A topological view of isomeric dendrimers

Article abstract of DOI:10.1002/ejoc.200800408

A topological approach to the analysis of isomeric dendrimers by considering their molecular graphs has been applied to correlate various properties of a series of new sulfonimide-based isomeric dendrimers with their structures. According to this approach

Full text of DOI:10.1002/ejoc.200800408

FULL PAPER
DOI: 10.1002/ejoc.200800408
A Topological View of Isomeric Dendrimers
[a]
[a]
[a]
[b]
[b]
ˇ
Dirk Schubert, Mirza Corda, Oleg Lukin,* Boris Brusilowskij, Evgenij Fiskin, and
Christoph A. Schalley[b]
Keywords: Dendrimers / Topology / Sulfonimides / Mass spectrometry / Isomers
A topological approach to the analysis of isomeric dendri-
mers by considering their molecular graphs has been applied
to correlate various properties of a series of new sulfonimide-
based isomeric dendrimers with their structures. According
to this approach isomeric dendrimers are referred to as either
isographic, that is, having the same graph, or non-isographic,
that is, having different graphs. Six sets of non-isographic
isomeric dendrimers with four to ten peripheral groups and
one set of three isographic isomers with eight peripheral
groups have been designed, synthesized and investigated
with regard to their melting, solubility, separation, NMR
spectroscopic and mass spectrometric characteristics. The re-
sults are discussed in the light of the graph-dependent and
-independent properties of the isomers.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
tus from successful syntheses of myriads of molecular cate-
nanes, rotaxanes and knots[6] as well as endohedral fuller-
enes[7] and orientational isomers of guests encapsulated in
self-assembled capsules.[8] Their structures and properties
depend not only on their geometry, but also on their topo-
logical and stereochemical features.[9] Nowadays, topologi-
cal theories of graphs and knots[10] are widely used to ade-
quately describe many types of isomerism and chirality. In
addition, graph theory has successfully been applied to the
systematic analysis of hydrogen-bonding patterns in a large
variety of organic crystal structures.[11]
In this contribution, we propose a topological approach
to the analysis of isomeric dendrimers by representing their
molecular structures as planar graphs. This consideration
has been applied to isomeric series of sulfonimide-based
dendrimers of different sizes and shapes which were then
examined with respect to their graph-dependent and -inde-
pendent physicochemical and NMR spectroscopic proper-
ties, as well as their fragmentation reactions as observed in
tandem MS experiments.
Introduction
Dendrimers play an increasingly important role in many
areas of chemistry.[1] The usually applied synthetic routes
can be classified into divergent[2] and convergent[3] synthe-
ses, although both have also been combined for the synthe-
sis of POPAM dendrimers decorated with peripheral
Fréchet dendrons.[4] Recently, we reported the synthesis of
sulfonimide-based dendrimers,[5] which in principle allows
us to program dendrimers with individual branching points
and peripheral units. The direction of growth of the dendri-
mers can be determined by the reaction conditions because
of the clearly distinguished reactivities of the building
blocks, that is, the branching aromatic amines and the inter-
mediate sulfonamides. The synthesis of a particular den-
drimer can thus be programmed by selecting the appropri-
ate series of reaction steps.
The deliberate preparation of highly unsymmetrical den-
drimers with a specific structure rather than the unintended
formation of some minor defects emphasizes the question
of how to accurately describe different structural isomers.
One way to do this is to use topology by defining and fol-
lowing molecular graphs. The application of topological
techniques to the analysis and classification of molecular
structures has become increasingly important over the past
30 years. The development in this area gained much impe-
Results and Discussion
Topological Considerations
Irrespective of their detailed chemical structure, all den-
dritic molecules contain a core and one or more shells of
branching points, both of a certain valence. In the periph-
ery, they are substituted with terminal groups. These
broadly accepted descriptors result in a tree-like graph (Fig-
ure 1). This simplified two-dimensional visualization of a
dendritic structure is a planar molecular graph consisting
of a set of linked symbols for the core, the branching points
and the terminal groups. Therefore, the dendrimer graph,
[a] Department of Materials, Institute of Polymers, HCI G 527,
ETH Zurich
8093 Zurich, Switzerland
Fax: +41-44-633-13-97
E-mail: oleg.lukin@mat.ethz.ch
[b] Institut für Chemie und Biochemie – Organische Chemie, Freie
Universität,
Takustr. 3, 14195 Berlin, Germany
Supporting information for this article is available on the
WWW under http://www.eurjoc.org/ or from the author.
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A Topological View of Isomeric Dendrimers
or simply the dendrograph, in Figure 1 maps the infor- different elemental compositions. Hawker et al.[15] reported
mation on the numbers of branching points and terminal the first detailed study of pairs of Fréchet-type dendrons
groups and on their connectivities. This graphical represen- and their truly isomeric linear analogues. Noticeable differ-
tation is often used in general discussions on dendrimers ences in hydrodynamic volumes as well as in solubility and
because the dendrograph does not contain any data on the crystallinity were observed only for isomers with high
chemical composition of the dendrimer. Figure 2 provides
the chemical structures and the corresponding dendro-
graphs of two different pairs of isomeric dendrimers (den-
droisomers). Dendroisomers can be described by either the
same dendrograph or different dendrographs. Isographic
isomers can be depicted by the same dendrograph, whereas
non-isographic isomers have different dendrographs.[12]
Figure 1. Example of a planar graph of a dendrimer, a dendro-
graph.
Figure 2. (a) Examples of isographic dendroisomers (Gorman and
co-workers[13]). These are depicted by the same graph, but differ
with respect to their constitution. (b) Non-isographic dendroiso-
mers 1 and 2 (this work) have different dendrographs.
Figure 3. Dendrographs of a series of non-isographic isomers of
the sulfonimide dendrimers (R = n-octyl; spheres represent the
sulfonimide branching points, lines the p-phenylene linkers, and
Several precedents for isomerism in dendrimer research
have been reported in the literature.[13–17] Sometimes, the
rectangles represent the 2-naphthyl peripheral groups). See also
term “isomers” has not been used in a strict sense[14] to
Figure 2 for reference to compounds 1 and 2. Melting points and
denote compounds with similar structural formulae, but solubilities (in parentheses in mgmL–1) in chloroform are given.
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molecular weights (Ͼ5 kDa). More recently, Gorman and solubility in chloroform. Both trends point to significantly
co-workers[13] investigated the effect of constitutional differ- strengthened crystal packing. Most strikingly, the melting
ences on the encapsulation ability of Fréchet dendrimers. point for 8 is more than 100 °C higher than that of its iso-
For this purpose, Fréchet-type polyether dendrons were mer 7, and a difference in solubility of more than 200 is
synthesized with either ortho- or meta-substituted observed for dendroisomers 4 and 5. The low solubility of
branching points, such as the ones shown in Figure 2a. Iso- some of the dendrimers studied here may ultimately limit
mers with ortho-substituted branching points were more the scope of the programmed synthesis developed pre-
hydrophobic at their cores. Finally, a light-induced (E)/(Z) viously.[5] The melting points and the solubilities of the
isomerization of azobenzene-containing dendrimers was available dendroisomer pairs with seven, eight and ten pe-
used to either decrease[16] or increase[17] their hydrodynamic ripheral groups differ less significantly. As a general trend,
volumes. The dendroisomers mentioned in the two last ex- we note that the differences between isomers vanish almost
amples contain either constitutional or configurational dif- completely with increasing dendrimer size. This is likely due
ferences at their branching points. Despite these differences, to the more spherical, globular structure of the larger den-
they, however, have the same molecular graphs and are thus drimers in which the individual dipole moments cancel each
isographic dendrimers. As the number of terminal groups other more efficiently compared with smaller structures.
increases, the number of possible non-isographic isomers
grows almost exponentially. This is why in the present work
Chromatographic Behaviour
complete sets of non-isographic isomers were synthesized
only for compounds with four and five peripheral groups
(1–5; see Figure 3), whereas for compounds with a higher
molar mass only representative isomeric pairs were pre-
pared.
A series of non-isographic isomers with up to ten periph-
eral 2-naphthylsulfonyl groups 1–14, as depicted by their
dendrographs in Figure 3, were investigated with regard to
their melting points, solubility, NMR spectroscopic charac-
teristics, separability and mass spectrometric fragmentation
patterns. Additionally, two isographic isomers of the den-
drimer 11 with differences in their first (15) and second (16)
generation linkers were prepared and their properties com-
pared.
In this study, gel-permeation (GPC), high-performance
liquid (HPLC), and simple thin-layer (TLC) chromatog-
raphy were used to examine the separability of dendrimer
mixtures. Different stationary phases were examined for
GPC and HPLC.
Figure 4 shows the chromatograms of different pairs of
dendrimers of different molecular masses (including the
truly isomeric 9/10 pair for comparison). One component,
dendrimer 9, was the same in all mixtures for easy compari-
son. Although a mixture of 1 and 9 is separated almost to
the baseline of the two peaks, a mixture of 4 and 9, which
Synthesis and Physical Properties
In order to illustrate the practical utility of the topologi-
cal approach discussed here, a series of isomeric sulfon-
imide-based dendrimers (Figure 3) were tailored by choos-
ing the appropriate reaction sequence.[5] The comprehensive
synthetic schemes and the characterization data for all the
dendrimers mentioned in this work are collected in the Sup-
porting Information. Figure 3 shows the solubility in chlo-
roform of the dendrimers at room temperature and their
melting points. For the low-molar-mass dendrimers bearing
four to six peripheral groups, the highest melting points
were determined for compounds 2, 5 and 8, which have the
Figure 4. Analytical gel-permeation chromatograms of the den-
drimer pairs. Asterisks denote peaks from toluene added as a stan-
most linear shapes. These compounds also show the lowest dard.
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A Topological View of Isomeric Dendrimers
are closer in molecular mass, is much more difficult to sepa- and affinities to the stationary phase. It also means that
rate. Mixtures of 9 with larger dendrimers are inseparable mixtures of such isomers can be separated by conventional
on the GPC column used here (molecular weight range: 500 column chromatography although this is not the case for
to 3ϫ106 Da). Neither was separation obtained for mix- the non-isographic isomers. An HPLC analysis of mixtures
tures of isographic isomers 11, 15 and 16. This indicates of non-isographic isomers in Figure 3 performed using a
that routinely applied analytical GPC may fail as a tool to silica gel column revealed no separation. However, in con-
examine the structural purity of dendrimers.
trast to analytical GPC, all mixtures of differently sized
Unlike analytical GPC, recycling HPLC with a prepara- dendrimers, such as those indicated in Figure 4, were suc-
tive gel-permeation column (molecular weight range: 300– cessfully separated by HPLC. The HPLC analysis of mix-
3000 Da) shed more light on the separability of the dendro- tures of isographic 11 and 15 or 11 and 16 also showed
isomers. The device runs multiple cycles, increasing the peak separation.
probability of mixture separation. We analyzed 1:1 mixtures
of non-isographic isomers (11 + 12 and 9 + 10) as well as
NMR Spectroscopy
1:1 mixtures of isographic isomers (11 + 15 and 11 + 16).
The result is that the two mixtures of the non-isographic
isomers could not be separated even after 80 cycles, whereas
a mixture of the isographic pair 11 + 16 clearly showed
separating peaks after 20 cycles (Figure 5). The second pair
of isographic isomers 11 + 15 required 40 cycles to afford
complete separation. Other mixtures of differently sized
dendrimers were also successfully separated by recycling
GPC.
Most of the dendroisomers under study can be easily dis-
tinguished by high-resolution H NMR spectroscopy. Al-
1
though many signals of the dendrimers in the aromatic re-
1
gion of the H NMR spectra have very similar chemical
shifts, the signals for the naphthyl α-protons are highly in-
dicative of differences in symmetry. Figure 6 compares the
1
regions of the naphthyl α-protons in the H NMR spectra
of the three dendroisomers 3–5 which have five peripheral
substituents.
Figure 5. Recycling GPC chromatograms of (a) a 1:1 mixture of
non-isographic 11 and 12 (no separation even after 80 cycles); (b)
a 1:1 mixture of isographic 11 and 16.
Figure 6. Naphthyl α-proton signals in the 1H NMR (CDCl3) spec-
tra of compounds 3–5.
These results indicate the differences in hydrodynamic
volumes induced by the constitutional kinks within their
phenylene spacers, thus providing separability for the iso-
As seen in Figure 6, one can differentiate between the
graphic and geometrically closely related dendroisomers 11, ratios of non-equivalent peripheral naphthyl substituents in
15 and 16. At the same time, non-isographic dendroisomers the non-isographic dendroisomers. An additional interest-
exhibit matching hydrodynamic volumes despite their ing phenomenon is the multiplicity of the α-proton signals
strong geometrical dissimilarity. This latter finding is in and the variability of their coupling constants. Thus, as
marked contrast to the observations of Hawker et al. for shown in Figure 6, the corresponding signals in the 1H
the aforementioned isomeric polyethers.[15]
NMR spectrum of compound 3 appear as three singlets
Silica gel thin-layer chromatography (TLC) of all the with relative integral intensities of 2:2:1. The signals of
non-isographic isomer mixtures in Figure 3 revealed match- compound 4 are a singlet and a doublet (4JH,H = 1.5 Hz)
ing Rf values. Consequently, separation was impossible. In with relative intensities of 1:4. Finally, there are two dou-
contrast, a mixture of the three isographic dendroisomers blets (one proton each) with very large coupling constants
11, 15 and 16 showed noticeable separation on a TLC plate (4JH,H = 5.7 Hz) and two singlets (2:1) in this region of the
(CHCl3/SiO2; Rf = 0.40, 0.42 and 0.46, respectively). This 1H NMR spectrum of compound 5. Compounds 9 and 10
implies that the isographic dendroisomers not only have dif- were the only pair that could not easily be distinguished by
ferent hydrodynamic volumes but also different polarities 1H NMR spectroscopy. Both compounds have a low sym-
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FULL PAPER
metry that complicates the aromatic region of their 1H
NMR spectra, and both are expected to yield 1:2:4 ratios
for the three different sets of naphthyl α-proton signals.
Mass Spectrometry
Mass spectrometry is a routine tool for dendrimer char-
acterization: matrix-assisted laser desorption/ionization
(MALDI) is frequently used[18] due to the common coup-
ling to time-of-flight analyzers that offer a broad mass
range. Electrospray ionization (ESI) has also been applied
to several classes of dendrimers.[19] Mass spectrometry is
particularly useful for an analysis of dendrimer defects[20]
which are indistinguishable by most other methods. How-
ever, ionization artifacts can make mass spectrometric
analysis difficult.[21] In the context of this study, we at-
tempted to distinguish between pairs of non-isographic sul-
fonimide dendroisomers by analyzing their fragmentation
reactions as observed in tandem mass spectrometric experi-
ments.[22]
The dendrimers under study were first ionized by using
an electrospray ionization source coupled to a Fourier-
transform ion-cyclotron resonance (FTICR) mass spec-
trometer. All dendrimers described in this article can be
ionized as their sodium or potassium adducts, whereas pro-
tonation yields only a peak of marginal intensity. This be-
haviour is in agreement with MS studies on other per-
sulfonylated dendrimers with a polyaminoamine (POPAM)
scaffold. For these POPAM-based dendrimers, collision-in-
duced fragmentations indicate that they can be protonated
at the amine nitrogen atoms, whereas sodium adduct forma-
tion occurs through coordination of the cation to the
sulfonimide group. As the dendrimers under study in this
work do not bear amine nitrogen atoms, protonation is un-
likely. Sodium or potassium attachment, however, is quite
favourable. Figure 7a and d shows the ESI mass spectra of
dendrimers 1 and 2 as typical examples of the results ob-
tained with the ESI-FTICR instrument.
Only the sodium adducts gave intense enough peaks
for the MS/MS experiments. After mass selection of the
[M + Na]+ dendrimer ion of interest, a CO2 laser was used
to irradiate the ions in the IR region [infrared multiphoton
dissociation (IRMPD); 10.6 µm wavelength] and to induce
fragmentation (Figure 7b and c). The fragmentation pattern
is highly complex even for the smaller dendrimers under
study. Nevertheless, almost all fragments can be assigned to
combinations of a few different fragmentation pathways
that can be classified into three categories (Scheme 1).
Figure 7. (a) and (d) ESI-FTICR mass spectra of non-isographic
dendroisomers 1 and 2 sprayed from methanol/CH2Cl2 (3:1). (b)
and (c) tandem mass spectra (IRMPD) of dendrimers 1 and 2 after
mass selection of the [M + Na]+ ion. Vertical double arrows indi-
cate fragments that appear only in one of the mass spectra and are
thus indicative of structural differences.
The third category comprises direct S–N bond-cleavage
The first reaction gives rise to losses of SO2, presumably reactions that generate an N-centred radical that is stabi-
through an ipso substitution (process A), as found pre- lized by conjugation to the adjacent aromatic ring and two
viously.[5] The 1,2-elimination of octene (process B) from sulfonyl groups. The other fragment is a neutral S-centred
the alkyl side-chain at the focal point represents the second radical, which again is stabilized to some extent through
type of fragmentation reaction, which can occur through a conjugation. These effects weaken the S–N bond, thus pro-
simple 1,2-elimination process or, by analogy to ester pyrol- viding a predetermined breaking point. Depending on the
ysis, through a six-membered transition-state structure, as size of the substituent, the loss of terminal naphthylsulfonyl
depicted in Scheme 1; we cannot distinguish between these branches (process C), a whole first-generation dendron
two mechanisms on the basis of the mass spectrometric ex- (process D) or even a second-generation dendron (pro-
periments.
cess E) can occur.
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A Topological View of Isomeric Dendrimers
Figure 8. MALDI-TOF/TOF tandem mass spectra (matrix:
dithranol) of mass-selected isomers [2 + Na]+ (top) and [1 + Na]+
(bottom). Vertical double arrows indicate fragments that appear in
only one of the mass spectra.
Scheme 1. Three pathways that lead to the fragmentation patterns
in the tandem mass spectra.
The two MS/MS graphics compared in Figure 7b and c
are clearly different with respect to the intensities of the
major fragment signals (e.g., m/z = 1110, 840 and 776).
These differences are found irrespective of the laser power
chosen for the excitation of the ions in the experiment. The
fragmentation patterns even differ qualitatively in that sev-
eral mostly smaller signals are absent for one of the isomers,
but are observed in the spectrum of the other (e.g., m/z =
1046, 649 and 621).
The sodium adducts of dendrimers 3 and 4 gave similarly
complex IRMPD spectra which again differed significantly
from each other. However, dendrimer ions larger than [3
+ Na]+ and [4 + Na]+ hardly decomposed in the IRMPD
experiment, which indicates that they are highly stable
molecules sufficiently able to store the energy from the laser
beam without fragmenting. Therefore, MALDI-TOF/TOF
experiments were performed. Owing to rather high collision
energies, the larger dendrimer ions can be expected to frag-
ment more easily. At the same time, rearrangement reac-
tions are less prominent, and direct bond cleavage is pre-
ferred. Thus, structural information might be easier to ob-
tain by this technique.[23] In the following we therefore focus
on pairs of non-isographic isomers: 1/2, 3/4 and 9/10.
The differences observed in the ESI-FTICR tandem MS
experiment for [1 + Na]+ and [2 + Na]+ were confirmed by
the MALDI-TOF/TOF experiment (Figure 8). Qualita-
tively, the same fragmentation reactions appear, but quanti-
tatively direct bond cleavages (e.g., formation of the frag-
ment at m/z = 1031) are much more pronounced in the
MALDI MS/MS experiments.
Figure 9. MALDI-TOF/TOF tandem mass spectra (matrix:
dithranol) of mass-selected isomers [3 + Na]+ (top) and [4 + Na]+
(bottom). Vertical double arrows indicate fragments that appear in
After assignment of the peaks of the next largest pair
of dendrimers 3 and 4 (Figure 9), the two isomers can be only one of the mass spectra.
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Conclusions
distinguished by close analogy to the smaller dendrimers 1
and 2. Eye-catching qualitative differences (m/z = 1121, 967
and 931) as well as quantitative changes in intensities (m/z
= 1376, 1031, 776 and 495) are clear. The two spectra thus
differ significantly and clearly help to differentiate between
them.
As expected, the MALDI tandem mass spectra of the
largest investigated pair of dendrimers, that is, 9 and 10,
differ in respect of several key fragments (Figure 10) (e.g.,
m/z = 1812, 1403, 1278, 1188, 932 and 777).
On the basis of the results obtained in this work we have
been able to draw conclusions about the graph-dependent
and -independent properties of isomers of sulfonimide-
based dendrimers. Isographic sulfonimide-based dendroiso-
mers are distinguishable by recycling GPC, silica gel HPLC
and silica gel TLC. The non-isographic sulfonimide dendro-
isomers cannot be separated by any of the chromatographic
techniques applied in this work. Therefore, for both types
of isomers, chromatographic separation is the graph-inde-
pendent property. The graph-dependent properties of the
non-isographic isomers are their melting points, solubility,
NMR spectroscopic and mass spectrometric characteristics.
1
The number of sets of signals in the H NMR spectra per-
mits a clear-cut assignment of the symmetry of the dendri-
mers under study. In addition, tandem MS of pairs of non-
isographic isomers exhibits differences in their fragmenta-
tion pathway. Both methods are complementary to each
other. Isomers 9 and 10, for example, which could not easily
1
be distinguished by H NMR spectroscopy, still give rise
to different MS/MS patterns. The dendroisomers and their
mixtures are therefore a good benchmark for modern ana-
lytical tools and separation science. The topological de-
scription of dendrimers from the standpoint of the graph
theory proposed in this work is handy and will certainly be
helpful in the analysis of their structure–property relation-
ship.
Experimental Section
Electrospray Mass Spectrometry: The mass spectrometry experi-
ments were performed with a Varian/IonSpec QFT-7 FTICR mass
spectrometer equipped with a superconducting 7 Tesla magnet and
a micromass Z-spray ESI ion source utilizing a stainless steel capil-
lary with a 0.75 mm inner diameter. The different dendrimers were
dissolved in either CH2Cl2/MeOH (3:1) or CH2Cl2/MeOH (1:3) de-
pending on the solubility of the sample. These solutions were intro-
duced into the source with a syringe pump (Harvard Apparatus)
at flow rates of approximately 2 µLmin–1. Parameters were ad-
justed as follows: capillary voltage: 3.8 kV; extractor cone: 10 V;
sample cone: 10 V; source temperature: 40 °C; temperature of de-
solvation gas: 40 °C. No nebulizer gas was used for the experi-
ments. The ions were accumulated in the instrument’s hexapole
long enough to obtain useful signal-to-noise ratios. Next, the ions
were introduced into the FTICR analyzer cell, which was operated
at pressures below 10–9 mbar, and detected by a standard exci-
tation and detection sequence. After mass selection of the [M +
Na]+ dendrimer ion of interest, a CO2 laser was used to irradiate
the ions in the IR region [infrared multiphoton dissociation
(IRMPD); 10.6 µm wavelength] and to induce fragmentation. For
each measurement, 5–10 scans were averaged to improve the signal-
to-noise ratio.
Figure 10. MALDI-TOF/TOF tandem mass spectra (matrix:
dithranol) of mass-selected [10 + Na]+ (top) and [9 + Na]+ (bot-
tom). Vertical double arrows indicate fragments that appear in only
one of the mass spectra.
Although virtually all the peaks in the fragmentation
spectra can be assigned, it is not clear in which order they
are formed. MS3 experiments, which would provide clarity
here, can only be performed with the FTICR instrument.
However, the intensities are not sufficiently high, and the
larger dendrimer ions do not easily decompose.
With the MALDI-TOF/TOF instrument we are limited
to MS2 experiments. For the time being we can only draw
the following conclusions from these experiments. (i) Non-
isographic dendrimer ions with the same elemental compo-
sition give distinguishable MS/MS fingerprints. (ii) ESI-
FTICR and MALDI-TOF/TOF mass spectra support each
other in that similar fragmentation patterns and similar dif-
ferences between non-isographic isomer ions are observed.
Nevertheless, quantitatively, direct bond cleavage is shown
to be preferred in the MALDI-TOF/TOF spectra. (iii) Ow-
ing to the complexity of the spectra, it is not yet possible
to determine the structure of an unknown dendrimer di-
rectly from the fragmentation pattern.
MALDI Mass Spectrometry: Mass spectrometry analyses were per-
formed with a Bruker Daltonics ultraflex II MALDI-TOF/TOF
mass spectrometer. The three most important parameters for suc-
cessful MALDI-TOF/TOF mass spectra are all related to sample
preparation. (i) The use of dithranol as the matrix gave the highest
intensities (dihydroxybenzoic acid, sinapinic acid and others did
not give satisfying results). (ii) A small amount of sodium chloride
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A Topological View of Isomeric Dendrimers
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was added and increased the abundances of the sodium adducts
significantly. (iii) It was most efficient to directly mix a dichloro-
methane/methanol solution (ratios of 3:1 to 1:3 depending on the
sample solubility) of the sample with a chloroform solution of the
matrix (700-fold excess of the matrix over the sample).
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details including synthesis and characterization
data for all compounds.
[7]
[8]
[9]
Acknowledgments
We would like to thank Mr. M. Colussi and Dr. C. Münzenberg for
their professional assistance with the GPC analyses of the isomeric
mixtures. The Berlin group thanks Dr. Weise for the MALDI-TOF/
TOF measurements. The Zurich group thanks Professor A. D.
Schlüter (ETH Zurich) for his advice and continuous support.
C. A. S. thanks the Deutsche Forschungsgemeinschaft (DFG) and
the Fonds der Chemischen Industrie (FCI) for financial support,
and the FCI in particular for a Dozentenstipendium.
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Received: April 24, 2008
Published Online: July 9, 2008
4156
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