The effects of structural and microenvironmental disorder on the electronic properties of poly[2-methoxy,5-(2′-ethyl-hexoxy)-1,4-phenylene vinylene] (MEH-PPV) and related oligomers

In this study, electroabsorption (Stark) spectroscopy is used to determine the trace of the change in polarizability (tr (Delta x) over bar) and the absolute value of the change in dipole moment (/Delta(mu) over right arrow/) of the electroluminescent polymer poly [2-methoxy,5-(2'-ethyl-hexoxy)- 1,4-phenylene vinylene] (MEH-PPV) and several model oligomers in L solvent glass matrixes. We find a value of tr (Delta alpha) over left right arrow of similar to2000 Angstrom(3) for the polymer and for a 9-ring substituted oligomer in both toluene and 2-methyl tetrahydrofuran matrixes at 77 K with smaller values being obtained for 3- and 5-ring unsubstituted oligomers. Although gas-phase calculations of tr /(Delta alpha) over left right arrow/. using INDO/SCI yield values that are about a factor of 8 smaller than the experiment, excellent agreement is obtained when the effects of solid-state dielectric screening are included. Screening increases tr /(Delta x) over left right arrow/ by bringing the energy gap between the 1B(u) and mA(g) states into agreement with solid-state measurements. Substantial values of /Delta(mu) over right arrow/ are observed experimentally both for the polymer and for the oligomers (6-11 D). Because in a planar (C-2h) geometry the oligomer and polymer are centrosymmetric, the observed /Delta(mu) over bar/ is an indication of disorder-induced symmetry breaking in the material. Calculations indicate that disorder in the ground-state geometry of the polymer (inner-sphere disorder) can account for nearly half of the observed /Delta(mu) over right arrow/. Disorder in the glassy environment (outer-sphere disorder) leads to a nonuniform electrostatic environment, and calculations show that this is a substantial contributor, accounting for the remainder of the observed /Delta(mu) over right arrow/.