Femtosecond Fluorescence Anisotropy Studies of Solvation-Induced Intraligand Charge Transfer in Photoexcited Aluminum(III) Tris(8-hydroxyquinoline)

Abstract

For the organic light emitting diode compound aluminum(III) tris(8-hydroxyquinoline) (Alq3), dissolved in dimethylformamide (DMF), the time dependence of the fluorescence anisotropy has been studied using the femtosecond fluorescence upconversion technique. Upon excitation within the first absorption band, near 364 nm, with polarized laser pulses of duration less than 150 fs, an initial fluorescence anisotropy of about 0.2 is found to rapidly decay with a time constant of 2.0 ± 0.2 ps. The observed fast anisotropy decay component is concomitant with the solvation-induced dynamic Stokes shift of about 1000 cm-1. When excitation is at wavelengths below 330 nm, the fluorescence of Alq3 in liquid solution does not show any anisotropy effect. It is discussed that the emissive lowest excited electronic state of Alq3 is ligand localized and that its electronic wave function is affected by solvation. More specifically, the electronic wave function is considered to be an admixture of wave functions belonging to se...