Along with the metastable 2-(2′,4′-dinitrophenylmethylidene)-1,2-dihydropyridine (NH) and the unstable 6-aci-nitro-2-nitro-5-(2′-pyridylmethylene)-1,3-cyclohexadiene (OH), the stable form of 2-(2′,4′-dinitrobenzyl)pyridine (DNBP), CH, is photochemically converted into small amounts of 1,2-bis(2′,4′-dinitrophenyl)-1,2-bis(2′-pyridyl)ethane, trans-bis[5-nitro-2-(pyridine-2-carbonyl)phenyl]diazene N-oxide, 6-nitro-3-(2′-pyridyl)-2,1-benzisoxazole and 3-nitropyrido[1,2-b]quinolin-6-ium-11-olate. The latent photochromism of DNBP, as shown by x-ray analysis of the structures of the side-products and ESR/IR measurements, is attributed to open-shell reactions that are initiated by hydrogen photoabstraction and subsequent creation of two monoradicals, NH ̇ and OḢ. Large amounts of the radicals (ca 50% NḢ and 70% OḢ) confined in the crystalline interior are persistent under ambient conditions. Through quasi-periodic reactions, the remaining radicals partially recover the ground-state isomers CH, NH and OH, or decay to the side-products, which results in crystalline photofatigue. Together with proton tunneling from the excited CH, the radical reactions represent dominant mechanism for the creation of NH and OH in the low-temperature regimes, but are successfully competed by the closed-shell reactions at higher temperatures. The precursor state, whose existence was assumed previously from transient absorption spectroscopy, may be identified as the radical OḢ. The present work represents the first study of the photofatigue of a 2-(2′,4′-dinitrobenzyl)pyridine compound and extends the 'classical' mechanism of the photochromic reactions of nitrobenzylpyridines with a set of open-shell radical reaction routes.
- Photochromic ortho-nitrobenzylpyridine
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry