Correlation between molecular packing and optical properties in different crystalline polymorphs and amorphous thin films of mer-tris(8-hydroxyquinoline)aluminum(III)

dSince 1987, high-luminance low-voltage driven devices based on tris(8-hydr oxyquinoline)aluminum(III) (Alq(3)) opened the route to design low-cost lar ge area displays and illuminators. Despite the large number of studies devo ted to this material, very little is known about its basic structural and o ptical properties in the solid state. Therefore, we have investigated the s tructure(s) and the correlation between intermolecular interactions and opt ical properties in various Alq3 systems, including solution, amorphous thin films, and different crystalline forms. Two novel unsolvated polymorphs of Alq3, namely, alpha-Alq(3) and beta-Alq(3), have been synthesized and thei r crystalline structures determined from X-ray diffraction data on powders (cr) and single crystals (beta). Crystals of alpha-Alq(3) are triclinic, sp ace group P-1, a = 6.2586(8) Angstrom, b = 12.914(2) Angstrom; c = 14.743(2 ) Angstrom, alpha = 109.66(1)degrees; beta = 89.66(1)degrees, and gamma = 9 7.68(1)degrees; crystals of beta-Alq(3) are triclinic, space group p-1, a = 8.4433(6) Angstrom, b = 10.2522(8) Angstrom; c = 13.1711(10) Angstrom, alp ha = 108.578(1)degrees, beta = 97.064(1)degrees and gamma = 89.743(1)degree s Both these crystal structures consist of a racemic mixture of the mer iso mer, but are characterized by different molecular packings involving well-d efined short contacts between quinoxaline ligands belonging to symmetry-rel ated Alq3 molecules with interligand spacings in the 3.5-3.9 8 Angstrom ran ge. A third "high-temperature" phase, gamma-Alq(3), was found to contain or ientationally disordered mer-Alq(3) molecules, lying about a 32 position of the trigonal P-31c space group, with a = 14.41(1) Angstrom and c = 6.22(1) Angstrom. In addition, a hemichlorobenzene adduct of Alq(3) was also prepa red and structurally characterized (monoclinic, space group P2(1)/n, a = 10 .786(1) Angstrom, b = 13.808(2) Angstrom, c = 16.928(2) Angstrom, beta = 97 .90(2)degrees). Investigations of the different crystal phases, as well as of amorphous thin films and solutions by absorption, fluorescence excitatio n, fluorescence, and Raman spectroscopy, allowed the effect of the molecula r packing on the emission properties to be elucidated, the nature of the ph otoexcitations to be clarified, and the vibrational fingerprints of the alp ha and beta crystalline forms to be highlighted. The spectral position of f luorescence is found to be correlated with both the molecular density of th e packing and the length of interligand contacts between neigboring Alq(3) molecules as a consequence of different dispersive and dipolar interactions as well as different pi-pi orbital overlaps (the shorter the contacts, i.e ., the denser the crystal, the more the fluorescence is red-shifted). The l ow-temperature (4.2 K) vibronic structure of the fluorescence spectrum of A lq(3) is resolved for the first time. It is assigned to the Franck-Condon a ctivity of an in-plane bending mode at ca. 525 cm(-1), and it is symptomati c of the ligand-centered nature of the optical transitions. From the analys is of the vibronic progression the existence of a strong electron-phonon co upling involving the 525 cm(-1) mode with a Huang-Rhys factor of ca. 2.6 +/ - 0.4 is inferred. The origin of the amorphous nature of the vacuum-sublime d thin films is here explained on the basis of the accessibility of many di fferent pi-pi links between homo- and heterochiral Alq(3) molecules.