Reversible self-assembly of dendrimer based on polyhedral oligomeric silsesquioxanes (POSS)

Article abstract of DOI:10.1039/c0cc03359f

An acid-base switchable dendritic complex was constructed by self-assembly between dibenzo-24-crown-8 terminated T₁₀-POSS dendrimer and dibenzylammonium hexafluorophosphate salt based on T₈-POSS. The formation and its threading-dethr

Full text of DOI:10.1039/c0cc03359f

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Reversible self-assembly of dendrimer based on polyhedral oligomeric
silsesquioxanes (POSS)w
Xin Wang,* Vuthichai Ervithayasuporn, Yanhong Zhang and Yusuke Kawakami
Received 19th August 2010, Accepted 1st November 2010
DOI: 10.1039/c0cc03359f
An acid–base switchable dendritic complex was constructed
by self-assembly between dibenzo-24-crown-8 terminated
T10-POSS dendrimer and dibenzylammonium hexafluoro-
phosphate salt based on T8-POSS. The formation and its
dendrimer (host 1) and T8-POSS-based dibenzylammonium
hexafluorophosphate salt (guest 2-HÁPF6) were successfully
prepared in an excellent yield employing a convergent
approach via the click reaction. It was found that the host
shows excellent complexation abilities toward the guest and
consequently self-assembles into stable complex through non-
covalent bonding. The formation of dendritic complex and its
threading–dethreading property under molecularly acid–base
1
threading–dethreading property were characterized by H NMR
and UV-visible absorption spectroscopy.
Benefiting from their unique architectural, structural, and
functional features, dendrimers1 have attracted increasing
attention because of their potential applications in supra-
molecular chemistry.2 In this field, self-assembled dendrimers,3
especially, mechanically interlocked dendrimers,4 have been
extremely interesting to scientists as they offered reversible,
structural control and relatively large three dimensional
architectures starting from building blocks within nanometre
range. The self-assembled functionalization usually requires
high-affinity binding recognition motifs between molecules.
The threading property of secondary dialkylammonium salts
through the cavity of crown ethers,5 which was the genesis of a
diverse range of interlocked molecules,6 just meets this
requirement. With the development of various template
strategies, some classes of such topologically intriguing super-
molecules are now becoming more easily available than
before.7 However, how to construct the high order interlocked
assemblies, and what building blocks to choose, still remain as
challenges for switchable physical and chemical properties on
materials science.
1
switching were also investigated by H-NMR and UV-visible
absorption spectroscopy.
How to synthesize 1, our initial attempt on the attachment
of crown ether units to octakis(dimethylsilyl)silicate through
hydrosilylation reaction under toluene was considered to be a
failure. We suggest that the catalytic activity on Karstedt’s
catalyst [(Pt2(dvds)3] was possibly deactivated by the strong
complexation between crown ether moiety and Pt(0). To
achieve this goal, the Cu(I)-catalyzed Huisgen 1,3-dipolar
cycloaddition between deca(3-azidopropyl)silsesquioxane12
bearing ten functional azide groups and macrocyclic alkynyl
crown ether was successfully introduced at 40 1C for 48 h
leading to the formation of 1 in 89.0% yield (Scheme 1).
Synthesis of guest 2-HÁPF6 was also achieved by the click
reaction between freshly prepared monoazido-functionalized
POSS12 and dibenzylammonium alkyne, then treated with
conc. HCl and NH4PF6 solution to give 87% of total yield
(Scheme 2).
By a simple mixing of 1 with 2-HÁPF6 in 1 : 10 molar ratio,
the formation of a non-covalent dendritic complex has been
proposed as an illustrative dendrimer in Fig. 1. We explain
that holding the strong [N+–HÁ Á ÁO] hydrogen bonds between
Polyhedral oligomeric silsesquioxanes (POSS) molecules are
unique nanometre-sized structures.8 POSS molecules have a
polyhedral core which can be multifunctional and serve as a
platform that can be synthetically modified to contain groups
for various reactions.9 Since the click reaction between azide
and alkyne molecules has been considered to be an efficient
synthetic route, [3 + 2] dipolar cycloaddition reactions were
usually reported by the high yield and purity.10 The use of
POSS as cores for dendrimers is particularly attractive,
because their polyhedral structures produce spherically
symmetric dendrimers with smaller generation numbers than
conventional cores.11
+
the acidic NH2 protons and the oxygen atoms in the crown
ether ring, together with [C–HÁ Á ÁO] and p–p stacking van der
Waals interactions, as well as electrostatic forces, greatly
impacts the stability of the complex formation.
To the best of our knowledge, there is not one reported
example of synthesis of a POSS-based crown ether dendrimer.
Herein, the new dibenzo-24-crown-8 terminated T10-POSS
School of Materials Science, Japan Advanced Institute of Science and
Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan.
E-mail: tsukixin116@hotmail.com; Fax: +81 761-51-1635;
Tel: +81 761-51-1630
w Electronic supplementary information (ESI) available: Experimental
details and 1H, 13C, 29Si NMR, MALDI-TOF MS for all compounds
1
and H NMR of different ratio of 1 and 2-HÁPF6. See DOI: 10.1039/
c0cc03359f
Scheme 1 Synthesis of host 1.
c
1282 Chem. Commun., 2011, 47, 1282–1284
This journal is The Royal Society of Chemistry 2011

View Article Online
Scheme 2 Synthesis of guest 2-HÁPF6.
Fig. 3 Partial 1H NMR spectra (500 MHz, CDCl3, 298 K) of (a) free
1, (b) 1 and 10 equiv. of 2-HÁPF6 and (c) free 2-HÁPF6. [1]0 = 3.9 mM
(u = uncomplexed, c = complexed, and * = solvent residue).
Meanwhile, Fig. 2a confirmed the hyperchromic effect was
noticeable with increasing of the ratio of 2-HÁPF6 to 1.
However, no clear isosbestic point has been observed in this
case. We explain that the molecular self-assembly might
perturb to those aromatic rings by a small electronic influence
after complexation and isosbestic absorptions were likely
overlapped under spectra as well.
The complex of two molecules causes a considerable change
in their electronic environment, which leads to a shift of
signals in the NMR spectrum. The 1H NMR spectra were
obtained after 15 min of mixing of 1 and 2-HÁPF6. As shown in
Fig. 1 Structure of dendrimer by self-assembly of 1 and 2-HÁPF6.
1
Fig. 3b, the H NMR spectrum showed a dispersed array of
well-defined resonances and a great difference between 1 and
2-HÁPF6 (Fig. 3a and c). The partial 1H NMR spectrum of the
complex revealed that the characteristic benzylic methylene
proton (Ha and Hb) signals adjacent to the ammonium ion
center observed at 4.42 and 4.01 ppm in the 2-HÁPF6 were
shifted downfield to 4.63 and 4.49 ppm upon complexation.
Furthermore, the characteristic peaks for the protons on the
crown ether (Ha, Hb and Hg), which resonated at 4.13,
3.89 and 3.81 ppm in the spectrum of 1, were shifted upfield
to 4.11, 3.73 and 3.39 ppm in the spectrum of the complex,
respectively. These shifts of the NMR signals indicated that
complexation was taking place by threading of guest 2-HÁPF6
through dibenzo-24-crown-8 cavities of host 1, thus forming
the dendritic complex. The number of 2-HÁPF6 attaching onto
1, which originated from their NMR signal intensities of the
characteristic peaks, were calculated to be ten, by comparing
the ratio of average integrals between the complexed and
uncomplexed NMR signals. When similar complexation
experiments were carried out using an excess of 2-HÁPF6 to 1,
the degree of complexation remained almost unchanged from the
The support for the formation of the complex have been
investigated from its UV-visible absorption spectroscopy. The
absorption spectrum (Fig. 2a) of 1 (5.0 Â 10À6 mol LÀ1, fixed
concentration of 1, CH2Cl2, 298 K), besides a very strong
absorption peak at 230.5 nm, shows a weak peak at 279 nm
produced by the aromatic rings of crown ether. In contrast,
the UV-visible absorption spectrum (Fig. 2b) of 2-HÁPF6
(5.0 Â 10À5 mol LÀ1, CH2Cl2, 298 K) shows a weak absorption
peak at 221 nm and another stronger peak at 256 nm. When 1
and 2-HÁPF6 were mixed at different molar ratios from 1 : 4 to
1 : 18, the UV-visible absorption spectrum of the dendritic
complex (Fig. 2a) revealed gradual blue-shifts on the aromatic
rings of crown ethers due to a change of electronic level, as
well as slight red-shifts in another strong absorption peak
region with rising the ratios as a result of conformational
changes in the 1, induced by the formation of complex.
1
NMR spectra (ESIw). Moreover, the H NMR spectra of the
complex did not show any significant change after prolonged
standing, at least for one week, an observation which revealed
that the dendrimer complex was stable in CHCl3 solution.
One of the reasons for assembling mechanically interlocked
compounds is to construct molecular machines on the nano-
scale level, wherein their operation can be controlled through
external input, such as acid and base. The acid–base switching
Fig. 2 UV-visible absorption spectra of (a) 1 and complex in different
molar ratio of 1 and 2-HÁPF6 ([1]0 = 5.0 Â 10À6 mol LÀ1, CH2Cl2,
298 K) and (b) 2-HÁPF6 (c = 5.0 Â 10À5 mol LÀ1, CH2Cl2, 298 K).
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 1282–1284 1283

View Article Online
Notes and references
1 (a) E. Buhleier, W. Wehner and F. Vogtle, Synthesis, 1978, 155;
(b) C. J. Hawker and J. M. J. Frechet, J. Am. Chem. Soc., 1990,
112, 7638; (c) T. M. Miller and T. X. Neenan, Chem. Mater., 1990,
2, 346.
2 (a) J. M. Lehn, Angew. Chem., Int. Ed. Engl., 1988, 27, 89; (b) J. M.
Lehn, Angew. Chem., Int. Ed. Engl., 1990, 29, 1304; (c) G. R. Newkome,
Pure Appl. Chem., 1998, 70, 2337; (d) G. R. Newkome, E. F. He and
C. N. Moorefield, Chem. Rev., 1999, 99, 1689.
3 (a) F. Zeng and S. C. Zimmerman, Chem. Rev., 1997, 97, 1681;
(b) V. Balzani, H. Bandmann, P. Ceroni, C. Giansante, U. Hahn,
F.-G. Klarner, U. Muller, W. M. Muller, C. Verhaelen, V. Vicinelli
and F. Vogtle, J. Am. Chem. Soc., 2006, 128, 637.
4 (a) N. Yamaguchi, L. M. Hamilton and H. W. Gibson,
Angew. Chem., Int. Ed., 1998, 37, 3275; (b) A. M. Elizarov,
S.-H. Chiu, P. T. Glink and J. F. Stoddart, Org. Lett., 2002, 4,
679; (c) H. W. Gibson, N. Yamaguchi, L. Hamilton and
J. W. Jones, J. Am. Chem. Soc., 2002, 124, 4653; (d) K. C.-F.
Leung, F. Arico, S. J. Cantrill and J. F. Stoddart, J. Am. Chem. Soc.,
2005, 127, 5808; (e) K. C. -F. Leung, P. M. Mendes, S. N. Magonov,
B. H. Northrop, S. Kim, K. Patel, A. H. Flood, H. R. Tseng and
J. F. Stoddart, J. Am. Chem. Soc., 2006, 128, 10707.
5 (a) C. J. Pedersen, Angew. Chem., Int. Ed. Engl., 1988, 27, 1021;
(b) P. R. Ashton, P. J. Campbell, E. J. T. Chrystal, P. T. Glink,
S. Menzer, D. Philp, N. Spencer, J. F. Stoddart, P. A. Tasker and
D. J. Willianms, Angew. Chem., Int. Ed. Engl., 1995, 34, 1865.
6 (a) D. B. Amabilino and J. F. Stoddart, Chem. Rev., 1995, 95, 2725;
(b) J. D. Badjic, C. M. Ronconi, J. F. Stoddart, V. Balzani, S. Silvi
and A. Credi, J. Am. Chem. Soc., 2006, 128, 1489.
Fig. 4 Partial 1H NMR spectra (500 MHz, CDCl3, 298 K) of
(a) complex of 1 and 10 equiv. of 2-HÁPF6, (b) TEA base-treated,
and (c) TFA acid-treated. [1]0 = 3.9 mM (u = uncomplexed,
c = complexed, and * = solvent residue).
property of the complex (3.9 mM, calculated as the concentration
of 1) was monitored by 1H NMR spectroscopy. Upon
treatment of the complex with 1.5 equiv. (per crown ether)
of triethylamine (TEA), the 1H NMR spectrum (Fig. 4a)
indicated that the characteristic signals at 4.63 and 4.49 ppm
originating from the benzylic methylene protons adjacent to
+
the NH2
ion centers disappeared. The corresponding
characteristic peaks for the methylene protons adjacent to the
POSS-based secondary dibenzylammonium (–CH2–NH–CH2–)
center, resonating at 3.83 and 3.81 ppm occurred (Fig. 4b). At
the same time, the characteristic peaks for the protons on the
crown ether (Hac, Hbc and Hgc) in the complex were shifted
downfield from 4.11, 3.73 and 3.39 ppm to 4.13, 3.88 and
3.79 ppm again, respectively. These observations suggested
that 2-HÁPF6 were deprotonated and subsequently dethreaded
from the cavities of the crown ether in 1 to give the separate
products, that is, 1 and deprotonated 2-HÁPF6. Furthermore,
upon addition of 1.6 equiv. (per crown ether) of trifluoroacetic
acid (TFA) to the same solution, which is a slight excess to the
NMR sample previously treated with base, the original
1H NMR spectrum (Fig. 4c) was regenerated in every detail,
indicating that the switching process is pH-controlled and
completely reversible.
7 (a) K. S. Chichak, S. J. Cantrill, A. R. Pease, S. Chiu, G. W. V. Cave,
J. L. Atwood and J. F. Stoddart, Science, 2004, 304, 1308; (b) Y. H. Ko,
K. Kim, J. Kang, H. Chun, J. W. Lee, S. Sakamoto, K. Yamaguchi,
J. C. Fettinger and K. Kim, J. Am. Chem. Soc., 2004, 126, 1932;
(c) Y. Liu, P. A. Bonvallet, S. A. Vignon, S. I. Khan and J. F. Stoddart,
Angew. Chem., Int. Ed., 2005, 44, 3050; (d) K. Ghosh, J. M. Hu,
H. S. White and P. J. Stang, J. Am. Chem. Soc., 2009, 131, 6695.
8 G. Z. Li, L. C. Wang, H. L. Ni and C. U. Pittman Jr, J. Inorg.
Organomet. Polym., 2001, 11, 123.
9 (a) C. Zhang and R. M. Laine, J. Organomet. Chem., 1996, 521, 199;
(b) F. J. Feher, D. Soulivong, A. G. Eklund and K. D. Wyndham,
Chem. Commun., 1997, 1185; (c) I. M. Saez, J. W. Goodby and
R. M. Richardson, Chem.–Eur. J., 2001, 7, 2578; (d) I. Imae and
Y. Kawakami, J. Mater. Chem., 2005, 15, 4581; (e) K. Wada,
N. Watanabe, K. Yamada, T. Kondo and T. A. Mitsudo, Chem.
Commun., 2005, 95; (f) R. M. Laine, M. Roll, M. Asuncion,
S. Sulaiman, V. Popova, D. Bartz, D. J. Krug and P. H. J. Mutin,
J. Sol-Gel Sci. Technol., 2008, 46, 335.
10 (a) R. Huisgen, Angew. Chem., Int. Ed. Engl., 1963, 2, 633;
(b) Q. Wang, T. R. Chan, R. Hilgraf, V. V. Fokin,
K. B. Sharpless and M. G. Finn, J. Am. Chem. Soc., 2003, 125,
3192; (c) M. Meldal and C. W. Tornøe, Chem. Rev., 2008, 108,
2952; (d) V. Ervithayasuporn, J. Abe, X. Wang, T. Matsushima,
H. Murata and Y. Kawakami, Tetrahedron, 2010, 66, 9348.
11 (a) G. R. Newkome, C. N. Moorefield and F. Vogtle, in
Dendritic Molecules, VCH, Weinheim, New York, 1996;
(b) B. Trastoy, M. E. Perez-Ojeda, R. Sastre and J. L. Chiara,
Chem.–Eur. J., 2010, 16, 3833; (c) D. Heyl, E. Rikowski,
R. C. Hoffmann, J. J. Schneider and W.-D. Fessner, Chem.–Eur.
J., 2010, 16, 5544.
In summary, we have synthesized a novel dibenzo-24-
crown-8 terminated T10-POSS dendrimer and demonstrated
that it could form a stable dendritic complex with dibenzyl-
ammonium hexafluorophosphate salt based on T8-POSS.
Moreover, the complex processes could be controlled by
changing the solution pH, which would be useful for the
design of chemically controlled molecular machines.
Financial support from MEXT to X. Wang and V.
Ervithayasuporn, Grant-in Aid for Scientific Research is
gratefully acknowledged.
12 V. Ervithayasuporn, X. Wang and Y. Kawakami, Chem. Commun.,
2009, 5130.
c
1284 Chem. Commun., 2011, 47, 1282–1284
This journal is The Royal Society of Chemistry 2011