Synthesis and characterization of bent-rod liquid crystals

Article abstract of DOI:10.3390/50300383

Two series of diesters with bend-rod shapes were synthesized and their thermal properties characterized by POM and DSC. The central group conformational mobility and polarity as well as the length of the mesogenic groups were varied in order to correlate these parameters with mesophase stability. Results indicate that series II diesters are enantiotropic and that their mesophase sensitivity to central group structural changes is limited.

Full text of DOI:10.3390/50300383

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Molecules 2000, 5
Synthesis and Characterization of Bent-rod Liquid Crystals
Pablo Del Rosso, Sandra A. Hernandez and Raúl O. Garay*
INIQO, Universidad Nacional del Sur, Avenida Alem 1253, 8000 Bahía Blanca, Argentina
Tel/Fax: +54 (291)-459-5187, E-mail: rgaray@criba.edu.ar
* Author to whom correspondence should be addressed.
Abstract: Two series of diesters with bend-rod shapes were synthesized and their thermal
properties characterized by POM and DSC. The central group conformational mobility and
polarity as well as the length of the mesogenic groups were varied in order to correlate these
parameters with mesophase stability. Results indicate that series II diesters are enantiotropic
and that their mesophase sensitivity to central group structural changes is limited.
Keywords: liquid crystals, bend-rod, diesters.
Introduction
Most of the liquid crystals synthesized to present have the conventional anisodiametric rod-like
structure. As yet, only this conventional type of liquid crystals have been used in technological appli-
cations. Liquid crystals with non-conventional structures that apart from the lineal geometry are im-
portant for the theoretical understanding of the liquid crystalline phenomena and they have potential
new applications. It was found recently that two achiral bent-rod shape liquid crystalline systems
spontaneously generate helical structures in smectic arrangements [1]. Likewise, nematics arrange-
ments of this type of mesogens are also interesting while the origin of this ferroelectric behavior is still
under investigation [2]. The goal of this study was the synthesis and characterization of the mesomor-
fic properties of a series of diesters were the length of the mesogenic groups as well as polarity and
conformational mobility of the central group was varied as shown in Scheme 1.
O
O
O
C O
O C
O
Ar
Ar
G
C
F3
C
CF3
O
H3
CH3
serie I Ar =
serie II Ar =
C
S
b =
d =
C
a =
c =
G
O
S
Scheme 1.

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Molecules 2000, 5
Experimental
The diesters were synthesized by reaction of either 4-n-octiloxybenzoyl choride or 4’-n-
octiloxybifenil-4-carbonyl chloride with the corresponding diphenols at near reflux temperature in
pyridine as shown in Scheme 2. Structural characterization and purity checks of the diesters were per-
form by tlc and 1H RMN. Thermal transitions were characterized by differential scanning calorimetry,
DSC, (scanning rate 10ºC/min, temperature range: 50-260ºC) and polarizing optical microscope, POM
(Temperature range: r. t.-260ºC).
COOH
COCl
G
a) H+/MeOH
b) K2CO2/ DMF/C8H17Br
HO
OH
IIa-d
c) NaOH/EtOH
d) SOCl2
o
pyridine, 100 C
OH
OC8H17
Scheme 2.
Results and Discussion
The mesogen deviation from the preferential geometry to generate mesophases due to the introduc-
tion of a non lineal central group, G, does not preclude mesomorphic organisation in the heating cy-
cles, HC, or cooling cycles, CC, as long as the mesogenic arms have a minimum length. Thus, none of
series I compounds showed mesomorfic properties. On the contrary, all series II compounds are enan-
tiotropic liquid crystals. The preliminary results indicate that the mesophase stability (Ti). is not very
sensitive to variations either in the angle or in the polarity of G. Likewise, mesophase range (Ti-Tm)
present only small variations.
Thermal Properties of diesters II
IIb
175
260
259
170
IIa
182
249
248
180
IIc
175
245
244
170
IId
160
240
220
175
HC
CC
Tm
Ti
Td
Tc
Acknowledgements: Financial support for this research was provided by ANPCyT and SGCyT-UNS.
P. D. R. thanks SGCyT-UNS for a fellowship.

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Molecules 2000, 5
References and Notes
1. Pelzl, G.; Diele, S.; Jakli, A.; Lischka, Ch.; Wirt, I.; Weissflog, W. Helical superstructures in
novel smectic mesophase formed by achiral banana-shaped molecules. Liquid Cristals 1999, 26,
135.
2. Kishikawa, K.; Harris, M. C.; Swager, T. M. Nematic Liquid crystals with bent-rod shapes:
mesomorphic thiophenes with lateral dipole moments. Chem. Mater. 1999, 11, 867.