Electric field-induced isomerization of azobenzene by STM

Article abstract of DOI:10.1021/ja065449s

The electric field applied between the tip of a scanning tunneling microscope and a metallic surface is shown to induce the reversible trans-cis isomerization of single azobenzene derivatives adsorbed on a Au(111) surface. The investigated molecule is sym

Full text of DOI:10.1021/ja065449s

Published on Web 10/21/2006
Electric Field-Induced Isomerization of Azobenzene by STM
Micol Alemani,† Maike V. Peters,‡ Stefan Hecht,*,‡ Karl-Heinz Rieder,† Francesca Moresco,† and
Leonhard Grill*,†
Institut fu¨r Experimentalphysik, Freie UniVersita¨t Berlin, Arnimallee 14, 14195 Berlin, Germany, and
Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mu¨lheim an der Ruhr, Germany
Received July 28, 2006; E-mail: leonhard.grill@physik.fu-berlin.de; hecht@mpi-muelheim.mpg.de
Understanding and controlling conformational changes of mol-
ecules adsorbed on solid substrate surfaces is currently one of the
most actively pursued goals related to fundamental electronic device
miniaturization.1 Molecular devices can exhibit new operating
mechanisms with technological advantages. In particular, the
realization of a molecular device operable without electric current
is of interest. In the search for molecules suitable for molecular
electronics, present research interest is focusing on molecular
switches.2,3 Azobenzene represents a very interesting example of
such a molecular switch, and its operating mechanism is based on
the trans-cis isomerization of a double bond.4 In the electronic
ground state, azobenzene adopts two configurations: a nearly planar
trans and a nonplanar, three-dimensional cis form. The reversible
switching of azobenzene is well studied in solution and in the gas
phase, where the ground state barrier for isomerization is typically
overcome by photoexcitation.4,5 Adsorption and lateral manipulation
of azobenzene have been investigated by STM.6 Moreover, very
recent experiments have provided evidence that the isomerization
of azobenzene7 and of the azobenzene derivative Disperse Orange
38 can be induced by resonant or inelastic tunneling of electrons.
In this communication, we show for the first time that the
reversible isomerization of specifically designed azobenzene mol-
Figure 1. (a) trans-3,3′,5,5′-Tetra-tert-butylazobenzene (trans-TBA). (b)
ecules can be induced on Au(111) by the electric field in the STM
Corner of a molecular island (70 × 40 Å2). The molecular adsorption
junction, without tunneling of electrons.
geometry is indicated by structural models. (c and d) Switching experiment.
(c) Island of trans-TBA containing about 400 molecules (370 × 370 Å2).
Subsequent voltage pulses (20 s, Vm ) 2 V, tip height ) 6 Å) are applied
at the position indicated by the cross. (d) STM image (370 × 370 Å2) after
nine pulses: 43 molecules have been switched to the cis form. (e) Zoomed
STM image of a molecular island (35 × 35 Å2). One TBA molecule has
been switched to the cis-configuration (bright lobe), while the other TBA
molecules are in the trans-configuration. (f) After applying a voltage pulse
(Vm ) 1.5 V, t ) 5 s, tip height ) 5 Å), the molecule is switched back to
the trans-configuration (35 × 35 Å2). (g) Schematic model of an adsorbed
cis-TBA molecule (side view). In all images, the STM parameters are I )
0.1 nA, V ) 1 V, T ) 5 K.
We chose to investigate 3,3′,5,5′-tetra-tert-butylazobenzene (TBA),
an azobenzene molecule carrying four lateral tert-butyl groups (Fig-
ure 1a). This substitution pattern increases the separation between
surface and the azobenzene π-system, leading to increased surface
mobility and potentially lower electronic coupling. It does not signif-
icantly alter the electronics of the azobenzene chromophore and does
not impart steric hindrance upon the isomerization process, and its
set of four symmetrically placed labels facilitates conformational
analysis. TBA was synthesized by oxidative dimerization of 3,5-
di-tert-butylaniline and exhibits the photochemical and thermal iso-
merization behavior typical for azobenzene derivatives in solution.9
The experiments were performed in ultrahigh vacuum at a
temperature of 5 K using a home-built STM.9 Upon adsorption onto
a clean Au(111) surface at room temperature, the molecules are
mobile and form ordered islands consisting of parallel rows (Figure
1b, c). Each molecule appears as four lobes with a rhombic shape
and an apparent height of 2.7 ( 0.1 Å. From the dimensions of the
molecule in the gas phase, we infer that the lobes correspond to
the tert-butyl groups while the central azobenzene part is not visible.
All observed molecules were found in the same planar configuration
that we assign to the thermodynamically favored trans-isomer
(Figure 1a).
feedback loop switched off and applying a voltage Vm to the sample.
After applying subsequent voltage pulses, with the STM tip always
at the same position above the island, a collection of STM images
was recorded. The images before and after nine equivalent pulses
are shown in Figure 1c,d (the complete series of 48 pulses is
available as a movie9). As one can see, many molecules have
changed their appearance, showing a larger apparent height of 4.1
( 0.3 Å. Typically, the application of a single voltage pulse causes
the switching of several molecules in a radius up to 500 Å around
the position of the tip. The bright molecules are stable, and their
initial appearance can be precisely restored by applying another
pulse, as shown in Figure 1e,f. Such switching experiments can be
reproduced several hundred times, allowing us to conclude that the
observed changes are due to the isomerization of single molecules
from the trans form to the cis form, and back to the trans form, as
also indicated by the observed shift in the molecular LUMO.9
The isomerization experiments were performed by positioning
the STM tip at a fixed height above a molecular island with the
† Freie Universita¨t Berlin.
‡ Max-Planck-Institut fu¨r Kohlenforschung.
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J. AM. CHEM. SOC. 2006, 128, 14446-14447
10.1021/ja065449s CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
f cis isomerization the threshold voltage varies with the distance
as 0.1 and 0.7 V/Å, while for the cis f trans it amounts to 0.3 and
0.2 V/Å. Small tip heights are not included because in this region
there is a change in slope, probably related to electronic effects
that come into play at small tip heights (i.e., large currents). These
values are smaller than those calculated for switching TBA
molecules by the electric field in the gas phase (about 1-2 V/Å11),
which is likely due to the presence of the surface that enhances the
permanent dipole and the polarizability of the molecule. Addition-
ally, the exact variation of these parameters along the reaction path
was not included in the model. The different slopes for positive
and negative voltages in the case of trans f cis isomerization as
compared to the cis f trans process (Figure 2) are likely due to
the delicate balance between the intrinsic dipole moment of the
molecule and its polarizability for the different adsorption geom-
etries and reaction paths.
In conclusion, we have shown that the trans-cis isomerization
of TBA molecules can be reversibly induced on Au(111). The
relationship between the applied bias voltage necessary for switch-
ing and the tip height shows that the process is caused by the electric
field between the STM tip and the sample surface.
Figure 2. Plot of the threshold voltage as a function of tip height for the
trans f cis isomerization (top) and for the cis f trans isomerization
(bottom) for positive (electric field points toward the STM tip) and negative
voltage polarities. The experimental points have been measured by fixing
the tip at a specific height (referred to the bare Au(111) surface and
calibrated by recording I(z) curves) and applying a voltage pulse for a fixed
time of t ) 30 s. Starting from V ) 0, we increased the voltage by
increments of 0.1 V until we observed a switching event in a lateral area
of 80 × 80 Å2 from the tip position. Each sequence was repeated 10 times.
The linear fit of the data is done for points above 9.5 and 11 Å (cis f
trans at negative voltages). The tunneling current is 7 ( 1 nA (trans f
cis) and 8 ( 1 nA (cis f trans) at a tip height of 3.7 Å and 52 ( 6 pA
(trans f cis) and 67 ( 6 pA (cis f trans) at 6.7 Å tip height.
Acknowledgment. We thank Christian Roth for technical
support, and Peter Saalfrank for fruitful discussions. Partial funding
by the European Integrated Project PICO INSIDE and the Deutsche
Forschungsgemeinschaft (DFG GR 2697/1-1 and SFB 658) is
gratefully acknowledged. M.P. and S.H. acknowledge generous
support by the Max Planck Society (MPG).
The cis-isomer appears with a bright central intensity maximum,
while three lateral lobes in triangular shape can be resolved (Figure
1e). From the STM images, we propose a structural model in which
one phenyl moiety remains on the surface while the second phenyl
ring is pointing upward (Figure 1g). As observed by STM7 and in
agreement with the conformation in the gas phase,4 the cis-isomer
is not planar.
Supporting Information Available: Movie of an isomerization
sequence and experimental details. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
We have investigated the dependence of the voltage (necessary
for the isomerization) on the tip height for both polarities and
isomerization processes (Figure 2). The data show that switching
is possible with positive and negative voltage polarities, and the
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of electron resonant tunneling,10 but clearly increases (at large tip
heights) when retracting the tip. The isomerization process occurs
also at very large tip distances where no tunneling current is flowing
(the tunneling current decreases exponentially with the tip-surface
distance). Even in the extreme case when the tip is 36 Å above the
surface, we were able to switch molecules from trans to cis (after
applying a voltage pulse of 6.8 V).9 Moreover, it is also possible
to isomerize molecules when the tip is positioned above the bare
surface nearby the island.
All these observations, together with the large lateral extension
over which switched molecules are observed, let us conclude that
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isomerization barrier. This effect, based on the (de)stabilization of
certain molecular configurations, depends on the presence and
orientation of an intrinsic dipole moment and also on the polariz-
ability of the molecule.11
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By performing a linear regression of the data in Figure 2
considering the large tip height regime, we found that for the trans
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