Aluminium–salen luminophores as new hole-blocking materials for phosphorescent OLEDs

Abstract

The organic light-emitting diodes (OLEDs) employing complex [salen(tBu)4Al(OC6H4-p-C6H5)] (4) as a hole-blocking layer produced stable green EL emission of Ir(ppy)3 irrespective of changing current density and showed higher brightness and device efficiency than the BAlq-based device.

Synthesis and Properties of Salen–Aluminum Complexes as a Novel Class of Color‐Tunable Luminophores

“…In contrast to the high solubility of 1-6 in common organic solvents such as toluene, THF, and chloroform, due to its ionic nature 7 is soluble only in more polar solvents such as acetonitrile. All the complexes 1-7 are stable in air in both the solid and solution states, similarly to the aryloxide derivatives of 2, [21] probably because of the steric protection of the Al center by two tert-butyl groups as well as the strong bonding of the 4-phenylphenoxy ancillary ligand to the Al center. The identities of 1-7 have been fully characterized by 1 H, 13 C NMR spectroscopy, and elemental analysis.…”

“…TGA measurements exhibit high T d5 values (5 wt % decomposition temperatures) of 313-338 8C, indicating high thermal stability of the salen-Al complexes. It is also known that a large difference in T d5 values ( % 100 8C) is observed for the derivatives of 2, upon changing an aryloxy ancillary ligand, [21] and the tetracoordinate salen-Pt complexes, which lack the ancillary ligand show higher T d5 values ( % 400 8C); [19] therefore the similar high T d5 values for 1-7 may indicate not only that the salenAl bond is thermally robust, but also that the thermal stability originates mainly from the pentacoordinated bonding around the Al center. Most interestingly, DSC analyses reveal that the complexes are able to form amorphous glasses with reasonably high glass transition temperatures (T g = 95-132 8C) upon cooling of melt samples.…”

“…[13,21] The absorption maximum wavelength (l abs ) is gradually red-shifted as the substituent is varied from H to NMe 2 (from 1 to 6). These red shifts upon electronic alteration at the C5 position of the phenoxide ring in the salen moiety are quite similar to those observed for the 8-hydroxy-A C H T U N G T R E N N U N G quinoline-based aluminum luminophores, [4,5] as well as the recently reported salophen-Zn (H 2 salophen = N,N'-phenylene-bis(salicylidene)phenylenediamine) [33] and zinc bis(2-oxazolylphenoxide) [34] derivatives.…”

“…The bond between the Al atom and the 4-phenylphenoxy group is nearly perpendicular to the salen-Al coordination plane, revealing distorted pentacoordination geometry around the Al center. Geometric analysis with the index of the degree of "trigonality" t = (bÀa)/60 (t = 0 for perfectly square-pyramidal geometry and t = 1 for perfectly trigonal-bipyramidal geometry) [28,29] for 1, 2, [21] 4, 6, and 7 afford t values of 0.38, 0.61, 0.36, 0.44, and 0.19, respectively, indicating that the complexes 1, 4, 6, and 7 adopt the square-pyramidal geometry typically observed for other salen-Al complexes [11, 26-28, 30, 31] whereas 2, containing a 5-tertbutyl group, is more in favor of trigonal-bipyramidal geometry. The Al-O3-C biph angles of 126.3-136.58 and the Al-O3 bond lengths of 1.749-1.766 are in a similar range to those found in salen-Al phenoxide derivatives.…”

“…Furthermore, the salen ligands have often been employed in the phosphorescent emitters based on platinum. [17][18][19][20] All these reports have prompted us to consider whether the salen-Al complexes could be utilized as versatile materials for OLEDs; our group has recently demonstrated that the pentacoordinated salen-Al complexes with an aryloxy ancillary ligand can serve not only as efficient hole-blocking materials [21] for phosphorescent OLEDs, but also as hosts in the dopant assembly OLEDs based on a heterodinuclear Al/Ir complex. [22] They found that the salen-Al complexes possess suitable HOMO and LUMO energy levels, high thermal stability, and high PL efficiency, all of which may render them promising as emitting materials in OLEDs.…”