Cyano-substituted polyparaphenylenevinylene: description of the nature of the lowest singlet and triplet excited states

The lowest singlet and triplet excited states in cyano-substituted phenylene vinylene oligomers are characterized by means of configuration interaction calculations. First, the vertical singlet-singlet, S-0 --> S-1, singlet-triplet, S-0 --> S-1, and triplet-triplet T-1 --> T-n, excitation energies are evaluated in oligomers ranging in size from two to five phenylene rings; the spatial extent of the S-1, T-1, and T-n excited states is estimated on the basis of a simple analysis of their wavefunctions. We then pay attention to the lattice distortions taking place in the lowest singlet and triplet excited states of these model oligomers. In each case, the results are compared to those obtained for the corresponding unsubstituted oligo(phenylene vinylene)s. Besides the bathochromic shift associated with the electroactive character of the substituents, an overall localization of the excited state wavefunction is found when going from unsubstituted to cyano-substituted oligomers.