Phenyltetraene-based nonlinear optical chromophores with enhanced chemical stability and electrooptic activity

Article abstract of DOI:10.1021/ol701814r

(Chemical Equation Presented) "Push-pull" phenyltetraene-based chromophores are too sensitive to be incorporated into Diels-Alder-type cross-linkable polymers due to the reactivity of its diene segment with maleimides. A facile synthetic route has been ex

Full text of DOI:10.1021/ol701814r

ORGANIC
LETTERS
2007
Vol. 9, No. 22
4471-4474
Phenyltetraene-Based Nonlinear Optical
Chromophores with Enhanced Chemical
Stability and Electrooptic Activity
Jingdong Luo,† Su Huang,† Yen-Ju Cheng,† Tae-Dong Kim,† Zhengwei Shi,†
Xing-Hua Zhou,† and Alex K.-Y. Jen*,†,‡
Department of Materials Science & Engineering and Department of Chemistry,
UniVersity of Washington, Seattle, Washington 98195
ajen@u.washington.edu
Received July 27, 2007
ABSTRACT
“Push
−pull” phenyltetraene-based chromophores are too sensitive to be incorporated into Diels
−Alder-type cross-linkable polymers due to
the reactivity of its diene segment with maleimides. A facile synthetic route has been explored to incorporate a methoxy group into the R
position of such chromophores, which reduces their diene reactivity during the poling and lattice hardening process. The poled polymers with
one of such chromophores doped in a cross-linked polymer lattice showed ultrahigh electro-optic activities, up to 306 pm/V at 1310 nm.
Recently, the research of organic and polymeric electro-optic
(EO) materials has made tremendous progress. EO coef-
ficients (r33 values) of greater than 300 pm/V have been
demonstrated through controlled molecular self-assembly and
lattice hardening.1 These material properties have been
further translated into very low half-wave voltage (Vπ) in
Mach-Zehnder and phase modulators.2 In addition to their
applications in photonic devices, organic EO materials offer
great potential for integrating with nanotechnologies for new
generation of photonic systems with much smaller size and
lower power consumption.3
conjugated (4-dialkylamino)phenyltetraene bridge with ter-
minal substitueted CF3-TCF acceptors.4 This is a primary
class of high-µâ chromophores with certain degree of
synthetic efficacy. For general uses, it is very critical to
improve chemical stability and maintain all-trans conforma-
tion of these phenyltetraene-based chromophores to optimize
their nonlinear optical (NLO) properties.5
As an example, we have recently developed a new scheme
to synthesize 2-alkyl-isophorone-embeded phenyltetraene
chromophores (AJL24-type, Chart 1).6 The alkylation steri-
cally rigidified the polyenic bridge to ensure its all-trans
conformation, and the end of the alkyl groups can be further
To achieve ultrahigh EO activity, one of the key compo-
nents in these newly developed organic EO materials is the
(3) (a) Hochberg, M.; Baehr-Jones, T.; Wang, G.; Shearn, M.; Harvard,
K.; Luo, J.; Chen, B.; Shi, Z.; Lawson, R.; Sullivan, P.; Jen, A. K. Y.;
Dalton, L.; Scherer, A. Nat. Mater. 2006, 5, 703. (b) Baehr-Jones, T.;
Hochberg, M.; Wang, Guangxi; Lawson, R.; Liao, Y.; Sullivan, P. A.;
Dalton, L.; Jen, A. K.-Y.; Scherer, A. Optics Express 2005, 13, 5216.
(4) Jen, A.; Luo, J.; Kim, T.-D.; Chen, B.; Jang, S.-H.; Kang, J.-W.;
Tucker, N. M.; Hau, S.; Tian, Y.; Ka, J.-W.; Haller, M.; Liao, Y.; Robinson,
B. H.; Dalton, L.; Herman, W. Proc. SPIE Int. Soc. Opt. Eng. 2005, 5935,
49.
(5) (a) Zhang, C.; Wang, C.; Dalton, L. R.; Zhang, H.; Steier, W. H.
Macromolecules 2001, 34, 235. (b) He, M.; Leslie, T.; Garner, S.; DeRosa,
M.; Cites, J. J. Phys. Chem. B 2004, 108, 8731.
(6) Luo, J.; Cheng, Y.-J.; Kim, T.-D.; Hau, S.; Jang, S.-H.; Shi, Z.; Zhou,
X.-H.; Jen, A. K.-Y. Org. Lett. 2006, 8, 1387.
† Department of Materials Science & Engineering.
‡ Department of Chemistry.
(1) (a) Kim, T.-D.; Luo, J.; Ka, J.-W.; Hau, S.; Tian, Y.; Shi, Z.; Tucker,
N. M.; Jang, S.-H.; Kang, J.-W.; Jen, A. K.-Y. AdV. Mater. 2006, 18, 3038.
(b) Kim, T.-D.; Kang, J.-W.; Luo, J.; Jang, S.-H.; Ka, J.-W.; Tucker, N.;
Benedict, J. B.; Dalton, L. R.; Gray, T.; Overney, R. M.; Park, D. H.;
Herman, W. N.; Jen, A. K.-Y. J. Am. Chem. Soc. 2007, 129, 488. (c) Kang,
H.; Facchetti, A.; Jiang, H.; Cariati, E.; Righetto, S.; Ugo, R.; Zuccaccia,
C.; Macchioni, A.; Stern, C. L.; Liu, Z.; Ho, S.-T.; Brown, E. C.; Ratner,
M. A.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 3267.
(2) Enami, Y.; DeRose, C. T.; Mathine, D.; Loychik, C.; Greenlee, C.;
Norwood, R. A.; Kim, T. D.; Luo, J.; Tian, Y.; Jen, A. K.-Y.; Peygham-
barian, N. Nature Photonics 2007, 1, 180.
10.1021/ol701814r CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/29/2007

Chart 1. “Push-Pull” Structure of Phenyltetraene-Based NLO
Chromophores with Strong CF3-TCF Acceptorsa
Scheme 1. Synthesis of the Methoxy-Substituted
Phenyltetraene Chromophore AJL34
a The carbon atoms were labeled from Ca to Ch along the methine
skeleton.
functionalized with dendrons to provide shape modification.
In a guest-host polymer containing one such chromophore,
a very large r33 value of 262 pm/V at 1.31 µm has been
demonstrated. However, this approach has its own design
challenges. In an attempt to incorporate this type of chro-
mophores into Diels-Alder (DA) cross-linkable polymers
for better temporal stability, we observed a considerable
decrease in EO activities.1a,6 Spectroscopic studies on these
new systems revealed that these chromophores tend to react
with dienophiles such as maleimides through cycloaddition,
which lead to decomposition of chromophores and interfer-
ence of DA lattice hardening.7
In general, diene can only react with dienophile in its s-cis
conformation through the overlapped p-orbital.8 The above
result revealed that a relatively high diene reactivity exist in
these all-trans phenyltetraene-based compounds. This is not
unusual since the s-trans conformers could rotate through
their σ-bonds easily to form the s-cis conformers. In order
to tackle this problem, it is essential to understand the
conformational rigidity of the methine skeletons, and know
how it can be adjusted by the substitution at the R position.
In this regard, we have introduced a methoxy group to the
R position of typical phenyltetraene π-conjugates to make a
new type of polyene chromophores. The methoxy group can
be viewed as a good electron donor through its resonance
structure, however, it also can be viewed as an inductive
acceptor when compared to the H and sp3-C substituents at
the R position of AJL6- and AJL24-type chromophores,
respectively. Therefore, the methoxy group may create
asymmetry of electron density and introduce energy pertur-
bation to the rotational barrier of the polyene chains. This
could be used to tune the physical properties of chromophores
in an exquisite way which may not be easily achievable by
steric modification. Here, we report the synthesis of this type
of chromophores, and its chemical stability and EO property
in poled polymers during DA lattice hardening.
aminobenzaldehyde 3 was added and condensed with 2 after
refluxing overnight to form the methoxy-substituted amino-
phenyldienone 4. Then two hydroxyl groups on the donor-
end of 4 were protected by tert-butyldimethylsilyl group to
afford compound 5. The dienone 5 could be converted to
the key intermediate, trienal 7, in good yield by a three-step
route in the sequence of nucleophilic addition, dehydration,
and reduction. This route has been previously explored to
synthesize the alkylated tienal analogues.6 Finally, compound
8, a strong CF3-TCF-type acceptor, was condensed with 7
to afford compound AJL34 as a new phenyltetraene-based
chromophore with the methoxy group substituted at the R
position.
The chromophore AJL34 was obtained as a highly
amorphous organic solid with its glass transition temperature
around 70 °C. It is soluble in common solvents such as
methylene chloride, acetone, THF, hexane, and methanol.
However, AJLS102 and YLD124 (Chart 2), which are
Chart 2. Selected “Push-Pull” Phenyltetraene-Based
Chromophores for the Comparison Study of This Research
As shown in Scheme 1, 4-bromoisophorone 1 could be
converted to 2-methoxyisophorone 2 in quantitative yield by
treating with excess amount of sodium methoxide through
the allylic rearrangement.9 To the same reaction pot, dialkyl-
(7) Shi, Z.; Luo, J.; Tian, Y.; Cheng, Y.-J.; Kim, T.-D.; Huang, S.; Jen,
A. K.-Y. Chem. Commun., submitted for publication.
(8) Dienes in Diels-Alder Reactions; Fringuelli, F., Taticchi, A., Eds.;
John Wiley & Sons, Inc.: New York, 1990.
(9) Tsuboi, S.; Kurihara, Y.; Watanabe, T.; Takeda, A. Synth. Commun.
1987, 17, 773.
similar AJL6-type chromophores without the methoxy
modification, are highly crystalline and could not be dis-
solved in hexane or methanol. These comparisons indicate
4472
Org. Lett., Vol. 9, No. 22, 2007

that the modification of methoxy group on AJL34 can
prevent close packing of molecules, which in turn, may
attenuate the strong dipole-dipole electrostatic interactions
bewteen chromophores to improve poling efficiency of EO
polymers.
The UV-vis-NIR absorption spectra of AJL34 are
essentially the same as those for AJLS102 and YLD124 in
organic solvents or in polymeric thin films. The λmax of the
π-π* charge-transfer bands all have similar absorption
wavelengths. With the same donor/acceptor pair, AJL34
showed a slightly greater solvatochromic shift than that of
YLD124. These linear absorption data suggest that the
addition of methoxy group to the R position does not
interfere the efficient charge-transfer property of phenyl-
tetraene-based chromophores.
protocol to three phenyltetraene-based chromophores, namely
AJL24, AJL28, and AJL34, respectively. After the samples
were annealed for 1 h, both AJL24 and AJL28 chro-
mophores were almost totally decomposed, while less than
20% decomposition was observed for AJL34. This result
suggests significant difference in their diene reactivity toward
NPM. Its reactivity can be ranked by the following order,
AJL6-type ≈ AJL24-type . AJL34.
The dienic reactivity of chromophores was further evalu-
ated using the typical processing procedures for EO poly-
mers. AJL28 and AJL34 were mixed with a maleimide-
containing copolymer (PMI)7 in 1,1,2-trichloroethane. The
resultant solutions were filtered, spin-coated onto glass
substrates, and baked overnight at 50 °C in vacuum oven to
afford thin films of AJL28/PMI and AJL34/PMI, respec-
tively. After cured at 130 °C for 30 min, the intensity of the
absorption spectra of AJL28/PMI decreased significantly
(∼35%), which corresponds reasonably well with the per-
centage of dye decomposition. However, AJL34/PMI showed
much improved stability under the same curing condition,
with the intensity of its chromophoric absorption only
dropped by 2%. This head-to-head comparison clearly
showed that the diene reactivity of AJL34 chromophore was
significantly reduced toward maleimido dienophiles.
Our previous analysis indicated that the chemical sensitiv-
ity of AJL28 is mainly due to the butadiene segment next
to the terminal of its strong dialkyaminophenyl donor. This
diene structure has been labeled as the skeleton from Ca to
Cd in Chart 1. The cycloaddition between such a inner-outer-
ring diene and maleimido dienophile is determined by the
rotational energy barrier (∆E, Scheme 2) of the σ-bond
To study the poling and EO property of AJL34 in guest-
host polymers, amorphous polycarbonate (APC) is selected
as the host since it is commonly used in the literature, and
the chromophore loading is 25 wt %. This standardized
formulation has been also applied to another efficient
chromophore, AJLS102, for the fabrication of low Vπ and
low optical loss EO modulators.10 By following the typical
poling protocols for guest-host EO polymers,10,11 a relatively
large r33 value of 137 pm/V was obtained at 1310 nm for
AJL34/APC at a poling electric field of 1.0 MV/cm. This
represents a 30% improvement over that obtained for
AJLS102/APC. As we discussed earlier, this could be
attributed to the reduced intermolecular electrostatic interac-
tions by substituting the methoxy group on AJL34. Further-
more, the film of AJL34/APC poled at 163 °C (1.25 MV/
cm) has shown an r33 value of 166 pm/V at 1310 nm. The
poled sample was isothermally annealed at 85 °C to study
its temporal alignment stability. Under such testing condi-
tions, 90% of its original r33 value could be retained over
500 h. With both large EO activity and good temporal
stability, we believe that AJL34/APC EO polymer can be
used as an excellent material candidate for device exploration.
Another critical study is to examine the reactivity of diene
backbone on the AJL34 chromophore in the presence of
maleimide dienophile. The accelerated testing was conducted
as follows: To a small glass vial was added a few milligrams
of recrystallized N-phenylmaleimide (NPM) and a polyene
chromophore with the molar ratio of 25:1 together with
methylene chloride as the solvent. After the mixture was
completely dissolved, most of the solvent was evaporated,
and the vial was put on top of a hot plate with its temperature
set ∼100 °C. A melt mixture was obtained, in which the
polyene dye was dispersed homogeneously into a mixture
with excessive NPM dienophile. The mixture was then
annealed on top of the hot plate, and its composition change
vs annealing time was analyzed by thin layer chromatography
(TLC). This protocol is very convenient and only uses
minimal amount of reagents. We have applied this testing
Scheme 2. Proposed Mechanism for the Decomposition of
Polyenic Chromophores in the Presence of Maleimides
inbetween Cb and Cc (hereafter Cb-Cc). Although the s-trans
conformation is energetically favored for these polyenic
chromophores, the population distribution of different con-
formers still exists and follows the Boltzmann’s law.8,12 At
elevated temperatures, the equilibrium can be shifted to the
formation of meta-stable s-cis conformers such as the one
with respect to Cb-Cc. In the presence of dienophiles, this
shift is irreversibly driven by DA cycloaddition and further
amplified by severe decomposition of chromophores.
The reduced reactivity of diene segment on AJL34
indicates that the methoxy group does not significantly
(10) (a) Dinu, R.; Jin, D.; Huang, D.; Koenig, M. K.; Barklund, A. M.;
Fang, Y.; Parker, T. C.; Shi, Z.; Luo, J.; Jen, A. K.-Y. Proc. SPIE Int. Soc.
Opt. Eng. 2006, 6243, 62430G/1. (b) Enami, Y.; DeRose, C. T.; Loychik,
C.; Mathine, D.; Norwood, R. A.; Luo, J.; Jen, A. K.-Y.; Peyghambarian,
N. Appl. Phys. Lett. 2006, 89, 143506/1.
(11) Teng, C. C.; Man, H. T. Appl. Phys. Lett. 1990, 56, 1734.
(12) Orlandi, G.; Zerbetto, F.; Zgierski, M. Z. Chem. ReV. 1991, 91, 867.
Org. Lett., Vol. 9, No. 22, 2007
4473

increase the electron density on diene through the electronic
effect that is nomally observed in linear conjugated polyenes.8
Rather, the methoxy group acts as a polar substitution in
the middle of the highly polarized phenyltetraene bridges
with reduced bond order alternation (BOA).13
found to be around 2.37 to 2.42 Å, which is shorter than the
sum of van der Waals radii of H and O atoms (2.72 Å). We
suspect that such an interaction may rigidify the all-trans
π-electron bridge of AJL34 and elevate the rotational energy
barrier (∆E) of Cb-Cc, thereby reducing the diene reactivity
of the chromophore.8,15
We have also observed that the two vinylic protons, Hb
and Hf, have shown substantial long-range deshielding effect
Finally, poling and E-O properties of AJL34 in DA cross-
linkable polymers were studied. The host polymer used is
poly(methyl methacrylate-co-anthracen-9-ylmethyl meth-
acrylate) (PMMA-AMA), in which the anthraceyl pendant
groups can be thermally cross-linked with 1,6-bismale-
imidohexane (BMI) through the DA cycloaddition reactions.
By following the similar procedure used for AJ309,1a,2 40
wt % of AJL34 was formulated into the mixture of PMMA-
AMA/BMI. Thin films were poled and thermally cured at
around 108 °C under a poling field of 1.12 MV/cm. An
ultralarge r33 value of 306 pm/V can be achieved,11 which
represents a 25% improvement over that of AJ309 with the
same loading of a AJL6-type chromophore. This result,
again, is a great demonstration of the advantages offered by
the methoxy substitution, which improves the poling ef-
ficiency and chemical stability of highly nonlinear phenyl-
tetraene-based chromophores.
1
in their H NMR spectra. In this series of methoxylated
compounds, these two protons have all shown downfield shift
around 0.4-0.5 ppm relative to those of nonsubstituted
analogues (Table 1). Such a deshielding effect is due to the
1
Table 1. Chemical Shift of Protonsa Hb and Hf from the H
NMR Data of Methoxy-Substituted Molecules in CCDl3
δH b (ppm)
δH b (ppm)
b
f
dienone 5
trienenitrile 6
trienal 7
7.28 (+0.55)
7.14 (+0.44)
7.24 (+0.48)
7.27 (+0.49)
5.50 (+0.44)
6.33 (+0.46)
6.75 (+0.38)
chromophore AJL34
a The Hb and Hf represent the vinylic protons attached to the Cb and Cf,
respectively (Figure 1). b The values in parentheses are the shifted values
relative to those from their analogues without the methoxy substitution,
respectively; a positive value denotes a downfield shift.
Acknowledgment. We thank Dr. Yi Liao and Prof. Bruce
H. Robinson of the Department of Chemistry, University of
Washington, for their supply of the YLD124 chromophore
for comparison. Dr. Liao is now with the Department of
Chemistry, University of Central Florida. This work was
financially supported in part by the National Science
Foundation (the NSF-STC Program under Agreement No.
DMR-0120967), the Defense Advanced Research Projects
Agency (DARPA) MORPH program, and the Boeing-
Johnson Foundation.
fact that these two protons are in close vicinity with the polar
methoxy group (Figure 1).14 While the real nature of such a
Supporting Information Available: Experimental pro-
cedures and characterization of new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 1. Illustration of the deshielding effect of the polar oxygen
function on protons (Hb and Hf) in its vicinity.
OL701814R
(13) Marder, S. R.; Cheng, L. T.; Tiemann, B. G.; Friedli, A. C.;
Blanchard-Desce, M.; Perry, J. W.; Skindhoej, J. Science 1994, 263, 511.
(14) (a) Tomoeda, M.; Inuzuka, M.; Furuta, T.; Shinozuka, M.; Taka-
hashi, T. Tetrahedron 1967, 24, 959. (b) Wittstruck, T. A.; Malhotra, S.
K.; Ringold, H. J.; Cross, A. D. J. Am. Chem. Soc. 1963, 85, 1699. (c)
Wittstruck, T. A.; Malhotra, S. K.; Ringold, H. J.; Cross, A. D. J. Am.
Chem. Soc. 1963, 85, 3038. (c) Collins, D. J.; Hobbs, J. J. Sternhell, S.
Tetrahedron Lett. 1963, 197.
deshielding effect is not yet well understood, the intra-
molecular interaction between these H and O atoms, probably
the van der Waals interaction, could account for it. By using
the MOPAC 7.0 (AM1) program in Chem3D Ultra package
to perform a simple calculation of molecular conformation,
the distance between Hb (or Hf) and the oxygen atoms was
(15) (a) Bradley, A. Z.; Kociolek, M. G.; Johnson, R. P. J. Org. Chem.
2000, 65, 7134. (b) Sakai, S. J. Phys. Chem. A 2000, 104, 922.
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Org. Lett., Vol. 9, No. 22, 2007