Discrete multiporphyrin pseudorotaxane assemblies from di- and tetravalent porphyrin building blocks

Article abstract of DOI:10.3762/bjoc.11.85

Two pairs of divalent and tetravalent porphyrin building blocks carrying the complementary supramolecular crown ether/secondary ammonium ion binding motif have been synthesized and their derived pseudorotaxanes have been studied by a combination of NMR spectroscopy in solution and ESI mass spectrometry in the gas phase. By simple mixing of the components the formation of discrete dimeric and trimeric (metallo)porphyrin complexes predominates, in accordance to binding stoichiometry, while the amount of alternative structures can be neglected. Our results illustrate the power of multivalency to program the multicomponent self-assembly of specific entities into discrete functional nanostructures.

Full text of DOI:10.3762/bjoc.11.85

Discrete multiporphyrin pseudorotaxane assemblies from di-
and tetravalent porphyrin building blocks
Mirko Lohse1, Larissa K. S. von Krbek2, Sebastian Radunz1, Suresh Moorthy2,
Christoph A. Schalley*2 and Stefan Hecht*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 748–762.
1Department of Chemistry, Humboldt-Universität zu Berlin,
Brook-Taylor-Str. 2, 12489 Berlin, Germany. Fax: +49 (0)30
2093-6940; Tel: +49 (0)30 2093-7308 and 2Institut für Chemie und
Biochemie, Freie Universität Berlin, Takustraße 3, 14195 Berlin,
Germany. Fax: +49(0)308385-5366; Tel: +49(0)308385-2639
doi:10.3762/bjoc.11.85
Received: 28 February 2015
Accepted: 29 April 2015
Published: 12 May 2015
Email:
This article is part of the Thematic Series "Multivalency as a chemical
organization and action principle".
Christoph A. Schalley* - christoph@schalley-lab.de; Stefan Hecht* -
sh@chemie.hu-berlin.de
Guest Editor: R. Haag
* Corresponding author
© 2015 Lohse et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
crown ethers; multicomponent assembly; multivalency; porphyrins;
pseudorotaxanes
Abstract
Two pairs of divalent and tetravalent porphyrin building blocks carrying the complementary supramolecular crown ether/secondary
ammonium ion binding motif have been synthesized and their derived pseudorotaxanes have been studied by a combination of
NMR spectroscopy in solution and ESI mass spectrometry in the gas phase. By simple mixing of the components the formation of
discrete dimeric and trimeric (metallo)porphyrin complexes predominates, in accordance to binding stoichiometry, while the
amount of alternative structures can be neglected. Our results illustrate the power of multivalency to program the multicomponent
self-assembly of specific entities into discrete functional nanostructures.
Introduction
Supramolecular chemistry [1], the chemistry “beyond the mole- tivity [25,26] have attracted significant attention mediated in
cule“ [2], has immensely reshaped the concepts of chemistry by particular by the desire to understand biological phenomena,
putting the intermolecular interaction into the focus. Different such as virus docking to cells [27], toxin inhibition [28], or
fields of chemistry, from materials [3-6] and analytical sciences leucocyte recruitment in inflammation processes of the endothe-
[7-12] to life science [13-17] have benefited from the develop- lium [29]. Multivalency has also inspired synthetic supra-
ment of the basic concepts of molecular recognition, templation molecular architecture as it not only contributes to binding
[18], self-assembly [19], or self-sorting [20,21], just to name a enhancement, but also helps to exert control over complex for-
few. More recently, multivalent binding [22-24] and coopera- mation. For example, “molecular elevators” have been
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constructed by Stoddart et al. [30,31] and giant porphyrin cules. Beyond being a mere spacer and scaffold connecting the
wheels were prepared by Anderson and co-workers [32,33], binding sites, porphyrins also offer interesting physical and
both using a multivalent template strategy.
optical properties [51,52]. Therefore, they have played a pivotal
role in supramolecular chemistry [53-66], for example as
The crown ether/secondary ammonium ion binding motif [34] is potential candidates for artificial light-harvesting systems [67-
a powerful tool to create well-defined pseudorotaxane struc- 73].
tures [35-39], which have also served as precursors in rotaxane
syntheses [40-42] thus providing access to interlocked, mechan- Here, we report the synthesis of two new porphyrin-based di-
ically bound molecules. Based on these structures, functional and tetravalent ammonium guest molecules A2 and A4 and
supramolecular architectures such as molecular switches and their complementary porphyrin-based di- and tetravalent crown
motors [43-45] as well as artificial muscles [46-50], have been ether hosts C2 and C4 (Figure 1). The selection of these
synthesized.
building blocks is based on force-field calculations, which
suggest a good geometric fit between the crown ether hosts and
Due to their four-fold symmetry, porphyrins are excellent the ammonium ion guests. The two monovalent building blocks
candidates to extend these concepts to tetravalent supramole- A1 and C1 serve as control compounds. Based on this
Figure 1: Mono-, di-, and tetravalent axles A1, A2 and A4 and mono-, di-, and tetravalent hosts C1, C2 and C4. Numbers and letters are assigned to
specific H atoms as discussed later in the main text.
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Results and Discussion
“toolbox”, which can be expanded in the future with other func-
tional building blocks, the formation of specific multiply Synthesis
threaded pseudorotaxanes was achieved, thereby demonstrating The synthesis of the two ammonium-substituted porphyrins A2
the ability to program complex multicomponent self-assembly and A4 was performed convergent by first preparing two
[74,75].
different (zinc)porphyrin cores 1 and 2 (Scheme 1), which are
Scheme 1: Overview of the synthesis of the guests A2 and A4. a) Pyrrole (4), BF3·Et2O, DDQ, CHCl3, rt; b) Zn(OAc)2, CHCl3/MeOH, rt; c) dipyrro-
methane 6, BF3·Et2O, DDQ, CHCl3, rt; d) Zn(OAc)2, CHCl3/MeOH, rt; e) 1. benzylamine, trimethyl orthoformate, rt, 2. NaBH4, THF/MeOH, rt;
f) Boc2O, triethylamine, CH2Cl2, rt; g) 1. ethynyltrimethylsilane, CuI, PPh3, Pd(PPh3)4, TEA, toluene, 80 °C, 2. KOH, THF, rt; h) precursor 8, CuI,
PPh3, Pd(PPh3)4, TEA, toluene, 80 °C; i) TFA, CH2Cl2, rt; j) 1. HCl, MeOH/CHCl3, rt, 2. NaBArF, MeOH.
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equipped with two and four bromine atoms in the m-position of tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBArF), the
the meso-phenyl substituents, respectively, for further function- target compounds A2 and A4 were obtained. The weakly coor-
alization. Zinc porphyrins 1 and 2 have been synthesized dinating BArF counter-ion has been used to overcome solu-
following standard protocols for symmetrical [76] A4 and trans- bility problems in organic solvents. It should be noted that the
disubstituted [77] A2B2 meso-functionalized porphyrins. The porphyrin is demetalated to yield the free base porphyrin during
tetrabrominated core 1 was synthesized from aldehyde 3 and the deprotection of the Boc group. Furthermore, NMR integra-
pyrrole (4) to form the free base porphyrin 5, which is subse- tion of signals corresponding to the BArF protons relative to
quently converted into its zinc complex 1. On the other hand the those corresponding to the macrocycle indicates that the por-
difunctional core 2 was obtained through the condensation of phyrin core is protonated (three BArF anions per divalent guest
aldehyde 3 with mesityldipyrromethane (6) followed by metala- A2; five BArF anions per tetravalent guest A4). Based on the
tion of the intermediately formed free base porphyrin 7 to give assumption that protonation of the porphyrin core, which is
its respective zinc complex 2. In the next step, axle precursor 8 rather remote to the primary binding sites, does not influence
was synthesized by reductive amination of 4-bromobenzalde- the association strongly, no selective deprotonation of the por-
hyde (9) and benzylamine yielding amine 10, which was subse- phyrin core has been attempted.
quently Boc-protected, then reacted with trimethylsilylacety-
lene in a Sonogashira cross-coupling followed by desilylation. The preparation of the corresponding crown ether hosts
Finally, the porphyrin cores 1 and 2 were combined with axle (Scheme 2) involved an initial Williamson ether synthesis in
precursor 8 in another two and four-fold Sonogashira cross- which catechol (17) was first extended with 2-[2-(2-
coupling reaction. After deprotection of the termini of the at- chloroethoxy)ethoxy]ethanol to diol 18, which was then
tached axles with trifluoroacetic acid (TFA), protonation of the converted in dibromide 19 by an Appel reaction. Macrocycliza-
free amines with HCl, and anion exchange with sodium tion of 19 with 3,4-dihydroxybenzaldehyde under “pseudo high-
Scheme 2: Synthesis of crown ether hosts C4 and C2: a) K2CO3, LiBr, 17, 2-[2-(2-chloroethoxy)ethoxy]ethanol, DMF, 100 °C; b) CBr4, PPh3, CH2Cl2,
rt; c) Cs2CO3, 3,4-dihydroxybenzaldehyde, DMF, 85 °C; d) 1. pyrrole (4), propionic acid, 140 °C, 2. Zn(OAc)2, MeOH/CHCl3, rt; e) 1. dipyrromethane
(6), BF3·Et2O, DDQ, CHCl3, rt, 2. Zn(OAC)2, MeOH/CHCl3, rt.
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Beilstein J. Org. Chem. 2015, 11, 748–762.
dilution” conditions, i.e., slow addition of the two reactants into of the complexes present in solution. As all stereoisomers have
a solution of Cs2CO3 in DMF at 100 °C provides the corres- the same elemental composition, their presence as a mixture
ponding crown ether aldehyde 20. Porphyrin synthesis using 20 does not obscure the mass spectrometric results. For these
and pyrrole (4) following the Lindsey protocol [77] for A4 por- reasons, we describe our NMR spectroscopic data, but focus on
phyrins gives the desired tetravalent porphyrin host as the free ESI–MS of the complexes under study starting with the four
base 21, which is subsequently converted into the desired possible combinations of A2 and A4 with monovalent
product C4 by metalation using zinc(II) acetate. Host C2 was dibenzo[24]crown-8 C1 as well as of C2 and C4 with monova-
synthesized according to the above-mentioned standard lent dibenzylammonium A1 (Figure 2, top), followed by the
procedure [76] for trans-A2B2-porphyrins from 20 and results obtained for the multivalent 1:1 and 2:1 complexes
mesityldipyrromethane 6 to form the divalent free base por- A2@C2, A22@C4, A4@C22 and A4@C4 (Figure 2, bottom).
phyrin 22. Final zinc insertion provides the desired host C2.
For further detailed synthetic procedures and characterization
data the reader is referred to Supporting Information File 1.
Formation and characterization of complexes
NMR spectroscopy of simple pseudorotaxanes prepared from
crown ether wheels and secondary ammonium axles provides
complexation-induced shift data, which can be easily inter-
preted and yield insight into complexation. Earlier experiences
with divalent crown/ammonium pseudorotaxanes however also
demonstrated that the NMR spectroscopic approach is often
rather limited for more complex structures [78], as very compli-
cated spectra are obtained with typically overlapping signals
that prevent further (straightforward) analysis. Another compli-
cation, which makes the NMR analysis difficult, is the fact that
the di- and tetravalent crown ethers C2 and C4 are achiral
themselves, but become chiral, when complexed to axle compo-
nents A2 and A4. Consequently, the signals for all methylene
protons of the crown ethers split into two diastereotopic ones
not only producing another set of signals, but also more compli-
cated splitting patterns. Furthermore, the crown ethers are
connected to the porphyrin core by single bonds, around which
they can easily rotate in the non-complexed state. This rotation
is, however, fixed upon complexation and two possible orienta-
tions of each of the crown ethers on its corresponding axle are
possible. One can therefore expect a mixture of stereoisomers to
Figure 2: Schematic representation of the host–guests complexes.
Top: complexes A2@C12, A4@C14, A12@C2 and A14@C4, which
form. In the simplest case, A2@C2, two enantiomers and one
meso-form are expected to exist, which should result in two
overlapping sets of signals. For the other three complexes, the
situation is even more complicated. Therefore, a straightfor-
ward and easy analysis of the NMR spectra will likely be
impossible.
are built from one multi- and several monovalent building blocks.
Bottom: complexes A2@C2, A22@C4, A4@C22 and A4@C4, which
are built from di- or tetravalent building blocks.
[3]- and [5]pseudorotaxanes from monova-
lent building blocks
In our earlier studies [37,78,79], however, electrospray ioniza- First the association of A2 and A4 with monovalent C1 as well
tion (ESI) mass spectrometry (MS) turned out to be a perfectly as C2 and C4 with monovalent A1 was investigated and it can
suited method to characterize the complexes present in solution. be seen that in all four cases successful complexation with the
The formation of unspecific complexes as well as fragmenta- expected stoichiometry was achieved. For instance, upon addi-
tion upon ionization have been found to be quite limited tion of C1 to a 3 mM solution of A2 (Figure 3a) a continuous
so that the picture obtained from the mass spectra can be complexation, indicated by the appearance of a new set of
expected to provide realistic insights into the composition signals due to slow exchange rates on the NMR-time scale,
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Beilstein J. Org. Chem. 2015, 11, 748–762.
Figure 3: 1H NMR (500 MHz, 298 K, CD2Cl2, 3 mM) of a) C1 (top), A2@C12 (middle) and A2 (bottom); b) C1 (top), A4@C14 (middle) and A4
(bottom) showing clear evidence of the complexation. The red lines indicate the shift of the proton signals upon addition. The inserts show the titra-
tion curve of each complexation with the expected ratio of the complex formed.
could be observed. Upon association the benzyl signals Hb/c Figure 3a, inset), clearly proving the desired host–guest ratio in
shift downfield by approximately +0.3 ppm and split into two the supramolecular structure. Similar results are obtained for the
separate pair of signals, which is typical for a complexation of other [3]- and [5]pseudorotaxanes (Figure 3b and Figure 4a,b).
C1 with a dibenzylammonium moiety [36]. The aromatic However, it should be noted that despite extensive titration
signals of C1 H1/2 shift slightly upfield by −0.1 ppm and split as experiments (see Supporting Information File 1 for details) a
well. The signals of the crown ether region shift upfield by detailed analysis of the binding constants of these systems
−0.05, −0.14, and −0.38 ppm due to complexation. An overstoi- cannot be obtained as the binding constants are too high for a
chiometric addition of C1 results in no further association (see NMR-based method.
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Figure 4: 1H NMR (500 MHz, 298 K, CD2Cl2, 3 mM) of a) C2 (top), A12@C2 (middle) and A1 (bottom) and b) C4 (top), A14@C4 (middle) and A1
(bottom) showing clear evidence of the complexation. The red lines indicate the shift of the proton signals upon addition. The inserts show the titra-
tion curve of each complexation with the expected ratio of the complex formed.
Guests A2 and A4 as well as the hosts C2 and C4 show typical band) and less intense absorption bands between 500 and
absorption behavior for porphyrin-based molecules. All four 600 nm (Q-bands). However, A4 shows rather strong aggrega-
have pronounced absorption maxima at around 420 nm (Soret tion even in the µM concentration regime likely caused by elec-
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Beilstein J. Org. Chem. 2015, 11, 748–762.
mixtures (Figure 6). In the cases of the 1:2 and 1:4 mixtures of
A2 and A4 with C1, respectively, the respective pseudorotax-
anes A2@C12 and A4@C14 give rise to the second and third
most abundant species (Figure 6a,b). One signal represents the
desired doubly, respectively quadruply charged pseudorotaxane
([A2@C12]2+ at m/z 1094 and [A4@C14]4+ at m/z = 898). In
addition, a second set of signals for the triply, respectively five-
fold charged species ([A2@C12 + H]3+ at m/z = 729 and
[A4@C14 + H]5+ at m/z = 719) could be observed. The most
abundant species – most probably due to its high ESI response
factor – is the one sodium ion containing molecular ion of C1
([Na@C1]+ at m/z 471, see Supporting Information File 1). The
spectra of the di- and tetravalent hosts C2 and C4 and the
monovalent guest A1 show a more complex signal pattern
(Figure 6c,d). In the mixture of divalent crown ether C2 with
A1 three different species in a statistical distribution of 1:2:1
were detected: the host with two axles [A12@C2]2+ (m/z =
948), the host with one axle and one sodium ion [NaA1@C2]2+
(m/z = 861) and the host loaded with two sodium ions
([Na2@C2]2+ m/z = 773). This can be easily explained with the
nature of the ESI spray process, which is known to cause the
dissociation in multiply charged non-covalently bound
complexes. The results of the NMR titrations, however, clearly
indicate the doubly bound pseudorotaxane A12@C2 to be the
most prominent species in solution (Figure 4a). The fact that the
desired pseudorotaxane A12@C2 can be detected by mass spec-
trometry despite the likely dissociation of the multiply charged
Figure 5: Normalized UV–vis absorption spectra (CH2Cl2, 3 μM) of
A2, A4, C2 and C4 and their complexes formed with the monovalent
building blocks A1 and C1 showing no significant batho- or
hypsochromic shift. Absorption spectrum of A4 was normalized to 0.5
because of the strong self-aggregation and the resulting broad Soret
band.
trostatic interactions mediated by the closely associated BArF complex in the ion source shows that this technique gives
counter-ions that are expected to be significant as rather non- reasonable results for determining the species present in solu-
polar solvents are being used. This aggregation results in a tion. The 4:1 mixture of A1 and C4 gives rise to an even more
broad red-shifted absorption band. Upon complexation this complex signal pattern (Figure 6d). Due to the four binding
aggregate is broken, resulting in the recovery of a typical sharp sites of C4, there are numerous possibilities of A1 and sodium
Soret band at 420 nm. Note that UV–vis titration shows no cations to bind. There are species with three or four guest ions
significant batho- or hypsochromic shift upon association detected with an approximately statistic distribution:
(Figure 5) of neither di- and tetravalent guests A2 and A4 with [Na(4−x)A1x@C4]4+ (x = 1, 2, 3, 4) and [Na(3−y)A1y@C4]3+
monovalent host C1 nor of monovalent guest A1 to the di- and (y = 1, 2, 3). The desired [5]pseudorotaxane is not very stable at
tetravalent hosts C2 and C4. The lack of such optical signature the ionization conditions, but is nevertheless detected
of the complexation event in the characteristic porphyrin ([A14@C4]4+ at m/z = 737). As explained above, this shows
absorption can be explained by the fact that the binding sites are that mass spectrometry gives a reasonable image of the species
electronically decoupled from the porphyrin core.
present in solution, because we already know from NMR titra-
tion studies that the [5]pseudorotaxane A14@C4 is the predom-
The [3]- and [5]pseudorotaxanes with the monovalent building inant species in solution (Figure 4b).
blocks were further investigated by ESI-Q-TOF mass spectrom-
etry. Separate solutions of hosts and guests were prepared (A1/ To summarize, all four desired [3]- or [5]pseudorotaxanes could
C1: 4 mM, A2/C2: 2 mM, A4/C4: 1 mM all in CH2Cl2), and be detected by mass spectrometry despite the likeliness of
the same aliquots of the individual solutions were combined to A12@C2 and A14@C4 to dissociate upon electrospray ioniza-
obtain equal concentrations of ammonium ion functions and tion. These results show that mass spectrometry should be a
crown ether moieties in each solution. The solutions of the well suited method for the investigation of the multivalent
pseudorotaxanes were allowed to equilibrate for 24 hours at pseudorotaxanes under study. These usually show much higher
room temperature and diluted to 0.2 µM prior to analysis. The binding constants than the monovalent analogue and should
respective [3]- or [5]pseudorotaxanes could be detected for all therefore very likely survive the ionization process.
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Figure 6: ESI-Q-TOF-MS spectra (CH2Cl2, 0.2 µM; left hand side) and respective experimental and calculated isotopic patterns of the desired [3]- or
[5]pseudorotaxanes (right hand side): a) 1:2 mixture of A2 and C1, b) 1:4 mixture of A4 and C1, c) 2:1 mixture of A1 and C2, d) 4:1 mixture of A1 and
C4. For reasons of clarity not all of the peaks are assigned (see Supporting Information File 1 for details).
[2]- and [3]pseudorotaxanes from di- and
tetravalent building blocks
based on the present NMR spectroscopy data (Figure 7 and
Figure 8) one cannot exclude the formation of polymeric aggre-
Subsequently, we investigated the di- and tetravalent pseudoro- gates or only partially threaded structures. For this reason the
taxanes formed between A2, A4, C2, and C4. As already formed complexes were analyzed in detail using mass spec-
mentioned above, NMR spectroscopy is limited for the given trometry.
systems because of the numerous isomers that can be formed.
However, some general conclusion can be made. In all four Comparing the absorption of the complexes (Figure 9), one can
cases one can observe a shift of the benzylic protons Hb/c down see that the tetravalent A4@C4 complex shows the strongest
field by approximately 0.5 ppm, which is typical for the blue shift while the divalent A4@C22 shows almost no change
threading in a crown ether/secondary ammonium ion binding in the spectrum (except breaking the A4 aggregate). The
motif. Furthermore, the signals for the crown ether region hypsochromic shift indicates a parallel alignment of the por-
broaden significantly, which is in agreement with the assump- phyrin moieties, which is in good agreement with the hypothe-
tion that upon complexation the number of signals increases sized structure. However, since the observed shifts are rather
because the methylene protons become diasterotopic and small the interactions, i.e., exciton coupling, between the two
different supramolecular stereoisomers can form. However, porphyrin chromophores seems to be rather weak.
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Figure 7: 1H NMR (500 MHz, 298 K, CD2Cl2, 1 mM) of a) C4 (top), A22@C4 (middle) and A2 (bottom); b) C2 (top), A2@C2 (middle) and A2
(bottom). Disappearance and shift of the signals (red lines) suggest complexation. Due to the presence of a complex stereoisomeric mixture only
qualitative information of the complexation is possible.
For mass spectrometric analysis (ESI-Q-TOF MS) of the A species with only one guest A2 in host C4 [Na2A2@C4]4+
desired pseudorotaxanes separate solutions of hosts and guests (m/z = 873) could also be detected but with very low intensity.
were prepared (CH2Cl2, A2/C2: 0.6 mM, A4/C4: 0.3 mM). This partly bound species A2@C4 could in principle allow
They were mixed in the respective 1:1, 1:2, and 2:1 molar ratios formation of small oligomers, if present in solution. The
and allowed to equilibrate for 14 hours at 6 °C, after which no fact that no oligomers could be detected and the very small
further changes in the mass spectra were observed and thus abundance of the signal of the partly bound state
equilibrium was reached. The pseudorotaxane solutions were [Na2A2@C4]4+ (m/z = 873) leads to the conclusion, that this
diluted to 0.2 µM prior to analysis. The respective mass spectra partly bound pseudorotaxane is most probably a product of the
are shown in Figure 10. Guest A2 was combined with host C2 electrospray ionization process.
as well as C4 in 1:1 and 2:1 ratios, respectively. The expected
pseudorotaxanes [A2@C2]2+ (m/z = 1396) and [A22@C4]4+ In cases of the 1:1 mixture of A4 and C4 and the 1:2 mixture
(m/z = 1185) are detected as the major species (Figure 10a,b). of A4 and C2 the desired pseudorotaxanes [A4@C4]4+
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Beilstein J. Org. Chem. 2015, 11, 748–762.
Figure 8: 1H NMR (500 MHz, 298 K, CD2Cl2, 1 mM) of a) C4 (top), A4@C4 (middle) and A4 (bottom) and b) C2 (top), A4@C22 (middle) and A4
(bottom). Disappearance and shift of the signals (red lines) suggest complexation. Due to the presence of a complex stereoisomeric mixture only
qualitative information of the complexation is possible.
(m/z = 989) and [A4@C22]4+ (m/z = 1200) are the most abun- In summary, the formation of all desired multivalent pseudoro-
dant species and there are again only traces of the possible taxanes of building blocks A2, A4, C2, and C4 could be veri-
1:1 pseudorotaxane [A4@C2]4+ (m/z = 825) detected fied by mass spectrometry. The defined stoichiometry for the
(Figure 10c,d). As mentioned above, this is most probably a observed pseudorotaxanes in the gas phase ([A2@C2]2+,
product of the ionization process. The free hosts C4 and C2 are [A22@C4]4+, [A4@C4]4+, [A4@C22]4+), the only slight abun-
detected in only small amounts or traces. Again, in both cases dance of partly bound pseudorotaxanes ([Na2A2@C4]4+,
no oligomers are observed.
[A4@C2]4+) and the absence of any oligomeric species gives
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Beilstein J. Org. Chem. 2015, 11, 748–762.
Figure 9: Normalized UV–vis absorption spectra (CH2Cl2, 2 μM) of the guests A2 and A4 (black), the hosts C2 and C4 (blue) and of the mixtures
(red), showing a slight hypsochromic shift of the absorption maxima upon complexation.
Figure 10: ESI-Q-TOF-MS spectra (CH2Cl2, 0.2 µM; left hand side) and respective experimental and calculated isotopic patterns of the desired [2]- or
[3]pseudorotaxanes (right hand side): a) 1:1 mixture of A2 and C2, b) 2:1 mixture of A2 and C4, c) 1:1 mixture of A4 and C4, d) 1:2 mixture of A4 and
C2.
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Conclusion
The successful synthesis of di- and tetravalent porphyrin-based
guests A2 and A4 as well as their complementary di- and
tetravalent hosts C2 and C4 could be achieved. All four mole-
cules show strong binding even to simple monovalent building
blocks A1 and C1, respectively, which could be shown by
NMR-titration experiments as well as mass spectrometry.
Furthermore, the formation of the di- and tetravalent pseudoro-
taxanes A2@C2, A22@C4, A4@C22, and A4@C4 could be
demonstrated qualitatively by NMR spectroscopy and was
investigated in detail by mass spectrometry. Since the associ-
ation constants in the monovalent cases are already too high to
be determined by NMR-titration experiments, currently ongoing
work is dealing with the daunting task to quantify the binding
constants for the di- and tetravalent multiporphyrin complexes
for example using isothermal calorimetry (ITC), in order to
analyze the thermodynamics and kinetics of multivalent binding
in these architectures in detail. In the future, we will continue to
exploit the concept of complementary multivalent binding to
program the increasingly complex self-assembly of multiple
different chromophore components into functional supra-
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Acknowledgements
The authors thank the Deutsche Forschungsgemeinschaft for
generous financial support (SFB 765). L.v.K. is grateful to the
Studienstiftung des Deutschen Volkes for a Ph.D. fellowship.
S.M. acknowledges the Alexander-von-Humboldt foundation
for a postdoctoral fellowship.
doi:10.1002/anie.201201114
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