Hydrogen-bonding induced melamine-core supramolecular discotic liquid crystals

Article abstract of DOI:10.1039/c7tc03059b

Recognition of melamine is not only of importance for the food industry but also an interesting scientific research topic. The objective of this work is to screen suitable hydrogen-bonding complementary compounds for the construction of melamine-core supr

Full text of DOI:10.1039/c7tc03059b

View Article Online
View Journal
Journal of
M at erials Chemistry C
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: L. Guo, Y. Liu, L.
Wang, M. Wang, B. Lin and H. Yang, J. Mater. Chem. C, 2017, DOI: 10.1039/C7TC03059B.
Volume
4
Number
1
7
January 2016 Pages 1–224
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
www.rsc.org/MaterialsC
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

Please do not adjust margins
Journal of Materials Chemistry C
Page 1 of 11
View Article Online
DOI: 10.1039/C7TC03059B
Journal of Materials Chemistry C
ARTICLE
Hydrogen-Bonding Induced Melamine-Core
Supramolecular Discotic Liquid Crystals
fReceived 00th January 20xx,
Accepted 00th January 20xx
a
Ling-Xiang Guo, Yu-Han Liu, Li Wang, Meng Wang, Bao-Ping Lin, and Hong Yang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Recognition of melamine is not only of food industry importance but also an interesting scientific research topic. The
objective of this work is to screen suitable hydrogen-bonding complementary compounds for the construction of
melamine-core supramolecular discotic liquid crystal (LC) materials which might provide a LC perspective for detection and
separation of melamine in the future. We design and synthesize five categories of long-alkyl-tailed compounds bearing
different hydrogen-bonding complementary functional groups, including benzoic acid, cyanuric acid, homophthalimide,
succinimide and thymine derivatives. The experimental results demonstrate that the imidodicarbonyl unit is the best
hydrogen-bonding complementary functional group of melamine. Furthermore, the heterocyclic ring size markedly
influences the hydrogen-bonding strength of melamine-imidodicarbonyl unit complex. Most importantly, liquid
crystallinity can be generated from these hydrogen-bonding supramolecular complexes. In particular, a variety of
columnar mesomorphic orders (such as hexagonal columnar, rectangular columnar, square columnar phases) can be
effectively achieved by functionalization of thymine or succinimide units and modulation of the molar ratios of melamine
and these hydrogen-bonding complementary compounds.
Melamine (Mel), a common chemical used primarily in
melamine resin productions, is however extremely famous in
China due to the 2008 milk contamination scandal. Thereafter,
Introduction
Supramolecular liquid crystals (LCs), combining both the
fluidity, anisotropic molecular orders of LC and the dynamic
assembly, structural complexity of supramolecular chemistry,
have been considered as novel exciting functional materials
detection of melamine was of food industry importance and
1
7-20
such as high performance liquid chromatography
or HPLC
low-temperature plasma
probe combined with tandem mass spectrometry
2
1-23
hyphenated mass spectroscopy,
1
-6
and attracted intense scientific attention recently. Among
them, hydrogen-bonding induced supramolecular discotic LCs
that were usually formed from hydrogen bonding
complementary molecules containing a π-conjugated aromatic
2
4,25
(
LTP/MS),
recognition-induce
gold
29,30
nanoparticle
2
6-28
aggregation,
and enzymatic essay kits.
As a C -symmetric compound, each melamine molecule
3
7
-10
has 6 H-donors (-NH) and 3 H-acceptors (=N-), and can form
maximally 9 H-bonds per molecule. Taking advantage of
melamine’s hydrogen-bonding capability, plenty of pioneering
works about supramolecular self-assembly behaviours of
core and several flexible long-chain alkyl tails,
were
particularly attractive due to their predominant columnar
arrangements of mesogens which have great potential for
applications ranging from ion-conductive materials to organic
3
1,32
1
1-15
melamine derivatives
3-44
in collaboration with cyanuric acid
45-50
photovoltaic devices.
In particular, nanoporous cross-
3
derivatives,
benzoic acid derivatives,
or aromatic imide
linked polymers with columnar channels constructed from
hydrogen-bonded supramolecular LCs, could be used to
effectively recognize and isolate the original heterocyclic
5
1-54
derivatives
have been published. However, to our surprise,
there was barely any relevant research reporting liquid
crystallinity appearing in the hydrogen-bonded supramolecular
complex systems comprised of pure melamine (not
functionalized melamine derivatives).
1
6
cores, such as benzotri(imidazole), etc. This interesting
supramolecular imprinting function reminded us of melamine.
Inspired by all the above works, herein we designed and
synthesized a series of hydrogen-bonding complementary
compounds bearing flexible long aliphatic chains, which were
further combined with melamine in either a 3:1 or 2:1
stoichiometry, and investigated the self-assembly behaviours
of the corresponding supramolecular complexes. As
schematically illustrated in Fig. 1, the aim of this present work
a.
School of Chemistry and Chemical Engineering, Jiangsu Key Laboratory for Science
and Application of Molecular Ferroelectrics, Jiangsu Province Hi-Tech Key
Laboratory for Bio-medical Research, Jiangsu Optoelectronic Functional Materials
and Engineering Laboratory, Southeast University, Nanjing, 211189, China
E-mail: yangh@seu.edu.cn
Electronic Supplementary Information (ESI) available: synthetic procedures, NMR
spectra. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 2 of 11
View Article Online
DOI: 10.1039/C7TC03059B
ARTICLE
Journal Name
was to screen suitable hydrogen-bonding complementary
compounds for the construction of melamine-core discotic LC
materials, in particular room-temperature enantiotropic
columnar LC materials, which might provide a LC perspective
for detection and separation of melamine.
Results and discussion
Melamine-benzoic acid derivative complex
3,4,5-Trialoxybenzoic acids, as the perfect candidates bearing
not only hydrogen bond donor (O-H) but also hydrogen bond
acceptor (C=O), are the most widely used complementary
compounds for hydrogen-bonding induced supramolecular
1
-6
discotic LCs.
Moreover, most of previous successful
observations of mesomorphism of melamine-derivatives were
also achieved in the supramolecular complex systems
4
6-50
comprised of benzoic acid-derivatives.
Based on this
common sense, we first mixed 3,4,5-tris-dodecyloxy-benzoic
acid (Compound X1, Fig. 1) with melamine in a 3:1
stoichiometry to form a mixture which was added in
anhydrous N,N-dimethylformamide (DMF) and further
sonicated until fully dissolved. Evaporation of the solvent
1
provided a white powder, which was investigated by H-NMR
and polarized optical microscopy (POM). To our surprise,
compound X1 was completely immiscible with melamine. This
failure might derive from melamine’s self-complementary
hydrogen-bonding ability, which resulted in extremely high
melting point and poor solubility of melamine, and was
apparently stronger than the hydrogen-bonding strength of
benzoic acid. This result might also explain why previous
4
6-50
examples
used functionalized melamine-derivatives,
instead of melamine, to build supramolecular LCs.
Melamine-cyanuric acid derivative complex
Alternatively, a classical cyanuric acid derivative 1,3-didodecyl-
3
9-43
[
1,3,5]triazinane-2,4,6-trione
(compound X2, Fig. 1) was
Fig. 1 Schematic illustration of the desired hydrogen-bonding induced melamine-
core supramolecular discotic LCs and the molecular structures of all the
complementary compounds examined in this work.
chosen to mix with melamine in a 3:1 molar ratio to form Mel-
1
X2 complex, which was characterized by H-NMR, differential
scanning calorimetry (DSC) and POM.
1
Fig. 2 (A) H NMR spectra of compound X2 (bottom) and Mel-X2 complex (top). (B) DSC thermogram of compound X2. (C) DSC thermogram of Mel-X2 complex. (D)
POM image of compound X2 at 75 °C. (E) POM image of Mel-X2 complex at 60 °C.
As shown in Fig. 2A, the chemical shift of the imide hydrogen signals of amino groups of melamine could not be
1
hydrogen (H ) of X2 appeared at δ~8.105 ppm, while the recorded in H-NMR spectra possibly due to the fast proton
a
corresponding hydrogen signal of Mel-X2 complex shifted exchange. Fig. 2B,C presented the DSC diagrams of compound
upfield to δ~7.031 ppm. The chemical shift variation suggested X2 and Mel-X2 complex. As it could be seen, X2 possessed two
o
a stable Mel-X2 complex has been successfully enantiotropic phase transitions (23.8 C, 92.3 C, on heating)
o
that
constructed through hydrogen bonding. Interestingly, the while Mel-X2 complex had only one phase transition and
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 11
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03059B
o
raised its clearing point to 108.9 C. Based on POM
observations (Fig. 2D and E), both samples showed a typical
crystalline spherulite growth in the temperature range from
room temperature to their isotropic phases. In one word,
melamine-cyanuric acid derivative couple Mel-X2 did not have
liquid crystallinity.
Melamine-homophthalimide derivative complex
Although Mel-X2 complex could not generate liquid
crystallinity, the imidodicarbonyl group of cyanuric acid
derivatives demonstrated
a superior hydrogen bonding
capability. Based on the molecular structure of
imidodicarbonyl group, we designed two homophthalimide
compounds X3 (5,6,7-tris-dodecyloxy-4H-isoquinoline-1,3-
dione, Fig. 1) and X4 (6,7-bis-dodecyloxy-4H-isoquinoline-1,3-
dionet, Fig. 1) with three flexible alkyl tails, as melamine’s H-
bonding complementary compounds. The logic behind the
design was to decorate the six-membered imidodicarbonyl
heterocyclic ring with one extra planar aromatic ring so that
the core size of the formed supramolecular disc would
increase which might impede the crystallinity of the alkyl Scheme 1 Synthetic routes of (A) compound X3 and (B) compound X4.
chains of complex molecules.
1
Fig. 3 (A) H NMR spectra of compound X3 (bottom) and Mel-X3 complex (top). (B) DSC thermogram of compound X3. (C) DSC thermogram of Mel-X3 complex. (D)
o
POM image of compound X3 at 25 C. (E) POM image of Mel-X3 complex at 25 C.
o
The synthetic protocols of compound X3 and compound X4 a cyclization reaction to give the desired compound X4,
5
5
are shown in Scheme 1. Methyl 3,4,5-trihydroxybenzoate was possibly because of the poor regioselectivity.
first etherified with dodecyl bromide to provide intermediate As shown in Fig. 3A, after compound X3 and melamine were
bearing three long alkyl tails, which underwent saponification mixed in a 3:1 stoichiometry, the hydrogen of aromatic ring
reaction to provide intermediate (compound X1). After a (Ha) underwent a small shift from δ~7.320 ppm to δ~7.313
1
2
series of consecutive reactions including chloroformylation, ppm. The proton of imide group (Hb) showed an obvious
ammonolysis, amidation and nucleophilic substitution, the key upfield shift from δ~6.608 ppm to δ~6.111 ppm, the
xanthate
intermediate
6
was
prepared.
Finally, methylene protons (Hc) of the six-membered heterocyclic ring
homophthalimide compound X3 was successfully obtained by shifted from δ~3.408 ppm to δ~2.871 ppm. The chemical shift
an intramolecular radical cyclization onto the aromatic ring of variation suggested that a stable Mel-X3 complex has been
intermediate
B, methyl 3,4-dihydroxybenzoate was subjected to successive interaction.
alkoxylation, saponification, chloroformylation, ammonolysis, The mesomorphic properties of compound X3 and complex
6 (Scheme 1A). Similarly, as presented in Scheme successfully constructed through hydrogen bonding
1
amidation and nucleophilic substitution reaction to provide a Mel-X3 were investigated by DSC and POM. As illustrated in
xanthate intermediate 12, which however could not carry out Fig. 3B,C compound X3 and Mel-X3 both showed one phase
transition peak on the first cooling process and had clearing

Please do not adjust margins
Journal of Materials Chemistry C
Page 4 of 11
View Article Online
DOI: 10.1039/C7TC03059B
ARTICLE
Journal Name
temperatures at 50.1 °C and 46.6 °C on the second heating which was then met with maleimide and went through a
process respectively. According to POM experiment (Fig. 3D,E), tributyltinhydride-induced sequential atom transfer radical
neither compound X3 nor complex Mel-X3 did show any fluid addition-dehalogenation to give the desired molecules X5 and
LC textures, but unfortunately appeared as needle-like crystals X6.
and spherulite crystals.
Melamine-succinimide derivative complex
The failure of homophthalimide system might derive from the
very rigid fused aromatic heterocycle core which could be
tightly packed to induce crystallization. Thus we further
designed two succinimide (five-membered imidodicarbonyl
heterocyclic ring) derivatives X5 (3-(3,4,5-tris-dodecyloxy-
benzyl)-pyrrolidine-2,5-dione, Fig. 1) and X6 (3-(3,4-bis-
dodecyloxy-benzyl)-pyrrolidine-2,5-dione, Fig. 1), with the
tri(alkoxyl)benzyl unit laterally attached onto the aromatic
core.
The synthetic protocols of compound X5 and compound X6
are shown in Scheme 2. The benzyl ester group of compound
was reduced by lithium aluminum hydride, brominated by
phosphorus tribromide to provide the key intermediate 15
2 2
Scheme 2 Synthetic routes of compound X5 (R = H) and compound X6 (R =
OC12H25).
13
,
1
Fig. 4 (A) H-NMR spectra of compound X5 (bottom) and Mel-X5 mixture (top). (B) DSC thermogram of compound X5. (C) DSC thermogram of Mel-X5 mixture. (D)
o
WAXS patterns of compound X5. (E) POM image of compound X5 recorded at 25 C. (F) POM image of Mel-X5 mixture recorded at 47 C. (G) POM image of Mel-X5
o
o
mixture recorded at 200 C.
1
Unlike X2 or X3 lacking of liquid crystallinity, compound X5 obvious differences in the H NMR spectra and DSC curves,
itself was a room-temperature discotic LC with columnar between Mel-X5 mixture and the pure compound X5. The
phase, which showed a broken focal conic fan-shaped texture failure of forming hydrogen-bonding complex was intuitively
(
Fig. 4E) in the temperature range from room temperature to observed under POM. As presented in Fig. 4F, when
o
o
its clearing point (49.3 C, Fig. 4B). When the temperature of temperature was raised to above 49 C which was the LC-to-
X5 was cooled down from the isotropic phase, the isotropic phase transition of X5, the majority of birefringence
o
birefringence recovered at 45.5 C under POM observation, faded while some leftover birefringent spots never turned to
o
although the cooling cycle plot of DSC diagram did not show an dark even heated to above 200 C (Fig. 4G) which was clearly
apparent phase transition peak. The WAXS pattern (Fig. 4D) of consistent with melamine whose melting point was over 345
o
o
X5 at 20 C showed three reflections at 32.1
Å
, 22.9
in the small-angle region, with a reciprocal d-spacing ratio of imidodicarbonyl heterocyclic system could not form a stable
: √2 : 2. The three peaks were indexed as 100, 110 and 020 hydrogen bonding complex with melamine. In another word,
Å
and 16.3
C. These above experiments implied that the five-membered
Å
1
reflections, which were characteristic reflections for square six-membered imidodicarbonyl heterocyclic systems (i.e.
columnar (Col ) phase, with lattice parameters of a = 32.4 cyanuric acid, homophthalimide) would arrange the O=C-N-H
in the wide- unit of the imidodicarbonyl group at right positions which had
angle region confirmed the presence of liquid-like orders of perfect matches with H-N-C-N unit of melamine, while in the
the alkyl tails. five-membered imidodicarbonyl heterocyclic system (i.e.
Å
.
s
Besides, a broad halo appearing around 4.2
Å
However, when mixing the compound X5 with melamine in succinimide), the ring strain would enlarge the distance
a 3:1 stoichiometry, we could not get the desired hydrogen- between oxygen atom and hydrogen atom which would
bonding complex. As shown in Fig. 4A-C, there were no dramatically weaken the O-H hydrogen bond strength.
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 11
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03059B
To further investigate the steric hindrance of the formed Mel-X6-3 mixture could not generate a stable
surrounding flexible alkyl tails influencing the formation of hydrogen bonding complex. Interestingly, if compound X6 and
hydrogen bonding supramolecular complex system, we mixed melamine were mixed in a 2:1 stoichiometry, we could obtain
the two-tail succinimide analogue X6 with melamine in a 3:1 relatively stable hydrogen bonding supramolecular complex
stoichiometry. Similar to the previous Mel-X5 mixture, the Mel-X6-2.
1
Fig. 5 (A) H NMR spectra of the compound X6 (bottom) and complex Mel-X6-2 (top). (B) DSC thermogram of compound X6. (C) DSC thermogram of Mel-X6-2
o
o
complex. (D) POM images of compound X6 recorded at 43 C. (E) POM images of complex Mel-X6-2 recorded at 86 C. (F) WAXS patterns of compound X6. (G) WAXS
patterns of complex Mel-X6-2.
Compound X6 showed two enantiotropic phase transitions a square columnar symmetry. One intense scattering peak
in DSC diagrams (Fig. 5B). The two distinct peaks were the appeared at ~ 42.7 Å, and another peak appeared at ~ 30.4 Å
crystal-to-LC and LC-to-isotropic phase transitions respectively. were observed in the small-angle region (Fig. 5G). The two
The mesomorphic texture exhibited a typical broken focal scattering peaks were indexed as 100 and 110, with a
conic fan-shaped texture as shown in Fig. 5D. The reciprocal d-spacing ratio of 1 :
representative WAXS patterns of compound X6 were shown in lattice parameter was a = 42.7
√
2. The average value of
Å
. Interestingly, at lower
Fig. 5F. It consisted of a sharp and intense scattering peak at d temperatures, the WAXS pattern of Mel-X6-2 showed two
44.9 , along with two weak reflections at d ~ 35.3 and ~ peaks corresponding to the 110 and 200 diffractions at d ~
4.8 . These peaks were indexed as 100, 110, and 300, with a 42.2 Å and 34.0 (Fig. 5G), respectively. The reciprocal d-
reciprocal d-spacing ratio of 1 : 1.3 : 3.0, indicating a spacing ratio of these two peaks was ca. 1 : 1.24 implying that
rectangular columnar (Col ) phase with lattice constants of a = Mel-X6-2 complex at lower temperatures possessed
4.9 and b = 57.1 . In addition, a broad scattering at 4.3 rectangular columnar phase. The liquid-like behaviours of the
was observed in wide-angle region, which was compliance aliphatic chains were corroborated by the presence of the
with liquid-like order of the peripheral aliphatic tails. diffused scattering band at 4.2~4.4 . Such a Col -to-Col phase
When two molecules of X6 met one melamine molecule, transition of complex Mel-X6-2 has been previously observed
~
Å
Å
1
Å
Å
r
a
4
Å
Å
Å
Å
r
s
5
6
stable hydrogen bonding supramolecular complex Mel-X6-2 in other notched-disc-like LC molecules, which would tilt at
formed. As shown in Fig. 5A, the imino hydrogen (H ) of X6 had higher temperatures and transform from a side-by-side
a
r s
a slight upfield shift from 7.856 ppm to 7.736 ppm due to the packing (Col ) way to a more stable edge-by-side packing (Col )
hydrogen bonding interactions with melamine. DSC curves (Fig. approach.
5
C) of Mel-X6-2 presented two enantiotropic phase transitions
o
at 39.1 C and 149.2 C, which were its melting point and
o
Melamine-thymine derivative complex
clearing point respectively. In the LC temperature range, POM Learned from the above three sections, we realized that a
recorded an uncharacteristic texture (Fig. 5E), and beyond 150 loosely packed six-membered imidodicarbonyl heterocyclic
o
C, all the birefringence vanished which implied that all the ring system might find optimal hydrogen bonding
melamine molecules have been fixed in the hydrogen bonding complementary compounds of melamine for forming a stable
supramolecular complex Mel-X6-2.
melamine-core supramolecular discotic LC and providing the
WAXS experiments were conducted to further analyze the desired room-temperature enantiotropic LC phases. Based on
structures of the Mel-X6-2 complex. As illustrated in Fig. 5G, at this logic, we designed and synthesized a thymine-derivative
higher temperatures, the WAXS pattern of Mel-X6-2 exhibited compound X7 (N-(3,4,5-tris-dodecyloxy-benzyl)-thymine, Fig.

Please do not adjust margins
Journal of Materials Chemistry C
Page 6 of 11
View Article Online
DOI: 10.1039/C7TC03059B
ARTICLE
) as shown in Scheme 3. Starting with thymine, we selectively
Journal Name
1
When compound X7 and melamine were mixed in a 3:1
57
protected the imidodicarbonyl group with Boc- group, and stoichiometry, very stable hydrogen bonding supramolecular
functionalized another imino group with benzoic alcohol 14 via complex Mel-X7 formed. As shown in Fig. 6A, the imino
a
Mitsunubo coupling reaction. The desired compound X7 was hydrogen (H ) of X7 underwent an obvious upfield shift from
obtained after removal of the Boc- group.
8.652 ppm to 6.811 ppm because of the hydrogen bonding
interactions of Mel-X7 complex. Although DSC data (Fig. 6D)
showed that compound X7 possessed two enantiotropic phase
o
o
transitions (83.9 C, 103.4 C, on heating), POM experiments
presented a typical crystalline spherulite growth (Fig. 6B)
cooled from isotropic melt, which implied that compound X7
had no liquid crystallinity. DSC curves (Fig. 6E) of Mel-X7
complex implied a room-temperature LC phase with one
o
enantiotropic phase transition at 86.1 C, which was its
clearing point. An uncharacteristic birefringent texture was
observed under POM in the LC temperature range (Fig. 6C).
o
Meanwhile, all the birefringence vanished beyond 86 C, which
Scheme 3 Synthetic route of compound X7.
implied that all melamine molecules have been fixed in the
hydrogen bonding supramolecular complex Mel-X7.
1
Fig. 6 (A) H NMR spectra of compound X7 (bottom) and complex Mel-X7 (top). (B) POM images of compound X7 recorded at 86 C. (C) POM images of complex Mel-
o
o
X7 recorded at 25 C. (D) DSC thermogram of compound X7. (E) DSC thermogram of Mel-X7 complex. (F) WAXS patterns of complex Mel-X7.
WAXS experiments were conducted to further analyze the
mesomorphic structure of Mel-X7 complex. The WAXS pattern Mel-X3, Mel-X6-2 and Mel-X7.
o
Table 1. Phase transition temperatures ( C) of X1, X2, X3, X5, X6, X7, Mel-X2,
[
a]
of Mel-X7 exhibited one intense scattering peak at ~ 44.1
and two other peaks (ca. 25.5 and 21.8 ) in the small-angle
region (Fig. 6F). The three peaks were indexed as 100, 110 and
00, with a reciprocal d-spacing ratio of 1 : √3 : 2, which were
characteristic feature of hexagonal columnar (Col ) phase. The
liquid-like behaviours of the aliphatic chains were confirmed
by the presence of the diffused scattering band at ca. 4.2
Å
Compound
X1
Heating
K 55 → I
Cooling
Å
Å
I 53 → K
X2
K
1
24 → K
2
92 → I
I 80 → K
2
6 → K
1
2
X3
K 50 → I
I 22 → K
h
X5
K 16 → Col
K 40 → Col
84 → K
s
47 → I
I 45 → Col
s
12 → K
29 → K
X6
r
70 → I
I 67 → Col
r
X7
K
1
2
103 → I
2 1
I 88 → K 70 → K
Å
.
Mel-X2
Mel-X3
Mel-X6-2
K 109 → I
K 47 →I
I 95 → K
I -4 → K
Eventually after tedious syntheses and plenty of
experimental trials, we obtained the desired room-
K 39 → Col
r
70 → Col
s
149
I 94→ Col
s
65 → Col
r
24
h
temperature enantiotropic Col phase derived from a very
→
I
→ K
stable melamine-core supramolecular discotic LC material Mel-
X7. The detailed phase transitions and lattice parameters of all
the above tested compounds determined by DSC, POM and
WAXS were listed in Table 1 and Table 2.
Mel-X7
Col
h
86 →I
I 49 → Col
h
[
a]
h r
K = crystal phase; Col = hexagonal columnar phase; Col = rectangular columnar
s
phase; Col = square columnar phase; I = isotropic phase.
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 11
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03059B
Table 2. X-ray scattering data of the liquid crystalline phases of X5, X6, Mel-
Thus, large transition temperature hysteresis and
uncharacteristic textures were often observed in
X6-2 and Mel-X7.
3
,8,16,46-50,61-63
Compound
mesophase
lattice
spacing(Å)
a = 32.1
b = 32.4
c = 32.6
a = 44.9
b = 57.1
d-spacing
(Å)
Miller
indices
100
supramolecular LC materials.
Sometimes, the
recovery of mesomorphic property of the hydrogen-bonded
supramolecular LC molecules on the cooling process was
extremely slow, it could take even several days upon standing
o
X5
Col
s
at 20 C
32.1
22.9
110
16.3
020
16
o
at room temperature to reassemble into a mesophase.
X6
Col
r
at 60 C
44.9
100
Actually, this unusual large phase transition temperature
hysteresis phenomenon has already been concluded by
Beginn: “the more complex structure of supramolecular
columnar phases is more closely related to crystalline systems,
and it is frequently difficult to distinguish a columnar phase
35.3
110
1
4.8
300
o
Mel-X6-2
Col
Col
Col
r
at 50 C
a = 68.0
b = 53.8
a = 42.7
42.2
34.0
42.7
110
200
o
s
at 140 C
100
3
0.4
110
from a crystalline structure based only on the DSC
o
Mel-X7
h
at 45 C
a = 50.9
44.1
100
thermogram. In addition to their complex structure, the high
melt viscosity makes it simple to supercool the mesophase for
2
2
5.5
1.8
110
200
o
more than 15 C”.
3
In summary, the molecular structure of hydrogen-bonding
complementary compound, and the stoichiometry of
melamine and the complementary compound, markedly
influenced the mesomorphic property of the corresponding
supramolecular complex. First, the ideal complementary
compound must have an imidodicarbonyl functional group to
destruct melamine’s self-complementary hydrogen-bonding
matrix and form a stable hydrogen-bonding supramolecular
complex with melamine. Meanwhile, the steric hindrance of
the surrounding flexible alkyl tails also played a vital role on
the formation of hydrogen bonding supramolecular complex.
Secondly, the formed supramolecular complex must have a
suitable sized aromatic core, small-sized (e.g. Mel-X2) or very
rigid (e.g. Mel-X3) aromatic cores would easily induce the
crystallization of the supramolecular complex. Thirdly, the
stoichiometry of melamine and the complementary compound
would decide the specific mesophase of the supramolecular
complex. A complex of 1:3 stoichiometry usually formed a
hexagonal columnar structure while 1:2 stoichiometry would
induce rectangular columnar phase or square columnar phase.
Conclusions
In conclusion, to explore for suitable hydrogen-bonding
complementary compounds for the construction of melamine-
core supramolecular discotic LC materials, we designed and
synthesized five categories of long-alkyl-tailed compounds
bearing different hydrogen-bonding complementary functional
groups,
including
benzoic
acid,
cyanuric
acid,
homophthalimide, succinimide and thymine derivatives. Based
on experimental results, we discovered that benzoic acid could
not efficiently bind to melamine through hydrogen bonding
interactions, while the imidodicarbonyl unit was undoubtedly
the best hydrogen-bonding complementary functional group
of melamine. Furthermore, the heterocyclic ring size markedly
influenced the hydrogen-bonding strength of melamine-
imidodicarbonyl unit complex. For example, compared with
five-membered succinimide derivatives, six-membered
imidodicarbonyl heterocyclic systems (i.e. cyanuric acid,
homophthalimide, thymine) could form much more stable
hydrogen-bonding supramolecular complex with melamine.
Most importantly, liquid crystallinity could be generated
from these hydrogen-bonding supramolecular complexes. In
particular, a variety of columnar mesomorphic orders (such as
Phase transition hysteresis
As can be seen in Table 1, the calorimetric data of
supramolecular complexes Mel-X6-2 and Mel-X7 show large
hysteresis of the clearing points. It is well known that
thermotropic LC molecules usually have small phase transition
temperature hysteresis between the heating and cooling
cycles. However, as to hydrogen bond induced supramolecular
LC materials, their mesophases are not thermodynamically
Col
h
,
Col
r
,
Col
s
)
could be effectively achieved by
functionalization of thymine or succinimide units and
modulation of the molar ratios of melamine and these
hydrogen-bonding complementary compounds. Overall, this
work exhibited the art and power of synthetic chemistry in
controlling of the physical properties of matter. We hope these
findings might provide a unique LC perspective for detection
and separation of melamine in the future.
5
0
equilibrated since the dynamic nature of hydrogen bond
interactions are deeply involved in the formation and stability
of the supramolecular LC molecules’ mesophases. As
5
0
investigated previously, the hydrogen-bond donor and
hydrogen-bond acceptor components dissociated in the
isotropic liquid to some considerable extent, and when cooling
back from the isotropic phase, must coordinate together again
to provide mesomorphic character to the supramolecular
complex. Any other combination of the supramolecular
synthons might result in nonmesomorphic complexes or
crystalline.
Experimental Section
General considerations. The materials and instrumentation
information are described in the Supporting Information.
5
Compound X1
8-60
39
and compound X2 were synthesized
following literature protocols. The detailed synthetic
procedures of compounds X3, X4, X5, X6, X7 and all the

Please do not adjust margins
Journal of Materials Chemistry C
Page 8 of 11
View Article Online
DOI: 10.1039/C7TC03059B
ARTICLE
Journal Name
intermediates, NMR spectra and mass spectrometry data are 16 H.-K. Lee, H. Lee, Y. H. Ko, Y. J. Chang, N.-K. Oh, W.–C. Zin
and K. Kimoon, Angew. Chem. Int. Ed., 2001, 40, 2669-2671.
listed in the Supporting Information.
1
1
7 P. Cabras, M. Meloni and L. Spanedda, J. Chromatogr. A,
990, 505, 413-416.
8 S. S. Chou, D. F. Hwang and H. F. Lee, J. Food Drug Anal.,
2003, 11, 290-295.
1
A typical preparation protocol of hydrogen-bonded complex
Mel-X. Mel-X complexes were prepared by dissolving a
mixture of melamine and the corresponding heterocyclic 19 G. Venkatasami and J. R. Sowa Jr, Anal. Chim. Acta., 2010,
6
65, 227-230.
compound X in a molar ratio of 1 : 3 or 1 : 2 in anhydrous N,N-
dimethylformamide. The resulting mixture solution was
sonicated for 2 min at room temperature and the solvent was
evaporated under reduced pressure subsequently. The
2
2
0 H. W. Sun, L. X. Wang, L. F. Ai, S. X. Liang and H. Wu, Food
Control, 2010, 21, 686-691.
1 J. V. Sancho, M. Ibáñez, S. Grimalt, Ó. J. Pozo and F.
Hernández, Anal. Chim. Acta., 2005, 530, 237-243.
obtained mixture was heated to isotropic melt and then slowly 22 K. Xia, J. Atkins, C. Foster and K. Armbrust, J. Agric. Food
Chem., 2010, 58, 5945-5949.
cooled to room temperature to form the desired complex Mel-
2
2
3 J. Ge, L. W. Zhao, C. Y. Liu, S. Jiang, P. W. Lee and F. M. Liu,
Food Control, 2011, 22, 1629-1633.
4 Y. Liu, Z. Lin, S. Zhang, C. Yang and X. Zhang, Anal. Bioanal.
Chem., 2009, 395, 591-599.
25 J. D. Harper, N. A. Charipar, C. C. Mulligan, X. R. Zhang, R. G.
X.
Acknowledgements
Cooks and Z. Ouyang, Anal. Chem., 2008, 80, 9097-9104.
6 K. Ai, Y. L. Liu and L. H. Lu, J. Am. Chem. Soc., 2009, 131
This research was supported by National Natural Science
Foundation of China (Grant No. 21374016), Jiangsu Provincial
2
,
9
Natural Science Foundation of China (BK20170024), 27 W. J. Qi, D. Wu, J. Ling and C. Z. Huang, Chem. Commun.,
496-9497.
2
010, 46, 4893-4895.
8 M.W. Zhang, X. Y. Cao, H. K. Li, F. R. Guan, J. J. Guo, F. Shen,
Y. L. Luo, C. Y. Sun and L. G. Zhang, Food Chem., 2012, 135
894-1900.
Fundamental Research Funds for the Central Universities, and
the Open Research Fund of State Key Laboratory of
Bioelectronics, Southeast University.
2
,
1
2
3
3
9 J. X. Liu, Y. B. Zhong, J. Liu, H. C. Zhang, J. Z. Xi and J. P. Wang,
Food Control, 2010, 21, 1482-1487.
0 B. Kim, L. B. Perkins, R. J. Bushway, S. Nesbit, T. Fan,
R. Sheridan and V. Greene, J. AOAC Int., 2008, 91, 408-413.
1 S. Yagai, J. Photochem. Photobiol. C: Photochem. Rev., 2006,
Keywords: melamine • discotic liquid crystal • hydrogen
bonding • supramolecular chemistry • columnar
7
32 B. Roy, P. Bairi and A. K. Nandi, RSC Adv., 2014, 4, 1708-1734.
, 164-182.
References
3
3
3
3
3
3
3
4
4
4
3 J. A. Zerkowski, C. T. Seto, D. A. Wierda and G. M.
Whitesides, J. Am. Chem. Soc., 1990, 112, 9025-9026.
4 J. M. Lehn, M. Mascal, A. Decian and J. Fischer, J. Chem. Soc.,
Chem. Commun., 1990, 6, 479-481.
5 K. Hanabusa, T. Miki, Y. Taguchi, T. Koyama and H. Shirai, J.
Chem. Soc. Chem. Commun., 1993, 18, 1382-1384.
1
2
C. M. Paleos and D. Tsiourvas, Angew. Chem. Int. Ed., 1995,
4, 1696-1711.
T. Kato, N. Mizoshita and K. Kanie, Macromol. Rapid
Commun., 2001, 22, 797-814.
3
3
4
U. Beginn, Prog. Polym. Sci., 2003, 28, 1049-1105.
T. Kato, N. Mizoshita and K. Kishimoto, Angew. Chem. Int.
Ed., 2006, 45, 38-68.
6 W. J. Sang, K. Murata and S. Shinkai, Supramol. Sci., 1996,
3-86.
3,
8
7 L. J. Prins, F. D. Jong, P. Timmerman and D. N. Reinhoudt,
Nature, 2000, 408, 181-184.
8 X. Huang, C. Li, S. Jiang, X. Wang, B. Zhang and M. Liu, J.
Colloid Interf. Sci., 2005, 285, 680-685.
9 F. J. Hoeben, M. J. Pouderoijen, A. P. Schenning and E. W.
Meijer, Org. Biomol. Chem., 2006, 4, 4460-4462.
0 S. Yagai, T. Seki, T. Karatsu, A. Kitamura and F. Würthner,
Angew. Chem. Int. Ed., 2008, 47, 3367-3371.
1 S. Yagai, S. Kubota, K. Unoike, T. Karatsu and A. Kitamura,
Chem. Commun., 2008, 40, 4466-4468.
5
T. Kato, T. Yasuda, Y. Kamikawa and M. Yoshio, Chem.
Commun., 2009, 40, 729-739.
6
7
C. Tschierske, Angew. Chem. Int. Ed., 2013, 52, 8828-8878.
M. Suárez, J. M. Lehn, S. C. Zimmerman, A. Skoulios and B.
Heinrich, J. Am. Chem. Soc., 1998, 120, 9526-9532.
A. Kraft, A. Reichert and R. Kleppinger, Chem. Commun.,
8
9
1
2
000, 36, 1015-1016.
J. H. Lee and J. Y. Jho, Mol. Cryst. Liq. Cryst., 2015, 622, 55-
0.
6
0 T. Yasuda, H. Ooi, J. Morita, Y. Akama, K. Minoura, M.
Funahashi, T. Shimomura and T. Kato, Adv. Funct. Mater.,
2 S. Yagai, H. Aonuma, Y. Kikkawa, S. Kubota, T. Karatsu, A.
Kitamura, S. Mahesh and A. Ajayaghosh, Chem. Eur. J., 2010,
16, 8652-8661.
2009, 19, 411-419.
1 B. Soberats, M. Yoshio, T. Ichikawa, X. B. Zeng, H. Ohno, G.
Ungar and T. Kato, J. Am. Chem. Soc., 2015, 137, 13212-
1
1
4
4
3 L. J. Prins, C. Thalacker, F. Wurthner, P. Timmerman and D.
N. Reinhoudt, Proc. Natl. Acad. Sci. U. S. A., 2001, 98, 10042-
1
2 X. L. Feng, V. Marcon, W. Pisula, M. R. Hansen, J. Kirkpatrick,
3215.
1
0045.
4 A. Piermattei, M. Giesbers, A. T. M. Marcelis, E. Mendes, S. J.
Picken, M. Crego-Calama and D. N. Reinhoudt, Angew. Chem.
Int. Ed., 2006, 45, 7543-7546.
F. Grozema, D. Andrienko, K. Kremer and K. Müllen, Nat.
Mater., 2009, 8, 421-426.
1
1
3 K. Petritsch, R. H. Friend, A. Lux, G. Rozenberg, S. C. Moratti
and A. B. Holmes, Synth. Met., 1999, 102, 1776-1777.
4 L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R.
H. Friend and J. D. MacKenzie, Science, 2001, 293, 1119-
4
4
4
5 J. Barbera, L. Puig, J. L. Serrano and T. Sierra, Chem. Mater.,
004, 16, 3308-3317.
6 M. Xu, L. Chen, Y. Zhou, T. Yi, F. Li and C. Huang, J. Colloid
Interf. Sci., 2008, 326, 496-502.
2
1
122.
5 S. Sergeyev, W. Pisula and Y. H. Geerts, Chem. Soc. Rev.,
007, 36, 1902-1929.
7 X. Cheng, J. Jin, Q. Li and X. Dong, Chin. J. Chem., 2010, 28
957-1962.
,
1
1
2
8
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 11
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03059B
4
8 A. A. Vieira, H. Gallardo, J. Barbera, P. Romero, J. L. Serrano 56 S. Mir Sayed, B. P. Lin and H. Yang, Soft Matter, 2016, 12
and T. Sierra, J. Mater. Chem., 2011, 21, 5916-5922. 6148-6156.
9 F. Vera, R. M. Tejedor, P. Romero, J. Barbera, M. B. Ros, J. L. 57 M. F. Jacobsen, M. M. Knudsen and K. V. Gothelf, J. Org.
,
4
Serrano and T. Sierra, Angew. Chem. Int. Ed., 2007, 46, 1873-
877.
Chem., 2006, 71, 9183-9190.
58 J. F. Zheng, X. Liu, X. F. Chen, X. K. Ren, S. Yang and E. Q.
1
5
5
5
5
0 J. Barbera, L. Puig, P. Romero, J. L. Serrano and T. Sierra, J.
Am. Chem. Soc., 2006, 128, 4487-4492.
1 N. Kimizuka, T. Kawasaki, K. Hirata and T. Kunitake, J. Am.
Chem. Soc., 1995, 117, 6360-6361.
2 F. Wurthner, C. Thalacker and A. Sautter, Adv. Mater., 1999,
Chen, ACS Macro Lett., 2012, 1, 641-645.
59 N. T. Pokhodylo, O. Y. Shyyka and N. D. Obushak, Chem.
Heterocycl. Compd., 2015, 50, 1748-1755.
60 J. L. Havet, C. Porte and A. Delacroix, Tetrahedron Lett.,
2003, 44, 4399-4402.
61 K. Zhao, X. Bai, B. Xiao, Y. Gao, P. Hu, B. Wang, Q. Zeng, C.
11, 754-758.
3 R. F. M. Lange, F. H. Beijer, R. P. Sijbesma, R. W. W. Hooft, H.
Kooijman, A. L. Spek, J. Kroon and E. W. Meijer, Angew.
Chem. Int. Ed., 1997, 36, 969-971.
Wang, B. Heinrich and B. Donnio, J. Mater. Chem. C, 2015,
11735-11746.
62 J. Miao and L. Zhu, Chem. Mater., 2010, 22, 197-206.
3,
5
5
4 J. A. Theobald, N. S. Oxtoby, M. A. Phillips, N. R. Champness 63 B. Feringan, P. Romero, J. L. Serrano, R. Gimenez and T.
and P.H. Beton, Nature, 2003, 424, 1029-1031. Sierra, Chem. Eur. J., 2015, 21, 8859-8867.
5 B. Quiclet-Sire and S. Z. Zard, Chem. Commun., 2002, 34
306-2307.
,
2

Journal of Materials Chemistry C
Page 10 of 11
View Article Online
DOI: 10.1039/C7TC03059B

Page 11 of 11
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03059B
Table of Contents Graphic
Hydrogen-Bonding Induced Melamine-Core Supramolecular Discotic
Liquid Crystals
Ling-Xiang Guo, Yu-Han Liu, Li Wang, Meng Wang, Bao-Ping Lin, and Hong Yang*
A variety of hydrogen-bonding induced melamine-core supramolecular discotic liquid
crystals possessing hexagonal columnar, rectangular columnar and square columnar
phases are described.