Solvent vapor-induced polarity and ferroelectricity switching

Article abstract of DOI:10.1039/d0cc04497k

Vapor-induced crystal to crystal transformation between non-polar [Fe(sap)(acac)(sol)] (H2sap = 2-salicylideneaminophenol, acac = acethylacetate, sol = MeOH, pyridine) and polar [Fe(sap)(acac)(DMSO)] was demonstrated. It provides an example of switchable ferroelectric behaviour attributted to the structural phase transition triggered by solvent vapour.

Full text of DOI:10.1039/d0cc04497k

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DOI: 10.1039/D0CC04497K
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Solvent Vapor-Induced Polarity and Ferroelectricity Switching
Fumiya Kobayashi,a Ryohei Akiyoshi,b Daisuke Kosumi,c,e Masaaki Nakamura,b Leonard F. Lindoyd
and Shinya Hayami*b,e
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Vapor-induced crystal to crystal transformation between non- type
polar [Fe(sap)(acac)(sol)] (H2sap = 2-salicylideneaminophenol, acac Cl3PhCO2− = 2,3,5-trichlorobenzoate; TCNQMe2 = 2,5-dimethyl-
acethylacetate, sol
MeOH, pyridine) and polar 7,7,8,8-tetracyanoquinodimethane; DCM = dichloromethane).5
[Fe(sap)(acac)(DMSO)] was demonstrated. It provides an example Reports describing the switching of ferroelectric properties
of switchable ferroelectric behaviour attributted to the structural are extremely limited compared to those describing the
[{Ru2(O2CPh-2,3,5-Cl3)4}2(TCNQMe2)]·4DCM
(2,3,5-
=
=
phase transition triggered by solvent vapour.
switching of magnetic properties. Xiong and coworkers have
reported the reversible switching of the ferroelectric and SCO
The development of flexible responsive systems with high
structural order, as represented by the concept of “soft
crystals”, has received much recent attention.1 In contrast to
conventional hard crystals, such systems exhibit some
flexibility and are typically able to respond to particular
macroscopic stimuli, such as exposure to vapor as well as to
physical rubbing.2 Such responses may include changes in
optical properties such as luminescence. Obtaining a deep
understanding of such systems clearly provides the basis for
the development of new sophisticated responsive materials.
Several such vapor-induced functional switching systems have
been reported. These were shown to exhibit facile structural
transformations in response to specific vapor stimuli, that led
to reversible luminescent properties, colours and spin-state
changes.3
The development of functional switching molecules that
exhibit bistability with respect to ferroelectric, ferromagnetic
and spin crossover (SCO) properties has attracted considerable
interest for use in new functional devices that include memory
devices and sensors.4 For example, Miyasaka and coworkers
have reported that a reversible magnetic change is associated
with solvation/desolvation cycles for a “magnetic sponge” of
behaviour
of
the
mononuclear
iron(II)
complex,
[Fe(bpp)2](isonic)2·2H2O (bpp = 2,6-bis(pyrazol-3-yl)pyridine;
isonic = isonicotinate) on hydration/dehydration.6 This was
attributed to changes in the complex’s hydrogen bonding
network (and polarity) following the absorption/desorption of
lattice water. While such a functional switching system
induced by absorption/desorption of guest molecules is an
attractive candidate for use in the construction of molecular
devices, the synthesis of new systems of this type clearly
remains challenging.
In the present study, we have focused on investigating
mononuclear iron(III) complexes of type [Fe(sap)(acac)(sol)]
(H2sap = 2-salicylideneaminophenol; acac = acetylacetate and
sol = MeOH (1), DMSO (2), pyridine (3)) incorporating a
substitution-prone coordination site. In this context it is well
established that six-coordinated octahedral metal complexes
in which a coordination site is occupied by a solvent molecule
show a propensity for solvent ligand exchange with a second
solvent having a higher coordination ability, giving rise to the
prospect that polarity-dependent solvatochromism may
occur.7 Herein, we report the occurrence of ferroelectric
behaviour and solvent vapor-induced polarity switching
involving mononuclear iron(III) complexes of type
[Fe(sap)(acac)(sol)] (sol = MeOH (1), DMSO (2), pyridine (3)): in
each of these the coordinated solvent corresponds to a
substitution prone coordination site (Scheme 1).
a. Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3
Kagurazaka, Shinjuku-ku, Tokyo 162-8601 Japan.
b. Department of Chemistry, Graduate School of Science and Technology,
Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555 Japan.
c. Department of Physics, Graduate School of Science and Technology, Kumamoto
University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555 Japan.
d. School of Chemistry, The University of Sydney, NSW 2006, Australia.
e. Institute of Industrial Nanomaterials (IINa), Kumamoto University, 2-39-1
Kurokami, Chuo-ku, Kumamoto 860-8555 Japan.
Electronic Supplementary Information (ESI) available: Experimental sections,
Crystallographic data, PXRD, TGA, DSC, SHG, Dielectric properties. See
DOI: 10.1039/x0xx00000x
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arising from the H2sap and acac ligands (Fig. S3, ESI†), while
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DOI: 10.1039/D0CC04497K
the axial positions are occupied by O,O donors form acac and
MeOH. The Fe-O and Fe-N bond lengths for 1 at 123 K are
agreed well with values observed for typical high-spin iron(III)
ion in octahedral environments.9 Complexes 2a and 2b have
similar structures to 1 except that the central iron(III) in 2 is
coordinated by a DMSO molecule rather than a MeOH
molecule as in 1; both structures belong to the monoclinic
polar space group Cc (Fig. 1 and Figs. S5S8, ESI†). 2a and 2b
form polymorphic crystals displaying different lattice
parameters that are attributed to the presence of slightly
different intermolecular interactions (Table S1, Fig. S9, ESI†).
The crystal structure of 2b is in accordance with the previously
reported structure.10 The respective crystals can be
distinguished by their different morphologies and crystal sizes
(Fig. S2, ESI†). Although 3 crystallized in the monoclinic non-
polar space group P21/n with a similar molecular structure to 1,
the asymmetric unit incorporates half of a lattice pyridine
molecule (Figs. S10 and S11, ESI†).
Scheme 1 Solvent vapor-induced structural transformations for 1-3.
[Fe(sap)(acac)(MeOH)] (1) was synthesized by mixing
Fe(acac)3 with H2sap in methanol in a 1:1 molar ratio to yield
When the methanol-containing complex 1 was exposed to
the product as a brown powder (Fig. S1, ESI†). Black plate-like DMSO or pyridine as vapours for 1 day, then in each case the
initial PXRD pattern for 1 changed completely to the patterns
obtained directly for 2b and 3, respectively (Fig. 2). Clearly this
result is in accord with the coordinated MeOH in 1 having
exchanged with DMSO or pyridine upon exposure to the
DMSO or pyridine atmospheres. In addition, by undertaking
time-dependent PXRD measurements during the uptake of
DMSO enabled the time-dependent structural transformations
to be followed (Fig. S12, ESI†). The observed PXRD pattern for
1 after exposing to DMSO vapor for 1 day indicated that a
mixture of 2a and 2b was present. Further exposure to DMSO
vapor for 5 days resulted in a change to a simpler PXRD
pattern which corresponded to the pattern for 2b (Fig. S13,
ESI†).
single crystals were obtained on recrystallizing the powder
from MeOH/CH2Cl2 (see the Supporting Information, ESI†).
[Fe(sap)(acac)(DMSO)] (2) was obtained by recrystallizing 1
from MeOH/CH2Cl2/DMSO, resulting in a mixture of two
crystalline polymorphs: quartz-like (2a) and block shape (2b)
products (Fig. S2, ESI†). The pyridine-substituted complex,
[Fe(sap)(acac)(py)]·0.5py (3) was synthesized by recrystallizing
1
from MeOH/pyridine. 1, 2a, 2b and 3 were each
characterized by elemental analysis and single-crystal X-ray
diffraction (XRD) measurements.
Fig. 1 Packing structures of [Fe(sap)(acac)(MeOH)] (1) and [Fe(sap)(acac)(DMSO)]
(2b). Color code: orange, Fe; red, O; blue, N; grey, C; yellow, S. Hydrogen atoms
are omitted for clarity.
The single-crystal X-ray structural analyses of 1, 2a, 2b and 3
were carried out at 123 or 298 K. Crystallographic data and
selected bond lengths are displayed in Tables S1 and S2 (ESI†).
1 crystallized in the triclinic non-polar space group P-1. The
structure of 1 is in accordance with the previously reported
structure (Table. S1, ESI†).8 The iron(III) centre has a NO5-
donor coordination sphere with distorted octahedral geometry.
The metal centre has a NO3-donor set coordinated equatorially
Fig. 2 PXRD patterns for 1 (red), 2b (blue), 3 (pink) and 1 after exposure to DMSO
and pyridine vapours (black).
In the case where 2b was exposed to MeOH vapor, the PXRD
pattern for 2b did not change even after the exposure had
continued for 5 days in agreement with the coordination
ability of a MeOH being lower than DMSO.
2 | J. Name., 2012, 00, 1-3
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In order to compare the thermal stability of 2a relative to 2b,
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In an attempt to investigate the structuraVilew tArrtaicnlesOitniloinne
the thermogravimetric analysis (TGA) of each of these behaviour, differential scanning DsOpIe:c10tr.1o0s3c9o/Dp0yCC0(4D4S97CK)
polymorphs was performed (Fig. S14, ESI†). For both measurements for 2a and 2b were carried out from 125 K to
400 K (Fig. S20, ESI†). No peaks were observed over this
temperature region, indicating that no structural transitions
occurred for either crystal type. We also measured the
temperature-dependent dielectric constants for single
crystalline samples of 2a and 2b within the frequency range 10
Hz to 1 kHz using inductance capacitance and resistance (LCR)
measurements in order to investigate the electric field
responses to the phase transitions (Fig. S21, ESI†). The
dielectric constants of 2a and 2b remained almost unchanged
over 300400 K. These results indicate that no structural
transitions occur over this temperature range for both crystal
types. The observed behaviour is thus consistent with the
results of the DSC analyses mentioned above.
complexes, 17% weight loss was observed above ca. 170C.
This shows good agreement with the DMSO content of 2
(17.6%). Following the initial thermal treatment of crystalline
2b at 150C for 1 h under reduced pressure to remove the
coordinated DMSO, the PXRD pattern of the desolvated
sample, following its exposure to MeOH vapor, was again
measured (Fig. S15, ESI†). A relatively fast response to the
MeOH vapor was observed; the resulting PXRD pattern
corresponded well to the PXRD pattern for 1. Notably, the
observed PXRD transformation was found to be reversible;
that is, on exposing the above sample to DMSO vapor resulted
in regeneration of the original pattern for 2b. Related
behaviour was also observed on exposing pyridine-desolvated
3 to MeOH vapor. Again this yielded the PXRD pattern for 1
(Figs. S16 and S17, ESI†). On exposing 3 to DMSO vapor or 2b
to pyridine vapor, resulted in the PXRD patterns for 2b and 3,
respectively (Fig. S18, ESI†). Thus in contrast to the case of
MeOH vapor, the reversible structural transformations
between 2b and 3 are occurred spontaneously without
The ferroelectric properties for single crystalline samples of 2a
and 2b were studied by analysing polarization vs. electric field
(P–E) curves using a TF Analyzer1000. The observed P–E curves
for these compounds show well-defined hysteresis loops (Fig.
4). The low leakage currents (10-8–10-7 A cm-2) further
demonstrate that each hysteresis loop originates from the
presence of ferroelectricity. At 298 K, the remnant
desolvation
process.
The
vapor-induced
structural
transformations corresponding to the above PXRD changes polarizations (Pr) for 2a and 2b are 3.2 μC cm2 and 7.5 μC
cm2, with a coercive field (Ec) of 14.2 kV cm1 and 5.1 kV cm1,
respectively. Because 2a and 2b were structurally stable below
ca. 430 K, clear ferroelectric hysteresis loops with the remnant
polarization (Pr) values of 1.6 μC cm2 and 7.4 μC cm2, and
with coercive fields (Ec) of 10.9 kV cm1 and 5.5 kV cm1, were
able to be observed respectively, even at 400 K, (Fig. S22, ESI†).
These results confirm that 2a and 2b exhibit ferroelectric
behaviour even under high temperature conditions (up to at
least 400 K). Reported examples of molecular ferroelectric
systems with high Curie temperatures are quite limited; the
above complexes represent new examples of this rare
category.11
were also supported by second harmonic generation (SHG)
experiments for the respective structures using an excitation
wavelength of 1080 nm. A strong SHG intensity was evident at
ca 540 nm for 2a and 2b, both of which belong to the polar Cc
space group, while no SHG occurred for 1 (Fig. 3a and Fig. S19a,
ESI†). The exposure of 1 and 3 to DMSO vapor resulted in the
expression of polarity along with SHG behaviour (Figs. 3b and
S19b, ESI†).
Fig. 3 Results of SHG experiments for (a) 1, 2a and (b) 1 after exposure to DMSO
vapor.
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Fig. 4 P–E hysteresis loops for (a) 2a and (b) 2b with an EC-axis at 298 K (0.1
Hz).
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In conclusion, we have demonstrated the occurrence of
solvent vapor-induced crystal to crystal structural
transformations between iron(III) complexes of type
[Fe(sap)(acac)(sol)] (sol = MeOH (1), DMSO (2), pyridine (3)).
This involved the use of a mononuclear iron(III) complex
belonging to polar Cc space group which is SHG active. Thus,
complexes 2a and 2b exhibit ferroelectric behaviour up to high
temperature (at least to 400 K), in part, reflecting both their
polarity and their high thermal stabilities. These polymorphs
represent new examples of vapor-induced reversible
ferroelectric switching systems in which coordinated solvent
substitution is the ‘trigger’ for the corresponding structural
rearrangements. The study provides new insights towards
solvent-exchange strategies for obtaining new molecular
multifunctional materials – not only for achieving ferroelectric
switching - but also for solvent-exchange induced spin-state
and luminescent switching.
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This work was supported by a KAKENHI Grant-in-Aid for
Scientific Research (A) JP17H01200. F.K. is grateful to JSPS
Research Fellowships for Young Scientists (No. JP19J11651).
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Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/D0CC04497K
A new example of vapor-induced reversible polarity and ferroelectricity switching
system has been demonstrated in mononuclear iron(III) complexes.