Effect of intercalation and chromophore arrangement on the linear and nonlinear optical properties of model aminopyridine push-pull molecules

Article abstract of DOI:10.1039/c5tc03499j

Three push-pull aminopyridine derivatives having D-π-A, D-(π-A)₂, and D-(π-A)₃ arrangements were examined as model organic chromophores capable of intercalation into inorganic layered materials (alpha modification of zirconium hydrogen phosphate, zirconium 4-sulfophenylphosphonate, and gamma modification of titanium hydrogen phosphate). The fundamental properties of these dyes, their methylated analogues as well as their intercalates were studied by X-ray analysis, electrochemistry, UV/Vis absorption spectra, TGA, IR spectra, SHG, and were completed by DFT calculations. The synthesis of tripodal tris(pyridin-4-yl)amine is given for the first time. The HOMO-LUMO gap, the position of the longest-wavelength absorption maxima, and the dipole moment of aminopyridines can easily be tuned by attaching/removing pyridin-4-yl electron withdrawing units and their quaternization (pyridine vs. pyridinium acceptors). Their intercalation proved to be feasible affording novel inorganic-organic hybrid materials. The intercalation is accompanied by protonation of the guest, which enhances its ICT and strongly anchors the aminopyridines into the confined space of the layered host. Moreover, this process results in ordering of the organic chromophores and also brings improved thermal and chemical robustness. As a result, the measured SHG efficiencies of the intercalates are larger than those observed for the pure organic push-pull chromophores. Hence, the methodology of intercalation turned out to be very useful strategy for property tuning of NLO-active organic molecules.

Full text of DOI:10.1039/c5tc03499j

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Effect of intercalation and chromophore arrangement on the
linear and nonlinear optical properties of model aminopyridine
push-pull molecules
Received 00th January 20xx,
Accepted 00th January 20xx
Filip Bureš,*,a Daniel Cvejn,a Klára Melánová,b Ludvík Beneš,c Jan Svoboda,b Vítězslav Zima,b
Oldřich Pytela,a Tomáš Mikysek,d Zdeňka Růžičková,e Iwan V. Kityk,f Artur Wojciechowski,f and
Nasser AlZayedf
DOI: 10.1039/x0xx00000x
www.rsc.org/
Three push-pull aminopyridine derivatives having D-π-A, D-(π-A)2, and D-(π-A)3 arrangements were examined as model
organic chromophores capable of intercalation into inorganic layered materials (alpha modification of zirconium hydrogen
phosphate, zirconium 4-sulfophenylphosphonate, and gamma modification of titanium hydrogen phosphate). The
fundamental properties of these dyes, their methylated analogues as well as their intercalates were studied by X-ray
analysis, electrochemistry, UV/Vis absorption spectra, TGA, IR spectra, SHG, and were completed by DFT calculations. The
synthesis of tripodal tris(pyridin-4-yl)amine is given for the first time. The HOMO-LUMO gap, the position of the longest-
wavelength absorption maxima, and the dipole moment of aminopyridines can easily be tuned by attaching/removing
pyridin-4-yl electron withdrawing units and their quaternization (pyridine vs. pyridinium acceptors). Their intercalation
proved to be feasible affording novel inorganic-organic hybrid materials. The intercalation is accompanied by protonation
of the guest, which enhances its ICT and strongly anchors the aminopyridines into the confined space of the layered host.
Moreover, this process results in ordering of the organic chromophores and also brings improved thermal and chemical
robustness. As a result, the measured SHG efficiencies of the intercalates are significantly larger than those observed for
the pure organic push-pull chromophores. Hence, the methodology of intercalation turned out to be very useful strategy
for property tuning of NLO-active organic molecules.
renders the molecule unique linear and nonlinear optical (NLO)
properties. Hence, organic push-pull chromophores with large
nonlinearities are of substantial interest for all-optical signal
Introduction
Owing to their easy and modular synthesis as well as peculiar
properties such as distinct color, dipolar character, well-
defined and tunable structure, and supramolecular
arrangement, organic push-pull molecules have become an
extensively explored, and burgeoning area of organic and
materials chemistry.1 In such D-
intramolecular charge-transfer (ICT) from the electron donor
processing applications such as optical triggering, light
frequency transducers, optical memories, modulators and
deflectors.2 On the molecular level, the rational design and
property tuning in D-
the donating/withdrawing behavior of the D and A parts and
extension of the -system or production of d- metal-ligand
π-A systems rely mostly on modulation of
π-A systems direct
π
π
charge transfer complexes.1-3 At the same time, there exist
fundamental limits for enhancement of the second-order
hyperpolarizabilities of organic chromophores. These limits are
mostly determined by energy of the transition from the
ground to the first excited states and by the number of
effective conjugated electrons.4 However and as a rule,
enhanced hyperpolarizability decreases effective energy gap
leading to lower transparency. Based on such property tuning,
admirable number of organic push-pull molecules with large
intrinsic (hyper)polarizabilities were developed.5 However, it
turned out to be rather difficult to translate their high
molecular optical nonlinearity to macroscopic nonlinear- and
electro-optic activity.6 The traditional approach to overcome
this issue involves incorporation of the chromophore into a
polymeric backbone (either covalently bound or doped),
dendrimers, organic glasses, films as well as host-guest
(D) to the acceptor (A) takes place through the π-system and
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systems in which layered inorganic compounds might serve as herein its first successful preparation. All aminopyridines
the host matrix. The host matrix of the latter inorganic-organic APY1 and their intercalates were characterized
-3
hybrid systems comprises clays (laponite, saponite, experimentally by X-ray diffraction, TGA, IR, electrochemistry,
montmorillonite, hydrotalcite, kaoline etc.),7 silicates,8 and UV/Vis spectroscopy as well as theoretically by DFT
layered metal oxalates and MPS3 (M = Mn, Cd, Zn).9 Formerly, calculations. Their second order susceptibilities (SHG
organic NLOphores capable of intercalation were limited to efficiencies) were examined as well.
simple and linear D-π-A systems such as nitroanilines or
azobenzenes but, more recently, it was found that large
heterocyclic molecules such as (di)azines, porphyrines and
some branched chromophores undergo intercalation into a
layered material as well.10 The incorporation of organic dyes
into a restricted space of an ordered inorganic matrix
represents important strategy for achieving materials with the
following features and properties:
•
the periodical ordering of the inorganic matrix, which
can induce preferential orientation of the dye
reduction of random 3D-orientation of the dye into a
2D-arrangement
•
•
•
planarization of the π-system of the dye
confined environment with reduced internal motion
of the dye enhances its photophysical properties
encapsulation of less stable organic molecules by a
very robust and transparent inorganic carrier
increased chemical and thermal stability.
•
•
Hence, intercalation may open an additional reserve to further
enhance and optimize the second-order hyperpolarizabilities
without substantial decrease of the energy gap. Based on our
recent contribution on intercalation of tripodal push-pull
molecule bearing pyridin-4-yl terminus (tris[4-(pyridin-4-
yl)phenyl]amine, TPPA),11 we report herein combination of
both micro/macroscopic approaches mentioned above. In this
respect, three model push-pull molecules with central amino
donor and peripheral pyridine acceptor(s) were designed and
Fig. 1 Schematic representation of the intercalation process of APY2
.
Results and discussion
Synthesis of aminopyridines
The synthesis of APY compounds and their quaternized
analogues MeAPY is shown in Scheme 1. Dipyridylamine APY2
was synthesized via one-pot pyridine chlorination with PCl3
and subsequent nucleophilic aromatic substitution with
synthesized. These molecules (4-
differ in the number of terminal acceptors and overall
molecular arrangement – D- -A (APY1), D-( -A)2 (APY2), and
D-( -A)3 APY3). Due to the presence of peripheral basic
pyridin-4-yl moieties, the supramolecular arrangement of
APY1 can be controlled via intercalation into acidic layered
AminoPYridines, Scheme 1)
commercially available APY1 15
Introduction of the third
.
π
π
π
(
pyridine branch was initially attempted via Buchwald-Hartwig
reaction16 with 4-iodopyridine yielding no desired product.
However, by adopting copper(I)-catalyzed strategy of Sarges et
al.17 and upon optimization of the reaction condition, APY3
was prepared in 47% isolated yield. Electron withdrawing
behavior of azines can easily be improved via quaternization.18
Hence, in order to mimic the anticipated products generated
-
3
inorganic materials. Protonation of the peripheral pyridine
rings will generate pyridinium moieties, which are i) stronger
electron acceptors (ICT enhancement), ii) strongly anchors the
whole chromophore between the host layers, and iii) organizes
the dye molecules in bulk (Fig. 1).
upon intercalation into acid layered materials, APY1
-3 were
Aminopyridines APY1-3 were intercalated into α-modification
of zirconium hydrogen phosphate (Zr(HPO4)2·2H2O), zirconium
further N-methylated with iodomethane to afford pyridinium
salts MeAPY1-3 in almost quantitative yields.
4-sulfophenylphosphonate (Zr(HO3C6H4PO3)2·2H2O) and γ-
X-ray analysis
modification
of
titanium
hydrogen
phosphate
Among other spectral characterizations (see the ESI), the
molecular structure of APY3 was also confirmed by single
crystal X-ray analysis. Suitable crystals were obtained by slow
evaporation of its methanol solution. The ORTEP plot shown in
Fig. 2 confirms the proposed molecular structure of APY3 and
can also be used to determine the extent of the ICT.
(Ti(PO4)(H2PO4)·2H2O). It has recently been verified that
commercially available APY1 undergo intercalation into
α
-modification
of
titanium
hydrogenphosphate
(Ti(HPO4)2·2H2O) and generates significant NLO response on
condition of its protonation with maleic acid.12 While APY2 is
increasingly being used for construction of metal-organic
frameworks (MOF),13 APY3
, despite it features simple
molecular structure, has hypothetically been envisaged as
lately as in 2010.14 To the best of our knowledge, we report
2 | J. Name., 2012, 00, 1-3
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N
I
H
N
H2N
N
N
N
N
N
N
1. PCl3/ꢀ
2. HCl
3. NaOH
44 %
CuBr/Cs2CO3/NMP
47 %
APY2
APY1
N
N
APY3
MeI
MeI
>99 %
>99 %
MeI
>99 %
H2N
H
N
I
N
I
N
N
I
I
N
N
MeAPY1
MeAPY2
I
I
N
N
MeAPY3
Scheme 1 Synthesis of aminopyridines (APY) and aminopyridiniums (MeAPY)
triphenylamine derivatives, e.g. TPPA.21 The quinoid character
and aromaticity of pyridin-4-yl rings were calculated to be
within the range of
δr = 0.009-0.010 and I6 = 94.4-95.9.
Unsubstituted benzene has
δr and I6 equal to 0 and 100,
respectively, whereas unsubstituted pyridine has I6 = 85.7.
Both indicators imply that all pyridine rings possess relatively
low/high quinoid/aromatic character and, thus are involved in
less ICT. However, these findings are in accordance with
known tripodal triphenylamine derivatives.1,11
Electrochemistry
Electrochemical behaviour of APY1-3 and MeAPY1-3 were
studied by CV, RDV and polarography. The acquired data are
summarized in Table 1. The measured electrochemical
behavior of APY1 is consistent with the data reported in the
literature.22 The oxidations of APY2
-3 were not observed in the
Fig. 2 The molecular structure (ORTEP 50% probability level) and the side view of APY3
potential window of the given electrode, electrolyte and
solvent (up to +1.7 V vs. SCE). The reduction potentials E1/2(red1)
were recorded by polarography. The DME (dropping mercury
electrode) allowed a cathodic measurement up to -3 V vs. SCE,
while on the Pt-electrode it is only -2 V vs. SCE. Oxidation was
In this respect, the involvement of the particular pyridine rings
in the ICT can be assessed by their spatial arrangement and
bond length alternation (quinoid character
δ
r)19 and
aromaticity evaluated by Bird index I6.20 Central nitrogen atom
has perfectly planar environment with three nitrogen atoms of
pyridin-4-yl substituents, only N4 atom is slightly shifted out of
that central plane. The values of interplanar angles of the
central plane and pyridyl rings were found in the narrow
observed for pyridinium salts MeAPY1-3 but this is caused by
oxidation of the I- anions. The first reductions, which are
reversible one-electron processes, were measured only with
the Pt-electrode as interactions of MeAPY1-3 with the DME
region of 37.7-39.6°, which is similar to that found for
were observed.
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Table 1 Experimental and calculated parameters of APY1-3 and MeAPY1-3
Comp.
E1/2(ox1)
E1/2(red1)
(V)a
EHOMO
ELUMO
(eV)b
-
λmax
(nm(eV))c
ε
EHOMO
(eV)d
-6.52
-6.35
-6.30
-7.36
-7.75
-7.96
ELUMO
(eV)d
-0.66
-1.39
-1.58
-1.96
-3.21
-3.55
ꢀE
ꢁ
(V)a
(eV)b
(103 M cm-1)c
1.72
(eV)d
5.86
4.95
4.72
5.40
4.54
4.41
(D)d
5.36
3.64
0.02
0.58
0.16
0.06
APY1
APY2
+1.50
-
-5.85
267(4.64)sh
-
-
-
-
-
-2.52
-2.26
-1.36
-1.62
-0.90
-
-
-
-
-
-1.83
-2.09
-2.99
-2.73
-3.45
297(4.18)
29.56
APY3
302(4.11)
18.43
MeAPY1
MeAPY2
MeAPY3
273(4.54)
17.29
333(3.72)/403(3.08)sh
334(3.71)/404(3.07)
23.63/22.97
30.52/0.62
a E1/2(ox1) and E1/2(red1) are half-wave potentials of the first oxidation and reduction, respectively. All potentials were recorded in N,N-dimethylformamide containing 0.1
b
c
d
M Bu4NPF6 and are given vs. SCE. –EHOMO/LUMO = E1/2(ox1/red1) + 4.35 (Ref.23). Measured in DMSO (c = 2×10-5 M); sh denotes shoulder. DFT calculated by
(B3LYP/6-311++G(2d,p)//B3LYP/6-311++G(2d,p)) in DMF.
contrary, MeAPY2 was reduced at the most negative potential
(-1.62 V) among other pyridinium salts - MeAPY1 (-1.36 V) and
MeAPY3 (-0.90 V). This is most likely given by a facile loss of HI
from MeAPY2 and formation of an imine structure MeAPY2q
as shown in the ESI (Scheme S1). This quinoid cation has also
been detected by HR-MALDI-MS as a main peak (Fig. S2). In
MeAPY2/MeAPY2q, the observed most negative reduction
potential reflects enhanced electron density of the central
amino donor and subsequent higher saturation of the
pyridinium acceptor as expressed by the resonant structures
shown in Scheme S1.
UV/Vis absorption spectra of APY1-3 and MeAPY1-3
The optical properties of APY1-3 and MeAPY1-3 in liquid
media were investigated by electronic absorption
spectroscopy measured in DMSO (Fig. 3). The measured
longest-wavelength absorption maxima λmax and the molar
absorption coefficients
absorption spectra, except for APY1, are dominated by intense
CT-bands with the maxima appearing at 267-302 nm for APY1
and 273-404 nm for MeAPY1 . As expected, these CT-bands
ε are given in Table 1. All of the
-
3
-3
are red-shifted with the increasing number of pyridin-4-yl units
as well as with their replacement by pyridinium acceptors.
MeAPY2 showed two particularly developed CT-bands
appearing at 333 and 403 nm. The latter corresponds to facile
formation of the quinoid structure MeAPY2q with extended
Fig. 3 UV/Vis absorption spectra of APY1-3 (a) and MeAPY1-3 (b) measured in DMSO
(c = 2×10-5 M)
π
-conjugated system and stronger electron donor (ICT
enhancement). This observation is in accordance with other
betaine dyes, especially the Reichardt’s dye used to determine
the solvent polarity - ET(30).24 The position of the longest-
wavelength absorption maxima and the band-width and shape
of betaine dyes are strongly solvent-dependent and methanol
seems to be exceptional solvent in this respect.25 Hence, the
absorption spectra measured in methanol are shown in the ESI
(Fig. S3). In contrast to DMSO, the spectra of both MeAPY2
When going from APY2 to APY3, the first reduction potentials
decrease by 260 mV, most likely as a result of introducing
additional
electron
withdrawing
pyridin-4-yl
unit.
Quaternization of the pyridine to pyridinium moieties
significantly shifted the first reduction potentials to more
positive values by 0.9 (APY2 vs. MeAPY2) and 1.36 V (APY3 vs.
MeAPY3). This clearly reflects the improved electron
withdrawing ability and thus enhanced ICT from the central
amino donor to peripheral pyridinium acceptors. On the
and
3
measured in methanol are dominated by intensive low-
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energy bands appearing at 390 nm. These pronounced bands increasing number of pyridine and pyridinium acceptors
can be ascribed to a more efficient ICT from the central amino appended to the central amino donor. The calculated HOMO-
donor to the peripheral pyridinium acceptors and implies LUMO gaps decrease from 5.86/5.40 to 4.72/4.41 eV for APY1
-
higher stabilization of the formed quinoid structure by protic
solvents.
3
/
MeAPY1
The visualization of frontier molecular orbitals for APY1
MeAPY1
HOMO, and partial charge separation, which confirms their ICT
character (Fig, 4). In tripodal molecules APY3 and MeAPY3
-3.
-
3
and
-3 show acceptor-centered LUMO and donor-centred
,
the third pyridin-4-yl branch is occupied by the LUMO+1, the
HOMO-1 remained on the central amino donor. This is a
common feature of tripodal push-pull molecules.26 As
expected, the ground state dipole moment
increasing symmetry of the molecule.
Composition of intercalates
ꢁ vanishes with
The amount of the intercalate species x in each host is given in
Table together with the content of water y. The
2
compositions of the prepared intercalates were calculated on
the basis of elemental analysis and thermogravimetric data
(thermogravimetry curves are given in Figs. S5 to S7 in the ESI).
The general formula can be written as H·xG·yH2O (where H =
ZrP
Table 2 shows that the amount of the intercalated guest in the
hosts generally decreases in the order ZrSPP ZrP TiP. The
, ZrSPP, or TiP and G = APY1, APY2, or APY3). The data in
>
>
ratio of the amount the intercalated species in each host is
roughly the same and is xAPY1 : xAPY2 : xAPY3 = 6 : 3 : 2. This ratio
is inversely proportional to the charge present at each of the
protonated guest species (1:2:3). The higher amount of the
guest in the ZrSPP intercalate than in ZrP is in accordance with
our previous findings for ZrSPP and ZrP intercalates of tris[4-
(pyridin-4-yl)phenyl]amine.11 There is no distinct relation
between the amount of the intercalated guest and the amount
of water y.
Fig. 4 Optimized geometries and HOMO (red) and LUMO (blue) localizations in APY1-3
and MeAPY1-3.
On heating, all intercalates release water at relatively low
temperature, the weight decrease in all cases starts below 100
°C. For the APY1 intercalates, the second decrease starting at
about 300 °C is most probably due to the release of the guest
from the interlayer space. Intercalates with APY2 and APY3
release their less volatile guests at higher temperature (500 °C)
and the decrease corresponding to this release merges with
the weight decrease due to the decomposition of the host.
DFT calculations
The spatial and electronic properties of APY1-3 and MeAPY1-3
were also investigated by calculations. Their geometry was
optimized by PM3 and subsequently DFT B3LYP methods with
B3LYP/6-311++G(2d,p) basis set. The energies of the highest
occupied molecular orbital (HOMO) and lowest unoccupied
Table 2 Composition of prepared intercalates
molecular orbital (LUMO), HOMO-LUMO gap (ꢀE), and ground
state dipole moment were calculated by using the
(ꢁ)
B3LYP/6-311++G(2d,p) base, and are summarized in Table 1
Host H
ZrSPP
ZrSPP
ZrSPP
α-ZrP
α-ZrP
α-ZrP
γ-TiP
Guest G
APY1
APY2
APY3
APY1
APY2
APY3
APY1
APY2
APY3
Intercalate
ZrSPP-APY1
ZrSPP-APY2
ZrSPP-APY3
ZrP-APY1
ZrP-APY2
ZrP-APY3
TiP-APY1
x
y
1.20
0.50
0.35
0.86
0.43
0.30
0.69
0.35
0.22
0.70
1.50
1.60
1.00
2.25
1.75
1.00
0.75
2.50
(see the ESI for more details). Only the cationic parts of
MeAPY1
calculated energies of the LUMO differ slightly from those
obtained by electrochemical measurements ( ELUMO = 0.1-1.03
eV), the DFT-derived HOMO-LUMO gap ( E) correlate tightly
-3 were considered in DFT calculations. Although the
ꢀ
ꢀ
with the energy of the electron transition calculated from the
γ-TiP
TiP-APY2
position of the longest-wavelength absorption maxima
γ-TiP
TiP-APY3
(1240/λmax) as shown in Fig. S4. Hence, the used DFT
calculations are obviously capable of properly describing the
trends seen by the optical measurements and can be
considered as a reasonable tool for the electronic property
Structure of the host
description of APY1-3 and MeAPY1-3. The principal changes
The intercalation process is driven by acid-base interactions of
acidic active sites present on the surface of the host layers
are seen in the energies of the LUMO, which reflects the
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with basic guests APY1-3 (Fig. 1). The intercalation is therefore guest the interlayer space the guest molecule tends to be
accompanied by protonation of the peripheral pyridine more inclined with respect to the plane of the host layer.
moieties and generation of pyridinium(s). The gallery height in all APY1 intercalates (7.48 Å, 6.35, 6.50 Å
All three hosts are layered. All contain acidic active sites, for ZrSPP-APY1 ZrP-APY1 and TiP-APY1, respectively) is larger
,
formed by oxygen atoms carrying acid protons. In ZrP and TiP than or equal to the van der Waals length of the APY1
these active sites are formed by POH groups whereas in ZrSPP molecule (6.35 Å). It means that this molecule can be arranged
by SOH groups. The surface density of the active sites on one in the interlayer space with the plane of the pyridine ring
side of the layer can be expressed as the number of sites per perpendicular to the plane of the host layer most probably in
area unit of the layer surface.27 A reciprocal value of the an interdigitated manner.
surface density then gives an area available for one active site, Roughly the same gallery height (7.1 Å) can be observed for
which is called "free area". From comparison of the free area the ZrSPP-APY2 and ZrP-APY2 intercalates while it is much
with dimension of the intercalated molecule (cross sectional smaller for TiP-APY2 (5.9 Å). In all cases the gallery height is
area) we can estimate a maximum amount of the guest which smaller than the dimension of the APY2 molecule along its
can be intercalated into a given host.
The structure of ZrSPP in its inorganic part is based on the the possibility that the guest molecules are bonded to both
structure of the alpha modification of ZrP 28
From this it layers of the host through their pyridinium groups, forming a
longest axis (which is about 10.5 Å). Therefore, we can rule out
.
follows that ZrP and ZrSPP have the same number of active kind of pillared structure. The more probable arrangement is
acidic protons per unit of surface that is the same free area on that in which each guest molecule is bonded to one layer of
the layer (24 Å2) per active site, that is 48 Å2 per formula unit the host by both pyridinium nitrogen atoms of APY2 as shown
of the host.29
in Fig. 1.
The gamma modification of TiP has a different structure.30 Two High gallery height found for ZrSPP-APY3 (9.6 Å) indicates that
phosphate groups are present in this gamma modification, one this molecule is in an upright position in ZrSPP, whereas in ZrP
3-
is PO4 in which all oxygen atoms are bonded to titanium, the and TiP (gallery height 6.5 and 6.3 Å, respectively) it is tilted
-
second is H2PO4 with two oxygen atoms bonded to titanium with respect to the host layer plane under an estimated angle
while other two bear acidic protons. The formula of the of 45°.
gamma modification should therefore be written as
Table 3 Basal spacings d (in Å) of intercalates – an overview
Ti(PO4)(H2PO4)·2H2O. The free area surrounding each P(OH)2
group is 32.9 Å2.31
Intercalate
d
d after
heating to
210 °Ca
24.14
d after
Accessible
rehydrationb
volume (Å3)
Thickness of the layer defined as the shortest distance
between the barycenters of the oxygen atoms situated at the
opposite sites of the layer was calculated to be 16.8 Å, 6.3 Å,
ZrSPP-APY1
ZrSPP-APY2
ZrSPP-APY3
ZrP-APY1
ZrP-APY2
ZrP-APY3
TiP-APY1
24.282
23.905
26.390
12.653
13.418
12.794
15.704
15.156
15.480
24.22
23.98
359.136
341.04
and 9.2 Å for ZrSPP 28 ZrP 32 and γ-TiP 31
α- , , respectively. By
,
23.61
24.80
26.43
460.32
subtraction of these values from the interlayer distances of the
intercalates, we can get the height of the gallery available for
the incorporation of the intercalated species. When the gallery
height is multiplied by the half of the free area (i.e., 24 Å2 for
ZrSPP and ZrP; 16.5 Å2 for TiP) we get the volume which is
accessible for the intercalated species in the corresponding
host. The corresponding values are given in Table S4 in the ESI.
Structure of intercalates
11.95
12.64
304.944
341.664
311.712
213.9816
195.9524
206.612
11.62
13.58 + 13.36
12.75
10.92
14.52
15.02
TiP-APY2
13.77
15.16
TiP-APY3
14.29
15.45
a
Measured in situ on
a heated brass block in the diffractometer.
b The samples were rehydrated by standing in a closed vessel over water for one
week.
The intercalated guest molecules can be placed in the
interlayer space in several ways: (i) with the plane of the
pyridine rings parallel with the plane of the host layer, either
as a one layer of the guest molecules (a "monomolecular"
arrangement) or as two layers (a "bimolecular" arrangement);
(ii) with the plane of the pyridine ring either perpendicular or
tilted with respect to the plane od the host layers. Also in this
case the guest molecules can adopt monomolecular or
bimolecular arrangement.
Change on dehydration
To study the influence of dehydration on the interlayer
distance the change of the powder X-ray patterns of the
intercalates on heating to 200 °C was studied. Almost no
change of the interlayer distance was observed for ZrSPP-APY1
intercalate, which contains only 0.7 molecule of water per
formula unit. In this case the high degree of intercalation
ensures quite rigid arrangement of the guest in the interlayer
space which does not allow its change on dehydration. Also
small decrease of basal spacing (0.3 Å) was observed for
ZrSPP-APY2, even the content of water in this intercalate is
higher (y = 1.5). The ZrP-APY1 intercalate exhibits a slight
decrease of the basal spacing (0.7 Å), while the decrease for
the TiP-APY1 is 1.2 Å. Both these intercalates contain 1
The basal spacings/interlayer distances for all intercalates are
given in Table 3. In all cases the value of the interlayer distance
is increased on intercalation, which indicates that the guest
molecules are not placed with the pyridine ring planes parallel
to the host layers. In that case the height of the gallery should
be about 2 or 4 Å only. With the decrease of amount of the
6 | J. Name., 2012, 00, 1-3
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molecule of water per formula unit and show roughly the with the spectra of all three APY and MeAPY, confirm
same increase of the basal spacing on intercalation (4.3 and therefore that these guests are protonated in the intercalates.
4.5 Å). The decrease of the basal spacing on dehydration is
given by different amount of the intercalated guest. On the Solid state UV/Vis spectra
other hand, the intercalates of APY3 are most influenced by
Beside the measurements in liquid media (see above), the
dehydration, especially ZrP-APY3 (1.9 Å, y = 1.75) and ZrSPP-
optical properties of APY1-3, MeAPY1-3, and APY1-3
APY3 (1.6 Å, y = 1.6). Also high decrease of the basal spacing
was observed for the ZrP-APY2 intercalate (1.8 Å) which has
the highest amount of water (y = 2.25) among all the
intercalates.
intercalates were also investigated in the solid-state (see the
ESI, Figs. S17-S20).
Unprotonated APY1 has an absorption maximum (
nm, whereas methylated APY1 MeAPY1) showed a maximum
at 272 nm. The maxima of all three APY1 intercalates are
λmax) at 248
(
Infrared spectra
The way how the intercalated guest molecules interact with roughly at the same position, around 260 nm (Fig. S18) The
the host material i.e., the deprotonation of the host and ZrSPP-APY1 intercalate showed slightly bathochromically
protonation of the guest molecules, was determined on the shifted λmax at 262 nm, which reflects that ZrSPP is a host
basis of the IR spectra (see the ESI, Figs. S8-S16). The peaks in material with the strongest acidity of the interlayer
the region from 1150 to 850 cm-1 are narrower in the environment among all three hosts. APY1 protonated by the
intercalates compared to those in the pristine hosts. The band exposition of this compound to HCl vapours shows a maximum
at 1231 cm-1 of
δP-OH found in the spectrum of the original TiP at around 265 nm and its spectrum is similar to those of all
disappears in the spectra of all three TiP intercalates.12a three intercalates.
Analogously this band at 1247 cm-1 in the spectrum of ZrP Significant changes in the UV/Vis spectra were observed when
disappears in the spectra of ZrP-APY1 and ZrP-APY2; this band going from APY2 to methylated MeAPY2
.
Whereas
is retained in the spectrum of ZrP-APY3, nevertheless, with a unprotonated APY2 showed a single peak at 293 nm, MeAPY2
lower intensity.
In pure APY1
possesses two maxima at 326 and at 392 nm. This is similar to
NH scissoring band is observed at 1644 cm-1 and that observed by measurement in DMSO (see above).
,
δ
peak of ring stretching appears at 1590, 1558, 1504, 1434 Compared to the APY1 intercalates, the difference between
cm-1.33 In this region, a very broad intensive band at 1649 cm-1 the spectra of all three APY2 intercalates is clearly
and quite intensive band at 1535 cm-1 are observed in distinguishable (Fig. S19). The ZrP-APY2 intercalate has the
methylated APY1
(
MeAPY1). The two bands appear also in the maximum at the lowest wavelength (297 nm) almost at the
same position as the unprotonated APY2 guest, with a
TiP-APY1 intercalate. In spectra of ZrSPP-APY1 and ZrP-APY1
a
couple of bands at 1650 and 1675 cm-1 and a very broad band shoulder at around 320-330 nm. This means that in ZrP-APY2
at 1540 cm-1 are observed. In addition, in the spectrum of ZrP- we can distinguish between the unprotonated and protonated
APY1 a very weak peak at 1597 cm-1 is observed which is forms of APY2. This claim is further supported by the fact that
observed also in the pristine APY1. It indicates that in this APY2 protonated in HCl vapours has a maximum at 321 nm,
intercalate a part of the guest remains unprotonated.
which is the same region as found for the shoulder in the ZrP-
In the spectrum of ZrSPP-APY2, the most distinct feature, APY2 spectrum. The TiP-APY2 and ZrSPP-APY2 intercalates
besides the peaks belonging to the host material, is the showed one envelope band with the maxima at 305 and 317
existence of peaks at 1624 and 1508 cm-1 (see Figure S9 in the nm, respectively. These bands seem to be superposition of
ESI). These peaks are not present in the spectrum of APY2 but that found for APY2 and its protonated form. From these data
appear in the spectrum of MeAPY2 at 1654 and 1500 cm-1. In we can deduce that APY2 undergoes partial protonation
the ZrP-APY2 intercalate, these peaks appear at 1620 and during intercalation into all three hosts and is being most
1506 cm-1 (see Figure S12 in the ESI) and in TiP-APY2 at 1626 protonated in ZrSPP (most bathochromically shifted maxima).
and 1510 cm-1 (Figure S15 in the ESI). In the case of Thus, the acidity of the hosts increases in the order
ZrSPP-APY3
,
the analogous peaks corresponding to ZrP < TiP < ZrSPP.
methylated (protonated) APY are less distinct (Figure S10 in In the solid-state, APY3 and MeAPY3 showed the UV/Vis
the ESI). While the positions of these peaks are at 1634 and spectra similar to those observed in methanol (see above) with
1512 cm-1 in MeAPY3, only a weak peak at 1628 cm-1 appears the maxima appearing at 307 and 323/390 nm, respectively.
in ZrSPP-APY3 and the second peak is shifted to 1498 cm-1. In The ZrP-APY3 intercalate possesses almost the same maximum
the ZrP-APY3 intercalate these peaks are more distinct, with as APY3 with a shoulder at around 325 nm (Fig. S20). The
the positions at 1628 cm-1 and 1500-1508 cm-1, nevertheless, a spectrum of the TiP-APY3 intercalate shows a very broad
peak of the host material (ZrP) observed at 1614 cm-1 might unstructured band covering the range of 300-320 nm with the
contribute to the intensity of the peak at 1628 cm-1 (Figure S13 maximum reaching the position observed for APY3 exposed to
in the ESI).
HCl vapors
(
λmax
=
324 nm). The longest-wavelength
APY3 323 nm)
Generally, the IR spectra of the intercalates in the region from absorption maximum of ZrSPP
1800 to 1150 cm-1 represent a superposition of the peaks corresponds tightly to the maximum measured for protonated
-
(λmax =
found in the host and in the corresponding methylated guest APY3, but in contrast to MeAPY3, shows no shoulder. This
(MeAPY). The IR spectra of the intercalates, when compared implies that APY3 intercalated into ZrSPP is fully protonated.
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ARTICLE
Journal Name
NLO properties
intercalated molecule Vguest and is documented in Table S4 in
the ESI together with the corresponding calculations. Thus, the
arrangement of the guest molecules in TiP turned out to be
the most rigid among all three hosts. In agreement with our
assumptions given in the Introduction this reduced internal
motion of the guests leads to an increased NLO response.
To create a long-range ordered non-centrosymmetry the
investigated chromophores were embedded into liquid
photocomposition of oligoetheracrylate photopolymers,34
which were photo-solidified by nitrogen 337 nm cw laser with
power density 55 W cm-2 at an applied electric field
successively increasing up to 4 kV cm. The orientation control
was done using a polarized absorption. Typical time kinetics of
the fundamental and the SHG signals is shown in the ESI (Fig.
S21). The measured principal second-order susceptibility
parameters deff jointly with the calculated hyperpolarizabilties
Table 4 Second-order susceptibilities and DFT calculated hyperpolarizabilities of APY1-
3, MeAPY1-3, and intercalates
Comp.
deff
(pm V-1)a
1.34
1.56
0.35
1.42
1.67
1.04
1.78
1.89
1.21
1.67
1.72
1.45
2.01
2.21
1.56
β(-2ω;ω,ω)
(10-30 esu)b
APY1
APY2
1.06
β
Table 4.
(-2ω;ω,ω) of APY1-3, MeAPY1-3 and intercalates are listed in
1.44
APY3
0.01
The NLO data in Table 4 allows evaluation of the structure-
property relationships caused by the chromophore
arrangement, N-methylation as well as intercalation. The
effect of the chromophore arrangement can easily be assessed
MeAPY1
MeAPY2
MeAPY3
ZrSPP-APY1
ZrSPP-APY2
ZrSPP-APY3
ZrP-APY1
ZrP-APY2
ZrP-APY3
TiP-APY1
TiP-APY2
TiP-APY3
1.37
2.61
0.13
-
-
-
-
-
-
-
-
-
on the series APY1-3. The linear push-pull aminopyridine APY1
showed a SHG response with deff equal to 1.34 pm V-1. An
introduction of the second pyridin-4-yl branch as in APY2
increased the second-order susceptibility to 1.56 pm V-1. On
the contrary, tripodal APY3 showed a significantly diminished
NLO response with deff = 0.35 pm V-1. This drop reflects the
symmetrical arrangement of APY3 (see the X-ray analysis
above) and the very low calculated ground-state dipole
moment (Table 1). The extent of the ICT and resulting NLO
a
b
Measured in oligoetheracrylate at 1064 (±0.15 pm V-1). DFT calculated by
(B3LYP/6-311++G(2d,p)// B3LYP/6-311++G(2d,p)) in vacuum at 1064 nm.
properties of the heteroaromatic D-
improved via alkylation. Thus, the comparison of MeAPY1
APY1 allows evaluation of the effect of the
pyridine/pyridinium acceptors. Whereas, the pairs of
corresponding chromophores MeAPY1 APY1 and
MeAPY2 APY2 showed slightly improved nonlinearities by a
π
-A systems can be
-
3
vs.
-
3
Conclusions
A series of model organic push-pull aminopyridines with
various spatial arrangements has been synthesized to examine
their intercalation into inorganic layered materials (alpha
modification of zirconium hydrogen phosphate, zirconium
4-sulfophenylphosphonate, and gamma modification of
titanium hydrogen phosphate). The prepared inorganic-
organic hybrids were further investigated as ordered materials
for nonlinear optics.
/
/
factor of 1.06/1.07, the nonlinearity of MeAPY3 is almost
three times higher than that found for APY3. This observation
is most likely given by
a synergistic effect of the ICT
enhancement via N-methylation and the symmetry loss caused
by iodide counter ions. In general, the SHG response of
aminopyridines decreases in the order of APY2
>
APY1
>
APY3
A facile synthetic pathway has been developed to novel
and MeAPY2>MeAPY1>MeAPY3 and MeAPY
>
APY
.
The
tripodal tris(pyridin-4-yl)amine
(APY3). Its structure was
calculated hyperpolarizabilities
β
(-2ω ) showed very
;
ω
,
ω
unambiguously confirmed by single crystal X-ray analysis,
which revealed almost perfectly planar environment of all
three peripheral nitrogen atoms. The fundamental properties
of all push-pull aminopyridines were investigated by
electrochemistry, UV/Vis absorption spectra and were
completed with DFT calculations. It was demonstrated that
with increased number of the pyridin-4-yl acceptor units, the
HOMO-LUMO gap decreases steadily by more than 1 eV and
similar trends.
The intercalation of APY1
-
3
into ZrSPP, ZrP, and TiP is
accompanied by their protonation as well organization in the
bulk. Whereas the effect of protonation can be considered
similar to quaternization, the significant SHG improvement
seen for all intercalates over MeAPY1-3 must be elucidated as
the impact of their organization in the layered host. The
measured SHG responses reflect the acidity of the host, where
intercalates with the most acidic ZrSPP showed larger deff than
ZrP (except APY3). The highest nonlinearities were measured
the
longest-wavelength
absorption
maxima
shifts
bathochromically (
ꢀλmax ~ 30 nm). N-Quaternization, which
takes place exclusively on the peripheral pyridines, caused
further reduction of the HOMO-LUMO gap and red-shifts the
CT-band. Structural analysis of MeAPY compounds revealed
facile formation of the quinoid structures, which significantly
affected their electrochemical and optical properties.
for APY1-3 intercalated into gamma modification of titanium
hydrogen phosphate (TiP). In the TiP intercalates, the lowest
volume left in the interlayer space of the host after
intercalation of APY1-3 were found. This can be expressed as
the lowest difference between the accessible volume in the
Three types of intercalates were prepared with the ratio of the
interlayer space of the host VH and the volume of the
amount of intercalated APY1-3 molecules being 6 : 3 : 2. This
8 | J. Name., 2012, 00, 1-3
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ARTICLE
ratio is inversely proportional to the charge generated at each Three layered host materials were used for the intercalation,
of the aminopyridine guest (1 : 2 : 3). IR and solid-state UV/Vis namely alpha modification of zirconium hydrogen phosphate
spectra showed that all aminopyridines underwent with formula Zr(HPO4)2·H2O (denoted further as ZrP),
protonation during the intercalation process, which extent zirconium
depends on the number of guest basic centers as well as Zr(HO3SC6H4PO3)1.8(C6H5PO3)0.2·2H2O
acidity of the host. Taking spectra of APY1 and MeAPY1 as modification of titanium hydrogen phosphate with formula
limiting, the measured IR spectra of the intercalates are Ti(PO4)(H2PO4)·2H2O (TiP). The syntheses of the host materials
superposition of the peaks found for the host and methylated ZrP ZrSPP and TiP) and the intercalation of APY1 APY2 and
4-sulfophenylphosphonate
with
formula
(
ZrSPP
) and gamma
-3
-3
(
,
,
guest. This observation confirms intercalation process as well APY3 into them are described in the ESI.
as guest protonation. From the UV/Vis spectra, the extent of Materials and methods
the protonation can further be assessed. Whereas APY2
Reagents and solvents were reagent grade and were used as
1
received. H and 13C NMR spectra were recorded at 500 and
underwent only partial protonation upon intercalation, full
protonation of APY3 was observed in the most acidic ZrSPP
and TiP hosts. Powder X-ray diffraction measurements
revealed that the guest molecules are arranged in the
interlayer space of the hosts as interdigitated monolayers in
the way schematically shown in Figure 1 for APY2 intercalated
in ZrP. In view of a generally low thermal stability of organic
push-pull molecules, the prepared inorganic-organic materials
resisted thermal decomposition up to 300 (APY1) and 500 °C
125 MHz, with a Bruker AVANCE 500 instruments at 25 °C
equipped with Prodigy CryoProbe. High resolution MALDI MS
spectra were measured on a MALDI mass spectrometer LTQ
Orbitrap XL (Thermo Fisher Scientific, Bremen, Germany)
equipped with nitrogen UV laser (337 nm, 60 Hz). Elemental
analyses were performed on an EA 1108 Fisons instrument.
UV/Vis spectra were recorded on a Hewlett–Packard 8453
spectrophotometer. Electrochemical measurements were
(
APY2
Second-order nonlinear optical properties of APY1
quaternized MeAPY1 and APY1 intercalates were
-3). This feature makes them thermally very robust.
carried out in N,N-dimethylformamide containing 0.1
M
-3,
Bu4NPF6 in a three electrode cell by cyclic voltammetry (CV),
rotating disk voltammetry (RDV) and polarography. The
working electrode was platinum disk (2mm in diameter) for CV
and RDV experiments. As the reference and auxiliary
electrodes were used saturated calomel electrode (SCE)
separated by a bridge filled with supporting electrolyte and Pt
wire, respectively. All potentials are given vs. SCE.
-
3,
-3
examined by SHG. The measured as well as calculated optical
susceptibilities/hyperpolarizabilities were affected by the
following structural features:
•
arrangement of the push-pull aminopyridine (a
general decrease in the SHG response was observed
in the order of APY2>APY1>APY3)
Voltammetric measurements were performed using
a
•
•
quaternization/protonation (pyridine vs. pyridinium
acceptor moieties, ICT enhancement)
potentiostat PGSTAT 128N (AUTOLAB, Metrohm Autolab B.V.,
Utrecht, The Netherlands) operated via NOVA 1.10 software.
The X-ray data for the crystals of APY3 were obtained at 150 K
using an Oxford Cryostream low-temperature device on a
intercalation into layered materials (all intercalates
showed significantly higher SHG responses than pure
organic push-pull guests)
Nonius KappaCCD diffractometer with Mo Kα radiation (
λ =
•
•
acidity of the host material used for intercalation
0.71073 Å), a graphite monochromator, and the
mode.
φ and χ scan
(ZrSPP and TiP proved to be more efficient acids than
ZrP
)
Powder X-ray diffraction data were obtained with a D8
Advance diffractometer (Bruker AXS, Germany) with Bragg-
Brentano θ-θ geometry (40 kV, 30 mA) using CuKα radiation
with secondary graphite monochromator. The diffraction
structure of the host (compared to ZrSPP and ZrP, the
TiP intercalates have more tightly arranged APY
molecules and showed higher SHG response).
In this work, we have verified a new strategy on property
tuning of organic push-pull molecules. Three aminopyridines
angles were measured at room temperature from 2 to 70° (2
in 0.02° steps with a counting time of 15 s per step. Powder
X-ray diffraction measurements at 210 1 °C were carried out
on a heated brass block equipped with a thermocouple in the
range from 2 to 35° (2 ) in 0.025° steps with a counting time of
θ)
with D-π-A, D-(π-A)2, and D-(π-A)3 arrangements were
±
designed, synthesized, and investigated as model guest
molecules capable of intercalation into inorganic layered
materials providing access to new inorganic-organic hybrid
materials. We believe that this multidisciplinary study would
serve as useful guide in designing novel NLO-active materials
with tailored properties.
θ
15 s per step. The size of the crystallites of the intercalates was
calculated according to the Scherrer formula35 using an EVA
software.36
The 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 in air between 30 and 960 °C at a heating rate of
5 °C min−1.
Experimental
Aminopyridines
Infrared spectra in the range of 600–4000 cm–1 were recorded
at 64 scans per spectrum at 2 cm–1 resolution using HATR
adapter on a Perkin-Elmer FTIR Spectrum BX spectrometer on
APY1 is commercially available; the synthesis of APY2 and
APY3 is outlined in Scheme 1 and described in the ESI.
Intercalates
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DOI: 10.1039/C5TC03499J
ARTICLE
Journal Name
10 (a) Y. Suzuki, Y. Tenma, Y. Nishioka, J. Kawamata, Chem.
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neat samples. All spectra were corrected for the presence of
moisture and carbon dioxide in the optical path.
7
Aloisi, U. Costantino, F. Elisei, Inorg. Chim. Acta, 2007, 360
728; (c) M. Ogawa, K. Kuroda, Chem. Rev., 1995, 95, 399.
,
Second-order non-linear optical susceptibilities were
measured by a method similar to that described in the ref.37
where a polymer supported the crystalline powder. The pulsed
Q-switch Nd:YAG laser operating at 1064 nm wavelength with
a pulse duration of 18 ns and frequency repetition 10 Hz,
power about 1 MW and a pulse repetition about 10 Hz was
applied as the fundamental one. The samples were rotated on
the travel in order to achieve a maximal SHG which was
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Beneš, T. Mikysek, O. Pytela, P. Knotek, Dalton Trans., 2014,
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a monochromator connected to
photomultiplier. The optimal content of chromophore was
equal to about 30 % in weighting units. LiNbO3 crystals with
known parameters of a second-order optical susceptibility
were used as reference samples. Using the ratio of the
intensities for the reference and the studied samples we have
defined the second order susceptibilities with accuracy up to
0.15 pm V-1.
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Acknowledgements
This work has been supported by the Czech Science
Foundation (13-01061S).
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Notes and references
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