Dual emissive dinuclear Iridium(III) azomethine complexes: Synthesis, luminescence, thermal stability and antibacterial studies

Highlights: • Three new dinuclear Ir(III) azomethine [N,N,O,O] complexes. • Structure elucidation through elemental analysis, IR, ¹H and ¹³C NMR, UV–Visible, EDX and PXRD. • Single crystals X-ray analysis of HL² azomethine ligand showing significant twisting about central S producing a warped molecule with [N,O] moieties on opposite sides. • Photoluminescence investigation in solid and liquid states demonstrating dual emissive properties in the solid state with large Stoke's shifts. • Evaulation of thermal stabilities and bactericidal studies. A set of three novel dinuclear Ir(III) azomethine complexes was effectively synthesized from Schiff bases namely N,N-bis-(3,4-dihydroxybenzaldehyde)-4,4-diaminodiphenylsulphide (HL¹), N,N-bis-(2-hydroxy-5-nitrobenzaldehyde)-4,4-diaminodiphenylsulphide (HL²) and N,N-bis-(2-hydroxy-5-chlorobenzaldehyde)-4,4-diaminodiphenylsulphide (HL³). Structural elucidation was carried out through (CHNS) elemental analysis, FT-IR, NMR, energy dispersive X-ray diffraction (EDX), powder X-ray diffraction (PXRD), UV–Visible and photoluminescence spectroscopy. The FTIR and NMR spectra showed that the ligands contain dioxygen-dinitrogen donor atom system, forming dinuclear complexes IrHL¹, IrHL² and IrHL³ upon coordination with Ir(III) ion. X-ray analysis of a single crystal HL² confirmed the [NNOO] donor system and demonstrated a bending geometry with the torsion angle of 50.98° for C10–C11–S1–C14 linkage as the result of a significant twist at the central sulphur. The low molar conductivity values revealed the absence of electrolytic anions and cations in the complexes. The powder X-ray diffraction studies indicated the presence of crystalline phase in all the Ir(III) complexes. Photophysical characteristics of the compounds were thoroughly investigated in solid as well as liquid states at ambient conditions. The solid complexes manifested two strong emissions in the visible and near infra-red (NIR) regions. Primarily, at the visible region of 610–615 nm, the emission peak is assigned to ligand centered photoluminescence. The secondary emission peak was detected in the NIR region at 1119–1177 nm, ascribed to the triplet excited state emission of the iridium center. All the synthesized complexes were subjected to thermogravimetric analysis revealing thermal stability exceeding 230 °C before undergoing decomposition. Remarkable solid state photoluminescence activities of IrHL³ and IrHL², in that order, was exhibited through the Stoke's Shift (Δλ) values of 285 and 289 nm, respectively, consequently merit the complexes as promising keystone for luminescent materials in solid state. The bactericidal activity of the test compounds was investigated on three Gram-positive and three Gram-negative bacteria, unveiling mild to moderate antibacterial activity. The IrHL² complex with p-nitro substituent showed the highest activity against Streptococcus mutant, Staphylococcus aureus and Pseudomonas aeruginosa, succeeded by IrHL³ complex with p-chloro substituent for Streptococcus mutant.