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

Abstract

Since Tang and VanSlyke first reported efficient electroluminescence from a device based on tris(8-hydroxyquinoline)aluminium(III) (Alq 3 ) as the luminescent layer, 1 organic light-emitting diodes (OLEDs), based on these materials, are now at the stage where they are being used as monochrome displays. This results from emission from a ligand centered π, π* excited state, formed in the transition of an electron from a highest occupied molecular orbital (MO) lying mainly on the phenoxide ring to a lowest unoccupied MO predominantly on the nitrogen atom. 2 By doping with the appropriate dyes, the emission can be tuned through the visible spectrum, 3,4 leading to the possibility of full color OLED displays. The basic processes involved are reasonably well understood, 5,6 with electroluminescence involving generation of singlet excitons from electron-hole recombination. From spin statistics, theoretical calculations, and experiment, 7 the singlet:triplet exciton ratio is 1:3, limiting the maximum internal OLED efficiency from fluorescent materials to e25%. Nevertheless, by doping with heavy atom-containing complexes, such as platinum(II) octaethylporphyrin (PtOEP), it is possible to capture triplet excitons (or charge carriers) to give electrophosphorescence, increasing internal quantum yields dramatically. 8 Although of interest for display applications, efficient exploitation requires detailed knowledge of the properties of the lowest triplet state of Alq 3 , which is not available. We are reporting here what we believe to be the first detailed characterization of this state.There is relatively little detailed information on Alq 3 photophysics. 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. The triplet state of Alq 3 is characterized in the same way.Following pulse radiolysis 15 of an Ar saturated solution of Alq 3 (2.25 × 10 -4 M) and biphenyl (0.01 M) in benzene, the absorption of the biphenyl triplet state 16 was observed, with a maximum below 380 nm. This decayed within a few microseconds to be replaced by a new absorption maximum around 510 nm (Figure 1). This is similar to spectra reported by Ballardini et al. 11 for triplet states of various heavy metal 8-quinolinol chelates and is assigned to the Alq 3 triplet state, which decayed by good first-order kinetics with a lifetime τ ) 56.3 ( 0.4 µs. From the analysis of transient diffusion measurements in doped films, 12 a lifetime of 25 ( 15 µs was estimated for this triplet state. In contrast, a value of ca. 10 ms was reported from delayed luminescence measurements on polycrystall...

Magnetic‐Field Effects in Organic Semiconducting Materials and Devices

“…[29] Figure 6b shows the MFE PC for three typical organic semiconducting materials, poly[2-methoxy-5-(2 0 -ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV), poly(3-methylthiophene) (P3HT), and fac tris(2-phenylpyridine) iridium Ir(ppy) 3 . The triplet exciton densities are about 1.0% in MEHPPV, [97] 70% in P3HT, [98] and 100% in Ir(ppy) 3 . [99] It can be seen in Figure 6b that MEHPPV shows only the positive component, due to a nearly 100% singlet exciton fraction, while P3HT contains both positive and negative components, due to significant triplet exciton fraction in the MFE PC .…”

Vibrational spectroscopic studies and computational study of quinoline-2-carbaldehyde benzoyl hydrazone

“…The experimental NAN bond length of hydrazone [78] is reported as 1.449 Å and the electron diffraction NAN bond length of tetramethylhydrazone [79] is reported at 1.401 Å. Kostova et al [80] À179.9 D (30,26,28,24) 179.5 D (30,26,28,31) 0.0 D (20,32,33,36) À0.5 D (21,32,33,36) 178.4 D (32,33,36,19) 2.0 D (32,33,36,35) 157.4 D (16,35,36,19) À6.9 D (16,35,36,33) À157.7 D (37,35,36,19) 128.1 D (37,35,36,33) À22.6 Table 5 Calculated vibrational wavenumbers (scaled), measured infrared band positions and assignments for quinoline-2-carbaldehyde benzoyl hydrazone. [89] 121.0°and 123.6°.…”

Isotope effect in the spin response of aluminum tris(8-hydroxyquinoline) based devices

“…3 recombination. In order to examine this hypothesis we exposed the Alq 3 based OLED devices to oxygen atmosphere, which is known to quench TE species 20 . Figure 4 shows the MEL(B)…”