2‐(2,4‐Dinitrobenzyl)pyridine (DNBP): A Potential Light‐Activated Proton Shuttle

The well‐known photochromic tautomerism of 2‐(2,4‐dinitrobenzyl)pyridine ( 1 ; CH; Scheme  1) was re‐investigated by flash photolysis in aqueous solution in view of its potential application as a light‐activated proton pump. Irradiation of 1 yields the enamine tautomer NH ( λ max =520 nm) that rapidly equilibrates with its conjugate base CNO − ( λ max =420 nm). The pH–rate profile for the first‐order decay of NH and CNO − provides a direct determination of the acidity constant of NH, p K $\rm{ {_{a,c}^{NH}}}$ =5.94±0.12 ( I =0.1 M ) and serves to clarify the mechanisms of proton transfer prevailing in aqueous solutions. The acidity constant of protonated 1 (CHNH + ), p K $\rm{ {_{a,c}^{CHNH}}}$ =4.18±0.02, was determined by spectrophotometric titration.