Orthogonal bis(terpyridine)-Fe(ii) metal complex oligomer wires on a tripodal scaffold: Rapid electron transport

Article abstract of DOI:10.1039/c3cc42478b

The present work reports a tripodal scaffold for bis(terpyridine)-Fe(ii) oligomer wires on an Au(111) surface: the tripodal scaffold realised both orthogonality of the oligomer wires, and fast interfacial electron transfer through the oligomer wires.

Full text of DOI:10.1039/c3cc42478b

Chemical Communications
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Volume 49 | Number 64 | 18 August 2013 | Pages 7097–7178
ISSN 1359-7345
COMMUNICATION
Hiroshi Nishihara et al.
Orthogonal bis(terpyridine)−Fe(II) metal complex oligomer wires on a
tripodal scaffold: rapid electron transport
1359-7345(2013)49:64;1-V

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Orthogonal bis(terpyridine)–Fe(II) metal complex
oligomer wires on a tripodal scaffold: rapid electron
transport†
Cite this: Chem. Commun., 2013,
49, 7108
Received 5th April 2013,
Accepted 10th April 2013
Ryota Sakamoto, Yuuki Ohirabaru, Ryota Matsuoka, Hiroaki Maeda,
Shunsuke Katagiri and Hiroshi Nishihara*
DOI: 10.1039/c3cc42478b
www.rsc.org/chemcomm
The present work reports a tripodal scaffold for bis(terpyridine)– with two distinctive concepts. One is the employment of an sp3 Si
Fe(II) oligomer wires on an Au(111) surface: the tripodal scaffold atom as the origin of the tripodal architecture. The sp3 Si is known
realised both orthogonality of the oligomer wires, and fast inter- to show s–p conjugation with aromatic substituents.7 We expected
facial electron transfer through the oligomer wires.
that this feature of Si could accelerate interfacial electron transfer.
The other concept involves the attaching entity; a p-conjugated
The gold–thiol protocol is the most widely and frequently used benzenethiol group is employed. We note that all tripodal adsor-
procedure for the preparation of self-assembled monolayers bates reported thus far adopted non-p-conjugated aliphatic thiol
(SAMs). It produces dense and ordered monolayers of adsorbates groups,6 such as benzenemethanethiol6a,b,d and adamantylthiol.6e
in a short time without a laborious process.1 The Au–S bond The p-conjugated benzenethiol group was also expected to increase
forms a good molecular junction between adsorbates and gold the interfacial electron transfer rate.
electrodes, providing high electrical conductivity2 and rapid inter-
Scheme 2 shows the synthetic procedure for AT–Ac; see the ESI for
facial electron transfer.3 Taking advantage of the virtues of the details.† Then we considered the optimal conditions for the for-
gold–thiol protocol, we fabricated bis(terpyridine)–M [M = Fe(II) mation of SAMs of AT on a gold electrode surface using X-ray
and Co(III)] complex oligomer wires on terpyridine-terminated photoelectron spectroscopy (XPS). Solid AT–Ac showed a doublet at
gold–thiol SAMs, and observed excellent long-range intrawire 163.5 eV (doublet a, Fig 1a). This is typical of the S 2p1/2/S 2p3/2 couple
electron transport (Scheme 1a).4 One of the drawbacks of the with the 1 : 2 area ratio for an acetyl-protected thiol group.8 On the
gold–thiol protocol is that the adsorbates possess inclined con- other hand, a gold substrate immersed into a chloroform solution of
figurations. For example, the tilt angle of benzenethiol was AT–Ac for 3 hours showed that two additional doublets emerged at
reported to range from 301 to 761.5 Several efforts have been 162.9 eV and 161.7 eV (doublets b and c, respectively; Fig. 1b). An
made to compensate for this weakness. Tripodal surface attaching increase in immersion time to 3 days showed the disappearance of
molecules possessing three thiol, thioacetate, or sulfide groups doublet a, but doublets b and c were persistent. The area ratio
have been synthesized and chemisorbed on gold surfaces.6 This between doublets b and c was 1 : 2 (Fig. 1c). IR-ATR spectroscopy for
series of tripodal ligands highlights the firm and perpendicular an Au substrate immersed in a chloroform solution of AT–Ac for
chemisorption of adsorbates, however, no attention has been paid 3 days (Fig. S1b, ESI†) showed neither CQO stretching of the thio-
to functionality, including the pursuit of good junctions with a acetate group (1704 cmÀ1 for AT–Ac; Fig. S1a, ESI†) nor S–H stretching
gold electrode for fast interfacial electron transfer phenomena.
(ca. 2580 cmÀ1).9 The IR-ATR spectroscopy result helped us assign the
The purpose of this work was to realise a tripodal scaffold for two new peaks in XPS as follows. Doublet c was attributable to a
bis(terpyridine)–Fe(II) complex oligomer wires that would afford tightly bound Au–S species, because its binding energy was com-
both vertical configuration and fast interfacial electron transfer. To parable to the reported values for a SAM of benzenethiol (162.0 eV).10
achieve this, we designed a tripodal terpyridine anchor ligand AT On the other hand, we attributed doublet b to a weakly bound Au–S
(obtained as the form of thioacetate-protected AT–Ac, Scheme 1b) species. The immobilisation of AT on the Au(111) surface with the
three sulphur atoms was reflected in the orthogonality of the
bis(terpyridine)–Fe(II) oligomer wires constructed on AT (vide infra).
Single molecules of AT were identified in an STM image of a
sparse SAM of immobilised AT (Fig. 2).
Next we fabricated bis(terpyridine)–Fe(II) oligomer wires on the
SAM of AT. Scheme 1a illustrates the schematic procedure (see the
ESI† for details). Hereafter, the bis(terpyridine)–Fe(II) oligomer wire
with n layers is abbreviated as Au–[AT(FeLH)n]. First, we verified
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1,
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: nisihara@chem.s.u-tokyo.ac.jp;
Tel: +81 3 5841 8072
† Electronic supplementary information (ESI) available: Experimental details,
IR spectra of solids AT and AT on a gold substrate (Fig. S1), estimation of the
length of Au–[AT(FeLH)n] (n = 5 and 40, Fig. S2), and current (i)–time (t) and (b) ln
i–t plots for Au–[AT(FeLH)nÀ1FeT1] and Au–[A2(FeLH)nÀ1FeT1] (Fig. S3 and S4). See
DOI: 10.1039/c3cc42478b
c
7108 Chem. Commun., 2013, 49, 7108--7110
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Scheme 1 (a) Bottom-up fabrication of bis(terpyridine)–Fe(II) oligomer wires Au–[AT(FeLH)n] and Au–[AT(FeLH)nÀ1FeT1]. (b) Anchor (A), bridging (L), and terminal (T)
terpyridine ligands.
Fig. 3 (a) Cyclic voltammograms of Au–[AT(FeLH)n] (n = 1, 2, 4, 6, 8, and 10) in
1 M Bu4NClO4–dichloromethane at a scan rate of 0.1 V sÀ1. (b) G[Fe(tpy)2]-n plot.
ferrocenium/ferrocene (Fc+/Fc), assignable to the Fe(III)/Fe(II) redox
couple of the bis(terpyridine)–Fe unit.4 Fig. 3b shows surface
Scheme 2 Synthesis of AT–Ac. TsOH = p-toluenesulphonic acid.
coverage of the bis(terpyridine)–Fe unit, G[Fe(tpy)2], determined
by the quantity of the electricity of the redox wave depicted in
Fig. 3a. A linear relationship between G and n confirms that the
bottom-up fabrication of Au–[AT(FeLH)n] proceeded quantitatively.
We verified the orthogonal configuration of Au–[AT(FeLH)n] by
means of atomic force microscopy (AFM) and cross-sectional
scanning electron microscopy (SEM). Fig. 4 shows AFM topological
and phase images of Au–[AT(FeLH)5] after scratching using an AFM
Fig. 1 Experimental (black) and simulated (red) XPS focusing of the S2p region. (a)
Solid AT–Ac; (b) SAM of AT (immersion time: 3 h); (c) SAM of AT (immersion time: 3 d).
tip. From the topological image the depth of the tip was found to
be 9–10 nm. This value was in good agreement with the height of
Au–[AT(FeLH)5] estimated by molecular modelling (Fig. S2, ESI†).
In addition, Au–[AT(FeLH)40] was subjected to cross-sectional
The simulated spectra were reproduced by pair(s) of doublets. Orange, green, and
blue doublets correspond to doublet a, doublet b, and doublet c, respectively.
Fig. 2 STM topological image of a SAM of AT sparsely immobilised on an
Au(111) surface.
the quantitative formation of the bis(terpyridine)–Fe(II) unit
by means of cyclic voltammetry. Fig. 3a shows voltammo-
grams of Au–[AT(FeLH)n] (n = 1, 2, 4, 6, 8, and 10). The
Fig. 4 (a) AFM topological and (b) phase images of Au–[AT(FeLH)5] after
scratching using an AFM tip. The white dashed squares indicate the scratched
voltammograms feature a reversible redox wave at 0.64 V vs. area. (c) Cross-sectional analysis of image (a) at the intersecting white line.
c
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As indicated in the introductory section, the s–p conjugation
between the Si centre and arenes could account for the fast
electron transport. Another possibility is that an electron goes
through the three benzenethiol legs in Au–[AT(FeLH)nÀ1FeT1]. We
reported previously that doubly-anchored biferrocene on an
Au(111) surface through Au–S bonds underwent a faster inter-
facial electron transfer than in the singly-anchored case.12
In conclusion, we designed and synthesised tripodal terpyridine
ligand AT (AT–Ac). The SAM formation of AT on a gold electrode
surface was optimized to ensure that all S atoms of AT were
chemisorbed. We constructed bis(terpyridine)–Fe(II) oligomer wires
on the SAM of AT, the orthogonality of which was confirmed by
means of AFM and cross-sectional SEM. AT realised fast intrawire
electron transfer behaviour, showing a large k0et value.
Fig. 5 Cross-sectional SEM image of Au–[AT(FeLH)40].
The authors acknowledge Grants-in-Aid from MEXT of Japan
(No. 24750054, 21108002, and 25107510, and areas 2107 [Coordina-
tion Programming] and 2406 [All Nippon Artificial Photosynthesis
Project for Living Earth]), and a JSPS Research Fellowship for Young
Scientists. R. S. is grateful to the Tokuyama Science Foundation, and
the Iketani Science and Technology Foundation for financial support.
Notes and references
Fig. 6 ln ket–d plots for Au–[AT(FeLH)nÀ1FeT1] (red) and Au–[A2(FeLH)nÀ1FeT1]
(green). Dashed lines were obtained by least-squares fitting and solid lines by
fitting assuming that both plots had the same slope (À0.018 ÅÀ1).
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ket = k0et exp(Àbdd)
(1)
where ket is the rate constant for the electron transfer between the
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(2)
¨
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¨
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the ln i–t plot gives ket, and Fig. 6 shows a ln ket–d plot for Au–
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mer wires underlain by another anchor ligand, Au–[A2(FeLH)nÀ1FeT1]
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7110 Chem. Commun., 2013, 49, 7108--7110
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c
This journal is The Royal Society of Chemistry 2013