Thermodynamics of the helical, supramolecular polymerization of linear self-asembling molecules: Influence of hydrogen bonds and π stacking

Article abstract of DOI:10.1002/chem.201300553

The cooperative supramolecular polymerization of 1 and 2 show very similar thermodynamic parameters (ΔH_elo⁰, ΔS ⁰, and ΔH_nucl⁰), but very different organization of the self-assembling units to constit

Full text of DOI:10.1002/chem.201300553

COMMUNICATION
Helical Structures
F. Aparicio, L. Sꢀnchez* .... &&&& — &&&&
Thermodynamics of the Helical,
Supramolecular Polymerization of
Linear Self-Asembling Molecules:
Influence of Hydrogen Bonds and p
Stacking
The cooperative supramolecular poly-
merization of 1 and 2 show very simi-
units to constitute helical structures.
The results presented allow quantify-
ing the influence of the type and
number of non-covalent interactions in
supramolecular polymerization.
lar thermodynamic parameters (DHe0lo
DS0, and DH0nucl), but very different
organization of the self-assembling
,
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DOI: 10.1002/chem.201300553
Thermodynamics of the Helical, Supramolecular Polymerization of Linear
Self-Asembling Molecules: Influence of Hydrogen Bonds and p Stacking
Fꢀtima Aparicio and Luis Sꢀnchez*[a]
Supramolecular polymers integrate order and dynamics to
achieve important functions such as high and efficient re-
sponse to stimuli, environmental adaptation, amplification
of chirality, and self-repair ability.[1] These functions are the
consequence of the hierarchical organization of relatively
small, and simple molecules that exhibit defined molecular
properties that are strongly amplified upon their organized
self-assembly process.[2] Artificial membranes,[3] polyvalent
scaffolds for binding and detecting bacteria,[4] aggregates ca-
pable to transport charges or with optical wave-guiding
properties,[5] organogels with light transmittance and refrac-
tive index suitable to fabricate flexible optical lens,[6] are
some of the outstanding applications shown by supramolec-
ular polymers generated upon the self-assembly of the ap-
propriate organic molecules. To achieve such applications,
the supramolecular polymerization process of the constitu-
tive building blocks must be efficiently controlled and,
hence, an accurate understanding of this process is of great
significance. The influence on the stability and dynamics of
relevant factors such as the solvent—a crucial parameter in
the performance of supramolecular materials[7]—or the
pathways that yield kinetically metastable aggregates that
subsequently evolve into their thermodynamically favored
Figure 1. Chemical structure of the self-assembling molecules 1–3.
form have been recently examined for a number of supra-
molecular polymers.[8]
Herein, we report on the influence of small structural
changes on the supramolecular polymerization and amplifi-
cation of chirality of simple linear, self-assembling molecules
1–3 (Figure 1). These compounds, being structurally similar,
differ in the number and class of non-covalent interactions
that favor their self-assembly into organized linear struc-
tures. At relatively high concentrations, all compounds in-
vestigated—except chiral 1b—self-assemble efficiently and
form transparent gels with non-polar solvents. At high-dilut-
ed conditions, only compounds 2 and 3, with dissimilar
number of H-bonding interactions and p surface, self-assem-
ble into helical structures in a cooperative manner.[2,9] The
elongation and nucleation thermodynamic parameters of the
aggregating systems are very similar but these aggregates
differ in stability. The internal structure of the aggregates
formed from compounds 2 or 3 is very different as suggested
by the corresponding sergeants-and-soldiers (SaS) experi-
ments performed to investigate the amplification of chirality
phenomenon.[10,11] The dissimilar organization of the constit-
utive self-assembling molecules to yield the final helical ag-
gregates has been demonstrated by NOE experiments. The
results presented herein increase the knowledge and control
on supramolecular polymerization processes and could con-
tribute to develop optimized strategies to achieve functional
supramolecular systems.
Compounds 1 and 3 were prepared in five synthetic steps
starting from commercially available 3,4,5-trihydroxymethyl-
benzoate following a similar procedure to that reported pre-
viously for compounds 2 and other closely related naphtha-
lenediimides (NDIs).[12,13] The chemical structure of all new
compounds has been unequivocally demonstrated by NMR,
FTIR and mass spectrometry (Scheme S1 and Supporting
Information).
[a] F. Aparicio, Prof. Dr. L. Sꢁnchez
Departamento de Quꢂmica Orgꢁnica, Facultad de Ciencias Quꢂmicas
Ciudad Universitaria s/n, 28040 Madrid (Spain)
Fax : (+34)913944103
E-mail: lusamar@quim.ucm.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300553.
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Helical, Supramolecular Polymerization
COMMUNICATION
To investigate the influence of the different non-covalent
interactions participating in the supramolecular polymeriza-
tion and amplification of chirality of the reported self-as-
sembling molecules, we have utilized compounds 2 as refer-
ence systems. We have recently reported that these linear
compounds self-assemble into helical, columnar supramolec-
ular structures that further aggregate to form bundles of
fibers. These bundles capture solvent molecules to originate
the corresponding organogels.[14] Analogously to 2, the achi-
ral ester 1a and both NDIs 3 form organogels with different
critical gelation concentration (CGC) in non-polar solvents
such as methylcyclohexane (MCH) or toluene (Table S1 and
Figure S1) which imply that the reported self-assembling
molecules are able to form organized supramolecular struc-
tures. These supramolecular structures have been visualized
by scanning electron microscopy (SEM). As it was observed
for compounds 2,[14] the SEM images display uniform, long,
and intertwined wires for achiral 1a and 3a (Figure S1) but
globular aggregates joint together by an undefined, viscous
mass for chiral 3b (Figure S1). The fine structure of the ag-
gregates have been visualized at more diluted conditions by
using atomic force microscopy (AFM) (Figure 2, S2–S4).
thicker rods with height of 12 and 16 nm (Figure S3). A
closer inspection on the AFM images of 3a demonstrates
the helical character of the nanowires formed by this achiral
NDI that self-assembles into a mixture of right- and left-
handed helical structures (Figure 2b). The peripheral chiral
chains in 3b induce the aggregation of this NDI into twisted
filaments of 1.5 nm height that further interdigitate into
thicker fibers (Figure 2c, d and S4). The dissimilar critical
gelation concentration (CGC) required to form the corre-
sponding organogels and also the differences observed in
the AFM images of the aggregates formed by compounds 1,
2, and 3 suggest the influence that the number and/or type
of non-covalent interactions exert on the aggregation phe-
nomenon.
The self-assembly of the compounds in solution has been
investigated by applying the novel equilibrium model re-
ported for BTA-based systems in which the dynamic equili-
brium between free monomers and the supramolecular poly-
mer and the growth of the corresponding polymer is consid-
ered.[15] The formation of supramolecular polymers is simpli-
fied by a two-reaction model: the nucleation and the elonga-
tion. In the former, the equilibrium between the free
monomers and the nucleated seed is described. The latter
describes the fast growth of the nucleus that extends to
gives rise to the final aggregate. These two reactions are de-
scribed by two different constants, K1 and K2, and the pa-
rameter s that expresses the degree of cooperativity. If s=1
the mechanism is isodesmic being cooperative for values of
0
s ¼ 1.[2,9] The Arrheniusꢄ law, K=eꢀDG /RT, relates the equili-
brium constants K1 and K2, as well as s with an enthalpy of
elongation DH0elo, an entropy of elongation DS0, a nucleation
penalty DH0nucl, and a mismatch penalty DHm0 m. DS0 is consid-
ered independent of the supramolecular process, but the
value of DHe0lo is penalized by the DH0nucl and/or DHm0 m. The
former term, which is independent of the chirality of the ag-
gregate, expresses the energetic cost required to generate
active nucleus, and the latter penalizes a mismatch when a
chiral monomer is introduced into a stack of its unpreferred
helicity.[15] The three equations that relate all these parame-
ters are:
Figure 2. AFM images of the diluted gels of 3a (a and b) and 3b (c and
d) (HOPG, 1ꢃ10ꢀ5 m, toluene; the z scale values are 35, 20, 20, and 8 nm
for a), b), c), and d), respectively). The black lines in b) are to guide the
eye in the chirality of the fibers.
ꢀðDH0ꢀTDS0Þ=RT
ð1Þ
ð2Þ
ð3Þ
K1 ¼ e
0
mm
=RT
K2 ¼ K1eDH
0
nucl
=RT
s ¼ eDH
The ester derivatives 1 exhibit a low trend to form organ-
ized supramolecular structures at concentrations as low as
1ꢃ10ꢀ5 m. Achiral 1a self-assembles into isolated straight
rods of several micrometers length (Figure S2a) but chiral
1b form smaller intertwined fibrils together with deposits of
thin cylindrical micelles underneath (Figure S2b,c). Analo-
gously to compounds 2,[12] NDIs 3 exhibit a stronger trend
to aggregate into well-defined fibrillar structures at highly
diluted conditions than esters 1 (Figure 2a, b and S3). The
aggregates of achiral NDI 3a appear as long nanowires of
4 nm height (Figure S3) that intertwine efficiently to form
The new equilibrium model reported by ten Eikelder,
Meijer and co-workers[15] has allowed us to perform an accu-
rate analysis of the supramolecular polymerization of com-
pounds 1–3 quantifying the influence of the number and
type of non-covalent interactions. The cooling curves ob-
tained by variable temperature circular dichroism (VT-CD)
measurements of pristine chiral compounds yield the corre-
sponding values of DH0elo, DS0, and DHn0ucl. We started these
studies recording the CD spectra of compound 1b in MCH
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L. Sꢁnchez and F. Aparicio
as solvent. Although the AFM images of compounds 1 show
the formation of aggregates, the CD spectra of 1b at a con-
centration as high as 1ꢃ10ꢀ4 m did not show any dichroic re-
sponse (Figure S5). These spectroscopic features contrast
with that reported for compound 2b that displays a bisignat-
ed dichroic signal—with a positive maximum at 222 nm and
a negative maximum at 266 nm[12]—and are a clear indica-
tion of the enormous effect exerted in the self-assembly
process by the presence of two additional H-bonding amide
units in 2b in comparison to 1b. The global analysis of the
cooling curves obtained by VT-CD experiments at different
concentrations of 2b (Figure S6) shed values for DH0elo, DS0,
and DH0nucl of ꢀ104.9, ꢀ0.21, and ꢀ36.7 kJmolꢀ1, respective-
ly (Table 1).
but changes sensibly the stability of the aggregates as indi-
cates the corresponding Te values (Table 1).
However, the different type and/or number of non-cova-
lent forces does exert a remarkable effect in the supramo-
lecular polymerization as demonstrate the experiments of
amplification of chirality. The lack of dichroic response in
compound 1b impedes performing the corresponding stud-
ies of amplification of chirality. Even by mixing achiral 1a
with chiral 2b or vice versa resulted in a null dichroic re-
sponse thus indicating the strong effect that the presence of
only two H-bonding amide functionalities has on the ability
to form aggregates with a preferred helical orientation in
solution.[16] This situation is very different for compounds 2
and 3. We have previously described that the helicity of the
racemic mixture formed by achiral 2a can be biased by
adding increasing amounts of chiral 2b being the maximum
CD effect obtained upon the addition of 30% of the chiral
sergeant (Figure 3a).[12]
Table 1. Thermodynamic parameters calculated for the cooperative su-
pramolecular polymerization of compounds 2b and 3b in MCH.
DH0elo
[kJmolꢀ1
DS0
[kJmolꢀ1
DH0nucl
[kJmolꢀ1
s
Te
[K]
The SaS experiments accomplished for NDIs 3 show that
the maximum handedness value is achieved in the racemic
mixture of aggregates formed from achiral 3a by adding
only 10% of the chiral sergeant 3b (Figure 3b). The differ-
ent amount of chiral sergeant required to achieve the maxi-
mum chirooptical response in compounds 2 and 3 indicates
that the internal structure of the aggregates formed from
tetra-amides 2 or NDIs 3 is very different and the intercala-
tion of the chiral sergeant in the aggregates formed from 2a
is more impeded in comparison to the aggregates of 3a.
To elucidate the molecular organization of compounds 2
and 3 during aggregation, we have carried out selective
NOE experiments with solutions of 2a and 3a at different
concentrations (Figure 4 and S9).[17] The irradiation of the
protons corresponding to the ethylene spacer in a concen-
trated CDCl3 solution of 2a (20 mm) resulted in clear NOE
effects with both protons at the central and peripheral aro-
matic units as well as the expected interaction with the
amide functional groups. These NOE effects suggest the ag-
gregation of compounds 2 into columnar, rotated aggregates
G
]
E
]
G
]
2b
3b
ꢀ104.95
(ꢂ0.96)
ꢀ0.21
(ꢂ0.002)
ꢀ36.75
(ꢂ2.06)
3.6ꢃ10ꢀ7
355.1 (5ꢃ10ꢀ5);
G
G
ACHTUNGTRENNUNG
339.7 (1ꢃ10ꢀ5);
333.5 (5ꢃ10ꢀ6
)
ꢀ101.99
(ꢂ1.84)
ꢀ0.23
(ꢂ0.005)
ꢀ36.43
(ꢂ6.05)
4.1ꢃ10ꢀ7
322.8 (5ꢃ10ꢀ5);
309.7 (1ꢃ10ꢀ5);
E
E
ACHTUNGTRENNUNG
308.9 (9ꢃ10ꢀ6
)
A first indication of the efficient self-assembly of NDIs 3
is inferred from the corresponding UV/Vis spectra in poor
and good solvents like MCH or CHCl3, respectively (Fig-
ure S7). In CHCl3, a solvent in which the self-assembling
molecules are in a molecularly dissolved state, the UV/Vis
spectra of compounds 3 appear intense and well defined. In
contrast, in MCH the UV/Vis spectra appear less intense
and poorly defined due to the efficient aggregation.[7] To
achieve the complete thermodynamic analysis, we have per-
formed VT-CD of compound 3b in MCH. Compound 3b is
also CD-active at high diluted conditions showing two sig-
nals of opposite sign at 227 and 260 nm together with two
other smaller signals at 339 and 365 nm (Figure S8). The bis-
ignated Cotton effect observed in the CD spectra of NDI
3b at room temperature and the non-sigmoidal shape of the
corresponding cooling curve express the cooperative aggre-
gation of 3b into helical stacks (Figure S8).[9,12] The calculat-
ed values of DHe0lo, DS0, and DHn0ucl—extracted from the
global fitting of the cooling curves obtained at different con-
ꢀ
by the operation of amide C=O···H N H-bonds and with
negligible p stacking between the small aromatic units
(Figure 4, top). The columnar aggregates grow longitudinally
to form the fibers observed in the AFM images.[12] The irra-
diation of the ethylene spacer protons in a diluted solution
of 2a (2 mm) only show NOE effects with the amide N–H
resonances being the NOE effects with the aromatic units
completely cancelled, which unambiguously demonstrate the
intermolecular character of the interactions (Figure S9, top).
The irradiation of a concentrated solution of NDI 3a
(20 mm in CDCl3) at dꢁ8.7 ppm, corresponding to the aro-
matic protons of the NDI moiety, shows NOE effect with
the resonances at dꢁ6.9 and 3.9 ppm, ascribable to the pe-
ripheral trialkoxybenzene unit and the methylene group
joint to the oxygen in the peripheral chains (Figure 4,
bottom). These NOE effects disappear by using a diluted
solution of 3a (2 mm) that demonstrates the intermolecular
character of the NOE effect (Figure S9, bottom). The spatial
proximity of the NDI and trialkoxybenzene aromatic moiet-
centrations
of
3b—are
of
ꢀ101.9,
ꢀ0.23
and
ꢀ36.43 kJmolꢀ1, respectively (Table 1). These values are
very similar to that calculated for 2b, which suggest that
these two self-assembling molecular building blocks exhibit
an analogous trend to form supramolecular polymers re-
gardless the non-covalent interactions participating in this
process. Apparently, the replacement of two H-bonding
amides by a relatively large p surface has no influence on
the supramolecular polymerization thermodynamic parame-
ters and also in the degree of cooperativity of the process
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Helical, Supramolecular Polymerization
COMMUNICATION
ies can be justified by consider-
ing the formation of linear ag-
gregates in a head-to-tail fash-
ion by the H-bonding interac-
tion of the amide functional
groups (Figure 4, bottom). The
further p–p interactions be-
tween the aromatic units of
these linear aggregates give rise
to the intertwined fibers ob-
served in the AFM images. The
dissimilar internal organization
of the aggregates formed from
2 and 3 justifies the differences
found in the incorporation of a
chiral sergeant into the corre-
sponding helical aggregates and
demonstrates the strong influ-
ence exerted by the number
and type of non-covalent inter-
Figure 3. Sergeants-and-soldiers experiments performed by mixing a) achiral 2a and chiral 2b and b) achiral
3a and chiral 3b (MCH, 298 K, 1ꢃ10ꢀ5 m).
actions involved in the forma-
tion of supramolecular poly-
mers.
In summary, we have applied
the mathematical model recent-
ly described for cooperative
self-assembly—that
considers
the dynamic equilibrium be-
tween the growth of the supra-
molecular polymer and the
monomer pool—to quantify the
influence of the different
number and/or types of non-co-
valent forces on the supramo-
lecular
polymerization
of
simple linear molecules 1–3.
The presence of only two H-
bonding units allows the aggre-
gation into fibrillar structures
of esters 1, however, the self-as-
sembly of compounds 1 in solu-
tion is so weak that it cannot be
investigated by the above men-
tioned model. Compounds 1a,
2, and 3 also form fibers that
further interact to constitute
gels. The elongation and nucle-
ation thermodynamic parame-
ters of 2 and 3 are very similar
but the corresponding SaS ex-
periments show that an increas-
ing amount of the chiral ser-
geant with increasing the
number of H-bonds functionali-
ties is required to achieve the
maximum handedness of the
helical supramolecular struc-
Figure 4. Partial 1H NMR spectra (300 MHz, 298 K) (black) and NOE experiments (gray) of 2a (top) and 3a
(bottom) irradiating at d=3.7 and 8.7 ppm for 2a and 3a, respectively (298 K, CDCl3, 20 mm). Curved arrows
represent the NOE contacts and a schematic model of the aggregation of 2a and 3a is depicted.
Chem. Eur. J. 2013, 00, 0 – 0
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L. Sꢁnchez and F. Aparicio
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Acknowledgements
Financial support by the MINECO of Spain (CTQ2011-22581) and UCM
(UCM-SCH-PR34/07-15826) is acknowledged. F.A. is indebted to MEC
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Keywords: chirality
elongation · self-assembly · supramolecular polymers
·
helical structures
·
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Received: February 12, 2013
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