Thermally Twistable, Photobendable, Elastically Deformable, and Self-Healable Soft Crystals

Article abstract of DOI:10.1002/anie.201802785

The first example of a smart crystalline material, the 2:1 cocrystal of probenecid and 4,4′-azopyridine, which responds reversibly to multiple external stimuli (heat, UV light, and mechanical pressure) by twisting, bending, and elastic deformation without fracture is reported. This material is also able to self-heal on heating and cooling, thereby overcoming the main setbacks of molecular crystals for future applications as crystal actuators. The photo- and thermomechanical effects and self-healing capabilities of the material are rooted in reversible trans–cis isomerization of the azopyridine unit and crystal-to-crystal phase transition. Fairly isotropic intermolecular interactions and interlocked crisscrossed molecular packing secure high elasticity of the crystals.

Full text of DOI:10.1002/anie.201802785

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Accepted Article
Title: All-in-One: Thermally Twistable, Photobendable, Elastically
Deformable and Self-Healable Soft Crystal
Authors: Poonam Gupta, Durga Prasad Karothu, Ejaz Ahmed, Pance
Naumov, and Naba Kamal Nath
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802785
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10.1002/anie.201802785
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All-in-One: Thermally Twistable, Photobendable, Elastically
Deformable and Self-Healable Soft Crystal
Poonam Gupta,[a] Durga Prasad Karothu,[b] Ejaz Ahmed,[b] Panče Naumov*[b] and Naba K. Nath*[a]
Abstract: The first example of a smart crystalline material, the 2:1
cocrystal of probenecid and 4,4′-azopyridine, which responds
reversibly to multiple external stimuli (heat, UV light, and mechanical
pressure) by twisting, bending and elastic deformation without
fracture is reported. This material is also able to self-heal on heating
and cooling, thereby overcoming the main setbacks of molecular
crystals for future applications as crystal actuators. The photo- and
thermomechanical effects and self-healing capabilities of the
material are rooted in reversible trans‒cis isomerization of the
azopyridine unit and crystal-to-crystal phase transition. Fairly
isotropic intermolecular interactions and interlocked criss-crossed
molecular packing secure high elasticity of the crystals.
Several recent reports of self-healing molecular crystals[29‒31]
open prospects for further exploration into the self-repairing
mechanisms in crystals. Here we report the first example of a
molecular material, a cocrystal of 4,4′-azopyridine and the
uricosuric agent probenecid (Figure 1a), that responds reversibly
to three different external stimuli—heat, light and mechanical
force. Molecular crystals of probenecid are known to be
plastically bendable, and cocrystallization with 4,4′-azopyridine
was carried out to introduce an additional property of
photoinduced mechanical effects in the same system. The
crystals twist and untwist when they are heated or cooled, bend
reversibly when they are exposed to UV light, and deform
elastically when they are subject to mechanical force. Perhaps
most importantly, these crystals can also self-heal during
heating and cooling, thus displaying the most diverse set of
functions in a single soft crystalline material reported to date.
Nature has always amazed materials scientists with its highly
sophisticated biomolecular systems, and some of these
structures have already been translated by researchers and
engineers into useful advanced technologies.[1] Particularly
appealing are the mechanisms for motion or dispersal found in
plants and insects, the basic kinematic traits of which can be
replicated by the motility of the recently discovered dynamic
crystals, which are able to jump,[2‒6] bend,[7‒14] twist,[15,17] crawl,[18]
roll,[19] and even walk[18] when they are excited by heat or
light.[20‒23] The kinematic effects of these advanced crystalline
materials comply to many of the requirements for
micromachinery gears, such as those used in microrobotics and
microfluidics. Moreover, when impacted by external force
(application of localized pressure) many mechanically complaint
molecular crystals dissipate the applied stress and undergo
reversible (elastic)[24‒26] or irreversible (plastic)[27] bending
without disintegration.
Amidst the rapid rise of the adaptive crystalline materials, the
recent research efforts revolve around optimization of their
performance aimed to accomplish fast, reversible and
fatigueless response to applied stimuli after prolonged operation.
The capability of recovery after damage during operation, akin to
the biological systems, is as equally as important if these
materials are to be considered for future applications. Yet, due
to difficulties with predictability of molecular packing,
intermolecular interactions and crystal habit, combining multiple
mechanical responses to different stimuli in a single molecular
crystal remains as one of the most formidable challenges.[28]
Figure 1. (a) Molecular structures of probenecid and 4,4′-azopyridine, (b)
optical microscopic image showing the crystal habits of the two polymorphs of
the cocrystal, form 1 (acicular crystals) and form 3 (blocky crystals), and (c)
thermal microscopy images showing progression of the habit plane (phase
front) during the reversible phase transition form 1 → form 2 and during the
irreversible phase transition form 2 → form 3. The habit plane is highlighted
with broken white line, and is more easily visible in Supporting Information
Videos S1 and S2.
Cocrystallization of 4,4′-azopyridine with probenecid in 1 : 2
molar ratio by slow evaporation from acetonitrile afforded orange
crystals with acicular and blocky habit (Figure 1b; for synthesis
characterization and crystal morphology, see Supporting
Information, Scheme S1, Figure S1). Single crystal X-ray
diffraction analysis confirmed that the two crystalline habits
correspond to different polymorphs, hereafter referred to as form
1 (acicular crystals) and form 3 (blocky crystals). To our surprise,
these two polymorphs exhibited very different response to
heating, application of mechanical stress and exposure to UV
light: in contrast with form 3, which was unaffected by these
[a]
[b]
P. Gupta, Prof. N. K. Nath
Department of Chemistry, National Institute of Technology
Meghalaya (India)
Emails: nabakamal.nath@gmail.com
nabakamal.nath@nitm.ac.in
Dr. D. P. Karothu, Dr. E. Ahmed, Prof. P. Naumov
New York University Abu Dhabi, Abu Dhabi (United Arab Emirates)
Email: pance.naumov@nyu.edu
Supporting information for this article is given via a link at the end of
the document.
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stimuli, the single crystals of form 1 responded to each in three
distinct ways. When heated on a thermal stage coupled with a
microscope at a rate of 20 ºC min‒1, crystals of form 1 underwent
two thermal events. Between ~62 and 64 ºC a phase front (habit
plane) was observed to propagate through the crystal (Figure
1c; Supporting Information, Figure S2, Videos S1 and S2). Upon
subsequent cooling below ~58 ºC, the phase front moved in the
opposite direction. This was occasionally accompanied by rapid
and reversible distortion or motion of the crystal (Supporting
Information, Figure S2 and Video S2). On further heating above
~76 ºC, progression of a phase front was observed again,
however it occurred without any observable macroscopic motion
of the crystal (Figure 1c; Supporting Information, Figure S2,
Video S1 and S2). When cooled from 76 ºC to room temperature,
the crystal did not display any of the events observed during
heating, indicating that the change around 76 ºC is irreversible.
The rate of progression of the first transition was calculated from
the progressing habit plane to be approximately 2.5 mm s‒1, and
it is much slower compared to the rate of phase transitions
associated with the thermosalient crystals (typically on the order
of several m s‒1)[4‒6], however it is much faster than the
mechanically inactive second transition (0.08 mm s‒1).
(Figure 3; Supporting Information, Figure S5, Videos S3, S4 and
S5). None of these 14 crystals was found to splinter or crack
during twisting, and the macroscopic integrity and crystallinity
was preserved even after cooling. The other crystals were found
to exhibit other mechanical effects (Supporting Information,
Figure S6, S7, Video S6).
Figure 3. A cocrystal (size: length, 6.12 mm; width, 0.26 mm; thickness, 0.12
mm) twists when heated, whereupon one end lifts off the base (pointing
toward the reader), and untwists after cooling, regaining its original shape. The
black arrow indicates the region where the twisting occurs.
To gain mechanistic insight into the origin of the mechanical
effects, the structure of form 1 was determined by using X-ray
diffraction analysis (Supporting Information, Figure S8 and Table
S1). A crystal of form 1 was transformed to form 2 in situ, by
heating to 63 ºC with a stream of nitrogen gas on the
diffractometer. Although the heating caused some deterioration
of crystal quality, the diffraction quality was sufficient to
determine the structure of form 2 at 63 ºC. The same crystal was
further heated above 76 ºC, however the crystal quality of form 3
obtained in situ was insufficient for structure solution. Therefore
the same crystal was cooled to room temperature, and the
structure determination showed that it is in form 3. The
calculated powder diffraction pattern of thus obtained form 3 was
identical to that of form 3 obtained by crystallization (Supporting
Information, Figure S9). Form 1 crystallizes in the monoclinic
space group P21/c with one molecule of probenecid and half
molecule of 4,4′-azopyridine in the asymmetric unit whereas, the
Figure 2. Heating-cooling cycle in the DSC thermogram of the cocrystal
showing effects due to the reversible phase transition between forms 1 and 2.
Differential scanning calorimetry (DSC) carried out on crystals
of form 1 at 5 ºC min‒1 showed endothermic peaks at 59‒65 ºC
(Figure 2) and 76‒86 ºC (Figure S3) corresponding to the two
phase transitions. The first phase transition, from form 1 to
another polymorph, form 2, occurs at 59‒65 ºC on heating and
at 57‒53 ºC on cooling. The second transition (~76 ºC) from
form 2 to form 3 is irreversible, and no phase transition was
observed on cooling of form 3 (Supporting Information, Figure
S3).
structure of form 2 is triclinic, space group P
with two
1
molecules of probenecid and two half-molecules of 4,4′-
azopyridine molecules in the asymmetric unit (Supporting
Information, Table S1). The structure of form 3 is triclinic, space
group P with one molecule of probenecid and half molecule of
azopyridine in the asymmetric unit.
1
The mechanical response observed with thermal microscopy
prompted us to further investigate the heat-induced reshaping of
crystals of form 1. The thermomechanical response of 35
crystals was inspected by heating to 68 ºC each crystal in turn
on its wider face, (001) (for face indexing, see Supporting
Information, Figure S4) and subsequent cooling to room
temperature. A total of 14 crystals were found to twist,
occasionally by lifting at one end off from the base. The crystals
retained their twisted shape as long as they were kept above the
phase transition temperature; they untwisted and recovered their
original shape after they were cooled below the phase transition
The basic building block of both polymorphs consists of one
azopyridine molecule bonded by O—H···N and auxiliary C—
H···O hydrogen bonds to two probenecid molecules, one on
either of its two termini (Supporting Information, Figure S10).
These hydrogen-bonded entities are stacked over one another
by C—H···O and C=O···π interactions and form infinite stacks
along the b axis. The molecular stacks are closely packed and
form infinite 2D layers. The resulting 2D layers are criss-crossed
and interact through C—H···O hydrogen bonds.
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On heating and cooling over the transition 1 ↔ 2 the
molecular arrangement changes by small reorganization of the
2D layers. The interplanar angle between the two nearly
perpendicular probenecid benzene rings decreases from
89.3(3)º in form 1 to 85.4(2)º in form 2, while the angle between
the two azopyridine planes expands from 84.7(3)º in form 1 to
In many cases it was observed that the bi- or multi-furcate
branches of the split crystal advance towards each other after
the phase change and tend to rejoin. To inspect the details of
this phenomenon, several well-shaped single crystals were
examined with temperature-controlled optical microscopy. A
crystal was first heated over the transition 1 → 2 and cooled
below the transition 2 → 1 at rate of 5 ºC min‒1. (Supporting
Information, Video S7), during which a small crack appeared on
the (001) face of the crystal. The crystal visibly separated
around its central cracked section (Figure 5a; Supporting
Information, Video S8) during a second heat cool cycle. The
cracked crystal was heated above 1 → 2, whereupon the crack
disappeared without any visible trace of the original crack
(Supporting Information, Videos S8 and S9). Another instance
showed that two planks that separated out of the crystal were
rejoined by heating, thereby partially recovering the original
crystal (Supporting Information, Figure S12, Video S10). These
experiments indicate that after the crystal is cracked by
heating—at least macroscopically—it appears to heal when
cycled thermally in the temperature range 52‒68 ºC. Although
the effects are clearly facilitated by heating and the phase
transition, these observations resemble the recently reported
examples of self-healing crystals.[29‒31]
87.4(9)º in form
2 (Figure 4a). The centroid-to-centroid
disposition in the stacking direction of the criss-crossed 2D
layers changes from 0.0(6)º in form 1 to 6.0(4)º in form 2 (Figure
4b). Apart from these major changes, the transition to form 2 is
also associated with a change in conformation of the n-propyl
chains in one of the symmetry-independent probenecid
molecules. These changes in molecular packing result in
expansion along the a axis of 5.2%, expansion along the b axis
of 1.6% and contraction along the c axis of ‒2.8% when the
crystal transitions from form 1 to form 2. These changes in
molecular packing and conformation during the phase transition
generate anisotropic strains and result in macroscopic distortion
that ultimately appears as twisting of the crystal (Figure 4c,d).
Consistent with most single-crystal-to-single-crystal phase
transitions that are not accompanied by observable mechanical
effects, the second phase transition, 2 → 3, which was not
accompanied by mechanical effects, is related to significant
structural change (for the structure of form 3, see Supporting
Information, Figure S11).
Figure 5. Thermally-assisted self-healing effects. A crystal, cracked during
first heat-cool cycle over the phase transition 1 ↔ 2 (a) partially heals on
second heating as seen by the regained translucency under microscope (b).
The black arrow shows the region where crack occurred and disappeared.
Figure 4. Crystal structures of forms 1 and 2, and proposed mechanism for
the mechanical effect that accompanies the reversible phase transition 1 ↔ 2.
(a) Subtle changes in the angles between the aromatic rings of two criss-
crossed probenecid molecules and between the molecular planes of two criss-
crossed azopyridine rings. (b) Change in the angle between the centroid-to-
centroid π···π stacking direction of the criss-crossed 2D layers of about 6º
during the transition. (c,d) Non-uniform strain generated during the transition
results in reversible twisting of the crystals, shown here with a cartoon (c) and
twisting of actual crystal (d) (size: length, 5.90 mm; width, 0.36 mm; thickness,
0.15 mm).
Figure 6. (a) Slender crystals of form 1 can be elastically bend up to 360º to
form a loop. (b) Indices of the prominent faces of the crystal. (c‒e) View of the
molecular packing as seen normal to the (001) face (c), the (100) face (d) and
1 1
the ( , ,0) face (e), showing the interlocked molecular packing.
In addition to these properties, the cocrystals are also
extraordinarily elastic. Crystal of form 1 (typical size: 7.5 mm ×
0.3 mm × 0.2 mm) held on the wider face, (001), can be bent up
360º and shaped in a loop without breaking, similar to some
previously reported examples[25,26] (Figure 6a; Supporting
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Information, Video S11). After the stress is released, the crystals
regain their original straight shape. In the crystal structure of
form 1, the building blocks of probenecid and azopyridine are
connected by weak intermolecular interactions of comparable
length, that is, C=O···π (~2.8 kcal mol‒1), C—H···O (~2.8 kcal
mol‒1) and van der Waals interactions (~0.9 kcal mol‒1 per atom
pair) in all three directions. The criss-crossed 2D layers
consisting of hydrogen-bonded probenecid/azopyridine units are
interconnected by weak intermolecular C—H···O hydrogen bond
(Figure 6c‒e). Such interlocked molecular packing and isotropic
intermolecular interactions in crystal structures were earlier
related to the elastic bendability of other molecular crystals,[20,21]
and could be the main contributor to the extraordinary elasticity
observed with the cocrystal in this work.
Keywords: actuators • azobenzene • photomechanical effects •
self-healing • single crystal
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a
multifunctional, soft molecular crystalline material. An
additional trait of this material is the ability to heal by heat, which
turns this and related cocrystals into prospective candidates for
robust and durable singe crystal actuators.
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startup grant (N.K.N). This work was financially supportd by DST
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(NYUAD) for his support with the DSC experiments. We thank
NIT Meghalaya for the instrumental facilities. This research was
partially carried out using Core Technology Platform (CTP)
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A cocrystal of probenecid
and 4,4′-azopyridine is the
first truly multifunctional
polymorphic adaptive
Poonam Gupta, Durga Prasad Karothu, Ejaz Ahmed, Panče
Naumov* and Naba K. Nath*
molecular crystalline
Page No. – Page No.
material that responds to UV
light, heat and mechanical
pressure by elastic bending
and twisting, and is also
capable for self-healing.
All-in-One: Thermally Twistable, Photobendable, Elastically
Deformable and Self-Healable Soft Crystal
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