Self-assembling depsipeptide dendrimers and dendritic fullerenes with new cis- and trans-symmetric Hamilton receptor functionalized zn-porphyrins: Synthesis, photophysical properties and cooperativity phenomena

Article abstract of DOI:10.1002/ejoc.201000233

Two novel supramolecular building blocks, namely cis- and trans-symmetric zinc-porphyrins bearing Hamilton receptors as their focal points (10 and 15), were self-assembled with (i) first-, second- and third-generation depsipeptide cyanurates (21-23); (ii) second-generation dendrofullerene (24), and (iii) cyanuric acid substituted fullerene (20), and probed in a series of photophysical investigations. Importantly, an overall 1:2 stoichiometry was confirmed with binding con-stants Kn, and Hill coefficients nH that indicate remarkable cooperativities. As a general feature, stronger binding evolves from the trans-symmetric porphyrin isomer, although the increased steric demand of the depsipeptide ligands means that the binding constants decrease with each generation. The self-assembly of 10 or 15 with 20 affords novel electron donor-acceptor hybrids that reveal, upon excitation, intra-hybrid charge-transfer events.

Full text of DOI:10.1002/ejoc.201000233

FULL PAPER
DOI: 10.1002/ejoc.201000233
Self-Assembling Depsipeptide Dendrimers and Dendritic Fullerenes with New
cis- and trans-Symmetric Hamilton Receptor Functionalized Zn–Porphyrins:
Synthesis, Photophysical Properties and Cooperativity Phenomena
Katja Maurer,[a] Bruno Grimm,[b] Florian Wessendorf,[a] Kristine Hartnagel,[a]
Dirk M. Guldi,*[b] and Andreas Hirsch*[a]
Keywords: Dendrimers / Fullerenes / Porphyrinoids / Hydrogen bonds / Self-assembly / Cooperative effects
Two novel supramolecular building blocks, namely cis- and
trans-symmetric zinc–porphyrins bearing Hamilton receptors
as their focal points (10 and 15), were self-assembled with
stants Kn, and Hill coefficients nH that indicate remarkable
cooperativities. As general feature, stronger binding
evolves from the trans-symmetric porphyrin isomer, although
a
(i) first-, second- and third-generation depsipeptide cyan- the increased steric demand of the depsipeptide ligands
urates (21–23); (ii) second-generation dendrofullerene (24),
and (iii) cyanuric acid substituted fullerene (20), and probed
in a series of photophysical investigations. Importantly, an
overall 1:2 stoichiometry was confirmed with binding con-
means that the binding constants decrease with each genera-
tion. The self-assembly of 10 or 15 with 20 affords novel elec-
tron donor–acceptor hybrids that reveal, upon excitation,
intra-hybrid charge-transfer events.
Fréchet-type dendrons, were also studied with respect to
unidirectional transfer of excited-state energy.[6] Here, a su-
perb example is a dendritic architecture, where the use of
up to eight boron-dipyrrin chromophores has been used to
mimic the basic functions of the natural photosynthetic an-
tenna complexes.[7]
Introduction
Combining the rapidly evolving fields of nanostructured
materials and supramolecular chemistry is an attractive
strategy for constructing large and complex, yet highly or-
dered, molecular and supramolecular entities with defined
functions. Specifically, developing novel super- and supra-
molecular electron donor–acceptor architectures – based on
functional dyes – through careful design have emerged as
viable tools for efficient conversion of solar energy.[1] In this
context, porphyrins, owing to their biological relevancy[2]
and remarkable photoelectronic properties,[3] have stimu-
lated great interest in devising, designing, synthesizing and
testing porphyrin-containing supramolecular electron do-
nor–acceptor architectures. To this end, several facts of
multifunctional porphyrin assemblies – built on hydrogen-
bonding – have been explored with great success.[4] Like-
wise, covalent motifs have been employed for the prepara-
tion of porphyrin–dendrimer conjugates that function as
synthetic mimics of globular heme proteins.[5] Dendrimer-
like structures, i.e, zinc–porphyrin cores bearing different
Porphyrins endowed with Hamilton receptors have been
integrated into enzymatic models[8] and/or into photoactive
systems.[9] Our own contribution to this highly interdisci-
plinary field started with first, second, and third genera-
tions of depsipeptide dendrons and their self-assembly with
a homotritopic Hamilton receptor.[10a] Here, chiroptical in-
vestigations stood at the forefront. Conditions that enforce
the aggregation of these depsipeptide cyanurates with, for
example, the Hamilton receptor carrying porphyrin led to
the formation of different chiral porphyrin dendrimers.[10b]
The photophysical and (opto)electronic properties of
metalloporphyrins are truly unique, especially when they
are combined with fullerenes.[11] What renders fullerenes ex-
ceptional is their capability to reversibly accept up to six
electrons,[12] while exhibiting a low reorganization energy
that is associated with each of these electron-transfer
[a] Department of Chemistry and Pharmacy, and Interdisciplinary steps.[13] As a matter of fact, a considerable number of elec-
Center of Molecular Materials (ICMM), Friedrich-Alexander-
tron donor–acceptor conjugates – based on combinations
Universität Erlangen-Nürnberg,
of porphyrins and fullerenes – emerged in recent years that
combine all the key functions of the photosynthetic reac-
tion center, that is, light-harvesting and charge-transfer.[14]
Hydrogen bonding is, however, hardly ever employed to in-
tegrate the different building blocks (i.e., porphyrins and
fullerenes) into fully operational reaction center models.[15]
One of the few examples is a chiral fullerene dendron.[16]
Henkestrasse 42, 91054 Erlangen, Germany
E-mail: dirk.guldi@chemie.uni-erlangen.de
andreas.hirsch@chemie.uni-erlangen.de
[b] Department of Chemistry and Interdisciplinary Center of
Molecular Materials (ICMM), Friedrich-Alexander-Universität
Erlangen-Nürnberg,
Egerlandstrasse 3, 91058 Erlangen, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000233.
View this journal online at
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Self-Assembling Depsipeptide Dendrimers and Dendritic Fullerenes
Figure 1. Supramolecular complexes of the Hamilton receptor functionalized Zn–porphyrins 10·L2 and 15·L2.
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Placing cyanuric acid functionalities at the focal points of
these dendrons guarantees control of the directed self-as-
sembly of Hamilton receptor functionalized metalloporphy-
rins [i.e., tin porphyrin (SnP) and zinc porphyrin (ZnP)].[17]
In this context, we reported the modular self-assembly of
depsipeptide dendrons on a Hamilton receptor modified
Results and Discussion
Synthesis of the Hamilton Receptor Substituted Porphyrin
Derivatives
Scheme 1 sketches the synthetic route to the iodine-sub-
porphyrin platform – realized with an SnP functionalized stituted Hamilton receptor derivative 3,[19] which was used
with two axial Hamilton receptors.[17b] With the help of a as a starting material for the targeted porphyrin guest mole-
detailed time-resolved fluorescence and transient absorp- cules. In particular, heating of 5-iodobenzene-1,3-dioic acid
tion investigation we documented that, in the correspond- (1)[20] with thionyl chloride (SOCl2) at reflux, followed by
ing SnP/fullerene hybrids, energy transfer rather than elec- coupling of the dichloride with N-(6-aminopyridin-2-yl)-
tron transfer dominates the excited-state features.
3,3-dimethylbutanamide (2)[21] in tetrahydrofuran (THF) in
In the current work, we wish to introduce two novel sup- situ afforded the iodine-substituted Hamilton receptor 3 in
ramolecular ZnP building blocks 10 and 15, namely cis- 73% isolated yield.
and trans-zinc–porphyrin isomers bearing two Hamilton re-
The synthesis of the cis-configured Hamilton receptor
ceptor functionalities as their focal points as a complement porphyrin is summarized in Scheme 2. 3,5-Dimethoxybenz-
to the previously published Hamilton receptor function- aldehyde (4), 4-[2-(trimethylsilyl)ethynyl]benzaldehyde (5)
alized porphyrins.[17] The cis versus trans symmetry of 10 and pyrrole (6) were treated with trifluoroacetic acid (TFA)
and 15 represents a substitution pattern that deserves par- in dichloromethane (CH2Cl2) under Lindsey conditions.[22]
ticular attention with respect to the generation of novel sup- After 1 h, TFA was neutralized with an excess of triethyl-
ramolecular architectures. When, for example, a cis-sym- amine (NEt3), and the reaction mixture was subjected to
metric Hamilton receptor and a complementary bis(cyan- oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone
uric acid) building block are used, it is likely that rectangu- (DDQ). The porphyrin cis isomer was purified by repeated
lar architectures should evolve. This would, however, de- column chromatography on silica (SiO2) using a mixture of
pend on the conformational flexibility/rigidity of the supra- hexane/ethyl acetate (8:2) as eluent. Metal insertion with
molecular aggregates. Photophysical events such as elec- zinc acetate [Zn(OAc)2] followed by desilylation under inert
tron- and energy-transfer reactions have been shown to conditions using a 1  solution of tetrabutylammonium
depend considerably on the character and flexibility of the fluoride (TBAF) in THF led to the formation of porphyrin
linkage between the electron/energy donor and the electron/ derivative 9 in good yield. Finally, 10 was obtained by a
energy acceptor. Notably, high flexibility of the supramolec- Sonogashira coupling reaction of 9 with the Hamilton re-
ular hybrid might result in substantial inter/intramolecular ceptor derivative 3 using tris(dibenzylidenacetone)dipalla-
folding. Further consequences, such as shorter lifetimes of dium(0) [Pd2(dba)3] and triphenylarsane (AsPh3) as cata-
the intermediately formed radical ion pair states or loss of lysts. The target compound 10 was isolated by repeated col-
control over supramolecular architectures, might be ex- umn chromatography on SiO2 in 38% yield. TLC monitor-
pected.[18] In light of the aforementioned considerations, ri- ing of the Sonogashira reaction was impossible; therefore,
gid acetylene spacers were chosen for the covalent synthesis the progress of the reaction was monitored by FAB MS.
of the molecular building blocks 10, 15, and 20 (Figure 1). After 24 h, the FAB mass spectra of the crude reaction mix-
These are likely to restrict the flexibility of the Hamilton ture revealed a peak corresponding to the [M]+ ion of target
receptor and cyanuric acid functionalities and, at the same molecule 10 with, however, low intensity. During the pro-
time, provide enhanced levels of electronic communication. gress of the reaction, the intensity of the [M]+ peak corre-
The self-assembly processes of 10 and 15 with the depsipep- sponding to 10 barely changed. Therefore, longer reaction
tide dendrons (21–23) and second-generation dendrofuller- times were required to guarantee completion of the cou-
ene (24) and cyanuric acid substituted fullerene (20) were pling reaction.
studied and, based on a detailed photophysical investiga-
The synthetic route to the trans-configured Hamilton re-
tion, we conclude intrahybrid charge-transfer.
ceptor substituted porphyrin 15 barely differs from that of
Scheme 1. Synthesis of the iodine-substituted Hamilton receptor derivative 3. Reagents and conditions: (a) SOCl2, DMF, NEt3, THF,
room temp., 72 h, 73%.
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Self-Assembling Depsipeptide Dendrimers and Dendritic Fullerenes
Scheme 2. Synthesis of Hamilton receptor substituted porphyrin 10. Reagents and conditions: (a) TFA, CH2Cl2, 1 h; NEt3, DDQ, room
temp., 2 h, 9%; (b) Zn(OAc)2, THF, reflux, 1.5 h, 93%; (c) TBAF (1 , THF), THF, r.t., 12 h, 93%; (d) Pd2(dba)3, AsPh3, THF/NEt3
(2:1), 6 d, 38%.
10 (Scheme 3). Dipyrromethane 11[23] was chosen for two 12 is favored. Again, the Sonogashira coupling reaction of
reasons: (i) The tert-butyl groups should guarantee good 3 and 14 was monitored by FAB MS, and the target com-
solubility and low polarity of the porphyrin intermediates. pound 15 was isolated in 31% yield by repeated column
(ii) Literature reports[23] have documented the stability of chromatography on SiO2. The Hamilton receptor substi-
11 during scrambling processes, which leads to the conclu- tuted zinc–porphyrins 10 and 15 were characterized spec-
1
sion that the formation of statistical side products is in- troscopically (i.e., H and 13C NMR, UV/Vis and IR spec-
hibited and that the formation of the desired trans isomer troscopy), complemented by elemental ananlysis, FAB and
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Scheme 3. Synthesis of the Hamilton receptor substituted porphyrin 15. Reagents and conditions: (a) TFA, CH2Cl2, 1 h, room temp.;
NEt3, DDQ, 2 h, r.t., 10%; (b) Zn(OAc)2, THF, reflux 12 h, 83%; (c) TBAF (1 , THF), THF, 12 h, r.t., 79%; (d) 3, Pd2(dba)3, AsPh3,
THF/NEt3 (2:1), 6 d, r.t., 31%.
MALDI TOF mass spectrometry. All 1H and 13C NMR
Compound 13 was also investigated by using tempera-
assignments were guided by HETCOR and COSY analyses. ture-dependent 1H NMR measurements in the range be-
1
The characterization of 13, 14, and 15 (i.e., ZnP) by H tween –40 and +50 °C in CDCl3 (Figure 2). At very low
NMR spectroscopy in deuterated chloroform (CDCl3) led temperatures, all resonances were broad and unresolved.
to unresolved and broad resonance signals. However, the Importantly, over the entire temperature range the TMS
1H NMR spectra of 12 (i.e., H2P) showed the expected reso- resonance signal at δ = 0.4 ppm remained unaffected. Upon
lution and splitting pattern. The low solubility of 13, as a increasing the temperature above +50 °C the signals of the
possible rationale, is ruled out on the basis that the corre- aromatic protons (i.e., δ = 7.8–8.7 ppm) sharpened, while
sponding solutions prior to and after the measurements the signal of the pyrrol protons (i.e., 11 at δ = 8.5 ppm)
were clear with no appreciable signs of aggregate formation, remained unresolved. At 30 °C, the resonances of the
etc. Less concentrated CDCl3 solutions of 13 led to the phenyl protons 4 at δ = 7.5 ppm become visible in the form
same features. The same phenomenon was also observed in of broad, unresolved signals, which shift slightly to lower
the 13C NMR spectra.
fields with further increasing temperature. Concerning the
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Self-Assembling Depsipeptide Dendrimers and Dendritic Fullerenes
1
Figure 2. Temperature-dependent H NMR spectra (400 MHz, CDCl3) of 13.
alkyl protons 1, 6, 7 and 21, no clear tendency was, how- and CH2Cl2 were found to increase in the order S, P Ͻ O Ͻ
ever, discerned. For example, the resonance signals of the N[24]and, therefore, the addition of amines such as pyridine
alkyl protons 6 and methyl protons 7 are barely visible at δ should lead to ligand-exchange reactions. Only a few exam-
= 3.5 and 2.4 ppm, respectively, between –40 and –20 °C. ples of six-coordinate Zn–porphyrin complexes are de-
At higher temperatures, a notable broadening and slight scribed in the literature.[25] Nickel, on the other hand, favors
shifts to higher fields were observable. In the range of the formation of square-planar complexes.[11] This
–10 °C to +20 °C it was impossible to differentiate between prompted us to synthesize 16 and characterize it by means
the signal and the baseline. At +30 °C the resonances were of NMR spectroscopy (Figure 3). As expected, sharp and
visible again but remained unresolved even at +50 °C. In well-resolved peaks evolve in the 1H and 13C NMR spectra
the low-temperature regime, the resonances of the tert-butyl of 16.
protons 1 could be observed as a broad signal at δ =
According to PM3 calculations[26] on the geometry-opti-
1.4 ppm. When the temperature was increased to +50 °C, mized structure of 13, intramolecular donor–acceptor
the resonances became sharp and were slightly shifted to bonding involving the methoxy oxygen atom and the axial
lower fields.
coordination site of the zinc ion is restricted. It is notable
Because the free-base porphyrin 12 did not exhibit the that the free rotation of the tert-butyl-substituted phenyl
same temperature-dependent NMR spectral changes, we rings is hindered. Steric repulsion between the α-pyrrol pro-
considered the coordination chemistry of zinc as a possible ton and the methoxy substituents is likely to be responsible
rationalization. Zinc–porphyrins are known to coordinate for this phenomenon.[23] Hence, we excluded the presence
neutral ligands, such as N, O, S, P donors, and charged of different rotational isomers in solution. Further semiem-
ligands through axial coordination. However, the binding pirical PM3 calculations[27] performed on a Zn–porphyrin
strengths in nonpolar solvents such as chloroform (CHCl3) dimer of 13/13, resulted in a calculated 2.0 Å distance be-
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Figure 3. Ni–porphyrin 16.
Scheme 4. Synthesis of the cyanuric acid substituted fullerene 20. Reagents and conditions: (a) Pd(PPh3)2Cl2, CuI, THF/NEt3, room
temp., 73%; (b) fullerene, sarcosine, toluene/THF, reflux 29%.
tween the zinc ion and the coordinated methoxy oxygen emerge (see Figures 4 and 5). Complexation of the corre-
atom. Therefore, particularly strong intermolecular interac- sponding cyanurate guest, on the other hand, promotes
tions are considered likely.
dissociation of the intermolecular aggregates and is ac-
Scheme 4 depicts the synthesis of cyanuric acid substi- companied by a noticeable sharpening of the resonance sig-
tuted fullerene 20. Sonogashira coupling of the 4-iodo- nals. The NH1 and the NH2 resonances of the Hamilton
phenyl isocyanuric acid 17[28] with aldehyde 18 in the pres- receptor undergo a continuous downfield shift until the 1:2
ence of bis(triphenylphosphane)palladium(II) dichloride complexes prevail. After the addition of 0.4 equiv. of 21,
[Pd(PPh3)2Cl2] as a catalyst led to the formation of the the signal of the NH3 protons of the cyanuric moiety is
cyanuric acid substituted aldehyde 19 in good yields. The observed at δ = 13.4 ppm. In contrast to the amide protons
azomethine ylide of 18, which was formed in situ, and of the Hamilton receptor, the resonance of the NH3 protons
sarcosine were treated with fullerene to afford the target experiences a high-field shift and increased broadening due
compound 20, which was purified by column chromatog- to fast exchange processes between free and bound cyan-
raphy on SiO2.
urate. Nevertheless, the average signal disappears after the
addition of 2.4 equiv. of the guest molecule. The supra-
molecular hydrogen-bonded assemblies undergo continuous
self-assembly and disassembly processes[30] and the dynamic
character of these systems is revealed by temperature-de-
Determination of Association Constants and Cooperativity
of Binding
The association constants and the cooperativity of the pendent 1H NMR spectroscopy. Figure 5 shows the coordi-
supramolecular 10·L2 and 15·L2 complexes (L = 21, 22, and nation motif and the H NMR spectra of a 1:8 mixture of
1
23)[10a] were determined by 1H NMR titration experi- 10 and 21 at various temperatures. Below 0 °C, the NH1
ments[29] in CDCl3. The experiments were performed by the and NH2 protons of the Hamilton receptor are observed as
stepwise addition of 40 µL of a 2.5 m solution of 21–23 broad and unresolved signals. Upon increasing the tem-
to 0.5 mL of a 0.5 m solution of 10 or 15 in CDCl3. Ham- perature to 20 °C these resonances sharpen. In the tempera-
ilton receptors tend to form rather stable intermolecular ag- ture range between –40 and 20 °C the two resonance signals
gregates through multiple hydrogen bonds in apolar, nonco- relate to the free and bound NH3 protons of the cyanuric
ordinating solvents such as CDCl3.[10a,17b,19] As a conse- moiety. Here, a slow (on the NMR time-scale) association/
quence, rather broad and unresolved 1H NMR peaks disassociation equilibrium is inferred. Above 0 °C, the NH3
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Self-Assembling Depsipeptide Dendrimers and Dendritic Fullerenes
Figure 4. Chemical shifts of the NH1, NH2, and NH3 signals during the titration of 10 with the guest molecule 21 (1H NMR spectra,
400 MHz, CDCl3). After the addition of 21: (a) 0 equiv.; (b) 0.4 equiv.; (c) 0.8 equiv.; (d) 1.2 equiv.; (e) 1.6 equiv.; (f) 2.0 equiv.;
(g) 2.4 equiv.
resonances of the cyanuric moiety become very broad and Table 1. Association constants, R factor (R), rmax and Hill coeffi-
can no longer be detected. Temperature-dependent 1H
NMR spectroscopy analyzing similar supramolecular com-
plexes in the range of –60 to 90 °C using deuterated dichloro-
ethane as a solvent verified that the temperature interval of
0–50 °C represents a coalescence regime.[10a]
cients (nH) for the 1:2 complexes of 10 and 15 with dendrons 21,
22, and 23.
logK1 [Lmol–1] LogK2 [Lmol–1] R [%]
rmax
nH
10·212
10·222
10·232
15·212
15·222
15·232
3.42
4.02
3.80
4.31
4.67
4.14
8.14
7.80
7.27
8.97
8.78
8.11
0.48
0.30
0.46
0.38
0.56
0.45
0.88
0.53
0.41
0.86
0.60
0.61
0.79
0.35
0.26
0.75
0.43
0.44
For 10, the chemical shifts of the NH1 and NH2 signals
are displayed in Figure 4 as a function of the concentration
of 21. Implicit is that the equilibrium between the free and
bound cyanurate is fast on the NMR time-scale. Neverthe-
less, overcoming the intermolecular hydrogen bonds (i.e., in
15 and 10) takes time. Thus, to guarantee the establishment
of a stable equilibrium, the 1H NMR spectra were recorded
45 min after addition. The NMR titration plots for 10·212
and 15·212 gave rise to sigmoidal shapes with inflection
points at around 0.6 equiv., which confirms an overall posi-
tive cooperativity.[31–33] Nevertheless, the cooperativity is af-
fected by the nature of the dendron (see Figures 6 and 7),
with 10·212 Ͼ 15·212. The NMR assays for the first-genera-
tion dendron 20 revealed the most pronounced downfield
shifts, reflecting the higher association constants (Table 1).
Calculation of the association constants (Table 1) was
performed with the Chem-Equi program.[34,35] These calcu-
lations were based on assuming two equilibria [Equa-
tions (1) and (2)], namely between ZnP 10 and 15[36] (C)
and the depsipeptide dendrons 21, 22, and 23 (L).
K1 = C + L h CL
(1)
K2 = CL + L h CL2
(2)
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1
Figure 5. H NMR spectra (400 MHz, CDCl3) of a 1:8 mixture of 10 and 21 at various temperatures.
If identical and independent binding sites are considered, generation number. In general, the association constants for
multiple binding proceeds under statistical conditions and, the second binding decrease with increasing generation
in turn, Equation (3) becomes relevant (t = total number of number of the depsipeptide dendrons 21, 22, and 23. How-
binding sites, t = 2).
ever, the K1 values obtained for the second-generation den-
drons 22 prevail over those of the self-assemblies involving
the first-generation dendrons 21. A different spatial ar-
rangement is likely to be responsible for this trend. For ex-
ample, open-chain Hamilton receptors such as 10 or 15 may
adopt three different conformations depending on the de-
gree of intermolecular aggregation and complexation. Only
the cis/cis configuration ensures an effective complex-
ation.[21] Binding of the first depsipeptide dendron weakens
intermolecular aggregation and, in turn, facilitates binding
of the second depsipeptide dendron. This trend scales with
the generation number and the steric loading of the depsi-
peptide dendron. Importantly, the latter is also seen to de-
crease K1.
Kn +1
Kn
n(t – n)
=
(3)
(n + 1)(t – n + 1)
Positive cooperativity leads us to expect higher binding
strength of the (n+1)st ligand – relative to a statistical distri-
bution – and higher Kn+1/Kn values.[29] Remarkably, the ex-
perimental Kn+1/Kn values were found to be even higher
than those estimated on the basis of Equation (3). In all the
cases Kn+1/Kn Ͼ 1, which clearly demonstrates the pro-
nounced positive cooperativity for the supramolecular C/L2
complexes.[37]
In light of the steric aspects, the association constants for
15·L2 were higher than those for 10·L2 (L = 21, 22, and
All of the self-assembly processes feature dynamic char-
23). The large difference between K1 and K2 reflects the acter (see also Figure 5) and, as a consequence, the presence
pronounced positive cooperativity for these systems. The of three different species has to be assumed. Despite this,
aforementioned tendency is inversely proportional to the successful formation of the 1:2 complexes C·Ln (C = 10 or
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Self-Assembling Depsipeptide Dendrimers and Dendritic Fullerenes
Figure 7. NMR titration reflecting the chemical shifts of NH1 and
Figure 6. NMR titration reflecting the chemical shifts of NH1 and
NH2 in 15 as a function of added equivalents of 21, 22, and 23.
NH2 in 10 as a function of added equivalents of 21, 22, and 23.
15; n = 0–2; L = 21, 22 and 23) is shown in Figures 8 and 1.6 or more equivalents have been added. Interestingly, the
9. Even at low concentrations of 21, 22, and 23 the C·L2 cis configuration in 10 evokes increased steric loadings. As
complexes dominate, which guarantees that, upon addition a matter of fact, in the presence of 2 equiv. of 23, the con-
of 2 equiv., the C·L2 complexes range between 45–90%. The centration of 15·232 is close to 70%, whereas that of 10·232
only exception is 23, where the C·L complexes prevail until is only 45%.
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Figure 8. Distribution of free and complexed 10 in 10·Ln (n = 0–2;
L = 21, 22, and 23) as a function of added equivalents of dendrons
21, 22, and 23. The data was obtained from NMR analysis of the
corresponding titrations by using Chem-Equi.[34,35]
Figure 9. Distribution of free and complexed 15 in 15·Ln (n = 0–2;
L = 21, 22, and 23) as a function of added equivalents of dendrons
21, 22, and 23. The data was obtained from NMR analysis of the
corresponding titrations by using Chem-Equi.[34,35]
Further confirmation for the positive cooperativity was (i.e., C = 10 or 15; L = 21, 22, and 23). Initially, the occu-
obtained from the Scatchard[38] plots of the C·L2 systems pancy r as a reflection of the number of bound ligands was
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Self-Assembling Depsipeptide Dendrimers and Dendritic Fullerenes
determined from Equation (4) prior to analyzing the Scat-
chard Equation (5).
Figure 10 shows leading Scatchard plots. In all cases the
plots feature convex curves, which points to pronounced
positive cooperativity. Notably, statistical binding would re-
sult in a straight line, whereas concave Scatchard plots
would suggest pronounced negative cooperativity.[29] A size-
able measure of the cooperativity is expressed by the Hill
constants nH,[39] which were determined from the maxima
of the Scatchard plots rmax by using Equation (6).
tQx
r =
r =
(4)
(5)
(1 + Qx)
[CL] + 2[CL2]
[C] + [CL] + [CL2]
Assuming a statistical binding, the binding polynomial
and r equal (1 + Qx)t and tQx(1 + Qx)t–1, respectively.
Here, x refers to the concentration of L.
rmax
nH
=
(6)
t – rmax
In fact, the largest nH values (Table 1) were associated
with the formation of the C·L2 complexes when the first-
generation dendron 21 was probed. Increasing the genera-
tion number of the dendron leads to smaller nH values.
Finally, Job’s plot analysis was performed with 1H NMR
titration data (see Figures 11 and 12) for the C·L2 systems
(i.e., C = 10 and L = 22; C = 15 and L = 21). Overall, the
obtained Job’s plots give rise to maxima at a mol fraction
of 0.33, which confirms the underlying 1:2 stoichiometry.
1
Figure 11. Job’s plot analysis of the H NMR titration of 10 with
22 in CDCl3.
1
Figure 12. Job’s plot analysis of the H NMR titration of 15 with
21 in CDCl3.
Figure 10. Scatchard plots for 15·L2 (L = 21, 22, and 23).
Eur. J. Org. Chem. 2010, 5010–5029
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5021

D. M. Guldi, A. Hirsch et al.
FULL PAPER
Figure 13. Depsipeptide fullerene 24 with all-(S) configuration.
Photophysical Investigations
With the changes in the absorption features in hand, we
then quantified the binding constants. Considering the data
at 423 nm (where the absorption diminishes) as a function
of concentration of 24, led to logK values of 3.30 Lmol–1
and 6.71 Lmol–1 for the first and second binding events,
respectively.[41] Of note is the excellent agreement with the
calculated values that were obtained based on the NMR
titrations. Decisive information on the stoichiometry was
obtained by Job’s plot titration experiments. Figure 15 illus-
trates that the maximum occurs at 0.33, which points to a
1:2 stoichiometry.
As a complement, Zn–porphyrins 10, 15, and fullerene
derivatives 20 and 24 (Figure 13) were subjected to a series
of photophysical investigations.
To this end, solutions of Zn–porphyrin 10 (in dichloro-
methane, or ortho-dichlorobenzene) were titrated with vari-
able concentrations of fullerene derivative 24. We note – in
perfect agreement with previous investigation – that adding
24 to solutions of 10 evoked a gradual redshift of the ab-
sorption features. For example, the Soret band shifts as
much as 3 nm (see Figure 14). Moreover, when subtracting
the fullerene absorption, a net decrease in the intensity of
the Soret and Q-bands evolves. The presence of isosbestic
points (418 and 428 nm) implies effective ground-state in-
teractions between the Zn–porphyrin and fullerene.[40]
A
newly developing absorption band at 780 nm further corro-
borates this assumption.
Figure 15. Job’s plot analysis of the absorption titration of 10 with
24 in CH2Cl2.
In contrast, titrations involving Zn–porphyrin 15 and ful-
lerene 24 in ortho-dichlorobenzene/pyridine (1000:1) re-
vealed only a slight decrease of the Soret-band and no ap-
preciable shift. A reasonable assumption can be made that –
in addition to axial coordination – weaker binding in the
presence of pyridine occurs. Nevertheless, the isosbestic
points at 430 and 440 nm provided evidence for mutually
interacting Zn–porphyrin 15 and fullerene. A similar con-
clusion can be derived for Zn–porphyrin 10 and fullerene
derivative 20 in chloroform/carbon disulfide (5:1). However,
Figure 14. Changes in the Zn–porphyrin electronic absorption
spectra of 10 (9.0ϫ10–7 ) in CH2Cl2 upon successive addition of
24 (0 to 2.0ϫ10–5 ).
5022
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Self-Assembling Depsipeptide Dendrimers and Dendritic Fullerenes
including a p-phenyleneethynylene spacer hampers sizeable
Using the sigmoidal dependency between IF/I0 and C60
concentration described by Equation (7) led to binding con-
ground-state interactions.[42]
Decisive confirmation on the nature of the electron do- stants logK1 = 3.37 Lmol–1 and logK2 = 7.30 Lmol–1.
nor–acceptor interactions came from complementary
2
(I0 + c0·b·K1·cD + Iϱ·β12·cD
)
steady-state fluorescence experiments. Here, the characteris-
tic fluorescence of Zn–porphyrin 10 (ΦF = 0.04) was used
and monitored while adding variable amounts of fullerene
24. As shown in Figure 16, the Zn–porphyrin fluorescence
is subjected to strong quenching when 24 is present. Tenta-
tively, we invoke for this new deactivation pathway an elec-
tron transfer that evolves from the photoexcited Zn–por-
phyrin. Spectroscopic evidence in favor of the electron-
transfer hypothesis came from transient absorption mea-
surements (see below). The gradual quenching was used to
estimate the binding constants (Figure 16b).
IF
=
(7)
2
(1 + K1·c0 + β12·cD
)
In Equation (7), I0 and Iϱ refer to the initial and the final
fluorescence intensity, respectively, whereas c0 is the total
porphyrin concentration; the added fullerene concentration
is given by cD. K1 is the first binding constant and β12
equals K1·K2.[43] The perfect agreement with the NMR
analysis and absorption assays strengthens our hypothesis
for positive cooperativity.
An important control experiment focused on the non-
photoactive dendron 22,[10a] which was added in variable
amounts to a mixture of 10 and 24 in ortho-dichloroben-
zene. Upon increasing the concentration of 22 a ligand-ex-
change reaction replaced 24, and the initial fluorescence of
10 was quantitatively restored.
The Zn–porphyrin fluorescence also enabled a determi-
nation of the extent of electronic communication between
Zn–porphyrin 15 and 24: logK1 = 3.92 Lmol–1 and logK2 =
5.80 Lmol–1 (Table 2). Interestingly, the difference between
these two binding constants was much less when compared
with the difference observed for 10·24. A weaker cooper-
ativity might arise from the trans positioning of the two
Hamilton receptors.
In the next step, the fluorescence quantum yields at the
plateau values – corresponding to a quantitative conver-
sion – were taken to gather preliminary information about
the excited-state deactivation dynamics. In 10·24, for exam-
ple, the new deactivation pathway (7.6ϫ109 s–1) outper-
forms the intrinsic decay of the singlet excited state of 10,
namely intersystem crossing (4.7ϫ108 s1).
Once the steady-state characterization – absorption and
fluorescence – was completed, we turned to time-resolved
fluorescence measurements. The fluorescence of Zn–por-
phyrin 10 was best fitted by a mono-exponential decay
function with a lifetime that typically ranged from 1.5 to
1.8 ns, when dichloromethane and ortho-dichlorobenzene
were used as solvents. In the presence of 24, a reasonable
fit of the fluorescence decay was only achieved when a bi-
exponential fitting function was used. In particular, a long
lifetime of ca. 1.5 ns – corresponding to free, uncomplexed
10 – and a short lifetime of 0.4 ns – corresponding to com-
plexed 10·24 – emerged. The pre-exponential factor of the
long and the short lifetimes decreased and increased with
increasing concentration of 24, respectively. Importantly,
Figure 16. (a) Changes in the steady-state fluorescence of Zn–por-
phyrin 10 (8ϫ10–7 ) upon successive addition of 24 (0 to
1.02ϫ10–5 ) in ortho-dichlorobenzene; λexc = 424 nm. (b) Fluores-
cence intensity at 600 nm of 15 and 15·24 with non-linear fit ac-
cording to Equation (7).
Table 2. Association constants obtained from fluorescence titration for the 1:2 complexes of 10 and 15 with fullerene derivative 24 and
fluorescence lifetimes of 10·24.
logK1
logK2
Fluorescence lifetime [τ]
10·24 [ns]
Solvent
[Lmol–1]
[Lmol–1]
10·24
10·24
15·24
3.54
3.37
3.92
6.37
7.30
5.80
1.6:0.4
1.8:0.3
dichloromethane
ortho-dichlorobenzene
ortho-dichlorobenzene/pyridine (1000:1)
Eur. J. Org. Chem. 2010, 5010–5029
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5023

D. M. Guldi, A. Hirsch et al.
FULL PAPER
the two lifetimes remained unchanged. From the short de- 2.0 eV) decays slowly (4.8ϫ108 s–1) to the energetically low-
caying component – attributed to complexed 10·24 – the lying triplet excited state (ETriplet = 1.53 eV) through in-
rate of excited-state deactivation was estimated to be tersystem crossing (ΦTriplet = 0.88). In terms of spectral
2.8ϫ109 s–1. This is in good agreement with the deactiva- characteristics of the latter, a maximum at around 840 nm
tion rate obtained from the steady-state fluorescence experi- should be considered.
ments (7.6ϫ109 s–1). In a control experiment, redox- and
photoinactive 22 was added to 10·24. In this case, the pre-
exponential factors were completely reverted, namely the
short-lived component decreased gradually and finally dis-
appeared, while that of the long-lived component increased
likewise (Figure 17).
Figure 18. Differential absorption spectrum obtained upon femto-
second flash photolysis (420 nm) of zinc–porphyrin 24 in argon-
saturated ortho-dichlorobenzene with time delays of 2 and 2892 ps.
Next, transient absorption measurements were carried
out with 24·10, and the results were compared with the
spectra of 24. At early time delays, transient absorption
spectra of 24·10 were practically identical with those of 24.
However, at time delays of 100 ps the fingerprint absorp-
tions of the one-electron-oxidized Zn–porphyrin radical
cation and of the one-electron-reduced fullerene radical
anion appeared at 680 and 1030 nm,[44] respectively. These
radical ion pair features decayed over a time period of 3 ns
to recover quantitatively the singlet ground state without
populating any triplet excited states. In summary, electron
transfer is responsible for the fast deactivation of the photo-
excited Zn–porphyrin and, in turn, affords the formation of
the radical ion pair state. By analyzing the kinetics of the
time absorption profiles, we can derive rate constants for
the charge separation and the charge recombination of
9.0ϫ109 s–1 and 7.8ϫ108 s–1, respectively. Notably good is
the agreement between the charge separation rate deter-
mined from steady-state fluorescence (7.6ϫ109 s–1) and
Figure 17. Normalized pre-exponential factors of the two lifetimes
transient absorption (9.0ϫ109 s–1) measurements (Fig-
observed by time-correlated single-photon counting (TCSPC) as a
ure 19).
function of the number of equivalents of fullerene 24 and dendron
22.
Complementary nanosecond transient measurements,
which were performed with 532 nm laser excitation of dif-
Finally, we turned to transient absorption measurements ferent mixtures of 24·10, shed further light on the electron-
to find spectroscopic and kinetic evidence in support of the transfer process. All the spectra were characterized by
hypothesized electron transfer. Representative femtosecond strong triplet–triplet absorptions at 710 nm, which corre-
transient absorption spectra, recorded after a 150 fs laser spond to that of the fullerene triplet excited state, without,
pulse at 420 nm in ortho-dichlorobenzene solutions of 24, however, giving rise to any radical ion pair features at 680
are displayed in Figure 18. The spectra, obtained at time or at 1030 nm. We must assume that the fullerene triplet
delays shortly after the laser pulse, revealed broadly absorb- excited state originates from a locally excited state of free,
ing features between 600 and 1100 nm. Superimposed onto uncomplexed fullerene. Implicit in this analysis is that in
that are features of transient bleach, especially around 420 the femtosecond experiments, charge recombination occurs
and 550 nm. The latter features correspond to the loss of on a time-scale faster than in the nanosecond experiments,
ground-state absorption. The singlet excited state (ESinglet
=
that is, less than 6 ns.
5024
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 5010–5029

Self-Assembling Depsipeptide Dendrimers and Dendritic Fullerenes
association constants to be calculated; these were in excel-
1
lent agreement with the estimated values based upon H
NMR titration analysis. In contrast to these outcomes, the
self-assembly between Zn–porphyrin 15 and fullerene deriv-
ative 24 (ortho-dichlorbenzene/pyridine, 1000:1) was found
to be much less pronounced. We believe that, in addition to
intermolecular aggregation (axial coordination through the
Zn2+ ion), weaker binding in the presence of pyridine can
be considered to be a reasonable assumption. A similar
conclusion can be reached for the self-assembly between
Zn–porphyrin 10 and fullerene derivative 20 in a mixture
of CHCl3/carbon disulfide (5:1). A series of photophysical
experiments – ranging from steady-state and time-resolved
fluorescence experiments to transient absorption measure-
ments on the femtosecond and nanosecond time-scales –
provided decisive confirmation of the nature of electron do-
nor–acceptor interactions because they are operative be-
tween Zn–porphyrin 10 and fullerene derivative 24 in the
supramolecular assembly 24·10.
Figure 19. Differential absorption changes obtained upon femtose-
cond flash photolysis (420 nm) of 24·10 in argon-saturated ortho-
dichlorobenzene with time delays of 1 and 97 ps.
Conclusions
We have investigated the aggregation and photophysical
properties of supramolecular self-assemblies between cis-
and trans-configured Hamilton receptor functionalized Zn–
porphyrins and chiral depsipeptide (fullerene) cyanurates
based upon the Hamilton receptor bonding motif. The cor-
responding association constants were determined by me-
ans of NMR and fluorescence titration experiments. In ge-
neral, NMR titration analysis confirmed the largest com-
plex stability, and most pronounced cooperativity for the
1:2 aggregates involved the first-generation depsipeptide
dendron 21; this was reflected in the values of the first and
second binding steps (K1 and K2) and the Hill constant
(nH). The self-assembly of the supramolecular first- to
third-generation complexes with trans geometry was found
to be much more pronounced compared to their cis-substi-
tuted analogues. Furthermore, the association constants K2
were found to decline with increasing generation number of
the depsipeptide dendrons 21, 22, and 23 due to increased
steric loading of the ligands. The graphical interpretations
of all the possible C·21n complexes (C = 10 or 15; n = 1–2)
in solution as a function of the concentration of 21 revealed
that the C·212 complexes prevail over the C·211 aggregates
even at low concentrations. The formation of the C·L2 as-
semblies involving the more bulky third-generation dendron
23 were found to be much less pronounced, and the percen-
tage of the C·232 complex reached only 45–70%, whereas
the concentration of the C·212 species varied between 95
and 100%. The 1:2 stoichiometry of the supramolecular
first- to third-generation aggregates was confirmed by Job’s
Experimental Section
General Remarks: Commercially available chemicals were pur-
chased from Aldrich, Fluka, Sigma, or Acros Organics. Com-
pounds 1, 2, 3, 11, and 17 were synthesized according to literature
procedures.[19–21,23,25] The preparation of chiral depsipeptide den-
drons 21–23 and the fullerene derivative 20 was described previous-
ly.[10a,16] Solvents were dried by using standard techniques, anhy-
drous DMF was obtained from Acros. HPLC-grade solvents were
purchased from Acros Organics or SDS. HPLC-grade CHCl3 was
freshly distilled from potassium carbonate (K2CO3) prior to use, to
avoid protonation of the porphyrins 10 and 15. For the same
reason, CDCl3 was stirred with K2CO3 and filtered prior to use.
Reactions were monitored by thin-layer chromatography (TLC)
using Riedel-de-Haën silica gel 60 F254 aluminum foil, detection at
254 nm by UV lamp. 1H and 13C NMR spectra were recorded with
JEOL JNM EX 400, JEOL JNM GX 400, JEOL A 500, Bruker
Avance 300, or Bruker Avance 400 spectrometers. The chemical
shifts are given in ppm relative to tetramethylsilane (TMS) or to
the solvent peak as a standard reference. The resonance multiplici-
ties are indicated as: s (singlet), d (doublet), t (triplet), q (quartet),
and m (multiplet), unresolved signals as broad (br.) or very broad
(v. br.). Mass spectra were measured with a Micromass Lab Spec
(FAB) using a Finnigan MAT 900 spectrometer with 3-nitrobenzyl
alcohol as a matrix. IR spectra were recorded with a Bruker Vector
22 instrument with an ATR/RFS 100/S detector using liquid or
powder substances. MALDI TOF mass spectra were measured
with a Shimadzu Axima Confidence spectrometer (Version 3.04,
Kratos Analytical) using 2,5-dihydroxybenzoic acid (DHTB), sina-
pinic acid (SIN), dithranol (DIT) or trans-2-[3-(4-tert-butylphenyl)-
2-methyl-2-propynylidene]malononitrile (DCTB) as matrices. UV/
Vis spectroscopy was performed by using a Specord S 600 spectro-
photometer. Elementary analyses were performed by combustion
and gas chromatographic analysis with an EA 1110 CHNS ana-
lyzer (CEInstruments). Products were isolated by flash column
chromatography (FC) (silica gel 60, particle size 0.04–0.063 nm,
Merck).
1
plot analysis on the collected data from the H NMR ti-
tration experiments. As a complement, supramolecular ag-
gregates consisting of the Hamilton receptor functionalized
Zn–porphyrins 10 or 15 with fullerene derivatives 20 or 24
as ligands were subjected to a series of photophysical in-
vestigations. Fluorescence titrations of solutions of Zn–por-
phyrin 10 (in CH2Cl2 or ortho-dichlorobenzene) with vari-
Synthesis of Compound 7: TFA (0.20 mL, 3.00 mmol) was added to
a solution of 6 (759.00 mg, 3.75 mmol), 5 (623.00 mg, 3.75 mmol)
able concentrations of fullerene derivative 24 allowed the and pyrrole 4 (0.50 mL, 7.50 mmol) in CH2Cl2 (150 mL). The reac-
Eur. J. Org. Chem. 2010, 5010–5029
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5025

D. M. Guldi, A. Hirsch et al.
(3400 Lmol–1 cm–1) nm. IR (ATR): νmax = 3287, 3004, 2970, 2360,
FULL PAPER
tion mixture was stirred at room temp. for 1 h. After neutralization
˜
with NEt3 (0.55 mL, 3.95 mmol), DDQ (1.28 g, 5.63 mmol) was 2341, 1738, 1591, 1525, 1494, 1455, 1421, 1366, 1348, 1229, 1216,
added, and the solution was stirred at room temp. for a further 2 h.
The crude reaction mixture was concentrated and filtered through
SiO2 by using CH2Cl2/EtOAc (9:1) as eluent. The desired porphyrin
compound was separated by repeated column chromatography
[SiO2; hexane/EtOAc (8:2), Rf = 0.42]. Yield: 144.00 mg (9%);
dark-violet powder. 1H NMR (300 MHz, CDCl3, 25 °C): δ = –2.78
(br., 2 H, NH), 0.43 (s, 18 H, CH3), 4.00 (12 H, OCH3), 6.95 [t,
1203, 1154, 1065, 1001, 950, 933, 858, 811, 797, 767, 745, 719 cm–1.
Synthesis of Compound 10: Compound 3 (73.60 mg, 110.00 µmol)
was dissolved in anhydrous THF/NEt3 (2:1, 12 mL). Pd2(dba)3
(77.00 mg, 84.10 µmol), AsPh3 (129.00 mg, 421.00 µmol) and 9
(30.00 mg, 36.60 µmol) were added, and the reaction was allowed
to proceed in the dark under inert conditions for 6 d. The solution
was concentrated to dryness and purified by repeated column
chromatography [SiO2; CH2Cl2 Ǟ CH2Cl2/EtOAc (1:1), CH2Cl2/
MeOH (99:1 Ǟ 98:2); Rf = 0.18 (CH2Cl2/EtOAc, 98:2)]. The dark-
red powder was dissolved in CH2Cl2 and precipitated with n-pen-
tane. Yield: 27.00 mg (38%); dark-violet powder. 1H NMR
(400 MHz, [D8]THF, 25 °C): δ = 1.10 (s, 36 H, CH3), 2.58 (s, 8 H,
CH2), 3.95 (s, 12 H, OCH3), 6.93 (t, 3JH,H = 2.1 Hz, 2 H, Bn), 7.39
(d, 3JH,H = 2.4 Hz, 4 H, Bn), 7.76 (t, 3JH,H = 8.1 Hz, 4 H, Py), 8.07
3
3JH,H = 2.1 Hz, 2 H, Bn], 7.44 (d, JH,H = 2.1 Hz, 4 H, Bn), 7.91
3
3
(d, JH,H = 8.1 Hz, 4 H, Bn), 8.21 (d, JH,H = 8.1 Hz, 4 H, Bn),
8.87 (br., 4 H, pyrrole), 9.00 (br., 4 H, pyrrole) ppm. 13C NMR
(75 MHz, CDCl3, 25 °C): δ = 0.49 (CH3), 56.04 (OCH3), 96.01
(CϵC), 100.55 (Bn), 105.39 (CϵC), 114.26, 119.77, 120.46, 123.04,
130.78, 134.79, 142.81, 144.30, 159.27 (Bn) ppm. C58H54N4O4Si2
(927.26): calcd. C 75.13, H 5.87, N 6.04; found C 72.29, H 4.80, N
5.83. MS (FAB): m/z = 927 [M]+. UV/Vis (CH2Cl2): λ (ε) = 422
(377800), 516 (16900), 552 (7400), 590 (5300), 646
3
(overl. m, 12 H, Bn, Py), 8.30 (d, JH,H = 8.1 Hz, 4 H, Py), 8.45
3
(d, JH,H = 1.8 Hz, 4 H, Bn), 8.55 (br.,2 H, Bn), 8.88–9.01 (overl.
(3900 Lmol–1 cm–1) nm. IR (ATR): νmax = 3002, 2970, 2360, 2342,
˜
m, 8 H, pyrrole), 9.08 (br., 4 H, NH), 9.77 (br., 4 H, NH) ppm.
13C NMR (100.5 MHz, CDCl3, 25 °C): δ = 29.99 (CH3), 31.64
(CCH3), 50.71 (CH2), 55.62 (OCH3), 89.47, 91.67 (C=C), 98.63,
100.14 (Bn), 110.21, 110.42 (Py), 114.65, 120.30, 121.73, 122.55,
124.96, 127.53, 130.48 (Bn), 131.93, 132.38 (pyrrole), 134.29,
135.52, 136.80, 140.57, 145.29, 145.87 (Bn), 150.36, 150.60, 150.82,
151.01 (pyrrole), 151.23, 151.76, 159.86 (Bn), 164.88, 170.75 (C=O)
ppm. C112H104N16O12Zn·1.5CH2Cl2 (2058.91): calcd. C 66.21, H
5.24, Cl 5.17, N 10.86, O 9.32, Zn 3.18; found C 66.44, H 4.80, N
10.32. MS (FAB): m/z = 1932 [M]+. UV/Vis (CH2Cl2): λ (ε) = 303
(127700), 424 (590700), 550 (26500), 589 (6400 Lmol–1 cm–1) nm.
2157, 1738, 1591, 1558, 1499, 1456, 1421, 1397, 1356, 1249, 1228,
1217, 1204, 1155, 1064, 1021, 974, 930, 859, 800, 762, 735 cm–1.
Synthesis of Compound 8: Zn(OAc)2 dihydrate (72.00 mg,
330.00 µmol) was added to a solution of 7 (60.00 mg, 66.60 µmol)
in THF (15 mL). The reaction was mixture was heated at reflux for
90 min. The solution was concentrated to dryness and purified by
column chromatography (SiO2; CH2Cl2, Rf
= 0.85). Yield:
79.00 mg (81%); pink powder. 1H NMR (300 MHz, CDCl3, 25 °C):
3
δ = 0.39 (s, 18 H, CH3), 3.91 (s, 12 H, OCH3), 6.84 (t, JH,H
=
3
3
2.1 Hz, 2 H, Bn), 7.36 (d, JH,H = 2.4 Hz, 4 H, Bn), 7.87 (d, JH,H
3
= 8.1 Hz, 4 H, Bn), 8.16 (d, JH,H = 7.8 Hz, 4 H, Bn), 8.87 (m, 4
IR (ATR): ν
= 3003, 2970, 2360, 2341, 1738, 1586, 1522, 1507,
˜
max
H, pyrrole), 9.00 (m, 4 H, pyrrole) ppm. 13C NMR (75 MHz,
CDCl3, 25 °C): δ = 0.07 (CH3), 55.58 (OCH3), 95.38 (CϵC), 99.97
(Bn), 105.07 (CϵC), 113.69, 120.30, 120.96, 122.31, 130.21 (Bn),
131.70, 131.79, 132.10, 132.20 (pyrrole), 134.24, 143.09, 144.53
(Bn), 149.75, 149.86, 149.98, 150.09 (pyrrole), 158.64 (Bn) ppm.
C58H52N4O4Si2Zn·0.25CH2Cl2 (1011.58): calcd. C 69.14, H 5.23,
N 5.54; found C 69.17, H 5.56, N, 5.08. MS (FAB): m/z = 990
[M]+. UV/Vis (CH2Cl2): λ (ε) = 423 (513500), 549 (20400), 588
1446, 1366, 1352, 1298, 1229, 1217, 1204, 1154, 1066, 999, 950,
904, 858, 797, 752, 719 cm–1.
Synthesis of Compound 12: TFA (1.00 mL, 13.10 mmol) was added
to a solution of 11 (3.00 g, 8.19 mmol) and 4-[2-(trimethylsilyl)eth-
ynyl]benzaldehyde (6; 1.66 g, 8.19 mmol) in CH2Cl2 (655 mL). The
reaction mixture was stirred at room temp. for 1 h. After neutral-
ization with NEt3 (2.40 mL, 17.03 mmol), DDQ (2.79 g,
12.28 mmol) was added, and the solution was stirred at room temp.
for a further 2 h. The crude reaction mixture was concentrated and
filtered through SiO2 by using CH2Cl2/EtOAc (95:5) as eluent. The
desired porphyrin isomer was separated by repeated column
chromatography (SiO2; CH2Cl2, Rf = 0.22). Yield: 448.00 mg
(10%); violet powder. 1H NMR (400 MHz, CDCl3, 25 °C): δ =
–2.67 (br., 2 H, pyrrole NH), 0.39 (s, 18 H, SiCH3), 1.65 (s, 18 H,
CH3), 2.78 (s, 12 H, OCH3), 3.93 (s, 8 H, CH2), 7.89 (m, 8 H, Bn),
(2900 Lmol–1 cm–1) nm. IR (ATR): νmax = 3002, 2970, 2360, 2342,
˜
2157, 1739, 1592, 1524, 1494, 1455, 1421, 1366, 1350, 1249, 1228,
1217, 1204, 1154, 1065, 1001, 950, 932, 859, 843, 811, 797, 760,
719 cm–1.
Synthesis of Compound 9: A solution of TBAF in THF (1 ,
0.40 mL) was added dropwise to a solution of compound 8
(66.00 mg, 68.40 µmol) in anhydrous THF (5 mL). The reaction
mixture was stirred under inert conditions at room temp. for 12 h.
The solution was concentrated to dryness, the residue was dissolved
in CH2Cl2 (15 mL) and extracted with aq. NaHCO3 (5%;
2ϫ10 mL). The organic layer was separated, washed with water
(2ϫ10 mL) and dried with Na2SO4. The solution was filtered, con-
centrated to dryness and purified by column chromatography
(SiO2; CH2Cl2, Rf = 0.66). Yield: 62.00 mg (93%); pink powder. 1H
NMR (300 MHz, CDCl3, 25 °C): δ = 3.32 (s, 2 H, CϵH), 3.91 (s,
3
3
8.17 (d, JH,H = 8.4 Hz, 4 H, Bn), 8.70 (d, JH,H = 4.8 Hz, 4 H,
pyrrole), 8.78 (d, JH,H = 4.8 Hz, 4 H, pyrrole) ppm. 13C NMR
3
(100.5 MHz, CDCl3, 25 °C): δ = 0.47 (SiCH3), 32.12 (CH3), 35.65
(CCH3), 58.51 (OCH3), 73.45 (CH2), 96.11, 105.34 (CϵC), 115.79,
119.56, 122.87, 123.08, 130.84, 134.87, 135.39, 139.93, 142.40,
152.46 (Bn) ppm. C70H78N4O4Si2·1.5CH2Cl2 (1166.86): calcd. C
69.48, H 6.31, Cl 9.12, N 4.80, O 5.48, Si 4.81; found C 69.51, H
7.40, N 4.65. MS (FAB): m/z = 1095 [M]+. UV/Vis (CH2Cl2): λ (ε)
3
12 H, OCH3), 6.85 (br., 2 H, Bn), 7.37 (d, JH,H = 2.1 Hz, 4 H,
=
422 (394500), 517 (17300), 552 (7300), 592 (5300), 647
3
3
Bn), 7.88 (d, JH,H = 8.1 Hz, 4 H, Bn), 8.18 (d, JH,H = 8.1 Hz, 4
H, Bn), 8.92 (m, 4 H, pyrrole), 9.03 (m, 4 H, pyrrole) ppm. 13C
NMR (75 MHz, CDCl3, 25 °C): δ = 55.58 (OCH3), 87.16 (CϵCH),
83.72 (CϵC), 99.95, 113.73, 120.09, 120.98, 121.32, 130.36 (Bn),
131.68, 131.77, 132.11, 132.21 (pyrrole), 134.31, 143.47, 144.55
(3800 Lmol–1 cm–1) nm. IR (ATR): νmax = 2970, 2360, 2342, 2157,
˜
1738, 1474, 1457, 1365, 1251, 1228, 1217, 1205, 1103, 1022, 981,
967, 862, 847, 802, 761, 741, 714 cm–1.
Synthesis of Compound 13: Zn(OAc)2 dihydrate (200.00 mg,
(Bn), 149.70, 149.83, 149.97, 150.11 (pyrrole), 158.62 (Bn) ppm. 0.91 mmol) was suspended in MeOH (2 mL) and added to a solu-
C52H36N4O4Zn·1.5C6H14 (975.53): calcd. C 73.80, H 4.29, N 6.62,
O 7.20, Zn 7.35; found C 74.89, H 5.95, N 5.51. MS (FAB): m/z =
tion of 12 (200.00 mg, 0.18 mmol) in THF (60 mL). The reaction
mixture was heated at reflux for 12 h. The solution was concen-
844 [M]+. UV/Vis (CH2Cl2): λ (ε) = 422 (502900), 549 (20900), 588 trated to dryness and purified by column chromatography (SiO2,
5026
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 5010–5029

Self-Assembling Depsipeptide Dendrimers and Dendritic Fullerenes
CH2Cl2/EtOAc, 9:1). Yield: 175.00 mg (83%); pink powder. 1H
NMR (400 MHz, CDCl3/[D5]pyridine, 25 °C): δ = 0.38 (s, 18 H,
SiCH3), 1.64 (s, 18 H, CH3), 2.71 (s, 12 H, OCH3), 3.88 (s, 8 H,
[M]+. C124H128N16O12Zn·2CH2Cl2 (2213.59): calcd. C 66.20, H
5.65, Cl 6.41, N 10.12, O 8.67, Zn 2.95; found C 66.74, H 5.87, N
10.04. UV/Vis (CH2Cl2): λ (ε) = 303 (103400), 424 (432700), 552
3
CH2), 7.84 (m, 8 H, Bn), 8.17 (d, JH,H = 8.0 Hz, 4 H, Bn), 8.66
(22800), 591 (5700 Lmol–1 cm–1) nm. IR (ATR): νmax = 2970, 2360,
˜
3
3
(d, JH,H = 4.8 Hz, 4 H, pyrrole), 8.76 (d, JH,H = 4.8 Hz, 4 H,
pyrrole) ppm. 13C NMR (100.5 MHz, CDCl3/[D5]pyridine, 25 °C):
δ = 0.50 (SiCH3), 32.16 (CH3), 35.59 (CCH3), 58.33 (OCH3), 73.52
(CH2), 95.58, 105.68 (CϵC), 115.90, 119.81, 122.29, 122.49, 130.41
(Bn), 130.41, 131.32 (8 C, pyrrole), 132.28, 134.97, 136.83, 139.71,
144.07 (Bn), 150.16, 150.23 (pyrrole), 151.77 (Bn) ppm.
C70H76N4O4Si2Zn·0.5H2O (1167.95): calcd. C 71.30, H 6.26, N
5.04, O 6.48, Zn 5.88; found C 71.60, H 6.55, N 5.32. MS (FAB):
m/z = 1158 [M+]. UV/Vis (CH2Cl2): λ (ε) = 423 (482500), 551
2342, 1738, 1586, 1523, 1447, 1366, 1299, 1230, 1217, 1205, 1154,
1118, 996, 889, 797, 720 cm–1.
Synthesis of Compound 16: Nickel(II) acetylacetonate (29.31 mg,
114.10 µmol) was added to a solution of porphyrin 12 (25.00 mg,
22.82 µmol) in toluene (20.0 mL), and the reaction mixture was
heated at reflux for 12 h. The solution was concentrated to dryness
and purified by column chromatography [SiO2; CH2Cl2/hexane
(8:2); Rf = 0.46]. Yield: 21.0 mg (80%); cherry-red powder. 1H
NMR (400 MHz, CDCl3, 25 °C): δ = 0.37 (s, 18 H, CH3), 1.61 (s,
18 H, CH3), 2.86 (s, 12 H, CH3), 3.90 (s, 8 H, CH2), 7.83 (m, 8 H,
(21800), 589 (3200 Lmol–1 cm–1) nm. IR (ATR): νmax = 2970, 2360,
˜
2157, 1738, 1460, 1365, 1248, 1228, 1217, 1203, 1115, 996, 967,
3
3
Bn), 8.02 (d, JH,H = 8.4 Hz, 4 H, Bn), 8.61 (d, JH,H = 5.2 Hz, 4
H, pyrrole), 8.70 (d, 3JH,H = 5.2 Hz, 4 H, pyrrole) ppm. 13C NMR
(100.5 MHz, CDCl3, 25 °C): δ = 0.46, 32.09 (CH3), 35.60 (CCH3),
58.58 (CH3), 73.29 (CH2), 96.00, 105.30 (CϵC), 115.07, 118.72,
122.93, 123.12, 130.95 (Bn), 132.14, 123.69 (pyrrole), 134.03,
134.30, 139.53, 141.54, 143.16, 143.17, 152.40 (pyrrole, Bn) ppm.
MALDI TOF/MS: m/z = 1150 [M]+. C70H76N4NiO4Si2·1.5CH2Cl2
(1276.41): calcd. C 67.11, H 6.22, Cl 8.31, N 4.38, Ni 4.59, O 5.00,
Si 4.39; found C 66.69, H 6.19, N 4.15. UV/Vis (CH2Cl2): λ (ε) =
858, 843, 797, 758, 721 cm–1.
Synthesis of Compound 14: A solution of TBAF in THF (1 ,
0.86 mL) was added dropwise to a solution of 13 in anhydrous
THF (10 mL). The reaction mixture was stirred under inert condi-
tions at room temp. for 12 h. The solution was concentrated to
dryness, and the residue was dissolved in CH2Cl2 (30 mL) and ex-
tracted with aq. NaHCO3 (5%, 2ϫ30 mL). The organic layer was
separated, washed with water (2ϫ30 mL) and dried with Na2SO4.
The solution was filtered, concentrated to dryness and purified by
column chromatography [SiO2; CH2Cl2/EtOAc (97:3); Rf = 0.52].
Yield: 115.00 mg (79%); pink powder. 1H NMR (400 MHz, CDCl3/
[D5]pyridine, 25 °C): δ = 1.63 (s, 18 H, CH3), 2.71 (s, 12 H, OCH3),
3.33 (s, 2 H, CϵH), 3.88 (s, 8 H, CH2), 7.84 (m, 8 H, Bn), 8.17 (d,
417 (264200), 528 (20900 Lmol–1 cm–1) nm. IR (ATR): νmax = 2961,
˜
2924, 2822, 2159, 1605, 1499, 1393, 1351, 1290, 1250, 1220, 1194,
1102, 1003, 952, 930, 863, 843, 815, 803, 761, 716 cm–1.
Synthesis of Compound 19: (4-Iodophenyl)isocyanuric acid (17;
150.00 mg, 0.45 mmol), Pd(PPh3)2Cl2 (3.00 mg, 5.00 µmol) and
CuI (3.00 mg, 15.00 µmol) were dissolved in THF (10 mL). To this
solution, NEt3 (4 mL) and 4-ethynylbenzaldehyde (18; 70.00 mg,
0.54 mmol) were added, and the reaction mixture was stirred at
room temp. for 16 h. The precipitate was filtered off, the solvent
was evaporated, and the crude product was purified by column
chromatography [SiO2; CH2Cl2 Ǟ CH2Cl2/THF (1:1)]. Yield:
3
3JH,H = 8.0 Hz, 4 H, Bn), 8.67 (d, JH,H = 4.4 Hz, 4 H, pyrrole),
8.77 (d, 3JH,H = 4.4 Hz, 4 H, pyrrole) ppm. 13C NMR (100.5 MHz,
CDCl3/[D5]pyridine, 25 °C): δ = 32.16 (CH3), 35.59 (CCH3), 58.31
(OCH3), 73.52 (CH2), 78.47 (CϵC), 84.30 (CϵCH), 115.96,
119.69, 121.37, 122.52, 130.56 (Bn), 131.37, 132.30 (pyrrole),
135.01, 136.10, 136.84, 139.72, 144.36 (Bn), 150.13, 150.27
(pyrrole), 151.80 (Bn) ppm. MS (FAB): m/z
=
1012 [M]+.
1
110.00 mg (73%). H NMR (400 MHz, [D8]THF, 25 °C): δ = 7.17
C64H60N4O4Zn·0.5H2O (1023.59): calcd. C 74.49, H 5.52, N 5.79,
O 7.44, Zn 6.76; found C 74.08, H 5.88, N 5.16. UV/Vis (CH2Cl2):
λ (ε) = 423 (636200), 551 (20000), 588 (16600 Lmol–1 cm–1) nm. IR
3
3
(d, JH,H = 7.02 Hz, 2 H, Bn), 7.45 (d, JH,H = 8.5 Hz, 2 H, Bn),
3
3
7.80 (d, JH,H = 8.7 Hz, 2 H, Bn), 7.98 (d, JH,H = 6.7 Hz, 2 H,
Bn), 10.93 (s, 1 H, HC=O), 11.59 (br., 2 H, NH) ppm. 13C NMR
(100.5 MHz, [D8]THF, 25 °C): δ = 87.54, 90.56 (CϵC), 120.16,
126.26, 130.14, 130.38, 130.51, 133.43, 134.03, 136.06, 147.95,
(ATR): ν
= 3003, 2970, 2360, 2341, 1738, 1456, 1365, 1229,
˜
max
1217, 1205, 1102, 1065, 998, 908, 809, 797, 720 cm–1.
Synthesis of Compound 15: Compound 3 (87.20 mg, 130.04 µmol)
was dissolved in anhydrous THF/NEt3 (2:1, 17.5 mL). Pd2(dba)3
(114.20 mg, 124.71 µmol), AsPh3 (191.40 mg, 625.02 µmol) and 14
(55.00 mg, 542.10 µmol) were added to this solution, and the reac-
tion was allowed to proceed in the dark under inert conditions
for 6 d. The solution was concentrated to dryness and purified by
repeated column chromatography [SiO2; CH2Cl2 Ǟ CH2Cl2/
147.99 (Bn), 192.78 (HC=O) ppm. IR (ATR): ν
= 3406, 2361,
˜
max
1765, 1714, 1294, 1177, 1152, 1025, 1004, 821, 763, 733 cm–1.
Synthesis of Compound 20: Compound 19 (50.00 mg, 0.15 mmol)
was dissolved in THF (10 mL) before a solution of C60 (152.00 mg,
0.22 mmol) in toluene (300 mL) was added. After stirring under
the exclusion of light for 30 min, sarcosine (18.00 mg, 0.22 mmol)
MeOH (99:1) Ǟ CH2Cl2/MeOH (98:2) Ǟ CH2Cl2/MeOH (98:5)]. was added, and the reaction mixture was heated at 110 °C under
The dark-red substance was dissolved in CH2Cl2 and precipitated
the exclusion of air and light for 22 h. The monoadduct was sepa-
rated from C60 and higher adducts by column chromatography
with n-pentane. Yield: 35.00 mg (31%); violet powder. 1H NMR
(400 MHz, [D8]THF, 25 °C): δ = 1.10 (s, 36 H, CH3), 1.65 (s, 18 [SiO2; toluene Ǟ toluene/EtOAc (1:1)]. The resulting brown solid
H, CH3), 2.29 (s, 8 H, CH2), 2.78 (s, 12 H, OCH3), 3.95 (s, 8 H, was dissolved in CS2 and precipitated with n-pentane to remove
3
CH2), 7.77 (t, JH,H = 8.0 Hz, 4 H, Py), 7.94 (s, 4 H, Bn), 8.06
solvent residues. Yield: 46 mg (29%). 1H NMR (400 MHz, CS2/
3
(overl. d, 20 H, Bn, Py), 8.29 (d, JH,H = 7.9 Hz, 4 H, Bn), 8.45 (s,
4 H, Bn), 8.57 (s, 2 H, Bn), 8.69 (d, JH,H = 4.5 Hz, 4 H, pyrrole),
[D8]THF, 25 °C): δ = 4.77 (d, 2JH,H = 9.5 Hz, 1 H, NCH2), 2.94 (s,
3
2
3 H, CH3), 4.97 (s, 1 H, NCH), 4.98 (d, JH,H = 9.5 Hz, 1 H,
3
3
8.84 (d, JH,H = 4.5 Hz, 4 H, pyrrole), 9.09 (br., 4 H, NH), 9.79
NCH2), 7.12 (d, 3JH,H = 8.6 Hz, 2 H, Bn), 7.46 (d, JH,H = 8.8 Hz,
(br., 2 H, NH) ppm. 13C NMR (100.5 MHz, [D8]THF, 25 °C): δ =
2 H, Bn), 7.54 (d, JH,H = 8.55 Hz, 2 H, Bn), 7.77 (d, JH,H =
3
3
30.00 (CH3), 31.63 (CCH3), 31.97 (CH3), 35.70 (CCH3), 50.71 6.7 Hz, 2 H, Bn), 10.51 (s, 2 H, NH) ppm. 13C NMR (100.5 MHz,
(CH2), 57.94 (OCH3), 73.84 (CH2), 89.51, 91.97 (CϵC), 110.20, CS2/[D8]THF, 25 °C): δ = 62.01 (CH3), 69.10, 70.40 (sp3C C60),
110.41 (Py), 116.52, 120.28, 122.54, 122.73, 124.95, 127.54, 130.44
(Bn), 131.39, 132.39 (pyrrole), 134.27, 135.68, 136.80 (Bn), 140.57
(Py), 145.03, 150.58, 150.67, 151.25 (Bn, Py), 151.66, 151.78 (pyr-
role), 164.88, 170.77 (C=O) ppm. MALDI TOF/MS: m/z = 2736
78.16 (NCH), 90.12, 90.57 (CϵC), 123.50, 123.67, 129.08, 129.37,
131.80, 133.72, 135.66, 136.02, 136.47, 136.64 (Bn), 140.07, 140.28,
140.35, 141.34, 141.66, 141.75, 141.97, 142.17, 142.28, 142.69,
142.75, 143.13, 143.26, 144.45, 144.70, 144.73, 144.77, 144.84,
Eur. J. Org. Chem. 2010, 5010–5029
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5027

D. M. Guldi, A. Hirsch et al.
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Supporting Information (see footnote on the first page of this arti-
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Acknowledgments
We the thank the SFB 583 and the Cluster of Excellence “Engineer-
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Received: February 20, 2010
Published Online: July 26, 2010
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