Photoactuators based on the dynamic molecular crystals of naphthalene acrylic acids driven by stereospecific [2+2] cycloaddition reactions

Article abstract of DOI:10.1039/c9tc06689f

The photomechanical effects of the dynamic molecular crystals of halogen-substituted naphthalene acrylic acids (1FNaAA, 1ClNaAA, 1BrNaAA, 1INaAA and 6BrNaAA) have been investigated. Upon UV irradiation, the needle-like crystal of 1FNaAA curls away from the light source, while the slice-like crystal of 6BrNaAA bends towards the light source. Moreover, the light-induced bending, flipping and bursting are observed for the elongated needle-like crystals of 1FNaAA, and the slice-like crystals of 1ClNaAA and 1BrNaAA show bending, cracking, coiling, rotating and twisting triggered by 365 nm light. It is found that stereospecific [2+2] cycloaddition reactions take place in the crystals to afford one stereoisomer of β-type cyclobutanes, since 1FNaAA, 1ClNaAA, 1BrNaAA and 6BrNaAA pack in a head-to-head mode, which satisfies the Schmidt's topo-photochemical criteria. The strain can be generated and accumulated during the photodimerization, and the release of the strain leads to the photomechanical effects. This provides new clues for the development of photomechanical molecular crystals based on acrylic acids bearing halogen-substituted aromatic units.

Full text of DOI:10.1039/c9tc06689f

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Photoactuators based on the dynamic molecular
crystals of naphthalene acrylic acids driven by
stereospecific [2+2] cycloaddition reactions†
Cite this:DOI: 10.1039/c9tc06689f
Jiaxi Liu, Kaiqi Ye,
Yanbing Shen, Jiang Peng, Jingbo Sun
and Ran Lu
*
The photomechanical effects of the dynamic molecular crystals of halogen-substituted naphthalene acrylic
acids (1FNaAA, 1ClNaAA, 1BrNaAA, 1INaAA and 6BrNaAA) have been investigated. Upon UV irradiation, the
needle-like crystal of 1FNaAA curls away from the light source, while the slice-like crystal of 6BrNaAA bends
towards the light source. Moreover, the light-induced bending, flipping and bursting are observed for the
elongated needle-like crystals of 1FNaAA, and the slice-like crystals of 1ClNaAA and 1BrNaAA show bending,
cracking, coiling, rotating and twisting triggered by 365 nm light. It is found that stereospecific [2+2]
cycloaddition reactions take place in the crystals to afford one stereoisomer of b-type cyclobutanes, since
1FNaAA, 1ClNaAA, 1BrNaAA and 6BrNaAA pack in a head-to-head mode, which satisfies the Schmidt’s
topo-photochemical criteria. The strain can be generated and accumulated during the photodimerization,
and the release of the strain leads to the photomechanical effects. This provides new clues for the development
of photomechanical molecular crystals based on acrylic acids bearing halogen-substituted aromatic units.
Received 6th December 2019,
Accepted 15th January 2020
DOI: 10.1039/c9tc06689f
rsc.li/materials-c
twisting34,43 or swimming52 of the crystals. Although the
solid-state [2+2] cycloaddition reactions were reported a long
Introduction
The transformation of light, heat, electric or chemical energy time ago,53 the exploration of the photomechanical effects
into motion is one of the fundamental processes in nature. The driven by photodimerization of ethylene derivatives remains
development of mechanically responsive materials stimulated challenging. Naumov et al. reported the first example of a
by heat,1–3 light4–7 and pressure8–10 is of importance in the photosalient effect of the crystals based on coordination metal
fields of actuators,11–15 artificial muscles,16,17 soft robotics,18,19 complexes bearing styrylpyridine ligands, in which the [2+2]
microscopic pumps,20,21 etc. In recent decades, numerous photocycloaddition reaction occurred.54 Zhang et al. presented
solid-state organic reactions have been reported,22 and parti- the light-induced bending of the crystal of 1,2-bis(4-pyridyl)-
cularly the photochemical reactions in the crystalline state are ethylene-based pyridinium salt.46 Recently, our group found
usually accompanied by the obvious movement of the atoms, that the molecular crystals of (E)-2-(2,4-dichlorostyryl)benzo-
causing morphological changes on the crystal surface.23–26 [d]oxazole showed light-induced bending, curling, rolling, and
It has been increasingly recognized that photochemical reactions salient behaviors depending on the shapes of the crystals.55
are ideal means of converting the absorbed photon energy into Besides, the needle-like crystals of (E)-5-chloro-2-(naphthalenyl-
motion for the photomechanical materials.27–30 For example, the vinyl)benzoxazols were found to exhibit rapid elastic bending
light-induced electrocyclic31–34 and cycloaddition35–38 reactions, triggered by topo-photochemical reactions,56 and the high
as well as cis–trans isomerizations39–43 in highly ordered mole- dimerization efficiencies of naphthalenylvinylbenzoxazol in
cular crystals may lead to the macroscopic motions of rod-like and needle-like crystals led to the bursting and bending
bending,44,45 jumping,46,47 cracking,48 curling,49 coiling,50,51 of the crystals.57 Notably, as one kind of photochemically active
compound, cinnamic acid underwent photodimerization in
State Key Laboratory of Supramolecular Structure and Materials, College of
the solid-state, which was first reported by Schmidt,58,59 who
Chemistry, Jilin University, Changchun, P. R. China. E-mail: luran@mail.jlu.edu.cn
proposed the geometric criterion of molecular packing for the
† Electronic supplementary information (ESI) available: Syntheses of the acrylic
acid derivatives; 1H NMR spectra, 13C NMR spectra and HRMS; photophysical light-induced [2+2] cycloaddition reaction.53 Since then, the
data; single crystal structure data; UV-vis absorption and fluorescence emission
photodimerization of cinnamic acid derivatives has been inves-
tigated widely.22,60 However, the photomechanical effect driven
spectra. CCDC 1948533 (1FNaAA), 1948658 (1ClNaAA), 1948592 (1BrNaAA),
1948594 (1INaAA), 1948602 (6BrNaAA), 1955133 (b-type D-1FNaAA) and 1955134
by [2+2] cycloaddition of cinnamic acids was rarely reported,1,61
(b-type D-1ClNaAA); crystallographic data in CIF; movies for the photomechanical
and there is no example of the motion of the crystal based on
naphthalene acrylic acid triggered by photodimerization.
processes of the crystals. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c9tc06689f
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In addition, the halogens of fluorine,62 chlorine63 and bromine64 1-chloro-2-naphthaldehyde (3), 2-(1-bromonaphthalen-2-yl)-1,3-
have been used as steering groups for the arrangement of dioxolane (4), 1-iodo-2-naphthaldehyde (5), methyl 6-bromo-2-
p-systems in crystal engineering. For example, Bardeen et al. found naphthoate (6), (6-bromonaphthalen-2-yl)methanol (7) and
that the microribbons composed of 4-chlorocinnamic acid showed 6-bromo-2-naphthaldehyde (8) were prepared according to the
light-induced twisting.61 Barrett et al. demonstrated that irradia- methods reported previously.66–71 The halogen substituted
tion with visible light transformed a perhalogenated cis-azo- acrylic acid derivatives (1FNaAA, 1ClNaAA, 1BrNaAA, 1INaAA
benzene single crystal into a polycrystalline aggregate of its trans- and 6BrNaAA) were obtained via Knoevenagel condensation
isomer in a photomechanical transformation that involved reactions of halogen substituted naphthaldehyde with malonic
a significant, controllable, and thermally irreversible change acid catalyzed by piperidine in yields of 57–98%.72 The target
of crystal shape.43 The studies on the halogen-substituted molecules were characterized by 1H NMR, 13C NMR, and FT-IR
coumarins and cinnamic acids have also promoted the under- spectroscopy, and high-resolution mass spectroscopy. In the
standing of the effect of halogens on the molecular packing 1H NMR spectra, no signals at ca. 6.5 ppm assigned to the cis-
modes, and suggested that halogen can be used as a useful form vinyl were observed, meaning that the vinyls were in trans-
‘‘crystal engineering’’ group for topochemical reactions.65 With forms. The single crystal structures of 1FNaAA (CCDC 1948533),
these in mind, the naphthalene acrylic acids, in which different 1ClNaAA (CCDC 1948658), 1BrNaAA (CCDC 1948592), 1INaAA
halogens were introduced, are synthesized so as to fabricate (CCDC 1948594) and 6BrNaAA (CCDC 1948602) further
the photomechanical crystals. It is interesting that the mole- confirmed the trans-configuration of the vinyls.
cular crystals of 1FNaAA, 1ClNaAA, 1BrNaAA and 6BrNaAA
(Scheme 1) showed light-induced curling, flipping, bending,
Photophysical properties in solutions
bursting, coiling, twisting and cracking. The occurrence of the
The normalized UV-vis absorption and fluorescence emission
stereospecific [2+2] cycloaddition reactions was revealed to
spectra of the synthesized halogen-substituted 2-naphthalene
be responsible for the above motions. This provides a new
platform for photomechanical materials.
acrylic acid derivatives in THF (1.0 Â 10À5 M) are shown in
Fig. 1 and the corresponding photophysical data are listed in
Table S1 (ESI†). 1FNaAA gave the absorption bands at 267 nm,
297 nm and 308 nm ascribed to p–p* transitions. When
chlorine, bromine and iodine were introduced into the
1-position of the naphthyl ring to replace fluorine, the absorption
Results and discussion
Synthesis
The synthetic routes for the halogen-substituted 2-naphthalene bands in the relatively long-wavelength region for 1ClNaAA,
acrylic acid derivatives are shown in Scheme 1. 1-Fluoro-2-naphth- 1BrNaAA and 1INaAA were red-shifted to 301 nm/312 nm,
aldehyde (1), 1-chloro-3,4-dihydronaphthalene-2-carbaldehyde (2), 302 nm/312 nm and 307 nm/319 nm, respectively, due to the
Scheme 1 Synthetic routes for halogen-substituted 2-naphthalene acrylic acid derivatives.
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On the other hand, the absorption wavelength (labs), oscillator
strength ( f ) and dominant excitation characters of 1FNaAA,
1ClNaAA, 1BrNaAA, 1INaAA and 6BrNaAA were calculated with
the TD-DFT/B3LYP/3-21G(d) function. As listed in Table S2
(ESI†), the absorption bands at ca. 310 nm for the above
compounds mainly originated from the electronic transitions
from HOMOÀ2 to LUMO or HOMOÀ1 to LUMO, owing to p–p*
transitions.
Photomechanical effects
To investigate the photomechanical effects of the molecular
crystals, the needle-like crystals of 1FNaAA, the slice-like crystals
of 1ClNaAA, 1BrNaAA and 6BrNaAA, and the block-like crystal
of 1INaAA were obtained by slow evaporation of solvents under
ambient conditions. Firstly, the light-induced curling of the
needle-like crystal of 1FNaAA was observed. As shown in Fig. 2a
and Video S1 (ESI†), before UV irradiation, a needle-like crystal
(1200 mm  67 mm  18 mm) was vertical to the horizon. Upon
irradiation by 365 nm light from left to right, the lower end
(marked as ‘‘L’’) of the crystal moved rapidly away from the
light source, and the crystal exhibited light-induced backlight
curling. Upon UV irradiation for 3 s, the needle-like crystal
Fig. 1 Normalized UV-vis absorption (a) and fluorescence emission
(b, lex = 320 nm) spectra of 1FNaAA, 1ClNaAA, 1BrNaAA, 1INaAA and
6BrNaAA in THF (1.0 Â 10À5 M).
p–p conjugation of Cl, Br and I atoms with naphthalene. As for
6BrNaAA, the absorption bands in the low-energy region
appeared at 303 nm and 315 nm, similar to those of 1BrNaAA.
The emission of 1FNaAA appeared at 383 nm with a shoulder at
368 nm in THF, and the maximal emission of 1ClNaAA,
1BrNaAA and 1INaAA was red-shifted to 387 nm, 403 nm and
392 nm, respectively, similar to the results of the absorption
spectra. 6BrNaAA gave emissions at 359 nm and 376 nm
in THF.
Theoretical chemical calculations
We carried out density functional theory (DFT) calculations for
the 2-naphthalene acrylic acid derivatives using the Gaussian
09W program73 and using the DFT/B3LYP/3-21G(d) method to
reveal the electronic structures of 1FNaAA, 1ClNaAA, 1BrNaAA,
1INaAA and 6BrNaAA. As shown in Fig. S1 (ESI†), the HOMOs
and LUMOs of 1FNaAA, 1ClNaAA, 1BrNaAA, 1INaAA and
6BrNaAA were distributed over the whole conjugated systems,
indicating the better conjugation. The energy levels of the
HOMOs were À6.34 eV, À6.48 eV, À6.33 eV, À6.37 eV and
À6.35 eV for 1FNaAA, 1ClNaAA, 1BrNaAA, 1INaAA and
6BrNaAA, respectively, whereas their LUMO energy levels were
À2.21 eV, À2.33 eV, À2.20 eV, À2.24 eV and À2.35 eV, respectively.
Fig. 2 Optical microscopy photos during (a) the curling of the needle-like
crystal (1200 mm  67 mm  18 mm) and (b) the bending, flipping and
bursting of the elongated needle-like crystals (2600 mm  65 mm  20 mm)
of 1FNaAA upon irradiation with 365 nm light (3 W) for different times. The
irradiation direction is from left to right and the letters of ‘‘U’’ and ‘‘L’’ in (a)
represent the two ends of the needle-like crystal.
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curved into a semi-circular shape with an opening to the upper
right side. During the curling of the crystal of 1FNaAA, the light
source was moved up to follow the crystal without changing the
irradiation direction. Therefore, when the light was irradiated
for 5 s, a loop in which the two ends intersected was formed.
Upon prolonging the UV irradiation time to 9 s, the intersection
part gradually opened, and the original end of ‘‘L’’ of the
needle-like crystal turned to become the upper end of the loop.
After irradiation for 37 s, the cross section of the loop was
completely opened and further curved into another semicircle
with an opening to the left. Due to the continuous movement of
the needle-like crystal of 1FNaAA, the phototropic faces of the
crystal would be changed during the light-induced curling,
so that the crystal moved to the upper right side of the field
of the vision. When the UV lamp was turned off, no movement
was observed. Besides the curling of the needle-like crystal of
1FNaAA, it was found that the UV irradiation could induce the
elongated needle-like crystals (2600 mm  65 mm  20 mm) to
bend and subsequently burst, and some fragments jumped
away from their original positions significantly. As shown in
Fig. 2b and Video S2 (ESI†), two needle-like crystals were next to
each other before UV irradiation. Upon irradiation with 365 nm
light from left to right, they exhibited a slight slipping towards
the light source. When irradiated for 3 s, the crystals bent
backwards from the light source. After UV irradiation for 6 s,
the crystals bent into an arc with the opening to the right, and
the two crystals stacked almost in parallel. Interestingly, the
crystals flipped rapidly at 7 s of irradiation time, leading to
the direction of the opening for the arc to turn to the left. The
crystals continuously bent away from the light source and
began to burst into several fragments as the irradiation time
was prolonged to 11 s. Furthermore, the crystals exploded into a
Fig. 3 UV-vis absorption (a) and fluorescence emission (b; lex = 320 nm)
spectra of 1FNaAA in the microcrystals before and after irradiation by
365 nm light (3 W) for different times.
number of fragments, some of which moved out of the visual of irradiation time to 100 s, a new emission at 370 nm was
field after irradiation for 15 s. It is worth noting that the observed. The above photochromic behavior of 1FNaAA in the
bursting behavior of the crystals completely stopped after microcrystals suggested that photochemical reaction might
15 s of UV irradiation, while the formed smaller needle-like occur.74
crystals exhibited bending backwards from the light upon
Accordingly, the samples for 1H NMR measurements were
further increasing the irradiation times. Besides the above gained by dissolving the microcrystals of 1FNaAA, which were
light-induced curling, bending and bursting were observed first irradiated by UV light for different times, in DMSO-d6.
for the needle-like crystals of 1FNaAA, and light-induced sliding As shown in Fig. 4, two new doublets at 4.75 ppm and 4.15 ppm
was also observed (Video S3, ESI†).
appeared when the microcrystals of 1FNaAA were irradiated by
To reveal the driving forces for the above photomechanical 365 nm light for 30 s, and they could be ascribed to the signals
effects, fluorescence emission and UV-vis absorption spectral for H3 and H4, respectively, in cyclobutane formed by [2+2]
changes before and after UV irradiation of the microcrystals cycloaddition. Upon prolonging the irradiation times, the
of 1FNaAA were monitored. As shown in Fig. 3a, 1FNaAA in intensities for the signals at 4.75 ppm and 4.15 ppm increased
the microcrystals gave a broad absorption in the range of gradually, and those at 7.92 ppm and 6.76 ppm corresponding
213–240 nm and another absorption band located at 267 nm. to H1 and H2 in the vinyl unit decreased. The ratio of the
After UV irradiation for 2 s, a new absorption centered at integral values for the peaks at 6.76 ppm and 4.15 ppm was
230 nm emerged, and the absorption at 267 nm decreased. ca. 1/7.14, illustrating that ca. 88% 1FNaAA molecules were
When the irradiation time was prolonged to 6–10 s, the newly transformed into the photodimerization product (named
emerged absorption at 230 nm was intensified, and the absorp- D-1FNaAA) when the microcrystals were UV irradiated for
tion at 267 nm became a shoulder. Additionally, before irradia- 10 min.75 In addition, the newly emerging peaks in the range
tion, the microcrystals of 1FNaAA gave an emission band at of 7.3–7.8 ppm for D-1FNaAA were also intensified upon
435 nm, which was decreased and blue-shifted gradually upon prolonging the irradiation times. Notably, only one kind of
exposure to 365 nm light. For instance, it was blue-shifted to stereoisomer of D-1FNaAA was gained via the photodimeriza-
377 nm after UV irradiation for 50 s (Fig. 3b). With the increase tion of 1FNaAA in microcrystals since no signals for any other
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Fig. 4 1H NMR spectra of the samples before (a) and after irradiation of the microcrystals of 1FNaAA by 365 nm light (3 W) for 30 s (b), 1 min (c), 3 min (d),
5 min (e) and 10 min (f), followed by dissolution in DMSO-d6; (g) 1H NMR spectrum of D-1FNaAA in DMSO-d6.
kind of cyclobutane moiety were detected in Fig. 4b–f. There- in parallel viewed from the b-axis (Fig. 5b and c). The H-bonded
fore, the dimerization of 1FNaAA in the microcrystals was dimer based on two strong H-bonds of O–HÁ Á ÁOQC (1.425 Å)
stereospecific. Accordingly, D-1FNaAA was purified by column between carboxyl groups was involved (Fig. 5a and d). Besides,
chromatography from the microcrystals, which were exposed to the weak H-bonds of C–HÁ Á ÁO (2.638 Å, 2.737 Å, 2.845 Å,
UV light for 30 min. The single crystal structure illustrated that 2.981 Å, 3.020 Å, 3.043 Å, 3.820 Å and 3.913 Å) and C–HÁ Á ÁF
D-1FNaAA was of b-type (Fig. 5e and Table S3, ESI†), and its (2.397 Å, 2.714 Å, 2.814 Å and 3.615 Å) were formed. Hirshfeld
1H NMR spectrum is shown in Fig. 4g and Fig. S37 (ESI†). Thus, surface analysis is known to be very valuable in the visualiza-
[2+2] cycloaddition reaction was the driving force for the tion and exploration of the packing modes and intermolecular
transformation of the light energy into motility in the molecular close contacts of a structure, so we calculated the Hirshfeld
crystals of 1FNaAA.
surfaces of 1FNaAA with the program CrystalExplorer, which
It is known that stereoselective photodimerization could have been mapped in dnorm mode.76 The red spots showed the
take place in pre-arranged molecular crystals, so the single shortest intermolecular contacts, the blue regions represented
crystal structure of 1FNaAA was determined and found to the longest intermolecular contacts and the white ones indi-
belong to a monoclinic crystal system with the space group cated the contacts around van der Waals radii.77 As depicted
P21/c (Table S3, ESI†). Four molecules were involved in one-unit in Fig. 6a, the shortest intermolecular contacts corresponded
cell, and the dihedral angle between the plane of the double to the interactions of O–HÁ Á ÁOQC and C–HÁ Á ÁF, which was
bond and naphthalene was 0.561, meaning a planar conforma- consistent with the conclusions made from the single crystal
tion of 1FNaAA. Viewed from the a-axis, the molecules showed a structure. Moreover, the intermolecular interactions between
wavelike stacking, and the potential reactive molecules packed 1FNaAA were also illustrated in the two-dimensional (2D)
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Fig. 5 (a) The H-bonding in the single crystal of 1FNaAA; the single crystal structure viewed along the a-axis (b) and b-axis (c); (d) the packing of
H-bonded dimer in a single crystal; (e) the dihedral angle between the plane of the double bond and naphthalene and the distance between two adjacent
parallel molecules; (f) the molecular configuration of b-type D-1FNaAA in the single crystal.
fingerprint plots, which showed the contacts contributing to
the Hirshfeld surface represented in dnorm mode (Fig. 6b). The
contributions of HÁ Á ÁH contacts (37.0%), HÁ Á ÁO contacts
(20.3%) and HÁ Á ÁF contacts (10.5%) suggested that van der
Waals forces and H-bonding played key roles in the molecular
arrangement in the single crystal of 1FNaAA. In addition, we
deduced that the H-bonded dimers formed from 1FNaAA in the
crystal were favourable for the topo-photodimerization of
naphthylvinylcarboxylic acid.78 The adjacent molecules were
aligned in parallel separated by a distance of 3.769 Å (Fig. 5e),
less than 4.20 Å, which was one of the criteria for [2+2]
cycloaddition.66 Besides the distance in ‘‘olefin pairs’’, the
angles y1, y2 and y3 were also considered to be geometric
criteria for photodimerization (Chart S1, ESI†). The closer that
y1, y2 and y3 are to 01, 901 and 901, the easier it is for the
photochemical reaction to take place. As listed in Table S5
(ESI†), y1 and y2 were 01 and 103.731, while y3 for the dihedral
angles between naphthalene and cyclobutane, and between
carboxyl group and cyclobutane was 69.391 and 70.251,
respectively. As a result, the head-to-head packing of 1FNaAA
in the single crystal satisfied the Schmidt’s topo-photochemical
criteria, and the light-induced [2+2] cycloaddition reaction
could afford b-type D-1FNaAA in a mirror-symmetric manner,
which could be further confirmed by the single crystal structure
of D-1FNaAA (Fig. 5f). It should be noted that the H-bond of
Fig. 6 (a) Hirshfeld surface mapped with dnorm of 1FNaAA; (b) the two-
dimensional fingerprint plots of crystal stacking for 1FNaAA; (c) the
percentage of individual atomic contact contributions to the Hirshfeld
surface. The surfaces are shown as transparent to allow visualization of the
orientation and conformation of the functional groups in the molecules.
O–HÁ Á ÁOQC (1.340 Å) still existed in the single crystal of ring were 1.589 Å and 1.553 Å, and the bond angles in
D-1FNaAA. The bond lengths of C–C in the four-membered the cyclobutane ring were 87.711, 90.271, 88.651 and 88.631,
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respectively. It can be seen that the photodimerization led to appeared at 386 nm and ca. 438 nm. As shown in Fig. S4 (ESI†),
the distance between the carbon atoms in the potential reaction two new doublets appeared at 5.04 ppm and 4.16 ppm desig-
double bond decreasing from 3.769 Å to 1.589 Å. Meanwhile, nated as the signals of H3 and H4, respectively, in cyclobutane
the formation of cyclobutane caused the naphthalene and upon UV irradiation of the microcrystals of 1ClNaAA, and
carboxyl units to stretch out of their original position, like a they were enhanced upon prolonging UV irradiation times.
bird spreading its wings.55 The distance between the distal Therefore, the photodimerization of 1ClNaAA occurred in
hydrogen atoms in the two naphthyls in D-1FNaAA became the crystals. Meanwhile, the integral values for the peaks at
7.854 Å, so the [2+2] cycloaddition reaction resulted in signifi- 8.17 ppm (H1) and 6.76 ppm (H2) corresponding to the protons
cant movement on the molecular level. In addition, 21% in the vinyl unit decreased gradually. Accordingly, ca. 78%
1FNaAA molecules transformed into D-1FNaAA upon UV (Fig. S4f, ESI†) of the molecules were converted into the
irradiating the microcrystals for 30 s (Fig. 4b, the ratio of the photodimerization product (D-1ClNaAA) after UV irradiation
integral values for the peaks at 6.75 ppm and 4.15 ppm was of the microcrystals for 10 min. Notably, no other cyclobutane
1/0.26), suggesting the high photodimerization reaction activity skeleton was obtained, as shown in Fig. S4 (ESI†), confirming
of 1FNaAA in microcrystal form. On account of the significant the stereospecific nature of the dimerization. Moreover,
movement on the molecular level during photodimerization, D-1ClNaAA was separated and purified via column chromato-
strain was produced on the phototropic surface of the crystal graphy from the microcrystals, which were exposed to UV light
in the short period of time. Therefore, when the strain accu- for 30 min (Fig. S41, ESI†). To reveal the topo-photochemical
mulated during light-induced structural transformation was reaction of 1ClNaAA in the crystals, X-ray crystallographic
released slowly, the needle-like crystals would bend or curl analysis was performed. As depicted in Fig. S5 (ESI†), the single
away from the light source. In the case of the elongated needle- crystal of 1ClNaAA belonged to a triclinic crystal system with
%
like crystal, besides light-induced bending, they might burst the space group P1, and two molecules were involved in one
into several fragments if the accumulated strain was released unit cell. The dihedral angle between the plane of the double
rapidly.54
bond and naphthalene in 1ClNaAA was 19.251, meaning that
On the other hand, it was found that the slice-like crystal of the molecules accumulated in a non-planar conformation
1ClNaAA could bend backwards upon exposure to UV light and (Fig. S5e, ESI†). The H-bonded dimer directed by two strong
cracked into many pieces under 365 nm light. As shown in H-bonds of O–HÁ Á ÁOQC (1.791 Å) was involved (Fig. S5a, ESI†),
Fig. S2 and Video S4 (ESI†), before UV irradiation, the lower half and it arranged into a layered structure (Fig. S5d, ESI†).
of a needle-like crystal (360 mm  33 mm  15 mm) was attached Besides, the weak H-bonds of C–HÁ Á ÁO (2.637 Å, 2.794 Å,
to the upper end of a slice-like crystal (700 mm  110 mm  2.824 Å, 2.859 Å, 2.977 Å, 3.069 Å, 3.416 Å and 3.667 Å) and
15 mm) that was vertical to the horizon. Upon irradiation by C–HÁ Á ÁCl (3.030 Å, 3.122 Å, 3.160 Å and 3.606 Å) were formed
365 nm light from left to right, both crystals bent together (Fig. S5a, ESI†). In Fig. S6 (ESI†) the intermolecular interactions
quickly backwards from the light source. Except for light- of C–HÁ Á ÁO and O–HÁ Á ÁOQC (18.7%) are shown as the bright
induced bending, the light-induced cracking of slice-like crystals red areas on the Hirshfeld surface. In addition, the proportion
of 1ClNaAA was observed in Video S5 (ESI†). Before UV of HÁ Á ÁCl interaction was 12.0% in the intermolecular inter-
irradiation, the slice-like crystals of 1ClNaAA (1300 mm  actions. Thus, H-bonding played a major role in the crystal
200 mm  20 mm, 760 mm  80 mm  10 mm) were vertical to packing. When viewed from the b-axis, the molecules featured
the horizon. Upon UV irradiation, the crystals bent slightly a face-to-face stacking (Fig. S5c, ESI†).79 The nearest neigh-
away from the light source, while the lower part of the crystal bouring parallel molecules were separated by a distance of
was lifted up from the glass substrate. When the irradiation 3.771 Å in the single crystal, and y1, y2 and y3 were 01, 112.941
time was prolonged, some cracks emerged parallel to the and 68.351/87.981, respectively (Table S5, ESI†). The molecular
bending direction of the crystal, and the slice-like crystal packing in the crystal of 1ClNaAA satisfied Schmidt’s topo-
exploded into fragments after irradiation for 30 s. Moreover, photochemical criteria. Accordingly, the single crystal structure
the newly emerged slice-like crystals could further bend during of the photodimerization product revealed the stereospecific-
the cracking, while the lower needle-like crystals twisted. The tion of the topo-[2+2] cycloaddition reaction. As shown in
changes in the absorption, emission and 1H NMR spectra upon Fig. S5f (ESI†), only one isomer of b-type D-1ClNaAA was
UV irradiation of the microcrystals of 1ClNaAA were similar to afforded. Similarly, each carbon atom in the original CQC
those of 1FNaAA. In detail, before irradiation, 1ClNaAA gave had to move ca. 1.1 Å during photodimerization (from 3.771 Å
three absorption bands at 213 nm, 276 nm and 315 nm with a to 1.587 Å). At the same time, the units of naphthyl and carboxyl
shoulder at 390 nm in the microcrystals (Fig. S3a, ESI†). Upon groups departed from their original positions and stretched
prolonging the UV irradiation time, the absorption at 276 nm outside the molecular long axis. As a result, the distance
and 315 nm was decreased significantly, and the absorption at between the outermost hydrogen atoms in naphthyl became
213 nm was raised and red-shifted. Notably, an isoabsorptive 9.726 Å. Thus, the photodimerization reaction was the driving
point at 258 nm appeared, meaning a new species was formed. force for the photomechanical effects of the molecular crystal
Meanwhile, the crystals of 1ClNaAA exhibited two emission of 1ClNaAA, and the mechanism for the light-induced bending,
bands centered at 367 and 452 nm before irradiation (Fig. S3b, cracking and twisting of the molecular crystals of 1ClNaAA was
ESI†). After irradiation with 365 nm UV light for 100 s, emission similar to that of the needle-like crystal of 1FNaAA.
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Moreover, the slice-like crystals of 1BrNaAA exhibited light- clockwise along its long axis for ca. 901, the accumulated strain
induced bending, rotating and coiling. As shown in Fig. S7 and was slowly released so that the bending and coiling of the
Video S6 (ESI†), the crystal of 1BrNaAA (1640 mm  60 mm  crystals were observed.
20 mm) lay on the substrate vertically and emitted orange-red
Light-induced bending and twisting of the slice-like crystal
fluorescence before UV irradiation. Upon irradiation by 365 nm of 6BrNaAA were observed. As shown in Fig. S12 and Video S7
light from left to right for 10 s, the upper end of the crystal (ESI†), the longest slice-like crystal of 6BrNaAA (2400 mm Â
slanted to the right slightly. Upon irradiation by UV light for 80 mm  40 mm) was vertical to the horizon before UV
32 s to 45 s, the crystal rotated clockwise along its long axis irradiation. Upon irradiation with 365 nm light from left to
until the rotation angle reached ca. 901. During 48–60 s of right for 40 s, the crystal bent slightly toward the light source
irradiation time, the upper part of the crystal bent away from and simultaneously lifted up perpendicular to the flat surface.
the light source. When the irradiation time was prolonged to After UV irradiation for 130 s, the lifted crystal fell on the
62–100 s, the crystal continued to rotate clockwise along its plane and continued to bend toward the light source as the
long axis, accompanied by coiling and cracking. The UV irradiation time was prolonged. After irradiation for 150 s,
1
absorption and H NMR spectral changes upon UV irradiation the deflection reached a maximum and the crystal stopped
of the microcrystals of 1BrNaAA suggested that photodimeriza- bending. The slice-like crystal bent into a semi-circular shape
tion took place. In detail, UV light caused a decrease in with an opening to the lower side. Subsequently, when the
absorption bands in the range of 280–600 nm and the increase irradiation direction was reversed, the bent crystal began to
of the absorption at 219 nm, and the later one was red-shifted bend to the right side. During the irradiation time of 160–190 s,
to 231 nm upon exposure to UV light for 5 s (Fig. S8, ESI†). the crystal repeated the previous motions that lifted it
1H NMR spectral changes illustrated that ca. 36% of the perpendicular to the plane, while bending toward the light
1BrNaAA molecules were converted into the photodimerized source. Upon prolonging the irradiation time to 340 s, the
product of D-1BrNaAA after UV irradiation of the microcrystals longest crystal continued to bend toward the light source,
for 10 min (Fig. S9, ESI†). To reveal the reason why the accompanied by twisting. Upon irradiation for 380 s, a few
dimerization efficiency of 1BrNaAA was lower than that of cracks appeared perpendicular to the long axis of the crystal.
1FNaAA, X-ray crystallographic analysis of a single crystal of On the other hand, the light-induced [2+2] cycloaddition reac-
1BrNaAA was performed. It revealed that the crystal belonged to tion of 6BrNaAA was confirmed to be responsible for the
%
a triclinic crystal system with the space group P1 and two transformation of the light energy into motility in crystals
molecules were involved in one unit cell (Table S4, ESI†). The based on the changes in the absorption (Fig. S13a, ESI†),
dihedral angle between the plane of the double bond and emission (Fig. S13b, ESI†) and 1H NMR spectra (Fig. S14, ESI†)
naphthalene was 17.411, indicating a nonplanar conformation upon UV irradiation. Around 39% of 6BrNaAA molecules in the
(Fig. S10e, ESI†). The strong H-bond of O–HÁ Á ÁOQC (1.731 Å) microcrystals were converted into D-6BrNaAA upon irradiation
and the weak H-bonds of C–HÁ Á ÁO (1.731 Å, 2.853 Å, 2.861 Å, with UV light for 10 min. The single crystal structure of
2.926 Å, 2.930 Å, 3.014 Å, 3.062 Å, 3.603 Å and 3.719 Å) and 6BrNaAA revealed that the crystal belonged to a triclinic crystal
%
C–HÁ Á ÁBr (3.263 Å, 3.301 Å, 3.321 Å and 3.774 Å) were formed in system with the space group P1 and two molecules were
the crystal (Fig. S10a, ESI†). The Hirshfeld surface illustrated involved in one unit cell (Table S4, ESI†). The dihedral angle
that the predominant intermolecular interactions in the crystal between the plane of the double bond and naphthalene was
of 1BrNaAA was H-bonds (the proportion of HÁ Á ÁO interactions 3.031, meaning a planar conformation of 6BrNaAA in the single
was 19.6%) (Fig. S11, ESI†). As a result, the adjacent molecules crystal. The H-bonds of O–HÁ Á ÁOQC (1.708 Å), C–HÁ Á ÁO
were arranged in parallel directed by the above intermolecular (1.708 Å, 2.618 Å, 2.825 Å, 2.904 Å, 2.991 Å, 2.999 Å, 3.119 Å
interactions. Moreover, the distance of ‘‘olefin pairs’’ was and 3.600 Å) and C–HÁ Á ÁBr (3.241 Å, 3.388 Å and 3.758 Å)
3.895 Å (Fig. S10e, ESI†), and y1, y2 and y3 were 01, 112.171 (Fig. S15a and d, ESI†) were proved to be essential for the crystal
and 67.691/85.681, respectively (Table S5, ESI†). Although the packing (Fig. S16, ESI†). The adjacent molecules were aligned
[2+2] cycloaddition of 1BrNaAA could occur in the crystal, the in parallel separated by a distance of 3.974 Å (Fig. S15e, ESI†),
bromine in the naphthyl group enlarged the distance of the which was less than 4.20 Å, and y2 and y3 in the single crystal of
olefin pairs compared with that of 1FNaAA (3.769 Å). Therefore, 6BrNaAA were 96.351 and 64.041/62.781, respectively, so [2+2]
the dimerization efficiency in the crystals of 1BrNaAA might be cycloaddition reaction took place in the crystals. We deduced
lower than that of 1FNaAA. The strain accumulated in a crystal that the gradient of the strain would be generated from the
by structural change in response to photoexcitation can be phototropic surface to the backlight surface of the crystals, and
released slowly, which leads to bending, twisting or coiling. the light-induced bending towards UV light of the crystal of
If the relaxation does not occur immediately, the strain is accumu- 6BrNaAA originated from the contraction of the phototropic
lated during the induction period and is released suddenly, surface.45
whereby the crystals rotate, crack, explode or jump.80 Hence, the
To understand the different bending directions of the crystals
relaxation of the strain did not occur immediately in the slice-like of 1ClNaAA (backwards light source) and 6BrNaAA (towards light
crystals of 1BrNaAA during the early period of the photoexcitation. source) upon UV irradiation, the index of the highly bendable
Once the accumulated strain in the induction period was released crystal faces was determined through calculating the morphology
suddenly, the crystals began to rotate. After the crystal rotated of 1ClNaAA and 6BrNaAA crystals by the software of Materials
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Studio 8.0 with the crystallographic data as the input information.81 only exhibited bending backwards from UV light, but also
Based on the crystalline-plane attachment energies, which deter- cracking, coiling, rotating and twisting in response to UV light.
mine the growth rates of crystalline planes, the crystallization Differently, the slice-like crystal of 6BrNaAA bent towards the
morphology and the indexes of the side faces were obtained. light source. The changes in 1H NMR spectra induced by
The calculated morphology reproduced the experimentally UV light indicated that the light-induced [2+2] cycloaddition
observed prismatic shapes of the 1ClNaAA and 6BrNaAA crystals reactions took place in the above photomechanical molecular
(Fig. S16 and S17, ESI†). According to the calculation, for both crystals. It should be noted that the single crystal structures
1ClNaAAand 6BrNaAA crystals, the widest face corresponded to illustrated that the H-bonded dimers were formed from
the (001) plane (Tables S6 and S7, ESI†), and the length direction 1FNaAA, 1ClNaAA, 1BrNaAA and 6BrNaAA, which packed in a
is along the a-axis. Therefore, the bending process observed in head-to-head mode, so that only one isomer of b-type photo-
Fig. S2 and S12 (ESI†) is the bending of the (001) plane along the dimerization products was yielded. The strain would be gener-
a-axis. However, we did find a difference in the arrangement of ated in crystals induced by the movements on the molecular
the molecules on the phototropic surfaces of the 1ClNaAA and level during the stereospecific photodimerization. If the accu-
6BrNaAA crystals. In detail, the carboxyl and naphthyl groups in mulated strain was released slowly, the light-induced bending,
1ClNaAA were on the outer side and the inside of the phototropic curling, coiling, and twisting would be observed. The flipping,
surface of the crystal, respectively. When the molecules on the bursting and cracking of the crystals might result from
phototropic surface underwent cycloaddition reaction to yield the rapid release of strong strain originating from the photo-
b-type D-1ClNaAA, the movement of the naphthyl group might triggered structural transformations. This provided new clues
be more significant compared with the carboxyl group. for the development of photomechanical molecular crystals
Therefore, the 1ClNaAA crystal bent backwards from the light based on acrylic acids.
source due to the expansion of the phototropic surface upon
irradiation by UV light for a few seconds. In contrast, the
carboxyl and naphthyl groups in 6BrNaAA were on the inside Conflicts of interest
and the outer side of the phototropic surface of the crystal,
respectively, so that the phototropic surface would contract
There are no conflicts to declare.
under UV light, resulting in the bending toward the light source.
Differently, no photomechanical effect was observed for the
crystal of 1INaAA, which could be explained from the X-ray
Acknowledgements
crystallographic analysis. As shown in Table S5 and Fig. S21
(ESI†), the single crystal of 1INaAA belonged to a triclinic crystal
This work was financially supported by the National Natural
Science Foundation of China (51773076) and the Open Project
of State Key Laboratory of Supramolecular Structure and
Materials (SKLSSM2019019).
%
system with the space group P1, and two planar molecules were
involved in the crystal lattice. Although the H-bonds of
O–HÁ Á ÁOQC (1.754 Å), C–HÁ Á ÁO (1.754 Å, 2.640 Å, 2.838 Å,
2.844 Å, 3.848 Å and 3.949 Å) and C–HÁÁÁI (3.141 Å and 3.250 Å)
were observed (Fig. S21a and S22, ESI†) and the molecules were
Notes and references
arranged in parallel, the distance of the ‘‘olefin pair’’ was 4.814 Å,
more than 4.20 Å. Therefore, the dimerization of 1INaAA cannot
take place, and no changes in the absorption, emission, and
1H NMR spectra were observed for 1INaAA in the block-like
crystal, even when it was exposed to UV light for 1 h (Fig. S19
and S20, ESI†). It was reasonable that the photomechanical effects
cannot be observed from the crystal of 1INaAA because of the
absence of [2+2] cycloaddition reaction.
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