Porphyrin-oligothiophene-fullerene triads as an efficient intramolecular electron-transfer system

Article abstract of DOI:10.1021/ol016511n

(Equation Presented) Por-nT-C60 (x = 1-3) A series of porphyrin-oligothiophene-fullerene triads containing quaterthiophene, octithiophene, and dodecithiophene spacers has been synthesized. The fluorescence of the porphyrin chromophore in benzonitrile is e

Full text of DOI:10.1021/ol016511n

ORGANIC
LETTERS
2002
Vol. 4, No. 3
309-311
Porphyrin−Oligothiophene−Fullerene
Triads as an Efficient Intramolecular
Electron-Transfer System
Junya Ikemoto,† Kazuo Takimiya,† Yoshio Aso,* Tetsuo Otsubo,*
,†
,†
,‡
Mamoru Fujitsuka,‡ and Osamu Ito*
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima
UniVersity, Higashi-Hiroshima 739-8527, Japan, and Institute of Multidisciplinary
Research for AdVanced Materials, Tohoku UniVersity, Katahira, Aoba-ku,
Sendai 980-8577, Japan
otsubo@hiroshima-u.ac.jp
Received July 27, 2001
ABSTRACT
A series of porphyrin−oligothiophene−fullerene triads containing quaterthiophene, octithiophene, and dodecithiophene spacers has been
synthesized. The fluorescence of the porphyrin chromophore in benzonitrile is efficiently quenched by electron transfer to the fullerene moiety.
This process shows a weak distance dependence of the oligothiophene spacer with an attenuation factor â ) 0.11 Å-1.
Conjugated nanoscale oligomers have received current at-
tention as molecular wires in the fields of molecular
electronics1 and light-harvesting molecular antenna-type
architecture.2 The rates of electron and energy transfer
through oligomeric molecules exponentially decrease with
their increasing lengths, and the preexponential coefficients
(attenuation factor) are closely related to their chemical
bonding.3 Well-defined oligothiophenes are of special interest
in terms of their ready accessibility, structural modifications,
high π-conjugation, low oxidation potentials, and environ-
mental stability,4 but how efficiently oligothiophenes can
function as a molecular wire is a still unclear key issue. In
this regard, Effenberger and co-workers recently reported
that in anthryloligothienylporphyrins5 and anthryloligo-
thienylfullerenes6 photoinduced electron or energy transfer
smoothly occurred from the excited anthracene as a donor
to the porphyrin or fullerene as an acceptor via the oligo-
thiophene spacer. However, the spacer is limited to only short
oligomers up to the 5-mer, so that the distance dependence
of oligothiophenes remains unsolved. Extending our study
of an intriguing charge separation dyad system nT-C60,7,8
† Hiroshima University.
‡ Tohoku University.
(1) For recent reviews on conjugated nanoscale oligomers, see: (a) Tour,
J. M. AdV. Mater. 1994, 6, 190-197. (b) Tour, J. M. Chem. ReV. 1996, 96,
537-553. (c) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999,
38, 1350-1377. (d) Tour, J. M. Acc. Chem. Res. 2000, 33, 791-804.
(2) For recent reviews on intramolecular electron and energy transfer,
see: (a) Panetta, C. A.; Heimer, N. E.; Hussey, C. L.; Metzger, R. M. Synlett
1991, 301-309. (b) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461.
(c) Imahori, H.; Sakata, Y. AdV. Mater. 1997, 9, 537-546. (d) Mart´ın, N.;
Sa´nchez, L.; Illescas, B.; Pe´rez, I. P. Chem. ReV. 1998, 98, 2527-2547.
(e) Imahori, H.; Sakata, Y. Eur. J. Org. Chem. 1999, 2445-2457.
(3) Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L. R.; Smalley, J. F.;
Newton, M. D.; Feldberg, S. W.; Chidsey, C. E. D. J. Am. Chem. Soc.
1997, 119, 10563-10564, and references therein.
(4) For recent reviews on oligothiophenes, see: (a) Ba¨uerle, P. In
Electronic Materials: The Oligomeric Approach; Mu¨llen, K., Wegner, G.,
Eds; Wiley-VCH: Weinheim, 1998; Chapter 2. (b) Fichou, D. Handbook
of Oligo- and Polythiophenes; Wiley-VCH: Weinheim, 1998.
(5) (a) Wu¨rthner, F.; Vollmer, M. S.; Effenberger, F.; Emele, P.; Meyer,
D. U.; Port, H.; Wolf, H. C. J. Am. Chem. Soc. 1995, 117, 8090-8099. (b)
Vollmer, M. S.; Wu¨rthner, F.; Effenberger, F.; Emele, P.; Meyer, D. U.;
Stu¨mpfig, T.; Port, H.; Wolf, H. C. Chem. Eur. J. 1998, 4, 260-269.
(6) (a) Effenberger, F.; Grube, G. Synthesis 1998, 1372-1379. (b) Knorr,
S.; Grupp, A.; Mehring, M.; Grube, G.; Effenberger, F. J. Chem. Phys.
1999, 110, 3502-3508.
10.1021/ol016511n CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/15/2002

we have prepared the novel triad system Por-nT-C60, which
comprises three components of porphyrin, oligothiophene,
and fullerene. The involvement of a series of long oligo-
thiophene spacers, such as quarterthiophene, octithiophene,
and dodecithiophene, in the system has allowed us to study
a long-range electron transfer and to evaluate its distance
dependence. Here we would like to report the synthesis and
photophysical properties of Por-nT-C60.
pyrrole in the presence of 4 molar equiv of trifluoroacetic
acid according to the Lindsey method.9 Subsequent oxidation
with p-chloranil formed hybrid porphyrins containing both
oligothienyl and phenyl substituents together with tetra-
phenylporphyrin, which were separated from one another by
a column chromatographic technique (silica gel/hexane-
dichloromethane). Although the yield is low (2-9%), this
access to Por-nT is straightforward. Subsequently, Por-nT
were converted to Por-nT-CHO by the Vilsmeier reaction
(55-65% yield) and finally treated with fullerene and
N-methylglycine in toluene according to the Prato method10
to give the target Por-nT-C60 (40-50% yield).11
The electronic absorption spectra of Por-nT-C60 in
benzonitrile, as shown in Figure 1, are understood in terms
The porphyrin-oligothiophene-fullerene triads were syn-
thesized as shown in Scheme 1. The formyl oligothiophenes,
readily accessible by the Vilsmeier reaction of oligothio-
phenes,7a were subjected together with 3 molar equiv of
benzaldehyde to hybrid condensation with 4 molar equiv of
Scheme 1a
Figure 1. Electronic absorption spectra of Por-4T-C60 (solid line),
Por-8T-C60 (dashed line), and Por-12T-C60 (dotted line) in
benzonitrile.
of simple superimposition of the electronic transitions of the
three constituent chromophores: the porphyrin component
shows a very strong absorption at 426 nm (Soret band) and
(7) (a) Yamashiro, T.; Aso, Y.; Otsubo, T.; Tang, H.; Harima, Y.;
Yamashita, K. Chem. Lett. 1999, 443-444. (b) Fujitsuka, M.; Ito, O.;
Yamashiro, T.; Aso, Y.; Otsubo, T. J. Phys. Chem. A 2000, 104, 4876-
4881. (c) Fujitsuka, M.; Matsumoto, K.; Ito, O.; Yamashiro, T.; Aso, Y.;
Otsubo, T. Res. Chem. Intermed. 2001, 27, 73-88.
(8) van Hal, P. A.; Knol, J.; Langeveld-Voss, B. M. W.; Meskers, S. C.
J.; Hummelen, J. C.; Janssen, R. A. J. J. Phys. Chem. A 2000, 104, 5974-
5988.
(9) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.;
Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827-836.
(10) Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519-526.
(11) All new compounds Por-nT-C60 were characterized by spectro-
scopic and elemental analyses. Por-4T-C60: brown powder; mp 255 °C
1
(dec); MS (MALDI-TOF) m/z 1810 (M+); H NMR (CDCl3) δ -2.71 (s,
2H), 0.88 (t, J ) 6.8 Hz, 6H), 1.27 (m, 12H), 1.84-1.92 (m, 2H), 1.96
(quin, J ) 7.7 Hz, 2H), 2.80 (t, J ) 7.7 Hz, 2H), 2.92 (s, 3H), 3.12 (t, J )
7.7 Hz, 2H), 4.11 (d, J ) 9.4 Hz, 1H), 4.87 (d, J ) 9.4 Hz, 1H), 5.08 (s,
1H), 7.07 (d, J ) 3.9 Hz, 1H), 7.18 (d, J ) 3.9 Hz, 1H), 7.21 (s, 1H), 7.22
(d, J ) 3.9 Hz, 1H), 7.28 (d, J ) 3.9 Hz, 1H), 7.76 (s, 1H), 7.74-7.78 (m,
9H), 8.19-8.22 (m, 6H), 8.82 (s, 4H), 8.87 (d, J ) 4.6 Hz, 2H), 9.22 (d,
J ) 4.6 Hz, 2H). Anal. Calcd for C129H63N5S4: C, 85.55; H, 3.51; N, 3.89.
Found: C, 85.23; H, 3.58; N, 3.80. Por-8T-C60: brown powder; mp 181
a Reagents and conditions: (i) 4 equiv of pyrole, 3 equiv of
benzaldehyde, 4 equiv of trifluoroacetic acid, dichloromethane, rt,
2 h, then 3 equiv of chloranil, reflux, 1 h, finally potassium
carbonate, rt, 2 h; (ii) 10 equiv of DMF, 10 equiv of POCl3, 1,2-
dichloroethane, 40 °C, 12 h; (iii) C60, 5 equiv of N-methylglycine,
toluene, reflux, 48 h.
1
°C (dec); MS (MALDI-TOF) m/z 2309 (M+); H NMR (CDCl3) δ -2.71
(s, 2H), 0.81-0.95 (m, 12H), 1.22-1.43 (m, 24H), 1.59-1.69 (m, 6H),
1.94-1.99 (m, 2H), 2.72-2.90 (m, 6H), 2.89 (s, 3H), 3.13 (t, J ) 7.8 Hz,
2H), 4.07 (d, J ) 9.5 Hz, 1H), 4.84 (d, J ) 9.5 Hz, 1H), 5.03 (s, 1H),
7.000 (s, 1H), 7.001 (s, 1H), 7.02 (d, J ) 3.9 Hz, 2H), 7.07 (d, J ) 3.9 Hz,
1H), 7.095 (d, J ) 3.9 Hz, 1H), 7.101 (d, J ) 3.9 Hz, 1H), 7.18 (s, 1H),
310
Org. Lett., Vol. 4, No. 3, 2002

four weak absorptions at 521, 562, 591, and 655 nm (Q-
band), the oligothiophene shows a π-π* band in the region
of 350-470 nm, whose wavelength is red-shifted and
intensity is enhanced with the chain extension, and the
fullerene shows a strong π-π* band at 328 nm accompanied
with weak long-wavelength bands tailing to 704 nm.
Evidently there is no electronic interaction among the three
chromophores in the ground state. On the other hand, the
emission spectra are interactive, as demonstrated for Por-
4T-C60 in Figure 2. When the porphyrin chromophore is
to the respective porphyrin-linked oligothiophenes Por-nT.
On the basis of the quenching and the fluorescence lifetimes
of Por-nT (n ) 4, τ 3.3 × 10-9 s; n ) 8, 1.4 × 10-9 s; n
) 12, 1.3 × 10-9 s), we can estimate from the relation kET
) [Φ(Por-nT)/Φ(Por-nT-C60) - 1]/τ(Por-nT) that the rate
constants for electron transfer reaction (kET) are the follow-
ing: Por-4T-C60, 5.7 × 109 s-1; Por-8T-C60, 6.2 × 108
s-1; Por-12T-C60, 2.0 × 108 s-1. Molecular modeling shows
that the donor-acceptor distances (R) are 1.4 nm for Por-
4T-C60, 3.0 nm for Por-8T-C60, and 4.6 nm for Por-12T-
C60. A plot of ln kET vs R gives a good straight line
according to equation kET ) A exp(-âR). From the slope of
the line, an attenuation factor â ) 0.11 Å-1 is obtained. This
value for electron transfer is much smaller than 0.6-1.2 Å-1
for saturated hydrocarbon bridges12 and 0.32-0.66 Å-1 for
conjugated phenylenes,13 and comparable to 0.04-0.2 Å-1
for polyenes3,14 and 0.04-0.17 Å-1 for polyynes.14b,15
Evidently, the efficient electronic coupling between the donor
and the acceptor of Por-nT-C60 occurs through the oligo-
thiophene spacer. Taking accessibility to extraordinarily long
oligothiophenes into account,16 oligothiophenes are most
promising for long-distance molecular wires.
Acknowledgment. This research was suported by Grants-
in-Aid of Scientific Research (12440180, 12042263, and
13304051) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Figure 2. Fluorescence spectra of Por-4T (dashed line) and Por-
4T-C60 (solid line) in benzonitrile with excitation at 558 nm.
Supporting Information Available: Detailed experi-
mental procedures and characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL016511N
excited in benzonitrile, 95% of the porphyrin fluorescence
from Por-4T-C60 as compared to Por-4T is quenched by
the additionally attached fullerene. Considering no ap-
preciable emitting from the fullerene, this result indicates
that a substantial amount of electron transfer occurs from
the porphyrin moiety to the fullerene. Similar quenching is
also observed for the extended homologues, but reduced with
the extended chain length; 46% from Por-8T-C60 and 21%
from Por-12T-C60 in benzonitrile are quenched as compared
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114, 3173-3181. (f) Paulson, B.; Pramod, K.; Eaton, P.; Closs, G.; Miller,
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Tamai, N. J. Am. Chem. Soc. 1990, 112, 4958-4959. (b) Helms, A.; Heiler,
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7.20 (d, J ) 3.9 Hz, 1H), 7.24 (d, J ) 3.6 Hz, 1H), 7.31 (d, J ) 3.6 Hz,
1H), 7.76 (s, 1H), 7.74-7.80 (m, 9H), 8.20-8.22 (m, 6H), 8.82 (s, 4H),
8.87 (d, J ) 5.0 Hz, 2H), 9.23 (d, J ) 5.0 Hz, 2H). Anal. Calcd for
C
157H95N5S8: C, 81.32; H, 4.24; N, 3.10. Found: C, 81.06; H, 4.04; N,
2.94. Por-12T-C60: brown powder; mp 119 °C (dec); MS (MALDI-TOF)
m/z 2804 (M+); H NMR (CDCl3) δ -2.71 (s, 2H), 0.89-0.95 (m, 18H),
1
1.23-1.45 (m, 36H), 1.67-1.71 (m, 10H), 1.97 (quin, J ) 7.8 Hz, 2H),
2.72-2.90 (m, 10H), 2.93 (s, 3H), 3.14 (t, J ) 7.8 Hz, 2H), 4.12 (d, J )
9.6 Hz, 1H), 4.88 (d, J ) 9.6 Hz, 1H), 5.07 (s, 1H), 7.00 (s, 2H), 7.02 (s,
2H), 7.01-7.05 (m, 4H), 7.08 (d, J ) 3.9 Hz, 1H), 7.104 (d, J ) 3.7 Hz,
1H), 7.108 (d, J ) 3.7 Hz, 1H), 7.13 (d, J ) 3.7 Hz, 3H), 7.215 (s, 1H),
7.21 (d, J ) 3.9 Hz, 1H), 7.24 (d, J ) 3.9 Hz, 1H), 7.30 (d, J ) 3.9 Hz,
1H), 7.76 (s, 1H), 7.74-7.80 (m, 9H), 8.20-8.22 (m, 6H), 8.82 (s, 4H),
8.87 (d, J ) 5.0 Hz, 2H), 9.23 (d, J ) 5.0 Hz, 2H). Anal. Calcd for
C185H127N5S12: C, 79.22; H, 4.56; N, 2.50. Found C, 79.12; H, 4.42; N,
2.46.
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