Photo- and electro-chromism of diarylethene modified ITO electrodes - Towards molecular based read-write-erase information storage

Article abstract of DOI:10.1039/b608502d

Molecular memory devices based on dithienylethene switch modified ITO electrodes undergo reversible ring opening/closing both photo- and electro-chemically with non-destructive electrochemical readout. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b608502d

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Photo- and electro-chromism of diarylethene modified ITO electrodes—
towards molecular based read–write–erase information storage{
Jetsuda Areephong, Wesley R. Browne, Nathalie Katsonis and Ben L. Feringa*
Received (in Cambridge, UK) 16th June 2006, Accepted 13th July 2006
First published as an Advance Article on the web 9th August 2006
DOI: 10.1039/b608502d
coupling with 3-aminopropyl-triethoxysilane and n-propylamine,
respectively (Scheme 3).
Molecular memory devices based on dithienylethene switch
modified ITO electrodes undergo reversible ring opening/
closing both photo- and electro-chemically with non-destructive
electrochemical readout.
As for Ho,14 2o undergoes efficient photochemical ring
closing and opening in solution upon irradiation with 312 nm
and .400 nm light, respectively. The redox chemistry of 2o is
characterised by an irreversible oxidation at Ep,a = 1.15 V [vs. SCE]
leading to oxidative ring closure to 2c2+, which can then be
reduced, first to 2c+ at 0.78 V and finally 2c at 0.42 V (Fig. 1a).
The general strategy employed15 for immobilisation of 1o on
ITO electrode surfaces is summarized in Scheme 4. The ITO
electrodes were activated by a procedure, described by Markovich
et al.16 The treatment did not affect, significantly, the electro-
chemical properties or the hydrophobicity of the surface (see
ESI).{ Diarylethene 1o was immobilised on the activated ITO
surface in toluene at reflux for 24 h to give 1o-ITO (Scheme 4).
The increase in surface hydrophobicity upon immobilisation was
confirmed by contact angle measurement, with the contact
angle changing from 30u (for the activated ITO surface) to 80u
(for 1o-ITO).7
Self assembled monolayers of molecular switches1 hold consider-
able potential in the development of molecular electronic2 and
optoelectronic memory devices.3 Amongst the many photochromic
molecular systems reported, spiropyrans,4,5 azobenzenes,6,7 and
dithienylethenes8 have demonstrated their applicability as photo-
switchable molecular systems for immobilisation on surfaces due
to their excellent photochemical characteristics, which can be tuned
by synthetic modification. However, the development of practical
read/write memory devices depends, ultimately, on additional
functions other than the molecular switching ‘write/erase’ function
(e.g. photochromism). That is, to achieve a read/write memory
device a secondary physical signal (electrochemistry,9,10 IR,11
Raman spectroscopy12 etc.) is required, to read the state of the
switch non-destructively.13 Recently, we10 have reported that
dithienylethenes such as Ho can undergo an electrocyclic ring
closure to form Hc, via electrochemical oxidation and subsequent
reduction, in addition to the well-known photoswitching
(Scheme 1).
Here we report the first chemisorbed diarylethene based
molecular switch monolayers on ITO electrodes. The immobilised
switch can undergo multi-cyclic ring-opening and ring-closing
reactions both electrochemically and photochemically and, impor-
tantly, the state of the modified surface can be read ‘non-
destructively’ by electrochemical means (Scheme 2).
Scheme 2 Write–Read–Erase system based on the photochemical/
electrochemical switching of 1o-ITO to 1c-ITO.
Diarylethene 5 was prepared via Suzuki coupling of 314 with
methyl-4-bromobenzoate, followed by alkaline hydrolysis of the
methyl ester. Subsequently 5 was converted to 1o and 2o by
Scheme 1 Photochemical/electrochemical switching of Ho/Hc.
Organic and Molecular Inorganic Chemistry, Stratingh Institute,
University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The
Netherlands. E-mail: b.l.feringa@rug.nl; Fax: +31 50 3634296;
Tel: +31 50 3634235
{ Electronic supplementary information (ESI) available: Synthesis and
characterisation of 1o–5o, AFM, cyclic voltammetry and experimental
details. See DOI: 10.1039/b608502d
Scheme 3 Synthesis of 1o and 2o.
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1o-ITO is estimated as 1.2 by atomic force microscopy.{ No
change in the separation of the anodic and cathodic peak
potentials centred at y0.45 V was observed indicating that at
the experimental timescales (i.e. maximum scan rate) employed in
the present study electron transfer (ET) kinetics are not rate
limiting. The lower limit for the kET from the electrode to the
diarylethene units in the monolayer can be set to 10 s21 based on
the highest scan rate employed (10 V s21).
The cyclic voltammogram of the 1o-ITO shows no Faradaic
processes from 20.2 to 0.6 V (Fig. 2a). Irradiation of 1o-ITO
(l312nm for 5 min) results in photocyclisation of 1o-ITO to 1c-ITO
(Fig. 2a) and the appearance of a reversible redox wave at 0.45 V,
i.e. the first oxidation of 1c-ITO. Irradiation of the electrode with
l . 420 nm, for 30 min sees a complete loss in the redox process
at 0.45 V, i.e. restoration of 1o-ITO. This process can be continued
over several cycles of photochemical switching of the monolayer
between the states 1o-ITO and 1c-ITO, however it is apparent that
with each cycle the signal in the closed state diminishes
considerably (Fig. 2b). In addition the contact angle of the surface
decreases from 80u before irradiation to 58u after the four cycles.
The decrease in the intensity of the redox wave of 1c-ITO over
several cycles can be attributed to instability of the anchoring silyl
group17 of the dithienylethene switch on the surface due to the
liberation of F2 from the supporting electrolyte, rather than any
inherent electrochemical instability of 1c. Indeed, this instability
can be circumvented by using TBA(CF3SO3) as the supporting
electrolyte. The decrease in intensity of the first oxidation wave of
1c-ITO was not observed over four photochemical ‘close–open–
close’ cycles (Fig 3 and ESI{). Moreover, the contact angle of ITO
Fig. 1 Cyclic voltammetry of (a) 2o (inset cyclic voltammetry of 2c) and
(b) 1o-ITO at 0.1 Vs21 in CH2Cl2 (0.1 M TBAPF6).
Fig. 2 (a) Cyclic voltammetry of (i) 1o-ITO and (ii) 1c-ITO (formed
after irradiation of 1o-ITO at 312 nm for 5 min). (b) Repetitive
photochemical switching of 1o-ITO to 1c-ITO, Ip,c at 0.38 V. In 0.1 M
TBAPF6–CH2Cl2 at 2 Vs21. For Cyclic Voltammetry at 0.1 V s21 see
ESI.{
Scheme 4 Covalent attachment of 1o on an ITO electrode surface (to
form 1o-ITO), which subsequently can be switched reversibly to 1c-ITO
either photo- or electrochemically.
Cyclic voltammetry of 1o-ITO in 0.1 M TBAPF6–CH2Cl2
shows an irreversible oxidation wave at 1.10 V [1o-ITOA1c2+
-
ITO], which gives rise to two reversible redox waves at 0.73 and
0.46
vs. SCE [1c2+-ITOA1c1+-ITOA1c-ITO] (Fig. 1b).
V
Essentially the cyclic voltammetry of 1o-ITO/1c-ITO is equivalent
to that observed for 2o/2c in solution, however, as is expected for a
modified surface the intensity of the Faradaic current is directly
dependent on the scan rate (0.5–10 Vs21) and for 1c-ITO Ep,a
2
Ep,c , 59 mV at 0.1 Vs21.{ Coulometric analysis of the 1o-ITO
redox wave yields a surface density of the diarylethene on the
electrode surface of 5.5 6 10211 mol cm22; the roughness factor of
Fig. 3 Left: Cyclic voltammetry of (a) 1o-ITO and (b) 1c-ITO after
irradiation at 312 nm for 5 min. Right: Repetitive photochemical switching
of 1o-ITO to 1c-ITO, in 0.1 M TBA(CF3SO3)–CH2Cl2, scan rate 2 V s21
.
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Esch, B. L. Feringa and B. J. van Wees, Phys. Rev. Lett., 2003, 91,
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glass after the four cycles with TBA(CF3SO3) as electrolyte showed
only a modest change from 80u to 70u. In addition, the use of
TBA(CF3SO3) allows for oxidative ring opening of 1c-ITO by
cycling at scan rates , 1 V s21, between 0 and 0.5 V vs. SCE{
without loss in surface coverage.
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The electrochemical/photochemical properties of the diary-
lethene modified ITO electrodes reveal a robust system, where
redox and/or optical switching allows for write–read–erase
function (Scheme 2). The open form 1o-ITO can act as an
information recording interface. The information is ‘written’ either
photochemically or electrochemically (i.e. to produce the closed
form 1c-ITO) and the information stored can be erased
subsequently by either photochemical or electrochemical conver-
sion from 1c-ITO to 1o-ITO. The information is ‘read out’ non-
destructively by monitoring the reversible first oxidation of the
closed form electrochemically in the potential interval of 0.0 to
0.5 V as shown in Scheme 2 and Fig. 3. In the same potential
window the open form 1o-ITO is electrochemically inert.
In summary, the present communication demonstrates that
robust immobilisation of monolayers of dithienylethene switches
can be achieved on non-metallic conductive interfaces, without loss
of functionality. Furthermore the ability to drive ring-opening and
closing reactions oxidatively provides increased functionality to
these photochromic surfaces and a basis for read–write–erase
systems.
The authors thank the University of Groningen (Ubbo Emmius
scholarship J.A.), the NRSC-C program (W.R.B.) and the
Koerber Foundation (Hamburg, Germany, N.K.) for financial
support. We thank Dr. Johan Hjelm (Risoe National Laboratory,
Denmark) for suggestions and discussion.
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