Synthesis and unusual physical behavior of a photorefractive polymer containing tris(bipyridyl)ruthenium(II) complexes as a photosensitizer and exhibiting a low glass-transition temperature

Article abstract of DOI:10.1021/ja9825130

This paper reports the synthesis and characterization of a low-T(g) photorefractive (PR) polymer which contains ionic ruthenium complexes as the photogenerator of charge carriers. It was expected that a combination of the highly efficient photocharge gene

Full text of DOI:10.1021/ja9825130

12860
J. Am. Chem. Soc. 1998, 120, 12860-12868
Synthesis and Unusual Physical Behavior of a Photorefractive
Polymer Containing Tris(bipyridyl)ruthenium(II) Complexes as a
Photosensitizer and Exhibiting a Low Glass-Transition Temperature
Qing Wang, Liming Wang, and Luping Yu*
Contribution from the Department of Chemistry and The James Franck Institute,
The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
ReceiVed July 16, 1998
Abstract: This paper reports the synthesis and characterization of a low-Tg photorefractive (PR) polymer which
contains ionic ruthenium complexes as the photogenerator of charge carriers. It was expected that a combination
of the highly efficient photocharge generation of the Ru complex, the efficient charge transporting of the
conjugated backbone, and the orientational enhancement associated with low-Tg materials would lead to further
improvement of PR performances. However, it was very interesting to find that the PR performance of this
polymer was limited by the local internal field formed in response to the external electric field by the alignment
of ion pairs around the photocharge generation sites. The effects of such a local field on photocharge generation
and photorefractivity were investigated through the analyses of the dependence of photocurrent, photorefractive
gain, diffraction efficiency, electrooptic activity, and birefringence on the applied field. The dynamic behavior
of the PR grating formation was also investigated.
Polymeric photorefractive (PR) materials have been the
“orientational enhancement effect”,7 in which NLO chro-
mophores can be reoriented under the influence of the space
charge field at ambient temperature. This effect leads to a spatial
modulation of the birefringence coming from both the optical
anisotropy of chromophore and electrooptic (EO) effect, which
greatly improves the magnitude of the refractive index grating.
In fact, for the low-Tg PR materials, the orientational effect
makes a greater contribution to the photorefractive gain than
does the linear EO effect.8
However, our work mainly focuses on the synthesis and
characterizations of fully functionalized materials, in which all
functional species are covalently attached to a polymer back-
bone. Compared with composite PR polymers, fully function-
alized PR polymers enjoy the advantage of long-term stability
and minimized phase separation. Several systems have been
successfully explored, such as functionalized polyurethanes,9
functionalized conjugated polymers,10 and polyimides containing
porphyrin and NLO chromophore units.11 More recently, we
developed a new PR polymer system, i.e., a hybridized PR
polymer that contains an ionic tris(bipyridyl)ruthenium complex
as the charge-generating species, a conjugated polymer backbone
as the charge transporting channel, and an NLO chro-
mophore.12,13 The ruthenium complex was introduced to utilize
its efficient photoinduced metal-to-ligand charge-transfer (MLCT)
subject of intense research since the first PR polymer system
was disclosed.1 Part of the stimulation for these research efforts
is the wide variety of potential applications of these materials
in image processing and optical data storage.2 These are
multifunctional materials which usually contain the following
functional moieties: charge generating and transporting agents,
charge trapping sites, and second-order nonlinear optical (NLO)
chromophores. Their mechanisms of photorefractivity are very
different from those of the conventional inorganic crystals and
thus render a great challenge to gain deep insight into their
mechanistic aspect.3,4
A popular approach to the preparation of PR polymers is to
mix all of the necessary functional species into polymer
matrices, forming composites.5 For example, large photorefrac-
tivity has been achieved in a PR composite based on poly(N-
vinylcarbazole) (PVK) polymers. Some of the characteristic
parameters have matched or even exceeded those of their
inorganic counterparts, for example, a nearly 100% diffraction
efficiency and 200 cm-1 net optical gain have been obtained.6
A general observation in the composite PR polymeric materials
is that only those systems with low glass transition temperatures
(Tg) (below or slightly above room temperature) give rise to
large net optical gain. This phenomena is referred to as the
(1) Ducharme, S.; Scott, J. C.; Twieg, R. J.; Moerner, W. E. Phys. ReV.
Lett. 1991, 66, 1846.
(7) Moerner, W. E.; Silence, S. M.; Hache, F.; Bjorklund, G. C. J. Opt.
Soc. Am. B 1994, 11, 320.
(8) Moerner, W. E,; Grunnet-Jepsen, A.; Thompson, C. L. Annu. ReV.
Mater. Sci. 1997, 27, 585.
(9) Yu, L. P.; Chan, W. K.; Bao, Z. N.; Cao, S. Macromolecules 1993,
26, 2216.
(10) Chan, W. K.; Chen, Y. M.; Peng, Z. H.; Yu, L. P. J. Am. Chem.
Soc. 1993, 115, 11735.
(11) Peng, Z. H.; Bao, Z. N.; Chen, Y. M.; Yu, L. P. J. Am. Chem. Soc.
1994, 116, 6003.
(2) (a) Gu¨nter, P.; Huignard, J.-P. Ed. PhotorefractiVe Materials and
Their Applications; Springer-Verlag: Berlin, 1989; Vol. 2. (b) Schloter,
S.; Haarer, D. AdV. Mater. 1997, 9, 991. (c) Meerholz, K. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 945. (d) Poga, C.; Lundquist, P. M.; Lee, V.; Shelby,
R. M.; Twieg, R. J.; Burland, D. M. Appl. Phys. Lett. 1996, 69, 1047.
(3) Moerner, W. E.; Silence, S. M. Chem. ReV. 1994, 94, 127.
(4) Yu, L. P.; Chan, W. K.; Peng, Z. H.; Gharavi, A. R. Acc. Chem.
Res. 1996, 29, 13.
(5) Zhang, Y.; Burzynski, R.; Ghosal, S.; Casstevens, M. K. AdV. Mater.
1996, 8, 111.
(12) Peng, Z. H.; Gharavi, A. R.; Yu, L. P. Appl. Phys. Lett. 1996, 69,
4002.
(6) Meerholz, K.; Volodin, B. L.; Sandalphon; Kippelen, B.; Peygham-
barian, N. Nature 1994, 371, 497.
(13) Peng, Z. H.; Gharavi, A. R.; Yu, L. P. J. Am. Chem. Soc. 1997,
119, 4622.
10.1021/ja9825130 CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/26/1998

Low-Tg Polymer Containing Ionic Ru Complexes
J. Am. Chem. Soc., Vol. 120, No. 49, 1998 12861
Scheme 1. Synthesis of Monomer A
process so that problems of low quantum yield for the
photogeneration of charge carriers in organic materials can be
addressed. This PR polymer system has displayed greatly
enhanced PR performance; a net optical gain of about 200 cm-1
was obtained. That is one of the largest net optical gains reported
so far. This PR response, however, is mainly due to the linear
EO contribution because the polymer exhibits a high glass
transition temperature (130 °C) and the dipoles of the NLO
chromophores cannot be reoriented in responding to the
periodical space-charge field. We reasoned that, if the Tg of
the polymer was lowered without a large change in the polymer
structure, we may have the chance to further enhance the PR
performance by combining a highly efficient photocharge
generator, an efficient charge transportor, the large EO contribu-
tion, and the orientational enhancement associated with low-Tg
materials. We succeeded in synthesizing such a polymer
containing transition metal complexes and a conjugated system
and exhibiting a Tg of 11 °C. However, detailed physical studies
revealed an interesting phenomenon: at high external electric
field, the PR efficiency of this polymer was hindered by the
local internal field which is induced by ion dipole moment
formed between the Ru(II) complex and its counterion (PF6-).
To our knowledge, this is the first time that such a local field
effect on the photocharge generation and the PR response was
observed. This kind of local field effect on the photocharge
generation efficiency may also be a general issue in other fields.
Any electric field assist charge separation processes, such as
those in solar cells, xerographic layer, may be subject to such
an effect. In this paper, we report the synthesis and physical
characterization of a low-Tg conjugated polymer containing Ru-
(II)-tris(bipyridiyl) complexes and pendant NLO chromophores.
later one.14 Instead of using alkoxy side chains, long alkyl side
chains were introduced into monomers. These side chains help
to increase the solubility and processibility of the resulting
conjugated polymers and to lower the Tg of polymers. Another
advantage of introducing alkyl chain is that the absorption the
poly(p-phenylenevinylene) (PPV) backbone of the resulting
polymer is blue shifted compared to those with alkoxy substi-
tutes. This blue-shift minimizes the absorption overlap between
the π-π* transition of PPV backbone and the MLCT transition
of the Ru complex.13 Thus, charge carriers can be selectively
generated from the Ru complexes center by using a longer
wavelength laser (He-Ne, 632.8 nm).
The polymerization was carried out according to Scheme 3,
using a catalytic system composed of Pd(OAc)2/P(o-tolyl)3/n-
Bu3N (4%/20%/250%, mole percentage vs monomers). The
resulting polymer is soluble in most common organic solvents,
such as THF, chloroform, DMF, etc. GPC measurements in
THF, using polystyrene as a standard, indicated a number-
averaged molecular weight (Mn) of approximately 18 000 with
a polydispersity (PD) of ca. 1.9.
The structural characteristics of polymers were provided by
1H NMR, UV/vis spectroscopy, and elemental analysis. Since
the resulting polymers contains only 1% of the Ru complexes,
1
the H NMR spectrum of the polymer is dominated by the
chemical shift of the chromophore and the PPV backbone.
However, chemical shifts due to the bipyridyl ligand protons
(8.0, 8.4 ppm) still can be observed (Figure 1), indicating the
incorporation of the ruthenium complex into the polymer. Some
small peaks at 5.2 and 5.8 ppm, which are believed to be
introduced by side reactions of the Heck reaction,15 were also
found (about 1-2%). Figure 2 shows the UV/vis spectrum of
the thin film of the polymer (polymer I). The major absorption
band around 380 nm is attributed to the absorption of the PPV
backbone overlapping with that of the chromophore, which is
supported by the similarity of this spectrum with that of the
polymer without Ru complexes (polymer II) as also shown in
Figure 2. For polymer I, there is an absorption tail extending
Results and Discussion
Synthesis and Structural Characterization. The syntheses
of monomers are outlined in the Schemes 1 and 2. 5,5′-
Dimethyl-2,2′-bipyridine (2) was prepared by homocoupling of
2-bromo-5-picoline (1) using a nickel catalyst which was
generated in situ by reduction of NiBr2(PPh3)2 with zinc in the
presence of Et4NI. This approach is advantageous over the
method catalyzed by the Raney nickel because its reaction
condition is mild and reaction yield is normally better than the
(14) Iyodaa, M.; Otsuka, H.; Sato, K.; Nisato, N.; Oda, M. Bull. Chem.
Soc. Jpn. 1990, 63, 80.
(15) Brenda, M.; Greiner, A.; Heitz, W. Makromol. Chem. 1990, 191,
1083.

12862 J. Am. Chem. Soc., Vol. 120, No. 49, 1998
Scheme 2. Synthesis of Monomer B
Wang et al.
Scheme 3. Synthesis of Polymers I and II
beyond 600 nm, which can be assigned to the MLCT absorption
of the ruthenium complexes. This extending tail is the most
interesting feature because it enables us to photoexcite the
polymer mainly through MLCT processes by using a He-Ne
laser (i.e., 632.8 nm).
III) exhibited an Mn of 18 000 with a PD of 1.88 determined
by GPC. The Tg of the polymer was measured by DSC to be
16 °C. The structure of the polymer was studied by elemental
1
analysis, H NMR, and UV/vis absorption spectra. The para-
1
magnetic property of Cu(II) severely broadened the H NMR
spectra of the monomer and the polymer. In the UV/vis spectrum
(in THF), Q-bands between 500 and 600 nm could be clearly
identified. These results indicate that the metalporphyrin moiety
was incorporated into the polymer backbone.
Physical Characterization. A. Photorefractive Gain. To
study the PR properties of the polymer, the first (also the most
important) experiment is the two-beam coupling (2BC) experi-
ment which can determine the PR nature of a material. In this
As we expected, the resulting polymer exhibits a relatively
low Tg of 11 °C, determined by differential scanning calorimetry
(DSC). This low Tg and the good solubility in organic solvents
make this polymer easy to be processed into high optical quality
films of thickness over 100 mm from its solution.
For the purpose of comparison, a neutral conjugated polymer
containing a Cu(II)-porphyrin moiety was synthesized under
similar conditions (Scheme 4). The resulting polymer (polymer

Low-Tg Polymer Containing Ionic Ru Complexes
J. Am. Chem. Soc., Vol. 120, No. 49, 1998 12863
Figure 1. 1H NMR spectrum of Ru-containing polymer.
Scheme 4. Synthesis of Cu-Porphyrin-Containing Polymer (Polymer III)
experiment, two coherent laser beams intersect inside a PR
polymer film and a refractive index grating can be generated.
Since the phase of the refractive index grating is shifted in
comparison to the illumination pattern, the two writing beams
are diffracted into each other’s direction by these very gratings,
accompanied with asymmetric energy exchange. This feature
is cited as the signature of photorefractivity. We performed the
2BC experiment on our polymer film with two p-polarized He-
Ne laser beams (632.8 nm) of equal intensity (2 × 1.6 W/cm2).
The normal of the sample was tilted 52° with respect to the
bisector of the writing beams to provide a projection of the
grating wave vector along the poling axis. With this geometry,
a holographic grating with a spacing of 2.7 µm was created in
the material (the refractive index of the polymer at 632.8 nm is
1.63). With the applied field on, clear asymmetric energy
transfer between the two beams was observed as shown in
Figure 3: one beam gained energy and the other lost energy.
As the electric field was turned off, the beam coupling
disappeared and the intensities of the both beams returned to
their original levels. When the polarity of the applied field was
reversed, the gain and loss beams were also switched, as
expected, which is due to reversal of the dipole orientation.
These experimental results are clear indications that the grating
is due to photorefractive effect and not due to thermal or
absorption grating.16 The 2BC gain coefficient (Γ) was calcu-
lated according to the following equation:2a
1
Γ ) ln
L
1 + R
1 - âR
(
)
(16) Wiederrecht, G. P.; Yoon, B. A.; Wasielewski, M. R. Science 1995,
270, 1794.

12864 J. Am. Chem. Soc., Vol. 120, No. 49, 1998
Wang et al.
Figure 2. Solid UV/vis spectrum of polymers I and II.
Figure 5. Dependence of photoconductive sensitivity (S) and photo-
current (I) on the applied field.
Figure 3. Asymmetric energy transfer at the field of 90 V/µm in the
2BC experiment.
Figure 6. Magnitude of the index modulation versus the applied field.
similar to that of conventional low-Tg composite PR polymers,3,5
while the photocurrent shows the same trend in the field
dependence as that of the 2BC gain coefficient: at a field of
50 V/µm, the photoconductivity became saturated. It seems that
the photoconductive process is the limitation on PR performance.
These results can be attributed to the existence of ionic
ruthenium and the low-Tg nature of the material. The counterion
-
pairs of PF6 and the Ru(II)-tris(bipyridyl) segment form the
ionic dipole moment pointing from Ru(II) to (PF6-)2. The dipole
moment is randomly oriented in the absence of an external field.
When an external electric field is applied to the PR polymer
film, the dipoles of both NLO chromophores and the ionic pairs
are readily aligned. The effect of alignment of NLO chro-
mophore dipoles on the local field is uniform throughout the
film and is limited due to the thermal randomization. The dipole
alignment becomes significant for ionic species in a low-Tg
Figure 4. Photorefractive gain coefficient as a function of the applied
field.
polymer because of the large freedom of local motion of the
where R is the ratio of the intensity modulation (∆Is/Is) and â
is the intensity ratio of the two incident laser beams (Is/Iq).
B. Local Field Effect. It is interesting, however, to observe
that the optical gain coefficient increases with the external field
initially and then levels off when the applied field surpasses
about 50 V/µm, as shown in Figure 4. This behavior is quite
different from those of the low-Tg composite PR materials, in
which the gain coefficient increases nonlinearly with the applied
field. Unlike the high-Tg version of this polymer, no net optical
gain was observed. At the field of 80 V/µm, an optical gain of
26.6 cm-1 was detected, while the absorption coefficient, R, is
-
polymer chains and the high mobility of the ions, PF6
.
Meanwhile, the magnitude of the ionic dipole moment also
increases with the field due to the increase in the distance
between the ion pair and the decrease in the angle between the
two subdipoles formed by the ionic Ru complex center with
-
two PF6 ions which overcomes the repelling force between
-
the two PF6 ions. This magnitude change of the dipole with
the applied field makes the ion-pair field increases superlinearly
in response to the external field. As a result, the higher the
external field applied, the stronger the counter internal field
generated from the ion dipoles, and this counter internal field
partially screens Ru(II)-tris(bipyridyl) complex sites from the
applied external field.
It is well-known that photogeneration of charge carriers in
organic polymers involves a two-step mechanism: the photo-
excitation and dissociation of the bound electron-hole pair.17
28 cm-1
.
To understand this deviation in the behavior of the field
dependence of the 2BC gain coefficient, two necessary elements
for the PR effect, photoconductivity and field-induced birefrin-
gence (which reflects the EO effect of the sample), were
investigated. The photocurrent and birefringence as a function
of the applied field are shown in Figures 5 and 6. The
birefringence exhibits a typical field enhancement behavior
(17) Mylinikov, V. In AdVances in Polymer Science; Spring-Verlag:
Berlin Heidelberg, 1994; Vol. 115.

Low-Tg Polymer Containing Ionic Ru Complexes
J. Am. Chem. Soc., Vol. 120, No. 49, 1998 12865
To assist the separation of the bound pairs, an applied field is
always needed. Therefore, the photogeneration efficiency of
charge carriers is strongly dependent on the local field around
the photocharge generation sites. Since the ruthenium complex
acts as a photocharge generator, the screen effect due to this
ionic dipole field will reduce the photocarrier separation
efficiency and cause the photocurrent to saturate. The photo-
conductive sensitivity (S) could be estimated from the photo-
current (Iph) by the following expression:18
IphL
S )
AVI0
where L is the sample thickness, A is the illumination area, V
is the applied voltage, and I0 is the intensity of illumination of
the beam. As a result of the decrease in photogeneration
efficiency of charge carriers, the photoconductive sensitivity
decreases as the increase of the applied field (Figure 5).
According to the “standard PR theory” developed for
inorganic ferroelectric single crystals, the saturation of optical
gain can possibly be explained by the decrease of the effective
trap density at the high field. But, the decrease in the effective
trapping centers should lead to the increase of the photoconduc-
tive sensitivity with the external field.19 This is clearly in contrast
with our experimental results that the photoconductive sensitivity
decreased dramatically with the increase of the external field
(Figure 5). The most reasonable explanation must be that the
dipole field induced by ion pairs limits charge generation rate
and further the steady-state space-charge field. Unfortunately,
the “standard PR theory” does not predict the relationship of
the space-charge field with the charge generation rate.20 This
theory was obtained under many simplified assumptions,
especially under the condition that no thermal and electric-
induced detrapping occurs after the trapping of the carrier. In
reality, for organic PR materials, especially for those low-Tg
materials, the trapped charge could be more easily detrapped
than that in inorganic PR materials because of the amorphous
nature. If the detrapping rate is large enough, the condition of
space-charge fields being limited by trap density could never
be reached and the space-charge field would be a function of
photoconductivity.21 The PR optical gain therefore became
saturated at high field.
This screen effect was also reflected in degenerate four-wave
mixing (DFWM) experiments. In this experiment, the two
s-polarized beams (632.8 nm, 2 × 1.45 W/cm2) were used as
writing beams and the grating formed in the material was read
out with a weak p-polarized reading beam (632.8 nm, 230 mW/
cm2) counter-propagating to one of the writing beams. The
diffraction efficiency, η, defined as the ratio of the diffracted
to incident reading beam power, was recorded. Because the
diffraction efficiency is determined by the amplitude of the index
grating, a behavior similar to the optical gain is expected if the
index grating is formed mainly due to the photorefractive effect.
As shown in Figure 9, the value of η increased with the applied
field and became saturated at 0.5% at the field of 50 V/µm.
When the pump beam was blocked, the diffracted signal dropped
immediately.
Figure 7. Second-harmonic coefficient, d33, value as a function of the
applied field.
Figure 8. Field dependence of photoconductive sensitivity (S) and
photocurrent (I) of Cu-porphyrin-containing polymer.
than the diameter of the polymer backbone. Therefore, this
internal field induced by ionic dipole moment is a local effect
compared to the variation of the space-charge field. Because
the Ru(II)-tris(bipyridyl) complex is only 1 mol % in the
polymer system, large amounts of the segments with the NLO
chromophore are not influenced by this local internal field.
Therefore, the birefringence due to the alignment of the dipole
moments of the NLO chromophores and Pockel effect should
not exhibit any saturation. This is indeed the case as shown in
Figure 6. Second-harmonic generation (SHG) experiments
indicate a field-dependent behavior of the SHG signal similar
to that of most of the NLO polymers. The NLO chromophores
could be effectively aligned at room temperature by applying
an external field, and the effect of the ionic dipole field was
not observed (Figure 7).
To confirm experimentally that such a local filed effect is
significant in the ionic metal-containing polymer, another novel
low-Tg polymer containing an neutral copper porphyrin as the
photocharge generator has been synthesized (Scheme 4). This
polymer exhibits an absorption coefficient of 23 cm-1 at 632.8
nm. The field dependence of the photocurrent and photoconduc-
tive sensitivity are shown in Figure 8. In contrast to the behavior
of the ionic ruthenium containing polymer, this Cu-porphyrin
polymer displays field enhancement behavior in both photo-
current and photoconductive sensitivity. It is worth pointing out
that the absolute value of the photoconductive sensitivity of the
Cu-porphyrin polymer is about 2 orders of magnitude smaller
than that of polymer containing Ru(II) complexes. This may
be attributed to the less efficient MLCT process in the Cu-
porphyrin polymer and the fast relaxation of the excited states
caused by the unpaired spin in Cu(I) center.
The spacing of the space-charge field in the 2BC experiment
is about 2.7 µm, which is more than 3 orders of magnitude larger
(18) Schildkraut, J. S. Appl. Phys. Lett. 1991, 58, 340.
(19) Pfisher, G.; Mort, J.; Grammatica, S. Phys. ReV. Lett. 1976, 37,
1360.
(20) (a) Valley, G. C.; Klein, M. B. Opt. Eng. 1983, 22, 704. (b)
Schildkraut, J. S.; Cui, Y. J. Appl. Phys. 1992, 72, 5055.
(21) Moharam, M. G.; Gaylord, T. K.; Magnusson, R.; Young, L. J.
Appl. Phys. 1979, 50, 5642.
Although the internal field induced by the ion dipole limits
the PR performance in our low-Tg materials, this limitation

12866 J. Am. Chem. Soc., Vol. 120, No. 49, 1998
Wang et al.
Figure 9. Dependence of the diffraction efficiency on the applied field.
Figure 11. Electric field dependence of the fast component of the
time constants of the grating formation (a) and the slow component of
the time constants of the grating formation (b).
Figure 10. Time dependence of the diffraction signal (E ) 30 V/µm)
probed by DFWM.
the speed of the initial grating formation (Figure 11a). At a field
of 80 V/µm, the initial grating writing time constant is around
0.21 s, which is comparable with that of the fastest known PR
polymers. Such a fast response time may be attributed to large
charge carrier mobility and facile NLO chromophore alignment
in this type of material. It was known that there are many
different channels that contribute to photoconductivity and
photorefractive charge storage.22 Since the charge-transporting
processes in organic amorphous materials are usually very
dispersed, the formation of space-charge field does not bear an
exponential relationship to the photoconductivity. In our poly-
mer, although the screening from the ion pairs limited the
photocharge generation efficiency, the transporting of charge
carriers is not affected since the ion-pair field is still a local
effect compared to the drift length, which is typically in the
order of submicrons. The mobility of the charge carrierssthe
hopping processscan still be greatly enhanced by the applied
field. Therefore, the initial space-charge field could be formed
through certain fast carrier transporting channels without the
limitation by the carrier generation rate. The behavior of the
slow rate is interesting and reflects the effect of the ionic dipole
field on the photogeneration rate of the charge carriers (Figure
11b). The formation speed slows down because of the limitation
on the photocharge generation and makes the slow component
almost independent of the applied field at a high external field
(Figure 11b).
should not present in high-Tg materials since the local motion
of the polymer chain is so small that the ion dipole cannot be
effectively aligned by the applied field at room temperature. In
fact, the photocarrier separation and mobility at zero external
field could be further enhanced by the internal field produced
by the aligned ion pairs and the dipoles of NLO chromophores
in PR polymer films after corona poling at an elevated
temperature. It seems to us that the enhancement of photocon-
ductivity and the stability of the aligned NLO chromophore are
responsible for the extraordinarily large PR optical gain at zero
field in our previously reported PR polymer that contains an
ionic Ru(II)-tris(bipyridyl) complex and possesses a Tg as high
as 130 °C.13
C. Dynamics of Grating Formation. The dynamics of the
holographic grating formation were studied by measuring the
time constants of the grating formation and their electric field
dependence in the DFWM experiment. A typical behavior of
grating formation is illustrated in Figure 10, in which the writing
beams are turned on at time t ) 0. A fast initial rise in the
diffraction signal is observed, and then slows down after
approximately 5 s of writing time. The rapid initial rise accounts
for about 80% of the saturated value of diffraction efficiency.
Quantitative information about the grating growth can be
obtained by an empirical two-exponential fit of the following
form to the data of diffraction intensity:
Conclusions
η(t) ∼ {Escf[1 - exp(-t/τf)] + Escs[1 - exp(-t/τs)]}2
A novel low-Tg PR polymer which contains tris(bipyridyl)-
ruthenium(II) as a photosensitizer has been synthesized. Its PR
properties have been studied through the analyses of the
dependence of photoconductivity, optical gain coefficient,
birefringence, EO activities, and diffraction efficiency on the
where Escf, Escs, τf, and τs are the four fitting parameters. The
fast component of the diffraction efficiency is indicated by the
first term of the equation, while the slow component is
represented by the second term of the equation. Figure 11 shows
the dependence of the initial grating growth rate and the slow
rate on the applied field. High electric field markedly increases
(22) Jone, B. E.; Ducharme, S.; Liphard, M.; Goodnessekera, A.; Takacs,
J. M.; Zhang, L.; Athalye, R. J. Opt. Soc. Am. B 1994, 11, 1064.

Low-Tg Polymer Containing Ionic Ru Complexes
J. Am. Chem. Soc., Vol. 120, No. 49, 1998 12867
was added dropwise into the solution. The resulting mixture was stirred
at room temperature for 2 h and poured into water. The organic layer
was collected, and the aqueous layer was extracted with ether. The
combined organic layer was washed with water and dried over MgSO4.
After removal of the solvent, the crude product was chromatographed
(silica gel, hexane/ethyl acetate (20:1)) to give a colorless solid (3.37
g, 70%). 1H NMR (CDCl3, ppm): δ 0.88 (t, J ) 6.59 Hz, 6H, -CH3),
1.26-1.35 (m, 28H, alkyl protons), 1.58 (m, 4H, alkyl protons), 2.71
(t, J ) 8.03 Hz, 2H, benzyl protons), 2.88 (t, J ) 7.99 Hz, 2H, benzyl
protons), 7.59 (s, 1H, aromatic protons ortho to CHO), 7.75 (s, 1H,
aromatic protons meta to CHO), 10.21 (s, 1H, -CHO).
Compound 8. Sodium hydride (0.37 g, 15.42 mmol) was added to
a solution of compound 7 (5.27 g, 10.29 mmol) in 25 mL of
1,2-dimethoxyethane (DME). The resulting suspension was stirred for
10 min at room temperature. Compound 3 (2.35 g, 5.15 mmol) in DME
(10 mL) was then added dropwise. The mixture was refluxed overnight.
After the solution was cooled to room temperature, water and
dichloromethane were added. The crude product was precipitated out
and separated by filtration. Recrystallization from dichloromethane gave
a bright yellow solid (5.07 g, 84%). 1H NMR (CDCl3, ppm): δ 0.85-
0.89 (m, 12H, -CH3), 1.25-1.55 (m, 56H, alkyl protons), 1.69 (m,
8H, alkyl protons), 2.64-2.72 (m, 8H, benzyl protons), 7.05 (d, J )
16.14 Hz, 2H, vinyl protons), 7.39 (d, J ) 16.12 Hz, 2H, vinyl protons),
7.44 (s, 2H, aromatic protons meta to I), 7.64 (s, 2H, aromatic protons
ortho to I), 7.98 (d, J ) 8.38 Hz, 2H, 4-pyridine protons), 8.43 (d, J )
8.30 Hz, 2H, 3-pyridine protons), 8.79 (s, 2H, 6-pyridine protons). Anal.
Calcd for C66H98N2I2: C, 67.60; H, 8.36; N, 2.39. Found: C, 67.41;
H, 8.30; N, 2.40.
field. It was found that the ionic dipole moments in this low-Tg
ionic PR polymer are easily aligned and generate an internal
field to screen the photocharge generation site from the external
applied field. Such a screen effect limits the photocharge
generation efficiency and PR performance at a high applied field.
Saturation was observed in the field dependence of photocon-
ductivity, PR optical gain coefficient, and diffraction efficiency.
We believe that this investigation of the local field effect on
photocharge generation should be useful enlightenment for
development and optimization of new PR polymers. The above
results indicate that to synthesize high-performance PR poly-
mers, a low-Tg PR polymer containing neutral Ru complexes
should be explored to fully utilize the efficient MLCT process
and orientational enhancement.
Experimental Section
Tetrahydrofuran (THF) and ethyl ether were purified by distillation
over sodium chips and benzophenone. The p-divinylbenzene was
separated from a mixture of p-divinylbenzene and m-divinylbenzene
according to the literature procedure.23 All of the other chemicals were
purchased from Aldrich Chemical Co. and used as received unless
otherwise noted. Compound 3 and monomer C were prepared according
to refs 13 and 24.
Synthesis of Monomers: Compound 2. To the nickel catalyst
prepared from NiBr2(PPh3)2 (14.38 g, 19.35 mmol), zinc (6.33 g, 96.83
mmol), and Et4NI (16.60 g, 64.55 mmol) in THF (90 mL) was added
a solution of 11.1 g (64.5 mmol) of 2-bromo-5-picoline (1) in THF
(40 mL). After being stirred at 50 °C for 20 h, the mixture was poured
into 2 M aqueous ammonia (500 mL), followed by addition benzene
(250 mL) and ethyl acetate (250 mL). A brown cloudy solution was
given. After filtration, the filtrate was extracted with benzene/AcOEt
(1:1). The organic layer was washed with water, dried over anhydrous
MgSO4, and evaporated in vacuo. The residue was separated by flash
chromatography (silica gel, benzene/AcOEt (10:1)) to give 5,5′-
Monomer A. A solution of compound 8 (0.306 g, 0.261 mmol),
cis-dichlorobis(2,2′-bipyridine)ruthenium(II) hydrate (0.126 g, 0.261
mmol), and 25 mL of methoxyethanol was stirred at 140 °C for 4 h.
After being cooled to room temperature, the solution was added into
an (NH4)PF6 (0.425 g, 2.61 mmol) aqueous solution. The solid
precipitated out and was purified by chromatography (silica gel,
1
dichloromethane/methanol (20:1)) (0.274 g, 56%). H NMR (CDCl3,
ppm): δ 0.85 (t, J ) 6.54 Hz, 12H, -CH3), 1.18-1.48 (m, 64H,
aliphatic protons), 2.53-2.64 (m, 8H, benzyl protons), 6.79 (d, J )
16.42 Hz, 2H, vinyl protons), 7.30 (d, J ) 16.42 Hz, 2H, vinyl protons),
7.42 (s, 2H, aromatic protons meta to iodo), 7.46 (m, 4H, aromatic
protons), 7.56 (s, 2H, aromatic protons ortho to iodo), 7.63 (s, 2H,
aromatic protons), 7.76 (dd, J ) 5.10 Hz, 4H, aromatic protons), 7.95
(d, J ) 8.37 Hz, 4H, aromatic protons), 8.38 (d, J ) 8.79 Hz, 2H,
aromatic protons), 8.41 (m, 6H, aromatic protons). Anal. Calcd for
C86H114N6I2P2F12Ru: C, 55.04; H, 6.07; N, 4.48; I, 13.52. Found: C,
55.06; H, 6.09; N, 4.47; I, 13.60.
Compound 9 was obtained from 1.4-dibromobenzene (4) in 78%
yield, following a procedure similar to that described for compound 5.
1H NMR (CDCl3, ppm): δ 0.88 (t, J ) 6.70 Hz, 3H, -CH3), 1.28 (m,
26H, alkyl protons), 1.57 (m, 2H, alkyl protons), 2.55 (t, J ) 7.60 Hz,
2H, benzyl protons), 7.05 (d, J ) 8.30 Hz, 2H, aromatic protons), 7.38
(d, J ) 8.30 Hz, 2H, aromatic protons).
1
dimethyl-2,2′-bipyridine (2) (4.3 g, 73%). H NMR (CDCl3, ppm): δ
2.35 (s, 6H, -CH3), 7.61 (d, J ) 8.08 Hz, 2H, aromatic protons), 8.24
(d, J ) 8.09 Hz, 2H, aromatic protons), 8.49 (s, 2H, aromatic protons).
Compound 5. 1-Bromodecane (22.92 g, 0.1035 mol) in 18 mL of
ether was added to a suspension containing Mg (2.5 g, 0.1028 mol)
and ether (25 mL) at a rate to maintain the refluxing of the reaction
mixture. After the addition was complete, the mixture was further heated
to reflux for half an hour. The solution was then added dropwise into
a mixture containing 1,4-dibromobenzene (4) (11.0 g, 0.0466 mol),
PdCl2(dppf) (0.76 g, 0.93 mmol), and 40 mL of ether. The resulting
mixture was refluxed overnight and then poured into water. After
removal of the catalyst residue (red precipitate) by filtration, the filtrate
was extracted with ether. The combined organic layer was then washed
with water and dried by MgSO4. Evaporation of the solvent gave a
brown oil which was distilled under vacuum, yielding a slight yellow
1
Compound 10. Compound 9 (7.18 g, 18.8 mmol) in 20 mL of THF
was added dropwise into a mixture of Mg (0.55 g, 22.2 mmol), 10 mL
of THF, and a small crystal of iodine at such a rate that the reaction
mixture maintained self-refluxing. After the addition was complete,
the mixture was heated to reflux for half an hour. The resulting Grignard
reagent was transferred by a needle to a mixture containing 1.12-
dibromododecane (9.25 g, 28.18 mmol), Li2CuCl4 (2.8 mL of 0.1 M
THF solution, 28.18 mmol), and 20 mL of THF. The resulting mixture
was stirred overnight at room temperature and then poured into water.
The mixture was extracted with dichloromethane. The combined organic
layer was washed successively with saturated aqueous NaHCO3 solution
and water and dried over MgSO4. The solvent was removed by rotary
evaporation, and the residue was recrystallized from acetone to give a
colorless solid of compound 10 (6.71 g, 65%). 1H NMR (CDCl3,
ppm): δ 0.87 (t, J ) 6.59 Hz, 3H, -CH3), 1.25-1.29 (m, 40H, alkyl
protons), 1.38 (m, 2H, alkyl protons), 1.55 (m, 4H, alkyl protons), 1.85
(m, 2H, alkyl protons), 2.58 (t, J ) 7.65 Hz, 4H, benzyl protons), 3.41
(t, J ) 6.88 Hz, 2H, -CH2Br), 7.08 (s, 4H, aromatic protons).
Compound 11 was obtained from compound 10 in 60% yield,
oil (14.19 g, 85%, bp 213-214 °C at 0.2 mmHg). H NMR (CDCl3,
ppm): δ 0.87 (t, J ) 6.58 Hz, 6H, -CH3), 1.25-1.29 (m, 32H, alkyl
protons), 1.57 (m, 4H, alkyl protons), 2.55 (t, J ) 7.81 Hz, 4H, benzyl
protons), 7.07 (s, 4H, aromatic protons).
Compound 6. A mixture of compound 5 (11.48 g, 0.032 mol), iodine
(10.17 g, 0.04 mol), H5IO6 (3.8 g, 0.017 mol), acetic acid (30 mL),
30% sulfuric acid (15 mL), and chloroform (15 mL) was stirred at 80
°C for 48 h and then poured into water. The crude product was collected
by filtration and washed with water and cold ethanol. Recrystallization
from ethanol/ethyl acetate (6:1) gave a colorless solid (14.66 g, 75%).
1H NMR (CDCl3, ppm) δ 0.88 (t, J ) 6.58 Hz, 6H, -CH3), 1.27-
1.35 (m, 28H, alkyl protons), 1.52-1.57 (m, 4H, alkyl protons), 2.59
(t, J ) 7.81 Hz, 4H, benzyl protons), 7.59 (s, 2H, aromatic protons).
Compound 7. n-BuLi (3.8 mL, 2.5 M solution in hexane, 9.424
mmol) in 15 mL of ether was added dropwise in the 35 mL of an ether
solution of compound 6 (5.75 g, 9.425 mmol) at 0 °C. After the addition
of BuLi was completed, DMF (1.09 mL, 14.13 mmol) in 5 mL of ether
(23) Strey, B. T. J. Polym. Sci., Part A 1965, 3, 265.
(24) Bao, Z.; Chen, Y.; Yu, L. P. Macromolecules 1994, 27, 4629.
1
following a procedure similar to that described for compound 6. H

12868 J. Am. Chem. Soc., Vol. 120, No. 49, 1998
Wang et al.
NMR (CDCl3, ppm): δ 0.87 (t, J ) 6.60 Hz, 3H, -CH3), 1.26-1.32
(m, 40H, alkyl protons), 1.39 (m, 2H, alkyl protons), 1.55 (m, 4H,
alkyl protons), 1.85 (m, 2H, alkyl protons), 2.59 (t, J ) 7.82 Hz, 4H,
benzyl protons), 3.41 (t, J ) 6.85 Hz, 2H, -CH2Br), 7.59 (s, 2H,
aromatic protons).
remove the catalyst residue. The filtrate was concentrated and precipi-
tated into methanol, followed again by filtration and reprecipitation.
The resulting polymer was further purified by extraction in a Soxhlet
extractor with methanol for 24 h and then was dried under a vacuum
at 40 °C for 24 h.
Compound 12. A mixture of compound 11 (4.13 g, 5.15 mmol),
N-ethylaniline (1.30 mL, 10.31 mmol), potassium carbonate (2.14 g,
15.48 mmol), tetrabutylammonium bromide (0.166 g, 0.515 mmol),
and sodium iodine (7 mg, 0.047 mmol) in toluene (12 mL) was refluxed
overnight. The mixture was then poured into water and extracted with
dichloromethane. The organic layer was separated and washed with
water and dried over MgSO4. After the solvent was evaporated, the
residue was purified by flash chromatography (silica gel, hexane/
dichloromethane (2:1)) (3.37 g, 78%). 1H NMR (CDCl3, ppm): δ 0.88
(t, J ) 6.58 Hz, 3H, -CH3), 1.14 (t, J ) 7.04 Hz, 3H, -NCH2CH3),
1.22-1.31 (m, 42H, alkyl protons), 1.54 (m, 6H, alkyl protons), 2.58
(t, J ) 7.83 Hz, 4H, benzyl protons), 3.24 (t, J ) 7.63 Hz, -CH2N-),
3.34 (quintet, J ) 7.04 Hz, 2H, -NCH2CH3), 6.64 (m, 3H, aromatic
protons), 7.20 (t, J ) 7.24 Hz, 2H, aromatic protons), 7.59 (s, 1H,
aromatic protons).
Polymer I. Anal. Calcd for C66.3H95.27N1.05O1.98S0.99P0.02F0.12Ru0.01:
C, 81.79; H, 9.78; N, 1.51; Ru, 0.10. Found: C, 81.07; H, 9.87; N,
1.41; Ru, 0.08.
Polymer II. Anal. Calcd for C66H95NO2S1: C, 82.08; H, 9.84; N,
1.45. Found: C, 81.90; H, 9.92; N, 1.31.
Polymer III. Anal. Calcd for C66.54H15.68N1.04O2SCu0.01: C, 82.04;
H, 9.82; N, 1.49. Found: C, 81.41; H, 9.57; N, 1.44.
Characterization. The 1H NMR spectra were collected on a Bruker
500-MHz FT NMR spectrometer. A Shimadzu UV-2401PC UV/vis
was used to record the UV/vis spectra. Thermal analyses were
performed by using the DSC-10 system from TA Instruments with a
heating rate of 10 °C/min under a nitrogen atmosphere. Elemental
analyses were performed by Atlantic Microlab, Inc., except for the
ruthenium analyses, which were done by Galbraith Laboratories, Inc.
Molecular weights were measured with a Water RI GPC system using
polystyrene as the standard and THF as the eluent.
Compound 13. To a ice-cooled DMF (2 mL, 25.83 mmol) was
added phosphorus oxychloride (0.762 g, 4.96 mmol) dropwise. The
solution was stirred at 0 °C for 1 h and at room temperature for another
1 h. Compound 12 (3.8 g, 4.517 mmol) in 7 mL of DMF was then
added dropwise to the mixture, and the resulting solution was stirred
at 90 °C overnight. The solution was poured into water and extracted
with dichloromethane. The separated organic layer was washed with
water and dried. After the solvent was evaporated, the residue was
chromatographed (silica gel, hexane/ethyl ether (2:1)) to give a colorless
liquid (2.40 g, 61%). 1H NMR (CDCl3, ppm): δ 0.88 (t, J ) 6.53 Hz,
3H, -CH3), 1.18-1.33 (m, 45H, alkyl protons), 1.61 (m, 6H, alkyl
protons), 2.58 (t, J ) 7.73 Hz, 4H, benzyl protons), 3.25 (t, J ) 7.60
Hz, -CH2N-), 3.32 (quintet, J ) 7.01 Hz, 2H, -NCH2CH3), 6.67 (d,
J ) 8.44 Hz, 2H, aromatic protons), 7.59 (s, 1H, aromatic proton),
7.72 (d, J ) 8.44 Hz, 2H, aromatic protons), 9.69 (s, 1H, -CHO).
Monomer B. Sodium hydride (0.145 g, 6.04 mmol) was added to a
solution of compound 13 (2.91 g, 3.348 mmol) in 10 mL of DME.
The suspension was stirred for 10 min at room temperature, followed
by dropwise addition of the solution of diethyl 4-(hexyl sulfone)benzyl
phosphate (1.26 g, 3.348 mmol) in 5 mL of DME. The resulting solution
was stirred at 80 °C overnight and then poured into water. The mixture
was extracted with dichloromethane. The combined organic layer was
washed with water and dried. After removal of the solvent, the residue
was separated by chromatography (silica gel, hexane/ethyl ether (4:1))
to give a greenish yellow solid (1.756 g, 48%). 1H NMR (CDCl3,
ppm): δ 0.87 (m, 6H, -CH3), 1.17-1.69 (b, 59H, alkyl protons), 2.58
(t, J ) 7.63 Hz, 4H, benzyl protons), 3.07 (t, J ) 8.14 Hz, 2H, -SO2-
CH2-), 3.23 (t, J ) 7.31 Hz, 2H, -CH2N-), 3.35 (quintet, J ) 6.85
Hz, 2H, -NCH2CH3), 6.64 (d, J ) 8.82 Hz, 2H, aromatic protons),
6.85 (d, J ) 16.20 Hz, 1H, vinyl proton), 7.12 (d, J ) 16.15 Hz, 1H,
vinyl proton), 7.40 (d, J ) 8.73 Hz, 2H, aromatic protons), 7.59 (m,
4H, aromatic protons), 7.80 (d, J ) 8.40 Hz, 2H, aromatic protons).
Anal. Calcd for C56H87SNI2O2: C, 61.58; H, 7.96; N, 1.28. Found: C,
61.65; H, 7.98; N, 1.23.
The films for PR characterization were made by sandwiching
polymers between two indium-tin oxide (ITO) covered glass substrates.
The thickness of the film was fixed around 104 mm with the help of
polyimide spacers.
The photoconductivity measurements were performed on about 26
mm thick film sandwiched between Au and ITO electrodes at a
wavelength of 632.8 nm using a photocurrent method.25 The photo-
current was measured by monitoring the voltage drop on a resistor
which is in series with the film capacitor.
Second-order NLO properties of polymeric films were characterized
by a second-harmonic generation experiment. A mode-lock Nd:YAG
laser (Continuum-PY 61 C-10, 10-Hz repetition rate) was used as the
light source. The second harmonic of the fundamental wave (1064 nm)
generated by the polymer sample was detected by a photomultiplier
(PMT) and then amplified and averaged in a boxcar integrator. The
d33 value was obtained by assuming d33 ) 3d31 with a quartz crystal as
the reference sample.
The electric field induced birefringence was measured using an
ellipsometric method with a crossed-polarizer geometry on the same
sample as in PR measurements.26 The sample normal was titled at 45°
with respect to the incident light. The polarization of the incident light
was 45° with respect to the incident plane.
Two-beam coupling experiments were performed using a He-Ne
laser (632.8 nm, 30 mW) as the light source. The laser beam
(p-polarized) was split into two beams with equal intensity (2 × 1.6
W/cm2), which were intersected in the polymer film at 24.5°. The
transmitted intensities of the two beams were monitored by two
calibrated diode detectors, and the data were recorded by a computer.
Acknowledgment. This work was supported by the National
Science Foundation and Air Force of Scientific Research.
Support from the National Science Foundation Young Inves-
tigator program is gratefully acknowledged. This work also
benefited from the support of the NSF MERSEC program at
the University of Chicago.
Polymerization. A typical polymerization procedure follows: Tri-
n-butylamine (0.32 mL, 1.34 mmol) was added to the mixture of
monomer A (0.010 g, 0.00540 mmol), monomer B (0.5890 g, 0.5400
mmol), p-divinylbenzene (0.070 g, 0.545 mmol), palladium acetate (4.9
mg, 0.0217 mmol), and tri-o-tolylphosphine (32.9 mg, 0.108 mmol) in
5 mL of DMF. The reaction mixture was stirred at 90 °C overnight
under a nitrogen atmosphere and then poured into methanol. The
precipitate was collected, redissolved in chloroform, and filtered to
JA9825130
(25) Li, L.; Lee, J. Y.; Yang, Y.; Kumar, J.; Tripathy, S. K. Appl. Phys.
B 1991, 53, 279.
(26) Grunnet-Jepsen, A.; Thompson, C. L.; Twieg, R. J.; Monerner, W.
E. J. Opt. Soc. Am. B 1998, 15, 901.