Localized π electron states in polydiacetylene oligomers

Abstract

It is shown that the particular bond alternation pattern of short chain polydiacetylene intermediates gives rise to the occurrence of two nonbonding and nearly degenerate π molecular orbitals (MOs) which are localized at the chain ends of the longer oligomers. These orbitals evolve with increasing chain length from the highest bonding and the lowest antibonding π MOs leading to a loss of the closed shell structure of the π system, which is characterized by an increasing admixture of a doubly excited configuration to the ground configuration. We present a Hartree-Fock scheme which optimizes the coefficients of this configurational mixing and the MO coefficients simultaneously. The configuration interaction (CI) calculation is reformulated in terms of a simple two-site Hubbard model in a localized basis.