The response of compound 2 to CN− (Fig. S4a) is similar to that of 1 and the absorbance at 422 nm slightly decreases from 0.47 to 0.25, upon the addition of CN−. The detection limits (Fig. S5) are 2.42 μM (1) and 0.51 μM (2) with absorption as detected signal, respectively. The detection limits (are 0.0004 μM(1) and 0.0002 μM(2)with fluorescence as detected signal, respectively.