In this study, formazans 5a-5h were synthesized by coupling the reactions of substituted phenylhydrazone compounds 3a-3h with diazonium salt of 4-chloro-2-phenoxybenzenamine (4). The substituted phenylhydrazones 3a-3h were obtained from the condensation of substituted phenylhydrazines 1a-1h with 4-methoxybenzaldehyde (2). The structures of the formazans were characterized by using elemental analysis, FTIR, 1 H NMR, 13 C NMR, LC-MS and the crystal structure of compound 5e was determined by X-ray crystallography. The absorption spectra of all compounds were investigated in various solvents. Although their absorption bands are only slightly dependent on the polarities of solvents, the significant change was observed by altering the electronic characteristics of the substituents. The fluorescence quantum yield properties and Stokes shifts of compounds 5a-5h in DMSO were also explored. Electrochemical properties of the new products were studied by CV measurements. In addition, computational studies were conducted at the PBE1PBE/6-311g (2d,2p) level to shed light on the structures of possible tautomers and intramolecular H-bonds.Scheme 1 Synthetic route for the formation of formazan derivatives 5a-5h.Scheme 2 Intramolecular hydrogen bonding and tautomerisation of the formazan system. View Article Onlinea Oxidation and reduction potentials from cyclic voltammograms. b Onsets of oxidation and reduction. c HOMO = À(E onset (ox) + 4.4) (eV), LUMO = À(E onset (red) + 4.4).Scheme 4 (a) Schematic representation of the structure of 5c for possible oxidation mechanisms of 5c, 5d, 5g and 5h; (b) structure of 5a for possible oxidation mechanism of 5a, 5b, 5e and 5f.

New phenylsulfanyl and aryl carbonyl containing formazan derivatives 4a–4h and 5a–5h have been developed, and investigated the effect of substituents on their photophysical and electrochemical properties.

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The acid dissociation constants (pK a ) of eight biologically active pyridyl Shiff bases were determined using a UV-vis spectroscopic technique at 25 ( 0.1 °C. The first acidity constants (pK a1 ) of the 2- 7), and 2-[2-aza-2-(5-methyl-(2-pyridly))vinyl]-4,6-dinitrophenol (8) are found to be associated with the protonation of phenolate oxygen. The second acidity constants (pK a2 ) are found to correspond to protonation of pyridine nitrogen for compounds 5, 6, and 7 and oxoprotonation for molecules 1, 2, 3, 4, and 8. The third acidity constants (pK a3 ) are found to be associated with the protonation of pyridine nitrogen for compounds 1, 3, 7, and 8 and amino protonation for molecules 2 and 5. For molecules 4 and 6, it is associated with oxo protonation. The contribution of the ketoamino tautomeric form was found to be considerably important.