Crystal-to-Gel Transformation Stimulated by a Solid-State E→Z Photoisomerization

Article abstract of DOI:10.1002/anie.201907454

The molecule (E)-(5-(3-anthracen-9-yl-allylidene)-2,2-dimethyl-[1,3] dioxane-4,6-dione) (E-AYAD) undergoes E→Z photoisomerization. In the solid state, this photoisomerization process can initiate a physical transformation of the crystal that is accompanied by a large volume expansion (ca. 10 times), loss of crystallinity, and growth of large pores. This physical change requires approximately 10 % conversion of the E isomer to the Z isomer and results in a gel-like solid with decreased stiffness that still retains its mechanical integrity. The induced porosity allows the expanding gel to engulf superparamagnetic nanoparticles from the surrounding liquid. The trapped superparamagnetic nanoparticles impart a magnetic susceptibility to the gel, allowing it to be moved by a magnetic field. The photoinduced phase transition, starting with a compact crystalline solid instead of a dilute solution, provides a new route for in situ production of functional porous materials.

Full text of DOI:10.1002/anie.201907454

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
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Accepted Article
Title: Crystal-to-Gel Transformation Stimulated by a Solid-State E→Z
Photoisomerization
Authors: Christopher John Bardeen, Fei Tong, Shaolong Chen, Zhiwei
Li, Liu Mingyue, Rabih O Al-Kaysi, Umar Mohideen, and
Yadong Yin
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201907454
Angew. Chem. 10.1002/ange.201907454
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http://dx.doi.org/10.1002/ange.201907454

Angewandte Chemie International Edition
10.1002/anie.201907454
Crystal-to-Gel Transformation Stimulated by a Solid-State EZ
Photoisomerization
(1)
(2)
(1)
(2)
(3)*
Fei Tong , Shaolong Chen , Zhiwei Li , Mingyue, Liu , Rabih O. Al-Kaysi , Umar
(2)
(1)
(1)*
Mohideen , Yadong Yin , Christopher J. Bardeen
(1)
Department of Chemistry
University of California, Riverside
01 Big Springs Road
5
Riverside, CA 92521 (USA)
(2)Department of Physics & Astronomy
University of California, Riverside
9
00 University Ave
Riverside, CA 92521 (USA)
(3)
College of Science and Health Professions-3124,
King Saud bin Abdulaziz University for Health Sciences, and King Abdullah International
Medical Research Center, Ministry of National Guard Health Affairs, Riyadh 11426,
(Kingdom of Saudi Arabia)
Abstract
The molecule (E)-(5-(3-anthracen-9-yl-allylidene)-2,2-dimethyl-[1,3] dioxane-4,6-dione) (E-
AYAD) undergoes an EZ photoisomerization. In the solid-state, this photoisomerization can
initiate a physical transformation of the crystal that is accompanied by a large volume expansion
(
~10 times), loss of crystallinity, and the growth of large pores. This physical change requires
conversion of the ~10% conversion of the E to Z isomer and results in a gel-like solid with
decreased stiffness that still retains its mechanical integrity. The induced porosity allows the
expanding gel to engulf superparamagnetic nanoparticles in the surrounding liquid. The trapped
superparamagnetic nanoparticles impart a magnetic susceptibility to the gel, allowing it to be
moved by a magnetic field. The photoinduced phase transition, starting with a compact crystalline
solid instead of a dilute solution, provides a new route for the in situ production of functional
porous materials. (Keywords: photochemistry, crystal, gel, isomerization, magnetism)
1
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The concept of molecular crystals as stiff and fragile objects has been upended by recent
work demonstrating their ability to mechanically deform in response to external stimuli like heat,
electrical, chemicals, or light.[1] These mechanical motions reflect chemical changes inside the
crystal that generate internal stress. In the case of light exposure, a photochemical reaction can
create neighboring regions of reactant and product crystal phases, and the stress generated at the
interface between these incommensurate phases powers a change in crystal shape.[2] This
“
bimorph” mechanism underlies most photomechanical motions observed in crystals, like bending,
twisting, jumping and peeling.[3]
If the photochemical reaction does not induce a crystal-to-crystal phase change, a
mechanical response can still be observed. For example, when exposed to a pulse of visible light
crystalline nanowires composed of the photoreactive divinylanthracene molecule dimethyl-2(3-
anthracen-9-yl)allylidene)malonate (DMAAM) undergo a rapid coiling.[4] This coiling is driven
by EZ or ZE photoisomerization reactions and could be induced after 10-20% conversion of
the starting isomer as long as a surfactant like sodium dodecyl sulfate (SDS) is present in solution.
Without the surfactant present, the photoreaction proceeds much more slowly and no rapid shape
change or amorphization can be observed.[5] Subsequent characterization of this phenomenon
suggested that the surfactant enables the molecules at the crystal surface to isomerize more easily,
and the accompanying surface disruption then allows the surfactant to penetrate to the next layer
and accelerate the transition to an amorphous mixture. A similar photoinduced amorphization
process occurs in block-like microcrystals of DMAAM, in which the amorphous layers can be
sequentially peeled off using short pulses of visible light.[6] Unfortunately, in all these cases the
amorphous DMAAM photoproduct phase dissolved in the aqueous surfactant solution within 2-3
hours, preventing its detailed characterization.
2
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In order to preserve the amorphous product phase, we hypothesized that lowering the
conformational freedom of the ester groups by linking them together would reduce the molecular
solubility. These considerations led us to synthesize the new divinylanthracene, (E)-5-(3-
anthracen-9-yl-allylidene)-2,2-dimethyl-[1,3]dioxane-4,6-dione), denoted (E)-AYAD, whose
structure and photochemistry are shown in Scheme 1. This modification had the added benefit of
shifting the absorption spectrum further into the visible region, from 450 nm to 480 nm. In solution,
this molecule exhibits photophysical behavior similar to that of other divinylanthracene molecules
(Supporting Information), in which the E and Z photoisomers can be interconverted using different
wavelengths of light.[5-7] In Figure 1a, we show the results of an experiment in chloroform where
irradiation at 462 nm partially transforms the E isomer into the Z isomer, which can be partially
converted back into the E isomer by illumination by 365 nm light. In the solid-state, the crystal
packing of (E)-AYAD (Figure 1b) is similar to that of (E)-DMAAM, with the anthracene and
divinyl moieties too far apart and misaligned to support intermolecular [2+2] or [4+4]
photodimerization reactions.[8] The crystal packing leaves the E↔Z isomerization around the
anthracene-adjacent double bond as the only viable chemical reaction pathway. Similar to
DMAAM, irradiation of crystalline (E)-AYAD at 500 nm leads to rapid conversion of the E isomer
into the Z isomer (Figure 1c). The main difference between the two derivatives is that the solubility
of (E)-AYAD in alkane solvents is approximately 100× lower than that of DMAAM, as measured
by the absorption of a saturated solution in hexanes (Supporting Information, Figure S10).
As with DMAAM, (E)-AYAD microcrystals do not undergo any discernible shape change
when exposed to light in pure water, as shown in Figures 2a-d. Polarized light microscopy images
indicate a gradual loss of crystalline order after 30 min of irradiation at 532 nm with an intensity
-2
of 430 mW cm (Supporting Information, Movie S1 and Figure S11). When the same experiment
3
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is done on microcrystals in the presence of a surfactant (0.025 M SDS), the crystal rapidly expands
into a gelatinous mass (Figures 2e-h). This expansion is accompanied by a loss of the sharp edges
and the optical birefringence (Supporting Information, Figure S12). Videos of this expansion are
provided in the Supporting Information (Movies S2 and S3). The light-induced morphology
o
changes are not due to simple melting. First, the melting point of (E)-AYAD is 231-234 C, so a
temperature increase of this magnitude would vaporize the surrounding water. In the absence of
o
light, the crystal suspension can be heated to around 100 C without any change in shape
(Supporting Information, Figure S13). This temperature is much higher than that caused by the
microscope lamp illumination. Second, there is a clear boundary that expands outward, albeit in
random directions as the shape distorts, with no indication of liquification that would lead to the
formation of hydrophobic droplets or tendrils. There is also no sign of fragmentation or the
detachment of smaller particles during the shape change. Third, when the light is removed, the
expansion stops immediately, maintaining its shape without collapsing into a spherical droplet to
minimize surface area. Simple heating would lead to shrinkage after removal of the light as the
object cools. Lastly, the total volume occupied by the gelatinous product phase is roughly 10×
greater than that of the original crystal. Melting without solvent incorporation would not be
expected to generate such a large volume change, as the solid and melt densities are typically
within 10% of each other.
Instead of melting, the morphology changes in Figures 2e-h are consistent with a crystal-
to-gel transition, in which the solid becomes porous and incorporates the solvent into its interior.
Scanning electron microscopy (SEM) images of crystals before and after illumination confirm that
the smooth surfaces of the unreacted crystal transform into an irregular sponge-like surface with
pores on the order of 100 nm in diameter (Figures 3a-d). The SEM images are taken after drying
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and sputter-coating, and we were concerned that this processing might affect the appearance of the
product phase. To confirm the porous nature of the reacted crystal surface, we used atomic force
microscopy (AFM) to image the expanded solid in air. Figures 3e and 3f show AFM images of
the surface before and after irradiation. They confirm that the smooth surface of the unreacted
crystal gives way to a corrugated surface with deep indentations whose depth was greater than 300
nm. The expanded solid is moderately stable in air, retaining its overall shape even after drying.
Incorporation of water into the porous solid is consistent with the observed loss of adhesion of
many crystals to the glass surface during irradiation. In many cases, a previously stationary crystal
would detach from the microscope slide and start drifting through the solution after the light was
turned on as it expanded (Supporting Information, Figure S14).
The AFM measurements also provide a preliminary way to characterize the mechanical
properties of the expanded solid. Measurements on gelated crystals in air show that the surface
stiffness decreases from 9.0 N/m to 3.6 N/m after reaction, consistent with a softening due to loss
of crystallinity and solvent incorporation. The stiffness is proportional to the Young’s modulus,
although it also depends on sample thickness and tip area (Supporting Information). The same tip
was used for both measurements, and assuming no change in thickness, the stiffness decrease
implies a decrease of 60% in the Youngs modulus. This probably underestimates the actual
decrease due to 1) the loss of water in the air-exposed sample and 2) the likelihood that the
expanded gel is thinner than the original crystal. Despite this softening, the expanded solid does
retain some degree of mechanical coherence. After expansion, the gel-like produce maintains its
shape as it drifts and rotates in solution (Supporting Information, Figure S15 and Movie S3) and
can interact with rigid surfaces like an alumina filter or glass sidewall without deforming or
breaking apart.
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NMR analysis of the reacted crystal indicated that only ~12% of the (E) isomer was
converted to the (Z) form even after prolonged illumination (Supporting Information, Figure S16).
The overlap of the (E) and (Z) charge-transfer bands in the absorption spectra in Figure 1 likely
leads to a photostationary state that limits conversion to the (Z) isomer. The loss of crystallinity
can be initiated by the presence of a relatively small amount of photoproduct, as observed
previously for DMAAM nanowires and microblocks.[4-6] Furthermore, (E)-AYAD only acts as a
gelator when starting from the crystal form. Attempts to generate a solution-to-gel (sol-gel)
transition starting with dilute (E)-AYAD/SDS solutions were unsuccessful. (E)-AYAD is not
sufficiently soluble for us to observe any sign of gel formation by changing temperature, irradiation
or by slow solvent evaporation. It appears that the only route to the gel is by starting with
undissolved (E)-AYAD, i.e. the crystal, and inducing gelation using photochemical disruption.
The observation of a photoinduced crystal-to-gel transition is remarkable for two reasons.
First, it is a unique type of physical transformation. Photoisomerization has been used to induce
[9]
[10]
swelling in an already formed gel and to induce gel-liquid transitions in solution.
Both cases
involve a transition between two disordered phases with the solvent already incorporated. The
photoinduced gelation of (E)-AYAD begins with a dense, ordered single crystal. We hypothesize
that the surfactant facilitates the molecular rearrangements needed for photoisomerization, perhaps
analogous to the acid-induced melting recently observed in acetaldehyde phenylhydrazone
crystals.[11] This surface photoisomerization then opens up fissures in the crystal, in effect
increasing the surface area, both water and surfactant can then penetrate into the interior and cause
further reaction and amorphization. The idea that gelation begins at the surface is supported by
time-resolved imaging of the crystal, which shows the morphology changes starting at the outside
edges of the crystal and migrating inward (Figure 2f). Figure 4 shows a cartoon of this process, in
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which porosity develops through a combination of amorphization and solvent penetration. This
picture of gelation based on networks of molecular aggregates (either crystalline or non-crystalline)
is consistent with current pictures of how low molecular weight gels are formed.[12]
The interaction of a crystalline solid with a solvent is generally assumed to have only two
possible outcomes: dissolution or incorporation of the solvent through specific host-guest
interactions. The (E)-AYAD crystal-to-gel transformation appears to involve “solvation” that lies
between these two extremes, since the solvent disrupts the crystal integrity and facilitates the
growth of disorder but cannot overcome the lipophilic interaction between AYAD molecules to
fully dissolve them. It is possible that this “partial solvation” may be due to a time averaging of
reactant and product solubilities. (Z)-AYAD has a slightly higher solubility in alkane solvents (×3)
and surfactant aqueous (×2) solution than (E)-AYAD (Supporting Information, Figure S17 and
S18). Coupled with the overlapping absorption spectra of both isomers at the irradiation
wavelength, the differing solubilities could lead to a dynamic photoequilibrium between the
soluble Z isomer and the less soluble E isomer, accompanied rapid dissolution/deposition at the
surface of the crystal. A similar light-driven dynamic equilibrium between E and Z isomers has
been invoked to explain photoinduced pattern formation in polymers that contain azobenzene
chromophores.[13]
The second remarkable feature of this crystal-to-gel transition is its large volume change.
Large reversible expansions of crystalline solids induced by hydration have been observed.[14] In
these cases, the volume change is induced by the addition of a separate chemical species that drives
the system to a new equilibrium structure. The (E)-AYAD crystal is not porous and does not
support any type of host-guest interaction that could attract H O or surfactant guest molecules in
2
the absence of photoisomerization. The large volume change occurs only under highly
7
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nonequilibrium conditions, made possible by the light-driven isomerization reactions that facilitate
penetration of the solvent into the crystal. Typical minimum gelation concentrations are on the
[15]
order of 1% , which would suggest that even large expansions (up to 100×) may be possible
using this strategy.
Finally, we briefly touch on possible applications of this photoinduced gelation. The
physical expansion of the gel can be used to generate work or to disrupt it surroundings. For
example, if the crystal was enclosed inside a membrane, the light-induced expansion could be used
to burst the membrane. A second application is to take advantage of the expanding gel to entrap
objects from the surrounding medium and acquire their properties. From the images in Figure 3
and the Supporting Information (Figure S19), gelation opens up channels with diameters on the
order of 50-150 nm. One could imagine a light-activated sponge that soaks up nanoparticles in a
hard-to-access environment. We first tried to use the gel to entrap fluorescent microparticles from
the surrounding solution, but the absorption of the solid-state (E)-AYAD extends out to 700 nm
and either quenched or absorbed the nanoparticle emission, leaving only surface adsorbed particles
visible. We then performed an experiment using superparamagnetic colloidal nanocrystal clusters
(
CNCs) with an average diameter of 70 nm.[16] Before irradiation, intact crystals can be imaged
using transmission electron microscopy (TEM), as shown in Figure 5a. Some CNCs rest outside
the (E)-AYAD microcrystals, suggesting CNCs cannot be entrapped by the (E)-AYAD crystals
without light exposure. After irradiation, aggregated CNCs were visible, encased in an organic
layer, as shown in Figure 5b. Clear interfacial boundaries can be seen between magnetic CNCs
and (E)-AYAD samples, indicating the CNCs are incorporated into the AYAD gel after light
irradiation.[17] Their incorporation into the gel is consistent with the relatively large pores (70
nm diameter) resulting from the light-induced gelation.
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The trapped CNCs allow the expanded gel to be manipulated by an external magnetic field.
The microcrystals without light irradiation barely move under external magnetic field even after a
collision with a huge aggregate of magnetic CNCs, as shown in Figure 5c and Movie S4. On the
other hand, an exposed gel with entrapped CNCs could be pulled over a distance of 180 μm or
more by a magnet with a field strength on the order of 100 Gauss with an average speed of about
-1
1
.4 μm s , as shown in Figure 5d and Movie S5.
In conclusion, we have shown that the newly synthesized molecule (E)-AYAD can undergo
a photoinduced crystal-to-gel transition in aqueous surfactant solutions. The gelation is
accompanied by a large volume expansion (>10×) that enables it to engulf surrounding
nanoparticles. We hypothesize that the gelation is driven by the synergistic action of molecular
photoisomerization that disrupts the local crystal packing, followed by solvent penetration that
facilitates further photoisomerization reaction deeper inside the crystal, eventually leading to a
porous, amorphous gel. We have demonstrated how this crystal-to-gel transformation can enable
new types of photo-controlled processes like spatially selective nanoparticle trapping or
environmental disruption. The discovery of a new route to expanded porous solids, based on
photoismerization in a compact crystal starting material, opens up new opportunities for the
application of low-molecular weight gels as dynamic materials.
Acknowledgments
This research was supported by the National Science Foundation grants DMR-1810514 (CJB)
PHY-1607749 (UM), and the King Abdullah International Medical Research Center (KAIMRC)
through grant RC10/104 (ROK). We thank Professor Imadul Islam, senior scientist with KAIMRC,
9
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for helpful and inspiring discussions and we also thank the NSF grant CHE-0541848 for High
Resolution Mass Spectrometroy support.
Graphic abstract
The molecule (E)-(5-(3-anthracen-9-yl-allylidene)-2,2-dimethyl-[1,3] dioxane-4,6-dione) ((E)-
AYAD) undergoes an EZ photoisomerization in the crystal state that drives a dramatic physical
transformation characterized by a large volume expansion, loss of crystallinity, and high porosity.
This photoinduced crystal-to-gel transition can be used to entrap superparamagnetic nanoparticles
in the surrounding liquid.
1
0
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O
O
O
O
O
O
O
O
hν1
hν2
Scheme 1. Reaction scheme of (E)-AYAD that undergoes E and Z photoisomerization respectively.
1
1
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Figure 1. (a) UV-Vis absorption spectra of (E)-AYAD in solution (chloroform as solvent,
-
5
2
concentration = 6.9 × 10 M, intensity of 462 nm light is 15 mW/cm and the intensity of 365 nm
2
light is about 2.85 mW/cm ). (b) Crystal structure of (E)-AYAD viewed along a axis. (c) UV-Vis
absorption spectra of solid-state polycrystalline thin film of (E)-AYAD under light irradiation (532
2
nm light at 50 mW/cm )
1
2
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Figure 2. (a)-(d) Optical microscope images of (E)-AYAD microcrystals in pure water under light
-2
irradiation for about 30 min (532 nm, 430 mWcm ). (e)-(h) Optical microscope images of (E)-
AYAD microcrystals suspended in aqueous surfactant solution under light irradiation with the
same time scale of (a)-(d). The red rectangle indicates the original crystal. Scale bar: 50 μm
1
3
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Figure 3. Scanning electron microscopy (SEM) images of (E)-AYAD microcrystals before (a) and
after light irradiation (b)-(d), showing various phases of gelation. The images in (b)-(d) are of
different crystals. Scale bar: 5 μm. Atomic force ficroscopy (AFM) images of surface of (E)-
AYAD microcrystals before (e) and after (f) light irradiation.
1
4
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Figure 4. Schematic illustration of the changes of (E)-AYAD microcrystals under light irradiation
in aqueous surfactant solution.
1
5
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Figure 5. (a) Transmission electron microscopy (TEM) image of (E)-AYAD microcrystals with
magnetic CNCs before light irradiation. (b) TEM image of (E)-AYAD incorporated with CNCs
after light irradiation. Black scale bar: 200 nm (c) (E)-AYAD crystal without UV light irradiation.
As an external magnetic field is applied (purple arrow), the superparamagnetic CNCs aggregate
and migrate along the direction of the field, but the (E)-AYAD crystal is barely affected. The drift
of position is due to the collision between the crystal and a big aggregate of CNCs. (d) (E)-AYAD
crystal after UV light irradiation and becomes a gel. When the magnetic field is applied, the CNCs
go inside the body of (E)-AYAD and the gel can move in the direction of the magnetic field. The
yellow dash line indicates the position of AYAD gel. White scale bar: 50 μm.
1
6
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