Engineering of supramolecular H-bonded nanopolygons via self-assembly of programmed molecular molecular modules

Article abstract of DOI:10.1021/ja807530m

Discrete and multicomponent nanoscale noncovalent assemblies on surfaces featuring polygonal porous domains are presented. The molecular engineering concept involves multivalent molecular modules that are preprogrammed to undergo heteromolecular recogniti

Full text of DOI:10.1021/ja807530m

Published on Web 12/23/2008
Engineering of Supramolecular H-Bonded Nanopolygons via
Self-Assembly of Programmed Molecular Modules
Anna Llanes-Pallas,† Carlos-Andres Palma,‡ Luc Piot,‡ Abdelhalim Belbakra,§
Andrea Listorti,§ Maurizio Prato,† Paolo Samor`ı,*,§,‡ Nicola Armaroli,*,§ and
Davide Bonifazi*,†,|
UniVersita` degli Studi di Trieste, Dipartimento di Scienze Farmaceutiche and INSTM UdR di
Trieste, Piazzale Europa 1, 34127 Trieste, Italy, ISIS-CNRS 7006, UniVersite´ Louis Pasteur,
8 alle´e Gaspard Monge, 67000 Strasbourg, France, Istituto per la Sintesi Organica e la
FotoreattiVita` (ISOF), Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna,
Italy, and UniVersity of Namur (FUNDP), Department of Chemistry, Rue de Bruxelles 61,
5000 Namur, Belgium
Received June 20, 2008; E-mail: samori@isis-ulp.org; nicola.armaroli@isof.cnr.it; davide.bonifazi@fundp.ac.be
Abstract: Discrete and multicomponent nanoscale noncovalent assemblies on surfaces featuring polygonal
porous domains are presented. The molecular engineering concept involves multivalent molecular modules
that are preprogrammed to undergo heteromolecular recognition by exploiting complementary multiple H
bonds. Two types of molecular modules have been engineered: (i) a linear unit of twofold symmetry exposing
two 2,6-di(acylamino)pyridyl [donor-acceptor-donor (DAD)] recognition sites at its extremities with a 180°
orientation relative to each other and (ii) an angular unit constituted by a 1,3,6,8-tetraethynylpyrene core
peripherally functionalized with four uracil groups [acceptor-donor-acceptor (ADA)] positioned at 60° and
120° relative to each other. These molecular modules self-assemble through H-bonds between the
complementary recognition sites, forming supramolecular architectures. Their symmetry depends upon
the type of each individual subunit and the stoichiometry as well as on the combination and distribution of
the main symmetry axes. These so-formed two-dimensional (2D) supramolecular oligomers have been
studied in solution by optical spectroscopy and on highly ordered pyrolitic graphite (HOPG) substrates by
scanning tunneling microscopy (STM) at the solid-liquid interface. Steady-state UV/vis absorption and
emission titration measurements suggest the reversible formation of multiple oligomeric species with slightly
modulated fluorescence spectra. This likely reflects the presence of various aggregates between the two
polytopic receptors, which exhibit somewhat different electronic delocalization as a function of the aggregate
size. The presence of multiple species is further confirmed by time-resolved luminescence measurements:
lifetime values are fitted as double/multiple exponentials and are always shorter than 6.5 ns. The formation
of several oligomeric species is further supported by in situ STM measurements at the solid-liquid interface
that provided evidence, with submolecular resolution, for the formation of multicomponent and discrete 2D
polygon-like assemblies. We highlight the role of accurate control of the concentration required to image
on the surface the 2D oligomeric species formed in solution, which allows us to bypass the determinant
role of the substrate-molecule interactions in forming the thermodynamically stable monocomponent
architectures at the solid-liquid interface.
fields, such as electronics,7-12 optoelectronics,13-16 photonics,17
and energy storage.18,19 The ability to control matter at the
Introduction
Organic-based materials originate from the need to engineer
matter at the molecular level,1-6 resulting in advanced materials
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molecular level with specific molecular functional systems
constitutes one of the challenges in the field of nanoscience.20-22
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9
10.1021/ja807530m CCC: $40.75
2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 509–520 509

A R T I C L E S
Llanes-Pallas et al.
To develop such materials, the hierarchical self-assembly of
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Engineering of Supramolecular H-Bonded Nanopolygons
A R T I C L E S
Figure 1. (a-c) Possible polygonal networks obtained by combining the tetratopic and ditopic modules 1 (red) and 2 (green), respectively. (d) Chemical
structures and representative cartoons of the modules.
solvent and molecule-substrate interactions governing the
process of self-assembly at surfaces.72-75
cence lifetime decays that are fitted as double/multiple expo-
nentials. The generation of several oligomeric assemblies
featuring hollow structures has been observed at surfaces using
in-situ STM measurements at the solid-liquid interface. Despite
the fact that the thermodynamics of formation of supramolecular
oligomers in solution and on the surface are significantly
different in nature, it is usually very difficult to direct the
deposition of two or more molecules on a surface, consequently
driving the formation of discrete geometrical assemblies because
of the difference in adsorption energies78,79 of the components
and their tendency to minimize the occupied area. To this end,
low-concentration solutions have been used as a strategy to
prevent competitive adsorption,80-82 thus operating in a regime
where the packing is ruled only by the intermolecular interac-
tions, thereby neglecting the role of substrate-molecule interac-
tions. This allowed the simultaneous physisorption of molecules
1 and 2 at the interface, which revealed the formation of discrete
polygon-like nanoassemblies featuring hollow structures with
a submolecular resolution, therefore providing a direct insight
into the complex self-recognition behavior on highly oriented
pyrolitic graphite (HOPG).
Here we describe the engineering of discrete, molecular-scale,
H-bonded assemblies nucleated in solution that have been
studied at the solid-liquid interface using a concentration-based
approach that allows the visualization of bicomponent 2D hollow
structures of different geometries. Two important factors have
been considered in the rational design of such supramolecular
architectures: (i) the symmetry and (ii) the binding capabilities
(and thus the functionality) of the building blocks. The overall
shape of the resulting supramolecular assembly is then a
consequence of these parameters.76 To accomplish this goal,
two types of conformationally rigid geometrical modules
(angular and linear) equipped with multiple complementary
H-bonding sites have been synthesized (Figure 1). Specifically,
a 1,3,6,8-tetraethynylpyrene 1 with four exposed uracil groups
[acceptor-donor-acceptor (ADA)] positioned at 60° and 120°
relative to each other77 has been designed to interact with a
complementary linear module, a 1,4-diethynyl benzene deriva-
tive 2 bearing two 2,6-di(acylamino)pyridyl units [donor-
acceptor-donor (DAD)], through triple H bonds. Steady-state
UV/vis absorption and emission titration measurements suggest
the reversible formation of multiple oligomeric species with
slightly modulated fluorescence spectra, reflecting the presence
of various assemblies between the two polytopic receptors. The
presence of multiple species is further confirmed by lumines-
Results and Discussion
Synthesis. The synthesis of modules 1 and 2 is outlined in
Scheme 1. Both modules were synthesized via palladium-
catalyzed Sonogashira cross-coupling reactions83 between the
complementary H-bonding recognition units and the corre-
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9
J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009 511

A R T I C L E S
Llanes-Pallas et al.
Scheme 1a
a (a) 1-Bromohexane, K2CO3, DMSO, 40 °C, 20 h; (b) LDA, THF, -78 °C, 1.5 h, then I2, 2 h, then AcOH, rt, 10 h; (c) TMSA, [Pd(PPh3)2], CuI, PPh3,
i-Pr2NH, THF, 80 °C, 12 h; (d) 1 M (aq) KOH, 1:1 MeOH/CH2Cl2, rt, 40 min; (e) [Pd(PPh3)4], CuI, Et3N, THF, 45 °C; (f) DMSO, KOH, MeI; (g) Ac2O,
pyridine, rt, 10 h; (h) TMSA, [Pd(PPh3)4], CuI, NEt3, THF, reflux, 12 h; (i) 1 M (aq) KOH, MeOH, rt, 40 min; (j) 1,4-diiodobenzene, [Pd(PPh3)4], CuI, NEt3,
THF, reflux, 12 h. Abbreviations: DMSO, dimethyl sulfoxide; LDA, lithium diisopropylamine; THF, tetrahydrofuran; TMSA, trimethylsilylacetylene.
sponding aromatic cores. Module 1, bearing four uracil recogni-
tion sites, was prepared in the following way. 1-Hexyluracil
(4) was obtained by reaction of unsubstituted uracil with
1-bromohexane in the presence of K2CO3 and DMSO. Subse-
quent deprotonation of 4 with LDA and addition of I2 afforded
the C6-iodinated derivative 5. In parallel, tetraethynylpyrene 7
was prepared by Sonogashira cross-coupling between 1,3,6,8-
tetrabromopyrene (6) and trimethylsilylacetylene (TMSA) fol-
lowed by cleavage of the TMS protecting groups upon addition
of a 1 M aqueous solution of KOH. Reaction of 1,3,6,8-
tetraethynylpyrene (8) with 5 in the presence of CuI and
[Pd(PPh3)4] afforded the final tetratopic 1,3,6,8-tetrakis[(1-
hexylurac-6-yl)ethynyl]pyrene 1 as a dark-red powder, which
was purified by repeated precipitation cycles and characterized
protected acetylene derivative 10. Subsequent treatment of 10
with aqueous KOH in MeOH afforded 11, which was coupled
to 1,4-diiodobenzene via Sonogashira reaction. All of the
compound structures were assessed by electrospray ionization
1
(ESI) mass spectrometry and H and 13C NMR, UV-vis, and
IR spectroscopies. Module 1, possessing pseudofourfold sym-
metry with two different angularities (120° and 60°), could lead
to several polygonal arrangements when assembled with mol-
ecule 2 (Figure 1). In particular, the assembly of module 2 with
(a) six angular modules 1 through twelve latitudinal uracil sites,
(b) three angular modules 1 through six longitudinal uracil sites,
and (c) four angular modules 1 through four longitudinal and
latitudinal uracil sites, could in principle yield hexagonal,
triangular, and rhomboidal structures, respectively.
Self-Recognition Behavior of 2D Oligomers in Solution:
UV/Vis Absorption and Emission Measurements. The poor
solubility of the H-bonding modules 1 and 2 (Figure 1) in aprotic
solvents did not allow us to study the self-assembly process in
solution by 1H NMR spectroscopy. Nevertheless, the low
concentration (∼10-6 M) required by the spectroscopic analysis
allowed a titration study of the self-assembly process in solution
using UV/vis and fluorescence spectroscopies. Molecules 1 and
2 are strong absorbers in the UV region, with maxima at 345
nm (ε ) 37 500 M-1 cm-1) and 324 nm (ε ) 40 600 M-1 cm-1),
respectively, in DMSO (Figure 2). 1,3,6,8-Tetrasubstituted
pyrene 1 also exhibits an extended absorption envelope in the
visible spectral region peaked at 481 nm (ε ) 37 700 M-1
cm-1)84 that does not overlap with the spectral features of linear
module 2. This mismatching proved to be of crucial importance
1
by H and 13C NMR spectroscopy in pyridine-d5. Molecule 1
proved to be scarcely soluble in most organic solvents because
of the extended homomolecular double H-bonding interactions
established between the peripheral uracil moieties. In order to
comprehensively characterize the compound, we methylated the
imidic functionalities by reacting the tetrauracil module 1 with
MeI in the presence of KOH. As expected, the introduction of
the methyl groups at the imidic positions afforded the tetra-
methylated derivative 3, which proved to be extremely soluble
in CHCl3, and thus easily characterized. Ditopic module 2 was
synthesized following a similar approach to that utilized for 1,
using the Pd-catalyzed cross-coupling reaction between 4-eth-
ynyl-2,6-diacetylaminopyridine (11) and 1,4-diiodobenzene.
4-Ethynyl-2,6-diamidopyridine was prepared in three steps
starting from 4-bromo-2,6-diaminopyridine. Acetylation of
4-bromo-2,6-diaminopyridine with Ac2O in pyridine yielded
product 9, which was subsequently cross-coupled with TMSA
using [Pd(PPh3)4]/CuI in the presence of Et3N to yield the TMS-
(84) Maeda, H.; Maeda, T.; Mizuno, K.; Fujimoto, K.; Shimizu, H.; Inouye,
M. Chem.sEur. J. 2006, 12, 824.
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512 J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009

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Figure 2. (a) Absorption spectra of 1 (gray line) and 2 (black line) in
DMSO. (b) Normalized fluorescence spectra of 1 (gray line) and 2 (black
line) in DMSO using λexc ) 308 nm.
for performing optical titrations because it enabled excitation
selectivity and easy attribution of spectral changes. Both 1 and
2 are relatively strong emitters, with fluorescence bands exhibit-
ing maxima at 503 nm (Φfluor ) 0.26) and 423 nm (Φfluor
)
0.10), respectively, in DMSO (Figure 2). The related singlet
lifetimes are 1.5 ns for 1 and 1.2 ns for 2.
The H-bonding-based molecular recognition process between
modules 1 and 2 can be monitored in solution by taking
advantage of the strong chromophoric and emissive character
of the two molecules, provided that a solvent that does not
compete with formation of H-bond is utilized. This turned out
to be particularly challenging because of the poor solubility of
1,3,6,8-tetrasubstituted pyrene 1 in many organic solvents.
Indeed, attempts made in mixtures of CCl4 or CH2Cl2 with
DMSO, in which molecule 1 is soluble, gave no evidence of
association. Finally, a clear proof of molecular recognition was
obtained in a 1:50 DMSO/1,2,4-trichlorobenzene solution. Under
these conditions, addition of increasing amounts of linear module
2 to a 8.4 × 10-6 M solution of pyrene 1 caused dramatic
changes in the absorption and fluorescence spectra. Two
different stages can be traced: 0-1 equiv (Figure 3a) and 1-2.4
equiv (Figure 3c) of compound 2.
Upon addition of 1 equiv of molecule 2, the visible absorption
spectrum of pyrene 1 undergoes significant changes, and clean
isosbestic points are found at 435 and 497 nm (Figure 3a). The
former is slightly shifted to 427 nm upon further addition of
molecule 2 (up to 2.4 equiv, Figure 3c). At 2.5 equiv or higher,
extensive formation of suspended aggregates occurs. Parallel
to the changes in the absorption spectra, substantial variations
of the fluorescence profiles occur during the titration. During
the initial stage (Figure 3b) the strong green fluorescence signal
of the pyrene derivative is decreased by 80% and a novel
emission band, peaked at ∼570 nm, grows in with an isoemis-
sive point at 593 nm. During the second titration step, the
pyrene-centered fluorescence signal is further decreased to less
than 4% of its initial value, whereas the intensity of the new
emission band is affected only slightly or not at all (Figure 3d).
Notably, the isoemissive point observed during the first stage
of the titration is lost, pointing to the formation of more than
one emissive species.
Figure 3. Changes of the absorption (left) and fluorescence (left) spectra
of a 8.4 × 10-6 M solution of module 1 in 1:50 DMSO/1,2,4-trichloroben-
zene upon addition of increasing amounts of linker 2 (from 0 to 2.0 ×
10-5 M): (a, b) stage 1; (c, d) stage 2. Dashed arrows in (a) and (b) indicate
isosbestic and isoemissive points, respectively. λexc ) 430 nm, at which
the absorbance value is virtually constant throughout the entire titration.
complementary DAD binding units. During the initial stage of
the titration, the substantial decrease in the pyrene-centered
fluorescence intensity (∼80%, Figure 3b) suggests that a large
majority of the pyrene derivatives are bound to at least one unit
of the linear module86 (Scheme 2). This likely occurs through
a variety of associations between molecules 1 and 2 that give
rise to clean isosbestic points because the binding sites are
independent of each other.87 The supramolecular adducts are
soluble up to the addition of ∼2 equiv of the linear ditopic
module 2. However, when larger amounts of 2 are added to a
solution of pyrene 1, the progressive formation of a solid
suspension is observed. Starting from 2.5 equiv of molecule 2,
the aggregation process becomes so extensive that the optical
transparency of the solution is no longer suitable for performing
reliable spectroscopic measurements, and precipitation also
occurs. This finding underpins the progressive formation of large
supramolecular oligomers/networks that are insoluble in solution.
(86) The observed luminescence quenching of the pyrene derivative can
occur only via intramolecular interactions, since intermolecular
quenching processes must be ruled out because of the short singlet
lifetime of 1 (1.5 ns) and the low concentration of the added component
2 (< 1.0 × 10-5 M). Moreover, energy-transfer quenching of 1 by 2
is thermodynamically not allowed, as can be derived by the relative
spectral location of the fluorescence bands.
(87) It is worth emphasizing that optical studies in solution do not evidence
self-aggregation of 2. No absorption and emission spectral changes
were detected in the DMSO/trichlorobenzene solvent mixture when
the concentration of 2 was increased from 8.4 × 10-6 to 2.1 × 10-5
M (the same concentration range as in the titration experiment).
The interpretation of these data in solution is not straight-
forward since tetratopic (1) and ditopic (2) assembling modules
are involved, with the H-bonding recognition sites rather distant
from each other, thus favoring independent complexation of four
(85) Deleted in proof.
9
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Scheme 2. Tentative Interpretation of the H-Bonding Recognition Process during the Titration Studies and Formation of Noncyclic Dimeric
1·2 and Trimeric 1·(2)2 Assemblies
The red-shifted, weaker emission band that grows up during
the titration could be assigned to H-bonded structures in solution.
It could be argued, however, that its shape and position are
reminiscent of excimer-type bands, which are normally observed
for pyrene derivatives in solution at relatively high concentration
(>10-4 M).88 This excimer assignment can be ruled out because
the excitation spectrum recorded at the end of the titration at
585 nm (see Figure S1 in the Supporting Information) matches
the features observed in the absorption spectrum (e.g., the 515
nm peak), supporting the hypothesis that the novel emitting
species are also found in the electronic ground state and not
only in the excited state, as would be observed for excimers.
As far as excited-state lifetimes are concerned, the singlet
lifetime of the residual pyrene-centered signal during the first
titration stage (0-1 equiv of 2) is unchanged relative to the
initial value (1.5 ns), whereas the new red-shifted signal exhibits
a multiexponential emission decay. The related lifetime values
are slightly dependent on the monitored emission wavelength
but are never longer than 6.5 ns; thus, they are shorter than the
lifetime of the intrinsic excimer emission of pyrene derivative
1 recorded at 10-3 M (29 ns). These results indicate that the
H-bonded complexes, albeit exhibiting identical absorption
spectra (see above), do show slightly different excited-state
properties. This trend is confirmed at the end of stage 2 (at ∼2
equiv), where it is possible to selectively excite the supramo-
lecular assemblies at their new absorption features, e.g., at 515
nm (Figure 4). The wide, irregular emission profile suggests
the formation of multiple H-bonded oligomeric species with
slightly modulated fluorescence spectra, reflecting the presence
of various aggregates between the two polytopic receptors that
exhibit different electronic delocalization as a function of the
aggregate size. The presence of multiple species is confirmed
by luminescence lifetime decays that are fitted as double/
multiple exponentials with wavelength-dependent amplitudes
(Figure 4).
The noticeable red shift of the pyrene-centered absorption
and emission features suggests substantial electronic delocal-
ization in the supramolecular structures formed upon titration.
This might occur via either H-bonding interactions between
molecules 1 and 2 or π-π stacking between the 1,3,6,8-
(88) Birks, J. B. Photophysics of Aromatic Molecules; John Wiley and Sons:
New York, 1970.
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Engineering of Supramolecular H-Bonded Nanopolygons
A R T I C L E S
the substrate plays a negligible role in dictating the final
symmetry of the assembly. Unambiguous interpretation of the
self-assembled patterns can be made provided that submolecular
resolution is achieved. Furthermore, most studies involve self-
assembled patterns that are dictated by the surface and do not
exist (or have not been proved to exist) in solution. A strategy
based on concentration control can be employed to accurately
visualize the formation of bicomponent 2D supramolecular
oligomers at an interface and compare them to the 2D supramo-
lecular oligomeric motifs nucleated in solution. Given the
difference in adsorption energies of the components and the
tendency to minimize the occupied area, it is possible to set a
threshold of concentration in which there cannot be competitive
adsorption. Thus, we operate in a regime where the assembly
is mostly dominated by the highly directional intermolecular
interactions and where substrate-molecule interactions con-
tribute less to the determination of phase formation.80-82 This
approximation holds as long as the molecule-solvent interac-
tions are negligible. By setting the concentration around or
below that necessary to form a densely packed monolayer of
the different molecular patterns, we can ensure that the
minimization of the global interfacial energy (yielding the
thermodynamically stable phase) depends only on the intermo-
lecular energies between assemblies 1·1, 2·2, and 1·2. It is
noteworthy that the interface concentration is a function of the
interfacial area, which depends experimentally on the area
wetted by the solution. This approach is somewhat analogous
to that used for ultrahigh vacuum STM experiments, where
polymorphism in multicomponent networks is avoided through
a careful control of the stoichiometry of the components
deposited on the surfaces.24 In order to prove this concept, we
have imaged the constitutive components of the supramolecular
2D oligomers at submonolayer concentrations. Notably, the
concentrations are in the micromolar range, and the solvent used
for imaging is 1,2,4-trichlorobenzene; these are the same
conditions used for the formation of the bicomponent oligomers
in solution. The DMSO content in the STM solutions, which
was needed to solubilize the starting material, was kept near
that used for the optical studies (1-0.1%). For the sake of
comparison, the patterns at high concentration have also been
imaged and reported. Figure 5a,b displays the surface-confined
pattern of a monolayer of pyrene conjugate 1 prepared by
depositing 5 µL of a diluted solution (concentration <10 µM).
In the high-resolution image (Figure 5b), individual molecules
within the ordered pattern can be identified by their characteristic
cross shape: the central bright spot corresponds to the pyrene
aromatic core and the lateral protrusions correspond to the uracil
aliphatic chains. The contrast is ruled by resonant tunneling
between the Fermi level of the HOPG substrate and the frontier
orbitals of the adsorbed molecules.91 From the proposed model,
it clearly appears that neighboring molecules interact with each
other through the formation of homomolecular coupling that
occurs via the coexistence of two parallel H-bonds between their
peripheral uracil moieties. The same packing was obtained from
more concentrated solutions (>100 µM) (image not shown). In
view of the area of the unit cell occupied by a single molecule
of 1, which amounts to 4.95 ( 0.06 nm2, and the fact that the
crystalline domains are extended over several hundreds of square
nanometers, it is possible to estimate that 5 µL of a 10 µM
solution contains ∼1.5 times the number of molecules needed
to form a densely packed monolayer on 1 cm2 of HOPG. The
Figure 4. (a) Fluorescence spectrum (λexc ) 515 nm) of a solution
containing pyrene derivative 1 (8.4 × 10-6 M) and linker 2 (2.0 × 10-5
M) in 1:50 DMSO/1,2,4-trichlorobenzene at the end of the titration
experiment (2.4 equiv of 2 added). (b) Luminescence decays as a function
of the emission wavelength. (c) Emission decays recorded at three selected
wavelengths: (red) 520, (magenta) 600, and (orange) 700 nm.
tetrasubstituted pyrene moieties belonging to different oligo-
meric assemblies. The sharp bands of the absorption spectrum
at the end of the second stage tend to support the former
hypothesis. On the other hand, π-π stacking aggregates of
aromatic molecules usually give weak and broad absorption tails
on the red edge of the spectrum,89 which, in our case, could be
masked by the absorption background tail caused by the
formation of a solid suspension (see above). It cannot be
excluded that insoluble aggregates could be generated by the
combined formation of larger and larger H-bonded oligomers
that also undergo intermolecular stacking between the pyrene
units.90 At any molar ratio, the H-bonding interactions are
essential for inducing the formation of supramolecular as-
semblies, as evidenced by a control experiment carried out with
the methyl-capped 1,3,6,8-tetrasubstituted pyrene derivative 3,
which cannot undergo triple H-bonding interactions with linker
2. Upon titration of 3 with linear module 2 under the same
conditions as described in Figure 3 for molecules 1 and 2, neither
absorption/emission spectral changes nor formation of insoluble
aggregates are observed, also ruling out the possibility of
stacking interactions between the two different molecules (see
Figure S2 in the Supporting Information). Very importantly,
all of the spectral changes observed during the titrations are
fully reversible. By addition of a few drops of DMSO, which
disrupts the intermolecular H-bonds, a full recovery of the
pyrene-centered absorption and fluorescence features is ob-
served, and the precipitated assemblies are promptly and
completely dissolved (see Figure S1 in the Supporting Informa-
tion).
Self-Recognition Behavior of 2D Oligomers at the Solid-
Liquid Interface. STM Investigations. While there are many
reports on 2D supramolecular oligomers/polymers imaged via
STM, whose structures rely on the ability of the molecules to
form self-assembled crystalline patterns, rare are those reports
in which the organization is solely dictated by the structural
information embedded within the molecular modules, i.e., where
(89) Wu¨rthner, F.; Thalacker, C.; Sautter, A. AdV. Mater. 1999, 11, 754.
(90) Wu¨rthner, F.; Hanke, B.; Lysetska, M.; Lambright, G.; Harms, G. S.
Org. Lett. 2005, 7, 967.
(91) Lazzaroni, R.; Calderone, A.; Bre´das, J. L.; Rabe, J. P. J. Chem. Phys.
1997, 107, 99.
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Figure 5. Constant-current STM images of monocomponent monolayers of physisorbed molecules: (a, b) 1; (c, d) 2. The high-resolution images show the
unit cells: (b) a ) 2.3 ( 0.1 nm, b ) 2.2 ( 0.1 nm, R ) 80 ( 4°; (d1) a ) 1.0 ( 0.2 nm, b ) 2.5 ( 0.1 nm, R ) 65 ( 4°; (d2) a ) 3.3 ( 0.2 nm, b )
3.9 ( 0.1 nm, R ) 47 ( 2°. The average tunneling current was It ) 20 pA, and the sample bias Ut was between 400 and 800 mV.
molecules in excess (i.e., those not adsorbed at the solid-liquid
interface) are solvated in the 3D supernatant solution. On the
other hand, molecule 2 was found to physisorb from diluted
(<10 µM) solutions on HOPG into two different and coexisting
self-assembled networks (Figure 5c1,c2). In the STM image
(Figure 5d), individual molecules 2 can be identified in view
of their characteristic linear shape, where three aligned bright
lobes can be discerned (Figure 5d1). Each lobe can be attributed
to an aromatic core: the central one corresponds to the 1,4-
disubstituted phenyl moiety, whereas the peripheral functions
correspond to the 2,6-di(acylamino)pyridyl substitutents. The
first network (Figure 5c1,d1; unit cell: a ) 1.0 ( 0.2 nm, b )
2.5 ( 0.1 nm, R ) 65 ( 4°) shows a lamella-type motif. While
molecule-molecule interactions at the intralamellar level are
of the van der Waals type, at the interlamellar level they consist
of H-bonding. In the second network (Figure 5c2,d2; unit cell:
a ) 3.3 ( 0.2 nm, b ) 3.9 ( 0.1 nm, R ) 47 ( 2°), the
molecules arrange in a distorted hexagonal structure. The self-
assembly is likely to be driven by the interplay of van der Waals
and H-bonding interactions, but the exact packing could not be
elucidated from the STM images. Statistical grain analysis of
the STM constant-current images reveals that the apparent pores
possess a projected surface area of 2.2 nm2. Notably, the same
two patterns were also observed by applying highly concentrated
solutions (>100 µM) to the surface.
) 3.1 ( 0.2 nm, b ) 3.7 ( 0.2 nm, and R ) 50 ( 3°. This
indicates that the formation of a molecule 2-based monolayer
is thermodynamically favored. It should be noted that no
coexisting structured domains containing molecule 1 were found
in over 100 survey images (i.e., mapping an area of 104 nm2).
Therefore, under these conditions, molecule 1 remains in the
supernatant solution. To avoid such a fractionation, we opted
to operate in a kinetically controlled regime, i.e., by using a
concentration below that needed to form a full monolayer.40
The use of very low concentration solutions (<10 µM) allowed
the simultaneous physisorption of the solutes at the solid-liquid
interface, preventing unfavorable thermodynamically driven
competitive adsorption between modules 1 and 2. As expected,
for concentrations below 10 µM, a new 1·2 heterogeneous
hybrid phase appears (Figure 6; large-area images are shown
in Figure S3c,d in the Supporting Information). Such a
heterogeneous phase was also found to coexist with the two
packing modes of molecule 2, which in this case are in the
minority. We remark that the different phase coverages of the
1·2 phase varied in different experiments and that a trend could
not be established. This was true even when employing ratios
of 1:1, 1:2, 1:4, and 1:6 at different concentrations (from ∼0.02
to ∼6 µM) of molecules 1 and 2, respectively. When some
samples were annealed (∼50 °C, ∼2 min), phases of molecule
2 only were encountered. This further confirms that a mono-
component assembly is the thermodynamically stable phase and
that the imaging of the heterogeneous phase is kinetically
controlled. This kinetic control at low concentrations further
extends the hypothesis that the 2D oligomeric species are created
in solution and then directly transferred to the surface.
Significantly, the STM images of the self-assembled pattern
obtained from films prepared by depositing equimolar mixtures
of modules 1 and 2 on HOPG from concentrated solutions
(concentration >100 µM) (see Figure S3a,b in the Supporting
Information) exactly match that obtained for neat films com-
posed of linear unit 2 (Figure 5c2), featuring a unit cell with a
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Engineering of Supramolecular H-Bonded Nanopolygons
A R T I C L E S
Figure 6. (a) Submolecularly resolved supramolecular aggregates featuring discrete oligomeric species (1)m ·(2)n prepared from diluted equimolar solutions.
Each yellow circle identifies a single molecule of tetrasubstituted 1,3,6,8-pyrene derivative 1. (b) Model assembly of a hexameric hybrid complex (1)2 ·(2)4
showing the distance values as estimated by MM2-based computational geometry optimization. (c, d) Heterogeneous 1·2 phase images highlighting the
presence of rhomboidal and rectangular nanopolygons, respectively, formed as a consequence of complementary H-bonding motifs between molecules 1 and
2. Each yellow circle identifies a single molecule of 1 and each red rectangle a single molecule of 2. The inset in (d) shows the underlying graphite. All of
the constant-current STM images where recorded with It ) 20 pA and Ut ) 400 mV.
Only when high-resolution images of the 1·2 heterogeneous
phase (Figure 6) are obtained modules 1 and 2 can be
unequivocally assigned after close inspection. The images reveal
several polygon-like oligomeric species featuring hollow struc-
tures in line with the geometries shown in Figure 1. The high
adsorption energies of the modules are also evidenced: polygons
featuring large pore areas (Figure 6c) contain adsorbed mol-
ecules of 2. Pores featuring small areas appear to be empty of
solute molecules (Figure 6d). As described in the Introduction,
the structures of the supramolecular complexes are dictated by
the predesigned geometries of the constituent molecular mod-
ules. In particular, the structural fingerprints of the single
modules can be easily discerned within the formed oligomers.
Tetratopic pyrene 1 is imaged (yellow circles) as a bright spot
exposing four arms, whereas ditopic linker 2 (red rectangles)
features three aligned bright lobes. The formation of polygonal
assemblies composed of complementary molecules 1 and 2
indicates that H-bonding interactions have been established.
However, STM image analysis revealed a low percentage of
molecules forming H-bonded pentamers (i.e., one molecule 1
surrounded by four H-bonded molecules of type 2), as depicted
in Figure 6b, namely, ∼0.08 pentamers/nm2 as estimated from
10 different images from different samples. The average angles
γ and ꢀ in the pentamers relative to the pyrene center, as
determined from the STM images, amount to 71 ( 8 and 110
( 7°, respectively. Indeed, there is some slight discrepancy from
the theoretical angles of 60° and 120° defined relative to the
uracil groups because of the elongated pyrene unit. Further-
more, the average distance between two pyrene units in a
linear 1 · 2 · 1 oligomer was found to be 4.3 ( 0.4 nm, which
is in good agreement with the theoretical value of 4.2 nm
estimated by MM2-based geometry optimization (Figure 6b).
It is noteworthy that once they were formed, the supramo-
lecular oligomeric motifs revealed to be stable for several
minutes; thus, no self-healing phenomena were observed on
the time scale of the experiment (see Figures S4 and S5 in
the Supporting Information). Notably, some homomolecular
1 · 1 and 2 · 2 complexes were also observed as coexisting
phases. Our STM studies have provided fundamental infor-
mation on the design and engineering of supramolecular
discrete patterns at interfaces. Even when competitive
adsorption is prevented by using low concentrations, the
supramolecular (1 · 2) oligomers cannot ripen into a thermo-
dynamically stable self-assembled 2D crystalline pattern for
two reasons: (i) module 1 is not centrosymmetric, so
competing phenomena involving the different nucleating
nanopolygons in Figure 6 exist; (ii) the monocomponent
phase formed by 2 intrinsically appears to be the thermody-
namically favored assembly. We have revealed that it is
possible to resolve submolecularly amorphous 2D phases,
which can serve as a powerful tool for the study of
crystallization phenomena. In such phases, STM can locally
probe the dynamics of single molecules (see Figures S4 and
9
J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009 517

A R T I C L E S
Llanes-Pallas et al.
S5 in the Supporting Information), which are relatively
decoupled from a densely packed monolayer.
selectively incorporate suitable designed functional molecules
that can interact with an external signal, exploiting desired
functional activities.
Conclusions
Experimental Section
In summary, we have engineered nanoscale assemblies in
solution and on surfaces through the design of programmed
molecular modules that undergo heteromolecular recognition
through complementary triple H-bonds. In particular, two
molecular modules have been synthesized: a linear unit 2 having
twofold symmetry with two exposed 2,6-di(acylamino)pyridyl
(DAD) recognition sites at its extremities and an angular unit 1
constituted by a 1,3,6,8-tetraethynylpyrene core peripherally
functionalized with four uracil groups (ADA) positioned at 60°
and 120° relative to each other. Steady-state UV/vis absorption
and emission titration measurements showed the reversible
formation of multiple H-bonded oligomeric species in solution.
At the end of the titration of 1 with 2, novel absorption and
fluorescence features were detected; by means of several control
experiments including excitation spectra, these were unambigu-
ously attributed to supramolecular aggregates, likely exhibiting
a variety of association ratios between the two components.
Careful analysis of electronic spectroscopy data allows us to
conclude that in the ground state, H-bond formation between
the ditopic molecule 2 and the tetratopic module 1 is substan-
tially independent for the various sites, which are rather far apart.
On the other hand, fluorescence spectra and singlet lifetimes
indicate the presence of multiple species, suggesting distinctive
properties in the excited state. After the addition of more than
2.4 equiv of linear module 2 to solutions of pyrene 1, suspended
aggregates and precipitates attributed to the generation of
increasingly larger supramolecular arrays were observed. Despite
the fact that other intermolecular interactions can occur (e.g.,
π-π stacking), it has been demonstrated that complementary
H-bonding is essential for the formation of supramolecular
adducts in solution, because unique spectral variations are
observed when both DAD and ADA fragments are present.
Control of the experimental conditions allowed us to transfer
such supramolecular oligomers from solution into the solid-liquid
interface. This made possible the imaging of the 2D oligomers
in real space, allowing the identification of higher-order
structures and showing the expected formation of nanoscale
polygons. Images of the 1·2 heterogeneous phase revealed that
the overall formation of all of the supramolecular systems is
primarily driven by the noncovalent yet highly directional and
selective triple H-bonding interactions established between the
complementary DAD and ADA recognition sites and by the
geometrical molecular constraints, leading to several polygon-
like arrays matching the preprogrammed geometries. We have
highlighted the distinctive roles of self-healing, interfacial free
energy, nucleation, and competing phenomena within 2D
assemblies, which in turn dictate the ripening of the oligomeric
species into ordered or disordered phases. Our observations
allowed us to suggest that oligomers are nucleated in solution
and transferred to interfaces without altering the nature of their
directing interactions. A formal study of the mechanisms
underlying the adsorption of single molecules or complete
oligomeric species requires the development of in-situ spectro-
scopic techniques at the solid-liquid interface. Our findings
and suggestions are of paramount importance for the continuous
development of the field of 2D crystal engineering. Although
further improvements are needed in order to strengthen the
stability and the homogeneity of the assemblies, these nanopo-
lygons could be employed as two-dimensional receptors to
Synthesis. NMR spectra were obtained on a Varian Gemini 200
1
spectrometer (200 MHz H NMR and 50 MHz 13C NMR) and on
a JEOL JNM-EX400 spectrometer (400 MHz 1H NMR). Chemical
shifts (δ) are reported in parts per million using the solvent residual
signal as an internal reference (CDCl3, δH ) 7.26 ppm, δC ) 77.16
ppm; CD3OD, δH ) 3.31 ppm, δC ) 49.00 ppm; C5D5N, δH
)
7.19, 7.55, 8.71 ppm, δC ) 123.5, 135.5, 149.5 ppm; (CD3)2SO,
δH ) 2.50 ppm, δC ) 39.52 ppm). Coupling constants (J) are given
in hertz. Each resonance multiplicity is described as s (singlet), d
(doublet), t (triplet), q (quartet), dd (doublet of doublets), m
(multiplet), or br (broad signal). IR spectra (KBr) were recorded
on a Perkin-Elmer 2000 spectometer by Mr. Paolo de Baseggio. In
regard to mass spectrometry (MS) measurements, ESI was per-
formed on a PerkinElmer API1 instrument at 5600 eV and electron
impact (EI) on a GCQ Finnigan Thermoquest ion trap at 70 eV,
both at Universita` degli Studi di Trieste by Dr. Fabio Hollan.
Melting points were measured on a Bu¨chi SMP-20 apparatus.
Chemicals were purchased from Aldrich, Fluka, and Riedel and
used as received. Solvents were purchased from J.T.Baker and
Aldrich and deuterated solvents from Cambridge Isotope Labora-
tories. The solutions were degassed usin “freeze-pump-thaw”
cycles. The solvents CH2Cl2, toluene, THF, and NEt3 were distilled
from CaH2, Na, Na/benzophenone, and CaH2 respectively. Syn-
theses of 4-bromo-2,6-diaminopyridine,92 1,3,6,8-tetrabromopy-
rene,93 and 1,3,6,8-tetraethynylpyrene94 were performed according
to literature procedures.
1-Hexyluracil (4). To a suspension of uracil (2.8 g, 25 mmol)
in DMSO (30 mL), dry K2CO3 (3.8 g, 27.5 mmol) was added, and
the suspension was stirred for 15-20 min. 1-Bromohexane (3.5
mL, 25 mmol) was added and the reaction mixture stirred for 20 h
at 40 °C. The suspension was diluted with CHCl3, washed with
0.1 M HCl solution (20 mL × 3), H2O (20 mL × 2), and brine (20
mL), and dried over Na2SO4. The organic layer was concentrated
and poured into cold hexane with vigorous stirring. The resulting
precipitate was filtered and washed with cold hexane to afford
1
compound 4 (1.79 g, 35%) as a white solid. Mp: 91-95 °C. H
NMR (200 MHz, CDCl3): δ 10.1 (br, 1H, CONHCO); 7.1 (d,
3
3J(H,H) ) 7.9 Hz, 1H, COCH); 5.7 (d, J(H,H) ) 7.9 Hz, 1H,
3
NCH); 3.7 (t, J(H,H) ) 7.4 Hz, 2H, NCH2(CH2)4CH3); 1.7 (m,
3J(H,H) ) 7.4 Hz, 2H, NCH2CH2(CH2)3CH3); 1.3 (m, 6H,
N(CH2)2(CH2)3CH3); 0.9 (t, 3H, N(CH2)5CH3). 13C NMR (50 MHz,
CDCl3): δ 164.14; 150.93; 144.37; 101.92; 48.78; 31.24; 28.92;
25.98; 22.39; 13.91. IR ν (cm-1): 3417.2; 3155.1; 3098.2; 3044.9;
2951.0; 2931.8; 2862.1; 2821.1; 1975.3; 1694.0; 1646.9; 1467.1;
1420.6; 1368.8; 1250.3, 1179.3; 989.5; 887.7; 816.5; 761.0; 726.0;
558.5. MS (70 eV, EI): found, 196 (M+); calcd for C10H16N2O2,
196.09.
1-Hexyl-6-iodouracil (5). To a THF solution (55 mL) of
1-hexyluracil 4 (1.7 g, 8.7 mmol), LDA (24 mL of a 1.8 M solution,
43.5 mmol) was added dropwise, and the resulting solution was
stirred under Ar at -78 °C for 1.5 h. I2 (11 g, 43.5 mmol) was
added and the reaction mixture stirred for 2 h. The solution was
then treated with acetic acid (1.2 mL) and allowed to warm to room
temperature (r.t.). The organic phase was diluted with CHCl3 (30
mL), washed with saturated aqueous (sat. aq.) NaHCO3 solution
(30 mL × 3), sat. aq. Na2SO3 solution (30 mL × 3), and brine (30
mL), and dried over Na2SO4. Evaporation of the solvents in vacuo
and purification of the crude product by column chromatography
(CC) (5:5 AcOEt/cyclohexane) yielded compound 5 (2.02 g, 72%)
(92) Nettekoven, M. Synlett 2001, 1917.
(93) Vollmann, H.; Becker, H.; Correll, M.; Streeck, H. Justus Liebigs Ann.
Chem. 1937, 531, 1.
(94) Venkataramana, G.; Sankararaman, S. Eur. J. Org. Chem. 2005, 4162.
9
518 J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009

Engineering of Supramolecular H-Bonded Nanopolygons
A R T I C L E S
1
as a white solid. Mp: 123-125 °C. H NMR (200 MHz, CDCl3):
2,6-Diacetylamino-4-[(trimethylsilyl)ethynyl]pyridine (10).
Dry NEt3 (40 mL), THF (6 mL), and DMF (1 mL) were added to
a Schlenk tube, and the solution was degassed by one freeze-pump-
thaw cycle. Diacetylamino pyridine derivative 9 (0.44 g, 1.6 mmol),
[(Pd(PPh3)4] (0.074 g, 0.06 mmol), and CuI (0.024 g, 0.128 mmol)
were added and the solution degassed a second time. TMSA (0.44
mL, 3.2 mmol) was then added, and the reaction mixture was
degassed one last time and stirred overnight at 85 °C under Ar.
The resulting dark mixture was filtered over Celite and washed
with toluene (10 mL), CH2Cl2 (20 mL), and MeOH (20 mL).
Removal of the solvents under vacuum and purification of the crude
product by CC (5:5 cyclohexane/AcOEt) yielded compound 10 (0.4
3
δ 8.6 (br, 1H, CONHCO); 6.4 (s, 1H, COCH); 4.0 (t, J(H,H) )
8.1 Hz, 2H, NCH2(CH2)4CH3); 1.7 (m, J(H,H) ) 8.1 Hz, 2H,
3
NCH2CH2(CH2)3CH3); 1.3 (m, 6H, N(CH2)2(CH2)3CH3); 0.9 (t, 3H,
N(CH2)5CH3). 13C NMR (50 MHz, CDCl3): δ 161.04; 148.02;
115.73; 113.92; 53.84; 31.46; 28.85; 26.14; 22.69; 14.17. IR ν
(cm-1): 3173.7; 3043.6; 1953.7; 2932.6; 2917.1; 2855.1; 1684.9;
1567.8; 1431.0; 1394.5; 1353.7; 1220.4; 1169.4; 1071.9; 1002.7;
827.4; 792.1; 750.9; 720.9; 635.4; 572.8; 537.0. MS (70 eV, EI):
found, 322 (M+); calcd for C10H15IN2O2, 322.14.
1,3,6,8-Tetrakis[(1-hexylurac-6-yl)ethynyl]pyrene (1). To a
degassed solution of dry THF (5 mL) and NEt3 (5 mL), 1-hexyl-
6-iodouracil 5 (0.270 g, 0.84 mmol), [Pd(PPh3)4] (4 mg, 0.034
mmol), and CuI (13 mg, 0.068 mmol) were added, and the resulting
mixture was degassed a second time. 1,3,6,8-Tetraethynylpyrene
8 (0.05 g, 0.17 mmol) was added, and the solution was degassed
one last time and stirred overnight at 45 °C under Ar. After some
minutes, the reaction color changed and a red precipitate appeared.
The suspension was then concentrated in vacuo, and the crude
product purified by several precipitations from CHCl3 upon addtion
of MeOH, yielding compound 1 (0.082 g, 46%) as a dark-red solid.
1H NMR (400 MHz, C5D5N): δ 9.03 (s, 4 H, pyrene-H); 8.90 (s,
1
g, 85%) as a yellow crystalline solid. Mp: 85-90 °C. H NMR
(200 MHz, CDCl3): δ 8.4 (br, 2H, CH3CONH-Ar); 7.8 (br, 2H,
Ar-H); 2.0 (s, 6H, CH3CONH-Ar); 0.2 (s, 9H, Si(CH3)3). 13C
NMR (50 MHz, CDCl3): δ 168.36; 149.49; 135.88; 111.91; 102.44;
99.97; 24.83; 0.11. IR ν (cm-1): 3422.9; 3276.1; 2960.2; 2161.9;
1681.5; 1611.9; 1557.9; 1416.0; 1370.0; 1276.1; 1249.7; 1206.9;
1145.0; 1033.4; 996.7; 983.5; 953.7; 848.1; 760.2; 705.1; 640.7;
625.8; 569.6; 539.3. MS (70 eV, EI): found, 289 (M+); calcd for
C14H19N3O2Si, 289.41.
2,6-Diacetylamino-4-ethynylpyridine (11). To a solution of
TMS-protected ethynylpyridine derivative 10 (0.4 g, 1.38 mmol)
in MeOH (15 mL), a 1 M KOH aq. solution was added, and the
mixture was stirred at r.t. for 40 min. H2O (10 mL) was added,
and the organic phase was extracted with CHCl3 (10 mL × 5) and
dried over Na2SO4. Evaporation of the solvent under vacuum
yielded compound 11 as a yellow crystalline solid in a quantitative
yield. Mp: 208-213 °C. 1H NMR (200 MHz, 1:1 CD3OD/CDCl3):
δ 7.8 (br, 2H, Ar-H); 3.7 (br, 2H, CH3CONH-Ar); 3.1 (s, 1H,
CH); 2.0 (s, 6H, CH3CONH-Ar). 13C NMR (50 MHz, 1:1 CD3OD/
CDCl3): δ 169.79; 149.79; 134.16; 111.72; 81.31; 24.09. IR ν
(cm-1): 3319.0; 3253.2; 3124.4; 2115.1; 1715.6; 1669.5; 1612.1;
1561.0; 1518.5; 1415.9; 1365.6; 1278.1; 1235.7; 1202.7; 1037.5;
998.8; 949.9; 876.0; 853.6; 724.9; 703.3; 673.7; 636.7; 562.4. MS
(70 eV, EI): found, 217 (M+); calcd for C11H11N3O2, 217.22.
2H, pyrene-H); 6.64 (s,
4 H, COCH); 4.37 (br, 8 H,
NCH2(CH2)4CH3); 2.01 (m, 8H, NCH2CH2(CH2)3CH3); 1.53 (m,
8H, N(CH2)2CH2(CH2)2CH3); 1.31 (m, 16H, N(CH2)3(CH2)2CH3);
0.81 (t, 12H, N(CH2)5CH3). 13C NMR (50 MHz, C5D5N): δ 169.67;
152.14; 137.85; 133.64; 131.88; 126.68; 121.41; 118.21; 109.09;
96.41; 88.86; 47.09; 32.13; 27.16; 26.32; 23.26; 14.53. IR ν (cm-1):
3442.6; 3164.0; 3034.1; 2954.8; 2919.3; 2850.8; 2202.9; 1690.2;
1600.0; 1581.9; 1456.2; 1243.3; 1170.5; 1114.1; 825.2; 618.7. MS
(5600 eV, ESI): found, 1075.4(M+), 1098.3 (M + Na)+; calcd for
C64H66N8O8, 1075.26.
1,3,6,8-Tetrakis[(1-hexyl-3-methylurac-6-yl)ethynyl]pyrene
(3). To a suspension of 1,3,6,8-tetrakis[(1-hexylurac-6-yl)eth-
ynyl]pyrene 1 (0.03 g, 0.028 mmol) in DMSO, KOH powder (0.1
g, 1.78 mmol) was added, and the resulting mixture was stirred at
r.t. for 15 min. MeI (0.04 mL, 0.64 µmol) was then added and the
whole mixture allowed to stir at r.t. for 5 h. The resulting red
solution was then diluted with CHCl3 (5 mL), copiously washed
with H2O (5 mL × 5) and brine (5 mL × 2), and dried over Na2SO4.
Removal of the solvent under vacuum and purification of the crude
product by CC (9.8:0.2 CH2Cl2/MeOH) yielded compound 3 (0.015
4,4′-[(Phen-1,4-diyl)diethynyl]bis(2,6-diacetylaminopyridine)
(2). To a degassed solution of dry Et3N (10 mL) and THF (10 mL),
1,4-diodobenzene (0.07 g, 0.22 mmol), [Pd(PPh3)4] (0.01 g, 0.009
mmol), and CuI (3 mg, 0.018 mmol) were added and the mixture
degassed a second time. Ethynylpyridine derivative 11 (0.120 g,
0.55 mmol) was then added, and the reaction mixture was degassed
one last time and stirred overnight at 85 °C under Ar. After 1 h, a
precipitate appeared. The solvent was then concentrated under
vacuum and the precipitate purified by several precipitations from
CHCl3 upon addition MeOH, yielding compound 2 (0.9 g, 83%)
as a pale yellow solid. Mp: >260 °C. 1H NMR (400 MHz, DMSO-
d6): δ 10.25 (s, 4H, CH3CONH-Ar); 7.9 (s, 4H, Ar-H); 7.7 (s,
4H, benzene-H); 2.1 (s, 12H, CH3CONH-Ar). 13C NMR (50 MHz,
DMSO-d6): δ 169.53; 150.69; 132.82; 132.13; 122.10; 110.26;
91.63; 89.54; 24.09. IR ν (cm-1): 3296.7; 2217.3; 1672.3; 1612.1;
1557.6; 1416.4; 1370.3; 1278.8; 1239.6; 1205.7; 1026.9; 1002.3;
859.8; 629.3; 568.3. MS (5600 eV, ESI): found, 531.2 (M + Na)+;
calcd for C28H24N6O4, 508.53.
1
g, 48%) as a red-orange oil. H NMR (200 MHz, CDCl3): δ 8.67
(s, 4H, pyrene-H); 8.45 (s, 2H, pyrene-H); 6.28 (s, 4H, COCH);
4.23 (t, 8H, NCH2(CH2)4CH3); 3.41 (s, 12H, N-CH3); 1.89 (m,
8H, NCH2CH2(CH2)3CH3); 1.6 (m, 8H, N(CH2)2CH2(CH2)2CH3);
1.33 (m, 16H, N(CH2)3(CH2)2CH3); 0.82 (t, 12H, N(CH2)5CH3).
13C NMR (50 MHz, CDCl3): δ 161.90; 151.37; 135.59; 134.95;
133.11; 127.86; 123.75; 117.51; 107.46; 95.97; 87.71; 48.00; 31.62;
29.16; 28.21; 26.64; 22.72; 14.15. IR ν (cm-1): 3404.1; 2858.8;
2363.8; 2207.5; 1707.0; 1664.3; 1452.7; 1369.9; 1265.8; 1104.7;
810.7. MS (5600 eV, ESI): found, 1131.5 (M+), 1153.5 (M + Na)+;
calcd for C68H76N8O8, 1131.36.
Photophysical Measurements. Solutions containing molecule
1 were prepared by dissolving a weighed amount of sample (1 mg)
in a few drops of DMSO, in which it is soluble. Next, a much
larger volume of 1,2,4-trichlorobenzene was added, in order to
achieve a 1:50 volume ratio of the two solvents. Electronic
absorption and emission measurements, respectively, were carried
out on a Lambda 950 UV/vis/NIR spectrophotometer (PerkinElmer)
and an Edinburgh FLS920 spectrofluorometer equipped with a
continuous 450 W Xe lamp and a Peltier-cooled Hamamatsu R928
photomultiplier tube (185-850 nm). Emission quantum yields were
determined according to the approach described by Demas and
Crosby95 using quinine sulfate as the standard (Φem ) 0.546 in
4-Bromo-2,6-diacetylaminopyridine (9). 4-Bromo-2,6-diami-
nopyridine (0.45 g, 2.14 mmol) was dissolved in a solution of
pyridine (1.4 mL, 17.5 mmol) and Ac2O (2.5 mL, 26.4 mmol),
and the reaction mixture was stirred overnight at r.t. The mixture
was then diluted with CHCl3 (10 mL), washed with H2O (10 mL
× 5) and brine (20 mL), and dried over Na2SO4. Solvent evaporation
and precipitation with Et2O yielded compound 9 (0.44 g, 68%) as
a yellow solid. Mp: 210-215 °C. 1H NMR (200 MHz, 1:1 CD3OD/
CDCl3): δ 7.7 (s, 2H, Ar-H); 4.3 (s, 2H, CH3CONH-Ar); 1.9 (s,
6H, CH3CONH-Ar). 13C NMR (50 MHz, 1:1 CD3OD/CDCl3): δ
171.08; 151.20; 135.53; 112.67; 23.93. IR ν (cm-1): 3311.5; 3124.7;
1685.7; 1577.9; 1538.1; 1415.5; 1367.2; 1280.8; 1237.7; 1199.7;
1035.0; 996.7; 861.4; 777.4; 746.9; 693.0; 603.0; 561.3; 549.1. MS
(70 eV, EI): found, 273 (M+); calcd for C9H10BrN3O2, 272.
(95) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
9
J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009 519

A R T I C L E S
Llanes-Pallas et al.
air-equilibrated 1 N H2SO4 solution).96 Emission lifetimes were
determined with the time-correlated single-photon counting tech-
nique using an Edinburgh FLS920 spectrometer equipped with a
laser diode head as the excitation source (1 MHz repetition rate,
λexc ) 337 nm, 200 ps time resolution upon deconvolution) and a
Hamamatsu R928 PMT as the detector (details of the setup are
reported elsewhere).97 The fitting of the experimental data to single-
or multiple-exponential decays was carried out with the software
provided by the instrument manufacturer. Phosphorescence spectra
and related long-lived decay signals were recorded with a Perki-
nElmer LS-50B spectrofluorimeter equipped with a Hamamatsu
R928 PMT. The solvents used were 1,2,4-trichlorobenzene (Sigma-
Aldrich, 99+% spectrophotometric grade) and dimethyl sulfoxide
(Carlo Erba for UV-Fluo spectroscopy, 99.8%).
STM Studies at the Solid-Liquid Interface. Solutions were
prepared by dissolving ∼0.2 mg of 1 (1075 g/mol) or 2 (508 g/mol)
in 50 µL of DMSO and 450 µL of trichlorobenzene (99%
spectrometric grade, Alpha-Aesar). The solutions were heated at
90 °C to achieve complete dilution. The solutions were then diluted
5× in trichlorobenzene to obtain the high-concentration solutions
(>100 µM) and 100× to obtain the low-concentration solutions
(<10 µM). To prepare the high-concentration (>100 µM) 1·2
solutions, 100 µL of the starting solution of 1 was mixed with 50
µL of the 2 solution, and the volume was taken to 500 µL. To
prepare the low-concentration (<10 µM) 1·2 solutions, 100 and
10 µL of the corresponding <10 µM solutions of 1 and 2,
respectively, were mixed. Next, 5 µL of the solutions were drop-
cast onto HOPG. STM imaging at the solid-liquid interface was
performed using a commercial apparatus (multimode Nanoscope
III, Veeco) equipped with a low-current amplifier. First, the lattice
of a freshly cleaved HOPG (001) surface (ZYH grade, Advanced
Ceramics) was examined until thermal equilibrium was reached,
after which a drop of the solution of interest was applied to the
basal plane of the substrate. The Pt/Ir tip was then immersed in
the organic solution. It was possible to visualize either the first
organic layer physisorbed on the basal plane of the substrate or
the HOPG lattice underneath by varying the tunneling parameters.
All of the phases were imaged within the first 10 min of scanning
and remained stable until complete solvent evaporation (∼3 h).
Submolecularly resolved images were recorded with a scan rate
between 0.6 and 0.8 µm/s. The STM images were recorded in
constant-current mode under ambient conditions. Unit cells were
averaged over a minimum of three images after correction for the
piezo drift, which was performed using the underlying graphite as
reference and applying the correction to the image by means of
the Scanning Probe Image Processor (SPIP) version 2.0 software
(Image Metrology ApS, Lyngby, Denmark). Other image treatment
included plane correction, contrast change, and low-weight low-
pass filtering. Untreated images are shown in the Supporting
Information. The reported error bars in the unit cells correspond to
the standard deviation multiplied by a factor of 2. The size of the
pores estimated by grain analysis did not take into account tip
convolution effects. MM2-based optimization was performed with
Chem3D Ultra 5.0.
Acknowledgment. This work was financially supported by the
European Union through the Marie-Curie Research Training
Network PRAIRIES (MRTN-CT-2006-035810) and EST-SUPER
(MEST-CT-2004-008128), the CNR (Commessa PM.P04.010,
MACOL), MIUR (Firb RBIN04HC3S), the Belgian National
Research Foundation (FRS-FNRS, through Contract 2.4.625.08 F),
the University of Namur, the ERA-Chemistry Project SurConFold,
the ESF-SONS2-SUPRAMATES project, and the Regione Emilia-
Romagna PRIITT Nanofaber Net-Laboratory. A.L.-P. thanks Uni-
versita` di Trieste for the doctoral fellowship.
Supporting Information Available: Figures S1-S5 and NMR
and mass spectra of the synthesized compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
(96) Meech, S. R.; Phillips, D. J. Photochem. 1983, 23, 193.
(97) Armaroli, N.; Accorsi, G.; Song, F.; Palkar, A.; Echegoyen, L.;
Bonifazi, D.; Diederich, F. ChemPhysChem 2005, 6, 732.
JA807530M
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520 J. AM. CHEM. SOC. VOL. 131, NO. 2, 2009