Excimers in light-emitting conjugated polymers

The electronic structures of the low-lying excited states for two parallel chains of poly$(p$-phenylene-vinylene) with arbitrary relative positions are studied. The interchain interaction is treated microscopically. The energy and charge transfer fraction of the (light-emitting) lowest singlet state are found to depend sensitively on the horizontal shift of the two chains, which is in turn determined by the packing geometry. Our predictions of the excimer-forming geometry and the relative (dipole-forbidden) photoluminescence lifetimes are in excellent agreement with experiments.