Zirconium phosphate/phosphonate multilayered films based on push-pull stilbazolium salt: Synthesis, characterization and second harmonic generation

Article abstract of DOI:10.1039/b801194j

The preparation of materials featuring enhanced second harmonic generation (SHG) values by self-assembly of molecules characterized by high second-order non-linear optic (NLO) activity is nowadays an important and challenging field of research. In order to show SHG the material must have an acentric structure with the dipoles of the molecular components oriented in the same direction and this is synthetically fairly difficult to achieve. This study describes the synthesis of the push-pull stilbazolium salt 1 and its assembly in multilayered acentric thin films, on quartz glass surface, by using the zirconium phosphate/phosphonate (Zr-PO_x) technique. A particular care has been paid to the optimization of the surface preparation and of the deposition conditions. This allows to obtain highly homogeneous lamellar inorganic-organic materials showing satisfactory second harmonic generation (SHG) values together with high chemical, thermal and mechanical stabilities which are necessary for their integration in optoelectronic devices. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b801194j

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Zirconium phosphate/phosphonate multilayered films based on push–pull
stilbazolium salt: synthesis, characterization and second harmonic generation
Tamara Morotti,a Valentina Calabrese,a Marco Cavazzini,a Danilo Pedron,b Matteo Cozzuol,b
Antonino Licciardello,c Nunzio Tuccittoc and Silvio Quici*a
Received 22nd January 2008, Accepted 25th March 2008
First published as an Advance Article on the web 24th April 2008
DOI: 10.1039/b801194j
The preparation of materials featuring enhanced second harmonic generation (SHG) values by
self-assembly of molecules characterized by high second-order non-linear optic (NLO) activity is
nowadays an important and challenging field of research. In order to show SHG the material must have
an acentric structure with the dipoles of the molecular components oriented in the same direction and
this is synthetically fairly difficult to achieve. This study describes the synthesis of the push–pull
stilbazolium salt 1 and its assembly in multilayered acentric thin films, on quartz glass surface, by using
the zirconium phosphate/phosphonate (Zr-POx) technique. A particular care has been paid to the
optimization of the surface preparation and of the deposition conditions. This allows to obtain highly
homogeneous lamellar inorganic–organic materials showing satisfactory second harmonic generation
(SHG) values together with high chemical, thermal and mechanical stabilities which are necessary for
their integration in optoelectronic devices.
promising, e.g.: Langmuir–Blodgett films,10 layered cationic–
Introduction
anionic polymer,11 trifunctionalized siloxane12 and zirconium
phosphate/phosphonate (Zr-POx) technique.13 In these cases,
an appropriate design of the molecular organic components
ensures a rigid control over their alignment on the substrate
surface. The Zr-POx technique is based on the consideration
that organic phosphonic acids form highly insoluble, layered
salts with Zr(IV) and other transition metals.14 By using this
procedure Mallouk and co-workers showed that a series of
different bis-phosphonic acids could be assembled as Zr(IV) salts
on a substrate one layer at time and that this iterative method
allowed to reproduce on the surface the lamellar structure of the
Zr(IV) phosphate/phosphonate salt.15–17 A few years later Katz
and co-workers, at the AT & T Bell Laboratories, by using a
modification of Mallouk’s method assembled a family of push–
pull azo-dyes into noncentrosymmetric multilayer films.13 The key
of the sequential deposition technique used by Katz et al. has been
to design a three-step adsorption sequence involving molecules
with a phosphonate group in one end and a hydroxyl at the other.
The three steps are: chemisorption of the phosphonate end of
the molecule on the Zr4+ phosphate layer, phosphorylation of
the exposed hydroxyl group with POCl3 to afford phosphate and
zirconation of the latter to regenerate the surface. The reiteration
of this sequence led to multilayered hybrid organic/inorganic films
that showed second-order non-linear optic coefficients of the same
order of magnitude of LiNbO3 together with very high chemical,
mechanical and thermal stabilities, which are prerequisites for their
integration in optoelectronic devices. In this paper, together with
the synthesis of the stilbazolium salt 1 and the preparation of
oriented multilayers by self-assembly with the Zr-POx technique,
a particular attention has been paid to the improvement of
deposition parameters that are essential for the reproducibility
of the materials either from the structural or functional point of
view.
Nowadays the preparation of functional and multifunctional
devices by nano-organization of single molecular components
represents a demanding goal in the development of nanoscience
and nanotechnology.1,2 In particular, in recent years increasing
efforts have been focused on materials showing high second
harmonic generation (SHG) values, which are prepared from
second-order non-linear optical (NLO) organic molecules. These
materials could find application in many fields such as telecom-
munications, data storage and processing and laser technologies
and in perspective could efficiently replace usually exploited
inorganic matrices.3 It is well known that high second-order NLO
activity is observed in dipolar compounds characterized by the
presence of an electron-donating and an electron-withdrawing
group connected by p-conjugated linker “push–pull molecules”.4
For this reason materials showing high SHG values should feature
an acentric structure which means that the dipole moment of
molecular components should be oriented in the same direction.
To this end, several nano-organization techniques have been
developed, for instance growing of non-centrosymmetric molec-
ular crystals,5 inclusion of NLO compounds into organic or
inorganic guest,6,7 electric field induced poling of polymer8 and
hydrogen-bonding assisted physical vapour deposition (PVD).9
Moreover the “layer by layer” approaches towards the prepa-
ration of non-linear optic materials seem to be the most
aCNR - Istituto di Scienze e Tecnologie Molecolari, Via C. Golgi 19, 20133,
Milano, Italy. E-mail: silvio.quici@istm.cnr.it; Fax: +39-02-50314159;
Tel: +39-02-50314164
bDipartimento di Scienze Chimiche, Universita` di Padova, Via Marzolo 1,
35131, Padova. E-mail: danilo.pedron@unipd.it; Fax: +39-049-8275135;
Tel: +39-049-8275121
cDipartimento di Scienze Chimiche, Universita` di Catania, Viale A. Doria 6,
95125, Catania, Italy. E-mail: alicciardello@dipchi.unict.it; Fax: +39-095-
580138; Tel: +39-095-7385086
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the synthesis is the condensation of 3 and 5 promoted by piperidine
in refluxing ethanol for 18 h, which afforded the completely
protected stilbazolium salt 6 in 73% yield after purification by
column chromatography. Stilbazolium salt 1 was readily obtained
from 6 through two subsequent hydrolysis steps. The cleavage of
phosphonic ester was achieved by treatment of 6 with trimethylsilyl
bromide in CH2Cl2 afforded the benzoyl protected derivative 7 in
77% yield. This latter was hydrolyzed with aqueous 2% NaOH to
give 1 in 91% yield after purification with Amberlite XAD 1600T
in order to remove all inorganic salts.
Results and discussion
Synthesis of the chromophore 1
The stilbazolium salt 1 (Scheme 1) has been designed in order to
allow the deposition of one Zr-POx monolayer at time. Indeed
it contains a phosphonic group, at one end of the molecule,
which is ready for binding with the surface functionalized with
zirconium phosphate, and a primary hydroxyl group on the other
end that can be easily converted to a phosphate residue after
phosphorylation with POCl3. In comparison with the push–pull
azo-dyes reported by Katz and co-workers13 the stilbazolium
salt 1 should show an enhanced second-order NLO activity
because of the presence of the pyridinium cation, that behaves
The simultaneous removal of both protecting groups is also
possible by treating 6 with concentrated hydrochloric acid at
70 ◦C overnight and subsequent purification with Amberlite XAD
1600T.
=
as stronger electron-withdrawing group, and a C C spacer, which
allows a better conjugation inside the molecule with respect to
an azo spacer.18 Moreover chemical, physical and photochemical
stabilities should be substantially the same for 1 and published
azo-dyes. The preparation of 1 has been carried out according to
Scheme 1.
The hydroxyl protected benzaldehyde 3 was prepared in
76% yield by Vilsmeier reaction of N-(3-benzoyloxypropyl)-N-
methylaniline 2, which was obtained by alkylation of commer-
cially available N-methylaniline with 3-bromopropyl benzoate
carried out in refluxing CH3CN in the presence of catalytic
amounts of KI and solid K2CO3 as base. The pyridinium salt
5 was obtained in 70% yield by alkylation of 4-picoline with 3-
iodopropyl phosphonic acid isopropyl ester 4.19 The key step of
Preparation of multilayer films by the Zr-POx technique
The protocols reported in the literature for the preparation of
Zr-POx multilayer films seem all very similar, but a careful
insight of the experimental procedures evidences deep differences
concerning mostly the priming of the surface and the reaction
conditions used (solvents, temperatures, pH, reaction times and
so on). The aim of the priming step is to produce a properly
functionalized surface monolayer from which the chemisorption-
based film deposition can be initiated. In the case of the Zr-POx
assembly technique the primed surface consists of a Zr4+ phosphate
or phosphonate monolayer. The priming of hydroxyl groups
containing surfaces used by Katz and co-workers was carried
Scheme 1 Synthesis of push–pull stilbazolium salt 1.
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out by treating the substrate with 3-aminopropyltrimethoxysilane,
phosphorylation of the amino-terminated monolayer with POCl3
and subsequent zirconation of the resulting phosphoramide end
groups.12 In our protocol the priming of substrate has been
carried out directly reacting the superficial hydroxyl groups
with POCl3 and the so-obtained phosphate residues zirconated
with ZrOCl2 (Fig. 1). In this way it was possible to avoid
the drawbacks associated with the very high reactivity of 3-
aminopropyltriethoxysilane12 that requires strictly controlled re-
action conditions in order to avoid undesired polymerization on
the surface.
molecules. In this case the substrate was transferred into a 20 mM
POCl3 and 20 mM collidine acetonitrile solution for 10 min
at room temperature and then rinsed with CH3CN and water.
Each monolayer was considered completed after the zirconation
step, which ensured the capping of the organic/inorganic lamellar
structure, thus avoiding possible molecular reorientation in the
chromophore layer. It is worth stressing that with glass quartz
substrate the growth of the multilayer occurs at both faces of the
sample.
Structural characterization of the films
Dipping the primed substrate in a 1.5 mM aqueous solution
of stilbazolium salt 1 at pH 2–3, adjusted by addition of 10%
aqueous HCl, for 15 min at 70 ◦C led to formation of the first
monolayer by reaction of the phosphonic acid end group of 1
with the superficial Zr4+ phosphate. Layered zirconium phosphate
phosphonate salts are normally prepared by simple precipitation
reaction in which a source of Zr4+ cations is mixed with a
solution of the appropriate organic phosphoric or phosphonic
acid. Since the reaction involves the loss of protons from weak
acids this step is pH dependent.17 Indeed the optimum pH for the
chemisorption step is in the range 2–3 because the phosphoric
or phosphonic acid groups are dissociated and ligating at this
pH where the metal ions are Lewis acidic. In order to maintain
the optimum conditions we believe that water is the best solvent
for the chemisorption of the chromophore although EtOH,20
dimethylacetamide–EtOH,19 DMF–EtOH21 and DMSO22 have
been used as solvents. Although push–pull molecules, because of
the strong repulsion between dipoles, should spontaneously array
in a centrosymmetric arrangement, the strong reactivity of Zr(IV)
towards phosphonate groups of 1 precluded this phenomenon. A
modified procedure with respect of the priming step was used
in phosphorylation of hydroxyl groups of the anchored NLO
Mass spectrometry. Time of Flight Secondary Ion Mass
Spectrometry (ToF-SIMS) analysis has been used at the beginning
of this work in order to verify the monolayer formation by
investigating the surface after each deposition step.
The positive secondary ion mass spectrum (acquired in “static”
conditions) after deposition of the chromophore showed a clear
peak at m/z 391 for (1)+ together with several fragmentation peaks
(Fig. 2).
After the phosphorylation step the same behavior was observed
in the case of a phosphonate monolayer, where the occurred
reaction on the substrate was confirmed by the presence of a
positive peak at m/z 457 for a (C20H25O2N2P2Cl2)+ fragment in
addition to a number of other phosphorylated species. Interesting
information was collected by analysis of the negative secondary
ions mass spectrum (Fig. 3), which evidenced the presence of a
characteristic pattern due to repetitive losses of (C4H9NO4P)+
fragments of m/z 166 Da. This gives evidence of the presence on
the surface of sufficiently large domains in which the chromophore
molecules are tightly packed.
Similar mass spectra have been obtained also during the
preparation of subsequent monolayers thus indicating that the
Fig. 1 Schematic representation of adopted deposition protocol.
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Fig. 2 Part of a typical positive ToF-SIMS spectrum of the molecular
film after the first chromophore adsorption.
Fig. 4 (a) Typical ToF-SIMS profiles of Si+ signal acquired on chro-
mophore multilayers of different thickness. (b) Number of sputter cycles
necessary for reaching the interface as a function of chromophore nominal
number of layers. The position of the interface is obtained from the
inflection point of profiles in (a).
inflection point of the sigmoidal curve of the Si+ signal indicates
the point where all the deposited film has been removed and the
naked quartz surface is reached by the sputtering ion beam. The
good linearity observed in Fig. 4(b) indicates that the thickness
of the film under investigation is directly proportional to the
number of layers and is a further proof of the reproducibility
of this deposition technique.
UV-Vis Analysis. UV-Vis spectroscopy is the simplest ana-
lytical technique that allows to follow the growth by measuring
the absorption of the multilayers. Indeed a linear increase of the
absorbance by increasing the number of layers indicates that a
constant amount of chromophore is anchored to the substrate
after each deposition cycle. Fig. 5(a) shows the UV-Vis absorption
spectrum of chromophore 1 in 4.37 × 10−5 M aqueous solution
together with those of four samples having 10, 20, 30 and 35 layers,
respectively.
The absorption profile of the chromophore 1 in aqueous
solution is characterized by a large maximum at 461 nm, probably
influenced by a relevant charge-transfer contribution between the
electron-withdrawing and electron-donating groups. This peak
shifts to 476 nm in the case of the push–pull molecule fixed to the
quartz surface, indicating the possible formation of J-aggregates of
1 within the monolayer. Indeed when polar compounds are packed
in a monolayer strong interactions occur between molecules thus
allowing the formation of aggregates which are characterized by
a blue-shift (H-aggregates) or by a red shift (J-aggregates).23,24
Furthermore an increasing absorption in the UV range can be
Fig. 3 Portion of the negative ion ToF-SIMS spectrum of the phospho-
rylated layer.
reaction occurring at each step is highly reproducible. Dual-
beam ToF-SIMS depth profiles have been acquired in order to
obtain indirect evidence of the multilayer growth. Such profiles are
obtained by sputtering the surface with a relatively large current
ion beam and analyzing the crater bottom by a second ion beam.
In the obtained depth profiles the molecular information from the
organic layer is lost, due to the extensive ion beam-induced damage
of the organic system occurring in depth profiling conditions
(dynamic SIMS). However the elemental signals are still present
and can be followed as a function of the number of sputter cycles
(proportional to the sputter time and also to the film thickness
under the assumption of constant sputter rate).
In particular we used the rise of the Si+ signal from the substrate
for locating the interface in samples of different thickness. By
following this signal as a function of sputter cycles it is possible
to extract indirect evidence of the film growth (Fig. 4). Indeed the
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Fig. 6 1 lm × 1 lm AFM images of samples having 10 (a) and 35 (b)
layers. Also shown are an AFM image of a scratched 35 layer sample (c)
and measurement of the corresponding thickness (d).
affected by the presence of aggregates with height in the range 12–
15 nm, probably due to adventitious precipitation of insoluble zir-
conium salts occurring during the multilayer deposition process.23
Comparing the surface morphologies of the two samples, the 35-
layer sample seems to have more extended homogeneous areas
together with fewer zirconium salt aggregates, although they seem
to be larger. By scratching the surface with a metallic tip it is
possible to completely remove the multilayer Zr-POx film from the
quartz surface as shown in Fig. 6(c). It should be stressed that
the complete detachment of the film occurs only with difficulty
with in general the Zr-POx film of the chromophore 1 being very
compact and showing good adherence to the quartz surface. This
observation is also proved by the presence of vitreous chips well
evidenced at the border of the scratch. Fig. 6(d) shows the cross
section profile obtained by the AFM tip along the white line shown
in Fig. 6(c). The highest peak of this profile near to the border of
the scratch is due to the removed material, while the roughness of
the surface seems to be very low. The cross section measurement
indicates that the thickness of the 35-layer film is 70 nm.
From this value and from ellipsometric measurements carried
out on several samples having different numbers of layers an
average thickness of 2 nm per monolayer could be obtained.
By considering that the thickness of the Zr-POx inorganic layer
obtained by crystallographic data is 0.7 nm,14 the length of the
chromophore 1 is 2 nm (molecular mechanic calculations) and
that the average thickness of the multilayer is 2 nm, the tilt angle
calculat◦ed for the multilayer samples is 50◦, a value very close to
that (46 ) reported in the literature for a similar azo-dye.21
Fig. 5 (a) UV-Vis absorption spectra of 1 in aqueous solution and
of multilayer samples; (b) absorbance of multilayer samples plotted vs
nominal layers.
envisaged that is due to the presence of the inorganic layers.
Fig. 5(b) shows a plot of the absorption maximum of the four
multilayer samples of Fig. 5(a) as a function of the number
of layers. The linear dependence of the absorption from the
number of layers evidences that the single layer deposition is
highly reproducible in the deposition of the multilayer film. The
slope of the best fit line through the origin was 0.0109 per layer
which corresponds to the mean absorption of the single layer.
We have also estimated the molar extinction coefficient for our
chromophore at the maximum of absorbance in aqueous solution
giving a value of 27820 l cm−1 mol−1. From the mean single
layer absorption and the chromophore molar extinction coefficient
value, it is possible to calculate the surface chromophore density
r by the expression:
r = mabsNA/1000e
(1)
where mabs (layer−1) is the slope of the line obtained by plotting
absorbtion maxima vs nominal layer numbers, and NA is Avogadro
constant. We obtained a value for r of 2.36 × 1014 molecules cm−2
layer−1, which is comparable with values reported in the literature
for Zr-POx films of some azo-dyes.20
Second harmonic generation (SHG) measurements
The SHG signal of the same series of films, from 10 to 35 layers, was
measured with the experimental set up described in the following
section. The Maker fringes for the 35-layer film (Fig. 7) were
integrated and the subtending areas plotted as function of the
number of layers.
Atomic force microscopy (AFM) analysis
1 mm × 1 mm AFM micrographs of samples having 10 and 35
layers of 1 are shown in Fig. 6(a) and (b).
The images show similar topographies with no evidence of
cracks or pinholes. The surfaces appear relatively homogeneous
although the average roughness obtainable from these images is
As it was supposed the signals were quadratically proportional
to the number of layers (Fig. 8) and it follows that the square
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to write the induced non-linear polarization in terms of the tensor
dIJK which is related to the second-order susceptibility by eqn (2).8
dIJK = 0.5v2IJK
(2)
In this equation IJK indices represent the coordinate system of
the bulk material. Tensor d components were calculated by fitting
experimental curves collected for three different polarizations (see
Experimental section). For the system under investigation only
three of the d tensor elements are non-zero: d15, d31 and d33. The val-
ues of d15 and d31 could be obtained directly through two different
measurements in SP→S and SS→P polarization configuration,
respectively. These values were then used to calculate d33 which
represent the SHG values along the direction perpendicular to the
sample surface (z axis).
A mean d33 value of 12 pm V−1 was found and the results were
in good agreement for all films. This value was comparable to that
obtained by Katz et al. (19 pm V−1) for a similar chromophore
and is in the range of those found for LiNbO3 (2–44 pm V−1).25
As it is evidenced in Fig. 10 the d33 values obtained for samples of
10, 20, 30 and 35 layers are all very similar and fall within the error
bars, thus giving further evidence that the deposition technique
ensures a regular orientation of chromophores by increasing the
number of layers.
Fig. 7 Example of SHG angle dependence intensity response for a
35-layer zirconium phosphate/phosphonate film.
Fig. 8 SHG integrated area plotted vs. nominal layers.
root of areas increases linearly (Fig. 9). This result was in accord
with the linear growth of the absorption spectra and it suggested a
good reproducibility of deposition technique and a constancy of
polar order of chromophores even after several layer depositions.
The second-order NLO properties of bulk materials are generally
defined by the susceptibility tensor v2IJK . For second harmonic
generation (SHG) and optical rectification it is a common practice
Fig. 10 d33 (pm V−1) vs. nominal layers.
Experimental
Materials and methods
All commercially available solvents and reagents were used as
received. TLC was carried out on silica gel Si 60-F254. Column
chromatography was carried out on silica gel Si 60 mesh size
0.040–0.063 mm (Merk, Darmstadt, Germany). 1H NMR spectra
were recorded with a Bruker AC300 spectrometer operating at
300 MHz. UV-Vis spectra were recorded on Nicolet Evolution
500 (Thermo Electron Corporation) spectrometer, ellipsometric
measurements were carried out at CIVEN laboratories with a
GES 5 (Sopra) ellipsometer and AFM images were obtained from
NTEGRA Probe NanoLaboratory (NT-MDT) microscope. ToF-
SIMS measurements were performed with ToF-SIMS IV (ION-
ToF, Muenster, Germany) instrumentation. Static SIMS spectra
were obtained with 69Ga+ primary ions (25 keV, PI fluence <3 ×
1011 ions cm−2). Depth profiles were obtained in dual beam mode,
Fig. 9 Square-root of SHG integrated area plotted vs. nominal layers.
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using Ar+ ions (1 keV) for sputtering and 69Ga+ (25 keV) for
analysis. The sputter beam (1020 nA) was rastered over 500 mm ×
500 mm, and the analysis beam (1 pA) over a concentric area
of 100 mm × 100 mm. A slow (18 eV) pulsed electron flood
was used for charge compensation when acquiring profiles from
insulating samples. Determination of second-order coefficients
was performed by the Maker fringes technique.26 Second harmonic
generation (SHG) measurements of samples were carried out with
a Q-switched Nd:YAG (Quanta System HYL-101) laser operating
at 1064 nm, 10 Hz repetition rate. The laser intensity on the sample
was measured by a pyroelectric detector Gentec-eo QE25SP-S-MB
and was about 0.2 mJ pulse−1. The light beam was focalized on
the sample by a 400 mm lens. The focus was positioned a few
millimeters in front of the sample to avoid damage to the films.
The polarization of the incident beam was controlled by a k/2
waveplate. Polarization of a duplicated beam was selected by a
beam-splitter polarizer cube mounted on a rotation stage.
For calculation of the absolute value of the second-order NLO
coefficient, the punctual symmetry of the multilayer films was
assumed to be C∞v. This was reasonable in consideration of
deposition technique and of literature references,30 and was also
confirmed by experimental evidences. For a C∞v system only three
of the tensor elements are nonzero: d15, d31 and d33. The values
of d15 and d31 could be obtained directly through two different
measurements in SP→S and SS→P polarization configuration,
respectively. These values were then used to calculate the value of
the d33 tensor from PP→P configuration measurement. In order
to correctly construct a theoretical model to fit the experimental
data it was necessary to evaluate the linear optical properties of the
material and this was done by performing UV-Visible absorption
spectra (Perkin-Elmer Lambda 2 Spectrometer) and ellipsometric
measurement (GES 5–Sopra) in the 200–1100 nm range. As
mentioned previously the theoretical model was constructed
starting from the assumptions of Herman and Hayden.27
The samples were mounted on a motorized rotation stage
Newport SR50. The rotation stage was controlled by a Newport
ESP300 motion driver. Care was taken to make the incident beam
to precisely pass through the rotation axis; the minimum step
resolution available was 0.001◦. The second harmonic signals in-
tensity (532 nm) was measured by a H9307–03 Hamamatsu photo-
multiplier (PMT) connected to a Tektronix 3054B oscilloscope. A
visible interference filter (centre wavelength 532 nm) was mounted
in front of the PMT to remove the infrared transmitted funda-
mental beam and to reveal just second harmonic (SH) signals.
In order to take in to account both films on the opposite side of
the quartz substrate and to obtain the NL coefficients a modified
Hermann & Hayden formula27 was used for interpreting the SHG
phenomenon in nonzero absorption films. To correctly calculate
the angular dependence of the signal it was necessary to consider
the difference of phase between the beams generated in the two
different layers on both faces of the substrate. This difference of
phase Du depends on both the thickness and the light dispersion
of the substrate and can be written as:
N-Methyl-N-(3-benzoyloxypropyl)aniline (2)
Solid potassium iodide (0.85 g, 5.1 mmol) and potassium car-
bonate (7.07 g, 51.2 mmol) were added to a solution of N-
methylaniline (1.37 g, 12.8 mmol) and 3-bromopropyl benzoate
(3.42 g, 14.1 mmol) in acetonitrile (50 mL) and the mixture was
refluxed under magnetic stirring for 48 h. The mixture was cooled
at room temperature, diluted with acetonitrile (50 mL), filtered
on a fritted glass and the filtrate was evaporated to afford a
residue. This product was dissolved in dichloromethane (100 mL)
and washed with water (2 × 50 mL). The organic phase, dried
over magnesium sulfate, was evaporated under reduced pressure
to afford an orange oil residue which was purified by column
chromatography on silica gel using 6 : 4 (v/v) light petroleum–
dichloromethane as eluent to afford 3 (2.51 g, 73%) as a colourless
oil; dH (200 MHz, CDCl3) 8.06 (2H, m), 7.49 (1H, m), 7.45 (2H,
m), 7.22 (2H, m), 6.71 (3H, m), 4.38 (2H, t, J 6.23), 3.52 (2H, t, J
7.11), 2.96 (3H, s), 2.06 (2H, quintet, J 7.01).
(3)
3-[(4-Formylphenyl)(methyl)amino]propyl benzoate (3)
where nx and n2x are the refractive indexes for the fundamental
and the duplicate frequencies in the substrate and hx and h2x are
the propagation angles of the two beams inside the quartz.
Accordingly the SH intensity could be defined by the expression:
A mixture of anhydrous dimethylformamide (30 mL) and POCl3
(1.49 g, 9.7 mmol) was stirred under nitrogen atmosphere at 0 ◦C
for 1.5 h, then at room temperature for 1 h to give a yellow
solution that was then heated at 95 ◦C. A sample of N-methyl-
N-(3-benzoyloxypropyl)aniline 2 (2.5 g, 9.3 mmol) dissolved in
dimethylformamide (6 mL) was added and the resulting mixture
was stirred at 95 ◦C for 4 h. The solvent was removed under
reduced pressure, the residue was taken up with water (50 mL)
and the pH was adjusted at 7–8 by addition of sodium bicarbonate
saturated aqueous solution, then the organic phase was extracted
with n-hexane (6 × 50 mL). The organic phase was washed with
water (100 mL), dried with magnesium sulfate and the solvent
evaporated under reduced pressure to afford a brown oil residue
that was purified by column chromatography on silica gel using
pure dichloromethane first and subsequently dichloromethane–
ethyl acetate 10 : 0.3 (v/v) as eluent to afford 3 (2.11 g, 76%) as
a colourless oil; dH (200 MHz, CDCl3) 9.73 (1H, s), 8.03 (2H, m),
7.74 (2H, d, J 9.00) 7.60 (1H, m), 7.47 (2H, m), 6.74 (2H, d, J 1.0),
4.39 (2H, t, J 6.09), 3.63 (2H, t, J 7.12), 3.09 (3H, s), 2.11 (2H,
quintet, J 7.48).
(4)
2x
where P is the power of SHG signal detected by the PMT, P12x
TOT
and P22x is the power of second harmonic generated at the front and
back faces of the sample. As a matter of fact P2TxOT is strongly angle
dependent and the number and the position of the minima are
strictly related to the thickness of the quartz substrate. The Maker
fringes technique needs a well-known reference to calculate NL
coefficients. For this reason a quartz crystal wafer (X-cut 1 mm
thick, d11 is 0.46 pm V−1 28,29) was used as reference. The presence of
a reference substrate was also necessary for completely removing
the laser beam power long term drift and to take in account the
beam waist width. Thus the Maker fringes measurement of the
reference was performed just before each measurement. Short-
term power fluctuations were reduced by choosing an adequate
averaging time.
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1-[3-(Diisopropoxyphosphoryl)propyl]-4-methylpyridinium
iodide (5)
(91%) of pure product 1 as a red powder (Found: C, 55.7; H, 6.8;
N, 6.6. C20H28N2O4P requires C, 56.3; H, 6.6; N, 6.6); dH (CD3OD,
300 MHz) 8.63 (2H, d, J 6.88), 7.93 (2H, d, J 6.89), 7.79 (1H, d,
J 15.98), 7.58 (2H, d, J 8.93), 7.05 (1H, d, J 16.01), 6.79 (2H, d,
J 8.98), 4.51 (2H, t, J 7.36), 3.62 (2H, t, J 6.08), 3.55 (2H, t, J
7.27), 3.05 (3H, s), 2.20 (2H, m), 1.82 (2H, quintet, J 7.05), 1.48
(2H, m); m/z (ToF-SIMS) 391 (M+. C20H28N2O4P requires 391);
kmax(EtOH): 488 nm (e/dm3 mol−1 cm−1 36 996); kmax(H2O)/nm
461 (e/dm3 mol−1 cm−1 27 820).
A solution of diisopropyl 3-iodopropyl phosphonate (4) (1.82 g,
5.4 mmol) and 4-methylpyridine (0.81 g, 8.7 mmol) in anhydrous
tetrahydrofurane (20 cm3) was heated at reflux for 72 h. The mix-
ture was cooled at room temperature and the solvent evaporated
to dryness under reduced pressure to afford a deep yellow thick
oil. This product was stirred overnight with diethyl ether (40 cm3)
to afford a solid that was filtered on a fritted glass in a nitrogen
atmosphere and dried under vacuum to afford 5 (1.62 g, 70%) as
a yellow hygroscopic solid; dH (200 MHz, CDCl3) 9.33 (2H, d, J
6.70), 7.85 (2H, d, J 6.50), 5.08 (2H, t, J 7.18), 4.68 (2H, m), 2.69
(3H, s), 2.40 (2H, m), 1.76 (2H, m), 1.32 (12H, d, J 6.20).
Film preparation
Layers were deposited on quartz glass HSQ 300 which surfaces
were cleaned and oxidized by immersion for 15 min at 60 ◦C in a
freshly prepared Piranha solution (3 : 1 mixture of 96% H2SO4 and
30% H2O2), followed by rinsing with deionized water for 5 min
and drying under a nitrogen stream. CAUTION! a mixture of
concentrated sulfuric and hydrogen peroxide reacts violently with
many organic compounds and should be handled with care. The
hydroxyl-functionalized surface was then immersed in neat POCl3
at room temperature for 18 h, washed with acetonitrile and water
and dried under nitrogen stream to convert –OH groups into –
OPO(OH)2 residues. The deposition of a Zr(IV) layer was achieved
by immersion of the substrate in a 5 mM ZrOCl2·8H2O aqueous
solution at room temperature for 10 min. Again the substrate
was rinsed by dipping into deionized water for 5 min and dried
under a nitrogen stream to complete priming procedure. The
formation of the first monolayer of chromophore 1 was fulfilled
by dipping the sample in a 1.5 mM aqueous solution of the
stilbazolium salt 1 at pH value, adjusted by addition of 10%
HCl, in the range 2–3 for 15 min at 70 ◦C. After careful washing
with water for 5 min the substrate was transferred into a 20 mM
POCl3 and 20 mM collidine acetonitrile solution for 10 min at
room temperature and then rinsed with CH3CN and water. The
third step of deposition was accomplished again by immersion of
substrate into a 5 mM ZrOCl2·8H2O aqueous solution at room
temperature for 10 min. Each deposition cycle was considered
completed after the zirconation step, which ensured the capping
of the organic/inorganic lamellar structure, thus avoiding possible
molecular reorientation in the chromophore layer. It is worth
stressing that with glass quartz substrate the growth of the
multilayer occurs at both faces of the sample.
(E)-4-{4-[(3-Benzoyloxypropyl)(methyl)amino]styryl}-1-[3-
(diisopropoxyphosphoryl)propyl]pyridinium chloride (6)
A solution of 5 (3.68 g, 12.4 mmol), 3 (3.73 g, 12.37 mmol) and
20 drops of piperidine in ethanol was heated at reflux for 18 h.
The mixture was cooled at room temperature and the solvent
evaporated to dryness under reduced pressure to afford a residue
that was purified by column chromatography using 9 : 1 (v/v)
dichloromethane–methanol as eluent to give pure 6 (4.72 g, 70%)
as orange powder; dH (CDCl3, 300 MHz) 9.04 (2H, d, J 6.83), 8.04
(2H, d, J 7.23), 7.80 (2H, d, J 6.86), 7.59 (2H, m); 7.47 (4H, m),
6.84 (1H, d, J 15.9), 6.73 (2H, d, J 8.9), 4.87 (2H, t, J 7.02), 4.67
(2H, m), 4.39 (2H, t, J 6.07); 3.61 (2H, t, J 7.01), 3.08 (3H, s), 2.34
(2H, m), 2.11 (2H, quintet, J 6.62), 1.77 (2H, m), 1.32 (12H, d, J
6.20).
(E)-4-{4-[(3-Benzoyloxypropyl)(methyl)amino]styryl}-1-(3-
phosphonopropyl)pyridinium chloride (7)
A solution of 6 (4.72 g, 6.68 mmol) and trimethylsilyl bromide
(3.68 g, 24.05 mmol) in 40 ml of anhydrous CH2Cl2 was stirred
overnight at room temperature and under nitrogen atmosphere.
The solvent was evaporated to dryness under vacuum and the
residue was dissolved in water (200 cm3) at pH 8.5, by adding
a saturated solution of sodium carbonate, to afford a deep red
solution. This solution was washed with diethyl ether (2 × 70 cm3)
and acidified at pH 3–4 by addition of 10% aqueous hydrochloric
acid to afford a solid precipitate which was filtered on a fritted
glass, washed with dioxane (50 cm3) and dried under vacuum to
give 2.74 g (77%) of product as red powder; dH (CD3OD, 200 MHz),
8.62 (2H, d, J 6.86), 8.01 (4H, m), 7.82 (2H, d, J 16.03), 7.54
(5H, m), 7.06 (2H, d, J 15.99), 6.81 (2H, d, J 8.98), 4.52 (2H, t,
J 7.29), 4.38 (2H, t, J 6.04), 3.67 (2H, t, J 7.09), 3.08 (3H, s), 2.14
(4H, m), 1.55 (2H, m).
Conclusions
The results here reported indicate that the Zr-POx technique repre-
sents a useful way to nanorganize push–pull NLO active molecules
into multilayered inorganic–organic thin films in which the polar
order is ensured by the design of the chromophore. Indeed only
the phosphonate end of the chromophore can be chemisorbed by
the zirconium phosphate primed surface of the substrate leaving
the hydroxyl ended part available for the regeneration of the
surface through two simple chemical steps, i.e. phosphorylation
and zirconation. Although the steps required for the preparation
of each monolayer are very simple, the optimization of the
deposition conditions, namely solvent, pH, temperature and time
of reaction, proved to be very important in determining the
homogeneity and the chemical–physical properties of the obtained
materials. Another important point rests on the careful chemical
(E)-4-{4-[(3-Hydroxypropyl)(methyl)amino]styryl}-1-(3-
phosphonopropyl)pyridinium chloride (1)
A solution of 7 (0.806 g, 1.3 mmol) in 2% sodium hydroxide
aqueous solution (30 cm3) was stirred overnight at room tem-
perature. The solution was adjusted at pH 7–8 by addition of
10% aqueous hydrochloric acid and the product was purified by
column chromatography on Amberlist XAD 1600T by eluting the
inorganic salts with pure water and the product with 8/2 (v/v)
water/acetonitrile. Evaporation of the solvent afforded 0.61 g
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characterization of the surface during the preparation of the
multilayer in order to guarantee the reproducibility of the steps
allowing the formation of each monolayer. Taking care of these
considerations and by using the Zr-POx technique it has been
possible to prepare highly homogeneous and reproducible thin
films of the stilbazolium salt 1, that are characterized by superior
mechanical stability and NLO properties. The results obtained
clearly evidence the great potential of the ZrPOx assembly
technique for the preparation of multilayered films in which
a defined alignment of molecules should be maintained. An
important aspect which needs to be investigated is the possibility
of automating the layer by layer deposition thus allowing the
preparation of micrometer thick films that are often required
for NLO operating devices. Further investigations aimed to use
the Zr-POx technique for the assembly of new and more active
chromophores are under way in our laboratories.
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2982 | Dalton Trans., 2008, 2974–2982
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The Royal Society of Chemistry 2008
©