STRUCTURE AND OPTICAL-ABSORPTION OF OLIGO(P-PHENYLENEVINYLENE AND M-PHENYLENEVINYLENE) AND THEIR RADICAL-ANIONS - A COMPARATIVE THEORETICAL-STUDY

We report on a comparative quantum-chemical study of oligo(p- and m-ph enylenevinylene)s (OPPV and OMPV, respectively) isomers and their radi cal anions in order to explain their optical absorption spectra. The A M1 semiempirical approach is employed to assess the structural changes from neutral to monocharged states. The optimized geometries are furt her used in PPP-CI calculations of the optical transitions for the clo sed and open shell pi-systems. With increasing number of repeat units, OPPV exhibits significant splitting of the frontier pi-states, which remain almost degenerate for OMPV. As a consequence, reduction is foun d to result in an extended charged defect for the former oligomers. In contrast, self-trapping of the excess charge into a single stilbene u nit is predicted for OMPV. This result serves further as a basis to re produce and interpret in detail the several characteristic absorptions of the title compounds.