Hydrogen bond-directed supramolecular polymorphism leading to soft and hard molecular ordering

Article abstract of DOI:10.1039/d0cc01636e

Transformation of metastable supramolecular stacks of hydrogen-bonded rosettes composed of an ester-containing barbiturated naphthalene into crystalline nanosheets occurs through the rearrangement of hydrogen-bonding patterns. The involvement of the ester group in the crystalline hydrogen-bonded pattern is demonstrated, guiding us to a new molecular design that can afford supramolecular polymorphs with soft and hard molecular packing.

Full text of DOI:10.1039/d0cc01636e

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Hydrogen bond-directed supramolecular polymorphism leading to
soft and hard molecular ordering
Received 00th January
20xx,
Takumi Aizawa,a Keisuke Aratsu,a Sougata Datta,b Takaki Mashimo,c Tomohiro Seki,c
Takashi Kajitani,d Fabien Sillye and Shiki Yagai*b,f
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Transformation of metastable supramolecular stacks of hydrogen- hydrogen bonding unit that gives rise to specific polymorphisms
bonded rosettes composed of an ester-containing barbiturated derived from a variety of hydrogen bonding patterns depending
naphthalene into crystalline nanosheet occurs through the on the external environment. Most reported structures are
rearrangement of hydrogen-bonding patterns. The involvement of based on open-ended tapelike hydrogen-bonded motifs. In
the ester group in the crystalline hydrogen-bonded pattern is contrast, in recent years we have developed unique
demonstrated, guiding us to a new molecular design that can afford supramolecular systems organized from cyclic hydrogen-
supramolecular polymorphs with soft and hard molecular packing. bonded
hexamer
of
barbituric-acid-functionalized
bearing
(barbiturated) π-conjugated molecules such as
1
wedge-shaped aliphatic tails.6 These cyclic hexamers, referred
to as “rosettes”,7 stack through π–π interactions to afford
supramolecular polymers with an intrinsic curvature. Extending
this hierarchical process has also led to supramolecular
polymers with diverse topologies such as rods,8 toroids (by
A phenomenon wherein a molecule has various stable and
metastable assembled states can be called supramolecular
polymorphism, and is recently increasing attention in the field
of supramolecularly engineered soft materials.1 The
phenomenon is caused by an interplay of different self-
assembly pathways, bringing in time-evolving dynamic nature.
Because the properties of supramolecular materials are highly
dependent on the molecular packing structure,2 designing
supramolecular polymorphism beforehand is crucially
significant. In particular, it has been rarely reported for single
supramolecular building blocks that can form polymorphs
whose softness/hardness are dramatically different.3
1
),8a,9 and helicoids,10 and even chimeric topologies.11 Almost all
the barbiturated π-conjugated molecules with an aliphatic
wedge12 have provided rosette-based supramolecular polymers,
but very recently we encountered an exceptional molecule that
provides crystalline ribbons which are totally different from
rosette-based supramolecular polymers in terms of morphology
as well as chemical properties.13 Although we could not define
the underpinning hydrogen-bonded patterns, it was obvious
that the presence of an ester linkage in the molecular structure
is the key to form such a dramatically different structure.
A pragmatic approach is to use building blocks that can
produce distinctly different hydrogen-bonded architectures
through directional multiple hydrogen bonds.4 Griesser and co-
workers5 reported that barbituric acid is one of the multiple
a. Division of Advanced Science and Engineering, Graduate School of Science and
Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-85223, Japan.
b.
Department of Applied Chemistry and Biotechnology, Graduate School of
Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan.
c. Division of Applied Chemistry and Frontier Chemistry Center (FCC) Faculty of
Engineering, Hokkaido University Kita 13 Nishi 8 Kita-ku, Sapporo, Hokkaido 060-
8628, Japan.
d. Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of
Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
e. Université Paris-Saclay, CEA, CNRS, SPEC, TITANS, F-91191 Gif sur Yvette, France.
f. Institute for Global Prominent Research (IGPR), Chiba University, 1-33 Yayoi-cho,
Inage-ku, Chiba 263-8522, Japan.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Fig. 1 (a) Molecular structures of compounds
1
,
2
and
3. (b,c) Schematic
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DOI: 10.1039/D0CC01636E
representation of supramolecular polymorph of
2
. Photographs in (b)
and (c) show gel of TF and precipitates of PN formed by 2
(c = 1.0 × 10−2
M).
Toward a new direction of the self-assembly of barbiturated
π-conjugated molecules, herein we investigated self-assembly
of simple naphthalene molecule
2 involving an ester linkage. We
reveal that molecule kinetically organizes into supramolecular
2
polymers, which in turn transforms into thermodynamically
stable crystalline assembly through the rearrangement of
hydrogen-bonding pattern (Fig. 1). Remarkably, we successfully
revealed the single crystal structure of a reference compound
3
lacking wedge unit, which provides clear evidence that the ester
unit is involved in the formation of thermodynamically attained
hydrogen-bonded polymeric arrays.
According to our accumulated protocol that organizes
barbiturated molecules through the formation of rosettes, we
Fig. 2 (a,b) Temperature-dependent changes of (a) UV–Vis and (b)
in MCH (c = 1.0 × 10−4
fluorescence spectra (lex = 346 nm) of 2 M) upon
cooling from 80 to 20 °C at a rate of 1 °C min−1. Inset in (a) shows a plot
of absorbance at 470 nm versus temperature. Inset in (b) is a
photograph of the cooled solution under 365-nm light illumination. (c)
AFM image of TF spin-coated immediately after cooling a MCH solution
−4
initially examined self-assembly of
2
(c = 1.0 × 10 M) in
methylcyclohexane (MCH) by cooling a molecularly-dissolved
−1
solution from 80 °C to 20 °C at a rate of 1 °C min . At 80 °C,
2
showed a vibronic absorption band with a maximum at 362 nm
(Fig. 2a). Upon cooling, this absorption band hypsochromically
shifted to 350 nm with the emergence of a new absorption in
420–480 nm. This spectral change by cooling proceeded non-
sigmoidally as a function of temperature descent, which is
characteristic of cooperative supramolecular polymerization
mechanism (inset in Fig. 2a).14 Because the subsequent heating
of the as-cooled solution did not show thermal hysteresis, we
analysed the cooling curves obtained at several concentrations
with a nucleation-elongation model (Fig. S2).14b A van’t Hoff
analysis of the collected data provided an elongation enthalpy
of
2
(c = 1.0 × 10−4 M) from 80 to 20 °C at a cooling rate of 1 °C min−1.
Inset in (c) is another AFM image of TF (c = 2.5 × 10−4 M). Scale bar = 100
nm. (d) Schematic representation of TF formed by rosettes.
Despite the fragility of TF in the diluted condition, a kinetic
supramolecular polymerization of
2 in MCH by quenching hot
solutions resulted in gelation at concentrations above 2.0 × 10−
3
M (Fig. 1b). Since this observation implies the formation of
well-defined one-dimensional aggregates, we assessed the
H° of −58 kJ mol−1, which is smaller than that of
D 1 (DH° = −72
packing structure of
calorimetry (DSC) and polarized optical microscopy revealed
that forms a thermotropic liquid crystalline mesophase in the
2 in the bulk state. Differential scanning
kJ mol−1). At 20 °C, the solution showed a green emission with
the emission maxima at 530 nm (fluorescence quantum yield:Ff
= 0.022) (Fig. 2b). When the solution was spin-coated onto
highly oriented pyrolytic graphite (HOPG) substrate, atomic
2
temperature range of 109–194 °C upon cooling from isotropic
state (Fig. S5a,b). The X-ray diffraction (XRD) measurement of
the mesophase revealed the formation of a hexagonal columnar
structure (Colh) with lattice constant a of 5.71 nm (Fig. S5c). This
lattice constant is in good agreement with the width of TF
estimated by AFM. We further found a periodicity of 0.34 nm,
which could be attributed to the center-to-center distance of
force microscopy (AFM) image showed bundled thin fibers (TF
)
with the width and thickness of 5.98 ± 1.01 nm and 2.32 ± 0.49
nm, respectively (Fig. 2c). The width is in good accordance with
the diameter of rosette model of
S1a, S3). The observed morphology is quite different from well-
defined toroidal morphology of with highly uniform
curvature.9a,10c Density functional theory (DFT) calculation
revealed that the trialkoxyphenyl wedge unit of has a larger
dipole moment compared to that of due to the electron-
withdrawing ester group (Fig. S4). We thus inferred that the
dipole dipole interaction between wedge units competes the
p-p stacking interaction of the rosettes of , which prevents
2 (ca. 7 nm) (Fig. 2d and Fig.
1
stacked disks (intracolumnar periodicity, or lattice constant c)
−3 8a
(Fig. S5c). Assuming the density of
2 to be 1.0 g cm , the
2
number of molecule per columnar slice of 0.34 nm is calculated
to be 6.2, which strongly supports the formation of hexameric
1
rosettes (Fig. S5d,e).8a Accordingly, the rosettes of
2 can be
—
densely packed into the hexagonal columnar structure in the
condensed phases, whereas in the diluted solution such
columnar structures cannot maintain their shapes as shown by
the ambiguous AFM image of TF, probably due to competition
between the wedge units and naphthalene unit.
2
uniform rotational and translational displacements that are
prerequisite to form intrinsic curveture.15
Reflecting the weakened interaction between rosettes, TF is
−4
stable only kinetically. At c = 5.0 × 10 M, the TF solution
remained homogeneous for 10 h at room temperature,
thereafter it completely precipitated within 12 h. Similarly, the
gel solution shown in Fig. 1b resulted in precipitation within a
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day (Fig. 1c). AFM images of the precipitates visualized layered dotted lines in Fig. 4a). The dimer units are then further
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DOI: 10.1039/D0CC01636E
platelike nanostructures (PN) with constant thickness of about connected through double hydrogen bonds between barbituric
3.64 ± 0.39 nm (Fig. 3a). Precipitated PN exhibited more acid units (orange doted lines in Fig. 4a). In this structure, the
yellowish emission at 570 nm (Ff = 0.097) compared with TF (Fig. periodicity along to the hydrogen-bonded array of 3 was
3b). Powder XRD analysis of PN displayed a number of peaks (Fig. estimated to be 1.27 nm (Fig. 4a). This periodicity might
3c and S6), suggesting the crystalline nature of this correspond to the in-plane periodicity of PN at d = 1.33 nm (Fig.
nanostructure. The diffractions in the small-angle region could 3c).
be assigned as two sets of lamellar structures with periodicities
of 3.74 and 1.33 nm (yellow and pink diffractions in Fig. 3c, supramolecular packing structure of
respectively). The former value showed good agreement with dimensional organization of at the liquid−solid interface by
To further correlate the crystal structure of
3 and the
2, we investigated a two-
2
the thickness of PN obtained by the AFM images, and the latter using scanning tunnelling microscopy (STM). STM imaging
is considered to be the in-plane diffraction (vide infra). In the showed the formation of two types of lamellar domains, one
Fourier transform infrared (FT-IR) spectra, some C=O stretching major and the other minor domains, with different lamellar
bands of PN were observed at frequencies considerably molecular arrays (Fig. 4b and S8). In the major domain (domain
different from those of TF (Fig. 3d). Among three bands a, c and I), hydrogen bond formation seems to be neglected to form a
d that can be assigned to each of the three C=O groups of the dense molecular packing (Fig. S8a,b). In the minor domain
barbituric acid unit, the bands a and d displayed low and high (domain II), a tilted molecular arrangement could be seen (Fig.
frequency shifts, respectively. These shifts suggest that TF and 4c and S8c,d), which can be fitted with the ester-mediated
PN are based on different hydrogen bonding patterns. hydrogen-bonded supramolecular array found in the crystal
Furthermore, the C=O stretching band b derived from the ester structure of
3. This finding demonstrates that molecule 2 can
group displayed a low frequency shift in PN (Fig. 3d), suggesting also organize through a similar ester-mediated hydrogen-
its involvement in the supramolecular interactions to form PN
.
bonding motif. In the homogeneous solution phase, the
resulting hydrogen-bonded chains presumably assemble into
two-dimensional direction with face-to-face slipped π-stacking
motif to form PN (Fig. 4d), which is also supported by the two-
dimensional organization of 3 in the crystal (Fig. S7).
Fig. 3 (a) AFM image of PN. Inset shows cross-sectional analysis of PN
between the blue dots. (b) Fluorescence spectra of TF solution and PN
suspension in MCH (c = 1.0 × 10−4 M). Inset in (b) is a photograph of
precipitated PN under 365 nm-light illumination. (c) Powder XRD
pattern of PN. (d) FT-IR spectra of TF solution and PN suspension in MCH
(c = 5.0 × 10−4 M).
To obtain more detailed insight into the supramolecular
Fig. 4 (a) Single-crystal structure of
3 along x axis (hydrogen-bonding
packing structure of PN, we performed X-ray crystallographic
direction). Green doted lines show hydrogen bonds between a NH of
barbituric acid and the C=O of ester group. Orange doted lines show
hydrogen bonds between the other NH and a C=O of two barbituric acid
analysis for a single crystal of an analogous molecule
lacks the aliphatic tails (Fig. 4a and S7, Table S1). The single
crystals of were obtained by diffusing toluene (poor solvent)
3 which
3
units. (b) Large scale and (c) high-resolution STM images of
2 at a
vapor to a THF (good solvent) solution. The single-crystal
structure revealed the formation of an antiparallel dimeric unit
liquid−solid interface between 1-phenyloctane and HOPG. In (b),
domains I and II are surrounded by blue and red lines, respectively. (d)
of
3 held together by double hydrogen bonds between a NH
Schematic representation of proposed packing structure of
2 in PN.
group of barbituric acid unit and the ester C=O group (green
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Indeed, when the transformation kinetics was observed under
an isothermal condition by monitoring the emission intensity at
650 nm (I650), the lag time became shorter upon decreasing
initial monomer concentration (Fig. S9).16
In conclusion, we have demonstrated that the introduction
of an ester unit into a barbiturated naphthalene supramolecular
building block destabilizes gel-forming columnar stacks through
the hydrogen-bonded rosettes, and could open a new pathway
leading to crystalline two-dimensional nanostructures. Role of
the ester group is remarkable because it not only destabilizes
the interaction between rosettes but also allows the formation
of new molecular sequences by intervening in hydrogen bond
formation. The rosette-based supramolecular polymers
covered with long alkyl chains show sufficient solubility in MCH,
thus act as gel-forming nanostructures. In contrast, the ester-
mediated hydrogen-bonded chains promote their two-
6
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9
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which is on-going in our group.
p-conjugated units,
This work was supported by the Japan Society for the
Promotion of Science (JSPS) KAKENHI Grant Number 19H02760.
S.Y. also acknowledges financial support from the The Murata
Science Foundation. K. A. also acknowledges financial support
from JSPS for a research fellowship for young scientists
(17J02520).
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There are no conflicts to declare.
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