Bis‐Bipyridinium Gemini Surfactant‐Based Supramolecular Helical Fibers and Solid State Thermochromism

Ionic Covalent Organic Framework as a Dual Functional Sensor for Temperature and Humidity

Visual sensing of humidity and temperature by solids plays an important role in the everyday life and in industrial processes. Due to their hydrophobic nature, most covalent organic framework (COF) sensors often exhibit poor optical response when exposed to moisture. To overcome this challenge, the optical response is set out to improve, to moisture by incorporating H-bonding ionic functionalities into the COF network. A highly sensitive COF, consisting of guanidinium and diformylpyridine linkers (TG-DFP), capable of detecting changes in temperature and moisture content is fabricated. The hydrophilic nature of the framework enables enhanced water uptake, allowing the trapped water molecules to form a large number of hydrogen bonds. Despite the presence of non-emissive building blocks, the H-bonds restrict internal bond rotation within the COF, leading to reversible fluorescence and solid-state optical hydrochromism in response to relative humidity and temperature.

Iron ore production using a new Gemini surfactant at 273 K

Herein we present the use of a Gemini surfactant and reverse froth flotation to efficiently separate magnetite from quartz and produce iron ore at 273 K. This surfactant achieved an obviously superior flotation performance (TFe recovery increased by 48.18%), and the dosage of the Gemini surfactant was three times less than that of a conventional monomeric surfactant. Our findings are expected to serve as a general guide to design a new and excellent collector for high-efficiency mineral flotation and to lead to an efficient and clean development of mineral resources.

Light-Controlled Aggregation and Gelation of Viologen-Based Coordination Polymers

Ditopic bis-(triazole/pyridine)viologens are bidentate ligands that self-assemble into coordination polymers. In such photo-responsive materials, light irradiation initiates photo-induced electron transfer to generate π-radicals that can self-associate to form π-dimers. This leads to a cascade of events: processes at the supramolecular scale associated with mechanical and structural transition at the macroscopic scale. By tuning the irradiation power and duration, we evidence the formation of aggregates and gels. Using microscopy, we show that the aggregates are dense, polydisperse, micron-sized, spindle-shaped particles which grow in time. Using microscopy and time-resolved micro-rheology, we follow the gelation kinetics which leads to a gel characterized by a correlation length of a few microns and a weak elastic modulus. The analysis of the aggregates and the gel states vouch for an arrested phase separation process, a new scenario to supramolecular systems.

Unexpected three-state hydrochromism of a donor-acceptor self-complex with fluctuations in relative humidity

Water in our environment is ever present, particularly in our atmosphere, from which it may be adsorbed by materials hygroscopically. At the molecular level, the binding of water molecules to various materials is driven by weak interactions but can have profound effects on physical properties, including the donor-acceptor interactions in charge transfer (CT) salts. Herein we present the unexpected three-state hydrochromatic switching of a bipyridinium-based donor-acceptor self-complex with changes in relative humidity (RH) and subsequent stable hydrate formation. RH is typically an overlooked variable that can vary greatly. These findings suggest that care should be taken to consider fluctuations in RH when characterizing the solid state optical band gap and CT absorption bands for organic donor-acceptor CT salt complexes.

The Unusual Photochromic and Hydrochromic Switching Behavior of Cellulose-Embedded 1,8-Naphthalimide-Viologen Derivatives in the Solid-State

Stimuli-responsive chromic materials such as photochromics, hydrochromics, thermochromics, and electrochromics have a long history of capturing the attention of scientists due to their potential industrial applications and novelty in popular culture. However, hybrid chromic materials that combine two or more stimuli-triggered color changing properties are not so well known. Herein, we report a design strategy that has led to a series of emissive 1,8-naphthalimide-viologen dyads which exhibit unusual dual photochromic and hydrochromic switching behavior in the solid-state when embedded in a cellulose matrix. This behavior manifests as reversible solid state fluorescence hydrochromism upon changes in atmospheric relative humidity (RH), and reversible solid state photochromism upon generation of a cellulose-stabilized viologen radical cation. In this design strategy, the bipyridinium unit serves as both a water-sensitive receptor for the hydrochromic fluorophore-receptor system, and a photochromic group, capable of eliciting its own visible colorimetric response, generating a fluorescence quenching radical cation with prolonged exposure to ultraviolet (UV) light. These dyes can be inkjet-printed onto cellulose paper or drop-cast as cellulose powder-based films and can be unidirectionally cycled between three different states which can be characteristically visualized under UV light or visible light. The material's photochromism, hydrochromism, and underlying mechanism of action was investigated using computational analysis, dynamic vapor sorption/desorption isotherms, electron paramagnetic resonance spectroscopy, and variable humidity UV-Vis adsorption and fluorescence spectroscopies.

Stimuli-Sensitive Self-Assembled Tubules Based on Lysine-Derived Surfactants for Delivery of Antimicrobial Proteins

Drug delivery vectors based on amphiphiles have important features such as versatile physicochemical properties and stimuli-responsiveness. Amino acid-based surfactants are especially promising amphiphiles due to their enhanced biocompatibility compared to conventional surfactants. They can self-organize into micelles, vesicles and complex hierarchical structures, such as fibers, twisted and coiled ribbons, and tubules. In this work, we investigated the self-assembly and drug loading properties of a family of novel anionic double-tailed lysine-derived surfactants, with variable degree of tail length mismatch, designated as mLys10 and 10Lysn, where m and n are the number of carbon atoms in the tails. These surfactants form tubular aggregates with assorted morphologies in water that undergo gelation due to dense entanglement, as evidenced by light and electron microscopy. Lysozyme (LZM), an enzyme with antimicrobial properties, was selected as model protein for loading. After the characterization of the interfacial properties and phase behavior of the amphiphiles, the LZM-loading ability of the tubules was investigated, under varying experimental conditions, to assess the efficiency of the aggregates as pH- and temperature-sensitive nanocarriers. Further, the toxicological profile of the surfactants per se and surfactant/LZM hydrogels was obtained, using human skin fibroblasts (BJ-5ta cell line). Overall, the results show that the tubule-based hydrogels exhibit very interesting properties for the transport and controlled release of molecules of therapeutic interest.

Classical and non-classical melatonin receptor agonist-directed micellization of bipyridinium-based supramolecular amphiphiles in water

The addition of molecular recognition units into structures of amphiphiles is a means by which soft matter capable of undergoing template-directed micellization can be obtained. These supramolecular amphiphiles can bind with molecular templates using non-covalent bonding interactions, forming host-guest complexes that hold the amphiphiles together as they undergo micellization. In most cases, such templates are synthesized and designed for a specific molecular recognition motif. It is not clear, however, to what extent these types of amphiphile systems are responsive to members of a biologically derived class of molecular targets, for example, melatonin receptor agonists and their numerous isosteres. Herein, we describe the template-directed micellization and arrangement at the air-water interface of a bipyridinium-based gemini surfactant, driven by the influence of donor-acceptor CT interactions with a series of bioactive classical and non-classical melatonin isosteres. Under the conditions of templation by either 5-methoxytryptophol, N-acetylserotonin, N-acetyltryptamine, or the pharmaceutical agent agomelatine, favorable Gibbs free energies of micellization were observed with decreases in CMC by up to 70%, and concomitant increases of 28% in surface pressure, and decreases of 20% in contact angle versus untemplated solutions. Solid state thermochromic transition temperatures for inkjet-printed patterns of the templated amphiphile solutions were inversely correlated with trends observed for their respective CMCs, and exhibited no correlation to their binding constants. These findings contend for the generalizable use of melatonin receptor agonists as targets and/or templates for chemical systems, which rely on π-stacking donor-acceptor CT interactions in water to facilitate the actions of binding, sequestration, or template-directed self-assembly.

Augmented polyhydrazone formation in water by template-assisted polymerization using dual-purpose supramolecular templates

Template-assisted polymerization using donor–acceptor supramolecular templates results in higher M_w and M_n values, decreased critical hydrogelation concentrations, and increased gel recovery velocity following shear-induced breakdown.