Intrachain photoluminescence dynamics of MEH-PPV in the solid state

The photoluminescence (PL) dynamics of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) blended in host polymer (polypropylene, PP) matrix as well as that in the neat film has been studied. The concentration of MEH-PPV in the PP blend is designed to be fairly low (0.01 wt %) in order to observe the intrinsic intrachain PL property of MEH-PPV in the solid state. The steady-state 0-0 PL band of the blend sample shows a blue-shift of 0.12 eV with respect to that of the neat film of MEH-PPV. The PL-excitation (PLE) spectra of the blend sample exhibit definite vibronic structure, and hence we can determine the magnitude of the Stokes shift as 0.06 eV. The blend sample shows a single-exponential PL decay at 4 K with a time constant of 850 ps. We emphasize that this single-exponential-type PL decay is an intrinsic property of the intrachain PL species. Time-resolved PL measurements confirm dynamical red-shift of the PL band in the neat film, whereas this trend is not found in the case of the PP blend. These observations indicate that the energy transfer between finite segments, which can cause exciton migration, is much less efficient within the isolated MEH-PPV polymer chain compared to the case of the interchain transfer. The time-resolved measurements further demonstrate that the Stokes shift identified in the blend sample takes place at the early stage within 50 ps following photoexcitation. We attribute this Stokes shift to the rapid increase of the planarity of the MEH-PPV chain caused by the torsion of some constituent phenyl rings following photoexcitation. Finally, based on an argument on the different magnitudes of Stokes shift between the blend sample and the neat film, we conclude that the PL of MEH-PPV in the neat film predominantly occurs at the site of interchain excitations via the interchain migration of excitons.