As expected, the corresponding absorption spectrum exhibits a new band at 505 nm, which increases in intensity as the concentration of TPAPyBP:ZnCl2 (1:1 equiv.) increases. As shown in Figure , the detection limit of ZnCl2 is around 5.0 ´10 -5 M: at this concentration, the presence of the 1:1 adduct with TPAPyBP can be confirmed. Furthermore, we highlight the fast reaction time, which occurs within several seconds (ESI Video 1). This rapid response is highly desirable for sensing applications.