Donor-acceptor-substituted phenylethenyl bithiophenes: Highly efficient and stable nonlinear optical chromophores

Article abstract of DOI:10.1021/ol991118r

(formula presented) For practical applications of poled electrooptic polymers, highly efficient and thermally stable nonlinear optical (NLO) chromophores are required. We describe here a concise synthesis and characterization of a series of donor-acceptor

Full text of DOI:10.1021/ol991118r

ORGANIC
LETTERS
1
999
Vol. 1, No. 11
847-1849
Donor−Acceptor-Substituted
Phenylethenyl Bithiophenes: Highly
Efficient and Stable Nonlinear Optical
Chromophores
1
Chengzhi Cai,*,†,§ Ilias Liakatas, Man-Shing Wong, Martin B o1 sch,†
†
†,|
,†
†
‡,⊥
‡,#
Christian Bosshard,* Peter G u1 nter, Simona Concilio, Nicola Tirelli, and
‡
Ulrich W. Suter
Nonlinear Optics Laboratory, Institute of Quantum Electronics, Swiss Federal Institute
of Technology (ETH), CH-8093 Zurich, Switzerland, and Institute of Polymers,
Department of Materials, ETH, CH-8092 Zurich, Switzerland.
bosshard@iqe.phys.ethz.ch
Received October 4, 1999
ABSTRACT
For practical applications of poled electrooptic polymers, highly efficient and thermally stable nonlinear optical (NLO) chromophores are
required. We describe here a concise synthesis and characterization of a series of donor−acceptor chromophores incorporating a bithiophene
moiety in the conjugated bridge. They display a suitable thermal stability and significantly enhanced molecular nonlinearity as compared to
their monothiophene analogues and are among the most efficient yet stable NLO chromophores prepared so far.
The continuous interest in poled electrooptic (EO) polymers1
for device applications in information processing and tele-
communications has resulted in the development of several
types of second-order nonlinear optical (NLO) chromophores
that are highly efficient, as characterized by their high scalar
product of the dipole moment (µ) and the molecular second-
order polarizability (â). To obtain a macroscopic order, these
2
dipolar chromophores are usually aligned by poling with a
high electric field at the glass transition temperature (T ) of
the polymer. To reduce randomization of the order after the
poling, it is necessary to use high T polymers such as
g
3
g
2f
polyquinolines, and the chromophores should be stable
†
Nonlinear Optics Laboratory.
Institute of Polymers.
Future address: Department of Chemistry, University of Houston,
‡
(2) (a) Jen, A. K.-Y.; Rao, V. P.; Wong, K. Y.; Drost, K. J. Chem.
Commun. 1993, 90. (b) Marder, S. R.; Gorman, C. B.; Meyers, F.; Perry,
J. W.; Bourhill, G.; Bredas, J.-L.; Pierce, B. M. Science 1994, 265, 632.
(c) Wu, X.; Wu, J.; Liu, Y.; Jen, A. K.-Y. J. Am. Chem. Soc. 1999, 121,
472. (d) Rao, V. P.; Jen, A. K.-Y.; Wong, K. Y.; Drost, K. J. Chem.
Commun. 1993, 1118. (e) Jen, A. K.-Y.; Cai, Y.; Bedworth, P.; Marder, S.
AdV. Mater. 1997, 9, 132. (f) Cai, Y.; Jen, A. K. Y. Appl. Phys. Lett. 1995,
67, 299. (g) Blanchard-Desce, M.; Alain, V.; Bedworth, P. V.; Marder, S.
R.; Fort, A.; Runser, C.; Barzoukas, M.; Lebus, S.; Wortman, R. Chem.
Eur. J. 1997, 3, 1091.
§
Houston, TX 77204-5641. Current e-mail: cai@iqe.phys.ethz.ch.
|
Current address: Department of Chemistry, Hong Kong Baptist
University, Hong Kong.
⊥
Current address: Dipartimento di Chimica, Universit a´ degli Studi di
Napoli “Federico II”, Via Mezzocannone 4, 80134 Napoli, Italy.
#
Current address: Institute of Biomedical Engineering, Department of
Materials, ETH, CH-8044 Z u¨ rich, Switzerland.
(1) (a) Dalton, L. R. Chem. Ind. 1997, 511. (b) Bosshard, C.; Sutter, K.;
Pr eˆ tre, P.; Hulliger, J.; Fl o¨ rsheimer, M.; Kaatz, P.; G u¨ nter, P. Organic
Nonlinear Optical Materials; Gordon & Breach: Amsterdam, 1995; Chapter
(3) Pretre, P.; Meier, U.; Stalder, U.; Bosshard, C.; G u¨ nter, P.; Kaatz,
P.; Weder, C.; Neuenschwander, P.; Suter, U. W. Macromolecules, 1998,
31, 1947.
6
.
1
0.1021/ol991118r CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/02/1999

under the poling conditions. Jen et al. showed that incorpora-
tion of thiophene rings into the conjugated units in e.g. 1a-f
The tricyanovinyl-substituted dyes 2a-d were synthesized
in three steps from bithiophene (3) through the common
intermediate 4, as demonstrated with 2a in Scheme 1.
4
(Figure 1) can greatly enhance µâ while permitting a high
Scheme 1a
a
Conditions: (a) (1) BuLi, THF; (2) CuI; (3) ICH
N-C -CHO (5a), NaH, 15-crown-5
cat.), THF, 100%. (c) BuLi, THF, (CN) CdC(CN) , 80%. (d)
BuLi, THF, I , 93%. (e) HCCPh, Pd (dba) /CuI (cat.), Et N, THF,
8%. (f) (CN) CdC(CN) , CHCl , 80%.
2 2
PO(OEt) ,
DMSO, 73%. (b) p-Bu
2
6 4
H
(
2
2
2
2
3
3
9
2
2
3
Diethyl thienylmethyl phosphonate, analogue of 4, was
previously prepared in two steps by chloromethylation of
2b
3
thiophene followed by Arbuzov reaction with P(OEt) .
However, all our attempts to prepare 2-(chloromethyl)-
bithiophene failed probably due to its instability. To cir-
cumvent this problem, we prepared 4 in 73% yield by
Figure 1.
2 2
nucleophilic substitution of ICH PO(OEt) with 2-bithienyl-
copper generated in situ by treatment of bithiophene (3) with
BuLi followed by CuI. The relatively low reactivity of
bithienylcopper was overcome by addition of a polar aprotic
solvent (DMPU or DMSO) and running the reaction at
elevated temperatures. Conversion of phenyl and naphthyl
bromides to the arylmethylphosphonates via the arylcoppers
thermal stability.2a According to the general guideline for
optimization of µâ,2b we reason that using bithiophene
moieties in e.g. 2a-f (Figure 1), having a longer conjugation
length than 1a-f, should further increase µâ. Also, the robust
bithiophene moiety may maintain the thermal, chemical, and
photochemical stability. In this letter, we present the synthesis
and characterization of 2a-f and the comparison with their
analogues 1a-f.
5
has been described by Poindexter and Katz. Here, we
extended this method to the preparation of electron-rich
thienylmethylphosphonates that are useful intermediates for
the synthesis of low band gap thienylvinylene derivatives.
Wittig-Horner reaction of 4 with 4-(dibutylamino)benzal-
dehyde (5a) afforded exclusively the (E)-alkene 6a in
quantitative yield. The tricyanovinyl group was easily
introduced to provide 2a in 80% yield.
The 2-phenyl-tetracyanobutadienyl-substituted dyes 2e and
2f were synthesized according to the reported procedure2d
for 1e and 1f. For example, iodination of 6a (Scheme 1) led
to 7a in 93% yield. Sonogashira reaction of 7a readily
afforded the electron-rich acetylene 8a in 98% yield. A
(
4) Synthesis of 2a (more details are in Supporting Information): At
-
76 °C, n-BuLi (2.35 M in hexane, 35 mmol) was slowly added to
bithiophene (3, 6 g, 35 mmol) in THF (140 mL) under N2. After 1.5 h of
stirring at -76 °C, the solution was added to a suspension of CuI (6.67 g,
3
5 mmol) in THF (4 mL) at ca. -40 °C. The mixture was stirred at 0 °C
for 1 h, treated with diethyl iodomethylphosphonate (8.757 g, 31.5 mmol)
and DMSO (30 mL), and stirred at 56 °C for 20 h. Workup and FC (AcOEt)
afforded 4 (7.32 g, 73%). A solution of 4 (1.142 g, 3.609 mmol) and
4
-(dibutylamino)benzaldehyde (5a, 720 mg, 3.085 mmol) in THF (25 mL)
was added to NaH (4.15 mmol) and 15-crown-5 (40 mg, 0.18 mmol) at
room temperature. The suspension was stirred for 4 h. Workup and FC
(
0
toluene) afforded 6a (1.22 g, 100%). To obtain 2a, BuLi (2.1 M in hexane,
.63 mmol) was added to 6a (239 mg, 0.604 mmol) in THF (5 mL) at
[2 + 2] cycloaddition of tetracyanoethylene with 8a followed
-
76°. The solution was stirred at -76 °C to 0 °C for 1.5 h, recooled to
76 °C, treated in one portion with tetracyanoethylene (94.48 mg, 737
-
by ring opening gave rise to chromophore 2e in 80% yield.
The molecular nonlinearity (µâ) of the dyes 2a-f were
determined by electric field induced second-harmonic gen-
eration (EFISH) at a fundamental wavelength of 1907 nm
mmol) in THF (1.3 mL), stirred at -76 °C to room temperature for ∼2 h.
After workup and FC (toluene), 2a was obtained as a dark green solid (242
1
mg, 80%). H NMR (300 MHz, CDCl3): 7.94 (d, J ) 4.4, 1 H); 7.42 (d,
J ) 4.0, 1 H); 7. 34 (d, J ) 8.9, 2 H); 7.28 (d, J ) 4.4, 1 H); 6.98 (d, J )
4
2
.0, 1 H); 6.97 (d, J ) 16.0, 1 H); 6.92 (d, J ) 16.0, 1 H); 6.62 (d, J ) 9.0,
H); 3.31 (t, J ) 7.6, 4 H); 1.65-1.54 (m, 4 H); 1.43-1.31 (m, 4 H); 0.97
using a quartz reference (nonlinear optical coefficient d11
0.27 pm/V). The decomposition temperature (T ) of the
d
)
1
3
6
(
t, J ) 7.3, 6 H). C NMR (75 Hz, CDCl3, DEPT): 152.99 (s), 150.99 (s),
1
48.81 (s), 141.59 (d), 133.13 (d), 131.72 (s), 131.11 (s), 130.77 (s), 130.22
chromophores was measured by thermal gravimetric analysis
(d), 128.58 (2d), 126.41 (d), 124.67 (d), 122.89 (s), 115.25 (d), 112.97 (s),
1
1
12.91 (s), 112.60 (s), 111.66 (2d), 79.13 (s), 50.80 (t), 29.49 (t), 20.33 (t),
3.99 (q). EI-MS m/z: 496 (1.6, M ), 471 (1.7), 453 (1.9), 428 (1.6), 149
+
(5) Poindexter, M. K.; Katz, T. J. Tetrahedron Lett. 1988, 29, 1513.
(6) Bosshard, C.; Knopfle, G.; Pr eˆ tre, P.; G u¨ nter, P. J. Appl. Phys. 1992,
71, 1594.
(
3.9), 86 (56), 84 (86), 49 (100). Anal. Calcd for C29H28N4S2: C, 70.13;
H, 5.68; N, 11.28; S, 12.91. Found: C, 70.28; H, 5.86; N, 11.04; S, 12.78.
1848
Org. Lett., Vol. 1, No. 11, 1999

(
N
TGA) in air and differential scanning calorimetry (DSC) in
1f) improves the thermal and chemical stability.2c The
molecular nonlinearities (µâ) of 1e and 1f have not been
reported. According to our measurements, at least for the
bithiophene-based chromophores, the use of Ph-TCBD as
the acceptor did not bring up the µâ value: while the µâ
value remains about the same for 2f and 2b, it decreased
considerably from 2a to 2e. Also, the Ph-TCBD acceptor
improves thermal stability only if the diphenylamino group
is used as the donor (2f vs 2e and 2a). This suggests that
the thermal stability of 1a, 2a, 1e, and 2e can be limited by
the dialkylamino groups but not by the acceptor groups.
To address the thermal stability of the chromophores in
2
, both with a heating rate of 20 °C/min. The results of
these measurements and the electronic absorption data are
summarized in the Table 1. Also listed for a comparison are
Table 1. Electronic Absorption, Molecular Nonlinear Optical
Properties, and Thermal Stability of Selected Compoundsa
λmax
(nm)
µâ
Td
48
compound
(10- esu)
(°C)
1
2
1
2
1
2
1
2
1
2
1
2
a
a
b
b
c
c
d
d
e
e
f
640
661
601
611
662
655
718
712
604
623
566
575
473
6 700
10 600
3 510
3 940
10 600
14 300
7 450
-
-
6 840
-
4 120
1 130
240
249
315
308
-
238
200
-
245
250
340
343
308
the presence of high T
compound 2a and 2f were dispersed in PQ100, and heated
at 200 °C for 30 min under N . Thin-layer chromatography
TLC) showed no sign of decomposition for 2f, while around
g
polymers such as polyquinolines,
8
2
(
2
0% decomposition for 2a. Therefore, most of the chro-
mophores should be able to withstand poling at 200 °C that
was usually completed within 10 min.
In summary, we have developed an efficient synthesis of
a series of donor-acceptor substituted chromophores (2a-
f) based on arylvinyl bithiophene as the conjugated unit.
Nonlinear optical and thermal measurements showed that
except for 2d they are among the most efficient, yet stable,
NLO chromophores reported to date. In particular, besides
the high µâ value and thermal stability, chromophore 2a also
possesses a high photochemical stability that exceeds DR1
by a factor of 2, as indicated by our preliminary experiments.
Unfortunately, the poled thin films of 2a-f in PQ100 or
PMMA displayed low electrooptic coefficient (r33) values
in the order of 3-10 pm/V. This is not unexpected for such
rodlike chromophores with very high µâ values. Dalton et
al. have reported that such molecules are difficult to pole
due to the strong intermolecular electrostatic interaction that
facilitates centrosymmetrical aggregation of the dipolar
f
DR1
a
λmax and µâ at λ ) 1907 nm were measured in dioxane. λmax and Td
values of 1a, 1b, and 1d are from ref 2e, 1c from ref 2f, 1e and 1f from ref
2
same value for the quartz reference (d11 ) 0.27 pm/V) and the power series
definition to enable comparison with our results. Td is the onset temperature
of TGA in air with a scanning rate of 20 °C/min.
c. The µâ values of 1a-d in these references were adjusted using the
the data of their monothiophene analogues 1a-f and the
standard NLO chromophore Disperse Red 1 (DR1). Indeed,
the bithiophene-based chromophores display considerably
higher µâ values than their monothiophene analogues,
probably due to the longer and more polarizable π-conjuga-
tion. Significantly, except 2d, the thermal stability remains
nearly the same; 2d dissolved only in polar solvents such as
DMSO and DMPU, and decomposed in the presence of day
light and air.
The effect of substituents on the µâ value and thermal
stability are similar for both types of chromophores. Thus,
µâ values increase with the addition of one more ethenylene
moiety into the bridge (1c and 2c vs 1a and 2a) which,
interestingly, did not lead to a red shift of the absorption
9
molecules. Fortunately, this interaction can be greatly
reduced by introducing side chains to the chromophores to
9
separate the dipoles. We expect that incorporation of side
chains to the bithiophene moiety should not decrease the
efficiency and stability of the chromophores. Further struc-
tural modification, e.g. replacing the dibutylamino-phenyl
group in 2a with diphenylaminothienyl group, could further
2
e
improve µâ while keeping a good stability. Together with
the need to properly bond the chromophores to a suitable
polymer to ensure durability of the poling induced order,
considerable synthesis efforts still lie ahead before practical
EO polymers based on such promising chromophores can
be developed.
λ
max. However, this unfortunately results in the decrease of
thermal stability (2c vs 2a). The high thermal stability of 1a
and 2a can be enhanced even further by using the diphenyl-
amino group as the donor (in 1b and 2b), however, at the
expense of a significant reduction of µâ probably due to the
lowering of the electron donating strength. Jen et al. reported
that replacement of tricyanovinyl with 2-phenyltetracyano-
butadienyl (Ph-TCBD) as the acceptor (1a vs 1e, and 1b vs
7
Acknowledgment. This work is supported by the Swiss
National Science Foundation.
Supporting Information Available: Synthetic procedures
and characterization for the new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
(7) Moylan, C. R.; Twieg, R. J.; Lee, V. Y.; Swanson, S. A.; Betterton,
K. M.; Miller, R. D. J. Am. Chem. Soc. 1993, 115, 12599.
(8) We thank Hitachi Co. for providing the sample.
(9) Dalton, L. R.; Harper, A. W. Polym. News 1998, 23, 114.
OL991118R
Org. Lett., Vol. 1, No. 11, 1999
1849