Femtosecond site-selective probing of energy relaxing excitons in poly(phenylenevinylene): Luminescence dynamics and lifetime spectra

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 (epsilon=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 [phi(tau;epsilon,tau(0))] 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 subassemblies to low-energy tail states of the DOS. We quantitatively show that the absence of significant rise terms in the S-0(v=0) <-- S-1(v=0) luminescence transition of low-lying tail states is due to the spectral superpostion of the S-0(v=0) <-- S-1(v=0) luminescence decay from states near the center of the DOS which, 1 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 [phi(tau;epsilon=2.4 eV, tau(0))] has been reconstructed in an unbiased fashion by inverting the high-precision, picosecond, single-photon counting luminescence data at epsilon=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 pi-bond conjugation lengths. (C) 1997 American Institute of Physics.