Electric field effects on state energy and molecular orientation of 2-hydroxyquinoline in solid polymer films

External electric field effects on the electronic absorption spectra have been examined for 2-hydroxyquinoline (2-HQ) doped in polymer films of poly(methyl methacrylate) (PMMA) and poly(vinyl alcohol) (PVA). The electroabsorption (E-A) spectra of 2-HQ observed at 295 K and at 50 K show the change both in the permanent dipole moment (Δμ) and in the molecular polarizability (Δα) associated with the absorption transitions to the lower lying electronic states. The polarization dependence and the temperature dependence of the E-A spectra show that 2-HQ is oriented along the direction of applied electric fields at room temperature both in PMMA films and in PVA films. At a low temperature of ∼50 K, on the other hand, the field-induced orientation of 2-HQ is restricted in both films, and only the Stark shifts induced by changes in Δμ and Δα are observed. 6-Hydroxyquinoline (6-HQ) also orients along the applied electric field in PMMA, but the field-induced orientation of 6-HQ is restricted in PVA even at room temperature [M.S. Mehata, T. Iimori, T. Yoshizawa, N. Ohta, J. Phys. Chem. A 110 (2006) 10985]. The difference of the field-induced orientation in PVA between 2-HQ and 6-HQ suggests that 2-HQ has a cis enol form in the ground state.