Polarized-electroabsorption spectroscopy of a soluble derivative of poly(p-phenylenevinylene) oriented by gel processing in polyethylene: Polarization anisotropy, the off-axis dipole moment, and excited-state delocalization

We present the polarized electric field modulation spectra of dilute blends of the conjugated polymer poly[2-methoxy,5-(2’-ethyl-hexoxy)-p-phenylenevinylene] (MEH-PPV) oriented in ultrahigh-molecular-weight polyethylene (PE). The photoluminescence and electric-field-modulation spectra of tensile drawn MEH-PPV/PE blends are highly anisotropic, with preferred polarization parallel to the draw axis; the field-induced absorption and photoluminescence demonstrate polarization anisotropies in excess of 150:1 and 60:1, respectively. Because of the orientation, the enhanced order and the dilution of the MEH-PPV conjugated polymer in the polyethylene matrix, the MEH-PPV/PE blend is an ideal system in which to investigate the intrinsic linear and nonlinear optical properties of oriented conjugated polymer chains. The absorption is directly proportional to the imaginary part of ${\mathrm{\chi }}^{\mathrm{}\left(1\right)\mathrm{}}$(ω), and the electroabsorption is directly proportional to the imaginary part of ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(ω,0,0) of the ordered MEH-PPV π-electron system. The optical data obtained from the oriented blends are therefore directly relevant to existing theoretical models which do not account for disorder. Using a model for the off-axis transition dipole moment of the conjugated polymer, which is based on the measured off-axis transition dipole moment of trans-stilbene and the geometric structure of MEH-PPV, we demonstrate that the observed anisotropy of 150:1 in the field-induced absorption requires that the instantaneous excited-state wave functions are delocalized over a minimum of 50 unit cells (400 Å).