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.
Chem. Eur. J. 2011, 17, 3262 – 3273
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3271