Piezo-Responsive Hydrogen-Bonded Frameworks Based on Vanillin-Barbiturate Conjugates.

A concept of piezo-responsive hydrogen-bonded π-π-stacked organic frameworks made from Knoevenagel-condensed vanillin-barbiturate conjugates was proposed. Replacement of the substituent at the ether oxygen atom of the vanillin moiety from methyl (compound 3a) to ethyl (compound 3b) changed the appearance of the products from rigid rods to porous structures according to optical microscopy and scanning electron microscopy (SEM), and led to a decrease in the degree of crystallinity of corresponding powders according to X-ray diffractometry (XRD). Quantum chemical calculations of possible dimer models of vanillin-barbiturate conjugates using density functional theory (DFT) revealed that π-π stacking between aryl rings of the vanillin moiety stabilized the dimer to a greater extent than hydrogen bonding between carbonyl oxygen atoms and amide hydrogen atoms. According to piezoresponse force microscopy (PFM), there was a notable decrease in the vertical piezo-coefficient upon transition from rigid rods of compound 3a to irregular-shaped aggregates of compound 3b (average values of d33 coefficient corresponded to 2.74 ± 0.54 pm/V and 0.57 ± 0.11 pm/V), which is comparable to that of lithium niobate (d33 coefficient was 7 pm/V).

Melamine-Barbiturate Supramolecular Assembly as a pH-Dependent Organic Radical Trap Material.

In the last two decades, a large number of self-assembled materials were synthesized and they have already found their way into large-scale industry and science. Hydrogen-bond-based supramolecular adducts are found to have unique properties and to be perfect host structures for trapping target molecules or ions. Such chemical systems are believed to resemble living matter and can substitute a living cell in a number of cases. Herein, a report on an organic material based on supramolecular assembly of barbituric acid and melamine is presented. Surprisingly, the structure is found to host and stabilize radicals under mild conditions allowing its use for biological applications. The number of free radicals is found to be easily tuned by changing the pH of the environment and it increases when exposed to light up to a saturation level. We describe a preparation method as well as stability properties of melamine-barbiturate self-assembly, potentiometric titration, and hydrogen ions adsorption data and EPR spectra concerning the composite.

Melamine Barbiturate as a Light-Induced Nanostructured Supramolecular Material for a Bioinspired Oxygen and Organic Radical Trap and Stabilization.

Use of coantioxidant systems is a prospective way to increase the effectiveness of antioxidant species in tissue repair and regeneration. In this paper, we introduce a novel scheme of a reactive oxygen species (ROS) trap and neutralization during self-assembly of supramolecular melamine barbiturate material. The performed reaction chain mimics the biological process of ROS generation in key stages and enables one to obtain stable hydroperoxyl and organic radicals in a melamine barbiturate structure. Melamine barbiturate also neutralizes hydroxyl radicals, and the effectiveness of the radical trap is controlled with ROS scavenger incorporation. The number of radicals dramatically increases during light-inducing and depends on pH. The proposed scheme of the ROS trap and neutralization opens a way to the use of supramolecular assemblies as a component of coantioxidant systems and a source of organic radicals.

Biocompatible pH-Degradable Functional Capsules Based on Melamine Cyanurate Self-Assembly.

Development of adaptive self-regulating materials and chemical-biological systems-self-healing, self-regulating, etc.-is an advanced modern trend. The very sensitive pH-controlled functionality of supramolecular assemblies is a very useful tool for chemical and biochemical implementations. However, the assembly process can be tuned by various factors that can be used for both better functionality control and further functionalization such as active species, e.g., drugs and dyes, and encapsulation. Here, the effect of a dye, sodium fluorescein (uranine) (FL), on the formation of a self-assembled melamine cyanurate (M-CA) structure is investigated and calculated with density functional theory (DFT) and molecular dynamics. Interestingly, the dye greatly affects the self-assembly process at early stages from the formation of dimers, trimers, and tetramer to nucleation control. The supramolecular structure disassembly and subsequent release of trapped dye occurred under both high- and low-pH conditions. This system can be used for time-prolonged bacterial staining and development of supramolecular capsules for the system chemistry approach.