Influence of bromoalkyloxy side chain on mesomorphic behavior in heterocyclic 7-(4-bromoalkyloxy)-3-(4′-decyloxyphenyl)-4H-1-benzopyran-4- ones

Article abstract of DOI:10.1016/j.cclet.2011.03.006

A new homologous series of isoflavone-based ethers, 7-(4-bromoalkyloxy)-3- (4′-decyloxyphenyl)-4H-1-benzopyran-4-ones were synthesized and characterized. The mesomorphic properties of all homologues were investigated by differential scanning calorimetry (

Full text of DOI:10.1016/j.cclet.2011.03.006

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 947–950
www.elsevier.com/locate/cclet
Influence of bromoalkyloxy side chain on mesomorphic
behavior in heterocyclic 7-(4-bromoalkyloxy)-3-(40-
decyloxyphenyl)-4H-1-benzopyran-4-ones
a
a
Guan Yeow Yeap a, , Tze Nee Chan , Wan Sinn Yam ,
*
Daisuke Takeuchi b, Masato M. Ito c
a Liquid Crystal Research Laboratory, School of Chemical Sciences, University Sains Malaysia, 11800 Minden, Malaysia
b Chemical Resources Laboratory (Mailbox R1-03), Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
c Department of Environmental Engineering for Symbiosis, Faculty of Engineering, Soka University, Hachioji, Tokyo 192-8577, Japan
Received 16 November 2010
Available online 18 May 2011
Abstract
A new homologous series of isoflavone-based ethers, 7-(4-bromoalkyloxy)-3-(40-decyloxyphenyl)-4H-1-benzopyran-4-ones
were synthesized and characterized. The mesomorphic properties of all homologues were investigated by differential scanning
calorimetry (DSC) and polarizing optical microscopy (POM). Enantiotropic smectic A (SmA) phase was observed for all
homologues. The structure–property study was carried out by comparing the mesomorphic behavior of the homologues with those
previously reported analogues. The bromine atom from the alkyloxy side chain of the benzene ring fused to the heterocyclic moiety
was found to be capable of changing the mesomorphic properties.
# 2011 Guan Yeow Yeap. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Heterocyclic; Isoflavone; Enantiotropic
Over the years, the classical calamitic isoflavone-based thermotropic liquid crystals consisting of a heterocyclic
ring within the central core and one or two terminal chains have been studied and reported [1–3]. In order to study and
understand the effect of different structural changes on the mesomorphism of these compounds, various modifications
on existing molecules have been made keeping in view the molecular geometry of the isoflavone moiety [4,5]. The
influence of different functional linkages, terminal chain length and polar terminal groups on the mesogenicity of
liquid crystalline compounds has been reported [6–8]. Many hitherto reported works focused on the influence of polar
lateral and terminal groups in the central moiety rather than in the terminal alkyloxy chain. Hence, we report herein the
synthesis, characterization and effect of terminal bromine atom from the alkyloxy side chain (which is rare, at least to
our knowledge) on the mesomorphic properties of newly derived isoflavone ethers, 7-(4-bromoalkyloxy)-3-(40-
decyloxyphenyl)-4H-1-benzopyran-4-ones 1–5. The synthetic routes toward the formation of all intermediates and
title compounds are shown in Scheme 1 and their structures were elucidated by FT-IR and FT-NMR spectroscopic
techniques [9].
* Corresponding author.
E-mail address: gyyeap@usm.my (G.Y. Yeap).
1001-8417/$ – see front matter # 2011 Guan Yeow Yeap. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.03.006

9[()TD$FIG] 48
G.Y. Yeap et al. / Chinese Chemical Letters 22 (2011) 947–950
OH
OC
H
10 21
HO
OH
O
(a)
HO
OH
(b)
+
COOH
COOH
OC
H
10 21
(c)
BrCnH nO
O
O
HO
O
O
2
(d)
n=4, 6, 8, 10, 12
OC10H21
OC
H
10 21
Scheme 1. Reactions and reagents. (a) KOH (2 equiv.), 1-bromodecane (1.2 equiv.), refluxing in methanol, 18 h; yield: 52%; (b) resorcinol
(1.1 equiv.) in BF3/Et2O, N2, 70–75 8C, 4 h; yield: 60%; (c) BF3/Et2O (4 equiv.) in DMF, N2, 55 8C, 1 h; then in MeSO2Cl (3 equiv.), N2, 85 8C,
1.5 h; yield: 63%; (d) dibromoalkane, BrCnH2nBr (5 equiv.) (n = 4, 6, 8, 10, 12), K2CO3 (2 equiv.), refluxing in acetone, 6 h; yield: 70–78%.
Table 1
Phase transition temperatures and enthalpy change for compounds 1–5 upon heating and cooling.
Compound
Phase transition, 8C (enthalpy change, kJ molꢀ1
)
1 (n = 4)
Cr 105.3 (33.5) Cr2 118.3 (29.0) SmA 169.7 (11.0) I
I 167.0 (10.8) SmA 108.2 (30.6) Cr2 98.4 (33.6) Cr
Cr 101.9 (55.0) SmA 152.8 (8.4) I
2 (n = 6)
3 (n = 8)
4 (n = 10)
5 (n = 12)
I 149.3 (8.8) SmA 80.4 (52.5) Cr
Cr 108.0 (42.9) SmA 140.9 (10.5) I
I 137.5 (8.8) SmA 79.7 (46.2)
Cr 111.6 (49.9) SmA 131.4 (10.4) I
I 128.1 (9.9) SmA 91.7 (51.8) Cr
Cr 102.9 (7.4) Cr2 111.8 (44.3) SmA 125.8 (8.0) I
I 122.9 (7.6) SmA 85.0 (49.2) Cr
Cr, crystal; SmA, smectic A; I, isotropic.
The phase transition temperatures and associated enthalpy changes of compounds 1–5 were measured by
differential scanning calorimetry (DSC). The results obtained were listed in Table 1. The mesomorphic textures were
determined by employing the polarizing optical microscope equipped with a hot stage and temperature controller.
Phase identification was made by comparing the textures observed with those reported in the literature [10,11].
Observation under crossed polarizers showed that regardless of the brominated alkyloxy chain length, all homologues
exhibited enantiotropic SmA phase which can be characterized by the formation of batonnets that coalesce to form
focal conic fan-shaped textures. This suggests that the molecules are arranged in orthogonal layers. A typical
morphology of these compounds can be shown by compound 2 in Fig. 1.
The mesomorphic behavior exhibited by this system was compared to those of the reported homologous series I [4]
and II [5] in order to understand the structure–property relationships of these compounds. All homologues of this series
and those of series I and II have a heterocyclic isoflavone moiety in the central core. The difference of structures between
this series and series I can be ascribed to the presence of brominated alkyloxy chain in n-alkyloxy chain attached to the
benzene ring fused with the heterocyclic moiety (ring A) whilst the present series differs from series II in term of the
geometry of the terminal chain attached to the free benzene ring (ring B) (Fig. 2). Nematic (N) and smectic C (SmC)
phases were observed in addition to SmA phase in series I. The introduction of a Br atom in place of H atom (present
series) or a branched methyl group (series II) in the flexible terminal chain gives rise to a change in the mesomorphic
behavior as compared to series I. Homologues in the present series exhibit monomorphic SmA phase independent of
chain length. Since the mesomorphic properties of a compound are a consequence of the balance between the attractive
(due to polarizability andpolarity) andrepulsive forces (governed bysteric effect), thepresenceof a polar andpolarizable

[()TD$FIG]
G.Y. Yeap et al. / Chinese Chemical Letters 22 (2011) 947–950
949
Fig. 1. Focal conic fans of SmA phase exhibited by compound 2.
[()TD$FIG]
BrC
H
O
2n
O
n
A
A
C
(Present series)
B
O
O
C
OC 10 H 21
C O
H
H
2n+1
n
(I)
B
O
OC
H
10 21
C
O
n
O
2n+1
C
A
(II)
B
O
n=4, 6, 8, 10, 12
OCH CH CH(CH )CH
3
2
2
3
Fig. 2. Comparison between the molecular structures of compounds 1–5 and their analogues.
Br atom facilitates lamellar packing (due to polarizability and polarity) of molecules leading to the formation of SmA.
However, this phenomenon also prevents and spatially destabilizes the tilted arrangement of molecules (due to steric
effect when molecules are tilted) which could lead to the formation of SmC phase. A reduced compatibility of geometry
when the terminal chain is branched (series II) may disrupt the tilted arrangement that is essential for the formation of
SmC phase. The average smectic thermal stability of the present series is similar to that of series I. Large mesomorphic
ranges (>30 8C) and high clearing temperatures (>120 8C) were recorded for compounds in both series. However, the
average mesomorphic range of series II (<10 8C) is much narrower than the two formerly mentioned series which can be
rationalized in term of a reduced surface area for intermolecular van der Waals interactions when one of the terminal
chains is branched which results in a decreased stability of the mesophase [5].
A graphical presentation of the clearing temperatures as a function of the number of carbon atoms in the brominated
alkyloxy terminal chain for the present series is shown in Fig. 3. There is a gradual decrease in clearing temperatures as
the length of the brominated alkyloxy side chain increases in which the homologue having the longest side chain
possesses lowest thermal stability. This phenomenon can be explained in term of dilution effect of the flexible chains
as reported for a series of laterally branched azobenzenes [13]. This observation is also in good agreement with the
previously reported homologous series, N,N0-bis(3-methoxy-4-alkoxybenzylidene)-1,4-phenylenediamines [14]. It
suggests that the flexibility of the long terminal chain prevents the long molecular axis from remaining in a fully
stretched geometry but tends to coil resulting in the non-linearity of the molecule when the terminal chain length
increases [12]. Furthermore, this could probably be due to the repulsive (steric) forces leading to larger intermolecular
distance. This can be considered as a consequence of deviation from linearity of the terminal alkyloxy chains with
increasing length.

9[()TD$FIG] 50
G.Y. Yeap et al. / Chinese Chemical Letters 22 (2011) 947–950
Fig. 3. Plot of clearing temperatures of compounds 1–5 as a function of the number of carbon atoms.
In conclusion, all homologues of this series exhibit stable, enantiotropic liquid crystalline smectic A phase which
was found to be independent of side chain. The stability of the mesophase is attributable to the polar Br atom at the end
of the alkyloxy chain together with the planarity and enhanced polarity of the oxygen atoms of the ether linkages as
well as the heterocyclic central moiety.
Acknowledgments
The authors would like to thank Universiti Sains Malaysia for USM RU Grants (No. 1001/PKIMIA/811127 and
1001/PKIMIA/811140). Chan T.-N. would like to acknowledge USM for the research fellowship.
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´
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