Excitonic relaxation in a conjugated polymer: Initial coherence beyond energy-dispersive population transfer

In this contribution, we deal with the pathways of femtosecond excitonic relaxation in conjugated polymers, focusing on the prominent, ground-state nondegenerate model system poly(p-phenylenevinylene) (PPV). After a brief discussion of the molecular exciton picture of optical excitations in organic semiconductors, we present exemplary results, categorized in two regimes. On early picosecond time scales, the dynamics of incoherent population relaxation is investigated by combining selectively tuned femtosecond excitation pulses with the technique of fluorescence up-conversion. As an initially prepared excitonic distribution gradually lowers its energy by diffusion to the bottom of the density of states (DOS), an excitation energy-dependent bathochromic shift dynamics is observed, accompanied by spectral intensity rearrangements. Motivated by the perception that intersite electronic coupling is absent in the lowest energy regime of the DOS, we further devise a strategy that enables us to measure the very early electronic oscillations and their quantum-stochastic relaxation within approximately the first 200 fs. By using femtosecond wave packet interferometry with appropriately tuned but otherwise freely propagating pulses, we observe fluorescence interferograms with strongly damped, low-frequency beatings, which stem from the spatial interference of two wave packets launched by two-pulse excitation. The results can be explained, semiquantitatively, in terms of a second-order perturbational approach and open up a new perspective on the complex puzzle of PPV optical dynamics.