Abstract Model series of thiophosphinic acid derivatives were scrutinized at the B3LYP/6–311 + + G(2d,2p) level of theory, and the resulting geometries were compared with experimental crystal structure data obtained from the Cambridge Structure Database (CSD). Typical bond length and bond angle values were selected for a series of similar fragments from CSD, and mean dimensions were also calculated from the experimental data accumulated. Linking the results of theoretical calculations and the evidences of data mining allowed the reconsideration of a few past experimental models with (thio)phosphinate-anion-(thio)phosphinic acid-type associations. It seems also from this survey that thiophosphinic acid derivatives may be suitable vehicles for transgressing boundaries from a single molecule to supramolecular relations and vice versa. The study shows that a few intramolecular distances such as the P-O bond lengths both in the anionic and in the neutral forms may significantly depend on intermolecular contacts as well. The tautomerism involving the thionic acid and thiolic acid forms was studied by theoretical methods at the B3LYP/6–311 + + G(2d,2p) level of theory assuming monomolecular and different bimolecular mechanisms. The shift toward the thionic actor was supported by the kinetic and thermodynamic energetics.

Abstract The crystal structures of isoelectronic 4,11-diaza[5]helicene (DNH) and 4,7,8,11-tetraaza[5]helicene (TNH) are accurately determined at room temperature by single crystal X-ray diffraction. Analysis of crystal packing with computational chemistry tools, including the evaluation of intermolecular and cohesive energies with force field and charge density (PIXEL) methods, highlights the effect of skeletal replacement of two aromatic C-H with nitrogen atoms. Both crystals are centrosymmetric racemates, with asymmetric units (ASUs) exploiting the same conformation. The ASU of TNH bears two symmetry-independent enantiomers, and the different settings of symmetry operators mirror in different packing motifs with respect to DNH. Both structures exhibit columnar stacking arrangements, which differ in their mutual orientation, distance and strength. Stacking interactions are more frequent and stronger in the tetraaza compound, owing to the greater intrinsic polarization of its ground-state electron density that favors antiparallel arrangements of neighboring molecular dipole moments. Finally, the possible existence of thermodynamically stable chiral forms of the title compounds is investigated by symmetry-constrained Monte Carlo (SC-MC) calculations.

The solid-state photochromism of viologen derivatives offers significant potential for the development of optical and multifunctional materials; however, the structural factors governing their color fading behavior remain poorly understood. In this study, we synthesized and characterized four viologen-based ionic compounds featuring different cationic substituents (H or tert-butyl) and counter anions (Br⁻ or p-toluenesulfonate, TsO⁻) to systematically investigate the correlation between crystal structure and the thermal fading rate of photoinduced color changes. Upon UV irradiation, all compounds exhibited characteristic coloration due to the formation of viologen radical cations, followed by thermal decolorization in the dark. The fading rate was evaluated using time-dependent diffuse reflectance UV-vis spectroscopy. Structural analyses via single-crystal X-ray diffraction revealed that, in bromide salts, shorter donor–acceptor (D–A) distances between the bromide anion and viologen cation corresponded to faster fading rates, consistent with a distance-dependent electron back-transfer mechanism. In contrast, for TsO⁻ salts, one compound exhibited faster fading despite a longer D–A distance. Crystal packing analysis and DFT calculations indicated that this behavior stems from a π–π stacking-mediated electron transfer pathway between the viologen cation and the aromatic TsO⁻ anion. These findings highlight that, while ionic D–A distances dominate in systems with atomic anions, supramolecular interactions such as π–π stacking significantly influence photochromic fading in systems with molecular anions. This work provides new insights into structure–property relationships in solid-state photochromic systems and contributes to the rational design of viologen-based materials with tunable optical responses.

The simplest amide, formamide, forms no less than five different phases between ambient pressure and about 2 GPa. The online version contains supplementary material available at 10.1007/s11224-026-02744-2.