Fabrication of Photoactuators: Macroscopic Photomechanical Responses of Metal–Organic Frameworks to Irradiation by UV Light

Article abstract of DOI:10.1002/anie.201903757

Photoreactive olefinic species are incorporated into a metal–organic framework (MOF), [Zn(bdc)(3-F-spy)] (1). Single crystals of 1 are shown to undergo three types of photomechanical macroscopic deformation upon illumination by UV light. To demonstrate the practical potential of this system, the inclusion of 1 in a PVA (polyvinyl alcohol) composite membrane, by exploiting hydrogen-bonding interactions, is presented. Using this composite membrane, the amplification of mechanical stress to achieve macroscopic actuation behavior is demonstrated. These results pave the way for the generation of MOF-based soft photoactuators that produce clearly defined mechanical responses upon irradiation with light. Such systems are anticipated to have considerable potential in photomechanical energy harvesting and conversion systems.

Full text of DOI:10.1002/anie.201903757

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Accepted Article
Title: Fabrication of New Photoactuators: Macroscopic
Photomechanical Responses of Metal-Organic Frameworks to
Irradiation by UV Light
Authors: Yi-Xiang Shi, Wen-Hua Zhang, Brendan F. Abrahams, Pierre
Braunstein, and Jian-Ping Lang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903757
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10.1002/anie.201903757
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Fabrication of New Photoactuators: Macroscopic
Photomechanical Responses of Metal-Organic Frameworks to
Irradiation by UV Light
Yi-Xiang Shi, Wen-Hua Zhang, Brendan F. Abrahams, Pierre Braunstein, and Jian-Ping Lang*
Abstract: Photoactuators capable of converting light energy into
mechanical work at the macroscopic level are of considerable
interest in a wide variety of soft robotic systems. Although stimulus-
responsive materials, which result in geometric deformations, have
been developed it is only recently that photo-triggered actuators,
which allow self-propelled motion, have been included in metal-
organic frameworks (MOFs). Herein we describe the incorporation of
photoreactive olefinic species into a MOF, [Zn(bdc)(3-F-spy)] (1).
response is of great relevance to both fundamental research and
practical applications toward the prompt and effective
conversion of external stimuli to mechanical movement.[12]
As a branch of stimuli-responsive actuation, photoactuators
is particularly attractive for energy conversion as they can be
triggered externally in a non-invasive, non-contact manner, and
as such they are particularly attractive for practical
applications.[13] Recent progress in the field of photoactuation
has focused on deformation by photochemical reactions and
photothermal effects.[10a,14] The actuation mechanism for
Single crystals of
1 are shown to undergo three types of
photomechanical macroscopic deformation upon illumination by UV
light. In an effort to demonstrate the practical potential of this system, photoinduced motion is well established for photo-responsive
the inclusion of 1 in a PVA (polyvinyl alcohol) composite membrane,
by exploiting hydrogen-bonding interactions, is presented. Using this
novel composite membrane the amplification of mechanical stress to
achieve macroscopic actuation behavior is demonstrated. Our
results pave the way for the generation of MOF-based soft
photoactuators that produce clearly defined mechanical responses
upon irradiation with light. Such systems are anticipated to have
considerable potential in photomechanical energy harvesting and
conversion systems.
molecules including diarylethene,[15] anthracene,[16] Schiff
bases[17] and azobenzene[18] which undergo reversible shape
deformations upon photoirradiation. As
a
consequence,
actuators have been designed and manufactured by
incorporating photo-responsive molecules into liquid crystalline
polymer networks, hydrogels or molecular crystals. Within these
matrices the photoresponsive molecules serve as molecular
photo-switches that are able to produce rapid and reversible
macroscopic motion upon irradiation.[19] Despite these
responses upon exposure to light, the anisotropy of macroscopic
mechanical movement is strongly associated with the degree of
crystallinity, free volume, and molecular orientation of the
photoresponsive molecule.[20] Thus, the design and fabrication of
anisotropic photoactuation systems by a bottom-up approach
leading to a hierarchical structure that can convert photonic
energy into mechanical energy at the macroscopic scale still
remains a formidable challenge.[5b,21]
Actuation materials that effectively convert external controllable
stimuli to mechanical energy have recently attracted attention in
the fields of soft robotics,[1] biomedicince,[2] biomimetic systems[3]
and molecular machines[4] with synthetic materials having the
potential to undergo intended deformations involving twisting,
bending, swimming, and elongation/contraction.[5] Various types
of smart actuation driven by external physical or chemical stimuli
(e.g. heat,[6] electricity,[7] magnetism,[8] humidity[9] or light[10]
)
have been extensively investigated and developed. The design
and fabrication of a smart actuation that can achieve multiple
desired deformations and perform target functions represent
essential prerequisites.[4a,11] Accordingly, the manipulation and
deformation of actuation associated with rapid and precise
[*] Y. X. Shi, Prof. W. H. Zhang, Prof. J. P. Lang
College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, No.199 RenAi Road, Suzhou 215123, Jiangsu
(P. R. China)
E-mail: jplang@suda.edu.cn
Prof. J. P. Lang
State Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai
200032 (P. R. China)
Prof. B. F. Abrahams
School of Chemistry, University of Melbourne, Victoria 3010 (Australia)
Prof. P. Braunstein
Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, 4 rue
Blaise Pascal-CS 90032, 67081 Strasbourg (France)
Figure 1. Structural representation of photo-induced lattice contraction. a, b)
View along the b axis of 1 and 1A. (c) View of the head-to-tail molecular
packing of photoreactive ligands in 1. d) The structure of photodimerization
ligand in 1A. Sky blue arrows represent significant separations in 1 and 1A.
Supporting information for this article is given via a link at the end of
the document.
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Metal-organic frameworks (MOFs) constructed from metal-
containing secondary building units and organic linkers have
emerged as a class of hierarchically ordered porous crystalline
materials with widespread applications in the fields of gas
storage and separation,[22] chemical sensing,[23] drug delivery,[24]
heterogeneous catalysis[25] and solar energy harvesting[26] owing
much to their thermal and chemical stability, functional diversity
and structural tenability. The incorporation of photoactive units
into a MOF structure offers the prospect of producing a
crystalline material in which photoresponsive molecules are not
only arranged in a periodic manner but also aligned in a way that
will deliver macroscopic movement upon irradiation.[27] Herein
we report a self-assembly strategy for the fabrication of
hierarchical photoactuator system in which photoresponsive
units are incorporated into a MOF structure. Irradiation of
[Zn(bdc)(3-F-spy)] (1, 3-F-spy = 4-(3-fluorostyryl)pyridine; H2bdc
= 1,4-benzenedicaboxylic acid) with 365 nm light results in [2+2]
photodimerization reactions that in turn lead to geometrical
changes to individual crystals. Photomechanical twisting and
bending have been investigated in slender and rod-like crystals.
A detailed understanding of the photomechanical actuation
mechanism of single crystals of 1 has been obtained. Of
particular importance is the fabrication of a photoactuation
system involving a composite membrane formed from the
combination of PVA and 1. The membrane exhibits a clear
photomechanical response to irradiation with UV light. To the
best of our knowledge, the findings in the present work
demonstrate, for the first time, a photoactuation system based
on photoresponsive MOFs.
Colorless block crystals of 1 were derived from the
solvothermal reactions of ZnSO4·7H2O, H2bdc and 3-F-spy in
DMF-H2O mixed solvents at 145 °C for 12 h. The phase purity of
bulk crystalline material was well confirmed by comparison of
the experimental powder X-ray diffraction pattern with the
simulated powder X-ray diffraction pattern derived from the
single crystal data (Figure S1). Single crystal X-ray diffraction
(SCXRD) analysis revealed that 1 crystallized in the monoclinic
space group C2/c. The crystallographic asymmetric unit of 1 is
comprised of one Zn(II) ion, two distinct half bdc2− linkers and
one 3-F-spy ligand (Figure S2). The combination of Zn(II)
centres and bdc2- anions yields a polymeric structure in which
pairs of Zn(II) centres are linked by four carboxylate groups to
form a [Zn2(OOC)4] paddle-wheel type unit that serves as a 4-
connecting unit in a 2D square grid-type polymeric structure.
The coordination environment of each Zn centre is completed by
the pyridyl nitrogen atom of a 3-F-spy ligand to produce a
slightly distorted square pyramidal coordination geometry with
the four carboxylate oxygen atoms forming the square base and
the nitrogen donor of the 3-F-spy ligand occupying the apex of
the pyramid. With each of the Zn centres of the paddle-wheel
unit coordinated by a 3-F-spy ligand, the structure consists of a
2-D network with the terminal 3-F-spy ligand extending above
and below the sheet (Figures S3-S5). Each coordinated 3-F-spy
ligand passes through a (Zn2)4(bdc)4 rhombic-shaped hole of an
adjacent parallel sheet where it associates, in a head-tail
arrangement, with a 3-F-spy ligand originating from the opposite
direction. Intermolecular π–π interactions exist between the pair
of 3-F-spy ligands located within the rhombic-shaped hole of an
intermediate Zn2(bdc)2 sheet (Figures S6 and S7). The
alignment of the olefinic bonds, which are ca. 3.76 Å apart,
places them in an arrangement suitable for photodimerisation to
occur (Figure 1a, c).[28] Thermal gravimetric analysis (TGA) of 1
indicates no weight loss up to 300 °C under N2 after which,
thermal decomposition occurs (Figure S8a). In addition, no
appreciable N2 adsorption could be observed with a Brunauer–
Emmett–Teller (BET) specific surface area of 6.4 m2 g–1, which
is consistent with the calculated results of the total accessible
volumes by PLATON software (about 7.8%) (Figure S8b).
As part of an evaluation of the photoactivity of 1 the
ultraviolet–visible (UV-vis) absorption spectra of both 3-F-spy
and 1 were recorded. The UV-vis absorption spectra of 3-F-spy
and 1 indicated a broad absorption band in the range 200−425
nm (Figure S9). A significant enhancement of the absorbance of
1 was observed compared to the free ligand (3-F-spy) is
attributed to either metal to ligand or ligand to ligand charge
transfer (LLCT/MLCT). The broad absorption band of
1
containing photoreactive units encouraged us to investigate its
photo-responsive behavior. Upon irradiation at 365 nm
wavelength light equipped with internal heat filter at room
temperature on its (0-10) plane for ca. 1 min, the aligned 3-F-spy
ligands undergo photodimerisation to form a rctt-1,3-bis(4-
pyridyl)-2,4-bis(3-fluorophenyl)cyclobutane (rctt-ppcb) which
provides a covalent link between Zn2bdc2 sheets. Each rctt-ppcb
ligand passes through a (Zn2)4bdc4 square hole and thus the 2D
structure of 1 is transformed into two crystallographically
equivalent interpenetrating frameworks of composition
[Zn(bdc)(rctt-ppcb)0.5] (1A) in a single-crystal to single-crystal
(SCSC) transformation (Figure 1b, d, Figure S10 and Figure
S11). The transformation of 1 to 1A results in a contraction of
the unit cell volume by 6.1% (3908.3 Å3 to 3669.7 Å3). In regards
to the cell axes, the most significant decrease is in the a axis
(6.67%) whilst the b axis shows only a modest decrease (0.87%).
A slight increase is noted in the c axis from 18.73 to 18.87 Å.
This is in contrast with the anti-parallel arrangement of the
unreacted ligands depicted in Figure 1c. The dimerisation of the
3-F-spy ligands is accompanied by geometrical changes in the
Zn2(bdc)2 sheets. Firstly, the aromatic rings which adopted a
conformation in which they were almost normal to the mean
plane of the Zn2(bdc)2 network in 1 are significantly inclined in
1A (Figure S12). In addition, the Zn2(bdc)2 network undergoes a
shear distortion with Zn···Zn···Zn angles within a rhombic cavity
changing from 75.41 and 104.59° in 1 to 71.3 and 108.62° in 1A.
The type of anisotropic lattice contraction demonstrated in
the photo-induced transformation of 1 to 1A has been identified
in actuators that exhibit mechanical responses to external
stimuli.[29] In order to investigate whether the lattice contraction
that occurs when 1 is transformed to 1A is able to produce a
measurable macroscopic change, a long single crystal of 1 (0.11
 0.13  2.62 mm) was irradiated UV-light and the physical
appearance of the crystal was monitored. The changes
accompanying the exposure to UV light are depicted in Figure
2a and in Movie S1. Initially, the crystal bent towards the
incident 365 nm light, reaching a bending angle of approximately
80° within 1 second. The crystal then rapidly twisted into the
right-hand helical shape within 15 seconds as the photoreaction
involving the dimerisation of coordinated 3-F-spy ligands
transforms the 2D structure of 1 to the interpenetrating 3D
structure of 1A. Noticeably, the crystal retained its helical
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morphology when the light is switched off (Figure S14). Further
irradiation experiments revealed that the generation of the right-
handed helix morphology was independent of the direction of the
light source. The rapid and substantial change in the crystal
shape (80° s–1) is attributed to the lattice contraction of 6.1%
upon irradiation and is in contrast to photoresponsive shape-
ray analysis reveals that 1B adopts the same monoclinic space
group, C2/c, as both 1 and 1A (Table S3). The crystal structure
refinement reveals the presence of both the unreacted and
dimerised 3-F-spy ligands which appear as overlapping
molecules each with partial occupancies. Refinement of site
occupancies indicate that approximately 23% of the ligands
have undergone dimerisation (Figure 3). The partial conversion
of 1 to 1A results in a contraction mainly along the a axis (1.14%)
which is accompanied by a unit cell volume contraction of about
1.26% from 3908.3 Å3 to 3859.1 Å3 (Figure 3b). It is proposed
that the partial photoconversion of the crystal, as indicated by
the structure determination of 1B, is responsible for the
introduction of forces across interfacial regions within the crystal,
leading to the observed macroscopic deformations of the type
depicted in Figure 2.
memory polymer-based actuators[30] that exhibit
mechanical response upon irradiation.
a slower
Figure 2. Optical microscope images of photomechanical deformations of
crystal 1 induced by UV irradiation light. a‒c) The photo-induced helix, twisting
and splitting morphology transformation of single crystal 1 with difference size.
The crystal was fixed to a pin sample holder and UV irradiation was conducted
from the left side on the image.
Figure 3. Light-induced [2+2] cycloaddition reaction occurring within a single
crystal of 1. a‒c) Portions of the structures of 1, 1B and 1A with indicated
C···C distance. Sky blue and orange represent the structure of 3-F-spy and
rctt-ppcb, respectively. For clarity, hydrogen atoms have been omitted.
Previous work described in the literature indicates that the
magnitude of mechanical deformation of a photo-sensitive
crystal is sensitive to the dimensions of the crystal.[31] To
ascertain whether the extent and type of mechanical distortion is
affected by the crystal dimensions in the case of 1, an
investigation was undertaken whereby different sized crystals
were irradiated with UV light. As crystal of dimensions 0.18 
0.21  2.29 mm, which is significantly thicker than the crystal
depicted in Figure 2a, was found to bend towards the incident
light source before undergoing a transformation to a helically
twisted needle (Figure 2b, Figure S15 and Movie S2). When a
slightly thicker and longer crystal (0.26  0.21  3.25 mm) was
used, the crystal gradually bends away from the incident light.
With prolonged irradiation a sudden release of strain energy
resulted in the crystal splitting into several pieces as depicted in
Figure 2c, Figure S16 and Movie S3. It has been proposed that
these types of transformations result from the alleviation of long-
order interfacial strain in crystals arising from changes in
molecular structure that occur upon irradiation.[32] The results
imply that the observed transformation is a consequence of a
delicate balance of factors including crystal dimensions, tensile
stress and the exposure time to the incident light source.
In addition to single crystal data, in situ PXRD was
employed to confirm the transformation of 3-F-spy to rctt-ppcb
upon exposure to light. As shown in Figure 4, the sharp pristine
(022), (40-2), (22-1) and (221) peaks at 2 = 10.7°, 16.5°, 18.0°
and 20.6° diminish and almost completely disappear during
irradiation over a period 15 seconds, while the (200), (202) and
(20-2) peaks gradually split into two new peaks. However, two
distinct new peaks appeared at 2 = 19.5° and 21.4° over the
1
irradiation period (Figure S17). H or 13C NMR a sample of 1
after exposure for more than 40 seconds indicate complete
photochemical conversion (Figures S18-S21).
To elucidate the photomechanical actuation mechanism, a
high quality, single crystal of 1 was exposed to 365 nm light for
15 seconds. This irradiation leads to the generation of 1B which
may be considered to be a crystal representing an intermediate
form of 1 and 1A i.e. one in which only a proportion of the 3-F-
spy ligands has undergone photodimerisation. Single crystal X-
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Figure 4. In situ PXRD spectra of crystalline 1 as a function of irradiation time
at room temperature. The as-prepared 1 was converted to 1A phase by
irradiation for 20 seconds.
seconds upon exposure to 365 nm light irradiation (Movie S4).
To gain further insight into the kinematics and mechanism of the
photomechanical deformation, we quantified the bending angles
() as a function of exposure time (t) by employing a high-speed
video camera (1200 frames per second) which record a series of
photodeformation snapshot images (Figure 6b, Figure S25). As
evident in Figure 6c, the bending angle  increased from 0° to
58° within 12 seconds. The motion effectively stops after 20
seconds of irradiation. The bending process can be described
using the following equation,
Generally, inorganic and crystalline materials are brittle in
comparison with soft materials[33] and thus in isolation they are
unlikely to fulfill a useful technological role in which predictable
macroscopic responses to irradiation are required. We describe
here
a macroscopic photoactuation device in which the
photoactive MOF is combined with polyvinyl alcohol (PVA) to
form a composite membrane. It is known that, the orientation
and distribution of photo-responsive moieties in composite
membranes can play
a critical role in modulating the
Where k is the response time constant, t is the irradiation
time, and  is the actuation response time coefficient. Further
details on the modelling is presented in the Note S1.
photomechanical actuation. It is proposed that the hydrophilic
PVA polymer, with abundant OH groups, can interact with F
atoms of the MOF through the formation of OH···F hydrogen
bonds.[34a] The fabrication process for the photoactuator is
presented schematically in Figure 5. The homogeneous 1-PVA
composite membrane with uniform thickness (~3 μm) was
successfully constructed and characterized by scanning electron
microscopy (SEM) image and PXRD patterns (Figures 5b-d). In
the IR of 1-PVA, the OH stretching vibration was shifted to the
lower wavenumber at 3250 cm1 compared with that of pure
PVA (3404 cm-1), suggesting the formation of stronger OH···F
bonding interactions[34b] between the OH groups of PVA and the
F atoms of 3-F-spy ligands in 1 (Figure S22). The SEM image
with EDX of 1-PVA composite membrane indicated the
formation of a homogeneous composite membrane (Figures 5c,
S23 and S24). The PXRD patterns of the 1-PVA membrane
matched the combined patterns of PVA and 1 (Figure 5d).
Figure 6. Mechanism of the light-driven bent actuation. a) Time-dependent
photomechanical deformation of 1-PVA upon exposure to 365 nm light. b)
Schematic illustration of the experimental set-up including an indication of
bending angle  in schematic structure. c) A plot of the bending Vs exposure
time with a photograph of the membrane before and after irradiation (inset).
In order to obtain more insights into the bending deformation
behaviors of membrane, atomic force microscopy (AFM) was
employed to probe the surfaces of the 1-PVA and blended 1-
PVA membranes. The typical surface morphological evolution is
presented in Figure S26. Following irradiation of UV light, the
heterogeneous wrinkle patterns of the 1-PVA membrane
provided a contrast with the smooth surface of the pure
membrane (Figure S27). The surface of the irradiated
membrane has a root mean-square roughness of 201.1 nm
compared to the relatively smooth pure membrane with a
corresponding value of 5.1 nm. The surface roughness,
indicated by the AFM images, appears to reflect the
deformations resulting from the exposure to UV light.
Figure 5. Stimuli-responsive behaviour of 1-PVA membrane. a) Schematic
illustration of the fabrication process of 1-PVA photoactuator. b) Optical photo
showing the flexibility of the actuator. c) Scanning electron microscopy (SEM)
image of the 1-PVA membrane surface. d) PXRD profile of 1-PVA membrane.
In summary, we have succeeded in developing
a
photomechanical, single crystal actuator that exhibits fast
response time and high sensitivity. The photomechanical
behavior of this MOF-based crystal arises from the
photodimerisation of olefin molecules which participate in [2+2]
cycloaddition reactions. The photomechanical deformations
include bending, twisting and splitting. Detailed examination of
In order to examine the photomechanical behavior,
polymeric strips (0.5 cm  2.0 cm) were exposed to 365 nm LED
light and the photomechanical deformation was recorded. As
illustrated in Figure 6a, the 1-PVA membrane bends rapidly
towards the incident light source with an angle of ~61° within 20
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Grants Nos. 21531006 and
21773163), and the State Key Laboratory of Organometallic
Chemistry of Shanghai Institute of Organic Chemistry (Grant No.
2018kf-05), the Priority Academic Program Development of
Jiangsu Higher Education Institutions and the Postgraduate
Research & Practice Innovation Program of Jiangsu Province for
financial support (KYCX18_2495).
Conflict of interest
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The authors declare no competing financial interests.
Keywords: Photoactuators • stimulus-responsive materials •
metal-organic frameworks • lattice contraction • photomechanical
deformation
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10.1002/anie.201903757
Angewandte Chemie International Edition
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COMMUNICATION
Yi-Xiang Shi, Wen-Hua Zhang, Brendan
F. Abrahams, Pierre Braunstein, and
Jian-Ping Lang,*
Page No. – Page No.
Fabrication of New Photoactuators:
Macroscopic Photomechanical
Responses of Metal-Organic
Frameworks to Irradiation by UV
Light
A self-assembly strategy for the fabrication of a hierarchical photoactuator system
in which a photoresponsive moiety is incorporated into a MOF crystal. Additionally,
we have successfully fabricated a photoactuation system which involves a MOF-
PVA composite membrane which exhibits a macroscopic response upon exposure
to UV light irradiation.
This article is protected by copyright. All rights reserved.