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-STATEDELOCALIZATION

We present the polarized electric field modulation spectra of dilute b lends of the conjugated polymer 2-methoxy,5-(2'-ethyl-hexoxy)-p-phenyl enevinylene] (MEH-PPV) oriented in ultrahigh-molecular-weight polyethy lene (PE). The photoluminescence and electric-field-modulation spectra of tensile drawn MEH-PPV/PE blends are highly anisotropic, with prefe rred polarization parallel to the draw axis; the field-induced absorpt ion and photoluminescence demonstrate polarization anisotropies in exc ess of 150:1 and 60:1, respectively. Because of the orientation, the e nhanced order, and the dilution of the MEH-PPV conjugated polymer in t he polyethylene matrix, the MEH-PPV/PE blend is an ideal system in whi ch to investigate the intrinsic linear and nonlinear optical propertie s of oriented conjugated polymer chains. The absorption is directly pr oportional to the imaginary part of chi(1)(omega), and the electroabso rption is directly proportional to the imaginary part of chi(3) (omega , 0, 0) of the ordered MEH-PPV pi-electron system. The optical data ob tained from the oriented blends are therefore directly relevant to exi sting theoretical models which do not account for disorder. Using a mo del for the off-axis transition dipole moment of the conjugated polyme r, 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 delo calized over a minimum of 50 unit cells (400 angstrom).