Electronic structure of diradical and dicarbene intermediates in short-chain polydiacetylene oligomers

Semiempirical molecular orbital (MO) calculations on short-chain (n=2 to n=8 monomer units) diradical and dicarbene intermediates obtained in the solid state polymerization reaction of diacetylene monomer crystals have been performed. With increasing chain length a transition from the diradicals with two unpaired electrons to the dicarbenes with four unpaired electrons is found in agreement with experimental results. The energetic ordering of the low-lying multiplets (singlet and triplet for the diradicals; singlet, triplet, and quintet for the dicarbenes) is discussed in terms of a model Hamiltonian containing a Hubbard term which takes into account the electron transfer between the radical electrons on different chain ends, and a Heisenberg term describing the exchange interaction of radical electrons on the same chain end in the case of the dicarbenes. All intermediates have a singlet ground state. The small energy gaps to the higher triplet and quintet states decrease rapidly with increasing chain length due to a decrease in the ratio t/U, with t and U being the transfer integral and the on-site Coulomb repulsion of the Hubbard model, respectively. The fine structure arising from the magnetic dipolar interaction of the unpaired electrons is discussed.