Surface-assisted assembly of discrete porphyrin-based cyclic supramolecules

Article abstract of DOI:10.1021/nl9029994

We employed de novo synthesized porphyrin modules to construct discrete cyclic supramolecular architectures supported on a copper surface. The programmed geometry and functionality of the molecular modules together with their conformational flexibility an

Full text of DOI:10.1021/nl9029994

pubs.acs.org/NanoLett
Surface-Assisted Assembly of Discrete
Porphyrin-Based Cyclic Supramolecules
Daniel Heim,† Knud Seufert,† Willi Auwa¨rter,*,† Claudia Aurisicchio,‡ Chiara Fabbro,§
Davide Bonifazi,*,‡,§ and Johannes V. Barth†
† Physik Department E20, Technische Universita¨t Mu¨nchen, D-85748 Garching, Germany, ‡ Department of
Chemistry, University of Namur, Rue de Bruxelles 61, B-5000, Namur, Belgium, and § Department of
Pharmaceutical Sciences, University of Trieste, Piazzale Europa 1, I-34127, Trieste, Italy
ABSTRACT We employed de novo synthesized porphyrin modules to construct discrete cyclic supramolecular architectures supported
on a copper surface. The programmed geometry and functionality of the molecular modules together with their conformational
flexibility and substrate interaction yields symmetric discrete assemblies, including dimers and chains as well as three- to six-membered
cyclic structures. The area of the molecular cavities is extended by creating bicomponent structures combining building blocks with
different symmetry.
KEYWORDS Porphyrin, supramolecules, scanning tunneling microscopy, metal-organic coordination, copper surface
upramolecular engineering on surfaces provides ac-
cess to a multitude of nanoscale architectures, includ-
ing clusters of distinct symmetry and size,1,2 one-
5,10 (cis-like) and 5,15 (trans-like) positions, respectively
(Chart 1). The synthesis of pyridyl-substituted porphyrins
1-3 is outlined in Scheme 1. Mixed condensation25-27 of
pyrroles with the appropriate aromatic aldehydes and selec-
tive meso-functionalization of preformed25,27 disubstituted
porphyrin scaffolds are, by far, the most used synthetic
approaches toward the synthesis of “asymmetrically”28-32
meso-substituted porphyrins. Due to the easy separation of
our porphyrin derivatives, in this work we opted for the
mixed condensation route. Therefore, precursor 4, bearing
a free terminal alkyne has been obtained via statistical
macrocyclization of pyrrole in the presence of aldehydes 5
and 6 in a 3:1 ratio (BF3, CHCl3) followed by oxidation (DDQ)
and removal (K2CO3, MeOH) of the TMS protecting group.33
Monosubstituted derivative 1 was then prepared in good
yield following a Sonogashira-type cross-coupling reaction34
with 4-iodopyridine in the presence of [PdCl2(PPh3)2] and CuI
in (i-Pr)2NH (Scheme 1, path c). In parallel, the synthesis of
the two dipyridyl-porphyrin modules 2 and 3 bearing two
pyridyl groups at the 5,10 (cis-like) and 5,15 (trans-like)
positions, respectively, has been carried out. In this case,
following a modified procedure as that described in the
literature,35 the statistical condensation (BF3, CHCl3) between
5 and 6 in a 1:1 ratio with 4 equivalents of pyrrole, gave rise
to an isomeric mixture of disubstituted porphyrins. Metala-
tion (Zn(OAc)2, MeOH) and removal (K2CO3, MeOH) of the
TMS protecting group afforded an isomeric mixture contain-
ing porphyrin derivatives 7 and 8, which were isolated by
extensive chromatography (Scheme 1, path f). Subsequently,
terminal bisacetylene poprhyrins 7 and 8 reacted separately
with 4-iodopyridine under Sonogashira-type reaction condi-
tions34 leading to targeted molecules 2 and 3 (Scheme 1,
paths g and h). Detailed information on the synthetic
S
dimensionalgratings,3orporoustwo-dimensionalnetworks.4-9
A variety of noncovalent interactions and bonding motifs has
been explored,10-12 but to date pathways to realize discrete
surface-confined cyclic structures are lacking, albeit regular
arrays of metallacycles have been obtained by segregation of
prefabricated entities at solid-liquid interfaces.13-16 A particu-
lar versatile class of molecular building blocks is provided by
porphyrins, where meso-substituted derivatives have been
employed to steer the formation of specific assemblies in
solution,17-19 bulk materials,20 and on surfaces.2,21-23 Here
we make use of three ingredients for the surface-assisted
assembly of supramolecular cyclic architectures with
distinct shape and size: (i) the geometry and chemical
properties of the molecular building blocks providing
functional end groups to promote directional intermo-
lecular interactions via substrate-mediated metal-ligand
bonding, (ii) their conformational flexibility24 bestowing
a discrete set of bonding motifs, and (iii) molecule-surface
interactions to guide the azimuthal orientation of the
supramolecules.
Synthesis of the Mono- and Disubstituted Pyridylporphy-
rin Derivatives 1-3. The self-assembling molecular modules
studied in this work are based on a tetrapyrrolic core
differently substituted at the four meso-positions: porphyrin
module 1 features three 3,5-di-tert-butyl (t-Bu2Ph) substitu-
ents and one pyridyl function (Chart 1), while porphyrins 2
and 3 bear two t-Bu2Ph and two pyridyl substitutents at the
*Authors to whom correspondence should be addressed: davide.bonifazi@
fundp.ac.be and wilhelm.auwaerter@ph.tum.de.
Received for review: 09/10/2009
Published on Web: 11/04/2009
© 2010 American Chemical Society
122
DOI: 10.1021/nl9029994 | Nano Lett. 2010, 10, 122-128

CHART 1. Schematic Structures of the Investigated Porphyrin Compounds
SCHEME 1. Synthesis of Mono- and Disubstituted Pyridylporphyrins 1-3a
a Reagents and conditions: a, 5 (3 equiv), 6 (1 equiv), BF3·Et2O, DDQ, CHCl3, rt, 2 h; b, K2CO3, MeOH/THF, rt, 2 h; c, 4-iodopyridine, [PdCl2(PPh3)2],
CuI, (i-Pr)2NH, rt, 16 h, 50%; d, 5 (2 equiv), 6 (2 equiv), BF3·Et2O, DDQ, CHCl3, rt, 2 h; e, K2CO3, MeOH/THF, rt, 2 h; f, Zn(OAc)2, CHCl3/MeOH, rt, 2 h,
7 23%, 8 34%; g, 4-iodopyridine, [PdCl2(PPh3)2], CuI, (i-Pr)2NH, rt, 16 h, 36%; h, 4-iodopyridine, [PdCl2(PPh3)2], CuI, (i-Pr)2NH, rt, 16 h, 48%.
protocols and spectroscopic characterizations are available
as Supporting Information.
Scanning Tunneling Microscopy (STM) Observations.
All STM experiments were performed in a custom-designed
© 2010 American Chemical Society
DOI: 10.1021/nl9029994 | Nano Lett. 2010, 10, 122-128
123

FIGURE 1. (a) Molecular structure of porphyrin 1 showing the three t-Bu2Ph groups and the pyridyl substituent. The molecular dimensions
are extracted from PM3 calculations as implemented in the HyperChem38 molecular modeling package and are consistent with values39
reported in literature. (b) STM image of self-assembled dimers (1)2 on a Cu(111) surface (data acquired with I ) 0.12 nA, U ) -1 V). (c)
Detailed view of a single (1)2 dimer exhibiting the three double lobes representing the t-Bu2Ph groups of each molecular module and the
head-on aligned pyridyl substituents (I ) 0.4 nA, U ) -0.9 V). (d) The t-Bu2Ph groups follow the direction of the dense-packed Cu(111) atomic
rows (nearest neighbor distance between atoms: 2.55 Å; I ) 2.9 nA, U ) -31 mV, low-pass filtered). The right panel summarizes a statistical
analysis of the orientations of the dimers. (e) Molecular structure of porphyrin 2. (f) Coexistence of triangular and rhombic supramolecules
after deposition of porphyrin 2 (I ) 0.1 nA, U ) -0.5 V). The arrows and lines mark the distinguishable azimuthal orientations of the
supramolecules. (g-j) Detailed view and structural properties of supramolecular cycles (2)n (g, n ) 3; h, n ) 4; i, n ) 5; j, n ) 6). The two
possible opening angles of the t-Bu2Ph groups are indicated (f, I ) 0.2 nA, U ) -0.22 V; g, I ) 0.1 nA, U ) -1 V; i, I ) 0.2 nA, U ) -1 V; j,
I ) 0.1 nA, U ) 1 V).
ultrahigh vacuum (UHV) system providing a base pressure
below 1 × 10-10 mbar.36 The monocrystalline Cu(111)
substrate was cleaned by repeated Ar+ sputtering cycles at
an energy of 800 eV, followed by annealing at 730 K for ∼10
min. Subsequently, a submonolayer coverage of the por-
phyrin modules (1-3) was deposited by organic molecular
beam epitaxy from thoroughly degassed quartz crucibles
held at 680 K. During deposition, the Cu(111) surface was
kept at 340 K and the pressure remained < 5 × 10-10 mbar.
The STM images were acquired employing a low-tempera-
ture CreaTec-STM37 with the sample held at 6 K using
electrochemically etched W tips. In the figure captions, U
refers to the bias voltage applied to the sample.
The first step in our study is the deposition of a submono-
layer of porphyrin module 1 onto a Cu(111) surface under
UHV. While very few single protrusions corresponding to
individual porphyrin molecules are observed in the resulting
STM images (cf. Figure 1b), the surface mainly hosts larger
units characterized by two bright terminations and a dim
central connection. Specifically, high-resolution images (Fig-
ure 1c) reveal that each assembly consists of two molecules
1 interconnected through their pyridyl substitutents and thus
resulting in a dimeric (1)2 species. This structural assignment
is based on the molecular dimensions and on previous STM
work on 5,10,15,20-tetra(t-Bu2Ph)porphyrin molecules2,40 in
which each double-lobed feature corresponds to the t-Bu2Ph
groups. Besides the six bright t-Bu2Ph groups, the two pyridyl
substitutents are clearly resolved. They align in an antipar-
allel head-on configuration, where the pyridyl groups face
each other. Importantly, the combination of the 6-fold
symmetry of the first Cu layer with the C2h symmetry of the
porphyrin core in the gas phase, together with a considerable
molecule-substrate interaction, leads to a conformational
adaptation of the molecules upon adsorption. A rotation of
the t-Bu groups results in an alignment of the double-lobed
features with the dense-packed substrate directions. Accord-
ingly, the double lobes representing the t-Bu2Ph substituents
are not perpendicular to each other, but enclose an angle of
either 60° or 120° (see Figure 1c, d). Nevertheless, a close
inspection of Figure 1b reveals that the (1)2 dimer orienta-
tions, defined by the axis connecting the centers of the two
constituting molecules, do not exclusively follow the Cu(111)
© 2010 American Chemical Society
DOI: 10.1021/nl9029994 | Nano Lett. 2010, 10, 122-128
124

dense-packed rows but are frequently rotated by an angle
of (15° (see right panel in Figure 1d). These multiple
orientations of the dimers relative to the underlying atomic
lattice are induced by an apparent flexibility of the pyridyl
legs allowing for some lateral bending (vide infra). The
coupling mechanism of the dimers apparently induced by
the pyridyl groups is mediated by copper atoms (vide infra),
whereby the center-to-center distance in straight dimers
amounts to ∼32 Å.
To further explore the controlled aggregation of porphy-
rins guided by the their pyridyl substituents, we investigated
a porphyrin module bearing two functional pyridyl moieties
in a cis-like configuration and proceed to molecule 2 (cf.
Figure 1e). The large-scale image in Figure 1f shows that
different aggregates of well-defined shape and orientation
coexist on the surface after deposition of building block 2.
Triangular and rhombic architectures (Figure 1g, h) consist-
ing of three and four molecular modules 2, (2)3 and (2)4, are
clearly resolved. Again, the molecular modules are interact-
ing via the head-on pyridyl functions as observed for dimers
(1)2. While the trimeric species (2)3 dominate at low cover-
age, the tetrameric (2)4 assemblies prevail at higher surface
coverages where additionally pentameric (Figure 1i) and
hexameric (Figure 1j) assemblies occur. This trend indicates
that the spontaneous self-assembly of the supramolecules
is based on a statistical process related to the probability that
a given number n (n ) 3, 4, 5, ...) of molecules meets while
diffusing on the surface and is not solely induced by an
energy difference favoring either triangular or rhombic
architectures.
FIGURE 2. (a) Molecular structure of porphyrin 3. (b) STM image of
the one-dimensional supramolecular assembly following deposition
of 3 on Cu(111) (I ) 77 pA, U ) -0.1 V). (c) Schematic model of the
(2·3·2)2 binary cyclic assembly shown in (e). The black dots
represent Cu adatoms embedded in between the terminal pyridyl
N atoms. (d) STM image showing the coexistence of homomolecular
assemblies (2)3 and (2)4 with bicomponent rhombic (2·3·2)2 and
(2·3)4 triangular (2·3)3 architectures formed upon mixing of the
respective modules. (e, f) Detailed view of the bicomponent cyclic
architectures: (e) (2·3·2)2 and (f) (2·3)4. (d, f, I ) 0.1 nA, U ) -1 V;
e, I ) 0.2 nA, U ) -1 V).
Together with the pyridyl-to-pyridyl coupling, the confor-
mational flexibility of the macrocyclic structure and thus its
adaptability and interaction with the underlying surface
governs the geometrical properties of the supramolecules.
Notably, the different interactions between the porphyrin
and the Cu(111) surface induce marked changes in the
conformation of the central tetrapyrrolic core (Figure S1 in
the Supporting Information) strongly affecting the opening
angles of both t-Bu2Ph and pyridyl groups. This results in
opening angles of the t-Bu2Ph groups of either 60° or 120°.
In addition, the pyridyl substitutents can also laterally
bend.41 Besides the intrinsic opening angle of 90° rarely
observed on the surface, we determined opening angles of
either 80 ( 5° or 100 ( 5° (Figure S1 in the Supporting
Information). The small t-Bu2Ph angles favor the small
pyridyl angle and vice versa. As a consequence, to assemble
a supramolecular trimeric cycle by a nearly straight head-
on coupling of the pyridyl groups, the molecules have to
adapt to the t-Bu2Ph opening angle of 60° and, as a result,
symmetric trimeric species can only exist in two distinguish-
able azimuthal orientations on the surface, rotated by 180°
with respect to each other (Figure S2 in the Supporting
Information). This is visualized in Figure 1f, where trimeric
supramolecules (2)3 are observed exclusively in two orienta-
tions with respect to the substrate atomic lattice. Exceptions
constituted by triangles exhibiting t-Bu2Ph opening angles
of 120° were rarely spotted. Their assembly follows the
same empirical rules, but they exhibit a more complex,
chiral structure. The area of the regular triangular cavities
amounts to ∼6 nm2.
Following these rules, the only possible geometry for
symmetric and quadrangular supramolecules coupled by the
pyridyl groups is a rhombic shape, where two opposing
corner molecules exhibit a t-Bu2Ph angle of 60°, whereas
the other pair has a t-Bu2Ph opening angle of 120°. Conse-
quently, the rhombic (2)4 supramolecule exists exclusively
in three distinguishable azimuthal orientation on the surface
(compare Figure 1f and Figure S2 in the Supporting Informa-
tion) and regular square or pentagonal structures cannot
form. Over 95% of the observed quadrangular supramol-
ecules (2)4 are regular rhombs exhibiting 60°/120° t-Bu2Ph
opening angles and a cavity area of 10.5 nm2. For the
pentagons and the hexagons (cf. Figure 2i,j) the area amounts
to 15 and 19 nm2, respectively.
To complete the characterization of the self-assembly of
our basic building blocks, we proceed to the trans-like isomer
3 depicted in Figure 2a. The STM image reproduced in Figure
2b reveals that the positioning of the pyridyl groups (trans
© 2010 American Chemical Society
DOI: 10.1021/nl9029994 | Nano Lett. 2010, 10, 122-128
125

versus cis position) has a drastic impact on the resulting
assemblies. As expected from the head-on coupling of the
pyridyl groups, molecules 3 align in extended chains,
whereby straight segments have a stride of 32.7 ( 0.5 Å (see
Figure 2b).
After characterizing the homomolecular supramolecular
architectures and developing empirical rules to rationalize
their diverse yet highly regular shapes, we now proceed to
the construction of larger supramolecular cavities by mixing
ditopic building blocks with different symmetry. In order to
probe the recognition properties of the different molecular
modules, we thus studied bicomponent assemblies. As an
example, Figure 2d shows the nanoarchitectures evolving
after subsequent deposition of 2 and 3 with a 3.6:1 ratio.
Besides the homomolecular triangles (2)3 and rhombs (2)4
discussed above, unprecedently large cyclic architectures
incorporating modules 3 are also clearly discernible. Figure
2e shows a cyclic assembly (2·3·2)2 with a 2:1 ratio of 2:3
and a cavity area of 20.6 nm2. Accordingly, when passing
from assembly (2·3·2)2 to (2·3)4 (Figure 2f) the area of the
cavities is markedly extended to 41.2 nm2. Again, the
statistical assembly process inhibits a uniform size distribu-
tion of the supramolecules. To our knowledge this is the first
observation of individual supramolecular cavities at surfaces
formed by multiple molecular modules. An overview of
possible assemblies originating from combinations of por-
phyrins 1, 2, and 3 is given in Figure S2 in the Supporting
Information.
In order to obtain further insight into the flexibility of both
the porphyrin modules and the cyclic supramolecules as well
as the strength of the intermolecular coupling, molecular
manipulation experiments have been performed. To this
end, the STM tip was approached to the surface by decreas-
ing the tunneling resistance and laterally moved across the
supramolecules in constant current mode. A similar proce-
dure has been applied previously to translate individual
porphyrins substituted by four identical t-Bu2Ph substitutents
on a Cu(100) surface.41 Figure 3 compares states prior and
after such manipulation steps. Panels a-c of Figure 3
indicate that the shape of (2)4 supramolecules can be com-
pletely changed without breaking the intermolecular bonds,
evidencing a conformational deformation of the cyclic as-
sembly without apparent disruption of the noncovalent
intermolecular recognition site. The yellow color in Figure
3b originates from a superposition of the initial state (green)
and the final state (red) and highlights regions molecules
overlap prior to and after the manipulation step. Neverthe-
less, it is possible by STM manipulation to rupture the
noncovalent interactions and to transfer molecules to the
STM tip: Figure 3d shows an example where a transforma-
tion from a rhombic to a triangular assembly was induced
by the extraction of one module.
FIGURE 3. Molecular manipulation experiments with (2)4 assem-
blies. Between the initial (a, green) and final states (c, red) the STM
tip was laterally moved at low tunneling resistance across the
supramolecule following the path indicated by the arrow in (b). The
experiments indicate a high flexibility of both the molecular building
blocks and the supramolecular assemblies combined with stable
intermolecular bonds and show that it is possible to induce a
conformational change of the supramolecules without breaking
them (a-c: imaging, I ) 0.1 nA, U ) -0.5 V; manipulation, I ) 10
nA, U ) -50 mV). (d) Transition of a rhombic to a triangular
structure by extraction of one molecule by STM manipulation
(imaging, I ) 0.1 nA, U ) -0.5 V; manipulation, I ) 8 nA, U ) -50
mV). The dark dots representing adsorbed gas molecules can be used
as markers.
intermolecular distance of 32.7 ( 0.5 Å for straight 3 chain
segments, where the distance measurement and calibration
are the most precise. This value is consistent with the center-
to-center spacing in the other aggregates. Subtracting the
length of molecule 3 of 29.1 Å as determined from a PM3
optimization and preliminary crystallographic data of the
molecular structure and in agreement with ref 39, yields an
intermolecular N-N separation of 3.6 ( 0.5 Å between the
two terminal pyridyl groups. From electrostatic consider-
ations, a straight head-on coupling of the two opposite
pyridyl groups is energetically highly unfavorable, as the
terminating N atoms carry negative charge.42 Accordingly,
on noble metal surfaces as Ag(111), the head-on coupling is
not observed. Instead, the molecules form dense-packed
islands where the pyridyl groups avoid each other even at
low submonolayer coverages (see Figure S4 in the Support-
ing Information). Thus, the Cu surface plays a decisive role
in promoting a head-on alignment of the pyridyl groups. In
this context, the strong bonding interactions of pyridyl to
Cu are well documented both on surfaces43-45 and for the
solid state,46 where typical N-Cu bond distances range from
1.9 to 2.2 Å. These attractive interactions were reported to
induce a rotation and bending of the terminal pyridyl groups
of a porphyrin molecule toward the Cu(111) surface.44
Accordingly, a possible explanation of the head-on pyridyl
coupling consistent with the determined N-N separation of
3.6 ( 0.5 Å would be that the two N atoms coordinate to a
Cu surface atom by bending toward the substrate, possibly
inducing a displacement of the Cu atom.47 However, this
scheme is excluded because diffusion experiments show
that the (1)2 dimers translate at 220 K without breaking (see
Figure S5 in the Supporting Information). It is highly unlikely
that two molecules of 1 would coherently form and break
bonds to underlying Cu atoms embedded in the surface while
Finally, we detail the origins of the pyridyl coupling
mechanism resulting in the appreciable intermolecular bond
strength evidenced by the experiments. Our data reveal an
© 2010 American Chemical Society
DOI: 10.1021/nl9029994 | Nano Lett. 2010, 10, 122-128
126

wandering large distances without drifting apart. On the other
hand, it is well-known that appreciable Cu adatom densi-
ties exist on Cu surfaces even at moderate temperatures,
enabling stabilization of supramolecular aggregates by
metal-ligand bonding between pyridyl (or other) groups
and Cu adatoms.16,21,45,48 The observed N-N distance of
3.6 ( 0.5 Å would yield a N-Cu bond length of 3.6 Å/2 )
1.8 Å, which is slightly shorter than the values reported
above, but consistent with calculated values for a pyridyl-
Cu-pyridyl complex.21 However, this number is a lower
limit, as it only considers the projection of the pyridyl-Cu
bond length onto the surface plane. Geometric effects as the
separation of the terminal N atom from the surface dictated
by the molecular backbone or a lateral offset of the coupling
adatom can sensitively increase the real pyridyl-Cu dis-
tance. For example, assuming a typical vertical position of
an adatom 2 Å above the surface and a characteristic
separation of an aromatic backbone from the surface plane
of 3 Å,49 the N-Cu bond length would increase to 2.1 Å, a
value in full agreement with literature. The fact that metal
atoms coordinating organic ligands are hardly seen in STM
images because of electronic effects is well documented in
the literature.50,51 The 2-fold pyridyl-Cu-pyridyl coordina-
tion48 motif suggested here indicates that the terminal
pyridyl rings are rotated toward the surface plane. As a
consequence, steric repulsions between H atoms of neigh-
boring pyridyl rings prevents the 3-fold coordination motif
observed for related metal-carbonitrile bidimensional
metal-organic networks6 (cf. Figure 2c). The same mech-
anism disfavors pyridyl-pyridyl bonding angles that deviate
strongly from the head-on configuration of 180° present in
the regular (2)3 and (2)4 structures. Accordingly, the lowest
observed angle in supramolecules assembled from intact
modules amounts to 160° as measured in the regular (2)6
structures. This excludes for example (2)2 dimers or (2)6
hexagons exhibiting t-Bu2Ph opening angles of 60° (see
Figure S2 in the Supporting Information).
building blocks, bicomponent cyclic supramolecules of
unprecedented extended size could be engineered. Thus
our study explores design criteria for the surface engi-
neering of supramolecular cyclic compounds and reveals
an intricate interplay of conformational adaptation and
self-assembly to be generally considered when complex
and flexible species are employed. The fabricated large
cavities bear promises as hosts for molecular guest
species,7,11 whereas the constituting functional porphyrin
modules can serve as platforms for the axial coordination
of additional ligands52 or as catalytic centers for activating
organic reactions.20
Acknowledgment. Work supported by the Munich Center
for Advanced Photonics (MAP) and by the European Union
through the Marie-Curie Initial Training Network “FINELU-
MEN”, Grant Agreement PITN-GA-2008-215399, the Univer-
sity of Trieste, the University of Namur, the Belgian National
Research Foundation (FRS-FNRS, through the contracts No.
2.4.625.08 F, 2.4550.09 F, and F.4505.10), the “Loterie
Nationale”, and the Re´gion Wallonne through the “SOL-
WATT” program (Contract No. 850551). Dr. A. Llanes-Pallas
is gratefully acknowledged for help with the organic synthesis.
Note Added in Proof. Related work has been reported
recently: Fendt, L-A.; Stöhr, M.; Wintjes, N.; Enache, M.;
Jung, T. A.; Diederich, F. Chem. Eur. J. 2009, DOI 10.1002/
chem.200901502.
Supporting Information Available. Schematic illustration
of the molecular conformation and the empirical rules
describing the molecular assembly, overview of the sym-
metric supramolecular aggregates emerging from mixing
building blocks 1, 2, and 3, STM data highlighting the
packing of building block 1 on Ag(111), and image sequence
showing the diffusion of (1)2 dimers on Cu(111). This
information is available free of charge via the Internet at
http://pubs.acs.org.
REFERENCES AND NOTES
In conclusion, we synthesized novel porphyrin deri-
vates exhibiting either one or two functional pyridyl
recognition groups, the latter being cis-like and trans-like
isomers. The design of these molecular modules promotes
the spontaneous self-assembly of individual cyclic su-
pramolecules on a Cu(111) surface. All supramolecular
architectures are stabilized by attractive pyridyl-pyridyl
interactions, mediated by Cu surface atoms. The strength
of the intermolecular interactions allows for conforma-
tional changes of the entire assemblies by STM manipula-
tion. The self-assembly of both the mono and the trans-
like porphyrins results in distinct supramolecular dimers
and chains, respectively. In contrast, the molecular sym-
metry in combination with the molecular flexibility and
surface interactions explains the multiformity of the
supermolecules formed by the cis species, expressed in
open three- to six-membered cyclic assemblies. Further-
more, upon combination of the different ditopic porphyrin
(1)
(2)
(3)
Bo¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R.;
Mauri, F.; De Vita, A.; Car, R. Phys. Rev. Lett. 1999, 83, 324.
Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko,
S. Nature 2001, 413, 619–621.
Schiffrin, A.; Riemann, A.; Auwa¨rter, W.; Pennec, Y.; Weber-
Bargioni, A.; Cvetko, D.; Cossaro, A.; Morgante, A.; Barth, J. V.
Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 5279–5284.
Schull, G.; Douillard, L.; Fiorini-Debuisschert, C.; Charra, F.;
Mathevet, F.; Kreher, D.; Attias, A.-J. Nano Lett. 2006, 6, 1360–
1363.
Stepanow, S.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.;
Delvigne, E.; Lin, N.; Deng, X.; Cai, C.; Barth, J. V.; Kern, K. Nat.
Mater. 2004, 3, 229–233.
Schlickum, U.; Decker, R.; Klappenberger, F.; Zoppellaro, G.;
Klyatskaya, S.; Ruben, M.; Silanes, I.; Arnau, A.; Kern, K.; Brune,
H.; Barth, J. V. Nano Lett. 2007, 7, 3813–3817.
(4)
(5)
(6)
(7)
(8)
(9)
Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.;
Beton, P. H. Nature 2003, 424, 1029–1031.
Kudernac, T.; Lei, S.; Elemans, J. A. A. W.; De Feyter, S. Chem.
Soc. Rev. 2009, 38, 402–421.
Bonifazi, D.; Mohnani, S.; Llanes-Pallas, A. Chem.sEur. J. 2009,
15, 7004–7025.
(10) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671–679.
© 2010 American Chemical Society
DOI: 10.1021/nl9029994 | Nano Lett. 2010, 10, 122-128
127

(11) Barth, J. V. Annu. Rev. Phys. Chem. 2007, 58, 375–407.
(12) Treier, M.; Ruffieux, P.; Gro¨ning, P.; Xiao, S.; Nuckolls, C.; Fasel,
R. Chem. Commun. 2008, 4555–4557.
(13) Li, S.-S.; Northrop, B. H.; Yuan, Q.-H.; Wan, L.-J.; Stang, P. J. Acc.
Chem. Res. 2009, 42, 249–259.
(33) Zeitouny, J.; Aurisicchio, C.; Bonifazi, D.; De Zorzi, R.; Geremia,
S.; Bonini, M.; Palma, C. A.; Samori, P.; Listorti, A.; Belbakra, A.;
Armaroli, N. J. Mater. Chem. 2009, 19, 4715–4724.
(34) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
16, 4467–4470.
(14) Safarowsky, C.; Merz, L.; Rang, A.; Broekmann, P.; Hermann,
B. A.; Schalley, C. A. Angew. Chem., Int. Ed. 2004, 43.
(15) Jeong, K. S.; Kim, S. Y.; Shin, U.-S.; Kogej, M.; Hai, N. T. M.;
Broekmann, P.; Jeong, N.; Kirchner, B.; Reiher, M.; Schalley, C. A.
J. Am. Chem. Soc. 2005, 127, 17672–17685.
(16) Lin, N.; Stepanov, S.; Ruben, M.; Barth, J. V. Top. Curr. Chem.
2009, 287, 1–44.
(35) Taylor, P. N.; Anderson, H. L. J. Am. Chem. Soc. 1999, 121, 11538–
11545.
(36) Auwa¨rter, W.; Schiffrin, A.; Weber-Bargioni, A.; Pennec, Y.;
Riemann, A.; Barth, J. V. Int. J. Nanotechnol. 2008, 5, 1171–1193.
(37) CreaTec, D-74391 Erligheim, Germany .
(38) HYPERCHEM; Hypercube Inc.: 1115 NW 4th St, Gainesville, FL 32601.
(39) Wintjes, N.; Bonifazi, D.; Cheng, F.; Kiebele, A.; Sto¨hr, M.; Jung,
T.; Spillmann, H.; Diederich, F. Angew. Chem., Int. Ed. 2007, 46,
4089–4092.
(17) Drain, C. M.; Varotto, A.; Radivojevic, I. Chem. Rev. 2009, 47,
1630–1658.
(18) Tsuda, A.; Nakamura, T.; Sakamoto, S.; Yamaguchi, K.; Osuka,
A. Angew. Chem., Int. Ed. 2002, 41, 2817–2821.
(19) Tsuda, A.; Hu, H.; Tanaka, R.; Aida, T. Angew. Chem., Int. Ed. 2005,
44, 4884–4888.
(40) Moresco, F.; Meyer, G.; Rieder, K.-H.; Ping, J.; Tang, H.; Joachim,
C. Surf. Sci. 2002, 499, 94–102.
(41) Jung, T. A.; Schlittler, R. R.; Gimewski, J. K.; Tang, H.; Joachim,
C. Science 1996, 271, 181–184.
(20) Piot, L.; Bonifazi, D.; Samori, P. Adv. Funct. Mater. 2007, 17, 3689–
3693.
(21) Eichberger, M.; Marschall, M.; Reichert, J.; Weber-Bargioni, A.;
Auwa¨rter, W.; Wang, R. L. C.; Kreuzer, H. J.; Pennec, Y.; Schiffrin,
A.; Barth, J. V. Nano Lett. 2008, 8, 4608–4613.
(22) Bonifazi, D.; Kiebele, A.; Sto¨hr, M.; Cheng, F.; Jung, T.; Diederich,
F.; Spillmann, H. Adv. Funct. Mater. 2007, 17, 1051–1062.
(23) Wintjes, N.; Hornung, J.; Lobo-Checa, J.; Voigt, T.; Samuely, T.;
Thilgen, C.; Sto¨hr, M.; Diederich, F.; Jung, T. Chem.sEur. J. 2008,
14, 5794–5802.
(24) Yokoyama, T.; Kamikado, T.; Yokoyama, S.; Mashiko, S. J. Chem.
Phys. 2004, 121, 11993–11997.
(25) Lindsey, J. S.; Prathapan, S.; Johnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941–8968.
(26) Osuka, A.; Tanabe, N.; Nakajima, S.; Maruyama, K. J. Chem. Soc.,
Perkin Trans. 2 1996, 199–203.
(27) Felber, B.; Diederich, F. Helv. Chim. Acta 2005, 88, 120–153.
(28) Rao, P. D.; Littler, B. J.; Geier, G. R.; Lindsey, J. S. J. Org. Chem.
2000, 65, 1084–1092.
(29) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J.; Lindsey, J. S. J. Org.
Chem. 2000, 65, 7323–7344.
(42) Wang, Y.; Ge, X.; Schull, G.; Berndt, R.; , C.; Bornholdt; Koehler,
F.; Herges, R J. Am. Chem. Soc. 2008, 130, 4218–4219.
(43) Wu, D. Y.; Hayashi, M.; Chang, C. H.; Liang, K. K.; Lin, S. H.
J. Chem. Phys. 2003, 118, 4073–4085.
(44) Auwa¨rter, W.; Klappenberger, F.; Weber-Bargioni, A.; Schiffrin,
A.; Strunskus, T.; Wo¨ll, C.; Pennec, Y.; Riemann, A.; Barth, J. V.
J. Am. Chem. Soc. 2007, 129, 11279–11285.
(45) Klappenberger, F.; Weber-Bargioni, A.; Auwa¨rter, W.; Marschall,
M.; Schiffrin, A.; Barth, J. V. J. Chem. Phys. 2008, 129, 214702.
(46) Dietrich-Buchecker, C. O.; Guilhem, J.; Pascard, C.; Sauvage, J.-
P. Angew. Chem., Int. Ed. 1990, 29, 1154–1156.
(47) Bedwani, S.; Wegner, D.; Crommie, M. F.; Rochefort, A. Phys. Rev.
Lett. 2008, 101, 216105.
(48) Tait, S. L.; Langner, A.; Lin, N.; Stepanow, S.; Rajadurai, C.; Ruben,
M.; Kern, K. J. Phys. Chem. C 2007, 111, 10982–10987.
(49) Stadler, C.; Hansen, S.; Kro¨ger, I.; Kumpf, C.; Umbach, E. Nat.
Phys. 2009, 5, 153–158.
(50) Classen, T.; Fratesi, G.; Constantini, G.; Fabris, S.; Stadler, F. L.;
Kim, C.; de Gironcoli, S.; Baroni, S.; Kern, K. Angew. Chem., Int.
Ed. 2005, 44, 6142–6145. Kühne, D.; Klappenberger, F.; Decker,
R.; Schlickum, U.; Brune, H.; Klyatskaya, S.; Ruben, M.; Barth,
J. V. J. Am. Chem. Soc. 2009, 131, 3881–3883.
(51) Breitruck, A.; Hoster, H. E.; Meier, C.; Ziener, U.; Behm, R. J. Surf.
Sci. 2007, 601, 4200–4205.
(52) Williams, F. J.; Vaughan, O. P. H.; Knox, K. J.; Bambos, N.;
Lambert, R. M. Chem. Commun. 2004, 1688.
(30) Blake, I. M.; Anderson, H. L.; Beljonne, D.; Bredas, J. L.; Clegg,
W. J. Am. Chem. Soc. 1998, 120, 10764–10765.
(31) Burrell, A. K.; Officer, D. L.; Plieger, P. G.; Reid, D. C. W. Chem.
Rev. 2001, 101, 2751–2796.
(32) The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978.
© 2010 American Chemical Society
DOI: 10.1021/nl9029994 | Nano Lett. 2010, 10, 122-128
128