Entropy-Enthalpy Compensation in Solvent Geometry Regulated Supramolecular Polymerization of Luminescent Napthalimide via a Non-Cooperative, Isodesmic Mechanism

Supramolecular polymers of π-conjugated systems are an important class of materials with fascinating functions and properties originated from the dynamic behavior and highly ordered molecular organizations. Here, a donor-π-acceptor based functionalized luminescent napthalene monoimide (NMI) undergoes J-type self-assembly by non-covalent interactions via a non-cooperative, isodesmic mechanism to form supramolecular 1D nanowire. The fundamental insights into the thermodynamics regulating the supramolecular polymerization were derived through the fitting of the isodesmic model to variable temperature UV/Vis data in linear (dodecane) and nonliner hydrocarbon (decalin) based solvents. This shows a significant role of entropy-enthalpy compensation in solvent geometry-regulated formation and stabilization of supramolecular polymer. Furthermore, we have quantitively estimated the influence of solvent geometry and found that NMI forms stronger self-assembly and spontaneous gel in linear hydrocarbon based solvent compared to nonliner one and thereby substantially increases the degree of polymerization in linear hydrocarbon solvent (dodecane). This is accredited to the effective influence of the linear hydrocarbon solvent molecules in the polymerization process by favourable van der waals interactions with the peripheral alkyl chains of the NMI monomers in contrast to unfavourable interaction of nonliner hydrocarbon solvent due to geometry mismatch.

K. AK, Sagar S, Das S, Varghese R. Blue-emissive two-component supergelator with aggregation-induced enhanced emission

Two-component organogels offer several advantages over one-component gels, but their design is highly challenging. Hence, it is extremely important to design new approaches for the crafting of two-component organogels with interesting optical and mechanical properties. Herein, we report the design of a new class of two-component supergelators obtained from the assembly between acid functionalized tetraphenylethylene (TPE)-based dendrons and alkylated melamine. No gelation behaviour is observed for the individual components, but interestingly, remarkable gelation behaviour is observed for their hydrogen-bonded complex. The primary driving force responsible for the gelation is the strong π–π stacking interaction of TPE units. Because of the strong π-stacking of TPEs in the gel state, the C(sp²)–C(sp²) bond rotation of the TPE segment is completely arrested in the gel state, which results in intense fluorescence emission of the gels. Furthermore, excellent elastic response is observed for the gels as evident from their high storage modulus compared to loss modulus values. Our results clearly demonstrate that by the appropriate selection of the molecular components, this approach can be applied for the creation of functional nanomaterials with emergent properties absent in the individual blocks.

Design of a novel class of two-component, highly emissive, low molecular weight supergelator is reported.

Photoresponsive Circular Supramolecular Polymers: A Topological Trap and Photoinduced Ring-Opening Elongation

Topological features of one-dimensional macromolecular chains govern the properties and functionality of natural and synthetic polymers. To address this issue in supramolecular polymers, we synthesized two topologically distinct supramolecular polymers with intrinsic curvature, circular and helically folded nanofibers, from azobenzene-functionalized supramolecular rosettes. When a mixture of circular and helically folded nanofibers was exposed to UV light, selective unfolding of the latter open-ended supramolecular polymers was observed as a result of the curvature-impairing internal force produced by the trans-to-cis photoisomerization of the azobenzene. This distinct sensitivity suggests that the topological features of supramolecular polymers define their mechanical stability. Furthermore, the exposure of circular supramolecular polymers in more polar media to UV irradiation resulted in ring opening followed by chain elongation, thus demonstrating that the circular supramolecular polymer can function as a topological kinetic trap.

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.

H-Aggregated π-Systems Based on Disulfide-Linked Dimers of Dipyrrolyldiketone Boron Complexes

Dipyrrolyldiketone boron complexes linked by a disulfide bond were synthesized, forming H-aggregated dimers assisted by intramolecular π-π and hydrogen-bonding interactions. The conformations of the dimers, with small C-S-S-C dihedral angles, were examined by UV-vis absorption and ¹H NMR spectra as well as single-crystal X-ray analysis and theoretical studies.

Self-assembled chiral helical nanofibers by amphiphilic dipeptide derived from d- or l-threonine and application as a template for the synthesis of Au and Ag nanoparticles

The discovery of a class of self-assembling peptides that spontaneously undergo self-organization into well-ordered structures opened a new avenue for molecular fabrication of biological materials. In this paper, the structure controlled helical nanofibers were prepared by two artificial β-sheet dipeptides with long alkyl chains derived from l- and d-threonine (Thr) and sodium hydroxide (NaOH). These helical nanofibers have been characterized using transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), circular dichroism (CD), Fourier transform infrared (FT-IR) spectroscopy, and X-ray powder diffraction (XRD). It was demonstrated that the helicity of the nanofibers could be easily controlled by changing the chirality of the constituent amino acids in the peptide species (d- or l-threonine). Moreover, the hydrogen bonding interactions between the amide groups as well as the hydrophobic interactions among the alkyl chains play important roles in the self-assembly process. It also can be observed that with the passage of time, the hydrogen bonding interactions between the individual nanofiber induced the conversion from nanofibers to nanobelts. Particularly, gold and silver nanoparticles performed good catalytic ability were synthesized using the assembled nanofibers as template.

Superstructure Formation and Topological Evolution Achieved by Self-Organization of a Highly Adaptive Dynamer

The adaptive property of supramolecular building blocks facilitates noncovalent synthesis of soft materials. While it is still a challenging task, fine-tuning and precise control over topological nanostructures constructed from the self-assembly of low-molecular-weight building blocks are an important research direction to investigate the structure-property relationship. Herein, we report controlled self-assembly evolution of a low-molecular-weight building block bearing cholesterol and naphthalene-dicarboximide moieties, showing ultrasensitivity to solvent polarity. In low-polarity solvents (<4), it could form an M-type fiber-constituted organogel (supergel) with high solvent content, columnar molecular packing, and self-healing property. Highly polar solvents (>7.8) favor the formation of P-type helical nanostructures terminated by nanotoroids, having lamellar molecular packing. With a further increase in solvent polarity (up to 9.6), unilamellar and multilamellar vesicles were generated, which could undergo an aggregation-induced fusion process to form branched nanotubes tuned by the concentration. Self-attractive interactions between aggregates were found to be responsible for the formation of superstructures including helix-nanotoroid junctions as well as membrane-fused nanotubes.

Helical supramolecular organization of a 1,2-diol appended naphthalene diimide organogelator via an extended intermolecular H-bonding network

Hydrogen bonded self-assembly of a 1,2-diol linked naphthalene diimide derivative features M-helical and J-type aggregation. In MCH/CHCl₃, the compound exhibits intense yellow excimer and thermoreversible “sol–gel” behavior.

Mixed diacetylene/octadecyl melamine nanowires formed at the air/water interface exhibit unique structural and colorimetric properties

Polydiacetylene (PDA) assemblies exhibit interesting photophysical properties, specifically, visible colorimetric transformations. A considerable body of work has focused on the formation and characterization of PDA Langmuir monolayer systems, and the overwhelming majority of reports so far have indicated that the adoption of 2D sheetlike structures associated with a hydrogen bond network between the diacetylene headgroups is a prerequisite for polymerization and chromatic properties. Here we report for the first time on the assembly of nanowire networks in mixed Langmuir monolayers comprising diacetylene monomers and octadecyl melamine surfactants. Structural and physical analysis indicates that the nanowires are composed of a helical organization of stacked diacetylene/octadecyl melamine building blocks assembled through hydrogen bonds between the melamine residues and the carboxylic termini of the diacetylenes. Following ultraviolet-induced polymerization, the PDA/octadecyl melamine nanowires exhibited unusual chromatic properties, specifically, an absence of the ubiquitous "blue" phase, rather transforming into a new "purple" PDA phase. This study demonstrates that the incorporation of surfactant constituents within diacetylene frameworks provides a means for modulating the structural and chromatic features of PDA assemblies, giving rise to new morphologies and unique optical properties.

Gathering nanorings via Fe2+–bipyridine coordination

Fe²⁺–bipyridine coordination was used to assemble nanorings into supernanostructures.

Hierarchical preferences of hydroxylated oxanorbornane-based achiral amphiphiles

Achiral amphiphiles with hydroxylated oxanorbornane headgroups showed specific morphological characteristics and hierarchical preferences depending upon the nature of lipophilic units. Detailed scanning electron microscopic (SEM) studies showed that twisted ribbonlike aggregates are characteristic of monoalkoxyaryl lipids with hydrocarbon chain length in the range C10-C13; these systems also had a preference toward lamellar arrangement. Asymmetric packing of these lipids is a unique occurrence and shows that the presence of molecular chirality is not an absolute requirement for curvature effects in their supramolecular assemblies. Aryl units in these systems were found to be important for the observed morphological preferences, which became evident from comparative studies involving simple long chain esters without this moiety. Single-crystal X-ray diffraction analysis of one of the lipids from the latter group gave finer details of strong and weak secondary interactions, which operate during their assembly process. Introduction of more than one alkyl chain on the aromatic ring caused a notable shift in the packing propensity toward columnar arrangement. Most of these cone-shaped molecules were found to give doughnut-shaped aggregates from acetone solution through the intermediary of fibrous structures, which was confirmed through SEM, transmission electron microscopic, and atomic force microscopic studies.

Control over hierarchy levels in the self-assembly of stackable nanotoroids

We report a precise control over the hierarchy levels in the outstanding self-organization process shown by chiral azobenzene dimer 1. This compound forms uniform toroidal nanostructures that can hierarchically organize into chiral nanotubes under the control by temperature, concentration, or light. The nanotubes further organized into supercoiled fibrils, which finally intertwined to form double helices with one-handed helical sense.

Determinants of cyanuric acid and melamine assembly in water

While the recognition of cyanuric acid (CA) by melamine (M) and their derivatives has been known to occur in both water and organic solvents for some time, analysis of CA/M assembly in water has not been reported (Ranganathan, A.; Pedireddi, V. R.; Rao, C. N. R. J. Am. Chem. Soc.1999, 121, 1752-1753; Mathias, J. P.; Simanek, E. E.; Seto, C. T.; Whitesides, G. M. Macromol. Symp.1994, 77, 157-166; Zerkowski, J. A.; MacDonald, J. C.; Seto, C. T.; Wierda, D. A.; Whitesides, G. M. J. Am. Chem. Soc.1994, 116, 2382-2391; Mathias, J. P.; Seto, C. T.; Whitesides, G. M. Polym. Prepr.1993, 34, 92-93; Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc.1993, 115, 905-916; Zerkowski, J. A.; Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc.1992, 114, 5473-5475; Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc.1990, 112, 6409-6411; Wang, Y.; Wei, B.; Wang, Q. J. Chem. Cryst.1990, 20, 79-84; ten Cate, M. G. J.; Huskens, J.; Crego-Calama, M.; Reinhoudt, D. N. Chem.-Eur. J.2004, 10, 3632-3639). We have examined assembly of CA/M, as well as assembly of soluble trivalent CA and M derivatives (TCA/TM), in aqueous solvent, using a combination of solution phase NMR, isothermal titration and differential scanning calorimetry (ITC/DSC), cryo-transmission electron microscopy (cryo-TEM), and synthetic chemistry. While the parent heterocycles coprecipitate in water, the trivalent system displays more controlled and cooperative assembly that occurs at lower concentrations than the parent and yields a stable nanoparticle suspension. The assembly of both parent and trivalent systems is rigorously 1:1 and proceeds as an exothermic, proton-transfer coupled process in neutral pH water. Though CA and M are considered canonical hydrogen-bonding motifs in organic solvents, we find that their assembly in water is driven in large part by enthalpically favorable surface-area burial, similar to what is observed with nucleic acid recognition. There are currently few synthetic systems capable of robust molecular recognition in water that do not rely on native recognition motifs, possibly due to an incomplete understanding of recognition processes in water. This study establishes a detailed conceptual framework for considering CA/M heterocycle recognition in water which enables the future design of molecular recognition systems that function in water.

Synthesis, characterization, self-assembly, and physical properties of 11-methylbenzo[d]pyreno[4,5-b]furan

Synthesis, structure, and physical properties of a novel 11-methylbenzo[d]pyreno[4,5-b]furan (BPF) and its self-assembly in water have been reported. The performance of nanowire-based films in organic light-emitting diodes is much better than that of the thin film deposited by directly drop-coating BPF molecules in THF solution. SEM study indicates that the well-organized structure (nanowires) is an important factor in enhancing the performance of OLED devices.

Fluorescence and morphology modulation in a photochromic diarylethene self-assembly system

Switchable supramolecular self-assemblies on the basis of interaction between melamine group containing photochromic diarylethene unit (DTE) and naphthalimide derivate (1) were designed and fabricated. 1 can gelate several aprotic solvents with different morphologies. The gel turned into partial gel in ethyl acetate with the addition of DTE as a guest molecule. Both the absorption and fluorescence spectra of the assembly can be reversibly switched by alternating UV/visible light irradiation. Meanwhile, the morphology of the coassembly of 1(2)·DTE changed to film from original pieces of gel 1 in ethyl acetate. When 1(2)·DTE was irradiated by UV light, the film morphology was converted into aggregated flakes. Moreover, the surface wettability of the complex can also be switched by light irradiation. The photochromic diarylethene unit is able to modulate the fluorescence and morphology of the assembled system only by virtue of light irradiation. Therefore, these results provide further insights into fluorescence and morphology controlling, especially application in upscale smart responsive materials.

Rational construction of perylene bisimide columnar superstructures with a biased helical sense

Discotic supramolecular complexes bearing six perylene bisimide (PBI) chromophores were prepared by mixing monotopically triple-hydrogen-bonding melamines equipped with two PBI chromophores and two 3,7-dimethyloctyl chiral handles with tritopically triple-hydrogen-bonding cyanuric acid (CA). UV/Vis and fluorescence titration experiments demonstrated that the discotic complexes were formed in methylcyclohexane by the 3:1 complexation between the melamines and CA. TEM and AFM studies revealed that the complexes hierarchically organize into fibrous columnar assemblies, which eventually results in the formation of organogels. Circular dichroism (CD) and flash-photolysis time resolved microwave conductivity measurements revealed the presence of extended chiral stacks of PBI chromophores within the columns. The anisotropy factors of the columnar assemblies are remarkably high (g=1.5×10(-3)) when considering the presence of only one 3,7-dimethyloctyl chiral handle per perylene chromophore, suggesting that the columnar structures have a biased helical sense. The fact that the chiral centers are located inside the discotic complexes rather than at their peripheries might be unique structural property responsible for the rather strong optical activities for the assemblies of this chromophore. The effective transcription of the molecular chirality to the extended columnar assemblies through the formation of unique discotic complexes enables the expression of "majority-rules" chiral amplification effect, which is unprecedented for the supramolecular assemblies of PBIs.