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

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Abstract

Color-tunable Zn(II) complexes of the type Zn( N,O-OPh (OxZ)ArX) 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 = NMe 2 a, OMe b, Ph c, Cl d, F 2 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, NMe 2 < OMe < Ph < Cl < F 2 < 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/m (2) 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/Alq 3/LiF/Al with an efficiency of 7000 cd/m (2) at 13 V with 2.0 cd/A.

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