Arylazopyrazoles for Long-Term Thermal Energy Storage and Optically Triggered Heat Release below 0 °c

Article abstract of DOI:10.1021/jacs.0c00374

Arylazopyrazole derivatives based on four core structures (4pzMe, 3pzH, 4pzH, and 4pzH-F2) and functionalized with a dodecanoate group were demonstrated to store thermal energy in their metastable Z isomer liquid phase and release the energy by optically triggered crystallization at -30 °C for the first time. Three heat storage-release schemes were discovered involving different activation methods (optical, thermal, or combined) for generating liquid-state Z isomers capable of storing thermal energy. Visible light irradiation induced the selective crystallization of the liquid phase via Z-to-E isomerization, and the latent heat stored in the liquid Z isomers was preserved for longer than 2 weeks unless optically triggered. Up to 92 kJ/mol of thermal energy was stored in the compounds, demonstrating remarkable thermal stability of Z isomers at high temperatures and liquid-phase stability at temperatures below 0 °C.

Full text of DOI:10.1021/jacs.0c00374

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Article
Arylazopyrazoles for Long-Term Thermal Energy
Storage and Optically-Triggered Heat Release below 0 °C
Mihael A. Gerkman, Rosina S. L. Gibson, Joaquín Calbo, Yuran Shi, Matthew J Fuchter, and Grace G. D. Han
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c00374 • Publication Date (Web): 22 Apr 2020
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Arylazopyrazoles for Long-Term Thermal Energy Storage and Opti-
cally-Triggered Heat Release below 0 °C
Mihael A. Gerkman1†, Rosina S. L. Gibson2†, Joaquín Calbo3, Yuran Shi1, Matthew J. Fuchter2*,
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Grace G. D. Han1*
1Department of Chemistry, Brandeis University, 415 South Street, Waltham, MA 02453, USA
2Molecular Sciences Research Hub, Department of Chemistry, Imperial College London, Wood Lane, London W12
0BZ, United Kingdom
3Instituto de Ciencia Molecular, Universidad de Valencia, 46890 Paterna, Spain
Keywords: photo‐switch, arylazopyrazole, phase transition, heat storage, sub‐zero heat release
Supporting Information
upon UV irradiation. One challenge that azobenzene deriva‐
ABSTRACT: Arylazopyrazole derivatives based on four core
tives face as MOST storage materials is their short Z isomer
half‐life (t1/2) ranging from seconds to days depending on azo‐
benzene functionalization.29‐30 The thermal stability of the Z
isomer is determined by both steric and electronic effects on
both the E/Z ground states and isomerization transition
state.18, 31 Such thermal stability governs the thermal energy
storage lifetime in MOST materials, and various molecular de‐
signs have been developed to increase thermal stability of az‐
obenzene Z isomers.
structures (4pzMe, 3pzH, 4pzH, and 4pzH‐F2) and function‐
alized with a dodecanoate group were demonstrated to store
thermal energy in their metastable Z isomer liquid phase and
release the energy by optically triggered crystallization at −30
℃ for the first time. Three heat storage‐release schemes were
discovered involving different activation methods (optical,
thermal, or combined) for generating liquid‐state Z isomers
capable of storing thermal energy. Visible light irradiation in‐
duced the selective crystallization of the liquid phase via Z‐to‐
E isomerization, and the latent heat stored in the liquid Z iso‐
mers was preserved for longer than two weeks unless optically
triggered. Up to 92 kJ/mol of thermal energy was stored in the
compounds demonstrating remarkable thermal stability of Z
isomers at high temperatures and liquid‐phase stability at
temperatures below 0 ℃.
One method to increase the half‐life of azobenzenes is to
replace one of the aryl rings with a pyrazole: the arylazopyra‐
zoles.32‐41 While there are several factors that underpin the in‐
creased stability of the azopyrazoles, it is notable that these
azo switches have exceptionally long thermal half‐lives (days
to years). In principle, the increased thermal stability of the Z
isomer presents a potential for generating MOST materials
with elongated total heat storage time and a wider range of
temperatures at which heat can be stored. Additionally, aryla‐
zopyrazoles can undergo Z‐E isomerization in the presence of
protons42 and metal ions,43 which opens up the possibility of
creating dual‐responsive materials that can release heat when
exposed to chemical stimuli as well as light.
INTRODUCTION
Photo‐switching molecular systems including azoben‐
zenes,1‐3 norbornadienes,4‐7 dihydroazulenes,8‐11 and ful‐
valenediruthenium complexes12‐14 have been recognized as
molecular solar thermal (MOST) energy storage materials that
convert photon energy to thermal energy by reversible isom‐
erization and energy storage in a metastable isomeric state.
This class of molecules complements traditional and emerging
photothermal materials which directly convert photon energy
into heat, for example, from sunlight to significant thermal
energy for vaporizing water, despite the lack of the energy
storage capability.15 Among the photo‐switching molecular
systems listed, azobenzene derivatives have been particularly
well explored as MOST compounds due to a plethora of func‐
tionalization methods16‐17 available and their remarkable E‐Z
isomerization cyclability.18 Various forms of MOST materials
have been developed incorporating azobenzene groups, in‐
cluding small molecules,19‐20 oligomers,21‐22 polymers,23‐24 car‐
bon nanotubes,25‐26 and graphene oxides,27‐28 and have
demonstrated energy storage in the metastable Z isomer state
In addition to addressing the stability of Z isomers, contin‐
uous effort has been made to increase the total heat storage
density in MOST compounds, particularly azobenzenes that
show low energy density (41 kJ/mol for pristine).44 A promi‐
nent strategy harvests the latent heat of azobenzene deriva‐
tives as well as the Z‐E isomerization energy by optically
changing the phase of the molecules during the photo‐isom‐
erization process (i.e. solid E ↔ liquid Z).45‐48 The azobenzene
derivatives have also been used as a dopant in organic phase
change materials to manipulate the solid‐liquid phase transi‐
tion of the composite by optical means.49 Despite the in‐
creased energy storage density in these materials, the heat
storage time is still limited to hours due to the thermal rever‐
sion of the azobenzene derivatives, even at room temperature.
To maximize the usefulness of MOST materials, it is im‐
portant to extend beyond the limited range of temperatures at
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which optically controlled heat release can occur currently.
solid E state for thermal energy release applications. The azo‐
benzene derivative (a) reported by Grossman and coworkers49,
53 was integrated with other traditional organic phase‐change
materials such as fatty acids and paraffins to optically regulate
the heat storage and release from the phase‐change composite
materials. Azobenzene derivatives exhibit a short Z isomer
half‐life in both solution and in the condensed phase at room
temperature, which limits the long‐term storage of latent heat
in the Z isomer liquid phase. Arylazopyrazoles functionalized
with an alkyl or terminal alkenyl ether group on benzene ring
were recently reported by Li and coworkers.50 This work
showed the light‐triggered crystallization of Z isomers at room
temperature, which was enabled by the relatively low melting
points of select Z isomers (19–37 ℃). However, their design
still renders the Z isomers to be highly crystalline below room
temperature, which prevents the liquid‐to‐solid phase change
and Z‐to‐E isomerization below room temperature.
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Most of the reported systems measure heat release at around
room temperature, due to the facile Z‐to‐E reversion at high
temperatures (above 60 ℃).49 Moreover, triggered heat re‐
lease at low temperatures, especially below 0 ℃, remains a
challenge because the Z liquid phase crystallizes and uncon‐
trollably loses latent heat storage.50 According to a recent re‐
view by Wu and co‐workers on the melting points of mono‐
and di‐substituted E and Z azobenzenes, most of the reported
Z isomers show high crystallinity and melting points in the
range of 20–200 ℃,45 which implies the uncontrolled crystal‐
lization of Z isomer below room temperature.
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Herein, we present molecular design strategies for azo
MOST materials that address both thermal stability of the Z
isomers and the stability of liquid phase Z isomers under 0 ℃
by using dodecanoate‐appended arylazopyrazoles. For the
first time, we demonstrate the visible‐light‐triggered crystalli‐
zation of photo‐switches at −30 ℃ and the heat storage in sta‐
ble liquid phase of the Z isomer for longer than two weeks.
The heat release at such low temperatures has significant im‐
plications in de‐frosting of engine oils and mechanical parts,
and personal heating under extreme cold conditions where
other traditional heat generation, such as combustion, is lim‐
ited.
In order to achieve the light‐triggered phase transition and
heat release at extreme cold, we introduce a new functionali‐
zation strategy that stabilizes Z liquid phase at very low tem‐
peratures (−30 ℃, for example) while retaining E crystalline
phase. The arylazopyrazole parent structures (Figure 1, center)
previously reported by Fuchter and coworkers were selected
for our study.33‐34 DSC measurements (Figures S1‐S3) and
photo‐switching studies (Figure S4) first revealed that all of
the parent compounds 1‐4 were unable to release thermal en‐
ergy via light‐triggered crystallization for various reasons.
Compounds 1 and 2, after initial melting and subsequent cool‐
ing, could not be optically triggered to crystallize as a result of
RESULTS AND DISCUSSION
Structural Design.
Figure 1 shows previous examples of (a, b) azobenzene,46, 49, 51‐
53 and (c) arylazopyrazole50 derivatives that are reported to un‐
dergo photo‐triggered crystallization from a liquid Z state to a
Figure 1. Previous azo‐derivatives (azobenzene and arylazopyrazole) that have shown photo‐triggered crystallization and heat
release. R (or R′) represents a linear alkyl (Grossman et al.), linear alkyl and oligoether‐based ammonium group (Kimizuka et al.),
or linear alkyl/terminal alkenyl group (Li et al.). Compounds 1′‐4′ are the derivatives of previously synthesized parent azopyrazoles
1‐4, functionalized with dodecanoate group. Thermal Z isomer half‐lives (t1/2) of previous compounds are given in solution at room
temperature, except for an azobenzene derivative (Figure 1b) which was measured as a solid. Red, purple, and blue circles show
the structural modification from pristine 4pzH (3).
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relatively similar thermal characteristics for both isomers. Af‐
4′ at a rate of 1 °C/min by DSC (Figure S2). The E isomers of
1′–4′ show sharper melting and crystallization peaks, with
minimal changes in phase transition temperatures and ener‐
gies. By running slower measurements, we were able to re‐
solve two crystallization peaks at 32 and 37 °C for the poly
morphs of compound 3′. The Z isomer of 1′ exhibits new fea‐
tures including cold crystallization at −4 °C and melting at 47
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ter melting and cooling, they essentially form a stable liquid
for both the E and Z isomeric forms. Compound 4 shows fa‐
vorable thermal characteristics (i.e. liquid Z and crystalline E,
as individually determined by DSC), but the Z‐to‐E switching
below 39 ℃ (Tm of E form) did not result in any nucleation.
Compound 3 shows crystallization by the reverse E‐to‐Z
photo‐isomerization, since the E form of compound 3 is less
crystalline than its Z isomer. However, the photo‐induced
crystallization did not lead to the overall heat release, because
of the larger thermal energy uptake by the isomerization pro‐
cess than the exothermicity from the crystallization. Nonethe‐
less, this intriguing Z isomer crystallization is illustrated and
discussed in more detail below (schemes compared between
Figure 2 and 3) as an alternative new method for potentially
storing and releasing thermal energy with reduced activation
steps.
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The parent arylazopyrazole structures were modified to in‐
corporate a dodecanoate group (1′‐4′). It was hypothesized
that such a substituent would increase the crystallinity of E
isomers via the enhanced van der Waals interactions, while
producing stable liquid phases for the Z isomers. Compounds
1′‐4′ were particularly designed to preserve the core electronic
structure of the patent arylazopyrazoles, with a methylene
linker between the alkyl ester group and the photoswitching
moiety (Figure 1, right). Comparative UV‐Vis spectra of the
compounds 1‐4 and 1′‐4′ (Figure S4) show that light absorp‐
tion of each parent compound is mostly identical to that of
respective alkyl‐functionalized derivative. The degrees of E‐
to‐Z and Z‐to‐E photo‐isomerizations and the absorption
spectra of photo‐stationary state (PSS) are essentially un‐
changed after the functionalization of parent compounds (Ta‐
ble S1). We note that the E isomer contents in the photo‐irra‐
1
diated samples are minimal (2‐3%) as measured by H NMR
(Table S1 and Figure S5).
Phase Transition of Photo‐isomers.
The drastic difference in crystallinity between E and Z forms
of our designs was directly measured by differential scanning
calorimetry (DSC) as shown in Figure 2b and S1 with a rate of
10°C/min. The E isomers of compounds 1′‐4′ exhibit sharp
melting peaks and subsequent crystallization peaks upon
cooling, whereas such melting and crystallization features are
absent (compounds 1′, 3′, and 4′) or significantly reduced
(compounds 2′) for the Z isomer counterparts. These Z iso‐
mers were found computationally to have significantly higher
polarity than the E isomers (Figures S6‐S7), which likely con‐
tributes to Z isomer liquid phase stability upon cooling, along
with the increased bulkiness of Z isomers. The high polarity
and bulkiness of Z isomers effectively disrupt the π–π interac‐
tions between aromatic groups and van der Waals interactions
between the alkyl chains. This disruption reduces the ability
of Z isomers to pack and form ordered crystals. The alkyl func‐
tionalized Z isomers were found to remain in the liquid phase
either between −45 ℃ and 85 ℃ (compounds 1′, 3′, and 4′) or
between −30 ℃ and 10 ℃ (compound 2′). Compounds 1′, 3′,
and 4′ undergo glass transition below −45 ℃ and Z‐to‐E re‐
verse isomerization above 85 ℃. Compound 2′ partially crys‐
tallizes at temperatures lower than −30 ℃ and cold‐crystal‐
lizes at temperatures above 10 ℃.
Figure 2. (a) Schematic illustrations of activation methods I
and II for arylazopyrazoles derivatives which store latent heat
in stable liquid phase upon activation. (b) Differential scan‐
ning calorimetry (DSC) plots of E and Z isomers of compound
4′ as a representative example of group I and II compounds.
Tm: melting point, Tc: crystallization point, Tg: glass transition
point (c) A series of optical images of compound 1′ in a powder
form that undergoes melting, UV irradiation, cooling, and vis‐
ible‐light‐induced crystallization according to the activation
method I. Compound 1′ also directly absorbs UV at room tem‐
perature (i.e. in the absence of the first melting process) to
form liquid‐state Z isomer, according to activation method II.
In order to examine the impact of heating and cooling rates
on the observed phase transitions, we studied compounds 1′–
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°C, indicating that the prolonged exposure of the amorphous
the cold‐crystallization temperature of the E isomer to −8 °C
and minimal change in the Z isomer crystallization (Figure
S2). The higher crystallinity of the Z isomer over the E isomer
allows for the latent heat storage in the E isomer liquid phase
and the optical triggering of crystallization by UV‐induced E‐
to‐Z isomerization. The advantage of this activation method
(III) is the absence of photon absorption requirement for the
generation of the stable liquid phase.
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solid to very low temperatures (below −50 °C) generates nu‐
cleation seeds which propagates upon thermal activation. The
Z isomer of 2′ exhibits a broad crystallization around 19 °C in‐
stead of cold‐crystallization as previously seen at the faster
scans. This emphasizes that compound 2′ is more prone to
crystallization compared to compounds 1′, 3′, and 4′. We
found no impact of scan rate change for the Z isomers of 3′ and
4′.
Regardless of the heating and cooling rate, there is a distinct
difference observed between E and Z isomers of compounds
1′‐4′; E isomers are highly crystalline while Z isomers are liquid
at room temperature. This difference between E and Z forms
presents an exciting opportunity for storing latent heat in liq‐
uid form and releasing the thermal energy upon photoswitch‐
ing of the liquid, resulting in crystallization of the photogen‐
erated isomer. For long‐term latent heat storage, high Z sta‐
bility of liquid form is required. The arylazopyrazoles, partic‐
ularly the 4pzH derived compounds 3 and 4 (and thus 3′ and
4′), are predicted to retain the Z isomer phase for significantly
longer periods of time, before being optically triggered to re‐
lease heat via Z‐to‐E isomerization.
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Heat Storage and Release Mechanism.
Upon investigation of the mechanisms of latent heat storage
and triggered release, three types of energy storage‐release cy‐
cles were discovered (Figures 2 and 3). Most compounds (2′,
3′, and 4′) undergo both heat and photon absorption to pro‐
duce stable liquid phase of Z isomers (activation method I).
Initially, crystalline E isomers absorb heat to become liquid,
which in turn absorbs UV photons to isomerize to liquid Z iso‐
mers. The Z liquid‐phases are very stable within a wide range
of temperatures (−45 ℃ to +85 ℃) and can be cooled below 0
℃ without losing latent heat. Visible light irradiation isomer‐
izes Z back to E isomers that readily crystallize even at low
temperatures, releasing thermal energy. This process is illus‐
trated visually using compound 1′ in Figure 2c.
Figure 3. (a) Schematic illustrations of activation method III
for arylazopyrazole derivatives which store latent heat in sta‐
ble liquid phase upon activation. (b) DSC plots of E and Z iso‐
mers of compound 3, the only group III compound that does
not require photon absorption for activation. Tcc: cold‐crystal‐
lization point.
In addition to the method previously described (I), com‐
pound 1′ can also be activated by an alternative method (II).
Method II (Figure 2a) involves the solid‐to‐liquid phase
change upon direct UV irradiation at room temperature. Com‐
pound 1′ has the lowest heat of fusion (ΔHm) in E form (Table
1) among the dodecanoate derivatives together with a rela‐
tively low melting point (Tm). This suggests that poorer pack‐
ing of compound 1′ in the solid state enables the direct phase
transition upon UV photon uptake. As the Z isomers in the
stable liquid phase form, compound 1′ can be cooled to low
temperatures and optically triggered to release heat, as with
compounds 2′‐4′.
Table 1. Thermal parameters of E and Z isomers
We note that an intriguing activation method III (Figure 3a)
was discovered with parent compound 3 (4pzH). This com‐
pound shows the opposite thermal behaviors for E and Z iso‐
mers (Figure 3b): the Z isomer melts and crystallizes without
significant supercooling, similar to the E isomers of the other
compounds. The E isomer of compound 3, on the other hand,
exhibits large supercooling and no crystallization process dur‐
ing cooling to −85 ℃. The amorphous solid can then be heated
up to 39 ℃ to recover crystallinity. This cold‐crystallization
behavior indicates the lack of strong driving force for the mol‐
ten E isomer to crystallize during the cooling cycle. For com‐
pound 3, at a scan rate of 1 °C/min, we observed lowering of
ΔHm: heat of fusion, ΔHc: heat of crystallization, superscript
cc: cold‐crystallization process. Tc of compounds 1 and 2 Z iso‐
mers could not be accurately measured due to the concomi‐
tant Z‐to‐E isomerization during the melting of Z isomer and
the formation of liquid consisting of Z and E mixtures.
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Figure 4. (a) Optical images of a stable liquid film (compound 1′ as an example of group I and II compounds) that is irradiated
with visible light (530 nm) at −30 ℃ and the subsequent crystallization of E isomers. The left side of the film is covered by a mask
to preserve Z isomers in stable liquid state. Optical microscope images taken on the left (L), center (C), and right (R) spots of the
film in (3a), showing liquid phase, interface between liquid and crystalline solid, and solid phase, respectively. (b) UV‐Vis absorp‐
tion spectra of E and Z forms of compound 1′ obtained in solution. Z‐to‐E isomerization is induced by the absorption of 530 nm
light by Z isomer (n–π* transition). (c) Optical images of a stable liquid film (compound 3 in group III) that is irradiated with UV
light (365 nm) at 0 ℃ and the subsequent crystallization of Z isomers. The left side of the film is covered by a mask to preserve E
isomers in stable liquid state. Optical microscope images taken on a film of compound 3 after selective crystallization by UV
irradiation. (d) UV‐Vis absorption spectra of E and Z forms of compound 3 and the process of UV‐induced crystallization.
Table 1 summarizes thermal properties of all compounds
measured by DSC (Figure S1). The lowest temperature at
which thermal energy can be stored in Z isomer liquid phase
is determined by Tc of Z isomers (highlighted red). Com‐
pounds 1′, 3′, and 4′ in Z isomeric forms are confirmed to be
liquid above −45 ℃, and compound 2′ can preserve liquid
phase of Z isomers above −33 ℃. The highest temperature at
which the optical triggering can induce crystallization via Z‐
to‐E isomerization is defined by Tc of E isomers (highlighted
blue).
triggered crystallization (Figure S3). The exothermic peaks ap‐
pear during the thermal activation of Z isomers in DSC, and
the integrated area under the peaks represent ΔHiso. As sum‐
marized in Table 2, the thermally‐induced isomerization oc‐
curs at higher temperatures for compounds with higher ther‐
mal stability or longer t1/2 (Tiso of compound 4 > 3 > 2 > 1; com‐
pound 4′ > 3′ > 2′ > 1′). This indicates the higher thermal acti‐
vation energy (Ea) of Z‐to‐E isomerization for more stable Z
isomers. ΔHiso, on the other hand, decreases as the Z isomer
stability is increased (ΔHiso of compounds: 1 > 2 > 3 > 4; 1′ > 2′
> 3′ > 4′), indicating the lower energy of the more stable Z iso‐
mers and corroborating their higher Ea.
Demonstration of Phase Transition and Heat Release at
Low Temperatures.
Maximum thermal energy release from the optically trig‐
gered crystallization process (ΔHtotal) is calculated by the inte‐
gration of crystallization energy (ΔHc) of E isomers and Z‐to‐
E isomerization energy (ΔHiso) for compounds 1′, 2′, 3′, and 4′
following the activation method I and II (marked blue in Table
2). We note that in the case of incomplete Z‐to‐E conversion,
only a smaller fraction of the ΔHc and ΔHiso will be harnessed
upon optically triggered crystallization. We examined the
long‐term stability of liquid films at −20 ℃ observing no crys‐
tallization after two weeks in the case of compounds 3′ and 4′
(Table S2). Compounds 1′ and 2′ were stable for 1 day and 7
hours, respectively, due to the lower Z thermal stability (1′)
and crystallization behavior of Z at −33 ℃ (2′).
The temporal and spatial control over latent heat release
can be demonstrated by the selective crystallization of stable
liquid films involving optical masks (Figure 4). The films were
partially covered by the mask (typically thick and dense card‐
board or plastic) on the left side (L), and crystalline phase for‐
mation was found to occur only within the area (right side, R)
exposed to light for 5–15 min. The translucent liquid films be‐
come opaque after crystallization (right side, R) observed by
darker coloring under optical microscope due to increased dif‐
fraction. Compounds 1′‐4′ (represented in Figure 4a by com‐
pound 1′) show a clear interface between the solid and liquid
phase and distinct crystalline features of micro scale grains.
Figure 4b shows distinct absorption spectra of E and Z isomers
of compound 1′, particularly the blue‐shifted and diminished
n–π* band of E isomer in the visible light region, which leads
to the prominent color change from orange to yellow upon Z‐
to‐E isomerization. Comparable data for compounds 2′‐4′ are
provided in the Figure S8. The color change of the compound
3 film (Figure 4c) after irradiation is not as significant as that
for compound 1′ (Figure 4a) because of the similar absorption
profile and intensity in visible light range (Figure 4d) between
E and Z isomers of compound 3. Images for compounds 1, 2,
and 4 are also provided in the Figure S8, demonstrating no
crystallization induced by light irradiation. As aforemen‐
tioned, these compounds remain liquid after Z‐to‐E switching
due to the high supercooling of melt for both isomers.
We note that compound 3 that is activated by method III
results in a negative value of calculated ΔHtotal due to the en‐
dothermic E‐to‐Z photo‐isomerization required for the crys‐
tallization. Since ΔHiso is larger than ΔHc for compound 3, the
photo‐induced crystallization does not lead to the overall heat
release. However, this energy storage scheme can be further
developed for net heat release by the judicious design of
photo‐switches that possess small ΔHiso and large ΔHc. This
scheme generates a stable liquid phase without requiring
photo‐activation and effectively release latent heat by one‐
step optical triggering. This will potentially open up opportu‐
nities to investigate other switches including spiropyrans,54 di‐
arylethenes,55‐57 or various molecular motors58‐60 which have
not been considered as MOST molecules due to their negligi‐
ble ΔHiso. We envision the potential of utilizing photoinduced
phase transitions of the aforementioned classes of switches
and motors for latent heat storage by careful structural design.
DSC was additionally utilized to measure the isomerization
energy (ΔHiso) of Z‐to‐E switching accompanied by the light‐
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For example, the installation of long alkyl chains can induce
spectra, dipole moment calculations, optical images of films,
thin film stability measurement, and NMR spectra supplied as
Supporting Information. The Supporting Information is avail‐
able free of charge via the Internet at http://pubs.acs.org. Raw
data can be found at 10.14469/hpc/6767.
1
2
3
4
5
6
7
8
crystallinity to materials which can be disrupted by ring open‐
ing and closing through photo‐irradiation. The substitution
pattern of chains on other photo‐responsive core structures
that would determine the degree of such phase transition is
currently under investigation.
AUTHOR INFORMATION
Corresponding Author
Table 2. Thermal Parameters Obtained from DSC
1 gracehan@brandeis.edu
2 m.fuchter@imperial.ac.uk
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Author Contributions
†These authors contributed equally.
Notes
A U.S. provisional patent (62/969,634) was filed on this intel‐
lectual property.
ACKNOWLEDGMENT
ΔHtotal is the total thermal energy release from the optically
triggered crystallization process. Tiso is measured as peak tem‐
perature of exothermic curve observed for thermally‐induced
Z‐to‐E isomerization. (a) ΔHtotal = ΔHiso for compounds 1, 2,
and 4. (b) ΔHc of compound 3 is the crystallization energy of
Z isomers. (c) ΔHtotal for compound 3 is defined as ΔHc − ΔHiso
due to the endothermic E‐to‐Z photo‐isomerization occurring
for the crystallization. DSC plots of Z‐to‐E thermal isomeriza‐
tion are shown in Figure S3.
We acknowledge the SPROUT award (2019‐042) from
Brandeis Office of Technology Licensing and Provost Research
Award from Brandeis University. We thank Prof. Ben Rogers
and his laboratory in the Department of Physics at Brandeis
University for the usage of optical microscope. R.S.L.G. would
like to thank the Faculty of Natural Sciences for the
Schrӧdinger Scholarship. M.J.F. would like to thank the
EPSRC for an Established Career Fellowship (EP/R00188X/1).
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