PHOTOCHROMISM AND THERMOCHROMISM DRIVEN BY INTRAMOLECULAR PROTON-TRANSFER IN DINITROBENZYLPYRIDINE COMPOUNDS

The photoinduced proton transfer reaction taking place in 2-(2',4'-din itrobenzyl)pyridine (alpha-DNBP) and in some of its derivatives is cha racterized by LR, visible, and NMR spectroscopy. The enamine ''NH'' st ructure of the blue phototautomer is confirmed by the analysis of the IR spectra of alpha-DNBP and its deuterated analogue. 2D NOESY H-1 NMR data indicate that this tautomer is predominantly in the cis configur ation. In the 2-(2',4'-dinitrobenzyl)phenanthroline derivative, the st abilization of the phototautomer is sufficient to make it thermally ac cessible. A quantitative analysis of the resulting thermochromism indi cates 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 al pha-DNBP compound.