studies by polarizing

Fig. 4 Static dielectric permittivity of compound 4 measured during

slow cooling.

6060 | J. Mater. Chem., 2010, 20, 6057–6079

This journal is ª The Royal Society of Chemistry 2010

View Article Online

Fig. 5 (a) 2D X-ray diffraction pattern (XRD) for a partially surface-aligned sample of compound 4 at 120 ^ꢁC on cooling, (b) theta-scans for the 2D

patterns depending on the temperature.

substituted bent-core mesogens. Besides the locally steric and

Table 2 Isomeric ﬁve-ring bent-core compounds different in the direc-

tion of the carboxylic connecting groups V, X, Y, W

electronic inﬂuence, lateral substituents can additionally change

the bending angle of the mesogenic core, and furthermore, can

hinder the rotation of the bent molecules about their long axes

which is of importance on the formation of polar phases.^{26–28,35–37}

Previous studies on the substance class under discussion have

shown that in many cases the attachment of lateral atoms and

small substituents, respectively, to the central phenyl ring results

in liquid crystalline materials (e.g. Z ¼ 2-NO₂, 2-Br, 4-Cl, 4-Br,

5-F, 4,6-Cl₂).³⁰That means the search for new constellations

resulting in non-liquid crystalline materials was clearly limited

Mesophase

behaviour^a

No.

V

X

Y

W

8³⁰

OOC

COO

OOC

COO

CrI 144.0

CrII 156.0 Is

Cr 121 Is

Cr 123.6 [35.0]

SmC 220 [^b]

Table 3 The inﬂuence of alkyl groups attached to different positions Z

of the central phenyl ring on the phase behaviour

9³⁰

10

COO

OOC

COO

OOC

COO

OOC

COO

SmA 241.7 [11.1] Is

Cr 119.6 [59.6]

SmC 173.7 [1.9]

N 211.9 [1.30] Is

Cr 120.7 [23.5]

SmC 139.5 [1.3]

SmA 179.5 [8.4] Is

11

12

COO

OOC

COO

OOC

COO

Phase

behaviour^a

No

13³⁰

1²⁹

n

Z

SHG^b

(lc)

a

b

SmA, SmC: smectic A and C phases; N: nematic phase. SmC–SmA

transition could not be detected by DSC.

16

H

Cr 151.0 [110.4]

USmCP 160.5 [24.3] Is

CrI 135 [19.0]

CrII 165 [58.0] Is

Cr 146 [80.0] Is

Cr 112 [87.3] Is

Cr 102 [70.6]

SmCP_A133 [22.1] Is

Cr 137 [58.1] Is

Cr 158.1 [47.4] Is

Cr 153 [49.5] Is

CrI 112 [20.0]

CrII 149 [50.6] Is

CrI 118 [13.6]

CrII 136 [7.6]

2-CH₃

+

microscopy show textures typical for nematic, smectic A and

smectic C phases. Electro-optical measurements are under study

and will be published with further experimental details elsewhere.

This means the esters of the 2-methylisophthalic acid and of the

2-methyl-3-hydroxybenzoic acid are unsuitable materials con-

cerning the objective under study. Therefore, the inﬂuence of

lateral substituents on the phase behaviour and electro-optical

properties has been studied using the chemical constitution of the

compounds 1 and 8, respectively.

14

15

16

2-C₂H₅

2-C₃H₇

4-CH₃

—

(lc)

17

18

19

20

16

8

12

14

5-CH₃

—

+

2,5-(CH₃)₂

+

21

22

16

18

2,5-(CH₃)₂

+

CrIII 148 [58.6] Is

CrI 111.5 [6.7]

CrII 119.3 [17.1]

CrIII 145.3 [55.3] Is

Variation of lateral substituents Z attached to the central phenyl

ring of 1,3-phenylene bis[4-(4-n-alkyloxyphenoxycarbonyl)-

benzoates]

a

SmCP_A: polar antiferroelectric smectic C phase; USmCP: undulated

polar smectic C phase. + means a SHG intensity d_eff> 10^ꢀ6measured

in the crystalline solid phase; (lc) Measurement of the spontaneous

polarization but no SHG studies were used to prove polar properties in

the liquid crystalline state.

b

The inﬂuence of lateral substituents on the mesophase behaviour

of banana-shaped liquid crystals is an object of great interest.

Many of the new ‘banana-phases’, of unusual phase sequences,

and of exciting physical properties have been found on laterally

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6061

View Article Online

concerning the type and position of substituents. In the

following, this chapter will be subdivided. In the ﬁrst part only

small alkyl groups will be attached to different positions of the

central phenyl ring, in the second part more polar substituted

resorcinols are esteriﬁcated to form ﬁve-ring bent-core meso-

gens. All compounds contain alkyloxy chains at the terminal

positions.

4-Methyl-substituted compound 16. In contrast to the

compound 1 (the lateral methyl group is attached at the 2-posi-

tion) the isomeric compound 16 bearing the lateral methyl group

in the 4-position exhibits liquid crystalline properties. On cooling

the isotropic liquid a fan-shaped texture with a lot of small

transition bars can be observed which is transformed to a smooth

one on application of an electric ﬁeld (see Fig. 6a, b). The

polarity of the electric ﬁeld has no inﬂuence on the texture

formed. The current response presents two peaks per half period

that proved to have antiferroelectric behaviour, see Fig. 6c. X-

Ray investigations indicate a smectic phase without in-plane

order. The layer thickness is clearly smaller than the molecular

length, a tilt angle of about 30^ꢁcan be estimated. Summarizing

the experimental results, the mesophase of compound 16 can be

assigned as SmC_sP_Aphase. Since the texture of the switched

states are independent of the polarity of the applied ﬁeld and it is

nearly identical with the original texture, it can be assumed that

the switching is based on the collective rotation of the molecules

around their long axes.

Variation of lateral substituents Z—Part I: Compounds derived

from alkyl-substituted resorcinols

Previous studies have shown that the liquid crystalline behaviour

reported for the laterally non-substituted resorcinol parent

compounds 13 (see Table 3) is lost by the attachment of a methyl

group in position 2 of the central phenyl ring resulting in

compound 1. The question arises what happens if the methyl

group is lengthened to n-propyl or if it is shifted to the positions

4 and 5. Furthermore, the inﬂuence of two methyl groups

attached to the positions 2 and 5 will be reported in this chapter.

5-Methyl-substituted compound 17. The 5-methyl-substituted

compound 17, which is isomeric to the methylresorcinol deriva-

tives 1 and 16 does not show _ꢁliquid crystalline properties. The

melting point amounts to 138 C. The nucleation of a colourful

crystal modiﬁcation can be observed if the isotropic liquid is

slowly cooled down to 131 ^ꢁC in the presence of a sufﬁciently

high electric ﬁeld (about 10 V mm^ꢀ1), see Fig. 7. Because a similar

behaviour has also been observed for the crystalline polar

compound 1, we believed that such ﬁeld-induced formation of

2-Ethyl- and 2-propyl-substituted compounds 14, 15. With the

preparation of the next homologues of compound 1 by length-

ening the lateral 2-methyl group to ethyl (compound 14) and

propyl (compound 15), respectively, materials have been

obtained which exhibit one crystalline modiﬁcation only (melting

points at 146 ^ꢁC and 112 ^ꢁC). Electro-optical and SHG

measurements do not deliver any hint for ferro- or pyroelectric

properties. The disappearance of the ferroelectric properties

caused by this slight structural change is an unexpected result.

Fig. 6 Compound 16: (a) Texture of the SmCP_Aphase on cooling the isotropic liquid at 121 ^ꢁC; (b) after applying an electric ﬁeld (ꢂ13 V mm^ꢀ1); (c)

current response curve using a triangular voltage 13 V mm^ꢀ1, 5 Hz, cell thickness 6 mm, P_S¼ 170 nC cm^ꢀ2

.

6062 | J. Mater. Chem., 2010, 20, 6057–6079

This journal is ª The Royal Society of Chemistry 2010

View Article Online

arcs could be a general hint for polar properties. However no

polarization peak was observed even at 0.1–1 Hz, at 380 V_pp

,

using a 6 mm cell. Furthermore, information suggesting a polar

structure could not be found by SHG measurements.

The small step of the dielectric permittivity at 117 ^ꢁC, see

Fig. 8, indicates the solid I/solid II transition and the higher

permittivity in the solid II phase probably related to the fast

dynamics of the polar end groups. A dielectric relaxation could

not be detected in the investigated frequency range.

2,5-Dimethyl-substituted compounds 18–22. The melting

behaviour of the ﬁve homologues 18–22 is listed in Table 3, their

chain length grows from C8 to C18. Liquid crystalline phases do

not exist. The tendency to form more than one crystalline

modiﬁcation increases with lengthening the terminal alkyloxy

chains.

Fig. 7 Nucleation of ribbons and arcs on cooling the isotropic liquid of

compound 17 in the presence of an electric ﬁeld of 10 V mm^ꢀ1; temper-

ature 131 ^ꢁC.

X-Ray powder diffraction investigations on the hexadecyloxy-

substituted compound 21 clearly show the crystalline nature of

all occurring phases by Bragg reﬂections found in the whole

scattering range, see Fig. 9. Slight changes of the patterns

between 138 and 120 ^ꢁC and at about 110 ^ꢁC are in line with the

phase transitions indi_ꢁcated by calorimetric measurements at

ꢁ

about 127 C and 112 C, all on cooling.

It is interesting that the crystallisation process can be inﬂu-

enced by an electric ﬁeld. To give an example, the tetradecyloxy

derivative 20 preferably forms plate-like crystals on cooling of

the isotropic liquid (Fig. 10a), however, in the presence of an

electric ﬁeld the formation of circular domains, ribbons and

loops was induced as demonstrated in Fig. 10b,c.

In some cases the nucleation of the last mentioned modiﬁca-

tions can be induced by electric ﬁelds at temperatures about 1–

2 K higher than in the ﬁeld-free state. Such a ﬁnding was also

Fig. 8 Static dielectric permittivity of substance 17.

Fig. 9 2D X-ray diffraction patterns for a powder-like sample of compound 21 at (a) 138, (b) 120, (c) 100, and (d) 25 ^ꢁC on cooling (insets: small angle

patterns). (e) The q-scans show the changes on cooling, (f) more detailed for the wide angle region.

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6063

View Article Online

Fig. 10 Compound 20: Cooling the isotropic liquid in an electro-optic cell, thickness 6 mm at 145 ^ꢁC: (a) 0 V, plate-like crystals; (b) and (c) dc ﬁeld,

150 V, plate-like crystals and circular domains, loops and ribbons in different optical areas of the same sample.

reported for compound 1 and can be interpreted by the existence

of polar clusters already in the isotropic liquid.³⁰

on the temperature on cooling and heating can result in different

curves, which can be compared in the Fig. 11a and 11b.

For the dodecyloxy homologue 19 (see Fig. 11a), the SHG

intensity increases suddenly on cooling the isotropic liquid at the

crystallization point and slowly grows with decreasing temper-

ature to a maximum value. The curve takes a similar trace,

however the inverse direction, on heating the crystalline state,

breaks down during the melting process. The temperature

behaviour of the hexadecyloxy-substituted compound 21 is

comparable to that of compound 19, see Fig. ESI 3.†

Although the crystallization could be inﬂuenced in a similar

way as reported for the crystalline polar parent compound 1,

electro-optical switching was not found on the compounds of this

series independent of the frequency and power of the triangular-

wave electric ﬁeld.

However, SHG measurements give clear evidence for a polar

structure in the crystalline state of all homologues 18–22. The

polar packing is formed spontaneously on cooling of the

isotropic liquid without ﬁeld. The dependence of the SHG signal

On cooling the isotropic liquid of the octyloxy derivative 18,

the SHG signal makes a jump at the crystallization temperature,

after that the intensity remains nearly constant, see Fig. 11b. On

heating, however, the curve clearly takes a maximum before it

goes down to zero during the melting. This could mean that some

degrees below the melting temperature a relatively soft crystalline

state exists which allows more molecules or molecular clusters to

form by polar packing.

Looking at the dependence of the SHG intensity as a func-

tion of the chain length of the terminal alkyloxy groups of the

compounds 19–22, we can notice that the homologues with the

length 12–18 have similar values of d_eff(Fig. 12). The homo-

logue 18 having the shortest chain length of 8 carbon atoms

falls out of the sequence. It can be caused by a poor orientation

in the cells. However, compared to the CrIII and CrII phases of

the homologues with the length 12–18, the SHG signal can not

be switched by the inversion of the electric ﬁeld, which indicates

Fig. 11 Dependence of the SHG signal on the temperature (a)

compound 19; (b) compound 18.

Fig. 12 Dependence of the maximal SHG signal intensity on the length

of the hydrocarbon chain of the homologous compounds 18–22.

6064 | J. Mater. Chem., 2010, 20, 6057–6079

This journal is ª The Royal Society of Chemistry 2010

View Article Online

days later. A clear step of 3₀at about 135 ^ꢁC indicates quite better

a solid/solid transition, see Fig. 13. Dielectric relaxation as an

indication of a dynamics at molecular level could not be observed

in our frequency window.

Summarizing the results of this series, we can conclude, that

compound 18 exhibits pyroelectric properties and the

compounds 19–22 show ferroelectric behaviour in different

crystalline modiﬁcations.

Variation of lateral substituents—Part II: Compounds derived

from non-alkyl-substituted resorcinols

5-Methoxy-substituted compound 23. The attachment of

a methoxy group to the top of the bent mesogenic core that

means in position 5 of the central phenyl ring, results in

compound 23. Although the methoxy group is larger than

a methyl group, the compound exhibits liquid crystalline prop-

erties in contrast to 21. X-Ray studies on a partially aligned

sample reveal a simple layer structure (Fig. 14a). From the c-

scan at 115 ^ꢁC a tilt angle of 27^ꢁwas derived (Fig. 14b). Together

Fig. 13 Static dielectric permittivity 3₀of substance 21 measured on

cooling and heating.

that the phase is not switchable (similar to the CrI phases of

the other homologues). In contrast, the CrII and CrIII phases

of the compounds 19–22 show a clear switching of the SHG

signal, but the optical textures of the states of opposite polarity

are the same.

ꢁ

with the layer thickness d ¼ 52 A an effective molecular length of

ꢁ

Supercooling of the isotropic liquid–crystalline transition of

about 5 ^ꢁC was observed during the dielectric measurements. The

further cooling results at about 132 ^ꢁC in a small decrease of the

dielectric permittivity which may be related to a CrII–CrI tran-

sition. The measurements during heating were performed two

about 58 A could be calculated using the equation L ¼ d / coss.

This effective molecular length corresponds to the average length

measured at models for two molecular conformations with

extreme orientations of the nearly stretched alkyloxy chains at

both terminal ends (Fig. 15).

Fig. 14 (a) 2D X-ray diffraction pattern for a partially surface-aligned sample of compound 23 at 115 ^ꢁC on cooling; (b) c-scan for the outer diffuse

ꢁ

scattering (relative intensity I_rel¼ I₁₁₅/ I_{118 C}) showing the curves for a Gaussian ﬁt to determine the maxima of the intensity distribution.

C

Fig. 15 Molecular length of compound 23 estimated for two extreme chain conformations (Chem3D, Cambridge Soft.Com, 2000.

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6065

View Article Online

Table 4 The inﬂuence of polar substituents attached to different posi-

tions Z of the central phenyl ring on the phase behaviour

No

23

n

Z

Phase behaviour^a

SHG^b

(lc)

16

12

5-CH₃O

5-Cl

Cr 109 [53.8]

SmCP_A120 [24.5] Is

CrI 90 [34.2]

24

+

CrII 108 [23.4]

CrIII 138 [40.6] Is

CrI 99.0 [40.4]

CrII 112.4 [28.7]

CrIII 135.8 [43.0] Is

CrI 106 [44.8]

CrII 117 [33.5]

CrIII 134 [47.6] Is

CrI 110.0 [48.8]

CrII 118.6 [36.9]

CrIII 130.2 [44.9] Is

CrI 116 [14.0]

25

26

27

28

29

30

31

14

16

18

12

14

16

18

5-Cl

+

Fig. 16 Compound 23: current response at 111 ^ꢁC, 12 V mm^ꢀ1

Frequency 2 Hz, P_S¼ 240 nC cm^ꢀ2, cell gap 6 mm.

,

5-Cl

+

By applying a dc ﬁeld, the fan-shaped texture characterized by

many perpendicular stripes is changed to a more smooth texture.

The optical change is comparable to that of compound 16, see

Fig. 6. The switching process is independent of the polarity of the

electric ﬁeld. Two peaks per half period have been found as

shown in Fig. 16. The experimental results allow an assignment

of the mesophase as a SmC_sP_Aphase.

5-Cl

+

5-CN

—

CrII 143 [7.6]

CrIII 173 [48.6] Is

CrI 126.8 [18.7]

CrII 139.5 [5.1]

CrIII 167.1 [51.0] Is

CrI 129 [27.9]

5-Chloro-substituted compounds 24–27. The four homologues

24–27 derived from 5-chlororesorcinol are not able to form liquid

crystalline phases but exhibit crystal trimorphism, see Table 4.

With increasing length of the hydrocarbon chains the CrI–CrII

and the CrII–CrIII transition temperatures increase sligh_ꢁtly

whereas the CrIII–isotropic temperatures decrease from 138 C

to 130 ^ꢁC. The enthalpies are typical for transitions between

crystalline or higher ordered states. To prove that the materials

really exhibit different crystalline modiﬁcations but not higher

ordered smectic phases, extensive X-ray studies have been per-

formed using the hexadecyloxy-substituted compound 26.

X-Ray diffraction patterns of the three crystalline phases of

the hexadecyloxy derivative 26 on cooling are shown in Fig. 17.

All of them exhibit Bragg reﬂections over the whole recorded

scattering range as typical for the crystalline state. But the

difference between the high-temperature and the two low-

temperature phases is obvious: CrIII causes considerable diffuse

scattering due to a rather high degree of disorder within the

CrII 141 [9.6]

CrIII 167 [54.4] Is

CrI 130 [33.3]

CrII 137 [6.9]

CrIII 163 [51.1] Is

a

b

SmCP_A: polar antiferroelectric smectic C phase. : + means a SHG

intensity d_eff> 10^ꢀ6measured in the crystalline solid phase; (lc)

measurement of the spontaneous polarization but no SHG studies were

used to prove polar properties in the liquid crystalline state.

structure, whereas the patterns of CrII and CrI are those of

conventional crystalline phases.

To get further insight into the nature of the high-temperature

phase, 2D X-ray patterns of a surface-aligned sample have been

recorded. As already found in the powder patterns, the CrIII

phase displays a lot of sharp reﬂections in the small to wide angle

range, se_ꢁe Fig. 18a. The diffuse scattering with its maximum at

ꢁ

2q ¼ 9.3 (d ¼ 4.8 A) may be explained by ‘‘partially molten’’

Fig. 17 Guinier ﬁlm patterns of a powder-like sample of compound 26 on cooling showing the X-ray diffraction of three crystalline phases: 130–110 ^ꢁC

CrIII, 105 ^ꢁC CrII, RT (CrI at room temperature).

6066 | J. Mater. Chem., 2010, 20, 6057–6079

This journal is ª The Royal Society of Chemistry 2010

View Article Online

Fig. 18 X-Ray patterns of an aligned sample of compound 26: (a) CrIII

phase at 132 ^ꢁC; (b) CrI phase at 80 ^ꢁC.

Fig. 20 Experimental data and the ﬁtted curve of the complex dielectric

permittivity in the CrI phase of compound 24.

chains. The low to medium angle reﬂections on the meridian of

the pattern are equidistant (2nd to 13th order reﬂections

measured) and point to a layered structure with a spacing of

5.9 nm. Most of the non-layer reﬂections are smeared out

parallel to the meridian of the pattern. This feature suggests

a structure with a comparatively high order within the layers,

but weaker correlations between adjacent layers. The two

streaks next and parallel to the meridian have a distance of 2q ¼

6.14^ꢁ(d ¼ 7.2 A) to the meridian on the equator. Since the

ꢁ

intensity maxima on these streaks are situated on a 2D rect-

angular reciprocal lattice and the maximum of the outer-most

ꢁ

streaks lies on the equator at 2q ¼ 12.3 (d ¼ 3.6 A) and can be

assigned as second order of the ﬁrst streaks, the second edge of

the real lattice is supposed to be perpendicular to the layer

ꢁ

normal with 7.2 A length. Because the maxima of the other two

Fig. 21 Relaxation frequencies versus temperature in the CrI phase of

compound 24.

streaks are also found on the equator, the third edge of the 3D

unit cell is likely to be normal to the other two and an ortho-

rhombic lattice results for CrIII. This is in agreement with the

layer spacing d ¼ 5.9 nm being nearly identical with the esti-

mated length of the molecules L z 6.0 nm.

In the following, dielectric properties of the dodecyloxy-

substituted compound 24 are described in detail.

Comparing the patterns at 132 ^ꢁC (Fig. 18a) and 80 ^ꢁC

(Fig. 18b) the higher order of the low-temperature CrI phase

predicted by the powder patterns is clearly proven. The latter

shows a pattern with a lot of sharp reﬂections in the small angle

and in the wide angle range without considerable diffuse scat-

tering, i.e. the chains are also long-range ordered as typical for

a highly ordered crystalline phase.

Fig. 19 shows the dielectric permittivities obtained during slow

cooling. In the low-temperature CrI phase a dielectric relaxation

process in the frequency range of 0.1 MHz was detected. The

intensity of this process is very weak as demonstrated in Fig. 20.

Fig. 21 presents the relaxation time as a function of the

temperature. With increasing temperature the motion becomes

faster and is out of our experimental range in the CrII phase. The

static dielectric permittivity increases stepwise at the transition

Fig. 19 Static dielectric permittivities of sample 24 versus temperature

(closed squares) and the high frequency limit (open circles) in the different

crystalline phases.

Fig. 22 Compound 24: current response at 129 ^ꢁC, EHC, cell gap 6mm,

f ¼ 1 Hz, V_pp¼ 250 V (triangular wave voltage).

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6067

View Article Online

CrII/CrIII of 0.4 units (see Fig. 19). This means that the intensity

for the fast dynamics considerably increases. Therefore, one can

conclude that in the high temperature phase bigger parts of the

bent-core molecule are movable. It has to be also noted that this

dynamic is not forced by the weak electrical ﬁeld used for

dielectric measurements.

Measurements of the electro-optical behaviour have been

performed on compound 24. Fig. 22 shows the electric response

in the CrIII phase. A clear switching current corresponding to

P_S¼ 400 nC cm^ꢀ2was recorded. The proﬁle of the curve of the

current response is unusual because the maximum of the current

response is directly positioned with the peak top of the applied

voltage. The peak disappears in the isotropic phase. The

switching process can be clearly observed with the polarizing

microscope, positive and negative polarity of the ﬁeld results in

the same texture. It should be mentioned, however, that the

optically visible switching behaves slower and slower on

decreasing the temperature, it can be observed to about 10–15 K

below the CrIII–isotropic transition.

Fig. 24 Hysteresis effect of the SHG signal in dependence on the applied

dc ﬁeld in the ferroelectric state of the CrIII phase of compound 24.

To obtain more information about the polar properties of

these crystalline phases, SHG measurements have been per-

formed starting with compound 24. On cooling the sample from

the isotropic liquid the crystalline phase CrIII developed a weak

SHG signal. Near to the threshold voltage the switching can be

observed by ﬂickering/ﬂuctuation with the frequency of the ac

ﬁeld. Looking at the SHG signal, a strongly increasing intensity

can be detected above the threshold voltage. Switching off the

ﬁeld the SHG signal remains and the optical texture remains

unchanged, too. Fig. 23a shows the dependence of the SHG

signal of the applied ac ﬁeld. If the temperature is increased

above the transition temperature CrIII–isotropic liquid, the

polar state is lost as demonstrated in Fig. 23b.

To obtain further information, a square-wave ﬁeld was applied

to compound 25 to demonstrate the optical behaviour together

with the current response. Fig. 25 shows the change of the texture

after switching from ꢀ12 V mm^ꢀ1(Fig. 25a) to +12 V mm^ꢀ1

(Fig. 25c). The cohesive point can be observed at +4 V mm^ꢀ1

(Fig. 25b). That means the hysteresis effect typical for ferro-

electric materials and found by SHG measurements (see Fig. 24)

could also be proved by electro-optical studies. The corres-

ponding current response is shown in Fig. 25d, the peak disap-

pears below the threshold voltage or on heating to the isotropic

liquid phase.

As already discussed for the 2,5-dimethyl-substituted

compounds 18–22, the SHG signals can differently behave in

dependence on the temperature. On cooling the sample, the SHG

signal can suddenly appear and remains nearly constant, as

observed for compounds 25 and 27. It can slowly increase with

decreasing temperature, as for compound 24. For the hexa-

decyloxy-substituted homologue 26, the SHG signal appears at

the transition to the crystalline state, but decreases with

decreasing temperature, see Fig. 26. The decrease could be due to

Applying of dc ﬁelds of different amplitudes allows one to

observe hysteresis of the SHG signal during the switching

between states of positive and negative polarity, which can be

seen in the following Fig. 24. Going from the positive ﬁeld to the

negative one (black arrow) or reverse (red arrows) the switching

between the two polar states (which is characterized by a sudden

decrease of the SHG signal) occurs below and above 0 V,

respectively. Switching off the ﬁeld the SHG signal remains

which is characteristic for ferroelectric materials.

Fig. 23 SHG signal after switching into the ferroelectric state, compound 24: (a) dependence of the SHG signal on the applied voltage (square wave, f ¼

10 Hz); (b) dependence of the SHG signal on the temperature on heating the sample above the CrIII–isotropic transition temperature.

6068 | J. Mater. Chem., 2010, 20, 6057–6079

This journal is ª The Royal Society of Chemistry 2010

View Article Online

Fig. 25 Compound 25: current response using a square-wave voltage at 125 ^ꢁC, EHC, cell gap 10 mm, R ¼ 50 K; f ¼ 0.2 Hz, E_pp¼ 24 V mm^ꢀ1

.

Fig. 26 Decreasing SHG intensity on cooling compound 26.

Fig. 27 Dependence of the maximal intensity of the SHG in ﬁeld-aligned

cells of the chloro-substituted compounds 24–27.

a kinetic reason, or could be caused by a reduction of polar order

at the transition to the other crystalline modiﬁcation.

difﬁcult to ﬁnd a current response, which can be observed using

a square-wave technique at sufﬁciently high temperatures.

All three crystalline phases CrIII–CrI of the compounds 24–27

show a comparable intensity of SHG, which occurs spontane-

ously on cooling the isotropic phase. Treating the cells with an

electric ﬁeld, we could improve the alignment of the crystals.

Now, the intensity reaches a plateau a few degrees below the

recrystallization point and shows relatively weak temperature

dependence. This allows us to compare the maximal SHG signals

in compounds with different chain lengths (Fig. 27). There is not

much difference between the CrII and CrIII phases: they both

show a spontaneous generation of SH, they show electro-optical

switching which is reﬂected by hysteresis behaviour of the SHG

signal as a function of the applied electric voltage (see Table 5).

The switching is quite slow with characteristic times ranging

from 0.5 s (CrIII) to 30 s (low temperature CrII). This is why it is

5-Cyano-substituted compounds 28–31. DSC measurements

and microscopic investigations prove the existence of three

crystalline modiﬁcations for all homologues 28–31 under study,

see Table 4. Cooling the isotropic liquid the high temperature

crystalline phase CrIII grows as needles and bundles resulting in

a non-speciﬁc or grainy texture. Fig. ESI 4† shows the micro-

graph of the CrIII phase of compound 31. The application of an

electric ﬁeld during the cooling process leads to the formation of

chiral areas in the sample which can be seen in the background of

the grainy texture, see Fig. ESI 5.†

The investigation of the crystalline modiﬁcations is of interest

because there are clear differences in their behaviour. The XRD

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6069

View Article Online

Fig. 28 Guinier ﬁlm patterns of a powder-like sample of compound 30 on cooling showing the X-ray diffraction of two high temperature (at 160, 150,

140 ^ꢁC and 130, 120 ^ꢁC, respectively) and two crystalline phases (at 110 ^ꢁC and room temperature, respectively).

Fig. 29 2D X-ray diffraction patterns for a powder-like sample of compound 30 on cooling at (a) 160 ^ꢁC, (b) 120 ^ꢁC, and (c) 90 ^ꢁC along with (d) theta-

scans for several temperatures and (e) plot of the layer spacing depending on the temperature.

powder patterns recorded for compound 30 on cooling (Fig. 28–

29) can be assigned to three solid-like phases found in the DSC

measurements and the room temperature crystalline phase

(Guinier powder pattern in Fig. 28). All phases show 1st to 6th

order reﬂections at small and medium 2q angles indicative of

pronounced layer structures. The layer spacing changes from

5.0 nm at 160 ^ꢁC to 4.6 nm at 30 ^ꢁC (Fig. 29e). The high

temperature phase exh_ꢁibits a diffuse outer scattering with its

The distribution of the diffuse scattering changes at the tran-

sition to the next phase on cooling and shows two maxima at

120 ^ꢁC (Fig. 29b,d) which could be explained by an enhanced

order within the layers and/or by stronger correlations between

the layers.

At the next phase transition this scattering condenses to more

or less broadened reﬂections (Fig. 29c,d) as found for crystalline

phases with a high degree of disorder and/or a very small size of

the domains or crystallites. Unfortunately no alignment could be

achieved, so further conclusions on the molecular packing in the

three phases would be highly speculative.

ꢁ

maximum at 2q ¼ 20.0 (d ¼ 4.4 A) and two broadened reﬂec-

ꢁ

tions at 2q ¼ 16.0^ꢁ(d ¼ 5.5 A) and 2q ¼ 24.5 (d ¼ 3.6 A),

respectively. These reﬂections are characteristic for layer struc-

tures with a long-range order within the layers but no or only

weak correlations between the molecules of adjacent layers as

described for hexatic SmB, I, or F phases of calamitic

compounds. The additional diffuse scattering indicates a liquid-

like lateral order of parts of the structure, most probably of the

terminal chains as already assumed for compound 26.

ꢁ

Although the crystalline high-temperature phase could be

discussed as soft crystals, the inﬂuence of an electric ﬁeld on the

CrIII phase is non-speciﬁc. Applying >150 V_pp(cell thickness

6 mm) the birefringence is a little increased and a slight optical

switching can be observed. That is not sufﬁcient to serve as

evidence for a polar switching because the current response is not

6070 | J. Mater. Chem., 2010, 20, 6057–6079

This journal is ª The Royal Society of Chemistry 2010

View Article Online

sufﬁciently characteristic. SHG measurements do not deliver

clear signals for polar behaviour, the measured intensity is very

low and furthermore ﬂuctuating. Therefore we have to conclude

that the 5-cyano-substituted homologues do not form polar

crystalline structures.

Increasing the mesogenic core by insertion of C]C bonds:

compounds 32, 33

Starting from the 2-methyl-substituted compound 1 and the 5-

chloro-substituted compound 26 for which polar properties in

a crystalline phase was proved the question arises what happens

if the mesogenic core is a little enlarged by insertion of a C]C

bond within each leg of the bent ﬁve-ring compound. The effect

should be a little smaller than that caused by an increase of the

number of phenyl rings from 5 to 6. 2-Methylresorcinol and 5-

chlororesorcinol have been esteriﬁcated with 4-(4-n-hexadecy-

loxyphenoxycarbonyl)cinnamic acid to obtain the compounds 32

and 33. Both compounds exhibit enantiotropic mesophases up to

relatively high temperature as listed in Table 6.

Fig. 30 Texture of the columnar phase of compound 32 on cooling the

isotropic liquid.

phase, respectively, and an outer diffuse scattering at 2q ¼ 18.6^ꢁ

ꢁ

(d ¼ 4.8 A) and 2q ¼ 18.8 (d ¼ 4.7 A), respectively. A close

inspection of the small angle region at the far limit of our

measuring range reveals the most striking difference in the

patterns: the high temperature phase shows an additional reﬂec-

tion on the meridian of the pattern at a d value for approximately

twice the layer spacing indicating a kind of superstructure.

Unfortunately, an accurate measurement and hence a further

interpretation of this reﬂection is not possible with our experi-

mental equipment. There are no other hints to a columnar

structure in the XRD patterns. At the transition to the low

temperature mesophase, the reﬂection disappears on cooling. The

c-scans for the diffuse outer scattering exhibit maxima, from

which average tilt angles for the molecules of 23 and 27^ꢁcan be

2-Methyl-1,3-phenylene bis[4-(4-n-hexadecyloxyphenoxycar-

bonyl)cinnamate] 32. Compound 32 derived from 2-methyl-

resorcinol exhibits two mesophases. The transition enthalpy

between the two mesophases is very low. On cooling the isotropic

liquid a texture appears which gives a strong hint for a columnar

phase, see Fig. 30. At the transition to the low-temperature phase

the texture is transformed to a grainy one.

The 2D XRD patterns of surface aligned samples for the two

liquid crystalline phases of compound 32 closely resemble each

other on ﬁrst sight, and they show layer reﬂections up to the 5th

ꢁ

calculated for 180 and 160 C, respectively. The effective mole-

cular lengths calculated from these tilt angles and the layer spac-

ꢁ

orde_ꢁr on the meridian yielding layer spacings of 55.4 A (180/

ings according to L ¼ d / coss ¼ 60.2 and 61.5 A, respectively, are

ꢁ

178 C) and 54.8 A (160 C) for the high and low temperature

in the range measured at molecular models for two extreme chain

conformations (Fig. 33). For comparison, Fig. 31c,f shows the 2D

XRD pattern of the crystalline phase formed on further cooling.

Electro-optical studies demonstrate a different behaviour of

both phases. The texture of the high temperature phase remains

nearly unchanged on applying an electric ﬁeld independent of the

polarity of the ﬁeld. One polarisation peak per half period in the

current response shows ferroelectric phase behaviour. The

process can be explained by a rotation of the molecules around

their long axis. The high temperature phase can be deﬁned to be

a Col_Fphase.

Table 5 Second harmonic generation in crystalline phases of the chloro-

substituted compounds 24–27

CrI

CrII

CrIII

SHG

No

SHG

Switching

SHG

Switching

24

25

26

27

+

—

+

The low-temperature phase exhibits a layer structure. The

electro-optical behaviour is shown in Fig. 32. By application of

an electric ﬁeld the grainy picture is transformed in a fan-shaped

texture, which is identical for both polarities of the electric ﬁeld,

see Fig. 32a–c. Two peaks per half period prove together with the

optical behaviour a racemic SmC_sP_Aphase, Fig. 32d.

In a small transition range between the ferroelectric columnar

and the antiferroelectric smectic C phase the current response

shows three peaks per half period, which means a coexistence of

both phases.

Table 6 Mesophase behaviour of the cinnamic esters 32 and 33

No

32

Z

Mesophase behaviour

2-CH₃

5-Cl

Cr 153.7 [27.6] SmCP_A180.5 [0.4]

Col_F188.3 [23.6] Is

5-Chloro-1,3-phenylene

bis[4-(4-n-hexadecyloxyphenox-

ycarbonyl)cinnamate] 33. The 5-chlororesorcinol derivative 33

exhibits two mesophases. The phase transition is characterized

by a relatively high enthalpy of about 12 kJ mol^ꢀ1. Cooling the

33

Cr 107.2 [12.6] SmXP 137.7 [11.9]

SmCP_A172.4 [27.0] Is

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6071

View Article Online

Fig. 31 2D X-ray diffraction patterns for a surface-aligned sample of compound 32 on cooling: (a,d) at 180/178 ^ꢁC; (b,e) 160 ^ꢁC; and (c,f) 130 ^ꢁC; (a,b,c)

wide-angle patterns, (d,e,f) small-angle patterns along with the theta-scans for the wide-angle (g) and small-angle regions (h) and the c-scans for the

outer diffuse scattering in the liquid crystalline phases (black arrow in (d) and (h) pointing to the additional small-angle reﬂection in the high temperature

phase).

isotropic liquid textures can be observed typical for SmCP pha-

ses, the growing process is shown in Fig. 34.

derived from the distribution of the outer diffuse scattering with

ꢁ

its maximum in 2q ¼ 19.3 (d ¼ 4.6 A) (Fig. 35). The effective

ꢁ

From the XRD studies for the SmCP phase at 160 C a layer

ꢁ

molecular length L ¼ d / coss ¼ 61.3 A is almost the same as that

spacing of 52.0 A can be calculated and an average tilt of the

molecules with respect to the layer normal of about 32^ꢁcan be

for compound 32. The diffuse scattering changes at the transition

to the lower temperature mesophase and shows two maxima in

6072 | J. Mater. Chem., 2010, 20, 6057–6079

This journal is ª The Royal Society of Chemistry 2010

View Article Online

Fig. 32 Switching behaviour of the SmC_sP_Aphase of compound 32. (a,c): texture on applying an electric ﬁeld of ꢂ 27 V; (b) without electric ﬁeld, (d)

current response at 145 ^ꢁC, cell thickness 5 mm, 10 Hz, 76 V_pp, P_S¼ 260 nC cm^ꢀ2

.

Fig. 33 Molecular length of compounds 32 (above) and 33 (below) estimated for two extreme chain conformations (measurement as in Fig. 15 for

compound 23, Chem3D, Cambridge Soft.Com, 2000; energy minimized by MM2 starting from all-trans conformations of the chains).

160 ^ꢁC gives a hint to a higher degree of short-range order within

the layers. Using all X-ray data we could not assign the type of

mesophase beyond all doubt, therefore we prefer to say SmX.

Electro-optical studies prove a polar behaviour for both

mesophases. Measurements on the high-temperature phase of

compound 33 provide evidence of the occurrence of two separate

repolarisation peaks in each half period of the applied voltage

ﬁeld, which is characteristic for antiferroelectric switching

(SmCP_A). The application of an electric ﬁeld leads to the

formation of a smooth fan-like texture. After applying the ﬁeld,

all extinction crosses are aligned along the directions of the

crossed polarizers and their positions do not change on applying,

Fig. 34 Optical micrographs of compound 33 with growing SmCP

fragments from the isotropic liquid. EHC cells 6 mm polyimide-coated,

U ¼ 0 V.

reversal or on terminating the applied ﬁeld, see Fig. 36a–c. This

ﬁnding suggests that the antiferroelectric ground state as well as

the switched ferroelectric states have an anticlinic tilt and the

switching corresponds to a transition from a SmC_aP_Astate to

a SmC_aP_Fstate and that the chirality of the layers switches by

a collective rotation of the molecules around their long axes on

reversing the applied ﬁeld.

ꢁ

2q ¼ 19.1 (d ¼ 4.6 A) and 2q ¼ 23.9 (d ¼ 3.7 A), while the layer

ꢁ

spacing slightly increases to 54.1 A at 130 C. These two maxima

with somewhat smaller full width at half maximum as the one at

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6073

View Article Online

Fig. 35 2D X-ray diffraction patterns for a surface-aligned sample of compound 33 on cooling at (a) 160 and (b) 130 ^ꢁC, insets: small-angle patterns

along with (c) the theta-scans and (d) the c-scans for the outer diffuse scattering in the liquid crystalline phases (layer reﬂection for comparison).

The electro-optical behaviour of the low-temperature SmXP

phase seems to be nearly unchanged in comparison to the SmCP

phase. Two repolarisation peaks per half period exist, and the

extinction crosses remain in the same position after switching the

ﬁeld, as described above. The texture is identical for both signs of

the electric ﬁeld, see Fig. 36d–e. There are two differences,

however. The texture of the SmX phase can present additional

birefringent stripes, especially in the case of alternating current,

running parallel to the smectic layers. Furthermore, optical and

polar answers under an electric ﬁeld could be observed only

Fig. 36 Electro-optical measurements on compound 33, 6 mm polyimid-coated cell. (a–c): SmCP phase, 146 ^ꢁC: (a) 0 V, b) ꢂ 24 V, c) ac current

response, 20 Hz, 230 V_pp, P_S410 nC cm^ꢀ2; (d–f) SmXP phase, 130 ^ꢁC: (a) 0V, (b) ꢂ112 V, c) ac current response, 30 Hz, 238 V_pp, P_S280 nC cm^ꢀ2

.

6074 | J. Mater. Chem., 2010, 20, 6057–6079

This journal is ª The Royal Society of Chemistry 2010

View Article Online

Increasing the mesogenic core by substitution of the central 2-

methyl-1,3-phenylene unit by a 1-methyl-2,7-naphthylene moiety;

compound 34

Another possibility to enlarge the mesogenic core slightly, but to

retain the number of aromatic rings constantly at ﬁve, is the

insertion of a naphthalene unit in the central position. Several

bent-core naphthalene derivatives exhibiting mesophases have

been reported in the literature.^26–28,38

In relation to our intention the question arises whether the

liquid crystalline properties disappear by the attachment of

a lateral methyl group to position 1 of the naphthalene unit in

a similar way as found before for the resorcinol derivatives

(compare compounds 1 and 13 in Table 3).

Fig. 37 SHG measurements on compound 33, SHG intensity in

dependence on the applied electric ﬁeld.

down to 125 ^ꢁC, which means to about 10 K below the transition

temperature SmXP/SmCP, but not in the whole SmX existence

range (107–138 ^ꢁC).

Compound 34 exhibits two non-polar crystalline phases beside

an enantiotropic mesophase, which will be characterized in the

following. Only a weak partial surface-alignment of a sample

could be achieved for the X-ray measurements on compound 34

(Fig. 38). The high temperature mesophase exhibits a layer

structure phase as proven by the diffuse outer scattering with

By SHG measurements a polar order of the SmXP phase could

be clearly proved, too. Applying a relatively low electric ﬁeld,

a SHG signal could be observed. The threshold voltage is shown

in Fig. 37.

Fig. 38 2D X-ray diffraction patterns for a weakly surface-aligned sample of compound 34 at (a) 185 ^ꢁC (inset: small-angle region) and (b) 160 ^ꢁC,

along with (c) the theta-scans for the wide-angle and (d) the small angle regions (all on cooling). The arrow in (d) points to one of the weak additional

reﬂections indicating the modulation/undulation of the layer structure in the high temperature phase. The small-angle scan at 180 ^ꢁC (red line) shows

peaks of this phase and the phase below 180 ^ꢁC because of a small temperature gradient within the sample.

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6075

View Article Online

Fig. 39 Switching behaviour of compound 34 at 193 ^ꢁC, cell thickness 6mm. (a) ꢂ 22 V mm^ꢀ1, (b) 0 V mm^ꢀ1; (c) current response using a triangle voltage

of 21 V mm^ꢀ1, frequency 7 Hz, P_S¼ 200 nC cm^ꢀ2; (d) ﬁeld-induced texture at 200.5 ^ꢁC, 17 V mm^ꢀ1

.

ꢁ

a maximum at 2q ¼ 18.4 (d ¼ 4.8 A) and a strong layer reﬂection

mechanism on applying an electric ﬁeld. Single-component

organic ferroelectrics have been observed more or less by chance.

Up to now, there was no general conception to develop such new

materials.

ꢁ

with weak 2nd and 3rd order yielding a layer spacing of 55.9 A at

185 ^ꢁC. Calculation of the length of the molecules estimated for

two extreme chain conformations is shown in Fig. ESI 7.† Some

very weak additional small-angle reﬂections speak for a modu-

lated or undulated layer structure with a rather long period

Working in the ﬁeld of bent-core mesogens, recently we could

observe ferroelectric behaviour in the crystalline phase of

a homologous series of compound 1, which do not form liquid

crystalline phases.^29,30In the present paper, this result was used as

a starting point to search for a general design of polar single-

component organics on the base of non-liquid crystalline bent-

core molecules. As sketched in Fig. 1, different parts of the

bent-core molecules have been varied. It could be shown that—to

create polar crystalline phases—the chemical structure shown in

Fig. 40 proved to be successful with a high probability.

Because the chemical structure corresponds to the classical

conception of banana-shaped liquid crystals, the problem was to

ﬁnd non-liquid crystalline materials. Using four carboxylic

groups between the ﬁve phenyl rings, any change of the direction

of the ester groups results in materials exhibiting mesophases, in

some cases up to very high temperatures (compounds 9–12).

Furthermore, the slight enlargement of the bent aromatic core by

ꢁ

(at least 120 A, the resolution of our SAXD patterns is too low

for an exact calculation). There is a sharp phase tra_ꢁnsition to

a crystalline phase on cooling the sample below 180 C, which

probably remains layer-like showing 1st to 5th order reﬂections

with a maximum on the meridian of the pattern, the outer diffuse

scattering condensed to Bragg reﬂections, as typical for crystals

(Fig. 38b–d).

Electro-optical measurements prove the polar character of the

mesophase. The texture is independent of the polarity of the

electric ﬁeld (see Fig. 39a), however they relax after switching off

the ﬁeld (see Fig. 39b). Because the main peak of the current

response shows a small shoulder, antiferroelectric switching can

be assumed, see Fig. 39c. The spontaneous polarization amounts

to 200 nC cm^ꢀ2

.

Applying an electric ﬁeld (dc, 17 V mm^ꢀ1) to the isotropic phase

about 1 K above the clearing point, the texture of the polar

mesophase was induced, see Fig. 39d. Such ﬁeld-induced meso-

phase formation has been reported for banana-shaped liquid

crystals in few cases up to now.^39,40The results are in agreement

with the assignment of the mesophase as an antiferroelectric

USmCP phase.

Conclusion

Fig. 40 Chemical structure of bent-core compounds (with selected

lateral substituents Z) which are able to form solid single-component

organic ferroelectrics and pyroelectrics.

As shown in the introduction, there are different types of organic

ferroelectrics depending on the number of components and the

6076 | J. Mater. Chem., 2010, 20, 6057–6079

This journal is ª The Royal Society of Chemistry 2010

View Article Online

insertion of C]C-units and the use of the 1-methyl-2,7-naph-

thylene moiety in the central part gives liquid crystals, too

(compounds 32–34). This means the chemical structure should

correspond to that shown in Fig. 40. The terminal groups have

been varied (see Table 1) however only saturated and unsatu-

rated hydrocarbon chains connected by ether oxygen resulted

(in the case of 2-methylresorcinol derivatives) in ferroelectric

solids (compounds 1, 4). More game for variation is possible by

the use of different substituted resorcinols in the central part of

the bent molecules. This way, the inﬂuence of the number and

position of lateral substituents Z has been investigated. As

known before, a substitution by Z ¼ 2-NO₂, 2-Br, 4-Cl, 4-Br,

5-F, 4,6-Cl₂) yields liquid crystalline materials.³⁰Now we have

learnt that 4-CH₃(16) and 5-CH₃O groups (23) also leads to

banana-phases. This is surprising because the 5-methyl-

substituted derivatives do not show mesophases, and the

methoxy group is larger than the methyl group. This means not

only the size but also electronic properties of the lateral groups

are of inﬂuence. Looking at the alkyl-substituted resorcinol

derivatives (see Table 3), the enlargement of the lateral methyl

group attached to the obtuse angle of the bent molecules to ethyl

and propyl, respectively, results in a loss of polar properties (14,

15). Shifting the lateral methyl group to the top of the bent

molecule, polar behaviour could not be found (17). A combi-

nation of both positions that means the methyl groups in the 2-

and 5-position results in a series of solid single-component

ferroelectrics (compounds 19–22) or pyroelectric (compound 18).

The substitution of the central phenyl unit with halogen atoms is

also of interest. While the compounds with Z ¼ 5-F, 4-Cl, 4,6-

dichloro form banana-phases, the 5-chloro-substituted

compounds do not show liquid crystalline behaviour, but

ferroelectric properties could be proven in the crystalline solid

phases of the compounds 24–27. In contrast to the 5-chloro-

substituted compounds, the 5-cyanoresorcinol esters 28–31 do

not form mesophases, however, polar properties could not be

found unfortunately. Cyano-substituted aromatic molecules

have a strong tendency to compensate the dipole moments by

antiparallel orientation. Such packing would prevent the

formation of polar domains, necessary for macroscopic polar

properties.

X-ray studies give evidence for the crystalline-solid state, which

is important for the conclusion that we have polar properties in

the solid state and not in a higher ordered liquid crystal.

Furthermore, crystalline modiﬁcations existing in several

compounds under study could be characterized and the degree of

order with decreasing temperature has been shown. At the

beginning of our work, we believed that crystalline poly-

morphism could be helpful for the formation of polar solids as

found for the 2-methyl- and 5-chloro-substituted series. But we

have learnt, there is no correlation. For instance, all 5-cyano-

substituted compounds 28–31 exhibit crystalline trimorphism,

proven by DSC and X-ray measurements, but they do not show

ferro- or pyroelectric properties. In contrast, the ﬁrst members

18, 19 of the 2,5-dimethyl-substituted series have one crystalline

phase only (see Table 3), but show pyroelectric and/or ferro-

electric properties. Furthermore, no relationships could be

observed between the microscopic images of the crystalline

modiﬁcations formed on cooling the isotropic liquid and polar

behaviour. Our X-ray studies do not allow conclusions con-

cerning polar properties of the crystalline materials on the base of

the diffraction pattern because the exact space groups are

unknown.

Dielectric measurements give results of limited signiﬁcance for

our subject. In the case of different crystalline modiﬁcations

a change or jump of the dielectric permittivity could be measured

at the phase transitions. But there are only a few connections

between results obtained by dielectric spectroscopy and the

existence of polar domains in the crystalline state. It should be

mentioned, however, that two conditions for the detection of

ferroelectricity by dielectric spectroscopy are necessary: a ferro-

electic long range order and a certain dynamic within the phase.

These conditions seem to be fulﬁlled in substance 24.

By electro-optical measurements polar behaviour can be

proven. It is a well-established method in the ﬁeld of liquid

crystals to characterize the switching behaviour of polar and

non-polar mesophases. In our ﬁrst series of crystalline polar

bent-core compounds (homologues of compound 1), ferroelec-

tric switching has been optically observed and recorded by

a corresponding current response. The electro-optical investi-

gation of the compounds under study demonstrated the limits of

this method. An interesting point is the inﬂuence of an electric

ﬁeld during the crystallization process at the phase transition

isotropic liquid to crystalline solid. The ﬁeld-induced growing of

arcs and ribbons seemed to be a ﬁrst hint for the formation of

polar crystals (homologues of compound 1),³⁰could be esti-

mated for the 2,5-dimethyl-substituted series 18–22 (see Fig. 10),

but is not true for the 5-methyl-substituted derivative 17

(see Fig. 7).

Enlarging the aromatic core of the 5-chloro-substituted

compound 26 by insertion of C]C-bonds (33) and of a 2-

methyl-substituted compound 1 (to give 32) results in liquid

crystalline materials again, in a similar way as the insertion of

a 1-methyl-2,7-naphthalene fragment in the central position of

the bent-core (34).

It is important to mention that the relationships between

chemical structure and the mesophase behaviour discussed

before are only applicable for the substance class under study. If

the chemical structure of the aromatic bent core is changed,

liquid crystalline phases have been reported in the literature for

2-methyl-, 5-methyl- and 5-methoxy-substituted ﬁve-ring

compounds and 5-chloro- and 5-cyano-substituted seven-ring

bent-core mesogens.^{26–28,41–46}

The electro-optical behaviour of the crystalline compounds is

very different. In some cases there is no response to an electric

ﬁeld. For other compounds a change of the texture on applying

a ﬁeld can be observed, which means a change of the birefrin-

gence and/or small optical ﬂuctuations, but no current response

could be measured. In some cases, an optical answer together

with a current response could be found. These compounds can be

characterized as solid organic ferroelectrics. It should be

mentioned that the optically observable switching process only

occurs up to 10–20 K below the isotropic–crystalline transition

temperature, however it is reversible on cooling and heating.

The investigation of the liquid crystalline phases of the

compounds 16, 23, 32–34 has been done as usual by DSC,

polarizing microscopy, X-ray diffraction, and electro-optical

methods. The physical characterization of the crystalline phases,

however, is not simple.

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6077

View Article Online

3 G. H. Haertling, Ferroelectrics, 1992, 131, 1.

SHG measurements have been performed because the charac-

terization of the materials with the methods discussed before

were not sufﬁcient. Using SHG we could check the results

obtained by electro-optical measurements, and more important,

pyroelectric and/or ferroelectric properties could be proven for

compounds which do not show any optical switching. Pyro-

electrics exhibited a macroscopically polar order, however the

SHG signal could not be reversed by changing the polarity of the

ﬁeld. The intensity of the SHG signal depended on the domain

structure of the crystalline modiﬁcation which was spontane-

ously formed on re-crystallization. This was a process which

could hardly be inﬂuenced by our equipment. Therefore, the

quantitative comparison of the intensity values of different

compounds was difﬁcult, because the detailed structure of the

crystalline modiﬁcations and the size of polar domains were

unknown.

4 Ferroelectric Polymers: Chemistry, Physics and Application, ed. H. S.

Nalwa, Marcel Dekker Inc., New York, Basel, Hong Kong, 1995.

5 C. Rankin, Chun-Han Chu, D. Conklin and D. A. Bonnell, ACS

Nano, 2007, 1, 234.

6 S. Horiuchi and Y. Tokura, Nat. Mater., 2008, 7, 357.

7 Z. Zikmund, P. Vanek, M. Havrankova, B. Brezina, M. Cermak and

M. Vasa, Ferroelectrics, 1994, 158, 223.

8 J. Sworakowski, Ferroelectrics, 1992, 128, 295.

9 J. Fousek, Ferroelectrics, 1991, 113, 3.

10 S. Horiuchi, F. Ishii, R. Kumai, Y. Okimoto, H. Tachibana,

N. Nagaosa and Y. Tokura, Nat. Mater., 2005, 4, 163.

11 S. Horiuchi, R. Kumai and Y. Tokura, J. Mater. Chem., 2009, 19,

4421; S. Horiuchi, R. Kumai and Y. Tokura, Angew. Chem., Int.

Ed., 2007, 46, 3497.

12 S. Horiuchi, Y. Tokunaga, G. Giovannetti, S. Picozzi, H. Itoh,

R. Shimano, R. Kumai and Y. Tokura, Nature, 2010, 463, 789.

13 J. Valasek, Phys. Rev., 1921, 17, 475.

14 T. Mitsui, I. Tatsuzaki, and E. Nakamura, An Introduction to the

Physics of Ferroelectrics, Gordon and Breach Science Publishers,

New York, London, Paris, 1976.

Finally, some remarks should be given on the mechanism of

ferroelectric switching in the crystalline phases of the bent-core

compounds. The ﬁnding that not all of the pyroelectric

compounds showed additionally a ferroelectric switching is

related to the constrained system of the different crystalline

modiﬁcations. We assign the switching process in the solid state

to a cooperative rotation of fragments of the molecules or of the

whole molecules around their long axes—perhaps also around

a cone.^29,30It should be mentioned that movement of the whole

molecule is the accepted mechanism during the switching in ﬂuid

liquid crystalline phases. Furthermore, we can exclude a change

of the shape of the molecules between a bent one and ﬂat one

during the switching process as proposed for a charge-transfer

complex containing 5,10-dihydro-5,10-dimethylphenazine.⁴⁷For

the compounds under study, the driving force to build up a polar

order is caused by the preferred packing of the bent molecules in

layers in such a way that the transversal dipoles show in one

direction, giving rise to macroscopic polarization. If the structure

of the crystal is too conﬁned to allow any motion of the mole-

cules or parts of the molecules by an electric ﬁeld, the materials

possess polar properties, they are pyroelectrics but not ferro-

electrics.

15 A. L. Solomon, Phys. Rev., 1956, 104, 1191.

16 D. Bordeaux, J. Bonard, A. Campiomont, J. Lajazerowicz-

Bonneteau, J. Lajazerowicz and J. F. Legrand, Phys. Rev. Lett.,

1973, 31, 314.

17 J. Kroupa, P. Vanek, R. Krupkova and Z. Zikmund, Ferroelectrics,

1997, 202, 229.

18 M. Szafranski, A. Katrusiak and G. J. McIntyre, Phys. Rev. Lett.,

2002, 89, 215507.

19 D. Semmingsen and J. Feder, Solid State Commun., 1974, 15, 1369.

20 P. Gruner-Bauer and E. Dormann, J. Phys.: Condens. Matter, 1992,

4, 5599.

21 Y. Kamishima, Y. Akishige and M. Hashimoto, J. Phys. Soc. Jpn.,

1991, 60, 2147.

22 P. Szklarz and G. Bator, J. Phys. Chem., 2005, 66, 121.

23 S. V. Jablonski, T. Weyrauch, W. Haase, J. Ponti, A. Strigazzi,

C. A. Veraccini and C. Gandolfo, Ferroelectrics, 1996, 188, 175.

24 R. B. Meyer, L. Liebert, L. Strzelecki and P. Keller, J. Phys. Colloq.,

1975, 36, C5-165.

25 S. T. Lagerwall, Ferroelectric and Antiferroelectric Liquid Crystals,

Wiley-VCH, Weinheim 1999.

26 G. Pelzl, S. Diele and W. Weissﬂog, Adv. Mater., 1999, 11, 707.

27 R. A. Reddy and C. Tschierske, J. Mater. Chem., 2006, 16, 907.

28 H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys., 2006, 45, 597.

€

29 G. Pelzl, H. N. Shreenivasa Murthy, M. W. Schroder, S. Diele,

Z. Vakhovskaya, H. Kresse and W. Weissﬂog, J. Mater. Chem.,

2006, 16, 1702.

30 W. Weissﬂog, H. N. Shreenivasa Murthy, S. Diele and G. Pelzl,

Philos. Trans. R. Soc. London, Ser. A, 2006, 364, 2657.

Summarizing the results, a design strategy to produce solid

single-component organic polar materials has been found for the

ﬁrst time. 34 compounds have been investigated in the present

paper. Although the molecules exhibit a chemical structure

characteristic for banana-shaped liquid crystals, 24 compounds

exhibit only crystalline modiﬁcations. Ferroelectric and/or

pyroelectric properties have been proved for 12 compounds,

which is a high success rate. Generally it seems that bent-core

compounds which do not exhibit liquid crystalline phases are

good candidates to form single-component organic ferroelectrics

and/or pyroelectrics.

€

31 Broadband Dielectric Spectroscopy, ed. F. Kremer and A. Schonhals,

Springer, Berlin, 2000, p. 61.

32 K. Miyasato, S. Abe, H. Takezoe, A. Fukuda and E. Kuze, Jpn. J.

Appl. Phys., 1983, 22, L661.

€

33 W. Weissﬂog, G. Naumann, B. Kosata, M. W. Schroder, A. Eremin,

S. Diele, H. Kresse, R. Friedemann and S. Ananda Rama Krishnan,

J. Mater. Chem., 2005, 15, 4328.

€

34 M. W. Schroder, K. Brand, G. Pelzl, U. Baumeister, S. Diele and

W. Weissﬂog, Liq. Cryst., 2008, 35, 325.

35 A. Eremin, H. Nadasi, G. Pelzl, S. Diele, H. Kresse, W. Weissﬂog and

S. Grande, Phys. Chem. Chem. Phys., 2004, 6, 1290.

36 W. Weissﬂog, H. Nadasi, U. Dunemann, G. Pelzl, S. Diele, A. Eremin

and H. Kresse, J. Mater. Chem., 2001, 11, 2748.

€

37 U. Dunemann, M. S. Schroder, R. Amaranatha Reddy, G. Pelzl,

S. Diele and W. Weissﬂog, J. Mater. Chem., 2005, 15, 4051.

38 J. Svoboda, V. Novotna, V. Kozmik, M. Glogarova, W. Weissﬂog,

S. Diele and G. Pelzl, J. Mater. Chem., 2003, 13, 2104.

Acknowledgements

39 G. Pelzl and W. Weissﬂog, in Thermotropic Liquid Crystals, Recent

Advances, ed. A. Ramamoorthy, Springer, 2007, 1–58.

€

40 W. Weissﬂog, M. W. Schroder, S. Diele and G. Pelzl, Adv. Mater.,

2003, 15, 630.

The DFG (Deutsche Forschungsgemeinschaft) is acknowledged

for their generous ﬁnancial support.

41 J. Mieczkowski, J. Szydlowska, J. Matraszek, D. Pociecha,

E. Gorecka, B. Donnio and D. Guillon, J. Mater. Chem., 2002, 12,

3392.

42 K. Gomola, L. Guo, S. Dhara, Y. Shimbo, E. Gorecka, D. Pociecha,

J. Mieczkowski and H. Takezoe, J. Mater. Chem., 2009, 19, 4240.

References

1 M. E. Lines and A. M. Glass, Principles and Applications of

Ferroelectrics and Related Materials, Clarendon Press, Oxford, 1977.

2 J. Fricke, Phys. uns. Zeit, 1989, 20, 33.

6078 | J. Mater. Chem., 2010, 20, 6057–6079

This journal is ª The Royal Society of Chemistry 2010

View Article Online

43 S. Umadevi, S. Radhika and B. K. Sadashiva, Liq. Cryst., 2006, 33,

139.

46 V. Prasad, S. W. Kang, K. A. Suresh, L. Joshi, Q. Wang and

S. Kumar, J. Am. Chem. Soc., 2005, 127, 17224.

44 S. Umadevi and B. K. Sadashiva, Liq. Cryst., 2005, 32, 287.

45 R. A. Reddy and B. K. Sadashiva, Liq. Cryst., 2004, 31, 1069.

47 S. Horiuchi, R. Kumai, Y. Okimoto and Y. Tokura, J. Am. Chem.

Soc., 1999, 121, 6757.

This journal is ª The Royal Society of Chemistry 2010

J. Mater. Chem., 2010, 20, 6057–6079 | 6079

Article Doi

In search of a new design strategy for solid single-component organic ferroelectrics: Polar crystalline phases formed by bent-core molecules

DOI: 10.1039/c0jm00322k

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c0jm00322k

Full text of DOI:10.1039/c0jm00322k

Products guided by the article

R&D Labs maybe for 1242610-38-3

Relevant to this article

Hot Product