Phosphine sulfides as an anchor unit for single molecule junctions

Article abstract of DOI:10.1246/cl.2011.174

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.

Full text of DOI:10.1246/cl.2011.174

doi:10.1246/cl.2011.174
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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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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.
1
,2
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
6
7
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
8
9
1
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,
4
anchoring groups has been investigated, such as pyridine,
2
3
isocyanide, carboxylate, selenide,8 amine,
5
,6
7
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.
1
5
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
10
11
S
c
P
P
S
2
1
9,20
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
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Figure 2. Examples of conductance traces of gold nano-
contacts measured in the presence (red line) and absence (black
line) of (a) 1 and (b) 2 in mesitylene.
Figure 3. Conductance histograms of gold nanocontacts con-
structed from 2000 traces measured in the presence (red line)
and absence (black line) of (a) 1 and (b) 2 in mesitylene. The
insets show the conductance histograms on the log­log plot.
generated a P-4-(dichlorophosphanyl)phenyl derivative 8 by the
successive treatment with n-BuLi and PCl and then reacted this
3
compound with 2,2¤-dilithiobiphenyl. The oxidation of the
produced mixture afforded the corresponding phosphine oxide
9
, which was further converted to 1 using Lawesson reagent
(Scheme 1). As a reference compound, we also synthesized a
dibenzophosphole oxide (DBPO)-terminated biphenyl 3 by a
method similar to the synthesis of 2. The details of these
2
1
syntheses are described in Supporting Information (SI).
Conductance measurements of the DBPS derivatives 1 and
were performed using a modified STM (Figure 1a). Thus, a Au
2
STM tip was repeatedly moved into and out of contact with a Au
substrate in a 0.1 or 0.2 mM sample solution in mesitylene.
Conductance traces were recorded during breaking under an
applied bias of 20 mV between the tip and substrate. The
statistical data were obtained from 2000 conductance traces. The
individual traces of 1 and 2 exhibited steps at the molecule-
dependent conductance values less than the quantum of con-
Figure 4. (a) The PDOS of 2 (black) and the projection onto
the terminal sulfur atoms (red) and phosphorus atoms (blue).
(
b) The PDOS of 5 (black) and the projection onto the terminal
sulfur atoms (red).
2
(3(1) © 10¹5 G0) was two-orders-of-magnitude lower than that
ductance G0 = 2e /h. The conductance plateaus correspond to
¹3
22
the conduction through a molecule bonded between the two
Au point contacts (Figures 2a and 2b). Figures 3a and 3b are
conductance histograms of 1 and 2, respectively, both of which
showed obvious peaks. Such peak structures were not observed
in either the conductance traces nor histograms obtained in the
absence of molecules (Figures 2 and 3). In addition, as shown in
the insets in Figures 3a and 3b, both conductance histograms of
compounds 1 and 2 on the log­log plot exhibited significant
peaks before the contacts were broken under our experimental
conditions. Therefore, the last plateau before the contact is
broken in the conductance trace and a peak observed in the
conductance histogram likely originates from the formation of
stable single Au­1­Au and Au­2­Au junctions. The conduct-
ance values of the single molecule junctions were determined to
of the benzene-1,4-dithiol 4 (3(1) © 10 G0), the difference
in conductance between the DBPS-terminated phenylene 1 and
biphenyl-4,4¤-diyl 2 was only one-order-of-magnitude. These
results imply that the conduction mechanism in the DBPS-
anchored systems is different from that in the thiol-anchored
systems.
To elucidate the difference in the conduction properties
between the DBPS- and thiol-anchored systems, we performed
theoretical calculations using the nonequilibrium Green’s func-
2
3­25
tion combined with density functional theory (NEGF-DFT).
PDOS (phonon density of states) and zero-bias transmission
spectra of the Au­molecule­Au junctions of the DBPS- and
thiol-terminated biphenyls 2 and 5 are shown in Figures 4 and 5,
respectively. Whereas in the thiol 5 the Fermi level (where
E = 0) is located in the middle of the HOMO­LUMO gap, the
DBPS-biphenyl 2 has a rather low-lying LUMO, which is closer
to the Fermi level. According to differential pulse voltammetry
(DPV) measurements in THF, compounds 1 and 2 indeed
exhibited the first reduction waves with low peak potentials at
¹1.90 and ¹1.91 V vs. ferrocene/ferrocenium couple, respec-
tively. These results are due to the electron-withdrawing nature
of the P=S moiety. Notably, this low-lying LUMO plays an
important role in the electron-transport properties. In the
transmission spectra (Figure 5), while 5 has a broad peak below
the Fermi level, 2 shows a sharp transmission peak close to the
Fermi level. This suggests that while the electron transport of
¹
4
¹5
be 5(2) © 10 and 5(2) © 10 G for 1 and 2, respectively.
0
These results demonstrated that the phosphine sulfide indeed
acts as an anchoring group for the Au­molecule junction.
In order to assess these values, we also determined the
electron conductance values for DBPO-terminated biphenyl 3
as well as thiol-terminated analogs 4 and 5 under the same
measurement conditions.21 Notably, the conductance histogram
of 3 did not show any significant peak, indicative of either its
poor electron-conductance or poor adsorption ability on the gold
electrode. The use of the P=S group is likely crucial for the
formation of the Au­molecule contact. On the other hand,
while the conductance of the the biphenyl-4,4¤-dithiol 5
Chem. Lett. 2011, 40, 174­176
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Figure 6. MPSH states of (a) 2 and (b) 5 for applied bias of 0 V.
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1 For the conductance histograms of compounds 3, 4, and 5,
see SI. Supporting Information is available electronically on
the CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
In summary, we demonstrated the utility of phosphine
sulfide as an anchoring group at the Au­molecule junction. The
high stability as well as strong electron-withdrawing effect
makes this functional group a distinctive anchor unit. In
particular, the DBPS-anchored biphenyl provides a good
example that allows electron conduction through the reso-
nance-tunneling mechanism.
2
2 Recently, several reports regarding the observation of different
conductance peaks ascribed to different contact geometries
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3
18. b) W. Haiss, C. Wang, R. Jitchati, I. Grace, S. Martín, A. S.
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Jensen, R. J. Nichols, J. Phys.: Condens. Matter 2008, 20,
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74119. Although compounds 4 and 5 may also adopt several
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (No. 17069011; M.K. and S.Y.) and
Research and Education Funding for Inter-University Research
Project (Kyoto Univ./Nagoya Univ./Kyushu Univ.; K.Y. and
S.Y.) from the MEXT, Japan, and PRESTO, Japan Science and
Technology Agency (M.K.).
different contact geometries, we observed only broad signals
corresponding to the molecular conductance under our experi-
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References and Notes
25 ATK Version 2008.07, quantumwise; http://www.quantumwise.
com.
26 For the transmission spectra upon applying an external bias, see
SI.
³
Present address: Department of Chemistry, Graduate School
of Science and Engineering, Tokyo Institute of Technology,
2
-12-1 Ookayama, Meguro-ku, Tokyo 152-8551
Chem. Lett. 2011, 40, 174­176
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/