Highly Elastic Organic Crystals for Flexible Optical Waveguides

Article abstract of DOI:10.1002/anie.201802020

The study of elastic organic single crystals (EOSCs) has emerged as a cutting-edge research of crystal engineering. Although a few EOSCs have been reported recently, those suitable for optical/optoelectronic applications have not been realized. Here, we r

Full text of DOI:10.1002/anie.201802020

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Accepted Article
Title: Highly Elastic Organic Crystals for Flexible Optical Waveguides
Authors: Hongyu Zhang, Huapeng Liu, Zhuoqun Lu, Zuolun Zhang,
and Yue Wang
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10.1002/anie.201802020
Angewandte Chemie International Edition
Highly Elastic Organic Crystals for Flexible Optical Waveguides
Huapeng Liu, Zhuoqun Lu, Zuolun Zhang,* Yue Wang, and Hongyu Zhang*
Abstract: The study of elastic organic single crystals (EOSCs) has
emerged as a cutting-edge research of crystal engineering. Although
a few EOSCs have been reported recently, those suitable for
optical/optoelectronic applications have not been realized. Here, we
expanded/compressed to a certain degree and thus serve as a
somewhat designable packing feature contributing to the
bending property of single crystals.[10,11] In addition, the π-
stacking is usually observed to associate with some fascinating
functions of crystalline materials, such as the high carrier
mobility and waveguiding capability.[12]
report an elastic crystal of
a
Schiff base, (E)-1-(4-
The
(dimethylamino)phenyl)iminomethyl-2-hydroxyl-naphthalene.
crystal is highly bendable under external stress and able to regain
immediately its original straight shape when the stress is released. It
In our exploration of functionalized EOSCs, we attempted the
use
of
a
Shiff
base,
(E)-1-(4-
displays bright orange-red emission with
a
high fluorescence
(dimethylamino)phenyl)iminomethyl-2-hydroxyl-naphthalen
(DPIN in Figure 1a). The formation of intramolecular hydrogen
bond between the OH and CH=N groups is expected because
similar interactions have been commonly observed in the crystal
structures of analogs compounds.[13] The hydrogen bond should
result in a relatively planar conjugated skeleton for π-stacking. It
was found that brightly fluorescent ultra-long EOSCs which are
highly bendable under external stress can be prepared using
DPIN (Figures 1b and c). Intriguingly, taking advantage of the
good elasticity and bright luminescence, the crystal works as a
flexible optical waveguide which indicate the possibility of
realizing flexible optoelectronics based on bulk single-
component organic single crystals.
By layering hexane on the top of a CH2Cl2 solution (0.1 M) of
DPIN and subsequent solvent diffusion, ultra-long rod-like single
crystals were easily obtained. The typical lengths of the crystals
were 15–25 mm, and the widths and thicknesses were in the
range of 40–200 μm. Under microscope, a prismatic body of the
DPIN crystal with three pairs of parallel prism faces was
observed (Figure 1d). Two faces in each pair had nearly
identical widths. One pair of the faces was wider than the other
two pairs.
quantum yield of 0.43. Intriguingly, it can serve as a low-loss optical
waveguide even at the highly bent state. Our result highlights the
feature and utility of “elasticity” of organic crystals.
As a class of organic solid materials, organic single crystals
have attracted increased attention in optical/optoelectronic
applications, such as optical waveguides,[1] solid lasers,[2] field-
effect transistors[3] and light-emitting diodes.[4] One of the current
developing trends of optical/optoelectronic technology is the
realization of flexibility. However, organic crystals are generally
brittle, leading to a serious problem for their application in
flexible devices. To make single-crystal materials applicable in
flexible devices, the exploration of crystals with both elastic
(reversible) bending ability and optical/optoelectronic functions is
essential. Although being a challenging target, elastic organic
single crystals (EOSCs) have been realized in recent years.[5]
Ghosh and Reddy gave a pioneer contribution to the EOSCs.
They reported that needle-like cocrystals comprising of three
components display elastic bending under applied stress,[6]
rather than plastic (irreversible) deformation.[7] Subsequently,
based on the study of a series of N-benzylideneanilines, Ghosh
et. al. proposed a structural feature of EOSCs: interlocked
packing with weak and dispersive interactions in crystal.[8]
Hayashi et. al. reported an elastic crystal of 1,4-bis[2-(4-
methylthienyl)]-2,3,5,6-tetrafluorobenzene with fluorescence
property.[9] Very recently, the elasticity was observed in
macrophage-synthesized biocrystals by Rosania et. al.[10] All of
these works significantly advanced the study of EOSCs.
However, the number of known EOSCs is still very limited.
Especially, the EOSCs with functions for use in the fast
developing optical/optoelectronic technology have not been
achieved.
To realize EOSCs with optical/optoelectronic functions, π-
conjugated molecules with a relatively planar donor-acceptor
structure would be a good choice. Such structure on one hand
facilitates efficient solid-state emission. On the other hand, it
allows the intermolecular π-stacking, which can be
[*]
H. Liu, Z. Lu, Dr. Z. Zhang, Prof. Dr. Y. Wang, Prof. Dr. H. Zhang
State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University
Figure 1. Molecular structure of DPIN (a), photographs of the rod-like single
crystals of DPIN taken under room light (b) and 365 nm UV light (c), the
prismatic crystal body observed under microscope (top: view perpendicular to
the widest face, which is white due to the light reflection; bottom: view alone
the widest face) and schematic elucidation of the cross-section shape of the
crystal (d), and absorption and emission spectra of the DPIN crystal (e).
Qianjin Street, Changchun (P. R. China)
E-mail: zuolunzhang@jlu.edu
hongyuzhang@jlu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201802020
Angewandte Chemie International Edition
Figure 3b shows the molecular packing structure viewed down
the (010) plane. Infinite π···π interactions are formed along the a
axis (red arrows), with an overlapping area between neighboring
molecules being about two third of a molecule and a π···π
vertical distance of about 3.44 Å. These π···π interactions,
together with the intermolecular C–H···π interactions formed
between the CH3 and C6H4 groups (dashed green line; H···π:
2.88 Å), result in the formation of a molecular chain along the a
axis. Two adjacent molecular chains interact through C–H···O
hydrogen bonds between the CH3 and OH groups (dashed
yellow line; H···O: 2.61 Å, C–H···O: 160.78°) to form a chain
dimer along the c axis. The cross arrangement of chain dimers
with the closest inter-dimer distance of about 3.0 Å can be
observed.
The elastic bending of the crystal requires the expansion and
contraction of the outer and inner arcs, respectively (Figure 3b).
As the molecular chains are formed along the crystal length
direction (a axis), the π···π interactions within the chains can
respond to the expansion of the outer arc by enlarging the π···π
distance and to the contraction of the inter arc by decreasing the
π···π distance. Therefore, these interactions are a key factor for
the realization of crystal elasticity. A tiny relative slippage of the
adjacent and parallel π-systems along the bending direction,
accompanied by the formation of a small angle between these
π-systems, should also occur in the bending process to fit the
Figure 2. Elastic bending under mechanical stress applied through a pair of
tweezers: (a) the initial straight crystal with a length of about 11 mm; (b–f) the
bending process; (g) the recovery of crystal shape upon removing the stress,
and the photographs taken under room light (h) and UV light (i) for a crystal
screwed on a glass tube.
Upon applying stress at the two tips of a straight crystal with a
pair of tweezers, the widest faces of the crystal were easily bent
(Figures 2a–f and S1). A half loop could be formed, without
cracking or breaking the crystal. Once the stress was released,
the original straight shape was recovered immediately (Figure
2g). The bending-relaxing process could be repeated many
times. These results indicate the highly elastic nature of the
crystal. The high elasticity is also reflected by the feasibility of
screwing the crystal on a glass tube with a diameter of 1.20 mm
(Figures 2h and i). Overbending led to the direct cracking of
crystal, but not the plastic (irreversible) deformation.
Single-crystal X-ray diffraction analysis indicates that the
crystal belongs to the monoclinic space group P21/c. One DPIN
molecule exists in the asymmetric unit, and there are no solvent
molecules within the crystal lattice. As expected, O–H···N
hydrogen bond (H···N: 1.77 Å, O–H···N: 148.26°) is formed
between the OH and CH=N groups, resulting in a nearly
coplanar arrangement of the naphthalene, CH=N and OH
moieties (Figure 3a). The C6H4 group is twisted out of the CH=N
plane by a small angle of 23.07°. Above structural features
indicate a relatively planar molecular skeleton, which facilitates
the intermolecular π-stacking.
To understand the elasticity, the index of the highly bendable
crystal faces was firstly determined through calculating the
morphology of DPIN crystal by the software of Materials Studio
8.0 with the crystallographic data as the input information. Based
on the crystalline-plane attachment energies, which determine
the growth rates of crystalline planes, the crystallization
morphology as well as the indexes of the side faces was
obtained. The calculated morphology reproduced the
experimentally observed prismatic shape of the crystal body
(Figure S2). According to the calculation, the two widest faces of
the crystal correspond to the (002) plane, and the length
direction of the crystal is along the a axis. Therefore, the
bending process observed in Figure 2 is the bending of (002)
plane along the perpendicular (010) plane.
Figure 3. (a) Structure of DPIN in the single crystal (the dashed pink line
represents the intramolecular hydrogen bond); (b) molecular packing structure
viewed down the (010) plane along the b axis (the red arrows and green
dashed line represent the π···π and C–H···π interactions within a molecular
chain, respectively; the dashed yellow lines show the C–H···O hydrogen
bonds between molecular chains).
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10.1002/anie.201802020
Angewandte Chemie International Edition
equation, the thickest crystal able to form a half loop with a fixed
inner-arc
diameter
corresponds
to
the
largest
expansion/contraction ratio of the outer/inner arc. By winding the
crystals with different thicknesses on a ballpoint pen refill
(external diameter: 4.46 mm) to form a half loop, the largest
applicable thickness was determined to be around 103 μm
(Figure S3). Correspondingly, the largest extension/contraction
ratio of the outer/inner arc of our crystal is estimated to be
2.26% according to the above-mentioned equation. Accordingly,
the π···π distance is approximately expanded from 3.44 to 3.52
Å at the outer arc and contracted from 3.44 to 3.36 Å at the inner
arc when the bending limit is reached. Notably, 3.52 and 3.36 Å
are just in the middle of the distance range of normally observed
π···π interactions (ca. 3.2–3.7 Å).[15] Therefore, the cracking of
the crystal upon overbending seems to be not directly caused by
the over-expansion of π···π interaction.
Due to the relatively small extension/contraction ratio of π···π
distance and the synergic movement of molecules caused by
the cooperation of multiple factors, i.e. π···π interactions,
hydrogen bonds and the cross and close stacking of chain
dimers, relative arrangement of molecules in the bending
process only alter slightly from that of the original straight crystal.
This is supported by the existence and elongation of diffraction
spots for a bent crystal in the single-crystal X-ray diffraction
experiment (Figure S4).[6] When the external stress applied on
the crystal is released, the original packing structure is
recovered, as confirmed by the nearly identical cell parameters
of the straight crystal and the crystal recovered from the bending
experiment. Thus, the morphology of the crystal is restored. The
easy recovery of the packing structure in turn supports the slight
variation of molecular arrangements in the bending process.
Under UV light, the crystal emits bright orange-red emission.
The emission spectrum shows the maximum at 607 nm and a
Figure 4. Graphical presentation of the packing-structure variation when the
crystal is bent to form a half loop. The short blue lines represent the parallel
stacked π-systems within molecular chains. The dashed lines represent the
neutral plane. t and L correspond to the thickness and length of the crystal,
respectively, while d is the diameter of the inner arc.
curvature of the bent faces (Figure 4). In addition to the infinite
π-stacking, the C–H···π interactions within molecular chains
could also serve as a factor that suppress the breaking trend of
molecular chains towards expansion and thus benefit the
bending resistance of the crystal. Moreover, C–H···O hydrogen
bonds within the chain dimers and the cross and close
arrangement of chain dimers restrict, respectively, the relative
slippage of neighboring chains and chain dimers in the bending
process. Therefore, plastic (irreversible) deformation of the
crystal which might be caused by such relative slippage[14] can
be avoided during bending. Consequently, the C–H···O
hydrogen bonds and the cross and close packing feature also
contribute to the high elasticity.
As the expansion and contraction of π···π distance are closely
related to the bending process, one question is if the over-
expansion of π···π interaction directly caused the creaking of the
crystal upon overbending. To investigate the question, the
expansion and contraction limits of the π···π distance in our
crystal was studied. During the elastic bending process, the
expansion ratio of the outer arc and the contraction ratio of the
inner arc, relative to the neutral plane with a constant length
during the bending process (Figure 4), are equal to each other.[6]
When the crystal is bent to form a half loop, the molecular
chains can be expanded/contracted uniformly and the bending-
caused angle between originally parallel and adjacent π-
systems should be fairly small because of the existence of a
huge number of molecules in a molecular chain. In this case, the
π···π distance at the outer/inner arc is approximately
expanded/contracted to a degree same as that of the outer/inner
arc itself. The extension/contraction ratio (ε) of the outer/inner
arc can be estimated by the equation of ε = t/(d+t), where t is the
thickness of the crystal and d is the diameter of the inner arc
(Figure 4 and the Supporting Information).[8] According to the
shoulder peak at 563 nm (Figure 1e).
A high absolute
fluorescence quantum yield of 0.43 was determined for the
crystal despite the self-absorption suggested by the overlap
between the emission and absorption spectra. Interestingly, the
crystal possesses optical waveguiding property, which was
initially detected from the brighter emission of the crystal tips
than the body when the straight crystal was irradiated with a UV
lamp. Further examination indicated that the waveguiding
behavior could be realized not only by the straight crystal, but
also by the highly bent crystal (Figure 5b). To examine the
influence of elastic bending on waveguiding performance, optical
loss coefficients (OLCs) of a single DPIN crystal (length: 5 mm)
at both the straight and the highly bent (close to a half loop)
states were studied. By irradiating different positions of the
crystal using the same excitation light of 355 nm laser and
collecting the emission spectrum corresponding to each
irradiation position at one tip of the crystal, distance-dependent
emission spectra were firstly obtained (Figure 5c and d). The
emission intensity at the tip decreases gradually when the
distance between the tip and the irradiated position is increased,
due to the loss of more emitted light for a longer propagation
distance. In addition, with increased propagation distance, the
emission spectra show a slight redshift because of heavier
reabsorption in the overlapping range of the emission and
absorption spectra (ca. 530‒620 nm; Figure 1e). By fitting the
data of Figure 5c and d, respectively, according to literature
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10.1002/anie.201802020
Angewandte Chemie International Edition
procedures,[16] the OLCs at 615 nm are estimated to be 0.270
dB mm‒1 for the straight state and 0.274 dB mm‒1 for the bent
state (Figure S5). These values are among the smallest OLCs
ever reported for organic crystals and comparable to or even
better than those of some polymers (Table S3), suggesting the
good waveguiding performance for both states of the crystal.
Notably, the OLC of the bent state is only slightly higher than
that of the straight state. The retaining of a low OLC at the highly
bent state is intriguing considering that the optical loss of
waveguides is usually serious in the bent state.[17]
In order to further demonstrate the potential utility of elastic
DPIN crystal as flexible materials, a crystal was repeatedly
bended and recovered, and multiple measurements of optical
waveguide were conducted. The optical waveguiding property of
the crystal, in terms of emission maximum and intensity, did not
change upon repeatedly bending and recovering (Figure S6),
highlighting the good elasticity of DPIN crystal as well as its
potential application in flexible optical/optoelectronic devices.
could form elastically bendable crystals. Among these kinds of
elastic crystals, the DPIN crystal is the only choice which further
exhibits optical waveguiding property.
In conclusion, a π-conjugated molecule with a relatively planar
donor-acceptor structure has been applied to develop
functionalized elastic organic bulk crystals. The structural feature
of the compound endows its single crystal with not only bright
emission, but also infinite π···π interactions that serve as a key
factor for the high elasticity of the crystal. Taking advantages of
the high elasticity and bright luminescence, a flexible optical
waveguide based on single-component organic bulk crystals has
been achieved. The structural feature of the untilized compound
is of guiding significance for the design of organic compounds
for EOSCs with optical/optoelectronic functions. The easier
preparation of centimeters-long EOSCs and their use in low-loss
flexible optical waveguides shown in this study suggest the
application potential of the single-crystal materials in flexible
optical/optoelectronic devices.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51622304, 51773077, 51603082, and
51773079). We thank the staff from BL17B beamline of National
Facility for Protein Science in Shanghai (NFPS) at Shanghai
Synchrotron Radiation Facility for assistance in crystal data
collection.
Keywords: fluorophore • organic crystal • optical waveguide •
elastic bending• crystal structure
[1]
[2]
a) C. Zhang, Y. S. Zhao, J. Yao, Phys. Chem. Chem. Phys. 2011, 13,
9060; b) T. Liu, Y. Li, Y. Yan, Y. Li, Y. Yu, N. Chen, S. Chen, C. Liu, Y.
S. Zhao, H. Liu, J. Phys. Chem. C 2012, 116, 14134; c) B. Tang, Z.
Zhang, H. Liu, H. Zhang, Chin. Chem. Lett. 2017, 28, 2129.
a) P. Baronas, G. Kreiza, P. Adomenas, O. Adomeniene, K.
Kazlauskas, J. Ribierre, C. Adachi, S. Jursěnas, ACS Appl. Mater.
Interface 2018, 10, 2768; b) W. Zhang, Y. Yan, J. Gu, J. Yao, Y. S.
Zhao, Angew. Chem. Int .Ed. 2015, 54, 7125.
[3]
[4]
a) E. Ahmed, A. L. Briseno, Y. Xia, S. Jenekhe, J. Am. Chem. Soc.
2008, 130, 1118; b) S. Mun, Y. Park, Y. K. Lee, M. M. Sung, Langmuir
2017, 33, 13554.
a) R. Ding, J. Feng, X. Zhang, W. Zhou, H. Fang, Y. Liu, Q. Chen, H.
Wang, H. Sun, Adv. Funct. Mater. 2014, 24, 7085; b) R. Ding, J. Feng,
F. Dong, W. Zhou, Y. Liu, X. Zhang, X. Wang, H. Fang, B. Xu, X. Li, H.
Wang, S. Hotta, H. Sun, Adv. Funct. Mater. 2017, 27, 1604659.
a) S. Saha, G. Desiraju, Chem. Commun. 2016, 52, 7676; b) S.
Takamizawa, Y. Miyamoto, Angew. Chem. Int. Ed. 2014, 53, 6970; c) S.
Saha, G. R. Desiraju, Chem. Eur. J. 2017, 23, 4936.
[5]
Figure 5. Fluorescence photographs of straight (a) and bent (b) crystals
excited with a UV lamp (uppermost line) and with 355 nm laser focused at
different positions, and fluorescence spectra collected at a tip of a single DPIN
crystal in either the straight (c) or the bent (d) state, with the distance between
this tip and the excitation site of the laser changed gradually.
[6]
[7]
S. Ghosh, C. M. Reddy, Angew. Chem. Int. Ed. 2012, 51, 10319.
a) G. R. Krishna, R. Devarapalli, G. Lal, C. M. Reddy, J. Am. Chem.
Soc. 2016, 138, 13561; b) P. Naumov, S. Chizhik, M. K. Panda, N. K.
Nath, E. Boldyreva, Chem. Rev. 2015, 115, 12440; c) M. K. Panda, S.
Ghosh, N. Yasuda, T. Moriwaki, G. D. Mukherjee, C. M. Reddy, P.
Naumov, Nat. Chem. 2015, 7, 65; d) M. K. Panda, K. B. Pal, G. Raj, R.
Jana, T. Moriwaki, G. D. Mukherjee, B. Mukhopadhyay, P. Naumov,
Cryst. Growth Des. 2017, 17, 1759.
In addition, we also tested the crystal elasticity of some
analogs of DPIN (Table S4). Several strategies, e.g. π-extension
and introduction of hydrogen bonds, have been adopted to tune
the intermolecular interactions in crystal, and thus change the
mechanical property. We found that only a small part of analogs
[8]
[9]
S. Ghosh, M. K. Mishra, S. B. Kadambi, U. Ramamurty, G. R. Desiraju,
Angew. Chem. Int. Ed. 2015, 54, 2674.
S. Hayashi, T. Koizumi, Angew. Chem. Int. Ed., 2016, 55, 2701.
This article is protected by copyright. All rights reserved.

10.1002/anie.201802020
Angewandte Chemie International Edition
[10] E. M. Horstman, R. K. Keswani, B. A. Frey, P. M. Rzeczycki, V. Lalone,
J. A. Bertke, P. J. Kenis, G. R. Rosania, Angew. Chem. Int. Ed. 2017,
56, 1815.
[15] a) M. Linares, S. Stafström, P. Norman, J. Chem. Phys. 2009, 130,
104305; b) H. Li, J. A. Boatz, M. S. Gordon, J. Am. Chem. Soc. 2008,
130, 392.
[11] S. Hayashi, T. Koizumi, N. Kamiya, Cryst. Growth Des. 2017, 17, 6158.
[12] a) X. Fang, X. Yang, D. Yan, J. Mater. Chem. C 2017, 5, 1632; b) R.
Chandrasekar, Phys. Chem. Chem. Phys. 2014, 16, 7173.
[13] Z. Zhang, X. Song, S. Wang, F. Li, H. Zhang, K. Ye, Y. Wang, J. Phys.
Chem. Lett. 2016, 7, 1697.
[16] Y. Li, Z. Ma, A. Li, W. Xu, Y. Wang, H. Jiang, K. Wang, Y. Zhao, X. Jia,
ACS Appl. Mater. Interfaces 2017, 9, 8910.
[17] a) S. Lee, S. Kim, H. Lim, Opt. Express 2009, 17, 19435; b) J.-M. Lee,
S. Park, M. Kim, S. K. Park, J. T. Kim, J.-S. Choe, W.-J. Lee, M.-H. Lee,
J. J. Ju, Opt. Express 2009, 17, 228.
[14] G. R. Desiraju, S. Saha, J. Am. Chem. Soc. 2017, 139, 1975.
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Angewandte Chemie International Edition
Entry for the Table of Contents (Please choose one layout)
Organic Crystal
Functionalized elastic organic
crystals: centimeters-long elastic
organic crystals of a Schiff base have
been facilely prepared and display
bright orange-red emission with a
high fluorescence quantum yield of
0.43. These elastic crystals show
waveguiding property with low optical
loss coefficient not only in the straight
state, but also in the highly bended
state, demonstrating the usability of
“elasticity”.
Huapeng Liu, Zhuoqun Lu, Zuolun
Zhang,* Yue Wang and Hongyu Zhang*
Page No. – Page No.
Highly Elastic Organic Crystals for
Flexible Optical Waveguides
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