New low polar tolane cholesterics designed for infrared applications

Article abstract of DOI:10.1039/c6ra16023a

We have designed, synthesized and evaluated the physical properties of new fluorinated phenyltolane based chiral liquid crystal materials with 2-methylbutyl terminal chain. The 2- or 2,6-fluorosubstitution in combination with the phenyltolane core brings surprising improvement of mesomorphic properties. The investigated compounds are characterized by a broad temperature range of the cholesteric (chiral nematic N?) phase, very low melting temperatures and easy thermal control of selective reflection in the near infrared region. The moderate helical twisting power β, high optical anisotropy and source of a blue phase make these compounds attractive for cholesteric mixture formulations.

Full text of DOI:10.1039/c6ra16023a

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New low polar tolane cholesterics designed for infrared
applications.
Received 00th January 20xx,
Accepted 00th January 20xx
Jakub Herman and Przemysław Kula
We have designed, synthesized and evaluated physical properties of new fluorinated phenyltolane based chiral liquid
crystal materials with 2-methylbutyl terminal chain. The 2-or 2,6-fluorosubstitution in combination with phenyltolane core
brings surprising improvement of mesomorphic properties. Investigated compounds are characterized by a broad
temperature range of cholesteric (chiral nematic N*) phase, very low melting temperatures and easy to thermal control
selective reflection in the near infrared region. The moderate helical twisting power β, high optical anisotropy and a
source of blue phase make these compounds attractive for cholesteric mixtures formulation.
DOI: 10.1039/x0xx00000x
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Introduction
to synthesize, moreover solubility of such structures is also usually
poor. For these reasons, the fabrication of novel cholesteric liquid
crystals with increased birefringence, hence broad bandgap is
Cholesteric structures attract much attention due to number of
unique applications.1-5 Advanced studies on blue phase materials6
have also brought recently an increased interest in chiral liquid
crystal field. These fields provide fresh impact in the area of the
design and synthesis of new cholesteric liquid crystals.7 Selective
light reflection given by the cholesteric LCs may be placed in the UV,
visible or IR wavelengths. Materials with reflection of infrared light
and transparency in the visible region have already found many
application areas such as an infrared-light reflective plates,8 films,9
reflectors,10 followed by numerous patent applications. They are
used mainly to reduce the energy consumption or as a window
heat-shielders (energy-saving smart windows).11 These and other
innovative applications create a need for broadband IR reflector,
where characteristic reflection bandwidth Δλ should be wide. Δλ
measured as a width of the reflection bandgap at half height is
normally limited to ~100nm in the visible spectrum. This value
strongly depends on the birefringence of the material used.
Therefore, continuous efforts in broadening of the reflection band
bring the need for increasing the birefringence Δn of cholesteric
systems.12 Increase of Δλ, obtained as a result of birefringence
increase may be done only by the synthesizing tailored liquid crystal
compounds. Molecules with large electron polarizability anisotropy
Δα, which is induced by the presence of the long conjugated π
electron bond systems, have high birefringence. The most
promising are molecules with large length to breadth ratio and rigid
cores composed of aromatic rings. The best ones comprise of
benzene rings connected directly, or better, via the ethynyl (–C≡C–)
group.13 Highly birefringent liquid crystals are often difficult
challenging. Typical commercially available chiral compounds show
rather poor solubility in the liquid crystal host (several wt %), mainly
because of structural incompatibility with widely used nematic
hosts. Additionally, for a vast majority of cholesteric liquid crystals
used nowadays Δn is less than 0.3.14 In this work we focused on
synthesis of new class of liquid crystal materials based on
phenyltolane core, which assures increased birefringence Δn>0.3.
Lateral fluorine substitution, 2,6-difluorophenylacetylene unit in
particular brings unique and valuable nematic stability,15-17 which in
connection with
a chiral centre broadens significantly the
cholesteric (N*) phase range, even up to 100K. As a chiral source
(S)-2-methylbutyl alkyl chain was chosen, mainly because it’s easily
accessible. Although the selected chiral chain, due to lack of strong
polar substituents in vicinity to asymmetric carbon, does not
provide high helical twisting power (β<20µ-1), still great solubility
and miscibility with various achiral hosts make these kind of
structures promising materials. Compounds were designed to show
structural similarity to widely used nematic achiral hosts, in that
way the solubility and interactions with hosts could be enhanced.
As a result, solutions of different concentrations of chiral compound
(5÷20wt %) were composed, which shows reflection in the near
infrared region.
Experimental
Synthesis
17
Synthetic approach described deeply in is generally based on the
method of a copper (Li2CuCl4) catalysed cross coupling of an aryl
Grignard reagents with alkyl bromides. This method is a simple
modification of known procedure,18 gives much greater yields and
besides of non-branched alkyl chains can be easily used for chiral
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branched (S-2-methylbutyl) alkyl chains incorporation as well. For
the synthesis commercially available (S)-(+)-1-bromo-2-
methylbutane (99% ee) was used. Following this one-pot procedure
we obtained both alkyl fluorosubstituted benzene (1,2) and 4-
alkylbiphenyl (5) derivatives that were main starting materials to
final mesogens’ synthesis, see Figure 1. Subsequent synthetic
process was generally based on Sonogashira cross coupling
reaction, after which purification of final compounds could be
carried out.
Purity and structural analysis
The purity of intermediates and final products was confirmed by
thin-layer chromatography (TLC), gas chromatography equipped
with flame ionization and mass selective detector GC–FID/MS
(Shimadzu GCMS–QP2010S), and by high-performance liquid
chromatography HPLC–PDA–MS (APCI–ESI–dual source) (Shimadzu
LCMS–2010 EV, column Kinetex C18 and PFP; 50 mm × 3.00 mm ×
2.6 μm) in water/acetonitrile or water/methanol. Products were
purified by crystallization and column liquid chromatography (or in
the case of liquid products column chromatography only) to
confirm final purity greater than 99.5%. Optical purity: 99% ee for
compounds 10, 11, 12 and 13 and 98% ee for compounds 8, 9
respectively.
Mesomorphic properties
Phase transition temperatures and enthalpy data were determined
by polarising optical microscopy (POM) with an OLYMPUS BX51
equipped with a Linkam hot stage THMS-600 and differential
scanning calorimeter (SETARAM DSC 141) during heating/cooling
cycles (rate 2 K/min).
Figure 1. Synthetic route to investigated compounds. Synthetic
conditions: a: Mg, THF anhydrous; b: (S)-(+)-1-bromo-2-
methylbutane, Li2CuCl4; c: n-BuLi or sec-BuLi, THF anhydrous, -70°C,
then iodine; d: I2, HIO3, CH3COOH, H2SO4, H2O; e: 2-methyl-3-butyn-
2-ol, PdCl2(PPh3)2, CuI, TEA, DBU, THF anhydrous; f: NaH, toluene; g:
PdCl2(PPh3)2, CuI, TEA, DBU, THF anhydrous.
Selective light reflection measurements and
Helical Twisting Power calculations
Selective reflection measurements were conducted with UV-3600
Shimadzu spectrophotometer using Peltier’s temperature
controller.
The helical twisting power β [μm-1] was determined using a
19
selective light reflection method
and evaluated according to
Equation (1):
2 | J. Name., 2012, 00, 1-3
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pentyl alkyl chain. Temperatures and enthalpies of phase
transitions are listed in Table 2. Compounds are distinguished by a
broad N* phase temperature range. Additionally, homologues with
two fluorine atoms (9, 11, 13) have much lower melting and
clearing temperatures than compounds mono fluoro-substituted (8,
10, 12), however N* phase range is broader for difluoro
homologues. This is the phenomena of 2,6-difluorophenylacetylene
unit, which strongly stabilizes nematic phase. Melting temperatures
below 30°C allow to broaden the range of N* phase even up to 100
K, which is still difficult to obtain for single liquid crystal chiral
compounds. It is also worth to add that BP-N* transitions were
observable in DSC thermograms with its fine enthalpies. This basic
property hardly ever met in liquid crystal systems, in our case was
obtained only for dichiral homologues 8 and 9 (See Figure 2).
β
[μm-1] = (p·ee·xe)−1
,
(1)
where xe is the mole fraction of chiral compound and p is the helical
pitch, ee is the enantiomeric excess.19 The helical pitch as a linear
function of selectively reflected light wavelength λmax is defined by
the relationship (2):
λmax= p·naverage
, (2)
where naverage is the average refractive index, approximately 1.5 for
liquid crystals. β values were determined at several different weight
solutions: 5÷10 wt% in three different nematic hosts (See Table 1)
by the method described in 7b. First host mixture 1980 is based on
structurally similar to the investigated in this article compounds:
fluoro-substituted alkyl-alkyl phenyltolanes, described in 16, with
the following properties: TN–Iso= 106°C, melting point Tm< –10°C,
birefringence Δn= 0.31 measured at 20°C for sodium line D, ∆ε=
2.04 measured at 1KHz and 20°C. Helical twisting power values
measured in this host are indicated β1. The second host mixture
1816, the dielectrically positive nematic, six component material
containing two- and three-ring isothiocyanates and cyanates has a
Table 2. Phase transition temperatures and enthalpies (J/mol) of
investigated phenyltolanes. Temperatures of melting are taken
from heating, while N*-BP and clearing temperatures are listed
from cooling.
Temperature [°C] and enthalpy [J/mol]
no.
of phase transition
nematic range TN–Iso
= 87.6°C, melting point Tm< –10°C, and
8 a
Cr
Cr
Cr
Cr
Cr
Cr
N*
N*
N*
N*
N*
N*
BP
BP
Iso
Iso
Iso
Iso
Iso
Iso
102.2
(18 329)
124.8
(13.1)
126.6
(366.2)
birefringence Δn= 0.22 measured at 20°C for sodium line D. Helical
twisting power values measured in this host are indicated β2. The
third host composition was the dielectrically negative nematic
mixture 1754 (∆ε= -1.82 measured at 1KHz and 20°C), containing
fluorinated terphenyls. Temperatures of phase transitions are the
following Tm= 16.7°C, TN-Iso= 114.2°C, optical anisotropy is ∆n= 0.25
measured at 20°C for sodium line D. Twisting power values
measured in this host are indicated β3.
9 a
56.5
(17 988)
105.1
(10.0)
106.5
(339.9)
10 b
11 b
12 b
13 b
89.2
(18 134)
156.7
(831.5)
30.9
(18 715)
133.5
(841.3)
Table 1. Nematic host mixtures for helical twisting power
measurements.
75.2
(16 497)
151.0
(1 108)
Dielectric
Nematic
Indicator
anisotropy
(Δε)
Structure
host
27.4
130.8
(24 530)
(889)
β1
1980
+ 2.5
a obtained from DSC
b obtained from polarizing thermomicroscope
β2
β3
1816
1754
+ 15.0
- 1.8
Results and discussion
Mesomorphic properties
Characterized liquid crystal compounds possess (S)-2-methylbutyl
chain as a chiral source either at one or both terminal sides of
molecule. In case where only one branched chain is presented, the
second side of molecule is ended with carbon atom equivalent, n-
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Figure 2a. DSC thermogram of compound 8.
Figure 3. Transmission spectra of thin layers of compound 9 in 1980
nematic host mixture recorded at 25°C for three different
concentrations: green: 18.5wt %; red: 14wt %; black: 7wt %.
For all investigated compounds helical twisting power was
determined every 5K step in temperature range: 10°C to 90°C. β1
values were obtained for all synthesized homologues, while β 2 and
β
only for dichiral compounds 8 and 9. Results for two main
3
temperatures 20°C and 50°C are listed in Table 3. Figures 4 and 5
show temperature dependence of experimental results over the
whole range. Helical twisting power, as one of the parameters that
informs us of chiral dopants ability to induce the helical structure in
achiral liquid crystalline host, helps to categorize chiral dopants.20 It
is also proven that achiral nematic medium has strong influence on
helical twisting power values for chiral liquid crystalline
compounds.21 We decided to select three structurally different
achiral nematic hosts, to sufficiently describe synthesized
phenyltolanes. Investigated compounds are characterized by
moderate values of the helical twisting power indicator. It is clearly
seen that compounds 8 and 9 with two chiral chains have much
higher values of β 1 than single-chiral homologues 10-13 (See Figure
4). Results for 1816 (β2) and 1754 (β3) do not bring significant
difference. Twisting power is higher for compound 9 than for
compounds 8 in three different hosts. We can also find, that
twisting power values decrease with temperature for 1980 and
1816 hosts (β1 and β2), whereas for dielectrically negative nematic
1754 host, β3 values increase, especially at lower temperature
regions.
Figure 2b. DSC thermogram of compound 9.
Selective light reflection and Helical Twisting
Power
Synthesized compounds and their mixtures in achiral nematics
show selective reflection in the near infrared region 0.7÷2.5 µm
(See Figure 3). Additionally, compounds are characterized by an
excellent miscibility with nematic hosts - primarily due to low
enthalpies of crystalline-nematic transitions (17÷24 kJ/mol – Table
2) and their structural compatibility with nematic hosts, we were
able to prepare even up to 20 weight % solutions. By the
formulation of different concentrations, it is shown in Figure 3, that
selective reflection is still placed in NIR region. Therefore
compounds should stand as perfect ingredients of cholesteric
mixtures, especially when precisely fixed reflection band is needed.
4 | J. Name., 2012, 00, 1-3
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Figure 4. Temperature dependence of β1 values for compounds 8-
13.
Acknowledgements
Work was supported by the PBS 23-651 grant and Military
University of Technology grant 08-794.
Notes and references
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Figure 5. Temperature dependence of helical twisting power values
β2 (red symbols) and β 3 (blue symbols) for compounds 8 and 9.
Table 3. Comparison of helical twisting power β values for
compounds 8 and 9 in different nematic achiral hosts, obtained at
20°C and 50°C.
Compound number
8
9
Helical Twisting Power β [μm-1]
20°C
50°C
20°C
50°C
20°C
50°C
9.17
8.68
9.53
8.97
8. US Pat., US20120094118 A1, 2010.
β 1
β 2
β 3
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11.10
11.04
10.19
10.33
11.98
11.64
12.08
12.01
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Summary
13. R. Dabrowski, P. Kula and J. Herman, Crystals, 2013, 3, 443-
482.
We have synthesized and evaluated physical properties of new
fluorinated phenyltolane based chiral liquid crystal materials
with (S) 2-methylbutyl chains. Investigated compounds are
characterized by a broad temperature range of N* phase, very
low melting temperatures and easy to achieve selective
reflection in the near infrared region. That makes them
excellent candidates for a cholesteric mixture formulation,
designed for example for energy-saving exploitation. High
stability of the chiral nematic mesophase originates mainly
from the rigid core which consists of 2,6-difluorophenyl
acetylene structural substructure, investigated in detail.15-18 Its
phenomenon leads to a perfect miscibility of investigated
compounds in different achiral nematics. Moderate values of
twisting power come from the origin of chiral source used
here, but its accessibility should be found as a huge advantage.
14. a. R. Eelkema and B. L. Feringa, Org Biomol Chem, 2006, 4,
3729-3745. b. Handbook of Liquid Crystals, ed. D. Demus, J.
Goodby, G. W. Gray, H.-W. Spiess and V. Vill, Wiley-VCH,
Weinheim, 1998, vol. 1–3. c. J. W. Goodby, Mol. Cryst. Liq.
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15. a. R. Buchecker, G. Marck and M. Schadt, Mol Cryst Liq Crys A,
1995, 260, 93-105. b. F. Seils and M. Schadt, Mol Cryst Liq Crys
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16. P. Kula, A. Aptacy, J. Herman, W. Wojciak and S. Urban, Liq
Cryst, 2013, 40, 482-491.
17. J. Herman and P. Kula, Tetrahedron Lett, 2013, 54, 3621-3623.
18. S. Liang, Z. Zhi-Yong, Y. Hong-Jun, D. Zhi-Qun, W. Ben-Mei, X. Li
and P. Zeng-Hui, Chinese Journal of Liqud Crystals and Displays,
2010, 6, 784-791.
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19. Ch. Mauguin, B Soc Fr Mineral Cr, 1911, 34,71–117.
20. a. W. Rejmer, K. Czuprynski and M. Zurowska, Mol Cryst Liq
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Turk and A. Schonhofer, J Mater Chem, 1995, 5, 2167-2174.
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Cryst, 2011, 547, 33-38.
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graphical abstract
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