Electron-spin-resonance experiments and theory of the butatriene-to-acetylene transition in short-chain polydiacetylene molecules

We report on electron-spin-resonance experiments and theoretical calculations concerning the electronic structure of short-chain polymerization intermediates in diacetylene crystals. The structural transition from diradicals to dicarbenes observed at a critical chain length of six repeat units is quantitatively explained by the generation of ``up'' and ``down'' radical electrons at the chain ends of the reaction intermediates. The best fit of the fine-structure and hyperfine-structure parameters is obtained by an energy difference of 0.4 eV per unit cell between the butatriene and acetylene structure and by a transfer integral of 0.5 eV.