A new crystalline phase of the electroluminescent material tris(8-hydroxyquinoline) aluminum exhibiting blueshifted fluorescence

Abstract

We report on two different crystalline phases of tris͑8-hydroxyquinoline͒ aluminum (Alq 3) which were obtained by thermal sublimation in a horizontal glass tube. These phases are investigated by x-ray powder diffraction, Raman and infrared spectroscopy, and low temperature photoluminescence measurements. Apart from the already known ␣ phase we could identify a new crystalline phase of Alq 3 (␦-Alq 3) showing blueshifted fluorescence. As compared to the ␣ phase this new phase is characterized by a larger unit cell volume, a reduced number of Raman lines in the energy range between 70 and 700 cm Ϫ1 , a blueshift of the photoluminescence maximum by about 0.2 eV, and a decreased intersystem crossing to the triplet state. These differences are interpreted in terms of the isomery of the Alq 3 molecule. It is supposed that the new phase contains the facial isomer, whereas in the other phases only the meridianal isomer was reported. Low temperature photoluminescence spectra show a well-resolved vibronic progression with about the same spacing of 550 cm Ϫ1 for both crystalline phases of Alq 3. Site-selective photoluminescence measurements reveal the existence of an additional redshifted featureless emission, which is ascribed to energy relaxation into low-lying states.

Characterization of the Triplet State of Tris(8-hydroxyquinoline)aluminium(III) in Benzene Solution

“…In contrast, a value of ca. 10 ms was reported from delayed luminescence measurements on polycrystalline Alq 3 at 6 K. 13 This may indicate a marked temperature dependence for the triplet decay, possibly due to triplet hopping to quenching sites at higher temperatures. From the initial absorbances of biphenyl triplet and Alq 3 transient absorptions, and the value ) 27 100 M -1 cm -1 for the biphenyl triplet state, 16 a molar absorption coefficient 510 nm ) 3350 ( 350 M -1 cm -1 was determined for the Alq 3 triplet state, within the range of values reported for triplets of heavy metal quinolates.…”

“…This is close to φ T ≈ 0.25 estimated for the meridianal isomer at low temperatures. 13 This, coupled with the fluorescence yield, 3,9,10 shows the importance of nonradiative decay of singlet excitons in Alq 3 , with internal conversion being particularly important. 20 Energy transfer is a valuable method for determining triplet energies.…”

“…Various values have been reported for the fluorescence quantum yield 3,9,10 and lifetime, 3,11 and only indirect information exists on the triplet state. 12,13 Attempts to observe phosphorescence have proved unsuccessful, 11,12 although some information has been inferred from data on heavy metal quinolates. 11 We have shown 14 that pulse radiolysis of benzene solutions selectively produces triplet states of conjugated organic polymers by energy transfer from appropriate sensitizers following pulse radiolysis of benzene solutions.…”

Formation of Flower-Like Crystals of Tris(8-hydroxyquinoline)aluminum from 8-Hydroxyquinoline on Anodic Porous Alumina

“…The preparation and characterization of Alq 3 have been widely studied [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Because of the bidentate ligand, Alq 3 complexes form two isomers, as shown in Figure 1: meridional and facial.…”

“…Because of the bidentate ligand, Alq 3 complexes form two isomers, as shown in Figure 1: meridional and facial. Interestingly, the two isomers exhibit different types of photoluminescence (PL): specifically, green [2,3] in the case of the meridional isomer and blue [4][5][6][7] for the facial isomer. To date, five crystalline phases ( , , , , and ) have been identified for Alq 3 [8][9][10].…”