Interplay between H-Bonding and Preorganization in the Evolution of Self-Assembled Systems

Article abstract of DOI:10.1002/anie.201813955

Cooperative π–π interactions and H-bonding are frequently exploited in supramolecular polymerization; however, close scrutiny of their mutual interplay has been largely unexplored. Herein, we compare the self-assembly behavior of a series of C₂

Full text of DOI:10.1002/anie.201813955

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Accepted Article
Title: Interplay Between H-bonding and Preorganization in the
Evolution of Self-Assembled Systems
Authors: Divya S. Philips, Kalathil K. Kartha, Antiope Politi, Timo
Krüger, Rodrigo Q. Albuquerque, and Gustavo Fernandez
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201813955
Angew. Chem. 10.1002/ange.201813955
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Interplay Between H-bonding and Preorganization in the
Evolution of Self-Assembled Systems
Divya S. Philips,[a] Kalathil K. Kartha,[a] Antiope T. Politi,[b] Timo Krüger,[a] Rodrigo Q. Albuquerque*[b,c]
and Gustavo Fernández*[a]
Abstract: Cooperative --interactions and H-bonding are frequently
exploited in supramolecular polymerization, however, close scrutiny
of their mutual interplay has been largely overlooked. Herein, we
compare the self-assembly behavior of a series of C2- and C3-
symmetrical oligophenyleneethynylenes differing in the amide
topology (N- or C-centred). This subtle structural modification brings
about drastic changes in their photopysical and supramolecular
properties, highlighting the reciprocal impact of H-bonding vs.
preorganization on the evolution and final outcome of supramolecular
systems.
carried out for the linear OPEs (1 and 2), whose results were then
compared with those obtained for the C3-symmetrical derivatives
(3 and 4) to confirm the generalization of the trends found.
Numerous life processes including protein-ligand recognition,
cell-cell interaction and cytoskeletal formation are governed by
the subtle interplay of multiple non-covalent interactions.[1] Among
them, cooperative H-bonding and π-π interactions are
overwhelmingly exploited for the construction of ordered arrays of
artificial functional molecules.[2] This approach provides detailed
structural and mechanistic insights into the underlying self-
assembly pathways.[3] However, there is still a lack of fundamental
understanding of the reciprocal influence of H-bonding and
preorganization on molecular conformational changes preceding
aggregation and their subsequent evolution into the final
supramolecular structures.
Oligomeric π-systems[4] appear to be archetypal building
blocks to bridge this knowledge gap due to their variable torsional
flexibility, which in turn allows versatile molecular reorganization
upon aggregation. In this context, oligo(p-phenyleneethynylene)s
(OPEs) represent a particularly relevant class of molecules with
excellent optical and electronic properties, which have been
extensively applied in the fields of optoelectronics,[5] sensors[6]
and stimuli-responsive materials.[7] OPEs have also been
exploited to create hierarchical self-assembled structures driven
by -stacking and H-bonding.[8] Herein, we reveal how the subtle
interplay between H-bonding and preorganization determines the
evolution and final outcome of supramolecular nanostructures. To
address this challenge, we compare the self-assembly behavior
of a series of C2– and C3–symmetrical OPEs (1–4, Fig. 1),
differing only in the amide topology (N- or C-centred, for synthesis
see the Supporting Information, S.I.). The investigations were first
Figure 1. Chemical structures of OPEs 1-4. The highlighted portions show the
nature of amide connectivity (N-centred, blue; C-centred, pink).
Initial semi-empirical calculations at the PM7 level anticipate
marked differences in the electron density distribution brought
about by the inversion of the amide topology. While the highest
occupied molecular orbital (HOMO) is delocalized over the
aromatic core for the N-centred OPEs 1 and 3, it is localized more
on the peripheral phenyl rings for the C-centred derivatives 2 and
4 (Fig. S1-S2). On the other hand, the lowest unoccupied
molecular orbital (LUMO) is largely localized on the central
aromatic core for all molecules. These findings have two main
implications: i) a strong charge-transfer (CT) character for HOMO-
LUMO excitations is expected for the C-centred systems, where
the outer phenyl rings behave as donor and the central aromatic
core acts as the acceptor. This CT character can significantly
affect the emission characteristics; ii) The N-centred molecules (1,
3) show a much stronger HOMO-LUMO overlap compared to the
C-centred counterparts (2, 4), which is an indication of more
allowed electronic transitions for the former OPEs. In fact, the
absorption and emission studies match well with the theoretical
predictions. The molar absorptivity in a good solvent as
chloroform (CHCl3, c = 1 × 10-5 M) at the respective absorption
maxima is ca. 35 % higher for 1 compared to 2 (89,000 vs. 66,000
Lmol-1cm-1, Fig. S3 a-b) and about 45 % higher for 3 compared to
4 (140,000 vs. 95,000 Lmol-1cm-1, Fig. S4). The emission
spectrum of 1 in CHCl3 (c = 1 × 10-5 M, λex = 300 nm) revealed two
distinct vibronic transitions at 381 nm and 401 nm with almost
equal intensities, while 2 was nearly non-emissive, probably due
to charge transfer from the donor side groups to the acceptor core
(Fig. S3 d-e).[9] 4 was also nearly non-emissive compared to 3 for
the same reason (Figs. S7 and S9).
[a]
Dr. D. S. Philips, Dr. K. K. Kartha, Timo Krüger, Prof. Dr. G.
Fernández
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität
Münster, Corrensstraße 40, 48149 Münster (Germany)
E-mail: fernandg@uni-muenster.de
Significant differences in the optical properties caused by
aggregation are observed for all molecules in a poor solvent such
as methylcyclohexane (MCH). For 1, variable temperature (VT)
UV-Vis studies at 1 × 10-4 M (for other experiments at multiple
concentrations, see Fig. S5) using a cooling rate of 1 K/min
showed an initial enhancement in absorption upon decreasing the
temperature from 363 to 327 K along with a slight red-shift from
340 to 343 nm without any isosbestic point (Fig. 2a, inset).
[b]
[c]
A. T. Politi, Prof. Dr. R. Q. Albuquerque
School of Pharmacy and Biomolecular Sciences
Liverpool John Moores University (LJMU), Liverpool (UK)
E-mail: ralbuque@uni-muenster.de
Prof. Dr. R. Q. Albuquerque
São Carlos Institute of Chemistry, University of São Paulo (Brazil)
Supporting information for this article is given via a link at the end of
the document.
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Figure 2. Temperature-dependent absorption (a, e) and emission (b, f) studies of 1 and 2 (c = 1  10-4 M, MCH), respectively. The inset shows the increase in
absorbance of 1 upon cooling from 363 K () to 327 K (). Top and side views of PM7-optimized hexamers of 1 (c, d) and 2 (g, h).
Further cooling to 283 K revealed a 2 nm blue shift of the
absorption maximum and the simultaneous appearance of a red-
shifted broad contribution at 385 nm through clear isosbestic
points at 265, 332 and 371 nm (Fig. 2a). Identical absorption
changes upon aggregation were observed for the C3-symmetrical
N-centred homologue 3 (Fig. S7a). Emission studies of 1 showed
a slight fluorescence enhancement upon decreasing temperature
from 363 to 327 K followed by a significant quenching between
327 and 283 K (Figs. 2b and S6). The observed optical features
between 363 and 327 K can be ascribed to the restricted
molecular rotation of the OPE core,[10] which results in a co-planar
π-backbone with improved conjugation leading to an increment in
the absorbance and emission due to restriction of the non-
radiative pathways. Below 327 K, the planarization-induced
preorganization (Fig. 2d) ultimately promotes a subsequent
aggregation event via aromatic interactions of the OPE units, as
evident from fluorescence and UV/Vis studies. For both N-centred
OPEs 1 and 3, the blue shift of the absorption maximum along
with the appearance of a red-shifted shoulder are in accordance
with a face-to-face stacking of the OPE units with a sligthly twisted
arrangement in order to favor H-bonding interactions. This is
indeed supported by the PM7 geometry optimizations performed
for hexameric stacks of 1 (Fig. 2c,d) and pentameric stacks of 3
(Fig. S7 c-d). The calculations reveal a highly planar nature of the
OPE cores within the stacks, which are stabilized by van der
Waals interactions involving the chains, amide H-bonds and
aromatic interactions.
On the other hand, VT absorption and fluorescence
measurements of the C-centred OPEs in MCH gave rather
different observations. Unlike 1, 2 experiences no increase in
absorption, i.e. no planarization on cooling from 363 to 313 K, but
rather a concomitant decrease in intensity and red-shift from 335
to 338 nm (Fig. 2e; for an overview at various concentrations see
Fig. S8). Simultaneously, a red-shifted shoulder at 380 nm and a
clear isosbestic point at 362 nm become evident. The lack of
planarization is also found from PM7 calculations (see Figs 2g,h)
and can be attributed to the slight mismatch between H-bonds
and aromatic interactions caused by the inversion in the amide
topology, which results in a less compact packing of the OPE units.
This is evident from the twisting and translational displacement of
the π-backbones observed in the PM7-optimized hexameric
stacks of 2 (Fig. 2g,h) and pentameric stacks of 4 (Fig. S9c,d),
which is in agreement with the red-shift in the absorption upon
aggregation. These experiments also reveal a gradual increase in
absorbance but no changes in the spectral shape below 313 K
(Fig. 2e), most likely indicating non-specific aggregation of the
previously formed assemblies. The new shoulders observed in
the low-temperature absorption spectra of 1 and 2 (380-385 nm)
and of 3 and 4 (ca 360 nm) were reproduced by the theoretical
UV-Vis spectra predicted for small aggregates using the ZINDO/S
methodology (Fig. S10). Regarding the emission properties, 2
and 4 (Fig. 2f and S9b, respectively) are also nearly non-emissive
in MCH regardless of the temperature, as already discussed.
The overall results bring to light the importance of H-bonding
to promote the self-assembly of the OPE units, as also
demonstrated by the fact that a previously reported derivative with
the same OPE core (OPEM[11] in Fig. S11) but without amide
groups showed no signs of aggregation under equivalent
conditions. Fourier-transform Infrared spectroscopy (FTIR)
measured for all OPEs 1-4 in MCH shows typical values of H-
bonded amide groups, i.e. N–H and C=O stretching frequencies
in the range of 3290-3269 cm-1 and 1645-1653 cm-1,
respectively.[12] Furthermore, lower stretching frequencies were
observed in CHCl3 upon cooling, indicating a strengthening in the
intermolecular H-bonds upon aggregation (Figs. S12 - S15). This
synergy between H-bonding and aromaticinteractions in all OPEs
is responsible for a cooperative supramolecular polymerization,
as revealed by global fitting of the cooling curves (αagg vs. T)
extracted from UV/Vis at different concentrations (Figs. S16 - S17
and Table S1) to the equilibrium (EQ) model.[13] For the C2-
symmetrical systems, the degree of cooperativity  is slightly
lower for the C-centred OPE 2 compared to N-centred 1, a trend
that is even more pronounced for the C3-symmetrical OPEs (~one
order of magnitude lower for 4, Table S1). This higher
cooperativity found for our C-centred vs. N-centred systems is
also in line with the behavior observed for benzenetrisamide
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derivatives.[3d,14] but shows an opposite trend compared to related
C3-symmetrical systems derived from OPE.[15] This difference
may be due to the different nature of the substituents attached to
the amide groups (for our systems both the C=O and the N-H
groups of the amides are attached to an aromatic ring whereas
the previously reported C3-symmetrical OPEs feature an alkyl
chain attached to the amide).
which implies that the temperature range 363-343 K corresponds
to the pre-nucleation regime (Fig. 3c). In this temperature range,
the amide groups are already involved in H-bonding whereas the
protons Ha and Hb slightly shift to lower and upper magnetic fields,
respectively, well before the aggregation is initiated. Possibly,
disorganized pre-nuclei dimers (we can make this assumption
considering the approximation of the EQ model) are formed above
Te. A plausible explanation would be the initial association of two
monomers via an H-bond through the amide groups into a V-
shaped pre-nucleus dimer (Fig. 3c, inset). Within this
arrangement, only the outer phenyl ring Ha would be involved in
-stacking resulting in the observed shielding, while Hb protons
might weakly interact with the carbonyl oxygen of the adjacent
molecule leading to a weak downfield, as suggested by PM7
calculations. Cooling below 343 K promotes a molecular
rearrangement into a stable, more organized nucleus dimer with
a compact parallel arrangement, which acts as a seed for the
supramolecular growth into twisted stacks through cooperative H-
bonding and π-stacking interactions below Te. This is supported
theoretically by PM7 calculations and experimentally by the
significant broadening of all NMR signals and the fact that the
central OPE ring is strongly shielded, the adjacent ring is only
slightly affected while the outer ring signal remains completely
unaltered. VT-NMR and semi-empirical calculations performed for
3 gave similar trends as found for 1 (Fig. S18), although in this
case CDCl3 had to be selected as solvent due to severe signal
broadening in MCH-d14.
In order to gain insights into the reciprocal influence of H-
bonds and aromatic interactions on the evolution of the self-
assembled structures, we monitored the chemical shifts of 1 and
1
2 by VT H-NMR measurements in MCH-d14 upon cooling the
monomer solutions from 363 to 283 K at 2.5 × 10-4 M. For 1, the
amide N-H signal initially shifts downfield from δ = 7.43 to 7.50
ppm on cooling from 363 to 343 K, and then broadens with further
decrease in temperature (Fig. 3a). In contrast, the proton signals
corresponding to the peripheral phenyl rings experience a less
pronounced broadening with insignificant (0.01 ppm) shifts in the
same temperature window. Interestingly, the signals
corresponding to the central phenyl ring of the OPE core remain
initially unaffected between 363 and 343 K, however further
cooling to 323 K induces a significant broadening and upfield shift.
On the other hand, the doublet at 7.60 ppm undergoes a meagre
downfield shift between 363 and 343 K and ultimately moves
upfield below 343 K due to the initiation of aggregation. These
results suggest that the self-assembly process is separated into
pre-nucleation, nucleation and elongation phases. Analysis of the
cooling curve extracted from UV/Vis studies under identical
conditions affords an elongation temperature (Te) of 334 K for 1,
Figure 3. Partial variable temperature 1H-NMR measurement of 1 (a) and 2 (b) in MCH-d14 at 2.5  10-4 M. c,d) Cooling curves determined from VT-UV
measurements at 2.5  10-4 M for 1 (c) and 2 (d) obtained by monitoring the absorbance at 385 nm and 380 nm, respectively. The geometries of the proposed pre-
nuclei and dimer structures shown in c) and d) are the initial (pre-nucleus) and optimized (dimer) structures obtained from PM7 calculations.
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For 2 under the same experimental conditions, signal
broadening occurs at a higher temperature (353 K) compared to
1 (343 K), which closely matches with the higher calculated value
of Te (352 K) (Fig. 3b,d). For the C3-symmetrical homologs, Te is
also higher for the C-centred 4 and the NMR signals also broaden
at higher T (Fig. S19). With regards to the specific chemical shifts,
the amide N-H signal at 7.33 ppm shifts slightly downfield to 7.36
between 363 and 353 K, while further cooling causes significant
broadening and ultimately disappearance of the signal. These
observations suggest a higher tendency of the C-centred
monomers to associate into pre-nuclei, which however appear to
have a lower stabilization through H-bonds. In contrast to 1, the
protons corresponding to the peripheral phenyl rings of 2
experience only a minor downfield shift (0.01 ppm) upon cooling
while all other aromatic protons remain nearly unaffected but are
likewise broadened as a result of aggregation. This might suggest
that aromatic interactions negligibly influence the pre-nucleus
formation of 2, which is primarily stabilized by weaker H-bonds
compared to 1. Further cooling down to the Te initiates the
formation of a dimer nucleus, whose π-backbone is considerably
more distorted than the 1 dimer due to less efficient H-bonding,
as predicted by PM7 calculations (Fig. 3d). Ultimately, the nuclei
undergo further growth into higher order aggregates that are
responsible for the signal broadening below 351 K. All these
disorganized stacks of 2 are arranged edge-on on the HOPG
substrate (see the side view of 2 stacks in Fig. 2h). This distortion
explains why further growth of the system in a preferred direction
into 1D fibers is hindered. The C3-symmetrical 3 and 4 also form
long fibres and small disordered objects, respectively (Fig. S22).
In conclusion, we have elucidated the reciprocal impact of
H-bonding and preorganization on the evolution of
supramolecular systems by detailed analysis of the molecular
conformational changes preceding aggregation. This was
achieved by comparison of appropriate C2- and C3-symmetrical
OPE systems that exclusively differ in the amide topology.
Various experiments and theory reveal how this subtle structural
change brings about differences in their photophysical and
supramolecular properties, i.e. molecular packing and aggregate
morphology. Our understanding on the subtle interplay of different
non-covalent interactions in self-assembled structures might
serve as a stepping stone for the rational design of molecular
systems that can evolve via intricate processes into functional
superstructures.
Experimental Section
observations confirm
a better match between H-bonding
The experimental and computational details can be found in the
Supporting Information.
arrangement and preorganization for 1 compared to C-centred 2,
which ultimately results into a more compact and ordered
organization of the π-backbones in the supramolecular structures.
This is in excellent agreement with the hydrogen bond trends
found from the theoretical calculations, the H-bonding distances
being shorter for 1 aggregates (Fig. S20).
Acknowledgements
We acknowledge the Humboldt Foundation (Sofja-Kovalevskaja)
and the European Commission (SUPRACOP ERC-StG-2016) for
funding.
Keywords: • Self-assembly • -conjugated systems •
Oligophenyleneethynylene • H-bonding • π-π interactions
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10.1002/anie.201813955
Angewandte Chemie International Edition
COMMUNICATION
Table of Contents
COMMUNICATION
To match or not to match: Detailed
self-assembly studies of a series of
C2- and C3-symmetrical π-systems
that differ in the amide connectivity
(N- vs. C-centred) allowed us to
examine the reciprocal impact of H-
bonding and preorganization on the
D. S. Philips, K. K. Kartha, A. T. Politi,
T. Krüger, R. Q. Albuquerque* and G.
Fernández*
Page No. – Page No.
Interplay Between H-bonding and
Preorganization in the Evolution of
Self-Assembled Systems
evolution
of
self-assembled
structures.
This article is protected by copyright. All rights reserved.