Unraveling Concomitant Packing Polymorphism in Metallosupramolecular Polymers

Article abstract of DOI:10.1021/jacs.8b11011

The phenomenon of polymorphism is ubiquitous in biological systems and has also been observed in various types of self-assembled materials in solution and in the solid state. In the field of supramolecular polymers, different kinetic vs thermodynamic self

Full text of DOI:10.1021/jacs.8b11011

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Article
Unraveling Concomitant Packing Polymorphism
in Metallosupramolecular Polymers
Anja Langenstroer, Kalathil K. Kartha, Yeray Dorca, Jörn Dröste, Vladimir Stepanenko,
Rodrigo Q. Albuquerque, Michael Ryan Hansen, Luis Sanchez, and Gustavo Fernández
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11011 • Publication Date (Web): 20 Feb 2019
Downloaded from http://pubs.acs.org on February 21, 2019
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Unraveling Concomitant Packing Polymorphism in
Metallosupramolecular Polymers
Anja Langenstroer,§ Kalathil K. Kartha,§ Yeray Dorca,† Jörn Droste,¥ Vladimir Stepanenko,‡ Rodrigo
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Q. Albuquerque,¤ Michael Ryan Hansen, ¥ Luis Sánchez,*† Gustavo Fernández*§
§Organisch-Chemisches Institut, Universität Münster, Corrensstraße 40, 48149 Münster, Germany
†Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain
¤São Carlos Institute of Chemistry, University of São Paulo 13560-970 São Carlos, Brazil
¥Institut für Physikalische Chemie, Universität Münster, Corrensstraße 28/30, 48149 Münster, Germany
‡Institut für Organische Chemie, Universität Würzburg, Am Hubland 16, 97074 Würzburg, Germany
ABSTRACT: The phenomenon of polymorphism is ubiquitous in biological systems and has been also observed in various types
of self-assembled materials in solution and the solid state. In the field of supramolecular polymers, different kinetic vs.
thermodynamic self-assembled species may exist in competition, a phenomenon termed as pathway complexity. In these examples,
the transient kinetic species often has a very short lifetime and rapidly converts into the thermodynamic product. In this article, we
report a π-conjugated Pt(II) complex 1 that self-assembles in nonpolar medium into two supramolecular polymers with distinct
molecular packing (slipped (A) vs. pseudo-parallel (B)) that do not interconvert over time in a period of at least six months at room
temperature. Precise control of temperature, concentration and cooling rate enabled us to elucidate the stability conditions of both
species through a phase diagram. Extensive experimental studies and theoretical calculations allowed us to elucidate the packing
modes of both supramolecular polymorphs A and B, which are stabilized by unconventional N-H···Cl-Pt and N-H···O-alkyl
interactions, respectively. Under a controlled set of conditions of cooling rate and concentration, both polymorphs can be isolated
concomitantly in the same solution without interconversion. Only if A is annealed at high temperature for prolonged time, a slow
transformation into B takes place via monomer formation. Our system, which in many respects bears close resemblance to
concomitant packing polymorphism in crystals, should help bridge the gap between crystal engineering and supramolecular
polymerization.
number of -conjugated systems and metal complexes.11 In
1. INTRODUCTION
these examples, the transient kinetic species ultimately
converts into the thermodynamic product, which can be
accelerated by the addition of seeds.12 This pioneering seeded-
growth approach enables size control in one and two
dimensions leading to sophisticated self-assembled
structures.13
The phenomenon of polymorphism is a very broad term that
has been subject of investigation across different research
disciplines including biology, chemistry, computer and
materials sciences among others. Although the first
manifestation of polymorphism in a chemical system dates
back more than two centuries, its definition is still a matter of
ongoing debate.1 Polymorphism has been defined as the ability
of a chemical compound to exist as two or more forms and/or
crystalline phases.2 Polymorphs that differ in their molecular
arrangement can be termed as packing polymorphs,1 whereas
conformational polymorphism3 results from a molecule
exhibiting several possible shapes or conformations. In many
cases, two or more concomitant polymorphs of similar
formation energies are isolated simultaneously in the same
crystallizing medium,4 which makes polymorph control and
prediction a challenging subject matter.5 This serendipitous
phenomenon is often encountered in crystal engineering, and it
has also been observed in various types of self-assembled
materials including lyotropic liquid crystals,6 block
copolymers7 and self-assembling dendrimer systems.8 The
existence of pathway complexity9 in supramolecular
polymers,10 where kinetic vs. thermodynamic self-assembly
pathways are in competition, has also been reported for a
Very recently, Matsumoto et. al. have reported an intriguing
example of polymorphism in aqueous supramolecular
assemblies based on benzene-1,3,5-tricarboxamides (BTAs).14
The authors have investigated a series of BTAs that feature
two identical amphiphilic arms and a third arm consisting of a
variable length alkyl chain with a terminal carboxyl group.
The synergy of cryogenic transmission electron microscopy
and small-angle X-Ray scattering shows the coexistence of
different aggregates including ribbons, membranes and
nanotubes. This was supported by a structural model for one
of the BTAs that considers the combination of a hollow
cylinder and a rectangular parallelepiped model. Notably, a
reversible temperature-dependent polymorphism is observed,
in which short tubular fragments elongate into well-ordered
longer nanotubes upon heating. Therefore, this BTA series
exemplify a beautiful phenomenological investigation of
polymorphism in supramolecular polymers, remaining still
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elusive a comprehensive study about mechanistic features (S.I.)), in which amide groups are introduced to allow for a
more versatile molecular reorganization via H-bonds.18 1 self-
assembles in solution, under controlled conditions of
temperature, concentration and cooling rate, into two
coexisting, stable supramolecular polymers with different
molecular packing (slipped vs. pseudo-parallel) that do not
interconvert over time in a period of at least six months.
Further proof of the stability of the two competing pathways is
the absence of interconversion when both species A and B are
allowed to coexist. Only if A is annealed at 337 K (between
the critical temperatures of pathways A and B), a clear, fast
transformation to the monomer is observed and, keeping
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responsible for this phenomenon.
Even though the concomitant formation –often for a very short
time– of different supramolecular polymers has been observed
for a number of -systems, the isolation of two aggregate
species (formed via two different pathways) that remain stable
over time (e.g. months) has not been realized so far.
Oligo(phenyleneethynylene) (OPE)-based dichloro(bis)pyridyl
square-planar complexes15 appear to be ideal building blocks
to achieve this goal due to well-balanced attractive and
repulsive interactions, which is a prerequisite for competing
packing modes to occur concomitantly. In crystal engineering,
this is often achieved by the exploitation of hydrogen bond
synthons,16 as “free” unbound X-H (X=heteroatom) groups
will almost always search for multiple potential H-bonding
partners.17
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constant the temperature for prolonged time,
a slow
aggregation takes place leading to B. Such behavior in
supramolecular polymers bears close resemblance to the
previously described phenomenon of concomitant packing
polymorphism that has been often observed in crystals, which
is thoroughly described in this paper.
Based on these considerations, we have designed a new, S-
chiral dichloro(bis)pyridyl OPE-based Pt(II) complex (1 in
Scheme 1, for synthetic details, see the supporting information
Scheme 1. Chemical structure of 1 and dispersion-corrected PM6 calculations depicting its concomitant supramolecular polymerization.
2. RESULTS AND DISCUSSION
planarization and subsequent restricted rotational motion of
the OPE backbone.19 Decreasing the cooling rate to 0.5 K
min-1 showed an identical trend (Figure 1b). These spectral
features are reminiscent of those found in previously reported
assemblies of dichloro(bis)pyridyl Pt(II) complexes15a,b and
indicate a slipped (translationally displaced) arrangement of
the monomer units within the assembly (State A, Scheme 1).
Synthesis. The synthesis of the Pt(II) complex 1 was
accomplished using a multistep synthetic protocol, similar to
that described for a referable Pd(II) complex, in which the
reaction of the chiral OPE-based ligand endowed with a
pyridine ring with [PtCl2(PhCN)2] is the key step (Scheme S1
in the S.I.).19 1 has been completely characterized using
spectroscopic techniques (NMR, IR, HRMS-ESI, UV/Vis, and
fluorescence spectroscopy) and elemental analysis.
A
battery of spectroscopic experiments (concentration-
1
dependent H NMR, 2D-ROESY; see below) demonstrate the
significant contribution of intermolecular Pt(II)-Cl···H-N
interactions to stabilize the stacks, as recently observed for
structurally analogous Pd(II) complexes.19
Competing pathways at diluted conditions. Solvent-
dependent UV/Vis studies allowed us to identify
methylcyclohexane (MCH) as the suitable solvent for
aggregation studies (Figure S1). Variable Temperature (VT)-
UV/Vis in MCH revealed significant spectral changes upon
cooling hot solutions of 1 from 347 K to 298 K at various
concentrations between 7.5 and 100 µM, which is suggestive
of aggregation (Figures 1 and S2-S6). To our surprise, the
spectral signatures of the aggregate species at 7.5 µM strongly
depend on the selected cooling rate. At relatively fast (2 K
min-1) rates, a bathochromic shift in the absorption maximum
from 365 to 387 nm and a concomitant formation of a sharp
red shifted band at 401 nm were observed (Figure 1a). VT-
photoluminescence (VT-PL) studies under these conditions
(Figure S7) showed a fluorescence enhancement suggesting a
Decreasing the cooling rate to 0.1 K min-1 led to remarkably
different spectroscopic results. For example, a significant
hypochromism with negligible shifts in the absorption
maximum along with a broad shoulder covering the region up
to 500 nm were observed in VT-UV/Vis (Figure 1c).
Monitoring this aggregation process by VT-PL studies
revealed a 2-fold fluorescence enhancement compared to the
more rapidly cooled (2 K min-1) solution (Figure S8),
indicating a more significant contribution of π-stacking of the
aromatic moieties at slower rates. These spectral changes
cannot be assigned to typical face-to-face H-type aggregates.20
Furthermore, the occurrence of short Pt-Pt contacts can be also
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discarded for this second pathway, as supported by The curves corresponding to the pure self-assembled species A
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photophysical studies (Figures S9-S14).
show a clear sigmoidal shape whereas considerably sharper
non-sigmoidal transitions are found for species B (Figure 2a
and ESI for details). Even though some deviations can be
observed under conditions where both aggregates coexist, all
curves generally fall into either of the two categories
depending on the aggregate species (A or B) that is in excess.
While the shallow sigmoidal curves are well described by the
isodesmic model,10f the nucleation-elongation model21 can be
successfully used to fit the non-sigmoidal cooperative curves.
None of the curves show a superposition of two species
aggregating independently of each other, as the transition
temperatures of both states are very similar. The
corresponding thermodynamic parameters are displayed in
Tables S1-2.
This cooling rate-dependent self-assembly process becomes
also evident at higher concentration (100 µM), where these
two pathways can be also identified at 2 and 0.1 K min-1
(Figures 1d,f). Notably, at intermediate cooling rates (0.5 K
min-1), the obtained aggregate spectrum at 298 K is nearly the
superposition of the absorption spectra of both pure aggregate
species A and B (see Figure 1e, green spectrum). These
findings
suggest
the
occurrence
of
concomitant
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supramolecular polymers –two coexisting aggregate species A
and B in competition– under these conditions. In-depth
analysis of the spectral changes and cooling curves (plot of
αagg vs. temperature at 420 and 400 nm) at multiple
concentrations and cooling rates allowed us to ascertain the
stability conditions of the two states (see Figures S15-S16).
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Figure 1. Variable temperature UV-Vis experiments of 1 in MCH. (a,b,c) 7.5 µM; (d,e,f) 100 µM cooled from 347 K to 298 K, with a
cooling rate of (a,d) 2 K min-1, (b,e) 0.5 K min-1, (c,f) 0.1 K min-1. The red spectra correspond to the monomer species, which converts into
aggregate type A (light blue), type B (royal blue) or a mixture of A and B (green) on cooling.
We noticed that higher concentrations (˃50 µM) and slow
cooling rates (0.1 K min-1) favor state B, whereas lower
concentrations (≤50 µM) and faster rates (2 K min-1)
preferentially stabilize pathway A (Figures 2c, S17). At
intermediate concentrations (50 µM) and cooling rates (0.5 K
min-1), both pathways operate concomitantly indicating the
narrow energy distribution between them. This energy
difference (ΔGo) lies in the range of ~4 kJ mol-1 (see Tables
S1,2), which is within the expected energy difference range
(maximum ca. 10 kJ mol-1) observed for most conventional
crystal polymorphs.1 For the calculation of this energy
difference, the values of ΔGo extracted from the fits to the
isodesmic model for A and the values of ΔGo elongation
extracted from the fits to the cooperative model for B using a
concentration of 50 M have been compared. Detailed
analysis of the overall cooling curves enabled us to create an
experimental phase diagram showing the conditions under
which each species can be isolated in its pure form or in
competition (Figure 2c and S17). In several of these
experiments, both aggregates coexist to a greater or lesser
extent. Thus, for the derivation of the thermodynamic
parameters, we selected extreme cases to ensure that only pure
species A or B are present (Figures S15,16). Interestingly, the
supramolecular polymorph A does not convert into polymorph
B at room temperature over a period of at least six months.
This behavior is typical of monotropic crystal polymorphs,
which lack a transition point below the melting points of both
polymorphs.4 To rationalize our results, we overlaid two
cooling curves of pure A and B obtained from the same
monomer solution (see Figure 2a). The temperature at which
both curves intersect lies between the melting (Tm) and
elongation (Te) temperatures of species A and B, respectively
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(gray area in Figure 2a), which suggests the monotropic nature favored assemblies in the solid state has been previously
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of our supramolecular polymorphs. Only after heating
aggregate A to a selected temperature within this range (e.g.
337 K) and keeping this solution over time, leads to the
dissociation of aggregate A and, subsequently, to its
transformation into state B via monomer formation (Figure
2b). Importantly, we have also inspected the stability of the
mixture of the two aggregates A and B. Time-dependent
UV/Vis studies demonstrate that the mixture of A+B, created
either by cooling the monomer solution using an intermediate
cooling rate of 0.5 K min-1 (Figure S18), or by mixing
equimolar amounts of both pure polymorphs A and B (Figure
S18), also remains invariant over time. Thus, no conversion of
the mixture of A and B to solely B occurs in the investigated
period of time (three weeks).
elucidated in detail for various systems including perylene
bisimides,22a dendronized cyclotriveratrylenes22b and dendritic
dipeptides.22c,d
The decoration of 1 with chiral side chains can be utilized to
induce an efficient transfer of chirality from the stereogenic
centers to the aggregates. Thus, we studied the assemblies
with electronic circular dichroism (CD) spectroscopy. The CD
response is strongly contaminated with linear dichroism (LD)
upon cooling a hot MCH solution of 1 from 347 to 298 K
(Figures S20-S24). These results can be ascribed to an
anisotropy effect due to the formation of intertwined 1D
nanostructures.23 To examine the effect of cooling rate on the
aggregate formation, we prepared thin films on a quartz plate
using a 7.5 M solution with different cooling rates (2 K min-1
and 0.1 K min-1, Figure S25). Assuming that the 1D structures
are oriented along the dropping direction, the LD signature of
the thin film was similar to the corresponding CD signature of
1 from MCH solution (Figures S20-S24). A vertical coating of
the quartz plate was also tested, resulting in a negative LD
response without a remarkable change in the signature.
Rotation of the film 90° resulted in an inversion of the signal
whereas a 45° rotation showed no LD signal. Irrespective of
the solution dropping or measuring angle, the chiroptical
signature remains unaltered, indicating the absence of helical
structures but rather the formation of 1D nanostructures.
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Structural elucidation of polymorphs A and B. The
elucidation of the packing modes of both aggregates A and B
has been carried out combining UV-Vis experiments, Fourier-
transform infrared (FTIR) spectroscopy, 1H NMR studies, both
in solution and in the solid state, and quantum chemical
calculations. Figure 3 depicts the partial FTIR spectra of two
samples prepared by heating a 200 M solution of 1 in MCH
and applying different cooling rates. The spectra shown as
cyan line has been obtained using a cooling rate of 2 K min-1,
conditions under which UV-Vis experiments demonstrated the
formation of aggregate A. FTIR spectroscopy reveals a strong
band at 3318 cm-1 and a weaker band at 3421 cm-1 ascribable
to the amide N-H stretching (cyan spectrum in Figure 3a).
These two bands can be assigned to free N-H and N-H···Cl
bonded, respectively, which is in accordance with the
formation of aggregate A. A different behavior is observed
when a cooling rate of 0.1 K min-1 is applied (see royal blue
spectra in Figure 3). Notably, a single band at 3325 cm-1 is
observed for the slowly cooled sample with no free N-H group
band at 3421 cm-1 thus confirming the formation of a different
aggregate species (B, Figure 3a). The higher value observed
for the N-H band (3325 cm-1 vs. 3318 cm-1) indicates slightly
weaker N-H bonds for B compared to A.
Figure 2. (a) Cooling curves (αagg vs T) of 1 (7.5 M) with a rate
of 2 K min-1 and its isodesmic fit (cyan) with corresponding Tm
and 0.1 K min-1 and its cooperative fit (royal blue) displaying Te.
(b) Time-dependent UV/Vis spectrum of 1 (7.5 M) at 337 K
showing the transformation from A to B via monomer. (c) Phase
diagram illustrating the competitive self-assembly behavior of 1
with respect to concentration and cooling rates. Inset: Energy
landscape depicting pathways A and B.
Interestingly, the values of the amide N-H stretching for
both aggregates (3318 cm-1 for A and 3325 cm-1 for B) differ
significantly from those typically observed for amide-amide
C=O···H-N hydrogen-bonded systems (ca. 3290 cm-1).18 In the
carbonyl stretching region, the Amide I band appeared at
1670 cm-1 and 1678 cm-1 for aggregate A and B, respectively
(Figure 3b). These values are in good correlation with free
(non-H-bonded) C=O groups, as they differ from the position
of the amide I stretching of typical amide-amide C=O···H-N
H-bonded systems (1635 cm-1).18 The wavenumbers observed
for the Amide I band indicate that the carbonyl groups of both
aggregate species (A and B) are not involved in the formation
of H-bonding arrays. Consequently, the formation of amide-
Differential scanning calorimetry (200 µM in MCH,
2 K min-1, heating from 298 K to 360 K) showed a melting
transition of 335 K and 355 K for A and B, respectively
(Figure S19). Upon cooling to 298 K at a rate of 2 K min-1 and
recording the second heating cycle, only one exothermic
transition at 335 K was observed for both samples. These
results underline the formation of state A from the isotropic 1
in the solution phase with fast cooling rate (2 K min-1). In
addition, DSC also showed that a conversion of aggregate A
into B occurs in the solid state upon heating (Figure S38). This
reorganization of stable assemblies to more energetically
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amide C=O···H-N hydrogen-bonded, i.e. parallel stacks, for signals diagnostic of a very slow exchange and, consequently,
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both aggregate species A and B can be ruled out. An intriguing
question is, hence, why the N-H band groups are involved. To
disclose this interaction, the band of the aryl-O-side chain,
around 1207 cm-1 was monitored. Remarkably, the solution of
aggregate B in MCH (0.1 K min-1; royal blue plot) showed a
splitting into two bands at 1214 cm-1 and 1207 cm-1 (Figure
3c). This may be explained by the formation of intermolecular
H-bonds between one out of three alkoxy oxygen atoms of
each molecule and the H-N group of another molecule via N-
H···O(aryl) interactions. In contrast, the alkoxy side chains do
not interact significantly with the neighboring molecules upon
forming the aggregate A (2 K min-1; cyan plot), which is
evident from the single broad band at 1207 cm-1 (Figure 3c).
of a high tendency to aggregate. Heating this 500 M solution
to 347 K allows the resonances being more defined due to the
partial disassembly by the effect of heating (Figure S29). In
any case, the observed shifts appear to follow the same trend
as in CDCl3.
A rotating-frame Overhauser effect spectroscopy (ROESY)
NMR experiment in a concentrated solution of 1 (60 mM,
CDCl3, 298 K) has been also used to obtain more information
about the supramolecular organization of the aggregates of 1.
Together with the expected intramolecular contacts, this
experiment also shows cross-peaks that can only by justified
considering intermolecular contacts. Thus, correlation signals
between the protons of the pyridine ring (Ha and Hb in Figure
4 and S28) and those of the benzamide unit (Hd and He in
Figure 4 and S30) are observed. In addition, the intermolecular
cross-peaks between the proton of the trialkoxybenzamide
moiety (Hg in Figure 4 and S30) and Ha of the pyridine ring
together with the contacts observed for the peripheral side
chains and the aromatic protons (Figure 4 and S30) support the
slipped packing of the molecules of 1 to form aggregate A.
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Furthermore,
the
involvement
of
the
peripheral
trialkoxybenzamide moieties in the formation of aggregates B
is also confirmed by monitoring the shift corresponding to the
meta-substituted ring vibration. In aggregate A (light blue
plot; 2 K min-1), this band appeared at 1066 cm-1. On the
contrary, this band shifts to lower wavenumbers (1060 cm-1,
Figure 3d) indicating a weakening of ring stability for B
compared to A.
Figure 4. Zoomed region of the 2D-ROESY spectrum of 1 in
CDCl3 (60 mM, 298 K, pulse sequence: p1 ~ 8 s, p15 = 500 ms).
The dotted black and red circles highlight intra- and
intermolecular through-space coupling signals, respectively. The
upper part of the panel shows the slipped arrangement of two
molecules of complex 1 depicting the different anisochronous
aromatic protons.
Figure 3. Partial FT-IR spectra of 1 in MCH (200 M) from a)
3500 to 3250 cm-1; b) 1700 to 1540 cm-1; c) 1260 to 1160 cm-1
and d) 1100 to 1000 cm-1 at 298 K. The sample was prepared by
cooling from 363 K to 298 K at 2 K min-1 (cyan trace, pathway A)
and 0.1 K min-1 (royal blue trace, pathway B).
To complement the FTIR studies, we have also developed a
series of H NMR studies. Initially, we have checked the
1
All the above H NMR experiments contribute to shed light
1
on the aggregation mode followed by complex 1 to form
polymorph A. However, at this stage, the aggregation mode of
polymorph B still needs to be addressed to corroborate the
findings observed by FTIR. Previous work on Pt(II)
complexes showed the participation of oxygen atoms of side
chains in the consecution of stable aggregates.15c Considering
this previous report, we hypothesized the possibility of the
operation of non-covalent interactions between the N-H group
and one of the outer oxygens present in the side
trialkoxybenzamide moiety, as suggested by FTIR (vide
supra).
variation of the resonances upon increasing the concentration
in CDCl3. As in some other examples of self-assembling units
involving amide functional groups, increasing the
concentration results in a deshielding of the proton ascribable
to the N-H group. Concomitantly, the opposite trend is
observed for most of the aromatic resonances that shift upfield
at high concentration. However, this trend is not observed for
the proton corresponding to the 3,4,5-trialkoxybenzamide
moiety that slightly deshields upon increasing the
concentration (Figure S28). We have also checked the
1
influence of solvent on the aggregation of 1. Registering a H
To confirm the existence of these intermolecular non-
covalent interactions for aggregate B, 1H high-resolution
NMR spectra of 1 in d14-MCH at a concentration as low as 500
M results in a highly undefined spectrum with very broad
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magic-angle spinning (HR-MAS) NMR in d14-MCH was Starting from the premise that only two decyloxy chains per
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employed. For these experiments, aggregate B was created
using the same method and conditions as for previous UV/Vis
studies (cooling a 100 L solution of 1 from 347 K to 298 K
using a rate of 0.1 K min-1) in d14-MCH. Then the sample was
slightly concentrated by partial removal of the solvent using
N2 and finally introduced in the 4.0 mm HR-MAS rotor (50
molecule of aggregate B may form H-bonds, there would be
four O-CH2 groups that do not participate in H-bonding and,
1
therefore, will show no change in their H chemical shift, i.e.,
1
they give rise to the H signal at  4.2 ppm. Both O-CH2
groups affected by hydrogen bonding show a 1H chemical shift
corresponding to  3.1 ppm. Likewise, the O-CH2-CH2 groups
1
1
L). The resulting H NMR spectrum includes both splitting
taking part in H-bonded decyloxy chains contribute to the H
and shielding/deshielding of the (O-CH2-CH2-) proton signals
(Figure 5) as will be elucidated in the following, confirming
the spatial proximity between the N-H and O-CH2-CH2-
groups of the side chains. Moreover, the formation of H-
bonding interactions between the amide group and the
decyloxy oxygen affects the (C(Ar)-O-C) bond as seen in the
FTIR spectrum, changing the O-CH2 bond geometry as
signal at 2.3 ppm. These considerations show that for one
molecule of aggregate B, a total of 16 protons, corresponding
to four O-CH2 (red), two O-CH2 (blue), and two O-CH2-CH2
(green), with a ratio of 2:1:1 have to be regarded. Such a ratio
corresponds to the formation of two H-bonds per molecule on
average, in excellent agreement with deconvolution results and
the stacking motif shown in Scheme 1.
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1
sketched in the insert of Figure 5. In the H HR-MAS NMR
The proposed packing modes for polymorphs A and B on
spectrum for aggregate B, two additional 1H signals ( 3.1 and
1
the basis of FTIR, ROESY and H HR-MAS NMR have been
2.3 ppm, blue and green labels in Figure 5) are visible when
ultimately complemented by semi-empirical calculations at the
dispersion-corrected PM6 level in vacuum (Scheme 1 and
Figures S32-S34). Intermolecular Cl···H-N and R-O···H-N
interactions involving amide groups were observed for the
slipped packing (State A) and pseudo-parallel packing (State
B), respectively (Scheme 1). Interestingly, the rather unusual
R-O···H-N interaction involves only one of the three alkoxy
chains, nicely matching NMR and FTIR measurements (vide
supra). Both packings are also stabilized by aromatic
interactions between the OPEs, and only state B is further
stabilized by intermolecular van der Waals interactions
involving the alkyl chains, which help understand the lower
Gibbs energy predicted for state B (Figure 2c, inset), as well
as the slightly larger local disorder observed for state A (see
Figure S32). The thickness of the 1D fiber (ca 1.6 nm, Figure
S34) represented by a dodecamer of A assembled using the
PM6-optimized tetramer structures nicely matches the height
of a single fiber measured by atomic force microscopy (AFM)
(vide infra), besides being in agreement with extensive NMR
investigations.
1
compared to the conventional liquid-state H NMR spectrum
(cf. Figure 5 and S31). The 1H resonance at  3.1 ppm
corresponds to the O-CH2 group in α position with respect to
the alkoxy oxygen participating in H-bonding, whereas the O-
CH2 groups that are not involved in H-bonding are at a higher
1H chemical shift of  4.2 ppm (red labels in Figure 5). This
difference in 1H chemical shift is a result of the changes in the
O-CH2 bond geometry associated with a higher proton
shielding. Additionally, the bond of the methylene group in β
position with respect to the H-bonded alkoxy oxygen
(-O-CH2-CH2) is shortened, resulting in a deshielding effect
and, thereby, an increase of  to 2.3 ppm. The non-bonded O-
CH2-CH2 groups are not resolved due to an overlap with the
Morphological analysis of polymorphs A and B. Ultimately,
the structural dissimilarity of both supramolecular polymorphs
was examined by AFM on highly-oriented pyrolytic graphite
(HOPG). Thin flexible fibers with a height of 1.5 nm and
width of 4(±0.3) nm were visualized for species A (Figures
6a,c and S35). As the π-backbone of 1 has a length of ca. 5 nm
(7 nm assuming outstretched chains), a parallel organization is
therefore excluded. Considering the low height and the
spectroscopic signatures of the thin fibers, the units of 1 are
arranged in a slipped fashion via N-H···Cl contacts and
oriented almost parallel to the longitudinal axis of the fibers
(Scheme 1, State A).
solvent signal (d14-MCH) in the aliphatic region below 2 ppm.
A deconvolution of all three H signals at  4.2, 3.1, and
2.3 ppm thus gives information about the fraction of molecules
that participate in H-bonding. This deconvolution shows a
1.99:1.04:0.97 ratio for the resonances at  4.2, 3.1, 2.3 ppm,
respectively.
In sharp contrast, significantly longer (≤10 µm), more rigid,
and thicker fibers with a strong tendency to bundle could be
distinguished for pathway B (Figures 6b,d and S36). The good
match between the height of the individual fibers (6-7 nm, see
inset of Figure 6d) and the molecular length supports our
proposed pseudo-parallel packing with a slight rotational
displacement (Pathway B, Scheme 1).24 Such stacks can be
stabilized by π-π and N-H···O-alkyl interactions as described
earlier. The arrangement of the alkyl chains on the peripheral
part of a single fiber facilitates the interdigitation with
neighboring fibrils, resulting into the observed bundles of
thick fibers (Figure S36). Finally, we also examined the
nanoscale morphology by AFM at the phase boundary (see
1
1
Figure 5. Aliphatic part of the H HR-MAS NMR spectrum for
aggregate B (Pathway B, Scheme 1) recorded at 298 K in d14-
MCH. The dashed spectrum illustrates the deconvolution. The
inset illustrates the hydrogen bonding of 1 to the amide group of
neighboring molecules and the assignment scheme.
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green region in the experimental phase diagram, Figure 2c) enabled us to elucidate the stability conditions of both species
1
2
3
4
5
6
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8
between both aggregate states using intermediate
concentrations and cooling rates (50 µM; 0.5 K min-1). The
images clearly reveal the coexistence of both thin flexible (1-
2 nm height) and thick rigid fibers (11-55 nm, see Figure S37),
providing final evidence of the concomitant packing
polymorphism. Overall, we have experimentally observed that
an isodesmic process leads to the slipped packing (Pathway A)
whereas the pseudo-parallel stacks (Pathway B) are formed in
a cooperative fashion. This can be rationalized in terms of a
different contribution of the groups involved in H-bonding.
While each OPE-based pyridyl ligand in the slipped species
can interact intermolecularly with just one neighboring OPE-
based ligand, this number is doubled in the case of a pseudo-
parallel arrangement. Additionally, the pseudo-parallel stacks
can be laterally stabilized into bundles by alkyl chain
interdigitation, which is not possible in the less compact
slipped arrangement. These effects might contribute to a more
pronounced molecular preorganization in the state B that
ultimately enable the needed arrangement of the H-bonding
groups to act cooperatively.10e,f
through a phase diagram. The packing modes of both
supramolecular polymorphs A and B have been elucidated by
extensive experimental studies (UV/Vis, emission, FTIR, 1D,
2D and MAS NMR, CD and LD, DSC, AFM) and theoretical
calculations. Notably, while N-H···Cl-Pt(II) interactions,
recently observed by our group,19 stabilize the slipped
pathway, the pseudo-parallel stacks are assembled via
unconventional N-H···O-alkyl interactions. Interestingly,
under a controlled set of conditions of cooling rate and
concentration, both polymorphs can be isolated concomitantly
in the same solution. Kinetic UV/Vis studies show that the
supramolecular polymers A and B do not interconvert over
time at room temperature for a period of at least six months,
revealing the narrow energy distribution between them. Only
if A is annealed at a temperature between the critical
temperatures of pathways A and B, i.e. 337 K for prolonged
time, a slow transformation into B takes place via monomer
formation. This rearrangement from A to B also occurs in the
solid state upon thermal annealing at high temperatures. Our
system, which in many respects bears close resemblance to
concomitant packing polymorphism in crystals, should help
bridge the gap between crystal engineering and
supramolecular polymerization.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Synthesis and characterization; variable temperature (VT) UV-Vis
studies; 1D and 2D NMR studies; fluorescence studies; cooling
curves and thermodynamic analysis, theoretical calculations; CD
experiments; DSC; AFM imaging.
AUTHOR INFORMATION
Corresponding Author
fernandg@uni-muenster.de; lusamar@quim.ucm.es
Author Contributions
The manuscript was written through contributions of all authors. /
All authors have given approval to the final version of the
manuscript.
ACKNOWLEDGMENT
Figure 6. Height AFM images of 1 at 298 K prepared by spin-
coating of a 100 µM MCH solution onto HOPG using a cooling
rate of 2 K min-1 (a, c) and 0.1 K min-1 (b, d). Z scale is 30 nm (a),
60 nm (b), 6 nm (c) and 180 nm (d). Inset: cross-section analysis
from dashed lines 1-1’ and 2-2’ in images (c, d).
We thank the University of Münster, the Deutsche
Forschungsgemeinschaft (SFB 858) and the Humboldt
Foundation (Sofja Kovalevskaja Program) for financial support.
Financial support by the MINECO of Spain (CTQ2017-82706-P)
and the Comunidad de Madrid (NanoBIOSOMA, S2013/MIT-
2807) is acknowledged. Y. D. is thankful to the Comunidad de
Madrid for his predoctoral fellowship. We acknowledge the group
of Prof. Cristian Strassert for lifetime measurements
CONCLUSIONS
In summary, we have unraveled the concomitant packing
polymorphism in the supramolecular polymerization of a π-
conjugated Pt(II) complex 1 in nonpolar medium. 1 self-
assembles, by careful selection of temperature, solution
concentration and cooling rate, into two different
supramolecular polymers with distinct molecular packing
(slipped (pathway A) vs. pseudo-parallel (pathway B)) and
high stability. While higher concentrations and slow cooling
rates favor the pseudo-parallel pathway B, slipped aggregates
A can be preferentially isolated using lower concentrations
and faster cooling rates. Precise control of these variables
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