Theoretical studies of localization phenomena in excited states of polyene chains

We present π‐electrons RHF‐SCF and CI calculations of the evolution of the first singlet excited state transition energy as a function of chain length, in polyene oligomers ranging from C₂H₄ up to C₄₂H₄₄. The results indicate that in small oligomers, the excitation is delocalized over the whole molecule. Localization phenomena are present in the larger chains (larger than ∼ C₁₄H₁₆) thereby explaining the failure of the RHF‐SCF technique to properly describe the first singlet excited state energies in the extended oligomers. Consequences in the use of so‐called OAO techniques are pointed out.