A novel approach to achieve molecular switching in solid-state driving by thermal treatment: A photochromic zinc-coordination polymer

Article abstract of DOI:10.1016/j.ica.2020.119879

The molecular switching of complexes based on diarylethenes in a solid-state is their most frequently studied property. This switching becomes possible when the active carbon atoms acquire a suitable interatomic separation in the molecule. Even if the structural analysis may suggest conditions disfavouring the occurrence of such switching, it may still be enabled by post-synthetic modification of the sample, like dehydration. In the present study, a novel one-dimensional coordination polymer with composition {[Zn(μ₃-HA)(H₂O)₃]·H₂O}_n (I) was prepared and characterized. In its solid state, however, its thiophene rings were arranged in the non-switchable conformation yielding a C···C separation 4.148(6) ? between the active carbon atoms. After fully dehydrating the sample, this interatomic separation decreased to 3.814 ?, making it susceptible to photochromism by UV radiation.

Full text of DOI:10.1016/j.ica.2020.119879

Inorganica Chimica Acta 512 (2020) 119879
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
A novel approach to achieve molecular switching in solid-state driving by
thermal treatment: A photochromic zinc-coordination polymer
a,⁎
b
c
d
Miroslav Almáši , Martin Walko , Jana Boržíková , Róbert Gyepes
a
Department of Inorganic Chemistry, Faculty of Science, P. J. Šafárik University, Moyzesova 11, SK-041 54 Košice, Slovak Republic
School of Chemistry, University of Leeds, GB-LS2 9JT Leeds, United Kingdom
State Veterinary and Food Institute in Dolny Kubin, Veterinary and Food Institute in Košice, Hlinkova 1, SK-043 65 Košice, Slovak Republic
Department of Inorganic Chemistry, Faculty of Science, Charles University, Albertov 8, CZ-128 43 Prague 2, Czech Republic
b
c
d
A R T I C L E I N F O
A B S T R A C T
Keywords:
Zinc-coordination polymer
Diarylethene
Molecular switching
Photochromism
DFT calculations
The molecular switching of complexes based on diarylethenes in a solid-state is their most frequently studied
property. This switching becomes possible when the active carbon atoms acquire a suitable interatomic se-
paration in the molecule. Even if the structural analysis may suggest conditions disfavouring the occurrence of
such switching, it may still be enabled by post-synthetic modification of the sample, like dehydration. In the
3 2 3 2 n
present study, a novel one-dimensional coordination polymer with composition {[Zn(μ -HA)(H O) ]·H O} (I)
was prepared and characterized. In its solid state, however, its thiophene rings were arranged in the non-
switchable conformation yielding a C···C separation 4.148(6) Å between the active carbon atoms. After fully
dehydrating the sample, this interatomic separation decreased to 3.814 Å, making it susceptible to photo-
chromism by UV radiation.
1. Introduction
between the metallic ions can be switched by photo-irradiation, be-
cause the π-conjugated bond structures between the two aryl groups are
In general, photochromism is defined as a reversible transformation
different in the two isomers. Based on published results it has been
shown that in the case of diarylethenes, the distance between the active
carbon atoms should be within 3.45–3.80 Å [6–8].
two isomers having different absorption spectra induced in one or both
directions by photoirradiation. Photochromic compounds have at-
tracted remarkable attention because of their potential applicability in
optical memory media, optical switching devices and as control ele-
ments in molecular motors [1,2].
In the present work, we report the synthesis and characterization of
one-dimensional coordination polymer with composition {[Zn(μ
3
-HA)
(H O) ]·H O} (I, HA = 1,2-bis(5-carboxy-2-methylthien-3-yl)cyclo-
2
3
2
n
Among them, diarylethenes with heterocyclic aryl groups are the
most promising compounds for application, due to their resistance
against fatigue and their thermally irreversible photochromic proper-
ties. Moreover, it has been recently found that some diarylcyclo-
pentenes derivatives with thiophene or benzothiophene rings undergo
photochromic reactions even in the single-crystalline phase [3–5].
Upon irradiation with UV light, diarylethene molecules undergo pho-
tocyclization and this reaction is accompanied by the colour change
that is dependent on the chemical structure of the molecules. Upon
irradiation with visible light, the closed coloured form turns to the in-
itial colourless open form and this process can be repeated many times.
Photocyclization is a reversible process and can be observed in solution
and rarely in solid-state, because in crystals large geometrical structure
changes are prohibited. However, with metal cations coordinated at
both ends of the diarylethene switch aryl groups, the interaction
pentene) and its photochromic reaction in solid-state before and after
dehydration process. It was shown, that after the dehydration process of
the sample, the interatomic C···C separation decreased from the original
value of 4.148 to 3.814 Å, making it susceptible to photochromism by
UV radiation.
2. Experimental
2.1. Materials
Zinc(II) nitrate tetrahydrate and chemicals used in the synthesis of
1,2-bis(5-carboxy-2-methylthien-3-yl)cyclopentene (HA) were pur-
chased from Sigma-Aldrich or Acros Organics and used without further
purification.
⁎
Corresponding author.
E-mail address: miroslav.almasi@upjs.sk (M. Almáši).
https://doi.org/10.1016/j.ica.2020.119879
Received 26 May 2020; Received in revised form 2 July 2020; Accepted 4 July 2020
Available online 11 July 2020
0020-1693/ © 2020 Elsevier B.V. All rights reserved.

M. Almáši, et al.
Inorganica Chimica Acta 512 (2020) 119879
2
2
.2. Synthesis
(400 MHz, CDCl
3
): δ 1.92 (s, 6H), 2.01 (p, J = 7.5 Hz, 2H), 2.77 (t,
13
J = 7.5 Hz, 4H), 7.42 (s, 2H) ppm. C NMR (100.6 MHz, DMSO‑d ): δ
6
.2.1. Synthesis of 1,2-bis(5-carboxy-2-methylthien-3-yl)cyclopentene
14.3, 22.2, 37.8, 130.4, 133.8, 134.2, 136.3, 141.6, 162.5 ppm.
Corresponding 1H and C NMR spectra of 4 with assignments of
hydrogen and carbon atoms are depicted in Fig. S4 in ESI.
13
(HA)
2
.2.1.1. 2-Chloro-5-methylthiophene (1). 2-methylthiophene (50 ml,
0.52 mol) and N-chlorosuccinimide (76 g, 0.57 mol) were added to a
stirred solution of benzene (200 ml) and glacial acetic acid (200 ml).
The suspension was stirred for 30 min at room temperature, then
heated to reflux for 1 h. The cooled mixture was poured into 3 M NaOH
solution (300 ml). The organic phase was washed with a solution of 3 M
2.2.2. Synthesis of {[Zn(μ -HA)(H O) ]·H O} (I)
3
2
3
2
n
150 mg (0.57 mmol) of Zn(NO ) ·4H O and 50 mg (0.14 mmol) of
3
2
2
H HA were added to 20 ml of distilled water. For deprotonation of
2
3
dicarboxylic acid 2.8 cm of 0.1 M NaOH (0.28 mmol) solution was
NaOH (3 × 200 ml), dried with Na
evaporated in vacuum to yield a light yellow liquid. The final product
was purified by vacuum distillation at pressure 50 mbar and 70 °C to
give a colourless liquid (57.65 g, 84%). H NMR (400 MHz, CDCl
2
SO
4
, filtered and the solvent was
added to the reaction mixture. The reaction mixture was refluxed for
3.5 h and after cooling to the laboratory temperature, the solution was
left for isothermal crystallization for two weeks. During this time, col-
ourless crystals of I were formed (see Fig. S3 in ESI), which were filtered
off, washed with water and dried in a stream of air (yield: 48 mg, 71%
1
3
): δ
2
(
.30 (s, 3H, H(3)), 6.40–6.41 (m, 1H, H(2)), 6.58 (d, J = 3.6 Hz, 1H, H
1)) ppm. Corresponding 1H NMR spectrum of 1 with assignments of
hydrogen atoms is depicted in Fig. S1 in ESI.
−1
based on H HA). EA (485.89 g.mol ) and FAAS, clcd: C 42.02%; H
2
4.98%; S 13.20%; Zn 13.46%; found: C 41.54%; 4.84%; S 13.04%; Zn
13.75%.
2
.2.1.2. 1,2-bis(5-chloro-2-methylthien-3-yl)pentane-1,5-dione (2). To a
stirred solution of glutaryldichloride (12.08 g, 71.5 mmol) and 2-
chloro-5-methylthiophene (1, 18.84 g, 142 mmol) in CH Cl (150 ml)
in ice bath, AlCl (22.88 g, 172 mmol) was carefully added. After
addition of AlCl , the reaction mixture was stirred for 2 h at room
temperature. Then 50 ml of water was added to the reaction mixture
and extracted with CH Cl (3 × 150 ml). The combined organic phases
were washed with water (2 × 100 ml), dried with Na SO , filtered off
and the solvent was evaporated in vacuum to yield a brown tar
2.3. Methods and characterization
2
2
3
The elemental analysis was performed with CHNOS Elemental
Analyzer vario MICRO from Elementar Analysensysteme GmbH with a
mass sample approximately 3 mg.
3
2
2
The determination of the amount of zinc(II) ions in compound I
have conducted on AAS PERKIN-ELMER 410 at wavelength 214 nm.
Before measurement, the sample was mineralized in aqua regia upon
heating.
2
4
1
(
2
2
25.35 g, 98%). H NMR (400 MHz, CDCl
3
): δ 2.06 (p, J = 6.9 Hz,
1H NMR and 13C NMR spectra were recorded with a Varian Mercury
H, H(3)), 2.66 (s, 6H, H(4)), 2.86 (t, J = 6.9 Hz, 4H, H(2)), 7.18 (s,
1
1
H, H(1)) ppm. Corresponding H NMR spectrum of 2 with assignments
Plus 400 spectrometer at a frequency of magnetic field 400 ( H NMR)
and 100 (13C NMR) MHz, respectively using TMS as the internal re-
of hydrogen atoms is depicted in Fig. S2 in ESI.
ference. Samples were dissolved in CDCl
3 6
or DMSO‑d solvents.
2
.2.1.3. 1,2-bis(5-chloro-2-methylthien-3-yl)cyclopentene (3). In a three-
neck flask, zinc powder (10 g, 153 mmol) was added to anhydrous THF
200 ml) under nitrogen atmosphere. Afterwards, TiCl (11.85 ml,
08 mmol) was added to this suspension very carefully using a syringe
Infrared spectra were recorded with Avatar FT-IR 6700 spectro-
meter in the wavenumber range 4000–650 cm using ATR (attenuated
total reflectance) technique.
−1
(
1
4
The TG measurement was carried out using a TGA Q500 instrument
−
1
with needle (!be careful!) and the solution was refluxed for 1 h. In the
next reaction step, the mixture was cooled in an ice bath and 1,2-bis(5-
chloro-2-methylthien-3-yl)pentane-1,5-dione (2) (25.35 g, 70 mmol)
was added. The final mixture was refluxed for 2 h. The reaction mixture
was then cooled down, 200 ml 10% aq. K
was extracted with Et O (3 × 200 ml). The combined organic phases
were dried with Na SO , filtered off and evaporated under vacuum to
yield a brown tar. After purification by column chromatography on
silica gel (mobile phase: petroleum ether 40–60) a slightly yellow
crystalline solid was obtained (16.43 g, 71%). H NMR (400 MHz,
under dynamic conditions with a heating rate of 10 °C.min . The
sample has been heated under air atmosphere with an air flow rate of
3
−1
50 cm .min
in the temperature range from 30 to 800 °C.
The crystallinity of the sample I before and after dehydration pro-
cess was determined by PXRD measurements on a Bruker AXS D8
ADVANCE diffractometer in the Bragg–Brentano geometry using CuKα
(λ = 1.54056 Å) radiation with a NaI dynamic scintillation detector
(Vantec-1) in the 2θ range of 2-30°. In order to achieve parallel and
clean X-ray beam for PXRD experiments, initially divergent CuKα ra-
diation emitted from X-ray lamp was further guided through the multi-
layered mirror and set of slits.
2 3
CO was added and then it
2
2
4
1
CDCl
J = 7.5 Hz, 4H, H(2)), 6.56 (s, 2H, H(3)) ppm. C NMR (100.6 MHz,
CDCl ): δ 14.1 (C(8)), 22.8 (C(1)), 28.1 (C(2)), 123.2 (C(6)), 126.6
C(5)), 133.3 (C(4)), 134.4 (C(3)), 134.8 (C(7)) ppm. Corresponding H
3
): δ 1.90 (s, 6H, H(4)), 2.03 (p, J = 7.5 Hz, 2H, H(1)), 2.73 (t,
13
UV/VIS measurements of the free ligand in methanol and compound
́
3
I and I in solid-state were performed on SPECORD 250 spectrometer by
1
(
Analytic Jena. As a source of ultraviolet and visible light for switching
experiments in solution and solid-state 6 W UV/VIS lamp by KRÜSS
Optronic with wavelengths, 256/578 nm was used.
1
3
and C NMR spectra of 3 with assignments of hydrogen and carbon
atoms are depicted in Fig. S3 in ESI.
DFT computations have been carried out using Gaussian 16,
Revision A.03. Computations employed the M11 functional and the
6–31 + G(d,p) basis set for all atoms [9]. The Hessians for geometry
optimizations were estimated before the initial step of optimizations.
Geometry optimizations were performed using redundant internal co-
ordinates with all coordinates free to vary.
2
2
.2.1.4. 1,2-bis(5-carboxy-2-methylthien-3-yl)cyclopentene (4). 3 (9 g,
7.3 mmol) was dissolved in dry THF (100 ml) under nitrogen
atmosphere. To this solution, n-butyllithium (43.2 ml of a 1.6 M
solution in hexane, 69 mmol) was added dropwise at room
temperature while stirring. A brown precipitate of the organyl
lithium salt was formed. Then solid CO
2
was added into the mixture
Diffraction data for I were collected at 150 K using graphite
to give a colourless precipitate of the lithium salt of 7. Then the solution
was allowed to warm up to room temperature and consequently
quenched with an aqueous solution of NaOH (10 wt%, 150 ml). The
water layer was washed with diethyl ether (3 × 50 ml) and then
acidified with conc. HCl until pH = 1 was reached. The precipitate was
collected and recrystallized from 300 ml (methanol:diethyl ether /1:4/
v:v) to yield (7.28 g, 76%) as a slightly brown powder. 1H NMR
α
monochromated MoK radiation on Nonius KappaCCD diffractometer
equipped with a Bruker APEX-II detector. The phase problem was
solved by intrinsic phasing and the structure model was refined by full-
2
matrix squares on F using the SHELX program package [10]. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms on
carbon atoms were included in idealized positions, while all other hy-
drogen atoms were refined with no positional constraints. All hydrogen
2

M. Almáši, et al.
Inorganica Chimica Acta 512 (2020) 119879
atoms were refined with isotropic thermal parameters fixed to 1.2Ueq of
After two weeks (in darkness), colourless crystals were formed, which
were further characterized and their photochromic properties studied.
their parent atoms. Crystal data for {[Zn(μ
491694, colourless block, crystal size: 0.29 × 0.14 × 0.05 mm ,
monoclinic crystal system, space group P2 /c, a = 20.333(3) Å,
b = 10.3696(14) Å, c = 9.3867(11) Å, β = 99.726(3)°, V = 1950.7(4)
3 2 3 2 n
-HA)(H O) ]·H O} , CCDC
3
1
1
3 2 3 2 n
3.2. Crystal structure of {[Zn(μ -HA)(H O) ]·H O}
3
−3
−1
Å , Z = 4, D
(
c
= 1.647 g.cm , μ = 1.516 mm , Goodness of Fit
A view of the structural model is shown in Fig. 2a, selected bond
distances and angles are summarized in Table S1 in ESI. The compound
crystallizes in the monoclinic space group P2 /c with four formula units
GooF) = 0.993. Of 14,460 total reflections collected (-24 ≤ h ≤ 25,
−
data R
12 ≤ k ≤ 12, −11 ≤ ℓ ≤ 11), 3819 were unique. Based on these
/R1,all (wR2/ wR2,all) = 0.0532/0.1041 (0.0876/0.1174) for 275
parameters. Structure figures were drawn using DIAMOND 3.0 [11].
1
1
in the unit cell. The asymmetric unit is built from one zinc(II) ion, one
HA ligand, three coordinated and one crystallization water molecules.
6
The Zn(II) ions are hexacoordinated with donor set [ZnO ] and the
3
. Results and discussion
.1. Synthesis
The H HA ligand was prepared by modification of organic synthetic
coordination polyhedron can be described as a distorted tetragonal
bipyramid. The equatorial plane of tetragonal bipyramid is formed by
four Zn-O bonds with lengths in the range of 1.986(3)–2.187(4) Å, (O2,
O4, O5, O7) and axial positions are occupied by two oxygen atoms O1
and O3. The tetragonal bipyramid is distorted with the angle O1ax-Zn-
O3ax 171.32(13)°, deviating slightly from the ideal octahedral 180°.
3
2
procedures described in [12–14]. In the first step, chlorination of 2-
methyltiophene was followed by Friedel-Crafts acylation with glutar-
2
Both carboxylic groups of the H HA ligand are deprotonated and co-
yldichloride and using AlCl
TiCl led to the formation of a cyclopentene ring. The final dicarboxylic
acid was prepared by substituting the chlorine atoms using n-BuLi and
3
as a catalyst. The cyclization reaction with
ordinated to zinc(II) ions in different coordination fashion. The car-
boxylate group including C16 is coordinated in a monodentate mode,
the carboxylate group including C1 in syn-anti mode and thus one HA
4
1
13
CO
2
, then followed by acidification. H and C NMR spectra of the final
3
linker bridges three central atoms (μ -bridge).
product used further as a ligand for the synthesis of I, together with the
assignment of hydrogen and carbon atoms are depicted in Fig. 1b.
Compound I was prepared by isothermal crystallization using zinc
Compound I can be included into the group of coordination poly-
mers and exhibits a chain-like structure propagating parallel with the c
crystallographic axis. The 1D polymeric chains are stabilized on one
side by hydrophobic effects formed between the cyclopentene rings of
2
nitrate tetrahydrate, H HA as the ligand and NaOH in a 4:1:2 M ratio.
Fig. 1. Preparation of H
2
HA ligand (1,2-bis(5-carboxy-2-methylthien-3-yl)cyclopentene): (1) NCS, reflux, benzene, acetic acid, 84%; (2) glutaryldichloride, AlCl
, reflux, THF, 71%; (4) n-BuLi, CO (s), RT, THF, 76%.
3
,
CH Cl , 98%; (3) Zn, TiCl
2
2
4
2
3

M. Almáši, et al.
Inorganica Chimica Acta 512 (2020) 119879
Fig. 2. a) Coordination environment of Zn(II) ion and coordination modes of HA ligand. b) A view showing final crystal packing of 1D polymeric chains, which are
stabilized by hydrogen bonds formed between water molecules and hydrophobic effect of cyclopentene rings.
the HA ligands (hydrophobic part in Fig. 2b) and by several weak and
medium hydrogen bonds formed between coordinated and crystal-
lization water molecules (hydrophilic part in Fig. 2b) on the other. The
list of intermolecular hydrogen bonds is summarized in Table S2 in ESI.
The distance between reactive carbons of HA ligand in I is 4.148(2) Å,
sufficiently long to interact in solid-state. Based on literature data, an
effective C···C distance for UV driven photochromism ranges from
deduced from a plateau appearing on the TG curve (see Fig. S6 in ESI).
At higher temperatures, the decomposition of {[Zn(HA)]} chains takes
place in several overlapping steps. The final product of thermal de-
composition was ZnO (obs. residual mass 18.4%, clcd. residual mass
17.8%). All observed decomposition steps and mass changes are in good
agreement with the crystal structure and the chemical composition of I.
n
3.45 Å to 3.80 Å [6–8].
3.5. Powder X-ray diffraction measurements
3.3. Infrared spectroscopy
Based on TG measurement it was possible to determine the dehy-
dration temperature of compound I. The phase purity of as-synthesized
and dehydrated material was studied by powder X-ray diffraction
(PXRD) experiments. Fig. S7 in ESI shows a comparison of the experi-
mental PXRD pattern of I and the pattern calculated from the single-
crystal X-ray data. Very good agreement between the measured and
simulated pattern was observed, indicating the presence of pure phase
of I. Differences observed in the diffraction line intensities can be at-
tributed to the variation of the preferred orientation of crystallites in
the powdered as-synthesized sample. The pattern of the dehydrated
compound demonstrates, that the crystal structure of the compound
does not collapse after removal of the solvent molecules and a forma-
tion of novel phase was observed.
IR spectroscopy confirms the presence of HA ligand and water
molecules in I (see Table S3 and Fig. S5 in ESI). The presence of co-
ordinated and crystallization water molecules is evident from the broad
−
1
absorption band located at 3428 cm
corresponding to ν(OH)
stretching vibration. At lower wavenumbers, the presence of HA mo-
lecules is evident from several weak absorption bands appearing below
−1
3
2 3
000 cm , which correspond to the stretching vibrations of CH /CH
−1
groups. The absence of a strong absorption band at 1660 cm
corre-
sponding to υ(C]O) vibration of the carboxylic group of the free HA
indicates complete deprotonation of HA carboxyl groups during the
–
synthesis. The band of antisymmetric carboxylate stretch, υas(COO )
−
1
–
was found at 1564 and 1555 cm , the band of symmetric υ
s
(COO )
−
1
–
stretch was found at 1337 cm and the band of deformation δ(COO )
was observed at 795 cm , respectively.
3.6. Photochromic properties
−1
Initially, the switching properties of the free ligand were in-
−
3
3
.4. Study of thermal robustness
vestigated in its methanol solution (0.575 μmol.dm
concentration)
and the corresponding time-dependent spectra after irradiation at
254 nm are depicted in Fig. 3. Before irradiation, the colourless solution
For the investigation of thermal robustness and the dehydration
process of the synthesized polymer, thermal analysis was conducted in
an air atmosphere (see Fig. S6 in ESI). Compound I is thermally stable
up to 60 °C; above this temperature in the range 60–155 °C dehydration
takes place in two decomposition steps. In this temperature range, a
total mass loss of 14.2% was observed, corresponding to the release of
four water molecules (clcd. mass loss 14.8%). The observed decom-
position step could be separated into two distinct processes: the first
one in temperature range 70–110 °C (mass loss obs. 7.2%, clcd. 7.4%)
corresponding to the release of two water molecules, then the second in
range 110–155 °C (mass loss obs. 7.0%, clcd. 7.4%) corresponding also
2
of H HA had no absorption in the visible region and exhibited a single
intensive band at 256 nm corresponding to π → π* transition. After
irradiating the solution with UV (254 nm) in different periods 5, 10, 15,
25, 35, 45 min, the intensive peak at 256 nm decreased and two new
strong absorption bands at 352 and 537 nm appeared with gradually
increasing intensity. UV–VIS spectra indicated that after 45 min of ir-
radiation, the open form was transformed to the closed-form (inset of
Fig. 3a) and a photo-stationary state was reached. Upon irradiation
with visible light (578 nm), the violet solution of closed-form was
completely bleached, and the absorption spectrum returned to that of
open form.
́
to the release of another two water molecules. The dehydrated form (I)
is thermally stable in the temperature range 155–240 °C, which was
Despite the application of suitable UV light for an extended period,
4

M. Almáši, et al.
Inorganica Chimica Acta 512 (2020) 119879
exceedingly large for possible photochemical reaction, with respective
values 4.639 Å and 4.653 Å. Therefore, a dimeric unit was chosen for
DFT modelling, which was closer in its composition to the polymeric
crystal structure of I. As evident from Fig. 4e, this dimeric unit un-
dergoes structural changes after dehydration, through which the C···C
distance becomes reduced to 3.814 Å. This distance is sufficiently short
to react by UV irradiation. The results of DFT modelling confirm the
possibility of ligand switching in the solid phase, which was also con-
firmed by optical microscopy (see Fig. 3b and Fig. S8 in ESI) and UV/
VIS measurements (see Fig. 3b above).
4. Conclusion
3
In conclusion, the compound with composition {[Zn(μ -HA)
2 3 2 n
(H O) ]·H O} was synthesized and structurally characterized. The
chemical composition of the as-synthesized form was analysed by CHNS
elemental analysis) and FAAS (flammable atomic mass spectrometry)
analysis, IR (infrared spectroscopy), TG (thermogravimetry), PXRD
powder X-ray diffraction) and SXRD (single-crystal X-ray diffraction).
(
(
Based on SXRD, the solid-state arrangement of I is formed by a linear
chain polymer composed of 1,2-bis(5-carboxylate-2-methylthien-3-yl)
cyclopentene (HA), water molecules and Zn(II) ions. The switching
2
properties of H HA ligand in methanol solution, the as-synthesized
́
coordination polymer I and its dehydrated form I in the solid-state were
investigated by switching experiments driven by UV irradiation. The as-
synthesized form does not show switching after irradiation with UV
light in solid-state, which is consistent with the diffraction results (C···C,
d = 4.148(6) Å). Surprisingly, the compound undergoes a photo-
chromic reaction after dehydration, confirmed by optical microscopy,
solid-state UV/VIS measurements and DFT studies. We conclude that
although structural features may sometimes prove to be unfavourable
for switching conditions, this process can be achieved by post-synthetic
modification of the sample, such as dehydration.
2
Fig. 3. a) Time dependant UV–VIS spectra of H HA in methanol solution upon
CRediT authorship contribution statement
irradiating with 254 nm. The inset shows the ring-open and ring-closed form of
the ligand. b) Solid-state UV–VIS spectra of H HA (red line for open form and
black line for closed-form) and dehydrated compound Í
before (green line) and
2
Miroslav Almáši: Writing - original draft, Writing - review &
editing, Synthesis, Characterization. Martin Walko: Synthesis,
Characterization. Jana Boržíková: Synthesis, Characterization. Róbert
Gyepes: Characterization, Writing - review & editing.
after UV irradiation (orange line). The inset shows optical images corre-
sponding to the colour changes of the crystals. (For interpretation of the re-
ferences to colour in this figure legend, the reader is referred to the web version
of this article.)
Declaration of Competing Interest
the synthesized sample of I remained without any colour change, which
was consistent with its C···C distance 4.148(6) Å of the active carbons.
Surprisingly, the compound underwent a photochromic reaction after
dehydration, which was confirmed by optical microscopy, UV/VIS
measurements and also by DFT computations. Within TG measure-
ments, a sample of I was heated initially to 200 °C, whereupon it
transformed to the dehydrated form I. The particles of I turned to violet
upon irradiating with 254 nm UV light, while they still retained their
shape and the observed colour change was attributed to the formation
of the closed-ring form isomer of the HA linker.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
́
́
Acknowledgements
This work was supported by the VEGA project no. 1/0745/17 and
project from P.J. Safarik University no. VVGS-2019-1033. M.A. thanks
to the Ministry of Education, Science, Research and Sport of the Slovak
Republic and the Accreditation Commission for the financial support of
the TRIANGEL team in the frame of the scheme “Top Research Teams in
Slovakia”. R.Gy. is grateful to the Charles University Centre of
Advanced Materials (CUCAM) OP VVV Excellent Research Teams,
project number CZ.02.1.01/0.0/0.0/15_003/0000417.
3.7. DFT calculations
The possibility of switching process occurring after dehydration was
examined using DFT computations [9]. Fig. 4a-d show changes in in-
teratomic separation between the active carbon atoms by releasing
gradually water from the monomeric unit. In the case of the as-syn-
thesized complex, the C···C distance was 4.148 Å, which decreased to
3.880 Å after removing one water molecule, resulting from the for-
Appendix A. Supplementary data
mation of an intramolecular hydrogen bond between the carboxylate
oxygen atoms and the coordinated water molecule. In the models of
monohydrate and anhydrous complexes, the C···C separations were
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2020.119879.
5

M. Almáši, et al.
Inorganica Chimica Acta 512 (2020) 119879
Fig. 4. Results of DFT modelling for a-d) step-by-step dehydration process of the monomeric unit and e) dehydrated dimeric unit with corresponding C···C distance
between active carbon atoms.
References
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
Vreven, K. Throssell, J.A. Montgomery, Jr., J.E. Peralta, F. Ogliaro, M.J. Bearpark,
J.J. Heyd, E.N. Brothers, K.N. Kudin, V.N. Staroverov, T.A. Keith, R. Kobayashi, J.
Normand, K. Raghavachari, A.P. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M.
Cossi, J.M. Millam, M. Klene, C. Adamo, R. Cammi, J.W. Ochterski, R.L. Martin, K.
Morokuma, O. Farkas, J.B. Foresman, D.J. Fox, Gaussian 16 Revision A.03,
Gaussian, Inc., Wallingford CT, 2010; M. Almáši, V. Zeleňák, R. Gyepes, S.
Bourrelly, M.V. Opanasenko, P.L. Llewellyn, J. Čejka, Inorg. Chem., 57 (2018)
[
[
[
1] J. Zhang, Q. Zou, H. Tian, Chem. Mater. 25 (2013) 378.
2] B.L. Feringa, Angew. Chem. Int. Ed. 56 (2017) 11060.
3] W.R. Browne, J.J.D. de Jong, T. Kudernac, M. Walko, L.N. Lucas, K. Uchida,
J.H. van Esch, B.L. Feringa, Chem. Eur. J. 11 (2005) 6414.
[4] M. Irie, T. Fukaminato, K. Matsuda, S. Kobatake, Chem. Rev. 114 (2014) 12174.
[5] T. Fukaminato, S. Ishida, R. Métivier, NPG Asia Mater. 10 (2018) 859.
[6] K. Matsuda, K. Takayama, M. Irie, Inorg. Chem. 43 (2004) 482.
[7] J. Han, M. Maekawa, Y. Suenaga, H. Ebisu, A. Nabei, T. Kuroda-Sowa,
M. Munakata, Inorg. Chem. 46 (2007) 3321.
1
774.
[
[
10] G.M. Sheldrick, A. Crystallogr, C. Sect, Struct. Chem. 71 (2015) 3.
11] K. Brandenburg, H. Putz, DIAMOND–Crystal and Molecular Structure Visualization,
version 4.5, Crystal Impact GbR, Bonn, 2017.
[
[
8] K. Matsuda, Y. Shinkai, M. Irie, Inorg. Chem. 43 (2004) 3774.
9] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
G. Scalmani, V. Barone, G.A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A.V.
Marenich, J. Bloino, B.G. Janesko, R. Gomperts, B. Mennucci, H.P. Hratchian, J.V.
Ortiz, A.F. Izmaylov, J.L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F.
Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V.G.
Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R.
[
[
[
12] L.N. Lucas, J.J.D. de Jong, J.H. van Esch, R.M. Kellogg, B.L. Feringa, Eur. J. Org.
Chem. (2003) 155.
13] M. Akazawa, K. Uchida, J.J.D. de Jong, J. Areephong, M. Stuart, G. Caroli,
W.R. Browne, B.L. Feringa, Org. Biomol. Chem. 6 (2008) 1544.
14] D.J. van Dijken, J.M. Beierle, M.C.A. Stuart, W. Szymański, W.R. Browne,
B.L. Feringa, Angew. Chem. Int. Ed. 53 (2014) 5073.
6