Role of topological charge stabilization in protomeric tautomerism

Polarization-Independent Second Harmonic Generation in 2D Van Der Waals Kagome Nb3 SeI7 Crystals

Second harmonic generation (SHG) of 2D crystals has been of great interest due to its advantages of phase-matching and easy integration into nanophotonic devices. However, the polarization-dependence character of the SHG signal makes it highly troublesome but necessary to match the laser polarization orientation relative to the crystal, thus achieving the maximum polarized SHG intensity. Here, it is demonstrated a polarization-independent SHG, for the first time, in the van der Waals Nb₃ SeI₇ crystals with a breathing Kagome lattice. The Nb₃ triangular clusters and Janus-structure of each Nb₃ SeI₇ layer are confirmed by the STEM. Nb₃ SeI₇ flake shows a strong SHG response due to its noncentrosymmetric crystal structure. More interestingly, the SHG signals of Nb₃ SeI₇ are independent of the polarization of the excitation light owing to the in-plane isotropic arrangement of nonlinear active units. This work provides the first layered nonlinear optical crystal with the polarization-independent SHG effect, providing new possibilities for nonlinear optics.

Experimental and theoretical study on structure-tautomerism among edaravone, isoxazolone, and their heterocycles derivatives as antioxidants

Edaravone is a heterocyclic pyrazolone compound. It has pronounced effect against free radicals, however renal and hepatic disorders have been reported. Isoxazolones are considered bioisosteric analogues of pyrazolones and may have comparable properties. Thus, we investigated the structural and electronic influences for edaravone, isoxazolone, and their tautomers on antioxidant process. Structure and tautomerism study among edaravone, isoxazolone and their heterocycles derivatives were related to antioxidant mechanisms by using the hybrid DFT method B3LYP with the basis sets 6-31++G(2d,2p). The C—H tautomer was the most stable and energetically favored among them. Intramolecular N—H—N hydrogen bonds and polar medium were responsible for the low energy differences among all possible tautomers. N—H tautomers in both systems proved to be better antioxidant by SET (single electron transfer), while O—H tautomers were better antioxidant on HAT (homolytic hydrogen atom transfer) mechanism. Theoretical calculation showed that edaravone is more potent than phenylisoxazolone, however, both has similar antioxidant scavenging on experimental DPPH. The carbonyliminic system played a very important role in the antioxidant activity for both studied classes.

Designing Self-Assembled Rosettes: Why Ammeline is a Superior Building Block to Melamine

In supramolecular chemistry, the rational design of self-assembled systems remains a challenge. Herein, hydrogen-bonded rosettes of melamine and ammeline have been theoretically examined by using dispersion-corrected density functional theory (DFT-D). Our bonding analyses, based on quantitative Kohn-Sham molecular orbital theory and corresponding energy decomposition analyses (EDA), show that ammeline is a much better building block than melamine for the fabrication of cyclic complexes based on hydrogen bonds. This superior capacity is explained by both stronger hydrogen bonding and the occurrence of a strong synergy.

Some structural aspects of ammeline - Keto preference and dimerization

Subtle tautomerism in ammeline is clarified by means of IR- and UV-spectroscopic measurements and DFT calculations. While the enol form is narrowly preferred as an isolated molecule, dimerization of the keto form is plausible in solid phase due to double hydrogen bonds. In aqueous solutions, the keto form is proved to be more stable than the enol form due to the large electric dipole moment of the peptide group. The keto preference is also consistent with the acidity and basicity of the related heterocycles.

A photochromic-acidochromic HCl fluorescent probe. An unexpected chloride-directed recognition

Non-classical protomerism of Schiff bases offers several advantages; for example, specific interactions in the -C[double bond, length as m-dash]N- linkage can be controlled and differentiated because the interactions are not governed by keto-enol tautomerism. Herein, the pH sensing properties of a new protomeric Schiff base probe () are reported. In particular, among several acids, the probe displays significant optical responses upon interaction with hydrochloric acid (HCl). X-ray structural analysis confirmed the existence of an intermolecular interaction with HCl through a -C[double bond, length as m-dash]NH-ClO- linkage. Moreover, an optical response via a second channel is manifested as photochromic fluorescence behavior. The properties of were investigated by UV-vis and fluorescence spectroscopy in a solution and the solid state. Its strong acidofluorochromic behavior was analyzed and its pKa and values were determined, which revealed a photobasic character. Positive solvatochromism that resulted from specific interactions taking place in was studied using four different solvent scales, namely, Lippert-Mataga, Kamlet-Taft, Catalán and the recently proposed scale of Laurence et al., which yielded consistent results. Finally, theoretical calculations were conducted to analyze the mechanism of the probe in terms of natural transition orbitals (NTOs) and the spatial extent of charge transfer excitations.