Catrin Goeschen, Rainer Herges, Josef Richter, Bogdan Tokarczyk, and Jakob Wirz,
"2-(2',4'-Dinitrobenzyl)pyridine: A light-activated proton shuttle", Helv. Chim. Acta. 2009, 92, 1909-1922.
Abstract: The well-known photochromic tautomerism of 2-(2',4'-dinitrobenzyl)pyridine (1, CH, Scheme 1) was reinvestigated 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 (lambda-max = 520 nm) that rapidly equilibrates with its conjugate base CNO- (lambda-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, pKa,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+), pKa,c(CHNH) = 4.18 ± 0.02, was determined by spectrophotometric titration.