Persistent, well-defined, monodisperse, π-conjugated organic nanoparticles via G-quadruplex self-assembly

Article abstract of DOI:10.1021/ja908537k

Several oligo(p-phenylene-vinylene) oligomers capped with a guanosine or a guanine moiety have been prepared via a palladium-catalyzed cross-coupling reaction. Their self-assembly, in both the absence and presence of alkaline salts, has been studied by means of different techniques in solution (NMR, MS, UV-vis, CD, fluorescence), solid state (X-ray diffraction), and on surfaces (STM, AFM). When no salt is added, these --conjugated molecules self-associate in a mixture of hydrogen-bonded oligomers, among which the G-quartet structure may be predominant if the steric hindrance around the guanine base becomes important. In contrast, in the presence of sodium or potassium salts, well-defined assemblies of eight functional molecules (8mers) can be formed selectively and quantitatively. In these assemblies, the --conjugated oligomers are maintained in a chirally tilted (J-type) stacking arrangement, which is manifested by negative Cotton effects, small bathochromic absorption and emission shifts, and fluorescence enhancements. Furthermore, these self-assembled organic nanostructures, ～1.5-2.0 nm high and 8.5 nm wide, exhibit an extraordinary stability to temperature or concentration changes in apolar media, and they can be transferred and imaged over solid substrates as individual nanoparticles, showing no significant dissociation or further aggregation.

Full text of DOI:10.1021/ja908537k

Published on Web 12/29/2009
Persistent, Well-Defined, Monodisperse, π-Conjugated
Organic Nanoparticles via G-Quadruplex Self-Assembly
David Gonza´lez-Rodr´ıguez,*,† Pim G. A. Janssen,‡ Rafael Mart´ın-Rapu´n,‡
Inge De Cat,§ Steven De Feyter,*,§ Albertus P. H. J. Schenning,*,‡ and
E. W. Meijer*,‡,|
Departamento de Qu´ımica Orga´nica, Facultad de Ciencias, UniVersidad Auto´noma de Madrid,
E-28049 Madrid, Spain, Laboratory for Macromolecular and Organic Chemistry, EindhoVen
UniVersity of Technology, 5600 MB EindhoVen, The Netherlands, and DiVision of Molecular and
Nanomaterials, Laboratory of Photochemistry and Spectroscopy, and Institute of Nanoscale
Physics and Chemistry, Katholieke UniVersiteit LeuVen, B-3001, LeuVen, Belgium
Received October 14, 2009; E-mail: david.gonzalez.rodriguez@uam.es; a.p.h.j.schenning@tue.nl;
e.w.meijer@tue.nl; steven.defeyter@chem.kuleuven.be
Abstract: Several oligo(p-phenylene-vinylene) oligomers capped with a guanosine or a guanine moiety
have been prepared via a palladium-catalyzed cross-coupling reaction. Their self-assembly, in both the
absence and presence of alkaline salts, has been studied by means of different techniques in solution
(NMR, MS, UV-vis, CD, fluorescence), solid state (X-ray diffraction), and on surfaces (STM, AFM). When
no salt is added, these π-conjugated molecules self-associate in a mixture of hydrogen-bonded oligomers,
among which the G-quartet structure may be predominant if the steric hindrance around the guanine base
becomes important. In contrast, in the presence of sodium or potassium salts, well-defined assemblies of
eight functional molecules (8mers) can be formed selectively and quantitatively. In these assemblies, the
π-conjugated oligomers are maintained in a chirally tilted (J-type) stacking arrangement, which is manifested
by negative Cotton effects, small bathochromic absorption and emission shifts, and fluorescence
enhancements. Furthermore, these self-assembled organic nanostructures, ∼1.5-2.0 nm high and 8.5
nm wide, exhibit an extraordinary stability to temperature or concentration changes in apolar media, and
they can be transferred and imaged over solid substrates as individual nanoparticles, showing no significant
dissociation or further aggregation.
Introduction
patibility, and a more flexible material preparation. However,
the conventional techniques for producing ONPs1,5 allow limited
The successful development of nanosized technologies relies
on the ability of researchers to efficiently manufacture persistent
structures of nanometer dimensions that are endowed with a
particular function. Within this context, organic nanoparticles
(ONPs) have become the subject of increasing attention in the
past years.1 In addition to their current use in inks, drugs, or
cosmetics, they are promising materials for optoelectronics,2
semiconductors,3 and biological sensing4 applications. Compared
to inorganic semiconductor or metal nanoparticles, ONPs enjoy
a much wider structural versatility, due to their diversity in
molecular constitution and arrangement, their higher biocom-
control on particle size and distribution (usually >20 nm) as
well as on the relative arrangement of the molecular components.
These are key issues that determine many properties of the
nanostructures. Similar to their inorganic counterparts, the
special properties of ONPs, lying between those of individual
molecules and bulk materials, exhibit confinement effects caused
by their finite size.3,6 For instance, recent studies have shown a
size-dependent enhancement of fluorescence emission,4,7 which
is believed to have its origin in the stacking arrangement within
the ONP. On one hand, aggregation causes a significant
planarization of the molecules, and nonradiative decays are
reduced due to a restricted conformational motion. On the other
hand, some intermolecular orientations lead to J-type aggregates
that display bathochromic absorption and enhanced, red-shifted
emission features.4,7a,c,d
† Universidad Auto´noma de Madrid.
‡ Laboratory for Macromolecular and Organic Chemistry, Eindhoven
University of Technology.
§ Katholieke Universiteit Leuven.
| Present address: Institute for Complex Molecular Systems, Eindhoven
University of Technology, P.O. Box 513, 5600 MB Eindhoven, The
Netherlands.
(5) ONPs can be prepared by rapid expansion of supercritical solutions
(see ref 5a), stabilization by surfactants (refs 5b,5c), laser ablation of
microcrystals (ref 5d), or by the most common reprecipitation method
(see refs 6,7). (a) Sane, A.; Thies, M. C. J. Phys. Chem. B 2005, 109,
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Y.; Xu, H.-P.; Bai, J. J. Colloid Interface Sci. 2008, 322, 491–496.
(d) Asahi, T.; Sugiyama, T.; Masuhara, H. Acc. Chem. Res. 2008, 41,
1790–1798.
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10.1021/ja908537k  2010 American Chemical Society

Organic Nanoparticles via G-Quadruplex Self-Assembly
A R T I C L E S
Molecular self-assembly8 offers an excellent alternative tool
to cheaply and easily construct, Via a “Bottom-Up” approach,
diverse supramolecular architectures smaller than 100 nm in
which the morphology and the relative organization of individual
molecules can be controlled by rational selection of different
self-assembling motifs.9 This supramolecular approach often
fails, however, in providing nanostructures that are monodisperse
and persistent in size and shape. Many noncovalent interactions,
such as π-π stacking or solvophobic interactions, promote
supramolecular polymerizations,10 and size control is generally
complex to achieve. In addition, the weak nature of the
noncovalent interactions that hold the molecular building blocks
together make these self-assembled architectures very sensitive
to concentration and temperature variations, which affects
dramatically their homogeneity. The supramolecular synthesis
of discrete, uniform, stable nanoobjects11 is therefore a chal-
lenging objective that must be addressed for practical applica-
tions, so that the physical properties can be related to individual
well-defined assemblies and the molecular organization is not
altered upon deposition and manipulation over solid substrates.12
In this context, recent studies have disclosed the singular
characteristics of guanine (G) self-assembly in the presence of
alkaline salts,13 where multiple noncovalent interactions (i.e.,
(7) (a) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem.
Soc. 2002, 124, 14410–14415. (b) Xiao, D.; Xi, L.; Yang, W.; Fu,
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Org. Lett. 2008, 10, 281–284. (h) Yao, H.; Yamashita, M.; Kimura,
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Palayangoda, S. S.; Neckers, D. C. Langmuir 2009, 25, 2402–2406.
(8) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418–2421.
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J. C.; de Greef, T. F. A.; Vos, M. R. J.; Bomans, P. H. H.; Sommerdijk,
N. A. J. M.; Christianen, P. C. M.; Lecle`re, Ph.; Lazzaroni, R.; van
der Schoot, P.; Meijer, E. W.; Schenning, A. P. H. J. J. Am. Chem.
Soc. 2009, 131, 1222–1232. (b) Janssen, P. G. A.; Ruiz-Carretero,
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Angew. Chem., Int. Ed. 2009, 48, 8103–8106. (c) For a general review
see: Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning,
A. P. H. J. Chem. ReV. 2005, 105, 1491–1546.
Figure 1. (a) Levels of organization in the formation of a G-quadruplex
(8mer). In a first level of organization, a tetramer (G-quartet) is formed by
a circular array of eight self-complementary hydrogen bonds. This arrange-
ment leaves the four carbonyl dipolar groups pointing toward a small central
cavity in which a cation can be strongly complexed. Complexation is even
more efficient if another quartet layer is added on top, so the cation is now
coordinated to the eight carbonyl groups of two π-π stacked G-quartets
whereas the anion is left out at the complex periphery. (b) Structure of the
three oligophenylenevinylene-guanine (OPV-G) molecules studied in this
work.
(10) (a) Ciferri, A. Macromol. Rapid Commun. 2002, 23, 511–529. (b)
Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem.
ReV. 2001, 101, 4071–4098. (c) de Greef, T. F. A.; Meijer, E. W.
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125, 15941–15949.
hydrogen-bonding, π-π stacking and cation-dipole interac-
tions) can work in concert to provide discrete14-16 and relatively
stable16,17 G-quadruplexes (see Figure 1a). We and other
research groups have therefore considered the G-quadruplex
structure as an ideal scaffold to which photoactive organic
molecules can be attached to construct functional assemblies.18,19
Recently, the equilibrium between linear G-oligomers (G-
ribbons)20 and G-quadruplex species has been studied in
porphyrin-G,19a pyrene-G,19b or oligothiophene-G19c conjugates.
Upon addition of sodium or potassium salts, this equilibrium is
(13) (a) Davis, J. T. Angew. Chem., Int. Ed. 2004, 43, 668–698. (b) Davis,
J. T.; Spada, G. P. Chem. Soc. ReV. 2007, 36, 296–313.
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X.; Mullaugh, K. M.; Fettinger, J. C.; Jiang, Y.; Hofstadler, S. A.;
Davis, J. T. J. Am. Chem. Soc. 2003, 125, 10830–10841. (d) Ma, L.;
Iezzi, M.; Kaucher, M. S.; Lam, Y.-F.; Davis, J. T. J. Am. Chem. Soc.
2006, 128, 15269–15277.
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2008, 130, 10492–10493. (b) Betancourt, J. E.; Mart´ın-Hidalgo, M.;
Gubala, V.; Rivera, J. M. J. Am. Chem. Soc. 2009, 131, 3186–3188.
(16) Gonza´lez-Rodr´ıguez, D.; van Dongen, J. L. J.; Lutz, M.; Spek, A. L.;
Schenning, A. P. H. J.; Meijer, E. W. Nat. Chem. 2009, 1, 151–155.
(17) The impressive thermodynamic and kinetic stability of this kind of
assemblies has already been evidenced in a number of papers. See
refs 14c-d, 16, and: (a) Davis, J. T.; Kaucher, M. S.; Kotch, F. W.;
Iezzi, M. A.; Clover, B. C.; Mullaugh, K. M. Org. Lett. 2004, 6, 4265–
4268. (b) Kaucher, M. S.; Lam, Y.-F.; Pieraccini, S.; Gottarelli, G.;
Davis, J. T. Chem.sEur. J. 2005, 11, 164–173.
(18) Lena, S.; Masiero, S.; Pieraccini, S.; Spada, G. P. Chem.sEur. J. 2009,
15, 7792–7806.
(19) (a) Masiero, S.; Gottarelli, G.; Pieraccini, S. Chem. Commun. 2000,
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2438.
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A R T I C L E S
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Scheme 1. Synthesis of the Three OPV-G Molecules Studied in This Work and Their Precursorsa
a (a) Synthesis of 8-bromoguanosines G1 and G2 and 8-ethynylguanine G3. (b) Synthesis of the OPV core precursors (10-13). (c) Cross-coupling
reaction between the G and the OPV precursors.
shifted toward the formation of quadruplexes (typically octamers
(8mers)) due to the high affinity of the stacked quartets to
coordinate the cations.
ethynyl spacer employed is essential to transmit a defined
orientation to the stacked OPV units in the octameric assemblies,
which is mainly determined by the guanine-cation coordination.
The OPVs are therefore maintained in a chirally tilted (J-type)
stacking arrangement, which is manifested by negative Cotton
effects, small bathochromic absorption and emission shifts, and
enhancement of the fluorescence in the ONPs.
Here we demonstrate not only that this approach can provide
selectively and quantitatively well-defined assemblies of eight
functional molecules but also that the discrete ONPs formed,
∼1.5-2.0 nm high and 8.5 nm wide, exhibit an extraordinary
uniformity and stability toward temperature or concentration
changes in apolar media. As a result, they can be transferred
and studied over solid substrates as single entities, showing no
significant dissociation or further intermolecular stacking into
larger objects. In particular, we focused on the study of the self-
assembly, in the absence and presence of alkaline salts, of
oligo(p-phenylene-vinylene) (OPV), well-known semiconduct-
ing and fluorescent π-conjugated oligomers that are linked to a
guanine or guanosine base (Figure 1b). The rigid nature of the
Results and Discussion
Synthesis. The synthesis of the three OPV-G compounds
(Scheme 1) was envisaged via a convergent approach that
involved a palladium-catalyzed Sonogashira coupling of the
conjugated oligomer and the base. The latter unit contains most
of the information for the self-assembly process, including the
array of hydrogen-bonding groups and the chiral center.
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Organic Nanoparticles via G-Quadruplex Self-Assembly
A R T I C L E S
Self-Assembly in the Absence of Cations. Solution Experi-
ments. Among the natural nucleobases, guanosine certainly
occupies a central role due to its versatile supramolecular
chemistry. The G base offers a rich array of hydrogen-bonding
donor and acceptor groups that has been exploited both by nature
and by synthetic chemists to build diverse supramolecular
architectures.13 In the absence of some specific additives
(alkaline cations, cytosine derivatives, ...) guanine tends to self-
associate into multiple oligomeric species by formation of pairs
of self-complementary hydrogen bonds (some representative
examples are shown in Figure 2a). Depending on the participat-
ing hydrogen bonds, the resulting oligomers can be linear
(ribbons) or circular (quartets). Although in some conditions
the formation of empty quartets31 or a particular kind of ribbon20
is favored in concentrated samples (principally due to steric
effects), the weak nature of these pairs of hydrogen bonds
typically produces a complex mixture of loosely bound, rapidly
exchanging oligomeric species in dilute nonpolar solutions at
room temperature.
The preparation of bromoguanines G1, G2, and 5 required
two different synthetic routes (Scheme 1a). Bromoguanosines
G1 and G216 were prepared from guanosine Via protection of
the diol moiety as an acetonide,21 bromination at the 8-position
of the guanine in the presence of NBS,22 and functionalization
of the 5′-OH in the ribose with a tert-butyldimethylsilyl14a,23
(G1) or a decanoyl24 (G2) group. Compound 5 was in contrast
prepared from commercial 2-amino-6-chloropurine by an N-
alkylation reaction25 with (-)-2-octyl-p-toluenesulfonate (3),26
followed by hydrolysis25 and subsequent bromination at the
8-position.22 Compound 5 was then transformed to the 8-ethynyl
derivative G3 by Sonogashira coupling27 with triisopropylsilyl
acetylene and deprotection of the silyl group in the presence of
tetrabutylammonium fluoride.
The synthesis of the conjugated OPV core (Scheme 1b)
started from dialdehyde 6,28 which was subjected to a
Horner-Wadsworth-Emmons reaction, first with 1.1 equiv of
phosphonate 729 and then with either commercial phosphonate
8 or with tridodecyloxy-phosphonate 9.30 This procedure led
to the corresponding unsymmetrically substituted iodo-OPVs
10 and 11 in acceptable yields. Ethynyl derivatives 12 and 13
were then synthesized from 10 and 11, respectively, by
Sonogashira coupling with triisopropylsilyl acetylene and
subsequent deprotection of the silyl group.
Our OPV-G compounds are quite soluble in solvents of
medium-low polarity such as THF, CHCl3, CH2Cl2, or toluene
and, in the case of OPV-G2 and OPV-G3, in apolar solvents
such as methylcyclohexane or dodecane. 1H NMR experiments
in such solvents indicate that, like their guanosine precursors,16
these OPV-G molecules self-associate in a mixture of oligomeric
species in the absence of alkaline cations. The degree of self-
association is clearly evidenced in the amide (NH) and amine
(NH2) proton signals, which shift downfield when decreasing
the solvent polarity (Figure S1), the temperature (Figure 2b),
or when increasing the concentration, as a result of a higher
participation in hydrogen-bonding. At the same time, ESI
Q-TOF MS analysis of CHCl3 solutions of OPV-G1 showed
several peaks that correspond to cluster ions of the general
The final key step, the coupling of the chromophore and base
units (Scheme 1c), was carried out using a copper-free Sono-
gashira procedure27 in the presence of catalytic amounts of
Pd(PPh3)4 and under strictly oxygen-free conditions, to minimize
the formation of the corresponding homocoupling products. The
formation of these diacetylene byproducts was still quite difficult
to avoid for alkoxy-OPV 13, especially when the coupling was
performed with the rather insoluble bromoguanine 5. Therefore,
the coupling strategy was inverted for OPV-G3, with the
reaction between ethynyl-guanine G3 and iodo-OPV 11 leading
to higher yields. All compounds were purified and characterized
by 1H and 13C NMR techniques, mass spectrometry, and
UV-vis and IR spectroscopies (see the Supporting Information).
formula [OPV-G1n+Hx(+Nay)](x+y)+
.
Compound OPV-G3, which bears a guanine substituent,
clearly exhibits a much higher tendency to self-associate than
compounds OPV-G1 or OPV-G2, substituted with a guanosine
derivative. For instance, 10-2 M solutions of OPV-G3 in
CDCl2CDCl2, or in more apolar solvents like toluene-d8 or
cyclohexane-d12, display the aforementioned downfield shift of
the NH and NH2 proton signals and a very pronounced
broadening of all proton signals (Figure S2), which is attributed
to the formation of ribbon-like aggregates. Such aggregation is
prevented in CDCl2CDCl2 at high temperatures (>50 °C), upon
addition of small amounts of CD3OD, or using more polar
solvents, such as THF-d8 (Figure S2). On the contrary,
compounds OPV-G1 or OPV-G2 display in general much
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Samor`ı, P.; Spada, G. P. Chem.sEur. J. 2000, 6, 3242–3248. (b)
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sharper features in the H NMR spectra, indicating a lower
degree of aggregation. In agreement with this interpretation,
DOSY experiments at 25 °C yielded comparable diffusion
values for OPV-G1 or OPV-G2 10-2 M CDCl3 solutions (D
) (6.91 ( 0.05) × 10-10 m2 s-1 and (6.42 ( 0.04) × 10-10 m2
s-1, respectively), whereas somewhat lower values were ob-
served for OPV-G3 under the same conditions (D ) (3.89 (
0.09) × 10-10 m2 s-1).
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The lower tendency of OPV-G1 or OPV-G2 to form
oligomeric aggregates, when compared to OPV-G3, is attributed
to the larger steric volume of the ribose substituent. It is known
that, in the case of 8-substituted guanosines, the ribose unit is
forced to adopt a syn conformation, which prevents the
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protons are equivalent due to fast spinning around the C-N
bond and therefore appear as a broad singlet between 6.5 and
5.5 ppm. Lowering the temperature below 25 °C, however,
causes a considerable broadening of this signal that then, below
0 °C, splits into two signals as the quartet is stabilized and the
two amino protons exchange slowly on the NMR time
scale. The downfield-shifted signal at approximately 9 ppm is
assigned to the hydrogen-bonded proton in the quartet (NH2(b)),
whereas the signal around 5 ppm belongs to the “free”, solvent-
exposed amino proton (NH2(f)).
It is also interesting to see that, parallel to these changes
observed when decreasing the temperature, the water signal32
undergoes a considerable downfield shift (from 1.5 to 3.5 ppm;
green arrow in Figure 2c). This seems to indicate, as has been
proposed before,33 that water molecules are involved in the
stabilization of “closed” oligomers, which eventually lead to
cyclic tetramers, by hydrogen-bonding to the guanine carbonyl
groups (see Figure 2a).
The aggregation of these compounds in nonpolar solvents at
lower concentrations (4 × 10-4-10-5 M) was also studied by
circular dichroism (CD), absorption, and fluorescence spectro-
scopic techniques (Figure S4). The combined results indicate
that, at the concentrations employed in these experiments, these
OPV-G molecules, including OPV-G3, do not show a strong
tendency to π-π stack in ordered aggregates. For instance, only
a small positive Cotton effect with a zero-crossing at the
maximum of the OPV absorption (∼420 nm) was observed in
very apolar solvents as methylcyclohexane or dodecane, indicat-
ing that the degree of chiral order is low. This CD signal did
not change considerably with the concentration of the sample
or the temperature. In addition, the normalized absorption
and emission spectra at different concentrations and in
different solvents were virtually identical, whereas in tem-
perature-dependent experiments we only observed the typical
red shift in the absorption and emission spectra due to
planarization of the OPVs, which increases linearly when
decreasing the temperature (see Figure S4).
The low degree of aggregation observed in our OPV-G
molecules at low concentrations is not entirely surprising in view
of the dense substitution of our molecules (especially OPV-
G1 and OPV-G2), the low rotational barriers around triple
bonds, and the relatively weak and unspecific self-association
by pairs of hydrogen bonds, which may give rise to a complex
mixture of oligomers in fast exchange. These results contrast
with comparable OPV molecules studied previously in our group
that can form strong and specific quadruple hydrogen-bonded
dimers, which show a large driving force to stack in long helical
fibers.34
Figure 2. (a) Different modes of self-association of the OPV-G molecules
by pairs of hydrogen bonds. When the group R on the guanine is a bulky
ribose, as in the case for OPV-G1 and OPV-G2, the formation of the so-
called G-ribbon B is prevented by steric hindrance, leading to the preferential
formation of cyclic oligomers that eventually form G-quartets. Traces of
water can participate in the process of stabilization of these cyclic oligomers
1
by hydrogen-bonding to the carbonyl groups. (b) Portion of the H NMR
Self-Assembly in the Absence of Cations. Surface Experi-
ments. Scanning Tunneling Microscopy (STM) experiments
provided a clearer picture on the self-assembly of our OPV-G
molecules in the absence of cations, and the results are consistent
with the interpretations drawn from solution experiments. For
instance, 10-4-10-6 M solutions of OPV-G3 in 1-phenyloctane
or octanoic acid yielded very disordered monolayers in which
spectra of OPV-G1 (CDCl2CDCl2; 10-2 M) as a function of the temperature.
In less polar solvents or at lower temperatures, both the amide (NH; red
arrow) and amine (NH2; blue arrow) proton signals experience a downfield
shift, as a result of a higher participation in hydrogen-bonding. The water
signal is indicated by a green arrow.
formation of hydrogen-bonded G-ribbons by steric blockage of
the NH2(f) proton (see Figure 2a).16,31 As a result, the formation
of circular G-quartets, where the NH2(f) proton does not
participate in hydrogen-bonding, must become dominant at high
self-association degrees (see Figure 2a). The stabilization of
quartet structures is evidenced, as has been described before,31
in the evolution of the NH2 proton signal upon cooling OPV-
G1 or OPV-G2 solutions in, for instance, CDCl2CDCl2 (Figure
2c; see also Figure S3 for G1). At high temperatures these
(32) The traces of water mainly come from the residual water in the
deuterated solvent and are difficult to eliminate totally.
(33) Kotch, F. W.; Sidorov, V.; Lam, Y.-F.; Kayser, K. J.; Li, H.; Kaucher,
M. S.; Davis, J. T. J. Am. Chem. Soc. 2003, 125, 15140–15150.
(34) (a) Schenning, A. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W.
J. Am. Chem. Soc. 2001, 123, 409–416. (b) Jonkheijm, P.; van der
Schoot, P.; Schenning, A. P. H. J.; Meijer, E. W. Science 2006, 313
(5783), 80–83.
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Organic Nanoparticles via G-Quadruplex Self-Assembly
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Figure 3. Scanning tunneling microscopy (STM) images at the 1-phenyloctane/HOPG interface of (a) OPV-G3 (6 × 10-4 M; 20 °C), with tunneling
parameters Iset ) 0.026 nA, Vset ) -0.680 V, and (b) OPV-G2 (7 × 10-6 M; 20 °C), with tunneling parameters Iset ) 0.028 nA, Vset ) -0.600 V. The insets
are tentative models of the corresponding features in the STM images.
we could distinguish diverse oligomeric species, such as small
ribbons and quartets (Figure 3a). In general, high quality images
were difficult to obtain due to the lack of two-dimensional
ordering and the rapid reorganization of the adsorbed molecules,
as observed in consecutive scans. This mixture of disordered
and fast exchanging oligomers contrasts with previous studies
on oligothiophene-G molecules connected Via the 5′-position
of the ribose that do show a long-range ordering in G-ribbons.19c
Now, when we moved to compound OPV-G2, substituted
with a guanosine moiety, we found that the morphology of the
monolayers is totally different. Despite the two-dimensional
ordering is still poor, all OPV-G2 molecules are mostly
assembled in individual quartets on the HOPG surface (Figures
3b and S5), a kind of organization that, to the best of our
knowledge, has not been observed before using STM.35 This
divergence in the self-assembly between OPV-G2 and OPV-
G3,36 as previously evidenced in the NMR experiments, agrees
with the notion of steric blocking of the NH2(f) proton by the
bulky ribose group, which prevents the formation of G-ribbons
and thus stabilizes the circular tetramers.37
Self-Assembly in the Presence of Cations. Solution Experi-
ments. The whole supramolecular picture changes dramatically
upon complexation of the OPV-G compounds with Na+ or K+
salts. The strong affinity of the G-quartet cavity for alkaline
cations shifts the equilibrium between these loosely bound
hydrogen-bonded species toward the formation of well-defined
G-quadruplexes (see Figure 1). In these assemblies, the cation
is efficiently coordinated by eight carbonyl groups and is
typically positioned between two stacking quartets, whereas the
anion is left out at the complex periphery. In organic solvents,
lipophilic G derivatives usually assemble into discrete G-
quadruplexes (2 to 6 stacked quartets; 8mers to 24mers). The
kind of complex obtained is very sensitive to the experimental
conditions employed, depending particularly on the nature of
the anion or the guanosine derivative employed, as well as on
the solvent polarity.14-17
In the present study, the complexation of alkaline salts by
our OPV-G molecules always produced octamers of D4-
symmetry (D4-8mers). For example, stirring a 10-1 M THF-d8
solution of OPV-G1 in the presence of 0.25 equiv of KPF6
results in the quantitative formation of a D4-8mer, which
reproduces exactly the self-assembly of the bromoguanosine
precursor G1 (Figure S6).16 As shown in Figure 4a, the 1H NMR
spectrum of the D4-8mer presents characteristic proton signals
that differ from the uncomplexed OPV-G species. The amide
proton signal becomes sharper and downfield-shifted upon
complexation, the amino protons split into two signals (NH2(b)
and NH2(f)) that become more perceptible at low temperatures,
and the signals of the protons on the OPV fragment show a
small broadening. Temperature-dependent NMR experiments
(Figure 4b) revealed that these 8mers are still stable at
temperatures close to the boiling point of the THF-d8 solvent,
and no significant dissociation is detected at, for instance, 50
°C. In this relatively polar solvent, the OPV-G D4-8mers are,
in contrast, very sensitive to the concentration and, as observed
for G1,16 dissociation into the uncomplexed OPV-G species
takes place upon dilution of the sample until, below 5 × 10-3
M, only uncomplexed OPV-G (mostly in the form of monomeric
species or small, loosely bound oligomers) is present in solution
(Figure 4c). The two species in equilibrium (8mer and uncom-
plexed OPV-G) could also be detected in DOSY NMR
experiments in THF-d8 at concentrations between 10-1 and 5
× 10-3 M (Figure S7).
The formation of a D4-symmetric 8mer seems to be general
for all the three OPV-G molecules in different conditions.
-
Changes in the nature of the cation (K+, Na+) or the anion (PF6 ,
BPh4 , I-, MeDNP- (4-methyl-2,6-dinitrophenolate)) had virtu-
-
ally no influence on the self-assembly of our OPV-G com-
pounds. Aggregation is therefore limited to octameric assemblies
in all the solvents we studied (THF-d8, CDCl3, CDCl2CDCl2,
toluene-d8, or cyclohexane-d12).38
(35) Hydrogen-bonded networks of quartets of pristine guanines have,
however, been reported before. See: Otero, R.; Scho¨ck, M.; Molina,
L. M.; Lægsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F.
Angew. Chem., Int. Ed. 2005, 44, 2270–2275.
(36) The tri(dodecyloxy)benzene wedge of compounds OPV-G2 and OPV-
G3 is usually important to obtain adsorbed monolayers, and therefore
compound OPV-G1 was not analyzed.
(38) It has been recently demonstrated that the growth of these G-
quadruplex assemblies, up to a 12mer or a 16mer, can be finely
controlled in organic media by a subtle interplay between anion-
complex (see refs 14,16) and anion-solvent interactions (see refs
15b,16). In particular, more polar solvents, such as acetone or
acetonitrile, can stabilize more efficiently the anion in solution and
tend to lead to higher complexes. Unfortunately, our OPV-G molecules
were not sufficiently soluble in those solvents.
(37) It is interesting to note that neither the controlled addition of sodium
or potassium salts to these 1-phenyloctane or octanoic acid solutions
nor the analysis of previously assembled OPV-G 8mers led to the
observation of adsorbed molecules. This result is not entirely surprising
in view of the increase in height of the complexed OPV-G assemblies,
which would lead to longer tip-substrate distances.
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A R T I C L E S
Gonza´lez-Rodr´ıguez et al.
Figure 4. (a) Schematic representation of the equilibrium between uncomplexed OPV-G (M) and the D4-symmetric octamer (8mer) in the presence of
potassium salts. (b, c) Changes in the NH/NH2 and tert-butyl regions of the 1H NMR spectra of OPV-G1 in THF-d8 in the presence of 0.25 equiv of KPF6
as a function of (b) the temperature ([OPV-G1] ) 10-1 M) or (c) the OPV-G1 concentration (T ) 25 °C).
However, we noticed that reducing the solvent polarity had
an enormous effect on the stability of the octameric nanoobjects
formed toward temperature or concentration changes. For
instance, 1H NMR experiments in toluene or cyclohexane
solutions revealed that they can be heated to 75 °C (Figure 5a)
or diluted up to 10-4 M (Figure 5b) without any evidence of
significant dissociation. In agreement with these observations,
in DOSY NMR experiments at different concentrations we
detected the 8mer complex as the only species in solution.
The persistence of the OPV-G 8mer structures in diluted
apolar solutions allowed us to overlap the NMR studies (up to
10-4 M) with spectroscopic studies (4 × 10-4-10-5 M), using
CD, absorption, and emission techniques. Compared to the
uncomplexed oligomers, OPV-G2 and OPV-G3 8mers display
distinct features that originate from the well-defined stacking
arrangement of the chromophores. As shown in Figure 6a,
complexation brings about a small red shift and tailing of the
OPV absorption band, as well as a 2- to 3-fold enhancement
and bathochromic shift of the fluorescence emission (see also
Figure S8). For example, the OPV-G2/OPV-G3 fluorescence
quantum yields increase from 0.32/0.37 in the samples without
cations to 0.84/0.81 in their respective KBPh4-8mers in toluene.
A similar trend was observed in methylcyclohexane, though the
quantum yield values derived were somewhat lower for both
the monomeric and octameric OPV-G2/OPV-G3 systems (0.30/
0.29 and 0.68/0.72, respectively). Moreover, the self-assembled
nanostructures exhibit a negative Cotton effect with a zero-
crossing (λz-c) at the OPV absorption maximum, when compared
to the uncomplexed oligomers, which has a similar shape for
both OPV-G2 and OPV-G3 despite the different nature of the
chiral substituents at the G unit. It is interesting to note that
related oligothiophene-G D4-symmetric octamers, where the
oligomer was attached to the 5′-position of the G ribose, showed
a similar negative Cotton effect.19c
Further spectroscopic studies confirmed the outstanding
stability of these G-quadruplex nanoparticles in nonpolar media.
For instance, the 8mers can be diluted by a factor of 40 (up to
10-5 M) without any change in the CD, absorption, or emission
spectrum (Figure 6b). Furthermore, the diluted (4 × 10-5-10-5
M) solutions can be heated up to 75 °C without any other
absorption change than the typical red shift assigned to the
planarization of the OPV units (Figure 6c). Returning to 20 °C
after these heating-cooling cycles resulted in identical absorp-
tion, fluorescence, and CD spectra compared to those of freshly
prepared samples. Even the addition of small amounts (<5%
v/v) of polar additives, such as diisopropylethylamine or
N-methyl-2-pyrrolidinone, produced no spectroscopic change.
The G-quadruplex particles were, however, immediately dis-
sociated upon dilution in a more polar solvent, such as THF
(Figure 6b), which is consistent with the results obtained in the
NMR experiments (see Figure 4c).
These results suggest that the π-conjugated oligomers are
strongly maintained in a well-defined chiral arrangement within
the complex, which must be determined by the G-K+ coordina-
tion and steric interactions between the chiral side groups on
the G unit, and further transmitted to the OPVs through the
rigid ethynyl spacer. The molecular models39 of the OPV-G
8mers (Figure 7), obtained by using some geometrical param-
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Organic Nanoparticles via G-Quadruplex Self-Assembly
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Figure 6. Absorption, emission (λexc ) 420 nm) and circular dichroism
(CD) spectra of (a) OPV-G3 or OPV-G2, in the absence or in the presence
of 0.25 equiv of KBPh4, in toluene or MCH, as indicated in the figure
legends ([OPV-G] ) 4 × 10-5 M; 20 °C), (b) OPV-G3 in MCH before
and after 10-fold dilution with MCH or THF (20 °C), or (c) OPV-G3 as a
function of the temperature (MCH; 4 × 10-5 M).
Figure 5. Changes in a region of the 1H NMR spectra of OPV-G2 in the
presence of 0.25 equiv of KBPh4 in toluene-d8 as a function of the
temperature at an OPV-G concentration of (a) 10-3 M or (b) 10-4 M. For
comparison, we are also showing the spectra of OPV-G2 in the absence of
cation and in a THF-d8-toluene-d8 mixture, where only uncomplexed
species are present (M). The amide (NH) and amine (NH2(b)) proton signals
are indicated by red and blue arrows, respectively. The integration of the
signal of the protons of complexed KBPh4 (brown arrow) supports the
formation of a 1:8 complex.
Figure 7. Top and side views of the D4-8mer model of OPV-G2 with
KBPh4. The BPh4- anion (only shown in the side view) has been arbitrarily
positioned on top of the complex, since it leads to the shorter cation-anion
distance. Complex dimensions were derived from these models.40
were derived from these models taking into account the alkyl
tails and the anion.
Self-Assembly in the Presence of Cations. Solid-State X-ray
Diffraction Experiments. X-ray diffraction (XRD) studies were
conducted at room temperature on bulk samples of OPV-G1,
OPV-G2, and OPV-G3, as well as of their respective Na+- or
K+-D4-8mers. In the wide angle region, only OPV-G3 samples
showed a diffraction maximum at 0.35 nm, corresponding to
the π-π stacking distance, which is sharper and more intense
in the complexes, indicating that the addition of a cation favors
the stacking (see Figure S9). The samples of guanosine-OPVs
OPV-G1 and OPV-G2, however, did not display this
eters from the reported crystal structures of related G-
quadruplexes,14a,16,31 indicated that the OPVs are positioned at
a suitable distance for π-π stacking (3.3 Å), but their
π-conjugated surfaces overlap only partially, due to the ca. 30°
angle imposed by the two quartets upon K+ complexation.
Particle dimensions of ∼1.5-2.0 nm high40 and 8.5 nm wide
(39) Structural modeling was performed using the MM+ method imple-
mented on the Hyperchem 8.03 software package (Hypercube, Inc.)
for Windows. Some of the geometrical parameters (quartet-quartet
distance, position of the cation, relative orientation of quartets) were
kept within the values observed in the crystal structures reported in
the literature (see refs 14a,16,31). In these models, the anion was
positioned on top of one of the quartets, since this leads to the shorter
cation-anion distance due to the dense peripheral crowding.
(40) Small variations on the calculated particle height mainly come from
the size of the anion employed and also from the conformation of the
substituents on the G unit and the OPV tri(dodecyloxy)benzene wedge.
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Figure 8. AFM images of the [OPV-G3]8-KMeDNP nanoparticles deposited from 10-5 M solutions in toluene on (a) graphite or (b) mica surfaces.
maximum, which demonstrates the detrimental effect of the
pending bulky ribose on the stacking. The diffraction patterns
of the samples containing OPV-G2 and OPV-G3 exhibit as
well a diffuse halo corresponding to ∼0.45 nm, which is the
typical distance between the molten long alkyl tails in liquid
crystals (Figure S9).41
Self-Assembly in the Presence of Cations. AFM Surface
Experiments. To confirm the integrity and uniformity of the
OPV-G2 and OPV-G3 complexes formed in the presence of
K+ salts, we deposited diluted (10-4-10-5 M) solutions in apolar
solvents such as toluene or methylcyclohexane over graphite
or mica surfaces and, after solvent evaporation, imaged the self-
assembled nanoobjects by atomic force microscopy (AFM;
Figure 8).
We think that the degree of nanoparticle aggregation observed
on both surfaces is simply a result of the interaction of the
lipophilic alkyl tails with the surface, which provides the
particles with some mobility upon physisorption. This interaction
is much lower on mica than on graphite, and therefore, the
nanoparticles can rearrange to a certain extent and form
aggregates during solvent evaporation. In contrast, on graphite
the 8mers display a much reduced mobility. Aside from this
nanoparticle aggregation phenomenon, presumably occurring
upon sample drying, we never detected the formation of
elongated, fiber-like structures that could have been formed by
further aggregation of OPV-G quartets. This fact underscores
the uniformity and supramolecular identity of our octameric
OPV-G particles.
The atomic force micrographs of drop-casted OPV-G2 and
OPV-G3 D4-8mer solutions show nanoparticles whose dimen-
sions, considering the convolution with a tip of radius of
curvature of 15-20 nm (10 ( 5 nm width and 1.8 ( 0.4 nm
height),42 nicely match those estimated for the OPV-G D4-8mers
(8.5 nm width and 1.8 nm height). On graphite (Figures 8a and
S10a), we could generally observe the individual nanoparticles
at low sample concentrations,43 together with some larger objects
probably formed by further aggregation of these particles. On
mica surfaces (Figures 8b and S10b), the OPV-G 8mers
aggregate into mono- and bilayers with a height of 1.8 ( 0.4
nm and 3.2 ( 0.4 nm, respectively, and we could only see a
few isolated nanoparticles. In the absence of potassium salts,
we observed in contrast very different features. For instance,
OPV-G3 molecules formed mono- and bilayers on graphite,
together with larger objects, but the characteristic 8mer nano-
particles were not detected.
Conclusions
Molecular self-assembly is a powerful tool that may be
used to construct, in a bottom-up approach, persistent and
monodisperse nanoobjects where a discrete number of
functional molecules are organized in a well-defined arrange-
ment. For such a goal, one must be able to modulate the
interplay between multiple noncovalent interactions so that
they work in concert to impart stability to the assemblies
and, at the same time, limit their growth to a certain extent.
We have shown that the special characteristics of G-
quadruplex self-assembly meet these requirements and, in
certain conditions, allow the supramolecular synthesis of disk-
shaped ONPs comprising exactly eight π-conjugated mol-
ecules. The rigid, chirally tilted arrangement of the OPVs
within the octameric complexes results in negative Cotton
effects, bathochromic shifts of the absorption and emission
bands, and a 2- to 3-fold enhancement of the fluorescence
emission. These features are also characteristic of larger
ONPs (>25 nm), typically produced by reprecipitation
methods,4,7 that show size-dependent emission quantum
yields, but for which particle size and chromophore organiza-
tion is more difficult to control. Future research will be
directed to gradually increasing the size (particularly the
height) of these self-assembled nanoobjects,38 which might
(41) However, optically polarized microscopy experiments revealed that
none of the OPV-G compounds, in the presence or absence of Na+/
K+ cations, displayed liquid crystalline behavior in the temperature
range 20-220 °C.
(42) Bustamante, C.; Vesenka, J.; Tang, C. L.; Rees, W.; Guthold, M.;
Keller, R. Biochemistry 1992, 31, 22–26.
(43) More concentrated samples led to thick layers where individual
nanoparticles could not be observed. However, we expect that these
layers consist of assembled nanoparticles.
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Research-Flanders (FWO). I.D.C. thanks the Institute for the
Promotion of Innovation by Science and Technology in Flanders
(IWT).
result in a progressive fluorescence enhancement as a function
of the number of stacked quartets, as well as to the study of
the dynamics of these systems.
Acknowledgment. The authors are grateful for the financial
support from the Council for the Chemical Sciences of The
Netherlands Organization for Scientific Research (CW-NWO) and
the EURYI scheme. D.G.R. and R.M.-R. would like to acknowledge
a Marie Curie Intraeuropean Fellowship. The authors would also
like to thank K. U. Leuven (GOA), the Belgian Federal Science
Policy Office through IAP-6/27 and the Fund for Scientific
Supporting Information Available: Experimental section,
synthetic procedures, characterization data, and some selected
supplementary figures (Figures S1-S10). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA908537K
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