ELECTRON DELOCALIZATION IN HELICAL BIS(QUINONE) ANION-RADICALS

Ab initio quantum chemical calculations were employed to determine the extent of charge localization in the anion radicals of helical bis(quinones). Recent experimental evidence has suggested that the electronic charge in these systems is delocalized, in contrast to that in their linear analogs. The optimized molecular geometries of the neutral and radical anion five- and seven-ring bis(quinones) reveal unusual supershort nonbonded contacts between the outermost rings and large positive electron affinities. The regular helicenes, in contrast, lack the short contacts and have no electron affinities at the SCF level of theory Of central importance to these short contacts is the interaction between the 2 pi* orbitals of the carbonyls on the Inner edge of the helicene, characteristic only of the five-ring complex, or, for the larger systems, between the pi systems of the overlapping rings at the opposite end of the helicene, of which the inner carbonyl 2 pi* orbitals are principal components. The theoretical results suggest that the electronic charge in the helical bis(quinone) anion radicals is best described as delocalized.