Influence of functional groups on the self-assembly of liquid crystals

Article abstract of DOI:10.1016/j.cclet.2020.09.016

Functional groups in the molecule play an important role in the molecular organization process. To reveal the influence of functional groups on the self-assembly at interface, herein, the self-assembly structures of three liquid crystal molecules, which only differ in the functional groups, are explicitly characterized by using scanning tunneling microscopy (STM). The high-resolution STM images demonstrate the difference between the supramolecular assembly structures of three liquid crystal molecules, which attribute to the hydrogen bonding interaction and π-π stacking interaction between different functional groups. The density functional theory (DFT) results also confirm the influence of these functional groups on the self-assemblies. The effort on the self-assembly of liquid crystal molecules at interface could enhance the understanding of the supramolecular assembly mechanism and benefit the further application of liquid crystals.

Full text of DOI:10.1016/j.cclet.2020.09.016

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Chinese Chemical Letters xxx (2020) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Influence of functional groups on the self-assembly of liquid crystals
c,
a,***
*
,
Shanchao Tana,b,1, Jiayu Taoc,1, Wendi Luod,f,1, Hao Jiangc, Yuhong Liu
, Haijun Xu
a,b,**
Qingdao Zengb,e, , Hongyu Shi
*
a
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
b
CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience
and Technology (NCNST), Beijing 100190, China
c
Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, College of Chemical Engineering, Nanjing Forestry University,
Nanjing 210037, China
d
Laboratory of Theoretical and Computational Nanoscience, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology,
Beijing 100190, China
e
Center of Materials Science and Optoelectonics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
f
Sino-Danish Center for Education and Research, University of Chinese Academy of Sciences, Beijing 100190, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Available online xxx
Functional groups in the molecule play an important role in the molecular organization process. To reveal
the influence of functional groups on the self-assembly at interface, herein, the self-assembly structures
of three liquid crystal molecules, which only differ in the functional groups, are explicitly characterized
by using scanning tunneling microscopy (STM). The high-resolution STM images demonstrate the
difference between the supramolecular assembly structures of three liquid crystal molecules, which
Keywords:
Self-assembly
Liquid crystal
Functional group
Hydrogen bond
attribute to the hydrogen bonding interaction and
p-p stacking interaction between different functional
groups. The density functional theory (DFT) results also confirm the influence of these functional groups
on the self-assemblies. The effort on the self-assembly of liquid crystal molecules at interface could
enhance the understanding of the supramolecular assembly mechanism and benefit the further
application of liquid crystals.
p-pStacking interaction
© 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Liquid crystals are basically dynamic functional materials that
ion transporting [7,8], sensing [9,10], catalysis [11,12], templates
[13–15] and biomedicine [16,17], etc.
combine order and mobility at molecular and macroscopic levels
[1]. Since the discovery by Reinitzer in 1888 [2], liquid crystals have
been studied in multiple disciplines as prototype self-assembling
supramolecular soft materials, because of their involvement of
almost all kinds of interactions including van der Walls interaction,
hydrogen bonding interaction, dipolar interaction, metal coordi-
nation and so on [1,3]. Other than the well-known technological
applications in the liquid-crystal display (LCD) devices [4–6], liquid
crystals can also be used as functional materials for electron and
Self-assembly is a spontaneous organization of components
into regular patterns or structures, which has been extensively
investigated for decades with the main goal of providing access to
functional materials [18–22]. During the molecular organization
process, the supramolecular structure is primarily determined by
the intermolecular interactions, which correlate with the func-
tional groups to a great extent. Hence the effect of functional
groups on self-assembly has drawn more and more attention.
Valera et al. [23] investigated the supramolecular assembly of two
pyrene derivatives, governed by the C–HÁ Á Á
p interactions and the
H-bonding interaction between the amide groups and the
imidazole moieties. The pyrene-based aggregate preserves the
blue-emitting properties of the monomer, showing the potential
application in Organic Electronics. Aparicio et al. [24] synthesized
three cyano-substituted divinylene arene-based luminogens as
* Corresponding author at: CAS Key Laboratory of Standardization and
Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience,
National Center for Nanoscience and Technology (NCNST), Beijing, 100190, China.
** Corresponding author at: State Key Laboratory of Tribology, Tsinghua
University, Beijing, 100084, China.
fluorescent
p-gelators. The donor-acceptor interaction in self-
*** Corresponding authors.
assembly can be modulated by changing the position of cyano
groups in the conjugated backbone, thus achieving the modulation
of the emission color. Recently, Rödle et al. [25] have also compared
E-mail addresses: liuyuhong@tsinghua.edu.cn (Y. Liu), xuhaijun@njfu.edu.cn
(H. Xu), zengqd@nanoctr.cn (Q. Zeng), shihongyu1991@126.com (H. Shi).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.cclet.2020.09.016
1001-8417/© 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Tan, et al., Influence of functional groups on the self-assembly of liquid crystals, Chin. Chem. Lett. (2020),
https://doi.org/10.1016/j.cclet.2020.09.016

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the supramolecular polymerization of two liquid crystal molecules
that only differ in the linking group to investigate the influence of
amide and ester groups on the molecular arrangement. They
revealed the rotationally displaced organization for the amide-
containing molecules and the parallel arrangement for the ester-
containing molecules. However, by using spectroscopic methods,
the assembly structures of aggregates were too complicated to be
characterized directly and clearly enough.
Therefore, to further investigate the impact of functional groups
on the supramolecular assembly mechanism, herein we report the
explicit self-assembly structures of three liquid crystal molecules
which are similar in molecular structure. As shown in Scheme 1,
we employed three liquid crystal molecules which only differ in
the functional groups of backbones as the research objects. The
difference between LC-amide-1 and LC-ester is the amide/ester
groups at both ends of the backbone, while LC-amide-1 and LC-
amide-2 differ in the presence/absence of central BODIPY group.
The LC-amide-1, LC-amide-2 and LC-ester are separately dissolved
in 1-phenyloctane, wherein the concentrations are less than 10À3
mol/L. Then the assembly samples were prepared by depositing a
Fig. 1. STM images of LC-amide-1 assembly structure at the HOPG/1-phenyloctane
interface: (a) Large scale; (b) high resolution. Tunneling conditions: Iset = 219.7 pA,
Vbias =734.3 mV. (c) The simulated molecular packing structure.
both ends of the molecule are almost perpendicular to the
corresponding benzene rings of another molecule in the dimer,
which can form
p-p stacking interaction. It can also be
distinguished that the alkyl chains are distributed in a staggered
form and stretched in different directions. The van der Waals
interaction between these alkyl chains, the
tion between corresponding benzene rings and the
p
-
p
stacking interac-
stacking
p-p
interaction between two BODIPY moieties in the dimer are the
primary assembly motifs of the linear structure. The parameters of
the unit cells overlaid in Fig. 1a are measured as follows:
a = 4.8 Æ 0.1 nm, b = 3.5 Æ 0.1 nm, α = 85 Æ1ꢀ.
droplet of the corresponding solution (0.1 mL) onto the freshly
cleaved highly oriented pyrolytic graphite (HOPG) surface,
respectively. Afterwards, the uniform and regular assembly
structures were precisely revealed at the HOPG/1-phenyloctane
interface by using STM under ambient conditions. The details of
sample preparation and STM characterization procedures can be
found in Supporting Information. In combination with DFT
calculation, the self-assembly mechanism was deeply explored.
First, the assembly structure of LC-amide-1 at the HOPG/1-
phenyloctane interface is characterized by using STM. Fig. 1a
presents the large scale STM image of the assembly structure, in
which the bright rods are aligned in parallel and form the uniform
linear pattern. The average length of bright rods is measured to be
4.0 Æ 0.1 nm, which shows good agreement with the theoretical
size of the backbone of LC-amide-1 molecules. And the alkyl chains
of LC-amide-1 molecules correspond to the lines distributed in the
shade, which cannot be explicitly characterized due to the much
lower density of electric states. Further structural details are
displayed in high resolution STM image, as shown in Fig. 1b,
indicating that each rod element is a dimer formed by two closed-
packed LC-amide-1 molecules. The two molecules in the dimer can
interact with each other through N-HÁÁÁO hydrogen bonding
between amide groups. Besides, the simulated molecular packing
structure as shown in Fig. 1c indicates that the benzene rings at
In the next step, the self-assembly of LC-ester at the HOPG/1-
phenyloctane interface is investigated. Fig. 2a presents the large
scale STM image of LC-ester assembly structure. It can be observed
that the butterfly-shaped dimers are arranged in parallel into the
linear pattern. And the orientation of dimers is at an angle to the
direction of the rows. As shown in the high resolution STM image
in Fig. 2b, each dimer consists of two parallel bright rods. The
average length of bright rods is measured to be 3.8 Æ 0.1 nm, which
is consistent with the theoretical size of the backbone of LC-ester.
Therefore, it can be concluded that the butterfly-shaped dimers are
composed of two opposite LC-ester molecules, which mostly
interact through
p-p stacking interaction between BODIPY
moieties and benzene rings. Combined with the simulated
molecular packing structure in Fig. 2c, it can be concluded that
the parallel lines distributed between the neighbor rows are
attributed to the alkyl chains of LC-ester molecules. The assembly
motif derives from both intermolecular
p p stacking interaction
À
and the van der Waals interaction between alkyl chains. The
parameters of the unit cells overlaid in Fig. 2b are measured as
follows: a = 4.3 Æ 0.1 nm, b = 4.3 Æ 0.1 nm, α = 105 Æ1ꢀ.
LC-amide-2 molecules can also form regular self-assembly
structure at the HOPG/1-phenyloctane interface. Fig. 3a presents
the large scale STM image of LC-amide-2 assembly structure, in
which the bright rods aligned in parallel can be observed. As shown
in the high resolution STM image in Fig. 3b, the rods containing two
bright spots at both ends are measured to be 2.7 Æ 0.1 nm, which
shows good agreement with the theoretical size of the backbone of
LC-amide-2 molecules. Therefore, it can be inferred that the bright
rods correspond to the LC-amide-2 molecules and the bright spots
correspond to the benzene rings at both ends of the backbone. The
Fig. 2. STM images of LC-ester assembly structure at the HOPG/1-phenyloctane
interface: (a) Large scale; (b) high resolution. Tunneling conditions: Iset = 228.9 pA,
Vbias =714.7 mV. (c) The simulated molecular packing structure.
Scheme 1. Chemical structures of (a) LC-amide-1, (b) LC-ester and (c) LC-amide-2.
Please cite this article in press as: S. Tan, et al., Influence of functional groups on the self-assembly of liquid crystals, Chin. Chem. Lett. (2020),
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3
(-152.124 kcal/mol) due to the
p
-p
stacking interaction between
BODIPY groups and graphite substrate. The interaction energies
between molecules and substrate of three assembly systems are all
much lower than the interaction energies between molecules,
indicating that the absorption between molecules and HOPG
substrate is quite strong.
The total energies (including the interaction energies between
molecules and the interaction energies between molecules and
substrate) of three self-assembly systems are presented in Table 2.
In general, the total energy can be compared to evaluate the
relative thermodynamic stability of different assembly systems
with the same unit cell. However, the effect of the unit area should
be considered when comparing two systems with different unit
cells. For the assembly system with the smaller unit cell, more
molecules would be adsorbed on the surface within the same area,
thus contributing more interaction energy to the system. [26]
Therefore, the total energy per unit area is also calculated to avoid
such effect. As displayed in the last column in Table 2, the total
Fig. 3. STM images of LC-amide-2 assembly structure at the HOPG/1-phenyloctane
interface: (a) Large scale; (b) high resolution. Tunneling conditions: Iset = 222.7 pA,
Vbias =693.2 mV. (c) The simulated molecular packing structure.
Table 1
Experimental (Expt.) and calculated (Cal.) unit cell parameters of the self-
assemblies on the HOPG surface.
Sample
a (nm)
b (nm)
α (ꢀ)
LC-amide-1
Expt.
Cal.
Expt.
Cal.
Expt.
Cal.
4.8 Æ 0.1
4.8
3.5 Æ 0.1
3.5
85 Æ 1ꢀ
85
energy per unit area of LC-amide-2 system (-0.234 kcal molÀ1 Å-2
)
LC-ester
4.3 Æ 0.1
4.3 Æ 0.1
105 Æ 1ꢀ
105
is the highest and the total energy per unit area of LC-amide-1
system (-0.338 kcal molÀ1 Å-2) is slightly lower than that of the LC-
ester system (-0.303 kcal molÀ1 Å-2), which suggests that the self-
assembly of LC-amide-1 is most thermodynamically stable.
As mentioned above, the three liquid crystal molecules all
assemble into the linear patterns at the HOPG/1-phenyloctane
interface. However, the differences in the backbone of molecules
lead to different linear structures. As for the self-assembly of LC-
amide-1 and LC-ester, the linear patterns are both formed by the
arrangement of dimers. The two liquid crystal molecules in the
4.3
4.3
LC-amide-2
3.0 Æ 0.1
3.0
2.5 Æ 0.1
2.5
110 Æ 2ꢀ
108
acetylene linkages and alkyl chains cannot be explicitly character-
ized due to the much lower density of electric states. Combined
with the simulated molecular packing structures shown in Fig. 3c,
it can be inferred that the alkyl chains are aligned into one
direction and the amide groups are too far apart to form hydrogen
bonds. Therefore, the assembly motif mostly derives from the van
der Waals interaction between these alkyl chains. The parameters
of the unit cells overlaid in Fig. 3b are measured as follows:
a = 3 Æ 0.1 nm, b = 2.5 Æ 0.1 nm, α = 110 Æ 2ꢀ.
dimer can interact through p-p stacking interaction. Moreover, the
amide moieties in LC-amide-1 molecules can form hydrogen
bonds, while the ester moieties in LC-ester molecules cannot.
Therefore, it can be distinguished that the two LC-amide-1
molecules in the dimer are arranged much closer comparing to
the arrangement of LC-ester dimer, which agrees well with the
result that the interaction energy between LC-amide-1 molecules
(-149.536 kcal/mol) is lower than that between LC-ester molecules
(-134.914 kcal/mol). As for the self-assembly of LC-amide-2, the
linear pattern is formed by the arrangement of molecules instead
of dimers. The van der Waals interaction between alkyl chains is
the primary assembly motif, and the alkyl chains are arranged into
one direction to maximize the van der Waals interaction.
Moreover, as shown in Fig. 3, the lowermost end of the backbone
of the LC-amide-2 molecule is aligned with the uppermost end of
the backbone of the lower right molecule to make the alkyl chains
be aligned and interact more with each other. Hence the amide
groups are too far apart to form the hydrogen bonds. Considering
that the only difference between the molecular structures of LC-
amide-1 and LC-amide-2 is the BODIPY group, it can be inferred
To better understand the self-assembly structures of three
liquid crystal molecules, the unit cell parameters and interaction
energies are calculated by the DFT method based on the STM
characterizations. The details of DFTcalculation can be found in the
Supporting Information. Table 1 lists the calculated unit cell
parameters of three assembly systems, which agree well with the
corresponding experimental results. The interaction energies of
three self-assemblies are presented in Table 2 and the lower energy
indicates the stronger interaction herein. It can be observed that
the interaction energy between LC-amide-1 molecules
(-149.536 kcal/mol) is lower than that between LC-ester molecules
(-134.914 kcal/mol), which is largely due to the extra N-HÁÁÁ
O
hydrogen bonds between amide groups of LC-amide-1 molecules.
And the interaction energy between LC-amide-2 molecules
(-16.254 kcal/mol) is much higher because LC-amide-2 molecules
mainly interact through the weak van der Waals interaction.
Besides the interaction between assembled molecules, the
interaction between molecules and substrate also plays a crucial
part in the surface assembly. As shown in the second column in
Table 2, the interaction energy between LC-amide-1 molecules and
substrate (-418.567 kcal/mol) is similar to that between LC-ester
molecules and substrate (-421.721 kcal/mol), which are both lower
than that between LC-amide-2 molecules and substrate
that the
p-p stacking interaction is also important in forming
dimers. Combined with the DFT result that the interaction energy
between LC-amide-1 molecules (-149.536 kcal/mol) is much lower
than that of LC-amide-2 molecules (-16.254 kcal/mol), it can be
inferred that the interaction between LC-amide-2 molecules is not
strong enough to form dimers with the absence of
p-p stacking
interaction and hydrogen bonding interaction. Considering the
Table 2
Total energies and energies per unit area of self-assemblies on the HOPG surfacea.
Sample
Interactions between
molecules (kcal/mol)
Interactions between molecules
and substrate (kcal/mol)
Total energy
(kcal/mol)
Total energy per unit
area (kcal molÀ1 Å-2
)
LC-amide-1
LC-ester
LC-amide-2
À149.536
À134.914
À16.254
À418.567
À421.721
À152.124
À568.103
À561.635
À168.378
À0.338
À0.303
À0.234
a
The total energy includes the interaction energies between molecules and the interaction energies between molecules and substrate.
Please cite this article in press as: S. Tan, et al., Influence of functional groups on the self-assembly of liquid crystals, Chin. Chem. Lett. (2020),
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experimental results and DFT calculations of three self-assembly
systems, it is indicated that the simultaneous presence of N-HÁÁÁ
hydrogen bonding between amide groups, van der Waals interac-
tion between alkyl chains, stacking interaction between
Appendix A. Supplementary data
O
Supplementary material related to this article can be found, in
p
-p
the
online
version,
at
doi:https://doi.org/10.1016/j.
BODIPY groups and between benzene rings results in the closest
molecular arrangement and the best thermodynamic stability of
LC-amide-1 system in three liquid crystal self-assemblies.
cclet.2020.09.016.
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Declaration of competing interest
The authors report no declarations of interest.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (Nos. 51875303, 21773041, 21972031),
the National Basic Research Program of China (No.
2016YFA0200700) and the Strategic Priority Research Program
of Chinese Academy of Sciences (No. XDB36000000).
Please cite this article in press as: S. Tan, et al., Influence of functional groups on the self-assembly of liquid crystals, Chin. Chem. Lett. (2020),
https://doi.org/10.1016/j.cclet.2020.09.016