Semicarbazides as gel forming agents for common solvents and liquid crystals

Article abstract of DOI:10.1039/b510474b

This paper describes the synthesis of 14 new gelling agents with semicarbazide groups as H-bonding motifs and alkyl- and/or azobenzene side groups. They gel solvents like decaline, 1,2-dichlorobenzene and toluene and liquid crystalline mixtures. X-Ray structure analysis shows that the semicarbazides are connected by H-bonds, each molecule to four neighbours. As a result a ribbon is formed with a core of H-bonded semicarbazide groups and alkyl chains sticking to the side. IR measurements show an unchanged H-bonding motif in large crystals and in the gel fibres, even in LC-mixture. During heating the gel melts (rheology), while the H-bonding motif of the crystal disappears (IR and DSC measurements). First experiments show that these gel-forming agents can be used to gel LC-phases and to stabilise the director pattern present during gel formation. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b510474b

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www.rsc.org/materials | Journal of Materials Chemistry
Semicarbazides as gel forming agents for common solvents and liquid
crystals
Pia Deindo¨rfer,a Thomas Geiger,b Dieter Schollmeyer,a Jian Hui Yea and Rudolf Zentel*a
Received 21st July 2005, Accepted 19th October 2005
First published as an Advance Article on the web 10th November 2005
DOI: 10.1039/b510474b
This paper describes the synthesis of 14 new gelling agents with semicarbazide groups as
H-bonding motifs and alkyl- and/or azobenzene side groups. They gel solvents like decaline,
1,2-dichlorobenzene and toluene and liquid crystalline mixtures. X-Ray structure analysis shows
that the semicarbazides are connected by H-bonds, each molecule to four neighbours. As a result
a ribbon is formed with a core of H-bonded semicarbazide groups and alkyl chains sticking to the
side. IR measurements show an unchanged H-bonding motif in large crystals and in the gel fibres,
even in LC-mixture. During heating the gel melts (rheology), while the H-bonding motif of the
crystal disappears (IR and DSC measurements). First experiments show that these gel-forming
agents can be used to gel LC-phases and to stabilise the director pattern present during gel
formation.
one gel-forming agent, which reduces the synthetic afford to
Introduction
synthesise various cross-linkable LC–polymers (LC–elasto-
The self-assembly of small functional molecules into supra-
mers). So far LCs have been gelled mostly with trans-(1R, 2R)-
diamino cyclohexane derivates.3,5,17–19 We observed now that
molecular structures compromises a powerful approach toward
the development of new materials and devices of nanoscale
semicarbazides with one long alkyl chain can be rather good
dimensions. The control of these self-organising processes by
gel forming agents. They can be easily synthesised and offer a lot
chemical or physical elements is a major challenge. One example
of possibilities for structural variation. According to this, the
of materials with this potential are low molecular mass gel
group of semicarbazides seems to be a perfect choice to create
forming agents (gelators). They self-assemble from solution by
controlled aggregation to form anisotropic aggregates, which
build up three-dimensional networks. These networks—formed
by only a few mass percent of the solution—extend through
the whole liquid, which gets a macroscopically solid-like
appearance (the gel). In combination with liquid crystalline
(LC) materials systems with a self assembly on two levels
(gelator–gel network, LC-molecules–anisotropic phase in the
voids of the gel network) are accessible.
new gel-forming agents, even for liquid crystals. The possibility
to introduce azo groups makes also the synthesis of photo
addressable or more generally ‘‘stimuli responsive gels’’ feasible.
As a result we synthesised various types of semicarbazides
(including semicarbazides with azo groups as photochromic
groups) and investigated their gelling abilities in organic
solvents in detail. In addition first results, which show their
potential to gel LCs, are presented.
The capacity to act as a gel-forming agent results, for most
compounds, from strong formanisotropic molecular forces
like hydrogen bonding (H-bonding) or p-stacking, which direct
the aggregation.1,2
Results and discussion
Synthesis
The semicarbazide group (Table 1) offers the possibility
for several H-bonding structures with 3 N–H groups, two
carbonyl groups (CLO) and one ester oxygen. It can be
combined with simple alkyl groups (alkyl semicarbazides 1–9)
and with photochromic azo groups (azo semicarbazides 10–14)
according to the Schemes 2 and 3 (Experimental). The
resulting semicarbazides with the general structure shown
in Scheme 1 are compiled in Table 1.
Besides classical solvents, low molar mass liquid crystals
(LC) can also be gelled by gel-forming agents,3–9 although such
investigations are still rather rare. These investigations have
shown that it is, for example, possible to stabilise the director
pattern present during gel formation in the LC. In so far, the
resulting gelled liquid crystals resemble network-stabilised
liquid crystals,10–12 or LC-elastomers,13–16 which are both
stabilised by chemical cross-linking. Gelled liquid crystals
have, however, the advantage that the network formation is
reversible under heating and cooling cycles.17 In addition,
many different liquid crystalline compounds can be gelled with
The azo semicarbazides can be isomerised photochemically
in solution from the trans to the cis state (irradiation with UV
light, 265 nm, Fig. 1a). The reisomerisation (cis–trans) happens
thermally in solution with a half-life time of 21 h at room
temperature (compound 12 in THF, Fig. 1B). Alternatively the
cis–trans isomerisation can be induced by irradiation with
visible light (l larger than 420 nm, Fig. 1 A). However no
photo isomerisation could be induced in the crystalline or the
gelled state, presumably because of topological restrictions.
aInst. of Organic Chemistry, University of Mainz, Duesbergweg 10-14,
D-55099, Mainz, Germany. E-mail: zentel@uni-mainz.de;
Fax: +49 6131 3924778; Tel: +49 6131 3920361
b
¨
Empa, Laboratory for Functional Polymers, Uberlandstrasse 129,
CH-8600, Du¨bendorf, Switzerland
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Table 1 Synthesized alkyl semicarbazides 1–9 and azo semicarbazides
10–14
Compound
R9
R
n
1
2
3
4
5
6
7
8
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C6H13
C8H17
C9H19
C10H21
C11H23
C12H25
C14H29
C16H33
C18H37
C4H13
C4H13
C6H13
C6H13
C7H15
—
—
—
—
—
—
—
—
—
3
9
10
11
12
13
14
5
3
5
3
Fig. 1 photo isomerisation of 12 in THF (0.02 g L21): (A) cis–trans
isomerisation by irradiation; (B) thermal reisomerisation at room
temperature (half-life time 21 h).
Gel formation
At first the gel forming potential was investigated for several
organic solvents. For a quick test a few milligrams of the
semicarbazide are dissolved in the heat in an organic solvent
and cooled down in an ice bath. After some minutes the test
tube is turned over. If the liquid is solidified, gelation has
occurred (Fig. 2). These results are summarised in Table 2
Generally the alkyl semicarbazides were found to be good
geling agents for solvents like decaline, 1,2-dichlorobenzene,
toluene, tetraline and cyclohexane. Concentrations of 0.5 to
5 wt% are usually sufficient for the formation of a macroscopic
gel. The gels with most solvents are slightly opaque but stable
over some weeks. Gels in toluene are transparent and the ones
from semicarbazides 2 and 4 are especially stable for several
months. Values for these semicarbazides are included in
Fig. 2 Pictures of gels with organic solvents: (A) a: 2 in toluene
(2 wt%); b: 2 in decalin (2 wt%); c: 12 in decaline (8 wt%); (B) 2 in
decalin (2 wt%).
Scheme 1 Compounds of the LC mixture and its composition. Phase
transition: X 9 SmC* 63 SmA 65 N 76 I.
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Table 2 Gelling and non gelling behaviour of various semicarbazides: p = precipitate, s = soluble, g = gel (min wt%)
Semicarbazide
Solvent
2
4
10
11
12
13
14
Toluene
Decaline
1,2-Dichlorobenzene
Cyclohexane
Tetraline
Heptane
Water
Ethanol
g (0.5%)
g (6%)
g (2.5%)
g (10%)
g (9%)
p
g (0.5%)
g (5.5%)
g (2.3%)
g (9%)
g (9%)
p
g(9%)
g (8.3%)
g (4.8%)
p
g (10.6%)
p
p
s
g (8.5%)
g (7.2%)
g (6.4%)
p
g (13.2%)
p
p
s
g (10%)
g (4.8%)
g (4.4%)
p
g (9.3%)
p
p
s
g (7.7%)
g (6%)
g (6.7%)
p
g (8.4%)
p
p
s
g (5.4%)
g (5.7%)
g (5.4%)
p
g (6.6%)
p
p
s
p
s
p
s
sticking to the side. Such a structure explains a preferably one-
dimensional crystal growth, as the strong interaction along the
core should favour crystal growth in this direction, whereas the
weaker interactions between the alkyl chains slow down crystal
growth perpendicular to the fiber axis. At the same time the
interdigitation between the alkyl chains allows some crystal
growth in perpendicular directions, which gives the resulting
fibers enough strength to support a load (Fig. 3A).
The question to be answered next is: does the structure
formed in the bulk of one gelling agent also apply to the fibers
in the gel? IR measurements in the range of the N–H vibration
allow to test it, if the same H-bonding motif exists in the bulk
and in the gel. The results of these measurements are displayed
in Fig. 4. Fig. 4A shows the IR spectra of the crystalline
powder of 2 and its gels in toluene. IR measurements of a
LC-mixture (Scheme 1) to be discussed later are also included.
All show the same N–H vibrations (peaks between 3600–
2600 cm21) and therefore the same H-bonding motif. The
same applies to the other alkyl semicarbazides. For the azo
semicarbazides (Fig. 4B) the N–H vibrations are only slightly
shifted presumably due to some interactions with the azo
benzene groups. As a result it can be concluded that the crystal
structure presented in Fig. 3 does also apply for the gel fibers
formed in organic solvents and even in liquid crystals, at least
with respect to the H-bonded core structure.
To determine the gel–sol transition temperature, we per-
formed IR measurements, DSC measurements and rheological
measurements as function of the temperature. The results are
presented in Fig. 5 to 7. Fig. 5 shows, for example, the tem-
perature dependent IR measurements of a gel of semicarbazide
2 in toluene. With increasing temperature the height of the
N–H vibrations decreases. Between 70 and 80 uC this drop is
largest, which indicates that the H-bonding network breaks. At
90 uC the typical pattern of the N–H vibrations has dis-
appeared. This happens about 27 uC below the melting point
of the bulk crystal without solvent, which melts at 117 uC.
As an additional method DSC measurements were per-
formed (Fig. 6). They show that the gel has a broad melting
transition, which starts at about 50 uC and is finished at 95 uC.
The melting of the gel shows two steps, the first from 50 uC to
65 uC, the second from 65 uC to 95 uC. A possible explanation
for this could be (i) higher melting points for long fibers with
few defects and (ii) lower melting points for areas rich in
defects, like the joints between different fibers.
Fig. 3 X-Ray structure of a single crystal of 2. Part of the pack-
˚
ing diagram: H-bonds between H9–O18: 2.14(2) A and H12–O17:
˚
1.91(2) A are shown with dashed lines.
Table 2.The azo-containing semicarbazides 10 to 14 are also
good gelling agents for the same solvents beside cyclohexane
(Table 2), but usually a higher concentration is needed to form
a gel (5 to 8 wt%).
To get a better understanding of the gelling process, it
was essential that we succeeded in growing single crystals of
compound 2 by slowly cooling a solution of 2 in toluene. The
X-ray crystal structure obtained from this single crystal is
displayed in Fig. 3. A short summary of the X-ray data is given
in Table 3 (Experimental) and further details are included in
the cif, available as supporting information{. It shows that the
semicarbazides are connected by H-bonds, each molecule to
four neighbours (Fig. 3B). As a result a ribbon is formed with
a core of H-bonding semicarbazide groups and alkyl chains
Rheological measurements show generally the same onset of
the melting of the gels. For a gel of the azo semicarbazide 11 in
1,2-dichlorobenzene rheological measurements could be done,
{ CCDC reference number 151383. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b510474b
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Table 3 Crystal data for compound 2a,b
˚
Empirical formula
FW/g mol21
Crystal system
Space group
Z
C12H25N3O3
259.35
Orthorhombic
Pna21
4
298
1.121
0.66
568
a/A
36.062(2)
˚
b/A
9.1053(3)
4.6794(3)
1536.5(1)
3454
3126
2620
0.0487
1.026
0.25, 20.13
˚
c/A
3
˚
V/A
Refl. meas.
Unique refl.
refl |F| . 4s(F)
Temperature/K
calc/g cm23
r
m/mm21
F(000)
R [|F| . 4s(F)]c
Goodness of fit
Largest diff. peak and hole/e23
Crystal size/mm3
h-range
0.03 6 0.38 6 0.90
1.5u ¡ h ¡ 75.0u
0 ¡ h ¡ 45
Limiting indices
a
0 ¡ k ¡ 11
0 ¡ l ¡ 5
b
˚
Cu Ka radiation with l = 1.54178 A was used. Crystallographic data for the structure have been deposit with the Cambridge
Crystallographic Data Centre (CCDC 151383). R = S||F0| 2 |Fc||/S|F0|.
c
which cover the whole range of the melting process (Fig. 7).
They also show a two step process, but with the opposite
magnitude of steps 1 and 2. At 56 uC the elastic properties
drop strongly and a soft solid (G9 larger than G0) is transfered
into a viscoelastic liquid (G0 larger than G9). At 70 uC G0 drops
a second time and G9 gets unmeasurably small. Compared to
the DSC measurements the first small transition in the DSC
run shows up very strongly in the rheological measurements,
whereas the second signal in the DSC is very weak in the
rheological measurements. A possible explanation of this
behavior could be as follows. The gel is based on long
crystalline fibers with connecting joints, which build a three-
dimensional network. With increasing the temperature the
joints between the fibers melt first. The network breaks apart
and fibers remain floating in a liquid. At higher temperature
the fibers melt, which has little influence on the mechanical
properties, but because of their mass, a big influence in IR and
DSC measurements.
As a result of these investigations we found a new class
of organic gel forming agents, alkyl and azo semicarbazides,
which phase separate on cooling from many liquids forming
a physical gel. Since we are aiming at the gelation of liquid
crystalline materials,17 we have already performed the first
experiments in this direction to check the potential of the new
gelling system. As a matrix, which is liquid crystalline at room
temperature, we especially investigated the mixture presented
in Scheme 1, which offers two smectic phases (smectic A and
C*) and a nematic (cholesteric) phase. A solution of the gel-
forming agents in the isotropic phase of this mixture can be
quenched to 10 uC without a crystallisation of the matrix.20
The ferroelectric smectic C* phase allows then switching
experiments to determine the influence of network formation
on the dynamics and on the director field. A first example
of this ongoing research is presented below to show the
potential of the new gel-forming agents for the preparation of
gelled liquid crystals.
Fig. 4 IR spectra of (a) 2 in the bulk, gel with toluene and gel
with LC mixture; (b) 12 in the bulk, as gel with decalin and as gel with
LC-mixture.
Fig. 5 Temperature dependent IR spectra of 2 in toluene (5 wt%).
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Fig. 6 DSC measurements of 11 in bulk and as gel in 1,2-dichlorobenzene (20 wt%).
switching states. If, on the contrary, a DC field is applied
during cooling, then the director is fixed (polarization either up
or down depending on the sign) while the network is formed.
Under such conditions the gel network may stabilise the polar
structure present during gel formation. To determine the
difference between both samples, the switching times between
the polar states up and down and vice versa were measured
separately. The results are presented in Fig. 8.
Gelled liquid crystals
The influence of gel formation on the ferroelectric switch-
ing was investigated in the quasi-book-shelf geometry between
two ITO covered glass slides with a distance of 10 mm.10,17
At first the ferroelectric switching times increase strongly
for LC mixtures, which contains 2.5 to 11 wt% of the gel-
forming agents 2 or 12. The switching times are about
1.5 ms at 50 uC (10 V) for the gel-free mixture. This value
increases, e.g., to about 6 ms at 50 uC for a mixture with
2.5 wt% of the gelling agent 2. This increase of the switching
time measures an unspecific hindrance of the rotation of the
director field.
Fig. 8A shows the result for a sample cooled (gelled) in an
AC field. For this sample the switching times between both
polar states are identical within experimental error. Fig. 8B
shows the results for the same cell, after cooling (gelled) in a
DC field. This time both switching times differ strongly. As a
result it is possible to use such gels to stabilize the director
configuration present during gel formation and to stabilize
subsequently different director configurations by new heating
and cooling cycles.
To determine if gel formation can be used to stabilise the
director field,17 we performed a second set of experiments.
After having dissolved the gel forming agent 2 in the isotropic
phase, we cooled the sample slowly to 60 uC and then, after
mechanical shearing for a good alignment, further to 40 uC,
while either an AC or a DC field was applied. The application
of an AC field (2 Hz) during cooling leads to a continuous
switching of the ferroelectric LC between the two polar states
with the polarization up or down. As the network is formed
during switching, it should not differentiate between both
Experimental
Instrumental data
Chemicals: commercial available chemicals from Acros and
Fluka were used without any further purification. Calorimeter:
DSC measurements were performed with a DSC 7 from Perkin
Elmer. UV–vis spectrometer: For UV-vis spectra the UV-vis
Scanning Spectrometer UV2101PC from Shimazu was adapted
to an arrangement for maintaining the temperature of the
sample holder named TCC260. NMR spectrometer: The
NMR spectra were measured with a 300 MHz NMR from
Bruker. IR spectrometer: The IR spectra were measured with a
FT-IR Vetor 22 from Brucker with an ATR-Unit (attenuated
total reflection). The temperature chamber was from Specac,
England. Rheometer: The rheolgical measurement were
performed with the TA 1000 from TA-Instruments.
Polarizing Microscope: The electro-optical measurements were
performed with
a polarizing microscope from Olympus
mounted with a photo diode.
1. Synthesis of alkyl semicarbazides
1.1 n-Alkylisocyanates. 470 mmol n-alkylamine is dissolved
in 500 ml abs. toluene and a solution of 160 mmol triphosgene
Fig. 7 Rheological measurement of 11 in 1,2-dichlorobenzen
(20 wt%).
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solvent is evaporated and the isocyanates are used without any
purification.
1.2 n-Alkyl semicarbazide. A solution of 470 mmol alkyl
isocyanates in 50 ml abs. toluene is dropped into a fresh
solution of 470 mmol ethyl carbazate and 400 ml abs. toluene
under vigorous stirring. A colorless precipitate is formed
immediately. Afterwards the reaction mixture is refluxed for
two hours. The cooled mixture is filtrated and the white solid is
washed with ice cold toluene. Yield: 75%.
NMR data for alkyl semicarbazide compounds 1–9
1H-NMR in CDCl3: d/ppm = 6.9–6.7 (s,1H, –NH–, 4); 6.8–
6.6 (s, 1H, –NH–, 3); 5.5–5.4 (s, 1H, –NH–, 5); 4.18 (q, 2H,
–CH2–, 2); 3.18 (q, 2 H, –CH2–, 6); 1.4 (q, 2H, –CH2–, 7); 0.8–
1.2 (m, 2r + 3H, –CH2– and –CH3).
Crystal structure setermination of alkyl semicarbazide 2. The
diffraction experiment was performed on a Nonius CAD4
diffractometer. The crystal structure was solved by direct
methods and difference Fourier technique, structural refine-
ment was against F2. Details of the crystal structure deter-
mination and some crystal data for 2 are given in Table 3.
2. Synthesis of azobenzene containing semicarbazides
2.1 4-Hydroxy-49-n-alkyl azobenzene. Diluted HCl (36 g
conc. HCl and 120 ml H2O) is added dropwise to 0.12 mol of
the appropriate 4-n-alkyl anilin in 48 ml H2O. The mixture is
cooled below 10 uC and a solution of 0.12 mol NaNO2 in 20 ml
H2O is dropped into it.
After stirring for two hours at y10 uC, a buffered solution
of phenolate (0.12 mol phenol, 0.12 mol NaOH and 0.17 mol
Na2CO3 in 160 ml H2O) is added dropwise. The mixture is
neutralized (pH = 8) and filtered. The orange solid is solved in
ether, dried with Na2SO4 and evaporated.
Fig. 8 Switching times of the gel-stabilzed LC mixture (2.5 wt% 2)
measured from 40 to 60 uC in a AC field (2 Hz, 10 V).
If necessary the compound can be purified by recrystallisa-
tion in hexane. Yield about 30–50%.
in 100 ml abs. toluene is added dropwise at room temperature
under vigorous stirring. For total conversion the reaction is
stirred over night, while a white precipitate is formed. The
General structure:
Scheme 2 Synthesis of alkyl semicarbazides 1–9.
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Scheme 3 Synthesis of azo semicarbazides 10–14.
1H-NMR in CDCl3: d/ppm = 0.91 (t, 3H, CH3, 9); 1.3 (m,
2m H, CH2, 8); 1.62 (m, 2H, CH2, 7); 2.66 (t, 2H, CH2, 6) 5.26
(s, 1H, OH, 1); 7.8 (q, 4H, CH, ring 4, 3); 7.25 (t, 2H,CH,
ring 5); 6.9 (d, 2H, CH, ring 2).
2.3 Ether synthesis. 1 mol of 4-hydroxy-49-alkyl azobenzene,
1 mol of the appropriate bromine alkyl phthalimide and
1.2 mol KOH are solved in boiling ethanol and refluxed for
3 days. The hot reaction mixture is filtered and afterwards
cooled to 8 uC. The product precipitates from cold ethanol.
Yield about 30–40%.
2.2 Synthesis of bromine alkyl phthalimide. 0.3 mol
1,n-dibromine alkane (n = 4, 6) are dissolved in 600 ml
acetone. 0.15 mol phthalimide potassium salt is added in 4
parts to the boiling solution during 6 h. After an additional
24 h reaction time, the solution is cooled to room temperature
and a white solid, KBr, is removed by filtration and the
acetone is evaporated. The rest of the dibromine alkane can be
removed by vacuum destillation until a chain length of n = 5.
Longer alkyl chains require purification with column
chromatography in PE–EE 8 : 1 as solvent. If necessary the
product can be recrystallised in toluene.
1H-NMR in CDCl3: d/ppm = 0,9 (t, 3H, CH3, 13); 1.3 (m,
2mH, CH2, 12); 1.6 (m, 2H,CH2, 11); 1.8 (t, 2(n21) H, CH2,
4); 2.66 (t, 2H, CH2, 10); 3.7 (t, 2H, CH2, 3); 4.0 (t, 2H, CH2,
5); 6.9 (d, 2H, CH, ring 6); 7.3 (t, 2H, CH, ring 9); 7.6–7.8 (q,
8H, CH, ring 7, 8, 1, 2).
General structure:
2.4 Amines. 1 mol phthalimides and 10 mol N2H4?H2O
are dissolved in boiling ethanol under N2 atmosphere
and refluxed for 16 h. Then 15 mol conc. HCl is added.
After an additional hour of refluxing, the reaction mixture
is allowed to cool to room temperature. The white precipitate
is removed by filtration and the orange coloured solution is
evaporated to dryness. The product is an orange–yellow
powder.
1H-NMR in CDCl3: d/ppm = 1.5–1.9 (m, 2(n 2 1)H, CH2,
4); 3.4 (t, 2H, CH2, 5); 3.7 (m, 2H, CH2, 3); 7.7(q, 2H, CH,
ring 2); 7.8 (q, 2H,CH, ring 1).
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General structure:
(t, 2(n22)H, CH2, 4); 2.66 (t, 2H, CH2, 10); 3.0 (t, 2H, CH2, 3);
4.0 (t, 2H, CH2, 5); 4.2 (m, 2H, CH2, 2); 6.4 (s, 1H, NH, c); 6.9
(d, 2H, CH, ring 6); 7.2 (d, 2H, CH, ring 9); 7.7 (s, 1H, NH, b);
7.8 (q, 4H, CH, ring 7, 8); 8.7 (s, 1H, NH, a).
Acknowledgements
Dr. R. Wingen from Clariant GmbH is thanked for the gift of
the low molecular mass liquid crystals. Thanks are due to the
DFG (Ze230/14-1) for the financial support.
1H-NMR in DMSO-d6: d/ppm = 0.9 (t, 3H, CH3, 13); 1.3
(m, 2mH, CH2, 12); 1.6 (m, 2H,CH2, 11); 1.8 (t, 2(n22)H,
CH2, 4); 2,66 (t, 2H, CH2, 10); 2.8 (t, 2H, CH2, 3); 4.0 (t, 2H,
CH2, 5);7.0 (d, 2H, CH, ring 6); 7.4 (t, 2H, CH, ring 9); 7.6–7.8
(q, 8H, CH, ring 7,8); 7.9 (s, 3H, NH3, 1)
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2.6 Semicarbazides
2 mmol isocyanates are solved in abs. toluene and a solution
of 2.1 mmol ethylcarbazate in 50 ml abs. toluene is added
dropwise under vigorous stirring. Afterwards the mixture is
refluxed for 2 h. After cooling down, the product precipitates.
Yield: 77% of the theory.
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General structure:
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achiral pyrimidines in the ratio of 39%–26%–35% has a phase
transition temperature of I 68 N 60 SA 51 SC 7,5(-3)X.
1H-NMR in DMSO-d6: d/ppm = 0.9 (t, 3H, CH3, 13); 1.1–
1.3 (m, 2m + 3H, CH2, 1, 12); 1.6 (m, 2H,CH2, 11); 1.8
358 | J. Mater. Chem., 2006, 16, 351–358
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