Dynamic control of dendrimer-fullerene association by axial coordination to the core

Article abstract of DOI:10.1039/c3cc43249a

The effect of axial coordination of pyridine derivatives to the core porphyrin on the fullerene encapsulation of the 4th generation carbazole-phenylazomethine dendrimer (ZnPG2-2) was investigated. The axial coordination of large (bulky) pyridine derivativ

Full text of DOI:10.1039/c3cc43249a

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Dynamic control of dendrimer–fullerene association by
axial coordination to the core†
Cite this: Chem. Commun., 2013,
49, 6861
Ken Albrecht, Yuto Kasai, Yasunori Kuramoto and Kimihisa Yamamoto*
Received 2nd May 2013,
Accepted 7th June 2013
DOI: 10.1039/c3cc43249a
www.rsc.org/chemcomm
The effect of axial coordination of pyridine derivatives to the core
porphyrin on the fullerene encapsulation of the 4th generation
carbazole–phenylazomethine dendrimer (ZnPG2-2) was investi-
gated. The axial coordination of large (bulky) pyridine derivatives
affects the cavity in an allosteric manner, and the size-selectivity of
the fullerene association could be controlled.
Molecular recognition plays an important role in natural pro-
teins,1 and especially, dynamic (allosteric) molecular recognition
is attracting many scientists. Naturally, several trials to construct
artificial hosts2 and studies on allosterism have been com-
pleted.3 For definite molecular recognition, several macrocyclic
or double-decker type molecules are widely used, because these
molecules can determine the size of the binding site (cavity).
There are also several polymeric molecular recognition systems.
These systems introduce low molecular weight hosts into the
polymer chain4 or imprint guest structure into the crosslinked Fig. 1 Structure of the compounds (dendrimer and pyridine) that are used in
polymer network,5 and are used for chemosensing applications.6
this study.
However, a methodology to develop an artificial macromolecular
system with allosterism using the protein-like cavity that is
fullerenes has size-selectivity (C60 o C70 o C84) and this result
provided by the polymer chain is not yet established.
demonstrates that high-generation dendrons do not only act as a
Dendrimers7 are often compared with proteins because of
steric hindrance. ZnPG2-2 has two cavities at the axial position of
their monodispersity, distinct interior space, and relatively stable
the porphyrin and only one is occupied when fullerenes bind.
conformation. Actually, encapsulation,8 catalytic reactions,7a
Therefore, it should be possible to bind another guest such as
molecular recognition,9,10 and imprinting11 have been studied.
pyridine16 at the same time and control the fullerene association
However, many reports have shown that dendrons simply act as a
(Fig. 2). Now we report allosteric fullerene recognition of ZnPG2-2
steric hindrance in these applications. The fourth-generation
that is controlled by pyridine complexation to the core porphyrin.
carbazole–phenylazomethine double layer-type dendrimer with a
To confirm the concept of the allosteric control of the
zinc porphyrin core has a p-electron rich surface with a rigid
fullerene association (Fig. 2), the UV-vis titration of a fullerene
(carbazole12) and a semi-rigid (phenylazomethine13) backbone
(ZnPG2-2, Fig. 1).14 According to this structure, this dendrimer
has an ca. 1 nm diameter cavity on top of the porphyrin plane and
can form a 1 : 1 complex with fullerenes.15 This association with
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta
Midori-ku, Yokohama 226-8503, Japan. E-mail: yamamoto@res.titech.ac.jp;
Fax: +81-45-924-5260; Tel: +81-45-924-5260
† Electronic supplementary information (ESI) available: Experimental section,
synthesis, titrations, and thermodynamic parameters. See DOI: 10.1039/
Fig. 2 Co-binding of pyridines and fullerenes to the ZnPG2-2 dendrimer.
Chem. Commun., 2013, 49, 6861--6863 6861
c3cc43249a
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show that the fullerene binding can be controlled by co-binding
of pyridine derivatives to the core of ZnPG2-2. This can be
explained by (1) the steric change in the second binding site, (2)
the electric change in the second binding site, and (3) the
solvent polarity change upon the addition of an excess amount
of pyridine (solvent polarity strongly affects the binding15).
To exclude the third reason, a free base porphyrin cored
double layer-type dendrimer (H2PG2-2) was synthesized and
the titration with C60 with and without 4-phenylpyridine was
performed. Without pyridine, the binding constant of H2PG2-2
and C60 was nearly the same as that of ZnPG2-2. Under the
condition of 1000 eq. of 4-phenylpyridine, the binding constant
of H2PG2-2 and the fullerene decreased slightly. This slight
decrease might be explained by the weak pyridine–fullerene
interaction17 and is in contrast to the behaviour of ZnPG2-2
(Fig. S2, ESI†). H2PG2-2 and 4-phenylpyridine do not form a
complex, therefore, this result clearly indicates that the addi-
tion of an excess amount of 4-phenylpyridine did not affect
the polarity of the solvent. This means that the increase in the
binding constant of ZnPG2-2 and C60 is derived from the
electronic or steric change upon the complexation of pyridine
to the porphyrin.
To study the electronic effect of the complexation of pyridine
to the porphyrin, the complexation constants were examined
(UV-vis titration Fig. 5, and ITC (isothermal titration calorime-
try, Fig. S3 and Table S1, ESI†)). Interestingly, the binding
constants between the DPA pyridines and ZnPG2-2 increased
when the generation of DPA increased up to the third genera-
tion. However, G4Py did not show any interaction with the
dendrimer. On the other hand, the binding constants of the
simple zinc tetraphenylporphyrin (ZnTPP) and DPA pyridines
were almost constant (G1 to G4). This means that the electronic
states of the pyridine (nitrogen) of each DPA pyridine are
similar and the electronic effect after complexation should also
be similar. Usually, dendritic hosts have a negative steric effect
due to the bulky dendron that covers the core binding site.9 The
unusual behaviour of DPA pyridines (larger binding constant
with larger guests) was also observed in the case of fullerenes
(the order of binding constants was C60 o C70 o C84),15 and
definitely proves the existence of a stable cavity. The size-selective
Fig. 3 Binding constants of ZnPG2-2 with fullerenes in the presence of 1000 eq.
DPA pyridines. The binding constants were determined by UV-vis titrations in
toluene : acetonitrile = 2 : 1 at 20 1C.
(C60) in an excess amount of 4-phenylpyridine (1000 eq.) was
performed (under these conditions, all ZnPG2-2 are bound with
4-phenylpyridine). Surprisingly, the binding constant of C60
increased ca. 1.5 times (Fig. S1, ESI,† 9.4 Æ 1.7 Â 103 MÀ1 to
1.5 Æ 0.2 Â 104 MÀ1). For further studies, similar titrations were
performed in the presence of bulkier phenylazomethine den-
dron substituted pyridine derivatives (DPA pyridines (GnPy,
n = generation), Fig. 1). Intriguingly, when the generation of
the DPA pyridines increased, the binding constants of C60 and
C
70 increased. On the other hand, the binding constants of C84
were almost the same or slightly decreased with a peak at G1Py
(Fig. 3). Within the experimental error it is difficult to predict,
but notably the binding constant of C70 is higher than that of
C84 in the presence of G3Py. To confirm this result, MALDI-
TOF-MS measurements were performed (Fig. 4). As reported
previously,15 when ZnPG2-2, C70 (10 eq.), and C84 (10 eq.) are
mixed, the MALDI-TOF-MS spectra show only the m/z peak that
corresponds to [ZnPG2-2 + C84]À due to the thermodynamic
stability of ZnPG2-2–C84 in the solid state. Upon addition of
G3Py (1 eq.) to this mixture, the peak of the ZnPG2-2–C84
complex decreased and the peak that corresponds to the
ZnPG2-2–C70 complex appeared. Upon further addition of
G3Py (5 eq.), the peak of the ZnPG2-2–C84 complex disappeared.
These binding constants and MALDI-TOF-MS results clearly
Fig. 4 MALDI-TOF-MS spectra of ZnPG2-2 with (a) 10 eq. of C70, and C84, (b)
10 eq. of C70 and C84, and 1 eq. of DPAG3py, and (c) 10 eq. of C70 and C84, and
5 eq. of DPAG3py.
Fig. 5 Binding constants of ZnTPP and ZnPG2-2 with DPA pyridines. The
binding constants were determined by UV-vis titrations in toluene : acetonitrile =
2 : 1 at 20 1C.
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complexation of pyridines to ZnPG2-2 indicates that G3Py well
matches the cavity size of ZnPG2-2 and has a positive interaction
with the dendron (probably p–p interaction) and the solvent
exclusion effect.
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From the discussion above, the behaviour of the binding
constants of fullerenes in the presence of pyridine derivatives
cannot be explained by electronic change or the solvent effect.
However, it can be understood from the steric effect, i.e., the
size of the second cavity for the fullerene becomes narrower
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the dendrimer. The large increase in DS0 indicates that the
second cavity changes the conformation and decreases the
vibration after the initial complexation with G3Py.
In conclusion, it was found that the size-selective fullerene
association of the fourth-generation carbazole–phenylazomethine
dendrimer can be controlled by axial coordination to the core
porphyrin. The thermodynamic studies and experiments using
bulky pyridine derivatives revealed that this modulation happens
mainly in a steric (allosteric) manner. As a consequence, the
selectivity of the fullerene could be changed by coordination of
G3Py (C70 o C84 to C70 > C84). This is one of the first examples of
an allosteric macromolecular host, and further studies on the
higher allosteric effect, higher selectivity, and applications to
fullerene isolation are underway.
This work was supported in part by the CREST program of
the Japan Science and Technology (JST) Agency, Grant-in-Aids
for Scientific Research on Innovative Areas ‘‘Coordination
Programming’’ (area 2107, no. 21108009) and by a Grant-in-Aid
for Encouragement of Young Scientists (B) (no. 24750099) from
the Japan Society for the Promotion of Science (JSPS).
15 K. Albrecht, Y. Kasai, Y. Kuramoto and K. Yamamoto, Chem.
Commun., 2013, 49, 865.
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