The intriguing observation of this selective ZnCl2 sensing prompted us to explore the underlying mechanism. We first investigated the detection limit of ZnCl2, which is correlated with the concentration of the emitter. As shown in Figure , the fluorescence spectra of different concentrations of TPAPyBP in a mixture of ethanol and toluene (0.0012/1 v/v) upon addition of 1 equivalent of ZnCl2 were measured. As the concentration of TPAPyBP:ZnCl2 (1:1 equiv.) increases, the fluorescence intensity at 555 nm increases until the concentration reaches 1.3´10 M. When the concentration increases further, the intensity of 555 nm emission band decreases while concomitantly a new emission band at 680 nm emerges and gradually becomes the principal emission band, reflecting the observed color change from green to deep red (Figure and).