Third-order nonlinear optical properties of unsymmetric pentamethine cyanine dyes possessing benzoxazolyl and benzothiazolyl groups

Article abstract of DOI:10.1016/j.dyepig.2012.07.007

A series of unsymmetric pentamethine cyanine dyes featuring benzoxazolyl and benzothiazolyl groups were synthesized, the structures were characterized by ¹H NMR, UV-vis absorption spectroscopy and mass spectrometry. The dyes with different anions such as chloride, tetraphenylborate and tetra(4-fluorophenyl)borate were also obtained and their solubility can be changed by the choice of the anion part. The third-order nonlinear optical properties were measured by the Z-scan technique with a picosecond laser beam at 532 nm in DMF solution. The third-order nonlinear optical susceptibilities χ⁽³⁾ and the second-order hyperpolarizabilities γ' were obtained. The results reveal that these dyes exhibit strong reverse saturable absorption and nonlinear refraction.

Full text of DOI:10.1016/j.dyepig.2012.07.007

Dyes and Pigments 96 (2013) 189e195
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Third-order nonlinear optical properties of unsymmetric pentamethine cyanine
dyes possessing benzoxazolyl and benzothiazolyl groups
,
a
a
b
b
Ru Sun a, Bao-Long Yan a, Jian-Feng Ge a , Qing-feng Xu , Na-Jun Li , Xing-Zhi Wu , Ying-Ling Song ,
*
,
Jian-Mei Lu a
*
a Key Laboratory of Organic Synthesis of Jiangsu Province, Soochow University, Suzhou 215123, China
b School of Physical Science and Technology, Soochow University, 1 Shizi Street, Suzhou 215006, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of unsymmetric pentamethine cyanine dyes featuring benzoxazolyl and benzothiazolyl groups
were synthesized, the structures were characterized by 1H NMR, UVevis absorption spectroscopy and
mass spectrometry. The dyes with different anions such as chloride, tetraphenylborate and tetra(4-
fluorophenyl)borate were also obtained and their solubility can be changed by the choice of the anion
part. The third-order nonlinear optical properties were measured by the Z-scan technique with a pico-
Received 4 April 2012
Received in revised form
10 July 2012
Accepted 10 July 2012
Available online 20 July 2012
second laser beam at 532 nm in DMF solution. The third-order nonlinear optical susceptibilities
the second-order hyperpolarizabilities ’ were obtained. The results reveal that these dyes exhibit strong
reverse saturable absorption and nonlinear refraction.
c
(3) and
g
Keywords:
Cyanine dye
Third-order
Ó 2012 Elsevier Ltd. All rights reserved.
Nonlinear optical material
Z-Scan
Functional dyes
Optical material
1. Introduction
focused on the symmetric cyanine dyes with the four-wave mixing
method [19]. Recently, the third-order NLO properties of cyanine
dyes were re-investigated and designs were focused on the
heteroatom containing symmetric cyanines. The selenium con-
taining symmetric cyanines with different anion parts were looked
as potential optical switch materials [20]; and sulfur containing
symmetric cyanines with pentamethines were used for the third-
order NLO research in the solution and the solid state [21].
Although some attention was directed towards the second-order
NLO properties for the unsymmetric cyanines [22], little attention
has been focused on the unsymmetric heterocyclic cyanine dyes
[23,24]. The unsymmetric pentamethine cyanine dyes with either
an oxygen or sulfur containing heterocycle were synthesized in this
paper, and their third-order NLO properties using the picosecond Z-
scan technique at 532 nm in N,N-dimethylformamide (DMF) solu-
tion are discussed.
Functional dyes play an important role in material science.
Cyanine dyes, which were discovered in the nineteenth century,
were used in the field of photography [1]. The developments of
cyanine dyes have come into limelight both in experimental and
theoretical aspects; they present favorable optical properties and
have been explored as important organic functional materials in
many fields, such as data storage materials [2], laser disks materials
[3], sensitizers in solar cells [4], fluorescence labels in DNA [5e8],
protein [9e12] detection and nonlinear optical material [13e16].
It is believed that the existence of intermediate diradical and
zwitterionic character in the molecules and the large coupling
between these two covalent band resonance forms are responsible
for the third-order nonlinear optical (NLO) susceptibilities
the second-order hyperpolarizabilities ’ [17,18] The candidates
c
(3) and
g
with intermolecular charge transfer abilities and large polarization
were good selections for the third-order NLO materials. The earlier
reported research of the third-order NLO properties of cyanine dyes
2. Experimental section
2.1. General information
All the starting materials were purchased from TCI (Shanghai)
Development Co., Ltd. or Sinopharm Chemical Reagent Co., Ltd. All
analytic grade solvents (A.R.) were obtained from commercial
* Corresponding authors. Tel./fax: þ86 (0) 512 6588 4717; þ86 (0) 512 6588 0367.
E-mail addresses: ge_jianfeng@hotmail.com (J.-F. Ge), lujm@suda.edu.cn
(J.-M. Lu).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.07.007

190
R. Sun et al. / Dyes and Pigments 96 (2013) 189e195
suppliers and used directly. Dyes 3b-f were synthesized according
to the similar procedure of 3a, dyes 4b-f were obtained by the
similar procedure described for dye 4a, dyes 5b-f were prepared by
the same procedure as that of 5a. Melting points were determined
on X-4 microscope electron thermal apparatus (Taike, China)
without correction. Absorption spectra were taken on U-3900
UVeVis spectrophotometer. 1HNMR spectra were recorded on
Varian-300 or 400 NMR spectrometer, and TMS or solvent peak
were used as an internal standard. Mass spectra were recorded on
Finnigan MAT95 mass spectrometer (ESI). IR spectra were recorded
on a Ni-colet 5200 FT-IR instrument using solid samples dispersed
in KBr pellets. The third-order NLO properties were measured by
the Z-scan technique. A Nd:YAG 532 nm laser (EKSPLA) with a pulse
width of 21 ps (fwhm) and repetition rate of 10 Hz was used for
picosecond Z-scan measurements.
d 8.05e7.91 (m, 2H), 7.74e7.60 (m, 4H), 7.53e7.41 (m, 2H),
7.29e7.16 (m, 1H), 6.56e6.37 (m, 2H), 6.21 (d, J ¼ 13.6 Hz, 1H), 3.78
(s, 3H), 3.71 (s, 3H). HRMS (ESIþ): m/z calcd for C21H18FN2OSþ
365.1124 [M-Clꢂ]þ, found 365.1118.
2.2.5. 5-Chloro-3-methyl-2-(5-(3-methylbenzothiazol-2(3H)-
ylidene)penta-1,3-dien-1-yl)benzothiazol-3-ium chloride (3d)
Dark blue powder, yield 68%, mp 217e218 ꢁC; IR (KBr, cmꢂ1):
1654, 1577, 1498, 1463, 1356, 1323, 1190, 871. 1H NMR (300 MHz,
DMSO):
d
8.05e7.96 (m, 1H), 7.91 (d, J ¼ 8.4 Hz, 1H), 7.82e7.53 (m,
5H), 7.43 (t, J ¼ 7.6 Hz, 1H), 7.34 (d, J ¼ 8.3 Hz, 1H), 6.63 (d,
J ¼ 13.4 Hz, 1H), 6.53e6.38 (m, 2H), 3.86, 3.80 (s, 3H), 3.77, 3.71 (s,
3H). HRMS (ESIþ): m/z calcd for C21H18ClN2Sþ2 397.0600 [M-Clꢂ]þ,
found 397.0594.
2.2.6. 5-Fluoro-3-methyl-2-(5-(3-methylbenzothiazol-2(3H)-
ylidene)penta-1,3-dien-1-yl)benzothiazol-3-ium chloride (3e)
Dark blue powder, yield 58%, mp 227e228 ꢁC; IR (KBr, cmꢂ1):
1577, 1495, 1467, 1357, 1318, 1188, 932, 826. 1H NMR (300 MHz,
2.2. Synthesis and characterization of compounds 2c-5f
2.2.1. 5-Chloro-3-methyl-2-(4-(N-phenylacetamido)buta-1,3-dien-
1-yl)benzothiazol-3-ium iodide (2c)
DMSO):
d 8.06e7.94 (m, 2H), 7.83e7.53 (m, 5H), 7.47e7.37 (m, 1H),
A suspension of 1c (1.63 g, 5.0 mmol) and 3-(phenylamino)allyli-
dene aniline hydrochloride (1.50 g, 5.0 mmol) in acetic anhydride
(7.0 mL) and acetic acid (7.0 mL) was heated under reflux for 2 h. The
reaction was cooled to room temperature. After most of the solvent
wasremoved by vacuum distillation, ethyl ether (50.0mL) was added.
The suspension was heated under reflux for 30 min. The solid was
collected by filtration and washed with ethyl ether (3 ꢀ 15.0 mL) to
afford 2c as a red solid. Yield 59%, mp 213e214 ꢁC; 1H NMR (300 MHz,
7.31e7.19 (m, 1H), 6.66e6.42 (m, 3H), 3.87, 3.81, 3.78, 3.73 (s, 6H).
HRMS (ESIþ): m/z calcd for C21H18FN2S2þ 381.0895 [M-Clꢂ]þ, found
381.0900.
2.2.7. 2-(5-(5-Chloro-3-methylbenzothiazol-2(3H)-ylidene)penta-
1,3-dien-1-yl)-5-fluoro-3-methylbenzothiazol-3-ium chloride (3f)
Dark blue powder, yield 45%, mp 223e224 ꢁC; IR (KBr, cmꢂ1):
1655, 1578, 1500, 1353, 1219, 1118, 933, 872. 1H NMR (300 MHz,
DMSO):
d
8.57 (d, J ¼ 13.4 Hz, 1H), 8.34e8.18 (m, 3H), 7.75e7.72 (m,
DMSO): d 8.04e7.96 (m, 2H), 7.84e7.69 (m, 4H), 7.45e7.34 (d,
1H), 7.65e7.54 (m, 3H), 7.41 (d, J ¼ 7.0 Hz, 2H), 7.15 (d, J ¼ 14.7 Hz, 1H),
5.47e5.39 (m,1H), 3.99 (s, 3H),1.99 (s, 3H). HRMS (ESIþ): m/z calcd for
C2oH18ClN2OSþ 369.0828 [M-Iꢂ]þ, found 369.0826.
J ¼ 8.4 Hz, 1H), 7.32e7.21 (s, 1H), 6.57e6.42 (m, 3H), 3.77 (s, 6H).
HRMS (ESIþ): m/z calcd for C21H17ClFN2Sþ2 415.0506 [M-Clꢂ]þ,
found 415.0500.
2.2.2. 3-Methyl-2-(5-(3-methylbenzoxazol-2(3H)-ylidene)penta-
1,3-dien-1-yl)benzothiazol-3-ium chloride (3a)
2.2.8. 3-Methyl-2-(5-(3-methylbenzothiazol-2(3H)-ylidene)penta-
1,3-dien-1-yl)benzoxazol-3-ium tetraphenylborate (4a)
To a mixture of 2a (0.22 g, 0.50 mmol) and 1b (0.15 g,
0.50 mmol) in ethanol (12 mL), Et3N (0.3 mL) was added in
portions, and then the mixture was refluxed for 1 h. The reaction
was cooled to room temperature, and ethyl ether (20 mL) was
added slowly during 0.5 h. The solid was filtrated, washed with
ethyl ether (3 ꢀ 10.0 mL) and water (3 ꢀ 10.0 mL) to afford the
iodide salt. A solution of the iodide salt in methanol was passed
through the anion-exchange resin (Amberlite IRA-400, chloride
form), and the resin was washed with methanol. After the
concentration of the eluent chloride, the residue was collected to
give 3a as dark blue powder. Yield 59%, mp 182e183 ꢁC; IR (KBr,
cmꢂ1): 1636, 1577, 1488, 1464, 1357, 1317, 1152, 818. 1H NMR
The iodide salt was synthesized in the same way as described in
the synthesis of dye 3a. To a clear solution of the iodide salt
(0.060 g, 0.125 mmol) under reflux sodium tetraphenylborate
(0.083 g, 0.25 mmol) was added in portions, and then the mixture
was heated under reflux for 1 h. The solid was filtrated and washed
by ethanol to afford 4a as dark green powder. Yield 67%, mp
230e231 ꢁC; IR (KBr, cmꢂ1): 2925, 1619, 1579, 1480, 1431, 1357,
1322, 1175, 824. 1H NMR (300 MHz, DMSO):
d 7.96e7.93 (m, 2H),
7.73e7.64 (m, 4H), 7.52e7.35 (m, 4H), 7.16 (s, 8H), 6.93e6.89 (m,
8H), 6.79e6.75 (m, 4H), 6.54e6.43 (m, 2H), 6.11e6.05 (m, 1H), 3.77
(s, 3H), 3.71 (s, 3H). HRMS (ESIþ): m/z calcd for C21H19N2OSþ
347.1218 [M-Clꢂ]þ, found 347.1225; HRMS (ESIꢂ): m/z calcd for
C24H20Bꢂ 319.1658 [B(C6H5)4]ꢂ, found 319.1664.
(300 MHz, DMSO):
d
7.96 (d, J ¼ 6.6 Hz, 2H), 7.71e7.64 (m, 3H),
7.55e7.23 (m, 5H), 6.46 (d, J ¼ 12.2 Hz, 2H), 6.09 (d, J ¼ 12.5 Hz, 1H),
3.78 (s, 3H), 3.73 (s, 3H). HRMS (ESIþ): m/z calcd for C21H19N2OSþ
347.1218 [M-Clꢂ]þ, found 347.1225.
2.2.9. 2-(5-(5-Chloro-3-methylbenzothiazol-2(3H)-ylidene)penta-
1,3-dien-1-yl)-3-methylbenzoxazol-3-ium tetraphenylborate (4b)
Dark green powder, yield 70%, mp > 250 ꢁC; IR (KBr, cmꢂ1):
2926, 1627, 1582, 1479, 1430, 1345, 1182, 1085, 872. 1H NMR
2.2.3. 5-Chloro-3-methyl-2-(5-(3-methylbenzoxazol-2(3H)-
ylidene)penta-1,3-dien-1-yl)benzothiazol-3-ium chloride (3b)
Dark blue powder, yield 67%, mp 193e194 ꢁC; IR (KBr, cmꢂ1):
1623, 1577, 1500, 1466, 1351, 1303, 1182, 873. 1H NMR (300 MHz,
(300 MHz, DMSO):
d 8.04e7.96 (m, 1H), 7.90e7.84 (m, 1H),
7.73e7.61 (m, 4H), 7.51e7.40 (m, 2H), 7.33 (d, J ¼ 8.6 Hz, 1H), 7.15 (s,
8H), 6.92e6.87 (m, 8H), 6.78e6.74 (m, 4H), 6.53e6.32 (m, 2H), 6.21
(d, J ¼ 13.6 Hz, 1H), 3.75 (s, 3H), 3.68 (s, 3H). HRMS (ESIþ): m/z calcd
for C21H18ClN2OSþ 381.0828 [M-Clꢂ]þ, found 381.0821; HRMS
(ESIꢂ): m/z calcd for C24H20Bꢂ 319.1658 [B(C6H5)4]ꢂ, found
319.1664.
DMSO):
d 8.05e7.85 (m, 2H), 7.74e7.61 (m, 4H), 7.48e7.40 (m, 2H),
7.33 (d, J ¼ 8.3 Hz, 1H), 6.55e6.35 (m, 2H), 6.23 (d, J ¼ 13.4 Hz, 1H),
3.79 (s, 3H), 3.71 (s, 3H). HRMS (ESIþ): m/z calcd for C21H18ClN2OSþ
381.0828 [M-Clꢂ]þ, found 381.0821.
2.2.4. 5-Fluoro-3-methyl-2-(5-(3-methylbenzoxazol-2(3H)-
ylidene)penta-1,3-dien-1-yl)benzothiazol-3-ium chloride (3c)
Dark blue powder, yield 45%, mp 197e198 ꢁC; IR (KBr, cmꢂ1):
1623, 1580, 1466, 1351, 1340, 1147, 922. 1H NMR (300 MHz, DMSO):
2.2.10. 2-(5-(5-Fluoro-3-methylbenzothiazol-2(3H)-ylidene)penta-
1,3-dien-1-yl)-3-methylbenzoxazol-3-ium tetraphenylborate (4c)
Dark green powder, yield 65%, mp > 250 ꢁC; IR (KBr, cmꢂ1):
3047, 1581, 1518, 1471, 1431, 1346, 1175, 930. 1H NMR (300 MHz,

R. Sun et al. / Dyes and Pigments 96 (2013) 189e195
191
DMSO):
d
8.00e7.90 (m, 2H), 7.69 (s, 4H), 7.45 (s, 2H), 7.14 (s, 8H),
found 381.0821; HRMS (ESIꢂ): m/z calcd for C24H16BFꢂ4 391.1281
6.89 (m, 8H), 6.78e6.75 (m, 4H), 6.53e6.34 (m, 3H), 6.20 (s, 1H),
3.74 (s, 3H), 3.68 (s, 3H). HRMS (ESIþ): m/z calcd for C21H18FN2OSþ
365.1124 [M-Clꢂ]þ, found 365.1118; HRMS (ESIꢂ): m/z calcd for
C24H20Bꢂ 319.1658 [B(C6H5)4]ꢂ, found 319.1664.
[B(C6H4F)4]ꢂ, found 391.1287.
2.2.16. 2-(5-(5-Fluoro-3-methylbenzothiazol-2(3H)-ylidene)penta-
1,3-dien-1-yl)-3-methylbenzoxazol-3-ium tetrakis(4-fluorophenyl)
borate (5c)
2.2.11. 5-Chloro-3-methyl-2-(5-(3-methylbenzothiazol-2(3H)-
ylidene)penta-1,3-dien-1-yl)benzothiazol-3-ium tetraphenylborate
(4d)
Dark green powder, yield 22%, mp 216e217 ꢁC; IR (KBr, cmꢂ1):
2927, 1578, 1465, 1338, 1209, 1150, 921, 834. 1H NMR (400 MHz,
DMSO):
d 8.02e7.93 (m, 2H), 7.79e7.60 (m, 4H), 7.52e7.42 (m, 2H),
Dark green powder, yield 79%, mp > 250 ꢁC; IR (KBr, cmꢂ1):
7.29e7.17 (m, 1H), 7.05 (s, 8H), 6.77e6.72 (m, 8H), 6.55e6.37 (m,
2H), 6.20 (d, J ¼ 13.0 Hz, 1H), 3.77 (s, 3H), 3.71 (s, 3H). HRMS (ESIþ):
m/z calcd for C21H18FN2OSþ 365.1124 [M-Clꢂ]þ, found 365.1118;
HRMS (ESIꢂ): m/z calcd for C24H16BF4ꢂ 391.1281 [B(C6H4F)4]ꢂ, found
391.1287.
2925, 1570, 1450, 1342, 1303, 1116, 1032, 863. 1H NMR (300 MHz,
DMSO):
d
7.96 (d, J ¼ 7.9 Hz, 2H), 7.84 (s, 2H), 7.73 (t, J ¼ 12.7 Hz,
2H), 7.41 (d, J ¼ 8.1 Hz, 2H), 7.16 (s, 8H), 6.91 (s, 8H), 6.77 (s, 4H),
6.53e6.40 (m, 4H), 3.76 (s, 6H). HRMS (ESIþ): m/z calcd for
C21H18ClN2Sþ2 397.0600 [M-Clꢂ]þ, found 397.0594; HRMS (ESIꢂ): m/
z calcd for C24H20Bꢂ 319.1658 [B(C6H5)4]ꢂ, found 319.1664.
2.2.17. 2-(5-(5-Chloro-3-methylbenzothiazol-2(3H)-ylidene)penta-
1,3-dien-1-yl)-3-methylbenzothiazol-3-ium tetrakis(4-fluorophen-
yl)borate (5d)
2.2.12. 5-Fluoro-3-methyl-2-(5-(3-methylbenzothiazol-2(3H)-
ylidene)penta-1,3-dien-1-yl)benzothiazol-3-ium tetraphenylborate
(4e)
Dark green powder, yield 37%, mp 230e231 ꢁC; IR (KBr, cmꢂ1):
2927, 1573, 1463, 1345, 1212, 1122, 983, 870. 1H NMR (400 MHz,
Dark green powder, yield 77%, mp > 250 ꢁC; IR (KBr, cmꢂ1):
DMSO):
d
7.99 (d, J ¼ 8.4 Hz, 2H), 7.86 (s, 2H), 7.77 (t, J ¼ 12.7 Hz,
2924, 1572, 1463, 1352, 1320, 1177, 931. 1H NMR (300 MHz, DMSO):
2H), 7.43 (d, J ¼ 7.0 Hz, 2H), 7.05 (s, 8H), 6.77e6.72 (m, 9H),
6.55e6.47 (m, 3H), 3.78 (s, 6H). HRMS (ESIþ): m/z calcd for
C21H18ClN2Sþ2 397.0600 [M-Clꢂ]þ, found 397.0594; HRMS (ESIꢂ): m/
z calcd for C24H16BF4ꢂ 391.1281 [B(C6H4F)4]ꢂ, found 391.1287.
d
8.03e7.91 (m, 2H), 7.81e7.54 (m, 5H), 7.44e7.40 (m, 1H), 7.15 (s,
8H), 6.93e6.88 (m, 8H), 6.79e6.75 (m, 4H), 6.63e6.40 (m, 4H), 3.84,
3,79 (s, 3H), 3.76, 3.71 (s, 3H). HRMS (ESIþ): m/z calcd for
C21H18FN2Sþ2 381.0895 [M-Clꢂ]þ, found 381.0900; HRMS (ESIꢂ): m/z
calcd for C24H20B ꢂ 319.1658 [B(C6H5)4]ꢂ, found 319.1664.
2.2.18. 2-(5-(5-Fluoro-3-methylbenzothiazol-2(3H)-ylidene)penta-
1,3-dien-1-yl)-3-methylbenzothiazol-3-ium tetrakis(4-
fluorophenyl)borate (5e)
2.2.13. 2-(5-(5-Chloro-3-methylbenzothiazol-2(3H)-ylidene)penta-
1,3-dien-1-yl)-5-fluoro-3-methylbenzothiazol-3-ium
tetraphenylborate (4f)
Dark blue powder, yield 34%, mp 231e232 ꢁC; IR (KBr, cmꢂ1):
2927, 1577, 1460, 1352, 1207, 1148, 1112, 932, 815. 1H NMR
Dark green powder, yield 82%, mp > 250 ꢁC; IR (KBr, cmꢂ1):
(400 MHz, DMSO): d 8.05e7.93 (m, 2H), 7.85e7.57 (m, 5H),
2925, 1572, 1465, 1344, 1254, 1172, 993, 873. 1H NMR (300 MHz,
7.46e7.39 (m, 1H), 7.27e7.19 (m, 1H), 7.07e7.06 (s, 8H), 6.78e6.73
(m, 8H), 6.65e6.42 (m, 3H), 3.86, 3.81 (s, 3H), 3.78, 3.73 (s, 3H).
HRMS (ESIþ): m/z calcd for C21H18FN2S2þ 381.0895 [M-Clꢂ]þ, found
381.0900; HRMS (ESIꢂ): m/z calcd for C24H16BFꢂ4 391.1281
[B(C6H4F)4]ꢂ, found 391.1287.
DMSO):
d 7.98 (s, 2H), 7.82e7.69 (m, 4H), 7.41 (s, 1H), 7.26 (s, 1H),
7.14 (s, 8H), 6.90 (s, 8H), 6.76 (s, 4H), 6.55e6.40 (m, 3H), 3.75 (s, 6H).
HRMS (ESIþ): m/z calcd for C21H17ClFN2Sþ2 415.0506 [M-Clꢂ]þ,
found 415.0500; HRMS (ESIꢂ): m/z calcd for C24H20Bꢂ 319.1658
[B(C6H5)4]ꢂ, found 319.1664.
2.2.19. 5-Chloro-2-(5-(5-fluoro-3-methylbenzothiazol-2(3H)-
ylidene)penta-1,3-dien-1-yl)-3-methylbenzothiazol-3-ium
tetrakis(4-fluorophenyl)borate (5f)
2.2.14. 3-Methyl-2-(5-(3-methylbenzothiazol-2(3H)-ylidene)
penta-1,3-dien-1-yl)benzoxazol-3-ium tetrakis(4-fluorophenyl)
borate (5a)
Dark green powder, yield 39%, mp 225e226 ꢁC; IR (KBr, cmꢂ1):
The iodide salt was synthesized in the same way as described in
the synthesis of dye 3a. To the solution of the iodide salt (0.060 g,
0.125 mmol) under reflux sodium tetra(4-fluorophenyl)borate
(0.52 g, 0.125 mmol) was added in portions, then mixture was
heated under reflux for 1 h. The residue was purified via silica gel
chromatography (CH2Cl2/CH3OH ¼ 10/1) to afford 5a as dark green
powder. Yield 29%, mp 231e232 ꢁC; IR (KBr, cmꢂ1): 2927, 1580,
1460, 1357, 1321, 1149, 923, 815. 1H NMR (300 MHz, DMSO):
2925, 1577, 1464, 1344, 1211, 1153, 931, 871. 1H NMR (300 MHz,
DMSO):
d 7.97 (s, 2H), 7.80e7.71 (m, 4H), 7.39 (s, 1H), 7.24 (s, 2H),
7.03 (s, 8H), 6.71 (s, 8H), 6.50 (s, 2H), 3.75 (s, 6H). HRMS (ESIþ): m/z
calcd for C21H17ClFN2Sþ2 415.0506 [M-Clꢂ]þ, found 415.0500; HRMS
(ESIꢂ): m/z calcd for C24H16BF4ꢂ 391.1281 [B(C6H4F)4]ꢂ, found
391.1287.
3. Results and discussion
d
7.98e7.93 (m, 2H), 7.72e7.63 (m, 4H), 7.54e7.35 (m, 4H), 7.03 (s,
8H), 6.76e6.70 (m, 8H), 6.53e6.42 (m, 2H), 6.09e6.05 (m, 1H), 3.76
(s, 3H), 3.71 (s, 3H). HRMS (ESIþ): m/z calcd for C21H19N2OSþ
347.1218 [M-Clꢂ]þ, found 347.1225; HRMS (ESIꢂ): m/z calcd for
C24H16BFꢂ4 391.1281 [B(C6H4F)4]ꢂ, found 391.1287.
3.1. Synthesis
The synthesis of dyes 3e5 were represented in Scheme 1, the
iodide 1a-d were synthesized by the reaction of starting materials
with methyl iodide in DMF solution for 24 h as reported in our
previous work [25]. The intermediates 2a-c were synthesized from
1a-c with 3-(phenylamino)allylidene aniline hydrochloride
according to the reported references [26] with some modification.
The iodide dyes 3a-f were obtained from the reaction of 2a-c with
1b-d, then ion-exchange procedures were taken using IRA-400(Cl),
NaB(C6H5)4 and NaB(C6H4F)4 to afford the final products according
to references [27,28]. Dyes 4a-f were collected by filtration and
washed by hot ethanol. Dyes 5a-f were obtained by silica gel
chromatography (CH2Cl2/CH3OH ¼ 10/1) with Rf values were nearly
2.2.15. 2-(5-(5-Chloro-3-methylbenzothiazol-2(3H)-ylidene)penta-
1,3-dien-1-yl)-3-methylbenzoxazol-3-ium tetrakis(4-fluorophenyl)
borate (5b)
Dark green powder, yield 31%, mp 222e223 ꢁC; IR (KBr, cmꢂ1):
2927, 1580, 1463, 1341, 1216, 1174, 1083, 871. 1H NMR (300 MHz,
DMSO):
d 8.04e7.84 (m, 2H), 7.73e7.61 (m, 4H), 7.51e7.39 (m, 2H),
7.32 (dd, J ¼ 8.4, 1.8 Hz, 1H), 7.07e6.99 (m, 8H), 6.75e6.69 (m, 8H),
6.53e6.32 (m, 2H), 6.21 (d, J ¼ 13.7 Hz,1H), 3.76 (s, 3H), 3.68 (s, 3H).
HRMS (ESIþ): m/z calcd for C21H18ClN2OSþ 381.0828 [M-Clꢂ]þ,

192
R. Sun et al. / Dyes and Pigments 96 (2013) 189e195
X1
X1
N
X1
N
.
CH3I
DMF
C15H14N2 HCl
O
N
R1
N
R1
R1
HAc, Ac2O
I
I
1a: X1=O, R1=H
2a: X1=O, R1=H
X1=S, R1=H
X1=S, R1=H
1b:
2b:
1c: X1=S, R1=Cl
1d: X1=S, R1=F
2c: X1=S, R1=Cl
IRA-400(Cl)
NaB(C6H5)4
NaB(C6H4F)4
with 1a-d
EtOH, Et3N
R1
X1
X2
X
R2
N
N
3a-f, 4a-f, 5a-f
1
1
2
2
1
1
2
2
1
1
2
2
4a:
3a:
X =O,R =H,X =S,R =H, X=B(C6H5)4
X =O, R =H,X =S, R =H, X=Cl
5a:
X =O,R =H,X =S,R =H, X=B(C6H4F)4
4b: X1=O,R1=H,X2=S,R2=Cl,X=B(C6H5)4
3b: X1=O, R1=H,X2=S, R2=Cl,X=Cl
1
1
2
2
5b:
X =O,R =H,X =S,R =Cl,X=B(C6H4F)4
5c: X1=O, R1=H,X2=S, R2=F,X=B(C6H4F)4
X =O, R =H,X =S, R =F,X=B(C6H5)4
1
1
2
2
1
1
2
2
4c:
3c:
X =O, R =H,X =S, R =F,X=Cl
4d: X1=S, R1=H,X2=S, R2=Cl,X=B(C6H5)4
4e: X1=S, R1=H,X2=S, R2=F,X=B(C6H5)4
3d: X1=S, R1=H,X2=S, R2=Cl,X=Cl
3e: X1=S, R1=H,X2=S, R2=F,X=Cl
1
1
2
2
5d:
X =S, R =H,X =S, R =Cl,X=B(C6H4F)4
5e: X1=S, R1=H,X2=S, R2=F,X=B(C6H4F)4
1
1
2
2
1
1
2
2
1
1
2
2
4f:
3f:
X =S, R =Cl,X =S, R =F,X=B(C6H5)4
X =S, R =Cl,X =S, R =F,X=Cl
5f:
X =S, R =Cl,X =S, R =F,X=B(C6H4F)4
Scheme 1. The synthesis of dyes 3e5.
at 0.60, while the corresponding iodide salts cannot be washed out.
Their structures were characterized by 1H NMR spectroscopy and
mass spectrometry (ESI). The 1H NMR spectra of dyes 3b, 4b and 5b
with different anions are shown in Fig. 1. The numbers of the
protons indicated the full changes of the anion parts of 4b and 5b.
Dyes 5a-f with a tetra(4-fluorophenyl)borate anion possess
favorable solubility in methanol, ethanol and DMF, while dyes 4a-f
featuring a tetraphenylborate anion have poor solubility in the same
solvents. Dyes 3a-f with a chloride counter ion could be dissolved in
DMF, but possessed very low solubility in methanol and ethanol.
Fig. 1. The 1H NMR spectra of dyes 3b, 4b and 5b.

R. Sun et al. / Dyes and Pigments 96 (2013) 189e195
193
3.2. Absorption spectra
The UVevis absorption spectra of dyes 3e5 were measured in DMF
solution at a concentration of 1 ꢀ10ꢂ5 mol Lꢂ1. The absorption spectra
of dyes 4a-f were presented in Fig. 2; and the absorption spectra of dyes
3a-f and5a-f were presented in Fig. S1 (supplementary data). As shown
in Fig. 2 and Fig. S1, the absorption bands and absorption intensities of
these dyes were similar. Comparing the influence of different anions,
the absorption bands did not show any obvious changes with the
variation of anion from Clꢂ to [B(C6H5)4]ꢂ and [B(C6H4F-p)4]ꢂ. Mean-
while, the fluorine (3c-5c, 3e-5e and 3f-5f) or chlorine (3b-5b, 3d-5d
and 3f-5f) substituents on the benzothiazolyl or benzoxazolyl rings just
made a slightly shift. However, the maximum absorption bands were
obviously affected by the different heteroatoms, and the maximum
absorption bands red-shifted by about 20 nm with the change of
oxygen heteroatom to sulfur as expected [32].
3.3. Nonlinear optical (NLO) properties
The measurements of third-order NLO properties were per-
formed on all compounds using the Z-scan technique at 532 nm in
DMF solution. In order to get an appropriate transmittance (T0),
Dyes 3d-f, 4d-f and 5d-f were measured at the concentration of
1 ꢀ 10ꢂ4 mol Lꢂ1, dyes 3a-c, 4a-c and 5a-c were measured at the
Fig. 3. The open-aperture Z-scan signals of dye 3d (Cycles are the test data points and
line is the fit curve).
The close-aperture Z-scan signals of dyes 3a-f, 4a-f and 5a-5f are
shown in Fig. 4 and Fig. S3 (supplementary data) respectively. The
strong nonlinear refractions were exhibited for dyes. The valley-
peak shape presented a positive nonlinear refraction for the
samples, which indicated a self-focusing behavior. The nonlinear
refractive data can be obtained from the ratio of the closed aperture
transmittance divided the open aperture transmittance. The effec-
tive third-order nonlinear refractive index n2 can be calculated from
concentration of 5 ꢀ 10ꢂ5 mol Lꢂ1
.
The open-aperture Z-scan signals of dyes 3a-f, 4a-f and 5a-5f
are shown in Fig. 3 and Fig. S2 (supplementary data) respectively.
As can be seen from Fig. 3 and Fig. S2, there is a dip in the signal
around the sample position of Z ¼ 0 mm, indicative of reduced
transmission of 532 nm pulses at the focus, caused by a reverse
saturable nonlinear absorption. The light transmittance (T) is
a function of the sample’s Z position (against focal point Z ¼ 0). The
nonlinear absorption data can be obtained from Eq. (1) [29], where
the equations:
D
ZVꢂP
¼
1.72pu20
/
l
,
ne2ff
¼ la0DTVꢂP/[0.812pI
a
(1ꢂeꢂ L)], where
DZVꢂP is the difference between the normalized
a0 and
a
are linear and effective third-order NLO absorptive coef-
transmittance values at the valleyand peak positions, L is the sample
ficients.
s
is the time, and L is the optical path.
thickness,
l is the wavelength of the laser, I is the peak irradiation
intensity at focus and a0 is the linear coefficient [30]. The effective
"
#
þN
Z
a0
L
a0
1 ꢂ eꢂ
third-order nonlinear optical (NLO) susceptibility values of cyanine
s2
dyes 3e5 can be derived from the equations: c(I3) ¼ 9 ꢀ 108
3
0n20c2
b/
À
Á
pffiffiffi
TðZÞ ¼
ln 1 þ IiðZÞ
a
eꢂ
d
s
a0
L
paIiðZÞ 1 ꢂ eꢂ
a0
ꢂN
(4up), c(R3) ¼ cn02n2/(80
p
), c(3) ¼ [(cI(3))2þ(c(R3))2]1/2. The second-
order hyperpolarizabilities g’ of the compounds were obtained by
(1)
equation:
g
’ ¼
c
(3)/[N ((n20þ2)/3)4], where N is the density of
Fig. 4. The close-aperture Z-scan signals of dye 4d (Cycles are the test data points and
line is the fit curve).
Fig. 2. UVevis spectra of dyes 4a-f.

194
R. Sun et al. / Dyes and Pigments 96 (2013) 189e195
Table 1
Key parameters related to NLO properties of dyes 3e5.
n2c(10ꢂ18 m2 Wꢂ1
)
b
d(10ꢂ11 m Wꢂ1
)
c(R3) (10ꢂ12 esu)
c(I3) (10ꢂ12 esu)
c(3) (10ꢂ12 esu)
g
’ (10ꢂ32 esu)
c(R3) c(I3)
/
b
Dyes
T0
3aa
3ba
3ca
3da
3ea
3fa
0.82
0.79
0.82
0.75
0.80
0.82
0.78
0.68
0.73
0.81
0.75
0.79
0.76
0.73
0.77
0.80
0.77
0.80
2.40
2.80
2.40
4.50
4.50
3.20
4.00
5.50
3.90
3.40
4.90
3.90
4.30
4.40
3.10
4.10
4.90
3.90
0.75
1.30
0.82
5.60
3.80
3.60
1.80
2.60
1.50
4.10
4.60
4.60
2.20
1.90
1.30
5.20
4.80
3.80
1.86
2.17
1.86
3.48
3.48
2.48
3.09
4.26
3.02
2.63
3.79
3.02
3.33
3.40
2.40
3.17
3.79
3.02
0.24
0.42
0.26
1.83
1.24
1.17
0.59
0.85
0.49
1.34
1.50
1.50
0.72
0.62
0.42
1.70
1.57
1.24
1.87
2.21
1.88
3.94
3.70
2.74
3.15
4.34
3.06
2.95
4.07
3.38
3.41
3.46
2.44
3.61
4.09
3.27
1.88
2.04
1.88
1.98
1.86
1.37
3.16
4.32
3.06
1.47
2.04
1.69
3.42
3.46
2.44
1.81
2.06
1.64
7.75
5.17
7.15
1.90
2.80
2.11
5.23
5.11
6.06
1.96
2.52
2.01
4.62
5.48
5.71
1.86
2.41
2.43
4aa
4ba
4ca
4da
4ea
4fa
5aa
5ba
5ca
5da
5ea
5fa
a
With picosecond laser beam at 0.2
The light transmittance.
The third-order nonlinear refractive index.
The third-order nonlinear absorptive index.
mJ.
b
c
d
molecules in the unit of number of molecules per cm3 and n0 is the
linear refractive index of the DMF (n0 ¼ 1.4305). The detailed
parameters of nonlinear optical (NLO) properties of these dyes were
collected in Table 1.
indicated that the terminal cation structures would be hopeful
candidates for the next stage.
4. Conclusion
From Table 1, it is found that the third-order nonlinear optical
(NLO) susceptibility c(3) and second-order hyperpolarizabilities
g’
A series of novel unsymmetric pentamethine cyanine dyes were
synthesized and their UVevis absorption spectra were reported.
of dyes 3e5 are up to 10ꢂ12 and 10ꢂ32 esu under picosecond laser
bean irradiation at 532 nm, respectively. To dyes 3d-f, 4d-f and 5d-
f, the values of c(R3) were at the level of 10ꢂ12 esu, with the imagi-
nary parts at the same magnitude. While the c(R3) values of dyes 3a-
c, 4a-c and 5a-c were at the level of 10ꢂ12 esu, the imaginary parts
Dyes with
a tetra(4-fluorophenyl)borate counter anion have
favorable solubility in common solvents. The third-order nonlinear
optical properties were measured with Z-scan technique at 532 nm
in DMF solution. The third-order nonlinear optical susceptibility
were nearly an order of magnitude lower. The c(R3) cI(3) values of
/
c(3) and the second-order hyperpolarizabilities
g
’ of these dyes
were obtained under picosecond laser bean irradiation with the
energy of 0.2 J. It was found that these dyes exhibit reverse
dyes 3a-c, 4a-c and 5a-c were from 4.62 to 7.75, nearly two times
more than those of dyes 3d-f, 4d-f and 5d-f, whose c(R3) cI(3) values
/
m
were from 1.90 to 2.80. Comparing the influence of different anions
saturable nonlinear absorption and strong nonlinear refraction. The
from Clꢂ to [B(C6H5)4]ꢂ and [B(C6H4F)4]ꢂ, the third-order NLO
third-order NLO susceptibility c(3) and second-order hyper-
susceptibility c(3) and second-order hyperpolarizabilities
g’ values
polarizabilities
one oxygen atom was replaced by one sulfur atom, the values of
c(R3) c(I3) increased, while the c(I3) values decreased.
g
’ were at the level of 10ꢂ12 and 10ꢂ32 esu. When
do not show any obvious change. When the different substituents
Cl and F were taken into consideration, it can be seen that the
different substituents have little influence on the third-order NLO
properties of these dyes. However, the influence of different
heteroatoms could not be neglected, when one oxygen atom was
replaced by one sulfur atom, the c(I3) values decreased, while the
/
Acknowledgements
The project is financially supported from National Natural
Science Foundation of China (51273136, 20902065, 21071105), the
National High Technology Research and Development Program of
China (863 Program, SQ2009AA06XK1482331), the Natural Science
Fund (BK2009113) in Jiangsu Province and the Project funded by
the Priority Academic Program Development of Jiangsu Higher
Education Institutions.
c(R3) c(I3) values increased.
/
The c(3) values of cyanine dyes with two sulfur atoms were
higher than cyanine dyes with one oxygen atom and one sulfur
atom. As many references reported, sulfur atoms possessed
stronger electron donation properties (less electronegative)
compared with oxygen atoms [31,32]. Therefore, the distribution of
the electronic density will be more equally shared between sulfur
and carbon than between oxygen and carbon. Consequently, the
orbitals in sulfur-containing aromatic heterocycles are better
delocalized than in molecules containing oxygen. The effect of this
p
Appendix A. Supplementary data
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.dyepig.2012.07.007.
contribution will also be a reduction in the energy of the
pep*
transition, and a higher hyperpolarizability would be conse-
quently obtained. Besides, with ion-exchange procedures, dyes 5a-f
with anion [B(C6H4F)4]ꢂ possessed good solubility. This advantage
will be helpful in fabrication of nonlinear optical device.
References
The results of the c(R3) cI(3) values do not show the big difference
/
[1] Panigrahi M, Dash S, Patel S, Mishra BK. Syntheses of cyanines: a review.
Tetrahedron 2012;68:781e805.
[2] Heilemann M, Margeat E, Kasper R, Sauer M, Tinnefeld P. Carbocyanine dyes
as efficient reversible single-molecule optical switch. J Am Chem Soc 2005;
127:3801e6.
with different anion parts [20], and the third-order NLO properties
of the title compounds show moderate abilities comparing with
recently reported sulfur containing polymethine dyes [21]. This

R. Sun et al. / Dyes and Pigments 96 (2013) 189e195
195
[3] Kameyama A, Kaneda Y, Yamanaka H, Yoshimine H, Narimatsu H,
Shinohara Y. Detection of oligosaccharides labeled with cyanine dyes using
matrix-assisted laser desorption/ionization mass spectrometry. Anal Chem
2004;76:4537e42.
[4] Fan B, de Castro FA, Heier J, Hany R, Nüesch F. High performing doped cyanine
bilayer solar cell. Org Electron 2010;11:583e8.
[5] Hilal H, Taylor JA. Determination of the stoichiometry of DNAedye interaction
and application to the study of a bis-cyanine dyeeDNA complex. Dyes Pigm
2007;75:483e90.
[6] Zhang X-H, Wang L-Y, Nan Z-X, Tan S-H, Zhang Z-X. Microwave-assisted
solvent-free synthesis and spectral properties of some dimethine cyanine
dyes as fluorescent dyes for DNA detection. Dyes Pigm 2008;79:205e9.
[7] Peng X, Wu T, Fan J, Wang J, Zhang S, Song F, et al. An effective minor groove
binder as a red fluorescent marker for live-cell DNA imaging and quantifica-
tion. Angew Chem Int Edit 2011;50:4180e3.
substituents, conformations, spacer size, and basis sets. J Phys Chem A 2007;
111:8241e9.
[19] Johr T, Werncke W, Pfeiffer M, Lau A, Dähne L. Third-order nonlinear polar-
izabilities of a homologous series of symmetric cyanines. Chem Phys Lett
1995;246:521e6.
[20] Hales JM, Matichak J, Barlow S, Ohira S, Yesudas K, Brédas J-L, et al. Design of
polymethine dyes with large third-order optical nonlinearities and loss
figures of merit. Science 2010;327:1485e8.
[21] Scarpaci A, Nantalaksakul A, Hales JM, Matichak JD, Barlow S, Rumi M, et al.
Effects of dendronization on the linear and third-order nonlinear optical
properties of bis(thiopyrylium) polymethine dyes in solution and the solid
state. Chem Mater 2012;24:1606e18.
ꢀ
[22] Cigán M, Gáplovský A, Sigmundová I, Zahradník P, Dĕdic R, Hromadová M.
Photostability of d-p-a nonlinear optical chromophores containing a benzo-
thiazolium acceptor. J Phys Org Chem 2011;24:450e9.
[8] Peng X, Yang Z, Wang J, Fan J, He Y, Song F, et al. Fluorescence ratiometry and
fluorescence lifetime imaging: using a single molecular sensor for dual mode
imaging of cellular viscosity. J Am Chem Soc 2011;133:6626e35.
[9] Zheng H, Mao YX, Li DH, Zhu CQ. Dye-binding protein assay using a long-
wave-absorbing cyanine probe. Anal Biochem 2003;318:86e90.
[10] Volkova KD, Kovalska VB, Balanda AO, Vermeij RJ, Subramaniam V,
Slominskii YL, et al. Cyanine dyeeprotein interactions: looking for fluorescent
probes for amyloid structures. J Biochem Bioph Meth 2007;70:727e33.
[11] Volkova KD, Kovalska VB, Balanda AO, Losytskyy MY, Golub AG, Vermeij RJ,
[23] Kasatani K. Large electronic third-order optical nonlinearities of cyanine dyes
measured by resonant femtosecond degenerate four-wave mixing. Opt Mater
2003;21:93e7.
[24] Werncke W, Pfeiffer M, Johr T, Lau A, Grahn W, Johannes HH, et al. Increase
and saturation of the third order hyperpolarizabilities in homologous series of
symmetric cyanines. Chem Phys 1997;216:337e47.
[25] Shi Q-Q, Sun R, Ge J-F, Xu Q-F, Li N-J, Lu J-M. A comparative study of
symmetrical and unsymmetrical trimethine cyanine dyes bearing benzox-
azolyl and benzothiazolyl groups. Dyes Pigm 2012;93:1506e11.
[26] Ikeda S, Kubota T, Yuki M, Okamoto A. Exciton-controlled hybridization-
sensitive fluorescent probes: multicolor detection of nucleic acids. Angew
Chem Int Edit 2009;48:6480e4.
[27] Zhao CF, He GS, Bhawalkar JD, Park CK, Prasad PN. Newly synthesized dyes
and their polymer/glass composites for one- and two-photon pumped solid-
state cavity lasing. Chem Mater 1995;7:1979e83.
et al. Specific fluorescent detection of fibrillar
a-synuclein using mono- and
trimethine cyanine dyes. Bioorgan Med Chem 2008;16:1452e9.
[12] Qiao X, Wang L, Ma J, Deng Q, Liang Z, Zhang L, et al. High sensitivity analysis
of water-soluble, cyanine dye labeled proteins by high-performance liquid
chromatography with fluorescence detection. Anal Chim Acta 2009;640:
114e20.
[13] Lacroix PG, Malfant I, Payrastre C, Wolf J-G, Bonvoisin J, Nakatani K. Synthesis,
crystal structure, and second-order nonlinear optical properties of a new
phase-matchable cyanine dye. Chem Mater 1998;10:1135e40.
[14] Raos G, Del Zoppo M. Substituent effects on the second-order hyper-
polarisability of cyanine cations. J Mol Struc-theochem 2002;589e590:439e45.
[15] Tolmachev OI, Pilipchuk NV, Kachkovsky OD, Slominski YL, Gayvoronsky VY,
Shepelyavyy EV, et al. Spectral and non-linear optical properties of cyanine
bases’ derivatives of benzo[c, d]indole. Dyes Pigm 2007;74:195e201.
[16] Gao F, Wang C, Zeng H, Ma S. Linear and nonlinear optical properties in
langmuireblodgett multilayers of new cyanine dye. Colloid Surf A 2008;321:
7e10.
[28] Kang M, Nag OK, Nayak RR, Hwang S, Suh H, Woo HY. Signal amplification by
changing counterions in conjugated polyelectrolyte-based fret DNA detection.
Macromolecules 2009;42:2708e14.
[29] Zhang C, Song Y, Wang X, Kuhn FE, Wang Y, Xu Y, et al. Large third-order
optical nonlinearity of two cubane-like clusters containing oxo-
trithiometalate anions and silver: synthesis, characterization, reactivity, and
NLO properties-structure correlation. J Mater Chem 2003;13:571e9.
[30] Niu Y, Song Y, Hou H, Zhu Y. Synthesis, structure, and large optical limiting
effect of the first coordination polymeric cluster based on an {I@[AgI(inh)]6}
hexagram block. Inorg Chem 2005;44:2553e9.
[31] Greve DR, Schougaard SB, Geisler T, Petersen JC, Biørnholm T. Large third-
order nonlinear optical response from molecules with effective multidirec-
tional charge-transfer transitions: new design of third-order nonlinear chro-
mophores. Adv Mater 1997;9:1113e6.
[32] Reinhardt BA, Unroe MR, Evers RC, Zhao M, Samoc M, Prasad PN, et al. Third-
order optical nonlinearities of model compounds containing benzobisthiazole,
benzobisoxazole, and benzbisimidazole units. Chem Mater 1991;3:864e71.
[17] Fukui H, Kishi R, Minami T, Nagai H, Takahashi H, Kubo T, et al. Theoretical
study on second hyperpolarizabilities of singlet diradical square planar nickel
complexes involving o-semiquinonato type ligands. J Phys Chem A 2008;112:
8423e9.
[18] Orimoto Y, Aoki Y. Strong electron correlation effects on first- and second-order
hyperpolarizabilities in zwitterionic
s-conjugated systems: its dependence on