Key words
two-dimensional materials - supramolecular polymers - hydrogen bonding - π–π interactions
- hydrophobicity - curvature
Introduction
Two-dimensional (2D) materials are a tantalizing area of research for materials scientists
because of their potential applications in next-generation electronics, sensors and
photovoltaics.[1] Interest in these systems arises from, amongst others, their large surface/volume
ratio and tunable band gap, making them suitable candidates for energy storage and
transport.[2] The ability to interact with light and to carry charge, as well as their thinness,
offers potential to efficiently harvest energy, prompting appeal to photovoltaics.[3] 2D porous materials have shown capability to act as sensors and catalysts, allowing
for specific adsorption of different guests on their thin surfaces.[4]
However, creating large, defect-free 2D lattices suitable for these purposes is nearly
impossible. Graphene is a system that came close with its discovery in 2004 (Nobel
prize 2010).[5] This landmark achievement was the result of a top-down approach where simple exfoliation
of graphite yielded a single layer of covalently linked, conjugated carbon atoms.[5b] However, graphene cannot easily be modified – chemical changes occur randomly, if
they occur at all – and its covalent build-up from smaller fragments remains challenging.
Supramolecular approaches are a possible alternative,[6] where tailored fragments are made to assemble through weaker interaction forces
rather than via the carbon–carbon bond. Early supramolecular materials used the hydrogen
bond,[7] a particularly powerful tool, which showcased the reversibility and tunability of
the formed polymers that is not achievable with regular covalently linked polymers.[8] These same interactions were then used to create soft 2D materials, where the directional
nature of the hydrogen bond guides the assembly. Similar strides were made in the
field of metal–organic frameworks, where incorporating directional coordination bonds
between the metal and the ligand yielded thin 2D layers,[9],[1e] although the suppression of layer-by-layer growth in the third dimension remains
a challenge to this day. Despite the clear design approach, it is often not a sole
interaction that drives the assembly, but a combination of several interactions, which
leaves much room for serendipity. Within this short review, we detail recent conceptual
approaches in hydrogen-bonded systems that have resulted in 2D supramolecular assemblies,
as well as a brief overview of design principles that have succeeded in overcoming
the necessity for strong directional interactions in such architectures.
Highly Directional Interactions
Highly Directional Interactions
One of the most common methods to facilitate an assembly is to use monomers with components
possessing strong dipoles that are therefore capable of sustaining a prolonged interaction
between one another. This mandates that the interaction originates from specific sites
within the monomer, acting as a focal point for the polymerization mechanism. Consequently,
there is a clear preference for directionality through donor and acceptor components.
It is well documented that not all types of non-covalent interactions are similar
in strength and, although difficult to quantify, we may single out hydrogen bonding
due to its highly directional nature.[10] Coordination bonds are another outstanding example of these strong directional interactions,
but we see them outside the scope of this short review.
Hydrogen Bonding
In 2020, Montenegro et al. reported the assembly of a macrocyclic peptide that forms
2D layers in water by using a strategy where hydrogen bonding is supplemented by weaker
interactions.[11] Initially, a one-dimensional (1D) assembly occurs, forming hollow columns of alternating
D/L stacked monomers held together through hydrogen bonding between the residues.
The conformation of the individual nanotubes is rigidified by complementary π–π interactions
between tryptophan residues that allow alignment of the entire tube. Leucine residues
flank the tryptophan and promote inter-tube hydrogen bonding, which generates a tertiary
sheet structure in solution. The hydrophobicity of tryptophan is then used to create
a flattened 2D sheet where the residue is buried within the structure to reduce exposure
to the aqueous environment, resulting in the formation of thin sheets of lengths greater
than 100 µm and with consistent thicknesses of 3.2 nm. This shows that combining hydrogen
bonding with hydrophobicity is a potent method to create rigid, flat structures following
a 1D-to-2D self-assembly approach ([Figure 1]).
Figure 1 Schematic representation of a tubular bilayer reported by Montenegro et al. in which
D/L-alternating peptides undergo one-dimensional self-assembly into hollow amphiphilic
nanotubes that subsequently form tubular bilayer sheets through inter-tube hydrogen
bonding. STEM images reprinted with permission from Ref. [11]. Copyright 2020 American Chemical Society.
A less elaborate example of building 2D sheets using a combination of hydrogen bonding
and hydrophobicity was reported by Stevens and co-workers using linear instead of
cyclic peptides ([Figure 2]).[12] Hydrogen bonding between phenylalanine groups occurs to knit the structure together
in two dimensions, which allows for direct 2D assembly as there is no directional
preference for interaction between rods within the plane of the sheet. By incorporating
an alkyl chain which is linked to a phenylalanine segment, the peptides undergo self-sorting
in the assembly process where separation based upon hydrophobicity occurs, providing
single-layer sheets of micrometer length and with thicknesses of approximately 5 nm.
Furthermore, the acyclic nature of the monomer allows for functionalization of the
sheets from the respective N- or C-termini, increasing the potential application of
the created surface in fields such as catalysis, drug delivery and sensing.
Figure 2 Schematic representation of free-standing nanosheets formed via hydrogen-bonded peptide
assembly in two dimensions supplemented by hydrophobic effects from aliphatic chains.
TEM image reprinted from Ref. [12] published under a creative commons licence (CC BY).
Work from De Yoreoʼs group has further shown that peptides are a strong choice for
2D assemblies.[13] A peptide containing seven amino acid residues acylated at the N-termini and amidated
at the C-termini with high binding affinity to MoS2 assembles when incubated on this inorganic surface. Sheets of 0.7 nm thickness and
1 µm in length are formed and appear to have a bias of direction for assembly that
can be explained by their affinity to sulfur. The peptides first form dimers through
hydrogen bond pairing at the C-terminus, which are then propagated through further
hydrogen bonding and van der Waals interactions between internal residues of the dimers
to form a 2D assembly. These sheets orient preferentially in three directions within
the plane of the MoS2 surface with an offset of 60° between one another, which relates to the densest sulfur
packing axes of the MoS2. In contrast to the previous example, peptides align themselves parallel to the surface
to maximize association with sulfur and between one another.
Following a more generalized approach, Li et al. reported in 2021 that a hydrogen-bonded
organic framework (HOF) can assemble to create layers with micrometer dimensions and
a thickness of around 1 nm.[14] A double imidization reaction on a napthalenic dianhydride with two diaminotriazoles
forms imide monomers, which can then self-assemble in solution through hydrogen bonding.
The network is propagated through free amino groups on the triazole moiety interacting
with the carbonyl groups on the naphthalenes, showcasing the importance of the repeating
unit structure. Like many hydrogen-bonded structures, treatment with a strong acid
or base disassembles the HOF into its monomers, highlighting the reversibility of
these non-covalent interactions. The sheets are reinforced with π–π interactions between
the naphthalene groups to generate thin layers.
Weaker Interaction Modes
Unlike strong directional interactions, which originate from a specific site, weaker
interactions arise from a fragment within the molecule and often have a lower directionality
than hydrogen bonding. This results in a less defined conformation of assembly and
a weaker association across the structure due to the variability in constituency.
Thus, these approaches are often labelled as non-conventional methods for self-assembly
as they usually rely more on cooperative forces that emerge as a function of the tertiary
structure or the environment, lacking the strength to hold together suprastructures.
However, recent advances in supramolecular chemistry, illustrated by the examples
included in this short review, showcase 1D and 2D supramolecular structures formed
from monomers that have to date not been classical choices for self-assemblies.
Preorganized π–π Interactions
Preorganized π–π Interactions
Molecules with an abundance of π-electrons can interact with one another to provide
a driving force for assembly, usually in tandem with another weak interaction. Whilst
we acknowledge that the strength of these interactions is size-dependent and may not
always be sufficient to be termed as “π–π interaction”, we include π-systems of any
size that exhibit coplanar arrangement in this section of the review.[15] Preorganization of the monomers via covalent bonding to a rigid backbone that holds
the π-surfaces in close proximity can greatly contribute to a successful assembly,
forming highly ordered 2D structures.
One reported example of 2D sheets includes the supramolecular “push–pull” synergetic
strategy used by Huang and co-workers in which an attractor/(steric)repeller monomer
adopts a propeller structure ([Figure 3]).[16] This approach suppresses the interlayer 3D stacking through sterics between repellers
while maintaining the assembly of the intralayer for 2D growth, driven by π–π interactions
between pyrene units. Formation of well-defined 2D nanosheets obtained from a solution
of THF was observed, with a side-length ratio of 1.93 and a height of 56 nm. In addition,
a different assembly was obtained as a 2D crystal film from surface-assisted layer-by-layer
assembly from a bilayer solution (toluene/water) with a thickness of 25 nm. These
self-assembled 2D materials show notable charge mobility, a high photoluminescence
quantum yield and deep-blue laser characteristics.
Figure 3 Schematic representation of a lipid bilayer mimic reported by Huang and co-workers.
Well-defined crystal nanosheets and millimeter-sized crystal films with layered amphiphile-like
packing were observed from the assembly of propeller-shaped monomers. SEM image reprinted
with permission from Ref. [16]. Copyright 2021 John Wiley and Sons.
Another example of preorganized π–π interactions using a covalent linkage was reported
by Wennemers et al. in 2016.[17] Formation of hierarchical supramolecular self-assemblies occurs from the π–π association
of sterically demanding perylene monoimides (PMIs) connected to an oligoproline backbone
acting as an insulating shell, and the effect of structural modifications on these
monomers is studied. Formation of fibers was observed in the case of two repeating
units of the monomer and sheets in the case of three, which were obtained from a solution
of THF/water with widths up to 200 nm. In 2017, the same group reported a triaxial
supramolecular weave consisting of organized organic threads formed by self-assembly
of building blocks containing a rigid oligoproline backbone with additional residues
between two PMI π-surfaces ([Figure 4]).[18] π-Stacking of chromophores leads to spatially defined threads stabilized by cross
CH–π interactions between adjacent PMI groups leading to further assembly of the threads
into a triaxial woven structure. The self-assembled weave was precipitated from a
THF/water solution of the monomer, revealing a highly ordered morphology of flat hexagonal
structures. These sheets showed diameters over 1 µm and thicknesses over 100 nm, with
formation of defined hexagonal pores within the interwoven network. Iridium nanoparticles,
which are of interest in catalysis and gas storage, can be prepared from Ir(cod)acac
that acts as a guest to the pores.
Figure 4 Schematic representation of a Kagome weave reported by Wennemers et al. prepared
from organic threads formed by π–π self-assembly of building blocks containing two
π-surfaces connected to a rigid oligoproline backbone. TEM image adapted with permission
from Ref. [18]. Copyright 2017 Springer Nature.
The last set of examples in this section highlights work from Schlüter and co-workers,
which results in 2D materials after photo-irradiation of a preorganized π–π network
to form covalent bonds. Whilst we would consider these materials to no longer be supramolecular
in nature, the underlying design principle is supramolecular and highly relevant.
In several works, tripodal monomers with blades containing π-surfaces (anthracenes,
alkynes) stack face-to-face to form a hexagonal lattice to favor maximized packing
([Figure 5]).[19] The π–π interactions are sufficient to hold these layers in place, but do not prevent
growth in a third dimension, affording a crystalline material. Following photo-irradiation
to induce cycloaddition between adjacent monomers and exfoliation from 1-methyl-2-pyrrolidone,
2D covalent sheets are obtained. In a more recently developed method with the purpose
of avoiding the exfoliation process, extended triptycenes with a hydrophilic group
at one end were used at an air/water interface.[20] Single-layer supramolecular preorganization into a hexagonal packing array followed
by photocyclization and compression at the interface directly yielded a 2D covalent
sheet.
Figure 5 Model of a π–π preorganized 3D crystal composed of tripodal monomers that is later
photo-irradiated and exfoliated to yield 2D covalent sheets. AFM image adapted with
permission from Ref. [19a]. Copyright 2012 Springer Nature.
Hydrophobic Interactions
Another approach to build 2D supramolecular materials in the absence of strong directional
interactions is based on the hydrophobicity of the monomers. Hydrophobic interactions
can become the driving force to build self-assemblies of different dimensions and
properties when association of a monomer containing a hydrophobic core with the appropriate
side chains takes place in a hydrophilic solvent.
In 2016, Lee et al. reported the self-assembly of monomers based on a design combining
anthracene units (hydrophobic π-core) and polyethylene glycol chains (hydrophilic
outer shell) that gives rise to the formation of static and dynamic sheets prepared
from two separate geometric isomers.[21] Aggregation takes place in a two-step process from an aqueous solution. The weak
π–π association between anthracenes yields primary nanofibers that then undergo lateral
association driven by hydrophobicity in order to reduce contact between aromatic units
and water ([Figure 6]). The cis-isomer yields static planar sheets and the trans-isomer forms dynamic sheets, which can be reversibly rolled as a result of thermal
dehydration of dendrimer side chains that causes twisting of adjacent macrocyclic
cores. The mixed solution of both isomers exhibits self-sorting behavior.
Figure 6 Schematic representation of thermoresponsive sheets reported by Lee et al. where
lateral association of primary nanofibers is based on geometric macrocyclic isomers
and driven by hydrophobicity of the aromatic cores in an aqueous solution. Cryo-TEM
image reprinted with permission from Ref. [21]. Copyright 2016 John Wiley and Sons.
Stupp et al. reported in 2014 the formation of self-assembling hydrogel scaffolds
for photocatalytic hydrogen production in which the design of the monomer is based
on a PMI unit connected to a carboxylic acid through a five-carbon linker.[22] This has shown to pack in an anti-parallel fashion into a highly interdigitated
bilayer, forming supramolecular ribbons of around 40 nm in width and several micrometers
long. The formation of these 2D structures takes place in aqueous solution and is
a result of the hydrophobic collapse of the aromatic core. Additionally, when an aqueous
solution of the nanoribbons was doped with different salts, formation of a gel was
observed with a network of flat sheet-like structures.
Space-Filling Design
Formation of large-area molecular films with long-range 2D structural integrity up
to the centimeter length scale was reported in 2015 by Fukushima et al. relying on
a space-filling design ([Figure 7]).[23] By taking advantage of the C3 symmetry of a triptycene monomer, previously reported for its liquid crystalline
properties,[24] and its inherent geometry, three-fold interdigitation of π-surfaces for 2D growth
is permitted. The three-bladed propeller shape of the monomer, with benzene rings
at 120° to one another, allows for interpenetration of propeller parts in a 2D hexagonal
packing array. In a second step, these assemblies stack vertically to form multilayers
with long-range structural order. The assembled structures were observed by vacuum
evaporation, spin-coating and cooling of the isotropic liquid of the triptycene.
Figure 7 Model of a hexagonally packed 2D array of three-bladed monomers held together through
a space-filling design and segregating alkyl chains. Image adapted with permission
from Ref. [23]. Copyright 2015 American Association for the Advancement of Science.
Curvature-Assisted Interactions
Curvature-Assisted Interactions
The influence of curvature in an aggregation-competent monomer has not been studied
systematically to a large extent. With the improved access to non-planar π-systems
in recent years, π-bowls[25] have found their way into the supramolecular world. They are mostly biased scaffolds
that facilitate growth in one direction, where they can be functionalized at specific
points to optimize polymerization; however, little consideration has been given to
the shape as an ordering factor in self-assembly. Saddles have been studied to a much
lesser degree in self-assembling systems due to their challenging syntheses. In the
rare cases where their assembly was carefully studied, mainly 1D stacks were observed.
Itami and co-workers have shown that warped nanographenes can stack into nanofibers
solely based on dipolar π–π stacking and are assisted by a negative curvature.[26]
We have recently reported the first example of a saddle-shaped monomer able to self-assemble
into monolayered 2D nanosheets of over 1 µm in length and single-molecule thickness
formed from a solution of toluene ([Figure 8]).[27] This example showcases the impact topography can have to enforce order within assemblies
through restriction of rotation and translation between monomers. The simplicity in
the monomer design implies that association into 1D stacks is primarily driven by
π–π interactions and assisted by shape, such that the individual effects alone would
be insufficient. Weak van der Waals interactions then drive the lateral association
of linear stacks into sheets and short alkyl chains enable layer segregation. Pairing
these modes of association results in highly defined sheets, demonstrating that the
curvature should not be overlooked as a possible approach towards supramolecular polymers.
Figure 8 Shape-assisted nanosheet formation reported by Rickhaus and co-workers where monomers
are forced into columnar stacks by π–π interactions that are assisted by the shape
of the monomer. Lateral association of the columns into nanosheets takes place through
van der Waals interactions. TEM images reprinted from Ref. [27] published under a creative commons licence (CC BY).
Complementary Interaction Modes
Complementary Interaction Modes
Successful assemblies are dependent on many factors, of which the encoded “strongest”
interaction is one of them. Sometimes, however, multiple interactions are introduced
in a balanced design, as exemplified by the following examples. On almost all occasions,
a combination of most of the previously mentioned interactions within the same monomer
allows access to suprastructures that would not be possible with a single interaction
alone. However, this can be most clearly seen with the following assemblies.
One such case is a porphyrin-based monomer reported by Sugiyasu and co-workers in
2017 incorporating hydrogen bonding motifs and alkyl chains on the periphery ([Figure 9]).[28] The authors observed formation of fibers following a nucleation–elongation model
with hydrogen bonding between amide groups and π–π stacking between porphyrin cores
being the main driving forces. Although the influence of the alkyl chain length was
considered negligible for 1D assembly, van der Waals forces among the alkyl chains
play a vital role leading to the formation of 2D self-assemblies. The thermodynamic
pathway for forming sheets was facilitated through the use of longer alkyl chains
for lateral assembly, overcoming the kinetic preference of fiber formation. This example
showcases the drastic change in assembly morphology at the expense of one methylene
group.
Figure 9 Model of oblique porphyrin layers obtained by Sugiyasu et al. When the alkyl chain
is shorter than hexyl, fibers are obtained as the thermodynamic minimum, whereas sheets
are obtained with longer side chains. AFM image adapted with permission from Ref.
[28]. Copyright 2017 Springer Nature.
Feng et al. reported in 2013 the formation of 2D nanostructures by the assembly of
n-type thiophene-armed tetraazaanthracene molecules with peripheral alkyl chains of
different lengths.[29] π–π stacking, S–S interactions and weak hydrogen bonding from electron-rich N heteroatoms
in the molecular backbone primarily lead to assembly, which is then further enhanced
by van der Waals interactions arising from the side chains to form 2D layers ([Figure 10]). Self-assemblies of these layers in binary solvent systems yield alkyl-length-dependent
2D nanosheet morphologies (sheets obtained from C6H13 and C12H25 alkyl chains, spherical aggregates obtained from C18H37, and rods obtained from a branched C8H17).
Figure 10 Schematic representation of lamellar sheets obtained by Feng et al. from π–π stacking
of a tetraazaanthracene monomer in which electron-rich heteroatoms create multiple
weak intermolecular interactions and alkyl chains define the morphology of the sheets
by van der Waals interactions. TEM image reprinted with permission from Ref. [29]. Copyright 2013 John Wiley and Sons.
Finally, the last example that highlights the power of orthogonal interactions is
an amphiphilic hexa-perihexabenzocoronene (HBC) derivative that was reported by Aida et al. in 2004.[30] The monomer bears two long alkyl chains on one side of the aromatic core and two
triethylene glycol chains on the other side. Assembly of this derivative from THF
and THF/water was studied, affording 1D columns of π-stacked HBC units which form
graphitic layers that further stack onto a bilayer tape connected by interpenetration
of the alkyl chains. Tight rolling-up of the 2D structure results in nanotubes where
both the internal and external surfaces are covered by the hydrophilic triethylene
glycol chains.
Conclusions and Outlook
In summary, the examples presented above show how hydrogen-bonded supramolecular polymers
are amongst the most studied 2D materials, which is attributed to the highly directional
interactions they possess. However, this does not mean that other modes of association
should be overlooked. With advances in synthesis and greater understanding of weaker
interactions, new routes towards 2D materials have been uncovered using π–π stacking,
hydrophobicity and the effects of curvature or topography. Combining a number of these
weaker effects provides a powerful toolkit to balance competing interactions in order
to form new materials we would otherwise think impossible. With these design concepts,
greater effort can be put to preparation of new 2D materials with the purpose of expanding
their scope and accessibility. It is not unimaginable that novel 2D organic electronics
may one day be used to replace more scarcely available resources (e.g., yttrium, neodymium),
that porous 2D materials may help to capture and catalytically remove harmful agents
(e.g., CO2, CFCs, pathogens) and that 2D devices may be prototypes for more efficient renewable
energy production (e.g., photovoltaics).
Funding Information
M. R. gratefully acknowledges funding from the Swiss National Science Foundation (grant
PZ00P2_180 101).
