[1,2,3]-triazole derivatives: Mesomorphic property dependence on the molecular shape

Article abstract of DOI:10.1080/15421406.2017.1289614

Nowadays, heterocyclic units have been extensively studied for the preparation of thermotropic liquid crystals due to their ability to impart lateral and/or longitudinal dipoles and induce changes in molecular shape. Key parameters to accede to stable mesophases are the position of the heterocycle and the nature of terminal and linking functions. We present in this paper, an overview of several series of 1,2,3-triazole liquid crystalline derivatives that we designed and characterized. We highlight the effect of different connecting functions and the geometry of mesogenic core on the mesomorphic behavior of these derivatives.

Full text of DOI:10.1080/15421406.2017.1289614

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
[1,2,3]-triazole derivatives: Mesomorphic property
dependence on the molecular shape
Souria Benallou, Salima Saidi-Besbes, Eric Grelet & Ahmed Bentaleb
To cite this article: Souria Benallou, Salima Saidi-Besbes, Eric Grelet & Ahmed Bentaleb (2017)
[1,2,3]-triazole derivatives: Mesomorphic property dependence on the molecular shape, Molecular
Crystals and Liquid Crystals, 647:1, 290-298, DOI: 10.1080/15421406.2017.1289614
To link to this article: http://dx.doi.org/10.1080/15421406.2017.1289614
Published online: 23 May 2017.
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Date: 24 May 2017, At: 01:18

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
[,,]-triazole derivatives: Mesomorphic property dependence
on the molecular shape
Souria Benalloua, Salima Saidi-Besbesa, Eric Greletb, and Ahmed Bentalebb
aUniversité d’Oran  Ahmed Benbella, Laboratoire de Synthèse Organique Appliquée (LSOA), Département de
chimie, Faculté des sciences exactes et appliquées, BP  EL Mnaouer, Oran, Algérie; bCentre de Recherche
Paul-Pascal, CNRS UPR , Université de Bordeaux, F- Pessac, France
ABSTRACT
KEYWORDS
,,-triazole; catalyzed
,-dipolar cycloaddition;
X-ray diffraction; smectic
mesophases
Nowadays, heterocyclic units have been extensively studied for the
preparation of thermotropic liquid crystals due to their ability to impart
lateral and/or longitudinal dipoles and induce changes in molecular
shape. Key parameters to accede to stable mesophases are the position
of the heterocycle and the nature of terminal and linking functions. We
present in this paper, an overview of several series of 1,2,3-triazole liquid
crystalline derivatives that we designed and characterized. We highlight
the effect of different connecting functions and the geometry of meso-
genic core on the mesomorphic behavior of these derivatives.
Introduction
Heterocyclic units attracted considerable attention as building blocks for the preparation of
liquid crystalline compounds due to their particular properties and their structural diversity
allowing for various mesogenic designs. Different supramolecular organizations have been
targeted and tuned through appropriate functionalization of the heterocyclic core [1–5]. The
rich polymorphism usually observed of such structures was attributed to their medium to
strong lateral dipole, high polarizability and anisotropy, low symmetry and/or non-planar
structure caused by nitrogen, oxygen, sulfur heteroatoms, etc [6–7].
Six membered aromatic heterocyclic rings containing one or two nitrogen atoms such as
pyridine [8], tetrazine [9], pyrimidine [10] are known to be conducive to liquid crystalline
organizations and in particular to smectic mesophases. Even if, such heterocycle ring changes
slightly the geometry of mesogenic molecules in comparison with benzene homologous, the
lone pairs of electrons on the nitrogen atoms introduce attractive forces which favor layer
formation. The position of nitrogens may have a substantial effect on the polarisability and
polarity of mesogenic molecules and thus on the mesophase stabilities. For calamitic meso-
gens, the mesophase is even more stable than the polarity and polarisability are high due to
the intensification of side-chain interactions.
For a long time, the nonlinearity of five-membered heterocyclic rings was considered as
a brake on the achievement of mesomorphic properties. The unfavorable deviation from
linearity could be compensated by [11]: a) the lengthening of the mesogenic core, b) the
introduction of functional goups allowing for the straightening out of the molecules (as imine
CONTACT Salima Saidi-Besbes
saidi.salima@univ-oran.dz; salima_saidi@yahoo.fr
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[559]/291
function) or, c) the introduction of the heterocyclic ring at the end of the mesogenic core.
Several five-membered heterocyclic rings have been investigated such as pyrrole, thiophene,
isoxazole [12], tetrazole [13] and 1,2,4-oxadiazole [14] in the aim to establish a structure-
liquid crystalline properties relationship. Nematic and smectic mesophases have been usually
obtained with lower melting points than those obtained for classical 1,4-phenylene core owing
to the reduced packing efficiency of this type of molecules.
For instance, nitrogen based heterocyclic liquid crystals [14–17] that exhibit tilted smectic
liquid crystalline packing (SmC) have been explored, as achiral host LC, in mixtures with
chiral dopants having a large spontaneous polarization to prepare ferroelectric liquid crystal
materials [13]. The aim was to accede to materials with low viscosities and melting points and
a wide range SmC phase for use in “guest-host” approach to ferroelectric mixtures.
1,2,3-triazole mesogenic derivatives
Since the development of copper-catalyzed azide-alkyne cyloaddition also known as ‘Click
Chemistry’, [1,2,3]-triazole compounds have attracted much more attention in different fields
of pharmaceutical industry, supramolecular chemistry and material chemistry for realizing
highly efficient phosphorescent OLEDs [18–19] or for nonlinear optical applications [20–21].
1,2,3-triazole combines multiple interesting properties including a high chemical and ther-
mal stabilities, a strong dipole moment, an aromatic character and a good hydrogen-bond-
accepting ability [22–23]. Despite, these outstanding features, 1,2,3-triazole heterocyclic has
gained less interest to devise mesogenic compounds.
In view of this lack of 1,2,3-triazole LC materials, we have developed different triazole based
mesogenic compounds with different shapes [24–26] (Figure 1).
The majorly of reported calamitic derivatives in literature contains a central rigid core com-
posed of a phenyl or biphenyl ring directly linked to triazole heterocyclic at N1 and/or C4 posi-
tion. We have shown in our previous studies the positive effect of the introduction of an ester
function group within the triazole heterocycle, as a connector group, on the enhancement of
thermal stability of mesophases (1a-d, 2a-c) [24]. This can be explained by the contribution of
Figure . Synthesized ,,-triazole mesogenic derivatives [–].

292/[560]
S. BENALLOU ET AL.
the ester group on the enhancement of dipole moment of mesogen molecules favoring layer
organization of molecules within smectic A mesophase. It is worth mentioning that this type
of organization was obtained even for short mesogens bearing only three aromatic rings.
This feature was exploited to prepare unsymmetrical tetracatenar compounds containing
ester type [1,2,3]-triazole derivatives (3a-d, 4a-d) by the esterification of 1H-[1,2,3]-triazole-
4-carboxylic acid compounds bearing octyl, decyl, dodecyl or tertadecyl oxychain with a tri-
catenar alcohol.
Stable hexagonal columnar organization of these derivatives was evidenced from X-ray
diffraction (XRD) analyses with a very weak correlation of the molecules within the columns
[25]. The building block was composed of a disc-like structure containing two molecules lying
side by side. The lamellar ordering was maintained even in the solid structure.
The effect of alkene function as a linking group has been also investigated (5a-c, 6a-c). We
demonstrated that the nature of obtained mesophases (SmA, SmC and N) and their stability
could be tuned through a suitable terminal functionalization [26]. The elongation of the aro-
matic rigid core decreased by a factor of 2.5 the temperature range of smectic A mesophase sta-
bility of and induced the disappearance of SmC in favor of less ordered nematic phase. Trans
isomers were more conducive to liquid crystalline packing in comparison with cis homolo-
gous and induced significantly higher stable smectic A mesophase and higher melting points.
This type of molecules presents an extended conjugated structure and can potentially act as
electron-transporting materials.
Recently, as part of our continuing effort in triazole derivatives, we investigate the effect
of oxymethylene function as a connector group on mesomorphic behavior of triazole based
mesogens. Symmetrical bis-1,2,3-triazole ether derivatives with a biphenyl central core were
designed that exhibit an enantiotropic smectic C mesophase at high temperature.
These compounds (11a-d) were prepared according to the synthetic pathway described in
Figure 2. They present a dimeric structure consisting of a 4,4ꢀ-biphenyl central core connected
on both sides, through an oxymethylene unit, to 1-(4-(alkyloxy)phenyl)-1H-[1,2,3]-triazoyl
group. The precursor used to accede to these compounds is the 4-azido-1-alkyloxybenzene
(9a-d) which was obtained from 4-aminophenol using a three-step synthesis strategy as
reported in a previous work [25]. Meanwhile, 4,4ꢀ-bis(propargyloxy)biphenyl (10) was syn-
thesized by etherification of 4,4ꢀ-biphenol with two equivalents of propargylbromide in the
presence of sodium hydride. Catalytic 1,3-dipolar cycloaddition of the arylazide derivatives
(9a-d) with 4,4ꢀ-bis(propargyloxy)biphenyl (10) in the presence of CuI and catalytic amount
of triethyamine affords the final products (11a-d). Under click chemistry condition, only the
1,4-regioisomers of compounds (11a-d) were obtained with good yields (between 56 and
1
77%). Their structures were supported by H and 13C NMR analyses and High Resolution
Mass Spectroscopy.
The synthesized derivatives showed enantiotropic liquid crystalline behavior with very
high transition temperatures (above 220°C) that cause partial thermal decomposition of few
compounds after extensive annealing at high temperature (Table 1, Figure 3). On cooling from
the isotropic liquid, elongated “bâtonnets” nucleated from the dark background and grew up
to broken fan-shaped texture (Figure 4c-d). Schlieren textures were also observed for com-
pounds 11b and 11c in cooling runs indicating the existence of SmC phase (Figure 4a-b).
Differential scanning calorimetry analysis confirmed the mesomorphic behavior. Three
endothermic peaks, corresponding to crystallization, melting and isotropic transitions were
observed. A typical thermogram obtained by DSC analysis of compound 11a is presented in
Figure 5.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
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Figure . Reagents for the synthesis of triazole derivatives (i) . acetic anhydride, HO, . CnHn+Br (n = ,
, , ), KCO, butanone; (ii) HCl, HO; (iii) . HCl, NaNO, HO, . NaN, HO; (iv) propargyl bromide, NaH,
DMF; (v) CuI, N(CH), water/EtOH :.
Table . Transition temperatures (°C) and transition enthalpies ꢀH (KJ mole−) of compounds 5 a-d deter-
mined by DSC (°C min−) during the second cycle on heating.
Compound
n
Transitions temperatures (°C) [transition enthalpies ꢀH (KJ mole−
)
11a
11b
11c
11d

Cr(.)[.]Cr’(.)[.]SmC(.)[.]I



Cr(.) Cr’(.)[Cr+Cr’= .]Cr”(.)[.]SmC(.)[.]I
Cr(.)[.]Cr’(.)[.]SmC(.)[.]I
Cr(.)[.]Cr’(.)[[.]Cr”’(.)[.]SmC(.)[.]I
The thermal stability of the mesophase is independent of the length of the alkoxy terminal
chain and is in the range of (12.4 to 15.6°C).
Cristano et al. reported analogue derivative of general formula 4-(4ꢀ-Decyloxybiphenyl-4-
yl)-1-(4-decyloxyphenyl)-1H-[1,2,3]-triazole, that showed SmC phase in the thermal range
of 184.9°C-246.1°C [27]. The structural difference with our compounds and especially com-
pound 11b bearing the same decyloxy chain is in the number of triazole ring and the nature of
the connector linking the latter to biphenyl core (Cristano’s compound contains one triazole
ring and no connector). Compound 11b exhibits higher melting transition temperature and

294/[562]
S. BENALLOU ET AL.
Figure . Comparative thermal behavior of compounds (11a-d).
narrow mesomorphic range. Two factors should be at the origin of the short mesophase stabil-
ity: the rotation around the oxymethylene connector group that is free to take place and con-
tributes to the loss of linearity and planarity of the mesogenic core disfavoring the mesophase
formation and the linearity deviation caused by the two 1,2,3-triazole units. The bis-triazole
Figure . Optical textures observed on cooling (°C/min) between cover slip and glass slide by polarized
microscopy for (a) 11b at °C, (b) 11c at °C, (c) 11d at °C and (d) 11d at °C.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
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Figure . Thermogram of compound 11a during the first and second cycles. The heating and cooling rates
are °C min−.
mesogens reported in this paper are less linear than monotriazolic derivative; the heterocyclic
ring may acts as a polar terminal substituent rather than a part of the central rigid core.
XRD experiments were carried out on compound 11b to determine the nature of the
mesophase. The XRD pattern for compound 11b (Figure 6) showed two reflection peaks char-
acterized by the main Bragg peaks q100 = 1.61 nm−1 and the second-order reflection (200) at
3.19 nm−1 with ratio q200/q100 ࣈ 2, confirming the lamellar organization. The signal of the
disordered aliphatic chains corresponds to the broad reflection at about 13.7 nm−1, is charac-
teristic of a liquid-like order. Due to the instability of investigated compounds to prolonged
exposure at high temperatures, we were not able to carry out X-ray diffraction experiments
to determine the temperature dependence of the layer spacing and therefore the nature of
the local smectic order [25]. However, the comparison of the interlayer spacing d = 3.90 nm
(d = 2π/q100) with the molecular length L = 4.61 nm estimated from the energy minimized
structure obtained using semi-empirical Hartree-Fock AM1 (Austin Model 1) level, indicated
that the molecules are tilted within the layer. In this case, the tilted angle estimated from this
Figure . XRD pattern of compound 11b at °C in the SmC mesophase.

296/[564]
S. BENALLOU ET AL.
molecular length using the following equation cosθ = d/L will be around 32° and is consistent
with SmC orgnaization deduced from POM textures.
Conclusion
Several liquid crystalline 1,2,3-triazole derivatives with different molecular shapes have been
synthesized in order to investigate the relationship between molecular structure and meso-
morphic properties. Rich polymorphism was exhibited by this heterocyclic synthon such as
nematic, smectic and columnar phases. We demonstrated that the introduction of a polar
connector group within the heterocyclic unit may have a significant effect on the nature of
observed mesophases and their stability. Ester and alkene fucntion were more conducive to
liquid crystalline phases than the oxymethylene group probably due to the lack of linearity of
the latter mesogenic core.
Many scientific issues in this field still need to be explored in the feature in particular
for the development of 1,2,3-triazole based discotic liquid crystals that have been very little
reported in the litterature. This class of materials will be potentially interesting for optoelec-
tronic devices and solar cell applications due to the thermal stability of these derivatives and
their long-range self-assembly.
Experimental section
Characterization
1H and 13C NMR spectra were recorded on a Bruker 300 MHz spectrometer (Wissembourg,
France). Tetramethylsilane was used as an internal reference for chemical shifts. Column
chromatographies were carried out using Merck, Darmstadt silica gel (Kieselgel 60, 230–400
mesh) as the stationary phase (Merck Millipore, Gernsheim, Germany). Thin-layer chro-
matography was carried out on aluminum plates pre-coated with Merck silica gel 60F254
and visualized by means of ultraviolet fluorescence quenching. The melting points, transition
temperatures and phase transition enthalpies were determined using a differential scanning
calorimetry (DSC Q200 , TA Instruments) at a heating and cooling rate of 5°C min−1. Meso-
®
morphic textures were observed using a Zeiss Scope. A1 polarizing microscope equipped with
Mettler Toledo heating stage.
X-ray diffraction experiments were performed by using a rotating anode generator (Rigaku
Nanoviewer MicroMax 007HF) coupled to a confocal Max-Flux Osmic mirror (Applied
®
˚
Rigaku Technologies, Austin, USA) producing beam with a wavelength of 1.54 A together with
a homemade heating stage with a thermal stability of 0.1°C. The spectra were recorded with a
bi-dimensional detector (MARResearch-345) located at 308 mm from the sample, which was
introduced as a powder in glass capillary tubes (Glas, Muller, Germany) having a diameter of
1.5mm. The spectra were analyzed using FIT2D software (ESRF; http://www.esrf.eu/).
Synthesis
All the reagents were purchased from Aldrich (Prochima Sigma, Tlemcen, Algeria) and used
as received. The solvents were of commercial grade quality and were dried and distilled before
use. Methylene chloride was distilled over calcium hydride. 4-azido-1-alkyloxybenzene (9a-d)
were prepared according to our previous study from 4-aminophenol [25].

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[565]/297
,ꢀ-bis(propargyloxy)biphenyl ()
In a two-necked round bottom flask, is placed (0.97 g, 5.3 mmol) of 4,4ꢀ-biphenol solubi-
lized in a minimum of DMF and (0.25 g, 10.6 mmol) of NaH. The reaction is stirred during
2 hours at room temperature under nitrogen atmosphere. The solution is cooled to 0°C then
10.6 mmol of propargyl bromide are added dropwise. The reaction is stirred at room temper-
ature during 48 h. Afterwards, distilled water is added and the obtained precipitate is filtered,
abundantly washed with water then recrystallized from ethanol.
Yield: 93%, mp = 176°C, 1H NMR (C2D6O): 2.85 (s, 2H, CࣕCH), 4.83 (s, 4H, HCࣕC–
CH2), 7.07(d, 4H, Ar-H, 3J = 8.61 Hz), 7.56(d, 4H, Ar-H, 3J = 8.62 Hz). 13C NMR (C2D6O):
55.42, 76.18, 78.93, 115.18, 127.49, 156.97.
,ꢀ-bis (-(-alkyloxyphenyl)-H-[,,]-triazole)oxymethylenebiphenyl (a-d)
(0.76 mmol) of the appropriate 4-azido-1-alkyloxybenzene (9a-d) and (0.1 g, 0.38 mmol) of
4,4ꢀ-bis(propargyloxy)biphenyl (10) are dissolved in 15 mL of a water/ethanol (1:1) mixture.
Afterward, (14 mg, 0.076 mmol) of cupper iodide (CuI) and a catalytic quantity of triethy-
lamine are added. The mixture is stirred at 70°C during 48 hours. After cooling to room tem-
perature, the reaction mixture is filtered and the obtained solid is washed with ethanol then
purified by flash column chromatography using acetone then dichloromethane as eluents.
,ꢀ-Bis (-(-octyloxyphenyl)-H-[,,]-triazole)oxymethylenebiphenyl (a)
1
3
Yield: 77%, H NMR (CDCl3, 50°C): 0.91 (t, 6H, CH3, J = 6.33 Hz), 1.31 (m, 20H,
3
CH3(CH2)5), 1.83 (m, 4H, OCH2CH2); 4.02 (t, 4H, OCH2CH2 , J = 6.90 Hz), 5.35 (s,
4H,OCH2), 7.00 (d, 4H, Ar-H, 3J = 8.96 Hz), 7.11 (d, 4H, Ar-H, 3J = 8.37 Hz), 7.50 (d, 4H, Ar-
H, 3J = 8.37 Hz, 7.62 (d, 4H, Ar-H, 3J = 8.96), 7.99 (s, 2H, H5-triazole). 13C NMR (CDCl3):
14.09, 22.41, 25.75, 28.90, 28.97, 29.09, 31.55, 61.85, 68.43, 114.84, 115.06, 122.25, 127.62,
129.97, 133.95, 140.29, 157.15, 159.29. ESI-HRMS [M+H]+: calculated for C46H57N4O6:
757.4436; found 757.4426.
,ꢀ-Bis (-(-decyloxyphenyl)-H-[,,]-triazole) oxymethylenebiphenyl (b)
1
3
Yield: 56%, H NMR (CDCl3, 50°C): 0.91 (t, 6H, CH3, J = 6.33 Hz), 1.31 (m, 28H,
3
CH3(CH2)7), 1.83 (m, 4H, OCH2CH2); 4.03 (t, 4H, OCH2CH2, J = 6.9 Hz), 5.35 (s, 4H,
OCH2), 7.03 (d, 4H, Ar-H, 3J = 8.81 Hz), 7.11 (4H, Ar-H, 3J = 8.54 Hz), 7.52 (d, 4H, Ar-H, 3J =
8.54 Hz), 7.64 (d, 4H, Ar-H, 3J = 8.81 Hz), 7.96 (s, 2H, H5-triazole). 13C NMR (CDCl3):13.92,
22.57, 25.95, 29.22, 29.30, 29.38, 29.56, 29.97, 31.82, 68.58, 68.68, 115.31, 115.50, 119.58,
122.31, 127.91, 134.15, 136.18, 140.67, 157.62, 159.70. ESI-HRMS [M+H]+: calculated for
C50H67N6O4: 813.5062; found 813.5067.
,ꢀ-Bis (-(-dodecyloxyphenyl)-H-[,,]-triazole)oxymethylenebiphenyl (c)
Yield: 75%, 1H NMR (CDCl3): 0.91 (t, 6H, CH3, 3J = 6.33 Hz), 1.30 (m, 36H, CH3(CH2)9),
1.83 (m, 4H, OCH2CH2), 4.03 (t, 4H, OCH2CH2, 3J = 6.9 Hz), 5.35 (s, 4H, OCH2), 7.01 (d,
4H, Ar-H, 3J = 7.57 Hz), 7.11 (d, 4H, Ar-H, 3J = 7.80 Hz), 7.49 (d, 4H, Ar-H, 3J = 7.80 Hz);
7.64 (d, 4H, Ar-H, 3J = 7.57); 7.96 (s, 2H, H5-triazole). 13C NMR (CDCl3): 14.02, 22.67, 26.05,
29.24, 29.34, 29.38, 29.59, 29.64, 29.66, 29.70, 31.93, 62.48, 68.69, 115.33, 115.51, 119.07,
122.32, 125.32, 127.92, 134.18, 146.12, 157.63, 159.72. ESI-HRMS [M+H]+: calculated for
C54H73N6O4: 869.5688; found 869.5695.

298/[566]
S. BENALLOU ET AL.
,ꢀ-Bis (-(-tetradecyloxyphenyl)-H-[,,]-triazole)oxymethylenebiphenyl (d)
Yield: 66%, 1H NMR (CDCl3): 0.91 (t, 6H, CH3, 3J = 6.33 Hz), 1.31 (m, 44H, CH3(CH2)11),
1.84 (m, 4H, OCH2CH2), 4.04 (t, 4H, OCH2CH2 , 3J = 6.9 Hz), 5.35 (s, 4H, OCH2), 7.03 (d,
4H, Ar-H, 3J = 8.93 Hz), 7.10 (d, 4H, Ar-H, 3J = 8.66 Hz), 7.50 (d, 4H, Ar-H, 3J = 8.66 Hz),
7.61 (d, 4H, Ar-H, 3J = 8.93 Hz), 7.96 (s, 2H, H5-triazole). 13C NMR (CDCl3): 13.98, 22.66,
26.07, 29.28, 29.35, 29.40, 29.58, 29.60, 29.58, 29.60, 29.67, 29.68, 31.95, 62.62, 68.79, 115.44,
115.61, 121.02, 122.36, 127.94, 130.59, 137.6, 149.00, 157.74, 159.79. ESI-HRMS [M+H]+:
calculated for C58H81N6O4: 925.6314; found 925.6329.
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