New class of imidazoles incorporated with thiophenevinyl conjugation pathway for robust nonlinear optical chromophores

Article abstract of DOI:10.1016/S0040-4039(00)02143-2

A new series of thermally stable heterocyclic imidazole-based nonlinear optical chromophores has been developed. These chromophores possess a thiophene based stilbene conjugation pathway with a nitro acceptor group attached to the phenyl end. This feature leads to robust chromopores with high thermal stability and enhanced molecular nonlinearity.

Full text of DOI:10.1016/S0040-4039(00)02143-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 805–808
New class of imidazoles incorporated with thiophenevinyl
conjugation pathway for robust nonlinear optical chromophores
Javier Santos,a Eric A. Mintz,a Oliver Zehnder,b Christian Bosshard,b Xiu R. Bua,* and
Peter Gu¨nterb
aDepartment of Chemistry & NASA Center for High Performance Polymers and Composites Clark Atlanta University, Atlanta,
GA 30314, USA
bNonlinear Optics Laboratory, Institute of Quantum Electronics, Swiss Federal Institute of Technology (ETH),
CH-8093 Zu¨rich, Switzerland
Received 21 November 2000; accepted 22 November 2000
Abstract—A new series of thermally stable heterocyclic imidazole-based nonlinear optical chromophores has been developed.
These chromophores possess a thiophene based stilbene conjugation pathway with a nitro acceptor group attached to the phenyl
end. This feature leads to robust chromophores with high thermal stability and enhanced molecular nonlinearity © 2001 Elsevier
Science Ltd. All rights reserved.
prompted us to design a new system, which possesses
thiophene in a long conjugation pathway, but at the
same time to have nitro groups attached to phenyl
instead of thiophenyl. The new class of NLO imida-
zoles has been found to possess improved molecular
nonlinearity, high thermal stability, and excellent
solubility.
In order to be compatible with existing semiconductor
processing technology, organic nonlinear optical (NLO)
chromophores for device application are required to be
highly thermally stable for sustaining the processing
and operating temperatures.1 Heterocyclic imidazole
based NLO chromophores have received increasing
interest due to their excellent thermal stability in guest–
host systems, and good miscibility with high perfor-
mance polymers.2 The imidazole ring is easily tailored
to accommodate functional groups,3 which allows the
covalent incorporation of the NLO chromophores into
polyimides leading to NLO side chain polymers.3
Owing to synthetic challenge, most of the NLO imida-
zoles developed so far only possess short conjugation
pathways (spacers) such as phenyl.4,5 No report has
appeared yet about NLO imidazoles with long conjuga-
tion pathways.
The general route for the preparation of new chro-
mophores 13–16 is developed according to Scheme 1.
The key development is the aldehydes 9–12, which are
versatile precursors which could lead to a variety of
imidazole-based NLO chromophores.7 Condensation of
2-formylthiophene with benzil derivatives in the pres-
ence of ammonium acetate led to the formation of the
imidazole 1–4 in a yield from 70 to 87%. N-methylation
was achieved by the treatment of 1–4 with iodomethane
in the presence of potassium carbonate to give 5–8 with
a yield from 65 to 90%. It was found that the exact 1:1
ratio of iodomethane to imidazoles 1–4 was needed,
and even slightly excessive MeI would lead to the
formation of salts. For example, treatment of 3 with 1.2
equiv. iodomethane gave 17 exclusively (Scheme 2). The
aldehydes 9–12 were prepared by the treatment of 5–8
with n-butyl lithium at low temperature in situ,
followed by the addition of anhydrous DMF. Yields
are moderate to good with 9 of 68%, 10 of 40%, 11
of 75% and 12 of 53%. The direct formulation of
1–4 using 1–3 equiv. n-butyl lithium had been
attempted, and in no case had any of the corresponding
aldehydes been obtained, suggesting the N-methylation
Extension of conjugation pathways is desirable to
enhance molecular nonlinearity. Jen and co-workers
also revealed that molecular nonlinearity can be further
improved when thiophene is used in the conjugation
pathways.6 However, simple annexation of conjugation
pathways often leads to a compromise of thermal sta-
bility and solubility. In particular, when a nitro group
is used as an acceptor, it is likely that a direct place-
ment of the nitro group on the thiophenyl end will
severely compromise thermal stability.5 This has
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02143-2

806
J. Santos et al. / Tetrahedron Letters 42 (2001) 805–808
of 1–4 is very necessary for the successful formulation
made to manifest the contribution of thiophenevinyl
conjugation in 13–16. The synthesis started with the
direct condensation of benzil derivatives and 4-nitrobe-
naldehyde in the presence of ammonium acetate, fol-
lowed by N-methylation by iodomethane in the
presence of potassium carbonate (Scheme 3). The chro-
mophore 20 absorbs at 399 nm and 21 at 443 nm.
Compared with 20, the chromophore 15 has a red shift
of 37 nm while 16 is red shifted by 20 nm relative to
21. These large shifts clearly indicate that the present
system with larger delocalized conjugation pathway
facilitates charge transfer from donors to acceptors,
leading to more effective D–A chromophores. In addi-
tion, the transparency of chromophores 13–16 in the
visible region has also been determined. The chro-
mophore 13 is transparent from 550 nm while 14, 15
and 16 are transparent from 560, 570, and 650 nm,
respectively.
in the next step.
The new NLO chromophores 13–16 were initially pre-
pared by the condensation of the aldehydes 9–12 with
4-nitrophenylacetic acid, but with less satisfactory yield
and more time-consuming separation due to several
undesirable by-products. It was found that the reaction
of a Wittig reagent with 9–12 in the presence of potas-
sium tert-butoxide generated the chromophores in
moderate to good yield (13, 75%; 14, 65%; 15, 67%; 16,
43%). The reaction was promoted by 15-crown-5 in
methylene chloride. The use of 18-crown-6 led to a
similar result of the yield.
Chromophores 18–21 have been prepared which bear
identical donors and acceptors to those in 13–16 but
without thiophenevinyl units. The comparison has been
Scheme 1. i: NH4OAc/HOAc, 120°C; ii: MeI/K2CO3, 55°C; iii: BuLi/THF, −78°C to 0°C; iv: DMF; v: 15-crown-5/t-BuOK/
CH2Cl2/1,4-O2NC6H4-CH2PPh3Br.
Scheme 2.

J. Santos et al. / Tetrahedron Letters 42 (2001) 805–808
807
Scheme 3.
Table 1. EFISH data for second-order nonlinear properties of 13–16 and 18–21a
Chromophores
vi (10−48)esu
Chromophores
vi (10−48)esu
D(vi) (10−48)esu
13
14
15
16
370
945
475
590
18
19
20
21
100
145
130
360
270
800
345
230
a vi @ 1907 nm using a quartz reference (d11=0.277 pm/V).
Table 2. Electronic absorption and thermal properties of 13–16
Chromophores
umax (nm)8
m (M−1cm−1
)
ucut off (nm)
Td(°C)
T5(°C)
Tm (°C)9
13
14
15
16
426
432
436
463
3.05×104
3.43×104
3.04×104
4.32×104
550
560
570
650
340
320
332
242
317
316
337
251
213
236
119
242
The second-order nonlinear optical properties of the
two series of chromophores have been evaluated by vi,
the scalar product of the dipole moment (v) and the
molecular first-order hyperpolarizability (b). The values
of vi for both series as determined by electric field-
induced second-harmonic generation (EFISH) at a fun-
damental wavelength of 1907 nm10,11 are listed in Table
1. While both series show second-order nonlinear opti-
cal properties, the one with the longer conjugation
pathway was found to have much enhanced properties.
The chromophores 18–21 exhibited vis in the range of
100 to 360×10−48 esu. In marked contrast, the chro-
mophores 13–16 showed significantly larger vis which
range from 370 to 945×10−48 esu. The parallel compari-
son was made between 13 and 18, 14 and 19, 15 and 20,
and 16 and 21, revealing that the vi value of 13 is 3.7
times that of 18, and so is the value of 15 compared to
that of 20. The vi value of 16 is 1.6 times that of 21
while the value of 14 is 6.5 times that of 19. If we
characterize the contribution of the conjugation path-
ways (series 18–21 vs series 13–16) in terms of the
difference in vi values we see that the contribution of
the thiophenylvinyl conjugation pathway ranges from
230 to 800×10−48 esu. The largest difference of 800×
10−48 esu was found between compounds 14 and 19.
Thermal stability for this new series of chromophores
has been evaluated by means of differential scan
calorimetry (DSC) and thermogravimetric analysis
(TGA). The onset decomposition temperature, Td, at a
heating rate of 10°C min−1 in air is obtained from DSC
by the intercept of the positive slope of the decomposi-
tion peak with the baseline, and a 5% weight loss
temperature, T5, is obtained from TGA with the same
heating rate of 10°C min−1 in air (Table 2). All the
chromophores except 16 exhibit high thermal stability
with the onset decomposition temperatures at or higher
than 320°C. These chromophores also show 5% weight
loss at 316°C or higher. The most impressive chro-
mophore is 15 which does not start decomposition until
332°C, and just loses 5% weight up to 337°C. The
overall thermal stability remains high in spite of present
expansion of conjugation pathway designed for high
nonlinearity. In addition, all the new chromophores are
soluble in common organic solvents such as THF,
chloroform and DMF. Therefore, the addition of thio-
phenevinyl to the conjugation pathway and the attach-
ment of the nitro acceptor to the phenyl end lead to
enhanced nonlinearity without compromising thermal
stability and solubility.

808
J. Santos et al. / Tetrahedron Letters 42 (2001) 805–808
In summary, a new class of heterocyclic imidazole
based NLO chromophores has been developed. The
new chromophores possess large conjugation pathway
to facilitate charge transfer and enhance nonlinearity.
They also exhibit high thermal stability, excellent solu-
bility, and good transparency.
3. Li, H. Y.; Bu, X. R.; Ahmed, M.; Mintz, E. A. Polym.
Prep. 1996, 37 (1), 599.
4. Moylan, C. R.; Miller, R. D.; Twieg, R. J.; Betterton,
K. M.; Lee, V. Y.; Matray, T. J.; Nguyen, C. Chem.
Mater. 1993, 5, 1499.
5. Twieg, R. J.; Betterton, K. M.; Burland, D. M.; Lee, V.
Y.; Miller, R. D.; Moylan, C. R.; Volksen, W.; Walsh,
C. A. SPIE 1993, 2025, 94.
6. Jen, A. K.-Y.; Rao, V. P.; Wong, K. Y.; Drost, K. J. J.
Chem. Soc. Chem. Commun. 1991, 90.
Acknowledgements
7. Other types of chromophores from the aldehydes 9–12
are being developed and will be reported in due course.
8. Concentration used in this study is in the range of 10−5
M/L in chloroform.
The support of this project by NASA(NCC-533) and by
Ballistic Missile Defense Organization managed by
Office of Naval Research (N00014-99-1-0962) is grate-
fully acknowledged. We also thank Dr. Gafai Lai and
Haiying Li for the early preparation of some of the
compounds.
9. Melting point (Tm) is obtained in DSC run at a heating
rate of 10°C min−1 in air.
10. Bosshard, C.; Kno¨pfle, G.; Preˆtre, P.; Gu¨nter, P. J.
Appl. Phys. 1992, 71, 1594.
11. Bosshard, C.; Bo¨sch, M.; Liakatas, I.; Ja¨ger, M.; Gu¨n-
ter. P. In Nonlinear Optical Effects and Materials; P.
Gu¨nter, Ed. Second-order nonlinear optical organic
materials: recent developments. Springer Series in Opti-
cal Sciences, Springer Verlag: New York, 2000; Vol. 72,
pp. 163–299.
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