The Effects of Structural and Micro-Environmental Disorder on the Electronic Properties of MEH-PPV and Related Oligomers

In this study, electroabsorption (Stark) spectroscopy is used to determine the trace of the change in polarizability (Tr α ∆ ) and the absolute value of the change in dipole moment ( μ ∆ ) of the electroluminescent polymer MEH-PPV and several model oligomers in solvent glass matrices. We find a value of Tr α ∆ of ~ 2000 Å for the polymer and for a 9-ring substituted oligomer in both toluene and 2-methyl tetrahydrofuran matrices at 77K with smaller values being obtained for 3 and 5 ring unsubstituted oligomers. Although gas-phase calculations of Tr α ∆ 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 α ∆ by bringing the energy gap between the 1Bu and mAg states into agreement with solid-state measurements. Substantial values of μ ∆ are observed experimentally both for the polymer and the oligomers (6-11 D). Because in a symmetric