Controlled storage of ferrocene derivatives as redox-active molecules in dendrimers

Article abstract of DOI:10.1021/ja9064377

Dendritic polyphenylazomethines (DPA) could encapsulate ferroceniums by complexation of the electron-donating skeleton of the DPA imines. Upon addition of ferroceniums to a series of dendritic polyphenylazomethines (DPAGX, where X is the generation number, X = 1-4), the UV-vis spectra showed changes in a manner similar to that observed for the complexation of metal ions with DPAGX. Stepwise shifts in the isosbestic point were consistently observed with the number of imine groups in the first and second layers of the generation-4 dendrimer (DPAG4). DPAG2 and DPAG3 were also found to trap 6 equiv of ferroceniums. To investigate the complexation, UV-vis spectroscopy, ⁵⁷Fe MoIssbauer spectroscopy, electrospray ionization-mass spectroscopy (ESI-MS), cyclic voltammetry (CV), and fluorescence spectroscopy were performed. We confirmed that neutral ferrocenes cannot complex with the imine group while ferroceniums can. Utilizing the redox property of ferrocenes, we were able to electrochemically control the encapsulation and release of ferrocenes into the DPA in a manner similar to redox-responsive proteins such as ferritin. In addition to ferrocenes, oligoferrocenes could also be trapped in the DPA. The biferrocene cation(1+) was particularly suitable for electrochemical switching due to its stable mixed valence condition. The terferrocene dication(2+) encapsulated into DPAG4 could be fabricated into a thin film, which exhibited the near-infrared absorption of an intervalence charge-transfer (IV-CT) band, pointing the way toward the use of such systems in material science.

Full text of DOI:10.1021/ja9064377

Published on Web 03/19/2010
Controlled Storage of Ferrocene Derivatives as Redox-Active
Molecules in Dendrimers
Yousuke Ochi,† Mana Suzuki,† Takane Imaoka,†,§ Masaki Murata,‡
Hiroshi Nishihara,‡ Yasuaki Einaga,† and Kimihisa Yamamoto*,†,§
Department of Chemistry, Faculty of Science and Technology, Keio UniVersity,
Yokohama 223-8522, Japan and Department of Chemistry, School of Science, The UniVersity of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received September 14, 2009; E-mail: yamamoto@res.titech.ac.jp
Abstract: Dendritic polyphenylazomethines (DPA) could encapsulate ferroceniums by complexation of the
electron-donating skeleton of the DPA imines. Upon addition of ferroceniums to a series of dendritic
polyphenylazomethines (DPAGX, where X is the generation number, X ) 1-4), the UV-vis spectra showed
changes in a manner similar to that observed for the complexation of metal ions with DPAGX. Stepwise
shifts in the isosbestic point were consistently observed with the number of imine groups in the first and
second layers of the generation-4 dendrimer (DPAG4). DPAG2 and DPAG3 were also found to trap 6
equiv of ferroceniums. To investigate the complexation, UV-vis spectroscopy, 57Fe Mo¨ssbauer spectroscopy,
electrospray ionization-mass spectroscopy (ESI-MS), cyclic voltammetry (CV), and fluorescence spectros-
copy were performed. We confirmed that neutral ferrocenes cannot complex with the imine group while
ferroceniums can. Utilizing the redox property of ferrocenes, we were able to electrochemically control the
encapsulation and release of ferrocenes into the DPA in a manner similar to redox-responsive proteins
such as ferritin. In addition to ferrocenes, oligoferrocenes could also be trapped in the DPA. The biferrocene
cation(1+) was particularly suitable for electrochemical switching due to its stable mixed valence condition.
The terferrocene dication(2+) encapsulated into DPAG4 could be fabricated into a thin film, which exhibited
the near-infrared absorption of an intervalence charge-transfer (IV-CT) band, pointing the way toward the
use of such systems in material science.
Chart 1. (a) Fourth Generation of Dendritic Polyphenylazomethine
(DPAG4) and (b) Ferrocenium Hexafluorophosphate (FcPF6)
Introduction
Dendrimers1 are highly branched macromolecules often
spherical in geometry that contain a nanospace, making them
highly useful as nanocapsules or nanoreactors. They have a
single molecular weight and unique dendritic structure, in which
the density of each layer increases radially outward. The
nanospace has been used to encapsulate drugs,2 nanoparticles,3
and electronic materials4 among other applications. In particular,
† Keio University.
‡ The University of Tokyo.
§ Current Address: Chemical Resources Laboratory, Tokyo Institute of
Technology, Yokohama 226-8503, Japan.
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Chem. 1985, 50, 2003–2004. (b) Tomalia, D. A.; Baker, H.; Dewald,
J.; Hall, H.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P.
Polym. J. 1985, 17, 117–132. (c) Grayson, S. M.; Fre´chet, J. M. J.
Chem. ReV. 2001, 101, 3819–3868. (d) Bosman, A. W.; Janssen, H. M.;
Meijer, E. W. Chem. ReV. 1999, 99, 1665–1668. (f) Fischer, M.;
Vo¨gtle, F. Angew. Chem., Int. Ed. 1999, 38, 884–905.
dendrimers incorporating metal ions5 have received much
attention as a type of novel organometallic catalyst. For instance,
we reported that metal ions are complexed to the imine groups
of dendritic polyphenylazomethines (DPA, Chart 1a) in a
stepwise radial fashion based on the basicity gradient of imine
groups.6 In addition to their application as nanocapsules,
dendrimers show interesting potential as metal-organic hybrid
materials.
Ferrocene is a famous organometallic used for a variety of
applications due to its useful redox property, stability, and
synthetic convenience. This redox property has been utilized
for sensors, surfactants, electronic materials, and even molecular
machines.7 In addition, the accumulation of the redox center
has been investigated in relation to multielectron transfer,
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macol. 2005, 2, 264–272. (b) Copper, A. I.; Londono, J. D.; Wignall,
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9
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A R T I C L E S
Ochi et al.
magnetic, and nonlinear optical devices.8 While ferrocene is
easily oxidized into ferrocenium, which acts as a 1-electron
acceptor, its noncovalent sensitivity for various anions based
on the hydrogen-bonding, electrostatic attraction, and topological
effects has been investigated in previous studies of the ferrocenyl
dendrimers.9
The redox property of ferrocenes is also related to such redox
phenomena in organisms. One example is ferritin,10 iron storage
proteins found in the liver. In this system, the mechanism of
encapsulation and release of iron is mediated by the redox
properties of the iron within the protein shell. We previously
reported the ferritin-like redox switching irons complexed on
DPA.11 Similarly, other researchers have attempted to create
redox-active bioorganometallics or metalloproteins.12 In com-
parison to the use of iron ions, ferrocene offers a convenient
alternative for creating novel redox-responsive systems, because
of the potential for the molecular design through common
organic synthesis.
insight of the dendritic effect to the electron transfer,13 and
applications such as catalysts.5b While these covalent approaches
were performed, the noncovalent approach to incorporate
ferrocenes into dendrimers, which produces a reversibility and
simplicity for a molecular design, is a rare and interesting field.
Various researches of reversible molecular recognition using
famous molecular cages through noncovalent bonding have been
reported, such as cyclodextrin, cucurbit[7]uril, and cyclophane,
among others.14 The application of noncovalent interaction and
equilibrium becomes advantageous and interesting to utilize the
fine reversible redox property of the ferrocene. In fact, the
assembly of ferrocenes into a supramolecular dendrimer for
anion sensors has been studied using hydrogen bonding without
the multistep synthetic procedure by Astruc’s group.15 It is
possible that the noncovalent approach of ferrocene assembly
using dendrimers creates novel electrofunctional materials.
Moreover, the electrochemically controlled storage of ferrocenes
and its derivatives using the dendrimer interior and redox-active
ferrocene is scientifically interesting and unprecedented to the
best of our knowledge.
We demonstrated the novel and precise assembly of fer-
rocenes into DPAs utilizing the unique interaction between the
nucleophilic π-conjugated Schiff bases, which is advantageous
to holding cationic molecules16 and oxidized ferroceniums (Fc+,
Chart 1b). We found that our higher generation of DPAs could
encapsulate ferroceniums into the electron-donating layer of the
dendrimer. Using the useful redox property of ferrocene, we
succeeded in fine control of the electrochemical encapsulation
and release of ferrocene and its derivatives, analogous to redox-
responsive proteins, i.e., ferrtins. This reversible encapsulation/
release switching using a dendrimer is a unique characteristic
of this ferrocene assembly using its noncovalent approach and
equilibrium condition. In addition to ferrocene, we were also
able to encapsulate oligoferrocenes within DPAs. We were able
to significantly obtain thin films of the amorphous DPA
including the oligoferrocenes, which absorbed in the near-
infrared. Biferrocenes, in particular, were found to be suitable
for the fine reversible switching. The overall assemblies of the
DPA complexes were investigated on the basis of the UV-vis
absorption spectroscopy, 57Fe Mo¨ssbauer spectroscopy, ESI-
MS, CV, and fluorescence spectroscopy.
In terms of constructing metalloprotein-like macromolecules,
a dendritic framework is ideal as it allows for a precise design.
To date, the incorporation of a ferrocene into a dendrimer has
mainly been attempted through covalent bonding to produce
redox-active dendrimers, which produced the anion sensors and
electrochromic batteries9 using the enhanced redox ability, the
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Results and Discussion
Assembly of Ferroceniums. We found that the ferrocenium
in our system complexed with the nucleophilic DPA imines.
The UV-vis spectra revealed that the ferrocenium complexed
with DPA. When the UV-vis titration of ferrocenium hexafluo-
rophosphate (FcPF6) into a DPA was carried out, a distinct
change was observed in the UV-vis spectra; that is, the
absorption around 445 nm increased while the absorption around
300 nm, attributed to the imines, decreased.
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Encapsulation/Release of Ferrocene and Oligoferrocene
A R T I C L E S
In this study, the charge-transfer (CT) interaction18 is the driving
force of the ferrocenium assembly. In fact, similar CT complexes
including ferrocene compounds or other metallocene compounds
have been reported.19 High equilibrium constants similar to the
ferrocenium DPA complex (∼105 M-1) have been reported in
CT ion pairs.18a,19c,20 Moreover, π-conjugated molecules con-
taining nitrogen can also make CT complexes.21 In addition to
these previous studies, the change in the UV-vis spectra of
DPA (Figure 1) and the ferroceniums22 (Figure S3, Supporting
Information) and 57Fe Mo¨ssbauer spectra described in the
following section indicated their CT interaction. The increase
in the UV-vis spectra of the DPA complexed with ferroceniums
was observed at almost the same wavelength (445 nm) as that
of the metal-complexed DPAG4. This similar change indicated
that an unshared electron pair of the imine nitrogen interacts
with a ferrocenium, which supports the stepwise radial com-
plexation based on the higher electron density of the inner
imines.6
Figure 1. UV-vis spectra of DPAG4 complexed with (a) 0-6, (b) 0-2,
and (c) 3-6 equiv of FcPF6 (solvent, CHCl3/CH3CN ) 1/1, [DPAG4] )
5 × 10-6 M). (d) Schematic representation of the encapsulation of DPAG4
with FcPF6. (e) Job plot of FcPF6 and DPAG1 based on the changes in the
UV-vis spectra. The equilibrium constant of complexation, K, obtained
by curve-fitting using a theoretical simulation of the experimental data was
1.3 × 105 M-1. (f) Schematic representation of the complexation of a
DPAG1 imine and FcPF6.
In terms of the ferrocenium counteranion, weakly coordinat-
ing anions23 were found to be suitable for the complexation of
ferrocenium with DPA to avoid any competition due to an
electrostatic attraction.9d The equilibrium constants of FcPF6,
FcBF4, and FcBPh4 were determined by curve fitting using a
titration curve with DPAG1 (Figure S4, Supporting Informa-
A UV-vis titration was performed by the addition of FcPF6
to DPAG4 in a CHCl3/CH3CN ) 1:1 solution (Figure 1a).
During the addition of 6 equiv of FcPF6, isosbestic points were
observed at 366 nm from 0-2 equiv of FcPF6 and at 370 nm
from 3-6 equiv of FcPF6, indicating complexation within the
first and second layer imines, respectively, due to the higher
electron density of the inner imines.6 After the addition of 6
equiv of FcPF6, there were few additional spectral changes,
which is in contrast to that for metals. A similar assembly
process occurred in the cases of DPAG2 and DPAG3 (Figure
S1, Supporting Information). For DPAG2, two isosbestic points,
at 343 and 351 nm, were apparent after the addition 0-2 and
3-6 equiv of FcPF6, respectively. In the case of DPAG3, two
isosbestic points, centered at 375 nm and 368 nm, were apparent
after the addition of 0-2 and 3-6 equiv of FcPF6, respectively.
A Job plot17 of DPAG1 and FcPF6 showed a maximum at the
0.5 mol fraction of FcPF6, which indicates that the imine forms
a 1:1 complex with FcPF6 (Figure 1e). The equilibrium constant
of the complexation, K, was determined to be 1.3 × 105 M-1
by curve fitting using a theoretical simulation of the experimental
data by the Job plot and titration curve (Figure S2, Supporting
Information). In contrast, the UV-vis spectra of DPA with
neutral ferrocenes did not change, which suggested no interac-
tion. In addition in the change of the DPA absorption, the
ferrocenium absorption at 620 nm decreased in proportion to
the complexation with an imine (Figure S3, Supporting Infor-
mation).
tion): KFcPF6 ) 1.3 × 105 M-1 > KFcBF4 ) 1.5 × 104 M-1
>
KFcBPh4 ) 7.0 × 102 M-1. In addition, the 19F-NMR chemical
-
-
shifts of PF6 and BF4 showed the shifts before and after
complexation of the ferrocenium with DPA (Figure S5, Sup-
-
-
porting Information). PF6 and BF4 showed the upfield shift
of 0.7 ppm and the downfield shift of 2.0 ppm, respectively,
the chemical shifts of which after complexation became almost
the same ppm as tetrabutylammonium (TBA)PF6 and TBABF4.
These results suggest that the moderate dissociation of a
counteranion from a ferrocenium24,25 is important for the
complexation of ferroceniums with a DPA.
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(22) Similar disappearance of ferrocenium cation absorption (Figure S4,
Supporting Information) has been observed, for instance, in the case
of the CT complex of Fc/TCNE system (ref 19d). Unfortunately, a
new CT absorption could not be identified clearly because the new
CT absorption would exist near the π-π* large absorption of imines
(445nm, ε ) 104 M-1 cm-1), much smaller and broader than DPA
absorption (∼500 nm, ε ) ∼102 M-1 cm-1), expected from similar
CT complexes (refs 19a and 19c).
For the metal assembly, an imine coordinates with a metal
salt based on the Lewis acid and base interaction. However,
ferrocenium is a 17-electron system complex, which cannot
make a complete coordinate bond with a 2-electron donor imine.
(23) Strauss, S. H. Chem. ReV. 1993, 93, 927–942.
(24) We anticipate that the counteranions are in a relatively free state inside
or near a DPA molecule within the electrostatic limitation shown by
ESI-MS (Figure S8b, Supporting Information) due to the weakly
coordinating anion, the wide space inside DPA (Table S1, Supporting
Information, refs 25a and 25b), and less back folding of the DPA
rigid structure (refs 25c and 25d).
(17) F(x) ) Abs/(CG0 + CFcPF6)-(εG0-εFcPF6)x-εFcPF6, x ) CG0/(CG0
+
CFcPF6); x is the molar fraction of DPAG1. Chloroform/acetonitrile
(1/1) solution of DPAG1 (2.0 × 10-4 M based on imine) and FcPF6
with the same concentration were mixed in various proportions. The
plot shows a maximum at a 0.5 mol fraction of DPAG1, which means
that the imine forms a 1:1 complex with FcPF6.
(25) (a) Imaoka, T.; Tanaka, R.; Yamamoto, K. Chem.sEur. J. 2006, 12,
7328–7336. (b) Fujii, A.; Ochi, Y.; Nakajima, R.; Yamamoto, K.
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Figure 2. UV-vis spectra of DPAG4 complexed with (a) 6 equiv of FcPF6 and 24 equiv of AuCl3, (b) 0-2 equiv of FcPF6 and 2equiv of FcPF6, (c) 3-6
equiv of FcPF6 and 4 equiv of FcPF6, (d) 7-14 equiv of FcPF6 and 8 equiv of AuCl3, and (e) 15-30 equiv of FcPF6 and 16 equiv of AuCl3 (solvent,
CHCl3/CH3CN ) 1/1. [DPAG4] ) 5 × 10-6 M). (f) Schematic representation of the stepwise radial complexation of DPAG4 with FcPF6 and AuCl3.
Figure 3. (a) UV-vis spectra of DPAG4 complexed with 14 equiv of FcPF6 and 8 equiv of AuCl3, 6 equiv of FcPF6 and 8 equiv of AuCl3 (solvent,
CHCl3/CH3CN ) 1/1. [DPAG4] ) 5 × 10-6 M). (b) Schematic representation of the heterocomplexation: 14FcPF6 + 8AuCl3 and 6FcPF6 + 8AuCl3.
We reported that various kinds of metal salts can be located
on a DPAG4 through a stepwise radial complexation.26 Using
this unique heteroassembly, we further investigated the com-
plexation of ferroceniums on the first and second layers of
DPAG4. To confirm the assembly of ferroceniums on the first
and second layers, we demonstrated that DPAG4 encapsulating
6 equiv of ferroceniums was complexed with a quantity of
AuCl3, which forms a 1:1 complex with DPA imines,27
consistent with the filling of the third and fourth layers of
DPAG4 (Figure 2). That is, after adding 6 equiv of ferroceniums
to DPAG4, giving rise to two isosbestic points, 24 equiv of
AuCl3 could be added with two isosbestic points, observed at 8
and 16 equiv, respectively, corresponding to the filling of the
third and fourth layers. As a control experiment, after adding
14 equiv of ferroceniums to DPAG4, 24 equiv of AuCl3 was
added, and again, two isosbestic points were observed at 8 and
16 equiv. In addition, the UV-vis spectra of the 14FcPF6 system
with 8AuCl3 corresponded to that of the 6FcPF6 system with
8AuCl3 (Figure 3). These results confirm that 6 equiv of
ferrocenium is complexed with the first and second layers of
DPAG4. The same conclusion was obtained using SnCl2 instead
of AuCl3.
and acceptor is 3-4 Å.19 From the equilibrium condition of
the ferrocenium and imine with a relatively high interaction (K
) 105 M-1), a complexed ferrocenium is probably located within
3-4 Å from an inner imine site. The radius of DPAG4 is 15
Å, and the average distances from the first and second layers
of the imine site to the outer shell are 14 and 8 Å, respectively.10
Taking these molecular distances into consideration, the ferro-
ceniums complexed with the inner imines of DPAG4 are
sufficiently encapsulated into the dendrimer molecules.
Next, we performed the titrations using the half-substituted
DPA, represented as Half-DPAGX (X ) 3, 4), which has less
dendritic effects on the difference between the inner and the
outer microenvironments28 than those for DPAG4 (Chart S1,
Supporting Information). Although the change in absorption was
lower than that observed for metal salts, four changes in the
position of the isosbestic points could be observed for Half-
DPAG4 (Figure S6, Supporting Information). This result
suggests that the ferroceniums are able to interact with the outer
layers of the Half-DPAG4 and Half-DPAG3 to some degree.
Considering these results of Half-DPA, the limited Fc+ as-
sembly on the third and fourth layers of DPAG4 is attributed
to the dendritic shielding effects28 or the different molecular
The previously reported CT complexes of the ferrocene
compounds show that the average distance between the donor
(26) Takanashi, K.; Fujii, A.; Nakajima, R.; Chiba, H.; Higuchi, M.; Einaga,
Y.; Yamamoto, K. Bull. Chem. Soc. Jpn. 2007, 80, 1563–1572.
(28) (a) Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74–
91. (b) Harth, E. M.; Hecht, S.; Helms, B.; Malmstrom, E. E.; Fre´chet,
J. M. J. J. Am. Chem. Soc. 2002, 124, 3926–3938.
(27) Takanashi, K.; Ochi, Y.; Yamamoto, K. Manuscript in preparation.
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Encapsulation/Release of Ferrocene and Oligoferrocene
A R T I C L E S
oxidation seems to be less sensitive than that of the release for
the previous sensors of ion-pairing ferrocenyl dendrimers,9
probably due to the electrostatic competition by anions and the
dendritic shielding effect, the release of the ferrocenes by the
electrochemical reduction rapidly occurs.
In order to clarify the oxidation state and charge-transfer (CT)
interaction of the ferroceniums encapsulated in DPA, we
performed 57Fe Mo¨ssbauer spectroscopy. To obtain the
(57FcPF6)2@ DPAG4 species, we formed a complex of 57FcPF6
and DPAG4, allowed it to reach an equilibrium, and then
precipitated it into hexane. In the Mo¨ssbauer spectrum,
(57FcPF6)2@DPAG4 showed a doublet spectrum, which indi-
cated different isomeric shift (IS) and quadrupole splitting (QS)
from that of FcPF6 (IS ) 0.31 mm·s-1) or Fc (IS ) 0.32
mm·s-1, QS ) 2.39 mm·s-1) in isolation (Figure 5). The IS
and QS values of (57FcPF6)2@DPAG4 were IS ) 0.24 mm ·s-1
and QS ) 0.79 mm ·s-1 at RT with the features broadening at
150 and 9 K (Figure S7, Supporting Information). This broad
peak was caused by spin relaxation of the 3d5 orbital and
30
suggests the low-spin state (S ) 1/2) of an Fe3+
.
Almost the
same IS and QS values were observed in the case of
(57FcPF6)6@DPAG4, which confirmed that 6 equiv of FcPF6
could be encapsulated within DPAG4 (Figure S7, Supporting
Information). In the case of (57FcPF6)2@DPAG1, a mixed
spectra of (57FcPF6)6@DPA and FcPF6 was observed due to
its poor encapsulating ability as a result of reprecipitation.
This change in the 57Fe Mo¨ssbauer spectra confirms the CT
interaction between a ferrocenium and a DPA imine, which is
suggested by the discussion of the UV-vis spectrum investiga-
tion. The change in QS by the complexation is attributed to the
charge delocalization and orbital overlap31a,19c between a
ferrocenium and a DPA imine. In fact, similar 57Fe Mo¨ssbauer
spectra of the ferroceniums have been reported for the CT
complexes of the previous ferrocenium compounds.31 Moreover,
the approach of nucleophilic species to the ferrocenophaniums
has been also reported having similar 57Fe Mo¨ssbauer spectra,32
supported by the possibility of a nucleophilic interaction with
the 2a1 molecular orbital of the ferrocene.33,9d These studies
sufficiently support the CT interaction of the ferrocenium and
DPA.
Characterization of the FcPF6/DPA complex was also carried
out by ESI-MS spectroscopy. It is difficult to detect the
complexes of higher generation DPAs because of their low
ionization activity. However, in the case of DPAG1, the
complex of [(FcPF6)@DPAG1+H+] (calcd 768.2) was ob-
served at 768.3 using a sample of the DPAG1 solution in which
1 mol equiv of FcPF6 was added (Figure S8, Supporting
Information). The observed peak pattern (Figure S8b, Supporting
Information) corresponded to the simulated peak pattern includ-
Figure 4. (a) Electro-UV-vis spectral changes of DPAG4 complexed with
2 equiv of ferroceniums, (FcPF6)2@DPAG4. [DPAG4] ) 5 × 10-5 M,
[Fc] ) 1 × 10-4 M, [TBAPF6] ) 0.2 M, from +0.2 to -0.25 V vs Ag/
Ag+. (b) Absorption changes at 450 nm. The absorption change during
release is fitted to be first-order kinetics (k1 ) 8.0 × 10-4 s-1). The
complexation is fitted to be second-order kinetics (k2 ) 32 s-1 M-1). (c)
Schematic representation of the electrochemical encapsulation/release of
the ferrocenes.
motion between the inner and outer backbones.29 We expect
that this different Fc+ behavior from that of the metal assembly
is also due to the environmental sensitivity19a or the greater
distance between the Fc+-imine than the metal-imine coor-
dination, which are typical characteristics of a charge-transfer
complex.
Whereas a ferrocenium could be coordinated to the DPA
imines, the neutral ferrocene was not. By taking advantage of
this property, we could control the reversible electrochemical
encapsulation and release of the ferrocenes. Specifically, the
UV-vis absorption at around 445 nm of the DPA imines was
increased at a potential of 0.2 V vs Ag/Ag+ for 15 min, whereas
the absorption decreased at -0.25 V for 10 min, indicating the
encapsulation and release of the ferrocenes (Figure 4). A
relatively long equilibration was required to complete the
assembly and release because of the diffusion-limited access
to the electrode. As a control experiment, a DPAG4 solution
was measured without ferrocenes, and no changes were observed
in the UV-vis spectra under the same conditions. Although
switching behavior was observed, the switching gradually
became irreversible as a result of the exposure to high voltage
over a long time or after many cycles of switching, presumably
due to decomposition. Thus, switching of the encapsulation/
release of ferrocene was limited, occurring only for a relatively
short time and after only a few cycles, two at most. Considering
the reaction rate of the switching, the release by the Fc+
reduction (k1) 8.0 × 10-4 s-1) was faster than that of the
encapsulation by the Fc oxidation (k2 ) 32 s-1 M-1). A similar
behavior was also observed in the case of biferrocene switching
(60 min for encapsulation; k2 ) 10 s-1 M-1; 30 min for release;
k1 ) 3.0 × 10-4 s-1) described in the following section. While
the responsiveness of the complexation by the electrochemical
(30) Birchall, T.; Drummond, I. Inorg. Chem. 1971, 10, 399–401.
(31) (a) Kowalski, K.; Linseis, M.; Winter, R. F.; Zabel, M.; Za´lisˇ, S.;
Kelm, H.; Kru¨ger, H. J.; Sarkar, B.; Kaim, W. Organometallics 2009,
28, 4196–4209. (b) Mochida, T.; Takazawa, K.; Matsui, H.; Takahashi,
M.; Takeda, M.; Sato, M.; Nishio, Y.; Kajita, K.; Mori, H. Inorg.
Chem. 2005, 44, 8628–8641. (c) Kramer, J. A.; Hendrickson, D. N.
Inorg. Chem. 1980, 19, 3330–3337. (d) Kaufmann, L.; Breunig, J. M.;
Vitze, H.; Scho¨del, F.; Nowik, I.; Pichlmaier, M.; Bolte, M.; Lerner,
H. W.; Winter, R. F.; Herber, R. H.; Wagner, M. Dalton Trans. 2009,
2940–2950. (e) Liu, Y.; Tang, H.; Qin, J.; Inokuchi, M.; Kinoshita,
M. J. Inorg. Organomet. Polym. Mat. 2007, 17, 111–119.
(32) (a) Watanabe, M.; Sato, K.; Motoyama, I.; Sano, H. Chem. Lett. 1983,
11, 1775–1778. (b) Watanabe, M.; Motoyama, I.; Sano, H. J.
Radioanal. Nucl. Chem. 1985, 6, 585–592. (c) Ogino, H.; Tobita, H.;
Habazaki, H.; Shimoi, M. J. Chem. Soc., Chem. Commun. 1989, 13,
828–829.
(29) (a) Higuchi, M.; Shiki, S.; Yamamoto, K. Org. Lett. 2000, 2, 3079–
3082.
(33) Lauher, W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729–1742.
9
J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010 5065

A R T I C L E S
Ochi et al.
Figure 5. 57Fe Mo¨ssbauer spectra of (a) (FcPF6)2@DPAG4 (IS ) 0.24 mm ·s-1, QS ) 0.79 mm ·s-1), (b) FcPF6 (IS ) 0.32 mm ·s-1), and (c) ferrocene
(IS ) 0.32 mm ·s-1, QS ) 2.39 mm ·s-1) at 294 K.
Figure 6. (a) Cyclic voltammograms of DPAGX (X ) 1-4) complex with 1 equiv of FcBF4 taken at the fast scan rate of 20 V/s. The curves are displayed
in order from the inner curve to the outer one, DPAG4, DPAG3, DPAG2, and DPAG1, respectively. (b) Nichlson plots36 for the electron-transfer rate
constants ks of (FcBF4)@DPAGX (X ) 1-4).
ing Fe isotopes (Figure S8c, Supporting Information). Although
other fragment peaks were observed (Figure S8a, Supporting
Information), such as DPAG1, (DPAG1)2, and (DPAG1)2PF6,
into ferrocenes. To detect the influence of ferrocenes being
released from DPA, a solution of (FcBF4)@DPAGX (X ) 1-4)
was swept at a relatively fast scan rate (0.2-50 V·s-1) by cyclic
voltammetry (CV). FcBF4, which complexed in a fashion similar
to DPA, was selected in this experiment due to its reproduc-
ibility. In the CV, the higher generation dendrimers gave rise
to a lower electric current and a wider peak separation (Figure
6a). These tendencies were attributed to the decrease in the
diffusion constant and the electron-transfer rate constant caused
by the increase in molecular size (DPAG4, 3.2 nm; DPAG3,
2.4 nm; DPAG2, 1.8 nm; DPAG1, 1.2 nm diameter). The
electron-transfer rate constants ks were estimated using Nichl-
son’s method36 based on the peak separations (Figure 6b,
this result suggested that a ferrocenium complexed with the DPA
-
which was electrostatically stabilized by PF6
.
These results exclude the possibility of a side reaction of the
ferroceniums, such as a simple oxidation or decomposition.
Ferrocenium is known as a 1-electron oxidant in its reaction
from ferrocenium to ferrocene. Significantly, no peaks corre-
sponding to ferrocene were detected in the 57Fe Mo¨ssbauer
spectra or ESI-MS spectrum, which shows that the ferrocenium
was not reduced through a 1-electron transfer. Decomposition
of the ferrocenium by nucleophilic species was also excluded
based on the absence of any ferrocene signals because the
decomposition of ferrocenium is known to involve the formation
Fc@DPAG4, 0.03 cm s-1; Fc@DPAG3, 0.05 cm s-1
;
Fc@DPAG2, 0.08 cm s-1, Fc@DPAG1, 0.10 cm s-1). The
diffusion constants estimated by DigiSim37 were as follows:
Fc@DPAG4, 6.5 × 10-6 cm2 s-1; Fc@DPAG3, 7.5 × 10-6
cm2 s-1, Fc@DPAG2, 9.5 × 10-6 cm2 s-1; Fc@DPAG1, 10.5
× 10-6 cm2 s-1. Although the tendency was slightly weak due
to the release of the ferrocenes and the noncovalent interaction,
these results correspond to those observed for ferrocene-cored
dendrimers13b,d in regard to the electron-transfer rate constant
and the diffusion constant found in the encapsulation of a
ferrocene.
of neutral ferrocene following reduction by C5H5 ions.34
-
-
Fc+FeX4 , a typical decomposed compound of ferrocenium, has
asymmetric 57Fe Mo¨ssbauer peaks, which clearly differs from
our results.35
Ferrocene shows a reversible redox wave in the cyclic
voltammogram. The encapsulation of ferrocenium into DPA was
monitored by cyclic voltammetry, taking into account the
difficulty in measuring the generation effect of dendrimers
because of the rapid release by the reduction of ferroceniums
(34) (a) Prins, R.; Korswagen, A. R.; Kortbeek, A. G. T. G. J. Organomet.
Chem. 1972, 39, 335–344. (b) Holecek, J.; Handlir, K.; Klikorka, J.;
Nguyen, D. B. Collect. Czech. Chem. Commun. 1979, 44, 1379–1387.
(35) Birchall, T.; Drummond, I. Inorg. Chem. 1971, 10, 399–401.
(36) Nicholson, R. J. Anal. Chem. 1965, 37, 1351–1355.
(37) Rudolph, M.; Reddy, D. P.; Felberg, S. W. Anal. Chem. 1994, 66,
589A.
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Encapsulation/Release of Ferrocene and Oligoferrocene
A R T I C L E S
Figure 7. (a) Fourth generation of a phenylazomethine dendrimer with a zinc tetraphenylporphyrin core (DPAG4-ZnP). (b) UV-vis spectra of DPAG4-
ZnP complexed with 0-12 equiv of FcPF6 (solvent, CHCl3/CH3CN ) 1:1. [DPAG4-ZnP] ) 2.5 × 10-6 M).
Table 1. Complexation Constants and Redox Potentials of
Ferroceniums and Oligoferroceniums
E1/2 (V vs Ag/Ag+)
K (M-1
)
Fc(+)
BiFc(+)
TFc(2+)
0.14
0.055, 0.40
-0.023, 0.18, 0.55
1.3 × 105
1.5 × 103
1.0 × 106
promoted due to the higher reduction potential of the ferrocenes,
in the same way as does FeCl3 (Figure 8c). The difference in
the quenching rate between the ferrocenium and FeCl3 would
depend on both the complexation constant and the reduction
potential. As a control experiment, no quenching was observed
when FcPF6 was added to a solution of a zinc tetraphenylpor-
phyrin (Figure S9, Supporting Information), which shows that
the quenching effect was mediated through the DPA backbone.
These results support the encapsulation of ferroceniums within
the interior of the dendrimer.
Expanding to Oligoferrocene Assembly. In addition to fer-
rocenes, we found that mixed valence oligoferrocenes could also
be encapsulated into DPA. In particular, the biferrocene
cation(1+) and terferrocene dication(2+), represented as BiFc(1+)
and TFc(2+), respectively, were found to complex with DPA,
whereas neutral oligoferrocenes did not interact with the imines.
For the assembly of BFc(1+), BiFcPF6 was synthesized and
added to a solution of DPAG4. The UV-vis spectra of the
resulting solution showed that the absorption at around 445 nm
increased while the absorption attributed to the imine decreased,
which confirmed the interaction between BiFcPF6 and the
Figure 8. (a) Change in normalized fluorescence intensities of DPAG4-
ZnP during the addition of FcPF6 (O) or FeCl3 (2). (b) Fluorescence spectra
of DPAG4-ZnP complexed with 0-8 equiv of FcPF6. [DPAG4-ZnP] ) 1
× 10-5 M. (c) Energy diagram for the oxidative quenching of a zinc
porphyrin S1 state by FcPF6.
The assembly of ferroceniums into a phenylazomethine
dendrimer with a zinc tetraphenylporphyrin core (DPAG4-ZnP,
Figure 7) was also performed. In the case of DPAG4-ZnP,
although there was no obvious shift in the isosbestic point
because of the small Soret-band shift, it was noted that the
absorption at around 445 nm increased while the absorption at
around 300 nm, attributed to the imine, decreased. No further
changes were observed after the addition of 12 equiv of FcPF6,
which suggests that the DPAG4-ZnP also encapsulated ferro-
ceniums up to the second layer, consistent with the DPAG4
results (Figure 7).
The influence of the ferrocenium assembly on the fluorescence
of DPAG4-ZnP was also investigated. The singlet excited state
(S1) of DPAG4-ZnP is a weak oxidant (E ) 0.4 V vs Ag/
Ag+) and a strong reductant (E ) -1.7 V vs Ag/Ag+). In
addition, the ferrocene (E ) 0.1 V vs Ag/Ag+) is known to
quench fluorescence.38 Indeed, the fluorescence spectrum of
DPAG4-ZnP upon excitation at the Soret band was completely
quenched by the addition of the ferroceniums (Figure 8ab). This
quenching behavior was similar to a previous report on the
complexation of FeCl3 (E ) -0.1 V vs Ag/Ag+) with DPAG4-
ZnP.39 These results indicate that electron transfer from the
excited zinc porphyrin to the coordinating ferroceniums is
imines. BFc(1+) has a lower complexation constant (KBiFc
)
ca. 1.5 × 103 M-1, Figure S10, Supporting Information, Table
1) than the ferrocenium due to delocalization of a cation
(E1/2 Fc+ ) 0.14 V, E1/2 BiFc+ ) 0.06 V vs Ag/Ag+). The UV-vis
titration was performed by addition of BiFcPF6 using DPAG4
(Figure 9a). During addition of 6 equiv of BiFcPF6, isosbestic
points were observed at 375 nm after 0-2 equiv of BiFcPF6
and at 378 nm after 3-6 equiv of BiFcPF6. Similar results were
observed in the titration of DPAG3 and DPAG2 (Figure S11,
Supporting Information). Although the complexation constant,
K, of BiFcPF6 is slightly lower than that of FcPF6, it was also
effectively encapsulated within the DPA in a manner similar
to FcPF6 due to the stronger basicity of the inner DPA imines
when compared to the outer DPA imines.6
(38) (a) Cao, W.; Ferrance, J. P.; Demas, J.; Landers, J. P. J. Am. Chem.
Soc. 2006, 128, 7572–7578. (b) Geiber, B.; Alsfasser, R. Inorg. Chim.
Acta 2003, 344, 102–108.
In terms of controlling the encapsulation and release, bifer-
rocenes were found to be more suitable for reversible switching
(39) Imaoka, T.; Tanaka, R.; Arimoto, S.; Sakai, M.; Fujii, M.; Yamamoto,
K. J. Am. Chem. Soc. 2005, 127, 13896–13905.
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DPAG4 is able to encapsulate relatively large TFc or BiFc
molecules due to the inner wide space of the DPA,25a,b which
was supported by the estimated free volume in a DPAG4
molecule using the van der Waals volumes and hydrodynamic
volume (Table S1, Supporting Information). However, TFc(+)
had a weak complexation behavior because of its low redox
potential, thus making it unsuitable for assembly.
In the encapsulation of TFc(2+), an IV-CT band in the near-
infrared was observed. The IV-CT band gradually disappeared
over a long time due to electron donation or charge localization
by the DPA imine. As compared to BiFc(+), the IV-CT band
of TFc(2+) was found to be relatively stable (Figure S12,
Supporting Information). Interestingly, TFc(2+), which could
not form a self-standing film, was able to form a thin film after
encapsulation into DPAG4 exhibiting an IV-CT band. In this
process, a solution of TFc(2+) and DPAG4 (1.5 mM, C6H5Cl/
CH3CN ) 1:1 as solvent) was cast on a glass plate to form a
thin film (Figure S13, Supporting Information), the thickness
of which was ca. 1 µm in this experiment. We confirmed that
the IV-CT band of the mixed valence terferrocenes in this film
was stable for over 7 days. Consequently, the TFc(2+)-
assembled DPAG4 was shown to be suitable for application as
near-infrared responsive materials, and further investigations are
now in progress.
Figure 9. UV-vis spectra of DPAG4 complexed with 0-6 equiv of (a)
BiFc(1+) and (b) TFc(2+).
Conclusion
DPA is able to encapsulate ferroceniums into their inner
layers. Specifically, 6 equiv of ferrocenium was trapped in the
first and second layers of DPA. Electrochemical switching of
the encapsulation/release of ferrocene was accomplished simi-
larly to the redox-responsive protein. Not only ferrocenes but
also oligoferrocenes, which form mixed valence states, are able
to complex with the DPA imines and improve its functionality
as an electrofunctional material. In terms of encapsulation/
release switching, biferrocenes are proved to be the most suitable
among the investigated ferrocenes. Moreover, a thin film of
terferrocenes encapsulated in DPA exhibited an IV-CT band,
indicating the potential for application of such complexes as
infrared-responsive materials.
Figure 10. (a) Electro-UV-vis spectral changes in (BiFc(1+))2@DPAG4.
[DPAG4] ) 5 × 10-5 M, [BiFc] ) 1 × 10-4 M (solvent CHCl3:CH3CN
) 1:1 with [TBAPF6] ) 0.2 M, the voltage switched from +0.2 to -0.25
V vs Ag/Ag+). (b) Time-dependent change in DPA absorption at 450 nm
with voltage switching. The absorption change during the release is fitted
using first-order kinetics (k1 ) 3.0 × 10-4 s-1). Complexation is fitted using
second-order kinetics (k2 ) 10 s-1 M-1). (c) Schematic representation of
the electrochemical encapsulation/release of biferrocenes.
This study expands the range of species which can be
encapsulated into DPA from metal ions to organometallics.
Ferrocene is a redox-active metallocene and can be easily
modified by various functional groups for the encapsulation/
release of other functional molecules into DPA. Utilizing the
electron density gradient of DPA, the storage of ferroceniums
into DPA is unique from the standpoint of the electrochemical
control of the reversible encapsulation/release system. Taking
advantage of the noncovalent approach, we showed the con-
venient versatility of this system for other ferrocene derivatives,
biferrocenes and terferrocenes, in this study. The application
of this system for other metallocenes, ferrocene-attached deriva-
tives, and various kinds of acceptors will be expected. Such
methods will offer a route to the creation of novel intelligent
materials, such as drug delivery systems, catalysis, or electro-
functional materials, as systems analogous to redox-responsive
proteins.
than the ferrocenes. As previously stated, the switching of
ferrocenes was demonstrated only under a certain condition.
Similarly, terferrocenes(2+) did not effectively release due to
a higher redox potential than the other two (E1/2 TFc2+ ) 0.18
V). In contrast, the electro-UV-vis spectra showed that
biferrocenes were stable and allowed many cycles (4 times at
least) of the encapsulation and release (Figure 10) due to its
charge delocalization and mild complexation. BiFc(1+) is
indicated to be the most suitable species for a reversible
encapsulation and release among the investigated ferrocene
derivatives.
TFc(2+) was oxidized by 2 equiv of FcCl2PF6, before
addition to the solution of DPAG4, because TFc(2+) was
difficult to isolate as TFc(PF6)2. The UV-vis spectra of the
DPAG4 mixture showed that the absorption at around 445 nm
increased while the absorption attributed to the imine decreased,
which confirmed the interaction of the TFc(2+) and the imines
(Figure 9b). The equilibrium constant of the complexation, K,
was determined to be 1.0 × 106 M-1 by curve fitting of the
DPAG1 UV-vis spectra (Figure S10, Supporting Information),
which had a relatively high complexation constant due to its
Experimental Section
Methods. UV-vis spectra were recorded using a Shimadzu UV-
3100PC spectrometer with a sealed quartz cell (optical path length
1 cm) at 293 K. Details of the UV-vis titration method are
described in the following section General Method of UV-vis
Titration.
high redox potential (Table 1, E1/2 TFc2+ ) 0.14 V, E1/2 TFc+
)
-0.02 V vs Ag/Ag+). The UV-vis spectral results indicate that
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A R T I C L E S
Electro-UV-vis spectra monitoring of the encapsulation and
release was carried out in chloroform/acetonitrile ) 1:1 solvent
mixture (DPAG4 5.0 × 10-5 M, 0.2 M TBAPF6) after N2 bubbling
for 5 min. An UV-vis spectrometer (Shimazu UV-3100), a
potentiostat (Hokuto Denko Co., Ltd., HA-501G), and a specially
made thin layer cell (optical path length 1 mm) with a platinum-
mesh electrode were used for the experiments. In the case of
ferrocene, the oxidation was done at 0.2 V (vs Ag/Ag+) for 15
min and reduction at -0.25 V (vs Ag/Ag+) for 10 min. In the case
of biferrocene, the oxidation was done at 0.2 V (vs Ag/Ag+) for
60 min and reduction at -0.25 V (vs Ag/Ag+) for 30 min.
The cyclic voltammetry experiments were carried out using an
ALS 445 electrochemical analyzer. As the working electrode, a
glassy carbon electrode (φ ) 3 mm) was used, which was polished
with 0.05 mm alumina paste before analysis. The counter and the
reference electrodes were Pt wire and Ag/Ag+, respectively. All
measurements were carried out with the same Ag/Ag+ reference
after N2 bubbling for 5 min. The measurement solution was 2 mL
of CHCl3:CH3CN ) 1:1 including 0.1 M TBABF4. The concentra-
tion of FcBF4 in each solution was 0.5 mM, and equimolar
quantities of DPAGX (X ) 1-4) were added.
ferrocene were purchased from Kanto Kagaku Co. Ferrocenium
hexafluorophosphate (FcPF6) and ferrocenium tetrafluoroborate
(FcBF4) were purchased from Aldrich. Ferrocenium tetrafluorobo-
rate (FcBPh4) was synthesized by a previous method.41 57Fe was
purchased from TANGO OVERSEAS, and 57Fe(Cp)2 was synthe-
sized from 57FeCl2 using a previous method.42 Biferrocene and
terferrocene were synthesized by a literature method.43 Biferroce-
nium(1+) hexafluorophosphate and FcCl2PF6 were also synthesized
by a literature method.44,45 The terferrocene(2+) dication was
prepared by addition of 2 equiv of FcCl2PF6 to an acetonitrile
solution. All other chemicals were purchased from the Kanto Kaga-
ku Co.
General Method of UV-vis Titration. Solutions of DPAG4
(chloroform/acetonitrile ) 1:1, 5:0 × 10-6 M) and FcPF6 (aceto-
nitrile, 2.5 × 10-3 M) were prepared in volumetric flasks. To a 3.0
mL amount of the DPAG4 solution in a quartz cell was immediately
added 6 µL of a freshly prepared FcPF6 solution (i.e., 1 equiv for
DPAG4). The UV-vis spectra were measured until the absorbance
change was converged. Measurements were repeated after each
addition of 6 µL of FcPF6 solution to achieve the UV-vis titration.
After measurement of the UV-vis spectra, the absorption spectra
of the additives (FcPF6, BiFcPF6, TFc(2+), AuCl3) were subtracted
to identify the changes in the DPA absorption and isosbestic
points.10,17 In the titration curves of DPAG1, the curve fitting with
a theoretical simulation as a second-order reversible reaction was
used.26
The 57Fe Mo¨ssbauer spectra were measured using a Topologic
systems model 222 constant-acceleration spectrometer. The data
were recorded on a 1024-channel pulse height analyzer. The spectra
were analyzed by fitting a Lorentzian line shape using MOSSWINN
version 3 software. All samples were prepared using 57FcPF6. The
samples of (FcPF6)2@ DPAG4 were prepared by reprecipitation
from hexane.
Acknowledgment. The authors extend their gratitude to JEOL,
Ltd., for the ESI-mass spectroscopy measurements and to Mr. K.
Oshida for his help with the 57Fe Mo¨ssbauer spectroscopy measure-
ments. We also thank Dr. N. Satoh, Mr. K. Albrecht, and Ms. R.
Nakajima for their useful suggestions. This work was supported
by Grants-in-Aid for Scientific Research (nos. 19205020, 21108009,
21750152), and Mitsubishi and Futaba Foundations.
The ESI-mass spectra were recorded by a JEOL JMS-T100CS
using the (FcPF6)2@DPAG1 complex in chloroform/acetonitrile
) 1:1 solvent mixture at 1 mM concentration.
The fluorescence spectra were measured by a Hamamatsu
Photonics C9920-02 absolute PL quantum yield measurement
system at room temperature. The concentration of the chloroform/
acetonitrile solution was 10 µM, and the excitation wavelengths
were 436 (DPAG4-ZnP) and 423 nm (ZnTPP) (Soret band of
porphyrin). The titration method was similar to the UV-vis titration
method.
Supporting Information Available: Additional UV-vis spec-
tra, 57Fe Mo¨ssbauer spectra, ESI-MS and NMR data, and
additional data for the ferrocenium and oligoferrocene cation
assembly. This material is available free of charge via the
Internet at http://pubs.acs.org.
The 19F-NMR spectra were recorded using a JEOL ECX-400
FT-NMR spectrometer (400 MHz) in a CH2Cl2/CD3CN ) 4:1
solution for obtaining a sufficient solubility, and TFA was used as
the external standard. The 1H NMR spectra were recorded using a
JEOL JMN400 FT-NMR spectrometer (400 MHz) in a CDCl3/
CD3CN ) 1:1 + TMS (internal standard) solution. The concentra-
tion of DPAG0 was 2 mM in the BiFcPF6 experiment.
Chemicals. All DPAs were synthesized by a previously reported
method.40 The dehydrated chloroform was purchased from Wako
Pure Chemical Industries, and the dehydrated acetonitrile and
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