Effects of crystal packing on photoinduced proton-transfer processes of 2,4-dinitrobenzylpyridine derivatives

Photoinduced and thermally activated proton-transfer processes taking place in crystals of 2-(2,4-dinitrobenzyl)pyridine and some of its derivatives are highly sensitive to molecular packing. Small differences in the way the molecules are packed in the crystal are found to dominate molecular properties in controlling the photoactivity of the different phototautomers, leading, for example, to photoactive or photoinert systems. Three compounds, 2-(2,4-dinitrobenzyl)-4-methylpyridine, 1-(2,4-dinitrophenyl)-1-(2-pyridine)ethane and 4'-(2,4-dinitrobenzyl)-4-methyl-2,2'-bipyridine, having different photochemical properties, were prepared and their crystal structures characterized by means of X-ray analysis. In the photoinert crystals the 2,4-dinitrophenyl group is pi-stacked with other aromatic rings of neighboring molecules. This arrangement may open some deactivation channels to the excited state which are faster than the proton-transfer process, leading to photoinert crystals. The absence of pi-stacking between the chromophore and other aromatic rings leads to photoactive systems. An O atom of the o-nitro group is the only basic atom that is systematically found to interact with the abstracted proton. It seems that this atom is responsible for the photoinduced proton abstraction of the benzylic H atom, while the role of the N atom of the pyridine ring in the proton-abstraction process is mainly inductive.