Star-shaped oligobenzoates: Non-conventional mesogens forming columnar helical mesophases

DOI: 10.1002/chem.200700922

Star-Shaped Oligobenzoates: Non-conventional Mesogens Forming

Columnar Helical Mesophases

Matthias Lehmann,*^[a]Michael Jahr,^[a]Bertrand Donnio,^[b]Robert Graf,^[c]

Sibylle Gemming,^[d]and Igor Popov^[d]

Abstract: Star-shaped mesogens with a

phloroglucinol or a trimesic acid core

and oligobenzoate arms with upto five

repeating units have been synthesised.

These non-conventional mesogens

form various columnar mesophases

over a broad temperature range. The

liquid-crystal phases were characterised

by optical microscopy, differential scan-

ning calorimetry, X-ray diffraction, di-

latometry and solid-state NMR spec-

troscopy. In addition to the high-tem-

perature hexagonal columnar phases,

the columnar self-assemblies undulate

upon cooling and consequently form

higher-ordered body-centred ortho-

rhombic columnar 3D structures. A

model of E-shaped folded conformers

helically displaced along the columns is

proposed. Helical preorganisation in

the hexagonal phase precedes the tran-

sition to the low-temperature phases.

Space filling and nano-segregation

compete in the self-organisation pro-

cess, thus aliphatic chains and the polar

oligobenzoate scaffold are not perfectly

separated in these star-shaped meso-

gens.

Keywords: liquid crystals · oligo-

benzoates · solid-state NMR spec-

troscopy · star-shaped mesogens ·

X-ray diffraction

Introduction

ducting and opto-electronic materials. Nano-segregation of

chemically and/or physically different molecular segments

has been shown to play a key role in self-assembly of meso-

gens in liquid-crystal (LC) structures,^[7–9]as previously ob-

served with lyotropic systems and block copolymers. One

family of non-conventional mesogens are star-shaped mole-

cules with three arms. Many examples are known that have

various three-armed cores as well as different numbers and

positions of flexible aliphatic chains.^[10–18]Molecules with no

lateral chains^[17b]or internal short alkoxy chains^[11c]form

nematic phases, whereas mesogens with more than six long

peripheral aliphatic chains frequently assemble into colum-

nar structures.^{[10a,b,g,11a,b,d–g,12a,c,d,13–16,17c,f,18]}With only three ali-

phatic chains on the periphery, a broad range of phases can

be found: crystalline phases as well as nematic, smectic and

columnar mesophases. Evidently, flexibility of the core is an

important parameter with respect to supramolecular self-as-

sembly. Oligobenzoates can be characterised as semi-flexible

molecules (Figure 1). The individual oxyphenylcarboxy

In the last decade, increasing numbers of non-conventional

liquid crystals have been synthesised.^[1]Among the different

structures are multiarm-mesogens, dendrons, dendrimers,^[2–4]

and supramolecular mesogens, such as metallomesogens^[5]

and hydrogen-bonded mesogens.^[6]If adequately designed,

these compounds can be used as soft functional materials,

for example, anisotropic electron/hole-, ion-, proton-con-

[a] Dr. M. Lehmann, M. Jahr

Institute of Chemistry, Chemnitz University of Technology

Straße der Nationen 62, 09111 Chemnitz (Germany)

Fax : (+49)371-531-1839

E-mail: matthias.lehmann@chemie.tu-chemnitz.de

[b] Dr. B. Donnio

Institut de Physique et Chimie des MatØriaux de Strasbourg - IPCMS,

UMR 7504 - CNRS/UniversitØ Louis Pasteur

23, rue Loess, BP 43, 67034 Strasbourg Cedex 2 (France)

[c] Dr. R. Graf

Max-Planck-Institut of Polymer Research

Ackermannweg 10, 55128 Mainz (Germany)

ꢀ

groups are flat and shape-persistent; however, the single C

O bonds to the ester oxygen atoms allow the formation of a

large number of different star-, E- and T-shaped conform-

ers.^[19]If most extended star-shaped conformers are drawn

schematically with C₃symmetry, as shown in Figure 1, there

is an unoccupied volume between the arms. In crystal struc-

tures of star-shaped molecules with propyloxy chains, the

[d] Dr. S. Gemming, I. Popov

Institute of Physical Chemistry and Electrochemistry

Dresden University of Technology

Mommsenstraße 13, 01062 Dresden (Germany)

Supporting information for this article is available on the WWW

under http://www.chemeurj.org/ or from the author.

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Chem. Eur. J. 2008, 14, 3562 – 3576

FULL PAPER

cedes the transition to the

highly ordered mesoACHTREUNGphases.

Star-shaped compounds with

different cores have also been

synthesised

(Figure 2

and

Scheme 1) to study the steric

and electronic effects of the

structural changes on the con-

former and thus on self-assem-

bly in the columnar mesophase.

Results and Discussion

Synthesis: The synthesis of the

various star-shaped derivatives

is shown in Scheme 1. Star-

shaped mesogens 1 were pre-

pared as previously describe-

ꢀ

Figure 1. Star-shaped oligobenzoates. Semi-flexibility is illustrated by the rotation about the C O single bonds.

void can be filled by solvent or by neighbouring molecules,

or even by mixing the different molecular segments.^[20,21]

Such compensation is not possible in the neat liquid crystal.

Assuming nano-segregation and coplanar stacking of these

extended molecules, thus with only one molecule in the unit

cell, the model structure would possess an unreasonably low

density.^[21]Consequently, self-assembly in which disk-shaped

molecules stack on topof each other, as observed with dis-

cotic mesogens in hexagonal mesophases,^[22]is unlikely. A

series of C₃-symmetric oligobenzoates has been prepared to

study the packing behaviour of star-shaped mesogens.^[18a]

The increasing size of the oligobenzoate arms also increases

the void within these structures in the C₃-symmetric, star-

shaped compounds. Nevertheless, all of the molecules in the

series have been found to form columnar mesophases: a

high-temperature hexagonal columnar phase (Col_h) and a

low-temperature columnar mesophase. Column diameters

were very small compared with the diameters of the star-

shaped conformers^[18a]and shape-persistent mesogens of

comparable structure and size.^[11e,f]Thus, other derivatives

were proposed, such as E-shaped conformers, which explain

the X-ray results. Evidence for E-shaped conformers was

obtained from the discovery of a helical crystal phase

formed from the monotropic hexagonal mesophase upon an-

nealing at ambient temperature.^[23]Helical mesophases were

recently found for similar compounds, for example within a

series of stilbenoid derivatives,^[11a,b]cyclotriphosphazene-

s^[15a,b]and wedge-shaped molecules.^[3]In this contribution,

we explore the different mesophases, which includes the

previously described helical crystal phase, by X-ray scatter-

ing on oriented fibres (wide-angle X-ray scattering (WAXS)

and small-angle X-ray scattering (SAXS)), dilatometry stud-

ies and solid-state NMR spectroscopy. We demonstrate that

the low-temperature phases of star-shaped semi-flexible oli-

gobenzoates generally consist of columns with a 3D organi-

sation, which is based on undulated structures best described

by helically packed E-shaped mesogens. There is evidence

that a partial helical assembly in the hexagonal phase pre-

d.^[18a]Triiodo derivatives 2 were obtained analogously by

using triiodophloroglucinol^[24]as the core. Compound 3 was

synthesised by threefold esterification of 1,3,5-benzenetricar-

bonyl trichloride (trimesic acid trichloride) with 4-(3,4,5-tri-

dodecyloxybenzoyloxy)phenol. The latter compound is an

arm derivative that was prepared from a mono-benzyl-pro-

tected hydroquinone and 3,4,5-tridodecyloxybenzoic acid.

All of the compounds were successively purified and ana-

lysed by using standard methods (see the Experimental Sec-

tion). The materials were freeze-dried prior to their thermal

study to prevent a rapid transesterification reaction at high

temperatures.^[25]

Molecular models—different conformations and the influ-

ence of different cores: Star-shaped mesogens consist of a

highly flexible periphery, the alkyl chains, and a polar semi-

flexible oligobenzoate interior. Previous investigations indi-

cate that nano-segregation of these two different segments

leads to LC columnar assemblies.^[18]However, the column

diameters were shown to be relatively small compared with

the diameters of all-s-trans conformers (Figure 2, confor-

mer A). Conformational analysis suggests that a folded con-

former may play a role in the aggregation process.^[19]Differ-

ent conformations for the molecules can be obtained by ro-

tation about the single bonds of the benzoate scaffold.

ꢀ

There are three types of s bonds 1) C O bonds that connect

ꢀ

aromatic rings with the carboxy groups, 2) C C bonds that

join the carboxy groups to the aromatic carbon atoms and

ꢀ

3) internal C O bonds of the carboxy groups. Only rotation

around the third type of s bonds leads to a significant

change in the molecular shape and size. Minimisation of the

conformational energy of a single molecule of 1a results in

an almost planar structure with an all-s-trans conforma-

tion.^[19]Only the middle benzoate groups are turned out of

the plane and give rise to a dipole moment perpendicular to

this plane. Such a conformation has recently been found for

a star-shaped molecule in its single crystal obtained from

acetone.^[21]The crystal structure is stabilised by antiparallel

Chem. Eur. J. 2008, 14, 3562 – 3576

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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3563

M. Lehmann et al.

Figure 2. A) Different possible conformers of compound 1a: star-shaped conformer A, folded star-shaped conformer B and E-shaped conformer C.

B) Energy-minimised core structures (AM1). Core (i) is based on triiodophloroglucinol, core (ii) on phloroglucinol and core (iii) on trimesic acid.

aligned dipoles, and the ben-

zoate arms interdigitate com-

pletely. However, such a confor-

mer does not correctly explain

the small column diameter mea-

sured. The change in shape and

size is realised by rotation about

the single bond of the carboxy

groupand is shown by confor-

mers B and C (Figure 2), which

might be possible structures;

however, no proof has been ob-

tained so far. Therefore, we syn-

thesised iodine derivatives 2 and

trimesic acid derivative 3, which

should show strong steric and

electronic effects on the confor-

mers B and C and, consequently,

on the LC properties. Figure 2B

visualizes the results on energy-

minimised structures by the

semi-empirical AM1 method.

Iodine at the centre of the mole-

cule introduces steric conges-

tion, therefore, the middle ben-

zoate units turn out of the plane

by 908 relative to the central ar-

Scheme 1. Synthesis of star-shaped mesogens (R=C₁₂H₂₅).

omatic unit. Thus, core (i) pos-

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Chem. Eur. J. 2008, 14, 3562 – 3576

Star-Shaped Oligobenzoates

FULL PAPER

sesses larger local dipoles perpendicular to the molecular

plane than the core (ii), without iodine. If the inner carboxy

groupis inverted, that is, trimesic acid is used as central

building block, the planar part of core (iii) is enlarged by the

three carboxy groups, which are conjugated with the aro-

matic ring. The middle benzene rings of the benzoate arms

can then freely rotate without producing a local dipole.

Whereas the larger, planar core should stabilise the meso-

phases, the reduced dipole along the column could show the

opposite effect.

and 3 also stay in the hexagonal phase when stored below

the glass transition temperature as a LC glass. However, an-

nealing for several days above the glass transition tempera-

ture converts the mesophases to phases of higher order with

a waxy consistency that are characterised as soft crystalline

materials. The hexagonal columnar mesophase of 1a is thus

monotropic. Enantiotropic Col_hphases are found for com-

pound 3 and the extended mesogens 1b–d. The temperature

interval for the Col_hphases varies with the length of the oli-

gobenzoate arms from 198C (1b), 98C (1d) to 28C (1c). At

lower temperatures, higher-ordered body-centred ortho-

rhombic mesophases are formed.

Thermotropic properties: All of the materials were investi-

gated by means of differential scanning calorimetry (DSC)

and polarised optical microscopy (POM). Table 1 summaris-

POM investigations of compounds 1a–c, 2b and 3 show

pseudo-focal-conic textures typically observed for columnar

phases.^[26]Figure 3 presents exemplary textures of materials

Table 1. DSC data of star-shaped mesogens.^[a]

Compound

Onset phase transition temperatures [8C]

(transition enthalpies [kJmol^ꢀ1])

1a

g

I

20

Col_h

I

53 (5.5)

20

I

g

48 (6.0)

55 (32.7)

58 (6.9)

50 (5.0)

34

88 (4.5)

74 (31.2)

79 (11.6)

95 (2.8)

82 (3.4)

78 (2.5)

170 (10.1)

170 (4.2)

241 (11.4)

247 (4.8)

[b]

Cr_borh

Col_h

I

g

2a

3

I

[d]

Col_h

I

Col_h

89 (4.9)

I

[b]

Cr_borh

Col_borh

I

Col_h

I

Col_borh

I

88 (4.8)

98 (3.0)

70 (7.8)

I^[c]

I

Col_borh

1b

2b

1c

1d

Figure 3. Textures of compounds 1a and 1b. A) Pseudo-focal-conic tex-

ture of 1a in the Col_hphase at RT. B) Columnar Cr_borhphase of 1a after

20 h at RT. C) Cr_borhof 1a after shearing. D) Pseudo-focal-conic texture

of 1b in the Col_hphase at 928C. E) Texture at 748C in the Col_borhphase

of 1b. The inset shows the left dark brush at increased camera sensitivi-

ty.^[28]

172 (5.4)

166 (9.6)

251 (7.5)

234 (8.2)

I

Col_borh

I

Col_borh

I

[a] Col_h: high-temperature hexagonal phase, Col_borh: low-temperature or-

thorhombic body-centred phase, Cr_borh: soft crystal. [b] Annealing at

room temperature leads to the formation of a soft crystal. [c] After sever-

al weeks at room temperature, compound 3 forms an additional non-

shearable phase that transforms at 498C to the soft crystal. [d] Heating

1a and 1b. Compound 2a exhibits a mosaic texture, which is

also frequently reported for hexagonal columnar mesophas-

es (see the Supporting Information),^[26c]whereas mesogen

1d does not facilitate the formation of a characteristic tex-

ture owing to its thermal instability above 2008C. The tex-

tures of 1b and 1c become more birefringent at the transi-

tion to the higher-ordered phases (Figure 3D, E). Interest-

ingly, a completely dark image occurs during the transition

between the low-temperature phase and the hexagonal mes-

ophase of 1b, which indicates a high degree of disorder in

the reorganisation process. However, the almost identical

textures in the high- and the low-temperature phases point

to a close relationship between the two phases; for example,

for the transformation of 1a from the hexagonal columnar

liquid-crystal phase to the soft crystal it was shown that the

columnar structure remains.^[23c]Despite the close relation-

ship between the coarse structures, a fine pattern appears on

the pseudo-focal-conic texture at low temperatures (Fig-

ure 3B and E); in the areas of the dark brushes it is similar

to features found for undulated phases.^[26a,27]The structure-

less, smeary texture obtained by shearing of the crystalline

phase of 1a demonstrates its softness (Figure 3C).

rate 28Cmin^ꢀ1

.

es the results of star compounds 1 to 3. The data for the

groupof molecules with only one repeat building block be-

tween the central benzene ring and the peripheral gallic

acid unit is shown in the first three entries. Inverting the

inner carboxy groupleads to a stabilisation of the columnar

mesophase by 358C. This can be rationalised by a larger

planar core for the latter compound (Figure 2). Consequent-

ly, iodo substitution, which causes the centre to be non-

planar, should destabilise the liquid-crystal phase. Surpris-

ingly, introduction of three iodo atoms at the phloroglucinol

centre again results in the formation of a columnar meso-

phase, which is still stabilised by 38C compared to parent

molecule 1a. Similarly, larger molecule 2b assembles in a

columnar mesophase; however, the clearing temperature is

lower than that of parent molecule 1b. In contrast with

mesogens 1a, 1b and 3, which transform to mesophases of

higher order at low temperature, the iodine derivatives

always remain in hexagonal columnar phases. Mesogens 1a

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3565

M. Lehmann et al.

X-ray diffraction

columns. Similar results were obtained with the high-tem-

perature phase of 3 at 808C or with a freshly extruded

sample at room temperature (see the Supporting Informa-

tion).

X-ray diffraction of fibres of iodo derivatives 2a and 2b

in their single mesophase show typical patterns for columnar

phases, which consist of a halo at wide angles and reflections

Hexagonal phases: X-ray diffraction studies were performed

on powder samples and/or on oriented fibres obtained by

extrusion from the high-temperature LC phases.^[29]The

series 1a–d has been recently investigated by powder X-ray

diffraction by using a synchrotron source.^[18]Herein, the hex-

agonal phases of 1c and 1d have not been investigated fur-

ther because at these high temperatures transesterification

reactions irreversibly damage the LC self-assembly during

prolonged heating. Figure 4A and B show the WAXS pat-

at the equator (Figure 4C). The corresponding reciprocal

p

distances of the latter are related by 1: 3 and thus substan-

tiate a hexagonal 2D-structure (Figure 4D). A set of four

diffuse reflections on the halo of the X-ray pattern is indica-

tive for mesogens tilted with re-

spect to the columnar axis. The

set of four diffuse reflections at

small angles points to addition-

al order along the columns,

which will be discussed later.

Table 2 summarises the diame-

ters of the columns in the hex-

agonal phases. The values in-

crease with the size of the

mesogens. Although, the differ-

ent steric and electronic influ-

ences in derivatives 1a, 2a, 3,

1b and 2b, respectively, could

lead to different conformers in

the LC phases and consequent-

ly to different column diame-

ters, the latter values are almost

matching for all molecules with

the same number of repeating

units.

Body-centred

orthorhombic

Figure 4. WAXS and SAXS pattern of extruded fibres of 1a, 1b and 2a. A) Compound 1a freshly extruded at

mesophases (Col_borh): Figure 4E

and F show the WAXS patterns

of material 1a, annealed for

four days at ambient tempera-

ture, and 1b at 608C in the

cooling scan. For all other de-

rivatives 1c, 1d, and 3, similar

patterns were recorded in their

308C. B) Compound 1b at 928C (2nd heating cycle). C) Compound 2a at 308C. D) Integration along the equa-

p

tor of the patterns in A) to C); 1/d₁₀:1/d₁₁:1/d₂₀=1: 3:2, which indicates a hexagonal structure. The 11 and

20 reflections of 1a and 1b are very broad with low intensity in WAXS, the SAXS of 1b shows clearly the

11 reflections. E) Compound 1a annealed for four days at ambient temperature. F) Compound 1b at 608C

after cooling from the hexagonal phase (B); i) 002 reflection, ii) set of four 211 reflections, iii) set of four reflec-

tions attributed to tilted mesogens.

terns of extruded fibres of 1a and 1b in their high-tempera-

ture columnar LC phases. Intense reflections at the equator

are related to columnar 2D order. Integration of the WAXS

low-temperature phases (see the Supporting Information).

Table 2. Cell parameter a of hexagonal columnar phases.

and SAXS patterns along the equator clearly shows a ratio

Compound

n^[a]

T [8C]

a (Col_h) []^[b]

p

of 1: 3:2 for the reciprocal Bragg distances, which is evi-

1a

2a

3

1b

2b

1c

1d

1

(1)

2

40

30

80

91

70

42.6

dence for hexagonal packing of the columns (Figure 4D). At

wide angles, a diffuse halo is attributed to liquid-like ali-

phatic chains. For 1a, the intensity on the halo is highest at

the meridian, which points to an average orthogonal orien-

tation of the mesogen cores with respect to the columnar

axis. In contrast, the set of four maxima for 1b on the halo

indicates that the molecules are tilted by an average of 358.

Interestingly, for both materials a very diffuse X-ray intensi-

ty at small angles was observed on the meridian of the pat-

tern. This may be attributed to some periodicity along the

42.1; 43.4^[c]

49.9

50.6

3

4

171

247

57.4^[d]

65^[b]

[a] n=Number of benzoate repeating units (Figure 1); the number in pa-

renthesis for compound 3 indicates the different hydroquinone repeating

unit (Scheme 1). [b] Values obtained from SAXS investigations. [c] This

value was obtained at RT in the supercooled Col_hphase of 3 from

WAXS measurements. [d] Parameters from ref. [18].

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Chem. Eur. J. 2008, 14, 3562 – 3576

Star-Shaped Oligobenzoates

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As for the hexagonal phase, reflections at the equator reveal

the maintained orientation of the material. Indeed, recent

AFM studies of 1a demonstrated that the transformation

from the hexagonal phase into the low-temperature phase

proceeds along the liquid-crystal columns that act as templa-

tes.^[23c]Therefore, the equatorial reflections are attributed to

2D packing of columns and can be indexed according to a

centred rectangular unit cell (see the Supporting Informa-

tion). The halo at approximately 4.4 is characteristic for

the liquid-like aliphatic chains. In contrast to the Col_h

phases, in the rectangular

broad and correlation lengths calculated by the Scheerer for-

mula^[31]are thus relatively low (10–20 periods). Moreover,

solid-state NMR experiments provide evidence for the mo-

bility of molecular segments. Therefore, we assign the

phases of 1a and 3 to soft columnar crystals that are only

obtained after prolonged annealing. In contrast, for the low-

temperature phases of 1b–d, which show reversible transi-

tions with low hysteresis in cooling and heating cycles, we

refer to undulated columnar liquid crystals. The cell parame-

ters are summarised in Table 3. Parameters a/2 for 1a, and b

phases many reflections are ob-

Table 3. Orthorhombic unit cells of low-temperature phases.

served on layer lines along the

meridian. These reflections in-

dicate the correlation of the

mesogens along the columns.

The number of signals is larger

for the small mesogens. Howev-

er, the diffraction patterns of all

materials have common meri-

dional reflections (i), a set of

four very intense reflections (ii)

and a second set of four diffuse

reflections at the halo (iii), as

shown in Figure 4. The reflec-

tions indicated by (iii) may be

Mesogen T [8C] Unit cell parameter (Col_borh

)

d_meridian[]

d_halo[] (xd_halo^ꢀ1; a [8])^[d,e]

ꢀ1 [d]

[]

(xd_meridian

)

[b]

a

b

c

d₀₀₂

d₂₁₁

Set of signals on the halo Halo^[f]

1a

3

1b

1c

1d

31

30

60

150

190

77.2

64.3

69.7

82.1

93.3

35.6

47.0

50.5

60.0

68.6

34.0

64.6

30.4

37.0

43.0

17.0 (21)

32.7 (9)

15.2 (20)

18.5 (17)

21.5 (9)

20.8 (11)

37.8 (8)^[c]

30.6 (10)

37.9 (8)

4.4 (10; 608)

3.8 (11)^[a], 4.4 (21)

4.4 (20;588)

4.7 (6; 628)

4.7 (6; 668)

4.4 (9)

4.4 (6)

4.5 (6)

4.6 (6)

4.8 (6)

41.9 (7)

[a] Diffuse signals at the meridian. [b] First set of split reflections at small angles. [c] d₀₁₁. [d] x Correlation

length calculated by the Scheerer formula; the value of x/d in parenthesis provides an estimate for the number

of correlated repeating units. [e] a is the angle between the diffraction vector and the column direction and

corresponds to the inclination of the mesogens. [f] Halo attributed to the liquid-like chains.

related to inclined molecules within the columns. The other

reflections, (i) and (ii), can be indexed as 002 and 211; of

which the only exception is compound 3, in which reflec-

tions (ii) correspond to the (011) planes. Reflections with

Miller indices h+k+l¼2n are not detected. Therefore the

3D assembly can be described as body-centred orthorhom-

bic unit cells (Col_borh). Because the columnar organisation

remains during the transition from the hexagonal to the or-

thorhombic phase, and also the reverse, 3D organisation

into the orthorhombic cell may be achieved by undulation

of columns.^[30]Undulation correlates the neighbouring col-

umns, although the aliphatic chains separating the polar

cores (nano-segregation) are still liquid-like. Figure 5 illus-

trates a possible model.

for all of the other compounds, compare well with the diam-

eter of the corresponding hexagonal unit cells (Table 2),

thus showing their relationshipto the hexagonal columnar

liquid crystals.

1

2

Solid-state NMR spectroscopy: H and H solid-state NMR

spectroscopy studies have been performed on mesogen 1e,

which is a derivative of 1a deuterated at the central phloro-

glucinol ring. To identify the conformation and the packing

1

of the aromatic mesogens, H double-quantum (DQ) NMR

correlation spectra^[32]have been recorded at T=270 and

300 K in the soft crystal phase. Because the 2D DQ spectra

did not show any temperature dependence other than a sig-

nificant line narrowing at higher temperature that resulted

from increased molecular mobility and led to a better spec-

tral resolution of the different proton sites, only the spec-

trum recorded at T=300 K is given in Figure 6.

Mixed indices h,k,l¼0 often indicate an underlying crys-

talline phase. The present materials are waxy and highly vis-

cous; however, they are still shearable. The reflections are

The aliphatic, OCH₂and the aromatic protons are well re-

solved, and all spatial proximities due to the chemical struc-

ture were confirmed (grey, horizontal lines in Figure 6a).

The signal intensity of the aromatic signals and that of the

OCH₂-protons is significantly enhanced by the DQ filtration

compared to the aliphatic chains, as can easily be seen from

Figure 6b. This indicates a lower molecular mobility of these

moieties in contrast to the highly mobile aliphatic chains.

The fact that the OCH₂signal is even more pronounced

than the aromatic signal arises from the closer spatial prox-

imity (0.18 nm) of the two protons in this group compared

with the closest proton–proton distance in a phenyl ring

(0.24 nm) and cannot be directly attributed to a difference

in molecular mobility.

Figure 5. A) Section through a unit cell of the Col_borhphase orthogonal to

the columnar axis. The elliptic cross-section of columns symbolises the in-

clined mesogens. B) Correlation of columns along direction c by undula-

tion. The light grey columns are shifted by a/2 behind the plane of the

drawing.

Chem. Eur. J. 2008, 14, 3562 – 3576

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3567

M. Lehmann et al.

H_BDQ coherences, if both aro-

matic proton sites experience

the same shift owing to their

spatial proximity to aromatic p-

electron systems. Ridges along

these lines would be expected

in a heterogeneous distribution

of packing arrangements of the

benzoate chains. The presence

of local maxima on these lines

indicates that preferred packing

arrangements are found in the

sample. However, the chemical

shift information alone is not

sufficient to propose such pack-

ing arrangements.

In contrast to the discussion

above, the local maximum at

d=14.1 (7.3 + 6.9) ppm does

not follow the dashed lines. In

fact there are different ways to

interpret this signal: it could

result from local packing in

which H_Aand H_Bexperience

significantly different p-shifts or

from local packing with a spa-

tial proximity between H_Band

H_Cas a result of substantial

bending of the oligobenzoate

branch.

Figure 6. a) ¹H DQ correlation spectrum recorded at T=300 K sample temperature and 30 kHz MAS using

1

1 rotor period back-to-back (BaBa)^[33]for DQ excitation. b) Comparison of signal intensities in H DQ-filtered

(c, 1 tR BaBa, 30 kHz MAS) and ¹H MAS NMR spectrum (c), normalised to the signal intensity of the

1

aromatic protons. The solution H NMR spectrum of 1e (c) was recorded in CDCl₃(400 MHz).

Most informative with respect to molecular conformations

and the packing of the oligobenzoate molecules in the col-

umnar phase is the aromatic region of the two-dimensional

It should be noted that neither the signals of the aliphatic

chains nor the signals of the OCH₂groups show a noticeable

influence on the p-shift distributions, which indicates nano-

segregation between the benzoate branches and the aliphat-

ic side-chains. The quality of this nano-segregation is direct-

ly reflected in the two-dimensional DQ spectrum. In a per-

fectly segregated system, the excitation of DQ coherences

between aromatic and aliphatic protons should not be feasi-

ble. However, in Figure 6a the signal at d=8.5 (7.2 +

1.3) ppm demonstrates the presence of a spatial proximity

between H_B, H_Cand the protons of the aliphatic chain of

within 0.4 nm, which may be explained by back-folding of

the aliphatic chains if it would have been only a weak

signal. In fact, the substantial intensity of this signal dis-

proves the assumption of perfect nano-segregation. The

weak DQ signal at d=9.5 (8.2 + 1.3) ppm, which has to be

assigned to a DQ coherence between aromatic H_Aand an

aliphatic proton, finally indicates that any nano-segregation

is not compatible with the assumption of a planar arrange-

ment of the star-shaped oligobenzoate molecules with the

central phloroglucinol ring located in the centre of the aro-

matic columns.

1

DQ spectrum. Based on the liquid-state H NMR spectrum,

in which three aromatic resonances at d=8.35 (ꢁ), 7.37

A

(ꢁ) and 7.42 ppm (ꢁ) are resolved (Figure 6b), only a

B

C

single DQ coherence between H_Aand H_Bat d=15.72, 8.35–

7.37 ppm should be observed in this region of the DQ spec-

trum because the aromatic protons (H_C) of the outer

OC₁₂H₂₅-substituted phenyl ring are too far from the nearest

aromatic protons to be involved in DQ coherences for short

DQ excitation times (t_exc=33.6 ms). However, the ¹H–¹H

DQ correlation NMR spectrum exhibits a complex pattern

in the aromatic region. Local maxima are found for the ex-

pected (H_A–H_B) DQ coherence at d=15.4 (8.2 + 7.2) ppm,

the chemical shift values of which were already slightly shift-

ed to lower ppm values, and also at d=14.5 (7.8

+

6.7) ppm, where every single proton site then experiences a

high-field shift of about 0.6 ppm. The high-field shift of the

aromatic resonances can be explained by the spatial proxim-

ity of the proton sites to aromatic ring currents that, de-

pending on the distance and the orientation of the proton

site with respect to the plane of the aromatic p-electron

system, causes an additional shift in the range of +0.2 to

ꢀ1.5 ppm.^[34]In the two-dimensional DQ NMR spectrum,

the dashed lines parallel to the diagonal of the spectrum

(solid diagonal line) indicate the expected positions of H_A–

1

In comparison with H DQ NMR spectroscopy, deuteron

NMR spectroscopy in the solid state is not as sensitive to

local packing, but it does provide detailed information on

dynamic processes. This is of particular interest in LC sys-

tems or soft crystals. Therefore, the dynamic behaviour of

3568

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Chem. Eur. J. 2008, 14, 3562 – 3576

Star-Shaped Oligobenzoates

FULL PAPER

1e was studied by heating the sample from the solid phase

at T=300 K to the isotropic phase at T=330 K. Deuteration

of the central ring was chosen because it assembles all three

benzoate arms, and therefore, reflects the overall mobility

of the molecule. The spectra measured are shown in

Figure 7. At room temperature, a typical static powder line

In summary, the overall molecular mobility of 1e is low

and not well-defined. Only in the hexagonal phase, after

cooling from the isotropic liquid, librational dynamics may

explain the low intensity of the signals in ²H solid-state

NMR spectroscopy. The deuterated core in the soft crystal

phase is almost solid. In contrast, the intensity distribution

of the DQ spectra in the same phase clearly shows segment

mobility of the aliphatic chains and aromatic building

blocks. Although most of the aromatic units and aliphatic

chains are nano-segregated, which is determined from the

absent influence of p-shift distributions on CH₂, CH₃, or

OCH₂groups, the strong signals for DQ coherence between

aromatic and aliphatic protons clearly points to spatial prox-

imity of such protons. Thus, nano-segregation is not perfect,

even in the soft crystal phase.

Dilatometry and model of the LC phases: Dilatometry is an

important tool that has been used to support structural stud-

ies on lyotropic and thermotropic liquid crystals, which as-

sumes that nano-segregation is the driving force of self-as-

sembly.^[36]Mesogens 1, 2 and 3 consist of an aliphatic pe-

riphery and a polar benzoate core. Previous results point to

nano-segregation as the driving force for molecular assem-

bly because only mesogens with the right number of long

alkoxy chains form liquid-crystal phases.^[18,37]Thus, the unit

cell volume V_cellis the sum of the volumes of the polar core

and the aliphatic chains. Because the latter is known,^[18,38]

the volume of the benzoate cores can be estimated in the

different mesophases by measuring the molecular volume

V_mby dilatometry. In this case, the number of molecules (Z)

in a columnar unit or the unit cell can be obtained from the

quotient V_cell/V_m=hS/V_m(in which h=height of the unit

cell, S=cross-section of the unit cell). V_cellis calculated from

X-ray data. In the orthorhombic unit cell (Col_borh), Z

amounts to Z=abc/V_m. The height h in the hexagonal unit

cell is not known and consequently, V_celland Z are not

easily available. For this reason, we have chosen an arbitrary

value h, for which a molecular equivalent Z is calculated by

Z=ha²sin608/V_m. Note that only for the sake of comparison,

h is kept constant at 4.5 for all compounds in their hexag-

onal columnar phases. Table 4 lists the dilatometry results

for the different phases of compounds 1a and 1b. The corre-

sponding graphs can be found in the Supporting Informa-

tion.

Figure 7. a) ²H NMR spectra of 1e recorded on heating (solid echo, 25 ms

echo delay). b) 45 ²H NMR spectra recorded over a period of 90 h while

2

cooling from the isotropic phase to T=318 K. c) Twelve H NMR spectra

recorded over a period of 24 h at 298 K, with the two last spectra taken

again at T=318 K.

shape with a quadrupolar coupling d_Q=136 kHz and asym-

metry parameter h=0.07 is observed. Heating to T=320 K

led to only a minor dropin intensity. On further heating, an

isotropic line appears in the centre of the spectrum that co-

existed at T=325 K with a remaining part of the powder

line shape visible mainly at the position of the singularities.

The singularities completely vanish on heating to T=330 K.

While cooling from the isotropic phase to T=318 K (Fig-

ure 7b), only the two singularities with a slightly reduced

splitting of d_Q=120 kHz and a remaining broadened iso-

tropic line are observed, which indicates a highly mobile

state with isotropic molecular tumbling and a broad distribu-

tion of correlation times. Although a static powder line

shape has been detected at the same temperature in the

heating run, the highly mobile state is stable for at least

90 h. The highly mobile phase returns slowly (within 24 h)

to the solid phase only after cooling to T=298 K (Fig-

ure 7c). This observation is in agreement with the mono-

tropic nature of the columnar hexagonal mesophase.^[23c]

It should be noted that the molecular motion of the cen-

tral phloroglucinol ring in the mobile phase is not rotation

around the C₃-symmetry axis of the molecule, as expected

for a star-shaped conformation of the molecule, but rather

librational dynamics with a broad distribution of correlation

times.^[35]This type of motion only leads to a minor reduction

in the quadrupolar splitting in the slow motion regime,

cannot be observed in the intermediate motion regime and

appears to be isotropic at the fast motion limit. The ob-

served dynamic behaviour, however, is consistent with the

proposed E-shaped conformation of the molecule.

Comparing the phases with the same structure (Col_hfor

1a and 1b) and at the same temperature, the volume of

three benzoate units is estimated to be approximately

433 ³. From the temperature-dependence of compound 1b,

the molecular volume of the larger mesogens can be esti-

mated as shown in Table 4. The data is discussed in more

detail below in which models for the different phases are de-

veloped, based on X-ray diffraction, dilatometry and solid-

state NMR spectroscopy results.

Body-centred orthorhombic columnar soft crystal phases of

1a and 3: A model of the low-temperature phase of 1a has

been previously presented based on a hexamer cluster heli-

Chem. Eur. J. 2008, 14, 3562 – 3576

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3569

M. Lehmann et al.

Table 4. Dilatometry results for 1a and 1b, and estimation of molecular

volumes of 1c and 1d.

Compound^[a]

T [8C]

V_m[³]

Z (h [])

Temperature

[c]

dependence [³]

1a (soft Cr_borh

1a (Col_h)

)

31

40

60

91

4080

4117

4620

4707

22.9 (34.0)

1.7 (4.5)

23.2 (30.4)

2.1 (4.5)

2.695T+3995.9

2.864T+4002.2

0.972T+4561.8

3.121T+4422.5

1b (Col_borh

1b (Col_h)

)

1c (Col_borh

1c (Col_h)

1d (Col_borh

1d (Col_h)

)

150

171

190

247

5128^[b]

5376^[b]

5587^[b]

6056^[b]

35.5 (37)

2.4 (4.5)

49.3 (43)

2.7 (4.5)

–

)

[a] Dilatometry was not performed for compound 3. Because molecules

1a and 3 only differ by the inverted inner carboxy group, their molecular

volumes are assumed to be identical. [b] The molecular volume is esti-

mated from the temperature dependence of 1b and the volume incre-

ment for three benzoate units, which amounts to ꢂ433 ³. Thus, for ex-

ample, V_m(1c, Col_h)=(3.121T+4422.5+433) ³; in which T=1718C

V_m=5376 ³. [c] T in 8C.

Figure 8. Model of the self-assembly in the soft crystal phase of 1a.

A) Side view of the double helix. B) Topview of the column. C) Columns

represented by dimers of a CPK model. Section through the unit cell (a,b

plane, c=0).

cally packed in the column.^[23a]This model is in agreement

with density measurements, X-ray data and AFM pictures.

However, aliphatic chains mix intensively with the polar

core of the columns. Herein, we developa helical model

based in principal on a dimer of E-shaped molecules. Densi-

the column direction and the pairs of molecules are separat-

ed by 6.2 .^[40]The next dimer is rotated by 338.^[41]This

leads to a double helix with a red (A1–A6) and a blue (B1–

B6) strand. The model explains the repeating unit along the

column because the first E-conformer (B1) of the blue helix

is at the same angular position as the sixth mesogen (A6) of

the red helix. The periodicity is then (56.2)+3.1 =

34.1 because the blue and the red strand are offset by

3.1 . Thus, the unit cell contains 5.5 mesogens from the

blue strand and 5.5 from the red strand, which is in agree-

ment with X-ray and dilatometry data. The arrangement of

dimers (CPK model) in the unit cell is shown in Figure 8C.

The benzoate core has a length of approximately 27 and a

breadth of 22 . The column in the middle is shifted by c/2,

thus the long half-axis of the elliptical cross-section is rotat-

ed by 908 versus the long half-axes of the four other col-

umns (Figure 5). The aliphatic chains are in a liquid state

and not necessarily inclined with respect to the column axis,

and consequently, may fill the free space.

Compound 3 is distinguished from 1a only in the inverse

inner carboxy groups. This small detail leads to a significant

change in the transition temperatures, although the packing

behaviour seems to be similar to that of 1a. The soft crystal-

line phase is characterised by a body-centred orthorhombic

unit cell (Table 3). The columnar structure remains upon

crystallisation by annealing, which is evidenced by the X-ray

diffractogram of an aligned enantiotropic hexagonal phase

after the transition from the soft crystal (see the Supporting

Information). The packing of the undulated columns in the

a,b plane is slightly different from 1a, and the repeating unit

along the column axis is increased by a factor of two (i.e.,

64.6 ). The number of molecules per columnar unit also

doubles and amounts to approximately 24 mesogens, if the

same molecular volume as for 1a is assumed. The diffuse set

3

ty measurements give a molecular volume of 4079 for

the soft crystal phase at 318C. Thus, 22.9 molecules fill the

repeating unit of the body-centred orthorhombic unit cell.

As the unit cell contains two columns, 11 mesogens form a

column with a length of 34 , which is the periodicity along

the column. How can these mesogens be arranged into a

columnar structure with a minimised packing energy, that is,

with maximum nano-segregation and space-filling? At the

halo a set of four maxima (Figure 4) indicates that the meso-

gens are inclined by 608 versus the columnar axis with an

intra-columnar separation of 4.4 . The repeat unit of 34

along the column clearly shows a 3D periodicity and might

be explained with a helical arrangement, which is frequently

reported for star-shaped molecules.^{[11a,b,15a,b]}The high order

of the mesostructure and solid-state NMR spectroscopy re-

sults indicate that a certain conformer constitutes the build-

ing block. As discussed recently, a large number of different

conformers are possible for these semi-flexible star-meso-

gens.^[19]If star-shaped molecules arrange in a similar manner

to those recently reported for the crystal structure of a de-

rivative with short chains, in which space filling dominates,

then 11 mesogens on topof each other would result in a

column with a length of 114.4 =48.4 .^[39]This result

shows that the column must comprise more than one mole-

cule in the columnar unit of a thickness of 4.4 . If we

assume nano-segregation as the main driving force towards

the supramolecular structure, then a dimer of E-shaped con-

formers can segregate aliphatic chains and the benzoate

core and efficiently fill the space. A model of a possible ar-

rangement is given in Figure 8A and B. The E-shaped con-

former is inclined by 608 and covers an azimuthal angle of

about 1698. A pair of mesogens (red and blue) form a

dimer. One of the mesogens (blue) is offset by 3.1 along

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Star-Shaped Oligobenzoates

FULL PAPER

of four reflections at the meridian point to an inclination of

the molecules by only 308; the smaller tilt of the mesogens

may be attributed to the larger planar core of the trimesic

acid centre. A helical organisation can be rationalised by

12 dimers separated by 5.4 and rotationally displaced by

308. In contrast to compound 1a, the periodicity relates to a

full turn of the double helix.

lattice symmetry, but also two other interesting features.

The increased intensity at the halo is very broad and the in-

tensity is highest at the meridian, which indicates that the

mesogens are, on average, oriented orthogonally to the col-

umnar axis and are no longer regularly inclined. The small-

angle meridional peak, which was relatively intense and

sharpfor the orthorhombic helical phase (17 , xd^ꢀ1=20 re-

peating units), is now diffuse and shifted to a Bragg distance

of 19.3 . Consequently, a periodicity along the columns

still exists in the hexagonal phase, but with a very small cor-

relation length x of approximately three repeating units.

This is probably rationalised by a preceding helical order

before the material transforms into the low-temperature

phase.

These features can also be observed for the hexagonal

phase of 3. On heating the soft crystal to 808C, the helical

arrangement disappears. At 808C, no meridional reflection

can be determined in the X-ray pattern of the hexagonal

phase. Nevertheless, if the Col_hphase is supercooled, a meri-

dional reflection is present at room temperature, which indi-

cates periodicity along the columns and which may be ra-

tionalised by a helical preorganisation. Similar to 1a, the re-

lated Bragg distances of the meridional reflections increase

from 32.3 in the orthorhombic cell to 37.9 in the hexag-

onal cell at room temperature. The smaller distance in the

rectangular helical soft crystal points to optimised packing

compared to the hexagonal phase.

Body-centred orthorhombic columnar liquid-crystal phase of

1b–d: X-ray diffraction of the extended oligobenzoates

shows diffraction patterns with similar features as those ob-

tained with 1a or 3 (see the Supporting Information); 1) the

mesogens are inclined by almost 608, 2) the reflections on

the equator can be attributed to a rectangular organisation

of columns and 3) the set of four signals at the meridian can

be indexed according to (211) planes. Thus, the LC unit cells

are described as body-centred orthorhombic cells and con-

tain two undulated columns, which are formed by approxi-

mately 12, 18 and 24 molecules per column in 1b, 1c and

1d, respectively. It is evident that the number of molecules

per columnar unit increases more rapidly than the height of

the unit cells (30.4, 37.0, 43.0 , see Table 3). This observa-

tion can be rationalised if more molecules are needed to

form a columnar slice of a defined thickness with larger

column diameter. Although the length of the individual

arms, and therefore, the diameter of the columns increases

from 1a to 1d, the volume of the peripheral groups remains

the same. Consequently, when the benzoate groups in the

centre increase the radius of the column, a single molecule

occupies a smaller columnar fraction, which is reflected by

the decreasing angle enclosed by the two folded arms of the

E-shaped conformer with increasing size of the mesogens

(Figure 9). The number of molecules needed to pack into a

columnar slice thus increases.

Interestingly, the hexagonal cell parameters are of the

same size for both molecules. This may raise the question as

to whether the same type of conformers participates in the

molecular assembly. Before tackling the latter problem, a

model based on nano-segregation should be discussed for

1a that visualises the size of cross-sections for benzoates

and aliphatic chains. The results are shown in Figure 10; the

detailed calculations can be found in the Supporting Infor-

mation.

Hexagonal columnar phase of 1a and 3: The WAXS pattern

of 1a of a freshly extruded fibre shows not only hexagonal

Figure 10. Model of the hexagonal phase. A) E-shaped conformer of 1a.

The dark grey background with the light grey circle indicates the cross-

section of the benzoate core of a column. B) Possible organisation of two

molecules of 1a. The lines highlight the hexagonal unit cell and the light

grey background shows the cross-section of the aliphatic chains. C) As-

sembly of nine unit cells. The free space on the upper part of the cell is

filled by segments of molecules positioned below the conformer in A.

D) E-shaped conformers of 1a, 3 and 2a.

Figure 9. E-shaped conformers of oligobenzoates (CPK models) based on

a phloroglucinol core.

Chem. Eur. J. 2008, 14, 3562 – 3576

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3571

M. Lehmann et al.

Table 5. Comparison of the experimental radius of the benzoate core of

the column with the measured radius of E-shaped and C₃-symmetric star-

shaped mesogens.

The black area with the light grey border corresponds to

the cross-section of the core of the column filled with ben-

zoates. The grey background of the hexagonal unit cell is

the cross section of the aliphatic chains. The model demon-

strates that a single E-shaped conformer cannot fill the

space of such a columnar slice as shown in Figure 10. There-

fore, about 40% of the cell (h=4.5 ) is occupied by mole-

cules below and above the reference mesogen in panel A,

which is probably the driving force towards the helical pre-

organisation in the hexagonal phase. Only a folded confor-

mer, such as the E-shape type, can account for all of the ob-

served features: 1) the small benzoate diameter of the

column, 2) the number of molecules per columnar slice,

3) no large angle mobility (rotation) of mesogens in the

liquid-crystal phase owing to the fact that the centre of mass

does not coincide with the centre of the column and 4) the

helical structures.

As discussed earlier, the core of 3 is larger than that of 1a

owing to conjugation between the carboxy groups and the

central benzene. Nevertheless, the column diameters are the

same for the mesogens. Figure 10D shows E-shaped con-

formers (CPK models) of molecules 1a and 3. Mesogen 3

can adopt such a conformer in spite of the larger planar

core.

Compound

r_{benzoate core}

r_E

r_C3stars

shape

1a

1b

1c

1d

11.8

16.0

20.0

23.3

8.9

15.2

21.7

28.2

34.6

11.9

15.0

18.1

(1d). As the periphery of these mesogens always contains

three gallic acid derivatives, they occupy a segment on the

circumference of the circular cross-section of the Col_hphase

with constant length L. The circumference of a circle in-

creases with 2pr and should contain Z molecules, thus LZ=

2pr. In the hexagonal phases with a circular cross-section of

the column cores completely surrounded by peripheral

alkoxy-substituted groups, a linear relationship between the

number of molecules in a columnar slice Z and the radius r

is expected, namely, Z=2prL^ꢀ1. The diagram in Figure 11

presents an excellent linear correlation between the experi-

mental radius of the core and the number of molecules in a

columnar slice with a height of 4.5 . Comparison of the

ideal curve (dashed line), in which the interface between the

aliphatic and benzoate segments is formed by peripheral

units, with the experimental curve (solid line) shows that the

experimental number of molecules contained in a columnar

slice increases less rapidly than proposed. The mismatch be-

tween the columnar radius and the number of molecules Z

becomes larger as the length of the oligobenzoate arms in-

creases. This is related to the fact that the oligobenzoates of

one molecule fill a larger fraction of the columnar core than

the alkoxy groups at the periphery. The inset of Figure 11

shows a model for mesogens 1a, in which individual mole-

cules occupy a segment of a columnar slice. This model

Hexagonal columnar phases of 1b–d: Owing to transesterifi-

cation reactions of 1c and 1d in their hexagonal phases at

high temperatures, the cell parameters of previously pub-

lished synchrotron results are used for the following analysi-

s.^[18a]Similar to 1a, the meridional diffuse intensity for 1b

(22.4 , xd^ꢀ1=5.4 repeating units) points to a helical preor-

ganisation of mesogens. We assume a closely related self-or-

ganisation for 1c and 1d, as a consequence of the similar

structures for the low-temperature phases of 1a–d. Table 5

summarises the core radii of these compounds obtained

from dilatometry and X-ray

studies. As emphasised earlier,

the radius of the benzoate core

of the column is much smaller

than the radius of a C₃-symmet-

ric

star-shaped

conformer

(Table 5, entries in the last

column). The length of the ben-

zoate segment of the E-shaped

conformers, r_E-shape, is smaller

than the radius of the column

core. This is in agreement with

the

model

presented

in

Figure 10 in which molecules

from below and above the ref-

erence mesogen contribute to

the columnar slice. The calcu-

lated number of molecules in

the columnar unit for 1b and

the estimated values for 1c and

1d (Table 4) show that Z in-

Figure 11. Relationshipbetween the benzoate core radius r and the number of molecules Z in a columnar slice

&

for mesogens 1a–d. Experimental data ( ) and the fit function for Z=0.458ꢃ0.046+(0.102ꢃ0.002)r (R=

0.9994) (c) are shown. The ideal relationshipwhen the peripheral units of an E-shaped molecule perfectly

cover the interface between the aliphatic chains and the oligobenzoate core (Z=2pr/L=0.180r with L=35 )

is also shown b. The parameter L is taken from a molecular model of 1a. The inset illustrates a hypothetical

segment model, in which E-shaped molecules pack in segments of a circular columnar slice, and therefore, op-

~

creases from 1.7 (1a) to 2.8 timise the nano-segregation. Much larger radii and numbers of molecules Z should be expected ( ).

3572

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Chem. Eur. J. 2008, 14, 3562 – 3576

Star-Shaped Oligobenzoates

FULL PAPER

would optimise nano-segregation; however, a void remains

in the centre of the column. Moreover, such a model would

predict much larger numbers Z per columnar slice and

umnar stack was investigated with the helpof density-func-

tional-based tight-binding calculations^[42], and included dis-

persion interaction between non-covalently bonded mole-

cules or fragments.^[43]This approach has already been suc-

cessfully applied to analyse stacking of the more rigid and

ideally planar 1,3,5-tris-pyrazolylbenzene derivatives in col-

umnar phases.^[44]A similar analysis for the present star-

shaped mesogens with a phloroglucinol core and oligoben-

zoate arms is more difficult owing to the higher flexibility

and non-planarity of the molecules. Hence, the thorough

analysis of the single star-shaped mesogens yielded the core

structure of 1a in good agreement with experimental crystal

structure data,^[19,21]but only limited information could be

obtained on the preferred conformation and orientation of

such a mesogen within a columnar phase.

Of special interest is the relative rotation angle f between

two adjacent mesogens along the column, which can give

rise to a helical order. The theoretical investigation of this

quantity was performed in a two-step process: the lower rel-

ative rotation angles f<208 were studied with the helpof a

non-periodic stack of four molecules. For larger values of f,

a periodically repeated stack was employed that consisted of

N=3608/f molecules or 1208/f for the threefold-symmetric

structures. The simulations were performed for several fixed

average intermolecular distances along the stack, ranging

from 3 to 6 . Both the three-fold symmetric star-shaped

molecule and the less symmetric folded star in the E confor-

mation were studied. Figure 12 shows the relative total ener-

gies per single mesogen as a function of the rotation angle

f; the energy of the most stable arrangement was chosen as

the reference. Figure 12A displays the values calculated for

the nearly planar three-fold symmetric stars, and Figure 12B

shows the curves for stacks of non-planar folded mesogens.

In both cases, the curves for the experimental intermolecu-

lar distance of ꢂ4.4 is displayed (b in Figure 12) along

~

larger core radii, which are not found experimentally ( in

Figure 11). Consequently, the experimental data indicates

that the peripheral gallic acid residues cannot perfectly

cover the interface between aliphatic chains and oligoben-

zoates, which is in agreement with the results from solid-

state NMR spectroscopy that demonstrate non-perfect

nano-segregation. The self-assembly of these non-conven-

tional mesogens is obviously governed by a compromise be-

tween nano-segregation and space filling.

Hexagonal columnar phases of 2a and 2b: Compounds 2a

and 2b have been designed to hinder the formation of

folded conformers. Indeed, the central benzoate units have

to rotate 908 out of the molecular plane. Such steric hin-

drance has been thought to prevent the formation of liquid

crystals. Nevertheless, compounds 2a and 2b assemble in

columnar liquid-crystal phases, which possess almost the

same thermal stability as related mesogens 1. Obviously, the

impact of steric congestion from the iodine atoms on colum-

nar aggregation is small. Moreover, it is surprising that pa-

rameters a for the hexagonal unit cells of 2a or 2b are again

identical to those of 1a and 3 or 1b, respectively. Because

the central benzoates are forced into a C₃-symmetric inner

part of the mesogen in the AM1-minimised model

(Figure 2), larger radii comparable to the values of r_C3starsin

Table 5, and consequently, larger column diameters were ex-

pected. In contrast, the column diameters of 2a and 2b are

comparable to the diameters of 1a and 1b, respectively, and

can be rationalised with E-shaped conformers. Figure 10D

shows that such conformers are indeed reasonable for mole-

cules with a triiodophloroglucinol core. The two lateral arms

enclose a wider angle than those of mesogens 1a or 3. For

this reason, the benzoate core

becomes broader. The size of

mesogen 2a increases to

ꢂ28 , which is larger than the

diameter of the column core

measured for 1a. This is evi-

dently compensated by a 438

tilt of the mesogens in the LC

phase, which is confirmed by

the X-ray pattern. The butterfly

pattern at small angles points to

a helical arrangement of meso-

gens (Figure 4C). In contrast to

1a, the Col_hphase of 2a is

enantiotropic. An analogous

behaviour has been observed

for mesogen 2b.

Figure 12. Relative total energies per mesogen in a stack of four molecules as a function of the rotation angle

f at various intra-stack distances; results from periodic calculations are included from 208 onward. A) Calcula-

tions for C₃-symmetric conformers at 3.6 (g), 4.0 (c) and 4.4 (b). B) Calculations for E-shaped con-

formers at 3.6 (g), 4.0 (c), 4.4 (b) and 6.0 (d); the value proposed by the model best fitting the

Packing

motifs—theoretical

considerations: The optimum

relative orientation of star-

shaped mesogens within a col- experimental results is also indicated (*).

Chem. Eur. J. 2008, 14, 3562 – 3576

ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

3573

M. Lehmann et al.

(FD MS) instrument. Elemental analyses were carried out in the micro-

analytical laboratory at the University of Mainz (Germany).

with other prototypical distances. The most stable structure

for a single column is formed by three-fold symmetric meso-

gens with an intermolecular distance of 4.4 and a rotation

angle f of 128, that is, with about 10 molecules along the

repeat unit of the column. However, other threefold-sym-

metric arrangements with f values between 10 and 208 and

oscillations of the intermolecular distances between 4 and

5 are energetically very close, so that they may also be

stabilised by packing and/or template effects. It is equally

likely that the same mechanisms can stabilise a stack of de-

formed mesogens, as displayed in Figure 12B, although the

angle dependence is more severe at the experimental inter-

molecular distance. Yet, with an optimum angle of 358,

stable stacks are obtained that are consistent with the analy-

sis of the experimental data. Deviations from helical stack-

ing are disfavoured by energy barriers of at least 0.04 eV

per molecule at f=08 (or 608) at the experimental intermo-

lecular distance.

X-ray diffraction: The WAXS measurements of aligned samples obtained

by extrusion were made with a standard copper anode (2.2 kW) source

with a pinhole collimator equipped with an X-ray mirror (Osmic type

CMF15-sCu6) and a Bruker detector (High-star) with 10241024 pixels.

The SAXS measurements of aligned samples were performed with a ro-

tating anode (mikromax 007, copper, Rigaku) source with a pinhole colli-

mator equipped with an X-ray mirror (Osmic type 140–004012) and a

Bruker detector (High-star) with 10241024 pixels. Calibration was per-

formed with silver behenate.^[45]The X-ray patterns were evaluated with

the datasqueeze software.^[46]The correlation length was determined by

using the Scheerer formula and the half-width and reflection maximum

were obtained from the fit function.

1,3,5-Tris-[4-(3,4,5-tridodecyloxybenzoyloxy)benzoyloxy]-2,4,6-trideuter-

iobenzene (1e): Compound 5 (n=1; 1.5 g, 1.89 mmol), 1,3,5-trihydroxy-

2,4,6-trideuteriobenzene (0.08 g, 0.60 mmol),^[47]dicyclohexylcarbodiimide

(DCC; 0.79 g, 3.77 mmol) and 4-dimethylaminopyridinium toluylsulfo-

nate (DPTS; 0.22 g, 0.76 mmol) were dissolved in dry CH₂Cl₂(10 mL)

and stirred at ambient temperature overnight. The solvent was removed

and the product was extracted with hexane. Evaporation of the solvent

and recrystallisation from acetone gave a colourless waxy solid (1.05 g,

1

74%). The degree of deuteration was 95%. H NMR (400 MHz, CDCl₃):

d=0.88 (t, 27H; CH₃), 1.26–1.53 (m, 162H; CH₂), 1.73–1.88 (m, 18H;

CH₂), 4.06 (2t, 18H; OCH₂); 7.20 (residual proton signal of the core,

0.15H), 7.37 (AA’BB’, 6H; ArH), 7.41 (s, 6H; ArH); 8.29 ppm (AA’BB’,

6H; ArH); ¹³C NMR (100 MHz, CDCl₃): d=14.1 (CH₃), 22.7–31.9

(CH₂), 69.3, 73.6 (OCH₂), 108.6 (Ar CH), 113.1 (br; Ar CD), 122.2 (Ar

CH), 123.2, 126.5 (Ar C_q), 131.9 (Ar CH), 143.3, 151.4 153.0, 155.5 (Ar

C_q), 163.7, 164.4 ppm (C=O); FD MS: m/z (%): 2460.3 (100, [M+1]⁺);

elemental analysis calcd (%) for C₁₅₆H₂₄₃D₃O₂₁: C 76.15, H 10.20; found:

C 75.94, H 10.08.

Conclusion

Non-conventional star-shaped mesogens with various cores

(phloroglucinol, triiodophloroglucinol and trimesic acid) and

different benzoate arm lengths have been synthesised. The

compounds form high-temperature hexagonal phases (Col_h)

and low-temperature orthorhombic body-centred phases

(Col_borh) or soft crystals (Cr_borh). Dilatometry, X-ray diffrac-

tion data and solid-state NMR spectroscopy results of a

core-deuterated derivative are in agreement with a model of

undulated columns formed by a helical arrangement of

tilted E-shaped conformers in the orthorhombic soft crystal

and columnar liquid crystal. The data indicate that the mate-

rial does not undergo perfect nano-segregation, instead,

nano-segregation and space-filling are two competing factors

in the self-organisation of star-shaped oligobenzoates. Ben-

zoate core diameters in the hexagonal phases can be esti-

mated and the small values also point to E-shaped conform-

ers. The different electronic and steric effects of the various

central building blocks do not affect the benzoate core di-

ameters, they only affect the mesogen tilt with respect to the

column axis. Interestingly, X-ray diffraction data point to a

helical preorganisation of mesogens in the Col_hphase. Only

in derivative 3, in which the existence of the hexagonal col-

umnar phase extends over a large temperature interval of

548C, the intracolumnar preorganisation disappears close to

the clearing temperature.

1,3,5-Tris(3,4,5-tridodecyloxybenzoyloxy-4’-benzoyloxy)-2,4,6-triiodoben-

zene (2a): Compound 5 (n=1; 150 mg, 0.19 mmol), 2,4,6-triiodophloro-

glucinol 4 (X=I; 31 mg, 0.06 mmol), DCC (230 mg, 0.28 mmol) and

DPTS (10 mg, 0.04 mmol) were dissolved in dry CH₂Cl₂(10 mL) and

stirred at ambient temperature overnight. The solvent was removed and

the product was purified by column chromatography (silica gel, hexane/

ethyl acetate). Recrystallisation from acetone gave a colourless waxy

solid (126 mg, 72%). T_Cl(clearing temperature; onset DSC)=588C;

¹H NMR (300 MHz, CDCl₃): d=0.86, 0.88 (2t, 27H; CH₃), 1.26–1.55 (m,

162H; CH₂), 1.80 (m, 18H; CH₂), 4.06, 4.08 (2t, 18H; OCH₂), 7.416

(AA’BB’, 6H; ArH), 7.421 (s, 6H; ArH), 8.38 ppm (AA’BB’, 12H;

ArH); ¹³C NMR (100 MHz, CDCl₃): d=14.1 (CH₃), 22.7–31.9 (CH₂),

69.3, 73.6 (OCH₂), 86.0 (Ar C_q), 108.6, 122.3 (Ar CH), 123.2, 125.9 (Ar

C_q), 132.4 (Ar CH), 143.3, 153.0, 154.3, 155.8 (Ar C_q), 161.6, 164.4 ppm

(C=O); FD MS: m/z (%): 2835.2 (100, [M+1]⁺); elemental analysis calcd

(%) for C₁₅₆H₂₄₃I₃O₂₁: C 66.08, H 8.64; found: C 66.12, H 8.55.

1,3,5-Tris(3,4,5-tridodecyloxybenzoyloxy-4’-benzoyloxy-4’’-benzoyloxy)-

2,4,6-triiodobenzene (2b): Compound 5 (n=2; 172 mg, 0.19 mmol), 2,4,6-

triiodophloroglucinol

4 (X=I; 31 mg, 0.06 mmol), DCC (230 mg,

0.28 mmol) and DPTS (10 mg, 0.04 mmol) were dissolved in dry CH₂Cl₂

(10 mL) and stirred at ambient temperature overnight. The solvent was

removed and the product was purified by column chromatography (silica

gel, hexane/ethyl acetate). Recrystallisation from acetone gave a colour-

less waxy solid (126 mg, 66%). T_Cl(onset DSC)=828C; ¹H NMR

(400 MHz, CDCl₃): d=0.879, 0.883 (2t, 27H; CH₃), 1.26–1.88 (m, 180H;

CH₂), 4.06, 4.08 (2t, 18H; OCH₂), 7.39, 7,46 (AA’BB’, 24H; Ar CH),

7.42 (s, 6H; Ar CH), 8.31, 8.40 ppm (AA’BB’, 12H; Ar CH); ¹³C NMR

(100 MHz, CDCl₃): d=14.1 (CH₃), 22.7–31.9 (CH₂), 69.3, 73.6 (OCH₂),

86.0 (Ar C_q), 108.6, 122.3 (Ar CH), 123.2, 126.1, 126.4 (Ar C_q), 132.0,

132.5 (Ar CH), 143.3, 153.0, 154.3, 155.6 (Ar C_q), 161.6, 163.8, 164.4 ppm

(CO); FD MS: m/z (%): 3197.2 (100, [M+2+H]⁺); elemental analysis

calcd (%) for C₁₇₇H₂₅₅I₃O₂₇: C 66.53, H 8.04; found: C 67.02, H 7.90.

Experimental Section

General: Chemicals were obtained from Acros and Sigma-Aldrich and

were used as received. The synthesis of compounds 1a to 1d was de-

scribed previously.^[19]Column chromatography was carried out on

silica 60 gel (Merck, mesh 70–230). PFT ¹H and ¹³C NMR spectra were

recorded in CDCl₃by using Varian 300 MHz and Varian Oxford

400 MHz spectrometers with the residual solvent signal at d=7.26 ppm

as a reference. Mass spectra were obtained by using a Finnigan MAT95

1,3,5-Tris(3,4,5-tridodecyloxybenzoyloxy-4’-phenyloxycarbonyl)benzene

(3): Compound 7 (205 mg, 0.27 mmol) and 6 (24 mg, 0.09 mmol) were

dissolved in dry CH₂Cl₂(10 mL) and triethylamine (0.2 mL) was added.

The mixture was stirred at ambient temperature overnight. The solvent

3574

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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2008, 14, 3562 – 3576

Star-Shaped Oligobenzoates

FULL PAPER

was removed and the product was purified by column chromatography

(silica gel, hexane/ethyl acetate). Recrystallisation from acetone gave a

colourless waxy solid (125 mg, 57%). T_Cl(Onset DSC)=888C; ¹H NMR

(300 MHz, CDCl₃): d=0.92 (m, 27H; CH₃), 1.27 (m, 162H; CH₂), 1.50

(m, 18H; CH₂), 1.81 (m, 18H; CH₂), 4.06, 4.07 (2t, 18H; OCH₂), 7.31

(AA’BB’, 12H; ArH), 7.42 (s, 6H; ArH), 9.27 ppm (s, 3H; ArH);

¹³C NMR (75 MHz, CDCl₃): d=14.1 (CH₃), 22.6–31.9 (CH₂), 69.2, 73.5

(OCH₂), 108.5, 122.4, 122.9 (Ar CH), 123.6, 131.1 (Ar C_q), 136.1 (Ar

CH), 143.1, 147.9, 148.8, 152.9 (Ar C_q), 163.1, 164.8 ppm (C=O); FD MS:

m/z (%): 2458.0 (100, [M]⁺), 1424.2 (77, [M]²⁺); elemental analysis calcd

(%) for C₁₅₆H₂₄₆O₂₁: C 76.24, H 10.09; found: C 75.88, H 9.89.

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Acknowledgements

We thank Prof. H. Meier, A. Oehlhof and Dr. N. Hanold for the elemen-

tal analyses and FD mass spectra completed at the University of Mainz.

The Deutsche Forschungsgemeinschaft (DFG) and the Bundesministeri-

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M. Lehmann et al.

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would be 5.58.8 =48.4 , which is not in agreement with the ex-

perimentally observed distance of 34 . A separation of dimers by

6.2 explains the observed periodicity along the column perfectly.

Coplanar molecules inclined by 608 with respect to the columnar

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increases by rotational displacement of the aromatic segments

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Received: June 17, 2007

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Published online: February 25, 2008

3576

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Chem. Eur. J. 2008, 14, 3562 – 3576

Article Doi

Star-shaped oligobenzoates: Non-conventional mesogens forming columnar helical mesophases

DOI: 10.1002/chem.200700922

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