Polarized electroabsorption spectroscopy of highly ordered poly(2-methoxy,5-(2’-ethyl-hexoxy)-p-phenylene vinylene)

We present a quantitative analysis of the absorption and field-induced absorption (electroabsorption) spectra of dilute blends of the conjugated polymer poly[2-methoxy,5-(2’-ethyl-hexoxy)-p-phenylene vinylene] (MEH-PPV) oriented in ultrahigh-molecular-weight polyethylene (PE). 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 oriented, ordered MEH-PPV π-electron system. The optical data obtained from the oriented blends are, therefore, directly relevant to theoretical models which describe the intrinsic electronic structure of the conjugated polymer, i.e., models which do not account for disorder. Using a model based on the continuum version of the Su-Schrieffer-Heeger theory, we simultaneously fit to the absorption and electroabsorption spectra, and demonstrate that the intrinsic optical line shape of MEH-PPV oriented in polyethylene is highly asymmetric, and can be modeled accurately in terms of a Gaussian-broadened one-dimensional square-root singularity in the joint density of states.