Multichromophoric polyphenylene dendrimers: Toward brilliant light emitters with an increased number of fluorophores

Article abstract of DOI:10.1021/ja067174m

Two routes for the introduction of highly fluorescent peryleneimide chromophores into the scaffolding of polyphenylene dendrimers via iterative Diels-Alder cycloadditions are presented. The key intermediates for the divergent dendrimer buildup were two cy

Full text of DOI:10.1021/ja067174m

Published on Web 03/23/2007
Multichromophoric Polyphenylene Dendrimers: Toward
Brilliant Light Emitters with an Increased Number of
Fluorophores
Ingo Oesterling and Klaus Mu¨llen*
Contribution from the Max-Planck-Institute for Polymer Research, Ackermannweg 10,
D-55128 Mainz, Germany
Received October 6, 2006; E-mail: muellen@mpip-mainz.mpg.de
Abstract: Two routes for the introduction of highly fluorescent peryleneimide chromophores into the
scaffolding of polyphenylene dendrimers via iterative Diels-Alder cycloadditions are presented. The key
intermediates for the divergent dendrimer buildup were two cyclopentadienone branching units carrying
two peryleneimides and two masked terminal alkynes. The difference between the two reagents is the
mode of incorporation of the chromophores. In the first case, the chromophores were attached to the
R-position of the tetraphenylcyclopentadienones. In the second case, peryleneimides are used as a “spacer”
in the â-position of the cyclopentadienones giving rise to dendrimers with extended molecular diameters
(up to 12 nm) and 24 chromophores within their scaffold. Absorption and emission characteristics of the
new multichromophoric nanoparticles were investigated and compared to those of the parent dyes.
Additionally, an asymmetrically substituted first-generation dendrimer with six perylene diimide chromophores
and one ester functionality is reported. The ester serves as a potential anchor group, and this nanoemitter
paves the way to a multichromophoric fluorescence label. All dendrimers have good solubility in common
organic solvents, high fluorescence quantum yields, and defined distances between the chromophores,
making them attractive candidates for single-molecule spectroscopy.
to an active center has been imitated.7 Other examples are
multichromophoric polymers used as active components in light-
Introduction
In recent years, there has been an increasing interest in
multichromophoric systems.1 The concept of concentrating many
chromophores in a confined volume is used by nature in light-
harvesting complexes2 and fluorescent proteins.3 Important
representatives of artificial multichromophoric systems are
supramolecular assemblies,4 fluorescent latex nanoparticles,5 and
polymers, in which the chromophores are covalently attached
to the polymer backbone or as a side chain.6 In many cases,
the optical functions depend sensitively upon the distances and
relative spatial orientations of the individual chromophore
centers. In artificial multichromophoric dyads or triads, the
vectorial energy transfer of natural light-harvesting complexes
emitting diode (LED) materials.8 The increased fluorescence
intensity of multichromophoric systems has been used to
increase the sensitivity of biological labels.9 A group of
macromolecules that is particularly qualified for the design of
multichromophoric systems is that of dendrimers.10 Especially
for biological labels and for molecular probes, where the
sensitivity often correlates directly with the number of fluoro-
phores, dendritic structures have distinct advantages over
supramolecular or polymeric multichromophores. As dendrimers
are monodisperse molecules, it is possible to tailor the number
of chromophores within these nanoobjects and thus to quantify
the concentration of the analyte. In the case of flexible
dendrimers, intramolecular aggregation of the dyes is not always
suppressed and can result in excimer formation with the
(1) (a) Wolfbeis, O. S.; Bo¨hmer, M.; Durkop, A.; Enderlein, J.; Gruber, M.;
Klimant, I.; Krause, C.; Kurner, J.; Liebsch, G.; Lin, Z.; Oswald, B.; Wu,
M. AdVanced luminescent labels, probes and beads and their application
to luminescence bioassay and imaging; Springer-Verlag: Berlin-Heidelberg,
2002; Vol. 2. (b) Balzani, V.; Ceroni, P.; Maestri, M.; Saudan, C.; Vicinelli,
V. Luminescent dendrimers. Recent advances. In Dendrimers V: Functional
and Hyperbranched Building Blocks, Photophysical Properties, Applications
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A R T I C L E S
Oesterling and Mu¨llen
Scheme 1. Variation of the Chromophore Location in the AB2
Branching Agent: (A) Side Chains and (B) Backbone
Figure 1. Chemical structures of unsubstituted peryleneimide chro-
mophores: perylene-3,4-dicarboximide (PMI) and perylene-3,4,9,10-tetra-
carboxdiimide (PDI).
concomitant decrease in fluorescence intensity.11 Hence, the
chromophore interactions within these dendrimers are rather
complex, and their study is further inhibited by the often low
photostability and fluorescence quantum yields of the chosen
chromophores.12
coefficients, and a fluorescence quantum yield of ∼1.18 These
properties render them attractive candidates for single-molecule
spectroscopy.
The insertion of fluorescent dyes into the scaffold of a rigid
dendrimer can lead to a variety of new and desirable proper-
ties: (i) the number of dyes within the nanoemitter is not
restricted to the number of the possible surface functions; (ii)
the distance between individual dyes in the same or different
generations is defined by the design of the branching units; and
(iii) as the number of chromophores increases within the
dendrimer generations, the intensity of the overall absorption
or emission increases, as does the sensitivity of molecular probes
and biological labels as well. Accordingly, a large number of
chromophores within a confined, well-defined volume is desir-
able.
Herein, we report two different methods of inserting peryle-
neimide dyes into the scaffolding of polyphenylene dendrimers.
In each route, functionalization of the tetraphenylcyclopenta-
dienone branching reagent with peryleneimides is the key step,
resulting in a dendritic branching unit carrying two perylene-
imides and two triisopropylsilyl (TIPS)-protected terminal
alkynes for further dendritic growth. In the first case (route A,
Scheme 1), a chromophore was attached to the phenylene rings
at the R-position of the cyclopentadienone. In the second case
(route B, Scheme 1), the dyes were inserted between the
phenylene rings at the â-position and the alkyne. The dye units
thereby act as stiff spacers between the diene and the dienophile,
maintaining the rigid nature of the resulting dendrimer. The
spacer extends the diameter of the resulting dendrimers com-
pared to the parent polyphenylene dendrimer by approximately
a pentaphenyl moiety per generation. This approach has been
successfully carried out with a p-terphenyl spacer up to the fifth
generation to yield a diameter above 20 nm (see Figure 2).19
Using PDI as a spacer, the buildup of even larger nanoparticles
with an even higher number of generations should be possible.
Furthermore, the insertion of the PDI as a “functional” spacer
permits the material to be used in a number of applications not
accessible to the dendrimer containing the p-terphenyl spacer,
We have recently shown that the use of shape-persistent
polyphenylene dendrimers as rigid dendritic scaffolds ensures
the spatial separation of chromophores. Thus, after loading a
polyphenylene dendrimer with perylenedicarboxmonoimide
(PMI, see Figure 1), the chromophores on the surface do not
show detectable intramolecular aggregation.13 Furthermore, the
interchromophore distances are well defined, which makes them
perfect candidates for investigating excitation energy transfer,
especially at the single-molecule level.14,15
Polyphenylene dendrimers are made by repetitive Diels-
Alder reactions using an ethynyl-functionalized core and a
tetraphenylcyclopentadienone derivative as the branching agent,
leading to pentaphenylbenzene repeating units. The benzene
rings are twisted out-of-plane, and the polyphenylene dendrimers
show high thermal and photochemical stabilities as well as shape
persistence.16
Besides the surface functionalization of polyphenylene den-
drimers with PMI, a variety of polyphenylene dendrimers have
been synthesized carrying perylenetetracarboxdiimide (PDI, see
Figure 1) as the core.17 These dendronized dyes have excellent
thermal, chemical, and photochemical stabilities, high extinction
(11) Stewart, G. M.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 4354-4360.
(12) Neuwahl, F. V. R.; Righini, R.; Adronov, A.; Malenfant, P. R. L.; Fre´chet,
J. M. J. J. Phys. Chem. B 2001, 105, 1307-1312.
(13) (a) De Belder, G.; Jordens, S.; Lor, M.; Schweitzer, G.; De, R.; Weil, T.;
Herrmann, A.; Wiesler, U. K.; Mu¨llen, K.; De Schryver, F. C. J. Photochem.
Photobiol. A: Chem. 2001, 145, 61-70. (b) Weil, T. Ph.D. thesis, Johannes
Gutenberg-University of Mainz, 2002.
(14) (a) Liu, D. J.; De Feyter, S.; Cotlet, M.; Stefan, A.; Wiesler, U. M.;
Herrmann, A.; Grebel-Koehler, D.; Qu, J. Q.; Mu¨llen, K.; De Schryver, F.
C. Macromolecules 2003, 36, 5918-5925. (b) Masuo, S.; Vosch, T.; Cotlet,
M.; Tinnefeld, P.; Habuchi, S.; Bell, T. D. M.; Oesterling, I.; Beljonne,
D.; Champagne, B.; Mu¨llen, K.; Sauer, M.; Hofkens, J.; De Schryver, F.
C. J. Phys. Chem. B 2004, 108, 16686-16696. (c) Christ, T.; Kulzer, F.;
Weil, T.; Mu¨llen, K.; Basche´, T. Chem. Phys. Lett. 2003, 372, 878-885.
(15) Jordens, S.; De Belder, G.; Lor, M.; Schweitzer, G.; Van der Auweraer,
M.; Weil, T.; Herrmann, A.; Wiesler, U. M.; Mu¨llen, K.; De Schryver, F.
C. Photochem. Photobiol. Sci. 2003, 2, 1118-1124.
(16) Zhang, H.; Grim, P. C. M.; Foubert, P.; Vosch, T.; Vanoppen, P.; Wiesler,
U. M.; Berresheim, A. J.; Mu¨llen, K.; De Schryver, F. C. Langmuir 2000,
16, 9009-9014.
(17) (a) Herrmann, A.; Weil, T.; Sinigersky, V.; Wiesler, U. M.; Vosch, T.;
Hofkens, J.; De Schryver, F. C.; Mu¨llen, K. Chem. Eur. J. 2001, 7, 4844-
4853. (b) Qu, J. Q.; Pschirer, N. G.; Liu, D. J.; Stefan, A.; De Schryver, F.
C.; Mu¨llen, K. Chem. Eur. J. 2004, 10, 528-537.
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Rademacher, A.; Markle, S.; Langhals, H. Chem. Ber. 1982, 115, 2927-
2934. (c) Nagao, Y.; Misono, T. Dyes Pigm. 1984, 5, 171-188.
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Angew. Chem., Int. Ed. 2005, 44, 6348-6354.
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Multichromophoric Polyphenylene Dendrimers
A R T I C L E S
Figure 2. Introduction of different spacers into a tetraphenylcyclopentadienone branching agent.
Figure 3. Multichromophoric polyphenylene dendrimers 4, 5, 6, and 27.
e.g., as high-fluorescence molecular probes and as single-photon
sources at a single-molecular level.
The two peryleneimide-functionalized cyclopentadienones, 3
(Figure 2) and 13 (Scheme 2, below), are subsequently used to
build up multichromophoric first-generation polyphenylene
dendrimers 4 and 5 (Figure 3), each carrying eight peryleneimide
chromophores. Remarkably enough, in the case of the tetraphe-
nylcyclopentadienone branching reagent 3, the second-genera-
tion dendrimer 6 (Figure 3), containing 24 PDI chromophores
and 16 terminal ethynyl functions, can be achieved. Branching
agent 3 was also used for the construction of an asymmetric
first-generation dendrimer (27, Figure 3 and Scheme 6, below)
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A R T I C L E S
Oesterling and Mu¨llen
Scheme 2. Synthesis of the “Dye-Loaded” Tetraphenylcyclopentadienone Branching Agent 13a
a Reagents and conditions: (i) 1,3-bis(4-bromophenyl)acetone (8), KOH, ethanol, 20 min, 80 °C, 70%; (ii) bis(pinacolato)diboron (11), potassium acetate,
[Pd(ddpf)], dioxane, 16 h, 70 °C, 75%;24 (iii) compound 9, PMI 12, K2CO3, [Pd(PPh3)4], toluene/ethanol, 16 h, 80 °C, 47%.
with six PDI chromomophores and one aliphatic ester. The ester
function is a potential anchor group which could be attached to
different kinds of substrates, e.g., surfaces, polymers, or
biologically active moieties. With its multiplied fluorescence
intensity, its use as a fluorescence label is anticipated.
coupling reaction under Suzuki conditions21,25 to afford 13 in
47% yield. Chromatographic purification was performed after
each step.
The “Extended” Tetraphenylcyclopentadienone Branch-
ing Unit 3. To use PDI as a rigid spacer, the two functions
were introduced into the imide structures of the PDI chro-
mophore, whereby the inherent stiffness of the chromophore is
retained. In an imidization reaction, 1,6,7,12-tetrachloroperylene-
3,4,9,10-tetracarboxy dianhydride26 (14) was reacted with
4-bromo-2,6-dimethylaniline (15) in propanoic acid for 16 h at
160 °C (Scheme 3). For characterization, 16 was purified by
column chromatography using CH2Cl2 as an eluent, which, due
to the low solubility, led to significant losses. In general, the
crude product was used directly after precipitation from water.
After subsequent substitution of the chlorides in the so-called
“bay region” of the chromophore (see Figure 1) with an excess
of the bulky tert-octyl-substituted phenol 17 in N-methylpyr-
rolidone (NMP) at 90 °C, the product was purified by column
chromatography due to its increased solubility. Compound 18
was obtained as a red solid with a yield of 80%.
The desymmetrization of 18 was achieved via a statistical
palladium-catalyzed Hagihara reaction27 with 1.15 equiv of
triisopropylsilylacetylene (19). This resulted in two products:
the doubly substituted PDI 21 and the singly substituted PDI
20. The two products were separated by column chromatography
using petroleum ether/CH2Cl2 (3/2) as the eluent to obtain 21
and the desired desymmetrized PDI chromophore 20 in 11%
and 32% yield, respectively, with recovery of the starting
material.
Results and Discussion
We have already described the synthesis of tetraphenylcy-
clopentadienones bearing rylene chromophores as functional
moieties.20 There, the Pd-catalyzed Suzuki cross-coupling
reaction21 appeared as a straightforward synthetic tool for
introducing chromophores into the tetraphenylcyclopentadienone
branching units, as it is easy to handle and results in good yields.
Here we use this synthetic strategy of introducing peryleneimide
chromophores into tetraphenylcyclopentadienones via the Suzuki
coupling reaction for the substitution at both the R- and
â-positions. Whereas in the case of the R-coupling the required
chromophore carries only the boronic ester function needed for
a Suzuki coupling, in the case of the â-linkage a chromophore
with two different functions is needed: one aryl halide for the
Suzuki coupling with the cyclopentadienone and one TIPS-
protected ethynyl group to ensure further dendritic growth. The
resulting dendrimers were synthesized using a divergent route,
which we have used repeatedly in the past,22 starting from
tetrakis(4-ethynylphenyl)methane (24).
Syntheses. The r-Substituted Tetraphenylcyclopenta-
dienone Branching Unit 13. Cyclopentadienone 9 was prepared
via a Knoevenagel condensation of 4,4′-bis(triisopropylsilyl-
ethynyl)benzil (7) and 1,3-bis(4-bromophenyl)acetone (8),23
bearing two bromides in the para position of the phenyl ring in
the R-position and two TIPS-protected ethylene functions at the
phenyl ring in the â-position of the cyclopentadienone (Scheme
2). Cyclopentadienone 9 was then treated with the boronic ester-
functionalized PMI derivative 1224 in a palladium-catalyzed
Cyclopentadienone 22 was prepared according to literature
procedures.23,28 The cyclopentadienone derivative 23 was
achieved by a palladium-catalyzed reaction of commercially
available bis(pinacolato)diboron (11) and 22 in a modification
of the Miyaura procedure21 (Scheme 4). The pure product could
be obtained by using column chromatography with CH2Cl2 as
eluent. However, since this purification leads to a loss of
(20) (a) Weil, T.; Wiesler, U. M.; Herrmann, A.; Bauer, R.; Hofkens, J.; De
Schryver, F. C.; Mu¨llen, K. J. Am. Chem. Soc. 2001, 123, 8101-8108. (b)
Gensch, T.; Hofkens, J.; Herrmann, A.; Tsuda, K.; Verheijen, W.; Vosch,
T.; Christ, T.; Basche´, T.; Mu¨llen, K.; De Schryver, F. C. Angew. Chem.,
Int. Ed. 1999, 38, 3752-3756.
(25) Watanabe, T.; Miyaura, N.; Suzuki, A. Synlett 1992, 3, 207-210.
(26) Eilingsfeld, H.; Patsch, M. (BASF AG). Ger. Offen DE 2519790, 1976; 9
pp.
(21) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483.
(22) Wiesler, U. M.; Berresheim, A. J.; Morgenroth, F.; Lieser, G.; Mu¨llen, K.
Macromolecules 2001, 34, 187-199.
(27) (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 627-630. (b) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron
Lett. 1975, 50, 4467-4470. (c) Dieck, H. A.; Heck, F. R. J. Organomet.
Chem. 1975, 93, 259-263. (d) Cassar, L. J. Organomet. Chem. 1975, 93,
253-257.
(23) Morgenroth, F.; Reuther, E.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1997,
36, 631-634.
(24) Weil, T.; Reuther, E.; Beer, C.; Mu¨llen, K. Chem. Eur. J. 2004, 10, 1398-
1414.
(28) Morgenroth, F.; Reuther, E.; Mu¨llen, K. Angew. Chem. 1997, 109, 647.
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Multichromophoric Polyphenylene Dendrimers
A R T I C L E S
Scheme 3. Synthesis of the Desymmetrized PDI Chromophore 20a
a Reagents and conditions: (i) 2,6-dimethyl-4-bromoaniline (15), propanoic acid, 16 h, 160 °C, 52%; (ii) 4-(1,1,3,3-tetramethylbutyl)phenol (17), K2CO3,
NMP, 16 h, 90 °C, 80%; (iii) triisopropylsilylacetylene (19), CuI, PPh3; [Pd(PPh3)Cl2], THF/triethylamine (1/1), 15 h, 80 °C, 14%.
Scheme 4. Synthesis of the Branching Agent 3, Bearing a PDI Chromophore as a Spacer between Diene and Dienophilea
a Reagents and conditions: (i) bis(pinacolato)diboron (11), potassium acetate, [Pd(ddpf)], dioxane, 16 h, 75 °C; (ii) PDI chromophore 20, K2CO3/H2O,
[Pd(PPh3)4], toluene/ethanol, 16 h, 80 °C, 44%.
product, crude 23 was used without further purification. The
formation of the PDI-substituted cyclopentadienone derivative
3 was achieved by a palladium-catalyzed Suzuki coupling.25
Purification by column chromatography with petroleum ether/
CH2Cl2 (3/2 f 1/1) gave the branching agent 3 in 44% yield.
To prevent the photooxidation of the branching unit, 3 was
stored in the dark under an argon atmosphere at 4 °C.
The Multichromophoric Dendrimers 4, 5, and 6. For the
synthesis of the multichromophore 4 with the chromophores in
the side chain, the tetrahedral core molecule tetrakis(4-ethy-
nylphenyl)methane (24) was treated with branching agent 13
in diphenyl ether at 180 °C (Scheme 5). After 2 days, the
reaction mixture was precipitated into methanol and purified
by column chromatography to give the first-generation multi-
chromophore 4, bearing eight PMI and eight protected alkynes,
in 69% yield.
The first-generation dendrimer 5, with the chromophores in
the backbone, was formed by Diels-Alder reaction of the
tetrahedral core 24 with an 8-fold excess of the branching unit
3 in m-xylene at 150 °C. Dendrimer 5 was isolated by column
chromatography with petroleum ether/CH2Cl2 (1/1) as eluent
in 86% yield. After facile deprotection of 5 with tetrabutylam-
monium fluoride, the ethynyl-functionalized dendrimer 25 was
treated with 24 equiv of 3 to give the second-generation
dendrimer 6, bearing 24 PDI chromophores and 16 terminal
TIPS-protected alkynes. Column chromatography on silica gel,
as well as purification by dialysis (in dimethylformamide), failed
to separate dendrimer 6 from branching agent 3. Molecular
mechanics calculations (see following section) suggest that the
â-substituted dendrimers possess large cavities, so we assume
that the branching unit 3 can penetrate into these cavities,
thereby inhibiting purification by column chromatography and
dialysis. However, pure 6 was obtained by size exclusion
chromatography with tetrahydrofuran (THF) as eluent.
All the dendrimers described in this paper are highly soluble
in common organic solvents such as dichloromethane, toluene,
and THF. Characterization by MALDI-TOF mass spectrometry,
1H NMR, and 13C NMR spectroscopy was therefore easily
accomplished.
The MALDI-TOF spectrum of 4 (Figure 4a) showed three
peaks: one at m/z 7122, corresponding to the molecular ion
mass, and two below the molecular ion peak. As they do not
correspond to any potential growth imperfections and their
intensities increase with increasing laser power (in comparison
to the molecular ion peak), they are likely the result of
fragmentation during ionization. The perfect agreement between
the calculated and experimentally determined m/z ratios strongly
supports the monodispersity of the dendrimer 4.
The MALDI-TOF spectrum of 6 (Figure 4b) shows only one
peak, at m/z 41 674. The broad appearance of the molecular
ion peak results from the linear detection mode of the MALDI
measurement, which, due to the high molecular mass of the
molecule, is necessary for detection. However, the broad
molecular ion peak does not overlap with the peak of any
potential growth imperfection due to unreacted ethynyl groups
during the Diels-Alder reaction. This side product would cause
a mass difference of 3546 g/mol.
1
The H NMR spectrum of 4 (see Supporting Information)
indicates that the eight PMI chromophores are not identical.
Due to the rigid nature of the dendrimer backbone, the two
chromophores at one dendrimer arm are located in substantially
different environments. The signals of the benzylic CH protons
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A R T I C L E S
Oesterling and Mu¨llen
Scheme 5. Synthesis of the “Dye-Loaded” First- and Second-Generation Polyphenylene Dendrimers 4, 5, and 6a
a Reagents and conditions: (i) branching agent 13, diphenyl ether, 180 °C, 2 d, 69%; (ii) branching agent 3, m-xylene, 150 °C, 5 d, 86%; (iii)
tetrabutylammonium fluoride, THF, 1 h, 90%; (iv) branching agent 3, diphenyl ether, 200 °C, 18 d.
of the diisopropylphenyl group appear as two overlapping
equally intense septets at 2.86-2.66 ppm. The signals of the
corresponding methyl groups are also nonequivalent.
solvent, column chromatography with petroleum ether/CH2Cl2
(1/1) gave the mixture of 27a and 27b in 52% yield. Separation
of these two isomers by column chromatography was not
possible.
The 1H NMR spectra of 5 and 6 (see Supporting Information)
show well-separated and assignable signals for the protons of
the bay region of the PDI chromophore at 8.11 ppm as well as
for the aliphatic protons between 2.05 and 0.71 ppm. Further,
the signals of the single proton on the central benzene ring of
the pentaphenylbenzene units show generation-dependent chemi-
cal shifts.22 The relative intensities of aromatic and aliphatic
signals correspond well with the expected values.
The Monofunctional “PDI-Loaded” Polyphenylene Den-
drimer 27. For the design of a biological label, not only a high
fluorescence quantum yield and a high absorption cross section
but also one anchor group are needed. The â-substituted
branching agent 22 was applied to the synthesis of a polyphe-
nylene dendrimer decorated with six PDI chromophores and
one aliphatic carboxylic acid group (27). The introduction of
the aliphatic ester, which can serve as an anchor, succeeds by
application of the monofunctional core building block 26. This
core was recently introduced by our group29 and carries one
protected carboxylic acid and three terminal ethyne functions,
which are needed for Diels-Alder reaction during the dendritic
growth. As a result of the [4+2] cycloaddition mechanism
during the formation of 26, this product exists in two chro-
matographically inseparable regioisomers (26a and 26b).29
In the synthesis of the monofunctional first-generation
dendrimer 27, the asymmetrically substituted core molecule 26
(the product of both isomers) was reacted with the cyclopen-
tadienone branching agent 3 (Scheme 6). After removal of the
1
The presence of the regioisomers was verified by H NMR
(see Supporting Information). As the polar esters of the two
isomers are localized in substantially different environments,
the protons of the methoxy groups appear as two separate
singlets at 3.64 and 3.63 ppm. The ratio of 27a:27b, calculated
from the intensities of these peaks, is about 1:1.
Visualization and Simulation. The structures of the three
dendrimers 4, 5, and 6 (Figure 5) have been optimized by
applying the MM+ force field and the Conjugate Gradient
algorithm implemented in the HyperChem 6.0 program pack-
age.30 Although all the dendrimers were built around the same
core molecule, the two types of dendrimers have different
appearances. Whereas the â-substituted dendrimers 5 and 6 show
an overall tetrahedral structure, the R-substituted dendrimer 4
has a dense “pancake-like” shape with no large cavities. The
calculated diameters of 5 and 6 are larger than those of 4 and
the corresponding parent polyphenylene dendrimers, which are
built up from core molecule 24 and branching agent 1 (see Table
1). The diameter of the second-generation dendrimer 6 is about
7 nm larger than that of the second generation of the parent
polyphenylene dendrimer22 and about 2.5 nm larger than that
(30) The hypersurfaces of the dendrimer branches were first calculated separately
by using the MM+ force field method and the “Fletcher-Reeves” algorithm
until a global minimum was evaluated. In the next step, they were attached
to the separately minimized core molecule, and the geometry of the
complete molecule was calculated again with the MM+ force field and
the “Fletcher-Reeves” algorithm. Because the hypersurfaces generally
obtained for more complex dendrimers were fairly flat, the three-
dimensional structures presented here are one of several possible local
minima. However, these structures were found to be sufficient to evaluate
the dimension of the molecules.
(29) Mihov, G.; Scheppelmann, I.; Mu¨llen, K. J. Org. Chem. 2004, 69, 8029-
8037.
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Multichromophoric Polyphenylene Dendrimers
A R T I C L E S
Figure 4. (a) MALDI-TOF mass spectrum of the first-generation dendrimer 4 (m/z 7122 [M]+, calcd for 4 m/z 7121.78; dithranol matrix, reflector mode).
(b) MALDI-TOF mass spectrum of second-generation dendrimer 6 in the presence of sodium triflate (m/z 41 715 [M]+, calcd for 6 m/z 41 705; dithranol
matrix, linear mode).
of the second generation of the previously presented “extended”
dendrimer built from branching unit 2.19 Calculations of the
theoretical dendrimer density31 (Table 1) show significantly
lower densities for the â-substituted dendrimers 5 and 6 than
for 4 and the parent polyphenylene dendrimers.19 This suggests
much higher porosities. Indeed, large cavities can be seen in
the calculated structures. In selected cases of polyphenylene
dendrimers, we have already verified the porosity by the uptake
of guest molecules as measured by using an ultramicro
balance.32 Due to the small diameter and the compact shape of
4, several chromophores within this multichromophore are close
together; distances between the “midpoints” 33 of the two
chromophores are less than 9 Å, making chromophore-
chromophore interactions very likely (see the following section).
The distances between chromophores within dendrimers 5 and
6 are larger. The smallest distance is that between the two
chromophores within the same repeat unit. Here, the two PDI
chromophores are almost face-to-face, and the distance between
the “midpoints” 33 of the chromophores is larger than 14 Å.
We showed for many examples that the results of molecular
dynamics simulations provide a good first approximation of the
three-dimensional structures of polyphenylene dendrimers. The
results of these simulations are supported by experimental data
such as fluorescence correlation spectroscopy (FCS),34 dynamic
light scattering (DLS),19,34 transmission electron microscopy
(TEM),19,22 and atomic force microscopy (AFM).16,35 However,
(31) The dendrimer densities were calculated by dividing the molecular mass
by the volume determined from the radius given by the molecular mechanics
calculation.
(32) Schlupp, M.; Weil, T.; Berresheim, A. J.; Wiesler, U. M.; Bargon, J.;
Mu¨llen, K. Angew. Chem., Int. Ed. 2001, 40, 4011-4015.
(33) In this calculation, the center of gravity of the aromatic perylene system
was chosen as the “midpoint” of the chromophore.
(34) Minard-Basquin, C.; Weil, T.; Hohner, A.; Ra¨dler, J. O.; Mu¨llen, K. J.
Am. Chem. Soc. 2003, 125, 5832-5838.
(35) Shifrina, Z. B.; Rajadurai, M. S.; Firsova, N. V.; Bronstein, L. M.; Huang,
X. L.; Rusanov, A. L.; Mu¨llen, K. Macromolecules 2005, 38, 9920-9932.
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J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007 4601

A R T I C L E S
Oesterling and Mu¨llen
Scheme 6. Synthesis of the Ester-Functionalized Multichromophoric Dendrimer 27a
a Reagents and conditions: (i) ester-functionalized core 26, diphenyl ether, 170 °C, 5 d, 52%.
Figure 5. Calculated (MM+) 3D structures of first-generation dendrimer 4, first-generation dendrimer 5, and second-generation dendrimer 6.
Table 1. Molecular Mass, Theoretical Diameter, and Fluorescence Quantum Yields of 10, 20, 27, 4, 5, and 6
extinction
molecular
mass
FD/MALDI-TOF
MSb
SEC
theor
radiusd
[nm]
theor
dendrimer densitye
[g/mL]
molecular
formula
no. of
dyes
coefficienta
M
P (PS)c
SEC
PD
compd
[L/(mol
‚
cm)]
[g/mol]
[g/mol]
[g/mol]
Φflf
10
20
27
4
5
6
C34H26BrNO2
C107H125BrN2O8Si
1
1
6
8
8
35 320
45 970
201 600
282 900
275 400
1 090 000
560
1 675
11 565
7 122
14 596
41 705
0.94
0.98
0.97
0.95
0.96
0.95
1 676
11 588 (Na)
7 122
14 619 (Na)
41 674 (Na)
C
794H855N13O51Si6
C505H460N8O16Si8
2.1
3.2
6.7
0.30
0.18
0.09
C
C
1001H1092N16O64Si8
2865H3076N48O192Si16
24
33633
1.04
a Extinction coefficients were measured in CHCl3. The extinction coefficient of 10 is known36 and was measured in CH2Cl2. b All MALDI-TOF mass
spectra were acquired by irradiating the 1:250 analyte/matrix mixture with a nitrogen laser (337 nm) using dithranol as matrix and, if specified, sodium
trifluoroacetate salt as cationization agent. c SEC was performed at room temperature in THF using SEC columns with 500, 104, and 106 Å porosities and
calibrated against narrowly disperse, linear polystyrene (PS) standards. d Theoretical radii were determined by using MM+ molecular mechanics calculations.30
e The dendrimer densities were calculated by dividing the molecular mass by the volume determined by the radius given by the molecular mechanics
calculation. f Fluorescence quantum yields were measured in chloroform by excitation at 540 nm with cresyl violet (in MeOH) as the reference chromophore.
to obtain insight into the dynamic behavior, a molecular
dynamics conformational search is necessary, which is still under
investigation.
Optical Characterization. The UV/vis absorption spectra
of all dendrimers exhibit two components: an absorption
between 400 and 650 nm due to the chromophores and one at
much shorter wavelength arising from the dendritic framework,
which consists of strongly twisted polyphenylene units. In order
(36) Geerts, Y.; Quante, H.; Platz, H.; Mahrt, R.; Hopmeier, M.; Bo¨hm, A.;
Mu¨llen, K. J. Mater. Chem. 1998, 8, 2357-2369.
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Multichromophoric Polyphenylene Dendrimers
A R T I C L E S
Figure 6. Normalized UV/vis (solid lines) and fluorescence spectra (dashed lines) of PMI 10 (red), dye-functionalized cyclopentadienone 13 (blue), and
first-generation dendrimer 4 (black). Inset: Extinction coefficient of PMI chromophore 10 (9), branching agent 13 (2), and first-generation dendrimer 4
([).
to determine the influence of the spatial arrangement of the
chromophores within the dendrimers, the absorption and emis-
sion spectra of the dendrimers were compared with those of
individual dyes.
number of chromophores and the extinction coefficients at the
absorption maximum (Figure 7, inset). Dendrimer 6, with 24
chromophores, exhibits an extinction coefficient of 1 090 000
M-1 cm-1 and possesses almost the same absorption per
chromophore as an isolated chromophore.
The absorption and emission spectra of the monofunctional
first-generation dendrimer 27 (Figure 8) show signal patterns
similar to those of the spectra of the monomeric chromophore
20 and the first-generation dendrimer with eight PDI chro-
mophores, 5. Compared to these latter spectra, the emission
spectrum of 27 is slightly hypsochromically shifted. The
fluorescence quantum yield of 27 is 0.97 (Table 1) and thus
remains as high as those of the chromophore 20 and the first-
generation dendrimer 5. No quenching effect, with its associated
reduction of the fluorescence quantum yield, is detected, and
the fluorescence intensity of the dendrimer is 6 times higher
than that of the single PDI dye 20.
In the case of the precursors to dendrimer 4, substitution of
the bromine atom of 10 with an aryl group (to give cyclopen-
tadienone 13) results in a red-shift of both the absorption and
emission spectra of ∆λmax ) 23 nm (Figure 6) and a loss of the
fine structure of both spectra. Formation of the dendrimer 4
results in a small hypsochromic shift of the absorption maxima
but a bathochromic shift of the emission maximum. In the
emission spectra of 4, nearly no fine structure is detectable. The
loss of the fine structure and the bathochromic shift of the
fluorescence maxima indicate interaction between neighboring
chromophores. This was also suggested by the molecular
mechanics calculations (see above). Due to the small distances
between the chromophores in 4 (∼0.9 Å), excimer formation
becomes more likely. However, a direct correlation exists
between the number of chromophores and the extinction
coefficients at the absorption maximum (Figure 7, inset), and
the dendrimer 4 shows a fluorescence quantum yield close to
unity (see Table 1).
For dendrimers 5 and 6 (with the chromophores in the
backbone), the absorption spectra show spectra comparable to
that of the monomeric dye 20 (Figure 7). The emission spectra
of the first-generation dendrimer 5 and the monomeric dye 20
are also nearly identical. The fluorescence maximum for both
molecules is located at 628 nm. The fluorescence band of the
second-generation dendrimer 6 is hypsochromically shifted, with
an emission maximum at 623 nm. The retention of the fine
structures for both the absorbance and the emission spectra
indicates the absence of strong interactions of neighboring
chromophores within the dendrimers 5 and 6. The fluorescence
quantum yields of 20 and 5 and 6 are nearly unity (Table 1).
As in dendrimer 4, a direct correlation exists between the
Conclusion
We have presented new ways to create multichromophoric
nanomaterials that possess a large number of chromophores in
a confined volume. In this process, peryleneimide chromophores
were inserted into the scaffold of shape-persistent polyphenylene
dendrimers 4, 5, and 6 using two novel tetraphenylcyclopen-
tadienone branching agents, 13 and 3. By linking the chro-
mophores at different positions of the tetraphenylcyclopenta-
dienone branching agent, dendrimers with diverse topologies
were obtained. Thus, connecting the chromophores at the
R-position of the tetraphenylcyclopentadienone branching agent
13 gave a densely packed dendrimer, 4, in which the chro-
mophores are in close proximity. Loss of the fine structure of
the absorption and emission spectra indicates chromophore-
chromophore interaction. In contrast, linking chromophores at
the â-position of the branching unit 3 led to the drastically
enlarged dendrimers 5 and 6, which are distinguished by
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A R T I C L E S
Oesterling and Mu¨llen
Figure 7. Normalized UV/vis (solid lines) and fluorescence spectra (dashed lines) of PDI 20 (blue), first-generation dendrimer 5 (red), and second-generation
dendrimer 6 (black). Inset: Extinction coefficient of PDI chromophore 20 (9), branching agent 3 (2), first-generation dendrimer 5 ([), and second-
generation dendrimer 6 (b).
Figure 8. Absorption (solid line) and emission spectrum (dashed line) of the monofunctional multichromophore 27 in chloroform (excitation at 540 nm).
increased interchromophore distances. Thus, both dendrimers
show no loss of the fine structure of the absorption and emission
spectra compared to the single dye 20.
By using the core molecule 26, it was possible to build up
the first-generation dendrimer 27, bearing six PDI chromophores
and one aliphatic ester. The ester represents a potential anchor
group which could potentially be attached to various substrates,
such as surfaces, polymers, or biologically active moieties. The
high extinction coefficient and a fluorescence quantum yield
near unity make it an attractive candidate for use as a
fluorescence label.
All dendrimers possess fluorescence quantum yields near
unity. Due to the high photochemical stability of the chro-
mophore units and their incorporation into well-defined positions
within the dendrimers, the dendritic multichromophores reported
herein can be used for single-molecule spectroscopy, leading
to a better insight into interactions between chromophores in
close proximity. In addition, peryleneimide-functionalized den-
drimers have high potential as single-photon sources, as several
single-molecule studies have already shown.15,37
(37) (a) Vosch, T.; Cotlet, M.; Hofkens, J.; Van der Biest, K.; Lor, M.; Weston,
K.; Tinnefeld, P.; Sauer, M.; Latterini, L.; Mu¨llen, K.; De Schryver, F. C.
J. Phys. Chem. A 2003, 107, 6920-6931. (b) Tinnefeld, P.; Weston, K.
D.; Vosch, T.; Cotlet, M.; Weil, T.; Hofkens, J.; Mu¨llen, K.; De Schryver,
F. C.; Sauer, M. J. Am. Chem. Soc. 2002, 124, 14310-14311.
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Multichromophoric Polyphenylene Dendrimers
A R T I C L E S
Acknowledgment. Financial support by the Deutsche For-
schungsgemeinschaft (SFB 625), the “Fonds der Chemie”, the
EU Integrated Project NAIMO (No. NMP4-CT-2004-500355),
BASF AG, and Roche Deutschland Holding GmbH is gratefully
acknowledged.
Supporting Information Available: Experimental section and
1H NMR spectra of 4, 5, 6, 20, and 27. This material is available
free of charge via the Internet at http://pups.acs.org.
JA067174M
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J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007 4605