Liquid Crystals
ISSN: 0267-8292 (Print) 1366-5855 (Online) Journal homepage: https://www.tandfonline.com/loi/tlct20
Exploring the mesomorphic potential of 2,4disubstituted thiophenes: a structure–property
study
A. S. Matharu , P. B. Karadakov , S. J. Cowling , Gurumurthy Hegde & L.
Komitov
To cite this article: A. S. Matharu , P. B. Karadakov , S. J. Cowling , Gurumurthy Hegde &
L. Komitov (2011) Exploring the mesomorphic potential of 2,4-disubstituted thiophenes: a
structure–property study, Liquid Crystals, 38:2, 207-232, DOI: 10.1080/02678292.2010.539786
To link to this article: https://doi.org/10.1080/02678292.2010.539786
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Liquid Crystals,
Vol. 38, No. 2, February 2011, 207–232
Exploring the mesomorphic potential of 2,4-disubstituted thiophenes: a
structure–property study
A.S. Matharua*, P.B. Karadakova, S.J. Cowlinga, Gurumurthy Hegdeb and L. Komitovb
a
Department of Chemistry, University of York, Heslington,York, UK; b Department of Physics,
University of Gothenburg, Gothenburg, Sweden
(Received 5 October 2010; final version received 9 November 2010)
The synthesis and mesomorphic properties of 25 novel esters derived from 4-substituted thiophene-2-carboxylic
acids and either (S)-1-methylheptyl 4-hydroxybenzoate or (S)-1-methylheptyl 4 -hydroxybiphenyl-4-carboxylate
are described. A structure–property relationship has been deduced, and mesogenic properties are found to be
dependent on the total number of rings in the molecular core and the nature of the substituent in the 4-position of
the thiophene ring. Four-ring systems have greater thermal stability and are more likely to form mesophases than
their three-ring counterparts. A 1,4-phenylene substituent in the 4-position of the thiophene ring (Series I and II)
gives better thermal properties than does a 2,5-thienyl substituent (Series III and IV). A rationale is suggested in
terms of conjugative effects that may be important in mesophase thermal stability. A preliminary molecular modelling study (B3LYP/6–31G(d)) reveals that 2,4-disubstituted systems offer a slight preference for a folded geometry
structure compared with their 2,5-counterparts. Homologues (n = 7–10,12) in Series II show an interesting chiral
phase behaviour between the SmA and SmC∗ ferrielectric phase, tentatively assigned SmX∗ , which then gives a
SmC∗ antiferroelectric phase. Textures have been characterised by thermal polarising microscopy using a conventional glass slide cover-slip and free-standing film. Although the SmX∗ phase has still to be fully characterised, this
is the first reported occurrence of such a phase in 2,4-thiophene systems.
Keywords: 2,4-thiophene;
unknown SmX
synthesis;
characterisation;
1. Introduction
At the present time the molecular architecture of
many thermotropic liquid crystals is still based on
Vorländer’s century-old concept of a linear lath-like
geometry [1, 2]. As expected, many non-linear deviants
have since emerged, of which thiophene is of particular
interest both in terms of its ability to impart mesomorphic properties and its possible use in a polymeric
medium for optical data storage [3–6].
Thiophene, an aromatic five-membered heterocycle
containing sulfur, produces molecular architectures of
non-linear geometry, due to its propensity to undergo
aromatic electrophilic substitution in the C–2 and C–5
positions, giving an exocyclic bond angle of 147.5◦
(Figure 1). Despite a 32.5◦ deviation from linearity (1,4phenylene bond angle is 180◦) suitably 2,5-disubstituted
thiophenes are known to exhibit liquid crystalline
character and some have been mentioned by Seed
in his review of thermotropic liquid crystals derived
from thiophene and related five-membered heterocycles
[7]. Suitable disubstituted thiophenes are of particular
interest, since they tend to have lower melting points
than their non-heterocyclic counterparts, and the presence of a highly polarisable electronegative sulfur atom
∗ Corresponding author. Email: avtar.matharu@york.ac.uk
ISSN 0267-8292 print/ISSN 1366-5855 online
© 2011 Taylor & Francis
DOI: 10.1080/02678292.2010.539786
http://www.informaworld.com
structure–property;
antiferroelectric;
ferrielectric;
imparts a transverse dipole moment, avoiding the need
for additional lateral substituents.
In our previous work we have shown that, depending upon the disposition of the thiophene ring and the
total number and nature of additional rings within the
molecular core, esters derived from chiral 2-octanol
and comprising 2,5-disubstituted thiophene (Figure 2)
are liquid crystalline [8–11]. In certain cases suitably
2,5-disubstituted thiophene-based chiral esters comprising a (S)-1-methylheptyl 4-hydroxybenzoate or
(S)-1-methylheptyl 4 -hydroxybiphenyl-4-carboxylate
moiety exhibit SmC∗ ferroelectric, ferrielectric and
antiferroelectric phase character, providing potential
interest for use in electro-optical switching devices
[8–11].
Despite increasing reports in the literature of the
ability of thiophene to form liquid crystals, the use of
2,4-disubstituted thiophenes as potential liquid crystals remains relatively unexplored [7, 12]. This might
be due either to their difficult synthesis or because the
exocyclic bond angle for 2,4-disubstitution of 140.5◦
(see Figure 1) is even less conducive to mesophase formation than 2,5-disubstitution [13]. Seed has reported
an exocyclic bond angle of 131.5◦ [7].
208
A.S. Matharu et al.
2,5-
1,4-
In contrast to the 2,5-disubstituted thiophenes
reported previously, the present article describes
our results on the mesomorphic capability of compounds comprising a 2,4-disubstituted thiophene moiety and pendant 1-methylheptyl groupings, Series I–V
(Figure 3). The synthesis and characterisation of novel
isomeric 2,4-disubstituted thiophenes in Series I–V are
summarised, and the influence of 2,4-disubstitution on
mesomorphic properties in terms of the relationship
between molecular structure and the thermal stability
of the mesophases in discussed.
2,4S
S
180°
148°
140.5°
Figure 1. Exocyclic bond angles for 1,4-phenylene, 2,5thienyl and 2,4-thienyl.
CnH2n+1O
CnH2n+1
S
S
*
CO2
S
CO2CHC6H13
CH3
x
*
CO2
CO2CHC6H13
CH3
x
Refs [10] and [11]
Refs [8] and [9]
where x = 1 or 2
Figure 2. Examples of suitably 2,5-disubstituted thiophene esters comprising the 1-methylheptyl moiety.
S
CnH2n+1O
*
CO2
CO2CHC6H13
CH3
CO2
CO2CHC6H13
CH3
S
CnH2n+1O
*
S
CnH2n+1O
S
CnH2n+1
S
S
CO2
Series III
2,4-Isomer
Exchange phenyl ring LHS with
thiophene
*
CO2CHC6H13
CH3
S
*
CO2
S
CO2
C10H21O
Series I
2,4-Isomer
Series II
*
CO2CHC6H13 2,4-Isomer
Extra phenyl ring RHS
CH3
CO2
CnH2n+1
2,5-Parent
CO2CHC6H13
CH3
*
CO2CHC6H13
CH3
Series IV
2,4-Isomer
Exchange phenyl ring LHS with
thiophene and extra phenyl ring
RHS
Series V
2,4-Isomer
Extra phenyl ring LHS
Figure 3. Proposed systematic set of three- and four-ring structures (Series I-V) based on 2,4-disubstituted thiophenes, with
2,5-parent shown for reference.
Liquid Crystals
The molecular core of Series I varies in the substitution of the left-hand terminal 1,4-disubstituted
benzene ring on the 2,5-disubstituted thiophene ring,
and Series III enables investigation of the influence of
substituents in the heteroaromatic ring on mesomorphic properties. Additionally, the molecular core has
been extended from three to four rings by the inclusion of an additional 1,4-disubstituted benzene ring,
either on the right-hand side of the structure (Series II)
or on the left-hand side (Series V). Similarly, by the
inclusion of an extra phenyl ring on the right-hand
side Series III has been transformed into a four-ring
counterpart (Series IV). It was envisaged that due to
reduction in non-linearity the thermal stability of the
2,5-substituted compounds would be lower than that
of their 2,5-isomeric counterparts.
To the best of our knowledge there have been no
previous reports of the occurrence of SmC∗ ferro-, ferriand antiferroelectric phase types in 2,4-disubstituted
thiophenes containing a chiral 1-methylheptyl moiety.
Kiryanov et al. [14] have however reported the synthesis
of a 3-fluoro-2,4-substituted thiophene ester containing a pendant 1-methylheptyl moiety which melted to
an isotropic liquid at 77.9◦ C without mesomorphic
properties. We also wish to acknowledge the work of
Seed’s group at Kent State University, who are expected
in the near future to report their own findings on
2,4-disubstituted thiophenes [15].
2. Results and discussion
2.1 Synthesis of Series I, 9a–f and Series II,
10a–f (Scheme 1)
The synthetic route to the chiral thiophenebased esters, Series I, 9a–f, and Series II, 10a–f,
derived from the 4-(4-n-alkoxyphenyl)thiophene-2carboxylic acids, 7a–f, with either (S)-1-methylheptyl
4-hydroxybenzoate, 8a, or (S)-1-methylheptyl 4 hydroxybiphenyl-4-carboxylate, 8b, is illustrated in
Scheme 1.
The preparation of 4-bromothiophene-2-carbox
aldehyde, 2, was the key step in the synthesis of
Br
Br
i
S
Br
ii
CHO
S
1
iii
CHO
S
2
CnH2n+1O
209
CO2H
S
3
CO2CH3
4
CnH2n+1O
v
S
iv
CO2H
S
7a–f
CO2CH3
6a–f
CnH2n+1O
B(OH)2
5a–f
CnH2n+1 O
vi
S
HO
*
CO2 CHC6H13
CH3
x
8a (x = 1)
8b (x = 2)
CO2
*
CO2 CHC6H13
CH3
x
where x = 1, n = 6–10, 12; Series I, 9a–f;
x = 2, n = 6–10, 12; Series II, 10a–f
Scheme 1. Reagents and conditions: i. a. AlCl3 , CH2 Cl2 . b. Br2 ; ii. a. NaOH, AgNO3 . b. 4M HCl; iii. (CH3 )2 SO4 , acetone,
K2 CO3 , reflux; iv. Pd(PPh3 )4 , DME, 2M Na2 CO3 , reflux; v. a. ethanolic KOH, reflux. b. 4M HCl; vi. DCC, DMAP, CH2 Cl2 .
210
A.S. Matharu et al.
both Series I and Series II. Substituted thiophenes
are readily brominated, but purification can be troublesome due to the formation of regioisomers and
polybrominated derivatives. To control polybromination while regulating the position of substitution, 4-bromothiophene-2-carboxaldehyde was prepared by the methods reported by Goldfarb et al.
[16] and Chadwick et al. [17]. Swamping commercial 2-formylthiophene, 1, with an excess of anhydrous aluminium(III) chloride directs electrophilic
attack to C–4 in preference to C–3 and C–5,
which are deactivated, as confirmed by their canonical structures (Figure 4). Subsequent treatment of
the chloroaluminothiophene complex with elemental
bromine produces the required 4-bromothiophene-2carboxaldehyde, 2.
In the absence of anhydrous aluminium(III) chloride, a mixture of 4- and 5-bromothiophene-2carboxaldehyde resulted which was difficult to separate.
Oxidation of 2 (NaOH/AgNO3 ) furnished the
intermediate 4-bromothiophene-2-carboxylic acid, 3,
which was then converted in the presence of
dimethyl sulfate to the methyl ester, 4, in excellent yield (81%). Palladium-catalysed cross-coupling
of 4 with the appropriate 4-n-alkoxyphenylboronic
acids, 5a–f, yielded the required methyl 4-(4-nalkoxyphenyl)thiophene-2-carboxylates, 6a–f, in good
yield (62–74%). Hydrolysis of compounds, 6a–f, with
aqueous ethanolic KOH to the intermediate acids,
7a–f, followed by dicyclohexylcarbodiimide(DCC)mediated esterification [18, 19] with the appropriate chiral phenol, 8a or 8b, gave the desired chiral
thiophene-based esters, Series I, 9a–f and Series II,
10a–f, respectively.
2.2 Synthesis of Series III, 17a–f and Series IV,
18a–f (Scheme 2)
The preparation of the esters in Series III, 17a–f and
Series 1V, 18a–f, derived from 5 -n-alkyl-2,4-bithienyl2-carboxylic acids, 16a–f, and either (S)-1-methylheptyl
4-hydroxybenzoate, 8a, or (S)-1-methylheptyl 4 hydroxybiphenyl-4-carboxylate, 8b, is shown in
Scheme 2.
It had been hoped to be able to prepare members
of Series III and Series IV via boronic acid crosscoupling to provide the 2,4 -substituted bithiophenes,
O AlCl3
S
H
15a–f, as outlined in Figure 5, since the synthesis 5-nalkylthiophene-2-boronic acids had been reported in
good to high yield [12]. Despite numerous attempts
we were unable to produce the appropriate thiophene
boronic acids in satisfactory yield. We are also aware
that Gronowitz et al. [20] reported poor coupling of
thiophene-2-boronic acid and organic halides due to
proto-deboronation.
The Stille cross-coupling method [21] was therefore
employed to form the 2,4 -disubstituted bithiophene
moiety, 15a–f, as illustrated in Scheme 2. Titanium(IV)
chloride-mediated Friedel–Crafts acylation of commercial thiophene, 11, with the appropriate alkanoyl
chloride yielded the 2-n-alkanoylthiophenes, 12a–f, in
good yield (68–80%). The Huang Minlon modification
of the Wolff–Kishner reduction [22] of 12a–f gave the
intermediate 2-n-alkylthiophenes, 13a–f, which upon
lithiation using 2.5 M n-BuLi at 0◦ C, and quenching with tri-n-butyltin chloride at −78◦ C, yielded 2(tri-n-butyl)-5-n-alkylthiophenes, 14a–f, in high yield
(72–88%). Stille cross-coupling of compounds, 14a–f,
with methyl 4-bromothiophene-2-carboxylate, 4, in
the presence of Pd(PPh3 )4 as catalyst gave methyl
5 -n-alkyl-2,4-bithienyl-2-carboxylates, 15a–f, in good
to high yield (64–72%). Subsequent base hydrolysis of the methyl esters, 15a–f, using ethanolic
KOH gave the corresponding 5 -n-alkyl-2,4-bithienyl2-carboxylic acids, 16a–f, which on esterification
(DCC and 4-N,N-dimethylaminopyridine, DMAP)
with the appropriate chiral phenol, either 8a or 8b,
gave Series III, 17a–f and Series IV, 18a–f, respectively.
2.3 Synthesis of Series V, 22 (Scheme 3)
The synthetic route to the ester, 22 (Series V), derived
from 4-(4-n-decycloxybiphenyl-4-yl)thiophene-2-car
boxylic acid, 21, and (S)-1-methylheptyl 4-hydro
xybenzoate, 8a, is shown in Scheme 3. Palladiumcatalysed cross-coupling of 4 -n-decyloxybiphenyl-4boronic acid, 19, with methyl 4-bromothiophene-2carboxylate, 4, gave methyl 4-(4 -n-decyloxybiphenyl4-yl)thiophene-2-carboxylate, 20. Subsequent base
hydrolysis of 20 using ethanolic KOH gave the 4-(4n-decyloxybiphenyl-4-yl)thiophene-2-carboxylic acid,
22, which on DCC-mediated esterification with (S)-1methylheptyl 4-hydroxybenzoate, 8a, yielded the chiral
ester, 22.
O AlCl3
O AlCl3
S
S
H
O AlCl3
S
H
Figure 4. Canonical structures for the chloroaluminothiophene σ -complex.
H
Liquid Crystals
i
ii
CmH2m+1CO
S
where m = n–1
12a–f
S
11
211
CnH2n+1
S
13a–f
iii
Br
S
CO2CH3
CnH2n+1
iv
4
S
Sn(C4H9)3
14a–f
CnH2n+1
S
CO2CH3
S
15a–f
v
CnH2n+1
S
CO2H
S
16a–f
vi
HO
CnH2n+1
S
*
CO2 CHC6H13
CH3
x
8a (x = 1)
8b (x = 2)
*
CO2 CHC6H13
CH3
x
where x = 1, n = 6–10, 12: Series III, 17a–f;
x = 2, n = 6–10, 12: Series IV, 18a–f
S
CO2
Scheme 2. Reagents and conditions: i. TiCl4 , Cm H2m+1 COCl, CH2 Cl2 ; ii. NH2 NH2 .H2 O, KOH, dieethylene glycol, reflux;
iii. a. 2.5 M n-BuLi, −78◦ C, Sn(C4 H9 )Cl. b. H2 O; iv. Pd(PPh3 )4 , DMF, 80◦ C; v. a. ethanolic KOH, reflux. b. 4M HCl; vi. DCC,
DMAP, CH2 Cl2 .
2.4 Optical, thermal and preliminary electro-optical
properties
2.4.1 Polarising optical microscopy: conventional
glass slide and cover-slip
The mesomorphic transition temperatures and
enthalpy data for members of the homologous series
of (S)-4-(1-methylheptyloxycarbonyl)phenyl 4-(4-nalkoxyphenyl)thiophene-2-carboxylates (Series I) and
(S)-4 -(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-(4n-alkoxyphenyl)thiophene-2-carboxylates (Series II)
are summarised in Tables 1 and 2, respectively.
The thermal properties of six members, n =
6–10 and 12, of a homologous series of (S)4 -(1-methylheptyloxycarbonyl)phenyl 5 -n-alkyl-2,4bithienyl-5-carboxylates (SeriesIII) are listed in Table 3.
Unfortunately, no mesophase formation was detected
either on heating to isotropic liquid or on subsequent
cooling to the crystalline solid. On the other hand, the
thermal properties of the homologous series of (S)-4 (1-methylheptyloxycarbonyl)biphenyl-4-yl 5 -n-alkyl2 ,4-bithienyl-5-carboxylates (SeriesIV) given in Table 4
indicate that all members (n = 6–10 and 12) exhibited
a monotropic SmA phase. The thermal properties
of (S)-4-(1-methylheptyloxycarbonyl)phenyl 4-(4 n-decyloxybiphenyl-4-yl)thiophene-2-carboxylate, 22
212
A.S. Matharu et al.
Br
CnH2n+1
CnH2n+1
B(OH)2
S
'thiopheneboronic acid'
S
13a–f
S
CO2CH3
4
CnH2n+1
S
CO2CH3
S
15a–f
Figure 5. Intended boronic acid cross-coupling strategy for intermediates leading to the formation of members of Series III
and Series IV.
Br
C10H21O
B(OH)2
CO2CH3
S
4
19
i
S
CO2CH3
C10H21O
20
ii
S
CO2H
C10H21O
21
iii
HO
8a
*
CO2 CHC6H13
CH3
S
CO2
C10H21O
*
CO2 CHC6H13
CH3
Series V, 22
Scheme 3. Reagents and conditions: i. Pd(PPh3 )4 , DME, 2M Na2 CO3 , reflux; ii. a. ethanolic KOH, reflux. b. 4M HCl; iii. DCC,
DMAP, CH2 Cl2 .
Liquid Crystals
213
Table 1. Transition temperaturesa (◦ C) and enthalpy valuesb (kJ mol−1 ) for the homologous series of (S)-4-(1-methylheptyloxycarbonyl)phenyl 4-(4-n-alkoxyphenyl)thiophene2-carboxylates, Series I (9a–f).
S
CO2
CnH2n+1 O
n-alkoxy
*
CO2 CHC6H13
CH3
Series I (9a–f)
Compound
Cr –I
I –SmA
SmA–Cr
9a
58.2a
(44.3)c
[37.50]b
64.8
[31.49]
49.6
[29.17]
59.3
[48.51]
53.1
[29.71]
49.5
[4.94]
[3.88]
(43.5)
[29.24]d
(46.7)
[4.42]
(46.1)
[5.73]
(47.7)
[4.43]
(48.0)
[4.92]
(44.2)
[17.68]
(43.4)
[29.24]d
(28.1)
[18.62]
(29.9)
[13.70]
(33.1)
[18.74]
(27.3)
[30.11]
C6 H13
C7 H15
9b
C8 H17
9c
C9 H19
9d
C10 H21
9e
C12 H25
9f
Notes: a Determined by thermal polarising microscopy using conventional glass slide and coverslip
method.
b Determined by DSC.
c Monotropic transition.
[ ]d – Enthalpy data for individual I–SmA and SmA–Cr transitions could not be resolved. A
combined enthalpy value is reported.
Table 2. Transition temperaturesa (◦ C) and enthalpy valuesb (kJ mol−1 ) for the homologous series of (S)-4 -(1methylheptyloxycarbonyl)biphenyl-4-yl 4-(4-n-alkoxyphenyl)thiophene-2-carboxylates, Series II (10a–f).
S
CO2
CnH2n+1 O
n-alkoxy
Compound
C6 H13
10a
C7 H15
10b
C8 H17
10c
C9 H19
10d
C10 H21
10e
C12 H25
10f
*
CO2CHC6H13
CH3
Series II (10a–f)
Cr–SmA/SmC∗ ferri/
SmC∗ antiferroe
I–SmA
SmA–SmX∗
SmX∗ –
SmC∗ ferrie
SmC∗ ferri–
SmC∗ antiferroe
SmA/SmC∗
antiferroe –Cr
132.1a
[31.79]b
125.0
[37.32]
108.2
[31.13]
110.8
[30.38]
107.9
[31.02]
107.8
[30.87]
145.5
[7.41]
140.1
[6.74]
139.1
[6.85]
136.5
[6.01]
134.8
[6.36]
133.5
[6.50]
–
–
–
(105.7)c
[ – ]d
114.6
[–]
120.4
[–]
124.0
[0.15]
126.1
[0.26]
(101.4)
[–]
111.6
[–]
(106.5)
[–]
111.8
[–]
(105.0)
[–]
(98.4)
[–]
109.5
[–]
(102.4)
[–]
110.4
[–]
(102.7)
[–]
111.1
[27.01]
96.1
[27.99]
92.2
[27.11]
90.2
[29.43]
84.5
[26.03]
78.5
[25.71]
Notes: a Determined by thermal polarising microscopy using conventional glass slide and coverslip method.
b Determined by DSC.
( )c Monotropic transition.
[ – ]d Enthalpy of transition too low to be determined.
e Tentative assignment based on thermal polarising optical microscopy.
(Series V) are reported in the Experimental section,
3.2.15.
The members of Series I exhibited only a
monotropic SmA phase. The I–SmA transition
temperatures of members of Series I show an
odd–even behaviour, with the even-numbered
members showing higher thermal stability than
their odd-numbered homologues. The intensity of
modulation of this effect decreased as the series was
ascended.
On cooling from the isotropic liquid, all the homologues of Series II (Table 2) exhibited an enantiotropic
214
A.S. Matharu et al.
Table 3. Transition temperaturesa (◦ C) and enthalpy valuesb (kJ mol−1 ) for
the homologous series of (S)-4 -(1-methylheptyloxycarbonyl)phenyl 5 -n-alkyl-2 ,4bithienyl-5-carboxylates Series III (17a–f).
S
S
CnH2n+1
*
CO2CHC6H13
CH3
CO2
n-alkyl
Series III (17a–f)
Compound
Cr–I
I–Cr
17a
67.9a
30.0
[29.29]
35.5
[29.67]
38.4
[44.48]
38.3
[32.25]
45.9
[29.71]
31.1
[36.05]
C6 H13
C7 H15
17b
C8 H17
17c
C9 H19
17d
C10 H21
17e
C12 H25
17f
[38.04]b
59.5
[34.95]
64.2
[45.68]
56.0
[35.15]
47.5
[29.71]
38.1
[35.94]
Notes: a Determined by thermal polarising microscopy using conventional glass slide and
cover-slip method.
b Determined by DSC.
Table 4. Transition temperaturesa (◦ C) and enthalpy valuesb (kJ mol−1 ) for the
homologous series of (S)-4 -(1-methylheptyloxycarbonyl)biphenyl-4-yl 5 -n-alkyl-2 ,4bithienyl-5-carboxylates, Series IV (18 a–f).
CnH2n+1
S
S
*
CO2CHC6H13
CH3
CO2
Series IV (18a–f)
n-alkyl
Compound
Cr–I
I–SmA
SmA–Cr
C6 H13
18a
C7 H15
18b
C8 H17
18c
C9 H19
18d
C10 H21
18e
C12 H25
18f
125.8a
[37.37]b
120.0
[44.36]
116.0
[41.07]
114.8
[48.24]
113.8
[41.78]
114.5
[32.71]
(110.0)c
[37.82]d
(109.3)
[5.24]
(107.6)
[37.56]
(105.6)
[8.33]
(104.6)
[7.17]
(103.4)
[6.94]
110.1
[37.82]
104.3
[18.41]
104.1
[37.56]
99.7
[11.24]
103.9
[25.98]
95.4
[22.01]
Notes: a Determined by thermal polarising microscopy using conventional glass slide and coverslip method.
b
Determined by DSC.
( )c Denotes monotropic transition.
[ ]d Enthalpy data for individual I–SmA and SmA–Cr transitions could not be resolved.
A combined enthalpy value is reported.
SmA phase (enthalpy values ranging between 6.01
and 7.41 kJ mol−1 ), appearing initially as short rods,
or bâtonnets, which thereafter coalesced to a classic
focal–conic texture interspersed with optically inactive
homeotropic regions (Figure 6(a)). The I–SmA transition temperatures also showed an odd–even effect,
with the even-numbered homologues being thermally
more stable; in this instance the modulation decreased
Liquid Crystals
as the series was ascended. The n = 6 homologue
exhibited no other phase before the onset of crystallisation. The remaining homologues (n = 7–10, 12)
exhibited an interesting phase on further cooling from
(a)
Figure 6(a). Digital photomicrograph of (S)-4 -(1methylheptyloxycarbonyl)-biphenyl-4-yl
4-(4-n-decyloxy
phenyl)thiophene-2-carboxylate (n = 10; 10e; Series II) at
131◦ C exhibiting focal–conic fan and homeotropic texture.
215
the SmA phase. Initial observations of the appearance
of dechiralisation lines across the backs of the focal–
conic fans (Figure 6(b)) suggested a SmC∗ ferroelectric
phase. However, no visible change was seen in the
homeotropic region and, as is shown later, our preliminary electro-optical results contradicted our initial
assignment of SmC∗ ferroelectric character. Thereafter,
on further cooling, the onset of a rapidly moving, shimmering, milky-white texture was observed
in the pseudo-homeotropic region, characteristic of
the SmC∗ ferrielectric phase (Figure 6(c)). This texture usually persisted over a short temperature range
and then disappeared, to leave a pseudo-homeotropic
region with almost total loss of dechiralisation lines
across the backs of the focal–conic fans (Figure 6(d)),
resembling the formation of the SmC∗ antiferroelectric
phase.
2.4.2 Polarising optical microscopy: free-standing film
To lend clarity to the preliminary textural assignments
made from observations of liquid crystals sandwiched
(b)
Figure 6(b). Digital photomicrographs (LHS, middle of sample; RHS, edge of sample) of (S)-4 -(1-methyl
heptyloxycarbonyl)biphenyl-4-yl 4-(4-n-decyloxyphenyl)thiophene-2-carboxylate (n = 10; 10e; Series II) at 116.5◦ C, exhibiting
faint dechiralisation lines within focal–conic fans and a pseudo-homeotropic texture.
(c)
Figure 6(c). Digital photomicrographs (LHS, middle of sample; RHS, edge of sample) of (S)-4 -(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-(4-n-decyloxyphenyl)thiophene-2-carboxylate (n = 10; 10e; Series II) at 111.5◦ C exhibiting milky-white,
shimmering, Schlieren-like texture characteristic of SmC∗ ferroelectric phase.
216
A.S. Matharu et al.
(d)
Figure 6(d). Digital photomicrographs (LHS, middle of sample; RHS, edge of sample) of (S)-4 -(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-(4-n-decyloxyphenyl)thiophene-2-carboxylate (n = 10; 10e; Series II) at 103.6◦ C pseudo-homeotropic and focal–
conic fan-like texture resembling SmC∗ antiferroelectric phase.
between glass slide and cover-slip, an additional study
was carried out using free-standing film. A small
amount of liquid crystal material was placed adjacent
to a 1 mm hole pre-drilled in a copper strip (0.5 cm
× 3 cm), which was then taped to the lower heating
plate of a Mettler FP52 hot-stage and the hole positioned within the optical light path of the microscope.
The liquid crystal material was heated to its SmA
phase and a free-standing film was formed by dragging the liquid crystalline fluid across the hole using
the edge of a glass cover-slip. The lid of the Mettler
FP52 hot stage was closed carefully, in order not to
break the film, and textural observations were made
at heating and cooling rates of 1◦ C min−1 . Between
crossed polarisers the film appeared optically negative
(homeotropic) in the SmA phase. The following is a
summary of the observations on the homologue, 10e
(n = 10), of Series II and taken as representative of the
series.
On cooling from the optically negative SmA phase,
a very faint bluish Schlieren-like texture developed in
the film at 126.5◦ C, which was difficult to capture but
nevertheless an image was taken at 124.5◦C and is
shown in Figure 7(a). At 122.3◦ C a sweeping motion
was seen across the film, leaving a very faint Schlierenlike texture, which again was difficult to capture but an
image was obtained at 117.9◦ C (Figure 7(b)). Changes
in the nature of defect lines within the film were noticeable. In comparison with the changes and textures
observed from the sandwich cell investigation only
one transition was detected at this temperature, and
we have tentatively regarded this as the conversion of
SmA to SmX∗ . It was seen as the onset of dechiralisation lines across the back of the focal conic fans (cf.
Figure 6(b)).
Investigation using free-standing film revealed the
presence of an additional phase not encountered
previously, which may be SmC∗ alpha. Anticlockwise
rotation of the stage by a few degrees showed a redbrown colouration in the film, indicating the presence
of a helical structure. On continued cooling a dramatic change became apparent at 116.7◦C, with the
onset of transition lines emanating from the edge of
the film (Figure 7(c)) and the formation of mobile,
shimmering, Schlieren-like texture. When the temperature was held at 116.7◦ C the transition lines
disappeared but the shimmering texture remained
(Figure 7(d)). In contrast with our investigation using
the sandwich cell, this transition corresponded to a
SmX∗ –SmC∗ ferrielectric transition (cf. Figure 6(c)).
This texture remained until 115.7◦C, when the transition lines reappeared (Figure 7(e)) and the mobile
shimmering texture disappeared (Figure 7(f)), marking the onset of another transition, most probably SmC∗ ferrielectric–SmC∗ antiferroelectric. In this
instance a change in helicity was noted, as clockwise
rotation of the stage revealed a red-brown colouration
in the film.
On heating the free-standing film similar reversible
transitions were observed, as illustrated in Figures 8(a)–
8(f). In addition rotation of the stage, either clockwise
or anticlockwise, gave similar colouration to that noted
earlier. On heating from the SmC∗ antiferroelectric
phase (Figure 8(a)) transition lines were observed,
marking the onset of the SmC∗ ferrielectric phase
(Figure 8(b)). Immense movement was noted, followed
by the appearance of transition lines marking the end of
the SmC∗ ferrielectric phase (Figures 8(c) and 8(d)) and
the onset of the SmX∗ phase (Figure 8(e)). A Schlierenlike texture was observed in the film and an additional
change in texture was noted at 124.5◦ C (Figure 8(f)),
which may be the SmC∗ alpha phase. These observations mirrored the changes seen on heating, and also
confirmed the possibility of sub-phases within SmX∗ .
Liquid Crystals
(a) 124.5°C:
onset of SmX*–note bluish colour
(b) 117.9°C: possible change in SmX*–
note increase in defect lines/Schlieren
(c) 116.7°C: onset SmX*–SmC*ferri
note transition lines and shimmering
(d) 116.7°C: fully developed SmC*ferri
note constant shimmering and movement
(e) 115.7°C: onset SmC*ferri–SmC*anti
note transition lines
(f) 115.7°C: fully developed SmC*antiferro
note loss of shimmering
217
Figure 7. Digital photomicrographs of a free-standing film of (S)-4 -(1-methylheptyloxycarbonyl)-biphenyl-4-yl 4-(4-ndecyloxyphenyl)thiophene-2-carboxylate (n = 10; 10e; Series II) on cooling from 124.5◦ C (a) to 115.7◦ C (f).
2.5 Preliminary electro-optical studies
From the textural observations reported above only tentative assignments are made at present since, although
we observed textural changes by sandwich cell investigation (Figure 6(b)) which appeared to show that
SmX∗ might in fact be SmC∗ ferroelectric, preliminary
electro-optical studies appeared to be contradictory.
For instance, the SmA–SmX∗ transition deduced
from preliminary optical response data looked more like
a SmA–SmC∗ alpha, a SmA–SmC∗ antiferroelectric,
or a SmA–SmC∗ ferrielectric transition. If a SmA–
SmC∗ ferroelectric transition was occurring, then the
SmC∗ ferroelectric phase was probably behaving as an
extremely short-lived material since such compounds
give an electro-optic response similar to that of antiferroelectric materials. It could be that the region governed
by SmX∗ comprised two ferrielectric phases but no
SmC∗ ferroelectric phase. There has been an indication
218
A.S. Matharu et al.
(a) 108.1°C:
SmC* antiferroelectric
(b) 117.6°C:
onset of SmC* anti–SmC*ferrielectric
(c) 118.1°C:
onset of SmC* ferrielectric–SmX*
(d) 118.1°C:
completion of SmC*ferrielectric–SmX*
(e) 119.1°C:
development SmX* phase
(f) 124.5°C:
textural change noted in SmX* phase
Figure 8. Digital photomicrographs of a free-standing film of (S)-4 -(1-methylheptyloxycarbonyl)-biphenyl-4-yl 4-(4-ndecyloxyphenyl)thiophene-2-carboxylate (n = 10; 10e; Series II) on heating from 108.1◦ C (a) to 124.5◦ C (f).
in the literature of the difficulties and uncertainties
associated with the electro-optical characterisation of
such phases, especially when confined within cells [23].
To ascertain the identity of SmX∗ further, extensive and
detailed electro-optical investigations are in progress.
At the present stage we merely wish to draw attention to
the fact that an interesting chiral behaviour was taking
place below the SmA phase in the case of homologues
n = 7–10 and 12 of Series II. We have in fact deter-
mined the spontaneous polarisation, Ps (Figure 9) and
tilt angle θ (Figure 10) by probing the region below the
expected Curie point, in our case SmA–SmX∗ , such
that T c –T corresponds to the extent to which the SmX∗
phase has been adopted.
Commercial EHC cells were filled and alignment
was achieved by cooling at 0.1◦ C min−1 from the
isotropic liquid into the SmA phase, using an applied
square waveform of 20 V μm−1 at 50 Hz. The
Liquid Crystals
75
2.6 Three-ring compounds: 2,5- versus
2,4-disubstitution and replacement of the left-hand
1,4-phenylene ring with a 2,5-thienyl ring
C7
C8
C9
C10
C12
70
65
60
55
Ps (nC cm–2)
50
45
40
35
30
25
20
15
10
5
0
50
45
40
35
30
25
20
15
10
5
0
Reduced temperature (Tc-T) (°C)
Figure 9. Spontaneous polarisation data for n = 7–10 and
12 homologues of Series II (colour version online).
C7
C8
C9
C10
C12
35
Tilt angle / degrees
30
25
20
15
10
5
50
45
40
35
30
25
20
15
10
5
219
0
Reduced temperature (Tc-T) (°C)
Figure 10. Tilt angle data for n = 7–10 and 12 homologues
of Series II (colour version online).
measurements were carried out at 10 V μm−1 at 30 Hz.
The value of Ps was measured using a triangular waveform, and the tilt angle using a square waveform.
Figures 9 and 10 show a clear trend, in that both
the tilt angle, θ , and the spontaneous polarisation,
Ps , increase with the length of the alkyl chain from
n = 7 to 12. However, no significant discontinuities
were detected that might indicate phase transitions
within this region.
Resonant polarised X-ray diffraction (RPXRD)
may be an ideal probe technique to ascertain the
complex behaviour in the SmX∗ region since thiophene, with its large polarisable sulfur atom, possesses
an absorption edge energy that can be probed with
suitable X-ray sources [24].
The influence of structure on mesophase thermal stability for a series of three-ring thiophene-based esters
(where R = C10 H21 ), measured by melting point and
clearing point temperatures, is shown in Figure 11.
Altering the substitution pattern from 2,5disubstituted (parent) to 2,4-disubstituted (Series I)
has a major effect on both melting point (reduced
by 13.3◦ C) and mesophase thermal stability (clearing
point reduced by 53.2◦ C). A monotropic SmA phase
alone is observed, unlike the 2,5-parent compound
which displays SmC∗ ferro-, ferri- and antiferroelectric
phases. This significant change in thermal properties
may be related to the reduction in exocyclic bond
angle from 147.5◦ (parent, 2,5-disubstituted) to
140.5◦ (Series I, 2,4-disubstituted). A much narrower
substitution angle produces an even more non-linear,
or bent, structure which reduces packing efficiency
and melting point, making it is less conducive to
mesophase formation.
Replacing the left-hand terminal 1,4-phenylene
ring in Series I with a 2,5-disubstituted thienyl ring
(Series III) completely destroys mesophase formation.
Compared with the 2,5-parent structure, both melting point and clearing point decreased, by 18.9◦ C and
53.4◦ C, respectively. However, compared with Series I
(phenyl-2,4-thiophene), ring replacement from 1,4phenyl to 2,5-thiophene still has a detrimental effect
on molecular packing, but to a slightly lesser extent,
as the melting point decreases by only 5.6◦ C. It is
important to note that strictly we are not making
a like-for-like comparison when considering Series I
with Series III, because the left-hand terminal chain
in Series I is decyloxy (C10 H21 O–) and in Series III
it becomes decyl (C10 H21 –). The loss of an oxygen
atom will affect polarisability and mesomeric relay,
which will in turn affect mesophase thermal stability (see later). The deterioration in 5,5 -disubstituted2,4 -bithiophenes, e.g. Series III, is further illustrated
by comparison with their isomeric counterparts, the
5,5 -disubstituted-2,2-bithiophenes [8, 9]. As shown
in Figure 11, the melting point and clearing point of
the C10 H21 homologue, comprising 2,2 -bithiophene,
were higher by 12.5◦ C and 8.6◦ C, respectively. In
addition, the 2,2 -bithiophene homologue exhibited
monotropic SmA and SmC∗ ferro-, ferri- and antiferroelectric phase types prior to the onset of crystallisation [8, 9].
We have so far only considered the possibility of
a non-linear kinked structure to explain mesophase
thermal stability. 2,4-Disubstitution, compared with
2,5-disubstitution, significantly affects the extent of
mesomeric relay of electrons through the molecular
220
A.S. Matharu et al.
Variation in m.p., °C
Variation in cl. pt., °C
O
S
Cr 66.4 SmC*antiferro
CO2R*
I 100.9 SmA
O
RO
–53.2
–13.3
S
–18.9
Cr 53 I
CO2R*
I (47.7) SmA
monotropic
O
RO
–5.6
R
O
S
S
Cr 47.5 I
–53.4
–0.2
O
CO2R*
Cr 47.5 I
not liquid crystalline
O
–12.5
–8.6
R
Cr 59.0 I
O
S
S
CO2R*
I (56.1) SmA
monotropic
O
where R = C10H21
Figure 11. Variation in melting point (m.p.) and clearing point (cl. pt.) for a series of three-ring thiophene-based esters.
O
O
O
RO
S
O
O R*
RO
O
S
O R*
O
O
S
O
S
O
O R*
RO
O
O
O R*
RO
O
Figure 12. Mesomeric relay of electrons in 2,4-disubstituted systems (bottom) with respect to the parent structure (2,5disubstituted, top).
core, as shown in Figure 12. The extent of conjugation will affect the molecular polarisability along
the molecular axis, and will ultimately influence the
anisotropy of molecular polarisability.
For the parent 2,5-disubstituted system (Figure 12,
top) mesomeric relay is possible by donation of an
electron pair (+ M effect) from the left-hand terminal ether oxygen through to the carbonyl oxygen
atom of the central ester group (– M effect). Similarly,
mesomeric relay is possible on the opposite side of the
central ester group, as shown.
In the case of 2,4-disubstituted systems, for example, Series I and III, mesomeric relay is much more
fragmented and cannot extend continuously from the
terminal oxygen to the central ester group. This break
in conjugation remains, irrespective of the nature of
the left-hand aromatic ring, either 1,4-phenylene or
2,5-thienyl, and may help explain the loss of, or reduction in, mesomorphic properties.
The importance of mesomeric relay is further shown by the 5,5 -disubstituted-2,2-bithiophenes
(Figure 13) which as discussed previously are mesomorphic, unlike their 2,4 -counterparts, in which the
extent of conjugation emanates from the sulfur atom
in the left-hand terminal ring and extends through to
the central ester group.
2.7 Four-ring compounds: Three-ring
2,4-disubstitution and inclusion of an extra
1,4-phenylene ring either on the left- or
the right-hand side
Four-ring compounds are more conducive to
mesophase formation. Inclusion of an additional 1,
Liquid Crystals
O
O
O
R
O
O R*
S
R
O
S
O R*
S
S
O
O
O
O
S
R
S
O R*
S
221
R
O
S
O
O R*
O
Figure 13. Mesomeric relay within 2,4-substituted (for example, Series III, top) and 2,5-substituted bithiophenes (bottom).
4-disubstituted phenyl ring either on the left-hand side
of the molecular core, adjacent to a 2,4-disubstituted
thiophene moiety (Series V), or on the right-hand
side, adjacent to a 1,4-disusubstituted phenyl ring
(Series II), induced enantiotropic mesophase formation and increased both melting and clearing
point.
As seen in Figure 14, the magnitude of such
changes is dependent on the disposition of the phenyl
ring. The greatest enhancement with respect to the
three-ring compound was observed when the additional phenyl ring is located on the left-hand side
(Series V), increasing both melting and clearing point,
by 67.1◦ C and 125.3◦ C, respectively. Comparison
between the two four-ring isomeric counterparts
shows a difference of 12.3◦ C and 38.2◦ C in melting and
clearing point temperatures, respectively. The inclusion of an additional phenyl ring and its disposition
influences linearity, molecular packing and the extent
of conjugation.
Variation in m.p., °C
Figure 15 shows that the presence of an additional
phenyl ring on the right-hand side appears to give
a pseudo-symmetrical non-linear bent geometry with
two rings placed either side of the central ester linking
group, with conjugation extending within the phenolic portion. Non-linearity seems to be reduced by the
inclusion of an extra ring on the right-hand side, giving
an arrangement of three rings on one side separated
from the fourth ring by the central ester linkage. The
kinked or bent portion is effectively shifted to the end
of the structure, leaving a sufficiently long three-ringed
aromatic region of extended conjugation and linearity.
2.8 Preliminary molecular modelling study:
2,5-disubstituted thiophene (Parent 2) versus
2.4-disubstituted thiophene (10e, Series II)
The results of an initial high-level molecular modelling
study, conducted to tease out any further differences in the structure and electronic properties
Variation in cl. pt., °C
S
CO2R*
O
Cr 53.1 I
I (47.7) SmA
monotropic
O
RO
+87.1
+54.8
S
+67.1
Cr 107.9
SmC*antiferro
RO
CO2R*
O
I 134.8 SmA
+125.3
O
+38.2
+12.3
S
Cr 120.2 SmC*
O
CO2R*
I 173.0 SmA
O
RO
Figure 14. Variation in melting point (m.p.) and clearing point (cl. pt.) for a series of four-ring thiophene-based esters.
222
A.S. Matharu et al.
S
O
O
RO
O R*
O
S
O
O
RO
O R*
Pseudo-symmetrical
geometry
two ring
two ring
portion
portion
S
O
O
O
RO
O R*
O
S
O
O
RO
O R*
O
Asymmetrical geometry
off-centre structure
relatively long
3-ring portion
short onering portion
Figure 15. Postulated geometry of structures, showing extent of mesomeric relay.
of 2,5-disubstituted thiophene (Parent 2) [11] with
respect to 2,4-disubstituted thiophene (10e, Series II),
are given below. Parent 2 [11] was chosen as the reference since it is a direct isomeric counterpart of 10e,
differing only in respect of its thiophene substitution
pattern (Figure 16).
In the calculations on molecules Parent 2 and
10e we have used two sets of initial geometry. The
first corresponds to the ‘most extended’ structures, in
which the side-chains adopt an all-trans conformation. The initial ‘most extended’ geometries for Parent
2 and 10e were constructed using PCModel [25] and
Liquid Crystals
C10H21O
CO2
S
*
CO2CHC6H13
CH3
223
Parent 2
m.p. 88.1(°C)
I 193.2 SmA 183.2 SmC*ferro 159.6 SmC*ferri 158.3 SmC*antiferro 108.9 SmI* 46.1 Cr
S
CO2
C10H21O
*
CO2CHC6H13
CH3
10e
m.p. 107.9(°C)
I 134.8 SmA 124.0 SmX* 111.8 SmC*ferri 110.4 SmC*antiferro 84.5 Cr
Figure 16. Structures and transition temperatures (◦ C) for Parent 2 [11] and 10e.
optimised using the MMX force field included in the
program. Another set of initial geometries for Parent 2
and 10e were obtained by performing GMMX molecular mechanics conformer searches starting from
the MMX ‘most extended’ geometries and allowing
PCModel to test at least 5000 alternative conformations before selecting those of lowest energy. The ‘most
extended’ and ‘lowest-energy conformer search’ structures were then optimised at the B3LYP/6–31G(d)
level of theory by means of GAUSSIAN03 [26], using
the gradient-based GDIIS procedure under the ‘tight’
convergence criteria. The static polarisabilities of the
‘most extended’ and ‘lowest-energy conformer search’
structures of Parent 2 and 10e, optimised at the
B3LYP/6–31G(d) level, were evaluated at the same
level of theory, again with GAUSSIAN03, by means
of analytical vibrational frequency calculations.
The B3LYP/6–31G(d) optimised geometries of the
‘most extended’ and ‘lowest-energy conformer search’
structures of Parent 2 and 10e are shown in Figures 17
and 18, respectively, together with the corresponding
dipole moments.
The B3LYP/6–31G(d) energies and exact static
polarisabilities of the structures are summarised in
Table 5.
It is interesting to note that whereas the molecular mechanics geometry optimisation (MMX force
field) favours folded or partially folded conformers
for both Parent 2 and 10e, in the case of Parent
2 the more accurate B3LYP/6–31G(d) calculations
give preference, by a relatively small margin (of 10.2
kJ mol−1 ), to the ‘most extended’ structure. In the
case of 10e the B3LYP/6–31G(d) results suggest that
partial folding leads to a more significant energy lowering, of 33.6 kJ mol−1 . This may be related to the
lower thermal stability of 2,4-disubstituted thiophenes
with respect to their 2,5-disubstituted counterparts. A
preference for a slightly folded structure reduces linearity and the overall dipole moment along the molecular axis. The anisotropy of molecular polarisability,
α, will be affected, and probably lowered. The total
energy surfaces corresponding to Parent 2 and 10e
are very flat, which suggests that single-molecule (gas
phase) calculations using any theoretical approach will
be able to locate large numbers of conformers that are
reasonably close in energy. The values of some specific
angles between bonds, and dihedral angles characterising the structures shown in Figures 17 and 18, are
collected in Tables 6 and 7.
The results of a more detailed quantum-chemical
analysis of the characteristic features of Parent 2 and
10e and related molecules, utilising localised molecular
orbitals to highlight trends in electronic structure, will
be published separately.
3. Experimental
3.1 Instrumental
The structural integrity of the intermediates and final
products was confirmed by 1 H NMR spectroscopy
(JEOL FX60Q 270MHz spectrometer), using tetramethylsilane as internal standard, and infrared spectroscopy (Perkin–Elmer FT1605 spectrophotometer).
Compounds 8a and 8b were obtained from a previous
synthesis, and this also allowed a direct comparison to
be made with parent 2,5-disubstituted thiophene and
2,2 -bithiophene analogues described in the discussion
section [11]. Transition temperature measurements
were made using an Olympus BH–2 polarising microscope in conjunction with a Mettler FP52 hot-stage
and FP5 control unit. Complementary differential
scanning calorimetry (thermal analysis) was carried
out on a Perkin–Elmer DSC7 at heating and cooling
rates of 5 and 10◦ C min−1 . Instrument calibration was
224
A.S. Matharu et al.
(a)
(b)
Figure 17. B3LYP/6–31G(d) optimised geometries of the (a) ‘most extended’ and (b) ‘lowest-energy conformer search’
structures of Parent 2. Dipole moments in D (colour version online).
checked against an indium standard (measured H In ,
28.37 J g−1 ; required H In , 28.45 J g−1 ). Mass spectrometric data were obtained on a Perkin–Elmer 8500
GC–MS, comprising 30 m BP1 column connected to
a Perkin–Elmer ion-trap detector. The microanalysis
department at the University of Canterbury, Kent,
UK, conducted the elemental analyses. The Centre
of Excellence in Mass Spectrometry at the University
of York performed mass spectrometry and mass ion
determinations.
(Note that extreme care must be exercised due to
the evolution of HBr gas.) The reaction mixture was
stirred overnight at room temperature, poured on
to ice/concentrated hydrochloric acid (100 ml) and
allowed to hydrolyse for 1 h. The crude product
was extracted with dichloromethane (2 × 150 ml),
washed with water (2 × 100 ml), brine (2 × 100 ml),
and dried over MgSO4 . The solvent was removed
in vacuo and the crude residue recrystallised from
aqueous methanol to yield pure 4-bromothiophene-2carboxaldehyde 2 as a white crystalline solid.
3.2 Synthesis
3.2.1 4-Bromothiophene-2-carbaldehyde,
2 (Scheme 1)
Elemental bromine (8.8. ml, 0.17 mol) was added
drop-wise to a stirred mixture of freshly distilled
commercial thiophene-2-carboxaldehyde, 1 (13.1 ml,
0.14 mol), and anhydrous powdered aluminium(III)
chloride (37.3 g, 0.28 mol) maintained below 50◦ C.
Yield: 19.8 g (73%), m.p. 45–47◦ C.
IR ν max (KBr)/cm−1 : 3095m (Ar C–H str.),
2923, 2852w (aldehydic C–H str.), 1667 (aldehyde C=O str.), 1408, 1228, 1163, 783, 662s
(C–H out of plane deformation (o.o.p.d)).
δ H (270 MHz; CDCl3 ; Me4 Si): 7.6 (1 H, d, ThH,
J = 2 Hz), 7.7 (1 H, d, ThH, J = 2 Hz), 9.8 (1 H,
s, ThCHO) ppm.
Liquid Crystals
225
(a)
(b)
Figure 18. B3LYP/6–31G(d) optimised geometries of the (a) ‘most extended’ and (b) ‘lowest-energy conformer search’
structures of 10e. Dipole moments in D (colour version online).
Table 5. B3LYP/6–31G(d) energies (in Hartree) and exact static polarisabilities (isotropic α iso and elements of
the polarisability tensor, all in Bohr3 ) for the structures shown in Figures 17 and 18.
Structure
Parent 2 (a)
Parent 2 (b)
10e (a)
10e (b)
Energy
α iso
α xx
α xy
α yy
α xz
α yz
α zz
−2406.1565
−2406.1526
−2406.1540
−2406.1668
544.30
507.12
530.11
519.66
828.33
564.83
835.66
771.4
−16.36
58.92
22.99
−23.54
443.07
575.93
456.8
433.56
51.38
21.82
−17.43
56.36
−23.98
−5.09
−14.94
66.11
361.48
380.60
298.41
354.08
3.2.2 4-Bromothiophene-2-carboxylic acid, 3
(Scheme 1)
4-Bromothiophene-2-carboxaldehyde 2 (17.3 g, 0.091
mol) was added in a single portion to a stirred mixture
of aqueous silver nitrate solution (30.9 g, 0.182 mol in
water, 50 ml) and aqueous sodium hydroxide (14.6 g,
0.36 mol in water, 50 ml). After stirring for 1 h the
inorganic salts were filtered off and discarded, and the
filtrate acidified (4M HCl). The resulting white precipitate was extracted with diethyl ether (2 × 100 ml),
washed with water (2 × 50 ml), dried over MgSO4
and the solvent removed in vacuo. The resulting crude
residue was recrystallised from cyclohexane to give
4-bromothiophene-2-carboxylic acid, 3, as a white
crystalline solid.
Yield: 14.9 g (80%), m.p. 122–124◦ C.
IR ν max (KBr)/cm−1 : 3449–3050br s (O–H str.,
H-bonded), 3086w (Ar C–H str.), 2917, 1675
(acid C=O str.), 1525, 748s (C–H o.o.p.d).
δ H (270 MHz; CDCl3 ; Me4 Si): 7.55 (1 H, d,
ThH, J = 2 Hz), 7.8 (1 H, d, ThH, J = 2
Hz), 10.4 (1 H, s, ThCO2 H, disappears on D2 O
shaking) ppm.
226
A.S. Matharu et al.
Table 6. Angles between bonds attached to the thiophene ring (labels
A–B and C–D shown below) for the structures shown in Figures 17
and 18 (in degrees).
Structure
Angle between A–B and C–D
Parent 2 (a)
Parent 2 (b)
10e (a)
10e (b)
153.66
155.89
135.65
140.67
Table 7. Dihedral angles between atoms forming the thiophene and phenyl rings (as
labelled in Table 6) for the structures shown in Figures 18 and 19 (in degrees).
Structure
Parent 2 (a)
Parent 2 (b)
10e (a)
10e (b)
E–C–D–G
E–C–D–H
F–C–D–G
F–C–D–H
26.13
−26.40
−32.39
29.97
−153.67
155.95
147.78
−150.21
−153.20
152.91
147.70
−150.25
27.00
−24.74
−32.12
29.57
3.2.3 Methyl 4-bromothiophene-2-carboxylate, 4
(Scheme 1)
4-Bromothiophene-2-carboxylic acid, 3 (14.6 g, 0.071
mol) was added to a stirred mixture of anhydrous
potassium carbonate (19.6 g, 0.142 mol), dimethyl sulphate (6.7 ml, 0.071 mol) and dry propanone (100 ml).
The reaction mixture was heated under reflux for 4 h,
cooled and filtered to remove any insoluble residues.
The filtrate was evaporated to dryness and the crude
residue purified by vacuum distillation (Kugelrohr) to
yield methyl 4-bromothiophene-2-carboxylate, 4, as a
colourless oil.
Yield: 12.6 g (81%), b.p. 95◦ C at 0.02 mm Hg
(lit. [27] 113–115◦ C at 9 mm Hg).
IR ν max (thin film)/cm−1 : 3031w (Ar C–H str.),
2953, 1716 (ester C=O str.), 1517, 1435, 1182,
1097, 769s (C–H o.o.p.d).
δ H (270 MHz; CDCl3 ; Me4 Si): 4.0 (3 H, s,
ThCO2 CH3 ), 7.4 (1 H, d, ThH, J = 2 Hz), 7.7
(1 H, d, ThH, J = 2 Hz) ppm.
3.2.4 Methyl 4-(4-n-alkoxyphenyl)thiophene-2carboxylate, 6a–f (Scheme 1)
Under an atmosphere of nitrogen a solution of an 4-n-alkoxyphenylboronic acid, 5a–f
(0.01 mol), in dimethoxyethane (30 ml) was
added to a vigorously stirred mixture of methyl
4-bromothiophene-2-carboxylate, 4 (1.5 g, 0.007
mol), tetrakis(triphenylphosphine)palladium(0) (0.3
mol%), 2M aqueous sodium carbonate solution
(30 ml) and dimethoxyethane (30 ml). The mixture was heated under reflux until all the methyl
4-bromothiophene-2-carboxylate 4 had been consumed (TLC). Thereafter, the reaction mixture was
cooled, filtered through a short plug of glass wool to
remove particulate matter and extracted with diethyl
ether (2 × 50 ml). The extract was washed with brine
(50 ml), dried (MgSO4 ) and the solvent removed
in vacuo to yield the crude product as a brown
solid. Purification was achieved by flash column
chromatography (SiO2 ), eluting with 2 : 1 petroleum
ether (b.p. 40–60◦C): dichloromethane, followed by
recrystallisation from aqueous methanol to give the
relevant methyl 4-(4-n-alkoxyphenyl)thiophene-2carboxylate, 6a–f (62–74%), as a white crystalline
solid. Melting points: C6 H13 O, 6a, 74–76◦C; C7 H15 O,
6b, 79–81◦C; C8 H17 O, 6c, 84–86◦C; C9 H19 O, 6d,
88–90◦C; C10 H21 O, 6e, 82–84◦C; C12 H25 O, 6f,
88–90◦C.
The following data refer to methyl 4-(4-ndecyloxyphenyl)thiophene-2-carboxylate, 6e, and are
typical of the series:
Liquid Crystals
IR ν max (KBr)/cm−1 : 3010w (ar. C–H str.),
2954, 2920, 1715 (ester C=O str.), 1512, 1297,
1096, 835s (C–H o.o.p.d).
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (3 H, t,
–CH3 ), 1.2–1.4 (14 H, m, alkyl), 1.8 (2 H, quint.,
ArOCH2 CH2 CH2 –), 3.9 (3 H, s, ThCO2 CH3 ),
4.0 (2 H, t, ArOCH2 CH2 –), 6.9 (2 H, d, ArH,
J = 8 Hz), 7.5 (2 H, d, ArH, J = 8 Hz), 7.55 (1
H, d, ThH, J = 2 Hz), 8.0 (1 H, d, ThH, J = 2
Hz) ppm.
3.2.5 4-(4-n-Alkoxyphenyl)thiophene-2-carboxylic
acids, 7a–f (Scheme 1)
A mixture of the appropriate methyl 4-(4-nalkoxyphenyl)thiophene-2-carboxylate, 6a–f (0.003
mol), potassium hydroxide (1.5 g, 0.027 mol) and
80% aqueous ethanol (50 ml) was heated under reflux
for 2 h. The reaction mixture was cooled, acidified
(4M aqueous HCl) and extracted with diethyl ether
(2 × 50 ml). The combined extract was washed with
water (50 ml), dried (MgSO4 ) and evaporated to
dryness in vacuo. The resulting crude material was
purified by recrystallisation from aqueous methanol
to give the respective 4-(4-n-alkoxyphenyl)thiophene2-carboxylic acid, 7a–f (78–92 %), as white crystalline
solids. Melting points and transition temperatures1 :
C6 H13 O, 7a, Cr–I 124.7; I–N/Cr 110.3; C7 H15 O, 7b,
Cr–I 133.1; I–N/Cr 124.0; C8 H17 O, 7c, Cr–I 111.0;
I–N/Cr 108.5; C9 H19 O, 7d, Cr–I 114.7; I–N/Cr 111.0;
C10 H21 O, 7e, Cr–I 112.3; I–N/Cr 106.8; C12 H25 O, 7f,
Cr–I 116.6, I–N/Cr 110.3◦ C.
The following data refer to methyl 4-(4-nnonyloxyphenyl)thiophene-2-carboxylic acid, 7d, and
are typical of the series:
IR ν max (KBr)/cm−1 : 3502–3256m (br OH str.),
2952w, 1677s (acid C=O str.), 1258, 773s (C–H
o.o.p.d).
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (3 H,
t, –CH3 ), 1.2–1.4 (14 H, m, alkyl), 1.8 (2
H, quint., ArOCH2 CH2 CH2 –), 4.0 (2 H, t,
ArOCH2 CH2 –), 6.95 (2 H, d, ArH, J = 8 Hz),
7.5 (2 H, d, ArH, J = 8 Hz), 7.55 (1 H, d, ThH,
J = 2 Hz), 8.1 (1 H, d, ThH, J = 2 Hz) ppm.
OH proton not detected.
3.2.6 (S)-4-(1-Methylheptyloxycarbonyl)phenyl
4-(4-n-alkoxyphenyl)thiophene-2-carboxylates, 9a–f
(Series I) (Scheme 1)
A mixture of the appropriate 4-(4-n-alkoxy
phenyl)thiophene-2-carboxylic acid, 7a–f (0.00064
mol), dicyclohexylcarbodiimide (0.16 g, 0.00077
mol), DMAP (2–3 crystals), (S)-1-methylheptyl
4-hydroxybenzoate, 8a (0.16 g, 0.00064 mol) and
227
dry dichloromethane (25 ml) was stirred at room
temperature overnight. The resulting white precipitate
was filtered off and discarded. The filtrate was
evaporated to dryness in vacuo and subjected to
flash column chromatography on silica gel, eluting
with 1 : 1 dichloromethane : petroleum ether (b.p.
40–60◦C). The solvent was removed from the relevant fractions and the crude product purified by
repeated recrystallisation from ethanol, giving the
required
(S)-4-(1-methylheptyloxycarbonyl)phenyl
4-(4-n-alkoxyphenyl)thiophene-2-carboxylate,
9a–f
(Series I), as a white crystalline solid (yield, 52–56%).
Melting points and transition temperatures are listed
in Table 1.
The following analytical and spectroscopic data
refer to (S)-4-(1-methylheptyloxycarbonyl)phenyl
4-(4-n-nonyloxyphenyl)thiophene-2-carboxylate, 9e,
and are typical of the series:
Found: C, 72.34; H, 7.96%; C35 H46 O5 S requires:
C, 72.63; H, 8.01%.
m/z: 578 (M +): 467, 341, 329.
IR ν max (KBr)/cm−1 : 2952, 2881, 1743s (C=O
str.), 1714 (C=O str.), 1289, 1239, 777s (C–H
o.o.p.d).
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (6 H, t, 2 ×
–CH3 ), 1.2–1.5 (25 H, m, alkyl + C∗ (H)CH3 ),
1.8 (2 H, quint., ArOCH2 CH2 CH2 –), 4.0 (2 H,
t, ArOCH2 CH2 –), 5.2 (1 H, m, C∗ (H)CH3 ), 7.0
(2 H, d, ArH, J = 8 Hz), 7.3 (2 H, d, ArH, J =
8 Hz), 7.5 (2 H, d, ArH, J = 8 Hz), 7.7 (1 H, d,
ThH, J = 2 Hz), 8.1 (2 H, d, ArH, J = 8 Hz),
8.2 (1 H, d, ThH, J = 2 Hz) ppm.
3.2.7 (S)-4 -(1-Methylheptyloxycarbonyl)biphenyl4-yl 4-(4-n-alkoxyphenyl)thiophene-2-carboxylates,
10a–f (Series II) (Scheme 1)
The members of Series II were prepared according
to the method described in 3.2.6 for Series I, with
quantities as follows: 4-(4-n-alkoxyphenyl)thiophene2-carboxylic acid, 7a–f (0.00077 mol), DCC
(0.19 g, 0.00092 mol), DMAP (2–3 crystals), (S)1-methylheptyl 4 -hydroxybiphenyl-4-ylcarboxylate,
8b (0.25 g, 0.00077 mol), and dry dichloromethane
(25 ml), giving 10a–f as white crystalline solids, yield
52–60%. Melting points and transition temperatures
are listed in Table 2.
The following analytical and spectroscopic data
refer to (S)-4-(1-methylheptyloxycarbonyl)bipheny
l-4-yl 4-(4-n-hexyloxyphenyl)thiophene-2-carboxylate,
10a, and are typical of the series:
Found: C, 72.24; H, 7.2 6%; C38 H44 O5 S
requires: C, 74.48; H, 7.24%.
m/z 612 (M+), 501, 287.
228
A.S. Matharu et al.
IR ν max (KBr)/cm−1 : 2952, 2881, 1741s (C=O
str.), 1716 (C=O str.), 1284, 766s (C–H o.o.p.d).
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (6 H, t, 2 ×
–CH3 ), 1.2 – 1.5 (19 H, m, alkyl + C∗ (H)CH3 ),
1.8 (2 H, quint., ArOCH2 CH2 CH2 –), 4.0 (2 H,
t, ArOCH2 CH2 –), 5.2 (1 H, m, C∗ (H)CH3 ), 7.0
(2 H, d, ArH, J = 8 Hz), 7.3 (2 H, d, ArH, J =
8 Hz), 7.6 (2 H, d, ArH, J = 8 Hz), 7.7 (5 H,
comp. m, ArH/ThH), 8.1 (2 H, d, ArH, J = 8
Hz), 8.2 (1 H, d, ThH, J = 2 Hz) ppm.
3.2.8 2-(Tri-n-butyl)-5-n-alkylthiophenes, 14a–f
(Scheme 2)
2.5 M n-BuLi (21.0 ml, 0.053 mol) was added dropwise under nitrogen to a stirred, cold (–78◦C), solution
of the appropriate 5-n-alkylthiophene, 13a–f (0.048
mol), at such a rate that the temperature did not rise
above −70◦ C. After the addition was complete the
reaction mixture was maintained at −78◦ C for a further 1 h. Tri-n-butyltin chloride (14.4 ml, 0.053 mol)
was then injected drop-wise and the reaction mixture allowed to stand overnight at room temperature.
The reaction mixture was quenched with water and the
product extracted with diethyl ether (2 × 100 ml). The
combined extract was washed with water (2 × 100 ml),
dried (MgSO4 ) and the solvent removed in vacuo.
The resulting crude residue was purified by vacuum
distillation (Kugelrohr) to yield the relevant 2-(tri-nbutyl)-5-n-alkylthiophene, 14a–f, as a pale yellow oil
(yield, 72–88%). B.p.: C6 H13, 14a, 180◦C at 0.5 mm Hg;
C7 H15 , 14b, 190◦ C/0.5; C8 H17 , 14c, 200◦ C/0.5; C9 H19 ,
14d, 180◦ C/0.1; C10 H21 , 14e, 210◦C/0.1; C12 H25 , 14f,
250◦ C/0.1.
The following data refer to 2-(tri-n-butyl)-5-noctylthiophene, 14c, and are typical of the series:
IR ν max (thin film)/cm−1 : 3010w (ArC–H str.),
2954, 2922, 1463, 1379, 1072, 935, 795.
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (12 H, t, 4
× –CH3 ), 1.0 (6 H, m, 3 × Sn(CH2 CH2 CH2
CH3 )3 ), 1.2–1.4 (16 H, m, alkyl), 1.6 (6
H, quint., 3 × Sn(CH2 CH2 CH2 CH3 )3 ), 1.8
(2 H, quint., ThCH2 CH2 –), 2.9 (2 H, t,
ThCH2 CH2 –), 6.9 (1 H, d, ThH, J = 3 Hz), 7.0
(1 H, d, ThH, J = 3 Hz) ppm.
3.2.9 Methyl 5 -n-alkyl-2 ,4-bithienyl-2-carboxylates,
15a–f (Scheme 2)
Methyl 4-bromothiophene-2-carboxylate 4 (2.0 g,
0.009 mol) was added under nitrogen in a single portion to a stirred mixture of the appropriate
2-(tri-n-butyltin)-5-n-alkylthiophene, 14a–f (0.01 mol)
and tetrakis(triphenylphosphine)palladium(0) (0.5 g,
0.0004 mol) in dry N,N-dimethylformamide (DMF; 40
ml). The reaction mixture was heated at 80◦ C for 2
h and then cooled to room temperature. Water was
added and the crude product extracted with diethyl
ether (3 × 50 ml), dried (MgSO4 ) and the solvent
removed in vacuo2 . The resulting white solid was
purified by flash chromatography (SiO2 , eluent 1 : 1
dichloromethane : petroleum ether, b.p. 40–60◦C) to
yield the relevant methyl 5 -n-alkyl-2,4-bithienyl-2carboxylate, 15a–f (64–72%), as a white crystalline
solid. Product purity was confirmed by thin-layer
chromatography (silica gel, single spot). M.p.: C6 H13 ,
15a, 47–49◦C; C7 H15 , 15b, 53–55◦C; C8 H17 , 15c,
57–59◦C; C9 H19 , 15d, 52–54◦C; C10 H21 ,15e, 58–60◦C;
C12 H25 , 15f, 60–62◦C.
The following data refer to methyl 5 -n-octyl-2,4bithienyl-2-carboxylate, 15c, and are typical of the
series:
IR ν max (KBr)/cm−1 : 3010w (ArC–H str.),
2954, 2920, 2855, 1715s (C=O str.), 1512, 1297,
1086, 771.
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (3 H, t,
–CH3 ), 1.2–1.4 (10 H, m, alkyl), 1.8 (2 H, quint.,
ThCH2 CH2 –), 2.8 (2 H, t, ThCH2 CH2 –), 3.9
(3 H, s, ThCO2 CH3 ), 6.7 (1 H, d, ThH, J = 3.5
Hz), 7.0 (1 H, d, ThH, J = 3.5 Hz), 7.4 (1 H, d,
ThH, J = 2 Hz), 8.0 (1 H, d, ThH, J = 2 Hz)
ppm.
3.2.10 5 -n-Alkyl-2 ,4-bithienyl-2-carboxylic acids,
16a–f (Scheme 2)
The 5 -n-alkyl-2,4-bithienyl-2-carboxylic acids, 16a–f,
were prepared according to the method described for
compounds 7a–f (3.2.5, above), but with quantities as
follows: methyl 5 -n-alkyl-2,4-bithienyl-2-carboxylate,
15a–f (0.0006 mol); potassium hydroxide (3.0 g,
0.054 mol); and 80% aqueous ethanol (50 ml).
Purification was by repeated recrystallisation from
aqueous methanol until constant sharp melting points
were achieved3 . M.p.: C6 H13 , 16a, 115.8◦ C; C7 H15 ,
16b, 115.1◦C; C8 H17 , 16c, 109.9◦ C; C9 H19 , 16d,
101.6◦ C; C10 H21 , 16e, 107.9◦ C; C12 H25 , 16f, 103.9◦ C.
The following spectroscopic data refer to 5 -nnonyl-2,4-bithienyl-2-carboxylic acid, 16d and are
typical of the series:
IR ν max (KBr)/cm−1 : 3400–3310m (br O–H
str.), 2954, 1677s (C=O str.), 1466, 1257, 773.
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (3 H, t,
–CH3 ), 1.2–1.4 (12 H, m, alkyl), 1.8 (2 H, quint.,
ThCH2 CH2 –), 2.8 (2 H, t, ThCH2 CH2 –), 4.0 (1
H, s, ThCO2 H, disappears on D2 O shaking), 6.7
(1 H, d, ThH, J = 3.5 Hz), 7.0 (1 H, d, ThH, J
= 3.5 Hz), 7.5 (1 H, d, ThH, J = 2 Hz), 8.0 (1
H, d, ThH, J = 2 Hz) ppm.
Liquid Crystals
3.2.11 (S)-4-(1-Methylheptyloxycarbonyl)phenyl
5 -n-alkyl-2 ,4-bithienyl-2-carboxylates, 17a–f
(Series III) (Scheme 2)
Members of Series III were prepared according to the
method described in 3.2.6 for (S)-4-(1-methyl
heptyloxycarbonyl)phenyl
4-(4-n-alkoxyphenyl)
thiophene-2-carboxylates, 9a–f (Series I), but with
quantities as follows: 5 -n-alkyl-2,4-bithienyl-2carboxylic acid, 16a–f (0.00088 mol), DCC (0.22 g,
0.0011 mol), DMAP (2–3 crystals), (S)-1-methylheptyl
4-hydroxybenzoate, 8a (0.22 g, 0.00088 mol), and dry
dichloromethane (25 ml). The crude residue was
purified by flash chromatography (SiO2 , 1 : 1 CH2 Cl2 :
petroleum ether b.p. 40–60◦C), followed by recrystallisation from ethanol to give the required members
of Series III (48–54%) as white crystalline solids.
Melting points and transition temperatures are listed
in Table 3.
The following analytical and spectroscopic data
refer to (S)-4-(1-methylheptyloxycarbonyl)phenyl
5 -n-dodecyl-2,4-bithienyl-2-carboxylate, 17f, and are
typical of the series:
Found: C, 70.56; H, 8.10%; C36 H50 O4 S2
requires: C, 70.78; H, 8.25%.
m/z: 610 (M+); 499, 361.
IR ν max (KBr)/cm−1 : 2920, 2885, 1730s (C=O
str.), 1715s (C=O str.), 1284, 1037, 767.
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (3 H, t,
–CH3 ), 1.2–1.5 (34 H, m, alkyl + C∗ (H)CH3 ),
1.8 (2 H, quint., ThCH2 CH2 CH2 –), 2.8 (2 H,
t, ThCH2 CH2 –), 5.2 (1 H, m, C∗ (H)CH3 ), 6.7
(1 H, d, ThH, J = 4 Hz), 7.1 (1 H, d, ThH,
J = 4 Hz), 7.3 (2 H, d, ArH, J = 8.5 Hz), 7.6
(1 H, d, ThH, J = 2.5 Hz), 8.1 (3 H, comp. m,
ArH/ThH) ppm.
3.2.12 (S)-4 -(1-Methylheptyloxycarbonyl)biphenyl4-yl 5 -n-alkyl-2 ,4-bithienyl-2-carboxylates, 18a–f
(Series IV)
Members of Series IV were prepared according to
the method described in 3.2.6 for (S)-4-(1-methylhepty
loxycarbonyl)phenyl 4-(4-n-alkoxyphenyl)thiophene2-carboxylates, 9a–f (Series I), with quantities as follows: 5 -n-alkyl-2,4-bithienyl-2-carboxylic acid, 16a–
f (0.00086 mol), DCC (0.21 g, 0.001 mol), DMAP
(2–3 crystals), (S)-1-methylheptyl 4 -hydroxybiphenyl4-ylcarboxylate, 8b (0.28 g, 0.00086 mol), and dry
DCM (25 ml). The crude residue was purified by
flash chromatography (SiO2 ; 1 : 1 dichloromethane :
petroleum ether, b.p. 40–60◦C), followed by recrystallisation from ethanol, to give the required members of
Series IV (48–54%), as white crystalline solids. Melting
points and transition temperatures are listed in
Table 4.
229
The following analytical and spectroscopic data
refer to (S)-4-(1-methylheptyloxycarbonyl)biphenyl-4yl 5 -n-octyl-2,4-bithienyl-2-carboxylate, 18c, and are
typical of the series:
Found: C, 72.16; H, 7.37%; C38 H46 O4 S2
requires C, 72.34; H, 7.35%.
m/z: 630 (M +), 391, 305.
IR ν max (KBr)/cm−1 : 2923, 2854, 1741s (C=O
str.), 1713s (C=O str.), 1285, 1053, 774.
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (6 H, t, 2 ×
–CH3 ), 1.2–1.4 (23 H, m, alkyl + C∗ (H)CH3 ),
1.8 (2 H, quint., ThCH2 CH2 CH2 –), 2.8 (2 H, t,
ThCH2 CH2 –), 5.2 (1 H, m, C∗ (H)CH3 ), 6.7 (1
H, d, ThH, J = 4 Hz), 7.1 (1 H, d, ThH, J = 4
Hz), 7.3 (2 H, d, ArH, J = 8.5 Hz), 7.6 (1 H, d,
ThH, J = 2.5 Hz), 7.7 (4 H, unresolved mult.,
ArH), 8.1 (3 H, comp. m, ArH/ThH) ppm.
3.2.13 Methyl 4-(4 -n-decyloxybiphenyl4-yl)thiophene-2-carboxylate 20
(Scheme 3)
4 -n-Decyloxybiphenyl-4-ylboronic acid, 19 (3.2 g,
0.009 mol), in 1,2-dimethoxyethane (30 ml) was
cross-coupled with methyl 4-bromothiophene-2carboxylate, 4 (2.0 g, 0.009 mol), in the presence
of tetrakis(triphenylphosphine)palladium(0) (0.3
mol%) under similar conditions to those described
in 3.2.4 for methyl 4-(4-n-alkoxyphenyl)thiophene2-carboxylates, 6a–f. After work-up, products were
purified by column chromatography (SiO2 , 2 : 1
CH2 Cl2 : petroleum ether, b.p. 40–60◦C), followed
by recrystallisation from toluene to give the required
methyl
4-(4-n-decyloxybiphenyl-4-yl)thiophene-2carboxylate, 20, 2.5 g (yield 62%), as a white crystalline
solid. Melting points and transition temperatures:
Cr–SmA, 143.7◦C; I: 186.1◦C; SmA, 127.7◦C Cr.
Found: C, 74.67; H, 7.66%; C28 H34 O3 S requires:
C, 74.63; H, 7.60%.
IR ν max (KBr)/cm−1 : 2954, 2918, 1715s (C=O
str.), 1443, 1289, 1086, 774.
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (3 H, t,
–CH3 ), 1.2–1.4 (14 H, m, alkyl), 1.8 (2 H, quint.,
ArOCH2 CH2 CH2 –), 3.9 (3 H, t, ThCO2 CH3 ),
4.0 (2 H, t, ArOCH2 CH2 –), 7.0 (2 H, d, ArH,
J = 9 Hz), 7.5 (2 H, d, ArH, J = 9 Hz), 7.6 (2
H, d, ArH, J = 9 Hz), 7.7 (2 H, d, ArH, J = 9
Hz), 7.8 (1 H, d, ThH, J = 2.5 Hz), 8.1 (1 H, d,
ThH, J = 2.5 Hz) ppm.
3.2.14 4-(4 -n-Decyloxybiphenyl-4-yl)thiophene2-carboxylic acid 21 (Scheme 3)
4-(4 -n-Decyloxybiphenyl-4-yl)thiophene-2-carboxylic
acid, 21, was prepared according to the methods described in 3.2.5 and 3.2.10, respectively,
230
A.S. Matharu et al.
for compounds 7a–f and 16a–f, with quantities
as follows: methyl 4-(4-n-decyloxybiphenyl-4yl)thiophene-2-carboxylate, 20 (1.8 g, 0.004 mol),
potassium hydroxide (2.0 g, 0.036 mol), and 80%
aqueous ethanol (50 ml). After work-up, the crude
acid, 21, was used in the step following without
purification.
IR ν max (KBr)/cm−1 : 3430–3200v br (O–H str.),
2952, 2848, 1677s (C=O str.), 1275, 810.
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (3 H,
t, –CH3 ), 1.2–1.4 (14 H, m, alkyl), 1.8 (2
H, quint., ArOCH2 CH2 CH2 –), 2.9 (1 H, s,
ThCO2 H, disappears on D2 O shaking), 4.0
(2 H, t, ArOCH2 CH2 –), 7.0 (2 H, d, ArH,
J = 9 Hz), 7.5 (2 H, d, ArH, J = 9 Hz), 7.6
(2 H, d, ArH, J = 9 Hz), 7.7 (2 H, d, ArH,
J = 9 Hz), 7.8 (1 H, d, ThH, J = 2.5 Hz), 8.1 (1
H, d, ThH, J = 2.5 Hz) ppm.
3.2.15 (S)-4-(1-Methylheptyloxycarbonyl)phenyl 4(4 -n-decyloxybiphenyl-4-yl)thiophene-2-carboxylate,
22 (Series V) (Scheme 3)
Compound 22 was prepared according to the method
described in 3.2.6 for (S)-4-(1-methylheptyloxycarbo
nyl)phenyl 4-(4-n-alkoxyphenyl)thiophene-2-carboxy
lates, 9a–f (Series 1), with quantities as follows:
4-(4-n-decyloxybiphenyl-4-yl)thiophene-2-carboxylic
acid, 21 (0.28 g, 0.00062 mol), DCC (0.15 g, 0.00074
mol), DMAP (2–3 crystals), (S)-1-methylheptyl
4-hydroxybenzoate, 8a (0.16 g, 0.00062 mol), and
dry DCM (25 ml). The crude residue was purified by
flash chromatography (SiO2 , 1 : 2 dichloromethane :
petroleum ether, b.p. 40–60◦C), followed by repeated
recrystallisation from ethanol to yield the required
(S)-4-(1-methylheptyloxycarbonyl)phenyl
4-(4-ndecyloxybiphenyl-4-yl)thiophene-2-carboxylate,
22
(Series V) as a white crystalline solid (yield, 0.24 g,
52%). Melting points and transition temperatures: Cr,
120.2◦ C; SmC, 158.5◦ C; SmA, 173.0◦C, I.
Found: C, 75.21; H, 7.74%; C42 H52 O5 S requires:
C, 75.41; H, 7.84%.
m/z: 668 (M +): 419.
IR ν max (KBr)/cm−1 : 2923, 2853, 1742s (C=O
str.), 1715s (C=O str.), 1284, 1238, 1161, 777.
δ H (270 MHz; CDCl3 ; Me4 Si): 0.9 (6 H, t, 2 ×
–CH3 ), 1.2–1.4 (27 H, m, alkyl + C∗ (H)CH3 ),
1.8 (2 H, quint., ArOCH2 CH2 CH2 –), 4.0 (2 H,
t, ArOCH2 CH2 –), 5.2 (1 H, m, C∗ (H)CH3 ), 7.0
(2 H, d, ArH, J = 9 Hz), 7.3 (2 H, d, ArH, J =
9 Hz), 7.6 (2 H, d, ArH, J = 9 Hz), 7.63 (2 H,
d, ArH, J = 9 Hz), 7.7 (2 H, d, ArH, J = 9 Hz),
7.8 (1 H, d, ThH, J = 2.5 Hz), 8.1 (2 H, d, ArH,
J = 9 Hz), 8.3 (1 H, d, ThH, J = 2.5 Hz) ppm.
4. Summary and conclusions
2,4-Disubstituted thiophenes can be incorporated into
a variety of molecular cores to form mesogenic compounds. In the present study a range of esters derived
from 4-substituted thiophene-2-carboxylic acids,
and either (S)-1-methylheptyl 4-hydroxybenzoate or
(S)-1-methylheptyl 4 -hydroxybiphenyl-4-carboxylate,
exhibited mesogenic properties which were dependent
on the total number of rings in the molecular core and
the nature of the substituent in the 4-position of the
thiophene ring. Conjugative effects may be important
in the thermal stability of the mesophase.
Four-ring systems are more thermally stable and
are more prone to form mesophases than their threering counterparts. For example, the n = 10 homologue
of Series III (three rings) was not liquid crystalline,
whereas its four-ring counterpart in Series IV exhibited a monotropic SmA phase. Within the four-ring
systems (Series II, IV and V) a non-symmetrical
disposition of the rings on either side of the central ester linkage improved thermal stability (Series
V > Series II > Series IV). A 1,4-phenylene substituent in the 4-position of the thiophene ring (Series I
and II) gave better thermal properties than a 2,5thienyl substituent in this position (Series III and
IV).
The n = 7–10 and 12 homologues of Series II
showed interesting chiral phase behaviour, tentatively
assigned as SmX∗ . Thermal polarising microscopy
showed a sequence of phase changes that may
involve ferroelectric and antiferroelectric transitions.
If this is the case, this is the first reported occurrence of such phase types in 2,4-thiophene systems. Further detailed electro-optical studies are being
undertaken.
A structure–property relationship has been developed for a series of esters comprising 2,4-thiophene
and chiral 1-methylheptyl moieties and a summary is
given in Figure 19.
Acknowledgements
ASM is grateful to Dr Chrissie Grover for synthesis of the
compounds discussed. Sincere thanks are also due to Dr
Gunnar Andersson for his valuable assistance and guidance
with the preliminary electro-optical studies.
Notes
1. I–N and N–K transitions occur simultaneously.
However, the existence of the N phase is observed in
supercooled unco- vered droplets.
2. Ultra-high vacuum was used to remove both DMF and
the tri-n-butyltin bromide by-product. DMF may also be
removed by washing with pentane.
Liquid Crystals
231
Thermal stability order
Most stable
S
CO2
*
Series V
2,4-Isomer
Extra phenyl ring LHS
CO2CHC6H13
CH3
C10H21O
S
*
CO2
CO2CHC6H13
CH3
CO2
CO2CHC6H13
CH3
C10H21O
C10H21
S
S
*
S
CO2CHC6H13
CH3
CO2
CO2CHC6H13
CH3
C10H21O
C10H21
S
*
CO2
S
*
Series II
2,4-Isomer
Extra phenyl ring RHS
Series IV
2,4-Isomer
Exchange phenyl ring LHS with
thiophene and extra phenyl ring
RHS
Series I
2,4-Isomer
Series III
2,4-Isomer
Exchange phenyl ring LHS with
thiophene
Least stable
Figure 19. A structure–property thermal stability relationship for a series of 2,4-disubstituted thiophenes.
3. Melting points were determined using a Mettler FP52
hot-stage. No liquid crystal mesophases were observed,
either on heating to the isotropic melt or on cooling to
the crystalline state.
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