Multistep photoinduced proton transfer in crystalline 2-(2′,4′-dinitrobenzyl)pyridine Original Research Article

Photochromism in crystalline 2-(2′,4′-dinitrobenzyl)pyridine is associated with a photoinduced proton transfer reaction leading to a metastable colored tautomer of several hours lifetime at room temperature. Transient absorption experiments performed between 10 and 300 K show two competitive proton transfer routes: a fast direct and temperature-independent one, assigned to an excited state process, and a much slower one, thermally activated, involving a multistep proton transfer in the ground state via a precursor state, assigned to an additional tautomer. An energy level diagram, including energy barriers, is afforded by a temperature-dependent study of the photochromic reaction.