charge distribution induced by the hydrogen bonds between CN−and HNA could be responsible for the “turn-on” fluorescence and the red-shift of emission peak from 462 nm to 504 nm. Absorption spectral changes of HNA in the presence of various anions provided further evidence to above speculation. Upon addition of CN−, the absorption band at 350 nm diminished, while a new red-shifted absorption band appeared at 450 nm (Supplementary data, Figs. S8 and S9).