Thieno[3,4-d]imidazolium-containing molecular wire: Switching behavior of photoinduced intramolecular electron transfer

Article abstract of DOI:10.1246/cl.2006.1366

Zinc-porphyrin-oligothiophene-fullerene triad, where diethynylthieno[3,4-d] imidazolium is incorporated as a central unit of oligothiophene, has been synthesized. A dramatic enhancement of fluorescence from the zinc-porphyrin unit was observed on displaci

Full text of DOI:10.1246/cl.2006.1366

1366
Chemistry Letters Vol.35, No.12 (2006)
Thieno[3,4-d]imidazolium-containing Molecular Wire:
Switching Behavior of Photoinduced Intramolecular Electron Transfer
Yutaka Ie,1 Tetsuro Kawabata,1 Takahiro Kaneda,1 and Yoshio Asoꢀ1;2
1The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047
2CREST, Japan Science and Technology Agency (JST)
(Received September 14, 2006; CL-061061; E-mail: aso@sanken.osaka-u.ac.jp)
Zinc–porphyrin–oligothiophene–fullerene triad, where di-
ethynylthieno[3,4-d]imidazolium is incorporated as a central
unit of oligothiophene, has been synthesized. A dramatic en-
hancement of fluorescence from the zinc–porphyrin unit was ob-
served on displacing counter anion from Iꢁ to Fꢁ in the imida-
zolium salt, indicating that photoinduced intramolecular electron
transfer is controllable by the electronic perturbation to this unit.
C4H9
N
N
TPP-I
+
Bu3Sn 4T
Si(iPr)3
I
I
4T CHO
+
S
1
2
3
c (44%)
C4H9
a (69%), b(quant)
Ph
N
N
N
N
Thiophene-based ꢀ-conjugated systems have attracted
much attention as electron-transporting molecular wires for
nanoscale molecular electronics.1,2 One particularly important
chemical aspect in molecular electronics is to control electron
transfer through molecular wire by the installed switching unit
that responds to an external stimulus and switches the electronic
state of the conjugated system. The recent advances in the devel-
opment of switching molecules are mostly based on the structur-
al change of conjugated system leading to interconversion
between two distinct electronic states.3,4 We have previously
reported on the porphyrin–oligothiophene–fullerene triad and
its efficient photoinduced electron transfer from the porphyrin
chromophore to the fullerene,2 and recently reported that this
electron transfer can be controlled by complexation/decomplex-
ation of a sodium cation in the crown ether ring integrated with
the oligothiophene part.5 This switching function is also caused
by a geometrical change of the oligothiophene conjugated sys-
tem. In contrast, the switching without concomitant of structural
change must be very useful but has received little attention,6 in
part due to the difficulty of choosing the appropriate external
stimuli and controlling the electron transfer, except for the mod-
ulation with an electrical gate.7 In this regard, we anticipated that
an organic ionic unit incorporated into the molecular wire can in-
duce different electronic perturbations to the conjugated system
depending on the nature of counter ions without interrupting the
conjugation. In this study, we have synthesized the porphyrin–
oligothiophene–fullerene triad bearing a thieno[3,4-d]imidazoli-
um salt in the oligothiophene part and elucidated the switching
properties in photoinduced electron transfer.
Ph
Zn
4T
+
I
4T CHO
S
N
N
I-Im-4T-CHO
Por-4T
Ph
Ph
d (44%)
C4H9
N
N
N
N
N
Ph
Zn
4T
4T
CHO
S
N
Por-4T-Im-4T-CHO
Ph
Ph
e (45%)
C4H9
N
N
N
N
N
Zn
N
Ph
4T
4T
S
N
Por-4T-Im-4T-C60
Ph
Ph
f (68%)
C4H9
I −
N
N
N
N
N
Zn
N
Ph
4T
Por-4T-Im+I–-4T-C60
4T
S
N
Ph
H13C6
S
S
4T =
S
S
C6H13
The synthesis of the triad Por-4T-ImþIꢀ-4T-C60 is sche-
matically presented in Scheme 1. To avoid the steric influence
of methyl substituents in the imidazolium salt upon the conjuga-
tion of the backbone, ethynyl spacers were introduced between
quaterthiophene and 2-butyl-1,3-dimethylthieno[3,4-d]imidazo-
lium iodide. The Stille coupling of zinc iodotetraphenylporphy-
rin (TPP-I)8 with ꢁ-stannylquaterthiophene (1) followed by
the treatment with TBAF gave Por-4T in 69% yield for two
steps. The counter part I-Im-4T-CHO was synthesized by the
Sonogashira coupling reaction between 1H-2-butyl-4,6-diiodo-
1-methylthieno[3,4-d]imidazole (2) and ꢁ-ethynylquaterthio-
phene (3). Detailed synthetic procedures of 1, 2, and 3 are
Scheme 1. Reagents and conditions: a) Pd(PPh3)4, toluene,
reflux; b) TBAF, THF/MeOH, rt; c) Pd(PPh3)4, NEt3, THF, rt;
d) Pd(PPh3)4, NEt3, THF, 60 ꢂC; e) C60, N-methylglycine,
C6H5Cl, reflux; f) MeI, 50 ꢂC.
given in Supporting Information.9 The palladium-catalyzed
Sonogashira coupling of Por-4T with I-Im-4T-CHO furnished
Por-4T-Im-4T-CHO in 44% yield. The azomethyne ylide gen-
erated in situ from Por-4T-Im-4T-CHO and N-methylglycine
was allowed to react with [60]fullerene in refluxing chloro-
benzene to give the fullerene adduct Por-4T-Im-4T-C60 in a
moderate yield.10 In the final step, treatment of Por-4T-Im-
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.12 (2006)
1367
dazolium cation takes place, so that the cationic charge of the
imidazolium is effectively neutralized to suppress its accelera-
tion effect on the photoinduced electron transfer.
In conclution, we have successfully synthesized the Por-4T-
ImþIꢀ-4T-C60 triad containing an imidazolium salt and dem-
onstrated that the photoinduced intramolecular electron transfer
is controllable by displacing the counter anion, realizing a new
type of the chemical-gate system.6
650
700
750
800
This work was supported by a Grant-in-Aids for Scientific
Research (Nos. 16350022, 18028016, and 18041012) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan.
300
400
500
600
700
800
Wavelength/nm
Figure 1. Electronic absorption spectrum of Por-4T-ImþIꢀ-
4T-C60 in benzonitrile.
References and Notes
1
a) T. Otsubo, Y. Aso, K. Takimiya, Bull. Chem. Soc. Jpn. 2001,
74, 1789. b) T. Otsubo, Y. Aso, K. Takimiya, J. Mater. Chem.
2002, 12, 2565.
4T-C60 with methyl iodine gave the target triad Por-4T-ImþIꢀ-
4T-C60.11
2
a) J. Ikemoto, K. Takimiya, Y. Aso, T. Otsubo, M. Fujitsuka,
O. Ito, Org. Lett. 2002, 4, 309. b) T. Nakamura, M. Fujitsuka,
Y. Araki, O. Ito, J. Ikemoto, K. Takimiya, Y. Aso, T. Otsubo,
J. Phys. Chem. B 2004, 108, 10700.
The UV–vis spectrum of Por-4T-ImþIꢀ-4T-C60 in benzo-
nitrile showed the porphyrin Soret band at 432 nm and Q-band
absorption at 563 and 604 nm, accompanied with the ꢀ–ꢀꢀ band
of oligothiophene in the region of 450–550 nm as shown in
Figure 1. The inset of Figure 1 shows the typical weak absorp-
tion of fulleropyrrolidine derivative at 710 nm. The essentially
identical spectral features of the triad with the superposition of
its component chromophores indicate that there is no electronic
interaction among them in the ground state.
3
4
a) A. S. Lukas, M. R. Wasielewski, in Molecular Switches, ed. by
B. L. Feringer, Wiley-VCH, Weinheim, Germany, 2001, Chap. 1,
pp. 1–35. b) V. Balzani, M. Venturi, A. Credi, Molecular Devices
and Machines, Wiley-VCH, Weinheim, Germany, 2003.
a) A. Osuka, D. Fujikane, H. Shinmori, S. Kobatake, M. Irie,
J. Org. Chem. 2001, 66, 3913. b) J. Nishida, T. Miyagawa, Y.
Yamashita, Org. Lett. 2004, 6, 2523. c) T. A. Golovkova, D. V.
Kozlov, D. C. Neckers, J. Org. Chem. 2005, 70, 5545. d) T.
Kawai, Y. Nakashima, M. Irie, Adv. Mater. 2005, 17, 309. e) Y.
Moriyama, K. Matsuda, N. Tanifuji, S. Irie, M. Irie, Org. Lett.
2005, 7, 3315.
In contrast, the fluorescence from the porphyrin chromo-
phore of Por-4T-Im-4T-C60 in benzonitrile upon excitation
at the Q band (563 nm) was quenched to 73% in comparison
with the strong fluorescence of Por-4T-Im-4T-CHO,9 caused
by occurrence of a photoinduced intramolecular electron transfer
from the porphyrin to the fullerene.2 The fluorescence intensity
of Por-4T-ImþIꢀ-4T-C60 was further reduced to 35%,9,12
indicating a strong electronic perturbation of the imidazolium
unit to accelerate the electron transfer. The controlling
ability of counter anions for the photoinduced electron transfer
5
6
7
T. Oike, T. Kurata, K. Takimiya, T. Otsubo, Y. Aso, H. Zhang, Y.
Araki, O. Ito, J. Am. Chem. Soc. 2005, 127, 15372.
X. Xiao, L. A. Nagahara, A. M. Rawlett, N. Tao, J. Am. Chem.
Soc. 2005, 127, 9235.
For recent examples, see: a) W. Hu, H. Nakashima, K. Furukawa,
Y. Kashimura, K. Ajito, Y. Liu, D. Zhu, K. Torimitsu, J. Am.
Chem. Soc. 2005, 127, 2804. b) B. Xu, X. Xiao, X. Yang, L. Zang,
N. Tao, J. Am. Chem. Soc. 2005, 127, 2386. c) J. Park, A. N.
Pasupathy, J. I. Goldsmith, C. Chang, Y. Yaish, J. R. Petta, M.
Rinkoski, J. P. Sethna, H. D. Abrun˜a, P. L. McEuen, D. C. Ralph,
Nature 2002, 417, 722.
was evaluated by intensity changes of the fluorescence upon
ꢀ
addition of excess ammonium salts (TBAþFꢀ, TBAþPF6
,
TBAþBF4ꢀ, and TBAþClO4ꢀ) to a benzonitrile solution of
Por-4T-ImþIꢀ-4T-C60. A marked enhancement of the fluores-
cence intensity was observed on addition of TBAþFꢀ as shown
8
9
O. Mongin, A. Gossauer, Tetrahedron 1997, 53, 6835.
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
ꢀ
in Figure 2, while the other tested anions (PF6ꢀ, BF4
,
and ClO4ꢀ) showed almost insignificant changes. This result
suggests that the strong coordination of fluoride anion to the imi-
10 a) M. Maggini, G. Scorrano, M. Prato, J. Am. Chem. Soc. 1993,
115, 9798. b) M. Prato, M. Maggini, C. Giacometti, G. Scorrano,
`
G. Sandona, G. Farnia, Tetrahedron 1996, 52, 5221.
5
4
3
2
1
0
11 Synthesis of Por-4T-ImþIꢀ-4T-C60: A solution of Por-4T-Im-
4T-C60 (25 mg, 0.009 mmol) in MeI (excess) was stirred at
60 ꢂC for 4 h in a sealed tube. After removal of the volatile matter,
the residue was purified by a reprecipitation method with CHCl3/
hexane. The resulting solid was filtered and washed with hexane
to give Por-4T-ImþIꢀ-4T-C60 as a brown solid (17 mg, 68%);
MS (MALDI-TOF, 1,8,9-trihydroxyanthracene matrix) m=z
[M–I]þ, calcd for C178H114N7S9Zn, 2704.6; found, 2704.5;
1H NMR (CDCl3) ꢂ 0.83–1.05 (m, 15H), 1.21–1.70 (m, 34H),
2.75–2.87 (m, 13H), 3.88 (d, 1H, J ¼ 9:6 Hz), 4.12 (br s, 6H),
4.62 (d, 1H, J ¼ 9:9 Hz), 4.93 (s, 1H), 7.06–7.15 (m, 11H),
7.42 (s, 1H), 7.74 (m, 9H), 7.95 (d, 2H, J ¼ 8:2 Hz), 8.16–8.22
(m, 8H), 8.88–8.98 (m, 8H).
600
650
700
750
12 Upon conversion of Por-4T-Im-4T-C60 to Por-4T-ImþIꢀ-4T-
C60, the absorption band of oligothiophene shifted to longer
wavelength, while no change was observed in the absorption
bands of the porphyrin and fullerene chromophores as shown in
Figure S01.
Wavelength/nm
Figure 2. Fluorescence spectra of Por-4T-ImþIꢀ-4T-C60
(dashed line) and after addition of TBAF (solid line) in benzoni-
trile with excitation at 563 nm.
Published on the web (Advance View) November 4, 2006; doi:10.1246/cl.2006.1366