High-fidelity self-assembly pathways for hydrogen-bonding molecular semiconductors

Phase Transitions of Naphthalene-2,3-carbonitride Steered by Solvent Effects and Metal Ion Concentration Variation

The delicate regulation of structural phase transition can provide advanced approaches for fabricating desired and well-organized nanoarchitectures on surfaces. Introduction of metal ions into pure organic systems can facilitate the phase transition from hydrogen-bonded structures to metal-organic structures by coordinating with organic molecules. However, it remains a challenge to attain a phase transition dominated by variable metal coordination configurations through adjustment of the metal ion concentration. Herein, we report the phase transitions of naphthalene-2,3-carbonitride (2,3-DCN) molecules on highly oriented pyrolytic graphite (HOPG) under varying solvents and Cu2+ ion concentrations. By integrating data from scanning tunneling microscopy imaging and density functional theory calculations, it is demonstrated that phase transitions of 2,3-DCN occur through forming diverse coordination configurations where Cu2+ ions can coordinate with 2,3-DCN and 1-nonanoic acid or Cl- ions to form different ligand components with a coordination number of 4 when varying the molar ratios of 2,3-DCN to Cu2+ ion in the 1-nonanoic acid solvent. However, in the case of 1-heptanoic acid as a solvent, the self-assembly structure of 2,3-DCN only changes via the alteration of hydrogen bonding sites and Cu2+ ions do not coordinate with 2,3-DCN molecules. These findings provide valuable insights into the coordination behavior of metal ions in different solvents.

A Coformer Approach for Supramolecular Polymerization at High Concentrations

Insolubility of functional molecules caused by polymorphism sometimes poses limitations for their solution-based processing. Such a situation can also occur in the preparation processes of supramolecular polymers formed in a solution. An effective strategy to address this issue is to prepare amorphous solid states by introducing a "coformer" molecule capable of inhibiting the formation of an insoluble polymorph through co-aggregation. Herein, inspired by the coformer approach, we demonstrated a solubility enhancement of a barbiturate π-conjugated compound that can supramolecularly polymerize through six-membered hydrogen-bonded rosettes. Our newly synthesized supramolecular coformer molecule features a sterically demanding methyl group in the π-conjugated unit of the parent molecule. Although the parent molecule exhibits low solubility in nonpolar solvents due to the formation of a crystalline polymorph comprising a tape-like hydrogen-bonded array prior to the supramolecular polymerization, mixing with the coformer compound enhanced the solubility by inhibiting mesoscopic organization of the tapes. The two monomers were then co-polymerized into desired helicoidal supramolecular polymers through the formation of heteromeric rosettes.

Anion-assisted supramolecular polymerization of luminescent organic π-conjugated chromophores in a moderately polar solvent: tunable nanostructures and their corresponding effects on electronic properties

Supramolecular polymers of π-conjugated organic chromophores have emerged as promising candidates in organic electronics because of their dynamic and highly ordered molecular organization. Herein, we demonstrate the formation of luminescent, highly conducting supramolecular polymers of a functionalized naphthalimide π-chromophore-based organic semiconductor in a moderately polar organic solvent (tetrahydrofuran) by overcoming solute-solvent H-bonding via assistance from fluoride anions. The polymerization is exclusively guided by the synergistic effects of cascade H-bonding (F-⋯H-N- of primary amines, followed by -CO⋯H-N- of amides), π-π stacking and hydrophobic interactions. An increasing molar equivalent of anions leads to a morphology transition from 1D nanowires to 2D nanosheets via nanotubes and nanorings, but above a particular threshold of the same anion, depolymerization-mediated disruption of long-range order and formation of non-luminescent spherical particles was observed. Such significant impacts of anions in supramolecular polymerization-depolymerization were utilized in modulating the electronic properties of this naphthalimide-based organic semiconductor.

Two-dimensional molecular networks at the solid/liquid interface and the role of alkyl chains in their building blocks

Nanoarchitectonics has attracted increasing attention owing to its potential applications in nanomachines, nanoelectronics, catalysis, and nanopatterning, which can contribute to overcoming global problems related to energy and environment, among others. However, the fabrication of ordered nanoarchitectures remains a challenge, even in two dimensions. Therefore, a deeper understanding of the self-assembly processes and substantial factors for building ordered structures is critical for tailoring flexible and desirable nanoarchitectures. Scanning tunneling microscopy is a powerful tool for revealing the molecular conformations, arrangements, and orientations of two-dimensional (2D) networks on surfaces. The fabrication of 2D assemblies involves non-covalent interactions that play a significant role in the molecular arrangement and orientation. Among the non-covalent interactions, dispersion interactions that derive from alkyl chain units are believed to be weak. However, alkyl chains play an important role in the adsorption onto substrates, as well as in the in-plane intermolecular interactions. In this review, we focus on the role of alkyl chains in the formation of ordered 2D assemblies at the solid/liquid interface. The alkyl chain effects on the 2D assemblies are introduced together with examples documented in the past decades.

Supramolecular polymerization of thiobarbituric acid naphthalene dye

2-Thiobarbituric acid-functionalized naphthalene dye selectively self-assembles into crystalline fibers to show material properties that are different from those of a previously reported oxo-barbituric acid derivative affording curved supramolecular polymers via...

Peptide-Protein Interactions: From Drug Design to Supramolecular Biomaterials

The self-recognition and self-assembly of biomolecules are spontaneous processes that occur in Nature and allow the formation of ordered structures, at the nanoscale or even at the macroscale, under thermodynamic and kinetic equilibrium as a consequence of specific and local interactions. In particular, peptides and peptidomimetics play an elected role, as they may allow a rational approach to elucidate biological mechanisms to develop new drugs, biomaterials, catalysts, or semiconductors. The forces that rule self-recognition and self-assembly processes are weak interactions, such as hydrogen bonding, electrostatic attractions, and van der Waals forces, and they underlie the formation of the secondary structure (e.g., α-helix, β-sheet, polyproline II helix), which plays a key role in all biological processes. Here, we present recent and significant examples whereby design was successfully applied to attain the desired structural motifs toward function. These studies are important to understand the main interactions ruling the biological processes and the onset of many pathologies. The types of secondary structure adopted by peptides during self-assembly have a fundamental importance not only on the type of nano- or macro-structure formed but also on the properties of biomaterials, such as the types of interaction, encapsulation, non-covalent interaction, or covalent interaction, which are ultimately useful for applications in drug delivery.

Cooperative assembly of H-bonded rosettes inside a porphyrin nanoring

The melamine·barbiturate H-bonded rosette motif is of comparable dimensions and symmetry to the cavity of a butadiyne-linked 6-porphyrin nanoring. Functionalisation of each of the barbiturate components and the pyrimidine components of a H-bonded rosette with a pyridine ligand leads to a self-assembled hexapyridine ligand, which binds cooperatively to the zinc porphyrin nanoring. UV-vis-NIR and ¹H NMR experiments show that the 7-component assembly forms at concentrations at which neither the H-bonding interactions nor the zinc porphyrin–pyridine interactions are formed in the absence of one of the three components. The mean effective molarities of these rosette complexes are around 200 mM in chloroform at 298 K.

Mixing barbiturates and pyrimidines equipped with pyridine ligands to leads to self-assembly of a hexadentate rosette ligand, which is complementary to a hexameric zinc porphyrin macrocycle.

Diacetylene-Functionalized Dendrons: Self-Assembled and Photopolymerized Three-Dimensional Networks for Advanced Self-Healing and Wringing Soft Materials

The physical properties of supramolecular soft materials strongly depend on the molecular packing structures constructed by thermodynamically and kinetically controlled molecular self-assembly. To investigate the relationship between molecular function and self-assembled molecular packing structure, a series of diacetylene (DA)-based supramolecules was synthesized by chemically connecting flexible dendrons to DA with amide (aDA-D) or ester (eDA-D) functions. The three-dimensional (3D) organogel network of amide-functionalized aDA-D was prepared in both polar and nonpolar solvents due to the intermolecular hydrogen bonding. 3D networks of aDA-D can be further stabilized by topochemical photopolymerization. The self-healing behavior of aDA-D was observed in the sheet-like structure formed in n-dodecane by the hydrophobic interaction between the gelator and solvent. The wringing behavior of aDA-D was also demonstrated using the dynamic interaction of amide function with n-butanol solvent. Kinetically controlled and photostabilized 3D networks can be a key component from biomedical devices to soft robotic applications.

Hydrogen Bonding as a Supramolecular Tool for Robust OFET Devices

In the present study, we demonstrated the effect of hydrogen bonding in the semiconducting behaviour of a small molecule used in organic field-effect transistors (OFETs). For this study, the highly soluble dumbbell-shaped molecule, Boc-TATDPP based on a Boc-protected thiophene-diketopyrrolopyrrole (DPP) and triazatruxene (TAT) moieties was used. The two Boc groups of the molecule were removed by annealing at 200 °C, which created a strong hydrogen-bonded network of NH-TATDPP supported by additional π-π stacking. These were characterised by thermogravimetric analysis (TGA), UV/Vis and IR spectroscopy, XRD and high-resolution (HR)-TEM measurements. FETs were fabricated with the semiconducting channel made of Boc-TATDPP and NH-TATDPP separately. It is worth mentioning that the Boc-TATDPP film can be cast from solution and then annealed to get the other systems with NH-TATDPP. More importantly, NH-TATDPP showed significantly higher hole mobilities compared to Boc-TATDPP. Interestingly, the high hole mobility in the case of NH-TATDPP was unaffected upon blending with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁ BM). Thus, this robust hydrogen-bonded supramolecular network is likely to be useful in designing efficient and stable organic optoelectronic devices.

Supramolecular Polymers Capable of Controlling Their Topology

One important class of supramolecular materials is one-dimensionally elongated supramolecular polymers, in which monomers are associated by reversible intermolecular interactions, yielding a fibrous morphology. Unlike frequently reported conventional supramolecular polymers based on, for instance, host-guest interactions, those composed of one-dimensionally stacked π-conjugated molecules can be encoded with high degrees of internal order by cooperative association of the rigid aromatic monomers, endowing such supramolecular polymers with extraordinary properties and functionality. However, their internal order has not yet been exploited to manipulate the complex landscape of well-defined states of the supramolecular polymer backbone, which may induce new functionalities beyond the intrinsic properties of the backbones. This Account will focus on the inceptive phase of our research on supramolecular polymers with high degrees of internal order able to impart intrinsic curvature to their backbones. Initially, we developed a naphthalene molecule functionalized with barbituric acid, which forms uniform toroidal short fibers with diameters of approximately 16 nm via the formation of hydrogen-bonded cyclic hexamers (rosettes). As we thought the uniformity of the toroid size to arise from the intrinsic curvature generated upon stacking of the rosettes, we exploited this intrinsic curvature to design continuously curved extended supramolecular polymers by extension of such molecular π-systems. The intrinsic curvature produced by the monomers with more expanded π-systems indeed gave us access to higher-order structures (topologies) ranging from randomly folded to helically folded coils in extended supramolecular polymers. We will discuss the kinetic aspects of the generation of intrinsic curvature for topology control, including the formation of toroidal structures resulting from ring-closing processes. For extended supramolecular polymers with well-defined topologies, we will discuss manipulation of a complex landscape of well-defined states by external stimuli. The incorporation of a photoresponsive azobenzene chromophore in the original naphthalene molecular scaffold allowed us to reversibly destroy or recover the curvature of the main chain through trans- cis photoisomerization. By means of this photocontrollable curvature, we have demonstrated light-induced unfolding of helically folded structures into entirely stretched structures. Furthermore, a direct extension of the π-conjugated core provided us with access to unprecedented supramolecular polymers with emergent time-dependent topology transitions. Molecules with a naphthalene core conjugated with two phenylene units kinetically afforded supramolecular polymers that consist of helically folded and misfolded domains. Upon aging the supramolecular polymer solution, we observed spontaneous folding of the misfolded domains in a time scale of days, eventually obtaining a supramolecular polymer topology analogous to the tertiary structure of proteins. These supramolecular polymers with unrivaled and active topologies provide new prospects for supramolecular polymers as one-dimensional nanomaterials.

Oligoprolines guide the self-assembly of quaterthiophenes

Oligoprolines of differing lengths control the self-assembly of quaterthiophenes into mono-layered or double-layered sheets, or helically twisted ribbons.

Control over the molecular organization of π-conjugated oligothiophenes into different types of supramolecular assemblies is key to their use in organic electronics but difficult to achieve as these chromophores have a pronounced tendency to aggregate. Herein we show that oligoprolines, which do not self-assemble on their own, control the self-assembly of quaterthiophenes. Spectroscopic, microscopic, and diffraction studies with quaterthiophene–oligoproline conjugates revealed the formation of mono- or double-layered sheets or, alternatively, helically twisted ribbons – depending on the length of the oligoproline. The dimensions of the nanoscopic objects, which extend into the micrometer regime, correlate with the molecular dimensions of the quaterthiophene–oligoproline building blocks.

Probing functional self-assembled molecular architectures with solution/solid scanning tunnelling microscopy

Over the past two decades, solution/solid STM has made clear contributions to our fundamental understanding of the thermodynamic and kinetic processes that occur in molecular self-assembly at surfaces. As the field matures, we provide an overview of how solution/solid STM is emerging as a tool to elucidate and guide the use of self-assembled molecular systems in practical applications, focusing on small molecule device engineering, molecular recognition and sensing and electronic modification of 2D materials.

Native DNA electronics: is it a matter of nanoscale assembly?

The genomic DNA is enveloped by nanotubes formed by the nuclear aggregates of polyamines (NAPs) that induce DNA conformational changes and provide protection and increased interaction abilities for the double strands. In a physiological environment, the nanotube arrangement is initiated by spontaneous interaction among the terminal amino groups of the polyamines and the phosphate ions, with the consequent formation of cyclic monomers that hook at the DNA grooves. The polymer thus formed has the morphological features of an organic semiconductor and therefore, it can be considered to be able to conduct electric charges. Phosphate ions positioned on the NAP external surface could regulate, as in a physical electric circuit, both linear and rotational (histones) protein motion, in accordance with the basilar principles of the electronics. A model of a carrier system for protein motion along the polymer wrapping the DNA strands, based on the phosphate-phosphate complexation, is proposed.

Towards the Prediction of Global Solution State Properties for Hydrogen Bonded, Self-Associating Amphiphiles

Through this extensive structure-property study we show that critical micelle concentration correlates with self-associative hydrogen bond complex formation constant, when combined with outputs from low level, widely accessible, computational models. Herein, we bring together a series of 39 structurally related molecules related by stepwise variation of a hydrogen bond donor-acceptor amphiphilic salt. The self-associative and corresponding global properties for this family of compounds have been studied in the gas, solid and solution states. Within the solution state, we have shown the type of self-associated structure present to be solvent dependent. In DMSO, this class of compound show a preference for hydrogen bonded dimer formation, however moving into aqueous solutions the same compounds are found to form larger self-associated aggregates. This observation has allowed us the unique opportunity to investigate and begin to predict self-association events at both the molecular and extended aggregate level.

Impact of helical organization on the photovoltaic properties of oligothiophene supramolecular polymers

Higher order structures of semiconducting supramolecular polymers have a huge impact on their BHJ-OPV device performance.

Helical self-assembly of functional π-conjugated molecules offers unique photochemical and electronic properties in the spectroscopic level, but there are only a few examples that demonstrate their positive impact on the optoelectronic device level. Here, we demonstrate that hydrogen-bonded tapelike supramolecular polymers of a barbiturated oligo(alkylthiophene) show notable improvement in their photovoltaic properties upon organizing into helical nanofibers. A tapelike hydrogen-bonded supramolecular array of barbiturated oligo(butylthiophene) molecules was directly visualized by STM at a liquid–solid interface. TEM, AFM and XRD revealed that the tapelike supramolecular polymers further organize into helical nanofibers in solution and bulk states. Bulk heterojunction solar cells of the helical nanofibers and soluble fullerene showed a power conversion efficiency of 4.5%, which is markedly high compared to that of the regioisomer of butyl chains organizing into 3D lamellar agglomerates.

Hydrogen-bonded rosettes comprising π-conjugated systems as building blocks for functional one-dimensional assemblies

Hydrogen-bonded supermacrocycles (rosettes) are attractive disk-shaped noncovalent synthons for extended functional columnar nanoassemblies. They can serve not only as noncovalent monomer units for supramolecular polymers and discrete oligomers in a dilute solution but also as constituent entities for soft matters such as gels and lyotropic/thermotropic liquid crystals. However, what are the merits of using supramolecular rosettes instead of using expanded π-conjugated covalent molecules? This review covers the self-assembly of photochemically and electrochemically active π-conjugated molecules through the formation of supramolecular rosettes via directional complementary multiple hydrogen-bonding interactions. These rosettes comprising π-conjugated covalent functional units stack into columnar nanoassemblies with unique structures and properties. By overviewing the design principle, characterization, and properties and functionalities of various examples, we illustrate the merits of utilizing rosette motifs. Basically, one can easily access a well-defined expanded π-surface composed of multi-chromophoric systems, which can ultimately afford stable extended nanoassemblies even in a dilute solution due to the higher association constants of supermacrocyclized π-systems. Importantly, these columnar nanoassemblies exhibit unique features in self-assembly processes, chiroptical, photophysical and electrochemical properties, nanoscale morphologies, and bulk properties. Moreover, the stimuli responsiveness of individual building blocks can be amplified to a greater extent by exploiting rosette intermediates to organize them into one-dimensional columnar structures. In the latter parts of the review, we also highlight the application of rosettes in supramolecular polymer systems, photovoltaic devices, and others.

Towards quantifying the role of hydrogen bonding within amphiphile self-association and resultant aggregate formation

The role of hydrogen bonding within aggregate formation and CMC: can these properties be predicted by low level computational modelling?

Herein, we present a series of five tetrabutylammonium (TBA) sulfonate–urea amphiphilic salts. In solution these amphiphilic salts have been shown to form a variety of self-associated species. The proportion and type of which are both solvent and concentration dependent. In DMSO-d₆ a variety of NMR experiments provide evidence towards the formation of mainly dimeric over larger aggregate species. Increasing the percentage of water was shown to increase the concentration of the larger aggregates over dimers in solution. A correlation was established between critical micelle concentration (CMC) values obtained in a 1 : 19 EtOH : H₂O mixture, dimeric self-association constants obtained in a DMSO-d₆ – 0.5% H₂O and the results of simple semi-empirical PM6 computational modelling methods. This approach begins to quantify the role of hydrogen bonding in amphiphile self-association and the effects it imparts on surfactant properties. This consequently provides preliminary evidence that these properties maybe predicted by simple low level computational modelling techniques.