Correlation between Molecular Packing and Optical Properties in Different Crystalline Polymorphs and Amorphous Thin Films of mer-Tris(8-hydroxyquinoline)aluminum(III)

Since 1987, high-luminance low-voltage driven devices based on tris(8-hydroxyquinoline)aluminum(III) (Alq₃) opened the route to design low-cost large area displays and illuminators. Despite the large number of studies devoted to this material, very little is known about its basic structural and optical properties in the solid state. Therefore, we have investigated the structure(s) and the correlation between intermolecular interactions and optical properties in various Alq₃ systems, including solution, amorphous thin films, and different crystalline forms. Two novel unsolvated polymorphs of Alq₃, namely, α-Alq₃ and β-Alq₃, have been synthesized and their crystalline structures determined from X-ray diffraction data on powders (α) and single crystals (β). Crystals of α-Alq₃ are triclinic, space group P-1, a = 6.2586(8) Å, b = 12.914(2) Å; c = 14.743(2) Å, α = 109.66(1)°; β = 89.66(1)°, and γ = 97.68(1)°; crystals of β-Alq₃ are triclinic, space group P-1, a = 8.4433(6) Å, b = 10.2522(8) Å; c = 13.1711(10) Å, α = 108.578(1)°, β = 97.064(1)°, and γ = 89.743(1)°. Both these crystal structures consist of a racemic mixture of the mer isomer, but are characterized by different molecular packings involving well-defined short contacts between quinoxaline ligands belonging to symmetry-related Alq₃ molecules with interligand spacings in the 3.5−3.9 Å range. A third “high-temperature” phase, γ-Alq₃, was found to contain orientationally disordered mer-Alq₃ molecules, lying about a 32 position of the trigonal P-31c space group, with a = 14.41(1) Å and c = 6.22(1) Å. In addition, a hemichlorobenzene adduct of Alq₃ was also prepared and structurally characterized (monoclinic, space group P2₁/n, a = 10.786(1) Å, b = 13.808(2) Å, c = 16.928(2) Å, β = 97.90(2)°). Investigations of the different crystal phases, as well as of amorphous thin films and solutions by absorption, fluorescence excitation, fluorescence, and Raman spectroscopy, allowed the effect of the molecular packing on the emission properties to be elucidated, the nature of the photoexcitations to be clarified, and the vibrational fingerprints of the α and β crystalline forms to be highlighted. The spectral position of fluorescence is found to be correlated with both the molecular density of the packing and the length of interligand contacts between neigboring Alq₃ molecules as a consequence of different dispersive and dipolar interactions as well as different π−π orbital overlaps (the shorter the contacts, i.e., the denser the crystal, the more the fluorescence is red-shifted). The low-temperature (4.2 K) vibronic structure of the fluorescence spectrum of Alq₃ is resolved for the first time. It is assigned to the Franck−Condon activity of an in-plane bending mode at ca. 525 cm^-1, and it is symptomatic of the ligand-centered nature of the optical transitions. From the analysis of the vibronic progression the existence of a strong electron−phonon coupling 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-sublimed thin films is here explained on the basis of the accessibility of many different π−π links between homo- and heterochiral Alq₃ molecules.