Structure and Optical Properties of a Thermochromic Schiff Base. Low-Temperature Structural Studies of the N,N′-Disalicylidene-p-phenylenediamine and N,N′-Disalicylidene-1,6-pyrenediamine Crystals

Abstract The crystal structure of N,N′-disalicylidene-p-phenylenediamine (BSP) has been determined at 108 K. This compound shows a thermochromic-type behavior in the crystalline state; this chromism has been interpreted in terms of an intramolecular proton transfer from the hydroxyl oxygen to the imine nitrogen through the O–H···N hydrogen bond. In comparison with the room-temperature structure, no significant differences in the molecular structure and the molecular packing in the lattice have been observed. The room-temperature and low-termperature structures of another N,N′-salicylideneaniline derivative, N,N′-disalicylidene-1,6-pyrenediamine (DSPY), have also been determined. In contrast to BSP, the absence of a thermal proton-transfer process in DSPY is suggested by optical measurements. Comparative studies of these two compounds indicate that a conjugation system which is not included in a π-electron configurational change by the proton transfer must be coplanar with the hydrogen-bonded chelate ring for the thermal proton transfer.