Bromine-bromine interactions enhanced plasticity for the bending of a single crystal without affecting fluorescent properties

Article abstract of DOI:10.1039/C8CE02086H

Ethyl 3-(3-bromo-4-methoxy-phenyl)-2-cyano-acrylate crystals exhibited very superior plasticity and shape persistent, self-standing properties compared to their non-bromo analogue. The crystal plasticity, controlled by a bromine-bromine interaction, has b

Full text of DOI:10.1039/C8CE02086H

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Bromine–bromine interactions enhanced plasticity
for the bending of a single crystal without
affecting fluorescent properties†
Cite this: DOI: 10.1039/c8ce02086h
Received 10th December 2018,
Accepted 19th December 2018
Arpita Paikar, Debasish Podder, Srayoshi Roy Chowdhury,
*
DOI: 10.1039/c8ce02086h
Supriya Sasmal and Debasish Haldar
rsc.li/crystengcomm
Ethyl 3-(3-bromo-4-methoxy-phenyl)-2-cyano-acrylate crystals
exhibited very superior plasticity and shape persistent, self-
standing properties compared to their non-bromo analogue. The
crystal plasticity, controlled by a bromine–bromine interaction, has
been found to play a crucial role in generating a slip plane and
thus, under mechanical force, the crystals undergo bending with-
out affecting their fluorescent properties.
by introducing active slip planes within molecules and
showed that some noninterfering supramolecular weak inter-
actions like hydrogen bonding, π–π stacking and van der
Waals interactions are the key elements to introducing the
slip plane within a crystal.23
Solid-state packing-induced photoluminescence is ex-
tremely important in the area of crystal engineering,24 and it
has potential applications in the field of lasers, OLEDs, opti-
cal data storage,25 two-photon photoluminescence (PL)
microscopy,26 optical switching27 and limiting.28 It is well
known in the literature that various synthetic organic dyes
show different fluorescent colours due to their different solid
state packing.29,30 It was also reported that solid-state molec-
Flexible crystals of small organic molecules with
optoelectronic properties are highly important for device
fabrication and engineering.1 Previously, it was reported that
molecular crystals can bend,2 twist,3 curl,4 or change their
shape5–8 under external stimuli like heat, light, mechanical
force, etc.6–8 Previously, most of the reported bending crystals
were obtained by serendipity until a proper explanation was
given by Reddy et al. on the strategy for the design and
ular packing can enhance
a luminescent phenomenon
through aggregation-induced emission (AIE).31 Presently,
non-dopant emitters with an enhanced emission property in
the solid state are highly desirable to avoid the complicated
doping process during the fabrication in OLEDs.32,33 So, the
discovery of a novel solid-state red-emissive chromophore is
in high demand as well as challenging,34 as they are very rare
in the literature.35 It has also been reported that a compound
having Br⋯Br interactions can control the luminescence
property of the compound.36 T. Koizumi and his group
reported a fluorescent organic crystal which shows elastic
bending.37 Recently, we have reported a halogen bond in-
duced self-assembly and solid state phosphorescence of a
bromo-substituted capped γ-amino acid foldamer.38 We have
also reported the effect of packing on the solid state fluores-
cent properties of peptidic luminophores and their
thermochromic behavior.39
synthesis
of
bendable
crystals.9
Both
plastically
(irreversible)10–12 and elastically (reversible) bendable organic
crystals13–16 have been well-studied in recent years. Plastic
crystals are the type of crystal in which molecular rotation is
enabled under sheer pressure without the loss of crystallinity.
They are useful in the field of electrolytes,17–19 ferroelec-
trics,20,21 and molecular rotors.22 Recently, Reddy and his
group reported various mechanically flexible organic crystals
Department of Chemical Sciences, Indian Institute of Science Education and
Research Kolkata, Mohanpur, West Bengal 741246, India.
E-mail: deba_h76@yahoo.com, deba_h76@iiserkol.ac.in; Fax: +91 33 2502 8002;
Tel: +91 9748487503
† Electronic supplementary information (ESI) available: Synthesis and characteri-
zation of compounds 1 and 2, 1H NMR, 13C NMR, solid state FTIR spectra, ESI
Fig. 1–6, Fig. S1–S6. Crystallographic data: Compound 2: C13H12BrNO3, MW =
310.15, monoclinic, space group P21/n, a = 11.5570IJ4), b = 4.03422IJ16), c =
27.4354IJ15) Å, α = 90, β = 95.906(4), γ = 90, V = 1272.35IJ10) Å3, Z = 4, dm = 1.619
g cm−3, T = 100 K, R1 0.0393 and wR2 0.0866 for 2241 data with I > 2σ(I). Inten-
sity data of 2 were collected with MoKα radiation using a Bruker APEX-2 CCD
diffractometer. Data were processed using the Bruker SAINT package and the
structure solution and refinement procedures were performed using SHELX-
2014/7.42 A Hirshfeld surface (HS) analysis43 was carried out by using Crystal Ex-
plorer 17.5.44 CCDC 1877329 contain the crystallographic data. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c8ce02086h
Scheme 1 The schematic structures of compounds 1 and 2.
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Inspired by previous reports, here we aim to investigate
the effect of a bromine atom on molecular packing, optical
and mechanical properties of a small organic molecule
(Scheme 1). For that purpose, we have designed and synthe-
sized compound 1, (E)-ethyl 2-cyano-3-(4-methoxyphenyl) acry-
late, and its meta substituted bromo derivative, compound 2.
Surprisingly, we have observed plastic bending (Scheme 1) of
compound 2 due to a unique Br⋯Br interaction which gener-
ates a slip plane between the two stacks of molecules in a
higher order assembly. As that kind of Br⋯Br interaction is
absent in compound 1, it has failed to exhibit a bending
property. Rather, the compound 1 crystals broke under me-
chanical stress. Furthermore, due to this unique Br⋯Br inter-
action, compound 2 exhibits green and red fluorescence in
the solid state upon excitation at 490 nm and 540 nm, respec-
tively. So, herein, we attempt to explore both the crystalline
solid-state properties that deal with the bromine-induced
plastic bending of crystals, as well as the packing-induced
solid state green and red emissions of the crystal of com-
pound 2. This proves that the Br⋯Br interactions have an ex-
plicit effect on the unique combination of crystal packing of
the compound 2 molecules.
pound 1, crystals break into multiple parts under the same
mechanical stress. Thus, we are able to tune the mechanical
behaviour, as well as the supramolecular assembly, of com-
pound 2 by introducing a heavy atom effect. As seen in
Fig. 1c and d, the crystals of compound 2 bend upon applica-
tion of local pressure at the end of the crystal and the bend-
ing is highly plastic in nature, i.e. the deformation is
irreversible.
To investigate the nature of the surface of the crystals of
compounds 1 and 2 under mechanical stress, we have used
field emission scanning electron microscopy (FE-SEM). The
FE-SEM image (Fig. 2a) shows that the compound 1 crystals
are broken under the mechanical stress. However, under the
same condition, the crystals of compound 2 depict a striped
surface with rims along the length of the crystal (Fig. 2b).
Fig. 2b shows that various macroscopic layers are stacked
along the longitudinal axis of the crystal surface which are
not clearly noticeable in the straight section of the crystal
(Fig. 2c). Thus, we understood that layers have generated as a
result of the shear stress by the mechanical force during the
bending. The SEM images of the poked crystal clearly showed
the slices of separated macroscopic layers. This type of
macroscopic layer formation in a plastically bendable crystal
was previously reported by Reddy and co-workers.40
Compound 1 was synthesized with a 79.4% yield by a
microwave-assisted
Knoevenagel
reaction
between
p-anisaldehyde and ethylcyanoacetate in the presence of am-
monium formate. Further, compound 2 was obtained from a
NaIO4- and LiBr-mediated meta position bromination of com-
pound 1 in AcOH (ESI† Scheme S1). Both the compounds
To examine the topographic features, as well as the rough-
ness of the surface of the bent crystals, we have performed
atomic force microscopy (AFM). From the AFM, we have also
observed distinct stacking layers at a bent section of the crys-
tals (ESI† Fig. S1a). From the 3D view, it is very clear that the
layers are very rough in nature (ESI† Fig. S1b) and the rough-
ness of the surface was ca. 0.67 μm, as calculated from the
AFM.
The effect of bending on the molecular arrangement and
intermolecular interactions were investigated by micro-
Raman spectroscopy (Fig. 2d). The symmetry-equivalent com-
pound 2 molecular arrangement became dissimilar by the
1
were characterized by H-NMR (nuclear magnetic resonance),
13C-NMR, FT-IR (Fourier-transform infrared) and mass
spectrometry (MS) analysis.
Colourless crystals of compounds 1 and 2 were obtained
from a methanol–water solution by slow evaporation. Using a
needle and forceps, crystals of the compounds 1 and 2 were
subjected to bend manually in
a three-point geometry
(Fig. 1a), under an optical microscope. Surprisingly, the crys-
tals of compound 2 are flexible and bend under mechanical
stress (Fig. 1b). However the non-bromo derivative, i.e. com-
Fig. 2 (a) FE-SEM image of a sharply broken compound 1 crystal un-
der mechanical stress. (b) FE-SEM image of a compound 2 bent crystal
showing various macroscopic layers stacked along the longitudinal axis
of the crystal surface. (c) FE-SEM image of the straight section of a
compound 2 crystal. (d) The micro-Raman spectra of bent (red) and
straight (black) crystals of 2.
Fig. 1 (a) Crystal of compound 2 bent on the 010 face. The arrows
show the point of the applied forces. (b), (c) and (d) step-by-step pic-
tures of 360° crystal bending.
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formation of layers on the crystal surface (Fig. 2d). Moreover,
the symmetry lowering was reflected as significant intensity
changes in the respective Raman spectra of the bent and
straight crystals.
compound 2 (highlighted in colour in Fig. 3c). It is evident
from the crystal structure of compound 2 that there is an
intermolecular Br⋯Br interaction which generates the slip
plane between the two stacks of molecules (Fig. 3d and ESI†
Fig. S4). Due to this Br⋯Br interaction, the two molecules of
compound 2 come closer compared to 1, and for this reason
the π⋯π interaction is also stronger in the case of compound
2 (centroid–centroid distance = 3.56 Å).
Then, we studied a concentration-dependent emission for
both the compounds to understand their self-assembly pro-
pensities. The emission spectra (λex = 340 nm) show that with
an increasing concentration the emission intensity decreases
gradually (ESI† Fig. S5) for both the compounds. This is due
to an aggregated form that causes the molecules to return to
their ground state in a non-radiative pathway. This phenome-
non is known as aggregation-induced fluorescence
quenching. This result clearly indicates that both compounds
have a similar type of aggregation in the solution state as
they both possess a similar chromophore.
Colourless needle-shape crystals of compound 1 were suit-
able for X-ray diffraction, as was previously reported by S.
Sreenivasa et al.41 Compound 1 crystallizes in the monoclinic
space group P21/n with one molecule in the asymmetric unit.
As expected, compound 1 adopts a planar conformation
(Fig. 3a, red). In the crystal, molecules are interlinked into
anti-parallel dimers through two C—H⋯O interactions
(Fig. 3b). Further, the compound 1 molecules are stabilized
by weak C—H⋯π interactions, and weak π⋯π interactions
(centroid–centroid separation = 4 Å) along the c axis.41 Thus,
the compound 1 molecules are arranged in a herringbone-
like pattern in a higher order assembly, but there is a lack of
a slip plane.
Compound 2 crystallizes in the monoclinic space group
P21/n with one molecule in the asymmetric unit (ESI† Fig.
S2). As seen in Fig. 3a, compound 2 also adopts a planar con-
formation and there is no significant difference from the
backbone conformation of compound 1. In the packing of
compound 2, individual subunits are also interlinked into
anti-parallel dimers, through two C—H⋯O interactions (ESI†
Fig. S3). However, the 2D fingerprint plot (a visual summary
of the frequency of each combination of de and di across
compound 2's surface) shows Br⋯Br interactions present in
Next, to understand the effect of the Br⋯Br interaction,
here we have studied and compared the solid state fluores-
cence behaviour of brominated and non-brominated com-
pounds. In solution, both the compounds exhibit a similar
type of aggregation pattern, however, they differ in the solid
state. Crystals of compound 2 show emission peaks at 590
Fig. 3 (a) The overlay of the molecular conformations of compounds
1 (red) and 2 (green) in the solid state showing the structural similarity.
(b) The packing diagram of compound 1 in crystal (carbon: grey;
oxygen: red; nitrogen: blue). C–H⋯O and C–H⋯N interactions are
shown as black dotted lines. (c) 2D fingerprint plot with Br⋯Br
interactions highlighted in colour for compound 2. (d) Br⋯Br
interactions in the crystal of compound 2 with a slip plane (green box).
π⋯π interactions between aromatic rings oriented along different
directions (red and blue arrows).
Fig. 4 (a) and (b) Green and red fluorescence of the straight section
of the crystal of compound 2 under a fluorescence microscope. (c)
and (d) Green and red fluorescence of the bent section of the crystal
of compound 2 under a fluorescence microscope showing the striped
surface with stacked layers along the length of the crystal. (e) Effect of
bending on the Br⋯Br interaction. (f) The origin of fluorescence.
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nm and 610 nm upon excitation at 490 nm and 540 nm, re-
spectively (ESI† Fig. S6). But crystals of compound 1 failed to
emit in that particular wavelength (ESI† Fig. S6). We have
also done fluorescence microscopy imaging of the crystals of
compound 2 and they show intense green and red emissions
on excitation at 490 nm and 540 nm, respectively
(Fig. 4a and b). Due to the conjugate effect of a Br⋯Br inter-
action and strong π–π stacking, the compound 2 crystals
show a solid state packing-induced emission. Fig. 4c and d
show the green and red fluorescence of the bent section of
the crystal of compound 2 on excitation at 490 nm and 540
nm under fluorescence microscope. Moreover, Fig. 4c and d
show the striped surface with stacked layers (marked with a
white arrow) along the length of the crystal. However, these
kind of stacked layers are absent in the straight section of
the crystal. Thus, under mechanical force, the bending of the
compound 2 crystals does not affect their fluorescent proper-
ties due to heavy atom effect (Fig. 4e and f).
In conclusion, we have reported that a Br⋯Br interaction
significantly enhanced the plasticity of the crystal and helped
with bending of the crystal. The studies showed that stacked
layers slid on top of one another, but ultimately bound to
each other due to a restorative effect of the Br⋯Br interac-
tion. Raman spectra of bent and straight crystals showed sig-
nificant intensity changes on bending, but amorphous mate-
rial did not appear. Furthermore, due to a heavy atom effect
compound 2 also showed green and red fluorescence under
suitable conditions which did not change under mechanical
stress. This crystal with plasticity and optoelectronic proper-
ties has potential for device fabrication.
6 P. Naumov, S. C. Sahoo, B. A. Zakharov and E. V. Boldyreva,
Angew. Chem., Int. Ed., 2013, 52, 9990.
7 D. P. Karothu, J. Weston, I. T. Desta and P. Naumov, J. Am.
Chem. Soc., 2016, 138, 13298.
8 N. K. Nath, T. Runcevski, C. Y. Lai, M. Chiesa, R. E.
Dinnebier and P. Naumov, J. Am. Chem. Soc., 2015, 137,
13866.
9 G. R. Krishna, R. Devarapalli, G. Lal and C. M. Reddy, J. Am.
Chem. Soc., 2016, 138, 13561.
10 L. O. Alimi, P. Lama, V. J. Smith and L. J. Barbour, Chem.
Commun., 2018, 54, 2994.
11 H. Liu, Z. Bian, Q. Cheng, L. Lan, Y. Wang and H. Zhang,
Chem. Sci., 2019, 10, 227–232.
12 C. M. Reddy, R. C. Gundakaram, S. Basavoju and M. T.
Kirchner, Chem. Commun., 2005, 3945.
13 S. Hayashi and T. Koizumi, Angew. Chem., Int. Ed., 2016, 55,
2701.
14 S. Ghosh, M. K. Mishra, S. B. Kadambi, U. Ramamurty and
G. R. Desiraju, Angew. Chem., Int. Ed., 2015, 54, 2674.
15 S. Ghosh and C. M. Reddy, Angew. Chem., Int. Ed., 2012, 51,
10319.
16 E. M. Horstman, R. K. Keswani, B. A. Frey, P. M. Rzeczycki,
V. LaLone, J. A. Bertke, P. J. A. Kenis and G. R. Rosania,
Angew. Chem., Int. Ed., 2017, 56, 1815.
17 D. R. MacFarlane and M. Forsyth, Adv. Mater., 2001, 13, 957.
18 P. Wang, Q. Dai, S. M. Zakeeruddin, M. Forsyth, D. R.
MacFarlane and M. Grätzel, J. Am. Chem. Soc., 2004, 126,
13590.
19 F. Chen, J. M. Pringle and M. Forsyth, Chem. Mater.,
2015, 27, 2666.
We acknowledge the IISER Kolkata, India, for financial as-
sistance. A. Paikar acknowledges the UGC, India for fellow-
ship. D. Podder thanks CSIR, India for research fellowship. S.
Roy Chowdhury and S. Sasmal acknowledge IISER-Kolkata for
fellowship.
20 J. Harada, T. Shimojo, H. Oyamaguchi, H. Hasegawa, Y.
Takahashi, K. Satomi, Y. Suzuki, J. Kawamata and T. Inabe,
Nat. Chem., 2016, 8, 946.
21 J. Harada, M. Ohtani, Y. Takahashi and T. Inabe, J. Am.
Chem. Soc., 2015, 137, 4477.
22 Y. Z. Sun, B. Huang, W. J. Xu, D. D. Zhou, S. L. Chen, S. Y.
Zhang, Z. Y. Du, Y. R. Xie, C. T. He, W. X. Zhang and X. M.
Chen, Inorg. Chem., 2016, 55, 11418.
Conflicts of interest
There are no conflicts to declare.
23 G. R. Krishna, R. Devarapalli, G. Lal and C. M. Reddy, J. Am.
Chem. Soc., 2016, 138, 13561.
24 X. Wang, O. S. Wolfbeis and R. J. Meier, Chem. Soc. Rev.,
2013, 42, 7834–7869.
Notes and references
1 (a) A. G. Shtukenberg, Y. O. Punin, A. Gujral and B. Kahr,
Angew. Chem., Int. Ed., 2014, 53, 672; (b) S. Saha and G. R.
Desiraju, J. Am. Chem. Soc., 2017, 139, 1975; (c) M.
Owczarek, K. A. Hujsak, D. P. Ferris, A. Prokofjevs, I. Majerz,
P. Szklarz, H. Zhang, A. A. Sarjeant, C. L. Stern, R. Jakubas,
S. Hong, V. P. Dravid and J. F. Stoddart, Nat. Commun.,
2016, 7, 13108.
2 S. Saha and G. R. Desiraju, Chem. Commun., 2018, 54, 6348.
3 T. Kim, L. Zhu, L. J. Mueller and C. J. Bardeen, J. Am. Chem.
Soc., 2014, 136, 6617.
25 B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E.
Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S.
Lee, D. M. Maughon, J. Qin, H. Rockel, M. Rumi, X. L. Wu,
S. R. Marder and J. W. Perry, Nature, 1999, 398, 51.
26 (a) L. A. Bagatolli and E. Gratton, Biophys. J., 2000, 78, 290;
(b) M. Ipuy, Y. Y. Liao, E. Jeanneau, P. L. Baldeck, Y.
Bretonniere and C. Andraud, J. Mater. Chem. C, 2016, 4, 766.
27 R. Petermann, M. Tian, S. Tatsuura and M. Furuki, Dyes
Pigm., 2003, 57, 43.
28 J. Wang, C. Wang, Y. Gong, Q. Liao, M. Han, T. Jiang, Q.
Dang, Y. Li, Q. Li and Z. Li, Angew. Chem., Int. Ed., 2018, 57,
16821–16826.
29 T. Bullard, K. L. Wustholz, E. D. Bott, M. Robertson, P. J.
Reid and B. Kahr, Cryst. Growth Des., 2009, 9, 982.
4 T. Kim, M. K. Al-Muhanna, S. D. Al-Suwaidan, R. O. Al-Kaysi
and C. J. Bardeen, Angew. Chem., Int. Ed., 2013, 52, 6889.
5 S. Kobatake, S. Takami, H. Muto, T. Ishikawa and M. Irie,
Nature, 2007, 446, 778.
CrystEngComm
This journal is © The Royal Society of Chemistry 2018

View Article Online
CrystEngComm
Communication
30 T. Zhou, T. Jia, S. Zhao, J. Guo, H. Zhang and Y. Wang,
Cryst. Growth Des., 2012, 12, 179.
38 S. K. Maity, S. Bera, A. Paikar, A. Pramanik and D. Haldar,
Chem. Commun., 2013, 49, 9051.
31 (a) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem.
Commun., 2001, 1740; (b) Y. Hong, J. W. Y. Lam and B. Z.
Tang, Chem. Soc. Rev., 2011, 40, 5361.
39 A. Pramanik and D. Haldar, RSC Adv., 2017, 7, 389.
40 M. K. Panda, S. Ghosh, N. Yasuda, T. Moriwaki, G. D.
Mukherjee, C. M. Reddy and P. Naumov, Nat. Chem.,
2015, 7, 65.
32 C. T. Chen, Chem. Mater., 2004, 16, 4389.
33 Z. Q. Guo, W. H. Zhu and H. Tian, Chem. Commun.,
2012, 48, 6073.
34 (a) A. C. Grimsdale and K. Müllen, Angew. Chem., Int. Ed.,
2005, 44, 5592; (b) A. Ajayaghosh and V. K. Praveen, Acc.
Chem. Res., 2007, 40, 644.
41 (a) P. A. Suchetan, B. S. Palakshamurthy, N. R. Mohan, S.
Madan Kumar, N. K. Lokanath and S. Sreenivasa, Acta
Crystallogr., Sect. E: Struct. Rep. Online, 2013, 69, 1610; (b)
M. J. Kim, J. Y. Mang and I. H. Suh, J. Korean Phys. Soc.,
1988, 21, 405; (c) J. T. Mague and S. K. Mohamed, CSD
Communication, 2016.
35 M. Shimizu, R. Kaki, Y. Takeda, T. Hiyama, N. Nagai, H.
Yamagishi and H. Furutani, Angew. Chem., Int. Ed., 2012, 51,
4095.
42 G. M. Sheldrick, SHELXL-2014/7, Program for the Refinement
of Crystal Structures. University Göttingen, Göttingen,
Germany, 2014.
36 S. Lv, D. M. Dudek, Y. Cao, M. M. Balamurali, J. Gosline and
H. Li, Nature, 2010, 465, 69.
37 S. Hayashi, A. Asano, N. Kamiya, Y. Yokomori, T. Maeda and
T. Koizumi, Sci. Rep., 2017, 7, 9453.
43 H. L. Hirshfeld, Theor. Chim. Acta, 1977, 44, 129.
44 M. J. Turner, J. J. McKinnon, S. K. Wolff, D. J. Grimwood,
P. R. Spackman, D. Jayatilaka and M. A. Spackman,
CrystalExplorer17, The University of Western Australia, 2017.
This journal is © The Royal Society of Chemistry 2018
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