Self-assembly of periodic bicomponent wires and ribbons

Article abstract of DOI:10.1002/anie.200604083

Extended order: Co-deposition of two molecular species with complementary hydrogen-bonding motifs on a naturally patterned template surface has been shown by STM studies to promote the formation of the anticipated three hydrogen bonds per heteromolecular

Full text of DOI:10.1002/anie.200604083

Communications
DOI: 10.1002/anie.200604083
Supramolecular Chemistry
Self-Assembly of Periodic Bicomponent Wires and Ribbons**
Marta E. Caæas-Ventura, Wende Xiao, Daniel Wasserfallen, Klaus Müllen, Harald Brune,
Johannes V. Barth, and Roman Fasel*
The success of modern supramolecular chemistry is largely
based on the enhanced stability and enlarged number of
distinct heteromolecular species generated from relatively
weak, noncovalent interactions such as hydrogen bonding and
metal–ligand interactions.[1] In recent years, the transfer and
adaptation of the corresponding concepts to the formation of
two-dimensional (2D) supramolecular structures on surfaces
has been intensively explored.[2] A number of studies on the
adsorption of binary molecular mixtures on well-character-
ized single-crystal surfaces have been performed.[3] In a few
cases, the successful formation of regular heteromolecular
species through hydrogen-bonding interactions has been
reported.[4] The controlled organization of surface-supported
supramolecular nanosystems over large length scales, how-
ever, has not been achieved so far.
Ordered arrays of supramolecular wires are among the
most desirable artificially structured nanosystems, because of
their potential as elemental building blocks in future device
applications. The growth of one-dimensional (1D) adsorbed
supramolecular structures has been achieved, albeit with
chain lengths restricted to the size of atomic terraces.[5] Long-
range ordered 1D supramolecular structures require a
template surface where steps induce order instead of destroy-
ing it. The fabrication of a regular superlattice of specific 1D
surface-supported supramolecular wires based upon hydro-
gen-bonding interactions thus relies on a rational choice of
both 1) the complementary building blocks for 1D multitopic
hydrogen-bonding interactions, and 2) an appropriate tem-
plate surface to guide the long-range growth of the supra-
molecular structures.
Herein we present ultrahigh vacuum (UHV) scanning
tunneling microscopy (STM) investigations at 40 K (unless
otherwise stated) of bimolecular wires and ribbons that form
along the equidistant and parallel steps of an Au vicinal
surface. Internal as well as long-range order of the binary
wires and ribbons are reported. We use molecules exhibiting
complementary end-group functionalities designed to form
three hydrogen bonds. Deposition of both molecular species
on Au(11,12,12) is shown to promote the anticipated for-
mation of three hydrogen bonds per heteromolecular pair, in
contrast to the two hydrogen bonds observed for single-
component deposits. Depending on coverage, the bicompo-
nent system gives rise to a regular superlattice of 1D
heteromolecular wires consisting of one or two molecular
rows, as well as 2D supramolecular ribbons.
The chosen molecular species are 1,4-bis-(2,4-diamino-
1,3,5,-triazine)-benzene (BDATB; Figure 1a) and 3,4,9,10-
perylenetetracarboxylic diimide (PTCDI; Figure 1d). PTCDI
exhibits a -CO-NH-CO- (imide) sequence on the two
opposite sides with a NH hydrogen-bond donor (D) and
two CO hydrogen-bond acceptors (A), thus giving rise to the
well-known A-D-A sequence.[1f,6] BDATB was synthesized
(see the Supporting Information) to provide interaction
selectivity by means of complementary end-group function-
alities (NH-N-NH) on the opposite sides of the molecules,
which correspond to a D-A-D hydrogen-bonding sequence.
The high suitability of the Au(11,12,12) surface as a template
for the growth of long-range-ordered molecular structures is
derived from its large extended double periodicity: 5.8-nm-
wide Au(111) terraces separated by monoatomic steps as well
as periodic truncated “V”-shape discommensuration lines
which indicate the border between face-centered cubic (fcc)
and hexagonal close-packed (hcp) stacking domains (see the
Supporting Information).[7] The low reactivity of Au(111) is
well suited for the formation of organic molecular nano-
structures.[8] Deposited molecules are generally highly mobile
at room temperature, and at higher coverages form islands in
which the order is predominantly determined by intermolec-
ular interactions. Furthermore, reconstruction patterns and
low-coordinated Au atoms at step edges represent preferen-
tial nucleation sites for the growth of supramolecular
structures.[5a,9]
[*] M. E. Caæas-Ventura, Dr. W. Xiao, Dr. R. Fasel
nanotech@surfaces Laboratory
Empa, Swiss Federal Lab. for Materials Testing and Research
Feuerwerkerstrasse 39, 3602 Thun (Switzerland)
Fax: (+41)33-228-6465
E-mail: roman.fasel@empa.ch
M. E. Caæas-Ventura, Prof. Dr. H. Brune
Institut de Physique des Nanostructures
Ecole Polytechnique FØdØrale de Lausanne
Station 3, 1015 Lausanne (Switzerland)
Dr. D. Wasserfallen, Prof. Dr. K. Müllen
Max-Planck-Institut für Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
Prof. Dr. J. V. Barth
Departments of Chemistry and Physics and Astronomy
University of British Columbia
Vancouver, BC V6T1Z4 (Canada)
[**] We gratefully acknowledge the financial support from the European
Commission (NMP3-CT-2004-001561 RADSAS). We thank P. Ruf-
fieux, A. Arnau, and E. Ortega for stimulating discussions and for
critically reading the manuscript.
In a first step, we investigated the homomolecular
supramolecular structures resulting from the self-assembly
of each molecular species deposited as a unique adsorbate on
the Au(11,12,12) template surface (Figure 1). At coverages of
approximately 0.5 monolayer (ML), BDATB and PTCDI
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
deviation from the nominal terrace
width in some regions. This effect
might be related to the propensity
ꢁ
of gold to act as an acceptor in N
H···Auꢁ hydrogen bonds.[11]
Deposition of PTCDI leads to
domains oriented along the steps
which can extend over several
terraces (Figure 1e). As indicated
in Figure 1 f, the long axis of each
PTCDI molecule is canted by
ꢀ (11 ꢀ 2)8 with respect to the
high-symmetry direction. Both ori-
entations can alternate from row to
row and seem to adapt such that
there is always space for an integer
number of PTCDI rows. The aver-
aged intermolecular distance along
the rows is (14.2 ꢀ 0.2) Š. Pairs of
ꢁ
N H···O hydrogen bonds (length
of (2.7 ꢀ 0.2) Š) are formed at both
ends of the PTCDI molecules.
Relatively weak hydrogen-bonding
interactions between adjacent
ꢁ
PTCDI rows is suggested (C
H···O distance of (3.0 ꢀ 0.2) Š).
We believe that the molecular
canting results from intermolecular
forces rather than the substrate,
since similar arrangements have
been found for the same molecule
adsorbed on other surfaces.[12] The
main difference with our system is
given by the anisotropy of the
surface which arises from the pres-
ence of a regular array of mono-
atomic steps. The close agreement
between the PTCDI rows observed
on this surface and the row motif
observed in crystalline PTCDI fur-
ther confirms the importance of
intermolecular hydrogen-bonding
interactions in surface ordering.
Figure 2 shows STM images of
Figure 1. Single-component superlattice structures. a) Chemical structure of BDATB. b) STM image of
0.5 ML BDATB adsorbed at RT on Au(11,12,12) showing several domains with different high-symmetry
directions (white arrows) a and b. Mirror domain boundaries (dashed lines) and the high-symmetry
directions of the underlying template surface are indicated (bias voltage 0.7 V, tunneling current
!
!
0.1 nA, scanned at RT). c) High-resolution STM image. Unit-cell vectors: a and b . A proposed
model for the hydrogen bonds stabilizing the structure is superposed (1.55 V, 0.025 nA). d) Chemical
structure of PTCDI. e) STM image of a part of a PTCDI island covering several Au(11,12,12) terraces
(ꢁ0.5 V, 0.04 nA). f) High-resolution STM image. The brighter molecular rows on the right-hand side
lie on the upper terrace. Both canted configurations (ꢀ118) are defined with respect to the direction
of the step edge (0.06 V, 0.09 nA).
a highly ordered heteromolecular
both self-assemble into extended domains. Two different
orientational domains (a and b) are identified for BDATB,
with left (L) and right (R) staggering of the molecules along
the high-symmetry direction. The long molecular axes of
BDATB are canted by ꢀ (10 ꢀ 2)8 with respect to the high-
symmetry directions. By averaging the intermolecular dis-
tances determined from several STM images, the unit-cell
superlattice resulting from the deposition of PTCDI (50%)
and BDATB (50%) slightly below 1 ML. The deposition
sequence of the two components is unimportant for the
formation of the mixed network—both molecules are mobile
enough to intermix homogeneously whenever the stoichiom-
etry is close enough to 1:1. Annealing further enhances the
molecular mobility and increases the length scale on which
the order of the mixed layer prevails; the optimum annealing
temperature is 390 K, for which an average of only one
stacking fault per 10000 nm2 is found. The desorption
temperature of approximately 490 K for BDATB (> 520 K
for PTCDI) defines the temperature stability limit of the
system. In Figure 2, the brighter spots are the fingerprints of
individual PTCDI molecules, which alternate with dimmer
!
!
!
vectors a and b are found to be j a j = (14.5 ꢀ 0.2) Š and
!
j b j = (10.1 ꢀ 0.2) Š, with an angle of (50 ꢀ 2)8 in between. A
twofold hydrogen-bonding pattern along the high-symmetry
ꢁ
direction is suggested (N H···N distance of (3.1 ꢀ 0.2) Š)
together with four additional hydrogen bonds across the rows
(N H···N distance of (3.3 ꢀ 0.2) Š).[10] Deposition of BDATB
ꢁ
results in a rearrangement of the step edges, which leads to a
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Communications
also offers a greater level of coordination between molecules
and substrate atoms. Interestingly, the lateral interaction
between molecules (in the direction perpendicular to the step
edges) is also rather strong, and even extends across the step
edges: It can frequently be observed that a stacking fault (for
example, two PTCDI molecules in a row) does not only
propagate across the entire terrace, but propagates to many
neighboring terraces across several step edges (see Figure 2
and the Supporting Information).
Deposition of small amounts (around 0.3 ML) of PTCDI
and BDATB in a 1:1 stoichiometry results in the formation of
the anticipated 1D bimolecular wires. Figure 3a clearly shows
alternating brighter and darker protrusions along the steps
which correspond to PTCDI and BDATB, respectively.
Electronic effects, such as the local density-of-states modu-
lation at steps or between fcc and hcp stacking areas, can
generate preferential adsorption sites.[5g,9c,e] In the present
Figure 2. BDATB and PTCDI bicomponent supramolecular organiza-
tion on Au(11,12,12) at 390 K. STM image of seven terraces fully
covered with the heteromolecular superlattice. One terrace (middle)
shows only two molecular rows. The small arrows indicate a stacking
fault propagating over several terraces (ꢁ0.9 V, 0.034 nA). Inset: High-
resolution STM image (ꢁ0.4 V, 0.084 nA) and corresponding structural
model of the binary supramolecular lattice with three hydrogen bonds
between the heteromolecular pairs.
spots of the BDATB molecules. The alternating PTCDI-
BDATB sequence along both directions of the 2D ribbons can
be described by a rectangular lattice, with (nonprimitive)
!
!
unit-cell vectors a ’ and b ’ as shown in the inset. The lengths
!
of these vectors are found to be j a ’ j = (28.1 ꢀ 0.2) Š and
!
j b ’ j = (15.2 ꢀ 0.1) Š, which correspond to hydrogen-bonding
ꢁ
ꢁ
distances of (2.8 ꢀ 0.2) Š for the N H···O and N···H N
bonds, which are similar to hydrogen-bond lengths reported
in other studies.[1c,13] Only a single terrace near the middle of
the image is not completely covered by molecules in Figure 2.
A closer inspection reveals that it contains exactly two rows of
alternating PTCDI and BDATB molecules. STM images show
that terraces are either covered by one or two rows depending
on the coverage, starting from the lower part of the step edges,
or they are fully covered at higher coverages. Since both
molecular species are geometrically symmetric and exhibit
complementary end-group functionalities, a comparison with
the single-component adsorption behavior leads us to attrib-
ute the directionality of the bicomponent system to the
anticipated formation of the threefold hydrogen-bonding
pattern. A tentative model is superimposed in the inset.
In addition to the threefold hydrogen-bonding pattern,
the regular array of steps on the Au(11,12,12) template
surface plays an important role in imprinting the unidirec-
tionality of the bicomponent supramolecular structure, which
is not observed on a Au(111) surface (see the Supporting
Information). Au atoms at the lower step edges exhibit an
enhanced negative charge density, which is thought to be
responsible for their different (often enhanced) chemical
reactivity.[14] Furthermore, adsorption at the lower step edge
Figure 3. Double- and single-row heteromolecular wires running along
the lower step edges of the Au(11,12,12) template surface. a) Overview
STM image (ꢁ1.7 V, 0.2 nA). Inset: STM image of a defect-free area.
Brighter spots correspond to PTCDI molecules, darker ones to BDATB
(ꢁ1.1 V, 0.05 nA). b) Well-resolved individual molecules within the
double-row wires (ꢁ2.1 V, 0.13 nA). c) Model of the threefold hydro-
gen-bonding pattern suggested to promote the directionality of the
structure. d) Close-up STM image of a well-ordered single-row wire
exhibiting a perfect 1:1 stoichiometry, obtained after deposition of
0.15 ML of PTCDI and BDATB (ꢁ0.9 V, 0.13 nA).
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Angewandte
Chemie
heteromolecular system, however, intermolecular interac-
tions appear to overrule the influence of the substrate
reconstruction, since there is no apparent preference for
either PTCDI or BDATB locating at fcc or hcp areas. This
finding can be explained by the significant energy penalty of a
gap in the hydrogen-bonding sequence (> 2 ” 0.37 eV, see the
Supporting Information) compared to the small electronic
potential energy difference (ca. 0.092 eV[15]) between fcc and
hcp regions. Figure 3b highlights the intramolecular features
already detectable on the larger scale images. The corre-
sponding model in Figure 3c clarifies the structure that is
stabilized by the targeted triple hydrogen-bonding pattern
along the wire axis.
A further lowering of the surface coverage to 0.15 ML
leads to single-row bicomponent wires decorating the step
edges (Figure 3d). In these single-row wires, the alternating
order of PTCDI and BDATB along the wire axis is
significantly lower than for the double-row wires; the average
defect-free wire length is on the order of 10 nm for single-row
wires and 30 nm for double-row wires (see the Supporting
Information). Single-row wires are also significantly less
straight than double-row wires because of the increased
presence of homomolecular pairs, which give rise to the
previously described canted structures. Therefore, we con-
clude that the presence of a second row stabilizes the ordering
within the molecular wires, possibly as a consequence of some
degree of cooperativity in the hydrogen-bonding interac-
tions.[1f,16] This observation reinforces the idea that lateral
adsorbate–adsorbate interactions play a non-negligible role.
Nevertheless, there are still areas where the periodic order is
preserved also for single-row bicomponent wires (Figure 3d).
To get a further understanding of the experimental
observations, we performed molecular mechanics calculations
at the AMBER level of theory for homo- and heteromolec-
ular pairs (see the Supporting Information).[17] The exper-
imentally determined canted configurations are confirmed to
be the most stable ones for homomolecular pairs, and the
calculations rationalize the observed preference for hetero-
versus homomolecular formation which underlies the forma-
tion of the bicomponent wire: The three hydrogen bonds
between PTCDI and BDATB are significantly stronger than
the two hydrogen bonds between homomolecular BDATB
and PTCDI pairs.
Keywords: adsorption · hydrogen bonds · nanostructures ·
scanning probe microscopy · self-assembly
.
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