5-Phenyl-1,3,4-thiadiazole-2-amine has been synthesized by single step reaction. A series of heterocyclic azodyes were synthesized by diazotisation of 5-phenyl-1,3,4-thiadiazole-2-amine by nitrosyl sulphuric acid followed by coupling with different coupling compounds such as 8-hydroxyquinoline, 2,6-diaminopyridine, 2-naphthol, N,N-dimethyl aniline, resorcinol, and 4,6-dihydroxypyrimidine. The dyes were characterized by UV-Vis, IR, 1H-NMR, 13C NMR, and elemental analysis. The synthesized compounds were also screened for biological activity. 1. Introduction Azo dyes are the most widely used class of colouring materials because of their massive applications in various fields of science and technology [1–3]. The azo dyes are synthesized by diazotization of aromatic amines and coupling reagent, which include one or more azo groups (–N=N–) attached to one or more aromatic moieties (Karci et al. [4]). These dyes play a major role in textile, printing, leather, papermaking, drug (Torres et al. [5]) and food industries (Yousefi et al. [6]). Heterocyclic azo dyes have wide applications as high level-dying agents in the dyestuff industries (Hallas and Choi [7]). It has been known that the activity of azo linkage increases with the incorporation of suitable heterocyclic moiety. The increasing usage of these dyes in electronic industry, such as colorimetric sensors, nonlinear optical (NLO) devices and liquid crystalline displays (LCDs) used as potential sensitizers for photodynamic therapy (PDT) has attracted much attention (Demirbas et al. [8]). Nowadays, much attention has been focused on 1,3,4-thiadiazole derivatives as a very important class of nitrogen-containing aromatic heterocyclic compounds due to their diverse biological activities such as antitumor [9, 10], antibacterial [11, 12], anti-inflammatory, antimycotic (Fernandez et al. [13]), and powerful antifungal agents (Waring and Hallas [14]). In contrast, 1,3,4-thiadiazole derivatives exhibits a broad spectrum of biocidal activities possibly due to the presence of toxophoric-N-C-S moiety (Mavrova et al. [15]). With these objects in view and also work carried out in our lab on this class of azo dyes [16, 17], we now focus on synthesis and screening for antimicrobial and antioxidant activities of 5-phenyl-1,3,4-thiadiazole-2-amine containing azo group in their structure. 5-phenyl-1,3,4-thiadiazole-2-amine was synthesized by single step reaction and it was transformed to its corresponding diazonium salt by diazotization reaction and was further coupled with various coupling agents (8-hydroxy quinoline,
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