Synthesis of siloxane-based PAMAM dendrimers and luminescent properties of their lanthanide complexes

Article abstract of DOI:10.1016/j.jorganchem.2010.09.022

The 0.5-2 generations of siloxane-based PAMAM dendrimers with 1, 3-bis(3-aminopropyl) tetramethyldisiloxane (G0) as core unit were synthesized by two different methods. Their structures were characterized by FTIR, ¹H NMR, ¹³C NMR, LC/MS, TGA, and DSC. Results show that method two is more suitable as its synthetic procedure is simple and it provides higher yield than method one. DSC analysis indicates that the introduction of the siloxane linkage into the interior of the dendrimers has significant effect on the flexibility of the dendrimer structures. Lanthanide complexes of the newly designed siloxane-based PAMAM dendrimers were obtained by complexing with Eu(III) and Tb(III), respectively. The luminescent properties of the complexes in the solution were investigated. Narrow-width red and green emissions were observed from the complexes of G0.5, G1.5, and G2.0, indicating intramolecular energy transfer process takes place between ligands and lanthanide ions.

Full text of DOI:10.1016/j.jorganchem.2010.09.022

Journal of Organometallic Chemistry 696 (2011) 544e550
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis of siloxane-based PAMAM dendrimers and luminescent properties
of their lanthanide complexes
Yuzhong Niu a , Haifeng Lu , Dengxu Wang , Yuanzhi Yue , Shengyu Feng
,
b
a
a
a
a,
*
a
Key Laboratory of Special Functional Aggregated Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China
School of Chemistry and Materials Science, Ludong University, Yantai 264025, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2010
Received in revised form
The 0.5e2 generations of siloxane-based PAMAM dendrimers with 1, 3-bis(3-aminopropyl) tetrame-
thyldisiloxane (G0) as core unit were synthesized by two different methods. Their structures were char-
1
13
acterized by FTIR, H NMR, C NMR, LC/MS, TGA, and DSC. Results show that method two is more suitable as
its synthetic procedure is simple and it provides higher yield than method one. DSC analysis indicates that
the introduction of the siloxane linkage into the interior of the dendrimers has significant effect on the
flexibility of the dendrimer structures. Lanthanide complexes of the newly designed siloxane-based
PAMAM dendrimers were obtained by complexing with Eu(III) and Tb(III), respectively. The luminescent
properties of the complexes in the solution were investigated. Narrow-width red and green emissions were
observed from the complexes of G0.5, G1.5, and G2.0, indicating intramolecular energy transfer process
takes place between ligands and lanthanide ions.
26 August 2010
Accepted 7 September 2010
Available online 21 September 2010
Keywords:
Synthesis
Siloxane
PAMAM dendrimers
Luminescent properties
Lanthanide complexes
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
Dendrimers are hyperbranched polymers with well-defined
geometries and surface functionality [19e21]. Polyamidoamine
Luminescent lanthanide complexes have attracted considerable
attentions for their excellent emission properties such as long
luminescence lifetime, photostability, and narrow emission band-
widths [1e4]. These properties make them promising candidates for
a wide variety of biological applications, including drug discovery,
biochemical sensors, medical diagnostics, and fluoroimmuno-assays
(PAMAM) dendrimers have drawn considerable interest for their
potential biological applications in drug delivery, gene transfection,
and imaging [22e26]. These macromolecules can also be function-
alized to incorporate transition metal complexes into the dendrimer
backbone. PAMAM dendrimers, whose branches contain internal
carbonyl and amide groups, are promising candidates for lumines-
cent materials [10]. However, with the increasing of the generation,
the increased steric hindrance at the dendrimer periphery make it
hard for metal ions to diffuse into the interior of the dendrimer,
which will affect the binding abilities of PAMAM dendrimers for
lanthanide ions [22,27,28]. Therefore, the design and synthesis of
PAMAM dendrimers based on novel cores to alleviate the surface
congestion in higher generation, will facilitate the binding of
lanthanide ions more efficiently and promote their applications in
biological systems. It is particularly interesting to note that siloxane-
based compounds, which exhibit unique properties such as excel-
lent biocompatibility, high degree of flexibility, and an exceptionally
low glass transition temperature, have a remarkable effect on
supramolecular geometry [29e32]. The incorporation of siloxane
into the interior of PAMAM dendrimers may dramatically alter the
morphology and properties of these compounds by making the
overall morphology less compact, and thus allowing the access of
lanthanide ions to the internal carbonyl and amide groups more
easily even at larger generations.
[5e8]. Especially europium (III) and terbium (III) complexes, which
show luminescence in the visible region, have been extensively
investigated [6e11].
In general, excellent luminescence properties of lanthanide
complexes are attributed to the energy transfer from ligands to
lanthanide ions, that is, the intense lanthanide ion luminescence
originates from the intramolecular energy transfer from the excited
state of the ligand to the emitting level of lanthanide ion, which is
known as “antenna effect” [12e14]. In recent years, considerable
efforts have been devoted to the design of suitable ligands which
can efficiently optimize the luminescent properties of lanthanide
ions for their biological applications. A promising strategy is to
incorporate the chromophores into a multidentate chelating ligand
containing carbonyl or amide groups with strong binding abilities
to lanthanide ions [15e18].
*
Corresponding author. Tel.: þ86 531 88364866; fax: þ86 531 88564464.
E-mail address: fsy@sdu.edu.cn (S. Feng).
0
022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.09.022

Y. Niu et al. / Journal of Organometallic Chemistry 696 (2011) 544e550
545
In the present study, we described the synthesis of the 0.5e2
generations PAMAM dendrimers with 1, 3-bis(3-aminopropyl)
tetramethyldisiloxane (G0) as core unit by two different methods
according to the procedure described in Scheme 1. Their structures
were fully characterized by FTIR, H NMR, C NMR, LC/MS, TGA,
and DSC. The newly designed siloxane-based PAMAM dendrimers
were then complexed with Eu(III) and Tb(III) to form lanthanide
complexes, and their luminescent properties in solution were
investigated.
the samples were determined with thermogravimetric analyzer
(TGA) and different scanning calorimeter (DSC). TGA was performed
with a MettlerToledo SDTA-854 TGA system. The analysis was per-
formed with approximately 10 mg of samples in a dynamic nitrogen
1
13
ꢁ
atmosphere (flow rate 50 ml/min) at a heating rate of 10 C/min. DSC
measurements were carried out in MettlerToledo DSC822 series.
ꢁ
About 40 mg sample panwas ramped from ꢀ100 to 100 C at the rate
ꢁ
of 10 C/min in a steady flow of nitrogen (20 ml/min). The fluores-
cence spectra were determined with ISS K2-Digital spectropho-
tometer: excitation slit width ¼ 10 nm, emission slit width ¼ 3 nm.
Methyl acrylate (MA), ethylenediamine (EDA), allylamine (AA),
2
. Experimental
N
hexamethyl disilizane (MM ), 1,1,3,3-tetramethyldisiloxane
H
H
(
M M ), tetrahydrofuran (THF) were redistilled just before use.
2.1. Materials and methods
Europium and terbium nitrates were obtained from their corre-
sponding oxides in dilute nitric acid. All of the other reagents
used were of analytical-regent grade.
Fourier transform infrared spectra (FTIR) were recorded on
a Bruker Tensor 27 spectrophotometer (Bruker, Switzerland). Test
ꢀ
1
conditions: potassium bromide pellets, 32 scans, resolution: 4 cm
.
The datawere treated with OPUS spectroscopysoftware of version 6.
H NMR and C NMR were measured in chloroform on a Bruker
2.2. Preparation of allylaminotrimethylsilane (ATMS)
1
13
AVANCE-400 NMR Spectrometer, and chemical shifts were recorded
in parts per million (ppm) without internal reference. Molecular
weights were determined by Aglient HP1100 LC-Applied Biosystems
API4000 TQ Mass Spectrometer (LC/MS). Thermal characteristics of
ATMS was prepared according to the procedure described in
references [33,34]. A suspension of 11.40 g AA (0.2 mol), 19.34 g
N
(0.12 mol) of MM and 0.07 g anhydrous (NH
4
)
2
SO
4
was charged
into a 100 ml flask. The reaction mixture was heated under argon
Scheme 1. The synthetic routes of siloxane-based PAMAM dendrimers.

546
Y. Niu et al. / Journal of Organometallic Chemistry 696 (2011) 544e550
ꢁ
atmosphere and refluxed over night until the temperature reached
50 C using a rotary evaporator. The excess EDA was removed using
an azeotropic mixture of toluene and methanol (v/v ¼ 9:1). The
remaining toluene was removed by azeotropic distillation using
ꢁ
106 C. Then the reaction mixture was cooled to room temperature
and the precipitate was filtered off. The liquid mixture was distilled
and 18.83 g (0.15 mol) ATMS was obtained. (Yield: 73%). b.p.
methanol. Finally,1.38 g (1.96 mmol) G1.0 was obtained. (Yield: 98%).
ꢁ
ꢁ
1
110e112 C (b.p. ¼ 110e112 C in Ref. [34]).
H
NMR (CDCl
H, SiCH CH CH
NH ), 2.36 (t, 8H, NCH
H, NCH
8H, NCH CH
3
):
N), 1.38e1.46 (m, 4H, SiCH
CH CO), 2.42 (t, 4H, SiCH
CO), 2.82 (t, 8H, NCH CH NH
), 7.66 (t, 4H, CONH). C NMR (CDCl
d
(ppm) 0.04 (s, 12H, SiCH
CH CH N), 1.88 (s, 8H,
CH CH N), 2.71 (t,
), 3.25e3.31(m,
): (ppm) 0.31,
3
), 0.41 (t,
4
2
2
2
2
2
2
2.3. Preparation of DMAA
2
2
2
2
2
2
8
2
CH
2
2
2
2
13
DMAA was prepared by Michael addition reaction of MA with
2
2
NH
2
3
d
AA according to the procedure described elsewhere [27]. Under
argon atmosphere, a mixture of 2.85 g (0.05 mol) AA and 12.90 g
MA (0.15 mol) in 70 ml of methanol was stirred at 50 C for 36 h.
15.95, 20.12, 33.83, 41.19, 41.90, 49.85, 56.75, 172.92. LC/MS: m/z
706.0 [M] (calcd. 705.1).
þ
ꢁ
Then the solvent and excess MA was removed under reduced
2.7. Preparation of G1.5
ꢁ
pressure at 50 C using a rotary evaporator, and 11.45 g (0.05 mol)
1
DMAA was obtained. (Yield: 100%). H NMR (CDCl
3
):
d
(ppm)
CH CO),
), 5.07e5.15 (t,
N).
A suspension of 0.98 g (1.39 mmol) G1.0, 3.84 g (44.6 mmol) MA
and 50 ml of methanol was stirred under argon atmosphere at 50 C
ꢁ
2
3
2
.38e2.42 (t, 4H, NCH
.04e3.06 (d, 2H, CH ]CHCH
H, CH ]CHCH N), 5.70e5.80 (m, 1H, CH
2
CH
2
CO), 2.72e2.75 (t, 4H, NCH
N), 3.62 (s, 6H, OCH
]CHCH
2
2
2
2
3
for about 3d. Then the solvent and excess MA was removed under
ꢁ
2
2
2
2
reduced pressure at 50 C using a rotaryevaporator. The product was
redissolved in methanol and redistilled by rotary evaporator again,
1
2
.4. Preparation of G0
1.86 g (1.33 mmol) G1.5 was obtained. (Yield: 96%). H NMR (CDCl
3
):
N), 1.43e1.47
CO), 2.53 (t,
CO), 3.28e3.29 (m, 8H,
), 7.19e7.21 (m, 4H, CONH).
d
(ppm) 0.04 (s, 12H, SiCH
3
), 0.42 (t, 4H, SiCH
N), 2.37e2.48(m, 24H, NCH
CH
2
CH
2
CH
CH
2
Similar to the method described in Ref. [35], a suspension of
5.48 g (0.12 mol) ATMS and three drops of Karstedt catalyst was
added into the flask under argon atmosphere. The mixture was
(m, 4H, SiCH
12H, CH NCH
CONHCH CH
C NMR (CDCl
2
CH
2
CH
2
2
2
1
2
2
), 2.74e2.80 (m, 24H, NCH
2
2
2
2
NCH
):
2
), 3.67 (s, 24H, OCH
3
ꢁ
H
H
13
heated to 95 C and then 6.7 g (0.05mol) M M was added dropwise
3
d (ppm) 0.28, 15.85, 20.00, 32.51, 37.03, 49.41,
into the flask. After the reactionwas initiated, the temperature of the
50.19, 51.45, 52.11, 52.87, 56.53, 172.21, 172.94. LC/MS: m/z 1394.2
[M] (calcd. 1393.8).
ꢁ
ꢁ
þ
mixture was maintained between 110 C and 125 C. When the
reaction was complete, 30 ml of anhydrous ethanol was added and
the resulting mixture was refluxed for 4 h, and then the lower
boiling components were distilled off from the mixture. The residue
was further distilled at reduced pressure to give 7.19 g (0.029 mol)
2.8. Preparation of G2.0
Under argon atmosphere, a solution of 0.80 g (0.57 mmol) G1.5,
ꢁ
1
ꢁ
G0. (Yield: 58%). b.p. 136e142 C at 11.5mmHg. H NMR (CDCl
(ppm) 0.03 (s, 12H, SiCH ), 0.45e0.50 (t, 4H, SiCH CH CH NH
CH CH NH ), 2.61e2.66 (m, 4H,
3
):
4.32 g (72 mmol) EDA and 50 ml methanol was stirred at 50 C for
d
3
2
2
2
2
),
about 6d. After the reaction is complete, the purification procedures
were similar to that of G1.0. Afterward, 0.87 g (0.54 mmol) G2.0 was
1
.37e1.47 (m, 8H, SiCH
2
2
2
2
1
SiCH
2
CH
2
CH
2
NH
2
).
obtained. (Yield: 95%). H NMR (CDCl
3
):
N), 1.36e1.44 (m, 4H, SiCH
), 2.39e2.48 (m, 24H, NCH CH CO), 2.69e2.78 (m, 28H,
CH N, NCH CH CO), 2.83e2.94 (m, 24H, NCH CH N),
3.31e3.23(m, 24H, NCH CH NH ), 3.62 (s, 3H, OCH ), 7.85e7.92 (m,
12H, CONH). C NMR (CDCl ): (ppm) 0.24, 15.60, 19.87, 32.07,
d
(ppm) 0.03 (s,12H, SiCH
3
),
0
.38 (t, 4H, SiCH
(s, 16H, NH
SiCH CH
2
CH
2
CH
2
2
CH CH N), 2.30
2
2
2.5. Preparation of G0.5
2
2
2
2
2
2
2
2
2
2
According to the procedure described in Scheme 1, G0.5 was
prepared by two different strategies.
2
2
2
3
13
3
d
Method one: A suspension of 1.34 g (5.40 mmol) G0, 3.72 g
34.05, 37.50, 41.21, 42.06, 44.25, 49.45, 51.62, 52.68, 56.57, 172.92,
173.22. LC/MS: m/z 1590.9 [M] (calcd. 1590.2).
þ
(
43.20 mmol) MA and 50 ml of methanol was stirred under argon
ꢁ
atmosphere at 50 C for about 2d. Then the solvent and excess MA
ꢁ
was removed under reduced pressure at 50 C using a rotary
2.9. Preparation and luminescent properties of siloxane-based
evaporator, and 3.06 g (5.16 mmol) G0.5 was obtained. (Yield: 95%).
Method two: A suspension of 5.50 g (0.024 mol) DMAA and three
drops of Karstedt catalyst was added into the flask, and then 1.34 g
PAMAM dendrimerselanthanide complexes
The luminescent properties of siloxane-based PAMAM den-
drimerselanthanide complexes were investigated according to the
similar procedures described elsewhere [18,36]. A typical proce-
dure is that: Under argon atmosphere, a known amount of
G0.5eG2.0 (0.050 mmol) was dissolved in 10 ml dry THF, and
H
H
(
0.01 mol) M M was added dropwise with stirring. The reaction
ꢁ
was carried out under argon atmosphere at 70 C for 48 h. After the
reaction completed, the solvent and excess DMAA were removed
under reduced pressure using a rotary evaporator. To ensure the
complete evaporation of DMAA, the product was redissolved in
a stoichiometric proportion of Ln(NO
3
)
3
$6H
2
O (Ln ¼ Eu, Tb) was
methanol and removed under reduced pressure again, 5.46 g
added to the solution. The mixture was stirred and refluxed for 2h.
The resulting solution was filtered, and the filtrate was allowed to
stand at room temperature for 24h, and then transferred to a quartz
cell for optical measurements.
1
(
9.22 mmol) G0.5 was obtained. (Yield: 92%). H NMR (CDCl
3
):
d
(ppm) 0.02 (s, 12H, SiCH
3
), 0.41 (t, 4H, SiCH
2
CH
2
CH
CH
). C NMR (CDCl
2
N), 1.33e1.44
(
m, 4H, SiCH
2
CH
2
CH
CH
2
N), 2.34e2.44 (m, 12H, SiCH
CO), 3.63 (s, 12H, OCH
2
2
CH
2
NCH
2
),
1
3
2
d
.74 (t, 8H, NCH
2
2
3
3
):
(ppm) 0.32, 15.77, 20.88, 32.51, 49.26, 51.46, 57.19, 172.97. LC/MS:
3. Results and discussion
þ
m/z 593.9 [M] (calcd. 592.9).
3.1. Optimization of reaction condition
2.6. Preparation of G1.0
In general, the synthesis of PAMAM dendrimers based on novel
A solution of 1.18 g (1.99 mmol) G0.5, 12 g (200 mmol) EDA and
0 ml methanol was stirred under argon atmosphere at 50 C for
about 3d. Then the solvent was removed under reduced pressure at
cores was commonly achieved by divergent method through the
following steps: (1) Introduction of the amino groups to the core
unit. (2) Michael addition of MA to the amino groups. (3) Amidation
ꢁ
5

Y. Niu et al. / Journal of Organometallic Chemistry 696 (2011) 544e550
547
of the terminal ester groups with EDA. Then repeat the last two
steps in sequence to obtain the following series of products. Based
on this idea, G0 was developed as core unit for the construction of
the siloxane-based PAMAM dendrimers as shown in Scheme 1. To
obtain G0.5, two different approaches were adopted: (1) In method
one, the amino groups were firstly introduced to the core unit via
core unit. For ester-terminated siloxane-based PAMAM dendrimers
ꢀ
1
G0.5 andG1.5, theadsorption at 1735cm suggested thepresence of
ester group (eCO CH ), while the amino-terminated products G1.0
and G2.0 present the typical amino group (NH ) adsorption peaks at
2
3
2
ꢀ
1
ꢀ1
ꢀ1
3286 cm , 3317 cm , and 1640 cm . In the convergent method for
the synthesis of G0.5, the hydrosilylation reaction can also be
monitored by the disappearance of vibration peak at 2110 cm and
H
H
ꢀ1
the hydrosilylation of ATMS with M M in the presence of karstedt
catalyst. Then, Michael addition of MA to the amino groups of the
core unit was followed to obtain G0.5. (2) In method two, the
reaction precursor DMAA was firstly synthesized by the Michael
addition reaction of MA with AA. Then G0.5 was obtained by the
ꢀ
1
3070 cm for
bonds (eCO CH
y
(SieH) and
y(C]C), and the appearance of ester
) adsorption peaks at 1735 cm .
The H NMR of DMAA exhibits the characteristic resonances for
ꢀ
1
2
3
1
allyl protons in the region of 5.07e5.15 and 5.70e5.80 ppm, and for
ester protons (OCH ) at 3.62 ppm. For G0, SieCH protons exhibit
3 3
H
H
hydrosilylation of DMAA with M M in the presence of karstedt
H
H
catalyst. In the first method, hydrosilylation of ATMS with M M ,
and the subsequent alcoholysis to give the key starting material G0
in 58% yield. It is noticeable that during the reaction process, the
amino group of AA should be protected by trimethylsilyl groups to
reduce the side-reaction between amino group and the SieH bond
resonances in the region of 0.00e0.07 ppm, while the protons in
the region of 1.37e1.47 ppm attributed to the protons of amino
1
groups. The H NMR spectrum of G0.5 exhibits resonances for
SieCH
3
protons between ꢀ0.01 and 0.07 ppm and for ester protons
1
(OCH ) at 3.63 ppm. The H NMR spectrum of G1.5 is almost
3
[
34]. Furthermore, ATMS must be added in excess to ensure the
identical to that of G0.5 for analogous protons, although broader
resonances are observed with the increasing generation. The
reason for this can be ascribed to the slightly different chemical
environments for the protons in different generations and the
restricted mobility of the respective protons in the outer shells
[37,38]. For the amino-terminated product G1.0, the resonances
H
H
complete reaction with M M due to its relative lower activity. To
overcome this drawback and the tedious protection-deprotection
procedures, method two was developed. In method two, DMAA
was obtained by the Michael addition of MA with AA at 50 C for
3
ꢁ
6h in nearly 100% yield. Then, hydrosilylation of DMAA with
H
H
M M in the presence of karstedt catalyst at relatively lower
temperature (70 C) for 48h to obtained G0.5 in higher yield (92%).
appeared in the range of 0.00e0.08, 1.88, and 7.71e7.75 ppm are
ꢁ
1
corresponding to the protons of SieCH
3
, eNH
2
, and eCONH. The H
Then, G1.0eG2.0 was synthesized by the divergent method
following the alternate amidation and Michael addition reactions.
Due to its simple synthetic procedures and much higher yield,
method two was more suitable for the synthesis of siloxane-based
PAMAM dendrimers.
NMR spectrum of G2.0 exhibits identical resonance patterns to
those observed in its counterpart G1.0, although broader signals
were observed with increasing generation.
Furthermore, the molecular structures of the siloxane-based
13
PAMAM dendrimers are also proved by C NMR and LC/MS. The
13
spectroscopic data of C NMR are consistent with the assignment
1
obtained by the H NMR. The molecular weights determined by LC/
3.2. The characterization of siloxane-based PAMAM dendrimers
MS are in well agreement with the theoretical results, further
supporting that the desired siloxane-based PAMAM dendrimers
have been successfully synthesized.
The incorporation of siloxane on the thermal properties of the
resulting PAMAM dendrimers was investigated by DSC and TGA. The
glass transition temperature (T ) of G0.5eG2.0 and their corre-
g
sponding amino-terminated products using EDA as core molecular
were present in Table 1. As shown in Table 1, the siloxane-based
1
The structures of the compounds were characterized by FTIR, H
NMR,13C NMR, LC/MS, and the thermal properties of the dendrimers
were evaluated by DSC and TGA. Fig. 1 shows the FTIR spectra of
H
H
H
H
M M , DMAA, and G0eG2.0. For M M , a characteristic band at
ꢀ1
2
1
110 cm
corresponding to
y
(SieH) and intense bands near
) were observed. After the hydro-
silylation reaction of M M with ATMS, the vibration peak at
ꢀ
1
251 cm attributed to (SieCH
d
3
H
H
ꢀ
1
PAMAM dendrimers all possess low T
g
values. The T
g
values for the
2
110 cm for
vibration peak at 1251 cm for
Furthermore, the new adsorption peaks at 3292 cm , 3366 cm
y(SieH) disappears in FTIR spectrum of G0, while the
ꢁ
ꢀ
1
ester-terminated products G0.5 and G1.5 are ꢀ83.52 C and
d
3
(SieCH ) can still be observed.
ꢁ
ꢀ
1
ꢀ1
ꢀ52.76 C, respectively. The T
g
values of the amino-terminated
,
ꢁ
ꢁ
ꢀ1
products G1.0 and G2.0 are ꢀ59.45 C and ꢀ3 C, which are lower
and 1598 cm corresponding to
2 2
y(NH ) and d(NH ) were observed,
than those of the corresponding EDA-cored PAMAM [39]. The low T
g
suggested that the amino groups were successfully introduced to the
values of siloxane-based dendrimers indicate that the introduction
of siloxane into the interior of the dendrimer has greatly improve
the flexibility of dendrimer structures, and would alleviate the
surface congestion of high generation products.
Fig. 2 shows the TGA curves of the dendrimers in nitrogen
atmosphere. As can be seen from Fig. 2, the ester-terminated
Table 1
g
Glass transition temperature (T ) of siloxane-based
PAMAM dendrimers and EDA-cored PAMAM
dendrimers.
Dendrimers
T
g
G0.5
G1.0
G1.5
G2.0
EDAeG1.0
EDAeG2.0
ꢀ83.52
ꢀ59.45
ꢀ52.76
ꢀ3.21
ꢀ3
a
a
0
H
H
Fig. 1. FTIR spectra of M M , DMAA, and siloxane-based PAMAM dendrimers. (a)
a
H
H
Cited from Ref. [39].
M M (b) DMAA (c) G0 (d) G0.5 (e) G1.0 (f) G1.5 (g) G2.0.

548
Y. Niu et al. / Journal of Organometallic Chemistry 696 (2011) 544e550
products G0.5 and G1.5 are thermally stable when the temperature
ꢁ
below 200 C. The amino-terminated products G1.0 and G2.0 show
relatively low thermal stability, and began to decompose at the
start of heating. The TGA curves of G1.0 and G2.0 indicate two major
weight loss stages. For the first stage, the maximum rate of weight
ꢁ
loss occurs around 120 C, corresponding to the residual solvent
and EDA encapsulated in the amino-terminated products. The
formation of hydrogen bond between the amino groups leading to
the viscosity increased in G1.0 and G2.0, which made it hard for the
completely removal of methanol and EDA, and thus the first stage
occurred. The second stage accounts for the decomposition of the
ꢁ
dendrimer structures with the maximum rate occurs around 250 C
similar to their counterparts using EDA as the core unit in the
previous study [39,40].
3.3. Luminescent properties of siloxane-based PAMAM
dendrimerselanthanide complexes
The luminescent properties of siloxane-based PAMAM den-
drimerselanthanide complexes were investigated in THF solution
at room temperature. Figs. 3 and 4 show the excitation and emis-
sion spectra of the dendrimers complexes with Eu(III).
Fig. 3. The excitation spectra of siloxane-based PAMAM dendrimers-Eu(III) complexes
a) G0.5 (b) G1.0 (c) G1.5 (d) G2.0.
(
The excitation spectra of these complexes were all obtained by
monitoring the emission peak of Eu(III) at 617 nm. As can be noticed
in Fig. 3, broad excitation bands located at 350e400 nm can be
found for all the complexes and 395 nm is the most appropriate
excitation band. As can be seen from the emission spectra in Fig. 4,
the intense narrow band located at 617 nm corresponding to the
(III) site symmetry [41]. The asymmetry factor for the complexes of
G0.5, G1.5, and G2.0 are 5.6, 6.2, and 5.9, indicating that Eu(III) ion
in these complexes has lower symmetric coordination environ-
ment. Compared with the complexes of G0.5, G1.0, and G2.0, the
luminescent properties of G1.0 with Eu(III) was relatively poor. As is
shown in Fig. 4(d), the ligand emission (located at 450e500 nm)
intensity was stronger than those of the characteristic Eu(III) ion
emission. The reason for this might be that, amino groups of G1.0
also take part in the coordination process with Eu(III), the high
energy vibrations of NeH may quench the excited state of Eu(III)
ions, resulting in the poor luminescent properties of G1.0 complex
[42]. Although G2.0 also contains amino groups at peripheral, there
are more carbonyl groups in the interior of G2.0 compared with
G1.0. Since the coordination ability of oxygen with lanthanide ions
is stronger than that of the nitrogen, the luminescent properties of
G2.0 complex were better than that of G1.0 complex [43].
Furthermore, Eu(III) ions can also be encapsulated in the interior of
G2.0 structure, while it is hard for G1.0 to encapsulate Eu(III) ions as
the early generation dendrimer G1.0 possesses relatively open
structure. Another phenomenon can also be observed from Fig. 4,
with the increasing of the generation number, the ligand emission
located at 450e500 nm nearly vanish in G2.0 complex, indicated
that the energy transfer between the ligand and Eu(III) ions tend to
be perfect at that generation, further indicating the synthesized
dendrimers are promising candidate for luminescent materials.
The emission spectra of Tb(III) complexes with G0.5eG2.0
excited at 250 nm were recorded in Fig. 5. The emission spectra of
the Tb(III) complexes with G0.5, G1.5 and G2.0 exhibit the charac-
teristic narrow-width emission bands, which were related to the
transition from the triplet state energy level of Tb(III) to different
5
7
hypersensitive D
0 2
/ F transition of Eu(III) ion was found for the
complexes of G0.5, G1.5, and G2.0. The presence of bands at 593 nm,
6
of D
of
50 nm and 688 nm associated with the characteristic transitions
5
7
5
7
5
7
0
1
/ F , D
0
3 0 4
/ F and D / F , respectively. The transitions
5
7
D
0
/
F
0
at 580 nm could not be observed for all the cases,
which means no inverse center for Eu(III) ion in those complexes
5
7
[18,41]. It is known that the
0 2
D / F transition induced by the
electric dipole moment is very sensitive to the coordination envi-
5
7
ronment of the Eu(III) ion, while the
0 1
D / F transition arose
from the magnetic dipole is less sensitive to the coordination
environment. Thus, the emission spectra were normalized with
5
7
respect to the D
0 1
/ F transition [10,36]. The asymmetry factor,
5
7
which is the ratio of the integrated intensity of D
to D / F transition, has been widely used as an indicator of Eu
0
2
/ F transition
5
7
0 1
5
7
single state levels and were assigned to the
D
4
/ F
6
(487 nm),
5
7
5
7
5
7
D
4
/ F
5
(545 nm), D
4
/ F
4
(585 nm), and D
4
/ F
3
(620 nm)
transitions. Compared with the other complexes, the complex of
G1.0 exhibits poor luminescent properties, as weaker emission
peaks corresponding to characteristic emission bands of Tb(III) ions
and stronger emission peaks belong to the ligand were observed.
The reason for this was similar to that of its Eu (III) complex. It is
noteworthy that the ligand emission background in the region
400e450 nm nearly could not be detected in the emission spectra
of these complexes with the generation number increasing from
G0.5, G1.5 to G2.0, indicated that the energy transfer between the
higher generation dendrimer and Tb (III) ions is more efficient. A
Fig. 2. TGA curves of siloxane-based PAMAM dendrimers. (a) G0.5 (b) G1.0 (c) G1.5 (d)
G2.0.

Y. Niu et al. / Journal of Organometallic Chemistry 696 (2011) 544e550
549
Fig. 4. The emission spectra of siloxane-based PAMAM dendrimers-Eu(III) complexes (
l
ex ¼ 395 nm) (a) G0.5 (b) G1.0 (c) G1.5 (d) G2.0.
Fig. 5. The emission spectra of siloxane-based PAMAM dendrimers-Tb(III) complexes (
l
ex ¼ 250 nm) (a) G0.5 (b) G1.0 (c) G1.5 (d) G2.0.

550
Y. Niu et al. / Journal of Organometallic Chemistry 696 (2011) 544e550
reasonable explanation for this is that: the binding ability of
G0.5eG2.0, which is mainly due to the functional groups, increased
with the increasing generation; furthermore, the number of inte-
rior cavities is also increased with increasing generation number,
since the metal ions can be easily encapsulated within the cavities,
the binding ability of dendrimer also increased.
[6] A. Picot, A. D’Aléo, P.L. Baldeck, A. Grichine, A. Duperray, C. Andraud, O. Maury,
J. Am. Chem. Soc. 130 (2008) 1532e1533.
[7] G.L. Law, K.L. Wong, C.W.Y. Man, W.T. Wong, S.W. Tsao, M.H.W. Lam,
P.K.S. Lam, J. Am. Chem. Soc. 130 (2008) 3714e3715.
[8] J. Hynes, T.C. O’Riordan, A.V. Zhdanov, G. Uray, Y. Will, D.B. Papkovsky, Anal.
Biochem. 390 (2009) 21e28.
[
9] M. Elbanowski, B.M. Kowska, J. Photochem. Photobiolo. A 99 (1996) 85e92.
10] J.P. Cross, M. Lauz, P.D. Badger, S. Petoud, J. Am. Chem. Soc. 126 (2004)
6278e16279.
11] C.P. Montgomery, B.S. Murray, E.J. New, R. Pal, D. Parker, Acc. Chem. Res. 42
2009) 925e937.
12] J.M. Lehn, Angew. Chem. Int. Edit 29 (1990) 1177e1370.
[
[
[
1
4
. Conclusions
(
In conclusion, the 0.5e2 generations of siloxane-based PAMAM
dendrimers with 1,3-bis(3-aminopropyl) tetramethyldisiloxane
G0) as core unit were prepared via two different methods. Results
[13] D. Dong, S. Jiang, Y. Men, X. Ji, B. Jiang, Adv. Mater. 12 (2000) 646e649.
[
[
[
14] Y.T. Yang, S.Y. Zhang, Acta A 60 (2004) 2065e2069.
15] N. Arnaud, J. Georges, Spectrochim. Acta A 59 (2003) 1829e1840.
16] J.G. Bünzli, C. Piguet, Chem. Soc. Rev. 34 (2005) 1048e1077.
(
indicate that method two is more suitable for the synthesis of the
dendrimers for its simple synthetic procedure and higher yield.
DSC analysis shows that the introduction of the siloxane into the
interior of the dendrimers has great effect on the flexibility of the
dendrimer structures as evidenced by DSC analysis. Except for G1.0,
narrow-width red and green emissions were observed for the
complexes of G0.5, G1.5, and G2.0 with Eu(III) and Tb(III), indicating
intramolecular energy transfer process takes place between the
lanthanide ions and the ligands. The improved structure flexibility
and surface congestion after the incorporation of siloxane not only
make it easy for the metal ions to diffuse into the interior of the
dendrimer structures, but also the provide the possibility to
synthesize the higher generation dendrimers or peripherally
functionalize the dendrimers with certain groups according to the
applications.
[17] X.F. Wen, M.Y. Li, Y. Wang, J.P. Zhang, L.M. Fu, R. Hao, Y. Ma, X.C. Ai, Langmuir
4 (2008) 6932e6936.
18] L. Liu, H.F. Lu, H. Wang, Y.L. Bei, S.Y. Feng, Appl. Organomet. Chem. 23 (2009)
29e433.
2
[
4
[19] D.A. Tomalia, A.M. Naylor, W.A. Goddard III, Angew. Chem. Int. Edit 29 (1990)
138e175.
[20] A.W. Bosman, H.M. Janssen, E.W. Meijer, Chem. Rev. 99 (1999) 1665e1688.
[21] S.M. Grayson, J.M.J. Fréchet, Chem. Rev. 101 (2001) 3819e3868.
[22] G.R. Newkome, C.D. Shreiner, Polymer 49 (2008) 1e173.
[23] R. Esfanda, D.A. Tomalia, Drug Discov. Today 6 (2001) 427e436.
[
[
24] D. Luo, K. Haverstick, N. Belcheva, E. Han, W.M. Saltzman, Macromolecules 35
2002) 3456e3462.
25] M.K. Calabretta, A. Kumar, A.M. McDermott, C.Z. Cai, Biomacromolecules 8
(2007) 1807e1811.
(
[
[
26] S.H. Medina, M.E.H. El-Sayed, Chem. Rev. 109 (2009) 3141e3157.
27] R.J. Qu, Y.Z. Niu, C.M. Sun, C.J. Ji, C.H. Wang, G.X. Cheng, Micropor. Mesopor.
Mat 97 (2006) 58e65.
[28] R.J. Qu, Y.Z. Niu, J.H. Liu, C.M. Sun, Y. Zhang, H. Chen, C.N. Ji, React. Funct.
Polym. 68 (2008) 1272e1280.
[
[
29] I. Gill, A. Ballesteros, J. Am. Chem. Soc. 120 (1998) 8587e8598.
30] K. Deguchi, K. Tsuru, T. Hayashi, M. Takaishi, M. Nagahara, S. Nagotani,
Y. Sehara, G. Jin, H.Z. Zhang, S. Hayakawa, M. Shoji, M. Miyazaki, A. Osaka,
N.H. Huh, K. Abe, J. Cerebr. Blood F. Met 26 (2006) 1263e1273.
31] D.M.L. Goodgame, P.D. Lickiss, S.J. Rooke, A.J.P. White, D.J. Williams, Inorg.
Chim. Acta 343 (2003) 61e73.
Acknowledgements
[
The authors are grateful for the financial support by the National
Natural Science Foundation of China (No. 20574043 and 20874057),
the China Postdoctoral Science Foundation Funded Project
[32] Y. Abe, T. Gunji, Prog. Polym. Sci. 29 (2004) 149e182.
[33] J.L. Speier, R. Zimmerman, J.S. Webster, J. Am. Chem. Soc. 78 (1956)
2278e2281.
(
2008041135), and the Key Natural Science Foundation of Shan-
[
[
34] C.F. Li, D.X. Li, S.Y. Feng, Polym. Int. 54 (2004) 1041e1046.
35] J. Saam, J. Speier, J. Org. Chem. 24 (1959) 119e120.
dong Province (Z2007B02). The authors are also grateful to the
editor and the reviewers for their valuable comments and
constructive suggestions
[36] Z.H. Zhang, Y. Song, T. Okamura, Y. Hasegawa, W.Y. Sun, N. Ueyama, Inorg.
Chem. 45 (2006) 2896e2902.
[
37] P. Ortega, J.F. Bermejo, L. Chonco, E. Jesus, F. Javier de la Mata, G. Fernández,
J.C. Flores, R. Gómez, M.J. Serramía, M.A. Muñoz-Fernandez, Eur. J. Inorg.
Chem. 2006 (2006) 1388e1396.
References
[38] K. Lorenz, R. Muelhaupt, H. Frey, U. Rapp, F.J. Mayer-Posner, Macromolecules
28 (1995) 6657e6661.
[
[
1] N. Sabbatini, M. Guardigli, J.M. Lehn, Coord. Chem. Rev. 123 (1993) 201e228.
2] D. Parker, R.S. Dickins, H. Puschmann, C. Crossland, J.A.K. Howard, Chem. Rev.
[39] H.M. Tan, Y.J. Luo, Dendritic Polymer. Chemical Industry Press, Beijing, 2002,
pp. 199e201.
102 (2002) 1977.
[
[
3] E.G. Moore, A.P.S. Samuel, K.N. Raymond, Acc. Chem. Res. 42 (2009) 542e552.
4] L. Armelao, S. Quici, F. Barigelletti, G. Accorsi, G. Bottaro, M. Cavazzini,
E. Tondello, Coord. Chem. Rev. 254 (2010) 487e505.
[40] P. Zheng, L.X. Gao, X.G. Sun, S.G. Mei, Iran. Polym. J. 18 (2009) 257e264.
[41] S. Samikkanu, K. Mellem, M. Berry, P.S. May, Inorg. Chem. 46 (2007)
7121e7128.
[42] G.R. Choppin, D.R. Peterman, Coord. Chem. Rev. 174 (1998) 283e299.
[43] Y. Li, B. Yan, H. Yang, J. Phys. Chem. C 112 (2008) 3959e3968.
[5] N. Weibel, L.J. Charbonnière, M. Guardigli, A. Roda, R. Ziessel, J. Am. Chem. Soc.
126 (2004) 4888e4896.