Photoswitching of conductance of diarylethene-Au nanoparticle network

Article abstract of DOI:10.1039/b617246f

A network composed of gold nanoparticles covered with diarylethene dithiophenols was prepared on an interdigitated nanogapped gold electrode to show the reversible photoswitching of the conductance due to the photochromism of the diarylethene molecules in

Full text of DOI:10.1039/b617246f

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Photoswitching of conductance of diarylethene-Au nanoparticle
network{
Masumi Ikeda,a Naoki Tanifuji,b Hidehiro Yamaguchi,a Masahiro Irie*a and Kenji Matsuda*ab
Received (in Cambridge, UK) 27th November 2006, Accepted 20th December 2006
First published as an Advance Article on the web 15th January 2007
DOI: 10.1039/b617246f
attributed to the fast photoreaction of diarylethenes in comparison
with the quenching by the Au metal surface.
A network composed of gold nanoparticles covered with
diarylethene dithiophenols was prepared on an interdigitated
nanogapped gold electrode to show the reversible photoswitch-
ing of the conductance due to the photochromism of the
diarylethene molecules induced by UV and visible light.
Considering that conjugated molecules have a much better
conductance than non-conjugated molecules,9 sulfur atom should
be attached directly to the p-conjugated system despite the
anticipated quenching of the excited state. Thus, diarylethene
dithiophenol 1a was designed and synthesized (Scheme 1) (for
details see ESI{). The two introduced thiophenol units bridge the
Au nanoparticles and create a conducting path between electrodes
(Fig. 1). Au nanoparticles covered with monothiophenol 2a were
synthesized to see the effect of the direct attachment on the
photoreactivity. During the synthetic procedure, trimethylsilylethyl
protecting groups were found to work effectively for the
thiophenol moiety (compounds 4–6 in Scheme 1).10 Although
the thiophenols 1a and 2a were sufficiently characterized, they
were easily oxidized to form disulfides, so that the preparation of
the diarylethene-covered gold nanoparticles was carried out
immediately after the deprotection. For reference studies, non-
photoreactive dithiophenol 3 was also prepared.
‘‘Molecular electronics’’ is a rapidly expanding field in nanoscience
and nanotechnology. Molecular electronics can be defined
narrowly as the study of electrical and electronic processes by
accessing the individual molecules with electrodes and exploiting
the molecular structure to control the flow of electrical signals
through them.1
Recently, we have been studying the photoswitching of the
intramolecular magnetic exchange interaction by using a diary-
lethene photochromic spin coupler carrying a nitroxide radical at
each end.2 The photoisomerization of the p-conjugated spin
coupler gives rise to a switching of the exchange interaction by
more than 150-fold. The spins are localized in the nitroxide moiety
in these studies, and so the exchange interaction between spins can
be regarded as an analogue of the conductance between electrodes.
Both the exchange interaction3 and the tunneling current4 show an
inverse exponential dependence on the distance.
Dithiophenol 1a and monothiophenol 2a underwent reversible
photochromic reactions in ethyl acetate upon alternate irradiation
with 313 and 578 nm light. The open-ring isomers 1a and 2a
showed absorption maxima at 287 and 278 nm, respectively, and
had no absorption in the visible region, while the closed-ring
isomers 1b and 2b both showed absorption at 577 nm (see ESI{).
Preparation of the Au-1a nanoparticle network was carried out
according to the procedure of Tamada and co-workers11 Au
nanoparticles protected by tetraoctylammonium bromide (TOAB)
were first prepared and then the dithiophenol 1a was reacted with
the TOAB-protected Au nanoparticles. In this method, the
Although there are increasing numbers of reports dealing with
the conductance of single molecules, studies on photoswitchable
molecules are very rare.5 Besides, studies on the conductance of
networks prepared with organic molecules and Au nanoparticles
on interdigitated nanogapped electrodes are attracting interest
because of the relatively easy preparation and the applicability to
the small number of molecules.6 In this paper, the photoswitching
of the conductance of a diarylethene-Au nanoparticle network is
reported. We have already reported the photochromic reaction of
gold nanoparticles covered with diarylethenes with mercaptopentyl
groups7 and Feringa and co-workers have recently reported that a
diarylethene connected to Au surface without alkyl spacers also
undergoes the photochromic reaction.8 Although the excited state
of organic molecules on noble metal surfaces is considered to be
easily quenched by the surface plasmon resonance, the diarylethene
on Au nanoparticles showed reversible photochromism. This is
aDepartment of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-
0395, Japan. E-mail: kmatsuda@cstf.kyushu-u.ac.jp (K. M.);
irie@cstf.kyushu-u.ac.jp (M. I.); Fax: +81 92 802 2927;
Tel: +81 92 802 2927
bPRESTO, JST, Japan
{ Electronic supplementary information (ESI) available: Experimental
procedures, syntheses, UV-vis and IR spectra, TEM, SEM, and optical
images. See DOI: 10.1039/b617246f
Scheme 1 Photochromic reactions of diarylethenes
1 and 2. The
structures of the precursors and the reference compounds are also shown.
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Fig. 3 Difference absorption spectra of the Au-1 network prepared from
Fig. 1 Schematic drawing of the diarylethene-Au nanoparticle network
the closed-ring isomer 1b. The spectra were obtained by subtraction of the
open-ring isomer spectrum which was measured after irradiation with
578 nm light. Measurement was performed in a KBr matrix. Closed: as
prepared; PSS: after irradiation with 578 and then 313 nm light.
on an interdigitated nanogapped Au electrode.
formation of the nanoparticles occurred before the reaction with
the dithiol, so that the size and shape of the nanoparticles could be
controlled.12 The prepared Au-1a network was characterized by
transmission electron microscopy (TEM) (Fig. 2). TEM images
show clearly the formation of the network structure. The size of
the nanoparticles was obtained as 3.6 ¡ 0.7 nm. The IR spectrum
of the Au-1a network showed the characteristic peaks of 1a,
suggesting the incorporation of the diarylethene.
prepared solution of 1a. Scanning electron microscopy (SEM)
measurement showed the formation of bridging between the
electrodes (see ESI{).
The Au-1 network showed photochromism in the solid state
(Fig. 3). The absorption spectra of the Au-1 network was measured
in KBr. The spectrum showed reversible changes along with the
photochromism. From the difference spectra the absorption
maximum of the closed-ring isomer Au-1b was determined to be
628 nm, which thus showed a large bathochromic shift compared
with free 1b, indicating the perturbation of the electronic structure
of the closed-ring isomer 1b by Au. The gold nanoparticles
prepared from monothiophenol 2a also showed photochromism
though it was less reactive than the previously reported alkyl-
containing analogue. The absorption maximum of Au-2b mea-
sured in ethyl acetate was 609 nm, which also showed a
bathochromic shift on the Au surface.
The Au-1a nanoparticle network was prepared on the
interdigitated nanogapped Au electrode (NTT-AT, 5 mm gap).
The preparation of the network was performed on the electrode by
mixing the TOAB-protected Au nanoparticles and a freshly
Fig. 4 (a) Change of the I–V curve of the Au-1 network and (inset) the
Au-3 network on the interdigitated Au nanogapped electrode. UV light
(290 nm , l , 380 nm) and visible light (560 nm , l) was used for the
experiment. (b) The same measurement was performed with a shorter time
of the UV irradiation. In this case, the conductance showed clear
reversibility.
Fig. 2 (a) TEM image of the Au-1a network (100 kV, 6 50000). (b)
Histogram of the size of the nanoparticles in the Au-1a network.
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Conductance was measured during the alternate irradiation with
UV and visible light (Fig. 4). Upon irradiation with UV light
(290 nm , l , 380 nm), the conductance increased significantly,
while the conductance decreased upon irradiation with visible light
(560 nm , l) (Fig. 4(a)). The Au-3 network, prepared for
reference, did not show any change upon photoirradiation
(Fig. 4(a), inset). The photoreaction of the Au-1a nanoparticle
network was very slow, especially for the ring-opening reaction
due to the severe quenching, which is also observed in the Au-2a
nanoparticles. When the UV irradiation was carried out for 10 h,
the maximum ON/OFF ratio of the conductance was more than
five-fold, but the conductance was not completely restored by
visible irradiation. To check the reversibility, the same measure-
ment was performed with 30 min of UV irradiation, which is
shorter than in the previous experiment (Fig. 4(b)). In this case, the
ON/OFF ratio was as small as 1.2-fold, but complete reversibility
was observed. The absolute value of the conductance varied with
the different setups because the network formed on the electrode
varied with the different setups. However, the photochemical
change of the conductance did not vary.
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In conclusion, by preparing a diarylethene-Au nanoparticle
network made of diarylethene dithiophenol, the photoswitching of
conductance though the organic molecule has been achieved. By
using narrower gapped electrodes, digital-type switching should be
possible.
We acknowledge PRESTO, JST and a Grant-in-Aid for
Scientific Research (S) (No. 15105006) from Japan Society of the
Promotion of Science for financial support.
11 A. Manna, P.-L. Chen, H. Akiyama, T.-X. Wei, K. Tamada and
W. Knoll, Chem. Mater., 2003, 15, 20.
12 When the dithiol was added before the addition of NaBH4, the formed
networked Au nanoparticles precipitated immediately, so that homo-
geneity of the nanoparticles was not achieved.
Notes and references
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Rev., 1996, 96, 537.
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Chem. Commun., 2007, 1355–1357 | 1357