Photoinduced energy transfer in a fullerene-oligophenylenevinylene conjugate

Article abstract of DOI:10.1039/b000564i

A fullerene derivative in which two oligophenylenevinylene (OPV) groups are attached to C₆₀ through a pyrrolidine ring has been prepared and photophysical studies in CH₂Cl₂ solution show that photoinduced energy transfer f

Full text of DOI:10.1039/b000564i

Photoinduced energy transfer in a fullerene–oligophenylenevinylene conjugate†
Nicola Armaroli,*a Francesco Barigelletti,a Paola Ceroni,b Jean-François Eckert,c Jean-François Nicoudc and
Jean-François Nierengarten*c
a Istituto di Fotochimica e Radiazioni d’Alta Energia (FRAE) del CNR, via Gobetti 101, 40129 Bologna, Italy.
E-mail: armaroli@frae.bo.cnr.it
b Dipartimento di Chimica ‘G. Ciamician’, Universita` di Bologna, via Selmi 2, 40126 Bologna, Italy
c
Groupe des Mate´riaux Organiques, Institut de Physique et Chimie des Mate´riaux de Strasbourg, Universite´ Louis
Pasteur et CNRS, 23 rue du Loess, 67037 Strasbourg, France. E-mail: niereng@ipcms.u-strasbg.fr
Received (in Cambridge, UK) 19th January 2000, Accepted 25th February 2000
A fullerene derivative in which two oligophenylenevinylene
(OPV) groups are attached to C60 through a pyrrolidine ring
has been prepared and photophysical studies in CH2Cl2
solution show that photoinduced energy transfer from the
OPV moieties to C60 occurs, and not electron transfer.
Following the observation of electron transfer from conducting
oligomers and/or polymers derived from poly(phenyleneviny-
lene) or poly(thiophene),1 and the successful preparation of
photovoltaic cells from such bulk heterojunction materials,2
several examples of covalent fullerene derivatives bearing a
conjugated oligomer substituent have been synthesized in the
past few years.3,4 As a part of this research, we have recently
shown that such hybrid compounds can be incorporated in
photovoltaic devices.4 Whereas the synthesis and applications
of such hybrid compounds have been investigated, their
electronic properties have been probed to a much lesser
degree.
In this paper, we report on the preparation and the electronic
properties of fullerene derivative 1 in which two OPV groups
are attached to C60 through a pyrrolidine ring using the related
OPV derivative 2 and the fulleropyrrolidine FP4 as reference
compounds. Interestingly, the photophysical studies of 1 in
solution show that photoinduced energy transfer from the OPV
moiety to C60 is the main pathway and not electron transfer.
Fig. 1 Reagents and conditions: i, CF3CO2H, CH2Cl2, H2O, room temp., 4
h (96%); ii, DIBAL-H, CH2Cl2, 0 °C, 1 h (90%); iii, TsCl, LiCl, DMAP,
CH2Cl2, 0 °C to room temp., 12 h (80%); iv, 3,5-dihydroxybenzyl alcohol,
K2CO3, KBr, 18-crown-6, acetone, D, 24 h (69%); v, MnO2, CH2Cl2, room
temp., 2 h (95%); vi, C60, N-methylglycine, toluene, D, 16 h (40%).
comparison of the E1/2 potentials of 1 and FP clearly shows that
the first three waves correspond to fullerene-centred reductions
and the slightly negative shift observed for 1 compared to FP
could be the result of a small interaction between the electron-
accepting C60 unit and the electron-donating OPV groups. The
fourth reduction process seen for 1 appears to be difficult to
assign unambiguously as either fullerene- or OPV-based. In the
anodic region, 1 presents two chemically irreversible and ill-
defined peaks, corresponding to the transfer of three electrons.
They can likely be attributed to the simultaneous oxidation of
the two OPV groups and the bridging dialkyloxybenzene unit,6
since these groups present in both model compounds 2 and FP
are irreversibly oxidized in the same potential region.
The ground state electronic absorption spectrum of 1 is
reported in Fig. 2 and, within the experimental error, it matches
the profile obtained by summation of the spectra of the
component units FP and 2. Importantly, on each moiety of 1, a
fairly good excitation selectivity can be achieved. Indeed, at l >
The synthesis of 1 is depicted in Fig. 1 and the functionalisa-
tion of C60 is based on the 1,3-dipolar cycloaddition5 of the
azomethine ylide generated in situ from aldehyde 7.
The electrochemical properties of 1, 2 and FP were
investigated by cyclic voltammetry (CV) in CH2Cl2–0.1 M
Bu4NPF6 solutions (Table 1). In the cathodic region, 1 shows
three reversible one-electron processes followed by a bielec-
tronic peak (IVred), visible only at low temperature. A
Table 1 E1/2 or Ep values (V vs. SCE) determined by CV on a glassy carbon
electrode at 298 K, unless otherwise noted,b of compounds 1, 2 and FP in
CH2Cl2–0.1 M Bu4NPF6 solutions
Compound
IIox
Iox
Ired
IIred
IIIred
IVred
1
2
FP
+1.7a,c +1.4a,c 20.66
+1.82a +1.28a 21.88b
+1.60a +1.38a 20.62
21.04
21.56 21.98b,d
21.54 21.90b
22.11a,b
21.01
† Electronic supplementary information (ESI) available: CV curves for 1, 2
and FP, and calculation of energy transfer rate constants. See http://
www.rsc.org/suppdata/cc/b0/b000564i/
a Chemically irreversible processes, peak potentials measured at 0.2 V s21
.
b T = 208 K. c Trielectronic process. d Bielectronic process.
DOI: 10.1039/b000564i
Chem. Commun., 2000, 599–600
This journal is © The Royal Society of Chemistry 2000
599

View Article Online
Fig. 2 Absorption spectra of 1 (full line), FP (dashed line) and 2 (dotted line)
in CH2Cl2 at 298 K; above 400 nm a multiplying factor of 10 is used. Inset:
fluorescence spectra of solutions of 2, FP and 1 (for all samples, optical
density = 0.50 at lexc = 360 nm). Above 660 nm the spectrofluorimeter
sensitivity is increased by three orders of magnitude, due to the weakness of
the FP fluorescence relative to that of 2.
Fig. 3 Sensitized fullerene triplet–triplet transient absorption spectrum of 1
at 298 K in CH2Cl2 air equilibrated solution, upon laser excitation at 355 nm
(energy = 5 mJ pulse21). The spectra were recorded at delays of 100, 300,
600, 900, 2000 ns following excitation (from top to bottom). The inset
shows the time profile of DA (690 nm) from which the spectral kinetic data
were obtained; the fitting is monoexponential and gives a lifetime of 540
ns.
530 nm light is exclusively directed to the fullerene fragment,
whereas at l = 360 nm at least 85% of the incident light is
absorbed by the OPV moieties.
absorption spectrum of the fulleropyrrolidine acceptor unit
allows calculation of the overlap integrals JF = 1.4 3 10214
cm6 mol21 and JD = 1.4 3 1024 cm for the two approaches,
Upon selective excitation of 1 on the fullerene fragment, the
typical fulleropyrrolidine fluorescence and triplet–triplet tran-
sient absorption spectra are observed.6,7 This indicates that the
excited state properties of the C60 fragment are not affected by
the presence of the nearby OPV moieties. On the other hand,
when excitation is directed to the latter (e.g. at l = 360 nm, see
above), intercomponent processes are evidenced. Under such
conditions, the intense fluorescence band characteristic of the
OPV moiety (Fem = 1.0, t = 1.0 ns) is not observed (Fig. 2),
whereas the typical fluorescence band of the fulleropyrrolidine
fragment (lmax = 710 nm, t = 1.3 ns) is detected; in addition,
the fullerene fluorescence quantum yields of 1 and FP obtained
at lexc = 360 nm are identical (Fem = 5.5 3 1024), although
in the former at least 85% of the incident light is absorbed by the
and results in estimated rate constants, kenF; ken 9 1012 s21 up
D
to 10 Å of intercenter separation. These estimates suggest that
the energy transfer step is so fast that the competing charge
separation path is not effective.
In conclusion, 1 appears to be a multicomponent array
containing a fullerene unit as a terminal receptor of excitation
energy.10 These results suggest the possibility of building up
more complex arrays where a larger number of OPV fragments
allow an even more pronounced excitation selectivity with an
antenna effect, and the attachment of a suitable electron donor
to the C60 unit would result in multicomponent artificial
photosynthetic systems where light energy harvesting is
followed by charge separation.11
OPV fragments. The excitation spectrum of 1, taken at lem
=
Notes and references
715 nm, matches the absorption profile throughout the UV–
VIS, range including the diagnostic band of the OPV moieties
around 360 nm. These findings are consistent with quantitative
occurrence of singlet–singlet energy transfer from the OPV unit
to the fullerene in the multicomponent array 1.
1 L. Pasimeni, A. L. Maniero, M. Ruzzi, M. Prato, T. Da Ros, G.
Barbarella and M. Zambianchi, Chem. Commun., 1999, 429; R. A. J.
Janssen, M. P. T. Christiaans, K. Pakbaz, D. Moses, J. C. Hummelen and
N. S. Sariciftci, J. Chem. Phys., 1995, 102, 2628; R. A. J. Janssen, J. C.
Hummelen, K. Lee, K. Pakbaz, N. S. Sariciftci, A. J. Heeger and F.
Wudl, J. Chem. Phys., 1995, 103, 788; N. S. Sariciftci, L. Smilowitz,
A. J. Heeger and F. Wudl, Science, 1992, 258, 1474.
2 L. Ouali, V. V. Krasnikov, U. Stalmach and G. Hadziioannou, Adv.
Mater., 1999, 11, 1515; L. S. Roman, M. R. Andersson, T. Yohannes
and O. Ingana¨s, Adv. Mater., 1997, 9, 1164; G. Yu, J. Gao, J. C.
Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789.
3 S.-G. Liu, L. Shu, J. Rivera, H. Liu, J.-M. Raimundo, J. Roncali, A.
Gorgues and L. Echegoyen, J. Org. Chem., 1999, 64, 4884; J. L. Segura
and N. Martin, Tetrahedron Lett., 1999, 40, 3239; T. Yamashiro, Y.
Aso, T. Otsubo, H. Tang, Y. Harima and K. Yamashita, Chem. Lett.,
1999, 443; F. Effenberger and G. Grube, Synthesis, 1998, 1372.
4 J.-F. Nierengarten, J.-F. Eckert, J.-F. Nicoud, L. Ouali, V. Krasnikov
and G. Hadziioannou, Chem. Commun., 1999, 617; J.-F. Nierengarten,
J.-F. Eckert, D. Felder, J.-F. Nicoud, N. Armaroli, G. Marconi, V.
Vicinelli, C. Boudon, J.-P. Gisselbrecht, M. Gross, G. Hadziioannou, V.
Krasnikov, L. Ouali, L. Echegoyen and S.-G. Liu, Carbon, 2000, in
press.
5 M. Prato and M. Maggini, Acc. Chem. Res., 1998, 31, 519.
6 M. Goez and G. Eckert, J. Phys. Chem., 1991, 95, 1179.
7 C. Luo, M. Fujitsuka, A. Watanabe, O. Ito, L. Gan, Y. Huang and C.-H.
Huang, J. Chem. Soc., Faraday Trans., 1998, 94, 527.
8 F. Prat, R. Stackow, R. Bernstein, W. Qian, Y. Rubin and C. S. Foote,
J. Phys. Chem. A, 1999, 103, 7230.
9 V. Balzani and F. Scandola, Supramolecular Photochemistry, Ellis
Horwood, Chichester, UK, 1991, p. 44.
10 D. M. Guldi, G. Torres-Garcia and J. Mattay, J. Phys. Chem. A, 1998,
102, 9679.
In order to monitor the fate of the lowest triplet state centred
on the fullerene moiety, following excitation of the OPV
counterpart, we performed a series of transient absorption
experiments by setting the excitation wavelength of an
Nd+YAG laser to 355 nm. The reference compound FP displays
a triplet–triplet transient absorption spectrum with lmax = 690
nm and t = 0.54 ms in air-equilibrated solution, which becomes
31 ms in deaerated solution due to the suppression of the well-
known, very efficient quenching of fullerenes by dioxygen.7,8
A
quite similar behaviour is observed for 1, which displays a
fullerene triplet yield formation equal to that of FP and the same
triplet lifetimes. In other words, preferential excitation of the
OPV moieties ( > 85%) quantitatively sensitizes the formation
of the lowest fullerene singlet state, which then populates the
lower lying triplet level (Fig. 3) via intersystem crossing.
From the electrochemical data one can place the energy of the
charge separated state9 of 1 at about 2 eV, i.e. well below the
energy of the lowest singlet excited state of the OPV moieties
(3.1 eV, as derived from the 77 K fluorescence spectrum).
However, even if the population of the charge separated state
following photoexcitation of the OPV units is thermodynam-
ically allowed, this process is not evidenced in CH2Cl2 solution.
The use of polar solvents does not change the observed pattern;
for instance, quantitative evidence for OPV?C60 singlet–
singlet energy transfer is also observed in benzonitrile solution.
We have performed model calculations on the OPV?C60
singlet–singlet energy transfer step by following both the
dipole–dipole and double-electron exchange approaches (for
details, see supplementary data).† The spectral overlap between
the luminescence spectrum of the donor OPV unit and the
11 H. Imahori and Y. Sakata, Eur. J. Org. Chem., 1999, 2445; D.
Kuciauskas, P. A. Liddel, S. Lin, T. E. Johnson, S. J. Weghorn, J. S.
Lindsey, A. L. Moore, T. A. Moore and D. Gust, J. Am. Chem. Soc.,
1999, 121, 8604.
Communication b000564i
600
Chem. Commun., 2000, 599–600