Exciton dynamics in a disordered conjugated polymer: Three-pulse photon-echo and transient grating experiments

We study the sub-picosecond exciton dynamics of the prototypical soluble conjugated polymer poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) by comparative three-pulse photon-echo peak-shift and transient grating measurements, performed under excitation energy tuning conditions. The strong sensitivity of signal decays to the chosen excitation energy is ascribed to the interplay of inter- and intra-segmental relaxation events in an ensemble of electronically coupled sites. As a result, the energetic site-to-site disorder and its contribution to the spectral lineshape is most reliably determined for excitation of the redmost part of the absorption spectrum. Comparing the data for the two solvents toluene and tetrahydrofurane, we conclude a weak sensitivity only to the specific type of solute-solvent interactions. Within our time-resolution, we surprisingly recover weak but reproducible modulations of the third-order signals that are damped out on the early picosecond timescale.