Excited State Relaxation in pi-Conjugated Polymers

We study ultrafast relaxation processes of odd (B sub u) and even parity (A sub g) exciton states in poly(p-phenylene vinylene) derivatives. The B sub u states are studied using a regular two-beam pump-and-probe spectroscopy, which can monitor vibronic relaxation and exciton diffusion. In order to observe the A sub g states, a novel three-beam fsec transient spectroscopy is developed, in which two different excitation pulses successively generate odd parity (1 B sub u) excitons at 2.2 eV and then re-excite them to higher A sub g states. We are able to distinguish two different classes of A sub g states: one class (mA sub g) experiences ultrafast internal conversion back to the lowest singlet exciton, whereas the other class (k A sub g) in violation of Vavilov-Kasha's rule undergoes a different relaxation pathway. The excitons subsequently dissociate into long-lived polaron pairs, which results in emission quenching with the action spectrum similar to that of the intrinsic photoconductivity. We conclude that the Asub g states above 3.3eV (k A sub g) are charge transfer states, which mediate carrier photogeneration.