Large photocurrent generation of gold electrodes modified with [60]fullerene-linked oligothiophenes bearing a tripodal rigid anchor

Article abstract of DOI:10.1021/ja016703d

[60]Fullerene-linked quarter- and octithiophenes bearing a tripodal rigid anchor have been synthesized as components of highly efficient SAM-based photovoltaic cells. It has been demonstrated that the tripodal anchor plays an important role in preparing a

Full text of DOI:10.1021/ja016703d

Published on Web 01/05/2002
Large Photocurrent Generation of Gold Electrodes Modified with
[60]Fullerene-Linked Oligothiophenes Bearing a Tripodal Rigid Anchor
Daigo Hirayama,† Kazuo Takimiya,† Yoshio Aso,*,† Tetsuo Otsubo,*,† Taku Hasobe,‡
Hiroko Yamada,‡ Hiroshi Imahori,*,‡ Shunichi Fukuzumi,‡ and Yoshiteru Sakata§
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima UniVersity,
Kagamiyama, Higashi-Hiroshima 739-8527, Japan, Department of Material and Life Science,
Graduate School of Engineering, Osaka UniVersity, CREST, Japan Science and Technology Corporation,
2-1 Yamada-oka, Suita, Osaka 565-0871, Japan, and The Institute of Scientific and Industrial Research,
Osaka UniVersity, Mihoga-oka, Ibaraki, Osaka 567-0047, Japan
Received July 25, 2001
Scheme 1
The preparation of fullerene-based thin films has been an active
research field, with promise for practical applications to molecular
devices and sensors.1 In particular, self-assembled monolayers
(SAM) of fullerenes have merited current attention from both fun-
damental and practical points of view.2 In this context, donor-linked
or functionalized fullerenes have frequently been fabricated on
electrodes to develop photovoltaic cells.3,4 However, it has still been
difficult to control the structures of the SAMs themselves, which
notably would affect the performance of the devices. Oligothio-
phenes are rigid rodlike π-conjugated molecules which are ideal
materials to be densely packed normal to the electrodes when a
thiol is attached as a terminal group. Oligothiophene-linked fulle-
renes have been reported to generate a long-lived charge-separated
state via photoinduced electron transfer (ET).5,6 Thus, oligothio-
phene-linked fullerenes with a thiol group at the end show great
promise for being assembled on gold electrodes for efficient pho-
tocurrent generation.
We report herein newly designed [60]fullerene-linked quater-
and octithiophenes (1 and 2) bearing a tripodal rigid anchor, which
is a tetraphenylmethane core with three mercaptomethyl arms,
allowing such molecules to be well-organized on the electrodes. A
similar protocol using three-armed anchors for SAM formations
on gold or TiO2 was recently proposed,7 but the practical advantage
of such SAMs has not been demonstrated yet, to our knowledge.
Our strategy to employ a tripodal rigid anchor has led to remarkable
enhancement of the photocurrent generation [(Au/1 or 2/MV2+/Pt
cell)], as compared to the reference systems with one-armed
anchoring compounds 3 and 4 [(Au/3 or 4/MV2+/Pt cell)].8 The
to the bromo derivative 6 by the Sandmeyer reaction, which was
then brominated with 3 equiv of NBS to the tris(bromomethyl)
derivative 7, and finally treated with potassium thioacetate to afford
the tris(acetylthiomethyl) derivative 8. The Stille coupling reactions
of 8 with tributylstannyl-substituted quater- and octithiophenes (9
and 10), prepared by a combination of lithiation and tributyl-
stannylation of the corresponding oligothiophenes, gave 11 and 12,
respectively. They were converted to the formyl derivatives 13 and
14 by the Vilsmeier reaction, and then treated with C60 and
N-methylglycine10 to give the corresponding fullerene-linked oli-
gothiophenes 15 and 16. Mild alkaline hydrolysis of the acetyl
protecting groups afforded the desired fullerene-linked oligo-
thiophenes 1 and 2 with the free mercaptomethyl groups.
SAMs of 1 (1/Au) and 2 (2/Au) were prepared by immersion of
a hydrogen-frame-annealed Au(111)/mica substrate in their 2 ×
10-5 mol dm-3 CH2Cl2 solutions at 25 °C for 24 h. The samples
were rinsed with CH2Cl2 and EtOH, and then dried with a stream
of nitrogen. To confirm the SAM formation, cyclic voltammetric
experiments with the modified Au electrodes were carried out in
CH2Cl2 containing 0.1 mol dm-3 of Bu4NPF6 as an electrolyte at
scan rate of 100 mV s-1. The cyclic voltammograms of 1/Au and
2/Au exhibited two well-resolved reversible cathodic waves at
-0.66 and -0.97 V corresponding to the successive one-electron
compounds 1 and 2 were synthesized according to Scheme 1.
4-Aminophenyltris(4-methylphenyl)methane (5)9 was first converted
* Corresponding author. Phone: +81-824-24-7732. Fax: +81-824-24-5494.
E-mail: aso@hiroshima-u.ac.jp.
† Hiroshima University.
‡ Osaka University and CREST.
§ Osaka University.
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532 VOL. 124, NO. 4, 2002 J. AM. CHEM. SOC.
10.1021/ja016703d CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
and by the gold electrode, leading to increasing the photocurrent.
The quantum yields of 1/Au and 2/Au were estimated based on
the number of absorbed photons being 17 ( 7% and 35 ( 8%, re-
spectively.13 The remarkably large quantum yield for 2/Au as com-
pared to the value for 1/Au indicates that the long oligothiophene
chain can facilitate the generation and charge transport of photo-
current.
In summary, we have demonstrated an important role of the
tripodal rigid anchor in preparing well-organized SAMs of [60]-
fullerene-linked long oligothiophenes on the electrode to construct
highly efficient photovoltaic cells.
Figure 1. Photocurrent response of 1/Au irradiated with 400 nm light (0.65
mW cm-2) at -100 mV of bias vs Ag/AgCl.
Acknowledgment. This research was supported by Grants-in-
Aid for Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
reductions of the C60 moiety (Figure S2). The observation of these
well-defined redox waves indicates that the molecules are strongly
adsorbed on the surface very probably due to three-point connection.
In contrast to the three-point connection systems, the voltammo-
grams of SAMs of 3 (3/Au) and 4 (4/Au) with one-point connection
show poorly reversible waves, suggesting that the molecules are
sparsely attached to the surface and tend to be detached during the
reduction sweep. On the basis of the integrated charge of the first
reduction peak, surface coverages (Γ) are calculated as follows:
1/Au, 1.7 × 10-10 mol cm-2; 2/Au, 1.6 × 10-10 mol cm-2; 3/Au,
Supporting Information Available: The experimental procedures
and spectral data for all new compounds, cyclic voltammograms, ab-
sorption spectra, and action spectra of the SAMs. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Mirkin, C. A.; Caldwell, W. B. Tetrahedron 1996, 52, 5113-5130.
(b) Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593-601.
(c) Diederlich, F.; Go´mez-Lo´pez, M. Chem. Soc. ReV. 1999, 28, 263-277.
(2) (a) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993,
115, 1193-1194. (b) Chupa, J. A.; Xu, S.; Fischetti, R. F.; Strongin, R.
M.; McCauley, J. P., Jr.; Smith, A. B., III; Blasie, J. K. J. Am. Chem.
Soc. 1993, 115, 4383-4384. (c) Shi, X.; Caldwell, W. B.; Chen, K.;
Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 11598-11599. (d) Arias, F.;
God´ınez, L. A.; Wilson, S. R.; Kaifer, A. E.; Echegoyen, L. J. Am. Chem.
Soc. 1996, 118, 6086-6087. (e) Liu, S.-G.; Martineau, C.; Raimundo,
J.-M.; Roncali, J.; Echegoyen, L. Chem. Commun. 2001, 913-914.
(3) (a) Luo, C.; Guldi, D. M.; Maggini, M.; Menna, E.; Mondini, S.; Kotov,
N. A.; Prato, M. Angew. Chem., Int. Ed. 2000, 39, 3905-3909. (b) Kamat,
P. V.; Barazzouk, S.; Hotchandani, S.; Thomas, K. G. Chem. Eur. J. 2000,
6, 3914-3921. (c) Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F.; Liu,
S.-G.; Echegoyen, L.; Barigelletti, F.; Armaroli, N.; Ouali, L.; Krasnikov,
V.; Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 7467-7479. (d) Ikeda,
A.; Hatano, T.; Shinkai, S.; Akiyama, T.; Yamada, S. J. Am. Chem. Soc.
2001, 123, 4855-4856.
(4) (a) Imahori, H.; Azuma, T.; Ajavakom, A.; Norieda, H.; Yamada, H.;
Sakata, Y. J. Phys. Chem. B 1999, 103, 7233-7237. (b) Imahori, H.;
Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y. J. Phys. Chem. B
2000, 104, 2099-2108. (c) Imahori, H.; Norieda, H.; Yamada, H.;
Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc.
2001, 123, 100-110.
(5) (a) Segura, J. L.; Mart´ın, N. J. Mater. Chem. 2000, 10, 2403-2435. (b)
Knorr, S.; Grupp, A.; Mehring, M.; Grube, G.; Effenberger, F. J. Chem.
Phys. 1999, 110, 3502-3508. (c) 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. (d) Yamashiro, T.; Aso, Y.;
Otsubo, T.; Tang, H.; Harima, Y.; Yamashita, K. Chem. Lett. 1999, 443-
444.
3.5 × 10-11 mol cm-2; 4/Au, 8.9 × 10-11 mol cm-2 11
. The small
surface coverage for 3/Au is due to its lying structure. Judging from
the other three surface coverages, it turns out that the three-armed
molecules 1 and 2 are more densely adsorbed than the one-armed
molecule 4. In addition, it is worth noting that the adsorbed amounts
of 1 and 2 are virtually the same, independent of the oligothiophene
length. These results indicate that the three-armed anchor molecules
can stand up on the surface by themselves, forming highly ordered
SAMs.
Photoelectrochemical measurements were performed in an argon-
saturated 0.1 mol dm-3 Na2SO4 solution containing 5 × 10-3 mol
dm-3 of methyl viologen (MV2+) as an electron carrier using the
modified Au electrode as a working electrode and a platinum
counter electrode at -100 mV bias against an Ag/AgCl reference
electrode under illumination of 400 nm light for 1/Au or 440 nm
for 2/Au, which is mainly absorbed by the oligothiophene chro-
mophore. The Au/1/MV2+/Pt cell showed a cathodic photoelectro-
chemical response as shown in Figure 1. The amount of generated
photocurrent for 1/Au (close to 1 mA in Figure 1) is remarkable as
compared to several nA scale usually observed for this type of
photovoltaic cells.12 The action spectrum of the Au/1/MV2+/Pt cell
(Figure S3) roughly agrees with the absorption spectrum on the
gold electrode (Figure S4). The cathodic photocurrent increases with
an increase of the negative bias of the gold electrode (Figure S5).
On the basis of the reported photodynamics of the oligo-
thiophene-[60]fullerene dyads,6 together with the results on similar
photoelectrochemcial cells,4 we can conclude that photoexcitation
of the quaterthiophene chromophore first induces ET from the
quaterthiophene to the C60 moiety to generate a net vectorial electron
flow from the gold cathode to the Pt counter electrode via MV2+
with the aid of applied bias. The photocurrent density is much larger
by a factor of about 190 than that observed for the Au/3/MV2+/Pt
cell. The Au/2/MV2+/Pt cell also revealed a large photoelectro-
chemical response, and its photocurrent density is about 6 times as
large as that observed for the Au/4/MV2+/Pt cell. The enhanced
photoresponses of 1/Au and 2/Au are primarily attributable to the
densely packed SAMs. However, it is interesting to note that the
enhancement ratios of the photocurrent much exceed the increased
ratios of the Γ values. The self-reliant standing of 1 and 2 on the
gold surface probably may suppress quenching of the excited states
of the oligothiophene chromophore by intermolecular interactions
(6) (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.
(7) (a) Whitesell, J. K.; Chang, H. K. Science 1993, 261, 73-76. (b) Fox, M.
A.; Whitesell, J. K.; McKerrow, A. J. Langmuir 1998, 14, 816-820. (c)
Tao, Y.; Tour, J. M. J. Org. Chem. 1999, 64, 1968-1971. (d) Guo, W.;
Galoppini, E.; Rydja, G.; Pardi, G. Tetrahedron Lett. 2000, 41, 7419-
7421. (e) Hu, J.; Mattern, D. L. J. Org. Chem. 2000, 65, 2277-2281. (f)
Galoppini, E.; Guo, W.; Qu, P.; Meyer, G. J. J. Am. Chem. Soc. 2001,
123, 4342-4343.
(8) Hirayama, D.; Yamashiro, T.; Takimiya, K.; Aso, Y.; Otsubo, T.; Norieda,
H.; Imahori, H.; Sakata, Y. Chem. Lett. 2000, 570-571.
(9) Heim, C.; Affeld, A.; Nieger, M.; Vo¨gtle, F. HelV. Chim. Acta 1999, 82,
746-759.
(10) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115, 9798-
9799.
(11) The occupied areas per molecule of 1/Au and 2/Au agree largely with
the calculated areas, assuming 1 and 2 are well-packed on the gold surface
with a perpendicular orientation.
(12) Lahav, M.; Heleg-Shabtai, V.; Wasserman, J.; Katz, E.; Willner, I.; Du¨rr,
H.; Hu, Y.-Z.; Bossmann, S. H. J. Am. Chem. Soc. 2000, 122, 11480-
11487.
(13) Photocurrent density/nA cm-2 (absorbance): 1/Au, 783 (0.006 at 400 nm);
2/Au, 1570 (0.006 at 440 nm). For comparison, it was estimated that the
quantum yield of 3/Au was less than 0.1% and that of 4/Au was about
5%.
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