Introducing a π-Skeleton Perpendicular to the Central Methylene Carbon in Alkanediamides: Design of Supramolecular Polymers with an Offset π-Stacking Arrangement

The self-assembly behavior of a heptanediamide derivative that contains a four-ring fused π-skeleton on its central methylene carbon atom has been examined. This molecule, which also contains two octyl chains, gelated the nonpolar solvent methylcyclohexane through the formation of fibrous nanostructures with hydrogen-bonding networks through a cooperative nucleation-elongation process. The supramolecular polymerization is accompanied by bathochromic shifts of both the absorption and fluorescence bands while maintaining a fluorescence quantum yield comparable to that of the monomeric state. Theoretical calculations provided an energetically stable structure, in which the π-skeletons are stacked with an offset of more than 8.0 Å, replicating the experimentally observed absorption change due to exciton coupling. Moreover, a slow transition with an inversion of the chiral arrangement of the π-conjugated moieties was induced by replacing the octyl chains with chiral alkyl chains. Our molecular-design strategy was further applied to a five-ring fused π-skeleton, which also forms an offset π-stacking arrangement and exhibits more effective chiral exciton coupling in the aggregated state.

Handedness-Inverted and Stimuli-Responsive Circularly Polarized Luminescent Nano/Micromaterials Through Pathway-Dependent Chiral Supramolecular Polymorphism

The precise manipulation of supramolecular polymorphs has been widely applied to control the morphologies and functions of self-assemblies, but is rarely utilized for the fabrication of circularly polarized luminescence (CPL) materials with tailored properties. Here, this work reports that an amphiphilic naphthalene-histidine compound (NIHis) readily self-assembled into distinct chiral nanostructures through pathway-dependent supramolecular polymorphism, which shows opposite and multistimuli responsive CPL signals. Specifically, NIHis display assembly-induced CPL from the polymorphic keto tautomer, which become predominant during enol-keto tautomerization shifting controlled by a bulk solvent effect. Interestingly, chiral polymorphs of nanofiber and microbelt with inverted CPL signals can be prepared from the same NIHis monomer in exactly the same solvent compositions and concentrations by only changing the temperature. The tunable CPL performance of the solid microbelts is realized under multi external physical or chemical stimuli including grinding, acid fuming, and heating. In particular, an emission color and CPL on-off switch based on the microbelt polymorph by reversible heating-cooling protocol is developed. This work brings a new approach for developing smart CPL materials via supramolecular polymorphism engineering.

Pathway-dependent supramolecular polymerization by planarity breaking

In controlled supramolecular polymerization, planar π-conjugated scaffolds are commonly used to predictably regulate stacking interactions, with various assembly pathways arising from competing interactions involving side groups. However, the extent to which the nature of the chromophore itself (planar vs. non-planar) affects pathway complexity requires clarification. To address this question, we herein designed a new BOPHY dye 2, where two oppositely oriented BF2 groups induce a disruption of planarity, and compared its supramolecular polymerization in non-polar media with that of a previously reported planar BODIPY 1 bearing identical substituents. The slightly non-planar structure of the BOPHY dye 2, as evident in previously reported X-ray structures, together with the additional out-of-plane BF2 group, allow for more diverse stacking possibilities leading to two fiber-like assemblies (kinetic 2A and thermodynamic 2B), in contrast to the single assembly previously observed for BODIPY 1. The impact of the less rigid, preorganized BOPHY core compared to the planar BODIPY counterpart is also reflected in the stronger tendency of the former to form anisotropic assemblies as a result of more favorable hydrogen bonding arrays. The structural versatility of the BOPHY core ultimately enables two stable packing arrangements: a kinetically controlled antiparallel face-to-face stacking (2A), and a thermodynamically controlled parallel slipped packing (2B) stabilized by (BF2) F⋯H (meso) interactions. Our findings underscore the significance of planarity breaking and out-of-plane substituents on chromophores as design elements in controlled supramolecular polymerization.

Increasing Dimensionality in Self-Assembly: Toward Two-Dimensional Supramolecular Polymers

Different approaches to achieve 2D supramolecular polymers, as an alternative to the covalent bottom-up approaches reported for the preparation of 2D materials, are reviewed. The significance of the operation of weak non-covalent forces to induce a lateral growth of a number of self-assembling units is collected. The examples of both thermodynamically and kinetically controlled formation of 2D supramolecular polymers showed in this review demonstrate the utility of this strategy to achieve new 2D materials with biased morphologies (nanosheets, scrolls, porous surfaces) and showing elegant applications like chiral recognition, enantioselective uptake or asymmetric organic transformations. Furthermore, elaborated techniques like seeded or living supramolecular polymerizations have been demonstrated to give rise to complex 2D nanostructures.

Two-Dimensional Supramolecular Polymerization of a Bis-Urea Macrocycle into a Brick-Like Hydrogen-Bonded Network

We report on a dendronized bis-urea macrocycle 1 self-assembling via a cooperative mechanism into two-dimensional (2D) nanosheets formed solely by alternated urea-urea hydrogen bonding interactions. The pure macrocycle self-assembles in bulk into one-dimensional liquid-crystalline columnar phases. In contrast, its self-assembly mode drastically changes in CHCl3 or tetrachloroethane, leading to 2D hydrogen-bonded networks. Theoretical calculations, complemented by previously reported crystalline structures, indicate that the 2D assembly is formed by a brick-like hydrogen bonding pattern between bis-urea macrocycles. This assembly is promoted by the swelling of the trisdodecyloxyphenyl groups upon solvation, which frustrates, due to steric effects, the formation of the thermodynamically more stable columnar macrocycle stacks. This work proposes a new design strategy to access 2D supramolecular polymers by means of a single non-covalent interaction motif, which is of great interest for materials development.

Amphiphile desymmetrisation-induced steric relief governs self-assembly pathways in aqueous media

Herein, we show that a straightforward desymmetrisation of a bolaamphiphilic chromophore can tune aromatic interactions and exciton coupling upon self-assembly. As a result, multiple assembled states become accesible offering a facile approach to induce pathway complexity in aqueous media.

Pathway-Controlled Aqueous Supramolecular Polymerization via Solvent-Dependent Chain Conformation Effects

Solute-solvent interactions play a critical role in multiple fields, including biology, materials science, and (physical) organic, polymer, and supramolecular chemistry. Within the growing field of supramolecular polymer science, these interactions have been recognized as an important driving force for (entropically driven) intermolecular association, particularly in aqueous media. However, to date, solute-solvent effects remain poorly understood in the context of complex self-assembly energy landscapes and pathway complexity. Herein, we unravel the role of solute-solvent interactions in controlling chain conformation effects, allowing energy landscape modulation and pathway selection in aqueous supramolecular polymerization. To this end, we have designed a series of oligo(phenylene ethynylene) (OPE)-based bolaamphiphilic Pt(II) complexes OPE2-4 bearing solubilizing triethylene glycol (TEG) chains of equal length on both molecule ends, but a different size of the hydrophobic aromatic scaffold. Strikingly, detailed self-assembly studies in aqueous media disclose a different tendency of the TEG chains to fold back and enwrap the hydrophobic molecular component depending on both the size of the core and the volume fraction of the co-solvent (THF). The relatively small hydrophobic component of OPE2 can be readily shielded by the TEG chains, leading to only one aggregation pathway. In contrast, the decreased capability of the TEG chains to effectively shield larger hydrophobic cores (OPE3 and OPE4) enables different types of solvent quality-dependent conformations (extended, partly back-folded and back-folded), which in turn induce various controllable aggregation pathways with distinct morphologies and mechanisms. Our results shed light on previously underappreciated solvent-dependent chain conformation effects and their role in governing pathway complexity in aqueous media.

Living supramolecular polymerization of an amphiphilic aza-BODIPY dye realized by water-assisted kinetic trapping

The kinetic assembly pathways of a newly synthesized amphiphilic aza-BODIPY dye 1 were tuned by using H₂O as a co-solvent in MeOH. Accordingly, the biphasic aggregation pathways resulting in kinetically-trapped and thermodynamically stable aggregates of 1 were established and the multiple cyclic seeded living supramolecular polymerization of this dye was realized.

Anti-cooperative Self-Assembly with Maintained Emission Regulated by Conformational and Steric Effects

Herein, we present a strategy to enable a maintained emissive behavior in the self-assembled state by enforcing an anti-cooperative self-assembly involving weak intermolecular dye interactions. To achieve this goal, we designed a conformationally flexible monomer unit 1 with a central 1,3-substituted (diphenyl)urea hydrogen bonding synthon that is tethered to two BODIPY dyes featuring sterically bulky trialkoxybenzene substituents at the meso-position. The competition between attractive forces (H-bonding and aromatic interactions) and destabilizing effects (steric and competing conformational effects) limits the assembly, halting the supramolecular growth at the stage of small oligomers. Given the presence of weak dye-dye interactions, the emission properties of molecularly dissolved 1 are negligibly affected upon aggregation. Our findings contribute to broadening the scope of emissive supramolecular assemblies and controlled supramolecular polymerization.

Macroscopic Supramolecular Assembly Strategy to Construct 3D Biocompatible Microenvironments with Site-Selective Cell Adhesion

Three-dimensional (3D) scaffolds with chemical diversity are significant to direct cell adhesion onto targeted surfaces, which provides solutions to further control over cell fates and even tissue formation. However, the site-specific modification of specific biomolecules to realize selective cell adhesion has been a challenge with the current methods when building 3D scaffolds. Conventional methods of immersing as-prepared structures in solutions of biomolecules lead to nonselective adsorption; recent printing methods have to address the problem of switching multiple nozzles containing different biomolecules. The recently developed concept of macroscopic supramolecular assembly (MSA) based on the idea of "modular assembly" is promising to fabricate such 3D scaffolds with advantages of flexible design and combination of diverse modules with different surface chemistry. Herein we report an MSA method to fabricate 3D ordered structures with internal chemical diversity for site-selective cell adhesion. The 3D structure is prepared via 3D alignment of polydimethylsiloxane (PDMS) building blocks with magnetic pick-and-place operation and subsequent interfacial bindings between PDMS based on host/guest molecular recognition. The site-specific cell affinity is realized by distributing targeted building blocks that are modified with polylysine molecules of opposite chiralities: PDMS modified with films containing poly-l-lysine (PLL) show higher cell density than those with poly-d-lysine (PDL). This principle of selective cell adhesion directed simply by spatial distribution of chiral molecules has been proven effective for five different cell lines. This facile MSA strategy holds promise to build complex 3D microenvironment with on-demand chemical/biological diversities, which is meaningful to study cell/material interactions and even tissue formation.

Pathway and Length Control of Supramolecular Polymers in Aqueous Media via a Hydrogen Bonding Lock

Programming the organization of π-conjugated systems into nanostructures of defined dimensions is a requirement for the preparation of functional materials. Herein, we have achieved high-precision control over the self-assembly pathways and fiber length of an amphiphilic BODIPY dye in aqueous media by exploiting a programmable hydrogen bonding lock. The presence of a (2-hydroxyethyl)amide group in the target BODIPY enables different types of intra- vs. intermolecular hydrogen bonding, leading to a competition between kinetically controlled discoidal H-type aggregates and thermodynamically controlled 1D J-type fibers in water. The high stability of the kinetic state, which is dominated by the hydrophobic effect, is reflected in the slow transformation to the thermodynamic product (several weeks at room temperature). However, this lag time can be suppressed by the addition of seeds from the thermodynamic species, enabling us to obtain supramolecular polymers of tuneable length in water for multiple cycles.

Hydrophobic domain flexibility enables morphology control of amphiphilic systems in aqueous media

In this work, we unravel the impact of hydrophobic domain flexibility on the self-assembly pathways and aggregate morphology of amphiphilic systems in aqueous media.

Alignment of supramolecular J-aggregates based on uracil-functionalized BODIPY dye for polarized photoluminescence

The H-bonding directed supramolecular J-aggregates of a uracil-functionalized BODIPY dye are uniaxially aligned through the rubbing method to generate highly polarized photoluminescence.