nent architectures with fullerenes, SOSIP was inconceivable at
this point because the method is suited for stacks of planar ar-
omatics and highly sensitive toward structural changes. TSE, in
sharp contrast, is very tolerant to structural changes and is, in
principle, compatible with the construction of strings of molec-
ular spheres. Here we report facile access to oriented strings of
fullerenes along oriented stacks of oligothiophenes, that is,
double-channel photosystem 3, by SOSIP-TSE with fullerene 4.
Fullerene 4 was designed based on difficulties to achieve
stack exchange with fullerene 5 (not shown). It contains an ar-
omatic aldehyde to give stable hydrazones and two tri(eth-
ylene glycol) (TEG) solubilizers to reach the concentrations in
the polar aprotic solvents needed for TSE. The synthesis was
based on protocols from the Nierengarten group. One TEG sol-
ubilizer 6 was attached to malonic acid 7 as described
(Scheme 1).[2c] The resulting malonate monoester 8 was reacted
Figure 2. A) UV/Vis absorption spectra, B) photocurrent generation and
C) action spectra of oligothiophene SOSIP photosystem 1 (c) and oligo-
thiophene-fullerene photosystem 3 (c). Incident photon-to-current effi-
ciency (IPCE) values are normalized against the IPCE of 1 at 420 nm (A=ab-
sorbance, AU=absorbance units, J=photocurrent density, rel=relative).
action in 2 h, the yield of stack exchange was estimated by
comparing the absorbance of thiophene at 420 nm and fuller-
ene at 320 nm before and after the stack exchange (see Sup-
porting Information). The obtained 50% yield is reasonable
considering the three-dimensional bulk of the fullerenes. The
fate of the remaining acyl hydrazines, if any, is unknown. Cova-
lent addition to the fullerenes and oxidation during photocur-
rent generation are less likely given the mild conditions[17] and
the test-retest reliability of the photocurrent kinetics (see
below).
Scheme 1. Synthesis of formyl-fullerene 4. Reagents and conditions: a) 1108C,
4 h, 80%;[2c] b) 2,2-dimethyl-1,3-propanediol, C6H6, pTosOH cat., D, Dean–
Stark trap, 97%; c) tBuLi (4 equiv), THF, À78–08C, then DMF, À78–08C, then
aq. 2m HCl, 58%; d) DIBAL-H, DCM, 08C, 97%;[2a] e) DCC, DMAP, DCM, 08C to
RT, 43%; f) C60, DBU, I2, toluene, RT, 39%; g) TFA, H2O, DCM, RT, 50%.
Photocurrent generation was examined under standard
assay conditions. In brief, the photosystem was used as a work-
ing electrode together with a platinum wire counter electrode
and a silver/silver chloride reference electrode. Triethanolamine
(TEOA) was used as a mobile sacrificial hole acceptor; activities
found with alternative hole acceptors such as the reversible
carrier p-methoxyaniline di(2-ethylsulfonic acid) (MDESA) or
ascorbic acid were analogous but overall clearly weaker.[15] Irra-
diated with a solar simulator, oligothiophene–fullerene conju-
gate 3 generated much higher photocurrent than the photo-
system 1 with only oligothiophenes (Figure 2B). Repeated pho-
tocurrent generation gave unchanged kinetic profiles, suggest-
ing that the decrease observed with 3 is not due to instability
of the photosystem. Although conceivable with a HOMO
energy around the À5.7 eV reported for the quaterthio-
phenes,[14] irreversible oxidation of unreacted acyl hydrazines, if
occurring, does therefore not account for the phenomenon.
Repeatable photocurrent decrease with time thus originates
most likely from biphasic saturation behavior somewhere
along the charge-transporting pathways.
with the primary alcohols in diol 9 to give bis-malonate 10.
Diol 9 was prepared as described by Nierengarten et al.[2a]
Namely, 3,5-dibromobenzaldehyde 11 was protected as
acetal 12 and formylated by treatment with first tert-butyl lithi-
um (tBuLi) and then N,N-dimethylformamide (DMF). Reduction
of the obtained dialdehyde 13 gave diol 9. The regioselective
double Bingel cyclopropanation[2d] of C60 with bis-malonates
such as 10 has been developed early on in the Diederich
group.[2b] The obtained macrocyclic bis-adduct 14 was depro-
tected to afford the final formyl-fullerene 4.
Oligothiophene SOSIP was prepared, and the benzaldehyde
template was removed as reported previously.[14] Hydrazone
bond formation between formyl-fullerene 4 and the photosys-
tem 2 was evidenced spectroscopically by the appearance of
characteristic fullerene absorption bands (Figure 2A, Figure S7).
Unlike the previously tested fullerene 5, facile incorporation of
fullerene 4 demonstrated the importance of its high solubility
in polar organic solvent, and the tolerance of stack exchange
to the structural variation. After apparent completion of the re-
The action spectra (Figure 2C) revealed about 15-times in-
creased charge generation by the oligothiophene stack upon
conjugation with fullerenes. Absorption and action spectra are
ꢀ 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemistryOpen 2013, 2, 55 – 57 56
Bolag, Altan
