Three series of azo dyes derived from 2-amino-5-aryl-1,3,4-thiadiazoles and aniline, N,N-dimethylaniline and phenol were synthesized in high yields by a conventional diazotization-coupling sequence. The chemical structures of the prepared compounds were confirmed by ¹H-NMR, ¹³C-NMR, IR, UV-Vis spectroscopy, mass spectrometry and elemental analysis. In addition, the X-ray single crystal structure of a representative azo dye was presented. For explicit determination of the influence of a substituent on radiation absorption in UV-Vis range, time-dependent density functional theory calculations were performed.

Extremely extensive calculations of potential energy surfaces for the parallel-displaced configuration of pyridine dimer systems have been carried out using a dispersion-corrected density functional. Instead of focusing on stationary geometries these calculations provide much deeper insight into the “landscape” of the interaction energies of the particular systems—one can learn how the pyridine dimer stability changes along with various geometrical parameters. Other calculations such as natural bond orbital and energy decomposition have also been applied. The interplay of two significant factors, electrostatic forces and electron correlation effects, have been evaluated. The role of π···π interactions in the stacked pyridine systems has also been confirmed, and surprisingly, this happened to be true even for the geometries where the formation of C-H···π interactions might be proposed instead. The combination of many different methods has revealed the complexity of the stacking interactions. Apart from providing a “literal new look” into pyridine interaction patterns another picture has emerged. A stacking interaction in a pyridine dimer system is perceived as a combination of many different sources of the interaction energy, including orbital ones, and this is true for many different geometries.

The magnesium sulphate complex compounds of general formulae [Mg(H₂O)₆]²⁺·2(C₆H₁₂N₄)·SO₄ ²⁻·5(H₂O) (1) and Mg(C₁₂H₈N₂)(H₂O)₃SO₄ (2) have been synthesized, characterised by elemental and thermal analysis, IR, UV–VIS and fluorescence spectroscopy, and X-ray crystallography. The obtained compounds are air stable at room temperature and well soluble in water. In the structures of the investigated complex compounds the O–H⋯O, O–H⋯N, and C–H⋯O hydrogen bonds exist, and they create N₂C ²₂ (8), R ²₂ (8) (compound 1) and N₁C ¹₁ (6), N₁R ²₂ (12) (compound 2) patterns. Their thermal decomposition processes in the investigated atmospheres (air and helium) are different. After the slightly similar dehydratation, the observed transitions and the obtained final products are different (in helium atmosphere the sulphate ion of studied compounds undergoes decomposing what does not take place in air atmosphere). The UV–VIS spectrum of 2 shows maxima that are typical for π → π* and n → π* transitions, and fluorescence spectrum of the same compound displays its great fluoresce properties. The 1 does not exhibit absorption in the investigated region of electromagnetic spectrum due to the absence of respect chromophore groups. The IR spectrum of 2 shows typical vibrations for chelating amine molecule. An interesting fact is that in 1 the SO stretching vibrations (existing at 1119 and 1182 cm⁻¹) are doubled in comparison to the magnesium sulphate whilst in 2 these vibrations are absent.

Extensive calculations of potential energy surfaces for parallel-displaced configurations of pyrrole–pyrrole systems have been carried out by the use of a dispersion-corrected density functional. System geometries associated with the energy minima have been found. The minimum interaction energy has been calculated as −5.38 kcal/mol. However, bonding boundaries appeared to be relatively broad, and stacking interactions can be binding even for ring centroid distances larger than 6 Å. Though the contribution of the correlation energy to intermolecular interaction in pyrrole dimers appeared to be relatively small (around 1.6 smaller than it is in a benzene–benzene system), this system’s minimum interaction energy is lower than those calculated for benzene–benzene, benzene–pyridine and even pyridine–pyridine configurations. The calculation of the charges and energy decomposition analysis revealed that the specific charge distribution in a pyrrole molecule and its relatively high polarization are the significant source of the intermolecular interaction in pyrrole dimer systems.

The fluorescence of phenylhydrazones: understanding the critical role of substituents in influencing the intensity of fluorescence and the evolution of emissive materials.