Zn(II) complexes based on N-[2-(Hydroxyalkyliminomethyl)Phenyl]-4-Methylbenzenesulfonamides and their application as luminescent emitters for blue organic light-emitting diodes

Abstract

This work presents the synthesis and comprehensive study of a novel series of zinc(II) coordination compounds based on azomethine ligands derived from N-[2-(hydroxyalkyliminomethyl)phenyl]-4-methylbenzenesulfonamides. The ligands incorporate aliphatic spacers of varying length (CH₂)ₙ (n = 2–6), enabling a systematic investigation of the spacer’s influence on the structural, thermal, photophysical, and electroluminescent properties of the resulting complexes. The complexes were characterized by elemental analysis, and IR and NMR spectroscopy. Single-crystal X-ray diffraction analysis confirmed the formation of chelate complexes, where the Zn(II) ion adopts a distorted tetrahedral geometry, coordinated by two bidentate ligands via the nitrogen atoms of deprotonated sulfonamide and azomethine groups. Elongation of the methylene spacer does not significantly alter the core coordination geometry (Zn–N bond lengths, bond angles) or the dominant intermolecular interactions (hydrogen bonds, π–π stacking) governing crystal packing. These structural findings are strongly supported by density functional theory (DFT/B3LYP) calculations. In the solid state, the complexes exhibit intense blue photoluminescence with emission maxima ranging from 435 to 466 nm. The photoluminescence quantum yields (PLQY) vary from 7.4 to 21.5%, with excited-state lifetimes of 4.6–5.5 ns. The most promising complexes, featuring longer alkyl spacers (n = 4–6), were successfully implemented as emitting layers in organic light-emitting diodes (OLEDs) with the architecture ITO/MoO₃/mCP/emitter/TSPO1/TPBi/LiF/Al. All fabricated devices emitted blue electroluminescence with CIE coordinates in the blue region. The OLEDs achieved maximum luminance values up to 1017 cd m⁻², current efficiencies up to 4.4 cd A⁻¹, and external quantum efficiencies (EQE) up to 2.2% at turn-on voltages of approximately 5 V. The device based on the complex with the hexamethylene spacer (n = 6) exhibited the best overall performance. These results underscore the potential of this class of zinc(II) azomethine complexes as efficient blue emitters for OLED applications.