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A. Gégout et al.
LETTER
triad 17, compared to reference 3. By contrast, the same acts as a light harvesting antenna unit and electron transfer
comparison shows a substantial quenching of the fluores- only takes place upon direct or indirect excitation of the
cence band (ca. 70%) in more polar CH2Cl2 and benzo- fullerene fragment.
nitrile. The fullerene singlet lifetime of 17 is shortened
In conclusion we have synthesized a novel OPV deriva-
accordingly to 0.7 0.1 ns in both solvents. This medium
tive substituted with two fullerene subunits, which exhibit
polarity dependent quenching is attributable to intra-
light induced energy and electron transfer processes from
molecular photoinduced electron transfer from the OPV
the OPV central unit to the appended C60 fragments. The
moiety to the carbon sphere.5c–5e The experimental rate for
charge separation process shows a strong solvent-polarity
charge separation, as determined with lifetime data, turns
dependence.
out to be 7.6·108 s–1. In an attempt to detect the charge re-
combination processes we carried out transient absorption
Acknowledgment
measurements on the nanosecond timescale to detect
fingerprints of the cationic (OPV+) and anionic species
(C60–) in the charge separated state.
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). We further thank the Région Alsace – ADEME for a
fellowship to A.G.
Down to a scale of 10 ns no such signals were detected
indicating that back electron transfer occurs on the sub-
nanosecond timescale, as observed earlier for OPV-C60
arrays.5b,c,e,13 Nanosecond time resolved absorption spec- References and Notes
tra of 17 in the three solvents show only the features of the
(1) Müllen, K.; Wegner, G. Electronic Materials: The Oligomer
lowest fullerene triplet level (3C60*). In air equilibrated so-
lution the lifetimes of the excited states of 3 and 17 are
identical in each solvent (670 ns in CH2Cl2). In oxygen
free solution they turn out to be shorter for 17 compared
to 3 (8 and 30 ms, respectively, in CH2Cl2), probably due
to intramolecular excited-state interactions among the two
fullerene subunits of 17.14 3C60* is a potent singlet oxygen
(1O2*) sensitizer5e and the sensitization process was mon-
itored in air equilibrated solutions of 17 and 3 by record-
ing the singlet oxygen luminescence spectra in the near
infrared region (Figure 2). Notably, the ratio of 1O2* lumi-
nescence intensity between 17 and 3 is pretty much the
same as that measured for C60 fluorescence spectra. These
findings show that the lower extent of triplet formation in
17 is only due to reduced intersystem crossing from the
singlet level, which is depleted due to electron transfer
(see above). It must be pointed out that the trend of emis-
sion spectra in Figure 2 is exactly the same either by exci-
tation of the C60 (l = 520 nm) or of the OPV moiety (l =
415 nm), clearly indicating that the OPV rod of 17 simply
Approach; Wiley-VCH: Weinheim, 1998.
(2) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning,
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(12) Compound 17: 1H NMR (300 MHz, CDCl3): d = 7.69 (s, 2
H), 7.48 (m, 16 H), 7.32 (d, 3J = 16.0 Hz, 2 H), 7.15 (d,
3J = 16.0 Hz, 2 H), 7.08 (s, 4 H), 7.04 (d, 3J = 16.0 Hz, 2 H),
6.94 (d, 3J = 16.0 Hz, 2 H), 6.72 (s, 4 H), 6.57 (d, 4J = 1.5 Hz,
Figure 2 Fluorescence spectra (left) and sensitized singlet oxygen
luminescence (right) of 17 (triangles) and 3 (empty circles) in three
different solvents. From top to bottom toluene, CH2Cl2 and benzo-
nitrile; in all cases lexc = 520 nm, 298 K.
Synlett 2006, No. 18, 3095–3099 © Thieme Stuttgart · New York