Polyviologen dendrimers as hosts and charge-storing devices

Article abstract of DOI:10.1002/chem.200800773

Two dendrimers were designed and synthesized that contain a 1,3,5-trisubstituted benzenoid core and incorporate 9 and 21 viologen (4,4′-bipyridinium) units in their branches in addition to hydrophilic (aryloxy) terminal groups. For comparison purposes, model compounds containing one and two viologen units were also studied. These polycationic dendrimers form strong host-guest complexes with the dianionic form of the red dye eosin in dilute CH₂Cl₂ solutions. Titration experiments, based on fluorescence measurements, showed that each viologen unit in the dendritic structures becomes associated with an eosin dianion. Electrochemical (in MeCN) and photosensitization (in CH₂Cl₂) experiments revealed that only a fraction of the viologen units present in the dendritic structures can be reduced. This fraction corresponds to the number of viologen units present in the outer shells of the dendrimers. The reasons for incomplete charge pooling are discussed. Comparison with the behavior of polyviologen dendrimers that are terminated with bulky tetraarylmethane groups and were studied previously enabled the role played by the terminal groups in the redox and hosting properties to be elucidated.

Full text of DOI:10.1002/chem.200800773

FULL PAPER
DOI: 10.1002/chem.200800773
Polyviologen Dendrimers as Hosts and Charge-Storing Devices
CØlia M. Ronconi,[b, d] J. Fraser Stoddart,*[a] Vincenzo Balzani,[c] Massimo Baroncini,[c]
Paola Ceroni,[c] Carlo Giansante,[c] and Margherita Venturi*[c]
Abstract: Two dendrimers were de-
signed and synthesized that contain a
1,3,5-trisubstituted benzenoid core and
incorporate 9 and 21 viologen (4,4’-bi-
pyridinium)units in their branches in
addition to hydrophilic (aryloxy)termi-
nal groups. For comparison purposes,
model compounds containing one and
two viologen units were also studied.
These polycationic dendrimers form
strong host–guest complexes with the
dianionic form of the red dye eosin in
dilute CH2Cl2 solutions. Titration ex-
periments, based on fluorescence meas-
urements, showed that each viologen
unit in the dendritic structures becomes
associated with an eosin dianion. Elec-
trochemical (in MeCN)and photosen-
sitization (in CH2Cl2)experiments re-
vealed that only a fraction of the violo-
gen units present in the dendritic struc-
tures can be reduced. This fraction cor-
responds to the number of viologen
units present in the outer shells of the
dendrimers. The reasons for incomplete
charge pooling are discussed. Compari-
son with the behavior of polyviologen
dendrimers that are terminated with
bulky tetraarylmethane groups and
were studied previously enabled the
role played by the terminal groups in
the redox and hosting properties to be
elucidated.
Keywords: dendrimers
·
electro-
chemistry · host–guest systems ·
photosensitization · viologens
Introduction
can lead to unusual, sometimes unpredictable, physicochem-
ical properties that result in a wide range of potential appli-
cations.[2] Dendrimers containing photoactive and/or electro-
active moieties are currently attracting much attention,
since they can be designed to perform useful functions, such
as light harvesting,[3] charge pooling,[4,5,6] ion sensing with
signal amplification,[7] and molecular recognition.[8]
Dendrimers are repetitively branched, yet constitutionally
well-defined, macromolecules exhibiting high monodispersi-
ty.[1] These nanoscopic compounds can contain selected
chemical units in predetermined sites of their structure
(core, branches, periphery), mutual interactions of which
Dendrimers B918+ and B2142+ (Scheme 1), discussed in
this paper, are based on a 1,3,5-trisubstituted benzenoid
core and contain 9 and 21 4,4’-bipyridinium (viologen)units,
respectively, in their branches, and 6 and 12 aryloxy groups,
respectively, at their peripheries. Viologen is a well-known
electroactive species that undergoes two successive one-elec-
tron, reversible reduction processes[9,10] and exhibits peculiar
spectroscopic features in both its dicationic and radical-cat-
ionic forms. Viologen units are also known to give strong
donor–acceptor complexes with electron-donating species.[10]
We report 1)the synthesis of dendrimers B918+ and
B2142+ as their hexafluorophosphate salts, 2)their interac-
tion with the eosin dianion (Ey2ꢀ, Scheme 1), and 3) their
electrochemical and photosensitized reduction. For compari-
son purposes, we also investigated the properties of mono-
[a] Prof. J. F. Stoddart
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, IL 6028 (USA)
Fax : (+1)847-491-7713
E-mail: stoddart@northwestern.edu
[b] Prof. C. M. Ronconi
Department of Chemistry and Biochemistry
University of California, Los Angeles
405 Hilgard Avenue, Los Angeles, CA90095 (USA)
[c] Prof. V. Balzani, M. Baroncini, Prof. P. Ceroni, C. Giansante,
Prof. M. Venturi
Dipartimento di Chimica “G. Ciamician”
Università di Bologna, Via Selmi 2, 40126 Bologna (Italy)
Fax : (+39)051-209-9456
E-mail: margherita.venturi@unibo.it
viologen compounds 1,1’-dibenzyl-4,4’-bipyridinium (dbV2+
)
[d] Prof. C. M. Ronconi
Present address: Instituto de Quimica
and 1,1’-dioctyl-4,4’-bipyridinium (doV2+), as well as den-
Universidade Federal do Rio de Janeiro
Centro de Tecnologia, Bloco A, 68 Andar, sala 637
Cid. Universitµria, Rio de Janeiro, RJ CEP 21.941.590 (Brasil)
dron B24+
, which contains only two viologen units
(Scheme 1).
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The results are discussed in
comparison with those reported
previously for two analogous
dendrimers (A918+ and A2142+
)
terminated with tetraarylme-
thane bulky moieties instead of
aryloxy groups.[5,11] The proper-
ties of dendron A24+, analo-
gous to B24+, were also investi-
gated in order to complete the
comparison between the two
families of compounds.
Results
Synthesis: The synthetic routes
to dendrimers B9·18PF6 and
B21·42PF6 and their intermedi-
ates 5·4PF6 and 8·12PF6 are
outlined in Schemes 2 and 3.
Bromide 5·4PF6 (Scheme 2)
was obtained in 61% yield by
treating alcohol B2·4PF6 with
CBr4 in dry MeCN in the pres-
ence of PPh3. Alkylation of salt
6·3PF6 with bromide 5·4PF6 in
MeCN
gave
dendrimer
B9·18PF6 in 25% yield after
counterion exchange. Reaction
of bromide 5·4PF6 with salt
4·2PF6 followed by counterion
exchange
7·12PF6
afforded
in 65%
alcohol
yield
(Scheme 3). Alcohol 7·12PF6
was treated with CBr4 in dry
MeCN in the presence of PPh3
to obtain bromide 8·12PF6,
which was then treated with
salt 6·3PF6 to give dendrimer
B21·42PF6 in an overall yield
of 15% after counterion ex-
change.
Scheme 1. Structural formulas of the investigated compounds. Symbols Bn2n+ indicate the number of viologen
units in the dendritic structure (n)and the overall electric charge (2 n)on each compound.
messo in evidenza che non tutte le unità viologeno contenute
nei dendrimeri possono essere ridotte; in particolare, sembra
che siano riducibili solo quelle presenti nel guscio dendritico
piffl esterno. Il confronto con il comportamento di composti
modello, contenenti una e due unità viologeno, e di dendri-
meri precedentemente investigati, che differiscono da quelli
discussi in questo lavoro per la presenza di gruppi terminali
piffl ingombranti di tipo tetraarilmetano, ha fatto luce sui
motivi che inibiscono la riduzione di tutte le unità viologeno
contenute nelle strutture dendritiche e ha chiarito se e come
la natura dei gruppi terminali influenza le proprietà redox e
complessanti di questi dendrimeri.
Abstract in Italian: Sono stati progettati e sintetizzati due
dendrimeri contenenti unꢀunità benzenica sostituita nelle po-
sizioni 1,3,5 come core, 9 e 21 unità viologeno (4,4’-dipiridi-
nio), rispettivamente, nelle ramificazioni e gruppi idrofilici di
tipo arilossi come unità periferiche. In CH2Cl2 questi dendri-
meri policationici interagiscono con la forma dianionica del-
lꢀeosina dando complessi host–guest in cui, come mostrato
chiaramente dalle titolazioni basate su misure di fluorescen-
za, ogni unità viologeno contenuta nella struttura dendritica
si associa con un anione eosina. Esperimenti di riduzione
elettrochimica (in MeCN) e fotochimica (in CH2Cl2 e in pre-
senza di un opportuno fotosensibilizzatore) hanno inoltre
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Polyviologen Dendrimers
FULL PAPER
with that of dbV2+, employed
as model compound of the den-
drimer viologen units, at con-
centrations
9 and 21 times
higher than those of B918+ and
B2142+, respectively. Evidently,
the absorption spectra of the
dendrimers are not identical to
that of the model compound,
and the differences are much
more pronounced for B2142+
.
In particular, its spectrum is
less intense than that of the
model compound in the spec-
tral region around the maxi-
mum and shows broad and
weak absorption features that
are also present to a minor
extent in the spectrum of B918+
and emerge from the low-
energy tail of the intense UV
band.
Eosin complexation: 4,4’-Bipyr-
idinium
dications
interact
strongly with the dianionic
Scheme 2. Synthesis of dendron B2·4PF6 and dendrimer B9·18PF6.
form of eosin (Ey2ꢀ)to yield
charge-transfer
plexes.[11,12] We observed previously that addition of doV2+
as its hexafluorophosphate salt, to a solution of Ey2ꢀ in
(CT)com-
Absorption spectra: Figure 1 shows the absorption spectra
in MeCN of dendrimers B918+ and B2142+ in comparison
,
Scheme 3. Synthesis of dendrimer B21·42PF6.
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J. F. Stoddart, M. Venturi et al.
ates with one eosin dianion. Titration of dendron B24+
(9.0”10ꢀ6 m viologen units)confirms the stoichiometric for-
mation of 1:1 complexes between the viologen units and
eosin (Figure 2). The quenching of eosin fluorescence by as-
sociation with the quencher (static mechanism)is confirmed
by the fact that dynamic quenching can be ruled out consid-
ering the short lifetime (t=3.8 ns)of the eosin excited state
and the low concentration (<5”10ꢀ5 m)of the quencher.
Electrochemical reduction: Electrochemical experiments
were carried out in argon-purged MeCN. The half-wave po-
tentials, number of exchanged electrons, and diffusion coef-
ficients of dbV2+ and dendrimers B918+ and B2142+ are
listed in Table 1, in which results previously obtained[5] for
dendrimers A918+ and A2142+ are also reported for compar-
ison purposes (see Discussion section).
Figure 1. Absorption spectra in MeCN of dendrimers B918+ (dashed line)
and B2142+ (solid line)compared with the spectra of 9 (dotted line)and
21 (dashed and dotted line) dbV2+ units. Inset: Enlarged view (l>
300 nm)of the spectra of the dendrimers.
Table 1. Half-wave reduction potentials, numbers of exchanged electrons,
and diffusion coefficients in argon-purged MeCN solution at 298 K.
[a]
[a]
[b]
E11=2
E21=2
nel
n[c]
105 D
[cm2 sꢀ1
]
[V vs. SCE]
[V vs. SCE]
A
dbV2+
ꢀ0.35
ꢀ0.30
ꢀ0.30
ꢀ0.29
ꢀ0.30
ꢀ0.77
1
6
11
5
1
9
21
9
1.60
0.45
0.27
0.32
0.27
B918+
ꢀ0.78[d]
ꢀ0.73[d]
ꢀ0.75[d]
ꢀ0.76[d]
CH2Cl2, as its tetrabutylammonium salt, causes 1)noticeable
perturbations in the visible absorption band of Ey2ꢀ, and
2)complete quenching of its fluorescence because of the for-
mation of a strong 1:1 complex (Kass >106 mꢀ1).[11]
B2142+
A918+[e]
A2142+[e]
14
21
[a] Glassy carbon working electrode, TBAPF6 supporting electrolyte.
[b] Number of exchanged electrons obtained by chronoamperometric ex-
periments (estimated error ꢁ20%). [c] Overall number of viologen units
present in the compound, as confirmed by eosin complexation experi-
ments. [d] Process affected by adsorption. [e] From reference [5].
Fluorescence titration experiments were performed to in-
vestigate the interaction between Ey2ꢀ and dendrimers
B918+ and B2142+. The experimental data, obtained by ti-
trating solutions of B918+ and B2142+ in CH2Cl2 (each con-
taining ca. 1”10ꢀ5 m viologen units)with a solution of Ey2ꢀ
in acetonitrile (1 mm)(Figure 2,) show that the eosin fluo-
rescence signal appears only when the number of added
Ey2ꢀ exceeds the number of viologen units contained in
each dendrimer.
Compounds dbV2+, B918+, and B2142+ exhibit the two re-
duction processes typical of viologens. Cyclic voltammetric
patterns show that the reducible viologen units in each den-
drimer are equivalent and that the first reduction is reversi-
ble with Nernstian behavior in all cases (Figure 3), whereas
the second process, particularly for B2142+, is affected by
adsorption of the reduced species on the electrode.
These results demonstrate that B918+ and B2142+ can
quench the fluorescence of 9 and 21 eosin dianions, respec-
tively; that is, each viologen unit of the dendrimers associ-
The diffusion coefficients and number of exchanged elec-
trons (Table 1)were determined by chronoamperometric ex-
periments performed at the potential of the first reduction
process by using ultramicroelectrodes.[13] As expected, the
diffusion coefficient of B918+ is larger than that of B2142+
.
For both dendrimers the number of exchanged electrons is
smaller than that expected on the basis of the number of the
viologen units in the molecular structure. Apparently, only
the external viologen units—6 for B918+ and 12 for B2142+
—are reduced in electrochemical experiments.
Unfortunately, spectroelectrochemical experiments aimed
at obtaining the absorption spectra of the reduced com-
pounds could not be performed because of adsorption of the
reduced species on the platinum minigrid used as working
electrode.
Figure 2. Fluorescence titration experiments performed in CH2Cl2 solu-
tions. Intensity of the Ey2ꢀ fluorescence band (lex =500 nm, lem
=
560 nm)as a function of the [ Ey2ꢀ]/
[Bn2n+] ratio: B24+ (solid triangles),
ACHTREUNG
Photosensitized reduction: One-electron reduction of violo-
gen compounds can be conveniently performed by using
B918+ (solid circles), B2142+ (open circles).
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Polyviologen Dendrimers
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Scheme 4. Scheme of photosensitized doV2+ reduction.
Figure 3. Cyclic voltammetric behavior as a function of sweep rate for
the first reduction process of a)dendrimer B918+ (1.2”10ꢀ4 m)and
b)dendrimer B2142+ (7”10ꢀ5 m). Argon-purged MeCN, glassy carbon
working electrode, TBAPF6 (0.1m)supporting electrolyte.
Figure 4. Spectral changes (optical path 1 cm)observed on irradiation
with 365 nm light of a solution of 9-methylanthracene (1.2”10ꢀ4 m),
TEOA (5.0”10ꢀ2 m), and B2142+ (4.0”10ꢀ6 m)or doV2+ (6.7”10ꢀ5 m;
inset)in degassed CH Cl2. The solid line corresponds to the photostation-
2
ary state.
A
solution containing 9-methylanthracene (1.2”10ꢀ4 m)in
CH2Cl2 was used as reference.
suitable photosensitizers in the presence of sacrificial reduc-
tants. Light excitation leads the photosensitizer to a long-
lived excited state that can transfer an electron to a viologen
during an encounter. The back-electron-transfer reaction be-
tween the oxidized photosensitizer and the reduced viologen
is prevented by fast reduction of the oxidized photosensitiz-
er by the sacrificial reductant. Such photoinduced processes
have been exploited extensively for photogeneration of hy-
drogen from aqueous solutions[3d,14] and to power artificial
molecular devices and machines.[15]
Figure 4 shows the spectral changes observed on irradia-
tion with 365 nm light of a solution of 9-methylanthracene
(1.2”10ꢀ4 m), TEOA (5.0”10ꢀ2 m), and B2142+ (4.0”10ꢀ6 m)
in degassed CH2Cl2. The spectral features that arise on irra-
diation are characteristic of the formation of both monomer-
ic and dimeric reduced viologen species; similar changes
were obtained for dendrimer B918+. The total number of
electrons exchanged by dendrimers B918+ and B2142+ at the
photostationary state (Table 2), determined from absorb-
ance measurements, is slightly smaller than that obtained in
the chronoamperometric experiments. This discrepancy can
be attributed to small amounts of oxidizing impurities con-
tained in the solvent and/or the samples used or, most likely,
to underestimation of the dimer concentration because the e
values[17] used refer to a different solvent.
Previous investigations[5] showed that the photochemical
reduction of viologen compounds can be performed effi-
ciently in degassed CH2Cl2 by using 9-methylanthracene[16]
as photosensitizer and triethanolamine (TEOA)as sacrificial
reductant. Light excitation of a solution of 9-methylanthra-
cene in CH2Cl2 in the presence of TEOA causes reduction
+
of doV2+ to its monomeric radical cation doV (Scheme 4,
Spectra recorded at different irradiation times showed
that the fraction of monomeric and dimeric reduced violo-
C
Table 2, inset of Figure 4).
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J. F. Stoddart, M. Venturi et al.
Table 2. Ratio between dimerized (VCdþ)and monomeric (V mCþ)monore-
Absorption spectra: The absorption spectra of dendrimers
B918+ and especially B2142+ are different from that of
dbV2+. The differences could originate from the fact that
1) dbV2+ is not a fully satisfactory model, at least from the
spectroscopic viewpoint, for the viologen units in the den-
dritic structure, since each unit in the dendrimer shares
benzyl groups with other units (Scheme 1)and some benzyl
groups carry bismethyleneoxy substituents, and 2)CT inter-
actions between the electron-accepting viologen units and
the proximate (through-bond)or remote (through-space)
electron-donating aryloxy units, as suggested by the appear-
ance of an absorption tail for l>320 nm.
duced viologen units, percentages of the total monoreduced viologen
Cþ
units (V ), and numbers of exchanged electrons (nphot)at the photosta-
tot
tionary state.[a]
Cþ
[b]
VCdþ/VCmþ
% V
nphot
n[c]
tot
doV2+
B24+
0
100
75
53
41
72
48
60
1.0
1.5
4.7
8.5
1.4
1
2
9
21
2
9
0.15
1.32
1.10
0.21
0.85
0.82
B918+
B2142+
A24+
A918+[d]
A2142+[d]
4.3
12.6
21
[a] Degassed CH2Cl2, irradiation with 365 nm light, 1.2”10ꢀ4 m 9-methyl-
anthracene, 5.0”10ꢀ2 m TEOA. [b] Estimated error ꢁ20%. [c] Overall
number of viologen units present in compounds, as confirmed by eosin
complexation experiments. [d] Adjusted from reference [5].
Previously studied dendrimers A918+ and A2142+ exhibit-
ed similar behavior; however, a quantitative analysis shows
that A2142+ has an absorption due to CT interactions that is
less intense (about one half)and shifted towards lower
energy compared to B2142+. These results show that the ter-
minal stoppers play some role in determining the spectro-
scopic properties of such dendrimers.
gen species changes with time (Figure 5). At the photosta-
tionary state, dendrimer B918+ shows prevalence of dimer-
ized (ca. 57%)over monomeric (ca. 43%)viologen units;
Host properties toward eosin: Both A- and B-type dendrim-
ers can host a number of eosin dianions equal to the
number of viologen units in their branches. Thus, neither the
dendrimer generation nor the nature of the peripheral
groups play a role in the formation of viologen/eosin com-
plexes. Clearly, eosin anions can penetrate into the interior
ꢀ
of the positively charged dendrimers, replacing the PF6
counterions. The dendrimers can be viewed as polyvalent
scaffolds with a well-defined number of independent sites
where single eosin dianions can be hosted.
Charge pooling: The electrochemical and photosensitization
experiments performed on A- and B-type dendrimers
(Tables 1 and 2)revealed that, in all cases, only a fraction of
the viologen units can be reduced. Within experimental
error, this fraction corresponds to the number of viologen
units in the outer shell (6 for A918+ and B918+, 12 for
A2142+ and B2142+).
Figure 5. Changes in the percentages of the monomeric (VCmþ, solid sym-
bols)and dimerized (V Cdþ, open symbols)monoreduced viologen units
with increasing irradiation time for dendrimers B918+ (circles)and
B2142+ (triangles).
The fact that the number of reducible viologen units is
smaller than that expected cannot be attributed to lack of
branches in the structures of the dendrimers, because the re-
sults obtained for eosin complexation show clearly that the
compounds examined do contain 9 (A918+ and B918+)and
21 (A2142+ and B2142+)viologen units, in agreement with
for dendrimer B2142+, the fraction of dimerized viologen
units is slightly lower.
For comparison purposes, we performed photochemical
reduction of dendrons B24+ and A24+ (Table 2)and found
that 1)the number of reducible viologen units is slightly
smaller than expected and that 2)the percentage of dimeri-
zation is low and almost unaffected by the nature of the
stoppers.
1
the H NMR spectroscopic and mass-spectrometric charac-
terization.
The electrochemical reduction experiments showed that,
in each dendrimer, the first reduction process of all of its re-
ducible viologen units occurs at the same potential and that
the first reduction process of all of the reducible viologen
units in all of the dendrimers occurs at the same potential.
Interestingly, the reduction potential of a reducible unit is
not affected by the state of the other reducible units, and
this is an ideal property for a charge-pooling system. Fur-
thermore, the nature of the peripheral groups—tetraarylme-
thane and aryloxy units for A- and B-type families, respec-
tively—affects neither the number of reducible viologen
Discussion
The results presented here are now discussed and compared
with those reported previously for similar dendrimers, termi-
nated with bulky tetraarylmethane groups (A918+ and
A2142+, Tables 1 and 2).[5,11]
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Polyviologen Dendrimers
FULL PAPER
units nor the first reduction potential. Two concomitant ef-
fects can be taken into account to explain the lack of com-
plete electrochemical reduction: 1)on reduction of the ex-
ternal viologen shell, shrinkage of the dendrimer structure,
favored by dimerization of the reduced units, prevents the
internal viologen units “seeing” the electrode, and 2)the in-
ternal viologen units, engaged in tight ionic couples with the
hexafluorophosphate counterions, become more difficult to
reduce, whereby electron hopping from external to internal
viologen units is prevented.
twice that of the viologen units. Although it has been report-
ed[4] that in dendrimers very similar to those described here
all the viologen units are reducible, our results show clearly
that only a fraction of these units can be reduced. This ob-
servation throws light on the reasons why charge pooling is
incomplete and on the role played by the terminal groups.
The fact that the number of reducible viologen units is
smaller than that expected cannot be attributed to lack of
branches in the structures of the dendrimers, because the re-
sults obtained for eosin complexation indicate that the com-
pounds examined do contain 9 (A918+ and B918+)and 21
(A2142+ and B2142+)viologen units.
Photosensitized reduction experiments revealed that, for
both the A- and B-type families of dendrimers, a fraction of
the monoreduced viologen units undergoes dimerization
and that this process prevails for the dendrimers in which
the bulky tetraarylmethane terminal groups have been re-
moved.
We have also identified a dendrimer effect in the dimeri-
zation process involving the reduced viologen units. The
dendritic structure of compounds B918+, B2142+, A918+, and
A2142+ provides an environment that favors dimerization,
since it forces the reduced viologen units to occupy close po-
sitions yet is flexible enough to enable interactions not only
between the reduced units belonging to the same branch (in-
tradendron interactions), but also between reduced units lo-
cated in different branches (interdendron interactions).
Experiments on photosensitized reduction showed that
the numbers of viologen units reducible under the photo-
chemical conditions are in reasonable agreement with those
obtained by chronoamperometric experiments, that is, only
the viologen units in the external shells can be reduced.
These experiments also reveal that formation of the one-
electron reduced viologen units is accompanied by their di-
merization. The lack of reduction of all the viologen units in
such experiments can be explained by considering that
1)the uncharged photosensitizer cannot displace the coun-
terions assembled in the proximity of the highly charged
core of the dendrimer, and 2)the interaction between the
photosensitizer and the internal viologen units can be pre-
vented by dimerization of the external reduced units, a phe-
nomenon which shrinks the dendrimer structure. We have
also discovered that dimerization is not a strongly favored
process, as shown by the fact that it does not occur for the
mono-viologen doV2+ species and leads to only 15–20% of
associated species in the case of dendrons B24+ and A24+
(Table 2), which are structurally preorganized for dimeriza-
tion. For the dendrimers, the fraction of dimerized viologen
units is higher, by about 45 and 57% for the A- and B-type
families, respectively, than expected because of the possibili-
ty of producing dimers also between reduced units belong-
ing to different dendrons. The results also show that the
bulky tetraarylmethane peripheral moieties disfavor forma-
tion of dimers compared with the smaller aryloxy groups.
Experimental Section
Electrochemical experiments: Cyclic voltammetric (CV)experiments
were carried out in argon-purged MeCN (Romil Hi-Dry)at room tem-
perature with an Autolab 30 multipurpose instrument interfaced to a per-
sonal computer. The working electrode was a glassy carbon electrode
(0.08 cm2, Amel); its surface was routinely polished with 0.3 mm alumina/
water slurry on a felt surface immediately prior to use. In all cases, the
counterelectrode was a Pt spiral, separated from the bulk solution by a
fine glass frit, and an Ag wire was used as a quasireference electrode.
Ferrocene (E1/2 =+0.395 V vs. SCE)was present as an internal standard.
In all the electrochemical experiments the concentration of the com-
pounds was in the range 1”10ꢀ4 to 7”10ꢀ5 m, and tetrabutylammonium
hexafluorophosphate (TBAPF6)with 100 times higher concentration was
added as supporting electrolyte. Cyclic voltammograms were obtained
with sweep rates in the range 0.02–1.0 Vsꢀ1; the IR compensation imple-
mented within Autolab 30 was used, and every effort was made through-
out the experiments to minimize the resistance of the solution. At any in-
stance, the full reversibility of the voltammetric wave of ferrocene was
taken as an indicator of the absence of uncompensated resistance effects.
The reversibility of the observed processes was established by using the
criteria of 1)separation of 60 mV between cathodic and anodic peaks,
2)a ratio of the intensities of the cathodic and anodic currents close to
unity, and 3)constancy of the peak potential on changing sweep rate in
the cyclic voltammograms. The experimental error in the potentials was
estimated to be ꢁ10 mV. The diffusion coefficients and the numbers of
exchanged electrons were obtained independently by chronoamperome-
try, as described in the literature.[13] A Pt disk with a diameter of 50 mm
was used as working electrode, and the experiments were carried out for
5 s, with 0.05 s sample time, at potentials of ꢀ0.40 V for dbV2+ and
ꢀ0.35 V for B918+ and B2142+. The current intensities under steady-state
conditions were determined from CV experiments by using the same
Conclusion
We have found that polyviologen dendrimers, independently
of the nature and bulkiness of the terminal groups (A918+
,
A2142+, B918+, and B2142+), can act as polytopic receptors
toward electron-donor substrates and host a number of
eosin dianions equal to the number of viologen units in their
branches. The data obtained show clearly that the host–
guest interactions that drive complex formation in low-po-
larity media are affected by neither the steric hindrance of
the terminal groups nor the electronic interactions that
these groups establish with the viologen units. These results
are of interest for the design and construction of dendrimers
capable of performing functions related to drug-delivery or
sensing applications.
In principle, polyviologen dendrimers can behave as mo-
lecular batteries,[4–6] as they are potentially capable of stor-
ing, at easily accessible potentials, a number of electrons
Chem. Eur. J. 2008, 14, 8365 – 8373
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8371

J. F. Stoddart, M. Venturi et al.
working electrode as the chronoamperometric experiments and a sweep
rate of 10 mVsꢀ1
stirred under argon at room temperature to completion, during which
1.25 equiv of CBr4 and Ph3P were added at 30 min intervals until TLC
showed no starting material. The reaction mixture was then poured into
H2O, extracted with CH2Cl2 (3”25 mL), and the combined extracts were
evaporated to dryness. The residue was subjected to column chromatog-
raphy (SiO2, Me2CO, and then 5 mm solution of NH4PF6 in Me2CO). The
resulting product was washed with H2O and dried to afford the desired
product 5·4PF6 (1.27 g, 61%)as a dark brown solid. M.p. >2508C;
1H NMR (500 MHz, (CD3)2CO): d=9.42 (d, J=10 Hz, 4H), 9.37 (d, J=
9 Hz, 4H), 8.72 (d, J=6 Hz, 8H), 7.86 (s, 1H), 7.84 (s, 2H), 7.61 (d, J=
10 Hz, 4H), 7.04 (d, J=6 Hz, 4H), 6.16 (s, 4H), 6.15 (s, 4H), 4.58 (s,
2H), 4.14–4.13 (m, 4H), 3.79–3.77 (m, 4H), 3.61–3.60 (m, 4H), 3.47–3.45
(m, 4H), 3.25 ppm (s, 6H): HRMS (ESI): m/z: 1363.26 [MꢀPF6]+.
.
Photochemical reduction: The experiments were performed in solutions
of doV2+, A24+, B24+, B918+, or B2142+ in CH2Cl2 with 9-methylanthra-
cene as photosensitizer and triethanolamine as sacrificial reductant. The
solutions (3 mL)were degassed by repeated freeze–pump–thaw cycles
and irradiated at 365 nm with light from a medium-pressure Hg lamp
equipped with an interference filter.
Concentrations of the monomeric and dimeric forms of the monoreduced
viologens: The monomer (cM)and dimer ( cD)concentrations of the mon-
oreduced viologen units were determined from the absorbance at 605
and 537 nm (A605 and A537)by using Equations (1)and (2)
[20]
B9·18PF6: A solution of 6·3PF6 (73.5 mg, 0.072 mmol)in dry MeCN
cM ¼ ðA605eD537ꢀA537eD605Þ=ðeM605eD537ꢀeM537eD605
Þ
ð1Þ
(1 mL)was added dropwise to
a
solution of
5·4PF6 (647.3 mg,
0.43 mmol)in dry MeCN (2 mL)under reflux and an argon atmosphere
over 2 h. The reaction mixture was stirred for 48 h and then cooled to
ambient temperature. The solvent was removed under vacuum and the
residue subjected to HPLC (Hypersil BDS C18 column, flow rate
1.0 mLminꢀ1; mobile phase, pump A 0.1% CF3CO2H in H2O, pump B
MeCN/0.1% CF3CO2H in H2O (95/5); time [min]/pump A [%] =0/100, 8/
100, 28/0, 42/0, 45/100). Excess MeCN and H2O were evaporated under
vacuum such that the product still remained dissolved, and a saturated
aqueous solution of NH4PF6 was added dropwise. The solid was collected
by filtration, washed with H2O (4”20 mL), and dried under vacuum to
afford B9·18PF6 as a light brown solid (616 mg, 25%). M.p. >2508C;
1H NMR (500 MHz, (CD3)2CO): d=8.91–8.87 (m, 36H), 8.38–8.31 (m,
36H), 7.62 (brs, 9H), 7.60 (brs, 3H), 7.44 (d, J=8.5 Hz, 12H), 7.00 (d,
J=7.5 Hz, 12H), 5.80–5.72 (m, 36H), 4.11–4.09 (m, 12H), 3.76–3.74 (m,
12H), 3.60–3.58 (m, 12H), 3.47–3.45 (m, 12H), 3.26 ppm (s, 18H);
13C NMR (125 MHz, CD3CN): d=160.1, 150.5, 150.4, 149.9, 145.7, 145.6,
145.2, 134.7, 134.6, 131.7, 131.2, 127.3, 127.2, 124.2, 115.3, 71.4, 70.0, 68.9,
67.6, 63.5, 57.8 ppm; HRMS (ESI): m/z: 1289.80 [Mꢀ4PF6]+.
cD ¼ ðA537eM605ꢀA605eM537Þ=ðeM605eD537ꢀeM537eD605
Þ
ð2Þ
in which eD537 and eD605 represent the molar absorption coefficients of the
dimer at the two selected wavelengths; their values were taken from the
literature[17] and correspond to 29000 and 2350mꢀ1 cmꢀ1, respectively. For
eM605 and eM537, the molar absorption coefficients of the monomer, the
values used were 14000 and 5770mꢀ1 cmꢀ1, respectively, in agreement
with those reported in reference [17]. The concentration of the dimerized
one-electron-reduced viologen units corresponds to 2”cD.
Materials and methods: All chemicals were purchased from Aldrich and
used without further purification. Solvents were purchased from Aldrich
and purified according to literature procedures. Thin-layer chromatogra-
phy (TLC)was performed on aluminum sheets coated with silica gel 60F
(Merck 5554). The plates were inspected by UV light. Column chroma-
tography was performed on silica gel 60 (Merck 40–60 nm, 230–
400 mesh). HPLC-grade MeCN was purchased from Aldrich and de-
gassed by bubbling He. Known amounts of the compounds were dis-
solved in MeCN (HPLC grade)to afford stock solution of known con-
centration (0.001m). The resulting solutions (injected volume 10 mL)
[18]
7·12PF6: A solution of 4·2PF6 (0.20 g, 0.27 mmol)in dry MeCN (2 mL)
[19]
was added dropwise to a solution of 5·4PF6 (1.44 g, 0.95 mmol)in dry
MeCN (2 mL)under reflux and an argon atmosphere over 2 h. The mix-
ture was stirred for 48 h and then cooled to ambient temperature, the sol-
vent removed under vacuum, and the residue subjected to column chro-
matography (SiO2, Me2CO, and then 5 mm, 10 mm, and saturated solu-
tions of NH4 PF6 in Me2CO consecutively). The resulting product was
washed with H2O and dried to afford alcohol 7·12PF6 (2.47 g, 65%). M.p.
>2508C; 1H NMR (500 MHz, (CD3)2CO): d=9.05–8.90 (m, 24H), 8.50–
8.30 (m, 24H), 7.75–7.60 (brs, 9H), 7.50 (d, J=6 Hz, 8H), 7.05 (d, J=
6 Hz, 8H), 5.90–5.70 (m, 24H), 4.70 (s, 2H), 4.25–4.10 (m, 8H), 3.85–3.70
(m, 8H), 3.65–3.60 (m, 8H), 3.55–3.45 (m, 8H), 3.30 ppm (s, 12H);
HRMS (ESI): m/z: 1843.2 [Mꢀ2PF6]+, 1180.1 [Mꢀ3PF6]+, 849.5
[Mꢀ4PF6]+.
were analyzed at ambient temperature by HPLC (flow rate 1.0 mLminꢀ1
;
mobile phase, pump A 0.1% CF3CO2H in H2O, pump B MeCN/0.1%
CF3CO2H in H2O (95:5); time [min]/pump A [%] =0/100, 8/100, 28/0, 42/
0, 45/100)by employing a Hypersil BDS C18 column (length 25 cm,
inside diameter 5.6 cm)operated by HP 1090 (Hewlett-Packard)connect-
ed to a thermo surveyor UV/Vis diode-array detector. Melting points
were determined on an Electrothermal 9200 apparatus and are uncorrect-
ed. 1H and 13C spectra were recorded on Bruker Avance500 (500 and
125 MHz, respectively)with residual proton signals of solvents as internal
standard. Samples were prepared in CD3CN or (CD3)2CO purchased
from Cambridge Isotope Laboratories. Electron impact mass spectra
(EIMS)were obtained on a VG Prospec mass spectrometer.
B21·42PF6: Alcohol 7·12PF6 (500 mg, 0.126 mmol)was dissolved in dry
THF:MeCN (3:1, 4 mL), and CBr4 (62 mg, 0.188 mmol)and Ph P (49 mg,
3
B2·4PF6: A solution of 4·2PF6 (1.0 g, 1.38 mmol), prepared by the litera-
ture procedure,[18] in dry MeCN (10 mL)was added dropwise to a solu-
tion of 3[19] (1.0 g, 3.45 mmol)in dry MeCN (10 mL)under reflux and an
argon atmosphere for 2 h. The reaction mixture was stirred for 2 d and
then cooled to ambient temperature; the solvent was removed under
vacuum and the residue subjected to column chromatography (SiO2,
MeOH:2m NH4Claq:MeNO2 7:2:1). Excess MeNO2 and MeOH were
then removed under vacuum such that the product still remained dis-
solved, and a saturated solution of NH4PF6 was added dropwise. The
solid was filtered off, washed with H2O (4”50 mL), and dried to afford
B2·4PF6 as a yellow solid (1.3 g, 66% in two steps). M.p.>2508C;
1H NMR (500 MHz, (CD3)2CO): d=9.45 (d, J=10 Hz, 4H), 9.41 (d, J=
10 Hz, 4H), 8.76 (d, J=10 Hz, 8H), 7.93 (s, 1H), 7.89 (s, 2H), 7.66 (d,
J=8.5 Hz, 4H), 7.08 (d, J=6.5 Hz, 4H), 6.21 (s, 4H), 6.11 (s, 4H), 4.74
(s, 2H), 4.21–4.17 (m, 4H), 3.86–3.82 (m, 4H), 3.68–3.65 (m, 4H), 3.52–
3.50 (m, 4H), 3.31 ppm (s, 6H); HRMS (ESI): m/z: 1299.32 [MꢀPF6]+.
0.188 mmol)were added under vigorous stirring over 30 min. The reac-
tion mixture was stirred under argon at room temperature to completion,
during which 1.25 equiv of CBr4 and Ph3P were added in 30 min intervals
until TLC showed no starting material. The reaction mixture was poured
into H2O, extracted with CH2Cl2 (3”25 mL), and the combined extracts
were dried and evaporated to dryness. The residue was subjected to
column chromatography (SiO2, Me2CO, and then 5 mm, 10 mm, and satu-
rated solutions of NH4PF6 in Me2CO consecutively). The resulting prod-
uct was washed with H2O and dried to afford the desired product
(203 mg, 40%)as a dark brown solid, which was added to a solution of
[20]
6·3PF6
(9 mg, 0.0084 mmol)in MeCN (1 mL.) The reaction mixture
was stirred for 5 d under reflux and an argon atmosphere and then
cooled to ambient temperature. The solvent was removed under vacuum
and the residue subjected to HPLC on a Hypersil BDS C18 column, flow
rate 1.0 mLminꢀ1
; mobile phase, pump A 0.1% CF3CO2H in H2O,
pump B MeCN/0.1% CF3CO2H in H2O (95/5); time [min]/pump A [%] =
0/100, 8/100, 28/0, 42/0, 45/100). Excess MeCN and H2O were evaporated
off under vacuum such that the product still remained dissolved, and a
saturated aqueous solution of NH4PF6 was added dropwise. The solid
5·4PF6: Alcohol B2·4PF6 (2 g, 1.38 mmol)was dissolved in dry
THF:MeCN (3:1, 4 mL), and CBr4 (2.3 g, 6.9 mmol)and Ph 3P (1.82 g,
6.9 mmol)were added under vigorous stirring. The reaction mixture was
8372
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Chem. Eur. J. 2008, 14, 8365 – 8373

Polyviologen Dendrimers
FULL PAPER
was filtered off, washed with H2O (4”20 mL), and dried under vacuum
to afford B21·42PF6 as a light brown solid (0.1, 15%). M.p. >2508C;
1H NMR (500 MHz, (CD3)2CO): d=8.91–8.67 (m, 84H), 8.36–8.11 (m,
84H), 7.60 (brs, 27H), 7.56 (brs, 3H), 7.42 (d, J=8.5 Hz, 24H), 6.90 (d,
J=7.5 Hz, 24H), 5.78–5.70 (m, 84H), 4.00–3.69 (m, 24H), 3.70–3.68 (m,
24H), 3.56–3.45 (m, 24H), 3.42–3.37 (m, 24H), 3.24 ppm (s, 36H);
13C NMR (125 MHz, CD3CN): d=160.9, 146.4, 146.0, 135.6, 135.5, 132.5,
132.0, 128.2, 128.0, 125.0, 118.0, 117.5, 116.1, 72.3, 70.8, 69.8, 68.4, 65.1,
64.3, 58.6 ppm.
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Received: April 23, 2008
Published online: July 30, 2008
Chem. Eur. J. 2008, 14, 8365 – 8373
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8373