Tweezering the core of a dendrimer: A photophysical and electrochemical study

Article abstract of DOI:10.1002/anie.200501025

(Chemical Equation Presented) A large guest for a small host: The tweezering process about the bipyridinium core of a dendrimer by a molecular tweezer (see picture) has been studied electrochemically and by fluorescence spectroscopy and is found to be quite similar to the threading/dethreading processes observed with wire-type bipyridinium-based units and ring-shaped molecules.

Full text of DOI:10.1002/anie.200501025

Communications
Host–Guest Chemistry
Tweezering the Core of a Dendrimer: A
Photophysical and Electrochemical Study**
Vincenzo Balzani,* Paola Ceroni, Carlo Giansante,
Veronica Vicinelli, Frank-Gerrit Klärner,*
Carla Verhaelen, Fritz Vögtle,* and Uwe Hahn
Dendrimers[1] are complex, yet well-defined, branched com-
pounds that exhibit a high degree of constitutional order, with
the possibility of containing selected chemical units at
predetermined sites of their structure (core, branches, periph-
[*] Prof. Dr. V. Balzani, Dr. P. Ceroni, C. Giansante, Dr. V. Vicinelli
Dipartimento di Chimica “G. Ciamician”
Universitàdi Bologna
Via Selmi 2, 40126 Bologna (Italy)
Fax: (+39)051-209-9456
E-mail: vincenzo.balzani@unibo.it
Prof. Dr. F.-G. Klärner, Dipl.-Chem. C. Verhaelen
Institut für Organische Chemie
Universität Essen-Duisburg, Campus Essen
Universitätsstrasse 5, 45117 Essen (Germany)
Fax: (+49)201-183-4252
E-mail: frank.klaerner@uni-essen.de
Prof. Dr. F. Vögtle, Dr. U. Hahn
KekulØ-Institut für Organische Chemie und Biochemie
Universität Bonn
Gerhard-Domagk Strasse 1, 53121 Bonn (Germany)
Fax: (+49)228-735-662
E-mail: voegtle@uni-bonn.de
[**] We thank Professors Mauro Maestri and Margherita Venturi for
useful discussions, and Heinz Bandmann for NMR measurements.
This work was supported by the MIUR (Supramolecular Devices
Project) and the Deutsche Forschungsgemeinschaft (SFB 452).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
4574
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200501025
Angew. Chem. Int. Ed. 2005, 44, 4574 –4578

Angewandte
Chemie
ery). As a result of their nanoscopic size and fractal
structure, dendrimers resemble biocomponents such as
viruses, enzymes, and proteins.[2] Dendrimers that con-
tain redox-active units are currently attracting much
attention.[3] When a large number of redox-active units
are placed in the periphery and/or along the branches,
dendrimers can undergo multiple electron-transfer
processes and perform, for example, as molecular
batteries, sensors, and catalysts.[4] Dendrimers with a
single redox site as a core are the simplest examples of
systems with an encapsulated redox center,[5] and their
redox properties are usually modulated by the size and
nature of the dendritic branches.[6]
Despite their large structures, which usually render
dendrimers attractive as host molecules, suitably
designed dendrimers can be equally involved as guests
in molecular-recognition phenomena.[7] In such cases,
the host species does not interact with the whole
dendritric structure, but only with specific component
units. Usually, the guest behavior of the dendrimer is
connected with the threading of dendritic branches by
ring-shaped molecules such as crown ethers,[8] cyclo-
dextrins,[9] and cucurbiturils.[10]. More recently, Kaifer
and co-workers[7,11] prepared several dendrimers that
contain a single-site, potential guest unit and have
investigated their adducts with cucurbit[7]uril, b-cyclo-
dextrin, and bis-para-phenylene[34]crown-10.
When the potential guest unit constitutes the core of
a dendrimer, the formation of a host–guest complex
through threading of ring-shaped hosts cannot take
place. It occurred to us that in such a case, tweezer-
shaped molecules could be used to enclose the dendritic
core into a host structure. Herein, we show that this
strategy works in solution when suitably designed host
and guest units are used.[12]
Several molecular tweezers and clips that exhibit
electron-donating properties have been prepared
recently in one of our laboratories.[13] Molecular
tweezer T, which comprises one naphthalene and four
benzene components bridged by four methylene units,
was prepared and purified as previously reported
(Figure 1).[14] NMR spectroscopy (see Supporting Infor-
mation for details) showed that tweezer T forms a stable
1:1 complex with 4,4’-N,N-bis(3,5-di-tert-butylbenzyl)-
bipyridinium [D0]2+. Solvent-dependent binding con-
stants (Ka = 730mꢀ1 in [D6]acetone/CDCl3 (2:1) and
Ka = 8400mꢀ1 in CD2Cl2), and complexation-induced
upfield shifts of the chemical shifts for the protons of
the guest (Ddmax = 1.01 or 0.91 ppm (Ho), and 4.12 or
1
3.22 ppm (Hm)) were determined by H NMR titration
experiments (500 MHz, Bruker DRX 500) at 258C, and
the 1:1 stoichiometry of the complex was determined
through Job plot analysis. The observed large Ddmax
values and the specific broadening of the 1H NMR
signals assigned to the bridgehead protons of the complexed
tweezer at low temperature are evidence that one of the
pyridinium rings of [D0]2+ is positioned inside the cavity of
the tweezer and that tweezer T shuttles from one pyridinium
ring of [D0]2+ to the other. At room temperature the shuttling
Figure 1. Formulae of the investigated tweezer (T) and dendrimeric compounds (Dn
where n indicates the dendron generation: n=0,1,2,3).
process is fast and leads to an averaging of the 1H NMR
signals of the guest molecule lying inside/outside of the
tweezer cavity, while at ꢀ758C the process is slow with respect
to the NMR timescale (DG°(shuttle) = 10 kcalmolꢀ1, esti-
mated from the line broadening). As the 3,5-di-tert-butylben-
Angew. Chem. Int. Ed. 2005, 44, 4574 –4578
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4575

Communications
zyl stopper groups in [D0]2+ are already too bulky to form a
[Dn]2+ dendrimers[15,19] show two one-electron-transfer proc-
esses that correspond to the successive reductions of the 4,4’-
bipyridinium core: bpy2+!bpy+ and bpy+!bpy. The values of
the potentials are gathered in Table 1, and the cyclic
voltammetric curve for [D2]2+ is shown in Figure 3.
complex with tweezer T by threading and as the complexation
has to proceed by clipping of the guest molecule through the
tips of the tweezer, we thought that T should also be able to
interact with the bipyridinium core (bpy2+) of the first-,
second-, and third-generation dendrimers [Dn]2+ (n = 1, 2 and
3; see Figure 1). NMR spectroscopy, however, could not
provide information on the formation of a complex in the case
of the dendrimers because of solubility problems and line
broadening. Therefore we carried out a photophysical and
electrochemical investigation of the interactions.
Table 1: Half-wave potentials versus SCE (saturated calomel electrode)[a]
and association constants.[b]
bpy2+!bpy+ [V]
bpy+!bpy [V]
Ka ”10ꢀ3 [mꢀ1
]
[D1]2+ [c]
[D1]2+·T[c]
[D2]2+
ꢀ0.25
ꢀ0.30
ꢀ0.24
ꢀ0.31
ꢀ0.24
ꢀ0.30
ꢀ0.73
ꢀ0.73
ꢀ0.72
ꢀ0.72
ꢀ0.72
ꢀ0.72
The hexafluorophosphate salts of the [Dn]2+ dendrimers
had been prepared during the course of previous work.[15] The
equipment used for the photophysical and electrochemical
measurements has been described elsewhere.[16] All the
experiments were performed at room temperature (293 K)
in air-equilibrated solution. Quantum yields of fluorescence
were measured following the methods of Demas and
Crosby[17], using terphenyl in cyclohexane (F = 0.82) as the
standard.[18] Dichloromethane was used as the solvent for the
photophysical experiments, and mixtures of dichloromethane/
acetonitrile (3:1 or 9:1 v/v) were used for the electrochemical
experiments. The association constants were obtained from
fluorescence titration experiments.
27
18
9
[D2]2+·T
[D3]2+
[D3]2+·T
[a] Solvent: dichloromethane/acetonitrile (9:1), unless otherwise noted;
(NBu4)PF6 (0.1m); under these conditions more than 95% of the
electroactive species are in the complexed form. [b] Solvent: dichloro-
methane; from fluorescence titration experiments. [c] Solvent: dichloro-
methane/acetonitrile (3:1).
The absorption and emission spectra of tweezer T in
dichloromethane are shown in Figure 2. The strong fluores-
cence emission band at lmax = 344 nm (t = 9.5 ns, F = 0.53)
Figure 3. Cyclic voltammetry (CV) curves for a solution of [D2]2+ at
1.0”10ꢀ3 m in dichloromethane/acetonitrile (9:1) in the absence (c)
and presence (a) of tweezer T (3.2”10ꢀ3 m). Scan rate: 0.2 Vsꢀ1
(NBu4)PF6 (0.1m).Under such conditions, the association constant
(Table 1) shows that about 98% of the species are associated.
;
Titrations of solutions of tweezer T at approximately
10ꢀ5 m in dichloromethane with the dendrimers did not cause
any changes in the absorption spectra relative to the spectra
expected from the absorptions of the two components. The
Figure 2. Absorption spectra of tweezer T (c) and dendrimer [D3]2+
(as PF6ꢀ salt; a) in dichloromethane at room temperature. Inset:
Emission spectrum of tweezer T in dichloromethane at 293 K
(lex =334 nm).
fluorescence band of
T (lmax = 344 nm), however, was
quenched, as shown in Figure 4 for the case of [D3]2+. As
dynamic quenching can be ruled out because of the short
excited-state lifetime of T (t = 9.5 ns), these results indicate
that the tweezer and dendrimers give rise to adducts. We also
verified that the fluorescence of T is not quenched upon
addition to the solution of the dendrons used to construct the
dendrimers. Therefore we conclude that adduct formation
must involve an interaction between the tweezer and the
bipyridinium dendritic cores.
can be assigned to the naphthalene unit. The absorption
spectra of dendrimers [Dn]2+ (see Figure 2 for an example)
display a strong absorption band around l = 260 nm from the
bipyridinium unit, while the 1,3-dimethyleneoxybenzene
units of the dendrons show an absorption around l =
280 nm. The fluorescence of the 1,3-dimethyleneoxybenzene
units is completely quenched in the dendrimers.[15]
In a solution of 9:1 dichloromethane/acetonitrile, tweezer
T shows an irreversible oxidative process at + 1.6 V, whereas
no reduction process could be observed in the potential
window of the solvent used (up to ꢀ2.0 V vs. SCE). The
To elucidate the stoichiometry and the strength of adduct
formation, fluorescence titration experiments were per-
formed by taking into account[20] the fraction of light absorbed
4576
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 4574 –4578

Angewandte
Chemie
units. The assembly/disassembly process can be monitored by
fluorescence and electrochemical measurements. The twee-
zering process between T and [Dn]2+ is quite similar to the
threading/dethreading processes observed with bipyridinium-
based wire-type units and ring-shaped molecules,[21] with the
advantage that tweezering can occur also when the wire-type
unit terminates with bulky groups.
Received: March 21, 2005
Published online: June 27, 2005
Keywords: dendrimers · electrochemistry · fluorescence ·
.
molecular tweezers · supramolecular chemistry
[1] a) G. R. Newkome, C. Moorefield, F. Vögtle, Dendrimers and
Dendrons: Concepts, Syntheses, and Perspectives, VCH, Wein-
heim, 2001; b) Special issue on Dendrimers and Nanoscience, D.
Astruc, C. R. Chim. 2003, 6, 709; c) V. Balzani, P. Ceroni, M.
Maestri, C. Saudan, V. Vicinelli, Top. Curr. Chem. 2003, 228, 159;
d) J-P. Majoral, A.-M. Caminade, V. Maraval, Chem. Commun.
2002, 2929; e) S. Hecht, J. M. J. FrØchet, Angew. Chem. 2001, 113,
76; Angew. Chem. Int. Ed. 2001, 40, 74; f) M. W. P. L. Baars,
E. W. Meijer, Top. Curr. Chem. 2001, 210, 131.
Figure 4. Changes in the fluorescence spectrum of a solution of T at
1.6”10ꢀ5 m in dichloromethane at room temperature upon addition of
10 equivalents of [D3]2+ (lex =334 nm). Inset: The titration curve
obtained by plotting the emission intensity at l=356 nm as a function
of the equivalents of [D3]2+ added, after correction for the fraction of
light absorbed by T. The solid line shows the fit based on formation of
a 1:1 complex. a.u.=arbitrary units.
[2] a) B. Alonso, D. Astruc, J.-C. Blais, S. Nlate, S. Rigaut, J. Ruiz, V.
Sartor, C. ValØrio, C. R. Acad. Sci. Ser. IIc 2001, 173; b) L. Liu, R.
Breslow, J. Am. Chem. Soc. 2003, 125, 12110; c) M. F. Ottaviani,
S. Jockusch, N. J. Turro, D. A. Tomalia, A. Barbon, Langmuir
2004, 20, 10238.
[3] a) P. A. Chase, R. J. M. K. Gebbink, G. van Koten, J. Organomet.
Chem. 2004, 689, 4016; b) D. Astruc, Pure Appl. Chem. 2003, 75,
461; c) F. Diederich, B. Felber, Proc. Natl. Acad. Sci. USA 2002,
99, 4778; d) A. Juris, M. Venturi, P. Ceroni, V. Balzani, S.
Campagna, S. Serroni, Collect. Czech. Chem. Commun. 2001, 66,
1; e) C. B. Gorman, J. C. Smith, Acc. Chem. Res. 2001, 34, 60; f) I.
Cuadrado, M. Morµn, C. M. Casado, B. Alonso, J. Losada,
Coord. Chem. Rev. 1999, 193–195, 395.
[4] a) H. D. Abruæa, Anal. Chem. 2004, 76, 310A; b) M.-C. Daniel,
F. Ba, J. A. Ruiz, D. Astruc, Inorg. Chem. 2004, 43, 8649; c) B.
Alonso, C. M. Casado, I. Cuadrado, M. Morµn, A. E. Kaifer,
Chem. Commun. 2002, 1778.
[5] C. M. Cardona, S. Mendoza, A. E. Kaifer, Chem. Soc. Rev. 2000,
29, 37.
[6] a) T. L. Chasse, C. B. Gorman, Langmuir 2004, 20, 8792; b) F.
Marchioni, M. Venturi, P. Ceroni, V. Balzani, M. Belohradsky,
A. M. Elizarov, H.-R. Tseng, J. F. Stoddart, Chem. Eur. J. 2004,
10, 6361; c) N. D. McClenahan, R. Passalacqua, F. Loiseau, S.
Campagna, B. Verheyde, A. Hameurlaine, W. Dehaen, J. Am.
Chem. Soc. 2003, 125, 5356; d) Y. Rio, G. Accorsi, N. Armaroli,
D. Felder, E. Levillain, J.-F. Nierengarten, Chem. Commun.
2002, 2830; e) P. J. Dandliker, F. Diederich, M. Gross, C. B.
Knobler, A. Louati, Angew. Chem. 1994, 106, 1821; Angew.
Chem. Int. Ed. Engl. 1994, 33, 1739.
[7] W. Ong, M. Gomez-Kaifer, A. E. Kaifer, Chem. Commun. 2004,
1677.
[8] H. W. Gibson, N. Yamaguchi, L. Hamilton, J. W. Jones, J. Am.
Chem. Soc. 2002, 124, 4653.
[9] K. J. C. van Bommel, G. A. Metselaar, W. Verboom, D. N.
Reinhoudt, J. Org. Chem. 2001, 66, 5405.
[10] J. W. Lee, Y. H. Ko, S.-H. Park, K. Yamaguchi, K. Kim, Angew.
Chem. 2001, 113, 769; Angew. Chem. Int. Ed. 2001, 40, 746.
[11] a) W. Ong, J. Grindstaff, D. Sobransingh, R. Toba, J. M.
Quintela, C. Peinador, A. E. Kaifer, J. Am. Chem. Soc. 2005,
127, 3353; b) M. Moon, J. Grindstaff, D. Sobransingh, A. E.
Kaifer, Angew. Chem. 2004, 116, 5612; Angew. Chem. Int. Ed.
2004, 43, 5496; c) W. Ong, A. E. Kaifer, Angew. Chem. 2003, 115,
by the dendrimer. The titration plot obtained for a solution of
Tat 1.6 ” 10ꢀ5 m in dichloromethane upon addition of [D3]2+ is
shown in the inset of Figure 4. As the fluorescence lifetime
(t = 9.5 ns) is not affected upon addition of the dendrimer and
as there is no evidence of a double-exponential decay, we
conclude that even after addition of an excess of dendrimer a
significant fraction of tweezer remains uncomplexed in such a
dilute solution and that the adducts do not show any
appreciable fluorescence.
The results obtained from the titration plots show that 1:1
[Dn]2+·T adducts are formed, with values for the association
constants of the order of 104 mꢀ1 which decrease with
increasing dendrimer generation (Table 1). A similar depend-
ence of the stability of the complex on the size of the
dendrimer was observed in the gas phase by mass spectros-
copy.[12] Apparently, even in solution the bipyridinium core is
stabilized by “intramolecular solvation”, which results from
the back-folding of the electron-donor branches, an effect that
increases with increasing dendrimer generation.
The CV patterns for reduction of the dendritic cores are
affected by the presence of tweezer T (Table 1 and Figure 3)
which shows that the formation of the adduct involves an
interaction between the tweezer and the bipyridinium den-
dritic cores. In particular, both the cathodic and anodic peaks
which correspond to the first one-electron-reduction process
of the dendritic core move to more negative values upon
addition of the tweezer, whereas the peaks that correspond to
the second reduction process are almost unaffected. Such a
behavior shows that 1) formation of the adduct is caused by a
charge-transfer interaction and 2) the adduct dissociates upon
one-electron reduction of the dendritic core.
In conclusion, we have shown that the 4,4’-bipyridinium
core of first-, second-, and third-generation [Dn]2+ dendrim-
ers that bear FrØchet-type dendrons can be hosted by the
molecular tweezer T, which comprises a naphthalene moiety
and four benzene components bridged by four methylene
Angew. Chem. Int. Ed. 2005, 44, 4574 –4578
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4577

Communications
2214; Angew. Chem. Int. Ed. 2003, 42, 2164; d) R. Toba, J. M.
Quintela, C. Peinador, E. Romµn, A. E. Kaifer, Chem. Commun.
2002, 1768; e) C. M. Cardona, T. D. McCarley, A. E. Kaifer, J.
Org. Chem. 2000, 65, 1857.
[12] For a somewhat related gas-phase study of these systems, see:
C. A. Schalley, C. Verhaelen, F.-G. Klꢀrner, U. Hahn, F. Vögtle,
Angew. Chem. 2005, 117, 481; Angew. Chem. Int. Ed. 2005, 44,
477.
[13] a) F.-G. Klꢀrner, B. Kahlert, Acc. Chem. Res. 2003, 36, 919; b) F.
Marchioni, A. Juris, M. Lobert, U. P. Seelbach, B. Kahlert, F.-G.
Klꢀrner, New J. Chem. in press.
[14] F.-G. Klꢀrner, U. Burkert, M. Kamieth, R. Boese, J. Phys. Org.
Chem. 2000, 13, 604.
[15] P. Ceroni, V. Vicinelli, M. Maestri, V. Balzani, W. M. Mꢁller, U.
Mꢁller, U. Hahn, F. Osswald, F. Vögtle, New J. Chem. 2001, 25,
989.
[16] D. B. Amabilino, M. Asakawa, P. R. Ashton, R. Ballardini, V.
Balzani, M. Belohradsky, A. Credi, M. Higuchi, F. M. Raymo, T.
Shimizu, J. F. Stoddart, M. Venturi, K. Yase, New J. Chem. 1998,
22, 959.
[17] J. N. Demas, G. A. Crosby, J. Phys. Chem. 1971, 75, 991.
[18] I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic
Molecules, 2nd ed., Academic Press, New York, 1971.
[19] R. Toba, J. M. Quintela, C. Peinador, E. Roman, A. E. Kaifer,
Chem. Commun. 2001, 857.
[20] A. Credi, L. Prodi, Spectrochim. Acta Part A 1998, 54, 159.
[21] V. Balzani, A. Credi, M. Venturi, Molecular Devices and
Machines, A Journey into the Nano World, Wiley-VCH, Wein-
heim, 2003.
4578
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 4574 –4578