Push-pull chromophores based on 2,2′-bi(3,4-ethylenedioxythiophene) (BEDOT) π-conjugating spacer

Article abstract of DOI:10.1016/S0040-4039(00)02317-0

Replacement of 2,2′-bithiophene by BEDOT in push-pull NLO-phores produces a red shift of the absorption maximum accompanied with a large increase of the quadratic nonlinear optical susceptibility.

Full text of DOI:10.1016/S0040-4039(00)02317-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1507–1510
Push–pull chromophores based on
2,2%-bi(3,4-ethylenedioxythiophene) (BEDOT) p-conjugating
spacer
Jean-Manuel Raimundo,a Philippe Blanchard,a Pierre Fre`re,a Nicolas Mercier,a Isabelle Ledoux-Rak,b
Rolland Hierleb and Jean Roncalia,*
aInge´nierie Mole´culaire et Mate´riaux Organiques, CNRS UMR 6501, Universite´ d’Angers, 2 Bd Lavoisier,
49045 Angers Cedex, France
bLaboratoire de Photonique Quantique et Mole´culaire, Ecole Normale Supe´rieure, 61 av. du Pre´sident Wilson,
F-94235 Cachan, France
Received 18 December 2000; accepted 19 December 2000
Abstract—Replacement of 2,2%-bithiophene by BEDOT in push–pull NLO-phores produces a red shift of the absorption maximum
accompanied with a large increase of the quadratic nonlinear optical susceptibility. © 2001 Elsevier Science Ltd. All rights
reserved.
Push–pull chromophores for 2nd order nonlinear optics
involve electron-donor and -acceptor groups interacting
through a p-conjugating spacer.1 Whereas the optimiza-
tion of the donor2 and acceptor3 groups has led to
considerable progress in the development of stable and
efficient systems, the relationships between the structure
of the spacer and 2nd order hyperpolarizability seem to
have attracted less attention. Polyenic spacers allow
high nonlinearities to be reached as they provide the
most effective way to achieve charge redistribution
between the donor and the acceptor.4 As shown in
recent work, thiophene-based spacers can lead to stable
NLO-phores with large second order nonlinearities.2,3,5–
8 These high performances can be related to the moder-
ate aromatic resonance energy of thiophene which
allows a better p-electron delocalization than e.g. ben-
zene-containing spacers.
Me
Me
NC
N
N
N
S
CN
S
NC
NC
1c
O
O
Me
Me
S
S
CN
NC
O
O
O
2a
O
O
Me
Me
S
O
N
S
Thus, NLO-phores built around thiophene-based con-
jugating spacers such as thiophene and its
oligomers,2,3,6 fused ring systems,7 and oligo-
thienylenevinylenes,5,8 have been synthesized. Recently,
we have shown that the covalent bridging of a
dithienylethylene (DTE) spacer leads to a considerable
O
O
O
N
S
2b
O
Me
Me
NC
N
S
CN
S
NC
O
O
Keywords: push–pull chromophores; optical properties; conjugated
systems.
NC
* Corresponding author. Fax: 33(0)2 41 73 54 05; e-mail:
jean.roncali@univ-angers.fr
2c
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02317-0

1508
J.-M. Raimundo et al. / Tetrahedron Letters 42 (2001) 1507–1510
R
R
R
R
S
S
i
CHO
S
S
R
R
R
R
1f
2f
1 R = H
2 R = O-CH2CH2-O
PPh3, I
ii
N
R
R
S
S
N
R
R
1e
2e
i
R
R
1c
iii
S
2a
OHC
S
2b
2c
N
R
R
1d
2d
Scheme 1. Reagents and conditions: (i) POCl3/DMF; (ii) t-BuOK/MeCN–THF; (iii) 1c, 2c bis(dicyanomethylidene)indane, Ac2O,
reflux 1 h and 1.5 h. 2a malononitrile, CHCl3, reflux 18 h. 2b diethylthiobarbituric acid, Ac2O, 80°C, 24 h.
dithienothiophene.7b On the other hand, comparing the
enhancement of the molecular 2nd order hyperpolariz-
ability of the derived push–pull NLO-phores.8 We now
report new NLO-phores (2a–c) in which BEDOT is
used as p-conjugating spacer.
data for compounds 1c and 2c which bear the same
acceptor group clearly shows that replacement of BT by
BEDOT produces a 118 nm bathochromic shift of umax
and more than a twofold increase of vi, up to 11600×
10−48 esu. However, this high vi value may be, in part,
related to resonance effect.
The synthesis of the NLO-phores is depicted in Scheme
1. Vilsmeier formylation of 2,2%-bithiophene (BT) (1),
and BEDOT (2),9 gave aldehydes 1f and 2f in 94 and
80% yields, respectively. Wittig olefination of 1f and 2f
with a phosphonium iodide bearing the N,N-dimethyl-
aniline group led to compounds 1e and 2e as a mixture
of E and Z isomers (ꢀ7/3) (yields 93 and 73%). After
separation of the Z isomer, a second Vilsmeier formyla-
tion gave aldehydes 1d and 2d in 60 and 66% yields and
the target compounds 1c, 2a–c were obtained in 60–
90% yield by Knoevenagel condensation of 1d and 2d
with malononitrile, diethylthiobarbituric acid and 1,3-
bis(dicyanomethylidene)indane.10
While these results suggest that BEDOT is a more
effective electron relay than BT, the evaluation of the
relative contribution of electronic11 and geometric
effects on the enhancement of vi is not straightfor-
ward. Whereas theoretical work in progress should
contribute to clarify the role of electronic factors, fur-
ther insights on the effects of the ethylenedioxy group
Table 1. Absorption maxima,a quadratic hyperpolari-
zabilitiesb and decomposition temperaturesc of chromo-
phores 1–2
Table 1 lists the UV–vis absorption maxima (umax),
second order nonlinear hyperpolarizabilities (vi), and
decomposition temperatures (Td) of the new BEDOT
based NLO-phores and of compound 1c synthesized as
reference compound.
Compd
umax (nm)a
vi (10−48 esu)b
Td (°C)c
1c
2a
2b
2c
712
588
649
830
5000 (1900)
2120 (1200)
2000 (950)
215
308
300
206
11600 (2400)
As expected, the increase of the acceptor strength
induces a bathochromic shift of the umax accompanied
by an increase of vi. Comparison of the data for
compound 2a to those of other systems having the same
donor/acceptor pair but a different spacer, shows that
the BEDOT spacer leads to an efficiency comparable to
that of DTE5 but inferior to that of bridged DTE8 or
a In CH2Cl2.
b Measured in CHCl3 at 1.9 mm by the electric field-induced second
harmonic generation (EFISH) technique, values in parentheses rep-
resent the zero-frequency hyperpolarizability product vi0.
c Determined by differential scanning calorimetry at a rate of 10°C/
min.

J.-M. Raimundo et al. / Tetrahedron Letters 42 (2001) 1507–1510
1509
References
1. (a) Dalton, L. R.; Harper, A. W.; Ghosn, R.; Steier, W.
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(b) Rao, V. P.; Cai, Y. M.; Jen, A. K.-Y. J. Chem. Soc.,
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Bedworth, P. V.; Marder, S. R. Adv. Mater. 1997, 9, 132.
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J. Chem. Soc., Chem. Commun. 1993, 1118; (b) Boldt, P.;
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Chem. Commun. 1999, 2391.
Figure 1. ORTEP view of BEDOT (H atoms omitted).
Selected bond lengths: S(1)ꢀC(1) 1.732(2); S(1)ꢀC(4) 1.715(3);
O(1)ꢀC(2) 1.368(3); O(1)ꢀC(5) 1.435(3); O(2)ꢀC(3) 1.374(3);
O(2)ꢀC(6) 1.450(4); C(1)ꢀC(2) 1.373(3); C(2)ꢀC(3) 1.421(3);
C(3)ꢀC(4) 1.347(4); C(5)ꢀC(6) 1.483(4).
4. Blanchard-Desce, M.; Alain, V.; Bedworth, P. V.;
Marder, S. R.; Fort, A.; Runser, C.; Barzoukas, M.;
Lebus, M. S.; Wortmann, R. Chem. Eur. J. 1997, 3, 1091.
5. Jen, A. K.-Y.; Rao, V. P.; Wong, K. Y.; Drost, K. J. J.
Chem. Soc., Chem. Commun. 1993, 90.
6. Hutchings, M. G.; Ferguson, I.; McGeein, D. J.; Morley,
J. O.; Zyss, J.; Ledoux, I. J. Chem. Soc., Perkin Trans. 2
1995, 171.
7. (a) Rao, V. P.; Wong, K. Y.; Jen, A. K.-Y.; Drost, K. J.
Chem. Mater. 1994, 6, 2210; (b) Kim, O. K.; Fort, A.;
Barzoukas, M.; Blanchard-Desce, M.; Lehn, J.-M. J.
Mater. Chem. 1999, 9, 2227.
8. Raimundo, J.-M.; Blanchard, P.; Ledoux-Rak, I.; Hierle,
R.; Michaux, L.; Roncali, J. Chem. Commun. 2000, 1597.
9. Akoudad, S.; Roncali, J. Synth. Met. 1998, 93, 111.
10. All final compounds were characterized by NMR, mass
spectrometry and elemental analysis. Selected data for 2c
black powder, mp 206°C (dec.), IR (KBr): 2220 cm−1
(CN), 1603 cm−1 (CꢁC), 1168 cm−1 (C-O-C). 1H NMR
on the geometry of the BT system have been gained by
analyzing the crystallographic structure of a single crys-
tal of BEDOT (Fig. 1).12 Examination of the distances
between the sulfur and the oxygen S(1)···O(1%) and
,
O(1)···S(1%) (2.92 A) shows that these distances are
significantly shorter than the sum of the van der Waals
,
radii of sulfur and oxygen (3.25 A), which evidences the
occurrence of strong intramolecular interactions. An
interesting consequence of this spontaneous rigidifica-
tion is that contrary to oligothiophenes,13 the p-conju-
gated system of BEDOT adopts a perfectly planar
geometry. Such a planarization of the structure can
be expected to provide a significant contribution to
the superior electron relay properties of the
BEDOT spacer compared to the BT one. Such a con-
clusion is in agreement with the enhanced p-electron
delocalization recently observed in thiophene–EDOT
hybrid oligomers incorporating a median BEDOT sys-
tem.14
(CDCl3) 8.62 (s, 1H, ꢁCH
7.78 (m, 2H, ꢁCH
Ar), 7.38 (d, 2H, 3J=8.8 Hz, 2
CHArꢁCsp2-CHE), 6.98 (s, 2H, CHEꢁCHE), 6.70 (d, 2H,
3J=8.8 Hz, 2 CHAr
ꢁCsp2-NR2), 4.45–4.32 (m, 8H, 4
CH2O), 3.00 (s, 6H, CH3N). Anal. calcd C, 67.09 (67.14),
6 -Cthio), 8.57 (m, 2H, ꢁCH6 Ar),
6
6
6
6
6
6
6
The compounds exhibit decomposition temperatures
(Td) ranging from 200°C to more than 300°C. Com-
pounds 1c, 2c containing the bis(dicyanomethyli-
dene)indane acceptor group have comparable Td
(210–230°C) suggesting that the stability of these
molecules is essentially limited by the nature of the
acceptor. In contrast, the high Td of compounds 2a and
2b (]300°C) clearly shows that stable NLO-phores can
be synthesized from BEDOT spacers.
H, 3.77 (3.71), N, 9.93 (10.30), O, 9.01 (9.41), S, 9.29
(9.43). MS (MALDI-TOF) calcd for C38H25N5O4S2
+
’
679.1348, found 679.1418 (M ).
11. Albert, I. D. L.; Marks, T. J.; Ratner, M. A. J. Am.
Chem. Soc. 1997, 119, 6575.
12. Crystal data for C12S2O4H10. C12S2O4H10 (MW=282.34)
is monoclinic P21/c, Z=2, a=9.284(5), b=8.165(1), c=
3
,
,
7.7706(9) A, i=90.43(2)°, V=589.1(5) A , calculated
density 1.59 g cm−3
.
Data collection: Data collection was carried out at 293 K
using the zig-zag ꢀ–2q scan technique in the 2–30° q
range, on an Enraf-Nonius MACH3 four-circles diffrac-
To summarize we have shown that the use of a BEDOT
p-conjugating spacer allows stable NLO-phores with
enhanced quadratic hyperpolarizability to be devel-
oped. As suggested by X-ray data, this effect could be
related to the non-covalent rigidification of the conju-
gating spacer by intramolecular S···O interactions.
Covalent fixation of these chromophores into high Tg
polyimides is in progress and the resulting materials will
be described in future publications.
tometer. From 1948 measured reflections (uMoKa
=
0.71069, h 0,10; k 0,11; l −13,13), 1062 independent
reflections with I/|(I)>3 were available for structure
refinement.
Structure refinement: After Lorentz-polarization correc-
tions and absorption correction (psi-scan procedure), the
structure was solved by direct methods (SIR) which

1510
J.-M. Raimundo et al. / Tetrahedron Letters 42 (2001) 1507–1510
reveal all the non-hydrogen atoms. Refinement of struc-
ture did not allow hydrogen atoms to be located which
1062 reflections). All calculations were performed using
the MolEN package.
,
were then fixed at 0.95 A from carbon atoms (Hydro
13. van Bolhuis, F.; Wynberg, H.; Havinga, E. E.; Meijer, E.
W.; Staring, E. G. J. Synth. Met. 1989, 30, 381.
14. Turbiez, M.; Fre`re, P.; Blanchard, P.; Roncali, J. Tetra-
hedron Lett. 2000, 41, 5521.
program). Finally, refinement of all atomic coordinates,
anisotropic thermal factors (non-H atoms) lead to R=
0.043, RW=0.060 (use of F magnitude, 97 parameters for
.