Columnar mesophase from a new hybrid siloxane-triphenylene

Article abstract of DOI:10.1039/b202677e

The design, the synthesis and the mesomorphic properties of a new disc-like mesogen based on a hexasubstituted triphenylene core terminated with pentamethyldisiloxane pendant groups are reported. The siloxane fragments were grafted to the six terminal olefinic branches of a triphenylene precursor by a Pt-catalysed hydrosilylation reaction. Both the hexaolefinic and the hexasiloxane derivatives are mesomorphic, showing a rectangular columnar (Col_R - p2gg) mesophase between 55 and 88°C for the former, and a broad hexagonal (Col_H - p6mm) mesophase between ambient temperature and 125°C for the latter. The liquid crystalline behaviour was investigated by means of differential scanning calorimetry, polarised-light optical microscopy, and X-ray diffractometry. The mesophase temperature range and stability were considerably extended for the hybrid compound with respect to the siloxane-free compound. This behaviour is explained on the basis of enhanced microsegregation between the chemically distinct constituent components of the molecule.

Full text of DOI:10.1039/b202677e

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Columnar mesophase from a new hybrid siloxane-triphenylene
Rachid Zniber,a Redouane Achour,a Mohammed Zouba¨ır Cherkaoui,a{
Bertrand Donnio,b Lionel Gehringerb and Daniel Guillon*b
aUniversite´ Mohammed V, Faculte´ des Sciences, Laboratoire de Chimie Organique
He´te´rocyclique, Avenue Ibn Batouta, B.P. 1014 R.P Rabat, Morocco
bInstitut de Physique et Chimie des Mate´riaux de Strasbourg, Groupe des Mate´riaux
Organiques (CNRS-ULP, UMR 7504), 23 rue du Loess, BP 20CR, 67037 Strasbourg
Cedex, France. E-mail: daniel.guillon@ipcms.u-strasbg.fr; Fax: 1 33388107246
Received 18th March 2002, Accepted 23rd April 2002
First published as an Advance Article on the web 11th June 2002
The design, the synthesis and the mesomorphic properties of a new disc-like mesogen based on a
hexasubstituted triphenylene core terminated with pentamethyldisiloxane pendant groups are reported.
The siloxane fragments were grafted to the six terminal olefinic branches of a triphenylene precursor by a
Pt-catalysed hydrosilylation reaction. Both the hexaolefinic and the hexasiloxane derivatives are mesomorphic,
showing a rectangular columnar (ColR—p2gg) mesophase between 55 and 88 uC for the former, and a broad
hexagonal (ColH—p6mm) mesophase between ambient temperature and 125 uC for the latter. The liquid
crystalline behaviour was investigated by means of differential scanning calorimetry, polarised-light optical
microscopy, and X-ray diffractometry. The mesophase temperature range and stability were considerably
extended for the hybrid compound with respect to the siloxane-free compound. This behaviour is explained on
the basis of enhanced microsegregation between the chemically distinct constituent components of the molecule.
as a wide range of symmetrically and non-symmetrically mono-
Introduction
meric systems7 have been reported to address these points.
However, hybrid molecular systems that combine a siloxane
part with an organic disc-like group have been scarcely con-
sidered. Only one hybrid siloxane–triphenylene (Fig. 1) has
been reported, showing a stable ColH mesophase from room
temperature up to 141 uC.8 In general, the attachment of a
flexible siloxane part to a calamitic mesogenic structure, via an
alkyl spacer, maintains the liquid crystalline properties but
considerably reduces the transition temperatures with respect
to the aliphatic analogues. Moreover, the bulkiness of these
groups disfavours crystallisation, and these hybrids show a
strong tendency to freeze-in the mesophase on cooling due to a
strong supercooling effect. Consequently, the mesophase
temperature range becomes more accessible than their free-
siloxane analogues. The tri-block molecular architecture of
such low-molar mass hybrids consists of three chemically
incompatible molecular species covalently linked together,
namely a polarisable rod-like mesogenic core, an aliphatic part
and the siloxane moieties. The most studied structure consists
of a calamitic mesogenic moiety with a chain (or a polar group)
at one end, and a linear siloxane group at the other extremity
connected via an aliphatic spacer.9 The way the siloxane groups
are combined with the mesogenic units has also been analysed.
Such groups can be attached laterally to a mesogenic unit,10,15g
fixed at both extremities of a calamitic unit (this topology was
found to lower the mesophase stability),11 or used as bridges
between two mesogenic groups.12 Liquid crystalline systems
were also obtained with more complex molecular topologies by
attaching mesogenic side groups to cyclic13 and tetrahedral
siloxanes,14 polyhedral silsesquioxanes15 and end-functiona-
lised polycarbosilane dendrimers.16 Recently, reticulation of
alternate co-oligomers of disiloxane calamitic mesogen with
cyclosiloxane yielded new liquid crystalline elastomers.17 In all
cases, the mesomorphism results, on the one hand, from a con-
flict between the preferential anisotropic order of the mesogenic
units and the flexibility and bulkiness of the siloxane moieties
(by reducing the packing efficiency of the molecules within the
Discotic molecules, already used in phase compensation films,
also offer a unique possibility as potential one-dimensional
charge carrier systems1 when self assembled in hexagonal
columnar mesophases (ColH). Indeed, electronic interactions as
well as electron and exciton migrations are strongly favoured
within the columns since the stacking periodicity along the
column is short compared to the intercolumnar distance. As
such, discotic liquid crystals are seen as promising organic
semiconductors for applications in the domain of molecular
electronics, optoelectronics, photoconductivity, photovoltaic,
and electro-luminescent devices.2 However, the limited effi-
ciency of the charge-carrier mobility (1023–0.5 cm2 V21 s21
compared to that found in graphite of 3 cm2 V21 s21), a
parameter which is essential to estimate for an optimal design,
is in part due to the lack of short-range order of the intra-
columnar stacking in the liquid-crystalline mesophase. The
improvement of these properties requires perfectly stable
monodomains of the materials and, ideally, it would also be
more favourable if they could operate at ambient temperature.
Liquid crystals offer many alternative advantages to organic
monocrystals in that they can be easily macroscopically
aligned (monodomains) and processed; moreover, structural
defects can be self-healed because of the molecular fluctua-
tions. Various methods for increasing the extent of ordering,
facilitating the processing and improving the performances of
the charge mobility have been employed. Among these, the
freezing-in of the columnar order into stable, room tempera-
ture glasses appears to be an attractive strategy since the aniso-
tropic properties and macroscopically aligned monodomains
can be easily vitrified into various shapes. Triphenylene-
containing liquid crystals have been the most studied for
such prospects, and examples of triphenylene-containing liquid
crystalline polymers,3 elastomers,4 networks,5 oligomers,6 as well
{Present address: ROLIC Research Ltd., Gewerbstrasse 18, 4123
Allschwil, Switzerland.
2208
J. Mater. Chem., 2002, 12, 2208–2213
This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b202677e

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Fig. 1 Chemical structure of the hybrid triphenylene siloxane tetramer.
mesophases) and, on the other hand, from the chemical
incompatibility between the siloxane tails and the hydrocarbon
mesogenic cores which will show a propensity to microsegre-
gate in space, and thus stabilise the mesophases.
Indeed, these materials offer a better possibility of supra-
molecular ordering with a good segregation between molecular
moieties of different natures, and moreover the ordering is not
constrained by the presence of a long polymeric backbone.
Low molar mass hybrid-containing liquid crystals thus form
an interesting class of mesogenic materials since they combine
the properties of mesomorphic materials with those of their
poly(dimethyl)siloxane polymer analogues,18 namely the low
melt viscosity of the former with the glass-forming tendency
and the mechanical stability of the latter. In this context we
were interested in designing and synthesising a new hybrid
discotic material, and in investigating its thermal behaviour.
Results
Materials
The outline of the synthetic route of the hexasubstituted
triphenylenes is indicated in the reaction scheme (Scheme 1).
The triphenylene central core was synthesised according to
Scheme 1
J. Mater. Chem., 2002, 12, 2208–2213
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Table 1 Transition temperatures and corresponding enthalpies of compounds 3 and 4
Compound
DSC Cyclea
h1
Transition
Temperature/uC TPOM
Temperature/uC TONSET (TPeak
)
Enthalpy DH/kJ mol21
3
Crys–ColR
ColR–IL
IL–ColR
Crys’–Crys
Crys–ColR
ColR–IL
IL–ColR
ColH–IL
IL–ColH
ColH–IL
IL–ColH
55
88
82
53 (56)
82 (87)
81 (80)
47 (51)
53 (58)
85 (89)
84 (83)
118 (121)
116 (113)
115 (122)
115 (112)
50.1
1.8
21.8
34.5
6.7
1.7
21.65
5.1
22.45
3.0
22.45
c1
h2
c2
h1
c1
h2
c2
4
125
118
ah1: first heating, c1: first cooling, h2: second heating, c2: second cooling.
the literature, using the trimerisation19 reaction of veratrol (0)
in the presence of iron(III) chloride to give 2,3,6,7,10,11-
hexamethoxytriphenylene (1) in 83% yield. The latter was
then subjected to a complete demethylation reaction using
boron tribromide followed by the esterification of the resulting
crude 2,3,6,7,10,11-hexahydroxytriphenylene (2) with dec-9-
enoic acid to give 2,3,6,7,10,11-hexakis(non-8-enylcarbonyl-
oxy)triphenylene (3). The targeted compound was obtained by
hexa-hydrosilylation reaction of the hexaester (4) with penta-
methyldisiloxane in the presence of a platinum catalyst.
Liquid crystalline properties
The mesomorphic behaviour of the two discotic materials 3
and 4 was examined by DSC and phase assignments were based
on the combination of powder X-ray diffraction (XRD) and
polarised optical microscopy (POM). The transition tempera-
tures, phase sequence and some thermodynamic data are listed
in Table 1.
Fig. 3 Optical texture of 4 in the hexagonal mesophase.
that the clearing temperature is much higher in the siloxane
derivative 4 (125 uC compared to 88 uC for the derivative
deprived of a siloxane part (3)), and that the temperature range
of the columnar mesophase is also much larger, despite the fact
that the presence of bulky siloxane groups may be thought to
prevent good stacking of the triphenylene cores.
Optical microscopy. Compound 3 was found to be meso-
morphic from 55 uC up to 88 uC. On slow cooling from the
isotropic liquid, the microscopic preparation revealed a texture
which is typical of that of a columnar mesophase with large
dendritic monodomains (Fig. 2). On cooling, the crystallisation
was confirmed by the rapid change of the LC texture into
very narrow domains with sharp boundaries; note that this
transformation occurred at a much lower temperature than the
crystal-to-mesophase transformation determined on heating.
Compound 4 was found to be mesomorphic at ambient tem-
perature, and its optical texture revealed characteristic features
shown by columnar mesophases including cylindrical and
developable domains, with large homeotropic areas (Fig. 3). It
cleared at 125 uC. No sign of crystallisation of the sample could
be detected on cooling, and the texture remained unchanged,
the mesophase appearing less fluid. It is interesting to note
DSC. Two endothermic peaks were observed for compound
3 during the first heating (h1) from room temperature up to
100 uC (Fig. 4). The broad peak at 53 uC corresponds to the
melting of the crystalline phase into the mesophase, followed
by a small one at 82 uC, associated to the clearing temperature.
However, during the second heating (h2), a crystal-to-crystal
phase transformation took place, which was hardly visible in
Fig. 2 Optical texture of 3 in the rectangular mesophase.
Fig. 4 DSC traces of 3.
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Fig. 7 X-Ray diffraction pattern of 4 recorded at 60 uC in the
hexagonal mesophase.
Fig. 5 DSC traces of 4.
refereed to as a pseudo-hexagonal phase. The lattice para-
˚
meters are a ~ 40.9, b ~ 23.6 A, with a/b ~ d3, and the area
2
˚
h1. This indicates that the compound has crystallised (or
partially crystallised) in a different crystalline form from the
melt. Nevertheless a very good reproducibility of the cooling
cycles was observed (c1 and c2), and no sign of crystallisation
was detected in neither of them. The DSC traces of 4 (Fig. 5)
were much simpler. They consist of a single endothermic peak at
ca. 111–120 uC, corresponding to the mesophase-to-isotropic
liquid transformation. This behaviour was perfectly repro-
duced during the following heating and cooling cycles. As
deduced from POM, no other transition (such as crystallisation
or glass transition) were seen down to room temperature.
of one column, S, is 483 A . The pattern recorded at 60 uC
(Fig. 5) was chosen as a representative example. The position
of the reflections remains unchanged in the mesomorphic
temperature range.
Between room temperature and up to 125 uC, the X-ray
diffraction patterns also confirmed the existence of a liquid
crystalline mesophase for the organosiloxane triphenylene (4).
The diffraction patterns obtained at 60 uC (Fig. 7), chosen as a
representative example, consisted of three, sharp small-angle
˚
reflections (28.35, 16.35 and 14.1 A) and of two diffuse and
broad scattering halos in the wide angle region. The reciprocal
spacings of the three sharp reflections were in the ratio 1, d3,
d4 corresponding to the indexation (hk) ~ (10), (11) and (20)
which are characteristic of a 2D hexagonal lattice, and thus to a
hexagonal columnar mesophase (ColH phase). One of the two
halos is associated with the liquid-like order of the molten
aliphatic chains and of the rigid flat aromatic parts (II), at ca.
X-Ray characterisation. The mesomorphism of the two tri-
phenylene derivatives 3 and 4 was confirmed and the nature
of the mesophase determined by means of small-angle X-ray
diffraction (XRD). The temperature–XRD measurements
were carried out over the whole mesomorphic temperature
range (starting from room temperature up to the isotropic
liquid). A good agreement was found between the transition
temperatures determined by both POM and DSC techniques
with those determined by XRD.
˚
4.5 A, while the other one, slightly more intense and diffuse (I)
˚
at ca. 6.4 A, corresponds to the interactions between the ter-
minal siloxane moieties. Identical X-ray patterns were obtained
at any other temperature of the mesomorphic range, with no
variation of position of the reflections. The inter-columnar
2
The X-ray patterns obtained for the olefinic compound 3
(Fig. 6) consisted of three sharp small-angle reflections (20.45,
˚
˚
distance, a, is 32.7 A and the lattice area, S, is 928 A .
˚
15.4 and 11.8 A), with decreasing intensity, and of a diffuse
˚
halo in the wide angle part, at ca. 4.5 A. The diffuse scattering
Discussion
in the wide angle region corresponds to the liquid-like con-
formation of the molten aliphatic chains and to the disordered
stacking of the flat aromatic parts in the columns. The three
sharp reflections were indexed as 11 (d11 ~ d11¯ ~ d20) for the
first reflection, 21 and 31 (d31 ~ d02) for the second and third
order reflections respectively. This corresponds to a 2D arrange-
ment of columns with a rectangular p2gg 2D space group (h 1
k | 2n), and thus to a rectangular mesophase, commonly
In 1979, Destrade et al.20 reported the mesomorphic behaviour
of the 2,3,6,7,10,11-hexakis(alkanoyloxy)triphenylenes, some
of the earliest discotic liquid crystalline materials. These
materials were found to exhibit a rather rich polymorphism
showing a ColR mesophase from the heptanoyloxy up to long
chain-length homologues, and from the undecanoyloxy homo-
logue onwards, two additional mesophases sandwiching the
ColR one, namely a high temperature ColH mesophase (with a
temperature range of ca. 10–20 uC), and an ordered uniden-
tified low-temperature one. The ColR existed over a wide
temperature range of ca. 60 uC on average. It is thus interesting
to observe the dramatic effect of the chemical modification
of the pendant groups on the mesomorphic properties. The
hexakis(alkanoyloxy)triphenylene homologue of 3 (i.e. with
saturated chains) displays a ColR mesophase (n ~ 10: Crys
75 ColR 125.5 I). The presence of the unsaturation at the chain
extremity leads to a decrease of both the melting and clearing
points, and in addition to a reduction of the mesomorphic
temperature range, as already observed in several cases of
calamitic compounds.21 It is important also to note the absence
˚
of any periodicity of 3.5–3.6 A related to a stacking of the
aromatic cores as in many other mesogenic triphenylene
derivatives.20 This clearly indicates a tilt of the aromatic cores
with respect to the columnar axis as reported in the crystalline
Fig. 6 X-Ray diffraction pattern of 3 recorded at 60 uC in the
rectangular mesophase (pseudo-hexagonal).
J. Mater. Chem., 2002, 12, 2208–2213
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phase of triphenylene substituted with six ethyleneoxy chains,22
very presumably induced by the dipolar interactions between
the carboxylate groups along the columns. Thus, the symmetry
of the mesophase remains rectangular.
In the same way, the introduction of siloxane moieties at the
end of the aliphatic chains leads also to a dramatic change of
the temperature of transition, compound 4 being columnar
mesomorphic even at room temperature. This is a general trend
already observed in organosiloxane calamitic liquid crystals.
As for 3, the aromatic cores are tilted with respect to the
columnar axis as revealed by the absence of any signal in the
vigorously until the ice melted. It was then extracted with
diethyl ether (6 6 150 ml) and the combined organic extracts
were washed with half saturated NaCl solution (200 ml),
dried over magnesium sulfate and evaporated to dryness. This
afforded 2,3,6,7,10,11-hexahydroxytriphenylene (2) as a grey
solid; the yield was quantitative. This compound was used
without further purification.
2,3,6,7,10,11-Hexakis(non-8-enylcarbonyloxy)triphenylene (3).
To a suspension of 2,3,6,7,10,11-hexahydroxytriphenylene (2)
(0.64 g, 2 mmol), dec-9-enoic acid (3.40 g, 20 mmol) and
DMAP (124 mg) in dichloromethane (40 ml) cooled to 0 uC, a
solution of DCC (4.12 g, 20 mmol) in dichloromethane (40 ml)
was slowly added. After complete addition, the reaction
mixture was stirred for 15 min at 0 uC and for 6 h at room
temperature. The urea was then filtered off and the obtained
yellowish solution was evaporated to dryness. The beige-yellow
residue thus obtained was purified by filtration over a short
silica gel column using dichloromethane as eluent. The beige
solid was then dissolved in 10 ml of dichloromethane followed
by a slow addition of 100 ml of ethanol. The obtained
precipitate was filtered off, washed with ethanol (50 ml) and
dried to give 2,3,6,7,10,11-hexakis-(dec-9-enylcarbonyloxy)-
triphenylene (3) as a white powder. Yield 1.4 g , 65%.
Elemental analysis: found (calculated for C78H108O12; MW ~
1237.72 g mol21): %C ~ 75.92 (75.69); %H ~ 8.95 (8.80); %O
˚
diffraction pattern at 3.5–3.6 A. However, in the case of 4,
the only columnar mesophase observed is hexagonal in nature.
As a matter of fact, the addition of terminal siloxane moieties
clearly leads to a more homogeneous crown around the central
triphenylene cores, thus reducing the cross-sectional anisotropy
and the hexagonal packing can then be achieved with a
complete rotational disorder along the columnar axis. It is
interesting to note that a similar molecular engineering inclu-
ding microsegregation effects23 has been applied to other
triphenylene derivatives by attaching terminal fluoroalkylated
chains24 or terminal peripheral sugar moieties,25 leading
to similar effects with in particular a broadening of the
mesomorphic range.
As a conclusion, the molecular engineering of compounds 3
and 4 proved to be useful to considerably decrease the tem-
perature of transitions of triphenylene derivatives, to enlarge
the mesomorphic range and to tune the type of columnar
mesophase (rectangular or hexagonal). In the present case
however, the tilted arrangement of the triphenylene cores
within the columns is not the best configuration for an
optimised photoconduction process along the columnar axis.
On the other hand, such molecular engineering can be useful
when tilted arrangements are needed for ferroelectric columnar
mesophases for example. The charge transport properties of
derivatives 3 and 4 are currently being investigated, and
molecular engineering of similar derivatives with non-tilted
arrangement of the aromatic cores within the columns is now
in progress.
1
~ 15.24 (15.51). H NMR; 300 MHz; CDCl3; dH (ppm): 1.40
(s, 48 H), 1.74–1.85 (m, 12 H), 2.03–2.09 (m, 12 H), 2.59–2.67
(m, 12 H), 4.94–5.06 (m, 12 H), 5.73–5.93 (m, 6 H), 8.22 (s, 6 H,
ArH).
2,3,6,7,10,11-Hexakis{[10-(pentamethyldisiloxanyl)decanoyl]-
oxy}triphenylene (4). To a suspension of 3 (1.24 g, 1 mmol) and
1,1,1,3,3-pentamethyldisiloxane (5 ml), divinyl(tetramethyl)di-
siloxane platinum complex (Pt:DVTMS) (5 ml) was added
under an argon atmosphere. The obtained mixture was refluxed
for 3 h, then a solution of Pt:DVTMS (5 ml) in toluene
(thiophene free, 15 ml) was added to the reaction mixture which
was further stirred for 3 h at 85 uC. It was then cooled, diluted
with 10 ml of cyclohexane and filtered over a short silica gel
column using cyclohexane–toluene (1 : 1 v/v) as eluent. After
evaporation of solvent pure 2,3,6,7,10,11-hexakis{[10-(penta-
methyldisiloxanyl)decanoyl]oxy}triphenylene (4) was obtained
as a slightly grey pasty material. Yield : 1.76 g, 83%. Micro-
analysis: found (calculated for C108H204O18Si12; MW ~
2127.85 g mol21): %C ~ 61.12 (60.96); %H ~ 9.82 (9.66);
Experimental
Synthetic methods
2,3,6,7,10,11-Hexamethoxytriphenylene (1). A solution of
veratrol (0) (13.82 g, 100 mmol) in dichloromethane (140 ml)
was added dropwise to a suspension of FeCl3?H2O (53 g)
in dichloromethane (300 ml) and concentrated sulfuric acid
(0.7 ml). After complete addition (15 min), the reaction mixture
was further stirred for 3 h at room temperature, then 400 ml
of methanol were slowly added under vigorous stirring.
The obtained mixture was further stirred for 30 additional
minutes and the precipitate was filtered off, washed with
methanol (5 6 100 ml) and dried under reduced pressure to
give 1 as a slightly beige powder (11.5 g, 83%). Elemental ana-
lysis: found (calculated for C24H24O6; MW ~ 408.46 g mol21):
%C ~ 70.81 (70.57); %H ~ 6.01 (5.92); %O ~ 23.22 (23.50).
1H NMR: 300 MHz; CDCl3; dH (ppm): 4.05 (s, 18 H, OMe),
7.80 (s, 6 H, ArH).
1
%O ~ 13.25 (13.53). H NMR; 300 MHz; CDCl3; dH (ppm):
1.33–1.57 (m, 150 H), 1.77–1.81 (m, 24 H), 2.59–2.67 (m, 24 H),
8.25 (s, 6 H, ArH). 13C NMR; 50 MHz; CDCl3; dC (ppm):
171.07, 142.11, 127.57, 118.12, 34.20, 33.43, 29.46, 29.39, 29.27,
24.93, 23.28, 18.38, 1.96, 0.33.
Materials
1,1,1,3,3-Pentamethyldisiloxane,
dicyclohexylcarbodiimide
(DCC), 4-dimethylaminopyridine (DMAP), dichloromethane,
toluene and boron tribromide (Fluka); divinyltetramethyldi-
siloxane platinum complex (Pt:DVTMS in xylene 2.1–2.4%,
ABCR); veratrol (Aldrich); dec-9-enoic acid (AGIPAL);
iron(III)chloride monohydrate (FeCl3?H2O) (Merck).
2,3,6,7,10,11-Hexahydroxytriphenylene (2). Hexamethoxy-
triphenylene (1) (1.02 g, 2.5 mmol) was dissolved in dichloro-
methane (20 ml) and the obtained solution was cooled to
270 uC and maintained under a nitrogen atmosphere. A
solution of BBr3 (1 M, CH2Cl2, 30 ml) was then added
dropwise to the reaction mixture over a period of 30 min. After
complete addition, the reaction temperature was gradually
allowed to reach room temperature and stirring was continued
for 8 h. The reaction mixture was then slowly poured into
crushed ice (100 g) and the obtained mixture was stirred
Characterisations
1H NMR spectra were recorded on a Bruker AC 300 MHz
spectrometer using CDCl3 as solvent.
DSC
The transition and enthalpies were determined by differential
scanning calorimetry with a Perkin-Elmer DSC-7 instrument
operating at a scanning rate of ¡ 5 and 10 min21
.
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POM
The optical textures were studied using a polarising microscope
Orthoplan (Leica) working in the transmission mode and
associated with a Mettler hot stage.
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XRD
The XRD patterns were obtained with two different experi-
mental set-ups; in the experiments, the crude powder was filled
in Lindemann capillaries of 1 and 0.5 mm diameter for 3
for 4 respectively. A linear monochromatic Cu-Ka1 beam was
obtained using a Guinier camera or a Debye–Scherrer camera
equipped with a bent quartz monochromator and an electric
oven. A first set of diffraction patterns was registered with a
gas curved counter ‘‘Inel CPS 120’’ associated with a data
˚
acquisition computer system; periodicities up to 60 A can be
measured, and the sample temperature is controlled within
¡0.05 uC. The second set of diffraction patterns was registered
on both photographic films and image plate; the cell para-
meters were calculated from the positions of the reflections at
the smallest Bragg angle, which was in all cases the most
˚
intense. Periodicities up to 90 A can be measured, and the
sample temperature is controlled within ¡0.3 uC. In each case,
exposure times were varied from 1 to 24 h.
14 (a) G. H. Mehl and J. W. Goodby, Chem. Ber., 1996, 129, 521;
(b) G. H. Mehl and J. W. Goodby, Chem. Commun., 1999, 13;
(c) A. Kowalewska, P. D. Lickiss, R. Lucas and W. A. Stanczyk, J
Organomet. Chem., 2000, 597, 111.
Acknowledgement
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The authors gratefully acknowledge M. L. Oswald for her kind
assistance in the chemical characterisation of the samples.
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