Matthias Lehmann, Christiane Kohn, Herbert Meier, Sabine Renker and Annette Oehlhof- Supramolecular order of stilbenoid dendrons: importance of weak interactions

8/3/2019 Matthias Lehmann, Christiane Kohn, Herbert Meier, Sabine Renker and Annette Oehlhof- Supramolecular order of s

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Supramolecular order of stilbenoid dendrons: importance of weakinteractions{

Matthias Lehmann,*a Christiane Kohn,a Herbert Meier,b Sabine Renkerb and Annette Oehlhofb

Received 28th July 2005, Accepted 17th October 2005

First published as an Advance Article on the web 8th November 2005DOI: 10.1039/b510713j

Stilbenoid dendrons with various donor and acceptor groups on the focal unit were synthesised by

a WittigHorner reaction, starting from an aldehyde functionalised dendron and various

substituted phosphonic acid esters. The target molecules are composed of meta-branched arms,

two of them with extended conjugation (distyrylbenzene) and three flexible dodecyloxy chains; the

focal group consists of a donor or acceptor substituted styryl unit. The cross-conjugation of

the arms prevents the strong electronic influence of substituents on the two extended

oligophenylenevinylene chromophores. However, intermolecular interactions mediated by the

focal unit allow control of the supramolecular stacking into liquid crystal phases. Simple weak

acceptors stabilise the formation of columnar phases, whereas the additional propensity to build

hydrogen bonds leads to a cubic mesophase. All acceptor substituted materials freeze at low

temperature into a glassy state. Soft crystals are then formed upon heating the glassy material.

Stilbenoid dendrons are photosensitive and degradation of the supramolecular order proceeds

even in the glassy liquid crystal state.

Introduction

Stilbenoid molecules attract increasing attention due to their

interesting photophysical, photochemical and electronic

properties.1 Their propensity for charge transport and

electroluminescence has led to applications in light emitting

diodes,2 field effect transistors and photovoltaic cells.3

Recently, the conjugated stilbenoid scaffold has been com-

bined with the concept of dendrimers.

4

These molecules arehighly symmetric, regularly branched and possess a well-

defined size. Their modular design allows the synthesis of

variable structures, i.e. the usage of different functional cores,

branching and peripheral units.5 These features together with

the good film-forming processability of dendrimers and the

optoelectronic properties of the conjugated scaffold make such

molecules eligible for application in electronic devices.5,6

The first two generations of stilbenoid dendrimers with long

peripheral alkoxy chains form liquid crystalline (LC) phases.4

Self-assembly in mesophases has been shown to be favourable

for electronic materials, because of a facile alignment of

functional units and the possibility of structural self-healing.7

The molecular mobility in LC phases of stilbenoid mesogens isalso correlated with photochemical and photophysical pro-

perties, e.g. molecular motion allows photoreactions to

proceed even when the molecules are photostable in crystalline

phases.8 The driving force of columnar mesophase formation

for stilbenoid dendrimers has been proposed to be micro-

segregation of the rigid conjugated scaffold and the flexible

aliphatic chains, due to the preorganisation of these molecular

units in the mesogen.5,9,10 If the number and the position of

aliphatic chains change, mesomorphic behaviour changes too

or is even lost. The mesophase is stabilised if a dipole11 or a

pushpull character12 is introduced by the core unit.

Dendrons are wedge-shaped building blocks from which

dendrimers can be obtained in a convergent synthesis. Theymay show LC behaviour, if the contrast between the flexible

chains and rigid scaffold allows micro-segregation.13,14

Different units can be easily attached to the focal position

and thus functional groups can be arranged in the centre of a

column by micro-segregation and hierarchical self-assembly.15

In the series of dendrons 1, made of stilbene building blocks,

enantiotropic LC phases are formed in the third generation

(Fig. 1).16 The larger distyrylbenzene scaffold already allows

self-organisation in columnar mesophases starting with the

second generation dendron 2. Thus, the size of the semi-rigid

scaffold seems to be important for self-aggregation. In contrast

to radially-symmetrical dendrimers, most dendrons can only

form columnar phases with more than one mesogen placed in acolumnar slice.13 Since the focal units will then meet in the

centre of the columns, substituents at this position should

strongly influence the mesogenic properties. Therefore,

dendrons 3 with different small substituents on the focal unit

have been designed based on precursor 2 (Fig. 2). The focal

aldehyde group of 2 allows the facile modification of the

molecular scaffold with donor or acceptor substituted phos-

phonic acid derivatives. Since 2 already possesses an enantio-

tropic mesophase, mesomorphic properties were also expected

for 3, which is enlarged by a styryl group. Leaving the number

of aliphatic chains constant, we studied the influence of weak

interactions, i.e. dipoledipole interactions and H-bonds, on

aNon-Classical Synthetic Methods, Institute of Chemistry, ChemnitzUniversity of Technology, Strasse der Nationen 62, 09111 Chemnitz,Germany. E-mail: [email protected];Fax: +49 371 531 1839; Tel: +49 371 531 1205bInstitute of Organic Chemistry, University of Mainz, Duesbergweg1014, 55099 Mainz, Germany{ Electronic supplementary information (ESI) available: Comparisonof 1H-NMR data, UV-Vis spectra, and the FT-IR spectrum of amide3f. See DOI: 10.1039/b510713j

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

This journal is The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 441451 | 441


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the supramolecular stacking of stilbenoid dendrons 3.

Synthesis, mesomorphic, photophysical and photochemical

properties of new stilbenoid materials 3 are presented.

Synthesis

The preparation of the target compounds 3, outlined in

Scheme 1, was performed starting from dendron 2.17 The

WittigHorner reaction of aldehyde 2 with the phosphonic

acid diethyl esters 4a,18 4b,19 4c20 and 4d21 in THF in the

presence of KOtBu furnishes the products 3ad in moderate

isolated yields. The aldehyde 3g was obtained from 4f22 by asubsequent acidic treatment to cleave the acetal protecting

group. The amide derivative 3f was prepared by the reaction

with the phosphonic ester bearing the cyano group 4e23 and

hydrolytic workup. In order to prevent the hydrolysis of

the cyano function and thus to obtain 3e, NaH was used

as the base for the WittigHorner coupling. All compounds

are characterised by NMR-, IR-, mass-spectroscopy and

elemental analysis. The different electronic influence of the

substituents is evident from the 1H NMR spectra. Electron

withdrawing groups (e.g. CHO, CN, CONH2) shift the

aromatic signals of the focal benzene to lower field, compared

to the signals of non-substituted (3c) or donor substituted

compounds (e.g. CH3 (3a), OCH3(3b)). Although bromine

introduces a dipole in 3d, its electronic effect is very weak,

since nearly no differences to the chemical shifts in 3c are

apparent. The influence of the different substituents is

strongest at the focal benzene ring; their effect decreases

strongly at the central benzene and is not present for signals

of the alkoxy substituted arms due to their cross-conjugation(meta-position).24 The vanishing electronic influence of the

focal group upon the more extended oligophenylenevinylene

arms can be also observed in the series of UV-Vis spectra.

As in other meta-branched stilbenoid molecules,5,25 the

absorption of 3 can be approximated as the sum of the

absorptions of single arms (i.e. one stilbene and two

distyrylbenzene units). Therefore the absorption maximum

at 473 nm (CH2Cl2) corresponding to the distyrylbenzene

unit remains unchanged within the series. Only a slight effect

is observed on the high energy side of the long-wavelength

band, which can be related to the differently substituted

focal stilbene unit.24

Fig. 1 Stilbenoid dendrons forming columnar mesophases.

Fig. 2 Structure of donor and acceptor substituted stilbenoid

dendrons 3.

Scheme 1 Synthesis of substituted stilbenoid dendrons 3.

442 | J. Mater. Chem., 2006, 16, 441451 This journal is The Royal Society of Chemistry 2006


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Thermotropic behaviour

The thermotropic properties of 2 and 3 were studied by means

of differential scanning calorimetry (DSC), polarised optical

microscopy (POM) and X-ray diffraction and are summarised

in Table 1.

DSC and POM investigations

Aldehyde 2, the starting material for the preparation of the

target molecules 3, forms a liquid crystalline phase within a

small temperature range in the second heating trace. In the

cooling curve this interval increases since the LC phase can be

supercooled by almost 20 uC. However, when the sample is

annealed during the heating scan at 96 uC in the LC phase a

different crystal phase forms. Subsequent melting of the crystal

and clearing of the LC phase are very close and cannot be

separately evaluated. POM studies show mosaic and pseudo-

focal-conic textures typically found for columnar phases

(Fig. 3). The hexagonal growing germ at 104 uC points to a

hexagonal two-dimensional order of columns in the meso-phase. When the aldehyde group is exchanged with the styrene

building block to yield compounds 3, a decreased crystal-

lisation tendency can be observed. Molecules 3a and 3c, either

with or without an electron donor substituent, do not

crystallise upon cooling but form a monotropic mesophase

at low temperature instead. Obviously, 3b does not show a

supramolecular aggregation to a liquid crystal phase, which

presumable has its origin in the additional steric interaction of

the methyl group in ortho-position to the double bond.

In contrast, all compounds with electron withdrawing sub-

stituents assemble in, at first sight, enantiotropic mesophases.

Even the small, local dipole of the bromine substituent

stabilises mesophase formation. At low temperature the

columnar liquid crystals do not crystallise but freeze into a

glassy state. Decreasing further the temperature results in

another first order transition. It is assumed that this is related

to a partial crystallisation of side chains, which can still be

mobile above the transition.6 The interval of the LC phase can

be further increased by stronger acceptors or dipoles. The

largest mesophase range of 74 uC is observed for the aldehyde

substituted compound 3g. The pseudo-focal-conic textures

of 3d, 3e and 3g point to columnar stacking of mesogens

(Fig. 4).26 In the case of 3g, conoscopy on a homeotropic

aligned sample indicates an uniaxial phase with a negative

optical anisotropy, which is additional evidence for a columnar

Table 1 Thermotropic data obtained by DSC (heating rate 10u min21,phase transitions: onset [uC], enthalpies [kJ mol21]), POM and X-raydiffraction

CompoundPhase transitions (Onset [uC]) and transitionenthalpies [kJ mol21]a

2 1. cooling: I 102/23 Colhd 77/249 Cr2. heating: Cr 89 Cr1 96/43 Colhd 104/4 I

3a 1. cooling: I 19/21 Colhd 215/216 Cr2. heating: Cr 242/11 Colhd 23/1 I 31/24 Cr1 40/4

Cr2 44/219 Cr3 86/36 I3b 1. cooling: I 210/226 Cr

2. heating: Cr 219/9 I 12/254 Cr1 46/44 I3c 1. cooling: I 16/21 Colhd 212/222 Cr

2. heating: Cr 221/15 Colhd 29/1 I 43/261 Cr1 80/70 I3d 1. cooling: I 64/21 Colhd 19(Tg) g 213/224 X

2. heating: X 248/15 g 12 (Tg) Colhd 65/1 I3e 1. cooling: I 88/21.6 Colhd 25(Tg) g 220/229 X1

2. heating: X1 241/20 g 28(Tg) Colhd/X2 92/1.9 I3. heating after annealing at 50 uC: X2 92/1.9 I4. heating after cooling only to 80 uC: Colhd 92/1.7 I

3f 1. cooling: I 186/21 Cub 55(Tg) g 23/235 X2. heating: X 242/22 g 55(Tg) Cub 189/1 I

3g 1. cooling: I 96/21.6 Colhd 19(Tg) g 213/28 X12. heating: X1 235/11 g 24(Tg) Colhd/X2 98/1.7 I

3. heating after annealing at 50u

C: X2/Colhd 98/1.2X2/I 103/4.5 I

a Colhd columnar hexagonal disordered phase; Cub cubic phase; gglassy state; I isotropic liquid; Cr crystalline phase; X partiallycrystalline or plastic crystalline phases with unknown phasestructure.

Fig. 3 Texture of 2 at 88 uC between crossed polarisers. Three

different aligned domains are visible: (a) pseudo-focal-conic domains;

(b) domains with homogenous aligned material and (c) a homeotropic

aligned region. The inset shows a hexagonal growing germ at the I-LCphase transition.

Fig. 4 Pseudo-focal-conic texture of 3g at 89 uC between crossed

polarisers. The inset A shows a homeotropic aligned region. The

conoscopic picture B taken with a l-wave plate proofs the optically

negative, uniaxial nature of the mesophase.



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self-assembly of mesogens 3 from POM investigations.However, the thermotropic properties of the acceptor sub-

stituted mesogens are more complex when investigated at

different heating rates in POM and DSC. Upon slow heating

from room temperature (RT) above the glass transition, the

mesophase crystallises, as it is evidenced by a texture change

(Fig. 5). Note that this observation depends on the thermal

history of the sample and is only monitored when the sample

has been pre-frozen to the columnar glass. The smooth

pseudo-focal-conic texture typically obtained for the meso-

phases is stable upon cooling from the isotropic phase.

Annealing at 50 uC for 1 h does not lead to any transforma-

tion. Compound 3f behaves differently compared to the

column forming mesogens. The particularity of its neat phasesis the optical isotropy when observed under POM. However,

DSC clearly indicates a first order transition between two

optical isotropic phases. The phase below the transition is

viscous but fluid and becomes liquid-like above 198 uC. These

observations point to the presence of a cubic LC phase for 3f.

X-Ray diffraction

X-Ray diffraction was performed on cooling the samples

from the isotropic liquid to the mesophase. The results are

collected in Table 2. All compounds show diffraction patterns

typically observed for mesophases in the temperature range

of LC phases and frozen, glassy mesophases. The wide anglescattering powder diffraction patterns of 3a,c,d,e,g present

up to three reflections at small angles and only a halo

corresponding to the mean distance of liquid-like aliphatic

chains and stilbenoid cores. Fig. 6 shows the results, obtained

for compound 3e. The diffraction pattern A from an oriented

fibre of3e exhibits reflections of the small angle region only on

the equator. A detailed investigation of this angular region in

panel D presents signals in the ratio 1 : !3 : 2, thusdemonstrating the hexagonal symmetry of the two-dimen-

sional lattice. At the meridian of pattern A, a broad halo can

be observed. Comparison of the integration curves B (integra-

tion along the meridian) and C (integration along the equator)

illustrates that the halo consists of two diffuse signals at 4.50

and 4.05 A, which are partially superimposed. The halo

corresponding to the mean distance of liquid-like aliphatic

chains is distributed over the whole angular range. The

corresponding d-value is acquired from curve C and amounts

to 4.50 A. The second signal with a maximum at d = 4.05 A is

related to the mean distance of chromophores along the

column. Thus, the diffraction pattern of the extruded fibre

aligned with the meridian shows intercolumnar distances only

on the equator and intracolumnar distances exclusively on the

meridian. This is clear evidence for the columnar nature of thephase. The absence of reflections attributed to mixed indices

Fig. 5 Right: pseudo-focal-conic texture of 3e at 35 uC between

crossed polarisers. Left: phase transition upon heating with 5 uC min21

to 60 uC.

Table 2 X-Ray data obtained for stilbenoid dendrons 2 and 3

Compound T [uC] hkl d exp/A dcalc/A ahexa/A

2 99 100 40.1 46.3110 23.2 23.2200 20.5 20.1Halo 4.7

3a 1 100 42.8 49.4200 21.8 21.4Halo 4.5

3c 5 100 41.4 47.8Halo 20.6 20.7

4.53d 50 100 39.1 45.1

110 22.3 22.6200 19.9 19.6Halo 4.4

3e 15 100 48.8 56.3110 28.0 28.2200 24.6 24.4Halo 4.4

3g 23 100 47.3 54.6200 23.9 23.7Halo 4.4

a The parameter ahex is calculated with ahex = 2d100/!3 andcorresponds to the diameter of a column.

Fig. 6 X-Ray scattering of 3e in the columnar phase. A: Wide angle

X-ray scattering pattern of an extruded fibre of 3e at 23 uC. B

Integrated intensity (q over azimuthal angle). C Integration along the

equator of the pattern. D Small angle region from a sample quenched

from the isotropic liquid to 15 uC.



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hkl with l ? 0 demonstrates the two-dimensional correlation

of columns and thus the liquid crystalline character. From

the half width of the broad halo attributed to the mean

intracolumnar separation of chromophores 3e, a correlation

length of 23.5 A is determined.27 This low value indicates the

relative high disorder of mesogens along the columnar axis.

For compounds 2 and 3d similar results where obtained,

identifying the formation of Colh phases. The diffractionpatterns of 3a, 3c and 3g show, besides the fundamental 100

reflection, only a broad signal indexed as 200. Consequently,

their symmetry can not be directly defined since such patterns

may be observed for hexagonal, tetragonal columnar and

lamellar mesophases. However, the closely related molecular

structure and similar POM textures compared with those of 3e

or 3d suggest the formation of Colh phases.

A model for the columnar assembly of 3e based on nano-

segregated different molecular units (stilbenoid scaffold and

aliphatic chains) gives more insight in the self-organisation of

mesogens. Information on the number of molecules forming a

columnar unit can be obtained from the X-ray density, given

by eqn (1);

r~z|M

NA|A|h(1)

where r = density, z = number of molecules in the columnar

unit, M = molecular weight, NA = Avogadros constant, A =

columnar cross section and h = height of the columnar unit.

The height of a columnar slice and the density are in

principal not known. However, the height can be estimated by

the mean distance given by the halo attributed to intracolum-

nar stacking (4.05 A) and the density can be set to 1 g cm23, a

value typically found for organic material. The columnar

cross-section filled by molecules 3e at RT can be calculated by

A = a2

6 sin 60 = 2696 A2

, which is the area of the hexagonaltwo-dimensional unit cell. By simple transformation of eqn (1),

the number of molecules z can be calculated and amounts to

3.8 molecules per columnar slice. Thus, four molecules form a

columnar unit. If antiparallel orientation of local dipoles is

assumed, two pairs of molecules fill the space given by A 6 h

as shown in Fig. 7.28 Further details can be obtained

considering the uniaxial nature of the hexagonal lattice which

presumes a circular cross section of the columnar core,

occupied by stilbenoid chromophores. With this model the

volume fraction of the stilbenoid core Vcore can be calculated if

the volume fraction of the aliphatic chains VCH is known; then

Vcore = A 6 h 2 VCH (eqn (2)). The calculation of VCH was

carried out according to data from dilatometry investigations.29 At23 uC VCH amounts to 7801 A

3, which is the volume of the dodecyl

chains of four molecules, and thus Vcore = 3120 A3. In this

model Vcore is assumed to be of cylindrical shape. Consequently,

the radius of such a cylinder can be calculated by rcore = !Vcore/(h6 p) = 15.7 A (eqn (3)). The value compares excellently with the

extension of the stilbenoid scaffold from the central benzene ring

to the middle oxygen of a distyrylbenzene arm (see Fig. 7), which is

16.4 A.28

A close inspection of the hexagonal parameter ahex in Table 2

clearly shows a large difference in column diameters of donor

and acceptor substituted mesogens of up to 10 A. This is

surprising since all molecules are based on the same scaffold.

However, results from temperature-dependent powder X-ray

investigations, summarised in Table 3, may explain the

unexpected large variation of cell parameters. For the mole-

cules with the largest temperature interval of the hexagonal

phase (3e, 3g), columnar diameters decrease with increasing

temperature. In the same temperature range the position of

the halo remains almost constant between 4.4 and 4.5 A. This

would imply an increasing density of the material with

increasing temperature, which is not reasonable. Data

corresponding to the columnar extension along the axis could

not be obtained from the X-ray patterns. However, an increasein the height of the columnar slice can be expected to be the

reason for the decreasing spatial requirements perpendicular to

the columnar axis. The height hcol can be calculated by using

eqn (2) and (3) when the volume fraction of the aliphatic

chains is known and a constant core diameter of 16.4 A is

assumed.30 The acceptor substituted mesogens 3e and 3g

exhibit an increasing height hcol with increasing temperature

Fig. 7 Premilinary apparent model of the supramolecular stacking of

3e in a columnar slice.28 In the space filling representation alkyl chains

are omitted for clarity.

Table 3 X-Ray parameters of 3 in the columnar liquid crystal phase

Compound T [uC] ahexa/A VCH

b/A3 hcolc/A rd/g cm23

3a 1 49.4 7669 6.04 0.903c 5 47.8 7693 6.78 0.843d 30 45.7 7845 8.12 0.80

50 45.1 7976 8.67 0.773e 23 55.8 7801 4.22 1.00

50 53.6 7976 4.86 0.9570 51.3 8115 5.64 0.8990 49.8 8263 6.33 0.84

3g 23 54.6 7801 4.49 0.9945 53.0 7943 5.00 0.9490 49.5 8263 6.45 0.83

a Diameter of columns in the hexagonal columnar phase (Colh).b Volume fraction of alkyl chains of four molecules 3 at temperatureT, calculated as described in ref. 29a. c Calculated columnar heighthcol = Vch/(A 2 r

2core 6 p) with rcore = 16.4 A obtained from

a molecular model. d Density calculated according to eqn (1) withh = hcol.



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(Table 3, column 5), e.g. in the mesophase of 3e at 90 uC, four

mesogens occupy the space of a columnar unit with hcol =

7.3 A; at 23 uC this value decreases to 4.2 A. The latter

number is in good agreement with the intracolumnar distance

obtained from the diffraction pattern of an oriented fibre

(Fig. 6) and monitors the quality of the estimation. In contrast,

all donor substituted mesogens show a significantly higher

value for the height of the columnar unit even at temperaturesas low as 1 uC. This behaviour can be explained by interactions

of local dipoles at the focal unit of the dendrons. Dipole

moments are relatively large for the cyanobenzene

(4.18 D)31a,32 and benzaldehyde units (2.97 D)31

b,32 in 3e and

3g, respectively. Dipoledipole interactions are most attractive

in antiparallel arrangements. According to the model in Fig. 7,

it is reasonable to assume antiparallel orientation of adjacent

mesogens. In such a configuration, attractive dipoledipole

interactions should hold mesogens closely together, which

should lead to decreasing intracolumnar distances with

increasing dipole strength. This is in agreement with the

calculated values, where 3e and 3g show the smallest distances

hcol compared with donor substituted mesogens 3a and 3c.Note, that although the cyano derivative 3e possesses the

larger dipole moment, the clearing temperature for aldehyde

3g is higher by 6 uC. This cannot be rationalised only with

dipole moments, but may have its origin in additional weak

contributions, e.g. CHp interactions.33 The dipoles do not

only force the molecules in columnar units of smaller extension

along the columns compared to donor substituted mesogens,

but also facilitate the nano-segregation and, thus, the

formation of mesophases over a large temperature range.

This is evidence that the interaction introduced by the different

focal substituents play a key role on structural parameters

of the mesophases. The last column in Table 3 lists

calculated densities, based on the measured diameters andthe heights hcol, assuming four molecules in a columnar

unit. The range of the values and the decreasing densities

with increasing temperature are reasonable for organic

molecules, thus they support the proposed model for columnar

self-organisation.

A different situation is observed for 3f. Although the amide

should have a dipole moment similar to benzamide (3.77 D)31c,

a value between those of benzaldehyde and cyanobenzene, the

clearing temperature is much higher than clearing tempera-

tures for mesophases of3e and 3g. POM observations point to

a cubic arrangement of mesogens. X-Ray investigations at

small angles were performed in the isotropic phase at 220 uC

and after cooling to 150 uC in the mesophase (Fig. 8). In theisotropic phase only a halo at 47 A is detected, which can be

related to the molecular size. In the mesophase, many

reflections appear at small angles. The shoulder at 426 A is

attributed to the 100 reflection. The other signals can then be

indexed according to a cubic phase. The different thermotropic

behaviour cannot be explained by simple dipole interactions,

but is attributed to the formation of H-bonds between the

amide functions, which manifests in the position of the NH

vibrations at 3356 and 3196 cm21 in the FT-IR-spectrum of a

thin film at RT.34 Shoulders at 3480 and 3400 cm21, however,

indicate, that not all NH functions are involved in hydrogen

bonding.24

In contrast to the cubic phase of mesogen 3f, the columnar

phases are all monotropic, as emphasised earlier. Even the

acceptor stabilised mesophases transform to a different phase

when heated from RT above the glass transition. A sample of

3g extruded at 80 uC shows an X-ray pattern with many signals

at small angles for such a phase (Fig. 9). All these reflections

are found at the equator of the pattern, thus the molecules are

aligned along the fibre axis. Since the material in the new phase

remains soft and can be oriented by extrusion, a soft columnar

crystalline nature is proposed for this phase. However, the still

relatively diffuse reflections at the meridian of the X-ray

pattern point to a considerably large intracolumnar disorder.The two-dimensional order of the columns, i.e. the 2D space

group, will be studied in more detail by SAXS measurements

and will be published elsewhere. A similar structural change

in the vicinity of the glass transition was recently observed

for star-shaped oligobenzoates, where the columnar liquid

crystalline phase serves as a template for the formation of

columnar crystals.35

Photophysical and photochemical properties

As mentioned earlier, the meta substitution of stilbenoid

molecules 3 decouples the three chromophores electronically in

Fig. 8 Small angle X-ray scattering of a powder sample 3f in the

isotropic phase at 220 uC and the LC phase at 150 uC.

Fig. 9 Panel A: X-ray scattering from compound 3g at 23 uC

extruded from the soft crystal phase (85 uC). Panel B: Pattern

obtained when annealing the LC phase of 3g at 40 uC. Positions of

reflections at the equator correspond well with diffraction rings of the

annealed sample.



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the first approximation. Therefore, the small influence of

substituents on the stilbene chromophore at the focal position

of the dendron can be considered as small variations of the

high energy side of the long-wavelength absorption band.

Fig. 10 presents the absorption and emission spectra of3e in

hexane, CH2Cl2 and solid state. The absorption band of 3e in

hexane is normalised to the intensity of the absorption band

in CH2Cl2. The hexane spectrum is only slightly hypsochro-

mically shifted by about 3 nm. Considerably larger effects are

observed for the emission spectra. The fluorescence spectrum

of 3e in CH2Cl2 shows a maximum at 474 nm, which is

bathochromically shifted to 500 nm in the glassy liquid crystal

state at RT for a thin film of a neat sample. In contrast, the

spectrum of3e in hexane shows not only a much smaller band

width, but also a hypsochromically shifted emission maximum

at 419 nm with a shoulder at 440 nm. Similar observations

were made for stilbenoid dendrimers with C3-symmetry and

were attributed to the formation of weak aggregates.4 The

simple exciton theory proposed by Kasha36 predicts a

hypsochromic shift of the emission maximum if the transition

dipole moments are aligned in parallel. The formation of

aggregates is supported by investigating the photodegradation

upon irradiation. Fig. 11 depictes the fast photodegradation

of 3e in hexane. The long-waved band corresponding to the

distyrylbenzene chromophores at 366 nm disappears rapidly.

After 20 s a new maximum at 352 nm emerges, which may be

related with an initial cistrans isomerisation of the stilbene

unit. The prolonged irradiation leads to an irreversible

formation of CC bonds and, consequently, a new band with

a maximum at 324 nm corresponding to remaining stilbene

Fig. 10 Absorption and emission spectra of 3e in hexane and CH2Cl2. For comparison, the absorption spectrum of 3e in hexane is normalised to

the maximum of the spectrum in CH2Cl2. Absorption maxima: 363 nm (hexane), 366 nm (CH2Cl2); emission maxima: 419 nm (hexane), 474 nm

(CH2Cl2), 500 nm (liquid crystal at RT).

Fig. 11 Irradiation of 3e in hexane with l > 300 nm (light of a Xn lamp was filtered by a 1 M NiSO4 solution).



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units develops. All chromophores have been degraded after

additional irradiation for 26 min. This process is considerably

slower in a solution of 3e in CH2Cl2. Thus hexane, which is a

poor solvent for the more polar aromatic scaffold of 3e,

assists the formation of aggregates, which then undergo fast

photooligomerisation, as observed previously for different

stilbenoid denrimers.4,5,37 Photopolymerisation and crosslink-

ing of stilbenoid compounds in thin films could be nicely

pursued by AFM measurements.38

The photosensitivity hasalso been investigated in the neat condensed phases of

mesogen 3e by means of POM (Fig. 12). The liquid crystal

phase at a temperature close to the LCI transition already

transforms after a few minutes to an isotropic phase when the

material is not protected from the UV part of the microscope

light. Interestingly, 6 h of irradiation of 3e in the glassy LC

state only led to a slight texture change. However, when

heated, the sample melts at a temperature below the clearing

point of the LC phase and does not return to an LC state in the

irradiated areas. Thus, the frozen glassy state does not prevent

photoreactions of compounds 3, as opposed to the crystalline

state of a first generation stilbenoid dendrimer.8

Summary and conclusions

C2-Symmetric dendrons 3 have been synthesised starting from

aldehyde 2 by a WittigHorner reaction with appropriate

phosphonic acid diethyl esters. They are composed of a 1,3,5-

substituted benzene core, two linear stilbenylethenylene arms

bearing three peripheral dodecyloxy chains each and one styryl

group with various electron withdrawing or electron donating

substituents (OCH3, CH3, H, Br, CN, CONH2, CHO). The

different arms are cross-conjugated, thus in first approxima-

tion the UV-spectra are superpositions of absorptions from

individual chromophores. NMR and UV data give evidence

that focal substituents only play a minor role for the

chemical shifts and the absorption of the two extended

oligophenylenevinylene units. However, the weak interactions

mediated by the focal unit have a large impact on the

supramolecular assembly of the mesogens. Donor substituents

destabilise the mesophases compared to the initial aldehyde.

Acceptor groups initiate the formation of more stable liquid

crystal phases with an increased mesophase range. A possiblemodel of the hexagonal columnar mesophases proposes four

molecules with antiparallel dipoles in a columnar slice, which

can be regarded as the smallest columnar unit. The local

dipoles at the focal position of the stilbenoid scaffold affect the

intracolumnar distances. The amide, as hydrogen bond donor

and acceptor, allows the creation of hydrogen bonds and,

instead of a columnar assembly, the amide substituted

mesogens stack into a cubic phase. All acceptor substituted

mesogens freeze at low temperature into a glassy state. Despite

the stabilisation effect of large dipoles, all columnar meso-

phases are monotrope; annealing above the glass transitions

transforms the mesophases into crystalline materials, which

are reminiscent of phases formed by different star-shapedoligobenzoates. Investigations are in progress to study the

crystallisation process and the packing of stilbenoid mesogens

in these soft crystalline phases.

Experimental section

General methods

Middle pressure liquid chromatography (MPLC) was per-

formed using a Buchi apparatus with silica (J. T. Baker H2272,

5 6 40 cm). Differential scanning calorimetry (DSC) was

performed on a Perkin Elmer DSC 7 instrument. Polarised

optical microscopy (POM) observations were made with a

Zeiss Axioscop 40 equiped with a Linkam THMS600 hot

stage. PFT 1H and 13C NMR spectra were recorded in CDCl3with Bruker AM400, AC200 and ARX400 spectrometers.

Mass spectra were obtained on Finnigan MAT95 (FD MS).

UV/Vis spectra were recorded with a Zeiss MCS 320/340

spectrometer and fluorescence spectra were obtained with a

Perkin Elmer LS 50B instrument. The X-ray diffraction was

measured on a Siemens D500 diffractometer or a Kratky

Compact Camara with a Braun detector (Cu Ka radiation,

l = 0.154 nm). The WAXS measurements on aligned samples

obtained by extrusion were made by using a rotating anode

(Rigaku 18 kW) source with pinhole collimation equipped with

a graphite double monochromator (l = 0.154 nm) and a

Siemens area detector with 1024 6 1024 pixels.

(E,E,E,E,E)-1,3-Bis{2-[4-(2-{3,4,5-tridodecyloxyphenyl}-

ethenyl)phenyl]ethenyl}-5-[2-(4-methoxyphenyl)ethenyl]benzene

(3a)

Potassium-tert-butylat (3.00 g, 26 mmol) was dissolved in

60 ml THF under argon and cooled with an ice bath to 0 uC.

A mixture of aldehyde 2 (1.50 g, 0.93 mmol) and diethyl

methoxybenzylphosphonate 4a18 (0.25 g, 0.97 mmol) in THF

was added dropwise. The solution was then stirred at room

temperature (RT) for 48 h, poured on 100 g crushed ice and

50 ml HCl (18%) were added. The crude solid product was

Fig. 12 Irradiation of 1e in the glassy LC state at RT. A before

irradiation; B after irradiation for 6 h 15 min; C irradiated area at

84 uC; D irradiated area after cooling from the isotropic liquid to the

glassy LC state at RT.



9/11

collected, dissolved in CHCl3 and precipitated by addition of

ethanol. A further purification step by column chromatogra-

phy (petrol ether (4070 uC)acetone = 50 : 1) furnished 456 mg

(29%) of a light yellow solid, mp 79 uC; 1H NMR (400 MHz,

CDCl3): d = 0.87 (m, 18H; CH3), 1.261.85 (m, 120H; CH2),

3.97, 4.02 (2t, 12H; OCH2), 6.71 (s, 4H; aromat. H), 6.92

(AA9BB9, 2H; aromat. H), 6.96 (d, 3J = 16.2 Hz, 2H; olefin.

H), 7.01, 7.15 (2d, 2H,

3

J = 16.4 Hz; olefin. H), 7.03 (d,

3

J =16.1 Hz, 2H; olefin. H), 7.14 (d, 3J = 16.2 Hz, 2H; olefin. H),

7.19 (d, 3J = 16.1 Hz, 2H; olefin. H), 7.488 (AA9BB9, 2H;

aromat. H), 7.493, 7.53 (AA9BB9, 8H; aromat. H), 7.52 (s,

3H; 2H, 4H, 6H); 13C NMR (100 MHz, CDCl3): d = 14.1

(CH3), 22.731.9 (CH2), 55.3 (OCH3), 69.3, 73.6 (OCH2),

105.6, 114.3, 123.6, 123.8 (aromat. CH), 126.3, 128.9 (olefin.

CH), 126.7, 126.9 (aromat. CH), 127.3 (olefin. CH), 127.8

(aromat. CH), 128.3, 128.9, 129.0 (olefin. CH), 130.1, 132.6,

136.5, 137.0, 138.1, 138.5, 138.7 (Cq, CqO), 153.4 (CqO); FD

MS: m/z (%): 1723.7 (76, M+?), 1724.7 (100), 1725.7 (82),

1726.7 (41); elemental analysis: calcd for C119H182O7: C 82.87,

H 10.64; found C 83.09, H 10.70.


ethenyl)phenyl]ethenyl}-5-[2-(2-methylphenyl)ethenyl]benzene

(3b)

Preparation analogous to 3a using diethyl 2-methylbenzyl-

phosphonate 4b.19 The precipitated product was purified by

column chromatography (hexaneacetone = 30 : 1). Yield

436 mg (28%) of a yellow solid, mp 46 uC; 1H NMR (400 MHz,

CDCl3): d = 0.87 (t, 18H, CH3), 1.251.85 (m, 120H, CH2),

2.47 (s, 3H, ArCH3), 3.96, 4.02 (t, 12H, OCH2), 6.71 (s, 4H;

aromat. H), 6.96 (d, 3J = 16.1 Hz, 2H; olefin. H), 7.03 (d, 3J=

16.1 Hz, 2H; olefin. H), 7.16, 7.20 (2d, 3J= 16.1 Hz, 4H; olefin.

H), 7.177.25 (m, 3H; aromat. H), 7.03, 7.41 (2d, 3J= 16.2 Hz,

2H; olefin. H), 7.50, 7.53 (AA9BB9, 8H; aromat. H), 7.54 (s,

2H; 4H, 6H), 7.58 (s, 1H; 2H), 7.62 (m, 1H; aromat. H); 13C

NMR (100 MHz, CDCl3): d = 14.1 (CH3), 20.0 (ArCH3),

22.731.9 (CH2), 69.4, 73.6 (OCH2), 105.6, 123.7, 124.1, 125.5,

126.2, 126.7, 126.9 (aromat. CH), 126.7, 129.8, 127.3 (olefin.

CH), 127.7 (aromat. CH), 128.2, 128.96, 129.02 (olefin. CH),

130.5 (aromat. CH), 132.5, 135.9, 136.37, 136.43, 137.1, 138.2,

138.5, 138.7 (Cq, CqO), 153.4 (CqO); FD MS: m/z (%): 1708.1

(56, M+?), 1709.1 (100), 1710.2 (79), 1711.1 (16); elemental

analysis: calcd for C119H182O6: C 83.65, H 10.74; found C

83.38, H 10.82.


ethenyl)phenyl]ethenyl}-5-(2-phenylethenyl)benzene (3c)

Preparation analogous to 3a using diethyl benzylphosphonate

4c.20 The precipitated product was purified by column

chromatography on basic alumina (petrol ether (4070 uC)

acetone = 95 : 1) and subsequent middle pressure liquid

chromatography (MPLC) on silica (petrol ether (4070 uC)

acetone = 50 : 1), which afforded 568 mg (36%) of a yellow

solid, mp 80 uC; 1H NMR (400 MHz, CDCl3): d = 0.88 (t,

18H; CH3), 1.261.86 (m, 120H; CH2), 3.97, 4.02 (2t, 12H;

OCH2), 6.71 (s, 4H; aromat. H), 6.97 (d,3J = 16.2 Hz, 2H;

olefin. H), 7.03 (d, 3J = 16.1 Hz, 2H; olefin. H), 7.14 (d, 3J =

16.4 Hz, 3H; olefin. H), 7.20 (d, 3J = 16.4 Hz, 2H; olefin. H),

7.21 (d, 3J= 16.4 Hz, 1H; olefin. H), 7.28 (m, 1H; aromat. H),

7.38 (m, 2H; aromat. H), 7.50, 7.53 (AA9BB9, 8H; aromat. H),

7.55 (s, 3H; 2H, 4H, 6H), 7.55 (m, 2H; aromat. H); 13C

NMR (100 MHz, CDCl3): d = 14.0 (CH3), 22.731.9 (CH2),

69.3, 73.5 (OCH2), 105.5, 123.9, 126.6, 126.7, 126.9 (aromat.

CH), 127.2 (olefin. CH), 127.7 (aromat. CH), 128.1, 128.4,

129.2 (olefin. CH), 128.7 (aromat. CH), 128.8, 129.0 (olefin.

CH), 132.5, 136.4, 137.0, 137.3, 138.1, 138.6 (Cq, CqO), 153.3(CqO); FD MS: m/z (%): 1693.6 (72, M+N), 1694.5 (100), 1695.5

(56), 1696.7 (44); elemental analysis: calcd for C118H180O6: C

83.63, H 10.71; found C 83.63, H 10.73.


ethenyl)phenyl]ethenyl}-5-[2-(4-bromophenyl)ethenyl]benzene

(3d)

NaH (0.30 g, 12 mmol) was given to 60 ml THF under argon

and cooled with an ice bath to 0 uC. A mixture of aldehyde 2

(1.52 g, 0.93 mmol) and diethyl 4-bromobenzylphosphonate

4d21 (0.32 g, 1.05 mmol) in 40 ml THF were added dropwise.

The reaction mixture was then stirred at RT for 24 h, poured

on 100 g ice and 50 ml HCl (2 N) were added. The precipitate

was collected and recrystallised from CHCl3ethanol = 1 : 1.

1.01 g of the yellow solid (1.50 g) was then further purified by

column chromatography (silica, CH2Cl2hexane = 4 : 6) Yield

0.42 g (37%) of light yellow solid, Tcl = 65 uC;1H NMR

(400 MHz, CDCl3): d = 0.87 (m, 18H; CH3), 1.251.85 (m,

120H; CH2), 3.96, 4.01 (2t, 12H; OCH2), 6.71 (s, 4H; aromat.

H), 6.96 (d, 3J= 16.1 Hz, 2H; olefin. H), 7.03 (d, 3J= 16.1 Hz,

2H; olefin. H), 7.12 (AB, 2H; olefin. H), 7.13 (d, 3J= 16.4 Hz,

2H; olefin. H), 7.19 (d, 3J = 16.4 Hz, 2H; olefin. H), 7.40

(AA9BB9, 2H; aromat. H), 7.50 (m, 12H; aromat. H), 7.56

(s, 1H; 2H); 13C NMR (100 MHz, CDCl3): d = 14.1 (CH3),

22.731.9 (CH2), 69.3, 73.6 (OCH2), 105.5 (aromat. CH),

121.5 (Cq), 124.0, 124.2, 126.8, 126.9 (aromat. CH), 127.2

(olefin. CH), 128.0, 129.2 (olefin. and aromat. CH partially

superimposed), 129.1 (olefin. CH), 131.8 (aromat. CH),

132.5, 136.2, 136.3, 137.1, 137.7, 138.2, 138.7 (Cq, CqO),

153.4 (CqO); FD MS: m/z (%): 1771.7 (76, M+?), 1772.9 (91),

1773.8 (100), 1774.9 (86), 1776.1 (69); elemental analysis:

calcd for C118H179BrO6: C 79.91, H 10.17; found C 79.72,

H 10.20.


ethenyl)phenyl]ethenyl}-5-[2-(4-cyanophenyl)ethenyl]benzene

(3e)

Preparation analogous to 3d using diethyl 4-cyanobenzyl-phosphonate 4e.23 The crude product was purified by column

chromatography on silica (toluene) and subsequent recrystal-

lisation from CHCl3ethanol = 1 : 1, which afforded 496 mg

(31%) of a yellow solid, Tcl = 92 uC;1H NMR (400 MHz,

CDCl3): d = 0.87 (m, 18H; CH3), 1.261.85 (m, 120H; CH2),

3.97, 4.02 (2t, 12H; OCH2), 6.71 (s, 4H; aromat. H), 6.96 (d,3J = 16.4 Hz, 2H; olefin. H), 7.03 (d, 3J= 16.2 Hz, 2H; olefin.

H), 7.13 (d, 3J = 16.4 Hz, 2H; olefin. H), 7.16, 7.24 (2d, 3J =

16.2 Hz, 2H; olefin. H), 7.19 (d, 3J = 16.4 Hz, 2H, olefin. H),

7.50, 7.53 (AA9BB9, 8H, aromat. H), 7.54 (s, 2H; 4H, 6H),

7.59 (s, 1H; 2H), 7.60, 7.65 (AA9BB9, 4H; aromat. H);13C

NMR (100 MHz, CDCl3): d = 14.0 (CH3), 22.631.9 (CH2),



10/11

69.3, 73.5 (OCH2), 105.5 (aromat. CH), 110.8, 118.9 (Cq),

124.2, 124.7, 126.7, 126.93, 129.90 (aromat. CH), 127.1, 127.3,

132.1, 127.8, 129.1, 129.2 (olefin. CH), 132.4 (aromat. CH and

Cq superimposed), 136.2, 137.1, 137.2, 138.3, 138.7, 141.7 (Cq,

CqO), 153.4 (CqO). FD MS: m/z (%): 1719.0 (86, M+?), 1720.0

(100), 1721.0 (70), 1722.0 (34), 1723.0 (12); elemental analysis:

calcd for C119H179NO6: C 83.11, H 10.49, N 0.81; found C

83.04, H 10.34, N 0.89.

(E,E,E,E,E)-4-{2-[3,5-Bis(2-{4-[2-(3,4,5-tridodecyloxyphenyl)-

ethenyl]phenyl}ethenyl)phenyl]ethenyl}benzoic acid amide (3f)

Preparation analogous to 3a using diethyl 4-cyanobenzyl-

phosphonate 4e.23 The precipitated product was purified by

column chromatography (MPLC) on silica (petrol ether (40

70 uC)acetone = 3 : 2), which yielded 430 mg (27%) of a waxy

yellow solid, Tcl = 189 uC;1H NMR (400 MHz, CDCl3): d =

0.86 (m, 18H; CH3), 1.251.85 (m, 120H; CH2), 3.96, 4.01 (2t,

12H; OCH2), 6.71 (s, 4H; aromat. H), 6.96 (d,3J = 16.1 Hz,

2H; olefin. H), 7.03 (d, 3J = 16.4 Hz, 2H; olefin. H), 7.13 (d,3J= 16.2 Hz, 2H; olefin. H), 7.19 (d, 3J = 16.2 Hz, 2H; olefin.

H), 7.19, 7.24 (2d, 3J = 16.4 Hz, 2H; olefin. H), 7.49, 7.53

(AA9BB9, 8H; aromat. H), 7.55 (s, 2H; aromat. H), 7.57

(s, 1H; aromat. H), 7.60, 7.82 (AA9BB9, 4H; 2H, 3H, 5H,

6H); 13C NMR (100 MHz, CDCl3): d = 14.0 (CH3), 22.731.9

(CH2), 69.3, 73.6 (OCH2), 105.5, 124.1, 124.4, 126.6, 126.8,

127.0 (aromat. CH), 127.2 (olefin. CH), 127.9 (aromat. CH),

128.0, 128.1, 132.2, 129.1 (olefin. CH), 130.7, 132.5, 136.3,

137.1, 137.5, 138.2, 138.7, 140.9 (Cq, CqO), 153.4 (CqO), 168.8

(1Cq, amide); FD MS: m/z (%): 1737.2 (100, M+?), 1738.2 (77),

1739.2 (53), 1740.3 (29); elemental analysis: calcd for

C119H181NO7: C 82.25, H 10.50, N 0.81; found C 82.05, H

10.62, N 0.76.

(E,E,E,E,E)-4-{2-[3,5-Bis(2-{4-[2-(3,4,5-tridodecyloxyphenyl)-

ethenyl]phenyl}ethenyl)phenyl]ethenyl}benzaldehyde (3g)

Preparation analogous to 3d using diethyl 4-diethoxymethyl-

benzylphosphonate 4f.22 After 24 h, the reaction mixture was

poured on 100 g ice, 50 ml HCl (18%) and 100 ml CHCl3 were

added. The mixture was stirred until the cleavage of the acetal

was completed (approx. 3 h). The crude product was purified

by column chromatography on silica (petrol ether (4070 uC)

toluene = 1 : 4). Yield 501 mg (35%) of a yellow solid, mp =

100 uC; 1H NMR (400 MHz, CDCl3): d = 0.88 (m, 18H; CH3),

1.261.85 (m, 120H; CH2), 3.97, 4.02 (2t, 12H; OCH2), 6.71 (s,

4H; aromat. H), 6.96 (d, 3J = 16.4 Hz, 2H; olefin. H), 7.03 (d,

3J= 16.2 Hz, 2H; olefin. H), 7.13 (d, 3J = 16.2 Hz, 2H; olefin.H), 7.18 (d, 3J = 16.4 Hz, 2H; olefin. H), 7.20, 7.28 (2d, 3J =

16.4 Hz, 2H; olefin. H), 7.49, 7.53 (AA9BB9, 8H; aromat. H),

7.55 (s, 2H; 4H, 6H), 7.57 (s, 1H; 2H), 7.67, 7.88

(AA9BB9, 4H; aromat. H), 9.98 (s, 1H, CHO); 13C NMR

(100 MHz, CDCl3): d = 14.1 (CH3), 22.731.9 (CH2), 69.4,

73.6 (OCH2), 105.7, 124.2, 124.7, 126.8, 127.0, 127.0, 130.3

(aromat. CH), 127.2, 127.9, 128.0, 132.0, 129.2, 129.3 (olefin.

CH), 132.5, 135.6, 136.3, 137.2, 137.5, 138.4, 138.9, 143.4

(Cq, CqO), 153.4 (CqO), 191.4 (CHO); FD MS: m/z (%): 1721.6

(67, M+?), 1722.6 (100), 1723.5 (55), 1724.6 (10); elemental

analysis: calcd for C119H180O7: C 82.97, H 10.53; found C

82.67, H 10.37.

Acknowledgements

We are grateful to Prof. Jochen Gutmann and Michael Bach

for their strong support during the X-ray measurements, to

Dr Volker Abetz for measurements with the Kratky camera

and to the Deutsche Forschungsgemeinschaft and the Fonds

der Chemischen Industrie for financial support.

References

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28 The preliminary apparent model of antiparallel stacked mesogensagrees well with the columnar diameter, however, the assembly offour molecules in a columnar slice of only 4.05 A in height (hcol) isnot yet evident. The experimental values (hcol and z) may beexplained, if two antiparallel aligned mesogens are inclined versusthe columnar axis (see also ref. 29a) and a helical arrangement isassumed.

29 (a) B. Donnio, B. Heinrich, H. Allouchi, J. Kain, S. Diele,

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31 (a) Beilstein E IV 9/2, 892; (b) Beilstein E IV, 7/2, 506; (c) BeilsteinE IV, 9/2, 725.

32 The dipoles were also calculated from AM1 minimised models(Chem3D Ultra 8.0): Dipoles along the bisect of 4.08 D for the

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Thi j l i Th R l S i t f Ch i t 2006 J M t Ch 2006 16 441 451 | 451

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