Ruthenium complexes containing fully conjugated ligands terminated with thiol groups

Article abstract of DOI:10.1246/cl.2002.610

Bipyridine- and terpyridine-based ruthenium complexes containing fully conjugated ligands terminated with thiol groups were prepared and immobilized in self-assembled monolayers of alkanethiolates on gold surface, single molecules of which were observed w

Full text of DOI:10.1246/cl.2002.610

610
Chemistry Letters 2002
Ruthenium Complexes Containing Fully Conjugated Ligands Terminated with Thiol Groups
Joe Otsuki,ꢀ Hiroyuki Kameda, Sota Tomihira, Hiroshi Sakaguchi,yꢀ and Toshio Takido
College of Science and Technology, Nihon-University, 1-8-14 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308
yResearch Institute of Electronics, Shizuoka University, 3-5-1 Jyohoku Hamamatsu-shi, Shizuoka 432-8011
(Received March 8, 2002; CL-020224)
Bipyridine- and terpyridine-based ruthenium complexes
containing fully conjugated ligands terminated with thiol groups
were prepared and immobilized in self-assembled monolayers of
alkanethiolates on gold surface, single molecules of which were
observed with scanning tunnelling microscopy.
The advent and rapid development of the scanning probe
microscopy (SPM) allow us to probe molecules at a molecular
resolution. Small organic conjugated molecules are targets of
intense studies by SPM in connection with potential applications
in molecular electronics.1;2 On the other hand, however, small
metal complexes have rarely been subjected to measurements at
the single molecule level,3 despite the fact that metal complexes
feature well-defined three-dimensional structures and often
favourable physical—such as photo-, electro-, and magnetic—
properties that are often exploited in solution-phase molecular
‘‘devices.’’4 Here we report on the synthesis, properties, and
preliminary scanning tunnelling microscopy (STM) observations
of bipyridine- and terpyridine-based Ru complexes (1) and (2)
designed to be suitable for single molecule characterization.
The ligands in 1 and 2 are fully conjugated and hence rigid.
Chart 1. Optimized structures of metal complexes 1 and 2.
They can be viewed as molecular wires that have a connecting site
to a metal ion in the middle. The substitution positions on the
pyridine rings were determined such that the phenylethynyl
groups stick out along, roughly, the xyz-coordinate axes for 1 and
in the tetrahedral directions for 2. These assumptions have been
supported by molecular mechanics calculations optimizing the
2þ
2þ
6
7
phenylethynyl parts with the [Ru(bpy)3Š
and [Ru(tpy)2Š
cores being locked to the crystallographically known structures
(Chart 1). When 1b or 2b is placed on an atomically flat surface, it
should ‘‘stand’’ on three phenylethynyl arms, leaving remaining
arm(s) sticking out upward. Thus our idea was that coordination
chemistry might provide a convenient means to construct
molecular tripods that stand on their own.8 The calculation
suggests that the height of 1b standing as such on a surface is
ꢀ
about 19 A.
The ligands for these complexes were prepared through the
repetitive use of Pd-catalyzed cross-coupling reactions (Scheme
1). Bromopolypyridines 3a9/4a10 were coupled with a TMS-
protected acetylene to give 3b11/4b. These compounds, after
deprotection, were coupled with 4-(acetylthio)iodobenzene12 to
introduce acetyl-protected thiol functionalities to give 3d/4d. The
complex formation was facilitated by converting RuCl3 into
Scheme 1. Synthesis of complexes 1 and 2. (i) TMSCCH,
(PPh3Þ2PdCl2, CuI, i-Pr2NH, THF. (ii) KF or K2CO3,
MeOH. (iii) 4-(acetylthio)iodobenzene, (PPh3Þ2PdCl2, CuI,
i-Pr2EtN, THF. (iv) For 1, Ru(DMSO)4Cl2, AgPF6; for 2,
Ru(CH3COCH3Þ6(BF4Þ2. (v) Au substrate, Et2NH, DMF.
13
either [Ru(DMSO)4](PF6Þ2 or [Ru(CH3COCH3Þ6](BF4Þ2.14
charge-transfer absorption band. In CH3CN, the ꢀmax (485 nm) is
red-shifted by as large as 35 nm from that of [Ru(bpy)3Š2þ and the
molar absorption coefficient (4:4 Â 104 MÀ1 cmÀ1) is threefold
larger than that of the model complex (1:4 Â 104 MÀ1 cmÀ1),
owing to the extended ligand ꢁ systems in 1a.16 The
luminescence from 1a is also red-shifted by ca. 30 nm compared
with that from the model. The luminescence quantum yield is
TheÀacetyl protected complexes 1a and 2a were obtained as
PF6 salt after purification with silica-gel column chromatogra-
phy eluted with a mixture of CH3CN and aqueous KNO3, whose
identity and purity were confirmed with TLC, spectroscopic
measurements, and elemental analyses.15
The complex 1a exhibits a broad and large metal-to-ligand
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
611
twice that of the model in air-saturated CH3CN. Hence, the
complex 1a is a significant improvement from the standard
In summary, we successfully immobilized and observed
single molecules of fully conjugated metal complexes on Au
surface. Ru complexes are photo- and redox-active. Hence, some
sort of switching phenomena may be envisaged in response to
photon and/or electron input, which are being pursued in our
laboratories with various techniques of SPM.20
2þ
[Ru(bpy)3Š as a photosensitiser. On the other hand, the visible
absorption of 2a shows a broadening without a significant red-
shift compared from the model complex [Ru(tpy)2Š2þ. Cyclic
voltammetry of 1a in CH3CN shows a redox couple at 0.99 V vs.
Fcþ/Fc corresponding to Ru2þ/Ru3þ, which is more positive by
0.10 V than that of [Ru(bpy)3Š2þ. Similarly, the oxidation wave
for Ru2þ/Ru3þ in 2a appears at 1.00 V vs. Fcþ/Fc, which is more
This work was supported by the Research Foundation for
Opto-Science and Technology and the High-Tech Research
Center in Nihon University.
2þ
positive than the model [Ru(tpy)2Š by 0.11 V.
Immobilized samples of 1b/2b on a Au surface were prepared
by immersing a gold wire (Nilaco) into a millimolar solution of
1a/2a in DMF with a drop of Et2NH for 10–20 min, followed by
washing with solvents and drying with a flow of N2. Et2NH
removes the acetyl groups to bare the thiol moieties that anchor
onto the gold surface. The immobilization was confirmed by
alkaline desorption measurements in 0.5 M aqueous KOH. Upon
sweeping the potential of the Au electrode treated with 1b/2b as
described above to a negative potential, a broad desorption peak
was observed in the range of À0:9–À1:0 V vs. Ag/AgCl.17 This
potential is less negative than the case for a self-assembled
monolayer (SAM) consisting of octanethiol, whose sharper peak
was centered at À1:02 V vs. Ag/AgCl. These observations
suggest that the attachment of 1b/2b on Au surface is rather
inhomogeneous and there is little organization that reinforces the
assembly.
References and Notes
1
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5
6
7
8
More defined samples for single-molecule observation were
prepared by using an alkanethiolate monolayer matrix.18
Exposure of a SAM of dodecanethiolate formed in advance on
Au toa millimolar solution of 1a inDMF in thepresence ofEt2NH
for 2 h resulted in the isolated complexes adsorbing at existing
SAM defect sites.19 A typical example of such complexes is
p
p
shown in Figure 1. The ( 3 Â 3)R30 ꢁ crystalline domains of
the host dodecanethiolates are clearly observed. The bright spot is
apparently higher than the dodecanethiolate host by several
angstroms as shown by the height profile in the figure. This value
is comparable with the difference of the geometrical height of 1b
9
10 U. Lehmann, O. Henze, and A. D. Schluter, Chem.—Eur. J., 5, 854
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modified procedure.
¨
ꢀ
and the dodecanthiolate monolayer (14 A). The width of the
ꢀ
bright spot is 14 A, which matches the calculated dimension of the
complex 1b, providing unequivocal evidence that the spot is a
single molecule of 1b.
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17 The desorption peak was too broad to determine the peak area
accurately.
18 The atomically flat Au(111) surface for STM observation was
prepared by the deposition of a 50-nm-thick Au layer at a rate of
1 A s at 350 ꢁC on a mica substrate pre-baked at 550 ꢁC for 5 h.
19 Z. J. Donhauser, B. A. Mantooth, K. F. Kelly, L. A. Bumm, J. D.
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À1
ꢀ
Figure 1. An STM image and its height profile of a
single molecule of 1b immobilized in a host SAM of
dodecanethiolate on Au(111).