Synergy of Electrostatic and π–π Interactions in the Realization of Nanoscale Artificial Photosynthetic Model Systems

Article abstract of DOI:10.1002/anie.202006014

In the scientific race to build up photoactive electron donor-acceptor systems with increasing efficiencies, little is known about the interplay of their building blocks when integrated into supramolecular nanoscale arrays, particularly in aqueous environments. Here, we describe an aqueous donor-acceptor ensemble whose emergence as a nanoscale material renders it remarkably stable and efficient. We have focused on a tetracationic zinc phthalocyanine (ZnPc) featuring pyrenes, which shows an unprecedented mode of aggregation, driven by subtle cooperation between electrostatic and π–π interactions. Our studies demonstrate monocrystalline growth in solution and a symmetry-breaking intermolecular charge transfer between adjacent ZnPcs upon photoexcitation. Immobilizing a negatively charged fullerene (C₆₀) as electron acceptor onto the monocrystalline ZnPc assemblies was found to enhance the overall stability, and to suppress the energy-wasting charge recombination found in the absence of C₆₀. Overall, the resulting artificial photosynthetic model system exhibits a high degree of preorganization, which facilitates efficient charge separation and subsequent charge transport.

Full text of DOI:10.1002/anie.202006014

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Synergy of Electrostatic and π-π Interactions in the Realization of
Nanoscale Artificial Photosynthetic Model Systems
Authors: Tomas Torres, Eduardo Anaya-Plaza, Jan Joseph, Stefan
Bauroth, Maximilian Wagner, Christian Dolle, Michael Sekita,
Franziska Gröhn, Erdmann Spiecker, Tim Clark, Andrés de la
Escosura, Dirk M. Guldi, and Tomás Torres
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202006014
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10.1002/anie.202006014
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RESEARCH ARTICLE
Synergy of Electrostatic and π-π Interactions in the Realization of
Nanoscale Artificial Photosynthetic Model Systems
Eduardo Anaya-Plaza, [a,b] ‡ Jan Joseph,[c] ‡ Stefan Bauroth,[c] Maximilian Wagner,[c] Christian Dolle,[c]
Michael Sekita,[c] Franziska Gröhn,[c] Erdmann Spiecker,[c] Tim Clark,[c] Andrés de la Escosura,*[a,d] Dirk
M. Guldi,*[c] Tomás Torres.*[a,d,e]
E. Anaya-Plaza wants to dedicate this paper to his first born daughter, Gaia.
[a]
Dr. E. Anaya-Plaza, Dr. A. de la Escosura, and Prof. T. Torres
Department of Organic Chemistry
Universidad Autónoma de Madrid (UAM)
c/ Francisco Tomás y Valiente 7, Cantoblanco, 28049 Madrid, Spain
E-mail: andres.delaescosura@uam.es; tomas.torres@uam.es
Dr. E. Anaya-Plaza
Department of Bioproducts and Biosystems
Aalto University
[b]
[c]
Kemistintie 1, 02150 Espoo, Finland
J. Joseph, Dr. S. Bauroth, M. Wagner, Dr. C. Dolle, Dr. M. Sekita, Prof. F. Gröhn, Prof. E. Spiecker, Prof. T. Clark, and Prof. D. M. Guldi
Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM)
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
91058 Erlangen, Germany
E-mail: dirk.guldi@fau.de
[d]
[e]
Dr. A. de la Escosura, and Prof. T. Torres
Institute for Advanced Research in Chemical Sciences (IAdChem)
Universidad Autónoma de Madrid (UAM)
28049 Madrid, Spain
Prof. T. Torres
IMDEA-Nanociencia
c/ Faraday 9, Campus de Cantoblanco, 28049 Madrid, Spain.
Supporting information for this article is given via a link at the end of the document.
Abstract: In the scientific race to build up photoactive electron donor-
acceptor systems with increasing efficiencies, little is known about the
interplay of their building blocks when integrated into supramolecular
nanoscale arrays, particularly in aqueous environments. Here, we
describe an aqueous donor-acceptor ensemble whose emergence as
a nanoscale material renders it remarkably stable and efficient. We
have focused on a tetracationic zinc phthalocyanine (ZnPc) featuring
pyrenes, which shows an unprecedented mode of aggregation, driven
by subtle cooperation between electrostatic and π-π interactions. Our
studies demonstrate monocrystalline growth in solution and a
symmetry-breaking intermolecular charge transfer between adjacent
ZnPcs upon photoexcitation. Immobilizing a negatively charged
fullerene (C60) as electron acceptor onto the monocrystalline ZnPc
assemblies was found to enhance the overall stability, and to
suppress the energy-wasting charge recombination found in the
absence of C60. Overall, the resulting artificial photosynthetic model
system exhibits a high degree of preorganization, which facilitates
efficient charge separation and subsequent charge transport.
A rather elegant and simple approach to modeling natural
antennae involves the use of either electron donors or acceptors
that feature high extinction coefficients throughout the visible
region of the solar spectrum.[8–11] Electron donor-acceptor
systems are equally important as models to mimic the events that
take place in the photosynthetic reaction center.[12–15] Both are of
utmost relevance for the modulation of solar energy conversion
and optoelectronic materials.
Phthalocyanines (Pc) and fullerenes (C60) stand out among
the many molecular building blocks employed in artificial
photosynthetic models. Pcs are thermally and chemically stable,
feature
a rich redox chemistry, and are tunable through
coordination of different metals and/or peripheral/axial
substitution.[16–20] Moreover, their intense optical absorption in the
red region of the solar spectrum renders them as excellent
antennae.
Such unique physicochemical properties have
provided the incentive to incorporate Pcs into a large number of
electron donor-acceptor systems using both covalent and non-
covalent methodologies.[21–26] The role of Pcs is to harvest light
efficiently and, once photoexcited, to act as electron donors. The
choice of C60 as electron acceptor, in turn, is mainly driven by the
fact that it exhibits small reorganization energies in charge-
transfer reactions.[27–29] Non-covalent approaches en route to
electron donor-acceptor systems is especially interesting:[30–34]
They enable the realization of thermodynamically reversible
assemblies constituted of different functional building blocks in a
straightforward manner. Overall, their stability is influenced
through the careful choice of the type of non-covalent interactions,
Introduction
Natural photosynthesis is a key, highly sophisticated biological
process that allows the conversion of light into chemical energy.[1]
The development of artificial photosynthetic systems attempts to
mimic photosynthesis at the molecular level and to understand
the interplay between fundamental processes such as light
harvesting, energy transfer, and charge transfer.[2–7]
1
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on the one hand, and their fine-tuning by external factors such as
temperature, solvent, etc., on the other.
quaternization of 7 with methyl iodide. In order to obtain a suitable
reference, ZnPc 2 was synthesized without the pyrenyl moieties
(see SI).[40]
The grand challenge in the field is to use aqueous media for
the design, synthesis and characterization of electron donor-
acceptor systems. A recent example is constituted by single-
walled carbon nanotubes (SWCNTs).[35–37] Solubility of artificial
photosynthetic systems in aqueous media is desirable, especially
to mimic their natural counterparts more realistically and with
respect to environmental issues.[37,38] To this end, a tetracationic
ZnPc bearing four peripheral pyrene moieties (1) was synthesized
and characterized (Figure 1). Synthetic design endows 1 with
different modes of aggregation, which can be finely tuned by the
amount of organic solvent in the aqueous medium. The
aggregation modes range from the monomeric state in DMSO, to
well-known H-type aggregation in 60:40 water/DMSO and, more
interestingly, an unprecedented mode of aggregation in 95:5
water/DMSO. This third mode of aggregation was further
characterized by electron microscopy and dynamic light
scattering, revealing crystals that form rod-like structures.
Importantly, a symmetry-breaking charge separation occurs in the
aggregates of 1, as a consequence of the greatly facilitated
transfer of electrons and holes between neighboring ZnPc
molecules. When these aggregates interact with either tetra- or
octaanionic water-soluble fullerenes (3 or 4), the resulting
unidirectional charge separation yields an appreciably longer-
lived charge-separated state.
The anionic C60 derivatives 3 and 4 were prepared by
adapting a previously described methodology, based on the
Bingel
reaction
between
tetraethyl
bromomethylenediphosphonate 8 and pristine C60 in different
stoichiometric ratios, yielding monoadduct 9 and bisadduct 10
(see SI, Scheme S2).[41] Bisadduct 10 was obtained as a mixture
of regioisomers, which could be purified by column
chromatography (with
a
gradient from chloroform to
chloroform/THF 1:1 as eluent) and fully characterized by 31P-NMR,
HPLC, and ESI-MS (Figure S1-S3, respectively). The electron-
accepting features of 9 and the different 10 regioisomers were
evaluated by cyclic voltammetry (CV) in an anhydrous o-DCB/1,4-
dioxane (4:1) mixture, employing TBAPF6 0.1 M as supporting
electrolyte, Pt electrode as working electrode, Pt wire as counter
electrode, and Ag/AgNO3 non-aqueous electrode as pseudo-
reference electrode. The voltammograms showed very similar
first reductions in all cases (Figure S4, Table S1), confirming that
the amount and distribution of bisphosphonate groups over the
C60 structure does not affect the electron accepting properties.
Therefore, the trans3 regioisomer was selected for further
experiments due to the higher yield obtained in its synthesis. In
order to render aqueous solubility, monoadduct 9 and bisadduct
10 were hydrolyzed in presence of trimethylsilyl iodide in CCl4,
yielding the phosphonic acid derivatives 3 and 4, respectively.
Aggregation studies. Despite carrying four positive charges, 1
shows no solubility in pure water, due to the hydrophobic nature
of the pyrenes in the ZnPc periphery. However, aqueous
dispersions can be achieved by injection of water-miscible
organic solutions of 1, such as in DMSO, showing remarkable
stability even below 1% of the organic solvent. Indeed, the
aggregation behavior of 1 depends strongly on the relative
water/DMSO ratio, as shown by steady-state absorption (Figure
2a) and fluorescence spectroscopies (Figure 2b). In particular,
the absorption spectrum of 1 in DMSO features a sharp Q-band
absorption centered at 678 nm, typical of molecularly dissolved
ZnPcs. Increasing the water content up to 60 vol% induces 1 to
aggregate (1aggA), as shown by the broadening of the Q-band and
appearance of a new band at 635 nm (Figure 2a, blue spectrum).
This behavior, typical of H-type aggregates,[42] is consistent with
the decrease of fluorescence observed in Figure 2b (blue
spectrum). Further increase of the water content to 99.8 vol%
results in the diminishment of the 635 nm aggregation band and
recovery of the Q-band narrowness (Figure 2a, red spectrum).
However, the fluorescence remains strongly quenched in this
case (Figure 2b, red spectrum), suggesting a new mode of
aggregation (1aggB). In stark contrast, references 2 and 7, lacking
the pyrene moieties and the positive charges, respectively, give
rise to only one form of aggregation, with features similar to 1aggA
(Figure S5). A clear trend is observed by plotting the ratio
between the absorption at 678 and 632 nm as a function of the
vol% of water. In contrast to ZnPc 2 (Figure 2c, grey solid dots),
which shows a single aggregation behavior, 1 (Figure 2c, black
solid dots) exhibits a decrease in ratio below 60 vol% water (1aggA),
followed by an increase at higher water contents (1aggB). The
latter aggregation state is, therefore, consequence of a delicate
interplay between the repulsive electrostatic forces between
pyridinium moieties and the attractive hydrophobic effects
induced by the pyrene units in aqueous media.
Figure 1. Chemical structures of tetracationic ZnPc 1, bearing four pyrene
moieties, and ZnPc reference 2. Chemical structures of electron acceptor tetra-
and octaanionic fullerenes 3 and 4, respectively. Iodide (ZnPc) and sodium (C60
)
counterions are omitted for clarity purposes.
Results and Discussion
Synthesis and characterization. ZnPc 1 was prepared as
shown in Scheme S1. Briefly, pyrene-1-boronic acid pinacol ester
was synthesized by dissolving pyrene-1-boronic acid in THF,
adding it to a pinacol solution in diethyl ether, and refluxing the
mixture for 2.5 h.[39] The resulting pyrene deriva-tive was
subsequently reacted with 5-bromo-3-hydroxypyridine via Suzuki
cross-coupling reaction in the presence of Pd(dppf)Cl2 as catalyst,
leading to the pyrenyl pyridyloxy derivative 5. Phthalonitrile 6 was
obtained by nucleophilic aromatic substitution of 4-
nitrophthalonitrile with 5 in refluxing DMF, and it was easily
purified by recrystallization from ethanol. ZnPc 7 was obtained by
cyclotetramerization of phthalonitrile
6 in the presence of
anhydrous zinc acetate as a metal salt template, and refluxing 2-
dimethylaminoethanol as solvent. ZnPc 1 was obtained by
2
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Figure 2. a) Absorption and b) fluorescence spectra (λexc = 615 nm, inner filter corrected) of 1 (2 x 10-6 M) recorded for different solvent ratios of DMSO and water,
from 100 vol% DMSO to 99.8 vol% water. c) Plot of the corresponding normalized Q-band ratios (678/632 nm) and normalized emission intensity at 685 nm of 1
and 2. d) Intensity autocorrelation function of 1, 1aggA, and 1aggB (2.0 x 10-5 M) at a scattering angle of Θ = 90° and a laser wavelength of 633 nm. e) and f) Bright
field transmission electron microscopy (BF-TEM) images of one crystal formed from 1aggB. Growth axis is indicated as [100]. White circle shows aperture placement
for the recording of the electron diffraction pattern shown in the inset. Perpendicular lattice planes are indicated by white lines in the overlay with the spacing as
extracted from SAED. Please note the diffraction pattern has been rotated to rotationally align with the bright field image.
The size of the different aggregates of 1 can be obtained from
dynamic light scattering (DLS) measurements (Figure 2d), in
which the autocorrelation function of the scattered light illustrates
the formation of 1aggA and 1aggB with increasing water content.
Importantly, no scattering intensity is detectable for molecularly
dissolved 1 in DMSO. The scattering intensity rises by gradually
increasing the water content to 60 and 95 vol%.[43] For 60 vol%
water, a clear bimodal decay function is found that shifts to higher
decay times for 95 vol% water. The sizes of the different
aggregates of 1 are derived from angle-dependent dynamic light
scattering (DLS) measurements. To this end, the hydrodynamic
radius RH is obtained by extrapolating the angle-dependent
apparent diffusion coefficients against θ = 0°. The extrapolated
diffusion coefficient can be converted to the hydrodynamic radius
(Figure S6). The data fitting for 60 vol% water results in two
distinct species, one with a hydrodynamic radius of RH1 = 200 nm
Figure 2f) with a lattice plane spacing of ca. 3.5 Å, suggesting π -
π stacking. The perpendicular lattice plane spacing is ca. 2.6 Å.
However, electron beam damage renders an orientation of the
crystallites in different zone axes impossible and, in turn, no
definite statements regarding the global structural parameters can
be made.
In order to show that the spectroscopic features depend on
the degree of aggregation, we focused on temperature-
dependent experiments. In the case of 1aggB, the fluorescence
spectra of 1 at 5.0 × 10-7 M in 99% water were recorded at
temperatures ranging from 363 to 273 K (Figure S9). At high
temperatures, the fluorescence centred at 685 nm is recovered.
It decreases and shifts to 705 nm upon lowering the temperature.
The ratio between the emission maxima of the aggregates and
the monomers, that is, 705 and 685 nm, respectively, is fit to a
supramolecular polymerization model (Figure S8),[44–46] yielding
an aggregation constant of 8.4 × 103 M-1. Absorption studies were
not taken into account due to the lack of differences in absorption
between 1aggB and 1. In the case of 1aggA, fluorescence assays in
a concentration regime of 5.0 × 10-6 M resulted in an aggregation
constant of 8.0 × 103 M-1.
and a second one in a size range of micrometers (formally RH2
=
5 µm). By addition of water, the autocorrelation function shifts
towards higher decay times and correspondingly the second
species becomes even larger. At the same time, RH1 splits into
two, which might be due to Ostwald ripening of the smaller
particles. The intermediate form is likely to grow to the size of the
larger particles. The existence of multiple species hinders data
interpretation with static light scattering (SLS). Interpreting the
structure is challenging in this case. Instead, further insights into
the morphology of 1aggB were obtained by transmission electron
microscopy (TEM). ZnPc 1 tends to form large needles of
micrometer dimensions in aqueous dispersion (Figures 2e and
S17), with size distributions that are in good agreement with the
DLS measurements. Selected area electron diffraction (SAED)
reveals crystalline stacking along the growth axis (termed [100] in
Theoretical calculations. Combined Dreiding force field[47] and
wB97xd/def2-SVP-density
functional
theory
(DFT)[48–50]
calculations with a polarizable continuum solvent model were
used to elucidate possible aggregation alignments of 1 (see SI for
all details). The most stable conformations of 1 and 2 in their
monomeric forms are achieved by facing the positively charged
methylpyridinium groups outwards towards the solvent (Figure
S19/20 and Table S2). This orientation also promotes pyrene-
pyrene or pyrene-ZnPc interactions in case of 1, resulting in either
3
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a planar (1e) or bent ZnPc-core (1c-d), respectively. In DMSO,
conformer 1e (Figure 3, top) is 37 kcal mol-1 more stable than 1a-
b and 3 kcal mol-1 more stable than the bended conformation 1c.
Thus, we expect the initial aggregation to be based on conformer
1e, which offers four possible pyrene-pyrene- and two ZnPc-
ZnPc-interactions. Based on a force-field prescreening, four
dimer arrangements of 1e were chosen and optimized with DFT.
The dimerization energy (ED) for ZnPc-ZnPc- (C) or ZnPc-pyrene-
(D) interacting dimers are -48 and -55 kcal mol-1, respectively, in
water. The two pyrene-pyrene interacting dimers A and B are
stabilized by -20 kcal mol-1 (Figure S22/23, Table S4-6). The
pronounced solvent dependence stems from the high Coulomb
repulsion in the dimers, as inferred from a solvent independent ED
for uncharged dimers of ZnPc 7. This trend is well in line with the
distances between neighboring pyridinium units at 8.2 Å for A and
5.8 Å for C.
dimers A and B, none of the four show significant changes in
energy. In stark contrast, the ZnPc-interacting dimers differ
substantially in energy. Although the S0-S1/2 transitions are
stabilized by 0.1 eV for both, a significant oscillator strength is only
predicted for dimer D. Further deviation from the monomeric
vertical energies are found for dimer C, where S0-S3/4 transitions
with significant oscillator strength are found at 1.95 and 1.99 eV.
Comparison with the hypsochromic shifted absorption essays
indicate that 1aggA is likely a direct ZnPc-ZnPc interaction as
proposed by dimer C. In contrast, the lack of absorption changes
for 1aggB can be rationalized by the increased ZnPc-ZnPc distance
in this aggregation regime. To evaluate the ZnPc-distance in the
aggregates, the second layer of aggregation needs to be
considered to account for the three-dimensional growth. Figures
S29-32 show the force-field optimized geometries for aggregates
based on dimers of A and C. While the latter is built around by
ZnPc-ZnPc columns with alternating, perpendicular pyrene motifs
for secondary coordination, aggregates of A feature planes of
primarily pyrene-interactions.
structure (MIX) was found.
In addition, an intermediate
It is best described as perpendicular arranged strings of A,
which are supported by ZnPc-interactions similar to what is found
for C (Figure S33). Here, the better pyrene overlap with respect
to C is achieved by decreasing the steric hindrance via increasing
the ZnPc-interplane distances from 3.5 to 5.0 Å (Figure S34/35).
As a consequence of reduced ZnPc-ZnPc interactions, the TD-
DFT predicted vertical excitations closer resemble monomeric
behavior (Figure S36).
Still, aggregates C and MIX are
structurally similar, which is in line with the experimentally
observed gradual change in aggregation. This makes MIX a more
likely arrangement for 1aggB than A, although we cannot rule out
the presence of the latter.
Complexation studies. The ground- and excited-state
interactions of cationic ZnPcs 1 and 2 with anionic fullerene
derivatives 3 and 4 were investigated by absorption and
fluorescence spectroscopies. First, monomeric 1 and 2 at
constant concentrations of 2.0 × 106 M were titrated in pure
DMSO with up to three equivalents of 3 and 4. Throughout these
titrations, the Q-band absorption decreases in intensity and
undergoes a 8 nm red shift. In addition, the fluorescence is
quantitatively quenched (Figure S11-14). We ascribe these
trends to the formation of ZnPc-C60 electron donor-acceptor
hybrids. The overall stoichiometry was determined by applying
1:1 or 2:1 binding model fits to our titration data and evaluating
the quality of each fit. 1:1 and 2:1 ratios were found for 1-3 and 1-
4 species, respectively, consistent with charge neutralization in
the resulting hybrid. This points to electrostatic interactions as the
main driving force for the binding. Taking this into account, ZnPc
complexes with 3 consists of a one-step process, as shown in
equation (1), while ZnPc complexes with 4 involve two steps for
their formation – see equations (1) and (2):[51]
Figure 3. Top: wB97xd/def2-SVP optimized structure of monomer 1e with a
total dipole moment of 31.4 Debye as indicated by the blue vector (left).
Electrostatic potential (0.15-0.35, blue-red) mapped on the electron density
(isovalue
= 0.02) of 1e (top right). Bottom: wB97xd/def2-SVP optimized
structures of dimers A-D.
Based on the rather large dipole moments of 31.3 Debye for
the monomers and their different spatial orientations, we further
divided the dimers by means of their total dipole moments. In
particular, 482 and 485 Debye are the sums for dimer B and D,
respectively, while the sums are 127 and 195 Debye for C and A,
respectively. Any further growth goes hand in hand with an
increase in dipole moment, especially for B and D; a finding that
assists in neglecting these arrangements. In addition, the weaker-
stabilizing pyrene-pyrene interactions outnumber possible ZnPc-
ZnPc interactions. They are based on less Coulomb repulsions
due to the fact that the pyridinium moieties are at larger distances.
A coarse linear extrapolation from dimer-derived interaction
energies towards larger aggregates indicates similar stabilization
per monomer, especially for A and C (Table S7, Figure S24).
Additional insight into the structural alignment came from the
comparison of time-dependent TD-DFT predicted vertical
excitations and experimental absorption data (Table S8/9, Figure
S25-28). The first two predicted vertical transitions (S0-S1/2) for
the monomer are both about 1.9 eV and in good agreement with
the experimental data. In case of the dimers, the two ZnPc-
centered transitions split into four. For the pyrene interacting
[
]
[ ][ ]
퐻 + 퐺 ↔ 퐻퐺
퐻퐺 + 퐻 ↔ 퐻ꢁ퐺
퐾ꢀ = 퐻퐺 /( 퐻 퐺 )
Eq. (1)
Eq. (2)
[
]
[
][ ]
퐾ꢁ = 퐻ꢁ퐺 /( 퐻퐺 퐻 )
Here, H is the ZnPc host, G the fullerene guest, HG the 1:1
complex, and H2G the 2:1 complex. K denotes the binding
constants for each of the processes described. With equations
(1) and (2) in hand, the binding constants for 1-3, 1-4, and 2-3
were determined in absorption and fluorescence titrations (Figure
S11-13), using Reactlab Equilibria as fitting software (Table 1).
4
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RESEARCH ARTICLE
Figure 4. a) Absorption and b) fluorescence spectra (λexc = 615 nm, inner filter corrected) of 1aggB at a constant concentration of 2.0 x 10-6
M in 95 vol% water, titrated with up to three equivalents of 4. Absorption and fluorescence spectra are corrected by subtraction of the
absorption and fluorescence of pure 4, respectively. c) Dynamic light scattering experiments of 1aggB and 1aggB-4 (1:1) at 2.0 x 10-5 M in 95
vol% water (at a scattering angle of Θ = 90°, laser wavelength 633 nm).
the Q-band absorption at 678 nm (Figure S16). A subsequent
Table 1. Binding constants of different ZnPc and fullerene combinations.
Titrations were carried out at constant ZnPc concentrations of 2.0 x 10-6 M in
DMSO.
decrease of the temperature results in the recovery of the initial
features, suggesting better stabilization for the H-type
aggregation relative to the 1-4 hybrid formation. For 1aggB, no
deaggregation is observed. Instead, an irreversible trend
a
Complex
1-3
Kabs (M-1)
Kfluor (M-1)
3.6 × 106
6.5 × 106
suggests another thermodynamically more stable form that is only
accessible upon heating (Figure S10). However, in the presence
of 4 at a 2:1 ratio, the 1aggB stability is increased to the point that
no significant absorption changes evolve within the temperature
and concentration ranges (Figure S16).
K1 = 3.6 × 106
K2 = 3.5 × 106
1-4
K1 = 7.0 × 106
K2 = 1.9 × 106
2-3
1.3 × 105
2.1 × 105
Further insights regarding the 1aggB-4 morphology were
obtained by TEM micrographs. The rod-like shape of the 1aggB
aggregates are still observable (Figure S18).[52] A more statistical
overview stems from DLS. With increasing concentration of 4, the
previously multimodal decay function changes to a monomodal
decay (Figure 4c). An RH = 95 nm was inferred from such data,
leading to the conclusion that the presence of 4 affects the size of
the crystals with its anionic charges and by increasing stability in
solution, compared to the crystals in the absence of 4. The radius
of gyration RG can be calculated from the angle-dependent
scattering intensity, while the shape of the particles can be
estimated from the ratio of RG to RH. With an RG/RH = 1.68, a
prolate ellipsoid is expected as in pristine 1aggB (Figure S7).[53]
Interactions of the 1aggB aggregates with 3 and 4 were also
theoretically modeled with DFT and are illustrated in Figures S37-
39. While for the A dimer arrangement (see above) the interaction
is limited to the periphery, the C and MIX arrangements offer
vacancies for interactions and direct intercalations of 3 and 4. In
case of monomeric 1, 1:1 and 2:1 complexation were investigated
with force-field annealing, followed by further optimization with
DFT. In all cases, direct C60-ZnPc interactions are accomplished
by the pyrenes wrapping around C60, and, thus, increasing the van
der Waals contact area.
Overall, the design and incorporation of an electrostatic
interaction motif enables high binding constants in DMSO with
values of around 105 M-1 (see Table 1 for 2-3). Incorporation of
pyrene leads to an increase of the binding constant by one order
of magnitude (see Table 1 for 1-3 and 1-4). This phenomenon
can be explained by the additional interactions between the
pyrene surface and C60 via π-π interactions. Remarkably, 1-4
formation shows similar K1 and K2 values, which implies that the
two processes are independent.
The interaction between the anionic fullerenes and 1 in the
aggregated state was also studied. To this end, 1aggA and 1aggB
(2.0 × 10-6 M) were titrated with up to three equivalents of 4 in 60
vol% and 95 vol% water, respectively. No significant data were
obtained from titrations of 1agg with 3, due to the poor solubility of
the latter in water. Even if the data cannot be fitted to any
theoretical model due to the intrinsic uncertainty of the
stoichiometry of the aggregates, qualitative information can be
obtained. For instance, both 1aggA and 1aggB show a red shift of
ca. 8 nm of both the aggregation and the Q-band in the absorption
spectra (Figure 4a and S15). However, no other significant
changes in the overall shape of the absorption spectrum are
observed, despite the higher binding affinity of, for instance, 1-4
compared to 1aggB: 106 M-1 vs 103 M-1. The fluorescence
emission-titration curves for both aggregates present, on the
other hand, a straight line up to 1 equiv. of C60, and then the
emission is 100% quenched (Figure 4b and S15). On these
bases, we hypothesize that, in aqueous media, 1 remains as
aggregate, while 4 binds to the periphery of 1agg through
electrostatic and π-π interactions.
Photophysical studies. Prior to transient absorption
measurements, spectroelectrochemical experiments served to
determine the spectroscopic fingerprints of the one-electron
oxidized and reduced forms of 1 and 2 in DMSO. At a voltage of
+1.2 V vs. Ag-wire, features of the one-electron oxidized form are
discernible (Figure S40 and S41, respectively). Besides the
ground-state bleaching (GSB) of the Soret- and Q-band
absorptions, new features arise at 422, 520, and 840 nm. These
bands fit the values found in the literature for the one-electron
oxidized form of ZnPcs perfectly.[54] At a voltage of -0.2 V vs. Ag-
wire, features of the one-electron reduced forms of 1 and 2 are
discernable (Figure 40 and S41, respectively). In particular, new
In order to determine the nature of the colloidal hybrid,
temperature-dependant absorption experiments were performed
at a ZnPc concentration of 2.0 × 10-5 M. Temperature increases
of a 1aggA solution in the presence of 0.5 equivalents of 4 trigger a
decrease of the aggregation band at 632 nm and an increase of
5
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10.1002/anie.202006014
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RESEARCH ARTICLE
features at 560-580, 820, and 960-980 nm arise, accompanied by
GSB, which are also found in the literature upon reduction of
ZnPcs.[54] Comparing the GSB and evolving features during the
first oxidation and reduction of 1 with 2, we exclude any
participation of the pyrenes in these redox processes.
The role of the aggregation mode in the excited state was
studied by deconvolution of the deactivation mechanism after
photoexcitation of 1aggA and 1aggB, by pump-probe experiments.
Solutions of 1 at 2.0 × 10-5 M were excited at 630 nm, and time
delays ranging from femto- to microseconds were recorded. The
selected excitation wavelength excites the ZnPcs in the Q-
band/aggregation-band absorption exclusively. This avoids the
excitation of pyrenes and C60.
Initially, reference measurements were performed in pure
DMSO, to examine the excited state deactivation of monomeric 1
(Figure 5a and S42-44) and 2 (Figure S45/46). Directly after
photoexcitation, the singlet-excited state characteristics of both
ZnPcs exhibit maxima at 455, 515, and 820 nm together with the
GSB, with lifetimes of 1.68 and 2.24 ns, respectively.[55] Next, the
triplet-excited state with features at 460 and 495 nm evolves from
the singlet-excited state with a yield of 45% and a lifetime of 498
µs for 1. Full recovery of the GSB due to the population of the
ground state is the last step in the kinetic model (Figure S44). For
2 in DMSO, the same triplet-excited state is found, but it decays
slowly to the ground state.
With this kinetic model in hand, we turned our attention to the
changes in terms of spectroscopy and kinetics upon addition of 4.
Due to the prominent aggregation in DMSO of the complexes
involving 2 (even leading to precipitation when is confronted with
3),[56] these studies were focused on the complex 1-4, for which
significant changes relative to 1 are observable (Figures S47 and
48). For example, the singlet-excited state of 1 is discernible after
photoexcitation, but it is short-lived with 1.7 ps. The back-to-back
formation of two transient species with lifetimes of 18 and 638 ps,
which feature two sets of spectroscopic signatures, commences
with the singlet-excited state decay. It is, on the one hand, those
of the one-electron oxidized form of 1 at 445, 520 and 841 nm,
which we observed in our spectroelectrochemical measurements
(vide supra). On the other hand, we note those of the one-
electron reduced form of 4 at 1000 nm, which are in sound
agreement with the literature.[57]
Figure 5. Species associated differential absorption spectra (SAS) (left and center) and population over time (right) obtained upon deconvolution of spectra with
Target Analysis (GloTarAn) of 1 (a and b), 1aggA (c and d), 1aggB (e and f), and 1aggB-4 (g and h, 1:2 ratio). Raw spectra were received upon nanosecond pump-probe
experiments (λex = 630 nm, c = 2.0 x 10-5 M) at time delays from 0-250 µs.
6
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The first of the two species, however, gives rise to slightly
reorganization of the pyrene-containing substituents. This is re-
sponsible for the fruitful interplay between light harvesting and
charge-separation in 1. Importantly, when the system interacts
with a tetra- or octaanionic water-soluble fullerene under the same
conditions, charges are funneled in the hybrid aggregates to the
strongest electron-accepting units, that is, the fullerenes, resulting
in longer-lived charge-separated states. In this case, both the self-
assembly of 1 and the complexation with fullerene 3 or 4 are
controlled by a subtle interplay between electrostatic and π-π
interactions, favored by the aqueous medium. Hence, our work
represents a versatile approach, which enables water to modulate
the synergy between different non-covalent interactions and, in
turn, to achieve multi-chromophore supramolecular assemblies.
In the latter, all the fundamental processes of natural
photosynthesis are operative and give rise to highly efficient
artificial photosynthetic systems.
broadened signatures and is assigned to a 1δ+-4δ- charge-transfer
δ-
state. This 1δ+-4 charge-transfer state then undergoes full
charge separation, which affords the 1●+-4●- charge-separated
state as the second of the two species. Only 25% of the 1●+-4●-
charge-separated state deactivates to the triplet excited state of
1. The later reinstates the ground state with a lifetime of 734 µs.
A detailed mechanistic model is shown in Figure S49.
Upon transitioning from 1 to 1aggA, significant changes in
transient features and lifetimes are observable (Figure 5c and
S50). It is highly relevant that the singlet-excited state lifetime of
1aggA is quenched by three orders of magnitude down to 3.5 ps.
As the singlet-excited state decay comes to an end, new features
develop at 445, 555, 834, and 960 nm. These features are exact
matches to the spectroscopic fingerprints recorded upon
reduction and oxidation of 1 (vide supra). In other words,
excitation of the H-type 1aggA triggers a symmetry-breaking
●+
charge separation and consequently yields a 1aggA●--1aggA
Acknowledgements
charge-separated state. Three lifetimes of 52, 176, and 3600 ps
●+
are derived for the 1aggA●--1aggA charge-separated state. We
‡E.A.-P. and J.J. contributed equally to this work.
infer a transfer of electrons and/or holes between neighboring 1
to assist in a spatial separation of charges and, consequently, a
slowing-down of the charge recombination. Each of the transfer
competes, however, with the intrinsic charge recombination.
Financial support from MINECO (CTQ2017-85393-P, CTQ-2014-
53673-P and CTQ-2017-89539-P) and ERA-NET/European
Commission/MINECO, (UNIQUE, SOLAR-ERA.NET Cofund 2 N°
008/ PCI2019-111889-2) is acknowledged. Funding is grate-fully
acknowledged from the Emerging Fields Initiative “Singlet Fission”
supported by Friedrich-Alexander-Universität Erlangen-Nürnberg
and the Solar Technologies Go Hybrid (SolTech) initia-tive of the
Bavarian State Government. IMDEA Nanociencia acknowledges
support from the “Severo Ochoa” Programme for Centres of
Excellence in R&D (MINECO, Grant SEV2016-0686).
Thus, the relative amplitudes decrease significantly and amounts
●-
to, for example, only 5% in the case of the 3600 ps-lived 1aggA
-
●+
1aggA charge-separated state. For 1aggB, the correspondingly
formed 1aggB●--1aggB
charge-separated state exhibits five
●+
●-
lifetimes; next to the three lifetimes discussed for the 1aggA
-
1aggA●+ charge-separated states, two additional lifetimes of 190 ns
and 12 µs (Figure 5e and S52). A greatly facilitated transfer
between neighboring 1aggB enables not only a remarkable lifetime
of 12 µs but also a 5% overall yield.[58] Details regarding the
kinetic models are shown in Figure S51/53.[59]
Keywords: Phthalocyanine • Fullerene • Artificial
At first glance, the transient absorption measurements with
Photosynthesis • Nanoscale Self-assembly
1aggB-4 (Figures 5g and S54) are identical to what was discussed
[60]
for 1aggB
.
We reach the same conclusions from both the
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at the data fittings revealed a notable difference: Best fits of the
experimental data necessitate the inclusion of another species in
the kinetic model: 132 µs is the lifetime of the sixth charge-
separated state. Importantly, the 840 nm signature of the one-
electron oxidized form of 1aggB is persistent over the entire
timescale.[61] Therefore, we conclude that the longest lifetime of
132 µs stems from a charge transfer that evolves from 1aggB to 4,
which is located at the periphery of the aggregates. Experiments
with various equivalents of 4 (Figures S54-59) suggest an
increase of the 1aggB●+-4●- charge-separated state up to a yield of
7% for a 1:2 ratio. Furthermore, the formation of the triplet-excited
state is fully inhibited in the presence of 4, corroborating the
formation of the 1aggB●+-4●- charge-separated state, which is lower
in energy than the 1aggB●--1aggB●+ charge-separated state.
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Angewandte Chemie International Edition
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8
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RESEARCH ARTICLE
Entry for the Table of Contents
This work represents a powerful approach to build up highly organized electron donor-acceptor photoactive systems in aqueous media.
Our strategy is based on modulating the synergy between different non-covalent interactions to achieve supramolecular hybrids of a
tetracationic zinc (II) phthalocyanine and anionic water-soluble fullerenes. In the resulting monocrystalline assemblies, all the
fundamental processes of natural photosynthesis are operative and give rise to highly efficient artificial photosynthetic systems.
9
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