  The reported enhanced singlet-exciton yields in light-emitting polymers over
the statistical limit of 25% has attracted wide experimental and theoretical
attention. Most theoretical estimates of the singlet-exciton yield depend
crucially on estimates of the inter-system crossing rates induced by spin-orbit
coupling. In this paper we use the experimentally determined phosphorescent
life-time and energy of the lowest-lying triplet state, as well as calculated
values of Huang-Rhys factors to estimate the spin-orbit matrix element and
inter-system crossing time between the lowest-lying singlet and triplet states.
