Structure and Optical Properties of a Thermochromic Schiff Base. Thermally Induced Intramolecular Proton Transfer in the N,N′-Bis(salicylidene)-p-phenylenediamine Crystals

A thermochromic derivative of salicylideneaniline, N,N′-bis(salicylidene)-p-phenylenediamine (BSP), has been prepared and subjected to structural and optical studies in the crystalline state. An X-ray crystallographic analysis has shown that the molecules are planar and are stacked in a parallel fashion to form one-dimensional columns. The interplanar spacing has been found to be quite short, suggesting the existence of an intermolecular charge-transfer interaction. The molecule contains fairly short O–H··· N hydrogen bonds, the strength of which is manifested in an OH stretching absorption band in the infrared region, showing considerable broadening and a low-frequency shift. The BSP crystals are remarkably thermochromic, and visible absorption and emission spectral changes with temperature have been interpreted in terms of an intramolecular proton transfer from the hydroxyl oxygen to imine nitrogen through the O–H··· N hydrogen bond. The emission spectra have also been examined under high pressures.