Photo tuning of thiophene-2,5-dicarbohydrazide derivatives for their photoalignment ability-molecular modelling studies

Article abstract of DOI:10.1039/c5ra12772f

In this work, we reported a photoalignment of a nematic liquid crystal, promoted by thin alignment layers made from novel thiophene-2,5-dicarbohydrazide derivatives after being illuminated with linear polarized UV light. A clear indication of the relationship between the molecular structure and alignment properties of the materials was obtained by introducing a methyl substituent group (-CH₃) in the molecular structure of some of the materials studied in this work. Interestingly, the illumination with linear polarized UV caused the preferred direction of liquid crystal alignment, promoted by these layers, to be oriented either parallel (in the case of the thiophene derivative without methyl substitution) or perpendicular (in the case of the thiophene derivative with methyl substitution). The mechanism behind the observed alignment effect is elucidated by molecular modelling studies. These studies suggested the change of the orientation of the molecular net dipole moment, when a methyl group is incorporated in the molecular structure of the materials, as a possible origin. Such information is indispensable for the design and synthesis of novel photo-alignment materials for liquid crystal displays of high quality.

Full text of DOI:10.1039/c5ra12772f

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Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
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Photo tuning of thiopheneꢀ2, 5ꢀdicarbohydrazide
derivatives for their photoalignment abilityꢀMolecular
modelling studies
Gurumurthy Hegde a*, A. A. Murausky,b A. A. Murauski,b I. N. Kukhta,b A. V.
Adhikari,c L.Komitov,d
In this work, we reported a photoalignment of a nematic liquid crystal, promoted by thin alignment
layers made from novel thiopheneꢀ2,5ꢀdicarbohydrazide derivatives after being illuminated with linear
polarized UV light. A clear indication about the relationship between molecular structure and alignment
properties of the materials were obtained by introducing a methyl substituent group (ꢀCH3) in the
molecular structure of some of the materials studied in this work. Interestingly, the illumination with
linear polarized UV caused the preferred direction of liquid crystal alignment, promoted by these layers,
to be oriented either parallel (in case of thiophene derivative without methyl substitution) or
perpendicular (in case of thiophene derivative with methyl substitution). Mechanism behind the observed
alignment effect is elucidated by molecular modelling studies. These studies suggested the change of the
orientation of the molecular net dipole moment, when methyl group is incorporated in the molecular
structure of the materials, as a possible origin. Such information is indispensable for the design and
synthesis of novel photoꢀalignment materials for liquid crystal displays of high quality.
There are a number of requirements to the conventional materials
used for preparation of alignment layers in LCDs such as long term
chemical, physical and electroꢀchemical stability as well as high
voltage holding ratio (VHR), low residual dc (RDC) and proper
anchoring properties (low or strong anchoring strength, depending
on the particular LCD application). In addition to these
requirements, the materials for photoalignment layers in LCDs need
to be highly photosensitive so that light with low light intensity and
short exposure time will give sufficiently large photoalignment
effect, which is stable with time and ambient illumination.
1. Introduction
The alignment quality of the liquid crystal in liquid crystal displays
(LCD) is of vital importance for the performance of these devices.
The conventional alignment technique used at present in the
fabrication of LCDs is the mechanical rubbing of the alignment
layers deposited onto the inner surface of the supporting glass
substrates. This alignment technique, however, generate mechanical
defects and electrostatic charges which may damage the TFT
elements in LCDs as well as the optical appearance of the device,
deteriorating the performance of LCDs.
Generally, there are two groups of materials for photoalignment
layers. One group consist of materials, which promote planar
alignment (PA) of the liquid crystal molecules due to directional
photoꢀdegradation of the alignment layer exposed to illumination
with linearly polarized UV light. The other group of photoalignment
materials exhibit a photoalignment effect due to photochemical
Another alignment technique, attracted recently a lot of interest from
the LCD industry, is the photoalignment (PhA). This alignment
method is a nonꢀcontact method and therefore it does not generate
any electrostatic charges or mechanical defects which is a big
advantage compared to the rubbing alignment method.
reaction,
such
as
photoisomerization, photodimerization,
photodirected crosslinking, etc., which take place in these materials
under illumination with linearly polarized UV light [7].
PhA method is based on new materials and techniques. Although in
a very limited scale, this alignment method is already employed in
the LCD industry [1–6]. Many materials capable to align liquid
crystals (LC) have been discovered lately and used for preparation of
alignment layers in LCDs [7, 8]. However, the lack of proper
materials for photoalignment layers in LCDs is still a serious
obstacle for application of this technique in the LCD industry in a
broader scale.
Photoalignment effect may give preferred orientation of the liquid
crystal molecules parallel or perpendicular to the polarization plane
of UV light [9, 10] depending [9, 10] on the molecular structure of
the material. Molecular structure is also of vital importance for the
magnitude of anchoring energy of the promoted PhA, which has
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very strong impact on the LCD performance. Therefore, the method and were uncorrected. Elemental analysis was performed on
Thermo Fin ningan FLASHEA 1112CHN analyser. IR spectra (KBr
pellets) were recorded on a Shimadzu FTꢀIR 157 spectrophotometer.
1HꢀNMR spectra were recorded on a BRUKER AMX 400MHz
spectrometer using TMS as internal standard. Mass spectra were
recorded on LCMSꢀAgilent 1200 series with MSD using 0.1%
aqueous TFA in acetonitrile system on C18ꢀBDS column for 10 min
duration. Ionization mode is E1 for all the compounds. The purity of
the compounds was checked by TLC silica coated plates obtained
from Merck.
knowledge of the relations between the molecular structure of the
materials for photoalignment layers and LCD characteristics is of
vital importance for achievement of the desired performance of
LCDs.
Thiophene derivatives have been investigated as liquid crystalline
materials [11] but very little work has been reported on their
application as photoalignment materials in LCD. The inherent
properties and structure of thiophene moiety, synthesis and liquid
crystalline properties of thiophene derivatives have been extensively
investigated and the results are reported in [12, 13]. The most
interesting feature is that the introduction of thiophene, a five
membered heterocycle, develops a kink or bend in the molecule
structure as an electrophilic substitution can be achieved at αꢀ
positions (Cꢀ2 and Cꢀ5) in preference to the βꢀpositions (Cꢀ3 and Cꢀ
4) there by producing nonlinear geometry in the molecule. An
exocyclic bond angle of 147.5o can be obtained by 2,5ꢀdisubstitution
giving rise to a kink or bend in the molecular structure. Hence,
incorporation of suitably substituted thiophene tends to lower both
the melting point and the clearing point in comparison with the
classical design of liquid crystals, based on 1,4ꢀphenylene rings
(exocyclic bond angle, 180o) in the molecular core [12] and the
presence of a highly polarisable electronegative sulfur atom imparts
a transverse dipole moment, avoiding the need for additional lateral
substituents. Seed [12] has mentioned liquid crystalline character of
some 2,5ꢀdisubstituted thiophenes in his review of thermotropic
liquid crystals derived from thiophene and related fiveꢀmembered
heterocycles. On the other hand, certain 2,5ꢀthienylene derivatives
are structurally planar and electrically conductive making them
useful in electronic applications. Liquid crystalline properties of
esters derived from chiral 2ꢀoctanol and comprising 2,5ꢀdisubstituted
thiophene have been reported [13, 14]. In certain cases, 2,5ꢀ
disubstituted thiopheneꢀbased chiral esters exhibited SmC*
ferroelectric, ferrielectric and antiferroelectric phase sequence,
suggesting application in electroꢀoptical devices [13, 14].
Four different thiophene derivatives, assigned as T1-T4, with
structure depicted in the supplementary file were studied here. Each
of thiophene derivatives, were dissolved separately in THF solvent
with concentration of 1 wt%. With these solutions, photoalignment
layers were prepared according to the following procedures. The
solution was spin coated onto the surface of previously cleaned glass
substrates for 10sec at 800rpm followed by 20sec at 2000 rpm. After
that the substrates were heated for 15min at 100oC, which resulted in
thin films of the thiophene derivatives on the glass substrates’
surface with thickness of about 50nm. The substrates covered with
photoresponsive thiophene films were exposed to linearly polarized
UV light using standard USHIO light exposure system equipped
with deep medium pressure mercury lamp giving 3mW/cm2 linearly
polarized UV light passing through polarizer and light filter cutting
the light below 290nm. After UV light illumination, conventional
liquid crystal sandwich cells were prepared with these glass
substrates with a cell gap of 3ꢀ4µm. Nematic liquid crystal MLC
6873 – 100 (ꢁε>0) was filled in the isotropic phase in the cell gap
via capillary forces. The quality of PhA in the cells, promoted of
each of the thiophene derivatives, was determined by polarizing
optical microscope with crossed polarisers. The direction of the
preferred liquid crystal alignment, promoted by the thiophene PhA
layer, was defined by means of
λ firstꢀorder red plate [16].
2.1. Molecular modelling studies
Recently, we reported study on fluorine based derivatives of 1,3,4ꢀ
oxadiazole where changes in the molecular structure of this
compound, such as incorporation of mono fluoro and difluroro
substituents, bring the molecules to parallel and perpendicular
orientation, respectively, with respect to the polarization plane of the
UV illumination [15]. We applied this very simple idea of changing
the alignment of the liquid crystal promoted by alignment layer
made from thiophene derivatives by means of insertion of functional
groups in their molecular structure. In light of this herein we
explored the PhA properties of differently substituted thiophene
composed bis hydrazones. Depending on their structure these
compounds promoted either parallel or perpendicular alignment of
the liquid crystal with respect to the polarization plane of UV
illumination. The molecular modeling study has been conducted to
explain their photoalignment behavior.
The theoretical calculations for thiophene derivatives T1-T4 have
been carried out using the Gaussian 03 quantum chemical package
[17]. The computations have been performed using the density
functional theory (DFT) with the popular hybrid B3LYP functional
at the basis set level of 6–31G. The gas phase ground state molecular
geometries were fully optimized without imposing any molecular
symmetry constraints. Next the general molecular dipole moments
and the intramolecular distribution of transverse and longitudinal
dipole moments localized on the side groups were computed and
analyzed.
2.2. Synthesis
Synthesis of 3,4-di(substituted)thiophene-2,5-carbonyldihydra-
zide
In this work we report the results of our study on the impact of the
molecular structure of several thiophene composed bis hydrazones
on the alignment properties of layers made from these materials.
Exactly 0.003 mole of diethyl 3,4ꢀpropyloxythiopheneꢀ2,5ꢀ
dicarboxylate was added to a solution of 0.03mol of hydrazine
hydrate in 30mL of absolute ethanol. The reaction mixture was
refluxed for 2 h. Upon cooling the reaction mixture, white
precipitate was obtained. The product was then filtered, washed
with alcohol and dried to get required compounds with good yield.
The compound was crystallized using ethanol/chloroform.
2. Experimental details
2.1. Materials and Instrumentation
1H NMR (400 MHz, CDCl3), d (ppm): 1.03 (t, 6H, ꢀCH3, J = 7.4
Hz), 1.76–1.85 (m, 4H, >CH2), 4.11 (t, 4H, ꢀOCH2ꢀ, J = 6.8 Hz),
All chemicals were purchased from Aldrich and used without further
purification. Melting points were determined by open capillary
2 | J. Name., 2012, 00, 1ꢀ3
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4.98 (s, 4H, ꢀNH2), 8.34 (s, 2H, >NH). FTIR (KBr, cmꢀ1): 3338,
3290(>NꢀH), 3195, 2966, 2931, 2875, 1656 (>C=O), 1503, 1375,
1306, 1062, 949, 637. Elemental analysis data’s for C12H20N4O4S: C,
45.56; H, 6.37; N, 7.71; S, 10.14. Found: C, 45.46; H, 6.41; N, 7.69;
S, 10.05.
3. Results and Discussion
The synthesis route of the thiophene derivatives (T1-T4) is presented
in the Scheme 1. The formation of thiophene derivatives (T1-T4) was
confirmed by their IR, NMR and mass spectral studies. IR spectra
of thiophene derivatives (T1-T4) exhibited the absorption band due
to NꢀH in the region 3308ꢀ3222 cmꢀ1, nꢀpropyl band at 2969ꢀ2962
cmꢀ1, C=O bond at 1678ꢀ1641cmꢀ1 along with all other bands. The
1HꢀNMR results supports the IR observations. The representative
1HꢀNMR spectrum of T1 is discussed here. The spectrum showed
triplet in the region of δ, 0.9ꢀ1.06 (J = 7 ꢀ 7.2). A multiplet appeared
at δ 1.7ꢀ1.9 ppm is due to the –CH2ꢀ group. A triplet appeared in the
region of δ 4.2 – 4.3 ppm due to protons of –OCH2ꢀ with the
coupling constant J = 6.7 – 6.9. A triplet appeared at the region of δ
7.0 – 7.5 due to protons of C4. A singlet appeared at the region of 8.6
due to protons of –N=CHꢀ. A characteristic singlet appeared at δ
11.2 is assigned to proton of –CONHꢀ. In the compounds (T1-T4),
the peak due to azomethine was not observed and a singlet around δ
2.3ppm due to protons of –CH3 group was observed along with all
other functional protons. The mass spectrum of representative
compounds T1 displayed the molecular ion peak m/z (M+) at 504.
The elemental analysis confirms the purity of the compounds.
Synthesis of chemical structures of the compounds T1, T2, T3 and T4
is given in Scheme 1. Individual chemical structure of the
compounds T1, T2, T3 and T4 is given in supplementary section
(Fig.S1).
General procedure for synthesis of bis-hydrazides (T1-T4)
A clear solution of 0.005 mole of 3, 4ꢀdisubstituted thiophene carboꢀ
hydrazide in 15 mL of absolute ethanol was mixed with 0.5 mL of
conc. hydrochloric acid. To this 0.01 mole of appropriate carbonyl
compound, dissolved in 10 mL of absolute ethanol was added slowly
while stirring. The reaction mixture was heated to reflux for 4 h and
cooled to 10oC. The precipitated product was separated by filtration
and reꢀcrystallized from an appropriate solvent. The physical and
characterization data of all newly synthesized compounds are
presented in Table 4.1. Spectral data of some of the compounds are
given below along with their reꢀcrystallization solvent.
3,4-Dipropoxy-N2,N5-bis(thiophen-2-ylmethylene)thiophene-2,5-
dicarbohydrazide (T1)
o
M.P: 259ꢀ260 C. FTIR (KBr, cmꢀ1) υ: 3222 (ꢀNHꢀ), 2966 (propyl),
1
1641 (>C=O) and 1592 (>C=Nꢀ); H NMR (DMSOꢀd6, 400 MHz) δ
in ppm: 0.9 (t, 6H, ꢀCH3, J=6.9Hz ), 1.7 (m, 4H, ꢀCH2ꢀ,), 4.2 (t, 4H, ꢀ
OCH2ꢀ J=6.7 Hz), 7.1 (t, 2H, C4ꢀthiophene), 7.4 (d, 2H, C5ꢀ
thiophene) and 7.6 (d, 2H, C3ꢀthiophene, J=4Hz), 8.6 (s, 2H, ꢀ
N=CHꢀ), 11.2 (s, 2H, ꢀCONHꢀ); MS (m/z, %): 504 (M+, 100), 378
(50), 352 (20), 294 (40), 227 (20), 169 (30), 154 (40). Elemental
analysis; C22H24N4O4S3: Estimated C, 52.36%; H, 4.79%; N,
11.10%; S, 19.06%. Found: C, 52.30 %; H, 4.68 %; N, 11.32%; S,
19.11%.
EtOOC
S
COOEt
HO
EtOOC
OH
COOEt
1) NaOEt, EtOH
2) HCl/H2O
+
O
O
S
3,4-dipropoxy-N2,N5-bis(1-(thiophen-2-yl)ethylidene)thiophene-
2,5-dicarbo hydrazide (T2)
1
EtO
OEt
K2CO3, DMF
C3H7Br
o
M.P:253ꢀ254 C. FTIR (KBr, cmꢀ1) υ: 3304 (ꢀNHꢀ), 2962 (propyl),
1
1678 (>C=O) and 1626 (>C=Nꢀ); H NMR (CDCl3, 300 MHz) δ in
ppm: 1.0 (t, 6H, ꢀCH3 propyl, J=7.2Hz ), 1.9 (m, 4H, ꢀCH2ꢀ, J=7.2Hz
), 2.3 (s, 6H, ꢀCH3), 4.3 (t, 4H, ꢀOCH2ꢀ, J=6.9 Hz), 7.0 (t, 2H, C4ꢀ
thiophene), 7.4 (m, 4H, C3 and C5ꢀthiophene). Elemental analysis;
C24H28N4O4S3, Calculated C, 54.11%; H, 5.30%; N, 10.52%; S,
18.06%. Found: C, 54.01%; H, 5.22%; N, 10.62%; S, 18.09%.
3,4-Dipropyloxy-N2,N5-bis[1-(3-thienyl)ethylidene]thiophene-2,5-
dicarbo hydrazide (T3)
H7C3O
EtOOC
OC3H7
COOEt
H7C3O
OC3H7
O
NH2NH2, H2O
EtOH
NH
O
H2N
S
2
S
3
NH
NH2
Conc HCl
o
M.P: 256ꢀ257 C. FTIR (KBr, cmꢀ1) υ: 3308 (ꢀNHꢀ), 2969 (propyl),
1675 (>C=O), 1535 (>C=Nꢀ); 1H NMR (CDCl3, 300 MHz) δ in
ppm: 1.06 (t, 6H, ꢀCH3 of propyl, J=7.2 Hz ), 1.88 (m, 4H, ꢀCH2ꢀ),
2.33 (s, 6H, ꢀCH3), 4.28 (t, 4H, ꢀOCH2ꢀ), 7.31 (d, 2H, C4ꢀof
thiophene, J=16.2 ), 7.62 (s, 2H, C2ꢀ of thiophene), 7.71 (d, 2H, C5ꢀ
of thiophene, J=4.5 ), 10.13 (s, 2H, ꢀNHꢀ). Elemental analysis data’s
for C24H28N4O4S3: C, 54.11%; H, 5.30%; N, 10.52%; S, 18.06%.
Found: C, 54.02%; H, 5.22%; N, 10.59%; S, 18.15%.
where R1 is H, CH3
H7C3O
NH
OC3H7
O
R2
N
S
S
S
Br
R1
R2 is
S
O
NH
N
4a - d
R1
R2
T1ꢀT4
3,4-Dipropyloxy-N2,N5-bis[1-(2-bromo-2-
thienyl)ethylidene]thiophene-2,5-dicarbohydrazide (T4)
Scheme 1. Synthesis of T1-T4, thiophene based derivatives.
3.1. Photoalignment studies
o
M.P: 214ꢀ215 C. FTIR (KBr, cmꢀ1) υ: 3307 (ꢀNHꢀ), 2967 (propyl),
1677 (>C=O), 1548 (>C=Nꢀ). 1H NMR (CDCl3, 300 MHz) δ in
ppm: 1.0 (t, 6H, ꢀCH3 propyl, J=7.2 Hz), 1.9 (m, 4H, ꢀCH2ꢀ,
J=7.2Hz), 2.3 (s, 6H, ꢀCH3), 4.3 (t, 4H, ꢀOCH2ꢀ, J=6.9 Hz), 7.0ꢀ7.5
(m,4H, C3 and C4ꢀthiophene). MS (m/z, %): 691 (M+1, 100), 473
(30), 387 (20). Elemental analysis; C24H26Br2N4O4S3,calcd:C,
41.75%; H, 3.80%; N, 8.11%; S; 13.93%. Found: C, 41.84%; H,
3.87%; N, 8.18%; S; 13.85%.
The UV/Vis absorption spectra of T1-T4 exhibited a peak at 366,
366, 363 and 363 nm, respectively, as shown in Figure 1. Due to the
similarity in the chemical structure one can observe similar
wavelength for all the reported compounds here.
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Materials
Duration of
Photoꢀ
Molecular
used in this UV exposure alignment
orientation
study
T1
(min)
15
results
Good
Parallel
T2
T3
T4
15
15
15
Good
Very good
Weak
Perpendicular
Perpendicular
Perpendicular
Table 1 Photoalignment promoted by alignment layers made from
T1-T4 thiophene based derivatives showing planar alignment with
preferred direction parallel to the UV light polarisation plane, for T1
(without methyl substitution), and perpendicular to it, for T2-T4
(with methyl substitution).
The evaluated quality of the PhA in the cells together with preferred
direction of PhA, with respect to the polarization plane of the UV
light illumination, is summarized in Table 1. By means of λ first
order red plate was found that the alignment layer made by the
compound T1 promoted planar alignment with preferred direction
parallel to the UV light polarization plane whereas the other PhA
layers made from the compounds T2, T3 and T4, respectively,
promoted planar alignment but with preferred direction
perpendicular to the UV light polarization plane. Except for the
compound T1 all other compounds possess methyl group at the
carbon of the HC=Nꢀ functional group. Hence, the presence of
methyl group seems to be of vital importance for the orientation of
liquid crystal alignment direction with respect to UV light
polarization plane. Detail discussion about this behavior in which
slight modification in the molecular structure results in such drastic
change in the PhA properties of the thiophene compounds is given in
modelling section of this paper.
Figure 1. Absorption spectra of the compounds (a) T1, (b) T2, (c) T3
and (d) T4 showing absorption peak around ~363ꢀ366 nm.
According to the anisotropic response of the bisꢀhydrazide
molecules under the excitation of resonant (with bisꢀhydrazide
molecules) light, photoꢀstationary orientational states can be
expected using proper excitation geometry.
In the present work, the liquid crystal alignment promoted by the
alignment layers made from the thiophene derivatives after
exposure of the layers with linear polarized UV light for about 15
minutes were investigated. The quality of the obtained liquid crystal
alignment in the experimental cells containing PhA layer made of
compounds T1,T2 T3, and T4 respectively, was studied by means of
polarizing microscope. The dark cell image, as shown in Figure 2,
corresponds to the cell position between crossed polarizes with the
its optic axis oriented along the transmission direction of one of the
crossed polarizer whereas the bright image has been taken after
rotating the cell at 450.
During the exposure of thiophene derivatives with linear polarized
UV light, their molecules adopt orientation perpendicular to the UV
light polarization plane with the molecular axes lying in the plane of
the liquid crystal layer (planar alignment). The interactions of
thiophene molecules with the liquid crystal molecules are complex
due to Tꢀshape of the thiophene molecules. These interactions could
generally be characterized by the interplay between two main
factorsꢀsteric and dipoleꢀdipole interactions. Via steric interactions,
the molecules of the thiophene compounds tend to orient liquid
crystal molecules, which are in the close contact with the PhA layer,
along the thiophene molecule long axis. Steric interactions are short
range interactions but their influence on the alignment of the liquid
crystal molecules penetrate in the bulk of the liquid crystal layer via
elastic forces. In present scenario, however, should be pointed out
that there are two alkyl chains in the central part of the thiophene
molecules, oriented orthogonally to the molecular axis, which
violate the orientation promoted by the steric interactions. Thus in
this case, the other long range interactions go to the forefront. These
are the dipoleꢀdipole interactions. As it was shown in the works [18]
the dipoleꢀdipole interactions are to be addressed not as the
interactions between the whole molecules, but as between the
localized polar groups of the molecules, which have the zero net
charge of the group and the size comparable with the intermolecular
distances. The side groups are mobile and thus are capable to orient
in one direction, supplying the longꢀrange orientational order with
characteristic distance many times greater than the size of the
molecule which results in either parallel or perpendicular orientation
of the liquid crystal molecules with respect to the molecules of the
alignment layer. It provides orientation of the liquid crystal
molecules near to the surface of the PhA thiophene layer. Herewith
the orientational effect of the dipoleꢀdipole interactions is
P
A
Figure 2. Photographs of experimental cell, containing PhA layer
made from compound T1, inserted between crossed polarisers. The
cell gap is about 5 ꢂm and it is filled with the nematic mixture MLC
6873ꢀ100 (ꢁ ε>0). The preferred alignment direction of the liquid
crystal in the cell is oriented at angle 45° with respect to the
transmission direction of one of the crossed polarisers (bright state)
and 0° with respect to it (dark state). P and A in the diagram
represents polarizer and analyser directions.
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determined by the value of the longitudinal component of the dipole The large value of the total molecular dipole moment can be
moment of the side groups of the thiophene molecules. Further we compensated within close molecular neighboring by formation of the
have analysed the molecular structures and their impact on the liquid two molecule pair with antiparallel orientation of the net dipole
crystal alignment from this point of view using molecular modelling moments of the molecules in the pair. The presence of the alkyl
studies.
chains in the central part of the thiophene molecule, however,
hinders the interaction of this part both with the other thiophene
molecules, as well as with the liquid crystal molecules. In that case
the dipole moments localized on the side groups of the thiophenes
[See Figure 4] are important for the long range interaction with
liquid crystals.
3.2 Molecular modelling studies
According to modelling results the optimized structures of T1, T2, T3
and T4 (Fig 3) possess strong dipole moment of 10.5D, 11.2D,
12.4D and 10.8D correspondingly, which are orthogonal to the long
molecular axis.
We conclude that the transverse dipole moment of the side groups is
mutually compensated for the T2, having too small long range
impact on the PhA ability of the alignment layer made from T2.
LC molecule
b)
4bT2
a)
T1
c)
T3
?
4c
d)
T4
Figure 3. Molecular structures with localized dipole moments of the compounds: a) T1, b) T2, c) T3 and d) T4. The direction of the UV
light polarisation and the promoted liquid crystal alignment by the PhA layers made from these compounds are also depicted. T4
represents very week anchoring energy and difficult to measure and alignment quality also very bad.
a)
b)
c)
d)
Figure 4. Side groups with dipole moments localized within the a) T1, b) T2, T3 and T4, c) T1, T2 and T3, d) T4 compounds.
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ARTICLE
DOI: 10.1R03S9C/C5ARAd1v2a77n2cFes
Therefore, the steric interactions take over and promote alignment of in the thiophene compounds under study caused turning the
the liquid crystal molecules along the long molecular axis of the preferred direction of alignment the nematic liquid crystal promoted
thiophene molecules and thus being oriented perpendicular with by the thiophene PhA layer from parallel with respect to the
respect to polarisation direction of UV light illumination. However, polarisation of UV light, for the thiophene compound without
the mutual compensation of the side groups localized transverse methyl group, to perpendicular to it, for the thiophenes compound
dipole moments for the T1 is lower, enhancing thus the ability of the with methyl group. Advancing the knowledge about the relations
alignment layer made from this compound to promote liquid crystal between molecular structure of the compounds, the liquid crystal
alignment due to the long range dipoleꢀdipole interactions. Thus the alignment layers are made of, will enable to design new PhA
PhA layer made from T1 is capable to orient nematic liquid crystals materials and to tailor their alignment characteristics.
with positive dielectric anisotropy along direction perpendicular to
Acknowledgments
Authors GH and LK would like to acknowledge BMSCE and also
BMS R and D Centre for their support and encouragement.
the long molecular axis of the thiophene molecule. This alignment
direction, however, coincides with the polarisation direction of the
UV light.
Compare the side groups shown on Figure 4a and 4b, the presence
of the CH3 radical donate the electron density thus increasing the
dipole moment of the group 2.16D and 2.66D, correspondingly. It
explains the difference of the localized transverse dipole moments of
these side groups and its’ ability to compensate the transverse dipole
moment localized in the terminal side group (Fig. 4c) of the
Notes and references
aBMS R and D Centre, BMS College of Engineering, Basavanagudi,
Bangalore, 560078, India.
bInstitute of Chemistry of New Materials, NAS of Belarus, Minsk,
Belarus.
compound T1 (Fig.3a) and T2 (Fig.3b).
Consider the longitudinal orientation of the side group in Fig 4c
within the T3 compound (Fig.3c). In contrast to the compound in
Fig.3a the presence of the longitudinal dipole moment localized on
the side group explains the ability to orient the nematic liquid crystal
with positive dielectric anisotropy along the molecular axis of the
thiophene due to the long range dipoleꢀdipole interactions. The latter
cDept of Chemistry, National Institute of Technology, Suratkal,
Karnataka, India.
dLiquid Crystal Physics Group, University of Gothenburg,
Gothenburg, Sweden
results in that the PhA layer made from
crystal alignment being perpendicular to the UV light polarisation
direction. The side group in Fig.4d of the 4 compound (Fig.3d) has
T1 compound promote liquid
*Author for correspondence: murthyhegde@gmail.com
T
negatively charged Br atom in the outside of the molecule. The
terminal radicals of such molecular structures are likely to repel,
which prevent from formation of anisotropic rotational distribution
of thiophene molecules in the PhA layer. Similar behavior was
observed for SD2 dye with CF3 terminal radicals [19]. Thus the long
range averaged dipoleꢀdipole interactions of the T4 compound layer
are not capable to form any photoalignment preferred direction of
the liquid crystal. Thus T4 compound does not possess such a liquid
crystal photoalignment ability as the other thiophene derivatives.
† Supplementary Information (SI) available: See DOI: 10.1039/b000000x/
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Our study demonstrated that small changes in the molecular
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6 | J. Name., 2012, 00, 1ꢀ3
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