title: Packing-dependent photochromism: the case of photoinduced intramolecular proton transfer in 6-(2',4'-dinitrobenzyl)-2,2'-bipyridine.
abstract: 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.
authors.affiliation: Department of Chemistry and Materials Science, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan.
authors.forename: Pance
authors.initials: P
authors.lastname: Naumov
authors.forename: Yuji
authors.initials: Y
authors.lastname: Ohashi
mesh_terms: D015082:2,2'-Dipyridyl / Q000031:analogs & derivatives / Q000138:chemical synthesis* / Q000528:radiation effects*; D003116:Color; D003460:Crystallization / Q000379:methods; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008968:Molecular Conformation; D010777:Photochemistry; D011522:Protons; D012399:Rotation; D013053:Spectrophotometry; D013056:Spectrophotometry, Ultraviolet; D014466:Ultraviolet Rays
chemical_list: C488201:6-(2',4'-dinitrobenzyl)-2,2'-bipyridine; D011522:Protons; D015082:2,2'-Dipyridyl