Structural Arrangement of 4-[4-(Dimethylamino)phenylazo]pyridine Push–Pull Molecules in Acidic Layered Hosts Solved by Experimental and Calculation Methods

Article abstract of DOI:10.1002/ejic.201601053

4-[4-(dimethylamino)phenylazo]pyridine (further denoted as G) representing a type of push–pull molecules can be intercalated into α- and γ-modifications of zirconium phosphate (α-ZrP and γ-ZrP) and into zirconium (4-sulfophenyl)phosphonate (ZrSPP). The ob

Full text of DOI:10.1002/ejic.201601053

A Journal of
Accepted Article
Title: Structural arrangement of 4-[4-(N,N-dimethylamino)phenylazo]pyridine push-
pull molecules in acidic layered hosts solved by experimental and calculation
methods
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Authors: Klara Melanova; Petr Kovar; Martina Gamba; Miroslav Posp´ısil; Ludv´ık Be-
ˇ
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nes; Vitezslav Zima; Jan Svoboda; David Mikl´ık; Filip Bures; Petr Knotek
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European Journal of Inorganic Chemistry
10.1002/ejic.201601053
Structural arrangement of 4-[4-(N,N-dimethylamino)phenylazo]pyridine push-pull
molecules in acidic layered hosts solved by experimental and calculation methods
Klára Melánová,*[1][a] Petr Kovář,[2] Martina Gamba,[2],[3] Miroslav Pospíšil,[2] Ludvík Beneš,[4][a]
Vítězslav Zima,[1][a] Jan Svoboda,[1][a] David Miklík,[4] Filip Bureš,[4] Petr Knotek[4]
[1]
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic,
Heyrovský Sq. 2, 162 06 Prague 6, Czech Republic, klara.melanova@upce.cz,
[2]
Charles University in Prague, Faculty of Mathematics and Physics, Ke Karlovu 3, 121 16
Prague 2, Czech Republic
[3]
CETMIC−CCT La Plata, CICBA. Camino Centenario y 506, 1897 M. B. Gonnet, La Plata,
Argentina
[4] Faculty of Chemical Technology, University of Pardubice, 532 10 Pardubice, Czech Republic
[a]
Present address: Joint Laboratory of Solid State Chemistry, Studentská 95, 532 10 Pardubice,
Czech Republic; https://www.upce.cz/english/fcht/jlssch.html
Keywords: Layered compounds / Intercalation / Molecular modeling / Push-pull molecules
Abstract
4-[4-(N,N-dimethylamino)phenylazo]pyridine (further denoted as G) representing a type of push-
pull molecules can be intercalated into α-and γ-modifications of zirconium phosphate (α-ZrP and
γ-ZrP) and into zirconium 4-sulfophenylphosphonate (ZrSPP). The obtained intercalates form
single phases with interlayer distances of 12.75, 16.31 and 24.11 Å for α-ZrP·0.2G·1.5H2O,
γ-ZrP·0.2G·1.5H2O and ZrSPP·0.5G·1H2O, respectively. The increase of the interlayer distance
on intercalation suggests that the molecules of the intercalated guest lie parallel to the host layers.
All intercalates were further characterized by infrared and UV-Vis spectroscopies.
The arrangement of the guests in the interlayer space was obtained by molecular simulations
methods. The calculations were performed separately for protonated and unprotonated forms of
the guest in the models of hydrated and dehydrated α-ZrP and γ-ZrP intercalates. In the case of
the α-ZrP intercalate, the presence of interlayer water stabilizes the head-to-tail arrangement of
the guest molecules. Dehydration of this intercalate leads to disturbing of their arrangement,
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European Journal of Inorganic Chemistry
10.1002/ejic.201601053
mainly in the case of protonated guest molecules. In the case of dehydrated γ-ZrP, the guest
molecules are head-to-tail ordered and the guest molecules in the hydrated form of γ-ZrP are
more disordered than in the dehydrated intercalate. The simulations also described a layer shift
present in the dehydrated γ-ZrP intercalate, which explained why the rehydration of this
intercalate is not possible.
Introduction
Photochemical and photophysical properties of dyes and chromophores can be modified by their
intercalation into various layered host materials such as clays[1] or metal(IV) phosphates and
phosphonates.[2] The cause for this change can be, among others, the modification of their
immediate environment represented by a two-dimensional confined system of the host.[3]
Chromophores of special interest from the point of their optical properties are those with a
delocalized system of  electrons end-capped with an electron donor (D) and an electron acceptor
(A), widely known as push–pull systems.[4] A D–A interaction, or a so-called intramolecular
charge-transfer, is responsible for the polarization of the chromophore and generation of a
molecular dipole. Organic molecules of this type show a second-order non-linear optical (NLO)
response that has attracted great interest thanks to their potential applications in electro-optic
modulators and in wavelength conversion of lasers.[5]
Nevertheless, the main obstacle in their wide application is the fact that their assemblies
or crystals usually have a center of symmetry that destructs their NLO properties.[6] One way how
to avoid the centrosymmetric arrangement of NLO active species is to incorporate them in a two-
dimensional interlayer space of a layered compound.[7] It is supposed that under certain
conditions the push-pull molecules spontaneously form a non-centrosymmetric arrangement in
this environment. In an ideal case, a head-to-tail alignment of the push-pull molecules is created,
in which all electric dipoles point to the same direction. One of such push-pull molecules is 4-[4-
(dimethylamino)--styryl]-1-methylpyridinium (usually denoted as DAMS), which was
[8]
intercalated into MnPS3 and into - and -modifications of zirconium hydrogen phosphate.[9]
An azo analogue of DAMS, 1-methyl-4-[4-(N,N-dimethylamino)phenylazo]pyridinium, was
[8a, 10]
intercalated into MnPS3
and layered metal oxalates[11] or prepared as hybrid nanoparticles
with MnPS3.[12] The optical properties of the prepared intercalates were investigated and the
relation between these properties and presumed arrangement of the intercalated molecules was
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European Journal of Inorganic Chemistry
10.1002/ejic.201601053
discussed. We have recently reported how the non-linear optical properties of NLOphores can be
influenced by their intercalation into layered host materials.[13]
The problem is that the arrangement of the guest molecules in these intercalates cannot be
determined directly by experimental methods. Therefore, the question is whether it is possible on
the basis of experimental data, such as the interlayer distance and the amount of intercalated
chromophore, to model an arrangement of the push-pull molecules in the interlayer space of the
host and thus predict optical properties of these materials. A suitable tool that might be used for
the structure calculations are classical molecular simulations. In combination with the
experimental methods (XRD, IR spectroscopy, TG measurements) these simulations allow us to
suggest the structure of the studied materials and to calculate the interactions between the guests
and the host layers and between the guests themselves. This combination of theoretical and
experimental methods can also help us to better understand the other properties of the intercalates
and the relationship between structure and properties. From the molecular simulations point of
view several papers dealing with the hybrid organo-inorganic materials based on zirconium
hydrogen phosphates were published. These papers focused on mutual interactions of species,
adsorption of guests or pillaring properties.[14]
Several new compounds, in which 4-[4-(N,N-dimethylamino)phenylazo]pyridine
molecules are intercalated in acidic layered host materials ( modification of zirconium hydrogen
phosphate,
4-sulfophenylphosphonate), were prepared and characterized. The main aim of this work is to
describe the arrangement of the intercalated molecules of
γ
modification of zirconium hydrogen phosphate and zirconium
4-[4-(N,N-dimethylamino)phenylazo]pyridine in  and γ modifications of zirconium hydrogen
phosphate with a special focus on the question whether these flat-lying molecules could be
arranged in a head-to-tail manner or not. This question is solved from the minimum energy point
of view using a combination of experimental data and molecular simulations.
Results and discussion
The guest, 4-[4-(N,N-dimethylamino)phenylazo]pyridine, can be intercalated very easily into
ZrSPP and γ-ZrP, the intercalates were obtained very simply by refluxing the host in an ethanol
solution of the guest. In the case of -ZrP such intercalation does not proceed and the -ZrP·G
intercalate was prepared by mixing the exfoliated host with an ethanol solution of the guest.
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European Journal of Inorganic Chemistry
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Diffraction pattern of -ZrP·G (see Figure S1 in the Electronic Supporting Materials)
contains two broad basal reflections corresponding to a basal spacing of 12.2 Å and several non-
basal reflections including those at 4.44 and 2.65 Å, which correspond to (200) and (020)
diffraction lines of parent -ZrP (JCPDS No. 04-016-8875).[15] The presence of these reflections
in the diffraction pattern of the -ZrP intercalate indicates that the structure of the host layers is
not changed during the intercalation. After the dehydration (Figure S1), the most intensive peak
at 2 = 7.23° (corresponding to the basal spacing of 12.2 Å) shifts to 7.36° (12.0 Å). Due to the
generally low quality of the pattern it is difficult to assign the other peaks. The broadness of the
basal reflections indicates that the particles of the intercalate formed are significantly smaller than
that of the parent host (112 Å for -ZrP·G compared to 382 Å for -ZrP, as found by using
Scherrer formula). It is given by the methods of preparation of the intercalate through exfoliation.
The dehydrated -ZrP·GD rehydrates back to the original hydrate as follows from its diffraction
pattern.
In the case of the γ-ZrP·G intercalate (see Figure S2 in the ESM), the diffraction pattern
contains two basal reflections corresponding to a basal spacing of 16.3 Å, several broad weak
reflections, and two intensive sharp reflections at 2 = 26.9° (3.32) and 33.4° (2.68 Å), which
correspond to (020) and (200) diffraction lines of parent γ-ZrP (JCPDS No. 04-012-1442).[15]
After the dehydration (see diffraction pattern in Figure S2) the basal spacing of -ZrP·GD is
shifted from 16.3 Å to 14.6 Å. The peaks at 2 = 26.9° and 33.4°, corresponding to (020) and
(200) reflections found also in the parent -ZrP[15] and in the hydrated -ZrP·G, confirm that the
structure of the host layers is retained both in the hydrated and dehydrated samples. In addition,
new diffraction peaks appear in the diffraction pattern of the dehydrated intercalate at 2 = 16.2°
(5.48 Å) and at 22.6° (3.94 Å). These peaks might be an indication that the neighboring host
layers are shifted to each other during the dehydration. This shift of the layers can be the reason
for the stability of this dehydrated compound in humid environment and impossibility of its
rehydration back to -ZrP·G.
The diffraction pattern of the ZrSPP·G intercalate (Figure S3 in the ESM) contains,
besides four basal reflections corresponding to a basal spacing of 24.11 Å, a number of non-basal
reflections typical for the host structure. After the dehydration (see diffraction pattern in Figure
S3 in the ESM) the basal spacing of ZrSPP·GD is shifted from 24.11 Å to 23.41 Å (calculated
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European Journal of Inorganic Chemistry
10.1002/ejic.201601053
from 3 basal reflections) and the non-basal reflections typical for the host structure are retained.
The dehydrated ZrSPP·GD rehydrates very slowly at high humidity, after 4 days the basal
spacing is 23.99 Å.
The composition of the intercalates was determined by elemental and TG analyses. All
three intercalates decompose in two steps (see Figure 1). The TGA curves of both ZrP
intercalates are nearly the same, which is in accordance with their composition found by the
elemental analysis. In the first step up to 200 °C, co-intercalated water is released. The observed
weight loss of 8%, corresponding to 1.5 molecule of water per formula unit, agrees well with the
calculated values of 7.60% and 7.65% for -ZrP·G and γ-ZrP·G, respectively. The second step
is caused by the decomposition of the intercalated guest. The end product of the decomposition is
ZrP2O7 (JCPDS No. 04-008-5867)[15] in both cases. The total weight loss of 26.5% is in
accordance with the value of 25.4% calculated for the presence of 0.2 guest molecule per formula
unit. In the case of ZrSPP·G, the first weight loss corresponds to a release of one water molecule
per formula unit (calculated value 2.66%, found 3%). The second weight loss is caused by the
decomposition of the intercalated guest and the organic parts of the host. The end product of the
thermal decomposition is again ZrP2O7. The total weight loss of 61% agrees well with the value
of 60.63% calculated for the presence of a half of the guest molecule per formula unit. The
thermogravimetric curves of the dehydrated intercalates have the same course as those of the
hydrated intercalates above the temperature of dehydration.
The character of the interactions between the guest molecules and the host layers in the
intercalates was studied using IR spectroscopy. In the spectrum of the pure guest, the bands at
3057 and 2868 cm-1 are probably caused by C-H stretching vibrations of the aromatic rings and
the methyl groups, respectively. Four bands at 1605, 1583, 1522 and 1411 cm-1 (see Figure 2,
curve (a)), corresponding to a ring stretching vibration of both benzene and pyridine rings,[16]
were found in the spectrum. A strong band at 1367 cm-1 can be assigned to a C-N stretching
vibration of the dimethylamino group bonded to the benzene ring. A group of bands in region
from 1300 to 1000 cm-1 is probably caused by ring C-H deformation vibrations, the most
intensive are bands at 1148 and 1067 cm-1. A couple of bands at 834 and 823 cm-1 correspond to
an out-of-plane deformation of both rings.
In order to determine whether the guest compound is present in the intercalates in its
protonated form, we measured also the IR spectrum of its methylated analogue, 1-methyl-4-[4-
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(N,N-dimethylamino)phenylazo]pyridinium iodide. The spectrum of the pure methylated guest
(see Figure 2, curve (e)) differs from the spectrum of G in the positions of the peaks of the ring
stretching vibrations; there are several bands at 1632, 1605, 1567 and 1498 cm-1. The positions of
the bands of the ring C-H deformation vibrations are also affected by methylation on the N atom
of the pyridine ring, the strong broad band with two maxima at 1061 and 1019 cm-1 appears and
also the position of the bands corresponding to an out-of-plane deformation of the ring changes
slightly to 843 and 823 cm-1. The sharp band corresponding to the C-N stretching vibration of the
dimethylamino group was observed at the same position at 1367 cm-1. In the IR spectra of all
three intercalates (see Figure 2, curves (b), (c) and (d)), the most distinct feature is the broad band
in the region of 1150 – 900 cm-1 belonging to the P-O vibrations of the host layers (and in
addition to S-O vibrations in the case of ZrSPP·G). These bands are narrower in the spectra of
the intercalates compared to those of the parent hosts. The peaks of the δP-OH vibration, found in
the spectra of -ZrP and γ-ZrP at 1247 and 1218 cm-1, respectively,[17] were observed also in the
spectra of both intercalates. In the region of the ring stretching vibrations, bands at 1640, 1605,
1578, 1513, and 1470 cm-1 appeared. The shoulder at about 1620 cm-1 is probably caused by a
δH-O-H deformation vibration of co-intercalated water. The IR spectra of all three intercalates in
the region from 1800 – 600 cm-1 represent a superposition of the peaks found in the
corresponding host and in the methylated guest, indicating that the guest molecules are at least
partially protonated in the intercalates.
UV-Vis spectra. The UV-Vis spectra of the guest and the intercalates measured in solid
state are shown in Figure 3. The UV-Vis maximum for G is 440 nm and the maxima for all the
intercalates are bathochromically shifted. The largest shift by 27 nm was observed for the
-ZrP·G intercalate, a similar shift by 22 nm to 462 nm was found for -ZrP·G. The lowest
bathochromic shift was observed for the ZrSPP·G intercalate, the peak with the maximum at 443
nm is also the broadest one of all three intercalates. The IR spectra suggest that the guest is
present in the intercalates in its protonated form. Therefore the UV-Vis spectra of the intercalates
were compared with those of an N-methylpyridinium analogue of the guest and the guest
protonated by its exposure to HCl vapors. Quarternization of the pyridine N atom by methylation
of G causes a shift of the maximum in the UV-Vis spectrum by 110 nm, as can be seen from the
comparison of curves (d) and (e) in Figure 3. On the contrary, only a small shift by 20 nm caused
by the protonation of the guest molecule was observed (see curves (d) and (f) in Figure 3 for G
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European Journal of Inorganic Chemistry
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and its protonated analogue, respectively). The absorption peak of the protonated guest is sharper
and its FWHM is 82 nm, while it is 131 nm for the guest. The broadening and shift of the peaks
of the intercalates might be caused by a coexistence of unprotonated and protonated forms of the
guest molecules in the interlayer space of the host. The spectrum of -ZrP·G shows a shoulder at
the longer-wavelength side of the peaks. After the dehydration by heating up to 150 °C for 1 h,
this shoulder disappears (see Figure S4 in the ESM) and reappears again when the intercalate is
hydrated back by standing in an atmosphere with 75% relative humidity for 3 days at room
temperature. This shoulder at the longer wavelengths is caused by a mutual interplay of the guest
and water molecules. The presence of the water molecules decreases the mutual interactions
between G and the host layers. This allows a regular head-to-tail arrangement of G and
subsequently a formation of an ordered superstructure composed of G molecules, which is the
reason for an existence of the shoulder at longer wavelengths in the absorption spectra. If we
compare the results from the molecular modeling with a detailed description of azo dye behavior
in the literature,[18] we can assume that the shoulder for -ZrP·G indicates a formation of a
J-dimers type arrangement of the guest species in the interlayer space of the host.
Molecular modeling. The arrangement of the guest molecules in the intercalates was modeled
for compounds without water (dehydrated) and those containing water in the interlayer space.
The volume available for the accommodation of the guest molecules in the interlayer space is
smaller in γ-ZrP·G than in -ZrP·G. In the case of -ZrP·G, the surface area on the host layer
(3a x 6b = 864 Å2 per 6 guests) related to one guest molecule is about 30% higher than in the case
of γ-ZrP (6a  6b = 1286 Å2 per 12 guests). Therefore, one can expect that the variability of the
guest arrangement in the case of the γ-ZrP·G intercalate is limited.
In the case of the -ZrP intercalate the supercell consisted of two host layers and two
interlayer spaces and every interlayer space contained 6 guest molecules. The supercell of the
γ-ZrP intercalate consisted of one host layer and one interlayer space containing 12 guests.
Models of the dehydrated -ZrP intercalate. The basal spacing of dehydrated sample,
-ZrP·GD, was found to be 12 Å. Taking into account that the thickness of the layer in -ZrP is
6.3 Å,[19] the height of the interlayer space (the so-called gallery height, see Figure 4a) is about
5.7 Å, which indicates a parallel or nearly parallel arrangement of the guest molecules in the
interlayer space. The longitudinal axes of the guest molecules (as defined in Figure 4b) tend to
be arranged in rows along the b cell vector (see Figure 5), keeping a head-to-tail arrangement.
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Mutual distances between the head (the N atom of the pyridine ring) and the tail (the nearest
carbon atom of the dimethylamino groups of the neighboring guest) of the guest molecules are
mostly between 4 and 5 Å. The distance between the guests (measured as a distance between
their longitudinal axes, along the a cell vector) ranges mostly between 5 and 9 Å. In most cases
the aromatic rings of the guests are in one plane, i.e., nearly no torsion along the N=N bond was
observed. In the case of the protonated guest molecules (G+), stronger mutual interactions
between the guests and the charge compensating host layers and between the guests themselves
are present. This leads to a rather disordered arrangement of the guests in the interlayer space
than in the case of unprotonated guest molecules. The longitudinal axes of the guests mostly do
not tend to be in rows and the guests are not in a head-to-tail arrangement.
The basal spacing of the calculated models ranged from 11.7 (for G+) to 12 Å (for G). If
one takes into account the presence of both G and G+ in the interlayer space, the calculated basal
spacing is in good agreement with the experimental data. The calculated X-ray diffraction pattern
is compared with the experimental one in Figure 6. In contrast to the calculated XRD pattern
which contains sharp reflection peaks stemming from the supposed ideal crystal structure of the
model, the experimental XRD profile contains poorly resolved peaks reflecting the poor
crystallinity of the real sample. This makes impossible to assign individual peaks in the
experimental pattern to those calculated from the model.
Models of the hydrated –ZrP intercalate. The presence of water molecules in the interlayer
space of –ZrP·G led to an increase of the basal spacing from 12 to 12.2 Å. The calculated basal
spacings are 12.1 Å and 12.2 Å for G+ and G models, respectively. The longitudinal axes of the
guests (for both G and G+ molecules) tend to be arranged in the rows and keep a head-to-tail
arrangement, see Figures 7a and 7b. The guests in the interlayer space are distributed more
evenly than in the case of the models without water. The distance between the longitudinal axes
of the guests (in the direction of the a cell vector) ranges mostly between 8 – 10 Å for G and
8.6 – 9.6 Å for G+. The distance between the head and the tail of the neighboring guest molecules
in the b cell vector direction ranges mostly from 4 to 5 Å. The aromatic rings of the guests lie
nearly in one plane – only a slight torsion around the N=N bond was observed.
As in the case of the models of the dehydrated compound, the guests can exhibit a certain
degree of disorder. One can see that the guest planes tend to be parallel with the host layers, but
in some cases their lateral axes (as defined in Figure 4b) adopt a tilted orientation, see Figure 8.
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The molecules of water are located in two positions: (i) near the host layers between the PO4
tetrahedra; the water molecules interact with hydrogen atoms of the PO4 tetrahedra via hydrogen
bonds, and (ii) between the guests as a consequence of their hydrophobicity. From the
comparison of the arrangements of the guests in the models without and with water we can
conclude that the water molecules in the interlayer space stabilize the head-to-tail arrangement of
the guests and lead to a more ordered arrangement in the interlayer space.
Models of the dehydrated -ZrP intercalate. After the dehydration of γ-ZrP·G, new peaks at
2 = 16.2° and 22.6° appear in the experimental powder X-ray pattern of the product of
dehydration, γ-ZrP·GD (see Figure S2 in the ESM and Figure 9a). These changes in the XRD
pattern can be simulated by a shift of the layers by about 3 Å along the b cell vector, which is
nearly a half of the distance (6.6 Å) between the apexes of the neighboring PO4 tetrahedra. In the
γ-ZrP·G intercalate the apexes of the PO4 tetrahedra are located between two neighboring
tetrahedra apexes of the opposite layer (see Figure S5a in the ESM). After the shift of the layers,
the apexes of the PO4 tetrahedra in one host layer of the γ-ZrP·GD intercalate are above the
apexes of the tetrahedra of the neighboring layer as shown in Figure S5b. The basal spacing of
+
the calculated models ranges from 14.6 Å (for γ-ZrP·GD ) to 14.8 Å (for γ-ZrP·GD) and is in
good agreement with the experimental basal spacing of 14.6 Å. The simulated powder X-ray
pattern (Figure 9b) then agrees quite well with the experimental one. On the other hand, when a
model without shifted layers is used, its powder X-ray pattern (Figure 9c) does not correspond to
the experimental pattern (Figure 9a).
Figure 10a shows a view looking down on the guest arrangement in the interlayer space of
γ-ZrP·GD. One can see an ordered head-to-tail arrangement of the guest molecules and their
position with respect to the PO4 tetrahedra of the "lower" host layer. The mutual distance
between the longitudinal axes of guests in the b cell vector direction ranges between 6 and 7 Å.
The head-to-tail distance of two neighboring guest molecules in the a vector direction ranges
between 4.5 and 5 Å. The guests adopt a parallel arrangement with respect to the host layers. Due
to a lower gallery height (4.3 Å) in comparison with -ZrP·G no guest molecules are tilted with
respect to the host layers, see Figure 11a. The PO4 tetrahedra form channels in the directions
along the a and b lattice vectors due to the layer shift. The lower gallery height and the smaller
interlayer area keep the guests in these channels; the methyl groups of the guests are located
between the PO4 tetrahedra. In the case of the γ-ZrP·G+ model, the guest molecules adopt a very
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European Journal of Inorganic Chemistry
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similar arrangement in the interlayer space as in γ-ZrP·G. The majority of the guest molecules
exhibit parallel arrangement with respect to the layers. The channels formed by the shift of the
layers can theoretically accommodate the guests with their lateral axes perpendicular or nearly
perpendicular with respect to the layers. We built a set of models where all the guests were
parallel with respect to the host layers, and the models where all the lateral axes of the guests kept
a perpendicular arrangement. While in the case of γ-ZrP·GD all the guest molecules adopted a
+
parallel arrangement, in γ-ZrP·GD some of G+ kept perpendicular arrangement, most probably
due to a mutual repulsion of the G+ cations, see Figures 10b and 11b.
Models of the hydrated -ZrP intercalate. Presence of the interlayer water led to an increase of
the basal spacing from 14.6 to 16.3 Å. The calculated basal spacing of 16.1 Å for both γ-ZrP·G
and γ-ZrP·G+ is in good agreement with the experimental value.
The increase of the gallery height led to the following differences in the guest
arrangement in comparison with the models without water: (i) Even the longitudinal axes of the
guest molecules G are parallel with the plane of the host layers, their lateral axes are not (see
Figure 12a). Most of the guest molecules keep a tilted orientation of the lateral axis with respect
to the layers and the angle, under which they are tilted, ranges from 20 to 30°. The G+ cations
also adopt a tilted orientation but they are arranged randomly with respect to each other and form
a disordered arrangement, see Figure 12b; (ii) the guest molecules tend to be located in the
channels formed by the PO4 tetrahedra, but their arrangement within them is not ordered as in the
models of the dehydrated intercalate. In the case of γ-ZrP·G+ some of the guests are not located
in the channels at all, due to mutual repulsive interactions. Water molecules in both protonated
and unprotonated models are located in two positions: (i) between the apexes of the host layer
tetrahedra (most of them) and (ii) between the guest molecules.
Conclusion
A typical representative of a push-pull molecule, 4-[4-(N,N-dimethylamino)phenylazo]pyridine,
was intercalated into three acidic layered host materials, namely -modification of zirconium
phosphate, -modification of zirconium phosphate and zirconium 4-sulfophenylphosphonate. The
interlayer distance of all three intercalates suggests that the guest molecules are arranged with
their longitudinal axes parallel to the host layers.
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The arrangement of the guests in the intercalates with both modifications of zirconium
phosphate was investigated using molecular simulations. Molecular simulations in combination
with X-ray diffraction patterns provided an insight into the interlayer arrangement of the guests
including the role of interlayer water. In the case of α-ZrP·G intercalate, the presence of
interlayer water stabilizes the head-to-tail arrangement of the guest molecules. Dehydration of
this intercalate leads to disruption of their arrangement, mainly in the case of the protonated guest
molecules. In the case of γ-ZrP·GD, the guest molecules are head-to-tail ordered, protonation of
the guest leads only to the tilting of their lateral axes. On the other hand, the guest molecules in
the γ-ZrP·G are more disordered than in the dehydrated intercalate. The suggested differences in
the ordering of the guest molecules in the interlayer space of the hosts is in good agreement with
the differences observed in the powder X-ray diffraction patterns. In α-ZrP·G with better ordered
guest molecules, the diffraction peaks are more pronounced than in less ordered dehydrated
α-ZrP·GD sample. Also, in the case of γ-ZrP the powder X-ray diffraction pattern with more
distinguishable peaks is that of γ-ZrP·GD, where the guest molecules are better arranged than in
γ-ZrP·G. The simulations also described a layer shift present in the dehydrated γ-ZrP intercalate
(γ-ZrP·GD), which is the reason why the rehydration of the γ-ZrP·GD is not possible. Based on
the presented calculated data, the α-ZrP·G intercalate seems to be a suitable material for
nonlinear optics applications.
Experimental
Synthesis of the host materials: Well-crystallized  modification of Zr(HPO4)2·H2O (further
denoted as -ZrP), was obtained according to the method proposed by Alberti and Torracca,[20]
modification of Zr(PO4)(H2PO4)·2H2O (further denoted as γ-ZrP) was obtained according to
Poojary et Zirconium 4-sulfophenylphosphonate with formula
al.[21]
γ
Zr(HO3SC6H4PO3)1.8(C6H5PO3)0.2·2H2O (further denoted as ZrSPP) was prepared according to
the procedure described by Zima et al.[22] The powder diffraction patterns (see Figures S1 to S3 in
the Electronic Supporting Materials) show that the crystallinity of all prepared host compounds is
quite good.
Synthesis of 4-[4-(N,N-dimethylamino)phenylazo]pyridine:
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European Journal of Inorganic Chemistry
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Scheme 1
The target guest compound (further denoted as G) was prepared by a modified Faessinger´s
method[23] (see Scheme 1). To a stirred solution of 4-aminopyridine (2 g, 21 mmol) in HBF4 (15
mL) cooled to -15 °C, sodium nitrite (1.6 g, 23 mmol) was slowly added keeping the reaction
temperature below -10 °C. The reaction mixture was stirred for additional 30 min, whereupon
N,N-dimethylaniline (5.2 mL, 41 mmol) was added in one portion. The reaction was stirred for 2
h, alkalized with a concentrated solution of sodium hydroxide. The resulting precipitate was
filtered, washed with water until the filtrate showed neutral pH and subsequently with hexane
(30 mL). The crude product was purified by column chromatography (SiO2; CHCl3/ Et2O; 3:1) to
give title compound as a bright orange solid: Yield: 3.8 g (80 %). M. p. 205.5-206.5 °C (207–
1
3
209 °C[23]). H-NMR (500 MHz, 25 °C, CDCl3):  = 3.10 (s, 6H, N(CH3)2), 6.72 (d, 2H, J = 9
3
Hz, ArH), 7.62 (d, 2H, 3J = 5.5 Hz, NPyH), 7.89 (d, 2H, J = 9 Hz, ArH), 8.70 ppm (d, 2H, 3J =
13
5.5 Hz, NPyH). C-NMR (125 MHz, CDCl3, 25 °C): = 40.30, 111.41, 116.06, 126.00, 143.51,
151.04, 153.33, 158.00 ppm. EI-MS: m/z (%) = 226 (100, [M+]), 148 (25), 120 (87), 105 (30), 77
(28), 51 (16).
Synthesis of 1-methyl-4-[4-(N,N-dimethylamino)phenylazo]pyridinium iodide: A solution of
the target guest compound G (226 mg, 1 mmol) in acetone (3 mL) and iodomethane (3 mL, 9.2
mmol) was stirred for 12 h at room temperature. The purple precipitate was filtered, washed with
hexane and dried under vacuum. Yield: 331 mg (90 %). 1H-NMR (400 MHz, 25 °C, d6DMSO): 
= 3.21 (s, 6H, N(CH3)2), 4.33 (s, 3H, NPyCH3), 7.03 (d, 2H, 3J = 8.8 Hz, ArH), 7.98 (d, 3J = 8.8
3
3
13
Hz, 2H, ArH), 8.17 (d, J = 6.0 Hz, 2H, PyH), 8.94 ppm (d, J = 6.0 Hz, 2H, PyH). C-NMR
(100 MHz, 25 °C, d6DMSO):  = 40.28, 46.83, 112.73, 118.19, 125.23, 143.64, 146.68, 155.62,
160.99 ppm.
Intercalation into α-ZrP: -ZrP (0.15 g) was exfoliated according to the procedure described
by Zhou et al.[24] An ethanol solution (10 mL) of the guest (0.1 g) was added to this exfoliated
host and shaken at room temperature for 5 days. The product (-ZrP·G) was separated by
centrifugation, extracted in a Soxhlet extractor until the extract was colorless and dried in air.
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European Journal of Inorganic Chemistry
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Elemental analysis: -ZrP·G calcd/found for Zr(HPO4)2·0.20(C13H14N4)·1.5H2O, Mr = 355.46;
C, 8.79/10.89 ± 0.02%; H, 2.21/2.51 ± 0.02%; N, 3.15/2.83 ± 0.02%.
Intercalation into γ-ZrP and ZrSPP: The host (0.1 g) was refluxed in ethanol solution (10 mL)
of the guest (0.1 g) for 8 hours. The products (further denoted as γ-ZrP·G and ZrSPP·G) were
separated by centrifugation, extracted in a Soxhlet extractor until the extract was colorless, and
dried
Zr(PO4)(H2PO4)·0.19(C13H14N4)·1.5H2O, Mr = 353.20; C, 8.40/8.63 ± 0.03%; H, 2.19/2.02 ±
0.01%; N, 3.01/2.79 0.01%. ZrSPP·G calcd/found for
in
air.
Elemental
analysis:
γ-ZrP·G
calcd/found
for
±
Zr(HO3SC6H4PO3)1.8(C6H5PO3)0.2·0.50(C13H14N4)·H2O, Mr = 678.65; C, 32.74/30.95 ± 0.02%;
H, 2.82/2.49 ± 0.01%; N, 4.13/4.05 ± 0.02%; S, 8.51/8.66 ± 0.03%.
Dehydration of -ZrP·G, γ-ZrP·G and ZrSPP·G: The -ZrP·G and ZrSPP·G intercalates
were dehydrated in a mixture with Al2O3 (2% w/w of the intercalates in Al2O3) by heating to 150
°C for 1 h. The γ-ZrP·G sample in the same mixture with Al2O3 was dehydrated by keeping in a
desiccator over P2O5 for 2 days. The intercalates were dehydrated in the mixture with Al2O3
because these samples were used directly for the measurement of the UV-Vis spectra. The
dehydrated samples are in the following text denoted as -ZrP·GD, γ-ZrP·GD and ZrSPP·GD.
To prove the reversibility of the dehydration process, the samples were rehydrated by keeping the
samples over a saturated solution of NaCl, which provides 75% relative humidity, at room
temperature for 3 days.
Characterization techniques: Powder X-ray diffraction data were obtained with a D8 Advance
diffractometer (Bruker AXS, Germany) with a Bragg–Brentano θ–θ geometry (40 kV, 30 mA)
using Cu Kα radiation with a secondary graphite monochromator. The diffraction angles were
measured at room temperature from 2 to 50° (2θ) in 0.02° steps with a counting time of 15 s per
step. Diffraction patterns of the dehydrated samples were measured at 150 °C on a heated copper
block equipped with a thermocouple.
Thermogravimetric measurements (TGA) were done using a home-made apparatus
constructed of a computer-controlled oven and a Sartorius BP210 S balance. The measurements
were carried out in air between 30 and 960 °C at a heating rate of 5 °C min−1.
Infrared spectra in the range of 600–4000 cm−1 were recorded at 64 scans per spectrum at
2 cm−1 resolution using a HATR adapter on a Perkin-Elmer FTIR Spectrum BX spectrometer on
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European Journal of Inorganic Chemistry
10.1002/ejic.201601053
neat samples. All spectra were corrected for the presence of moisture and carbon dioxide in the
optical path.
UV-Vis diffuse reflectance spectra of the powder materials diluted with Al2O3 to 2% w/w
were recorded in the range from 210 to 800 nm using a UV-Vis Lambda 20 spectrometer (Perkin-
Elmer, USA) equipped with a diffuse reflectance attachment with a 3-inch integrating sphere
with Al2O3 as a reference. The reflectance values were re-calculated using the Schuster–
Kubelka–Munk equation, F(R∞) = (1 − R∞)2/2R∞, where R∞ is the diffuse reflectance from a semi-
infinite layer. The details are given in the previous works.[25]
Molecular modelling: Molecular mechanics and classical molecular dynamics were carried out
in a Materials Studio modelling environment.[26] The unit cell parameters used for the modeling
of the host layers were taken from literature and were the following: -ZrP: a = 9.06 Å, b =
5.297 Å, α = γ = 90°, β = 101.71°, space group P21/n;[27] γ-ZrP: a = 5.38 Å, b = 6.64 Å, α = γ =
90°, β = 98.69°, space group P21.[21] We created a two-layered 3a  6b  2d002exp supercell for the
-ZrP·G intercalate, in which the composition of one layer was Zr36(PO4H)72. The experimental
basal spacings d002exp were 12.2 Å and 12.0 Å for the hydrated -ZrP·G and dehydrated -
ZrP·GD compounds, respectively. In the case of γ-ZrP·G, a 6a  6b  1dexp supercell was built
and the composition of the host layer was Zr72(PO4H)144. The experimental basal spacings dexp
were 16.3 Å and 14.6 Å for the hydrated γ-ZrP·G and dehydrated γ-ZrP·GD compounds,
respectively. The number of the guests in the interlayer space was taken on the base of the
experimental measurements and it was 12 guest molecules per supercell in our models. The
model of -ZrP·G contained six guest molecules in each interlayer space and in the case of
γ-ZrP·G the interlayer space of the model was filled with 12 guest molecules. The number of the
guest molecules in the interlayer space used for the modeling was slightly lower than the amount
calculated from the experimental data. Six molecules of the guest completely filled the interlayer
space and the calculated basal spacings for the models with six guest molecules were in a better
agreement with the experimental measurements than models with seven or more guest molecules
in one model layer. We suppose that a part of the guests can be also adsorbed on the surface.
The experimental results (IR and UV-Vis spectra, see below) showed that the guest can be
present in a neutral (unprotonated, G) or in a protonated form (as 1H-4-[4-(N,N-
dimethylamino)phenylazo]pyridinium, G+). Therefore we tested two extreme model cases in
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European Journal of Inorganic Chemistry
10.1002/ejic.201601053
which the guest molecules were present either as G or G+. In the models containing G+ cations in
the interlayer space, the corresponding amount of the hydrogen atoms was removed randomly
from the phosphate groups of the host layer. The negative charge of the host layer thus formed
was compensated by the positive charge of the G+ guest molecules. The initial mutual
arrangement of the guests and the arrangement of the guests with respect to the host layer was the
same for both the extreme model cases in order to investigate the differences in the arrangement
of G and G+. The amount of water was the same in both -ZrP and γ-ZrP intercalate hydrates –
1.5 H2O per formula unit, which corresponds to 108 water molecules per supercell. The geometry
of the models was optimized in Universal force field,[28] the charges were calculated by the Qeq
method.[29] The host layers were fixed or treated as rigid units and the positions of all atoms in the
interlayer space were variable. The molecular dynamics was carried out in an NVT (N – constant
number of particles, V – constant volume, T – constant temperature) Canonical statistical
ensemble at temperature of 298 K. The Canonical ensemble describes a system in contact with a
heat bath. One step of dynamics was 1 fs and 106 steps of dynamics were carried out.
Acknowledgment
The authors thank the Czech Science Foundation (project number 14-13368S) for financial
support.
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European Journal of Inorganic Chemistry
10.1002/ejic.201601053
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Figure captions
Figure 1. The thermogravimetric curves of the -ZrP·G (a), -ZrP·G (b) and ZrSPP·G (c)
intercalates.
Figure 2. The infrared spectra of the guest (a), the -ZrP·G (b), ZrSPP·G (c) and -ZrP·G (d)
intercalates and the methylated guest (e).
Figure 3. The UV-Vis spectra of the ZrSPP·G (a), -ZrP·G (b), and -ZrP·G (c) intercalates,
the guest (d), the methylated guest (e) and the protonated guest (f).
Figure 4. Side view on the interlayer arrangement of G in the -ZrP·GD intercalate (a).
Longitudinal and lateral axes of the guest molecule (b).
Figure 5. Top view on the guest arrangement (G left; G+ right) in the interlayer space of the
-ZrP·GD intercalate.
Figure 6. The experimental and calculated X-ray diffraction patterns of -ZrP·GD.
Figure 7. Top view on the arrangement of unprotonated guest molecules in -ZrP·G (a) and
protonated guest molecules in -ZrP·G+ (b).
Figure 8. Side view on the arrangement of the G (G+) molecules in the interlayer space of the
hydrated -ZrP intercalate.
Figure 9. XRD patterns of the dehydrated γ-ZrP intercalate: experimental data (a), the models of
γ-ZrP·GD with shifted layers (b) and without shifted layers (c).
Figure 10. Top view on interlayer arrangement of G in the γ-ZrP·GD intercalate (a) and G+ in
the γ-ZrP·GD intercalate (b). The violet tetrahedra are PO4 groups of the "lower" host layer.
+
Figure 11. Side view on the interlayer arrangement of G in γ-ZrP·GD (a) and G+ in γ-ZrP·GD
(b).
Figure 12. Top view on the interlayer arrangement of G in γ-ZrP·G (a) and G+ in γ-ZrP·G+ (b).
The violet tetrahedra are PO4 groups of the "lower" host layer.
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European Journal of Inorganic Chemistry
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Figures
Figure 1. The thermogravimetric curves of the -ZrP·G (a), -ZrP·G (b) and ZrSPP·G (c)
intercalates.
Figure 2. The infrared spectra of the pure guest (a), the -ZrP·G (b), ZrSPP·G (c) and -ZrP·G
(d) intercalates and the pure methylated guest (e).
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European Journal of Inorganic Chemistry
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Figure 3. UV-Vis spectra of the ZrSPP·G (a), -ZrP·G (b), and -ZrP·G (c) intercalates, the
guest (d), the methylated guest (e) and the protonated guest (f).
gallery height
interlayer
distance
a
b
Figure 4. Side view on the interlayer arrangement of G in the -ZrP·GD intercalate (a).
Longitudinal and lateral axes of the guest molecule (b).
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European Journal of Inorganic Chemistry
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Figure 5. Top view on the guest arrangement (G left; G+ right) in the interlayer space of the
-ZrP·GD intercalate.
Figure 6. Experimental (a) and calculated (b) powder X-ray diffraction patterns of -ZrP·GD.
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European Journal of Inorganic Chemistry
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Figure 7. Top view on the arrangement of unprotonated guest molecules in -ZrP·G (a) and
protonated guest molecules in -ZrP·G+ (b).
Figure 8. Side view on the arrangement of the G (G+) molecules in the interlayer space of the
hydrated -ZrP intercalate.
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European Journal of Inorganic Chemistry
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Figure 9. XRD patterns of the dehydrated γ-ZrP intercalate: experimental data (a), the models of
γ-ZrP·GD with shifted layers (b) and without shifted layers (c).
Figure 10. Top view on interlayer arrangement of G in the γ-ZrP·GD intercalate (a) and G+ in
the γ-ZrP·GD intercalate (b). The violet tetrahedra are PO4 groups of the "lower" host layer.
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+
Figure 11. Side view on the interlayer arrangement of G in γ -ZrP·GD (a) and G+ in γ -ZrP·GD
(b).
Figure 12. Top view on the interlayer arrangement of G in γ-ZrP·G (a) and G+ in γ-ZrP·G+ (b).
The violet tetrahedra are PO4 groups of the "lower" host layer.
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Table of Contents
An orientation of a selected azo dye representing a push-pull molecule intercalated between
layers of acidic layered hosts was modeled. The aim was to find whether the dye was able to form
a noncentrosymmetrical arrangement.
25
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