Chapter 17 – Tautomerism by Hydrogen Transfer in Anils, Aci-Nitro and Related Compounds

The reversible solid state photocoloration of anils of salicylaldehydes was first observed by Senier and co-workers, who noted that of the ring-substituted anils, only a few were photochromic and that polymorphic modifications of the same anil were not necessarily all photochromic. An alternate mechanism was suggested in which the H-transfer is to the oxygen of the nitro group. According to this mechanism, the pyridine ring is not an essential structural feature for photochromic activity, and is therefore replaceable by other electrophilic groups. Thus, experiments were accumulated indicating that this photochromic behavior is general for phenyl methanes, the structural requirement being a nitro group ortho to at least one benzylic hydrogen. Photochromism is probably an inherent property of the ligand because it occurs in the presence of a variety of metals. The central metal atom determines the photochemical stability, rate of the return reaction, and in some cases, the colors of the complexes.