Optical spectroscopy of unsolvated and solvated crystalline Alq(3)

Three novel unsolvated crystalline phases of tris-(8-hydroxyquinoline)-alum inum(III) (Alq(3)), namely alpha-, beta- and gamma -Alq(3), have been synth esized and their crystalline structure determined from X-ray diffraction da ta on powders and single crystals. Both alpha- and beta -Alq(3) crystals ar e 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 trigon al 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 o f these newly isolated crystal phases of Alq(3) have been investigated by a bsorption, photoluminescence excitation, photoluminescence and Raman spectr oscopy. The effect of the different molecular packing on the emission prope rties has been elucidated, the nature of the photoexcitations clarified, an d the vibrational fingerprints of the alpha- and beta -crystalline forms hi ghlighted. The origin of the amorphous nature of the vacuum sublimed thin f ilms 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.