Preparation, Antibacterial Activity and Absorption Spectra of Pyrazolo-Oxadiazine Cyanine Dyes Fayez M. Eissa Chemistry Department, Aswan Faculty of Science, Aswan 81528, Egypt A new pyrazolo[4,3-e]-1,3,4-oxadiazine heterocyclic ring system was prepared and employed to syn- thesize cyanine dyes of different methine types, including monomethine (simple cyanine) and trimethine (carbocyanine). The antibacterial activity of all the newly synthesized compounds was investigated against some bacterial strains. The electronic absorption spectra of the dyes were studied in absolute etha- nol. The structure of all the synthesized compounds was identified via elemental and spectroscopic analy- sis. Keywords: Cyanine dyes; Pyrazolo-oxadiazine; Synthesis; Antibacterial activity; Absorption spectra; Monomethine; Trimethine. 1. INTRODUCTION Fused 1,3,4-oxadiazine heterocyclic systems are an important class of organic compounds which have wide ap- plications, especially in antimicrobial activity, as reported in our previous work. 1 Also, cyanine dyes 2,3 are of great im- portance due to their useful applications, such as in nucleic acid detection, 4 as optical sensors, 5 photovoltaic devices, 6 near infrared laser dyes, 7 histological staining, 8 and dyes for polymers as well as sensitizers for various silver halide emulsions. 9 Hence, intense research activity has been de- voted to report new heterocyclic ring systems with antibac- terial activity, 10-15 and their derived cyanine dyes of relative stability, high molar extinction coefficients and high fluo- rescent intensity. This work reports the synthesis of newly synthesized cyanine dyes incorporating a pyrazolo-[4,3-e]-1,3,4-oxa- diazine heterocyclic ring system as a new synthesis contri- bution in this important field and investigates the antibacte- rial activity of all the synthesized derivatives against se- lected bacterial strains. Also, this work reports the elec- tronic absorption spectra of the newly synthesized cyanine dyes in absolute ethanol in order to elucidate their photo- sensitization activity. 2. RESULTS AND DISCUSSION 2.1. Synthesis 4-Bromo-3-methyl-1-pheny-pyrazol-5-one (1) was reacted with a bimolar ratio of acetic acid azide of hydra- zine or phenyl hydrazine (2a,b), in ethylene glycol as a sol- vent and sodium bicarbonate as a basic catalyst, and af- forded 2,5-dimethyl-4H(phenyl)pyrazolo[4,3-e]-1,3,4- oxadiazine (3a,b), Scheme I. Quaternization of (3a,b) using iodomethane pro- duced the 3,6-diquaternized compounds (4a,b). Further re- action of the 3,6-diquaternized compounds (4a,b) with a bimolar ratio of N-methylquinolinium iodide salt, in etha- nol and the presence of piperidine as catalyst, achieved the 2,5(4)-bis-monomethine cyanine dyes (5a,b) (Scheme I). Additionally, the 3,6-diquaternized compounds (4a,b) were reacted with a bimolar ratio of triethylorthoformate, in ethanol containing piperidine as a basic catalyst, and achieved the 2,5-di(2-diethoxy-ethane) compounds (6a,b) as intermediates. Further reaction of the intermediates (6a,b) with a bimolar ratio of N-methylquinaldinium io- dide salt, in ethanol and the presence of piperidine, resulted in the 2,5(2)-bis-trimethine cyanine dyes (7a,b), Scheme II. 2.2. Antibacterial activity The antibacterial activity of some selected newly syn- thesized compounds, as representatives, against some bac- terial strains, Pseudomonas, Serratia, Bacillus cereus, Escherichia coli, and Staphylococcus citrus, was investi- gated and the inhibition zone diameter IZD is recorded in Table 1. The results revealed that the selected compounds Journal of the Chinese Chemical Society, 2009, 56, 843-849 843