Mounting freestanding molecular functions onto surfaces: The platform approach

Full text of DOI:10.1021/ja807923f

Published on Web 12/29/2008
Mounting Freestanding Molecular Functions onto Surfaces: The Platform
Approach
Belinda Baisch,† Diego Raffa,† Ulrich Jung,† Olaf M. Magnussen,*,† Cyril Nicolas,‡ Jerome Lacour,‡
Jens Kubitschke,§ and Rainer Herges*,§
Institute of Experimental and Applied Physics, Christian-Albrechts-UniVersity Kiel, 24098 Kiel, Germany,
Department of Organic Chemistry, UniVersity of GeneVa, 1211 GeneVa, Switzerland, and Otto-Diels-Institute of
Organic Chemistry, Christian-Albrechts-UniVersity Kiel, 24098 Kiel, Germany
Received October 7, 2008; E-mail: rherges@oc.uni-kiel.de; magnussen@physik.uni-kiel.de
Attaching reversibly switchable molecular functions, such as
molecules that undergo photo-, redox-, or electric field-induced
isomerization, to surfaces is a topic of major interest for the preparation
of advanced nanosystems.1-3 A key requirement for preserving these
functions in an adsorbed monolayer film is sufficient free volume
availability for the functional group as well as an adsorption geometry
that avoids direct contact of this group with the surface. In particular
on metal substrates the latter can result in a strong degradation or even
complete quenching of the desired function.4 The most commonly
employed strategy to overcome these problems is incorporation of the
functional entities into a matrix of nonfunctional spacer molecules,
such as alkane thiols.1 However, in such mixed adlayers the distribution
and adsorption geometry of the functional groups on the surface is
ill-defined and a high, temporally stable density of functional groups
is difficult to obtain due to 2D phase separation.5 More recently,
alternative approaches have been suggested, such as increasing the
free volume of molecules in the self-assembled monolayer by integrated
bulky spacer groups6,7 or employing large tripod-shaped molecules,8
resulting in clearly improved switching properties. Nevertheless, also
in these cases structurally well-defined, ordered adlayers, as desirable
for the preparation of sophisticated nanosystems, are still difficult to
obtain.8i
To this end, we developed a molecular construction set by using
triazatriangulenium (TATA) ions as the platform, ethinyl or phenyl
units as the spacer, and a simple polar C-C bond formation to attach
the functional units R′ to the platform (Figure 2). TATA platforms
can be easily prepared with various side chains R of different steric
demand, which determine the size of the platform.9,10 As shown in
the following, both the TATA platform and the functionalized
platforms form close-packed chemisorbed layers on Au(111) surfaces
upon self-assembly from solution.
Here we introduce a novel concept where molecules are attached
to metal surfaces via customizable molecular platforms with a defined
adsorption geometry. These platforms act as pedestals that enforce a
controlled orientation of the functional groups relative to the surface
and determine by their size the spacing between these groups, resulting
in arrays of free-standing functional molecules (Figure 1).
Figure 2. Attachment of functional groups to the TATA platform.
Characteristic scanning tunneling microscopy (STM) images of 1
(Figure 3a and b) reveal that these novel molecular platforms form
hexagonally ordered adlayers with a defined in-plane orientation
relative to the Au substrate lattice and typical domain sizes of several
ten of nanometers. This is in accordance with a planar adsorption
geometry where the molecules bind, similar to the case in porphyrin
or phthalocyanine monolayers,11 via interactions between the π-system
and the metal substrate. Based on the unit cell parameters of the
molecular adlayer and on high-resolution images, which reveal the
triangular shape of the TATA moiety (see, e.g., insets in Figure 3), an
in-plane arrangement as shown in Figure 3c is proposed. Here the
molecules form a commensurate (ꢀ13 × ꢀ13)R13.9° superstructure
(unit cell indicated in Figure 3c) on the Au(111) substrate with all
molecules being oriented in the same direction. Although the molecular
orientation cannot be unambiguously identified, the tips of the TATA
triangles seem to point preferentially toward the centers of the sides
of three neighboring molecules in the high-resolution images. The
position of the propyl chains is currently unclear. However, the space
requirements of the molecules in this tightly packed structure cannot
be fulfilled by complete planar adsorption of these chains on the Au
Figure 1. Schematic view of the platform approach. The size of the platform
determines the distance between the functional molecules, the length of the
spacer defines the distance from the surface, and the reactive center allows a
“click type” attachment.
† Institute of Experimental and Applied Physics, Christian-Albrechts-University
Kiel.
‡ University of Geneva.
§ Otto-Diels-Institute of Organic Chemistry, Christian-Albrechts-University Kiel.
9
442 J. AM. CHEM. SOC. 2009, 131, 442–443
10.1021/ja807923f CCC: $40.75
2009 American Chemical Society

C O M M U N I C A T I O N S
surface, suggesting that the latter extend at least partially into the
adjacent solution.
species, i.e., a surface-parallel adsorption of the TATA platform with
the functional group pointing away from the surface. Further support
for this comes from spectroscopic data and photoelectrochemical
measurements, which reveal for 2aδ reversible electrochemical and
light-induced switching of the azobenzene unit.13 The resulting
molecular adlayers consist of a forest of perpendicularly oriented phenyl
or azobenzene units, respectively, with a uniform nearest neighbor
spacing that is clearly larger than that found in analogous thiol bound
self-assembled monolayers and obviously directly controlled by the
space requirement of the molecular platform rather than by packing
of the functional groups.
Adlayers of the bare (type 1) as well as functionalized (type 2)
TATA molecules on Au(111) were found to be stable for at least
several hours in air, solvents, and aqueous solutions of various pH’s,
supporting a strong adsorption. This was also supported by electro-
chemical studies in 0.1 M H2SO4 solutions using cyclic voltammetry
With increasing side chain length from propyl (1a) to octyl (1c)
the intermolecular spacing increases from 1.10 ( 0.05 (Figure 3a) to
1.30 ( 0.05 nm (Figure 3b), which corresponds to an ∼40% change
in the area per molecule. Although this change is somewhat smaller
than that expected for direct adsorption of the additional hydrocarbon
groups on the Au surface, an effect that again is attributed to partial
extension of side chains into the solution, it demonstrates clearly that
the surface density of the platforms can be controlled by the molecular
architecture. Even larger intermolecular distances should be achievable
by stiffer aromatic side chains.
Adlayer structures with a very similar in-plane molecular arrange-
ment are found for the functionalized, nonplanar species of type 2,
which carry groups that are perpendicularly oriented with respect to
the platform.12 This is illustrated in Figure 3d-f for molecules bearing
phenyl (2aγ) and azobenzene (2aδ) substituents, respectively, fixed
to the platforms by ethinyl spacers. Obviously, these adlayers also are
hexagonally ordered, although the domain sizes are noticeably smaller.
Furthermore, the intermolecular distances of 1.14 ( 0.05 nm for propyl
(2aγ, Figure 3d,f) and 1.35 ( 0.05 nm for octyl (2cγ, Figure 3e) side
chains, respectively, are almost identical to those found in adlayers of
the corresponding bare TATA molecules 1a and 1c. These STM data
strongly suggest a similar adsorption geometry as that for the type 1
and in situ STM, where identical adlayers were observed over a wide
13
potential range from the onset of hydrogen evolution up to ∼0.7 VSCE
.
In conclusion, we have proposed a novel, highly versatile concept
for the preparation of well-defined functional adsorbate layers on metal
surfaces using triazatriangulenium cations as basic building blocks.
Contrary to previous approaches, the requirements for maintaining
chemical functionality in terms of free volume and orientation are not
achieved by packing constrains within the self-assembled monolayer,
but by a broad molecular stand that allows (i) fixing of the functional
group at a given angle to the surface as well as (ii) precise tuning of
the intermolecular distances between the adsorbates, allowing prepara-
tion of adlayers with an optimized surface density of the functional
units. Due to its inherent great degree of control and flexibility, our
approach appears to be a promising, novel route toward functional
molecular nanostructures.
Acknowledgment. We gratefully acknowledge financial support
by the Deutsche Forschungsgemeinschaft via SFB 677.
Supporting Information Available: General preparation procedure
for the functionalization of the TATA platforms, and the preparation and
STM analysis of the monolayers. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) Shipway, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421.
(2) Balzani, V.; Credi, A.; Venturi, M. ChemPhysChem 2008, 9, 202.
(3) Kondo, T.; Uosaki, K. J. Photochem. Photobiol. C 2007, 8, 1.
(4) Morin, J.-F.; Shirai, Y.; Tour, J. M. Org. Lett. 2006, 8, 1713.
(5) (a) Tamada, K.; Akiyama, H.; Wei, T. X.; Kim, S.-A. Langmuir 2003, 19,
2306. (b) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992,
8, 1330.
(6) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.; Choi, I. S.;
Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371.
(7) Ito, M.; Wei, T. X.; Chen, P. -L.; Akiyama, H.; Matsumoto, M.; Tamada,
K.; Yamamoto, Y. J. Mater. Chem. 2005, 15, 478.
(8) (a) Li, Q.; Rukavishnikov, A. V.; Petukhov, P. A.; Zaikova, T. O.; Jin, C.;
Keana, J. F. W. J. Org. Chem. 2003, 68, 4862. (b) Kittredge, K. W.; Minton,
M. A.; Fox, M.-A.; Whitesell, J. K. HelV. Chim. Acta 2002, 85, 788. (c)
Kitagawa, T.; Idomoto, Y.; Matsubara, H.; Hobara, D.; Kakiuchi, T.;
Okazaki, T.; Komatsu, K. J. Org. Lett. 2006, 71, 1362. (d) Katano, S.;
Kim, Y.; Matsubara, H.; Kitagawa, T.; Kawai, M. J. Am. Chem. Soc. 2007,
129, 2511. (e) Jian, H.; Tour, J. M. J. Org. Chem. 2003, 68, 5091. (f) Zhu,
L.; Tang, H.; Harima, Y.; Yamashita, K.; Aso, Y.; Otsubo, T. J. Mater.
Chem. 2002, 12, 2250. (g) Sakata, T.; Maruyama, S.; Ueda, A.; Otsuka,
H.; Miyahara, Y. Langmuir 2007, 23, 2270. (h) Takamatsu, D.; Yamakoshi,
Y.; Fukui, K. J. Phys. Chem. B 2006, 110, 1968. (i) Wei, L.; Tiznado, H.;
Liu, G.; Padmaja, K.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Phys. Chem.
B 2005, 109, 23963.
(9) Laursen, B. W.; Krebs, F. C. Chem.sEur. J. 2001, 7, 1773.
(10) Nicolas, C.; Lacour, J. Org. Lett. 2006, 8, 4343.
(11) De Feyter, S.; De Schryver, F. Top. Curr. Chem. 2005, 258, 205.
(12) Lofthagen, M.; Chadha, R.; Siegel, J. S. J. Am. Chem. Soc. 1991, 113,
8785. Mobian, P.; Nicolas, C.; Francotte, E.; Bu¨rgi, T.; Lacour, J. J. Am.
Chem. Soc. 2008, 130, 6507.
Figure 3. (a,b,d-f) STM images (30 × 30 nm2, inset 7.5 × 7.5 nm2) of adlayers
of TATA platforms (a) 1a (R ) n-Pr) and (b) 1c (R ) n-Oct) on Au(111). (c)
Structural model for adlayers of 1a. (d) 2aγ (R ) n-Pr, R′ ) phenyl ethinyl)
and (e) 2cγ (R ) n-Oct, R′ ) phenyl ethinyl) as well as of (f) the azobenzene-
functionalized TATA platform 2aδ on Au(111).
(13) Jung, U. Dissertation, University of Kiel, 2008.
JA807923F
9
J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009 443