??(2) Grating in ru derivative chromophores incorporated within the PMMA polymer matrices

Article abstract of DOI:10.1021/jp048794h

We have found that Ru derivative chromophores incorporated into the PMMA polymer matrices might be considered as promising materials for optical poling. The maximally achieved effective second-order optical susceptibility for the wavelength 1.89 ??m is ab

Full text of DOI:10.1021/jp048794h

14942
J. Phys. Chem. B 2004, 108, 14942-14947
ø(2) Grating in Ru Derivative Chromophores Incorporated within the PMMA Polymer
Matrices
A. Migalska-Zalas, Z. Sofiani, and B. Sahraoui
UniVersite´ d’Angers, laboratoire POMA UMR CNRS 6136, 2. Bd LaVoisier, 49045 Angers, France
I. V. Kityk* and S. Tkaczyk
Institute of Biology and Biophysics, Technical UniVersity of Cze¸stochowa, Al. Armii Krajowej 36,
Cze¸stochowa, Poland, and Institute of Physics WSP Cze¸stochowa, PL-42217, Al. Armii Krajowej 13/15,
Cze¸stochowa, Poland
V. Yuvshenko
Institute of Physics, Ukrainian Academy of Science, KyjiV, pr. Nauki 144, Ukraine
J.-L. Fillaut and J. Perruchon
Institut de chimie de Rennes, UniVersite´ Rennes I- UMR CNRS 6509, Laboratoire Organome´talliques et
Catalyse, 35042 Rennes Cedex, France
T. J. J. Muller
Organisch-Chemisches Institut, UniVersita¨t Heidelberg, 69120 Heidelberg, Germany
ReceiVed: March 18, 2004; In Final Form: July 11, 2004
We have found that Ru derivative chromophores incorporated into the PMMA polymer matrices might be
considered as promising materials for optical poling. The maximally achieved effective second-order optical
susceptibility for the wavelength 1.89 µm is about 1.94 pm/V. The investigated composites possess metastable
trapping levels originating both from the d states of Ru and due to effective charge transfer due to the presence
of π-conjugated states. Because of incorporation of the chromophore into polymer matrices, we have revealed
that the maximal susceptibility is observed at 5-6% of the chromophore in weighting units. The further
increase of the second-order susceptibility is limited by the appearance of chromophore agglomerates
substantially restraining second-order susceptibilities. The investigated composites possess long-lived ø(2)
grating, which decreases less than 76% after 600 min of laser treatment. The electrostatic field causes additional
reanimation of the second harmonic generation during the decay regime. The electron microscopy pictures
clearly confirm appearance of the ø(2) grating.
I. Introduction
of space field distribution. But the absolute value of the induced
electric strength does not exceed 1 V/cm, which is too small
to align defect states. So several authors6,7 decided to use
conception of charge density asymmetry caused by asymmetric
photoionization due to multiphoton excitation. In our opinion,
a more appropriate explanation may be the idea of self-
organization for the transition dipole moments proposed by
Antonyuk et al.8,9 Particularly, a model of charge transfer
excitons was introduced. In ref 10, it was shown that a
substantial role of the photoinduced quasiphonons substantially
rescaled the observed phenomenon. Particularly, the photo-
induced piezoelectric and electrostricted quasiphonons may
play a crucial role for organic chromophores incorporated within
the polymer matrices.11,12
The second harmonic generation (SHG) described by third-
rank polar tensors is generally forbidden by symmetry in
randomly disordered media. However, in 1980, Fujii et al.
showed1 the possibility for weak SHG in the disordered media.
In ref 2, the effect of breaking the randomly oriented cen-
trosymmetry is slightly understandable for the near-the-surface
states;3 however, it is difficult to expect an occurrence of the
bulk noncentrosymmetry, which is a necessary attribute of the
ø
(2) grating. The first disordered optical materials in which SHG
was observed were optical fibers. However, physical insight of
the observed effect was not so simple to explain.4,5 There
appeared several theoretical approaches, particularly in ref 4; it
was shown that there occurs static polarization (see eq 1) and
the associated static electric field might be described by eq 2
From this point of view, chromophores possessing transition
metals may be of particular interest because they possess
relatively localized d states. Among the transition metals, Ru
and Cr derivatives may be of particular interest.
So in the present work we will explore ø(2) grating and
associated second-order optical effects in Ru-alkynyl com-
plexes incorporated within the polymer polymethyl methacrylate
(PMMA) matrices. In section 2, main details devoted to sample
preparation are given. Section 3 is devoted to measurement setup
Pi0 ) øijkl3EjEkEl
(1)
(2)
Ei0 ) 4πPi0
where the particular parameters correspond to the static electric
field induced by effective optical field.
Afterward, it was assumed that this field aligns defects
originating from trapping levels and finally creates recording
10.1021/jp048794h CCC: $27.50 © 2004 American Chemical Society
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ø(2) Grating in Ru Derivative Chromophores
J. Phys. Chem. B, Vol. 108, No. 39, 2004 14943
and aspects of the phenomenon under observation. Section 4
presents main experimental results including the time kinetics
of phenomenon and superposition of external field.
The chemical structure of studied compounds and their UV
spectra are presented in Figure 1c.
3. Optical Poling of Ru-Derivative Chromophores
2. Experimental Details
The optical poling has substantial advantages compared to
electric-field or corona poling consisting of satisfying phase-
matching conditions without the necessity of using electrodes.
Contrary to the azo dye chromophores,15 the investigated
composites do not break their structure due to orientation
relaxation. At the same time, they possess low mobility of the
carriers due to the presence of the d states.
So we try using the guest-host technique similar to that
described in ref 16 both to improve the stability of the grating
process and to enhance its efficiency, which should be mani-
fested through the decrease of time during which the optical
poling process achieves its saturation, determined by corre-
sponding steady-state processes. Simultaneously, we search
materials possessing a higher time of ø(2) grating decay.
The optical poling method consists of two stages: at the first
stage, a composite sample is optically treated by the bicolor
coherent light. The ø(2) grating of the DC electric field with the
proper wave vector K ) k2ω - 2kω is created by coherent
interaction of the fundamental and doubled frequency of the
same laser source. The resulting DC field is asymmetric in space
and results in appearance of the static electric field.17 At the
second stage, we continue the grating process by one of them,
particularly by the doubled frequency beam.
Sample Preparation. The syntheses of alkynyl complexes
C1 and C2-4 (see Figure 1) follow the literature procedures
of Dixneuf.13 The preparation of the new alkynes (A1 and A2-
4) follows the procedure outlined in Figure 1. Sonogashira
coupling14 was employed to obtain the trimethylsilyl-protected
alkynes Si1-4 in good yields, which were then reacted in the
standard manner to give the teminal alkynes A1-4. Isolation
of the trimethylsilyl-protected alkyne intermediates Si2-4
offered no advantage in terms of yield and purity. Conversely,
a great advantage in purity and yield of the final product A1
was gained by isolating the trimethylsilyl-protected alkyne Si1
and reacting it with the arene chrome tricarbonyle phosphonate
anion (Horner-Emmons-Wadsworth coupling) prior to the
deprotection step. Reaction of this anion with the terminal
alkynes 1 afforded a lower yield of 3, along with several
unidentified byproducts. These new alkynes were fully char-
acterized by classical methods.
Reaction of [RuCl(dppe)2][TfO] and these terminal alkynes
in dichloromethane at room temperature (r.t.) afforded vinyli-
dene species. These products were not isolated but instead
deprotonated with base in situ (using triethylamine) to afford
the desired alkynyl complex C1-4 in good yield (65%). The
new alkynyl complexes were characterized by liquid secondary
ion mass spectrometry (LSIMS), UV-vis, IR, 1H, 31P, and 13
C
Experimental Details. The optically induced experiment was
similar to that described in ref 9 (see Figure 2). The axial
symmetry beam sequence of the fundamental YAlO3:Er3+ laser
NMR spectroscopy. The LSIMS mass spectra of C2-4 contain
molecular ions with fragmentation proceeding by competitive
loss of chloro and alkynyl ligands to afford the base peak
corresponding to [Ru(dppe)2]. Fragmentation of C1 proceeded
also by loss of the Cr(CO)3 group. The IR spectra contain
characteristic bands assigned to ν(C≡C) close to 2035 (C2-4)
and 2040 cm-1 (C1). The 31P NMR spectra contain singlet
resonances close to 50 ppm, confirming the mutually trans
stereochemistry of the chloro and alkynyl ligands. The IR spectra
of complexes C2-4 contain characteristic bands assigned to
3
(λ ) 1.88 µm Er3+ 3H4 f H6 transitions) and its doubled
harmonic obtained by the KTP crystal that was cut to satisfy
phase-matching conditions were focused inside the sample in
the form of a ring with a diameter of about 1.0 cm. The choosing
of such lasers is caused by a necessity to avoid an overlap
between the fundamental and doubled frequency beams with
the absorption and luminescent bands. The duration of the pulses
was 320 ns, and the pulse energy was varied within 0.3-2.6
mJ during a repetition frequency of about 12 Hz. The funda-
mental beam was converted into the doubled frequency (λ )
0.94 µm) with a conversion efficiency of about 31%. The
diameter of the incident beams at the focal point was 90-110
µm, and the power density at the waist was about 1.1 and 0.29
GW/cm2 by fundamental and doubled harmonics, respectively.
The set of mirrors and lenses allow operation by the beam
parameters, and the shutters switch off several beams at the
desired moment of time. By use of the set of lenses, we varied
the diameters of the beams and the IR filters served for spectral
separation of the 0.94 µm doubled frequency beam from the
fundamental ones. The synchronized photomultipliers worked
in the fast-response regime (about 0.1 ns) and were connected
to the electronic boxcar integrator. The incident angles of the
two beams on the sample’s surface create a small angle (about
2-3°). This fact allowed us to control the grating period within
5-47 µm. The polarizations of the beams were linear and
were directed perpendicularly to the k wave vector of beam
propagation.
1
ν(CdO) close to 1590 cm-1. Their H NMR spectra contain
the characteristic signals from the formyl groups close to 10
ppm. The 13C NMR spectra also contain downfield resonances
at 190 ppm consistent with the formyl moiety. Complex C1, of
low solubility, was degraded gradually in solution. We could
not obtain satisfactory 13C spectra in this series. Meanwhile,
the η6 arene complexation is supported by the upfield shift of
1
the arene proton in the H NMR spectra of these complexes
and confirmed by characteristic infrared bands assigned to
ν(C≡O) close to 1960 and 1900 cm-1
.
The obtained compounds were incorporated into the PMMA
matrix. The investigated samples were prepared as a mixture
of 4 g of PMMA and 0.035 g of chromophore dissolved in 30
mL of chloroform in the argon atmosphere.
This mixture was mixed using a magnetical stirrer, and
afterward they were put in the Alumina tigle. By verification
of the evaporation rate after the 24 h, samples were obtained
under the investigations.
One of the problems of the complexes investigated is their
photodestruction under the light influence. The specially
performed NMR and IR investigations have shown that under
the influence of light, particularly of a power density higher
than 450 MW/cm2, there occurs a partial destruction of the
chromophore; however, in the PMMA matrix, this degradation
does not exceed 7.2%, and most of the molecule participates in
the desirable processes.
During this illumination of composites with coherent bicolor
radiation (during the optical preparing of composites), the
electrostatic field grating E(r) and correspondingly the ø(2)
grating were produced in the space region of the beam’s overlap.
The conversion of the fundamental frequency beam into the
photoinduced SHG output signal takes place on the accumulative
ø(2) grating. The propagation of the photoinduced SHG is in

14944 J. Phys. Chem. B, Vol. 108, No. 39, 2004
Migalska-Zalas et al.
Figure 1. (a) Synthetic procedure to obtain the alkynyl derivatives: (W) [RuCl(dppe)2][TfO] 0.25 mmol, A1 0.30 mmol, CH2Cl2 40 mL, r.t., 20 h;
(Wi) 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) 0.50 mmol, CH2Cl2 40 mL, r.t., 1 h; (Wii) [RuCl(dppe)2][TfO] 0.50 mmol, A2-4 0.60 mmol, CH2-
Cl2 50 mL, r.t., 20 h; (Wii) Et3N 1 mmol, CH2Cl2 20 mL, r.t., 1 h. (b) General access to the terminal alkynes A1-4: (i) PdCl2(PPh3)2, CuI,
trimethylsilylacetylene, THF, Et3N, 50 °C, 24 h; (ii) Bu4NF (solution 1 M in THF), 1 equiv THF, r.t., 1 h; (iii) [Cr(CO)3(η6-C6H5)CH2P(O)-
(OCH3)2], 1.1 equiv NaH, THF, 12 h, 65 °C; (iW) Bu4NF (solution 1 M in THF), 2 equiv THF, r.t., 1 h. (c) UV spectra of the Ru chromophore.

ø(2) Grating in Ru Derivative Chromophores
J. Phys. Chem. B, Vol. 108, No. 39, 2004 14945
between the fundamental and the doubled frequency writing
beam corresponding to maximal output SHG was equal about
30. A deviation from this ratio in both directions does not give
sufficiently good results. The temporary asymmetry may indicate
a substantial role of the nonequilibrium processes, described in
ref 6.
The direct observation of the ø(2) process is given in the phase
contrast electronic microscopic pictures presented in Figure 4.
By comparison of parts a-d of Figure 4 corresponding to
different stages of treatment, one can clearly see an occurrence
of typical ø(2) grating fringence. The rings are most clear at the
times of 10-12 min, which correlates with the grating observed
by the photoinduced PISHG. So one can say that we have shown
a correlation between the ø(2) grating observed by the PISHG
and observed by the direct phase contrast electronic microscopy
method. The estimated coherence length of the process is about
0.12-0.14 mm. The crucial point is that with increasing time
of treatment by the writing beam one can observe more clear
fringence corresponding to occurrence of the ø(2) grating.
Figure 2. Experimental setup for measurements of optical poling.
Another interesting fact consists of the occurrence of ad-
ditional rings, which seem like higher harmonics of the
fundamental grating. This may be a consequence of the
substantial role of the photoinduced polarons determining the
process of the grating. Occurrence of the periodical fringence
may reflect a substantial role of the d states in the chromophore
because for the chromohore without the d states the picture is
not so clear. To the best of our knowledge, it is a first direct
observation of correlation between the optical grating and output
PISHG. It may be a consequence of good photocontrast in the
observed materials.
To study the relaxation stability of the composites, we have
investigated a decay of the output PISHG with time. In Figure
5 are presented the corresponding dependences vs the number
of laser shots in the 125 Hz pulse repetition regime (samples
C4 and C3). One can see that, for the optimal fundamental/
writing beam ratio (about 30), the output SHG (deff) does not
decrease below the 0.82 pm/V (C4) and 0.75 pm/V (C3) with
respect to the initial noncentrosymmetric grating. For the higher
ratios, these decays are substantially higher. So one can conclude
that this ratio plays a substantial role in the observed stability
of the composites.
Figure 3. Time kinetics of the SHG for various ratios of power
densities for fundamental to writing beams: B, 30; C, 42; D, 16.
the direction of propagation of the incident beam with the
doubled frequency (the so-called writing beam). For the
registration of the ø(2) grating itself, we eliminated the incident
(writing beam) 2ω signal at the entrance of the sample for short
periods (about 0.1 s every 2-3 min). Finally we monitored the
peak power ISHG (2ω) on the ø(2) grating using as a detector a
photomultiplier in the remote zone. The output photoinduced
SHG signals were statistically averaged over 50 SHG pulses to
reduce the influence of the SHG signal instability. Indepen-
dently, instability of the source laser was monitored by an
independent fast-time photodiode. The sensitivity of the setup
by laser powers was about 0.12 µW/pulse.
At the same time, direction of the electrical field alignment
is maximally parallel to the directions of the dipole moments.
Because the molecules are prevailingly linearly prolonged, one
can expect that the corresponding directions should be parallel.
The observed process of the photoinduced SHG has traditional
behavior, like in refs 18-21. The efficiency of the photoinduced
SHG increases with phototreatment time and reaches the certain
steady-state value corresponding to maximum. However, there
is a region of the values of I2(2ω) for which the efficiency of
the SHG increases with time in the initial moment, but further,
the signal of the SHG decreases with time and reaches the same
steady-state value which is smaller than the maximum magni-
tude of the photoinduced SHG. This will be the subject of a
separate work in future.
The setup was supplied additionally by a phase contrast
microscopic setup allowing us to monitor the changes of the
phase during the optical poling.
4. Results and Discussion
We have found that a sufficiently good photoinduced SHG
signal was obtained only for the samples C4 and C3. In the
two remaining samples, the output signal was comparable with
the scattering background.
From Figure 3, one can see typical dependence of the
photoinduced SHG for the C4. One can see that the maximal
increase of the output photoinduced SHG is observed during
the first 1-12 min. The obtained output photoinduced SHG,
which is equal to about 10% compared to the fundamental beam
power, is higher than that for the inorganic glasses treated at
the same conditions and only a little less than that in the
azo dye chromophore. The evaluated maximum second-order
susceptibility is equal to about 1.96 pm/V. The optimal ratio
We have also done an additional experiment that consisted
of the study of the influence of a strong external electric field
(up to 104 V/cm) on the optical poling. We have found (see
Figure 6) that after switching off the writing beam (2ω) during
optical decay, application of a strong electric field (at 75 and
162 min) leads to a jump of the photoinduced SHG with the
following decay. Moreover, this effect was observed every time
during the repeating process. This may be explained within the
framework of theory developed for the self-organized systems.5,6

14946 J. Phys. Chem. B, Vol. 108, No. 39, 2004
Migalska-Zalas et al.
Figure 4. (a) The picture of grating after 2 min of optical treatment. The scale is given in µm. (b) The same after 3 min. (c) After 6 min. (d) After
8 min. (e) After 12 min.
Similar dependences were observed in ref 22. Within the frame-
work of this approach, every excitation of the system with
low mobility, to which belongs the considered Ru-derivative
composites, leads to the appearance of nonequilibrium states,
which according to the theory are responsible for the phenom-
enon observed.

ø(2) Grating in Ru Derivative Chromophores
J. Phys. Chem. B, Vol. 108, No. 39, 2004 14947
favoring additional polarization of the excited trapping levels.
The appearance of an additional number of trapping levels
leads to the occurrence of the larger number of the delocalized
states within the forbidden energy gap, which are responsible
for the appearance of the long-lived excitons, determining the
phenomenon.
5. Conclusions
In conclusion, in the present paper, we have shown that in
the Ru-derivative choromophores the effective maximally
achieved second-order optical susceptibility for the wavelength
1.89 µm is about 1.94 pm/V. We have revealed that maximal
increase of the output photoinduced SHG is observed during
the first 1-12 min. The obtained output photoinduced SHG,
which is equal to about 10% compared with the fundamental
beam power, is higher than that for the inorganic glasses treated
at the same conditions and only a little less than in the azo dye
chromophore. The optimal ratio between the fundamental and
doubled frequency writing beam corresponding to maximal
output SHG was equal to about 30. We have shown that for
the optimal fundamental/writing beam ratio (about 30) the output
SHG (deff) does not decrease below 0.82 pm/V (C4) and 0.75
pm/V (C3) with respect to the initial noncentrosymmetric
grating. We have found also that after switching off the writing
beam (2ω) during optical decay, application of a strong electric
field (up to 104 V/cm) leads to a jump of the PISHG with the
following decay.
Figure 5. Time decay of the photoinduced SHG after switching off
the writing beam field for the sample C4 (B) and C3 (C).
Figure 6. Behavior of SHG after application of the strong electrostatic
References and Notes
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