Highly luminescent glycocluster: silole-core carbosilane dendrimer having peripheral globotriaose

Article abstract of DOI:10.1016/j.tetlet.2007.04.100

A novel glycocluster periphery functionalized by globotriaose (Galα1-4Galβ1-4Glcβ1-) possessing a silole moiety as a luminophor was synthesized. The photoluminescence spectrum of the glycocluster in pure water showed extremely strong emission at 475 nm. A

Full text of DOI:10.1016/j.tetlet.2007.04.100

Tetrahedron Letters 48 (2007) 4365–4368
Highly luminescent glycocluster: silole-core carbosilane
dendrimer having peripheral globotriaose
Ken Hatano,a,* Hiroaki Aizawa,a Hiroo Yokota,a Akihiro Yamada,a Yasuaki Esumi,b
Hiroyuki Koshino,b Tetsuo Koyama,a Koji Matsuokaa and Daiyo Terunumaa
aDepartment of Functional Materials Science, Faculty of Engineering, Saitama University, 255 Shimo-Ohkubo, Sakura-ku,
Saitama 338-8570, Japan
bThe Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
Received 17 February 2007; revised 17 April 2007; accepted 20 April 2007
Available online 4 May 2007
Abstract—A novel glycocluster periphery functionalized by globotriaose (Gala1–4Galb1–4Glcb1–) possessing a silole moiety as a
luminophor was synthesized. The photoluminescence spectrum of the glycocluster in pure water showed extremely strong emission
at 475 nm. Analogous intense emission of the silole dendrimer was also observed in a lower water fraction of water/acetone mixture.
The water fraction of the silole dendrimer solution strongly affected the emission intensity; however, these luminescences were con-
stantly detected at around 475 nm.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Carbohydrate–protein interactions are of paramount
importance in the cell adhesion process. It is known that
the clustering effect of carbohydrates increases the indi-
vidual interactions between carbohydrates and pro-
teins.1 Today, the effect has been often applied for the
molecular design of artificial inhibitors of toxins, bacte-
ria, and viruses, and several forms of glycoclusters have
been developed.2 We recently reported syntheses of
glycoclusters in which carbosilane dendrimers were
employed as the scaffolds of carbohydrates3 and the
biological activities of some of these glycoclusters.4
For example, a carbosilane dendrimer having peripheral
globotriaose (Gb3: Gala1–4 Galb1–4Glcb1–) neutral-
ized Vero toxins produced by Escherichia coli O157:H7
with high affinity in in vivo experiments using mice.5
In the course of our investigation on glycoclusters, we
became interested in the synthesis of a novel carbo-
hydrate scaffold possessing a luminophor.
can be attributed to the low-lying LUMO level associ-
ated with the r*–p* conjugation arising from the inter-
action between the r* orbital of the silicon atom and
p* orbital of the butadiene moiety.7 In this Letter, we
report the first synthesis of a luminescent glycocluster
containing a silole moiety as a luminophor and its
unique optical properties in aqueous solution.
The silole core 2, 1,1-diallyl-2,3,4,5-tetraphenylsilole,
was synthesized via a known intermediate 18 in 50%
overall yield from 1,2-diphenylacetylene as shown in
Scheme 1. Hydrosilation of 2 with trichlorosilane using
H2PtCl6 Æ 6H2O as a catalyst and the succeeding Grig-
nard reaction with allyl magnesium bromide provided
the first generation of silole-core dendrimer 3 in 74%
yield. The resulting dendrimer 3 was treated with di-
cyclohexylborane followed by hydrolysis with hydrogen
peroxide in alkaline solution to afford a hexahydroxy
derivative 4 (64%), which further underwent successively
O-mesylation and replacement with bromo anions, giv-
ing 5 in 67% yield (2 steps). Although hydrosilation
and hydroboration of the butadiene moieties in 2 and
3, respectively, were suspected in this synthetic strategy,
these reactions regioselectively took place at the ex-
pected terminal olefins under such reaction conditions
Scheme 2.
Much interest has recently been shown in siloles (sila-
cyclopentadienes) because of their unique optical and
electronic properties and their potential applications in
organic electroluminescent devices.6 These properties
Keywords: Silole; Carbosilane dendrimer; Glycocluster; Photolumines-
cence; Globotriaose.
*
Coupling reaction between the silole-core dendrimer 5
and a peracetylated globotriaose derivative 63f was
Corresponding author. Tel./fax: +81 48 858 3535; e-mail: khatano@
fms.saitama-u.ac.jp
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.04.100

4366
K. Hatano et al. / Tetrahedron Letters 48 (2007) 4365–4368
Ph
Ph
Ph
Ph
SiCl4
Et2O, -196 o
Li
Ph
Ph
Ph
Ph
Ph
Ph
Et2O, rt, 4 h
C
r.t.
Si
Li
Li
Cl
Cl
1
Ph
Ph
Ph
Ph
.
Allyl Grignard
THF, reflux, 4 h
1) HSiCl3 H2PtCl6 6H2O, THF, rt, on
Ph
Si
Ph
Ph
Ph
Si
Si
Si
2) Allyl Grignard / THF, reflux, 2 h
3
3
3 (y. 74%)
2 (y. 50%)
Ph
Ph
1) c
-Hex2BH,THF, 0 oC, rt, 4h
MsCl
Pyr, -20 oC, 3 h
Ph
Ph
HO
Si
Si
Si
OH
2) 3M NaOH, 30% H2O2, 60 oC, 1 h
3
3
4 (y. 64%)
Ph
Ph
Ph
Ph
Ph
NaBr
Ph
Ph
Ph
MsO
OMs
Br
Br
Si
Si
Si
Si
Si
Si
DMF, 60 oC, on
3
3
3
3
5 (y. 67%)
Scheme 1.
AcO
AcO
OAc
O
Ph
Ph
Ac2O
28% NaOMe
MeOH, rt, on
AcO
AcO
+
5
O
OAc
O
OAc
Ph
Ph
Pyr, rt, on
(AcO)Gb3
Si
Si
Si
Gb3(OAc)
AcO
O
O
SAc
3
3
O
OAc
OAc
7 (y. 47%)
6
OR
O
RO
RO
Ph
Ph
RO
OR
O
O
1) 28% NaOMe, MeOH, rt, 1 h
2) 0.1M NaOH aq., rt, on
OR
O
RO
O
Ph
Ph
O
S
Gb3
Si
Si
Si
Gb3
RO
OR
OR
3
3
8 (y. 83%)
Gb3(OAc) : R = Ac
Gb3 : R = H
Scheme 2.
achieved by nucleophilic substitution of the terminal
bromide on the dendrimer 5 with a thiolate anion gener-
ated from 6 by treatment with sodium methoxide in
methanol.9 Since a part of the acetyl protecting group
was deprotected under such reaction conditions, the
resulting products were re-protected by the reaction with
acetic anhydride in pyridine for purification. After puri-
fication by means of recycling GPC, the silole-core
glycocluster 7 fully substituted by globotriaose was
obtained in 47% yield. Then the glycocluster 7 was
possessing a luminophor but also the first hydrophilic
silole derivative.11
A photoluminescence spectrum of the synthesized silole-
dendrimer 8 measured in water is shown in Figure 1 to-
gether with photoluminescence spectra of representative
hydrophobic silole-dendrimers 5 and 7 for comparison.
Although all of these silole dendrimers have an emission
band at around 475 nm from the silole moieties, the
hydrophilic dendrimer 8 displayed remarkably strong
emission in sharp contrast to hydrophobic dendrimers
5 and 7 measured in chloroform. Extremely bright blue
luminescence from dendrimer 8 in water is shown in
Figure 2. The difference between the emission intensity
of hydrophilic dendrimer 8 and emission intensities of
hydrophobic dendrimers 5 and 7 might be attributable
´
deprotected by a combination of Zemplen’s condition
and saponification to afford the corresponding silole-
core dendrimer 8 (83%), the structure of which was con-
firmed by NMR, UV–vis, and PL spectra, and MALDI-
TOF mass spectrometry.10 It should be noted that the
glycocluster 8 obtained is not only the first glycocluster

K. Hatano et al. / Tetrahedron Letters 48 (2007) 4365–4368
4367
10000
8000
6000
4000
2000
0
1000
λ
max = 475nm
8
7
5
Water fraction / vol %
800
600
400
200
0
100
90
80
70
60
380
400
420
440
460
480
500
520
380
430
480
Wavelength / nm
530
Wavelength / nm
Figure 1. PL spectra of silole-dendrimers 5 and 7 (in chloroform), and
8 (in water) at room temperature. Concentration: 10 lM; excitation:
360 nm.
Figure 3a. PL spectra of silole-dendrimer 8 in water and water/acetone
mixture with 90%, 80%, 70%, and 60% water fractions. Concentration:
1 lM; excitation: 360 nm.
1000
λ
max = 475nm
Water fraction / vol %
1
800
600
400
200
0
6
8
10
15
20
380
430
480
Wavelength / nm
530
Figure 3b. PL spectra of silole-dendrimer 8 in water/acetone mixture
with 1%, 6%, 8%, 10%, 15%, and 20% water fractions. Concentration:
1 lM; excitation: 360 nm.
Figure 2. Photoluminescence of silole-dendrimer 8 in pure water at
room temperature. Concentration: 1 mM; excitation: 360 nm.
influence on the emission intensity of 8. Interestingly,
however, the water fraction had little effect on lumines-
cence wavelength from silole 8, and PL spectra of silole
8 were constantly detected at around 475 nm (Fig. 3a
and 3b). The hydrophilic silole 8 in water/acetone mix-
ture showed different emission behavior than the behav-
ior of hydrophobic siloles previously reported AIE
effect.12,13
to their molecular aggregation form in each solvent.
Tang and co-workers recently reported aggregation-
induced emission (AIE) phenomena of a hydrophobic
simple silole molecule in an ethanol/water mixture whose
emission intensity was significantly enhanced by increas-
ing the water fraction above 50%.12 Analogous AIE
phenomena of some hydrophobic siloles also have been
observed in aqueous solutions.13
1
In the H NMR spectrum of 8 in D2O at 298 K broad
signals were observed, whereas in a mixture of D2O/ace-
tone-d8 (50/50) the relatively sharp signals compared
with the signals in D2O were detected at the same tem-
perature. In general, NMR spectrum of a compound
aggregated in a solution shows broadening signal due
to sluggish exchange on NMR time scale and increasing
Next, we examined the dependence of the emission
intensity of the hydrophilic dendrimer 8 on solvent com-
position using water/acetone mixture. The PL spectra of
8 are shown in Figure 3a (water fractions of 100–70%)
and Figure 3b (20–1%), and these emission intensities
vs water fraction in the solutions are plotted in Figure
4. In the range of 70 to 100% of water fraction, emission
intensity was enhanced by increasing the water fraction
in analogy with cases of previously reported hydropho-
bic siloles (Fig. 3a). Surprisingly, similar strong emission
from 8 was also observed in the case of water fraction of
less than 20%, and the intensity increased in inverse pro-
portion to the water content in the solution as shown in
Figure 3b. Consequently, silole 8 showed extremely in-
tense emission in 100 and 1% water fractions, the highest
and lowest water contents under the experimental condi-
tions (Fig. 4). The fluorescence quantum yields (UFL) in
100%, 50%, and 2% water fractions were estimated to be
0.65, 0.059, and 0.37, respectively. These results reveal
that the water fraction of the solution has a significant
1000
800
600
400
200
0
0
10
20
30
40
50
60
70
80
90
100
Water fraction / vol %
Figure 4. Emission intensity of 8 vs solvent composition of the water/
acetone mixture.

4368
K. Hatano et al. / Tetrahedron Letters 48 (2007) 4365–4368
Matsuoka, K.; Kurosawa, H.; Esumi, Y.; Terunuma, D.;
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308 K
4. Nishikawa, K.; Matsuoka, K.; Kita, E.; Okabe, N.;
Mizoguchi, M.; Hino, K.; Miyazawa, S.; Yamasaki, C.;
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7.0
6.5
Figure 5. Variable temperature 1H NMR spectra of 8 in D2O.
of the measurement temperature transform the peak
shape from broad to sharp.
7. Yamaguchi, S.; Tamao, K. Bull. Chem. Soc. Jpn. 1996, 69,
2327.
The aromatic proton signals of 8 measured in D2O at
between 298 K and 333 K are shown in Figure 5. The
broadening signals observed at 298 K progressively lead
to sharp signals with increasing of the temperature.
These NMR studies reveal that faster exchange of sil-
ole-dendrimer 8 in the state of aggregation is caused
by raising temperature. Analogous NMR experiment
of 8 in lower water fraction of D2O/acetone-d8 (2/98)
mixture failed because of the poor solubility.
8. (a) Joo, W.-C.; Hong, J.-H.; Choi, S.-B.; Son, H.-E. J.
Organomet., Chem. 1990, 391, 27; (b) Schuppan, J.;
Herrschaft, B.; Muller, T. Organometallics 2001, 20, 4584.
¨
9. Hatano, K.; Matsuoka, K.; Terunuma, D. Sci. Eng. Rep.
Saitama Univ. 2005, 38, 40.
10. Silole-core dendrimer 8; dH (400 MHz; D2O; HDO) 0.20–
0.75(br, 16H, 8 · SiCH2), 1.18–1.89 (br, 56H, 2 · SiCH2-
CH2, 6 · SiCH2CH2, 6 · SCH2CH2CH2CH2), 2.22–2.71
(br, 24H, 6 · CH2SCH2), 3.28–3.31 (br, 6H, 6 · H-2),
3.57–4.49 (br, 124H, 6 · CH2O, 6 · H-1, 6 · H-3, 6 · H-4,
6 · H-5, 6 · H-6ab, 6 · H-10, 6 · H-20, 6 · H-30, 6 · H-40,
6 · H-50, 6 · H-60 ab, 6 · H-200, 6 · H-300, 6 · H-400, 6 · H-
500, 6 · H-600 ab) 4.91 (br s, 6 H, 6 · H-100), 6.38–7.32 (br,
20 H, Ph); kmax (H2O)/nm 361 (e/dm3 molꢀ1 cmꢀ1 6 960);
mmax (KBr)/cmꢀ1 3361, 2922, 1151, 1074, 1049 and
Although account for the intense emission of 8 in the
lower water fraction solutions has remained uncertain
so far, we speculate that the silole moieties of 8 aggre-
gate in a solution with the higher water fractions from
30
1
1028; ½aꢁD +41.0 (c 0.80 in H2O); m/z (MALDI-TOF)
the results of variable temperature H NMR studies.
([M+ Na]+ 4432.84 C310H434O156S6Si3 requires 4432.73).
11. Recently, photophysical properties of silole-core dendri-
mers having a benzyl ether-type dendron in organic
solvent were reported by a group of Sanji and Tanaka,
the photophysical property of a hydrophilic silole-core
dendrimer has not yet been reported. Sanji, T.; Ishikawa,
H.; Kaizuka, T.; Tanaka, M.; Sakurai, H.; Nagahata, R.;
Takeuchi, K. Chem. Lett. 2005, 34, 1130.
Further investigations on elucidation of intense lumines-
cence of 8 in the lower water fractions and the applica-
tion to a visualization of pathogens are currently in
progress.
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