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 s pectra from perylene to pentarylene. Neutral OR have an aromatic struc ture caused by electron-lattice coupling and specific boundary conditi ons. Doping is predicted to favour the alternative quinoid form, found to be metastable for the corresponding pristine one-dimensional latti ce. Radical anion (negative polaron) and singlet excited state (excito n-polaron) species in finite OR are obtained as smooth segments of red uced CC bond length alternation. Crossover to the quinoid form, partic ularly for the innermost repeat units, is predicted to occur yet for t he dianions (negative bipolarons). Up to hexarylene, the structural re laxation affects the whole molecule, i.e. the characteristic length of the charged and neutral defects is not yet reached. The latter conclu sion is primarily inferred from the absorption spectra of OR in both r edox states. As for the neutral compounds, all characteristic absorpti ons show linear energy versus reciprocal length dependence. This featu re is well reproduced when employing the PPP-CI approach and accountin g for the doping-induced geometrical changes. The agreement between ex perimental and calculated transition energies and intensities is satis factory for the closed shell molecules and very good for the radical a nions. Particularly in the latter case, CI is essential, suggesting de ficiencies of the one-electron approach as far as optical transitions are concerned.