Effect of Crystal and Molecular Structure on the Thermochromism and Photochromism of some Salicylidene‐2‐Aminopyridines

Abstract Crystalline salicylidene‐2‐aminopyridine and its derivatives exhibit thermochromic phenomena, i.e. they show a reversible colour change as a result of variation in temperature. Photochromism, reversible colour change on ultraviolet irradiation, is not observed in this class of compounds. The appearance of the thermochromic phenomen and its generality depend upon the crystal structure and the molecular orientation of the compounds. In fluid solution, where the orientation of the molecules is random, the compounds exhibit photochromic phenomena. The process is so fast that it can be followed only by flash photolysis techniques or at liquid nitrogen temperature. The results and mechanisms of thermochromism and photochromism are compared with findings on salicylideneanilines.