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Published on the web January 15, 2011
Phosphine Sulfides as an Anchor Unit for Single Molecule Junctions
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2,³
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2
2
Aiko Fukazawa,* Manabu Kiguchi,* Satoshi Tange, Yasunori Ichihashi, Qiang Zhao, Takuya Takahashi, Tatsuya Konishi,
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Kei Murakoshi, Yuta Tsuji, Aleksandar Staykov, Kazunari Yoshizawa,* and Shigehiro Yamaguchi*
Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa-ku, Nagoya, Aichi 464-8602
Division of Chemistry, Graduate School of Science, Hokkaido University, N10W8, Kita-ku, Sapporo, Hokkaido 060-0810
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Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395
(
Received November 22, 2010; CL-100982; E-mail: yamaguchi.shigehiro@b.mbox.nagoya-u.ac.jp)
Phenylene and biphenyl compounds with dibenzophosphole
sulfide (DBPS) as an anchoring group for single molecule
junctions were synthesized. The conductance measurements
revealed that the phosphine sulfide indeed acts as an anchoring
group for Au electrodes. Theoretical calculations including
metal electrodes demonstrated that the LUMO level of the
DBPS-terminated biphenyl is close to the Au Fermi level,
leading to the electron conduction of the AumoleculeAu
junction based on the resonance-tunneling mechanism.
Figure 1. (a) Illustration of an assumed contact of the
DBPS-terminated oligo(p-phenylene)s in a broken gold gap.
Electron-transport properties through single molecules have
attracted growing attention as the basis for fabricating ultrasmall
(
b) Structures of the molecules studied in this study.
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electronic devices. Recent extensive studies on a variety of
single molecule junctions revealed that the electron conduction
through a molecule bonded between two metal electrodes highly
depends on the nature of anchoring groups, which affects not
only the HOMOLUMO energy gap of the whole molecular
a
b
O
c,d
Br
Br
Br
PCl2
P
Br
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system, but also the alignment of the energy levels of these
O
P
S
e
f
frontier MOs against the electrode Fermi level.2,3 The develop-
P
PCl2
P
P
P
O
S
ment of a new anchoring group as well as the deep under-
standing of the conduction mechanism are therefore urgent
subjects for the evolution of single molecule electronics. Besides
the combination of thiol (SH) and Au electrodes, a variety of
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Scheme 1. Reagents and conditions for the synthesis of 1: (a)
Mg, Et O, reflux, then PCl , 0 °C to rt; (b) 2,2¤-dilithiobiphenyl,
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anchoring groups has been investigated, such as pyridine,
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3
isocyanide, carboxylate, selenide,8 amine,
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,6
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913
phosphine,
14
Et O, 0 °C, then H O aq., 14% (over 2 steps); (c) HSiCl ,
2 2 2 3
C60, and carbon16,17 for various electrodes.
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toluene, 0 °C; (d) n-BuLi, Et2O, 0 °C, then PCl3, 0 °C to rt;
e) 2,2¤-dilithiobiphenyl, Et O, 0 °C, then H O aq., 3% (over 3
(
2
2
2
In this study, we explored the possibility of using a
phosphine sulfide (P=S) bond as a new anchoring group to form
single molecule junctions with Au electrodes. The P=S bond
has several characteristics. First, a sulfur atom in the P=S bond
has high affinities to various metals, such as Au, Ag, and Cu,
leading to strong adsorption.18 Second, phosphine sulfides have
high chemical and thermal stability among the various types of
organophosphorus compounds, which guarantees their versatile
use for the molecular junction. Moreover, the P=S group acts as
a strong electron-withdrawing group. We envisioned that the
incorporation of a P=S group as an anchor to a certain ³-
conjugated skeleton would decrease the LUMO level of the
steps); (f) Lawesson reagent, toluene, reflux, 36%.
Br
a
b
P
P
P
Br
Cl
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S
c
P
P
S
2
1
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molecular system,
and thus affect the electron-transport
mechanism. To minimize the steric hindrance around the P=S
moiety, we decided to employ a dibenzophosphole sulfide
Scheme 2. Reagents and conditions: (a) t-BuLi, THF, ¹78 °C,
then PCl3 (excess), ¹196 °C to rt; (b) 4,4¤-dilithiobiphenyl, THF,
¹
78 °C; (c) S8, rt, 16% (over 3 steps).
(DBPS) skeleton. As a model system, we synthesized the DBPS-
terminated phenylene 1 and biphenyl-4,4¤-diyl 2 (Figure 1b). We
now disclose the electron conductance of these molecular
systems and some insights into their conduction mechanism.
Compounds 1 and 2 were synthesized in different ways, as
shown in Schemes 1 and 2, respectively. The biphenyl deriv-
ative 2 was readily prepared by the reaction of P-chlorodi-
benzophosphole (DBPCl) (10) with 4,4¤-dilithiobiphenyl, fol-
lowed by the oxidation with elemental sulfur (Scheme 2).
However, a similar route based on the reaction between DBPCl
and 1,4-dilithiobenzene did not afford 1 at all. Alternatively, by
employing P-(4-bromophenyl)-DBP as a precursor, we in situ
Chem. Lett. 2011, 40, 174176
© 2011 The Chemical Society of Japan