Packing-dependent photochromism: the case of photoinduced intramolecular proton transfer in 6-(2′,4′-dinitrobenzyl)-2,2′-bipyridine
A new photoactive monoclinic polymorph (-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 α-form and photoinactive monoclinic -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.