In-Situ Photochromism Switching with Crystal Jumping through the Deammoniation of N-Salicylideneaniline Ammonium Salt

Article abstract of DOI:10.1021/acs.cgd.9b00039

N-Salicylideneaniline (SA) derivatives are traditional organic photochromic compounds whose properties are closely related to their molecular conformations and crystal structures. We herein present the photochromic property switching phenomenon of SA ammonium salt crystals using a deammoniation reaction. The N-salicylidene-4-carboxyaniline ammonium salt crystal (1-Sα) was found to exhibit photochromism upon irradiation with UV light. Upon heating, 1-Sα desorbed ammonia gas and transformed into a nonionic SA crystal (1-N) with non-photochromic properties. Surprisingly, this phase transition induced the "jumping crystal" phenomenon. Thus, the crystal structures both before and after the phase transition were determined, and it was revealed that the switching phenomenon of photochromism (photochromism switching) was caused by the crystal structural and molecular conformational changes that took place upon deammoniation. In addition, 1-N reverted to 1-Sα by exposure to aqua-ammonia vapor for a few minutes. These results suggested that such SA crystals could be applicable as switching materials or chemical sensors.

Full text of DOI:10.1021/acs.cgd.9b00039

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Article
In-situ Photochromism Switching with Crystal Jumping through
the Deammoniation of N-Salicylideneaniline Ammonium Salt
Haruki Sugiyama, Kohei Johmoto, Akiko Sekine, and Hidehiro Uekusa
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00039 • Publication Date (Web): 05 Jul 2019
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In-situ Photochromism Switching with Crystal Jumping through the
Deammoniation of N-Salicylideneaniline Ammonium Salt
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Haruki Sugiyama,1, 2 Kohei Johmoto,2 Akiko Sekine,2 and Hidehiro Uekusa2*
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1. Research and Education Center for Natural Sciences, Keio University, Hiyoshi 4-1-1, Kohoku, Yokohama
223-8521, Japan.
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2. Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro,
Tokyo 152-8551, Japan. (Phone) +81-3-5734-3529 (E-mail): uekusa@chem.titech.ac.jp
Abstract
N-Salicylideneaniline (SA) derivatives are traditional organic photochromic compounds whose properties
are closely related to their molecular conformations and crystal structures. We herein present the photochromic
property switching phenomenon of SA ammonium salt crystals using a deammoniation reaction. The N-
salicylidene-4-carboxyaniline ammonium salt crystal (1-Sα) was found to exhibit photochromism upon
irradiation with UV light. Upon heating, 1-Sα desorbed ammonia gas and transformed into a non-ionic SA
crystal (1-N) with non-photochromic properties. Surprisingly, this phase transition induced the “jumping
crystal” phenomenon. Thus, the crystal structures both before and after the phase transition were determined,
and it was revealed that the switching phenomenon of photochromism (photochromism switching) was caused
by the crystal structural and molecular conformational changes that took place upon deammoniation. In
addition, 1-N reverted to 1-Sα by exposure to aqua-ammonia vapor for a few minutes. These results suggested
that such SA crystals could be applicable as switching materials or chemical sensors.
Keywords: Photochromism switching, N-Salicylideneaniline, Deammoniation
1. Introduction
Photochromic compounds,1 which exhibit reversible color changes upon photo-irradiation, have
received significant attention due to their application in photo-switching materials,2 optical data
storage,3 and sensors.4 For example, N-salicylideneaniline (SA) is a well-known organic
photochromic compound,5–8 and the crystals of SA derivatives have been found to change color from
yellow to red upon UV light irradiation, and subsequently back to their original color upon heating
or visible light irradiation. This photochromic color change corresponds to a photo-isomerization
from the enol to the trans-keto form via the cis-keto form, as shown in Scheme 1.9–15
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The photochromic properties of such compounds have been found to depend on their crystal
structures, including their molecular conformations and arrangements.16 For example, SA crystals
exhibiting a non-planar molecular conformation and/or loose molecular packing exhibit
photochromism, while a planar molecular conformation and/or close packing leads to non-
photochromic crystals. In this context, studies into the threshold dihedral angle between two aromatic
rings of SA molecules to determine whether they are planar or non-planar in nature showed that a
dihedral angle of <20° leads to non-photochromic properties, while a dihedral angle >30° results in
photochromic properties (i.e., the “SA planarity rule”).17 Moreover, the molecular planarity affects
the population of enol and cis-keto tautomers within the crystal, which are colorless and yellow,
respectively. As non-planar molecules exhibit an overlap of the electron cloud between the lone pair
of an imine N atom and the π-electrons of an aniline ring, this overlap weakens the basicity of the
imine N atom and stabilizes the colorless enol form. Thus, SA crystals containing non-planar
molecules commonly exhibit a pale yellow color.18,19
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As an interesting relationship exists between the molecular conformation within the SA crystals and
their resulting photochromic properties, several advanced studies were performed to modify the SA
molecular conformations and in turn their photochromic properties.20–22 In terms of chemical
modification,23 SA derivatives bearing tert-butyl groups favored non-planar molecular conformations
due to the presence of these bulky groups. In addition, a change in the environment surrounding the
SA molecule can also induce a change in the molecular conformation of SA. Thus, the formation of
mixed materials with glasses24 or polymers,25 in addition to the production of clathrate compounds
with zeolites,26 bile acids,27 cyclodextrins,28 and molecular organic frameworks,29 has been reported.
Furthermore, mixed crystal formation30 and co-crystallization are key crystal engineering methods
for altering the crystalline environment via supramolecular synthons.31–33
Although the above methods have been successfully applied and reported in the literature, they lack
the ability to offer in-situ changes in the photochromic properties, since this property is permanently
fixed upon formation of the desired material or crystal complex. An alternative method to impart in-
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situ changes is therefore a desolvation-driven structural transformation of solvated crystals. Such a
transformation causes changes in the crystalline environment, which in turn alters the molecular
structure, and could cause in-situ changes in the physicochemical properties.34,35 Indeed, the synthesis
of an SA derivative composed of two SA moieties bridged by a methylene group has been reported,
and its methanol solvated crystal dynamically changed the photochromic properties through the
desorption and absorption of methanol molecules.36 However, the crystal structure of this species
could not be analyzed due to difficulties in preparing the necessary single crystals.
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We therefore expect that the solid-state phase transition induced by molecular desorption, such as
dehydration,37 desolvation, and deammoniation, could be a promising method to dynamically alter
the crystal structures and photochromic properties of such species. Moreover, it could allow
clarification of the mechanism of photochromic property changes with relation to the crystal
structures.
We herein report our study into the photochromic switching phenomenon of an SA derivative crystal
through the examination of two polymorphic ammonium salt crystals of N-salicylidene-4-
carboxyaniline (Chart 1). During heating of both polymorphs, a deammoniation phase transition was
induced that resulted in the jumping crystal phenomenon38,39 due to structural changes in the crystals;
both the deammoniation phase transition and the jumping crystal phenomenon were examined. The
photochromic properties of the deammoniated SA crystal was also determined. Furthermore, the
crystal structures of all forms were examined and the relationship between the crystal structure and
the physicochemical properties was discussed.
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2. Experimental Section
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Preparation of the N-salicylideneaniline derivatives
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Salicylaldehyde (1.0 mmol) and 4-aminobenzoic acid (1.0 mmol) were dissolved in methanol
(50 mL) and the resulting solution was stirred for 10 min at 25 °C. After this time, the solution was
evaporated under reduced pressure, and the precipitate containing the crude product was obtained.
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Pure crystals of N-salicylidene-4-carboxyaniline (1) were obtained by recrystallization from methanol.
Preparation of the ammonium salt crystals
N-Salicylidene-4-carboxyaniline ammonium salt crystals (1-Sα) were obtained by cooling the hot
methanolic solution of 1 with benzophenone imine in a molar ratio of 1:1. 1-Sα gradually transformed
over two weeks into the polymorphic crystal (1-Sβ) while standing in the methanolic solution.
Interestingly, crystal structure analyses revealed that the SA molecule formed salt crystals with
ammonia molecules, which were generated by the decomposition of benzophenone imine in the
solution. Elemental analysis also supported these results: 1-Sα (C28H25N3O6) expected: C, 67.33; H,
5.04; N, 8.41; O, 19.22; found: C, 67.28; H, 5.07; N, 8.40; O, 19.26, and for 1-Sβ (C28H25N3O6)
expected: C, 67.33; H, 5.04; N, 8.41; O, 19.22; found: C, 67.27; H, 5.05; N, 8.43; O, 19.25.
Identification of photochromism by UV-Vis measurements
The UV-Vis spectra of the SA crystals (1-Sα, 1-Sβ, and 1-N) were measured at 25 °C using a JASCO
V-560 spectrometer equipped with the option (ISV-469) for diffuse reflectance spectroscopy. The
measurement samples were prepared by mixing the SA crystals (7 mg) in barium sulfate powder
(350 mg). Photo-irradiation was then carried out at 365 nm through a glass filter (HOYA UV360)
using a high-pressure mercury lamp.
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Thermal analysis
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Thermal gravimetric and differential thermal analysis (TG-DTA) were performed using a Rigaku
Thermo Plus EVO (TG8120) instrument for 1-Sα and 1-Sβ. The TG-DTA curves were recorded up
to final temperatures of 250 and 300 °C, respectively (Figs. 3 and 4). Thermogravimetry-differential
thermal analysis and mass spectrometry (TG-DTA-MS) were performed using a Rigaku Thermo Plus
Photo instrument to analyze the thermal ammonia gas desorption for 1-Sα and 1-Sβ (Figs. S3 and
S4). Powder X-ray diffraction and differential scanning calorimetry (XRD-DSC) were
simultaneously carried out on a Rigaku D/MAX 2400 diffractometer coupled with a Rigaku Thermo
Plus 2 instrument (Figs. S5 and S6).
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Powder X-ray diffraction
To confirm the deammoniation of both 1-Sα and 1-Sβ, powder X-ray diffraction (PXRD) was carried
out using a Bruker D8 ADVANCE diffractometer with Cu-Kα radiation (λ=1.54184 Å) from a sealed
tube source.
Single-crystal X-ray diffraction
Single crystal X-ray diffraction data were collected in ω-scan mode with R-AXIS RAPID II Imaging
Plate cameras (Rigaku) using Cu-Kα radiation (λ=1.54186 Å) from a rotating anode source equipped
with a confocal multilayer mirror. The data were collected at 20 °C to correspond with the conditions
required for observing the photochromic properties. The integrated and scaled data were empirically
corrected for absorption effects using ABSCOR.40 The initial structures were solved by direct methods
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using SHELXS 97 and refined on F0 with SHELXL-2018/1.41 Non-hydrogen atoms were refined
anisotropically. All H atoms of the hydroxyl group, methine group, and ammonium cation were
picked from the differential Fourier map. The crystal structure of 1-Sα was solved with the space
group of C2/c, where an inversion center of symmetry was present. One independent SA molecule
and half an ammonium cation were present in an asymmetric unit. As shown in Fig. 1a, the SA
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molecules form a strong hydrogen bond (O∙∙∙O = 2.497(2) Å), and the H atom is on the inversion
center. Thus, the carboxyl group of the SA molecule has an average structure of a carboxylate and a
carboxyl group.
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In contrast, the crystal structure of 1-Sβ was solved with the space group of Cc, which does not
contain an inversion center. In Fig. 1b, the C-O bond distances and C-O-O angles clearly indicate that
the left SA molecule is neutral, while the other is cationic. The H atom of the carboxyl group was
also picked up from the differential Fourier map, and refined without any restraints or constraints.
Interestingly, although the local catemer-like structures of 1-Sα and 1-Sβ are similar, inversion
symmetry was present only in 1-Sα (C2/c). This was attributed a strong negative-charge-assisted
hydrogen bond forming a symmetric single well-type proton transfer profile, which generated a
higher symmetry in the crystal structure of 1-Sα than in that of 1-Sβ. In contrast, the corresponding
hydrogen bond is weaker in 1-Sβ than in 1-Sα (O∙∙∙O = 2.591(2) Å in 1-Sβ), and the H atom was
located on one of the SA molecules. Therefore, the carboxyl group and carboxylate were
distinguishable, and the space group was Cc without an inversion center. The molecular dihedral
angles (Φ) between two benzene rings of the SA molecule were calculated using the PLATON
program.42 Detailed crystal data are given in Table S1.
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3. Results and Discussion
Polymorphs of N-salicylidene-4-carboxyaniline ammonium salt
Polymorphic ammonium salt crystals of N-salicylidene-4-carboxyaniline, namely 1-Sα and 1-Sβ,
were recrystallized from methanol and their crystal structures were determined by single crystal X-
ray structural analyses. The crystals contained one neutral and one anionic SA molecule with one
ammonium cation. 1-Sα and 1-Sβ were found to belong to the different space groups of C2/c and Cc,
respectively. In the crystal structure of 1-Sα, the observed SA molecule was an average structure
between neutral and anionic SA molecules (Fig. 1a), where the C-O distances in the carboxyl group
were 1.290(2) and 1.236(2) Å, and the C-C-O angles were 116.9(2) and 120.4(2)°. However, the
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neutral and anionic SA molecules were distinguishable in the crystal structure of 1-Sβ (Fig. 1b), with
the neutral molecule having C-O distances of 1.314(5) and 1.227(6) Å, and C-C-O angles of 114.2(4)
and 122.5(4)°, while the anionic molecule had C-O distances of 1.284(5) and 1.256(6) Å, and C-C-O
angles of 118.3(4) and 117.4(4)°. These C-O distances and C-C-O angles of the carboxyl groups
differed significantly between the neutral and cationic SA molecules, which supported the assignation
of space group Cc.
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In addition, the carboxylic groups in 1-Sα and 1-Sβ exhibited catemer-like hydrogen bonding
patterns due to the presence of both neutral and anionic SA molecules. This catemer-like motif is not
uncommon, with 38 different crystal structures being found with this motif in the CSD database
(Version 5.40; search details are given in the Supporting Information). We also note that the hydrogen
bonding distance (O2···O2#1, #1: -x, y+2, -z+1) is 2.497(2) Å in 1-Sα, and which is similar to those
of previously reported structures (Fig. S7). However, in the case of 1-Sβ, a significantly longer
hydrogen bonding distance of 2.591(2) Å was detected.
Furthermore, the SA molecular pairs formed 1D chain structures with ammonium cations along the
c-axis via hydrogen bonding, and the interatomic distance between SA molecules and ammonium
cations was 2.776(2) Å in 1-Sα. In contrast, the distances between the neutral and cationic SA
molecules to the ammonium cations were 2.895(4) and 2.751(3) Å in 1-Sβ, respectively. This
difference in interatomic distances stabilizes the crystal structure where one neutral and one cationic
SA molecule are present independently.
We also found that the molecular arrangements viewed along the b-axis were similar (Figs. S1 and
S2). However, the molecular packing motifs viewed along the c-axis, as shown in Figs. 2a and 2b,
differed between 1-Sα and 1-Sβ. Although the SA molecules were arrayed almost parallel along the
b-axis in 1-Sα, they were arrayed in a herringbone pattern in 1-Sβ. Importantly, the molecular
conformations of SA differed significantly between the non-planar conformation in 1-Sα and the
planar conformation in 1-Sβ. When the 1-Sα crystals were allowed to stand in the methanol solution,
they gradually disappeared, and crystals of 1-Sβ were formed. This observation shows that the
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unstable kinetic polymorph (1-Sα) dissolved, and the thermodynamically stable polymorph (1-Sβ)
was formed. Indeed, the stabilities of these polymorphs were confirmed by the density rule, with
crystal densities of 1.386 g·cm−3 for 1-Sα and 1.444 g·cm−3 for 1-Sβ.
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Crystal jumping induced by deammoniation
TG-DTA measurements (Figs. 3 and 4) showed that 1-Sα and 1-Sβ deammoniated at 161.0 and
135.8 °C, respectively. In addition, TG-DTA-MS measurements supported the desorption of an
ammonium cation from the crystal structure as a neutral ammonia molecule upon heating (Figs. S3
and S4). This resulted in the SA molecule switching from a cationic to a neutral state. In addition,
Fig. 5 shows the PXRD patterns both before and after heating at the deammoniation temperatures,
and the changes in the PXRD patterns suggest that the salt crystals of 1-Sα and 1-Sβ transformed to
the same SA crystal of 1-N. The crystal structure of 1-N is well known and has been reported
previously.31 XRD-DSC measurements confirmed that 1-Sα and 1-Sβ directly transformed into 1-N
without passing through any other crystalline phase (Figs. S5 and S6). While heating at the
deammoniation temperatures using a hot plate, crystals of both 1-Sα and 1-Sβ exhibited the jumping
crystal phenomenon (Fig. 6). Movies of the jumping crystals are available in the Supporting
Information. These mechanical phenomena induced by thermal stimuli are classified as a
thermosalient effect.43 Indeed, in the past few decades, several crystals have been reported to exhibit
this interesting phenomenon. Such thermosalient crystals have attracted considerable attention due to
their potential use in the development of molecular actuators, molecular machines, and mechanical
sensors. Moreover, 1-Sα and 1-Sβ are classified into rare types of jumping crystals that are related to
molecular desorption.44,45
To account for the jumping crystal phenomena in 1-Sα and 1-Sβ, we compared crystal structures
before and after deammoniation. In the case of jumping crystal 1-Sα, a linear-chain structural motif
was formed by neighboring molecules along the a-axis (Fig. 7a). In contrast, in 1-N, the molecules
formed a zigzag-chain structural motif with neighboring molecules (Fig. 7b), thereby indicating that
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deammoniation produced a significant structural change in the crystal. In contrast, 1-Sβ, which also
exhibited the jumping crystal phenomenon upon heating, formed a zigzag-chain structural motif.
When 1-Sβ was deammoniated and transformed into 1-N, the zigzag-chain pattern, which is
highlighted in blue in Figs. 7c and 7d, slips half of a unit cell length. These large crystal structural
changes induced by deammoniation were therefore responsible for the jumping crystal phenomena.
Thus, we selected and examined the lattice parameters of single crystals of 1-Sα and 1-Sβ as soon as
the crystals jumped upon heating with a hot plate. Contrary to our expectations, these crystals
appeared to be present in their original ammonium salt phase, thereby suggesting that deammoniation
occurred on the crystal surface that was in contact with the hot plate. We note that the crystal structural
change taking place upon deammoniation causes a partial crystal deformation, which generates
mechanical stress, and results in the jumping crystal phenomenon. Indeed, not all crystals jumped
upon heating, which suggested that only crystals that had a suitable contact with the hot plate could
jump. In addition, 1-Sα and 1-Sβ were completely transformed into 1-N upon further heating, under
which conditions the crystals did not retain their single crystal state.
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In-situ photochromism switching via deammoniation and crystal color changes
We found that the photochromic properties differed between 1-Sα and 1-Sβ. More specifically, as
shown in Fig. 8a, 1-Sα exhibited photochromism from pale yellow to red upon UV light irradiation,
where the increase in the absorption band at approximately 500–600 nm corresponds to isomerization
from the colorless enol to the red trans-keto form. The dihedral angle of the SA molecule in 1-Sα
was 45.15(9)°, which is within the photochromic range (i.e., >30°). In contrast, 1-Sβ did not exhibit
photochromism (Fig. 8b), and its molecular dihedral angles were 2.1(2) and 1.9(2)°, which are lower
than the threshold value of 20°. These results indicate that both ammonia salt crystals conformed to
the previously mentioned SA planarity rule. Interestingly, 1-Sα lost its photochromic properties upon
heating due to deammoniation to 1-N. Actually, as shown in the UV-Vis. spectrum of 1-N, no
differences were observed following UV irradiation (Fig. 8c). In the case of 1-N, the molecular
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dihedral angle was 5.1(1)°, which is significantly lower than the non-photochromic threshold of 20°.
It was therefore apparent that the photochromic properties were switched off due to a change in the
SA molecular conformation from a photochromic non-planar conformation to a non-photochromic
planar conformation upon deammoniation from 1-Sα to 1-N (Fig. 9). As mentioned previously, SA
molecules switched their ionic state from anionic to neutral through deammoniation, which did not
affect the photochromic properties due to the photochromic mechanisms of SA.31,46
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To investigate the reversibility of the deammoniation reaction, we stored the crystalline sample of
1-N with a 2830% concentrated aqueous ammonia solution, such that 1-N was exposed to the aqua-
ammonia vapor. In this context, we note that a gas-solid reaction with aqua-ammonia vapor is a
common method for ammonium salt formation with an organic acid.47,48 Based on the PXRD results,
it was found that 1-N transformed into 1-Sα when exposed to aqua-ammonia vapor for a few minutes
(Fig. S8). Thus, the in-situ photochromism switching phenomenon appeared to be reversible upon
exposure to external stimuli, such as heating and aqua-ammonia vapor. Indeed, a few reports describe
the successful reversible transformation from ammonium salt crystals to deammoniated neutral SA
crystals.49,50 Recently, these crystals are called “soft crystals”,51 which can changes their structure
and properties flexibly by external stimuli.
As shown in Figs. 8a, 8b, and 8c, the crystal color of 1-Sα clearly differed from those of 1-Sβ and
1-N prior to UV light irradiation, and this was attributed to differences in the molecular conformations.
More specifically, 1-Sα exhibited a paler yellow color than the other samples, which was accounted
for by the fact that SA crystals containing non-planar molecules tend to exhibit a paler yellow color
than those containing planar molecules.18,19 The color differences between 1-Sα, 1-Sβ, and 1-N were
therefore supported by this observation. In addition, the UV-Vis spectra of 1-Sβ and 1-N showed
large absorption bands at ~500 nm, which were not observed for 1-Sα. Indeed, the absorption of light
by the cis-keto form was responsible for the orange crystal color, thereby confirming that the
molecular conformation change induced by deammoniation resulted in changes to both the crystal
color and the photochromic properties of 1-Sα.
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4. Conclusion
N-Salicylideneaniline (SA) derivatives are photochromic compounds in which the properties are
closely related to their molecular conformations and crystal structures. We herein demonstrated that
the photochromic properties of an SA derivative crystal 1-Sα were successfully switched via
deammoniation, which can be explained by the in-situ molecular conformation change from a
photochromic non-planar conformation to a non-photochromic planar conformation within the crystal.
Interestingly, the deammoniation phase transition with photochromism switching was reversible; 1-
N reverted to 1-Sα by exposure to aqua-ammonia vapor. In addition, the two polymorphs of the
ammonium salt crystal, namely the photochromic 1-Sα and the non-photochromic 1-Sβ, exhibited
different crystal colors that depended on the molecular conformation. Moreover, both polymorphs
exhibited the jumping crystal phenomenon upon heating. This thermosalient effect may be induced
by the partial mechanical strains caused by deammoniation. Our results therefore confirm that
ammonium salt crystals containing SA derivatives exhibit dynamic mechanical and chemical
property changes upon deammoniation. These crystals that can undergo changes in their structure and
properties upon exposure to external stimuli are known soft crystals, and could be potentially useful
in applications such as switching materials or chemical sensors.
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Supporting Information
The Supporting Information is available on the ACS Publications website at DOI: xxxxxxx.
TG-DTA-MS data, XRD-DSC data, X-ray crystallographic data, and powder X-ray diffraction
(PDF)
Accession Codes
CCDC 1884035 and 1884036 contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12
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Acknowledgments
9
This work was supported by JSPS KAKENHI Grant Numbers JP17K05745 and JP18H04504, and
by Keio Gijuku Academic Development Funds. The authors thank the referees for their valuable
suggestions on the crystal structure analysis. The authors also thank the Suzukakedai Materials
Analysis Division of the Technical Department at the Tokyo Institute of Technology for elemental
analyses, TG-DTA-MS, and TG-DSC measurements.
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In-situ Photochromism Switching with Crystal Jumping through the
Deammoniation of N-Salicylideneaniline Ammonium Salt
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Haruki Sugiyama,1, 2 Kohei Johmoto,2 Akiko Sekine,2 and Hidehiro Uekusa2*
N-Salicylideneaniline ammonium salt crystals 1-Sα exhibited the jumping crystal phenomenon, and
the photochromic properties were switched off due to the molecular conformation change through
the deammoniation transition upon heating.
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The photochromism mechanism in N-salicylideneanilines.
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The chemical structures of 1-Sα and 1-Sβ.
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The crystal structures of 1-Sα (a) and 1-Sβ (b). Hydrogen bonds are highlighted with blue dotted lines.
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The crystal structure of 1-Sα (a) and 1-Sβ (b) viewed along the c-axis.
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TG-DTA curves of 1-Sα. 1-Sα deammoniated to 1-N at 161.0 °C with a weight loss of 3.45 % (the ideal
mass percentage of 0.5 ammonia molecules is 3.40%).
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TG-DTA curves of 1-Sβ. 1-Sβ deammoniated to 1-N at 135.8 °C with a weight loss of 3.48 % (the ideal
mass percentage of 0.5 ammonia molecules is 3.40%).
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The PXRD patterns of 1-Sα (red), after the heating of 1-Sα (green), 1-Sβ (blue), after the heating of 1-Sβ
(green), and 1-N (dark green) calculated from the crystal data.
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Photographic images showing before (a, c) and after (b, d) crystal jumping of 1-Sα and 1-Sβ, respectively,
upon heating using a hot plate. The crystal circled by a red line jumped up to the position circled by a blue
line.
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(a) The straight chain in 1-Sα viewed along the c-axis. (b)The zigzag chain in 1-N viewed along the a-axis.
(c)The zigzag chain pattern in 1-Sβ viewed along the b-axis. (d) The zigzag chain in 1-N viewed along the
b-axis.
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The UV/Vis. diffuse reflectance spectra before (blue) and after (red) UV irradiation for (a) 1-Sα, (b) 1-Sβ,
and (c) 1-N. The inset photographs show the color changes of the powdery crystalline samples before and
after UV irradiation.
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Structural changes upon deammoniation from 1-Sα to 1-N. The molecular conformation converted from
non-planar to planar, and the photochromic properties changed.
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N-Salicylideneaniline ammonium salt crystal 1-Sα exhibited the jumping crystal phenomenon upon heating,
and the photochromic properties were switched off due to the molecular conformation change through the
deammoniation transition.
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