Synthesis and liquid crystalline properties of novel fluorinated N-benzoyl thiourea compounds. Effect of perfluoroalkyl chains on the thermal behavior and smectic phases stability

Article abstract of DOI:10.1016/j.jfluchem.2017.10.008

A series of novel N-benzoyl-N’-aryl thiourea derivatives (BTU) bearing different number of alkoxy groups in terminal positions of benzoyl unit and a perfluorooctyl group on the other side have been designed and prepared. Their liquid crystalline properties were investigated by a combination of three techniques: polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and variable-temperature powder X-ray diffraction (XRD). Their thermal stability was studied by thermogravimetric analysis (TG). It was found that only the compounds which have only one alkoxy chain attached to benzoyl unit, 1a and 1b, show calamitic mesomorphic behavior, with smectic A and C phases being displayed. The type and stability of these mesophases are greatly influenced by the alkyl chain length as well as by the presence of perfluorooctyl group. The clearing and the mesophase ranges are significantly increased with the incorporation of perfluoroalkyl chains when compared to non-fluorinated analogues, with almost 40 °C. The attachment of additional alkoxy groups on the benzoyl moiety led to a significant decrease of the clearing points and suppression of the mesogenic character.

Full text of DOI:10.1016/j.jfluchem.2017.10.008

Accepted Manuscript
Title: Synthesis and liquid crystalline properties of novel
fluorinated N-benzoyl thiourea compounds. Effect of
perfluoroalkyl chains on the thermal behavior and smectic
phases stability
Authors: Monica Ili s¸ , Marin Micutz, Iuliana Pasuk, Teodora
Staicu, Viorel C ˆı rcu
PII:
S0022-1139(17)30406-2
DOI:
Reference:
https://doi.org/10.1016/j.jfluchem.2017.10.008
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18-10-2017
Please cite this article as: Monica Ili s¸ , Marin Micutz, Iuliana Pasuk, Teodora
Staicu, Viorel C ˆı rcu, Synthesis and liquid crystalline properties of novel
fluorinated N-benzoyl thiourea compounds.Effect of perfluoroalkyl chains
on the thermal behavior and smectic phases stability, Journal of Fluorine
Chemistry https://doi.org/10.1016/j.jfluchem.2017.10.008
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Synthesis and liquid crystalline properties of novel fluorinated N-benzoyl
thiourea compounds. Effect of perfluoroalkyl chains on the thermal behavior
and smectic phases stability
1
2*
3
2
1*
Monica Iliş , Marin Micutz , Iuliana Pasuk , Teodora Staicu , Viorel Cîrcu
1
Department of Inorganic Chemistry, University of Bucharest, 23 Dumbrava Rosie st, sector 2, Bucharest
020464, Romania, e-mail: viorel.circu@chimie.unibuc.ro
2
Department of Physical Chemistry, University of Bucharest, 4-12 Elisabeta Blvd., Bucharest 030018,
Romania, e-mail: micutz@gw-chimie.math.unibuc.ro
3
National Institute of Materials Physics, P.O. Box MG-7, Magurele, 077125, Romania
Graphical Abstract
Synthesis and liquid crystalline properties of novel fluorinated N-benzoyl
thiourea compounds. Effect of perfluoroalkyl chains on the thermal behavior
and smectic phases stability

Highlights
•
•
•
•
Benzoyl thiourea compounds (BTU) with terminal pefluorooctyl group were designed
The new BTU compounds show SmA and SmC phases
0
Fluorinated BTU have higher mesophase ranges, ca. 40 C vs. non-fluorinated BTU
Compounds with higher number of alkoxy groups show no LC properties
Abstract
A series of novel N-benzoyl-N’-aryl thiourea derivatives (BTU) bearing different number of
alkoxy groups in terminal positions of benzoyl unit and a perfluorooctyl group on the other side
have been designed and prepared. Their liquid crystalline properties were investigated by a
combination of three techniques: polarizing optical microscopy (POM), differential scanning
calorimetry (DSC) and variable-temperature powder X-ray diffraction (XRD). Their thermal
stability was studied by thermogravimetric analysis (TG). It was found that only the compounds
which have only one alkoxy chain attached to benzoyl unit, 1a and 1b, show calamitic
mesomorphic behavior, with smectic A and C phases being displayed. The type and stability of
these mesophases are greatly influenced by the alkyl chain length as well as by the presence of
perfluorooctyl group. The clearing and the mesophase ranges are significantly increased with the

incorporation of perfluoroalkyl chains when compare to non-fluorinated analogues, with almost
00C. The attachment of additional alkoxy groups on the benzoyl moiety led to a significant
4
decrease of the clearing points and suppression of the mesogenic character.
Keywords: liquid crystals, smectic, perfluoroalkyl, N-benzoyl thiourea
Introduction
Liquid crystalline compounds have various applications such as the manufacturing of LCD,
molecular sensors and detectors, optical switches, spatial light modulator, etc.[1-3] There is a
growing interest to study the unique combination of properties of anisotropic fluids (anisotropy
of physical properties and fast orientational response to external fields) specific to liquid crystals
(
LC).[4-6] While the fluorine substitution at the aromatic core has direct consequences on the
physical properties of LC (transition temperature, birefringence, dielectric properties, etc. which
are relevant for various electro-optical applications), the alkyl chains substitution with semi or
perfluoro alkylated chains leads to a serious increasing of both the transition temperatures and
mesophases thermal ranges.[7-9] A very important feature of these mesogens containing
fluorocarbon moieties is that the stability of layered phases are markedly increased due to the
incompatibility between the aliphatic and perfluoroalkylated chains as well as the rigidity of the
latter.[10-19]
The N-benzoyl thiourea derivatives (BTU) are widely studied in connection with their excellent
properties to bind to various metal ions. These derivatives possess very strong donor groups
(carbonyl and thioamide) that afford the preparation of different transition metals complexes,
either in monoanionic bidentate form by deprotonation forming neutral homoleptic or
heteroleptic complexes with S, O – coordination or in neutral form only through S atom [20-28].
Such compounds have been used in the design of liquid crystalline materials, either in purely
organic liquid crystals, simple or connected to a ferrocene moiety, or metal-containing liquid
crystals – metallomesogens.[29-31] Moreover, the BTU compounds decorated with alkoxy
groups show interesting mesogenic behavior. Several examples of BTU derivatives having
terminal alkoxy groups at the both ends of the molecule were reported previously.[32] These

compounds were found to exhibit nematic, smectic A and C phases, depending on the position
and length of the alkyl chains. Furthermore, the BTU compounds with long alkyl chains have
found to display interesting gelation properties.[33] The effect of number and position of the
alkoxy chains grafted on BTU derivatives on the mesogenic behavior of cyclometalated square-
planar Pd(II) and Pt(II) complexes with different Schiff bases were reported by us.[34,35]
Employing BTU ligands with various number of alkoxy terminal groups can lead to mono- or
dinuclear palladium(II) complexes with lower transition temperatures and rich mesomorphic
behavior ranging from calamitic to discotic materials.[36,37] These new metallomesogens
display also interesting emission properties related to the cyclometalated fragment. On the other
hand, only monotropic nematic or smectic A phases were observed, if simple N-benzoyl thiourea
derivatives (with no mesogenic alkoxy groups) were used as auxilliary ligands for palladium(II)
and platinum metals accompanied by cyclometalated imine ligands.[38-42]
A deeper study of structure-liquid crystalline properties correlation for these BTU compounds is
necessary as they are a versatile platform for design of new liquid crystalline materials, both
organic and metallomesogens, with tailored properties specific to various electro-optical
applications. In this work we report the preparation and mesogenic behavior of a new series of
N-benzoyl thiourea derivatives (BTU) that have at one side a different number of alkyl chains (1,
2
or 3) with a different number of carbon atoms and, at the other side, a perfluorooctyl group.
Besides a systematic study regarding the effect of alkyl chain length, number and position of the
alkoxy groups attached to the benzoyl fragment on the mesomorphic properties, we aim to
explore further the influence of perfluoroalkyl group introduction on thermal properties.
Results and discussion
The preparation of BTU compounds along with their numbering is presented in Scheme 1. Their
synthesis was based on the general procedure described in literature.[43] The BTU compounds
were obtained in good yields ranging from 44 to 70%, as white or off-white microcrystalline
solid products, which are stable under normal conditions. These fluorinated compounds are
soluble in chlorinated solvents (methylene chloride, chloroform) and show a lower solubility in
alcohols (ethanol, propanol, etc.). These new compounds were structurally characterized by
1
13
elemental analysis, FT-IR, H and C NMR spectroscopy (see SI-supplementary information).

A high number of alkyl-substituted carboxylic acids are available through the Williamson O-
alkylation reaction of various alkyl bromides and different starting materials (methyl 4-
hydroxybenzoate, methyl 3,4-hydroxybenzoate or ethyl gallate). Therefore, there is a large
choice of such BTU compounds that can be synthesized according to LC properties and thermal
stability envisaged. Different number of alkoxy groups can be attached to the benzoyl unit,
varying both the position and the number of carbon atoms, in order to tailor the mesogenic
properties.
1
The chemical structure of these products could be easily assigned by H-NMR spectroscopy. The
1H-NMR spectra of all compounds contain two singlets, one in the 8.99 – 9.05 ppm range and a
second one in the 12.95 – 13.01 ppm range, which can be assigned to the two NH groups. The
first singlet can be assigned to NH proton located between the carbonyl and thiocarbonyl groups
while the second singlet can be assigned to the NH proton between the thiocarbonyl group and
1
benzene ring. Additionally, the H-NMR spectra contain a series of other peaks assigned to the
aromatic protons which show the normal splitting pattern corresponding to 1,4-, 1,3,4-, 1,3,4,5-
substitution of the aromatic ring. The chemical shifts values for the carbonyl and thiocarbonyl
carbons were found at about 166.0-167.0 ppm and around 178.5 ppm respectively, as reported
for the related BTU compounds.[44, 45]
By analysis of FT-IR spectra of all compounds, recorded in KBr discs, several regions are of
-1
-1
interest: 3200 – 3400 cm for N-H frequencies; 1500 – 1700 cm for strong C=O and N-H
frequencies, and, finally, 1100-1250 cm-1 where strong absorption bands assigned to a
combination of C-N and phenyl stretching frequencies can be found.[46-50]
Liquid crystals properties
The new partially fluorinated N-benzoyl-N’-aryl derivatives were investigated for their liquid
crystalline properties by a combination of hot stage polarizing optical microscopy (POM),
differential scanning calorimetry (DSC) and variable-temperature powder X-rays diffraction
analysis (XRD). The thermal parameters for all prepared BTU compounds 1-3a,b are presented
in Table 1.

The BTU compounds having only one aliphatic chain, 1a and 1b, both of them show two
enantiotropic SmA and SmC phases (Figure 1). Figures 1a and 1b show the first and second
heating-cooling DSC traces of compound 1b, taken as a representative example. Three
exothermic peaks assigned to three Cr-SmC, SmC-SmA and SmA-Iso transitions can be seen
during the first cooling run. They are reproducible during the second cooling step, but on the
second heating run one additional transition appeared from another crystalline state (Cr’) formed
during the first cooling run. The phase assignment was done based on the POM observations
when a typical fan shape or focal conic texture together with several homeotropic areas where
found for SmA phases, on cooling the samples from the isotropic state. Additionally, compounds
1
a and 1b, display on further cooling of SmA phase, a broken fan-shape texture which was
assigned unequivocally to a SmC phase (Figure 2).[51, 52] The comparison of the fluorinated
compounds 1a and 1b with the non-fluorinated analogues reported earlier by us,[30, 32] shows
that the melting points, clearing points and the mesophase ranges are significantly increased with
the incorporation of perfluoroalkyl chains. For instance, the clearing points for both fluorinated
0
compounds 1a and 1b are higher, with almost 40 C, compared to their related non-fluorinated
analogues (Table 1).
The related BTU compounds, investigated in this study, with two alkyl chains having twelve
0
carbon atoms, display only a single SmC phase stable over 17 C temperature range on heating
run, while its analogue, the compound 1b shows both the SmA and SmC phases stable from
0
0
0
1
18 C to 157 C, about 39 C.
All other BTU compounds having two or three aliphatic chains show no mesogenic properties
and their thermal behavior resembles the behavior of non-fluorinated analogues. A significant
0
drop of clearing temperature, ranging from 55 to 123 C, was seen for these fluorinated
compounds, similar to their non-fluorinated related analogues. It seems that addition of more
aliphatic chains on the benzoyl moiety leads to unfavorable molecular shape which cannot be
overcome by the presence of perfluoroalkyl groups in order to generate stable liquid crystalline
properties. There is one exception, compound 2b, that displays a very short-range, monotropic
0
SmA phase, about 2 C, which could be identified by POM observations (see Figures S4c and d

in SI) just before crystallization process. Its shorter alkyl chain analogue, compound 2a, didn’t
show the same behavior, not even at lower cooling rates.
In order to unequivocally confirm the mesophase assignment and understand the packing model
within the mesophase, the mesogenic compounds, 1a and 1b, and two other non-mesomorphic
compounds, 2a and 2b, with two alkoxy tails were selected as representative examples to be
subjected to variable temperature powder X-ray diffraction measurements. Powder X-ray
diffraction studies were performed by heating first to isotropic state and cooling back to
mesophase and crystalline state for all the selected samples. By recording the powder X-rays
diffractograms within the mesophase temperature ranges, typical patterns that are characteristic
of a lamellar packing were measured. For instance, in the mesomorphic temperature range, the
diffractograms of mesomorphic BTU 1a and 1b show a strong maximum from the (001)
reflection followed by additional sharp peaks with d-spacing ratio of d/2 and d/3 and from the
(
002), and (003) reflections, respectively, confirming a long-range ordering in the LC phase
Figure 3).
(
Additionally, the mesomorphic BTU compounds exhibit one diffuse signal with the maximum
located around 4.5 Å corresponding to the lateral short-range order of the molten alkyl chains.
Simple molecular calculations based on an all-trans extended model of 1a and 1b revealed
molecular lengths of 35.6 Å for the shorter alkyl chains 1a to 37.1 Å for the longer alkyl chains
1
b. These results show slightly higher values of molecular lengths for BTU compounds
compared to each of the experimental d-spacings, 32.5 Å for 1a and 34.2 Å for 1b, respectively,
0
as determined by powder-XRD studies at 120 C. Based on POM observations and DSC
0
measurements, both compounds, 1a and 1b, show a SmC phase at 120 C, and these results are in
agreement with a molecular packing of tilted molecules at a certain angle. These XRD results
suggest a monolayer organization for the smectic phases and no segregation of the fluorocarbon
and hydrocarbon chains into separate layers. The segregation of fluorocarbon and hydrocarbon
chains is reduced due to an antiparallel packing of molecules and it was commonly seen for
compounds having alkyl and perfluoroalkyl chains at opposite ends of a rod-like core.[9, 53] As
a consequence of reduced fluorophobic effect and lack of segregation, no broad signal assigned

to lateral interactions of fluorocarbon chains, around 5.5 Å, was found in the X-rays
diffractograms.
The XRD patterns of the compounds 2a and 2b, recorded on cooling from the isotropic phase,
contain a series of diffraction Bragg peaks which is indicative of a crystalline state (Figure S5 b
in SI).
The thermal stability of these new compounds was studied for the selected mesomorphic
0
compounds 1a and 1b, by mean of thermogravimetric analysis (TG) in the 25-550 C temperature
0
range (Figure S6 in SI). For both compounds, the decomposition started around 180 C,
temperature not too far from their izotropization values. From this point of view, the fluorinated
BTU compounds show lower thermal stability compared to their non-fluorinated analogues.[32]
It is clear that these two compounds cannot be heated above the izotropization values without
altering their properties, e.g. transition temperatures between mesophase and isotropic state.
0
These results could explain the slight shift of 1-2 C towards lower values of transition
temperatures recorded by DSC in the subsequent heating-cooling cycles for these two samples.
Conclusions
A series of N-benzoyl-N’-aryl thiourea derivatives bearing different number of alkoxy groups in
terminal positions on one side and a perfluorooctyl group on the other side have been
synthesized and characterized structurally by a combination of spectroscopic techniques.
The thermal behavior was investigated by a combination of DSC, polarizing optical microscopy
and powder X-ray diffraction. Only compounds having only one alkoxy chain and a
perfluorooctyl group show mesogenic properties, with smectic A and C phases being displayed.
The comparison of the fluorinated compounds 1a and 1b with the non-fluorinated analogues
revealed that the melting points, clearing points and the mesophase ranges are significantly
increased with the incorporation of perfluoroalkyl chains. The clearing points for both
0
fluorinated compounds 1a and 1b are higher, with almost 40 C, compared to their related non-
fluorinated analogues. The stability of the smectic phase is higher when longer alkyl chains are
present in the molecule. Additional alkoxy groups on the benzoyl moiety (two or three aliphatic
chains) led to a significant decrease of the clearing points and non-mesogenic properties; their
thermal behavior resembles the behavior of non-fluorinated analogues.

Acknowledgements
This work was supported by a grant of the Romanian Authority for Scientific Research, CNCS-
UEFISCDI, project number PN-II-ID-PCE-2011-3-0384. I.P. wishes to thank to Core
Programme PN 16 48-3/2016 for funding.
Appendix: Supplementary data
Supplementary data with this article (experimental details, POM textures, XRD patterns at
1
13
different temperatures, DSC traces, H- and C-NMR spectra and TG curves) can be found in
the online version.
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[
(

SmC-SmA
20
40
60
80
100
120
140
160
o
T( C)
Figure 1. The first heating-cooling DSC trace of compound 1b.
(a)
(b)
Figure 2. POM pictures of the mesophases of liquid crystalline compounds as observed on
0
cooling from the isotropic state: (a) fan shape texture of the SmA phase of 1a at 167 C; (b)
0
broken fan shape texture of the SmC phase of 1a at 125 C.

60000
50000
40000
30000
20000
10000
0
o
1
30 C
o
1
25 C
o
1
20 C
o
1
10 C
0
5
10
15
20
25
30
2
/deg
Figure 3. Variable-temperature powder X-rays diffractogram for the mesomorphic compound
a.
1
(
H)O
H)O
H)O
R1
R3
R1
R3
CnH2n+1Br
HCl
(
COOCH3
R2
COOCH3
R2
COOH
K2CO3/MEK
EtOH
(
R1
R2
R1
H2N
C8F17
SOCl2
DCM
KSCN
COCl
R2
CONCS
acetone
acetone
R3
R3
R1
R2
R3
H
H
N
N
O
S
C8F17
1: R1 = R3 = H, R2 = -OCnH2n+1, n = 10 (a); n = 12 (b)
2: R1=H, R2 = R3 = -OCnH2n+1, n = 9, (a); n = 12 (b)
3: R1 = R2 = R3 = - OCnH2n+1, n = 11, (a); n = 14 (b)
H2n+1CnO
H
H
N
N
O
S
OCnH2n+1
4: n = 10 (a); n = 12 (b)

Scheme 1. Synthesis of N-benzoyl-N’-aryl thiourea perfluoroalkyl terminated derivatives
BTU).The non-fluorinated compounds 4a and 4b were reported elsewhere.[32]
(
Table 1. Thermal parameters for fluorinated N-benzoyl-N’-aryl thiourea derivatives (first
heating-cooling cycle)
_
_____________________________________________________________________________
0
.
-1 a
Compound
_
1
Transitions, T/ C (H/kJ mol )
_____________________________________________________________________________
st
a
b
a
1 heating: Cr 91 (10.0) Cr’ 115 (25.2) SmC 127 (0.3) SmA 166 (6.5) Iso
st
1
2
2
cooling: Iso 166 (6.0) SmA 126 (0.2) SmC 104 (21.1) Cr’
heating: Cr’ 111 (20.6) SmC 126 (0.3) SmA 164 (6.0) Iso
cooling: Iso 163 (5.8) SmA 125 (0.2) SmC 101 (19.2) Cr’
nd
nd
st
1
2
1 heating: Cr 118(35.0) SmC 127(0.4) SmA 157(6.0) Iso
st
1
2
2
cooling: Iso 154(5.8) SmA 126(0.3) SmC 107(23.7) Cr 70(10.8) Cr’
heating: Cr’ 73 (12.1) Cr 116 (29.4) SmC 126 (0.4) SmA 154 (5.8) Iso
cooling: Iso 153 (5.5) SmA 125 (0.3) SmC 105 (22.0) Cr 69 (10.9) Cr’
nd
nd
st
1 heating: Cr 82 (4.3) Cr’ 106 (37.2) Iso
st
1
2
2
cooling: Iso 98 (39.6) Cr’ 80 (5.0) Cr
heating: Cr 82 (4.8) Cr’ 105 (36.5) Iso
cooling: Iso 98 (40.0) Cr’ 80 (5.0)
nd
nd

st
2
3
3
b
a
b
1 heating: Cr 80 (7.1) Cr’ 112 (19.6) Iso
st
b
1
2
2
cooling: Iso 108 SmA 106 (8.5) Cr” 90 (19.0) Cr’ 60 (2.8) Cr
heating: Cr 74 (2.9) Cr’ 92 (14.8) Cr” 121 (3.9) Iso
cooling: Iso 108 (11.5) Cr’ 81 (3.6) Cr
nd
nd
st
1 heating: Cr 45 (41.9) Iso
st
1
2
2
cooling: Iso 43 (10.4) Cr
heating: Cr 45 (41.2) Iso
cooling: Iso 43 (10.3)
nd
nd
st
1 heating: Cr 39 (35.7) Cr’ 58 (57.0) Iso
st
1
2
2
cooling: Iso 30 (21.2) Cr
heating: Cr 58 (56.8) Iso
cooling: Iso 30 (21.0) Cr
nd
nd
st
4
4
ac
bc
1 heating: Cr 113 (50.4) SmA 128 (7.6) Iso
st
b
1
cooling: Iso 122 (7.5) SmA 118 SmC 93 (55.9) Cr
st
1 heating: Cr 104 (32.6) SmC 121 (5.9) Iso
st
1
cooling: Iso 121 (6.2) SmC 90 (34.2) Cr
_
_____________________________________________________________________________
a
Cr, Cr’, Cr” = crystalline phases, Iso=isotropic phase, SmA, SmC = smectic A and C phases;
b
The Iso-SmA and SmA-Cr” transitions for 2b or SmA-SmC and SmC-Cr for 4a could not be
c
separated in the DSC and the given value represents the combined enthalpies; data taken from
reference [32].