Synthesis of tripodal anchor units bearing selenium functional groups and their adsorption behaviour on gold

Article abstract of DOI:10.1039/b906286f

The synthesis of new selenium-functionalized tripodal anchor units composed by a tetraphenylmethane core with three selenocyanate or selenol arms has been successfully accomplished and CV, XPS and UPS measurements of their monolayers on a gold surface wer

Full text of DOI:10.1039/b906286f

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COMMUNICATION
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Synthesis of tripodal anchor units bearing selenium functional groups
and their adsorption behaviour on goldw
Yutaka Ie,a Tomoya Hirose,a Atsushi Yao,b Taro Yamada,c Noriaki Takagi,b Maki Kawaibc
and Yoshio Aso*a
Received 7th February 2009, Accepted 6th April 2009
First published as an Advance Article on the web 22nd April 2009
DOI: 10.1039/b906286f
The synthesis of new selenium-functionalized tripodal anchor
units composed by a tetraphenylmethane core with three
selenocyanate or selenol arms has been successfully accomplished
and CV, XPS and UPS measurements of their monolayers on a
gold surface were investigated.
unit by transition-metal-catalyzed cross-coupling reactions
resulted in failure because of the catalytic poison of the
selenium atom. Thus, as shown in Scheme 1, the protected-triol
derivative 10 was used as a key intermediate for synthesizing 1.
The selenocyanato groups of 1 were converted to acetylseleno
groups by reduction with LiBEt3H followed by treatment with
Ac2O to give 11.13 After deprotection of its acetyl groups by
using H2SO4, obtained 2 was immediately used for monolayer
preparation. Compounds 4–6 (Chart 1) were synthesized by
similar transformations. The synthetic details and the
compound-identification data are described in the ESI.w
Monolayers of 1 and 2 on a gold surface were prepared by
immersing Au(111)/mica substrates in their CH2Cl2 solution
(5.0 Â 10À4 M) for 12 h. As shown in Fig. 1, cyclic voltammetry
(CV) with the monolayer-modified Au working electrodes in
CH2Cl2 displayed two successive one-electron redox waves
that correspond to the oxidation process of the phenyl-capped
quaterthiophene moiety. The integrated anodic charge of the
first oxidation peak gave surface coverage of the adsorbed
molecules as 1.1 Â 10À10 and 1.4 Â 10À10 mol cmÀ2 for 1 and
2, respectively, which are almost the same as that of tripodal
thiol 7 monolayer (1.4 Â 10À10 mol cmÀ2) prepared under the
same conditions.
Elucidating the nature of electronic communication between
metal electrodes and organic molecules via the interface is of
great importance for the development of single-molecular
devices.1,2 Especially, to investigate the electrical conduction
behaviour of long p-conjugated systems as ‘‘molecular wire’’,3
critical requirements for
a functional group attaching
molecules to electrodes include not only the appropriate
electronic coupling between them but also the robustness of
surface attachment and the control of molecular orientation.2
Recently, selenium functional groups have been attracting
much attention as a candidate4–7 alternative to thiol groups
most commonly used to be adsorbed on a gold surface. On the
other hand, the application of three-armed anchors to
controlled monolayer formation has been widely used,8–12
and the practical advantage of the tripodal structure with
three mercaptomethyl arms have been exemplified.10–12 In this
context, the combination of selenium functional groups with
the tripodal structure might be an ideal anchoring unit for
molecule–metal junctions. However, the realization of this
concept has not been accomplished yet owing to the difficulty
of synthesis. In this communication, we report on the
synthesis of selenium-functionalized tripodal anchor units
(1– 4, Chart 1) composed by a tetraphenylmethane core with
three selenocyanate or selenol arms and their monolayer
formation on a gold surface, which have allowed us to
investigate the electrochemical properties of the molecular
junction and the Se–Au bonding nature.
Interestingly, the initially rapidly increased surface coverage
for selenocyanate 1 was gradually decreased with increasing
immersion time (Fig. 2). In contrast, the surface coverage of
selenol 2 reached stable saturation within several minutes. As
it has been reported that benzeneselenocyanate can be
adsorbed on Au nanoparticle as selenolate with Se–CN bond
cleavage,14 it is assumed that energetically more favourable
adsorption of the CN species11 replaces the selenolate
species of 1 on Au and causes the gradual decrease of the
surface coverage then to reach
a steady state. These
In contrast to the synthesis of tripodal thiol systems,10–12 all
attempts to connect the selenocyanate-containing tripodal
unit 3, which was prepared similarly to the corresponding
acetylthio derivative,11a with a phenylquaterthiophene (Ph4T)
results apparently indicate the advantage of selenol groups
to homogeneous monolayer formation.15 An electrochemical
a The Institute of Scientific and Industrial Research, Osaka University,
8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan.
E-mail: aso@sanken.osaka-u.ac.jp
b Department of Advanced Materials Science, The University of
Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
c Surface Chemistry Laboratory, RIKEN, Wako, Saitama 351-0198,
Japan
w Electronic supplementary information (ESI) available: Detailed
experimental procedures and characterization data of all new com-
pounds, CV charts of selected compounds, and XPS spectra of 3. See
DOI: 10.1039/b906286f
Chart 1 Chemical structures of 1–8.
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the three-point chemisorption of 4 occurs on Au. It is
noticeable that the XPS spectral features of Se peaks for the
selenocyanate
3 monolayer were essentially the same
(160.7 and 166.4 eV) as those of 4, while the former spectrum
was accompanied by an N 1s peak at 398.3 eV (Fig. S2).w
These results support the assumption that selenocynato
groups are dissociatively adsorbed on Au(111) via
Se–CN bond cleavage to form Se–Au covalent bonds, which
is in good agreement with the results obtained from the
CV experiments.
Scheme 1 Reagents and conditions: (a) Pd(PPh3)4, Ph4TSnBu3,
toluene, reflux; (b) (1) nBu4NF, THF, rt, (2) PPh3, CBr4, CH2Cl2, rt,
(3) KSeCN, THF, rt; (c) LiBEt3H, Ac2O, THF, À78 1C.
To evaluate the electronic states of the anchoring groups on
Au(111), ultraviolet photoemission spectroscopy (UPS)
measurements for the selenol 4 and thiol 811a monolayers
have been also conducted. The energy scale was referenced
to the Fermi level of Au (EF). As shown in Fig. 3b, the UPS
spectrum of the monolayer 4 has a band at 1.0 eV. This
binding energy is smaller by 0.3 eV than that of the monolayer
8, which indicates that the Se–Au bond has an electronic
state more suitable to reducing the barrier for molecular
conductance. This result is consistent with the trends reported
by Taniguchi et al.6i
Fig. 1 Cyclic voltammograms of the monolayers 1 (a) and 2 (b) on Au.
Finally, monolayers prepared from single-armed 5 and 6 on
Au have been compared with those of 1 and 2 by means of CV
measurements. The surface coverage of selenocyanate 5
showed progressive desorption with increasing immersion time
(Fig. 2) like the monolayer 1, while that of selenol 6 on 12-h
immersion was 2.7 Â 10À10 mol cmÀ2. More importantly as
shown in Fig. 4a and d, the electrochemical responses of the
tripodal 2 monolayer are stable and remain essentially un-
changed after 50 scans within the range of 0–0.55 V, in
contrast to rapid fade-out for the single-armed 6 monolayer
(Fig. 4b and d). Furthermore, it was also shown that the
selenol 2 monolayer is electrochemically more stable when
compared with the thiol 7 monolayer (Fig. 4c and d). These
results clearly demonstrate the advantage of the selenol-
functionalized three-armed structure as an anchoring group
for gold electrodes.
Fig. 2 The changes of the surface coverage of 1, 2, and 5 on Au
depending on immersion time.
reductive-desorption experiment of 2 on Au in aqueous KOH
showed a clear desorption peak at À1.00 V vs. Ag/AgCl
(Fig. S1),w and its integrated cathodic charge was 3.0 times
larger than that of the first oxidation peak, suggesting that all
the three selenol groups of 2 are bound to Au.
To investigate the bonding characteristics of selenocyanate
and selenol groups, we have conducted X-ray photoelectron
spectroscopy (XPS) measurements of the monolayers formed
from 3 and 4 on Au(111). As shown in Fig. 3a, two photo-
emission peaks at 160.8 and 166.2 eV were observed for the
selenol 4 monolayer, and they can be assigned to Se 3p3/2 and
Se 3p1/2 peaks, respectively. These values are relatively lower
than those found for physisorbed selenium species (162.0 and
167.7 eV) and are consistent with those reported for chemically
bound selenium species (160.6 and 166.0 eV).16 The absence of
the peaks associated with the physisorbed species means that
Fig. 4 Cyclic voltammograms of 2/Au (a), 6/Au (b), and 7/Au (c) on
consecutive potential scans (1st, 10th, 20th, 30th, 40th, and 50th scans)
at 100 mV sÀ1. (d) The changes of the surface coverage depending on
scan number.
Fig. 3 Normalized Se 3p XPS spectrum of 4 on Au (a) and He I UPS
spectra of 4 and 8 on Au along with bare Au (b).
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In summary, we have successfully synthesized new tripodal
anchor units bearing selenium functional groups. CV, XPS,
and UPS measurements of their monolayers on gold
revealed the effectiveness of the three-armed structure as well
as selenol functional groups. Further studies of elucidating the
chemisorbed state on a gold surface and measuring the contact
resistance of the tripodal selenol anchor for the realization of
single-molecular devices are currently in progress and will be
reported in due course.
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