Dynamics in physisorbed monolayers of 5-alkoxy-isophthalic acid derivatives at the liquid/solid interface investigated by scanning tunneling microscopy

Article abstract of DOI:10.1002/1521-3765(20001016)6:20<3739::AID-CHEM3739>3.0.CO;2-1

Monolayers of isophthalic acid derivatives at the liquid/solid interface have been studied with scanning tunneling microscopy (STM). We have investigated the dynamics related to the phenomenon of solvent co-deposition, which was previously observed by our

Full text of DOI:10.1002/1521-3765(20001016)6:20<3739::AID-CHEM3739>3.0.CO;2-1

FULL PAPER
Dynamics in Physisorbed Monolayers of 5-Alkoxy-isophthalic Acid
Derivatives at the Liquid/Solid Interface Investigated by Scanning Tunneling
Microscopy
AndrØ GesquieÁre,[a] Mohamed M. Abdel-Mottaleb,[a] Steven De Feyter,*[a]
Frans C. De Schryver,*[a] Michel Sieffert,[b] Klaus Müllen,[b] Anna Calderone,[c]
Roberto Lazzaroni,[c] and Jean-Luc BreÂdas[c]
Abstract: Monolayers of isophthalic
acid derivatives at the liquid/solid inter-
face have been studied with scanning
tunneling microscopy (STM). We have
investigated the dynamics related to the
phenomenon of solvent co-deposition,
which was previously observed by our
research group when using octan-1-ol or
undecan-1-ol as solvents for 5-alkoxy-
isophthalic acid derivatives. This solvent
co-deposition has now been visualized in
real-time (two frames per second) for
the first time. Dynamics of individual
molecules were investigated in mixtures
of semi-fluorinated molecules with
video-STM. The specific contrast arising
from fluorine atoms in STM images
allows us to use this functionality as a
probe to analyze the data obtained for
the mixtures under investigation. Upon
imaging the same region of a monolayer
for a period of time we observed that
non-fluorinated molecules progressively
substitute the fluorinated molecules.
These findings illustrate the metastable
equilibrium that exists at the liquid/solid
interface,between the physisorbed mol-
ecules and the supernatant solution.
Keywords: liquid/solid interface
´
monolayers ´ scanning probe micros-
copy ´ self-assembly
STM does not only allow the visualization of the ordering
within monolayers,but also the study of the dynamics within
these monolayers. The range of dynamic phenomena that can
be investigated by STM is limited by the nature of the STM
experiment,which is generally slow compared with the
timescale of single molecule dynamics.
Several spontaneous dynamic phenomena have been vi-
sualized both in pure and mixed monolayer systems at the
liquid/solid interface. Stabel et al. investigated the two-
dimensional structure of 2-hexadecyl-anthraquinone mono-
layers.[4] The analysis suggests a rather loose packing with
nonoptimized intermolecular interactions. The authors pro-
pose that the STM-images present the average struc-
ture between two oscillating densely packed monolayer
structures.
Introduction
With scanning tunneling microscopy (STM) organic mono-
layers can be studied with submolecular resolution at the
liquid/solid interface. This type of monolayers,which are
formed by spontaneous self-organization of molecules from
solution on solid substrates,has been studied for a wide
variety of compounds.[1] Besides functionality,chirality and
photoreactivity have also been investigated at the liquid/solid
interface.[2],[3]
Á
[a] Dr. S. De Feyter,Prof. Dr. F. C. De Schryver,Drs. A. Gesquie re,
Drs. M. M. Abdel-Mottaleb
University of Leuven (KULeuven),Department of Chemistry
Laboratory of Molecular Dynamics and Spectroscopy
Celestijnenlaan 200-F,3001 Heverlee (Belgium)
E-mail: steven.defeyter@chem.kuleuven.ac.be
frans.deschryver@chem.kuleuven.ac.be
Ostwald ripening describes the growth of larger particles at
the expense of smaller particles. The thermodynamic driving
force in two dimensions is the reduction of the circumference-
to-area ratio and thereby the lowering of the interfacial or line
energy. Stabel et al. studied this phenomenon in two-dimen-
sional systems with STM.[5] They observed that small domains
in 2-hexadecyl-anthraquinone and tetradodecyl-octathio-
phene monolayers shrink and finally disappear. Reorientation
of molecules and lamellar fragments with respect to the
graphite substrate has been invoked to explain the observed
Ostwald ripening.
[b] Drs. M. Sieffert,Prof. Dr. K. Müllen
Max-Planck Institute for Polymer Research
Ackermannweg 10,55128 Mainz (Germany)
Â
[c] Dr. A. Calderone,Dr. R. Lazzaroni,Prof. Dr. J.-L. Bre das
Â
Service de Chimie des Materiaux Nouveaux
Â
Centre de Recherche en Electronique et Photonique Moleculaires
Â
Universite de Mons-Hainaut,
Place du Parc,20,7000 Mons (Belgium)
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3739

S. De Feyter,F. C. De Schryver et al.
FULL PAPER
‡
[M] ; elemental analysis calcd (%) for C16H20O5: C 75.74,H 6.90; found: C
A variable temperature fast STM was used to observe the
order-disorder transition in monolayers of alkanes at the
liquid/graphite interface.[6] At the interface between a neat
melt of a long-chain alkane and a solution of this long-chain
alkane disordered lamellae and a columnar phase are found.
Dynamic processes have also been reported for mixed
monolayers of triacontane/triacontanol by Venkataraman
et al.[7] They observed that triacontane and triacontanol form
separate domains on the graphite substrate. The alcohol
molecules are found to adsorb preferentially on the substrate
and they have a tendency to displace the alkane molecules off
the surface. This molecular motion in adlayers of the mixtures
is the main reason for the noisier images that were obtained
for the mixtures when compared with the single-component
solutions. Mixed octadecanol/tetracosanol monolayers phys-
isorbed on highly oriented pyrolytic graphite (HOPG) were
imaged by Elbel et al.[8] They found that the molecular
organization is dependent of concentration ratio between the
two components in the mixture. Dynamics could be observed
for an ordered arrangement consisting of mixed double
lamellae of octadecanol and tetracosanol. For this ordering
molecular dynamics could be detected. The composition of
the lamella was found to fluctuate,while maintaining an
overall densely packed double lamella structure. This was
explained by assuming a correlated desorption-readsorption
process of octadecanol/tetracosanol pairs.
75.66,H 6.89.
Dimethyl 5-(7,7,8,8,9,9,10,10,11,11,12,12,13,13,14,14,15,15,16,16,17,17,18,18,
18-pentacosafluoro-n-octadecyloxy)isophthalate:
1-Iodoperfluorodode-
cane (3.0 g,4.02 mmol) and 5-( w-hexenyloxy)isophthalate (1.17 g,
4.0 mmol) were added into a two-necked-round bottom flask equipped
with a reflux condenser,an argon inlet and a magnetic stirrer. The mixture
was homogenized by heating to 808C. During the following 2 h AIBN
(0.075 g,0.457 mmol) was added in small portions and the reaction mixture
1
was stirred for another 5 h. The reaction was followed by H NMR. After
cooling to room temperature,the mixture solidified to a yellow waxy solid.
The crude product (4.0 mmol) was dissolved in dry toluene (10 mL) under
argon. Tri-n-butyltin hydride (3.2 mL,12.0 mmol) and AIBN (0.082 g,
0.5 mmol) were added and the solution was stirred for 12 h at 808C.
Methanol was added to decompose the excess of hydride. After cooling to
room temperature,a white precipitate was isolated by suction filtration.
The product was isolated after column chromatography on silica gel
(dichloromethane/petroleum ether 3:2) as
a colorless solid (2.44 g,
2.67 mmol,67%). M.p. 97 8C; 1H NMR (250 MHz,CDCl 3,25 8C): d ˆ
8.02 (s,1H,ArH),7.71 (s,2H,ArH),4.05 (t,
3J(H,H) ˆ 6.3 Hz,2H,
OCH2),3.91 (s,6H,OCH 3),2.16 ± 1.95 (m,2H,CH 2CF2),1.86 ± 1.76 (m,
2H,OCH 2CH2),1.69 ± 1.40 (m,2H,CH 2); 13C NMR (75 MHz,CDCl
,
3
258C):d ˆ 166.18,159.11,131.74,122.86,119.78,68.27,52.37,30.81,28.84,
‡
27.01,25.70,20.08; MS-FD: m/z: 912.4 [M] ; elemental analysis calcd (%)
for C28H21F25O5: C 36.86,H 1.94; found: C 36.78,H 1.92.
Dimethyl 5-(7,7,8,8,9,9,10,10,11,11,12,12,13,13,14, 14,15,15,16,16,17,17,18,18,
18-pentacosafluoro-n-octodecyloxy)isophthalic acid (F12H6-ISA): The hy-
drolysis of dimethyl 5-((w-perfluorododecyl)hexyloxy)isophthalate (2.0 g,
2.2 mmol) was performed in a solution of sodium hydroxide (0.168. g,
3.0 mmol) in ethanol/water 2:1,which was stirred for 6 h under reflux. The
solution was cooled to room temperature. The white precipitate obtained
after addition of conc. HCl was filtered,washed with water,and dried
under high vacuum. The crude acid was recrystallized from ethyl acetate to
yield a pure colorless and crystalline product (1.84 g,2.08 mmol,95%);
M.p. 1978C; 1H NMR (250 MHz,DMSO,55 8C): d ˆ 8.02 (s,1H,ArH),7.56
(s,2H,ArH),3.96 (t, 3J(H,H) ˆ 6.0 Hz,2H),2.08 ± 1.90 (m,2H,CH 2,CF 2);
Molecular motion at domain boundaries in mixed mono-
layers of fatty acids adsorbed on graphite has recently been
observed.[9] Hibino et al. observed a dynamic exchange of the
two kinds of observed lamellae,which consist of myristic acid
and behenic acid; this indicates an adsorption ± desorption
process of molecules on the graphite surface.
1.70 ± 1.62 (m,2H,OCH 2CH2),1.47 ± 1.36 (m,6H,CH
2); 13C NMR
(75 MHz,DMSO,55 8C): d ˆ 165.93,158.44,132.41,121.89,118.66,67.70,
‡
29.65,27.91,27.65,24.56,19.14; MS-FD: m/z: 884.3 [M] ; elemental analysis
In this paper we report on dynamics occurring in mono-
layers of isophthalic acid (ISA) derivatives at the liquid/solid
interface,detected by scanning tunneling microscopy. The
presented images show the rearrangement of the molecules
adsorbed on the surface due to reorientation and adsorption-
desorption of the molecules in the monolayer.
calcd (%) for C26H17F25O5: C 35.31,H 1.94; found: C 35.23,H 2.02.
Scanning tunneling microscopy: Prior to imaging,all compounds under
investigation were dissolved in octan-1-ol or undecan-1-ol (Aldrich,99%)
and a drop of this solution was applied on a freshly cleaved surface of highly
oriented pyrolytic graphite. The STM images were acquired in the variable
current mode (constant height) under ambient conditions. In the STM
images,white corresponds to the highest and black to the lowest measured
tunneling current. STM experiments were performed using a Discoverer
scanning tunneling microscope (Topometrix Inc.,Santa Barbara,CA)
along with an external pulse/function generator (Hewlett Packard 8111 A),
with negative sample bias. Tips were electrochemically etched from Pt/Ir
wire (80%:20%,diameter 0.2 mm) in a 2 n KOH/6n NaCN solution in
water. The scanning speed of the Topometrix Discoverer STM in standard
mode is limited to seven seconds per image consisting of 200 lines and
200 pixels per line. After modification,the scanning speed improved to two
images per second (video-STM). Most home-built systems described in
literature can acquire several frames per second,although imaging rates up
to 20 frames per second have been reported.[1a,11] However,when frame
rates of ten frames per second or even higher are reported,one should keep
in mind that in these cases,the presented ªsnapshotsº are in fact several
averaged frames. This is done in order to reduce the considerable amount
of noise that is inherent to the high scanning speed.
Experimental Section
Synthesis of the compounds discussed in the paper: The synthesis of Hn-
ISA (Figure 1b) was reported previously.[10]
Dimethyl 5-(-hexenyloxy)isophthalate: Dimethyl 5-hydroxyisophthalate
(3.6 g,17 mmol) and potassium carbonate (2.8 g,20 mmol) were stirred in
dry N,N-dimethylformamide (100 mL) for one hour at 808C. To this
mixture,6-bromohex-1-ene (2.8 g,17 mmol) was added and the mixture
was stirred for 10 h. The solvent was then evaporated and the residue mixed
with water followed by extraction with dichloromethane (5 Â 50 mL). The
combined organic phases were dried over magnesium sulphate. The
mixture was concentrated through evaporation of dichloromethane and
purified by column chromatography on silica gel (ethyl acetate/petroleum
The feedback of the video-STM system is controlled by the Topometrix
hardware,the scanning and image acquisition is regulated by a home built
setup. Four HP 33120A function generators,set up as a phase-lock
assembly,control the scanning of the Stm tip as well as the storage of the
STM image in the image buffer of an Arlunya TF6000 image processor. In
this way,the tunneling current at each x,y-coordinate is mapped to the
correct pixel. The STM images are recorded in real-time (two frames per
second) on videotape. Individual images can be retrieved by PC.
ether 1:9) to obtain pure dimethyl 5-(-hexenyloxy)isophthalate as
a
,
colorless oil (4.02 g,13.7 mmol,81%);
1H NMR (250 MHz,CDCl
3
258C): d ˆ 8.23 (d, 4J(H,H) ˆ 1.55 Hz,1H,ArH),7.71 (d,
4J(H,H) ˆ
ˆ
1.55 Hz,2H,ArH),5.88 ± 5.72 (m,1H,CH
CH2),5.05 ± 4.93 (m,2H,
CH CH2),4.02 (t, 3J(H,H) ˆ 6.2 Hz,2H,OCH 2),3.91 (s,OCH 3,6H),
2.15 ± 2.06 (m,2H,CH 2),1.85 ± 1.74 (m,2H,CH 2),1.62 ± 1.49 (m,2H,CH 2);
13C NMR (75 MHz,CDCl 3,25 8C): d ˆ 166.19,159.16,138.35,131.68,
ˆ
The experiments were repeated in several sessions using different tips to
check for reproducibility and to avoid artifacts. Different settings for the
122.27,119.79,114.85,68.36,52.37,33.33,28.48,25.20; MS-FD:
m/z: 292.3
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Physisorbed Monolayers
3739±3746
tunneling current and the bias voltage were used,ranging from 0.3 nA to
1.3 nA and À10 mV to À1.5 V,respectively. During imaging,we detected
little drift of the STM system and care was taken that the same region of the
monolayer was imaged during consecutive scans. All STM images contain
raw data and are not subjected to any manipulation or image processing.
The presented snapshots are single frames.
Molecular mechanics calculations: The molecular mechanics calculations
have been performed on periodic systems with the Cerius2 package
developed by Molecular Simulations Inc. The potential energy is described
with the Universal Force Field (UFF),with partial atomic charges set to
zero.[12],[13] The long-range non-bonding interactions are calculated by using
the Ewald method (the parameters being estimated within the program).
The considered plane of graphite is composed of 20 Â 30 fused rings and
contains 1200 carbon atoms. The atomic positions are fixed during the
energy minimization processes. We emphasize that we have tested this
approach on small systems consisting of one CH4 or one CF4 molecule
adsorbed on graphite. The corresponding results are in agreement with
previous experimental and theoretical data obtained with post Hartree ±
Fock quantum chemical calculations.[18b]
Figure 1. Chemical structure of the compounds under investigation: (a) 5-
((w-perfluorododecyl)hexyloxy)isophthalic acid (F12H6-ISA); (b) 5-(w-alk-
oxy)isophthalic acid (Hn-ISA, n ˆ 18,20).
natant solution in the monolayer structure,is a dynamic
process. This is illustrated in Figure 2. At the left-hand side of
each picture one can see how solvent molecules are pro-
gressively incorporated between adjacent H20-ISA lamellae.
The newly formed solvent lamella interleaves the two
adjacent H20-ISA lamellae. A domain boundary at the left-
hand bottom corner is used as a reference point. The co-
deposition process was completed within two minutes. Not
surprisingly,this process is initiated at a domain boundary,
typically a region with free volume and hence increased
dynamics. In contrast to Ostwald ripening,which only
involves the reorientation of molecules and lamellar frag-
ments within the monolayer,co-deposition involves both
exchange with the supernatant solution as well as reorienta-
tion of the lamellar fragments. For instance,Figure 2 shows
that co-adsorption of octan-1-ol molecules induces a cooper-
Results and Discussion
Dynamic processes occurring at the liquid/solid interface,
between monolayers adsorbed on the substrate and the
supernatant solution,are illustrated by the systems inves-
tigated within our research group. Specifically,we report the
visualization of the exchange of molecules between mono-
layers of isophthalic acid derivatives adsorbed on HOPG and
the supernatant solution. The molecular structures of the
compounds under investigation are shown in Figure 1.
Insertion of solvent molecules
in H20-ISA monolayers: Upon
applying a drop of a solution of
H20-ISA in octan-1-ol onto a
freshly cleaved graphite surface
highly ordered monolayers,
spontaneously formed by phys-
isorption,can be observed with
STM. Figure 2a shows an STM
image of the structure of such a
monolayer. The aromatic moi-
ety of the H20-ISA molecules is
visible as a bright spot.[14] The
wide darker bands correspond
to the location of the alkoxy-
chains,which are fully interdi-
gitated,while the narrow ones
with a similar contrast reflect
the location of octan-1-ol mol-
ecules co-deposited between
adjacent H20-ISA lamellae.[15]
The short solvent molecules
can be observed on the sub-
strate since the carboxyl groups
of the ISA head groups form
hydrogen bonds with the hy-
droxyl group of the solvent
Figure 2. A series of STM pictures of the same area of a H20-ISA monolayer deposited from octan-1-ol obtained
at a) t ˆ 0 s, b) t ˆ 32 s,c) t ˆ 89 s,and d) t ˆ 120 s. The arrow indicates where the insertion of solvent molecules
between two adjacent lamellae is initiated. The wide lamellae consist of H20-ISA molecules. The narrow lamellae
are built up by solvent molecules. Image sizes are 23.0  23.0 nm2. Iset ˆ 1.0 nA, Vbias ˆ À0.8 V.
molecules. Co-deposition,in
this case the adsorption of sol-
vent molecules from the super-
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ative shift of the lamellae to the upper-left of the image. This is
expressed in the image by the appearance of a wavelike
pattern. The lamellae in the lower right of the image are
basically unaffected by the insertion process.
This local change of the monolayer structure also has an
impact on the neighboring lamella. Simultaneously with the
insertion of the solvent molecules,other undecan-1-ol mole-
cules desorb from the surface (solid arrow). Due to the extra
space taken up by the new co-adsorbed solvent molecules,the
adjacent H20-ISA lamella shifts towards the top of the image,
and thereby forces other undecan-1-ol molecules out of the
monolayer. Naturally,the distance over which the H 20-ISA
lamella is shifted corresponds to the width of a undecan-1-ol
lamella. The molecular motions during this rearrangement
were faster than the speed of image acquisition. Solvent
molecules adsorbed or desorbed between scans,thus the
adsorption and desorption process takes less than 0.5 s (the
time required to obtain one image). Based on the time
required to obtain one image and the number of scan lines
that build-up this image a rough estimate of the exchange rate
between the monolayer and the solution on top can be made.
An ISA group is seven scan lines in the presented images. The
time required to scan one line is 2.4 ms. This means that the
H20-ISA molecules have a residence time on the surface of at
least 16.8 ms. The undecan-1-ol molecules also appear stable
over several scan lines. Consecutive images usually show the
same general organization. This implies that the overall
structure of the monolayer in the observed region remains
stable for at least 0.5 s. Twenty-six seconds after the frame in
Figure 3b was obtained,the solvent lamella has progressed all
the way to the domain boundary,as shown in Figure 3c. The
actual residence time of individual molecules can not reliably
be determined. Although the overall structure appears to
remain unchanged,molecules are most probably exchanging
between consecutive images,even though it is not possible to
observe this. This makes a more detailed quantitative
discussion precarious. Therefore we moved to the study of
semi-fluorinated molecules. The specific contrast of the
fluorinated chain in the STM images serves as a label to
follow the molecular exchanges as a function of time. This will
be discussed further in the text.
A detailed quantitative analysis of this particular co-
deposition process reveals a linear relationship between the
number of octan-1-ol molecules that adsorb inbetween the
H20-ISA lamellae and the timescale of the process. It was
therefore possible to obtain the insertion rate that character-
izes the observed process,which was found to be 0.4 mole-
cules per second. Notably this rate is only applicable to this
particular example. In general,we did not find this kind of
linear relationship. For example,in the two systems described
hereafter in this section the number of solvent molecules that
build-up the solvent lamellae under consideration was found
to fluctuate.
These data were obtained with a commercial STM system
capable of collecting one STM image every seven seconds. We
took this study a step further by investigating dynamic
phenomena related to solvent co-deposition by means of a
video-STM,which enables us to acquire data in a continuous
mode (two frames per second).
Figure 3a shows an STM image of the structure of a
monolayer of H20-ISA dissolved in undecan-1-ol. The solid
arrow indicates a region where solvent molecules are incor-
porated in the H20-ISA monolayer. The dashed arrow
indicates a boundary between two H20-ISA lamellae,one
with (left) and one without (right) solvent molecules co-
deposited adjacent to it. The four ISA groups directly above
these solvent molecules are used as a reference to keep track
of the progress of the dynamic process (encircled in Fig-
ure 3a). With video-STM,we were able to observe dynamic
changes occurring at the location indicated by the dashed
arrow. Starting from this location,solvent molecules are
progressively inserted in between the H20-ISA lamellae. In
Figure 3b,which is a snapshot taken from a video tape forty-
one seconds after Figure 3a,the solvent lamella has advanced
towards the domain boundary in the top right section of the
image (indicated by the solid line). At the same time,the
isophthalic acid reference row has expanded along the growth
direction of the solvent lamella: now eight ISA groups
interacting with these solvent molecules can be distinguished.
On a few occasions,a double row of undecan-1-ol
molecules was observed. The width of such a double row is
3.1 nm,which is somewhat smaller than the width of an H
-
20
ISA lamella (3.7 nm). This explains why a double row of
undecan-1-ol molecules can adsorb in an H20-ISA adlayer
Figure 3. Snapshots taken from videotape,showing STM images of an H 20-ISA monolayer formed by physisorption from a solution in undecan-1-ol on
HOPG. The time of the first image is arbitrarily set to zero. a) Solvent molecules incorporate in between H20-ISA lamellae during imaging. The dashed arrow
indicates where the insertion process is taking place. The encircled area indicates the four isophthalic groups that serve as a reference to follow the observed
molecular motions. The solid arrow indicates where solvent molecules are consequently expelled from the monolayer (shown in b). c) The solvent lamella
extends all the way to the domain boundary (solid line). Image sizes are 15.0  15.0 nm2. Iset ˆ 0.9 nA, Vbias ˆ À0.6 V.
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Physisorbed Monolayers
3739±3746
without major disturbance of the monolayer structure. In
Figure 4a two adjacent solvent lamellae are visible in the
center of the image. Figure 4b and Figure 4c show how the
double solvent lamella breaks up. At this stage H20-ISA
molecules adsorb at the same location as the solvent
molecules,thereby replacing them. However,simultaneously
another double row of solvent molecules appears immediately
above the expanding H20-ISA lamella. At this time,we are not
able to determine if the solvent molecules stay adsorbed on
the surface and slide upwards over a distance corresponding
to the width of a H20-ISA lamella (thereby expelling the
neighboring H20-ISA molecules from the monolayer) or if the
two adjacent solvent lamellae and the neighboring H20-ISA
lamella desorb cooperatively and are replaced by molecules
from the solution on top of the monolayer. When the dynamic
process is completed,it appears as if the double lamella of
solvent molecules and the H20-ISA lamella directly above
have traded places,as shown in Figure 4d. The tip was then
translated during scanning in such a way that the rows of
solvent molecules again appear in the middle of the image
(Figure 4e). The images in Figure 4f ± h show that this less-
favorable packing is ultimately replaced by the more stable
arrangement of lamellae of the isophthalic acid derivative
interspersed by a single row of solvent molecules. Previous
reports from our research group,mentioned in the introduc-
tion,demonstrate that the latter arrangement is indeed the
most favorable for the systems considered in this paper.[15]
From these observations we find that double lamellae of
undecan-1-ol are stable within an H20-ISA monolayer on a
timescale of minutes. The time that the molecules remain on
the graphite surface was determined using the principle
explained above and was in this case at least 15 ms. The blurry
area in Figure 4f could be observed during two seconds. In this
time interval the double undecan-1-ol lamella and an H20-ISA
lamella (58 molecules) desorbed from the surface and the
structure as seen in Figure 4g was formed. The entire process
came about in 73 seconds.
Exchange of fluorinated and non-fluorinated isophthalic acid
derivatives in their mixed monolayers: The experiments in the
previous section clearly illustrate the dynamics and structural
changes that take place within monolayers at the liquid/solid
interface. However,these experiments do not provide in-
formation about the exchange or mobility of individual
molecules in domains that do not undergo major structural
changes. In view of that we introduced semi-fluorinated
isophthalic acid derivatives,which can act as markers that
allow us to study this type of dynamics.
In a previous paper we reported on the two-dimensional
organization of semi-fluorinated isophthalic acid derivatives
physisorbed on the basal plane of HOPG.[16] Upon mixing of a
fluorinated isophthalic acid derivative with its non-fluorinat-
ed analogues,mixed monolayers are formed. No separate
domains of non-fluorinated and semi-fluorinated molecules
could be observed. Individual lamellae are composed of a
random mixture of fluorinated and non-fluorinated mole-
cules. Now,this imaging process has been taken a step further
by studying dynamics in these mixed monolayers. Specifically,
monolayers physisorbed from a mixture of F12H6-ISA with
Figure 4. Snapshots taken from a video-STM movie. The time of the first
image is arbitrarily set to zero. a) A double row of undecan-1-ol molecules
can be recognized in the center of image (indicated by arrows). This
structure breaks up and re-emerges in the top of the image b) and c). d) ±
e) After the completion of this restructuring the scanning area (identical
size) was centered again around the double lamella. For clarity,an arrow
points to a reference point. f) At the end the double lamella disintegrates.
Because of the mobility of the molecules during the reorganization this
area appears distorted in the image. g) A more stable arrangement
composed of lamellae of the isophthalic acid derivative interspersed by a
single row of solvent molecules appears (indicated by arrows in h). The
domain visible in the top right of image e) and f) disappears and is replaced
by the expanding restructured domain,as can be seen in image h). Image
sizes are 16.5  16.5 nm2. Iset ˆ 0.9 nA, Vbias ˆ À0.85 V.
Chem. Eur. J. 2000, 6,No. 20
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3743

S. De Feyter,F. C. De Schryver et al.
FULL PAPER
H18-ISA (concentration ratio 1:1) in octan-1-ol have been
studied. When the same area of such a monolayer was imaged
during an extended period of time a dynamic process
occurring at the graphite surface could be observed,as shown
in the image series in Figure 5a ± f. Both semi-fluorinated and
non-fluorinated molecules can be distinguished by their
characteristic STM contrast. The perfluorinated part of the
alkyl chain appears as a black band,due to a relative decrease
in tunneling current compared with the non-fluorinated alkyl
chains. Hence,this functionality is a good marker that allows
us to study the exchange of molecules between the liquid
phase and the physisorbed monolayer quantitatively. The
observed contrast for the perfluorinated part of the alkyl
chain is in agreement with a previous study of a single fluorine
atom substituted stearic acid physisorbed on graphite,and
supported by theoretical calculations.[17,18] The bright spots
correspond to the location of the isophthalic acid groups. The
non-fluorinated part of the alkyl chain can be seen as a darker
band with respect to the isophthalic acid groups. The F12H6-
ISA molecules were found to be distributed sparsely over the
monolayer.
At first,two semi-fluorinated molecules are visible in the
STM image,shown in Figure 5a. These molecules are
indicated by the arrows. The bright edges around the black
bands of the semi-fluorinated molecules are scanning arti-
facts. These were always observed in the vicinity of a
fluorinated alkyl chain during the course of our experiments.
In Figure 5b,an additional fluorinated molecule has adsorbed
on the surface,as can be seen in the top left of the imaged
area. In Figure 5c,this semi-fluorinated molecule has des-
orbed,while the two initially imaged semi-fluorinated mole-
cules remain on the graphite surface. The residence time of
this third molecule on the surface was 8.0 s. While scanning
from the top of the image shown in Figure 5d to the bottom
only a part of the semi-fluorinated chain of the F12H6-ISA
molecule left in the image could be visualized,as can be seen
in the encircled area. The hind part of the fluorinated alkyl
chain is no longer visible. This is explained by the fact that
during the acquisition of a scanline over the F12H6-ISA
molecule,this molecule was exchanged for an H 18-ISA
molecule. Thus the exchange rate is faster than the time
needed to obtain one scanline,in this case 4.3 ms. In the
subsequent scan taken immediately after Figure 5d it is clear
that the F12H6-ISA molecule has indeed been substituted by a
non-fluorinated H18-ISA molecule.
In Figure 5f the remaining fluorinated molecule has also
desorbed from the graphite surface and has been replaced by a
non-fluorinated molecule. The entire process came about in 58 s.
In general,the residence time of a fluorinated molecule was
found to vary from several seconds to several minutes. In
some cases we even found the residence time to be tens of
minutes. These residence times are much higher than those
observed by Stevens et al. for mixtures of long-chain alcohols
and long-chain thiols.[19] This demonstrates the impact of the
hydrogen bonding between neighboring isophthalic acid
groups and between the isophthalic acid groups and the
solvent molecules in the monolayer. Because of this type of
hydrogen bonding highly stabilized two-dimensional crystals
are formed for which the exchange rate of molecules between
the monolayer and the solution is considerably reduced.
In order to reveal the relative importance of adsorbate-
substrate and adsorbate ± adsorbate interactions that lead to
Figure 5. Snapshots taken from an STM videotape. The time of the first image is arbitrarily set to zero. a) Two F12H6-ISA molecules are initially co-adsorbed
in an H18-ISA monolayer. The arrows indicate the two F12H6-ISA molecules. b) A third F12H6-ISA molecule has adsorbed in an H18-ISA lamella (indicated by
arrow). c) Eight seconds later this molecule is exchanged for an H18-ISA molecule. d) The encircled area indicates the partly imaged semi-fluorinated alkyl
chain. The F12H6-ISA molecule desorbs while the tip is scanning over it. e) The remaining F12H6-ISA molecule is replaced by an H18-ISA molecule as well,as
can be seen in f). Image sizes are 12.5  12.5 nm2. Iset ˆ 1.1 nA, Vbias ˆ À0.38 V.
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Chem. Eur. J. 2000, 6,No. 20

Physisorbed Monolayers
3739±3746
the gradual desorption of the fluorinated molecules and their
replacement by non-fluorinated ones,molecular mechanics
calculations have been performed. The relative stabilities
calculated for each kind of molecule (semi-fluorinated (F) or
non-fluorinated (H)) in terms of i) the adsorption of mole-
cules on graphite and ii) the interaction between the mole-
cules have been compared. In a first step,we have calculated
the adsorption energy of the single molecules on graphite. In a
next step,in order to obtain information on the intermolec-
ular interactions,the energy of several sets of three molecules
adsorbed on graphite has been determined. We have consid-
ered four possibilities: Two situations in which a set contains
three identical molecules (HHH or FFF) and two cases in
which a set contains both types of molecules (HFH or FHF).
Such a set reflects the orientation of three adjacent molecules
in a lamella,the alkyl chains are interdigitated and the
distance between the isophthalic acid groups is in the 9.585 ±
9.900 Š range,and allows for quantification of the intermo-
lecular interactions within a lamella. Note that this approach
does not take the hydrogen-bonding interactions between
ISA groups in adjacent lamellae into account. We assume that
these hydrogen bonds do not differ between semi-fluorinated
or non-fluorinated molecules,and as such do not contribute
to the difference in intermolecular interactions reported
below.
The total energies calculated for the systems with one
F12H6-ISA (F) or H18-ISA (H) molecule physisorbed on
graphite,hereafter noted as EF/graphite and EH/graphite,are À2.1
and À36.7 kcalmolÀ1,respectively. Thus,the interaction
between the non-fluorinated molecule and graphite is clearly
more favorable (DE ˆ 34.6 kcalmolÀ1),than that between the
semi-fluorinated compound and graphite. The difference in
stability can be explained on the basis of previous quantum-
chemical results (obtained at the ab initio level) which show
that besides induced dipole-induced dipole interactions
(present in all complexes) another phenomenon playing a
significant role in the stability of the alkyl chain/graphite
complex is the formation of weak ªhydrogen-bond-likeº
Finally,two cases of mixed lamellae of semi-fluorinated and
non-fluorinated compounds deposited on graphite have been
considered: one where a semi-fluorinated molecule is sur-
rounded by two non-fluorinated chains (HFH),the second
one corresponding to the reverse situation (FHF). The total
energies of these two systems (EHFH/graphite and EFHF/graphite) are
À109.3 kcalmolÀ1 and À77.2 kcalmolÀ1,respectively. The
intermolecular interaction energy between one semi-fluori-
nated molecule and two non-fluorinated molecules surround-
ing the semi-fluorinated one (EHFH) can be estimated with the
following Equation:
EHHH/graphite ˆ EHFH/graphite À EF/graphite ‡ EH/graphite À EHFH ‡ EHHH
(1)
In this way,we have calculated that
EHFH is about
À1
À33.8 kcalmolÀ1,that is 6.4 kcalmol
lower than the inter-
action energy between three non-fluorinated molecules
(EHHH). This difference in intermolecular interaction energy,
in combination with the relatively unfavorable adsorption
energy of a single semi-fluorinated molecule on graphite,
accounts for the gradual replacement of semi-fluorinated
molecules by non-fluorinated molecules.[20] It should be clear
that a desorption ± readsorption process also holds for non-
fluorinated molecules,which goes unnoticed except in those
rare cases where such a molecule is replaced by a semi-
fluorinated one. Based upon the previous calculations,one
might expect a longer residence time for the non-fluorinated
molecules.
One has to keep in mind that the influence of the solvent on
the energetics of adsorption and the entropic contributions
have been neglected in the present calculations.
Although the desorption sometimes occurs while the tip is
scanning over the molecule (Figure 5d),in general we find
that desorption takes place in between succesive scans
(Figure 5e ± f). Furthermore,on some occasions we could
observe that within one lamella a semi-fluorinated molecule
desorbs while a second one remains adsorbed. In combination
with the relatively long residence times,this suggests an only
minor influence of the STM tip on the observed molecular
dynamics.
À
interactions between the CH2 units and the p-system of
graphite.[18b] Moreover,the C H bond is slightly polarized as
À
the hydrogen atom points towards graphite,which is electron
rich. Obviously,no such interaction is established between the
All STM experiments described in this paper are performed
in solution. Hence,there is an equilibrium between the
molecules adsorbed on the graphite surface and those in the
supernatant solution. A mathematical model for such two-
component system has been devised by Venkataraman et al.[7]
The resulting equation that describes this system is reminis-
cent of the Langmuir model for the gas/solid interface. This
equation was further adapted by Venkataraman et al. with the
assumption that there is no interaction between the solute and
the solvent molecules. Hence,this model is not suitable to
evaluate our experimental findings,since the isophthalic acid
head groups can form hydrogen bonds with the hydroxyl
group of an alcohol molecule. Furthermore,in case of the
mixtures of fluorinated and non-fluorinated isopthalic acid
derivatives a three-component system has to be considered. In
addition,the fluorinated and non-fluorinated isopthalic acid
derivatives are interacting species since they do not assemble
on the graphite surface in separate domains.
À
À
CF2 units and graphite. Furthermore,the
CF2 units are
À
located slightly further from the carbon plane than the CH2
units (on the order of 0.4 Š for the C atoms and 0.2 Š for the
F atoms). In fact,the mean distance between the H and F
atoms and the graphite plane is on the order of 2.846 and
3.041 Š,respectively. This also points to a weaker interaction
À
of the CF2 units with the surface.
The total energies corresponding to three H molecules
(EHHH/graphite) or three F molecules (EFFF/graphite) assembled on
the carbon plane are on the order of À150.4 and
À42.8 kcalmolÀ1,respectively. We obtain,by combining these
results,that the intermolecular interaction energies between
three interdigitated H18-ISA molecules (EHHH) and between
three interdigitated F12H6-ISA molecules (EFFF) are À40.2
and À36.4 kcalmolÀ1,respectively. Note that these two values
are nearly the same,which is not surprising since the structure
of the molecules is quite similar.
Chem. Eur. J. 2000, 6,No. 20
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3745

S. De Feyter,F. C. De Schryver et al.
FULL PAPER
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Dynamics occurring in monolayers of isophthalic acid deriv-
atives at the liquid/solid interface have been investigated with
a commercial STM and with a home-built video-STM in real-
time (two frames per second). When using octan-1-ol or
undecan-1-ol as solvents for the 5-alkoxy-isophthalic acid
derivatives,these solvent molecules co-adsorb in the mono-
layer structure. In this manner a two-dimensional co-crystal is
formed. This solvent co-deposition has for the first time been
visualized in real-time (two frames per second). From a
quantitative analysis we find residence times of at least 15 ms
for the physisorbed molecules. The overall structure of the
monolayer was found to be stable for at least 0.5 s,while the
exchange rate of solvent molecules was determined to be
faster than 0.5 s. For these systems it was not possible to
determine an actual residence time of individual molecules.
This difficulty has been overcome by use of labeled molecules,
namely semi-fluorinated molecules.
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For mixtures of semi-fluorinated and non-fluorinated iso-
phthalic acid derivatives dynamic phenomena in seemingly
static monolayers which would otherwise go unnoticed could
be detected as well. The observations could be made easily
since the fluorocarbon segments serve as a probe for the
interpretation of the obtained images. While imaging the
same region of a monolayer for an extended period of time we
observed that non-fluorinated molecules progressively sub-
stitute the fluorinated molecules. From molecular mechanics
calculations we propose that this phenomenon can primarily
be attributed to the substantially smaller adsorption energy of
a semi-fluorinated molecule physisorbed on graphite com-
pared with a non-fluorinated molecule. In addition,there is
also a small difference in intermolecular interaction energy.
Furthermore,the perfluorinated part of the alkyl chain of a
semi-fluorinated molecule is located slightly further from the
graphite plane than the hydrogenated part of the alkyl chain.
The residence time of the semi-fluorinated molecules ranges
from a few seconds to several minutes. These relatively long
times illustrate the stabilizing interactions between the
molecules in the monolayer,which are to a large extent based
upon hydrogen bonding. However,the exchange process itself
is fast and takes less than 4.3 ms.
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The co-deposition processes were initiated in the vicinity of
domain boundaries,while the exchange of semi-fluorinated
molecules by non-fluorinated molecules also took place in
densely packed regions. The observed molecular motions and
rearrangements in the monolayers are attributed to the
process of adsorption and desorption of molecules on the
graphite substrate and to rotation and translation of mole-
cules on the surface.
Á
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Â
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Â
[19] F. Stevens,T. P. Beebe,Jr., Langmuir 1999, 15,6884 ± 6889.
[20] This difference in stabilization energy also accounts for the low ratio
of semi-fluorinated molecules with respect to non-fluorinated mole-
cules in the observed mixed monolayers,even though the solution
from which the monolayer is formed has a 1:1 molar ratio. Similarly,it
accounts for the preferential readsorption of non-fluorinated mole-
cules.
Acknowledgement
The authors thank the DWTC,through IUAP-IV-11,and ESF SMARTON
for financial support. A.G. thanks the IWT for a predoctoral scholarship.
S.D.F. is a postdoctoral fellow of the Fund for Scientific Research-Flanders.
R.L. is Maître de Recherches du Fonds National de la Recherche
Scientifique. The collaboration was made possible thanks to the TMR
project SISITOMAS and IUAP.
Received: September 22,1999
Revised version: February 11,2000 [F2050]
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