Photoequilibrium and the Decay Mechanism in Dinitrobenzyl Pyridine Photochromism

The maximum intensity of photochromic color change in 2- and 4-(2′, 4′-dinitrobenzyl) pyridine solutions is controlled by a photon equilibrium. The equilibrium has been approached from both sides by means of irradiation. Solutions of the compound almost entirely in the form of its colored modification have been prepared. In the absence of photon activated decay, which is controlling in experiments where the preparation is made photolytically, these solutions decay thermally to the ordinary form by two mechanisms, the relative importance of which depends on temperature and pH.