Nonsymmetric bent-core liquid crystals based on a 1,3,4-thiadiazole core unit and their nematic mesomorphism

Article abstract of DOI:10.1021/cm200643u

The synthesis and thermotropic properties of novel V-shaped molecules having a central 1,3,4-thiadiazole core with a bend-angle of 160° are reported. The compounds consist of a shape-persistent oligo(phenylene ethynylene) scaffold with lateral alkyloxy su

Full text of DOI:10.1021/cm200643u

ARTICLE
pubs.acs.org/cm
Nonsymmetric bent-core liquid crystals based on a 1,3,4-thiadiazole
core unit and their nematic mesomorphism
Jens Seltmann,† Alberto Marini,‡,§ Benedetta Mennucci,‡ Sonal Dey,^ Satyendra Kumar,^ and
Matthias Lehmann*,†,#
†Institute of Chemistry, Chemnitz University of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany
#Institute of Organic Chemistry, University of W€urzburg, Am Hubland, 97074 W€urzburg, Germany
‡Dipartimento di Chimica e Chimica Industriale, Universitꢀa di Pisa, via Risorgimento 35, 56126 Pisa, Italy
^Department of Physics, Kent State University, Kent, Ohio 44242, United States
ABSTRACT: The synthesis and thermotropic properties of novel V-shaped molecules
having a central 1,3,4-thiadiazole core with a bend-angle of 160ꢀ are reported. The
compounds consist of a shape-persistent oligo(phenylene ethynylene) scaffold with lateral
alkyloxy substituents. One of the terminal aromatic units possesses an alkoxy chain capped
by an ethyl ester group while the second terminus is a pyridyl group. They exhibit
enantiotropic nematic phases and are characterized by polarized optical microscopy,
differential scanning calorimetry, and X-ray diffraction. Results from conoscopy indicate a
biaxial nature of the nematic phase near room temperature. DFT calculations of dipole
moments and molecular polarizabilities are used to substantiate the experimental findings.
KEYWORDS: V-shaped nematogens, biaxial nematics, X-ray, conoscopy, DFT calculations
’ INTRODUCTION
the formation of the Nb phase. However, the large local dipole
moment of 1,3,4-thiadiazoles along its bisector (see Table 2)
is believed to favor the formation of the Nb phases.16 We attached
a pyridyl group to one of the arms of the molecules 1aꢀd
(Figure 1) instead of an ester terminus. It is known to destabilize
the nematic phase and lower the melting point relative to cyano
substituted derivatives of this mesogen family.17 Nevertheless,
we were confident to find a stabilization of the mesophases at low
temperature owing to the desymmetrisation of the molecules and
consequently a weaker crystallization tendency. Additionally, the
pyridyl group provides the possibility to supramolecularly
modify the molecular structure by hydrogen bonding with an
H-bond donor.
Although the first bent-core liquid crystals (LC) were synthe-
sized in the 1920s by Vorl€ander,1 they attracted much attention
only in the last fifteen years. Since the report of the existence of
new mesophases in banana molecules by Niori et al., ferro-
electric, antiferroelectric and ferrielectric behavior of these
materials has been established.2 Theoretical considerations also
predicted the existence of the elusive thermotropic biaxial
nematic (Nb) phase in bent-core molecules with a specific range
of the bend-angle.3,4 Materials forming such phases are highly
attractive not only to elucidate theory, but also for application in
a new generation of display technology. The Nb phase was
predicted by Freiser5 in 1970 and subsequently realized in a
lyotropic system by Yu and Saupe6 and in liquid crystalline
polymers.7 Until recently, all endeavors to design thermotropic
low molar mass systems led to materials for which the biaxiality
could not be substantiated.8 In 2004, V-shaped oxadiazole
derivatives were shown to exhibit the Nb phase at high tempera-
tures by X-ray and 2H NMR spectroscopy.9,10 Since then, various
bent-shape or banana mesogens with flexible molecular scaffolds
have been reported to form the Nb phase. However, the exact
nature of this phase and the interpretation of results from various
methods continue to evolve.11ꢀ14
’ EXPERIMENTAL SECTION
Reagents were obtained from Acros and Sigma-Aldrich and used
as received. The synthesis of compounds 2ꢀ4 was described
previously.15,17 The target compounds were obtained by analogous
procedures, detailed in reference 15. Column chromatography was
carried out on silica 60 (Merck, mesh 70ꢀ230). PFT 1H and 13C
NMR spectra were recorded in CDCl3 with a Varian Oxford 400 MHz
spectrometer with the residual solvent signal at 7.26 ppm as a reference.
Mass spectra were obtained on a Finnigan MAT95 (FD MS). Elemental
analysis was carried out in the microanalytical laboratory at the
University of Mainz. POM observations were made with a Zeiss
We report here the synthesis of unique shape-persistent bent-
core liquid crystals. Recently, enantiotropic nematic phases were
realized at low temperature in mesogens with a 1,3,4-thiadiazole
central core and ethyl (oxyalkyl)carboxylate groups on both
terminal aromatic units.15 The bend-angle θ of approximately
160ꢀ deviates from the theoretically predicted angle of 109ꢀ for
Received: January 27, 2011
Revised:
April 11, 2011
Published: April 28, 2011
r
2011 American Chemical Society
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Figure 1. Target structure of nonsymmetric V-shaped molecules 1aꢀd and comparable mesogens 1eꢀg reported previously.15,17
Figure 2. Synthesis of V-shaped thiadiazole derivatives 1aꢀd.
Axioscop 40 equipped with a Linkam THMS600 hot stage. DSC was
performed using a Perkin-Elmer Pyris 1.
Quantum mechanical calculations were performed at in vacuo DFT
level of theory, by employing the combination of B3LYP functional and
6-311 g(d,p) basis set. All the calculations were done with the Gaussian
09 package.18
X-ray measurements were made at the undulator beamline station
6IDB of the Midwestern Universities Collaborative Access Team’s
sector at the Advanced Photon Source. A triple bounce Si monochro-
mator was used to select 16.2 keV (λ = 0.76534 Å) X-rays. A pair of X-Y
slits was used to collimate the incident X-ray beam to 100 μm ꢁ 100 μm.
An image plate detector MAR345 was placed at a distance of 506.4 mm
from the sample. The incident flux was sufficiently attenuated to avoid
radiation damage and to keep the detector from saturating. A silicon
standard (NIST 640C) was used to calibrate the spectrometer. Back-
ground scattering was recorded using an empty capillary and subtracted
from diffraction data. The 2D diffraction patterns thus obtained were
used to deduce 2θꢀθ and χ-scans using the Fit2d software package19
and to calculate the structural parameters and correlations lengths,
respectively.
1a. 1H NMR (400 MHz, CDCl3): δ [ppm] = 0.91 (m, 12H, CH3);
1.26 (t, 3H, CH3, 3J = 7.2); 1.36 (m, 16H, CH2); 1.56 (m, 8H, CH2);
1.84 (m, 12H, CH2); 2.39 (t, 2H, CH2COOEt, 3J = 7.0); 3.96 ꢀ 4.08 (m,
10H, OCH2); 4.12 (q, 2H, COOCH2CH3, 3J = 7.2); 6.85 (2H, AA0BB0);
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7.01 (s, 1H); 7.02 (s, 1H); 7.03 (s, 1H); 7.05 (s, 1H); 7.37 (2H,
AA0BB0); 7.47 (2H, AA0BB0); 7.63 (4H, AA0BB0); 8.00 (4H, AA0BB0);
8.60 (2H, AA0BB0); 13C NMR (100 MHz, CDCl3): δ [ppm] = 14.18,
14.21, 14.38 (CH3); 21.7, 22.78, 22.80, 25.86, 25.89, 28.7, 29.37, 29.40,
29.43, 29.45, 31.70, 31.75, 34.0 (CH2); 60.5 (COOCH2CH3); 67.6;
69.6, 69.7, 69.8 (OCH2); 84.7, 88.7, 89.2, 90.7, 92.3, 94.0, 94.7, 95.6
(CtC); 112.9, 113.1 (Cq); 114.6 (Ct), 114.7 (Cq); 115.2, 115.5 (Cq);
116.7, 116.8, 117.0 (Ct); 125.5 (Ct); 126.4, 126.7 (Cq); 127.1, 127.94
(Ct); 129.5, 129.8, 131.7 (Cq); 132.3, 132.4 (Ct); 133.2 (Ct); 149.9
(Ct); 153.5, 153.8, 154.0, 154.1 (Cq, C-OC6H13); 159.2 (Cq, C-OCH2);
167.7, 167.8 (Cq, NdC-S); 173.5 (Cq, CdO); Elemental anal. [%]
Calcd for C76H85N3O7S: C 77.06; H 7.23; N 3.55; S 2.71. Found: C
76.86; H 7.27; N 3.53; S 2.73. FD MS: m/z [%]: 1183.8 (100, Mþ);
1184.8 (66.6, [Mþ1]þ); 1186.0 (24.5, [Mþ2]þ); 1187.7 (16.6,
[Mþ3]þ).
Table 1. DSC Data of 1a-g at a Scanning Rate of 10 ꢀC/min
1b. 1H NMR (400 MHz, CDCl3): δ [ppm] = 0.90 (m, 12H, CH3);
1.26 (t, 3H, CH3, 3J = 7.2); 1.37 (m, 16H, CH2); 1.55 (m, 10H, CH2);
1.71 (m, 2H, CH2); 1.85 (m, 10H, CH2); 2.34 (t, 2H, CH2COOEt, 3J =
7.2); 3.98 (t, 2H, OCH2, 3J = 6.4); 4.05 (m, 8H, OCH2); 4.13 (q, 2H,
COOCH2CH3, 3J = 7.2); 6.85 (2H, AA0BB0); 7.01 (s, 1H); 7.02 (s, 1H);
7.03 (s, 1H); 7.05 (s, 1H); 7.38 (2H, AA0BB0); 7.45 (2H, AA0BB0); 7.64
(4H, AA0BB0); 8.02 (4H, AA0BB0); 8.61 (2H, AA0BB0); 13C NMR (100
MHz, CDCl3): δ [ppm] = 14.19, 14.22, 14.4 (CH3); 22.8, 24.8, 25.8,
25.9, 29.0, 29.38, 29.42, 29.45, 31.7, 31.8 (CH2); 34.4 (CH2COOEt);
60.4 (COOCH2CH3); 67.8, 69.70, 69.77, 69.81 (OCH2); 84.7, 88.7,
89.2, 90.7, 92.3, 94.0, 94.7, 95.6 (CtC); 112.9, 113.2 (Cq); 114.6 (Ct);
114.7, 115.2, 115.4 (Cq); 116.8, 116.9, 117.0 (Ct); 125.6 (Ct); 126.4,
126.7 (Cq); 128.0 (Ct); 129.5, 129.8, 131.7 (Cq); 132.36, 132.42, 133.2
(Ct); 149.9 (Ct); 153.6, 153.9, 154.0, 154.1 (Cq, C-OC6H13); 159.3
(Cq, C-OCH2); 167.7, 167.9 (Cq, NdC-S); 173.8 (Cq, CdO); Elemental
anal. [%] Calcd for C77H87N3O7S C 77.16; H 7.32; N 3.51; S 2.68. Found
C 76.60; H 7.12; N 3.52; S 2.36. FD MS: m/z [%]: 1196.8 (100, Mþ);
1197.8 (96, [Mþ1]þ).
phase transitionsa
(onset (ꢀC)/ΔH (kJ molꢀ1))
compd
ΔSN [J molꢀ1 Kꢀ1
]
1a
1b
1c
1d
1e
1f
Cr 109.3/47.0 N 150.3/0.9 I
Cr 98.4/21.4 N 150.2/0.7 I
Cr 90.9/45.4 N 154.7/0.9 I
Cr 105.5/58.6 N 144.6/0.9 I
Cr 68.4/38.5 N 177.7/1.9 I
Cr 98.5/65.5 N 173.9/1.8 I
Cr 141.8/45.1 (N 123.3/-0.1)b I
2.1
1.7
2.1
2.2
4.2
4.0
0.3
1g
a Cr crystal, N nematic phase, I isotropic phase; CrꢀN transition data is
given for the first heating cycle; NꢀI transition data is reported for the
second heating cycle. b Data given for the first cooling.17
Table 2. DFT Calculated Bend Angle Θ, Dipole Moment
Components (μX, μY, μZ), Modulus (μ), and Angle γ, Formed
with Respect to the Longitudinal Axis X (see Figure 3 for
definition)a
1c. 1H NMR (400 MHz, CDCl3): δ [ppm] = 0.90 (m, 12H, CH3);
1.26 (t, 3H, CH3, 3J = 7.2); 1.37 (m, 18H, CH2); 1.55 (m, 10H, CH2);
1.67 (m, 2H, CH2); 1.85 (m, 10H, CH2); 2.32 (t, 2H, CH2COOEt,
3J = 7.2); 3.97 (t, 2H, OCH2, 3J = 6.4); 4.05 (m, 8H, OCH2); 4.13 (q, 2H,
COOCH2CH3, 3J = 7.2); 6.86 (2H, AA0BB0); 7.01 (s, 1H); 7.02 (s, 1H);
7.03 (s, 1H); 7.05 (s, 1H); 7.38 (2H, AA0BB0); 7.46 (2H, AA0BB0); 7.65
(4H, AA0BB0); 8.02 (4H, AA0BB0); 8.61 (2H, AA0BB0); 13C NMR (100
MHz, CDCl3): δ [ppm] = 14.2, 14.4 (CH3); 22.8, 25.0, 25.88, 25.90,
29.0, 29.2, 29.38, 29.42, 29.5, 31.7, 31.8 (CH2); 34.4 (CH2COOEt);
60.4 (COOCH2CH3); 68.0, 69.7, 69.76, 69.79 (OCH2); 84.7, 88.7, 89.2,
90.8, 92.3, 94.0, 94.8, 95.6 (CtC); 112.9, 113.1 (Cq); 114.6 (Ct); 114.7,
115.2, 115.4 (Cq); 116.8, 116.9, 117.0 (Ct); 125.6 (Ct); 126.4, 126.7
Θ
μX
μY
μZ
μ
γ
DTPy mod 1
162.1
159.9
159.3
159.2
158.3
5.64
5.54
7.41
7.00
0.2
3.84
3.87
ꢀ0.01
ꢀ0.01
0.00
6.8
6.8
8.0
7.0
4.8
34.2
34.9
21.8
1.5
DTPy mod 2
1c
1d
1e
2.96
ꢀ0.11
4.81
0.15
0.02
87.6
a The values relative to angles and dipole moment are expressed in
degree (ꢀ) and debye (D), respectively.
Figure 3. Molecular structures of DTPy models optimized at DFT level. Model 1 and model 2 differ in the length of the lateral aliphatic chains.
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(Cq); 128.0 (Ct); 129.5, 129.8, 131.8 (Cq); 132.36, 132.42, 133.2 (Ct);
149.8 (Ct); 153.5, 153.8, 154.0, 154.1 (Cq, C-OC6H13); 159.4 (Cq,
C-OCH2); 167.7, 167.8 (Cq, NdC-S); 173.9 (Cq, CdO); Elemental
anal. [%] Calcd for C78H89N3O7S: C 77.26; H 7.40; N 3.47; S 2.64.
Found: C 76.77; H 7.18; N 3.60; S 2.64. FD MS: m/z [%]: 1210.6 (100,
Mþ); 1211.6 (96, [Mþ1]þ).
temperature of 60 ꢀC to afford the neat target compounds 1aꢀd in
good over all yields of 43ꢀ73% (two steps) and high purity.
All molecules show enantiotropic nematic phases with a
relatively wide temperature range (Table 1), that can be super-
cooled to rt. The melting points are lowered from 1a to c and the
mesophases’ range continuously widens. The nonsymmetric
pyridyl derivative 1c has the lowest melting point and crystal-
lization tendency in the series and the mesophase is indeed more
stable at low temperatures when compared to the symmetric
compound 1f. The clearing point indicates an oddꢀeven effect
with the number of carbons in the terminal spacers. The enthalpy
and entropy changes at the clearing point are comparable to a
typical nematic to isotropic (I) transition suggesting a first order
transition. In contrast, mesogen 1g, with two pyridyl groups,
exhibits a monotropic phase.17 Two terminal chains result in
enantiotropic mesogens with low melting temperatures depen-
dent on the length of the second alkyl chain (1e,f).17 Evidently, the
length of terminal alkyl spacers has important influence on the
mesophase stability for this subfamily of shape-persistent bent-
core mesogens.
1d. 1H NMR (400 MHz, CDCl3): δ [ppm] = 0.90 (m, 12H, CH3);
1.26 (t, 3H, CH3, 3J = 7.2); 1.30 ꢀ 1.68 (m, 32H, CH2); 1.75 ꢀ 1.90 (m,
3
10H, CH2); 2.30 (t, 2H, CH2-COOEt, J = 7.2); 3.97 (t, 2H, OCH2,
3J = 6.4); 4.05 (m, 8H, OCH2); 4.13 (q, 2H, COOCH2CH3, 3J = 7.2); 6.86
(2H, AA0BB0); 7.01 (s, 1H); 7.02 (s, 1H); 7.04 (s, 1H); 7.05 (s, 1H);
7.38 (2H, AA0BB0); 7.46 (2H, AA0BB0); 7.65 (4H, AA0BB0); 8.01 (4H,
AA0BB0); 8.61 (2H, AA0BB0); 13C NMR (100 MHz, CDCl3): δ [ppm] =
14.2, 14.4 (CH3); 22.8, 25.0, 25.88, 25.90, 29.0, 29.2, 29.38, 29.42, 29.5,
31.7, 31.8 (CH2); 34.4 (CH2COOEt); 60.4 (COOCH2CH3); 68.0,
69.7, 69.76, 69.79 (OCH2); 84.7, 88.7, 89.2, 90.8, 92.3, 94.0, 94.8, 95.6
(CtC); 112.9, 113.1 (Cq); 114.6 (Ct); 114.7, 115.2, 115.4 (Cq); 116.8,
116.9, 117.0 (Ct); 125.6 (Ct); 126.4, 126.7 (Cq); 128.0 (Ct); 129.5,
129.8, 131.8 (Cq); 132.36, 132.42, 133.2 (Ct); 149.8 (Ct); 153.5, 153.8,
154.0, 154.1 (Cq, C-OC6H13); 159.4 (Cq, C-OCH2); 167.7, 167.8
(Cq, NdC-S); 173.9 (Cq, CdO); Elemental anal. [%] Calcd for
C79H91N3O7S C 77.35 ; H 7.48 ; N 3.43 ; S 2.61. Found: C 76.79 ; H
7.11 ; N 3.64 ; S 2.75. FD MS: m/z [%]: 1224.6 (100, Mþ); 1225.6 (86,
[Mþ1]þ).
DFT calculations18 of molecular properties of the symmetric
and nonsymmetric mesogens were performed to gain more
insight into the influence of the geometry, dipoles, polarizability
and molecular biaxiality on the nature and stability of the nematic
phases. Tables 2 and 3 summarize the data from simplified
molecular models DTPy mod1,2 (Figure 3) and selected
molecules (1c,d,e).
’ RESULTS AND DISCUSSION
The synthesis of 1a-d is based on two consecutive Sonogashira
Hagihara reactions of different terminal alkyne arms 3aꢀe with a
bromo and iodo substituted 1,3,4-thiadiazole core 2 (Figure 2).
The hockey stick compound 4 was first prepared with the pyridyl
terminated arm 3a in a temperature controlled regioselective
cross coupling at room temperature (rt).14,17 Subsequently, the
arms 3bꢀe with a terminal ester group were attached at an elevated
The analysis shows that the bend-angle is approximately 159ꢀ
for almost all mesogens and is slightly influenced by the lateral
aliphatic chains (cp. DTPy mod1, DTPymod2 and 1c). For the
symmetric mesogen 1e, the dipole points almost in the direction
of the apex bisector. Molecules 1c and 1d possess a considerably
larger dipole along the molecule’s long axis. However, the angle
γ, between the dipole and the molecular X-axis, is modified by
the length of the terminal spacer. This explains the slight
oddꢀeven effect observed for the clearing points of 1aꢀd
(Table 1). As expected the polarizability R is largest along the
longitudinal X-axis (Table 3). Owing to the large bending angle,
the asymmetry parameter η = [(RYY ꢀ RZZ)/(RXX ꢀ Riso)] =
0.30ꢀ0.44 (RXX, RYY, RZZ being the principle components of the
polarizability tensor and Riso = (RXX þ RYY þ RZZ)/3) is rather
small. However, it increases when the lateral chains are attached
(cp. DTPy mod1,2; Table 3). The asymmetry parameter
becomes slightly larger for nonsymmetric compounds 1c,d
(0.41) compared with the symmetric 1e (0.38).
Table 3. DFT Calculated Principal Components (rXX, rYY,
r
ZZ), isotropic component riso = (rXX þ rYY þ rZZ)/3,
anisotropy Δr = [rXX ꢀ (rYY þ rZZ)/2], and Asymmetry,
η = [(rYY ꢀ rZZ)/(rXX ꢀ riso)], Parameters Relative to the
Molecular Polarizability Tensor, r, in the Cartesian Refer-
ence Frame Reported in Figure 3a
RXX
RYY
RZZ
Riso
ΔR
ηR
DTPy mod 1
2303
2511
2716
2734
2993
618
964
250
434
506
515
572
1057
1303
1420
1433
1561
1869
1812
1944
1951
2148
0.30
0.44
0.41
0.41
0.38
DTPy mod 2
1c
1d
1e
1039
1050
1117
The nature of the mesophases was first probed with polarized
optical microscopy (POM). Compounds 1aꢀd reveal highly
fluid Schlieren textures in the enantiotropic (Figure 4A, B)
and monotropic phase regions with integer and half-integer
disclinations indicating a nematic phase. On glass substrates, all
a All polarizability components and the anisotropy parameter are
expressed in Bohr^3 (with 1 Bohr = 0.52917 Å).
Figure 4. (A, B) Schlieren textures of nematic mesophases of 1c at 145 ꢀC (A) and 1d at 67 ꢀC (B). (C) Conoscopic image of the homeotropic aligned
sample 1c at 130 ꢀC with a full wave retardation plate in the light path indicating the optical positive nature of the nematic phase.
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Figure 5. Orthoscopy and conoscopy of homeotropically oriented sample 1c at rt. Upper row: Sample oriented parallel to the polarizer; lower row:
Sample rotated by 45ꢀ. (A,B) Orthoscopic images; (C,D) conoscopic observations; (E,F) view on the optical axes by conoscopy using a circular
polarizer.
compounds formed large homeotropic areas. Conoscopy, with a
full wave retardation plate in the path of light, revealed an
optically positive medium (Figure 4C). Because the refractive
index is directly proportional to the polarization of the sample,
we assume that the molecular long axis (X axis, cp. Figure 3) with
the largest polarizability is oriented normal to the substrate. All
samples were easily homeotropically aligned on glass presumably
due to H-bonding between surface OH-groups and pyridyl
H-bond acceptors. The latter was further confirmed by the
absence of homeotropic alignment for 1:1 mixtures of pyridyl
derivatives with decanoic acid.
In high-temperature nematic phases, a conoscopic cross is
observed which did not change upon rotation of the sample. The
use of an additional circular polarizer revealed only one dark spot
pointing to a uniaxial nematic phase (not shown). At approxi-
mately 50 ꢀC, the uniaxial cross split into two separated isogyres
(Figure 5C, D). With a circular polarizer, the two dark spots can
be clearly seen (E, F) separately. Rotation of the sample by 45ꢀ in
the orthoscopic mode causes strong change of the birefringence
(A, B). These observations indicate a biaxial nature of the low-
temperature monotropic nematic phase.
X-ray diffraction pattern of magnetic field aligned samples
were recorded upon cooling the samples from the isotropic
phase. Figure 6 shows the diffraction pattern of 1c in the isotropic
liquid, in the enantiotropic nematic at 139.0 ꢀC and in the
monotropic temperature range of the nematic phase at 75.0 and
Figure 6. Temperature-dependent X-ray diffraction patterns of mag-
45.6 ꢀC. There are two halos in the isotropic phase; one
netic field aligned sample 1c in (A) the isotropic phase, (B) the
corresponds to the typical mean separation of aliphatic chains
of 4.5 Å and the other to a separation (17.5 Å) of shape-persistent
molecules along the bisector. A reflection corresponding to the
molecular long axis was not observed for this type of molecules.
enantiotropic nematic phase at 139.0 ꢀC and the monotropic nematic
phase at (C) 75.0 and (D) 45.6 ꢀC. The peaks in (D) are shown on
different scales so that each of these are clearly visible.
In the nematic phase, six diffuse reflections at (i) 15.1ꢀ14.3 Å,
domains (Figure 7) about the direction of the magnetic field.
(ii) 4.5ꢀ4.2 Å, (iii) 10.3ꢀ9.9 Å, (iv) 5.2 Å, (v) 4.6ꢀ4.4 Å, and
Within each domain, molecular long axes align along the
(vi) 3.1ꢀ3.0 Å are apparent at 139.0 and 45.6 ꢀC. The peak (i) at
magnetic field, whereas the director corresponding to the direc-
tion of the bisector has an orientation that varies randomly from
domain to domain in a plane perpendicular to the magnetic field.
Consequently, intensity of the peaks corresponding to the two
length scales and originating from various domains probed by the
X-ray beam lies along the meridian. All other peaks lie on the
equator. The intensity of peak (iii) at ∼10.3 Å corresponds to the
15.1 Å is positioned at the meridian and corresponds to the
average separation along the bisector (cp. Figure 7). The second
diffuse peak (ii) on the meridian at 4.5 Å arises from the mean
transverse distance between hydrocarbon molecules, i.e., the
separation in the direction normal to the plane of the conju-
gated scaffold. Simultaneous observations of reflections (i) and
(ii) can be understood assuming a distribution of different biaxial
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Figure 7. Model of a possible short-range order of 1c in its nematic phase. The larger d spacings are values measured at 139 ꢀC and the lower ones at
45.6 ꢀC. The vectors L, M, N correspond to the molecular axes. (A) Orientation resulting in reflection (i) and all equatorial reflections. (B) Orientation
leading to reflection (ii) and all equatorial reflections.
average separation of sulfur atoms along the long axis.14 The
diffuse reflection (v) at 4.6 Å along the equator may be attributed
to the average distance between the lateral aliphatic chains
oriented perpendicular to the magnetic field. Although the origin
of the very diffuse and weak signals (iv) at 5.2 visible at all nematic
temperatures is not known, it may belong to the average width of
the six aromatic cores in the two arms of the molecule. The
source of the weakest of the signals (vi) corresponding to ∼3.1 Å
is unclear. We will continue further investigations to better
understand it and to rule out its being an experimental artifact.
The correlation lengths from the X-ray signals are estimated
using ξ = 2π/fwhm, (fwhm = full width at half-maximum), and
are found to be 16.4, 8.3, 11.0, 9.5, 33.3, and 4.2 Å. Almost all of
these liquidlike correlation lengths are in the range of one to
three molecular units, which together with DSC and POM
results point to the nematic nature of the phase. An exception
is the signal (v) at 4.6 Å with a correlation length of seven to eight
subunits. This might be explained by steric constrain and
subsequently a relatively higher order of the lateral hexyl chains.
Upon cooling to ambient temperature the correlation lengths
increase but not significantly, the largest change being from 8 Å
(<1 subunit) to 14 Å (1.8 subunits) for the reflection at 10 Å.
This is consistent with the nematic nature of the phase over
entire temperature range. However, the reflections become
better defined along the meridian and at the equator, thus
pointing to an increasing orientational order.
mesogens (e.g., 1f) that were previously described did not reveal
any sign of biaxiality. This can be rationalized by quantum-
mechanical calculations showing a considerably larger dipole
tilted with respect to the molecular long axis, which may help to
stabilize the low temperature Nb phase. The nature of this phase
is thus highly interesting. Work is in progress to study the
precise nature of these nematic materials using dynamic light
scattering, dielectric measurements, Raman, and 13C solid-state
NMR spectroscopy methods.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: Matthias.Lehmann@uni-wuerzburg.de. Fax: þ49 (0)931
31ꢀ87350.
’ ACKNOWLEDGMENT
We thank the DFG for the financial support (LE 1571/2-1);
H. Meier, A. Oehlhof and N. Hanold (University of Mainz) for
FD MS and elemental analysis data; and J. Gutmann and M. Bach
for the available X-ray equipment at the Max-Planck-Institute of
Polymer Research, Mainz. This work was supported by the U.S.
Department of Energy (USDOE) grant DE-SC0001412. The
use of the Advanced Photon Source (APS) was supported by the
U.S. Department of Energy, Basic Energy Sciences, Office
of Science, under Contract W-31-109-Eng-38. The Midwest
Universities Collaborative Access Team (MUCAT) sector at
the APS is supported by the U.S. Department of Energy, Basic
Energy Sciences, Office of Science, through the Ames Laboratory
under Contract W-7405-Eng-82.
’ CONCLUSIONS
Shape-persistent thiadiazole mesogens 1 form nematic meso-
phases over a wide temperature range. They self-assemble home-
otropically, presumably because the pyridyl units interact with
the glass surface via hydrogen bonding. The nematic phase can be
supercooled down to room temperature. At ∼ 50 ꢀC, conoscopy
shows a splitting of the isogyres pointing to a possible biax-
ial nature of these nematic phases. Symmetrically substituted
DEDICATION
§This paper is dedicated to the memory of our dear friend
and colleague Alberto Marini, who passed away before the
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Chemistry of Materials
ARTICLE
submission of this manuscript. His scientific talents and enthu-
siasm will never be forgotten.
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