To develop a variety of algaecides for harmful algae control that cause water pollution, we synthesized a series of 79 derivatives based on the structure of N1-benzyl-N3, N3-diethylpropane-1,3-diamine and analyzed their structure-activity relationships toward a harmful and a harmless algal species. Among the derivatives, the best algicidal activities with LC50 values of 0.49 (compound 24) and 0.42 µM (compound 34) in Cyanobacteria, respectively were achieved when CH3 and F groups were substituted at the R2 position, with CH3CH3 group at the R3 position, and amine chain length 3 at the R4 position after fixing Cl substituent group at the R1 position of the benzaldehyde group. The compounds 24 and 34 decreased the chlorophyll content of treated groups by approximately 75–80% as compared to the control. Algicidal activities for harmful algal species were as follows, in the order of greatest to the least: Microcystis sp. > Microcystis aeruginosa > Hererocapsa circularisquama > Chattonella marina > Heterosigma akashiwo. The acute toxicities of compounds 24 and 34 against Daphnia magna showed EC50 values of 20.18 and 22.32 µM, respectively, which were approximately 46.4 and 44.69 times higher than the LC50 value against Microcystis sp., showing low ecotoxicity. In the case of Danio rerio, they showed very low ecotoxicity with EC50 values of 94.97 and 45.97 µM, respectively. These results indicate that compounds 24 and 34 could be potential agents for selectively controlling harmful algal blooms.
