Dendrimers based on a bis-cyclam core as fluorescence sensors for metal ions

Article abstract of DOI:10.1039/b501869b

We have synthesized a novel dendrimer (1) based on two covalently linked cyclam units as a core appended to six branches, each one consisting of one dimethoxybenzene and two naphthyl units (cyclam = 1,4,8,11- tetraazacyclotetradecane). Such a dendrimer shows three fluorescence bands which can be assigned to naphthyl localized excited states (λ_max = 336 nm), naphthyl excimers (λ_max ca. 390 nm), and naphthyl-amine exciplexes (λ_max = 510 nm). Protonation or complexation of the bis-cyclam core with Zn²⁺ does not affect the absorption spectrum of the dendrimer, but causes noticeable changes in the fluorescence intensity of the three component bands. Complexation with Cu ²⁺ not only causes changes in the relative intensities of the fluorescence bands, but also the appearance of a new absorption band in the near UV spectral region. Analysis of the titration curves has allowed us to obtain clear evidence for the formation of 1 : 1 (1(H⁺), [Zn(1)] ²⁺, [Cu(1)]²⁺) and 2 : 1 (1(2H⁺), [Zn ₂(1)]⁴⁺, [Cu₂(1)]⁴⁺) species. Comparison with the behaviour of a previously investigated parent monocyclam dendrimer (2) suggests that in the 1 : 1 species of 1 both the cyclam units are involved in the complexation with Zn²⁺ and Cu²⁺. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b501869b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Dendrimers based on a bis-cyclam core as fluorescence sensors for metal
ions
a
Giacomo Bergamini, Paola Ceroni, Vincenzo Balzani,* Leandra Cornelissen, Jeroen van Heyst,
a
a
a
b
b
Sang-Kyu Lee and Fritz V o¨ gtle*
b
Received 4th February 2005, Accepted 14th March 2005
First published as an Advance Article on the web 30th March 2005
DOI: 10.1039/b501869b
We have synthesized a novel dendrimer (1) based on two covalently linked cyclam units as a core
appended to six branches, each one consisting of one dimethoxybenzene and two naphthyl units
(
cyclam 5 1,4,8,11-tetraazacyclotetradecane). Such a dendrimer shows three fluorescence bands
which can be assigned to naphthyl localized excited states (lmax 5 336 nm), naphthyl excimers
max ca. 390 nm), and naphthyl–amine exciplexes (lmax 5 510 nm). Protonation or complexation
(
l
2+
of the bis-cyclam core with Zn does not affect the absorption spectrum of the dendrimer, but
causes noticeable changes in the fluorescence intensity of the three component bands.
2
Complexation with Cu not only causes changes in the relative intensities of the fluorescence
+
bands, but also the appearance of a new absorption band in the near UV spectral region. Analysis
+
of the titration curves has allowed us to obtain clear evidence for the formation of 1 : 1 (1(H ),
2+
2+
+
4+
4+
[
Zn(1)] , [Cu(1)] ) and 2 : 1 (1(2H ), [Zn
2 2
(1)] , [Cu (1)] ) species. Comparison with the
behaviour of a previously investigated parent monocyclam dendrimer (2) suggests that in the 1 : 1
2+
2+
species of 1 both the cyclam units are involved in the complexation with Zn and Cu .
2
+
fluorescence of all the dansyl units is quenched when a Co
ion is incorporated into the dendrimer.
Introduction
1
Dendrimers are well defined, iteratively branched com-
In the above mentioned example, coordination of the metal
ion takes place with some of the amine units of the dendrimer
branches in a rather undefined way. The presence in the
dendritic structure of a well defined metal coordinating unit
could be useful to further develop opportunities for signal
amplification offered by luminescent dendrimers. For this
reason, we have recently synthesized dendrimers consisting of
a 1,4,8,11-tetraazacyclotetradecane (cyclam) core appended
pounds, with a high degree of order, rigid or flexible skeletons
that can reach nanometric dimensions, and with the potential
to contain selected chemical units at predetermined sites in
their structure. Dendrimer chemistry is rapidly expanding both
2
in fundamental research and in technological applications.
Luminescent units can be easily incorporated in different
regions of a dendritic structure. Indeed, in the last few years it
has been shown that coupling luminescence with dendrimer
chemistry can lead to systems capable of performing very
1
0
with dimethoxybenzene and naphthyl units. Cyclam is
indeed one of the most extensively investigated ligands in
1
coordination chemistry.
3
4
1,12
interesting functions, including (i) light harvesting, (ii)
5
changing the ‘‘colour’’ of light, (iii) sensing with signal
6
amplification, (iv) quenching and sensitization processes, and
In a further effort to explore the potential of cyclam-based
fluorescent dendrimers as ligands for metal ions (and thus as
7
8
(
v) fluorescence at the level of a single molecule.
9
Fluorescent chemosensors are usually made of a fluorescent
fluorescent sensors), we have synthesized a novel dendrimer (1,
13,14
Scheme 1) based on two covalently linked cyclam units
as
unit connected with a receptor. Recognition of a substrate by
the receptor affects the fluorescent properties of the fluoro-
phore. For example, coordination of a metal ion with the
receptor can cause quenching of the excited state of the
fluorophore by energy or electron transfer, thereby switching
off the fluorescent signal. In suitably designed dendrimers
containing a great number of fluorescent units, coordination of
a metal ion can lead to the quenching of the fluorescent excited
state of a great number of fluorophores, thereby offering the
possibility of signal amplification. For example, it has been
a core appended to six branches, each one consisting of a
dimethoxybenzene and two naphthyl units. In this paper we
report the changes caused by protonation and complexation
2+
2+
with Zn and Cu on the fluorescent properties of this
dendrimer. The results obtained are discussed by comparison
1
with the properties of the parent monocyclam dendrimer 2.
0,15
Experimental
The bis-cyclam dendrimer 1 was synthesized starting from
,19-[1,4-phenylenebis(methylene)]bis(1,4,8,11-tetraazacyclo-
tetradecane), which was synthesized as reported in the
6
1
shown that in a fourth generation poly(propylene amine)
dendrimer decorated with 32 dansyl units at the periphery and
containing 30 aliphatic amine units in the interior, the strong
1
6,13
literature,
,4,8-tris(2,2,2-trifluoroacetyl)-1,4,8,11-tetraazacyclotetra-
decane (1.0 g, 2.05 mmol) was dissolved in DMF (25 ml)
together with K CO (0.708 g, 5.12 mmol). The mixture was
with the following modification:
1
*vincenzo.balzani@unibo.it (Vincenzo Balzani)
voegtle@uni-bonn.de (Fritz V o¨ gtle)
2
3
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Scheme 1
then cooled to 0 uC before adding a,a-dibromoxylene (0.270 g,
.02 mmol), dissolved in DMF (5 ml), dropwise. Stirring was
continued for 4 d at RT after which the solvent was removed
in vacuo. The rest was dissolved in CH Cl , washed with H O/
sat. NaHCO -sol./H O, dried and removed in vacuo.
The tetra-substituted cyclam (dendrimer 2) was obtained as
1
0
1
3 3 2
previously described. Trifluoroacetic acid and M(CF SO )
2
+
2+
salts (M 5 Cu , Zn ) were purchased from Aldrich.
Acetonitrile and dichloromethane Uvasol were purchased
from Merck. All the absorption and emission spectra were
2
2
2
3
2
The rough product was then purified by column chromato-
carried out in air-equilibrated CH CN–CH Cl 1 : 1 v/v
3 2 2
graphy (SiO , 43–60 mm, CH Cl /EA/MeOH: 10/1/0,7) to give
2
solution at 298 K, and in a rigid butyronitrile matrix at 77 K,
with a Perkin Elmer l 40 spectrophotometer and a Perkin
Elmer LS50 spectrofluorimeter. The estimated experimental
errors are: ¡2 nm for the band maximum, ¡5% for the molar
absorption coefficient and ¡10% for the relative intensities
of the emission bands. Titration curves were obtained by
2
2
the pure product as a white foam, as shown by NMR and mass
spectra.
The dendron 3,5-bis(29-oxymethylnaphthyl)benzylbromide
1
was synthesized as previously described.
7,18
1
9
1
,19-[1,4-phenylenebis(methylene)]-4,49,8,89,11,119-hexakis[3,5-
submitting the spectra to the SPECFIT software.
bis (29-oxymethylnaphthyl)benzyl]bis(1,4,8,11-tetraazacy-
clotetradecane): (5bis-cyclam dendrimer 1)
Absorption and emission spectra of the bis-cyclam dendrimer 1
1
,19-[1,4-phenylenebis(methylene)]bis(1,4,8,11-tetraazacyclote-
tradecane) (0.150 g, 0.30 mmol) was taken up in DMF (35 ml)
and K CO (0.800 g, 5,79 mmol) was added. The resulting
The absorption spectra of dendrimers 1 and 2 are displayed
in Fig. 1. The chromophoric groups present in the two
2
3
mixture was stirred for 10 min at RT under Ar, after
which 3,5-bis(29-oxymethylnaphthyl)benzylbromide (1.100 g,
2
.276 mmol), dissolved in DMF (35 ml), was added dropwise.
The resulting mixture was stirred at RT under Ar for 4 d. The
solvent was then removed in vacuo and the rest was taken up in
CH Cl . The organic layer was then washed with H O/sat.
2
2
2
NaHCO
The resulting rough product was then purified by column
chromatography (SiO , 43–60 mm, CH Cl /MeOH, 10/1) to
give the pure product as a white foam (0.109 g, 0.065 mmol,
3 2
-sol./H O, dried and removed in vacuo.
2
2
2
1
2
1
3
4
2
1
1
2
3
1,7%); H-NMR:(400 MHz, CDCl , 25 uC), d/ppm 5 1.75–
.90 (m, 2 maxima, 8 H), 2.38–2.63 (m, 2 maxima, 32 H), 3.28–
.47 (m, 16H), 4.92–5.10 (m, 24H), 6.52–6.83 (m, 22H) 7.33 (s,
1
H), 7.33–7.87 (m, 84H); C-NMR: 100.6 MHz, CDCl ,
3
3
5 uC), d/ppm 5 24.4, 70.0, 70.1, 70.2, 101.6, 106.0, 125.3,
25.4, 126.0, 126.1, 126.4, 127.7, 128.0, 128.2, 128.4, 133.0,
33.3, 134.6, 143.3, 143.6, 160.3; MALDI-TOF-MS: m/z:
+
Fig. 1 Absorption spectra of dendrimers 1 and 2 in air-equilibrated
916.6 (M + H ); MS calculated for C202
H
186
N
8
O
12: 2915,4.
3 2 2
CH CN–CH Cl 1 : 1 v/v solution at 298 K.
2
960 | J. Mater. Chem., 2005, 15, 2959–2964
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2
1–24
compounds (Scheme 1) are dimethoxybenzene (six and four,
respectively) and 2-methylnaphthalene (twelve and eight,
respectively). 2-Methylnaphthalene exhibits absorption bands
excited naphthyl units and amines.
The broad band
exhibited by 1 with a maximum at about 510 nm can indeed
be assigned to such exciplexes. A further indication of this
assignment is the disappearance of this band in the emission
spectrum of compound 1 in butyronitrile at 77 K, where
formation of exciplexes is prevented by the lack of solvent
repolarisation. Careful examination of the emission spectrum
of 1 shows, in fact, that it receives a contribution from a third
component band in the 400 nm region, which overlaps the
2
1
21
at 275 nm (S
A S transition, e 5 1100 M cm ), and the dimethoxy-
benzene unit has an absorption maximum at 275 nm (e 5
0
A S
2
transition, e 5 4800 M cm ) and 310 nm
2
1
21
(
S
0
1
2
1
21
2
200 M
cm ). The absorption spectra of the two com-
pounds are those expected on the basis of the component units.
The emission spectrum of 1 in acetonitrile–dichloromethane
1
0
1
: 1 v/v at 298 K shown in Fig. 2 is quite similar to that of 2.
2
other two bands. Such an emission, previously observed for
25,26
macrocyclic ligands bearing naphthyl chromophores,
2
Both 2-methylnaphthalene (lmax 5 335 nm, W 5 6.6 6 10
)
can
2
2
and dimethoxybenzene (lmax 5 300 nm, W 5 1.1 6 10 ) are
known to exhibit fluorescence. Since the fluorescent excited
state of dimethoxybenzene is higher in energy than that of
be assigned to naphthyl excimers. As expected, such an
excimer band is almost completely absent in a butyronitrile
rigid matrix at 77 K.
2
-methylnaphthalene, excitation of dimethoxybenzene should
We would like to note that the very low intensity of the
naphthyl band of 1, compared to the intensity of 2-methyl-
naphthalene, can be due not only to the direct engagement of
excited naphthyl units in exciplexes and excimers, but also to
the deactivation of such excited units by energy transfer to the
lower lying exciplex and excimer levels.
be followed by energy transfer to the naphthyl unit. In order to
elucidate this point, we have compared the emission intensities
observed for compound 1 upon excitation at 275 nm (where
2
3% of the light is absorbed by the dimethoxybenzene units)
and 305 nm (where absorption is only due to the naphthyl
units). The results obtained show that dimethoxybenzene
emission is almost completely quenched and that energy
transfer does occur with an efficiency ¢0.5.
In Fig. 2, the fluorescence spectrum of 2-methylnaphthalene
is also reported for comparison purposes. Since excitation has
been performed at the same wavelength (310 nm) on solutions
showing the same absorbance (0.50), the emission intensities
are directly comparable. As in the case of 2, the emission
intensity of 1 at 336 nm is less than 10% of that exhibited by
Before studying the effect of addition of acid or metal ions
to solutions containing 1, we have performed experiments on
its dimethoxybinaphthyl dendron and we have found that
neither protons nor metal ions cause any change in the
absorption and emission spectra.
Effects of protonation
It is well known that cyclam undergoes protonation in aqueous
27,28
solution
29
as well as in other solvents. In aqueous solution,
2
-methylnaphthalene, and other bands clearly contribute to
28
a
the four successive pK values are 11.6, 10.6, 1.61, and 2.42,
the spectrum at longer wavelengths.
showing that cyclam can be easily mono- and di-protonated,
but further protonation is difficult. It is also interesting to note
The quenching of naphthyl excited states by amine units is a
well known phenomenon, usually ascribed to photoinduced
2
electron transfer (PET) processes. In several cases, quenching
that the fourth pK value is larger than the third one, a result
a
0
related to protonation-induced structural rearrangements.
Protonation of amines engages the lone pair of the nitrogen
atoms and therefore moves the n(N) A p* charge-transfer
occurs via formation of intramolecular exciplexes between
2
0
(
CT) transitions to higher energy. The lack of any change in
the absorption spectrum upon addition of trifluoroacetic acid
2
5
to a 1.1 6 10 M solution of 1 shows that CT transitions do
not contribute to the absorption spectrum. This finding shows
that there is no appreciable interaction between amine and
aromatic moieties in the ground state.
Addition of trifluoroacetic acid, however, causes strong
changes in the emission spectrum of 1 (Fig. 2), which are
1
0
similar but not identical to those previously observed for 2.
As shown in the inset of Fig. 2, no further change was
observed after addition of two equivalents of protons per
+
dendrimer, i.e., after formation of a 1(2H ) species; the same
1
0
2
: 1 proton : dendrimer ratio was observed at the end of the
titration of 2 which, however, contains only one cyclam unit
Scheme 1). These results suggest that the two cyclam units of 1
(
+
do not behave independently and that in the 1(2H ) species the
two protons are likely shared by the two cyclam units in a
sandwich-type structure.
Fig. 2 Emission spectra of 1 (full line) and 2-methylnaphthalene
(
dotted line). The dashed and dotted-dashed lines are the spectra of
after addition of one and two equivalents of trifluoroacetic acid.
The inset shows the normalized fluorescence intensity changes at
1
The spectra reported in Fig. 2 show that protonation of 1
causes: (i) a linear increase in the intensity of the naphthyl
localized band with lmax 5 336 nm that, however, remains
much weaker than the band of 2-methylnaphthalene; (ii) a
3
36 (&), 390 ($), and 510 (m) nm. Experimental conditions:
acetonitrile–dichloromethane 1 : 1 v/v solution, 298 K, lexc 5 275 nm,
absorbance 5 0.50.
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2
1 : 1 complex [Zn(1)] , both the cyclam units are likely to be
+
decrease, but not the disappearance, of the exciplex band with
maximum at around 510 nm; (iii) a moderate, slightly non-
linear increase in intensity of the excimer band around 390 nm.
The non-linear increase in intensity of the excimer band
suggests that protonation causes a rearrangement in the
structure of the dendrimer and the relatively small recovery
of the intensity of the naphthyl fluorescence on protonation is
likely related to the lack of disappearance of exciplexes and the
further formation of excimers. From a SPECFIT analysis of
the observed spectral changes the following values were
obtained for the first and second protonation constants:
log b1:1 5 8.1 ¡ 0.6, and log b2:1 5 14.1 ¡ 0.6.
2
coordinated to Zn , as suggested by the formation of a 1 : 2
+
2
+
compound, [Zn(2) ] , on titration of the monocyclam
2
2
dendrimer 2 with Zn . In the 1 : 1 complex of Zn with
+ 30
2+
2
1, [Zn(1)] , the metal ion is likely to be sandwiched between
+
2
+
2
the two cyclam units, as is thought to happen in the [Zn(2) ]
compound. All the metal complexes of cyclam and of non-
dendritic cyclam derivatives reported so far have a 1 : 1
1
1
stoichiometry. Apparently, the dendrimer branches not only
do not hinder, but in fact favour coordination of cyclam units
to metal ions with respect to solvent molecules and counter
3
+
ions; indeed, even evidence for formation of [Eu(2)
3
3
]
has also
1
been reported. Presumably, the dendritic branches force the
cyclam cores to adopt a structure in which only three N atoms
2+
Complex formation with Zn
are available for metal complexation. As in the case of
2+ 30
Upon addition of Zn(CF SO ) (up to three equivalents) to a
3
3 2
[
Zn(2) ] , two limiting structures can be proposed for
2
2
5
1
.1 6 10 M solution of 1 no change was observed in the
2+
[
Zn(1)] : (i) an ‘‘inward’’ structure, in which the branches of
absorption spectrum, whereas strong changes were observed in
the emission spectrum (Fig. 3). Such changes are qualitatively
similar to those caused by protonation, but there are some
important differences: (i) the titration plot of the naphthyl
fluorescence clearly shows a discontinuity after addition of one
the two coordinated cyclam units are intermeshed, or (ii) an
‘outward’’ structure, in which the branches of the two
‘
coordinated moieties do not interact. An inward structure
with intermeshed branches should favour the formation of
excimers. The results obtained, however, show that in going
2+
equivalent of Zn per dendrimer and at the end of the
titration, which occurs after addition of about two equivalents
of metal ion, the naphthyl band is more intense than in the case
of protonation; furthermore, most of the increase in intensity
of the naphthyl fluorescence band takes place after addition of
2+
4+
2
from [Zn(1)] to [Zn (1)] the intensity of the excimer band
does not change. It seems, therefore, more likely that in these
sandwich-type complexes the dendrimer branches extend
4+
outward and maintain the same structure in the [Zn (1)]
2
species.
2+
one equivalent of Zn ; (ii) the exciplex band with a maximum
at around 510 nm decreases in intensity, shows a discontinuity,
and does not disappear; (iii) the increase in intensity of the
excimer band at 390 nm, once corrected for the contribution of
the naphthyl fluorescence, reaches a plateau after addition of
one equivalent of metal ion.
2+
Complex formation with Cu
Quite different results were obtained upon titration of a 1.1 6
2
0
5
1
3 3 2
M solution of 1 with Cu(CF SO ) . The absorption
spectrum (Fig. 4) showed the appearance of a broad tail in the
300–400 nm region. The absorbance values increase almost
linearly up to the addition of two equivalents of metal ion per
dendrimer (Fig. 4, inset). A quite similar absorption band,
assigned to ligand-to-metal charge-transfer (MLCT) transi-
2
+
These results clearly indicate that a 1 : 1 complex, [Zn(1)] ,
4
+
2
is first formed and then replaced by a 2 : 1 species, [Zn (1)] .
SPECFIT analysis yielded values of log b1:1 5 9.7 ¡ 0.7, and
logb2:1 5 16.1 ¡ 0.8 for these two species, respectively. In the
2
5
tions, is obtained when a 2.2 6 10 M cyclam solution is
Fig. 3 Emission spectrum of 1 before (full line) and after the addition
of one (dashed line) and two (dotted–dashed) equivalents of
Fig. 4 Absorption spectrum of 1 before (full line) and after the
Zn(CF SO
3
3
)
2
. Inset shows the normalized fluorescence intensity
addition of one (dashed line) and two (dotted–dashed) equivalents of
changes at 336 (&), 390 ($), and 510 (m) nm. Experimental
conditions: acetonitrile-dichloromethane 1 : 1 v/v solution, 298 K,
3 3 2
Cu(CF SO ) . Inset shows the normalized absorption changes at
319 nm. Experimental conditions: acetonitrile–dichloromethane
1 : 1 v/v solution, 298 K.
lexc 5 275 nm, absorbance 5 0.50.
2
962 | J. Mater. Chem., 2005, 15, 2959–2964
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titrated by Cu(CF
3
SO
3
)
2
. Therefore we can conclude that upon
between the 1 : 1 and 2 : 1 species. In other words, the two
cyclam units behave independently as far as the MLCT
absorption is concerned. From the view point of the naphthyl
units (Fig. 5), however, it is clear that a 1 : 1 complex,
addition of Cu(CF
SO ) to 1, both the cyclam units of the
3 3 2
2+
dendrimer coordinate a Cu ion. SPECFIT analysis based on
the changes in the absorption spectrum yielded a value of
log b2:1 5 11.9 ¡ 0.3.
2+
[Cu(1)] , is first formed, as suggested by the formation of a
2
+
More details were obtained from the changes observed in
the emission spectrum (Fig. 5), which can be summarized as
follows: (i) the intensity of the naphthyl band is almost
constant up to the addition of one equivalent of metal ion and
then decreases slightly; (ii) the intensity of the exciplex band
decreases linearly and disappears after addition of two
equivalents of metal ions; (iii) the intensity of the excimer
band increases at the beginning of the titration, and reaches a
maximum value after addition of one equivalent of metal ion
and then decreases. SPECFIT analysis was not performed in
the case of the emission spectrum because of the too complex
behaviour and the relatively small intensity changes.
1 : 2 compound, [Cu(2)
2
+ 32
]
on titration of the monocyclam
2
dendrimer 2 with Cu , and then replaced by a 2 : 1 species,
4
+
[Cu (1)] .
2
Conclusions
We have prepared for the first time a dendrimer with a bis-
cyclam core. Such a dendrimer (1) contains in its branches 12
naphthyl units and exhibits three fluorescence bands assigned
to naphthyl localized excited states (lmax 5 336 nm), naphthyl
excimers (lmax ca. 390 nm), and naphthyl-amine exciplexes
+
2+
2+
(l
max 5 510 nm). Titration with H , Zn , and Cu causes
strong changes in the emission spectrum and, in the case of
We would like to emphasise an important difference in the
+
2+
Cu , also in the absorption spectrum. Clear evidence for
+
behaviour of the emission spectrum of 1 upon titration with H
2+
2+
2+
2+
formation of 1 : 1 (1?(H ), [Zn(1)] , [Cu(1)] ) and 2 : 1
4+
or Zn (Figs. 2 and 3), and titration with Cu (Fig. 5). In the
+
4+
+
2+
(1?(2H ), [Zn (1)] , [Cu (1)] ) species has been obtained. The
2 2
case of H or Zn , the decrease in the intensity of the exciplex
emission, caused by the engagement of the cyclam N atoms by
protons or metal ions, is accompanied, as expected, by an
increase in the intensity of the naphthyl emission. This does
2
+
three luminescence bands and, in the case of Cu , also an
absorption band, offer a way to monitor not only the metal–
ligand coordination process, but also its effect on the inter-
action between the naphthyl units contained in the dendritic
branches. The spectral changes are probably too weak and too
complex to be used for sensory purposes. We would like to
note, however, that while the availability of dendrimers
possessing a well defined ligand unit, like cyclam, opens the
way for the construction of mixed dendritic-ligand complexes,
dendrimers containing two cyclam units can be used, in
principle, to construct dendrimers containing two different
types of metal ions.
2+
not happen, however, when titration is performed with Cu ;
4+
in particular, at the end of the titration, when [Cu (1)] is
2
formed, although exciplex formation is fully prevented by
coordination of two metal ions, the naphthyl emission is
clearly less intense than in the free dendrimer 1. This result can
2+
be easily rationalized considering that coordination of Cu ,
while preventing deactivation of the excited naphthyl units via
exciplex formation, introduces another deactivation channel
related to the presence of the low energy MLCT state.
Fig. 4 shows that from the view point of the interaction
between cyclam and metal ion (i.e., from the behaviour of the
MLCT absorption band), there are minor (if any) differences
Acknowledgements
We thank Prof. Mauro Maestri for useful discussions. This
work was supported by MIUR (Supramolecular Devices
Project), FIRB (Manipolazione molecolare per machine
nanometriche), and the European SUSANA network
(
HPRN-CT-2002-00185).
a
Giacomo Bergamini, Paola Ceroni, Vincenzo Balzani,*
a
a
b
a
b
Leandra Cornelissen, Jeroen van Heyst, Sang-Kyu Lee and
b
Fritz V o¨ gtle*
Dipartimento di Chimica ‘‘G. Ciamician’’, Universit a` di Bologna, via
Selmi 2, I-40126 Bologna, Italy. E-mail: vincenzo.balzani@unibo.it;
a
Fax: +39 051 2099456; Tel: +39 051 2099560
Kekul e´ -Institut f u¨ r Organische Chemie und Biochemie der Universit a¨ t
b
Bonn, Gerhard-Domagk Strasse 1, D-53121 Bonn, Germany.
E-mail: voegtle@uni-bonn.de; Fax: +49 228 735662; Tel: +49 228 733495
References
1
G. R. Newkome, C. Moorefield and F. V o¨ gtle, Dendrimers and
Dendrons: Concepts, Syntheses, Perspectives, VCH, Weinheim,
2
001.
Fig. 5 Emission spectrum of 1 before (full line) and after the addition
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of one (dashed line) and two (dotted–dashed) equivalents of
3 3 2
Cu(CF SO ) . Inset shows the normalized fluorescence intensity
changes at 336 (&), 390 ($), and 510 (m) nm. Experimental
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This journal is ß The Royal Society of Chemistry 2005