Cooperativity and tunable excited state deactivation: Modular self-assembly of depsipeptide dendrons on a Hamilton receptor modified porphyrin platform

Article abstract of DOI:10.1021/ja8018065

A series of novel supramolecular architectures were built around a tin tetraphenyl porphyrin platform 6 - functionalized by a 2-fold 1-ethyl-3-3-(3-dimethylaminopropyl)carbodiimide (EDC) promoted condensation reaction - and chiral depsipeptide dendrons of different generations 1-4. Here, implementation of a Hamilton receptor provided the necessary means to keep the constituents together via strong hydrogen bonding. Characterization of all architectures has been performed, including 4 which is the fourth generation, on the basis of NMR and photophysical methods. In particular, several titration experiments were conducted suggesting positive cooperativity, an assessment that is based on association constants that tend to be higher for the second binding step than for the first step. Importantly, molecular modeling calculations reveal a significant deaggregation of the intermolecular network of 6 during the course of the first binding step. As a consequence, an improved accessibility of the second Hamilton receptor unit in 6 emerges and, in turn, facilitates the higher association constants. The features of the equilibrium, that is, the dynamic exchange of depsipeptide dendrons 1-4 with fullerene 5, was tested in photophysical reference experiments. These steady-state and time-resolved measurements showed the tunable excited-state deactivations of these complexes upon photoexcitation.

Full text of DOI:10.1021/ja8018065

Published on Web 06/07/2008
Cooperativity and Tunable Excited State Deactivation:
Modular Self-Assembly of Depsipeptide Dendrons on a
Hamilton Receptor Modified Porphyrin Platform
Jan-Frederik Gnichwitz,† Mateusz Wielopolski,‡ Kristine Hartnagel,† Uwe Hartnagel,†
Dirk M. Guldi,*,‡ and Andreas Hirsch*,†
Department of Chemistry and Pharmacy, and Interdisciplinary Center of Molecular Materials
(ICMM), Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Henkestrasse 42,
91054-Erlangen, Germany and Department of Chemistry and Pharmacy, and Interdisciplinary
Center of Molecular Materials (ICMM), Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg,
Egerlandstrasse 3, 91058-Erlangen, Germany
Received March 11, 2008; E-mail: dirk.guldi@chemie.uni-erlangen.de; andreas.hirsch@chemie.uni-erlangen.de
Abstract: A series of novel supramolecular architectures were built around a tin tetraphenyl porphyrin
platform 6sfunctionalized by a 2-fold 1-ethyl-3-3-(3-dimethylaminopropyl)carbodiimide (EDC) promoted
condensation reactionsand chiral depsipeptide dendrons of different generations 1s4. Here, implementation
of a Hamilton receptor provided the necessary means to keep the constituents together via strong hydrogen
bonding. Characterization of all architectures has been performed, including 4 which is the fourth generation,
on the basis of NMR and photophysical methods. In particular, several titration experiments were conducted
suggesting positive cooperativity, an assessment that is based on association constants that tend to be
higher for the second binding step than for the first step. Importantly, molecular modeling calculations
reveal a significant deaggregation of the intermolecular network of 6 during the course of the first binding
step. As a consequence, an improved accessibility of the second Hamilton receptor unit in 6 emerges and,
in turn, facilitates the higher association constants. The features of the equilibrium, that is, the dynamic
exchange of depsipeptide dendrons 1-4 with fullerene 5, was tested in photophysical reference experiments.
These steady-state and time-resolved measurements showed the tunable excited-state deactivations of
these complexes upon photoexcitation.
Introduction
Porphyrins and other closely related tetrapyrrolic pigments
are widely spread in nature and play very important roles in
various biological processes. Owing to their macrocylic
molecular architectures, these molecules provide excellent
platforms for light-harvesting and electron/energy-transfer
performances.3 The implementation of these features into
practical applications requires, however, adapting a coherent
strategy, via molecular engineering, to reconstitute the natural
biological environment.
The unique redox and structural properties of C60 provide
valuable incentives for their use as electron accepting building
blocks.4,5 Remarkable is that when integrating C60, for example,
together with porphyrins into electron donor-acceptor arrays,
C60 commonly retains its characteristic low reorganization
energy in electron transfer reactions. Consequently, porphyrin-
C60 arrays are ideally suited to generate long-lived radical ion
pair states in high yields.6 Up to date, most porphyrin-C60
systems are, nevertheless, based on a covalent linkage. The role
Until now, nature provides the most efficient pathways for
solar energy conversion. For example, natural photoactive
centers transform solar light into useful chemical energy in
quantum yields close to unity. Generally speaking, these energy
conversion processes involve three primary photochemical
events: light absorption, excitation energy transduction, and
photoinduced electron or energy transfer, respectively.12 There-
fore, one of the most exciting topics in current research is to
further advance the comprehension and optimization of each
of these key steps, which are all essential for developing
photosynthetic systems that are capable of efficient solar-energy
conversion.
† Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg, Henkestrasse 42.
‡ Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3.
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Gnichwitz et al.
played by the linker is not just structural, since its chemical
nature governs the electronic communication between the
terminal units (i.e., porphyrin and C60). Another important
feature of the spacer is its modular composition, which allows
altering the separation without affecting the electronic nature
of the connection.
Noncovalent electron donor-acceptor nanohybrids based on
supramolecular interactions are rarely known.7 To this end, we
have published the synthesis of an electron-transfer system based
on noncovalent electrostatic interactions between C60 and a
porphyrin or cytochrome C complex. In the resulting coulomb
complexes, the interactions between, for instance, the dendritic
Figure 1. Schematic representation of the complementary hydrogen
bonding motif of a cyanuric acid derivative and a Hamilton receptor.
on electron- and energy-transfer processes, and (iii) high
solubility of the corresponding hybrid architectures.9
C60 oligocarboxylate and the octapyridinium porphyrin salt stem
Recently, we have introduced a series of first-, second-, and
third-generation depsipeptide dendrons, including the all-R
configured 1-3 that bear cyanuric acid moieties at their focal
points.10 Strong six-point hydrogen bonding (Ka ≈ 103-106
M-1) with, for example, complementary Hamilton receptors11
(Figure 1) attached to a variety of molecular platforms renders
these dendrons as excellent building blocks to achieve the facile
construction of new functional supramolecular architectures.10,12
These include self-assembled dendrimers involving porphyrins
as electroactive components, which, except for the dendritic
pocket receptors reported by Tsukube and co-workers,13 are the
first representatives of this hydrogen bonded class of com-
pounds.14 Covalent dendrimers with porphyrin cores, on the
other hand, have been extensively investigated as intriguing
mimics for globular heme proteins.15
from the oppositely charged head groups of the two building
blocks. This, in turn, provides sufficiently strong electronic
couplings to power intrahybrid charge-separation processes.8
With respect to photoinduced charge separation, this approach
provides benefits for the successful fabrication of photovoltaic
devices that perform efficiently on the basis of hierarchically
ordering the individual building blocks, namely, donors and
acceptors.
Among the many supramolecular binding motifs that are
available, hydrogen bonding is considered as the structurally
most potent one. We have already shown the enormous potential
that rests on the highly directional self-assembly of porphyrin-
C60 electron donor-acceptor hybrids via hydrogen bonding. In
fact, our approach ensures (i) fine-tuning of the complexation
The importance of noncovalent building principles in protein
chemistry is undisputed and inspired us to develop for the first
time a modular concept toward supramolecular heme-protein
models. Key features are chromophoric and nonchromophoric
depsidpeptide dendrons 1-5 as protein mimics, which are
interchangeable at the tin-tetraphenyl-porphyrin (SnP) platform
strength, (ii) regulation of the electronic coupling and its impact
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Modular Self-Assembly of Depsipeptide Dendrons
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Chart 1
(6) (Chart 1). The latter contains two axial Hamilton receptors.
This scenario allows investigating ligand dependent binding
cooperativity and successive fluorescence quenching via energy
transfer between porphyrins and fullerenes in the corresponding
1:2 complexes.
date within the context of constructing chiral self-assembled
dendrimers. The key synthetic step is the 2-fold peptide coupling
of a tartaric acid core to a third generation amine precursor
(Scheme 1).
In 6, two Hamilton receptor substituents are connected to the
two axial positions of SnIVP through single oxygen atoms
leading to octahedral coordination. As a consequence, a
comparatively rigid and a spatially tight arrangement of H-
bonded cyanurates and porphyrins are realized. The synthesis
of 6 was accomplished by 1-ethyl-3-3-(3-dimethylaminopro-
pyl)carbodiimide (EDC) promoted coupling of (OH)2SnIV
Results and Discussion
In the current work, we wish to introduce as a complement
to the series of recently synthesized depsipeptide dendrons 1-3
and 510 the fourth generation representative 4. The enantio-
merically pure dendron 4 contains 30 stereogenic centers with
all R-configuration and is the largest building block known to
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Scheme 1. Synthesis of the Fourth Generation Depsipeptide Dendron 4a
a (a) HCl, EtOAc, room temperature; (b) DCC, HOBt, NEt3, CH2Cl2, 0°C.
Scheme 2. Final Step in the Synthesis of the Porphyrin Platform 6
not known. It should be mentioned that the conditions of the
coupling reaction are extremely mild; in particular, no acid is
required. We are currently exploring the carbodiimide coupling
protocol for other metalloporphyrins carrying axial hydroxyl
ligands.
First, the self-assembly of 6 with depsipeptide ligands 1-5
to yield 1:2 complexes was investigated by NMR spectroscopic
assays.17 Although chloroform was used as a solvent, to promote
the self-assembly and to ensure the integrity of the hydrogen
1
bonding, the H NMR spectrum of 6 reveals comparatively
broad and unresolved peaks. Such a trend is indicative of
intermolecular aggregation instigated by hydrogen bonding, as
has been noted in other Hamilton receptor systems.10,12 How-
ever, upon successive addition of the dendritic cyanuric acid
derivatives 1-5, a remarkable sharpening of the signals is seen
which indicates the successful forming of discrete 6:Ln complex.
porphyrin 1016 with the new Hamilton receptor 11 containing
a hydroxyl group at the focal point and two peripheral neo-
pentyl substitutents to increase the overall solubility (Scheme
2). Most interestingly, this transformation of a hydroxo metal-
loporphyrin derivative is, to the best of our knowledge, so far
(16) Crossley, M. J.; Thordarson, P.; Wu, R. A.-S. J. Chem. Soc., Perkin
Trans. 1 2001, 2294.
(17) Chirality transfer from the depsipeptide dendrons to the porphyrins is
very weak and could not be determined by CD spectroscopy.
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Modular Self-Assembly of Depsipeptide Dendrons
A R T I C L E S
pronounced (Figure 3). The 6:L2 fraction does not become
predominant until 4 equiv or more of the ligands are added.
The Scatchard plots give rise to a convex shape and are
indicative of pronounced positive cooperativity during the
formation of the complexes (Figure 4).20 In contrast a concave
shape would indicate negative cooperativity, and a straight line
would suggest no cooperativity at all. To this end, the most
straightforward presentation of the cooperativity phenomena is
based on determining the occupancy r. In particular, the maxima
of the Scatchard plot (rmax) allow the determination of the Hill
coefficients (nH).21 The amount of cooperativity is directly
correlated to the value of nH. The largest nH values are found
for the third generation and the lowest nH for the first generation
system.
The observed association properties of the porphyrin 6 with
the dendritic ligands 1-4 can be explained when looking closely
at the intermolecular interactions of the porphyrin 6. In a
nonpolar solvent like toluene or chloroform the Hamilton
receptors of 6 tend to self-aggregate via hydrogen bonding. The
first binding step of a dendritic ligand leads to a weakening of
this intermolecular self-assembly, because the solubility of the
newly formed 6:L complex is improved by the bulky dendritic
unit. The higher solubility causes a higher accessibility for the
second Hamilton receptor which leads to a higher second
association constant. This behavior explains the observed
association phenomena such as the positive cooperativity of the
second binding step. In this case the highest positive cooper-
ativity was found for the third-generation dendrons. Very bulky
ligands, on the other hand, disfavor this subsequent binding step
because of their sterical demand. The most pronounced positive
cooperativity is achieved when these factors are well balanced.
Molecular modeling suggests for 6 a rigid arrangement with
an almost octahedral coordination around the tin atom leading
to an in-plane all-trans configuration of both receptor units
(Figure 5). Thus, initially both dendrimer binding sites are
considered to be equal.
Figure 2. 1H NMR titration plot of the chemical shifts of the NH1 and
NH2 protons as a function of the amount of added dendron 3.
Table 1. Association Constants, R-factor (R), rmax, and Hill
coefficients for the systems 6:1n, 6:2n, 6:3n, 6:4n, and 6:5n
log K1
[L mol-1
log K2
[L mol-1
]
R
rmax
nH
]
6:1n
6:2n
6:3n
6:4n
6:5n
4.45
3.87
3.46
3.76
3.73
7.89
7.54
7.21
6.95
7.46
(0.18
(0.19
(0.18
(0.50
(0.91
0.35
0.62
0.76
0.50
0.71
0.21
0.45
0.62
0.33
0.55
This observation agrees well with a characteristic low-field
shift10–12 of the NH1 and NH2 protons, which reflects the
successive binding of the complementary ligands 1-5 by 6. It
is important to note that establishing the equillibria necessitated
a 45 min induction period, in which the intermolecular self-
assembly of 6 is overcome and the dendritic ligands are bound.
The titration curves (Figure 2) are sigmoidal in shape with an
inflection point at 2 equiv of added dendron.
Rotational degrees of freedom of the sp3-carbon network in
2, 3, and 4 lead to many possible rotamers and a large
conformational space. In fact, the potential energy surface is
very complex exhibiting several local energy minima, which
are separated from the global energy minimum by very low
energy barriers. However, considerable π-π interactions be-
tween the phenyl end groups, as they have already been found
for 2 and 3 in the gas phase, enable a rather compact
arrangement of the dendrons with accessible cyanuric acid
binding sites. To assess the hydrogen bonding interactions in
the Hamilton receptor units, we have optimized the porphyrin
unit 6 in the presence of 2 lacking the phenyl rings. The resulting
geometries reflect the strength of the directional six-point
binding motif of the cyanuric acid moieties when interacting
with the Hamilton receptor units. In all cases, hydrogen-bonding
distances on the order of 2.0 to 2.2 Å (Figure 6) confirm
favorable interactions between the porphyrin unit and the
dendrimer binding site.
Implicit here is a positive cooperativity in the stepwise binding
process.18 The corresponding association constants of the
complexes were calculated with Chem-Equili and the Solov’ev
1
method by employing the H NMR titration data (Table 1).19
For our systems major differences are observed for the first
binding step. The smaller first and the bulky fourth generation
addend lead to the highest K1 values. In the case of the second
binding step, the association constants for the second and third
generation ligands are much higher compared to their first
binding step. Overall the values confirm those determined for
comparable systems.10–12
The preferred formation of 6:L2 (L ) 1-4) compared to the
corresponding 6:L complexes becomes also apparent upon
analyzing the distribution of the Hamilton receptor as free core
(HR) and as part of the complexes 6:Ln (n ) 1-2) as a function
of added dendron equivalents (Figure 3). For example, the
amount of 6:L2 (L ) 2, 3) complexes prevails already at low
concentrations of added dendrofullerene 5. After the addition
of 2 equiv of dendron 2 and 3, respectively, the amount of 6:L2
complexes ranges between 50 and 70%. On the other hand the
formation of 6:L2 complexes involving 1 and 4 is much less
Furthermore, we employed molecular mechanics calculations
to determine the global minimum and to explore the confor-
mational space of the complexes between the acceptor units
(18) Schneider, H.-J.; Yatsimirsky, A. Principles and Methods in Supramo-
lecular Chemistry; Wiley-VCH: Chichester, 2000.
(19) (a) Solov’ev, V. P.; Baulin, V. E.; Strahova, N. N.; Kazachenko, V. P.;
Belsky, V. K.; Varnek, A. A.; Volkova, T. A.; Wipff, G. J. Chem.
Soc., Perkin Trans. 2 1998, 1489. (b) Solov’ev, V. P.; Vnuk, E. A.;
Strakhova, N. N.; Reavsky, O. A.; VINITI, Moscow, 1991.
(20) (a) Badjic´, J. D.; Nelson, A.; Cantrill, S. J.; Turnbull, W. B.; Stoddart,
J. F. Acc. Chem. Res. 2005, 38, 723. (b) Ercolani, G. J. Am. Chem.
Soc. 2003, 125, 16097. (c) Perlmutter-Hayman, B. Acc. Chem. Res.
1986, 19, 90. (d) Scatchard, G. Ann. N.Y. Acad. Sci. 1949, 51, 660.
(21) (a) Hill, A. V. Biochem. J. 1913, 7, 471. (b) Hill, A. V. J. Physiol.
(London) 1910, 40, 4.
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Figure 3. Distribution in percent of the porphyrin as free core (HR) and within the complexes 6:Ln (n ) 1-2) as a function of the amount of added
dendrons 1-4.
(2, 3, 4) and the receptor (6). We investigated the three different
1:1 and 2:1 complexes of the cyanuric acid derivatives with 6.
We note the high flexibility of the sp3-carbon network in 2, 3,
and 4, and as a direct consequence many possible low-energy
conformations, with an energy difference of less than 2 kcal/
mol relative to each other, emerge. In order to preserve the
hydrogen-bonding interactions we have constrained the
structure of the binding site and carefully decreased the
constrain energy to maintain the binding properties of the
dendrimers and SnP 6.
All calculations revealed a fairly rigid arrangement between
the porphyrins and the H-bonded cyanurates. Also important is
that the porphyrin framework preserves its planarity throughout
the entire simulation and is independent of the size of the
dendrimer. In the 1:1 complexes rotational and translational
modes of the receptor units, which are involved in binding of
the cyanuric acid derivatives, are almost completely restrained,
whereas the free accessible binding sites exhibit considerable
flexibility during the entire simulation. On one hand, this implies
stable hydrogen bonding between the cyanurates and the
receptors in the 1:1 complexes. On the other hand, complexation
of only one ligand does not influence the conformation of the
second Hamilton receptor, namely all-cis in favor of all-trans.
Still a notable weakening of the intermolecular hydrogen bonds
is discernible during the complexation of the first ligand. This
trend is, however, dependent on the dendron size. In all 1:1
complexes significant π-π interactions between 6 and the first
ligand are observed with increasing dendron generation. In turn,
the ligand adopts a very compact conformation, in which the
phenyl units will interact with the porphyrin π-system due to
the closer contacts. As a consequence, enhanced shielding of
the porphyrin framework by the bulky ligands was noticed, and
hence, the unoccupied Hamilton receptor binding site becomes
more accessible toward a second ligand with increasing size of
the dendron. These interactions prevent a reaggregation via the
hydrogen bonds of 6. Moreover, steric hindrance forces the
second receptor into a cis-cis planar configuration, which (i)
makes it more accessible, (ii) is most favorable for ligand
binding, and (iii) propagates favorable π-π interactions between
the phenyl units of the receptor (Figure 7).
In summary, MD simulations show strong interactions
between the porphyrin framework and the dendritic ligands in
the 1:1 complexes. These interactions tend to increase with
increasing size of the ligand and facilitate the second binding
step due to deaggregation of the intermolecular hydrogen bond
network of 6. As a consequence, the second Hamilton receptor
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Modular Self-Assembly of Depsipeptide Dendrons
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Figure 4. Scatchard plots for the systems 6:L2 (L ) 1-4).
encounters significant steric hindrance during the second binding
step, which ratifies the unexpected decrease of the binding
constant (Figure 8).
Support for the dynamic character of the equilibrium, that
is, forming the 1:2 complexes of 6 and the depsipeptide ligands
1-5, came from temperature dependent NMR measurements.
Whereas at room temperature the NH protons of both species,
bound and free ligand, give rise to a broad signal at 11.5 ppm,
two signals appear upon cooling down to -30 °C at 9 and 13.5
ppm for the free and the bound dendrons, respectively.10
Porphyrin 6 has emerged as a versatile platform that enables,
through exchanging 4 and 5, the reversible interconversion
between 6:42 and 6:52. Qualitative support for such a dynamic
exchange came from titration assays, in which complex 6:42
was probed in the absence and presence of variable concentra-
tions of 5. Figure 9 documents that an incremental addition of
5 to 6:42 leads to a notable quenching of the fluorescence, which
was indeed fully reverted into the strongly fluorescing 6:42 upon
adding an excess of 4.
A series of photophysical measurements provided mechanistic
insights into the aforementioned excited-state interactions,
namely, the reversible replacement of 4 (i.e., 6:42) with 5 (i.e.,
6:52) and vice versa. The lack of considerably photoactivity and
redox activity of 4 evokes in 6:42 no appreciable trends. In fact,
the fluorescing features of 6:42 are virtually identical to those
Figure 5. AM1 optimized structure of 6.
becomes more accessible to the second ligand. A sterically more
demanding ligand prevents reaggregation of the 1:1 complex.
However, with increasing dendron generation, the positive
cooperativity tends to decrease. The second ligand in 6:42
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A R T I C L E S
Gnichwitz et al.
Figure 6. AM1 optimized structure of 6:2 representing the hydrogen-bonding interactions and the bond distance in Å.
Figure 7. Minimum-energy conformations of the 1:1 complexes 6:3 and 6:4 visualizing the flexibility of the sp3-carbon network of the ligands; the surface
represents the local-energy-minima conformational space.
seen for 6 in the absence of 4; see Figure S1. To name a few
photophysical features: fluorescence quantum yields and life-
times of 0.003 and 0.3 ns, respectively. Adding 5, however,
results in a substantial quenching of the porphyrin centered
fluorescence maxima at 610 and 665 nm. As illustrated in Figure
10, the quenching is exponential and depends exclusively on
the added concentration of 5, which points to the successful
formation of 6:52. From the underlying nonlinear relationship
we determined the association constants for binding 1 and,
subsequently, 2 equiv of 5. Reassuringly, both K values, log
K1 (i.e., 3.6 M-1) and log K2 (i.e., 6.9 M-1), are in excellent
agreement with those determined in the NMR titrations. When
reversing the titration, that is, adding variable concentrations
of 4 to 6:52, a gradual reactivation of the fluorescence is noted,
Figure S2. From the underlying fluorescence intensity versus 4
concentration relationship, log K1 (i.e., 2.3 M-1) and log K2
(i.e., 5.1 M-1) were derived.
Notably, at the end point of the titration (i.e., 6:52) the
porphyrin fluorescence is nearly quantitatively quenched. In-
stead, a fluorescence pattern (i.e., *0-0 emission around 700
nm) evolved that is reminiscent of that of 5.22 Please note that
this happens although the porphyrin component in 6:52 is
exclusively excited. To shed light onto the mechanism by which
the fullerene fluorescence is generated, an excitation spectrum
was taken. Importantly, the excitation spectrum is an exact match
of the ground-state absorption of the porphyrin with maxima at
(22) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695.
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Modular Self-Assembly of Depsipeptide Dendrons
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Figure 8. Structure of 6:32 and 6:42 resulting from a 50 ps quench molecular dynamics simulation in Vacuo using the COMPASS force field parameters
after prior conjugate-gradient minimizations; the surface represents the local-energy-minima conformational space.
spectral features of the resulting photoproducts and to determine
absolute rate constants for the intramolecular decay.
Up front, the excited-state properties of 6 shall be discussed,
since they emerge as important reference points for the
interpretation of the features expected in 6:42 and 6:52. Figure
11 illustrates that the differential spectrum recorded immediately
after the 420 nm excitation of 6 reveals the instantaneous (i.e.,
>1012 s-1) formation of the singlet excited state, for which the
following features have been gathered: maxima at 455 and 800
nm as well as minima at 560 and 603 nm. The triplet excited-
state features, on the other hand, include maxima at 500, 582,
and 640 nm.23 The interplay between both excited states is
documented by (i) the occurrence of an isosbestic point at 480
nm and (ii) the close resemblance of the decay and formation
of the singlet and triplet excited-state features, respectively (1.3
× 109 s-1).
Figure 9. (Upper part) Qualitative fluorescence quenching of 6 upon
variable additions of 5 to 6:42: (a) 0 equiv of 5, (b) 0.5 equiv of 5, (c) 1
For 6:42, the transient changes, spectroscopically and kineti-
cally, that develop are dominated in the visible and in the near-
infrared by features that correlate nicely with those seen for 6.
This, again, confirms the lack of photoactivity and redox activity
of 4.
Regarding the femtosecond transient absorption measurements
of 6:52 (Figure 12), immediately, after the laser excitation, the
strong singlet-singlet absorptions of 6 start to grow in with
about >1012 s-1, maxima at 455 and 800 nm as well as minima
at 560 and 603 nm. This confirms that, despite the presence of
5, the porphyrin singlet excited state is successfully formed.
Instead of seeing, however, the slow ISC dynamics the
equiv of 5, (d) 1.5 equiv of 5, (e) 2 equiv of 5, (f) 4 equiv of 5, (g) 6 equiv
of 5. (Lower part) Schematic representation of the ligand exchange reaction
between 6:42 and 6:52.
430, 560, and 600 nm (Figure S3). This leads us to hypothesize
that a thermodynamically driven transduction of singlet excited-
state energy from the porphyrin (1.98 eV) to the fullerene (1.79
eV) must govern the photoactivity of 6:52.
Conclusive information about the nature of the interactions
came from transient absorption measurements, femtosecond and
nanosecond, with 6 in the absence and presence of variable
concentrations of 4 and 5 following 420 nm excitation.
Following the time evolution of the characteristic singlet excited-
state features of 6, for instance, is a convenient mode to identify
(23) Complementary nanosecond experiments corroborated the formation
of the oxygen sensitive SnP triplet excited state (Figure S4).
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A R T I C L E S
Gnichwitz et al.
Figure 10. (Upper part) Room temperature fluorescence spectra of 6 (2.5
× 10-5 M) in the presence of variable concentrations of 5 (i.e., 0, 1.25 ×
10-5, 2.5 × 10-5, 3.75 × 10-5, 5.0 × 10-5, 6.25 × 10-5, 7.5 × 10-5
,
8.75 × 10-5, 1.0 × 10-4, 1.125 × 10-4, and 1.25 × 10-4 M) upon 410
nm excitation, after correction for competitive ground absorption. (Lower
part) I/I0 versus 5 relationship used to determine the association constant.
Figure 11. (Upper part) Differential absorption spectra (visible and near-
infrared) obtained upon femtosecond flash photolysis (420 nm) of 6 in
toluene with several time delays between 0 and 3000 ps at room temperature;
see color code for details. Arrows illustrate the changes. (Lower part)
Time-absorption profile of the spectra shown above at 457 nm (black
spectrum) and 495 nm (gray spectrum), reflecting the singlet excited-state
decay and triplet excited-state formation, respectively.
singlet-singlet absorptions decay in 6:52 with accelerated
dynamics. To this end we noted that with increasing time delay
the porphyrin singlet excited-state features change to a transient
that bears no resemblance with its triplet excited state at 497,
582, and 640 nm, the product of the intersystem crossing seen
in 6. Instead near-infrared features develop that reveal a
maximum at around 900 nm. Unequivocally, this signature has
been established as an attribute of the singlet excited state of
5.22 This finding supports formation of one transient species
(i.e., fullerene singlet excited state) at the expense of the other
transient species (i.e., porphyrin singlet excited state). Notably,
the porphyrin decay and the fullerene formation dynamics, 2.8
× 1010 s-1, resemble each other.
fullerene triplet excited state as observed at the end of the
femtosecond experiments. The spectral attributes of the fullerene
triplet excited state, precisely, two maxima located at 360 and
720 nm, were observed. Thus, our analysis clearly shows the
photosensitization effect of the porphyrin chromophores, acting
as an antenna system and transmitting its excited energy to the
noncovalently associated fullerene moieties.
For determining the fullerene triplet quantum yields in 6:52
we utilized the quantitative conversion of the triplet excited
states of the porphyrin (0.78) and fullerene (i.e., 0.98) fragments
into singlet oxygen. By analyzing the singlet oxygen emission
at 1275 nm (see Figure 13), we derived a quantum yield of
0.84.
Analysis of the newly formed fullerene singlet excited-state
features confirms the metastability in 6:52. The lifetime is 1.3
ns (7.7 × 108 s-1) identical to what has been derived for the
intersystem crossing in 5 to afford accordingly the triplet
manifold (ETriplet ) 1.5 eV). Moreover, the transient absorption
features exhibit at the conclusion of this decay a maximum at
720 nm, which is a known fingerprint for the triplet excited
state of C60.22,24
Conclusions
To summarize, we have successfully demonstrated the
hierarchical integration of a porphyrin core as well as chro-
mophoric and/or nonchromophoric dendrons through means of
hydrogen bonding. Insights into the nature of the interactions
came from diverse titration and photophysical experiments,
which revealed the dynamic character of the self-assembly in
the 1:1 and 1:2 complexes of porphyrin 6 and the dendrons.
In line with the modeling and calculations, titration and
photophysical experiments shed light onto the interactions,
In line with the proposed energy transfer mechanism the
differential absorption changes, recorded immediately after a 8
ns pulse with 6:52, showed the same spectral features of the
(24) Addition of variable amounts of the dendron, to yield 6:42, leads to
no notable changes.
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Modular Self-Assembly of Depsipeptide Dendrons
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Figure 13. Singlet oxygen emission, measured at 1275 nm, for 5 (red
spectrum), 6 (black spectrum), and 6:52 (orange spectrum) in toluene upon
355 nm excitation.
distance, orientation, and energy gap, to name a few important
variables. Cascades of energy transfer reactions, along well
devised energy gradients, are conceivable, when, for example,
different generations of dendrimers would be implemented.
Nevertheless, additional complexation experiments with, for
instance, branching units are needed to understand all the details
and facets of the self-assembly process. Important for this
understanding will be the development of a supramolecular
toolboxswith the presented 6:Ln complexes considered as one
of the simplest systems thereofsthat includes porphyrins,
fullerenes, dendrimers, and other functional molecules to
construct larger and more variable assemblies. An important
aspect will focus on surrounding the porphyrin core with a more
compact shell of higher generation dendrimers. Obviously, this
would impact ligand exchange reactions.
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft DFG (SFB 583: Redoxaktive Metallko-
mplexe - Reaktivita¨tssteuerung durch molekulare Architektur) and
by the Cluster of Excellence: Engineering of Advanced Materials.
Figure 12. (Upper part) Differential absorption spectra (visible and near-
infrared) obtained upon femtosecond flash photolysis (420 nm) of 6:52 in
toluene with several time delays between 0 and 3000 ps at room temperature;
see color code for details. (Lower part) Time-absorption profile of the
spectra shown above at 563, 582, 603, and 700 nm, reflecting the energy
transfer process.
Supporting Information Available: Photophysical data, con-
taining room temperature fluorescence spectra of 6 in the
presence of 4 and 5 (S1, S2), excitation spectrum of the 6:52
complex (S3), and differential absorption spectra of 6:42 (S4).
Experimental details, including synthesis, characterization, and
molecular modeling. This material is available free of charge
via the Internet at http://pubs.acs.org.
namely, the reversibility and the dynamic equilibrium of the
complexation behavior. Furthermore, the examination of excited-
state characteristics revealed significant energy transfer proper-
ties of these complexes upon photoexcitation. Implicit is a
mediation of singlet excited-state energy driven by noncovalent
hydrogen-bonding interactions. To this end, hydrogen bonding
interactions enable the facile exchange/variation of the binding
motifs/binding partner and, in turn, the flexible tuning of
JA8018065
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