Effect of molecular conformation on spectroscopic properties of symmetrical Schiff bases derived from 1,4-phenylenediamine

The relationship between the molecular conformation and spectroscopic properties of symmetrical bis-Schiff bases was explored experimentally. Seven samples of compounds p-X–C6H4CHNC6H4NCHC6H4–p-X (X = OMe, Me, Et, Cl, F, CF3, or CN) were prepared for this study, and their crystal structures were measured by X-ray diffraction. Their λmax values in ethanol, acetonitrile, chloroform and cyclohexane solvents were measured, and their δC(CN) values in chloroform-d were determined. The results show that the νmax is dependent on the substituents at the benzylidene ring and the dihedral angle τ of the titled molecules, and the term sin(τ) is suitable to modify the substituent effects on the νmax. However, experimental investigations indicate that the dihedral angle τ has a limited effect on the values of δC(CN). This study provides a new understanding for the molecular conformation on spectroscopic properties of symmetrical Schiff bases.