Solvent Molding of Organic Morphologies Made of Supramolecular Chiral Polymers

Article abstract of DOI:10.1021/jacs.5b02448

The self-assembly and self-organization behavior of uracil-conjugated enantiopure (R)- or (S)-1,1′-binaphthyl-2,2′-diol (BINOL) and a hydrophobic oligo(p-phenylene ethynylene) (OPE) chromophore exposing 2,6-di(acetylamino)-pyridine termini are reported. S

Full text of DOI:10.1021/jacs.5b02448

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Article
Solvent molding of organic morphologies
made of supramolecular chiral polymers
Luka #or#evi#, Tomas Marangoni, Tanja Mileti#, Jenifer Rubio-Magnieto, John Mohanraj, Heinz
Amenitsch, Dario Pasini, Nikos Liaros, Stelios Couris, Nicola Armaroli, Mathieu Surin, and Davide Bonifazi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b02448 • Publication Date (Web): 20 May 2015
Downloaded from http://pubs.acs.org on June 4, 2015
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Solvent molding of organic morphologies made of supramolecular
chiral polymers
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Luka Đorđević,†‡ Tomas Marangoni,†‡ Tanja Miletić,† Jenifer Rubio-Magnieto,§ John Mohanraj,⊥
Heinz Amenitsch,∥ Dario Pasini,# Nikos Liaros,▽,º Stelios Couris,▽,º Nicola Armaroli,⊥ Mathieu Su-
rin§,* and Davide Bonifazi†, ,*
⁊
†Department of Chemical and Pharmaceutical Sciences, INSTM UdR Trieste, University of Trieste, Piazzale Europa
1, 34127 Trieste, Italy
§Laboratory for Chemistry of Novel Materials, Center for Innovation in Materials and Polymers, University of Mons
– UMONS, 20 Place du Parc, B-7000 Mons, Belgium
⊥Istituto per la Sintesi Organica e la Fotoreattività, Consiglio Nazionale delle Ricerche (CNR-ISOF), Via Gobetti 101,
40129 Bologna, Italy
∥Institute of Inorganic Chemistry, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria
#Department of Chemistry and INSTM Research Unit, University of Pavia, Viale Taramelli 10, 27100, Pavia, Italy
▽Department of Physics, University of Patras, 26504 Patras, Greece
ºInstitute of Chemical Engineering Sciences (ICE-HT), Foundation for Research and Technology-Hellas (FORTH),
P.O. Box 1414, Patras 26504, Greece
⁊Namur Research College (NARC) and Department of Chemistry, University of Namur (UNamur) Rue de Bruxelles
61, 5000 Namur, Belgium
Supporting Information Placeholder
ABSTRACT: The self-assembly and self-organization behaviour of uracil-conjugated enantiopure (R) or (S) 1,1’-
binaphthyl-2,2’-diol (BINOLs) and an hydrophobic oligo(para-phenylene ethynylene) (OPE) chromophore ex-
posing 2,6-di(acetylamino)pyridine termini are reported. Systematic spectroscopic (UV-Vis, CD, fluorescence,
NMR and SAXS) and microscopic studies (TEM and AFM) showed that BINOL and OPE compounds undergo
triple H-bonding recognition generating different organic nanostructures in solution. Depending on the sol-
vophobic properties of the liquid media (Toluene, CHCl3, CHCl3/CHX and CHX/THF), spherical, rod-like, fi-
brous and helical morphologies were obtained, with the latter being the only nanostructures expressing chirali-
ty at the microscopic level. SAXS analysis combined with molecular modelling simulations showed that the
formation of the helical superstructures is composed of dimeric double-cable tape-like structures that, in turn,
are supercoiled at the microscale. This behaviour is interpreted as a consequence of an interplay between the
degree of association of the H-bonded recognition, the vapour pressure of the solvent and the solvopho-
bic/solvophilic character of the supramolecular adducts in the different solutions under static and dynamic
conditions, namely during solvent evaporation conditions at room temperature.
tectures is an important research topic for develop-
INTRODUCTION
ing nanostructured materials.2 In this respect, chem-
ists have developed several protocols to engineer
functional architectures featuring programmed op-
toelectronic properties at different scales.3 In par-
The control and understanding of the driving forc-
es governing the self-assembly and self-organization1
of functional organic molecules into complex archi-
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ticular, molecules possessing extended π-conjugated mer were ruled by local changes in the solution
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5
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surfaces, such as oligo(phenylene ethynylene)s
(OPEs),4 oligo(phenylene vinylene)s (OPVs)3c, 5 and
coronene derivatives,6 are versatile modules to pre-
pare functional nanostructured materials due to
their tunable electronic and optical band gap as well
as their aptitude to arrange in π-π stacks. A further
control in the organization can be achieved using
directional non-covalent interactions, such as multi-
ple H-bonds3a, 3c, 3e, 7 and transition-metal coordina-
tion bonds.8
properties leading to temperature-dependent non-
equilibrium diffusions of the chemical species.16 On-
ly a limited number of efforts have been invested to
elucidate the solvent effect on the self-organization
of organic morphologies.7d, 10i, 17 This is even more
important if one wanted to understand and control
the transfer of molecular chirality at higher scales.18
9
We have been focusing for several years on the ura-
cil・diamidopyridine (Ur・DAP) H-bonded complex
as a heteromolecular recognition motif for preparing
different hierarchically-organized materials.19 With
its moderate association constant, the Ur・DAP com-
plex ensures a high degree of dynamicity and reversi-
bility requisite for establishing long-range molecular
order20 in liquids19b, 21 at liquid-solid22 and vacuum-
solid interfaces.23 With the aim of adding an addi-
tional brick toward the understanding of the expres-
sion of molecular chirality at higher dimensions, we
prepared different molecular modules (Scheme 1)
and studied their self-assembly and self-organization
behavior in different solvents (Chloroform (CHCl3),
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Whereas through molecular recognition, the prep-
aration of architectures with controlled function and
shape can be easily reached at the molecular level by
inserting the right functional groups in the self-
assembling molecular constituents,9 the expression
of the supramolecular structure properties at higher
scales, and thus the full control of the morphological
properties of microscopic structures, remains chal-
lenging.7b, 10
In this respect, one of the significant problems to
be tackled is the establishment of protocols capable
of combining the effects of chemical structures and
those of the boundary conditions, namely the sol-
vent, the interface and temperature. From a mecha-
nistic point of view, organic nanostructures generat-
ed by self-organization processes in solution are the
result of a complex combination of several different
variables. In particular, when organic materials are
processed, the final morphologies are generally in-
fluenced by phenomena such as solvent dewetting
and evaporation,11 Ostwald ripening,12 Rayleigh-
Bénard instabilities,13 Marangoni effect,14 and phase
separations.15 Recently, we have shown that the geo-
metric and spatial features of the organic morpholo-
gies generated by a supramolecular H-bonded poly-
Toluene
(CHCl3/CHX)
(Tol),
(1:1
Chloroform/Cyclohexane
v/v) and Cyclohex-
ane/Tetrahydrofuran (CHX/THF)) by means of com-
plementary spectroscopic (NMR, optical and chirop-
tical characterization) and microscopic techniques
(TM-AFM and TEM). Instead of using chiral side
chains, we have employed atropoisomeric chiral
cores, which consists of a (R)- or (S)- 1,1’-binaphthyl-
2,2’-diol (BINOL).24 This enantiomeric couple bears
two Ur moieties, while the complementary DAP
units are fixed at the extremities of a chromophoric
OPE spacer with lateral solubilizing alkoxy chains.
Atomic Force Microscopy (AFM) and Transmission
Electron Microscopy (TEM) showed a solvent-
Scheme 1. (a) Molecules prepared for this study; (b) synthetic routes for molecules (R) or (S)-1, (R) or (S)-3, 2, 4 and 5: a) 1) TMSA,
Pd(PPh3)2Cl2, CuI, Tol/Et3N, rt; 2) K2CO3, THF, MeOH; b) 6 (2.5 eq.), Pd(PPh3)4, CuI, THF/Et3N, rt; c) 6 (0.2 eq.), Pd(PPh3)4, CuI, THF/Et3N, rt; d)
7, Pd(PPh3)4, CuI, THF/Et3N, rt; e) Hg(OAc)2, I2 (2.5 or 1 eq.) , CH2Cl2; f) 2,6-di(acetylamino)-4-ethynylpyridine, Pd(PPh3)4, CuI, Tol/DMF/Et3N.
Abbreviations: TMSA, trimethylsilylacetylene; Tol, toluene; THF, tetrahydrofuran; DMF, dimethylformamide.
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dependent formation of different organic morpholo-
gies. For instance, employing different CHX/THF
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solutions (80:20 – 95:5 v/v), nanostructures such as
spheres, rods or supercoiled helices were formed,
with the last being the only morphologies expressing
a certain degree of macroscopic chirality. Finally,
Small-Angle X-ray Scattering (SAXS) and molecular
modeling simulations were used to shed further light
on the molecular organization of the helical mor-
phologies, suggesting a tape-like hierarchical organi-
zation.
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1
Figure 1. a) H-NMR (400 MHz, 298 K) titration data of (S)-4 in
CDCl3 (c = 1.3 × 10-3 M) with different molar amounts of 5 (from 0 to
3.9 equiv). NH peaks are highlighted in red; (b) titration data and 1:1
nonlinear least-square fitting of the NH signals of (S)-4 plotted
against the molar ratio; (c) Job’s plot and fitting of the NH signals of
(S)-4 plotted against the molar fraction.
RESULTS AND DISCUSSION
Synthesis
also measured in the different solvents, revealing to
be 7, 19 and 60 M-1 in CDCl3, CDCl3/CHX-d12 and Tol-
d8 , respectively (see the dilution experiments as dis-
played in Supporting Information Figure S8 and the
fitting in S9, Table 1) and was found to have only a
negligible effect on the heteroassociation. This is in
Molecules (R)-1 and (S)-1 were synthesized by Pd-
catalysed Sonogashira-Hagihara cross-coupling reac-
tion between enantiomerically pure 3,3′-diethynyl-
2,2’dimethoxy-1,1′-binaphthalenes 925 and 1-hexyl-6-
iodouracil 6.23 BOC-protected compounds (R)-3 and
(S)-3 molecules were synthesized following an anal-
ogous pathway using N-BOC protected 1-hexyl-6-
iodouracil 7.19b While compound 2 was prepared ac-
cording to our previously-reported procedure,21 com-
pound 5 results from the Pd-catalysed cross coupling
of 2,6-di(acetylamino)-4-ethynylpyridine with 1,4-
bis(dodecyloxy)-2-iodobenzene 12.21 Synthetic details
are reported in Schemes 1b and S1 (Supporting In-
formation). The compounds were characterized by
27
line with the literature reports. Further NMR inves-
tigations confirmed the 1:2 and 2:1 binding stoichi-
ometry for complexes [1∙(5)2] and [(4)2∙2], respective-
ly (Supporting Information Fig. S10). Considering the
measured Ka values and assuming that the recogni-
tion motifs of the ditopic modules are independent,
a mixture of complementary 1 and 2 molecules
should lead to degrees of polymerization N28 of
about 0.8, 2.1 and 2.7 in CDCl3, Tol-d8 and
CDCl3/CHX-d12 (1:1 v/v), respectively, when working
under dilute conditions (c = 2.5 × 10-4 M).
13
1H- and C-NMR spectroscopies, IR, HRMS and by
optical activity (Supporting Information, Fig. S1-6).
NMR investigations
Table 1. Calculated association constants (Ka) for the H-bonded
heterodimer at 298 K for 4 + 5 ⟺[4・5], dimerization constant (Kdim
for 4 and the corrected association constant (Kc) 4 + 5 ⟺[4・5] con-
sidering the dimerization of 4 in the given solvent. Calculations
performed with Scientist package from MicroMath.
The self-assembly of the complementary monotop-
ic modules 4 and 5 was firstly studied to rationalize
the influence of the lateral groups on the association
behavior of the Ur・DAP recognition motif in solu-
tion (Ka, Table 1). Titration experiments in CDCl3
(Fig. 1a) showed a fast equilibrium, a progressive
downfield shift of the uracil NH protons upon the
incremental addition of 5. The association constant
(Fig. 1a and 1b) was found to be 1296 (±3) M-1 with a
1:1 stoichiometry (as evidenced by the continuous
variation method, Fig. 1c), a value which is compara-
ble to that reported in the literature for similar sys-
tems.20b, 22a In more apolar solvents like Tol-d8 and
CDCl3/CHX-d12 (1:1 v/v), Ka values of 8737 (±627) and
14819 (±3820) M-1 were measured, respectively (Fig.
S7 and Table 1). As expected, increasing the apolar
character of the solvent mixture favored an en-
hancement of the binding strength.26 Dimerization
constants of the Ur-bearing molecular modules were
)
Ka, M-1 Kdim, M-1
Kc, M-1
(sd)
1300
(29)
Stoichi-
ometry
Solvent
CDCl3
ℇr
4.81
2.44
/
(sd)
(sd)
1:1
1:1
1:1
1296 (3)
7.5 (0.1)
8737
(627)
14819
(3820)
8205
Tol-d8
60 (5)
19 (4)
(564)
14427
(3980)
CDCl3/
CHX-d12
NMR titrations in CHX/THF solutions could not
be performed due to the limited solubility of both
molecular components in the concentration interval
needed for NMR. In order to corroborate the associa-
tion of the supramolecular assemblies [1∙2]n, 1H-NMR
spectroscopy was performed on a 1:1 mixture of mo-
lecular modules (S)-1 and 2 (Fig. 2a, spectra at
CDCl3, 500 MHz, 293 K) in CDCl3. The addition of an
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Molding of the morphologies
Page 4 of 14
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Evaporation Induced Polymerization
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As mentioned in the introductory section, the
evaporation of a liquid solvent plays a crucial role in
the formation of various nanostructures. This effect
is known as Evaporation-Induced Self-Assembly
(EISA).30 As shown for inorganic architectures,30 one
can also expect that in supramolecular polymeriza-
tion, the vapor tension of a given liquid could play a
dramatic role on the degree of polymerization. In a
simple model, this depends on the association
strength of the recognition motif, the monomers
concentration and the liquid vapor pressure at a giv-
en temperature. To evaluate these effects on our sys-
tems, we have investigated the polymerization be-
havior as a function of the evaporation rate of the
solvent (Fig. 3, Supporting Information, Fig. S13).
Thus, we have measured the normalized time-
dependent shrinking of a drop-casted liquid on an
AFM support plate at room temperature in an open
environment under ambient pressure. Depending on
the vapor pressure of the liquid, a progressive in-
crease in the concentration of the monomers is ob-
served, thus dynamically affecting the degree of
polymerisation.31 As outlined in Figure 3, the higher
the liquid vapor pressure (pA), the larger the maxi-
mum degree of polymerization; for instance, at the
50 sec time point, the estimated N is ∼15 for a solvent
with higher pA (CHCl3) and ∼3 for a solvent with a
lower pA (Tol). This suggests that, in evaporating
CHCl3 solutions, the organic morphologies are main-
ly controlled by the formation of longer polymers,
while in Tol very shorts oligomers are preferentially
formed. On the other hand, solutions with similar
evaporation rates give degrees of polymerization es-
sentially controlled by the association strength of the
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Figure 2. NMR characterization of supramolecular polymer [(S)-
1∙2]n: (a) H-NMR (CDCl3, 500 MHz, 298K) of (S)-1 (top), 2 (bottom)
1
and supramolecular polymer [(S)-1∙2]n (middle); (b) DOSY - normal-
ized signal decay versus the diffusion weighing (b values) for (S)-1
(blue), 2 (red) and supramolecular polymer [(S)-1∙2]n (purple); (c)
2D-NOESY spectra (CDCl3, 500 MHz, 298 K) of [(S)-1∙2]n; (d) VT-
NMR (C2D2Cl4, 400 MHz, 298 K) of [(S)-1∙2]n heating and cooling
cycle.
equimolar quantity of 2 to a solution of (S)-1 induced
a drastic downfield of the NH proton resonances
suggesting the formation of the H-bonded complex
(from 8.65 to 11.10 ppm and 8.15 to 9.75 ppm for (S)-1
and 2, respectively), whereas all the other signals re-
1
mained unaltered. H-1H NOESY (Fig. 2c) analysis
displayed the through-space interactions between
the NH resonances arising from the adjacent protons
(NH-Ur and NH-DAP, respectively) suggesting, as
expected, a close spatial proximity (< 5 Å) between
the Ur and DAP moieties. The non-covalent and
reversible nature of the interaction was confirmed by
Variable Temperature (VT)-NMR (400 MHz,
C2D2Cl4, Figures 2d and S12). Indeed, upon increas-
ing the temperature from 293 to 373 K, a gradual re-
instatement of the NH chemical shifts characteristic
of molecules (S)-1 and 2 alone were observed, clearly
suggesting the disruption of the H-bonded complex,
and thus that of the supramolecular assembly. Final-
ly, Diffusion Ordered Spectroscopy (DOSY) (CDCl3,
500 MHz, Figures 2b and S11) disclosed the presence
of oligomeric species, in agreement with the predict-
ed degree of polymerization N, featuring small diffu-
sion coefficients D[1∙2]n = 2.00 × 10-6 cm2 s-1 (compared
to D1 = 4.37 × 10-6 cm2 s-1 and D2 = 4.70 × 10-6 cm2 s-1)
owing to the formation of high-molecular-weight
oligomeric assemblies.29
Figure 3. (a) Normalized evaporation of the solvents reported as
weight lost over time and (b) estimated polymerization degree dur-
ing evaporation. V and V0 are the volume of the liquid at a t and 0
sec, respectively, calculated by measuring the weight divide by the
liquid density. Vapor tension (pA) values at 20 °C and ambient pres-
sure: 22 mmHg for Tol, 159 mmHg for CHCl3 and 118 for the 1:1 v/v
CHCl3/CHX mixture (the latter value was calculated from the Ra-
oult’s law).
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recognition unit. For instance, in the 1:1 v/v consideration, we can interpret these fibers as
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CHCl3/CHX mixture higher degree of polymerization
can be achieved (∼40) than in CHCl3 after 50 sec,
owing to the stronger association of the Ur・DAP
motif in apolar solutions.
formed by short supramolecular polymers (N ≃ 15-
20) that upon formation undergo aggregation afford-
ing the macroscopic constitutional unit of the ob-
served morphologies. Relying on the onset condition
of a concentration-driven solutal instability,16 the
extensive aggregation phenomenon seems to be gov-
erned by the solvophobic lateral alkoxy chains of
compound 2. It can be hypothesized that the OC12H25
chains are somewhat exposed on the external side of
the polymeric assemblies forming a hydrophobic
coat that enables extensive van der Waals interac-
tions through chain-chain interdigitation, ultimately
driving the formation of the aggregated morpholo-
gies.
In this model, however, we did not consider the
solvophobic and solvophilic properties of the solvent
with respect to the monomers and the supramolecu-
lar assemblies. As previously observed for similar sys-
tems, the evaporation of the solvent can also lead to
concentration-driven solutal instabilities,16 inducing
precipitation phenomena, aggregation or polymeri-
zation termination. This effect will be qualitatively
considered for each solvent in the discussions of the
next paragraphs.
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CHCl3: from solution to nanorods
Toluene: from solution to nanoparticles
Due to the chromophoric nature of the OPE spac-
er, UV-Vis spectra of dilute solutions were measured
to gain insight into the self-assembly process. Under
dilute conditions (c = 2.5 × 10-4 M) in CHCl3, no dis-
cernible changes in the absorption profiles of [1∙2]n
compared to the individual compounds were noticed
(Supporting Information Fig. S14), due to the weak
degree of association of the Ur・DAP complex. How-
ever, when drop-casting a solution of [1∙2]n (1.3 × 10-3
M) onto freshly cleaved mica or carbon coated grids,
aggregation was observed and different nanoscopic
morphologies were visualized by TM-AFM and TEM.
Both techniques confirmed the presence of rod-like
microstructures featuring diameters between 150 and
500 nm and lengths ranging from 0.5 to 6.5 μm.
Through the combined use of both TEM and AFM
phase imaging (Fig. 4, Supporting Information Fig.
S27), it was possible to discern rod-like structures
composed of bundles of smaller fibers exhibiting a
diameter of approximately 6 nm. Based on the above
Subsequently, the influence of toluene on the self-
assembly was studied. The UV-Vis profile of [1∙2]n (c
= 2.5 × 10-4 M) showed a redshift (λ = 388 nm to λ =
397 nm) of the π-π* transition of the OPE derivative
2 and the formation of a shoulder at around 415 nm
(Fig. 5a). Variable temperature measurements
showed the appearance of an isosbestic point (λ =
403 nm) and an absorption profile that shifted to-
wards the algebraic sum of the individual compo-
nents (Fig. 5b, Supporting Information Fig. S18-19).
Notably, the spectra were more intense at high tem-
peratures, suggesting a scarce solubility of com-
pound 2 in toluene solutions at room temperature
(scarce solubility of compound 2 was confirmed by
VT-UV-Vis measurements, see Fig. S20).32
Owing to the (R)-1 or (S)-1 intrinsic chirality and
the favorable association of the Ur・DAP complex in
toluene, it was possible to study the chiral assemblies
by Circular Dichroism (CD) spectroscopy on both
the pure components and the relative assemblies.
The CD spectra (c = 2.5 × 10-4 M) of the BINOL de-
rivatives were characterized by a maximum at 352 nm
and an overall CD effect, which takes place in the
region between 290 and 380 nm (Fig. 5c). When lin-
ear module 2 was added to a solution of (R)-1 or (S)-
1, a new band appeared in the region 390-435 nm
with a peak around 415 nm and a shoulder around
400 nm (Fig. 5c), a spectral region in which the
BINOL derivatives are not CD active. To further in-
vestigate this induced CD effect in the region where
achiral 2 absorbs, the CD spectra were recorded as a
function of temperature and concentration for both
[(S)-1∙2]n and [(R)-1∙2]n assemblies (Fig. 5d and Fig.
S24). VT-CD experiments from 293 to 343 K for [1∙2]n
showed a gradual decrease of the induced CD effect
Figure 4. TM-AFM (a-c) and TEM (d-f) images of the morphologies
obtained from a drop-casted CHCl3 solution of [(S)-1∙2]n (c = 1.3 × 10-3
M); the other enantiomer showed the same type of nanostructuring
(Supporting Information, Fig. S27).
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changes in the absorption profile (Supporting In-
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formation Fig. S16, S17 and S21), indicating that the
spectral changes in [1∙2]n are attributed to that on of
the supramolecular assembly. The same behavior was
also observed by CD spectroscopy (Supporting In-
formation Fig. S22-23).
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In CHCl3/CHX (1:1 v/v), the CD spectra (c = 2.5 ×
10-4 M) of the BINOL derivatives were characterized
by CD signals between 250 and 390 nm (Fig. 6b).
When linear module 2 is added to a solution of (R)-1
or (S)-1, a new band appeared in the region 390-440
nm with a maximum at around 415 nm and a shoul-
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Figure 5. (a) Electronic absorption spectra of (S)-1, (R)-1, 2, [(S)-1∙2]n
and [(R)-1∙2]n in toluene at 293 K (c = 2.5 × 10-4 M); (b) VT absorption
spectral changes in toluene from 293 to 343 K (c = 2.5 × 10-4 M, rec-
orded each 5 K at a rate of 1 K/min); the same behavior was observed
for the other enantiomer; (c) CD spectra of (S)-1, (R)-1, 2, [(S)-1∙2]n
and [(R)-1∙2]n in toluene at 293 K (c = 2.5 × 10-4 M); (d) VT-CD spec-
tral changes of [(S)-1∙2]n and [(R)-1∙2]n in toluene from 293 to 343 K
(c = 2.5 × 10-4 M) recorded each 10 K at a rate of 1 K/min); the inset
shows the CD intensity (at λ = 415 nm) plotted versus temperature.
at 415 nm, which is in accordance with the VT-UV-
Vis measurements. Surprisingly, drop-casting from a
toluene solution of [1∙2]n (c = 2.5 × 10-4 M) onto mica
showed only the presence of globular, yet undefined,
aggregates (Supporting Information Fig. S28). This
suggests that as the solvent evaporates, the forming
supramolecular assemblies undergo solutal instabil-
ity already at short lengths, thus giving rise to non-
structured morphologies most likely controlled by a
fast matter-momentum transport in solution.
CHCl3/CHX: from solution to nanofibers
The absorption profile of [1∙2]n in CHCl3/CHX (1:1
v/v, c = 2.5 × 10-4 M) displayed a red-shift (λ = 383 nm
to λ = 390 nm) of the π-π* transition of compound 2
and the formation of a shoulder at λ = 410 nm (Fig.
6a). Increasing the temperature from 293 to 333 K,
the absorption spectra underwent a gradual hypso-
chromic shift. The spectral shape at 333 K reminded
that of the single components at room temperature,
thus suggesting the disruption of the aggregates
(Supporting Information Fig. S15). As expected, the
absorption profiles were identical when performing
the experiments with (R)-1 or (S)-1. Additional ex-
periments (VT-UV-Vis and CD) with reference mole-
cules 4 and 5, BOC-protected molecules 3, and mole-
cules 1 and 2 alone, did not show any significant
Figure 6. (a) Electronic absorption spectra of (S)-1, (R)-1, 2, [(S)-
1∙2]n and [(R)-1∙2]n in CHCl3/CHX (1:1 v/v) at 293 K (c = 2.5 × 10-4 M);
(b) CD spectra of (S)-1, (R)-1, 2, [(S)-1∙2]n and [(R)-1∙2]n in
CHCl3/CHX (1:1 v/v) at 293 K (c = 2.5 × 10-4 M); (c) VT-CD spectral
changes of [(S)-1∙2]n and [(R)-1∙2]n in CHCl3/CHX (1:1 v/v) from 293 to
333 K (c = 2.5 × 10-4 M, recorded each 5 K at a rate of 1 K/min); insets
show plot CD intensity (λ = 415 nm) versus temperature; (d) Variable
concentration CD spectral changes of [(R)-1∙2]n in CHCl3/CHX (1:1
v/v) at 293 K; inset shows plot CD intensity (λ = 415 nm) versus con-
centration. The same behavior was also observed with the other
enantiomer. TM-AFM (e-g) and TEM (h-j) images of the morpholo-
gies obtained from a drop-casted CHCl3/CHX (1:1 v/v) solution of
[(S)-1∙2]n (c = 2.5 × 10-4 M) on mica surface and on a carbon coated
grid, respectively.
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der around 400 nm (Fig. 6b), again showing an in- entangled network of fibers displaying heights rang-
1
duced CD effect in the region where achiral OPE 2
absorbs. The variation of the induced CD effect was
recorded as a function of temperature and concen-
tration for both [(S)-1∙2]n and [(R)-1∙2]n assemblies.
As expected, both enantiomers displayed analogous
spectroscopic trends (Fig. 6c). VT-CD experiments
from 293 to 333 K on a solution of [1∙2]n showed a
gradual reduction of the CD signal at 415 nm, which
is in accordance with the VT-UV-Vis measurements
(Fig. S15). Similar results were also obtained when
monitoring the CD profile during dilution experi-
ments (Fig. 6d, from c = 2.5×10-4 M to c = 1.9×10-5 M).
Variable concentration CD experiments show a sig-
moidal behavior of the intensity at 415 nm (inset of
Fig. 6d), which suggests that the Ur・DAP complex
governs the formation of the supramolecular archi-
tectures. This was further confirmed by control
steady-state UV-Vis adsorption experiments using
solutions containing reference BOC-protected mole-
cules 3 in the presence of linear module 1, which dis-
played an adsorption profile deriving from the single
components. This further indicates that the for-
mation of the Ur・DAP complex governs the for-
mation of the supramolecular architectures. Appre-
ciably, when performing the experiments with [(R)-
1∙2]n and [(S)-1∙2]n assemblies, the spectra were mir-
ror images of each other (Fig. 6c).
ing from 20 to 80 nm and lengths up to several µm.
Although the CD spectra suggested the formation of
a chiral secondary structure in a CHCl3/CHX (1:1 v/v)
solution, a microscopic helicity was not observed
using AFM or TEM, even though it might be present
at the nanoscopic level. This was also noticed for
other supramolecular systems.7b, 33 The formation of
fibers suggested that, the higher degree of polymeri-
zation in the 1:1 v/v CHCl3/CHX solution favors the
development of fibers. Most likely, the solvophilic
character of this solvent mixture disfavors aggrega-
tion phenomena and thus solutal instabilities.
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CHX/THF: from solution to chiral helices
A good compromise between solubility and
nanostructuration was found in a CHX/THF (95:5
v/v) solvent mixture. Although we could not estimate
the degree of association in this solvent mixture be-
cause of the presence of precipitate at the typical
1
concentration conditions for H-NMR, a chiroptical
profile similar to that obtained in CHCl3/CHX (1:1
v/v) and toluene solutions was also observed for a 1:1
mixture of 1 and 2 (Fig 7d). Luminescence spectra
were obtained at even more dilute concentrations (c
= 3.5 × 10-6 M). Both compounds exhibited strong
fluorescence, with related quantum yields (Фfl) of
0.12 and ≃ 1.0 for 1 and 2, respectively. The absorp-
tion and emission features of OPE derivative 2 alone
in the CHX/THF (95:5 v/v) mixture were red-shifted
Subsequently, the morphology of the supramo-
lecular aggregates made of [1∙2]n on mica surface
(transferred from a CHCl3/CHX (1:1 v/v) solution, c =
2.5 × 10-4 M) was studied. The TM-AFM and TEM
images in Figures 6e-j showed the formation of an
compared to more polar solvents such as MeOH and
22b
THF,19b,
indicating a strong solvophobic-driven
Figure 7. (a) Electronic absorption of (R)-1 (light blue), 2 (grey), [(R)-1∙2]n (blue) and algebraic sum of (R)-1 and 2 (dash grey ) in CHX/THF (95:5
v/v), c = 3.5 × 10-6 M; (b) Absorption spectral changes of 1:1 (c = 3.5 × 10-6 M) mixture of [(R)-1∙2]n during VT measurement; (c) Normalized fluores-
cence spectra of (R)-1 (light blue), 2 (grey), [(R)-1∙2]n (blue) and (inset) VT-emission spectral changes of [(R)-1∙2]n recorded each 10 K, at a rate of
1 K/min; d) CD spectra of (S)-1, (R)-1, 2, [(S)-1∙2]n and [(R)-1∙2]n in CHX/THF (95:5 v/v) at 293 K (c = 3.5 × 10-6 M).TM-AFM (e-i) and TEM (k,j)
images of nanofibers as obtained from drop casting a CHX/THF (95:5 v/v) solution of [(R)-1∙2]n on mica surface and on a carbon coated grid, re-
spectively (c = 1.7 × 10-3 M).
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self-aggregation of 2, most likely through J-type as- combination of non-covalent interactions. Similarly
sociation association of the aromatic rings.34 Moreo-
ver, the emission of [(R)-1∙2]n showed a weak shoul-
der at longer wavelengths and, compared to a solu-
tion containing only 2, a lower quantum yield (Фfl =
0.18) and a higher full width at half maximum
(FWHM), i.e. 80 nm (2: 62 nm, Fig. 7c). The slight
changes observed in the absorption and emission
spectra of [(R)-1∙2]n along with the lifetime values
(3.4 ns for (R)-1; τ1 = 1.5 ns; τ2 = 3.7 ns for 2; τ1 = 1.2 ns;
τ2 = 4.2 ns for [(R)-1∙2]n) were ascribed to the pres-
ence of H-bonded self-organized structures in
CHX/THF (95:5 v/v) (Fig. 7a-c, Supporting Infor-
mation Fig. S26c). Notably, the emission at high
temperature showed a blue shifted maximum with
substantially reduced FWHM (52 nm) and the dis-
appearance of the extended shoulder at longer wave-
lengths (Fig. 7c). This indicated the presence of iso-
lated units of molecule 2 in the sample, which also
exhibited much higher intensity. Owing to the high
to the case of the CHCl3/CHX (1:1 v/v) solution, the
formation of narrow fibers from CHX/THF (95:5 v/v)
suggested that under these conditions higher degree
of polymerization are achieved and no extensive ag-
gregation is observed. This is probably caused by the
solvophilic character of these supramolecular poly-
mers in this solvent mixture.
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Nanostructuration of the chiral helices
Additional evidences for the formation of the fi-
brous aggregates in solution were obtained from
SAXS, a powerful technique to determine the shape
and size of colloidal, diluted, and particulate sys-
tems.35 The SAXS pattern of a solution of [(R)-1∙2]n in
CHX-THF (95:5 v/v, c = 1.7 × 10-6 M) is reported in
Figure 8. In order to reconstruct the three-
dimensional structure adopted by the supramolecu-
lar polymer, the data have been fitted by assuming
homogeneous monodisperse structural models, such
as cylinders, hollow cylinders and helical bundles.
The best fitting result (Fig. 8a, red line) were ob-
tained using helical superstructure models36 result-
ing from the topological twisting (angle of 36°) of a
1D nanostructure featuring a ribbon-like structure
(width of 8 nm). Their structural and geometrical
features of which are depicted in Figure 8b.37
quantum yield of the monomer unit 2 (Φfl ≃ 1.0), its
emission peak masked the signal from weakly emis-
sive molecular species (i.e., (R)-1, Φfl = 0.12). In order
to corroborate the presence of H-bonds between mo-
lecular species (R)-1 and 2, a H-bond disrupting po-
lar solvent such as DMSO was added (5% v/v) to a
solution of [(R)-1∙2]n (Fig. S26a-b). Upon addition of
DMSO, the absorption spectrum of [(R)-1∙2]n shows
the appearance of blue shifted bands with discernible
signatures of the individual molecular units, (R)-1
and 2 (Fig. S26). The disruption of the assembly by
DMSO was also observed with the VT-fluorescence
experiments. Microscopic analysis of the deposition
of CHX/THF (95:5 v/v) solutions containing [(R)-
1∙2]n, also displayed the formation of fiber-like mor-
phologies (Fig. 7e). The fibrous structures displayed
small diameters ranging from 5 to 10 nm and average
lengths between 0.8 µm to 3 µm. The same morphol-
ogies were also obtained upon deposition of the chi-
ral supramolecular systems [(S)-1∙2]n. Reference AFM
experiments of reference racemic [1∙2] polymer re-
vealed the formation of nanofibers that macroscopi-
cally lacked of an apparent chirality (Fig. S29). As
previously mentioned, CD investigations of dilute
solutions containing [(R)-1∙2]n and [(S)-1∙2]n under
the same experimental conditions showed the for-
mation of a new band, centered at around 420 nm
but presenting opposite signs (Fig. 7d). This observa-
tion, together with the formation of fibrous mor-
phologies, can be considered as a clear proof of the
transfer of chirality from the molecular scale to
nanostructured nano-objects, in which the different
single polymeric structures are held together by the
Figure 8. (a) Experimental SAXS data (blue) obtained from a solu-
tion of [(R)-1∙2]n in CHX/THF (95:5 v/v) (298 K, c = 1.7 × 10-6 M) su-
perimposed with a fitting model (red line) of a helical tape; (b)
schematic representation of the supramolecular helicoidal fibers as
obtained from [(R)-1∙2]n.
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Figure 9. Molecular modeling of the supramolecular polymers of [(R)-1∙2]n and [(S)-1∙2]n.. a) Side and top views of conformers of (X)-1 along with
their sketch representation (on the right); (b) Sketch of the different possible assemblies considered for the molecular mechanics (MM) simula-
tions. (c) Proposed simulated model for self-organized nanofibers (CPK view, one helix in blue, the other in yellow ≃15 nm-long fiber as ob-
tained through several MM simulations cycles of the superstructure): at first, the self-assembly occurs through H-bonding between the comple-
mentary modules in a helix-type arrangement, which is right- and left-handed for [(R)-1∙2]n and [(S)-1∙2]n, respectively, followed by a self-
interdigitation of the single helices yielding double helix nanofibers. As observed by SAXS analysis in solution and AFM images on surfaces, it is
envisaged that the individual nanofibers are organized into the final superhelical nanofibers by a possible three step mechanism based on the
formation of a tape structure (i) that will eventually twist into a single helical tape (ii) and finally dimerize into the final nanofibers structure (iii)
observed by TEM and AFM.
In order to gain insight into the molecular organi-
zation of the nanofibers, molecular modeling simu-
lations were also carried out at the atomistic level
using Molecular Mechanics (MM). MM studies were
performed using the COMPASS force field, which
accurately describes the geometry and torsion poten-
tials of phenylene ethynylene oligomers, alkyl
groups, BINOL, and is appropriately optimized for
modeling organic condensed phase and H-bonded
structures.38 Figure 9 shows the structures of the
studied compounds and the molecular modeling of
the H-bonded bi-component supramolecular net-
works. Conformers of (R)-1 and (S)-1 possess a tor-
sional angle between the naphthyl planes of around
96° for the transoid conformer (see Fig. 9a for the
transoid conformation, where the 2,2’-methoxy
groups are pointing in opposite directions) and 72°
for the cisoid conformers. For linear module 2, the
para-phenylene ethynylene moiety has a flat torsion
potential between the phenylenes (0.5 kcal/mol bar-
rier), which dictate through the torsional angle of the
BINOL units the geometrical properties of supramo-
lecular polymers [(R)-1∙2]n and [(S)-1∙2]n. Depending
on whether one naphthyl substituent of the (R)-1 or
(S)-1 module is facing the neighbouring naphthyl at
each side of the linear linker (see sketch in Fig. 9b
where equivalent naphthyl moieties are depicted
with the same black tonality), the complementary H-
bonding arrangement yields two possible types of bi-
component networks, namely helix or zig-zag pat-
terns (Fig. 9b). Based on a series of modeled assem-
blies obtained by different cycles of MM simulations,
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we found that square-shaped networks are indeed
Page 10 of 14
1
forming a helical assembly at the supramolecular
level with a pitch made of tetrameric units of [(R)-
1∙2]n (Fig. 9c). In contrast, the zig-zag arrangement
yields the formation of one-dimensional polymer,
because in this setting two equivalent naphthyl sub-
stituents face each other at each termini of the linear
unit (Fig. 9b). Although this type of supramolecular
organization would leave large voids in the arrange-
ment, the presence of the C12 alkyl groups stabilizes
the formation of supercoiled double helical cables
(Fig. 9c) by lateral alkyl-alkyl interdigitation. In the
geometry-optimised model of [(R)-1∙2]n shown in
Figure 9c, both molecular geometries and length of
the alkyl groups govern the formation of compact
double helicoidal arrangement, with a single-helix
pitch of around 3.8 nm (i.e. 1.9 nm from helix blue to
helix yellow in Fig. 9c). The geometry of the double
helix has a rectangular cross section, with a mini-
mum distance of around 2.8 nm from side to side
and a maximum distance of around 5.0 nm from
corner to corner. Indeed, such compact packing is
very likely more stable than the zig-zag arrangement
in apolar solvents, as the alkyl groups point at the
outer surface of the helix. Notably, the geometrical
parameters obtained from the MM simulation are in
strict relationship with the structural data obtained
from the SAXS analysis of [(R)-1∙2]n in a CHX/THF
(95:5 v/v) solution. Indeed, the supramolecular di-
merization of one of these single rectangular units
through van der Waals interactions, dictated most
probably by multiple interdigitation of the solubiliz-
ing alkyl chains, would lead to the formation of a flat
ribbon nanostructure with dimension comparable
with the one reported by the SAXS results (4 nm of
height and 8 nm of width, Fig. 9c, model (i)). The
consequent twisting and dimerization (Fig. 9c, (ii)
and (iii) steps, respectively) of the resulting ribbon
would allow the formation of the supercoiled fibrous
structure observed at the microscale by both TEM
and AFM analysis.
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Figure 10. Nanostructuration of the supramolecular polymers into
organic morphologies depending on the polarity of the drop-casted
solution: (a,d) TM-AFM and TEM images for CHX/THF (80:20 v/v);
(b,e) images for CHX/THF (90:10 v/v) and (c,f) images for CHX/THF
(95:5 v/v).
finally drop-casted onto mica surfaces for microscop-
ic analysis and let to evaporate at room temperature.
With the more polar mixture (CHX/THF 80:20 v/v),
the formation of round shaped nanostructures was
observed by TEM and TM-AFM investigations (Fig.
10a and 10d). Through detailed TM-AFM analysis,
spherical nanoparticles characterized by heights
from 10 to 70 nm and thickness of nm to few tens of
nm were obtained (Fig. 10a). Interestingly, when a
solution of [1∙2]n was transferred from a CHX/THF
(90:10 v/v) solution to the mica surface, globular ag-
gregates were predominantly observed, but fibrous
material also started to appear (Fig. 10b and 10e).
Specifically, the deposited material is characterized
by the presence of spherical morphologies that pe-
ripherally exposes fibrous ramifications, with diame-
ters between 5 and 50 nm and length up to several
µm. With respect to the fibrous morphologies ob-
tained from the CHX/THF (95:5 v/v) solutions, the
ramifications displayed some substantial structural
differences, as they appear as bamboo-like struc-
tures. In both cases, SAXS measurements did not
reveal any periodicity or formation of homogenous
structures. Whereas in the more polar solvent mix-
ture (CHX/THF 80:20 v/v) the aggregation seems to
be essentially governed by the concentration-driven
solutal instability, increasing the apolar character of
the solvent enhances the degree of polymerization,
thus favoring the formation of long supramolecular
polymers as the solvent evaporates, thus the for-
mation of long fibers.
From nanospheres to helices
To shed further light on the role of the solvent mix-
ture giving the chiral morphologies, we investigated
various solvent mixtures between CHX and THF. As
in previous cases, an equimolar amount of molecular
modules (R)-1 or (S)-1 and 2 were first dissolved into
a minor volume of THF (solvent able to completely
solubilize both compounds) and then diluted to the
necessary volume with CHX. Aiming at favoring the
formation of the most thermodynamically stable ar-
chitectures, the resulting mixtures were heated and
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gree of association, solvent vapor pressure and the
CONCLUSIONS
1
solvophobic/solvophilic properties needs to be ac-
complished in order to control the formation of or-
ganic morphologies.
We have presented the synthesis and characteriza-
tion of novel molecular modules carrying hetero-
complementary H-bonding recognition motifs. The
building block is composed of a BINOL unit which,
through its atropoisomers (R) and (S), served as a
chiral unit. Together with a complementary H-bond
partner, an achiral linear OPE chromophore, the
modules underwent self-assembly through triple H-
bonding interactions. The association strength of the
triply bonded structures was determined in CDCl3,
CDCl3/CHX-d12 and Tol-d8, together with the binding
stoichiometry. Through a simple experimental mod-
el, we have shown that, together with the association
strength, the vapor tension of a given solvent governs
the degree of polymerization under evaporation
conditions and thus the nanostructuration of an or-
ganic morphology. In particular, solvents favoring
strong H-bonded complexes and displaying high va-
por pressures induce the formation of high degree of
polymerization. Owing to the chromophore unit, it
was possible to follow the self-assembly through UV-
Vis, CD and fluorescence spectroscopy. Notably,
through concentration-dependent CD spectroscopy,
an isodesmic polymerization mechanism was uncov-
ered. The self-assembled supramolecular polymer
has been transferred onto surface for microscopic
investigations through AFM and TEM microscopy
measurements. Nanostructuration afforded nano-
rods (from CHCl3), nanofibers (CHCl3/CHX), nano-
spheres (Tol, CHX/THF 80:20 v/v), hybrid nano-
sphere/fibers (CHX/THF 90:10 v/v) and finally heli-
cal nanofibers (CHX/THF 95:5 v/v). The self-
organization and the chiral transfer from solution to
surface were also studied with the aid of SAXS analy-
sis and molecular modeling in order to gain insight
into the nanostructuration mechanisms of the mate-
rial into superhelical nanofibers. These results sug-
gest that solvents exhibiting high vapor tension and
favoring high degrees of polymerization mold the
formation of fibrous morphologies apparently ex-
pressing chiral morphologies at the microscale. On
the other hand, solvents favoring low degree of
polymerization with low vapor tensions yield amor-
phous morphologies. The solvophilic character of a
given solvent mixture disfavors aggregation phe-
nomena, thus reducing solutal instability and driving
the formation of supramolecularly-controlled
nanostructures. Taken in concert, our experiments
showed that the choice of solvent is crucial to dictate
the polymorphism of the nanomaterial and the
transmission of molecular chirality at higher scales.
In particular, an optimal balance between high de-
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ASSOCIATED CONTENT
Supporting Information
Synthetic protocols and spectroscopic characterisa-
tions, titration experiments. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
davide.bonifazi@unamur.be; mathieu.surin@umons.ac.be
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
DB gratefully acknowledges the EU through the ERC
Starting Grant “COLORLANDS”, the FRS-FNRS (FRFC
contracts n° 2.4.550.09), the “TINTIN” ARC project
(09/14-023), the MIUR through the FIRB Futuro in Ri-
cerca “SUPRACARBON” (contract n° RBFR10DAK6).
NA thanks MIUR (PRIN 2010 INFOCHEM, contract n°.
CX2TLM), and Consiglio Nazionale delle Ricerche
(Progetto Bandiera N-CHEM). LD and TM thank the
University of Trieste for their doctoral fellowships. J.R.-
M. is a FNRS post-doctoral researcher and M.S. is a
FNRS research associate. The collaboration between
Mons and Namur is supported by the FNRS through the
FRFC-BINDER project (grant no 2.4615.11) and the Bel-
gian Government (IAP-PAI P7 network "Functional Su-
pramolecular Systems”). Mr. C. Gamboz is acknowl-
edged for the help with the TEM imaging. DB and LD
thank Prof. Dr. P. Tecilla (University of Trieste) for the
calculation of the dimerization constants.
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