Given that 3 shows a response in the presence of Al 3+ as well as Fe 3+ , we also sought to determine if 3 could also selectively detect Fe 3+ in the presence of the same competing metal ions. A mixture of 12 competing metal ions (each 1 equiv.) was thus added to 3, followed by titration of this solution with Fe 3+ (Figure ). Addition of the competing ions yielded a clear shift in emission wavelength from 461 to 411 nm and an increase in fluorescence intensity (F/F0 = 1.98). This indicates formation of the 3/Al 3+ complex that displays enhanced fluorescence intensity, with partial quenching of this complex by the other metal ions present. Stepwise addition of Fe 3+ subsequently gave significant decreases in fluorescence intensity (F/F0 at 1 equiv. of Fe 3+ = 0.57; F/F0 at 5 equiv. of Fe 3+ = 0.097). This demonstrates that 3 is also able to act as a selective fluorescent sensor for Fe 3+ . However, in the presence of Al 3+ , this occurs via initial formation of a 3/Al 3+ complex, with the fluorescence of this new species subsequently dynamically quenched by Fe 3+ . The limits of detection of 3 and 5 for Fe 3+ were subsequently calculated, both in the presence and absence of competing metal ions (see ESI). The significant quenching effect of Fe 3+ on 5 allows for detection limits in the sub-micromolar range. A detection limit of 0.39 μM was calculated in the absence of competing ions, and of 0.48 μM in the presence of competing ions. The increased fluorescence intensity of 3 relative to 5 allows for even lower limits of detection; a detection limit of 0.13 μM was calculated in the absence of competing ions, and of 0.16 μM in the presence of competing ions. These values are in line with the detection limits of other pyrazole-based probes for Fe 3+ . Furthermore, to our knowledge, the Stokes shifts of 3 and 5 in MeCN/H2O (1:1) are the largest reported Stokes shifts to date for a fluorescent phosphorus-based metal sensor.