Host-guest complexation of [60]fullerenes and porphyrins enabled by "click chemistry"

Article abstract of DOI:10.1002/chem.201300793

Herein the synthesis, characterization, and organization of a first-generation dendritic fulleropyrrolidine bearing two pending porphyrins are reported. Both the dendron and the fullerene derivatives were synthesized by Cu^I-catalyzed alkyne-azi

Full text of DOI:10.1002/chem.201300793

FULL PAPER
Click Chemistry
K.-H. L. Ho, I. Hijazi, L. Rivier,
C. Gautier, B. Jousselme, G. de Miguel,
C. Romero-Nieto, D. M. Guldi,
B. Heinrich, B. Donnio,
S. Campidelli* ................ &&&& — &&&&
Into the jaws… Two fullerene–por-
phyrin electron-donor–acceptor conju-
gates have been synthesized and char-
acterized. One of them possesses a par-
ticular shape that allows the formation
of supramolecular head-to-tail poly-
mers by the intermolecular encapsula-
tion of C60 within the ZnP jaws of
another molecule (see figure). The
strong electronic communication
between C60 and ZnP has been probed
by steady-state and time-resolved
spectroscopy as well as by cyclic
voltammetry. The nanostructures have
been studied by AFM and SEM
(ZnP=zinc–porphyrin).
Host–Guest Complexation of [60]Full-
erenes and Porphyrins Enabled by
“Click Chemistry”
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DOI: 10.1002/chem.201300793
Host–Guest Complexation of [60]Fullerenes and Porphyrins Enabled by
“Click Chemistry”
Khanh-Hy Le Ho,[a] Ismail Hijazi,[a] Lucie Rivier,[a] Christelle Gautier,[a]
Bruno Jousselme,[b] Gustavo de Miguel,[c] Carlos Romero-Nieto,[c] Dirk M. Guldi,[c]
Benoit Heinrich,[d] Bertrand Donnio,[d] and Stꢀphane Campidelli*[a]
Dedicated to Professor Maurizio Prato on the occasion of his C60th birthday
Abstract: Herein the synthesis, charac-
terization, and organization of a first-
generation dendritic fulleropyrrolidine
bearing two pending porphyrins are re-
ported. Both the dendron and the full-
erene derivatives were synthesized by
CuI-catalyzed alkyne–azide cycloaddi-
tion (CuAAC). The electron-donor–ac-
ceptor conjugate possesses a shape that
allows the formation of supramolecular
complexes by encapsulation of C60
within the jaws of the two porphyrins
of another molecule. The interactions
between the two photoactive units (i.e.,
C60 and Zn–porphyrin) were confirmed
by cyclic voltammetry as well as by
steady-state and time-resolved spec-
molecules, which indicates that C60 is
included in the jaws of the porphyrin.
The fulleropyrrolidine compound ex-
hibits a rich polymorphism, which was
corroborated by AFM and SEM. In
particular, it was found to form supra-
molecular fibrils when deposited on
substrates. The morphology of the fi-
brils suggests that they are formed by
several rows of fullerene–porphyrin
complexes.
ACHUTNGRENtNUG rosACHTUNGTRENcNGUN opy. For example, a shift of about
85 mV was found for the first reduction
of C60 in the electron-donor–acceptor
conjugate compared with the parent
Keywords: charge transfer · click
chemistry · fullerenes · porphyri-
noids · supramolecular chemistry
Introduction
complexity of the supramolecular assemblies to be continu-
ously increased and has led to, for example, fullerene-based
micelles, vesicles, and aggregates,[2–10] polymers,[11–17] and
liquid crystals,[18–27] as well as the organization of fullerene
on electrode or nanoparticle surfaces.[28–30]
Fullerene-based supramolecular assemblies have been ex-
tensively investigated in the last 15 years. The early work
was mainly concerned with the complexation of C60 with
molecular building blocks that favor the formation of inclu-
sion complexes.[1] Later, more sophisticated structures, in
which the encoding information was introduced onto fuller-
ene by using classical covalent functionalization, were re-
ported. This approach has permitted the size, order, and
The interest in fullerene-based materials stems from the
remarkable properties of C60, such as its electron-accepting
nature and the low re-organization energy in electron trans-
fer. With the aim of achieving efficient electron and/or
energy transfer, a wide variety of electron donors have been
associated with C60. In particular, combining porphyrins with
C60 has evolved as a powerful strategy for the synthesis of
mimics of natural photosynthetic systems and active materi-
als for photovoltaic applications.[28,29,31,32] Porphyrins, and in
particular face-to-face bis-porphyrin systems, are known to
form host–guest complexes with C60 that spontaneously lead
to discrete supramolecular assemblies[33–42] or polymers/
arrays of interleaved porphyrins and fullerenes.[43–48] The aim
of this work was the covalent linkage of jaw-like porphyrin
building blocks to fullerenes to create a key–receptor self-
assembly and allow the formation of self-assembled head-to-
tail polymers by the intermolecular complexation of C60
within the jaws of the bis-porphyrin construct. Among the
impressive literature on fullerenes and porphyrins, many ex-
amples of the synthesis of fullerene/porphyrin covalent
dyads have been reported;[32,49–53] however, to the best of
our knowledge, a molecular key–receptor for supramolecu-
lar assembly purposes has not been investigated. Note
[a] Dr. K.-H. L. Ho, Dr. I. Hijazi, L. Rivier, Dr. C. Gautier,
Dr. S. Campidelli
IRAMIS, Laboratoire dꢁElectronique Molꢂculaire
SPEC (URA 2464), CEA-Saclay, 91191 Gif-sur-Yvette (France)
Fax: (+33)1-69-08-66-40
E-mail: stephane.campidelli@cea.fr
[b] Dr. B. Jousselme
IRAMIS, Laboratoire de Chimie des Surfaces et Interfaces, SPCSI
CEA-Saclay, 91191 Gif-sur-Yvette (France)
[c] Dr. G. de Miguel, Dr. C. Romero-Nieto, Prof. D. M. Guldi
Department of Chemistry and Pharmacy
and Interdisciplinary Center of Molecular Materials
Friedrich-Alexander-Universitꢃt Erlangen-Nꢄrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
[d] Dr. B. Heinrich, Dr. B. Donnio
IPCMS - UMR 7504 (CNRS/Universitꢂ de Strasbourg)
23, rue du Loess, BP 43, 67034 Strasbourg CEDEX 2 (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300793.
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Host–Guest Complexation of C60 and Porphyrins
FULL PAPER
though, Martꢅn and co-workers recently reported the forma-
tion of polymers and dendrimers by using this concept by
combining C60 with dendrons bearing two or four extended
tetrathiafulvalenes (exTTF).[12,13]
Herein we report the synthesis, characterization, and or-
ganization of a first-generation dendritic fulleropyrrolidine
bearing two pendant porphyrins (2). Both the dendron and
the fullerene derivative were synthesized by the CuI-cata-
lyzed alkyne–azide cycloaddition (CuAAC) reaction,[54,55]
one of the most powerful reactions in “click chemistry”.[56]
We also report on the synthesis of the linear fulleropyrroli-
dine–porphyrin 1 and porphyrin 3, which were designed as
references for optical and electrochemical studies.
just before the CuAAC. Porphyrin 5 and dendron 6 were
synthesized according to literature procedures.[59] Fullerene
derivatives 1 and 2 were obtained by the reaction of fullero-
pyrrolidine 4 with 5 and 6, respectively, and porphyrin 3 by
the reaction of 5 with phenylacetylene.
The electrochemical and photophysical properties of 1
and 2 were fully characterized by cyclic voltammetry (CV)
and spectroscopy, including steady-state absorption and
emission spectroscopy, NMR spectroscopy, femtosecond
transient absorption spectroscopy, and nanosecond flash
photolysis. Moreover, the supramolecular organization of 2
was investigated by small-angle X-ray scattering, and atomic
force and scanning electron microscopy.
Figure 1a shows the cyclic voltammograms of 1–4 in THF,
and the oxidation and reduction potentials are reported in
Table 1. In the positive potential region, 3 gives rise to two
reversible one-electron oxidations, which have been attribut-
ed to the sequential oxidation of ZnP. In the negative poten-
tial region, no discernible reduction was observed. In con-
trast, 4 exhibited no signal in the oxidative region, whereas
two reversible reductions corresponding to the sequential
reduction of C60 were observed in the negative potential
Results and Discussion
The syntheses of C60–ZnPs 1 and 2 and porphyrin 3[57] are
depicted in Scheme 1. Fulleropyrrolidine derivative 4 was
synthesized by treating 4-(2-trimethylsilylethynyl)benzalde-
hyde and N-methylglycine with C60 in a 1,3-dipolar cycload-
dition reaction.[58] The acetylene moiety was deprotected
region. Compounds
1 and 2
both showed two reversible oxi-
dations and two reversible re-
ductions in the potential range
of 0.8 to ꢀ1.65 V. In fact, com-
parison of the integrations of
the oxidation signals of 1 and 2
corroborates the overall C60/
ZnP stoichiometries, that is,
ratios of 1:1 and 1:2, respective-
ly (see Figure S1 in the Sup-
porting Information).
The redox potentials of 1 are
similar to those of references 3
and 4, which suggests the ab-
sence of intra- or intermolecu-
lar interactions between ZnP
and C60 in solution. Different
behavior was observed for 2. In
this case, the first oxidation and
the first reduction were cathod-
ically shifted by about 25 and
85 mV, respectively, compared
with the corresponding referen-
ces. Such changes have been ra-
tionalized on the basis of p–p-
interacting C60 and porphy-
AHCTUNGTRENNUNG
rins.[60,61] This interaction im-
pacts the highest-occupied mo-
lecular orbital (HOMO) of ZnP
and, in turn, facilitates its oxi-
dation. Likewise, cathodic shifts
of the first reduction of C60
have been observed upon en-
capsulation.[36] CV showed that
Scheme 1. Reagents and conditions: a) NBu4F, THF, 08C, 1 h, then CuACHTUGNTRENNNUG
RT, for 1: 3 days, 54%; for 2: 4 days, 65%; b) phenylacetylene, CuACHTUGNTRENNNUG
20 h, 60%.
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S. Campidelli et al.
C60 is less susceptible towards reduction when it interacts
with the ZnP dendrons. The second oxidation and reduction
were, however, not subject to any appreciable changes due
to the loss of p–p interactions in the reduced and/or oxi-
dized form.
1
The H NMR spectra of 2 recorded in 1,1,2,2-tetra
N
AHCTUNTGREGN[NNU D2]ethane at different concentrations and different temper-
atures are shown in Figure 1b,c and Figure S2 in the Sup-
porting Information. The protons of the fulleropyrrolidine
are less affected than the aromatic protons by an increase in
concentration. In the aromatic region, the spectra show par-
tial coalescence and an upfield shift of 0.03 to 0.20 ppm for
the signals of the b-pyrrole and meso-phenyl protons as well
as the protons of the triazole and benzyl dendron core as
the concentration is increased. This behavior is likely due to
the formation of large aggregates in solution rather than to
the insertion of the fullerene moiety between the porphyrin
jaws. Indeed, even at low concentration (ca. 10ꢀ5 m), the
Soret band of the porphyrins in 2 shows the typical broaden-
ing and redshift characteristic of the complexation of fuller-
ene in the porphyrin jaws.
Steady-state absorption spectroscopy was used to probe
the interactions in 1 and 2 in the ground state by using three
different solvents, namely toluene, THF, and benzonitrile.
The absorption spectrum of 1 is best described as the sum of
the individual building blocks. In contrast, the absorption
spectrum of 2 shows appreciable differences compared with
1 and 3 (Figure 2a). The absorption peaks are noticeably
broader and are redshifted by around 3.5 nm in toluene and
by 1.5 nm in THF and benzonitrile. In addition, close inspec-
tion of the 700 nm region of the spectra in THF reveals a
new absorption, which has been assigned to charge-transfer
features and reflects a notable redistribution of charge den-
sity between the electron donor and the electron acceptor
(see Figure S3 in the Supporting Information).[39,62] We con-
cluded that notable electronic communication prevails in 2
between the two electro- and photoactive constituents.
Fluorescence measurements shed additional light on the
interactions between ZnP and C60 in the excited state. After
excitation at 425 nm, the fluorescence patterns of 1 and 2
match that of ZnP with maxima at 600 and 650 nm. The flu-
orescence quantum yields differ, however, markedly in tol-
uene (F=0.004 for 2, F=0.008 for 1, and F=0.04 for 3), a
trend that has tentatively been assigned to charge-transfer
quenching. Further evidence for this assignment has come
from the lack of notable C60 fluorescence in the red region,
at least for 2, which would have been indicative of an
energy-transfer product. For 2, the strong charge-transfer-in-
duced quenching is accompanied by additional emission fea-
tures in a range in which neither ZnP nor C60 emit apprecia-
bly. In toluene, the maximum of the new emission is at
945 nm (Figure 2b). Comparison with the ground-state ab-
sorption suggests charge-transfer character.[39,62] The quan-
tum yield of this newly developing emission band depends
on the solvent polarity, the yields decreasing in more polar
environments. Moreover, the effect of temperature is quite
interesting. In fact, gradually raising the temperature from
Figure 1. a) CV spectra of 1–4 at 10ꢀ3
m in 0.1m Bu4NPF6/THF at
100 mVsꢀ1 recorded with a platinum working electrode (S=0.2 mm2);
1
potentials determined versus ferrocene. b) Aromatic part of the H NMR
spectra of 2 and c) 1H NMR spectral region showing the fulleropyrroli-
dine protons of 2. The spectra were recorded at 328 K at different con-
centrations.
Table 1. Electrochemical potentials of ZnP 3 and C60 4 as well as C60–
ZnP 1 and 2 in 0.1m Bu4NPF6/THF with a platinum working electrode
(potentials are given vs. Fc+/Fc).
o0
o0
o0
E
[mV]
Eore0dð1Þ [mV]
E
[mV]
E
[mV]
redð2Þ
oxð1Þ
oxð2Þ
3
4
1
2
–
–
ꢀ995
385
–
705
–
ꢀ1565
ꢀ1560
ꢀ1555
ꢀ1010
ꢀ1080
385
360
705
715
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25 to 758C leads to a deactivation of the charge-transfer
emission at 945 nm and a reactivation of the intrinsic fluo-
rescence of ZnP at 600 and 650 nm (see Figure S4 in the
Supporting Information). Excess vibrational energy is
thought to interrupt the charge transfer in ZnP and C60. The
same effects were realized when adding DABCO to 2 to co-
ordinate the zinc center (not shown). In addition to the
aforementioned changes in emission, the Soret band under-
goes a significant narrowing, again an indication of diminish-
ing ZnP–C60 interactions.
Next, we looked for evidence of the charge transfer by
means of transient absorption measurements on the femto-
and nanosecond timescales. Beginning with 3, following the
photoexcitation at 420 nm, the transient spectra exhibit typi-
cal ZnP singlet excited-state features, that is, strong maxima
at 465, 575, and 630 nm as well as bleaching at 550 nm. The
ZnP singlet excited state is metastable and decays slowly by
intersystem crossing to the energetically lower-lying triplet
excited state (see Figure S5 in the Supporting Information).
Femtosecond experiments with 1 and 2 revealed again the
formation of the ZnP singlet excited state upon excitation at
420 nm. But instead of seeing intersystem crossing, we ob-
served new features corresponding to the one-electron-oxi-
dized radical cation of ZnP, namely a broad band at around
650 nm, and the one-electron-reduced radical anion of C60, a
peak at 1015 nm (Figure 2c). On the nanosecond timescale,
for 1, features of the radical ion pair state are observed in
THF and benzonitrile with lifetimes of 844 and 650 ns. Life-
times that are close to a microsecond and that are solvent-
dependent prompt charge-transfer dynamics driven by a
through-bond mechanism. In toluene, the C60 triplet excited
state evolves as a thermodynamically favored product of
charge recombination. In contrast, the lifetime of 2 is as
short as 286 ns, a finding that is rationalized on the basis of
a
through-space mechanism driven by intermolecular
charge-transfer interactions.
In short, strong electronic communications dominate both
the ground- and excited-state features in 2. As a necessity,
compact packing between the two electro- and photoactive
constituents is assumed. To this end, two scenarios, intramo-
lecular versus intermolecular complexation, are envisioned
and prompted
a concentration-dependent investigation
starting at 10ꢀ10 m and reaching 10ꢀ5 m. However, in this
range, no appreciable changes were noted in terms of ab-
sorption (i.e., position or width of the Soret and Q-bands)
or emission (i.e., relative ratio between the 600/650 and
945 nm bands). There is no doubt that in this concentration
range intramolecular forces govern the charge-transfer inter-
actions. In this regard, two structural details of 2 are crucial:
1) The overall flexibility around the triazole and 2) the
length of the connecting linker. Based on these observations,
we postulated that 2 can fold to ensure tight intramolecular
arrangements. Structures of the intramolecular complex
were simulated and relaxed by using HyperChem software
(MM+ method) and it appears that the C60 moiety can be
complexed in the porphyrin jaw of the same molecule (see
Figure S6 in the Supporting Information). It is only in the
Figure 2. a) Absorption spectra of 3 (black), 1 (light grey), and 2 (dark
grey) in toluene. b) Charge-transfer emission of 2 in toluene, THF, and
benzonitrile (PhCN) recorded at an excitation wavelength of 425 nm.
c) Differential absorption spectra (visible and near-IR) recorded upon
femtosecond flash photolysis (420 nm; 110 mJ) of 2 in THF after several
time delays between 0 and 125 ps at room temperature. The evolution of
time is shown by the curves of black to dark grey to light grey. d) Time–
absorption profile of the transient spectra in c) at 1010 nm, monitoring
the charge separation and charge recombination.
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S. Campidelli et al.
10ꢀ4 m range that the 600/650 to 945 nm ratio changes and
the 945 nm charge-transfer emission starts to take over. Ac-
cordingly, we hypothesize the commencement of intermolec-
ular complexation in line with the AFM experiments (see
below).
Complementary measurements by AFM helped to corrob-
orate the aforementioned conclusions. Dichloromethane sol-
utions of 2 deposited onto silicon surfaces revealed the pres-
ence of cylindrical fibrils. Two different concentrations of 2
were deposited and, in both cases, the AFM images showed
similar objects on the surface. Thus, compound 2 forms fi-
brils with diameters of 20–25 and 2–3 nm when deposited
from the more (0.40 mm) and less concentrated solutions
(0.15 mm), respectively (Figure 3). In contrast, 1 did not
show any nanostructures (see Figure S7 in the Supporting
Information) under the same conditions. Note that below
0.15 mm the formation of fibril structures of 2 was not clear-
ly observed.
The size of 2 was estimated by molecular modeling (Fig-
ure 3i). In particular, in the fully extended conformation 2
exhibits a conical shape with a length of 4.3 nm and a maxi-
mal height of about 1.8 nm. In this conformation, the size of
the cavity allows the complexation of C60 from a neighbor-
ing molecule. The postulated head-to-tail organization is
shown in Figure 3j. Due to the differences in the molecular
dimensions of 2 and the fibrils, one can suppose that the fi-
brils are formed of several rows of molecules.
SEM investigations were performed on 2 and fulleropyr-
rolidine 4. Our first intention was to crystallize 2. Despite
several attempts, we were unsuccessful in obtaining suffi-
ciently large monocrystals. However, important morphologi-
cal differences were observed when the crystallization was
induced by the slow diffusion of acetonitrile into THF solu-
tions of 2 or 4. For 2, an iridescent solution containing nano-
spheres of 2 evolved after 2 days. The suspension was drop-
cast on to silicon wafers and imaged by SEM (Figure 4a,b).
The spheres have an average diameter of around 400–
500 nm and are hollow. Under the same conditions of “crys-
tallization”, 4 showed radically different behavior. In this
case, nanoplates of 1–3 mm length and roughly 50–100 nm
thickness were formed (Figure 4c,d). Compound 4 is likely
to organize into bilayer structures in the nanoplates. In fact,
similar nano-objects have been reported by Nakanishi et al.
for fulleropyrrolidines containing long alkyl chains.[5,6] The
supramolecular organization of 2 into hollow spheres and
the role of the ZnP jaws appears to be less intuitive, but
seems to be driven by its overall conical-like shape and its
tendency to form long polymeric strands.
Figure 3. AFM images of 2 deposited by spin-casting on to Si/SiO2 sub-
strates different concentrations of 2 in dichloromethane. a,b) 0.40 mm 2;
views of the surface at different magnifications, that is, 5ꢇ5 and 2.5ꢇ
2.5 mm. c) 3D topography of b). d) Profile of the AFM image showing
the heights of the objects. e,f) 0.15 mm 2; views of the surface at different
magnifications, that is, 2.25ꢇ2.25 and 0.5ꢇ0.5 mm. g) 3D topography of
f). h) Profile of the AFM image showing the heights of the objects.
i) Representation of 2. j) Postulated organization of molecules of
2
through complexation of C60 in the cone formed by the two ZnPs of a
neighboring molecule.
Finally, to shed light on the supramolecular organization
of 2 and 4 in the nanostructures, we performed small-angle
X-ray scattering on both the nanospheres and nanoplates.
As expected, due to the isotropic shape and dimensions of
the nanospheres, only broad and weakly intense diffusions
instead of a diffraction pattern were seen for 2. In stark con-
trast, the diffractograms recorded for the nanoplates formed
by 4 show sharp signals with the following three sharp re-
flections: d1 =21.675 ꢆ, d2 =10.77 ꢆ, and d3 =7.085 ꢆ (Fig-
ure 4e). These reflections, with a spacing ratio 1:2:3, indi-
cates a long-range lamellar organization of the C60 moieties
with an average lamellar periodicity of d=21.5 ꢆ. The pres-
ence of numerous additional, sharp and intense diffraction
reflections with mixed hkl indices nevertheless indicates
sharp boundaries and the long-range three-dimensional ex-
tension of the supramolecular nanostructure formed by 4.
The d-layer spacing is in good agreement with the length of
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Host–Guest Complexation of C60 and Porphyrins
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tween the two electron-donor–acceptor conjugates. For ex-
ample, a shift of about 85 mV was found for the first reduc-
tion of C60 in 2 compared with the values for the parent mol-
ecules, which indicates that C60 is included in the jaws of the
porphyrin. Compound 2 exhibits a rich polymorphism,
which was investigated by AFM and SEM. In particular, 2
forms supramolecular fibrils; the morphology of the fibrils
suggests that they are formed of several rows of fullerene–
porphyrin complexes. Finally, the supramolecular organiza-
tion of 2 into hollow spheres and the role of the ZnP jaws
are still not clear and new investigations to understand the
formation of these structures are ongoing.
Experimental Section
Techniques: Absorption spectra were recorded in quartz cuvettes on a
Perkin–Elmer Lambda 900 UV-Vis-NIR or Cary5000 (Varian) spectro-
photometer. Emission spectra were recorded by using a FluoroMax-3
(HORIBA) or Cary Eclipse fluorescence spectrophotometer. FTIR spec-
1
tra were recorded on a Nicolet MAGMA-IR 860 spectrometer. H NMR
spectra were recorded with a Bruker AC-300 (300 MHz) spectrometer
with solvent used as internal reference; MS (MALDI-TOF): spectra
were recorded with a PerseptiveBiosystems Voyager DE-STR spectrome-
ter. Molecular modeling was performed by using the HyperChem soft-
ware in conjunction with the MM+ method. Cyclic voltamperometry was
performed in anhydrous tetrahydrofuran solutions (THF was distilled
just before use over K/benzophenone under N2). Electrochemical-grade
tetrabutylammonium hexafluorophosphate (0.1m as supporting electro-
lyte) from Fluka was used without purification. Solutions were deaerated
by bubbling nitrogen through them prior to each experiment, which were
performed under nitrogen. Experiments were performed in a one-com-
partment cell equipped with a platinum working electrode (S=0.2 mm2)
and a platinum counter electrode. An Ag/Ag+ (10 mm) electrode, cali-
brated against the ferrocene/ferricenium couple (Fc/Fc+) before and after
each experiment, was used as reference. The reference electrode was
equipped with a nonaqueous double bridge to avoid possible interference
with metal cations. Electrochemical experiments were carried out with an
EG&G potentiostat, model 273A. For the AFM analyses, the samples
were prepared by spin-coating on silicon wafers from a solution of fuller-
ene derivatives in dichloromethane and then investigated with a Veeco
Multimode Scanning Probe Microscope equipped with a Nanoscope IIIa
controller. For SEM analyses, the samples were drop-cast on silicon
wafers from a THF/acetonitrile suspension and then investigated with a
Hitachi S4500 scanning electron microscope. Femtosecond transient ab-
sorption studies were performed with 425 nm laser pulses (1 kHz, 150 fs
pulse width, ca. 100 nJ) from an amplified Ti:sapphire laser system
(Clark-MXR, Inc.). Nanosecond laser flash photolysis experiments were
performed with 532 nm laser pulses from a Quanta-Ray CDR Nd:YAG
system (6 ns pulse width) in a front-face excitation geometry. The experi-
ments were performed at room temperature.
Figure 4. a,b) SEM images of aggregates of 2 deposited by drop-casting
solutions of 2 in THF/ACN onto Si/SiO2 substrates. c,d) SEM images of
aggregates of fulleropyrrolidine 4 deposited by drop-casting solutions of
2 in THF/ACN onto Si/SiO2 substrates. e) Small-angle X-ray diffracto-
gram of the nanoplates formed by 4. f) Postulated organization of bilay-
ers of fulleropyrrolidines.
a fully extended conformation of a dimer of 4 (ca. 26.4 ꢆ)
and confirms a bilayer structure, provided that the molecu-
lar pairs are tilted by around 358 within the bilayer (Fig-
ure 4f).
Conclusion
We have described herein the synthesis and characterization
of two fullerene–porphyrin electron-donor–acceptor conju-
gates. The communication between the fullerene and por-
phyrin units was investigated by steady-state and time-re-
solved spectroscopy as well as by cyclic voltammetry. We
found that the two electron-donor–acceptor conjugates ex-
hibit slightly different behavior when photoexcited: For 1,
charge-separated features with a lifetime close to 1 ms were
detected, whereas for 2 the lifetime was as short as 286 ns.
This difference is explained by a through-bond charge-trans-
fer mechanism for 1 and a through-space mechanism for 2.
In fact, 2 possesses a shape that allows the formation of a
supramolecular polymer by complexation of the C60 moiety
within the jaws of two porphyrins of another molecule. The
electrochemical characterization also shows differences be-
Materials: C60 (99.9%) was purchased from MER Corporation. Chemi-
cals were purchased form Aldrich and were used as received. Solvents
were purchased form Aldrich or VWR and were used as received. For
synthesis, CH2Cl2 (CaH2, N2), toluene (K/benzophenone, N2), THF (K/
benzophenone, N2) were distilled before use. 4-(2-Trimethylsilylethynyl)-
benzaldehyde,[63] azido-porphyrin 5 and porphyrin dendron 6[59] were syn-
thesized according to literature procedures.
Syntheses
Fulleropyrrolidine 4: C60 (50 mg, 0.069 mmol) was dissolved in dry tol-
uene (50 mL) and then 4-(2-trimethylsilylethynyl)benzaldehyde (14 mg,
0.069 mmol) and N-methylglycine (62 mg, 0.693 mmol) were added. The
mixture was stirred overnight at reflux, and evaporated to dryness. Purifi-
cation of the residue by column chromatography (eluent toluene) and
precipitation (dissolution in CH2Cl2 and precipitation by pouring the sol-
Chem. Eur. J. 2013, 00, 0 – 0
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S. Campidelli et al.
ution into MeOH) gave pure 3 (45 mg, 68% yield) as a brown powder.
1H NMR (300 MHz, CDCl3): d=7.77 (brd, J=7.2 Hz, 2H; arom. H),
7.55 (d, J=8.4 Hz, 2H; arom. H), 4.99 (d, J=9.3 Hz, 1H; H pyrrolidine),
4.93 (s, 1H; H pyrrolidine), 4.26 (d, J=9.6 Hz, 1H; H pyrrolidine), 2.79
(s, 3H; NCH3), 0.24 ppm (s, 9H; SiMe3); FTIR (KBr): n˜ =2948, 2779,
2155, 1500, 1462, 1427, 1331, 1246, 1216, 1122, 1103, 864, 842, 758, 704,
through hydrophobic filter paper (Phase separator Whatman filter) and
evaporated to dryness. Purification of the residue by column chromatog-
raphy (eluent first CH2Cl2, then CH2Cl2/Et2O, 100:5 to 100:8) gave pure 2
(13 mg, 65% yield) as a brown-purple powder. 1H NMR (300 MHz,
C2D2Cl4, 558C): d=9.04 (d, J=4.8 Hz, 4H; arom. H), 8.98 (s, 8H; arom.
H), 8.92 (d, J=4.2 Hz, 4H; arom. H), 8.36 (s, 2H; arom. H), 8.30 (d, J=
7.5 Hz, 4H; arom. H), 8.20–8.05 (m, 16H; arom. H), 7.93 (d, J=8.1 Hz,
2H; arom. H), 7.88 (s, 1H; arom. H), 7.82–7.72 (m, 12H; arom. H), 7.62–
7.55 (br m, 2H; arom. H), 6.94 (brs, 2H; arom. H), 6.85 (brs, 1H; arom.
H), 5.57 (brs, 2H; CH2N), 4.49 (brs, 4H; CH2O), 4.49 (d, J=9.6 Hz, 1H;
H pyrrolidine), 4.31 (s, 1H; H pyrrolidine), 3.67 (d, J=9.6 Hz, 1H; H pyr-
582, 552, 526 cmꢀ1
; UV/Vis (toluene): lmax =328, 432, 703 nm; MS
(MALDI-TOF): m/z calcd for C74H19NSi: 949.13 [MꢀH]+; found: 948.13.
Porphyrin 3: Phenylacetylene (4mL, 0.034 mmol), [CuACHTNUTRGNEUNG(MeCN)4]PF6
(3 mg, 0.008 mmol), 2,6-lutidine (100 mL), and water (1 mL) were added
to a solution of porphyrin 5 (15 mg, 0.017 mmol) in THF (10 mL). The
reaction mixture was frozen and the oxygen was removed by several
vacuum/argon cycles. Finally, the solution was stirred at 608C for 20 h,
then water was added, and the mixture was extracted with CH2Cl2. The
organic phase was filtered through hydrophobic filter paper (Phase Sepa-
rator Whatman filter) and evaporated to dryness. Purification of the resi-
due by column chromatography (eluent CH2Cl2) gave pure 3 (10 mg,
rolidine), 2.65 (s, 3H; NCH3) 1.54 ppm (s, 54H; CACHTNUGTRNEUGN(CH3)3); FTIR (KBr):
n˜ =2954, 2923, 2902, 2863, 1596, 1524, 1492, 1460, 1393, 1362, 1337, 1267,
1205, 1151, 1109, 1067, 1039, 998, 852, 810, 796, 720, 668, 576 cmꢀ1; UV/
Vis (toluene): lmax =428, 553, 591 nm; MS (MALDI-TOF): m/z calcd for
C196H124N18O2Zn2: 2888.87 [M]+; found: 2888.80.
1
60% yield) as a purple powder. H NMR (300 MHz, CDCl3): d=8.04 (d,
J=4.5 Hz, 2H; arom. H), 9.00 (s, 4H; arom. H), 8.95 (d, J=4.8 Hz, 2H;
arom. H), 8.53 (s, 1H; arom. H), 8.43 (d, J=8.4 Hz, 2H; arom. H), 8.21
(d, J=8.4 Hz, 2H; arom. H), 8.16 (d, J=8.1 Hz, 6H; arom. H), 8.05 (d,
J=8.4 Hz, 2H; arom. H), 7.77 (d, J=8.4 Hz, 2H; arom. H), 7.55 (t, J=
7.5 Hz, 2H; arom. H), 7.48–7.40 (m, 1H; arom. H), 1.63 ppm (s, 27H; C-
Acknowledgements
This work was supported by ANR (projects f-DNA ANR-09-NANO-005-
01 and TRANCHANT-ANR 2010 BLAN 1009 4) and by the Region Ile-
de-France through the framework of CꢁNanoIdF, the Nanoscience Com-
petence Center of the Paris Region (projects ElecTubes and TENAPO).
K.-H.L.H. and H.I. acknowledge ANR and CꢁNanoIdF for doctoral and
postdoctoral grants. D.M.G. and C.R.N. acknowledge the Bavarian initia-
tive "Solar Technologies Go Hybrid" and the DFG "Excellence Clus-
ter—Engineering of Advanced Materials".
ACHTUNGTRENNUNG
;
UV/Vis (toluene): lmax =425, 550, 589 nm; MS (MALDI-TOF): m/z calcd
for C64H57N7Zn: 987.40 [M]+; found: 987.42.
Fullerene–porphyrin 1: A 1m TBAF solution in THF (50 mL) was added
to a solution of fullerene 4 (15 mg, 0.015 mmol) in dry THF (20 mL) at
08C under argon. The reaction was stirred for 1 h and then water was
added, and the mixture was extracted with CH2Cl2. The organic phase
was filtered through hydrophobic filter paper (Phase separator Whatman
filter) and evaporated to dryness. The fulleropyrrolidine was used directly
for the coupling reaction without further purification. The fullerene de-
rivative was redispersed in THF (30 mL) and then porphyrin 5 (12 mg,
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ACHTUNGTRENNUNG
;
(MALDI-TOF): m/z calcd for
C
127H62N8Zn: 1762.44 [M]+; found:
1762.41.
Fullerene–porphyrin 2: A 1m TBAF solution in THF (100 mL) was added
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Received: February 28, 2013
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Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!