Femtosecond fluorescence anisotropy studies of solvation-induced intraligand charge transfer in photoexcited aluminum(III) tris(8-hydroxyquinoline)

For the organic light emitting diode compound aluminum(III) tris(8-hydroxyq uinoline) (Alq(3)), dissolved in dimethylformamide (DMF), the time dependen ce of the fluorescence anisotropy has been studied using the femtosecond fl uorescence upconversion technique. Upon excitation within the first absorpt ion band, near 364 nm, with polarized laser pulses of duration less than 15 0 fs, an initial fluorescence anisotropy of about 0.2 is found to rapidly d ecay with a time constant of 2.0 +/- 0.2 ps. The observed fast anisotropy d ecay component is concomitant with the solvation-induced dynamic Stokes shi ft of about 1000 cm(-1). When excitation is at wavelengths below 330 nm, th e fluorescence of Alq(3) in liquid solution does not show any anisotropy ef fect. It is discussed that the emissive lowest excited electronic state of Alq(3) is ligand localized and that its electronic wave function is affecte d by solvation. More specifically, the electronic wave function is consider ed to be an admixture of wave functions belonging to several close-lying st ates. Upon pulsed optical excitation, the admixture is assumed to vary with time, the time dependence being determined by the time evolution of the ge neralized solvation coordinate. The solvation-induced changes of the excite d-state wave function effectuates a directional change of the emission tran sition dipole moment and thus gives rise to a temporal dependence of the fl uorescence anisotropy. The rotational motion's of the Alq(3) solute molecul es also contribute to the fluorescence anisotropy decay. However, these mot ions occur on a much slower time scale (about 100 ps). A comparative study of the fluorescence anistropy decay of Alq(3) in different solvents shows t hat the rotational motions of the Alq(3) molecules follow the Debye-Stokes- Einstein relation.