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-phenylenevinylene)s (OPPV and OMPV, respectively) isomers and their radical anions in order to explain their optical absorption spectra. The AM1 semiempirical approach is employed to assess the structural changes from neutral to monocharged states. The optimized geometries are further used in PPP-CI calculations of the optical transitions for the closed 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 found to result in an extended charged defect for the former oligomers. In contrast, self-trapping of the excess charge into a single stilbene unit is predicted for OMPV. This result serves further as a basis to reproduce and interpret in detail the several characteristic absorptions of the title compounds.