Facile synthesis of highly efficient phenyltetraene-based nonlinear optical chromophores for electrooptics

Article abstract of DOI:10.1021/ol060178b

A facile synthetic route has been developed to convert an electron-rich, sterically hindered dialkylaminodienone into a conjugated dialkylaminotrienal with good yield. The derived dialkylaminotetraene-type nonlinear optical chromophores possess an all-tra

Full text of DOI:10.1021/ol060178b

ORGANIC
LETTERS
2006
Vol. 8, No. 7
1387-1390
Facile Synthesis of Highly Efficient
Phenyltetraene-Based Nonlinear Optical
Chromophores for Electrooptics
Jingdong Luo,† Yen-Ju Cheng,† Tae-Dong Kim,† Steven Hau,† Sei-Hum Jang,†
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 January 21, 2006
ABSTRACT
A
facile synthetic route has been developed to convert an electron-rich, sterically hindered dialkylaminodienone into a conjugated
dialkylaminotrienal with good yield. The derived dialkylaminotetraene-type nonlinear optical chromophores possess an all-trans conformation
and can be functionalized with fluoro-dendron to provide proper shape modification for poling. Polymers doped with two examples of these
chromophores in high concentrations have been poled to afford ultra-large electrooptic coefficients (r33) of 208 and 262 pm/V, respectively, at
the measuring wavelength of 1.31
µm.
Integrated optical devices based on organic nonlinear optical
(NLO) polymers have emerged as one of the enabling
elements for a new generation of optical telecommunications,
which needs fast modulation and switching of optical signals
at the speed of 100 Gbit/s or above to accommodate
anticipated growth in data traffic.1 The macroscopic NLO
response of such materials arises from the poling-induced
polar order of noncentrosymmetric NLO chromophores in
polymers.2 The dipolar NLO chromophores used in poled
polymers are generally represented as D-π-A, where D is
an electron donor, A is an electron acceptor, and π is a
conjugating bridge connecting the D and A moieties. In such
molecules, the donor and acceptor substituents provide the
requisite ground-state charge asymmetry, whereas the π-con-
jugation bridge provides a pathway for the ultrafast redis-
tribution of electric charges under an applied external electric
field.
† Department of Materials Science & Engineering.
‡ Department of Chemistry.
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10.1021/ol060178b CCC: $33.50
© 2006 American Chemical Society
Published on Web 03/11/2006

The polyene systems, with few or no aromatic rings, are
the most effective π-conjugated bridges for achieving large
molecular first hyperpolarizabilities (â).3 The conformation
of all-trans polyene bridges can be preserved by incorporating
part or all of their methine skeletons into single or fused
aliphatic rings.4 This approach has been employed by
numerous groups to effectively improve both thermal and
photochemical stability of polyene-based chromophores
without compromising their NLO properties. Typical ex-
amples are the ring-locked phenyltetraene-based “push-pull”
compounds, known as the CLD type of chromophores (Chart
1).5 The ring-locked structure of these compounds is based
developed efficient side-chain dendronized polymer, the
center of the phenylthiophene stilbene bridge can be func-
tionalized with a dendron, through the process of Diels-
Alder “click chemistry”, to provide better site isolation for
poling.8 As a result, very large r33 values (up to 110 pm/V
at 1.31 µm) can be achieved.
To improve the performance of these organic NLO
materials further, the most efficient polyenic dyes are ideal
because of their large nonlinearities. However, several
parameters concerning synthesis and control of the molecular
shape of these chromophores should be addressed before they
can be applied for general use. A majority of the double
bonds on the polyene chain, except the one embedded in
the isophorone ring structure, are often kept as unlocked to
avoid twisting of the conjugating backbone and to provide
synthetic convenience. Accordingly, the dialkylaminophe-
nyltrienal intermediates and the obtained CLD-type chro-
mophores are often a mixture of trans and cis isomers.
Attempts to purify these isomers through column chroma-
tography for higher NLO activity are proven to be very
difficult. In the best case reported, CLD-4, an all-trans
n-hexyl-substituted analogue of CLD-1, can only be syn-
thesized with an overall yield of <1% because of the low
reactivity of the hexylated enones and aminophenylenone
intermediates.5d
Chart 1. Typical Examples of
Alkylaminophenyltetraene-Based NLO Chromophores
To solve this problem, we have developed a facile
synthetic route to generate all-trans dialkylaminophenyltet-
raene chromophores. Both sides of the π-conjugated bridge
of these chromophores can be protected through an iso-
phorone ring and a fluoro-dendron. A very strong acceptor,
2-(3-cyano-4-methyl-5-phenyl-5-trifluoromethyl-5H-furan-2-
ylidene)-malononitrile (CF3-TCF), can also be functional-
ized on these bridges to significantly enhance the molecular
nonlinearity of these chromophores.9 The poled guest-host
or cross-linked polymers using these chromophores showed
exceptionally high r33 values (208 and 262 pm/V) at 1.31
µm.
To obtain the target aminophenyltrienal donor bridges
(compound 6), the main challenge is to improve the reactivity
of the quite inert enones (such as compound 2 in Scheme 1)
and aminophenylenones (compound 3) which have an alkyl
group substituted on the isophorone ring. We found that
sodium ethoxide is quite effective in improving the Knoev-
on an isophorone-derived six-membered ring to furnish the
central core of D-π-A chromophores and to provide a nearly
coplanar conjugation and prolate pseudo-ellipsoidal shape.
To prevent close packing of such extended dipolar structures
and improve their solubility, bulky tert-butyldimethylsilyl
(TBDMS) groups and asymmetric substituents have been
used to functionalize on either the donor or the acceptor end
of the chromophores. As a result of these shape modifica-
tions, relatively high electrooptic (EO) coefficients (r33
around 40-70 pm/V measured at the wavelengths of 1.3
and 1.55 µm) have been achieved from poled polymeric
composites using these polyenic chromophores as dopants.
Recently, the results of using a Monte Carlo simulation6
and nanoscale tailoring of the size, shape, or conformation
of NLO polymers and dendrimers7 all show that the ideal
shape of chromophores is spherical. For example, in a newly
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1388
Org. Lett., Vol. 8, No. 7, 2006

Scheme 1. Synthesis of All-Trans Dendronized Alkylaminophenyltetraene-Type NLO Chromophores 10 and 11.
enagel condensation between dialkylaminobenzaldehyde 1
and 2-hydroxylethyl-substituted isophorone 2. The yield of
these reactions increased from 10-15% to ∼50% simply
by replacing the commonly used potassium hydroxide or
potassium tert-butoxide with sodium ethoxide,4,5 possibly
because of its proper combination of strong basicity and less
steric hindrance. Three different pathways have been ex-
plored to extend the conjugation of aminophenyldienone 3
to aminophenyltrienal 6. In the literature, the one-pot reaction
to extend the alkyl-substituted enone to form the extended
conjugated aldehyde using N-tert-butylacetimine/lithium di-
isopropylamide (LDA) as base has been reported to give only
12% yield because of the steric hindrance of the alkyl
group.5d Another alternative route is to use the trienenitrile
synthon in a Hornor-Emmons reaction sequence. Compound
3 is reacted first with the anion of diisopropyl cyanometh-
ylphosphonate to, if applicable, generate the intermediate of
aminophenyltrienenitrile, which can be reduced by DIBAL-H
to afford the aminophenyltrienal bridge.10 In the past, we
have employed this efficient two-step synthetic scheme to
synthesize extended aldehydes for donor-substituted conju-
gating bridges. However, this approach is difficult to apply
to dienone 3 or its tert-butyldimethylsilyl-protected analogues
because of their steric congestion or low reactivity toward
the Hornor-Emmons reaction conditions.
substituted bridge 6 to afford chromophore 8 with a hydroxyl
group. It should be noted that the conjugated skeleton of
such a highly polarizable polyenic chromophore is very prone
to the attack of nucleophiles such as pyridine or triethy-
lamine. To functionalize the all-trans chromophores 10 and
11 with a dendron to modify their shape, a one-pot
esterification was carried out in refluxing tetrahydrofuran
(THF) with an excess amount of acid chloride 9. Because
no base catalysts are involved, the solution mixture remains
acidic. It can in situ cleave the methoxymethyl (MOM)
protecting group and generate the second hydroxyl group at
the donor end of the chromophores to afford the diden-
dronized product 11. This procedure avoids the side reactions
that often occur to cause chromophore decomposition during
the typical reaction of MOM cleavage.7b,g The chromophores
10 and 11 can be obtained in excellent yield after being
purified through flash chromatography. The relative ratio
Because lithioacetonitrile is a less bulky nucleophile
compared to the anions generated from N-tert-butylacetimine
or diisopropyl cyanomethylphosphonate,11 it can be reacted
with dienone 3 easily to produce the carbinol 4 in quantitative
yield. After dehydration, the newly formed nitrile synthon 5
can be reduced with DIBAL-H to form the donor-substituted
bridge aldehyde 6. The total yield from 4 to 6 is 60%.
Trienenitrile 5, trienal 6, and the derived tetraene chro-
mophores (8, 10, and 11) possess all-trans conformation, as
shown in the 1H NMR spectra (Figure 1). Compound 7, one
of the strongest acceptors, was condensed with the donor-
(10) (a) Wada, A.; Fujioka, N.; Tanaka, Y.; Ito, M. J. Org. Chem. 2000,
65, 2438. (b) Durantini, E. N. Synth. Commun. 1999, 29, 4201.
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Figure 1. 1H NMR spectra of all-trans trienenitrile 5 (A) and trienal
6 (C), in comparison with cis- and trans-isomer mixtures of
isophorone-unsubstituted analogues B and D, respectively.
Org. Lett., Vol. 8, No. 7, 2006
1389

between 10 and 11 can be controlled easily by reaction time
and monitored by thin-layer chromatography (TLC).
To study the EO property of polymers using these
chromophores as dopant, 47 wt % of 10 was formulated into
poly(methyl methacrylate) (PMMA) using 1,1,2-trichloro-
ethane as solvent. The solution of this polymer (7 wt %
concentration) was filtered through a 0.2-µm PTFE-syringe
filter and spin-coated onto an indium tin oxide (ITO) glass
substrate. The resulting films were baked in a vacuum oven
at 60 °C overnight to ensure the removal of any residual
solvent. Then, a thin layer of gold was sputtered onto the
films as a top electrode for contact poling. The r33 values
were measured using the Teng-Man simple reflection
technique at the wavelength of 1.31 µm.12 The ITO glass
substrates were also examined at the near-infrared (NIR)
wavelength of 1.31 µm to ensure their optical transparency
(over 85% transmission). After we calibrated the poling
results with a standardized AJL8/APC EO polymer,13 the
measurement accuracy of r33 values at 1.31 µm was very
consistent with the results measured from other transparent
electrodes such as ZnO or In2O3.14
The films of 10/PMMA were poled at 90 °C with a DC
electric field of 0.80 MV/cm. After cooling to room
temperature and removing the electric field, the poled films
showed an exceptionally large EO coefficient (r33 ) 262
pm/V at 1.31 µm). This value is exceptionally large
compared to those reported (70-110 pm/V at 1.31 µm) for
guest-host EO polymers containing polyene-type chro-
mophores.15 Such a great increase is attributed to the purity
and structural features of chromophore 10. Its all-trans
conformation could provide a better conjugation and increase
the âµ value of the chromophores significantly.5c In addition,
the central polyene bridge is spatially isolated by both the
isophorone ring and the center-anchored perfluoro aromatic
dendron. This spatial arrangement also greatly increases the
solubility of 10 resulting in a higher number density of
chromophores that can be incorporated into the polymer
matrix. The dendron also provides effective site isolation to
decrease the strong electrostatic interactions among chro-
mophores.
Poling and EO properties of the double-dendron-function-
alized chromophore 11 were also studied. The host polymer
used was poly(methyl methacrylate-co-anthracen-9-ylmethyl
methacrylate) (PMMA-AMA), in which the anthracenyl
pendant groups can be thermally cross-linked with tris(2-
maleimidoethyl)amine (TMI) through the Diels-Alder cy-
cloaddition reactions.16 By following the similar procedure
used for 10/PMMA, 45 wt % of 11 was formulated into the
mixture of PMMA-AMA/TMI. The thin films were poled
and cured at around 105 °C under a poling field of 90 V/µm.
Again, a very large r33 value of 208 pm/V can be achieved.
Through efficient lattice hardening by using the Diels-Alder
reaction, over 70% of the original EO activity can be retained
at 85 °C for several hundred hours.
In summary, we have developed a facile route to synthe-
size all-trans dendronized dialkylaminotetraene-based NLO
chromophores in good overall yields. This new design
provides an efficient way to modify the shape of high âµ
extended polyenic chromophores. Two exemplary chro-
mophores (10 and 11) were incorporated in polymers with
high concentrations and efficiently poled to generate excep-
tionally large r33 values of 208 and 262 pm/V at 1.31 µm
with good temporal stability.
Acknowledgment. Financial support from the National
Science Foundation (NSF-NIRT and the NSF-STC Program
under Agreement Number DMR-0120967), the Defense
Advanced Research Projects Agency (DARPA) MORPH
program, and the Air Force office of Scientific Research
(AFOSR) under the MURI Center on Polymeric Smart Skins
is acknowledged. Alex K-Y. Jen thanks the Boeing-Johnson
Foundation for its support. Tae-Dong Kim thanks the
Nanotechnology Center at the University of Washington for
the Nanotechnology fellowship.
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Wang, L.; Zhu, P.; Marks, T. J.; Luo, J.; Jen, A. K-Y. Opt. Express 2005,
13, 7380. (c) Sinyukov, A. M.; Leahy, M. R.; Hayden, L.; Haller, M.; Luo,
J.; Jen, A. K-Y.; Dalton, L. R. Appl. Phys. Lett. 2004, 85, 5827.
(14) (a) Michelotti, F.; Belardini, A.; Larciprete, M. C.; Bertolotti, M.;
Rousseau, A.; Ratsimihety, A.; Schoer, G.; Mueller, J. Appl. Phys. Lett.
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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.
OL060178B
(16) (a) Haller, M.; Luo, J.; Li, H.; Kim, T.-D.; Liao, Y.; Robinson, B.
H.; Dalton, L. R.; Jen, A. K-Y. Macromolecules 2004, 37, 688. (b) Luo, J.;
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