Heterocyclic columnar hexacatenar bisthiazoles

Article abstract of DOI:10.1016/j.tetlet.2009.02.078

Three new series of hexacatenar liquid crystals derived from heterocyclic bisthiazoles and bisoxazoles exhibiting columnar phases were reported, and the formation of columnar phases was strongly sensitive to central heteroatoms and/or their positions inco

Full text of DOI:10.1016/j.tetlet.2009.02.078

Tetrahedron Letters 50 (2009) 1906–1910
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Tetrahedron Letters
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Heterocyclic columnar hexacatenar bisthiazoles
Hui-Hsu Gavin Tsai a, Lung-Chun Chou a, Sheng-Chia Lin a, Hwo-Shuenn Sheu b, Chung K. Lai a,
*
a Department of Chemistry, National Central University, Chung-Li 32001, Taiwan, ROC
b National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new series of hexacatenar liquid crystals derived from heterocyclic bisthiazoles and bisoxazoles
exhibiting columnar phases were reported, and the formation of columnar phases was strongly sensitive
to central heteroatoms and/or their positions incorporated.
Received 22 November 2008
Revised 1 February 2009
Accepted 3 February 2009
Available online 14 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Columnar
Hexacatenar
Bisthiazoles
Bisoxazoles
Polycatenarliquid crystals,1 representedas a newstructuralmotif
in exploring new mesogenic materials, have been extensively inves-
tigated during the past decades. The general structures of such mes-
ogens2 are virtually based on calamitic molecules, and often
constituted by a relatively long central aromatic core surrounded
by more-than-two terminal aliphatic chains on both ends. The core
structures of known examples are composed of rigid elongated poly-
aromatic1b or highly conjugated heterocyclic-fused rings (1,3,4-oxa-
diazole,1a oxazolines,1h bipyridine2a,3). On the other hand, the overall
molecular shapes are more likely considered as lath-like, that is,
between rod- and disc-shaped. A symmetric central core with four
and six terminal chains was often applied to generate mesophases.
Molecules with four, five, or six terminal chains (i.e., so-called
tetra-, penta-, or hexacatenar mesogens2a) often preferably gave
columnar phases. In a few cases, nematic1a, smectic,1a,f or cubic pha-
ses1f,3a were occasionally observed for tetracatenar mesogens. A
phase crossover1a,4 between lamellar and columnar phases4 was also
reported. A few interesting examples1d,5 of columnar phases induced
by lyotropic 1H-imidazoles was also reported. Some disc-shaped
molecules, particularly with an extended conjugated core have
emergedas potentialelectronicmaterialsintechnologyapplications,
such as organic field-effect transistors (OFETs) or photovoltaics6
(OPVs). Interestingly, N, SmC, cubic, and columnar phases in some
tetracatenar mesogens1a,7 were also observed depending on the ter-
minal chain lengths.
interaction8a were attributed to some novel physicochemical and
mechanical properties observed in such materials. Heterocyclic
derivatives were often considered as electron deficient moieties,
and an intermolecular dipole was often induced by donor–acceptor
(D–A; aromatic as donor, heterocycle as acceptor) interaction when
structurally fused to aromatic rings. However, known examples
derived from benzobixazoles or benzobisazoles exhibiting meso-
morphic behavior were relatively rare. A similar structure of 2,6-
bis-(4-[6-(3-ethyl-3-methylene
oxyoxetan)hexyl]phenyl)benzo
[1,2-d-4,5-d0]-bisthiazole9 was recently reported, and this com-
pound formed N, and SmA phase. In this work, we describe three
new series of hexacatenar bisthiazoles and bisoxazoles exhibiting
columnar phases. The formation of columnar phase, Colh or Colr
phase, was determined or controlled by the heteroatoms and/or
the position incorporated.
In order to understand the effect of heteroatoms incorporated
on the formation of the mesophases, five derivatives of bisthiazoles
1 (n = 8, 10, 12, 14, 16) were prepared. The synthetic pathways for
compounds 1–2 are listed in Scheme 1. All compounds were char-
acterized by 1H, 13C NMR spectroscopy, and elemental analysis,
and their mesomorphic properties were investigated by differen-
tial scanning calorimetry (DSC) and polarizing optical microscopy
(POM). The results were compared with those observed by struc-
turally similar bisoxazoles 2 (n = 12).
Under polarized optical microscope, upon heating all materials
melt to give birefringent fluid phases with columnar superstruc-
tures, and a large area of mosaic textures displaying prominent
wedge-shaped defect patterns (Fig. 1) was easily observed. All
compounds 1 were then investigated by powder XRD experiments
(Fig. 2) at various temperatures to examine their mesophase struc-
tures. Rectangular columnar phases10 often displayed two intense
On the other hand, efficient light-emitting diode (LED) materials
derived from heterocyclic benzoxazoles or benzobisazoles8a have
been studied. An efficient
p-stacking and strong intermolecular
* Corresponding author. Tel.: +886 03 4259207; fax: +886 03 4277972.
E-mail address: cklai@cc.ncu.edu.tw (C.K. Lai).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.078

H.-H. G. Tsai et al. / Tetrahedron Letters 50 (2009) 1906–1910
1907
RO
H2N
HX
XH
H2N
HO
NH2
OH
O
RO
OH
NH2
RO
a
a
RO
OR
OR
RO
RO
OR
N
X
X
N
N
O
N
O
RO
OR
RO
OR
RO
OR
R = (CH2)nH
1; X = S
2; X = O
3; R = (CH2)nH
Scheme 1. Reagents and conditions; SOCl2 (1.05 equiv), refluxed in NMP, 48 h, 31–43%.
diffraction peaks in the lower angle region, which were indexed as
(200) and (110) reflections. For example, the compound 1 (n = 12)
gave a diffraction of two strong reflections at d 28.43 Å and 26.13 Å
and also a broad diffuse halo peak at d 4.56 Å at 77.0 °C. This dif-
fraction pattern corresponded to rectangular lattice constants:
a = 52.26 Å and b = 33.88 Å. In other cases, higher orders of diffrac-
tion peaks; d020 ꢀ 16.89 Å, d310 ꢀ 15.42 Å, and d400 ꢀ 13.00 Å with
much lower intensities could be also observed. The cross section
of the disks in Colr arrangement can be circular or elliptical
projected when the disks are perpendicularly stacked or tilted to
the columnar axis. Two columnar Colr phases, c2mm and p2gg
groups, were commonly identified depending on the point symme-
try of the molecules. In compounds 1, a point symmetry of c2mm
group was observed since all reflections were matched to
h + k = 2n (i.e., the sum equal to an even number10b). A model for
the formation of columnar phases2a,3b,4b,10 has been proposed.
The phase behavior of the compounds 1–4 is summarized in
Table 1. The DSC analysis showed a typical columnar phase transi-
tion, crystal-to-columnar-to-isotropic (Cr?Col?I). All compounds
1 (except for shorter derivative, n = 8) and 2 exhibited an enantio-
tropic behavior. All compounds 1 exhibited rectangular columnar
phases (Colr), in contrast, compound 2 formed hexagonal columnar
phase (Colh). For the compounds 1, the melting temperatures
increased with carbon chain length (Tmp = ꢁ1.60?44.8 °C), how-
ever, the clearing temperatures decreased with carbon chain
length (Tcl = 111.7?84.9 °C). On the other hand, the temperature
range of columnar phase was decreasing with carbon chain length,
that is,
D
T = 110.1 °C (n = 10) >
DT = 40.1 °C (n = 16). In addition, a
relatively large enthalpy (
D
H = 11.0–17.7 kJ/mol on heating cycle)
for the Colr?I transition was observed, indicating that the molec-
ular order in the columnar phase was relatively high.
On the other hand, a Colh phase was observed in compound 2
(n = 12). The clearing temperature and transition enthalpy for com-
pound 2 (n = 12) were both dramatically lower than those in com-
pounds 1 (n = 12) by ca.
DTcl = 49.0 °C and DH = 10.48 kJ/mol. A
lowering in both values indicated a much weaker intermolecular
interaction existent in compound 2. Compound 1 (n = 12) has a
wider temperature range than that in compound 2 (n = 12), that
is,
D
Tcol = 89.2 ꢂ 2.0 °C. Compound 2 (n = 12) displayed an optical
texture of pseudo focal-conics with linear birefringent defects
(Fig. 1b), suggesting hexagonal columnar structures. A large area
of homeotropic domains was observed, leading to the conclusion
of preferred uniaxial character in such Colh phase. A variable tem-
perature powder XRD experiment confirmed the Colh phase
formed by compound 2 (see Table 2). A diffraction pattern of a hex-
agonal lattice (Fig. 2) of an intense peak at d 29.13 Å at 46 °C was
displayed, which corresponded to Miller indice 100. This reflection
pattern corresponds to an inter-columnar distance (a parameter of
the hexagonal lattice) of 33.64 Å. The pseudo-hexagonal lattice
constant, that is, a rectangular lattice with an axial ratio of a/b from
Figure 1. The optical textures (magnification ꢃ 100) observed under POM upon the
cooling cycle. Top plate: Colr phase at 95.0 °C of 1 (n = 10), middle plate: Colh phase
at 44.0 °C of 2 (n = 12), and bottom plate: Colh phase at 40.0 °C of 3 (n = 12).

1908
H.-H. G. Tsai et al. / Tetrahedron Letters 50 (2009) 1906–1910
Table 1
The phase transitions and temperaturesa of compounds 1–4
28.43(110)
26.13(200)
40000
30000
20000
10000
0
1
n = 8
10
Cr
I
3000
Cr
Cr
Cr
Cr
Colr
Colr
Colr
Colr
I
I
I
I
16.89(020)
2500
2000
1500
1000
15.42(310)
13.00(400)
12
halo
4.0
4.5
5.0
5.5
6.0
6.5
5
10
15
2θ
20
25
30
30
30
14
40000
30000
20000
10000
29.13(100)
16
2
3
12
Cr
Colh
I
10
12
14
16
Colh
Colh
Colh
Cr
I
I
I
halo
Cr
Cr
10
20
2θ
12000
10000
8000
6000
4000
2000
0
29.26(100)
I
4
8
Cr
Cr
Cr
I
I
I
600
16.94(110)
halo
400
4.0
4.2
4.4
4.6
4.8
5.0
12
16
5
10
15
20
25
2θ
Figure 2. The powder XRD diffraction patterns. Top plot: 1 (n = 12) measured at
77.0 °C, middle plot: 2 (n = 12) measured at 46 °C, and bottom plot: 3 (n = 12)
measured at 30.0 °C.
a
n = the carbon numbers of terminal alkoxy chains; Cr = crystal; Colr = rectangular
the ideal hexagonal of 31/2 a for compound 1 was ranged in 1.91
(n = 10)–1.54 (n = 12), and this value indicated that the structural
departure from the ideal hexagonal lattice was about 15–19% in
this system. A mesophase10 that changed from Colr to Colh has
been observed in other systems, which was generally attributed
to the smaller core interaction necessary for the formation of the
Colh phases. The tilted Colr phase reduced the interactions between
the bulky side chains and allowed closer contacts between the
cores. A larger atomic size of sulfur atom over oxygen atom was
more polarized and a mesophase easily induced, leading to an
improved mesophase. A proposed model for molecular arrange-
ments in such columnar phases was shown in Figure 3.
columnar, Colh = hexagonal columnar; I = isotropic phases.
bDetermined by optical microscope.
To understand the effect of heteroatom position incorporated in
core structure, compounds 3 were prepared. Both compounds 2
and 3 are structurally similar. Two N atoms and O atoms in com-
pounds 2 are incorporated in para-position, however, they are in
meta-position in compounds 3. Interestingly, the compounds 3
formed same Colh phases as compound 2. Both clearing and melt-
ing temperatures of 3 (n = 12) were lower than those in compound
2 (n = 12) by
DTcl = 3.9 °C and DTmp = 35.3 °C, respectively. In
contrast, the temperature range of the columnar phase in

H.-H. G. Tsai et al. / Tetrahedron Letters 50 (2009) 1906–1910
1909
Table 2
Detailed indexation at a given temperature of columnar phases for compounds 1–3
Compound Mesophase d-Spacing (Å) Lattice const. Miller indices a/b
obsd (calcd)
1 (n = 10)
Colr c2mm
at 75.0 °C
27.02
(27.02)
25.07
(25.07)
15.15
(15.20)
14.19
(14.15)
13.47
(13.51)
12.52
(12.54)
5.57
ar = 54.04
br = 28.30
200
110
310
020
400
220
1.91
O
O
O
O
O
O
O
O
O
S
N
N
S
S
N
N
S
S
N
N
S
h
O
O
4.55
Halo
O
O
O
O
O
O
1 (n = 12)
1 (n = 14)
1 (n = 16)
Colr c2mm
at 77.0 °C
28.43
(28.43)
26.13
(26.13)
16.89
(16.94)
15.42
(15.49)
13.00
(13.07)
5.61
ar = 52.26
br = 33.88
110
200
020
310
400
1.54
1.57
1.59
O
Figure 3. Schematic representing molecular arrangements of hexacatenar meso-
gens proposed in the hexagonal columnar phase.
h
Halo
4.56
Colr c2mm
at 63.0 °C
29.92
(29.92)
27.78
(27.78)
17.69
(17.75)
16.39
(16.42)
13.88
(13.89)
5.57
ar = 55.56
br = 35.50
110
200
020
310
400
HO
RO
O
H2N
NH2
RO
OH
RO
a
h
Halo
4.55
RO
Colr c2mm
at 47.0 °C
31.60
(31.60)
29.64
(29.64)
18.56
(18.67)
17.42
(17.47)
15.74
(15.80)
14.77
(14.82)
5.67
ar = 59.28
br = 37.34
110
200
020
310
220
400
HO
RO
OR
N
N
RO
OR
OH
5
OR
b
OR
h
Halo
N
4.54
RO
OR
O
O
2 (n = 12)
3 (n = 10)
3 (n = 12)
Colh p6mm
at 45.0 °C
29.13
(29.13)
4.53
ah = 33.64
ah = 31.34
ah = 33.80
100
1.73
1.73
1.73
RO
OR
N
Halo
100
RO
Colh p6mm
at 50.0 °C
27.14
(17.14)
4.48
4; R = (CH2)nH
Halo
100
Scheme 2. Reagents and conditions. (a) 3,30-Dihydroxybenzidine (0.5 equiv),
CH3COOH (drops), refluxed in absolute C2H5OH, 6 h, 71–98%; (b) Pb(OAc)4
(2.2 equiv), refluxed in dry CH2Cl2, 2 h, 90–96%.
Colh p6mm
at 30.0 °C
29.27
(29.27)
16.94
(16.90)
4.43
110
Halo
100
probably attributed to the larger intermolecular dipole–dipole
interactions than that in compound 2. Density functional theory
calculations11 at B3LYP/6-31G(d,p) theoretical level showed that
the optimized structures12 of compounds 2 and 3 have a dipole
moment of 3.88 and 6.06 D, respectively. The two electronegative
N and O atoms incorporated in trans-position of compounds 2
might have a cancellation of the two central dipoles leading to a
smaller net dipole moment. In contrast, the position effect of the
3 (n = 14)
Colh p6mm
at 50.0 °C
30.71
(30.71)
17.64
(17.73)
4.54
ah = 35.46
1.73
110
Halo
compounds 3 was much wider than that in compound 2. The
improved mesomorphic behavior observed in compounds 3 was

1910
H.-H. G. Tsai et al. / Tetrahedron Letters 50 (2009) 1906–1910
On the other hand, a substitution of sulfur atom over oxygen atom in
core structure 1 often led to a red shift, due the higher polarizability
0.20
0.15
0.10
0.05
0.00
-0.05
1
2
3
4
and basicity of sulfur relative to oxygen. The compound
4
(kmax = 344, kem = 407 nm) has both spectra blue shifted due to the
conjugated length interrupted by two non-coplanar biphenyl rings.
However, the quantum yields measured in CH2Cl2 estimated with
anthracene as a standard (uf = 0.27 in hexane) were ranged from
30.1% to 67.4%.
Acknowledgment
This work was supported by the National Science Council of
Taiwan, ROC (NSC-96-2113-M-008-003-MY2, NSC-96-2113-M-
008-006-MY2 & NSC-95-2752-M-008-010-PAE).
250
300
350
400
wavelength (nm)
Supplementary data
250
200
150
100
50
Supplementary data (synthetic procedures and characterization
data for all new compounds 1–4) associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2009.02.078.
1
2
3
4
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slightly red-shift than those of compound
2
(kmax = 355,
k
em = 407 nm) due to the larger size and more polarized sulfur atom.