Synthesis and photophysical properties of electron-rich perylenediimide-fullerene dyad

Article abstract of DOI:10.1021/ol061506a

An electron-rich perylenediimide-C₆₀ dyad has been prepared to explore a new type of donor-acceptor system. Time-resolved absorption measurements in benzonitrile revealed unambiguous evidence for the formation of a charge-separated state consis

Full text of DOI:10.1021/ol061506a

ORGANIC
LETTERS
2006
Vol. 8, No. 20
4425-4428
Synthesis and Photophysical Properties
of Electron-Rich
Perylenediimide-Fullerene Dyad
Yuki Shibano,† Tomokazu Umeyama,† Yoshihiro Matano,† Nikolai V. Tkachenko,*,‡
Helge Lemmetyinen,‡ and Hiroshi Imahori*,†,§
Department of Molecular Engineering, Graduate School of Engineering,
Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, Fukui Institute for
Fundamental Chemistry, Kyoto UniVersity, 34-4, Takano-Nishihiraki-cho, Sakyo-ku,
Kyoto 606-8103, Japan, and Institute of Materials Chemistry, Tampere UniVersity of
Technology, P.O. Box 541, Korkeakoulunkatu 8, FIN-33101, Tampere, Finland
imahori@scl.kyoto-u.ac.jp; nikolai.tkachenko@tut.fi
Received June 20, 2006
ABSTRACT
An electron-rich perylenediimide-C60 dyad has been prepared to explore a new type of donor−acceptor system. Time-resolved absorption
measurements in benzonitrile revealed unambiguous evidence for the formation of a charge-separated state consisting of perylene diimide
radical cation and C60 radical anion via photoinduced electron transfer, showing a new class of artificial photosynthetic models in terms of
charge separation.
Recently, a wide variety of donor-acceptor linked molecules
have been prepared to develop artificial photosynthetic
systems toward the realization of highly efficient solar energy
conversion.1 The key points for the efficient conversion are
a high light-harvesting ability and fast charge separation (CS)
and slow charge recombination (CR), which allow the
systems to generate a long-lived charge-separated state in a
high quantum yield. Fullerenes are one of the most attractive
electron acceptors2 because they bear small reorganization
energies3 that accelerate CS and decelerate CR. A disad-
vantage of fullerenes is small molar absorption coefficients
in the visible region. Thus, porphyrins have been frequently
employed as a donor of such combinations owing to the
intense Soret band in the blue and moderate Q-bands in the
green regions, as well as relatively small reorganization
energies.4 Actually, porphyrin-fullerene linked systems have
successfully exhibited the efficient formation of a long-lived
charge-separated state.2,3,5 However, taking into account solar
energy distribution on the earth surface, the light-harvesting
properties of porphyrins are insufficient as a donor.
(2) (a) Imahori, H.; Sakata, Y. AdV. Mater. 1997, 9, 537. (b) Mart´ın, N.;
Sa´nchez, L.; Illescas, B.; Pe´rez, I. Chem. ReV. 1998, 98, 2527. (c) Diederich,
F.; Go´mez-Lo´pez, M. Chem. Soc. ReV. 1999, 28, 263. (d) Guldi, D. M.;
Prato, M. Acc. Chem. Res. 2000, 33, 695. (e) Nierengarten, N.-F. New J.
Chem. 2004, 28, 1177. (f) Armaroli, N. Photochem. Photobiol. Sci. 2003,
2, 73.
(3) (a) Imahori, H.; Hagiwara, K.; Akiyama, T.; Aoki, M.; Taniguchi,
S.; Okada, T.; Shirakawa, M.; Sakata, Y. Chem. Phys. Lett. 1996, 263,
545. (b) Imahori, H.; Yamada, H.; Guldi, D. M.; Endo, Y.; Shimomura,
A.; Kundu, S.; Yamada, K.; Okada, T.; Sakata, Y.; Fukuzumi, S. Angew.
Chem., Int. Ed. 2002, 41, 2344. (c) Fukuzumi, S.; Ohkubo, K.; Imahori,
H.; Guldi, D. M. Chem. Eur. J. 2003, 9, 1585.
† Kyoto University.
‡ Tampere University of Technology.
§ Fukui Institute for Fundamental Chemistry.
(1) (a) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (b) Gust, D.;
Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198. (c) Holten, D.;
Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57. (d) Balzani,
V.; Venturi, M.; Credi, A. Molecular DeVices and Machines; Wiley-VCH:
Weinheim, 2003.
(4) The Porphyrin Handbook; Kadish, K. M., Smith, K., Guilard, R.,
Eds.; Academic Press: San Diego, CA, 2000.
10.1021/ol061506a CCC: $33.50
© 2006 American Chemical Society
Published on Web 08/29/2006

Scheme 1
Perylene diimides (PDI) are well-known as chemically,
substituents would shift the absorption bands due to the PDI
moiety to the longer wavelength regions, improving the light-
harvesting properties in the visible and near-infrared re-
gions.11 To increase the solubility, a swallow-tail secondary
alkyl group12 and long alkyl group were introduced into one
imide end of the PDI and the pyrrolidine ring on the C60,
respectively.
The synthetic scheme is shown in Scheme 1. According
to Wasielewski’s method,10a the bispyrrolidine-substituted
PDI 2 was synthesized by treating the N,N′-dicyclohexyl-
1,7-dibromoperylene-3,4:9,10-tetracarboxylic acid bisimide
1 with a large amount of pyrrolidine. Under our conditions,
2 was contaminated by monopyrrolidinated PDI. The desired
product was separated by alumina chromatography (CHCl3/
hexane ) 1:1) to give 2 in 52% yield. The saponification of
2 by using the reported procedure10b gave the 1,7-dipyrro-
lidinylperylene bisanhydride 3 in 83% yield. Cross-conden-
sation of 8-aminopentadecane, 3, and formyl-protected
aniline, followed by acid-catalyzed deprotection, afforded 4
in 17% yield. The perylenebisimide-C60 dyad PDI-C60 was
obtained in 56% yield by 1,3-dipolar cycloaddition using C60
and N-octadecylglycine in toluene.13 Compounds PDI-ref
and C60-ref14 (Figure 1) were also synthesized as references.
thermally, and photophysically stable and good light-harvest-
ing dyes. Owing to their outstanding properties, they have
been regarded as potential candidates for optical devices such
as organic light-emitting diodes,6 photovoltaic devices,7 and
optical switches.8 So far several perylenediimide-C60 linked
systems have been reported.9 However, energy transfer (EN)
to C60 is a main relaxation pathway of the excited singlet
state of PDI as a result of the poor electron-donating ability
of the perylenediimide. As such, no unambiguous evidence
for electron transfer (ET) from the excited singlet state of
PDI to C60 has been presented.
We report herein synthesis and photophysical properties
of a novel perylenediimide-C60 linked dyad PDI-C60. It is
known that the electronic structures of perylenediimides are
highly affected by introducing electron-donating or electron-
withdrawing groups at a perylene core.6-8 Bearing this in
mind, we introduced electron-donating amine substituents
into the perylene core.10 Such substitution is expected to
facilitate photoinduced ET from the excited singlet state of
the PDI to the C60 as a result of the low oxidation potential
of the PDI moiety. More importantly, the electron-donating
(5) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34,
40. (b) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem.
Photobiol. C. 2004, 5, 79. (c) Imahori, H. Org. Biomol. Chem. 2004, 2,
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Mu¨llen, H.; MacKenzie, J. D.; Silva, C.; Friend, R. H. J. Am. Chem. Soc.
2003, 125, 437. (b) Alibert-Fouet, S.; Dardel, S.; Bock, H.; Oukachmih,
M.; Archambeau, S.; Seguy, I.; Jolinat, P.; Destruel, P. ChemPhysChem
2003, 4, 983.
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Lu, F.; Zhu, D. J. Org. Chem. 2004, 69, 9049. (b) Shin, W. S.; Jeong,
H.-H.; Kim, M.-K.; Jin, S.-H.; Kim, M.-R.; Lee, J.-K.; Lee, J. W.; Gal,
Y.-S. J. Mater. Chem. 2006, 16, 384.
(8) (a) O’Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.;
Gaines, G. L., III; Wasielewski, M. R. Science 1992, 257, 63. (b) Zang, L.;
Liu, R.; Holman, M. W.; Nguyen, K. T.; Adams, D. M. J. Am. Chem. Soc.
2002, 124, 10640.
Figure 1. Reference compounds PDI-ref and C60-ref.
(9) (a) Hua, J.; Meng, F.; Ding, F.; Li, F.; Tian, H. J. Mater. Chem.
2004, 14, 1849. (b) Liu, Y.; Xiao, S.; Li, H.; Li, Y.; Liu, H.; Lu, F.; Zhuang,
J.; Zhu, D. J. Phys. Chem. B 2004, 108, 6256. (c) Go´mez, R.; Segura, J.
L.; Mart´ın, N. Org. Lett. 2005, 7, 717. (d) Wang, N.; Li, Y.; He, X.; Gan,
H.; Li, Y.; Huang, C.; Xu, X.; Xiao, J.; Wang, S.; Liu, H.; Zhu, D.
Tetrahedron 2006, 62, 1216.
(10) (a) Zhao, Y.; Wasielewski, M. R. Tetrahedron Lett. 1999, 40, 7047.
(b) Wu¨rthner, F.; Stepanenko, V.; Chen, Z.; Saha-Mo¨ller, C. R.; Kocher,
N.; Stalke, D. J. Org. Chem. 2004, 69, 7933.
1
The products were characterized on the basis of their H,
13C NMR, mass, and IR spectra (Supporting Information).
(11) Lukas, A. S.; Zhao, Y.; Miller, S. E.; Wasielewski, M. R. J. Phys.
Chem. B 2002, 106, 1299.
(12) Langhals, H. Heterocycles 1995, 40, 477.
4426
Org. Lett., Vol. 8, No. 20, 2006

UV-vis-NIR absorption spectra of PDI-C60, PDI-ref,
and C60-ref were measured in benzonitrile (Figure 2). The
tion wavelength (λex) of 430 nm where the absorbances are
identical. The shape of the spectrum of PDI-C60 is almost
identical to that of PDI-ref, indicating that there is no
significant interaction between the PDI and C60 moieties in
the excited state. The fluorescence from the C60 moiety could
not be confirmed because of the overlapping of the two
emission at 700-800 nm and the low fluorescence quantum
yield of the C60. The fluorescence intensity of PDI-C60 is
significantly reduced compared to that of PDI-ref in ben-
zonitrile, indicating that the excited singlet state of the PDI
moiety (1PDI*) is strongly quenched by the C60 moiety. The
energy level of the excited singlet state (E00) of the PDI is
determined as 1.70 eV in benzonitrile from the absorption
and fluorescence spectra of PDI-ref in benzonitrile.
The first oxidation potential (Eox) of the PDI moiety and
the first reduction potential (Ered) of the C60 moiety were
measured in benzonitrile containing 0.1 M Bu4NPF6 as
supporting electrolyte using differential pulse voltammetry.
Figure 2. Absorption spectra of PDI-C60 (solid line), PDI-ref
(dashed line), and C60-ref (dotted line) in benzonitrile.
The redox potentials of PDI-C60 (Eox ) +0.82 V, Ered
)
-0.41 V vs NHE) are almost the same as those of PDI-ref
(Eox ) +0.81 V vs NHE) and C60-ref (Ered ) -0.41 V vs
NHE), supporting only a minor electronic interaction between
the PDI and the C60 moieties in the ground state. The free
energy change (-∆GCS) for photoinduced ET from the
1PDI* to the C60 in benzonitrile is calculated to be 0.47 eV,
implying that CS is possible.
dyad PDI-C60 reveals strong absorption at 700 nm and
relatively weak absorption at 430 nm that come from the
PDI moiety, together with strong absorption at the UV region
from the C60 moiety. The spectrum of PDI-C60 is virtually
the superposition of the spectra of PDI-ref and C60-ref,
suggesting that there is no significant interaction between
the PDI and C60 moieties in the ground state. It is noteworthy
that the spectral shapes of PDI-C60 in the NIR region,
namely, the strong absorption at 700 nm and a shoulder at
650 nm, reveal that 6 exists as a monomer rather than as a
π-π stacked aggregate in solution, because the H-aggregates
of pyrrolidine-substituted PDI are known to exhibit the
inverse spectral shapes in the NIR region, stronger absorption
at 650 nm and a shoulder at 700 nm.15
Femto- to picosecond transient absorption spectra of the
dyad PDI-C60 in benzonitrile were recorded using the pump-
probe technique. The excitation wavelength (λex ) 415 nm)
ensures the selective excitation of the PDI moiety. We could
not excite the C60 moiety selectively because of the extensive
overlapping of the absorption spectra of the C60 and the PDI
moieties. Multiexponential global fittings were applied to
transient-absorption decay curves (Figure S3, Supporting
Information) at different wavelengths. The component spectra
are shown in Figure S4 (Supporting Information). Three-
exponential fitting gave a reasonably small mean square
deviation value. The minor short-lived component (τ ) 7
ps) stems from the thermal vibrational relaxation of the PDI
excited singlet state. The longest lifetime component (τ )
170 ps) exhibits a characteristic spectrum resembling that
of the major component of PDI-ref that arises from the
lowest singlet excited state of the PDI moiety. The middle
lifetime component (τ ) 60 ps) has negative absorption at
1020 and at 880 nm, respectively, and has positive absorption
at around 700 nm. This spectrum well agrees with the inverse
of the differential spectrum of the charge-separated state
(PDI•+-C60•-). The recalculated differential spectra of 1PDI*
and PDI•+-C60•- states (Figure 4) also confirm the formation
of the charge-separated state via photoinduced ET from the
excited singlet state of the PDI to the C60 (see S3-S5,
Supporting Information, for the details). Namely, the spec-
Figure 3. Fluorescence spectra of PDI-C60 (solid line) and PDI-
ref (dashed line) in benzonitrile at λex ) 430 nm where the
absorbances of the compounds are identical. The spectrum of PDI-
C60 is also normalized at 750 nm for comparison (dotted line).
•-
trum of PDI•+-C60 has ground-state bleaching at 720 nm
Figure 3 shows the steady-state fluorescence spectra of
PDI-C60 and PDI-ref measured in benzonitrile with excita-
and broad positive absorption bands at 880 and 1020 nm
due to the PDI radical cation15 and C60 radical anion,2,3,5
(13) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115,
9798.
(14) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am.
Chem. Soc. 2000, 122, 6535.
(15) (a) Giaimo, J. M.; Gusev, A. V.; Wasielewski, M. R. J. Am. Chem.
Soc. 2002, 124, 8530. (b) van der Boom, T.; Hayes, R. T.; Zhao, Y.;
Bushard, P. J.; Weiss, E. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2002,
124, 9582.
Org. Lett., Vol. 8, No. 20, 2006
4427

CS and fast CR may result from large reorganization energy
of the perylenediimide moiety.
In conclusion, we have successfully synthesized a novel
perylenediimide-C60 dyad in which electron-donating amine
substituents are introduced into the perylenediimide moiety
to facilitate photoinduced electron transfer. By femto- to
picosecond transient absorption measurements, the formation
of the charge-separated state has been demonstrated unam-
biguously in perylenediimide-C60 linked dyads for the first
time. Thus, our perylenediimide-C60 dyad is highly promising
as a new class of artificial photosynthetic models and
photovoltaic materials.
Figure 4. Recalculated transient absorption spectra of PDI-C60 in
Acknowledgment. H.I. is grateful for a Grant-in-Aid
from MEXT, Japan (Priority Area of Chemistry of Coordina-
tion Space (no. 18033025) and 21st Century COE on Kyoto
University Alliance for Chemistry) and Kurata and Sekisui
foundations for financial support. N.T. and H.L. thank the
Academy of Finland.
benzonitrile. Lines with open circles and close circles represent
1
•-
the spectra of PDI* and PDI•+-C60 states, respectively.
respectively, whereas the spectrum of 1PDI* exhibits ground-
state bleaching at 720 nm and broad positive absorption band
at 980 nm. Thus, the time constant of CS (τCS ) 170 ps) is
larger than that of CR (τCR ) 60 ps) (S5).16 The rather slow
Supporting Information Available: Experimental details
(S1-S2) and kinetic analysis of transient absorption spectra
of PDI-C60 (S3-S5). This material is available free of charge
via the Internet at http://pubs.acs.org.
(16) (a) Asahi, T.; Ohkohchi, M.; Takahashi, A.; Matsusaka, R.; Mataga,
N.; Zhang, R. P.; Osuka, A.; Maruyama, K. J. Am. Chem. Soc. 1993, 115,
5665. (b) Tan, Q.; Kuciauskas, D.; Lin, S.; Stone, S.; Moore, A. L.; Moore,
T. A.; Gust, D. J. Phys. Chem. B 1997, 101, 5214.
OL061506A
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