Preparation and photosensitizing property of novel Cd10S 16 molecular cluster dendrimer

Article abstract of DOI:10.1039/b713680c

A new Cd₁₀S₁₆ molecular cluster dendrimer has been prepared and characterized; photooxygenation reaction using the molecular cluster dendrimer as a photosensitizer was successful. The Royal Society of Chemistry.

Full text of DOI:10.1039/b713680c

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Preparation and photosensitizing property of novel Cd10S16 molecular
cluster dendrimer{
Takaaki Tsuboi, Yutaka Takaguchi* and Sadao Tsuboi
Received (in Cambridge, UK) 6th September 2007, Accepted 13th November 2007
First published as an Advance Article on the web 21st November 2007
DOI: 10.1039/b713680c
concentrated. The residue was washed copiously with ether, then
reprecipitated from a chloroform–methanol solution to remove
incomplete ligand exchange products, i.e. Cd10S16 clusters contain-
ing PhS ligands, and decomposed clusters. Further reprecipitation
from a chloroform–ether solution afford the Cd10S16 molecular
cluster dendrimer 1 (168 mg, 73%) as a yellow solid; mp 86–90 uC
(decomp.). Interestingly, molecular cluster dendrimer 1 was soluble
A new Cd10S16 molecular cluster dendrimer has been prepared
and characterized; photooxygenation reaction using the mole-
cular cluster dendrimer as a photosensitizer was successful.
Semiconductor nanoparticles of cadmium chalcogenides (CdS,
CdSe, CdTe) have attracted attention because of their optical and
electrochemical properties.1 In particular, semiconductive molecu-
lar clusters of cadmium chalcogenides stabilized by organic ligands
are useful for optical and electrochemical devices since they have
uniform structure, molecular size and characteristics.2 Although
many studies on an ammonium salt of CdS molecular cluster
anion have been reported,2 few applications of neutral CdS
molecular clusters have been reported due to their low solubility.
Furthermore, photosensitizing activity of a neutral CdS cluster to
generate singlet oxygen has never been reported. Recently, Konishi
and co-workers have reported the synthesis of a water-soluble
Cd10S16 molecular cluster having oligo(ethyleneglycol) units by the
ligand-exchange reaction of Cd10S16 cluster with thiol,3 and its
luminescence sensing of copper ions.3c Meanwhile, we have
reported the syntheses of dendron-functionalized fullerenes (full-
erodendrons), and found that dendritic substituents were effective
to make fullerene, a poorly soluble organic semiconductor, soluble
in various organic solvent and water.4a–d Because of high solubility
of the fullerodendrons, they can act as photosensitizers to generate
singlet oxygen in various solution systems.4a,d,e In this connection,
cadmium chalcogenides having dendritic wedges are of interest.
Although size-controlled CdX (X = S, Se) nanoparticles stabilized
in organic solvents such as acetonitrile (solubility ca. 2.9 g mL21
)
and chloroform (solubility ca. 3.1 g mL21), in contrast to the
original cluster 2 (solubility ca. 2.7 6 1026 g mL21 in chloroform).
The structure of molecular cluster dendrimer 1 was confirmed
by 1H NMR spectroscopy, elemental analysis, and ICP-AES
spectroscopy. The 1H NMR spectrum of Cd10S16 molecular cluster
dendrimer 1 showed broad signals at 2.3–3.8 ppm (aliphatic
protons) and 6.9–8.0 ppm (amide and aromatic protons) (Fig. 1a).
by dendrons were reported,5
a structure-defined cadmium
chalcogenide molecular cluster having dendritic ligands has never
been reported. In this paper, we describe the synthesis of a novel
Cd10S16 molecular cluster dendrimer 1 having twelve dendritic
thiolate ligands. Furthermore, photooxygenation reactions of
various organic sulfides using the dendrimer 1 as a photosensitizer
are also reported.
Cd10S16 molecular cluster dendrimer 1 was synthesized using the
ligand-exchange reaction, as reported by Konishi.3 Scheme 1
shows the synthesis of Cd10S16 molecular cluster dendrimer 1. A
neutral cluster Cd10S4(SPh)12 (2)2 (104 mg, 0.041 mmol) was added
to a solution of dendron thiol 3 (360 mg, 0.98 mmol) in MeCN
(4 mL), and the mixture was stirred at 45 uC for 2 days under Ar
atmosphere. After filtration of the solution, the filtrate was
Graduate School of Environmental Science, Okayama University,
Tsushima-Naka 3-1-1, Okayama 700-8530, Japan.
E-mail: yutaka@cc.okayama-u.ac.jp; Fax: +81-86251-8903;
Tel: +81-86251-8903
{ Electronic supplementary information (ESI) available: Experimental
details and LD-TOF-mass spectrum of Cd10S16 molecular cluster
dendrimer 1. See DOI: 10.1039/b713680c
Scheme 1 Synthesis of C10S16 molecular cluster dendrimer 1.
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Fig. 2 Emission spectra (lex = 360 nm) of (a) C10S16 molecular cluster
dendrimer 1 (7.8 mM) and (b) 2/TOAB (7.8/6.22 mM) in chloroform.
Fig. 2 shows emission spectra of molecular clusters 1 and 2. The
emission spectrum of molecular cluster dendrimer 1 in chloroform
showed a broad peak at 570 nm. Meanwhile, the emission
spectrum of neutral molecular cluster Cd10S4(SPh)12 (2) was
difficult to observe because of its low solubility. The chloroform
solution of complex 2/TOAB (tetraoctylammonium bromide)3a
showed weaker fluorescence than molecular cluster dendrimer 1.
Hamity and co-workers reported that addition of tetraalkylam-
monium salts induces quenching of fluorescence.7 This result
indicates that dendritic ligands are effective for dissolving CdS
clusters in organic solvents without changing their photochemical
properties. The absorption spectrum of molecular cluster dendri-
mer 1 shows a maximum at 272 nm, which is due to the absorption
of dendron groups (Fig. 3). However, excitation spectra of
molecular clusters 1 and 2 were similar in shape with a maximum
at 360 nm (Fig. 4). These results also suggest that the
photochemical properties of the CdS cluster core are not changed
by any correlation with dendron groups.
Fig. 1 1H NMR spectra of a) C10S16 molecular cluster dendrimer 1
(0.91 mM) and b) 1/CTAB (0.91/18.2 mM) in CDCl3. Triangles indicate
C10S16 molecular cluster dendrimer 1. Asterisks, daggers, and double
dagger indicate chloroform, CTAB, and water, respectively.
Usually such broadening of NMR signals of nanoparticles is due
to slow tumbling which stops the averaging of anisotropic dipole–
dipole interactions. However, in very small clusters, such as
Cd10S16, the tumbling of the whole particle is presumably fast
enough so that this effect is not noticeable. So we can conclude
that the main reason for the large linewidths of the NMR peaks is
the heterogeneity of the environment around the protons, due to
many different slowly-interconverting conformations as reported
by Konishi et. al.3a In order to remove such an heterogeneous
1
environment, we measured the H NMR of 1 in the presence of
20 equiv. of cetyltrimethylalkylammonium bromide (Fig. 1b).
Complexation with ammonium ions to anionic sulfur atom of 1
makes the structure more rigid and uniform. Hence the dendron
group signals at 2.3–3.8 ppm were isolated to four signals of
methylene protons (d 2.45, 2.64, 2.77, and 3.45) and one signal of
methyl ester protons (d 3.61). The signals at 6.9–8.0 ppm were
changed into one triplet of amide proton (d 7.21) and four
doublets of aromatic protons (d 7.29, 7.48, 7.59, and 7.75). It is
notable that phenyl protons (d 6.5–7.0)3a of original cluster 2 have
completely disappeared and the new four doublets are different
from dendron thiol 3 (d 7.28 and 7.77) and the corresponding
disulfide (d 7.51 and 7.83).6 The peak integration ratio of d 7.00–
7.95 (aromatic and amide protons) and d 2.28–2.90 (methylene
protons of dendritic wedge) is 5 : 10, which is consistent with the
structure of 1 (see Supporting Information{). Furthermore,
elemental analysis confirmed the molecular formula (Found: C,
43.1; H, 4.90; N, 5.69; S, 9.05%. Calc. for C204H276Cd10N24O60S16:
C, 43.3; H, 4.91; N, 5.94; S, 9.06%). Cd content of cluster-core
dendrimer 1 was 20.1% (calc. for dendrimer 1: 19.9%) determined
by ICP-AES after destruction of the cluster with analytical grade
HNO3. Meanwhile, LD-TOF-mass spectrum of molecular cluster
dendrimer 1 showed a broad peak centered at ca. 5200 (see
Supporting Information).
To clarify the utility of molecular cluster dendrimer 1 as a
photosensitizer, we subjected a series of sulfides to photooxygena-
tion sensitized by molecular cluster dendrimer 1 (Table 1). In a
typical run, a mixture of di-n-butyl sulfide (4a) (26 mg, 0.18 mmol)
Fig. 3 Absorption spectra of (a) C10S16 molecular cluster dendrimer 1
(7.8 mM) and (b) 2/TOAB (6.7/26.8 mM) in chloroform.
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Di-n-butyl sulfoxide (5a) was detected in 98% yield (entry 1). A
control experiment, which was conducted without cluster-core
dendrimer 1, did not afford photooxygenation products.
Oxidation reactions of aromatic and benzylic sulfides were also
examined. Photooxygenation of thioanisole (4b) in the presence of
molecular cluster dendrimer 1 (10 mol %) gave methyl phenyl
sulfoxide (5b) in 62% yield (entry 2). Benzyl methyl sulfide (4d) and
dibenzyl sulfide (4e) were also converted to benzyl methyl sulfoxide
(5d) and dibenzyl sulfoxide (5e) in 99% and 72% yield (entry 4 and
5), respectively. The photooxygenation of diphenyl sulfide (4c),
which has low reactivity upon oxidation, gave diphenyl sulfoxide
(5c) and unchanged diphenyl sulfide (4c) in 17% and 65% yields
(entry 3), respectively. To our knowledge, these are the first
examples of photooxygenation reaction using the Cd10S16
molecular cluster as a photosensitizer.
Fig. 4 Excitation spectra (lem = 570 nm) of (a) C10S16 molecular cluster
dendrimer 1 (7.8 mM) and (b) 2/TOAB (7.8/6.22 mM) in chloroform.
In conclusion, we have synthesized the novel Cd10S16 molecular
cluster having twelve dendritic thiolate ligands. By incorporation
of the dendrons, the Cd10S16 molecular cluster can acquire high
solubility in organic solvents, and exhibit fluorescence in chloro-
form solution. We have also shown the photosensitizing activity of
the CdS molecular cluster dendrimer 1 to generate singlet oxygen.
Future work is in progress to explore applications and advantages
of the molecular cluster dendrimer as a photosensitizer, as a
semiconductor, and as a photoluminescence reagent.
Table 1 Photooxygenation of sulfides using CdS molecular cluster
dendrimer 1 as a photosensitizer
Entry Sulfide
1
Solvent
Time Product
Yield
98%b
We thank Prof. M. Oshima for ICP-AES measurement. This
work was partly supported by a Grant-in-Aid for Scientific
Research (19010105) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan.
CHCl3/MeOH 2.5 h
(9 : 1)
2
CHCl3/MeOH 4 h
(9 : 1)
62%b
Notes and references
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3a
CHCl3/MeOH 4 h
(9 : 1)
17%b
(Starting sulfide 4c was recovered in 65% yield)b
CDCl3/CD3OD 3 h
(9 : 1)
4
5
99%c
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CDCl3/CD3OD 2 h
(1 : 5)
72%c
a
Reaction condition: Cd10S16 molecular cluster dendrimer
(15 mol%). GC yield. NMR yield.
1
b
c
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and 10 mol% of Cd10S4 molecular cluster dendrimer 1 (100 mg,
0.018 mmol) in chloroform–methanol (9 : 1, 4.3 mL) was
irradiated for 2.5 h with a high-pressure mercury lamp (l .
300 nm) through a Pyrex filter at room temperature, while oxygen
was passed through the reaction mixture. The resultant mixture
was analyzed by GC; n-decane was used as the internal standard.
78 | Chem. Commun., 2008, 76–78
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