Azobenzene-based supramolecular liquid crystals: The role of core fluorination

Article abstract of DOI:10.1016/j.molliq.2020.113252

The impact of core fluorination on the phase behaviour of supramolecular hydrogen-bonded liquid crystals (HBLCs) is investigated in detail. Therefore, different types of HBLCs were synthesized using two benzoic acid derivatives as proton donors, namely, 4-octyloxybenzoic acid and octylbenzoic acid. The two acids were combined through intermolecular hydrogen-bonding with alkoxyazopyridine derivatives as proton acceptors. Three different types of azopyridines were used either without fluorine substitution or with one lateral fluorine substituent at different positions. The study proved the importance of using core fluorination as a significant tool to modify the liquid crystalline behaviour of HBLCs, where all azopyridines are non-mesomorphic and almost all their complexes exhibit enantiotropic mesophases. The formation of the hydrogen bond between the complementary components was confirmed using FTIR and ¹H NMR spectroscopy, while the liquid crystalline self-assembly of the HBLCs was investigated in detail using polarized light microscope (PLM) and differential scanning calorimetry (DSC). Depending on the type of the terminal chain on the benzoic acid derivative and on the position of the lateral fluorine substituent different types of mesophases including nematic (N), smectic A (SmA) and smectic C (SmC) phases were observed. Finally, under UV light illumination all the prepared HBLCs show cis-trans photoisomerization resulting in tuning the liquid crystalline phases, which is of importance for industrial applications.

Full text of DOI:10.1016/j.molliq.2020.113252

Journal of Molecular Liquids 310 (2020) 113252
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Azobenzene-based supramolecular liquid crystals: The role of
core fluorination
b
a
b
Mohamed Alaasar a,b, , Jaques-Christopher Schmidt , Ahmed F. Darweesh , Carsten Tschierske
⁎
a
Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt
b
Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt Mothes Str. 2, D-06120 Halle (Saale), Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The impact of core fluorination on the phase behaviour of supramolecular hydrogen-bonded liquid crystals
(HBLCs) is investigated in detail. Therefore, different types of HBLCs were synthesized using two benzoic acid de-
rivatives as proton donors, namely, 4-octyloxybenzoic acid and octylbenzoic acid. The two acids were combined
through intermolecular hydrogen-bonding with alkoxyazopyridine derivatives as proton acceptors. Three differ-
ent types of azopyridines were used either without fluorine substitution or with one lateral fluorine substituent
at different positions. The study proved the importance of using core fluorination as a significant tool to modify
the liquid crystalline behaviour of HBLCs, where all azopyridines are non-mesomorphic and almost all their com-
plexes exhibit enantiotropic mesophases. The formation of the hydrogen bond between the complementary
components was confirmed using FTIR and 1H NMR spectroscopy, while the liquid crystalline self-assembly of
the HBLCs was investigated in detail using polarized light microscope (PLM) and differential scanning calorime-
try (DSC). Depending on the type of the terminal chain on the benzoic acid derivative and on the position of the
lateral fluorine substituent different types of mesophases including nematic (N), smectic A (SmA) and smectic C
(SmC) phases were observed. Finally, under UV light illumination all the prepared HBLCs show cis-trans
photoisomerization resulting in tuning the liquid crystalline phases, which is of importance for industrial
applications.
Received 24 March 2020
Received in revised form 23 April 2020
Accepted 28 April 2020
Available online 03 May 2020
Keywords:
Supramolecular liquid crystals
Azobenzene
Hydrogen-bonding
Nematic
Fluorination
Photosensitivity
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
like HBLCs [12,13], bent-shaped LCs [14–18] polymeric framework
[19], modular hierarchical [20], non-symmetric dimers having conven-
Liquid crystalline (LC) materials are of special interest for the display
technology due to their molecular self-assembly which can be tuned
under the effect of external stimuli, temperature or light. Since the
first discovery of LC, different classes of molecular structures have
been designed to produce several LCs with fascinating properties. A
huge number of LCs have been reported using different synthetic
methods, however the design of supramolecular liquid crystals by inter-
molecular interaction between complementary components is of great
importance. The intermolecular interactions could be either halogen-
bonding [1–6], or hydrogen-bonding [7–9], and both have the advan-
tage of the ease accessibility compared to covalently bonded LCs. Kato
et al. reported the first examples of hydrogen bonded liquid crystals
(HBLCs) designed by hydrogen-bond formation between pyridine-
derivatives and benzoic acid derivatives [10,11] later several HBLCs
were designed using different types of proton donors and proton accep-
tors to produce wide varieties of molecular architectures such as rod-
tional nematic phases [21] or exhibiting the heliconical twist-bend ne-
matic phase (NTB) [22] and supramolecular polycatenars capable of
displaying chiral isotropic liquids beside chiral cubic phases [23]. One
class of the commonly used proton acceptors is azopyridine-
derivatives [12,13,16,20,21,23–26]. Azopyridines are interesting be-
cause they combine the possibility of self-assembly through intermolec-
ular hydrogen- or halogen-bond formation together with the unique
phenomenon of trans-cis photoisomerization under light irradiation
due to the presence of the azo linkage. Azo functionalized LCs are of spe-
cial interest for the manufacture of light responsive materials for the
technological applications such as molecular scissors [27] and photo-
oscillators [28,29]. In recent years different types of azobenzene-based
LCs with fascinating properties were reported [30–32]. One way to
modify the liquid crystalline behaviour is aromatic core fluorination
[33–40]. This is caused by the unique combination of high polarity and
low polarizability of the fluorine atom, as well as steric and conforma-
tional effects. Due to its small size fluorine atom can be accommodated
by the molecular core affecting the strength of the electrostatic and π-
stacking core-core interactions. In 2003 X. Song et al reported HBLCs be-
tween 4-octyloxybenzoic acid and 4-(alkoxyphenylazo) pyridines with
⁎
Corresponding author at: Department of Chemistry, Faculty of Science, Cairo
University, Giza, Egypt.
E-mail address: malaasar@sci.cu.edu.eg (M. Alaasar).
https://doi.org/10.1016/j.molliq.2020.113252
0167-7322/© 2020 Elsevier B.V. All rights reserved.

2
M. Alaasar et al. / Journal of Molecular Liquids 310 (2020) 113252
Table 1
Phase transition temperatures (T/°C), mesophase types, and transition enthalpies [ΔH (J/g)] of the supramolecular HBLCs An, A3Fn and A2Fna.
Complex
n
X
Y
Phase sequence (T/°C [ΔH (J/g)])
A10 [41]
10
H
H
H: Cr 88 [62.2] SmC 128 [22.0] Iso
C: Iso 126 [−19.5] SmC 89 [−70.8] Cr
A12 [41]
A14 [41]
A3F8
12
14
8
H
H
F
H
H
H
H
H
H
F
H: Cr 92 [40.2] SmC 128 [22.0] Iso
C: Iso 126 [−19.5] SmC 82 [−43.3] Cr
H: Cr 90 [82.3] SmC 127 [25.5] Iso
C: Iso 126 [−23.5] SmC 82 [−39.3] Cr
H: Cr 93 [75.2] N 113 [5.9] Iso
C: Iso 111 [−5.3] N 92 [−1.8] SmC 84 [−73.4] Cr
H: Cr 84 [46.8] SmC 98 [11.2]b N 115 [11.2]b Iso
C: Iso 110 [−10.0]b N 95 [−10.0]b SmC 64 [−53.0] Cr
H: Cr 87 [59.7] SmC 111 [16.2]b N 116 [16.2]b Iso
C: Iso 114 [−14.8]b N 108[−14.8]b SmC 73 [60.6] Cr
H: Cr 88 [73.6] SmC 115 [18.9] Iso
A3F10
A3F12
A3F14
A2F8
10
12
14
8
F
F
F
C: Iso 111 [−17.6] SmC 73 [−73.5] Cr
H
H
H
H
H: Cr 96 [51.2] SmC 107 [3.6] N 116 [0.3] Iso
C: Iso 110 [−0.2] N 105 [−2.1] SmC 88 [−50.6] Cr
H: Cr 86 [80.5] SmC 102 [2.0] N 113 [5.3] Iso
C: Iso 111 [−5.5] N 100 [−1.8] SmC 79 [−47.6] Cr
H: Cr 93 [85.6] SmC 108 [16.4]b N 114 [16.4]b Iso
C: Iso 110 [−14.4]b N 106 [−14.4]b SmC 79 [−49.8] Cr
H: Cr 85 [66.7] SmC 110 [18.2] Iso
A2F10
A2F12
A2F14
10
12
14
F
F
F
C: Iso 108 [−15.4] SmC 75 [−48.5] Cr
a
Peak temperatures as determined from 2nd heating (H) and 2nd cooling (C) DSC scans with rate 10 K min−1; abbreviations: Cr = crystalline solid; SmC = smectic C phase;
SmA = smectic A phase; N = nematic phase; Cr = crystalline soild; Iso = isotropic liquid.
b
The enthalpy value of Iso-N phase transition could not be separated from that of the N-SmC phase transition (see Fig. S5 in the SI).
variable chain lengths (the supramolecules A10–A14 in Table 1) [41]
and it was found that all of An complexes exhibit smectic C (SmC)
phases regardless the alkoxy chain lengths in the proton acceptor. It
was interesting for us to check how lateral fluorine substitution could
affect the liquid crystalline behaviour of An complexes. To answer this
question, we prepared different types of azopyridine derivatives with-
out any fluorine substitution (Hn) [23] or with one fluorine substituent
either at ortho position with respect to the terminal alkoxy chain (3Fn)
[42] or at meta position with respect to the terminal alkoxy chain (2Fn)
(see Scheme 1). All of the synthesized azopyridine derivatives (Hn, 3Fn
and 2Fn) were found to be non-mesomorphic and were used as proton
acceptors to prepare new fluorinated HBLCs by intermolecular H-bond
formation with the mesomorphic 4-octyloxybenzoic acid (A3Fn and
A2Fn) to compare it with the previously reported An complexes. More-
over, the effect of removing the oxygen atom connecting the terminal
chain on the benzoic acid on the LC behaviour of the synthesized
HBLCs was investigated systemically by preparing other types of H-
bonded aggregates (Bn, B3Fn and B2Fn) using the nematogenic 4-
octylbenzoic acid as the proton donor instead of 4-octyloxybenzoic acid.
2. Experimental
2.1. Synthesis
The synthesis of the HBLCs is shown in Scheme 1. The benzoic acid
derivatives are commercially available, while the azopyridine
Table 2
Phase transition temperatures (T/°C), mesophase types, and transition enthalpies [ΔH (J/g)] of the supramolecular HBLCs Bn, B3Fn and B2Fna.
Complex
n
X
Y
Phase sequence (T/°C [ΔH (J/g)])
B8
8
H
H
H: Cr 100 [48.5] SmA 124 [23.7] Iso
C: Iso 121 [−23.4] SmA 88 [−47.4] Cr
H: Cr 89 [35.7] SmC 105 [−] SmA 123 [24.8] Iso
C: Iso 119 [−23.6] SmA 104 [−] SmC 77 [−34.9] Cr
H: Cr 90 [33.4] SmC 119 [−] SmA 124 [27.6] Iso
C: Iso 121 [−25.8] SmA 115 [−] SmC 81 [−16.6] Cr
H: Cr 88 [30.7] SmC 122 [−] SmA 124 [28.1] Iso
C: Iso 121 [−28.8] SmA 119 [−] SmC 89 [−8.4] SmX 79 [−13.3] Cr
H: Cr 88 [47] SmC 102 [11.7] Iso
C: Iso 99 [−11.7] SmC 71 [−43.6] Cr
H: Cr 87 [53.2] SmC 101 [9.5] Iso
C: Iso 96 [−10.9] SmC 72 [−55.3] Cr
H: Cr 83 [49.2] SmC 106 [16.3] Iso
C: Iso 102 [−14.9] SmC 68 [−52.2] Cr
H: Cr 89 [64.1] SmC 106 [19.4] Iso
C: Iso 103 [−18.0] SmC 74 [−63.8] Cr
H: Cr 94 [53.4] Iso
C: Iso 94 [−7.7] SmA 80 [−36.1] Cr
H: Cr 90 [42.0] SmA 106 [19.6] Iso
C: Iso 102 [−17.5] SmA 72 [−45.3] Cr
H: Cr 84 [45.7] SmC 93 [−] SmA 108 [22.8] Iso
C: Iso 105 [−21.6] SmA 92 SmC [−] 70 [−41.5] Cr
H: Cr 83 [43.0] 95 SmC [−] SmA 108 [19.1] Iso
C: Iso 104 [−20.6] SmA 94 SmC [−] 68 [−37.9] Cr
B10
10
12
14
8
H
H
H
F
H
H
H
H
H
H
H
F
B12
B14
B3F8
B3F10
B3F12
B3F14
B2F8
B2F10
B2F12
B2F14
10
12
14
8
F
F
F
H
H
H
H
10
12
14
F
F
F
a
Peak temperatures as determined from 2nd heating (H) and 2nd cooling (C) DSC scans with rate 10 K min−1; abbreviations: SmA = smectic A phase; SmX = unidentified smectic
phase. For other abbreviations see Table 1.

M. Alaasar et al. / Journal of Molecular Liquids 310 (2020) 113252
3
derivatives with or without fluorine substitution were synthesized as
described in the supporting information (SI) by a coupling reaction be-
tween the diazonium salt of 4-aminopyridine and phenol, 2-
fluorophenol or 3-fluorophenol followed by etherification of the
resulting azo dyes with different alkyl bromides to give the target pro-
6.5 Hz, 2H, O-CH2), 1.82 (m, 2H, O-CH2CH2), 1.51–1.42 (m, 2H, CH2),
1.41–1.21 (m, 16H, CH2), 1.05–0.73 (m, 3H, CH3). 13C NMR (126 MHz,
CDCl3) δ 164.46, 164.37, 163.37, 161.30, 157.47, 151.27, 134.74 (d,
J = 7.0 Hz), 118.66, 116.22, 111.61 (d, J = 2.7 Hz), 102.33 (d, J =
23.3 Hz), 68.98, 31.89, 29.63, 29.61, 29.55, 29.51, 29.32, 29.29, 28.95,
25.90, 22.66, 14.09. 19F NMR (470 MHz, CDCl3) δ −116.75 to −121.48
ton
acceptors.
The
analytical
data
for
and
4-(3-fluoro-4-
4-(2-fluoro-4-
dodecyloxyphenylazo)pyridine
3F12
(m). MS (ESI): m/z (%): positive: 408.2422 ([M
+ ,
Na]+
dodecyloxyphenylazo)pyridine 2F12 are given below as representative
examples. The supramolecular HBLCs (An, A3Fn, A2Fn, Bn, B3Fn and
B2Fn) were prepared by mixing equimolar amounts of each of the
azopyridine derivatives (Hn, 3Fn or 2Fn) and each of the benzoic acid
derivatives (OC8 or C8) and melting them together in a DSC pan with
stirring to give an intimate blend then cooling to room temperature.
The obtained crystals were then melted, and the process was repeated
two additional times. Homogenous melting and reproducible phase
transition temperatures were observed for all complexes (Table 1).
C
23H32FN3O + Na+, calc.: m/z = 408.24).
3. Characterization
Thin layer chromatography (TLC) was performed on aluminium
sheet precoated with silica gel. Analytical quality chemicals were ob-
tained from commercial sources and used as obtained. The solvents
were dried using the standard methods when required. The purity
and the chemical structures of all synthesized materials were confirmed
by the spectral data. The structure characterization of the prepared ma-
terials is based on 1H NMR, 13C NMR, 19F NMR (Varian Unity 400 spec-
trometers, in CDCl3 solution, with tetramethylsilane as internal
standard) and high resolution mass spectroscopy.
2.1.1. 4-(3-Fluoro-4-dodecyloxyphenylazo)pyridine, 3F12
Orange crystals. Melting point: 66 °C, 0.58 g, 68% yield. 1H NMR
(500 MHz, CDCl3) δ 8.86–8.68 (m, 2H, Ar\\H), 7.82 (ddd, J = 8.7, 2.4,
1.3 Hz, 1H, Ar\\H), 7.72 (dd, J = 11.9, 2.3 Hz, 1H, Ar\\H), 7.69–7.63
(m, 2H, Ar\\H), 7.09 (t, J = 8.6 Hz, 1H, Ar\\H), 4.14 (t, J = 6.6 Hz, 2H,
O-CH2), 2.07–1.70 (m, 2H, O-CH2-CH2), 1.55–1.15 (m, 18H, CH2), 0.88
(t, J = 6.9 Hz, 3H, CH3). 13C NMR (126 MHz, CDCl3) δ 157.07, 153.86,
151.88, 151.30, 151.24, 151.15, 146.23 (d, J = 5.6 Hz), 124.28 (d, J =
2.9 Hz), 116.15, 113.39 (d, J = 2.3 Hz), 107.89 (d, J = 19.5 Hz), 69.57,
31.89, 29.63, 29.61, 29.55, 29.51, 29.32, 29.30, 29.01, 25.86, 22.66,
14.09. 19F NMR (470 MHz, CDCl3) δ −132.38 (dd, J = 11.9, 8.4 Hz).
MS (ESI): m/z (%): positive: 408.2421 ([M + Na]+, C23H32FN3O + Na+,
calc.: m/z = 408.24).
Infrared absorption spectra were measured in dry KBr with a Perkin-
Elmer B25 spectrophotometer.
The mesophase behaviour and transition temperatures of the supra-
molecular complexes were measured using a Mettler FP-82 HT hot
stage and control unit in conjunction with a Nikon Optiphot-2 polariz-
ing microscope. The associated enthalpies were obtained from DSC-
thermograms which were recorded on a Perkin-Elmer DSC-7, heating
and cooling rate: 10 K min−1
.
4. Results and discussion
2.1.2. 4-(2-Fluoro-4-dodecyloxyphenylazo)pyridine, 2F12
4.1. FTIR and NMR measurements
Orange crystals. Melting point: 67 °C, 0.50 g, 56% yield. 1H NMR
(500 MHz, CDCl3) δ 8.91–8.61 (m, 2H, Ar\\H), 7.81 (m, 1H, Ar\\H),
7.71–7.62 (m, 2H, Ar\\H), 6.88–6.62 (m, 2H, Ar\\H), 4.04 (t, J =
As an effective tool to investigate the intermolecular interactions
[43,44,16], FTIR spectroscopy was performed for the supramolecule
Scheme 1. Synthesis of the azopyridines (Hn, 3Fn and 2Fn and 23F8) and the hydrogen-bonded supramolecules (An, A3Fn, A2Fn, Bn, B3Fn and B2Fn).

4
M. Alaasar et al. / Journal of Molecular Liquids 310 (2020) 113252
B2F14 as a representative example for the prepared HBLCs in crystalline
state (KBr) at room temperature to prove the formation of the
hydrogen-bond between the benzoic acid derivatives and the
azopyridines. Fig. 1a, b shows the IR spectra of B2F14 and its comple-
mentary components the azopyridine derivative 2F14 and the proton
donor C8 at two different regions (see Figs. S1–S3 in the SI for the com-
plete range of spectra).
enantiotropic LC phase with wider range compared to that observed for
A3F8 and on further chain elongation the range of the SmC phase be-
comes more wider on the expense of the nematic phase. For the
supramolecules with n = 10 and 12 the transition enthalpy values for
the SmC-N and N-Iso transitions on heating and cooling scans cannot
be separated from each other's (see Table 1 and Fig. S5 in the SI). For
the longest complex A3F14 the nematic phase is totally removed and
the SmC phase is the only observed mesophase. The range of the SmC
phase of A14 is wider than that of the fluorinated supramolecules
A3F14. This also applies for all An complexes compared to A3Fn irre-
spective of the terminal chain length on the azopyridine segment.
Changing the position of the lateral fluorine atom from ortho posi-
tion with respect to the terminal chain on the proton acceptors i.e. the
azopyridine derivatives to be at meta position leads to the A2Fn com-
plexes. The phase behaviour of A2Fn complexes is very similar to that
of A3Fn complexes (Fig. 5c and Fig. S6 in the SI), however the SmC
phase is more stabilized and is observed as enantiotropic phase starting
from the shortest homologue A2F8.
The absence of a band at around 3000 cm−1 together with the ap-
pearance of two bands at around 2479 cm−1 and 1888 cm−1 (black
curve in Fig. 1a) clearly indicate the formation of 1:1 hydrogen-
bonded complex between the azopyridine derivative 2F14 and the
benzoic acid C8 [26,31]. Moreover, the carbonyl stretching vibration
band of the pure benzoic acid C8 which is present in a dimeric form as
a result of the intermolecular hydrogen-bond formation between the
free carboxylic groups is observed at around 1680 cm−1 (blue curve in
Fig. 1b) and upon complexation with the proton acceptor 2F14 it is
shifted to higher value ~1693 cm−1 (i.e. blue shifted, black curve in
Fig. 1b). This indicates the formation of another intermolecular hydro-
gen bond between the monomeric form of C8 and the pyridine based
derivative 2F14 which also confirms the formation of the supramolecule
B2F14 [26,31,45].
4.3. Liquid crystalline behaviour of Bn, B3Fn and B2Fn
Whereas IR confirms complex formation in the solid state, 1H NMR
spectroscopy was used to prove the hydrogen-bond formation in solu-
tion. Fig. 2 shows the 1H NMR spectroscopy in the aromatic region of
the supramolecule A3F14 and its complementary components the pro-
ton donor OC8 and the proton acceptor 3F14 as representative exam-
ples (See Fig. S4 for the complete 1H NMR spectra).
In order to investigate the effect of removing the oxygen atom
connecting the terminal octyl chain to the benzoic acid on the
a)
100
The spectra of the proton acceptor 3F14 shows that the signal corre-
sponding to the hydrogen atoms at ortho positions to the nitrogen atom
in the pyridine ring appears at δ 8.86–8.68 ppm (red curve in Fig. 2) and
upon hydrogen bond formation it becomes broad and shifted to δ
8.89–8.74 ppm (black curve in Fig. 2). Also, the signal appearing at δ
7.68 ppm in the pure azopyridine-based component 3F14 which corre-
sponds to the protons at meta positions to the nitrogen atom in the pyr-
idine ring is slightly low field shifted and overlaps with the hydrogens at
the fluorinated ring after complexation and become more closer to each
other's (compare the red and black curves in Fig. 2).
90
2479 cm-1
1888 cm-1
80
70
2F14
C8
B2F14
60
50
4.2. Liquid crystalline behaviour of An, A3Fn and A2Fn
The phase transition temperatures and types of LC phases of the pre-
pared HBLCs are revealed based on DSC measurements and textural ob-
servations under PLM. The obtained data for An, A3Fn and A2Fn are
summarized in Table 1 and represented graphically on Fig. 3a–c, while
those of Bn, B3Fn and B2Fn are collected in Table 2 and plotted on
Fig. 3d–f. As representative examples the DSC traces of A3F14 and B14
are shown in Fig. 4.
The benzoic acid derivative used to prepare the first type of the
HBLCs i.e. 4-octyloxybenzoic acid forms smectic C (SmC) and nematic
(N) phases in the following sequence: Cr 101 °C SmC 107 °C N 147 °C
Iso, while all of the individual azopyridine derivatives are nonmesorphic
and melts directly to isotropic liquids (see Table S1 in the SI). As can be
seen from Table 1 the nonfluorinated HBLCs (An) exhibit only SmC
phases as enantiotropic mesophases and the nematic phase is not ob-
served for any of An derivatives [41].
The HBLCs A3Fn having a fluorine atom at ortho position next to the
terminal chain at the azopyridine derivatives show different phase be-
haviour (Fig. 3b). For the shortest complex A3F8 an enantiotropic LC
phase is observed over ~20 K, which show typical observation of the
conventional nematic phase (N) with a characteristic schlieren texture
containing two- and four-point brush singularities in untreated cells
(Fig. 5a). On cooling A3F8 from the isotropic liquid the nematic phase
is observed followed by a monotropic SmC phase (Fig. 5b). This transi-
tion is accompanied by an enthalpy transition value in the DSC cooling
curve of A3F8 indicating a first order transition (Table 1). On chain elon-
gation and for the next homologue (A3F10) the SmC phase becomes an
3400 3200 3000 2800 2600 2400 2200 2000 1800
Wavenumber / cm-1
b)
100
90
80
70
60
50
1693 cm-1
2F14
C8
1680 cm-1
B2F14
1750
1700
1650
Wavenumber / cm-1
Fig. 1. FTIR spectra of the supramolecule B2F14 (black) and its complementary
components C8 (blue) and 2F12 (red) in the crystalline state (KBr) at room
temperature: a) enlarged area between 1700 cm−1 and 3500 cm−1; b) enlarged area
between 1630 cm−1 and 1790 cm−1. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)

M. Alaasar et al. / Journal of Molecular Liquids 310 (2020) 113252
5
Fig. 2. 1H NMR spectra (500 MHz, CDCl3) in the aromatic region of the supramolecule A3F14 (black) and its complementary components OC8 (blue) and 3F14 (red). The blue numbers are
for the benzoic acid OC8 protons and the red numbers are for the azopyridine 3F14 protons before and after complexation. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
c) 120
120
a)
120
b)
N
SmC
Cr
110
100
90
110
100
90
100
80
80
80
70
70
60
60
A2F10
Complex
A2F8
A2F14
A2F12
A10
A12
Complex
A14
A3F10
A3F12
A3F14
A3F8
Complex
110
e) 110
d)
f)
120
100
80
100
90
100
90
80
80
SmA
70
70
60
SmC
SmX
Cr
60
60
B8
B10
B12
Complex
B14
B3F8
B3F14
B3F10
B3F12
B2F8
B2F10
Complex
B2F14
B2F12
Complex
Fig. 3. Phase diagrams of the supramolecular HBLCs: a) An [41], b) A3Fn, c) A2Fn, d) Bn, e) B3Fn and f) B2Fn. The phase types are shown in c) and d).

6
M. Alaasar et al. / Journal of Molecular Liquids 310 (2020) 113252
As shown in Fig. 6a, b at the transition to the SmC phase from the
a) A3F14
SmA phase the birefringence in the homeotropic cell suddenly increases
indicating the tilt of the molecules, while in the planar cell the fan
shaped textures become broken as typically observed for SmC phases.
The orientation of the extinctions becomes inclined with the directions
of polarizer and analyzer indicating a synclinic SmC phase. On further
cooling B14 from the SmC phase the birefringence changes again at
~89 °C on both homeotropic and planar cells indicating the transition
to another LC phase (Fig. 6c, f). The formation of this phase is associated
with a large enthalpy change ~8.4 J/g (Table 2), indicating the formation
of additional monotropic highly ordered unidentified smectic LC phase
assigned as SmX phase below the SmC phase for the longest homologue.
As the inclination of the extinctions is retained a transition to a tilted
low temperature hexatic (HexI) or crystalline mesophases (CrJ/G) is
likely. The schlieren texture in Fig. 6c is in favour for a HexI phase,
whereas the CrJ/G phases are known to prefer moasic like textures [46].
Using 4-(3-fluoro-4-alkoxyphenylazo)pyridines (3Fn) as proton do-
nors leads to B3Fn HBLCs having a fluorine substituent at ortho position
with respect to the terminal alkoxy chain at the azopyridine side. All de-
rivatives of B3Fn complexes show only enantiotropic SmC mesophases
with comparable melting and clearing temperatures. Changing the posi-
tion of the fluorine atom to be in meta position leads to the formation of
B2Fn complexes. The shortest complex B2F8 melts directly to the iso-
tropic liquid on heating from the crystalline solid and on cooling it ex-
hibits short range of the SmA phase. The SmA phase converts to an
enantiotropic phase for all next longer homologues (with n ≥ 10)
which appears as the only mesophase for the complex B2F10 and beside
SmC phases for B2F12 and B2F14.
Heating
Cr
SmC
Iso
Iso
SmC
90
Cr
Cooling
50
60
70
80
100 110 120
T / °C
b) B8
Heating
Cr
SmA
Iso
Iso
SmA
Cr
Cooling
60
70
80
90
100
T / °C
110
120
130
Fig. 4. DSC traces of a) A3F14 and b) B8 with heating and cooling rates 10 K/min.
The results obtained for the prepared supramolecular aggregates in-
dicate that for both types of complexes An or Bn without fluorine sub-
stitution core fluorination leads to lower clearing temperatures
(Fig. 3) i.e. lower mesophase stability. However, the type of the
mesophases exhibited by HBLCs could be modified by fluorination.
Therefore, for the fluorinated complexes A3Fn and A2Fn nematic and
SmC phases were observed, while the nonfluorinated supramolecules
An exhibit only SmC phases. The same effect was also observed for the
other types of the complexes Bn, B3Fn and B2Fn, where the SmA phases
are exhibited by the HBLCs or suppressed depending on the position of
the fluorine substitution (Fig. 3d-f).
mesophase behaviour of the HBLCs An, A3Fn and A2Fn we prepared an-
other related H-bonded complexes (Bn, B3Fn and B2Fn) using 4-
octylbenzoic acid as the proton donor instead of 4-octyloxybenzoic
acid. The results prove that this slight modification has a great effect
on the liquid crystalline behaviour of the HBLCs. The pure 4-
octylbenzoic acid (C8) exhibits only a nematic phase in the following se-
quence: Cr 100 °C N 109 °C Iso, while all of the HBLCs do not exhibit N
phases and form different types of mesophases.
As can be seen from Table 2 and Fig. 3d–f the nonfluorinated HBLCs
(Bn) exhibit three different types of mesophases depending on the
chain length and temperatures. For the shortest homologue B8 only
SmA phase is observed based on the textural observations (see Fig. 6
as an example), where totally isotropic texture is observed on the
homeotropic cell and a truncated focal conic fan texture is observed in
the planar cell confirming the presence of a SmA phase. For the next lon-
ger homologues B10–B14 (with n ≥ 10) SmC phase starts to appear
below the SmA phase as indicated from the DSC traces (Table 1) and
the change in the optical textures under the PLM. The range of the
SmC phase increases with chain elongation, while that of the SmA
phase is suppressed (see Fig. 3d).
5. Photosensitivity
The prepared supramolecular complexes were designed to be pho-
tosensitive due to the presence of the azopyridine segments in its mo-
lecular structures. We found that all of the prepared HBLCs are
photosensitive and undergo a fast and reversible isothermal phase tran-
sition upon illumination with a UV laser pointer (405 nm, 5 mW/mm2).
This is due to the trans-cis photoisomerization of the azo unit upon
photo illumination. The photosensitivity of the supramolecule B10 will
be discussed here in details as a representative example (see Fig. S7 in
c)
b)
a)
100 µm
100 µm
100 µm
Fig. 5. Photomicrographs of: A3F8 in a) the N phase at 108 °C and in b) the SmC phase at 89 °C and c) A2F10 in the N phase at 110 °C. The direction of the polarizers is shown in a).

M. Alaasar et al. / Journal of Molecular Liquids 310 (2020) 113252
7
a)
and the isotropic liquid phase. Therefore, the SmC-SmA or SmA-Iso
transitions achieved upon UV irradiation both resulting from trans-cis
photoisomerization between the more stable trans form of the
azopyridines before light irradiation and the less stable cis form after ir-
radiation [4].
d)
e)
6. Summary and conclusions
100 µm
100 µm
100 µm
100 µm
100 µm
100 µm
In summary, we have reported herein the design and synthesis of
new azopyridine derivatives which have been used to prepare new pho-
tosensitive supramolecular liquid crystals via intermolecular hydrogen
bond formation with two different types of benzoic acid derivatives.
The formation of the supramolecular HBLCs was confirmed by FTIR, 1H
NMR, DSC and PLM. Where all the azopyridine derivatives are crystal-
line solids, all their H-bonded aggregates exhibit LCs phases. They
show rich mesomorphism depending on the position of the lateral fluo-
rine substituent, on the terminal chain length and on the type of the
proton donor. The supramolecules derived from 4-octyloxybenzoic
acid and non-fluorinated azopyridines exhibit only SmC phases irre-
spective of the terminal chain length (An complexes). Using fluorine
substituted azopyridines either at ortho or meta positions with respect
to the terminal chains as the proton acceptors with 4-octyloxybenzoic
acid (A3Fn and A2Fn complexes) results in additional nematic phases
beside the SmC phases for short and medium chains supramolecules
(with n = 8–12). Removing the oxygen atom connecting the terminal
octyl chain to the benzoic acid results in the related HBLCs Bn, B3Fn
and B2Fn constructed using 4-octylbenzoic acid as the proton donor in-
stead of 4-octyloxybenzoic acid. The later complexes do not exhibit ne-
matic phases but form SmA, SmC phases, and in one case of a non-
fluorinated complex an additional unidentified low temperature smec-
tic phase assigned as SmX. The effect of changing the position of the lat-
eral fluorine substituent was found to be more pronounced in these
types of complexes, where ortho fluorinated complexes (B3Fn) exhibit
SmC phases and their related meta fluorinated complexes (B2Fn) ex-
hibit SmA and SmC phases. Moreover, all of the prepared complexes
show fast and reversible photoinduced LCs phase transitions under UV
irradiation due to trans-cis photoisomerization of the azopyridine
units. Therefore, the transition between different types of LCs phases
or between the LCs phases and the isotropic liquid could be affected
with UV light, which provide additional possibility of phase modulation
by interaction with light.
b)
c)
f)
Fig. 6. Optical textures of the supramolecular complex B14 a–c) in homeotropic cell and d–
f) in 10 μm ITO coated cell: a, d) in SmA phase at 120 °C; b),e in SmC phase at 95 °C and c,
f) in the SmX phase at 88 °C. The direction of the polarizers is shown in a).
the SI for UV–Vis spectroscopy in solution). B10 exhibits enantiotropic
SmC and SmA phases, therefore we investigated the photoinduced
SmC-SmA and SmA-Iso phase transitions in homeotropic cells. The
SmC phase has a birefringent homeotropic texture under crossed
polarizers prior to the UV illumination (Fig. 7a). Under UV irradiation
and at 5 K below the SmC-SmA transition temperature i.e. ~ 99 °C the bi-
refringent textures disappeared and converts within 3 s into the totally
dark homeotropic texture of the uniaxial SmA phase (Fig. 7b). On re-
moving the UV source, the dark texture relaxes back to the original
SmC texture in few seconds, indicating a fast and reversible photoin-
duced phase transition between the SmC and SmA phases. Similar pho-
toinduced phase transition was also achieved between the SmA phase
CRediT authorship contribution statement
Mohamed Alaasar: Conceptualization, Supervision, Writing - origi-
nal draft, Writing - review & editing. Jaques-Christopher Schmidt: In-
vestigation. Ahmed F. Darweesh: Investigation. Carsten Tschierske:
Writing - review & editing.
a)
b)
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
hν
Ahmed F. Darweesh acknowledges the support by the Alexander
von Humboldt Foundation for the research fellowship at Martin Luther
University Halle-Wittenberg, Germany.
100 µm
100 µm
Fig. 7. Textures of the supramolecular complex B10 under crossed polarizers: a) in the
homeotropic aligned SmC phase at T = 99 °C before UV irradiation and b) after UV
irradiation indicating the transformation from the biaxial SmC phase to the uniaxial
SmA phase. The blue colour in b) is due to the UV light. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2020.113252.

8
M. Alaasar et al. / Journal of Molecular Liquids 310 (2020) 113252
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