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

Abstract

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.

Fast and Slow Time Regimes of Fluorescence Quenching in Conjugated Polyfluorene−Fluorenone Random Copolymers: The Role of Exciton Hopping and Dexter Transfer along the Polymer Backbone

“…37,38 This last case may be quenched (if located close to a CTS defect) or not depending on the distance between them and the CTS defects.…”

Conformational disorder and energy migration in MEH-PPV with partially broken conjugation

“…17,[21][22][23][24][25][26][27] The Förster transfer rate is calculated by summing the dipole-dipole interactions between each individual pair of monomers on different spectroscopic units. The orientation and position of the single monomer within a spectroscopic unit is thus needed, even though the excitation is assumed to be delocalized over a spectroscopic unit.…”

Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films

“…Excitation into the inhomogeneous distribution results in rapid energy transfer to the lowest energy states. [33][34][35][36] Thus, the emission in a conjugated polymer can come from a completely different chromophore than the one that was originally excited. Our assignment of the existence of aggregates in solutions of MEH-PPV was predicated on the observation of a distinct ground-state feature, excitation of which led to different photophysics than excitation into the main exciton band.…”