Photochromism of ortho-nitrobenzylpyridines: A brief overview

Review is presented regarding all structural and spectroscopic results reported on the reaction mechanism of photochromic ortho-nitrobenzylpyridines from the discovery of photochromism of 2-(2′,4′-dinitrobenzyl)pyridine in 1925 until the present. Although interpreted in various ways, the structural and spectroscopic data appear to be consistent with the widely accepted mechanism in which the photochromic activity in ortho-nitrobenzylpyridines in the solid state results from the intramolecular transfer of benzylic proton to pyridyl nitrogen, assisted by the ortho-nitro group. At low temperatures, a qualitatively different mechanism is active, probably involving radical species. In addition to being convenient models for studying the photoreactions of nitro-based caged compounds, this photoreactive system has potential applications for photon-based electronics, due to favorable properties such as photochromic activity in the solid state, small structural change during photoreactions and inherent polystability.