Dendrimers with a photoactive and redox-active [Ru(bpy)3]2+-type core: Photophysical properties, electrochemical behavior, and excited-state electron-transfer reactions

Article abstract of DOI:10.1021/ja990430t

We report the synthesis of six new dendrimers built around a [Ru(bpy)₃]²⁺-type core (bpy = 2,2′-bipyridine) and bearing up to 24 4′-tert-butylphenyloxy or 48 benzyl units in the periphery. The metallodendrimers were obtained by compl

Full text of DOI:10.1021/ja990430t

6290
J. Am. Chem. Soc. 1999, 121, 6290-6298
Dendrimers with a Photoactive and Redox-Active [Ru(bpy)3]2+-Type
Core: Photophysical Properties, Electrochemical Behavior, and
Excited-State Electron-Transfer Reactions
Fritz Vo1gtle,*,† Marcus Plevoets,† Martin Nieger,‡ Gianluca C. Azzellini,§,# Alberto Credi,§
Luisa De Cola,*,§, Valeria De Marchis,§ Margherita Venturi,§ and Vincenzo Balzani*,|,§
Contribution from the Kekule´-Institut fu¨r Organische Chemie und Biochemie and the Institut fu¨r
Anorganische Chemie der UniVersita¨t Bonn, Gerhard-Domagk Strasse 1, 53121 Bonn, Germany, and
Dipartimento di Chimica “G. Ciamician”, UniVersita` di Bologna, Via Selmi 2, 40126 Bologna, Italy
ReceiVed February 10, 1999. ReVised Manuscript ReceiVed April 30, 1999
Abstract: We report the synthesis of six new dendrimers built around a [Ru(bpy)3]2+-type core (bpy ) 2,2′-
bipyridine) and bearing up to 24 4′-tert-butylphenyloxy or 48 benzyl units in the periphery. The metalloden-
drimers were obtained by complexation of ruthenium trichloride or Ru(bpy)2Cl2 with bipyridine ligands carrying
dendritic wedges in the 4,4′-positions. The absorption spectra and luminescence properties (spectra and lifetimes
at 77 and 298 K; quantum yields at 298 K) of the six novel compounds are reported. All of them show the
characteristic luminescence of the [Ru(bpy)3]2+-type core unit. The dendritic branches protect the luminescent
excited state of the core by dioxygen quenching. For the three compounds containing the 4′-tert-butylphenyloxy
peripheral units, the electrochemical behavior and the excited-state quenching via electron transfer were also
studied. The electrochemical experiments have evidenced an oxidation and three reduction one-electron processes
centered in the [Ru(bpy)3]2+-type core and two multielectron oxidation processes involving the dioxybenzene-
and oxybenzene-type units of the dendritic branches. The core of the largest dendrimer shows an electrochemical
behavior typical of encapsulated electroactive units. The reaction of the luminescent excited state of the [Ru-
(bpy)3]2+-type core with three electron-transfer quenchers (namely, methyl viologen dication, tetrathiafulvalene,
and anthraquinone-2,6-disulfonate anion) was found to take place by a dynamic mechanism in all cases. The
quenching rate constants, obtained by Stern-Volmer kinetic analysis, are compared with those found for the
simple [Ru(bpy)3]2+ complex. The results show that, for each quencher, the value of the rate constant decreases
with increasing number and size of the dendritic branches. For the second-generation dendrimer containing 24
4′-tert-butylphenyloxy units at the periphery, the rate constant of the reaction with methyl viologen is more
than 1 order of magnitude smaller than that of the “naked” [Ru(bpy)3]2+ complex. All the experiments were
performed in acetonitrile solution, except for luminescence experiments at 77 K where butyronitrile was used.
Introduction
photochemical and electrochemical properties of such a
unit.5-8,10,11 This effect may be exploited, for example, to
improve the luminescence properties of the core or to decrease
the rate of electron-transfer processes.
Dendrimers1 are well-defined macromolecules exhibiting a
tree-like structure, first derived by the “cascade molecule”
approach.2 Dendrimer chemistry is a rapidly expanding field
for both basic and applicative reasons.1 Such compounds are
particularly interesting when they carry units capable of
performing specific function (e.g., redox reactions and photo-
induced processes). Transition-metal complexes, especially Ru-
(II) complexes of polypyridine-type ligands,3 are useful com-
ponents for constructing dendrimers because of their outstanding
photochemical, photophysical, and electrochemical properties.4,5
In dendrimers, transition-metal units may be incorporated as a
core, as components of the branches, and/or as peripheral
units.4-9 When a dendrimer contains a photo- and/or electro-
active unit as a core, the dendritic branches may modify the
(1) (a) Tomalia, D. A.; Durst, H. D. Top. Curr. Chem. 1993, 165, 193.
(b) AdVances in Dendritic Macromolecules; Newkome G. R., Ed.; JAI
Press: London, 1994-1996; Vols. 1-3. (c) Fre´chet, J. M. J. Science 1994,
263, 1710. (d) Issberner, J.; Moors, R.; Vo¨gtle, F. Angew. Chem., Int. Ed.
Engl. 1994, 33, 2413. (e) Newkome, G. R.; Moorefield, C.; Vo¨gtle, F.
Dendritic Molecules: Concepts, Syntheses, PerspectiVes; VCH: Weinheim,
1996. (f) Zeng, F.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681. (g)
Peerling, H. W. I.; Meijer, E. W. Chem. Eur. J. 1997, 3, 1563. (h) Archut,
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Newkome, G. R. Top. Curr. Chem. 1998, 197, 19. (j) Majoral, J.-P.;
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† Kekule´-Institut fu¨r Organische Chemie und Biochemie der Universita¨t
Bonn. E-mail: voegtle@uni-bonn.de.
‡ Institut fu¨r Anorganische Chemie der Universita¨t Bonn.
§ Universita` di Bologna.
# On leave from the Instituto de Qu´ımica, Universidade de Sa˜o Paulo,
C.P. 26077, Sa˜o Paulo, Brazil.
Current address: University of Amsterdam, Institute of Molecular
Chemistry, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Nether-
lands. E-mail: ldc@anorg.chem.uva.nl.
(3) (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85. (b) Kalyanasundaram,
K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic
Press: London, 1992.
| Address correspondence to this author. E-mail: vbalzani@ciam.unibo.it
10.1021/ja990430t CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/19/1999

[Ru(bpy)3]2+-Type Dendrimers
J. Am. Chem. Soc., Vol. 121, No. 26, 1999 6291
Continuing our investigations in the field of photoactive,4,10fi,12
metal-containing4,10fi dendrimers, we report here (i) the synthesis
of five 2,2′-bipyridine (bpy) ligands carrying dendritic wedges
in the 4,4′-positions (Figure 1), (ii) the synthesis of six new
dendrimers built around a [Ru(bpy)3]2+-type core and bearing
up to 24 4′-tert-butylphenyloxy or 48 benzyl units in the
periphery (Figure 2), (iii) the absorption spectra and lumines-
cence properties (spectra and lifetimes at 77 and 298 K; quantum
yields at 298 K) of the six novel metal-based dendrimers, and
(iv) the electrochemical behavior and the excited-state quenching
via bimolecular electron transfer of the three metal-based
dendrimers containing the 4′-tert-butylphenyloxy peripheral
units.
alcohol 11 with 2 equiv of 1-bromo-3-(4′-tert-butylphenyloxy)-
propane (12)14 gives the benzylic alcohol of the first generation
13. The reaction was carried out in boiling acetone in the
presence of potassium carbonate and a catalytical amount of
[18]crown-6. After purification the benzylic alcohol was
obtained as colorless solid. The conversion to the benzylic
bromide 9 was achieved with phosphorus tribromide15 in dry
toluene at 0 °C. A colorless solid compound was obtained.
Reaction of the first generation benzylic bromide with 11 under
the same conditions described above gave the second generation
alcohol 14, which was again treated with PBr3 to give the
corresponding dendritic benzylic bromide 10.
(b) Dendritic Bipyridine Ligands. For the preparation of
the dendritic bipyridine ligands 1-5 (Figure 1), 4,4′-dimethyl-
2,2′-bipyridine (Me2bpy) was treated with an excess of LDA
in dry THF at -10 °C to give the orange dilithio derivative.
After 45 min the dendritic benzyl bromides, dissolved in a
minimum amount of dry THF, were added to the reaction
mixture and gave the desired symmetrical 4,4′-disubstituted
dendritic bipyridine ligands (Figure 1). The crude products have
been purified by column chromatography on silica gel. In all
cases the product yield amounts to 50-60%. The structures of
Results and Discussion
Synthesis. The synthesis of the new dendritic ligands 1-5
(Figure 1) was performed with use of 4,4′-dimethyl-2,2′-
bipyridine units as starting material, which were substituted with
dendritic wedges of different generations and different peripheral
groups.
1
the new bipyridine ligands could be readily deduced from H
and 13C NMR spectra as well as from MALDI-TOF mass
spectrometric analysis.
(a) Dendritic Benzyl Bromides. The dendritic benzyl
bromides 6-8 (Figure 1) have been prepared by the convergent
procedure indicated by Fre´chet.13 The same procedure was used
to prepare the benzylic bromides 9 and 10 containing more
flexible end groups with 4′-tert-butylphenyloxy peripheral units.
In the first step (Figure 3), the reaction of 3,5-dihydroxybenzyl
We were also able to get a single-cristal X-ray analysis of
the second-generation bipyridine ligand 2. As can be seen from
Figure 4 , the structure shows a trans configuration, which is
typical for 2,2′-bipyridine derivatives. The whole structure is
substantially planar and symmetrical with respect to the center
of the pyridine-pyridine bond.
(4) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi,
M. Acc. Chem. Res. 1998, 31, 26.
(c) Dendritic Bipyridine Complexes. The procedures used
to obtain the metallodendrimers shown in Figure 2 followed
those described in the literature for other Ru-bpy type
complexes.3,10fi,16 The dendritic bipyridine ligands 1, 2, and 3
and a corresponding amount of ruthenium trichloride were
dissolved in a mixture of chloroform and ethanol (2:1 (v/v))
and gently refluxed for 5 days. The reaction mixture turned from
dark violet to bright orange, which indicates the formation of
the trisbipyridine-type ruthenium complexes [Ru(1)3]2+, [Ru-
(2)3]2+, and [Ru(3)3]2+. Contrary to what happens for the ligands
with phenyl peripheral groups, the ligand with 4′-tert-butylphe-
nylpropyl end groups 5 is soluble in more polar solvents so
that the reaction was carried out in pure ethanol and was
completed after 2 days, yielding the [Ru(5)3]2+ compound. The
mixed-ligand compounds [Ru(bpy)2(4)]2+ and [Ru(bpy)2(5)]2+
were obtained by refluxing the dendritic bipyridine ligands 4
and 5 and a stoichiometric amount of [Ru(bpy)2Cl2]‚2H2O in
ethanol for 3 days.
The dendritic ruthenium complexes have been purified by
column chromatography. Independently from the dendritic
generation and the nature of the peripheral groups, all the
ruthenium-based dendrimers are obtained as orange, highly
viscous, oily compounds. Except for [Ru(bpy)2(4)]2+, which was
obtained as a chloride salt, the counterions were exchanged with
hexafluorophosphate ions to facilitate purification.
Because of their highly branched dendritic wedges, the
ruthenium complexes have almost lost any salt-like character,
and are soluble in a wide spectrum of organic solvents. With
increasing generation and depending on the end groups, they
(5) Venturi, M.; Serroni, S.; Juris, A.; Campagna, S.; Balzani, V. Top.
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Mora´n, M.; Kaifer, A. E. J. Am. Chem. Soc. 1997, 119, 5760. (c) Vale´rio,
C.; Fillaut, J.-L.; Ruiz, J.; Guittard, J.; Blais, J.-C.; Astruc, D. J. Am. Chem.
Soc. 1997, 119, 2588. (d) Marvaud, V.; Astruc, D.; Leize, E.; van Dorsselaer,
A.; Guittard, J.; Blais, J.-C. New J. Chem. 1997, 21, 1309. (e) Achar, S.;
Immoos, C. E.; Hill, M. G.; Catalano, V. J. Inorg. Chem. 1997, 36, 2314.
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Cola, L.; Balzani, V. Chem. Eur. J. 1997, 3, 706. (g) Devadoss, C.; Bharathi,
P.; Moore, J. F. Angew. Chem., Int. Ed. Engl. 1997, 36, 1633. (h) Kimura,
M.; Nakada, K.; Yamaguchi, Y.; Hanabusa, K.; Shirai, H.; Kobayashi, N.
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Balzani, V. New J. Chem. 1999, 23, 63.
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Chem. Eur. J. 1996, 2, 1085. (b) Newkome, G. R.; Gu¨ther, R.; Moorefield,
C. N.; Cardullo, F.; Echegoyen, L.; Pe´rez-Cordero, E.; Luftmann, H. Angew.
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Gross, M.; Knobler, C. B.; Louati, A.; Sanford, E. M. Angew. Chem., Int.
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6292 J. Am. Chem. Soc., Vol. 121, No. 26, 1999
Vo¨gtle et al.
Figure 1. Synthesis of the 2,2′-bipyridine (bpy) ligands carrying dendritic wedges.
1
have the tendency to become more and more insoluble in polar
organic solvents. For example, the third generation compound
[Ru(3)3]2+ is only poorly soluble in acetonitrile at room
temperature. It is noteworthy that [Ru(5)3]2+, thanks to its 4′-
tert-butylphenyloxy end groups, is soluble even in ether and
THF. The structures of the dendritic ruthenium complexes could
be readily deduced from H and 13C NMR spectra and, except
for [Ru(3)3]2+, also from MALDI-TOF mass spectrometry.
Spectroscopic and Photophysical Properties. The absorp-
tion and emission spectra for the six metal-based dendrimers
are shown in Figures 5 and 6, and the most relevant spectro-
scopic and photophysical data of the six compounds are gathered

[Ru(bpy)3]2+-Type Dendrimers
J. Am. Chem. Soc., Vol. 121, No. 26, 1999 6293
Figure 2. Dendritic metal complexes.
in Table 1, where the literature data of the [Ru(bpy)3]2+ model
compound17 are also shown for comparison purposes. For the
fully dendritic [Ru(1)3]2+, [Ru(2)3]2+, [Ru(3)3]2+, and [Ru(5)3]2+
compounds the absorption spectra in the visible region are quite
similar, showing that the shape and chemical nature of the
dendritic branches do not substantially affect the [Ru(bpy)3]2+
-
based chromophoric core. The absorption spectra of the [Ru-
(bpy)2(4)]2+ and [Ru(bpy)2(5)]2+ compounds are slightly dif-
ferent from the spectra of the other compounds, as expected
because of their mixed-ligand nature. As far as emission is

6294 J. Am. Chem. Soc., Vol. 121, No. 26, 1999
Vo¨gtle et al.
Figure 3. Synthesis of dendritic benzyl bromides.
Figure 4. X-ray structure of the second-generation bipyridine ligand 2.
concerned, all compounds exhibit the characteristic lumines-
cence of triplet metal-to-ligand charge-transfer (MLCT) excited
states of Ru-bpy type compounds,3 i.e., of their metal-based
cores. However, several differences can be noted, related to
different overall chemical composition.
(i) A slight red shift can be observed in the case of the [Ru-
(bpy)2(4)]2+ and [Ru(bpy)2(5)]2+ compounds, most likely
because of their mixed-ligand nature.3
(ii) In the homogeneous [Ru(1)3]2+, [Ru(2)3]2+, and [Ru-
(3)3]2+ family, the emission maximum moves slightly to higher
energy with increasing size of the branches, both at 293 and 77
K. This can be accounted for by a decrease in the number of
(polar) solvent molecules surrounding the core, with consequent
destabilization of the CT excited state.
(iii) In deaerated solution the lifetime of the luminescent
excited state is only slightly different for the various dendrimers
and also very close to that of the [Ru(bpy)3]2+ model compound.
In aerated solution, however, the lifetimes (and quantum yields)
are very different. The [Ru(bpy)2(4)]2+ compound exhibits
almost the same lifetime as [Ru(bpy)3]2+, whereas [Ru(bpy)2-
(5)]2+ and the fully dendritic [Ru(1)3]2+, [Ru(2)3]2+, [Ru(3)3]2+
,
and [Ru(5)3]2+ compounds exhibit considerably longer lifetimes.
It can also be noticed that in the homogeneous [Ru(1)3]2+, [Ru-
(2)3]2+, and [Ru(3)3]2+ family the lifetime in aerated solution
increases with increasing size of the branches. The lifetime of
the largest compound is three times longer than that of the
smallest one. An increase in lifetime with increasing size or
number of the dendritic branches occurs also for the [Ru(bpy)2-
(4)]2+, [Ru(bpy)2(5)]2+, and [Ru(5)3]2+ family. These results
show that the dendrimer branches protect the [Ru(bpy)3]2+-based
core from dioxygen quenching, as has been previously observed
for other similar compounds.10fi A more detailed discussion of
dioxygen quenching will be given below. A long lifetime of
the luminescent excited state in aerated solutions is important
for immunoassay applications since the signal of the label can
be read after the decay of the background fluorescence of the
sample, whose lifetime usually is in the nanosecond time scale.18
Electrochemical Behavior. The amount of compound needed
for electrochemical investigations is much larger than that
(17) Slightly closer model compounds for the dendrimer cores would
have been [Ru(Me2bpy)3]2+ and [Ru(bpy)2(Me2bpy)]2+. For such com-
pounds, however, full spectroscopic and photophysical data under our
experimental conditions are not available.3
(18) Bu¨nzli, J.-C. G. In Lanthanides Probes in Life, Chemical and Earth
Sciences, Theory and Practice; Bu¨nzli, J.-C. G., Choppin, G. R., Eds.;
Elsevier: New York, 1989; p 219.

[Ru(bpy)3]2+-Type Dendrimers
J. Am. Chem. Soc., Vol. 121, No. 26, 1999 6295
benzene-type units that are known to undergo oxidation at
moderately positive potentials.19 More specifically, the oxy-
benzene- and dioxybenzene-type units are 4 and 2 for [Ru(bpy)2-
(4)]2+, 8 and 6 for [Ru(bpy)2(5)]2+, and 24 and 18 for [Ru(5)3]2+
,
respectively. To assign the observed electrochemical processes,
we have taken as model compounds of the cores the [Ru(bpy)2-
(Me2bpy)]2+ and [Ru(Me2bpy)3]2+ complexes, whose electro-
chemical behavior, also displayed in Table 2, is known from
literature.20 They show three reversible one-electron reduction
processes involving the three ligands and a reversible one-
electron oxidation process localized on the metal ion. Meth-
oxybenzene and 1,3-dimethoxybenzene, which are reported19
to undergo a one-electron oxidation process at +1.76 and +1.50
V (vs SCE, acetonitrile solution), respectively, are close model
compounds of the electroactive groups present in the branches.
Figure 5. Absorption and (inset) emission spectra of the [Ru(1)3]2+
,
The small [Ru(bpy)2(4)]2+ dendrimer shows six redox
processes (Table 2).21 By comparison with the behavior of [Ru-
(bpy)2(Me2bpy)]2+, the three reduction processes and the
oxidation process at less positive potential, all reversible and
monoelectronic, can be straightforwardly assigned to the metal-
complex moiety. The two oxidation processes observed at more
positive potentials can be assigned to the oxidation of the
dioxybenzene- and oxybenzene-type units of the branches,
respectively. Since these processes are not fully reversible, it is
difficult to establish whether the number of exchanged electrons
corresponds to that of the involved units.
[Ru(2)3]2+, and [Ru(3)3]2+ compounds in deaerated acetonitrile solution
at 298 K (λex ) 450 nm). The absorption spectrum of [Ru(2)3]2+, not
shown for the sake of clarity, is identical with that of the other two
compounds for λ > 380 nm.
The first oxidation (metal-centered) and reduction (ligand-
centered) processes of [Ru(bpy)2(5)]2+ occur at the same
potentials as those of the smaller [Ru(bpy)2(4)]2+ compound.
The potential values for the second and third reduction
processes, however, cannot be determined since the third wave
in the cathodic scan and the wave corresponding to the second
process in the anodic scan are covered by a stripping peak. This
suggests that the uptake of two or three electrons by [Ru(bpy)2-
(5)]2+ causes adsorption on the electrode surface. Since such a
phenomenon is not present in the fully dendritic [Ru(5)3]2+ (vide
infra), it cannot be attributed to the peripheral units of the wedge
and is likely related to the particular “comet shape” of [Ru-
(bpy)2(5)]2+. On the oxidation side, the two not fully reversible
multielectron processes involving the electroactive units of the
wedge are also present. The area of the DPV peaks correspond-
ing to these processes is larger than that observed in the case
of [Ru(bpy)2(4)]2+, in agreement with the larger number of
Figure 6. Absorption and (inset) emission spectra of the [Ru(bpy)2-
(4)]2+, [Ru(bpy)2(5)]2+, and [Ru(5)3]2+ compounds in deaerated aceto-
nitrile solution at 298 K (λex ) 450 nm). The absorption spectrum of
[Ru(bpy)2(5)]2+, not shown for the sake of clarity, is identical with
that of [Ru(bpy)2(4)]2+ for λ > 380 nm.
Table 1. Luminescence Dataa
298 Kb
77 Kc
λ
max, nm
611
τ, µs
10-2
Φ
λ
max, nm τ, µs
electroactive units present in the wedge of [Ru(bpy)2(5)]2+
.
d
[Ru(bpy)3]2+
[Ru(1)3]2+
[Ru(2)3]2+
[Ru(3)3]2+
0.172
0.990e
0.191
1.16e
0.316
0.990e
0.562
1.04e
1.6
1.9
2.9
4.8
1.8
1.9
3.5
582
586
584
579
588
586
581
4.8
5.3
5.4
5.6
5.2
5.6
5.5
The closest model compound for the core of the fully dendritic
[Ru(5)3]2+ compound is the [Ru(Me2bpy)3]2+ complex (Table
2). The assignment of the observed processes is again straight-
forward, taking into account that the two multielectron oxidation
processes related to the numerous dioxybenzene- and oxyben-
zene-type units of the branches merge into a high and very broad
DPV peak. In CV experiments, such oxidation processes result
in a huge wave that does not permit a detailed investigation of
the core-based oxidation process. The potential value for this
process can be obtained from DPV, where the corresponding
peak seems to involve less that one electron. Comparison with
the behavior of the [Ru(Me2bpy)3]2+ model compound shows
that (i) oxidation occurs at a more positive potential, (ii)
reduction takes place at less negative potential, and (iii) the
614
611
609
620
616
611
[Ru(bpy)2(4)]2+
[Ru(bpy)2(5)]2+
[Ru(5)3]2+
0.170
1.08e
0.193
1.05e
0.415
0.992e
a Aerated solution, except otherwise noted. b Acetonitrile solution.
c Butyronitrile solution. d From ref 10f. e Deaerated solution.
needed for photophysical experiments. With the available
samples, we could study the electrochemical behavior only for
the compounds of the [Ru(bpy)2(4)]2+, [Ru(bpy)2(5)]2+, and
[Ru(5)3]2+ series. The electrochemical data are gathered in Table
2.
(19) (a) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in
Nonaqueous Systems; Dekker: New York, 1970. (b) Jacques, P.; Burget,
D.; Allonas, X. New J. Chem. 1996, 20, 933.
(20) Mabrouk, P. A.; Wrighton, M. S. Inorg. Chem. 1986, 25, 526.
(21) Since [Ru(bpy)2(4)]2+ was available as Cl- salt, oxidation of the
anion was also present at ca. +1.0 V vs SCE, but it did not interfere with
the other processes.
In the examined compounds, besides the electroactive [Ru-
(bpy)3]2+-type core,3 there are several oxybenzene- and dioxy-

6296 J. Am. Chem. Soc., Vol. 121, No. 26, 1999
Table 2. Electrochemical Dataa
Vo¨gtle et al.
oxidationb
reductionb
compd
branch-centered
Ru-centered
bpy-centered
c
[Ru(bpy)2(Me2bpy)]2+
+1.25 (1)
+1.13 (1)
+1.27 (1)
+1.27 (1)
+1.26h
-1.36 (1)
-1.44 (1)
-1.35 (1)
-1.35 (1)
-1.33h
-1.56 (1)
-1.62 (1)
-1.56 (1)
f
-1.81 (1)
c
[Ru(Me2bpy)3]2+
-1.86 (1)
[Ru(bpy)2(4)]2+
+1.68e
+1.80e
+1.54e
+1.56e
-1.81 (1)
f
i
d
[Ru(bpy)2(5)]2+
[Ru(5)3]2+
+1.57e,g
-1.52h
a Argon-purged acetonitrile solution, room temperature, TBAPF6 as supporting electrolyte, glassy carbon as working electrode. b Halfwave potential
values in V vs SCE, unless otherwise noted; the number of exchanged electrons is indicated in parentheses. c Data from ref 20. d An irreversible
oxidation process is observed at ca. +1.0 V, due to the chloride counterions of this complex. e Not fully reversible process; potential value estimated
from DPV peaks. f The observation of this process is precluded by adsorption phenomena. g Very broad DPV peak. h Less than one electron is
involved; see text for details. i A poorly defined process is observed at ca. -1.9 V.
A* + B f A+ + B-
A* + B f A- + B+
oxidative quenching
reductive quenching
(1)
(2)
observed processes are not fully reversible (as indicated by both
CV and DPV for reduction, and from DPV for oxidation). This
behavior, which has been observed for other dendrimers
containing an electroactive core,11 is that expected for encap-
sulated electroactive units.8
The thermodynamic driving force of the reaction can be
estimated from the approximate equations:
Quenching Reactions. It is well-known that electronically
excited molecules can be quenched via energy or electron
transfer by other molecules.22 In fluid solution a quenching
reaction usually occurs during an encounter between the excited
molecule and quencher. It follows that only the excited states
that exhibit a reasonably long lifetime may undergo quenching
processes. The lifetime of the 3MLCT luminescent excited state
of Ru(II)-polypyridine complexes is long enough, even in
aerated solutions,23 to allow the occurrence of encounters when
the quencher concentration is not extremely high.3
∆G° ) -∆E°° + E(A+/A) - E(B/B-)
∆G° ) -∆E°° - E(A/A-) + E(B+/B)
(3)
(4)
where ∆E°° is the excited-state spectroscopic energy (in eV)
and E(A+/A), E(B+/B), E(A/A-), and E(B/B-) are the energies
(in eV) of the respective one-electron reduction processes.29 The
[Ru(bpy)2(4)]2+, [Ru(bpy)2(5)]2+, and [Ru(5)3]2+ dendrimers and
the [Ru(bpy)3]2+ model compound have practically the same
excited-state energy (about 2.12 eV),30 and the same E(A+/A)
and E(A/A-) potential values (Table 2 and ref 3). The values
of E(B/B-) are -0.4426 and -0.57 V27 (vs SCE) for MV2+
and AQ2-, respectively, and the value of E(B+/B) is +0.33 V
for TTF.28 Insertion of these values in eqs 3 or 4 shows that (i)
both oxidative quenching by MV2+ and AQ2- and reductive
quenching by TTF are thermodynamically allowed processes
and (ii) the free energy change is practically the same for the
quenching of the three dendrimers by the same quencher.
The rate constant kq of a quenching process taking place by
a dynamic mechanism can be calculated from the Stern-Volmer
equation22
When a luminescent unit is buried into the core of a
dendrimer, the rate of the quenching reactions is expected to
be slower than for the free unit. Despite its potential interest
for a variety of applications,8 this effect has been the object of
only a few scattered investigations.10a-c,f-i More attention has
been paid to dendrimers as the supramolecular host environment
for quenching reactions involving excited Ru-bpy type com-
plexes and a variety of quenchers.24
3
We have studied the quenching of the MLCT excited state
of the [Ru(bpy)2(4)]2+, [Ru(bpy)2(5)]2+, and [Ru(5)3]2+ den-
drimers by three compounds which were already known to
3
quench the MLCT excited state of the [Ru(bpy)3]2+ model
compound:25 methyl viologen dication (MV2+), tetrathiaful-
valene (TTF), and anthraquinone-2,6-disulfonate anion (AQ2-).
These three molecules do not possess excited states below the
3MLCT level of the [Ru(bpy)3]2+ chromophoric unit, but are
easy to reduce (MV2+ and AQ2-) or to oxidize (TTF). As a
consequence, quenching can only take place by electron transfer.
The experiments have been performed in acetonitrile solutions
containing TBAPF6 at 298 K.
τ°/τ ) Ι°/Ι ) 1 + kqτ°[Q]
(5)
where τ° and Ι° are the excited-state lifetime and quantum yield
in the absence of quencher, and τ and Ι are the same quantities
measured in the presence of a quencher concentration [Q]. For
each one of the [Ru(bpy)2(4)]2+, [Ru(bpy)2(5)]2+, and [Ru(5)3]2+
dendrimers, we have measured the luminescence lifetime (λex
) 350 nm; λem > 500 nm) and intensity (λex ) 480 nm; λem
)
Electron-transfer quenching reactions may involve the oxida-
tion or the reduction of the excited state:
610 nm) for different concentrations of the three selected
quenchers. In the case of TTF and AQ2- the intensity measure-
ments gave less precise results because of spectral overlap at
high quencher concentration. When the interference by quencher
absorption was not exceedingly high, the linear behavior
(22) (a) Turro, N. J. Modern Molecular Photochemistry; Benjamin:
Menlo Park, 1978. (b) Gilbert, A.; Baggot, J. Essentials of Molecular
Photochemistry; Blackwell: Oxford, 1991.
(23) Since in deaerated solution the excited-state lifetime is longer than
in air-equilibrated solution (Table 1), it would appear that quenching can
be more profitably investigated in the former condition. It should be noted,
however, that the quenching constants are calculated from correlations of
results obtained from sets of experiments. A not uniform deaeration of the
various solutions is likely to introduce errors that are not present when the
experiments are performed in air-equilibrated solution.
(24) (a) Turro, N. J.; Barton, J. K.; Tomalia, D. A. Acc. Chem. Res.
1991, 24, 332. (b) Turro, C.; Bossmann, S. H.; Niu, S.; Tomalia, D. A.;
Turro, N. J. J. Phys. Chem. 1995, 99, 5512. (c) Schwarz, P. F.; Turro, N.
J.; Tomalia, D. A. J. Photochem. Photobiol. A: Chem. 1998, 112, 47.
(25) Hoffman, M. Z.; Bolletta, F.; Moggi, L.; Hug, G. L. J. Phys. Chem.
Ref. Data 1989, 18, 219.
(26) Summer, L. A. The Bipyridinium Herbicides; Academic Press:
London, 1980.
(27) In alkaline aqueous solution: Clark, W. M. Oxidation-Reduction
Potentials of Organic Systems; Williams and Winkins Company: Baltimore,
1960.
(28) Coffen, D. L.; Chambers, J. Q.; Williams, D. R.; Garrett, P. E.;
Canfield, N. D. J. Am. Chem. Soc. 1971, 93, 2258.
(29) (a) Balzani, V.; Bolletta, F.; Gandolfi, M. T.; Maestri, M. Top. Curr.
Chem. 1978, 75, 1. (b) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.;
Serroni, S. Chem. ReV. 1996, 96, 759.
(30) Approximately given29a by the energy of the emission maximum at
77 K.

[Ru(bpy)3]2+-Type Dendrimers
J. Am. Chem. Soc., Vol. 121, No. 26, 1999 6297
radius of the compound; (ii) protection of the dendritic branches
toward diffusion of the quencher within the dendrimer volume;
(iii) competition between solvent and dendritic branches for the
solvation of the core; (iv) Coulombic attraction/repulsion
between the positively charged core and the charged quenchers;
and (v) folding of the dendritic branches of [Ru(bpy)2(4)]2+ and
[Ru(bpy)2(5)]2+ around the unsubstituted bpy ligands. Clearly,
the dynamic quenching of an excited state situated in the core
of a dendrimer is a very complex process that needs further
investigations. Anyway, a protection effect by the dendritic
branches is certainly present and can perhaps be exploited for
practical applications requiring an increase in the excited-state
lifetime under particular experimental conditions.8
Conclusions
Figure 7. Stern-Volmer plots for the quenching of the [Ru(bpy)2-
(4)]2+, [Ru(bpy)2(5)]2+, and [Ru(5)3]2+ compounds by MV2+ in aceto-
nitrile solutions containing 5 × 10-2 M TBAPF6 at 298 K.
We have synthesized six new dendrimers built around a [Ru-
(bpy)3]2+-type core and bearing up to 24 4′-tert-butylphenyloxy
or 48 benzyl units in the periphery. All compounds exhibit the
characteristic 3MLCT luminescence of their [Ru(bpy)3]2+-type
core. However, several differences can be noted in the photo-
physical parameters (particularly in the emission lifetime in
aerated solution), related to different overall chemical composi-
tion of the compounds. For the three compounds containing the
4′-tert-butylphenyloxy peripheral units, the electrochemical
behavior and the excited-state electron-transfer quenching by
cationic (MV2+), neutral (TTF), and anionic (AQ2-) quenchers
have been investigated. The core of the largest dendrimer shows
an electrochemical behavior typical of encapsulated electroactive
units. Oxidation of the electroactive units contained in the
dendritic branches has also been observed. In general, the values
of the quenching constants decrease with increasing number and
size of the dendritic branches. For MV2+ the quenching constant
is more than 1 order of magnitude smaller than that of the
“naked” [Ru(bpy)3]2+ complex. The results show that several,
sometimes contrasting, factors intervene in determining the
actual value of the quenching rate constant.
Table 3. Quenching Rate Constants kq (M-1 s-1
)
MV2+ a TTFa AQ2- a
O2
b
[Ru(bpy)3]2+
1.4 × 109 1.2 × 1010 1.2 × 1010 3 × 109
[Ru(bpy)2(4)]2+ 7.8 × 108 9.9 × 109
[Ru(bpy)2(5)]2+ 5.8 × 108 8.3 × 109
7.3 × 109
8.6 × 109
2.7 × 109
3 × 109
2 × 109
7 × 108
[Ru(5)3]2+
1.0 × 108 5.4 × 109
a Acetonitrile solution containing 5 × 10-2 M TBAPF6; kq values
obtained from eq 5 by using lifetimes from at least five different
quencher concentrations; λex ) 350 nm, λem > 500 nm. b Acetonitrile
solution; kq values obtained from eq 5 by using the lifetimes of deaerated
and air-equilibrated solutions.
predicted by eq 5 was obtained from intensity measurements,
yielding the same kq values obtained from lifetime measure-
ments.31 Figure 7 shows the Stern-Volmer plots for the
quenching of [Ru(bpy)2(4)]2+, [Ru(bpy)2(5)]2+, and [Ru(5)3]2+
by MV2+. The values of the quenching constants are gathered
in Table 3, where approximate kq values for dioxygen quenching,
obtained from eq 5 by using the lifetimes of deaerated and air-
equilibrated solutions, are also shown.32 As one can see, there
is an almost generalized decrease of the quenching constant with
increasing number and size of the organic branches appended
to the [Ru(bpy)3]2+-type core. This effect, however, is not the
same in all cases. For the positively charged MV2+ quencher,
the rate constant decreases by more than 1 order of magnitude
in going from [Ru(bpy)3]2+ to the fully dendritic [Ru(5)3]2+
compound, whereas the effect is smaller for the negatively
charged AQ2- quencher and very small (only a factor of 2) for
the neutral TTF quencher. In going from [Ru(bpy)3]2+ to [Ru-
(bpy)2(4)]2+, a comparable decrease is observed for MV2+ and
AQ2-, and practically no effect for TTF and O2. On increasing
the length of the dendritic wedge in the Ru(bpy)2-type species,
a small decrease is observed for MV2+, TTF, and O2, whereas
for AQ2- the rate constant increases slightly. Finally, replace-
ment of the two unsubstituted bpy ligands of [Ru(bpy)2(5)]2+
with dendritic ligands has a much larger effect for AQ2- than
for TTF. These results are difficult to rationalize, presumably
because the rate constant of the quenching process is affected
by several, sometimes contrasting, factors including the fol-
lowing: (i) dependence of the diffusion rate constant on the
Experimental Section
Materials, Methods, and Measurements. Chemicals were pur-
chased from Aldrich and Fluka and used as received. 4,4′-Dimethyl-
2,2′-bipyridine, the monomeric branching units 6, 7, and 8, and
1-bromo-3-(4′-tert-butylphenyloxy)propane (12) were prepared accord-
ing to published literature.13 Thin-layer chromatography (TLC) was
carried out on aluminum sheets precoated with silica gel 60 F254
(Merck 1.05554). The sheets were examined by UV light (λ ) 254
nm). Column chromatography was carried out with silica gel 60 (Merck
15101). Melting points were determined on a Kofler microscope heater
(Reichert, Vienna) and are not corrected. Mass spectra were obtained
on an A.E.I. (Manchester, UK) MS 50 operating in electron impact
mode (EIMS). MALDI spectra were recorded on a TofSpec E
1
(Micromass, Manchester, UK). The H and 13C NMR spectra were
recorded on a Bruker AM 250 (250 MHz (1H), 62.9 MHz (13C)) or on
a Bruker AM 400 (400 MHz (1H), 100.6 MHz (13C)). A description of
the equipment and procedures employed for the absorption, lumines-
cence, and electrochemical experiments can be found in ref 33 and in
the Supporting Information.
X-ray Structural Analysis of 4,4′-Bis[[3′′,5′′-bis[3′′′,5′′′-bis(ben-
zyloxy)]benzyloxy]phenylethyl]-2,2′-bipyridine (2). Crystal data:
C110H96N2O12, M ) 1637.89 g mol-1, colorless crystal of 0.20 × 0.10
× 0.08 mm, triclinic, space group P1h (No. 2), a ) 10.096(1) Å, b )
11.270(1) Å, c ) 20.127(1) Å, R ) 81.47(1)°, â ) 75.94(1)°, γ )
73.40(1)°, V ) 2121.2(3) Å3, Z ) 1, dcalc ) 1.282 Mg m-3, µ(Mo KR)
(31) In the case of [Ru(5)3]2+, when very high concentrations of quencher
had to be used, the luminescence decay was not strictly monoexponential,
and the τ°/τ plot showed some upward curvature. In such conditions, the
very weak luminescence intensity, along with strong spectral interference
of the quencher, prevented a more accurate analysis.
(32) The O2 concentration in air-equilibrated acetonitrile solution is 1.9
× 10-3 M at 297 K: Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook
of Photochemistry; Dekker: New York, 1993.
(33) Amabilino, D. B.; Asakawa, M.; Ashton, P. R.; Ballardini, R.;
Balzani, V.; Belohradsky, M.; Credi, A.; Higuchi, M.; Raymo, F. M.;
Shimizu, T.; Stoddart, J. F.; Venturi, M.; Yase, K. New J. Chem. 1998, 22,
959.

6298 J. Am. Chem. Soc., Vol. 121, No. 26, 1999
Vo¨gtle et al.
) 0.083 mm-1, F(000) ) 866; Nonius Kappa CCD diffractometer; θ
range for data collection, 1.89-28.20°; index ranges, -12 e h e 12,
-14 e k e 14, -25 e l e 25; temperature, 123(2) K; wavelength,
0.71073 Å (Mo KR); monochromator, graphite; reflections collected,
12136, independent reflections, 6836 (Rint ) 0.061); refinement method,
full-matrix least squares on F2; data/restraints/parameters ) 6828/0/
560;goodness-of-fit on F2, 0.763; final R indices [I > 2σ(I)], R1 )
0.039, wR2 ) 0.071; R indices (all data), R1 ) 0.113, wR2 ) 0.081;
largest difference peak and hole, 0.147 and -0.179 eÅ-3. The structure
has been deposited at the Cambridge Crystallographic Data Centre
(CCDC 114186).
lecular Devices Project), University of Bologna (Funds for
Selected Research Topics), and EU (TMR grant FMRX-CT96-
0031). G. C. Azzellini wishes to thank Fapesp (Fundac¸a˜o de
Amparo a` Pesquisa do Estado de Sa˜o Paulo-Brazil) for financial
support.
Supporting Information Available: Synthetic procedures,
characterization, equipments and methods for absorption, lu-
minescence and electrochemical experiments, and X-ray crystal-
lographic data (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported in Germany
by Volkswagen Stiftung and in Italy by MURST (Supramo-
JA990430T