Optical spectroscopy of unsolvated and solvated crystalline Alq3

Three novel unsolvated crystalline phases of tris-(8-hydroxyquinoline)-aluminum(III) (Alq(3)), namely alpha-, beta- and gamma -Alq(3), have been synthesized and their crystalline structure determined from X-ray diffraction data on powders and single crystals. Both alpha- and beta -Alq(3) crystals are triclinic, space group P-l, but differ for the molecular packings. In the gamma -Alq(3) phase, Alq(3) molecules lie about a 3-position of the trigonal P-3 space group. A solvated Alq(3)(C6H5Cl)(1/2) phase (monoclinic, space group P2(I)/n) was also isolated. The optical and vibrational properties of these newly isolated crystal phases of Alq(3) 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 alpha- and beta -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 pi-pi links between homo- and hetero-chiral Alq(3) molecules. (C) 2001 Elsevier Science B.V. All rights reserved.