Intermolecular organisation of triphenylene-based discotic mesogens by interdigitation of alkyl chains

Article abstract of DOI:10.1039/b006916g

Two series of hexakis(alkyloxy)triphenylenes have been synthesised and characterised. All materials contain two different lengths of n-alkyloxy chains. Series I (C_mC_n) materials contain three -OC₅H₁₁ chains and three -OC_nH_2n+1 chains. Series II (C_mC_n) materials contain three -OC_mH_2m+1 chains and three -OC_nH_2n+1 chains, such that m+n = 10. Hexagonal columnar mesophases are observed in Series I for n≥3 and, in Series II, for C₅C₅ and C₆C₄. Investigation by XRD shows that, within Series I, increasing the C_n chain length increases the intercolumnar spacing. Furthermore, the intercolumnar spacings for constitutional isomers are identical. The difference in the lengths of the two types of alkyl chain at the periphery of the molecules is defined as the Interdigitation Length (IL) and is a measure of the maximum extent of interdigitation that is possible between neighbouring molecules. Series I and II materials have been studied to probe the effect of IL on the change in enthalpy (ΔH_Col-1) and entropy (ΔS_Col-1) for the mesophase to isotropic liquid phase transition. In contrast to C_xC_x materials, which show a decrease in ΔH_Col-1 and ΔS_Col-1 with increasing chain length, Series I and II mesogens all exhibit ΔH_Col-1 and ΔS_Col-1 values similar to those of long chain C_xC_x materials, irrespective of chain length.

Full text of DOI:10.1039/b006916g

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Intermolecular organisation of triphenylene-based discotic mesogens
by interdigitation of alkyl chains{
Matthew T. Allen,a Siegmar Diele,b Kenneth D. M. Harris,*a Torsten Hegmann,c Benson
M. Kariuki,b Dirk Lose,b Jon A. Preece*a and Carsten Tschierskec
aSchool of Chemistry, The University of Birmingham, Edgbaston, Birmingham, UK B15 2TT.
E-mail: K.D.M.Harris@bham.ac.uk, J.A.Preece@bham.ac.uk
bInstitute of Physical Chemistry, Martin-Luther-University Halle, MuÈhlpforte 1, D-06118
Halle, Germany
cInstitute of Organic Chemistry, Martin-Luther-University Halle, Kurt-Mothes-Strasse 2, D-
06120 Halle, Germany
Received 24th August 2000, Accepted 11th October 2000
First published as an Advance Article on the web 1st December 2000
Two series of hexakis(alkyloxy)triphenylenes have been synthesised and characterised. All materials contain two
different lengths of n-alkyloxy chains. Series I (CmCn) materials contain three ±OC5H11 chains and three ±
OCnH2nz1 chains. Series II (CmCn) materials contain three ±OCmH2mz1 chains and three ±OCnH2nz1 chains,
such that mzn ~10. Hexagonal columnar mesophases are observed in Series I for n¢3 and, in Series II, for
C5C5 and C6C4. Investigation by XRD shows that, within Series I, increasing the Cn chain length increases the
intercolumnar spacing. Furthermore, the intercolumnar spacings for constitutional isomers are identical. The
difference in the lengths of the two types of alkyl chain at the periphery of the molecules is de®ned as the
Interdigitation Length (IL) and is a measure of the maximum extent of interdigitation that is possible between
neighbouring molecules. Series I and II materials have been studied to probe the effect of IL on the change in
enthalpy (DHCol±I) and entropy (DSCol±I) for the mesophase to isotropic liquid phase transition. In contrast to
CxCx materials, which show a decrease in DHCol±I and DSCol±I with increasing chain length, Series I and II
mesogens all exhibit DHCol±I and DSCol±I values similar to those of long chain CxCx materials, irrespective of
chain length.
triphenylenes form the Colh mesophase22 and have been shown
1. Introduction
to exhibit photoconduction.8 Charge mobilities for hexakis(-
pentyloxy)triphenylene are in the region of 1023 cm2 V21 s21
(see ref. 23). In contrast, hexakis(hexylthio)triphenylene forms
a helical Colh phase, which provides increased order of the
mesophase24 and a charge mobility of 0.1 cm2 V21 s21 is
observed, the highest charge mobility for a triphenylene-based
photoconducting system (for comparison, we note that several
one-dimensional organic metals are known,25±27 the proto-
typical material of which is the co-crystal formed between
Since their discovery in 1977 by Chandrasekhar,1 discotic
mesogens have been of great interest to the liquid crystal
community. The ®rst discotic liquid crystal was a hexa-
substituted benzene derivative1 and now there is a wide range
of discotic cores, including, for example, triphenylene,2,3
phthalocyanine4 and pyrene5 derivatives. Discotic molecules
can form a variety of mesophases. The least ordered phase is
the nematic discotic (Nd),6,7 in which the molecular directors
are aligned but with a random distribution of the molecules
parallel to this director. More ordered phases are the hexagonal
tetrathiafulvalene
(TTF : TCNQ), which exhibits
1 cm2 V21 s21 and a conductivity of 56102 V21 cm21 (cf. Si
which has
conductivity of 161023 V21 cm21)16). An
and
tetracyano-p-quinodimethane
a
charge mobility of
columnar (Colh),8±12 rectangular columnar (Colr)13 and
oblique columnar (Colob
14
)
phases.
a
Many potential applications for discotic mesogens aim to
exploit the phenomenon of photoconductivity,15 in which
electrons are excited to conduction bands yielding a photo-
current.16 Photoconductivity has been observed in inorganic
materials16 such as crystalline and amorphous silicon, sele-
nium±tellurium alloys, cadmium sul®de and zinc oxide. Silicon,
for example, exhibits charge mobilities in the range of 0.5±
1.56103 cm3 V21 s21. Examples of organic materials that
exhibit photoconductivity include crystalline anthracene16±20
and the charge-transfer complex of poly(vinylcarbazole) and
trinitro¯uorenone (PVK : TNF).21
important aspect in using liquid crystalline materials as the
basis of photoconductive systems is the fact that, on aligning
the mesophase due to an external stimulus, the grain boundary
defects (which trap charge in crystalline samples) do not exist.
The liquid nature of the mesophase allows for a self-repair16
mechanism and also provides good contact to the surface of
metals, which is essential for device applications.
In view of the fact that increased charge mobilities appear to
be associated with increased ordering of the columnar
mesophases, the thrust of much research on triphenylene-
based liquid crystals has been to introduce chemical modi®ca-
tions designed to increase the columnar ordering in terms of the
average length of ordered stacking arrays along the column.
Many examples of structural variations based on the simple
hexakis(alkyloxy)triphenylenes8±12 have been reported, includ-
ing hexakis(alkylthio)triphenylenes,28,29 polymer systems30±33
and triphenylene oligomer formation,34,35 as well as halogena-
tion36 and nitration37,38 of the aromatic core. However, none of
Liquid crystalline materials have been shown to exhibit
photoconductive properties.8 For example, hexakis(alkyloxy)-
{X-Ray diffraction data, quantities of reagents used in the syntheses
and spectroscopic data for C5Cn compounds are available as
supplementary data. For direct electronic access see http://
www.rsc.org/suppdata/jm/b0/b006916g/
302
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This journal is # The Royal Society of Chemistry 2001
DOI: 10.1039/b006916g

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these chemical modi®cations has led to an increase in the
columnar ordering with a commensurate increase in photo-
conductivity. Recently, some of us have studied the adhesive
forces generated by the interdigitation of alkyl chains39 using
atomic force microscopy. It was shown that by tailoring the
interdigitating alkyl chains, the adhesive force between two
surfaces could be controlled. Clearly it may be possible to use
the idea of interdigitation of alkyl chains to in¯uence the
ordering in the hexagonal columnar mesophase of hexakis(alk-
yloxy)triphenylene liquid crystals. In this regard, the aim of the
work described in the present paper has been to design and
prepare appropriately derivatized hexakis(alkyloxy)triphenyl-
enes such that the alkyl chains in one column interdigitate into
the alkyl chain region of a neighbouring column.
In order to design an interdigitated system, at least two
different chain lengths are required,40±44 alternating around the
triphenylene core (Fig. 1) and leading to an idealised network
of molecules interlinked in a cog-like fashion (Fig. 2).
Here we report the ®rst synthesis of two systematic series of
such molecules, Series I and Series II45 (Fig. 1), together with
characterisation of their structural and thermal properties
using polarized optical microscopy (POM), differential scan-
ning calorimetry (DSC) and X-ray diffraction (XRD). The two
series differ in the selection of the two types of alkyl groups at
the periphery: CmH2mz1 and CnH2nz1. In the notation adopted
here, CmCn refers to a triphenylene molecule containing
CmH2mz1 and CnH2nz1 alkyl chains. Series I materials contain
three CmH2mz1 (m~5) chains and three CnH2nz1 chains
(n~1±9), whereas Series II materials contain three CmH2mz1
chains (m~5±8) and three CnH2nz1 (n~10±m) chains. For
comparison, we note that hexakis(alkyloxy)triphenylenes,
which contain six CxH2xz1 alkyl chains (CxCx in the notation
adopted here), exhibit Colh mesophases for x¢4. It may be
envisaged that the mesophases of Series I and II materials may
also display Colh mesophases. Furthermore, the synthetic route
adopted allows for the production of a CmCn molecule with C3
symmetry (denoted by the suf®x `.sym', e.g. CmCn.sym) and an
asymmetric isomer (denoted by `.asym', e.g. CmCn.asym),
Scheme 1. The difference between the alkyl chain lengths in a
molecule CmCn (|m2n|) is called the Interdigitation Length (IL)
and is a measure of the extent to which two molecules may
interpenetrate each other through interdigitation. In summary,
Series I provides a family of materials with an increasing
number of alkyl carbon atoms per molecule. Series II provides
a range of constitutional isomers, with varying interdigitation
lengths.
Fig. 2 Idealised packing of triphenylenes in the Colh mesophase: (a)
non-interdigitated C5C5; (b) interdigitated C5C3.sym.
mesophase packing object can be approximated by a disc
(Fig. 2a). Clearly the Series I and II materials represent a
modi®cation of this disc, with protrusions in the region of the
longer alkyl chains and voids in the region of the shorter alkyl
chains, and the packing object thus resembles a propeller
(Fig. 2b). In order to establish ef®cient packing, the protrusions
of one molecule should be directed towards the voids of its
neighbours (Fig. 2b). In effect, the short chains of one molecule
should be close to the longer chains of its neighbours, thus
interdigitating the alkyl regions of the two molecules. The
potential for interdigitation varies both within Series I (for
example, for C5C3 IL~2, for C5C5 IL~0 and for C5C9 IL~4)
and Series II (IL increases from 0 (C5C5), to 6 (C8C2)).
Clearly the formation of interdigitated arrays of triphenyl-
enes may be expected to affect the mobility and packing of the
molecules within the mesophase in terms of intracolumnar
ordering (Fig. 3a), intercolumnar lateral slippage (Fig. 3b) and
rotational motion (Fig. 3c), which may depend signi®cantly on
the intercolumnar interactions.
For CxCx triphenylenes, a space ®lling model shows that the
In this regard, we may de®ne two contrasting effects arising
as a result of interdigitation:
(1) As the extent of interdigitation determines the extent of
interlocking of molecules in neighbouring columns, interdigita-
tion may promote the lateral slippage of molecules from
column to column (Fig. 3b), and may therefore decrease the
stability of the mesophase.
(2) Interdigitation may be expected to hinder the rotational
mobility of the molecules (Fig. 3c), which may lead to an
increase in the intracolumnar p±p interactions, and may
therefore increase the stability of the mesophase.
Fig. 1 Molecular structures of Series I and Series II materials: Series
I:CmCn (m~5, n~1±9); Series II: CmCn (m~5±8, n~102m; such that
mzn~10).
Depending on the balance of these competing factors,
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Scheme 1 General procedure for the synthesis of Series I and Series II compounds: the trihydroxytris(alkyloxy)triphenylenes were synthesized
following preparations described in the literature.46±49 i, R1Br, K2CO3, CH3CN, 82 ³C, 20 h.; ii, D2SO4, D2O, MeOD, 80 ³C, 30 h.
interdigitation of the alkyl chains may be expected either to
increase or decrease the mesophase order. Thus, within a series
of molecules with increasing IL, the mesophase order may vary
in a non-linear fashion, depending on the relative contributions
of these competing factors for each member of the series.
2. Results and discussion
2.1. Synthesis
The two series of triphenylene-based mesogens are illustrated in
Fig. 1.
Series I materials comprise the triphenylene core, sur-
rounded by two different n-alkyloxy chains, ±OR1 and ±OR2.
In Series I, R1~CmH2mz1 (m~5) and R2~CnH2nz1 (n~1±
9). Series II materials again contain six n-alkyloxy chains:
R1~CmH2mz1 (m~5±8) and R2~CnH2nz1 (n~10±m) such
that mzn~10. Series I compounds were synthesised following
the general synthetic route shown in Scheme 1. Catechol was
alkylated with 1-bromopentane as described in the literature.46
Hexakis(pentyloxy)triphenylene, C5C5 (D3h point group), was
prepared by the trimerisation of 1,2-bis(pentyloxy)benzene as
described in the literature, using both the ferric chloride46 route
and molybdenum pentachloride47 route. The formation of the
two trihydroxy constitutional isomers (Scheme 1), C5C0.sym
(C3h point group) and C5C0.asym (Cs point group), was
achieved using 9-bromobicycloborononane, 9-Br-BBN,48 to
effect the selective tris-dealkylation. The two isomers were
separated by silica gel chromatography, allowing subsequent
alkylation of C5C0.sym and C5C0.asym to yield the constitu-
tionally pure compounds C5Cn.sym and C5Cn.asym. Series II
Fig. 3 (a) Intracolumnar molecular motion. (b) An illustration of the
compounds were synthesised in a similar manner to Series I
compounds, and the general procedure is shown in Scheme 1.
Trimerisation of the 1,2-bis(alkyloxy)benzene was achieved,
concept of intercolumnar `slippage'. (c) An example of the difference in
packing of D3h and C3h symmetry objects in a triangular array and the
effect on axial rotation.
304
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using either ferric chloride46 or molybdenum pentachloride47
and the hexakis(alkyoxy)triphenylene was dealkylated using
either 9-Br-BBN48 or B-bromocatecholborane,49 to afford the
trihydroxy constitutional isomers CmC0.sym and CmC0.asym.
Separation of these isomers was achieved by silica gel
chromatography and the subsequent re-alkylation yielded the
constitutional isomers CmCn.sym and CmCn.asym.
Aromatic deuteration of triphenylene materials has been
carried out for the purposes of investigating mesophase motion
by 2H NMR. The triphenylene material was re¯uxed for
72 hours in a mixture of D2SO4, D2O and MeOD. The degree
of deuteration, analysed by mass spectrometry, showed that,
on average, ®ve to six aromatic protons had been exchanged
for deuterium.
2.2. Series I
2.2.1. Optical characterisation. POM studies show that the
Series I materials exhibit a mesophase for n~3±9. These
materials display characteristic defect textures for the columnar
hexagonal (Colh) mesophase, examples of which are shown in
Fig. 4 for C5C3.sym, C5C7.sym and C5C9.sym. The materials
C5C1.sym and C5C2.sym do not exhibit transitions to
mesophases.
2.2.2. X-Ray diffraction analysis. Structural analysis in the
mesophases by X-ray diffraction has been carried out using
photographic and, for C5C3.sym and C5C9.sym, diffractometric
methods.
The X-ray diffraction photographs (Fig. S1 in Supplemen-
tary Information) con®rm the formation of a Colh mesophase
for the materials from C5C3.sym to C5C9.sym. From the X-ray
diffraction photographs (Fig. S1 in Supplementary Informa-
tion) the intercolumnar spacings for each material can be
determined. The d-spacing (denoted d100) of the intense, sharp
(100) peak at low diffraction angles gives the perpendicular
distance between adjacent lattice planes (Fig. 5). The inter-
columnar spacing for a hexagonal lattice is obtained by
division of d100 by (cos 30³). For the members of Series I that
form mesophases, there is a monotonic increase in the
intercolumnar spacing as n increases, as illustrated in Fig. 6a
and 6b. The increasing intercolumnar spacing is expected on
the basis of the interdigitation model (Fig. 2), as the total alkyl
chain length for the shorter and longer chains increases from 8
carbon atoms (C5C3.sym) to 14 carbon atoms (C5C9.sym)
within Series I and, assuming that the densities of the materials
in their mesophases are similar, should give rise to an
increasing intercolumnar spacing (Fig. 6a and 6b). Note that
the densities of all Series I and II materials, estimated from the
unit cell volumes, are close to 1 g cm23 (Fig. 6a), with evidence
for a small odd±even effect. Comparison of the intercolumnar
spacings for constitutional isomers of the Series I materials and
8
50
for the corresponding CxCx materials (C4C4 and C7C7
plotted in Fig. 6b) show very similar intercolumnar spacings
Ê
for C5C3.sym and C4C4 (18.5 A) and for C5C9.sym and C7C7
Fig. 4 Mesophase textures for (a) C5C3.sym, (b) C5C7.sym and (c)
C5C9.sym, viewed through crossed polarizers.
Ê
Ê
(22.2 A and 22.6 A respectively). This observation is consistent
with the interdigitation packing model being adopted in the
mesophases of the Series I materials.
(001) peak at h#12.5³ corresponds to the repeat distance
(dintra~d001) of the triphenylene cores along the columns. The
For CxCx discotic compounds, it is not common to observe
the (210) re¯ection for the mesophase, with the exception of
C4C4, which displays several re¯ections in the mesophase.51
However, the (210) re¯ection is observed for the following
C5Cn materials in Series I (Fig. S2 in Supplementary Informa-
tion): C5C3.sym, C5C4.sym, C5C7.sym and C5C9.sym. Further-
more, C5C3.sym also exhibits the (200) re¯ection (Fig. S2 in
Supplementary Information).
The X-ray diffraction photographs for the mesophases
(Fig. S1 in Supplementary Information) also provide informa-
tion about the intermolecular spacing within a column and thus
gives information on the nature of the p±p stacking. The broad
8
observed values of ca. 3.6 A are typical for p±p stacking. The
Ê
position of the broad (001) peak is temperature dependent,
such that dintra increases on increasing temperature. For Series
Ê
I materials, the value of dintra is 3.55 A at the low temperature
end of the mesophase, whereas close to the clearing
Ê
temperature dintra is ca. 3.65 A (Fig. 7). In addition, the
peaks become broader with increasing temperature, indicating
a decrease in the degree of ordering along the columns (for
example, Fig. 3a). This observation may be interpreted on the
basis that the average length of the ordered domains along the
column decrease as temperature is increased. Furthermore, on
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Fig. 5 De®nitions of d100 and intercolumnar spacing for the Colh
mesophase.
increasing n, we note that the width of the (001) peak increases
(Fig. 8) and exhibits an odd±even effect.
2.2.3. DSC Analysis. The phase transition temperatures
were initially established from the POM studies and then
measured accurately by DSC. The mesophase ranges of the
Series I and II materials are shown in Fig. 9a±d, and transition
enthalpies (DHCol±I) and transition entropies (DSCol±I) are
shown in Fig. 10a and 10b. Table 1 details the phase transition
data for Series I materials. Mesogenic behaviour is observed
for C5Cn materials with n¢3. This observation is interesting
because C5C3 is the constitutional isomer of C4C4, the member
of the CxCx series with the smallest value of x that forms a
mesophase. Fig. 9c illustrates an increase in clearing tempera-
ture from C5C3.sym (Tc~105 ³C) to C5C4.sym (Tc~129 ³C).
On further increasing n, the clearing temperature decreases to
122 ³C (C5C5) and ®nally to 59 ³C (C5C9). This decrease in
clearing temperature is similar to that observed in the CxCx (for
x¢4). C4C4 exhibits the highest Tc for the CxCx series, with Tc
decreasing as x increases (Fig. 9b).27 Fig. 9b also shows that the
mesophase ranges vary with n for C5Cn materials. The
mesophase range reaches a maximum for C5C4.sym (74 ³C)
and is then depressed as n increases. Additionally, direct
comparisons of constitutional isomers in the CxCx and C5Cn
series reveals in all cases that the CxCx compound displays a
larger mesophase range and a higher clearing temperature.
On comparing the `sym' and `asym' isomers of the
compounds in Series I, it is apparent that the molecular
symmetry has little effect on the melting points and clearing
temperatures, with the exception of C5C9.asym, which exhibits
lower melting and clearing temperatures. It is interesting to
note that C5C9.asym exhibits a mesophase at room tempera-
ture, which could provide the basis for potential applications
under ambient conditions.
Fig. 6 (a) Calculated densities for CmCn.sym (Series I and II) materials
and CxCx materials. (b) Intercolumnar spacings for CmCn.sym (Series I)
materials(6,fullline),CmCn.sym(SeriesII)materials,C6C4.sym(©),CxCx
CxCx triphenylenes corresponding to the average chain length of the
CmCn.sym material (%).
CxCx materials. Presumably, at least at short range, the `asym'
isomers are able to interdigitate to approximately the same
extent as the `sym' isomers. However, this interdigitated
arrangement may not be so readily propagated to long range
in the `asym' isomers. It would appear that, due to the ¯uid
nature of the mesophases, the short range interdigitation of the
`asym' isomers is suf®cient to obtain similar thermal properties
to the `sym' isomers.
Comparison of Fig. 9b and 9c highlights the difference
between the CxCx materials and the CmCn.sym isomers of
Series I materials. Looking at the crystal to mesophase
transition (K±Colh), for CmCn.sym materials in the range
n~3±8, there is an odd±even effect in the temperature of the
transition. This behaviour is not apparent in the CxCx series,
and may indicate one (or both) of the following effects: (i) the
crystalline packing dictates the melting to the mesophase, with
more favourable packing for the `even' (combined number of
carbon atoms in the two types of alkyl chain is even)
compounds and/or (ii) the mesophase interactions are more
favourable for `odd' compounds. Fig. 9c and 9d illustrate the
differences in the transition temperatures between CmCn.sym
and CmCn.asym materials. Although some differences are
observed in the mesophase range and transition temperatures
between the CmCn.sym and CmCn.asym isomers, they are not as
signi®cant as the differences between the CmCn.sym and the
Fig. 7 The temperature dependence of the (210) and (001) re¯ections
and dintra for C5C3.sym.
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ColhAI transition (Fig. 10b). These results suggest that,
relative to the isotropic liquid, the CmCn mesophases and the
higher homologues of the CxCx series have low enthalpic
stability and a low degree of order. We suggest that the
interdigitation may decrease the mesophase stability primarily
through the slippage process (Fig. 3b). This process may
become available to the longer chain members of the CxCx
series due to the fact that the amount of available `free space'
between the adjacent alkyl chains increases, and may be ®lled
by the alkyl chains of neighbouring molecules. In Fig. 11a, the
intercolumnar spacings8,50 of CxCx materials are plotted as a
function of the chain length x, and shows that the relationship
between intercolumnar spacing and x is not linear. In
particular, as the alkyl chain length (x) increases, the increment
in intercolumnar spacing per methylene group decreases (i.e.
the gradient of the plot in Fig. 11a decreases, although still
remains positive, as x increases). This observation is consistent
with the fact that, as shown in Fig. 11b, there is an increase in
the `free space' between alkyl chains in a CxCx molecule as x
increases, with the proposal this free space becomes ®lled by
alkyl chains of neighbouring molecules to a greater extent as x
increases. Thus, the mesophase stability of CxCx materials may
be expected to decrease as the alkyl chain length increases.
Fig. 8 Widths of (001) peak for Series I and Series II materials,
measured from photographic ®lm.
Analysis of the enthalpy and entropy data of Series I
materials for the ColhAI transition shows interesting patterns.
As shown in Fig. 10a, CmCn.sym and CmCn.asym materials
from Series I and II have similar values (y3 kJ mol21) of the
enthalpy change for this transition. In comparison, the
enthalpy change for CxCx materials is higher than that of
their constitutional isomers in Series I at short chain lengths,
but is similar (converging to y3 kJ mol21) at longer chain
lengths. The same pattern exists for the entropy changes for the
2.3. Series II
2.3.1. Optical characterisation. Only two members of Series
II exhibit mesogenic behaviour, namely C5C5 and C6C4.sym. In
POM studies, both of these compounds exhibit a defect texture
typical of the Colh phase, Fig. 12a and 12b.
Fig. 9 (a) Mesophase ranges for Series I and II materials (`sym' and `asym'), and their CxCx constitutional isomers. (b) Phase transitions of CxCx
triphenylene materials. (c) Phase transitions of symmetrical isomers of Series I and II mesogenic materials. (d) Phase transitions of asymmetrical
isomers of Series I and II mesogenic materials.
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Fig. 11 (a) Intercolumnar spacing of CxCx materials as a function of
chain length, x. (b) An illustration of the increase in `free-space'
(indicated by the white regions) between alkyl chains with increasing
alkyl chain length for (i) C4C4 and (ii) C9C9 triphenylene materials.
The intracolumnar stacking distance, determined from the d-
Ê
Fig. 10 (a) Phase transition enthalpy data for Series I and II materials,
compared to literature values for CxCx materials. (b) Phase transition
entropy data for Series I and II materials, compared to literature values
for CxCx materials.
spacing for the (001) peak is y3.6 A, with similar peak widths for
C5C6.sym (IL~1) and C5C7.sym (IL~2), as illustrated in Fig. 8.
2.3.3. DSC Analysis. Phase transition data for Series II
materials are shown in Table 1. Thermodynamic data are
included in Fig. 10a and 10b. The mesophase range of
C6C4.sym is comparable with Series I materials. Indeed, the
mesophase range and Tc value of C6C4.sym is similar to
C5C7.sym, which also has IL~2. The fact that only C6C4 and
C5C5 exhibit mesophases within Series II re¯ects the delicate
balance between anisotropy and isotropy. Clearly, when three
heptyloxy chains are present, three propyloxy chains are not
suf®cient to stabilise the material to the degree required to
exhibit a mesophase. Indeed, perhaps the additional interaction
present through interdigitation may prevent the formation of a
mesophase. Enthalpy and entropy arguments are similar to
those for Series I. Thus, the enthalpy change for C6C4 is around
y3 kJ mol21, similar to that for interdigitated materials in
Series I, with a slight difference between the symmetric and
asymmetric isomers.
2.3.2. X-Ray diffraction analysis. Identi®cation of the
mesophases of Series II compounds by XRD also veri®es
that they are Colh in nature. The intercolumnar spacing for
C6C4.sym, calculated from the d-spacing of the (100) re¯ection,
Ê
is 20.1 A, which is very similar to the intercolumnar spacing for
Ê
C5C5 (19.9 A), a constitutional isomer of C6C4.sym. This
observation suggests that the molecules adopt a reasonably
effective space ®lling motif, achieved by interdigitation of alkyl
chains.
Table 1 Averaged thermal data for Series I and Series II materials.
Transition temperatures, enthalpies and entropies were recorded by
DSC on heating, at rates of 5, 10 and 20 ³C min21. The heating cycles
gave phase transition data which agreed to ¡2%
Transition
temp./³C
DI/kJ
DS/J K21
Compound
Transition
mol21
mol21
C5C1
C5C2
C5C3
CrAI
142
108
68
105
55
129
69
122
48
106
58
91
46
67
44
59
58
56.03
40.66
30.71
5.44
21.71
10.09
34.48
8.20
28.28
5.76
31.73
4.87
21.61
2.11a
61.27
3.15
38.95
1.17
134.03
106.68
89.02
14.38
66.16
25.22
100.81
20.75
82.65
15.19
95.18
13.28
65.26
6.21a
191.44
9.41
117.62
3.17
133.19
141.09
3. Concluding remarks
CrAI
CrAColh
ColhAI
CrAColh
ColhAI
CrAColh
ColhAI
CrAColh
ColhAI
CrAColh
ColhAI
CrAColh
ColhAI
CrAColh
ColhAI
CrAColh
ColhAI
CrAI
In this paper we have presented the ®rst examples of
triphenylene-based liquid crystals, which interdigitate by
design. The successful synthesis of two systematic series of
materials has allowed structural and thermal properties to be
rationalised on the basis of the effects of varying the relative
and absolute lengths of the two types of alkyl chain in the
molecule. X-Ray diffraction and differential scanning calori-
metry have highlighted several interesting issues, which may be
interpreted on the basis of the effect and in¯uence of
interdigitation of alkyl chains. From these results, we conclude
that the interdigitation leads to a decrease in mesophase
stability, which may be interpreted in terms of the effects of the
slippage process dominating over the rotational dynamics.
Further aspects of these materials are under investigation,
including studies of dynamic properties by means of 2H NMR
spectroscopy of the mesophases. Photoconductivity measure-
ments will also be made in the near future on Series I and II
C5C4
C5C5
C5C6
C5C7
C5C8
C5C9
C6C4
96
66
65
C7C3
C8C2
45.17
47.71
CrAI
aData recorded on cooling cycle at 10 ³C min21
.
308
J. Mater. Chem., 2001, 11, 302±311

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ZabSpec mass spectrometer, equipped with a caesium ion
source.
Optical microscopy experiments were carried out using a
Boetius hot stage microscope. Photographs were taken on a
Nikon Optiphot 2 polarizing microscope, equipped with a
Mettler FP 82 HT heating stage and a camera. In addition,
video capture images were recorded on an Olympus BX40
optical microscope, with crossed polarizers, equipped with a
Linkam LT350 hot stage.
Differential Scanning Calorimetry (DSC) results were
recorded on a Perkin-Elmer 7 Series and a Perkin-Elmer
Pyris thermal analysis system. All samples were subjected to 2
heating and cooling cycles, heating to 150±180 ³C (at least
30 ³C above a sample's TC) and cooled to 230 ³C, at a rate of
10 K min21
.
Mesophase X-ray diffraction was carried out on a Huber
Guinier Pulverkammer 621 variable temperature diffract-
ometer, using Cu Ka radiation. Both photographic and
diffractometric measurements were used.
1,2-Bis(pentyloxy)benzene, 1
To a stirred solution of catechol (25.0 g, 0.227 mol) in MeCN
(500 mL) was added K2CO3 (160.0 g, 1.140 mol). The slurry
was heated under re¯ux for 30 minutes under a N2 atmosphere.
A solution of 1-bromopentane (76.0 g, 0.500 mol) in MeCN
(100 mL) was added dropwise to the slurry, maintaining the
re¯ux and stirring. The mixture was heated under re¯ux for
20 hours. The reaction was cooled to room temperature and
®ltered. The inorganic residue was washed with CH2Cl2
(2650 mL). The ®ltrate was concentrated in vacuo and
dissolved in CH2Cl2 (100 mL). The organic layer was washed
with NaOH solution (0.5 M, 50 mL), followed by water
(2650 mL). The organic layer was dried (MgSO4). The
drying agent was removed by ®ltration, the inorganic residue
washed with CH2Cl2 (2625 mL) and concentrated in vacuo,
yielding a light brown oil, 2 (48 g, 83%). m/z (EI) 250 [M]z; dH
(300 MHz, CDCl3) 6.89 (4H, s, Ar-H), 3.99 (4H, t, J~6.6,
OCH2), 1.82 (4H, quintet, J~7.4, CH2), 1.32±1.52 (8H, m,
CH2), 0.92 (6H, t, J~7.2, CH3); dC (75 MHz, CDCl3) 149.2,
121.0, 114.0, 69.2, 29.1, 28.3, 22.5, 14.1.
Fig. 12 Mesophase textures for (a) C5C5 and (b) C6C4, viewed through
crossed polarizers.
materials. In addition, we are currently synthesising a third
series of materials, which are constitutional isomers of C9C9
(i.e. C10C8, C11C7, C12C6 and C13C5), analogous to Series II. It
is anticipated that this series should generate a greater number
of mesogens than Series II, and will contribute substantially
towards progress in developing a complete understanding of
the structural and thermal effects introduced by incorporating
interdigitation as an organisational motif.
2,3,6,7,10,11-Hexakis(pentyloxy)triphenylene, C5C5
A solution of 1 (5.30 g, 21 mmol) in dry CH2Cl2 (40 mL) was
degassed with N2 for 15 minutes. The reaction ¯ask was
covered in aluminium foil and the solution stirred under a N2
atmosphere, with a CaCl2 drying arm attached. MoCl5 (6.37 g,
23 mmol) was added to the solution quickly but with care. The
reaction was stirred at room temperature for 20±30 minutes
and was carefully monitored by TLC [eluent: petroleum ether
(bp 60±80 ³C)±acetone (5 : 1)], until all of 2 had reacted. The
reaction vessel was placed in an ice bath and the reaction
quenched with ice-cold MeOH (75 mL). The mixture was
stirred for 60 minutes at 0 ³C, precipitating a white solid. The
reaction mixture was ®ltered and the residue washed with ice-
cold MeOH (2615 mL). The precipitate was puri®ed by 2
recrystallisations from MeOH, yielding a white, ¯aky solid of
C5C5 (3.35 g, 64%). m/z (EI) 745 [M]z; dH (300 MHz, CDCl3)
7.85 (6H, s, Ar-H), 4.25 (12H, t, J~6.6, OCH2), 1.92±1.99
(12H, m, CH2), 1.42±1.67 (24H, m, CH2), 0.99 (18H, t, J~7.4,
CH3); dC (75 MHz, CDCl3) 148.9, 123.6, 107.2, 69.7, 28.4, 22.6,
14.2 {Found: C 77.31, H 9.72. Calc. for C48H72O6: C 77.38 H
9.74}.
4. Experimental
General procedures
All chemicals were purchased from Aldrich and used without
further puri®cation. Dry CH2Cl2 was prepared by distillation
over CaH2 under a N2 atmosphere. Thin Layer Chromato-
graphy (TLC) was performed on aluminium sheets coated with
silica gel 60 F254 (Merck 5554, 230±400 mesh). Initial inspection
of TLC plates was by use of UV light, and subsequent
development by iodine vapour or 5% CeSO4 solution in EtOH,
followed by heating. Column chromatography was carried out
using silica gel 60 F254 (Merck 9385, 230±400 mesh).
1H NMR spectra were recorded on a Bruker AC300
(300 MHz) spectrometer. 13C NMR were recorded on a
Bruker AC300 (75.5 MHz) spectrometer using the pendant
pulse sequence. All chemical shifts are quoted in ppm on the d
scale, with all coupling constants expressed in hertz (Hz), using
residual solvent as the internal standard.
2,6,10-Tris(pentyloxy)-3,7,11-trihydroxytriphenylene,
C5C0.sym; 2,7,10-tris(pentyloxy)-3,6,11-trihydroxytriphenylene
C5C0.asym
Mass spectra were obtained using Electron Impact Mass
Spectrometry (EIMS), from a VG ProSpec mass spectrometer,
or Liquid Secondary Ion Mass Spectra (LSIMS), from a VG
A solution of C5C5 (2.23 g, 3.00 mmol) in dry CH2Cl2 (25 mL)
was degassed with N2 for 15 minutes. A solution of 9-Br-BBN
J. Mater. Chem., 2001, 11, 302±311
309

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in CH2Cl2 (1.0 M, 13.5 mL, 13.5 mmol) was added dropwise to
the solution over 60 minutes under an atmosphere of N2, whilst
the solution was stirred at 0 ³C. The reaction mixture was
stirred for a further 8 hours at 0 ³C. The reaction vessel was
then transferred to the freezer for a further 30 hours. The
reaction was carefully quenched with 2-aminoethanol (25 mL).
The mixture was diluted with water (25 mL) and the products
extracted into CH2Cl2 (2625 mL). The organic layer was dried
(MgSO4). The drying agent was removed by ®ltration and the
®ltrate was concentrated in vacuo. The two constitutional
isomeric products were isolated from the crude mixture using
silica gel chromatography [eluent:CH2Cl2±MeOH (100 : 1)].
1.88±1.98 (12H, br m, CH2), 1.03 (9H, t, J~7.4, CH3), 0.93
(9H, t, J~7.0, CH3); dC (75 MHz, CDCl3) 149.0, 123.6, 107.3,
69.7, 69.4, 25.9, 22.7, 19.4, 14.0 {Found: C 77.74, H 9.49. Calc.
for C48 H72 O6: C 77.42, H 9.68}.
2,6,10-Tris(heptyloxy)-3,7,11-tris(propyloxy)triphenylene, C7C3
The synthesis of 1,2-bis(heptyloxy)benzene, 3, was carried out
in a similar fashion to 1,2-bis(pentyloxy)benzene, 1, replacing
1-bromopentane with 1-bromoheptane, yielding a brown oil, 3
(4.8 g, 68%). The synthesis of hexakis(heptyloxy)triphenylene,
C7C7, was carried out in a similar fashion to hexakis(pentyl-
oxy)triphenylene, C5C5, using 3 in place of 1, yielding a ¯aky,
white solid, C7C7 (0.91 g, 30%). The synthesis of 2,6,10-
trihydroxy-3,7,11-tris(heptyloxy)triphenylene, C7C0.sym, was
carried out in a similar fashion to 2,6,10-trihydroxy-3,7,11-
tris(pentyloxy)triphenylene, C5C0.sym, using C7C7 in place of
C5C5, yielding a ®ne, white powder, C7C0.sym (0.08 g, 30%); dH
(300 MHz, CDCl3) 7.91 (3H, s, Ar-H), 7.79 (3H, s, Ar-H), 4.25
(6H, t, J~4.3, OCH2), 1.93 (6H, quintet, J~7.0, CH2), 1.34±
1.59 (24H, br m, CH2), 0.91 (9H, t, J~6.8, CH3). To a solution
of C7C0.sym (0.07 g, 0.11 mmol) in CH3CN (20 mL) was added
K2CO3 (0.13 g, 0.90 mmol). The slurry was heated under re¯ux
for 30 minutes under a N2 atmosphere. 1-bromopropane
(0.07 g, 0.57 mmol) was added dropwise to the slurry,
maintaining the re¯ux and stirring. The mixture was heated
under re¯ux for 20 hours. The reaction was cooled to room
temperature and ®ltered; the residue was washed with CH2Cl2
(2610 mL). The ®ltrate was concentrated in vacuo. The crude,
white solid was subjected to two recrystallisations from EtOH,
yielding a pale yellow solid, C7C3 (0.06 g, 74%). m/z (LSIMS)
745 [MzH]z; dH (300 MHz, CDCl3) 7.83 (6H, s, Ar-H), 4.23
(6H, t, J~6.6, OCH2), 4.19 (6H, t, J~6.6, OCH2), 1.89±2.03
(12H, br m, CH2), 1.33±1.61 (24H, br m, CH2), 1.14 (9H, t,
J~7.4, CH3), 0.90 (9H, t, J~6.8, CH3); dC (75 MHz, CDCl3)
149.0, 123.6, 107.4, 71.2, 69.7, 31.9, 29.2, 26.2, 14.2, 10.7
{Found: C 77.62, H 9.71. Calc. for C48 H72 O6: C 77.42, H
9.68}.
C5C0sym. (0.67 g, 42%); m/z (EI) 534 [M]z; dH (300 MHz,
CDCl3) 7.91 (3H, s, Ar-H), 7.78 (3H, s, Ar-H), 5.91 (3H, s,
OH), 4.24 (6H, t, J~6.6, OCH2), 1.89±1.98 (6H, quintet,
J~7.1, CH2), 1.38±1.62 (12H, br m, CH2), 0.98 (9H, t, J~7.2,
CH3); dC (75 MHz, CDCl3) 145.7, 145.2, 123.8, 122.8, 106.9,
104.5, 69.0, 29.0, 28.2, 22.6, 14.1.
C5C0asym. (0.48 g, 30%); dH (300 MHz, CDCl3); 7.70±7.95
(6H, m, Ar-H), 5.87 (3H, t, J~5.46, OH), 4.25 (6H, m, OCH2),
1.76±2.00 (6H, br m, CH2), 1.38±1.70 (12H, br m, CH2), 0.97
(3H, dt, CH3); dC (75 MHz, CDCl3) 145.3, 122.9, 107.7, 107.4,
107.2, 104.3, 104.2, 104.0, 69.1, 56.4, 28.3, 22.6, 14.1.
2,6,10-Tris(pentyloxy)-3,7,11-tris(alkyloxy)triphenylene, C5Cn
Typical quantities for this homologous set of reactions are
shown in Table S1 in Supplementary Information.
K2CO3 was added to a solution of C5C0.sym in MeCN. The
slurry was heated under re¯ux for 30 minutes under a N2
atmosphere. A solution of (R1±Hal) in MeCN was added
dropwise to the slurry, maintaining the re¯ux and stirring. The
mixture was heated under re¯ux for 20 hours. The reaction was
cooled to room temperature and ®ltered; the residue was
washed with CH2Cl2 (2625 mL). The ®ltrate was concentrated
in vacuo. The crude, white solid was subjected to two
recrystallisations from MeOH or EtOH, yielding white/
yellow solids of the triphenylene derivatives. Spectroscopic
data are shown in Table S2 in Supplementary Information.
2,6,10-Tris(octyloxy)-3,7,11-tris(ethyloxy)triphenylene, C8C2
The synthesis of 1,2-bis(octyloxy)benzene, 4, was carried out in
a similar fashion to 1,2-bis(pentyloxy)benzene, 1, replacing 1-
bromopentane with 1-bromooctane, yielding a brown oil, 4
(8.8 g, 58%). dH (300 MHz, CDCl3) 6.88 (4H, s, Ar-H), 3.99
(4H, t, J~6.8, OCH2), 1.81 (4H, quintet, J~7.0, CH2), 1.28±
1.51 (20H, br m, CH2), 0.88 (6H, t, J~6.8, CH3). The synthesis
of hexakis(heptyloxy)triphenylene, C8C8, was carried out in a
similar fashion to hexakis(pentyloxy)triphenylene, C5C5, using
4 in place of 1, yielding a ¯aky, white solid, C8C8 (1.07 g, 49%).
The synthesis of 2,6,10-trihydroxy-3,7,11-tris(octyloxy)triphe-
nylene, C8C0.sym, was carried out in a similar fashion to 2,6,10-
trihydroxy-3,7,11-tris(pentyloxy)triphenylene, C5C0.sym, using
C8C8 in place of C5C5, yielding a ®ne, white powder, C8C0.sym
(0.18 g, 27%); dH (300 MHz, CDCl3) 7.91 (3H, s, Ar-H), 7.78
(3H, s, Ar-H), 4.24 (6H, t, J~6.6, OCH2), 1.93 (6H, quintet,
J~7.0, CH2), 1.25±1.57 (30H, br m, CH2), 0.90 (9H, t, J~6.8,
CH3). To a solution of C8C0.sym (0.15 g, 0.22 mmol) in
CH3CN (20 mL) was added K2CO3 (0.30 g, 2.17 mmol). The
slurry was heated under re¯ux for 30 minutes under a N2
atmosphere. 1-bromoethane (0.18 g, 1.63 mmol) was added
dropwise to the slurry, maintaining the re¯ux and stirring. The
mixture was heated under re¯ux for 20 hours. The reaction was
cooled to room temperature and ®ltered; the residue was
washed with CH2Cl2 (2610 mL). The ®ltrate was concentrated
in vacuo. The crude, white solid was subjected to two
recrystallisations from EtOH, yielding a pale yellow solid,
C8C2 (0.10 g, 58%). m/z (LSIMS) 745 [MzH]z; dH (300 MHz,
CDCl3) 7.84 (3H, s, Ar-H), 7.83 (3H, s, Ar-H), 4.31 (6H, q,
J~7.0, OCH2), 4.23 (6H, t, J~6.6, OCH2), 1.90±1.99 (6H, q,
2,6,10-Tris(hexyloxy)-3,7,11-tris(butyloxy)triphenylene, C6C4
The synthesis of 1,2-bis(hexyloxy)benzene, 2, was carried out in
a similar fashion to 1,2-bis(pentyloxy)benzene, 1, replacing 1-
bromopentane with 1-bromohexane yielding a dark brown oil,
2 (9.1 g, 72%). The synthesis of hexakis(hexyloxy)triphenylene,
C6C6, was carried out in a similar fashion to hexakis(pentyl-
oxy)triphenylene, C5C5, using 2 in place of 1, yielding a ¯aky,
white solid, C6C6 (0.89 g, 43%). The synthesis of 2,6,10-
trihydroxy-3,7,11-tris(hexyloxy)triphenylene, C6C0.sym, was
carried out in a similar fashion to 2,6,10-trihydroxy-3,7,11-
tris(pentyloxy)triphenylene, C5C0.sym, using C6C6 in place of
C5C5, yielding a ®ne, white powder, C6C0.sym. (0.21 g, 35%);
dH (300 MHz, CDCl3) 7.92 (3H, s, Ar-H), 7.79 (3H, s, Ar-H),
4.25 (6H, t, J~6.6, OCH2), 1.93 (6H, quintet, J~7.1, CH2),
1.35±1.56 (18H, br m, CH2), 0.94 (9H, t, J~7.0, CH3). To a
solution of C6C0.sym (0.20 g, 0.35 mmol) in CH3CN (10 mL)
was added K2CO3 (0.41 g, 2.97 mmol). The slurry was heated
under re¯ux for 30 minutes under a N2 atmosphere. 1-
bromobutane (0.21 g, 1.48 mmol) was added dropwise to the
slurry, maintaining the re¯ux and stirring. The mixture was
heated under re¯ux for 20 hours. The reaction was cooled to
room temperature and ®ltered; the residue was washed with
CH2Cl2 (2610 mL). The ®ltrate was concentrated in vacuo.
The crude, white solid was subjected to two recrystallisations
from EtOH, yielding a pale yellow solid, C6C4 (0.23 g, 89%).
m/z (LSIMS) 745 [MzH]z; dH (300 MHz, CDCl3) 7.83 (6H, s,
Ar-H), 4.23 (6H, t, J~6.4, OCH2), 4.22 (6H, t, J~6.6, OCH2),
310
J. Mater. Chem., 2001, 11, 302±311

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P. Schuhmacher and K. Siemensmeyer, Phys. Rev. Lett., 1993, 70,
457.
24 D. Adam, K.-H. Etzbach, D. Haarer, L. HauÈûling, H. Ringsdorf,
P. Schuhmacher, J. Simmerer and K. Siemensmeyer, Nature, 1994,
371, 141.
25 M. R. Bryce, Chem. Soc. Rev., 1991, 20, 355.
26 J. B. Torrence, Acc. Chem. Res., 1979, 12, 79.
27 F. Wudl, Acc. Chem. Res., 1984, 17, 227.
28 D. Adam, D. Haarer, M. P. de Haas, P. Schuhmacher,
J. Simmerer, A. M. van de Craats and J. M. Warman, Adv.
Mater., 1996, 8, 823.
J~7.2, CH2), 1.25±1.58 (39H, br m, CH2, CH3), 0.89 (9H, t,
J~6.8, CH3); dC (75 MHz, CDCl3) 149.0, 148.7, 123.7, 123.5,
115.5, 107.5, 107.0, 70.9, 70.8, 69.8, 69.6, 65.3, 31.9, 29.5, 29.4,
26.2, 22.7, 15.9, 14.2 {Found: C 77.55, H 9.83. Calc. for
C48 H72 O6: C 77.42, H 9.68}.
Acknowledgements
We are grateful to EPSRC, HEFCE, Perkin Elmer and NATO
for ®nancial support.
29 P. Carroll, P. A. Heiney, S. H. J. Idziak, J. P. McCauley and
A. B. Smith III, Mol. Cryst. Liq. Cryst., 1993, 237, 271.
30 N. Boden, R. J. Bushby and A. N. Cammidge, J. Am. Chem. Soc.,
1995, 117, 924.
References
31 B. HuÈser, W. Kranig, W. Kreuder, H. Ringsdorf, H. W. Spiess and
H. Zimmermann, Adv. Mater., 1990, 2, 36.
32 M. M. Green, H. Ringsdorf, J. Wagner and R. WuÈstefeld, Angew.
Chem., Int. Ed. Engl., 1990, 29, 1478.
33 M. Erbert, H. Ringsdorf, J. H. Wendorff, R. WuÈstefeld and
E. Zerta, Angew. Chem., Int. Ed. Engl., 1989, 28, 914.
34 N. Boden, R. J. Bushby, A. N. Cammidge, A. El-Mansoury,
P. S. Martin and Z. Lu, J. Mater. Chem., 1999, 9, 1391.
35 S. Kumar, M. Manickam, J. A. Preece and N. Spencer, Liq. Cryst.,
2000, 5, 703.
36 D. Blunk, A. Eckert and K. Praefcke, Liq. Cryst., 1997, 22, 113.
37 S. Kumar and M. Manickam, Mol. Cryst. Liq. Cryst., 1998, 309,
291.
38 N. Boden, R. J. Bushby and A. N. Cammidge, Mol. Cryst. Liq.
Cryst., 1995, 260, 307.
39 P. R. Ashton, G. Hadziiannou, J. A. Preece and E. van der Vegte,
Langmuir, 2000, 16, 3249.
40 S. J. Cross, J. W. Goodby, A. W. Hall, M. Hird, S. M. Kelly,
K. J. Toyne and C. Wu, Liq. Cryst., 1998, 25, 1.
41 P. Henderson, H. Ringsdorf and P. Schuhmacher, Liq. Cryst.,
1995, 18, 191.
42 N. Boden, R. J. Bushby and A. N. Cammidge, J. Chem. Soc.,
Chem. Commun., 1994, 465.
1
2
3
4
S. Chandrasekhar, B. K. Sadashiva and K. A. Suresh, Pramana,
1977, 9, 471.
J. Billare, J. C. Dubois, N. H. Tinh and A. Zann, Nouv. J. Chim.,
1978, 2, 535.
C. Destrade, J. MaltheÃte and M. C. Mondon, J. Phys. (Paris),
1979, 40, C3±17.
R. J. M. Nolte, C. F. van Nostrum, S. J. Picken and A.-
J. Schouten, J. Am. Chem. Soc., 1995, 117, 9957.
H. Bock and W. Helfrich, Liq. Cryst., 1992, 12, 697.
V. Minter and T. J. Philips, Liq. Cryst., 1996, 20, 243.
J. W. Goodby, P. Hindmarsh, M. Hird and M. J. Watson,
J. Mater. Chem., 1995, 5, 2111.
H. Bengs, F. Closs, T. Frey, D. Funhoff, H. Ringsdorf and
K. Siemensmeyer, Liq. Cryst., 1993, 15, 565.
J. D. Brand, A. van de Craats, Y. Geerts, K. MuÈllen and
J. M. Warman, Adv. Mater., 1998, 10, 36.
5
6
7
8
9
10 C. K. Lai, Y.-S. Pang and C.-H. Tsai, J. Mater. Chem., 1998, 8,
1355.
11 M.-G. Choi, S. H. Kang, H. Kim, K. Kim, S. J. Kim, K.-M. Park
and W.-C. Zin, Chem. Mater., 1998, 10, 1889.
12 P. Espinet, M. C. Lequerica and J. M. Martin-Alverez, Chem. Eur.
J., 1999, 5, 1982.
13 C. Destrade, H. Gasparoux, F. Hardouin, A. M. Levelut and
N. H. Tinh, J. Phys. (Paris), 1981, 42, 147.
14 C. Destrade, A. M. Levelut, J. Malthete, L. Mamlock and
N. H. Tinh, J. Phys. Lett., 1982, 43, L-641.
15 F. Closs, T. Frey, D. Funhoff and K. Siemensmeyer, Liq. Cryst.,
1993, 14, 629.
16 M. G. Carter, in Electronic Materials, eds. L. S. Miller and
J. B. Mullin, Plenum Press, New York, 1991.
17 G. Castro and J. F. Hornig, J. Chem. Phys., 1965, 42, 1459.
18 R. H. Batt, C. L. Braun and J. F. Hornig, J. Chem. Phys., 1968, 49,
1967.
19 P. J. Bounds and W. Siebrand, Chem. Phys., 1980, 75, 414.
20 H. BaÈssler, G. Peter, L. Sebastian and G. Weiser, Chem. Phys.,
1983, 75, 103.
43 J. W. Goodby, M. Hird, K. J. Toyne and T. Watson, J. Chem.
Soc., Chem. Commun., 1994, 1701.
44 M. C. Bernaud, C. Destrade, G. Sigaud and N. H. Tinh, Mol.
Cryst. Liq. Cryst., 1981, 65, 307.
45 M. T. Allen, S. Diele, K. D. M. Harris, T. Hegmann, B. M. Kariuki,
D. Lose, J. A. Preece and C. Tschierske, Liq. Cryst., 2000, 27, 689.
46 N. Boden, R. C. Borner, R. J. Bushby, A. N. Cammidge and
M. V. Jesudason, Liq. Cryst., 1993, 15, 851.
47 S. Kumar and M. Manickam, Chem. Commun., 1997, 1615.
48 F. Closs, L. HaÈuûling, P. Henderson, H. Ringsdorf and
P. Schuhmacher, J. Chem. Soc., Perkin Trans. 1, 1995, 829.
49 S. Kumar and M. Manickam, Synthesis, 1998, 1119.
50 E. O. Arikainen, N. Boden, R. J. Bushby, J. Clements,
B. Movaghar and A. Wood, J. Mater. Chem., 1995, 5, 2161.
51 D. Adam, K.-H. Etzbach, B. GluÈsen, D. Haarer, A. Kettner,
W. Paulus, H. Ringsdorf, P. Schuhmacher, K. Siemensmeyer,
J. Simmerer and J. H. Wendorff, Adv. Mater., 1996, 8, 815.
21 R. M. Schaffert, IBM J. Res. Dev., 1971, 15, 71.
22 Handbook of Liquid Crystals, eds. D. Demus, J. Goodby,
G. W. Gray, H.-W. Speiss and V. Vill, Wiley-VCH, Weinheim,
1998.
23 D. Adam, F. Closs, T. Frey, D. Funhoff, D. Haarer, H. Ringsdorf,
J. Mater. Chem., 2001, 11, 302±311
311