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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 (-DNBP) by solid state nuclear magnetic resonance spectroscopy (NMR) is presented. Whereas similar high-resolution 1H and 13C spectra were obtained from polycrystalline powders before and after ultraviolet light irradiation, the corresponding spectral data obtained from PVC films doped with -DNBP show significant changes. The -DNBP phototautomer lifetimes in PVC films (2.4 h or 6 d) were determined from the proton NMR signal recovery. Scanning electron microscopy (SEM) and differential scanning calorimetry of the doped films show that, in addition to matrix isolated molecules, -DNBP is incorporated into the polymer matrix in the form of nanocrystalline aggregates. At high guest concentrations, the crystallites can be modified by controlled solvent evaporation, as monitored by NMR and SEM. The variation of the 1H spectrum with temperature shows the presence of at least two stable conformational isomers in the solid samples.