A route to redox-active nanoparticle-cored dendrimers: Post-encapsulation of ferrocene

Article abstract of DOI:10.1246/cl.2006.644

Redox-active nanoparticle-cored dendrimers were synthesized by a stepwise reaction, in which synthesis of functionalized nanoparticles was followed by organic reactions to build dendritic architecture around a nanoparticle core. Incorporation of redox mol

Full text of DOI:10.1246/cl.2006.644

644
Chemistry Letters Vol.35, No.6 (2006)
A Route to Redox-active Nanoparticle-cored Dendrimers: Post-encapsulation of Ferrocene
Young-Seok ShonÃ1 and Daeock ChoiÃ2
1Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101, U.S.A.
2Department of Chemistry, Sunchon National University, Suncheon 540-742, South Korea
(Received March 7, 2006; CL-060276; E-mail: young.shon@wku.edu)
Redox-active nanoparticle-cored dendrimers were synthe-
alcohol. Reduction of these ester-functionalized dendrons using
lithium aluminum hydride (LAH) generated the hydroxy-
sized by a stepwise reaction, in which synthesis of functionalized
nanoparticles was followed by organic reactions to build den-
dritic architecture around a nanoparticle core. Incorporation of
redox molecules on the surface of nanoparticles resulted in the
formation of a redox-active nanoparticle-cored dendrimer.
´
functionalized ‘‘Frechet-type’’ dendron ([G1]-OH). For the syn-
thesis of [G2]-OH, the Mitsunobu coupling reaction of methyl
3,5-dihydroxybenzoate with [G1]-OH followed by LAH reduc-
tion were performed.15
The convergent synthesis of NCDs was accomplished by the
ester coupling reaction of MUA MPCs with excess hydroxy-
´
functionalized ‘‘Frechet-type’’ dendrons ([G2]-OH) in the pres-
Significant progress has been made in research involving the
use of dendritic frameworks to surround functional core mole-
cules.1–6 Dendritic encapsulation of a functional molecule pro-
vides a site isolation which mimics principles from biomaterials
because dendritic scaffolds can provide the segregation of exter-
nal and internal functionality.1–3 The current approaches of den-
dritic encapsulation are mostly based on building dendritic
frameworks to surround active core using iterative reaction
sequence.1,3 Our new strategy provides an alternative way to
core-functionalized dendrimers (Scheme 1), which the nanopar-
ticle core in nanoparticle-cored dendrimers (NCDs)7–11 acts as a
template for redox molecules. Incorporation of electroactive
compounds on the surface of nanoparticles in dendritic architec-
ture should provide an opportunity to tune their electrochemical
properties.11
ence of ester coupling reagents (1,3-dicyclohexylcarbodiimide
(DCC) and 4-dimethylaminopyridine (DMAP)).14 This synthetic
approach provided an access to functionalized NCDs, which
contained multiple reactive functional groups (COOH) inside
dendritic wedges even after the coupling of dendrons. The
unreacted COOH groups were resulted from steric repulsion of
dendrons during the synthesis of NCDs. The incorporation of
redox labels in NCDs was achieved by a simple coupling
reaction of ferrocene methanol with the remaining COOH
groups inside dendritic wedges of NCDs ([G2] NCDs).15
The appearance of a new band at ca. 1735 cmÀ1 in FT-IR
spectroscopy after the reaction indicated a formation of ester
bonds due to successful coupling reaction between 11-mercapto-
undecanoic acids and [G2]-OH. IR spectrum of [G2] NCDs syn-
thesized by convergent method also showed C=C and aromatic
stretching bands at ca. 1660 and ca. 1600 cmÀ1, respectively
(Figure 1).
This was a clear evidence for the attachment of hydroxy-
functionalized dendrons by the coupling reaction. The presence
of the carbonyl band at ca. 1710 cmÀ1 even after the reaction
suggested that the [G2] NCDs still contained unreacted COOH
functional groups in the monolayer. This was due to the steric
repulsion of incoming dendrons by the dendrons attached to
the clusters. After additional ester coupling reaction between
[G2] NCDs and ferrocene methanol, the IR spectrum of the
NCD-encapsulated ferrocene (Fc@NCDs) showed an increase
in the intensity of a band at 1735 cmÀ1 in addition to the strong
We have synthesized NCDs by an approach in which
the synthesis of 11-mercaptoundecanoic acid functionalized
monolayer-protected clusters (MUA MPCs)12–14 was followed
by adding dendrons on MUA MPCs [ꢀAu201(SC6)28-
(SC10COOH)43].15
´
Hydroxy-functionalized ‘‘Frechet-type’’ dendrons were
synthesized by the modification of the previously published
method.2 Briefly, the synthesis started with Mitsunobu coupling
reaction of methyl 3,4,5-trihydroxybenzoate with homoallyl
Scheme 1. Synthesis of nanoparticle-cored dendrimers encap-
sulated ferrocene.
Figure 1. IR spectra of [G2] NCDs and Fc@NCDs.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.6 (2006)
645
band around 1450 cmÀ1 indicating the presence of cyclopenta-
dienyl groups.
the relative absorbance of two carbonyl bands at ca. 1735 and
ca. 1710 cmÀ1 in IR spectra (Figure 1).
The 1H NMR spectrum of [G2]-OH showed the signals
for aromatic protons (6.45–6.65 ppm, 7H), –CH2CH=CH2
(5.80–6.00 ppm, 6H), –CH2CH=CH2 (5.00–5.20 ppm, 12H),
–ArCH2OArCH2OH (4.92 ppm, 4H), –ArOCH2OH (4.64 ppm,
2H), –ArOCH2CH2CH=CH2 (3.90–4.10 ppm, 12H), –CH2CH=
CH2 (2.40–2.60 ppm, 12H). Compared to the NMR spectrum
of [G2]-OH, the spectrum of [G2] NCDs showed the signals
for all these protons with a few different features. Due to the
coupling reaction of [G2]-OH and MUA MPCs, the signals for
ArCH2OH shifted further downfield from 4.64 to 4.86 ppm.
The signals for CH2CO2– were also observed at 2.32 ppm. The
signals for CH2 (1.20–1.40 ppm) and CH3 (0.80–0.90 ppm) were
shown in the spectrum due to the presence of MUA and hexane-
thiolate ligands on the cluster surface. The results also showed
the same peak-broadening effect, as do monolayers of alkane-
thiolate-protected nanoparticles generated from alkanethiols.
This peak-broadening effect clearly suggested that [G2]-OH
was reacted with MUA MPCs and bonded onto the surface
of nanoparticles. The NMR spectrum of Fc@NCDs showed
additional broad bands at ca. 4.10 ppm, which corresponded to
the signals from cyclopentadienyl groups. No additional sharp
resonances corresponding to free ferrocene methanol were
seen.15
Cyclic voltammetry was employed to compare electrochem-
ical behaviors of ferrocene methanol and Fc@NCDs (Figure 2).
Both ferrocene methanol and Fc@NCDs exhibited well-defined
voltammetric peaks corresponding to ferrocenyl groups. From
the voltammogram of ferrocene methanol, it could be seen that
there was a pair of voltammetric waves with a peak splitting
of 233 mV at 100 mV/s indicating quasi-reversible electron-
transfer processes. Interestingly, the peak splitting was found
to be 112 mV at 100 mV/s for Fc@NCDs, which was only half
of that for ferrocene methanol. This might be due to either multi-
electron transfer or adsorption of the particles to the electrode.
In summary, NCDs and redox-active NCDs were synthe-
sized by a stepwise reaction, in which synthesis of functionalized
nanoparticles was followed by organic reactions to build dendrit-
ic architecture and to incorporate redox molecules on the surface
of nanoparticles. The preparation of redox-active NCDs using
new methods represents an important advance to the control
and preparation of new organized nanostructures. Synthesis of
redox-active NCDs with distinct generations and functional
moieties will be attempted for the structural control at the nano-
scopic level. The detailed electrochemical properties of various
redox-active NCDs will be also studied in the near future.
The increased organic fractions (42.43% from 25.95%) were
also observed by thermogravimetric analysis (TGA). The quan-
titative analysis using TGA data suggested that ca. 14 COOH
groups (out of ca. 43) on MUA MPCs have been reacted with
[G2]-OH dendrons. This analysis was in agreement with IR
results, showing the existence of unreacted COOH groups in
the interior of [G2] NCDs. This result suggested that the
produced [G2] NCDs has an average molecular formula of
Au201(SC6)28(SC10COOH)29(SC10COO[G2])14. The reaction
of ferrocene methanol with [G2] NCDs resulted in the incorpo-
ration of ca. 15 ferrocene units per each nanoparticle based on
We thank Research Corporation and Western Kentucky
University for support.
References and Notes
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Figure 2. Cyclic voltammogram of (a) ferrocene methanol
and (b) Fc@NCDs in CH2Cl2. A Pt electrode was used as the
working electrode, a Ag QR wire was used as the quasi reference
electrode, and the counter electrode was a Pt wire. The solution
was deaerated for at least 10 min with nitrogen before data
acquisition and blanketed with a nitrogen atmosphere during
the experiment. (scan rate = 100 mV/s).
Published on the web (Advance View) May 20, 2006; doi:10.1246/cl.2006.644