Anomalous Exciton Diffusion in the Conjugated Polymer MEH−PPV Measured Using a Three-Pulse Pump−Dump−Probe Anisotropy Experiment

A femtosecond pump−dump−probe anisotropy experiment is used to study the time-dependent motion of singlet excitons in the conjugated polymer poly[2-methoxy-5-((2-ethylhexyl)oxy)-1,4-phenylenevinylene] (MEH−PPV). The pump pulse prepares an unpolarized population of excitons at time t = 0, which is then depleted by a linearly polarized dump pulse at t = T 12 . The anisotropy decay of the remaining population is then monitored as a function of the probe delay, T 23 . At both room temperature and 4 K, the T 23 anisotropy decay is observed to slow as T 12 increases. Although interpretation of our results is complicated by the possible presence of excited-state absorption at 600 nm, the wavelength of the dump and probe pulses, the T 12 dependence is consistent with a slowing down of the diffusion of the luminescent exciton during its lifetime in the material. This experiment provides a way to directly probe anomalous diffusion of the exciton at different points during its lifetime, which is what ultimately determines the distance it can travel in the polymer.