Self-assembled monolayers of metal-assembling dendron thiolate formed from dendrimers with a disulfide core

Article abstract of DOI:10.1021/ol900137r

Novel phenylazomethine dendrimers with a disulfide core (SS-DPA G1 -4) were synthesized in nearly quantitative yields. Although the disulfide core is shielded by the rigid dendron shell, direct formation of the self-assembled monolayers of metal-assemblin

Full text of DOI:10.1021/ol900137r

ORGANIC
LETTERS
2009
Vol. 11, No. 8
1729-1732
Self-Assembled Monolayers of
Metal-Assembling Dendron Thiolate
Formed from Dendrimers with a
Disulfide Core
Norifusa Satoh and Kimihisa Yamamoto*
Department of Chemistry, Faculty of Science & Technology, Keio UniVersity,
Yokohama 223-8522, Japan
yamamoto@chem.keio.ac.jp
Received January 21, 2009
ABSTRACT
Novel phenylazomethine dendrimers with a disulfide core (SS-DPA G1-4) were synthesized in nearly quantitative yields. Although the disulfide
core is shielded by the rigid dendron shell, direct formation of the self-assembled monolayers of metal-assembling dendron thiolate was
observed by XPS and electrochemical reduction of the self-assembled monolayer substrates. The dendrimers showed a similar metal-assembling
manner with other derivatives. The metal assembly to the self-assembled monolayers of metal-assembling dendron thiolate was also confirmed.
Self-assembled monolayers (SAMs) have attracted much
attention for designing functional surfaces in the fields of
chemical and biochemical sensing and nanopatterning for
molecular electronics.1 In particular, the combination of
SAMs with metal assembly on the surfaces has been applied
to electron transporters and sensors.2 Additionally, dendrim-
ers have been strictly used as a functional surfactant because
of their regulated structure,3,4 which allow the construction
of finely controlled surfaces.
of the dendron thiolate SAMs from the dendrimers. The
π-conjugated structure would improve the thermostability5
and electron transport6,7 in the SAM structure. Additionally,
we found that the SS-DPAs showed a stepwise metal
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We now report the synthesis of phenylazomethine den-
drimers with a disulfide core (SS-DPAs) and direct formation
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10.1021/ol900137r CCC: $40.75
Published on Web 03/12/2009
2009 American Chemical Society

assembly.8 These properties should allow the construction
of novel functional surfaces.
Scheme 1. Synthesis of SS-DPAs
The phenylazomethine dendrimers (DPAs) with a phenyl-
based core were synthesized up to the fifth generation by a
convergent method via the reaction of aromatic ketones with
aromatic amines.9 The DPA derivatives are synthesized by
the reaction between the DPA dendrons containing an
aromatic ketone and the core compound having some
aromatic amines.7a,d,10 Thus, we dehydrated di-p-diami-
nophenyldisulfide with the DPA dendrons, G1-4, in the
presence of titanium(IV) tetrachloride and 1,4-
diazabicyclo[2.2.2]octane (DABCO) to synthesize the di-
sulfide core dendrimers (SS-DPA G1-4, Scheme 1).
The MALDI-TOF mass spectra of the dendrimers showed
two peaks of the calculated mass and half the mass (Table
1 and Figure S3a). This fact strongly indicates that the
dendrimers have a disulfide unit at the core.11 In contrast,
size exclusion chromatography (SEC) of the dendrimers
showed a single peak (Mw/Mn ) 1.02, Table 1 and Figure
S3b, Supporting Information). Additionally, the absolute
molecular weight, analyzed by a triple detector (a differential
viscometer, laser light scattering, and an RI detector:
Viscotek Corp.),12 agrees with the calculated mass (Table
1). Thus, we determined that the obtained dendrimers have
a single molecular weight and a disulfide unit.
The yield of all the generations of the dendrimers was
almost quantitative;13 this result means that the contact for
dehydration between the aromatic amines of the core and
the aromatic ketone of the dendron would not be three-
dimensionally hindered by the growth of the dendron size
because of the free rotation of the disulfide bond. The yield
of the previous dendrimer derivatives having a rigid core,
such as a phenyl and tris(thienylphenyl)amine core (DPAs
and TPA-DPAs), decreases with the generation number.7a,d,10
The molecular structure of the dendritic molecules closely
relates to the intrinsic viscosity ([η]).14 Figure S4a (Sup-
porting Information) shows Mark-Houwink plots of the SS-
DPAs (Mark-Houwink-Sakurada equation: [η] ) K Ma).
The slopes of these plots (Mark-Houwink exponent; a)
gradually change but do not reach zero. In contrast, we have
confirmed that the Mark-Houwink exponent of the higher
generations of DPAs and TPA-DPAs clearly changes to
nearly zero, which means that these dendrimers have a rigid
spherical structure like a globular protein.7a,d Thus, we can
conclude that only one flexible unit at the core dramatically
changes the structural conformation of the rigid dendrimers.
The changes in the structural properties were also observed
in the hydrodynamic radii (Rh) of these dendrimers, obtained
from the triple detector analysis (Table 1 and Figure S4b.
Supporting Information). The radius linearly increased with
the generation number because of the rigid branched units.
The generational increase in the radius is consistent with the
modeling size of the branched units. Thus, the rigid molecular
chain would radially grow and shield the core (the modeling
structure of SS-DPA G4 is shown in Figure S5, Supporting
Information).15,16 However, the difference in the radii
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Org. Lett., Vol. 11, No. 8, 2009

Figure 2. (A) STM image and (B) the cross section of SAM
fabricated from SS-DPA G4; I ) 50 pA, Vsample ) 1.0 V.
Figure 1. Schemetic representation of stepwise complexation into
2,5-dichlorophenyl cores.18 However, the “reversed” pattern
of complexation was observed in only SS-DPA G2 because
of the very weak electron withdrawing nature of the disulfide
core. We confirmed that the complexation to the disulfide
core is never detected by the H NMR spectral changes
(Figure S10, Supporting Information).
the SS-DPAs.
between the DPAs and SS-DPAs is quite small (0.48 Å as
average), compared with the difference in the chemical
structure of these cores.7a,d This also suggests the flexibility
of the disulfide core.
1
For SS-DPA G3 and SS-DPA G4, the radial stepwise
complexation of SnCl2 was observed until a stoichiometric
amount (per imine site) of SnCl2 had been added. Four and
three distinct transitions could be identified at the isosbestic
pointss363 nm, 0-2 equiv; 361 nm, 3-6 equiv; 358 nm,
7-14 equiv; 353 nm, 15-30 equiv for SS-DPA G4 (Figure
S7, Supporting Information); 363 nm, 0-2 equiv; 361 nm,
3-6 equiv; 358 nm, 7-14 equiv for SS-DPA G3 (Figure
S8, Supporting Information), respectivelysindicating that the
complexation proceeds in a stepwise manner rather than
randomly. The existence of an isosbestic point reveals the
quantitative transformation of the compound, so the observa-
tion of four and three distinct isosbestic points suggests the
successive formation of four and three different complexes
upon the addition of SnCl2, respectively. The number of
added equivalents of SnCl2 at each transition is consistent
with the number of imine sites present in the different layers
from the inners position of the SS-DPA G3 and SS-DPA
G4, respectively (Figure 1).
For SS-DPA G2, the two isosbestic points were observed
during the titrations363 nm, 0-4 equiv.; 361 nm, 5-6
equiv. (Figure S9, Supporting Information)sindicating that
the complexation for the second layer was completed first,
and then the first layer was filled (Figure 1). This stepwise
complexation reflects the basicity of the imines in each layer
of the system, which is confirmed in the 13C NMR spectra
(Figure S11, Supporting Information); the high magnetic field
shift of the imine carbon peaks attributed to the electron-
The rigid branched units provide a high thermostability
to the DPA derivatives (Table 1 and Figure S6, Supporting
Information).17 The 10 weight % decreasing temperatures
(Td-10%) and the glass transition temperatures (Tg) of the SS-
DPAs improved with the increase in the generation number,
reaching 505 and 158 °C, respectively. Thus, we noticed that
the SS-DPAs have a high thermostability as SAM molecules,
comparable to engineering plastics. However, the SS-DPAs
have a lower thermostability by 10-20 °C than the DPAs
and the other derivatives having a rigid and dense structure,
such as porprine, with a similar molecular weight.17a
A
similar tendency was also observed in the TPA-DPAs
because of the central space around the core.17b Thus, the
thermostability of the SS-DPAs would decrease due to the
mobility at the disulfide core.
The metal-assembling property of the SS-DPAs, based on
the complexation between the imine ligands and SnCl2 in
this case, was confirmed by spectroscopic measurements
(Figure S7-9, Supporting Information). The absorption band
around 400 nm, attributed to the complexation, increases with
a decrease in the absorption bands around 320 nm that are
attributed to the phenylazomethine unit. During the addition
of SnCl2, we found a stepwise complexation behavior of the
SS-DPAs similar to that of the DPA derivatives having
electron-withdrawing cores, such as the tetrafluorophenyl and
(17) (a) Tanaka, R.; Imaoka, T.; Yamamoto, K. J. Photopolym. Sci. Tech.
2004, 17, 323. (b) Satoh, N.; Cho, J.-S.; Higuchi, M.; Yamamoto, K. J.
Photopolym. Sci. Tech. 2005, 18, 55.
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Chem. Soc. 2003, 125, 9988.
Org. Lett., Vol. 11, No. 8, 2009
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Table 1. Physicochemical Characteristics of SS-DPAs
TOF-MSa
Tri-SECb
TG-DSCc
d
e
SS-DPA
yield (%)
Mcal.
M/z
Mn
Mw/Mn
Rh (nm)
log {[η] (dL/g)}
Td-10% (°C)
Tg (°C)
G1
G2
G3
G4
83
97
99
98
576.77
1293.64
2727.38
5594.86
576.4
1292.4
2727.1
5595.5
287.7
646.1
1363.1
2795.8
580
1290
2760
5600
1.02
1.02
1.03
1.02
0.64
0.93
1.30
1.66
-1.493
-1.374
-1.305
-1.260
-
419
495
505
19
77
128
158
a MALDI-TOF mass spectroscopy. b SEC analyzed with a triple detector. c Thermogravimetry/Differential scanning calorimetry. d Number average molecular
weight. e Molecular weight distribution.
donated imine, showing the faster complexation with SnCl2.18
The carbon peak of the first layer imines of the SS-DPAs
showed a downfield shift when compared with that of the
DPAs. The downfield shift, however, is smaller than that of
the DPA derivatives having a tetrafluorophenyl or 2,5-
dichlorophenyl core. Thus, the carbon peak of the first layer
imines of SS-DPA G2 appeared at a lower field than that of
the second layer ones, although the carbon peak of SS-DPA
G3 shifted to the highest field for the imine peaks because
of the electron donation by the outer layer’s imines. These
phenomena agree with the complexation manner in the SS-
DPAs.
Absorption of the dendrimer on a Au substrate was
observed by AFM (Figure S12, Supporting Information). The
disulfide core as a binding site to form the SAM structure is
shielded by the rigid dendron. Thus, the SAM formation was
delayed by the rigid shell as the generation number increases.
The amounts of the SAM molecules covering on Au
substrates after the 1 h, 2 h, and 3 h soaks in the solution of
SS-DPA G1, G2, and G3 were estimated from the AFM
images; the initial formation rate constants were calculated
to be 0.30, 0.21, and 0.06 h-1 at room temperature,
respectively. The full coverage under SS-DPA G4 conditions
was observed after a 2-days soak (STM image shown in
Figure 2), indicating that the rate constant was estimated to
be ca. 0.02 h-1 at room temperature. The observed molecular
size was estimated to be ca. 3.8 nm, comparable to the
hydrodynamic diameter of the SS-DPA G4. The highly
ordered structure would be obtained by annealing or a change
in the soak conditions.
Formation of the Au-S bond was confirmed by the XPS
spectra and the electrochemical reduction of the fully covered
SAM substrate. The binding energy of S(2p3/2) showed a
difference between the dendrimer cast film (163 eV; Figure
S13a, Supporting Information) and the SAM film (162 eV;
Figure S13b, Supporting Information). This shift indicates
the occurrence of the following reaction: 1/2 R-S-S-R +
Au(surface) f R-S-·Au+(surface). Additionally, the reduction
current of the Au+(surface) was observed at 0.6 V in the cyclic
voltammogram of the SAM substrate (Figure S14a, Sup-
porting Information). After three cycles, the amount of the
binging SAM molecules decreased to 5% because of the
release of the SAM molecules (Figure S14b, Supporting
Information). These results indicate that the dendron thiolate
SAM directly forms from the dendrimers shielding the
disulfide core at the center, although the formation is
disturbed by the rigid shell.
The metal assembly into the SAM structure was observed
in the differential UV-vis spectra after the addition of SnCl2
in acetonitrile (Figure S15, Supporting Information). The
increase in the absorption band (350-550 nm) is character-
istic of the complexation.19 The spectral changes indicate
that the dendron thiolates on Au substrate would maintain
the metal-assembling properties, although the assembling
manner is unclear.
In summary, we have synthesized up to the fourth
generation of the SS-DPAs in near quantitative yields. The
only one flexible unit at the core dramatically affects the
structural conformation of the π-conjugated dendrimers,
resulting in a high yield. The hydrodynamic radius linearly
increases with the generation number because of the rigid
dendron shell. The π-conjugated structure contributes to the
thermostabilities. The UV-vis spectra changes by the
addition of SnCl2 revealed that the metal assembly into the
dendrimers is similar to the other DPA derivatives. It is
supported by the chemical shift in the imine carbon peaks
of the 13C NMR spectra. Similar spectral changes were also
observed for the metal assembly into the dendron thiolate
SAMs.
Acknowledgment. This work was partially supported by
CREST from the Japan Science and Technology Agency,
Grants-in-Aid for Scientific Research (No. 19205020) and
the 21st Center of Excellence (COE) Program (Keio-LCC)
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT).
Supporting Information Available: Experimental details,
synthetic procedures, NMR spectra, TOF-MS spectra, SEC
charts, Mark-Houwink plots, modeling structure, TG curves,
DSC charts, UV-vis spectra changes, AFM images, XPS
spectra, cyclic voltammogram, and differential UV-vis
spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL900137R
(19) Men, Y.; Higuchi, M.; Yamamoto, K. Sci. Technol. AdV. Mat. 2006,
7, 139.
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