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Journal ArticleDOI

A parallel high-throughput approach to liquid crystal screening

23 May 2005-Review of Scientific Instruments (American Institute of Physics)-Vol. 76, Iss: 6, pp 062216
TL;DR: In this paper, a technique was developed for the high-throughput (HT) parallel screening of liquid crystal phase transitions, using optical anisotropic effects observed only in the LC state to allow the determination of melting and clearing points.
Abstract: A technique has been developed for the high-throughput (HT) parallel screening of liquid crystal (LC) phase transitions, using optical anisotropic effects observed only in the LC state to allow the determination of melting and clearing points. A camera was used to monitor optical changes observed in arrays of LC samples upon heating, when illuminated with plane polarized light and viewed through a second polarizer aligned perpendicular to the plane of polarized light. Data was processed using Image Pro Plus, extracting values for the melting point and clearing point of each sample in the array. When compared with literature and conventional differential scanning calorimetry the new technique was found to be in excellent agreement, establishing the technique as a viable HT alternative. The method was used to analyze a 75 formulation library prepared using liquid handling for HT clearing point determination, showing the scope and power of the technique in the analysis of larger libraries.

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A parallel high-throughput approach to liquid crystal screening

  • Tobias Cull School of Chemistry, Joseph Black Buildings, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ Mark Goulding Merck Chemicals Ltd., Chilworth Science Park, Southampton, SO16 7QD Mark Bradleya!.
  • Data was processed using Image Pro Plus, extracting values for the melting point and clearing point of each sample in the array.
  • The application of high-throughputsHTd methods to the synthesis of liquid crystal libraries is relatively recent; however success has been found using combinatorial methods to introduce diversity into to a central core so that structure property relationships can be explored.
  • To date, no methods have been developed that are suitable for HT LC screening, although recently hyper DSC12,13 techniques have emerged, allowing increased throughput of samples using faster thermal scan rates.

II. DEVELOPMENT OF A HIGH-THROUGHPUT LC SCREENING DEVICE

  • While within this range many mesophases can exist, depending on the structure of the LC.
  • The drawback to the traditional technique of determining mesophases, using DSC followed by polarized optical microscopy, is that the adAuthor to whom correspondence should be addressed; electronic mail: mbradley@staffmail.ed.ac.uk.
  • Reuse of AIP content is subject to the terms at: http://rsi.aip.org/about/rights_and_permissions time taken for screening can exceed the time taken for synthesis and/or formulation.
  • In contrast, light which passes through the LC sample loses its linear polarization and is transmitted through the second polarizer, to be observed as a light area through the film.
  • Thus by analysis of the temperatures at which this change occurs, the melting and clearing points of the LC sample can be determined.

III. DESCRIPTION OF APPARATUS

  • Mounted to the top of the heating element was a 100 mm3100 mm35 mm brass block, chrome plated to provide a resilient and reflective surface for sample loading.
  • The whole unit was painted matte black to prevent the ingress of any unpolarized light which could reduce the sensitivity of the apparatus.
  • Above the heating block and enclosure setup and above one polarizing film was fixed a cold cathode backlight, to provide an intense homogenous light source, and a webcam above the other.
  • The HT screening device was calibrated externally using a digital surface thermometer that was attached to the surface of the brass block with thermal paste and the the Calcomms controller was adjusted to match the temperature while an IR thermal camerasIRRIS 256STd was used to measure temperature uniformity.
  • Other settings were configured automatically using the auto setup routine of the Calcomms instrument.

IV. SAMPLE LOADING

  • LC samples are loaded onto the apparatus in one of two ways.
  • For smaller numberss,50d, samples could be spotted, using a pipette, directly onto the chromed plate.
  • The wells were filled and the slide placed onto the surface of the chromed plate.
  • Containment also facilitates later analysis as the positions of samples are fixed throughout the data collection sequence.
  • LC’s anisotropic effects are usually only seen when the director is orientated by a alignment layer or an external field.

V. PROCEDURE FOR MEASUREMENT OF MELTING AND CLEARING POINTS

  • Following sample loading the apparatus was assembled and the webcam arranged to give optimal images of the LC samples.
  • The samples were heated from 25 °C and 160 °C at a rate of 5°C/min, the fastest heating rate available which produced a linear response throughout the temperature range, while data was collected from both the webcam and the Calcomms controller simultaneously.
  • The presence of monotropic and enatiotropic LC phases was determined by data collection throughout the cooling cycle.
  • Ad Samples spotted directly onto the analysis surface.
  • The sequence was then advanced and the intensity of each spot at each frame in the sequence recorded automatically in an Excel spreadsheet to allow a plot of intensity versus temperature to be produced.

VI. PROCESSING

  • The data were processed to ease identification of phase transitions and eliminate noise from fluctuations in background light levels over the course of the experiment by evaluating the change in intensity between each set of points.
  • A scatter plot was then produced with the phases easily identified by the points at which the data is nonlinear and moves significantly from the standard.
  • Figure 6 shows the averaged data from three LCs.
  • The transitions can be identified clearly by the sharp peaks above or below the normal line.
  • The standard deviation is also shown in Fig. 6 as two thin horizontal lines.

VII. EXPERIMENTAL RESULTS

  • To validate the technique a range of well-known commercially available LCssMerckd were analyzed using this FIG.
  • The standard deviation can be seen as the two horizontal lines running above and below the data.
  • Reuse of AIP content is subject to the terms at: http://rsi.aip.org/about/rights_and_permissions.
  • The transition temperatures were compared with values obtained from conventional DSC analysis of the same materialsstaking the onset temperature as the phase transitiond and those gained from the literature.
  • The results were in complete agreementswithin experimental errord, establishing this method as a viable alternative to DSC for library screening.

VIII. SCREENING OF MIXTURE LIBRARIES

  • A large percentage of the LC materials produced annually are used in the manufacture of LC displaysLCDd devices, and such displays contain a complex mixture of individual components chosen and blended to provide properties that match exact specifications.
  • To illustrate this approach a library of 75 mixtures was constructed using three commercially available materials.
  • The library was loaded into the microwells of the contact photolithography matrix and heated from 25 °C to 160 °C.
  • The data collected was visualized in Fig. 8 as a four-dimensional plot relating the % composition of each component to its clearing pointsthe fourth dimension represented using color and sized.

IX. DISCUSSION

  • The development of a HT screening technique for LC phase transitions, which provides comparable results to con- crystal phase transitions and conventional DSC analysis, compared with those given in the literature.
  • The mesophases observed in the liquid crystal samples are crystalline to nematicsCr-Nd, nematic to isotropicsN-Id, crystalline to smecticsCr-Smd, and smectic to nematicsSm-Nd. FIG.
  • The relationship between the composition of a mixture, formulated from three components and its clearing point as obtained rom the high-throughput screening apparatus for liquid crystals.
  • The clearing point is visualised in the fourth dimension using color and size.
  • Reuse of AIP content is subject to the terms at: http://rsi.aip.org/about/rights_and_permissions ventional DSC data, has shown that LC synthesis or formulation libraries can be analyzed in a fraction of the time compared with conventional methods.

ACKNOWLEDGMENTS

  • Financial support from Merck and the EPSRC is gratefully acknowledged.
  • Thanks also go to Patrick Hendra of Ventacon Ltd.sWinchester, UKd for help in the design and the fabrication of equipment.
  • Reuse of AIP content is subject to the terms at: http://rsi.aip.org/about/rights_and_permissions.

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A parallel high-throughput approach to liquid crystal screening
Citation for published version:
Cull, T, Goulding, M & Bradley, M 2005, 'A parallel high-throughput approach to liquid crystal screening',
Review of Scientific Instruments, vol. 76, no. 6, 062216, pp. -. https://doi.org/10.1063/1.1906125
Digital Object Identifier (DOI):
10.1063/1.1906125
Link:
Link to publication record in Edinburgh Research Explorer
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Download date: 10. Aug. 2022

A parallel high-throughput approach to liquid crystal screening
Tobias Cull, Mark Goulding, and Mark Bradley
Citation: Rev. Sci. Instrum. 76, 062216 (2005); doi: 10.1063/1.1906125
View online: http://dx.doi.org/10.1063/1.1906125
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A parallel high-throughput approach to liquid crystal screening
Tobias Cull
School of Chemistry, Joseph Black Buildings, University of Edinburgh, West Mains Road,
Edinburgh, EH9 3JJ
Mark Goulding
Merck Chemicals Ltd., Chilworth Science Park, Southampton, SO16 7QD
Mark Bradley
a
School of Chemistry, Joseph Black Buildings, University of Edinburgh, West Mains Road,
Edinburgh, EH9 3JJ
Received 18 November 2004; accepted 14 March 2005; published online 23 May 2005
A technique has been developed for the high-throughput HT parallel screening of liquid crystal
LC phase transitions, using optical anisotropic effects observed only in the LC state to allow the
determination of melting and clearing points. A camera was used to monitor optical changes
observed in arrays of LC samples upon heating, when illuminated with plane polarized light and
viewed through a second polarizer aligned perpendicular to the plane of polarized light. Data was
processed using Image Pro Plus, extracting values for the melting point and clearing point of each
sample in the array. When compared with literature and conventional differential scanning
calorimetry the new technique was found to be in excellent agreement, establishing the technique as
a viable HT alternative. The method was used to analyze a 75 formulation library prepared using
liquid handling for HT clearing point determination, showing the scope and power of the technique
in the analysis of larger libraries. © 2005 American Institute of Physics. DOI: 10.1063/1.1906125
I. INTRODUCTION
The synthesis of large numbers of liquid crystals LCs
has been pursued for many years in an attempt to gain struc-
ture property relationships and to develop specific properties
for uses in commercial applications such as liquid crystal
displays
1
LCDs. The application of high-throughput HT
methods to the synthesis of liquid crystal libraries is rela-
tively recent; however success has been found using combi-
natorial methods to introduce diversity into to a central core
so that structure property relationships can be explored.
Deeg
2,3
applied a solution phase parallel synthetic route with
parallel purification to produce a library 225 members of
4,4
-dipropyl-p-quaterphenyl liquid crystal homologes in
high yield. Goulding
4
demonstrated the solution phase, one
pot synthesis of multiple 4-alkyl-4
cyanobiphenyls while
Hioki
5
developed the solid phase parallel synthesis of a 28
member azomethine-linked liquid crystal library. Kang
6
pre-
pared a 40 member library by the coupling of resorcinol
derivatives, 4-formylbenzoic acids and 4-alkoxyanalines, to
form bent- shaped mesogens. To assess LC properties of the
resulting libraries a screening process must be employed. In
traditional libraries
7–11
a combination of optical polarizing
microscopy followed by differential scanning calorimetry
DSC is routinely used to identify and confirm the presence
of phase transitions. Although these techniques are well es-
tablished and accurate and can be used to distinguish a wide
range of liquid crystalline phases, they are time consuming
and can only be performed on single compounds. In the
screening of a large library this severely restricts a rapid
discovery process, limiting the formulation of new LCs,
which is the major route to optimizing new LC materials,
and thus provides a major obstacle when larger libraries need
to be considered.
To date, no methods have been developed that are suit-
able for HT LC screening, although recently hyper DSC
12,13
techniques have emerged, allowing increased throughput of
samples using faster thermal scan rates. In materials combi-
natorial chemistry HT screening can take many forms de-
pending on the application, usually requiring adaptation of
conventional analytical tools or the development of novel
methods.
14
Optical imaging is often employed as a tool,
demonstrated in the HT screening of ternary-metal-alloy
libraries,
15
imaging of phosphor discovery libraries by UV
excited photoluminescence
16,17
and screening of catalyst li-
braries by IR imaging.
18
A device was therefore designed and
built that would allow the HT formulation and analysis of
liquid crystal libraries.
II. DEVELOPMENT OF A HIGH-THROUGHPUT LC
SCREENING DEVICE
One of the initial parameters to be explored in a LC
library is the liquid crystal range defined at the lower end by
its melting point and at the upper end by the transition from
a liquid crystal to an isotropic liquid, referred to as the clear-
ing point. While within this range many mesophases can
exist, depending on the structure of the LC. The drawback to
the traditional technique of determining mesophases, using
DSC followed by polarized optical microscopy, is that the
a
Author to whom correspondence should be addressed; electronic mail:
mbradley@staffmail.ed.ac.uk
REVIEW OF SCIENTIFIC INSTRUMENTS 76, 062216 2005
0034-6748/2005/766/062216/6/$22.50 © 2005 American Institute of Physics76, 062216-1
Downloaded 09 Aug 2013 to 129.215.221.120. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://rsi.aip.org/about/rights_and_permissions

time taken for screening can exceed the time taken for syn-
thesis and/or formulation. The development of a device able
to measure both melting and clearing points of LC sub-
stances in parallel would allow large libraries to be screened
quickly, allowing the rapid identification of materials with
desirable LC properties for further in-detail characterization.
In the design of the HT device, a property which is ex-
hibited only in the liquid crystalline phase was exploited,
allowing the phase transition from solid to LC and LC to
isotropic liquid to be determined. Optical anisotropy causes
the circular polarization of linear polarized light as it passes
through the material, a result of light traveling at different
speeds through the material depending on its orientation in
relation to the optical axis, in this case the LC director. As a
consequence the emerging ordinary and extraordinary rays
come out of phase, causing a change in polarization.
To visualize this effect two polarizers can be placed,
with their plane of polarization at 90° to each other, on either
side of a LC sample. One side of the apparatus is illuminated
and the opposite side is then observed as shown in Fig. 1. In
the area where there is no LC sample there will be no light
transmitted due to the polarizing films and hence it will ap-
pear dark. In contrast, light which passes through the LC
sample loses its linear polarization and is transmitted through
the second polarizer, to be observed as a light area through
the film.
It was this principle that was the basis for the HT screen-
ing device for determining LC melting and clearing points.
As only the LC phase possesses anisotropic properties, solids
and isotropic liquids will not be seen through the crossed
polarizers. As the temperature is increased and the LC me-
sophase begins to form the sample will appear as a white
spot. As the temperature is increased further a phase change
to an isotropic liquid will occur and the sample would be
observed to disappear as its anisotropic properties are lost.
Thus by analysis of the temperatures at which this change
occurs, the melting and clearing points of the LC sample can
be determined. Mesophases that occur between the melting
and clearing points, such as smectic phases, can also be iden-
tified by a change in intensity of the white spot.
III. DESCRIPTION OF APPARATUS
The HT LC screening device in Fig. 2, consisted of a
heating block containing a thermocouple which provided
feedback to a Calcomms 3300 temperature control unit for
thermostatic control, interfaced to a PC to provide data log-
ging. Mounted to the top of the heating element was a
100 mm 100 mm 5 mm brass block, chrome plated to
provide a resilient and reflective surface for sample loading.
To the top face of the heating block was mounted a remov-
able enclosure, designed to hold two sheets of polarizing film
in a fixed angled position above the heating block, the angle
and height of the enclosure optimized so that the reflection of
the opposite polarizing film on the chrome block could be
observed when looking into the enclosure through one film.
The films were aligned with the plane of polarization at 90°
to each other, and to ensure optimum extinction the polariz-
ers could be rotated by each way. To prevent the buildup
of heat and damage to the polarizers inside the enclosure a
fan was fitted, while holes were made in the front and back
of the enclosure for cooling. The whole unit was painted
matte black to prevent the ingress of any unpolarized light
which could reduce the sensitivity of the apparatus. Above
the heating block and enclosure setup and above one polar-
izing film was fixed a cold cathode backlight, to provide an
intense homogenous light source, and a webcam above the
other. The webcam was aligned such that the reflection of the
second film could be seen and was interfaced to a PC to
provide image capture.
The HT screening device was calibrated externally using
a digital surface thermometer that was attached to the surface
of the brass block with thermal paste and the the Calcomms
controller was adjusted to match the temperature while an IR
thermal camera IRRIS 256ST was used to measure tem-
perature uniformity. The IR images in Fig. 3 show a variation
in temperature of approximately 2.53 °C from the center of
FIG. 1. The path of unpolarized light through a pair of perpendicularly
aligned linear polarizers showing the interaction when a sample of liquid
crystal is introduced. Normally light which has been linearly polarized in
one plane cannot pass the second polarizer, however the presence of an
optically anisotropic liquid crystal between the polarizers causes linearly
polarized light to become circularly polarized and able to pass through the
second film.
FIG. 2. Schematic of the high-throughput screening apparatus for liquid
crystal phase transitions.
062216-2 Cull, Goulding, and Bradley Rev. Sci. Instrum. 76, 062216 2005
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the heating block to the outer edge, and a high temperature
ring on the extreme edges, which could not be used for
sample analysis. Other settings were configured automati-
cally using the auto setup routine of the Calcomms instru-
ment.
IV. SAMPLE LOADING
LC samples are loaded onto the apparatus in one of two
ways. For smaller numbers 50, samples could be spotted,
using a pipette, directly onto the chromed plate. For larger
numbers of samples 50–100 this method was not suitable
as the sample density was too high, resulting in the amal-
gamation of sample spots upon heating, and therefore a
method of sample containment was used by application of a
contact photolithography process reported by Cabral
19
and
Harrison.
20
In this process a UV cured adhesive Norland,
NOA81 was cast into a 10 10 grid with 2.5 mm
2.5 mm wells produced on a 32 mm 32 mm cover slide.
The wells were filled and the slide placed onto the surface of
the chromed plate. Figure 4 shows an example of the LC
samples loaded onto the plate directly Fig. 4a兲兴 and in the
microwell array Fig. 4b兲兴 as visualized by the webcam.
Containment also facilitates later analysis as the positions of
samples are fixed throughout the data collection sequence.
LC’s anisotropic effects are usually only seen when the di-
rector is orientated by a alignment layer or an external field.
In this example the natural alignment of the LC’s on either
the chrome or the glass surface appeared sufficient to ob-
serve the anisotropic effects, eliminating the need for an ad-
ditional surface alignment layer.
V. PROCEDURE FOR MEASUREMENT OF MELTING
AND CLEARING POINTS
Following sample loading the apparatus was assembled
and the webcam arranged to give optimal images of the LC
samples. The samples were heated from 25 °C and 160 °C
at a rate of 5 °C/min, the fastest heating rate available which
produced a linear response throughout the temperature range,
while data was collected from both the webcam and the Cal-
comms controller simultaneously. Images from the webcam
were recorded at 1 frame per second, producing a video se-
quence of the intensity change while the temperature was
recorded at 4 s intervals. Figure 4b shows a sequence of
images as materials pass the clearing point on the apparatus
and become transparent. The presence of monotropic and
enatiotropic LC phases was determined by data collection
throughout the cooling cycle.
The resulting video sequence was analyzed using the
image analysis software Image Pro Plus, sampling every
FIG. 3. Color online IR images of the high-throughput screening hot-plate.
The thermal distribution was measured by analysis of the temperatures at the
outer and inner sections of the heating plate. Analysis is carried out in
duplicate and found to differ by 2.53 °C from the center to the edge of the
plate. At the extremes of the plate lies a high-temperature area which should
not be used for sample analysis.
FIG. 4. Images of liquid crystal samples loaded onto the high-throughput
screening apparatus for liquid crystal phase transitions, as seen through per-
pendicularly aligned linear polarizers. a Samples spotted directly onto the
analysis surface. b Samples loaded into a “well” matrix, prepared using a
contact photolithography process. The glass coverslip is then placed onto the
analysis surface. 1=25 °C, 2=80 °C, 3=100 °C, 4=115 °C, 5=130 °C,
and 6=150 °C.
062216-3 High-throughput liquid crystal screening Rev. Sci. Instrum. 76, 062216 2005
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241 citations

Journal ArticleDOI
TL;DR: The tetrakis(triphenylphosphine) palladium (0)-catalysed coupling of arylboronic acids with aryal halides is used to prepare several 4,4″-dialkyl- and 4, 4″-alkoxyalkyl-1,1′:4′,1″-terphenyls with 2,3- or 2′,3′-difluoro substituents and their related biphenyl systems as mentioned in this paper.
Abstract: The tetrakis(triphenylphosphine) palladium (0)-catalysed coupling of arylboronic acids with aryl halides is used to prepare several 4,4″-dialkyl- and 4,4″-alkoxyalkyl-1,1′:4′,1″-terphenyls with 2,3- or 2′,3′-difluoro substituents and their related biphenyl systems. Lithiation ortho to a 1,2-difluoroaromatic unit provides the route to the 2,3-difluoroarylboronic acids.The 2,3-difluoro substituted terphenyls are low-melting liquid crystals with wide-range Sc phases and no underlying smectic phase; these compounds are excellent hosts for ferroelectric systems. The compounds with widest Sc ranges are those with the difluoro substituents in an end ring and the compounds with difluoro substituents in the central ring show more nematic character and so are useful for ECB devices.

232 citations