Linear and nonlinear optical properties of the conjugated polymers PPV and MEH-PPV

We have used absorption and electroabsorption spectroscopy to investigate the electronic structure of poly(para-phenylene vinylene) (PPV) and poly (2-methoxy, 5-${(2\mathrm{}}^{\prime }$-(ethyl)hexyloxy)-p-phenylene vinylene) (MEH-PPV). In particular we examine the often used assumption that the electronic structure of PPV and its dialkoxy substituted derivatives are essentially the same. The absorption spectrum of PPV consists of three peaks, while that of MEH-PPV has four peaks. We discuss the controversial origin of the extra peak as well as evidence for Davydov splitting effects in the absorption spectrum of PPV. The analysis of the nonlinear spectra shows further differences between the two materials. First, the binding energy of the ${1B}_u$ exciton for PPV is some 0.1 eV higher than for MEH-PPV. Second, the peak value of $\mathrm{Im}\{{\chi }^{\mathrm{}\left(3\right)\mathrm{}}(-\omega ;0,0,\omega )\}$ for PPV is approximately 40 times higher than that of MEH-PPV. We also found that the sum-over-states modeling of the electroabsorption spectra indicates that the transition dipole moment between the ${\mathrm{mA}}_g$ and ${\mathrm{nB}}_u$ states is of opposite sign in the two polymers.