Kanato et al.
parabola,5 thus leading to slow charge recombination.6
A variety of donors have been thus attached to fullerenes.7
Among them, we studied the [60]fullerene-oligothiophene
dyads 1a -c (x ) 1-3, abbreviated as n T-C60) (Chart
1), in which the fullerene is covalently bonded to a
terminal position of oligothiophenes.8 The photophysical
study of the dyads 1a -c revealed that, although energy
transfer from the excited oligothiophene chromophore to
the fullerene occurred in nonpolar toluene,8 highly ef-
ficient photoinduced electron transfer preferably occurred
in polar solvents9 and in the solid state.10 As the ensuing
step of our efforts directed toward the development of
oligothiophene-based optoelectronic materials, we have
focused on the ferrocene-oligothiophene-fullerene triads
2 (abbreviated as F c-n T-C60). Direct attachment of a
strongly electron-donating ferrocene at the unsubstituted
terminal site of the oligothiophene might promote pho-
toinduced electron-transfer due to the stabilization of the
resulting charge-separated states. For comparison, we
have also studied another type of triads 3 (abbreviated
as F c-tm -n T-C60) inserting a trimethylene spacer be-
tween the ferrocene and the oligothiophene moiety, which
interferes with direct conjugation between the two chro-
mophores. Here we would like to report the synthesis and
photophysical properties of these two types of triads 2a -c
and 3a -c.
CHART 1
between 5,5′-dibromo-2,2′-bithiophene (4)12 and 2 equiv
of Grignard reagent (5b) in-situ derived from 3-hexyl-2-
thienyl bromide (5a )13 in ether-benzene under reflux.
Then, 4T was lithiated with 1 equiv of butyllithium in
THF, and the resulting lithiated species was subjected
to oxidative coupling with copper(II) chloride at room
temperature to give a mixture of tetrahexyloctithiophene
(6b, 8T) (40% yield) and hexahexylduodecithiophene (6c,
12T) (10% yield), which were smoothly separated by
column chromatography.
Oligothiophenes 6a -c were subjected to Vilsmeier
reactions at one of the terminal positions with DMF and
phosphorus oxychloride in 1,2-dichloroethane at 40 °C
to give the corresponding formyl derivatives 7a -c in 40-
53% yields, and subsequent bromination at the other
terminal position with NBS in CS2-DMF at room tem-
perature led to the formation of the bromo-oligo-
thiophene-carbaldehydes 8a-c in 80-90% yields (Scheme
2). The Negishi coupling of 8a -c with ferrocenylzinc
chloride (9) in the presence of catalytic tetrakis(tri-
phenylphosphine)palladium in THF14 provided the fer-
rocenyl-oligothiophene-carbaldehydes 10a -c in 40-60%
yields. Finally, treatment of 10a -c with fullerene and
N-methylglycine (Prato method)15 in refluxing toluene
afforded the ferrocene-oligothiophene-fullerene triads
2a -c in 40-50% yields.
Resu lts a n d Discu ssion
Syn th esis. For the basic skeletons of the F c-n T-C60
and the F c-tm -n T-C60 triads, three oligothiophenes,
quaterthiophene (6a , abbreviated as 4T), octithiophene
(6b, 8T), and duodecithiophene (6c, 12T), were examined.
According to the route shown in Scheme 1,8,11 3,3′′′-
dihexyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (4T) was pre-
pared in 95% yield by a nickel-catalyzed coupling reaction
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The syntheses of another type of triads 3a -c (F c-tm -
n T-C60) are not so straightforward. The key to their
syntheses is how to approach the ferrocenylpropyl-
oligothiophene-carbaldehydes 15a -c. The initially at-
tempted approach shown in Scheme 3 was successful only
for the synthesis of the quaterthiophene derivative 15a .
Thus, ferrocene (11) was lithiated with t-butyllithium in
1:1 hexane-THF at 0 °C and then reacted with 1-bromo-
3-chloropropane at room temperature to give 3-chloro-
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