Thermal behavior, decomposition mechanism by TG/MS/FTIR technique and theoretical study of some symmetric and asymmetric bent-core liquid crystals based on 2,7-dihydroxynaphthalene

The thermal stability of some symmetric and asymmetric bent-core liquid crystals derived from 2,7-dihydroxynaphthalene as well as the degradation mechanism by the TGA (thermogravimetric analysis) and TG/MS/FTIR technique (simultaneous mass spectrometry and Fourier transform infrared spectroscopy of off-gases from thermogravimetric analyzer), in inert atmosphere (nitrogen and helium) is reported. The TGA of the three classes of compounds in the nitrogen atmosphere allowed the stability series to be established. TG/MS/FTIR technique indicated that the initiation of thermal decomposition probably that occurs at –N = N– groups continues with the fragmentation of both aromatic units and alkyl terminal chains and removal of CO 2 at temperatures above 400 °C. In order to gain insight into the mechanism of thermal decomposition and the factors influencing the different transition temperatures of liquid crystals, a series of quantum calculations were performed. Values of the dipole moment, total energy and energy of the highest occupied/lowest unoccupied molecular orbitals of the mesogens were calculated by the density functional theory method. These along with different shape and size descriptors of the mesogens were used as input data for statistical analysis to find the relationship between these values and mesophase behavior. In addition, the Fukui indices for the radical attack, also determined by quantum calculation, showed a decomposition mechanism similar to the experimental thermal decomposition one.