Energy diagram of triplet and singlet excited states in conjugated polymers

We have theoretically studied the triplet and singlet excited states of conjugated polymers for the purpose of further understanding the photophysics of these systems. Exact finite chain calculations within the Pariser-Parr-Pople and extended Hubbard models show that the triplet absorption energy is almost independent of the degree of bond alternation. Furthermore, the final state of this triplet-triplet transition lies below the so-called m1 Ag charge-transfer exciton due to correlation effects higher than single configuration-interaction. This indicates that the final state in triplet absorption is an exciton. Based on the exciton nature of the excited triplet state, and energy relationships between the 11 Bu, 21 Ag, and triplet ground state, we are able to determine the absolute lower limit for the exciton binding energy in poly(p-phenylene vinylene).