Cracking under internal pressure: Photodynamic behavior of vinyl azide crystals through N2release

Article abstract of DOI:10.1021/jacs.0c07830

When exposed to UV light, single crystals of the vinyl azides 3- azido-1-phenylpropenone (1a), 3-azido-1-(4-methoxyphenyl)propenone (1b), and 3-azido-1-(4-chlorophenyl)propenone (1c) exhibit dramatic mechanical effects by cracking or bending with the release of N2. Mechanistic studies using laser flash photolysis, supported by quantum mechanical calculations, show that each of the vinyl azides degrades through a vinylnitrene intermediate. However, despite having very similar crystal packing motifs, the three compounds exhibit distinct photomechanical responses in bulk crystals. While the crystals of 1a delaminate and release gaseous N2 indiscriminately under paraffin oil, the crystals of 1b and 1c visibly expand, bend, and fracture, mainly along specific crystallographic faces, before releasing N2. The photochemical analysis suggests that the observed expansion is due to internal pressure exerted by the gaseous product in the crystal lattices of these materials. Lattice energy calculations, supported by nanoindentation experiments, show significant differences in the respective lattice energies. The calculations identify critical features in the crystal structures of 1b and 1c where elastic energy accumulates during gas release, which correspond to the direction of the observed cracks. This study highlights the hitherto untapped potential of photochemical gas release to elicit a photomechanical response and motility of photoreactive molecular crystals.

Full text of DOI:10.1021/jacs.0c07830

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Article
Cracking Under Internal Pressure: Photodynamic
Behavior of Vinyl Azide Crystals Through N2 Release
Dylan J Shields, Durga Prasad Karothu, Karthik Sambath, Ranaweera
A. A. Upul Ranaweera, Stefan Schramm, Alexander Duncan, Benjamin
Duncan, Jeanette A. Krause, Anna D. Gudmundsdottir, and Pance Naumov
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c07830 • Publication Date (Web): 29 Sep 2020
Downloaded from pubs.acs.org on October 1, 2020
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Cracking Under Internal Pressure: Photodynamic Behavior of Vinyl
Azide Crystals Through N2 Release
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Dylan J. Shields, Durga Prasad Karothu, Karthik Sambath,‡ Ranaweera A. A. Upul Ranaweera,‡ Stefan
Schramm, Alexander Duncan, Benjamin Duncan, Jeanette A. Krause, Anna D. Gudmundsdottir,* and
Panče Naumov*
ABSTRACT: When exposed to UV light, single crystals of the vinyl azides 3-azido-1-phenylpropenone (1a), 3-azido-1-(4-
methoxyphenyl)propenone (1b), and 3-azido-1-(4-chlorophenyl)propenone (1c) exhibit dramatic mechanical effects by cracking or
bending with the release of N2. Mechanistic studies using laser flash photolysis, supported by quantum mechanical calculations, show
that each of the vinyl azides degrades through a vinylnitrene intermediate. However, despite having very similar crystal packing
motifs, the three compounds exhibit distinct photomechanical responses in bulk crystals. While the crystals of 1a delaminate and
release gaseous N2 indiscriminately under paraffin oil, the crystals of 1b and 1c visibly expand, bend, and fracture, mainly along
specific crystallographic faces, before releasing N2. The photochemical analysis suggests that the observed expansion is due to internal
pressure exerted by the gaseous product in the crystal lattices of these materials. Lattice energy calculations, supported by
nanoindentation experiments, show significant differences in the respective lattice energies. The calculations identify critical features
in the crystal structures of 1b and 1c where elastic energy accumulates during gas release, which correspond to the direction of the
observed cracks. This study highlights the hitherto untapped potential of photochemical gas release to elicit a photomechanical
response and motility of photoreactive molecular crystals.
O
O
1. Introduction
O
N3
N3
The quest for new smart organic materials capable of
transducing light and other stimuli into mechanical motion has
intensified in recent years because of their potential in a number
of applications, including electronics, biomedicine, and soft
robotics. Specifically, these materials show promise for
applications in smart sensors, electronics, and medical devices.1-
N3
Cl
MeO
1c
1b
1a
Chart 1. Molecular structures of 3-azido-1-phenylpropenone (1a),
3-azido-1-(4-methoxyphenyl)propenone (1b), and 3-azido-1-(4-
chlorophenyl)propenone (1c).
4
Historically, polymers and elastomers were among the first
organic materials to be studied and applied as smart materials;
however, owing to limitations associated with their disordered
structures, there is now a demand for alternative materials
endowed with long-range structural order that can transduce
energy and respond faster while still being light in weight.
Adaptive molecular crystals have recently been shown to be able
to perform many of the same mechanical feats as their polymeric
predecessors.59 These previously underexplored materials can
respond rapidly to external stimuli such as light, heat, or
mechanical force by switching between different crystal packing
arrangements (polymorphs) or by undergoing chemical
reactions such as cyclization, cis–trans isomerization, or
dimerization.10 The underlying molecular reconfiguration
generates a considerable strain in the crystal lattice, which is
manifested as macroscopically visible motion such as bending,
shattering, jumping, or flipping.11
of oxygen gas as a product of the reaction, the direct correlation
of the reaction mechanism with the kinematic traits of the
phenomenon was not conducted. Except for some early reports
in some inorganic compounds,1519 other reports on mechanical
movements caused by gas release are scarce, consisting of brief
mentions or anecdotal reports in presentations. Thus, this study
aimed to provide an in-depth understanding of the photodynamic
behavior of the crystals of three structurally related gas-releasing
azido compounds (Chart 1) using a combined experimental and
computational approach to study the underlying reaction
mechanism. The crystals of 3-azido-1-phenylpropenone (1a), 3-
azido-1-(4-methoxyphenyl)propenone (1b) and 3-azido-1-(4-
chlorophenyl)propenone (1c), whose molecular structures differ
only by the para-substituent with respect to the ketone group,
were found to have similar packing motifs but very different
photomechanical responses. The driving forces responsible for
crystal disintegration and motility were elucidated using video
microscopy and by correlating the observed deformation, gas
release, and motion to the structural and molecular modeling
results. In addition to the kinematic study, the mechanism of
denitrogenation was also investigated in solution and in the solid
state through product studies and laser flash photolysis.
An alternative, unexplored approach for eliciting motility in
crystalline materials is to acquire momentum by releasing a gas,
a mechanism that is analogous to the operation of a gasoline
engine. A prominent recent example of crystal motion by gas
release is the thermally induced coupling of a zwitterionic metal
complex reported by Ovcharenko et al.1214Although this
phenomenon was attributed to the release
2. Results and Discussion
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O
O
O
2.1. Product Analysis
ISC
h
1
2
3
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5
6
7
8
Ar
S1 of
N3
Ar
N3
Ar
N3
The irradiation of the vinyl azides 1a–c with UV light (Hg arc
lamp, Pyrex) in argon-saturated chloroform-d resulted in the
corresponding azirines 3, isoxazoles 4, and cyanides 5, as
1
1
32BR
a: Ar = Ph
b: Ar = p-MeO-Ph
c: Ar = p-Cl-Ph
-N2
O
1
depicted in Scheme 1. The reaction progress, followed by H-
NMR spectroscopy, showed that the photoproduct ratio changed
as a function of irradiation time, with continuous irradiation of
the reaction mixture increasing the yield of cyanides 5 at the
expense of azirines 3 and isoxazoles 4. These results are in
accord with the established photolysis behavior of 1a.20 In
contrast, the irradiation of the crystals of vinyl azides 1a–c
pressed between glass slides selectively yielded azirines 3a–c.
As azirines 3a–c are highly reactive, isolation using traditional
column chromatography was not feasible. Instead, products 3, 4,
and 5 were identified in the reaction mixture based on IR and
NMR spectroscopic evidence. The recorded spectra of 4a–c and
5a–c were consistent with those available in the literature.2125
To support the characterization of azirines 3a–c, we compared
their 1H-NMR spectra to those reported for 2-benzoyl-2-methyl-
2H-azirine and 2-methyl-2-phenyl-2H-azirine, in which the
vinylic hydrogen atom on the azirine ring is observed at 9.75 and
9.80 ppm, respectively,22 corresponding to that in azirine 3 at
9.67 ppm. Similarly, the -hydrogen atom in azirine 3 is located
at 3.5 ppm, which is similar to the chemical shift observed for
the -hydrogen atom of 2-benzoyl-3-methyl-2H-azirine at 3.50
ppm.26
O
O
h
ISC
N
O
CN
N
Ar
+
Ar
Ar
N

Ar
5
4
3
32N
9
Scheme 2. Proposed mechanism for the formation of photoproducts
3, 4, and 5 from vinyl azides 1.
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2.2. Laser Flash Photolysis
To verify that vinyl azides 1a–c form products through
vinylnitrene intermediates both in solution and in the solid state,
we employed nanosecond laser flash photolysis studies. Laser
flash photolysis of 1a–c in argon-saturated acetonitrile gave
transient spectra with absorption maxima between 340 and 360
nm (Figure 1A, Supporting Information Figures S1 and S2). The
3
transient absorption was attributed to triplet vinylnitrene 2N
based on a comparison with the TD-DFT-calculated absorption
spectrum (Figure 1B) and the observation that the absorption is
quenched in oxygen-saturated acetonitrile.
O
O
O
O
N
O
h
h
CN
Ar
N
Ar
N
Ar
N3
Ar
Ar
-N2
-N2
3
1
3
4
5
Crystals
CDCl3
a: Ar = Ph
b: Ar = p-OMe-Ph
c: Ar = p-Cl-Ph
Scheme 1. Photoproducts obtained by irradiation of vinyl azides 1
in chloroform-d and as crystals.
To further aid with characterization, thermolysis was performed
in chloroform-d solutions of vinyl azides 1a–c at 70 °C. The
progress of the reaction was followed by 1H-NMR spectroscopy,
which demonstrated that vinyl azides 1a–c were cleanly
transformed into azirines 3a–c and cyanides 5a–c. The 1H-NMR
signals for the vinyl protons of vinyl azides 1a–c at 6.5 and 7.5
ppm were depleted and new signals for the protons on the azirine
moiety appeared at 3.5 and 9.0 ppm. Thus, cyanides 5 are most
likely formed both thermally and photochemically from azirines
3. The product studies clearly demonstrated that the
photoreactions in the crystals are more selective than their
counterparts in solution. The formation of azirines in the solid
state is presumably more favorable than the formation of
isoxazoles as less molecular movement is required, which is
preferable in a spatially constrained, rigid environment. This
reasoning is consistent with the topochemical postulate, which
states that solid-state reactions should proceed via the pathway
of least molecular motion.27 Based on the product formation, we
theorize that upon exposure to light, vinyl azides 1a–c form their
corresponding singlet excited states (S1) that intersystem-cross
Figure 1. (A) Transient spectra obtained by laser flash photolysis of
1b in argon-saturated acetonitrile. (B) TD-DFT-calculated spectrum
of vinylnitrene 32Nb using C-PCM solvation and acetonitrile.
3
efficiently to triplet 1,2-biradicals 2BR, which then extrude a
N2 molecule to afford triplet vinylnitrene intermediates 2N
(Scheme 2). Generally, triplet vinylnitrenes are short-lived
intermediates that have significant 1,3-biradical character and
intersystem-cross efficiently to form products.28,29
3
The decay of all transient absorptions in argon-saturated
solutions were best fitted with a pseudo-first-order function,
resulting in lifetimes of 10, 12, and 8 μs for vinylnitrenes 32Na,
32Nb, and 32Nc, respectively. The transient absorptions formed
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with rate constants of ~2 × 107 s−1, which correspond to the
those obtained in solution with absorbance maxima between 350
decay of the precursor 1,3-biradicals (32BR). The laser flash
photolysis studies demonstrate that solution photolysis of vinyl
azides 1a–c results in the formation of triplet vinylnitrenes and
that the p-substituent does not significantly affect the reactivity
while only slightly affecting the absorption profile.
and 380 nm, which are attributed to triplet vinylnitrenes 32Na–
c. However, the nanocrystalline suspension spectra are not fully
quenched when the suspension is saturated with oxygen or air,
presumably because the quenching mechanism in crystals is less
efficient than in solution. Furthermore, an analysis of the
kinetics in argon-saturated suspensions revealed that
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3
3
vinylnitrenes 2Na, 2Nb, and 2Nc are longer lived than in
solution, with lifetimes of 14, 12, and 14 μs, respectively. In
comparison, in air- and oxygen-saturated nanocrystalline
suspensions, the lifetimes of the triplet vinylnitrenes are reduced
but not completely quenched. Importantly, the rate of formation
of the vinylnitrenes from nanocrystalline suspensions of vinyl
azides 1 became faster than the time resolution of the laser.
Altogether, the laser flash photolysis studies of vinyl azides 1a–c
in nanocrystals demonstrate that in crystals, triplet vinylnitrenes
are formed faster, are longer lived, and are less easily quenched
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by oxygen. However, vinylnitrenes 2Na–c have remarkably
similar lifetimes in the solid state.
2.3. Quantum Chemical Calculations
To further explore the photoreactivity of vinyl azides 1, TD-DFT
calculations of their reactions were carried out using the B3LYP
method in conjunction with the 6-31+G(d) basis set.31,32 The
optimized minimal energy conformer of vinyl azide 1a showed
that the molecule is fully conjugated and flat (Figure 3). The S1
state of 1a was calculated by TD-DFT to be 76 kcal mol−1 higher
in energy than its ground state (S0) (Figure 4, left). The
optimization of the triplet configuration of vinyl azide 1a
3
revealed that it is best described as a 1,2-biradical, 2BRa
(Figure 3). The minimal energy conformer of 2BRa is 55 kcal
mol−1 above its S0 state (Figure 4A), and spin density
calculations located the unpaired electrons on the C and C
atoms, which were rotated ~90° away from each other to
minimize the overlap between the radical centers (Figure 4B).
3
The optimization of the structure of vinylnitrene 2Na showed
that this species exists as two conformers (trans and cis, Figure
3) corresponding to two energy minima, with the trans isomer
being 5 kcal mol−1 more stable than the cis isomer (Figure 4A).
Furthermore, spin density calculations showed that vinylnitrene
32Na has a significant 1,3-biradical character, and the rotational
barrier around the vinylic bond is low (~14 kcal mol−1) (Figure
4B). The calculated transition state barrier for the transformation
of 32BRa to vinylnitrene 32Na was found to be very small (0.4
kcal mol−1). Therefore, the calculations support the hypothesis
that vinylnitrene 32Na is easily formed from the excited state of
vinyl azide 1a. Structural flexibility can explain the formation of
both azirine 3a and isoxazole 4a in solution. The stationary
points on the triplet surfaces of vinyl azides 1b and 1c,
calculated using DFT, resulted in similar diagram as for vinyl
azide 1a (Supporting Information Figures S3-S4). Furthermore,
3
the calculated rotational barriers for vinylnitrenes 2Nb and
32Nc were comparable to that for vinylnitrene 32Na (Supporting
Information Figures S5-S6).
Figure 2. Transient spectra obtained by laser flash photolysis of (A)
1a, (B) 1b, and (C) 1c in argon-saturated nanocrystalline
suspensions.
The laser flash photolysis of vinyl azides 1 was performed in
nanocrystalline suspensions in accordance with literature
procedures.30 The transient absorption spectra obtained by laser
flash photolysis of crystalline 1a–c (Figure 2) are similar to
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(Supporting Information Video 2). The bubbles formed and
evolved from the crystals without a clear preference with respect
to their edges or visible defect sites.
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Figure 3. Optimized structures of 1a, 2BRa, and two isomers of
32Na. The numbers in the parenthesis are the calculated spin
densities.
Figure 5. Optical microcopy images showing the photomechanical
effects, cracking, and disintegration during the photochemical
reactions of the crystals of 1a–c. (A) Splintering of a crystal of 1a
upon irradiation. The approximate size of the crystal is ~1.6 × 0.5 ×
0.1 mm. (B) Bending of the crystal of 1b and development of
parallel striations on the crystal surface due to cracking. The
approximate size of the crystal is ~1.5 × 0.3 × 0.1 mm. (C) Bending
and cracking of a crystal of 1c. The approximate size of the crystal
is ~2 × 0.5 × 0.2 mm. For recording of the effects, see Supporting
Information Videos 1−7.
In stark contrast, the crystals of 1b were observed to visibly
bend, expand, and then crack (Figure 5B, Supporting
Information Video 3). Moreover, if they were not restrained,
some of these crystals jumped off the surface (Supporting
Information Video 4); however, no splintering or violent
fragmentation was observed. At this point, it is not clear whether
the expulsion of gas or the disintegration of the lattice are
responsible for the acquired motility of the crystals and the
ensuing debris. Since the crystal explosion occurs almost
concomitantly with the gas release, these effects cannot be
resolved by simple kinematic analysis. Another distinct
difference was that the cracks in the crystals of 1b always
developed perpendicular to the (010) face. When the irradiation
of 1b was performed in mineral oil, the crystals retained the
gaseous N2 before eventually releasing it at one of the cracks on
the (010) face. Occasionally, the formation of a gas bubble could
be observed in the crystal prior to cracking (Figure 6, Supporting
Information Video 5). The crystals of 1c behaved similarly to
those of 1b, in that exposure to light resulted in bending,
expansion, and cracking (Figure 5C, Supporting Information
Video 6) as well as some jumping (Supporting Information
Video 7). The cracks in the crystals of 1c also formed
perpendicular to the long axis, allowing for the release of N2 gas.
The behavior of the crystals of 1c in oil resembles that of the
crystals of 1b, with no N2 released from the crystal until cracks
Figure 4. A) Calculated stationary points on the triplet surface of
1a. The energies are in kcal mol−1. B) calculated rotational barrier
for 32Na.
2.4. Photomechanical and Disintegrative Effects
When the crystals of vinyl azide 1a were exposed to a 400nm
LED and monitored using an optical microscope equipped with
a camera, they were observed to fragment, shatter, and splinter
violently as a result of the photochemical reaction (Figure 5A,
Supporting Information Video 1). As the reaction progressed,
the color of the translucent crystals and the debris changed from
colorless to opaque amber. Due to violent fracturing of the
crystals, the resulting debris had irregular and defective surfaces,
and was not amenable to nanoindentation to determine their
mechanical properties. To observe the evolution of gas, the
crystals of azide 1a were covered in mineral oil and irradiated.
The released N2 appeared as bubbles emanating from the crystal
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develop in the interior that appear as striations on the surface generated. The gas is expected to accumulate preferentially at
(Supporting Information Video 8).
1
locations in the crystal where the interactions are weakest or in
any voids or channels in the structure. As more gas is generated,
it eventually reaches the surface of the crystal, which is the (010)
and (001) face for 1b and 1c, respectively. With this qualitative
scenario in mind, the structure diagrams in Figure 7 indicate that
all three crystal structures contain channels owing to their sheet-
like packing motifs. The channels provide preferred sites for
accumulation of gas. Being effective concentrators of the
accumulated elastic energy, the first cracks are expected to
appear at surface imperfections or other crystal defects that
alleviate the stability of the lattice. This reasoning implies that
the origin of the photomechanical motions lies within the
difference in lattice energetics between the three crystals; 1a
probably lacks sufficient energy to prevent cracking, whereas 1b
and 1c likely possess more energy owing to their resistance to
cracking. In an attempt to determine the structure of the products
while they was generated inside the crystal interior, we resorted
to X-ray photodiffraction (photocrystallography) by using
different conditions. However, these efforts remained fruitless
due to the proclivity of the crystal to crack and partially or
completely disintegrate before significant amounts of the
product could be generated for data collection and structure
determination.
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Figure 6. Formation of a bubble inside a crystal of azide 1b and its
subsequent release. Owing to the transparent nature of the crystal,
red lines have been added to highlight a pair of parallel crystal
edges. See Supporting Information Video 5.
Because the deformation is related to mechanical compliance of
the crystal, nanoindentation experiments were performed to
determine if the crystal stiffness and hardness could explain the
differences in the mechanical motions with gas release. Vinyl
azides 1b and 1c were found to have elastic (Young’s) moduli
of 9.8 ± 0.5 GPa and 8.1 ± 0.7 GPa, respectively (Figure S11),
attempts to obtain the elastic modulus of 1a were unsuccessful
owing to the brittle nature of its crystals. This result indicates
that 1b and 1c are likely to have higher stiffnesses, which was
subsequently confirmed using lattice energy calculations.
2.5. Crystal Structures and Mechanical Properties
The crystal structures of vinyl azides 1a–c were determined and
inspected for clues that may help elucidate the origin of the
photomechanical effects. The crystals of vinyl azides 1a and 1b
were both found to belong to the monoclinic P21/n space group,
1
whereas the crystals of 1c were in the triclinic P space group.
The intramolecular atomic distances are very similar in all three
compounds, generally falling within the range of ±0.1 Å for
C=C, C–C(O), and C–N3 bonds (Crystallographic Supporting
Information). The main structural difference in molecular
geometries was the torsion angle between the ortho carbon of
the phenyl ring and the carbonyl oxygen; 1a and 1b have planar
torsion angles of ≤1°, whereas that in 1c is 11° (Supporting
Information Figure S7-S10). In the crystals, the molecules of
1a–c pack in sheet-like motifs (Figure 7). The centroid-to-
centroid distances between the phenyl rings are all in the range
3.80–3.87 Å, suggesting weak π–π interactions. The shortest
distance between the vinyl moieties (C-to-C, C-to-C, N1-to-
N1, N2-to-N2 and N3-to-N3) of adjacent molecules are also
similar for 1a and 1b. As the crystal packing of the three
compounds is very similar, it cannot account for the differences
in their photodynamic behavior.
The faces of typical crystal habits were indexed and related to
the respective crystal structures to identify any preferred
directions in the crystals for reactivity, bending, and gas release.
As expected from the crystal symmetries, the crystal faces of 1c
are different from those of 1a and 1b (Figure 7). A comparison
of the packing diagrams and crystal faces indicated that 1b and
1c first show cracking on the (010) and (001) faces, respectively,
whereas 1a does not show a preferential face for chipping or
cracking. The directionality of cracking in 1b and 1c can be
rationalized by considering the intermolecular interactions. If
the interactions are sufficiently strong to overcome the elastic
energy, the gas remains inside the crystal during the course of
the photochemical reaction. It accumulates until a threshold in
the accumulated elastic energy is reached and a crack is
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Figure 7. Molecular packing in the crystals of compounds 1a–c
and shattering. This observation is supported by the
1
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3
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before and during reaction, and correlation with the mechanical
effects. (A,C,E)ꢀSuperimposition of an optical images of single
crystals of 1a (A), 1b (C) and 1c (E) and the respective crystal
structures before irradiation. (B,D,F) Optical images of the crystals
of 1a (B), 1b (D) and 1c (F) after irradiation, and model of the
respective crystal structures at the initial stage of nitrogen release,
before significant structure perturbation occurs. The channels are
shown between the molecular columns where the nitrogen
molecules are likely to accumulate before pressure builds up and
induces peeling, bending and cracking of the crystals.
nanoindentation experiments, which confirmed that the crystals
of 1a were too brittle to provide a proper basis for crystal
indentation. Lastly, the void spaces in this crystal are located
near the area with the weakest lattice energy interactions. The
void space location is significant because as the pressure of N2
increases in these void spaces, the likelihood of overcoming the
weak forces in this area increases, allowing cracks to appear.
In contrast, when the crystals of 1b are irradiated, they
consistently break along their (010) face. When this observation
is compared to the lattice interaction calculations, as shown in
Figure 8, the repeated cracks can reasonably be attributed to the
breakdown of the weak interactions between the methoxy
groups, which are the weakest interactions identified by the
lattice energy calculations. Because these weak interactions are
stronger than those in 1a, the crystals of 1b maintain their form
and shape more effectively upon the release of nitrogen gas. This
behavior is supported by the nanoindentation experiments,
which showed that the crystals of azide 1b had the highest
Young’s modulus. Again, the location of the void space
corresponds well with the location of the weak interactions in
the crystal, suggesting that the growing N2 volume will
eventually overcome the weak interactions.
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2.6. Lattice Energy Calculations
As lattice energies and anisotropy of intermolecular interactions
appear to be the key to explaining the different photomechanical
effects, the so-called energy frameworks33 were calculated to
provide a more quantitative description and to visualize the
anisotropy of the intermolecular interactions in the crystals. The
results, in Figure 8, showed that 1a features significantly weaker
crystal interactions than 1b and 1c. In 1a, the strong interactions
ranged between 15.2 and 33.9 kJ mol−1 with weaker interactions
of 6 kJ mol−1. In comparison, 1b had strong interactions ranging
from 13.2 to 40.6 kJ mol−1 and weak interactions of 11.7–12.9
kJ mol−1. Similar to 1b, 1c had strong interactions of 18.4–41.5
kJ mol−1 and weak interactions of 7.8–10.1 kJ mol−1. These
differences in the energy framework are responsible for the
differences in the strength toward cracking between the weak,
flaky crystals of 1a and the strong, mechanical crystals of 1b and
1c. The observation that 1b and 1c crack along a specific crystal
face during irradiation implies that as N2 gas is released within
the crystals at locations where the intermolecular interactions are
weakest. Support for this hypothesis is drawn from the
correlation among the lattice energy calculations, crystal face
indexing, and nanoindentation measurements.
Similar results were obtained when this rationale was applied to
1c. However, unlike those of 1b, upon irradiation the crystals of
1c crack perpendicular to the (001) face. When the lattice energy
calculation is rotated perpendicular to the c axis to imitate this
cracking action, the spaces in the cracks again align with the
weak interactions between the chlorine substituents between the
layers. Furthermore, the chlorine interactions are weaker than
the methoxy interactions, which is supported by the
nanoindentation data, which showed that 1c had a lower
Young’s modulus than 1b but presumably a higher Young’s
modulus than 1a. These weaker interactions are evident in the
videos showing the decomposition of 1c (Supporting
Information Video 7), in which the crystal shape is mostly
maintained, similar to the crystals of 1b, but some crystals
fragment and shatter, like the crystals of 1a. This behavior is in
line with the weak lattice points identified by the lattice energy
calculations being intermediate between those in 1a and 1b.
3. CONCLUSIONS
The reaction mechanism and photomechanical effects of vinyl
azides 1a–c were thoroughly investigated. To the best of our
knowledge, this is the first in-depth study of crystal
disintegration and movements induced by gas release where the
photomechanical effects are directly correlated with the
structure in a set of structurally similar compounds. While the
mechanistic details of the photochemical reactions in these
crystals are well established from the transient spectra and the
crystal structures of the unreacted crystals provides a solid basis
to hypothesize the origin of the mechanical effects, the relation
with the mechanical effect should be considered with some
precaution. Namely, the three key processes—the
photochemical reaction that results in formation and
accumulation of gas, the formation of cracks, and finally the
crystal explosion—occur on very different time scales that cover
several orders of magnitude. The kinetics of the mechanical
response is unlikely to follow the kinetics of the photochemical
reaction because the formation of cracks requires accumulation
of gaseous product and buildup of pressure up to a threshold
value. Since the quantum yield of the reaction is low, this
process requires significant amount of time and occurs on a
Figure 8. Energy frameworks of vinyl azides 1a–c (tube size = 80,
energy cutoff = 5 kJ mol−1). (A) 1a has weak interactions of 6 kJ
mol−1 and strong interactions of 15.2−33.9 kJ mol−1. (B) 1b has
weak interactions of 11.7−12.9 kJ mol−1 and strong interactions of
13.2–40.6 kJ mol−1. (C) 1c has weak interactions of 7.8–10.1 kJ
mol−1 and strong interactions of 18.4–41.5 kJ mol−1
Vinyl azide 1a does not exhibit a preferential cracking direction.
Instead, crystal surface delamination or random shattering
occurs upon irradiation. Correspondingly, the lattice energy
calculations showed that this compound has the weakest
interactions. These very weak interactions cannot keep the
crystal together effectively, resulting in random delamination
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much longer time scale than the photochemical reaction removed under vacuum and the residue extracted with diethyl
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generating the gas. As a consequence, the mechanical response
is much slower and has it own kinetics, which is additionally
affected by the ensuing release of gas. This work highlights the
relevance of different intracrystalline forces that determine the
mechanical effects as a result of release of gas molecules. These
and other gas-releasing crystals could be of significant use to
researchers looking to create single-crystalline machines, where
the momentum generated by the gas propels a crystal as a
macroscopic entity. These “crystalline machines” would
represent the macroscopic versions of molecular and nanoscale
robots that operate via gas-generating chemical reactions.
ether (100 mL). The diethyl ether layer was washed twice with
water, once with brine (saturated NaCl) and dried over
anhydrous MgSO4. The diethyl ether was evaporated under
vacuum to yield crude 1-phenyl-prop-2-yn-1-ol (2.0 g, 15.2
1
mmol, 78% yield). H NMR, 13C NMR and IR spectra of the
product are consistent with those reported.41 1H NMR (400
MHz, CDCl3,):  2.38 (broad, 1H), 2.67 (d, J = 2.4 Hz, 1H), 5.46
(d, J = 2.4 Hz, 1H), 7.32 – 7.41 (m, 3H), 7.55 (d, J = 7.2 Hz, 2H)
ppm; 13C NMR (100 MHz, CDCl,):  64.4, 74.9, 83.5, 126.6,
128.6, 128.7, 140.0 ppm; IR (CDCl3): 3291, 3064, 3033, 2118,
1493, 1455, 1022, 1002, 947, 916, 698, 650 cm-1.
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4.3.2. Synthesis of 1-(4-methoxy-phenyl)-prop-2-yn-1-ol. The
same procedure was followed for the synthesis of 1-phenyl-
prop-2-yn-1-ol.41,42 A reaction of p-methoxybenzaldehyde (2.65
g, 19.5 mmol) with ethynylmagnesium bromide (48 mL, 24
mmol, 1.2 eq) resulted in 4-methoxy-phenyl)-prop-2-yn-1-ol
(2a) (2.73 g, 16.8 mmol) in 86% yield. The 1H NMR, 13C NMR
and IR spectra of the product compare well with those in the
literature.43,44 1H NMR (400 MHz, CDCl3):  2.49 (broad, 1H),
2.66 (d, J = 2 Hz, 1H), 3.80 (s, 3H), 5.40 (d, J = 2 Hz, 1H), 6.90
(d, J = 8.4 Hz, 2H), 7.46 (d, J = 8.4 Hz, 2H) ppm.; 13C NMR
(100 MHz, CDCl3,):  55.4, 64.0, 74.6, 83.8, 114.0, 128.1,
132.4, 159.8 ppm.; IR (CDCl3): 3397, 3288, 3005, 2959, 2838,
2116, 1611, 1588, 1512, 1250, 1175, 1112, 1031, 913, 833 cm-
4. EXPERIMENTAL SECTION
4.1. Calculations. The geometries of all structures were
optimized using the B3LYP level theory with a 6-31G+(d) basis
set with Gaussian16 software.31,32,34 Absorption spectra of
reactants, intermediates, and products were obtained using TD-
DFT calculations using C-PCM solvation method and
acetonitrile.3537 Transition states were confirmed by analysis of
one imaginary vibrational frequency as determined by the
second derivative of the energy with respect to the optimized
internal coordinates. The geometry of the transition state was
also determined to be at a maximum of energy between the
reactant and product by Internal Reaction Coordinate (IRC)
calculations.38,39 All calculations were performed at the Ohio
Supercomputer Center.
1
.
4.3.3. Synthesis of 1-(4-chloro-phenyl)-prop-2-yn-1-ol. 1-
Phenyl-prop-2-yn-1-ol was prepared in a similar manner as
described above.41,42 A reaction of p-chlorobenzaldehyde (1.50
g, 10.7 mmol) with ethynylmagnesium bromide (48 mL, 24
mmol, 1.2 eq) resulted in 4 chloro-phenyl)-prop-2-yn-1-ol (1.7
g, 10.3 mml) in 96% yield. The NMR spectra of this compound
were similar to previous literature values.45 1H NMR (400 MHz,
CDCl3): δ 2.45 (broad, 1H), 2.66 (d, J = 2.0 Hz, 1H), 5. 40 (d, J
= 2.0 Hz, 1H), 7.32 (d, J = 7.6 Hz, 2H), 7.46 (d, J = 7.6 Hz, 2H)
ppm. 13C NMR (100 MHz, CDCl3): 61.8, 75.0, 82.3, 128.3,
128.8, 132.8, 137.3 ppm. IR (CDCl3): 3297, 1594, 1575, 1273,
1191, 1032, 955, 818, 757, 645 cm-1. GC/MS (EI): m/z 168, 167,
166, 149, 139, 131 (100), 113, 103, 77, 65.
4.2. Laser Flash Photolysis. Laser flash photolysis in solution
and the solid-state were performed with a laser flash photolysis
apparatus connected to an excimer laser (308 nm, 17 ns) .40 The
system at the University of Cincinnati is a commercial XE-LMP
Luzchem system using a Lambda Physik LPX 100 Excimer laser
at 308 nm with a 17 ns resolution. For solution laser flash
photolysis, stock solutions of derivatives 1a, 1b, and 1c in
acetonitrile were prepared with an absorption between 0.3 and
0.8 at 308 nm. Solid-state nanocrystalline suspensions are
prepared following the procedure by Simoncelli et al.30
A
saturated acetone solution of 1a, 1b, or 1c was added to a water
solution in a volumetric flask placed in a ultrasound bath, until
the absorption at 308 nm was between 0.3 and 0.8. The samples
were placed in a quartz cuvette with a 10 mm x 10 mm cross
section, and for measurement obtained in argon- or oxygen-
saturated acetonitrile and suspentions, the cuvettes were capped
with rubber septum and the appropiate gas bubbled through
them for 5 minutes. Transient absorption spectra were produced
by plotting absorbance values that had been averaged from the
kinetic traces at 20 nm intervals from 300 to 600 nm over the
specified time intervals. Kinetic information was determined by
plotting averaged kinetic profiles from specified wavenlength
and fitting the kinetic profiles with Igor Pro data software
produced by WaveMetrics.
4.3.4. Synthesis of 1-phenyl-propynone. 1-Phenylpropyone was
prepared using a similar method to Pigge et al. with some
modification.42 Jones reagent was prepared by mixing CrO3 (4 g,
0.04 mol) with 4 mL of conc. H2SO4 to yield a slurry, to which
water (3 x 4 mL) was added.26 The Jones reagent was added
dropwise to 1-phenyl-prop-2-yn-1-ol (2.0 g, 15.2 mmol)
dissolved in acetone (25 mL) at 0C, over a period of 10 min
until the color of the solution changed to green and then to
orange. The mixture was stirred for 30 min and the solvent
removed under vacuum. The resulting residue was extracted
with diethyl ether (100 mL). The ether layer was washed twice
with water (100 mL), once with saturated NaHCO3 (100 mL),
once with saturated brine solution (100 mL), and dried over
anhydrous MgSO4. The diethyl ether was evaporated under
vacuum to yield crude 1-phenyl-propynone (1.38 g, 10.6 mmol)
70% yields. 1H NMR, 13C NMR and IR spectra of this compound
correspond with the literature.46,47 1H NMR (400 MHz, CDCl3):
 3.45 (s, 1H), 7.51 (t, J = 7.8 Hz, 2H), 7.66 (t, J = 7.4 Hz, 1H),
8.18 (d, J = 8.4 Hz, 2H) ppm.; 13C NMR (100 MHz, CDCl3): 
80.3, 80.8, 128.7, 129.7, 134.5, 136.2, 177.4 ppm; IR (CDCl3):
3237, 2094, 1638, 1264, 1072, 1072, 1031, 696 cm-1.
4.3. Preparation of starting materials
4.3.1. Synthesis of 1-phenyl-prop-2-yn-1-ol. We followed the
procedure reported in the literature by Chassaing et al. and Pigge
et al. with some modifications.41,42 To
a solution of
benzaldehyde (2.05 g 18.9 mmol) in 25 mL of anhydrous THF
at 0C was added a solution of ethynylmagnesium bromide (48
mL 24 mmol, 1.2 eq) in 15 mL of anhydrous THF. The mixture
was stirred for 20 hours at 0C and the reaction quenched by
adding NH4Cl (30 g of NH4Cl dissolved in 100 mL of H2O) and
the resulting mixture was stirred for 1 hour. The solvent was
4.3.5. Synthesis of 1-(4-methoxy-phenyl)-propynone. 1-(4-
Methoxy-phenyl)-prop-2-yn-1-ol (2.74 g, 16.8 mmol) was used
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as described above42 to produce crude 1-(4-methoxy-phenyl)-
7.99 (d, J = 7.8 Hz, 2H) ppm; 13C NMR (100 MHz, CDCl3,): δ
36.3, 57.3, 129.8, 130.8, 132.8, 137.6, 197.0 ppm; IR (CDCl3):
1
propynone (2.01 g, 12.5 mmol) in 74% yields. H NMR, 13C
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NMR and IR of this compound agree with those reported
previously.4648 1H NMR (400 MHz, CDCl3):  3.34 (s, 1H), 3.90
(s, 3H), 6.97 (d, J = 8.8 Hz, 2H), 8.14 (d, J = 8.8 Hz, 2H) ppm;
13C NMR (100 MHz, CDCl3):  55.6, 80.0, 80.4, 114.0, 129.6,
132.2, 164.8, 176.0 ppm; IR (CDCl3): 3251, 2991, 2093, 1640,
1599, 1024, 840, 686 cm-1.
1693, 1588, 1569, 1468, 1432, 1344, 1270, 1213, 1073, 1040,
979, 757, 626, 584 cm-1; GC/MS (EI): m/z 248, 246, 244, 218,
165, 139 (100), 133, 111, 105, 85, 75. RMS: m/z calculated for
C9H7ONaClBr2 [M+Na]+, 346.8450; found, 346.8452.
4.3.10. Synthesis of 3-azido-1-phenyl-propenone (1a). 3,3-
Dibromo-1-phenyl-propan-1-one (1.69 g, 5.8 mmol) was
dissolved in 20 mL of methanol and 20 mL of acetone at 0°C.
NaN3 (1.01 g, 15.5 mmol, 5 eq) dissolved in a minimal amount
of water (~5 mL) was added to the solution. The mixture was
stirred for ~1 hour. The solvent was removed under vacuum and
the resulting residue was extracted with diethyl ether (50 mL).
The ether layer was washed twice with water (100 mL), once
saturated brine (100 mL), and dried over anhydrous MgSO4. The
solvent was evaporated under vacuum to give crude 3-azido-1-
phenyl-propenone (1a) (0.710 g, 4.1 mmol, 71% yields). The
crude was recrystallized from dichloromethane to give pale
yellow crystals. The product was characterized using 1H NMR,
13C NMR, IR, and MS spectroscopy. mp (observed): 78 – 84°C
(lit.;51 87 - 87.5°C). 1H NMR (400 MHz, CDCl3):  6.77 (d, J =
13.2 Hz, 1H), 7.48 (t, J = 7.6 Hz, 2H), 7.50 (d, J = 13.2Hz, 1H),
7.58 (t, J = 7.4 Hz, 1H), 7.92 (d, J = 7.2 Hz, 2H) ppm; 13C NMR
(100 MHz, CDCl3):  113.2, 128.2, 128.7, 133.0, 137.7, 144.9,
188.8 ppm; IR: 3069, 2133, 1657, 1596, 1575, 1448, 1253, 1215,
1018, 950, 773, 682, 644 cm-1. HRMS: m/z calculated for
C9H7N3ONa+ [M+Na]+, 196.04813; found, 196.04816.
4.3.6. Synthesis of 1-(4-chloro-phenyl)-propynone. 1-(4-Chloro-
phenyl)-propynone (1.7 g, 10.3 mmol) was used as described
above42 to produce crude 1-(4-methoxy-phenyl)-prop-2-yn-1-ol
(1.46 g, 8.9 mmol) in 86% yields. The NMR spectra of this
compound were similar to those reported.4750 1H NMR (CDCl3,
400 MHz):  3.46 (s, 1H), 7.49 (d, J = 8 Hz, 2H), 8.10 (d, J = 8
Hz, 2H) ppm; 13C NMR (100 MHz, CDCl3): δ 81.2, 81.3, 131.8,
133.4, 133.9, 134.5, 175.9 ppm; IR (CDCl3): 3225, 2088, 1652,
1588, 1564, 1462, 1432, 1235, 1136, 1040, 999, 730, 675, 626
cm-1; GC/MS (EI): m/z 166, 165, 164, 139, 138, 137, 136 (100),
111, 101, 85, 75.
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4.3.7. Synthesis of 3,3-dibromo-1-phenyl-propan-1-one. 3,3-
Dibromo-1-phenyl-propan-1-one was synthesized following the
procedure reported by Sanseverino et al. with some
modifications.49 1-Phenyl-propynone (1.38 g, 10.6 mmol) was
dissolved in dichloromethane (25 mL) and kept at -8°C and SiO2
(6.36 g, 106 mmol, 10 eq) added, followed by dropwise addition
of PBr3 (1.924 g, 7.1 mmol, 0.66 eq) in dichloromethane (10
mL) using a dropping funnel over a period of 10 min. The
mixture was stirred for 20 hours and the SiO2 was filtered off
and the solution dried. The solvent was removed under vacuum
to yield crude 3,3-dibromo-1-phenyl-propan-1-one (2.82 g, 9.6
4.3.11. Synthesis of 3-azido-1-(4-methoxy-phenyl)-propenone
(1b). The synthesis for 1b was analogous to 1a. 3,3-Dibromo-
1-(4-methoxy-phenyl)-propan-1-one (1.43 g, 4.4 mmol) resulted
in crude 3-azido-1-(4-methoxy-phenyl)-propenone (1b) (0.790
g, 3.9 mmol, 88% yields). The crude was recrystallized from
dichloromethane and yielded pale-yellow crystals which were
characterized using 1H NMR, 13C NMR, IR and MS
spectroscopy. mp (observed): 105-108°C (lit.;51 109-110°C). 1H
NMR (400 MHz, CDCl3):  3.88 (s, 3H), 6.76 (d, J = 13 Hz, 1H),
6.95 (d, J = 8.8 Hz, 2H), 7.48 (d, J = 13 Hz, 1H), 7.92 (d, J = 8.8
Hz, 2H) ppm; 13C NMR (100 MHz, CDCl3):  55.5, 113.0,
113.9, 130.6, 144.0, 163.6, 187.1 ppm; IR: 3045, 2138, 1658,
1592, 1574, 1515, 1256, 1226, 1172, 1024, 954, 822, 779, 599
cm-1. HRMS: m/z calculated for C10H9N3O2Na+ [M+Na]+,
226.05870; found, 226.05873.
1
mmol, 91% yield). The product was characterized using H
NMR, 13C NMR, IR and MS spectroscopy. The NMR and IR
spectra are consistent with literature values.50 1H NMR (400
MHz, CDCl3):  4.19 (d, J = 6.7 Hz, 2H), 6.20 (t, J = 6.7 Hz,
1H), 7.51 (t, J = 7.6 Hz, 2H), 7.63 (t, J = 7.4 Hz, 1H), 7.96 (d, J
= 7.6 Hz, 2H) ppm; 13C NMR (100 MHz, CDCl3):  36.7, 53.3,
128.2, 128.9, 134.0, 135.7, 194.3 ppm. IR (CDCl3): 1684, 1598,
1580, 1347, 1263, 1219, 755, cm-1; HRMS: m/z calculated for
C9H8O2Br2Na+ [M+Na]+, 314.8819; found, 314.8738; m/z
calculated for C9H8O2Br2H+ [M+H]+, 292.9000; found,
292.9025.
4.3.8. Synthesis of 3,3-dibromo-1-(4-methoxy-phenyl)-propan-
1-one. Synthesis of 3,3-dibromo-1-(phenyl)-propan-1-one was
accomplished by a similarly modified Sanseverino procedure as
4.3.12. Synthesis of 3-azido-1-(4-chloro-phenyl)-propenone
(1c). The synthesis of 1c was similar to that described for 1a.
3,3-Dibromo-1-(4-chloro-phenyl)-propan-1-one (2.0 g, 6.1
mmol) resulted in crude 3-azido-1-(4-chloro-phenyl)-propenone
(1c) (0.68 g, 5.4 mmol) in 89% yield. mp (observed): 93-98°C
described above.49
A reaction of 1-(4-methoxy-phenyl)-
propynone (2.01 g, 12.5 mmol) resulted in crude 3,3-dibromo-
1-(4-methoxy-phenyl)-propan-1-one (3.54 g, 11.0 mmol, 88%
yield). The product was characterized using 1H NMR, 13C NMR,
1
(lit.;51 98-99°C). H NMR (400 MHz, CDCl3): δ 6.37 (d, J = 14
1
Hz, 1H), 7.47 (d, J = 8.8 Hz, 2H) 7. 53 (d, J = 14 Hz, 1H), 7.86
(d, J = 8.8 Hz, 2H) ppm; 13C NMR (100 MHz, CDCl3): δ 117.5,
129.6, 131.2, 136.0, 139.5, 145.4, 187.4 ppm; IR: 3051, 2126,
1660, 1590, 1568, 1254, 1213, 1091, 1013, 823 cm-1; GC/MS
(EI): m/z 181, 180, 179, 151, 141, 140, 139, 126, 125, 124, 116,
113, 112, 111, 89, 85, 75, 68, 64.
IR and MS spectroscopy. H NMR (400 MHz, CDCl3):  3.89
(s, 3H), 4.13 (d, J = 6.4 Hz, 2H), 6.19 (t, J = 6.4 Hz, 1H), 6.97
(d, J = 8.8 Hz, 2H), 7.94 (d, J = 8.8 Hz, 2H) ppm; 13C NMR
(CDCl3, 100 MHz):  37.3, 52.9, 55.6, 114.1, 128.8, 130.7,
164.2, 192.8 ppm; IR: 1673, 1599, 1574, 1463, 1421, 1398,
1346, 1318, 1311, 1258, 1223, 785, cm-1; HRMS: m/z calculated
for C10H10O2Br2Na+ [M+Na]+, 344.89193; found, 344.89206.
4.4. Photolytic studies
4.3.9. Synthesis of 3,3-dibromo-1-(4-chloro-phenyl)-propan-1-
4.4.1. Photolysis of 1a in chloroform-d. A solution of 1a (14 mg,
0.08 mmol) in CDCl3 (1 mL) was purged with argon for 5 min
and photolyzed via a Pyrex filter for up to 24 hours at 298 K. 1H
NMR and GC analysis of the reaction mixture showed the
formation of 3a, 4a and 5a. The products were characterized by
GC-MS chromatography, 1H-NMR, 13C-NMR and IR
spectroscopy of the reaction mixture. The spectral data of 4a and
one. We followed the same procedure as for synthesis of 3,3-
dibromo-1-(phenyl)-propan-1-one.49
A
reaction of 1-(4-
methoxy-phenyl)-propynone (2.01 g, 12.5 mmol) resulted in
crude 3,3-dibromo-1-(4-chloro-phenyl)-propan-1-one (2.0 g,
6.1 mmol, 69% yield). 1H NMR (400 MHz, CDCl3): δ 4.19 (d, J
= 6.8 Hz, 1H), 6.17 (t, J = 6.8 Hz, 1H), 7.48 (d, J = 7.8 Hz, 2H)
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1H-NMR and IR spectroscopy. In each case, the photoproduct
5a match with those reported in the literature.2125 2-Phenyl-2H-
azirine (3a): 1H NMR (400 MHz, CDCl3,):  3.47 (d, J = 2 Hz,
1H), 7.44 – 7.55 (m, 3H), 8.06 (d, J = 8.4 Hz, 2H), 9.57 (d, J =
2 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3,):  28.4, 128.3,
128.8, 133.6, 136.9, 152.4, 197.4 ppm; 5-Phenyl-1,2-oxazole
(4a): 1H NMR (400 MHz, CDCl3):  6.53 (d, J = 1.6 Hz, 1H),
7.44 – 7.55 (m, 2H), 7.61 – 7.69 (m, 1H), 7.79 – 7.82 (m, 2H),
8.30 (d, J = 1.6 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3): 
98.7, 125.9, 127.3, 129.0, 130.2, 150.9, 169.4 ppm; 3-Oxo-3-
phenylpropanenitrile (5a): 1H NMR (400 MHz, CDCl3):  4.10
(s, 2H), 7.53 (t, J = 7.2 Hz, 2H), 7.67 (t, J = 7.2 Hz, 1H), 7.93 (d,
J = 7.2 Hz, 2H) ppm; 13C NMR (100 MHz, CDCl3):  29.4,
113.8, 128.5, 129.2, 134.3, 134.8, 187.1 ppm. IR (CDCl3): 2263,
1689 cm-1.
1
formed was the corresponding azirine 3.
2
3
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5
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4. 5. Thermolysis of vinyl azides 1a-1c. NMR tubes were
charged with solutions of 1a-1c (4.0 mg) in CDCl3 (0.5 mL).
The NMR tubes were heated in a pre-heated oil bath at 70°C for
10 min. 1H NMR, 13C-NMR, and IR spectroscopy of the reaction
mixture showed the formation of the corresponding azirines 3
and cyanides 5.
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4.6. Nanoindentation. The nanoindentation measurements on
1b and 1c were performed with an Agilent G200 nanoindenter
equipped with an XP head, using Berkovitch diamond indenter.
Each indentation was performed using the continuous stiffness
method to a depth of 1000 nm with a strain rate of 0.05, an
amplitude of 2 nm, and a frequency of 45 Hz.52 Prior to the
indentation, the stiffness and the geometry of the tip were
determined by using Corning 7980 silica reference sample
(Nanomechanics S1495-25). In order to ensure that the tip was
fully engaged, the modulus was measured between 200 and 1000
nm. The value of the Poisson’s ratio was assumed to be 0.30. A
comparison of the young’s modulus of 1b and 1c crystals with a
very few relevant recent photoresponsive systems were provided
in Supporting Information Table S10.5354
4.4.2. Photolysis of 1b in chloroform-d. A solution of 1b (20
mg, 0.1 mmol) in CDCl3 (1 mL) was purged with argon for 5
min and photolyzed via a Pyrex filter for up to 24 hours at 298
K. GC and NMR analysis of the reaction mixture showed the
formation of 3b, 4b and 5b. The products were characterized by
GC-MS chromatography, 1H-NMR, 13C-NMR and IR
spectroscopy of the reaction mixture. The spectral data for 4b
and 5b match with those reported in the literature.2125 2-(4-
Methoxy-phenyl)-2H-azirine (3b): 1H NMR (400 MHz, CDCl3):
 3.43 (d, J = 2.0 Hz, 1H), 3.86 – 3.89 (m, 3H), 6.97 – 7.00 (m,
2H), 8.057 (d, J = 8.8 Hz), 9.57 (d, J = 2.0 Hz, 1H) ppm; 13C
NMR (100 MHz, CDCl3,):  28.0, 55.5, 114.0, 130.0, 130.6,
152.9, 163.9, 195.6 ppm. IR (CDCl3): 1663 cm-1. 5-(4-Methoxy-
4.7. Crystal Movement Video Specifications. Crystals of vinyl
azides 1a, 1b, and 1c were grown from a dichloromethane
solution at 0°C and placed on the stage of a Keyence VHX-
1000E digital microscope. The photolysis was initiated using a
LuzChem LED array with blue light, as well as two LED’s of
365 nm and 400 nm outputs. The photolysis was recorded over
the course of a few min of exposure for video 2-8 and 60 min for
video 1.
1
phenyl)-1,2-oxazole (4b): H NMR (400 MHz, CDCl3):  3.86
(s, 3H), 6.40 (d, J = 1.6 Hz, 1H), 6.97 – 7.00 (m, 2H), 7.74 (d, J
= 8.8 Hz, 2H), 8.25 (d, J = 2.0 Hz, 1H) ppm; 13C NMR (100
MHz, CDCl3):  55.4, 97.3, 114.4, 120.2, 127.5, 150.8, 161.1,
169.3 ppm.; IR (CDCl3): 3014, 2946, 2840, 1599, 1462, 826 cm-
1; GC/MS (EI): m/z 175 (M, 100%), 160, 135, 120, 107, 92, 77,
63, 51. 3-Oxo-3-(4-methoxy-phenyl) propanenitrile (5b): 1H
NMR (400 MHz, CDCl3,):  3.90 (s, 3H), 4.03 (s, 2H), 6.98 (d,
J = 8.8 Hz, 2H), 7.90 (d, J = 8.8 Hz, 2H) ppm; 13C NMR (CDCl3,
100 MHz):  29.0, 55.7, 114.1, 114.3, 127.5, 130.9, 164.7, 185.4
ppm. IR (CDCl3): 2260, 1682 cm-1.
ASSOCIATED CONTENT
The Supporting Information is available free of charge on the ACS
Publications website. https://pubs.acs.org/doi/10.1021/
4.4.3. Photolysis of 1c in chloroform-d. A solution of 1c in
CDCl3 was purged with argon for 5 min and photolyzed via a
Pyrex filter for up to 24 hours at 298 K. GC analysis of the
reaction mixture showed the formation of 3c, 4c and 5c. The
products were characterized 1H-NMR, 13C-NMR and IR
Spectra for characterization of 1a–c and the photoproducts; DFT
calculation data; CCDC 2015372-2015374 which contain the
crystallographic data in CIF format. These data can be obtained, free
of charge via http://www.ccdc.cam.ac.uk/products/csd/request/ (or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; Fax: 44-1223-336033 or e-mail:
deposit@ccdc.cam.ac.uk).
spectroscopy of the reaction mixture. The spectral data for 4c
2125
and 5c match with those reported in the literature.
2-(4-
Chloro-phenyl)-2H-azirine (3c): 1H NMR (400 MHz, CDCl3): 
3.42 (d, J = 2.4 Hz, 1H), 7.44 – 7.54 (m, 2H), 7.87 (d, J = 8.4
Hz, 2H), 9.56 (d, J = 2.0 Hz, 1H) ppm; 13C NMR (100 MHz,
CDCl3): 28.3, 129.8, 131.5, 135.1, 140.1, 152.2, 196.2 ppm. 5-
(4-Chloro-phenyl)-1,2-oxazole (4c): 1H NMR (400 MHz,
CDCl3):  6.52 (d, J = 2 Hz, 1H), 7.46 (d, J = 8.4, 2H), 7.74 (d,
J = 8.4, 2H), 8.30 (d, J = 1.6 Hz, 1H) ppm; 13C NMR (100 MHz,
CDCl3):  99.0, 125.7, 127.2, 129.3, 136.3, 150.9, 168.3 ppm. 3-
Oxo-3-(4-chloro-phenyl)propanenitrile (5c): 1H NMR (400
MHz, CDCl3):  4.06 (s, 2H), 7.51 (d, J = 8.4 Hz, 2H), 7.87 (d,
J = 8.4 Hz, 2H) ppm; 13C NMR (100 MHz, CDCl3,):  29.4,
113.4, 129.7, 129.9, 132.5, 141.5, 186.0 ppm.
AUTHOR INFORMATION
Corresponding Authors
Anna D. Gudmundsdottir – Department of Chemistry, University
of Cincinnati, Cincinnati, OH 45221-0172, United States; Email:
anna.gudmundsdottir@uc.edu
Panče Naumov – New York University Abu Dhabi, P. O. Box
129188, Abu Dhabi, United Arab Emirates; Email:
pance.naumov@nyu.edu
Authors
4.4.4. Solid-State Photolysis of vinyl azides 1a-1c. Crystals of
each vinyl azide (5−10 mg) were crushed between two
microscope slides and exposed to either a mercury arc lamp in
Pyrex or 400 nm LED for 1 h and the sample was analyzed by
Dylan J. Shields – Department of Chemistry, University of
Cincinnati, Cincinnati, OH 45221-0172, United States;
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(7) Karothu, D. P.; Halabi, J. M.; Li, L.; Colin-Molina, A.; Rodríguez-
Molina, B.; Naumov, P. Global performance indices for dynamic
crystals as organic thermal actuators. Adv.
Mater. 2020, 32, 1906216.
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memory effects in molecular crystals. Nat.
Commun. 2019, 10, 3723.
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P.; Halabi, J. M.; Al-Handawi, M. B.; Li, L. The Rise of the
Dynamic Crystals. J. Am. Chem. Soc. 2020, 142, 13256– 13272.
(10) Naumov, P.; Chizhik, S.; Panda, M. K.; Nath, N. K.; Boldyreva,
E. Mechanically responsive molecular crystals. Chem. Rev. 2015,
115, 12440–12490.
(11) Naumov, P.; Sahoo, S. C.; Zakharov, B. A.; Boldyreva, E. V.
Dynamic single crystals: Kinematic analysis of photoinduced
crystal jumping (the photosalient effect). Angew. Chem. Int. Ed.
2013, 52, 9990–9995.
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Russ. Chem. Bull. 2018, 67, 933– 948.
Durga Prasad Karothu – New York University Abu Dhabi, P. O.
Box 129188, Abu Dhabi, United Arab Emirates
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Karthik Sambath– Department of Chemistry, University of
Cincinnati, Cincinnati, OH 45221-0172, United State
Ranaweera A. A. Upul Ranaweera – Department of Chemistry,
University of Cincinnati, Cincinnati, OH 45221-0172, United
States
Stefan Schramm† – New York University Abu Dhabi, P. O. Box
129188, Abu Dhabi, United Arab Emirates
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Alexander Duncan – Department of Chemistry, University of
Cincinnati, Cincinnati, OH 45221-0172, United States
Benjamin Duncan – Department of Chemistry, University of
Cincinnati, Cincinnati, OH 45221-0172, United States
Jeanette A. Krause – Department of Chemistry, University of
Cincinnati, Cincinnati, OH 45221-0172, United States
Present Addresses
†Present address: BASF SE, Carl-Bosch-Strasse 38, 67056
Ludwigshafen am Rhein, Germany
Author Contributions
‡KS and RAAUR contributed equally.
Notes
(18) Oates, W. A.; Todd, D. D. Proc. First Australian Conference on
Electron Microscopy, Hobart, 1965, page 88.
(19) Bowden, F. P.; Yoffe, A. D. Fast Reactions in Solids. Angew.
Chem. 1959, 71, 752.
(20) Sato, S. Azirines. IV. The photolysis of β-azidovinyl ketones. Bull.
Chem. Soc. Jpn. 1968, 41, 2524–2525.
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the NSF (CHE-1800140 & MRI:CHE-1726092) and
OSC (PES0597) for their support. DJS acknowledges Doctoral
Enhancement, Stecke, and Twitchell Fellowships; RAAUR
acknowledges Harry B. Mark, Ann P. Villalobos, and URC
Fellowships. Funding for the SMART6000 CCD diffractometer
was through NSF-MRI grant CHE-0215950. Synchrotron X-ray
data were collected through the SCrALS (Service
Crystallography at Advanced Light Source) program at
Beamline 11.3.1 at the Advanced Light Source (ALS),
Lawrence Berkeley National Laboratory. The ALS is supported
by the U.S. Department of Energy, Office of Energy Sciences
Materials Sciences Division, under contract DE-AC02-
05CH11231.
(21) Rosa, F. A.; Machado, B.; Bonacorso, H. G.; Zanatta, N.; Martins,
M. A. P. Reaction of β‐dimethylaminovinyl ketones with
hydroxylamine: A simple and useful method for synthesis of 3‐
and 5‐substituted isoxazoles. J. Heterocycl. Chem. 2008, 45, 879–
885.
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Taniguchi, H. Unusual formation of oxazoles by base- or acid-
catalyzed ring opening of 2-acyl-2H-azirines. Heterocycl. 1978, 9,
1207–1216.
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S. R.; Thomson, S.; Allan, R. D.; Dolle, F.; Fulham, M. J.;
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