Synthesis, structural characterization, and thermoresponsivity of hybrid supramolecular dendrimers bearing a polyoxometalate core

Article abstract of DOI:10.1002/chem.201300289

A series of cationic dendrons bearing triethylene glycol monomethyl ether terminal groups of different generations have been synthesized and used to encapsulate an inorganic polyanionic cluster [K_12.5Na _1.5(NaP₅W₃₀O₁₁₀)] through electrostatic interactions. The resulting dendritic cation-encapsulated polyoxometalate (POM) complexes, cluster-dendrimers, are soluble in water and exhibit lower critical solution temperatures (LCST). The thermoresponsivities of these complexes in aqueous solutions were studied by turbidimetry and variable-temperature ¹H NMR spectroscopy. The observed cloud points show a remarkable dependence on the generation of the dendrons. Complexes composed of first-generation dendrons exhibit no obvious thermoresponsive properties, but for complexes bearing second-generation dendrons, the LCST decreases as the number of dendritic cations around the POM cluster increases. Complexes composed of third-generation cations underwent reversible aggregation and disaggregation upon heating and cooling, respectively. This thermally induced self-aggregation was characterized by DLS and TEM. In addition, the effects of salt and solvent on the LCST were investigated. This research demonstrates a new type of thermoresponsive dendritic organic-inorganic hybrid complex and provides a general route to the endowment of POMs with temperature-sensitive properties through electrostatic interactions.

Full text of DOI:10.1002/chem.201300289

FULL PAPER
Thermoresponsive dendrimers: A
series of cationic dendrons with glycol
terminal groups have been used to
encapsulate a typical polyoxometalate
polyanion through electrostatic inter-
actions to obtain a new type of hybrid
dendrimer complexes. An increase in
the generation of the dendrons leads
to a dramatic aggregation transition at
a lower critical solution temperature,
which can be modulated by salts and
solvent (see figure).
Hybrid Dendrimers
H. Chen, Y. Yang, Y. Wang,
L. Wu* ......................... &&&& — &&&&
Synthesis, Structural Characterization,
and Thermoresponsivity of Hybrid
Supramolecular Dendrimers Bearing a
Polyoxometalate Core
Chem. Eur. J. 2013, 00, 0 – 0
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DOI: 10.1002/chem.201300289
Synthesis, Structural Characterization, and Thermoresponsivity of Hybrid
Supramolecular Dendrimers Bearing a Polyoxometalate Core
Hailong Chen, Yang Yang, Yizhan Wang, and Lixin Wu*[a]
Abstract: A series of cationic dendrons
bearing triethylene glycol monomethyl
ether terminal groups of different gen-
erations have been synthesized and
used to encapsulate an inorganic polya-
nionic cluster [K12.5Na1.5(NaP5W30O110)]
through electrostatic interactions. The
resulting dendritic cation–encapsulated
polyoxometalate (POM) complexes,
cluster–dendrimers, are soluble in
water and exhibit lower critical solu-
tion temperatures (LCST). The ther-
moresponsivities of these complexes in
aqueous solutions were studied by tur-
bidimetry and variable-temperature
1H NMR spectroscopy. The observed
cloud points show a remarkable de-
pendence on the generation of the den-
drons. Complexes composed of first-
generation dendrons exhibit no obvious
thermoresponsive properties, but for
complexes bearing second-generation
dendrons, the LCST decreases as the
number of dendritic cations around the
POM cluster increases. Complexes
composed of third-generation cations
underwent reversible aggregation and
disaggregation upon heating and cool-
ing, respectively. This thermally in-
duced self-aggregation was character-
ized by DLS and TEM. In addition,
the effects of salt and solvent on the
LCST were investigated. This research
demonstrates a new type of thermores-
ponsive dendritic organic–inorganic
hybrid complex and provides a general
route to the endowment of POMs with
Keywords: aggregation · dendrim-
ers · polyoxometalates · salt effect ·
supramolecular chemistry
temperature-sensitive
properties
through electrostatic interactions.
Introduction
and oligomers, but also functional additives such as hydro-
gen-bonding multiplexes, metal-coordination complexes,
gold nanoparticles, and quantum dots.[8b,c] By combining
guest additives, thermoresponsive water-soluble polymers
have been widely investigated as nano-size vehicles for drug
delivery systems. On the other hand, the change in water
solubility of thermoresponsive matrices with temperature,
pH, and ionic strength can be used to modulate the func-
tions of these additives such as the assembly, color, and lu-
minescence of quantum dots.[8d] To obtain versatile thermo-
Thermoresponsive materials with significant physicochemi-
cal property alternations triggered by temperature have at-
tracted considerable attention in recent years[1] due to their
wide-ranging potential in fields such as smart chemical self-
assembly,[2] drug delivery and controlled release,[3] and sensi-
tive nanomaterials.[4] Thermoresponsive molecules are gen-
erally defined as those exhibiting upper critical solution
temperature (UCST) or lower critical solution temperature
(LCST) behavior.[5] For LCST molecules, their aggregation
at one temperature undergoes a phase transition from the
soluble to insoluble state in aqueous solution.[6] Among ther-
moresponsive materials showing LCST properties, the most
frequently used are amphiphilic polymers bearing N-isopro-
pylacrylamide (NIPAM) and polyethylene glycol (PEG)
units.[7] These artificial stimuli-responsive materials have di-
verse applications in the fields of biomedicine, catalysis, and
nanotechnology[8a] due to their ability to undergo large
changes in their properties triggered by a small stimulus.
The functional components that have been incorporated
into thermoresponsive materials include not only polymers
AHCTUNGTREGrNNUN esponsive materials, sensitive units are often grafted onto
the main matrix through covalent bonding, whereas nonco-
valent interactions are normally employed as a driving force
to control self-assembly and LCST behavior.[9] However,
many of the simple procedures that have been reported are
hard to follow for special systems. In addition, considering
the requirements of various sensitive functions, the introduc-
tion of inorganic nanoparticles and nanoclusters is eagerly
desired.[10] Therefore, the development of novel hybrid ther-
moresponsive materials is of importance for both fundamen-
tal research and technological applications.
Polyoxometalates (POMs) are a type of discrete, molecu-
larly defined, inorganic metal oxide cluster with varying
structures that exhibit multifunctional properties such as cat-
alysis, optics, magnetism, and medical.[11] To increase the
biocompatibility and stability with respect to pH and ionic
strength, covalent grafting and electrostatic binding of addi-
tional components have been used to improve the surface
properties of POMs. Many successful examples of organic–
POM hybrid materials based on covalent bonding have
[a] H. Chen, Dr. Y. Yang, Y. Wang, Prof. L. Wu
State Key Laboratory of Supramolecular Structure and Materials
College of Chemistry, Jilin University
Changchun 130012 (P.R. China)
Fax : (+86)431-8519-3421
E-mail: wulx@jlu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300289.
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been reported.[12] However, their methods of preparation
often have intrinsic limitations because few POMs can be
modified covalently and this modification often causes diffi-
culty in selecting functions.[13] A useful and alternative
method is to modify POMs with cationic surfactants through
electrostatic interactions.[14] The resulting surfactant-encap-
sulated POM (SEP) complexes are compatible with organic
matrices and display interesting self-assembly,[15] catalysis,[16]
and magnetic resonance imaging characteristics,[17] with the
basic physical and chemical properties of the POMs being
retained. Among SEP assemblies, smart aggregates are of
increasing interest for their ability to modulate self-assem-
bled structures, morphologies, and functions in response to
environmental stimuli such as light and pH.[18] Although
temperature is an important controlling factor, to the best of
our knowledge the only thermoresponsive POM hybrids re-
ported are those formed from the thermoresponsive poly-
mer PDEAAm-NH2 and Dawson-type POMs through cova-
lent bonding.[19] Thus, the fabrication of thermoresponsive
complexes containing common POMs through electrostatic
interactions is still a challenge.
In this work we have developed a new route to the prepa-
ration of a series of organically modified POM complexes,
through electrostatic interactions, that exhibit reversible
thermoresponsive properties (Scheme 1). These hybrid su-
pramolecular dendrimers possess a preyssler-type POM as
[20]
the core, K12.5Na1.5[NaP5W30O110
]
(the polyanionic cluster
is abbreviated as (P5W30)), and shells of cationic dendrons
bearing triethylene glycol (TEG) chains. Interestingly, on
the one hand, after electrostatic encapsulation, the POM is
hidden in the center of the dendrimers, maintaining its struc-
tural and chemical stability, so that the complexes retain the
intrinsic properties of the organic dendrimers. On the other
hand, the POM-containing dendrimers can still dissolve in
water, possessing a structure similar to the model of a cata-
lytic center in an enzyme with no ionization between the
cationic and anionic parts. In addition, the POMs provide
this kind of hybrid dendrimer with diverse cores to develop
precise branches and structures by controlling the charges
and ratios of both components. In this study we have charac-
terized the dendrimer complexes and investigated their
LCST properties and the effects of concentration and gener-
ation on the thermosensitive properties. The influences of
salts and solvent on the thermal behavior of the complexes
were also studied. From the results, one can see that the
POM clusters not only attract dendritic cations, but they can
also be predicted to exhibit their own functional properties,
such as controllable catalysis, template synthesis, and re-
sponsive imaging in designed systems.
Scheme 1. Chemical structures of the complexes, and the reversible ag-
gregation and disaggregation process for D-3 complexes in aqueous solu-
tion triggered by temperature. Note that the dendrons around the POM
(P5W30) in the schematic drawings do not represent the real numbers in-
dicated by the chemical measurements.
Results and Discussion
one, two, and three generations were carefully characterized
(see Figures S1–S6 in the Supporting Information) and are
denoted as D-1, D-2, and D-3, respectively, as shown in
Scheme 1. As for the POMs, we chose a preyssler-type clus-
ter, (P5W30), as the inorganic core to attract the cationic
dendrons and form a hybrid complex. We chose this POM,
Synthesis and structural characterization of dendritic com-
plexes: Dendritic surfactant cations with different numbers
of TEG units were synthesized step-by-step as described in
detail in the Experimental Section.[21] Three surfactants with
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1
which has a high number of negative charges, to build a den-
drimer complex system that can attract more cationic den-
drons. The hybrid dendrimer complexes were synthesized by
The H NMR and IR spectra reveal that (P5W30) clusters
have been successfully enwrapped by the dendritic cations
through electrostatic interactions. In contrast to the
1H NMR spectra of the dendritic cations alone, almost all
the proton peaks of the dendritic units in the complexes in
CDCl3 are significantly broadened and shifted upfield, espe-
cially those close to the positively charged nitrogen atoms.
following the procedures described in the literature.[15a]
A
chloroform solution of the dendritic cations was simply
mixed with an aqueous solution of (P5W30) in a specific
molar ratio according to the requirements of complexes
with different dendrons. After stirring at a suitable tempera-
ture (35–408C) for some time, the organic phase was sepa-
rated and the solvent was evaporated under reduced pres-
sure to obtain the target products. By changing the initial
By taking (D-3)13ACHTUNRGTNEUNG(P5W30) as an example (see Figure S9 in
the Supporting Information), we can see that the proton
peak of the N-methylene group is significantly broadened
and shifted by around 0.56 ppm in CDCl3, and the proton
peak of the N-methyl groups is broadened and slightly shift-
ed upfield. Similar behavior of the NMR signals of the den-
dritic cations was also observed for the other complexes.
These results suggest strong electrostatic attractions between
the dendritic cations and the (P5W30) cluster in these com-
plexes. Based on the structures of these dendritic complexes,
it is apparent that the strong interactions and the high num-
bers of dendritic cations on the periphery of the POM limit
the movement of the head groups of the dendron molecules
greatly, leading to broadening and upfield shifts of the
proton peaks. Furthermore, the characteristic vibration
bands of CH2 at 1380, 1347, 1325, and 1300 cmꢀ1 attributed
to TEG groups[23] are clearly observed in the IR spectra
(Figure S10 in the Supporting Information) of the dendritic
complexes, which indicates dendritic cations surrounding the
dendritic cation/ACHTUNGTRENNUNG(P5W30) cluster ratio, we obtained three
complexes bearing different numbers of D-2 cations. The
characterizations of the five complexes are described in
detail in the Experimental Section and Supporting Informa-
tion. Whereas most of the reported SEPs are insoluble in
water due to the high hydrophobicity of the surfactants, in
this work we introduced TEG groups as the end-groups of
the dendritic cations to increase water solubility and ther-
mosensitivity through hydrogen bonding between these
TEG groups and water. Meanwhile, the hydrophobic aro-
matic groups in the middle of the cationic dendron and
charge neutralization of the complexes together prevent the
dissociation of (P5W30) in aqueous solution. On the other
hand, the good solubility of the TEG groups allows com-
plexes to be prepared that are soluble in both water and
conventional organic solvents such as CHCl3, CH2Cl2, and
THF while driving the phase transition to higher tempera-
tures.[22]
ꢀ ꢀ
inorganic cluster. In addition, characteristic W O W
stretching vibration bands of (P5W30) appear at 945 (W Od),
ꢀ
ꢀ1
ꢀ
ꢀ
ꢀ ꢀ
908 (W Ob W), and 779 cm (W Oc W), which suggests
that the preyssler structure of (P5W30) is retained in the
complexes. The assignments of the vibration bands of the
complexes are summarized in Table S2 in the Supporting In-
formation. The data reveal that the (P5W30) clusters in the
complexes have been electrostatically enwrapped by differ-
ent generations and numbers of dendritic cations.
The results of the elemental analysis (see Table S1 in the
Supporting Information) indicate that the chemical formulae
of the (P5W30) hybrid complexes with first-, second-, and
third-generation dendritic cations have average chemical
compositions of (D-1)
N
7ACHTGNURTNEUN(GN P5W30), (D-2)10-
A
N
13A(P5W30), respectively, in
which the subscript to D-n represents the number of den-
dritic cations around one POM cluster. As a result of the
relatively small surface area and high number of charges on
(P5W30), we did not obtain complexes in which the inorganic
clusters were fully neutralized by cationic dendrons. Com-
plete neutralization of the complexes was achieved with K+
or Na+ ions or a mixture. Thermogravimetric analysis
(TGA) was also used to determine the chemical composi-
tions of the prepared complexes. Assuming that the organic
components completely decompose into the corresponding
oxides, and the final inorganic residual species are P2O5 and
WO3 at 9008C, the measured total residue of 20.33% for
It is known that the electrostatic interactions of ionic
compounds can be greatly weakened in water by hydration,
which may lead to dissociation of the prepared dendritic
complexes. To evaluate the stability of the complexes in
1
water, we compared the H NMR spectra of the dendritic
molecules and complexes in D2O. If dissociation occurred,
the chemical shifts of the dendritic cations in the solution
containing the complex should be similar to those of the
dendrimer alone. Otherwise, the dendritic cations should be
in their complex state. As shown in Figure S11 in the Sup-
porting Information, in comparison with the spectra of pure
dendritic molecules, almost all the proton signals of the
complex solutions are clearly broadened and shifted upfield,
which implies that the organic components in the hybrids
are still close to the periphery of (P5W30). In comparison
with the D-2 and D-3 cations alone, the chemical shifts of
both the hydrophobic phenyl moiety and the hydrophilic
TEG groups in the D-2 and D-3 complexes are shifted up-
field. These differences imply that the TEG groups are less
hydrogen bonded because of the formation of the core–shell
structures in the complexes. Because the electrostatic inter-
actions tend to be stronger in complexes, the proton peaks
(D-3)13ACHTUNGTRENNUNG(P5W30) matches well the value of 21.1% derived
from the predicted chemical formula. The TGA results ob-
tained for the other complexes (see Figures S7 and S8 in the
Supporting Information) were also in agreement with the
expected chemical compositions for each of the correspond-
ing complexes. Combining the elemental analysis and TGA
data, we confirmed that the chemical formulae of the syn-
thesized complexes are consistent with those of the products
that had been designed.
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of the N-methyl groups should move upfield due to dehy-
dration and the deshielding effect induced by POM binding
in water.[24] Because the simple counter ions of both the den-
dritic cation and the (P5W30) polyanion have been excluded
as inorganic salts during the preparation of the complexes
through phase separation, it is hard to separate the two
components. Moreover, the hydrophobic phenyl groups in
the dendritic unit can protect the (P5W30) from hydration in
water, which may also increase the stability of the com-
plexes. Fortunately, this behavior provides a good opportuni-
ty for further studying the thermosensitive properties and
possible applications of these complexes in water solution.
Thermoresponsive behavior of the dendritic complexes:
When dendritic cations cover the (P5W30) clusters, the TEG
tails of the dendrons point outwards and render the synthe-
sized complexes soluble in water. The aqueous solutions of
the synthesized complexes are transparent by visual inspec-
tion at room temperature. For aqueous solutions of the or-
ganic dendron cations and the hybrid dendrimer (D-1)13-
ACHTUNGTRENNUNG(P5W30), their transparency is maintained even when sam-
Figure 2. A) Plots of transmittance versus temperature for aqueous solu-
tions of the complexes at a heating rate of 0.58Cminꢀ1 and a complex
concentration of 1 mgmLꢀ1. B) Dependence of LCST on the concentra-
ples are heated in the range of 20–908C. In contrast, the sol-
utions of the other four dendritic complexes abruptly turned
milky when heated to a critical temperature (Figure 1).
tion of corresponding complexes. a) (D-1)
N
7ACHTUNGTRENNUNG(P5W30),
c) (D-2)10A(P5W30), d) (D-2)13A(P5W30), and e) (D-3) CHTUTGNRENNUNG
C
E
13A(P5W30). The transmit-
tance versus temperature graphs are shown in detail in Figure S12 in the
Supporting Information.
2)13CATHNUGETNR(NUGN P5W30), and (D-3)13ACHTNUGTREN(GNNU P5W30) at a concentration of
1.0 mgmLꢀ1 are 64, 58, 51, and 428C, respectively.
From these curves and estimated LCST values, we can
reach the following conclusions. (1) These complexes, with
the exception of (D-1)13ACHTNUGTRNE(UGN P5W30), exhibit sharp transitions
during both heating and cooling, which indicates high sensi-
tivity over the temperature range. Compared with the heat-
ing curve, the very small hysteresis (less than 28C) observed
during cooling reveals the complete reversibility of aggrega-
tion and disaggregation and high sensitivity at the given rate
of heating. The D-3 complex shows a much sharper transi-
tion and a smaller hysteresis than the D-2 complexes.
(2) The LCSTs decrease remarkably with increasing genera-
tion of the dendritic cations, whereas the D-1 complex
shows no LCST. For the D-2 complexes, the more dendritic
cations at the periphery, the lower is the LCST of the com-
plex. Thus, the LCST value is clearly related to the number
of dendrons at the surface of the (P5W30) cluster. Conse-
quently, in general, the LCST of the POM-based complexes
can be modified by simply changing the number or genera-
tion of the dendrons on the POM.
Figure 1. Photographs showing temperature-induced turbidity of (D-3)13-
ACHTUNGTRENNUNG(P5W30) in aqueous solution.
These turbid solutions become transparent again when they
are cooled to room temperature. Based on the fact that the
transmission curves during the cooling process match those
observed during the heating process, one can assume that
the opposite processes occur during heating and cooling.
Repeated experiments revealed the same behavior and indi-
cated that the complexes composed of D-2 and D-3 den-
drons undergo LCST transformation in aqueous solution
during the rise in temperature. These LCST transformations
can occur several times. Although the LCST phenomena
can be observed at lower concentrations, the transition is
occurs sharply at concentrations higher than 0.5 mgmLꢀ1
.
We measured the LCSTs of the complexes and studied
their thermoresponsive behavior by measuring their turbidi-
ty using UV/Vis spectrophotometry; the results are collected
in Figure S12 in the Supporting Information. The plots of
transmittance versus temperature during the heating and
cooling processes are shown in Figure 2A. The LCSTs of the
complexes were estimated in accord with a published proce-
Figure 2B shows the effect of concentration on the LCST
of POM complexes. For those complexes that exhibit ther-
moresponsive behavior, the LCST decreases as the concen-
tration increases. It has been reported that a higher concen-
tration makes it easier for the aggregation of dehydrated
polymer chains to take place, accompanied by a lower cloud
point.[26] It is also apparent that the LCST of (D-3)
is less dependent on concentration than the D-2 complexes.
13ACHTUNGTRENNUNG(P5W30)
dure.[25] The values for (D-2)
7ACTHNURTGENN(UG P5W30), (D-2)10AHCUTNTGREN(NUGN P5W30), (D-
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As will be discussed in the following section, we can attrib-
ute this phenomenon to the more densely packed TEG
groups on the periphery of the D-3 complex, which may
lead to enhanced dehydration caused by intramolecular in-
teractions between TEG groups instead of intermolecular
interactions with water.[27]
Thermally induced self-aggregation: We characterized the in
situ aggregation of the dendritic complexes in water through
dynamic light scattering (DLS) studies of the heating and
cooling cycles (see Figure S13 in the Supporting Informa-
tion). Because of the wide size distribution of the aggre-
gates, the change in scattering intensity versus particle size
is quite different to the variation in particle numbers versus
the particle size observed in the characterization of the ag-
gregates. To exclude the influence of strong scattering inten-
sity from larger particles, although their numbers are far
from dominant, we used the particle number as a measure
of aggregation in solution. The D-1 complex forms aggre-
gates in water, and the sizes of the aggregates vary with in-
creasing temperature. However, importantly, the change in
the aggregation of the D-1 complex versus temperature
does not trigger any turbidity in solution. As determined in
the following calculation, the good miscibility of the aggre-
gates at higher temperatures makes this complex nonther-
moresponsive. For D-2 complexes, at lower temperature, the
observed particle sizes are much larger than the estimated
molecular diameter of the complex dendrimers, which shows
the existence of aggregates. This is reasonable because the
POM complexes tend to form aggregates when they have
free space on their outside surfaces for the organic cations
to rearrange even if the POMs are fully neutralized by or-
ganic cations.[15] The miscibility of the complexes allows the
aggregates to readily dissolve in water. Interestingly, the par-
ticle diameter of the D-3 complex measured in water solu-
tion is approximately 5.6 nm at 258C. The diameter of the
(P5W30) cluster calculated from its crystal structure is
around 2.0 nm[28] and the molecular length of the D-3
cation, estimated from its stable structure, is around 1.5–
2.0 nm. Thus, the diameter of a single dendrimer is 5.0–
6.0 nm, very close to the size measured by DLS in water,
Figure 3. Hydrodynamic diameters of the complexes in aqueous solution,
determined from DLS measurements, as a function of temperature.
of all the D-2 complexes grow sharply. For example, the di-
ameter of the (D-2)
around 96 nm at 258C to 1713 nm at 608C. Meanwhile the
(D-3)13A(P5W30) complex aggregates, reaching size of
13ACTHNUGTREN(UNGN P5W30) aggregate increases from
C
a
around 2428 nm close to the LCST. For all the thermores-
ponsive hybrid dendrimers, the temperature at which the
abrupt changes in size occur is in the region at which the
turbidity, shown in Figure 2, starts to occur. Therefore, it can
be concluded that the hydrophobicity induced by the tem-
perature increase triggers the enhanced aggregation, and
both the hydrophobicity and increase in size of the aggre-
gates facilitate the solution turbidity. It can be observed that
further heating of the sample solutions above the LCST
leads to a decrease in the size of the aggregates, similarly to
other dendrimers.[7b] In accord with the conclusions of previ-
ously published results,[8b,29] such a phenomenon can be ra-
tionally attributed to dehydration-induced intrachain con-
traction and the exclusion of water molecules from the ag-
gregates. On cooling the hybrid dendrimer solutions from
above the LCST to room temperature, the sizes of the ag-
gregates return to the initial values. The heat-induced aggre-
gation and disaggregation are completely reversible, and
good reproducibility is observed even after five heating/
cooling cycles (see Figure S14 in the Supporting Informa-
tion).
which suggests a monodispersed state of (D-3)13ACHTNURGTENUNG(P5W30) at
room temperature. It is known from a previous study that
complexes in which the cores are fully covered by organic
cations mainly exist as isolated particles. Thus, the D-3 com-
plex bearing larger fan-shaped dendrons can adopt a mono-
dispersed state in the absence of the intermolecular driving
force.
By surveying the dependence of aggregate size on temper-
ature, the relationship between the size of the aggregates
and the LCST can be modulated and clearly understood.
The variations in the sizes of the aggregates with increasing
temperature are shown in Figure 3. During the heating proc-
ess, at temperatures below the LCST, the sizes of the aggre-
gates of the D-2 complexes remain constant, whereas the D-
3 complex maintains its monodispersed state. When the tem-
perature approaches the LCST, the sizes of the aggregates
The thermoresponsivities of the hybrid dendritic com-
plexes in aqueous solution were also evaluated by TEM.
Aggregates at temperatures above the LCST were observed
by casting sample solutions that had been preheated to a
certain temperature on to a hot copper grid covered with a
carbon film at the same temperature. As illustrated in
Figure 4, all the aggregates are nearly spherical in shape at
both room temperature and above the LCST. A clear size
distribution of the aggregates can be observed. The mean di-
ameter of (D-2)
to around 630 nm at 658C, very close to the value deter-
mined by DLS. For the (D-3)13A(P5W30) complex, no clear ag-
10ACTHNUGTREN(NUNG P5W30) is about 110 nm at 258C and grows
CTHUNGTRENNUNG
gregates were observed, but instead, even-sized nano spots
with a diameter of approximately 5–6 nm appear at room
temperature, which is in good accord with the Dh value ob-
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exists as a single solid sphere whereas the D-2 complex is in
an aggregated state. Because of the inadequate surface cov-
erage, the D-2 complex forms aggregates both below and
above the LCST due to the phase-separation-induced am-
phiphilicity and dehydration-induced hydrophobicity. In
contrast, because of the full surface coverage, the aggrega-
tion of the D-3 complex is a result of just dehydration-in-
duced hydrophobicity.
1
Temperature dependence of the H NMR spectra: To gain a
further understanding of the temperature sensitivity of the
intermolecular interactions between the hybrid dendritic
1
complexes and water, variable-temperature H NMR experi-
ments were performed. With the (D-2)
10ACHTUNGTNERN(UNG P5W30) complex in
deuteriated water as an example (Figure 5A), the proton
Figure 4. TEM images of aqueous solutions of (D-2)10ACHTUNGTRNE(NUNG P5W30) prepared at
A) 25 and B) 658C, and of aqueous solutions of (D-3)
at C) 25 and D) 508C.
CHTUNGTRENNUNG
served by DLS. At 508C, above the LCST, large spherical
aggregates of the D-3 complex with a diameter distribution
of 200–2000 nm were distinctly observed. These results re-
flect the size and morphologic changes of the dendritic com-
plex aggregates near the LCST. Whereas DLS is a measure-
ment carried out in situ, TEM is performed ex situ and in-
volves solvent evaporation. Because of the temperature de-
pendence of aggregates during the preparation of samples
for TEM, the observed difference in size distribution com-
pared with that determined by DLS is reasonable.
For many POM-based complexes, regular and ordered
substructures have been found due to the ability of these su-
pramolecular molecules to self-assemble. However, in addi-
tion to the even dispersion, it is hard to find any organized
structures of the hybrid dendrimers below and above the
LCST by analysis of the high-resolution TEM images. To
identify their structural characteristics, we also investigated
the substructures of these hybrid complexes at different
temperatures by X-ray diffraction. No clear ordered meso-
structures were identified from the diffraction patterns. The
wide scattering of the samples indicates a simple, tight pack-
ing of the complexes in their aggregates.
In reference to previous publications,[15c] the coniform
structure and packing style of cationic dendrons in filling
the cluster and complex surfaces should be the main factors
dominating the differences in aggregation of the D-2 and D-
Figure 5. Temperature-dependent 1H NMR spectra of A) (D-2)
and B) (D-3)13A(P5W30) in D2O.
10ACHTUNGTRENNUNG(P5W30)
3 complexes. Among the D-2 complexes, (D-2)
the highest number of TEG chains, with 52, whereas (D-
3)13A(P5W30) has 112, more than twice that of (D-2)13A(P5W30).
In the case of maximum charge replacement, the surfaces of
the cluster cores of both (D-2)13A(P5W30) and (D-3)13A(P5W30)
13ACHTUGNTERN(NUNG P5W30) has
CTHUNGTRENNUNG
C
CHTUNGTRENNUNG
peaks attributed to the organic components in the upfield
area are recognized as arising from the TEG groups and in
the downfield area as being from benzyl moieties. Because
of the electrostatic interactions between the quaternary am-
monium head group and the POM, the chemical shift of the
N-methyl at around 2.9 ppm becomes broadened, and the
signal of the N-methylene group almost disappears. In the
temperature range of 25–658C, with increasing temperature,
C
CHTUNGTRENNUNG
should be fully occupied because in each complex one metal
cation was not substituted. Suppose D-3 complex is fully
covered on its outer surface, the same surface for the D-2
complex should be incompletely covered due to the differ-
ences in the D-2 and D-3 dendrons. Therefore, one can see
that, with the same numbers of dendrons, the D-3 complex
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L. Wu et al.
these proton peaks move downfield relative to the chemical
shift of water. Given that the LCST is derived from the in-
compatibility of a sample with the solvent, the clear shift of
the proton signals at a higher temperature implies the
breakdown of hydrogen bonding between the organic com-
ponent in the complex and water.[30] In the heating process,
the clear downfield movement of the chemical shifts of
almost all parts of the organic dendrons is derived from the
dehydration of the TEG chains and the increased electro-
static interactions driven by the growing hydrophobicity. Be-
ACHTUNGTRENNUNG
cause (D-2)10ACHTUNGTRENNUNG(P5W30) exists as aggregates below and above
the LCST, no abrupt change was observed at temperatures
close to the LCST.
For the (D-3)13ACHTUNGTRENNUNG(P5W30) complex (Figure 5B), although the
signals of the TEG protons are similar to those of the D-2
complexes, the proton peaks of the phenyl moieties move
little compared with those of D-3 alone. Because the strong
electrostatic interactions in the complex cause a deshielding
effect, the proton signals of the N-methyl groups completely
disappear. With an increase in temperature, the proton
peaks of the organic component shift downfield due to de-
hydration and the electrostatic interactions increase induced
by enhanced hydrophobicity. Both effects contribute to the
deshielding, driving the chemical shifts downfield. Interest-
ingly, clear broadening of the proton signals of the TEG
groups occurs when the temperature rises from 40 to 458C,
close to the LCST. This indicates restricted molecular move-
ment due to aggregation of the D-3 complex. This result
supports the results of the DLS and TEM analysis. Notably,
similar changes were not observed in the NMR spectrum of
the D-1 complex (see Figure S15 in the Supporting Informa-
tion) because of its temperature insensitivity. This means
that an adequate number of TEG groups around the POM
are necessary for a LCST for POM complexes.
Based on the above results, the phase transitions of the
hybrid complexes in aqueous solution can be described. At
lower temperatures, strong hydrogen bonding between the
polar TEG groups and water dominates both the solubility
of the dendritic complexes in water and the interactions be-
tween the hydrophobic moieties in the complexes. Thus, the
complexes and their small aggregates readily dissolve in
water. With increasing temperature, the hydrogen bonding
is gradually weakened and the hydrophobic interactions be-
tween complexes are enhanced. Both these properties pro-
mote the formation and growth of aggregates. Therefore, at
temperatures above the LCST, the hydrophobic interactions
become stronger, thereby leading to phase separation and
the formation of larger aggregates.
The logarithm of the partition coefficient of 1-octanol/
water (logP) is often used to quantify the hydrophilicity of
organic molecules, whereby a larger logP indicates a larger
hydrophobic molecular segment.[32] We adopted Meylan and
Howardꢁs atom/fragment contribution method,[33] used most
often in the group contribution method, to estimate the
values of logP for the dendritic cations and complexes in
different generations by using Equation (1), in which ꢀACHTUNGTRENNUNG(fini)
is the summation of products of fi (the coefficient for each
atom/fragment) and ni (the number of times that the atom/
fragment appears in the structure), and 0.229 is the linear
equation constant.
X
log P ¼
ðfiniÞ þ 0:229
ð1Þ
The results for each dendritic cation and complex are
summarized in Table 1, based on the detailed group infor-
mation listed in Tables S3–S10 in the Supporting Informa-
tion. The corresponding logP values for the dendritic cati-
ons D-1, D-2, and D-3 are ꢀ4.0, ꢀ1.9, and 2.3, respectively,
which demonstrates that the hydrophobicity of the surfac-
tants increases in the order D-1<D-2<D-3. The estimated
Table 1. Values of logP for the dendritic complexes as well as the TEG
and phenyl groups.[a]
logP
(total)
logP
(TEG groups)
logP
(phenyl groups)
G
D-1
D-2
D-3
ꢀ4.0
ꢀ1.9
2.3
ꢀ1.5
ꢀ3.0
2.3
5.8
Regularity of LCST variation: The regularity of the LCST
was evaluated to construct a synergistic relationship be-
tween each part of the dendritic complexes and the alterna-
tion of solubility of the aggregates triggered by temperature.
As mentioned before, neither the dendritic cations in differ-
ent generations nor the POM alone show temperature sensi-
tivity due to their high water solubility. In contrast, all the
complexes composed of dendritic ions, except for (D-1)13-
ꢀ6.0
13.0
29.9
40.6
58.0
75.4
169.0
A
E
ꢀ20.2+C[b]
ꢀ19.5
ꢀ21.0
ꢀ30.0
ꢀ39.0
ꢀ78.0
E
CHTUNGTRENNUNG
3.7+C
5.5+C
6.9+C
61.7+C
A
E
G
N
C
G
[a] The logP of the ammonium head part was estimated to be ꢀ6.6 in the
cationic dendrons and ꢀ4.2 in complexes with POM. [b] C represents the
value of logP in the (P5W30) cluster.
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FULL PAPER
values of logP for the hydrophilic TEG groups and the hy-
drophobic phenyl groups in each dendritic ion lead to a
deeper understanding of why the hydrophobicity of the
complexes increases with the number of hydrophilic TEG
groups. For the TEG groups of the dendritic ions, logP de-
creases as the generation increases, which indicates that the
hydrophilicity of the peripheral TEG groups increases with
the number of these hydrophilic groups. For the phenyl
groups, the value of logP increases with increasing genera-
tion, which reveals that the hydrophobicity of the phenyl
groups increases with the number of these hydrophobic
groups. The increase in logP for the phenyl groups is appa-
rently faster than the reduction in logP for the TEG groups.
Consequently, the values of logP combined for these two
groups results in an increase in the overall hydrophobicity in
higher generation dendritic cations. Therefore, the hydro-
phobicity of the complexes increase in the order (D-1)13-
LCST responsivity for additives: To investigate the sensitivi-
ty of the LCST to the chemical environment in solutions
and to control the LCST in a suitable temperature range, we
studied the “salting out” and “salting in” behaviors of some
additives. The salt effects of PNIPAM derivatives and the
Hofmeister salt series[35] have been reported, the latter often
being used to rank the ability of ions to change the structure
of water.[36] Therefore, it is believed that Hofmeister salts
can strongly affect the LCST of the complexes because of
the high interfacial structuring of water commonly associat-
ed with TEG.[37] Thus, we employed NaCl, a strong kosmo-
trope that could assist in structuring water, and NaSCN, a
strong chaotrope known to disrupt the structure of water, to
investigate their effects on the LCST of the hybrid com-
plexes. As can be seen in Figure 6A, increasing concentra-
A
7ACHTUNGTRENNUNG(P5W30)<(D-2)10ACHUTNEGTR(GNNUN P5W30)<(D-2)13CATHNUGTREN(NGUN P5W30)<
CHTUNGTRENNUNG
analysis, it is clear that the LCST of the D-3 complex is
lower than those of the D-2 complexes owing to its greater
hydrophobicity. In addition, the D-1 complex shows no
LCST because it is too hydrophilic. For the D-2 complexes
with different numbers of dendritic ions, the more D-2 cati-
ons around POM, the greater the contribution of the phenyl
groups to logP, which gives rise to a more hydrophobic com-
plex and a lower LCST.
Figure 6. A) LCST dependence of (D-3)
concentration in aqueous solution and B) the variation of LCST with
methanol concentration for (D-3)13A(P5W30) in aqueous medium. Complex
concentration: 1 mgmLꢀ1
13ACHTUNGTRENN(NUG P5W30) on NaCl and NaSCN
CTHUNGTRENNUNG
It can be inferred from the chemical structures that, due
to changes in the surface density, the complexes can under-
go an overall change from a flexible shape for lower-genera-
tion dendrons to a more rigid spheroid-like shape for
higher-generation dendrons.[34] The flexible structure is help-
ful for aggregation, whereas the spheroid-like dense struc-
ture may lead to an isolated sphere or solid sphere packing.
For the complexes prepared from D-1 and D-2, large aggre-
gates were found. However, for the complex composed of
D-3, no obvious aggregation was observed at room tempera-
ture. Because the terminal TEG groups are located at the
periphery of the dendritic complexes, higher generation and
a larger number of dendritic cations on the (P5W30) cluster
lead to a denser surface of TEG groups. These TEG groups
lead to an increase in hydrophobicity during the heating
process. In comparison with the aggregates of the D-2 com-
plexes, the isolated or well-dispersed D-3 complex has a
larger surface. Therefore, combined with the value of logP,
we can conclude that the hydrophobicity of the phenyl
groups and the dehydration of the TEG groups in the com-
plexes together contribute to the reduction in the LCST.
The role of the POM in the fabrication of the thermores-
ponsive complexes can be summarized as follows. It attracts
dendritic ions to form hybrid dendritic complexes and be-
stows increased hydrophobicity on these dendritic compo-
nents. In addition, the TEG groups are directed outwards at
the periphery of both the complexes and the aggregates. All
these factors grant the nonthermosensitive cations and POM
clusters thermosensitivity.
.
tions of these salts have opposite effects on the LCST. In
the case that no apparent dissociation of the complex
occurs, the LCST rises, and the sensitivity to temperature is
lowered upon the addition and increase in the concentration
of NaSCN to a solution of the D-3 complex. The phase tran-
sition curve at the turning point becomes broader (see Fig-
ure S16a in the Supporting Information), which suggests a
typical salting-in effect. The presence of NaSCN leads to a
strengthening of the hydrogen bonds between the TEG
groups in the complex and water, which consequently results
in the breaking of the interfacial interaction, thus raising the
LCST. In contrast, a typical salting-out effect was observed
when adding NaCl to the complex solution. The LCST of
the D-3 complex exhibits an approximately linear decrease
with increasing NaCl concentration. However, in contrast to
NaSCN, the addition of NaCl shows very little influence on
the sensitivity of the LCST as the phase transition curve
maintains its sharpness (see Figure S16b in the Supporting
Information). Apparently, the addition of NaCl causes a par-
tial dehydration-induced dissociation of the hydrogen bonds
between the complex and water, giving rise to a lower
LCST. Moreover, a two-step transition is observed at a
NaCl concentration higher than 0.2m, and this behavior be-
comes evident as the salt concentration continues to in-
crease, which accords with prior reports on multistage poly-
mer interactions with kosmotropic salts.[38] It is likely that
the first phase transition is related to the detachment of
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L. Wu et al.
water from the TEG groups, and the second phase transition
corresponds to the release of structured water from the hy-
drophobic parts of the complex. When the NaCl concentra-
tion is sufficient, the hydrogen bonds between the TEG
groups and the water molecules become weak enough that
dehydration of the TEG groups occurs earlier at a different
time to the rest of the complex, thereby yielding a two-step
transition. It has been noted that the LCST of the D-3 com-
plex could be reduced to 378C, very close to body tempera-
ture, revealing a potential application of POM-containing
complexes in the biomedical field.[39]
transitions between small and big aggregates as well as be-
tween monodispersed complexes and large aggregates,
during which the POM structure always being well retained.
Interestingly, the LCST can also be regulated by adding
salts or using different solvents other than water. Moreover,
the salt-modulated phase transitions upon heating may en-
courage the study of potential applications of these types of
complexes in the fields of catalysis and biomedicine. The in-
troduction of POMs into thermoresponsive systems may
provide an opening to the development of smart functional
materials.
The effect of methanol on the LCST of the D-3 complex
(see Figure S17 in the Supporting Information) was also in-
vestigated in the concentration range of 1–30% (in v/v), and
the results are shown in Figure 6B. Normally, as a hydrogen-
bond-breaking solvent, methanol lowers the LCST.[40]
However, the LCST of the complex increases with the addi-
tion of increasing amounts of methanol. Notably, when the
concentration of methanol is more than 10%, a dramatic in-
crease in the LCST was observed. On the one hand, the ad-
dition of methanol can promote a decrease in LCST due to
the dehydration of TEG, and on the other hand, due to the
compatibility of methanol with TEG groups on the periph-
ery of hybrid complexes,[41] the addition of methanol increas-
es the solubility of the hybrid complexes even on heating.
Therefore, the overall behavior is derived from the competi-
tion between these two factors. Clearly, the increase in solu-
bility has a critical effect on the LCST. When the concentra-
tion of methanol is greater than 30% (v/v), the LCST can
no longer be observed, similarly to the case of thermores-
ponsive polymers based on PEG units.[42]
Experimental Section
Materials: K12.5Na1.5[NaP5W30O110] (P5W30) was prepared according to a
literature procedures.[43] 3,5-Dihydroxybenzylbenzoic acid was purchased
from Alfa Aesar and 3,5-dihydroxybenzyl alcohol was the product of
Sigma–Aldrich. All starting compounds for the syntheses were used with-
out further purification, and the solvents used in the reactions were ana-
lytical grade. Doubly distilled water was used in the experiments. Silica
gel (100–200 mesh) was used for column chromatography.
Synthesis of dendritic molecules D-1, D-2, and D-3: Different genera-
tions of the dendritic quaternary ammonium cations were synthesized by
using the convergent step-by-step method. The synthetic procedures are
described in Scheme 2. The chemical structures of dendritic molecules
were confirmed by FTIR and 1H NMR spectroscopy as well as by
MALDI-TOF MS. The synthetic procedures are summarized in detail in
the Supporting Information.
Synthesis of the complexes: All the complexes were prepared at room
temperature according to previously reported procedures, and their struc-
tures were confirmed by 1H NMR and IR spectroscopy, elemental analy-
sis, and TGA.
ACHUTNERGN(UNG D-1)13ACHUTGNTREN(NUNG P5W30): A solution of D-1 (0.4 g) in chloroform (10 mL) was
added dropwise to an aqueous solution of K12.5Na1.5[NaP5W30O110] (0.7 g)
under stirring at 508C, keeping the initial molar ratio of D-1 to POM
controlled at 10:1. The organic phase was separated, washed with water
Conclusions
(3ꢂ30 mL), and dried over anhydrous sodium sulfate. (D-1)13ACHTNURGTNENUG(P5W30) was
obtained by evaporating the chloroform to dryness. The product was
stored in a vacuum desiccator until the weight remained constant (yield:
0.7 g, 74%). H NMR (500 MHz, CDCl3): d=3.32 (s, 15H), 3.50–3.70 (m,
A series of new water-soluble dendritic POM complexes
have been designed and synthesized through simple electro-
static interactions between the inorganic polyanions and or-
ganic dendritic cations, a procedure that is generally applica-
ble to clusters. The ability of the POM cluster at the core of
the dendrimer to attract peripheral dendrons allows the
hybrid complexes to form not only stable intermolecular in-
teractions, but also leads to water solubility and thermosen-
sitivity. In contrast to conventional polymers, neither of the
isolated components of the dendritic complexes are ther-
moresponsive. It can be concluded that the synergistic inte-
gration of the POM and dendrons induces functionality,
which can be attributed to the branched structure and the
collecting effect of the TEG terminals. The critical number
of TEG groups in a complex for possessing a reversible
sharp LCST transition is between 26 and 28, based on the
1
16H), 3.86 (s, 4H), 4.15 (s, 4H), 4.46 (s, 2H), 6.49 (s, 1H), 6.69 ppm (s,
2H); IR (KBr): n˜ =2926, 1599, 1454, 1350, 1323, 1302, 1165, 1111, 935,
912, 796 cmꢀ1
;
elemental analysis calcd (%) for (D-1)13ACHTUNGTRENNUNG(P5W30)
(C24H44O8N)13K[NaP5W30O110] (13662.0): C 27.46, H 4.22, N 1.33; found:
C 26.98, H 4.17, N 1.26. TGA displayed a 48.0% (w/w) loss of mass be-
tween 40 and 7008C.
ACHTUNGRENN(UG D-2)7ACHUTNTGREN(NGUN P5W30): A solution of D-2 (0.2 g) in chloroform (10 mL) was
added dropwise to an aqueous solution of K12.5Na1.5[NaP5W30O110] (0.5 g)
under stirring at 508C, keeping the initial molar ratio of D-2 to POM
controlled at 3:1. The crude product was treated and purified by a similar
procedure to that used for (D-1)13ACTHUNTRGNEUNG(P5W30) to give the final complex
1
(yield: 0.3 g, 76%). H NMR (500 MHz, CDCl3): d=3.19 (s, 9H), 3.34 (s,
12H), 3.52–3.69 (m, 32H), 3.84 (s, 8H), 4.10 (s, 8H), 4.96 (s, 6H), 6.39,
(s, 2H), 6.59 ppm (s, 7H); IR (KBr): n˜ =2937, 2867, 1596, 1454, 1353,
1327, 1295, 1158, 1111, 950, 914, 801 cmꢀ1; elemental analysis calcd (%)
for (D-2)7ACHTNUGRTENU(GN P5W30) (C52H84O18N)7K7[NaP5W30O110] (14805.2): C 29.53, H
4.00, N 0.66; found: C 30.43, H 4.33, N 0.66. TGA displayed a 54.2% (w/
w) loss of mass between 40 and 7008C.
thermal properties of (D-1)13ACHTUNTRGENN(UG P5W30) and (D-2)7CAHTUNTGRENN(UNG P5W30). The
ACHUTNERGN(UNG D-2)10ACHUTGNTREN(NUNG P5W30): A solution of D-2 (0.2 g) in chloroform (10 mL) was
thermoresponsive temperature strongly depends on the gen-
eration and number of TEG groups in the dendritic cations
surrounding the POM. A higher number of TEG groups in
a complex leads to a lower LCST temperature. A series of
experiments proved the occurrence of fully reversible phase
added dropwise to an aqueous solution of K12.5Na1.5[NaP5W30O110] (0.2 g)
under stirring at 508C, keeping the initial molar ratio of D-2 to POM
controlled at 10:1. The crude product was treated and purified by a simi-
lar procedure to that used for (D-1)
13ACTHNUGTREN(UNGN P5W30) to give the final complex
(yield: 0.3 g, 82%). 1H NMR (500 MHz, CDCl3): d=3.17 (s, 9H), 3.32
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AHCUTNTGREG(NNNU D-3)13ACHTNUGTREG(NNUN P5W30): A solution of D-3
(0.2 g) in chloroform (10 mL) was
added dropwise to an aqueous solution
of K12.5Na1.5[NaP5W30O110
]
(0.06 g)
under stirring, keeping the initial
molar ratio of D-3 to POM controlled
at 13:1. The crude product was treated
and purified by a similar procedure to
that used for (D-1)13ACTHUNTRGNEUNG(P5W30) to give
the final complex (yield: 0.2 g, 78%).
1H NMR (500 MHz, CDCl3): 3.19 (s,
9H), 3.28 (s, 24H), 3.45–3.61 (m,
64H), 3.74 (s, 16H), 4.01 (s, 16H),
4.28 (s, 2H), 4.86 (s, 12H), 6.33 (s,
4H), 6.42 (s, 2H), 6.52 (s, 8H),
6.63 ppm (s, 7H); IR (KBr): n˜ =2922,
2876, 1597, 1450, 1375, 1348, 1323,
1298, 1173, 1146, 937, 841, 797 cmꢀ1
elemental analysis calcd (%) for
(C108H164O38N)13K[NaP5W30O110
;
]
(34589.8): C 48.75, H 6.21, N 0.53;
found: C 49.02, H 6.24, N 0.52. TGA
displayed a 80.1% (w/w) loss of mass
between 40 and 7008C.
Measurements: 1H NMR spectra were
recorded on
a Bruker Avance 500
spectrometer with TMS as an internal
reference. IR spectra of samples in
pressed KBr pellets were recorded
with a Bruker IFS66v FT-IR spectrom-
eter equipped with a DGTS detector
(32 scans) at a resolution of 4 cmꢀ1
Elemental analysis (C, H, N) was car-
ried out on ThermoQuest Flash
.
a
EA1112 Analyzer. TGA was per-
formed on a Perkin-Elmer Diamond
TG/DTA instrument at a heating rate
of 108Cminꢀ1 under
a flow of air.
MALDI-TOF mass spectra were re-
corded on a LDI-1700 mass spectrom-
eter (Linear Scientific) with chloro-
form as solvent. TEM images were ob-
tained with a Hitachi H8100 electron
microscope with an accelerating volt-
age of 200 kV without staining. DLS
Scheme 2. Synthetic route to dendritic cations D-1, D-2, and D-3. Reagents and conditions: a) K2CO3, 18-
was performed with
a
Zetasizer
crown-6, acetone, 708C, 24 h; b) AlLiH4, THF, 5 to 258C, 15 h; c) PBr3, benzene, ice bath, 6 h; d) trimethyl-
ACHTUNGTRENNUNGamine, ethanol, 808C.
NanoZS instrument (Malvern Instru-
ments). UV/Vis turbidity measure-
ments were conducted on
a Varian
Cary 100 Bio UV/Vis spectrophotome-
ter equipped with a thermostatically regulated water bath for the deter-
mination of the lower critical solution temperatures (LCSTs). Solutions
of the complexes in deionized water at concentrations of 0.5–4 mgmLꢀ1
(s,12H), 3.52–3.67 (m, 32H), 3.82 (s, 8H), 4.09 (s, 8H), 5.00 (s, 6H), 6.37
(s, 2H), 6.57 ppm (s, 7H); IR (KBr) : n˜ =2937, 2867, 1596, 1454, 1353,
1327, 1295, 1158, 1111, 950, 914, 801 cmꢀ1; elemental analysis calcd (%)
for (D-2)10ACHTUNGTRENNUNG(P5W30) (C52H84O18N)10K4[NaP5W30O110] (17721.6): C 35.24, H
4.77, N 0.79; found: C 34.72, H 4.79, N 0.72. TGA displayed a 60.8% (w/
were placed in the spectrophotometer and heated at
a rate of
0.58Cminꢀ1. The absorptions of the solutions were recorded at l=
500 nm every minute.
w) loss of mass between 40 and 7008C.
ACHTUNGTRENNUNG(D-2)13ACHTUNGTRENNUNG(P5W30): A solution of D-2 (0.2 g) in chloroform (10 mL) was
added dropwise to an aqueous solution of K12.5Na1.5[NaP5W30O110] (0.1 g)
under stirring at 508C, keeping the initial molar ratio of D-2 to POM
controlled at 13:1. The crude product was treated and purified by a simi-
Acknowledgements
lar procedure to that used for (D-1)
G
1
This work was supported by the National Basic Research Program
(2013CB834503), the National Natural Science Foundation of China
(91227110 and 21221063), 111 Project (B06009) for supporting the visit of
Prof. M. S. Lee at Seoul National University and fruitful discussions, and
the Open Project of the State Key Laboratory of Polymer Physics and
Chemistry of the Chinese Academy of Sciences.
(yield: 0.3 g, 80%). H NMR (500 MHz, CDCl3): d=3.21 (s, 9H), 3.36 (s,
12H), 3.54–3.70 (m, 32H), 3.84 (s, 8H), 4.11 (s, 8H), 4.79 (s, 2H), 5.01 (s,
4H),6.41, (s, 2H), 6.60 (s, 4H), 6.66 (s, 1H), 6.77 ppm (s, 2H); IR (KBr):
n˜ =2937, 2867, 1596, 1454, 1353, 1327, 1295, 1158, 1111, 950, 914,
801 cmꢀ1; elemental analysis calcd (%) for (C52H84O18N)13K[NaP5W30O110
(20637.9): C 39.34, H 5.33, N 0.88; found: C 39.48, H 5.56, N 0.83. TGA
displayed a 66.1% (w/w) loss of mass between 40 and 7008C.
Chem. Eur. J. 2013, 00, 0 – 0
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Received: January 25, 2013
Revised: May 3, 2013
Published online: && &&, 0000
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These are not the final page numbers!