Shape-Persistent Actuators from Hydrazone Photoswitches

Article abstract of DOI:10.1021/jacs.8b11558

Interfacing molecular photoswitches with liquid crystal polymers enables the amplification of their nanoscale motion into macroscopic shape transformations. Typically, the mechanism responsible for actuation involves light-induced molecular disorder. Here, we demonstrate that bistable hydrazones can drive (chiral) shape transformations in liquid crystal polymer networks, with photogenerated polymer shapes displaying a long-term stability that mirrors that of the switches. The mechanism involves a photoinduced buildup of tension in the polymer, with a negligible influence on the liquid crystalline order. Hydrazone-doped liquid crystal systems thus diversify the toolbox available to the field of light-adaptive molecular actuators and hold promise in terms of soft robotics.

Full text of DOI:10.1021/jacs.8b11558

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Shape-Persistent Actuators from Hydrazone Photoswitches
Alexander Ryabchun,†,§ Quan Li, Federico Lancia, Ivan Aprahamian,*
‡,§
†
,‡
,
†
and Nathalie Katsonis*
†Bio-inspired and Smart Materials, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 207, 7500 AE Enschede,
The Netherlands
‡
Department of Chemistry, Dartmouth College, 6128 Burke Laboratory, Hanover, New Hampshire 03755, United States
*
S Supporting Information
We recently reported on a novel family of negatively
12,13
ABSTRACT: Interfacing molecular photoswitches with
liquid crystal polymers enables the amplification of their
nanoscale motion into macroscopic shape transforma-
tions. Typically, the mechanism responsible for actuation
involves light-induced molecular disorder. Here, we
demonstrate that bistable hydrazones can drive (chiral)
shape transformations in liquid crystal polymer networks,
with photogenerated polymer shapes displaying a long-
term stability that mirrors that of the switches. The
mechanism involves a photoinduced buildup of tension in
the polymer, with a negligible influence on the liquid
crystalline order. Hydrazone-doped liquid crystal systems
thus diversify the toolbox available to the field of light-
adaptive molecular actuators and hold promise in terms of
soft robotics.
photochromic hydrazones,
whose bistability motivated us
to explore the implication of this effect on polymer actuation.
Here we show how hydrazone H1 (Figure 1a) can be used to
create a large range of stable shapes. Moreover, we show that
upon photoisomerization, only a minor disruption of the LC
order occurs, indicating that in the polymer counterpart, the
origin of the light-induced shape transformation is not
confined to light-induced order disruption, which is typically
the major effect observed with azobenzenes. Instead, the major
contribution originates from mechanical stresses that are
generated by photoinduced molecular shape changes.
We designed hydrazone switches H1 and H2 bearing
acrylate units in the rotor and/or stator part to facilitate their
integration into cross-linked liquid crystalline polymers (Figure
1
a). The rod-like shape and rigidity increase compatibility with
the liquid crystals, so that they can be used as dopants with
concentrations up to 5−6 wt %. The switchable monomers
were synthesized using a straightforward procedure (Scheme
S1).
etting soft matter in motion with molecular machines will
S
result in dynamic materials that can amplify chemical or
light inputs into mechanical effects. However, harnessing the
motion of artificial molecular motors and switches in
1
The photoisomerization of H1 was studied using H NMR
in toluene-d . Irradiating a sample of Z-H1 (>99%) with λ =
8
1
producing useful work remains a daunting challenge that
410 nm light yields a photostationary state (PSS) consisting of
2
9
0
3
5% E-H2 (Figure S26), with a quantum yield of Φ
= 2.3 ±
requires interfacing these dynamic molecules with soft matter.3
E→Z
.1% (Figure S30). Irradiating the resulting solution with λ =
65 nm light yields a mixture consisting of 68% of the Z isomer
Polymeric systems have been used successfully in this context,
and it is expected that the development of such materials into
shape-shifting actuators that display a broad range of stable
shapes will support innovation in soft robotics, photonics,
microfluidics and biomedical applications. In this context,
the action of molecular photoswitches has yielded large and
complex shape transformations in liquid crystal (LC) polymer
networks, where the anisotropy and collective behavior of LCs
facilitate the emergence of macroscopic effects. Azobenzene
photoswitches have played a major role in the advancement of
research in this area; however, literature examples point at
limitations in terms of kinetics, mechanical function, multi-
at PSS with Φ
= 9.1 ± 0.4% (Figure S31). The
E→Z
14
compositions for different PSSs are gathered in Table S1.
The isomerization was followed by UV/vis spectroscopy
Figure 1b), and no photodegradation was observed up to
4
,5
(
ten cycles (Figure S29). The determination of the thermal
isomerization half-life of E-H1 in solution was hampered by
the presence of the thermally reactive acrylate moiety.
Therefore, we synthesized a model compound lacking the
polymerizable groups and determined its half-life in DMSO to
be 3435 ± 50 years at 298 K (Figures S32−38 and Table S2).
Before forming the liquid crystal polymer networks, we
investigated how the switch aligns in the LC, to quantify the
extent of dis/order stemming from each configuration. We
doped a nematic LC with H1 (1 wt %) and introduced the
resulting liquid crystal into a planar cell. Linearly polarized
light was used to characterize the orientation of both Z-H1 and
E-H1 in the LC host (Figure 1c). The angular dependence of
6
,7
8
stability and multiactuation modes. Azobenzene-based
actuators also lack light-generated shape stability, because in
9
most cases the Z isomer is transient under dark. While we and
others reported on LC polymers containing bistable fluori-
10,11
nated azobenzenes,
the field calls for broadening the
spectrum of photoswitches that can permanently lock the
actuation of soft materials and, more generally, for light-
responsive switches with new energy landscapes, that can
mediate the versatile behavior of moving polymers.
Received: October 26, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b11558
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 1. (a) Hydrazones H1 and H2. The stator and rotor are drawn in red and blue, respectively. l and l are the calculated lengths of the
Z
E
−
5
4
−1
−1
4
−1
switching moiety. (b) Absorbance spectra of H1 in toluene (2.6 × 10 M). εmax(Z-H1) = 2.86 × 10 M cm , ε (E-H1) = 2.03 × 10 M
max
−
1
cm . (c) Polarized absorbance spectra of Z-H1 and E-H1 in the unidirectionally aligned nematic host, and d) their angular dependence. Parallel
and perpendicular directions are defined with respect to the LC alignment. The dichroism of the guest molecules is calculated as D = (A − A )/
⊥
∥
(
A + A ). (e) Calculated structures of Z-H1 and E-H1. The acrylic moieties are omitted for clarity. The purple arrows show the direction of the
⊥ ∥
0
1
moment associated with the S to S electronic transition; their direction indicates how the switch interacts with polarized light.
Figure 2. (a) Molecular components of the LC prepolymer. (b) Photoactuation of the polymer ribbons incorporating H1. Thickness 25 μm. (c)
Representation of the photoinduced shape-shifting.
the polarized absorbance indicates that the rod-shaped Z-H1
aligns preferentially along the LC director (Figure 1d). In
contrast, after photoisomerization, the polar plot indicates that
the maximum polarized absorption of E-H1 becomes
perpendicular to the LC director (Figure 1d). In Z-H1, the
dichroism values of H1 measured before and after irradiation
are similar (Figure 1d). Further, the dichroism values before
and after photoisomerization are also similar for a well-known
molecular alignment probe that is embedded in the liquid
crystal (see Figure S40).
As polymerization preserves the alignment of liquid crystals,
we conclude that, also in the polymer network, the
photoisomerization does not yield any significant disorder. In
liquid crystal networks, two effects can contribute to shape
changes: (i) polymer tension as a result of changes in the shape
of constituent molecules, and (ii) shrinkage/elongation of the
network because of reduction of the order parameter. In the
absence of light-induced disorder, we conclude that in the case
of hydrazones, any shape-shifting event should be rooted in the
molecular shape changes of the switch primarily.
0
1
electronic transition moment associated with the S → S
electronic transition is oriented along the stator, while in E-H1
it is localized along the rotor moiety (Figure 1e). Given the L-
shape of E-H1, we conclude that even after switching, the
mesogenic stator remains aligned along the LC director.
Because only the rotor reorients its position in the liquid
crystal, we conclude that the liquid crystal order is only
minimally affected by photoconversion of the switch.
The absence of photoinduced disorder by hydrazone
switching in liquid crystals is confirmed by the fact that the
B
DOI: 10.1021/jacs.8b11558
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Next, we incorporated the switches covalently in a
15,16
photopolymerizable LC (Figure 2a)
After photopolymeri-
zation, polymer ribbons were cut along the molecular
alignment. When Z-H2 was incorporated as a side chain, no
shape changes were ever observed under illumination (Figure
S41). In contrast, when the switch was incorporated as a cross-
linker, large shape transformations were observed under
illumination (Figure 2b). This observation points at a shape-
shifting mechanism based on photogenerated stresses, because
only cross-linked molecular switches induce such a mechanical
17
action, in contrary to pendant side-chains. Also considering
that the liquid crystalline order is not disrupted by photo-
isomerization, we conclude that, in hydrazone LC networks,
the mechanical stresses are imposed by the action of the
switches on the polymer network. With this mechanism, the
shortening of the cross-linking length upon isomerization and
subsequent contraction of the network likely become key (the
difference in lengths is ≈0.6 nm).
In the initial stages of illumination (λ = 405 nm), a polymer
ribbon incorporating Z-H1 bends toward the light, and as
irradiation proceeds, the ribbon bends back away. Switching off
the light does not result in further shape changes, which
confirms that the shape transformation is not related to any
light-induced temperature increase. Overall, we propose that
the back-and-forth bending originates from the isomerization
of the switches that are located next to the irradiated surface,
whereas the switches on the other side of the sample do not
isomerize. Such a gradient photogenerates mechanical stress
Figure 3. (a) Bending angle of the ribbon as a response to
illumination. The molecules are aligned along the long axis of the
ribbon. The dark gray areas correspond to stages where photo-
generated stress builds up. (b) Stability of the curvature mirrors that
of the photogenerated gradient.
gradients in concentration translate into strong gradients of
mechanical strain, and thus into robust and chiral deformation.
We illuminated a hydrazone-containing polymer (Mixture 2,
Table S3) using a two-step procedure. A flat ribbon doped
with Z-H1 (Figure 4a) was first irradiated with visible light to
(Figure 2c). With further irradiation, and thanks to the
negative photochromicity of the switch (i.e., going from
colored to transparent upon isomerization), isomerization
develops throughout the entire thickness of the sample until
reaching PSS, and the ribbon bends back to its original shape.
At PSS, the ribbon displays a minor curvature in comparison
with the original flat shape: a fact that we attribute to a small
gradient in the cross-linking density, which results from
photopolymerization. Subsequent irradiation with UV light
(λ = 365 nm) results in a sharp gradient in Z-isomer
concentration, because of the efficient absorption of the E
isomer, and thus in the rapid bending away from the light
source. Further exposure to UV light completes the conversion
into the Z-isomer and causes unbending (Figure 3a).
We note that increasing the light intensity accelerates the
actuation process (Figure S42): with high intensities, a higher
curvature is reached, likely as a result of heating, that reduces
the order of the network.
The stability of both the Z- and E-forms allows fixing any
isomeric composition, simply by adjusting the total illumina-
tion time, resulting in photogenerated shapes that are stable for
months (Figure 3b, S43). Moreover, hydrazone-activated
polymers display large shape transformations and versatility
in their actuation modes, e.g., by making use of splay
organization (Figure S44, S45).
Figure 4. Macroscopic chirality resulting from the introduction of
gradients of mechanical strain. The white stripes indicate the
molecular orientation in the sample. Opposite sides of the ribbon
are colored in red and blue as a guide for the eyes. (a, b) Flat ribbon
with molecular alignment tilted ∼22° with respect to the long axis.
The stepwise illumination this flat ribbon includes exposure to blue
light, then to UV light. Side A was irradiated during photo-
polymerization and is thus characterized by a higher cross-linking
density. LH and RH indicate left- and right-handedness, respectively.
convert the switches into E-H1, followed by irradiation with
UV light. Because ε365(E-H1) > ε405(Z-H1), this two-step
procedure creates a sharp gradient in the concentration of Z-
H1. Overall, the process leads to a gradient of mechanical
strain, which eventually translates into an out-of-plane chiral
deformation, the handedness of which is determined by which
In LC polymer networks, gradients in molecular orientation
determine the shape of the polymer and, to a certain extent,
also its shape-transformation properties. The latter can also be
designed by gradients in cross-linking density, by patterning, or
18−21
22
using multilayers.
We sought to take advantage of the
side of the ribbon is illuminated (Figure 4a). A variety of
bistability of hydrazones and of their strong absorbance in both
forms, to photogenerate strong gradients of hydrazone isomer
concentrations, across the polymer film. As the switches
change in length upon photoswitching, the photogenerated
shapes can be prepared (Figure 4b), simply by cutting ribbons
at different angles with respect to the direction of molecular
alignment (Figure S46), and the steeper the gradient of
mechanical strain, the higher the curvature of the ribbons
C
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Journal of the American Chemical Society
Figure S47). All these photogenerated shapes demonstrate a
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long-term stability that mirrors that of the switches.
In conclusion, a hydrazone photoswitch was integrated into
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light with large shape transformations. The original photo-
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the photogenerated shapes, versatility in shape-shifting modes,
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b11558.
■
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AUTHOR INFORMATION
Corresponding Authors
n.h.katsonis@utwente.nl
ivan.aprahamian@dartmouth.edu
■
*
*
ORCID
4
Alexander Ryabchun: 0000-0001-9605-3067
Federico Lancia: 0000-0003-3075-1465
Ivan Aprahamian: 0000-0003-2399-8208
Nathalie Katsonis: 0000-0003-1054-6544
Author Contributions
(
Switch Architectures. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 9414−
9422.
(
§
9) For exception, see: Knie, C.; Utecht, M.; Zhao, F.; Kulla, H.;
These authors contributed equally.
Kovalenko, S.; Brouwer, A. M.; Saalfrank, P.; Hecht, S.; Bleger, D.
́
Notes
ortho-Fluoroazobenzenes: Visible Light Switches with Very Long-
The authors declare no competing financial interest.
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S. P.; Katsonis, N. Fluorinated Azobenzenes for Shape-Persistent
Liquid Crystal Polymer Networks. Angew. Chem., Int. Ed. 2016, 55,
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ACKNOWLEDGMENTS
We acknowledge financial support from the Volkswagen
Foundation and the Dutch Science Foundation NWO
■
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̧ r,
G.; Knie, C.; Bleger, D.; Hecht, S.; Jonkheijm, P.; Broer, D. J.;
́
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(
Projectruimte Grant 13PR3105). We thank Dr. D. Morozov
University of Jyvaskyla, Finland) for calculations of molecular
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