"Janus-type" ruthenium complex bearing both phosphonic acids and pyrene groups for functionalization of ITO and HOPG surfaces

Article abstract of DOI:10.1246/cl.140979

A novel Janus-type ruthenium complex bearing both phosphonic acid and pyrene groups was tethered to both ITO and HOPG surfaces in different tethering modes. On the ITO surface, the phosphonic groups were selectively attached to the ITO, resulting in the h

Full text of DOI:10.1246/cl.140979

CL-140979
Received: October 24, 2014 | Accepted: November 5, 2014 | Web Released: November 12, 2014
“Janus-type” Ruthenium Complex Bearing Both Phosphonic Acids and Pyrene Groups
for Functionalization of ITO and HOPG Surfaces
Li Yang,1 Hiroaki Ozawa,*2 Mayuko Koumoto,2 Kai Yoshikawa,2 Mariko Matsunaga,3 and Masa-aki Haga*2
1Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, P. R. China
2Department of Applied Chemistry, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551
3Department of Electrical, Electronic, and Communication Engineering, Chuo University,
1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551
(E-mail: mhaga@kc.chuo-u.ac.jp)
A novel Janus-type ruthenium complex bearing both phos-
phonic acid and pyrene groups was tethered to both ITO and
HOPG surfaces in different tethering modes. On the ITO surface,
the phosphonic groups were selectively attached to the ITO,
O
O
O
O
HO
OH
resulting in the hydrophobic surface because of the exposed pyrene
groups on the top. On the other hand, on the HOPG surface, pyrene
groups interacted with the HOPG surface through noncovalent
interaction, resulting in a hydrophilic surface exposing phosphonic
acid groups. This selectivity makes it possible to fabricate redox-
active ruthenium complex and graphene composite by layer-by-
layer growth of molecular units.
i
ii
Me
Me
Me
Me
Me
Me
N
N
N
N
N
N
N
Ru
N
N
N
N
N
N
N
N
N
N
N
L1
N
N
(HO)2OP
PO(OH)2
(HO)2OP
PO(OH)2
1
Scheme 1. Synthetic route of ligand L1 and Ru-1 complex. (i) 1-(4-
Bromobutyl)pyrene and K2CO3 in DMF and (ii) [Ru(EtL)(CH3CN)-
Cl2] in DMF:t-BuOH (1:1)/Me3SiBr in DMF and then in MeOH.
Surface functionalization plays an important role in realizing
molecular electronics, catalysts, and photovoltaic technologies. In
particular, surface chemistry of self-assembled monolayers has been
widely studied with the aim of modulating interfacial properties
such as wettability, adhesion, tribology, and biocompatibility.1,2
Self-assembly is the spontaneous formation of ordered
structures on the basis of the spatial organization of their
components, and an effective bottom-up approach for fabricating
various functional materials by methods like sequential layer-by-
layer growth.3,4 By selecting a functional anchoring group such
as a thiol group, it is possible to selectively immobilize a self-
assembling monolayer (SAM) film on gold on an Au/SiO2
patterned electrode. Ruthenium complexes having a carboxylate
or phosphonate group can be chemisorbed selectively on the
surface of a TiO2 or indium tin oxide (ITO) electrode, which have
been widely studied for numerous applications in dye-sensitized
solar cells (DSSCs).5,6 Recently, nanocarbons such as graphene
and carbon nanotubes have been paid much attention from the
viewpoint of application as next-generation electrode materials.7-12
In fields such as catalysts and biosensors, therefore, it is becoming
necessary to fabricate well-defined structures on nanocarbon. Since
nanocarbon is composed of a π-extended system with a hydro-
phobic surface, a pyrene group is often used for modifying the
carbon surface through noncovalent π-π interaction.13 The
combination of pyrene groups with hydrophilic phosphonic acid
groups in the same molecule forms an amphiphilic molecule,
which can be utilized as a surface-tethering molecule. At the same
time, phosphonic acid groups can act as a coordination site for the
metal ions. It is therefore of great interest to design and synthesize
a complex bearing both hydrophilic phosphonate groups and
hydrophobic pyrene groups. By utilizing the surface selectivity of
such a complex, layer-by-layer growth of a redox-active ruthenium
complex together with graphene is feasible for building a
multilayer film on highly ordered pyrolytic graphite (HOPG) or
graphene.
In the present study, the synthesis, stability, and electrochemical
behaviors of a novel amphiphilic ruthenium complex 1 (Ru-1), as
shown in Scheme 1, which consists of phosphonic acids as a hydro-
philic group and pyrene as a hydrophobic group, were investigated.
To obtain the target Ru-1, 2,6-bis(N-methylbenzimidazol-2-yl)-
4-{3,5-bis[4-(1-pyrenyl)butyloxy]phenyl}pyridine (L1) was syn-
thesized by the reaction of the corresponding diol with 1-(4-
bromobutyl)pyrene. The L1 ligand was first reacted with [Ru(EtL)-
(CH3CN)Cl2] to generate the complex [Ru(EtL)(L1)](PF6)2. The
deprotection reaction of the ethyl group was then promoted by the
addition of trimethylsilyl bromide, which produces the desired Ru-1.
Similarly, symmetric [Ru(L1)2](PF6)2 (Ru-2) was obtained by the
reaction of RuCl3¢3H2O with two molar equivalents of L1 in DMF-
glycerol (1:1 v/v) (Figure 1). Both complexes were fully charac-
terized by elemental analysis, 1H NMR, and electrospray-ionization
mass spectrometry (ESI-MS) (see Supporting Information).
The surface of an ITO electrode was immobilized by the
immersion of the substrate into a pH-5 aqueous DMF mixed
solution of Ru-1 (50 ¯M) (see Supporting Information). Typical
cyclic voltammograms of the Ru-1 on the ITO electrode is shown
in Figure 2a. The Ru(II/III) peak was clearly observed at +0.47 V
vs. Fc/Fc+, and linear dependence of the peak current on the scan
rate suggested that the Ru-1 was immobilized on the ITO electrode.
The temporal change of surface coverage Γ was monitored by
cyclic voltammetry of the modified ITO electrode in CH3CN
(0.1 M TBAPF6), and the result is shown in Figure 2b. The surface
adsorption of Ru-1 followed the kinetic Langmuir equation:
ꢀðtÞ ¼ ꢀðsÞf1 ꢀ expðꢀkCtÞg
ð1Þ
where Γ(t), Γ(s), k, C, and t are the surface coverage amounts,
saturated surface coverage, rate constant, the concentration in the
bulk solution, and time, respectively. The saturated surface
160 | Chem. Lett. 2015, 44, 160–162 | doi:10.1246/cl.140979
© 2015 The Chemical Society of Japan

2+
O
O
O
O
Me
Me
N
N
N
Ru
N
O
O
Me
Me
N
N
N
N
N
N
N
Ru
N
N
N
N
Me
Me
N
N
N
N
N
Ru
N
N
N
N
N
N
N
N
Me
Me
PO3
PO3
N
N
PO3
PO3
ITO
O
O
PO3
PO3
PO3
Zr
PO3
PO3
PO3
Zr
PO3
Zr
Zr
Zr
PO3
PO3
PO3
PO3
PO3
2
N
N
N
N
N
N
Ru
N
N
Ru
N
N
N
N
N
N
N
N
N
Me
Figure 1. Structure of the complex, Ru-2.
N
Me
N
Me
N
Me
O
O
O
O
(a)
HOPG
Bilayer
HOPG
Monolayer
Figure 3. Proposed surface-tethered structures for the Ru-1 mono-
layer on ITO and HOPG. A bilayer connected by Zr-phosphonate
bond on HOPG.
4
3
2
1
0
(b)
Contact angles of water droplet on both the ITO and HOPG
surfaces modified by Ru-1 were measured. The measured contact
angles on the Ru-1-modified ITO and HOPG surfaces were 75 and
51.8°, respectively. Since the contact angles on bare ITO and
HOPG are 52.8 and 61.5°, we concluded that the surface property
with respect to hydrophilicity-hydrophobicity reversed after the
surface immobilization of Ru-1. Phosphonic groups were selec-
tively attached to the ITO surface, thereby producing a hydro-
phobic surface. Furthermore, pyrene groups can be indirectly
immobilized on the HOPG surface through noncovalent π-π
interaction, resulting in the appearance of the exposed phosphonic
acid groups with a hydrophilic property on the HOPG surface.
While the selective adsorption of Ru-1 is obvious, further study on
the molecular orientation of the Ru-1 adlayers is indispensable in
due course. Ru-1 possesses double-faced properties; therefore, it is
hereafter called a “Janus-type complex.”
0
1
2
3
4
5
6
Time / h
Figure 2. (a) Typical cyclic voltammograms of Ru-1 on the ITO
electrode in CH3CN (0.1 M TBAPF6) at various scan rates. (0.1, 0.2,
0.3, 0.4, and 0.5 V s¹1). (b) Temporal change of surface coverage of
Ru-1 in DMF on ITO electrode by use of the CV method. The
simulated curve by eq 1 is shown in dotted line.
coverage of Ru-1 on the ITO surface was obtained as 2.5 ©
10¹11 mol cm¹2. Similarly, a HOPG electrode was immersed in the
same solution of Ru-1. This modified HOPG electrode showed an
adsorbed Ru(II/III) oxidation peak at +0.45 V vs. Fc/Fc+ with
Layer-by-layer (LbL) assembly to fabricate well-defined
nanostructures for molecular functional devices on a surface has
received much attention.15 As shown in Figure 3, the Janus-type
complex (Ru-1) was used as a primer layer to fabricate novel
multilayer structures of redox-active Ru complexes on the HOPG.
Since free phosphonic acid groups are present on the uppermost
layer of the Ru-1-modified HOPG, a method of metal-phospho-
nate bonding developed by Mallouk and others16 was used to
construct a multilayer structure. At first, a HOPG electrode was
immersed in an aqueous DMF solution of the Ru-1 complex
(50 ¯M) for six hours, after which it was rinsed with copious
ultrapure water. Next, the HOPG substrate was dipped in a 20 mM
aqueous solution of ZrOCl2 for 30 min, rinsed with water, and
immersed in the Ru-1 solution again. A cyclic voltammogram of
the resulting bilayer film showed an oxidative wave at +0.49 V
¹2
surface coverage of 1.94 © 10¹11 mol cm in CH3CN (0.1 M
TBAPF6). However, this oxidative peak decreased under desorp-
¹1
tion rate of 6.8 © 10¹7 s in fresh CH3CN solution since the
nonbonded Ru-1 molecules were slowly desorbed from HOPG
surface, particularly in an easily soluble solvent for Ru-1. Several
examples of pyrene-anchored redox-active molecules have been
reported.8,11-14 In these examples, the desorption rate of the
nonbonded redox-active complexes was employed as a measure of
the adsorption stability of the complexes on a nanocarbon surface
through noncovalent π-π interactions.14 Compared with the
reported desorption rates, that of the present Ru-1 is much slower,
indicating Ru-1 is relatively stable on an HOPG surface.
¹2
vs. Fc+/Fc with a surface coverage of 2.83 © 10¹11 mol cm in
Chem. Lett. 2015, 44, 160–162 | doi:10.1246/cl.140979
© 2015 The Chemical Society of Japan | 161

Figure 6. SEM images of (a) graphene monolayer on Ru-1 complex
and (b) graphene uppermost layer on 7 layers on ITO.
layers show a linear relationship (Figure 5b), which suggests a
uniform layer structure is formed during the LbL process. The
surface morphology of each graphene uppermost layered film was
measured by SEM (Figure 6 and Figure S4). A relatively flat
surface packed with graphene flakes is seen in the SEM images.
While the present graphene flakes used are not a pure single layer, a
Ru-2 complex (having two pyrene groups at both ends) is strongly
adsorbed on the graphene surface and connects two interlayers
of graphenes. As a result, the Ru complexes immobilized in each
layer became redox-active. The fabricated graphene-Ru-complex
multilayer films are applicable to functional devices such as super
capacitors and photoelectrochemical solar cells.
In conclusion, a “Janus-type” ruthenium complex bearing both
phosphonic acid and pyrene groups was tethered on both ITO and
HOPG surfaces in different tethering modes. The Ru complex was
directly attached on ITO surface by phosphonate groups, while
indirect attachment of the Ru complex was feasible through
noncovalent π-π interaction of pyrene groups on the HOPG surface.
Figure 4. UV-vis spectral monitoring of successive LbL fabrication
of (graphene-flake/Ru-2)n above Ru-1 adlayer on ITO. The Ru-1 film
was referred to 1 layer and “n” stands for the number of the paired
layers (n = 0-4); (blue) Ru layer on the top and (green) graphene on
the top.
(a)
(b)
0.15
0.10
0.05
0.00
0
2
4
6
8
Number of layers
Figure 5. (a) Plots of absorbance (red: Ru on the top; black:
graphene on the top) and (b) surface coverage obtained from CV in
Figure S3 vs. the number of layers for (graphene-flake/Ru-2)n above
Ru-1 adlayer on ITO (The number of paired layers, n = 0-4).
CH3CN (0.1 M TBAPF6), which is 1.5 times larger than that of the
Ru-1 monolayer on the HOPG surface. Furthermore, the contact
angle was 80.6°, which indicates that a bilayer structure was
formed and pyrene groups were exposed to the uppermost layer
after the formation of Zr-phosphonate bonds.
This work was supported by a grant from Tokyo Ohka
Foundation for the Promotion of Science and Technology and Chuo
University Joint Project Grant 2013 for partial financial support.
Supporting Information is available electronically on J-STAGE.
References
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© 2015 The Chemical Society of Japan