Oligothiophene-derivatized azobenzene as immobilized photoswitchable conjugated systems

Article abstract of DOI:10.1039/c002072a

Immobilization of an azobenzene-bithiophene compound on a gold surface leads to self-assembled monolayers with photoswitchable electrical properties.

Full text of DOI:10.1039/c002072a

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Chemical Communications
www.rsc.org/chemcomm
Volume 46 | Number 21 | 7 June 2010 | Pages 3617–3804
ISSN 1359-7345
COMMUNICATION
Philippe Blanchard, Jean Roncali et al.
Oligothiophene-derivatized azobenzene as immobilized photoswitchable
conjugated systems

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COMMUNICATION
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Oligothiophene-derivatized azobenzene as immobilized photoswitchable
conjugated systemsw
a
b
b
Sandrine Karpe,a Maı
¨
tena Oc¸ afrain, Kacem Smaali, Stephane Lenfant,
´ ´
Dominique Vuillaume,b Philippe Blanchard*a and Jean Roncali*a
Received (in Cambridge, UK) 1st February 2010, Accepted 9th March 2010
First published as an Advance Article on the web 23rd March 2010
DOI: 10.1039/c002072a
Immobilization of an azobenzene-bithiophene compound on a
gold surface leads to self-assembled monolayers with photo-
switchable electrical properties.
In the continuation of our work on photo-stimulable
p-conjugated systems,13 we present here the synthesis and
characterization of the electronic properties of thioester 1,
the preparation of electroactive monolayers of 1-SH on a gold
surface and preliminary results on their photoswitchable
electrical properties.
Surface-immobilization of light-stimulable p-conjugated
molecules as self-assembled monolayers (SAMs) is the focus
of high current interest.1 Azobenzene is known to reversibly
switch between an extended trans to a shorter cis configuration
upon irradiation at ca. 360 nm and 480 nm, respectively.
Azobenzene derivatives have been grafted on surfaces in view
of various applications in molecular electronics,2 molecular
machines,3 photoswitchable wettability,4 photoinduced
magnetization5 and modulation of interactions of surfaces
with biomolecules6 or metal cations.7
The synthesis of compound 1 is described in Scheme 1.
Compound 2 was prepared from 2,20-bithiophene by a double
lithiation followed by successive additions of elemental sulfur
and 3-bromopropionitrile according to known procedures.14
Reaction of 2 with one equivalent of caesium hydroxide led to
the selective deprotection of one thiolate group which was
reacted with a slight excess of bromomethyl azobenzene 3 to
give compound 4 in 60–67% yields. Compound 3 was prepared
in 62% yield from 4,40-dimethylazobenzene (p-diMeAB) by
reaction with 2 equiv. of NBS in the presence of a catalytic
amount of ZrCl4 as Lewis acid (see SIw).
Azobenzene-based molecular or nano-junctions that exhibit
conductance switching with trans/cis photoisomerization have
been recently reported.2a,c,d,3,8 In this context, the develop-
ment of molecular architectures that could lead to high on/off
current ratio between the two configurational states appears as
a valuable goal.
Treatment of compound 4 with one equiv. of caesium
hydroxide and subsequent addition of a slight excess of thiol
ester 515 gave the target compound 1 in 74–78% yield.
Efficient reversible trans/cis isomerization of immobilized
azobenzene derivatives requires a decoupling of the azobenzene
unit from the surface in order to prevent or limit the quenching
of the photo-excited state by the metal substrate.8 To this end,
azobenzene units have been covalently fixed on metal surfaces
through different spacers such as alkane chains,2a,5,6b,9 aryl
units,2d,2f,10 or interfacial platforms.4a,11
In this work part of the long electrically insulating alkyl
spacers generally used to improve the formation of SAM from
thiol-terminated molecular systems has been replaced by an
electron-rich bithienyl p-conjugated system. Besides possible
p–p intermolecular interactions in the SAM, the incorporation
of a conjugated segment redox active at moderately positive
potentials will allow for preliminary characterization of the
immobilization process by means of simple electrochemical
experiments.12
a University of Angers, CNRS, CIMA, Linear Conjugated Systems
Group, 2 Bd Lavoisier, Angers 49045, France.
E-mail: philippe.blanchard@univ-angers.fr, jean.roncali@univ-angers.fr;
Fax: +33 2 41 73 54 05; Tel: +33 2 41 73 50 59
b Molecular Nanostructures & Devices Group, Institute for Electronics,
Microelectronics and Nanotechnology, CNRS & University of Lille,
B.P. 60069, 59652, Villeneuve d’Ascq, France
w Electronic supplementary information (ESI) available: Synthetic
procedures, structural characterization of all compounds. 1H and
13C NMR spectra. CV and UV-vis spectra. Preparation and CV
characterization of SAMs. See DOI: 10.1039/c002072a
Scheme 1 Synthetic route to 1 and 1-SH.
ꢀc
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Saponification of 1 by caesium hydroxide followed by addition
of HCl gave monothiol 1-SH in 80% yield.
The UV-vis absorption spectrum of bithiophene 2 shows a
p–p* absorption band at 339 nm whereas that of p-diMeAB
exhibits two absorption bands characteristic of the azobenzene
unit, an intense band (lmax = 336 nm) corresponding to the
p–p* transition and a broad weak band (lmax = 440 nm)
related to the n–p* transition. As expected, the spectrum of 1,
1-SH and 4 shows an intense band at 339–342 nm and a weak
band at 435–440 nm. The band around 340 nm corresponds to
the convolution of the p–p* transitions of the 5,50-disulfanyl-
2,20-bithiophene segment and the azobenzene moiety, in
agreement with the higher molar extinction coefficients of 1,
1-SH and 4 relative to those of 2 and p-diMeAB (Table S1w).
As shown in Fig. 1, irradiation of a solution of compound 1
at 360 nm produces a progressive decrease of the intensity and
a slight broadening of the 342 nm band with an increase of the
absorbance of the n–p* transition in the 400–450 nm region.
The presence of two isosbestic points at 297 nm and 380 nm
indicates the coexistence of two chemical species in equilibrium.
These results are in full agreement with the trans/cis isomeri-
zation of the azobenzene moiety of 1. Irradiation with 480 nm
monochromatic light induces the reverse cis/trans isomeriza-
tion although a photostationary state is reached before
complete return to the initial conditions (Fig. S2w).
Fig. 2 CV of a monolayer of 1-SH on gold immersed in 0.1 M
n-Bu4NPF6/CH3CN–CH2Cl2 1 : 1, scan rate 1000 mV sꢁ1. Insert:
variation of the intensity of the oxidation peak vs. scan rate.
1
oxidation wave at Epa = 0.98 V associated with a cathodic
peak at 0.95 V. The analysis of the CV response of
the monolayer at different scan rates ranging from 50 to
8000 mV sꢁ1 reveals a linear variation of the peak current
1
vs. scan rate with an invariance of Epa (Fig. 2, insert). These
The cyclic voltammogram (CV) of compounds 1, 2 and 4
exhibits two reversible one-electron oxidation waves corres-
ponding to the successive formation of the radical cation and
dication of the dialkylsulfanyl-bithiophene system at anodic
peak potentials of Epa1 = 1.00–1.16 V and Epa2 = 1.17–1.29 V
respectively (Fig. S3w). The stability of the oxidized states can
be explained by the strong +M electron-donating effect of the
two sulfide groups.16 In the case of 1-SH, the first oxidation
wave associated with the formation of the radical cation
was slightly broadened due to the concomitant irreversible
oxidation of the thiol group into disulfide (Fig. S3w).
results are consistent with a fast redox reaction of a surface-
immobilized species. The determination of the surface
coverage by integration of the CV wave gave (after correction
for double-layer charge) a value of 4 ꢂ 10ꢁ10 mol cmꢁ2
consistent with a densely close-packed monolayer.
Photo-switching of the SAM was tested by focusing mono-
chromatic light through an optical fiber. Preliminary results
show that the water contact angle reversibly switches from
93 ꢃ 21 after irradiation at 360 nm (cis isomer) to 98 ꢃ 21 after
irradiation at 480 nm (trans isomer). In parallel, the thickness
of the SAM measured by ellipsometry switches from 25 ꢃ 1 A
(cis isomer) to 30 ꢃ 1 A (trans isomer). These results are in full
agreement with the trans/cis photo-isomerization of the
immobilized 1-SH molecules.
Monolayers of 1-SH have been prepared by immersion of
gold electrodes in CH2Cl2 solutions of freshly generated 1-SH
in the absence of light. Fig. 2 shows the typical CV trace of a
monolayer of 1-SH. The CV exhibits a reversible one-electron
The photoinduced changes of the electrical properties of the
SAMs were analyzed by recording current vs. voltage (I/V)
curves of the junction formed between the gold substrate and
the metallic tip of a conducting-AFM. Fig. 3 shows the typical
I–V curves for a pristine SAM, after 90 min exposure to
480 nm light (trans isomer), and after 90 min irradiation at
360 nm (cis isomer). From these two latter curves, where the
higher conductance state is associated with the cis isomer,
a typical on/off ratio is determined with a maximum of
3–4 ꢂ 103 at ꢃ1.6 V, irrespective of the sign of the applied
bias. Mayor and coworkers2d have recently reported an on/off
ratio of about 25–30 for SAMs based on a biphenyl-based
azobenzene derivative and attributed the observed changes in
conductance to the variation of the layer thickness. However,
the much larger on/off ratio observed here suggests that in
addition to purely geometrical effects, photo-induced changes
in the electronic structure of the junction can also play a role.
In fact, a detailed analysis of the I–V curves of the trans and
the cis isomers shows that the difference between the electrode
Fermi energy and the LUMO energy is lowered by ca. 0.4 eV
Fig. 1 Changes in the UV-vis spectrum of 1 in CH2Cl2 (4 ꢂ 10ꢁ6 M)
at 20 1C after photoirradiation at 360 nm for 0, 20, 45, 105, 165, 225,
285 and 345 min.
ꢀc
This journal is The Royal Society of Chemistry 2010
3658 | Chem. Commun., 2010, 46, 3657–3659

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Fig. 3 Current vs. voltage (I–V) curves measured by C-AFM
(load force 20–30 nN, V applied on the Au substrate of the sample)
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360 nm for 90 min (cis isomer) and after irradiation at 480 nm for
90 min (trans isomer).
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calculations (0.53 eV).17
In summary, compound 1 has been readily synthesized by
connecting an azobenzene and a 2,20-bithiophene unit thanks
to thiolate chemistry. The corresponding thiol compound
forms stable electroactive monolayers on a gold surface with
fast surface-confined electrochemical response. Preliminary
results show that after surface immobilization, the molecules
undergo reversible trans/cis photoisomerization with
a
concomitant conductance switch with very high on/off current
ratio. The relationships between the molecular structure, the
properties of the molecular junction and the photo-induced
changes in electrical properties are now subject to detailed
experimental and theoretical investigations and will be
reported in a forthcoming publication.17
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This work was financially supported by ANR-PNANO
(OPTOSAM project ANR-06-NANO-016). We thank
O. Aleveque (CIMA) for technical assistance and the PIAM
´
of the University of Angers for analytical characterizations.
Notes and references
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Chem. Commun., 2010, 46, 3657–3659 | 3659