Exciton relaxation in poly(phenylenevinylene), PPV, has been probed by femtosecond luminescence-up-conversion. We report on excitonic luminescence profiles that depend on the spectral position of the detection window (ε̂=2.7,2.6,2.5, and 2.4 eV, respectively). In an attempt to reveal the transient steps implied in fs relaxation, we present a quantitative forward reconvolution fit procedure that is based on a microscopic incoherent transport model, including diagonal disorder, dipolar intersite coupling, and a density-of-states (DOS) of molecular site excitations. Special emphasis has been placed (i) on the analysis of luminescence lifetime distributions 〈φ(τ;ε̂,τ0)〉 which directly map out the spectra of hopping modes of energy-cascading neutral excitations, and (ii) on the rigorous evaluation of (radiationless) transfer population from high-energy subensembles to low-energy tail states of the DOS. We quantitatively show that the absence of significant rise terms in the S0ν=0←S1ν=0 luminescence transition of low-lying tail states is due to the spectral superposition of the S0ν=1←S1ν=0 luminescence decay from states near the center of the DOS which, owing to the predominantly positive amplitudes of their ultrafast decay channels, destructively interfere with the negative amplitudes (luminescence rise terms) of migratively prepared bottom states. Finally, the asymptotic branch of the luminescence lifetime distribution 〈φ(τ;ε̂=2.4 eV,τ0)〉 has been reconstructed in an unbiased fashion by inverting the high-precision, picosecond, single-photon counting luminescence data at ε̂=2.4 eV with the help of a regularized exponential series methodology. The measurements combined by numerical computing are in accord with the molecular picture of exciton relaxation in PPV in which neutral, localized excitations undergo predominantly incoherent excitation energy transfer among sites of fluctuating self-energies related to segmental subunits with distributed π-bond conjugation lengths.

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We present a comprehensive experimental and computational study on fs-relaxational dynamics of optical excitations in the conjugated polymer poly(p-phenylenevinylene) (PPV) under selective excitation tuning conditions into the long-wavelength, low-vibrational S1ν=0-density-of-states (DOS). The dependence of single-wavelength luminescence kinetics and time-windowed spectral transients on distinct, initial excitation boundaries at 1.4 K and at room temperature was measured applying the luminescence up-conversion technique. The typical energy-dispersive intra-DOS energy transfer was simulated by a combination of static Monte Carlo method with a dynamical algorithm for solving the energy-space transport Master-Equation in population-space. For various, selective excitations that give rise to specific S1-population distributions in distinct spatial and energetic subspaces inside the DOS, simulations confirm the experimental results and show that the subsequent, energy-dissipative, multilevel relaxation is hierarchically constrained, and reveals a pronounced site-energy memory effect with a migration-threshold, characteristic of the (dressed) excitation dynamics in the disordered PPV many-body system.

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