Two low cytotoxic fluorescence probes Rb1 and Rb2 detecting Fe3+ were synthesized and evaluated. Rb1 and Rb2 exhibited an excellent selectivity to Fe3+, which was not disturbed by Ag+, Li+, K+, Na+, NH4+, Fe2+, Pb2+, Ba2+, Cd2+, Ni2+, Co2+, Mn2+, Zn2+, Mg2+, Hg2+, Ca2+, Cu2+, Ce3+, AcO−, Br−, Cl−, HPO42−, HSO3−, I−, NO3−, S2O32−, SO32− and SO42− ions. The detection limits were 1.87×10−7M for Rb1 and 5.60×10−7M for Rb2, respectively. 1:1 stoichiometry and 1:2 stoichiometry were the most likely recognition mode of Rb1 or Rb2 towards Fe3+, and the corresponding OFF–ON fluorescence mechanisms of Rb1 and Rb2 were proposed.