ROLE OF INTERMOLECULAR NON-COVALENT INTERACTIONS IN THE STABILIZATION OF THE MESOPHASE OF ANISOTROPIC 1,2,5-THIADIAZOLE DERIVATIVES

By differential scanning calorimetry and polarized light thermal microscopy the phase transition temperatures are determined and the data on the mesomorphic properties are obtained for three anisotropic 1,2,5-thiadiazole derivatives: 4,7-bis(dodecylsulfanyl)[2,3,1]benzothiadiazole BTDA, 4,7-bis (dodecylsulfanyl)[1,2,5]thiadiazolo[3,4-c]pyridine PyTDA, 4,7-bis(dodecylsulfanyl)[1,2,5]thiadiazolo[3,4-c]pyridazine PrdTDA. It is shown that among the studied compounds only PrdTDA is mesogenic. The induction of the PrdTDA mesophase is observed in a temperature range of 95.63-91.62 °C on cooling from the isotropic liquid phase. Using the quantum chemistry method DFT (B3LYP, B97D) with the cc-pVTZ and 6-311++G** basis sets, the structure of a free PrdTDA molecule is studied, its conformational properties are determined, and possible intermolecular interactions occurring in the crystalline phase are simulated. The analysis of potential functions of internal rotation of alkylsulfanyl substituents relative to the cyclic core shows that the PrdTDA molecule has three conformers. Based on the structure of the most energetically favorable conformer of the molecule, the geometry of the dimeric moiety of PrdTDA···PrdTDA chain associate characteristic of the crystalline phase is optimized. The analysis of electron density distribution (NBO) suggests that intermolecular interaction is possible due to LP(S), LP(N) lone pairs and σ-type anti-bonding NBOs σ*(C–N), σ*(S–N). A direct relationship between the molecular structure and the possibility of inducing mesophase due to non-covalent intermolecular interactions–chalcogen bond–is shown.