Evidence of strong hydration and significant tilt of amphiphilic [2]rotaxane molecules in Langmuir films studied by synchrotron X-ray reflectivity

Article abstract of DOI:10.1021/jp0448494

Surface sensitive X-ray techniques have been used to elucidate the structures of amphiphilic [2]rotaxane and dumbbell monolayers at the air/water interface. The [2]rotaxanes were found to adopt highly hydrated tilted and/or folded conformations on the wat

Full text of DOI:10.1021/jp0448494

1063
2005, 109, 1063-1066
Published on Web 12/31/2004
Evidence of Strong Hydration and Significant Tilt of Amphiphilic [2]Rotaxane Molecules in
Langmuir Films Studied by Synchrotron X-ray Reflectivity
Kasper Nørgaard,† Jan O. Jeppesen,*,‡ Bo W. Laursen,§ Jens B. Simonsen,†
Markus J. Weygand,# Kristian Kjaer,# J. Fraser Stoddart,*,§ and Thomas Bjørnholm*,†
Nano-Science Center, UniVersity of Copenhagen, UniVersitetsparken 5, DK-2100 KøbenhaVn Ø, Denmark,
Department of Chemistry, UniVersity of Southern Denmark, CampusVej 55, DK-5230 Odense M, Denmark,
Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark, and California
NanoSystems Institute and Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles,
405 Hilgard AVenue, Los Angeles, California 90095-1569, USA
ReceiVed: NoVember 10, 2004; In Final Form: December 14, 2004
Surface sensitive X-ray techniques have been used to elucidate the structures of amphiphilic [2]rotaxane and
dumbbell monolayers at the air/water interface. The [2]rotaxanes were found to adopt highly hydrated tilted
and/or folded conformations on the water surface largely due to the hydrophilic nature of their tetracationic
ring component. This conformation was less pronounced in monolayers of the dumbbell precursors. Increasing
the surface pressure resulted in an expansion of [2]rotaxane monolayers in the vertical direction and decreased
hydration.
Redox-controlled switching of bistable [2]catenanes and
[2]rotaxanes, wherein an electron deficient tetracationic cyclo-
phane moves between two electron-donating recognition sites,
has been the subject of intense investigations1 in recent times.
One of the potentially useful attributes of these mechanically
interlocked molecular switches is the finding2 that Langmuir-
Blodgett monolayers of amphiphilic variants of these bistable
molecules can act as electronically controllable memory devices
when incorporated into solid-state crossbar circuits. Experi-
mental data suggest3 that the switching behavior of these
crossbar devices, at least those employing Si/SiO2 or semicon-
ducting carbon nanotube bottom electrodes4,5 and vapor depos-
ited Ti/Al top electrodes, relies on the intrinsic electromechanical
properties of the amphiphilic bistable [2]rotaxane molecules.
Yet, the detailed (super)structure of the active molecular
monolayers present in these devices is not understood, and even
less is known about the overall superstructural response during
the device operation. This lack of detailed (super)structural
information is not a specific problem for molecular electronic
devices incorporating bistable [2]catenanes and [2]rotaxanes,
but rather a general problem6 for the type of soft organic
materials sandwiched as one-molecule thick layers between
metallic/semiconductor electrodes. Complications arise for
several reasons, which include (i) the small number of mol-
ecules, e.g., 107-103, in a typical crossbar device, (ii) the minute
areas, e.g., 108-104 nm2, occupied by the monolayers in the
devices, and (iii) the difficulties of probing through the electrode
materials. These complications provide a compelling reason for
studying more accessible model systems which may provide a
better understanding of the (super)structure and dynamics
occurring in the more intricate molecular electronic devices.
Herein, we describe our investigations of the (super)structure
of amphiphilic bistable [2]rotaxane molecules in Langmuir
monolayers, which form the basis of the crossbar devices, using
highly sensitive X-ray techniques. Synchrotron source X-ray
radiation is a useful tool7 for obtaining structural information
at the molecular scale. In the present investigation, two
complementary X-ray techniques were used to study the (super)-
structure of Langmuir monolayers of some amphiphilic bistable
[2]rotaxanes at the air/water interface employing conditions
resembling those used in the preparation2d of the crossbar
memory devices: they were (i) grazing incidence X-ray
diffraction (GIXD), which establishes the in-plane crystalline
structure, and (ii) specular X-ray reflectivity, which probes the
out-of-plane electron density profile of the Langmuir film.8
The structural formulas of the amphiphilic bistable [2]rotax-
anes9,10 14+ and 24+, which we have focused our attention on
in this investigation, are shown in Figure 1. The [2]rotaxane
14+ consists of (i) a tetracationic cyclophane component11s
cyclobis(paraquat-p-phenylene)scontaining two π-electron-
deficient bipyridinium units and (ii) a linear rodlike segment
that includes two different matching π-electron-rich recognition
sites for the cyclophane, viz., a monopyrrolotetrathiafulvalene
(MPTTF) unit and a 1,5-dioxynaphthlalene (DNP) moiety: the
rodlike segment is terminated by a hydrophilic dendritic stopper
at one end and by a hydrophobic tetraarylmethane stopper at
the other, thus rendering the molecule amphiphilic and enabling
it to form Langmuir monolayers at the air/water interface.2 In
solution, the cyclophane is located predominantly12 around the
MPTTF recognition site, it being the stronger of the two
* Corresponding author. E-mail: tb@nano.ku.dk.
† University of Copenhagen.
‡ University of Southern Denmark.
§ UCLA.
π-electron donating units in the molecule. Oxidation of 14+
with excess of oxidant, yields the hexacationic species 16+ (not
,
# Risø.
10.1021/jp0448494 CCC: $30.25 © 2005 American Chemical Society

1064 J. Phys. Chem. B, Vol. 109, No. 3, 2005
Letters
Figure 1. Graphical representation and structural formulas for the
amphiphilic bistable [2]rotaxane 14+, the amphiphilic “monostable”
[2]rotaxane 24+, and its amphiphilic dumbbell precursor 3.
shown in Figure 1), resulting from the removal of two electrons
from the MPTTF unit. This oxidation, which can be affected
either chemically of electrochemically, of the MPTTF unit is
accompanied in solution by the movement of the cyclophane
from this unit to the DNP moiety. By contrast, in the [2]rotaxane
Figure 2. Synchrotron X-ray reflectivity measurements at the air/water
interface: (a) electron density profile of the [2]rotaxane 14+, inverted
from the reflectivity data for the starting compound (solid green line):
(b) electron density profiles of the [2]rotaxane 24+ (red line) and its
dumbbell precursor 3 (black line). The areas above the horizontal dotted
lines correspond to the number of electrons in one molecule at the
given molecular area (where Π ) 20 mN/m), and the shaded section
illustrates the approximate position of the interface to the bulk water.
2
4+, the cyclophane has to be located on the DNP moiety even
in its “reduced” form, i.e., when the MPTTF unit is neutral, on
account of the presence of a blocking ethylthio group situated
between the two recognition sites, thus preventing the cyclo-
phane from moving onto the MPTTF unit. Hence, it serves as
a reference compound for the oxidized form 16+ of the
switchable [2]rotaxane. The neutral compound 3, the dumbbell
precursor to the [2]rotaxane 24+, also contains the DNP and
MPTTF recognition sites.
Chloroform solutions of the compounds were spread onto the
pure water surface of a Langmuir trough and GIXD and X-ray
reflectivity were measured at several positions along the
compression isotherm to follow the (super)structural evolution
of the resulting monolayers as a function of surface pressure
and molecular area. Chemical oxidation of 14+ was achieved
using Fe(ClO4)3 as the oxidant (see Supporting Information for
details).
Figure 3. Schematic illustration of the proposed monolayer structure
of the amphiphilic bistable [2]rotaxane 14+ at the air/water interface.
a phenomenon that inevitably increases the number of elec-
trons.14 Inspection of the resulting profile for 14+ reveals that
the monolayer height is close to 33 Å. This height is notably
smaller than the approximately 60 Å expected for an extended
molecular conformation of vertically oriented molecules on the
water surface. We suggest that this difference is a result of the
molecules being folded and/or tilted on the water surface, a
situation caused presumably by favorable interactions between
the highly charged cyclophane and its counterions with the polar
water subphase. Hydrophobic and/or π-π interactions between
the free DNP moiety and the cyclophane in either the same
molecule or in neighboring molecules, as well as between the
cyclophane and the aromatic portions of the hydrophilic stopper,
could also influence the packing of the monolayer. This situation
is illustrated graphically in an idealized manner in Figure 3.
To study the structural evolution of the rotaxane monolayers
as a function of the applied surface pressure, X-ray reflectivity
was performed at different points along the compression
isotherm. A marked increase in the monolayer thickness is
observed as the surface pressure is increased. This dependence
is illustrated in Figure 4, which shows the electron density
As expected for such intricate and flexible molecules, the
GIXD measurements did not reveal any in-plane crystallinity
in the monolayers of any of the rotaxanes studied at either of
the measured surface pressures. The electron density profiles
of selected samples, resulting from fitting13 the measured
reflectivity data (see Supporting Information), are shown in
Figure 2. Combined with the molecular areas deduced from the
Langmuir isotherms, these data give a quantitative account
(∼10% accuracy) of the total electron distribution in the
monolayer molecules normal to the water surface. The area
above the horizontal dotted line in Figure 2 corresponds to the
number of electrons in one molecule at the given surface
pressure (Π ) 20 mN/m). The shaded area represents the
approximate position of the interface to bulk water and was
chosen at the point on the curve where the electron density is
3% larger than that for bulk water. The discrepancy between
these two lines (indicated in light blue in Figure 2) is caused
by the inclusion of a substantial number of water molecules
(∼60, corresponding to 28% of the total electron density) from
the subphase into the hydrophilic parts of the [2]rotaxane 14+
,

Letters
J. Phys. Chem. B, Vol. 109, No. 3, 2005 1065
backbone conformation. Clearly, the degree of hydration of 3
is much less (ca. 15% of the total electron density) than that
for 14+. The limiting area of 3 on the water surface at a surface
pressure Π of 20 mN/m is also much smaller than that for the
[2]rotaxanes, i.e., approximately 80 Å2 for 3 as opposed to 180
Å2 for 14+ and 24+. Hence, the absence of the tetracationic
cyclophane results in a noticeably different molecular arrange-
ment and degree of hydration in Langmuir layers, suggesting a
closer packed monolayer with a less tilted/folded conformation.
In an attempt to elucidate the structural response to oxidation,
the electron density profiles for oxidized Langmuir films of 14+
,
supposedly containing the species 16+, were found to differ only
insignificantly from those obtained for the starting (unoxidized)
state 14+. This observation indicates that no net reorganization
of the electrons takes place in the vertical direction. Hence,
movement of the cyclophane, if any, would have to either (i)
occur predominantly in the plane of the film or (ii) be
accompanied by an opposite rearrangement of the water
molecules and counterions. To address these issues, experiments
employing longer and less hydrophilic rotaxanes are in progress.
In summary, we provide, by use of surface sensitive
synchrotron X-ray techniques, evidence for strongly tilted and/
or folded conformations for the rotaxane molecules in the
Langmuir films of 14+ and 24+, which have been used2d in the
preparation of molecular crossbar memory devices. The folded
conformation is presumably a result of the hydrophilic nature
of the tetracationic cyclophane and its counterions. These
observations are also supported by recent Langmuir-Blodgett
studies15 of other related amphiphilic bistable [2]rotaxanes.
Furthermore, we have established that the monolayers are highly
hydrated and show no in-plane crystalline order, and that it is
possible, based on electron density profiles, to resolve and
differentiate between (super)structures of closely related [2]ro-
taxanes (14+ vs 24+). The electron density profiles may serve
as important future references for molecular modeling of the
switching mechanism in amphiphilic rotaxanes in condensed
Langmuir films. This work is currently in progress.16
Figure 4. Electron density profiles for Langmuir films of the
amphiphlic bistable [2]rotaxane 14+ derived from synchrotron X-ray
reflectivity measurements at the air/water interface at applied surface
pressures Π ) 0.5, 4, and 20 mN/m, corresponding to mean molecular
areas of 470, 265, and 180 Å2, respectively. The shaded section
illustrates the approximate position of the interface to the bulk water.
profiles of 14+ measured at three different surface pressures, Π
) 0.5, 4, and 20 mN m-1, corresponding to mean molecular
areas of ∼470, 265, and 180 Å2. Assuming that the monolayer
is homogeneous, Figure 4 shows that the thickness of the [2]-
rotaxane monolayers increases from 22 Å, in very expanded
monolayers (Π ) 0.5 mN m-1), to over 23 Å in slightly
compressed monolayers (Π ) 4 mN m-1), and ultimately to
33 Å in moderately condensed monolayers (Π ) 20 mN m-1).
This progression suggests that this [2]rotaxane, when spread
on the water surface, initially adopts a very tilted conformation
in which the molecule is more or less lying down with large
parts exposed to the water surface. This picture is also supported
by the absence of texture in the Brewster angle micrographs of
the Langmuir film and the absence of crystallographic order,
effects that are otherwise frequently observed in Langmuir films
of more hydrophobic molecules that spontaneously form
2-dimensional islands when spread at the water surface.
In the case of the sterically encumbered [2]rotaxane 24+, a
similar monolayer thickness (ca. 31 Å) and mean molecular area
(180 Å2) were found at 20 mN/m (Figure 2, right). However,
the detailed electron density profile and, in particular, the degree
of hydration differ significantly. Hence, for 24+ where the
cyclophane is confined to the “upper” station (DNP), less
hydration of the monolayer is observed than for 14+, where the
cyclophane is predominantly on the “lower” (MPTTF) station.
From comparison of the electron density profiles of 14+ and
Acknowledgment. This research was funded by the Danish
Natural Science Research Council (SNF, projects #21-03-0317,
#21-03-0014, and #21-02-0414) in Odense, Denmark, and by
the Defense Advanced Research Projects Agency (DARPA) in
the United States. We thank HASYLAB at DESY in Hamburg
for beam time at beam line BW1 and DANSYNC for financial
support.
Supporting Information Available: The synthesis of 24+
and 3, Langmuir film details and X-ray reflectivity data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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voltages ((2 V) and incorporating semiconductors (polysilicon, for example)
as the bottom electrode. To date, all of the available experimental evidence
suggests that these devices switch as a consequence of a molecularly based
electromechanical mechanism.
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excess electrons in the monolayer, i.e., electrons not originating from the
[2]rotaxane. These can be converted to the number of water molecules by
simple division (one water molecule contains 10 electrons). Similar
calculations were performed using values of F/Fwater ) 1.02, 1.04, and 1.05
to determine the position of the interface to bulk water. These calculations
yielded the same quantitative results as F/Fwater ) 1.03 except for a small
perturbation in the number of water molecules included in the monolayer.
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