Solvent codeposition and cis-trans isomerization of isophthalic acid derivatives studied by STM

Article abstract of DOI:10.1021/jp962737+

Scanning tunneling microscopy (STM) was used to study physisorbed monolayers of 5-octadecyloxyisophthalic acid (C₁₈ISA) and 5-[ω-(4′-dodecyloxy-4-azobenzeneoxy)dodecyloxy]isophthalic acid (C₁₂(AZO)C₁₂ISA) at a liquid/graphite interface. The acquired STM images exhibit the molecular packing of the monolayers with submolecular resolution. Monolayers formed by codeposition of solvent molecules and C₁₈ISA or C₁₂(AZO)C₁₂-ISA molecules are found when 1-octanol and 1-undecanol are used as a solvent. The cis and trans isomers of C₁₂(AZO)C₁₂ISA, the reagent and reaction product of a reversible photoinduced reaction, are simultaneously present and are both structurally identified at the liquid/graphite interface.

Full text of DOI:10.1021/jp962737+

19636
J. Phys. Chem. 1996, 100, 19636-19641
Solvent Codeposition and Cis-Trans Isomerization of Isophthalic Acid Derivatives Studied
by STM
P. Vanoppen, P. C. M. Grim, M. Ru1cker, S. De Feyter, G. Moessner,† S. Valiyaveettil,†
K. Mu1llen,† and F. C. De Schryver*
UniVersity of LeuVen (KUL), Department of Chemistry, Laboratory of Molecular Dynamics and Spectroscopy,
Celestijnenlaan 200-F, 3001 HeVerlee, Belgium
ReceiVed: September 9, 1996X
Scanning tunneling microscopy (STM) was used to study physisorbed monolayers of 5-octadecyloxyisophthalic
acid (C18ISA) and 5-[ω-(4′-dodecyloxy-4-azobenzeneoxy)dodecyloxy]isophthalic acid (C12(AZO)C12ISA) at
a liquid/graphite interface. The acquired STM images exhibit the molecular packing of the monolayers with
submolecular resolution. Monolayers formed by codeposition of solvent molecules and C18ISA or C12(AZO)C12-
ISA molecules are found when 1-octanol and 1-undecanol are used as a solvent. The cis and trans isomers
of C12(AZO)C12ISA, the reagent and reaction product of a reversible photoinduced reaction, are simultaneously
present and are both structurally identified at the liquid/graphite interface.
Introduction
Scanning tunneling microscopy (STM) was already success-
fully applied for imaging a wide range of physisorbed two-
dimensional organic films on a graphite surface.1-4 Various
aspects of physisorbed organic monolayers at the liquid/solid
interface have been investigated including domain dynamics,5-10
codeposition of two-component alcohol mixtures,11,12 and a
chemical reaction.13
Recently, certain properties of the class of isophthalic acid
derivatives were investigated such as their liquid crystalline
behavior14 and self-assembly in the solid state.15-17 Moreover,
STM was employed to investigate monolayers of 5-hexadecy-
loxyisophthalic acid (C16ISA) molecules at the liquid/graphite
interface. The obtained 2D structure of their packing was
compared with the 3D crystal structure determined by X-ray
analysis.19
In this paper we present a STM study of 5-octadecyloxy-
isophthalic acid (C18ISA) and 5-[ω-(4′-dodecyloxy-4-azoben-
zenexy)dodecyloxy]isophthalic acid (C12(AZO)C12ISA), which
self-assemble at a liquid/graphite interface in a reproducible
fashion. The chemical structure of the compounds is depicted
in Figure 1.
Figure 1. Chemical structure of C18ISA and C12(AZO)C12ISA.
(a) Dimethyl 5-(ω-bromododecyloxy)isophthalate (1). 1,-
12-dibromododecane (8.0 g, 0.024 mol), powdered K2CO3 (3.31
g, 0.024 mol), a catalytic amount of KI, and 80 mL of dry N,N-
dimethylformamide were placed in a two-necked round-bottom
flask equipped with a reflux condensor, a pressure-equalizing
funnel, and a magnetic stirrer. The mixture was stirred and
heated to 80 °C. Dimethyl 5-hydroxyisophthalate (2.56 g, 0.012
mol) in 30 mL of dry N,N-dimethylformamide was added
dropwise over a period of 6 h. After the mixture was stirred
for another 10 h at 80 °C, the solvent was removed. The crude
product was taken up in dichloromethane and twice washed with
water. After the organic fraction was dried over magnesium
sulfate and dichloromethane removed, the obtained product was
purified by column chromatography on silica gel (eluent: ethyl
acetate/n-heptane (1:5)). The pure product is white and crystal-
line. Yield: 3.1 g (56%). 1H NMR (CDCl3): δ 8.26 (s, 1 H),
7.73 (s, 2 H), 4.05 (t, 2 H, J ) 3 Hz), 3.95 (s, 6 H), 3.41 (t 2
H, J ) 4 Hz), 1.94-1.72 (m, 4 H), 1.62-1.23 (m, 16 H). 13C
NMR (CDCl3): δ 166.6, 159.7, 132.2 (2), 123.2 (2), 120.3 (2),
We focused on the structural investigation of physisorbed
monolayers using solvents capable of hydrogen bond forma-
tion with the isophthalic acid groups. We report for the first
time codeposition of solvent molecules with isophthalic acid
derivatives and cis-trans isomerization of the azobenzene-
containing C12(AZO)C12ISA. The cis and trans isomers,
respectively reagent and product of a photoinduced reversible
reaction, are structurally identified at the liquid/graphite inter-
face.
Experimental Section
The synthesis of C18ISA was reported previously.20 C12-
(AZO)C12ISA was synthesized following in sequence the steps
a-e described below.
† Max-Planck Institute for Polymer Research, P.O. Box 3148, D-55021
Mainz, Germany.
* To whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)02737-2 CCC: $12.00 © 1996 American Chemical Society

Isophthalic Acid Derivatives
J. Phys. Chem., Vol. 100, No. 50, 1996 19637
76.9, 69.1, 52.8, 34.4, 33.3, 30.0, 29.9, 29.8, 29.6, 29.2, 28.6,
1H NMR (DMSO-d6): δ 8.09 (s, 1 H), 7.85 (d, 4 H, J ) 4 Hz),
7.66 (s, 2 H), 7.09 (d, 4 H, J ) 4 Hz), 4.08 (t, 6 H, J ) 3 Hz),
1.85-1.62 (m, 6 H), 1.51-1.15 (m, 34 H), 0.89 (t, 3 H, J )
2.5 Hz). 13C NMR (DMSO-d6): δ 166.7, 161.2, 159.1, 156.3,
132.9, 124.8, 123.7, 119.3, 115.2, 77.2, 68.4, 55.6, 32.4, 30.7,
30.0, 29.8, 29.6, 29.2, 28.8, 26.5, 25.6, 23.2, 14.6. FD-MS:
m/z 731.9 [(M+1)+]
The steps a, b, and d were conducted under a dry argon
atmosphere. 1H and 13C NMR spectra were recorded on a
Varian Gemini 200 spectrometer. Chemical shifts are reported
in parts per million (ppm) and are referenced to residual proton-
containing solvent (δ(CDCl3) ) 7.26; δ(DMSO-d6) ) 2.49).
Mass spectra were determined on a ZAB2-SE-FPD instrument.
All commercially available reagents were used without further
purification.
26.4. EI-MS: m/z 458.1 [(M + 1)+].
(b) 4,4′-Dihydroxyazobenzene (2). 4-Hydroxyanilin (6.4 g,
0.058 mol) was dissolved in 100 mL of dilute hydrochloric acid
(1 M) and cooled to 0 °C. An aqueous solution of sodium nitrite
(4.03 g, 0.058 mol in 20 mL of water) was added dropwise
under constant stirring. The mixture is diluted by adding 200
mL of precooled methanol. In a separate batch, phenol (5.46
g, 0.058 mol) and potassium hydroxide (6.2 g, 0.11 mol) are
dissolved in 40 mL of methanol and also cooled to 0 °C. This
phenolate solution is added dropwise under constant stirring to
the first mixture. The red solution is stirred for another 2 h at
0 °C before the reaction is quenched with dilute hydrochloric
acid. The red solid was filtered, washed thoroughly with water,
and dried. The crude material was purified by recrystallization
from concentrated acetic acid. Yield: 4.48 g, 0.021 mol, 36%.
1H NMR: (DMSO-d6) δ 10.05 (br, 2 H), 7.74 (d, 4 H, J ) 4
Hz), 6.93 (d, 4 H, J ) 4 Hz); (CDCl3) δ 7.84 (d, 4 H, J ) 4
Hz), 6.95 (d, 4 H, J ) 4 Hz), 5.05 (br, 2 H). 13C NMR (DMSO-
d6): δ 160.3 (2), 145.6 (2), 124.4 (4), 116.1 (4). EI-MS: m/z
214.0 (M+).
STM experiments were performed using a Discoverer scan-
ning tunneling microscope (TopoMetrix Inc., Santa Barbara,
CA) along with an external pulse/function generator (Model HP
8111 A). Tips are electrochemically etched from Pt/Ir wire
(80%/20%, diameter 0.2 mm) in 2 N KOH/6 N NaCN solution
in water.
Prior to STM experiments, the compound under investigation
was dissolved in a solvent with a high boiling point. The
solvents employed were 1-phenyloctane (Aldrich, 99%), 1-oc-
tanol (Sigma, 99.6%), and 1-undecanol (Aldrich, 99%). Con-
centrations used were typically about 1 mg/mL. Samples were
prepared by spreading a drop of this solution on the basal plane
of highly ordered pyrolytic graphite (HOPG) (grade ZYB,
Advanced Ceramics Inc., Cleveland, OH).
All the presented STM images were acquired in the variable
current mode (constant height) under ambient conditions. The
speed of image acquisition was relatively slow. For example,
for an image consisting of 200 lines and 200 pixels per line,
approximately 7 s was the minimum acquisition time for one
image frame, which is limited by the Discoverer instrument.
Typically, a tunneling current of 1 nA and a bias voltage of
0.5-1.5 V referenced to the graphite surface were employed.
STM images obtained at low bias voltages reliably revealed
the atomic structure of HOPG, providing an internal calibration
standard for the monolayer studies. The presented STM data
were not subjected to image processing.
(c) 4-Hydroxy-4′-dodecyloxyazobenzene (3). 4,4′-Dihy-
droxyazobenzene (4.3 g, 0.02 mol) was mixed with powdered
K2CO3 and a catalytic amount of KI, and the mixture was
suspended in 80 mL of dry N,N-dimethylformamide as described
in procedure a. A solution of 1-bromododecane (2.72 g, 0.011
mol) in 30 mL of dry N,N-dimethylformamide was added
dropwise to the mixture and stirred for 10 h at 80 °C. The
workup was done as described above. The yellow product was
purified by column chromatography on silica gel (eluent: ethyl
acetate/n-heptane (1:1)) to obtain the pure monoalkylated
product (3). Yield: 2.9 g (0.0076 mol, 69.5%) (3). 1H NMR
(CDCl3): δ 7.86 (dd, 4 H, J ) 5 Hz, 4 Hz), 6.99 (dd, 4 H, J )
7 Hz, 5 Hz), 5.62 (br, 1 H), 4.05 (t, 2 H, J ) 3 Hz), 1.94-1.75
(m, 2 H), 1.57-1.12 (m, 18 H), 0.90 (t, 3 H, J ) 2.5 Hz). 13C
NMR (CDCl3): δ 161.8, 158.1, 147.7, 147.4, 125.0 (2), 124.8
(2), 116.2 (2), 115.2 (2), 66.9, 32.4, 30.0 (3), 29.8 (3), 29.7,
26.5, 23.1, 14.6. FD-MS: m/z 383.4 [(M+1)+].
(d) Dimethyl 5-[(ω-4′-dodecyloxy-4-azobenzeneoxy)dode-
cyloxy]isophthalate (4). The ω-bromodiester (1, 1.4 g, 0.0031
mol) and the phenol (3, 1.17 g, 0.0031 mol) were mixed with
2.5 equivalents of powdered K2CO3 and suspended in dry N,N-
dimethylformamide. The mixture is stirred at 80 °C for 12 h.
After most of the solvent was removed, the residue was taken
in dichloromethane and purified as described in step a.
Purification of the crude material was achieved by repeated
recrystallization from methanol. Yield: 1.6 g (0.021 mol, 68%).
1H NMR (CDCl3): δ 8.28 (s, 1 H), 7.88 (d, 4 H, J ) 4 Hz),
7.72 (s, 2 H), 6.99 (d, 4 H, J ) 4 Hz), 4.04 (t, 6 H, J ) 3 Hz),
3.94 (s, 6 H), 1.90-1.68 (m, 6 H), 1.62-1.16 (m, 34 H), 0.88
(t, 3 H, J ) 2.5 Hz). 13C NMR (CDCl3): δ 166.6, 161.7, 159.7,
147.5, 132.2, 124.8, 123.2, 120.2, 115.2, 76.9, 69.1, 52.8, 32.4,
30.0, 29.9, 29.7, 29.6, 29.2, 28.6, 26.5, 23.1, 14.6. FD-MS:
m/z 759.2 [(M+1)+].
Irradiation of the solution in a cuvette as well as on the
graphite surface was performed using a CAMAG universal lamp
with a wavelength of 366 nm.
Results and Discussion
Solvent Codeposition. Figure 2a shows a STM image of a
monomolecular layer of C18ISA adsorbed from a C18ISA
solution in 1-phenyloctane applied to the basal plane of HOPG.
The image reveals a closely packed arrangement of C18ISA
molecules on the graphite surface with submolecular resolution.
The functional groups of C18ISA molecules can clearly be
recognized. The bright spots appearing in the image can be
correlated with the isophthalic acid groups of the molecule.
Bright and dark refers to the black/white contrast in the images.
White corresponds to the highest and black to the lowest
measured tunneling current in the image. The occurrence of a
higher tunneling current above an aromatic moiety, as predicted
by theoretical calculations,21 is a general finding, which has been
observed for a large variety of organic adsorbates on graphite.
The darker regions in the image correspond to the alkyl groups
of the molecules.
(e) C12(AZO)C12ISA (5). The methyl ester (4) (1.3 g, 0.0017
mol) was hydrolyzed by refluxing it with 3.5 equivalents of
potassium hydroxide in a mixture of ethanol and water (2:1)
for 8 h. After evaporation of the alcohol, the alkaline aqueous
solution was rendered neutral using dilute hydrochloric acid (10
M) up to a pH of 4-5. The product precipitated as a yellow
solid, which was extracted from the aqueous phase with diethyl
ether. The organic phase was washed twice with water and
dried over magnesium sulfate. The obtained crude acid after
evaporation of diethyl ether was purified by double recrystal-
lization from methanol. Yield: 0.88 g (0.0012 mol, 70.6%).
On the basis of the found monolayer structure, a 2D unit cell
and an illustrative molecular model (Figure 2b) for the molecular
arrangement could be proposed. The unit cell as marked in

19638 J. Phys. Chem., Vol. 100, No. 50, 1996
Vanoppen et al.
Figure 2. (a) STM image of an ordered monolayer of C18ISA molecules formed by physisorption from 1-phenyloctane at a liquid/graphite interface.
Image size is 10.7 × 10.7 nm2. ∆L1 is the width of one lamella of interdigitated C18ISA molecules. (b) Molecular model for the two-dimensional
packing of C18ISA molecules and proposed unit cell. The unit cell contains two C18ISA molecules. The molecular model represents the area indicated
in the STM image. (c) STM image of an ordered monolayer of C18ISA molecules formed by physisorption from 1-octanol at a liquid/graphite
interface. Image size is 10 × 10 nm2. The value for ∆L1 is identical with the one in (a) and corresponds again to interdigitated C18ISA molecules,
while the value for ∆L2 corresponds to the width of the lamella built up by solvent molecules. (d) Molecular model for the two-dimensional
packing of C18ISA molecules incorporating 1-octanol molecules and proposed unit cell. The unit cell contains two C18ISA and two 1-octanol
molecules. The molecular model represents the area indicated in the STM image.
the molecular model contains two molecules. The unit cell
parameters a, b and R, as indicated in Figure 2, are 0.94 (
0.05 nm, 3.6 ( 0.3 nm, and 81 ( 2°, respectively. The unit
cell data are consistent with the previously reported results for
C16ISA.19 From the data analysis it can be concluded that the
molecules are oriented in such a way that hydrogen bond
formation between neighboring molecules within the same
lamella and between different lamellae is enabled. The alkyl
chains of the molecules are interdigitated over the full length
of the chains. By the interdigitation of the alkyl chains, a closely
packed arrangement of the C18ISA molecules is realized. This
closely packed arrangement minimizes the free space per unit
area within the monolayer on the graphite surface. This results
in a maximum stabilizing van der Waals interaction between
the C18ISA molecules on one hand and between the molecules
and the graphite surface on the other hand. In this way a stable
monolayer with a sufficiently low lateral mobility of the
molecules on the surface is assembled. This sufficiently low
lateral mobility is a prerequisite in order to perform high-
resolution STM imaging.
solvent molecules are incorporated in the monolayer. An
example of this solvent codeposition is presented in Figure 2c
where 1-octanol was used. The displayed STM image reveals
again the molecular arrangement of the adsorbed molecules with
submolecular resolution. Two different lamellar widths can be
found in contrast to the case of C18ISA monolayers physisorbed
from 1-phenyloctane. The largest lamellar width is identical
with the one observed in Figure 2a and is determined by the
structure and the packing of the C18ISA molecules. The other
lamella consists of only solvent molecules. Its width can be
correlated with the length of the alkyl chain of the solvent
molecules. The 2D unit cell consists of two C18ISA and two
1-octanol molecules, and the unit cell parameters a, b, and R
are 1.15 ( 0.05 nm, 4.9 ( 0.3 nm, and 81 ( 2°, respectively.
An illustrative model is shown in Figure 2d. The C18ISA as
well as the 1-octanol molecules are interdigitated. In this case,
however, hydrogen bonding between the acid functions of
isophthalic acid groups of molecules within a lamella occurs.
In addition hydrogen bonding of the other acid function of the
isophthalic acid group with the 1-octanol molecules is possible.
This results in a monolayer that consists of C18ISA lamellae
separated by 1-octanol lamellae.
When, instead of phenyloctane, alcohols with a long alkyl
chain such as 1-octanol and 1-undecanol are used as a solvent,

Isophthalic Acid Derivatives
J. Phys. Chem., Vol. 100, No. 50, 1996 19639
Figure 3. Cis and trans isomers of C12(AZO)C12ISA, reagent and product of a reversible photoinduced reaction.
Figure 4. (a) STM image of an ordered monolayer of trans-C12(AZO)C12ISA molecules formed by physisorption from 1-undecanol at the
liquid/graphite interface. Image size is 13 × 13 nm2. ∆L1 is the width of one lamella of interdigitated trans-C12(AZO)C12ISA molecules,
and ∆L2 is the width of one lamella of 1-undecanol molecules. (b) Molecular model for the two-dimensional packing of trans-C12(AZO)-
C12ISA molecules incorporating 1-undecanol molecules with proposed unit cell. The unit cell contains two trans-C12(AZO)C12ISA molecules
and two 1-undecanol molecules. The molecular model represents the area indicated in the STM image. (c) STM image of an ordered monolayer
of coexisting cis- and trans-C12(AZO)C12ISA molecules, which exhibits three domains. The domain boundaries are indicated by white lines.
Image size is 13.4 × 13.4 nm2. The molecular order in domain 1 is identical with the one observed in (a) and corresponds to trans-C12(AZO)C12-
ISA. The molecular order in domains 2 and 3 can be correlated with cis-C12(AZO)C12ISA. No codeposition with solvent molecules has been
observed in the molecular packing of the cis isomers. (d) Molecular model for the two-dimensional packing of cis-C12(AZO)C12ISA molecules
and proposed unit cell. The unit cell contains two cis-C12(AZO)C12ISA molecules. The molecular model represents the area indicated in the STM
image.
Preliminary experiments indicate that solvent codeposition
might be a rather general phenomenon for these molecules if
solvents capable of hydrogen bond formation are used. For
example, if a longer chain alcohol such as 1-undecanol is applied
as a solvent, a monolayer is assembled that shows a larger
separation of the C18ISA lamellae. This indicates that this
phenomenon might be used to vary the lamellar structure of a
monolayer in a controlled manner. Another important conse-

19640 J. Phys. Chem., Vol. 100, No. 50, 1996
Vanoppen et al.
Figure 5. STM image of an ordered monolayer consisting of several domains of C12(AZO)C12ISA molecules. Image size is 40 × 40 nm2. Domains
consisting of cis and trans isomers can be distinguished by the direction of the lamellae as indicated by the black arrows. The angle between the
orientation of the cis and trans lamellae is 30°. Solvent incorporation in the two-dimensional packing of the cis isomer is never observed, in contrast
to the monolayers of the trans isomer in which solvent codeposition is mostly found. The white arrows in the STM image indicate the areas within
the trans monolayer where no solvent is incorporated.
quence of these results is that the procedure of using small
solvent molecules offers an approach for immobilizing these
molecules, which cannot be investigated as such at the liquid/
graphite interface owing to their high mobility.
cell contains two C12(AZO)C12ISA and two 1-undecanol mole-
cules. The C12(AZO)C12ISA molecules again interdigitate, and
they form lamellae that are separated by lamellae consisting of
solvent molecules such as in the case of the C18ISA monolayers.
Cis-Trans Isomerization. With the aim of imaging the
reaction partners of a reversible photoreaction, the cis-trans
isomerization of C12(AZO)C12ISA (Figure 3) was investigated.
Figure 4c shows a STM image of a monolayer obtained by
physisorption from a photostationary mixture. Three domains
can be recognized in this image. The molecular packing in the
lower domain is identical with the packing observed in Figure
4a. Consequently, this domain should contain only trans isomer
molecules. The patterns observed in the two other domains must
originate from a different molecular arrangement. Again, the
bright spots correspond to the isophthalic acid groups and the
broader bright bands to the azobenzene moieties. The measured
lamellar distances and the observed molecular packing cor-
respond to a monolayer of self-assembled cis isomers without
solvent codeposition. Figure 4d depicts a 2D unit cell and an
illustrative molecular model that can explain the obtained STM
data. The unit cell contains two cis-C12(AZO)C12ISA molecules,
and the parameters a, b, and R are 1.03 ( 0.10 nm, 6.0 ( 0.4
nm, and 85 ( 4°, respectively. In the molecular model the
isophthalic acid groups point toward each other and the alkyl
chains are not interdigitated. This results in the specific
molecular pattern observed in the two upper domains of Figure
4c. The absence of solvent codeposition is probably due to the
spatial orientation of the acid functions in the molecular
arrangement of the cis isomer.
Upon irradiation with light of an appropriate wavelength, the
trans-to-cis isomerization can be induced. To achieve an
efficient conversion of the trans to the cis isomer, the irradiation
wavelength should match the absorption maximum of the trans
isomer. Consequently, the absorption spectrum of a dilute
solution of C12(AZO)C12ISA in 1-octanol was recorded. The
absorption maximum of the cis and the trans isomers is situated
at 317 and 360 nm, respectively. Upon irradiation at 366 nm,
a photostationary mixture with a high cis isomer content is
reached. The absorbance of this mixture at 360 nm is strongly
reduced owing to the increased concentration of the cis isomer.
When kept in the dark at room temperature, the cis isomer
spontaneously converts to the thermodynamically more stable
trans isomer and the absorbance is restored to its original value.
Owing to an overlap of the absorption bands, however, it is not
possible to prepare solutions containing only cis or trans isomer.
In view of its importance for the STM experiments the rate of
cis-to-trans isomerization was determined following the increase
of absorbance of the trans isomer at 360 nm as a function of
time until a photostationary state was reached. The rate constant
of cis-to-trans conversion in the absence of light at room
Another important aspect of the cis isomer monolayers is the
high number of crystallographic defects per unit area found in
these layers. Since they are composed of relatively small
domains, crystallographic defects in terms of domain boundaries
occur relatively often. The small sizes of the domains may
result from the fact that owing to the symmetry of the cis isomer
and its rather complex arrangement in the monolayers, a defect-
free packing is hindered. Interestingly, cis domains were not
observed without the presence of trans isomer domains, as
shown in Figure 5, and a fixed angle of 30° is always found
between the orientation of the cis and trans lamellae. Figure 5
further illustrates that occasionally areas within the trans-
C12(AZO)C12ISA monolayers can be found where no solvent
is incorporated. The coexistence of cis and trans domains
correlates not only with the relative amounts of cis and trans
isomers in the solution but also with the monolayer stability of
the isomers at the liquid/graphite interface.
temperature amounts to 2.7 × 10-5 s-1
.
In Figure 4a a STM image of a monomolecular layer of trans-
C12(AZO)C12ISA is shown. The layer is prepared by phys-
isorption from a solution of C12(AZO)C12ISA in 1-undecanol
that was shielded from light. Hence, the solution should contain
merely trans isomers as can be deduced from the absorption
spectrum. The closely packed monolayer consists of trans-
C12(AZO)C12ISA molecules codeposited with 1-undecanol
molecules. Distinct bright spots corresponding to the isophthalic
acid groups are easily recognized in the image. The somewhat
broader bright bands represent the azobenzene moieties. Also,
the alkyl chains can be recognized in the image. It was possible
to define a unit cell and to create a molecular model shown in
Figure 4b. The unit cell parameters a, b, and R are 0.97 (
0.10 nm, 6.6 ( 0.4 nm, and 66 ( 4°, respectively. The unit

Isophthalic Acid Derivatives
J. Phys. Chem., Vol. 100, No. 50, 1996 19641
References and Notes
Similarly, cis-trans isomerization could be induced and their
respective domains be imaged by irradiating in situ a droplet
of C12(AZO)C12ISA dissolved in 1-undecanol directly on the
graphite surface. During imaging, the cis isomer domains
disappear with time. Finally, only trans isomer domains could
be observed, illustrating the reversibility of the reaction at the
liquid/graphite interface.
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We have shown that STM can be used to structurally
investigate monolayers of C18ISA and C12(AZO)C12ISA with
submolecular resolution at a liquid/graphite interface. By
employment of solvents capable of hydrogen bond formation,
codeposition of solvent molecules is found. The phenomenon
of codeposition cannot only be used to stabilize monolayers
but it might also be an effective way for immobilizing small
solvent molecules that cannot be imaged in monomolecular
monolayers owing to their high mobility. By use of codepo-
sition phenomena, it might also be possible to predict the
lamellar structure of these monolayers.
We have further demonstrated that both reagent and product
of a reversible photoinduced reaction can structurally be
identified with submolecular resolution at the liquid/graphite
interface. Moreover, images containing both cis and trans
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Acknowledgment. The authors thank FKFO and DWTC
for continuing financial support through IUAP-II-16 and
IUAP-III-040. S. De Feyter is a predoctoral fellow of the
NFWO, and P. Vanoppen thanks the IWT for a predoctoral
scholarship.
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