Balakrishna Bugga

Chemically stable polyarylether-based covalent organic frameworks

Chemically stable polyarylether-based covalent organic frameworks, Published online: 08 April 2019; doi:10.1038/s41557-019-0238-5

Competition leading to partnership: Study of aromatic bis‐urea derivatives in DMSO reveals their unexpectedly high acidity (pK _a≈14). Consequently, partial proton transfer occurs in their reaction with basic anions (AcO⁻ and H₂PO₄ ⁻). This process was quantitatively accounted for in the course of anion binding studies. Reliable stability constants of anion complexes (1:1 and 1:2, receptor/anion) were determined. Factors defining the anion binding properties of the studied ligand series are discussed.

Abstract

A series of aromatic bis‐urea derivatives was prepared and their proton dissociation, as well as anion binding properties in DMSO were investigated. To this end, UV/Vis and ¹H NMR spectroscopies and computational methods were employed. The synthesized molecules differed in the relative position of the urea moieties (ortho‐ and meta‐derivatives) and in the functional groups (−H, −CH₃, −OCH₃, −NO₂) in the para‐position of the pendant phenyl groups. Remarkably high acidities of the compounds (logK ₁ ^H≈14), were ascribed primarily to the stabilizing effect of the aromatic subunits. Quantum chemical calculations corroborated the conclusions drawn from experimental data and provided information from the structural point of view. Knowledge regarding protonation properties proved to be essential for reliable quantitative determination of anion binding affinities. Studied receptors were selective for acetate and dihydrogen phosphate among several anions. Formation of their complexes of 1:1 and 1:2 (ligand/anion) stoichiometries was quantitatively characterized. Proton transfer was taken into account in the course of data analysis, which was especially important in the case of AcO⁻. ortho‐Receptors were proven to be more efficient acetate binders, achieving coordination with all four NH groups. The meta‐analogues preferred dihydrogen phosphate, which acted as both hydrogen bond donor and acceptor. Cooperative binding was detected in the case of 1:2 H₂PO₄ ⁻ complexes, which was assigned to formation of interanionic hydrogen bonds.

The influence that the insertion method (post‐insertion vs. pre‐insertion) of “two‐wall” and “four‐wall” aryl‐extended calix[4]pyrrole carriers into liposomal membranes exerts on their chloride transport properties using the HPTS assay is described. The pre‐insertion method allowed a more accurate and reliable comparison of the transport activities of these carriers, being greater for the “four‐wall” carrier.

Abstract

We disclose the results of our investigations on the influence that the insertion method of aryl‐extended calix[4]pyrrole into liposomal membranes exerts on their properties as anion carriers. We use the standard HPTS assay to assess the transport properties of the carriers. We show that the post‐insertion of the carrier, as DMSO solution, assigns better transport activities to the “two‐wall” α,α‐aryl‐extended calix[4]pyrrole 1 compared to the “four‐wall” α,α,α,α‐counterpart 2. Notably, opposite results were obtained when the carriers were pre‐inserted into the liposomal membranes. We assign this difference to an improved incorporation of carrier 2 into the membrane when delivered by the pre‐insertion method. On the other hand, carrier 1 shows comparable levels of transport independently of the method used for its incorporation. Thus, an accurate comparison of the chloride transport activities featured by these two carriers demands their pre‐incorporation in the liposomal membranes. In contrast, using the lucigenin assay with the pre‐insertion method both carriers displayed similar transport efficiencies.

Hierarchical titanium(IV) helicates represent lithium‐dependent molecular switches that can be used as molecular balances to determine noncovalent interaction energies. In a bridged system removal or addition of lithium cations allows for a reversible switching between a compressed and expanded state, which in the case of chiral ligands can be even performed in a stereospecific manner.

Abstract

Catechol ligands with aldehyde, ketone or ester groups attached in 3‐position form, in the presence of titanium(IV) triscatecholate, titanium(IV) complexes. If lithium cations are the counterions, they can bind in a successive step to the salicylate units of the complex and form a dimeric triple‐lithium‐bridged dinuclear helicate. In solution, the dimer is in equilibrium with the monomer and the thermodynamics of the dimerization can be easily evaluated. Thus, the hierarchically assembled titanium(IV) helicates represent a lithium‐dependent molecular switch. The investigation of different derivatives of the complex allows for an estimation of the influence of side chain functionalities on the energetics of the dimerization. Thus, the hierarchically assembled helicates can be used as a kind of molecular balance to determine weak interaction energies (solvophobic effects and even dispersive effects). In addition, tethering of two ligands leads to “classical” helicates. Removal or addition of lithium cations allows for a reversible switching between a compressed and expanded state, which in the case of chiral ligands can be even performed stereospecifically.

Chemical computer: A novel logic platform was constructed based on a programmable supramolecular system, which could implement the complete set of contrary logic pair functions and further be integrated into a network pattern to execute advanced operations, possessing great potential for practical application.

Abstract

Molecular computation is increasingly attractive as a tool for medical and biological research because of its programmability and controllability. Herein, a novel visibly observable supramolecular system that can execute multi‐level logic functions on a uniform platform was constructed. By employing some programming factors, we succeeded in not only constructing a whole set of contrary logic pairs, but also building up a logic network that can implement advanced functions. Further, the platform is applied to sense thiols in specific environments. The developed method can efficiently filter signals of thiols in intracellular conditions and measure cysteine levels quantitatively in serum conditions. The visual readout makes the method particularly suitable for point‐of‐care testing. The supramolecule‐based platform illustrates not only an incremental advance for the construction of programmable molecular logic systems, but also viable applications in intelligent thiol analysis.

Click polymerization: Self‐assembly of amphiphilic alternating copolymers into diverse structures by solvent switching or emulsion‐assisted polymerization‐induced self‐assembly methods is summarized (see figure). The unique characteristics of these structures are determined both experimentally and theoretically.

Abstract

Polymer self‐assembly has been a hot research topic for several decades. Different types of polymers with various architectures, like block copolymers, brush polymers, hyperbranched polymers and dendrimers, etc., are currently being investigated. Alternating copolymers (ACPs) are regular copolymers with an alternating monomeric unit structure in the polymer backbones. However, despite the great progress in the synthesis of ACPs, their self‐assembly is still in an infant stage. Very recently, our group reported a new type of amphiphilic ACPs through click copolymerization and obtained spheres, vesicles, nanotubes, and even hierarchical sea urchin‐like aggregates through the self‐assembly process. In addition, we have found some intriguing features in the self‐assembly of amphiphilic ACPs when compared with other copolymers, including their facile syntheses, readily functionalization, novel self‐assembly structures, new folding‐chain mechanisms, and uniform but ultrathin feature length. In this Concept article, we present the self‐assembly of amphiphilic ACPs together with their unique features by reviewing our latest results and related studies. Moreover, the future perspective on the self‐assembly of amphiphilic ACPs is also proposed. Our aim is to capture the attention and interest of chemists in this new area of polymerization.