Optical Waveguiding Organic Single Crystals Exhibiting Physical and Chemical Bending Features

Article abstract of DOI:10.1002/anie.201914026

Bendable (elastic and plastic) organic single crystals have been widely studied as emerging flexible materials with highly ordered packing structures. However, even though manifold bendable organic crystals have been recently reported, most of them bend in response to only one stimulus. Herein, we report an organic single crystal of (Z)-4-(1-cyano-2-(4-(dimethylamino)phenyl)vinyl)benzonitrile, which bends under external stress (physical process) and also hydrochloric acid atmosphere (chemical process). This observation indicates that a single organic crystal, whose structure has been optimized simultaneously at both the molecular and supramolecular levels, may display multiple crystal-bending modes. Furthermore, the crystals exhibit bright orange-yellow emission and can serve as an active low-loss optical waveguide in both the straight and the bent state, which indicates a potential optical application.

Full text of DOI:10.1002/anie.201914026

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Optical Waveguiding Organic Single Crystals Exhibiting Physical
and Chemical Bending Features
Authors: Hongyu Zhang, Zhuoqun Lu, Yuping Zhang, Hao Liu, Wentao
Liu, and Kaiqi Ye
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914026
Angew. Chem. 10.1002/ange.201914026
Link to VoR: http://dx.doi.org/10.1002/anie.201914026
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10.1002/anie.201914026
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Optical Waveguiding Organic Single Crystals Exhibiting Physical
and Chemical Bending Features
Zhuoqun Lu, Yuping Zhang, Hao Liu, Kaiqi, Ye, Wentao Liu, and Hongyu Zhang*
Abstract: Bendable (elastic and plastic) organic single crystals have
been widely studied as emerging flexible materials with highly
ordered packing structures. However, even though manifold
bendable organic crystals have been recently reported, most of them
only display bending ability under the single circumstance. Herein,
we report an organic single crystal of (Z)-4-(1-cyano-2-(4-
(dimethylamino)phenyl)vinyl)benzonitrile, which could bend under
external stress (physical process) or in the hydrochloric acid
atmosphere (chemical process). This observation indicates that
multiple crystal bending modes might be achieved in the same single
organic crystal by simultaneously optimizing the structure at both
molecular and supramolecular levels. Furthermore, the crystal
exhibits bright orange-yellow emission and possesses good property
of active low-loss optical waveguide in both the straight and the bent
state, which indicates a potential optical application.
assembly of different stimuli-response mechanical properties,
which is also instructive to the exploitation of new versatile
organic solid materials.[20,21] Although different bending forms of
organic crystals have been developed, few of these
masterpieces about organic crystals have accomplished the
combination of different mechanical stimuli-responses.[22]
Therefore, there emerges a new challenge to the extension of
multiple stimuli-responsive bending organic crystals with optical
applications. However, it is very difficult to achieve the
combination of multiple mechanical properties. On the one hand,
it is quite tough to build the rational construction of molecular
structure and predict the packing modes of crystals, which
characterizes crystals with bending ability and luminescence. On
the other hand, some special substituents that can alter the
molecular configuration under special conditions need to be
introduced in proper position and have to be impervious to
packing modes of crystals. Therefore, perfect compatibility
should be carefully considered in the design of multiple stimuli-
responsive organic bending crystals.
Based on this situation, herein we report a dual stimuli-
responsive bendable organic luminescent crystal of (Z)-4-(1-
cyano-2-(4-(dimethylamino)phenyl)vinyl)benzonitrile CN-DPVB
(Figure 1a). The high-quality needle-like single crystals with
lengths in centimeter scale are elastically bendable under
external applied stress and exhibit bright orange emission with a
quantum yield of 0.19 (Figure 1b‒d). Cyan and dimethyl amino
substituted on both ends of the π-conjugated framework
facilitate a donor-acceptor structure that favors luminescence.[23]
Weak intermolecular interactions such as π-π stacking account
for its mechanical bendability. Besides, dimethyl amino is a
common substituent that is prone to get protonation and thus is
more likely to alter the molecular structure. Therefore, it was
observed that the CN-DPVB could bend automatically under the
hydrochloric acid atmosphere, which has been not reported yet
(Figure 1e). Meanwhile, CN-DPVB crystals also display quite
good active optical waveguide in both the straight and the bent
state. This new accomplishment of bendable crystals may
particularly enrich the diversity of organic functional materials.
The study of the mechanical properties of organic crystals has
become a burgeoning orientation in crystal engineering for its
potential application in various fields.[1‒4] Recently, the research
of elastic organic single crystals (EOSCs) has become a cutting-
edge field because EOSCs tend to be applied in flexible devices
which is a focus in the development of current advanced
material and technologies.[5‒8] Ghosh and Reddy are precursors
in the study of EOSCs and proposed mature mechanisms.[9,10]
Then Koizumi and his coworkers firstly reported an elastic π-
conjugated
molecule
1,4-bis[2-(4-methylthienyl)]-2,3,5,6-
tetrafluorobenzene with intense fluorescence.[11] Based on these
findings, many EOSCs were reported constantly and further
potential applications were developed successively in recent
years. Our latest research gave a direction for realizing active
optical waveguides in the elastically bent crystals, which
developed an important optical application of organic crystals.
We designed elastic crystals of Schiff base and derivatives of
single-benzene compounds, which exhibit excellent properties of
flexible optical waveguide and even amplified spontaneous
emission.[12‒16] The flexibility of crystals thrives the research
topics in crystal engineering. Nevertheless, in addition to
physical stress, some other inducements like heat, solvent, and
irradiation also result in the deformation of crystals, which were
seriatim reported in the past decade. Naumov and coworkers
reported the spatial photocontrol over the optical output from
slender single crystals of an azo compound.[17] Hu and his
coworkers designed a solvatomechanical cocrystal of perylene
and TCNB which responses mechanically to THF solvent.[18]
Zhang reported a self-healing cocrystal of coronene and TCNB
that bends and splits when the crystal is heated and restores to
initial state after cooling.[19] All these works have revealed the
diversity of inducements. Hence, it is significant to realize the
[a]
Z. Lu, Y. Zhang, H. Liu, Prof. Dr. K. Ye, W. Liu, Prof. Dr. H. Zhang
State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University
Figure 1. a) Molecule Structure of CN-DPVB. Photograph of the needle-like
single crystals of CN-DPVB taken under room light b) and 365 nm UV light c).
d) Physical bending under external stress applied through a pair of tweezers.
e) Chemical bending process under the hydrochloric acid atmosphere.
Qianjin Street, Changchun (P.R. China)
E-mail: hongyuzhang@jlu,edu,cn
1
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When a CN-DPVB crystal was applied with external stress, it
was easily bent into an arc of almost 180° (Figure 1d) without
rupture and restored instantaneously upon the withdrawal of
force. And the bending-relaxing process could repeat reversibly
for many times. The single crystal structure of CN-DPVB was
determined for a further investigation of the observed elasticity.
CN-DPVB crystallizes in monoclinic space group P21/c with one
molecule in an asymmetric unit (Figure S1). Individual molecules
connect via two kinds of C‒H···N interaction (N···H distance
between cyan and dimethyl amino: 2.599 Å, distance between C
and N: 3.504 Å. N···H distance between cyan and benzene:
2.627 Å, distance between C and N: 3.338 Å). Two benzene
rings in one molecule twist about 8° in a trans conformation
across the C=C bond. Thus the whole molecule is relatively
planar, which conduces to the intermolecular π-π stacking. The
bendable face is determined in the morphology calculation of
CN-DPVB by Material Studio 8.0. Based on the calculation,
(001) plane is the widest and bendable plane of crystal that
grows along a axis. Moreover, the morphology of the crystal was
characterized to be an oblique hexagonal prism (Figure S2),
which almost corresponded to the observation by microscope.
The molecular arrangements in different directions are shown in
Figure 2a. Molecules form two subtle π-π interactions of cyan-
benzene (3.447 Å) and dimethyl amino-benzene (3.560 Å) in
each line which connects reciprocally via C‒H···N interaction
(N···H distance: 2.599 Å,) finally generating a layer structure of
molecules (Figure 2b). Each layer is perpendicular to the (001)
plane and connects via C‒H···N interaction. When the crystal is
applied with external stress, a far-range movement of parallel
molecules is prohibited because of the crisscrossing packing
mode observed along the plane (10-3) (Figure 2c) but a subtle
motion from the equilibrium position is allowed. Meanwhile, the
weak interactions of C‒H···N and C‒H···C are prone to rupture
and restore and therefore absorbs and releases the strain
energy during the entire bending process. When a crystal is
applied with stress, the distance between molecules increases in
the outer arc to disperse the external stress energy and
molecules in the inner arc become more contracted to each
other due to the compression (Figure 2b). However, the whole
structure and energy equilibrium will be destroyed when external
stress exceeds the threshold limit, eventually leading to the
crystal fracture. Three-point bending test was carried out to
measure the modulus (E) of CN-DPVB crystals, which is an
important index for the evaluation of elasticity (Figures 3 and S3).
The linear curve of stress and strain validates the elasticity of
CN-DPVB, which also indicates that there is no plasticity
exhibiting in the whole bending process (Figure 3a). Modulus E
is the quotient of the stress (σ) and the strain (ε) which were
calculated according to the derived formulas (Figure S4).[24] The
average value of elasticity modulus is calculated to be 0.905
GPa, indicating a quite good elastic bending ability of CN-DPVB
crystals.
become straight. It was observed that the surface of the crystal
got rough and lost transparency after exposure to HCl in the
SEM image (Figure S6), which indicated that the transformation
was not single-crystal to single-crystal. Different from the
physical stress, this process is irreversible and the bent crystal
could not restore to its initial state by alkali flow or heat. Apart
from hydrochloric acid, a sequel of volatile acids such as formic
acid, acetic acid, propionic acid and trifluoroacetic acid (TFA)
were also tested. Whereas crystals were found to dissolve in the
TFA and there was no other emergence of crystal bending
phenomenon in other acids, which showed a selectivity of
hydrochloric acid. In addition, different faces of crystal were
respectively fumed by HCl acid to find the bendable planes in
this process. Surprisingly, it was found that the widest face (001)
plane was the bendable plane under the hydrochloric acid
stimulation, which resembled the situation of physical stress.
Moreover, the fluorescence of the entire crystal faded as the
process progressed.[27,28]
Figure 2. a) The growth morphology of CN-DPVB crystal calculated by
Material Studio 8.0 and its crystal indices. b) The molecular arrangement of
(001) plane observed along a axis. c) The molecular arrangement observed
along (10-3) plane.
For
a further investigation, single crystals of CN-DPVB
hydrochloride (CN-DPVBHCl) were attained by recrystallization
of CN-DPVB in acetone with an extra addition of a small amount
of concentrated hydrochloric acid (Supporting Information). CN-
DPVBHCl were light yellow blocks and crystallize in the triclinic
space group P-1 with one molecule in an asymmetric unit
(Figure S7 and S8). Proton NMR tests for CN-DPVB, CN-
DPVBHCl, and CN-DPVB@HCl crystals were carried out
(Figure 3b and 3c). The peak’s shapes of three crystals were
almost the same but chemical shifts differed a little from each
other. Peaks of the fumed crystals CN-DPVB@HCl moved to the
low field and those of two hydrogens on the benzene which is
closest to nitrogen atom shifted obviously (from 6.72 and 6.74 to
7.71 and 7.31 ppm, respectively). Lower shielding factor caused
by the diminishing electron density of molecules is the reason for
the low-field motion. When nitrogen atoms get positively charged
by protonation, its electronegativity increased and was more
likely to attract electrons and then resulted in the decrease of
electron density and shielding factor. The chemical yield of salt
formation in the crystals was 18-35% which was determined by
weight changes (Table S1). Meanwhile, Raman spectroscopy
test was also carried out (Figure S9). The peak of 1370 cm-1
corresponding to the stretching vibration of C-N in the dimethyl
amino decreased conspicuously after the process.[29]
In addition to the external stress mechanical response, CN-
DPVB crystal also exhibits a bending feature when it was
exposed to a stream of HCl vapor dried by bubbling through
concentrated sulfuric acid. The mechanical property of a fumed
crystal CN-DPVB@HCl was tested and confirmed to be brittle
and could not be deformed (Figure S5). As shown in Figure 1e,
one single crystal that absorbed concentrated hydrochloric acid
formed a fumed crystal CN-DPVB@HCl. The crystal began to
bend slowly after two minutes accompanied by color fading.[25,26]
Then the crystal was kept in HCl vapor for 12 h to ensure the
complete reaction with HCl. Eventually, the crystal could not
2
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Furthermore, we attempted to analyze the crystal structure
after fuming but failed to obtain the crystal structure of the fumed
crystal. The fumed crystals CN-DPVB@HCl tended to be brittle,
exacerbating over time in the hydrochloric acid atmosphere,
which suggested a devastation of crystal structure. XRD studies
of initial crystals and fumed crystals were carried out to inspect
the fuming degree. In comparison, the XRD pattern of fumed
crystal was similar to the CN-DPVB but some new peaks
appeared (Figure S10 and S11). The result suggested that this
bending process was not a complete phase transformation. The
fumed crystal still possessed the crystal structure as CN-DPVB
crystal. Combing the results of XRD and NMR experiments, we
could conclude that the fumed crystal CN-DPVB@HCl was a
the subtle motion of protonated molecules. Both the drastic
rotation of dimethyl amino in the CN-DPVB molecules and the
space occupation of chlorine atoms enlarge the distance of
protonated molecules, which leads to the deformation of the
crystal structure. Therefore, a probable mechanism of the acid-
induced bendability is proposed as follows: when a single crystal
was fumed with hydrochloric acid, molecules on both sections
would initially get protonated accompanied with the drastic
rotation of dimethyl amino groups. Adjacent molecules
expanded on account of the deficient space between them and
then resulted in the deformation of partial crystal structure, which
accumulated with the process and eventually became strong
enough to bend the crystal. Because the degree of deformation
mixture of CN-DPVB crystal and its hydrochloride CN-DPVBHCl. was relevant to the number of molecules that got protonated, the
It basically maintained the crystal structure of CN-DPVB but
partially altered the molecular configuration to the hydrochloride
CN-DPVBHCl. Hence, the crystal structure of CN-DPVBHCl
could provide important information to explain the bending
mechanism of this unusual hydrochloric acid atmosphere
induced crystal bending.
largest plane of crystal containing the most molecules was
capable of bending in comparison with other faces.
Figure 4. a) Conformation of single crystal CN-DPVB. b) Distance between
the dimethyl amino of two adjacent molecules in CN-DPVB. c) Conformation of
CN-DPVBHCl single crystal. d) Distance between the dimethyl amino of two
adjacent molecules of hydrochloride CN-DPVBHCl. e) Schematic diagram of
the bending process in hydrochloric acid atmosphere.
Figure 3. a) Load-displacement curves of CN-DPVB single crystal. b)
Comparison of NMR spectrum between CN-DPVB, CN-DPVB@HCl and CN-
DPVBHCl. c) Amplified NMR spectrums between 6.5‒8.5 ppm.
In contrast with two different bending modes, they resemble
each other in the principle of bending but differ from the
approaches. In both forms, a portion of molecules in the crystal
deviate slightly from their equilibrium position. The microscopic
subtle alteration eventually magnifies to the macroscopic
deformation of crystal. However, they are disparate when it
comes to the approach of alteration. Physical strain merely
breaks the weak intermolecular interactions that could restore
easily. Thus, it is a reversible process. On the contrary, chemical
reaction completely alters the molecular structure and its
configuration. Therefore, the acid-stimuli bending process is
irreversible and destructive to crystal structure. On the other
hand, the chemical process is much slower than the physical
process. The microscopic force needs to accumulate as the
reaction proceeds until it is strong enough to generate a
deformation on two sections. Hence, the physical process is
instantaneous but the chemical process is time-consuming.
In addition to the study of mechanical properties of CN-DPVB
crystal, its potential optical application was also exploited in both
the straight and the bent state. The emission (558 nm) and
lifetimes (2.69 ns) in the straight and the bent state were
In the crystal of CN-DPVBHCl, chlorine atoms occupy all
vertexes of one cell and one chlorine atom forms an ion bond
with a nitrogen cation (4.180 Å) which leads to a 70° rotation of
dimethyl amino group. Two benzene rings in one CN-DPVBHCl
molecule twist about 24.41°, greatly larger than the angle of
7.56° in CN-DPVB molecule (Figure S12 ). Moreover, as shown
in Figure 4b and 4d, the distance between the dimethyl amino of
two adjacent molecules increases from 3.56 Å to 4.61 Å. This
situation may be the critic factor for the chemical HCl induced
bending process. During the fuming process, we took a video
(Supporting Information) to record the variation of crystal
fluorescence under UV light. It was found that the fluorescence
of both sections disappeared initially, suggesting the protonation
beginning on two sections of the crystal (Figure 4e).
When crystals were fumed, drastic rotation of dimethyl amino
would lead to an increase of the distance between adjacent CN-
DPVB molecules due to the insufficient space. Moreover, space
occupation of chlorine atoms in the crystal also contributes to
3
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unchanged due to the subtle alteration of molecular interactions
in the bending process. This consequence encouraged us to
investigate the optical waveguide performance of CN-DPVB
crystal in the straight and the bent state. When different
positions of straight CN-DPVB crystals were irradiated by a
pulse laser, the emissions generating at one end were recorded
(Figure 5a and 5c). According to the graph, emission intensities
gradually enhance with diminishing the distances between
recorded end and irradiated position. Based on fitting the
collected data of Figure 5c, according to the literature,[30] optical
loss coefficient (OLCs) at 558 nm was evaluated be 0.358
dB•mm‒1 (Figure S13), which is relatively small among the
reported organic crystals and polymers (Table S2), indicating a
promising good optical property. The identical crystal was bent
and adhered to a glass slide with glue (Figure 5b). After
implementing the same optical waveguide measurements, it has
realized flexible optical waveguide of CN-DPVB crystal in the
bent state. Notably, the OLCs of bent CN-DPVB crystal at 558
nm was evaluated to be 0.409 dB•mm‒1 (Figure S13), which was
only 0.051 dB•mm‒1 larger than the result determined in the
straight state. Therefore, it is corroborated that the optical
propagation is not influenced in the bent crystals. Furthermore,
the optical waveguide property could switch off upon detection of
HCl, which acts as a potential chemical sensor (Figure S14).
Moreover, it was discovered that the CN-DPVB@HCl could not
exhibit the ability of passive waveguide, which ascribed to the
defects generated in the protonation.
bending manifestations reveal the possibility of developing multi-
bending responsive organic crystals. By rational modification of
molecular structures and substituents, more different types of
multiple mechanical-responsive organic crystals will be
developed and may further extend their application in manifold
fields.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (grant numbers 51773077 and 51622304).
Keywords: elastic crystal • physical bending • chemical bending
• crystal structure • optical waveguide.
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5
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Z. Lu, Y. Zhang, H. Liu, K. Ye, W. Liu, H.
Zhang*
Page No. – Page No.
Optical Waveguiding Organic Single
Crystals Exhibiting Physical and
Chemical Bending Features
Functionalized bendable organic crystals: Organic crystals of a benzonitrile
derivate in centimetre size have been readily prepared and exhibit bright yellow
emission. These crystals display physical and chemical bending features, which
achieved the combination of multiple bending modes in one single crystal. In
addition, the elastic crystal shows waveguiding property in the straight and highly
bended state, indicating its application in flexible optical devices.
6
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