Modulating the Photochemistry of Bipyridylic Compounds by Symmetric Substitutions

A quantum electronic study of the effect of substituents on (2,2'-bipyridyl)-3,3'-diol and (2,2'-bipyridyl)-3,3'-diamine is presented. A large difference in the photochemical behavior between the original and the substituted selected systems is expected. For the sake of simplicity, the study is restricted to the symmetrically bi-substituted compounds: fluorine, the more electronegative atom and thus a strong sigma-acceptor but also a weak pi-donor group, and NO2, a strong pi-acceptor substituent. Among the large set of compounds studied, two receive special attention: 5,5'-dinitro-(2,2'-bipyridyl)-3,3'-diamine and 6,6'-difluoro-(2,2'-bipyridyl)-3,3'-diol. While in the former case the nitro substitution transforms (2,2'-bipyridyl)-3,3'-diamine, previously suggested to behave as a photomemory material, into a simple fluorescent species, the latter substitution turns (2,2'-bipyridyl)-3,3'-diol into a fresh new candidate for a photomemory device.