Nuclear magnetic resonance spectroscopy studies of 2-(2,4-dinitrobenzyl) pyridine and long-lived photoinduced products

A study of the photo-induced and thermally activated tautomerization of 2-( 2,4-dinitrobenzyl) pyridine (alpha-DNBP) by solid state nuclear magnetic re sonance spectroscopy (NMR) is presented. Whereas similar high-resolution H- 1 and C-13 spectra were obtained from polycrystalline powders before and af ter ultraviolet light irradiation, the corresponding spectral data obtained from PVC films doped with alpha-DNBP show significant changes. The alpha-D NBP phototautomer lifetimes in PVC films (2.4 h or 6 d) were determined fro m the proton NMR signal recovery. Scanning electron microscopy (SEM) and di fferential scanning calorimetry of the doped films show that, in addition t o matrix isolated molecules, alpha-DNBP is incorporated into the polymer ma trix in the form of nanocrystalline aggregates. At high guest concentration s, the crystallites can be modified by controlled solvent evaporation, as m onitored by NMR and SEM. The variation of the H-1 spectrum with temperature shows the presence of at least two stable conformational isomers in the so lid samples.