Synthesis and properties of water-soluble fluorescent 2-borylazobenzenes bearing ionic functional groups

Article abstract of DOI:10.1016/j.ica.2011.07.053

Water-soluble fluorescent azobenzenes have been synthesized by quaternization of 2-[bis(pentafluorophenyl)boryl]azobenzenes bearing dimethylamino groups with iodomethane or propanesultone. The 2-borylazobenzenes both dissolve and fluoresce in water. The double introduction of ionic moieties provides the azobenzenes with higher water solubility. Dynamic light scattering and scanning electron microscopy measurements indicate that the 2-borylazobenzenes aggregate in water. They are expected to be applied in aqueous environments.

Full text of DOI:10.1016/j.ica.2011.07.053

Inorganica Chimica Acta 381 (2012) 117–123
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and properties of water-soluble fluorescent 2-borylazobenzenes
bearing ionic functional groups
⇑
⇑
Hiroaki Itoi, Tetsuya Kambe, Naokazu Kano , Takayuki Kawashima
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-0033, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 11 August 2011
Water-soluble fluorescent azobenzenes have been synthesized by quaternization of 2-[bis(pentafluo-
rophenyl)boryl]azobenzenes bearing dimethylamino groups with iodomethane or propanesultone. The
2-borylazobenzenes both dissolve and fluoresce in water. The double introduction of ionic moieties pro-
vides the azobenzenes with higher water solubility. Dynamic light scattering and scanning electron
microscopy measurements indicate that the 2-borylazobenzenes aggregate in water. They are expected
to be applied in aqueous environments.
Fluorescence Spectroscopy: from Single
Chemosensors to Nanoparticles Science –
Special Issue
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Azobenzenes
N–B interactions
Fluorescence
Ionic groups
Water-solubility
1. Introduction
derivatization. Hydrophilic ionic moieties are attached to the
2-borylazobenzenes via an alkoxy spacer to keep the electron-
Fluorescent dyes have been focused on for many purposes, such
as ion sensors [1], organic light-emitting diodes [2] and analytical
methods [3]. Water solubility is required for fluorescent dyes to be
used in an aqueous medium as a water-soluble ion sensor [4] and
as a fluorescent vital stain [5]. One of the most common and fre-
quently used dyes is an azobenzene, and many azobenzene deriv-
atives have been synthesized as coloring materials or
photoswitching devices that utilize their photoisomerization prop-
erties [6]. Almost none of the azobenzenes emit light because of a
radiationless deactivation process such as photoisomerization [7].
We have synthesized some 2-[bis(pentafluorophenyl)boryl]azo-
benzenes and reported the intense fluorescence properties upon
irradiation [8–10]. In the 2-borylazobenzenes, the N–B interaction
effectively restricts conformational freedom, influences both the
HOMO and LUMO and produces the fluorescence property for the
azobenzenes. However, the previously reported 2-borylazobenz-
enes are insoluble in water because of the hydrophobic pentafluor-
ophenyl groups, which are essential for fluorescence emission.
Attaching hydrophilic functional groups would be effective in sol-
ubilization of the fluorescent azobenzenes in water.
donating effect as an alkoxy group. The solubilities, fluorescent
properties and aggregation property of the 2-borylazobenzenes
are also discussed.
2. Results and discussion
2.1. Synthesis of ionic fluorescent azobenzenes
Desilylation of 1 with hydrochloric acid gave 40-hydroxy deriv-
ative 3, which is insoluble in water, in 92% yield (Scheme 1).
Deprotonation of 3 with NaH and further aminoalkylation with
1-chloro-2-(dimethylamino)ethane hydrochloride gave azoben-
zene 5 bearing an aminoalkoxy group in 86% yield. Quaternization
of the amine moiety of 5 with iodomethane in methanol gave
trimethylammonium salt 7 in 98% yield [11]. Heating of 5 with
1,3-propanesultone in toluene, which is a good method to intro-
duce a zwitterionic fragment [12,13], afforded the ring-opened
product 8 in 54% yield.
To attach one more ammonium moiety, the 3-(dimethyl-
amino)propynyl group was introduced at the 4-position of the
2-borylazobenzene by Sonogashira coupling of 2-boryl-4-bromo-
40-siloxyazobenzene 2, which was synthesized similarly to 1 [10].
The siloxy group was desilylated during the coupling process to
give 4 in 81% yield. As in the case of derivatization of 3, azobenzene
4 was converted to 9 and 10 bearing two trimethylammonium and
two sulfobetaine moieties, respectively, via 6. The 11B NMR
Herein, we report the synthesis of 2-borylazobenzenes bearing
ionic moieties such as ammonium or sulfobetaine by postsynthetic
⇑
Corresponding authors. Present address: Faculty of Science, Gakushuin Univer-
sity, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan (T.Kawashima).
E-mail address: kano@chem.s.u-tokyo.ac.jp (N. Kano).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.07.053

118
H. Itoi et al. / Inorganica Chimica Acta 381 (2012) 117–123
aq. HCI
MeOH, 50 ºC
TBDMS
N
O
O
OH
1. NaH
2. Cl(CH2)2NMe2•HCl
N N
B
N N
B
N N
B
THF, reflux
C6F5
C6F5
C6F5
C6F5
C6F5
C6F5
N
R
R
R
Pd(PPh3)2Cl2
CuI
3
1 (R = H)
2 (R = Br)
(R = H, 92%)
5
6
(R = H, 86%)
(R = C CCH2NMe2, 69%)
4 (R =C CCH2NMe2, 81%)
Et3N, 60 ºC
N
O
MeI
I
N N
MeOH, r.t.
N
B
C6F5
C6F5
O
7 (98%)
N N
B
C6F5
C6F5
SO3
O
S
O
N
O
O
5
toluene, 60 ºC
N N
B
C6F5
C6F5
8
(54%)
N
O
2I
MeI
MeOH, 50 ºC
N N
B
N
C6F5
O
C6F5
N
N N
B
9
(98%)
C6F5
C6F5
SO3
N
N
O
S
O
O
6
O
N N
B
toluene, 60 ºC
C6F5
C6F5
N
O3S
10
(40%)
Scheme 1. Preparation pathways for the ionic fluorescent azobenzenes.
spectroscopy of the 2-borylazobenzenes 7–10 [7: dB ꢀ0.5, 8: dB
ꢀ0.9, 9: dB ꢀ0.4, 10: dB ꢀ0.5 ppm], whose chemical shifts are al-
most the same as the previously reported values [10], suggested
the tetracoordination state of the boron atom, meaning that their
N–B dative bonds are maintained after the quaternization.
2.2. Optical properties in organic solvents
Azobenzenes 7–10 bearing ionic moieties appeared yellow or
yellow-green in organic solvents, and emitted green fluorescence,
as described in 2-[bis(pentafluorophenyl)boryl]-40-methoxyazo-

H. Itoi et al. / Inorganica Chimica Acta 381 (2012) 117–123
119
Table 2
benzene (11) [8]. The optical properties of azobenzenes 7–10 in
Properties of the 2-borylazobenzenes 7–10 in water.
both dichloromethane and methanol are summarized in Table 1.
In the UV/Vis and fluorescence spectra of 7 and 8 in dichlorometh-
ane, their absorption maxima show a blue shift from 11, whereas
their emission maxima are close to that of 11 and their fluores-
cence quantum yields are less than that of 11. In methanol, which
is a more polar solvent, azobenzene 11 showed a large red shift of
the absorption maximum from that in dichloromethane, whereas
azobenzenes 7 and 8 show very small red shifts, and no fluores-
cence emission. In contrast, azobenzenes 7 and 8 showed relatively
high fluorescence quantum yields (UFL = 0.77 (7) and 0.58 (8)) even
in methanol. These phenomena showed that azobenzenes 7, 8 and
11 have excited states more polar than the ground states and intro-
duction of the hydrophilic substituents at the 40-position decreases
the polarity of the excited state significantly, and that the high
affinity of the hydrophilic parts for polar solvents is effective in
keeping their fluorescence properties.
Water solubility kabs (nm) kem (nm) Stokes shift (cmꢀ1
(g/L)
)
UF
7
8
9
0.54
0.62
1.15
433
432
456
460
536
535
552
545
4400
4500
3800
4100
0.031
0.028
0.10
10 1.19
0.092
In the cases of azobenzenes 9 and 10 in methanol, both the
absorption and emission maxima are admittedly red shifted com-
pared with 7 and 8, reflecting the extension of
p-conjugation by
introduction of ethynylene groups at the 4-position (Table 1).
The fluorescence quantum yield of azobenzene 10 bearing two
sulfobetaine moieties is relatively low (UFL = 0.14). Molecular mo-
tions of the alkyl chains of 10 would cause radiationless deactiva-
tion and partly account for the low quantum yields.
2.3. Properties of the azobenzenes in water
Fig. 1. The UV–Vis (dotted line) and fluorescence (solid line) spectra of 9.
We measured the solubility of the 2-borylazobenzenes in water
(Table 2). The previously reported fluorescent 2-borylazobenzenes
such as 11 are completely insoluble in water. However, the hydro-
philic ionic moieties can make the 2-borylazobenzenes soluble in
water. The 2-borylazobenzenes 9 and 10 bearing two ionic func-
tional groups were found to have about double the water solubility
of 7 and 8 bearing only one ionic group. The most water-soluble
derivative was 10 among 7–10.
The UV–Vis and fluorescence spectra of 9 in water are shown in
Fig. 1 and its photophysical data in water as well as those of other
water-soluble azobenzenes 7, 8 and 10 are summarized in Table 2.
Their absorption maxima and fluorescence emission maxima were
not so different from those in organic solvents. Their quantum
yields in water were much lower than those in other solvents.
Although the quantum yield of azobenzene 9 in water was 0.10
and still low, this is the highest value ever among those of the
2-borylazobenzenes in water.
The solubility of azobenzenes 7–10 was tested in a mixture of
dichloromethane and water. As shown in Fig. 2a, most of azobenz-
enes 7, 8 and 10 dissolved much better in dichloromethane than in
water. In contrast, azobenzene 9 dissolved in water much better
than in dichloromethane. Upon irradiation with a UV lamp, as
shown in Fig. 2b, the water phase dissolving 9 emitted fluores-
cence. Therefore, 9 can be recognized as a water-soluble fluores-
cent azobenzene.
Fig. 2. Photos of solutions of azobenzenes 7–10 (a) under room light and (b) under
UV light. The top phase of each vial is water and the bottom phase is
dichloromethane.
As their emissions in water looked different from those in or-
ganic solvents, an aqueous solution of 9 was evaporated on a sili-
con surface, and the thus-prepared sample of 9 was observed by
scanning electron microscopy (SEM) [14] to obtain information
on the morphology of the azobenzene in water (Fig. 3). The SEM
image showed spherical bodies and a particle size of 1000–
3000 nm, suggesting the formation of aggregates. Attaching of
the hydrophilic moieties may be a reason for the aggregation in
water (see Fig. 4).
To confirm aggregation and to obtain information about particle
sizes in the aqueous solution, we performed dynamic light-scatter-
ing (DLS) [15] measurements of the 2-borylazobenzenes in water
as well as in methanol (Fig. 4). DLS measurements revealed that
the particle sizes of azobenzenes 7 and 8 in water were 1388 nm
and 1368 nm, respectively, and those in methanol were 224 nm
Table 1
Optical properties of the 2-borylazobenzenes 7–11 in organic solvents.
Solvent
kabs (nm)
kem (nm)
Stokes shift (cm–1
)
UF
7
8
CH2Cl2
MeOH
CH2Cl2
MeOH
MeOH
MeOH
CH2Cl2
MeOH
430
434
431
435
458
468
446
522
533
529
535
530
546
555
536
4500
4100
4500
4100
3500
3300
3700
0.39
0.77
0.88
0.58
0.53
0.14
0.98
9
10
11
a
a
a
–
–
–
a
Not detectable.

120
H. Itoi et al. / Inorganica Chimica Acta 381 (2012) 117–123
Chemistry, Faculty of Science, The University of Tokyo. 2-[Bis(pen-
tafluorophenyl)boryl]-40-(tert-butyldimethylsiloxy)azobenzene (1)
was prepared according to the literature [10]. The fluorescence
quantum yields were measured with a Hamamatsu absolute
photo-luminescence quantum yield measurement system C9920-
02. Synthesis of compounds 8 and 10 was carried out following
the procedure reported previously [12].
4.2. Syntheses
4.2.1. Synthesis of 4-bromo-40-(tert-butyldimethylsiloxy)-2-
iodoazobenzene
To a diazonium salt prepared from 4-bromo-2-iodoaniline
(2.98 g, 10.0 mmol) and sodium nitrite (690 mg, 10.0 mmol) in
hydrochloric acid (0.24 M, 100 mL, 24 mmol) solution, an aqueous
sodium hydroxide (0.25 M, 40 mL, 10 mmol) solution of phenol
(942 mg, 10.0 mmol) was added at 0 °C. The reaction mixture
was allowed to warm gradually to room temperature, further stir-
red for 16 h and treated with an aqueous sodium hydroxide solu-
tion until it became basic. After washing with chloroform, the
aqueous layer was acidified with hydrochloric acid. The organic
layer was extracted with chloroform, dried with anhydrous mag-
nesium sulfate and evaporated to give deep red oil. After purifica-
tion with a silica gel column chromatography (chloroform), the
residual solids were dissolved in DMF. tert-Butyldimethylsilyl
chloride (TBDMSCl) (397 mg, 5.83 mmol) and imidazole (805 mg,
5.34 mmol) were added to this solution, and mixture was stirred
at room temperature for 13 h. After evaporation, purification with
a silica gel column chromatography (chloroform/hexane) gave 4-
Fig. 3. SEM image of 9 on the silicon surface.
and 204 nm, respectively. In the cases of 9 and 10, the sizes in
water are 212 nm and 424 nm, and those in methanol are
0.76 nm and 0.73 nm, respectively. These results clearly show that
the azobenzenes 7 and 8 aggregate both in water and in methanol;
however, 9 and 10 exist as discrete molecules in methanol and
aggregate in water. The differences in the morphology of the azo-
benzenes between water and organic solvents are considered to af-
fect the differences in the properties such as fluorescence quantum
yields.
3. Conclusions
This work highlights the synthetic conversion of hydrophobic
fluorescent azobenzenes into the corresponding water-soluble
derivatives by attachment of two types of ionic moieties. The fluo-
rescence quantum yields of the azobenzenes in organic solvents
are relatively high or moderate despite the introduction of the io-
nic moieties. Attaching one sulfobetaine or one ammonium moiety
to the azobenzene scaffolds could not prevent aggregation, both in
methanol and in water. In addition, it was shown that the azobenz-
enes bearing two ionic moieties show higher water solubility and
do not aggregate in methanol. They emitted fluorescence in water,
although their fluorescence quantum yields are not so large. There-
fore, use of the above methodology to confer water solubility on
fluorescent azobenzenes would provide hydrophilic 2-borylazo-
benzenes with a handle for applications such as biological use
and ion sensing.
bromo-40-(tert-butyldimethylsiloxy)-2-iodoazobenzene
(2.26 g,
2.70 mmol, 27%). Red solids, m.p. 68.1–69.0 °C. 1H NMR
(400 MHz, CDCl3) d 0.24 (s, 6H), 0.99 (s, 9H), 6.94 (d, 3J = 9.0 Hz,
2H), 7.51 (d, 3J = 8.6 Hz, 1H), 7.53 (dd, 3J = 8.6 Hz, 4J = 2.0 Hz, 1H),
7.89 (d, 3J = 9.0 Hz, 2H), 8.14 (d, 4J = 2.0 Hz, 1H). 13C{1H} NMR
(100 MHz, CDCl3) d ꢀ4.21 (s, CH3), 18.34 (s, CSi), 25.69 (s, CH3),
102.73 (s, CH), 118.00 (s, CH), 120.52 (s, CH), 124.86 (s, CH),
125.44 (s, CH), 131.94 (s, quaternary C), 141.53 (s, quaternary C),
146.96 (s, quaternary C), 150.12 (s, quaternary C), 159.35 (s, qua-
ternary C). MS (FAB+, NBA) m/z 517 [M+H]+. Anal. Calc. for
C18H22BrIN2OSi: C, 41.79; H, 4.29; N, 5.42. Found: C, 41.94; H,
4.31; N, 5.21%.
4.2.2. Synthesis of {5-bromo-2-[4-(tert-butyldimethylsiloxy)-
phenylazo]phenyl}bis(pentafluorophenyl)borane (2)
4. Experimental
n-BuLi (1.6 M in hexane, 2.1 mL, 3.4 mmol) was added to an
Et2O solution (100 mL) of 4-bromo-40-(tert-butyldimethylsiloxy)-
2-iodoazobenzene (2.01 g, 3.89 mmol) at ꢀ112 °C. The reaction
solution was stirred for 30 min and then added to an Et2O solution
4.1. General
Solvents were purified before use by reported methods. All
reactions were carried out under argon atmosphere otherwise
noted. The 1H NMR (400 MHz), 11B NMR (128 MHz), 13C NMR
(100 MHz) and 19F NMR (376 MHz) spectra were measured with
a JEOL AL400 spectrometer. The 13C NMR (126 MHz) spectra were
measured with a Bruker DRX-500 spectrometer. Tetramethylsilane
was used as an external standard for the 1H and 13C{1H} NMR spec-
(20 mL)
of
ethoxybis(pentafluorophenyl)borane
(1.06 mL,
4.30 mmol) at ꢀ112 °C. After the solution was allowed to warm
gradually to room temperature and the solvent was removed in va-
cuo, the residue was dissolved in chloroform and insoluble materi-
als were removed by filtration. Concentration of the filtrate
followed by separation with gel permeation chromatography (elu-
ent: hexane) afforded 2 (2.02 g, 2.75 mmol, 71%). Orange solids,
m.p. 86.3–86.9 °C. 1H NMR (400 MHz, CDCl3) d 0.24 (s, 6H), 0.96
(s, 9H), 6.84 (d, 3J = 8.8 Hz, 2H), 7.61 (dd, 3J = 8.8 Hz, 4J = 2.6 Hz,
2H), 7.69 (s, 1H), 7.92 (d, 3J = 8.8 Hz, 2H), 8.05 (d, 3J = 8.4 Hz, 1H).
11B NMR (128 MHz, CDCl3) d ꢀ0.5 (line width h1/2 = 802 Hz).
13C{1H} NMR (126 MHz, CDCl3) d ꢀ4.34 (s, CH3), 18.15 (s, CSi),
25.42 (s, CH3), 114.05 (br s, CB), 120.96 (s, CH), 124.75 (s, CH),
128.57 (s, CH), 131.54 (s, CH), 131.70 (s, quaternary C), 131.93 (s,
CH), 136.16–138.15 (m, CF), 138.01 (s, quaternary C), 139.24–
141.25 (m, CF), 146.62–148.58 (m, CF), 154.50 (s, quaternary C),
156.46 (br s, CB), 160.59 (s, quaternary C). 19F NMR (376 MHz,
CDCl3) d ꢀ162.27 to ꢀ162.11 (m, 4F), ꢀ155.69 to ꢀ155.51 (m,
tra. BF3ꢁOEt2 and CCl3F were used as external standards for the 11
B
and 19F NMR spectra, respectively. Low- and high-resolution mass
spectra were measured with a JEOL MStation JMS-700 spectrome-
ter. Absorption spectra were measured with a Hitachi U-3500 UV/
Vis spectrometer and JASCO V-530 and V-670 UV/Vis spectrome-
ters. Fluorescence spectra were measured with a JASCO FP-6500
fluorescence spectrophotometer. Preparative gel permeation liquid
chromatography was performed by LC-908 with JAIGEL H1+H2 col-
umns (Japan Analytical Industry) with chloroform as the eluent. All
melting points were uncorrected. Elemental analyses were per-
formed by the Microanalytical Laboratory of Department of

H. Itoi et al. / Inorganica Chimica Acta 381 (2012) 117–123
121
Fig. 4. Size spectra of (a) 7, (b) 8, (c) 9, and (d) 10 by DLS.
2F), ꢀ131.30 (d, 3JFF = 22 Hz, 4F). MS (FAB+, NBA) m/z 737 [M+, 100],
735 [84%]. Anal. Calc. for C30H22BF10N2OSi: C, 49.00; H, 3.02; N,
3.81. Found: C, 49.14; H, 3.26; N, 3.59%.
4.08 (s, 2H), 6.93 (d, 3J = 9.0 Hz, 2H), 7.56 (d, 3J = 8.4 Hz, 1H), 7.62
(s, 1H), 7.95 (d, 3J = 9.0 Hz, 2H), 8.18 (d, 3J = 8.0 Hz, 1H). 11B NMR
(128 MHz, CDCl3) d ꢀ0.5 (line width h1/2 = 834 Hz). 13C{1H} NMR
(126 MHz, CDCl3) d 43.67 (s, CH3), 48.20 (s, CH2), 87.26 (s, quater-
nary C), 87.63 (s, quaternary C), 114.71 (br s, CB), 117.03 (s, CH),
125.25 (s, CH), 126.79 (s, CH), 128.35 (s, quaternary C), 131.27
(s, CH), 131.91 (s, CH), 136.15–138.25 (m, CF), 137.00 (s, quater-
nary C), 139.08–141.08 (m, CF), 146.64–148.56 (m, CF), 153.53
(br s, CB), 155.53 (s, quaternary C), 162.71 (s, quaternary C). 19F
NMR (376 MHz, CDCl3) d ꢀ162.25 to ꢀ162.11 (m, 4F), ꢀ155.79
4.2.3. Synthesis of [2-(4-hydroxyphenylazo)phenyl]bis-
(pentafluorophenyl)borane (3)
To a solution of 1 (225 mg, 343 lmol) in methanol (10 mL), was
added hydrochloric acid (2.0 mL, 24 mmol) in methanol (10 mL) in
air. The solution was stirred at room temperature for 15 h, until the
complete consumption of the starting material was recognized by
TLC. After the reaction, evaporation of the solvent and successive
reprecipitation from chloroform and hexane gave 3 (235 mg,
3
3
(t, JFF = 18 Hz, 2F), ꢀ131.13 (d, JFF = 18 Hz, 4F). MS (FAB+, NBA)
m/z 624 [M+H]+. HRMS (FAB+, NBA) m/z Calc. for C29H17BF10N3O
[M+H]+: 624.1227, Found: 624.1194.
317 l
mol, 92%). Yellow solids, m.p. 236.1–236.7 °C. 1H NMR
(400 MHz, CDCl3) d 5.35 (s, 1H), 6.88 (d, 3J = 9.2 Hz, 2H), 7.46–
7.54 (m, 2H), 7.61 (d, 3J = 6.8 Hz, 1H), 7.97 (d, 3J = 9.2 Hz, 2H),
8.24 (d, 3J = 8.0 Hz, 1H). 11B NMR (128 MHz, CDCl3) d ꢀ0.61 (s, h1/
2 = 204 Hz). 13C{1H} NMR (100 MHz, CDCl3) d 114.59 (br s, CB),
116.54 (s, CH), 124.91 (s, CH), 127.78 (s, CH), 128.25 (s, CH),
128.40 (s, CH), 135.00 (s, CH), 135.85–141.26 (m, two signals over-
lapped with each other), 137.96 (s, quaternary C), 146.33–148.77
(m, CF), 154.24 (br s, CB), 156.12 (s, quaternary C), 159.45 (s, qua-
ternary C). 19F NMR (376 MHz, CDCl3) d ꢀ162.29 to ꢀ162.15 (m,
4F), ꢀ155.88 (t, 3J = 20.3 Hz, 2F), ꢀ131.19 to ꢀ131.11 (m, 4F). MS
(FAB+, NBA) m/z 542 (M+). Anal. Calc. for C24H9BF10N2O: C, 53.17;
H, 1.67; N, 5.17. Found: C, 52.94; H, 1.93; N, 4.97%.
4.2.5. Aminoalkylation of 3
To a solution of 3 (49.2 mg, 90.8
added sodium hydride (5.4 mg, 225
2-(dimethylamino)ethane hydrochloride (14.9 mg, 103
dry THF (10 mL). The solution was refluxed for 22 h, until the com-
plete consumption of 3 was recognized by TLC. After the solution
was allowed to cool down to 0 °C, evaporation of the solvent, sep-
aration by silica gel column chromatography (methanol), and suc-
l
mol) in dry THF (10 mL), were
mol) and 1-chloro-
mol) in
l
l
cessive reprecipitation from chloroform and hexane gave
5
(47.8 mg, 86%). Brownish yellow solids, m.p. 145.9–147.2 °C. 1H
NMR (400 MHz, CDCl3) d 2.32 (s, 6H), 2.73 (t, 3J = 5.6 Hz, 2H),
4.11 (t, 3J = 5.6 Hz, 2H), 6.95 (d, 3J = 9.2 Hz, 2H), 7.46–7.53 (m,
2H), 7.60 (d, 3J = 7.2 Hz, 1H), 8.00 (d, 3J = 9.2 Hz, 2H), 8.23 (d,
4.2.4. Sonogashira coupling of 2
To a degassed solution of azobenzene 2 (150 mg, 204
triethylamine (20 mL), were added [Pd(PPh3)2Cl2] (14.4 mg,
20.4 mol), CuI (16.7 mg, 68.2 mol), and 3-dim-
ethylaminopropyne (65 L, 682 mol). The mixture was stirred
lmol) in
3J = 7.0 Hz, 1H). 11B NMR (128 MHz, CDCl3)
d
ꢀ0.5 (s, h1/
2 = 332 Hz). 19F NMR (376 MHz, CDCl3) d ꢀ162.32 to ꢀ162.19 (m,
4F), ꢀ155.94 (t, 3J = 20.1 Hz, 2F), ꢀ131.22 to ꢀ131.13 (m, 4F).
13C{1H} NMR (126 MHz, CDCl3) d 45.85 (s, CH3), 57.95 (s, CH2),
66.59 (s, CH2), 114.61 (br s, CB), 115.46 (s, CH), 124.69 (s, CH),
127.71 (s, CH), 128.27 (s, CH), 128.43 (s, CH), 134.91 (s, CH),
136.13–141.02 (m, CF, two signals overlapped with each other),
137.80 (s, quaternary C), 146.67–148.58 (m, CF), 154.19 (br s,
CB), 156.23 (s, quaternary C), 162.52 (s, quaternary C). MS (FAB+,
NBA) m/z 614 [M+H+]. Anal. Calc. for C28H18BF10N3O: C, 54.84; H,
2.96; N, 6.85. Found: C, 55.00; H, 3.20; N, 6.64%.
l
l
l
l
at 60 °C for 3 h, until the complete consumption of the starting
material was confirmed by TLC. The mixture was evaporated, ex-
tracted with CH2Cl2, washed with water and brine and dried with
unhydrous magnesium sulfate. Removal of the solvent under re-
duced pressure and successive silica gel column chromatography
(AcOEt/CHCl3) of the residue gave 4 (103 mg, 81%). Brown solids,
m.p. 140.1–141.8 °C. 1H NMR (400 MHz, CDCl3) d 2.88 (s, 6H),

122
H. Itoi et al. / Inorganica Chimica Acta 381 (2012) 117–123
3
4.2.6. Aminoalkylation of azobenzene 4
(d, JFF = 17 Hz, 4F). MS (FAB+, NBA) m/z 736 [M+H]+. Anal. Calc.
for C31H24BF10B2N3O4S (1.5H2O): C, 48.84; H, 3.57; N, 5.51. Found:
C, 49.16; H, 3.65; N, 5.15%.
Similarly to the synthesis of 5, successive treatment of 4
(30.0 mg, 48.1 mol) with sodium hydride (1.51 mg, 114 mol)
and 1-chloro-2-(dimethylamino)ethane hydrochloride (7.62 mg,
l
l
52.9
l
mol) in dry THF (20 mL) gave 6 (23.1 mg, 69%). Brown solids,
4.2.9. Synthesis of diammonium salt (9)
Similarly to the synthesis of 8, the reaction of 6 (23.0 mg,
33.1 lmol) with iodomethane (1.0 mL, 16.1 mmol) in methanol
120.8–123.8 °C. 1H NMR (400 MHz, CD3OD) d 2.31 (s, 6H), 2.34 (s,
6H), 2.72 (t, 3J = 5.2 Hz, 2H), 3.47 (s, 2H), 4.10 (t, 3J = 5.2 Hz, 2H),
6.93 (d, 3J = 8.8 Hz, 2H), 7.50 (d, 3J = 8.0 Hz, 1H), 7.59 (s, 1H), 7.96
(d, 3J = 8.8 Hz, 2H), 8.13 (d, 3J = 8.0 Hz, 1H). 11B NMR (128 MHz,
(10 mL) gave 9 (32.5 mg, 98%). Brown solids, m.p. 197.9–198.2 °C.
1H NMR (400 MHz, CD3OD) d 3.29 (s, 9H), 3.31 (s, 9H), 3.91 (t,
3J = 3.2 Hz, 2H), 4.64 (br s, 4H), 7.23 (d, 3J = 8.4 Hz, 2H), 7.77 (s,
1H), 7.82 (d, 3J = 8.2 Hz, 1H), 8.14 (d, 3J = 8.4 Hz, 2H), 8.44 (d,
3J = 8.2 Hz, 1H). 11B NMR (128 MHz, CD3OD) d ꢀ0.4 (line width
h1/2 = 649 Hz). 13C{1H} NMR (126 MHz, CD3OD) d 53.34 (s, CH3),
54.84 (s, CH3), 57.96 (s, CH2), 66.26 (s, CH2), 63.96 (s, CH2), 82.53
(s, quaternary C), 92.08 (s, quaternary C), 115.75 (br s, CB),
117.32 (s, CH), 126.29 (s, CH), 128.49 (s, quaternary C), 129.01 (s,
CH), 132.52 (s, CH), 134.16 (s, CH), 137.55–139.54 (m, CF), 139.75
(s, quaternary C), 140.54–142.57 (m, CF), 148.03–149.89 (m, CF),
154.196 (br s, CB), 157.65 (s, quaternary C), 163.28 (s, quaternary
C). 19F NMR (376 MHz, CD3OD) d ꢀ164.66 (m, 4F), ꢀ158.10 (t,
CD3OD)
d
ꢀ1.1 (line width h1/2 = 463 Hz). 13C{1H} NMR
(126 MHz, CD3OD) d 44.34 (s, CH3), 45.77 (s, CH3), 58.77 (s, CH2),
67.44 (s, CH2), 86.77 (s, quaternary C), 91.48 (s, quaternary C),
116.22 (br s, CB), 116.87 (s, CH), 126.00 (s, CH), 128.71 (s, CH),
131.08 (s, quaternary C), 132.00 (s, CH), 133.31 (s, CH), 137.51–
139.49 (m, CF), 139.16 (s, quaternary C), 140.43–142.48 (m, CF),
147.95–149.88 (m, CF), 155.10 (br s, CB), 156.97 (s, quaternary
C), 164.50 (s, quaternary C). One signal of the secondary carbon
atom in aliphatic region was not assigned due to overlapping. 19F
NMR (376 MHz, CD3OD) d ꢀ162.19 to ꢀ162.06 (m, 4F), ꢀ155.77
3
3
(t, JFF = 16 Hz, 2F), ꢀ131.09 (d, JFF = 16 Hz, 4F). MS (FAB+, NBA)
m/z 695 [M+H]+. HRMS (FAB+, NBA) m/z Calc. for C33H26BF10N4O
[M+H]+: 695.3728, Found: 695.3788.
3
3JFF = 20 Hz, 2F), –132.79 (dd, JFF = 20 Hz, 4F). MS (FAB+, NBA) m/
z 362 [Mꢀ2Iꢀ]2+. Anal. Calc. for C35H31BF10I2N4O (4.5 H2O): C,
39.68; H, 3.81; N, 5.29. Found: C, 39.37; H, 3.56; N, 4.93%.
4.2.7. N-methylation of 5
To a solution of 5 (84.0 mg, 137
lmol) in methanol (5 mL) was
4.2.10. Synthesis of sulfobetaine (10)
added iodomethane (50 L, 0.79 mmol) at room temperature in
l
Similarly to the synthesis of 8, the reaction of 6 (60.2 mg,
the air. The resulting reaction mixture was stirred for 8 h, and then
evaporated under reduced pressure. The resulting residue was rep-
recipitated from diethyl ether and hexane to afford 7 (101 mg,
98%). Orange solids, m.p. 162.5–168.3 °C. 1H NMR (400 MHz,
CDCl3) d 3.58 (s, 9H), 4.36 (br s, 2H), 4.63 (br s, 2H), 7.04 (d,
3J = 9.0 Hz, 2H), 7.48–7.57 (m, 2H), 7.63 (d, 3J = 6.8 Hz, 1H), 8.05
(d, 3J = 9.0 Hz, 2H), 8.27 (d, 3J = 7.6 Hz, 1H). 11B NMR (128 MHz,
CDCl3) d ꢀ0.1 (s, h1/2 = 854 Hz). 19F NMR (376 MHz, CDCl3) d
ꢀ161.99 to ꢀ161.86 (m, 4F), ꢀ155.50 (t, 3J = 18.4 Hz, 2F),
ꢀ131.08 to ꢀ131.00 (m, 4F). 13C{1H} NMR (126 MHz, CDCl3) d
55.25 (s, CH3), 63.00 (s, CH2), 65.08 (s, CH2), 114.47 (br s, CB),
115.62 (s, CH), 124.86 (s, CH), 128.32 (s, CH), 128.38 (s, CH),
128.63 (s, CH), 135.60 (s, CH), 136.19–141.10 (m, CF, two signals
overlapped with each other), 138.84 (s, quaternary C), 146.64–
148.57 (m, CF), 154.78 (br s, CB), 156.26 (s, quaternary C), 159.92
(s, quaternary C). MS (FAB+, NBA) m/z 628 [MꢀI]+. Anal. Calc. for
C29H21BF10IN3O: C, 46.12; H, 2.80; N, 5.56. Found: C, 45.88; H,
3.08; N, 5.36%.
86.7 lmol) with 1,3-propanesultone (58.2 mg, 477 lmol) in meth-
anol (20 mL) gave sulfobetaine 10 (32.5 mg, 40%). Brown solids,
m.p. 363.8–369.3 °C. 1H NMR (400 MHz, CD3OD) d 1.85–1.89 (m,
2H), 2.00–2.06 (m, 2H), 2.82 (s, 6H), 2.88 (t, 3J = 7.8 Hz, 2H), 2.96
(s, 6H), 3.50–3.54 (m, 2H), 3.62–3.66 (m, 4H), 3.70–3.74 (m, 2H),
4.43–4.49 (m, 2H), 4.60 (s, 2H), 7.21 (dd, 3J = 8.8 Hz, 2H), 8.10–
8.16 (m, 4H), 8.37 (d, 3J = 7.2 Hz, 1H). 11B NMR (128 MHz, CD3OD)
d ꢀ0.5 (line width h1/2 = 881 Hz). 19F NMR (376 MHz, CD3OD) d
3
3
ꢀ165.04 (t, JFF = 18 Hz, 2F), ꢀ154.26 (d, JFF = 23 Hz, 2F), ꢀ131.08
to ꢀ131.14 (m, 4F). MS (FAB+, NBA) m/z 938 [M]+.
4.2.11. DLS analysis
DLS measurements were performed using a Malvern Zetasizer
Nano ZS instrument equipped with an He–Ne laser operating at
4 mW power and 633 nm wavelength, and a computer-controlled
correlator, at a 173° accumulation angle. Measurements were car-
ried out in a polystyrene or glass cuvette. The sample was equili-
brated for 5 min at the set temperature each time. The data were
processed using Dispersion Technology software version 4.10 to
give the particle size distribution and average particle sizes.
4.2.8. Synthesis of sulfobetaine (8)
To a solution of 5 (50.2 mg, 81.9
lmol) in dry toluene (10 mL)
was added 1,3-propanesultone (20.3 mg, 164
lmol). The resulting
4.3. SEM Analysis
reaction mixture was stirred at 60 °C for 14 h, and then evaporated
under reduced pressure. The resulting residue was reprecipitated
from ethanol and hexane, and washed with toluene to give 8
(32.5 mg, 54%). Orange solids, m.p. 261.8–262.3 °C. 1H NMR
(400 MHz, CD3OD) d 2.28–2.32 (m, 2H), 2.88 (t, 3J = 6.8 Hz, 2H),
3.22 (s, 6H), 3.62–3.66 (m, 2H), 3.86–3.90 (m, 2H), 4.55–4.59 (m,
2H), 7.21 (dd, 3J = 9.2 Hz, 4J = 2.4 Hz, 2H), 7.57–7.62 (m, 3H), 8.08
(dd, 3J = 9.2 Hz, 4J = 2.4 Hz, 2H), 8.37 (d, 3J = 7.6 Hz, 1H). 11B NMR
(128 MHz, CD3OD) d ꢀ0.9 (line width h1/2 = 911 Hz). 13C{1H} NMR
(126 MHz, CD3OD) d 20.06 (s, CH2), 52.42 (s, CH3), 63.47 (s, CH2),
63.84 (s, CH2), 65.00 (s, CH2), 116.29 (br s, CB), 116.95 (s, CH),
125.78 (s, CH), 129.19 (s, CH), 129.96 (s, CH), 136.61 (s, CH),
139.63 (s, quaternary C), 140.05–142.53 (m, CF), 147.65–150.03
(m, CF), 155.48 (br s, CB), 157.74 (s, quaternary C), 162.58 (s, qua-
ternary C). One signal of the carbon adjacent to the fluorine atom,
one signal of the aliphatic carbon, and one signal of the tertiary car-
bon were not assigned due to overlapping. 19F NMR (376 MHz,
SEM image was recorded an accelerating voltage of 15 kV using
a JEOL JSM-7400FNT. The silicon(111) wafer was cleaned in ace-
tone ultrasonically for 10 min then rinsed with Milli-Q water.
The silicon(111) surface was then exposed to 1% HF aqueous solu-
tion at 70 °C for 1 min and further exposed to 40% NH4F aqueous
solution at room temperature for 5 min to remove surface oxide
and make surface atomically flat. Finally the wafers were rinsed
thoroughly with Milli-Q water carefully to give a hydrogen-termi-
nated silicon surface. An aqueous solution of 9 was evaporated on
the silicon surface, and the sample was subjected to the SEM
analysis.
Acknowledgements
The authors thank Dr. Koji Harano and Ms. Eri Nishiyama of The
University of Tokyo for their help of the measurement of DLS and
Prof. Hiroshi Nishihara and Prof. Yoshinori Yamanoi, Dr. Ryota
3
CD3OD) d ꢀ165.04 (m, 4F), ꢀ158.76 (t, JFF = 17 Hz, 2F), ꢀ132.89

H. Itoi et al. / Inorganica Chimica Acta 381 (2012) 117–123
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123
Sakamoto, Mr. Kenta Okabe and Mr. Junya Sendo of The University
of Tokyo for the measurement of SEM. We thank Tosoh Finechem
Corp. for gifts of butyllithium. This work was partially supported
by Grants-in-Aid for the Global COE Program for Chemistry Innova-
tion and for Scientific Researches (Nos. 22108508 and 22685005)
from the Ministry of Education, Culture, Sports, Science and Tech-
nology, Japan and the Japan Society for the Promotion of Science.
N.K. thanks JGC-S Scholarship Foundation and Mistubishi Chemical
Corporation Fund for research grants.
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