Fluorine as a robust balancer for tuning the reactivity of topo-photoreactions of chalcones and the photomechanical effects of molecular crystals

Article abstract of DOI:10.1039/d1ce00086a

Fluorine-free chalcones and chalcones bearing different numbers of fluorine atoms have been synthesized. It is found that fluorine can tune the reactivity of photo-induced [2 + 2] cycloaddition reactions in crystals. The higher the number of fluorine atom

Full text of DOI:10.1039/d1ce00086a

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Fluorine as a robust balancer for tuning the
reactivity of topo-photoreactions of chalcones
and the photomechanical effects of molecular
crystals†
Cite this: DOI: 10.1039/d1ce00086a
Yuanhong Shu, Kaiqi Ye,
Yuan Yue, Jingbo Sun,
Haoran Wang,
*
Jiangbin Zhong, Xiqiao Yang, Hongqiang Gao and Ran Lu
Fluorine-free chalcones and chalcones bearing different numbers of fluorine atoms have been synthesized.
It is found that fluorine can tune the reactivity of photo-induced [2 + 2] cycloaddition reactions in crystals.
The higher the number of fluorine atoms, the higher the reactivity of the photodimerization. The single
crystal structures and Hirshfeld surface analysis illustrated that the introduction of fluorine not only
increased the molecular planarity but could also steer the potentially reactive double bonds in appropriate
positions of the crystal lattices to meet Schmidt's criteria. Therefore, the stereospecific [2 + 2] cycloaddition
reactions took place to afford one diastereoisomer from the reactive chalcones except for (E)-3-(4-
fluorophenyl)-1-phenylprop-2-en-1-one (1FChH). Additionally, the chalcone-based molecular crystals
exhibited various photomechanical behaviors, such as bending toward or away from the light source,
swinging, cracking and jumping, driven by topo-photoreactions. As expected, the more efficient photo-
induced [2 + 2] cycloaddition reactions in crystalline states led to more significant motions of the
molecular crystals. Powder X-ray diffraction results suggested that the solid state [2 + 2] cycloaddition
reactions changed the unit cell of the single crystals. The photo-induced bending toward or away from the
light source for the needle-like crystals originated from the contraction or the expansion of the
phototropic surface. Hence, the robust balancer fluorine in chalcones might play an important role in the
crystalline packing. This provides a facile approach in crystal engineering to fabricate photo-induced
mechanically responsive crystalline materials.
Received 19th January 2021,
Accepted 25th March 2021
DOI: 10.1039/d1ce00086a
rsc.li/crystengcomm
of green chemistry, solid-state photoreactions can be
performed in environmentally friendly solvent-free systems,
Introduction
Organic photochemical reactions in the crystalline state have
been extensively studied for decades. They provide felicitous
methods for synthesizing desired molecular skeletons with
regioselectivity and stereospecificity.1–3 From the point of view
and the clean energy of light was employed to activate the
reactant molecules.4,5 Among the solid-state photoreactions,
[2 + 2] cycloaddition reactions have received continuous
attention since the 1960s.6–11 The correlation of the molecular
packing arrangement of the reactant species with the
stereochemical structure of the product has been revealed, for
instance, the topo-photochemical criteria for the solid-state [2
+ 2] cycloaddition reactions were postulated by Schmidt and
coworkers.12,13 Although olefins, whose packing mode meets
Schmidt's criteria, are capable of undergoing topo-
photodimerization, the strategies for aligning the potentially
reactive double bonds are still a challenging topic.14–17
Recently, templates of halogen bonding donors,18–21
H-bonding donors,22–24 and metal ions25–27 have been
employed for controlling the molecular packing so as to
satisfy the topo-chemical requirements.
State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun 130021, P. R. China.
E-mail: luran@mail.jlu.edu.cn
† Electronic supplementary information (ESI) available: Syntheses of chalcones;
1H NMR, 13C NMR and HRMS spectra; 1H NMR spectra and XRD patterns of
chalcones upon UV irradiation of the crystals; single crystal structure data;
Hirshfeld surfaces and fingerprint plots of the crystals; images and videos of the
crystals during photomechanical processes. CCDC 1974664 for MeOChOMe-4,
1974692 for 1FChBr-4, 2063362 for 1FChH, 1974691 for 1FChBr-3, 1974663 for
2FChF-4, 2036527 for 2FChMe-4, 1974670 for 2FChOMe-4, 1974662 for 3FChMe-
4, 1974669 for D-1FChBr-4, 1974666 for D-2FChCl-4, 1974667 for D-2FChBr-4,
2036528 for D-2FChMe-4, 1974668 for D-2FChOMe-4 and 1974665 for D-
3FChMe-4. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/d1ce00086a
It is known that light energy is transformed into chemical
energy during photo-chemical processes, whereas the
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geometrical structural changes of the molecules during the
photoreactions may sometimes lead to macroscopic motions
of the crystals. Photoactuators based on molecular crystals
might have potential applications in soft robotic and
molecular machines, etc.28–35 It should be worth noting that
the photomechanical effects driven by [2 + 2] cycloaddition
reactions of green fluorescent protein, cinnamic acids,
photodimerization
to
yield
four-membered
rings
stereospecifically, which might be difficult to prepare via
classical synthetic routes. This provides a facile approach in
crystal engineering where the steering group plays an
important role in the crystal packing, which is favourable for
fabricating photo-induced mechanically responsive crystalline
materials.
styrylpyridines,
and
phenylbutadienes
have
been
investigated.36–43 Our group has observed the photo-induced Results and discussion
bending, curling, rolling, salient, slipping and swinging
behaviors of crystals based on styrylbenzoxazoles,
Synthesis
The synthetic routes for the chalcones are sketched in
styrylbenzothiazoles and naphthalene acrylic acids during
dimerization.44–48 Therein, it was found that the F atom
might play a key role in the photo-induced motions of the
crystals of diarylethenes. This encourages us to reveal why F
can steer the carbon–carbon double bonds in appropriate
positions of the crystal lattices to meet Schmidt's criteria.
Particularly, chalcones, as a class of photochemically active
olefins,49,50 are subjects of great interest due to their
biological and pharmacological activity.51–53 Nevertheless,
examples of photomechanical materials constructed from
chalcones are quite limited except for the crystal of cyclic
chalcone reported by Tang et al.54 With these in mind,
chalcones with various numbers of fluorine atoms or
fluorine-free chalcones have been synthesized (Scheme 1) to
reveal the effect of fluorine on the reactivity of topo-[2 + 2]
cycloaddition as well as on the photomechanical behaviors of
the molecular crystals. Excitingly, it has been found that
increasing the number of F improves the photochemical
reactivity of chalcones in crystals, which can also be
perceived in the photomechanical effects. Hence, fluorine
can be used as a robust balancer to set the potentially
reactive double bonds in the positions for the topological
Scheme S1.† The aldol condensation reactions between
benzaldehydes and acetophenone derivatives in ethanol
using potassium hydroxide as the catalyst afforded the target
compounds,55,56 which were characterized by 1H NMR, 13C
NMR, FT-IR and HRMS (Fig. S89–S177†). In the 1H NMR
spectra, the signals for vinyls in the synthesized chalcones
appeared in the range of 7.41–8.04 ppm, and the coupling
constants were ca. 16 Hz, meaning the trans-form of carbon–
carbon double bonds. Besides, the single crystal structures of
MeOChOMe-4 (CCDC 1974664), 1FChBr-4 (CCDC 1974692),
1FChH (2063362), 1FChBr-3 (CCDC 1974691), 2FChF-4 (CCDC
1974663), 2FChMe-4 (CCDC 2036527), 2FChOMe-4 (CCDC
1974670) and 3FChMe-4 (CCDC 1974662) could also confirm
the trans-configurations (Tables S1–S3†).
Photo-induced [2 + 2] cycloaddition reactions in crystals
Firstly, the UV-vis absorption spectral changes of the
chalcones in microcrystals before and after UV irradiation
were monitored. Taking 1FChBr-4 as an example, it gave a
maximum absorption located at 324 nm (Fig. S1a†). After the
irradiation by 365 nm light for 5 s, a new absorption centered
Scheme 1 Molecular structures of the fluorine-containing chalcones.
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at 262 nm emerged, and the absorption at 324 nm decreased.
When the irradiation time was prolonged to 10–20 s, the
absorption at 262 nm was intensified, and the absorption at
324 nm became a shoulder. Besides, the UV-vis absorption
spectra of the microcrystals based on 1FChH, 1FChF-4,
1FChCl-4, 1FChBr-3, 1FChBr-2, 2FChF-4, 2FChCl-4, 2FChBr-4,
2FChBr-3, 2FChBr-2, 2FChI-4, 2FChMe-4, 2FChOMe-4,
2FChSMe-4, 3FChH, 3FChF-4, 3FChCl-4, 3FChBr-3, 3FChBr-3,
3FChBr-2, 3FChI-4, 3FChMe-4, 3FChOMe-4, 3FChOMe-2 and
3FChSMe-4 also showed obvious changes after UV irradiation
(Fig. S1–S4†). This meant that the photochemical reactions of
the above chalcones might have taken place. Although
MeChBr-4, MeOChBr-4, MeOChOMe-4, 1FChBr-2, 1FChMe-4,
1FChOMe-4 and 2FChOMe-2 in the microcrystals exhibited
strong absorption at ca. 360 nm (Fig. S5†), no change in their
absorption spectra was observed upon irradiation by 365 nm
light, meaning that they were photostable.
In order to investigate the photochemical reactions of the
synthesized chalcones in crystals, the samples for 1H NMR
measurements were prepared by dissolving the microcrystals,
which were irradiated by UV light for different times, in
CDCl3. It was found that the fluorine-free chalcones MeChBr-
4, MeOChBr-4 and MeOChOMe-4 were photostable even after
120 min of UV irradiation (Fig. S6–S8†). Interestingly, the
introduction of F atoms in chalcones can initiate the solid-
state photo-induced [2 + 2] cycloaddition reactions. The
higher the number of F, the higher the reactivity of the
photodimerization. Firstly, the photochemical reactions of
monofluorine-containing chalcones in microcrystals were
studied. Taking 1FChBr-4 as an example, after irradiating the
microcrystals by 365 nm light for 5 min, two new doublets at
4.62 ppm and 4.35 ppm appeared, which could be ascribed
to the signals for H3 and H4 in cyclobutane formed via [2 + 2]
cycloaddition (Fig. 1). With prolonging the irradiation times
the integral values for the signals at 4.62 ppm and 4.35 ppm
increased, while those at 7.79 ppm and 7.41 ppm
corresponding to H1 and H2 in the vinyl unit of 1FChBr-4
decreased. The newly emerged signals in the range of 6.8–7.7
ppm for the sample obtained by irradiating the crystals for
5–60 min were the same as those of the photodimerization
product (D-1FChBr-4, Fig. 1d). Notably, only one kind of
diastereoisomer of D-1FChBr-4 was obtained since no signals
for other kinds of cyclobutane moieties were detected as
shown in Fig. 1b–e. Therefore, the dimerization of 1FChBr-4
in crystals was stereospecific. The ratios of the integral values
for the signals at 7.41 ppm and 4.35 ppm were ca. 12.73/1,
5.15/1, 4.96/1 and 3.55/1 illustrating that ca. 7%, 16%, 17%
and 22% of 1FChBr-4 molecules were transformed into D-
1FChBr-4 when the microcrystals were UV irradiated for 5
min, 10 min, 30 min and 60 min, respectively (Table 1).
Similarly, as depicted in Fig. S9 and S10,† two new
doublets in the range of 3.5–5.0 ppm emerged after the
microcrystals of 1FChF-4 and 1FChCl-4 were exposed to 365
nm light, illustrating the occurrence of [2 + 2] cycloaddition
reactions. For 1FChF-4 and 1FChCl-4, the conversion ratios
were ca. 10% and 29% after 5 min of irradiation, and
Fig. 1 1H NMR spectra of 1FChBr-4 before (a) and after irradiation of
the microcrystals for 5 min (b), 10 min (c), 30 min (d) and 60 min (e) by
365 nm light (16.7 mW cm−2), followed by dissolving in CDCl3; (f) 1H
NMR spectrum of D-1FChBr-4 in CDCl3 (400 MHz).
increased to ca. 23% and 34% when the irradiation time was
prolonged to 10 min. With further prolonging the irradiation
time to 60 min, the conversion ratios of 1FChF-4 and
1FChCl-4 increased to 34% and 39%, which were similar to
those at 30 min (33% and 37%) (Table 1). Differently, after
the microcrystals of 1FChH were irradiated by UV light for
more than 10 min, eight new signals at 3.90–4.90 ppm
emerged, suggesting the formation of four kinds of isomers
of D-1FChH (dimerization product of 1FChH, Fig. S11†). The
total conversion ratios of 1FChH to cyclobutanes were 5%
and 20% after 10 min and 30 min of 365 nm irradiation
(Table 1). On the other hand, when 4-fluorostyryl was linked
to the bromobenzoyl unit, in which Br was in the 3- or
2-position in phenyl chalcones 1FChBr-3 and 1FChBr-2 still
exhibited photochemical reactivity. About 31% and 2% of
1FChBr-3 and 1FChBr-2 were transformed into the
dimerization products after 10 min of UV irradiation, and the
conversion ratios increased to ca. 53% and 4% after 30 min
irradiation (Fig. S12 and S13† and Table 1). Except for
1FChH, the above photoreactions were stereospecific since
only one kind of isomer of cyclobutane was produced
according to the change in the 1H NMR spectra of each
chalcone induced by 365 nm light. This suggested that the
introduction of fluorine atoms into chalcones was essential
for the solid-state topo-[2
+ 2] cycloaddition reactions.
However, the chalcones 1FChI-4, 1FChMe-4 and 1FChOMe-4,
which contained iodine, methyl and methoxy in the benzoyl
moiety, respectively, were photostable (Fig. S14–S16† and
Table 1). This meant that the substituted groups with larger
van der Waals radii than bromine might deter the
photodimerization of 4-fluorostyryl-containing chalcones.
These results inspired us to further clarify if the fluorine
might be the robust balancer for the solid-state [2 + 2]
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Table 1 Conversion ratios for the [2 + 2] cycloaddition reactions of 1FChH, 1FChF-4, 1FChCl-4, 1FChBr-4, 1FChBr-3, 1FChBr-2, 2FChF-4, 2FChCl-4,
2FChBr-4, 2FChBr-3, 2FChBr-2, 2FChI-4, 2FChMe-4, 2FChOMe-4, 2FChSMe-4, 3FChH, 3FChF-4, 3FChCl-4, 3FChBr-4, 3FChBr-3, 3FChBr-2, 3FChI-4,
3FChMe-4, 3FChOMe-4, 3FChOMe-2 and 3FChSMe-4 in crystals irradiated by UV light for different times and the photomechanical effects of the
crystals in different shapes
Conversion ratio (%)
Photomechanical effects
Compound
5 min
10 min
30 min
60 min
Shape of crystals
Movement
Irradiation time
1FChH
∼0
10
29
7
18
0
26
34
14
66
30
12
60
22
5
23
34
16
31
2
32
46
37
68
31
32
70
66
20
33
37
17
53
4
67
64
56
68
35
35
71
71
21
34
39
22
57
6
83
76
58
68
52
46
—
72
Rod
Slice
Needle
Needle
Block
Bending toward light
Cracking
Splitting and salient
Bending away from light
Cracking
0–60 s
0–60 s
0–60 s
0–95 s
0–50 s
0–60 s
0–18 s
0–40 s
0–54 s
0–60 s
0–90 s
0–60 s
0–30 s
0–66 s
0–129 s
0–422 s
0–158 s
0–60 s
0–60 s
0–30 s
0–40 s
0–24 s
0–23 s
0–58 s
0–60 s
0–65 s
0–40 s
0–40 s
0–60 s
1FChF-4
1FChCl-4
1FChBr-4
1FChBr-3
1FChBr-2
2FChF-4
2FChCl-4
2FChBr-4
2FChBr-3
2FChBr-2
2FChI-4
Block
—
Needle
Needle
Needle
Needle
Needle
Sheet
Bending toward light
Bending toward light
Bending toward light
Bending away from light
Bending toward light
Bending away from light
Cracking
Bending toward light
Swinging
Bending and jumping
Cracking
2FChMe-4
2FChOMe-4
Block
Needle
Needle
Needle
Cuboid
Rod
Block
Branch
Rod
Needle
Crystal bundle
Needle
Block
Block
Block
2FChSMe-4
3FChH
3FChF-4
4
7
7
29
53
52
81
55
30
66
77
73
73
10
11
87
53
67
83
80
73
67
100
100
100
28
14
88
54
69
—
84
75
—
—
—
—
29
—
Cracking
Bending toward light
Cracking
Popping and bending
Swinging
Bending away from light
Bending away from light
Cracking
35
36
80
50
26
60
61
41
55
6
3FChCl-4
3FChBr-4
3FChBr-3
3FChBr-2
3FChI-4
3FChMe-4
3FChOMe-4
3FChOMe-2
3FChSMe-4
Cracking
Bending away from light
Cracking
Bundle of fibers
Block
cycloaddition of chalcones. Therefore, 3,5-difluorostyryl-
based and 3,4,5-trifluorostyryl-based chalcones have been
synthesized.
for 30 min (Fig. S25† and Table 1). Hence, F can indeed act
as a steering group to align chalcones for dimerization.
Among the synthesized 3,5-difluorostyryl-based chalcones,
only 2FChOMe-2 was unreactive in crystals under UV
irradiation (Fig. S26†).
Encouragingly, the chalcones bearing the 3,5-difluorostyryl
unit could undergo photodimerization even though iodine,
methyl and methoxy groups were present in the benzoyl
moiety. More importantly, the solid-state photochemical
reactivity of 3,5-difluorostyryl-containing chalcones was
higher than that of the corresponding monofluorine-
containing ones. For example, the conversion ratios of
2FChF-4, 2FChCl-4, 2FChBr-4, 2FChBr-3, and 2FChBr-2 into
the corresponding cyclobutanes were ca. 26%, 34%, 14%,
66% and 30% after 5 min of irradiation, and increased to ca.
67%, 64%, 56%, 68% and 35%, respectively, after UV
irradiation of the microcrystals for 30 min (Fig. S17–S21† and
Table 1). It was clear that the conversions of difluorostyryl-
containing chalcones were more rapid than those of the
similar monofluorostyryl-containing chalcones. Although
1FChI-4, 1FChMe-4 and 1FChOMe-4 were inert, 30 min of UV
irradiation led to transformation of ca. 35% of 2FChI-4, 71%
of 2FChMe-4 and 71% of 2FChOMe-4 into cyclobutane
derivatives (Fig. S22–S24† and Table 1). When a methylthio
group was introduced, 2FChSMe-4 was also photochemically
active, and the conversion was ca. 11% after UV irradiation
When 3,4,5-trifluorostyryl was introduced in chalcones,
the [2 + 2] cycloaddition reactivity was further improved. In
detail, 3FChH, 3FChF-4, 3FChCl-4, 3FChBr-4, 3FChBr-3,
3FChBr-2, 3FChI-4 and 3FChSMe-4 could transform into the
derivatives of cyclobutanes in yields of ca. 7%, 35%, 36%,
80%, 50%, 26%, 60% and 6%, respectively, after 5 min of
irradiation, which increased to 87%, 53%, 67%, 83%, 80%,
73%, 67% and 28% after 30 min of UV irradiation (Fig. S27–
S34† and Table 1). Particularly, the conversion ratios of
3FChMe-4 and 3FChOMe-4 reached 100% after 30 min of
irradiation (Fig. S35 and S36† and Table 1). Obviously, their
reactivity was higher than that of the chalcones bearing
4-fluorostyryl and 3,5-difluorostyryl groups. Although
2FChOMe-2 was photostable, 3FChOMe-2 with three F atoms
showed rapid conversion into D-3FChOMe-2. For instance,
ca. 55% and 100% of 3FChOMe-2 molecules underwent [2 +
2] cycloaddition in crystals upon exposure to UV light for 5
min and 30 min (Fig. S37† and Table 1). As a result, F can be
recognized as a robust balancer to tune the reactivity of topo-
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Fig. 2 Single crystal structure of 1FChBr-4 viewed along the a-axis (a) and b-axis (b); H-bonding in the single crystal of 1FChBr-4 (c); distance between
two adjacent parallel molecules and the dihedral angle between two phenyl planes (d); configuration of D-1FChBr-4 in the single crystal (e).
[2 + 2] cycloaddition of chalcones. In particular, the [2 + 2]
cycloaddition of the synthesized chalcones except for 1FChH
afforded only one kind of stereoisomer of cyclobutane,
meaning that the stereospecific reactions were concerted and
atom-economic.
indicated in bright red areas, where the fluorine atom and
carbonyl were located. Chart S2† shows the contributions to
the Hirshfeld surface area of each type of intermolecular
contact for 1FChBr-4, and C–H⋯F and C–H⋯O contacts
comprised 23.6% and 16.2% of the surface area, respectively.
As mentioned above, intermolecular H-bonding was essential
in arranging the molecules, which might be favourable for
shortening the distance between the reactive double bonds.
Furthermore, the molecular packing of 1FChBr-4 was aligned
in parallel and the distance of the ‘olefin pair’ was 4.01 Å,
less than 4.20 Å, which was essential for the [2 + 2]
cycloaddition proposed by Schmidt.12 Meanwhile, θ2 and θ3
as shown in Chart S1† also affected the dimerization
according to Schmidt's criteria. In the single crystal of
1FChBr-4, the θ2 and θ3 values are 72.41° and 55.30°,
respectively (Table 2). Although they deviated from the ideal
value (90°) to some extent, the dimer of D-1FChBr-4 was
yielded via the [2 + 2] cycloaddition reaction. On account of
Effect of crystalline packing on topo-[2 + 2] cycloaddition
reactivity
The above [2 + 2] cycloaddition reactions are stereospecific,
so it is expected that X-ray crystallography would provide
more information about the effect of molecular packing on
the photochemical reactions. As shown in Fig. 2, the dihedral
angle between two phenyl planes in 1FChBr-4 was 8.43°,
meaning an approximately planar conformation. The
molecules of 1FChBr-4 were packed in a head–head form via
π–π interactions viewed along the a-axis. The adjacent
molecules having strong interactions between atoms were
Table 2 Geometric parameters and the presence or absence of C–H⋯F interactions in the single crystals of MeOChBr-4, MeOChOMe-4, 1FChH-4,
1FChCl-4, 1FChBr-4, 1FChBr-3, 1FChMe-4, 1FChOMe-4, 2FChF-4, 2FChMe-4, 2FChOMe-4 and 3FChMe-4 (parameter definitions are given in Chart S1),
and the photoreactivity of the compounds
Compound
Parallel mode
d (Å)
5.846
7.403
4.050
3.737
4.012
3.894
10.750
7.457
3.798, 3.799
3.736
3.936
θ1 (°)
θ2 (°)
θ3 (°)
C–H⋯F
Photoreactivity
MeOChBr-4
MeOChOMe-4
1FChH-4
1FChCl-4
1FChBr-4
Parallel
Parallel
Parallel
Parallel
Parallel
Anti-parallel
Parallel
Parallel
Parallel
0
0
0
0
0
0
0
0
72.93
74.22
73.66
70.65
72.41
76.02
65.33
76.04
69.56
66.83
65.85
64.82
15.73
68.60
68.07
56.65
55.30
77.11
21.92
82.07
69.15
85.96
89.80
88.36
None
None
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
None
None
Low
Low
Low
Moderate
None
None
High
High
High
High
1FChBr-3
1FChMe-4
1FChOMe-4
2FChF-4
2FChMe-4
2FChOMe-4
3FChMe-4
0, 0.001
Anti-parallel
Parallel
Anti-parallel
0
0
0
3.762
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the head–head packing of 1FChBr-4 in the single crystal, it
was deduced that only β-type D-1FChBr-4 was afforded if the
reaction was topological. In fact, the topo-photodimerization
of 1FChBr-4 in the crystal took place, affording β-type D-
1FChBr-4 (CCDC 1974669), which was further confirmed by
the single crystal structure (Fig. 2e). In addition, the
crystalline structure of 1FChBr-4 revealed that F played an
important role in the molecular arrangement for satisfying
the dimerization criteria.10 As depicted in Fig. 2c, the
adjacent molecules were linked by H-bonds C–H⋯F (2.909 Å
and 2.761 Å) and C–H⋯O (2.494 Å and 2.673 Å), illustrating
that the molecules might be confined in the H-bonded
networks. So as to obtain further insight into the
intermolecular interactions, the Hirshfeld surfaces and 2D
fingerprint plots for the crystal were calculated using
CrystalExplorer.57 By the way, the Hirshfeld surfaces were
obtained by partitioning the space in the crystal into regions
where the contribution from the electron distribution of a
sum of spherical atoms for the molecule exceeds the
contribution from the corresponding sum over the
crystal.58,59 The molecular packing of 1FChH (Fig. S39†),
1FChCl-4 (ref. 60) (Fig. S40†), 1FChBr-3 (Fig. S41†), 1FChMe-4
(ref. 61) (Fig. S42†), 1FChOMe-4 (ref. 62) (Fig. S43†), 2FChF-4
(Fig. S44†), 2FChMe-4 (Fig. S45†), 2FChOMe-4 (Fig. S46†),
3FChMe-4 (Fig. S47†), MeOChBr-4 (ref. 63) (Fig. S48†), and
MeOChOMe-4 (Fig. S49†) in crystals were analysed based on
the reported60–63 and the obtained single crystal data. In the
case of 1FChH, the dihedral angle between two phenyl planes
was 10.26°. The arrangement of the molecules of 1FChH was
not exactly parallel since the plane of the carbon–carbon
double bond in a molecule was tilted with respect to the
plane of the double bond in the adjacent molecule by 59.45°
(Fig. S39c†). The distances between C1 and C1′ in one vinyl
and C2 and C2′ in another vinyl were 4.050 Å and 4.045 Å in
adjacent molecules (Fig. S39d†). The distances were shorter
than 4.2 Å, agreeing with Schmidt's criteria for solid-state
photodimerization. This suggested that the formation of
β-type D-1FChH was the most likely outcome. In comparison
with the chemical shifts for the protons in cyclobutane β-type
D-1FChBr-4, β-type D-1FChH was indeed the main product in
motion of 1FChH might enable the double bonds to become
close to each other in head–tail mode, yielding α-type D-
1FChH.65–68 Besides, the reaction at defect sites in
microcrystals might afford different stereoisomers of D-
1FChH.50
In addition, the molecular conformation and molecular
packing of 1FChCl-4 in the crystal were similar to those of
1FChBr-4. The adjacent molecules of 1FChCl-4 were linked
by H-bonds C–H⋯F (2.959 Å and 2.720 Å) and C–H⋯O (2.482
Å and 2.693 Å). The d value between adjacent molecules was
only 3.737 Å (Fig. S40†), so β-type D-1FChCl-4 was afforded
via a topo-photochemical reaction. In the single crystal of
1FChBr-3, the H-bonds C–H⋯F (2.959 Å and 2.720 Å) and C–
H⋯O (2.854 Å) were observed (Fig. S40†). The dihedral angle
between two phenyl planes was as large as 47.6°, and the
π-dimer was formed in a head–tail type viewed along the
a-axis. The d value between adjacent molecules was 3.894 Å.
It was deduced that only α-type D-1FChBr-3 might be
obtained via the topo-cycloaddition reaction. Moreover, the
dihedral angle between two phenyl planes in the single
crystal of 1FChMe-4 or 1FChOMe-4 was 27.13° or 33.64° (Fig.
S42 and S43†). No π-dimer was formed, and the d values
reached 10.750 Å and 7.457 Å for 1FChMe-4 and 1FChOMe-4,
respectively (Table 2). Therefore, photodimerization cannot
happen. The fact that the absence of H-bonding is associated
with the inertness of the chalcone meant that the C–H⋯F
interaction is essential for the photochemical reactivity of
chalcones.
In the case of 2FChF-4, it showed good planarity in the
single crystal (Fig. S44†). Viewed along the b-axis the
molecules were packed in parallel with a head–head type.
The H-bonded network was formed based on C–H⋯F (2.879
Å, 2.701 Å and 2.976 Å). The d values between adjacent
molecules were 3.798 Å and 3.799 Å (Fig. S44†). It was
deemed that β-type D-2FChF-4 would be obtained. 2FChMe-4
had also good planarity (the dihedral angle between two
phenyl planes was 5.47°) in the single crystal and the π–π
interactions were along the c-axis (Fig. S45†). The H-bond C–
H⋯F (2.584 Å, 2.765 Å and 2.873 Å) was observed. Besides,
the distance between the adjacent molecules packed in a
head–tail type was 3.737 Å, so α-type D-2FChMe-4 (CCDC
2036528) could be afforded (Fig. S45e†). As depicted in Fig.
S46,† the H-bonds formed in the forms of C–H⋯F (2.773 Å,
2.827 Å and 2.926 Å) and C–H⋯O (2.719 Å) in the single
1
the initial stage of the dimerization reaction (see the H NMR
spectrum of 1FChH after irradiation of the microcrystals for
10 min, Fig. S11†). However, 1FChH might be also converted
to α-type D-1FChH based on the appearance of the signals
for the protons in cyclobutane located at a relatively low-field
region (the chemical shifts for the protons in β-type or α-type
cyclobutane will be further discussed below). The possible
reasons for the formation of α-type D-1FChH included the
following points. On the one hand, the initial reaction to
crystal of 2FChOMe-4. The
d value between adjacent
molecules packed in a head–head type was 3.936 Å, so β-type
D-2FChOMe-4 (CCDC 1974668) was obtained (Fig. S46e†).
The above results suggested that the planarity of chalcones
bearing difluorostyryl was improved compared with that of
form β-type D-1FChBr-4 occurred with
a
potentially
chalcones
containing
monofluorostyryl.
Furthermore,
substantial loss in crystallinity since the non-parallel
arrangement between the adjacent molecules might increase
the possibility of molecular motions.64 As a result, the
disorder along the lattice would be responsible for the
formation of other kinds of cyclobutanes. On the other hand,
as the reaction progressed in the solid state, the pedal-like
3FChMe-4 showed better planarity in the single crystal than
2FChMe-4, the dihedral angle between two phenyl planes was
only 3.02° (Fig. S47†). The adjacent molecules were arranged
in a head–tail type with a distance of 3.762 Å, affording
α-type D-3FChMe-4 (CCDC 1974665) via [2 + 2] cycloaddition
(Fig. S47e†).
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For further insight into the intermolecular interactions,
the Hirshfeld surfaces of 1FChCl-4, 1FChBr-3, 2FChF-4,
2FChMe-4, 2FChOMe-4 and 3FChMe-4 are summarized in
Fig. S38b–g,† and the contributions to the Hirshfeld surface
areas of each type of intermolecular contact are shown in
Chart S2.† The contributions of H⋯F for 1FChCl-4 (13.7%),
1FChBr-3 (9.6%), 2FChF-4 (39.1%), 2FChMe-4 (20.6%),
2FChOMe-4 (23.6%) and 3FChMe-4 (28.7%) were also primary
ones. Thus, the C–H⋯F contacts facilitated the formation of
layers, with neighboring chalcones in the same layer being
arranged in a near planar fashion. Then, the planar layers
would like to stack on top of one another to facilitate the
dimerization. In other words, the C–H⋯F interactions aided
in achieving the planarity of the chalcones and overall
packing, which in turn, helped the occurrence of the
photodimerization. In the crystals of fluorine-free chalcones
MeOChBr-4 and MeOChOMe-4, the dihedral angle between
two phenyl planes was as large as 50.17° and 57.60°,
respectively, and the distance between two adjacent olefins
was 5.846 Å and 7.403 Å, respectively, indicating the absence
of π–π interaction (Fig. S48 and S49†). Meanwhile, H-bonding
was not found either. Therefore, MeOChBr-4 and
MeOChOMe-4 cannot undergo cycloaddition reactions in
crystals. Consequently, the H-bonding as well as π–π
S22†), D-2FChOMe-4 (Fig. S24†), D-2FChSMe-4 (Fig. S25†), D-
3FChCl-4 (Fig. S29†), D-3FChBr-4 (Fig. S30†), D-3FChBr-3
(Fig. S31†), D-3FChBr-2 (Fig. S32†) and D-3FChOMe-2 (Fig.
S37†) as well as α-type D-1FChBr-3 (Fig. S12†), D-1FChBr-2
(Fig. S13†), D-2FChMe-4 (Fig. S23†), D-3FChH (Fig. S26†), D-
3FChF-4 (Fig. S30†), D-3FChI-4 (Fig. S33†), D-3FChSMe-4 (Fig.
S34†), D-3FChMe-4 (Fig. S35†) and D-3FChOMe-4 (Fig. S36†)
might be generated via topo-photoreactions.69–72
Photomechanical effects
It has been found that the photo-induced [2
+ 2]
cycloaddition reaction can drive the molecular crystals to
exhibit macroscopic movements. Thus, molecular crystals in
different shapes have been prepared from the
photochemically active chalcones via slow evaporation of the
solvents, and most of the crystals showed obvious UV light-
induced mechanical motions. On the one hand, the photo-
induced bending toward light was observed in a needle-like
crystal of 2FChOMe-4. As shown in Fig. 3a and Video S1,† the
needle-like crystal of 2FChOMe-4 (2.2 mm × 0.05 mm × 0.05
mm) was placed vertically before irradiation. Upon
irradiation by 365 nm light (16.7 mW cm−2) from right to left,
the bottom of the crystal bent toward the light source
obviously, while the top of the crystal was motionless. When
the irradiation time was prolonged to 33 s, the bottom of the
crystal bent away from the original position by 0.8 mm.
Subsequently, when the irradiation direction was turned
around, the bent crystal began to be straightened without
breaking. On the other hand, the needle-like crystal of
2FChOMe-4 showed unique photo-induced swinging like a
pendulum clock even though the UV irradiation direction
remained unchanged. Upon irradiation by 365 nm light from
interactions
can
improve
the
reactivity
of
the
photodimerization of the fluorine-containing chalcones.
As listed in Table 2, the angles of θ2 in the crystals were a
little bit closer, so the effect of θ2 on the photodimerization
reactivity of chalcones could be ignored. The angles of θ3 in
the crystals of 3,5-difluorostyryl- and 3,4,5-trifluorostyryl-
based chalcones were closer to 90° than those of
4-fluorostyryl-based chalcones. It might be one of the reasons
why the photodimerization reactivity of difluorostyryl- and
trifluorostyryl-based chalcones was higher than that of
monofluorostyryl-based
chalcones. In
addition,
the
enhancement of molecular planarity might be another factor
in improving the reactivity. For example, the planarity
decreased in the order 3FChMe-4, 2FChMe-4 and 1FChMe-4,
which was in accordance with their photochemical reactivity.
On the other hand, the crystal structures of D-2FChCl-4
(CCDC 1974666) and D-2FChBr-4 (CCDC 1974667) suggested
that only β-type cyclobutane derivatives were obtained via
topo-[2 + 2] cycloaddition reactions (Fig. S50†). We deduced
that 2FChCl-4 and 2FChBr-4 might pack in a head–head type
in the single crystals according to the topo-photochemical
criteria12 although their crystal structures were not obtained.
In particular, it was found that in the 1H NMR spectra the
two doublets for the protons in β-type cyclobutanes D-
2FChOMe-4, D-2FChCl-4 and D-2FChBr-4 appeared in the
range of 4.35–4.65 ppm, while those for α-type cyclobutanes,
such as D-3FChMe-4, were located at a relatively low-field
region (4.80–5.00 ppm). Consequently, the chemical shifts of
the protons on four-membered rings suggested that β-type D-
1FChF-4 (Fig. S9†), D-1FChCl-4 (Fig. S10†), D-2FChF-4 (Fig.
S17†), D-2FChCl-4 (Fig. S18†), D-2FChBr-4 (Fig. S19†), D-
2FChBr-3 (Fig. S20†), D-2FChBr-2 (Fig. S21†), D-2FChI-4 (Fig.
Fig. 3 Images of bending of a needle-like crystal (2.2 mm × 0.05 mm
× 0.05 mm) (a) and swinging of a needle-like crystal (2.1 mm × 0.05
mm × 0.05 mm) (b) of 2FChOMe-4 before and after irradiation with
365 nm light (16.7 mW cm−2) for different times observed by optical
microscopy (the arrows indicate the irradiation directions).
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right to left within 14 s, the bottom of the needle-like crystal
of 2FChOMe-4 (2.1 mm × 0.05 mm × 0.05 mm) moved quickly
toward the light source and the top of the crystal was static
(Fig. 3b and Video S2†). After irradiation for 14 s, the
deflection reached the maximum and deviated from the
origin position by 0.7 mm. Further prolonging the UV
irradiation time without changing the irradiation direction,
the bottom of the needle-like crystal gradually bent away
from UV light and the crystal nearly became a straight one at
the irradiation time of 26 s. After that, the bottom of the
crystal further bent to the left side until it reached the
maximum and deviated from the origin position by 0.5 mm
at 33 s. By further exposure to UV light without changing the
irradiation direction, the bending direction of the crystal
turned around again, but the moving rate slowed down
obviously. The crystal became a straight one once again at
the irradiation time of 129 s. Hence, the photo-induced
bending–unbending of the needle-like crystals of 2FChOMe-4
could be operated via changing or unchanging the irradiation
direction.
Similarly, a crystal bundle of 3FChBr-3 (1.4 mm × 0.01
mm × 0.01 mm) swung when induced by UV light (Fig. S51
and Video S3†). Besides, photo-induced bending toward the
light source was also observed for needle-like crystals of
2FChF-4 (2.0 mm × 0.05 mm × 0.05 mm, Fig. S52 and Video
S4†), 2FChCl-4 (2.0 mm × 0.05 mm × 0.05 mm, Fig. S53 and
Video S5†), 2FChBr-4 (2.0 mm × 0.05 mm × 0.05 mm, Fig.
S54 and Video S6†), and 2FChBr-2 (2.8 mm × 0.05 mm × 0.05
mm, Fig. S55 and Video S7†) and a branch-like crystal of
3FChF-4 (3.0 mm × 0.05 mm × 0.01 mm, Fig. S56 and Video
S8†). It is noteworthy that the needle-like crystals of 2FChF-4,
2FChCl-4, 2FChBr-4 and 2FChBr-2 showed slight bending
toward the light source (Fig. S52–S55 and Videos S4–S7†) due
to the low efficiency of photodimerization. In contrast,
needle-like crystals of 1FChBr-4 (1.0 mm × 0.01 mm × 0.01
mm, Fig. S57 and Video S9†), 2FChBr-3 (3.2 mm × 0.01 mm ×
0.01 mm, Fig. S58 and Video S10†) and 3FChBr-2 (2.8 mm ×
0.01 mm × 0.01 mm, Fig. S59 and Video S11†), an irregular
sheet-like crystal of 2FChI-4 (2.2 mm × 0.01–0.4 mm × 0.05
mm, Fig. S60 and Video S12†), and a bundle of fibers of
3FChOMe-2 (2.2 mm × 0.2 mm × 0.1 mm, Fig. S61 and Video
S13†) bent away from the UV light source.
Fig. 4 Images of bending and jumping of a needle-like crystal (2.4
mm × 0.1 mm × 0.1 mm) (a) and cracking of a cuboid-like crystal (2.2
mm × 0.4 mm × 0.1 mm) (b) of 2FChOMe-4 before and after irradiation
with 365 nm light (16.7 mW cm−2) for different times observed by
optical microscopy (the arrows indicate the irradiation directions).
the left and the short crystal on the right) until the
irradiation time reached 95 s, and the horizontal short
needle-like crystal did not move. However, during the
irradiation times from 95 s to 158 s without changing the
irradiation direction, the bending directions of the two thin
needle-like crystals turned around, showing photo-induced
swinging, too. At the same time, the short needle-like crystal
exhibited more obvious bending capacity. In addition, the
horizontal short crystal that jumped away from the original
crystal jumped back to the left long needle-like crystal at the
irradiation time of 107 s. As a result, the long crystal bearing
the short crystal that jumped back bent unceasingly away
from UV light. Since the short crystal on the right bent more
quickly than the long one, the bent crystals became straight
ones at 158 s. The photo-induced splitting and salient
behaviors were also observed in needle-like crystals of
1FChCl-4 (2.2 mm × 0.05 mm × 0.05 mm, Fig. S62 and Video
S15†) and 3FChBr-4 (2.0 mm × 0.05 mm × 0.05 mm, Fig. S63
and Video S16†). In particular, the needle-like crystal of
3FChBr-4 exhibited dramatic popping after slight bending in
Besides the photo-induced bending and swinging, the
splitting and salient behaviors were observed for a thick
needle-like crystal of 2FChOMe-4 (2.4 mm × 0.1 mm × 0.1
mm). As shown in Fig. 4a and Video S14,† the crystal was
smooth without cracks before irradiation. The bottom of the
crystal starts to bend toward the light source at the UV
irradiation time of 20 s. When the irradiation was prolonged
to 25 s, the crystal was split into two thin needle-like crystals
with similar diameters, accompanied by the breaking of the
right thin needle-like crystal into two parts. One part jumped
away to the horizontal position, and the other part was
attached onto the long thin needle-like crystal. And then, the
bending toward the light source was continued for both the
nearly vertical thin needle-like crystals (the long crystal on
a
few seconds on account of high photodimerization
efficiency. A cuboid-like crystal of 2FChOMe-4 (2.2 mm × 0.4
mm × 0.1 mm) exhibited photo-induced cracking (Fig. 4b
and Video S17†). For instance, more and more cracks
appeared on the crystal with prolonging the UV irradiation
time. Meanwhile, several small pieces jumped away from the
crystal body. Finally, the transparent crystal became opaque
on account of the emergence of numerous cracks. Similarly,
the transparency of the following crystals also changed
because of the photo-induced cracking, including the slice-
like crystal of 1FChF-4 (0.6 mm × 0.4 mm × 0.1 mm, Fig. S64
and Video S18†), the block-like crystals of 1FChBr-3 (2.8 mm
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× 0.5 mm × 0.1 mm, Fig. S65 and Video S19†), 2FChMe-4 (0.8
mm × 0.8 mm × 0.2 mm, Fig. S66 and Video S20†), 3FChH
(0.8 mm × 0.4 mm × 0.1 mm, Fig. S67 and Video S21†),
3FChI-4 (1.6 mm × 0.5 mm × 0.1 mm, Fig. S68 and Video
S22), 3FChMe-4 (1.6 mm × 0.6 mm × 0.1 mm, Fig. S69 and
Video S23†), 3FChOMe-4 (2.2 mm × 0.4 mm × 0.1 mm, Fig.
S70 and Video S24†) and 3FChSMe-4 (0.8 mm × 0.6 mm × 0.1
mm, Fig. S71 and Video S25†), and the rod-like crystal of
3FChCl-4 (2.2 mm × 0.4 mm × 0.1 mm, Fig. S72 and Video
S26†). Particularly, the [2 + 2] cycloaddition efficiency affected
the rate of cracking. For example, the crystals of 3FChCl-4,
3FChOMe-4 and 3FChCl-4 showed rapid cracking in dozens
of seconds, but the slight cracking of the crystals of 3FChH
and 3FChSMe-4 was slow. This illustrated that the significant
photomechanical motion resulted from the high reactivity of
the chalcones. As a result, the rod-like crystals of 1FChH (1.6
mm × 0.2 mm × 0.1 mm, Fig. S73 and Video S27†) and
2FChSMe-4 (1.6 mm × 0.2 mm × 0.1 mm, Fig. S74 and Video
S28†), and the block-like crystal of 1FChBr-2 (0.6 mm × 0.6
mm × 0.1 mm, Fig. S75 and Video S29†) were motionless
under 365 nm irradiation due to the low photodimerization
activity.
Although the origin of the photomechanical behaviors can
be assigned to the transformation from light–chemical–
mechanical energy, it cannot explain the reasons why
different types of movement were observed, such as the
bending toward or away from light. Accordingly, the changes
in the XRD patterns of the microcrystals before and after UV
irradiation for different times were investigated. Taking
2FChOMe-4 as an example, it was clear that the XRD pattern
calculated from the single crystal structure was roughly in
agreement with the experimental one for the microcrystals of
2FChOMe-4 (curve a in Fig. S76†), and the Miller planes of
(002), (101), (006), (202) and (303) were assigned (curve b in
Fig. S76†). When the microcrystals were exposed to UV light
for 1 min, some diffraction peaks decreased and shifted. For
instance, the (002) plane shifted to 5.79° from 5.80°, the
(101) plane shifted to 9.10° from 8.98°, the (006) plane
shifted to 17.22° from 17.27°, the (202) plane shifted to
18.11° from 17.98° and the (303) plane shifted to 27.22° from
27.09° (curve c in Fig. S76† and Table 3). It is worth noting
that the UV irradiation induced the slight expansion of the
c-axis and the obvious contraction of the a-axis of the crystal
according to Bragg's law.75
As discussed above, the [2 + 2] cycloaddition was the
driving force for the photomechanical effects of the chalcone-
based molecular crystals. The movement at the molecular
level induced by photodimerization could be figured out
from the crystalline structures. As shown in Fig. 2e, the bond
lengths of carbon–carbon single bonds in the four-membered
ring were 1.590 Å and 1.554 Å in the single crystal of D-
1FChBr-4. Therefore, the carbon atoms in each reacting
carbon–carbon double bond were brought closer and moved
ca. 1.2 Å during photodimerization. Meanwhile, the newly
formed cyclobutane actuated two benzenes to stretch away
from the plane of the four-membered ring. Consequently, the
distance between the outmost F atoms reached 9.371 Å and
that between the outmost Br atoms became 9.910 Å. Hence,
strain would be generated in the crystal during the evident
movement of atoms induced by the photodimerization. Once
the accumulated strain was released, the mechanical motion
would be observed.73,74
The crystal faces were determined by calculating the
morphology of the crystal of 2FChOMe-4 using the software
Mercury 4.1.0 with the crystallographic data as the input
information.76 The calculated morphology reproduced the
observed needle-like shape of the crystal body (Fig. 5 and
S77†). The widest faces of the crystal of 2FChOMe-4
corresponded to the (002) plane and the long axis direction
of the crystal was along the b-axis. Therefore, the bending
processes observed in Fig. 3 were the bending of the (002)
plane and (101) plane along the longest axis of the crystal.
On the (011) plane, the head–head parallel stacking of
2FChOMe-4 molecules was arranged in a linear manner
(Fig. 5a). On the (002) plane, the adjacent molecules were
arranged in a linear manner to form a columnar structure
along the longest axis of the crystal. Upon irradiation with
UV light from the right side, the crystal bent toward the
incident light because the right side of the crystal contracted
along the longest axis of the crystal of 2FChOMe-4
Table 3 The diffraction peaks corresponding to lattice planes in the microcrystals of 2FChOMe-4, 2FChF-4, 1FChBr-4, 1FChBr-3, 2FChMe-4 and
3FChMe-4 before and after UV irradiation for 60 s
2θ (°) of the diffraction peak and the corresponding lattice plane
Microcrystal
Before irradiation
After irradiation
2FChOMe-4
5.79
(002)
6.59
(100)
7.64
(020)
11.68
(101)
5.94
(001)
5.80
9.10
17.22
(006)
19.80
(300)
16.83
(041)
25.63
(103)
18.11
(202)
26.50
(400)
17.51
(032)
30.33
(121)
27.22
(303)
5.80
(002)
6.72
(100)
7.79
(020)
11.72
(101)
6.12
(001)
5.96
8.98
(101)
13.36
(200)
15.38
(040)
21.71
(112)
17.27
(006)
19.84
(300)
16.88
(041)
25.62
(103)
17.98
(202)
26.54
(400)
17.54
(032)
30.35
(121)
27.09
(303)
6.72
(100)
20.32
(051)
33.16
(005)
(101)
13.27
(200)
15.27°
(040)
21.69
(112)
2FChF-4
1FChBr-4
1FChBr-3
2FChMe-4
3FChMe-4
20.22
(051)
32.78
(005)
30.27
(072)
30.30
(072)
11.79°
(002)
29.63
(005)
11.85
(002)
29.69°
(005)
(001)
(001)
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the expansion of the b-axis of the crystal of 1FChBr-4 (Fig.
S80† and Table 3). Meanwhile, the (041) plane shifted to
16.83° from 16.88°, the (032) plane shifted to 17.51° from
17.54°, the (051) plane shifted to 20.22° from 20.32°, and the
(072) plane shifted to 30.27° from 30.30°, illustrating that the
c-axis of the crystal might expand slightly (Fig. S80† and
Table 3).70 The crystal faces were proposed via calculating the
morphology of the crystal of 1FChBr-4 using the software
Mercury 4.1.0, and the two wider faces of the crystal
corresponded to the (110) plane and (0–20) plane (Fig.
S81†).76 Meanwhile, the calculated morphology reproduced
the observed needle-like shape of the crystal body. On the
(0−11) plane and (110) plane, the π–π stacking made 1FChBr-
4 molecules arrange along the longest axis of the crystal (Fig.
S81 and S83a–c†). When the needle-like crystal was irradiated
with UV light, the phototropic surface, such as the (110) and
(0−20) planes, expanded, leading to the bending of the crystal
away from the light source (Fig. S82e†). The changes in the
PXRD patterns illustrated that the UV irradiation led to the
contraction of the b-axis and the expansion of the c-axis of
the crystal of 1FChBr-3, and the contraction of the c-axis of
the crystals of 2FChMe-4 and 3FChMe-4 (Fig. S83–S85† and
Table 3). This indicated that the crystallinity degree of the
crystals of 2FChMe-4 and 3FChMe-4 decreased upon UV
irradiation because of the decrease of the diffraction
intensity. In addition, the calculated morphology fits with the
experimentally observed block shapes of the crystals of
1FChBr-3 and 3FChMe-4 (Fig. S86–S88† and Table 3).
Therefore, crystals with low aspect ratios showed photo-
induced cracking on account of the distortion of the unit
cells of the crystals.36
Fig.
5 Single crystal structure of 2FChOMe-4 (a); schematic
illustration of the photo-induced swinging of the needle-like crystal of
2FChOMe-4 without changing the irradiation direction (b–f) (the
irradiation direction was maintained from right to left).
(Fig. 5b).77,78 However, the needle-like crystal of 2FChOMe-4
exhibited photo-induced swinging when the UV irradiation
direction was maintained. This phenomenon can be
explained as follows. Firstly, the photo-induced bending
toward the light source (irradiating from right to left) in the
initial period of photomechanical processes was also due to
the contraction along the longest axis of the crystal of
2FChOMe-4 on the side facing light (Fig. 5b). It was worth
mentioning that the conversion rate of photodimerization in
the phototropic surface was gradually decreased because D-
2FChOMe-4 might shelter the unreacted 2FChOMe-4 and the
amount of unreacted 2FChOMe-4 was decreased. At the same
time, the photodimerization might also take place on the
wide (002) plane of the crystal with a lower conversion rate.
After continuous irradiation from the right side the
contraction force accumulated on the backlight surface might
be stronger than that on the phototropic surface of the
crystal, so the crystal gradually bent away from the incident
light (Fig. 5d–f). Once the contraction force accumulated on
the backlight surface was weaker than that on the
phototropic surface, reverse bending was observed although
the irradiation direction was not changed. As a result, the
bent crystal of 2FChOMe-4 gradually returned to a straight
one (Fig. 5f). Similarly, the a-axis of the crystal of 2FChF-4
contracted when induced by UV irradiation (Fig. S78† and
Table 3). Combined with the morphology of the crystal of
2FChF-4 obtained using the software Mercury 4.1.0, the
phototropic surface of the crystal contracted during the
photo-induced dimerization (Fig. S79†), so the crystal of
2FChF-4 also bent toward the light source upon irradiation.
However, in the microcrystals of 1FChBr-4 the (020) plane
shifted to 7.64° from 7.79°, and the (040) plane shifted to
15.27° from 15.38° after 1 min of UV irradiation, suggesting
Conclusions
In summary, we have synthesized a series of chalcones with
various fluorine atoms. As a contrast, several fluorine-free
chalcones were synthesized. It has been found that the fluorine
atom can be employed as a robust balancer to tune the photo-
induced topo-[2 + 2] cycloaddition reactivity. The crystalline
structure analysis illustrated that the introduction of more
fluorine atoms would not only increase the planarity of the
molecules but also afford H-bonding, so that the molecules
were arranged into π-dimers satisfying Schmidt's criteria
directed by π–π interaction and H-bonding. Hence, the
chalcones bearing 3,4,5-trifluorostyryl and 3,5-difluorostyryl
groups showed higher dimerization reactivity as compared to
4-fluorostyryl-based derivatives. On account of the motions at
the molecular level during [2 + 2] cycloaddition reactions, strain
in the crystals is produced and accumulated. The chalcone-
based crystals with different shapes exhibited bending toward
or away from the light source, swinging, cracking, splitting or
salient behaviors upon the release of the strain. It is
understandable that obvious photomechanical effects can be
observed in the crystals formed from highly photoactive
chalcones. In addition, it was suggested that the contraction or
expansion of phototropic surfaces actuated the bending of the
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needle-like crystals toward or away from the UV light source.
The photo-induced swinging also originated from the
dimerization of chalcones located in different regions in the
crystals. Herein, we provided a useful strategy for tuning the
topo-photochemical reactivity via the introduction of a robust
balancer group for steering the molecules in appropriate
positions of the crystalline lattice to afford novel molecular
skeletons with stereospecificity. Besides, the molecular crystals
formed from the molecules with high photo-chemical activity
are desired platforms for photoactuators, which may be
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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