Functional liquid crystal films selectively recognize amine vapours and simultaneously change their colour

Article abstract of DOI:10.1039/b517768e

During recent years, cholesteric liquid crystals have found only a few applications, one of them being temperature detection; in this study, however, we intend to show that the combination of cholesteric liquid crystals and molecules with a trifluoroacety

Full text of DOI:10.1039/b517768e

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Functional liquid crystal films selectively recognize amine vapours and
simultaneously change their colour
Nicole Kirchner, Linda Zedler, Thomas G. Mayerho¨fer and Gerhard J. Mohr*
Received (in Cambridge, UK) 12th December 2005, Accepted 31st January 2006
First published as an Advance Article on the web 2nd March 2006
DOI: 10.1039/b517768e
One reason for the success of liquid crystalline displays (LCDs)
is their high operational stability. We wanted to combine the
chemical stability of liquid crystalline materials with the chemo-
sensor characteristics of our trifluoroacetyl receptors. Therefore,
we synthesized cholesteryl benzoate with a trifluoroacetyl receptor
attached to the benzene moiety, termed LCR-262.{ This
chemosensor was synthesized by converting 4-trifluoroacetylben-
zoic acid with thionyl chloride into the corresponding benzoic
chloride6 and then was esterified with cholesterol in a mixture of
pyridine and chloroform. LCR-262 was added to various mixtures
of cholesteryl nonanoate (CN), cholesteryl oleyl carbonate (COC)
and cholesteryl chloride (CC). The corresponding chemical
structures are depicted in Fig. 1.
During recent years, cholesteric liquid crystals have found only
a few applications, one of them being temperature detection; in
this study, however, we intend to show that the combination of
cholesteric liquid crystals and molecules with a trifluoroacetyl
function can be applied to optically detect amine vapours.
Liquid crystalline materials have found widespread application in
laptops, mobile phones, or PC games, and are now used to
develop large flat screen television sets. Often they are composed of
linear molecules with (a) medium length alkyl chains, (b) benzene
or cyclohexane moieties and (c) nitrile, trifluoromethyl or fluoro
substituents. These types of liquid crystals are called nematic liquid
crystals.1 Their transparency to polarized light depends on their
alignment and orientation between two polarizers with a twist of
90 degrees.
In the next step, the reflectance of the liquid crystal film (LCF)
composed of the above components was investigated in the
absence and presence of gaseous amines. White light in a spectral
range 350–850 nm was directed onto a liquid crystal film at normal
incidence using a fibre-optic diode-array photometer; then the
reflected light was guided back to the detector of the photometer.
In the course of the evaluation it became apparent that liquid
crystal films composed of cholesteryl derivatives crystallized after
several hours and were too liquid to remain stable on the substrate.
Therefore, different amounts of structure-supporting polymers
were added to raise mechanical stability and enhance the shelf
lifetime. For this we used poly(methylmethacrylate) (PMMA) and
polyurethane hydrogel (EG80A). Different compositions of the
cholesteryl derivatives were finally used to prepare the LCFs,
namely LCF1 composed of 12.2% CC, 32.4% COC, 32.4% CN,
15.4% PMMA and 7.6% LCR-262. LCF2 contained 12.7% CC,
29.9% COC, 29.9% CN, 20.3% PMMA and 7.2% LCR-262.
LCF3 consisted of 15.3% CC, 33.4% COC, 36.1% CN, 6.8%
EG80A and 8.4% LCR-262. Solutions of these cholesteryl
derivatives in tetrahydrofuran were deposited onto planar
A special form of the nematic liquid crystals are chiral nematic
liquid crystals, also known as cholesteric liquid crystals.2 In this
phase the molecules are gradually twisted against each other, so
that they form a helical structure. The length of a 360 degree
rotation is called the pitch. If the length of the pitch corresponds to
the wavelength of light, then this light is reflected with different
efficiency. The colouration depends on ambient temperature as
with an increase of temperature the length of the pitch rises and
accordingly colour changes are observed. Consequently, the main
application of cholesteric liquid crystals is temperature detection.
The length of the pitch, however, may also be modified through
the embedding of foreign molecules. This approach has already
been used to detect vapours of organic solvents (e.g. acetone,
benzene, methanol or chloroform) albeit without selectivity in the
recognition process.3 Here, we have evaluated an alternative
procedure to selectively detect amine vapours.
Recently, different chromogenic and fluorogenic structures with
trifluoroacetyl functions were developed to detect biogenic amines
or alcohols.4 While these trifluoroacetyl structures are generally
sensitive to aliphatic and aromatic amines, the detection of
alcohols requires the addition of the catalyst tridodecylmethylam-
monium chloride.5 The trifluoroacetyl dyes have been modified in
chemical structure so as to obtain absorbing and fluorescing
chemosensors with tailored optical characteristics, sensitivity and
selectivity. However, one inherent limitation is their photochemical
stability. Generally speaking, long-wavelength absorbing and
fluorescing structures are often photochemically decomposed and
are no longer of use for optical evaluation.
Institute of Physical Chemistry, Lessingstrasse 10, D-07743 Jena,
Germany. E-mail: gerhard.mohr@uni-jena.de; Fax: (+49) 3641 948302;
Tel: (+49) 3641 948379
Fig. 1 Chemical structures of the cholesteryl derivatives and LCR-262.
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substrates.{ Films with a higher content of LCR-262 had a short
shelf-time and crystallized within a few hours. Similarly, a higher
content of polymer proved to be unsuitable because the film
became inhomogeneous, as shown in Fig. 2. When the polymers
were added to the cholesteryl derivatives, the films exhibited a
significantly improved shelf lifetime, hydrophobic PMMA being
superior over hydrophilic EG80A. Accordingly, LCF1 was stable
for 52 days and LCF2 for 59 days, while LCF3 had a shelf life of
17 days only. During these periods the films were repeatedly used
to detect amine vapours.
(6 nm), however, the reflectance decreased more strongly than in
the case of 1-butylamine.
In general, a higher sensitivity for 1-butylamine over diethyla-
mine was observed (Fig. 5). This is due to the fact that primary
amines such as 1-butylamine are less sterically hindered to interact
with the trifluoroacetyl group of LCR-262 to form a hemiaminal
than secondary amines such as diethylamine.7
In order to characterize the liquid crystal films with respect to
forward and reverse response, time-course measurements at a
defined wavelength were performed. The films generally showed
longer forward and reverse response times in the presence of
1-butylamine than in the case of diethylamine (Table 1). The
response behaviour of LCF2 upon exposure to diethylamine is
given in Fig. 6. The regeneration time of the films was significantly
longer for triethylamine (>24 h). We attribute this to the fact that
1-butylamine and diethylamine have a different reaction mechan-
ism compared to triethylamine: both react with the trifluoroacetyl
group to form a hemiaminal while triethylamine reacts with the
trifluoroacetyl group to form a quarternary ammonium ion.8
The experiments were not only performed with films based on
LCR-262 added but also with films having a similar amount of
native cholesteryl benzoate (CB) to evaluate whether or not
unspecific interactions would take place. None of these reference
films showed colour changes with the amines under investigation.
In order to show that films based on LCR-262 were selective for
gaseous amines, they were additionally exposed to gaseous
acetone, methanol and ethyl acetate. No signal changes were
observed with concentrations as high as 18 000 ppm of acetone,
8000 ppm of methanol and 7500 ppm of ethyl acetate.
The following aliphatic amines were investigated as the analytes:
1-butylamine, diethylamine and triethylamine. The reflectance
maximum of LCF1 was observed at around 477 nm. The
interaction with 1-butylamine caused this maximum to shift to
503 nm when the concentration of 1-butylamine was as high as
3000 ppm. A similar behaviour was observed for LCF2 with a shift
in the reflectance maximum from 473 nm to 523 nm and a
concomitant change in colour from blue to green (Fig. 3). Fig. 4
illustrates the different stages of the visual colour change. In the
case of LCF3, a shift from 474 nm to 520 nm was observed. When
detecting diethylamine with a concentration as high as 8000 ppm, a
comparable shift in the reflectance maximum was found. With
LCF1, the shift was only 26 nm while it was 46 nm in the case of
LCF2 and 49 nm in the case of LCF3. Only LCF1 did also react
with triethylamine and showed a minor shift in the reflectance
Addition of polymers to liquid crystals usually leads to a so-
called multi-domain polymer-dispersed liquid crystal, the indivi-
dual domains of which are separated by thin polymer borders. If
each domain is oriented perfectly with the helical axes normal to
Fig. 2 Effect of increasing the polymer content on the homogeneity of
the liquid crystal film LCF2 (from left to right: 6.8%, 15.4%, 20.3%, 26.7%,
35.3% PMMA).
Fig. 5 Shift in the reflectance maximum of LCF2 upon exposure to
1-butylamine and diethylamine.
Fig. 3 Reflectance of LCF2 upon exposure to gaseous 1-butylamine.
Table 1 Forward and reverse response times of the liquid crystal
filmsa
Forward response time/min
BA DEA TEA
Reverse response time/min
BA DEA TEA
LCF1 30 to 40 15 to 25 12 to 15 60
45 to 55 .24 h
LCF2 70 to 90 20 to 40
LCF3 15 to 30 12 to 15
—
—
115 60
30 to 40 30 to 40
—
—
Fig. 4 Colour change of LCF2 (1st: initial colouration without analyte,
2nd: exposure to gaseous 1-butylamine, 3rd–6th: gradual recovery after
removal of 1-butylamine).
a
1-Butylamine (BA), diethylamine (DEA), triethylamine (TEA).
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carbonate solution and three times with water, evaporated to dryness and
finally purified by flash-chromatography (60 g of silica gel 60) using
dichloromethane : hexane = 1 : 1 as the solvent. lmax (chloroform) =
262 nm, lmax (hexane) = 258 nm, MS (m/z): 588, 370.
1H-NMR (CDCl3) d: 0.62 (3H, s), 0.78 (3H, d), 0.79 (3H, d), 0.86 (3H,
d), 1.00 (3H, s), 1.05–1.98 (28H, m), 4.82 (1H, m), 5.36 (1H, m), 8.05 (2H,
d), 8.13 (2H, d).
{ Preparation of liquid crystal films.
The glass substrates with a size of 9 6 14 mm were silanized prior to
immobilisation of the liquid crystal films. They were cleaned in a mixture of
concentrated sulfuric acid and 30% hydrogen peroxide (3 : 1 v/v) and
silanized by exposing them to hexamethyldisilazane vapour for 15 h in an
isolated container at room temperature and at ambient pressure. Finally,
the lower side of the glass plates was blackened with matt black varnish.
For the preparation of the films 5.0% (w/v) stock solutions of CC, COC
and CN in tetrahydrofuran were added to the respective polymer and
LCR-262 to yield a 6.5% (w/v) solution of LCF1, a 6.9% solution of LCF2
and a 5.9% solution of LCF3. Then 50 ml of the respective solution was
pipetted onto the silanized glass plate. After the evaporation of
tetrahydrofuran a liquid crystal film was obtained.
Fig. 6 Response behaviour of LCF2 upon exposure to different
concentrations of diethylamine, measured at a wavelength of 477 nm.
The sensor film was fixed in contact with the optical fibre of the diode-
array photometer and placed into a 100 ml flask closed with a septum,
where different concentrations of gaseous amines were injected at
22 uC ¡ 1 uC.
the substrate surface (parallel to the Z-axis of the reference frame)
the resulting state of helix orientation is called planar. In such a
case, neither the reflected nor the transmitted part of linearly
polarized light (incident along the normal of such a film)
experiences depolarization.9 Since we observed a non-zero cross-
polarization (10% transmittance of the incident light through the
liquid crystal film between crossed polarizers, independent of the
wavelength), a purely planar alignment must be excluded.
In contrast to the planar alignment, there is also the possibility
for a uniformly lying (quasi-planar) alignment, where the helical
axes are oriented parallel to each other and parallel to the substrate
surface. For such an alignment, the depolarization is a function of
the relative orientation between the helical axes and the
polarization vector. Therefore, any rotation around the Z-axis
will lead to a change in the depolarization which was not observed
in the present case. We therefore conclude that our films possess a
focal conic alignment as is commonly observed for multi-domain
polymer-dispersed liquid crystals.9,10 Further investigation of this
issue is in progress.
Polarized transmittance spectra of samples lacking the black coating
have been recorded on a Varian Cary 5000 instrument in the wavelength
range 400–700 nm utilizing two Polaroid sheets, one acting as polarizer and
the second as analyzer. For the measurements, the samples were oriented in
a way that the substrate surface (i.e. the glass substrate which the liquid
crystal solution was deposited on) was perpendicular to the ray direction
(the Z-axis). A first series of measurements was carried out with the
polarization direction of the polarizer and the analyzer being parallel and
oriented either along the X- or the Y-axis. (XX- or YY-polarization), a
second series of spectra was recorded with crossed polarization (XY- or
YX-polarization).
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In conclusion, we have shown that by embedding selective
cholesterol-based receptor molecules into liquid crystalline materi-
als, we were capable of developing selective liquid crystal films for
gaseous analytes such as aliphatic amines. We consider this
approach to be generic in that by using other cholesteryl
derivatives with appropriate receptor moieties, it will become
possible to obtain liquid crystal sensor films for e.g. biogenic
aldehydes, alcohols or volatile toxic agents.
This work was supported by the Heisenberg Fellowship MO
1062/1-1 and the research grant MO 1062/2-1 of Deutsche
Forschungsgemeinschaft, and by Fluka Chemie GmbH. This
support is gratefully acknowledged. We would also like to thank
Antje Kriltz for stimulating discussions.
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Notes and references
{ Synthesis of LCR-262.
The amount of 0.4 g of 4-trifluoroacetylbenzoic acid was suspended in
2 ml of thionyl chloride and heated under reflux for 5 h. Then, the solution
was evaporated to dryness and dissolved in 1 ml of toluene. This solution
was carefully added to a solution of 0.71 g of cholesterol and 0.6 ml of
pyridine in 2 ml of dry toluene cooled to 5 uC. The mixture was heated to
40 uC and stirred for 5 h. After evaporation to dryness, the residue was
dissolved in 100 ml of dichloromethane, washed with 30 ml of 10% sodium
1514 | Chem. Commun., 2006, 1512–1514
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