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Reversible cisЦtrans Isomerization of a Single Azobenzene Molecule.

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Angewandte
Chemie
Single-Molecule Studies
DOI: 10.1002/anie.200502229
Reversible cis–trans Isomerization of a Single
Azobenzene Molecule**
of relaxation prevent the cis–trans isomerization of molecules
in direct contact with metal surfaces.
Disperse Orange 3 (NH2-C6H4-N=N-C6H4-NO2 ; structure
shown in the inset in Figure 1 a) is an azobenzene derivative
Jrg Henzl, Michael Mehlhorn, Heiko Gawronski,
Karl-Heinz Rieder, and Karina Morgenstern*
Since isomers may differ substantially in physical and
chemical properties, isomerization offers the possibility of
converting less desirable compounds into isomers with
desirable properties. The isomerization of dye molecules,
such as derivatives of azobenzene (C6H5N=NC6H5), has
attracted much attention owing to the high potential of
these compounds for applications in optoelectronic devices,
for example, optical memory, photooptical switches, and
displays.[1] Azobenzene has been used as a photoregulator to
form or dissociate a duplex that regulates the geometry and
function of biomolecules.[2] Azobenzene undergoes a reversible light-induced conformational transition of the N N
double bond between an extended (trans) and a compact (cis)
conformation in the gas phase and in solution.[1] In nature,
specific functions are frequently realized with the aid of such
cis–trans isomerization reactions of molecules. Here we
describe the cis–trans isomerization of individual molecules
of the azobenzene derivative Disperse Orange 3 on Au(111)
triggered by tunneling electrons. Inducing chemical reactions
of specific single molecules is one intriguing aspect. In a more
general context, the understanding and control of molecular
functions on a single-molecule level is essential in nanoscale
science.[3]
The photoswitching of fluorescence properties of single
molecules observed with single-molecule spectroscopy has
developed rapidly in recent years.[4] The controlled displacement of parts of a molecule might be of further interest for
future molecule-based electronic devices.[5] It has been
proposed based on calculations that an azobenzene molecule
between two gold electrodes can serve as a switch operable at
room temperature.[6] This integration of molecules into larger
circuits is hardly imaginable without their arrangement on
surfaces. However, it is not immediately obvious whether
steric hindrance, surface bonding, or alternative mechanisms
[*] Prof. K. Morgenstern
Institut f4r Festk6rperphysik Abteilung Oberfl7chen
Universit7t Hannover
Appelstrasse 2, 30167 Hannover (Germany)
Fax: (+ 49) 511-762-4877
E-mail: morgenstern@fkp.uni-hannover.de
J. Henzl, M. Mehlhorn, H. Gawronski, Prof. K.-H. Rieder
Institut f4r Experimentalphysik, FB Physik
Freie Universit7t Berlin
Arnimallee 14, 14195 Berlin (Germany)
[**] We acknowledge financial support from the VolkswagenStiftung
within the program “Physik, Chemie und Biologie mit Einzelmolek4len”.
Angew. Chem. Int. Ed. 2006, 45, 603 –606
Figure 1. Disperse Orange 3 on Au(111): a) Large-scale image after
deposition of 0.05 molecules nm 2 on Au(111) at 230 K; dimers are
marked with D, monomers with M; tunneling parameters: U = 228 mV,
I = 11 pA. Inset: Ball-and-stick model of the molecule; arrows point to
lone-pair orbitals. b) Dissociation of a dimer into two monomers by
application of 2.1 V for 200 ms; the position of the tip is indicated
with a cross; left: before manipulation; right: after manipulation.
U = 337 mV, I = 9.4 pA.
with two substituents, NO2 and NH2. From adsorption
experiments of substituted benzenes in connection with
tight-binding calculations[7] we expect the substituents to
dominate the scanning tunneling microscopy (STM) image of
the molecule. Thus we could identify dimers (four protrusions,
D) and monomers (two protrusions, M) in the STM image
(Figure 1 a). This assignment is corroborated by electroninduced dissociation of a dimer into two monomers (at 2.1 V;
Figure 1 b). While the monomers in Figure 1 a are straight,
those in Figure 1 b are bent. This immediately suggests that
the former are in the trans form, while the latter are in the cis
form.
By placing the STM tip above a molecule, we could
transfer energy into the molecule by means of inelastically
tunneling electrons. The resulting excitation of specific
chemical bonds may lead to rotation or translation of small
molecules,[8] to desorption,[9] to selective bond breaking,[10] to
dissociation within an assembly,[11] to polymerization (both
tip-induced and photoinduced),[13] or to the separation of
hydrogen-bonded molecules, as demonstrated in Figure 1 b.
Here, we use the same method to show for the first time that
the reversible electron-induced cis–trans isomerization of a
single molecule is feasible.
In Figure 2 a–c some images are shown of a monomer that
was switched successfully more than 70 times between three
configurations—two bent forms and one elongated form. A
steep change in the tunneling current (Figure 2 d) indicates
the success of the manipulation, and the manipulation yield
per electron can be calculated for different electron energies
(Figure 2 e). The threshold is 650 meV for changes from the
elongated (trans) and 640 meV from the bent (cis) configuration. The different threshold voltages are indicative of
changes to the electronic structure of the molecule upon
isomerization. These voltages lie far below the photoisomerization voltages of azobenzene in the gas phase, which
correspond to irradiation with ultraviolet (l 350 nm;
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
603
Communications
Figure 2. Isomerization of an azobenzene derivative: a–c) Images
recorded of a molecule that was switched with indicated electron
energy; left column: before manipulation, electrons are injected with
indicated bias voltage at the position marked with a cross; right
column: after manipulation. U = 71 mV, I = 9.4 pA. d) Plot of the
current during the manipulations (a)–(c); sharp changes are indicative
of the conductance change during isomerization. e) Plot of the isomerization yield Y vs. bias voltage V for trans–cis isomerization or for
diffusion which is also induced.
3.55 eV) and blue (l 440 nm; 2.82 eV) light for the isomerization from the stable trans configuration and metastable cis
configuration, respectively.[1] This suggests that the isomerization is not due to excitation into the S1 state of the molecule
as in photoisomerization but is induced in the ground state. In
the ground electronic state, the energy of the trans-azobenzene is 0.6 eV lower than the energy of the cis structure,[12] and
they are separated by an energy barrier of approximately
1.6 eV (measured from trans to cis).[14] This value is still far
more than the excitation energy in our experiment.
To understand the isomerization in more detail, we need
to identify the different parts of the molecules, which is
possible with the high resolution of our STM. Figure 3 shows
the two types of monomers in submolecular resolution along
with ball-and-stick models. In STM images of the trans
molecule (Figure 3 a) we observe a weak increase in intensity
between the two dominant protrusions, which are 1.15 nm
apart. In the gas phase the equilibrium structure of the
molecule is almost planar (Figure 3 b). This geometry is
favorable for adsorption on metal surfaces, because the
adsorption of aromatic p systems parallel to the surface is
energetically preferred. Comparison to the molecular structure calculated in the gas phase (see Figure 3 b) shows that the
maxima of the larger protrusions are centered above the
nitrogen atoms of the NH2 and NO2 substituents (cf. Ref.[9]);
the increase in intensity between these protrusions is indicative of the N N double bond. The high negative density in the
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Figure 3. High-resolution images and structural formula of the monomer. a) STM image of the trans isomer; U = 71 mV, I = 9.4 pA. b) Balland-stick model of the trans isomer as optimized in the gas phase with
ArgusLab 4.0;[19] the ESP density is indicated in color: red indicates
areas of high electron density. c) STM image of the cis isomer;
U = 337 mV, I = 9.4 pA. d) Ball-and-stick model of the cis isomer on the
surface as deduced from the STM image; correspondence of functional
groups to circles is extracted from (b); inset: cis isomer in the gas
phase as optimized with ArgusLab 4.0;[19] dashed circles are identical
in (a) and (b), and in (c) and (d).
electrostatic potential (ESP) corresponds to protrusions in
our STM image.
A planar molecular structure in the cis configuration is
complicated by the steric repulsion of the phenyl groups. In
the gas phase, the molecule circumvents this problem by
rotating the phenyl groups around the N C bond (see inset in
Figure 3 d). The STM images of the cis isomers do not
correspond to this nonplanar molecule (Figure 3 c). Alternative to rotational relaxation, an increase in the bond angle
could minimize steric hindrance while allowing the phenyl
groups to assume the energetically preferred planar adsorption geometry. Based on the relationship of the protrusions to
the molecular structure as identified in the trans configuration
(Figure 3 a, b), we deduce that the STM image of the molecule
in Figure 3 c corresponds to the modified geometry shown in
Figure 3 d. Thus, we find that the bond angle is increased from
1278 in the gas phase to 1508 on the surface. This allows
both phenyl groups to lie in parallel to the surface. The
increased bond angle indicates a partial charge transfer from
the molecule to the metal. Either the partial charge is
transferred from the lone pair or the double bond is
weakened. This in turn suggests that the azo group is
adsorbed on a on-top place. Thus it has surface contact.
Based on the structural model, the isomerization shown in
Figure 2 can be interpreted as an conversion from a planar cis
to the trans configuration (Figure 2 a) and vice versa (Figure 2 b). Azobenzene bears lone-pair orbitals at the nitrogen
atoms of the N N double bond. Therefore, isomerization
might be result from rotation around the N N double bond or
the inversion of one of the N C bonds with the lone pair of
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 603 –606
Angewandte
Chemie
the nitrogen. Rotation seems unlikely for a molecule adsorbed on a surface. Furthermore, calculations have shown that
inversion is the preferred pathway for the ground-state
isomerization in the gas phase.[15] We also exclude the socalled hula–twist pathway, which has been suggested for the
cis–trans isomerization of the retinal chromophore within its
limited volume[16] and observed recently for stilbene,[17]
because this necessitates an energetically unfavorable vertical
displacement of a phenyl group. We thus interpret the
observed isomerization as an inversion of one of the N C
bonds with the lone pair of the nitrogen. In addition, inversion
of both N C bonds simultaneously leads to a switching from
one cis to another mirror-symmetric cis configuration (Figure 2 c). The increased bond angle of the molecule in
connection with chemisorption to the surface lowers the
activation barrier to 650 meV, as compared to 1.6 eV in the
gas phase value. Vibrational modes that are likely to be
involved in the inversion are NNC bending modes, both inplane and out-of-plane. These lie below 65 meV in energy[18]
and are thus in the range of typical external vibrational modes
of aromatic molecules which are responsible for induced
diffusion. Thus, modes with similar energy for one vibrational
quantum are responsible for both the observed isomerization
and diffusion. The high activation energy suggests an
excitation to a highly excited state of n 10. As we are able
to induce the isomerization with these energies only when the
tip is in close proximity to the N=N group, it is possible that
the primary excited mode is a N=N stretching mode at 180–
190 meV.[18]
The images in Figure 4 supports the interpretation given
in the previous paragraph. In this experiment electroninduced manipulation was used to fix a molecule of Disperse
Orange 3 (upper two protrusions) to two other (NO2- or
NH2-) substituted benzene molecules. By using electrons with
energies above the gas-phase barrier, the molecule isomerizes
from a bent (Figure 4 a) to an elongated form (Figure 4 b) and
then to a differently bent form (Figure 4 c). This form shows a
much smaller angle, and the left part of the molecule is
unstable under imaging conditions, indicating a three-dimensional relaxation. Again, this isomerization is reversible
(Figure 4 c, d).
To complete the picture we return to the dimers in
Figure 1 a. A high-resolution image shows that the dimer
consists of two planar cis molecules and that the the two N=N
groups are in close proximity (Figure 5 a). The structural
Figure 5. High-resolution STM images and ball-and-stick model for the
dimer and conformational change due to hydrogen rearrangement:
a) STM image of the dimer (U = 10 mV, I = 20 pA), corresponding balland-stick model, and a 3D view of a different, mirror-symmetric dimer
(U = 10 mV, I = 200 pA); yellow circles in the model are taken from the
STM image; monomers including black circles are taken directly from
Figure 3 d; red arrow points to the two hydrogen bonds; dashed line
indicates the mirror plane. b) Conformational change of the dimer
demonstrating the change in the relative position of the N=N groups
(U = 6 mV, I = 8.5 pA); ball-and-stick model showing the change in the
relative orientation of the molecule and the corresponding change in
the hydrogen bonds.
Figure 4. Isomerization of an azobenzene derivative covalently connected to other (NO2- or NH2-)substituted benzene units: a) – d)
sequence of four manipulations (indicated voltage was applied for
200 ms). At this resolution only the end groups are clearly visible and
the position of the molecule is indicated clearly in (a) and (b). In (c)
the position of the cis isomer is less clear since the unbound group
(left protrusion) moves during scanning.
Angew. Chem. Int. Ed. 2006, 45, 603 –606
model superimposed over the maxima in the STM image
reveals that these dimers are stabilized by two hydrogen
bonds to the lone pairs of the nitrogen atoms with a hydrogenbond length of 230 pm. Two mirror-symmetric dimmers exist
(compare 2D to 3D image in Figure 5 a). Isomerization of
molecules within the dimer is sterically obstructed and indeed
was not possible at any voltage below the dissociation voltage
of the dimer (see Figure 1 b). Electron injection leads to a
change in relative positions of the two molecules. The high-
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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605
Communications
resolution images of the two conformations (Figure 5 b) show
that before the manipulation the N=N group of the upper
molecule lies to the left of the N=N group of the lower
molecule, and after the manipulation, vice versa. Thus, the
relative orientation of the two molecules through a rearrangement of the hydrogen bonds between two molecules was
achieved (see ball-and-stick model). This conformational
change is also reversible and it is bistable.
In summary, we have shown that controlled cis–trans
isomerization of a single molecule on a metal surface is
feasible by tunneling electrons. However, even on weakly
interacting surfaces such as Au(111) the potential energy
surface of this isomerization is not the same as in the gas
phase. For the azobenzene derivative Disperse Orange 3,
isomerization is based on inversion and occurs in the ground
state.
Experimental Section
Au(111) was cleaned by repeated cycles of Ne+ sputtering and
annealing to 800 K. Disperse Orange 3 was dosed from a Knudsen
cell onto the surface held at 230 K. The sample was then transferred
to a custom-built low-temperature STM, which was operated at 5 K
for this measurement. All preparations, transfers, and measurements
took place in a home-built ultra-high-vacuum system with a base
pressure below 5 A 10 10 mbar.
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Received: June 24, 2005
Revised: October 14, 2005
Published online: December 13, 2005
.
Keywords: isomerization · scanning probe microscopy ·
single-molecule studies · surface chemistry
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