Star-Shaped Azo-Based Dipolar Chromophores: Design, Synthesis, Matrix Compatibility, and Electro-optic Activity

Article abstract of DOI:10.1021/ja039768k

Three new azo-benzene-based push-pull chromophores with dendritic architecture were synthesized as active materials for electro-optic applications. These chromophores were synthesized in six or seven synthetic steps with an overall yield of around 80% per

Full text of DOI:10.1021/ja039768k

Published on Web 01/22/2004
Star-Shaped Azo-Based Dipolar Chromophores: Design,
Synthesis, Matrix Compatibility, and Electro-optic Activity
Padma Gopalan,*,†,§ Howard E. Katz,*,† David J. McGee,‡ Chris Erben,†
Thomas Zielinski,‡ Danielle Bousquet,‡ David Muller,† John Grazul,† and
Ylva Olsson†
Bell Laboratories, Lucent Technologies, 600 Mountain AVenue, Murray Hill, New Jersey 07974,
Department of Physics, Drew UniVersity, Madison, New Jersey 07940, and Materials Science
and Engineering, UniVersity of Wisconsin, Madison, Wisconsin 53706
Received November 24, 2003; E-mail: pgopalan@wisc.edu; hek@lucent.com
Abstract: Three new azo-benzene-based push-pull chromophores with dendritic architecture were
synthesized as active materials for electro-optic applications. These chromophores were synthesized in
six or seven synthetic steps with an overall yield of around 80% per step and high purity. UV-vis
spectroscopy showed significant influence of the transient dipole moment on the observed r33 values. The
chromophores were stable to photochemical oxidation in ambient light and air. The electrical poling conditions
were optimized for each chromophore as the Tg of the composite material varied significantly. The highest
EO coefficient achieved was 22-25 pm/V at 1550 nm wavelength. STEM analysis of the blends enabled
the correlation of the activity of these large chromophores with the blend morphology. An amorphous
polycarbonate host effectively disperses the chromophores in 2-20 nm aggregates in the active materials.
However, macrophase separation into 200-500 nm aggregates was observed in a methacrylate host matrix.
Introduction
One of the most challenging aspects of designing novel EO
chromophores is to balance the need for a high EO coefficient
In recent years, organic and polymeric materials have emerged
as viable alternatives to conventional inorganic crystalline
materials in active photonic components such as electro-optic
modulators. Polymer electro-optic modulators offer a number
of advantages over the currently used lithium niobate modulator.
In addition to the potential processing advantages and lower
cost, the lower dielectric constant and higher electro-optic (EO)
coefficient of organic/polymeric materials result in higher
bandwidth compared to lithium niobate-based modulators.
Recently we have demonstrated an organic EO modulator
architecture with a large bandwidth of 150 GHz with a
detectable terabit signal1 using a simple nonlinear optical
chromophore DR1 in a polymer host. This was based on the
analysis of dielectric constants of various core and cladding
materials to achieve the closest velocity match reported so far
in a modulator configuration.2 In terms of material properties
this modulator architecture allows us in principle to utilize
chromophores that provide an EO coefficient of at least 25-30
pm/V and still achieve Vπ below 1 V. Dalton et al. have recently
demonstrated modulators with Vπ below 1 V by developing
novel polyene-based chromophores (CLD dyes).3,4
with synthetic ease and photochemical stability. At a molecular
level, to achieve high r33 values through high electric field
orientability requires maximizing the dipole moment of the
chromophores, which in turn results in antiparallel aggregation
of the dipoles. Hence the translation of microscopic to macro-
scopic nonlinearity depends on minimization of the electrostatic
interaction between the dipoles.
Dendritic architecture is very effective in site isolation of the
chromophores, hence reducing electrostatic interaction between
the dipoles. With this approach there are dendritic chromophores
reported to have EO coefficients as high as 60 pm/V at 1550
nm wavelength with good alignment stability at 80 °C5 and a
side chain dendrimer NLO polymer with an EO coefficient of
97 pm/V at 1310 nm.6 Many of these chromophores with high
EO coefficients involve 10-12 synthetic steps with overall low
yields and poor photostability in air. Our interest is in
incorporating azo-benzene-based push-pull chromophores in
dendritic architectures as active materials for electro-optic
applications. Compared with polyene- or stilbene-based systems,
the advantages of using azo-benzene-based dendritic chro-
mophores are the simple high-yielding synthetic steps and
increased photostability in air.7a,b Azo-benzene-containing den-
dritic benzyl aryl ethers have been extensively studied by
† Bell Laboratories, Lucent Technologies.
‡ Drew University.
§ University of Wisconsin.
(5) Ma, H.; Chen, B.; Sassa, T.; Dalton, L. R.; Jen, A. K.-Y. J. Am. Chem.
Soc. 2001, 123, 986.
(1) Lee, M.; Katz, H. E.; Erben, C.; Gill, D. M.; Gopalan, P.; Heber, J. D.;
McGee, D. J. Science 2002, 298, 1401.
(6) Luo, J.; Liu, S.; Haller, M.; Liu, L.; Ma, H.; Jen, A. K.-Y. AdV. Mater.
(2) Lee, M.; Mitrofanov, O.; Katz, H. E.; Erben, C. Appl. Phys. Lett. 2002,
81, 1474-1476.
2002, 14, 1763.
(7) (a) Galvan-Gonzalez, A.; Belfield, K. D.; Stegeman, G. I. Appl. Phys. Lett.
2000, 14, 2083. (b) Galvan-Gonzalez, A.; Canva, M.; Stegeman, G. I. Appl.
Phys. Lett. 1999, 21, 3306.
(3) Shi, Y.; et al. Science 2000, 288, 119-122.
(4) Zhang, H.; et al. Appl. Phys. Lett. 2001, 78 (20), 3136-3138.
9
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J. AM. CHEM. SOC. 2004, 126, 1741-1747
1741

A R T I C L E S
Gopalan et al.
Scheme 1. Synthesis of the Tricyanofuran Acceptor Derivative
and the Diazonium Salt
Figure 1. Structure of chromophores A, B, and C.
McGrath et al.,8a mainly to study the photomodulation of
dendrimer properties, and Yokoyama et al., for second-harmonic
generation studies.8b Use of three-dimensional molecules or
dendrimers for control of functionality on nanometer length
scales is one of the fastest growing areas of research in polymer
chemistry.9 Attributes such as spherical shape, increased solubil-
ity, low viscosity, and monodisperse nature of dendritic
molecules have been exploited in nanoelectronics, drug delivery
systems,10 and nonlinear optics.5 We report the first example
of star-shaped azo-benzene-based EO chromophores and our
studies on some of the critical parameters such as blend
morphology, poling conditions, and chromophore loading.
purification since in the subsequent step the diazonium salt of
that dye is isolated from the reaction medium as a pure solid in
good yield. Once the diazonium salt is isolated, coupling in
acetic acid in the presence of sodium acetate at room temperature
with the core amine results in the desired star-shaped chro-
mophore (Scheme 1). As outlined in an earlier publication,11
isolation of the benzenediazonium salts derived from the
(dicyanovinyl)- or in our case (tricyanofuran)anilines avoids the
exposure of the aniline precursors to the reactive byproducts of
the diazotization process. The azo coupling has also been
successfully employed on polymer backbones such as in the
synthesis of methacrylate copolymers with azo dye side chains.
Hence it is not surprising that the isolated diazonium salt could
be efficiently coupled at room temperature in acetic acid to all
three amine functionalities of the core. This is further supported
by lack of significant amount of partially functionalized core
material as seen from TLC of crude product. However the
coupling of diazonium salt 9 was less efficient than the
diazonium salt 7 to the core, resulting in a yield of less than
65% of chromophore C as against 93% of chromophore B.
An alternate synthetic approach is to synthesize a monomeric
chromophore by diazotization of p-aminobenzaldehyde and
coupling to N-(1-bromohexyl)-N-butylaniline or N-(1-hydroxy-
hexyl)-N-butylaniline followed by Knoevenegal reaction with
tricyanofuran. Subsequent coupling of this chromophore to the
1,1,1-tris(4′-hydroxyphenyl)ethane core by Mitsunobu reaction
(using triphenylphosphine/DEAD) led to decomposition of the
chromophore.
Results and Discussions
Synthesis. Our synthetic approach to these new star-shaped
chromophores is to utilize simple high-yielding diazonium salt
chemistry. The three new chromophores A, B, and C are shown
in Figure 1. We synthesize the core starting from 1,1,1-tris(4′-
hydroxyphenyl)ethane by etherification with N-(6-bromohexyl)-
N-butylaniline in two steps (Scheme 2). The length of the
flexible linker connecting the aniline to the core was kept as
six carbons, which allows sufficient flexibility to allow the
alignment of the chromophores in response to an applied field.
Chromophore A is synthesized by a simple coupling of
commercially available 4-nitrophenyldiazonium tetrafluoroborate
salt with the core. For chromophores B and C the acceptor part
of the chromophore is synthesized by reacting either TCF or
its derivative by a simple Knoevenegal reaction with p-
aminobenzaldehyde. The resulting dye does not require any
(8) (a) Li, S.; McGrath, D. V. J. Am. Chem. Soc. 2000, 122, 6795. (b)
Yokoyama, S.; Nakahama, T.; Otomo, A.; Mashiko, S. J. Am. Chem. Soc.
2000, 122, 3174.
(9) Frechet, J. M. J.; Hawker C. J.; Gitsov, I. J. Macromol. Sci. Pure Appl.
Chem. 1996, 10, 1399.
(10) Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S. Chem. ReV. 1999, 11, 3181.
UV-Vis Studies and Evaluation of Dipole Moment of the
Chromophores. The measured dipole moment per azo unit
(11) Schilling, M. L.; Katz, H. E.; Cox, D. I. J. Am. Chem. Soc. 1988, 53, 5538.
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1742 J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004

Star-Shaped Azo-Based Dipolar Chromophores
A R T I C L E S
Scheme 2. Synthesis of the Core of the Chromophore and Subsequent Coupling to the Diazonium Salt to Yield the Star-Shaped
Chromophore
tricyanofuran derivative in C does not lead to a significant
difference in excitation-induced polarization or does not lead
to a difference in the differential stabilization of the ground and
excited states by the two solvents.
Poling Studies and Electro-optic Characterization. We
started with chromophore A, which is the simplest structure for
electro-optic characterization. This chromophore is essentially
Disperse Red-1 (DR-1) except that three DR-1 units are linked
together through an aromatic core. DR1 is known to show
electro-optic activity in both polycarbonate and methacrylate
matrix hosts. It is known that 20 wt % loading of DR-1 in a
methacrylate host results in r33 (at 1550 nm) of 5-6 pm/V.12
This amounts to a number density of 4.8 × 1020 molecules of
DR1 in 1 g of polymer host. Hence we blended 38 wt %
(equivalent to 4.8 × 1020 molecules) of chromophore A in 20
Figure 2. UV-vis spectrum of DR1 and chromophores A, B, and C in
chloroform.
wt % solution of a methacrylate copolymer in cyclopentanone
[PMMA-co-HEMA (9:1) where HEMA is hydroxyethyl meth-
increases significantly from 8.5 D for the chromophore with
acrylate] containing a diisocyanate cross-linking agent. Films
nitro as the acceptor group (chromophore A) to 13.3 D for the
of 4-5 µm thickness were spin coated on ITO glass substrate
tricyanofuran derivative as the acceptor group. This is as
and dried thoroughly in a vacuum oven overnight. These films
expected since the donor group is identical in these chro-
were poled at 130 °C by direct DC poling (120 V/µm), and the
mophores but the strength of the acceptor group increases in
electro-optic coefficient was measured immediately. Measure-
chromophores B and C. This is also consistent with expectations
ment of r33 at 1550 nm showed an activity of less than 1 pm/V.
from monomeric dyes.
The possible reason for low activity could be incompatibility
The absorption spectra for the three star-shaped chromophores
between the host polymer and the large chromophore molecules,
were obtained in a polar solvent (chloroform) and a nonpolar
resulting in phase separation. To confirm this, we changed the
solvent (toluene) to evaluate the solvatochromic effects. The
absorption maxima for the charge-transfer band are reported in
host matrix to amorphous polycarbonate [poly(bisphenol A
carbonate-co-4,4′-(3,3,5-trimethylcyclohexylidene)diphenol)], com-
Table 1. Longer conjugation length in chromophore B and
monly known as APC. Unlike the methacrylate host APC has
chromophore C leads to a red shift in the maximum absorption
an aromatic backbone similar to the chromophore core, which
peak (Figure 2). This is also apparent from the color of the
is likely to result in a more compatible blend. Chromophore A
chromophore itself. Chromophore A is reddish-orange; chro-
was dissolved in 12 wt % solution of APC in cyclopentanone
[4.8 × 1020 chromophore molecules/g of polymer], spin coated
mophores B and C are bluish-violet in color. In C the presence
of a p-methoxyphenyl substituent on the tricyanofuran ring,
instead of the methyl group in B, leads to a red shift of 7 nm.
The energy level difference in the two solvents is approximately
0.08 eV for all the chromophores. This indicates that variation
of the acceptor group from nitro in chromophore A to a
on ITO substrate, and dried overnight. Contact poling at 150
(12) Sohn, J. E.; Singer, K. D.; Kuzyk, M. G.; Holland, W. R.; Katz, H. E.;
Dirk, C. W.; Schilling, M. L.; Comizzoli, R. B. Nonlinear Optical Effects
in Organic Polymers; Messier, J., Ed.; Kluwer Academic Publishers:
Dordrecht, 1989; p 291.
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A R T I C L E S
Gopalan et al.
Table 1. UV-Vis Characterization of the Star-Shaped
months in ambient light, as no significant change (<10%)
occurred in their measured EO values. This is consistent with
the established photostability of azo dyes.7a,b This is in contrast
to polyenic dye samples, which lost most of their activity (and
color) under identical conditions.
Chromophores
dipole
λmax
(in CHCl3)
λmax
(in toluene)
change
(eV)
chromophore
moment (D)a
∆λ
A
B
C
8.5
10.5
13.3
497
585
592
480
563
569
17
22
23
0.088
0.083
0.084
Chromophore C was harder to handle in terms of both puri-
fication and poling than the other star-shaped chromophores,
and only preliminary conclusions can be drawn about it. Up to
a loading density of 46 wt % (4.8 × 1020 molecules/g of poly-
mer) the films could be poled at close to 140 °C. Beyond 46
wt % the contact gold electrodes on the film for poling start to
crack at 130 °C, leading to electrical shorting of the sample.
This indicated that the glass transition temperature Tg of the
composite material containing more that 46 wt % of chro-
mophore was significantly lower. We could not detect Tg by
DSC analysis of the composite blends. This could also be attri-
buted to possible thermal expansion mismatch between the
polymer film and the gold electrode. Hence on the basis of the
conditions under which the samples shorted, an optimal poling
temperature of 95 °C was used. This allowed successful poling
of the two higher concentrations with a number density of 9.6
and 11.9 × 1020 molecules of chromophore C/g of polymer.
As discussed earlier, chromophore C was designed to have more
solubility in the polymer host and reduce the electrostatic inter-
actions due to the bulky p-methoxyphenyl side group at the
acceptor end of the molecule. However, the measured values
were found to be significantly lower than chromophore B. Incor-
poration of compound 5 in the azo-based star chromophore as
the acceptor group instead of compound 4 has the following
effects.
a Measured in dioxane as solvent.
°C resulted in an r33 value of 6-7 pm/V. No significant
relaxation was observed at room temperature, as the r33 value
remained constant at 6-7 pm/V over the period of 3 months.
The quality of the films obtained with APC as matrix was
significantly better than the methacrylate matrix, as evidenced
by a reduced number of pinholes, improved film uniformity,
and more consistent poling conditions. Chromophores B and C
also formed incompatible blends with methacrylate host matrix;
hence all further studies were conducted with APC matrix.
Varying weight % of chromophore B was blended in APC
matrix, and films were spin coated (2-3 µm thick) and dried
overnight at high temperature in a vacuum oven. The chro-
mophore/APC blends were contact poled at 150 °C, and the
electro-optic coefficient was measured immediately. Figure 3
shows the EO coefficients of poled chromophore B/APC films
plotted as a function of number density of the chromophore.
Results on the compound can be summarized as follows.
(a) A maximum r33 value of 22-25 pm/V was achieved at
1550 nm wavelength for a chromophore loading of 43 wt %
(number density of 7.18 × 1020 chromophore molecules/g of
polymer).
(b) Beyond the maximum, r33 drops to lower values. It is
important to point out that at this stage we have a loading of
almost 52 wt % of the chromophore in the polymer host, which
will lead to increased electrostatic interactions. This observation
is in agreement for similar maxima reported in the work by the
Dalton group.13
(1) Some of the samples showed a value of 20 pm/V at 1550
nm immediately after poling, but stabilized in 30 min to a value
of 15 pm/V. The Tg of these blends is significantly depressed,
leading to relaxation of the poled chromophores and hence lower
r33 values.
(c) Chromophore B loading density (4.8 × 1020 molecules/
g) equivalent to 20 wt % of DR-1 gives approximately 2.5 times
the EO coefficient (15 pm/V) compared to DR-1 (6 pm/V).14
The two main factors that contribute toward the r33 are the dipole
moment and the oscillator strength. The higher dipole moment
of chromophore B compared to DR1 clearly would lead to
enhanced orientation during poling. Integration of the area under
the peak in the UV-vis spectrum for similar concentration in
Figure 2 shows that the transition moment or the oscillator
strength of the chromophore B is approximately double that of
DR1, which translates into a higher â and hence explains the
observed values of r33.
(d) Only six high-yielding synthetic steps are involved in the
synthesis of this simple star chromophore, which still provides
an EO coefficient of around 25 pm/V. Substitution of the phenyl
group by a thiophene ring is likely to approximately double
the µâ value, and hence the r33 value is likely to double (i.e.,
close to 40-50 pm/V) with just one additional synthetic step,
by close analogy with previous work.15 At room temperature
the chromophores maintained their activity over a period of 3
(2) Solubility of the chromophore C is much higher than
chromophore B in the host matrix. Up to 65 wt % of chromo-
phore C can be loaded in the polymer host without any
precipitation.
Previous theoretical work on the wavelength dispersion of
the electro-optic coefficient has indicated an enhancement of
r33 as the optical wavelength approaches a material resonance.
For chromophores with absorption resonances in the 480-580
nm range, a simple two-level model predicts a 20-30% increase
in r33 as the optical wavelength decreases from 1550 to 1310
nm.16,17 Experimentally, we have measured a 22% increase in
r33 between 1550 and 1310 nm for chromophore B (which
translates into a EO coefficient of around 30 pm/V at 1310 nm)
and a 47% increase for chromophore A. While r33 is indeed
observed to increase as the optical wavelength approaches a
material resonance, there is only fair agreement with two-level
predictions. This could be due to assumptions inherent in the
model that are clearly not met in practice, such as complete
dipolar alignment, as well as actual variations in molecular
alignment from sample to sample.
Influence of Guest-Host Blend Morphology on the
Electro-optic Activity. The drastic difference in the electro-
(13) Harper, A.; Sun, S.; Dalton, L. R.; Garner, S. M.; Chen, A.; Kalluri, S.;
Steier, W. H.; Robinson, B. H. J. Opt. Soc. Am. B 1998, 15, 329.
(14) Kuzyk, M. G., Dirk, C. W., Eds. Characterization Techniques and
Tabulations for Organic Nonlinear Optical Materials; Marcel Dekker: New
York, 1998; pp 249-252.
(15) Ledoux, I.; Zyss, J.; Barni, E.; Barolo, N.; Diulgheroff, N.; Quagliotto, P.;
Viscardi, G. Synth. Met. 2000, 115, 213.
(16) Mortazavi, M. A.; Knoesen, A.; Kowel, S. T.; Henry, R. A.; Hoover, J.
M.; Lindsay, G. A. Appl. Phys. B 1991, 53, 287.
(17) He, M.; Leslie, T. M.; Sinicropi, J. A.; Garner, S. M.; Reed, L. D. Chem.
Mater. 2002, 14, 4669.
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Star-Shaped Azo-Based Dipolar Chromophores
A R T I C L E S
diameter of the chromophore dispersed in the APC host. These
observations correlate well with the measured r33 values of <1
pm/V for the methacrylate host and 7 pm/V in the APC host.
From the STEM images it can be inferred that for maximizing
the EO activity of the chromophores it is essential to disperse
the chromophores on 2-20 nm length scales and prevent their
aggregation and subsequent macrophase separation. Even in the
best blends, we can see isolated larger aggregates, which are
around 100 nm in diameter. The presence of these larger
aggregates in the chromophore/APC blends suggests that use
of compatabilizing agents for uniform dispersion can further
improve the blend morphology. It should be noted that the free
hydroxy groups in the methacrylate copolymer host are com-
pletely cross-linked by the diisocyante in the final blend, yet
the overall higher polarity of the methacylate host compared to
the APC host could be one of the reasons for the phase
separation.
Figure 3. Measured EO coefficient for chromophores B and C as a function
of chromophore loading.
Conclusions
In this work we have demonstrated a simple high-yielding
synthetic procedure involving three to six steps for the first star-
shaped azo-based electro-optic chromophores with high r33
values. These chromophores could be poled in a polycarbonate
host to achieve an EO coefficient as high as 25 pm/V at 1550
nm. UV-vis spectroscopy shows a significant influence of
transition dipole moments on the observed r33 values. These
values were found to be stable at room temperature for at least
3 months and did not require stringent handling conditions.
STEM studies to correlate blend morphology with the electro-
optic activity indicates that these high molecular weight
chromophores form incompatible blends in methacrylate co-
polymer, whereas they disperse predominantly into 2-20 nm
domains in an amorphous polycarbonate matrix.
Currently we are designing hetereocycles containing azo-
benzene-based dendritic chromophores, which is likely to double
the observed r33 values. We are also investigating the introduc-
tion of cross-linking groups at the acceptor end of the chro-
mophore C, which would impart orientational stability at higher
temperature.
Figure 4. Annular dark field STEM images of chromophore A: (a and b)
in acrylate host; (c and d) in APC host. Scale bar is 0.2 µm in all images.
optic activity of these star-shaped chromophores in different
host matrixes prompted us to examine the morphology of these
blends using ADF-STEM. It is known that chromophore phase
separation is one of the most important factors governing the
activity in a guest-host system. However, to the best of our
knowledge, no systematic study has been conducted to correlate
the blend morphology and the electro-optic activity. We looked
at the simplest system of chromophore A in both methacrylate
and APC matrix. In both blends the chromophore loading was
the same (number density of 4.8 × 1020).
Experimental Section
General Methods of Material Characterization. All the chemicals
were purchased from Aldrich and were used without further purification.
1H NMR was taken on a Bruker 360 MHz with tetramethylsilane as
internal standard. UV-vis spectra were obtained from a HP 8453 UV-
vis system. Elemental analyses were performed by Robertson Microlit
Laboratories, NJ. Thermal analysis was performed under N2 on a Perkin-
Elmer DSC-7 at a heating rate of 10 °C/min. The dipole moment of
the chromophore was obtained by measuring the differential capacitance
as a function of concentration. Measurements using p-dioxane solutions
were performed at 1 kHz, using a high-precision capacitance bridge.
Electron microscopy was performed in a JEOL 2010F-ARP operated
at 200 kV as a scanning transmission electron microscope with a 0.16
nm spot size.18 Samples were prepared for electron microscopy by
microtoming to 50 nm thick sections, which were then placed on ac
support grids. The samples were not stained. The images are recorded
on a Fischione annular dark field (ADF) detector with single-electron
sensitivity.19 The ADF signal produces an incoherent image where the
scattered intensity scales approximately as the density times the atomic
The ADF-STEM images are shown in Figure 4. From Figure
4a and b it is clear that the chromophore macrophase separates
into oval aggregates of 100-200 nm length scales in the
methacrylate host. The bright region is the chromophore and
the dark region is the methacrylate host. These phase-separated
systems would have light-scattering losses that would make them
unusable in an actual device. Figure 4c with the chromophore
in APC host shows bright spots of length scale 2-20 nm, which
is drastically smaller than the oval aggregates seen in the
methacrylate host. Figure 4d is from a different spot of the same
sample, which shows string-like domains a few nanometers in
(18) Muller, D. A.; Grazul, J. J. Electron Microsc. 2001, 50, 219.
(19) Voyles, P. M.; Muller, D. A.; Grazul, J. L.; Citrin, P. H.; Gossmann, H.-J.
L. Nature 2002, 416, 826-829.
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A R T I C L E S
Gopalan et al.
number Z1.7 (close to the asymptotic Rutherford scattering limit of
Z2).19-21 Consequently heavier atoms or higher density regions appear
brighter than lower Z or lower density regions. The incoherent nature
of the imaging means there are no contrast reversals with defocus or
sample thickness, as can occur in phase contrast images.21
and 1,1,1-tris(4′-hydroxyphenyl)ethane (27.5 g, 0.09 mol) was added
to the flask under nitrogen purge. After stirring for 15 min potassium
hydroxide pellets (40.0 g, 0.60 mol) were slowly added to the flask.
The mixture was refluxed for 48 h with azeotropic removal of water.
The remaining solvent was distilled off, and the resulting light brown
potassium salt was dried in a vacuum oven at 100 °C overnight to
obtain a quantitative yield of the potassium salt. 1H NMR (DMSO-d6):
δ 1.70 (s, 3H), 6.11 (d, 6H), 6.47 (s, 6H).
Electro-optic Characterization. A thin-film ellipsometric reflection
technique14,22 was used to measure the linear electro-optic coefficient.
Polymer blends were first spun-cast into 3-5 µm thick films on indium
tin oxide coated glass substrates. After the films cured, gold electrodes
(500 Å) were thermally evaporated onto the exposed surface, and the
films were contact poled. A poling voltage of 150-200 V/µm was
applied. To measure the electro-optic coefficient, a linearly polarized
laser beam illuminated the film at a 45° angle with respect to the
substrate normal. The beam reflected from the gold electrode and was
directed through a polarizer rotated 90° with respect to the incident
laser polarization. A 29.0 kHz square wave voltage was applied to the
film, which caused a phase shift between the s and p polarization
components of the laser, and effectively changed the polarization state
of the laser beam as it traversed the film. An amplified InGaAs
photodiode and lock-in amplifier were used to detect the modulated
light intensity transmitted through the polarizer. An optical compensator
was used to bias the optical system so that the detected modulated
intensity was proportional to the modulating voltage amplitude.
The electro-optic coefficient r33 is related to the modulated intensity
Im by
1,1,1-Tris(N-hexyloxyphenyl-N-butylaniline)ethane (3). N-(6-Bro-
mohexyl)-N-butylaniline (10.10 g, 0.0323 mol), THPOK (4.14 g, 9.58
mmol), and 50 mL of anhydrous DMSO were place in a 100 mL round-
bottom flask and reacted at 40 °C for 9 h. The reaction mixture was
extracted with dichloromethane and the solvent removed under vacuum.
The residue was column purified on neutral alumina using a 1:1 mixture
of hexane/dichloromethane to obtain 6.90 g (72%) of viscous clear
liquid as product. The excess starting aniline was quantitatively
1
recovered and recycled. H NMR (CDCl3): δ 0.96 (t, 9H), 1.3-1.80
(m, 30H), 2.10 (s, 3H), 3.28 (t, 8H), 3.42 (t, 4H), 3.94 (m, 6H), 6.5-
6.7 (br, 9H), 6.76 (d, 6H), 6.97 (d, 6H), 7.22 (m, 6H). 13C NMR
(CDCl3): δ 157.08, 148.24, 141.74, 129.57, 129.15, 115.25, 113.62,
111.87, 67.74, 50.95, 50.81, 50.60, 30.80, 29.45, 29.36, 27.22, 26.95,
26.04, 20.34, 13.95. IR: γ C-H str 3100, 2930, 2870 (m), CdC str
1590 (m), C-C str 1500 (s), C-N str 1360 (s), C-O str 1240, C-H
def (s) 1470, 861. Anal. Calcd for C68H93N3O3: C, 81.63; H, 9.37; N,
4.20. Found: C, 81.20; H, 9.29; N, 4.05.
Synthesis of 2-Dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-
dihydrofuran (TCF) (4). Under anhydrous conditions 5 g (0.05 mol)
of 3-hydroxy-3-methyl-2-butanone and 6.63 g (0.10 mol) of malono-
nitrile were dissolved in 125 mL of absolute ethanol in a 500 mL round-
bottom flask equipped with a reflux condenser and Soxhlet extractor
(containing 3 Å molecular sieves). Lithium shot (6 mg, 0.85 mmol)
was added to the above mixture and refluxed for 8 h. Upon cooling to
room temperature crystals of TCF (compound 4) separated out and
were collected and dried in a vacuum oven. The final yield was 5.50
g (60%).251H NMR (CDCl3): δ 1.6 (s, 6H), 2.3 (s, 3H). 13C NMR
(CDCl3): δ 182.21, 175.03, 110.88, 110.25, 108.84, 104.86, 99.58,
58.63, 24.31, 14.03. IR: γ C-H str 3100 (m), CdC str 1590 (m),
C-O str 1150, C-H def (s) 861, CtN 2230 (s).
(n2 - sin2 θ)1/2
Im
3λ
2πVm
r33
)
n2 sin θ2
Imax
where Imax is the amplitude of the maximum transmitted intensity (with
no voltage applied to the film), n is the refractive index, λ is the laser
wavelength, and θ is the angle of incidence (in this case, θ ) 45°). In
a typical measurement, Vm is ramped from 0 to 10 V in 1 V increments.
At each voltage step, multiple measurements of Im were averaged over
an appropriate time interval. The coefficient r33 is then obtained from
the slope of Im versus Vm. Electro-optic coefficients were measured at
1310 and 1550 nm using wavelength- and power-stabilized solid state
lasers with an average output power of 3.0 mW. It should be noted
that the expression for r33 assumes that r33 ) 3r13, the validity of which
can vary according to the poling orientation distribution.14 Despite this,
the ellipsometric reflection technique is an efficient way to compare
r33 for different polymer blends and is used in a number of recent
electro-optic studies.23,24
Synthesis of N-(6-Bromohexyl)-N-butylaniline (1). To a 200 mL
round-bottom flask fitted with a reflux condenser N-butylaniline (3.0
g, 0.02 mol) and 1,6-dibromohexane (38.0 g, 0.16 mol) were added.
This mixture was refluxed for 36 h. The reaction mixture was cooled
and extracted with dichloromethane and washed sequentially with a
saturated solution of sodium bicarbonate, distilled water, and saturated
sodium chloride solution. The dichloromethane extract was dried over
MgSO4 and concentrated by rotary evaporation. The crude product was
purified by silica gel chromatography using 3:1 hexane/dichloromethane
Synthesis of 2-Dicyanomethylene-3-cyano-4,5-dimethyl-5-(p-
methoxyphenyl)-2,5-dihydrofuran (5). This compound was synthe-
sized following the general procedure outlined by the Corning group.17
4-Methoxyacetophenone. 4-Hydroxyacetophenone (5 g, 0.037 mol),
methyl iodide (18.9 mL, 0.29 mol), and potassium carbonate (25.4 g,
0.18 mol) were dissolved in 150 mL of acetone and refluxed overnight.
The reaction mixture was filtered to remove the potassium salt, and
the filtrate was evaporated to dryness. The crude product was purified
by a silica gel column using dichloromethane as solvent to obtain 5.51
1
g (98%) of light brown solid. H NMR (CDCl3): δ 2.49 (s, 3H), 3.8
(s, 3H), 6.88 (d, 2H), 7.86 (d, 2H).
1-Hydroxy-1-methyl-1-(4′-methoxyphenyl)-2-propanone. Ethyl
vinyl ether (3.2 mL, 0.44m) was dissolved in anhydrous THF (10 mL)
in a round-bottom flask fitted with a reflux condenser. The solution
was cooled to -78 °C, and tert-butyllithium (26 mL, 0.04 mol) solution
in n-pentane was added gradually using a double-tipped needle under
argon. The solution was allowed to warm to -10 °C. p-Methoxyac-
etophenone (3 g, 0.02 mol) was dissolved in 2 mL of THF and added
dropwise to the above solution. The reaction mixture was stirred
overnight at room temperature. A 20:60:20 mixture of HCl/MeOH/
H2O was added to the solution to acidify to pH 5. The acidic solution
was stirred for 2 h at room temperature followed by concentration under
vacuum to remove THF. The ketol was extracted with ethyl ether, and
the organic layer was washed with a saturated solution of NaHCO3,
brine, and water followed by drying over MgSO4. The R-ketol obtained
by evaporation of solvent was purified by a silica gel column using
1
as eluent to obtain 5.0 g (80%) of product as pale yellow liquid. H
NMR (CDCl3): δ 0.96 (t, 3H), 1.35-1.59 (m, 10H), 1.88 (m,2H), 3.26
(t, 4H), 6.66 (d, 3H), 7.29 (t, 2H). 13C NMR (CDCl3): δ 148.27, 129.21,
115.41, 111.99, 50.90, 33.69, 32.82, 33.67, 32.82, 29.51, 28.11, 27.19,
26.39, 20.41, 14.03. IR: γ CdO(s) 1717, CtN 2211.
Potassium Salt of 1,1,1-Tris(4′-hydroxyphenyl)ethane (THPOK)
(2). A 1 L three-necked round-bottom flask containing a stir bar was
fitted with a Dean-Stark trap topped with a reflux condenser fitted
with a guard tube. A mixture of 225 mL of DMSO, toluene (75 mL),
(20) Crewe, A. V.; Wall, J.; Langmore, J. Science 1970, 168, 1338.
(21) Kirkland, E. J.; Loane, R. F.; Silcox, J. Ultramicroscopy 1987, 23, 77.
(22) Teng, C. C.; Mann, H. T. Appl. Phys. Lett. 1990, 56, 1734.
(23) Khanarian, G.; Sounik, J.; Allen, D.; Shu, S. F.; Walton, C.; Goldberg, H.;
Stamatoff, J. B. J. Opt. Soc. Am. B 1996, 13, 1927.
(24) Sansone, M. J.; Khanarian, G.; Kwaitek, M. J. Appl. Phys. 1994, 75, 1715.
(25) This procedure was kindly provided by the Dalton group.
9
1746 J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004

Star-Shaped Azo-Based Dipolar Chromophores
A R T I C L E S
dichloromethane as solvent. The final yield was 3.97 g (50%). 1H NMR
(CDCl3): δ 1.74 (s, 3H), 2.03 (s, 3H), 3.81 (s, 3H), 4.48 (s, 1H), 6.87
(d, 2H), 7.32 (d, 2H).
MgSO4 and the solvent evaporated under vacuum. The crude solid was
purified by a silica gel column using a 1:1 mixture of dichloromethane/
ethyl acetate as solvent to obtain 0.42 g (93%) of chromophore B as a
blue, tacky solid. The analytical sample was treated with an additional
excess of diazonium salt and sodium acetate and purified using up to
5% ethyl acetate in dichloromethane as eluant on a silica gel column.
1H NMR (CDCl3): δ 0.96 (m, 9H), 1.25-1.75 (br, 36H), 1.87 (s, 18H),
2.08 (s, 3H), 3.20-3.30 (br, 8H), 3.3-3.5 (br, 8H), 3.94 (m, 6H), 6.59
(d, 6H), 6.64 (d, 4H), 6.75 (d, 6H), 6.98 (d, 6H), 7.08 (d, 3H), 7.16 (t,
6H), 7.64 (d, 3H), 7.73 (d, 4H), 7.86 (t, 6H). 13C NMR (CDCl3): δ
175.11, 173.24, 157.1, 148.24, 146.46, 143.47, 141.71, 133.83, 130.82,
130.06, 129.57, 126.18, 123.18, 115.25, 114.62, 113.62, 111.89, 111.45,
110.86, 110.19, 99.71, 97.46, 67.75, 57.72, 56.50, 50.93, 50.79, 50.60,
31.61, 30.79, 29.42, 29.34, 29.34, 27.36, 27.20, 26.93, 20.33, 13.95.
IR: γ C-H str 3100, 2930, 2298 (m), CtN 2220 (s), CdC str 1590
(m), C-C str 1510 (s), C-N str 1360 (s), C-O str 1240, 1180, 1130
C-H def (s), 1470, 834, 745. Anal. Calcd for C122H126N18O6: C, 75.51;
H, 6.54; N, 12.99. Found: C, 75.27; H, 6.48; N, 12.77.
Synthesis of compound 5 was carried out by an identical procedure
as TCF using 1.91 g (0.011 mol) of the above R-ketol, 1.45 g (0.02
mol) of malononitrile, and 0.6 mL of a 1 M solution of lithium in
ethanol. The crude product was purified by a silica gel column using
dichloromethane as solvent to obtain 1.8 g (80%) of a yellow solid.
An analytical sample was triturated with methanol and dried under
1
vacuum at 100 °C. H NMR (CDCl3): δ 1.92 (s, 3H), 2.19 (s, 3H),
3.81 (s, 3H), 6.94 (d, 2H), 7.11 (d, 2H). 13C NMR (CDCl3): δ 182.17,
175.46, 161.18, 126.71, 125.43, 114.98, 110.79, 110.28, 108.98, 104.70,
101.54, 58.88, 55.47, 22.27,14.41. IR: γ C-H str 3100, 3030 (m),
CtN 2220(s), CdC str 1590, 1510 (m), C-O str 1250, 1190, C-H
def (s) 832. Anal. Calcd for C17H13N3O2: C, 70.09; H, 4.50; N, 14.42.
Found: C, 69.98; H, 4.29; N, 14.17.
1,1,1-Tris[N-hexyloxy-N-butyl-4-(4-nitrophenylazo)phenylamino]-
ethane (Chromophore A). In a 100 mL flask 1 g (9.99 mmol) of 3
was dissolved in 40 mL of acetic acid at room temperature. This solution
was cooled to 5 °C, and 0.75 g (3.19 mmol) of 4-nitrophenyldiazonium
tetrafluoroborate was added followed by addition of 0.52 g (6.31 mmol)
of sodium acetate in two portions. The reaction was allowed to proceed
at room temperature overnight and diluted with dichloromethane. The
crude product was obtained by washing the organic layer with a
saturated solution of NaHCO3 and water followed by removal of solvent
under vacuum. The red residue was purified by silica gel column
chromatography using a 1:1 mixture of dichloromethane/ethyl acetate
to obtain 1.45 g (90%) of a red product. 1H NMR (CDCl3): δ 0.96 (m,
9H), 1.25-1.75 (br, 36H), 2.08 (s, 3H), 3.20-3.41 (br, 12H), 3.94 (m,
6H), 6.59 (d, 6H), 6.75 (d, 6H), 6.98 (d, 6H), 7.84 (m, 12H), 8.28(d,
6H). 13C NMR (CDCl3): δ 157.03, 151.65, 147.26, 143.39, 141.76,
129.59, 126.31, 124.62, 122.48, 113.60, 111.25, 67.62, 58.89, 51.14,
50.60, 30.81, 29.49, 29.29, 27.32, 26.81, 26.01, 20.24, 13.88. IR: γ
C-H str 3100, 2930, 2870 (m), CdC str 1590 (m), NdO str 1530,
1310 (s), C-C str 1500 (s), C-N str 1360 (s), C-O str 1240, C-H
def (s) 1470, 861. Anal. Calcd for C68H66N12O9: C, 68.33; H, 5.57; N,
14.06. Found: C, 68.21; H, 5.44; N, 13.92.
Chromophore (B): 3-Cyano-2-dicyanomethylidene-4-{trans-p-
aminostyryl}-5,5-dimethyl-2,5-dihydrofuran (6). In a round-bottom
flask fitted with a reflux condenser, p-aminobenzaldehyde (1 g, 8.26
mmol) and TCF (compound 4) (1.93 g, 9.9 mmol) were dissolved in
25 mL of absolute ethanol. NaOH (16.60 mg) was added to this mixture
and refluxed for 14 h. The deep violet solution was extracted with
dichloromethane from water, and the organic layer was dried with
MgSO4. The yield of the crude product (6) was quantitative and used
without further purification in the next step.
Chromophore C was synthesized by a procedure similar to that
for chromophore B.
Synthesis of 3-Cyano-2-dicyanomethylidene-4-{trans-p-aminostyr-
yl}-5-methyl-5-(p-methoxyphenyl)-2,5-dihydrofuran (8). This com-
pound was synthesized by a procedure similar to thar for 4. Typically
2.0 g (6.86 mmol) of compound 5, 0.80 g (6.60 mmol) of p-
aminobenzaldehyde, and 30 mg of NaOH were dissolved in 40 mL of
absolute ethanol and refluxed for 24 h. Workup was similar to
compound 4 to obtain 1.68 g (65%) of crude product.
The diazonium salt was synthesized by following a procedure similar
to that for compound 6 from 1.0 g (2.53 mmol) of 8, 0.9 mL of
concentrated HCl, 0.18 g (2.53 mmol) of NaNO2, and 0.42 g (2.49
mmol) of NaPF6. This gave 1.01 g (72%) of diazonium salt 9. 1H NMR
(CD3CN): δ 1.80 (s, 3H), 7.3 (d, 1H), 7.7 (d, 1H), 8.2 (d, 2H), 8.5 (d,
2H). Anal. Calcd for C24H16N5O2PF6: C, 52.28; H, 2.92; N, 12.76.
Found: C, 53.09; H, 2.89; N, 12.12.
Compound 3 (0.44 g, 0.44 mmol) was dissolved in 10 mL of AcOH
at room temperature, and 0.80 g (1.45 mmol) of the diazonium salt 9
was added to the above solution. NaOAc (0.24 g, 2.90 mmol) was
added in two portions and the mixture stirred overnight. The dark blue
solution was extracted with dichloromethane and washed with saturated
NaHCO3 solution and water. The organic layer was dried over MgSO4
and the solvent evaporated under vacuum. The crude solid was purified
by a silica gel column using a 1:1 mixture of dichloromethane/ethyl
acetate as solvent to obtain 0.60 g (62%) of chromophore C as a blue
1
solid. H NMR (CDCl3): δ 0.96 (m, 9H), 1.25-1.75 (br, 36H), 1.97
(s, 9H), 2.07 (s, 3H), 2.19 (s, 9H), 3.23 (br, 6H), 3.36 (br, 6H), 3.82 (s,
9H), 3.90 (m, 6H), 6.55 (d, 4H), 6.65 (d, 4H), 6.75 (d, 6H), 6.95 (m,
10H), 7.03 (d, 3H), 7.11 (d, 6H), 7.18 (m, 6H), 7.28 (m, 6H), 7.51 (d,
3H), 7.91 (t, 6H). 13C NMR (CDCl3): δ 182.19, 175.48, 161.19, 157.07,
148.24, 141.68, 139.56, 130.08, 129.56, 129.17, 127.65, 126.71, 126.71
126.10, 125.45, 123.03, 114.99, 113.61, 111.89, 111.33, 110.81, 110.29,
108.99, 104.71, 101.55, 99.33, 67.74, 58.87, 55.47, 51.02, 50.94, 50.60,
29.31, 26.90, 25.99, 24.55, 22.27, 20.29, 14.40, 13.92. IR: γ C-H str
2950, 2890, 2298 (br), CtN 2230(s), CdC str 1600 (m), C-C str
1510 (s), C-N str 1370 (s), C-O str 1230, 1192, 1130 C-H def (s),
915, 849, 730. Anal. Calcd for C140H138N18O9: C, 75.86; H, 6.27; N,
11.37. Found: C, 74.59; H, 6.11; N, 10.80.
Compound 6 (0.5 g,1.65 mmol) was dissolved in 5 mL of AcOH
cooled to 10 °C. Concentrated HCl (0.46 mL) was added and stirred
for 15 min. NaNO2 (0.14 g, 2.02 mmol) dissolved in a minimum amount
of water was added dropwise and stirred at room temperature for 30
min. NaPF6 (0.20 g) was added slowly and stirred for an additional 30
min. The light yellow diazonium salt 7 was filtered from the solution
1
and dried overnight in a vacuum oven at room temperature. H NMR
(CD3CN): δ 1.80 (s, 3H), 7.3 (d, 1H), 7.7 (d, 1H), 8.2 (d, 2H), 8.5 (d,
2H). Anal. Calcd for C18H12N5OPF6: C, 47.07; H, 2.63; N, 15.24.
Found: C, 47.86; H, 2.55; N, 14.82.
Compound 3 (0.23 g, 0.23 mmol) was dissolved in 5 mL of AcOH
at room temperature, and 0.34 g (0.76 mmol) of the diazonium salt 7
was added to the above solution. A 0.13 g (1.52 mmol) sample of
NaOAc was added in two portions and the mixture stirred overnight.
The dark blue solution was extracted with dichloromethane and washed
with NaHCO3 solution and water. The organic layer was dried over
Acknowledgment. We thank Mark Lee, Douglas Gill, Edwin
Chandross, and Elsa Reichmanis for valuable suggestions. David
J. McGee, Danielle Bousquet, and Tom Zielinsky acknowledge
the support of NSF-RUI grant no. 0103817.
JA039768K
9
J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004 1747