Published on Web 12/06/2003

Phase Switching of a Single Isomeric Molecule andAssociated Characteristic Rectification
Satoshi Yasuda,† Tohru Nakamura,‡ Mutsuyoshi Matsumoto,‡ and
Hidemi Shigekawa*,†
Contribution from the Institute of Applied Physics and 21st COE, UniVersity of Tsukuba,
Tsukuba 305-8573, Japan, and Nanotechnology Research Institute, National Institute of
AdVanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
Received August 30, 2003; E-mail: hidemi@ims.tsukuba.ac.jp

Abstract: Variations in molecular electronic structures related to conformational change are exceedingly
attractive because of their key role in the understanding and development of functional processes inmolecular electronics and biology. We observed, for the first time, the novel phase switching of a photoactive
isomeric molecule, N-(2-mercaptoethyl)-4-phenylazobenzamide (Azo molecule) at a single-molecule level,
which exhibits a distinctive change in the conductive characteristic under scanning tunneling microscope(STM) measurement. In comparison with the results obtained by the measurement of photoactive
isomerization of the isolated Azo molecule, which was performed also for the first time, the observed
characteristics are attributed to the results of the trans and cis phase transformation of the Azo molecule,under the condition of an external electric field and current flow. A specific point is that the potential landscapeof the system is controllable by the electric field and provides a conformational stability with asymmetricbias dependence resulting in rectification.

Introduction

The understanding and control of molecular functions at asingle-molecule level is extremely important in nanoscalescience and technology from both fundamental and practicalpoints of view.1-14 Recent intensive studies are leading to theclarification of the characteristics of novel molecular electronic
properties, such as asymmetric charge transport through individual molecules. On the other hand, for example, the Azo† University of Tsukuba, http://dora.ims.tsukuba.ac.jp.
‡ National Institute of Advanced Industrial Science and Technology.

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molecule (Figure 1a) is a typical photoactive isomer and has
been widely studied because of its simple structure andreversible cis-trans isomerization under photoillumination.
Irradiation with ultraviolet (UV) (∼350 nm) and blue (vis)(∼440 nm) lights induces isomerization to metastable “cis” andstable “trans” conformations, respectively.15 The absorption
characteristic is easily controllable by the addition of polargroups on the Azo molecule. From these characteristics, theAzo molecule is promising for not only photoswitching andstorage materials,16-23 but also for photoactive biomaterials that,for example, control DNA hybridization.24
In consideration of these characteristics, it is of extreme
interest to examine the coupling of the photoactive isomericmaterials and its electronic characteristics. In particular, thecontrollability of the photoactive isomeric material by electronic
processes is highly attractive for application to nanoscale
molecular devices. In this study, the effect of the external
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M. K.; Ringsdorf, H. J. Phys. Chem. 1996, 100, 2290-2301.
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2002, 296, 1103-1106.
(24) Asanuma, H.; Ito, T.; Yoshida, T.; Liang, X.; Komiyama. Angew. Chem.,
Int. Ed. 1999, 38, 2393-2395.
10.1021/ja038233o CCC: $25.00 © 2003 American Chemical SocietyPhase Switching of a Single Isomeric Molecule

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Figure 1. Schematic structures of (a) trans and cis isomerization of N-(2mercaptoethyl)-4-phenylazobenzamide (Azo) and n-dodecanethiol (C12) usedin this study and (b) isolated Azo molecules embedded in n-dodecanethiol(C12) self-assembled monolayer (SAM) films formed on a Au(111) substrate.

embedded in n-dodecanethiol (C12) self-assembled monolayer(SAM) films formed on Au(111) substrates (Figure 1b) wasinvestigated at a single-molecule level. Azo molecules weremodified with a polar amide group to have a dipole, and theconformation-related novel transition in their electronic structure
was observed for the first time. Photoisomerization of isolated
Azo molecules is also presented for the first time.
Methods
Au(111) substrates were prepared by vapor deposition of 50-75nm thick gold films onto freshly cleaved mica surfaces. During Au
deposition, the temperature of the mica substrates was kept at 400 °C.
The deposition rate was 0.08-0.1 nm/s, and the vacuum pressure was
1.0 × 10-6 Torr. Following the deposition, they were annealed at 400°C for 1 h. With this Au(111) substrate, we made samples by two
methods: (1) exchange reaction and (2) coadsorption reaction methods.
Detailed procedures of the two methods are as follows. (1) C12 SAM
films were formed by dipping the Au(111) substrate into toluene
solution with 1 mM C12 molecules for 18-24 h. After being rinsed toremove the physisorbed molecules, the C12/Au system was immersed
in toluene solution containing 0.5-5 µM Azo molecules for 0.5-2 h,resulting in the formation of C12 SAM including isolated Azo molecules(exchange reaction procedure). (2) Au(111) substrates were immersed
into a toluene solution with a 99:1 (v/v) mixture of 1 mM C12 and 1
mM Azo molecules for 10-15 h at room temperature (coadsorption
reaction process). The samples were rinsed and heated at 80 °C for
3-6 h in a vaporized C12 molecule atmosphere to improve the qualityof films. After this procedure, they were rinsed with toluene again.
Molecular density was controllable.25 The difference in the structure
between the samples prepared by the two methods is that Azo molecules
are inserted only at the boundary and pit sites in the case of the exchange
reaction method (see the Supporting Information). In the case of thecoadsorption reaction method, Azo molecules were inserted into theC12 domains in addition to the boundary and pit sites.
All STM images were obtained in N2 atmosphere at room temperature using a Pt/Ir tip. The measurements and preparations were
performed under dark conditions to avoid the effect of illumination on
Azo molecule isomerization.
(25) Campbell, D. J.; Herr, B. R.; Hulteen, J. C.; Van Duyne, R. P.; Mirkin, C.
A. J. Am. Chem. Soc. 1996, 118, 10211-10219.

Figure 2. Typical STM images of Azo-embedded C12 SAM film obtainedat (a) Vs ) +1.0 V, It ) 10 pA and (b) Vs ) -1.0 V, It ) 10 pA. (c) I-V
curve obtained over an Azo molecule. (d) I-V curve measured over an
Azo molecule which changed in brightness in (b). Histogram of the turnon voltage for the flip-flop motion is shown in the inset.

As shown in Figure 1a, an Azo molecule modified with a polar amidegroup has a dipole in the direction of the long molecular axis in thetrans form and tilted from the axis in the cis form. Therefore, the electricfield applied between the substrate and the STM tip is expected toinfluence the molecular conformation, as predicted in a recent workby Troisi and Ratner.26 Under the assumption of the molecular dipole
strength and the tip-substrate distance of ∼3 D and ∼2 nm,
respectively, the molecular dipole electric field interaction energy atroom temperature for the trans form becomes about 1.2-3.6 kBT for
the applied voltages of 0.5-1.5 V.

Results

Figure 2a shows a typical STM image of an Azo moleculeembedded C12 film at the sample bias voltage of +1.0 V. Brightprotrusions are Azo molecules, and they are embedded in anarea of densely packed C12, at phase boundaries of the C12
domains and in etch pits. On the other hand, Figure 2b shows
an STM image of the same area with a sample bias of -1.0 V.
As shown in Figure 2, parts a and b, the Azo molecules
surrounded by densely packed C12 molecules exhibit negligible(26) Troisi, A.; Ratner, M. A. J. Am. Chem. Soc. 2002, 124, 14528-14529.
J. AM. CHEM. SOC. 9 VOL. 125, NO. 52, 2003 16431

ARTICLES

Yasuda et al.

Figure 3. A series of STM images representing the cis-trans change inan isolated Azo molecule controlled by alternating UV (450 nm) and vis(325 nm) photoillumination (15 × 15 nm, Vs ) +1.0 V, It ) 10 pA).

change of brightness; however, those adsorbed in etch pits orat phase boundaries became darker at negative sample bias
voltage (an example is marked by a red circle). This result
suggests a conformational change of the loosely packed
molecules instead of a homogeneous charge transfer without
any conformational changes.25,27
Figure 2c shows I-V curves obtained over an unchanged Azo
molecule. It shows a symmetric characteristic for positive andnegative voltages. On the other hand, a typical I-V curvemeasured over an Azo molecule which changed in brightness
is shown in Figure 2d. In contrast to the symmetric characteristicof I-V curves in Figure 2c, a drastic change was observed.
Namely, tunneling current was almost flat between -1.0 V and+0.5 V, and rapid switching in the tunneling current between
two I-V curves (high- and low-current states indicated by red
and blue envelope curves) was observed in the high-positive
voltage region.
The high-current state I-V curve shows a characteristic
similar to that obtained for the unchanged Azo molecule shownin Figure 2c. The low-current state I-V curve has a shapesimilar to that obtained for negative voltage. The flip-flop motion
rarely appears at negative bias voltage, but the observed cases
are also between the two states. The inverse decay lengths
obtained by I-Z measurement for the Azo molecules with thehigh- and low-current structures in a sample were 8.5 ( 1.6
nm-1 and 5.2 ( 1.3 nm-1, respectively. The difference isattributed to the change in the characteristic electronic structureof the molecule. These results indicate that Azo molecules
loosely surrounded by C12 molecules change their conformation
between two distinct (high and low current) states during I-V
measurement.
The high- and low-current states might be attributed to thetrans and cis conformations, respectively. To examine thestructures, photoisomerization of the isolated Azo molecules wasinvestigated by illuminating the samples with ultraviolet (UV)(325 nm) and visual (vis) (440 nm) light using a He-Cd laser
during STM measurements. Figure 3 shows an example of aseries of STM images representing the structural change inisolated Azo molecules induced by alternating UV and vis
photoillumination. Optically stimulated conformational changeof Azo-SAM has been confirmed macroscopically in pioneer
works.16-22 This is the first direct observation of photoactive
isomerization of the isolated azobenzene molecules controlledby photoillumination.
(27) McCreery, R.; Dieringer, J.; Solak, A. O.; Snyder, B.; Nowak, A. M.;
McGovern, W. R.; DuVall, S. J. Am. Chem. Soc. 2003, 125, 10748-10758.
16432 J. AM. CHEM. SOC. 9 VOL. 125, NO. 52, 2003

Figure 4. Flip-flop motion analyzed by the newly developed stepwise I-V
measurement technique. The applied voltage was changed stepwise to be-1.5, -1.0, -0.5, -0.1, +0.1, +0.5, +1.0, and +1.5 V, as shown in (a).
(b) Change in the tunneling current due to the stepwise change in the appliedbias voltage, observed for an Azo molecule, the detail of which is shownin (c).

The brightness of the STM image of Azo molecules changes
with photoillumination. Some molecules changed less due totight packing, as was pointed out in the previous macroscopic
work,21 and the optimal voltage for the measurement depends
on the packing condition. However, loosely packed Azo
molecules showed cis and trans structures, as expected, under
UV and vis photoillumination, respectively. The observedchange in the STM image of the isolated Azo molecules between
trans and cis structures is similar to the change in the image
observed at positive and negative bias voltages (Figure 2),
namely, in the high- and low-current states.
In consideration of the experimental results, the structures
observed during the STM measurement, i.e., the high- and lowcurrent states, are attributed to the trans and cis conformational
structures of an Azo molecule. This is the first observation of
the conformational change of a photoactive isomer through anelectronic process at the single-molecule level.
To examine the effect of the external excitation in more detail,we analyzed the bias voltage dependence of the flip-flop processusing a newly developed stepwise I-V measurement technique.
Namely, the applied voltage was changed stepwise as -1.5,-1.0, -0.5, -0.1, +0.1, +0.5, +1.0, and +1.5 V, as shown in
Figure 4a, and the change in the corresponding current wasmeasured. Figure 4b shows the change in the tunneling current,
due to the stepwise change in the applied bias voltage, observed
for an Azo molecule, the detail of which is shown in Figure
4c. Switching between two definite structures at each bias
voltage is clearly shown. By the exponential fitting of thehistogram of the residence time of the flip-flop motion, thelifetimes of the Azo molecule in the two states were acquiredPhase Switching of a Single Isomeric Molecule

Figure 5. (a) Example of the bias-dependence of lifetime obtained for asample. (b, c) Lifetimes of the Azo molecule in the two states acquired foreach voltage, obtained by the exponential fitting of the residence time of
the flip-flop motion in each of the two states.

for each set voltage. An example of the bias dependence of
lifetime is shown in Figure 5a. The obtained lifetimes for thehigh- and low-current states (τ H and τ L) of several samples
are summarized in Figure 5, parts b and c. The obtained values
are scattered due to the effect of the packing condition of thesurrounding C12 molecules. Since the voltage range is limited
because of the damage to the molecules at higher voltage, it isdifficult at this stage to obtain the precise voltage dependenceof the lifetime. However, the lifetimes of the two states clearlychange in opposite directions. There is the tendency that the
lifetime of the high-current state becomes longer with anincrease in the applied positive voltage. The results strongly
indicate that the conformation of the Azo molecule is influenced
by the external electric field and tends to be transformed to thatof the high-current state as the positive sample voltage increases.
The change in the flip-flop rate between the two states, ∼1/(τH + τL), remains in the order of 103/s against the change ofthe tunneling current from the order of 107 (at +0.5 V) to 109(at +1.5 V) electrons per second. Therefore, 104-106 electrons
are related to the excitation of each event, if the electronic
process is included. The change in the number of electrons in
the order of 102 does not seem to strongly influence thefrequency of the flip-flop motion, and the residence ratio
between the two states seems to change with the applied voltage,
which is in good agreement with the process considered above.
It is difficult to evaluate the absolute potential barrier height.
However, in consideration of the Boltzmann factor for a typicalchange in the ratio of the lifetime, τL ) 4τH at 0.5 V and τL )
τH at 1.5 V, the change in the difference of the potential energy
is about 1-2 kBT (kB is the Boltzmann constant, and T is theabsolute temperature). These values are consistent with thoseestimated for the molecular dipole electric field interactionenergy (1.2-3.6 kBT for the applied voltages of 0.5-1.5 V).
The direction of the dipole moment is also consistent with theobserved results under the assignment of the two states.

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Since the low-current state is stable in the low bias voltage
region, there exists a barrier height for the flip-flop motion which
is sufficiently high compared to the thermal energy at room
temperature. To explain the fact, inelastic tunneling may be apossible mechanism.28-30 The stretching vibration modes ofbenzene-N and NdN in the Azo molecule are 1140 cm-1 (0.14
eV) and 1410 cm-1 (0.17 eV), respectively,31 which arecomparable to the rising edge of the histogram of the turn-on
voltage (0.15-0.2 V). The value of ∼0.2 eV (∼7 kBT) issufficient to explain the stability of the low-current state forthe measurement of the low negative bias voltage region. The
considerable point is that the low-current state, which was
attributed to the cis phase, is apparently the ground state underthe measurement condition not only for the negative bias voltage
but also for the positive bias voltage. In previous work, the transform was thought to be the ground state even for the Azo
molecular films.16 The tightly packed Azo molecules in the C12film domains seem to retain the trans phase, as shown in Figure
2. According to the recent theoretical study, for example, theasymmetry of the system prepared to realize rectification is
reduced by the charge transport itself, which is considered to
be a reason for the rarity of the molecular rectification process.32
The essential point is that the measurement condition influences
the fundamental process itself, namely, changes in the functions
themselves on undergoing the essential operating procedures.
Although it is difficult to characterize the details at this stage,
the understanding of this effect is important not only for thefundamental understanding of molecular characteristics but also
for the development of functional materials.
In conclusion, we observed, for the first time, the novel phaseswitching of an photoactive isomeric molecule, N-(2-mercaptoethyl)-4-phenylazobenzamide (Azo molecule), which exhibits
a distinctive change in the conductive characteristic under STM
measurement. In comparison with the results obtained by themeasurement of photoactive isomerization of the isolated Azo
molecule, which was performed also for the first time, theobserved characteristics are attributed to the results of the transand cis phase switching of the Azo molecule, under the conditionof an external electric field and current flow. A specific pointis that the potential landscape of the system is controllable by
the electric field and provides a conformational stability withasymmetric bias dependence, resulting in rectification.
Acknowledgment. This work was supported by a Grant-inAid for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology of Japan. One of theauthors (S.Y.) was financially supported by the Society for the
Promotion of Science (JSPS) Fellowships for Japanese Junior
Scientists.
Supporting Information Available: Synthesis of the Azo
molecule and sample preparation (PDF). This material isavailable free of charge via the Internet at http://pubs.acs.org.
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