The sensing phenomena were also monitored by fluorescence spectroscopy. As shown in Fig. 3, receptor 1 alone and other anions all displayed weak fluorescence emission bands except for CN−. Upon addition of CN−, receptor 1 exhibited a prominent fluorescence enhancement and accompanied by a new major band at 425 nm with two shoulder bands at 490 nm and 527 nm, respectively. The fluorescent enhancement efficiency observed at 425 nm was 2.5-fold greater than that of the control in the absence of CN−. This significantly enhanced fluorescence intensity was probably caused by the nucleophilic addition of CN− to imine functional group of the receptor 1 to afford a receptor 1–CN adduct.