Solvothermal synthesis of novel phenylpyrazole Schiff base fluorescent insecticides fused extended conjugate units for enhancing bioactivities, photophysical and electrochemical properties

In order to find a new type of fluorescent insecticides, a series of phenylpyrazole Schiff base derivatives have been synthesized by a facile one-pot solvothermal method based on the amino group of fipronil addition-elimination condensation reaction with the corresponding benzaldehyde derivatives in high yields, which were fully characterized by 1 H NMR, IR, MS and elemental analysis . Crystal structure determination indicated that 1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-5-(4-fluoro-benzylidene-amino)-4-(trifluoromethylsulfinyl)-1 H -pyrazole-3-carbonitrile ( 7 ) crystallized into zigzag chains by π···π packing interactions between phenyl rings of neighbouring molecules. The investigation for insecticidal bioactivities of these pesticides against 3rd instar larvae of Plutella xylostella exhibited better activities than that of fipronil, which can be elucidated by enhanced calcium ions' coordination abilities with C N double bonds and chloride ions , extended conjugated parent structure, steric hindrance of the stereoscopic structure and electronegativity of substituted groups. The electronic absorption, fluorescent and electrochemical properties of phenylpyrazole Schiff base derivatives have been systematically investigated for the first time, which all emitted blue-green light in both solution and solid state at room temperature . The relationships between extended conjugated moieties of parent structure, positions of substituent group and the kinds of substitutes at 4-position on the phenyl ring and photophysical properties have been discussed, which was confirmed by electrochemical analysis . The alteration of absorption and emission wavelengths can be rationalized by substitutes' electronegativity and Substituents’ Hammett constants. The fluorescent quantum yields of phenylpyrazole Schiff base insecticides were almost 4 times that of Fipronil, which laid a foundation for their natural degradation and fluorescence detection of pesticide residues . This approach proposes a unique insight to provide a great number of novel phenylpyrazole Schiff base fluorescent insecticides by a general green method.