SPIN-DENSITY DISTRIBUTION AND ELECTRONIC-STRUCTURE OF RADICAL-ANIONS OF LADDER-TYPE OLIGORYLENES

The radical anions of ladder-type oligorylenes have been obtained from a homologous series of soluble oligoperinaphthylenes and characterized by EPR and ENDOR spectroscopy. By these methods in combination with HMO/McLachlan and PPP calculations and isotopic labeling, all H-1 hyperfine coupling constants have been assigned to the individual positions in the molecules. Considerable spin density prevails at both ends of the oligoperinaphthylenes, making them ideally suited as building blocks in molecular electronics. In higher oligomers, a redistribution of the position of highest spin density to more inner naphthalene units is predicted theoretically. The experimental and theoretical results point to a close relationship to even polyenes, but with the more rigid polyperinaphthylenes possessing a much lower band gap. The spin density distribution within the oligomers is in contrast to that of linear conjugated pi-systems with phenyl end groups like oligo(p-phenylenevinylenes) and diphenylpolyenes, where the positions of highest spin density occur in the central parts of the molecules already for small oligomers. The chain length dependence of the spin density distribution and the largest H-1 hyperfine coupling constants are compared to data from optical absorption measurements of the neutral and monocharged species. While the former reflects the spin density at certain positions of the pi-system, the latter characterize the whole conjugated chromophore.