Engineering spherical nanostructures through hydrogen bonds

Article abstract of DOI:10.1039/b820462d

Chromophoric acetylenic scaffolds bearing complementary uracyl and 2,6-di(acetylamino)pyridyl moieties undergo supramolecular recognition and generate uniform nanoparticles, as observed by UV-Vis, AFM and TEM measurements.

Full text of DOI:10.1039/b820462d

View Article Online / Journal Homepage / Table of Contents for this issue
Chemical Communications
www.rsc.org/chemcomm
Number 20 | 28 May 2009 | Pages 2773–2952
ISSN 1359-7345
FEATURE ARTICLES
COMMUNICATION
Andrei Ghicov and Patrik Schmuki
Self-ordering electrochemistry
K. Yoosaf, Abdelhalim Belbakra,
Nicola Armaroli, Anna Llanes-Pallas
and Davide Bonifazi
Massimo Cametti and Kari Rissanen
Engineering spherical nanostructures Recognition and sensing of fluoride
1359-7345(2009)20;1-3
through hydrogen bonds
anion

COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Engineering spherical nanostructures through hydrogen bondsw
K. Yoosaf,a Abdelhalim Belbakra,a Nicola Armaroli,*a Anna Llanes-Pallasb
and Davide Bonifazi*bc
Received (in Cambridge, UK) 17th November 2008, Accepted 8th December 2008
First published as an Advance Article on the web 14th January 2009
DOI: 10.1039/b820462d
Chromophoric acetylenic scaffolds bearing complementary
uracyl and 2,6-di(acetylamino)pyridyl moieties undergo
supramolecular recognition and generate uniform nanoparticles,
as observed by UV-Vis, AFM and TEM measurements.
specific solvophobic/solvophilic interactions that are estab-
lished in binary or ternary supramolecular adducts.
Molecule 1 consists of a para-disubstituted central benzenic
ring that is connected at both sides to two 2,6-di(acetylamino)-
pyridyl terminal groups through ethynyl spacers, while module
2 bears only one uracyl unit equipped with an anthracenyl
tailing group. These molecular units are known to undergo
self-assembly through triple H-bonding with very high
association constants in apolar solvents, i.e. of the order of
Self-assembly of organic p-conjugated molecules into nano-
structured materials is gaining attention as a potentially facile
and effective route towards the development of functional
materials for electronic and biological applications.1 The
optical and electronic properties of organic-based materials
are highly dependent on their size and shape.2 Hence for any
applications, for instance in optoelectronic devices, fine
control of the structural characteristics by varying the mole-
cular constituents and functionalities is essential.3 In this
context, due to their directionality and spatial arrangement,
complementary multiple H-bonding interactions are attractive
candidates for engineering well-defined supramolecular
structures,4 which might undergo further self-organisation
promoting the formation of controlled organic nanoparticles.5
To this end, we have designed chromophores 1 and 2, which
are equipped with complementary uracyl and 2,6-di(acetylamino)-
pyridyl H-bonding moieties (Fig. 1). Under temperature and
solvent-polarity control, we show that supramolecular adducts
between 1 and 2 are formed in solution and further evolve as
spherical nanostructures, such as vesicles (Fig. 1). Most
notably, the nanoparticle size distribution and shape can be
tuned by the stoichiometry ratio of the two molecular compo-
nents. This behaviour is interpreted as a consequence of the
104–105 MÀ1 6
.
Linear ditopic module 1 was prepared via a three-step
synthesis starting from commercial 1,4-dihydroquinone
(Scheme 1). Alkylation of 1,4-dihydroquinone in 1-bromodo-
decane afforded molecule 4, which was subsequently reacted
with I2 in the presence of Hg(OAc)2 as catalyst to yield
bis-iodo derivative 5.7 Finally, molecule 5 was reacted via a
Pd-catalysed Sonogashira cross-coupling reaction with a small
excess (2.5 eq.) of 2,6-di(acetylamino)-4-ethynylpyridine to
obtain the linear ditopic molecule 1. Monotopic module 2
and 2,6-di(acetylamino)-4-ethynylpyridine were synthesised
following the experimental protocols recently developed by
us.8 Reference compound 3 displaying the same structure as
module 1 but incapable of establishing H-bonding inter-
actions, was obtained upon methylation of the amidic func-
tionalities of 1 by reacting it with MeI in the presence of NaH.
Scheme 1 (a) 1-Bromododecane, K2CO3, DMF, 60 1C, 12 h;
(b) Hg(OAc)2, I2, CH2Cl2, rt, 12 h; (c) 2,6-di(acetylamino)-4-ethynyl-
pyridine, [Pd(PPh3)4], CuI, Et3N, THF, 85 1C, 12 h; (d) THF, NaH,
MeI, rt, 12 h.
Molecules 1 and 2 exhibit intense absorption bands in the
UV region and are strong luminophores in cyclohexane
(CHX) solution (Fig. 2a and S1, ESIw); fluorescence quantum
yields are 9 and 63%, respectively. The absorption and
fluorescence bands of 1 in CHX exhibit fully reversible
temperature-dependent profiles in the range 10–80 1C
(Fig. 2b). On the contrary, small or no changes are observed
for anthracenyl derivative 2 under identical conditions. These
findings suggest that molecule 1 undergoes self-aggregation,
which might be driven by a combination of weak homo-
molecular H-bonding, dipolar interactions, and p–p stacking.
Fig. 1 Schematic representation of the self-organisation process.
a Molecular Photoscience Group, Istituto per la Sintesi Organica e la
`
Fotoreattivita, Consiglio Nazionale delle Ricerche (CNR-ISOF),
Via Gobetti 101, 40129 Bologna, Italy.
E-mail: nicola.armaroli@isof.cnr.it; Fax: +39 051 6399844;
Tel: +39 051 6399820
b
`
Dipartimento di Scienze Farmaceutiche, Universita di Trieste,
Piazzale Europa 1, 34127 Trieste, Italy
c University of Namur, Chemistry Department, Rue de Bruxelles 61,
5000 Namur, Belgium. E-mail: davide.bonifazi@fundp.ac.be;
Fax: +32 81 725452; Tel: +32 81 725433
A
temperature-dependent test experiment with tetra-
w Electronic supplementary information (ESI) available: Details for
the experimental procedures, spectroscopic and microscopic charac-
terisation. See DOI: 10.1039/b820462d
methylated molecule 3 evidences negligible spectral changes
(Fig. S2, ESIw) confirming that the di(acetylamino)pyridyl
ꢀc
This journal is The Royal Society of Chemistry 2009
2830 | Chem. Commun., 2009, 2830–2832

presence of both spherical micro- and nanoparticles with the
size ranging typically from B10 nm to over 1 mm. Structurally,
molecule 1 has a solvophilic (with respect to CHX) aromatic
and aliphatic central part and solvophobic polar end groups.
Hence, in CHX, molecule 1 tends to self-assemble through
electrostatic or weak H-bonding interactions involving the
terminal moieties, as well as p-stacking interactions concerned
with the central part of its structure. The occurrence of these
combined aggregation phenomena is supported by the
bathochromic shift of the absorption and emission maxima
in solution upon temperature lowering (Fig. 2b).9
Contrary to that observed for molecule 1, both TEM and
AFM images of a dropcasted solution of 1 and 2 (1 : 2 molar
ratio) showed the presence of vesicles of highly uniform size
distribution, which are spherical in shape and with a narrower
diameter range of 80–180 nm (Fig. 3b,e and S6–7, ESIw). By
passing from 1 to [1Á(2)2], the change of the end functionality
from solvophobic to solvophilic increases the solute–solvent
interaction, prompting the formation of more thermo-
dynamically favoured nanostructures with narrow size
distribution. Since the [1Á(2)2] adduct has both ends being
solvophilic, the vesicular-like structure10 will be more
favoured, which further helps to minimise the solvophobic
interaction of the polar di(acetylamino)pyridyl units. Under
these specific conditions, it is reasonable to assume that
p-stacking of the anthracene moieties occurs in a J fashion.11
This is corroborated by the marked red shift of the absorption
spectrum of the anthracene moiety when self-organisation
occurs (Fig. 2c). In addition, the geometrical constraints
imposed by the fact that both modules 1 and 2 self-assemble
in a J fashion also dramatically affect the nanoparticle
morphology.
Fig. 2 (a) Absorption spectrum of 1, 2 and of their molecular adduct
(1 : 2 ratio) experimental and calculated; (b) variable-temperature
absorption spectra of 1 in CHX; (c) absorption and (d) emission
variable-temperature spectral changes of molecular adduct of 1 and 2
in the ratio 1 : 2.
moieties are essential to promote self-aggregation of 1. We can
reasonably assume that the terminal units of the linear ditopic
module 1 initially foster aggregation that further induces p–p
stacking.
The absorption spectrum of [1Á(2)2] turns out to be rather
different from that obtained with the algebraic sum of the
separated components (Fig 2a). In particular, the observed
red-shift of the lower energy absorption band of anthracene
derivative 2 (not observed for 2 alone) points to the presence
of p–p stacking interactions, most likely promoted by the
preliminary formation of triple H-bonds between comple-
mentary uracyl and 2,6-di(acetylamino)pyridyl moieties.8 This
rationale is supported by a control experiment carried out with
molecules 2 and 3, where the experimental spectrum is identical
to the algebraic sum of the components (Fig. S3, ESIw).
Furthermore, only the sample containing both molecules 1
and 2 (1 : 2) exhibits a fluorescence spectrum with a broad tail
on the low energy side (Fig. 2d). This finding further supports
the formation of supramolecular complexes driven by triple
H-bonding, as also confirmed by a Job plot via 1H-NMR
titration studies, which leads to intermolecular p–p stacking
between adducts [1Á(2)2].
The above interpretation was further confirmed by investi-
gating the self-assembly between 1 and 2 in CHX at the 1 : 1
stoichiometric ratio. TEM and AFM micrographs showing the
morphology of self-assembled structures are presented in
Fig. 3c,f and S8–9, ESIw. The analysis of these images showed
that the 1 : 1 complexes also forms spherical nanoparticles, but
with larger size distribution compared to those formed from
1 : 2 solutions (as well as smaller if compared to those obtained
with module 1 alone). Interestingly, these nanoparticles have a
bigger average size, with a diameter in the range 150–500 nm.
In line with the above proposed rationale, this is explained
considering that, under these conditions, there is a mixture of
adducts of different stoichiometry exhibiting both solvophilic
and solvophobic terminals.
The novel absorption and emission features of [1Á(2)2] in
CHX are progressively lost upon increasing the temperature
up to 80 1C and are reversibly recovered by cooling down the
sample to 10 1C, also with the observation of clean isosbestic
and isoemissive points (Fig. 2c–d). These observations are
consistent with the reversible formation/decomposition of the
supramolecular adduct.
In order to rule out self-assembly of the anthracenyl deri-
vative 2 alone, we have carefully checked its behaviour.
Absorption and emission spectra of 2 alone in CHX as
a function of temperature did not evidence any clue of
aggregation/self-organisation. This was further confirmed by
AFM experiments following dropcasting of 2 onto a freshly
cleaved mica surface. The images recorded from different areas
of the sample (Fig. S10, ESIw) showed the presence of particles
of different shapes with sizes ranging widely from 10 nm to
over 2 mm. This is due to uncontrolled aggregation of the
molecules during solvent evaporation and confirms that
only the H-bonding interactions between 1 and 2 drive the
formation of nanoparticles.
The morphology of the self-assembled structures was
characterised using TEM and AFM microscopic techniques.
The samples for microscopic studies were prepared by heating
CHX solutions to 80 1C to break any kind of aggregates
formed and were then slowly cooled down to rt to induce the
formation of the thermodynamically favoured nanostructures.
TEM and AFM images of 1 recorded for different areas of
the same sample (Fig. 3a,d and S4–5, ESIw) showed the
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 2830–2832 | 2831

Fig. 3 TEM (a, b and c) and AFM (d, e and f) of the self-assembled structures of 1 (a and d), molecular adducts between 1 and 2 in the
1 : 2 (b and e) and 1 : 1 ratio (c and f).
In summary, we have shown that by tuning the solvo-
philicity of organic systems through the formation of
complementary hydrogen bonds it is possible to promote the
formation of spherical nanostructures with novel optical and
morphological properties. Very importantly, the supra-
molecular aggregates volume and size distribution can be
tuned by the stoichiometric ratio of the molecular moieties
in the parent solution. The strategy proposed here can be
further refined for the engineering of nanoparticles of different
size and shape by selecting appropriate molecular components
and by varying their composition. We are currently working
along these lines.
and D. K. Smith, Angew. Chem., Int. Ed., 2008, 47, 8002;
(e) T. Nakanishi, T. Michinobu, K. Yoshida, N. Shirahata,
¨
K. Ariga, H. Mohwald and D. G. Kurth, Adv. Mater., 2008, 20,
443.
2 (a) H.-B. Fu and J.-N. Yao, J. Am. Chem. Soc., 2001, 13, 1434;
(b) B.-K. An, S.-K. Kwon, S.-D. Jung and S. Y. Park, J. Am.
Chem. Soc., 2002, 124, 14410; (c) D. Xiao, L. Xi, W. Yang, H. Fu,
Z. Shuai, Y. Fang and J. Yao, J. Am. Chem. Soc., 2003, 125, 6740;
(d) Y. S. Zhao, H. Fu, A. Peng, Y. Ma, D. Xiao and J. Yao, Adv.
Mater., 2008, 20, 2859.
3 (a) B. S. Kim, D. J. Hong, J. Bae and M. Lee, J. Am. Chem. Soc.,
2005, 127, 16333; (b) X. Zhang, Z. Chen and F. Wurthner, J. Am.
Chem. Soc., 2007, 129, 4886.
¨
4 (a) M.-J. Brienne, J. Gabard, J.-M. Lehn and I. Stibor, J. Chem.
Soc., Chem. Commun., 1989, 1868; (b) D. S. Lawrence, T. Jiang
and M. Levett, Chem. Rev., 1995, 95, 2229; (c) L. J. Prins,
D. N. Reinhoudt and P. Timmerman, Angew. Chem., Int. Ed.,
2001, 40, 2382; (d) S. Yagai, J. Photochem. Photobiol. C, 2006, 7,
164.
5 (a) F. Ilhan, T. H. Galow, M. Gray, G. Clavier and V. M. Rotello,
J. Am. Chem. Soc., 2000, 122, 5895; (b) I. Yoshikawa, J. Sawayama
and K. Araki, Angew. Chem., Int. Ed., 2008, 47, 1038; (c) S. Yagai,
S. Mahesh, Y. Kikkawa, K. Unoike, T. Karatsu, A. Kitamura and
A. Ajayaghosh, Angew. Chem., Int. Ed., 2008, 47, 4691.
This work was supported by the CNR (PM.P04.010,
MACOL), EU RTN ‘‘PRAIRIES’’ (MRTN-CT-2006-035810)
and ITN ‘‘FINELUMEN’’ (PITN-GA-2008-215399), INSTM,
the University of Namur, the Belgian National Research
Foundation (contract no 2.4.625.08 and 2.4.550.09). Mr Claudio
Gamboz (Centro Servizi Polivalenti di Ateneo, Universita di
Trieste) is gratefully acknowledged for assistance with TEM
imaging. D.B. and A.L.P. thanks Prof. M. Prato for his
continuous support and the access to the laboratory facilities
in Trieste.
6 F. Wurthner, C. Thalacker, A. Sautter, W. Schartl, W. Ibach and
¨
O. Hollricher, Chem.–Eur. J., 2000, 6, 3871.
¨
7 Q. Zhou, P. J. Carroll and T. M. Swager, J. Org. Chem., 1994, 59,
1294.
8 A. Llanes-Pallas, C. A. Palma, L. Piot, A. Belbakra, A. Listorti,
M. Prato, P. Samorı, N. Armaroli and D. Bonifazi, J. Am. Chem.
´
Soc., 2009, DOI: 10.1021/ja807530m.
Notes and references
1 (a) F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer and A. P. H. J.
Schenning, Chem. Rev., 2005, 105, 1491; (b) D. M. Guldi,
F. Zerbetto, V. Georgakilas and M. Prato, Acc. Chem. Res.,
2005, 38, 38; (c) S. Cui, H. Liu, L. Gan, Y. Li and D. Zhu, Adv.
Mater., 2008, 20, 2918; (d) A. R. Hirst, B. Escuder, J. F. Miravet
9 A. Ajayaghosh, R. Varghese, V. K. Praveen and S. Mahesh,
Angew. Chem., Int. Ed., 2006, 45, 3261.
10 M. Antonietti and S. Forster, Adv. Mater., 2003, 15, 1323.
¨
11 S. Yagai, T. Seki, T. Karatsu, A. Kitamura and F. W., Angew.
Chem., Int. Ed., 2008, 47, 3367.
ꢀc
This journal is The Royal Society of Chemistry 2009
2832 | Chem. Commun., 2009, 2830–2832