Near-infrared fluorescent nanoparticles formed by self-assembly of lipidic (bodipy) dyes

Article abstract of DOI:10.1002/chem.201101407

Bodipy-iercing: A Bodipy dye decorated with an ammonium at the tip and gallate-shaped flexible chains at the periphery was found to self-assemble into fluorescent nanoparticles in the presence of water (see figure). These nanoparticles act as a host for o

Full text of DOI:10.1002/chem.201101407

COMMUNICATION
DOI: 10.1002/chem.201101407
Near-Infrared Fluorescent Nanoparticles Formed by Self-Assembly of Lipidic
(Bodipy) Dyes
Jean-Hubert Olivier,[a] Josiane Widmaier,[b] and Raymond Ziessel*[a]
Fluorescent organic nanoparticles (FONs) formed from
p-conjugated frameworks are of immense interest in materi-
als science,[1] most notably as advanced means for imaging
of intact biological systems and delivery vehicles.[2] In partic-
ular, fluorescent nanoparticles engineered from polymer-
doped dyes[3] or small molecules[4] allow fine-tuning of the
emissive properties, at least over a restricted wavelength
range in the visible window. The highpoint of this research
is the intercalation of multiple dyes of disparate optical
properties into one-dimensional heterostructures[5] or to pro-
mote fluorescence resonance energy transfer within the
FONs as a means to induce white-light emission.[6] Small
molecular dopants, desirable for cost effectiveness, present
special problems in this field since they are prone to segre-
gation into microstructures or aggregates that serve as dark
quenchers of local fluorescence.[7] However, in certain cases,
emissive aggregates can be assembled from rather small
molecules, such as hexaphenylsilole, that themselves are
nonemissive when dispersed as monomers.[8] The extension
of this work has led to the formulation of systems with
which organic dyes are translocated into FONs by light-
driven self-assemblies that provide a means of fluorescence
enhancement.[9]
angle governed by the presence of appendages along the
Bodipy core. Such aggregates, or dimers, are weakly fluores-
cent and usually exhibit a modest red-shifted absorption
spectrum, which can be well interpreted by conventional ex-
citonic coupling theory.[13] In specific cases, formation of
H- and J-aggregates can be controlled by host–guest com-
plexation,[14] macrocyclization,[15] multilayering in polyelec-
trolytes,[16] and by confinement in sol-gel matrices,[17] or
liquid crystalline phases.[18,19]
In seeking to manipulate the type of self-aggregation un-
dertaken by functionalized Bodipy dyes, attention has
turned to a central platform decorated with an ammonium
cation at the tip and multiple paraffin chains at the outer pe-
riphery (Scheme 1). The intention was to achieve a balance
between stacking induced by the hydrophobic chains and
electrostatic repulsion caused by the localized cationic resi-
dues. In particular, it was anticipated that attenuation of the
latter charges might be realized by adoption of a tilted ge-
ometry that restricted the number of monomers accreted
into the final structure. To attain this goal, preliminary ma-
terials were designed around trialkoxyphenyl fragments and
trimethylammonium head-groups.
Boron-dipyrromethene (Bodipy) dyes, being uncommonly
popular fluorescent labels, are susceptible to aggregation in
polar solvents and in the solid state. The resultant supra-
molecular assemblies can be used to tune the color of
OLEDs[10] or to examine structure–function relationships for
proteins and lipid membranes.[11] In general, the apolar
Bodipy dyes form H- or J-aggregates due to facile stacking
of the electron-rich dipyrromethene core. In the former
structures, the Bodipy planes stack in parallel to give essen-
tially nonfluorescent species featuring a blue-shifted absorp-
tion profile.[12] In the J-aggregate, the transition dipoles are
not parallel but oriented in planes mutually tilted by an
[a] Dr. J.-H. Olivier, Dr. R. Ziessel
Laboratoire de Chimie Molꢀculaire et Spectroscopies Avancꢀes
Ecole Europꢀenne de Chimie, Polymꢁres et Matꢀriaux, CNRS
25 rue Becquerel, 67087 Strasbourg Cedex 02 (France)
E-mail: ziessel@unistra.fr
Scheme 1. Outline of the synthesis protocols used to prepare key inter-
mediates leading to the isolation of the pivotal blue dye 4. (i) Toluene, pi-
peridine, pTsOH (tr), 1408C, 53%; (ii) dimethylaminopropyne, [Pd-
[b] J. Widmaier
AHCTUNGTREN(GNUN PPh3)2Cl2] (6 mol%), CuI (10 mol%), benzene, TEA, 508C, 82%;
Institut Charles Sadron, 23 rue du Loess BP 84047
67034 Strasbourg Cedex 2 (France)
(iii) CH3I, THF, room temperature, 3 h, 75%, followed by anion ex-
change with KPF6 in a mixture of THF/water. The insert gives the struc-
tures for the yellow dyes 5 and 6 needed for intercalation studies and as
reference compounds.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101407.
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The target molecule 4 was prepared in three steps from 1
through a double Knoevenagel reaction carried out on 3,4,5-
tridodecyloxybenzaldehyde under basic conditions leading
to 2 in good yield.[20] The 16.2 Hz proton–proton coupling
tion[21] and are in agreement with similar features reported
for cyanine dyes.[22] Above 65% water precipitation of the
dye occurred. On irradiation at 380 nm, a sharp emission
was observed at 743 nm, for which FF was 6% and tS was
reduced to 0.27 ns (Figure 1b). The changes in fluorescence
could be observed by the naked eye (Figure 1b, inset). The
excitation spectrum followed the absorption spectrum, and
thereby confirmed that the J-aggregate is responsible for the
fluorescence found at 743 nm. We could not exclude that
the shift in absorption and emission wavelength could be
due to a combination of conformational effects and charge
transfer contributions as currently scrutinized in oligoacene
molecular crystals.[23]
Dynamic light scattering (DLS) carried out on a solution
of 4 in THF containing water (65% v/v water) revealed the
presence of spherical nanoparticles with a narrow size distri-
bution of 1.4 to 1.6 nm. These objects are designated as
4-agg. During aging of the mother liquor for 4 weeks no no-
ticeable changes in the nanoparticle size were observed (Fig-
ure 2a). Larger particles (>10 nm) were observed upon
using greater quantities of water. The particle size and dis-
tribution was confirmed by transmission electron microscop-
ic (TEM) studies (Figure 2b and c) and reproduced with
several fresh solution of 4-agg (~10ꢀ6 m).
1
constant for the double bond at 7.27 ppm in the H NMR
spectrum confirmed the trans conformation of the vinylic
bond. Subsequent cross-coupling with dimethylaminopro-
pyne provided 3 in fair yield. Alkylation of the tertiary
amine appears facile by using iodomethane and affords the
stable cationic species 4. The observation of a singlet at
1
2.64 ppm in the H NMR spectrum, at the expense of the
singlet at 2.24 ppm for the tertiary amine 3, is diagnostic of
the course of the reaction. Metathesis of the counter anion
is performed at will.
The absorption and emission spectra of 4 are typical of a
blue distyryl-based Bodipy dye with a S0!S1 transition cen-
tered at 646 nm, a second absorption transition peaking at
370 nm and a fluorescence maximum at 669 nm. A fluores-
cence quantum yield (FF) of 40% and an excited-state life-
time (tS) of 4.1 ns are in keeping with a singlet emitter. In-
terestingly, progressive addition of water (from 0 to 65%,
v/v) to a tetrahydrofuran (THF) solution of 4 resulted in
shifting of the absorption maximum to 737 nm (Figure 1a).
The final solution remained transparent and homogeneous
over several weeks without apparent change of the absorp-
tion and emission properties (Figure 1a, inset). The shape
and the bathochromic shift are clear signatures of J-aggrega-
Figure 2. a) Dynamic light scattering analysis of a THF/water (65% v/v)
solution of 4-agg (5ꢃ10ꢀ6 m) and after aging the solution. b) TEM image
of the same solution drop-cast and evaporated to dryness, overnight, on a
copper grid with a carbon membrane; c) sample prepared from a fresh
solution of 4-agg (3.6ꢃ10ꢀ6 m); scale bars: 50 nm.
Figure 1. Spectroscopic changes observed during addition of water to a
THF solution of 4, the concentration of the dye in the final mixture was
5ꢃ10ꢀ6 m. a) Evolution of the absorption spectrum as the water content
increased from 0 to 65% (v/v); the insert shows photographs of the origi-
nal THF solution (left) and after addition of 65% (v/v) water (right)
The next challenge was to examine the ability of the
nanoparticle to sequester dye molecules from solution in
such a way as to form functional units. A convenient test
system involves decorating the nanoparticles with a yellow
Bodipy dye that absorbs predominantly at 500 nm, at which
under ambient light. b) Evolution of the fluorescence spectrum (lexc
=
380 nm) under the same conditions; the insert shows photographs of the
fluorescence stimulated by UV illumination.
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Fluorescent Bodipy Nanoparticles
COMMUNICATION
the blue dye is almost transparent. To this end, the nanopar-
ticles were prepared in the presence of dye 5, a less-conju-
gated Bodipy dye bearing paraffinic chains but lacking the
cationic residue, which absorbs at 500 nm and emits strongly
at 520 nm (FF =90%, tS =5.3 ns). The addition of dye 5
(20 mol%) to the 4-agg nanoparticles did not perturb the
absorption or emission profiles but slightly altered the emis-
sion wavelength (from 743 to 733 nm). Furthermore, the
particle size distribution as determined by DLS was similar
to that of untreated 4-agg (Figure S3 in the Supporting In-
formation). The excitation spectra recorded in the presence
of 5 (20 mol%) shows that photons absorbed by the yellow
dye are efficiently transferred to 4-agg (Figure 3a). We can
conclude from these observations that 5 (guest) is intercalat-
ed into the nanoparticle where it acts as the donor for elec-
tronic energy transfer to the blue aggregate 4-agg (host).
Furthermore, estimation of band intensities at 500 nm and
at 737 nm indicates that the energy transfer efficiency from
the guest to the host is close to 100%. Model calculations[24]
by using Fçrster theory indicate that the critical distance for
through-space energy transfer in this system is about 29 ꢄ,
and thereby precludes the possibility that the yellow dye lies
outside the nanoparticle. It is likely that the small amount
of residual yellow emission (FF =1.2%) arises from 5 not
assimilated into the nanoparticle (Figure 3a).
Support for this hypothesis was provided by replacing the
hydrophobic dye 5 with a related derivative 6 lacking the
paraffinic chains. Thus, when 6 (20 mol%) was added to the
4-agg nanoparticles under identical conditions to those em-
ployed above, energy transfer did not take place. Instead, 6
emitted strongly (FF =67%; ts =5.1 ns) at 520 nm, and emis-
sion from the aggregate at 743 nm was extremely weak. This
latter weak fluorescence arises from direct excitation of the
blue aggregate 4-agg at 490 nm as confirmed from inspec-
tion of the excitation spectrum (lem =760 nm), which shows
no contribution from the yellow dye in the region around
500 nm (Figure 3b). It is concluded on the basis of the
energy transfer results that paraffin chains are essential to
promote uptake of the dye into the nanostructure.
Returning now to the system comprising 5 intercalated
into the nanoparticles, it was shown that a new fluorescence
band appears with a maximum at 667 nm as the loading of 5
approaches 20 mol% (Figure 3a). This latter emission band
could be explained either: 1) by formation of a dimeric form
of 5 embedded in the blue nanoparticles, by analogy with
lipid domains in cell membranes,[25] that does not contribute
to the energy transfer process to the nanoparticles as indeed
expressed by the excitation spectra (Figure 3a); or 2) the in-
sertion of dye 5 in the nanoparticle likely modifies the ag-
gregation of 4 in the nanoparticle and restores the emission
that can be observed when the water content is lower, gen-
erating a signal at 670 nm. The emission quantum yield of
the nanoparticle (FF =4.8%) remains comparable to that
measured in the absence of 5; this means that the dimer
does not trap excitation energy from the aggregated blue
dye. More significantly, it was concluded from close exami-
nation of the corresponding excitation spectra that photons
absorbed by the dimer were not transferred to the aggregat-
ed blue Bodipy that forms the nanoparticles (Figure 3b).
In fact, the excitation spectrum recorded for emission
from the nanoparticle contains a small contribution from
monomeric 5, indicating that this species exists in equilibri-
um with the dimer. The excitation spectrum recorded for
the dimer is dominated by the absorption at 520 nm. It is
important to note that the size distribution of the nanoparti-
cle remains insensitive to the degree of loading of 5, even
when the latter is extensively aggregated (Figure S1 in the
Supporting Information). Addition of water to a solution of
pure 5 in THF (5ꢃ10ꢀ6 m) induced flocculation of the dye
within a couple of minutes. The supernatant exhibited sever-
al emission bands, most notably at 605 and 667 nm, that can
be assigned to dimeric species by comparison with other
simple Bodipy dyes (Figure S6 in the Supporting Informa-
tion). Note that the emission of the dimer in solution for 5
at 667 nm is the same as that of the dimer incorporated in
the blue aggregate, 4-agg. The nanoparticle protects 5
against large-scale flocculation but permits formation of the
emissive dimer at a site that is unfavorable for excitation
energy transfer to the nanoparticle. Presumably, the dimer is
located in lipid-like regions provided by the paraffinic
chains whereas the monomer is mobile and approaches
more closely to the Bodipy head-groups. The fact that the
Figure 3. Spectroscopic changes accompanying the doping of the blue
nanoparticles (5.5ꢃ10ꢀ6 m) in THF/water (65% v/v) with a yellow dye (5
or 6). a) Absorption spectra of 4-agg (5.5ꢃ10ꢀ6 m) in THF/water (65%)
containing dye 5 (20 mol%; c) and emission spectra obtained by exci-
tation at 490 nm (c). Excitation spectra were plotted by scaling the
emission at 760 nm (c). b) Absorption of the same solution of 4-agg
but after addition of 6 (20 mol%; c) and emission spectra by excita-
tion at 490 nm (c). Excitation spectra was plotted by scaling the emis-
sion at 760 nm (c).
Chem. Eur. J. 2011, 17, 11709 – 11714
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R. Ziessel et al.
presence of 4-agg prevents precipitation of dye 5 is good in-
dication that 5 is sequestered inside the self-assembled
nanostructure.
A final goal relates to the concepts of reversibility of the
accretion process and recovery of the entrapped yellow dye.
This objective was successfully achieved simply by raising
the concentration of THF from 35 to 50% in an aqueous
dispersion of 4-agg containing 20 mol% of 5 (Figure 4a).
This treatment regenerates the absorption spectrum charac-
studies showed that electronic energy transfer from 5 to 4 is
negligible in solution at micromolar concentrations.
In summary, we have reported on the self-assembly of a
new type of discrete nanoparticle emitting in the NIR and
formed from a fluorescent organic dye. The self-assembly
process is quantitative and the resultant nanoparticle has a
very narrow size distribution consistent with the accretion of
a small number (four to five) of dye molecules in which the
paraffin chains are shrunk at the center of the FONs and
the polar head-groups are at the periphery of the object and
solvated by water molecules. A snapshot of the resulting ag-
gregates is illustrated in Figure 5.
teristic of 4 (labs =646 nm) contaminated with 5 (labs
=
500 nm; Figure 4a). Under these conditions, DLS measure-
ments confirmed the absence of nanoparticles and larger ob-
jects. In parallel, steady-state emission spectra displayed the
expected red fluorescence at 670 nm of the blue dye 4 to-
gether with strong emission at 520 nm due to the yellow dye
5 (FF =90%). This reversibility could be easily followed
with the naked eye, with the apparition of the bright-red
emission at around 650 nm (Figure S3 in the Supporting In-
formation). By careful manipulation of the composition of
the water/THF mixture, it was possible to recycle many
times between nanoparticles and monomeric species without
obvious fatigue or degradation of the dyes. Finally, control
Figure 5. Schematic representation of five molecules (arbitrary number)
of dye 4 self-assembled in a THF/water solution.
Controlled aggregation of this type is driven by the coop-
erative balance of an electrostatic repulsion at the periphery
and lipophilic associations at the core, and leads to the ap-
pearance of emission in the near-IR. These particles are un-
expectedly stable over many weeks standing, without precip-
itation. The actual composition of the system is determined
by the amount of water added to a THF solution of the
monomeric blue dye and can be cycled many times by
changing the mole fraction of water. The nanoparticles act
as a sponge (host) for added dye molecules (guest), provid-
ed that the latter are equipped with complementary lipo-
philic chains. In this way, advanced photon collectors that
emit at 760 nm can be assembled and subsequently disman-
tled at will. These interactive nanoparticles are likely to pos-
sess important applications as delivery vehicles for living
cells in which internalization of the particle can be moni-
tored by fluorescence microscopy. Dispersion of the nano-
particles in flexible films has already been demonstrated
and will be used to address a single particle.
Figure 4. Spectroscopic changes observed on varying the solvent composi-
tion of 4-agg (2.7ꢃ10ꢀ6 m) THF/water solution containing dye 5 (5.3ꢃ
10ꢀ7 m). a) Absorption spectra recorded for water (65%; c), 60%
(c), 55% (c), and 50% (c). b) Fluorescence spectra recorded
under excitation at 490 nm of the same four solutions. The concentrations
of dyes were kept constant for all solutions.
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Fluorescent Bodipy Nanoparticles
COMMUNICATION
particles the same procedure was used but with the addition of 5 to
20 mol% of dyes 5 or 6.
Experimental Section
Reversibility tests: The mother solution of the nanoparticles was diluted
with THF by using volumetric flasks in order to maintain the concentra-
tion of dye 4 constant at around 2.7ꢃ10ꢀ6 m. The resulting solution was
left at ambient temperature, overnight, and then analyzed by spectrosco-
py and DLS.
Compound 2: Compound 1 (200 mg, 0.44 mmol, 1 equiv), 3,4,5-tridodecy-
lalkoxybenzaldehyde (1.2 g, 1.82 mmol, 4 equiv) and PTSA (5 mg,
0.03 mmol) were dissolved in toluene (25 mL) and piperidine (1 mL) in a
round-bottom flask equipped with a Dean Stark apparatus. The resulting
solution was heated at 1408C until all the solvents were collected by the
Dean Stark apparatus. Toluene (25 mL) and piperidine (1 mL) were
added to the solid reaction media and the dryness protocol was repeated
four times. Purification by chromatography on silica gel (50:50 to 100:0
dichloromethane/petroleum ether) followed by precipitation (CH2Cl2/
EtOH), afforded 2 as a dark-blue solid (400 mg, 0.23 mmol, 53%).
1H NMR (300 MHz, CDCl3): d=7.88 (d, 3J=15.9 Hz, 2H), 7.51 (d, 3J=
Acknowledgements
3
3
We thank the Centre National de la Recherche Scientifique (CNRS) for
financial support of this work and Professor Anthony Harriman (Univer-
sity of Newcastle, UK) for critical reading of this manuscript prior to
publication. We also warmly acknowledge one of the reviewers for sug-
gesting that the aggregation during the incorporation of dye 5 into the
nanoparticle could induce some modification in the dye organization re-
sulting in different deactivation channels.
8.1 Hz, 2H), 7.11 (d, J=15.9 Hz, 2H), 6.75 (s, 4H), 6.56 (d, J=15.9 Hz,
2H), 6.38 (s, 2H), 4.05 (t, 3J=6.3 Hz, 4H), 3.78 (t, 3J=6.30 Hz, 8H),
1.81–1.25 (m, 96H), 0.87 ppm (t, J=6.3 Hz, 18H); 13C NMR (75 MHz,
C6D6): d=153.8, 153.4, 141.2, 140.5, 138.2, 137.5, 136.9, 135.2, 133.3,
132.1, 130.9, 118.7, 118.3, 106.8, 94.2, 73.4, 69.1, 32.3, 30.2, 30.1, 30.0, 29.8,
26.7, 23.1, 14.4 ppm; UV/Vis (THF): l nm (e, mꢀ1 cmꢀ1): 319 (31400), 377
(49200), 598 (50500), 648 (121500); EI-MS m/z (nature of the peak):
1731.19 ([M], 100); calcd for C105H170BF2IN2O6: C 72.80, H 9.89, N 1.62;
found: C 73.06, H 9.79, N 1.42.
Keywords: energy transfer · fluorescent nanoparticles
host–guest interactions · microscopy · self-assembly
·
Compound 3: [PdACHTUNGTRENNUNG(PPh3)2Cl2] (2.5 mg, 0.004 mmol) and CuI (1.1 mg,
0.006 mmol) were added to an argon-degassed solution of 2 (100 mg,
0.06 mmol) and 1-dimethylamino-2-propyne (20 mL, 0.18 mmol) in ben-
zene/triethylamine (10/2 mL), and the reaction mixture was heated at
608C for 48 h. The solution was then poured into H2O (10 mL) and ex-
tract with CH2Cl2 (3ꢃ15 mL). The organic phase was washed with water
and brine and dried over sodium sulfate. The solvents were removed
under vacuum. Purification by chromatography on silica gel (99:1 to 99:5
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dichloromethane/methanol) afforded
3 as a dark-blue solid (83 mg,
82%). 1H NMR (300 MHz, C6D6): d=8.37 (d, 3J=16.2 Hz, 2H), 7.39 (d,
3J=8.1 Hz, 2H), 7.30 (d, 3J=16.2 Hz, 2H), 6.97 (s, 4H), 6.63 (d, 3J=
8.3 Hz, 2H), 6.38 (s, 2H), 4.29 (t, 3J=5.7 Hz, 4H), 3.81 (t, 3J=6.3 Hz,
8H), 3.34 (s, 2H), 2.24 (s, 6H), 2.02–1.92 (m, 4H), 1.78–1.64 (m, 12H),
1.53–1.33 (m, 80H), 0.93 ppm (t, J=6.3 Hz, 18H); 13C NMR (75 MHz,
C6D6): d=154.1, 153.5, 141.7, 140.8, 138.2, 137.7, 135.3, 133.7, 132.4,
132.3, 124.5, 119.1, 118.5, 107.1, 87.1, 85.0, 73.6, 69.2, 48.7, 44.2, 32.35,
31.09, 30.31, 30.30, 30.25, 30.21, 30.17, 30.01, 29.85, 29.68, 29.66, 26.76,
26.68, 23.11, 14.35 ppm; UV/Vis (THF): l nm (e, mꢀ1 cmꢀ1): 329 (30353),
381 (49777), 602 (39107), 648 (119500); EI-MS m/z (nature of the peak):
1686.25 ([M], 100); calcd for C110H178BF2N3O6: C 78.30, H 10.63, N 2.49,
O 5.69; found: C 78.62, H 10.30, N 2.20.
Compound 4: Iodomethane (10 mL, 0.15 mmol) was added to a solution
of 3 (70 mg, 0.04 mmol) in distilled THF (5 mL). The solution was stirred
at 208C for 3 h and then poured into 20 mL of an aqueous solution of
KPF6 (2 mmol, 368 mg). The organic phase was extracted with CH2Cl2
(3ꢃ20 mL) and washed with water and brine and dried over sodium sul-
fate. The solvents were removed under vacuum. Purification by chroma-
tography on silica gel (99:3 to 99:15 dichloromethane/methanol) afforded
4 as a dark-blue sticky solid (55 mg, 75%). 1H NMR (300 MHz, CDCl3):
d=7.66 (d, 3J=8.3 Hz, 2H), 7.53 (d, 3J=16.2 Hz, 2H), 7.40 (d, 3J=
7.8 Hz, 2H), 7.17 (d, 3J=15.9 Hz, 2H), 6.78 (s, 4H), 6.62 (d, 3J=8.3 Hz,
2H), 6.38 (s, 2H), 4.43 (s, 2H), 4.05–3.97 (m, 12H), 3.35 (s, 9H), 2.10–
1.99 (m, 12H), 1.82–1.51 (m, 108H), 0.93 ppm (t, J=6.3 Hz, 18H);
13C NMR (50 MHz, CDCl3): d=153.32, 152.87, 141.70, 139.85, 139.12,
137.52, 134.92, 133.11, 132.30, 131.73, 131.63, 128.62, 124.03, 117.99,
106.42, 86.30, 84.58, 73.59, 69.27, 53.175, 48.62, 44.38, 31.92, 31.89, 31.85,
30.35, 29.75, 29.72, 29.69, 29.63, 29.54, 29.48, 29.44, 29.37, 29.31, 26.20,
26.13, 26.10, 22.63, 14.91, 14.08 ppm; UV/Vis (THF): l nm (e, mꢀ1 cmꢀ1):
334 (34322), 382 (54200), 604 (49612), 651 (125690); EI-MS m/z (nature
of the peak): 1702.33 ([M]+, 100); calcd for C111H181BF8N3O6P: C 72.17,
H 9.88, N 2.27; found: C 72.40, H 10.12, N 2.04.
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Received: May 9, 2011
Published online: September 6, 2011
11714
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ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 11709 – 11714