Generation of blue light-emitting zinc complexes by band-gap control of the oxazolylphenolate ligand system: Syntheses, characterizations, and organic light emitting device applications of 4-coordinated bis(2-oxazolylphenolate) zinc(II) complexes

Color-tunable Zn(II) complexes of the type Zn(N,O-OPh^OxZArX) ₂ (5), where the ligand consists of an oxazolylphenolate ion connected at the 4-position by a 2,4-substituted aryl functional group with X = NMe₂ a, OMe b, Ph c, Cl d, F₂ e, and CN f, were prepared. X-ray structural studies of 5a, 5b, and 5e showed that a zinc atom was positioned in a distorted tetrahedral coordination environment created by two oxazolylphenolate ligands with N,O-chelation. Hammet plots of absorption and emission maxima, respectively, in UV and photoluminescence (PL) spectra with respect to electron-donating and electron-withdrawing groups of the substituents indicate a direct correlation between the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) band gaps and electronic alterations at the ligand sites. A similar correlation was also observed for the reduction and oxidation potentials in cyclic voltammograms (CVs). A gradual increase in the HOMO-LUMO band gap is seen from electron-donating to electron-withdrawing functional groups, NMe₂ < OMe < Ph < Cl < F₂ < CN. An emission peak with a maximum at 455 nm was achieved when the most electron-withdrawing group (cyano) was applied to the oxazolylphenolate ligand system. Density-functional theory (DFT) calculations on the HOMOs and LUMOs for this series lead to a conclusion similar to that arrived at from a blue-shift trend observed in UV data and trends in the CVs. The 4-coordinated zinc complex (5c) was shown to be a potential blue-emitting material, exhibiting a maximum efficiency of 1720 cd/m² at 17 V with 0.3 cd/A in a multilayered device structure of ITO/NPB/5c/BCP/Alq ₃/LiF/Al. On the basis of the low HOMO level of this series, 5a was tested as a hole-transporting material; this resulted in the successful fabrication of a multilayered device of ITO/5a/DPVBI/Alq₃/LiF/Al with an efficiency of 7000 cd/m² at 13 V with 2.0 cd/A.