Fourth, determining the quantum yield of the fullerene fluores-
cence (i.e., 6.0 6 1024 in toluene and o-dichlorobenzene) shows
that its formation is quantitative in 5, despite the exclusive
excitation of the oligomer fragment.
electron donor OPE unit and electron acceptor fullerene moieties.
Although energy transfer processes between C60 and the central
p-conjugated oligomer12 prevents an efficient photoconversion
performance, the combination of 5 and P3HT led to external
quantum efficiencies of 15%. Work is currently in progress
towards the preparation of new nanoarrays and nanoensembles
with better donor abilities and light-absorbing properties. This is
expected to improve their applicability as active components in
solar cells.
To gather mechanistic evidence for the fullerene fluorescence we
inspected the excitation spectrum of the 650–850 nm features.
Since, the resulting spectrum matches the ground state spectrum of
the oligomer (Fig. S-3, see ESI{), we postulate a quantitative
transfer of singlet excited state energy from the high lying oligomer
singlet state (2.45 eV) to the lower lying fullerene singlet (1.76 eV).
Undisputable evidence for the reaction sequence con-
sidered above came from time-resolved transient absorption
Financial support by the MCyT of Spain and Comunidad de
Madrid (Projects BQU2002-00855 and HSE/MAT0633-04), the
Dutch Polymer Institute (DPI#324) and the EU (RTN network
‘‘WONDERFULL’’ and ‘‘CASSIUS CLAYS’’), SFB 583, DFG
(GU 517/4-1), FCI, and the office of Basic Energy Sciences of the
U. S. Department of Energy (NDRL 4638), are gratefully
acknowledged. We thank the CAIs of the UCM. C. M. A. and
G. F. thank MCyT and MEC for a research grant.
measurements. Singlet and triplet features of
4
and
N-methylfulleropyrrolidines were described recently.5 Important
for the current work are only the singlet–singlet characteristics that
include maxima at around 600 and 880 nm, for compound 4 and
N-methylfulleropyrrolidines, respectively. Upon 387 nm femtose-
cond excitation of the oligomer–fullerene conjugate, where the
oligomer absorption is dominant, led initially to the singlet excited
absorption of the oligomer fragment with a strong maximum at
875 nm. In contrast to the reference, which shows a fairly stable
singlet–singlet signature, 5 reveals an ultrafast singlet deactivation
(Fig. S-4 and S-5, see ESI{). Hereafter, only the fullerene singlet–
singlet transitions are observed at 5 ps (Fig. S-4, see ESI{) with a
maximum around 890 nm. Again, a singlet energy transfer is
responsible for this observation. The kinetics of the energy transfer
can be (i) estimated from the y1000-fold fluorescence quenching
in 5 and the 1.14 ns fluorescence lifetime of 4 to be extremely fast,
i.e. on the order of 1012 s21 (1 ps) and (ii) determined from the
femtosecond experiments (Fig. S-5, see ESI{) as 8.3 ¡ 0.5 6
1011 s21. Singlet excited N-methylfulleropyrrolidines are known to
form the corresponding triplet state with near unit quantum yield
via the intersystem crossing dynamics (i.e., 6.6 6 108 s21). The
same process occurs in the nanoarray 5—Fig. S-6{ shows, for
example, the triplet–triplet transition in the 550–950 region, as
detected at the conclusion of the femtosecond experiments (i.e.,
1.5 ns). When extending the time-resolved measurements into the
nanosecond and microsecond time windows the same triplet
features were found—see Fig. S-6{—with an overall quantum
yield of 0.98. Under our experimental conditions the oxygen-
sensitive fullerene triplet decays with a lifetime of about 20 ms and
reinstates hereby—in the absence of molecular oxygen—the singlet
ground state of 5.
Notes and references
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This photophysical relaxation and the PV effects of pure 5 and
its blend with P3HT are confirmed by photoinduced absorption
experiments on thin solid-state films recorded at 80 K (Fig. S-7, see
ESI{). Under these conditions, pure 5 shows the broad featureless
spectrum of the fullerene triplet state and a vibronically resolved
photoluminescence at 1.68 and 1.52 eV. No photoinduced charges
are observed, consistent with the absence of a PV effect. In
contrast, a mixture of P3HT and 5, measured under identical
conditions shows the clear features of P3HT radical cations at
y0.5 and 1.3 eV. Importantly, the spectrum of the P3HT : 5 blend
differs considerably from that of neat P3HT, which is dominated
by a triplet band at 1.06 eV and that of 5.
In summary, a visible light-absorbing nanoarray—integrating
four C60 moieties and a single p-conjugated oligomer—has been
synthesized from bisaldehyde 1. The redox studies reveal an
amphoteric redox behavior stemming from the presence of both
516 | Chem. Commun., 2006, 514–516
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