Synthesis and crystal structures of fluorinated chromophores for second-order nonlinear optics

Article abstract of DOI:10.1246/bcsj.80.1413

Partially fluorinated skeleton of 4-[2-(4-dimethylamino-2,3,5,6- tetrafluorophenyl)ethynyl]-2,3,5,6-tetrafluoro- N,N,N-trimethylanilinium (2) was synthesized as a second-order nonlinear optical (NLO) material to decrease the absorption around 1.3 and 1.5

Full text of DOI:10.1246/bcsj.80.1413

ꢀ 2007 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 80, No. 7, 1413–1417 (2007) 1413
Synthesis and Crystal Structures of Fluorinated Chromophores
for Second-Order Nonlinear Optics
y
ꢀ1;
Hirohito Umezawa,
Shuji Okada,2 Hidetoshi Oikawa,1 Hiro Matsuda,3 and Hachiro Nakanishi1
1Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
2-1-1 Katahira, Aoba-ku, Sendai 980-8577
2Department of Polymer Science and Engineering, Faculty of Engineering, Yamagata University,
4-3-16 Jonan, Yonezawa 992-8510
3National Institute of Advanced Industrial Science and Technology, Central 5, 1-1-4 Higashi, Tsukuba 305-8565
Received August 31, 2006; E-mail: umezawa@fukushima-nct.ac.jp
Partially fluorinated skeleton of 4-[2-(4-dimethylamino-2,3,5,6-tetrafluorophenyl)ethynyl]-2,3,5,6-tetrafluoro-
N,N,N-trimethylanilinium (2) was synthesized as a second-order nonlinear optical (NLO) material to decrease the
absorption around 1.3 and 1.5 mm, which deteriorates the device efficiency. Several salts of 2 were prepared, and we
found that the trifluoromethanesulfonate salt of 2 (2b) had a shorter absorption-maximum wavelength compared to that
of 4-nitroaniline (pNA). From hyper-Rayleigh scattering (HRS) measurement, the ꢀ value of 2b was found to be about 8
times greater than that of pNA. We also performed X-ray crystallographic analysis of 2b and estimated NLO coefficient
(d) by the oriented gas model. We found that diagonal component of 2b (d11 ¼ 610 pm Vꢁ1) was about 14 times greater
than that of 2-methyl-4-nitroaniline and off-diagonal component of 2b (d12 ¼ 155 pm Vꢁ1) was about 6.5 times greater
than that of N-(4-nitrophenyl)-L-prolinol. Compound 2b should be an excellent candidate for NLO applications.
A variety of organic chromophores for second-order non-
(H3C)2N
N+(CH3)3
linear optical (NLO) materials have been studied over the last
two decades,1–3 because they have the potential to surpass
inorganic materials. Especially, organic ionic chromophores
have several advantages compared to non-ionic chromophores,
like a large hyperpolarizability (ꢀ), derived from charged ꢁ-
conjugation systems, crystal structure controllability by chang-
ing the counter ion, and high melting points and hardness
due to Coulombic interaction. However, most organic chromo-
phores have C–H bonds, which cause absorption at wave-
lengths around 1.3 and 1.5 mm, which are used for optical com-
munications, due to overtones of C–H stretching vibration,
limiting the performance in electro-optic (EO) applications.
In this work, we investigated an ionic chromophore having
C–F bonds instead of C–H bonds, since C–F bond shifts the
absorption to lower energy. Sonoda et al. have synthesized
deuterated compounds4–6 for the same reason. However, shift
range after C–D substitution is smaller than that after C–F sub-
stitution, because fluorine is heavier than deuterium. Thus, we
tried to synthesize a chromophore with fluorine instead of
hydrogen. In a previous study, we have synthesized 4-[2-(4-
dimethylaminophenyl)ethynyl]-N,N,N-trimethylanilinium iodide
(1a) (iodide salt of 1 as shown in Fig. 1). We have found that
its ꢀ value is 7 times greater than 4-nitroaniline (pNA), which
is traditionally used as a NLO material, in spite of having
almost the same absorption region.7 Therefore, we investigated
fluorine substitution of the hydrogen atoms attached to the
1
2
F
F
F
F
F
F
F
F
(H3C)2N
N+(CH3)3
Fig. 1. Chemical structures of 1 and 2.
benzene rings of 1. Namely, 4-[2-(4-dimethylamino-2,3,5,6-
tetrafluorophenyl)ethynyl]-2,3,5,6-tetrafluoro-N,N,N-trimethyl-
anilinium (2), as shown in Fig. 1, was synthesized, and its
optical properties were studied.
Experimental
Calculated ꢀ values (ꢀ0,calc), excitation energy (Eeg), and di-
pole moment difference between the ground and excited states
(ꢀꢂeg) of pNA, 1 and 2 were obtained by semiempirical molecu-
lar orbital (MO) calculations using MOPAC94 PM3 (CAChe ver.
4.1.1) on their optimized structure. Compound 2a was synthesized
according to Fig. 2. Experimental procedures are described below.
2,3,5,6-Tetrafluoro-4-iodo-N,N-dimethylaniline (4). To a
mixture of pentafluoroiodobenzene (3) (47 g, 160 mmol) and etha-
nol (150 mL), 40% dimethylamine solution (45 mL) was added,
and the solution was refluxed for 2 days. Then, the mixture was
poured into water (300 mL) and extracted with ether (300 mL ꢂ
3). The organic phase was dried over Na2SO4 and filtered. Solvent
in the filtrate was removed under reduced pressure to give 4 as a
y Present address: Department of Chemistry and Biochemistry,
Fukushima National College of Technology, 30 Aza Nagao
Taira Kamiarakawa, Iwaki 970-8034
Published on the web July 11, 2007; doi:10.1246/bcsj.80.1413

1414 Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007)
Fluorinated Chromophore for Nonlinear Optics
F
F
F
F
F
F
F
F
(PPh3)2PdCl2, CuCl
TMS, N(C2H5)3
HN(CH3)2 aq
(H3C)2N
I
F
I
C2H5OH
3
4
F
F
F
F
F
F
F
(PPh3)2PdCl2, CuCl
TBAF
THF
(H3C)2N
TMS
(H3C)2N
4, N(C2H5)3
F
5
6
F
F
F
F
F
F
F
F
F
F
F
F
F
F
-
CF3SO3
N+(CH3)3
CF3SO3CH3
(H3C)2N
N(CH3)2
(H3C)2N
F
F
7
2b
F
F
F
F
F
F
I-
X
SO3Ag
NaI
CH3OH
(H3C)2N
N+(CH3)3
F
F
2a
-O3S
X
F
F
F
F
F
F
F
(H3C)2N
N+(CH3)3
F
2c: X = NO2
2d: X = Cl
2e: X = H
2f : X = CH3
2g: X = NH2
Fig. 2. Synthetic procedures for 2a through 2g.
yellow liquid (47 g, 92%). 1H NMR (CDCl3, ꢃ) 2.97 (t, J ¼ 2:2
Hz, 6H); 13C NMR (CDCl3, ꢃ) 43.1 (t, JC{F ¼ 4:3 Hz), 60.5 (t,
JC{F ¼ 27:9 Hz), 131.8 (tt, JC{F ¼ 2:5, 10.5 Hz), 141.5 (dddd,
JC{F ¼ 4:6, 7.1, 13.9, 241 Hz), 147.4 (dddd, JC{F ¼ 4:6, 7.1, 13.9,
241 Hz); Found: C, 30.04; H, 1.93; N, 4.18%. Calcd for C8H6-
F4IN: C, 30.12; H, 1.90; N, 4.39%.
mixture of 5 (7.5 g, 26 mmol) and tetrahydrofuran (50 mL), 1
mol Lꢁ1 tetrahydrofuran solution of tetrabutylammonium fluoride
(52 mL) was added, and the mixture was stirred for 16 h. Then,
concentrated hydrochloric acid (20 mL) and water (200 mL) were
added, and the mixture was extracted with ether (150 mL ꢂ 3).
The organic phase was dried over Na2SO4 and filtered. Solvent in
the filtrate was removed under reduced pressure to give 6 (6.0 g,
88%) as pale-yellow liquid. 1H NMR (CDCl3, ꢃ) 3.00 (t, J ¼
2,3,5,6-Tetrafluoro-4-trimethylsilylethynyl-N,N-dimethyl-
aniline (5). To a mixture of 4 (25 g, 78 mmol), dichlorobis-
(triphenylphosphine)palladium(II) (1.1 g, 1.6 mmol), CuCl (0.078
g, 0.79 mmol), and triethylamine (150 mL), trimethylsilylacety-
lene (15 mL) was added, and the mixture was stirred for 6 days
at room temperature under a nitrogen atmosphere. Then, the mix-
ture was filtered, and the solvent in the filtrate was removed under
reduced pressure. The residue was purified by vacuum distillation
(100–110 ꢃC, 0.67 kPa) to give 5 (88 g, 71%) as pale-yellow liq-
2:4 Hz, 6H), 3.50 (s, 1H); 13C NMR (CDCl3, ꢃ) 42.8 (t, JC{F
¼
4:3 Hz), 69.3 (t, JC{F ¼ 3:6 Hz), 87.2 (t, JC{F ¼ 3:4 Hz), 93.3 (t,
JC{F ¼ 18:3 Hz), 132.5 (tt, JC{F ¼ 2:6, 10.4 Hz), 140.8 (dddd,
JC{F ¼ 4:2, 6.6, 12.7, 248 Hz), 148.0 (dddd, JC{F ¼ 4:2, 6.6, 12.7,
248 Hz); Found: C, 55.81; H, 3.64; N, 5.84%. Calcd for C10H7-
F4N: C, 55.31; H, 3.25; N, 6.45%.
4,40-Ethynediylbis(2,3,5,6-tetrafluoro-N,N-dimethylaniline)
(7). To a mixture of 6 (10 g, 46 mmol), dichlorobis(triphenyl-
phosphine)palladium(II) (0.64 g, 0.91 mmol), CuCl (0.046 g,
0.46 mmol), and triethylamine (150 mL), 4 (15 g, 47 mmol) was
added, and the mixture was stirred for 3 days at room temperature
under a nitrogen atmosphere. Then, the mixture was filtered, and
solvent in the filtrate was removed under reduced pressure. The
residue was recrystallized from chloroform to give 7 (7.0 g,
37%) as yellow crystals. Mp 195 ꢃC; 1H NMR (CDCl3, ꢃ) 3.02
1
uid. H NMR (CDCl3, ꢃ) 0.27 (s, 9H), 2.99 (t, J ¼ 2:4 Hz, 6H);
13C NMR (CDCl3, ꢃ) ꢁ0:2, 43.1 (t, JC{F ¼ 4:6 Hz), 89.4 (t, JC{F
¼
3:7 Hz), 95.4 (tt, JC{F ¼ 1:5, 18.5 Hz), 106.0 (t, JC{F ¼ 3:5 Hz),
132.0 (tt, JC{F ¼ 2:2, 11.1 Hz), 141.0 (dddd, JC{F ¼ 3:9, 6.5, 13.4,
248 Hz), 147.8 (dddd, JC{F ¼ 3:9, 6.5, 13.4, 248 Hz); Found: C,
53.94; H, 5.40; N, 4.64%. Calcd for C13H15F4NSi: C, 53.96; H,
5.23; N, 4.84%.
4-Ethynyl-2,3,5,6-tetrafluoro-N,N-dimethylaniline (6). To a

H. Umezawa et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007) 1415
(t, J ¼ 2:4 Hz, 12H); 13C NMR (CDCl3, ꢃ) 43.1 (t, JC{F ¼ 4:8 Hz),
Table 1. Calculated Eeg, ꢀꢂeg, and ꢀ0 Values of pNA, 1,
and 2
69.7 (t, JC{F ¼ 4:0 Hz), 82.6 (t, JC{F ¼ 4:0 Hz), 133.1 (tt, JC{F
¼
2:2, 10.5 Hz), 140.5 (dddd, JC{F ¼ 4:1, 6.6, 13.6, 242 Hz), 148.6
(dddd, JC{F ¼ 4:1, 6.6, 13.6, 242 Hz); Found: C, 52.34; H, 2.99;
N, 6.68%. Calcd for C18H12F8N2: C, 52.95; H, 2.96; N, 6.86%.
4-[2-(4-Dimethylamino-2,3,5,6-tetrafluorophenyl)ethynyl]-
2,3,5,6-tetrafluoro-N,N,N-trimethylanilinium Trifluorometh-
anesulfonate (2b). A mixture of 7 (4.5 g, 11 mmol) and methyl
trifluoromethanesulfonate (6.9 mL) were stirred for 2 days at room
temperature. Then, chloroform was added, and the solution was
filtered. The solid was washed with chloroform. After dissolving
the remaining solid part using ethanol, the precipitate was filtered
off. The solvent of the filtrate was removed under reduced pres-
sure. The residue was recrystallized from methanol to give 2b
Eeg,calc/eV
ꢀꢂeg/Debye
ꢀ
0,calc=10ꢁ30 esu
pNA
1
2
3.68
3.48
3.53
6.24
19.5
22.5
8.4
88.5
77.1
35000
30000
25000
20000
15000
10000
5000
0
1
(3.5 g, 55%) as pale-yellow crystals. Mp 238 ꢃC; H NMR (CD3-
OD, ꢃ) 3.08 (t, J ¼ 2:7 Hz, 6H), 3.94 (t, J ¼ 1:8 Hz, 9H). Found:
C, 41.52; H, 3.09; N, 5.14%. Calcd for C20H15F11N2O3S: C,
41.97; H, 2.64; N, 4.89%. Unfortunately, 13C NMR data of 2b
could not be obtained because of the low solubility.
4-[2-(4-Dimethylamino-2,3,5,6-tetrafluorophenyl)ethynyl]-
2,3,5,6-tetrafluoro-N,N,N-trimethylanilinium Iodide (2a).
To a mixture of 2b (3.5 g, 6.1 mmol) and methanol (100 mL), so-
dium iodide methanol solution (NaI 35 g, methanol 100 mL) was
added. The mixture was stirred for a few seconds and poured into
cold water (500 mL). The resulting precipitate was collected by
filtration and washed with chloroform. After drying, 2a was ob-
tained as light-yellow powder (2.0 g, 60%). This compound was
unstable and readily changed to 7. So, we confirmed that this com-
pound formed by the generation of silver iodide, by mixing it with
a methanol solution of silver benzenesulfonate.
300
350
400
450
500
550
Wavelength / nm
Fig. 3. UV and visible absorption spectra of 2b (solid line)
and pNA (dotted line) in methanol.
Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; Fax: +44
1223 336033; or e-mail: deposit@ccdc.cam.ac.uk).
Results and Discussion
MO calculations were performed on pNA, 1, and 2 as shown
in Table 1. The ꢀ0,calc of 2 was estimated to be about 9 times
greater than that of pNA, whereas the Eeg values were similar.
The iodide salt of 2 (2a) was found to be unstable, and half
of it was changed to 7 in a few days at room temperature. This
is due to the relatively strong nucleophilicity of iodide anion
compared to trifluoromethanesulfonate anion. An ammonio
methyl group of 2a is eliminated by nucleophilic attack of
the iodide anion to give 7, and the resulting methyl iodide
evaporates. Then, in order to estimate the linear and nonlinear
optical properties of cation 2, 2b was used instead of 2a. The
UV and visible absorption spectra of 2b were measured in a
methanol solution and compared with that of pNA. As shown
in Fig. 3, absorption maximum wavelength (ꢄmax) of 2b was
342 nm, which is 27 nm shorter than that of pNA. However,
absorption cutoff was almost the same, because 2b had a
shoulder at about 380 nm. In the solid state, the shoulder of
2b disappeared, and the cutoff wavelength of 2b was found
The melting points were determined by a differential scanning
calorimeter (Perkin-Elmer Pyres Diamond DSC). The chemical
structures of the compounds obtained were confirmed by H and
1
13C NMR spectroscopies (JEOL LAMBDA 400) and elemental
analysis (IMRAM, Tohoku University). UV–visible absorption
spectra were recorded on a Jasco V-570 spectrophotometer. IR
spectra were recorded on a Nicolet AVATAR 360 spectrometer.
Hyper-Rayleigh scattering (HRS) measurement21 for 1a and 2b
were performed by using a nano-second Nd:YAG laser (Coherent
Infinityꢁ 40–100) at 1064 nm. These measurements were execut-
ed in methanol and pNA was used as an external standard,22
of which the ꢀ value at 1064 nm in methanol is 3:45 ꢂ 10ꢁ29 esu.
According to the two-level model,23 the ꢀ values obtained were
adjusted to ꢀ at zero frequency (ꢀ0,expt). The SHG activities
of synthesized crystals were confirmed by the emission of green
light (539.5 nm) after irradiation with a Nd:YAP laser (Elmas
L-100) at ꢄ ¼ 1079 nm. This was conducted only for qualitative
purpose to remove the centrosymmetric crystals from further
investigation. X-ray crystallographic analysis was performed
using a Mac Science MXC3 diffractometer with a Mo Kꢅ source
to be shorter than that of pNA, as shown in Fig. 4. For ꢄmax
,
a similar tendency between two compounds was observed.
The IR spectra of 2b were also measured and compared with
trifluoromethanesulfonate salt of 1 (1b) as shown in Fig. 5.
Transmittances of 1b and 2b were normalized to each other
in relation to the absorption around 2200 cmꢁ1, which origi-
nates from stretching vibration of the CꢄC bond. Absorption
of C–H stretching vibration around 3000 cmꢁ1 of 2b was
found to be weaken compared to 1b, because of the reduced
number of C–H bonds in 2b. Accordingly, lower absorption
of 2b around 1.3 and 1.5 mm than 1b was expected, since ab-
sorption around these wavelength regions is assigned to be
overtones of C–H stretching vibration. Hyper-Rayleigh scatter-
˚
(ꢄ ¼ 0:71073 A) for a crystal of 2b grown in methanol solution
by the slow evaporation method. Cell parameters were determined
from 22 preliminary reflections. The crystal structure was deter-
mined by direct methods and refined by full-matrix least-squares
procedures using the CRYSTAN program. Non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were attached to
their parent atoms by fixed bond lengths and idealized bond angles
and were refined isotropically. Crystallographic data have been
deposited in Cambridge Crystallographic Data Centre as supple-
mentary publication No. CCDC-255977. Copies of the data can
be obtained free charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data

1416 Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007)
Fluorinated Chromophore for Nonlinear Optics
400
350
300
250
200
150
100
50
E
6.0
5.0
4.0
D
3.0
2.0
1.0
0
0
0.5 1.0 1.5 2.0 2.5 3.0
C
B
A
Number density / 1017 cm-3
300
350
400
450
500
550
Wavelength / nm
Fig. 4. UV and visible diffuse reflectance spectra of 2b
(solid line) and pNA (dotted line). Spectra were normal-
ized at the maximum.
0
0
200
400
600
800
1000
Fundamental intensity / a.u.
Fig. 6. The quadratic dependence between fundamental
and second-harmonic intensities of 2b in methanol at dif-
ferent ion-pair number densities: A, 5:17 ꢂ 1016 cmꢁ3
;
B, 1:03 ꢂ 1017 cmꢁ3; C, 1:55 ꢂ 1017 cmꢁ3; D, 2:07 ꢂ
1017 cmꢁ3; E, 2:59 ꢂ 1017 cmꢁ3. Inset: the linear plot of
the quadratic coefficient as function of the number density.
Table 2. Crystallographic Data of 2b
Formula
F.W.
C20H15F11N2O3S
572.4
Crystal system
Space group
Monoclinic
Pc
3500
3000
2500
2000
˚
a/A
8.093(2)
11.155(3)
14.255(7)
117.4(1)
1142.4(9)
2
1.728
2.474
2981
2398
Wavenumber / cm-1
˚
b/A
˚
c/A
Fig. 5. IR spectra of 1b (dotted line) and 2b (solid line).
Spectra were normalized using the absorption band around
2200 cmꢁ1
ꢀ/ꢃ
˚
V/A
3
.
Z
Dcalcd/Mg mꢁ3
ing (HRS) measurement was performed on 2b to clarify its
molecular nonlinear optical efficiency. As shown in Fig. 6,
quadratic dependence between fundamental and second-
harmonic intensities was observed for 2b. From fitted curves
of the data, quadratic coefficients were obtained. The inset
shows a linear plot of the quadratic coefficient as function
of the ion-pair number density in the corresponding solution.
By comparing these data to those for pNA as an external stan-
dard, the ꢀ value at 1064 nm (ꢀ1064) of 2b was determined to be
234 ꢂ 10ꢁ30 esu and ꢀ0 was calculated to be 123 ꢂ 10ꢁ30 esu
from ꢀ1064 and ꢄmax by the two level model.23 This ꢀ0 value
is about 8 times greater than that of pNA. The counter anion of
2a was changed to a 4-substituted benzenesulfonate (p-nitro-
benzenesulfonate (2c), p-chlorobenzenesulfonate (2d), ben-
zenesulfonate (2e), p-toluenesulfonate (2f), and p-aminoben-
zenesulfonate (2g)) to obtain the optimal crystals as studied
before.8–12 However, 2f was unstable like 2a for the same rea-
son described above. By powder test of the prepared crystals,
2b, 2c, 2d, and 2f were found to have noncentrosymmetric
structures. Single crystals of these four compounds were pre-
pared by slow evaporation of respective methanol solutions
under a nitrogen atmosphere. Good quality and reasonable size
crystals were only obtained for 2b so far. From the X-ray crys-
ꢂ (Mo Kꢅ)/mmꢁ1
No. of total reflections
No. of unique reflections
No. of reflections with I > 2ꢆðIÞ
R
1792
0.059
0.052
wR
tallographic analysis, the space group of the crystal of 2b was
found to be Pc, which is noncentrosymmetric, and the polar
axis of this crystal was in ac plane. Details of crystallographic
data and projection of the crystal structure are shown in
Table 2 and Fig. 7, respectively. The angle between the polar
axis and molecular long axis was found to be about 14ꢃ, if the
molecular long axis was taken as a line linking two nitrogen
atoms in the cation. Using this information, the second-order
NLO coefficient (d) was estimated by using the oriented-gas
model.13 The local field factors of 2b were set to be the same
as those of a crystal of 4-[2-(4-dimethylaminophenyl)ethenyl]-
1-methylpyridinium p-toluenesulfonate (DAST), which is a
known organic ionic chromophore for second-order NLO ma-
terial.14–16 As a result, the diagonal component (d11) and off-
diagonal component (d12) were estimated to be 610 and

H. Umezawa et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007) 1417
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Fig. 7. Crystal structure of 2b viewed along the a axis.
Hydrogen atoms were omitted.
8
155 pm Vꢁ1, respectively. 2-Methyl-4-nitroaniline (MNA)17
and N-(4-nitrophenyl)-L-prolinol (NPP)18–20 are known to be
typical second-order NLO materials, and they have a similar
absorption to 2b. Thus, the d values of these two crystals were
also estimated by the same manner. MNA was found to have
d11 ¼ 43:9 and d12 ¼ 21:8 pm Vꢁ1 and NPP had d22 ¼ 8:9 and
d21 ¼ 24:0 pm Vꢁ1. When the d values of 2b were compared
with those of MNA and NPP, 2b was found to have 14 times
greater diagonal component than that of MNA and 6.5 times
greater off-diagonal component than that of NPP. These results
are caused by the large ꢀ value, which represents microscopic
efficiency, and efficient cation alignment in the crystal of 2b
compared with MNA and NPP.
In conclusion, we successfully synthesized the partially
fluorinated skeleton 2. From the UV and visible absorption
spectrum, ꢄmax of 2b was shorter than that of pNA, while the
ꢀ0 value was about 8 times larger. In the IR spectra, absorption
originated from C–H stretching vibration of 2b was less
intense than that of 1b. Therefore, 2b should have a lower
absorption around 1.3 and 1.5 mm, where overtones of C–H
stretching vibration are most prominent, compared with 1b.
By using the oriented-gas model, the diagonal d component of
2b was estimated to be 14 times greater than that of MNA, and
the off-diagonal d component was estimated to be 6.5 times
greater than that of NPP. From these results, we think that
2b is an excellent candidate for second-order NLO device
applications. Since deuteration of hydrogen in methyl group
is not so difficult task and generally does not change the crystal
structure of a compound,24 the corresponding chromophore
without C–H bonds is being synthesized and investigated.
9
H. Umezawa, K. Tsuji, Anwar, X.-M. Duan, S. Okada, H.
Oikawa, H. Matsuda, H. Nakanishi, Nonlinear Opt. 2000, 24, 73.
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Matsuda, Nonlinear Opt. 2000, 25, 45.
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