STRUCTURE AND OPTICAL-ABSORPTION OF OLIGORYLENES UPON DOPING

We report in parallel a theoretical MNDO-PMS study of the structure of negatively doped oligorylenes (OR) and their visible/NIR absorption spectra from perylene to pentarylene. Neutral OR have an aromatic structure caused by electron-lattice coupling and specific boundary conditions. Doping is predicted to favour the alternative quinoid form, found to be metastable for the corresponding pristine one-dimensional lattice. Radical anion (negative polaron) and singlet excited state (exciton-polaron) species in finite OR are obtained as smooth segments of reduced CC bond length alternation. Crossover to the quinoid form, particularly for the innermost repeat units, is predicted to occur yet for the dianions (negative bipolarons). Up to hexarylene, the structural relaxation affects the whole molecule, i.e. the characteristic length of the charged and neutral defects is not yet reached. The latter conclusion is primarily inferred from the absorption spectra of OR in both redox states. As for the neutral compounds, all characteristic absorptions show linear energy versus reciprocal length dependence. This feature is well reproduced when employing the PPP-CI approach and accounting for the doping-induced geometrical changes. The agreement between experimental and calculated transition energies and intensities is satisfactory for the closed shell molecules and very good for the radical anions. Particularly in the latter case, CI is essential, suggesting deficiencies of the one-electron approach as far as optical transitions are concerned.