Surface junction effects on the electron conduction of molecular wires

Article abstract of DOI:10.1021/ja910462x

Surface junction effects on the electron conduction of p -phenylene-bridged bis(terpyridine)iron oligomers terminated with a ferrocene moiety were quantitatively analyzed by employing three different surface-anchoring terpyridine ligands. The dependence of the electron-transfer rate constant for oxidation of the ferrocene moiety, k _et, on the distance between the electrode surface and the ferrocene moiety, x , showed that the attenuation factor, β^d, which indicates the degree of reduction of k _et with x , was 0.018 in all cases. However, the absolute k _et value depended strongly on both electronic and steric factors of the surface-anchoring ligand.

Full text of DOI:10.1021/ja910462x

Published on Web 03/15/2010
Surface Junction Effects on the Electron Conduction of Molecular Wires
Tomochika Kurita, Yoshihiko Nishimori, Fumiyuki Toshimitsu, Satoshi Muratsugu, Shoko Kume, and
Hiroshi Nishihara*
Department of Chemistry, Graduate School of Science, UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received December 11, 2009; E-mail: nisihara@chem.s.u-tokyo.ac.jp
One goal of molecular electronics is to control electron conduc-
tion in molecular wires and networks by combining appropriate
molecular units.1 A requisite for constructing molecular electronic
devices is the connection of molecular wires to the electrode surface.
Thus, to evaluate the total performance of the molecular wires as
well as the conventional electronic circuit, the electron conduction
properties of internal molecular segments as well as the resistivity
at the electrode-molecular wire junction must be elucidated.2
Hence, the surface junction is a fundamentally important issue in
the performance of organic thin-film field-effect transistors and
light-emitting diode devices.3
Stepwise coordination reactions to lengthen molecular wires have
recently attracted much attention as a convenient method to connect
molecular wires to the surface because these reactions are program-
mable and connect different units at the desired positions.4 We
previously reported the synthesis of π-conjugated linear and
Figure 1. (A) Bottom-up construction of a metal-complex oligomer wire,
branched M(tpy)2 (M ) Fe, Co; tpy ) 2,2′:6′,2′′-terpyridine)
oligomer wires attached to Au using the stepwise coordination
method5 and clarified that the redox conduction of the Fe(tpy)2
oligomer films occurs along the molecular wires,5c implying that
intrawire electron transfer is much faster than interwire transfer.
Additionally, we found that the M(tpy)2 oligomer wires show
superior long-range electron-transport abilities.5e
Au-[Ax(FeL)n-1FeT]. (B) Molecular sizes of H-A2 and H-A3 estimated
from DFT calculations. (C) Chemical structures of Au-[AxFeLFeT]
molecular wires with (a) x ) 1, (b) x ) 2, and (c) x ) 3.
and 1610 cm-1 increased with increasing accumulation (Figures
S1 and S2 in the Supporting Information); these peaks are attributed
to B-F stretching in BF4-, CdC stretching in tpy, and CdN
stretching in tpy, respectively. The peak at 2200 cm-1 in the film
spectrum after connection of the terminal ligand T is ascribed to
the CtC stretching vibration.
The contact angles of water droplets on the films were measured
at each step of the coordination reactions. The films with A1 and
A2 showed similar variations in the contact angle as the structure
was formed, whereas the film with A3 exhibited a different behavior
(Figure S3). The contact angle immediately after attachment of a
surface ligand was larger for A3 than for A1 or A2, probably because
of the effect of the hydrophobic t-Bu2Ph moieties in A3. The contact
angle decreased when a ferrous ion was attached and increased
upon attachment of an L ligand, as expected from the hydrophilicity
of the ferrous ion and the hydrophobicity of L. The overall change
in the contact angle was greater for A3 than for A1 or A2, and the
contact angles of all three films approached a similar value at the
end of the reactions. This finding is reasonable because all three
cases are expected to produce similar surface structures.
In the present study, we used the stepwise coordination method
to construct a system to clarify how the electron transport behavior
of molecular wires depends on the surface junction at the molecular
level by changing only the surface-anchoring molecular unit. Herein
we describe the dependence of the electron transfer rate constant,
ket, and the electron transport ability of p-phenylene-bridged Fe(tpy)2
oligomer wires on three surface-anchoring tpy ligands. Although
the three anchoring ligands give the same value of ꢀd, which
indicates the degree of reduction of ket with wire length, the
significant difference in the ket values is due to the electronic and
steric factors of the surface-anchoring ligands.
As reported previously, [FeII(tpy)2]2+ oligomer wires with BF4
-
counterions were prepared by a simple bottom-up method using a
stepwise complexation reaction (Figure 1A).5 To construct molec-
ular wires on gold/mica, designated as Au-[Ax(FeL)n-1FeT] (x )
1, 2, 3), where n is the number of Fe(tpy)2 units, we employed
disulfide derivatives of azobenzene-linked tpy (A1 ), phenylene-
In the cyclic voltammograms (CVs) of Au-[Ax(FeL)n-1FeT],
the ferrocenium/ferrocene (Fc+/Fc) redox couple in T was observed
at E0′ ) 0.08-0.11 V versus Fc+/Fc, and the FeIII(tpy)2/FeII(tpy)2
couple was observed at E0′ ) 0.64-0.65 V versus Fc+/Fc. These
values depended slightly on both x and n because of the electron-
donating and -withdrawing effects of the substituents on the Fe(tpy)2
moiety (Figures S4-S7). The amounts of terminal ferrocene units
in the molecular wire films, Γ, were evaluated from the CVs to be
1.7 × 10-10, 1.4 × 10-10, and 6.0 × 10-11 mol cm-2 for the films
with x ) 1, 2, and 3, respectively. The smaller Γ value for the A3
film is reasonable because the A3 is larger than A1 and A2 as a
result of the existence of the two t-Bu2Ph moieties on the tpy unit.
2
linked tpy (A2 ), and a bulky t-Bu2Ph-substituted phenylene-linked
2
tpy (A3 ) as surface-anchoring ligands, tpy-C6H4-tpy (L) as the
2
bridging ligand, and tpy-CtC-Fc (T; Fc ) ferrocenyl) as the
terminal ligand (Figure 1B). The disulfide derivative A32 was newly
prepared (see the Supporting Information).
Quantitative accumulation of each fragment by stepwise coor-
dination was confirmed by infrared reflection absorption spectros-
copy (IRRAS), contact angle measurements, and cyclic voltam-
metry. Comparison of the IRRAS results for each step in the
preparation of Au-[A3(FeL)4FeT] with reference spectra of A3,
L, and T revealed that the intensities of the peaks at 1080, 1430,
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4524 J. AM. CHEM. SOC. 2010, 132, 4524–4525
10.1021/ja910462x  2010 American Chemical Society

C O M M U N I C A T I O N S
H-A2Fe(tpy) and H-A3Fe(tpy) have similar electronic structures,
and in both, the HOMO is localized at the phenylene moiety
(orbitals 157 and 261, respectively) and MOs near the Fe center
are below the HOMO (orbitals 153-156 and 253-256, respec-
tively). The energies of these orbitals are slightly higher for
H-A3Fe(tpy) because of the electron-donating effect of the t-Bu2Ph
groups. Although the electronic effect may contribute to some extent
to the significant difference in the k0et values, the steric effect of the
t-Bu2Ph groups is likely to have more influence on this difference.
As noted above, the molecular wires in the film containing A3 have
a lower value of Γ than do the molecular wires in the film containing
A2, indicating that electrolyte ions can come closer to the electrode
surface in the film containing A3 than in the film with A2. In fact,
the electron transfer rate constant for the [Fe(tpy)2]3+/[Fe(tpy)2]2+
couple was higher for Au-[A3FeL] (330 s-1) than for Au-[A2FeL]
(56 s-1) (Figures S13 and S14).
In conclusion, the surface-anchoring molecular unit does not
affect the dependence of the electron transport properties of the
molecular wire on the wire length, but it alters the absolute rate
constant. This finding indicates that the electron conduction kinetics
of molecular wires on the surface can be tuned by varying the
combination of the surface-anchoring and internal units of the
molecular wire. The confirmation of this fundamental issue of
molecular-wire electron conduction will contribute significantly to
developing the field of molecular electronics.
Figure 2. Plots of ln(ket) vs d for [Ax(FeL)n-1FeT] (blue, x ) 1; green,
x ) 2; red, x ) 3). Dashed lines were obtained by least-squares fitting for
each x and solid lines by fitting for all x assuming the same slope.
The areas occupied by single Au-[A2(FeL)FeT] and
Au-[A3(FeL)FeT] wires were evaluated by density functional
theory (DFT) calculations to be 100 and 250 Å2, respectively; these
correspond to densities of 1.7 × 10-10 mol cm-2 and 6.8 × 10-11
mol cm-2, suggesting that the molecular wires are relatively densely
packed on the surface. The number of Fe(tpy)2 units in the film
increased linearly with the number of coordination cycles, as
reported previously,5 indicating quantitative accumulation of the
molecular units.
Rate constants for electron transfer between the electrode and the
terminal ferrocene moiety of the molecular wires, ket, were measured
for Au-[Ax(FeL)n-1FeT] (x ) 1, 2, 3; n ) 1, 2, 3, 4) by potential-
step chronoamperometry for reduction of the terminal ferrocenium
moiety at -0.13 V versus Fc+/Fc. Each chronoamperogram showed
an exponential decay of current (i) as a function of time (t), and ket
was obtained from the slope of the plot of ln(i) versus t. From the
relationship of ket to the length of the molecular wire between the
electrode surface and the Fe center in the terminal ferrocenium moiety,
d, we estimated ꢀd and ke0t using eq 1:
Acknowledgment. This work was supported by Grants-in-Aid
from MEXT of Japan (20245013 and 21108002, Area 2107) and
the Global COE Program for Chemistry Innovation.
Supporting Information Available: Materials and methods, elec-
trochemical and spectral data, and results of molecular orbital calcula-
tions. This material is available free of charge via the Internet at http://
pubs.acs.org.
ket ) k0et exp[-ꢀd(d - d0)]
(1)
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where d0 is the shortest molecular length (when n ) 1) and ke0t is the
ket value at d0. Least-squares fits of the plots gave ꢀd ) 0.014 ( 0.004,
0.024 ( 0.007, and 0.020 ( 0.004 for the wires with A1, A2, and A3,
respectively (see Figure 2). We also fitted straight lines with the same
slope to the plots (see Figure 2) in order to analyze the results using
the same values of ꢀd for all three types of molecular wires. This
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also smaller than that for wires with A3, despite a similar
p-phenylene-bridged structure and thus similar d0.
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terminal SH moiety, the azobenzene bridge, and the Fe center; (2)
there are MOs attributed to the azobenzene moiety around the
HOMO (orbitals 180-184). These results indicate that the azoben-
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surface Au-S and the nearest Fe(tpy)2 unit and thus increase the
electron conduction rate in the molecular wire. In contrast,
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