Phenylazomethine dendrimers with soft aliphatic units as metal-storage nanocapsules and their self-assembled structures

Article abstract of DOI:10.1002/chem.201002632

New dendritic poly(phenylazomethine)s (DPAs) with dodecyl end groups (C12DPA) have been synthesized. C12DPA showed stepwise radial complexation with metals as a metal-storage nanocapsule. Through modification with dodecyl, the properties of environmental

Full text of DOI:10.1002/chem.201002632

DOI: 10.1002/chem.201002632
Phenylazomethine Dendrimers with Soft Aliphatic Units as Metal-Storage
Nanocapsules and Their Self-Assembled Structures
Yousuke Ochi,[b] Kozue Sakurai,[b] Keisuke Azuma,[a] and Kimihisa Yamamoto*[a, b]
Abstract: New dendritic poly(phenyl-
ACHTUNGTRENNUNGazomethine)s (DPAs) with dodecyl end
the first time, which is a nanocapsule
with a basic atmosphere for metal com-
plexes in a hydrophobic environment.
In addition, we found a suitable struc-
ture and conditions for the fibrous self-
assembly of DPA through the precise
design of its dendritic structure.
C12DPA, with an asymmetric structure,
showed a fibrous assembly by a solvent
drop-casting. The metal-complexed
C12DPA showed an assembly different
from the fibrous one through metal
complexation on DPA imines. Interest-
ingly, the vesicular assembly structure
of C12DPA has been observed by the
complexation of CuCl2 in toluene. In
this study, we investigated the unique
properties of C12DPA as a novel p-
conjugated soft material.
groups (C12DPA) have been synthe-
sized. C12DPA showed stepwise radial
complexation with metals as a metal-
storage nanocapsule. Through modifi-
cation with dodecyl, the properties of
environmental responsiveness and self-
assembly
become
apparent
for
C12DPA as a p-conjugated soft materi-
al. The dodecyl-modified DPAs
(C12DPAG4) were synthesized up to
the fourth generation dendrimer, for
Keywords: copper · dendrimers ·
macromolecules · N ligands · self-
assembly
Introduction
creation of functional nanocapsules, the end-group modifica-
tion is a good approach due to its importance for solubility,
molecular selectivity as a unique nanocapsule,[7] and control
of hierarchical structure.[8] Previously, DPAs were modified
with poly(ethylene glycol) (PEG)[9] or ferrocene terminal
groups;[10] these showed good redox properties or hydrophil-
ic solubility with precise metal assembly.
Dendrimers are geometrically branched macromolecules
with a single molecular weight similar to proteins.[1] This
unique structure becomes a useful building block for creat-
ing nanomaterials or as a unique nanocapsule using the
inner space in the molecule. Many researchers have investi-
gated the encapsulation behavior of dendrimers using cova-
lent bonds,[2] coordinative bonds,[3] ionic bonds,[4] or hydro-
phobic interactions.[5] Using coordinative bonds, we previ-
ously reported a new method for the precise assembly of
The use of hydrocarbon chains is an effective and easy
way for the creation of functional soft materials, which have
unique characteristics such as hydrophobic interactions, en-
vironmental sensitivity, and self-assembly. In recent studies,
functional soft materials, such as lipid membranes or pro-
teins have been studied and found in living organisms. Kuni-
takeꢀs group studied artificial lipid membranes similar to
biomembranes using hydrocarbon chains to pioneer recent
soft material development.[11] They reported that an amphi-
phile with dodecyl chains and rigid azomethine segments
produced stable vesicle structures in water. It is also notable
that organic solvent systems also formed similar bilayer as-
semblies.[12] In recent years, the groups of Meijer[13] and
Kato[14] developed intelligent materials using hydrogen
bonding and a soft unit by the precise control of supra-
molecular structures. A hybrid structure with a p-conjugated
backbone and an aliphatic unit also gives various supramole-
cules.[15] In addition to organic molecules, Kimizukaꢀs group
developed metal–organic hybrid soft materials.[16] In terms
metal ions using
a
dendritic poly(phenylazomethine)
(DPA).[6] Furthermore, dendrimers allow the precise modifi-
cation of the core, main chain, and end groups, which produ-
ces sophisticated functions as a nanocapsule. In terms of the
[a] K. Azuma, Prof. Dr. K. Yamamoto
Chemical Resources Laboratory, Tokyo Institute of Technology
4259, Nagatsuta, Midori-ku, Yokohama, 226-8503 (Japan)
Fax : (+81)459-245-260
E-mail: yamamoto@res.titech.ac.jp
[b] Y. Ochi, K. Sakurai, Prof. Dr. K. Yamamoto
Department of Chemistry, Faculty of Science and Technology
Keio University, 3-14-1, Hiyoshi, Kohoku-ku
Yokohama, 223-8522 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002632.
800
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FULL PAPER
of the artificial creation of novel soft materials, the hybridi-
zation of soft units, rigid segments, and metals is an attrac-
tive approach due to the stimulus response of the soft units
and the rigid structure for control of the desired functions.
As one of the famous building blocks of supramolecular
structures, the branching structure of precisely designed den-
drimers is an attractive unit.[17] We suggest that the DPA
dendrimer is an attractive p-conjugated unit for novel soft
materials and metal–organic hybrid materials[18] due to its
stepwise metal assembly, based on an electron-density gradi-
ent[6] as a unique nanocapsule.[19] In this study, the various
generations of DPAs modified with dodecyl chains, the
fourth generation at most, were synthesized. The previously
reported DPAs modified with PEG or ferrocene terminal
groups were synthesized up to the third generation dendri-
mer. Modification of the DPA end groups with dodecyl
chains produced a stable metal/DPA complex in a hydro-
phobic environment. Compared with poly(amidoamine)
(PAMAM) or Frꢂchet-type dendrimers, the DPA skeleton is
relatively rigid and has a wide space within the molecule.[20]
The alkyl modification produces an effective isolation of the
DPA interior from outside in the form of a nanocapsule. In
addition, environmental sensitivity and self-assembling prop-
erties are expected to be obtained in the DPA through
modification of the alkyl unit. We found suitable structures
and conditions for the fibrous self-assembly of C12DPA by
precise design of its dendritic structure. Interestingly, the as-
sembled structure has been changed to vesicles by metal
complexation on DPA. Herein, we show the unique proper-
ties of aliphatic-modified DPA as a novel soft material.
G3on) was synthesized by the same method used for the
normal DPA. The ester G3on was deprotected to the hy-
droxy G3on (OH-G3on), and then OH-G3on was treated
with 1-iodododecane to obtain dodecyl-modified G3on
(C12G3on). Through the condensation reaction of C12G3on
and the NH2-DPAG1 core using titanium(IV) chloride, we
succeeded in synthesizing the dodecyl-modified DPAG4
(C12DPAG4) with a single molecular weight, which was
confirmed by MALDI-TOF MS and size exclusion chroma-
tography (SEC) measurements (Figures S1 and S2 in the
Supporting Information). C12DPAG4 was synthesized, for
the first time, as the fourth generation end-modified DPA.
Other C12DPAGn (n=1–3) were also synthesized by using
a similar procedure.
A dynamic light scattering (DLS) measurement was per-
formed to observe the average diameter of C12DPA, which
has an environmental responsiveness. Whereas the diameter
of DPA did not change when different solvents were used
due to the rigid azomethine backbone, that of C12DPA did
depend on the type of solvent (Figure 1b). Figure 1a shows
that the diameter of C12DPA in heptane or chloroform
became much larger than that of normal DPA. This result
showed that alkyl chains stretched outward in aliphatic sol-
vents. The diameter of C12DPA in THF had only a slight in-
crease compared with normal DPA, which suggests that the
alkyl chains do not stretch very much. The maximum diame-
ter of C12DPAG4 measured by DLS was 5.4 nm in heptane,
which is close to the diameter of 6.5 nm estimated by molec-
ular modeling using MOPAC AM1-MOZYME (Figure 1b).
These results suggest that C12DPA has an environmental re-
sponsiveness through hydrocarbon chain modification.
DPA complexes with metal salts in a stepwise radial fash-
ion, which is due to the electron-density gradient in a mole-
cule.[6] Figure 2 and Figure S4 in the Supporting Information
show that C12DPA has a greater UV/Vis absorption near
300 nm attributed to imines than the normal DPA, which
suggests the electron-donating effect of the alkoxy groups.
We confirmed that the stepwise radial complexation behav-
ior is due to an electron-density gradient using UV/Vis spec-
Results and Discussion
We now report the synthesis of dodecyl-modified DPA
(C12DPA) using the “double-stage” convergent method
(Scheme 1). According to a previously reported method,[9]
the hydroxy end groups of DPA were substituted by benzoyl
esters. The benzoyl-substituted DPAG3 dendron (Ester
Scheme 1. Synthetic procedure for dodecyl-modified dendritic poly(phenylazomethine)s. R=C12H25. a) 1) KOH, THF/MeOH, RT, 12 h; 2) 1-iododode-
cane, K2CO3, DMF, 908C, 12 h; b) TiCl4, 1,4-diazabicyclo[2.2.2]octane. (DABCO), PhCl, 1208C, 12 h. Bz=benzoyl.
AHCTUNGTRENNUNG
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K. Yamamoto et al.
troscopy during a titration in which FeCl3 was added to
novel C12DPA. It was observed that the absorption at
around 400 nm increased, while the absorption at around
300 nm attributed to the imines decreased (Figure 3). An
Figure 3. a) UV/Vis spectra of C12DPAG4 complexed with 0–30 equiva-
lents of FeCl3 and isosbestic points during the addition of 0–2, 3–6, 7–14,
and 15–30 equivalents of FeCl3 after the subtraction of the FeCl3 absorp-
tion. [C12DPAG4]=5.0ꢃ10À6 m. b) Schematic representation of the step-
wise radial complexation of C12DPAG4 with FeCl3 at 349 (0–2 equiv
FeCl3), 336 (3–6 equiv FeCl3), 333 (7–14 equiv FeCl3), and 335 nm (15–
30 equiv FeCl3).
Figure 1. a) DLS diameters of DPAGn and C12DPAGn (n=1–4) in THF,
CHCl3, and heptane. b) 3D model of C12DPAG4 optimized by
MOPAC (AM1 MOZYME) calculation.
a
isosbestic point appears when one compound is quantitative-
ly transformed into another by complexation (see the Sup-
porting Information). Four changes in the position of the
isosbestic points were observed during the addition of the
FeCl3 to C12DPAG4, which indicated that four different
complexations are successively formed upon FeCl3 addition
(Figure 3). The spectra of C12DPAG4 gradually changed
with an isosbestic point at 349 nm observed up to the addi-
tion of 2 equivalents of FeCl3, but shifted to 336 nm between
3 and 6 equivalents. During the addition of 7–14 equivalents,
an isosbestic point appeared at 333 nm, which moved to
335 nm during the addition of 15–30 equivalents. The
number of equivalents of FeCl3 required to induce a shift
was commensurate with the number of imine sites present
in each generation layer of C12DPAG4. Similar changes in
the UV/Vis spectra were observed for C12DPAG2 and
C12DPAG3 (Figure S5). For DPAG2, two isosbestic points
appeared at 343 and 330 nm when adding 0–2 and 3–
6 equivalents of FeCl3, respectively. In the case of DPAG3,
Figure 2. UV/Vis spectra of C12DPAG4 (c)and DPAG4 (a).
[C12DPAG4]=[DPAG4]=5.0ꢃ10À6 m.
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Phenylazomethine Dendrimers
FULL PAPER
three isosbestic points centered at 350, 333, and 328 nm ap-
peared when adding 0–2, 3–6, and 7–14 equivalents of FeCl3,
respectively. These results show that the stepwise radial
complexation definitely occurs in the novel C12DPA modi-
fied with alkoxy groups.
showed uniform particles of 2 nm, which corresponds with
the DPA backbone complexed with AuCl3. These results
showed that C12DPA is able to transport metal salts to hy-
drophobic environments as a nanocapsule.
In addition to gold chlorides, the encapsulation of Rose
Bengal in C12DPA was observed for investigating the inner
environment of the dendrimer. Whereas normal unmodified
DPA did not interact with a Rose Bengal lactone, C12DPA
could disperse Rose Bengal in a hydrophobic heptane sol-
vent and showed a red color (lmax =569 nm), which was
caused by a basic environment (Figure S7). In contrast, the
red color of Rose Bengal was not observed in chloroform or
THF. These results insist that Rose Bengal existed in the
isolated basic nanospace when in hydrophobic heptane,
which has no solubility to DPA backbone. Compared with
Rose Bengal in a CH3CN solvent, the absorption maxima of
Rose Bengal with C12DPA showed a redshift of 6 nm,
which indicates that Rose Bengal exists in a more aprotic
and hydrophobic environment of DPA backbones[21] than
In addition to FeCl3, the stepwise assembly of AuCl3 on
C12DPAG4 was carried out. Although the complexation
constant of AuCl3 was weaker than that of FeCl3 and the
change in the absorption was less (KFeCl >108 mÀ1, KAuCl
=
3
3
105 mÀ1), the UV/Vis spectra of C12DPAG4 showed four
shifts in the isosbestic points at 326, 329, 318, 320 nm when
adding 0–2, 3–6, 7–14 and 15–30 equivalents of AuCl3, re-
spectively (Figure 4a). The encapsulation of AuCl3 into
C12DPA was also confirmed by using TEM. We found that
the (AuCl3)30@C12DPAG4 complex could dissolve in hep-
tane, whereas the AuCl3@DPAG4 complex could not. The
(AuCl3)30@C12DPAG4 complex in heptane was cast on a
carbon-coated copper grid and observed by TEM and
energy-dispersive X-ray (EDX) (Figure 4b and Figure S6 in
the Supporting Information, respectively). The TEM image
1
that in CH3CN. In addition, comparing the H NMR relaxa-
tion time of the inner proton in C12DPA with that of the
normal DPA, C12DPA showed a different generation effect
in the inner environment from that of DPA (Figure S8).
These results supported the fact that the specific p-conjugat-
ed space existed in C12DPA as a unique nanocapsule.
For accelerating the molecular assembly of C12DPA as a
soft-material, that is, an asymmetric C12DPA, C12DPAG3-
Gn (n=1–3), was also synthesized (Scheme 2 and Figure S3
in the Supporting Information). As a characteristic of DPA,
the lower generation of DPA, specifically DPAG2, has a
planar molecular structure. Utilizing this p-conjugated DPA
characteristic, C12DPAG3-Gn specifically showed unique
assembly behavior. The sizes of the C12DPAG3 unit and the
DPAG2 unit of C12DPAG3-G2 were estimated to be 3 and
1.4 nm by using a 3D model (Figure 5), respectively. This
naked DPAG2 unit of C12DPAG3-G2 will accelerate the
DPA assembly using the p-conjugated plane structure and
the metal assembling ability. We should mention that the
DLS diameter of C12DPAG3-G2 in heptane was relatively
large, namely, 5.3 nm, which suggests molecular assembly in
heptane. This diameter gradually increased in proportion to
the concentration.
The assembled structures of the C12DPAs were observed
by AFM in tapping mode. Samples were prepared by slow
evaporation after drop casting on mica. C12DPAGn (n=1–
4) showed similar isotropic thin membranes or islands (Fig-
ure S9a) regardless of their generation. Meanwhile, we
found that C12DPAG3-Gn (n=1–3) formed an anisotropic
assembly similar to fibers (Figure S9b and c). Specifically,
C12DPAG3-G2 showed a clear fibrous structure from solu-
tions in heptane or CHCl3 (Figure 6a). The stacking struc-
ture of C12DPAG3-G2 through solvent evaporation is pro-
posed in Figure 6b due to the planar structure of the
DPAG2 unit. The observed height of the C12DPAG3-G2
fibers was 1.9 nm, which corresponds with that of the
DPAG2 unit (2 nm) based on a 3D model. A similar fibrous
structure of C12DPAG3-G2 was also observed by TEM
Figure 4. a) UV/Vis spectra of C12DPAG4 with 0–30 equivalents of
AuCl3 and isosbestic points during the addition of 0–2 (at 326 nm), 3–6
(at 329 nm), 7–14 (at 318 nm), and 15–30 equivalents (at 320 nm) of
AuCl3 after the subtraction of the AuCl3 absorption in 1:1 toluene/
CH3CN. [C12DPAG4]=5.0ꢃ10À6 m. b) TEM image of C12DPAG4 com-
plexed with 30 equivalents of AuCl3 dissolved in heptane. Scale bar is
20 nm.
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K. Yamamoto et al.
Scheme 2. Synthetic procedure of C12DPAG3-G2. R=C12H25. a) TiCl4, DABCO, PhCl, 1208C, 6 h.
During the addition of 7–10 equivalents, an isosbestic point
appeared at 339 nm. In the case of C12DPAG3er, similar
shifts in the isosbestic points were observed (Figure S11),
which supports the C12DPAG3-G2 result. These results in-
dicated that CuCl2 complexes with a DPA imine in this tolu-
ene/acetonitrile solution, which is probably supported by
À
acetonitrile coordination on the other side of the Cu N co-
ordination.[22–23]
By using the complexation of copper chloride and
C12DPA, the self-assembling behavior of the copper/
C12DPA complex was investigated. We found the vesicular
assembly of specific C12DPAs complexed with CuCl2 in tol-
uene by the evaporation of acetonitrile and sonication; this
was different from the fibrous ones using only C12DPAG3-
G2. In the DLS measurement, aggregates of the
(CuCl2)6@C12DPAG3-G2 complexes with diameters of
(60Æ20) nm have been observed in toluene at a concentra-
tion of 0.5 mm[24] (Figure 8a), which was not observed in the
case of C12DPAG3-G2 (DLS diameter: 3 nm) or
(SnCl2)6@C12DPAG3-G2 complex (DLS diameter: (4Æ
2) nm) in toluene. In addition to C12DPAG3-G2,
C12DPAG3-G1 and C12DPAG3er complexed with 2–
6 equivalents of CuCl2 also showed a similar assembly with
a single peak in DLS and good reproducibility (Table S1).
Although the assembly of C12DPAG3er was unexpected,
C12DPAG3er would become a cis-type conformation of the
first layer imine, which allows the intermolecular cross-link-
ing by six equivalents of CuCl2. In contrast, C12DPAG1er,
C12DPAG2er, C12DPAG3-G3 C12DPAG4er did not show
any clear aggregates with a single DLS peak. These results
suggest that the dendritic branch of C12DPAG3 is important
for assembly. For the further investigation into the aggre-
gates structure, the TEM image in Figure 9a showed that
the (CuCl2)6@C12DPAG3-G2 complexes had vesicular
structures, the diameters of which corresponded with the
DLS results (Figure 9 and Figure S12 in the Supporting In-
formation). Figure 9b shows spherical aggregates with a
small transparent part in the center, which denotes a spheri-
Figure 5. A 3D model of C12DPAG3-G2 optimized by a MOPAC (AM1
MOZYME) calculation.
without staining (Figure S10). These AFM and TEM results
show that C12DPAG3-G2 could make a fibrous molecular
assembly of DPA owing to their asymmetric p-conjugated
structure.
For further investigation into the self-assembly behavior,
we utilized the metal complexation of DPA. Previously, our
group reported the application of a copper/DPA complex as
an effective catalysis.[22] Copper complexes are often used
for building supramolecular structures with square-type co-
ordination. We confirmed the complexation behavior of
C12DPAG3-G2 and C12DPAG3er with CuCl2. In a 1:1 mix-
ture of toluene/acetonitrile, UV/Vis spectroscopy was run
during a titration using CuCl2. Three shifts in the position of
the isosbestic points were observed during the addition of
10 equivalents of CuCl2 to C12DPAG3-G2, which indicated
stepwise complexation upon CuCl2 addition (Figure 7). The
number of equivalents of CuCl2 required to induce a shift
was commensurate with the number of imine sites present
in each layer of C12DPAG3-G2. The spectra of
C12DPAG3-G2 gradually changed with an isosbestic point
at 341 nm observed up to the addition of 2 equivalents of
CuCl2, but shifted to 337 nm between 3 and 6 equivalents.
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Phenylazomethine Dendrimers
FULL PAPER
Figure 7. UV/Vis spectra of C12DPAG3-G2 with 0–10 equivalents of
CuCl2 and isosbestic points during the addition of 0–2 (at 341 nm), 3–6
(at 337 nm), and 7–10 equivalents (at 339nm) of CuCl2 after subtraction
of the CuCl2 absorption. [C12DPAG3-G2]=2.0ꢃ10À4 m. The schematic
representation shows the stepwise radial complexation of C12DPAG3-G2
with CuCl2.
Figure 6. a) Image (5 mmꢃ5 mm) of C12DPAG3-G2 from tapping-mode
AFM on mica prepared from a dendrimer solution (20 mgmLÀ1) in hep-
tane, and b) proposed assembly structure of C12DPAG3-G2 using 3D
models.
cal or bowl-like vesicle,[25] also supported by AFM measure-
ments (Figure S13).
Figure 8. DLS measurement of 0.5 mm (CuCl2)6@C12DPAG3-G2 a) in
toluene after evaporation and sonication, and b) in 1:1 toluene/acetoni-
trile.
As shown for a previously reported [CuCl
2ACHTUNGTRENUN(NG imine)2] com-
plex with square-planar coordination,[22] the assembly was
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K. Yamamoto et al.
a template is expected to be utilized for novel electronic
materials or biomimetic materials.
Experimental Section
Methods: The UV/Vis spectra were recorded by using a Shimadzu UV-
3100PC spectrometer with a sealed quartz cell (optical path length: 1 cm)
at 208C. The details of the UV/Vis titration method are described below.
The NMR spectra were recorded by using a JEOL JMN400 FT-NMR
spectrometer operating at 400 (1H) or 100 MHz (13C) in a 1:1 mixture of
CDCl3/CD3CN with tetramethylsilane (TMS) as the standard. The
1H NMR chemical shifts were referenced to the solvent peak (CDCl3:
d=7.26 ppm) and TMS (d=0 ppm) was used as the internal standard.
The 13C NMR chemical shifts were referenced to the solvent peak
(CDCl3: d=77.0 ppm). MALDI-TOF MS data were obtained by using
KOMPACT MALDI (Shimadzu/Kratos) and Ultraflex MALDI TOF
mass spectrometers (Bruker Daltonics) in the positive ion mode. Dithra-
nol was used as the matrix. Preparative-scale gel permeation chromatog-
raphy (preparative GPC) was performed using a recycling preparative-
scale HPLC LC-908 instrument (Japan Analytical Industry).
Figure 9. TEM image of C12DPAG3-G2 complexed with six equivalents
of CuCl2 prepared by the drop casting of a toluene solution (5.0ꢃ10À4 m)
a) without staining (scale bar is 100 nm), and b) with RuO4 vapor staining
(scale bar is 50 nm).
due to the intermolecular cross-linking through CuCl2,[23]
which was supported by no aggregation in the case of
(SnCl2)6@C12DPAG3-G2. While no aggregation was ob-
served in a 1:1 solution of toluene/acetonitrile (Figure 8b),
the cross-linking of copper chlorides is accelerated by the
evaporation of acetonitrile. The solid film of
(CuCl2)6@C12DPAG3-G2 showed the XRD pattern at 10–
308 (Figure S14), in which C12DPAG3-G2 had no XRD
peaks under the same conditions. The XRD peaks suggest
the partial crystallization of CuCl2 for the intermolecular
cross-linking of C12DPAG3-G2. CuCl2 cross-linking would
give the DPA oligomers, which have soft and rigid units
complexed with CuCl2 to build vesicles. In addition, the
CuCl2 complexed unit enhances the DPA polarity in mole-
cule or cuprophilic interactions[26] for vesicular assembly.
Different from the fibrous assembly observed in
C12DPAG3-G2, copper complexation could build up the ve-
sicular structure based on DPA.
TEM images were obtained by using a TECNAI G2 (FEI) microscope at
an acceleration voltage of 100 kV. The TEM samples were prepared by
depositing a solution of AuCl3-complexed C12DPAG4 in heptane on
carbon-coated copper grids. Dynamic light scattering (DLS) measure-
ments were performed by using a Malvern/Sysmex High Performance
Particle Sizer HPP5001 instrument. The DLS samples were prepared by
filtering the solutions of C12DPAs through a membrane filter (0.2 mm)
into a clean quartz cell. Analytical size-exclusion chromatography (SEC)
was performed by using an HPLC instrument (Shimazu, LC-10AP)
equipped with a TSK-GEL CMHXL column (Tosoh) at the flow rate of
1 mLminÀ1. The detection line was connected to a triple detector (Visco-
tek, Tri-SEC model 302). The molecular mechanics calculations and opti-
mizations of the molecular conformation were done on a Cache Worksys-
tem ver. 5.04 (Fujitsu) employing the AM1-MOZYME or MM3 program.
AFM images were obtained by using an SPA-400 microscope in the tap-
ping mode with an SPI3800N controller (Seiko Instruments Industry).
The cantilevers were of the type SI-DF40P (spring constant: 58 NmÀ1
,
,
resonance frequency: 333 kHz) and SI-DF20 (spring constant: 12 NmÀ1
resonance frequency: 128 kHz) from Seiko Instruments Industry. The X-
ray diffraction patterns were obtained by using D8 ADVANCE
a
(Bruker, 40 kV, 40 mA) diffractor with CuKa radiation at room tempera-
ture. XRD samples were prepared as films by the evaporation of a solu-
tion in toluene cast on a glass substrate.
Conclusion
We have synthesized various new dendritic poly(phenylazo-
methine)s with aliphatic end groups that are able to com-
plex with metals in a stepwise fashion. We have succeeded
in synthesizing C12DPAs up to the fourth generation, for
the first time, which has an environmental responsiveness
and can disperse the Au complexes or Rose Bengal in hep-
tane as a unique hydrophobic nanocapsule. Furthermore, we
found a suitable structure and conditions for the self-assem-
bly of DPA through the precise design of its dendritic struc-
ture. Specifically, asymmetric C12DPAG3-G2 was found to
be suitable for fibrous self-assembly, which was different
from previous DPAs. Interestingly, the vesicular assembly of
C12DPA has been observed through CuCl2 complexation on
DPA imine. The properties of environmental responsiveness
and self-assembly are present for DPA as a p-conjugated
soft material. We can also show the possible development of
the DPA function of precise metal assembly for hierarchical
structures. The metal–organic hybrid material using DPA as
Chemicals: All DPAs were synthesized by
a previously reported
method.[27] The benzoyl ester protected DPAs (Ester DPA) and hydroxy-
modified DPA (OH-DPA) were synthesized as previously reported.[9] De-
hydrated chloroform was purchased from Wako Pure Chemical Indus-
tries, whereas dehydrated acetonitrile and toluene were purchased from
Kanto Kagaku. The Rose Bengal lactone and AuCl3 were purchased
from Aldrich. All other chemicals were purchased from Kanto Kagaku.
General method for UV/Vis titration: Solutions of DPAG4 (chloroform/
acetonitrile=1:1, 5.0ꢃ10À6 m) and metal chloride (acetonitrile, 5.0
ꢃ10À3 m) were prepared in volumetric flasks. Metal chloride solution
(3 mL; i.e., 1 equiv for DPAG4) was added to a quartz cell containing the
DPAG4 solution (3.0 mL). The UV/Vis spectra were measured until
agreement was obtained with the previously recorded spectrum. Meas-
urements were repeated after each addition of metal chloride solution
(3 mL) for the UV/Vis titration. After the UV/Vis spectra measurements,
the absorption of the metal chloride itself was subtracted to find the
DPA absorption changes and isosbestic points.
Preparation of CuCl2/C12DPAG3-G2 vesicles: C12DPAG3-G2 was dis-
solved in toluene (1 mL) at 0.5 mm, and a solution of CuCl2 (6 equiv) in
acetonitrile was added. After complexation, the solution was evaporated
by a flow N2 to remove the acetonitrile. The solid complex was dissolved
in toluene and the solution was sonicated for 10 min in a water bath.
806
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 800 – 809

Phenylazomethine Dendrimers
FULL PAPER
Synthesis of C12DPAG1: OH-DPAG1 (57 mg, 0.11 mmol), 1-iododode-
cane (211 mg, 0.711 mmol), and K2CO3 (920 mg) were placed in a round-
bottomed flask, and DMF (5 mL) was added. The reaction mixture was
heated in an oil bath at 908C. The color of the reaction mixture turned
from yellow to orange. After 14 h, the reaction mixture was filtered and
concentrated. C12DPAG1 was isolated by silica gel column chromatogra-
phy (chloroform/ethyl acetate/hexane 3:1:3) and preparative-scale GPC
(90 mg, 0.18 mmol, 68%). 1H NMR (400 MHz, CDCl3, 258C, TMS): d=
7.64 (d, J=8.3 Hz, 4H), 6.98 (d, J=8.8 Hz, 4H), 6.87 (d, J=8.8 Hz, 4H),
6.73 (d, J=8.3 Hz, 4H), 6.52 (s, 4H), 3.99 (t, J=6.3 Hz, 4H), 3.94 (t, J=
6.6 Hz , 4H), 1.78 (t, J=6.3 Hz, 8H), 1.45–1.43 (m, 8H), 1.28–1.25 (m,
64H), 0.88 ppm (t, J=6.1 Hz, 12H); 13C NMR (100 MHz, CDCl3, 258C,
TMS): d=161.1, 159.1, 147.0, 132.9, 132.2, 131.4, 131.0, 128.4, 121.7, 113.9,
113.6, 68.1, 67.9, 31.9, 29.66, 29.63, 29.61, 29.58, 29.55, 29.43, 29.36, 29.33,
29.3, 29.2, 26.1, 26.0, 22.7, 14.1 ppm; MALDI-TOF MS: m/z: 1174.2
[M+H]+; elemental analysis calcd (%) for C80 H120N2O4: C 81.86, H
10.30, N 2.39; found: C 81.74, H 10.37, N 2.28.
(100 MHz, CDCl3, 258C, TMS): d=168.5, 167.9, 161.3, 159.4, 155.6,
152.4, 132.3, 132.2, 131.3, 131.2, 130.8, 130.2, 120.7, 113.9, 113.7, 113.5,
68.2, 68.0, 32.0, 29.7, 29.68, 29.65, 29.62, 29.47, 29.44, 29.40, 29.2, 26.13,
26.07, 22.7, 14.2 ppm; MALDI-TOF MS: m/z: 2732.2 [M+H]+; elemental
analysis calcd (%) for C187H256N6O9: C 82.21, H 9.44, N 3.08; found: C
82.65, H 9.68, N 3.20.
Synthesis of C12DPAG4: C12DPAG3on (599 mg, 99.4 mmol), NH2-
DPAG1 (13.6 mg, 27.5 mmol), and DABCO (2.00 g, 18.2 mmol) were
placed in a round-bottomed flask equipped with a three-necked adapter
(reflux condenser, dropping funnel, rubber septa seal), then dissolved in
chlorobenzene (50 mL). Under an N2 atmosphere, the reaction mixture
was heated in an oil bath at 1208C, then chlorobenzene (5 mL) and tita-
nium(IV) tetrachloride (0.5 mL, 4.56 mmol) were added by using an ad-
dition funnel. The titanium(IV) tetrachloride solution was added drop-
wise. After reacting for 12 h, the reaction mixture was cooled to room
temperature and vigorously stirred in air to quench the reaction. The pre-
cipitate was removed by filtration and the filtrate was concentrated. The
same procedure for the condensation reaction and quenching was repeat-
ed once using the filtrate. C12DPAG4 was isolated by silica gel column
chromatography (chloroform/ethyl acetate/hexane 1.5:1:1.5) and prepara-
tive-scale GPC. (43 mg, 3.8 mmol, 14%). 1H NMR (400 MHz, CDCl3,
258C, TMS): d=7.65–7.62 (m, 32H), 7.47–7.45 (m, 24H), 7.04–6.37 (brm,
188H), 3.86–3.73 (m, 64H), 1.75–1.26 (brm, 640H), 0.86 ppm (m, 96H);
13C NMR (100 MHz, CDCl3, 258C, TMS): d=168.0, 167.6, 167.3, 162.3,
161.2, 159.3, 159.0, 154.1, 153.4, 152.4, 152.3, 137.5, 134.3, 134.0, 132.2,
132.0, 131.1, 130.7, 130.1, 129.9, 128.6, 128.1, 127.8, 127.7, 125.4, 124.7,
121.6, 120.6, 120.2, 113.8, 113.7, 113.4, 68.2, 68.0, 67.9, 38.7, 31.9, 30.4,
30.3, 29.62, 29.58, 29.4, 29.3, 29.2, 26.0, 22.68, 22.67, 14.1 ppm; MALDI-
TOF MS: m/z: 11345.1 [M+H]+.
Synthesis of C12DPAG2: OH-DPAG2 (74 mg, 57.5 mmol), 1-iododode-
cane (270 mg, 0.71 mmol), and K2CO3 (920 mg) were placed in a round-
bottomed flask, then DMF (5 mL) was added. The reaction mixture was
heated in an oil bath at 908C. After 14 h, the reaction mixture was fil-
tered and concentrated. C12DPAG2 was isolated by silica gel column
chromatography (chloroform/ethyl acetate/hexane 3:1:3 with 1% triethyl-
amine) and preparative-scale GPC (169 mg, 51.2 mmol, 90%). 1H NMR
(400 MHz, CDCl3, 258C, TMS): d=7.68 (d, J=8.8 Hz, 8H), 7.64 (d, J=
8.8 Hz,8H), 7.50 (d, J=8.8 Hz, 4H), 7.06 (d, J=8.3 Hz, 4H), 6.91–6.87
(m, 8H), 6.78–6.76 (m, 4H), 6.71–6.68 (m, 8H), 6.54 (d, J=8.3 Hz, 4H),
6.42 (s, 4H), 4.00–3.98 (m, 8H), 3.92 (t, J=6.3 Hz, 4H), 3.81 (t, J=
6.3 Hz, 4H), 1.80–1.77 (m, 16H), 1.59 (d, J=7.3 Hz, 4H), 1.44 (m, 12H),
1.27–1.24 (m, 128H), 0.88–0.87 ppm (m, 24H); 13C NMR (100 MHz,
CDCl3, 258C, TMS): d=167.82, 167.5, 167.44, 161.4, 161.3, 159.4, 153.9,
152.3, 147.1, 134.6, 132.4, 132.3, 131.5, 131.3, 131.2, 130.6, 130.5, 130.0,
128.0, 127.8, 121.6, 120.7, 120.3, 113.9, 113.7, 113.5, 68.1, 67.9, 31.9, 29.8,
29.63, 29.61, 29.58, 29.55, 29.44, 29.37, 29.35, 29.32, 29.23, 29.17, 26.04,
26.00, 22.66, 14.1 ppm; MALDI-TOF-MS: m/z: 2625.55 [M+H]+; ele-
mental analysis calcd (%) for C180 H252N6O8: C 82.27, H 9.67, N 3.20;
found: C 82.11, H 9.81, N 3.18.
Synthesis of C12DPAG3-G1: C12DPAG3on (140 mg, 53 mmol), 4-amino-
modified Half DPAG1 (NH2-Half DPAG1; 14.3 mg, 53 mmol) and
DABCO (450 mg, 4 mmol) were placed in
a round-bottomed flask
equipped with a three-necked adapter, then dissolved in chlorobenzene
(15 mL). Under an N2 atmosphere, the reaction mixture was heated in an
oil bath at 1258C, then chlorobenzene (5 mL) and titanium(IV) tetra-
chloride (0.1 mL, 0.8 mmol) were added by using an addition funnel. The
titanium(IV) tetrachloride solution was added dropwise. After an addi-
tional 3 h, the reaction mixture was cooled to room temperature and vig-
orously stirred in air to quench the reaction. The precipitate was re-
moved by filtration and the filtrate was concentrated. C12DPAG3-G1
was isolated by silica gel column chromatography (chloroform/ethyl ace-
tate/hexane 3:1:3 with 1% triethylamine) and preparative-scale GPC.
(84 mg, 28 mmol, 53%). 1H NMR (400 MHz, CDCl3, 258C, TMS): d=
7.69–6.41 (brm, 70H), 4.01–3.69 (m, 16H), 1.80–1.64 (m, 16H), 1.40–1.29
(m, 144H), 0.88–0.87 ppm (m, 24H); 13C NMR (100 MHz, CDCl3, 258C,
TMS): d=168.4, 168.3, 167.9, 167.7, 167.5, 161.4, 159.4, 159.2, 154.3,
154.1, 152.7, 151.8, 147.2, 146.6, 140.0, 136.4, 134.4, 134.5, 132.3, 132.1,
131.5, 131.3, 131.2, 130.7, 130.3, 130.2, 130.0, 129.7, 129.6, 129.3, 128.4,
128.0, 127.9, 127.8, 121.9, 121.2, 120.8, 120.6, 120.3, 113.9, 113.8, 113.6,
113.5, 68.1, 67.9, 31.9, 29.63, 29.61, 29.57, 29.54, 29.4, 29.3, 29.21, 29.17,
26.0, 22.7, 14.1 ppm; MALDI-TOF MS: m/z: 2987.1 [M+H]+; elemental
analysis calcd (%) for C206H270N8O8: C 82.85, H 9.11, N 3.75; found: C
83.13, H 9.18, N 3.76.
Synthesis of C12DPAG3: OH-DPAG3 (69 mg, 24.2 mmol), 1-iododode-
cane (270 mg, 0.71 mmol), and K2CO3 (920 mg) were placed in a round-
bottomed flask, then DMF (5 mL) was added. The reaction mixture was
heated in an oil bath at 908C. After 24 h, the reaction mixture was fil-
tered and concentrated. The dodecyl DPAG3 was isolated by silica gel
column chromatography (chloroform/ethyl acetate/hexane 3:1:3) and
preparative-scale GPC (107 mg, 19.4 mmol, 80%). 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=7.66 (d, J=7.3 Hz, 8H), 7.62–7.60 (m, 8H),
7.47–7.45 (m, 12H), 7.04 (d, J=7.3 Hz, 8H), 6.93–6.84 (brm, 24H), 6.77–
6.67 (m, 32H), 6.63 (d, J=8.3 Hz, 4H), 6.53 (d, J=7.3 Hz, 8H), 6.48 (d,
J=8.8 Hz, 4H), 6.44 (d, J=8.3 Hz, 4H), 6.40 (s, 4H), 3.91–3.73 (m,
32H), 1.82–1.59 (m, 32H), 1.25–1.23 (m, 288H), 0.87 ppm (t, J=3.7 Hz,
48H); 13C NMR (100 MHz, CDCl3, 258C, TMS): d=168.2, 167.8, 167.6,
167.5, 167.1, 162.4, 161.4, 159.4, 159.2, 154.3, 154.2, 153.9, 152.6, 152.3,
151.9, 146.9, 134.2, 132.4, 132.2, 131.3, 131.2, 130.8, 130.5, 130.2, 130.0,
129.8, 127.9, 127.8, 121.8, 120.8, 120.3, 113.93, 113.88, 113.79, 113.6, 68.2,
68.1, 67.8, 31.9, 31.7, 29.63, 29.61, 29.58, 29.55, 29.4, 29.3, 29.2, 26.0, 22.7,
14.1 ppm; MALDI-TOF MS: m/z: 5534.6 [M+H]+; elemental analysis
calcd (%) for C380H516N14O16: C 82.44, H 9.39, N 3.54; found: C 82.35, H
9.35, N 3.51.
Synthesis of C12DPAG3-G2: C12DPAG3on (800 mg, 293 mmol), NH2-
Half DPAG2 (185 mg,293 mmol) and DABCO (450 mg, 4 mmol) were
placed in a round-bottomed flask equipped with a three-necked adapter,
then dissolved in chlorobenzene (30 mL). Under an N2 atmosphere, the
reaction mixture was heated in an oil bath at 1258C, then chlorobenzene
(5 mL) and titanium(IV) tetrachloride (0.1 mL, 0.8 mmol) were added by
using an addition funnel. The titanium(IV) tetrachloride solution was
added dropwise. After an additional 3 h, the reaction mixture was cooled
to room temperature and vigorously stirred in air to quench the reaction.
The precipitate was removed by silica filtration and the filtrate was con-
centrated. C12DPAG3-G2 was isolated by silica gel column chromatogra-
phy (chloroform/ethyl acetate/hexane 3:1:3 with 1% triethylamine) and
preparative-scale GPC. (717 mg, 214 mmol, 73%). 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=7.79–6.48 (brm, 88H), 3.97–3.85 (m, 16H), 1.83–
1.74 (m, 16H), 1.50–1.33 (m, 144H), 0.93 ppm (d, J=6.8 Hz, 24H);
Synthesis of C12DPAG3on: OH-DPAG3on (530 mg, 0.38 mmol), 1-iodo-
dodecane (1414 mg, 4.77 mmol), and K2CO3 (920 mg) were placed in a
round-bottomed flask, then DMF (5 mL) was added. The reaction mix-
ture was heated in an oil bath at 908C. After 14 h, the reaction mixture
was filtered and concentrated. C12DPAG3on was isolated by silica gel
column chromatography (chloroform/ethyl acetate/hexane 3:1:3 with 1%
triethylamine) and preparative-scale GPC. (920 mg, 0.34 mmol, 88%).
1H NMR (400 MHz, CDCl3, 258C, TMS): d=7.67–7.59 (m, 12H), 7.52 (d,
J=7.8 Hz, 4H), 7.06 (d, J=6.3 Hz, 4H), 6.93–6.89 (m, 16H), 6.76–6.73
(m, 16H), 6.57 (d, J=6.8 Hz, 4H), 3.95–3.89 (m, 16H), 1.78–1.71 (m,
16H), 1.44–1.26 (m, 144H), 0.87 ppm (t, J=2.9 Hz, 24H); 13C NMR
Chem. Eur. J. 2011, 17, 800 – 809
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
807

K. Yamamoto et al.
13C NMR (100 MHz, CDCl3, 258C, TMS): d=168.4, 168.0, 167.7, 167.3,
167.1, 161.3, 161.2, 159.4, 159.3, 159.1, 153.1, 151.5, 146.8, 139.3, 139.2,
135.7, 135.6, 134.81, 132.2, 131.9, 131.2, 130.6, 130.1, 129.8, 129.34, 129.26,
128.6, 128.0, 127.9, 127.83, 127.75, 127.6, 121.7, 121.5, 120.7, 120.3, 120.1,
113.8, 113.7, 113.4, 67.9, 67.8, 31.8, 29.52, 29.48, 29.46, 29.3, 29.2, 29.1,
25.93, 25.89, 22.6, 14.0 ppm; MALDI-TOF MS: m/z: 3344.5 [M+H]+, el-
emental analysis calcd (%) for C232H288N10O8: C 83.31, H 8.68, N 4.19;
found: C 83.51, H 8.80, N 4.11.
2107, no. 21108009) from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan, Mitsubishi and Sumitomo Foundation. We
extend our gratitude to Dr. K. Takanashi for his help with synthesizing
Ester DPAs. We also thank Dr. T. Imaoka and Dr. K. Albrecht for their
useful suggestions. Y.O. was supported by the “Doctor-21” scholar pro-
gram of the Yoshida Scholarship Foundation and Keio Leading-Edge
Laboratory of Science and Technology Research Grant for Ph.D. Pro-
gram.
Synthesis of C12DPAG3-G3: C12DPAG3on (139 mg, 51 mmol), NH2-
Half DPAG3 (68.5 mg, 51 mmol), and DABCO (450 mg, 4 mmol) were
placed in a round-bottomed flask equipped with an three-necked adapter,
then dissolved in chlorobenzene (15 mL). Under an N2 atmosphere, the
reaction mixture was heated in an oil bath at 1258C, then chlorobenzene
(5 mL) and titanium(IV) tetrachloride (0.1 mL, 0.8 mmol) were added by
using an addition funnel. The titanium(IV) tetrachloride solution was
added dropwise. After an additional 3 h, the reaction mixture was cooled
to room temperature and vigorously stirred in air to quench the reaction.
The precipitate was removed by silica filtration and the filtrate was con-
centrated. C12DPAG3-G3 was isolated by silica gel column chromatogra-
phy (chloroform/ethyl acetate/hexane 3:1:3 with 1% triethylamine) and
preparative-scale GPC. (171 mg, 42 mmol, 83%). 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=7.76–6.43 (m, 124H), 4.00–3.68 (m, 16H), 1.79–
1.66 (m, 16H), 1.39–1.29 (m, 144H), 0.86 ppm (t, J=6.6 Hz, 24H);
13C NMR (100 MHz, CDCl3, 258C, TMS): d=169.0, 168.7, 168.4, 168.3,
168.2, 168.1, 168.0, 167.9, 161.3, 159.4, 159.2, 153.7, 153.59, 152.64, 152.00,
151.8, 146.9, 139.3, 139.1, 139.0, 135.8, 135.7, 134.5, 134.4, 132.3, 132.1,
131.3, 131.2, 130.8, 130.6, 130.3, 130.0, 129.7, 129.4, 129.3, 129.2, 128.8,
128.2, 128.0, 127.9, 127.8, 127.7, 121.8, 120.8, 120.5, 120.3, 119.9, 113.9,
113.7, 113.6, 113.5, 68.1, 67.9, 31.9, 29.60, 29.58, 29.54, 29.51, 29.37, 29.33,
29.29, 29.2, 26.0, 22.6, 14.1 ppm; MALDI-TOF MS: m/z: 4060.7 [M+H]+,
elemental analysis calcd (%) for C284H324N14O8: C 83.98, H 8.04, N 4.83;
found: C 83.60, H 8.26, N 4.61.
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then dissolved in chlorobenzene (25 mL). Under an N2 atmosphere, the
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(5 mL) and titanium(IV) tetrachloride (0.2 mL, 1.8 mmol) were added by
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tography (chloroform/ethyl acetate/hexane 1:1:1 with 1% triethylamine)
and preparative-scale GPC. (586 mg, 940 mmol, 47%).
NH2-Half DPAG1: 1H NMR (400 MHz, CDCl3, 258C, TMS): d=7.71 (d,
J=8.0 Hz, 2H), 7.46–7.26 (m, 6H), 7.13 (dd, J=8.0, 2.0 Hz, 2H), 6.58 (d,
J=8.0 Hz, 2H), 6.49 (d, J=8.0 Hz, 2H), 3.48 ppm (s, 2H); MALDI-TOF
MS: m/z: 272.76 [M+H]+.
NH2-Half DPAG2: 1H NMR (400 MHz, CDCl3, 258C, TMS): d=7.74 (d,
J=8.0 Hz, 4H), 7.50–7.39 (m, 10H), 7.30–7.26 (m, 4H), 7.14 (dd, J=8.0,
2.0 Hz, 2H), 7.08 (d, J=8.0 Hz, 2H), 6.86 (d, J=8.0 Hz, 2H), 6.70 (d, J=
8.0 Hz, 2H), 6.59 (d, J=8.0 Hz, 2H), 6.50 (d, J=8.0 Hz, 2H), 6.44 (d, J=
8.0 Hz, 2H), 3.48 ppm (s, 2H); MALDI-TOF MS: m/z: 631.32 [M+H]+.
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NH2-Half DPAG3: 1H NMR (400 MHz, CDCl3, 258C, TMS): d=7.74–
6.44 (m, 68H), 3.59 ppm (s, 2H); MALDI-TOF MS: m/z: 1349.01
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Acknowledgements
This work was supported by CREST of the Japan Science and Technolo-
gy Agency, by Grants-in-Aid for Scientific Research (nos. 19205020,
19022034), and an Innovative Area (Coordination Programming, Area
808
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Received: September 13, 2010
Published online: December 16, 2010
Chem. Eur. J. 2011, 17, 800 – 809
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