Synthesis and characterization of 3,5-bis((2-hydroxybenzylidene)amino)-N-(2-hydroxyphenyl)benzamide and Zn(II) complex: Investigation of chromic, fluorescence and DPPH radical scavenging behaviours

Novel amido-Schiff base 3,5-bis((2-hydroxybenzylidene)amino)-N-(2-hydroxyphenyl)benzamide (2a) has been obtained by the reaction of Schiff base 1a with 2-hydroxyaniline in the presence of coupling agent N,N′-dicyclohexylcarbodiimide. Schiff base (1a) which serves as the starting compound in amide synthesis was produced from the condensation of 3,5-diaminobenzoic acid with salicylaldehyde in a 1:2 mol ratio. Tetrahedral geometry with 1:1 [M:L] stoichiometry Zn(II) complex (3a) was newly synthesized by the complexation reaction of ligand 2a with zinc(II) nitrate. 1a and 2a displayed a broad emission band at λ_em = 456 and 472 nm with a blue fluorescent in pure DMSO, while 3a emitted a strong greenish-blue fluorescent with the λ_em = 485 nm (λ_exc = 365 nm). The fluorescence efficiency of 1a was poor compared to the others. The λ_em in aqueous DMSO solution (v/v, 1:1) had ∼8–33 nm red-shift with respect to that of pure solution. The λ_em gave ∼1–15 nm red-shift with very low emission efficiency in acidic DMSO media. The results suggested that the aggregation caused quenching properties of 1a–3a in pure, aqueous, and acidic DMSO system with a 50 % water fraction. The fluorescence of the compounds appeared to be visibly very bright in basic DMSO media. All the compounds were also screened for antioxidant activity using UV–Vis spectroscopy. They were found to be not effective DPPH radical scavengers in DMSO.