Excited-state relaxation in π-conjugated polymers -: art. no. 205209

We study ultrafast relaxation processes of odd- (B-u) and even-parity (A(g)) exciton states in poly(p-phenylene vinylene) derivatives. The B-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(g) states, a three-beam femtosecond transient spectroscopy is developed, in which two different excitation pulses successively generate odd-parity (1B(u)) excitons at 2.2 eV and then reexcite them to higher A(g) states. We are able to distinguish two different classes of A(g) states: one class (mA(g)) experiences ultrafast internal conversion back to the lowest singlet exciton, whereas the other class (kA(g)) in violation of the 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 A(g) states above 3.3 eV (kA(g)) are charge-transfer states, that mediate carrier photogeneration.