Uni- and bi-directional light-induced switching of diarylethenes on gold nanoparticles

Article abstract of DOI:10.1039/b609119a

Photochromic studies of diarylethenes with their switching unit linked to the surface of gold nanoparticles via a conjugated aromatic spacer show linker-dependent switching behavior. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b609119a

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Uni- and bi-directional light-induced switching of diarylethenes on gold
nanoparticles{
a
b
b
Tibor Kudernac, Sense Jan van der Molen, Bart J. van Wees and Ben L. Feringa*
a
Received (in Cambridge, UK) 27th June 2006, Accepted 17th July 2006
First published as an Advance Article on the web 2nd August 2006
DOI: 10.1039/b609119a
Photochromic studies of diarylethenes with their switching unit
linked to the surface of gold nanoparticles via a conjugated
aromatic spacer show linker-dependent switching behavior.
nanoparticles showing spacer-dependent uni- and bi-directional
light-induced switching. In a recent paper dealing with the
photochemistry of dithienylethenes attached to gold nanoparti-
10
cles, the photochromic unit was isolated from the metal core to
some extent by an alkyl chain. The same held for related studies
A major challenge of molecular electronics is to design useful
devices based on addressable molecular structures and to incor-
1
porate these into integrated circuits. As a result of decades of
1
1
concerning other types of photoswitches. However, to make
molecular switching devices feasible, low on-state resistances (and
high on–off ratios) are essential. This requires a certain level of
communication between the metal electrodes and the switching
unit. Therefore, we decided to explore the photochromism of
molecules with an anchoring sulfur atom which is connected to the
switching unit via an aromatic spacer in a linearly- or cross-
conjugated fashion (Scheme 1 and 2). In particular, assembly of
these molecules on gold nanoparticles provides a useful model
system mimicking molecule–gold electrode interactions.{
2
a
research into this area substitutes for conventional switches,
2b
2c
rectifiers, wires and transistors have been introduced. The
2d,e
necessity to respond to external stimuli places molecular switches
3
among the most challenging systems. Photochromic molecular
3
switches meet this requirement, and specifically 1,2-dithienyl-
ethenes hold a prominent position among others because of their
reversible photo-induced transformations that modulate elec-
trical conductivity and their exceptional thermal stability and
4
fatigue resistance.
Despite the noticeable progress in designing new molecular
electronic components their incorporation into integrated circuits
remains a challenge. A common approach is based on anchoring
2b,5
molecules to metal electrodes.
However, in the case of
photochromic molecules, the situation is more complicated since
the presence of a metal surface may influence optical processes in
6
photoactive molecules. Recently we and others published
7
experimental and theoretical papers focused on photochromic
and electronic properties and very recently on the electrochemical
8
properties of diarylethenes at the interface of metallic electrodes.
Switching behavior depends critically on the nature, length and
position of the spacer linking the switching unit to the anchoring
group. Uni- or bi-directional switching has been observed. This
triggers the demand for a fast and reliable method to test the
photochromic properties of molecules attached to a metal. Here
Scheme 1 Photoisomerization between 1o- and 1c-anchored on gold
nanoparticles.
9
we show that monolayer-protected metallic nanoparticles may
help us to explore events happening at the molecule–metal
interface. These systems hence fill the gap between ordinary
solution-phase measurements and highly advanced molecular
device techniques demanding expensive and time-consuming
nanofabrications.
We report a study of the photochromic properties of
three different dithienylethene derivatives, anchored to gold
a
University of Groningen, Organic and Molecular Inorganic Chemistry,
Strating Institute, Nijenborgh 4, 9747 AG, Groningen, The Netherlands.
E-mail: b.l.feringa@rug.nl; Fax: +31 50 363 42 96
University of Groningen, Physics of Nanodevices, Material Science
b
Center, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
Electronic supplementary information (ESI) available: synthetic details
{
and structural characterizations for all compounds; photoisomerization of
unbound molecules in solution; TEM and IR data for functionalized gold
nanoparticles. See DOI: 10.1039/b609119a
Scheme 2 Molecular structures and molecule-capped nanoparticle
systems used in this study.
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Molecular switches 1–3 (Scheme 2) used in this work all contain
the central dithienyl cyclopentane part responsible for light-driven
4
transformations. In order to achieve better switching properties,
conjugation is extended by an aromatic moiety on each side of the
central switching unit. Variation of meta- or para-substituted
benzene rings in compounds 1 and 2, respectively, and a thiophene
group in compound 3 provides slight modifications in the final
electronic structure of the switch. All three compounds display a
typical photochromic behavior when present freely in solution.
Initial colourless open isomers in toluene turn coloured when
exposed to light with a wavelength of 313 nm (see the solution
spectra in the ESI{). Similarly, an inverse process occurs upon
irradiation with light above 420 nm and the original open isomers
are regained, thus demonstrating the reversibility of the whole
process.§ Monolayer-protected gold nanoparticles were prepared
1
2
by a modified Brust’s two-phase procedure. Acetyl-protected
thiol groups of compounds 1–3 in both open and closed forms
were deprotected with ammonium hydroxide in THF under an
inert atmosphere immediately before the nanoparticle preparation
in order to avoid an undesired oxidative dimerization. The
preparation itself was performed in a mixture of toluene and
Fig. 1 UV/Vis spectra of a) Au-1o, b) Au-2o and c) Au-3o measured in
dry toluene before (—) and after (---) irradiation at 313 nm. UV/Vis
spectra of d) Au-1c, e) Au-2c and f) Au-3c measured in dry toluene before
4
water containing HAuCl , tetraoctylammonium bromide (TOAB),
(—) and after (---) an irradiation at .420 nm.
deprotected thiol and NaBH . Purification of nanoparticle samples
4
is essential since any traces of unbound switches could affect the
photochemical experiments. Multiple precipitation or size-
exclusion chromatography were employed to obtain a clean
sample of nanoparticles free from thiols and TOAB. Although
both methods lead to a high purity, the multiple precipitation
results in significant losses due to the partial solubility in
appropriate solvents. The functionalized nanoparticles have an
average diameter of 2 nm, as established by transmission electron
microscopy (TEM; see ESI{). The nature of the organic shell
encapsulating the nanoparticles was confirmed by reflection FT-IR
spectroscopy, showing similar spectral band characteristics com-
pared to those of unattached molecules.
Au-1c, Au-2c and Au-3cI exhibit a decrease in absorption around
560 nm (Fig. 1d–f), demonstrating that opening of the switching
unit takes place.
Nevertheless, the efficiency of the ring-opening process is also
influenced and slowed down compared with molecules in solution,
just as was observed in the case of ring-closure of Au-1o and Au-
2o. A number of factors probably contribute to this phenomenon
including a reduced effective interaction with light due to the
restricted motion in a packed monolayer and a deteriorated
transparency of the medium due to the presence of light absorbing
clusters. However, the most important contribution is presumably
partial and, in the case of the Au-3o ring-closure, complete
quenching of the excited state. Quenching of the excited states of
organic chromophores anchored to gold nanoparticles is well
Fig. 1a shows the absorption spectra of the Au-1o cluster in
toluene (see Scheme 2). Its absorption is a superposition of the
spectral bands of the organic shell and a broad absorption of the
gold colloids, extending from the UV into the visible region. An
additional absorption at 525 nm corresponds to the surface
plasmon absorption which is typical for gold nanoclusters of 2 nm
9b,15
known and is usually atributed to resonance energy transfer.
In
energy transfer the efficiency of the process is related to the
strength of the coupling between chromophores. This strength is
determined by the proximity of energy states. As the HOMO–
LUMO gap of 3 is expected to be smaller, compared with 1 and
1
3
diameter in dimension. Upon irradiation with light of 313 nm,
the visible spectral band increases and a new maximum is found at
7
b
558 nm, indicating the formation of the closed isomer. Compared
2, we can reasonably assume that its LUMO lies closer to the
lowest unoccupied orbital of gold nanoparticles, so that mixing is
stronger. We suggest that this effect is at the origin of the
differences in the observed switching quantum yields. More
detailed studies of the quenching mechanism are currently under
investigation. The fact that ring-closure of Au-1o and Au-2o has
the same quantum yield indicates that the effect of linear- versus
cross-conjugated attachment does not play an important role in
the switching enhancement on gold.
with the unbound molecules, the ring-closure process is less
efficient. The quantum yield of this process was found to be 0.07,
1
4
which should be compared with 0.4 for the unattached molecule.
Spectral characteristics and light-induced transformations of Au-2o
Fig. 1b) are similar to those observed for Au-1o with the same
(
quantum yield. The absorption profile of Au-3o (Fig. 1c) resembles
the absorption profiles of Au-1o and Au-2o with a UV absorption
maximum at 336 nm. Contrary to Au-1o and Au-2o, cluster Au-3o
does not show any absorption changes after the irradiation of
In conclusion, photochromic behavior of diarylethenes
directly linked to gold nanoparticles via an aromatic spacer has
been investigated. Depending on the spacer, uni- (3) or bi-
directionality (1,2) has been observed. This demonstrates that
small changes in the molecular structure can have an enormous
impact on the overall photochemistry. More generally, it shows
that monolayer-protected gold colloids form a convenient tool for
313 nm light. The quantum yield of %0.01 reflects this
observation." This indicates that each excitation of the molecules,
arising from light absorption, is effectively quenched by the
presence of the gold core and does not lead to the photoreaction.
In contrast to the ring-closure process, ring-opening does not
vary going from phenyl to thiophene spacer. All three clusters
3
598 | Chem. Commun., 2006, 3597–3599
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We thank FOM, The Netherlands Foundation for Scientific
Research (NWO) and the Material Science Center for financial
support of this research. The authors would like to thank Dr P. R.
Hania for quantum yield measurements and Professor J. van Esch
for helpful discussion.
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{
The solubility of nanoparticles in some organic solvents and a relatively
high ratio between molecules and gold atoms enables easily accessible
analytical tools such as absorption spectroscopy to be employed.
§
together with the fact that a ring-closure has a typically larger quantum
The absence of absorption in the visible region for the open form,
3
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