Photochromism and thermochromism driven by intramolecular proton transfer in dinitrobenzylpyridine compounds

The photoinduced proton transfer reaction taking place in 2-(2‘,4‘-dinitrobenzyl)pyridine (α-DNBP) and in some of its derivatives is characterized by IR, visible, and NMR spectroscopy. The enamine “NH” structure of the blue phototautomer is confirmed by the analysis of the IR spectra of α-DNBP and its deuterated analogue. 2D NOESY 1H NMR data indicate that this tautomer is predominantly in the cis configuration. In the 2-(2‘,4‘-dinitrobenzyl)phenanthroline derivative, the stabilization of the phototautomer is sufficient to make it thermally accessible. A quantitative analysis of the resulting thermochromism indicates that in toluene solutions the ground state energy of the “NH” form is lowered to 2.9 kcal mol-1 above the thermodynamically stable “CH” tautomer compared to more than 8 kcal mol-1 in the parent α-DNBP compound.