Packing-dependent photochromism: the case of photoinduced intramolecular proton transfer in 6-(2',4'-dinitrobenzyl)-2,2'-bipyridine.

A new photoactive monoclinic polymorph (gamma-form) of 6-(2',4'-dinitrobenzyl)-2,2'-bipyridine was obtained from an acetone-methanol solution. The crystal structure was compared with those of two polymorphs reported previously (photoactive orthorhombic alpha-form and photoinactive monoclinic beta-form) and with structures of related nitrobenzylpyridines to explain the relation between the structure and photochromism. The comparison of the reaction cavities around the reactive pyridyl-benzyl-nitro fragment among the polymorphs and related nitrobenzylpyridines revealed that a crucial factor for photochromic activity is the rotational freedom of the ortho-nitro group in their crystals and its accessibility from the proton-donor and proton-acceptor sites. This is because the ortho-nitro group should rotate around the N-C bond to transfer a proton from the methylene group to the N atom of the pyridine ring.