Synthesis and excited state properties of a [60]fullerene derivative bearing a star-shaped multi-photon absorption chromophore

Article abstract of DOI:10.1039/b601987k

The synthesis and excited state properties of a compound assembling C ₆₀ with a new multi-photon absorption chromophore are reported. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b601987k

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Synthesis and excited state properties of a [60]fullerene derivative
bearing a star-shaped multi-photon absorption chromophore{
Teresa M. Figueira-Duarte,a John Clifford,b Vincenzo Amendola,c Aline Ge´gout,a Jean Olivier,a
Franc¸ois Cardinali,a Moreno Meneghetti,*c Nicola Armaroli*b and Jean-Franc¸ois Nierengarten*a
Received (in Cambridge, UK) 9th February 2006, Accepted 23rd March 2006
First published as an Advance Article on the web 6th April 2006
DOI: 10.1039/b601987k
malonic acid mono-ester 3 under esterification conditions using
N,N9-dicyclohexylcarbodiimide (DCC) and 4-dimethylamino
pyridine (DMAP) in CH2Cl2 (Scheme 1). Treatment of the
bis-malonate thus obtained with C60, I2, and 1,8-diazabicyclo
[5.4.0]undec-7-ene (DBU) in toluene at room temperature afforded
the desired cyclization product 1 in 32% yield.
The synthesis and excited state properties of a compound
assembling C60 with a new multi-photon absorption chromo-
phore are reported.
The past several years have seen a considerable interest in the
development of photochemical molecular devices based on the
combination of C60 with p-conjugated oligomers.1 In particular,
such hybrid systems have shown interesting excited state properties
and have found applications in the field of solar energy
conversion.2 Whereas the photophysical properties of a wide
range of fullerene-(p-conjugated oligomer) dyads have been
reported so far,1 research focused on related assemblies combining
C60 and conjugated oligomers with strong multi-photon absorp-
tion (MPA) cross sections has been probed to a lesser degree.3
Such hybrid systems are however potentially interesting since they
should be capable of generating singlet oxygen upon multi-photon
absorption4 followed by energy transfer to the fullerene sensitizing
unit.5 This could broaden the applicability of C60 derivatives in
photodynamic therapy6 which is currently highly limited by both
the weak linear and the low induced absorptions of fullerene-based
sensitizers in the 650–750 nm region. As part of our research on
compounds combining C60 with p-conjugated oligomers, we now
report the synthesis and the excited state properties of compound 1
assembling C60 with a new MPA chromophore. In the design of
the MPA dye, we have selected a star-shaped system. Actually,
whereas the optimization of MPA like TPA compounds has
largely focused on 1-D structures, it has recently been shown that
increased dimensionality and branched structures lead to highly
effective multiphoton absorption.7
Compound 1 was characterized by 1H- and 13C-NMR, UV-vis
and IR spectroscopies. In addition, the structure of 1 was
confirmed by FAB mass spectrometry. The relative position of
the two cyclopropane rings in 1 on the C60 surface was determined
Compound 1 was prepared by taking advantage of the versatile
regioselective reaction developed in the group of Diederich8 which
led to C60 bis-adducts by a cyclization reaction at the C sphere
with bis-malonates in a double Bingel9 cyclopropanation. To this
end, bis-malonate was prepared by the reaction of diol 2 with
aGroupe de Chimie des Fullere`nes et des Syste`mes Conjugue´s,
Laboratoire de Chimie de Coordination du CNRS, 205 route de
Narbonne, 31077, Toulouse Cedex 4, France.
E-mail: jfnierengarten@lcc-toulouse.fr; Fax: 33 (0) 5 61 55 30 03;
Tel: 33 (0) 5 61 33 31 00
bIstituto per la Sintesi Organica e la Fotoreattivita`, Molecular
Photoscience Group, Consiglio Nazionale delle Ricerche, Via Gobetti
101, 40129, Bologna, Italy. E-mail: armaroli@isof.cnr.it;
Fax: 39 051 639 98 44; Tel: 39 051 639 98 20
cUniversity of Padova, Dept. of Chemical Sciences, Via Marzolo 1,
35131, Padova, Italy. E-mail: Moreno.Meneghetti@unipd.it;
Fax: 39 049 827 52 39; Tel: 39 049 827 51 27
{ Electronic supplementary information (ESI) available: Experimental
details for the preparation of 1. See DOI: 10.1039/b601987k
Scheme 1 Preparation of compound 1.
2054 | Chem. Commun., 2006, 2054–2056
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Fig. 1 A comparison of the absorption spectrum of the dyad 1 (–) in
CH2Cl2 with that of the summation (---) of the absorption spectra of the
reference compounds 2 and 4 in CH2Cl2. The inset shows the fluorescence
spectrum of 2 in CH2Cl2.
based on the molecular symmetry (C1) deduced from the 1H- and
13C-NMR spectra. The absorption spectrum of dyad 1 in CH2Cl2
is shown in Fig. 1, along with the profile obtained by summing the
spectra of its component units 2 and 4. This comparison suggests
substantial ground state interactions between the fullerene and
p-conjugated units in dyad 1, with the main absorption band of
chromophore 2, centered at around 335 nm, being reduced in
intensity. The ground state interactions in dyad 1 may be explained
by the close proximity of the two subunits as already observed for
a bis(phenanthroline)Cu(I) derivative substituted with cyclic C60
bis-adduct moieties.10 However, small conformational changes in
the hexasubstituted benzene chromophore resulting from the cyclic
structure involving two of its aromatic units may also explain, at
least in part, the observed differences.
Fig. 2 Sensitized singlet oxygen luminescence of dyad 1 (#) and the
fullerene 4 ($) in toluene (left) and benzonitrile (right) and while exciting
at 500 nm (top) and 355 nm (bottom).
oxygen than fullerene 4; (b) irrespective of solvent polarity or
excitation wavelength, the relative quantum yield of singlet oxygen
of dyad 1 is the same, pointing to identical yields of fullerene triplet
formation. The absence of any solvent effect on the yield of C60
triplet tends to discard any major role of electron transfer in the
cascade of photoinduced processes. Furthermore, no C60 anion
features were found in the NIR region down to 1200 nm, neither
for 1 nor during bimolecular quenching experiments between 2
and 4 in toluene and benzonitrile, down to a 20 ns timescale.
Therefore an occurrence of long-lived charge separated states is
excluded. These data suggest that quenching of the star-shaped
organic conjugated moiety is likely due to singlet–singlet energy
transfer to the fullerene fragment, followed by intersystem crossing
to generate the fullerene triplet, as observed for similar C60
arrays.11 The lifetime of the quenched fluorescence could not be
determined since it is shorter than our instrumental resolution
(200 ps). However, from the fluorescence quenching data,11 the
rate constant of the energy transfer process can be estimated as
¢1.3 6 1011 s21 in all solvents.
Data in Fig. 2 suggest that the fullerene moiety in 1 has an
inherently different yield of singlet oxygen sensitization, probably
related to its own structure where the organic conjugated fragment
is close to the carbon cage. This might lead to some protective
effects towards oxygen quenching as previously found in
bismethanofullerene dendrimers.12 The transient absorption spec-
tra of 1 and 4 (Fig. S1){ compare well with triplet spectra of
bismethanofullerenes reported earlier, however they are not fully
superimposable to each other (Dlmax ca. 10 nm). This could be
related to intramolecular interactions, as suggested above.
Notably, the fullerene triplet lifetimes of dyad 1 (557 ns and
39 ms in air-equilibrated and air-free toluene solutions, respectively)
are substantially longer than that of 4 (362 ns and 15 ms, Fig. S2).{
This confirms that the triplet features of the fullerene are inherently
different in 1 and 4 and protective effects of the organic fragment
toward quenching of O2 (see triplet lifetime in air-equilibrated
solutions) or solvent impurities (deaerated samples) could at least
partly explain this trend.12
The fluorescence spectrum of 2 in CH2Cl2 is shown in Fig. 1. It
displays a broad band with a maximum at 425 nm. The
corresponding singlet lifetime was measured to be 1.5 ¡ 0.2 ns
and the fluorescence quantum yield is 0.53 in CH2Cl2. Direct
excitation of the star-shaped organic moiety in dyad 1 at 335 nm
results in a 200-fold decrease of its emission intensity in toluene,
CH2Cl2 and benzonitrile, when compared to 4 under the same
conditions. Clear evidence on whether this process is followed by
sensitization of the fullerene moiety in dyad 1 cannot be obtained
because some residual emission from the appended fragment is still
present above 650 nm, and this is enough to mask the weak
fullerene fluorescence (W 5 0.0004). Monitoring the luminescence
of sensitized singlet oxygen allows us to indirectly evaluate the
relative yield of formation of the lowest triplet excited state of
C60.11 Fig. 2 shows the singlet oxygen emission spectra recorded
for fullerene 4 compared to dyad 1 under the same conditions in
toluene and benzonitrile and at two different excitation wave-
lengths (355 and 500 nm). These data provide the following
information: (a) Dyad 1 is always a poorer sensitizer of singlet
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transient spectra (Fig. S1){ show that other excited states such as
charge transfer are not involved in the process and that an energy
transfer between singlet states occurs. Accordingly, the fitting of
the experimental data of 1 can be obtained only with variation of
the parameter values of the fullerene. The fitting is reported in
Fig. 3 and the values of the fitting parameters in Table S1.{ In
particular, one finds larger excited states absorption cross sections
most probably due to the protective effect resulting from the large
substituent on the fullerene sphere in 1, but the overall picture
remains the same as deduced above.
In conclusion, the combination of C60 with a MPA chromo-
phore leads to improved optical limiting properties and owing to
the intercomponent photoinduced energy transfer, the excited state
deactivation dynamics is dominated by the fullerene chromophore.
These findings pave the way towards the design of new efficient
fullerene-based singlet oxygen sensitizer for photodynamic therapy
applications.
Fig. 3 Optical limiting of 2 (#), 4 ($) and 1 (n) in benzonitrile
solutions using 9 ns pulses at 532 nm. Straight lines starting from the axis
origin would indicate a linear behaviour. Non linear absorptions are
observed as deviations from straight lines. Fitting of the data is obtained
with models for the excited state dynamics (see text).
This work was supported by the CNRS, the CNR (commessa
PM-P04-ISTM-C1-ISOF-M5) and the EU (RTN Contract
HPRN-CT-2002-00171) and the Italian MIUR (FIRB/
RBNE033KMA, FIRB/RBNE01P4JF). We further thank fellow-
ships from the Re´gion Alsace - ADEME (A.G.) and the Re´gion
Alsace - CNRS (F.C.).
Fig. 3 shows the results of optical limiting measurements of
benzonitrile solutions of the two separated moieties 2 and 4, and of
dyad 1, using 9 ns pulses of a duplicated Nd : YAG at 532 nm. It is
well known that fullerene is a reverse saturable absorption system
with a greater absorption at high intensities due to the population
of triplet states.13 However, Fig. 3 shows that also 2 has a non
linear behavior which should be considered in evaluating the
behaviour of 1. A model which accounts for the non linear
behaviour of 2 is a sequential three photon absorption in which,
similarly to other cases,14 a first one-photon transition populates
an excited state of the molecule and then a two-photon absorption
from this excited state occurs (ES-TPA). The fitting is reported on
the figure and was obtained by solving coupled equations for the
population dynamics of excited states.15 We found a cross section
for the ES-TPA, s(2) 5 1.58 6 10243 cm4 s ph21 mol21, which is
three to four order of magnitude larger than values found for
ground state TPA, and is characteristic of excited states with very
polarizable electronic states14 (other fitting parameter values are
reported in the electronic supplementary information).{
The non linear behaviour of 4 is characteristic of a fullerene
derivative. In this case the fitting of the low intensity behaviour can
be obtained with a four level system with two singlet states and
two triplet states which absorb more than the singlet states.16 The
best fitting, using this model, is reported as a dotted curve in Fig. 3.
One finds that it fails to reproduce the large intensities data. The
simplest model, which accounts for the experimental data, must
take into account that another one-photon transition can occur
from the excited triplet state (values of the fitting are reported in
Table S1).{ In this case, we obtain a good fitting which also
accounts for the high intensity region (continuous line over full
circles). By combining the separated non linear behaviour of 2 and
4, we are able to obtain an overall non linear response which is
reported as a dashed line in Fig. 3. A small difference is found with
respect to the fullerene behaviour, but the experimental data of 1
are not reproduced. This is another clear indication for an excited
state interaction between the two moieties in 1. However the
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2056 | Chem. Commun., 2006, 2054–2056
This journal is ß The Royal Society of Chemistry 2006