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

Configuration-interaction calculations are performed to describe the single t and triplet excited states of oligothiophene and oligo(phenylene ethynyle ne) 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 S-1-T-i matrix elements, obtained from first-order perturbation theor y, are used to compute the rates of intersystem crossing from the lowest si nglet excited state, S-1, into low-lying triplet states, T-i. On the basis of these results, a general mechanism is proposed to describe the intersyst em crossing process in conjugated oligomers and polymers. The roles of chai n length, heavy-atom derivatization, and ring twists are evaluated.