An Unusual Solid State Reaction Leading to the Photochromic N-(3,5 Dichlorosalicylidenem)-4-Aminopyridine

The photochromic and thermochromic properties of N-salicylidene-2-aminopyridines, N-salicyli-dene-3-aminopyridines and N-salicylidene-4-aminopy-ridines were investigated in the crystalline state and the results are compared with each other and with findings on N-salicylideneanilines. The first and the second groups of compounds exhibit exclusively thermochromic properties while the third group contains compounds which are either photochromic or thermochromic. The appearance of the photochromic and thermochromic phenomena depends on the molecular structure in the solid state and is accompanied by intramolecular proton transfer. In a certain compound of the third group, namely the N-(3,5 dichlorosalicylidene)-4-aminopyridine there are indications of both photochromic and thermochromic properties contrary to N-salicylideneanilines which are either photochromic or thermochromic but not both. This compound results from an unusual solid state reaction which is discussed.