Nematogenic and smectogenic liquid crystals from new heterocyclic isoflavone derivatives: Synthesis, characterization and X-ray diffraction studies

Article abstract of DOI:10.1016/j.cclet.2010.09.018

A new series of liquid crystals comprising eight heterocyclic isoflavone esters, 7-alkanoyloxy-3-[4′-(3-methylbutyloxyphenyl)]-4H-1-benzopyran-4- ones exhibiting enantiotropic nematic (N) and smectic C (SmC) phases were synthesized and investigated. The m

Full text of DOI:10.1016/j.cclet.2010.09.018

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 229–232
www.elsevier.com/locate/cclet
Nematogenic and smectogenic liquid crystals from new heterocyclic
isoflavone derivatives: Synthesis, characterization and X-ray
diffraction studies
*
Wan Sinn Yam , Guan Yeow Yeap
Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, Minden 11800, Malaysia
Received 26 March 2010
Abstract
A new series of liquid crystals comprising eight heterocyclic isoflavone esters, 7-alkanoyloxy-3-[40-(3-methylbutyloxyphenyl)]-
4H-1-benzopyran-4-ones exhibiting enantiotropic nematic (N) and smectic C (SmC) phases were synthesized and investigated. The
mesomorphic properties of all derivatives were investigated by means of differential calorimetry and polarized optical microscopy.
Wide angle X-ray diffraction technique was employed to investigate the molecular packing associated with the intermolecular
interaction as well as the correlation between the thermal behaviour of these compounds with their anisotropy properties within a
mesophase.
# 2010 Published by Elsevier B.V. on behalf of Chinese Chemical Society.
Keywords: Heterocyclic; Isoflavone; Enantiotropic; Mesophase
It has been well documented that the study of liquid crystalline materials is very important for the continual
development and understanding of the field of molecular engineering [1,2]. The design and synthesis of thermotropic
liquid crystals (LCs), especially noteworthy in case of conventional calamitic (rod-like) LCs, discotic (disc-like)
mesogens, and to some extents, polymeric LCs have attracted much interest from both fundamental research and
practical application points of view, thus emerged as an area of active research [3–5]. Research focused on modifying
existing molecules, particularly natural products, has shown to be a viable approach leading to new compounds
showing liquid crystalline properties [6]. Our present interest on new natural product-based heterocyclic mesogens is
focused on isoflavone derivatives. Isoflavones are water-soluble chemicals found in many plants. They comprise a
class of often naturally occurring organic compounds related to the flavonoids and their derivatives constitute the
largest of compounds among the natural isoflavonoids [7,8]. The mesogenicity of several isoflavone derivatives as
potential mesogens and their ability to form calamitic LCs of which many were polymorphic, exhibiting wide
thermomorphic ranges have been studied and reported in recent years [9,10]. The introduction of heterocyclic rings
within the central core and the linking groups between the middle and the terminal fragments have claimed to be
responsible for the liquid crystalline behaviour of classical calamitic mesogens leading to various mesomorphism.
* Corresponding author.
E-mail address: wansinn@usm.my (W.S. Yam).
1001-8417/$ – see front matter # 2010 Published by Elsevier B.V. on behalf of Chinese Chemical Society.
doi:10.1016/j.cclet.2010.09.018

2[()TD$FIG] 30
W.S. Yam, G.Y. Yeap / Chinese Chemical Letters 22 (2011) 229–232
COOH
COOH
HO
OH
(b)
(a)
O
2
OR
OR
1
OH
(c)
HO
O
R'
O
O
O
(d)
O
O
3
4-11
OR
OR
R' = C n H2n+1 ; R = OCH 2 CH 2 CH(CH 3 )CH
3
Compounds : 4 (n=3), 5 ( n=5), 6 ( n=7), 7 ( n=9), 8 ( n=11), 9 ( n=13), 10 ( n=15), 11 ( n=17)
Scheme 1. Reactions and reagents. (a) KOH (2 equiv), 1-bromo-3-methylbutane (1.1 equiv), refluxing in methanol, 18 h; (b) resorcinol (1.1 equiv)
in BF3/Et2O, N2, 70–75 8C, 4 h; (c) BF3/Et2O (4 equiv) in DMF, N2, 55 8C,1 h; then in MeSO2Cl (3 equiv), N2, 85 8C, 1.5 h; (d) acid chlorides,
R0COCl (1.1 equiv) (R0 = CnH2n+1; n = 3, 5, 7, 9, 11, 13, 15, 17); triethylamine (catalytic), stirred in dry DMF/CH2Cl2, N2, rt, 6 h.
However, the study on the relationship between the difference in phase behaviour of this type of compounds and their
molecular structure particularly on the correlation study between the molecular anisotropy and to the mesogenic
behaviour of these compounds using X-ray diffraction was not well documented. As such, we are prompted to carry
out a comprehensive study on isoflavone-cored mesogens covering both aspects. In this paper, we describe the
synthesis, characterization and mesomorphic properties of a new series of calamitic isoflavone ester, 7-alkanoyloxy-3-
[40-(3-methylbutyloxyphenyl)]-4H-1-benzopyran-4-ones, 4–11 with various alkanoyloxy chain lengths in odd-parity.
The synthetic routes toward the formation of the intermediates and title compounds are shown in Scheme 1 and their
structures were elucidated via elemental analysis, FT-IR and NMR spectroscopic techniques [11].
The mesomorphic behaviour of compounds 4–11 was characterized and studied using differential scanning
calorimetry (DSC) and polarized optical microscope (POM). Their phase transition temperatures and associated
enthalpies were collated in Table 1. All derivatives exhibited enantiotropic mesophases regardless of the alkyl chain
length. Derivatives with short alkyloxy chains, 4 (n = 3) and 5 (n = 5) are nematogenic. Observation under crossed
Table 1
Phase transition temperatures and enthalpy change for compounds 4–11 during heating.
Compound
Phase transition
Temperature (8C)
Enthalpy change (kJ molꢀ1
)
4
Cr–N
124.0
140.0
126.0
132.0
119.0
133.0
109.0
135.0
135.0
139.0
100.0
129.0
96.0
32.4
0.8
N–I
5
Cr–N
29.6
0.9
N–I
6
Cr–SmC
SmC–I
Cr–SmC
SmC–I
Cr–SmC
SmC–I
Cr–SmC
SmC–I
Cr–SmC
SmC–I
Cr–SmC
SmC–I
24.8
5.0
7
27.8
6.7
8
35.5
7.1
9
29.0
7.5
10
11
27.6
7.9
131.0
94.0
31.9
7.7
123.0
Cr, crystal; N, nematic; SmC, smectic C; I, isotropic.

[()TD$FIG]
W.S. Yam, G.Y. Yeap / Chinese Chemical Letters 22 (2011) 229–232
231
Fig. 1. (a) Compound 5 displaying marble texture of the N phase upon heating. (b) Schlieren texture co-exists with broken focal-conic fans of SmC
phase exhibited by compound 6.
polarizers of compound 5 showed marble texture (Fig. 1a) The N phase for these homologues is thermally stable with
mesomorphic ranges of 16 8C and 6 8C, respectively at high clearing temperatures (exceeding 130.0 8C).
However, the N phase is not detectable in other derivatives of which the number of carbon atoms in the odd-parity
terminal chains, n ꢁ 7. This observation can be rationalized in terms of the presence of long terminal alkyl chains
which become interwined that leads to tilted and disordered lamellar packing as commonly found in SmC phase [12].
SmC phase exhibited by compounds 6–11 (n = 7, 9, 11, 13, 15 and 17) can be characterized by the formation of
batonnets that coalesce to form the less crisp, broken fan-shaped textures. A typical morphology of these compounds
can be shown by compound 6 in Fig. 1b of which the Schlieren texture co-exists with some fan-shaped domains which
are due to inhomogeneity in alignment of the sample.
It can be generalized that the clearing temperatures are found to increase with the length of the alkyloxy chains in
odd parity. This may probably be attributed to an enhanced dispersive interaction or van der Waals interaction between
the terminal alkyloxy chains. However, the melting temperatures of compounds 9–11 is found to be lower than those of
compounds 4–8 which could probably be due to the repulsive (steric) forces leading to larger intermolecular distance,
a consequence of deviation from linearity of the terminal alkyloxy chains in compounds 9–11. Although the molecular
structures of compounds 4–8 may be fully stretched, the long molecular axis of compounds 9–11 could be distorted
from linearity. Furthermore, the flexibility of long alkyloxy chains tends to disrupt molecular packing thus facilitating
molecular tilting and stabilizes the SmC phase formation when coupled with enhanced intermolecular forces of
attraction associated with the ester and ether linkages of the terminal chains. Hence, the molecules are assumed to be
able to pack effectively in a lamellar manner and in space leading to higher sustainability of SmC phase [13]. This is
indicated by a rather broad thermal mesomorphic range (ꢂ20.0 8C), high clearing temperatures (>100 8C) and large
isotropic transition enthalpies (>6 kJ/mol) for all derivatives.
The powder X-ray diffraction measurements were performed on compound 6 to confirm the structure of SmC phase
[12]. As can be seen in Fig. 2, the layer spacing values, d, of compound 6 in SmC phase are almost independent on
˚
˚
temperature. Upon cooling, the layer spacing decreased from 27.5 A at 130.0 8C and reached a minimum at 27.4 A
˚
around 110.0 8C, then it increased slightly and reached a maximum of 27.7 A upon crystallization at 75.0 8C. The
layer spacing of compound 6 was 0.8–1.1 A shorter than that of its effective molecular length, l (28.5 A) estimated
[()TD$FIG]
˚
˚
27.75
27.70
27.65
27.60
27.55
27.50
27.45
27.40
27.35
SmC phase
70
80
90
100
110
120
130
T / oC
Fig. 2. Layer spacing of compound 6 during a transition from SmC phase to crystal phase upon cooling.

232
W.S. Yam, G.Y. Yeap / Chinese Chemical Letters 22 (2011) 229–232
using CCDC Mercury 1.4.2 version. The observation in layer spacing reflected the conformations of molecules in
crystal and SmC phase, respectively. The molecules thus assumed to be almost fully stretched in crystal phase but are
slightly tilted in SmC phase. However, the tilt angle in SmC phase cannot be determined due to a first order transition, a
transition that involved a discontinuity in entropy, which is the first derivative of the Gibbs energy function [14,15].
In conclusion, all the target compounds exhibited stable, enantiotropic liquid crystalline nematic and smectic C
phases. The mesophases were found to be side chain dependent; compounds 4 (n = 3) and 5 (n = 5) were nematogenic
whilst compounds 6–11 (n = 7, 9, 11, 13, 15, 17) were smectogenic. The planarity and enhanced polarity of the oxygen
atoms of the ester and ether linkages as well as the heterocyclic central moiety may be attributed to the stability of the
mesophases.
Acknowledgments
The authors would like to thank Universiti Sains Malaysia for USM RU (No. 1001/PKIMIA/81140). Prof. Masato
M. Ito of Department of Environmental Engineering for Symbiosis, Faculty of Engineering, Soka University for
carrying out the DSC measurements and Prof. Ewa Gorecka of Dielectrics and Mechanics Department, Warsaw
University for carrying out the powder X-ray diffraction measurements and helpful discussions.
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