Modulating large-area self-assembly at the solid-liquid interface by pH-mediated conformational switching

Article abstract of DOI:10.1002/chem.200802566

The two-dimensional ordering of molecules adsorbed on surfaces at the solid-liquid interface that are capable to undergo large conformational changes upon the application of an external chemical stimulus was investigated. Large-area self-assembly at the solid-liquid interface was modulated using pH-mediated conformational switching. Scanning tunneling microscopy (STM) visualization was attempted for the first time to examine and visualize large conformational changes of a responsive molecular building block resulting in its altered self-assemble behavior at the solid-liquid interface. It was observed that protonation can work effectively to overcome the repulsive interaction between certain 2,6-bis (1-aryl-1,2,3-triazol-4-yl) pyridine (BTP) molecules and also in the formation of an extended conformation on a HOPG surface. The method has encouraged the efforts towards development of reversible pH triggered switches at the solid-liquid interface.

Full text of DOI:10.1002/chem.200802566

DOI: 10.1002/chem.200802566
Modulating Large-Area Self-Assembly at the Solid–Liquid Interface
by pH-Mediated Conformational Switching
Luc Piot,[a] Robert M. Meudtner,[b] Tamer El Malah,[b] Stefan Hecht,*[b] and
Paolo Samorꢀ*[a, c]
Dedicated to Professor Roeland J. M. Nolte on the occasion of his 65th birthday
Nanoscale control over mechanical movement of mole-
cules has been intensely investigated in this decade, aiming
at the development of molecular machines that can be im-
plemented in miniaturized systems based on single or few
molecules.[1] Molecular movement in such systems can be
used directly to create nanocavitands or nanotweezers that
can capture/release host molecules[2] or indirectly in order to
alter the electronic or optical properties of the systems, for
example for the creation of nanoswitches[3] or fluorescence
sensors.[4] In this context it is crucial to design molecular sys-
tems able to undergo large conformational changes as a
result of external stimuli such as light irradiation,[5] metal
complexation or change in pH. Among them, pH represents
a prototypical biological stimulus and is ideally suited for
studies performed in solution or at the solid–liquid interface.
A related approach has been already used to trigger confor-
mational switching of molecular grippers[6] that can open or
close, or in dynamic chemical devices,[7] where the change of
pH can preferentially lead to the complexation or release of
metal ions by the system, enabling the reversible contrac-
tion/extension of the molecule. However, to exploit such
molecular motions the individual building blocks have to be
organized into larger functional arrays on meso- and macro-
scopic length scales at interfaces. This can be accomplished
by making use of self-assembly, which is a reliable bottom–
up approach for the fabrication of functional surfaces.[8]
Nevertheless, in view of the development of future applica-
tions, there is a need for developing solutions to integrate
such a type of switchable system on a given solid substrate
and to achieve control of conformational switching over
large areas in order to take advantage of collective ef-
fects.[5c,e]
We describe herein the two-dimensional ordering of mole-
cules adsorbed on surfaces at the solid–liquid interface, that
are capable to undergo large conformational changes upon
the application of an external chemical stimulus, which can
be either a change of pH or the addition of metal ions. As a
model system we have chosen a 2,6-bis(1-aryl-1,2,3-triazol-4-
yl)pyridine (BTP) derivative (Scheme 1), which incorporates
a tridentate coordination site consisting of two triazole moi-
eties bridged by a central pyridine ring.[9] The BTP core is
decorated with three alkoxy side chains providing enhanced
solubility as well as improved propensity to physisorption
on highly oriented pyrolitic graphite (HOPG) at the solid–
liquid interface. We have focused on two BTP derivatives,
differing only in the length of their respective alkoxy chains
(R2 =n-C10H21 vs. n-C18H37) to modulate the strength of the
interaction with the HOPG surface. Due to favorable elec-
trostatic interactions, the “kinked” anti,anti conformation of
the BTP core dominates in solution at neutral pH, whereas
the repulsive interactions between the lone pair of the nitro-
gen atoms destabilize the alternative “extended” syn,syn
conformation.[9] This repulsive interaction can be switched
into an attractive one by the addition of acids or metal ions
to the solution, causing either protonation or metalation of
the BTP core followed by a large structural change from the
“kinked” to its corresponding “extended” conformation. In
this paper we provide the first scanning tunneling microsco-
py (STM) visualization, recorded on the single molecule
[a] Dr. L. Piot, Prof. Dr. P. Samorꢀ
ISIS/CNRS UMR 7006—Universitꢁ Louis Pasteur
8 allꢁe Gaspard Monge, 67000 Strasbourg (France)
Fax : (+33)390245161
E-mail: samori@isis-ulp.org
[b] Dipl.-Chem. R. M. Meudtner, M. Sc. T. El Malah, Prof. Dr. S. Hecht
Departement of Chemistry
Humboldt-Universitꢂt zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
Fax : (+49)3020936940
E-mail: sh@chemie.hu-berlin.de
[c] Prof. Dr. P. Samorꢀ
ISOF—Consiglio Nazionale delle Richerche
via Gobetti 101, 40129 Bologna (Italy)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802566.
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COMMUNICATION
Scheme 1. Conformational switching from the horseshoe-like “kinked” to the linear “extended” conformation of the BTP scaffold upon protonation or
metal complexation (the destabilized neutral extended conformation is shown on the left). Distances between terminal 4-C atoms are taken from single
crystal structure analyses of several BTP derivatives and complexes.[9a]
level and at the solid–liquid interface, of the conformational
changes triggered by protonation. The bistable intramolecu-
lar conformational rearrangement in our BTP derivatives,
expected to occur in the supernatant solution, was found to
induce a collective re-organization of the supramolecular
2D packing motif.
The investigated BTP derivatives were readily synthesized
from n-hexadecyloxy 2,6-diethynylisonicotinate and the two
different 2-(n-alkoxy)phenyl azides in almost quantitative
yields via click reactions.[10,11]
Figure 1a–c displays the STM image of the BTP-1 mole-
cule in a neutral pH 1–phenyloctane solution physisorbed at
the solid–liquid interface.[11] It reveals large ordered arrays
featuring a “chess-board” 2D packing motif (Figure 1a). In
the high-resolution STM image (Figure 1b) it is possible to
identify the different functionalities of the molecules. Due
to the resonant tunneling between the Fermi level of the
HOPG and the frontier orbital of the adsorbed molecule,[12]
in the STM image the conjugated moieties appear brighter
whereas the alkyl chains physisorbed on the surface appear
Figure 1. STM images of the self-assembled BTP molecules adopting a “kinked” conformation on HOPG. a)–c) Monolayer of BTP-1 molecules in their
“chess-board” motif and proposed model. Unit cell parameters (a=5.4Æ0.2 nm, b=6.8Æ0.2 nm, g=83Æ28, area per molecule = 4.6Æ0.4 nm2). d)–f)
Monolayer of BTP-2 in their “rosette” motif and proposed model. Unit-cell parameters (a=5.1Æ0.2 nm, b=5.0Æ0.2 nm, g=61Æ28, area per molecule
= 7.4Æ1.1 nm2). Tunneling parameters: a) bias voltage Ut =800 mV, average tunneling current It =8 pA. b) Ut =700 mV, It =10 pA. d),e) Ut =800 mV,
It =5 pA.
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P. Samorꢀ, S. Hecht et al.
darker. The derived packing model (Figure 1c) provides
strong evidence that all molecules immobilized on the sur-
face adopt the “kinked” conformation of the conjugated
BTP core. Hence, the thermodynamically most stable anti,-
anti conformation at neutral pH in solution is retained upon
physisorption on the HOPG surface. A detailed inspection
of Figure 1b reveals that the molecules adopt different ar-
rangements on HOPG as determined by the rotation of the
The proposed model in Figure 1 f reveals that all the alkoxy
chains are adsorbed on the surface.
In order to trigger the switching of the BTP molecules
from the “kinked” to the “extended” conformation, a small
amount of trifluoroacetic acid (0.75 vol% TFA) was added
to the solution covering the already formed monolayers on
the surface.[10,13] Figure 2 shows the resulting physisorbed
monolayer of BTP-1 and BTP-2 obtained after applying the
acidified solution. In this acidified environment the resulting
structure of BTP-1 (Figure 2a–c) displays a markedly differ-
ent 2D architecture consisting of a lamellar-like motif, which
appeared on the surface 5–10 min after the deposition of the
acidified solution. The lamellar domains extend over hun-
dreds of nanometers (see Figure S2 in Supporting Informa-
tion) and no part of the substrate surface was found to be
occupied by molecules adsorbed in the previously described
“chess-board” packing. The high resolution image (Fig-
ure 2b) reveals that the adsorbed molecules adopt an “ex-
tended” conformation, thereby unequivocally confirming
the occurrence of protonation. Rows of alkyl chains packed
parallel to each other, surrounded by rows of conjugated
cores are visible. In contrast to the “chess-board” motif, in
the lamellar motif all the BTP molecules carry two alkoxy
side-chains, which are not adsorbed on the surface. Impor-
tantly, the unit cell drastically changes upon protonation, as
highlighted by the variation in symmetry and the number of
molecules in the unit cell, that is, two instead of eight. In ad-
dition, the area occupied by a single molecule physisorbed
on HOPG in the lamellar motif is reduced from 4.6 nm2
(without TFA) to 3.1 nm2 (upon TFA addition), as a conse-
À
terminal alkoxybenzene moieties around the phenyl tria-
À
zole C N single bond. The different orientations of the al-
koxybenzene are the result of the interplay of molecule–
molecule and molecule–substrate interactions. Surprisingly,
the STM images also show that one half of the molecules
immobilized on HOPG have at least one alkoxy side-chain
not adsorbed on the surface, thus back-folded in the super-
natant solution, as previously described for other systems
exposing alkoxy side-chains.[13] The resulting rather complex
unit cell contains eight molecules.
Interestingly, when considering the rather conservative
structural change, that is, extension of the alkoxy side chains
by eight methylene units, monolayers of BTP-2 physisorbed
on HOPG from a solution in 1-phenyloctane appear mark-
edly different in the STM images (Figure 1d–f). The BTP-2
molecules adopt a “rosette” motif resembling a diamond ar-
rangement of cyclic trimers, containing three molecules per
unit cell. This nicely illustrates the decisive role that the
alkoxy chain length is playing in the packing process. Also
in this case, the BTP cores adopt a kinked conformation,
but are packed three by three resulting in the formation of
the bright 6-legs rosette-like features visible in Figure 1e.
Figure 2. STM images of the packing structure of BTP molecules in their “extended” conformation after the addition of trifluoroacetic acid. a)–c) STM
image of the lamellar monolayer of protonated BTP-1 and proposed model. Unit cell parameter (a=1.5Æ0.2 nm, b=4.1Æ0.1 nm, g=81Æ28, area per
molecule = 3.0Æ0.5 nm2). d)–f) Tetragon motif of the protonated BTP-2 and proposed model. Unit cell parameter (a=6.2Æ0.2 nm, b=8.1Æ0.2 nm,
g=70Æ18, area per molecule = 6.7Æ0.7 nm2). a) Ut =550 mV, It =5 pA. b) Ut =550 mV, It =5 pA. d),e) Ut =800 mV, It =10 pA.
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Modulating Large Area Self-Assembly
COMMUNICATION
quence of both the increased number of non-adsorbed
alkoxy side chains as compared to the “chess board” motif
and the enhanced density of packing, determined by the reg-
istry with the substrate. Interestingly, the same packing
motif has been observed in BTP-1 upon addition of one
equivalent of tetrakis(acetonitrile)copper(I) hexafluorophos-
phate to the solution, leading to the formation of CuACTHUNTGRNEUNG(BTP-
1)+ complexes (see Figure S5[11]).
Figure 2d–f shows the packing motif of molecules BTP-2
after their protonation. Again, major structural changes
took place on the surface, that is, from the “rosette” to the
“tetragon” motif. The proposed packing model (Figure 2 f),
extrapolated from high resolution STM measurements,
shows unambiguously that the BTP-2 cores adopt an “ex-
tended” conformation, in line with protonation. Despite
their different initial arrangement under neutral pH condi-
tions, both BTP derivatives undergo significant structural
transformations of the self-assembled monolayer packing
upon protonation. Given that the structure of the unit cell
notably changes for both BTP-1 and BTP-2 upon protona-
tion, it is most likely that switching takes place in the 3D su-
pernatant solution, through a desorption–readsorption pro-
cess. Significantly, virtually identical protonation-driven
transformations were observed both for ex situ and in situ
acidifications.
Figure 3. Consecutive STM images showing the structural evolution of a
monolayer of BTP-2 over 20 min after the addition of ca. 10 mL TFA.
The evolution of the shape of the domain containing the molecules in the
rosette packing is highlighted in white. a) Image taken right after the ad-
dition of TFA b) After 5 min c) After 10 min d) After 15 min. The unit-
cell found for the new packing corresponds exactly to the one of the tet-
ragon structure. Tunneling parameters: Ut =500 mV, It =10 pA.
The real-time conformational switch, and related reorgan-
ization of the self-assembled pattern, occurring at the solid–
liquid interface have been monitored in situ on the scale of
several tens of nanometers. The transformation from the
“chess-board” to the “lamellar” motif for BTP-1 based mon-
olayers was found to be too fast for being visualized in real-
time with our experimental set-up, due to hardware limita-
tions.[11] Gratifyingly, under the same experimental condi-
tions the structural reorganization of the BTP-2 monolayer
from “rosette” to “tetragon” motif could be monitored in
real-time on the timescale of several tens of minutes, taking
advantage of the slower nature of the process. The observed
slower switching kinetics are caused by the longer alkoxy
chains of BTP-2, providing an increased desorption energy
on graphite[14] and hence a partial hindrance towards reor-
ganization of the self-assembled motif. Figure 3 shows a set
of measurements where the “rosette” domains are gradually
converted into “tetragon” ones in the vicinity of a HOPG
step over a time scale of 20 min, the latter “tetragon” pack-
ing becoming the more favorable over time. The total cover-
age of the surface by the “tetragon” assembly is completed
a few hours after the deposition of the droplet of the acidi-
fied solution. The fuzzy parts on the STM images corre-
spond to areas where the considerable mobility of the mole-
cules, triggered by the protonation process, hinders high res-
olution STM imaging. Such areas may include solvent mole-
cules as well.
In summary, for the first time we have utilized STM to
visualize large conformational changes of a responsive mo-
lecular building block resulting in its dramatically altered
self-assembly behavior at the solid–liquid interface. Protona-
tion can successfully be used to overcome the repulsive in-
teraction between the adjacent N atoms present in the neu-
tral “kinked” heteroaromatic BTP molecule and result in
the formation of an “extended” conformation on a HOPG
surface. This represents the first yet crucial step towards the
development of reversible pH triggered switches at the
solid-liquid interface. Furthermore, more sophisticated,
functionalized (macro)molecules are being synthesized in
order to control the adsorption conformation of extended
foldamers on solid substrate surfaces[16] and to switch benefi-
cial physico-chemical properties of the monolayer, in partic-
ular fluorescence[4] and conductivity.[3,9]
Experimental Section
Scanning tunneling microscopy (STM) measurements at the solid–liquid
interface have been carried out both in constant height and constant cur-
rent mode using a DI Multimode microscope. The STM tips have been
mechanically cut from a Pt/Ir (80:20) wire. Samples have been prepared
by applying a droplet of solution on freshly cleaved highly oriented pyro-
lytic graphite (HOPG). The molecules were dissolved in 1-phenyloctane
with an approximate concentration of 1 mmolLÀ1. The protonation of the
molecules was performed ex situ, by addition of trifluoroacetic acid
(TFA) to the solution containing the molecules before the deposition,
and in situ, that is, when an acidified solution of 1-phenyloctane was de-
The observed molecular switch occurring in a dynamic
scenario, that is, at the solid–liquid interface, differs from
previous STM evidences of pH-mediated conformational
switching for physisorbed organic systems that were detect-
ed on dry films.[15]
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P. Samorꢀ, S. Hecht et al.
posited on top of the already formed monolayers, under the STM tip. It
is important to note that the ex situ and in situ protonation of the mole-
cules led to the same effects on the monolayers. The raw STM data have
been processed by the application of background flattening and the drift
has been corrected using the underlying graphite lattice as a reference.
The latter lattice is imaged underneath the molecules by lowering the
bias voltage to 20 mV and raising the current to 65 pA.
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Acknowledgements
This work was supported by the ERA-Chemistry project SurConFold,
the EU through the projects Marie Curie EST-SUPER (MEST-CT-2004-
008128), RTN Marie Curie RTNs PRAIRIES (MRTN-CT-2006-035810),
the ESF-SONS2-SUPRAMATES project, and the Fonds der Chemischen
Industrie. T.E.M. acknowledges the Egyptian Government for providing
a doctoral fellowship. The authors thank Wacker AG, BASF AG, Bayer
Industry Services, and Sasol Germany for generous donations of chemi-
cals.
Keywords: clickamers · molecular switches · self-assembly ·
solid–liquid interfaces · scanning probe microscopy
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Received: December 7, 2008
Published online: March 25, 2009
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