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 Δα^↔) and the absolute value of the change in dipole moment (|Δμ^→|) of the electroluminescent polymer poly[2-methoxy,5-(2′-ethyl-hexoxy)-1,4-phenylene vinylene] (MEH-PPV) and several model oligomers in solvent glass matrixes. We find a value of tr Δα^↔ of ∼2000 ų 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 Δα^↔ 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 Δα^↔ by bringing the energy gap between the 1B_u and mA_g states into agreement with solid-state measurements. Substantial values of |Δμ^→| 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 |Δμ^→| 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 |Δμ^→|. 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 |Δμ^→|.