Enhancing molecular recognition in electron donor-acceptor hybrids via cooperativity

Article abstract of DOI:10.1021/ja5052236

Herein, we report the synthesis of guanidinium bis-porphyrin tweezers 1 and fullerene carboxylate 3, their assembly into a novel supramolecular 1@3 electron donor-acceptor hybrid, and its characterization. In solution, the binding constant affording 1@3 is exceptionally high. 1@3, which features a highly confined topography, builds up from a combination of guanidinium- carboxylate hydrogen bonding and π-π stacking/charge-transfer motifs. The latter is governed by interactions between the electron-donating porphyrin and the electron-accepting fullerene. Importantly, positive cooperativity between the applied binding motifs is corroborated by a number of experimental techniques, such as NMR, absorption, fluorescence, etc. In addition, transient absorption experiments shed light onto electron-transfer processes taking place in the ground state and upon photoexcitation. In fact, porphyrin excitation powers an electron transfer to the fullerene yielding charge separated state lifetimes in the nanosecond regime.

Full text of DOI:10.1021/ja5052236

Article
pubs.acs.org/JACS
Enhancing Molecular Recognition in Electron Donor−Acceptor
Hybrids via Cooperativity
Rafael M. Krick Calderon,§,‡ Julian Valero,†,∥,‡ Bruno Grimm,§ Javier de Mendoza,† and Dirk M. Guldi*,§
́
†Institute of Chemical Research of Catalonia, Avgda. Països Catalans 16, 43007 Tarragona, Spain
§Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials (ICMM),
Friedrich-Alexander-Universitat Erlangen-Nurnberg, Egerlandstraße 3, 91058 Erlangen, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Herein, we report the synthesis of guanidinium
bis-porphyrin tweezers 1 and fullerene carboxylate 3, their
assembly into a novel supramolecular 1@3 electron donor−
acceptor hybrid, and its characterization. In solution, the
binding constant affording 1@3 is exceptionally high. 1@3,
which features a highly confined topography, builds up from a
combination of guanidinium-carboxylate hydrogen bonding and
π−π stacking/charge-transfer motifs. The latter is governed by
interactions between the electron-donating porphyrin and the
electron-accepting fullerene. Importantly, positive cooperativity
between the applied binding motifs is corroborated by a number
of experimental techniques, such as NMR, absorption,
fluorescence, etc. In addition, transient absorption experiments
shed light onto electron-transfer processes taking place in the ground state and upon photoexcitation. In fact, porphyrin
excitation powers an electron transfer to the fullerene yielding charge separated state lifetimes in the nanosecond regime.
in terms of charge separation and charge recombination
between electron donating porphyrins and electron accepting
fullerenes have been elucidated and many similarities relative to
the natural photosynthetic systems have been established.8 In
terms of noncovalent hybrids, viable strategies consist of a
careful design of porphyrin based receptors, which strongly
bind fullerene guests, and, which result in the assembly of stable
and robust architectures.9
We previously reported the synthesis and the study of stable
supramolecular electron donor−acceptor hybrids with unique
photophysical features.10 Our approach consisted of assembling
electron accepting C60 derivatives with the electron donor
tetrathiafulvalene (TTF) through complementary guanidi-
nium−carboxylate interactions. From a full fledged spectro-
scopic characterization formation of the charge separated
radical ion pair states C60•−·TTF•+ was inferred. The latter
features lifetimes in the range of hundreds of nanoseconds. As
such, the flexibility of the system and the spatial proximity
between the electron donor and the electron acceptor via
noncovalent interactions enable unidirectional and efficient
through space electron-transfer interactions.
INTRODUCTION
■
In light of the world’s demand regarding renewable energy
sources, solar energy, in general, and organic photovoltaics, in
particular, have emerged as key players in recent decades.1
Great abundance and low production costs are just a few of the
numerous incentives that organic photovoltaics feature over
conventional techniques, such as silicon-based solar cells.
Importantly, concerted efforts have been directed to develop
molecular systems for the mimicry of natural photosynthesis.2
In particular, cascades of light induced energy and electron-
transfer reactions assist in harvesting light and converting it into
free charge carriers, that is, electrons and holes. An essential
requirement for them to function effectively is the hierarchical
organization of the molecular building blocks by, for example,
noncovalent motifs.3 Leading examples are hydrogen-bonding,
π−π-interactions, n-π-interactions etc. On one hand, porphyr-
inoids have been widely employed as light harvester/electron
donor, owing to their favorable character as natural
chromophores and panchromatic light harvester.4 On the
other hand, fullerenes have revealed themselves as an electron
accepting complement to porphyrins.5 The rigid and well
confined structure of fullerenes renders them perfectly suitable
for single and multi electron-transfer reactions in, for example,
artificial photosynthetic reaction centers.6
En-route towards establishing hydrogen-bonding based
supramolecular electron donor−acceptor hybrids we describe
herein the synthesis and properties of guanidinium bis-
porphyrin tweezers 1, which has been specifically designed to
In light of the aforementioned, the combination of fullerenes
and porphyrins in the form of electron donor−acceptor
conjugates and hybrids have evolved into a thoroughly studied
field of research.7 As a matter of fact, electron-transfer processes
Received: May 25, 2014
Published: July 11, 2014
© 2014 American Chemical Society
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interact with (1,2-methanofullerene C60)-61-carboxylate salt 3
(Figure 1). Past studies have documented that tweezers like
orientation and, in turn, induce the right fitting with respect to
C60. From the analysis of 21 C60@ZnP cocrystal structures
taken from the Cambridge Structural database, the average
distances between the centroids of the ZnP planes and C60 is
6.28 0.08 Å. This value is consistent with that calculated for
3@1. For a meta-substitution, the distances between porphyrins
in parallel disposition would be larger than 14 Å, and therefore,
the porphyrins would need to bend in order to provide the
proper geometry for hosting C60. Another consideration about
meta-substituted (m-phenoxy) porphyrins is their relative close
distance to the guanidinium scaffold, which would hamper a
suitable orientation and/or distance to form simultaneously the
salt bridge with the carboxylate and the corresponding π−
electron interactions with C60.
Figure 1. Guanidinium bis-porphyrin tweezers 1 and fullerene
carboxylate 3.
Nevertheless, the acyclic tweezers 1 is highly adaptable, since
the porphyrins are only tethered from one side. Binding
fullerene carboxylate 3 should, however, restrict the overall
conformation of the molecule to a parallel orientation of the
two porphyrins as shown in the molecular models.
Synthesis of Guanidinium Bis-Porphyrin Tweezers.
Guanidinium bis-porphyrin tweezers 1 was prepared by O-
alkylation of Zn-meso-5-(4′-hydroxyphenyl)-10,15,20-triphenyl-
porphyrin (4)14 with guanidinium salt 5 in 42% yield. Previous
zinc metalation of the porphyrin is necessary in order to avoid
the undesired N-alkylation of the porphyrin nitrogens (Scheme
1).
geometries are a sound platform for efficient supramolecular
binding.11 Additional π−π interactions as they play a significant
role between porphyrins and fullerenes, in general, should
further strengthen the overall binding.12 Bicyclic guanidiniums
bind a wide variety of anions, especially oxoanions, due to the
creation of well oriented hydrogen-bonded ion pairs.13 Thus,
the guanidinium scaffold not only acts as a structural linker
between the two porphyrins, but it also plays an active role to
complement anionic guests. In fact, fullerene carboxylate 3 is
expected to match nicely the geometrical and electronic
requirements of bis-porphyrin tweezers 1 and to form 3@1.
We will demonstrate that the assembly of 1 and 3 is
characterized by an unprecedented high binding strength and
electron-transfer activity once photoexcited. Most importantly,
the contribution of each binding motif will be determined by
comparison with suitable models to allow estimating their
cooperativity and their impact on the rates and yields of
electron transfer.
Scheme 1. Synthesis of Guanidinium Bis-Porphyrin
a
Tweezers 1
RESULTS AND DISCUSSION
■
Design of Guanidinium Bis-porphyrin Tweezers. The
choice of the spacer and binding motif between the
guanidinium scaffold and the porphyrin moieties was evaluated
by molecular modeling. The short ether linkage fits nicely with
the geometrical and spatial requirements to form a complex
with C60. Longer spacers would increase the distance between
the porphyrin planes as well as with the bicyclic guanidinium
scaffold and, in turn, disfavor binding of fullerene carboxylate 3.
Indeed, subtle structural considerations are essential for the
design of this receptor.
a
Conditions: (i) 2.2 equiv of 4, 4.5 equiv K2CO3, acetone, reflux.
For solubility reasons, the tetrabutylammonium fullerene
carboxylate salt 3 was prepared by addition of equimolar
amounts of tetrabutylammonium hydroxide to a solution of
commercially available (1,2-methanofullerene C60)-61-carbox-
ylic acid in ortho-dichlorobenzene.
Guanidinium hexafluorophosphate salt 2 was prepared as a
control to dissect the contribution of the guanidinium-
carboxylate to the overall binding with 1. Synthesis of 2 was
achieved as previously described by O-alkylation of phenol with
guanidinium salt 5, see Scheme 2. Details are presented in the
Experimental Section, Supporting Information.
For instance, p-phenoxy-porphyrins enable the correct
geometry for C60 complexation. As shown in Figure 2, the
molecular model of 3@1 implies that the porphyrin planes are
at a 12.2 Å distance relative to each other with a parallel
Finally, guanidinium monoporphyrin 6 was synthesized as a
reference to gather insights into the thermodynamics of the
association, including the determination of the effective
Scheme 2. Synthesis of Bis-Aryl Guanidinium
a
Hexafluorophosphate Salt 2
Figure 2. Optimized structure (SCIGRESS, standard conditions, no
solvent) of 3@1 in top view (left) and front view (right); Zn/C60
distance is ca. 3.2 Å.
a
Conditions: (i) 2.9 equiv of phenol, 5 equiv K2CO3, acetone, reflux.
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molarity (EM) and the assessment of the cooperativity.
Following the methodology depicted above, O-alkylation of
Zn-phenolporphyrin 4 with monoprotected guanidine 715
afforded compound 6 in good yields (Scheme 3).
a
Scheme 3. Synthesis of Guanidinium Mono-Porphyrin 6
a
Conditions (i) 1.5 eq of 7, 1 equiv of 4, 1.5 equiv K2CO3, acetone,
reflux.
Complexation Studies of Bis-Porphyrin Tweezers 1
with C60 and Fullerene Carboxylate 3. To evaluate the
binding motif and the association constants for 1 with C60 and
with fullerene carboxylate 3 to afford C60@1 and 3@1,
respectively, 1H NMR experiments were performed. Only
minor changes were observed upon addition of C60 to the
guanidinium bis-porpyhrin tweezers 1 in deuterated toluene
(Figure S1).
Please note that C60 has a unique 13C signal due to the
equivalence of all its carbons. Indeed, the 13C NMR spectrum
of C60@1 in tetrachloroethane revealed a rather significant
upfield shift of the C60 signal. A likely rationale for the upshift is
an effective complexation of C60 as previously reported by Boyd
et al.11e As such, complexation results in a chemical shift as an
average signal corresponding to free/uncomplexed and
complexed C60, see Figure 3. Upon heating, the stability of
Figure 4. Upper part: Mass spectrum of C60@1. Lower part:
Calculated and experimental isotopic pattern of C60@1.
In addition, 1H NMR titrations were performed with
fullerene carboxylate 3 and tweezers 1. As in the case of C60,
1
the H NMR spectra gave rise to minor chemical shifts when
ortho-dichlorobenzene was used as a solvent, see Figure 5.
Figure 3. 13C NMR spectra in TCE-d2 of C60 (top) and C60@1 at
different temperatures.
Figure 5. 1H NMR spectra in ortho-dichlorobenzene-d4 of
guanidinium bis-porphyrin tweezers 1 in the presence (1 equiv) (a)
and the absence (b) of fullerene carboxylate 3.
C60@1 decreases, and the signal shifts toward the resonance
seen for free/uncomplexed C60. Upon cooling, the association
process becomes more favorable. We were, however, unable to
reach the coalescence temperature, that is, the point at which
the equilibrium between free/uncomplexed and complexed C60
is slow enough to discern both signals.
C60@1 was further characterized by mass spectrometry
revealing in ESI an isotopically enriched ion of 2273.5 m/z
relative to 1552.4 m/z for just the bis-porphyrin tweezers 1. As
gathered in Figure 4, the experimental and the calculated
isotopic patterns are in good agreement with each other.
Upfield shifts of the guanidinium protons are taken as evidence
for structural rearrangements of the flexible guanidinium
scaffold that induces maximum interactions with fullerene
carboxylate 3 in 3@1. It is interesting to note that the aromatic
protons in the ortho positions of the phenoxy moieties also
showed both upfield shifts and broadening. Moreover, the
formation of complex 3@1 was corroborated in mass
spectrometric experiments, MALDI, with a M+ of 2330 m/z
(Figure S2).
Finally, titrations of model receptor 2 with fullerene
carboxylate 3 were performed in chlorobenzene-d5, and the
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corresponding 1H NMR spectra were used to assess the
contribution of the guanidinium-carboxylate salt bridge to the
overall binding. From Figure 6, which illustrates the downfield
3, the ZnP Soret band at 425 nm undergoes a slight red shift
and a lowering in intensity, see Figure 7.
Figure 7. Absorption spectra of a dilute ortho-dichlorobenzene
solution of 1 (9.0 × 10−7 M) in the presence of variable concentrations
of 3 (0; 1.9 × 10−7, 3.7 × 10−7, 6.5 × 10−7, 1.0 × 10−6, 1.4 × 10−6, 1.9
× 10−6, 2.4 × 10−6, 3.0 × 10−6, 4.2 × 10−6, 4.8 × 10−6, 5.5 × 10−6, 6.2
× 10−6, 7.5 × 10−6, 8.3 × 10−6 M). Arrows indicate the progression of
the titration.
Importantly, addition of 2, a reference bearing the same
hydrogen-bonding motif present in 1 but lacking ZnP, the
original ZnP Soret peak intensity and position is recovered, see
Figure S3. In other words, 1 is successfully replaced in 3@1 by
2 to afford 3@2. In an extra set of titrations, an equimolar
mixture of 2 and 3 was added to 1. Again, a red shift and a
lowered intensity of the ZnP Soret band are observed. Based on
the aforementioned titrations we conclude that rather strong
interactions power the formation of 3@1. It is safe to assume
that positive cooperativity made out of hydrogen bonding and
π−π interactions prevail between guanidinium bis-porphyrin
tweezers 1 and fullerene carboxylate 3, vide infra.
Figure 6. Upper part: 1H NMR titration in chlorobenzene-d5 of model
guanidinium salt 2 with fullerene carboxylate 3. Lower part: plot of the
chemical shift of the guanidinium protons vs equivalents of 3 added.
Complementary titrations of 1 with C60 revealed lowered
Soret band intensity in the presence of C60 but no appreciable
red shift. Notable is the necessity to employ much higher C60
concentrations relative to 3 in order to induce the lowering in
Soret band intensity.
To shed light onto the excited-state interactions between 1,
on one hand, and 3 or C60, on the other hand, fluorescence
titrations were carried out. Here, upon addition of 3 (Figure 8)
or C60 (Figure S4) to a solution of 1 in ortho-dichlorobenzene,
a nearly quantitative quenching of the ZnP fluorescence evolves
when exciting at 419 and 416 nm, respectively, as a
consequence of forming 3@1 and C60@1. Like in the
absorption experiments, addition of 2 to 3@1 led to a
quantitative recovery of the ZnP fluorescence, see Figure S5.
The overall fluorescence quenching within the concentration
range from 10−7 to 10−5 M was much more pronounced for 3
when compared with C60, see Figure S4.
To quantify the binding strength, the intensity of the ZnP
fluorescence at 646 nm was plotted versus the concentration of
either 3 or C60. Nonlinear curve fitting allowed calculating the
association constants (Ka). The resulting Ka values were 3 × 103
and 2.3 × 106 M−1 for C60 and 3, respectively. We assign the
rather significant difference of 3 orders of magnitude to the
cooperativity stemming from π−π interactions and hydrogen-
bonding interactions in the case of 3. In line with the
expectation, the corresponding association constant for the
shift of the guanidinium protons upon complexation, an
association constant of 3.3 × 103 M−1 was derived for 3@2.
This value is in sound agreement with the association constants
determined for similar guanidinium-carboxylate systems in
nonpolar media.16
In summary, 1H NMR studies and mass spectrometry
analysis of the complexes confirmed that sizable interactions of
C60 and fullerene carboxylate guest 3 with bis-porphyrin
tweezers 1 are the inception to form a rather strong complex,
namely C60@1. By means of variable-temperature 13C NMR
studies the encapsulation of C60 by guanidinium bis-porphyrin
tweezers 1 was further demonstrated. The nature of this
complexation relies on the electronic complementarity between
the electron-donating porphyrin tweezers 1 and the electron-
accepting fullerene. With the fullerene carboxylate 3, the higher
chemical shifts corresponded to the guanidinium scaffold
protons. This prompts to a structural rearrangement of the
flexible backbone to properly adapt and dock with 3.
Photophysical Studies of Bis-Porphyrin Tweezers 1
with C60 and Fullerene Carboxylate 3. To gain further
insights into supramolecular ground-state interactions between
1 and 3 in solution, absorption assays were carried out. To this
end, 3 was added stepwise to a solution containing a constant
concentration of 1 in ortho-dichlorobenzene to reach molar
ratios of 5:1 between 3 and 1. With increasing concentration of
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formation of 3@2, which is exclusively built on hydrogen
bonding, is notably weaker with a value of 3.3 × 103 M−1 that
was determined by NMR titration. In a titration of
monoporphyrin 6 with 3, a binding constant of 1.1 × 106
M−1 was determined. Taking the aforementioned into context,
the differences between 3@1, C60@1, 3@6, and 3@2
unambiguously corroborate the positive cooperativity, vide
infra.
Analyses of the Equilibria of Bis-Porphyrin Tweezers
1 with C60 and Fullerene Carboxylate 3. With these data in
hand, the contribution of each individual interaction and the
possible cooperativity effects arising from these multivalent
systems can be analyzed. For the formation of 3@1, we should
consider four different equilibria, as depicted in Scheme 4.
Please note that the order by which the different equilibria are
considered should not alter the overall binding. Hence, initially
guanidinium-carboxylate pairing takes place (K1) followed by
the intramolecular (EM1) binding of one of the two ZnPs (K2).
Successive intramolecular (EM2) binding of the second ZnP
(K2) leads to the formation of 3@1.
By definition, the cooperativity factor (α) is the ratio
between the experimental association constant, which is
affected by cooperative interactions (KA), and the statistical
or the reference constant (Kref).17 As the association constant
and the reference constant have the same value in the absence
of cooperativity, a consistent cooperativity factor should be
close to 1, while positive or negative cooperativity leads to α >
1 or α < 1, respectively. In the current case, cooperativity,
which stems from a chelation effect, needs to be considered as
two or more intramolecular binding interactions interplay
collectively. However, chelate cooperativity depends on ligand
concentrations. For simplicity, we consider from now on eq 1.
Figure 8. Upper part: fluorescence spectra upon 419 nm excitation of
a dilute ortho-dichlorobenzene solution of 1 (9.0 × 10−7 M) in the
presence of variable concentrations of 3 (0; 1.9 × 10−7, 3.7 × 10−7, 6.5
× 10−7, 1.0 × 10−6, 1.4 × 10−6, 1.9 × 10−6, 2.4 × 10−6, 3.0 × 10−6, 4.2
× 10−6, 4.8 × 10−6, 5.5 × 10−6, 6.2 × 10−6, 7.5 × 10−6, 8.3 × 10−6 M).
Arrows indicate the progression of the titration. Lower part: plot of I/
I0 for the fluorescence of 1 observed at 646 nm vs concentration of 3.
KA
Kref
α =
(1)
Taking a binding constant K2 for C60/ZnP interactions in
toluene of 3 × 103 M−1 into account,18 we are able to
Scheme 4. Thermodynamic Equilibrium Processes Involving the Complexation of Fullerene-Carboxylate 3 with Bis-Porphyrin
Tweezers 1
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extrapolate the effective molarities, namely EM1 and EM2, from
the stability constant values of 3@6 and C60@1, respectively.
These data allow us to estimate a reference constant for this
system. The reference constant is the expected theoretical
constant − the sum of all contributions from the different
interactions in a multivalent system, neglecting any coopera-
tivity between those interactions. In our case, we estimate the
reference constant, as different experimental techniques were
employed (NMR, UV−vis absorption/emission). The latter is
kept constant as the denominator for deriving the cooperativity
factor. For the numerator we use the experimentally
determined stability constant for 3@1.
106 M−1) by Kref afforded a cooperativity factor of α ≈ 2. This
suggests that positive cooperativity governs the interactions
and, in turn, enhances the thermodynamic stability of 3@1. To
facilitate the overall interpretation, we have dismissed side-
processes, such as self-aggregation, higher complex stoichio-
metries, etc., which may impact the different equilibria.
To shed light onto the influence of ionic strength on the
equilibrium, additional titration experiments using absorbance
and fluorescence were conducted. Therefore, addition of
various ratios of NaOH (5 × 10−4 M in 2-propanol) was
probed, and the binding constants increased to 8.1 × 106 M−1
for a 1:1 ratio of NaOH and 1, see Figure S7. Notably, the
binding constant maximizes at a 1:1 stoichiometric ratio and
decreases upon further increasing the ratio. This 4-fold increase
of the binding constant meets the expectations, as NaOH
facilitates the solvation of the guanidinium cations and the
corresponding counteranions. At NaOH concentrations larger
than the concentration of 1, higher ionic strengths impact the
binding constant.
Scheme 5 depicts the equilibria for the complexation
between guanidinium monoporphyrin 6 and fullerene carbox-
Scheme 5. Thermodynamic Equilibrium Processes Involving
the Complexation of Fullerene-Carboxylate 3 with Mono-
Porphyrin Compound 6
To conclude, we have demonstrated the formation of a stable
and robust C60/ZnP electron donor−acceptor hybrid, which is
based not only on electronic complementarity but also on
hydrogen-bonding and electrostatic interactions between the
bicyclic guanidinium scaffold and the carboxylate moiety of C60.
Electron-Transfer Studies of Bis-Porphyrin Tweezers 1
with C60 and Fullerene Carboxylate 3. Insights into
excited-state interactions between 1 and 3 or C60 were gathered
from femtosecond transient absorption spectroscopy. 150 fs
laser pulses were used to probe 3@1 and C60@1. When 1 was
subjected to 420 nm pulses in ortho-dichlorobenzene transients
were observed immediately after the laser pulse that are
characteristical for ZnP singlet excited states, see Figure S8.19 In
particular, a broad transient with a maximum around 460 nm, a
minimum at 550 nm, and several weaker maxima between 570
and 750 nm is assigned to singlet−singlet transitions involving
the ZnP singlet excited state. The singlet state relaxes via
intersystem crossing within 0.9 ns to the corresponding triplet
state. Such a singlet excited-state lifetime is shorter than the
lifetime known for ZnP monomers (∼2.0 ns). A reasonable
rationale for these data involves electronic communication
between the two ZnPs present in 1.
When exciting equimolar mixtures of 1 and 3, presumably
3@1, in ortho-dichlorobenzene or toluene at 420 nm the same
ZnP singlet excited-state features develop that were seen in
experiments with 1, vide supra. However, the ZnP singlet
excited-state decay in 3@1 is faster and is of a different nature
when compared to photoexcited 1. In particular, transient
features in the visible range of the spectrum, that is, from 600 to
800 nm, and in the near-infrared range of the spectrum, that is,
from 800 to 1050 nm, appear. The earlier band bears great
resemblance with the one-electron oxidized form of ZnP,20
while the latter band bears great resemblance with the one-
electron reduced form of C60.21 We assign the overall features
to the radical ion pair state of 3@1, that is, 3•−@1•+. Similar
observations were made in toluene, see Figure S9. On the
contrary, 420 nm excitation in THF failed to yield any evidence
for 3•−@1•+, see Figure S10. A possible rationale is that THF
competes with 3 in terms of hydrogen bonding and, in turn,
decreases the association constant.
ylate 3 with KB of 1.1 × 106 M−1, see Figure S6, which are
employed to determine the effective molarity (EM1) needed to
describe the cooperativity between bis-porphyrin 1 and
fullerene carboxylate 3.
Notable, the guanidinium-carboxylate stability constant K1 is
taken as 3.3 × 103 M−1 from the titrations of guanidinium salt 2
with fullerene carboxylate 3. Using the above-mentioned K2, we
calculated an EM1 value of 0.1 M. Likewise, by using the
experimental association constant KC regarding the complex-
ation between 1 and C60 as 4.7 × 103 M−1 (Scheme 6), we
determined an EM2 value of 5 × 10−4 M.
With these data in hand, we estimated Kref as 1.5 × 106 M−1.
Dividing the experimental stability constant KA (for 3@1, 2.3 ×
Scheme 6. Thermodynamic Equilibrium Processes Involving
the Complexation of C60 with Guanidinium Bis-Porphyrin 1
When investigating the charge recombination dynamics by
means of multiwavelength analyses, lifetimes of 2.1 and 1.3 ns
for the 3•−@1•+ radical ion pair state were derived in toluene
and ortho-dichlorobenzene, respectively. The longer radical ion
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ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic procedures and characterization and additional
photochemical data. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
dirk.guldi@fau.de
■
Present Address
∥LIMES Institute, Chemical Biology and Medicinal Chemistry
́
Unit, c/o Kekule Institute of Organic Chemistry and
Biochemistry, Gerhard-Domagk-Strasse 1, 53121 Bonn,
Germany.
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Deutsche Forschungsgemeinschaft and
the Bavarian Initiative − Solartechnologies Go Hybrid for
generous funding of this work. Dedicated to Prof. Tim Clark on
the occasion of his 65th birthday.
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pair state lifetime in toluene relates to the lower solvent polarity
of toluene when compared to ortho-dichlorobenzene as a means
to push the corresponding kinetics into the Marcus inverted
region.22
Equimolar mixtures of 1 and C60, either in toluene, ortho-
dichlorobenzene, or THF, which were subjected to 420 nm
excitation did not result in any noticeable charge-transfer
activity. A reasonable rationale is the weak association constant
for C60@1 when compared to 3@1. Thus, the concentration of
C60 was increased to reach a molar ratio of 10:1, and in fact,
weak features of C60•− and 1•+ are discernible. For C60•−@1•+,
kinetic analysis of the charge recombination dynamics revealed
radical ion pair state lifetimes of 0.7, 1.1, and 1.8 ns in toluene,
ortho-dichlorobenzene, and THF, respectively.
CONCLUSION
■
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