Article pubs.acs.org/JPCC
Electron Transport Materials for Organic Light-Emitting Diodes: Understanding the Crystal and Molecular Stability of the Tris(8hydroxyquinolines) of Al, Ga, and In José C. S. Costa,*,† Carlos F. R. A. C. Lima,†,‡ and Luís M. N. B. F. Santos*,† †
CIQ, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, P-4169-007 Porto, Portugal QOPNA, Departamento de Química, Universidade de Aveiro, P-3810-193 Aveiro, Portugal
‡
S Supporting Information *
ABSTRACT: An experimental and theoretical study on the phase and molecular stability of Mq3 (M = Al, Ga, and In; q = 8-hydroxyquinoline) is presented. The results suggest that the energy difference between the meridional and facial isomers of Mq3 shall be significantly lower than previously though and provide an explanation for the reported mer−fac solid−solid transition. The first accurate vapor pressure measurements for Alq3, Gaq3, and Inq3 are presented, which are of great relevance for the manufacturing of thin films by vacuum deposition. Phase transition thermodynamics indicates higher cohesive energies of the mer-isomers and an entropic differentiation that can be associated with the different molecular symmetry of the mer- and facisomers.
1. INTRODUCTION Materials based on tris(8-hydroxyquinoline) belong to one of the most important classes of electroluminescent/electron transport materials largely used as constituents of organic light emitting diodes (OLEDs).1−3 With the formula M(C9H6NO)3, generally abbreviated Mq3, they are coordination complexes wherein the trivalent metal, M(III), is bonded to the conjugate base of three bidentate 8-hydroxyquinolinate ligands (Hq). Among the Mq3, tris(8-hydroxyquinoline)aluminum(III), Alq3, was the first to be used in OLEDs, since the pioneering work of Tang and VanSlyke.4 This compound is frequently used in organic electronics because of its thermal stability; ease of synthesis, purification, and deposition; and exceptional electron transport and electroluminescent properties as an organic thin film.5−7 According to the reports, tris(8-hydroxyquinoline)gallium(III), Gaq3, and tris(8-hydroxyquinoline)indium(III), Inq3, also provide good electroluminescence yields, which makes them promising alternatives as emitting and charge transport materials.8−10 It is well-established that Mq3 are octahedral complexes and can occur in two different stereoisomers: meridional (mer) and facial ( fac) (Figure 1). The mer-isomer belongs to the C1 symmetry point group, while the fac-isomer has one C3 rotation axis, thus belonging to the C3 symmetry point group. The isomerization of Mq3 is an important issue with respect to the stability and emission behavior of OLED devices; for instance, fac-Alq3 presents blue emission, instead of the typical green emission of mer-Alq3.11−13 Crystallographic studies and spectroscopic characterization have shown the existence of different stable crystalline phases of Alq3: α, β, γ, and δ.14−19 According to these reports, α and β © 2014 American Chemical Society
Figure 1. Structural configuration of the meridional (mer) and facial ( fac) isomers of tris(8-hydroxyquinoline)M(III): mer-Mq3 and facMq3; M = Al, Ga, In.
polymorphs contain the mer-isomer, while the polymorphs γ and δ consist of the fac-isomer. In OLEDs, fac-isomers are particularly desirable due to their blue-shifted fluorescence and high quantum yield.11,13,17 Gaq3 was found to be isostructural with Alq3, presenting only slightly different lattice parameters.20 The mer-isomers of Alq3 and Gaq3 are observed at ambient conditions and can be converted to the fac-isomers by annealing at T > 650 K.20−22 The case of Inq3 is more controversial, with the fac-isomer being easily observed at ambient conditions.23 There is a crucial, and still unsettled, Received: April 22, 2014 Revised: August 26, 2014 Published: August 27, 2014 21762
dx.doi.org/10.1021/jp503935k | J. Phys. Chem. C 2014, 118, 21762−21769
The Journal of Physical Chemistry C
Article
deionized water; solution C, 1.11 g of KOH (0.0198 mol) in 25 mL of deionized water] and for Inq3 [solution A, 0.97 g (0.0067 mol) of 8-Hq powder into 25 mL of ethanol; solution B, 0.41 g (0.0019 mol) of indium chloride (InCl3) in 25 mL of deionized water; solution C, 1.06 g of KOH (0.0189 mol) in 25 mL of deionized water]. The precipitates of Alq3, Gaq3, and Inq3 were washed with methanol and purified by sublimation under reduced pressure (
