A new class of second-order non-linear optical material: Stilbazolium benzimidazolate derived from alkylsulfonyl substituted stilbazole

Article abstract of DOI:10.1039/a708620b

A novel stilbazolium benzimidazolate derivative, i.e. 2-(4-{2-[4-(octylsulfonyl)phenyl]ethenyl}pyridinio)benzimidazolate 4a, was prepared by the quaternization reaction of 4-{2-[4-(octylsulfonyl)phenyl]ethenyl}pyridine with 2-chlorobenzimidazole, followed by deprotonation with aqueous ammonia. Poly(methyl methacrylate) (PMMA) thin films containing 10 or 20 mass% of 4a were obtained by spin-coating from their THF solutions on an ordinary cover glass substrate. Second-harmonic generation (SHG) measurements of the obtained thin films were carried out by the Maker fringe method using a Q-switched Nd : YAG laser (1064 nm) as an exciting beam after corona-poling. A poled PMMA film containing 10 or 20 mass% of 4a exhibited a second-order non-linear optical (NLO) coefficient, d₃₃, of 7.6 or 11 pm V^-1, respectively, which was much larger than the d₃₃ value of a PMMA thin film containing 10 mass% of 2-[4-(2-phenylethenyl)pyridinio]benzimidazolate 4c (d₃₃ 1.6 pm V^-1) or 2-{4-[2-(4-octyloxyphenyl)ethenyl]pyridinio}benzimidazolate 4d (d₃₃ 0.51 pm V^-1). There were no significant differences in linear absorption properties of PMMA films containing 4a, 4c or 4d. The introduction of an octylsulfonyl group at the 4′-position of the stilbazole moiety increases the second-order NLO coefficient.

Full text of DOI:10.1039/a708620b

J O U R N A L O F
A new class of second-order non-linear optical material: stilbazolium
benzimidazolate derived from alkylsulfonyl substituted stilbazole
Nobukatsu Nemoto,a Jiro Abe,b Fusae Miyata,a Yasuo Shiraib and Yu Nagase*a
aSagami Chemical Research Center, 4–4-1 Nishi-Ohnuma, Sagamihara, Kanagawa 229, Japan
bDepartment of Photo-optical Engineering, Faculty of Engineering, T okyo Institute of
Polytechnics, 1583 Iiyama, Atsugi, Kanagawa 243–02, Japan
C H E M I S T R Y
A novel stilbazolium benzimidazolate derivative, i.e. 2-(4-{2-[4-(octylsulfonyl)phenyl]ethenyl}pyridinio)benzimidazolate 4a, was
prepared by the quaternization reaction of 4-{2-[4-(octylsulfonyl)phenyl]ethenyl}pyridine with 2-chlorobenzimidazole, followed
by deprotonation with aqueous ammonia. Poly(methyl methacrylate) (PMMA) thin films containing 10 or 20 mass% of 4a were
obtained by spin-coating from their THF solutions on an ordinary cover glass substrate. Second-harmonic generation (SHG)
measurements of the obtained thin films were carried out by the Maker fringe method using a Q-switched Nd5YAG laser
(1064 nm) as an exciting beam after corona-poling. A poled PMMA film containing 10 or 20 mass% of 4a exhibited a second-
order non-linear optical (NLO) coefficient, d , of 7.6 or 11 pm V−1, respectively, which was much larger than the d value of a
33
33
PMMA thin film containing 10 mass% of 2-[4-(2-phenylethenyl)pyridinio]benzimidazolate 4c (d 1.6 pm V−1) or 2-{4-[2-(4-
33
octyloxyphenyl)ethenyl]pyridinio}benzimidazolate 4d (d 0.51 pm V−1). There were no significant differences in linear
33
absorption properties of PMMA films containing 4a, 4c or 4d. The introduction of an octylsulfonyl group at the 4∞-position of the
stilbazole moiety increases the second-order NLO coefficient.
Second-order non-linear optical organic molecules, which have
been the subject of many reports, are aromatic compounds
with a pair of electron-donor and electron-acceptor groups at
p-conjugating sites, so-called classical D-p-A system mol-
ecules.1,2 Recently, various types of NLO molecules classified
into non-classical molecular systems have been developed3 for
the improvement of the physical properties of NLO molecules
and for solving the problem of the trade-off between optical
non-linearity and cutoff wavelength, which is generally defined
as the wavelength where the value of the first deviation for the
absorbance becomes 0. Heterocyclic betaines have received
much attention because of their unusually high dipole
moments, which are ascribed to their zwitterionic character;
this is confirmed by the fact that they exhibite negative
solvatochromism.4 Pyridinium or stilbazolium benzimidazol-
ates, which consist of a negatively charged aromatic donor
group and a positively charged aromatic acceptor group, are
heterocyclic betaines. The NLO activities of the present pyridi-
nium or stilbazolium benzimidazolates are attributed to short-
range charge transfer through the s-bond from the charged
aromatic donor group to the charged aromatic acceptor group.
We have revealed that some pyridinium or stilbazolium
benzimidazolates classified as non-classical molecular systems
are applicable as second-order non-linear optically active
molecules from theoretical investigations5 and hyper-Rayleigh
scattering measurements.6 One distinct characteristic of pyri-
dinium betaine compounds is that the value of the first
hyperpolarizability, b, is enlarged with decreasing dipole
moment.7 Namely, from theoretical calculations,8 the introduc-
tion of an electron-acceptor substituent at the 4∞-position of
the stilbazole moiety in stilbazolium benzimidazolate decreased
the dipole moment in the ground state, but increased the b
value compared with stilbazolium benzimidazolate without a
substituent at the 4∞-position of the stilbazole moiety. In many
cases, an increase in b for classical D-p-A system molecules is
accompanied by an increase in the dipole moment.2 A large
polarizability often brings temporal or thermal relaxation to
the noncentrosymmetric alignment of NLO-phores in a poled
polymer film due to dipole-dipole interactions, which results
in the temporal or thermal relaxation of second-order NLO
activity. It may be possible for pyridinium betaine compounds
to overcome this problem, due to their characteristics as
mentioned above. We have also investigated poled thin films
of some pyridinium or stilbazolium benzimidazolates dispersed
in poly(methyl methacrylate) (PMMA) via Maker fringe
measurements.9 However, the limited solubility and centro-
symmetric aggregation of pyridinium betaines in polymeric
matrixes seemed to inhibit the increase of their second-order
NLO susceptibility.
To solve these problems, we have prepared two kinds of
novel stilbazolium benzimidazolates 4a and 4b, the phenyl
groups of which are substituted by electron-acceptor groups,
i.e. octylsulfonyl or perfluorooctylsulfonyl groups, which would
cause not only an increase in the b value but also render them
more soluble in organic solvents owing to their decreased
dipole moment in the ground state. Additionally, a poly(methyl
methacrylate) (PMMA) thin film containing 4a was prepared,
the linear optical and second-order NLO properties of which
are described.
Experimental
Materials
N,N-Dimethylformamide (DMF) was distilled over CaH
under reduced pressure. Butan-1-ol and triethylamine (Wako
2
Pure Chemical Industries, Ltd.) were used after distillation
over CaH . 4-Bromobenzenethiol, 1-bromooctane (Tokyo
* E-mail: yunagase@alles.or.jp
2
J. Mater. Chem., 1998, 8(5), 1193–1197
1193

Kasei Kogyo Co., Inc.), palladium() acetate, 30% aqueous
hydrogen peroxide and acetic acid (Kanto Chemical Co., Inc.)
were commercially available and used as received. 4-
Vinylpyridine was distilled under reduced pressure just before
use. 2-Chlorobenzimidazole was prepared by modifying the
column packed with silica gel with hexane–ethyl acetate (751)
as eluent. Evaporation of the solvent afforded the title com-
pound 2a (14.83 g, 94.5%) as colourless crystals: d (CDCl ,
H
3
90 MHz) 0.86 [t, J 6.9, 3H, CH (CH ) ], 1.1–2.0 [m, 12H,
2 7
CH (CH ) CH ], 3.0–3.3 (m, 2H, CH SO ), 7.73 (s, 4H, phen-
3
3
2 6
2
2
2
ylene protons); n
/cm−1 3090, 3065, 2955, 2925, 2850, 1910,
method reported by Harrison et al.10 PMMA (M 54.3×105,
max
n
determined by gel permeation chromatography; T 101 °C
g
1780, 1650, 1580, 1470, 1410, 1390, 1325, 1315, 1305, 1280,
1245, 1215, 1205, 1180, 1145, 1085, 1065, 1010, 985, 960,
930, 895, 855, 820, 795, 770, 735, 725, 620, 565, 550, 530,
495, 455; m/z 334 (M++2), 332 (M+), 317, 315, 291
[(M++2)−(C H )], 289 [M+−(C H )], 249, 247, 234, 232,
determined by differential scanning calorimetry) was purchased
from Nacalai tesque, Inc. and used as received.
Instrumentation
3
7
3 7
223, 221, 203, 201, 197, 195, 185, 183, 171, 169, 157
UV-VIS absorption spectra were measured by transmission
on a Shimadzu Model U-2100 spectrophotometer. 1H NMR
spectroscopy was conducted with a Hitachi R-90H FT NMR
(90 MHz) spectrometer or a Bruker AM-400 FT NMR
(400 MHz) spectrometer; J values are given in Hz. IR Spectra
were measured by transmission on a Jasco A-202 IR spec-
trometer. Mass spectrometry was conducted on a Hitachi
Mass Spectrometer M-80B by electron ionization method.
Differential scanning calorimetry (DSC) measurements were
carried out on a Shimadzu Model DSC-50 under a helium
flow rate of 20 ml min−1 and a heating rate of 10 °C min−1.
[(M++2)−(C H SO )], 155 [M+−(C H SO )], 141, 112, 93,
C, 50.29; H, 6.32%).
3
7
2
3
7
2
71, 57, 43, 29 (Found: C, 50.45; H, 6.35. Calc. for C H BrO S:
14 21
2
Compound 2b was prepared via a similar method as for 2a
using 1b instead of 1a. The product yield was 87%: d (CDCl ,
H
3
90 MHz) 7.86 (s, 4H, phenylene protons); n /cm−1 3095,
max
1575, 1470, 1395, 1365, 1330, 1285, 1205, 1175, 1150, 1080,
1070, 1010, 935, 830, 800, 750, 705, 660, 645, 615, 595, 550,
530; m/z (SIMS) 641 (M++3), 639 (M+ + 1) (Found: C, 26.2;
H, 0.4. Calc. for C H BrF O S: C, 26.31; H, 0.63%).
14
4
17
2
4-{2-[4-(Octylsulfonyl )phenyl]ethenyl}pyridine 3a
4-(Octylthio)bromobenzene 1a
Under an argon atmosphere, a mixture of 2a (9.998 g,
30.0 mmol), 4-vinylpyridine (4.206 g, 40.0 mmol), triethylamine
(3.036 g, 30.0 mmol), palladium () acetate (0.203 g, 0.90 mmol)
and 15 ml of dry acetonitrile was degassed, refluxed for 72 h
and cooled. To this reaction mixture was added chloroform
and water. The crude product was extracted with chloroform,
and the organic layer was dried over anhydrous sodium sulfate.
The chloroform was evaporated, and the residue was purified
by column chromatography, using a column packed with silica
gel with hexane–ethyl acetate (251) as eluent. Finally, recrys-
tallization of the product from of ethyl acetate–hexane afforded
the title compound 3a with a yield of 8.093 g (75.5%) as
white crystals: d (CDCl , 400 MHz) 0.86 [t, J=7.0, 3H,
Under an argon atmosphere, 4-bromobenzenethiol (9.45 g,
50.0 mmol) in dry DMF (10 ml) was added dropwise to sodium
hydride (2.40 g, 60.0 mmol, 60% in mineral oil) suspended in
dry DMF (30 ml) in an ice bath. The reaction mixture was
stirred at ambient temperature for 1 h, and 1-bromooctane
(10.62 g, 55.0 mmol) was added dropwise. After the reaction
mixture was stirred at ambient temperature for 2 h, DMF was
evaporated under reduced pressure. Water and ethyl acetate
were added to the residue, and the organic layer was washed
with water. The organic layer was dried with anhydrous
sodium sulfate, and the solvent was evaporated to dryness.
The crude product was purified by column chromatography,
using a column packed with silica gel with hexane as eluent.
The product yield was 14.97 g (99%) as a colourless liquid:
d (CDCl , 90 MHz) 0.88 [t, J 7.0, 3H, CH (CH ) ], 1.1–2.0
H
3
CH (CH ) ], 1.2–1.3 [m, 8H, CH (CH ) CH ], 1.36 [quintet,
2 2 2 2
J 7.0, 2H, CH (CH ) CH ], 1.7–1.8 [m, 2H, CH (CH ) CH ],
2 4 2 2
3.1 (m, 2H, CH SO ), 7.16 (d, J 16.4, 1H, NCH-pyridyl), 7.33
3
3
2
3
2
3
2
H
3
3
2 7
2
2
[m, 12H, CH (CH ) CH ], 2.88 (t, J 7.0, 2H, CH S), 7.22 (dt,
J 2.2, 8.8, 2H, phenylene protons), 7.39 (dt, J 2.2, 8.8, 2H,
(d, J 16.4, 1H, NCH-phenylene), 7.40 (dd, J 1.6, 4.6, 2H,
pyridyl protons), 7.71 (dt, J 1.7, 8.5, 2H, phenylene protons),
7.92 (dt, J 1.7, 8.5, 2H, phenylene protons), 8.63 (dd, J 1.6, 4.6,
3
2 6
2
2
phenylene protons); n /cm−1 2955, 2925, 2855, 1885, 1630,
max
1565, 1475, 1385, 1305, 1265, 1240, 1180, 1095, 1070, 1005, 805,
2H, pyridyl protons); n
/cm−1 3050, 3030, 2980, 2955, 2925,
max
725, 505, 480; m/z 302 (M++2), 300 (M+), 203
82, 71, 57, 55, 43, 41, 29 (Found: C, 55.6; H, 7.1. Calc. for
1930, 1670, 1635, 1595, 1565, 1550, 1495, 1465, 1380, 1300,
1285, 1260, 1245, 1215, 1195, 1140, 1120, 1090, 1045, 1015, 980,
970, 870, 830, 800, 770, 750, 725, 705, 665, 620, 590, 570, 565,
545, 530, 500, 480, 445, 415; m/z 357 (M+), 328 [M+−(C H )],
[(M++2)−(C H )], 201 [M+−(C H )], 188, 186, 122, 108,
C H BrS: C, 55.81; H, 7.03%).
7
15
7 15
14 21
2
5
Compound 1b was prepared via a similar method as for 1a
using perfluorooctyl iodide instead of 1-bromooctane. The
314 [M+−(C H )], 292, 270, 265, 245, 228, 208, 195, 181, 160,
3
7
152, 138, 127, 69, 57, 43, 28 (Found: C, 70.6; H, 7.6; N, 4.0; S
9.0. Calc. for C H NO S: C, 70.55; H, 7.61; N, 3.92; S, 8.97%).
product yield was 86% as colourless crystals. d (CDCl ,
90 MHz) 7.54 (s, 4H, phenylene protons); n /cm−1 2925,
H
3
21 27
2
Compound 3b was prepared via a similar method as for 3a
using 2b instead of 2a. The product yield was 35.9%: d (CDCl ,
max
1905, 1640, 1570, 1475, 1385, 1370, 1325, 1245, 1200, 1150,
H
3
1115, 1100, 1090, 1070, 1010, 935, 820, 800, 780, 745, 730, 710,
675, 655, 600, 560, 530, 510, 490; m/z 608 (M++2), 606
(M+), 589 [(M++2)−F], 587 (M+−F), 508, 239
[(BrPhSCF +)+2], 237 (BrPhSCF +), 189 [(BrPhS+)+2],
400 MHz) 7.24 (d, J 16.4, 1H, NCH-pyridyl), 7.35 (d, J 16.4,
1H, NCH-phenylene), 7.41 (dd, J 1.5, 4.6, 2H, pyridyl protons),
7.80 (d, J 8.5, 2H, phenylene protons), 8.05 (d, J 8.5, 2H,
phenylene protons), 8.66 (dd, J 1.6, 4.6, 2H, pyridyl protons);
2
2
187 (BrPhS+), 169 [(C F )+], 158, 131, 119 (C F +), 108, 82,
n
/cm−1 3025, 3010, 1595, 1570, 1555, 1495, 1415, 1375,
3 7
2 7
max
69 (CF +), 55, 43, 28 (Found: C, 27.7; H, 0.7. Calc. for
1330, 1260, 1230, 1160, 1145, 1120, 1085, 1055, 1015,
990, 975, 955, 940, 880, 860, 830, 805, 750, 710, 695, 680,
660, 605, 585, 555, 515, 480; m/z 663 (M+), 644 (M+−F),
244 [M+−(C F )], 228, 196 [M+−(C F SO)], 180
3
C H BrF S: C, 27.49; H, 0.43%).
14
4
17
4-(Octylsulfonyl)bromobenzene 2a
8 17
8 17
[M+−(C F SO )], 169 [(C F )+], 152, 131, 119 (C F +),
A mixture of 1a (14.19 g, 47.1 mmol), acetic acid (100 ml) and
30% aqueous hydrogen peroxide (16.02 g, 141.3 mmol) was
refluxed for 1 h. The reaction mixture was poured into 300 ml
of saturated aqueous sodium hydrogen carbonate. The crude
product was extracted with ethyl acetate, and the combined
ethyl acetate extracts were dried over anhydrous sodium
sulfate. The residue resulting from evaporation of the ethyl
acetate was purified by column chromatography using a
8 17
2
3 7
2 5
90, 69 (CF +), 51 (Found: C, 37.9; H, 1.3; N, 2.1. Calc. for
3
C H NO SF : C, 38.02; H, 1.52; N, 2.11%).
21 10
2
17
2-(4-{2-[4-(Octylsulfonyl)phenyl]ethenyl}pyridinio)benz-
imidazolate 4a
Under an argon atmosphere, a mixture of 2-chlorobenzimida-
zole (1.526 g, 10.0 mmol), 3a (3.575 g, 10.0 mmol) and 5 ml of
1194
J. Mater. Chem., 1998, 8(5), 1193–1197

dry butan-1-ol was stirred at 100 °C for 12 h and then cooled.
This reaction mixture was poured into 500 ml of diethyl ether,
and the resulting precipitate was collected by filtration. The
residual solid was washed with diethyl ether and dissolved in
100 ml of methanol at 60 °C. The solution was treated with
10 ml of aqueous ammonia at 60 °C. To this mixture was
added 400 ml of water. The resulting precipitate was collected
by filtration. The crude product was purified by recrystalliz-
ation from acetone–methanol affording the title compound 4a
with a yield of 2.77 g (58%) as red–brown crystals: d (CDCl ,
H
3
400 MHz) 0.87 [t, J 7.1, 3H, CH (CH ) ] 1.2–1.3 [m, 8H,
2 7
CH (CH ) CH ], 1.38 [quintet, J 7.1, 2H, CH (CH ) CH ],
2 4 2 4
1.7–1.8 [m, 2H, CH (CH ) CH ], 3.1–3.2 (m, 2H, CH SO ),
3
3
2
3
2
3
2 5
2
2
2
7.10–7.16 (m, 2H, benzimidazolate protons), 7.18 (d, J 16.3,
1H, NCH-pyridyl), 7.55 (d, J 16.4, 1H, NCH-phenylene),
7.62–7.68 (m, 2H, benzimidazolate protons), 7.72 (d, J 7.2, 2H,
pyridyl protons), 7.78 (d, J 8.4, 2H, phenylene protons), 8.00
(d, J 8.4, 2H, phenylene protons), 9.83 (d, J 7.2, 2H, pyridyl
protons); n /cm−1 3125, 3055, 2930, 2855, 1620, 1565, 1555,
max
1500, 1465, 1410, 1395, 1320, 1310, 1270, 1190, 1145, 1115,
1090, 1045, 1015, 1000, 970, 955, 900, 880, 845, 810, 770, 745,
730, 710, 660, 620, 600, 555, 545, 530; m/z 475 (M++2), 396,
384, 357, 299, 284, 273, 259, 220, 209, 183, 133, 118, 105, 90,
79, 64, 51, 39 (Found: C, 71.3; H, 6.7; N, 9.0; S 6.7. Calc. for
C H N O S: C, 71.01; H, 6.60; N, 8.87; S, 6.77%).
Scheme 1 Reagents and conditions: i, NaH, DMF, 0 °C, 1 h; ii,
Br(CH ) Br or I(CF ) F, DMF, room temp., 2 h; iii, H O , AcOH,
reflux, 1 h; iv, 4-vinylpyridine, Pd(OAc) , Et N, CH CN, reflux, 72 h;
2 8
2 8
2 2
2
3
3
28 31
3 2
v, 2-chlorobenzimidazole, BuOH, 12–24 h; vi, aq. NH , MeOH, 60 °C,
30 min
Compound 4b was prepared via a similar method as for the
preparation of 4a using 3b instead of 3a The product yield
was 63%: d (CDCl , 400 MHz) 7.14–7.22 (m, 3H, benzimida-
3
H
3
zolate protons and NCH-pyridyl), 7.38 (d, J 15.2, 1H, NCH-
phenylene), 7.68–7.72 (m, 2H, benzimidazolate protons), 7.90
(d, J 8.5, 2H, phenylene protons), 7.93 (d, J 7.2, 2H, pyridyl
protons), 8.14 (d, J 8.5, 2H, phenylene protons), 10.02 (d, J 7.2,
zole and novel stilbazole derivatives 3a and 3b, which were
prepared by the Heck reactions of 4-vinylpyridine with 4-
(octylsulfonyl)bromobenzene 2a and 4-(perfluorooctylsulfon-
yl)bromobenzene 2b, respectively. Stilbazolium benzimidazol-
ates 4a and 4b are soluble in common polar organic solvents
such as chloroform, methanol, ethanol, acetone, THF and so
on, however, the solubility of 4a was much better than that of
4b in the common organic solvents mentioned above. The
decomposition temperatures of 4a and 4b were estimated from
differential scanning calorimetry (DSC) measurements. Melting
was observed from 232 °C in the case of 4a with thermal
decomposition occurring at 235 °C on a heating scan; however,
no melting was observed for 4b, while thermal decomposition
occurred at 237 °C. The thermal stability of 4a and 4b seems
to be compatible with the processing temperatures of promising
NLO polymers.13
Optical-quality thin films of PMMA containing 10 or 20
mass% of 4a could be obtained by spin coating on an ordinary
glass substrate from a THF solution which contained PMMA
and 10 or 20 mass% of 4a with respect to PMMA. However,
optical-quality thin films of PMMA containing 10 mass% of
4b could not be obtained, because of the poor solubility in
organic solvents of 4b, possibly owing to the effects of the
perfluorooctyl moiety. On the other hand, other stilbazolium
benzimidazolate derivatives 4c (4∞-position not substituted)
and 4d (4∞-position substituted with an octyloxy moiety could
be dissolved in PMMA up to 10 mass% with respect to
PMMA. Taking these results into account, it can be seen that
the introduction of an octylsulfonyl group at the 4∞-position
of the stilbazole moiety contributes to an improvement in
processability.
2H, pyridyl protons); n /cm−1 3125, 3075, 3050, 1620, 1590,
max
1570, 1555, 1500, 1470, 1450, 1410, 1375, 1330, 1305, 1215,
1175, 1150, 1125, 1085, 1055, 1030, 1010, 970, 955, 880, 845,
810, 750, 710, 680, 660, 645, 600, 580, 555, 525, 490; m/z 779
(M+), 715, 663, 553, 489, 346, 296 [M+−(C F SO )], 244,
8 17
2
2 5
228, 209, 196, 180, 169 [(C F )+], 152, 131, 119 (C F +), 100,
3 7
85, 69 (CF +), 64, 51, 48 (Found: C, 43.0; H, 1.6; N, 5.3. Calc.
3
for C H N O SF : C, 43.15; H, 1.81; N, 5.39%).
28 14
3
2
17
SHG Measurement
PMMA thin films containing 10 or 20 mass% of 4a were
obtained by spin-coating on an ordinary cover glass plate at
a rate of 2000 rpm from a THF solution which contained
PMMA and 10 or 20 mass% of 4a with respect to PMMA.
The thickness of the obtained film was determined to be
ca. 0.5 mm. Poling was normal to the surface by corona dis-
charge. The distance of the tungsten needle from the surface
was 25 mm. The needle side was set to 10 kV negative to an
aluminum heating plate. After 20 min of poling at 110 °C, the
film was cooled to ambient temperature with continuous
corona poling.
The second harmonic generation (SHG) at 532 nm was
measured in transmission by means of the Maker fringe
method,11 using a Q-switched Nd5YAG laser (Spectron
SL404G, l=1064 nm, 10 Hz repetition rate, 6 ns pulse dur-
ation) as the exciting light source. Detailed experimental and
calculating procedures are described in our previous report.12
As a reference sample we used a 1 mm-thick y-cut quartz
Optical properties of PMMA thin films containing stilbazolium
benzimidazolate derivatives
(d =0.46 pm V−1).
11
Results and Discussion
The optical properties of betaine 4a in PMMA are summarized
in Table 1. We defined the cutoff wavelength (l
) as the
Preparation of novel stilbazolium benzimidazolate derivatives
cutoff
wavelength where the value of the first deviation for absorbance
becomes 0. Fig. 1 shows the UV–VIS spectrum of 20 mass%
of 4a dispersed in a PMMA thin film, l of which is 426 nm.
The synthetic pathways to novel stilbazolium benzimidazolate
derivatives 4a and 4b are described in Scheme 1. Stilbazolium
benzimidazolate derivatives 4a and 4b were prepared by the
formation of the betaine structure between 2-chlorobenzimida-
CT
The present l
CT
absorbance becomes maximal is in the visible region. The l
CT
means that the wavelength where the
J. Mater. Chem., 1998, 8(5), 1193–1197
1195

Table 1 Linear and non-linear optical properties for spin-coated films of PMMA containing stilbazolium benzimidazolate derivatives
run
compound
content of betaine/mass%
l
/nma
l
/nm
d
/pm V−1
CT
cutoff
33
1
2
3
4
4a
4a
4cb
4db
10
20
10
10
433
610
7.6
11
1.6
0.51
426
430
440
610
600
600
al =the wavelength in the visible region where the absorbance becomes maximal. bFrom ref. 9(b).
CT
ively. In the case of run 4, the centrosymmetric aggregation of
4d in PMMA, which is probably promoted by the large
polarizability of 4d,8 provides the lowest order parameter in
the present series. The order parameter in the case of run 1
was lower than that in the case of run 3. Our previous study
using ab initio and INDO/S calculations6 revealed that the
dipole moment of a stilbazolium benzimidazolate derivative,
the 4∞-position of which is substituted with a nitro moiety, is
smaller than that of 4c by ca. 7.5 D. According to this finding,
the dipole moment of 4a in the ground state was expected to
be smaller than that of 4c. Thus, the electric poling was more
effective in the case of run 3 than in the case of run 1.
The second harmonic generation (SHG) of the thin film was
measured in transmission by means of the Maker fringe
method.11,12 A Q-switched Nd5YAG laser (Spectron SL404G,
l=1064 nm, 10 Hz repetition rate, 6 ns pulse duration) was
used as the exciting light source. The p-polarized laser beam
was chosen using a l/4 wave plate and a linear polarizer. Fig. 2
describes the relationship between SH light intensity and the
incident angle of the exciting beam for the spin-coated film of
PMMA containing 20 mass% of 4a obtained after corona
poling.
Fig. 1 UV–VIS absorption spectra of 10 mass% of 4a dispersed in a
PMMA thin film; (a) before corona poling, (b) after corona poling
at 110 °C
The second-order NLO coefficients, d , of the spin-coated
33
films were determined by the mean-square method15 using the
relationship of SH light intensity and the incident angle of an
exciting beam proposed by Jerphagnon and Kurtz.11 It has
been reported9 that poled films of PMMA containing 10
of the PMMA thin film containing 10 mass% of 4a was
433 nm. The blue-shift of l in the case of the PMMA thin
film containing 20 mass% of 4a is probably due to the increase
in the content of 4a inducing the enlargement of the polariz-
ability around the chromophores. An increase in the polarity
of the medium has been known to induce the blue-shift of a
CT
mass% of 4c or 4d exhibit a d of 1.6 or 0.51 pm V−1,
33
respectively. In the present case, the d value of PMMA
33
containing 10 or 20 mass% of 4a is 7.6 or 11 pm V−1,
mass% of 4a is 5-fold larger than that of PMMA containing
respectively. Namely, the d value of PMMA containing 10
33
maximum absorption band of pyridinium betaines.4 The l
of the stilbazolium benzimidazolate without a substituent, 4c,
CT
10 mass% of 4c, although the order parameter in the case of
run 1 was lower than that in the case of run 3, as mentioned
above. This result is due to the substituent effect of the
dispersed in a PMMA thin film has been reported9b to be
430 nm, which is comparable to the l of the present stilbazol-
ium benzimidazolate 4a. Namely, the introduction of an octyl-
CT
sulfonyl group at the 4∞-position of the stilbazole moiety has
no significant effect on l . It is generally accepted that the
introduction of a strong electron-acceptor group into classical
CT
D-p-A molecular systems is accompanied by the red-shift of
the l . Such red-shifts of the l were not observed in the
stilbazolium benzimidazolate derivatives investigated. The
CT
CT
absorbance around l decreased after the sample was poled
CT
at 110 °C for 20 min, applying the voltage of 4 kV cm−1 with
the corona poling method as shown in Fig. 1, indicating the
promotion of chromophore orientation by electric poling.
Similar results were obtained in the case of 20 mass% of 4a
dispersed in a PMMA thin film. Additionally, a blue-shift of
the maximum absorption in the visible region was observed,
which could be interpreted as resulting from the polarity of
the environment around the chromophores being increased
due to the noncentrosymmetric alignment of 4a, as mentioned
above. The order parameters of poled samples were estimated
using spectroscopic measurements at l and eqn. (1),14
CT
ꢀPꢁ=1−A/A
(1)
0
where ꢀPꢁ is the order parameter, and A and A are the
0
absorbance l before and after poling at 110 °C for 20 min,
CT
Fig. 2 Relationship between SH light intensity and the incident angle
of an exciting beam for 10 mass% of 4a dispersed in a PMMA thin
film after corona poling at 110 °C
respectively. The order parameters in the cases of runs 1, 3
and 4 (Table 1) were estimated as 0.14, 0.19 and 0.10, respect-
1196
J. Mater. Chem., 1998, 8(5), 1193–1197

J. Hulliger, M. Florsheimer, P. Kaatz and P. Gunter, Gordon and
¨
¨
octylsulfonyl moiety. The introduction of the octylsulfonyl
group at the 4∞-position of the stilbazole moiety is of value not
only for improving the processability of stilbazolium benzimid-
azolates but also for increasing their second-order NLO suscep-
Breach Publishers, New York, 1995, vol. 1; (c) Polymers for
Second-Order Nonlinear Optics, ACS Symposium Series 601,
ed. G. A. Lindsay and K. D. Singer, American Chemical Society,
Washington, DC, 1995; (d) Molecular Nonlinear Optics: Materials,
Physics and Devices, ed. J. Zyss, Academic Press, Boston, 1994;
(e) D. M. Burland, R. D. Miller and C. A. Walsh, Chem. Rev., 1994,
94, 31; ( f ) L. R. Dalton, A. W. Harper, R. Ghosn, W. H. Steier,
H. Fetterman, Y. Shi, R. V. Mustacich, A. K.-Y. Jen and K. J. Shea,
Chem. Mater., 1995, 7, 1060.
(a) J. L. Oudar and D. S. Chemla, J. Chem. Phys., 1977, 66, 2664;
(b) J. L. Oudar and J. Zyss, Phys. Rev. A, 1982, 26, 2016.
(a) J. O. Morley, J. Chem. Soc., Faraday T rans., 1994, 90, 1853;
(b) C. Serbutoviez, J. F. Nicoud, J. Fischer, J. Ledoux and J. Zyss,
Chem. Mater., 1994, 6, 1358; (c) M. S. Wong, C. Bosshard, F. Pan
tibility. On the other hand, the d value of a poled PMMA
film containing ca. 8 mass% of Disperse Red 1 (DR1, 2-{[N-
33
ethyl-4-(4-nitrophenyl)azo]anilino}ethanol), which is a typical
example of a classical D-p-A molecule (l 490 nm) has been
CT
reported16 to be ca. 8.5 pm V−1 using a Nd5YAG laser (l=
containing 10 mass% of 4a exhibited a shorter l (433 nm)
2
3
1064 nm) as an exciting source. The present PMMA film
CT
and a comparable d value (7.6 pm V−1) using the same
33
exciting light source.
and P. Gunter, Adv. Mater., 1996, 8, 677; (d) X.-M. Duan, S. Okada,
¨
If the preparation of stilbazolium benzimidazolate covalently
bound to a polymeric backbone is achieved hereafter, the NLO
activity would be expected to increase, because of the uniform
distribution of the stilbazolium betaine in the matrix as well
as the inhibition of aggregation due to polymeric effects.17
Studies on this subject are in progress.
H. Oikawa, H. Matsuda and H. Nakanishi, Nonlinear Opt., 1996,
15, 119.
Typical review: E. Alcalde, Adv. Heterocycl. Chem., 1994, 60, 197.
J. Abe and Y. Shirai, J. Am. Chem. Soc., 1996, 118, 4705.
J. Abe, Y. Shirai, N. Nemoto, F. Miyata and Y. Nagase, J. Phys.
Chem. B, 1997, 101, 576.
J. Abe, N. Nemoto, Y. Nagase and Y. Shirai, Chem. Phys. L ett.,
1996, 261, 18.
J. Abe, Y. Shirai, N. Nemoto and Y. Nagase, J. Phys. Chem. B,
1997, 101, 1910.
4
5
6
7
8
9
Conclusions
(a) N. Nemoto, J. Abe, F. Miyata, M. Hasegawa, Y. Shirai and
Y. Nagase, Chem. L ett., 1996, 851; (b) N. Nemoto, J. Abe,
F. Miyata, Y. Shirai and Y. Nagase, Nonlinear Opt., in the press.
The preparation of a novel stilbazolium benzimidazolate
derivative, i.e. 2-(4-{2-[4-(octylsulfonyl)phenyl]ethenyl}pyrid-
inio)benzimidazolate 4a, has been achieved. The introduction
of an octylsulfonyl group at the 4∞-position of the stilbazole
moiety is of value not only in improving the processability of
stilbazolium benzimidazolates but also in increasing their
second-order NLO susceptibilities without significantly influ-
encing their linear absorption properties. The application of
pyridinium heterocyclic betaines in NLO is worthy of notice
and should lead to the development of a new class of
second-order NLO materials.
10 D. Harrison, J. T. Ralph and A. C. B. Smith, J. Chem. Soc., 1963,
203.
11 J. Jerphagnon and S. K. Kurtz, J. Appl. Phys., 1970, 41, 1667.
12 N. Nemoto, Y. Nagase, J. Abe, H. Matsushima, Y. Shirai and
N. Takamiya, Macromol. Chem. Phys., 1995, 196, 2237.
13 P. Boldt, T. Eisentrager, C. Glania, J. Goldenitz, P. Kramer,
¨
¨
¨
R. Matschiner, J. Rase, R. Schwesinger, J. Wichern and
R. Wortmann, Adv. Mater., 1996, 8, 672.
14 B. Guichard, C. Noel, D Reyx and F. Kajzar, Macromol. Chem.
¨
Phys., 1996, 197, 2185.
15 N. Nemoto, F. Miyata, Y. Nagase, J. Abe, M. Hasegawa and
Y. Shirai, Macromolecules, 1996, 29, 2365.
16 M. A. Mortazavi, A. Knoesen, S. T. Kowel, B. G. Higgins and
A. Dienes, J. Opt. Soc. Am. B, 1989, 6, 733.
References
17 N. Nemoto, J. Abe, F. Miyata, Y. Shirai and Y. Nagase, J. Mater.
Chem., 1997, 7, 1389.
1
Recent books and reviews: (a) Nonlinear Optics of Organic
Molecules and Polymers, ed. H. S. Nalwa and S. Miyata, CRC
Press, Boca Raton, 1997; (b) Organic Nonlinear Optical Materials,
Advances in Nonlinear Optics, ed. C. Bosshard, K. Sutter, P. Preˆtre,
Paper 7/08620B; Received 1st December, 1997
J. Mater. Chem., 1998, 8(5), 1193–1197
1197