Optical spectroscopy of unsolvated and solvated crystalline Alq3

Three novel unsolvated crystalline phases of tris-(8-hydroxyquinoline)-aluminum(III) (Alq3), namely α-, β- and γ-Alq3, have been synthesized and their crystalline structure determined from X-ray diffraction data on powders and single crystals. Both α- and β-Alq3 crystals are triclinic, space group P-1, but differ for the molecular packings. In the γ-Alq3 phase, Alq3 molecules lie about a 3-position of the trigonal P-3 space group. A solvated Alq3(C6H5Cl)1/2 phase (monoclinic, space group P21/n) was also isolated. The optical and vibrational properties of these newly isolated crystal phases of Alq3 have been investigated by absorption, photoluminescence excitation, photoluminescence and Raman spectroscopy. The effect of the different molecular packing on the emission properties has been elucidated, the nature of the photoexcitations clarified, and the vibrational fingerprints of the α- and β-crystalline forms highlighted. The origin of the amorphous nature of the vacuum sublimed thin films is discussed on the basis of the accessibility of many different π-π links between homo- and hetero-chiral Alq3 molecules.