From molecular to macroscopic engineering: Shaping hydrogen-bonded organic nanomaterials

Article abstract of DOI:10.1002/chem.201002103

The self-assembly and self-organization behavior of chromophoric acetylenic scaffolds bearing 2,6-bis(acetylamino)pyridine (1, 2) or uracyl-type (3-9) terminal groups has been investigated by photophysical and microscopic methods. Systematic absorption and luminescence studies show that 1 and 2, thanks to a combination of solvophilic/solvophobic forces and π-π stacking interactions, undergo self-organization in apolar solvents (i.e., cyclohexane) and form spherical nanoparticles, as evidenced by wide-field optical microscopy, TEM, and AFM analysis. For the longer molecular module, 2, a more uniform size distribution is found (80-200nm) compared to 1 (20-1000nm). Temperature scans in the range 283-353K show that the self-organized nanoparticles are reversibly formed and destroyed, being stable at lower temperatures. Molecular modules 1 and 2 were then thoroughly mixed with the complementary triply hydrogen-bonding units 3-9. Depending on the specific geometrical structure of 3-9, different nanostructures are evidenced by microscopic investigations. Combination of modules 1 or 2 with 3, which bears only one terminal uracyl unit, leads to the formation of vesicular structures; instead, when 1 is combined with bis-uracyl derivative 4 or 5, a structural evolution from nanoparticles to nanowires is observed. The length of the wires obtained by mixing 1 and 4 or 1 and 5 can be controlled by addition of 3, which prompts transformation of the wires into shorter rods. The replacement of linear system 5 with the related angular modules 6 and 7 enables formation of helical nanostructures, unambiguously evidenced by AFM. Finally, thermally induced self-assembly was studied in parallel with modules 8 and 9, in which the uracyl recognition sites are protected with tert-butyloxycarbonyl (BOC) groups. This strategy allows further control of the self-assembly/self-organization process by temperature, since the BOC group is completely removed on heating. Microscopy studies show that the BOC-protected ditopic modules 8 self-assemble and self-organize with 1 into ordered linear nanostructures, whereas BOC-protected tritopic system 9 gives rise to extended domains of circular nano-objects in combination with 1.

Full text of DOI:10.1002/chem.201002103

DOI: 10.1002/chem.201002103
From Molecular to Macroscopic Engineering: Shaping Hydrogen-Bonded
Organic Nanomaterials
K. Yoosaf,[a] Anna Llanes-Pallas,[b] Tomas Marangoni,[b] Abdelhalim Belbakra,[a]
Riccardo Marega,[c] Edith Botek,[c] Benoꢀt Champagne,[c] Davide Bonifazi,*[b, c] and
Nicola Armaroli*[a]
Abstract: The self-assembly and self-
organization behavior of chromophoric
acetylenic scaffolds bearing 2,6-bis(ace-
tylamino)pyridine (1, 2) or uracyl-type
(3–9) terminal groups has been investi-
gated by photophysical and microscop-
ic methods. Systematic absorption and
luminescence studies show that 1 and
2, thanks to a combination of solvo-
philic/solvophobic forces and p–p
stacking interactions, undergo self-or-
ganization in apolar solvents (i.e., cy-
clohexane) and form spherical nano-
particles, as evidenced by wide-field
optical microscopy, TEM, and AFM
analysis. For the longer molecular
module, 2, a more uniform size distri-
bution is found (80–200 nm) compared
to 1 (20–1000 nm). Temperature scans
in the range 283–353 K show that the
self-organized nanoparticles are revers-
at lower temperatures. Molecular mod-
ules and were then
which prompts transformation of the
wires into shorter rods. The replace-
ment of linear system 5 with the relat-
ed angular modules 6 and 7 enables
formation of helical nanostructures, un-
ambiguously evidenced by AFM. Final-
ly, thermally induced self-assembly was
studied in parallel with modules 8 and
9, in which the uracyl recognition sites
are protected with tert-butyloxycarbon-
yl (BOC) groups. This strategy allows
further control of the self-assembly/
self-organization process by tempera-
ture, since the BOC group is complete-
ly removed on heating. Microscopy
studies show that the BOC-protected
ditopic modules 8 self-assemble and
self-organize with 1 into ordered linear
nanostructures, whereas BOC-protect-
ed tritopic system 9 gives rise to ex-
tended domains of circular nano-ob-
jects in combination with 1.
1
2
thoroughly mixed with the complemen-
tary triply hydrogen-bonding units 3–9.
Depending on the specific geometrical
structure of 3–9, different nanostruc-
tures are evidenced by microscopic in-
vestigations. Combination of modules 1
or 2 with 3, which bears only one ter-
minal uracyl unit, leads to the forma-
tion of vesicular structures; instead,
when 1 is combined with bis-uracyl de-
rivative 4 or 5, a structural evolution
from nanoparticles to nanowires is ob-
served. The length of the wires ob-
tained by mixing 1 and 4 or 1 and 5
can be controlled by addition of 3,
Keywords: hydrogen bonds · nano-
structures · self-assembly · supra-
molecular chemistry
ACHTUNGTRENNUNGibly formed and destroyed, being stable
Introduction
based on the controlled assembly of molecular building
blocks.[1–8] Indeed, the geometric structure of molecules can
considerably influence their ability to self-organize into
more complex objects[9–14] and therefore tune the structure
and properties of the final material.[8,10,15,16] Functional mo-
lecular-ordered materials nowadays have potential applica-
tions in a wide range of fields, such as solar cells,[17,18] poly-
mer-based transistors,[19–21] and the production of new semi-
conductors.[22–25] Various approaches including colloidal,[5,26]
templated,[27,28] and self-assembly[29–31] methods have been
developed to prepare materials with properties distinct from
those of the single molecule or bulk phase. In this context,
the supramolecular approach,[32] which exploits molecular
recognition processes based on the combination of a variety
of noncovalent interactions, can give access to a large
number of self-assembled molecular architectures,[33–39]
which may undergo further self-organization with formation
of controlled macroscopic systems.[12,30,40–44] In the literature
the terms self-assembly and self-organization are often used
interchangeably, probably because there is no general con-
sensus on their definitions.[45,46] Herein, the term self-assem-
In the last decades, chemists have devoted a great deal of
effort to the preparation of novel nanostructured materials
[a] Dr. K. Yoosaf, Dr. A. Belbakra, Dr. N. Armaroli
Molecular Photoscience Group
Istituto per la Sintesi Organica e la Fotoreattivitꢀ
Consiglio Nazionale delle Ricerche (CNR-ISOF)
Via Gobetti 101, 40129 Bologna (Italy)
Fax : (+39)051-6399844
E-mail: nicola.armaroli@isof.cnr.it
[b] Dr. A. Llanes-Pallas, T. Marangoni, Prof. D. Bonifazi
Dipartimento di Scienze Farmaceutiche, Universitꢀ di Trieste
Piazzale Europa 1, 34127 Trieste (Italy)
[c] Dr. R. Marega, Dr. E. Botek, Prof. B. Champagne, Prof. D. Bonifazi
Chemistry Department, University of Namur
Rue de Bruxelles 61, 5000 Namur (Belgium)
Fax : (+32)8172-5433
E-mail: davide.bonifazi@fundp.ac.be
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem201002103.
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FULL PAPER
bly refers to the processes of molecular recognition through
hydrogen-bonding interactions between the single molecular
modules, whereas the self-organization processes are consid-
ered to be those in which the macroscopic pattern emerges
solely from numerous interactions among the molecular
components of the system, resulting in the formation of
larger submicrostructures with varying sizes and shapes.
Recognition sites based on complementary hydrogen-
bonding systems are versatile tools for organization and
tuning of the shape of materials at the nanoscale level.[47–50]
Recently, we showed that, under the control of temperature
and solvent polarity, supramolecular adducts between two
p-conjugated molecules bearing complementary hydrogen-
bonding groups can be formed in solution and further
evolve as hollow spherical nanoparticles with a vesicular
structure.[51] Most notably, we have demonstrated that the
size distribution and shape of the vesicles can be tuned by
means of the stoichiometric ratio of the two constituent mo-
lecular components. This behavior was interpreted to be a
consequence of the specific solvophobic/solvophilic interac-
tions established between the supramolecular adducts.
Herein we extend this strategy and report a systematic
study on several combinations of suitably designed molecu-
lar modules 1–9 that undergo complementary triple hydro-
gen-bonding interactions. Specifically, supramolecular recog-
nition is mediated by triple parallel hydrogen bonds estab-
lished between the NH–N–NH (donor–acceptor–donor,
DAD) groups of 2,6-di(acetylamino)pyridine and the CO–
NH–CO (ADA) imidic groups of a uracyl derivative.[52–54]
Selection of these two moieties is based on the fact that
they show very high hydrogen-bonding association constants
on the order of 103 mꢀ1 in polar solvents like CHCl3 and 104–
105 mꢀ1 in apolar solvents like cyclohexane (CHX) and
CCl4.[51,55,56] Our aim is to investigate how the structural
properties of the molecular modules affect the nanoscale
morphology of the self-assembled hydrogen-bonded aggre-
gates.
Molecular modules 1–9 can be classified into three differ-
ent groups: molecules bearing 1) bis(acetylamino)pyridyl
(DAD) groups (1, 2), 2) uracyl (ADA) groups (3–7), and
3) tert-butyloxycarbonyl (BOC)-protected uracyl (ADA)
groups (8, 9). Compound 2, an elongated analogue of
module 1, is essentially a linear conjugated phenyleneethy-
nylene oligomer (OPE) terminated with two DAD hydro-
gen-bonding recognition units. Molecule 3 bears only one
uracyl moiety and is terminated with an anthracenyl unit,
which is used to tune the solvophobicity of the resulting
supramolecular aggregate (vide infra). The other modules
bearing uracyl ADA units (4–7) were conceived as angular
modules in which the peripheral recognition sites occupy
para, meta, or ortho positions relative to the central alkoxy-
lated benzene ring. Finally, in modules 8 and 9, BOC-pro-
tected uracyl recognition sites provide better solubility in or-
ganic solvents and thus easier processability when nano-
structured under thermal control. Combination of comple-
mentary modules 1 and 2 with 3–9 gives rise to a variety of
nanostructured architectures such as nanoparticles, nanoves-
AHCTUNGTRENGNiUN cles, nanofibres, and nanorods.
Results and Discussion
Self-organization of individual molecules into nanoparticles:
The UV/Vis absorption and emission spectra of 1 and 2 in
different solvents show that, in most cases, there is little
effect of the solvent polarity, while the redshifted absorption
and emission profiles observed in H2O and CHX suggest
formation of aggregates (Figure 1). This could be due to a
self-organization process initiated by solvophilic/solvophobic
forces and p–p stacking interactions.[10,57]
The self-organization behavior of these molecules in CHX
was confirmed by spectroscopic investigations as a function
of temperature (Figure 2). On increasing the temperature
from 298 to 353 K the absorption spectra in CHX under-
went a gradual hypsochromic shift (Figure 2a and c). The
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D. Bonifazi, N. Armaroli et al.
inal fluorescence band above 500 nm (Figure S2, Supporting
Information). The observed redshifts at low temperatures
are attributed to p–p stacking, most likely in J-type aggre-
gates.[10,58,59]
Simulated spectra for 1 and 2 (Figure 3) match their ex-
perimental counterparts well (see Figure 1). In particular,
theory reproduces the redshift of the lowest energy band on
enlarging the conjugated segment (exptl: from 380 to
Figure 1. Absorption (left) and fluorescence (right) spectra of 3.3 mm so-
ACHTUNGTRENNUNGlutions of 1 (a, b; lexc =370 nm) and 2 (c, d; lexc =360 nm) in solvents of
different polarity.
Figure 3. Calculated absorption spectra of 1 (a) and 2 (b) with a manifold
of 50ꢂ50 states and shifted by 0.23 eV and 0.26 eV, respectively, to match
the lowest-energy experimental absorption peak.
420 nm; theory: from 355 to 384 nm). Nevertheless, for 1,
the relative position of the highest energy absorption band
with respect to the lowest one is not well reproduced: the
excitation energy is overestimated by about 0.7 eV. Calcula-
tions on J aggregates of both 1 and 2 showed an exciton
shift of about 1000 cmꢀ1, which can explain the change of
shape of the lowest energy absorption band.[60,61]
To determine the shape and size of the materials formed
through self-organization, various microscopic studies such
as wide-field optical microscopy (WFM), tapping mode
atomic force microscopy (TM-AFM), and transmission elec-
tron microscopy (TEM) were performed (Figure 4). In all
characterization experiments, the solution used was subject-
ed to several heating and cooling cycles in order to un-
Figure 2. Absorption (left) and emission (right) spectra of 3.3 mm solu-
tions of 1 (a, b; lexc =390 nm) and 2 (c, d; lexc =370 nm) as a function of
temperature in CHX.
spectral shape at the highest temperature is virtually identi-
cal to that in solvents in which the compounds are soluble at
room temperature (Figure 1) and thus indicates disruption
of the aggregates. Notably, these changes in the absorption
and emission spectra are completely reversible on cooling
the sample, with a gradual redshift of the absorption profile
(Figures S1 and S2, Supporting Information). During the
cooling cycle (353!298 K), in line with the absorption
trends, the initial fluorescence band of 1 is progressively
quenched, while a new redshifted feature appears with max-
imum at about 460 nm (Figure S1, Supporting Information).
In extended p-conjugated module 2, the growing emission
feature is weaker and appears only as a shoulder of the orig-
Figure 4. WFM (a, d), TM-AFM (b, e), and TEM (c, f) images of self-or-
ganized nanostructures formed from CHX solutions containing molecules
1 (a–c) or 2 (d–f).
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Shaping Hydrogen-Bonded Organic Nanomaterials
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ACHTUNGTRENNUNGambiguously probe the self-organization. The WFM images
recorded from different areas indicated that both 1 (Fig-
ure 4a) and 2 (Figure 4d) form bright fluorescent particles
of different sizes.
AFM and TEM images of 1 recorded from different areas
of the sample indicate the presence of both nano- and mi-
croparticles with sizes typically ranging from about 20 nm to
greater than 1 mm (Figure 4b and c and Figure S3, Support-
ing Information). In contrast, molecular module 2 alone
generates nanoparticles with a much more uniform size dis-
tribution ranging from 80 to 200 nm (Figure 4e and f and
Figure S4, Supporting Information). This difference in size
distribution can be rationalized by taking into account the
different lengths of the OPE spacing units of 1 and 2. These
molecules have a central apolar phenyleneethynylene core
functionalized with alkoxyl groups and terminal polar
Figure 5. a) Absorption spectra of 1, 3, their 1:2 adduct [1·(3)2] and alge-
braic sum of the absorption profiles of the individual compounds (1+
(3)2). b) Variable-temperature absorption and c) emission (lexc =347 nm)
spectral changes in CHX of adduct [1·(3)2]. d) WFM, e) TM-AFM, and
f) TEM images of nanostructures composed of [1·(3)2].
bis(aceACHTUNGTRENNUNGtylamino)pyridyl moieties. In an apolar solvent like
CHX, molecules 1 and 2 undergo self-organization through
either electrostatic or hydrogen-bonding interactions of the
terminal polar groups, leading to the formation of energy-
minimized spherical particles (Scheme 1). Compared to 1,
extended linear module 2 exhibits a larger percentage of sol-
vophilic structure and hence can form nanoparticles with a
narrower size distribution.
ure 5a). In particular, the observed redshift of the lower
energy absorption band of anthracenyl derivative 3 (not ob-
served for 3 alone, Figure S5 in Supporting Information)
points to the presence of p–p
stacking
interactions,
most
likely promoted by preliminary
formation of triple hydrogen
bonds between complementary
uracyl and 2,6-di(acetylamino)-
pyridyl moieties.[63] The emis-
sion spectrum at 353 K is due
to a combination of the fluores-
cence features of both isolated
1 and 3 (see the absorption and
emission profiles of 1 and 3 at
353 K in CHX, Figure S6 in the
Supporting Information), since
both chromophores are excited
at 347 nm. Most notably, only
the sample containing both 1
and 3 (1:2) exhibits a fluores-
cence spectrum with a broad
Scheme 1. Schematic representation of the combined self-assembly and self-organization processes of the
linear modules 1--4, leading to different nanostructures, namely, particles, fibers, vesicles, and rods. The use of
angular modules, such as 6 or 7, can also lead to helical nanofibres (vide infra).
From particles to vesicles: In a given solvent, the nature of
the functional groups (i.e., solvophobic or solvophilic) plays
a key role in the aggregation process,[62] and one can expect
to tune structural features by combining molecules with dif-
ferent functional groups. First, compound 3, which has only
one hydrogen-bonding site, was combined with ditopic
linear unit 1. The 2:1 self-assembly of 3 with 1 in CHX re-
sults in a dramatic change of the chemical nature of periph-
eral functionalities from solvophobic to solvophilic, due to
the presence of an anthracenyl unit in 3 (Scheme 2).
tail on the low-energy side (Figure 5c) at 298 K.
The simulated and experimental spectra for compounds 1
and 3 as well as for the trimeric assembly [1·(3)2] are pre-
sented in the Supporting Information (Figure S7). They con-
firm that the lowest-energy absorption band of the trimer
comes from the spectrum of compound 3 and that, in ab-
sence of aggregation, the spectrum of the supramolecular
adduct corresponds almost to the algebraic sum of the spec-
tra of its components.
Wide-field microscopic (WFM) images (Figure 5d) indi-
cate the presence of bright hollow, luminescent, circular
shapes, which are attributable to either rings or vesicular
structures; similar shapes are observable by AFM (Figure 5e
and Figure S8, Supporting Information). Conclusive support
In a typical experiment, 6.6 mm of compound 3 was com-
bined with 3.3 mm of 1 in CHX (2:1 ratio). The absorption
spectrum of the mixture is rather different from that ob-
tained by adding those of the separated components (Fig-
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D. Bonifazi, N. Armaroli et al.
for the formation of nanovesicles is obtained from TEM
images (Figure 5 f).
whereas it dissolves well in toluene and most of the other
solvents.
The formation of nanovesicles can be rationalized by the
changing nature of molecular end groups. In CHX com-
pound 1 alone, due to the presence of polar bis(acylamino)-
pyridyl groups, tends to self-organize through electrostatic
or weak hydrogen-bonding interactions involving the termi-
nal moieties and leading to spherical nanostructures (vide
supra). After the establishment of complementary hydro-
gen-bonding interactions with molecule 3, the nature of the
end functionality of the supramolecular adduct [1·(3)2] is in-
verted from solvophobic (hydrogen-bonding sites) to solvo-
philic (anthracene units, prompting the change from filled to
vesicular/hollow structures, Scheme 2).
The absorption spectrum of a mixture of 1 and 4 in tolu-
ene at room temperature ([1·4]n) is rather different from
that of their algebraic sum; in particular, a new redshifted
band around 430 nm is observed (Figure 6a). To understand
the origin of the new band and hence to prove the revers-
AHCTUNGTREGiNNUN bility, absorption and emission studies as a function of tem-
perature were carried out. On increasing the temperature
From nanoparticles to nanowires: To probe the effect of ex-
tended linear hydrogen bonding, we then investigated the
self-assembly of 1 with linear OPE derivatives of different
solubility equipped with uracyl termini (4 or 5). In principle,
the complementary hydrogen-bonding interaction can result,
in this case, in a supramolecular linear polymer (Scheme 3).
On mixing 1 and 4 in CHX in a 1:1 molar ratio, a precipitate
is immediately generated, indicating the formation of an in-
soluble supramolecular polymer aggregate, which cannot be
dissolved even at temperatures as high as 353 K. To avoid
formation of this intractable material, further studies were
carried out in toluene, which also exhibits low polarity but it
is more likely to dissolve organic conjugated aromatic sys-
tems. The absorption and emission spectra of 4 recorded in
different solvents (Figure S9, Supporting Information) show
that it undergoes self-organization in H2O and CHX,
Figure 6. a) Absorption spectra of 3.3 mm toluene solutions of 1, 4, [1·4]n,
and algebraic sum of the absorption profiles of the individual compounds
(1+4). b) Absorption and c) emission (lexc =380 nm) spectral changes of
[1·4]n in toluene as a function of temperature. The dashed line in c) is the
fluorescence spectrum at 293 K normalized in intensity relative to that at
358 K. d–f) TM-AFM images of the wirelike nanostructures, organized in
toluene and deposited on mica.
*
Scheme 2. Representation of H-bonded self-assembly between 1 and 3, adduct [1 (3)2].
Scheme 3. Molecular representation of linear hydrogen-bonded self-assembly between 1 and 4 to give adduct [1·4]n.
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from 298 to 358 K, the absorption band around 430 nm
gradually disappears with concomitant increase of the opti-
cal density at higher energy (Figure 6b). At 358 K, an ab-
sorption spectrum very similar to that of the algebraic sum
of the absorption profiles of the individual compounds is ob-
served. As the temperature rises, the corresponding emis-
sion spectra show a gradual increase of the band intensity at
430 nm (Figure 6c). At the highest temperature, 358 K, the
detected fluorescence spectrum corresponds to that of a
mixture of individual units 1 and 4, with a prevalent contri-
bution from 1, which exhibits the highest absorbance at the
excitation wavelength of 380 nm and almost unitary fluores-
cence quantum yield (see Figure 6a and Figure S10, Sup-
porting Information). On cooling the system from 358 to
293 K, complete restoration of the original absorption spec-
trum is observed, along with a steady decrease of the emis-
sion intensity to the initial value. Normalization of the spec-
tral profiles obtained at the highest and lowest temperatures
(at peak wavelength) shows widening on the low-energy
side at 293 K, and again indicates formation of aggregates at
lower temperatures (vide supra). The observed spectral
changes on heating are attributed to the disruption of the
supramolecular aggregates, which are formed by hydrogen-
bonding and p-stacking interactions at room temperature.
Indeed, AFM images of nanostructures deposited from a
toluene suspension onto mica recorded at different areas of
the sample indicate the presence of nanowires with lengths
greater than 400 nm and widths of about 30 nm (Figure 6d
and f).
A quite similar spectroscopic behavior in toluene is ob-
served for a 1:1 mixture of 1 and 5 ([1·5]n). Typical TM-
AFM images recorded for [1·5]n also show the formation of
nanofibres (Figure 7). Compared to assembly [1·4]n, [1·5]n
forms much longer nanofibres with lengths on the order of
several micrometers and typical widths between 40 and
80 nm (Figure S11, Supporting Information). The increase in
fiber length is probably attributable to the presence of the
alkoxyl chains, which enhance the solubility of the supra-
molecular polymer. At any rate, the hydrogen-bonding inter-
actions between 1 and either 4 or 5 cause the formation of
extended linear polymeric structures that, through p–p
stacking interactions between the aromatic units, prompt the
formation of nanowire-like architectures (Scheme 1).
Figure 7. TM-AFM images of self-organized nanostructures of [1·5]n, or-
ganized from toluene and deposited on mica.
From nanowires to nanorods: In an attempt to further mod-
ulate the structure of the self-organized nanomaterials, we
combined three different molecular components: the anthra-
cene derivative bearing only one uracylic unit (3) with mod-
ules 1 and 4, both with H-bonding sites at the ends. The ra-
tionale behind this choice is to control the length of the
polymers on addition of a predetermined amount of molecu-
lar stopper 3, which should reduce the extent of the linear
hydrogen-bonded assembly in [1·4]n (Scheme 4) and possibly
lead to size-controlled nanomaterials. To test this possibility
we investigated the self-organization behavior of mixtures
of 1, 4, and 3 in the molar ratio 10:10:1 in toluene.
In a typical experiment, modules 1, 4, and 3 were added
to toluene at 373 K, stirred for 3 min, and slowly cooled to
room temperature. About 20 mL each of this solution was
drop casted onto a freshly cleaved mica surface for AFM
imaging or onto a carbon-coated copper grid for TEM
measurements. Both the AFM and TEM images (Figure 8)
recorded from different areas of the sample clearly showed
the presence of a large portion of nanorods along with some
Figure 8. a, b) TMAFM and c) TEM images of nanostructures, organized
from a toluene solution of modules 1, 4, and 3 in molar ratio 10:10:1.
Scheme 4. Molecular representation of the anthracenyl-capped hydrogen-bonded self-assembly between 1, 4 and 3.
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D. Bonifazi, N. Armaroli et al.
spherical aggregates. The typical length and width of the
rods are 80–200 and 40–60 nm, respectively; their formation
is schematically depicted in Scheme 1. The absorption and
luminescence properties of the mixture of 1, 4, and 3 are
substantially identical to those of 1 and 4 due to the very
low concentration of 3.
Helical nanowires: Self-assembly between a linear module
(1) and molecules having angular structures (6 and 7) was
also investigated (Scheme 5). Molecular modules 5–7 are
similar in composition and in the number of hydrogen-bond-
ing sites, but the latter differ in their relative position. Mole-
cule 5 is a linear module with a twofold symmetry and the
reactive sites are placed at 1808 (para) relative to each
other; in molecules 6 and 7 the hydrogen-bonding sites are
located at 120 (meta) and 608 (ortho), respectively.
Compound 1 was mixed with either 6 or 7 (1:1 molar
ratio) in CHX/4% tetrahydrofuran (THF).[64] This solvent
composition was chosen to prevent self-organization of the
individual compounds, which was evidenced in pure CHX
(Figure 1 and Figure S12, Supporting Information). Similar
to previous cases, on mixing the two components, redshifting
of the absorption and emission features is observed. Further-
more, variable-temperature studies show reversible changes
of the absorption and emission spectra, with blueshifted fea-
tures at higher temperature, eventually shifted to lower en-
Figure 9. Absorption (left) and emission (right, lexc =380 nm) spectral
changes of [1·6]n (a, b) and [1·7]n (c, d) as a function of temperature in
CHX/THFACHTUNTRGNEU(GN 4%) solutions.
ergies on cooling (Figure 9). This trend can be again attrib-
uted to the formation of aggregates arising from comple-
mentary hydrogen-bonding and p–p stacking interactions.
The [1·6]n and [1·7]n assemblies were also characterized by
molecular modeling and compared with experiments
Scheme 5. Molecular representation of hydrogen-bonded self-assembly between 1 and either 6 or 7, giving adducts [1·6]n, and [1·7]n.
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Shaping Hydrogen-Bonded Organic Nanomaterials
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evident intensity enhancements as well as a shift with re-
spect to the monomeric species.
Thermally controlled polymerization: To achieve better sol-
ubility and thus improved processability of our molecular
modules in organic solvents (otherwise impossible for triur-
acyl molecule 9 without BOC groups), BOC-protected mo-
lecular modules 8 and 9 were also synthesized (Figure S15
and S16, Supporting Information). In these compounds, the
uracyl recognition sites are protected with BOC groups,
which prevent homomolecular self-assembly directed by
double hydrogen-bonding recognition. Since a BOC group
can be selectively removed by heat treatment,[65] the hydro-
gen-bonded assemblies could be generated in situ upon ther-
mal treatment (Scheme 6).
Figure 10. Calculated absorption spectrum for the a) [1·6] and b) [1·7]
dimers (manifold of 50ꢂ50 excited states and redshifted by 0.4 eV).
c) Spectral evolution (25nꢂ25n manifold) for 1·6 oligomers.
(Figure 10). For the dimer [1·6], the nature of the lowest
transition, with two peaks at 360 and 400 nm is well repro-
duced by the calculations whereas the peak around 320 nm
is predicted to appear at a shorter wavelength (280 nm).
Again, there is a good match between the spectrum of the
dimer and the algebraic sum of the spectra of the two moie-
ACHTUNGTRENNUNGties. Thus, calculations performed on trimers [6·1·6] and
[1·6·1] as well as on tetramer [6·1·6·1] show that size effects
are rather small but that the stoichiometry strongly impacts
on the relative intensities, since the lowest-energy band re-
sults from both molecules.
For both [1·6]n and [1·7]n assemblies, the AFM images in-
dicate the formation of nanofibres, but a close analysis of
the images revealed that they are entirely different from
those formed by combining 1 with either 4 or 5 (see above).
In the present case, each fiber exhibits humps and lumps
suggesting the formation of helicoidally organized nano-
structures (Figure 11).
Such helicoidal structures are indeed obtained from PM6
geometry optimizations, as illustrated in Figure S13 of the
Supporting Information, with three- and sixfold symmetry
for ortho and meta substitution, respectively. Figure S14 of
the Supporting Information further shows that these helical
structures display specific CD signatures, which could be re-
vealed provided the nonracemic mixtures are obtained. Typ-
ically the lowest Cotton absorptions of the helices display
Scheme 6. Schematic representation of the self-assembly process induced
by thermal removal of the BOC protecting group.
To test the stability and thermal cleavage as well as to
obtain quantitative data on BOC removal from modules 8
and 9, thermogravimetric analysis (TGA) under inert atmo-
AHCTUNGTREGsNNNU phere was carried out (Figure 12 and Figure S17, Support-
ing Information). As shown in Figure 12, a steep one-step
weight loss in the temperature interval of 388–423 K (115–
1508C), corresponding to about 28.4 wt% is observed for
Figure 11. TMAFM images of assemblies [1·6]n (a–c) and [1·7]n (d, e) de-
posited on mica from a CHX/THF (4%) solution. f) Height cross-section
profile along the delimited lines traced in e).
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D. Bonifazi, N. Armaroli et al.
Figure 12. Temperature-modulated (solid line) and differential (dashed
line) TGA plots of
9 (0.796 mg) recorded under a N2 flow of
60 mLminꢀ1. Insets: details of the 1H NMR spectra of 9 before and after
the thermal removal of the BOC moieties.
both compounds. This weight loss is attributed to the pyroly-
sis of the BOC moieties, since it completely matches the cal-
culated theoretical weight loss for the removal of these
groups. To further confirm selective thermal deprotection
and the integrity of the deprotected synthon after thermo-
ACHTUNGTRENNUNGgravimetric analysis, the organic residues were characterized
1
by H NMR spectroscopy in DMSO (Figure 12, insets). The
NMR spectra of the compounds show total disappearance
of the BOC proton resonances centered at 1.60 ppm and the
onset of a broad resonance at 11.5 ppm, which is unambigu-
ously attributed to the imidic protons of the free uracyl moi-
eties. The other signals are maintained, which confirms that
the molecular skeleton of both 8 and 9 is preserved and that
only BOC groups have been efficiently removed after ther-
mal treatment.
These results prompted a TM-AFM study of the nano-
structures on mica surfaces, by combining 8 or 9 with 1, fol-
lowed by a thermal treatment (Figure S18, Supporting Infor-
mation). In a typical experiment, the two mixtures (8/1 and
9/1, 1:1 molar ratio) were prepared in a CH2Cl2/o-xylene
(9:1) solution, drop cast on a freshly cleaved mica surface,
and heated at 1458C under Ar atmosphere for 15 min.
As intuitively expected from their molecular shape and
reciprocal positioning of the recognition sites, the combina-
tion of 1 and 8 leads to the formation of linear structures re-
sembling rods or blocks with nanoscopic dimensions
(Figure 13, Scheme 7). In every image, the observed rectan-
gular objects exhibit a flat surface with a linear shape and
well-defined borders. The average dimensions of the objects
range from 50 to 500 nm in length, from 40 to 200 nm in
width, and from 1 to 9 nm in height. By comparison with the
structures obtained by direct mixing of modules 1 and 4
(Figure 6), increased homogeneity of the nanostructures is
evidenced (Figure 13). We tentatively explained it by consid-
ering that the thermally induced self-assembly of 1 and 8
occurs directly on the surface and thus in a confined envi-
ronment, which has fewer degrees of freedom and reduces
Figure 13. TM-AFM height images, at different magnifications, of the
self-organized nano-objects obtained after thermal removal of the BOC
groups [1·8-BOC]n (a–c) and [1·9-BOC]n (d–f) assemblies on mica sur-
face. 3D representation of the AFM topographies for [1·8-BOC]n and
[1·9-BOC]n (g, h), respectively.
Scheme 7. Schematic representation of the thermally induced self-assem-
bly process of 1 with BOC-protected linear (8) and trigonal (9) modules.
3270
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Shaping Hydrogen-Bonded Organic Nanomaterials
FULL PAPER
the number of possible secondary aggregation phenomena.
Moreover, in solution the aliphatic lateral chains present on
both complementary modules can foster aggregation of the
supramolecular system by means of secondary solvophobic
interactions. At the solid/liquid interface not only may this
effect be reduced, but also the interaction between the
single molecules and the surface can increase the order of
the system and thus favor the formation of flat, well-defined
nanostructures. Remarkably, modulation of the self-organi-
zation process by the surface is strongly dependent on the
concentration of the molecular modules deposited on the
surface. At higher concentration (1.4 mm), for which the in-
teraction with the surface is less extensive and thus the in-
termolecular interactions dominate, no ordered nanostruc-
tures are observed (Figures S18–S26, Supporting Informa-
tion).
Unexpectedly, the combination of 1 and 9 (Scheme 7)
leads to the formation of nano-objects with circular shapes,
resembling a ring (Figure 13d and f). The average dimen-
sions of the objects range from 680 to 860 nm and 290 to
390 nm in outer and inner diameter, respectively, and from 1
to 4 nm in height. The unique geometrical features of assem-
bled objects can be tentatively attributed to a biphasic
nature of the self-assembly/self-organization processes,
which could originate from a nucleation step taking place
during the thermal polymerization process. As a conse-
quence of the two-dimensional molecular character of
module 9, assembly [1·9-BOC]n could give birth to bidimen-
sional nanostructures that, depending on the concentration
of the single modules, could cover the entire surface. At
higher concentration (1.2 mm), these nanostructured objects
cover the entire mica surface, and still conserve the circular
structures of the nucleation process (Figures S18–S26, Sup-
porting Information).
This example suggests how a univocal correlation between
the molecular structure of the organic module and the archi-
tecture of the nano-objects is difficult to achieve. Thus, the
elucidation of the mechanisms of formation of the observed
nanostructures would need an inquiry into the link between
molecular short-range and macromolecular long-range
scales. In principle, a promising framework for this approach
would be supplied by the theories of self-assembling pat-
terns and interfaces,[66] successfully exploited and improved
over the years to understand the complex energetic and geo-
metric issues of self-organized dispersions such as micelles,
vesicles, and microemulsions. Whether their present level of
understanding could suffice for this aim, or will require to
be somehow reformulated, is an interesting prospect left for
future work on these supramolecular systems.
drogen-bonding, p–p stacking, and solvophobic/solvophilic
interactions. Preparation of specific nanostructures was
achieved by solvent and temperature control of thoroughly
mixed binary or ternary mixtures of selected compounds.
Photophysical and microscopic studies allowed the forma-
tion of structures of different shapes and sizes to be evi-
denced and rationalized, such as nanoparticles, nanovesicles,
nanofibres, and nanorods, also with helicoidal or circular
variants.
The assortment of nanostructures obtained in this work
represents a first step towards the design and preparation of
functional macroscopic supramolecular materials with well-
defined shapes that, in the future, might be engineered to
express functionality at the macroscopic level. To this end,
we are now conducting qualitative and quantitative theoreti-
cal simulations for understanding in detail the formation
mechanisms of the observed nanostructures, merging the
theories of molecular self-assembly (i.e., molecular recogni-
tion phenomena) with those of self-organizing patterns and/
or interfaces, such as micelles, vesicles, and microemulsions.
Although no specific systems for a particular application
have been created yet, the systematic approach described
here demonstrates that thoroughly designed libraries of
novel organic modules can, under specific boundary condi-
tions, bring closer the perspective of a truly “bottom-up” ap-
proach toward the fabrication of tailored nanomaterials po-
tentially exploitable for practical technologies.
Experimental Section
Spectrometric characterization: The solutions for spectroscopic studies
were prepared by injecting microliter amounts (10/20 mL) of 1 mm solu-
tions in THF of each compound into 3 mL of various solvents such as cy-
clohexane (CHX), toluene, CCl4, THF, dichloromethane, DMSO, metha-
nol, and water. Electronic absorption and emission measurements were
carried out, respectively, on a Lambda 950 UV/VIS/NIR spectrophotom-
eter (Perkin-Elmer) and on a Edinburgh FLS920 spectrofluorimeter
(continuous 450 W Xe lamp), equipped with a Peltier-cooled Hamamatsu
R928 photomultiplier tube (185–850 nm). The temperature of the solu-
tions was varied with a Haake F3-C digital heated/refrigerated water
bath (Haake Mess-Technik GmbH u. Co., Germany), which can be man-
ually connected to a cuvette holder and controlled externally. Emission
quantum yields were determined according to the approach described by
Demas and Crosby[67] with quinine sulfate (Fem =0.546 in air-equilibrated
acidic aqueous solution, 1n H2SO4) as standard. All solvents were of
spectrophotometric grade (Carlo Erba, 99+ %) and were used as re-
ceived.
Wide-field optical microscopy (WFM): The samples for WFM were pre-
pared by spin coating about 20 mL of solution onto a glass microscope
slide and mounted on a inverted Olympus microscope. The samples were
then irradiated through a 100ꢂ objective lens (N.A. 1.4) with a 400 nm
laser. The emission from the sample was collected through the same ob-
jective lens and filtered from excitation light by using a dichroic mirror
and fed to a CCD camera.
TEM analysis: The samples for TEM studies were prepared by drop cast-
ing about 20 mL of self-organized solution onto a carbon-coated nickel
grid (3.00 mm, 200 mesh), air dried, and imaged on a Philips EM 208
TEM microscope (accelerating voltage of 100 kV).
Conclusion
A library of nine chromophoric acetylenic scaffolds periph-
erally equipped with 2,6-bis(acetylamino)pyridine or uracyl-
type terminal fragments was designed to take advantage of
self-assembly and self-organization processes driven by hy-
TM-AFM analysis: Tapping-mode AFM measurements on the mica sub-
strates were carried out in air at 298 K by using a Nanoscope IIIa (Digi-
tal Instruments Metrology Group, USA) instrument, model MMAFMLN.
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3271

D. Bonifazi, N. Armaroli et al.
The tips used in all measurements were phosphorus-doped silicon cantile-
vers (T=20–80 mm, L=115–135 mm, fo =200–400 kHz, k=20–80 Nmꢀ1
Asanuma, H. Li, T. Nakanishi, H. Mçhwald, Chem. Eur. J. 2010, 16,
9330.
,
Veeco, USA) at a resonant frequency of about 300 kHz. The collected
images were then analyzed with WsXm 4.0 software (Nanotec Electroni-
ca S. L.),[68] in order to evaluate the cross-sections of nanoassemblies and
to produce 3D images.
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Published online: February 9, 2011
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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