cis-trans Driven organized reorientation of an azobenzene derivative monolayer at the liquid/graphite interface

Article abstract of DOI:10.1039/b212060g

An azobenzene derivative, 2-hydroxy-4,4′-dihexyloxyazobenzene, in which a hydroxy group is introduced at the 2-position of the phenyl ring, forms an ordered monolayer at the liquid/graphite interface. Upon light irradiation, this monolayer can undergo a reversible cis-trans driven cooperative reorientation as observed by scanning tunneling microscopy.

Full text of DOI:10.1039/b212060g

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cis-trans Driven organized reorientation of an azobenzene derivative
monolayer at the liquid/graphite interfacey
ab
Jian Jin, Wensheng Yang,* Yingshun Li, Linsong Li, Yingying Zhao, Lei Jiang and
a
b
c
a
b
a
Tiejin Li
a
College of Chemistry, Jilin University, Changchun 130023, P. R. China.
E-mail: wsyang@mail.jlu.edu.cn
b
Center of Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing
00080, P. R. China
1
c
Chemical Science and Technology Division, Los Alamos National Laboratory, Los Alamos,
NM 87545, USA
Received (in Montpellier, France) 4th December 2002, Accepted 16th June 2003
First published as an Advance Article on the web 29th August 2003
0
An azobenzene derivative, 2-hydroxy-4,4 -dihexyloxyazobenzene, in which a hydroxy group is introduced at
the 2-position of the phenyl ring, forms an ordered monolayer at the liquid/graphite interface. Upon light
irradiation, this monolayer can undergo a reversible cis-trans driven cooperative reorientation as observed
by scanning tunneling microscopy.
Introduction
Experimental
Self-assembled monolayers physisorbed at liquid/solid inter-
faces have aroused growing interest due to their potential
applications in adhesion, wetting and as functional elements
in nanotechnology. A wide variety of molecules, such as
alkanes, alcohols, fatty acids, etc., have been found to form
The STM measurements were performed with a mechanically
sharpened Pt/Ir tip operating under ambient conditions with
a SPA 3700 STM/AFM system (Seiko Instruments, Japan).
Prior to imaging, the compound was dissolved in 1-undecanol
1
–5
ꢀ1
at a concentration of approximately 1 mg ml . One drop of
this solution was then applied to the basal plane of a piece
of HOPG. The tip was immersed in the solution and brought
close to the liquid/solid interface. Imaging was performed in
the current imaging mode with bias voltages of 252 mV (sam-
ple positive) and an average tunneling current of 0.14 nA.
STM images of the graphite surface, obtained at low tunneling
voltages, were used for calibrations. Immediately after the
sample image a calibration image of the graphite surface was
acquired at exactly the same position. Acquisition time for a
one-image frame (256 lines and 256 pixels per line) was
approximately 30 s (limits set by the STM instrument). For
the trans-cis isomerization experiment, the tip was first
retracted. Then the graphite, its surface still coated with solu-
tion, was irradiated by putting a 30 W UV lamp (wavelength
365 nm) approximately 5 cm above the sample surface. After
illumination for 20 min, the tip was immediately approached
and the image scanned continuously as before irradiation.
two-dimensional molecular lattices at the liquid/solid inter-
6
–10
face.
special attention due to their interesting photoresponsive beha-
Among them, azobenzene derivatives have received
1
1–13
vior.
It has been shown that suitably designed azobenzene
derivatives can form long-range ordered arrangements of
two-dimensional molecular lattices at the liquid/solid inter-
face. Generally, due to the lack of lateral interconnection
between molecules, the ordered arrangement of molecules will
be destroyed by a local cis-trans isomerization process upon
1
1,12
light irradiation.
From a technological viewpoint, it would
be very useful to design a system that can undergo a reversible
organized transition under irradiation.
In this work, we report the observation of a cis-trans driven
cooperative organized sliding motion of an azobenzene deriva-
tive monolayer at a liquid/solid interface by scanning tunnel-
ing microscopy (STM). The azobenzene derivative used here
0
is 2-hydroxy-4,4 -dihexyloxyazobenzene (1). In this molecule,
a hydroxyl group is introduced in the 2-position of the phenyl
ring. It is expected that the molecules can be held together by
a hydrogen bond network to form a two-dimensional molecu-
lar architecture and that the intermolecular hydrogen bonds
will endow the monolayer with new transition behavior upon
irradiation.
Results and discussion
After 1 was dissolved in 1-undecanol, a small droplet of the
solution was applied to the basal plane of newly cleaved highly
orientated pyrolytic graphite (HOPG). In this case, a physi-
sorbed monolayer formed spontaneously at the liquid/gra-
phite interface. Fig. 1 shows the STM image of a 1 adlayer
deposited from solution on the HOPG surface and the pro-
posed molecular packing pattern of the adlayer. The ordered
row-like structure with alternating bright and dark stripes
can be clearly observed. The periodicity of every row is
2
.9 ꢁ 0.1 nm, corresponding well to the length of one extended
y Electronic supplementary information (ESI) available: comparison
0
molecule of 1 in the trans form. The brighter stripes with a
width of 0.9 ꢁ 0.1 nm in the STM image originate from the
data with 4-hydroxy-3 -trifluoromethylazobenzene. See http://www.
rsc.org/suppdata/nj/b2/b212060g/
DOI: 10.1039/b212060g
New J. Chem., 2003, 27, 1463–1465
1463
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

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Fig. 3 The two types of trans configurations for molecule 1 used for
the Hartree–Fock calculation. To simplify the calculation, a methyl
group is used to replace the hexyl group.
II. For configuration I, HF ¼ ꢀ866.8526889, and for config-
uration II, HF ¼ ꢀ866.8240217. The difference of their ener-
Fig. 1 STM image of an ordered monolayer of 1 formed by physi-
sorption at the liquid/graphite interface from 1-undecanol solution.
ꢀ
1
gies is 75.2 kJ mol , indicating that configuration I is more
stable than II. So in our system, molecule 1 tends to adopt
the more stable configuration I and form lateral intermolecular
hydrogen bonds between neighboring molecules. That is to say
all molecules in one row are assumed to be connected by an
intermolecular hydrogen bond network.
2
Image size: 20 ꢃ 20 nm . The image was obtained with a 252 mV bias
and a 0.14 nA current setpoint in constant current mode. The inset is a
cut of Fig. 1 after 2-D fast Fourier transform. In every row, the pro-
˚
trusion part can be clearly seen with a spacing of 9 A, corresponding
well to the intermolecular spacing of the azobenzene moiety of one
molecule 1. Next to the STM image is the proposed molecular packing
pattern of 1 derived from this image.
After irradiation of the solution on the HOPG surface with
UV light (365 nm) for 20 min, the graphite surface was re-
examined immediately as before (Fig. 4). Owing to the trans-
cis isomerization, the image exhibits a different orientation of
the row-like structure. An ordered structure is also obtained
azobenzene moieties and correspond to a higher tunneling cur-
rent. The darker stripes in the image originate from the alkyl
1
4
ꢂ
but rotated by 60 compared with that before irradiation
[Fig. 4(a)]. Continuing the image scanning, the row-like struc-
chains and correspond to a lower tunneling current. The dis-
tance between neighboring molecules in a row was measured to
˚
be about 2.6 A.
ture slowly rotates clockwise [Figs. 4(b) and 4(c)]. Within 3
min [Fig. 4(d)] the orientation of the row-like structure returns
to that observed in Fig. 1. During the process of reorientation
and recover, the width of every row hardly changes and the
two-dimensional architecture exhibits a cooperative reorienta-
The formation of a physisorbed monolayer at the liquid/
graphite interface results from the interactions between the
molecules and the graphite surface. The positions of the mole-
cules need to match with the graphite lattice to maximize the
binding energy between the molecules and the graphite surface.
Fig. 2 is a model for the molecular arrangement of trans 1 on
a graphite surface based on STM observations. It shows that
1
5
tion of all molecules on the nanoscale. The sliding motion
ꢂ
ꢀ1
proceeds with a steady speed of about 1 s
.
For the realization of the cooperative sliding motion of the
monolayer, the formation of a two-dimensional hydrogen
bond network among neighboring molecules plays a key role
in the whole process. Commonly, for a conjugated planar
the molecules in adjacent rows are oriented along the [21]
ꢂ
HOPG
direction and that there is an inclination angle of 5 between
the row direction and the [12]HOPG direction. This is supposed
to be the energetically most favorable position of trans 1 on a
graphite surface.
In 1, the introduction of a hydroxyl group allows the forma-
tion of two types of trans configurations when taking the posi-
tion of the hydroxyl group into account. As shown in Fig. 3,
configuration I is prone to form intermolecular hydrogen
bonds while in configuration II it is easy to form an intramo-
lecular hydrogen bond. Hartree–Fock calculations were used
to compare the ground state energies of configurations I and
Fig. 4 STM images of 1 adsorbed from 1-undecanol solution after
2
UV light irradiation. The image size is also 20 ꢃ 20 nm , corresponding
to the same position as in Fig. 1. The bias and current parameters are
the same as those in Fig. 1. (a) Image obtained immediately after UV
irradiation; (b) and (c) continuously scanning images obtained after
(a), every image scanning is finished within 10 s; (d) image obtained
after 3 min.
Fig. 2 Proposed orientation of trans 1 on a graphite surface with 2D
packing vectors a
the direction of molecules in the adjacent row. g
vectors of graphite along the [12]HOPG and [21]HOPG directions.
1
and a
2
. a
1
is the direction of one row and a
2
is
1
and g are the lattice
2
1
464
New J. Chem., 2003, 27, 1463–1465

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molecule such as benzene, theoretical results showed that the
energetically most favorable position of a benzene ring is on
top of a C atom of the graphite lattice to form a planar-to-pla-
nar interaction with the graphite surface. Under UV light
irradiation, a part of the azobenzene molecules will be excited
liquid/graphite interface. In this cooperative system, the lat-
eral hydrogen bonds restrict the behavior of every individual
molecule and allow energy to be transferred efficiently among
molecules. The position-matched orientation of 1 on the gra-
phite surface determines where the cis-trans driven sliding
motion stops. Such a reversible and organized manipulation
of a monolayer at a liquid/graphite interface is of great tech-
nical interest in surface and interface phenomena.
1
6,17
and undergo a trans-cis transition. After excitation, the N=N
double bond will be converted to a N–N single bond and the
conjugated planar structure of the molecules will be destroyed.
This will cause a decrease of the binding energy between the
molecules and the graphite surface. As a result, the excited
molecules can depart from the graphite surface and the
trans-cis transition can be completed through rotation of the
N–N single bond; it is unnecessary to destroy the hydrogen
bonds. Due to the presence of intermolecular hydrogen bonds,
if one molecule in a row undergoes the trans-cis transition, the
neighboring molecules in this row need to adjust their relative
positions to provide enough space for the cis isomer. This will
result in an alteration of the orientation of the row. The cis iso-
mer is a thermodynamically unstable species and prone to con-
vert back to the thermodynamically more stable trans isomer.
In this experiment, the 20 min UV light irradiation time is so
long that every molecule has a chance to complete one or sev-
eral cycles of trans-cis or cis-trans transitions. During this time,
the orientation of the rows is greatly altered on the graphite
surface [Fig. 4(a)]. The experimental result shows that the
deviation of the row orientation angle between Fig. 1 and
Acknowledgements
This work was supported by the National Natural Science
Foundation of China and the Excellent Young Teachers
Program of MOE, P. R. C.
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0
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In summary, we report here the observation of a cis-trans
driven sliding motion of an azobenzene monolayer at a
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1465