On the assignment of the absorption bands in the optical spectrum of Alq₃

Abstract

The optical absorption and/or excitation spectrum of tris(8-hydroxyquinolinato) aluminium(III) (Alq₃) in thin solid film is similar to the spectra in organic solvents, in vapour at ~600 K and under jet-cooled conditions. The spectrum of Alq₃ shows two weak but prominent bands at ~334 and ~320 nm which have not been assigned before. These weak bands are now assigned to a vibronic progression due to the ring deformation modes at ~1300 cm^-1 associated with an electronic transition whose origin is at 346 nm (28862 cm^-1). This assignment is based on the one-to-one correspondence of the absorption bands of Alq₃ and the ligand, 8-hydroxyquinoline in its protonated form. In addition, the long wavelength absorption band of Alq₃ at ~390 nm is concluded to be a superposition of two or more electronic transitions to account for the large discrepancy between the calculated radiative lifetime of Alq₃ and measured fluorescence lifetimes in condensed phase and jet-cooled beam.