Self-Assembly of Metallo-Supramolecular Amphiphiles based on Azadipyrromethenes: Consecutive Pathways to Nanodiscs and Nanosheets

A new class of metallo-supramolecular amphiphilic dyes 1 a, b was constructed by using two azadipyrromethene units which were respectively modified with two hydrophobic alkyl and two hydrophilic oligo(ethylene glycol) chains. The spectroscopic and morphological studies revealed the consecutive self-assembly pathways of 1 a in EtOH/H2O mixed solvent. The monomers of 1 a firstly aggregated into the kinetic-controlled, nanodisc-shaped Agg. I upon cooling and the latter spontaneously transformed into the thermodynamically more stable Agg. II with a nanosheet morphology. While the non-fluorescent Agg. I displayed a broad absorption band (λmax=615 nm), the Agg. II displayed a more intensive and narrowed J-band (λmax=693 nm) and a fluorescence band with a maximum at 760 nm (Φfl=0.1), which could be ascribed to the J-aggregation induced emission enhancement. The kinetics of Agg. I to Agg. II transformation was further modulated by seed-initiated supramolecular polymerization with various ratios of Seedagg.II, in which the transformation rate was significantly increased by ca. 2 orders of magnitude compared to the spontaneous process. The supramolecular amphiphile 1 b bearing longer hydrophilic chains formed only one type aggregate, which exhibited spectroscopic and morphological properties that were highly comparable with that of Agg. I.

Colorimetric Polydiacetylene Nanotubes from Self-Assembly of a Barbituric Acid-Derived Diacetylene

Covalent organic nanotubes offer enhanced stability, robustness, and functionality, compared to their noncovalent counterparts. This study explores constructing polydiacetylene (PDA) nanotubes using a two-step process: self-assembly via noncovalent interactions followed by UV-induced polymerization of a diacetylene template. A promising building block PCDA-BA consisting of a hydrogen-bonding headgroup, barbituric acid, linked to a linear diacetylene chain was prepared. Through self-complementary hydrogen bonding arising from barbituric acid and π-π stacking of diacetylene template directs molecular ordering to form a tapelike molecular arrangement, which then transforms to bilayer lamellar sheets that scroll into nanotubes with increasing solvent polarity. Fourier transform infrared spectroscopy and powder X-ray diffraction patterns show both single-wall and multiple-wall nanotubes, depending on the scrolling pathway. These noncovalent structures convert into covalently linked blue-phase chromogenic nanotubes (P(PCDA-BA)) via UV-induced polymerization. The blue phase P(PCDA-BA) shows promising potential as a colorimetric sensor material with significant reversible thermoreversibility up to 160 °C for multiple thermal cycles and hydrazine sensing capabilities. This study highlights the significance of molecular integration design in constructing covalent nanotubes with chromogenic properties.

Design and Photonics of Merocyanine Dyes

Merocyanines, thanks to their easily adjustable electronic structure, appear to be the most versatile and promising functional dyes. Their D-π-A framework offers ample opportunities for custom design through variations in both donor/acceptor end-groups and the π-conjugated polymethine chain, and leads to a broad range of practical properties, including noticeable solvatochromism, high polarizability/hyperpolarizabilities, and the ability to sensitize various physicochemical processes. Accordingly, merocyanines are applied and extensively studied in various fields, such as light-converting materials for optoelectronics, nonlinear optics, optical storage, solar cells, fluorescent probes, and antitumor agents in photodynamic therapy. This review encompasses both classical and novel more important publications on the structure-property relationships in merocyanines, with particular emphasis on the results by A.  I. Kiprianov and his followers in Institute of Organic Chemistry in Kyiv, Ukraine.

Fluorescent supramolecular polymers of barbiturate dyes with thiophene-cored twisted π-systems

Because supramolecular polymerization of emissive π-conjugated molecules depends strongly on π-π stacking interaction, the formation of well-defined one-dimensional nanostructures often results in a decrease or only a small increase of emission efficiency. This is also true for our barbiturate-based supramolecular polymers wherein hydrogen-bonded rosettes of barbiturates stack quasi-one-dimensionally through π-π stacking interaction. Herein we report supramolecular polymerization-induced emission of two regioisomeric 2,3-diphenylthiophene derivatives functionalized with barbituric acid and tri(dodecyloxy)benzyl wedge units. In CHCl3, both compounds are molecularly dissolved and accordingly poorly emissive due to a torsion-induced non-radiative decay. In methylcyclohexane-rich conditions, these barbiturates self-assemble to form crystalline nanofibers and exhibit strongly enhanced emission through supramolecular polymerization driven by hydrogen-bonding. Our structural analysis suggests that the barbiturates form a tape-like hydrogen-bonding motif, which is rationalized by considering that the twisted geometries of 2,3-diphenylthiophene cores prevend the competing rosettes from stacking into columnar supramolecular polymers. We also found that a small difference in the molecular polarity originating from the substitutional position of the thiophene core influences interchain association of the supramolecular polymers, affording different luminescent soft materials, gel and nanosheet.

Asymmetric living supramolecular polymerization of an achiral aza-BODIPY dye by solvent-mediated chirality induction and memory

The kinetic self-assembly properties of an achiral aza-BODIPY dye 1 bearing two hydrophobic fan-shaped tridodecyloxybenzamide pendants through 1,2,3-triazole linkages was investigated in detail in chiral solvents (S)- and (R)-limonene by...

Supramolecular Nanowires from an Acceptor-Donor-Acceptor Conjugated Chromophore

Oligothiophene derivatives have been extensively studied as p-type semiconducting materials in organic electronics applications. This work reports the synthesis, self-assembly and photophysical properties of acceptor-donor-acceptor (A-D-A)-type oligothiophene derivatives by end-group engineering of quaterthiophene (QT) with naphthalene monoimide (NMI) chromophores that are further connected to a trialkoxy benzamide wedge. Conjugation to the NMI units reduces the HOMO-LUMO gap significantly, and consequently the absorption spectrum exhibits a bathochromic shift of about 50 nm compared with QT. Furthermore, extended H-bonding interactions among the amido groups of the peripheral wedges produce entangled fibrillar nanostructures and gelation in hydrocarbon solvents such as methylcyclohexane, wherein the A-D-A chromophore exhibits typical H-aggregation. On the contrary, the fact that the same chromophore, lacking only the amido units, does not produce gels or H-aggregates indicates strong impact of H-bonding on the self-assembly. Computational studies revealed the electronic properties of the chromophore and predicted the geometry of a dimer in the H-aggregate that reasonably matches with the experimental results. Bulk electrical conductivity measurements determined an excellent conductivity of 2.3×10^-2  S cm^-1 for the H-aggregated system (OT-1), which is two orders of magnitude higher than that of the same chromophore lacking the amido groups (OT-2).

Alkyl chain length effects on double-deck assembly at a liquid/solid interface

Controlled double-deck packing is an appealing means to expand upon conventional 2D self-assembly which is critical in crystal engineering, yet it is rare and poorly understood. Herein, we report the first systematic study of double-deck assembly in a series of alkylated aminoquinone derivatives at the liquid-solid interface. The competition between the fraction of alkyl chains adsorbed on the surface and the optimal conformation of the alkyl chains near the head group leads to a stepwise structural transformation ranging from complete double-deck packing to complete monolayer packing. Alkyl chains on the bottom or top layer of the double-deck assemblies were selectively visualized by carefully tuning the scanning tunneling microscopy settings. A method to easily identify mirror image domains was discovered based on the coincidence of domain boundaries with a graphite main axis. The effect of molecular symmetry and metal complexation on the formation of the double-deck assembly was also explored. Based on 2D crystal engineering principles, this bottom-up double-deck assembly can potentially provide an essential toehold for constructing precise 3D hierarchical structures.

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.

Gels with sense: supramolecular materials that respond to heat, light and sound

Advances in the field of supramolecular chemistry have made it possible, in many situations, to reliably engineer soft materials to address a specific technological problem. Particularly exciting are "smart" gels that undergo reversible physical changes on exposure to remote, non-invasive environmental stimuli. This review explores the development of gels which are transformed by heat, light and ultrasound, as well as other mechanical inputs, applied voltages and magnetic fields. Focusing on small-molecule gelators, but with reference to organic polymers and metal-organic systems, we examine how the structures of gelator assemblies influence the physical and chemical mechanisms leading to thermo-, photo- and mechano-switchable behaviour. In addition, we evaluate how the unique and versatile properties of smart materials may be exploited in a wide range of applications, including catalysis, crystal growth, ion sensing, drug delivery, data storage and biomaterial replacement.

Supramolecularly Engineered π-Amphiphile

This article describes self-assembly of supramolecularly engineered naphthalene-diimide (NDI)-derived amphiphiles NDI-1 and NDI-2. They have the same hydrophobic/hydrophilic balance but merely differ by a single functional group, amide or ester. They exhibit distinct self-assembly in water; NDI-1 forms hydrogel, which upon aging forms crystals, whereas NDI-2 forms micelles as revealed by in-depth structural analysis using cryo-TEM, dynamic light scattering, and small-angle X-ray scattering studies. These results suggest that the H-bonding among the amide groups fully regulates the self-assembly by overruling the packing parameters. Further, the present study elucidates sharp lower critical solution temperature exhibited by these π-amphiphiles, which has been extensively studied for many important applications of water-soluble polymers but hardly known in the literature of small-molecule surfactants. Control experiments with the same water-soluble hydrophilic wedge did not show such a property, confirming this to be a consequence of the supramolecular polymerization by extended amide-amide H-bonding and not inherent to the structure of the hydrophilic wedge containing oligo-oxyethylene chains.

Near-IR Absorbing J-Aggregate of an Amphiphilic BF2 -Azadipyrromethene Dye by Kinetic Cooperative Self-Assembly

A new amphiphilic BF₂ -azadipyrromethene (aza-BODIPY) dye 1 has been synthesized using a Cu^I -catalyzed "click" reaction. For this dye, two self-assembly pathways that lead to different type of J-aggregates with distinct near-infrared optical properties have been discovered. The metastable off-pathway product displays a broad, structureless absorption band while the thermodynamically stable on-pathway aggregate exhibits the characteristic spectral features of a J-aggregate, that is, red-shifted intense absorption band with significantly narrowed linewidth. The morphology and structure of the aggregates were studied by atomic force microscopy, transmission and scanning electron microscopy. The aggregation processes of 1 were investigated by temperature- and concentration-dependent UV/Vis spectroscopy and evaluated by models for cooperative self-assembly.

Exciton Coupling of Merocyanine Dyes from H- to J-type in the Solid State by Crystal Engineering

A key issue for the application of π-conjugated organic molecules as thin film solid-state materials is the packing structure, which drastically affects optical and electronic properties due to intermolecular coupling. In this regard, merocyanine dyes usually pack in H-coupled antiparallel arrangements while structures with more interesting J-type coupling have been rarely reported. Here we show that for three highly dipolar merocyanine dyes, which exhibit the same π-scaffold and accordingly equal properties as monomers in solution, the solid-state packing can be changed by a simple variation of aliphatic substituents to afford narrow and intense absorption bands with huge hypsochromic (H) or bathochromic (J) shifts for their thin films and nanocrystals. Time-dependent density functional theory calculations show that the energetic offset of almost 1 eV magnitude results from distinct packing motifs within the crystal structures that comply with the archetype H- or J-aggregate structures as described by Kasha's exciton theory.

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

The design of molecular systems with high-fidelity self-assembly pathways that include several levels of hierarchy is of primary importance for the understanding of structure-function relationships, as well as for controlling the functionality of organic materials. Reported herein is a high-fidelity self-assembly system that comprises two hydrogen-bonding molecular semiconductors with regioisomerically attached short alkyl chains. Despite the availability of both discrete cyclic and polymeric linear hydrogen-bonding motifs, the two regioisomers select one of the two motifs in homogeneous solution as well as at the 2D-confined liquid-solid interface. This selectivity arises from the high directionality of the involved hydrogen-bonding interactions, which renders rerouting to other self-assembly pathways difficult. In thin films and in the bulk, the resulting hydrogen-bonded assemblies further organize into the expected columnar and lamellar higher-order architectures via solution processing. The contrasting organized structures of these regioisomers are reflected in their notably different miscibility with soluble fullerene derivatives in the solid state. Thus, electron donor-acceptor blend films deliver a distinctly different photovoltaic performance, despite their virtually identical intrinsic optoelectronic properties. Currently, we attribute this high-fidelity control via self-assembly pathways to the molecular design of these supramolecular semiconductors, which lacks structure-determining long aliphatic chains.

Construction of Supramolecular Assemblies from Self-Organization of Amphiphilic Molecular Isomers

Amphiphilic coil-rod-coil molecules, incorporating flexible and rigid blocks, have a strong affinity to self-organize into various supramolecular aggregates in bulk and in aqueous solutions. In this paper, we report the self-assembling behavior of amphiphilic coil-rod-coil molecular isomers. These molecules consist of biphenyl and phenyl units connected by ether bonds as the rod segment, and poly(ethylene oxide) (PEO) with a degree of polymerization of 7 and 12 as the flexible chains. Their aggregation behavior was investigated by differential scanning calorimetry, thermal optical polarized microscopy, small-angle X-ray scattering spectroscopy, and transmission electron microscopy. The results imply that the molecular structure of the rod building block and the length of the PEO chains dramatically influence the creation of supramolecular aggregates in bulk and in aqueous solutions. In the bulk state, these molecules self-organize into a hexagonal perforated lamellar and an oblique columnar structure, respectively, depending on the sequence of the rod building block. In aqueous solution, the molecule with a linear rod segment self-assembles into sheet-like nanoribbons. In contrast, its isomer, with a rod building block substituted at the meta-position of the aryl group, self-organizes into nanofibers. This is achieved through the control of the non-covalent interactions of the rod building blocks.

Controlling the crystalline three-dimensional order in bulk materials by single-wall carbon nanotubes

The construction of ordered single-wall carbon nanotube soft-materials at the nanoscale is currently an important challenge in science. Here we use single-wall carbon nanotubes as a tool to gain control over the crystalline ordering of three-dimensional bulk materials composed of suitably functionalized molecular building blocks. We prepare p-type nanofibres from tripeptide and pentapeptide-containing small molecules, which are covalently connected to both carboxylic and electron-donating 9,10-di(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene termini. Adding small amounts of single-wall carbon nanotubes to the so-prepared p-nanofibres together with the externally controlled self assembly by charge screening by means of Ca(2+) results in new and stable single-wall carbon nanotube-based supramolecular gels featuring remarkably long-range internal order.

Nano- and microstructuration of supramolecular materials driven by H-bonded uracil·2,6-diamidopyridine complexes

In the last few decades, multiple H-bonded arrays have been shown to be versatile tools to prepare functional supramolecular materials. Supramolecular complexes formed by uracil (Ur) and 2,6-diamidopyridine (DAP) developed by Lehn are the first examples of multiple H-bonded systems governing the formation of supramolecular polymers in solution. Although a large variety of complementary multiple H-bonded complexes has been prepared, the use of the heteromolecular Ur·DAP complex still remains very promising due to its ease of preparation and its intermediate association strength that ensures a dynamical character to the self-assembly and self-organisation processes. In this feature article, we report a detailed account on the results that our group has obtained in this field by designing and engineering a novel library of shape persistent molecular modules able to transfer their geometrical information to the final supramolecular architectures through the formation of Ur·DAP complexes both at the nanoscopic and microscopic levels.