Effect of modified donor units on molecular hyperpolarizability of thienyl-vinylene nonlinear optical chromophores

Article abstract of DOI:10.1016/j.tetlet.2010.08.060

Highly efficient and thermally stable nonlinear optical chromophores based on the phenyl vinylene thiophene vinylene (FTC) donor-π-acceptor structure have been synthesized and investigated. The donor part of the chromophores was modified with additional d

Full text of DOI:10.1016/j.tetlet.2010.08.060

Tetrahedron Letters 51 (2010) 5873–5876
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Effect of modified donor units on molecular hyperpolarizability
of thienyl-vinylene nonlinear optical chromophores
Xianmin Zhang a,d, Isao Aoki b, Xianqing Piao a, Shinichiro Inoue a, Hidehisa Tazawa c,
b,
Shiyoshi Yokoyama a,b, , Akira Otomo
⇑
⇑
a Institute for Materials Chemistry and Engineering, Kyushu University, 6-1, Kasuga Koen, Kasuga 816-8580, Japan
b National Institute of Information and Communications Technology, 588-2 Iwaoka, Nishi-ku, Kobe 651-2492, Japan
c Sumitomo Electric Industries, Ltd, 1 Taya-cho, Sakae-ku, Yokohama 244-8588, Japan
d Department of Physics, Liaoning University, 66 Chongshan Middle Road, Shenyang 110036, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly efficient and thermally stable nonlinear optical chromophores based on the phenyl vinylene thi-
ophene vinylene (FTC) donor-p-acceptor structure have been synthesized and investigated. The donor
Received 16 June 2010
Revised 11 August 2010
Accepted 18 August 2010
part of the chromophores was modified with additional donor units, resulting in the enhanced nonlinear
optical property with large molecular hyperpolarizability. Hyper-Rayleigh scattering measurement indi-
cated nearly threefold increase of the molecular hyperpolarizability for novel chromophores compared
with the benchmark FTC chromophore. Furthermore, measurement of the electro-optic coefficient con-
firmed that enhancement of microscopic molecular hyperpolarizability of the chromophores can be effec-
tively translated into macroscopic electro-optic property. Measured electro-optic coefficients were nearly
twofold larger than that for the benchmark FTC. Thermal analysis indicated that the synthesized chro-
mophores showed the excellent temperature stability with decomposition temperatures up to 280 °C.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Nonlinear optics
Molecular hyperpolarizability
Chromophores
Donor–acceptor systems
Thermal stability
Over the past few decades, considerable progress has been
made in the development of organic and polymeric electro-optic
(EO) materials for applications in ultrafast optical modulators,
optical sensors, and information processors.1–3 One of the chal-
lenges encountered in preparing highly efficient EO materials is
the development of nonlinear optical (NLO) chromophores with
large molecular hyperpolarizability (b), high optical transparency,
and excellent thermal and chemical stabilities.4–9 Most of expected
bridges in the synthesis of chromophores, few studies have exam-
ined the use of various types of electron donors to obtain chro-
mophores with large molecular hyperpolarizability.15–17
This paper describes our experimental approach to the design
and synthesis of NLO chromophores containing strong donor sub-
units and with large molecular hyperpolarizability. The use of new
p-electron donors as the additional subunits to the donor part in
the donor- -acceptor molecules was examined (Fig. 1). The syn-
p
chromophores have a
p
-electron conjugated structure and are cou-
thesized chromophores contained a phenyl vinylene thiophene
vinylene bridge and a TCF acceptor part. The NLO property of the
synthesized chromophores was characterized using the hyper-
Rayleigh scattering (HRS) technique. The HRS method is a reliable
technique for measuring the molecular hyperpolarizability. The
chromophore denoted as FTC, having 4-(dibutylamino)aryl group
as the donor, was chosen as a benchmark chromophore for com-
parison. In order to determine the electro-optic property of the
chromophores, electric-poled polymer films containing synthe-
pled to electron donor and electron acceptor units, which indicate
large intramolecular charge-transfer transitions at high b values. A
successful approach for preparing chromophores with large b val-
ues is to consider their ground-state polarization and bond-length
alternation upon modification with electron donors, electron
acceptors, and
p
-electron bridges.10–14 In particular, high-b chro-
mophores with excellent EO responses have been obtained when
the strong electron acceptors such as 2-dicyanomethylene-
3-cyano-4,5,5-dimethyl-2,5-dihydrofuran (TCF) and 2-dicyano-
methylene-3-cyano-4,5-dimethyl-5-trifluoromethyl-2,5-dihydrofuran
(CF3-TCF) were used.12 Although many studies have been per-
sized chromophores were prepared and the EO coefficients (r33
)
were measured at 1.31 m using reflection technique. In addition
l
to NLO measurements, relevant linear optical spectra and thermal
formed on the investigation of electron acceptors and
p-electron
properties are presented and discussed herein.
The synthesis of donor parts of the chromophores is depicted in
Scheme 1. Modified 4-(dibutylamino)ary compounds 1b–1e were
synthesized from 4-(dibutylamino)salicylaldehyde 1a by treat-
ment with appropriate reagents and conditions. All the donor parts
were obtained with high yield and were characterized by 1H and
⇑
Corresponding authors. Address: Institute for Materials Chemistry and Engi-
neering, Kyushu University, 6-1, Kasuga Koen, Kasuga 816-8580, Japan (S.Y).
E-mail addresses: s_yokoyama@cm.kyushu-u.ac.jp (S. Yokoyama), akira_o@nict.
go.jp (A. Otomo).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.08.060

5874
X. Zhang et al. / Tetrahedron Letters 51 (2010) 5873–5876
Figure 1. The molecular structure of synthesized chromophores and FTC benchmark.
density at C1, resulting in a upfiled shift of C1 relative to
124.5 ppm.
For this study, five new NLO chromophores 5a–5e consisting of
modified donors, a conjugated thiophene bridge, and the TCF
acceptor were synthesized following the general route shown in
Scheme 2. The thiophene bridge was prepared in two steps for
obtaining Wittig salt 2. After Wittig condensation of the salt with
the aldehydes 1b–1e, the desired thiophene bridges were pro-
duced. The compounds 3b–3e were then formylated effectively
with n-butyl lithium and DMF to form 4b–4e. The TCF acceptor
was synthesized according to the literature method12 and was cou-
pled to the thiophene bridges by Knoevenagel condensation to af-
ford chromophores 5b–5e. Chromophore 5a was synthesized from
thiophene bridge 4a, which was obtained after the deprotection
reaction of 4e.
Thermal properties of chromophores were measured by differ-
ential scanning calorimetry and are summarized in Table 1. Chro-
mophore 5e containing TBDPS unit shows a clear glass transition
at 81 °C followed by the crystallization at 171 °C and a melting
point at 205 °C. Chromophores 5b and 5c only exhibit glass transi-
tion temperatures (Tg), suggesting that these chromophores tend
to form amorphous glasses. However, chromophores 5a and 5d
did not exhibit any glass transitions. Thermo gravimetry and differ-
ential thermal analysis indicated that synthesized chromophores
had high temperature resistance: their decomposition tempera-
tures (Td) were above 220 °C as summarized in Table 1. In particu-
lar, the highest Td was observed for the chromophore 5e (283 °C).
The excellent thermal stability of these chromophores makes them
suitable for practical device fabrication and EO applications.
The absorption spectra of the synthesized chromophores were
measured in chloroform as shown in Figure 2a. The spectrum data
Scheme 1. Synthesis of the donors 1b–1e.
13C NMR spectroscopy. To clarify additional unit’s
p-donating
interaction with the donor part, 13C NMR spectra were used to sys-
tematically compare their electronic effect. From 13C NMR, the
chemical shifts of C1 position in the phenyl (1a–1e) are located
at around 114–116 ppm. While, the chemical shift of C1 in 4-(dib-
utylamino)benzaldehyde is at 124.5 ppm and serves as a reference
value. 13C NMR analysis confirmed that any electron-donating
groups (RO-) introduced at C2 position would increase the electron
Scheme 2. Synthesis of chromophores 5a–5e.

X. Zhang et al. / Tetrahedron Letters 51 (2010) 5873–5876
5875
Table 1
wavelength of excitation laser could be selected between 1.3 and
1.9 m using an optical parametric oscillator with the Q-switched
Glass transition temperature, decomposition temperatures, absorption maximum,
and solvatochromism data of the chromophores
l
Nd:YAG laser. The experimental technique provides b values rela-
tive to the b of chloroform (0.49 Â 10À30 esu), where the chloro-
form solvent acts as the internal standard. We confirmed that the
b of benchmark FTC was comparable to the literature value at
D
kd (nm)
a
b
c
Compounds
Tg (°C)
Td (°C)
kmax (nm)
kmax (nm)
FTC
5a
5b
5c
5d
5e
—
—
76
61
—
220
220
274
253
262
283
685
709
717
717
720
721
624
643
643
644
643
648
61
66
74
73
77
73
the laser wavelength of 1.3 l
m.20 However, in order to compare
molecular hyperpolarizability among chromophores with different
absorption characteristics, it must be taken to account for reso-
nance effects.21–25 Thus, we performed HRS measurement on the
81
a
Td was the onset decomposition temperature measured by TG-DTA under
chromophores 5a–5e at the longer laser wavelength of 1.9
Since the HRS signal from pure chloroform is negligible at
1.9 m, HRS intensity from the chromophore solution was con-
lm.
nitrogen at a heating rate of 5 °C/min.
b
kmax were measured in chloroform.
c
l
kmax were measured in 1,4-dioxane.
d
D
k was the difference between kamax and kmb ax
.
verted to the b value using the external reference method using
the benchmark FTC, which has a known b value as the reference
at 1.9 l
m.20 The results of HRS measurement are presented in Ta-
ble 2. HRS analysis indicates that the addition of donor groups is
effective in increasing the b values. The result of the b enhance-
ment for 5a–5e was consistent with the result of absorption spec-
tral analysis. The factors of increase in the b value of 5a and 5b
were 1.51 and 1.78, respectively, by comparison with FTC. The
enhancement of b value in 5c and 5d was more than twofold. In
particular, the substantial increase in b value of 5e, an almost
threefold enhancement, was measured with the experimentally
obtained b value of 4430 Â 10À30 esu.
Polymer films for EO coefficient (r33) measurements were pre-
pared by mixing the chromophore in poly(methyl methacrylate)
(PMMA) at a number density of 2.1 Â 1020 molecules/cm3 by using
cyclopentanone as a solvent. The solution was filtrated through a
0.2
(ITO) glass substrate to produce a film with the thickness of
m. The films were then baked at 80 °C in a vacuum to remove
lm syringe filter and spin cast onto an indium-tin-oxide
2
l
the residual solvent. The result of absorption spectra of the chro-
mophores in PMMA films is summarized in Table 2. The kmax of
5a–5e shows at around 670 nm, which are 20 nm red-shifted com-
pared to that of FTC/PMMA film. The trend of this shift is similar to
the results measured in the chloroform and dioxane solutions.
After sputtering a gold electrode on to the polymer film, the elec-
tric poling was carried out by applying an electrical field of about
Figure 2. UV–vis absorption spectra in chloroform (a) and 1,4-dioxane (b).
100 V/
temperature of PMMA. The r33 values of poled films were mea-
sured by the Teng–Man reflection method26 with a 1.31
m laser
and are summarized in Table 2. As expected, all the films contain-
ing chromophores 5a–5e achieved higher r33 values than the r33 of
FTC benchmark. The results confirm that the enhancement of the
microscopic b values in chromophores can be translated into the
performance of macroscopic EO coefficient. Especially, 5c and 5e
showed nearly twofold increase in the r33 values compared with
that of FTC.
lm at elevated temperatures up to near the glass transition
is summarized in Table 1. The effect of the electric donor on the
l
p-conjugated bridges was characterized by shifting the absorption
maximum (kmax) compared with kmax of the benchmark FTC. When
an additional hydroxyl group was added to the donor part of 5a,
kmax was shifted 24 nm to the longer wavelength of 709 nm. A sim-
ilar shift in the case of 5b and 5c indicates that the addition of
methoxy and benzyloxy groups to the donor parts provides equiv-
alent electron-donating effect on the
p-conjugated bridge. Larger
shifts of kmax were measured for 5d and 5e, indicating the strong
electron-donating effect of the isopropoxy and TBDPS units. The
absorption spectra of the chromophores were also measured in
1,4-dioxane as shown in Figure 2b, so that solvatochromic behavior
of the chromophores could be explored; their spectroscopic prop-
erties are summarized in Table 1. On the basis of simple solvent
studies using polar chloroform and less polar 1,4-dioxane, it was
found that chromophores 5b–5e showed a larger energy shift of
In summary, five new chromophores based on strong aryl-ami-
no electron donors with FTC structure were synthesized. Efficient
Table 2
Optical properties and electro-optic coefficients of chromophores
b
c
Compounds
b/bFTC
ba (Â10À30 esu)
kmax (nm)
r33 (pm/V)
FTC
5a
5b
5c
5d
5e
1
1500 170
2260 250
2670 310
3160 390
3490 490
4430 660
648
676
672
669
674
670
29
38
45
57
49
55
5
3
5
4
5
5
1.51
1.78
2.11
2.33
2.95
the charge-transfer band (Dk = 73–77 nm) than FTC (Dk = 61 nm).
The resulting spectrum data confirm the concept that donor-en-
hanced FTC chromophores are more strongly polarizable than the
benchmark.11,12,14
The molecular hyperpolarizabilities of chromophores were
measured in chloroform using the hyper-Rayleigh scattering
a
b values were measured in chloroform using the hyper-Rayleigh scattering at
1.9
lm. FTC was used as an external standard.
(HRS) technique at a laser wavelength of 1.9
nique allows the direct evaluation of b values, employing the refer-
ence chromophore with known value. In our HRS setup,
l
m.18,19 The tech-
b
kmax was measured in PMMA film.
r33 values were measured by Teng–Man reflection method at the wavelength of
c
1.31 lm.
b

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X. Zhang et al. / Tetrahedron Letters 51 (2010) 5873–5876
5. 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, 3267–3286.
6. Dalton, L. R.; Philip, A. S.; Denise, H. B. Chem. Rev. 2010, 110, 25–55.
7. Zhang, C.; Wang, C.; Dalton, L. R.; Zhang, H.; Steier, W. H. Macromolecules 2001,
34, 235–243.
8. He, M.; Leslie, T.; Garner, S.; DeRosa, M.; Cites, J. J. Phys. Chem. B 2004, 108,
8731–8736.
9. Kim, T.; Juo, J.; Cheng, Y.; Shi, Z.; Hau, S.; Jang, S.; Zhou, X.; Tian, Y.; Polishak, B.;
Huang, S.; Ma, H.; Dalton, L. R.; Jen, A. K. J. Phys. Chem. C 2008, 112, 8091–8098.
10. Dalton, Larry. R. Opt. Eng. 2000, 39, 589–595.
synthetic approaches were developed to prepare these chromoph-
ores through simple chemical reactions. In addition to high Td data,
the linear and nonlinear optical properties of all chromophores
were thoroughly investigated. Synthesized chromophores showed
a nearly threefold enhancement in molecular hyperpolarizability.
The EO coefficient of novel chromophores in PMMA host were
nearly two times higher as compared to previously reported
benchmark FTC chromophore.
11. He, M. Q.; Leslie, T. M.; Sinicropi, J. A. Chem. Mater. 2002, 14, 4662–4668.
12. Zhang, C.; Dalton, L. R.; Oh, M.-C.; Zhang, H.; Steier, W. H. Chem. Mater. 2001,
13, 3043–3050.
Acknowledgments
13. Liu, S.; Haller, M. A.; Ma, H.; Dalton, L. R.; Jang, S.-H.; Jen, A. K.-Y. Adv. Mater.
2003, 15, 603–607.
14. Andrew, R.; Franco, S.; Galán, E.; Garín, J.; Martínez de Baroje, N.; Momblona,
C.; Orduna, J.; Alicante, R.; Villacampa, B. Tetrahedron Lett. 2010, 51, 3662–
3665.
15. Spraul, B. K.; Suresh, S.; Sassa, T.; Herranz, Á.; Echegoyen, L.; Wada, T.; Perahia,
D., ; Smith, D. W., Jr. Tetrahedron Lett. 2004, 45, 3253–3256.
16. Cheng, Y.; Luo, J.; Hau, S.; Bale, D. H.; Kim, T.; Shi, Z.; Lao, D. B.; Tucker, N. M.;
Tian, Y.; Dalton, L. R.; Reid, P. J.; Jen, A. K. Chem. Mater. 2007, 19, 1154–1163.
17. Davies, J. A.; Elangovan, A.; Sullivan, P. A.; Olbricht, B. C.; Bale, D. H.; Ewy, T. R.;
Isborn, C. M.; Eichinger, B. E.; Robinson, B. H.; Reid, P. J.; Li, X.; Dalton, L. R. J.
Am. Chem. Soc. 2008, 130, 10565–10575.
18. Clays, K.; Persoons, A. Phys. Rev. Lett. 1991, 66, 2980–2983.
19. Clays, K.; Persoons, A. Rev. Sci. Instrum. 1992, 63, 3285–3289.
20. Firestone, K. A.; Lao, D. B.; Casmier, D. M.; Clot, O.; Dalton, L. R.; Reid, P. J. Proc.
SPIE-Int. Soc. Opt. Eng. 2005, 5935, (Linear and Nonlinear Optics of Organic
Materials V), 49.59350P.
21. Oudar, J. L.; Chemla, D. S. J. Chem. Phys. 1977, 66, 2664–2668.
22. Meshulam, G.; Berkovic, G.; Kotler, Z. Opt. Lett. 2001, 26, 30–32.
23. Kauranen, M.; Verbiest, T.; Boutton, C.; Teerenstra, M. N.; Clays, K.; Schouten, A.
J.; Nolte, R. J. M.; Persoons, A. Science 1995, 270, 966–969.
24. Raposo, M. M. M.; Sousa, A. M. R. C.; Kirsch, G.; Cardoso, P.; Belsley, M.; de
Matos Gomes, E.; Fonseca, A. M. C. Org. Lett. 2006, 8, 3681–3684.
25. Cho, B.; Chajara, K.; Oh, H. J.; Son, H.; Jeon, S. Org. Lett. 2002, 4, 1703–1706.
26. Teng, C. C.; Man, H. T. Appl. Phys. Lett. 1990, 56, 1734–1736.
We acknowledge financial support from a Grant-in-Aid for
Young Scientists (S) and the Global COE (Centers of Excellence)
Program of the Ministry of Education, Culture, Sports, Science
and Technology, Japan.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.08.060.
References and notes
1. Shi, Y. Q.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.;
Steiner, W. H. Science 2000, 288, 119–122.
2. Lee, M.; Katz, H. E.; Erben, C.; Gill, D. M.; Gopalan, P.; Heber, J. D.; McGee, D. J.
Science 2002, 298, 1401–1403.
3. 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.; Peyghambarian, N. Nat. Photon. 2007, 1,
180–185.
4. 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, 1387–1390.