Wavelength and Temperature Dependence of the Femtosecond Pump−Probe Anisotropies in the Conjugated Polymer MEH-PPV:  Implications for Energy-Transfer Dynamics

Abstract

Energy transfer in the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is investigated using femtosecond degenerate pump-probe experiments at 298 and 4 K. The polarization anisotropy decays are of the form exp [-(t) 1/2 /T pol ], as predicted by theories of energy transfer in dilute chromophoric systems. At 4 K, these decays depend on the excitation wavelength, with T pol ) 26 fs -1/2 at the peak of the absorption (520 nm) and T pol ) 78 fs -1/2 at the low-energy side of the absorption (580 nm). This wavelength dependence becomes less pronounced at higher temperatures but is always present. We find that models for Förster energy transfer in dilute chromophore solutions cannot describe our data using a single energy-transfer rate calculated from the Förster overlap of the steady-state absorption and emission spectra. The Förster radius R 0 obtained from fitting the experimental anisotropy decays does not agree with that obtained from the steady-state absorption and fluorescence spectra. This fact, along with the wavelength dependence of the anisotropy decays, indicates that the steady-state spectral properties alone are insufficient to explain the energy-transfer properties of MEH-PPV. By use of a simple model to account for inhomogeneous broadening, vibrational line shape, and the intramolecular Stokes shift, we obtain semiquantitative agreement with the experimental results. The key quantity in this modeling is the ratio of inhomogeneous to homogeneous broadening. As the temperature increases, this ratio decreases, leading to less wavelength dependence in the anisotropy decays. The agreement between our modeling and the data suggests that models developed to describe incoherent energy transfer in dilute solutions may be useful for predicting the energy-transport properties of amorphous conjugated polymers, as long as they are modified to take factors such as vibrational structure and inhomogeneous broadening into account.

Origin of the Red Sites and Energy Transfer Rates in Single MEH‐PPV Chains at Low Temperature

“…The average value of 3.9 ps is in reasonable agreement with energy transfer times of MEH-PPV of the order of a few picoseconds that have been estimated on the basis of anisotropy decays at the ensemble level. [16][17][18] While we have no insights of how energy transfer times among red chromophores compare to those of blue ones (since no donor ZPLs could be observed in the latter case [46] ), the situation for red chromophores, given that they are located in densely packed chain regions, is likely to be to some extent similar to pristine MEH-PPV films. Thus, the results presented herein might be relevant for technical devices based on conjugated polymers, their performance often being crucially influenced by the efficiency of energy transfer.…”

Variable Temperature Single‐Molecule Dynamics of MEH‐PPV

“…[2-6, 10, 13, 21, 43] This interpretation is further supported by the observation that the relative intensity ratios for an ensemble of single-molecule spectra excited at different wavelengths (488/543 nm and 543/568 nm) are close to the absorption cross-section ratios in the absorption spectrum of the MEH-PPV film at 4 K for these wavelengths. [43] This is shown as the dashed lines in Figures 4 A-4 B and 5 A-5 B.…”

Exciton Delocalization Drives Rapid Singlet Fission in Nanoparticles of Acene Derivatives

“…The observation of a rapid decay of the anisotropy on a picosecond time scale is consistent with energy migration in a disordered, condensed phase medium. 106 The pump−probe anisotropy measurements enable further clarification of the extent of solid-state order in the nano- Overlaying the data is a linear fit to the first five data points, with r 2 = 0.9972. A linear fit including all six data points gave r 2 = 0.9860.…”