The Low-Lying Excited States of Luminescent Conjugated Polymers: Theoretical Insight into the Nature of the Photogenerated Species and Intersystem Crossing Processes

In this contribution, we first discuss the nature of the primary photoexcited species in the lowest excited state of luminescent conjugated polymers (for instance, polyparaphenylene vinylene, PPV) on the basis of quantum-chemistry calculations including both electron-phonon coupling and electron-electron interactions. We conclude that electronic excitations in long PPV chains lead to the formation of weakly bound polaron-excitons, in agreement with a large number of recent experimental measurements. We then describe singlet and triplet excited states in oligothiophenes and point to the importance of intersystem crossing as an efficient nonradiative decay process of the singlet excitations.