Ermüdungsprodukte des photochromen 2‐(2′,4′‐Dinitrobenzyl)‐pyridins

Fatigue Products of Photochromic 2‐(2′,4′‐Dinitrobenzyl)‐pyridine Under prolonged irradiation or frequent repetitions of its photochromic cycle, 2‐(2′,4′‐dinitrobenzyl)‐pyridine 1 shows fatigue phenomena both in solid state and in solution. These phenomens — yellow discoloration, depression of the melting point, decrease of band intensity of the coloured form, shorter lifetime of the coloured form — are caused by irreversible redox processes and subsequent reactions at the methylene and ortho‐nitro groups which are essential for photochromism. Azomethine 2, ketone 3 and amine 4 are isolated as the major fatigue products.