Theory of π-electron states in polydiacetylene molecules

We report on π-electron states in polydiacetylene chains using Hückel and self-consistent-field methods (SCF). The Hückel band-structure calculation for the infinitely long chain yields two valence and two conduction bands. The Peierls gap between the upper valence and the lower conduction band is dependent on the bond alternation within the conjugated π system described by a bond alternation parameter −1<q<1. It turns out that the acetylene structure (q>0) is energetically more favorable than the butatriene structure (q<0). Performing Hückel calculations for short chain polydiacetylene intermediates and including the σ electrons of the polymer backbone by a harmonic potential we obtain the chain length dependency of the bond alternation parameter q. The intermediates with two reactive chain ends (diradicals and dicarbenes) show a transition from the butatriene to the acetylene structure. To determine the spin multiplicity of the ground state of these intermediates we perform a SCF calculation for the singlet and the triplet state of the conjugated π system. According to the experiments we find a singlet ground state for chain lengths of less than six monomer units (diradicals) and a triplet ground state for longer chain lengths (dicarbenes). Optical excitation energies of the short-chain intermediates are determined using the method of Pariser, Parr, and Pople. The qualitative tendencies of the chain length dependency agree well with the experimental results of optical absorption measurements, the absolute values are too high as compared to the experiments.