Electric field induced reorientation of polar molecules in a poly(methyl methacrylate) film studied by electroabsorption spectroscopy

Electroabsorption (EA) or Stark effect spectroscopy is a widely used method for characterizing the dopant molecules in polymer films and the films themselves. In this method the spectrum of a small (∼0.1%) absorbance change induced by a strong (∼1 MV/cm) electric field is measured. Analysis of the spectra provides information about the change in dipole moment and the change in polarizability accompanying an electronic transition. We measured near-UV absorption and EA spectra of indole-doped poly (methyl methacrylate) (PMMA) film prepared by a spin coating method. In the normal-incidence EA spectrum a negative contribution, being proportional to the absorption, was observed. In the case of magic angle between the electric vectors of the absorbed light and the applied field this contribution disappeared. These observations have been explained by field-induced orientation/alignment of the ensemble of polar indole molecules. Since the ground state dipole of indole is nearly parallel with the transition dipole, the absorption rate of light, which is polarized perpendicularly to the applied field, should be smaller. Our study demonstrated that the angular movement of indole-sized molecules in PMMA at room temperature is hindered, but not stopped.