Zinc (II) complexes with Schiff bases obtained from N-[2-(cyclohexyliminomethyl)- or 2-(4-cyclohexylphenyliminomethyl) phenyl]-4-methylbenzenesulfonamides and their application as highly luminescent blue emitters for OLEDs

Abstract

Two new Schiff base compounds of N-{2-[(E)-сyclohexyliminomethyl]phenyl}-4-methylbenzenesulfonamide, N-{2-[(E)-(4-сyclohexylphenyl)iminomethyl]phenyl}-4-methylbenzenesulfonamide and their Zn(II) complexes have been synthesized and characterized by elemental analysis, FT-IR and UV–Vis spectra, and single crystal X-ray determination. In both complexes, Zn2+ ions have a tetrahedral environment with two nitrogen atoms of the tosylamide groups and two nitrogen atoms of the imine fragment. Time-dependent density functional theory calculations have been performed on two zinc(II) complexes in order to assign their experimental UV–visible absorption bands. Zinc(II) complexes showed thermal stability up to 335–340°C under a nitrogen atmosphere by thermogravimetric analysis (TGA). The photoluminescent spectra show that both Zn(II) complexes in the solid state at room temperature emit blue luminescence with high emission quantum yields of 20% and 29%. The doped devices with configurations of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD)/4,4′-N,N′ -dicarbazolebiphenyl (CBP):Zinc(II) complex (5%)/1,3,5-tris(N-phenylbenzimidazole-2-yl) benzene (TPBI)/LiF/Al have been fabricated and investigated. The doped device based on the complex with the сyclohexylphenyl substituent of the ligand showed the best electroluminescent characteristics with maximum brightness Lmax of 3415 cd/m2, maximum current efficiency of 2.8 cd/A, and power efficiency of 1.9 lm/W, while the doped device with emitter on the base of the complex with the сyclohexyl substituent showed slightly worse electroluminescence (EL) performance with Lmax of 2105 cd/m2, maximum current efficiency of 2.1 cd/A, and power efficiency of 1.6 lm/W.