Comparative study of electroabsorption spectra of polar and nonpolar organic molecules in solution and in a polymer film.

Electroabsorption (EA) spectra of polar and nonpolar molecules of coumarin 153 (C153) and pyrene in solution and in a polymer film of polymethylmethacrylate (PMMA) have been measured in the UV-visible region at room temperature. The shape of the EA spectra of C153 in benzene, 1,4-dioxane, or monochlorobenzene remarkably depends on the angle between the polarization direction of the absorption light and the applied electric field, whereas the EA spectra of C153 doped in PMMA show only the Stark shift and the field-induced change in spectral shape is negligible. These results demonstrate that C153 is reoriented by application of electric fields in solution, but the molecules are immobilized in a PMMA film. Based on the EA spectra, electric dipole moments both in the ground state and in the excited state have been evaluated for C153 in different solvents. In the EA spectra of pyrene, only the Stark shift is observed both in solution and in PMMA, indicating that the field-induced molecular reorientation does not occur both in solution and in PMMA. The change in dipole moment of C153 as well as the change in molecular polarizability of pyrene following absorption is much larger in solution than that in PMMA.