Synthesis and photophysical properties of ferrocene-oligothiophene- fullerene triads

Article abstract of DOI:10.1021/jo049137o

To promote photoinduced charge separation previously observed for the oligothiophene-fullerene dyads (nT-C60), we have designed an additional attachment with a strongly electron-donating ferrocene at the unsubstituted terminal site of the oligothiophene and synthesized two types of the ferrocene-oligothiophene-fullerene triads, Fc-nT-C60 directly linking the ferrocene to the oligothiophene and Fc-tm-nT-C60 inserting a trimethylene spacer between the ferrocene and the oligothiophene. For the central oligothiophene of the triads, a homologous series of quaterthiophene (4T), octithiophene (8T), and duodecithiophene (12T) are systematically examined. The cyclic voltammograms and electronic absorption spectra of Fc-nT-C60 indicate conjugation between the ferrocene and oligothiophene components. The emission spectra of Fc-nT-C60 measured in toluene demonstrate that the fluorescence of the oligothiophene is markedly quenched, as compared to that observed for the dyads nT-C60. This quenching is explained in terms of the involvement of intramolecular electron transfer in the photophysical decay process. The additionally conjugated ferrocene evidently contributes to the stabilization of charge separation states, thus promoting intramolecular electron transfer. This is corroborated by the observation that the emission spectra of the nonconjugated triads Fc-tm-nT-C60 are essentially similar to the corresponding dyads nT-C60.

Full text of DOI:10.1021/jo049137o

Syn th esis a n d P h otop h ysica l P r op er ties of
F er r ocen e-Oligoth iop h en e-F u ller en e Tr ia d s
Hiroki Kanato,† Kazuo Takimiya,† Tetsuo Otsubo,*,† Yoshio Aso,‡ Takumi Nakamura,§
Yasuyuki Araki,§ and Osamu Ito*,§
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,
Higashi-Hiroshima 739-8527, J apan, The Institute of Scientific and Industrial Research,
Osaka University, CREST, J apan Science and Technology Agency (J ST), Ibraki, Osaka 567-0047, J apan,
and Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
Katahira, Aoba-ku, Sendai, 980-8577, J apan
otsubo@hiroshima-u.ac.jp; ito@tagen.tohoku.ac.jp
Received May 22, 2004
To promote photoinduced charge separation previously observed for the oligothiophene-fullerene
dyads (n T-C60), we have designed an additional attachment with a strongly electron-donating
ferrocene at the unsubstituted terminal site of the oligothiophene and synthesized two types of the
ferrocene-oligothiophene-fullerene triads, F c-n T-C60 directly linking the ferrocene to the
oligothiophene and F c-tm -n T-C60 inserting a trimethylene spacer between the ferrocene and the
oligothiophene. For the central oligothiophene of the triads, a homologous series of quaterthiophene
(4T), octithiophene (8T), and duodecithiophene (12T) are systematically examined. The cyclic
voltammograms and electronic absorption spectra of F c-n T-C60 indicate conjugation between the
ferrocene and oligothiophene components. The emission spectra of F c-n T-C60 measured in toluene
demonstrate that the fluorescence of the oligothiophene is markedly quenched, as compared to
that observed for the dyads n T-C60. This quenching is explained in terms of the involvement of
intramolecular electron transfer in the photophysical decay process. The additionally conjugated
ferrocene evidently contributes to the stabilization of charge separation states, thus promoting
intramolecular electron transfer. This is corroborated by the observation that the emission spectra
of the nonconjugated triads F c-tm -n T-C60 are essentially similar to the corresponding dyads n T-
C60.
In tr od u ction
because they have small reorganization energies in
electron-transfer reactions and, therefore, force the charge
separation systems into the inverted region of the Marcus
Charge separation originating from photoinduced elec-
tron-transfer is a key function for the construction of
optoelectronic devices or artificial photosynthetic sys-
tems.1,2 In this context, molecular systems covalently
bonded between electron donors and electron acceptors
have been actively investigated, where intramolecular
through-bond charge separation readily takes place via
the excited singlet states of either the sensitizing electron-
donor chromophores or electron-acceptor chromophores.3
In general, however, such charge separation is followed
by fast intramolecular charge recombination, so it is
difficult to keep the generated electron or hole for enough
time to participate in redox reaction systems.4 Fullerenes
are electron acceptors of choice to solve this problem,
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B. J . Am. Chem. Soc. 1985, 107, 1080-1082. (b) Asahi, T.; Ohkohchi,
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Z.; Zheng, G.; Chen, Y.; Pandey, R. K.; Fujitsuka, M.; Ito, O.; Kadish,
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L.; Pe´rez, I.; Mart´ın, N.; Guldi, D. M. Chem. Eur. J . 2003, 9, 2457-
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* To whom correspondence should be addressed. Tel: +81-82-424-
7733. Fax: +81-82-424-5494.
† Hiroshima University.
‡ Osaka University.
§ Tohoku University.
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10.1021/jo049137o CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/22/2004
J . Org. Chem. 2004, 69, 7183-7189
7183

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
(5) (a) Marcus, R. A. J . Chem. Phys. 1956, 24, 966-978. (b) Marcus,
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545-550. (b) Guldi, D. M.; Luo, C.; Prato, M.; Dietel, E.; Hirsch, A.
Chem. Commun. 2000, 373-374. (c) Guldi, D. M.; Prato, M. Acc. Chem.
Res. 2000, 33, 695-703. (d) Imahori, H.; Tamaki, K.; Guldi, D. M.;
Luo, C.; Fujitsuka, M.; Ito, O.; Sakata, Y.; Fukuzumi, S. J . Am. Chem.
Soc. 2001, 123, 2607-2617. (e) Imahori, H.; Yamada H.; Guldi, D. M.;
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2445-2457. (e) Imahori, H.; Tamaki, K.; Yamada, H.; Yamada, K.;
Sakata, Y.; Nishimura, Y.; Yamazaki, I.; Fujitsuka, M.; Ito, O. Carbon
2000, 38, 1599-1605. (f) Schulter, D. I. Carbon 2000, 38, 1607-1614.
(g) Guldi, D. M.; Maggini, M.; Martin, N.; Prato, M. Carbon 2000, 38,
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13-25. (i) Guldi, D. M. Chem. Soc. Rev. 2002, 31, 22-36. (j) Nieren-
garten, J .-F.; Armaroli, N.; Accorsi, G.; Rio, Y.; Eckert, J .-F. Chem.
Eur. J . 2003, 9, 36-41.
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-
(8) Yamashiro, T.; Aso, Y.; Otsubo, T.; Tang, H.; Harima, Y.;
Yamashita, K. Chem. Lett. 1999, 443-444.
(9) (a) Fujitsuka, M.; Ito, O.; Yamashiro, T.; Aso, Y.; Otsubo, T. J .
Phys. Chem. A 2000, 104, 4876-4881. (b) Fujitsuka, M.; Matsumoto,
K.; Ito, O.; Yamashiro, T.; Aso, Y.; Otsubo, T. Res. Chem. Intermed.
2001, 27, 73-88.
(10) Fujitsuka, M.; Masuhara, A.; Kasai, H.; Oikawa, H.; Nakanishi,
H.; Ito, O.; Yamashiro, T.; Aso, Y.; Otsubo, T. J . Phys. Chem. B 2001,
105, 9930-9934.
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Eur. J . 2000, 6, 735-744.
(12) Ba¨uerle, P.; Wu¨rthner, F.; Go¨tz, G.; Effenberger, F. Synthesis
1993, 1099-1103.
(13) McCullough, R. D.; Lowe, R. D.; J ayaraman, M.; Anderson, D.
L. J . Org. Chem. 1993, 58, 904-912.
(14) (a) Iyoda, M.; Kondo, T.; Okabe, T.; Matsuyama, H.; Sasaki,
S.; Kuwatani, Y. Chem. Lett. 1997, 35-36. (b) Sato, M.; Sakamoto,
M.; Kashiwagi, S.; Suzuki, T.; Hiroi, M. Bull. Chem. Soc. J pn. 2001,
74, 1737-1742.
(15) (a) Maggini, M.; Scorrano, G.; Prato, M. J . Am. Chem. Soc. 1993,
115, 9798-9799. (b) Prato, M.; Maggini, M.; Giacometti, C.; Scorrano,
G.; Sandona`, G.; Farnia, G. Tetrahedron 1996, 52, 5221-5234.
7184 J . Org. Chem., Vol. 69, No. 21, 2004

Ferrocene-Oligothiophene-Fullerene Triads
SCHEME 1. Syn th esis of Oligoth iop h en es n T
SCHEME 2. Syn th esis of Tr ia d s F c-n T-C60
SCHEME 3. Syn th esis of F er r ocen ylp r op yl-Oligoth iop h en e-Ca r ba ld eh yd es 15a ,b
SCHEME 4. Syn th esis of F er r ocen ylp r op yl-Oligoth iop h en e-Ca r ba ld eh yd es 15b,c
propylferrocene (12) in 30% yield. Haloexchange of 12
with sodium iodide in refluxing butanone afforded 3-io-
dopropylferrocene (13) quantitatively. After the quater-
thiophene-carbaldehyde 7a was quantitatively protected
to the acetal 14a by treatment with ethylene glycol and
p-toluenesulfonic acid in refluxing benzene, the unsub-
stituted terminal of 14a was lithiated with butyllithium
in THF at 0 °C followed by treatment with 13 at room
temperature to produce the desired 15a in 24% yield. It
was, however, found that this approach could not be
applied to the higher oligothiophenes: a similar coupling
using the octithiophene 14b gave only a trace of 15b.
An alternative approach to 15b and 15c was devised
as shown in Scheme 4. First, quaterthiophene 6a (4T)
was brominated with 2 equiv of NBS in CS2-DMF at
room temperature to the dibromo derivative 16 (70%
yield), which was subjected to a metal-halogen exchange
with 1 equiv of butyllithium in THF at -70 °C, followed
by treatment with 3-iodopropylferrocene (13) at room
temperature, to give the bromo-ferrocenylpropyl-quater-
thiophene 17 in 15% yield. The acetals 14a and 14b were
converted to the respective tributyltin derivatives 18a
and 18b (50-58% yield) by treatment with tributyltin
chloride in THF at room temperature after lithiation with
butyllithium. The Stille coupling between 17 and 18a ,b
in the presence of catalytic tetrakis(triphenylphosphine)-
palladium in refluxing toluene gave the desired ferroce-
nylpropyl-oligothiophene-carbaldehydes 15b and 15c in
60-63% yield. Finally, the Prato reactions of 15a -c
afforded the triads 3a -c (F c-tm -n T-C60) in 39-42%
yields (Scheme 5).
J . Org. Chem, Vol. 69, No. 21, 2004 7185

Kanato et al.
SCHEME 5. Syn th esis of Tr ia d s F c-tm -n T-C60
SCHEME 6. Syn th esis of Dya d F c-4T
To examine the physical properties of the present
triads 2a -c and 3a -c, the oligothiophene-fullerene
dyads 1a -c (n T-C60), the ferrocene-quaterthiophene
dyad 20 (F c-4T), and the ferrocenylpropyl-oligothiophene
dyads 21a -c (F c-tm -n T) were used as reference com-
pounds. The dyads 1a -c were prepared in 46-60% yield
by utilizing the Prato reactions of the oligothiophene-
carbaldehydes 7a -c.8 F c-4T was synthesized in 68%
yield by coupling the ferrocenylzinc chloride (9) with the
bromo-quaterthiophene 19 prepared by monobromination
of 4T (Scheme 6).
The synthesis of F c-tm -n T was attempted by lithiation
of oligothiophenes with butyllithium, followed by coupling
with 3-iodopropylferrocene (13) (Scheme 7). This was
successful with quaterthiophene and octithiophene, but
the yields tended to decrease with long oligothiophenes:
30% for F c-tm -4T and 10% for F c-tm -8T. Instead, the
synthesis of the duodecithiophene derivative 21c was
accomplished in 80% yield by the Stille coupling between
the bromo-ferrocenylpropyl-quaterthiophene 17 and tribu-
tylstannyl-octithiophene 22, derived from octithiophene
6b.
Cyclic Volta m m ogr a m s. The F c-n T-C60 and F c-tm -
n T-C60 triads are characterized by amphoteric proper-
ties due to the presence of both donor and acceptor
components. As demonstrated in Figure 1, the cyclic
voltammograms of the F c-n T-C60 triads include multi-
oxidation waves due to the ferrocene and oligothiophene
components in the positive potential region and one
reduction wave due to the fullerene component in the
negative potential. The half-wave redox potentials of F c-
n T-C60 are summarized in Table 1, together with those
of F c-tm -n T-C60 and the reference compounds. Consid-
ering the half-wave oxidation potential (+0.45 V vs Ag/
AgCl) of ferrocene (11) itself, the lowest oxidation wave
(+0.48-0.50 V) of the F c-n T-C60 triads can be assigned
to be due to the ferrocene component. The following
oxidation waves are attributable to the oligothiophene
F IGURE 1. Cyclic voltammograms of (a) F c-4T-C60, (b) F c-
8T-C60, and (c) F c-12T-C60 in benzonitrile.
component. For F c-4T-C60, only the first one-electron
oxidation wave due to the quaterthiophene is observed
at +0.97 V. For the higher homologues F c-8T-C60 and
F c-12T-C60, the multioxidation waves of the long olig-
othiophenes are observed as a result of stabilization of
the cationic states due to the extended π-conjugation.
In these triads, it is evident that the attached ferrocene
component has a lower oxidation potential than the
oligothiophene component. From this result, it is expected
that an electron transfer from the ferrocene component
to the photoexcited oligothiophene may occur. On the
other hand, the half-wave reduction potentials due to the
fullerene component of F c-n T-C60 are quite identical to
those of the dyads n T-C60 and N-methyl-3,4-fulleropyr-
rolidine (23) (Chart 2). This reflects nonconjugated link-
age between the oligothiophene and fullerene compo-
nents, and the remote attachment of the ferrocene has
no influence on the reduction of the fullerene.
Apparently, the F c-tm -n T-C60 triads exhibit similar
cyclic voltammograms. However, on careful comparison,
there are some differences in half-wave oxidation poten-
tials between F c-tm -n T-C60 and F c-n T-C60, although
SCHEME 7. Syn th esis of Dya d s F c-tm -n T
7186 J . Org. Chem., Vol. 69, No. 21, 2004

Ferrocene-Oligothiophene-Fullerene Triads
TABLE 1. Ha lf-Wa ve Red ox P oten tia ls of th e
F er r ocen e-Oligoth iop h en e-F u ller en e Tr ia d s a n d
Rela ted Com p ou n d sa
compd
F c
n T
C60
F c-4T-C60
F c-8T-C60
F c-12T-C60
F c-tm -4T-C60
F c-tm -8T-C60
F c-tm -12T-C60
4T-C60
8T-C60
12T-C60
F c-4T
F c-tm -4T
F c-tm -8T
F c-tm -12T
ferrocene (11)
13
+0.48
+0.48
+0.50
+0.40
+0.40
+0.40
+0.97
+0.76
+0.63
+0.87
+0.76
+0.64
+0.92
+0.69
+0.59
+0.97
+0.86
+0.76
+0.64
-0.59
-0.59
-0.59
-0.59
-0.59
-0.59
-0.59
-0.59
-0.59
+0.48
+0.40
+0.40
+0.40
+0.45
+0.42
23
-0.59
a
Measurement conditions: solvent, benzonitrile; supporting
electrolyte, 0.1 M n-Bu4NPF6; working electrode, Pt wire; counter
electrode, Pt wire; reference electrode, Ag/AgCl; scan rate, 100 mV/
s.
F IGURE 2. Electronic absorption spectra of F c-4T-C60
(dashed line), F c-8T-C60 (dotted line), and F c-12T-C60 (solid
line) in toluene.
CHART 2
ꢀ 2.52) tailing to the near-infrared region.16 Thus, it
follows that the absorption peaks observed for F c-n T-
C60 mainly originate from the transition of the oligoth-
iophene chromophore and the two transitions of the
fullerene chromophore, and the absorption band due to
the ferrocene and the second absorption band of the
fullerene are concealed by the strong absorption band of
the oligothiophene. Table 2 summarizes the electronic
absorption and fluorescence data of the two types of
triads and the reference compounds. The structural
modification from n T-C60 to F c-n T-C60 results in red-
shifts for the absorption bands of oligothiophenes; in
particular, it is eminent for the quaterthiophene series.
On the other hand, the F c-tm -n T-C60 triads show no
such shifts. This evidently corroborates the presence of
an electronic interaction, presumably conjugation be-
tween the ferrocene and the oligothiophene, as estimated
above on the basis of the voltammetric study.
Em ission Sp ectr a . We previously discovered that the
n T-C60 dyads efficiently undergo photoinduced intramo-
lecular energy or electron transfer from the oligo-
thiophene component to the fullerene.8,9 The decay
process of n T-C60 is markedly solvent-dependent: en-
ergy transfer occurs in nonpolar toluene, whereas elec-
tron transfer is predominant in polar benzonitrile or
THF. It is considered that high solvation by polar
solvents stabilizes charge separation states much more,
and as a result, electron transfer takes precedence over
energy transfer. Figure 3 shows the emission spectra of
F c-4T-C60, 4T-C60, and F c-4T measured with excita-
tion in the region of the quaterthiophene chromophore
in toluene. For ready comparison, the emission spectrum
of 4T is also included on a low scale of 2 orders of
magnitude. In contrast to strong emission of 4T, these
linkage compounds 4T-C60 and F c-4T-C60 demonstrate
little or no emission from the oligothiophene moiety. In
4T-C60, an alternative emission is observed around 700
the half-wave reduction potentials of the fullerene are
the same (-0.59 V). The oxidation potentials due to the
ferrocene component of the F c-tm -n T-C60 triads are all
identical (+0.40) in the series and appreciably lower than
the potentials (+0.48-0.50 V) of the F c-n T-C60 series.
In addition, the oxidation potentials due to the quater-
thiophene between F c-tm -4T-C60 and F c-4T-C60 are
different, although this is not the case with the higher
oligothiophene systems. One should notice that the
oxidation potentials due to the ferrocene of the F c-tm -
n T-C60 triads are identical to those of the F c-t m -n T
dyads (21a -c) and nearly the same as that (+0.42 V) of
3-iodopropylferrocene (13). From these results, it is
reasonable to consider that the oxidation potentials of
the directly linked F c-n T-C60 triads are rather affected
by a definitive electronic interaction between the fer-
rocene and oligothiophene moieties. This leads to differ-
ent photophysical behaviors for these two types of triads,
as described later.
Electr on ic Absor p tion Sp ectr a . Figure 2 demon-
strates the electronic absorption spectra of the F c-n T-
C60 triads measured in toluene. Roughly speaking, these
spectra consist of superposition of the electronic transi-
tions of three component chromophores. Oligothiophenes
(6) show a strong π-π* absorption band in the ultraviolet/
visible region, which is red-shifted and intensified with
chain extension: 4T, λmax 379 nm (log ꢀ 4.64); 8T, 439
(4.77); 12T, 457 (4.91). Ferrocene (11) has an electronic
transition at 444 nm, but its intensity (log ꢀ 1.97) is very
weak as compared to those of oligothiophenes. N-Methyl-
3,4-fulleropyrrolidine (23) has a strong absorption band
at 330 nm (log ꢀ 4.59) accompanied with weak peaks at
434 nm (log ꢀ 4.55), 640 nm (log ꢀ 2.51), and 704 nm (log
(16) Obara, Y.; Takimiya, K.; Aso, Y.; Otsubo, T. Tetrahedron Lett.
2001, 42, 6877-6881.
J . Org. Chem, Vol. 69, No. 21, 2004 7187

Kanato et al.
TABLE 2. Sp ectr oscop ic Da ta of th e F er r ocen e-Oligoth iop h en e-F u ller en e Tr ia d s a n d Rela ted Com p ou n d s
abs max/nm (log ꢀ)
emission max/nma
C60 n T*
compd
solvent
toluene
n T
C60*
F c-4T-C60
402 (4.59)
408 (4.62)
449 (4.84)
454 (4.84)
459 (5.04)
465 (5.02)
379 (4.67)
395 (4.61)
436 (4.83)
452 (4.89)
459 (5.03)
463 (4.99)
379 (4.64)
384 (4.59)
436 (4.83)
450 (4.81)
456 (5.03)
463 (4.98)
378 (4.49)
391 (4.47)
442 (4.78)
448 (4.78)
459 (5.03)
462 (4.96)
335 (4.68), 703 (2.41)
327 (4.69), 705 (2.43)
331 (4.74), 706 (2.48)
327 (4.89), 704 (2.56)
331 (4.81), 702 (2.55)
329 (4.78), 702 (2.53)
335 (4.63), 708 (2.53)
327 (4.63), 701 (2.53)
335 (4.75), 706 (2.54)
327 (4.70), 708 (2.50)
334 (4.81), 705 (2.46)
327 (4.75), 708 (2.56)
335 (4.69), 708 (2.59)
327 (4.61), 703 (2.47)
335 (4.75), 708 (2.54)
330 (4.81), 702 (2.56)
334 (4.81), 706 (2.57)
332 (4.74), 703 (2.43)
nd
nd
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
F c-8T-C60
F c-12T-C60
F c-tm -4T-C60
F c-tm -8T-C60
F c-tm -12T-C60
4T-C60
549, 715
718
714
716
715
714
714
715
714
566, 713
460, 705
544, 714
566, 715
447, 698
benzonitrile
toluene
benzonitrile
toluene
8T-C60
546, 547 sh, 713
567, 712
12T-C60
benzonitrile
toluene
benzonitrile
toluene
benzonitrile
toluene
F c-tm -4T
462, 490, 524 sh
546, 588, 640 sh
567, 615, 655 sh
F c-tm -8T
F c-tm -12T
benzonitrile
a
Excited at 379 nm for 4T*, 437 nm for 8T*, 457 nm for 12T*, and 650 nm for C60*.
F IGURE 3. Emission spectra of 4T (× 0.1, dotted line), 4T-
C60 (solid line), F c-4T-C60 (bold solid line), and F c-4T
(dashed line) with excitation at 379 nm in toluene, where the
absorbances of all samples were kept constant.
F IGURE 4. Emission spectra of 4T-C60 (dashed line), F c-
4T-C60 (bold solid line), and N-methyl-3,4-fulleropyrrolidine
(solid line) with excitation at 650 nm in toluene, where the
absorbances of all samples were kept constant.
nm, being assignable to that from the fullerene chro-
mophore. This evidently indicates the involvement of
intramolecular energy transfer in the quenching process.
However, the spectrum of F c-4T-C60 gives little emission
not only from the quaterthiophene but also from the
fullerene. This quenching is interpreted in terms of the
precedence of intramolecular electron transfer even in
nonpolar toluene.
weak emission from the quaterthiophene moiety as
compared to quaterthiophene 4T itself. The other is that
the excited electron of the quaterthiophene transfers to
the LUMO orbital of the fullerene. Such electron transfer
from the quaterthiophene to the fullerene was also
observed by the experiment that, when the fullerene
chromophore is excited, the emission from the fullerene
of F c-4T-C60 is markedly depressed, as compared to that
of 4T-C60 (Figure 4). Presumably, the interaction be-
tween the ferrocene and the quaterthiophene of the F c-
4T-C60 triad contributes to the stabilization of the charge
The intramolecular electron transfer of F c-4T-C60 has
two possibilities. One is that the HOMO electron of the
ferrocene moiety transfers to the half-filled HOMO
orbital of the excited quaterthiophene. This is evidenced
by the observation that the dyad F c-4T also shows a
7188 J . Org. Chem., Vol. 69, No. 21, 2004

Ferrocene-Oligothiophene-Fullerene Triads
fer is presumably not large enough to overwhelm the
dominant energy transfer in the decay process.
Both triads F c-n T-C60 and F c-tm -n T-C60 exhibited
inappreciable emission either from the oligothiophene or
from the fullerene, when measured in benzonitrile.
Evidently, intramolecular electron transfer almost en-
tirely dominates the decay process as a result of the
stabilization of the charge separation states in the polar
solvent. Since even the dyads n T-C60 also favor in-
tramolecular electron transfer in benzonitrile, the effect
of the additional ferrocene of these triads on the photo-
physical properties cannot be elucidated only by the
emission study.
Con clu sion
To promote photoinduced electron transfer previously
observed for the oligothiophene-fullerene dyads (n T-
C60), we have developed two types of the ferrocene-
oligothiophene-fullerene triads, F c-n T-C60 and F c-tm -
n T-C60, designed by additional attachment of a strongly
electron-donating ferrocene to the dyads. The two types
of triads demonstrate different photophysical properties.
The directly linked F c-n T-C60 triads are characterized
by conjugation between the ferrocene and oligothiophene
components, as evidenced by an examination of their
cyclic voltammograms and electronic absorption spectra.
This conjugated interaction serves to promote electron
transfer either from the excited oligothiophene to the
fullerene or from the oligothiophene to the excited
fullerene. In particular, in the emission spectrum of F c-
4T-C60 in toluene, the quenching process is governed by
electron transfer, in contrast to energy transfer for 4T-
C60. On the other hand, the photophysical properties of
the trimethylene-inserted F c-tm -n T-C60 triads are not
so largely perturbed by the attached ferrocene as those
of F c-n T-C60. When measured in toluene, their emission
spectra, like those of n T-C60, are still dominated by
photoinduced energy transfer in the decay process.
However, the effect of the additional ferrocene of the two
types of triads on the stabilization and lifetimes of charge
separation states still remain unclear. Further investiga-
tion on these subjects by measuring transient absorption
spectra is under way and will be reported elsewhere.
F IGURE 5. Emission spectra of 8T-C60 (dashed line), F c-
8T-C60 (solid line), 12T-C60 (bold dashed line), and F c-12T-
C60 (bold solid line) with excitation at 437 nm for the
octithiophene chromophore and at 457 nm for the duodeci-
thiophene chromophore in toluene, where the absorbances of
all samples were kept constant.
separation state, and as a result, electron transfer takes
precedence over energy transfer.
The spectra of the dyads 8T-C60 and 12T-C60 are
characterized by dual fluorescence not only from the
fullerene chromophore but also from the oligothiophene
chromophore (Figure 5). The π-centers of these long
oligothiophenes become more distant from the fullerene,
so that energy transfer is retarded and competes with
direct fluorescence. The emission spectra of the triads F c-
8T-C60 and F c-12T-C60 also show dual fluorescence, but
the intensities of both bands are smaller than those of
the corresponding dyads. It is reasonable to consider that
intramolecular electron transfer also participates in the
decay process of these long oligothiophene-containing
triads.
The photophysical decay process of the F c-tm -n T-C60
system is quite different from those of the F c-n T-C60
system. Their fluorescence spectra measured in toluene,
with excitation of either the oligothiophene or the fullerene
chromophore, are essentially similar to the corresponding
dyads n T-C60. It seems that the nonconjugated ferrocene
component has apparently no influence on the quenching
process. However, it is worth noting that the reference
dyad F c-tm -4T shows very weak emission with an
intensity of 1/36 compared to quaterthiophene 4T. This
means that, even though the two chromophores are
separated by the trimethylene chain, electron transfer
from the ferrocene to the excited oligothiophene can
occur. In the F c-tm -n T-C60 system, such electron trans-
Ack n ow led gm en t. This research was partly sup-
ported by Grants-in-Aid of Scientific Research (Category
A, No. 1330405, and Priority Areas of Molecular Con-
ductors, No. 15073218) from the Ministry of Education,
Culture, Sports, Science and Technology, J apan.
Su p p or tin g In for m a tion Ava ila ble: All experimental
details for the syntheses of the triads F c-n T-C60 and F c-tm -
n T-C60, as well as reference compounds, and the electronic
absorption and emission spectra of the triads F c-tm -n T-C60
and F c-tm -4T. This material is available free of charge via
the Internet at http://pubs.acs.org.
J O049137O
J . Org. Chem, Vol. 69, No. 21, 2004 7189