Synthesis and photophysical properties of [60]fullerene adducts carrying oligocarbazole dendrons

Article abstract of DOI:10.1246/cl.2007.264

[60]Fullerene adducts carrying oligocarbazole moieties were successfully prepared by the Bingel reactions. Their fluorescence spectra and transient absorption spectra suggested the intramolecular electron transfer from the oligocarbazole moiety to the ful

Full text of DOI:10.1246/cl.2007.264

264
Chemistry Letters Vol.36, No.2 (2007)
Synthesis and Photophysical Properties of [60]Fullerene Adducts
Carrying Oligocarbazole Dendrons
Yosuke Nakamura,ꢀ1 Takashi Konno,1 Satoru Watanabe,1 Masato Suzuki,1
Toshitada Yoshihara,2 Seiji Tobita,2 and Jun Nishimuraꢀ1
1Department of Nano-Material Systems, Graduate School of Engineering, Gunma University, Kiryu 376-8515
2Department of Chemistry, Faculty of Engineering, Gunma University, Kiryu 376-8515
(Received November 17, 2006; CL-061352; E-mail: nakamura@chem.gunma-u.ac.jp)
O
O
[60]Fullerene adducts carrying oligocarbazole moieties
were successfully prepared by the Bingel reactions. Their
fluorescence spectra and transient absorption spectra suggested
the intramolecular electron transfer from the oligocarbazole
R
O
O
R
O
O
1
ꢀ
moiety to the fullerene moiety mainly via C60
.
C
60, CBr4, DBU
Toluene, rt
R
O
O
R
4, 5
[60]Fullerene–donor dyads linked by covalent bondings
have attracted much interest from the aspects of charge-separat-
ed states arising from the intramolecular photoinduced electron
transfer.1 Especially, porphyrins and related compounds have
been extensively utilized among a variety of donor moieties.1
In contrast, there have been only a few examples of carbazole-
linked fullerene adducts,2,3 although carbazole is a good electron
donor known as a component of photoconductive poly(N-vinyl-
carbazole) (PVCz).4 Quite recently, we have successfully pre-
pared [60]fullerene adduct 1 (Chart 1) bearing a carbazole resi-
due by using Bingel reaction.5 However, the photoinduced elec-
tron transfer via the excited states of [60]fullerene was not evi-
dently detected in adduct 1. Aiming at the construction of
further new fullerene–donor dyad systems, which can facilitate
the electron transfer, the introduction of two carbazole moieties
or oligocarbazole moieties onto the [60]fullerene surface was
undertaken. Herein, we have designed three [60]fullerene ad-
ducts 2 and 3 bearing two carbazole or oligocarbazole moieties
attached to the cyclopropane ring symmetrically. The trimeric
carbazole moieties in 3 have larger ꢀ-conjugated system and
more electron-donating ability than the single carbazole systems
in 1 and 2.6
2, 3
N
N
3,5: R =
N
2,4: R =
N
Scheme 1.
R
N
H
N
a)
80%
N
N
N
N
Both 2 and 3 are expected to be accessible by the Bingel re-
actions using malonate esters 4 and 5, respectively (Scheme 1).
The preparation of 4 and 5 was accomplished by the reaction of
malonyl dichloride with the precursor alcohols, which were
readily obtained by the NaBH4-reduction of the corresponding
aldehydes. Scheme 2 depicts the synthetic sequence of 5 as a
representative.
6: R = CHO
7: R = CH2OH
c)
b), 80%
5
50%
Scheme 2. Reagents and conditions: a) 4-BrC6H4CHO,
K2CO3, (t-Bu)3P, Pd(OAc)2, xylene. b) NaBH4, EtOH. c) Ma-
lonyl dichloride, pyridine, CH2Cl2.
The Bingel reaction of 4 and 5 with [60]fullerene was
carried out under conventional conditions;7 4 and 5 were allowed
to react with [60]fullerene, CBr4, and DBU in toluene at room
temperature for 6 h to give monoadducts 2 and 3 in 30 and
29% isolated yields, respectively (Scheme 1), along with bisad-
ducts as regioisomeric mixtures. Both 2 and 3 were isolated by
column chromatography (silica gel) with toluene as the eluent.
O
O
O
O
O
OEt
EtO
OEt
Et N
1
Monoadducts 2 and 3 were definitely characterized by H
and 13C NMR, APCI-mass, and UV–vis spectroscopies. The
1H NMR spectra, composed of carbazole and phenyl proton
peaks and benzyl proton peak (singlet), apparently demonstrated
their structures.8 Among these peaks, phenyl and benzyl proton
peaks were down-field shifted relative to those of precursors 4
1
8
Chart 1.
Copyright ꢀ 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.2 (2007)
265
quite reasonable, since the trimeric carbazole moiety in 3 has
more electron-donating ability than the monomeric carbazole
moiety in 2; the difference in their oxidation potentials is ca.
0.2 V.6
Transient absorption spectra of 2 and 3 were also measured
by 532-nm laser flash photolysis in benzonitrile at room temper-
ature along with reference monoadduct 8. All of 2, 3, and 8 ex-
hibited transient absorption spectra characteristic of the excited
triplet state (3C60ꢀ) of [60]fullerene moiety with a maximum
around 710–720 nm. However, the initial absorbance in 3 at
720 nm was considerably small compared to that in 2 and 8. This
observation is consistent with the results in fluorescence meas-
1
ꢀ
urements; for 3 in benzonitrile, C60 is significantly quenched
by the intramolecular electron transfer via C60ꢀ, leading to
1
3
ꢀ
the less extent of ISC ꢀto C60 in 3 than 2 and 8. Furthermore,
3
the lifetime of C60 in 3 (28 ms) determined at 720 nm
was shorter than those in 2 and 8 (42 ms), which may suggest
the possibility of the intramolecular electron transfer via C60
3
ꢀ
1
ꢀ
in addition to C60 in 3.
In summary, we have first successfully synthesized oligocar-
bazole-linked [60]fullerene adduct 3. The fluorescence spectra
and transient absorption spectra of 3 suggested the intramolecu-
lar electron transfer from the oligocarbazole moiety to the
fullerene moiety mainly via 1C60ꢀ. More detailed photophysical
investigation and synthesis of other oligocarbazole-linked
[60]fullerene adducts is now in progress.
Figure 1. Fluorescence spectra of (a) 2 and (b) 3 in benzene and
benzonitrile at room temperature (ꢂex ¼ 430 nm).
and 5, due to the deshielding effect of the fullerene moiety. The
APCI-mass spectra of 2 and 3 afforded a molecular ion peak of
m=z ¼ 1332 and 1992, respectively. The UV–vis spectra of these
adducts exhibited a sharp peak around 430 nm and a broad band
around 700 nm, which are characteristic of cyclopropanated
[60]fullerene monoadducts.9
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
References and Notes
1
D. M. Guldi, Chem. Soc. Rev. 2002, 31, 22; T. Otsubo, Y. Aso,
K. Takimiya, J. Mater. Chem. 2002, 12, 2565; H. Imahori,
Org. Biomol. Chem. 2004, 2, 1425; K. G. Thomas, M. V.
George, P. V. Kamat, Helv. Chim. Acta 2005, 88, 1291; J. L.
Segura, N. Martin, D. M. Guldi, Chem. Soc. Rev. 2005, 34, 31.
G. Yin, X.-P. Mao, Z.-Y. Suo, Z. Xu, Chin. J. Chem. 2001, 19,
822.
H. Yonemura, S. Moribe, K. Hayashi, M. Noda, H. Tokudome,
S. Yamada, Appl. Magn. Reson. 2003, 23, 289.
H. Hoegl, J. Phys. Chem. 1965, 69, 755.
Fluorescence spectra of 2 and 3 were measured at room tem-
perature under several conditions (Figure 1). In 292-nm excita-
tion, which can afford the excited state (1Carbazoleꢀ) of carba-
zole moiety as well as [60]fullerene moiety, the fluorescence
of carbazole moieties in 2 and 3 was almost completely
quenched in benzene, cyclohexane, or dichloromethane, where-
as precursors 4 and 5 afforded intense emission at this excitation
wavelength. This quenching in 2 and 3 is probably ascribed to
the intramolecular energy (and/or electron) transfer from the
carbazole moiety to the fullerene moiety, since the S1 energy
of carbazole is higher than that of C60. In order to produce the
excited state (1C60ꢀ) of [60]fullerene moiety exclusively, the
fluorescence spectra were also measured in 430-nm excitation.
In benzene, both 2 and 3 exhibited weak and broad fluorescence
around 700 nm, similar to [60]fullerene monoadducts without a
carbazole moiety such as 8. There was no appreciable difference
in the fluorescence intensity between 2 and 3. Thus, it is obvious
that the fluorescence of [60]fullerene moiety for 2 and 3 is not
quenched in benzene. In benzonitrile, however, the behavior of
2 and 3 remarkably contrasts with each other. While 2 showed
almost the same emission as that in benzene, the fluorescence
of 3 was extremely quenched. The quenching in 3 indicates that
the intramolecular electron transfer from the carbazole moiety
2
3
4
5
Y. Nakamura, M. Suzuki, Y. Imai, J. Nishimura, Org. Lett.
2004, 6, 2797.
6
7
8
F. Loiseau, S. Campagna, A. Hameurlaine, W. Dehaen, J. Am.
Chem. Soc. 2005, 127, 11352.
X. Camps, A. Hirsch, J. Chem. Soc., Perkin Trans. 1 1997,
1595.
1
Selected spectroscopic data of 2 and 3. 2: H NMR (CDCl3,
500 MHz) ꢁ 5.67 (s, 4H), 7.26–7.29 (m, 4H), 7.33–7.38 (m,
8H), 7.61 (d, J ¼ 8:0 Hz, 4H), 7.74 (d, J ¼ 8:0 Hz, 4H), 8.12
(d, J ¼ 7:5 Hz, 4H). 3: 1H NMR (CDCl3, 500 MHz) ꢁ 5.71
(s, 4H), 7.24–7.28 (m, 8H), 7.34–7.37 (m, 16H), 7.54 (d,
J ¼ 9:0 Hz, 4H), 7.61 (d, J ¼ 9:0 Hz, 4H), 7.77 (d, J ¼ 8:0
Hz, 4H), 7.85 (d, J ¼ 8:5 Hz, 4H), 8.13 (d, J ¼ 8:0 Hz, 8H),
8.24 (s, 4H).
1
ꢀ
to the fullerene moiety via C60 takes place, as previously
observed in the [60]fullerene adduct bearing an N,N-dimethyl-
aniline moiety.10 On the contraryꢀ, it is obvious that the intramo-
9
R. A. J. Janssen, J. C. Hummelen, F. Wudl, J. Am. Chem. Soc.
1995, 117, 544.
10 Y. Nakamura, T. Minowa, Y. Hayashida, S. Tobita, H.
Shizuka, J. Nishimura, J. Chem. Soc., Faraday Trans. 1996,
92, 377.
1
lecular electron transfer via C60 is hardly involved in 2. Such
difference in fluorescence in benzonitrile between 2 and 3 seems
Published on the web (Advance View) January 18, 2007; doi:10.1246/cl.2007.264