Excited-state relaxation in π-conjugated polymers

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.