Spin-orbit coupling and intersystem crossing in conjugated polymers: A configuration interaction description

Configuration-interaction calculations are performed to describe the singlet and triplet excited states of oligothiophene and oligo(phenylene ethynylene) conjugated chains. Intersystem crossing from the singlet to the triplet manifold is made possible by spin-orbit coupling, which leads to a mixing of the singlet (S_n) and triplet (T_n) wave functions. The electronic spin-orbit S₁-T_i- matrix elements, obtained from first-order perturbation theory, are used to compute the rates of intersystem crossing from the lowest singlet excited state, S₁, into low-lying triplet states, T_i-. On the basis of these results, a general mechanism is proposed to describe the intersystem crossing process in conjugated oligomers and polymers. The roles of chain length, heavy-atom derivatization, and ring twists are evaluated.