Symmetric bi-pyridyl banana-shaped molecule and its intermolecular hydrogen bonding liquid-crystalline complexes

Article abstract of DOI:10.1016/j.molstruc.2008.04.005

A new symmetric bi-pyridyl banana-shaped molecule 1,3-phenylene diisonicotinate (PDI) was designed and synthesized. Its molecular structure was confirmed by FTIR, Elemental analysis and ¹H NMR. X-ray crystallographic study reveals that there is an angle of approximate 118° among the centroids of the three rings (pyridyl-phenyl-pyridyl) in each PDI molecule indicating a desired banana shape. In addition, a series of liquid crystal complexes nBA:PDI:nBA induced by intermolecular hydrogen bonding between PDI (proton acceptor) and 4-alkoxybenzoic acids (nBA, proton donor) were synthesized and characterized. The mesomorphism properties and optical textures of the complex of nBA:PDI:nBA were investigated by differential scanning calorimetry, polarizing optical microscope and X-ray diffraction.

Full text of DOI:10.1016/j.molstruc.2008.04.005

Journal of Molecular Structure 891 (2008) 312–316
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Symmetric bi-pyridyl banana-shaped molecule and its intermolecular hydrogen
bonding liquid-crystalline complexes
Dan Sui a, Qiufei Hou a, Jia Chai a, Ling Ye a, Liyan Zhao a, Min Li b, Shimei Jiang a,
*
a State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun 130012, PR China
b Key Laboratory of Automobile Materials of Ministry of Education, Department of Materials Science, Jilin University, 2699 Qianjin Avenue, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new symmetric bi-pyridyl banana-shaped molecule 1,3-phenylene diisonicotinate (PDI) was designed
and synthesized. Its molecular structure was confirmed by FTIR, Elemental analysis and 1H NMR. X-ray
crystallographic study reveals that there is an angle of approximate 118° among the centroids of the three
rings (pyridyl–phenyl–pyridyl) in each PDI molecule indicating a desired banana shape. In addition, a ser-
ies of liquid crystal complexes nBA:PDI:nBA induced by intermolecular hydrogen bonding between PDI
(proton acceptor) and 4-alkoxybenzoic acids (nBA, proton donor) were synthesized and characterized.
The mesomorphism properties and optical textures of the complex of nBA:PDI:nBA were investigated
by differential scanning calorimetry, polarizing optical microscope and X-ray diffraction.
Ó 2008 Elsevier B.V. All rights reserved.
Received 23 January 2008
Received in revised form 31 March 2008
Accepted 1 April 2008
Available online 11 April 2008
Keywords:
Banana-shaped molecule
Liquid crystals
Intermolecular hydrogen bonding
1. Introduction
hydrogen bonding interactions. Recently, another symmetric or
unsymmetric hydrogen bonded banana-shaped liquid crystal
Banana-shaped liquid crystals have attracted much attention
since Niori et al. first reported in 1996 because they exhibit a vari-
ety of unique phases and respond as ferroelectric and antiferro-
electric materials [1]. These banana-shaped molecules can give
rise to a number of different mesophases provisionally labeled as
B phases and this nomenclature express the special features of
the liquid-crystalline phases formed by the banana-shaped mole-
cules. Until now, liquid-crystalline phases B1–B8 [2,3] were found.
Among them, B2, B5 and B7 are switchable by applied electric field
[4] while B4 phase is designated as the ‘blue phase’ [5]. The B2 is the
most common and the most widely studied phase. This tilted
lamellar polar phase can appear in four different supramolecular
packing arrangements, two of which form chiral conglomerates
and two of which appear as racemates [6–8]. In recent years, how-
ever, increasing research activity has focused on materials in which
the bent core is assembled via hydrogen bonding interactions.
Hydrogen bonding is one of the key interactions for chemical
and biological processes in nature due to its stability, directional-
ity, and dynamics. The intermolecular hydrogen bonding between
carboxyl and pyridyl moieties has been found to be extremely
fruitful for the formation of liquid crystals [9–11]. The hydrogen
bonding liquid crystal of rod shaped is investigated in detail by
Kato and Frechet [12,13]. The first hydrogen bonding banana-
shaped liquid crystal is reported by Gimeno et al. [14]. They proved
that it is possible to stabilize this type of mesophase through
materials have been investigated by Martin and Bruce [15].
Here, we report a novel series of hydrogen bonding banana-
shaped liquid crystals in which 1,3-disubstituted benzene with
4-pyridine connected via ester linkages instead of the two Schiff’s
base-containing units [16]. In this supramolecular complex, the
symmetric bent core 1,3-phenylene diisonicotinate (PDI) has no
mesophase and it acts as the proton acceptor, while 4-alkoxyben-
zoic acids (nBA) act as the proton donor and the series of nBA show
different mesophase such as smectic C or nematic. After introduc-
ing the hydrogen bonding interactions between PDI and nBA, the
complexes nBA:PDI:nBA show the typical B phase of banana-
shaped liquid crystals.
2. Experimental
2.1. Synthesis
The PDI was prepared by resorcinol (0.1 mol) and isonicoti-
nic acid (0.2 mol) which were dissolved in dry dichloromethane
with dicyclohexylcarbodiimide (DCC) (0.22 mol). The mixture
was stirred at room temperature for about 48 h. The product
was purified by flash chromatography on silica gel using chloro-
form/methanol (20:1) as eluent, and then recrystallized three
times from ethanol to provide a white crystalline solid. Yield:
30%. 1H NMR for PDI shown in Fig. 1 (DMSO, ppm): 8.89 (d,
4H, J = 6.0 Hz); 8.02 (d, 4H, J = 6.0 Hz); 7.60 (t, 1H, J = 8.2 Hz);
7.43 (s, 1H); 7.35 (d, d, 2H, J = 8.2, 2.2 Hz). Elemental analysis
for C18H12N2O4: calcd C 67.50, H 3.78, N 8.75; found C 67.67,
* Corresponding author. Tel.: +86 431 85168474; fax: +86 431 85193421.
E-mail address: smjiang@jlu.edu.cn (S. Jiang).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.04.005

D. Sui et al. / Journal of Molecular Structure 891 (2008) 312–316
313
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: +44 1223
336 033; e-mail: data_request@ccdc.cam.ac.uk].
c
e
O
O
b
a
O
O
2.3. Characterization
d
N
N
1H NMR spectrum was recorded on a Bruker Avance 500
instrument using CDCl3 as solvent and TMS as internal reference.
Elemental analysis (C, H and N) were performed on a Flash
EA1112 from ThermoQuest Italia S.P.A. Fourier transform infra-
red (FTIR) spectra were recorded on a BRUKER IFS-66V equipped
with a DTGS detector. The spectra were collected with a 4 cmꢀ1
resolution and at room temperature. Differential scanning calo-
b
a
e
rimetry (DSC) measurements were performed on
a Netzsch
c
DSC 204. The DSC analysis was carried out at rates of 5 °C/min
by applying several heating and cooling cycles from 40 to
140 °C. Optical textures were observed by a Leica DMLP polariz-
ing optical microscope equipped with a Leitz 350 microscope
heating stage. Variable-temperature XRD experiments were per-
formed on a Rigaku X-ray diffractometer (Dmax 2500 V, using
CuKa1 radiation of a wavelength of 1.54 Å) with a PTC-20A tem-
perature controller.
d
9.0
8.7
8.4
8.1
ppm
7.8
7.5
7.2
Fig. 1. 1H NMR spectrum of PDI (DMSO).
As for the X-ray Diffraction, diffraction data were collected on a
Rigaku RAXIS-PRID diffractometer using the x-scan technique with
graphite-monochromated MoKa (k = 0.71073 Å) radiation. The
structures were solved by direct methods and refined by the fullm-
atrix least-squares technique using the SHELXTL programs. Aniso-
tropic displacement parameters were applied to all nonhydrogen
atoms, hydrogen atoms were assigned isotropic displacement
coefficients.
H 3.89, N 8.72%. The 4-alkoxybenzoic acids (nBA) were pre-
pared according to literature methods [17]. The complex nBA:P-
DI:nBA were prepared by dissolving 1:2 molar amount of PDI
and nBA in anhydrous pyridine and evaporating pyridine slowly
under reduced pressure. The complexes were dried under vac-
uum for at least 24 h before characterizations [18].
3. Result and discussion
2.2. Supplementary data
3.1. Synthesis of the PDI
Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre (CCDC)
as supplementary data under deposition number CCDC 634149.
Copies of the data can be obtained free of charge on application
The synthetic route for the proton acceptor PDI and the struc-
tures of nBA and the complexes nBA:PDI:nBA are illustrated in
Scheme 1. The chemical structure of PDI is confirmed by FTIR, Ele-
mental analysis and 1H NMR (Fig. 1).
COOH
O
O
HO
OH
O
O
N
N
N
PDI
DCC CH2Cl2
O
OCnH2n+1
nBA, n=4,7,8,10,12
HO
O
O
O
O
O
O
N
N
OH
HO
H2n+1 CnO
OCnH2n+1
nBA: PDI: nBA
Scheme 1. Chemical structures of the PDI, nBA and nBA:PDI:nBA.

314
D. Sui et al. / Journal of Molecular Structure 891 (2008) 312–316
Table 1
3.2. X-ray crystallographic analysis of the PDI
Crystallographic data for compound PDI
The PDI (0.032 g, 0.1 mmol) was dissolved in a solution of eth-
anol (5 ml) and was allowed to evaporate slowly at room temper-
ature. Colorless crystals of PDI were obtained after one week.
The crystal structure of PDI is exhibited in Fig. 2. Systematic
absences in the diffraction data of PDI establish the space group
as P-1. The crystal details are summarized in Table 1. The main
intermolecular interactions in the crystal are weak hydrogen bonds
including C–HꢁꢁꢁN (pyridyl) and C–HꢁꢁꢁO (carbonyl) which are
exhibited in Fig. 2. It is noticeable that there is an angle of approx-
imate 118° among the centroids of the three rings (pyridyl–phe-
nyl–pyridyl) in each PDI molecule which indicating a desired
banana shape.
Formula
Formula weight
Temperature
Space group
a (Å)
b (Å)
c (Å)
a (°)
b (°)
C18H22N2O4
320.30
293(2) K
Pꢀ1
7.9838(16)
12.279(3)
30.891(6)
89.70(3)
90(3)
90(3)
3028.3
8
2.209
0.71
0.053 ꢂ 0.136 ꢂ 0.371
Colorless crystal
55.55
c (°)
v (Å3)
Z
Dc (g cmꢀ3
)
l (cmꢀ1
)
Crystal size (mm3)
Color
2hmax (°)
Reflns meads
3.3. Intermolecular hydrogen bonding of the complex nBA:PDI:nBA
12975
Reflns used (Rint
Final R [I > 2r(I)]
)
8421 (11.6060)
R1a = 0.0839, wR2b = 0.1976
r1 = 0.2888, wr2b = 0.3151
0.866
In order to confirm the intermolecular hydrogen bonding
between the PDI and nBA, we choose FTIR spectroscopy because
it is sensitive to the change of molecular conformation, inter and
intramolecular interactions [19,20]. The FTIR spectra of PDI,
12BA, and 12BA:PDI:12BA are shown in Fig. 3. The peak of the car-
boxylic carbonyl band of 12BA is 1687.0 cmꢀ1, this band shows
that the carboxylic acid dimer occurs by means of intermolecular
hydrogen bonding [21]. The spectrum of PDI has an ester carbonyl
band at 1747.3 cmꢀ1. In the IR spectrum of complex 12BA:PDI:12-
BA, the ester carbonyl band changes from 1747.3 cmꢀ1 to
1751.1 cmꢀ1 due to the complexation of PDI with nBA, and the car-
boxylic carbonyl peak changes from 1687.0 cmꢀ1 of (nBA)2 to
1695.2 cmꢀ1. These results mean that the intermolecular hydrogen
bonding between carboxylic acids has been substituted by the
intermolecular hydrogen bonding between the pyridyl and the car-
boxylic acid, especially the two new bands centered at
2537.0 cmꢀ1 and 1911.0 cmꢀ1 can been seen very clearly. These
two new bands are strong evidence of the intermolecular hydrogen
bonding, which is of an unionized type between the pyridyl and
the carboxylic acid [22,23]. The other complexes of the nBA:PDI:n-
BA show the same characteristic in FTIR spectra. Furthermore, the
R indices (all data)
Goodness-of-fit on F2
P
P
|F0| ꢀ |FC|/ |F0|.
P
aR1
=
P
bwR2 ¼ p wpðF20 ꢀ FC2Þ2y= wðF20Þ2y1=2; w ¼ 1=pr2ðF20Þ þ ð0:08PÞ2y where P ¼
pmaxðF20; 0Þ þ 2F2Cy=3.
formation of the complex was easily confirmed through polarizing
optical microscopy measurements; the solid samples melted
cleanly without the appearance of biphasic regions, which would
otherwise have indicated the presence of nonstoichiometric
complexes.
3.4. Mesomorphic properties of the complexes nBA:PDI:nBA
The detailed phase behaviour of the PDI and complexes nBA:
PDI:nBA were investigated by differential scanning calorimetry.
The DSC curves of 10BA:PDI:10BA and 12BA:PDI:12BA on cooling
are given in Fig. 4. The transition temperatures and enthalpies of
these complexes are summarized in Table 2. From the results of
Fig. 2. The crystal structure of PDI.

D. Sui et al. / Journal of Molecular Structure 891 (2008) 312–316
315
The complexes of n = 8, 10, 12 in the series exhibit enantiotropic li-
quid-crystalline phases.
We also studied the phase transitions by polarizing optical
microscopy to confirm the textures of the complexes. On the slow
cooling from the isotropic liquid of this series complex, a texture
with circular-domain can be found and it is the B2 phase. This kind
of fluid smectic phase was observed for most of the complexes. The
polarizing photomicrographs of complexes 10BA:PDI:10BA and
PDI
12BA
PDI-12BA
1911
2537
4000
3000
2000
1000
Wavenumber/cm-1
Fig. 3. FTIR spectra of compounds PDI, 12BA and 12BA:PDI:12BA at room
temperature.
PDI-12BA
PDI-10BA
40
50
60
70
80
90
100 110 120
O
Temperature/ C
Fig. 4. The DSC thermograms of 10BA:PDI:10BA and 12BA:PDI:12BA on cooling.
Table 2
Fig. 5. Optical textures of (a) 10BA:PDI:10BA on cooling at 75 °C (400ꢂ) and (b)
12BA:PDI:12BA on cooling at 70 °C (200ꢂ).
Thermal transitions of PDI and nBA:PDI:nBA series
Compound
Coolingab
I
Heating
Cr Cr
B2
B2
143.3(122.6) –
I
7000
6000
5000
4000
PDI
– –
– –
–
–
111.1(124.3) –
–
–
–
–
–
–
–
–
–
–
–
–
4BA:PDI:4BA
7BA:PDI:7BA
8BA:PDI:8BA
10BA:PDI:10BA – 84.9(10.5) –
12BA:PDI:12BA – 77.9(10.3) –
108.6(16.5)
52.4(22.8)
83.4(19.3)
72.3(38.2)
68.2(14.4)
–
–
–
–
–
120.5(16.0)
91.7(37.0)
90.4(13.6)
86.7(24.7)
90.8(11.3)
–
–
–
–
–
– 80.6(15.3) –
– 84.8(25.2) –
98.1(37.7) –
94.5(27.5) –
92.3(9.17) –
a
Phase transition temperatures (°C) and enthalpies of transition (kJ/mol) (in
parentheses).
10
15
20
25
30
b
Theta / deg
Cr, crystalline; B2, smectic C polar mesophase; I, isotropic liquid phase.
3000
2000
1000
0
DSC, we find the melting point of PDI is 143.3 °C and the banana-
shaped PDI has no mesophase, while the proton donor nBA has
shown different mesophases such as smectic C or nematic phases
[24]. The intermolecular hydrogen bonding induced complexes
nBA:PDI:nBA show mesophases which are different from the ones
of nBA. Among them, 4BA:PDI:4BA shows no liquid-crystalline
phase with a single peak at 120.5 °C. With the increase of the
alkoxy chain length, 7BA:PDI:7BA is monotropic and its
liquid-crystalline phase is only observed during cooling process.
0
2
4
6
8
10
12
Theta / deg
Fig. 6. X-ray diffraction pattern of complex 12BA:PDI:12BA at 75 °C on cooling.

316
D. Sui et al. / Journal of Molecular Structure 891 (2008) 312–316
O
O
O
O
O
O
O
O
RO
OR
R=CnH2n+1
n = 10 Cr 105 SmCP 113 I
n = 12 Cr 109 SmCP 119 I
Fig. 7. Chemical structures and the phase transitions of the molecules reported by Pelzl’s group (temperatures in °C) [25].
12BA:PDI:12BA on cooling are given in Fig. 5a with magnification
400 and in Fig. 5b with magnification 200, respectively.
tiotropic liquid crystals. In summary, intermolecular hydrogen
bonding is a successful way to obtain different multifunctional ba-
nana-shaped liquid crystals.
In order to identify the exact liquid phase, the X-ray diffraction of
nBA:PDI:nBA at elevated temperatures is examined. Fig. 6 shows the
X-ray diffraction of 12BA:PDI:12BA at 75 °C on cooling. In the small
angle region a single sharp peak at 2h of 2.78° was observed, whereas
in the wide angle region thereis a broad halo which is consistent with
the diffuse lamellar structure. This result indicates a layer structure
and the layer spacing d is 31.8 Å for the liquid-crystalline phase.
The measured interlayer distance (31.8 Å) is significantly shorter
than the calculated molecular length (51.6 Å) and indicating the
tilted lamellar structure of the complex with angle of around 52.4°.
All these facts are consistent with a B2 mesophase in which 12BA:P-
DI:12BA molecules are organized in layer structures and tilted.
We find an example of banana-shaped liquid crystals in the lit-
erature which could be considered as a covalent analogue of our
bent complexes and compared the mesomorphic properties of
nBA:PDI:nBA with the related liquid crystals reported by Pelzl’s
group [25] (shown in Fig. 7). From the data obtained, we can con-
clude that both of the molecules show the B2 phase, so it is success-
ful to form banana-shaped liquid crystals through hydrogen
bonding between pyridine and carboxyl. But the mesomorphic
temperature of the covalent analogue is higher 20 °C than the
nBA:PDI:nBA, so the intermolecular hydrogen bonding between
PDI and nBA decreases the phase transition temperature because
the hydrogen bonding is more flexible than the covalent bond.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (50573029 and 50520130316) and the
National Basic Research Program of China (2007CB936402), the
111 Project (B06009) and Program for Changjiang Scholars and
Innovative Research Team in University (PCSIRT0422).
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In this paper, we designed and synthesized a symmetric bana-
na-shaped molecule (PDI), and through the X-ray crystallographic
analysis, the bent angle of PDI is confirmed as 118°. We take the
symmetric PDI as the proton acceptor and nBA as the proton donor
and get the symmetric banana-shaped complexes of nBA:PDI:nBA
through intermolecular hydrogen bonding. The intermolecular
hydrogen bonding interactions between PDI and nBA were con-
firmed by FTIR. The mesomorphism of the complexes was charac-
terized through differential scanning calorimetry, polarizing
optical microscope and X-ray diffraction measurements. The re-
sults are as follows: for the homologue with short terminal chains
(n = 4), no liquid-crystalline phases were observed; the homologue
with intermediate length of terminal chains (n = 7) exhibited
monotropic liquid-crystalline phase, while ones with long terminal
chains (n = 8, 10 and 12) displayed B2 phases and they are all enan-
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