Abstract
AbstractThe 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 (λmax=520 nm) that rapidly equilibrates with its conjugate base CNO− (λmax=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, pK$\rm{ {_{a,c}^{NH}}}$=5.94±0.12 (I=0.1M) and serves to clarify the mechanisms of proton transfer prevailing in aqueous solutions. The acidity constant of protonated 1 (CHNH+), pK$\rm{ {_{a,c}^{CHNH}}}$=4.18±0.02, was determined by spectrophotometric titration.
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