Polarized electroabsorption spectroscopy of MEH-PPV oriented by gel processing in polyethylene: polarization anisotropy, the off-axis dipole moment and excited state delocalization

Summary form only given. We present the polarized electric field modulation spectra of dilute blends of MEH-PPV oriented in ultra-high molecular weight polyethylene, PE. The polarized electroabsorption spectra of the oriented blend demonstratesa field-induced nonlinear refractive index anisotropy in excess of 150:1, with preferred polarization parallel to the draw axis. 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 X/sup (10)/(w), and the electroabsorption is directly proportional to the imaginary part of X/sup (3)/(w, 0, 0) of the ordered MEH-PPV /spl pi/-electron system. The optical data obtained from the oriented blends is 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 transstilbene 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 wavefunctions are delocalized over a minimum of 50 unit cells (400 /spl Aring/).