Azobenzene crystal polymorphism enables tunable photoinduced deformations, mechanical behaviors and photoluminescence properties

Article abstract of DOI:10.1039/d1tc01143j

Stimuli-responsive molecular crystals are fascinating for their potential as adaptive smart materials. However, achieving crystals that could respond to multiple stimuli and perform multiple functionalities simultaneously is still a challenging task. Herein, we report the fabrication and preparation of polymorphic crystals with multiple stimuli of a photoactive azobenzene derivative,trans-4-cyano-4′-oxethyl azobenzene (AzC₂). Different polymorphic forms of AzC₂exhibit remarkable different photoinduced deformations, mechanical behaviors and photoluminescence properties. Upon UV irradiation, although both polymorphic forms undergo reversible photomechanical bending motions, Form I bent away from the light source while Form II bent towards the light source. Upon exertion of external mechanical stress, Form I tended to break whereas Form II exhibited elastic deformation. Furthermore, being excited at 325 nm, the block crystal of Form I exhibited a relatively strong green-yellow emission while a very weak red fluorescence emission was observed from the thicker Form II crystal. These different properties can be attributed to different molecular packings rather than molecular conformations. The present work provides an effective strategy to construct multiple stimuli-responsive crystal materials with potential applications in actuators, switches and sensors.

Full text of DOI:10.1039/d1tc01143j

Journal of
Materials Chemistry C
View Article Online
View Journal | View Issue
PAPER
Azobenzene crystal polymorphism enables
tunable photoinduced deformations, mechanical
behaviors and photoluminescence properties†
Cite this: J. Mater. Chem. C, 2021,
9, 8294
Yunhui Hao,a Lei Gao,a Xiunan Zhang,a Rongli Wei,a Ting Wang,a Na Wang,a
a
Xin Huang, *a Haifeng Yu*b and Hongxun Hao
*
Stimuli-responsive molecular crystals are fascinating for their potential as adaptive smart materials.
However, achieving crystals that could respond to multiple stimuli and perform multiple functionalities
simultaneously is still a challenging task. Herein, we report the fabrication and preparation of polymorphic
crystals with multiple stimuli of
a
photoactive azobenzene derivative, trans-4-cyano-40-oxethyl
azobenzene (AzC2). Different polymorphic forms of AzC2 exhibit remarkable different photoinduced
deformations, mechanical behaviors and photoluminescence properties. Upon UV irradiation, although
both polymorphic forms undergo reversible photomechanical bending motions, Form I bent away from
the light source while Form II bent towards the light source. Upon exertion of external mechanical stress,
Form I tended to break whereas Form II exhibited elastic deformation. Furthermore, being excited at
325 nm, the block crystal of Form I exhibited a relatively strong green-yellow emission while a very weak
red fluorescence emission was observed from the thicker Form II crystal. These different properties can
be attributed to different molecular packings rather than molecular conformations. The present work
provides an effective strategy to construct multiple stimuli-responsive crystal materials with potential
applications in actuators, switches and sensors.
Received 11th March 2021,
Accepted 28th May 2021
DOI: 10.1039/d1tc01143j
rsc.li/materials-c
and photodimerization of anthracenes.2,24 However, most of these
kinds of crystal materials are generally responsive to one stimulus
Introduction
Stimuli-responsive crystal materials can change their chemical or possess one responsive mode. Moreover, the complicated
or physical properties in response to external stimuli, such as synthesis of these different molecules also increases the produc-
temperature, light, mechanical stress, electricity, pH, and so tion cost, which apparently limits their practical applications.
on,1–14 which has aroused great interest in materials science and
To enhance the versatility and extend multifarious applications
engineering for their applications as actuators,15–18 sensors,19 of such materials, researchers have developed multi-stimuli
switches20 and biomimetic materials.21 The most widely used responsive and multi-functional crystal materials.25–30 Recently,
strategy to construct stimuli-responsive crystal materials is to take the synthesis of cocrystals via non-covalent interactions has been
advantage of specially designed functionalities of molecules. For proved to be an effective strategy for developing multi-stimuli
instance, macroscale photomechanical motions may arise from responsive and multi-functional crystal materials.31,32 Naumov
photoreactions of molecules such as trans–cis photoisomerization et al. reported a cocrystal with thermally twistable, photobend-
of azobenzene derivatives,5,6 photocyclization of diarylethenes,22,23 able, elastically deformable and self-healing properties by the
co-crystallization of 4,40-azopyridine with probenecid.33 However,
it could be a difficult and time-consuming task to find appropriate
host molecules and guest molecules to synthesize the target
cocrystal with ideal properties. Also, the stability of the cocrystal
a State Key Laboratory of Chemical Engineering, School of Chemical Engineering
and Technology, and Collaborative Innovation Center of Chemical Science and
Chemical Engineering, Tianjin University, Tianjin 300072, China.
could be another issue when the weak strength of the non-covalent
interactions is considered.34 In comparison, manipulating crystal
polymorphs of the same functional molecules by regulating the
conformation and packing during the crystal nucleation and
growth processes is more convenient and efficient. Generally, the
solid-state structure of functional molecules plays an important
role in their performance, and some polymorphs may exhibit
E-mail: hongxunhao@tju.edu.cn, x_huang@tju.edu.cn
b School of Materials Science and Engineering and Key Laboratory of Polymer
Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871,
China. E-mail: yuhaifeng@pku.edu.cn
† Electronic supplementary information (ESI) available: Details of experiments
and characterization, crystallographic information and movies. CCDC 1944178
and 1944179. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/d1tc01143j
8294 | J. Mater. Chem. C, 2021, 9, 8294–8301
This journal is © The Royal Society of Chemistry 2021

View Article Online
Paper
Journal of Materials Chemistry C
significantly different properties like colors,35 mechanics,36,37
luminescence38–40 and electrical features.37 Under appropriate
conditions, crystalline polymorphs could undergo a single-
crystal-to-single-crystal (SCSC) phase transition,22,31,40,41 which
provides the potential for making multi-stimuli responsive and
multi-functional crystal materials by manipulating different
polymorphs of the same compound. However, development
these kinds of crystalline materials has been scarcely reported.
Azobenzene derivatives are among the most promising
candidates to prepare multi-responsive crystal materials due
to their superior characteristics. First, they exhibit a quick and
reversible trans–cis photoisomerization, in which a large difference
in geometric shapes and molecular sizes often exists between the
isomers, leading to macroscale deformation in bulk. A variety of
molecular crystals of azobenzene derivatives have been prepared to
undergo various photomechanical deformations.42–45 Recently,
we reported mono-component molecular crystals of 4-cyano-40-
pentyloxy azobenzene with multi-directional deformation ability.46
Second, the multitude and moderate intermolecular interactions in
the molecular structures of azobenzene derivatives make them
potential candidates for designing elastic crystals.36,47 However,
mechanical behaviors of azobenzene crystals under mechanical
stress are also rarely studied. Third, azobenzene derivatives are
often non-fluorescent due to photoisomerization. Nevertheless,
relatively weak fluorescence has been occasionally observed when
photoisomerization is suppressed in the crystalline state,48–50 as a
Fig. 1 (a) Molecular structure and photoisomerization of AzC2. (b) Optical
microscopy images of the plate-like Form I crystals and (c) needle-like
Form II crystals. The scale bar is 100 mm. (d) Comparative PXRD patterns of
two polymorph crystals. Crystal packing in (e and f) Form I and (g and h)
Form II.
result of crystallization-induced emission (CIE).51,52 Herein, we two forms have almost identical conformation (Fig. S6, ESI†).
report two crystal polymorphs of 4-cyano-40-oxethyl azobenzene However, they adopt significantly different packing patterns to
(AzC2), exhibiting remarkable difference in photomechanical have different cell parameters. As shown in Fig. 1e, the mole-
deformation, mechanical properties and luminescence. This work cules in Form I are pꢀ ꢀ ꢀp stacked (3.44 and 3.47 Å) in alternately
may inspire more investigations on constructing multiple stimuli- anti-parallel patterns along the c-axis. Then, the stacked col-
responsive crystal materials through polymorph manipulation.
umns are connected by C–Hꢀ ꢀ ꢀH intermolecular interactions to
%
form parallel tapes along the [102] direction. The adjacent tapes
are connected via very weak dispersion interactions to further
form a herringbone arrangement (Fig. 1e and f). In Form II, on
the other hand, the molecules are pꢀ ꢀ ꢀp stacked (3.36 Å) in
parallel patterns along the a-axis. Then, the stacked columns
Results and discussion
Crystal growth and structural analysis of polymorphs
As shown in Fig. S1 (ESI†), trans-AzC2 was synthesized in 84% are connected by C–Hꢀ ꢀ ꢀN, C–Hꢀ ꢀ ꢀC and C–Hꢀ ꢀ ꢀO along the
yield.46 Here, AzC2 was selected as one representative example c-axis in a middle-to-middle way to form a crossing structure,
for several reasons: (1) its electrons are easily excited because of which is further expended along the b-axis via C–Hꢀ ꢀ ꢀNRC and
the stronger conjugation from the push–pull structure, (2) its C–Hꢀ ꢀ ꢀH intermolecular interactions (Fig. 1g and h). Their
para-substituents are stable enough to resist chemical reactions dominant surfaces were indexed based on SEM observations,
during experiments, (3) the oxethyl chain acts as the end-group combined with the results of preferred orientation from PXRD
for acquiring free space that permits isomerization, and the and the calculated Miller indices based on crystallographic data
chain flexibility is easily controlled to avoid the formation of (Fig. S8 and S9, ESI†).32,53
conformational polymorphs. Fig. 1 shows the crystallization
In order to investigate their structural stability, two poly-
of trans-AzC2 by sublimation on quartz slide-generated micro- morph crystals were grinded and heated from 25 1C to 120 1C.
crystals with plate habits, while microcrystals with good needle No obvious peak shifts were observed in the variable-
habits were obtained through epitaxial crystallization on the temperature PXRD patterns in Fig. S11 (ESI†), indicating that
(100) surface of the KBr substrate. More details on crystal- both polymorph crystals have good thermal stability. Moreover,
lization and characterization are given in the ESI.† X-ray even after continuous grinding for 10 min, the crystal structure
diffraction analyses confirm that the two crystalline habits of both polymorphs did not change, indicated by their PXRD
correspond to different polymorphs, hereafter referred to as patterns before and after grinding (Fig. S12, ESI†). This means
Form I (plate crystal) and Form II (needle crystal) (Table S1, that the external force of grinding does not affect the crystal
ESI†). Both of them belong to the monoclinic space group P21/n structure. Both polymorphs are relatively stable and cannot trans-
with one molecule in the asymmetric unit. The molecules in the form to each other, due to their far different molecular packings.
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 8294–8301 | 8295

View Article Online
Journal of Materials Chemistry C
Paper
These characteristics make these two polymorphs exhibit comple- irradiated surface upon photoirradiation. However, it is difficult to
tely different properties under the same stimulus conditions.
determine the structure after isomerization because this photo-
chemical reaction often occurs in a range of only a few microns on
the surface and the products have very short half-lives. Powder
X-ray diffraction (PXRD) is one of the most powerful methods to
identify and characterize the subtle variations of lattice in the
surface. As shown in Fig. 3a and b, all the PXRD characteristic peak
intensities of Form I and Form II microcrystals decreased and
broadened upon continuous illumination of UV light, indicating
that microstrain was generated in both crystal lattices due to
photoisomerization.5,6 In addition, the peak positions of Form I
slightly shifted to larger angles, while the peak positions of Form II
slightly shifted to smaller angles. These results suggest that the
interplanar spacing of the irradiated surfaces contracts in Form I
while expands in Form II upon UV irradiation (Bragg’s law,
Equation 1 in the ESI†). Particularly, new peaks appeared at
2y = 6.361 in Form I while new peaks appeared at 2y = 16.421 in
Form II, suggesting that a new crystalline phase formed in both
polymorphs on photoisomerization.
Photomechanical deformation
Like other azobenzene derivatives, AzC2 exhibited reversible
isomerization upon irradiation with UV light and thermal
relaxation (Fig. S13 and S14, ESI†), which played an important
role in photomechanical deformation. When the main micro-
crystal surface of Form I and Form II was exposed to the same
UV irradiation, respectively, photomechanical deformations
were observed in both forms, but in opposite direction. Form
I bent away from the light source, while Form II bent towards
the light source. As shown in Movie S1 (ESI†) and Fig. 2a, one
end of the plate-like microcrystal of Form I (size: 2926 mm ꢁ
228 mm ꢁ 10 mm) is free while the other end is fixed to a syringe
needle. Upon photoirradiation of the (020) surface from the left
front with UV light (365 nm, 150 mW cmꢂ2), the microcrystal
quickly bent away from the light source to reach a maximum tip
displacement of 346.64 mm within 0.2 s. Subsequent removal of
the illumination resulted in the immediate return of its initial
shape. A similar reversible sequence of motions in opposite
direction was observed for the needle-like microcrystal of Form
II (size: 2847 mm ꢁ 27 mm ꢁ 24 mm). It bent towards the light
source to reach a maximum tip displacement of 295.45 mm
within 1 s when UV light irradiated the (012) surface from the
left front (Movie S2, ESI† and Fig. 2b). After the irradiation was
terminated, the crystal of Form II instantly regained its original
straight shape. Both the reversible photomechanical bending
and unbending motions were repeated more than 20 cycles
(Fig. S15, ESI†) without any detectable fatigue observed. During
irradiation, the temperature of the two polymorphic micro-
crystals did not apparently increase, which excluded the possibility
of photothermal effects contributed to photomechanical defor-
mations (Fig. S16, ESI†). Additionally, the directions of UV
irradiation did not affect the photomechanical behaviors of the
two polymorphic microcrystals, and photoirradiation for longer
durations did not lead to any more significant deformation of
the two polymorphic microcrystals either (Fig. S17, S18 and
Movies S3, S4, ESI†).
This was further confirmed by the AFM measurements of
surface topological changes in the two crystal polymorphs. As
shown in Fig. 3c and e, smooth top surfaces were clearly
observed for the as-prepared crystals in both polymorphisms.
Upon UV irradiation, crinkle-like features appeared on both
crystal surfaces with an increased roughness from 0.193
to 3.86 nm in Form I (Fig. 3d), and from 0.091 to 7.11 nm in
Photoinduced bending has been the most frequently reported
motion of mechanical molecular crystals so far,10 and it has been
ascribed to the so-called bimetal mechanism.54 Generally, the
bending direction depends on the nonuniform change in the
crystal unit volume caused by the change of molecules on the
Fig. 3 Change of crystal structure and surface topology of two poly-
morphic microcrystals upon UV irradiation at room temperature: PXRD
patterns of (a) one plate-like Form I crystal and (b) one needle-like Form II
crystal before and during irradiation, respectively. AFM images of the (020)
surface of one plate-like Form I crystal (c) before illumination, and (d) upon
irradiation with 365 nm light during measurements. AFM images of the
(012) surface of one needle-like Form II crystal (e) before illumination, and
(f) upon irradiation with 365 nm light during measurement.
Fig. 2 Photoinduced reversible deformation: (a) one plate-like Form I
microcrystal bent away from the light source upon irradiation on the (020)
face, and (b) one needle-like Form II microcrystal bent towards the light
source upon irradiation on the (012) face.
8296 | J. Mater. Chem. C, 2021, 9, 8294–8301
This journal is © The Royal Society of Chemistry 2021

View Article Online
Paper
Journal of Materials Chemistry C
Form II (Fig. 3f). These results suggested that the molecular bending deformation away from the light source. On the other
arrangement in each polymorph becomes disordered on the hand, on the (012) face of Form II, the length and width
short-range length scale in the irradiated areas.5 In addition, of the trans-AzC2 molecule decreased by 23.39% and 24.40%,
Form II has a rougher surface compared to Form I. The respectively, while the thickness increased by 10.34% upon
narrower and higher crinkle-like feature of Form II indicates photoisomerization (Fig. S19e–h, ESI†). This should produce a
that its photoisomerization ability is better than that of Form I. contraction on the irradiated (012) surface, and expands in the
These phenomena are consistent with Raman experimental direction normal to it (Fig. 4b), which is consistent with
results (Fig. S14, ESI†).
the increased d-spacing observed by PXRD and the crinkle
Then, the possible mechanisms of photomechanical bending topologies of AFM (Fig. 3b and f). As a result, the photoinduced
behaviors of these polymorphic microcrystals are discussed below. bending motion occurs towards the light source.
According to the results of molecular simulations, the length,
width and thickness of the trans-AzC2 molecule on the (020) face of
Form I increased by 30.26%, and decreased by 9.03% and 36.33%,
respectively, after photoisomerization (Fig. S19a–d, ESI†). This
should produce an expansion along the c-axis and lead to the
crinkle topologies as surface bucking occurs to release the strain,
which agrees well with the results of the AFM measurements
(Fig. 3d). This also produces a contraction along the b-axis, which
is consistent with the decreased d-spacing observed by PXRD
(Fig. 3a). The overlay of packing arrangements of trans- and cis-
AzC2 is given in Fig. 4a. Hence, the irradiated microcrystal surface
would expand in the direction along the microcrystal growth,
and contract in the direction along the cross-section to induce a
Mechanical behaviors
As mentioned above, the two polymorphs exhibit distinct photo-
induced deformation behaviors upon photoirradiation. Interestingly,
the two crystal polymorphs also exhibited different elastic
mechanical properties. Upon application of external mechanical
stress, Form I tends to break while Form II underwent elastic
deformation.
As shown in Fig. 5a, the two ends of one plate-like Form I
crystal were bound to double-sided adhesive tape. Then, an
external force was applied at the middle of the major surface
(020) of Form I, and the crystal instantly broke into two pieces
(Movie S5, ESI†). Considering that the acicular Form II has two
main crystal surfaces, we tested them separately. When an
external force was applied perpendicular to the (012) surface,
a semi-circular arc formed without any detectable fracture and
crack observed simultaneously. This process was found to be
reversible as the crystal straightened immediately upon with-
drawal of the force (Fig. 5b and Movie S6, ESI†). This sequence
can be repeated many times. Besides, a similar elastic bending
deformation was observed when poked on the (020) surface of
the same Form II crystal (Fig. 5c and Movie S7, ESI†), indicating
that Form II has two-dimensional elastic mechanical properties.
Moreover, it was found that the elastic mechanical behavior
of Form II is related to the crystal size. As shown in Fig. 5d and
Movie S8 (ESI†), the much thinner crystal of Form II can easily
Fig. 4 Schematic illustration of the reversible photoinduced bending
motion of two polymorph crystals. (a) Overlaid molecular packing of Form
I (blue) and cis-isomer (green) on the (020) face and perpendicular to the
(020) face, respectively. (b) Overlaid molecular packing of Form II (red) and
cis-isomer (green) on the (012) face and perpendicular to the (012) face,
respectively. The colored arrows indicate the direction of dimension
changes. For clarity, all hydrogen atoms are omitted.
Fig. 5 (a) Brittle breaking of Form I crystal. Elastic bending of the same
Form II crystal from (b) the (012) face and (c) the (020) face. (d) Plastic
deformation of the much thinner Form II crystal.
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 8294–8301 | 8297

View Article Online
Journal of Materials Chemistry C
Paper
exceed the threshold of elastic deformation and undergo plastic
deformation when subjected to an external force. It was clearly
observed that the thinner acicular crystal was elongated along
its longest axis when it bent into an arc shape upon exerting
pressure. Upon removal of external force, the deformed crystal
gradually and slowly returned to its original shape but cannot
be completely recovered, which is quite different from the
fragmentation of Form I. However, the crystal would break into
two pieces rather than deform permanently when it was bent
beyond a threshold limit.
The surface morphology of Form II crystals was characterized
by scanning electron microscopy (SEM), as shown in Fig. 6. The
straight region of the crystals has a smooth surface, and one
terminal is thicker than the other due to the growth direction of
the crystals. In contrast, when one crystal was bent to the critical
point of fracture, the thickness of the bent section in the middle
was 12.66 mm, which is narrower than the straight section in
both terminals. In addition, stretching stripes appeared on the
surface of the bent section, while the straight sections still
remained smooth. These results indicate that the lattice of
Form II is elongated parallel to the long axis of the acicular
crystal and compressed in the vertical direction during bending
deformation.
Fig. 7 (a) An optical microscopy image showing one bent Form II crystal
and the locations where the micro-Raman spectra were recorded
(crosses). (b) Raman spectra were recorded for the concave (cc), convex
(cx) and straight (s) regions of the bent Form II crystal.
It is expected that both elongated and compressed deformations
of the lattice of Form II crystals would influence the strength of
the intermolecular interactions, which can be examined by
micro-Raman spectroscopy at the molecular level (Fig. 7).47 interactions between the layers was probed by the position of
The strength of the pꢀ ꢀ ꢀp stacking interactions was probed by the in-plane d(C–NQN) mode at 1132 cmꢂ1. Its intensity
the position of the out-of-plane d (C–NQN) mode55 at 1146 cmꢂ1
.
increased simultaneously on the cx and cc sides relative to
Its intensity decreased on the convex (cx) side relative to the the straight section, demonstrating that the C–Hꢀ ꢀ ꢀN and C–
straight section, indicating that the pꢀ ꢀ ꢀp interactions become Hꢀ ꢀ ꢀC interactions become stronger after bending deformation,
weaker after bending deformation, possibly in response to the which is consistent with contraction between the layers during
expansion on the cx side. On the other hand, its intensity did the bending process.
not increase much on the concave (cc) side, probably because
To further investigate the relationship between the crystal
this mode is much more sensitive to tensile stress than structure and the mechanical property quantitatively, nanoin-
to compression. The strength of the C–Hꢀ ꢀ ꢀN and C–Hꢀ ꢀ ꢀC dentation tests were performed on both polymorphs. In Form I,
the elastic modulus and hardness of the (020) surface are
199.16 ꢃ 23.81 MPa and 15.71 ꢃ 8.06 MPa, respectively
(Fig. 8a), which are consistent with those of the previously
reported organic molecular materials.56 The occurrence of pop-
ins is possibly due to a sudden slip during the breaking of the
Form I crystal. Form II is comparatively softer in nature. Its
elastic modulus is 5.42 ꢃ 1.88 MPa and its hardness is only
Fig. 8 Representative load–depth (P–h) curves obtained from nanoin-
Fig. 6 SEM images of one bending Form II crystal (a) and the corres-
ponding magnification (b–d).
dentation on (a) the (020) face of Form I and (b) the (012) face of Form II
crystals.
8298 | J. Mater. Chem. C, 2021, 9, 8294–8301
This journal is © The Royal Society of Chemistry 2021

View Article Online
Paper
Journal of Materials Chemistry C
1.74 ꢃ 0.45 MPa (Fig. 8b), which are in the same order of to it (Fig. 9c). Particularly, there is a distinct blue region on the
magnitude as those of soft materials such as rubber.57 Further- isosurface of the tape stacking. The Form I molecules are tightly
more, the elastic modulus of Form I is about 40 times higher connected along the (020) surface, resulting in a layered structure,
than that of Form II, and its hardness is about 10 times higher which may account for the high elastic modulus and hardness of
than that of Form II. These results indicate that the molecular nanoindentation. As these layers are weakly interconnected, they
arrangement has a significant influence on the elastic mechanical can therefore break irreversibly when the crystals are stressed in
properties of the crystals. Obviously, the cross-molecular arrange- the direction vertical to the molecular layers. In contrast, the Form
ment makes the crystals more flexible than the layered arrange- II molecules are connected isotropically by moderate interactions
ment. Considering that the mechanical properties along the long (Fig. 9d–f). The isosurfaces of the three directions are all large
axis of the crystal growth are not suitable for nanoindentation green sections with some small blue areas. These interactions are
tests, we measured the elastic modulus of Form II by stretching weak enough to achieve free molecular motion for releasing the
three individual single crystals (see Table S5, ESI†) by dynamic stress during deformation, yet strong enough to reform after the
mechanical analysis (DMA). The storage modulus was found to be removal of external force. The flexibility makes the elastic modulus
in the range of 14.479–16.159 MPa, and the loss modulus was in and hardness of Form II much smaller than those of Form I. The
the range of 3.745–11.070 MPa. The loss tangent (tan d) was same conclusion can be drawn from the energy framework
determined to be 0.259–0.685, suggesting that most of the energy analysis: the interactions in Form I are highly anisotropic
generated by the external force is stored as potential energy, (Fig. S21a, ESI†) and the interactions in Form II are almost
while another small part is dissipated as thermal energy during isotropic (Fig. S21b, ESI†).
stretching. These results are consistent with the phenomena of
plastic deformation of the thinner crystal in Form II.
Photoluminescence behaviors.
Such difference in elastic mechanical properties can be In addition to the different photoinduced deformations and
attributed to the weak intermolecular interactions, which can mechanical properties, the two different polymorphs also exhibited
be visualized by the independent gradient model (IGM) different photoluminescence behaviors. Generally, crystals with a
method, used in the cluster of the central molecule with other higher crystallinity have a narrower full width at half maximum
surrounding molecules within 3.8 Å. As shown in Fig. 9, the (FWHM) of the characteristic peaks in their PXRD. Here, we chose
intermolecular interactions around the central molecule (yel- the crystals with a higher crystallinity by comparing the FWHM to
low) are divided into three parts along the orthogonal direction, suppress the isomerization of AzC2 molecules as much as
respectively: forming pꢀ ꢀ ꢀp stacking with the top and bottom possible, so as to investigate the photoluminescence behavior
molecules (purple), forming tape stacking with the molecules of both polymorphs.
on the side (blue), and forming a slip plane with the head and
As mentioned above, molecular azobenzene is a non-emissive
tail molecules (pink). The large green sections of the isosurface p-conjugated group since the energy associated with the excited
should be interpreted as dispersion interactions, and the blue state is mainly dissipated for photoisomerization instead of
sections should be considered as hydrogen bond interactions. emission.48–50 In order to investigate the ability of photoisomeriza-
Obviously, the crystal of Form I shows high anisotropy, and the tion, the absorption spectra of AzC2 in the free state (solution) and
intermolecular interactions parallel to the poked (020) surface the crystalline state were characterized before and after irradiation,
(Fig. 9a and b) are significantly greater than those perpendicular respectively. In THF, the free AzC2 compound underwent effective
trans-to-cis isomerization upon UV irradiation (Fig. S22a, ESI†).
The maximum absorption peak at 365 nm quickly decreased and
shifted to 325 nm, and the peak at 448 nm increased after 3 s of UV
irradiation (80 mW cmꢂ2), corresponding to the (p - p*) and
(n - p*) excitation, respectively. Compared to the free state, the
UV-vis absorption spectra of the block Form I and thicker Form II
crystals after irradiation remained almost unchanged before and
after irradiation (Fig. S22b and c, ESI†). These results indicate that
the ability of isomerization of the azobenzene crystals with higher
crystallinity is suppressed since large conformational change
requires sufficient free space.
In this case, however, weak photoluminescence was detected
in both polymorphic crystals with higher crystallinity. Interestingly,
Fig. 9 Visualization of intramolecular interactions of (a–c) Form I and (d–
far different fluorescence emission properties were observed from
Form I crystals and Form II crystals due to their different molecular
packings (Fig. 10a and b). Green-yellow photoluminescence was
observed from Form I crystals under UV irradiation with emission
maxima at 564 nm (the absolute fluorescence quantum yield
FPL = 5.79%, lex = 325 nm) (Fig. 10c, blue solid line). In
comparison, Form II crystals exhibited a very weak red emission
f) Form II. The intermolecular interactions around the central molecule
(yellow) are divided into three parts along the orthogonal direction and
investigated respectively: (a and d) forming pꢀ ꢀ ꢀp stacking with the top and
bottom molecules (purple); (b and e) forming tape stacking with the
molecules on the side (blue); (c and f) forming a slip plane with the head
and tail molecules (pink). The large green sections of the isosurface should
be interpreted as dispersion interactions, and the blue sections should be
considered as hydrogen bond interactions.
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 8294–8301 | 8299

View Article Online
Journal of Materials Chemistry C
Paper
and optical behaviors were attributed to different molecular
packings rather than molecular conformations. Upon UV
irradiation, the thin microcrystals of Form I bent away from
the light source while Form II bent towards the light source,
because the photoisomerization-induced lattice changes on the
irradiated surface are different in both polymorphs. Upon
exertion of mechanical stress, Form I tends to break whereas
Form II exhibits elastic deformation, and the elastic modulus of
Form I is about 40 times higher than that of Form II, owing to
remarkable different intermolecular interactions generated
by different molecular packings. Furthermore, when the iso-
merization of AzC2 molecules is suppressed in the crystalline
state, the block crystal of Form I exhibits a relatively strong green-
yellow emission while a very weak red fluorescence emission was
observed for the thicker Form II crystal under the same irradiated
conditions. Moreover, the crystal structure of each polymorph is
stable at high temperatures and even after continuous grinding
for 10 min, which increases the durability of the crystals in
practical applications. The results in this work provide an effective
strategy for designing and preparing multiple stimuli-responsive
crystal materials towards desired potential applications.
Fig. 10 Fluorescent microscopy images of (a) Form I crystals and (b) Form
II crystals with higher crystallinity. (c) Absorption and photoluminescence
spectra displayed as solid and dashed lines, respectively.
at 616 nm (FPL = 1.53%) (Fig. 10c, red solid line) under the same
irradiation condition (lex = 325 nm). The CIE 1931 chromaticity
diagrams (Fig. S23, ESI†) indicate that the coordinate of
Form I is (0.402, 0.597) and that of Form II is (0.683, 0.317),
respectively. There is a blue-shift for the absorption spectrum of
Form I crystals (Fig. 6c, blue dash line) compared with that of
Form II crystals (Fig. 6c, red dash line), which is consistent with
the stronger p–p stacking structures in the Form I crystals.
Furthermore, both polymorphs exhibit very large Stokes shifts
(256 nm for Form I and 303 nm for Form II), which may be
because the nonradiative process makes them lose parts of the
excitation energy. Obviously, the Stokes shift of Form II crystals
is larger than that of Form I crystals. In the meanwhile, the FPL
of Form I crystals is about 4 times higher than that of Form II
crystals, indicating that the luminescence ability of Form I
crystals is better than that of Form II crystals. A possible reason
for the difference in luminescence between these two poly-
morphic crystals is that the extent of photoisomerization of
Form I is lower than that of Form II due to its fewer void space
along the two orthogonal directions (Fig. S24, ESI†). Therefore,
the non-radiative process of Form I is possibly lower than that of
Form II, thus exhibiting a higher radiative phenomenon.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research is financially supported by the Tianjin Natural
Science Foundation (grant number 18JCZDJC38100). Y. Hao
thanks Dr S. Wu for aiding in the AFM experiments, and
Dr T. Xue for helping in the micro-Raman experiments.
Notes and references
1 M. Irie, S. Kobatake and M. Horichi, Science, 2001, 291,
1769–1772.
2 L. Zhu, R. O. Al-Kaysi and C. J. Bardeen, J. Am. Chem. Soc.,
2011, 133, 12569–12575.
3 H. Koshima, K. Takechi, H. Uchimoto, M. Shiro and
D. Hashizume, Chem. Commun., 2011, 47, 11423–11425.
4 R. O. Al-Kaysi and C. J. Bardeen, Adv. Mater., 2007, 19, 1276–1280.
5 H. Koshima, N. Ojima and H. Uchimoto, J. Am. Chem. Soc.,
2009, 131, 6890.
6 H. Koshima and N. Ojima, Dyes Pigm., 2012, 92, 798–801.
7 S. C. Cheng, C. H. Wang, Y. C. Lin, Y. Tsuchido, Y. Suzaki,
Y. Sei, T. S. Kuo and M. Horie, ACS Appl. Mater. Interfaces,
2020, 12, 50002–50010.
8 R. Medishetty, A. Husain, Z. Bai, T. Runcevski, R. E.
Dinnebier, P. Naumov and J. J. Vittal, Angew. Chem., Int.
Ed., 2014, 53, 5907–5911.
Conclusions
In summary, the influence of crystal polymorphs of an azobenzene
derivative (AzC2) on the photoinduced deformation, mechanical
behaviour and photoluminescence properties was studied. By
regulating the crystallization conditions, two crystal polymorphs
of herringbone structure Form I and crossing structure Form II
were successfully developed and prepared. These two crystal
polymorphs show a remarkable difference in photomechanical
9 F. Tong, M. Al-Haidar, L. Zhu, R. O. Al-Kaysi and C. J.
Bardeen, Chem. Commun., 2019, 55, 3709–3712.
bending motions, elastic mechanical properties and photo- 10 P. Naumov, S. Chizhik, M. K. Panda, N. K. Nath and
luminescence under the same stimulus. These distinct mechanical
E. Boldyreva, Chem. Rev., 2015, 115, 12440–12490.
8300 | J. Mater. Chem. C, 2021, 9, 8294–8301
This journal is © The Royal Society of Chemistry 2021

View Article Online
Paper
Journal of Materials Chemistry C
11 S. Li and D. Yan, Adv. Opt. Mater., 2018, 6.
34 C. R. Taylor and G. M. Day, Cryst. Growth Des., 2018, 18,
12 M. Dharmarwardana, S. Pakhira, R. P. Welch, C. Caicedo-
892–904.
Narvaez, M. A. Luzuriaga, B. S. Arimilli, G. T. McCandless, 35 L. Yu, Acc. Chem. Res., 2010, 43, 1257.
B. Fahimi, J. L. Mendoza-Cortes and J. J. Gassensmith, J. Am. 36 K. B. Raju, S. Ranjan, V. S. Vishnu, M. Bhattacharya,
Chem. Soc., 2021, 143, 5951–5957.
B. Bhattacharya, A. K. Mukhopadhyay and C. M. Reddy,
13 M. Dharmarwardana, B. S. Arimilli, M. A. Luzuriaga, S. Kwon,
Cryst. Growth Des., 2018, 18, 3927–3937.
H. Lee, G. A. Appuhamillage, G. T. McCandless, R. A. 37 C. Cappuccino, L. Catalano, F. Marin, G. Dushaq, G. Raj,
Smaldone and J. J. Gassensmith, CrystEngComm, 2018, 20,
M. Rasras, R. Rezgui, M. Zambianchi, M. Melucci, P. Naumov
6054–6060.
and L. Maini, Cryst. Growth Des., 2019, 20, 884–891.
14 M. Dharmarwardana, R. P. Welch, S. Kwon, V. K. Nguyen, 38 Z. Lin, X. Mei, E. Yang, X. Li, H. Yao, G. Wen, C.-T. Chien,
G. T. McCandless, M. A. Omary and J. J. Gassensmith, Chem.
Commun., 2017, 53, 9890–9893.
15 F. Terao, M. Morimoto and M. Irie, Angew. Chem., Int. Ed.,
2012, 51, 901–904.
16 X. Dong, T. Guo, D. Kitagawa, S. Kobatake, P. Palffy-
Muhoray and C. J. Bardeen, Adv. Funct. Mater., 2019, 30.
17 J. Lee, S. Oh, J. Pyo, J. M. Kim and J. H. Je, Nanoscale, 2015,
7, 6457–6461.
18 K. J. Chen, Y. C. Tsai, Y. Suzaki, K. Osakada, A. Miura and
M. Horie, Nat. Commun., 2016, 7, 13321.
19 H. Wang, X. Gu, R. Hu, J. W. Y. Lam, D. Zhang and B. Z.
Tang, Chem. Sci., 2016, 7, 5692–5698.
20 D. Kitagawa and S. Kobatake, Chem. Commun., 2015, 51,
4421–4424.
21 R. O. Al-Kaysi, F. Tong, M. Al-Haidar, L. Zhu and C. J.
Bardeen, Chem. Commun., 2017, 53, 2622–2625.
22 D. Kitagawa, K. Kawasaki, R. Tanaka and S. Kobatake, Chem.
Mater., 2017, 29, 7524–7532.
23 A. Hirano, T. Hashimoto, D. Kitagawa, K. Kono and
S. Kobatake, Cryst. Growth Des., 2017, 17, 4819–4825.
24 T. Kim, L. Zhu, L. J. Mueller and C. J. Bardeen, J. Am. Chem.
Soc., 2014, 136, 6617–6625.
T. J. Chow and Q. Ling, CrystEngComm, 2014, 16, 11018–11026.
39 D. Liu, C. Li, S. Niu, Y. Li, M. Hu, Q. Li, W. Zhu, X. Zhang,
H. Dong and W. Hu, J. Mater. Chem. C, 2019, 7, 5925–5930.
40 H. Ito, M. Muromoto, S. Kurenuma, S. Ishizaka, N. Kitamura,
H. Sato and T. Seki, Nat. Commun., 2013, 4, 2009.
41 T. Taniguchi, H. Sugiyama, H. Uekusa, M. Shiro, T. Asahi
and H. Koshima, Nat. Commun., 2018, 9, 538.
42 O. S. Bushuyev, T. A. Singleton and C. J. Barrett, Adv. Mater.,
2013, 25, 1796–1800.
43 T. Taniguchi, J. Fujisawa, M. Shiro, H. Koshima and
T. Asahi, Chemistry, 2016, 22, 7950–7958.
44 N. K. Nath, L. Pejov, S. M. Nichols, C. Hu, N. Saleh, B. Kahr
and P. Naumov, J. Am. Chem. Soc., 2014, 136, 2757–2766.
45 O. S. Bushuyev, A. Tomberg, T. Friscic and C. J. Barrett,
J. Am. Chem. Soc., 2013, 135, 12556–12559.
46 Y. Hao, S. Huang, Y. Guo, L. Zhou, H. Hao, C. J. Barrett and
H. Yu, J. Mater. Chem. C, 2019, 7, 503–508.
47 A. Worthy, A. Grosjean, M. C. Pfrunder, Y. Xu, C. Yan, G. Edwards,
J. K. Clegg and J. C. McMurtrie, Nat. Chem., 2018, 10, 65–69.
48 M. Yamauchi, K. Yokoyama, N. Aratani, H. Yamada and
S. Masuo, Angew. Chem., Int. Ed., 2019, 58, 14173–14178.
49 M. Han, S. J. Cho, Y. Norikane, M. Shimizu, A. Kimura,
T. Tamagawa and T. Seki, Chem. Commun., 2014, 50,
15815–15818.
25 J. Guo, J. Fan, X. Liu, Z. Zhao and B. Z. Tang, Angew. Chem.,
Int. Ed., 2020, 59, 8828–8832.
26 S. Hayashi and T. Koizumi, Angew. Chem., Int. Ed., 2016, 55, 50 M. R. Han, Y. Hirayama and M. Hara, Chem. Mater., 2006,
2701–2704. 18, 2784–2786.
27 P. Commins, M. B. Al-Handawi, D. P. Karothu, G. Raj and 51 Z. Zhao, H. Zhang, J. W. Y. Lam and B. Z. Tang, Angew.
P. Naumov, Chem. Sci., 2020, 11, 2606–2613. Chem., Int. Ed., 2020, 59, 9888–9907.
28 P. Gupta, T. Panda, S. Allu, S. Borah, A. Baishya, A. Gunnam, 52 J. Mei, N. L. Leung, R. T. Kwok, J. W. Lam and B. Z. Tang,
A. Nangia, P. Naumov and N. K. Nath, Cryst. Growth Des.,
2019, 19, 3039–3044.
29 D. Kitagawa, H. Tsujioka, F. Tong, X. Dong, C. J. Bardeen
and S. Kobatake, J. Am. Chem. Soc., 2018, 140, 4208–4212.
30 G. Huang, Y. Jiang, S. Yang, B. S. Li and B. Z. Tang, Adv.
Funct. Mater., 2019, 29.
Chem. Rev., 2015, 115, 11718–11940.
53 A. Lynch, V. Verma, J. Zeglinski, P. Bannigan and Å. Rasmu-
son, CrystEngComm, 2019, 21, 2648–2659.
54 M. Yamada, M. Kondo, J. Mamiya, Y. Yu, M. Kinoshita,
C. J. Barrett and T. Ikeda, Angew. Chem., Int. Ed., 2008, 47,
4986–4988.
31 G. Liu, J. Liu, X. Ye, L. Nie, P. Gu, X. Tao and Q. Zhang, 55 C. M. Stuart, R. R. Frontiera and R. A. Mathies, J. Phys.
Angew. Chem., Int. Ed., 2017, 56, 198–202. Chem. A, 2007, 111, 12072–12080.
32 Y. Ye, L. Gao, H. Hao, Q. Yin and C. Xie, CrystEngComm, 56 G. R. Krishna, R. Devarapalli, R. Prusty, T. Liu, C. L. Fraser,
2020, 22, 8045–8053. U. Ramamurty and C. M. Reddy, IUCrJ, 2015, 2, 611–619.
33 P. Gupta, D. P. Karothu, E. Ahmed, P. Naumov and N. K. 57 A. Rouilly, L. Rigal and R. G. Gilbert, Polymer, 2004, 45,
Nath, Angew. Chem., Int. Ed., 2018, 57, 8498–8502.
7813–7820.
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 8294–8301 | 8301