Effects of excitation tuning on ultrafast exciton dynamics in poly(p-phenylene-vinylene) (PPV)

We investigate the relaxational dynamics of optical excitations in the conjugated polymer poly(p-phenylene-vinylene) (PPV) at low (1.4 K) and at room temperature, focussing on the femtosecond (fs) and early picosecond (ps) time-regime. Combining selectively tuned fs-excitation pulses with the technique of fluorescence up-conversion, we report a significant dependence of decay patterns and spectral transients on the energetic position of the central excitation energy. At 1.4 K, as an initially prepared excitonic population gradually lowers its energy by diffusion to the bottom of the density-of-states (DOS), excitation energy dependent bathochromic shift dynamics is observed, revealing an initial site-energy memory effect. Our measurements further unveil, at higher excitation energies, a sub-ps rise-term for one of the vibronic transitions, mainly attributed to a superposition of the transient red-shift and an initial reabsorption of the blue part of the fluorescence spectra. At room temperature, the energy-downhill gradient for exciton migration becomes thermally eroded, and only for higher excitation energies, considerably weaker spectral kinetics is detected. (C) 2004 Elsevier B.V. All rights reserved.