Wavelength and temperature dependence of the femtosecond pump-probe anisotropies in the conjugated polymer MEH-PPV: Implications for energy-transfer dynamics

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 Forster energy transfer in dilute chromophore solutions cannot describe our data using a single energy-transfer rate calculated from the Forster overlap of the steady-state absorption and emission spectra. The Forster 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-tran sport properties of amorphous conjugated polymers, as long as they are modified to take factors such as vibrational structure and inhomogeneous broadening into account.